Metal pyrazolate frameworks: crystal engineering access to stable functional materials

Abstract

As the focus evolves from structure discovery/characterization (what it is) to property/performance exploration (what it is for), the pursuit of stable functional metal–organic frameworks (MOFs) has been ongoing in terms of both fundamental research and industrial implementation. Under the guidance of crystal engineering principles, a plethora of research has developed pyrazolate MOFs (metal pyrazaolate frameworks, MPFs) featuring strong coordination M–N bonding. This attribution helps them retain their structures and functions under the alkaline conditions required for practical use. Based on poly-topic pyrazolate ligands, various classic MOFs, such as Co(bdp), Fe₂(BDP)₃, Ni₈L₆, PCN-601, and BUT-55, to name a few, have revealed fascinating architectures, intriguing properties, and record-breaking performances in applications during the past decade. This review will present the full scope of MPFs to date: (1) the superiority and significance of constructing MPFs through the crystal engineering approach, (2) synthetic strategies adopted in building and/or modifying MPFs, (3) structural features and stability of the MPF community, and (4) potential applications in energy and environmental related fields. The future opportunities of MPFs are also discussed for designing the next-generation of smart materials. Overall, this review attempts to provide insights and guidelines for the customization of pyrazolate-based MOFs for specific purposes, which would also promote the development of stable functional porous materials for addressing societal challenges.

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Xiang-Jing Kong

Xiang-Jing Kong received her PhD from Beijing University of Technology in 2020 under the supervision of Prof. J.-R. Li. From 2021, she was a postdoctoral researcher under the supervision of Prof. M. J. Zaworotko in the crystal engineering research group at the University of Limerick. She then joined Prof. O. K. Farha's group at Northwestern University as a postdoctoral researcher in 2023. Her research interest is focused on the design and synthesis of novel MOFs and their applications in adsorption, catalysis, etc.

Guang-Rui Si

Guang-Rui Si received his PhD degree from Beijing University of Technology in 2024 under the supervision of Prof. J.-R. Li. Now he is a postdoctoral researcher under the supervision of Prof. Z.-R. Nie at the same university. His research focuses on the design, synthesis, and application of functionalized MOFs.

Tao He

Tao He received his PhD from Beijing University of Technology in 2019 under the supervision of Prof. J.-R. Li. Then he pursued postdoctoral work under the supervision of Prof. Z.-R. Nie at the same university during 2019–2021 before joining Qingdao University in 2021. After completing postdoctoral research at the University of Limerick (2022–2025) under the supervision of Prof. M. J. Zaworotko, he joined Beijing University of Technology as an associate professor. His research interest is focused on stable porous materials for gas storage/separation and heterocatalysis.

Jian-Rong Li

Jian-Rong “Jeff” Li obtained his PhD in 2005 from Nankai University under the supervision of Prof. X.-H. Bu. Until 2008, he was an assistant professor at the same University. From 2008 to 2012, he worked as a postdoctoral research associate and then as an assistant research scientist at Texas A&M University in Prof. H.-C. Zhou's group. Since 2011, he has been a full professor at Beijing University of Technology. His research interests are focused on porous materials for chemical engineering, energy, and environmental science.

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1. Introduction

Metal–organic frameworks (MOFs)^1,2 have been a booming family of crystalline porous materials since the discovery of HKUST-1³ and MOF-5⁴ with permanent porosity. The modular nature of MOFs allows for their design and synthesis amenable to crystal engineering principles across a huge library of organic/inorganic building blocks.^5–8 Through decades of rapid development, MOFs have presented extraordinary structure diversity and application versatility spanning gas storage and separation, heterogeneous catalysis, sensing, biomedical diagnosis and therapy, and proton conduction.^9–18 For practical use, good stability under real conditions is a prerequisite.^19–21 Plenty of MOFs have shown great potential in applications but were eventually demonstrated unsuitable for practical use due to their limited stability. As the emphasis shifts from identifying and characterizing structures to exploring their properties and performances, the pursuit of stable functional MOFs continues to be a priority in both fundamental research and industrial applications.

Chemical stability refers to the ability of a MOF to retain crystallinity (intact coordination bonds) under chemical environments/upon chemical treatments, such as exposure to humidity/water, acids, bases, nucleophilic/electrophilic reagents, salt aqueous solutions, and so on.²⁰ The strength of coordination bonds is one of the main reasons among various thermodynamic and kinetic factors defining the stability of frameworks. According to the hard/soft acid/base (HSAB) principle, both combinations of carboxylate ligands with high-valent metals and azolate ligands with low-valent metals can form strong coordination bonds.¹⁹ Carboxylate MOFs built with high valent metal ions (i.e. Al(III), Cr(III), Zr(IV), and Ti(IV)) have attracted increasing attention due to their water/acid stability, as well as unique properties.^20,22–25 Many recent reviews have discussed the construction and application of carboxylates MOFs.^16,26–28 As their base-stable counterparts, MOFs based on azolate ligands are relatively less reported due to the limited ligand scope and immature synthetic protocols.²⁹ Zeolitic imidazolate framework-8 (ZIF-8, also named metal azolate framework-4, MAF-4), as a representative example, is composed of imidazolate linkers and Zn(II) ions. This framework exhibits remarkable stability in aqueous environments, being able to maintain its structure in an 8 M NaOH basic solution.^30,31 MAF-X27-Cl as a triazolate-based framework was able to retain its structural integrity in strong alkaline solution (1.0 M KOH) over one week.³² Reactions between pyrazolate ligands and divalent transition metal salts are expected to afford pyrazolate MOFs (metal pyrazaolate frameworks, MPFs) with higher base stability due to the higher pK_a value of pyrazoles among azoles.^33,34 Nevertheless, the strong metal–pyrazolate (M–N) bonding makes it harder to crystallize in a controlled manner and grow into big crystals before powder or even amorphous products form and precipitate.

In the past two decades, with continuous efforts, advances have been made in the design and application of MPFs. A series of polypyrazolate ligands have been designed, and a series of relevant MOFs, M(bdp) (M = Co, Fe, Zn, Ni, Cu; bdp²⁻ = benzenedipyrazolate), Fe₂(BDP)₃, Ni₈L₆, PCN-601, BUT-55, to name a few, have been synthesized, which reveal fascinating architectures, intriguing properties, and record-breaking performance in applications (Fig. 1).^35–40 Some of these materials have structures specific for azolate MOFs,⁴¹ such as M(bdp) with pts topology and BUT-55 with wuk topology, while others are isoreticular analogs of known carboxylate MOFs, including Fe₂(BDP)₃ (sct), Ni₈L₆ (fcu) and PCN-601 (ftw), highlighting the strength of crystal engineering principles in enlarging the MOF library.

Fig. 1 Timeline of representative works on MPFs.

With a better understanding of materials design principles from the perspective of crystal engineering and increasing attention on stable MOFs, research on MPFs has thrived during the last decade (Fig. 2). Although a few reviews have overviewed MPFs or their structures, no systematic review documenting MPFs has been published yet.^42–45 We intend to provide a rich palette of MPFs in this review, spanning the significance of their design and synthesis, common and distinct structure features, superior base stability and wide application scope. This review will start by classifying and introducing the structures of MPFs (building blocks and topologies), followed by summarizing synthetic strategies adopted to build and/or modify MPFs. Representative examples will be elaborated to discuss the stability of MPFs. And then, based on physical/chemical properties, potential applications will be explored. The structure–property relationship will also be discussed for insights into the underlying mechanism, which would in turn improve and perfect the design principles. Overall, this review attempts to contribute an up-to-date library for MPFs. It is expected to facilitate the design of next-generation MOFs by taking inspiration from the known structures, which could expedite the discovery of more advanced functional materials for addressing global challenges.

Fig. 2 The number of pyrazolate-MOF publications from 2008 to 2025.

2. Synthesis and structure of pyrazolate-based MOFs

As five-membered aromatic nitrogen-containing heterocycles, azoles are common building blocks of synthetic or natural compounds (Fig. 3a).²⁹ Pyrazole is an azole with two adjacent N atoms. The coordination behaviors of the sp² N-donors in pyrazoles are basically identical to pyridines, while the other sp³ N in pyrazoles could be deprotonated to form the corresponding pyrazolate anions. Both N atoms share the same environment as those in imidazoles, though their relative positions are different (1,2 for pyrazoles and 1,3 for imidazoles). Pyrazoles also exhibit similar stronger basicity to their imidazole isomers.^46,47 In contrast to imidazolate, which has a bridging angle around 140°, pyrazolate exhibits a much smaller bridging angle, approximately 70°, leaving two coordinated metal ions in close proximity (ca. 3.5–4.7 Å depending on the ion radius) (Fig. 3b).^48,49 The coordination geometry of exo-bidentate pyrazolate usually resembles the syn,syn-μ-η¹,η¹-bidentate mode of carboxylate.²⁹ This geometry comparability allows for accessing isoreticular pyrazolate counterparts by employing the crystal engineering approach.

Fig. 3 (a) Structure and pK_a value of prototypical azoles. (b) Comparison of the coordination modes of imizolate, pyrazolate, and carboxylate.

In addition to the similarity, pyrazolates also present some uniqueness in forming metal-based building blocks specific for pyrazolate MOFs, including zigzag chains (rod building blocks, RBBs) with square-planar/tetrahedral metal ions, helical chains with tetrahedral metal ions, triple zigzag chains with octahedral metal ions, and cubic clusters [M₈(OH)₆(Pz)₁₂] (Pz = pyrazolate, M = Ni or Co) (Table 1 and Fig. 4).^29,43 The structural configurations and internal surface properties of MOFs can be systematically tuned by varying the geometry of spacing ligands and metal ions, as well as building block modification with specific functional groups (Table 1 and Fig. 4, 5).⁴³ Actually, pyrazolate derivatives have been well explored as ligands for the construction of discrete, polynuclear complexes, which is not our focus here.^50–55 Similarly, there are numerous reports on the use of pyrazole-containing multifunctional ligands, such as pyrazole carboxylic acid, for the synthesis of MOFs, however, this is also beyond the scope of our review.^42,56,57 In this section, we will focus on the reported MPFs constructed from different chains/clusters.

Table 1 Summary of reported MPFs

Type	Name	Ligand	Metal	Stability	Application
Single-walled chain	Co(bdp)	1,4-Benzenedi(4′-pyrazolyl) (H2BDP)	Co	Thermally stable up to 260 °C	H2 adsorption,^35,58 catalytic oxidation of cyclohexene,^59 CO2/CH4 separation,^60 methane storage^38,61
	Ni(bdp), Ni(bpb), PCF-9		Ni	pH 3–12 (room temperature (RT), 24 h); thermally stable up to 400 °C	Adsorption of harmful organic vapors,^62 oxidation of 5-hydroxymethylfurfural,^63 heterogeneous catalysis,^64 C2H6/C2H4 separation,^65 photo-reduction^66
	Zn(bdp), Zn(bpb), AIST-1		Zn	pH = 10 (RT, 24 h); thermally stable up to 400 °C	Adsorption of harmful organic vapors,^62 amination of 4-chloropyridine,^67 hydrogen storage,^68 oxygen evolution reaction (OER),^69 drug delivery,^70 zinc–air batteries^71
	Fe(bdp)		Fe	—	Methane storage,^38 switchable thermal conductivity (simulation)^72
	Co(F-bdp)	H2(F-bdp)	Co	—	Gas storage and separation^61
	Co(p-F2-bdp)	H2(p-F2-bdp)
	Co(o-F2-bdp)	H2(o-F2-bdp)
	Co(D4-bdp)	H2(D4-bdp)
	Co(p-Me2-bdp)	H2(p-Me2-bdp)
	Zn-BDP-x	H2BDP-X, X = –NO2, –NH2, –OH	Zn	Thermally stable up to 400–450 °C	Gas separation and purification,^73 drug delivery,^74 and catalytic cycloaddition of CO2^75
	Ni-BDP-x		Ni
	BUT-31	H2BDP-CHO	Zn	Boiling water; 0.1 M HCl, 4 M NaOH solution (RT, 24 h); thermally stable up to 400 °C	Gas adsorption^76
	MFU-2	1,4-Bis(3,5-dimethyl-1H-pyrazol-4-yl)benzene (H2DMBDP)	Co	Ethanol, ethanol/water 7 : 3 (RT, 96 h); thermally stable up to 340 °C	Catalytic oxidation^77
	Zn(BPE)	1,2-Bis(1H-pyrazol-4-yl)ethyne (H2BPE)	Zn	Water (15 days)	Removal of pollutants in wastewater^78
	Ni(BPEB)	1,4-Bis(1H-pyrazol-4-ylethynyl)benzene (H2BPEB)	Ni	Water vapors, acetone (RT, 5 days); boiling water (2 days); pH 5–9 (RT, 8 h); thermally stable up to 422 °C	Synthesis and structure,^79 catalytic oxidation^63
	Zn(BPEB)		Zn	Thermally stable up to 410 °C	Synthesis and structure^79
	M-pyNDI	Naphthalene diimide bipyrazole (H2NDI)	M = Zn, Co, Ni	Thermally stable up to 300 °C	Gas sensing^80
	ZnNDI	H2NDI	Zn	—	Electrical conductivity enhancement,^81 semiconductor/insulator switching,^82 electrochromism,^83 redox conductivity,^84 electrical conductivity (computation)^85
	ZnPMDI	Pyromellitic diimide bis-pyrazolate (PMDI)		—	Redox conductivity^84
	Zn(NDI-X)	H2NDI-X (X = H, SC2H5, NH-C2H5)		Water stable	Electrochromism,^86 water adsorption^87
	Zn(dmpz)2NDI	(dmpz)2NDI		—	Electrode^88
	Zn-PDI-MOF	perylene diimide (PDI) pyrazolate		—	Electrochromic properties^89
	Fe-pyNDI	2,7-Bis(1H-pyrazol-4-yl)-benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetrone (H2pyNDI)	Fe	—	Electrosynthesis of ammonia^90
	Zn(BPZ)	4,4′-Bipyrazole (H2BPZ)	Zn	Air-stable; thermally stable up to 300–450 °C	Synthesis and structure,^91 gas separation^92,93
	Co(BPZ)		Co
	Cd(BPZ)		Cd
	Hg(BPZ)		Hg
	Cu(BPZ)		Cu
	Ni(BPZ)		Ni
	Pd(BPZ)		Pd
	Cu(H2BPZ)2(NO3)2		Cu
	Cd(H2BPZ)(CH3COO)2		Cd
	M-CFA-6		Fe	Thermally stable up to 250 and 300 °C	Synthesis and structure^94
			Ga
	Zn(BPZX)	H2BPZX, X = H, NO2,NH2	Zn	Thermally stable up to 435–453 °C	Adsorption of CO2^95
	M(BPZNO2)	H2BPZNO2	M = Co, Cu, Zn	Thermally stable up to 390 °C	CO2 capture^96
	Co(BPZX)	H2BPZX, X = H, NO2, NH2	Co	Thermally stable up to 400 °C	Catalytic oxidation^97
	M(BPZNH2)	H2BPZNH2	M = Zn, Ni, Cu	Water vapor stable; thermally stable up to 290–430 °C	CO2 capture and conversion^98
	MIXMOFs	3,5-Diamino-4,4′-bis(1H-pyrazole) (3,5-H2L)	Cu, Ni	Thermally stable up to 377 °C	CO2 electrochemical reduction (CO2RR)^99
	FICN-8	(5,10,15,20-Tetra(1H-pyrazol-4-yl) porphyrin) (H4TPP)	Cu	—	CO2RR^100
	PCN-1004	1,1,2,2-Tetrakis(4-(1H-pyrazol-4-yl)phenyl) ethene (H4TPPE)	Cu	pH 1–14; thermally stable up to 300 °C	Heterocatalysis^101
	Zn(Hazbpz)NO3	4,4′-Azobis(3,5-dimethyl-1H-pyrazole) (H2AZBPZ)	Zn	Thermally stable up to 340 °C	Synthesis and structure^102,103
	[Cu(NO3)2(H2O)2(H2azbpz)2]·2H2O		Cu	—	Synthesis and structure^102,103
	[Ni(H2O)4(H2azbpz)2](NO3)2·2H2O		Ni
	Zn(azbpz)		Zn	Water; thermally stable up to 400 °C	C2H2/CO2 separation^104
Double walled Chain	Zn(1,3-BDP)	1,3-Benzenedi(4′-pyrazolyl) (1,3-H2BDP)	Zn	pH = 3 (90 °C, 30 min); thermally stable up to 400–500 °C	Hydrogen storage^68
	Co(1,3-BDP)		Co
	BUT-53			pH = 5–14 (RT, 24 h); thermally stable up to 363–432 °C	Trace removal of benzene vapor,^40 SF6/N2 separation^105
	BUT-54	2,7-Di(1H-pyrazol-4-yl)naphthalene) (H2DPN)			Trace removal of benzene vapor^40
	BUT-55	H2BDP
	BUT-56	2,5-Di(1H-pyrazol-4-yl)pyridine) (H2DPP)
	BUT-57	2,7-Di(1H-pyrazol-4-yl)pyrene) (H2PDP)
	BUT-58	H2BDP	Zn
	JNU-204, HIAM-3004	4,7-Di(1H-pyrazol-4-yl)benzo[c][1,2,5]thiadiazole (H2PBT, H2BT-Pz)		pH = 3–14 (RT, 24 h); thermally stable up to 500 °C	Photocatalytic aerobic oxidation,^106 luminescence^107
	HIAM-3005	4,7-Di(1H-pyrazol-4-yl)benzo[c][1,2,5]-selenadiazole (H2DPBS, H2BS-Pz)		pH = 2–12 (RT, 24 h); thermally stable up to 500 °C	Luminescence^107
	HIAM-3006	5,6-Dimethyl-4,7-di(1H-pyrazol-4-yl)benzo[c][1,2,5]thiadiazole (DDPBT)
	HIAM-213	3,5-Di(1H-pyrazol-4-yl)aniline (H2DPA)		pH = 3–14 (RT, 2 days); thermally stable up to 350 °C	Gas separation^108
	HIAM-214	H3BTP		Thermally stable up to 180 °C
	Fe(II)(bppdi)(DMF)0.5	(2,6-Bis(1H-pyrazolyl)-pyromellitic diimide) (H2bppdi, H2TET)	Fe	Thermally stable up to 360 °C	NO reduction^109
Fe2(BDP)3 type	Fe2(BDP)3	H2BDP	Fe	pH = 2 to 10 (100 °C 2 weeks); thermally stable up to 360 °C,	Separation of hexane isomers,^37 C2H6/C2H4 adsorption separation,^110 CO2 adsorption^111,112
	A2Fe2(BDP)3 (A = Li +, Na +, K+			Thermally stable up to 360 °C	H2 adsorption^113
	AxFe2(BDP)3 (A = Na +, K +; 0 < x ≤ 2)			—	O2 adsorption^114
	Fe2(BPEB)3	1,4-Bis(1H-pyrazol-4-ylethynyl)benzene (H2BPEB, H2EBDP)		Water vapor (RT, 1 h); thermally stable up to 410 °C	Synthesis and structure^79
	Fe-PF1	H2EBDP-F, H2EBDP-2F, H2EBDP-4F		Water (RT, 24 h); thermally stable up to 300–350 °C	Adsorption of CO2^115
	Fe-PF2
	Fe-PF4
Cu3	CFA-2	3,3′,5,5′-Tetraphenyl-1H,1′H-4,4′-bipyrazole (H2PhBPZ)	Cu	Thermally stable up to 300, 500 °C	Synthesis and structure^116
	CFA-3		Ag
	Cu-BTPP	1,3,5-Tris((4-pyrozole)phenyl)-benzene) (H3BTPP)	Cu	Thermally stable up to 350 °C	Synthesis and structure^117
	FJU-66	H2NDI	Cu	pH = 3–14, 10 M NaOH (RT, 12 h); thermally stable up to 530 °C	Conductivity^118
M4 (M = Ni, Cu, Co, Cd, Pd)	Ni3(BTP)2	1,3,5-Tris(1H-pyrazol-4-yl)-Benzene (H3BTP)	Ni	pH = 2–14 (100 °C, 14 days); thermally stable up to 430–510 °C	Synthesis and structure,^33 adsorption and degradation of nerve agent simulant and pesticide,^119 electroreduction of CO2^120
	Cu3(BTP)2		Cu
	BUT-124(Cd)		Cd	Unstable	OER^121
	BUT-124(Co)		Co	H2O, pH = 14 (KOH) (RT, 24 h)
	BUT-32	2,4,6-Tris(4-(pyrazole-4-yl)phenyl)-1,3,5-triazine (H3TPTA)	Ni	pH = 3, 4 M NaOH, (RT, 1 day); thermally stable up to 398 and 380 °C	C–C coupling^122
	BUT-33				C–C,^122 Gold extraction^123
	BUT-33(Pd)		Pd	pH = 3, 8 M NaOH (RT, 1 day)	Suzuki and Heck coupling^124
	BUT-78	3,3′,5,5′-Tetra(1H-pyrazol-4-yl)-1,1′-biphenyl (H4BPTP)	Ni	pH = 2, 5 M NaOH, (RT, 1 day); thermally stable up to 430 °C	Capture of trace benzene and SO2^125
	MFU-1	H2DMBDP	Co	Ethanol, ethanol/water 7 : 3 (RT, 96 h); thermally stable up to 340 °C	Catalytic oxidation^126
Ni8/Co8/Zn9	[Ni8(OH)4(OH2)2(pbp)6]	4,4′-Bis(1H-pyrazol-4-yl)biphenyl (H2PBP)	Ni	Thermally stable up to 410 °C	Synthesis and structure^127
	[Ni8(OH)4(OH2)2(tet)6]	2,6-Bis(1H-pyrazol-4-yl)pyrrolo[3,4-f]isoindole-1,3,5,7(2H,6H)-tetrone (H2TET)
	[Ni8(OH)4(H2O)2(L)6]	H2BDP; H2BYDP; H2EBDP		Stable in base (24 h); thermally stable up to 300–350 °C	The capture of harmful volatile organic compounds,^36 SO2 adsorption,^128 synthesis and structure^129
	K[Ni8(OH)5(EtO)(H2O)2(BDP_X)5.5]	H2BDP_X, X = H, OH, NH2		Thermally stable up to 300 °C	CO2 capture from flue gas,^130 CO2^131 and SO2^132 adsorption
	K3[Ni8(OH)3(EtO)(H2O)6(BDP_O)5]
	Pd@NiBDP	H2BDP			Hydroamination of alkynes^133
	Ni8(BDP)6@Cu				Heterocatalysis^134
	Ni8(BDP)6@Fe				Oxidase-mimicking H2O2 activation^135
	Ni8(BDP-X)6	H2BDP-X, X = CHO, CN, COOH		pH = 3, 4 M NaOH (RT, 1 day); thermally stable up to 300 °C	CO2 adsorption and proton conduction^136
	NiBDP-AgS	H2BDP-NH2		pH 2–13, 1 M DBU solution; thermally stable up to 400 °C	Chemical fixation of CO2^137
	Pd@Ni-MOF	4,4′-(Benzene-1,4-diyldiethyne-2,1-diyl)bis(1-H-pyrazole) (H2EBDP)		—	Catalytic Heck arylation of functionalized olefins^138
	BUT-2	H2BDP		—	Synthesis and structure^139
	BUT-3	H2BDP-CN
	BUT-4	H2BDP-CHO
	BUT-48	1,3-H2BDP-CN		pH = 5, 1 M NaOH (RT, 24 h); boiling water (24 h); thermally stable up to 350 °C
	BUT-49	1,3-H2BDP-CF3
	BUT-123	H3BTPP
	Ni8(OH)4(H2O)2(L4)6	(4,4′-Buta-1,3-diyne-1,4-diyl)bis-pyrazole (H2BYDP)		—	C2H2/CO2 separation^140
	Ni8(OH)4(H2O)2(L5)6	4,4′-(Benzene-1,4-diyldiethyne-2,1-diyl)bis-pyrazole (H2EBDP)
	NiL1	4,4′-((2,5-Bis((4-(methylthio)phenyl)ethynyl)-1,4-phenylene)bis(ethyne-2,1-diyl))bis(1H-pyrazole) (H2EBDP-SMe)		pH = 3, 15 M NaOH (24 h); thermally stable up to 300 °C	Catalytic hydrogen evolution reaction (HER),^141 proton conduction^142
	NiL2	4,4′-((2,5-Di(hex-1-yn-1-yl)-1,4-phenylene)bis(ethyne-2,1-diyl))bis(1H-pyrazole) (H2EBDP-Prop)		pH = 1, 15 M NaOH (RT, 24 h), thermally stable up to 320 °C	Proton conduction^142
	NiL1	2,2′-((2,5-Bis((1H-pyrazol-4-yl)ethynyl)-1,4-phenylene)bis(ethyne-2,1-diyl))dianiline (H2EBDP-NH2)		pH = 3–13 (RT, 24 h)	Electrocatalytic hydrogen evolution^143
	NiL1-F	4,4′-((2,5-Bis((1H-pyrazol-4-yl)ethynyl)-1,4-phenylene)bis(ethyne-2,1-diyl))bis(1-(perfluorophenyl)-1H-1,2,3-triazole)		Thermally stable up to 300 °C	Remove perfluoro pollutants^144
	JNU-212	H2BDP		7 M NaOH (24 h, 100 °C); pH = 2 (RT, 24 h)	Catalytic oxidation^145
	JNU-213	H2BT-Pz
	JNU-214	H2BS-Pz
	JNU-215	4,9-Di(1H-pyrazol-4-yl)naphtho[2,3-c][1,2,5]selenadiazole (H2NS-Pz)
	CFA-25	1,4-Bis(1H-pyrazol-4-yl)benzoxadiazole (H2BPBO)		—	Ionic transport^146
	PCN-601	H4TPP		0.01 mM HCl (RT, 24 h), 10 M NaOH (100 °C, 24 h); thermally stable up to 300 °C	Synthesis and structure,^39 photoreduction,^147 electrocatalytic^148
	PCN-602	5,10,15,20-Tetrakis(4-(pyrazolate-4-yl)phenyl)porphyrin (H4TPPP)		pH 4-14, 1 M KF, Na2CO3 and K3PO4 (1 day); thermally stable up to 300 °C	C−H bond halogenation^149
	PCN-624	5,10,15,20-Tetrakis(2,3,5,6-tetrafluoro-4-(1H-pyrazol4-yl)phenyl)-porphyrin (H4TTFPPP)		Water, ethanol, HCl (1 M), Na2CO3 (3 M) and K3PO4 (3 M) (RT, 24 h); thermally stable up to 400 °C	Heterocatalysis^150
	TPE4Pz-Ni	H4TPPE		pH = 3, saturated NaOH solution (RT, 24 h); thermally stable up to 350 °C	C–S cross-coupling^151
	FICN-18	2,2′-((1E,1′E)-(ethane-1,2-diylbis(azaneylylidene))bis(methaneylylidene))bis(4-(1H-pyrazol-4-yl)phenol) (H2EBE)		Boiling water; pH = 1–14 (RT, 21 days); thermally stable up to 200 °C	Heterocatalysis^152
	FICN-19		Co
	fcu-L-Co	H2DPP, 3,6-di(1H-pyrazol-4-yl)pyridazine (H2L2), 2,5-di(1H-pyrazol-4-yl)pyrazine (H2L3), and 2,5-di(1H-pyrazol-4-yl)pyrimidine (H2L4)		pH = 5, 4 M NaOH solution (RT, 24 h); thermally stable up to 350–400 °C	Structural transformation and water sorption^153
	Zn9O2(OH)2(L)6	H2EBE	Zn	pH = 3, 2 M NaOH (RT, 210 days); pH = 3 and H2O (21 days, 100 °C); thermally stable up to 400 °C	Catalytic CO2 cycloaddition^154
Others	BUT-41	1,3,6,8-Tetra(1H-pyrazol-4-yl)-9H-carbazole (H4CTP)	Ni	Thermally stable up to 400 °C	Adsorption of CO2, N2, and CH4^155
	MROF-12	H2NDI	Cu	pH = 2–13 (RT, 12 h); thermally stable up to 481 °C	Separation of C2H2/CH4 mixture^156
	Cu-TPP	H4TPP	Cu	pH = 1–14 (RT, 24 h); thermally stable up to 300 °C	Fluorescence and detection^157
	MnTPzP-Mn		Mn	Thermally stable up to 300 °C	Li–CO2 battery cathode material^158
	TMBP·CuI	(3,3′,5,5′-Tetramethyl-4,4′-bipyrazol) (H2BPZ-Me)	Cu	Water-stable; thermally stable up to 300 °C	Immobilization of I2 and ICl^159
	[Mn(DMPMB)] (n = 1, 2)	H2DMPMB	M = Co, Zn, Cd, Cu, Ag	Thermally stable up to 300 °C	Synthesis and structure^160
	[Cu2(bpz)2(CN)X], (X = Cl; I)	H2BPZ-Me	Cu	—	Catalytic azide–alkyne “Click” reactions^161
	[Cu6(bpz)6(CH3CN)3(CN)3Br]·2OH
	M[CFA-4]	1,4-Bis(3,5-bis(trifluoromethyl)-1H-pyrazole-4-yl)benzene (H2TFPB)	M = Cu(I), K, Cs, Ca(0.5)	Thermally stable up to 300 °C	Gas sorption and fluorescence^162
	PCN-300	H4TPP	Cu	pH = 1–14 (RT, 24 h); thermally stable up to 300 °C	C–O cross-coupling reaction^163
	PCN-1003	H4TPPE	Ni	pH = 1–12 (RT, 24 h); thermally stable up to 450 °C	Perfluorooctanoic acid concentration and degradation^164
	SIFSIX-18-Cd	H2BPZ-Me	Cd	Boiling water (12 h); air (2 months); thermally stable up to 250 °C	Synthesis and structure,^165 hydrogen isotope separation^166
	SIFSIX-18-Ni		Ni	75% relative humidity (40 °C, 14 days); thermally stable up to 300 °C	Trace CO2 capture^167

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Fig. 4 Typical chain/cluster structures observed in MPFs.

Fig. 5 Typical pyrazolate ligands used in constructing MOFs.

2.1. Chains/RBBs (Zn/Co/Ni/Fe)

Divalent late transition metal ions with 4-coordinate geometries, including Fe(II), Co(II), Ni(II), Cu(II), and Zn(II), can link the simple pyrazolate (Pz⁻) ligand to construct infinite one-dimensional (1D) chains or RBBs, represented by the formula [M(Pz)₂], which could also be considered as extended vertex-sharing M₂(Pz)₂ rings.^50–53 Therein tetrahedral metal ions form planar M₂(Pz)₂ rings with two bridging pyrazolate moieties, while square-planar metal ions form bent ones.^50–53 Cu(II) is able to adopt both the square-planar and flattened tetrahedral geometries, enabling the formation of two distinct isomeric [Cu(Pz)₂] chains.⁵³ These chains/RBBs can be connected to form higher dimensional networks with polypyrazolate ligands by replacing the monopyrazolates.

2.1.1. Single-walled M(bdp) and derivatives. Based on the infinite 1D chains/RBBs Co(pz)₂ consisting of tetrahedrally coordinated Co(II) sites that are bridged by pyrazolate moieties, as reported by Masciocchi and co-workers,⁵² a new three-dimensional (3D) MPF was obtained when researchers replaced the simple pyrazole ligand with a linear bis-pyrazole ligand (Fig. 6a). In 2008, Long's and Volkmer's groups reported the syntheses of Co(bdp) using the H₂BDP ligand, 1,4-benzenedi(4′-pyrazolyl), under different solvothermal conditions, respectively.^58,59 Co(bdp) has a flexible framework and exhibits multiple phases with various structures. The open phase of Co(bdp) has a tetragonal structure featuring 1D chains/RBBs of Co(II) ions as the vertices (Fig. 4). This kind of motif has been observed previously in molecules such as [Co(dmpz)(Hdmpz)₂]₂ (Hdmpz = 3,5-dimethylpyrazole).¹⁶⁸ The Co(II) chains/RBBs are spaced by four BDP²⁻ ligands to form Co(bdp). There are in total five phases identified during the framework structure evolution (Fig. 6b). Except for the open phase, all other phases are in the monoclinic lattice, where Co(II) adopts a square planar coordination geometry along with the reorientation of the BDP²⁻ ligands. The structure suddenly transforms to a tetrahedral geometry in the open phase, which could be recovered by guest unloading. In terms of topology, the four monoclinic phases are closely related to the MIL-53(Fe) structure type.^169,170

Fig. 6 (a) Structure of Co(bdp). (b) N₂ adsorption isotherm measured at 77 K, indicating the five pressure-dependent phases.⁵⁸ Reprinted with permission from American Chemical Society 2010.

After the discovery of prototype Co(bdp), a great number of variants/analogs were synthesized under the guidance of crystal engineering principles in the following decades. The metal ions are amenable to substitution by various species, including Fe(II), Ni(II), Zn(II), and Cu(II), to yield isostructural compounds. In 2010, Long's group reported the synthesis of Zn(bdp) following similar procedures to that for Co(bdp).⁶⁸ Navarro's group also synthesized powder samples of Zn(bdp) and Ni(bdp) in the same year.⁶² The overall structures of as-synthesized Zn(bdp) and Ni(bdp) are isoreticular with Co(bdp), however, the coordination modes show slight differences.^35,62 The Zn(II) in Zn(bdp) exhibits a similar coordination geometry with Co(bdp), while Ni(II) in Ni(bdp) adopts a square-planner geometry in an orthorhombic structure. In addition, they show different degrees of flexibility upon gas adsorption/desorption, where Zn(bdp) is more rigid compared the other two analogs. In 2015, an iron analog Fe(bdp) was accessed in Long's group.³⁸ Fe(bdp) is also isoreticular with Co(bdp) with Fe(II) irons in tetrahedral coordination geometry as well. Another Cu-based variant Cu(bdp), also named Cu-dpb, was synthesized by Cao and co-workers in 2020.⁶⁴ There are no structural details about Cu(bdp) available, except that it matched the PXRD pattern of Ni(bdp).^38,64

With the M(bdp) platforms in hand, researchers have attempted to functionalize the bdp²⁻ ligand for new isostructural materials with diverse functionality. In 2011, Volkmer and co-workers used the four-methyl functionalized 1,4-bis(3,5-dimethyl-1H-pyrazol-4-yl)benzene (H₂dmbdp) ligand to synthesize the Co(bdp) analog, MFU-2, also denoted Co(dmbdp).⁷⁷ As opposed to the flexible nature of Co(bdp), MFU-2 was demonstrated to be a rigid network exhibiting type I gas sorption isotherms due to the steric interactions between adjacent methyl groups. Viewed along the Co(II) chains/RBBs, these methyl groups also lead to a torsional distortion of planes running through opposite pyrazole rings, while the planes through adjacent pyrazole ring systems are strictly orthogonal to each other in Co(bdp). In 2012, Navarro and co-workers synthesized two series, Mbdp-X, adopting the Ni(bdp) and Zn(bdp) structure, with the tagged organic linkers H₂bdp-X (X = –NO₂, –NH₂, –OH), respectively, as well as a structural isomer of Ni(bdp), Nibdp-SO₃H with a sulfonate tagged linker, H₂bdp-SO₃H.⁷³ In 2016, Long and co-workers introduced various functional groups, including fluorine, deuterium, and methyl to the H₂BDP ligand, affording five Co(bdp) analogs, Co(F-bdp), Co(p-F₂-bdp), Co(o-F₂-bdp), Co(D₄-bdp), and Co(p-Me₂-bdp).⁶¹ After functionalization, all the materials have a high and similar degree of flexibility but different pore size and pore chemistry. The introduction of functional groups has altered the edge-to-face π–π interactions in the collapsed phase, leading to divergent behaviours in methane storage.

In addition to the metal substitution and ligand functionalization, there are plenty of publications on reticular contraction/expansion by shortening or lengthening the ligand. The simple rigid spacer 4,4′-bipyrazole (H₂BPZ) has been employed to synthesize MOFs with late transition metal ions by Galli, Civalleri and co-workers in 2012.^91,171 A series of M(BPZ) (M = Zn, Co, Cu, Ni) were obtained under conventional routes (room-temperature stirring for Zn and Cu with base), solvothermal (for Co) or reflux (for Ni with base) conditions. The four materials are isoreticular 3D porous networks containing square and rhombic 1D channels, respectively. In 2019, Galli, Rossin, and co-workers used the amino-functionalized organic linker 3-amino-4,4′-bipyrazole (H₂BPZNH₂) to synthesize three MOFs M(BPZNH₂) (M = Zn, Ni, Cu).⁹⁵ The Zn(II) compound was formed as a 3D structure featuring tetrahedral Zn(II) nodes and bridging BPZNH₂²⁻, which define the vertices and edges of square-shaped channels. The Ni(II) and Cu(II) analogs, which are isostructural, feature square-planar metal centers and BPZNH₂²⁻ linkers acting as connectors at the vertices and edges of rhombic channels within a 3D framework. These findings are in agreement with their parent M(bdp) materials.

In 2013, Dincă and co-workers extended the dmbdp²⁻ linker with redox-active 1,4,5,8-naphthalene diimide (NDI) containing cores to get three new ligands with different functionalities, H₂NDI-X (X = H, S-C₂H₅, NH-C₂H₅).⁸⁷ Solvothermal reaction of these ligands with zinc salt yielded an extending isostructural series of MOFs, designated as Zn(NDI-X). These MPFs feature RBBs of tetrahedral Zn(II) and NDI linker functionalized channels (width of ∼16 Å). Post-synthetic oxidation of Zn(NDI–SEt) using dimethyldioxirane was deployed to introduce ethyl sulfoxide and ethyl sulfone groups, thereby increasing the hydrophilicity of the channels. In 2022, Diring and co-workers substituted the central phenyl group of dmbdp²⁻ linker with highly electro-deficient perylene diimide (PDI) cores, resulting in two new PDI ligands with their four-bay positions functionalized by either a chlorine (1,6,7,12-tetrachloro-perylene-3,4,9,10-tetracarboxylic dianhydride, PDI-Cl) or bridged ethyl-1,2-disulfone (PDI-SO₂eth) group.¹⁷² In 2023, based on these two ligands, they reported the in situ preparation of two Zn-PDI-MOF thin films with interesting electrochromic properties.⁸⁹ These two MOFs, Zn-PDI-Cl and Zn-PDI-SO₂ are expected to be isostructural to Zn(NDI-X) as reported by Dincă and co-workers, featuring the infinite chains/RBBs, in which Zn(II) ions are in a tetrahedral geometry and spaced by the long ligand.

In terms of alkynyl functionalization, Galli and co-workers employed a long ligand 1,4-bis(1H-pyrazol-4-ylethynyl)benzene (H₂BPEB) to react with a number of transition metal ions (M = Zn(II), Ni(II), Fe(II)).⁷⁹ Comprising the same structural motif (square planar metal ions), Ni(BPEB) and Ni(BDP-X) constitute an isoreticular family. While an attempt at the isoreticular expansion of Zn(BDP-X) failed with the insurgence of interpenetration in α-Zn(BPEB), which might be promoted by more space spared from the C≡C triple bonds due to the tetrahedral geometry adopted by Zn(II). The two interpenetrated networks finally cross between them. However, the reaction with Fe(II) in N,N′-dimethylformamide (DMF) yields a different phase Fe₂(BPEB)₃, which will be discussed later (the section on Fe₂(BDP)₃ and derivatives). In addition, two MOFs, Zn(BPE) and Zn(BPE)·nDMF (interpenetrated i-Zn and non-interpenetrated ni-Zn·S) were synthesized based on a 1,2-bis(1H-pyrazol-4-yl)ethyne (H₂BPE) ligand by Galli and co-workers in 2023.⁷⁸ Interestingly, a HgCl₂-triggered interpenetrated to non-interpenetrated phase transformation (i-Zn-to-HgCl₂@ni-Zn) was observed at the molecular level, representing the first example of pristine phase decomposition and concomitant assembly of the other phase upon the inclusion of a specific guest.

In 2015, Volkmer and co-workers synthesized a new type of chain/RBB based MOFs M-CFA-6 (M = Ga, Fe, CFA = termed Coordination Framework Augsburg University).⁹⁴ The structure motif of the M-CFA-6 framework is similar to that of MIL-53, featuring trivalent metal centers in octahedral coordination connected by the bifunctional BPZ²⁻ ligands and hydroxyl groups.¹⁷⁰ MIL-53 and CFA-6 show similar thermal stabilities, surface areas and pore sizes. The key difference lies in the breathing behavior of MIL-53, which is not observed in the more rigid network of CFA-6. In 2018, the authors reported the Mn(III)-based analog of M-CFA-6 (M = Ga, Fe), Mn-CFA-6.¹⁷³ The Jahn–Teller distortion of Mn(III) centers causes asymmetric distortion of the hydroxyl bonds and results in a distortion of the unit cell. As opposite to Fe-CFA-6 and Ga-CFA-6, Mn-CFA-6 exhibits similar structural flexibility with MIL-53.

In 2024, Cao and co-workers synthesized FICN-8 by combining a tetra-pyrazole ligand, (5,10,15,20-tetra(1H-pyrazol-4-yl) porphyrin) (H₄(H₂TPP)), with Cu RBBs and elucidated its structure using continuous rotation electron diffraction (cRED).¹⁰⁰ The analysis revealed that FICN-8 crystallizes in the orthorhombic Cmmm space group. The structure features fully deprotonated pyrazolate groups from CuTPP⁴⁻, which was formed by chelating the H₂TPP⁴⁻ ligand to a Cu(II) ion in the center. The CuTPP⁴⁻ ligand connects to eight different Cu(II) centers via eight pyrazolate N atoms. Each Cu(II) center, in turn, coordinates to four pyrazolate N atoms, forming square planar units. These units link into 1D [Cu(Pz)₂]_n chains, which are interconnected by CuTPP⁴⁻ ligands to create a 3D framework (Fig. 7a and b). This structure is notably different from the 2D MOF, PCN-300,¹⁶³ despite using the same porphyrinic ligand, marking the first instance of forming 1D pyrazolate chains with a tetratopic pyrazole ligand. In the same year, Zhou et al. reported the synthesis of PCN-1004 via a solvothermal reaction between Cu(OAc)₂ and 1,1,2,2-tetrakis(4-(1H-pyrazol-4-yl)phenyl) ethene (TPPE).¹⁰¹ TPPE is also a tetrapyrazole ligand, but its structure is entirely different from FICN-8. Each TPPE ligand connects to eight copper ions through four bidentate μ₂-pyrazolate. The copper ions exhibit square planar coordination geometry, with each copper ion coordinating to four nitrogen atoms from four TPPE ligands, resulting in Cu(Pz)₂ chains. The interconnection of TPPE with the Cu(Pz)₂ chains creates a 3D network, featuring two types of 1D rhombic channels with different sizes (Fig. 7c and d).

Fig. 7 Structures of FICN-8 (a) and (b) and PCN-1004 (c) and (d).^100,101 Reprinted with permission from Wiley-VCH 2024.

2.1.2. Double walled M(bdp) and derivatives. In the attempt to synthesize Zn(bdp) as an analog of Co(bdp), as reported by Long and co-workers, they also changed the para-substituted 1,4-H₂BDP ligand for its meta-substituted isomer 1,3-H₂BDP.⁶⁸ A Zn(bdp) isomer, double-walled structure Zn(1,3-BDP) was obtained with the 1,3-H₂BDP ligand, as well as its Co counterparts, Co(1,3-BDP).

In 2022, Li and co-workers also reported a series of MPFs, BUT-53 to BUT-58 (BUT = Beijing University of Technology; BUT-53 = CoDPB, H₂DPB = 1,3-H₂BDP; BUT-54 = CoDPN, H₂DPN = 2,7-di(1H-pyrazol-4-yl)naphthalene; BUT-55 = CoBDP; BUT-56 = CoDPP, H₂DPP = 2,5-di(1H-pyrazol-4-yl)pyridine; BUT-57 = CoPDP, H₂PDP = 2,7-di(1H-pyrazol-4-yl)pyrene; BUT-58 = ZnBDP), constructed from Co(II) or Zn(II) ions and dipyrazolate ligands under solvothermal reactions (Fig. 8).⁴⁰ They are all double-walled structures as pairs of pyrazolate moieties link adjacent tetrahedral metal cations to form helical chains/RBBs that are cross-linked by pairs of BDP²⁻ ligands. BUT-55/BUT-58 and Co(bdp)/Zn(bdp) share the same formula and are supramolecular isomers, yet they differ in pore structures and pore chemistry. The rigid BUT-55 exhibits smaller 1D square channels (width ∼8.0 Å) than those of the flexible Co(bdp) (width ∼13.2 Å). Replacing the BDP²⁻ ligand with longer ones DPP²⁻ and PDP²⁻ afforded isostructural BUT-56 and BUT-57 with larger pores, while using angular variants of BDP²⁻ and DPN²⁻ yielded BUT-53 and BUT-54 with smaller pores. Unlike the cross-wise arrangement of ligands in BUT-55 to BUT-58, the ligands in BUT-53 and BUT-54 adopt a nose-to-nose configuration with their central aromatic rings aligned in an eclipsed orientation. Li and co-workers developed another double-walled MOF, JNU-204 (JNU = Jinan University), with a 4,7-di(1H-pyrazol-4-yl)benzo[c][1,2,5]thiadiazole (H₂PBT) ligand, where a photosensitive pyrrolo[2,1-a]isoquinoline unit was integrated with coordinating pyrazolate moieties.¹⁰⁶ JNU-204 is a robust microporous Zn-MPF of the wuk topology, isostructural with the aforementioned BUT-55 to BUT-58.

Fig. 8 Double-walled MPFs featuring dipyrazolate ligands and Co(II)/Zn(II)-based RBBs.⁴⁰ Reprinted with permission from Springer Nature 2022.

In 2021, Wade and co-workers synthesized a novel Fe-MOF 1 with helical 1D Fe(II) chains/RBBs bridged by dipyrazolate linkers and terminal DMF ligands linking every other two adjacent Fe(II) centers (Fig. 9).¹⁰⁹ Solvothermal reaction of Fe(OTf)₂(THF)₂ (OTf = triflate) and 2,6-bis(1H-pyrazolyl)-pyromellitic diimide (H₂bppdi/H₂TET), with triethylamine as the modulator, results in the formation of 1. Similar synthesis in the presence of trace air/O₂ affords the oxidized analog 2-OH of 1. 2-OH is composed of helical Fe(III) chains/RBBs linked by linear bppdi²⁻ linkers as well as μ₂-OH ligands to form triangular channels (∼10 Å in diameter). This structure is reminiscent of Fe₂(BDP)₃, but presents a double-walled network where the μ₂-OH groups linking neighbouring Fe(III) ions are cis to each other. This arrangement leads to a disphenoidal arrangement of pyrazolate N atoms and the formation of helical chains. Fe₂(BDP)₃ contains octahedral Fe(III) centers coordinated by six pyrazolate linkers, forming a single-walled structure; however, each Fe(III) site in 2-OH adopts a pseudo-octahedral geometry with 4 coordination to pyrazolates and two to μ₂-OH ligands. 1 adopts a similar framework structure with 2-OH. For 1, the presence of Fe(II) centers necessitates the coordination of neutral DMF molecules as bridges along the chains, instead of the μ₂-OH groups present in 2-OH. The distinctive chains/RBBs have accessible coordination sites between adjacent metal centers, creating motifs that resemble the active sites in non-hemediiron enzymes.

Fig. 9 (a) Synthesis of 1 and 2-OH. (b) Single crystal structure of 2-OH.¹⁰⁹ Reprinted with permission from Wiley-VCH 2021.

These results indicate that the linear chains/RBBs are prone to forming single-walled structures with linear ligands, such as M(bdp), while helical chains/RBBs prefer to generate double-walled compounds, such as BUT-55 and Fe-MOF 1. Based on these findings, it is promising that the coordination number and geometry of metal nodes could be finely tuned by employing different ligands and synthetic conditions to regulate the composition, structure and thereby properties of the resulting MPFs.

2.1.3. Fe₂(BDP)₃ and derivatives. Fe(II) tends to form 1D chains with pyrazolate ligands in a 4-coordinate tetrahedral geometry, affording isostructural compounds with Co(bdp) under air-free conditions.³⁸ However, Fe(II) is easily oxidized to Fe(III) in the presence of air, and the latter prefers to octahedrally coordinate with six pyrazolate moieties (Fig. 4). In 2016, Long and co-workers synthesized Fe₂(BDP)₃, which was obtained upon solvothermal reaction of Fe(acetylacetonate)₃ and H₂BDP. Octahedral Fe(III) centers are linked via μ₂-pyrazolate moieties to form 1D chains/RBBs, defining sharply angled triangle channels with the BDP²⁻ ligands running along the triangle edges and the chains of Fe(III) centers forming the vertices (Fig. 10a).³⁷ Fe₂(BDP)₃ is structurally similar with the carboxylate-based MOF Sc₂(BDC)₃ (H₂BDC = 1,4-benzenedicarboxylic acid).¹⁷⁴

Fig. 10 (a) Structure of Fe₂(BDP)₃ as viewed down the (001) direction. (b) Chains of pyrazolate-bridged iron(III) ions in Fe₂(BDP)₃. (c) Schematic of the reduction of Fe₂(BDP)₃ with potassium naphthalenide to obtain K_xFe₂(BDP)₃, 0 < x ≤ 2. (d) and (e) Scanning electron microscopy (SEM) micrograph of Fe₂(BDP)₃ microcrystallites and a single crystal.¹⁷⁵ Reprinted with permission from Springer Nature 2018.

In 2018, Long's group established a novel strategy for partial reduction of Fe(III) centers in Fe₂(BDP)₃ to Fe(II) species, yielding a conductive mix-valence MPF, wherein chains of pyrazolate-bridged Fe(II)/Fe(III) ions would provide a pathway for electron transfer, while the large pores would accommodate stoichiometric intercalation of charge-balancing cations (Fig. 10).¹⁷⁵ The K_xFe₂(BDP)₃ (0 < x ≤ 2.0) materials were prepared by reducing the pre-synthesized Fe₂(BDP)₃ with potassium naphthalenide in tetrahydrofuran, exhibiting full charge delocalization within the framework and great charge mobility. The authors further demonstrated the growth of sizable single-crystals of the robust Fe₂(BDP)₃ and accessed the solvated A₂Fe₂(BDP)₃·yTHF (A = Li⁺, Na⁺, K⁺) series through chemical reductions.^113,114 The cation positions were observed to differ in the solvated and activated Na-containing structures: cations were located near the center of the triangular framework channels and were stabilized by weak cation–π interactions with the framework ligands in the former, while cation migration occurred upon solvent removal in the latter to maximize stabilizing cation–π interactions. They also partially reduced the expanded Fe₂(BPED)₃ to obtain K_xFe₂(BPED)₃, to support the generality of their proposed chemical reduction strategy to access conductive, guest-selective, and mixed-valence Fe-pyrazolate MOFs.¹¹⁴

As mentioned above, when Galli and co-workers tried to expand the M(bdp) (M = Zn(II), Ni(II), Fe(III)) compounds with an elongated H₂BPEB ligand, Fe₂(BPEB)₃ was synthesized as a different phase in the reaction with Fe(III), which could be regarded as the reticular expansion of Fe₂(BDP)₃.⁷⁹ The structure reported by Long's group is identical to this compound.¹¹⁴

2.2. Polynuclear metal cluster

In addition to the 1D chains/RBBs, a number of discrete high-symmetry polynuclear clusters formed, such as [Co₄(μ₄-O)(μ₂-dmpz)₆], (3,5-dmpz = 3,5-dimethylpyrazolate), structurally similar to those constructed by carboxylate ligands, Zn/Co(μ₄-O)(COOH)₆, as found in MOF-5.^4,126,176 Meanwhile, there are many clusters structurally specific to pyrazolate MOFs, including the cubic octanuclear [Ni₈(μ₄-OH)₆(pz)₁₂]²⁻ and [Co₈(μ₄-OH)₆(pz)₁₂]²⁻ clusters, square tetranuclear M₄(pz)₈ cluster, and triangular trinuclear Cu₃(μ₃-X)(μ₂-pz)₃]ⁿ⁺ (X = OH⁻, O²⁻, halide, etc.) cluster.^{33,127,152,177}

2.2.1. Cu ₃ . The small bridging angle between two N atoms on pyrazolate facilitates the construction of low-dimensional structures, such as the trigonal-planar [M₃(pz)₃] (M = Cu(I), Ag(I), Au(I)) complexes composed of linear univalent metal ions (2-coordinate) and pyrazolates with different substituents.^{54,55,178,179} In 2013, Volkmer and co-workers synthesized CFA-2 (Cu₂(phbpz), H₂phbpz = 3,3′,5,5′-tetraphenyl-1H,1′H-4,4′-bipyrazole) and CFA-3, Ag₂(phbpz), which were assembled from trinuclear [Cu(I)₃/Ag(I)₃(pz)₃] clusters and the H₂phbpz ligand. Thanks to the non-planar orientation of two pyrazolate moieties in the phbpz²⁻ ligand, CFA-2 features a flexible 3D structure with two-fold framework interpenetration.¹¹⁶ Upon exposure to different kinds of organic solvents (MeOH, EtOH, DMF, N,N-diethylformamide (DEF)), CFA-2 shows pronounced breathing effects. The [Cu(I)₃(pz)₃] cluster could be reversibly converted to Cu(II)-based [Cu₃(μ₃-OH)(pz)₃L3] units (L = monodentate neutral or anionic ligand) in the presence of strong oxidant, such as molecular oxygen. In contrast, CFA-3 features a layered 2D structure.

With bulky substituents, the reaction between univalent coinage metal ions and pyrazolates tends to form larger, distorted rings such as tetranuclear saddle-shaped cycles.¹⁸⁰ Alternatively, infinite zigzag chains/RBBs could be accessed without steric substituent groups. Reactions between Cu(II) salts and pyrazoles, in the presence of secondary ligands, typically result in trinuclear [Cu₃(μ₃-X)(μ₂-pz)₃]ⁿ⁺ complexes. These complexes feature a central μ₃-X entity that connects three 4-coordinate Cu centers, with X representing OH⁻, O²⁻, halides, and others.¹⁷⁷ Thompson and co-workers reported the heat-promoted preparation of a mixed-valence complex Cu(I)Cu(II)₂(F₆dmpz)₅ (F₆dmpz = 3,5-bis-(trifluoromethyl)-pyrazolate).¹⁸¹ In this compound, three Cu ions form a trigonal-planar Cu₃ cluster, where a F₆dmpz ligand bridges two adjacent Cu ions. The Cu(I) ion is linear 2-coordinate, while two Cu(II) ions additionally coordinate with another two F₆dmpz ligands above and below the trigonal planar array, respectively.

Starting from an extended ligand of 1,3,5-tris(1H-pyrazol-4-yl)benzene (H₃BTP), 1,3,5-tris (1H-pyrazol-4-yl)phenyl)benzene (H₃BTPP), a mixed-valence 2D MPF, Cu-BTPP, Cu(I)₂Cu(II)(OH)(BTPP) was obtained after temperature optimization.¹¹⁷ Three Cu ions form a trigonal-planar Cu₃ cluster with a central μ₃-OH entity, reminiscent of the Cu₃(μ₃-X)(μ₂-pz)₃]ⁿ⁺ (X = OH⁻, O²⁻, halide, etc.) complexes.¹⁷⁷

2.2.2. M₄ (M = Cu, Ni, Co, Cd, Pd). As mentioned above, univalent coinage metals and pyrazolates are likely to form tetranuclear saddle-like cycles when bulky substituents are present on the pyrazolate ring.¹⁸⁰ In 2016, using a H₂phbpz ligand bearing bulky phenyl groups on the pyrazolate ring, Volkmer and co-workers synthesized a flexible chiral MOF Cu(I)₂(phbpz)·(CFA-9) composed of the saddle Cu₄pz₄ cluster.¹⁸⁰ The open phase of CFA-9 features a 3D microporous framework structure with 1D channels.

In 2011, Long and co-workers constructed M₃(BTP)₂ (M = Ni and Cu) composed of the M₄ cluster and tritopic pyrazole ligand H₃BTP.³³ M₃(BTP)₂ presents an expanded sodalite-like structure, analogous to the tetrazolate-based MOFs Mn₃[(Mn₄Cl)₃(BTT)₈]₂ (Mn-BTT, H₃BTT = 1,3,5-tris(2H-tetrazol-5-yl)benzene) and triazolate-based H₃[(Cu₄Cl)₃(BTTri)₈] (Cu-BTTri, H₃BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene).^182,183 Mn-BTT was formed with chloride-centered 8-coordinate [Mn₄(μ₄-Cl)]⁷⁺ squares (in a cuboid coordination geometry) linked via triangular BTT³⁻ ligands, and the anionic framework is balanced by [Mn(DMF)₆]²⁺ cations included in the tetrahedral pores. Cu-BTTri is isostructural with Mn-BTT, whereas the anionic framework is balanced by protons. In contrast, M₃(BTP)₂ comprises of M₄ squares, representing a neutral framework. These compounds share a the-a topology with accessible metal centers.

In 2020, Li and co-workers developed a mesoporous Ni(II)-based MPF (BUT-33) as the reticular expansion of Ni₃(BTP)₂ (Fig. 11).¹²² BUT-33 is composed of an elongated planar ligand, 2,4,6-tris(4-(pyrazolate-4-yl)phenyl)-1,3,5-triazine (TPTA³⁻) and square Ni₄ cluster. The planar configuration of TPTA³⁻ is critical for the reticular expansion of Ni₃(BTP)₂, which was not available with the non-planar extending 1,3,5-tris((1H-pyrazol-4-yl)phenyl)benzene (H₃BTPP) ligand. BUT-32 has the same composition as BUT-33, but presents a two-fold network interpenetration, which was confirmed to be a thermodynamically stable product. In contrast, the kinetically favored BUT-33 is non-interpenetrated, which features preferred mesopores for catalysis. In the following year, the authors synthesized the first Pd(II)-azolate MOF, BUT-33(Pd), using a deuterated solvent-assisted metal metathesis approach.¹²⁴ BUT-33(Pd) preserves the sodalite network and mesoporosity of the original BUT-33(Ni) template while exhibiting improved chemical stability and excellent catalytic activity due to the robust Pd(II)–pyrazolate bond and accessible Pd(II) centers. In 2024, Li and co-workers reported the access to BUT-124(Co), as the Co-based variant of M₃(BTP)₂. Comprising the Co₄ cluster and BTP³⁻ ligand, BUT-124(Co) was obtained in a two-step synthesis approach,¹²¹ by performing a post-synthetic metal metathesis in a template framework BUT-124(Cd).

Fig. 11 Images of (a) the cubic crystals of BUT-32 and (c) the microcrystalline powder of BUT-33. (b) Ni₄pz₈ cluster and TPTA³⁻ ligand. Structure of (d) BUT-32 and (f) BUT-33. (e) The sodalite network (the-a) of BUT-32 and BUT-33.¹²² Reprinted with permission from American Chemical Society 2020.

In addition to the 8-connected Co₄ clusters, there were also 6-connected Co₄O clusters discovered in MPFs. In 2009, Volkmer and co-workers constructed MFU-1 with the dmbdp²⁻ ligand and Co₄O unit.¹²⁶ Isostructural with MOF-5 with a pcu topology, the MFU-1 network contains octahedral [Co₄O(dmpz)₆] clusters that are reminiscent of the [Zn₄O(CO₂)₆] clusters in MOF-5.⁴ This is a rare sample based on [Co₄O] units and pyrazolate linkers to date, against the big M(bdp) family amenable to elaborate reticular/crystal engineering contraction/expansion/functionalization/modification.

2.2.3. Ni₈/Co₈/Zn₉. In 2008, Journaux and co-workers discovered the octanuclear hydroxonickel(II) complex [Ni₈(μ₄-OH)₆(μ₂-pz)₁₂]²⁻ formed in a molecular cube with eight Ni(II) ions at each vertex, six μ₄-OH⁻ groups covering each face and twelve pyrazolates (μ₂-pz) spanning each edge.¹⁸⁴ The [Ni(bma)(H₂O)₃]²⁺ (bma = bis(2-benzimidazolylmethyl)amine) cation is in disorder over two positions above an inversion center to balance the charge.

In 2010, Bordiga and co-workers integrated the cubic Ni(II) node with pyrazolate linkers to build MPFs.¹²⁷ This marked the beginning of research prevalence in this family, as the pyrzolate counterparts of the famous carboxylate UiO series.¹⁸⁵ Two pyrazolyl ligands, 2,6-di(1H-pyrazol-4-yl)pyrrolo[3,4-f]isoindole-1,3,5,7(2H,6H)-tetraone (H₂TET) and 4,4′-di(1H-pyrazol-4-yl)-1,1′-biphenyl (H₂PBP) were designed to react with Ni(OAc)₂, affording [Ni₈(OH)₄(OH₂)₂(pbp)₆] [shortened as Ni₈(pbp)₆] and [Ni₈(OH)₄(OH₂)₂(tet)₆] [shortened as Ni₈(tet)₆]. Although the authors claimed that the octanuclear node resembles that of the [Ni₈(μ₄-OH)₆(μ₂-pz)₁₂]²⁻ anion reported by Journaux and co-workers,¹⁸⁴ this cluster had been defined as a neutral one [Ni₈(OH)₄(OH₂)₂(pz)₁₂] (shortened as Ni₈). Two new isostructural MPFs exhibit the fcu topology of the UiO series, but have different pore sizes and chemistry.

In 2013, Navarro and co-workers reported a series of isoreticular materials based on different pyrazole ligands: including two dual-functional pyrazole/carboxylic ligands, H₂L1 = 1H-pyrazole-4-carboxylic acid and H₂L2 = 4-(1H-pyrazole-4-yl)benzoic acid, and five di-pyrazole ligands, H₂L3 = H₂BDP, H₂L4 = 4,4′-buta-1,3-diyne-1,4-diylbis(1H-pyrazole), H₂L5= H₂BPEB, and H₂L5-R (R = methyl, trifluoromethyl).³⁶ The combination of pyrazolate linkers and 12-connected Ni₈ nodes yielded the series of Ni₈(L)₆ MPFs, which present highly porous 3D networks without interpenetration, therefore, allow for the accessibility of cavities (Fig. 12).

Fig. 12 (a) The crystal structure of Ni₈(L5-CF₃)₆ is viewed as a combination of octahedral and tetrahedral cavities. (b) View of the tetrahedral (top) and octahedral (bottom) cages found in the crystal structures of Ni₈(L3)₆, Ni₈(L4)₆, and Ni₈(L5)₆, and the corresponding metric descriptors.³⁶ Reprinted with permission from Wiley-VCH 2013.

Following this work, they developed a sequential post-synthetic strategy to introduce missing-linker defects and extra-framework cations into Ni₈(L)₆ to tune their properties in gas adsorption and conductivity.^{45,130,132,134,140} Li and co-workers also introduced three different functional groups (CHO, CN, and COOH) onto the backbone of H₂BDP to build the functionalized Ni₈(BDP-X)₆ (X = CHO, CN, and COOH) MOFs in 2017.¹³⁶ These MPFs share the fcu network, but exhibit different pore chemistry with various functional groups. In 2018, Fei and co-workers grafted an amino group on the bdp²⁻ ligand (2-amino-[1,4-bis(1H-pyrazol-4-yl)benzene], H₂BDP-NH₂).¹³⁷ Ni₈(BDP-NH₂)₆ was then synthesized, followed by post-synthetic mercaptoacetylation and Ag(I) anchoring, giving rise to the final product Ni₈(BDP-NH₂)₆-AgS with high alkaline resistance and CO₂ affinity.

In 2019, Li and co-workers obtained single crystals of six Ni₈ MPFs under solvothermal conditions for the first time, including three Ni₈(BDP-X)₆ analogues: Ni₈(BDP)₆ (BUT-2), Ni₈(BDP-CN)₆ (BUT-3), Ni₈(BDP-CHO)₆ (BUT-4); and two disordered pcu MOFs with functional 1,3-BDP linkers, Ni₈(1,3-BDP-CN)₆ (BUT-48, 1,3-BDP-CN²⁻ = 5-cyan-1,3-benzenedipyrozolate), Ni₈(1,3-BDP-CF₃)₆ (BUT-49, 1,3-BDP-CF₃²⁻ = 5-trifluoromethyl-1,3-benzenedipyrozolate), and one with a trigonal ligand, Ni₈(BTPP)₄ (BUT-123, BTPP³⁻ = 1,3,5-tris((4-pyrozolate)phenyl)-benzene).¹³⁹ BUT-123, featuring a 3-connected trigonal ligand and 12-connected Ni₈ cluster, exhibits a new topology, which has not been observed in carboxylate MOFs yet due to unmatched symmetry.

In 2023, Li and co-workers extended the Ni₈(BDP)₆ family with three new ligands (4,7-di(1H-pyrazol-4-yl)benzo[c][1,2,5]thiadiazole, H₂BT-Pz; 4,7-di(1H-pyrazol-4-yl)benzo[c][1,2,5]-selenadiazole, H₂BS-Pz; 4,9-di(1H-pyrazol-4-yl)naphtho[2,3-c][1,2,5]selenadiazole, H₂NS-Pz), where the central phenyl ring of H₂BDP was replaced by benzothiadiazole, benzoselenadiazole, and naphthoselenadiazole, respectively.¹⁴⁵ Four fcu MPFs were synthesized accordingly (JNU-212, JNU-213, JNU-214, and JNU-215), where JNU-212 represents Ni₈(BDP)₆ as the parent compound. The linker engineering by tuning the electron-accepting capacity of the pyrazole linkers was demonstrated to render these Ni₈-based MPFs with enhanced charge separation and transfer efficiency under visible-light irradiation. Based on these materials, He and co-workers performed systematic research on employing cruciform pyrazolate ligands featuring pyrazole donors for framework construction, and the presence of bulk arms facilitated the formation of defects (missing linker on the node) in the resulting Ni₈-MPFs.^141–144

In 2016, Zhou and co-workers brought the Ni₈ cluster to a tetra-pyrazolate MOF and constructed the first pyrazolate-porphyrinic MOF, PCN-601, which is composed of the Ni₈ cluster and TPP⁴⁻ ligand (Fig. 13).³⁹ PCN-601 is isostructural with PCN-221, both of which are formed in the ftw-a topology. PCN-601 has small cage windows (∼2.1 × 8.0 Å after deducting van der Waals radii) and strong base-resistance in aqueous media. In the next year, they expanded this MPF by using a longer ligand.¹⁴⁹ The 5,10,15,20-tetrakis(4-(1H-pyrazol-4-yl)-phenyl)porphyrin (H₄TPPP) ligand was designed by inserting one phenyl between the central porphyrin cycle and peripheral pyrazolate groups in TPP⁴⁻. TPPP⁴⁻, as one of the shortest elongated versions of TPP⁴⁻, maintains the D_4h symmetry. The resulting PCN-602 is isostructural with PCN-601 but shows greater pore dimensions (6.0 × 14.0 Å) without compromising the robustness of the framework.

Fig. 13 (a) Ftw-a topological net; (b) structure of PCN-601 and (c) PCN-602.¹⁴⁹ Reprinted with permission from American Chemical Society 2017.

Zhou and co-workers further functionalized H₄TPPP to obtain a fluorinated ligand 5,10,15,20-tetrakis-(2,3,5,6-tetrafluoro-4-(1H-pyrazol-4-yl)phenyl)-porphyrin (TTFPPP).¹⁵⁰ Based on this ligand, a perfluorophenylene functionalized metalloporphyrinic MPF, PCN-624, was synthesized. PCN-624 is isostructural with PCN-601 and PCN-602. Thanks to the pore surface of PCN-624 decorated with pendant perfluorophenylene groups, the framework stability was improved, and selective guest capture was endowed upon this material.

In 2024, Cao reported the synthesis and catalytic applications of two highly chemically stable isoreticular pyrazolate MOFs, FICN-18 and FICN-19.¹⁵² These MOFs are composed of 12-connected M₈ (M = Ni and Co) clusters and linear metalloligands, forming two-fold interpenetrated structures. The Co₈ cluster, Co₈(OH)₄(OH₂)₂(pz)₁₂, is rare in MOFs. Li and co-workers subsequently reported four isostructural MOFs, fcu-L-Co, built from anionic the Co₈ cluster, [Co₈(OH)₆(pz)₁₂]²⁻, and four dipyrazolate ligands, including one recently reported ligand: H₂DPP,⁴⁰ and three new ligands: 3,6-di(1H-pyrazol-4-yl)pyridazine (2), 2,5-di(1H-pyrazol-4-yl)pyrazine (3), and 2,5-di(1H-pyrazol-4-yl)pyrimidine (4).¹⁵³ These fcu-L-Co frameworks are able to spontaneously transform from Co(II)₈ to Co(III)₈ in the air, marking the first example of such behavior in MOFs.

In 2022, Cao and co-workers constructed a Zn₉ MPF composed of an unprecedented [Zn₉O₂(OH)₂(pyz)₁₂] cluster and a Ni(salen)-based bis(pyrazolate) ligand.¹⁵⁴ The Zn₉ cluster consists of eight zinc atoms positioned at the vertexes of a cube, with an additional Zn atom sitting at its geometric center. The central zinc atom features a distorted tetrahedral ZnO₄ coordination geometry, where the four oxygen atoms link eight peripheral zinc atoms in a μ₃-O coordination mode. The geometry and connectivity of the Zn₉ cluster are reminiscent of the Ni₈/Co₈ cluster, as well as the overall framework topology. In contrast, the distance between two adjacent vertex Zn atoms is 3.58 Å, longer than those reported in Ni₈ (2.96–3.01 Å) and Co₈ (3.02–3.11 Å) clusters, indicating the larger size of the Zn₉ cluster. In addition, a two-fold interpenetration occurred in this MOF.

2.3. Unconventional types (Ni/Cu/Cu₅/Cu₁₂)

In addition to the typical frameworks assembled with commonly encountered chains/RBBs or clusters, there are some intriguing structures formed by unconventional building blocks.^{102,103,155,156} In 2016, Li and co-workers designed a tetra-pyrazole carbazole ligand 1,3,6,8-tetra(1H-pyrazol-4-yl)-9H-carbazole (H₄CTP).¹⁵⁵ Solvothermal reaction of this ligand and NiCl₂·6H₂O gave a 3D MOF, [Ni(H₄CTP)Cl₂] (BUT-41), which crystallized in the cubic space group Pm3̄ although H₄CTP is in a low C_s symmetry. The framework of BUT-41 shows a rhr topology when both the organic ligand and the single metal center are considered as four-connected nodes. Nanocages are enclosed by interconnecting 12 H₄CTP ligands and 20 Ni(II) ions. Each nanocage connects with six adjacent ones through sharing hexagonal windows, resulting in 3D intersecting channels. In 2024, Zhou and co-workers reported a novel MOF named PCN-300, which is constructed from the H₄TPPP ligand and [CuPz₄Cl₂].¹⁶³ The framework adopts a layered architecture and forms 1D open channels.

In 2017, Volkmer and co-workers synthesized a fluorinated linker 1,4-bis(3,5-bis(trifluoromethyl)-1H-pyrazole-4-yl)benzene (H₂tfpb) and the Cu(I)-based MPF family M[CFA-4], M[Cu₅(tfpb)₃], (M = Cu(I), K(I), Cs(I), Ca(0.5)).¹⁶² Cu(I)[CFA-4] features a 3D framework composed of an unusual pentanuclear [Cu(I)₅pz₆]⁻ (Cu₅) cluster. The Cu₅ unit consists of two distorted trigonal-planar coordinated copper(I) ions, and three linear 2-coordinate copper(I) ions. Five Cu(I) ions are connected by twelve nitrogen atoms from six pyrazolate moieties of different tfpb²⁻ ligands. M[CFA-4] represents the first MOF family in which the copper(I)-based Cu₅ cluster was observed, showing a new chiral topology thanks to its distinct symmetry. The Cu₅ clusters of the M[CFA-4] family are interconnected by the tfpb²⁻ ligand, generating 1D channels along the c-axis direction. In the as-synthesized Cu(I)[CFA-4], the channels are filled with disordered DMF molecules and Cu(I) ions, which balance the framework's negative charge. The latter could be exchanged with various metal ions [K(I), Cs(I), Ca(II)] to get the substituted M[CFA-4] compounds via post-synthetic cation substitution.

In 2018, Xiang and co-workers reported a hierarchical pyrazolate framework MROF-12, which was constructed from an elongated rigid ligand (H₂NDI) and a metal–ligand ring cluster [Cu₁₂(μ₂-OH)₁₂(dmpz)₁₂] (dmpz = 3,5-dimethyl-pyrazole, shortened as Cu₁₂).¹⁵⁶ Adjacent Cu(II) ions are linked by one bidentate bridging dmpz moiety and one μ₂-OH group. Twelve Cu ions form a puckered cyclic Cu₁₂ cluster, where the μ₂-OH group resides on the inner surface, while dmpz is located on the outer surface. Each cluster is connected to 12 neighboring clusters through NDI²⁻ linkers to produce two types of polyhedral cages: dodecahedron and octahedron cages. The interconnection of two types of polyhedral cages extends to form a 3D framework, generating three different channels.

We discussed above representative examples to show the present scope of high-dimensional MPFs constructed by adopting or beyond the crystal engineering/reticular chemistry principles. It is inevitable to miss some publications out due to having a different focus. The enormous library of MPFs has contributed great structure diversity, which provides opportunities for inducting the general rules of stable compounds synthesis and exploring their properties.

2.4. Synthesis of pyrazolate-based MOFs

Coincident with the inherent high stability of MPFs, researchers have encountered great difficulties in their general synthesis and structure determination.^139,186 Unlike the well-developed synthetic protocols for carboxylate MOFs, such as the acid-modulated solvethermal synthesis to harvest large high-quality single crystals, it is still harder to get single crystals of MPFs to date, especially for those involving multinuclear clusters.^26,122,139 Possible reasons include the short bridging length and small bending angle of the N⋯NH moiety, the difficult deprotonation of the pyrazolate group, and the easy precipitation of insoluble and amorphous products due to the formation of strong M–N coordination bond. As a result, their structure analysis and prediction are facing great challenges.

Nonetheless, some design strategies and synthetic approaches have been established and some single crystals of MPFs have been harvested, offering pivotal references for the discovery of new compounds.^{35,59,122,139} In this section, we highlight some established synthetic strategies, which are frequently implemented to construct MPFs, including methods for the direct synthesis of powder samples and single crystals, isoreticular expansion, topology-guided design (crystal engineering design), and known structure functionalization/modification.

2.4.1. Powder synthesis. Powder samples are usually obtained in the synthesis of MPFs. Generally, the metal salt and pyrazole ligand are dissolved in a solvent/mixed solvents and subsequently sealed to a reaction vessel. The reactor will be heated at a specific temperature for a given time duration. The resulting MPFs could show high crystallinity, however, only microcrystalline powders can be obtained in most cases. In addition, the size and morphology of MPFs are generally hard to control properly.

H₂BDP is a simple and popular pyrazole linker, which has been used for the synthesis of a great number of classical MPFs, while most of them have only been obtained as powder samples. The flexible and stable MOFs, Ni(bdp) and Zn(bdp), were harvested as microcrystalline powders.⁶² When a solution of Ni(CH₃COO)₂ was refluxed with H₂BDP in acetonitrile in the presence of Et₃N, Ni(bdp) was separated as a dark orange precipitate. Zn(bdp) is a white product obtained by reacting H₂BDP in benzonitrile, also in the presence of Et₃N. Similarly, microcrystalline powders of Fe₂(BDP)₃ and Fe(II)(bppdi)(DMF)_0.5 were available from solvothermal reactions between Fe(II)/Fe(III) salts and pyrazole ligands in DMF.^37,109

Another classic example is Ni₈(BDP)₆, which was prepared from the reaction between Ni(OAc)₂·4H₂O and H₂BDP in mixed DMF and H₂O.³⁶ Noteworthy, extending/prolonging the H₂BDP ligand can improve the porosity of MOF, but also reduce the solubility and affect the crystallinity.^36,127 To facilitate the precipitation of powders with high crystallinity, boc-protection of pyrazole ligands was leveraged in the case of extended bipyrazole linkers (Boc₂L4, Boc₂L5-R; R = H, CH₃, CF₃; Boc = tert-butoxycarbonyl), which favor the slow release of ligands in the reaction system and enhance sample quality.

Based on the tri-topic H₃BTP ligand, Ni₃(BTP)₂ and Cu₃(BTP)₂, as yellow and brown microcrystalline powders, were obtained from the reaction of Ni(OAc)₂·4H₂O or Cu(OAc)₂·H₂O in DMF at 160 °C.³³ They feature sodalite-like frameworks with accessible open metal sites. To access pore-expanding analogs of the M₃(BTP)₂ type, Long and co-workers synthesized the prolonged ligand H₃BTPP by inserting one phenyl group between the central phenyl ring and the peripheral pyrazolate groups.¹¹⁷ Both Ni-BTPP and Cu-BTPP precipitated in high yields as air-stable powders by reacting H₃BTPP with metal salts. The diffraction patterns indicate that the two structures are not isostructural nor isomorphous with M₃(BTP)₂. In 2020, Li and co-workers synthesized another extended ligand, 2,4,6-tris(4-(pyrazol-4-yl)phenyl)-1,3,5-triazine (H₃TPTA), with a non-steric triazine core.¹²² The conventional reaction of Ni(OAc)₂·4H₂O and H₃TPTA under heating at 150 °C and stirring for 2 h yielded a yellow microcrystalline powder of BUT-33. c-RED¹⁸⁷ was applied to determine the structure of BUT-33, which is isostructural with Ni₃(BTP)₂ and the desired mesopores (Fig. 14).

Fig. 14 3D reciprocal lattice presenting the (a) hk0 and (b) hhl planes of the BUT-33 crystal from c-RED analysis, and (c) Pawley refinements of BUT-33.¹²² Reprinted with permission from American Chemical Society 2020.

2.4.2. Single crystal synthesis. Single-crystal X-ray diffraction (SCXRD) is the most straightforward technique for structural determination of MOFs. Therefore, synthesizing single crystals suitable for SCXRD analysis is usually a prerequisite for developing new MOFs. However, this is challenging for MPFs, relative to the synthesis of carboxylate MOFs, as the formation of strong bonds in most cases leads to rapid precipitation of powder products.³³ Modulated synthesis involves introducing modulators to regulate the reaction between metal ions and organic ligands, i.e. competitive coordination, which generally favors fine-tuning of the nucleation and crystal growth rates.²⁶ The competitive mechanism allows for selective coordination, enabling precise control over crystal morphology and promoting the growth of large MOF crystals. Despite the lack of versatile synthesis methods for MPFs, the use of various modulators has somewhat facilitated the expansion of the MPF library, contributing to some successful examples of crystal synthesis.

Although the authors were unable to obtain single crystals of Fe(II)(bppdi)(DMF)_0.5 suitable for X-ray diffraction as aforementioned above, the framework structure has been established from single crystals of an oxidized Fe(III) analogue, Fe(III)(bppdi)(OH).¹⁰⁹ In an N₂-filled glovebox, a borosilicate glass tube was charged with Fe(OTf)₂(THF)₂, H₂bppdi, DMF, as well as one drop of Et₃N. The tube was then sealed with a glass stopcock adapter, degassed, and flame-sealed under a static vacuum. After heating the sealed tube at 170 °C for 2 days, a few small red crystals of Fe(III)(bppdi)(OH) were obtained, offering great opportunities for the structure determination of this MPF. CFA-2 was also synthesized as colorless crystals by heating a DEF–MeOH solution of H₂phbpz and Cu(OAc)₂·H₂O in the presence of Et₃N, which is necessary for deprotonating the ligand.¹¹⁶ Mn-CFA-6 and Fe-CFA-6 were also synthesized from solvothermal reactions between trivalent metal salts and the H₂BPZ ligand.^94,173 For Mn-CFA-6, H₂BPZ, N,N-dimethylacetamide (DMA), and 0.1 mL of 2,6-lutidine were mixed in a tube, which was capped and heated at 130 °C for three days. Fe-CFA-6 crystals were obtained by adjusting the molar ratios of the ligand, metal salts, and base in a DMA solution.

The above reactions were performed with the assistance of trace base modulators. In some cases, acid modulators could also help get single crystals of MPFs. Single crystals of Fe₂(BDP)₃ were obtained by modifying the reported procedure for the synthesis of microcrystalline powder, where an acidic modulator was incorporated to increase the crystallite size.¹¹³ Reaction of Fe(acac)₃ with H₂BDP and acetylacetone in DMF produced dark yellow, needle-like crystals.

The Long group harvested single crystals of Co(bdp) by reacting Co(CF₃SO₃)₂ and H₂BDP in DEF at 130 °C for 4 days under air-free conditions along with programmable heating and cooling.³⁵ Volkmer's group obtained a similar crystal, named MFU-3, under different conditions: the ligand reacting with Co(NO₃)₂·6H₂O in DMF at 120 °C for 3 days, where a trace amount of diluted HCl aqueous solution was used as a modulator.⁵⁹ In 2022, Li's group further modified the synthesis of Co(bdp): reacting the H₂bdp ligand with Co(NO₃)₂·6H₂O in DMF at 130 °C, using deionized water as a modulator.⁴⁰ This method yielded single crystals suitable for SCXRD within 12 hours. Double-walled Co(bdp) analogous crystals of BUT-53 to BUT-58 were also harvested from solvothermal reactions using a DMF and water-mixed solvent in the temperature range of 100–150 °C.⁴⁰ Among these compounds, crystals of BUT-53, BUT-54 and BUT-57 were accessed with trace acetic acid as the modulator; otherwise, only tiny microcrystals were available. The double-walled isostructural framework, JNU-204, was synthesized as orange prismatic crystals via a solvothermal reaction in DMF and water at 120 °C for 72 h, where HNO₃ was added as the modulator.¹⁰⁶ In these syntheses, the addition of acidic modulators alters the pH value of the reaction systems, resembling their role in the reaction with carboxylic ligands, including inhibiting the deprotonation of pyrazole ligands and slowing down the nucleation rate. This favors their competitive coordination with the metal centers, contributing to the growth of large crystals.

Preparing single crystals for polynuclear cluster-based MPFs (such as Ni₈ MOFs) has consistently been challenging, and the Li group addressed this issue in 2019. Single crystals of Ni₈ MPFs (BUT-2 to BUT-4, BUT-48, BUT-49, and BUT-123) were obtained from the solvothermal reaction between Ni(NO₃)₂·6H₂O and respective ligands in a mixed solvent of DMF or DMA and H₂O (Fig. 15).¹³⁹ The formation and size of single crystals were significantly influenced by the metal-to-ligand ratio, solvent composition (particularly water content), and the reaction time, all of which also impacted crystal quality and yield. Similarly, a solvothermal reaction involving H₃TPTA and Ni(NO₃)₂·6H₂O in a mixture of DMF and H₂O at 150 °C for 72 h, with a small amount of acetic acid as the modulator, yielded yellow cubic crystals of BUT-32.¹²² They present the first Ni₈-/Ni₄-based MPFs whose structures have been unambiguously identified by SXRD.

Fig. 15 Optical microscopy images of six Ni₈ MOF single crystals. The radius of the crystal centering cycle is 0.1 mm.¹³⁹ Reprinted with permission from American Chemical Society 2019.

These successful examples are evidence that the synthesis of MPF crystals is highly sensitive to synthetic conditions, particularly for growing single crystals suitable for SCXRD. Subtle variations in synthetic parameters can significantly affect the quality of the resulting MPF crystals and even alter the product species. A wide range of modulators, including weak acids and bases such as acetic acid, Et₃N, ammonia solution and 2,6-lutidine, as well as some strong acids such as HCl, HNO₃, and trifluoroacetic acid, and even neutral water, would make a difference in yielding good crystals by facilitating the deprotonation of pyrazole ligands and regulating the reaction equilibrium through competitive coordination.

2.4.3. Crystal engineering/reticular chemistry. Upon the discovery of a parent compound, a family of isostructural materials would be accessible under the guidance of crystal engineering/reticular chemistry principles thanks to the modular/periodic nature of MOFs. The expansion of typical MOF series, such as M₃(BTP)₂, Ni₈(L)₆, and PCN-601, to name a few, by varying the ligand length, functionality, and metal species, has validated the feasibility of this strategy.^{33,36,39,122,149,150}

The development of solidate-type MPFs, including M₃(BTP)₂, BUT-33, and BUT-124(Co), demonstrates the power of these principles in varying the pore size and composition.^33,121,122 In 2011, Long and co-workers constructed M₃(BTP)₂ (M = Ni, Cu) with M₄ clusters and tritopic pyrazole ligand H₃BTP, creating a sodalite-like structure with accessible metal centers.³³ In 2020, Li and co-workers obtained the mesoporous analog BUT-33 using the elongated planar ligand TPTA³⁻, where the active sites are more accessible than in the microporous M₃(BTP)₂.¹²² In 2024, they synthesized BUT-124(Co) from the parent framework, BUT-124(Cd), via post-synthetic metal metathesis.¹²¹ The M₃(BTP)₂ family then has great variety in terms of nodal metal species, including Ni, Cu, Cd and Co.

Based on the first discovery of Ni₈ MPFs, Ni₈(pbp)₆ and Ni₈(tet)₆,¹²⁷ Navarro and co-workers expanded this type of structure by using pyrazolate ligands with various functionalities and lengths, to synthesize a series of isoreticular MPFs Ni₈(L)₆.³⁶ These MPFs exhibit highly porous, non-interpenetrated 3D networks, maintaining accessible pores with different size and chemical environments.

As a continuation of their interest in constructing pyrazolate-porphyrinic MPFs, Zhou and co-workers constructed PCN-601 composed of the Ni₈ cluster and H₄TPP ligand.³⁹ In 2017, they expanded it to PCN-602 by using an elongated ligand H₄TPPP, creating larger pores suitable for catalysis under basic conditions.¹⁴⁹ Using a fluorinated ligand, TTFPPP, they further developed PCN-624, which maintains the structure robustness of its parent framework PCN-602 and exhibits an inner pore surface covered with fluorine (Fig. 16).¹⁵⁰ All three MOFs share the ftw-a topology, highlighting the strength of crystal engineering/reticular chemistry principles in enriching the diversity of the MOF community, varying chemical composition and pore dimension, and attaching functionalities for specific purposes.

Fig. 16 (a) Ftw-a topological network. (b) Construction of PCN-602. (c) Synthesis and structure of PCN-624.¹⁵⁰ Reprinted with permission from American Chemical Society 2018.

2.4.4. Post-synthetic modification. The desired framework stability and suitable functionality required for the specific application do not always co-exist in one single MPF material.^19,20 Post-synthetic modification (PSM) provides an ideal approach to attaching the needed functionality and/or improving the stability of existing MPFs through covalent modification of organic ligands, ligand or metal exchange, and cluster modification.^9,16

A tandem post-synthetic modification approach was employed to build the first Ag(I)-functionalized MPF.¹³⁷ To introduce thiol functionality, the microcrystalline Ni₈(BDP-NH₂)₆ was incubated with mercaptoacetic acid in a CH₂Cl₂ solution at room temperature. The covalently modified MOF was then treated with an aqueous solution of 6 mM AgNO₃ to anchor Ag(I) sites. This resulting MOF catalyst exhibited high activity and efficiency in the cyclic carboxylation of propargyl amines and CO₂ under ambient conditions, due to the incorporation of Ag(I) centers.

An ideal crystal should have good structural ordering/periodicity, but thermodynamics favor deviations from this order, creating “intrinsic defects” that impact various properties of solid-state materials.^188,189 Navarro and co-workers investigated the effect of post-synthetic introduction of missing linker defects in Ni₈(OH)₅(EtO)(H₂O)₂(BDP-X)₆ (X = H, OH, NH₂) on their performance in SO₂/CO₂ sorption.¹³⁰ Treating MOFs with ethanolic KOH solutions deprotonated coordinated water and hydroxo tags, created defects, and added extra-framework potassium ions, yielding 1@KOH, 2@KOH, and 3@KOH (Fig. 17). The treated materials have higher SO₂ adsorption capacities and SO₂/CO₂ partition coefficients. The countering potassium ions can be further exchangeable with Ba and Cu, and the resulting materials behave differently against their K-based analogs.^132,134

Fig. 17 (a) Schematic representation of the successive PSMs, from pristine nickel pyrazolate frameworks to the missing linker defective 1@KOH, 2@KOH 3@KOH and subsequently, the ion-exchanged 1@Ba(OH)₂, 2@Ba(OH)₂), and 3@Ba(OH)₂. (b) Overview of the structures of the studied materials. Reprinted with permission from Springer Nature 2017.

To access the completely reduced A₂Fe₂(BDP)₃ (A = Li⁺, Na⁺, K⁺) compounds from the parent Fe₂(BDP)₃, single-crystal-to-single-crystal reductions were performed using lithium, sodium, or potassium naphthalenide.¹¹³ THF solutions of Na(C₁₀H₈) or K(C₁₀H₈) were slowly added to Fe₂(BDP)₃ crystals in THF, with excess naphthalenide to address the possible issues of incomplete solvent exchange and uncertainties in the formula unit. Single crystals of A₂Fe₂(BDP)₃·yTHF were obtained after suspensions were stored for one to two months. For the preparation of Li₂Fe₂(BDP)₃·yTHF, the direct reduction did not afford reduced single crystals suitable for diffraction. Alternatively, Fe₂(BDP)₃ crystals on a fritted filter were suspended in a vial containing a slurry of microcrystalline Fe₂(BDP)₃ stirred in a THF solution of lithium naphthalenide. Crystals of Li₂Fe₂(BDP)₃·yTHF could be isolated over half to one month.

Developing MOF-based noble metal catalysts typically involves incorporating metal nanoparticles or anchoring metal-containing molecules onto the framework, which can narrow the pore apertures and limit mass transfer, impairing catalytic performance.¹⁶ Uneven dispersion of catalysts may also lower activity. Constructing MOFs with noble metal nodes ensures a high density of uniformly distributed metal sites without compromising porosity. The similar square planar coordination of Pd(II) and Ni(II) allows Pd(II) to replace Ni(II) in BUT-33.¹²⁴ A six-day reaction in deuterated chloroform at 60 °C achieved this metathesis, resulting in the mesoporous Pd-MPF, BUT-33(Pd), with preserved crystallinity. BUT-33(Pd) retains the structure of BUT-33, demonstrates excellent chemical stability, and performs well as a heterogeneous Pd(II) catalyst.

3. Stability of pyrazolate MOFs

MOFs incorporating pyrazolate ligands with late transition metals generally exhibit high chemical robustness, attributed to the strong donating properties of pyrazolates.⁴⁷ Various pyrazolate ligands have been used to build highly base-resistant materials, including PCN-601, Ni₃(BTP)₂, and BUT-33.^33,122,149 These MPFs demonstrate significantly better stability in basic environments compared to carboxylate MOFs, which are able to withstand even saturated NaOH solutions at 100 °C. This underscores the power of pyrazolate ligands in the rational construction of base-stable MOFs. Here we introduce some typical examples and analyze the factors that influence their stability.

3.1. Base-stable pyrazolate MOFs

The group and the Navarro group reported MOFs featuring the linear bipyrazole ligand H₂BDP, such as Fe₂(BDP)₃, Co(bdp), Ni(bdp), and Zn(bdp).^{33,35,37,38,62} These MOFs contain linear chains of metal ions with varying coordination geometries, which affect not only the structural flexibility but also framework stability. Fe₂(BDP)₃, Ni(bdp), and Zn(bdp) are robust in alkaline aqueous solutions, among which Fe₂(BDP)₃ can notably maintain integrity in saturated NaOH.^33,37,62 However, Co(bdp) is not that stable, whose handling needs to be done in a glove box.³⁸ The isomer of Co(bdp), the double-walled BUT-55, as well as the whole BUT-55 series, BUT-53 to BUT-58, has helical RBBs of Co(II) ions linked by BDP²⁻ ligands and shows stability in pH 14 NaOH for 24 hours and maintains integrity in the air after a year.⁴⁰

Featuring cubic M₄Pz₈ clusters connected to triangular planar ligands, sodalite-type MPFs, M₃(BTP)₂ (M = Ni²⁺ and Cu²⁺) demonstrate exceptional stability in boiling water and alkaline solutions. For instance, Ni₃(BTP)₂ retains its crystalline structure in saturated NaOH.³³ The extended version of Ni₃(BTP)₂, BUT-33, is a rare example of mesoporous pyrazolate MOFs.¹²² Both BUT-33 and its interpenetrated microporous analog BUT-32 exhibit excellent resistance to highly basic solutions with a pH of 14.

The 12-connected Ni₈/Co₈ clusters are prominent inorganic nodes in base-stable pyrazolate MOFs. Integrating Ni₈/Co₈ nodes with dipyrazolate linkers, the typical M₈(L)₆ family shares the fcu topology but exhibits different pore sizes and functionalities.^36,127,139 Stability tests confirmed that these MOFs maintain their structure in NaOH aqueous solution, demonstrating excellent base resistance. In 2016, Zhou, Li, and co-workers developed PCN-601, the most base-stable pyrazolate-based MOF, using a top-down approach.³⁹ When the Ni₈ clusters meet tetratopic ligands, PCN-601 and PCN-602 were synthesized with the ftw-a topology (Fig. 18a and b), isostructural with MOF-525 (or PCN-221).^190,191 PCN-601 demonstrated exceptional stability in 20 M NaOH at 100 °C, highlighting its robustness under concentrated alkaline conditions. Although PCN-602 with larger pores is somewhat less resistant to bases compared to PCN-601, it maintains structural integrity in 1 M NaOH and remains stable in 1 M solutions of KF, Na₂CO₃, and K₃PO₄, which, as coordinating anions, can easily disintegrate MOFs (Fig. 18c and d).¹⁴⁹

Fig. 18 (a) Image of PCN-601 samples being treated with 0.1 mM HCl solution and saturated NaOH for 24 h, respectively. (b) UV-vis spectra of the samples in (a). (c) PXRD patterns and (d) N₂ adsorption/desorption isotherms of PCN-602 and of those treated in different aqueous solutions.^39,149 Reprinted with permission from American Chemical Society 2016 and 2017.

3.2. Factors contributing to the stability of pyrazolate MOFs

Similar to the whole MOF community, the stability of MPFs can be affected by multiple factors, including the strength of the coordination bond, connectivity of building blocks, length and rigidity of ligands, hydrophobicity of the pore surface, the operating environment, etc.

3.2.1. Coordination bond strength. Opposite to the case in which carboxylate MOFs have higher acid resistance, MPFs demonstrate significantly better stability in basic environments.^19,46 Low-valence metal ions tend to form strong bonds with pyrazolate groups while exhibiting relatively low affinity to H₂O/OH⁻, rendering MPFs featuring low-valence metal ions and pryazolate ligands to be stable in alkaline solutions.²⁰ Here, we will analyze the strength of coordination bonds between low-valence metal ions and pryazolate ligands from both thermodynamic and kinetic perspectives.

A strong coordination bond forms the thermodynamic foundation for the stability of MOFs, generally providing these materials with high resistance to various competing entities.¹⁹ The strength of coordination bonds is influenced by the charge density of the metal ions.²⁰ Higher metal ion charges and smaller ionic radii contribute to greater charge density, thereby increasing metal–ligand bond strength. This concept aligns with the HSAB principle and is supported by extensive research in MOF chemistry.^20,192 According to the HSAB principle, stable MPFs can be constructed using a combination of soft azolate ligands (such as imidazolates, pyrazolates, and triazolates) and soft divalent metal ions (like Zn²⁺, Cu²⁺, Ni²⁺, Mn²⁺, and Ag⁺). The metal–nitrogen (M–N) bonds typically exhibit greater resistance to bases compared to the M–O bonds formed in carboxylate MOFs.⁴⁷ Such enhanced base resistance is due to the stronger orbital interactions between nitrogen donors in azolates and the metal ions. Increasing the donor strength of the ligand, which is measured by its basicity, further strengthens the metal–ligand bonds. Among azolaes, pyrazolates exhibit the highest pKa values, i.e. the highest basicity, promising to form strong coordination bonds. This effect is particularly pronounced with late transition metals, leading to improved framework stability.

However, the stability of MPFs is not solely determined by the strength of the M–N bonds. The thermodynamic stability of the metal hydroxides formed in alkaline conditions also plays a significant role.⁴⁷ The pK_sp values of various metal hydroxides, which indicate the metal ions’ affinity for hydroxide ions (OH⁻), are critical in this context. Higher pK_sp values imply a stronger affinity for OH⁻, making MOFs with such metal ions more susceptible to degradation in alkaline environments. Late transition metals, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺, have a lower affinity toward OH⁻.⁴⁷ Therefore, MOFs incorporating pyrazolate ligands and late transition metals often exhibit exceptional base stability. This stability is attributed to both the strong metal–ligand interactions and the lower susceptibility of these bonds in competition with OH⁻ in highly alkaline conditions.

The kinetic inertness of MOFs is also a crucial indicator of their utility in practical applications, which is primarily influenced by the activation energy required to break the coordination bonds.⁴⁶ High kinetic inertness means that MOFs can resist decomposition under moderate conditions with less energy input, such as lower temperatures or less extreme chemical environments. In kinetics, this is related to the ligand exchange rates of the metal ions (Fig. 19).⁴⁶ For example, even though Pd and Ni have similar charge–radius ratios and affinities for hydroxide anions, Pd-MPFs exhibit slower ligand exchange rates compared to Ni-MOFs. Experimental data show that Pd-MPFs are more inert than their Ni-based counterparts.¹²⁴ Similarly, Co-MPFs display higher kinetic inertness compared to Cd-MPFs of similar structures.¹²¹ In addition to ligand exchange rates, other structural factors also affect the kinetic inertness of MOFs, which will be explored further in the next section.

Fig. 19 Water exchange rate of metal ions.⁴⁶ Reprinted with permission from Springer Nature 2019.

3.2.2. Structural factors. Research on the stability of MOFs reveals that MPFs composed of identical inorganic clusters and organic ligands with the same coordination groups can display markedly different stabilities under alkaline conditions. In addition to the strength of coordination bonds, certain factors also make a difference in the stability of MPFs.
3.2.2.1. Connectivity of building units. Higher connectivity in MPFs, whether from organic ligands or metal nodes, enhances structural stability by facilitating the repair of defects and preventing further decomposition.²⁰ This principle helps explain the remarkable chemical stability of PCN-601, which features a ftw-a network.³⁹ Both the Ni₈ cluster (12-connected node) and the porphyrinic ligand TPP⁴⁻ (4-connected linker) exhibit relatively high connectivity compared to other inorganic clusters and organic ligands. From a kinetic standpoint, MPF decomposition in solution can be viewed as a process where coordination moieties are replaced by small molecules or ions, leading to the formation and accumulation of structural defects. Higher connectivity in the framework helps suppress ligand dissociation and enhances the rate of ligand association, which promotes faster repair of these defects. This effect, analogous to the chelating effect observed with multi-dentate ligands, is referred to as the 3D chelating effect.¹⁹³ The same principles apply to highly connected metal nodes, reinforcing the stability of MPFs. Consequently, MPFs with more interconnected fragments are significantly more stable.
3.2.2.2. Length and rigidity of ligands. In addition to the 3D chelating effect, the length and rigidity of ligands play a significant role in determining the stability of MOF materials. PCN-601 demonstrated that the length and rigidity of the ligand are associated with the activation energy required for ligand dissociation, impacting the overall decomposition of the MPF.³⁹ As illustrated in Fig. 20, two isoreticular frameworks are compared, one constructed with a long ligand and the other with a short one. From a kinetic perspective, the decomposition of MPFs requires activation energy. Whether the substitution reactions proceed through a dissociative or associative pathway, metal ions or clusters and ligands undergo displacement in the transition states due to the addition of molecules or anions to the metal coordination sphere. For MPFs with isostructures, we can reasonably assume that the displacement of ligand coordination terminals remains consistent during these processes (d_s = d_l). However, shorter ligands will bend more significantly in the transition state (with a greater bending angle θ_s > θ_l), resulting in a higher energy barrier and making decomposition kinetically more challenging. Consequently, MPFs with shorter ligands exhibit greater chemical stability and higher kinetic inertness under alkaline conditions, a trend confirmed by experimental data. For instance, PCN-601 and Ni₃(BTP)₂, which have shorter ligands, retain their structural integrity even in saturated NaOH solutions.^33,39 In contrast, their reticular expansions PCN-602 and BUT33, which feature elongated ligands, completely decompose when exposed to the same conditions.^122,149 These findings demonstrate that shorter rigid ligands enhance the chemical stability of MOFs, especially in basic environments.

Fig. 20 Illustration of the kinetic stability of MOFs with ligands of different lengths.³⁹ Reprinted with permission from American Chemical Society 2016.

3.2.2.3. Hydrophobic groups. Modifying MOFs’ surfaces to increase their hydrophobicity has proven to be an effective strategy for enhancing their stability in basic environments.¹⁹⁴ In 2011, Volkmer and colleagues synthesized the Co(bdp) analog, MFU-2, utilizing the four-methyl functionalized dmbdp ligand.⁷⁷ Unlike the flexible Co(bdp), MFU-2 exhibits a rigid framework, characterized by a type I gas sorption isotherm and enhanced stability. Introducing hydrophobic groups, such as methyl groups, makes the materials less accessible to water molecules and hydroxide ions (OH⁻), which are common sources of framework degradation in aqueous solutions.^19,20,126 The presence of methyl groups has been shown to increase the relative pressure of water vapor condensation within the frameworks, demonstrating their impact on hydrophobicity. Consequently, these modifications reduce the probability that vulnerable sites within the MOFs are exposed to destructive species, leading to greater kinetic inertness and improved stability under basic conditions.

4. Applications of pyrazolate MOFs

Thanks to their customizable structures, expansive surface areas, adjustable pore sizes, and exceptional stability, MOFs have been extensively researched and have demonstrated remarkable performance across various applications. This section will summarize the application of MPFs in gas adsorption and separation, heterocatalysis, sensing, biomedicine, etc.

4.1. Gas adsorption/separation

Gas adsorption and separation have garnered significant interest among chemists and engineers due to their low energy requirements in addressing social, environmental, and industrial challenges.^{11,13,17,27,195} MOFs, with their adjustable porous structures and large specific surface areas, are ideal for molecular adsorption and separation. MPFs, in particular, have been extensively studied for these applications, demonstrating excellent performance. They have been applied in the adsorption and separation of hydrocarbons, the removal of harmful gases from the air (such as SO₂ and VOCs), and the storage of energy-related gases like CH₄ and H₂.^{37,38,40,62,65,113,114,132}

4.1.1. Toxic gas removal. The release of toxic pollutants into the atmosphere, including combustion products, chemical reaction byproducts, and industrial gas leaks, is a growing global concern. Hazardous compounds such as SO₂, diethylsulfide, tetrahydrothiophene, and volatile organic compounds like benzene are major sources of environmental air pollution, predominantly from human activities.²⁷ Effective capture and segregation of these harmful chemicals is crucial for environmental protection and human health. MPFs are leading the way in mitigating the adverse effects of human activities by enhancing the efficiency of industrial processes and pollutant remediation, thanks to their remarkable potential as physisorbents.

Navarro et al. demonstrated that the Ni₈(L)₆ series can serve as sorbents for the capture of warfare agents. Their performance is influenced by the pore size and surface polarity, which are determined by the length and functionality of the linkers.³⁶ Analysis of the adsorbate phase after the adsorption of diethylsulfide (DES) in 80% RH humid streams showed that only Ni₈(L5-CF₃)₆ can capture DES in high efficiency. Ni₈(L5-CF₃)₆ also outperforms the hydrophobic activated carbon Blücher-101408, demonstrating the superiority of the Ni₈(L)₆ family in DES capture. Furthermore, Navarro et al. introduced a defect engineering strategy that deliberately incorporated missing-linker defects and uncoordinated extra-framework Ba²⁺ into the Ni₈-MPF, resulting in 1@Ba(OH)₂, 2@Ba(OH)₂, and 3@Ba(OH)₂ to enhance SO₂ adsorption.¹³² These modified materials exhibited high SO₂ adsorption capacities at 303 K and low SO₂ partial pressures (0.025 bar). Dynamic adsorption experiments revealed that these treatments enhanced SO₂ capture capacity by increasing the basicity of the nickel hydroxide clusters and introducing more amino and hydroxyl functional groups on the linkers, which are binding sites of SO₂, as well as by the formation of stable MSO₃ (M = Ba, 2K) species near defect sites.

Earlier work by Navarro et al. also reported that Ni(bdp) can be used to capture benzene and cyclohexane.⁶² This material also demonstrated the ability to capture tetrahydrothiophene (up to 0.34 g per gram of material) from CH₄–CO₂ mixtures under dynamic conditions, even in the presence of 60% humidity. However, current regulations on the low concentration of aromatic-based VOCs in indoor air and industrial effluent streams have increased the demand for structurally designed MOF-based adsorbents, which need to show enhanced binding strength to capture VOCs at trace levels.^196–198 In this regard, the 1D channel apertures in the double-walled MPF family (BUT-53–58) were fine-tuned by modifying the linker length and geometry, resulting in channels ranging from 7.8 to 11 Å. These MOFs demonstrated high benzene uptakes at 298 K ranging from 2.47 to 3.28 mmol g⁻¹ at pressures below 10 Pa (Fig. 21). Notably, with an inlet concentration of 10 ppm and a flow rate of 10 mL min⁻¹, BUT-55 achieves a breakthrough time of approximately 8000 h g⁻¹. The benzene residual in the effluent streams was reduced to 2.82 ng L⁻¹, well below the indoor air limits of 3–100 μg m⁻³. SCXRD analysis revealed that the adsorption mechanism for benzene binding in BUT-55 involves C–H⋯N/C–H⋯π interactions within the cavities and C–H⋯N, C–H⋯π, and π⋯π interactions within the channels. Among this family, BUT-55 shows the highest adsorption capacity at pressures below 0.01 kPa, significantly surpassing the performance of the best carbon-based materials like carboxen for capturing aromatic-based VOCs. This kind of performance, combined with facile regeneration and good recyclability, makes BUT-55 a promising adsorbent for removing aromatic VOCs from indoor air.

Fig. 21 Benzene static isotherms of BUT-53 to BUT-58: (a) linear-scale plots and (b) logarithmic-scale plots of P/P₀. (c) Comparison of benzene uptakes with various representative porous sorbents at low relative pressures. (d) Breakthrough curves for BUT-55 and Co(bdp).⁴⁰ Reprinted with permission from Springer Nature 2022.

4.1.2. Gas storage. Since the establishment of permanent porosities, MOFs have been recognized as promising adsorbents for gas storage.¹⁷ Compared to traditional porous materials, MOFs offer prominent structural advantages. In particular, the flexibility of MPFs shows great potential in the storage of important gases such as CH₄.

To achieve high CH₄ working capacities in MOFs for onboard storage applications, maximizing CH₄ uptake at an adsorption pressure of 65 bar while minimizing uptake at desorption pressures of 5.0/5.8 bar is crucial. The Long group demonstrated high CH₄ working capacity in two flexible MOFs, Fe(bdp) and Co(bdp), and effectively regulated internal heat during both adsorption and desorption processes.³⁸ Specifically, these two flexible MOFs undergo a structural phase transition at certain CH₄ pressures, resulting in adsorption and desorption isotherms with a distinct ‘step’. This behavior enables these materials to achieve greater storage capacities than traditional adsorbents. Notably, Co(bdp) has a record-high CH₄ working capacity at 25 °C, with 155 cm³ STP cm⁻³ (v/v) at 35 bar and 197 v/v at 65 bar by then. This capacity is achieved by coupling with the heat offset resulting from the structural transformation, where the endothermic expansion reduces the heat released during adsorption and the exothermic contraction minimizes the cooling effect during desorption. In 2016, the same group further regulated the CH₄-induced framework opening pressure by systematic ligand functionalization.⁶¹ The resulting frameworks, functionalized Co(bdp), are isoreticular to the parent structure, Co(bdp), and exhibit similar phase transition behaviors. Mechanism studies reveal that the fluorine atoms on the aryl ring disrupt edge-to-face π–π interactions, which help stabilize the collapsed phase and induce framework opening to occur at lower pressure, while deuterium atoms preserve these interactions and methyl groups strengthen them, resulting in framework opening at the highest pressure. This work provides a powerful tool to optimize the functionality of flexible MOFs for better CH₄ storage performance.

The alkali cations of A_xFe₂(BDP)₃ (A = Li⁺, Na⁺, K⁺; 0 < x ≤ 2) reside in proximity to the framework after activation, and the Long group studied how the surroundings affect the Lewis acidity of these cations using H₂ as a probe molecule.¹¹³ Microcrystalline samples of Li_xFe₂(BDP)₃ (x = 1.18, 1.90), Na_xFe₂(BDP)₃ (x = 1.14, 2.06), and K_xFe₂(BDP)₃ (x = 0.68, 1.33) were synthesized through the stoichiometric reduction of Fe₂(BDP)₃ using lithium, sodium, or potassium naphthalenide. By varying the cation loadings, they assessed the effect of cation density on H₂ uptake. The hydrogen adsorption results demonstrated that the reduced frameworks generally have lower adsorption capacities than Fe₂(BDP)₃, due to the partially quenched charge density by interactions with the linkers and iron–pyrazolate chains. Interestingly, Mg_0.85Fe₂(BDP)₃ was obtained through reductive insertion with magnesium anthracene, incorporating charge-dense Mg²⁺ cations and endowing substantially improved H₂ uptake and affinity compared to Fe₂(BDP)₃.

Meanwhile, they uncovered that A_xFe₂(BDP)₃ (A = Na⁺, K⁺; 0 < x ≤ 2), which feature coordinatively saturated iron centers, could strongly and selectively adsorb O₂ over N₂ at ambient (25 °C) or even elevated (200 °C) temperatures, presenting a potential O₂-selective adsorbent to produce high-purity oxygen from the air.¹¹⁴ A selective adsorption mechanism was proposed, involving outer-sphere electron transfer from the framework to generate superoxide species, which are stabilized by alkali metal cations within the 1D triangular channels. They also observed similar O₂ adsorption behavior in an expanded-pore framework analogue, which supports the O₂ adsorption mechanism. This approach involves the chemical reduction of a robust MOF to expose active metal sites, which enable O₂ binding through an outer-sphere electron transfer mechanism, representing a promising and underexplored strategy for designing next-generation O₂ adsorbents.

4.1.3. Gas separation. Gas separation is a key and fundamental industrial process for manufacturing chemicals, fuels, plastics, and polymers, but traditional cryogenic distillations are highly energy-intensive.¹¹ MOFs have emerged as some of the most promising materials for addressing industrial gas separations, due to easy operation and low energy footprint.

In 2013, Long and co-workers reported utilizing Fe₂(BDP)₃ featuring sharp triangular channels to separate hexane isomers.³⁷ The static adsorption isotherms and isosteric heat of the isomers, n-hexane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane and 2,2-dimethylbutane, indicate that a higher degree of branching leads to weaker binding with the framework. In breakthrough experiments conducted with an equimolar mixture of n-hexane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane and 2,2-dimethylbutane, di-branched isomers were the first eluted, followed by the monobranched isomer, and the last linear isomer. Configurational-bias Monte Carlo simulations reveal the separation mechanism: the number of carbon atoms interacting with the pore surface of Fe₂(BDP)₃ decreases as the degree of branching increases. Therefore, dimethylbutane isomers exhibit the weakest interactions with the framework surface, allowing them to pass through the triangular channels quickly. In contrast, the linear n-hexane fits the corner of triangular channels with all six carbons, leading to the longest duration time.

In 2018, the Long group disclosed that the flexible Co(bdp) can achieve high CO₂/CH₄ selectivity across a broad range of pressures through a reversible CO₂ templating mechanism (Fig. 22).⁶⁰ At a CO₂ partial pressure of 3.6 bar (7.2 bar total pressure), diffraction studies show that the selective CO₂ adsorption is attributed to size exclusion: Co(bdp) forms a phase with pores large enough to permit the entrance of CO₂, while being too narrow to allow CH₄ to pass through. As pressure increases, Co(bdp) transfers to phases with expanded pores that can accommodate CH₄, whereas CO₂ clathrate formation remains energetically favorable, maintaining the exclusion of CH₄. When the CO₂/CH₄ ratio heavily favors CH₄ (6 : 94), the selectivity decreases, highlighting the need for comprehensive multicomponent equilibrium experiments to accurately assess the gas separation performance of flexible materials under varied conditions. In situ PXRD data further reveal that this selectivity resulted from a reversible guest-templating process, in which the framework expands to create a CO₂ clathrate and returns to its original phase once desorption occurs. In addition, single-component adsorption isotherms for CO₂, CH₄, N₂, and H₂ suggest that Co(bdp) also has potentially high selectivities and capacities in the separation of other gas mixtures, such as CO₂/N₂, CO₂/H₂, CH₄/N₂, and CH₄/H₂. These findings position Co(bdp) as a promising candidate for CO₂/CH₄ separation and provide valuable references for further research on flexible adsorbents addressing significant gas separations.

Fig. 22 (a) CO₂ adsorption over CH₄ in a mixture induces a structural change in Co(bdp). (b) Multicomponent adsorption experiments for CO₂/CH₄ mixtures in Co(bdp) under equilibrium CO₂/CH₄ molar ratios of 46 : 54, 42 : 58, and 43 : 57, respectively and (c) in a CH₄-rich atmosphere (with an equilibrium CO₂/CH₄ molar ratio of 6 : 94).⁶⁰ Reprinted with permission from American Chemical Society 2018.

Sulfur hexafluoride (SF₆) is widely used in the power industry, but its emission is a major concern for the concentration of greenhouse gases.¹⁹⁹ Efficient recovery of high-purity SF₆ from industrial waste gases is challenging.²⁰⁰ In 2024, Li and co-workers demonstrated that BUT-53, with its dynamic molecular traps, can enrich SF₆ from SF₆/N₂ mixtures, which can maintain performance under humid conditions (90% RH) over multiple cycles (Fig. 23).¹⁰⁵ BUT-53 showed outstanding SF6 adsorption of 2.82 mmol g⁻¹ at 0.1 bar and 298 K, and an exceptional SF₆/N₂ (10 : 90) selectivity of 2485. This high selectivity allows for the recovery of over 99.9% pure SF₆ from a 10% SF₆ mixture in breakthrough experiments.

Fig. 23 (a) Crystal structure of BUT-53 for trapping SF₆ in the pore, and (b) comparison of selectivity and SF₆ uptake of BUT-53 with reported materials.¹⁶³ Reprinted with permission from American Chemical Society 2024.

4.2. Heterogeneous catalysis

In addition to applications in gas capture, storage, and separation, MPFs are increasingly recognized for their potential in heterogeneous catalysis. These materials offer a solid framework that can incorporate functional molecular catalysts, such as porphyrins and salens, through rational design and modification of their ligands, and active metal species from the structural nodes or incoming sources.^100,152,163 The robust, periodic structure of MPFs ensures that these active centers are securely immobilized within the framework, minimizing aggregation and thereby enhancing catalytic efficiency. In this section, we will introduce the progress made in leveraging MPFs as heterogeneous catalysts.

4.2.1. Organic catalysis. Metalloporphyrins, which act as cofactors in many metallo-enzymes, are known for their ability to catalyze various organic reactions under relatively mild conditions with high selectivity.^201,202 To harness the exceptional chemical stability of PCN-602(Ni), Zhou and co-workers utilized PCN-602(Mn) to facilitate C–H bond halogenation in methylene dichloride containing 0.165 M NaClO as the nucleophilic reagent.¹⁴⁹ The integration of Mn³⁺ into the MOF framework enables PCN-602(Mn) to achieve a high yield of chlorocyclohexane (92%) after 5 hours of reaction, significantly outperforming the homogeneous catalyst Mn(TPP)Cl, which yields only 8%.

Following this, Zhou and co-workers developed and characterized a new perfluorophenylene-functionalized metalloporphyrin MOF, PCN-624.¹⁵⁰ This MPF, isostructural to PCN-602, features a pore surface decorated with perfluorophenylene groups, making it a highly efficient heterogeneous catalyst for the synthesis of fullerene–anthracene bisadducts. Thanks to its excellent chemical stability, PCN-624 can be reused more than five times with only a slight reduction in catalytic performance. In 2024, they also synthesized PCN-300, which has a lamellar structure with two distinct Cu centers and 1D open channels. PCN-300 demonstrated catalytic activity in the cross-dehydrogenative coupling reaction to form C–O bonds.¹⁶³ In the same year, PCN-1004 was developed as a catalyst for cross-dehydrogenative coupling reactions, enabling the formation of C–O and C–S bonds without requiring directing groups.¹⁰¹ This system achieved impressive yields up to approximately 99%, exhibiting good cycling durability and broad substrate versatility.

With rich active Ni(II) sites and excellent chemical stability, BUT-32 and BUT-33—extended versions of the sodalite-type Ni₃(BTP)₂—were employed as heterogeneous catalysts for C–C coupling reactions by Li and co-workers.¹²² The combination of superior chemical stability, large mesopores, and accessible Ni(II) sites makes BUT-33 particularly effective for a range of C–C coupling processes. While noble metal catalysts are known for their high activity, incorporating these metals into a MOF's skeleton can further improve their catalytic efficiency thanks to the isolation of active centers and the robust solid framework allows for catalyst recycling.^203–205 In this regard, BUT-33(Pd), featuring high chemical stability and rich open Pd(II) sites within a mesoporous framework, is anticipated to perform well in cross-coupling reactions as a recyclable single-site catalyst.¹²⁴ In model reactions including Suzuki and Heck coupling reactions, BUT-33(Pd) confirmed its potential as an effective catalyst with high conversion rates and yields, coupled with the ease of regeneration for consecutive reactions.

4.2.2. Photocatalysis. Porphyrin molecules possess exceptional photophysical properties, including strong visible light absorption and efficient energy transfer.^201,202 Incorporating porphyrin ligands in MOFs imparts these materials with unique optical and photocatalytic characteristics. The porphyrinic MPF, PCN-601, which combines attributes of light harvesting ability, catalytic activity, and high surface area, has been demonstrated to be an outstanding and stable photocatalyst for visible-light-driven CO₂ reduction in the presence of H₂O vapor at room temperature.¹⁴⁷ Kinetic studies indicate that the robust pyrazolyl coordination spheres and Ni-oxo clusters in PCN-601 facilitate effective energy band alignment and rapid ligand-to-node electron transfer. As a result, PCN-601 achieves a CO₂-to-CH₄ production rate that surpasses those of carboxylate-porphyrinic MOFs and the traditional Pt/CdS photocatalyst by over 3 and 20 times, respectively.

In 2021, Li and co-workers designed a pyrazole–benzothiadiazole–pyrazole organic molecule with a donor–acceptor–donor conjugated π-system for rapid charge separation, and its integration into MOFs resulted in the efficient heterogeneous photocatalyst JNU-204.¹⁰⁶ Under visible-light irradiation, JNU-204 can catalyze three aerobic oxidation reactions (arylboronic acid oxidation, enamine oxidation, oxysulfonylation of alkynes) with different oxygenation pathways (Fig. 24). Recycling tests confirmed JNU-204's stability and reusability, demonstrating its robustness as a heterogeneous photocatalyst. Additionally, JNU-204 was employed for the straightforward synthesis of pyrrolo[2,1-a]isoquinoline heterocycles, which are core structures in various marine natural products. They also used this MPF for photocatalytic C–N and C–C coupling reactions.²⁰⁶ Based on this type of photosensitizer ligands (Ph-Pz, BT-Pz, BT-Ph-Pz, and BT-Th-Pz), they synthesized a series of porous organic polymers (JNU-208, -209, -210, and -211) for photocatalytic aerobic oxidation of benzylamines. Featuring a donor–π–acceptor–π–donor structure with a more suitable band gap for the generation of reactive oxygen species, JNU-211 was found to exhibit the highest photocatalytic activity.²⁰⁷

Fig. 24 Schematic illustration of JNU-204 as a photocatalyst for aerobic oxygenation under visible-light irradiation.¹⁰⁶ Reprinted with permission from American Chemical Society 2021.

Subsequently, they developed a series of isostructural fcu topology Ni₈-MPFs (JNU-212, JNU-213, JNU-214, and JNU-215) using linear bipyrazolate ligands.¹⁴⁵ Experimental results show that modifying the electron-accepting properties of the pyrazolate-bridging units enhances charge separation and transfer efficiency under visible-light irradiation. Among these, JNU-214, which includes a benzoselenadiazole unit, demonstrated the highest photocatalytic activity for the aerobic oxidation of benzylamines, achieving a 99% conversion rate in 24 hours. Recycling tests further confirmed JNU-214's stability and reusability as a robust heterogeneous catalyst. Tuning the electron-accepting capacity in donor–acceptor–donor MOFs provides a promising route to developing effective noble-metal-free photo-catalysts for aerobic oxidation reactions, with JNU series pyrazolate MOFs exemplifying this approach through its excellent photon absorption, suitable band gap, rapid charge separation, and exceptional chemical stability under visible light.

4.2.3. Electrocatalysis. In addition to photocatalysis, the desired electronic transfer capabilities also enable porphyrin molecules to be suitable catalysts for electrochemical processes. Lan and co-workers utilized PCN-601 to develop a hybrid electrocatalytic system where a single catalyst serves both as an anode and a cathode for a full reaction, coupling CO₂ reduction with methanol oxidation (MOR) instead of oxygen evolution (OER) (Fig. 25).¹⁴⁸ The electron transfer from the electron-rich Ni₈ clusters to the electron-deficient Ni ions in the porphyrin centers results in significantly high electrocatalytic activities and faradaic efficiencies of 93.5% for the MOR and 94.5% for the CO₂RR. The performance further improves under light irradiation, with the faradaic efficiency for formic acid exceeding 90% across all applied potentials and formate efficiency nearing 100%. Notably, the hybrid electrolyzer operates at a low cell voltage to generate a current of 5 mA cm⁻² for the overall reaction, which is 310 mV lower compared to traditional CO₂RR systems and yields a high-value product (formic acid) at the anode instead of low-value oxygen.

Fig. 25 Structure of (a) Ni₈-TET, Ni-TPP, and PCN-601. (b) Potential active sites and (c) performance speculation of Ni₈-TET, Ni-TPP, and PCN-601 applied in electrolytic cells.¹⁴⁸ Reprinted with permission from Wiley-VCH 2022.

Metalloporphyrin complexes are also among the most extensively investigated molecular catalysts for electrochemical CO₂ reduction reactions (CO₂RRs).^208,209 While it is well recognized that the metal center plays a key role in determining the selectivity of CO₂RR products, the influence of electrolyte composition has received comparatively less attention. Recent studies on a copper-porphyrin MPF catalyst, FICN-8, demonstrated that adjusting the electrolyte composition could systematically control the product distribution.¹⁰⁰ Specifically, increasing the concentration of a proton source in an acetonitrile-based electrolyte shifted the primary CO₂RR product from CO to HCOOH. A combination of experimental and computational approaches revealed that the formation of a ligand-bound hydride intermediate, facilitated by proton-coupled electron transfer, is a critical step in the generation of formic acid, whose formation is significantly influenced by the concentration of the proton source.

4.3. Conductivity

Developing efficient hydroxide-ion conducting materials is critical for advancing anion-exchange membranes.^210,211 One strategy to enhance electronic mobility in poorly conductive MOFs is by introducing defects.²¹² Studies on the ionic conductivity of Ni₈-MPFs [Ni₈(OH)₄(H₂O)₂(BDP-X)₆] (X = H (1), OH (2), NH₂ (3)) and their post-synthetically modified forms 1@KOH, 2@KOH, 3@KOH), which feature missing-linker defects, revealed a significant increase in conductivity-up to four orders of magnitude higher compared to the pristine 1–3 systems.¹³¹ For instance, at 313 K and 22% RH, 2 exhibits a conductivity of 5.86 × 10⁻⁹ S cm⁻¹ with an activation energy E_a of 0.60 eV. When treated with KOH, the conductivity increases to 2.75 × 10⁻⁵ S cm⁻¹, accompanied by a reduced activation energy of 0.40 eV, and further to 1.16 × 10⁻² S cm⁻¹ with E_a reduced to 0.20 eV at 100% RH. The improved performance of the KOH-modified materials is attributed to their increased porosity, basicity, and hydrophilicity compared to the unmodified systems.

Aubrey et al. recently reported the reductive doping of the poorly conducting Fe(III) MPF [Fe₂(BDP)₃] with potassium naphthalenide, resulting in mix-valence Fe(II)/Fe(III) systems of K_x[Fe₂(BDP)₃] (0 ≤ x ≤ 2).¹⁷⁵ Such doping significantly enhances conductivity without compromising pore accessibility. Intermediate reduction levels show a characteristic inter-valence charge-transfer band at 0.52 eV, indicative of improved charge transport. Flash photolysis time-resolved microwave conductivity and single-crystal field-effect transistor measurements revealed an increase in charge mobility from 0.02 cm² V⁻¹ s⁻¹ to 0.29 cm² V⁻¹ s⁻¹ for K_0.8[Fe₂(BDP)₃], with a further 10 000-fold increase in conductivity to 7 × 10² S cm⁻¹ for K_0.98[Fe₂(BDP)₃], attributed to 1D conduction along the [Fe₂(pyrazolate)₆] chains.

Similarly, the proton-conducting properties of MPFs can also be enhanced through post-synthetic modifications. He and co-workers prepared Ni₈-MPFs, NiL1 and NiL2, and studied their proton conductivity in both pristine and imidazole-encapsulated forms (Im@NiL1 and Im@NiL2).¹⁴² The inclusion of imidazole increased the proton conductivity by 3 to 5 orders of magnitude, reaching up to 1.72 × 10⁻² S cm⁻¹ at 98% RH and 80 °C. Activation energy analyses indicated that NiL1, NiL2, and Im@NiL1 follow the Vehicle mechanism, while Im@NiL2 likely employs the Grotthuss mechanism due to the flexible alkyl side arms on L2 coordinating imidazole and water molecules.

4.4. Other applications

4.4.1. Sensing. Crystal engineering/reticular chemistry also offers versatile principles for designing MPFs that mimic the role and activity of enzymes. In 2021, Lu and co-workers developed a novel dual-readout sensing platform for dopamine detection using copper-porphyrinic MPFs, Cu-TPP, Cu-TPP(BA), and Cu-TPP(PA).¹⁵⁷ By employing a coordination regulation strategy, they enhanced the catechol oxidase mimic enzyme activity of these MPFs. In the presence of H₂O₂, the porphyrin MPFs catalyzed the oxidation of dopamine to produce catechol derivatives, specifically the o-quinone intermediate. This intermediate then reacted with 1,3-dihydroxynaphthalene, generating immediate fluorescent and colorimetric signals. The Cu-TPP MPF-catalyzed systems demonstrated a strong linear relationship with dopamine concentration under optimal conditions, achieving a detection limit of 2.5 nM for the fluorescent method and 4 nM for the colorimetric method.

4.4.2. Drug delivery. The Zn(bdp-X) series, assembled with Zn(II) ions and functionalized organic linker H₂BDP-X (X = H, NO₂, NH₂, OH), were synthesized and studied for the encapsulation and release of the antitumor drugs mitoxantrone and Ru(p-cymene)Cl₂(pta) (RAPTA-C).⁷⁴ They were evaluated to understand the effect of framework functionalization on drug incorporation and delivery. RAPTA-C (0.50–0.55 mmol mmol⁻¹) was successfully loaded using a straightforward and fast impregnation technique, whereas mitoxantrone could be incorporated by grinding. Drug loading differed in the Zn(bdp-X) series with variable functional groups, correlating with their different BET surface areas. RAPTA-C release in simulated body fluid occurred in two distinct steps, possibly due to the flexibility of the framework. Drug release from Zn(bdp-X) (X = H, NH₂, NO₂) was unaffected by the stability of the matrix, and RAPTA-C delivery from Zn(bdp-OH)@RAPTA-C was highly dependent on the kinetics of framework degradation. Among the stable matrices, the polar Zn(bdp-NH₂) showed a slower delivery rate, likely due to stronger drug–matrix interactions and hindered diffusion compared to Zn(bdp-NO₂) and Zn(bdp-H) analogues.

4.4.3. Gold extraction. With the growing accumulation of electronic waste and increasing demand for rare metals, developing technologies for the efficient recovery of metals like gold from waste, known as urban mining, is of particular interest and significance. A highly porous MOF–polymer composite, BUT-33–poly(p-phenylenediamine), was fabricated for this purpose.¹²³ This composite demonstrates a remarkable gold extraction rate, achieving over 99% Au³⁺ removal within 45 seconds, a capacity of 1600 mg g⁻¹, high selectivity, good stability and recyclability. This performance is attributed to its high porosity and redox adsorption mechanism, which also endows it with catalytic activity due to the formation of gold nanoparticles. This work highlights the potential of MPF–polymer composites for recycling precious metals from electronic waste, whose disposal will be demanding otherwise.

5. Conclusion and outlook

Under the guidance of crystal engineering/reticular chemistry principles, researchers have developed plenty of MPFs with great structure diversity and base resistance. Various classic MOF families such as Co(bdp), Fe₂(BDP)₃, Ni₈(L)₆, PCN-601, and BUT-55 demonstrated fascinating architectures and record-breaking performances over the past decade. This review comprehensively introduces MPFs, including the superiority and significance of construction through rational design, synthetic strategies, structural features, and stability. Potential applications have also been explored in energy and environmental fields, with insights gained into the mechanisms to shed light on underlying structure–property relationships.

Although significant progress has been made, there are still a few issues pending in the development of this field. Strong metal–pyrazolate bonds, despite being beneficial for stability, pose significant synthetic challenges, such as kinetic trapping and reduced crystallinity. The difficulty in obtaining high-quality single crystals further inhibits detailed structure analysis and new materials discovery, impeding a deeper understanding of their structure–property relationships. Due to the coordination constraints of pyrazolate ligands, and the relatively small library of inorganic chains and clusters, the number of matched topologies remains limited. Targeting at easy access to advanced functional materials, general design principles and further structure diversification are still actively sought after.

In addition to these scientific limitations, the scalability of MPFs presents another critical challenge for industrial applications. The synthesis and post-synthetic treatments (washing and activation) of MPFs, usually performed under controlled laboratory conditions, requires adaptation to scalable, cost-effective, and reproducible procedures suitable for mass production while ensuring the maintenance of structures and properties. Key considerations include the development of efficient, solvent-free or green synthetic routes (for both ligands and MFPs), as well as the optimization of reaction parameters to reduce energy and resource consumption. Furthermore, the design of synthetic workflows that enable continuous or modular manufacturing of MPFs, rather than batch processes, will be essential for meeting industrial demands. The integration of MPFs into industrial applications also necessitates the evaluation of their performance under real-world conditions, such as chemical robustness, thermal stability, possibility of shaping and processing, and reusability over extended operational cycles. Addressing these issues will be vital for their use in areas like gas storage and separation, catalysis, and environmental remediation.

Substantial interdisciplinary efforts are on demand to develop versatile synthetic strategies, advanced characterization/analysis techniques, computational/theoretical modeling, and artificial intelligence/machine learning-assisted materials design and screening, to unveil the full palette of MPFs. Bridging the gap between laboratory-scale synthesis and large-scale production will further rely on close collaborations between academia, industry, and government initiatives to foster standardization. Continued innovation in this area will not only advance the chemistry of porous frameworks but also inspire the development of next-generation materials to tackle global challenges in a more sustainable manner. By addressing both the fundamental scientific limitations and the practical challenges of scalability, MPFs have the potential to play a transformative role in shaping a greener future.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (No. 22225803, 22038001 and 22401168), the Beijing Natural Science Foundation (No. Z230023), and the Beijing Outstanding Young Scientist Program (Project No. JWZQ20240102008).

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