Hydrothermal synthesis, structures, and catalytic performance of five coordination compounds driven by 5-aminoisophthalic acid

Abstract

An amino-functionalized-dicarboxylic acid, 5-aminoisophthalic acid (H₂aipa), was used as a versatile building block to synthesize a series of five novel coordination compounds under hydrothermal conditions and formulated as [Co(μ₃-aipa)(2,2′-H₂biim)]_n (1), [Ni₂(μ-aipa)₂(2,2′-H₂biim)₂(H₂O)₄]·4H₂O (2), {[Cd(μ₃-aipa)(2,2′-H₂biim)]·H₂O}_n (3), {[Ni(μ-aipa)(μ-bpb)]·0.5bpb·H₂O}_n (4), and {[Ni₂(μ-aipa)(μ₃-aipa)(μ-dpea)₂(H₂O)][Ni(μ-aipa)(μ-dpea)(H₂O)]·8H₂O}_n (5). Three supporting ligands (2,2′-biimidazole (H₂biim),1,4-bis(pyrid-4-yl)benzene (bpb), and 1,2-di(4-pyridyl)ethane (dpea)) were used in the synthesis. The structures of the studied products 1–5 vary significantly, ranging from a 0D dimer (2), 2D sheets (1, 3 and 4) to 3D + 2D interpenetrated frameworks (5). Furthermore, these compounds were evaluated as heterogeneous catalysts for the Knoevenagel reaction, achieving high product yields under optimized conditions. In addition, we also investigated various reaction parameters, substrate scope, and assessed the feasibility of catalyst recycling. This thorough investigation highlights the versatility of H₂aipa as a dicarboxylate building block in the formation of functional coordination polymers.

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Introduction

Coordination polymers (CPs) are an important class of crystal materials that are constructed using self-assembly of metal ions with organic bridging ligands.^1–3 Their versatile topologies and chemical properties^4,5 make them ideal candidates for diverse applications such as gas adsorption and separation,^6–9 heterogeneous catalysis,^10–12 sensing,^8,13,14 and biomaterials.¹⁵ To study the process of mineral formation under supercritical conditions in the laboratory, researchers initially employed hydrothermal and solvothermal synthesis methods. These days, hydrothermal synthesis is extensively utilized in the construction of CPs, especially for the preparation of CPs crystals.^16–19 The self-assembly in these reactions is highly sensitive to several variables including the types of metal ion, the concentration of reactants as well as the pH value of the solution.^20–23 Polycarboxylate ligands are commonly used as versatile organic linkers in the synthesis of metal clusters and polymers.^24,25 This preference stems from their partially or fully deprotonated sites, which enable the development of different structural topologies and exhibit a variety of coordination modalities.²⁶ These ligands, in particular, can function as terminal unidentate, chelating, bridging bidentate and bridging tridentate ligands.²⁷

During the last few years, our research has focused on designing new functional coordination polymers using polycarboxylic ligands.^10,18,28 In this study, we chose 5-aminoisophthalic acid (H₂aipa, Scheme 1) as a amino-functionalized dicarboxylate building block.

Scheme 1 Structures of H₂aipa block and auxiliary ligands.

H₂aipa exhibits several intriguing properties that make it an attractive linker: (a) H₂aipa has five coordination sites, including one amino nitrogen and four carboxyl oxygen atoms. (b) The free amino nitrogen atom can act as a base site for catalysis, facilitating reactions such as the Henry reaction, cyanosilylation and Knoevenagel condensation.^28–30

Here, we report the synthesis and characterization of five novel coordination compounds derived from metal(II) salts, H₂aipa, and three auxiliary ligands. These compounds exhibit diverse structures, ranging from a 0D dimer (2), 2D sheets (1, 3 and 4) to 3D + 2D interpenetrated frameworks (5), specifically [Co(μ₃-aipa)(2,2′-H₂biim)]_n (1), [Ni₂(μ-aipa)₂(2,2′-H₂biim)₂(H₂O)₄]·4H₂O (2), {[Cd(μ₃-aipa)(2,2′-H₂biim)]·H₂O}_n (3), {[Ni(μ-aipa)(μ-bpb)]·0.5bpb·H₂O}_n (4), and {[Ni₂(μ-aipa)(μ₃-aipa)(μ-dpea)₂(H₂O)][Ni(μ-aipa)(μ-dpea)(H₂O)]·8H₂O}_n (5).

In addition, we have also evaluated the catalytic activities of these compounds in the Knoevenagel reaction between pyridine-3-aldehyde with propanedinitrile.

Experimental section

General methods

All chemicals were purchased commercially utilized exactly as supplied. To record the FTIR spectra (KBr discs), a Bruker EQUINOX 55 spectrometer was utilized. Elemental analyses (EA) for carbon, hydrogen, and nitrogen in compounds 1–5 were conducted using an ElementarVario EL elemental analyzer. Thermogravimetric analyses (TGA) were conducted using a LINSEIS STA PT1600 thermal analyzer under a nitrogen flow and heating at a rate of 10 °C min⁻¹. The powder X-ray diffraction patterns (PXRD) of the compounds were collected using a Rigaku-Dmax 2400 diffractometer (CuKα radiation; λ = 1.54060 Å, Rigaku Corporation, Tokyo, Japan). For ¹H NMR experiments, a JNM ECS 400 M spectrometer was used with CDCl₃ as the solvent.

Synthesis of [Co(μ₃-aipa)(2,2′-H₂biim)]_n (1)

A 20 mL Teflon cup was sealed with the following contents: H₂aipa (36.2 mg, 0.2 mmol), H₂biim (26.8 mg, 0.2 mmol), CoCl₂·6H₂O (47.6 mg, 0.2 mmol), NaOH (16.0 mg, 0.4 mmol), and H₂O (10 mL). After three days of heating the mixture at 160 °C, it was allowed to cool to ambient temperature. Purple block-shaped crystals were extracted and rinsed with water. Yield: 43% based on H₂aipa. Computed for C₁₄H₁₁CoN₅O₄: C 45.18, N 18.82, H 2.98%. Found: C 45.43, N 18.49, H 3.00%. FTIR (KBr, cm⁻¹): 3578 w, 3309 w, 1620 w, 1595 m, 1517 m, 1412 s, 1327 s, 1244 w, 1100 w, 1058 w, 995 w, 961 w, 920 w, 845 w, 778 m, 730 w, 666 w, 621 w 580 w, 547 w.

Synthesis of [Ni₂(μ-aipa)₂(2,2′-H₂biim)₂(H₂O)₄]·4H₂O (2)

Compound 2 was synthesized using a method analogous to CP 1 by substituting NiCl₂·6H₂O (47.6 mg, 0.2 mmol) for CoCl₂·6H₂O. The synthesis yielded green block-shaped crystals of compound 2, with a 42% yield based on H₂aipa. Computed for C₂₈H₃₈Ni₂N₁₀O₁₆: C 37.87, N 15.77, H 4.31%. Found: C 38.03, N 15.85, H 4.28%. FTIR (KBr, cm⁻¹): 3273 m, 3168 w, 3120 w, 1617 w, 1554 s, 1487 w, 1371 s, 1270 w, 1192 w, 1128 w, 1054 w, 991 w, 964 w, 924 w, 864 w, 818 w, 785 m, 748 w, 715 m, 610 w, 570 w.

Synthesis of {[Cd(μ₃-aipa)(2,2′-H₂biim)]·H₂O}_n (3)

CP 3 was synthesized using a method analogous to CP 1 by substituting CdCl₂·H₂O (40.2 mg, 0.2 mmol) for CoCl₂·6H₂O. The synthesis yielded yellow block-shaped crystals of CP 3, with a 45% yield based on H₂aipa. Computed for C₁₄H₁₃CdN₅O₅: C 37.90, N 15.78, H 2.95%. Found: C 38.15, N 16.01, H 2.96%. FTIR (KBr, cm⁻¹): 3304 m, 3266 m, 3180 w, 1613 w, 1580 m, 1546 s, 1397 s, 1349 s, 1323 s, 1252 w, 1114 m, 1021 w, 964 w, 920 w, 853 w, 782 m, 734 w, 673 w, 595 w, 540 w.

Synthesis of {[Ni(μ-aipa)(μ-bpb)]·0.5bpb·H₂O}_n (4)

A 20 mL Teflon cup was sealed with the following contents: H₂aipa (36.2 mg, 0.2 mmol), bpb (46.4 mg, 0.2 mmol), NiCl₂·6H₂O (47.6 mg, 0.2 mmol), NaOH (16.0 mg, 0.4 mmol), and H₂O (10 mL). After three days of heating the mixture at 160 °C, it was allowed to cool to ambient temperature. Green block-shaped crystals were extracted and rinsed with water. Yield: 46% based on H₂aipa. Computed for C₃₂H₂₅N₄NiO₅: C 63.61, N 9.27, H 4.17%. Found: C 63.90, N 9.23, H 4.20%. FTIR (KBr, cm⁻¹): 3359 m, 3309 m, 3083 w, 1614 s, 1546 s, 1480 w, 1386 s, 1226 w, 1110 w, 1073 w, 1013 w, 905 w, 804 m, 726 w, 678 w, 618 w, 573 w.

Synthesis of {[Ni₂(μ-aipa)(μ₃-aipa)(μ-dpea)₂(H₂O)][Ni(μ-aipa)(μ-dpea)(H₂O)]·8H₂O}_n (5)

A 20 mL Teflon cup was sealed with the following contents: H₂aipa (36.2 mg, 0.2 mmol), dpea (36.8 mg, 0.2 mmol), NiCl₂·6H₂O (47.6 mg, 0.2 mmol), NaOH (16.0 mg, 0.4 mmol), and H₂O (10 mL). After three days of heating the mixture at 160 °C, it was allowed to cool to ambient temperature. Green block-shaped crystals were extracted and rinsed with water. Yield: 43% based on H₂aipa. Computed for C₆₀H₇₁N₉Ni₃O₂₂: C 49.83, N 8.72, H 4.95%. Found: C 50.17, N 8.75, H 4.91%. FTIR (KBr, cm⁻¹): 3445 w, 3363 m, 3236 w, 1621 m, 1546 s, 1468 w, 1431 m, 1382 s, 1222 w, 1099 w, 1069 w, 1024 w, 957 w, 894 w, 831 w, 789 m, 726 m, 673 w, 606 w, 547 w.

Single crystal X-ray diffraction & topological analysis

A Bruker APEX-II CCD diffractometer was used to collect the crystal data of compounds 1–5 using graphite-monochromated Mo/CuK_α radiation; λ = 0.71073/1.54178 Å). SHELXS-97 and SHELXL-97 software were used for the determination of the structures.³¹ Detailed crystal parameters and structural refinements can be found in Table 1, with selected bond parameters are listed in Tables S1 and S2 (ESI).† The supplementary crystallographic data for compounds 1–5 are available in CCDC 2367606–2367610.

Table 1 Crystal data for 1–5

Compound	1	2	3	4	5
a R1 = Σ||Fo| − |Fc||/|Fo|.b wR2 = {Σ[w(Fo^2 − Fc^2)]^2/Σ[w(Fo^2)]^2}^1/2.
Chemical formula	C14H11CoN5O	C28H38Ni2N10O16	C14H13CdN5O5	C32H25N4NiO5	C60H71N9Ni3O22
Formula weight	372.21	888.10	443.68	604.27	1446.24
Crystal system	Triclinic	Monoclinic	Triclinic	Monoclinic	Triclinic
Space group	P1̄	P21/c	P1̄	P21/n	P1
a/Å	7.6951(3)	11.9238(3)	7.89650(10)	11.0968(4)	10.1262(3)
b/Å	8.0250(2)	8.0959(2)	8.16800(10)	22.3594(7)	13.2389(3)
c/Å	11.2161(6)	18.3412(5)	11.4250(2)	11.6156(4)	13.3575(3)
α/°	83.335(3)	90	83.7430(10)	90	88.8838(17)
β/°	76.981(4)	95.998(2)	77.408(2)	107.065(4)	75.6606(19)
γ/°	81.112(3)	90	81.2660(10)	90	74.270(2)
V Å^−3	664.34(5)	1760.85(8)	708.630(19)	2755.14(18)	1667.75(7)
T/K	303(2)	303(2)	285(2)	293(2)	303(2)
Z	2	2	2	4	1
Dc/g cm^−3	1.861	1.675	1.995	1.457	1.297
μ/mm^−1	1.326	1.159	12.657	0.754	1.530
F(000)	378	920	420	1252	674
Refl. measured	2470	3277	2577	5127	8599
Unique refl. (Rint)	2264 (0.0319)	2914 (0.0332)	2463 (0.0291)	4389 (0.0261)	8204 (0.0380)
GOF on F^2	1.136	1.031	1.037	1.051	1.066
R1[I > 2σ(I)]^a	0.0369	0.0324	0.0226	0.0300	0.0420
wR2[I > 2σ(I)]^b	0.0759	0.0864	0.0579	0.0788	0.1184

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Topological analysis of the obtained coordination polymers was conducted using ToposPro software. This involved generating a simplified underlying net, wherein bridging ligands were reduced to the centroids.^32,33

Catalytic activity in Knoevenagel reaction

The following ingredients were mixed in a suspension and stirred at 25 °C for a required reaction time: catalyst (2.0 mol%), aromatic aldehyde (0.50 mmol), propanedinitrile (1.0 mmol), and solvent (1.0 mL, usually CH₃OH). Centrifugation was subsequently used to extract the catalyst. A crude solid product was obtained by evaporating the filtrate using a rotary evaporator. The amount of this solid product was ascertained by ¹H NMR spectroscopy (JNM ECS 400M spectrometer) after it was dissolved in CDCl₃ (Fig. S7, ESI†). The catalyst was recovered by centrifugation, washed with methanol, allowed to dry at room temperature, and subsequently reused in further reactions, following the same procedure for recycling experiments.

Results and discussion

Hydrothermal synthesis of compounds 1–5

A series of reactions were conducted under hydrothermal conditions to assess the potential of 5-aminoisophthalic acid (H₂aipa) as a building block for synthesizing coordination polymers. These reactions involved a mixture of metal(II) salts, H₂aipa, sodium hydroxide, and an auxiliary ligand selected from the following H₂biim, bpb, and dpea. The hydrothermal synthesis was carried out at 160 °C for three days, followed by a gradual reduction in temperature to promote the crystallization of CPs. The resulting products were collected in substantial yields and analyzed using standard techniques. The synthesis repeatability of the five complexes is good and the yields are stable. The experimental PXRD patterns of products 1–5 agree with the computed diffractograms (Fig. S3†), suggesting their homogeneous phase purity. Structural discrepancies among compounds 1–5 could be attributed to the varying auxiliary ligands and the coordination properties of the metal(II) centers.

Description of structures

[Co(μ₃-aipa)(2,2′-H₂biim)]_n (1). The asymmetric unit of CP 1 contains a μ₃-aipa²⁻ block, a Co(II) center, and a H₂biim auxiliary ligand (Fig. 1a). The Co1 center is six-coordinate with the Co1 center being coordinated by three carboxyl oxygen and one N atoms from three μ₃-aipa²⁻ linkers and two N donors from the H₂biim moiety and forms a distorted octahedral {CoN₃O₃} geometry. The Co–O and Co–N bond lengths, which measure 2.044(2)–2.399(2) and 2.087(2)–2.230(3) Å, are within the typical ranges for Co(II) compounds.^18,28 In CP 1, the aipa²⁻ ligand exhibits the coordination mode I (Scheme 2) with two monodentate or bidentate COO⁻ groups. Furthermore, the nitrogen atom of the NH₂ is involved in coordination. The μ₃-aipa²⁻ blocks connect the Co(II) centers, forming a 2D sheet (Fig. 1b) with hcb topology (Fig. 1c).

Fig. 1 Structure of CP 1. (a) Coordination environment at cobalt(II) atom. (b) The two-dimensional layer viewed along the c axis. (c) The sheet exhibiting a hcb topology, observed along the c axis.

Scheme 2 The coordination modes of aipa²⁻.

[Ni₂(μ-aipa)₂(2,2′-H₂biim)₂(H₂O)₄]·4H₂O (2). One Ni(II) atom, one μ-aipa²⁻ block, one H₂biim, two H₂O ligands and two lattice water molecules form the asymmetric unit of compound 2 (Fig. 2a). The six-coordinate Ni1 atom shows a distorted octahedral {NiN₃O₃} environment. It comprises one carboxyl oxygen and one N atoms from two μ-aipa²⁻ blocks, two O donors from two H₂O ligands, and a pair of N atoms of a H₂biim moiety. The Ni–O [2.021(2)–2.120(2) Å] and Ni–N [2.072(2)–2.164(2) Å] bond lengths fall within the expected ranges.^18,28 The cpic²⁻ block functions as a μ-linker (mode II, Scheme 2). Fig. 2b shows that a Ni₂ molecule is formed when two μ-aipa²⁻ blocks link two Ni1 atoms. These Ni₂ molecules are further assembled into a 3D H-bonded supramolecule framework (Fig. 2c).

Fig. 2 Structure of dimer 2. (a) Coordination environment at nickel(II) atom. (b) The Ni₂ molecule. (c) A 3D H-bonded supramolecule framework; view along the b axis.

{[Cd(μ₃-aipa)(2,2′-H₂biim)]·H₂O}_n (3). CP 3 is an asymmetric unit with one μ₃-aipa²⁻ block, one cadmium(II) center, one H₂biim supporting ligand, and one lattice water molecule. The five-coordinate Cd1 atom has a distorted trigonal bipyramidal {CdO₂N₃} geometry (Fig. 3a), made up of one nitrogen and two carboxyl oxygen atoms from three distinct μ₃-aipa²⁻ blocks and two Ndonor from the H₂biim ligand. The Cd–O [2.248(2)–2.285(2) Å] and Cd–N [2.257(2)–2.447(3) Å] distances are comparable to those in Cd(II) derivatives.^28,29 In 3, the aipa²⁻ block behaves as a μ₃-spacer (mode III, Scheme 2). The adjacent cadmium(II) atoms are linked by μ₃-aipa²⁻ blocks, generating a 2D sheet (Fig. 3b) with a hcb topology (Fig. 3c).

Fig. 3 Structure of CP 3. (a) Coordination environment at cadmium(II) center. (b) The two-dimensional layer viewed along the c axis. (c) Sheet exhibiting a hcb topology, observed along the c axis.

{[Ni(μ-aipa)(μ-bpb)]·0.5bpb·H₂O}_n (4). CP 4 is an asymmetric unit with one Ni(II) center, one μ-aipa²⁻ block, a half of μ-bpb, moiety and one free water molecule. The compound forms a two-dimensional coordination polymer (Fig. 4a). As shown in Fig. 4a, the nickel center is six-coordinate and form a distorted octahedral {NiO₄N₂} geometry. The geometry is completed by two N_bpb atoms from two μ-bpb moieties and four carboxyl oxygen atoms from two μ-aipa²⁻. The Ni–O [2.052(2)–2.192(2) Å] and Ni–N [2.050(2)–2.062(2) Å] bond lengths fall within expected ranges.^18,28 The aipa²⁻ moiety adopts μ-coordination fashion (mode IV, Scheme 2). The aipa²⁻ and bpb ligands linked neighbouring Ni(II) centers, resulting a 2D layer (Fig. 4b) with a new topology and point symbol of (8⁴.12²)(8)₂ (Fig. 4c).

Fig. 4 Structure of 4. (a) Coordination environment at the nickel(II) atom. (b) The two-dimensional layer viewed along the a axis. (c) Sheet exhibiting a new topology, viewed along the a axis.

{[Ni₂(μ-aipa)(μ₃-aipa)(μ-dpea)₂(H₂O)][Ni(μ-aipa)(μ-dpea)(H₂O)]·8H₂O}_n (5). The structure consists of a [Ni(μ-aipa)(μ-dpea)(H₂O)]_n 2D sheet and a [Ni₂(μ-aipa)(μ₃-aipa)(μ-dpea)₂(H₂O)]_n 3D framework. CP 5 is an asymmetric unit with three distinct nickel(II) centers, three aipa²⁻ blocks, three dpea, two coordinated water molecules, and eight lattice water molecules. The Ni1 and Ni3 atoms exhibit distorted octahedral {NiO₄N₂} coordination geometries, which include three carboxyl oxygen atoms from two distinct aipa²⁻ blocks, one O atom from the H₂O ligand, and two N donors from two dpea ligands (Fig. 5a). The six-coordinate nickel 2 center forms a distorted octahedral {NiO₃N₃} configuration. It is surrounded by one nitrogen and three oxygen atoms from three distinct aipa²⁻ blocks, as well as two N atoms of two dpea ligands. The Ni–O [2.002(3)–2.158(4) Å] and Ni–N [2.070(4)–2.145(4) Å] bond lengths are similar to those observed in analogous Ni(II) derivatives.^18,28 The aipa²⁻ blocks act as μ- or μ₃-linkers (modes IV and V, Scheme 2). The dpea shows a bridging coordination fashion. The μ-aipa, μ₃-aipa and μ-dpea blocks linked Ni1 and Ni2 centers to form a 3D framework (Fig. 5b). Meanwhile, the Ni3 atoms were connected by the μ₃-aipa and μ-dpea ligands to generate a 2D layer (Fig. 5c). Finally, 3D + 2D interpenetrated frameworks were formed (Fig. 5d) with a new topology and point symbol of (8⁴·12²)(8)₂ (Fig. 5e and f).

Fig. 5 Structure of 5. (a) Coordination environment at nickel(II) atoms. (b) Three-dimensional framework along the c axis. (c) Two-dimensional metal–organic network along the b axis. (d) 3D + 2D interpenetrated frameworks along the c axis. (e) Topological representation of the framework along the c axis. (f) Two 3D + 2D interpenetrated frameworks shown by different colours (green and gray).

TGA & PXRD data

The assessment of thermal stability for compounds 1–5 was carried out using thermogravimetric analysis (TGA) under a N₂ atmosphere within a temperature range of 23–800 °C (Fig. 6). The CP 1 does not contain coordinated or crystallization water, so its network is stable up to 395 °C. At temperatures 72 to 292 °C, dimer 2 lost four lattice and four coordinated water molecules (exptl 15.9%; calculated 16.2%), whereas the dehydrated sample held steady until 316 °C. For CP 3, a weight loss of 4.3% (calcd 4.1%) occurred at temperatures from 28 and 193 °C, associated with the release of one lattice water molecule. In CP 4, one lattice water and a half of bpb were lost at 28–193 °C (exptl, 22.0%; calcd, 22.2%), with the dehydrated sample is stable up to 343 °C. For CP 5, a mass loss between 29–190 °C indicated the release of eight lattice water molecules and two coordinated water molecules (exptl 12.1%, calculated 12.4%), whereas the sample held steady until 335 °C.

Fig. 6 TGA plots of 1–5.

Power X-ray diffractograms were acquired at 25 °C for compounds 1–5 (Fig. S3†). Phase purity of all samples was established by analyzing the experimental patterns and comparing them with the simulated ones (based on CIF data).

Catalytic Knoevenagel reaction

We investigated compounds 1–5 for their potential as heterogeneous catalysts in the Knoevenagel reaction, which involves a combination of propanedinitrile and a variety of aldehydes, considering the possibility of different coordination complexes acting as catalysts in this reaction.^{28–30,34,35} Using pyridine-3-aldehyde as a model substrate, we reacted it with malononitrile at 25 °C in methanol to synthesize the related product. As a model substrate, pyridine-3-aldehyde was reacted with malononitrile at 25 °C in methanol (Scheme 3 and Table 2). Furthermore, we conducted a thorough investigation of various reaction parameters, which included the duration of the reaction, the type of solvent used, catalyst loading, potential for catalyst reuse, and the scope of substrates involved. CP 3 exhibited the highest activity, achieving a >99 conversion rate of pyridine-3-aldehyde (Table 2). It was then used to study the effects of different reaction parameters. The yield increased from 55 to >99% when the reaction time was extended from 10 to 60 min (Table 2, entries 1–6). The impact of catalyst amount was also examined, and the results showed that increasing the loading of catalyst from 1 to 2 mol% increased product yield from 95 to >99% (entries 6 and 11). It is worth mentioning that various solvents, including methanol, were tested in these reactions. Chloroform, acetonitrile, water, and ethanol all have lower product yields (66–98% product yields). Compounds 1, 2, 4 and 5 exhibit reduced activity when compared to CP 3, achieving product yields in the 82–90% range (entries 12–15, Table 2). It is worth mentioning that the Knoevenagel reaction of pyridine-3-aldehyde is much less efficient without a catalyst (only 20% product yield) or when H₂aipa or CdCl₂ are used as catalysts (28% yield or 24% yield), respectively under comparable reaction conditions (entries 16–18, Table 2). The improved performance of CP 3 is likely due to the presence of unsaturated coordination sites in the Cd(II) centers, which makes it easier to access the substrates.^28,30,32

Scheme 3 Knoevenagel reaction of pyridine-3-aldehyde with propanedinitrile.

Table 2 Knoevenagel reaction of pyridine-3-aldehyde with propanedinitrile^a

Entry	Catalyst	Reaction time (min)	Catalyst loading (mol%)	Solvent	Product yield^b (%)
a Conditions: pyridine-3-aldehyde (0.5 mmol), propanedinitrile (1.0 mmol), catalyst (1–2 mol%), solvent (1.0 mL), 25 °C.b Yield based on ^1H NMR analysis: [moles of product per mol of aldehyde substrate] × 100%.
1	3	10	2.0	CH3OH	55
2	3	20	2.0	CH3OH	75
3	3	30	2.0	CH3OH	84
4	3	40	2.0	CH3OH	91
5	3	50	2.0	CH3OH	96
6	3	60	2.0	CH3OH	>99
7	3	60	2.0	H2O	98
8	3	60	2.0	C2H5OH	96
9	3	60	2.0	CH3CN	86
10	3	60	2.0	CHCl3	66
11	3	60	1.0	CH3OH	95
12	1	60	2.0	CH3OH	82
13	2	60	2.0	CH3OH	87
14	4	60	2.0	CH3OH	85
15	5	60	2.0	CH3OH	90
16	Blank	60	—	CH3OH	20
17	CdCl2·2H2O	60	2.0	CH3OH	32
18	H2aipa	60	2.0	CH3OH	26

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A range of substituted benzaldehyde substrates was evaluated to determine the substrate scope in the Knoevenagel reaction with propanedinitrile. These reactions were carried out under optimized conditions (2.0 mol% 3, methanol, 25 °C, 1 h). Table 3 shows that the yields of the respective products ranged from 33 to >99%. Benzaldehydes with strong electron-withdrawing groups, such as nitro, chloro, bromo and fluoro substituents, demonstrated the highest efficiency (entries 2–7, Table 3). This increased efficiency is likely due to the higher electrophilicity of these substrates. However, benzaldehydes with electron-donating groups, such as methyl or methoxy groups, resulted in lower product yields (entries 9 and 10, Table 3).

Table 3 Substrate scope for Cd-catalyzed Knoevenagel reaction of substituted benzaldehydes with propanedinitrile^a

Entry	Substituted benzaldehyde	Product yield^b (%)
a Conditions: aldehyde (0.5 mmol), propanedinitrile (1.0 mmol), catalyst 3 (2.0 mol%), CH3OH (1.0 mL), 25 °C.b Yield on the basis of ^1H NMR analysis: [moles of product per mol of aldehyde substrate] × 100%.
1	Benzaldehyde	>99
2	2-Nitrobenzaldehyde	>99
3	3-Nitrobenzaldehyde	>99
4	4-Nitrobenzaldehyde	>99
5	4-Chlorobenzaldehyde	>99
6	4-Bromobenzaldehyde	>99
7	4-Fluorobenzaldehyde	>99
8	Pyridine-4-aldehyde	>99
9	4-Methylbenzaldehyde	97
10	4-Methoxybenzaldehyde	66
11	4-Hydroxybenzaldehyde	33

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The recyclability of catalyst 3 was evaluated. After every reaction cycle, the catalyst was separated by centrifugation, rinsed inmethanol, air-dried at approximately 25 °C, and reused in the subsequent cycle. The data show that CP 3 retains its activity for at least five more reaction cycles (Fig. S5†). Additionally, PXRD patterns show that the structure of CP 3 remains intact (Fig. S6†), despite the occurrence of multiple new signals or widened peaks. These changes are likely due to the existence of certain or a decrease in crystallinity after multiple catalytic cycles.

Referring to previous studies in this area,^36–40 we propose a possible reaction mechanism for the Knoevenagel condensation catalyzed by 3 (Scheme S1, ESI†). The unsaturated Cd(II) metal centers of the catalyst (5-coordinate cadmium centers) eventually act as the Lewis acid sites interacting with the H–C=O functionality of pyridine-3-aldehyde, leading to its polarization and an enhanced electrophilicity of the corresponding carbon atom. Such a polarization can facilitate a nucleophilic attack of this site by propanedinitrile acting as a nucleophile precursor. On the other hand, an interaction between the Lewis acid site and the –CN group of malononitrile augments an acidic character of the methylene functionality and enhances its deprotonation. The basic sites present in 3 (O-carboxylate sites) can easily abstract H⁺ from the –CH₂– group to give rise to a nucleophile that would attack the H–C=O moiety of pyridine-3-aldehyde and result in the C–C bond formation, followed by the dehydration to give the 2-(pyridin-3-ylmethylene)malononitrile product.

Conclusions

This study showed the use of H₂aipa as a amino-functionalized dicarboxylate precursor to synthesize five novel coordination compounds using a hydrothermal method. These compounds 1–5 exhibit diverse structural features, ranging from 0D dimer (compound 2), 2D sheets (CPs 1, 3 and 4) to 3D + 2D interpenetrated frameworks (CP 5). The catalytic potential of compounds 1–5 was evaluated in Knoevenagel reaction, which involved pyridine-3-aldehyde and propanedinitrile. CP 3 showed significant catalytic efficiency in this reaction.

Data availability

The data supporting this article have been included as part of the ESI.† CCDC-2367606–2367610 (compounds 1–5) contain the supplementary crystallographic data for this paper.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Science and Technology Projects by State Nickel Cobalt New Material Engineering Technology Research Center, Lanzhou, China. The grant number is GCZX2023JSKF0005.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: optical images (Fig. S1), FTIR spectra (Fig. S2), PXRD patterns (Fig. S3), additional catalysis data (Fig. S4–S7) and structural parameters (Tables S1 and S2). CCDC 2367606–2367610. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4ra05352d

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