Two 3D Mn-based coordination polymers: synthesis, structure and magnetocaloric effect

Abstract

Two three-dimensional (3D) coordination polymers, namely Mn^II₆(CH₃COO)₂(HCOO)₂(IN)₈(C₄H₈O)₂(H₂O) and Mn^III₆Mn^II₁₂(μ₃-O)₆(CH₃COO)₁₂(IN)₁₈(H₂O)_7.5 (abbreviated as Mn^II₆ and Mn^II₁₂Mn^III₆ respectively; HIN = isonicotinic acid), were synthesized by the reaction of Mn(CH₃COO)₂·4H₂O and isonicotinic acid under solvothermal conditions. Magnetic studies revealed that antiferromagnetic interactions may be present in compounds Mn^II₆ and Mn^II₁₂Mn^III₆. Moreover, the values of −ΔS_m (26.27 (Mn^II₆) and 37.69 (Mn^II₁₂Mn^III₆) J kg⁻¹ K⁻¹ at ΔH = 7 T) are relatively larger than those of the reported Mn-based coordination polymers. This work provides a great scope in the magnetocaloric effect (MCE) of pure 3d-type systems.

---

Introduction

Manganese (Mn) ions owing to their variety in valency and coordination numbers make the Mn-based compounds exhibit interesting structures, such as chain-like¹ and wheel-like,^2,3 as well promising applications in the proton exchange membrane,^4,5 catalysis,^6–8 adsorption and so on.^3,9–15 Moreover, since the discovery of the first case of single-molecule magnet behavior (SMM) in the compound Mn^III₈Mn^IV₄ in 1980,¹⁶ Mn-based compounds have played an active role in the field of magnetism.^17–21 For example, Mn_X (X = 7, 12, 16, 19, 26, 30, 32, 44, 70, and 84) demonstrate the single-molecule magnet (SMM) behaviour,^9,17,22–30 and Mn_X (X = 1, 4, 10, 14, and 17) demonstrate the magnetocaloric effect (MCE),^31–34 which can replace expensive and scarce ³He.^35–40

Undoubtedly, all molecular magnetic complexes can exhibit MCE to a certain extent.^35–40 However, only some Gd^III-based compounds displayed large magnetic entropy change (ΔS_m), which is an important criterion to evaluate MCE.^37,45–47 Studies manifest that the neglected magnetic anisotropy, large spin ground state (S) and low-lying excited spin states are beneficial to the MCE.^38,46 Although lanthanide (4f)-type compounds have made a breakthrough in magnetic cooling materials, the search for highly efficient MCE materials without lanthanide remains a crucial issue owing to the expensive and rare 4f compounds.^41–44 In 2014, [Mn(glc)₂(H₂O)] with ΔS_m = 60.3 J kg⁻¹ K⁻¹, the value of which is larger than that of the majority of pure Gd-style and 3d-Gd compounds, was prepared by Tong et al.³¹ This example inspires us to study the MCE of 3d-type systems. Thus, we aim at developing a procedure regarding the pure 3d-type systems with MCE.

According to the literature, using HIN as a ligand is a better choice to prepare metal coordination polymers.⁴⁸ The coordination sites (one N- and two O-donors) of HIN can connect at least one metal ion; thus, HIN ligands are beneficial to form high-nuclear metal clusters accompanied with large MCE, such as Gd₅₂Ni₅₂ with −ΔS_m = 35.6 J kg⁻¹ K⁻¹.³⁸

Herein, two six/eighteen-nuclear manganese coordination polymers Mn^II₆(CH₃COO)₂(HCOO)₂(IN)₈(C₄H₈O)₂(H₂O) and Mn^III₆Mn^II₁₂(μ₃-O)₆(CH₃COO)₁₂(IN)₁₈(H₂O)_7.5 (abbreviated as Mn^II₆ and Mn^II₁₂Mn^III₆, respectively), were successfully obtained by the reaction of Mn(CH₃COO)₂·4H₂O and HIN under solvothermal conditions. The magnetic studies reveal that antiferromagnetic interactions are present in Mn^II₆ and Mn^II₁₂Mn^III₆, and both show excellent MCE properties with −ΔS_m = 26.27 (Mn^II₆) and 37.69 (Mn^II₁₂Mn^III₆) J kg⁻¹ K⁻¹. The −ΔS_m values obtained in this work are relatively larger than those of the existing 3d-based compounds.^32,33 In 2018, [Mn^II₃]₆, with an interesting wheel structure and 18 metal Mn^II ions, was synthesized by Qin et al.³ In our study, comparing Mn^II₁₂Mn^III₆ with [Mn^II₃]₆, we found that (i) the valencies of Mn ions in Mn^II₁₂Mn^III₆ are +2 and +3, as determined by X-ray photoelectron spectroscopy (XPS); (ii) [Mn^II₃]₆ was mainly applied to sorption; however, we deeply studied the MCE for Mn^II₁₂Mn^III₆. Moreover, among the current pure Mn-type compounds, −ΔS_m (37.69 J kg⁻¹ K⁻¹) for Mn^II₁₂Mn^III₆ is nearly the largest. Our pure Mn-type materials are highly promising applications in MCE.

Experimental section

X-ray crystallography

A Bruker Apex II CCD detector was applied to collect the data of single-crystal X-ray diffraction (SCXRD) analyses under 296 K and 50 kV as well 30 mA with a sealed tube X-ray source (Mo-Kα radiation, λ = 0.71 Å). The structures of Mn^II₆ and Mn^II₁₂Mn^III₆ were resolved based on direct methods, and were refined based on full-matrix least-squares refinement using the SHELXL-2018/3 program package.

Synthesis of compounds

Preparation of Mn^II₆. A mixture of HCOOH (0.023 g 0.50 mmol), ethylenediamine (0.030 g, 0.50 mmol), Mn·(CH₃COO)₂·4H₂O (0.350 g, 2.00 mmol), HIN (0.246 g, 2.0 mmol) and tetrahydrofuran (8.0 mL) was stirred at room temperature for 12 h. Then, this suspension was heated to 170 °C and kept for 7 days in a 25 mL Teflon-lined autoclave (Schemes 1 and S1†). After cooling for 48 h, a colourless block product was obtained. The product was washed with CH₃CH₂OH, and the yield of pure Mn^II₆ was 68% (based on Mn). Anal. calcd for C₆₂H₅₈Mn₆N₈O₂₇ (FW = 1676.80): C, 44.37, H, 3.45, N, 6.67%. Found: C, 45.05, H, 3.64, N, 6.57%.

Scheme 1 Synthetic route of Mn^II₆.

Preparation of Mn^II₁₂Mn^III₆. A mixture of HCOOH (0.023 g, 0.50 mmol), ethylenediamine (0.03 g, 0.50 mmol), HIN (0.246 g, 2.00 mmol), NH₄VO₃ (0.005 g, 0.45 mmol), Gd₂O₃ (0.182 g, 0.50 mmol), Mn·(CH₃COO)₂·4H₂O (0.350 g, 2.00 mmol) and tetrahydrofuran (8.0 mL) was stirred at room temperature for 12 h. Then, this suspension was heated to 170 °C and kept for 10 days in a 25 mL Teflon-lined autoclave (Schemes 2 and S2†). After cooling for 48 h, a colourless block product was obtained. The product was washed with CH₃CH₂OH, and the yield of pure Mn^II₁₂Mn^III₆ was 38% (based on Mn). Anal. calcd for C₁₃₂H₁₂₃N₁₈O₇₃Mn₁₈ (FW = 4118.40): C, 38.32, H, 3.10, N, 6.05%. Found: C, 38.46, H, 3.00, N, 6.12%.

Scheme 2 Synthetic route of Mn^II₁₂Mn^III₆.

Results and discussion

Synthesis

Developing a synthetic process for desired products with higher yields is much harder for the following reasons. The growth of target composition is sensitive for the initial reaction circumstances and conditions, such as the ratio of reactants, the type and ratio of solvents and the temperature.^36,49–56 In this work, HCOOH, tetrahydrofuran, ethanediamine, Mn·(CH₃COO)₂·4H₂O, and HIN were chosen as ideal reactants. In addition, we also tried out other organic solvents (ethyl alcohol, methyl alcohol, acetonitrile and N,N-dimethyl formamide) in the synthetic process, but no products could be synthesized. Ethanediamine was not observed in the final structure, but played an indispensable role in the construction of 3D Mn-clusters. Besides, HCOOH and ethanediamine must be measured at first. Interestingly, while adding NH₄VO₃ and Gd₂O₃ as the reactants, a beautiful structure with 18 Mn ions was obtained. We also optimized the reaction, and Mn^II₁₂Mn^III₆ was successfully synthesized in the presence of NH₄VO₃ and Gd₂O₃. During the cooling process, the yield and morphology of products were unproductive for a cooling time of less than 48 h. Consequently, on cooling for over 48 h, two compounds were obtained with yields of 68% and 38%. In order to study the related properties regarding the two compounds, purification is an important step. The products were washed with CH₃CH₂OH and then filtrated to obtain target products (Table 1).

Table 1 Summary of crystal data and structure results for compounds Mn^II₆ and Mn^II₁₂Mn^III₆

a R 1 = Σ||Fo| − |Fc||/Σ|Fo|. b wR2 = Σ[w(Fo^2 − Fc^2)^2]/Σ[w(Fo^2)^2]^1/2.
Compound	Mn ^II 12 Mn ^III 6	Mn ^II 6
Empirical formula	C132H123Mn18N18O73	C62H58Mn6N8O27
Formula weight	4118.40	1676.80
Crystal system	Trigonal	Monoclinic
Space group	P3̄	C2/c
a (Å)	24.293(8)	15.513(2)
b (Å)	24.293(8)	10.486(2)
c (Å)	9.537(5)	22.374(4)
α (deg)	90	90
β (deg)	90	107.311(3)
γ (deg)	120	90
Volume (Å^3)	4874(4)	3474.8(11)
Z	1	2
Dc (Mg m^−3)	1.403	1.603
μ (mm^−1)	1.204	1.146
F(000)	2075	1704
Crystal size (mm^3)	0.160 × 0.130 × 0.130	0.150 × 0.100 × 0.100
Radiation type	Mo Kα	Mo Kα
2θ range for data collection (°)	0.968–25.484	1.907–25.099
Limiting indices	−29 ≤ h ≤ 27	−17 ≤ h ≤ 18
	−28 ≤ k ≤ 29	−12 ≤ k ≤ 12
	−11 ≤ l ≤ 11	−26 ≤ l ≤ 26
Reflections collected	35 185	12 170
R int	0.0763	0.0321
Data/restraints/parameters	6053/66/388	3100/19/252
Goodness-of-fit on F^2	1.081	1.037
Final R indices, R1^a, wR2^b	R 1 = 0.0365	R 1 = 0.0268
[I > 2σ(I)]	wR2 = 0.1071	wR2 = 0.0670
R indices (all data)	R 1 = 0.0459	R 1 = 0.0308
	wR2 = 0.1114	wR2 = 0.0692

---

Structure

Single-crystal X-ray analysis was performed, which made it clear that Mn^II₆ and Mn^II₁₂Mn^III₆ crystallize in the monoclinic space group C2/c and the trigonal space group P3̄, respectively. As shown in Fig. 1a and b, the asymmetric unit of Mn^II₆ contains six Mn^II ions, two CH₃COO⁻ groups, two HCOO⁻ groups and eight IN⁻ ligands, one free H₂O and two free tetrahydrofuran. Moreover, the asymmetric unit of Mn^II₁₂Mn^III₆ contains twelve Mn^II ions, six Mn^III ions (determined by XPS, Fig. 6), twelve CH₃COO⁻, eighteen IN⁻ ligands, six μ₃-O²⁻ and 7.5 free water molecules (Fig. 2a). All Mn ions in Mn^II₆ and Mn^II₁₂Mn^III₆ are hexa-coordinated, displaying a near-octahedral geometry. As shown in Fig. S1,† Mn1 of Mn^II₆ is coordinated to two N atoms (from two HIN) and four O atoms (from one HCOO⁻ and one CH₃COO⁻ and two IN⁻ groups); Mn2 of Mn^II₆ is coordinated to six O atoms (from one HCOO⁻ and one CH₃COO⁻ and four IN⁻ groups). For Mn^II₁₂Mn^III₆ (Fig. 2b–d and S2†), Mn1 is coordinated to two N atoms (from two IN⁻ ligands) and four O atoms (from one μ₃-O²⁻ group, two IN⁻ ligands and one CH₃COO⁻ group); Mn2 is coordinated to one N atom (from one IN⁻ ligand) and five O atoms (from one μ₃-O²⁻ group), two Mn3 is coordinated to six O-donors (from one μ₃-O²⁻ group, three HIN ligands and two CH₃COO⁻ groups). The Mn–O bond lengths are between 2.131(3) and 2.277(2) Å as well as the Mn–N bond lengths are between 2.312(3) and 2.391(3) Å, which are approximate to the data obtained in the previously reported Mn-based compounds.^3,31–34 However, in Mn^II₆ and Mn^II₁₂Mn^III₆, the coordination modes of IN⁻ and CH₃COO⁻ ligands have only one mode, as indicated in Fig. S3,† which are both coordinated with three Mn ions.

Fig. 1 The stick-ball pattern of Mn^II₆ (a and b); the ball and stick of HIN (c and d); the ball and stick of CH₃COOH (e and f); the ball and stick of HCOOH (g and h); 1D chain-like structure unit in Mn^II₆ (i). (O: red, N: blue, C: black, H: white, Mn: purple for (a)–(h)).

Fig. 2 The basic building block (a) and coordination models of Mn ions (b–d) in compound Mn^II₁₂Mn^III₆. [Symmetry code: (i) x, y, 1 + z; (ii) −y, 1 + x − y, z; (iii) −1 + y, −1 − x + y, 2 − z. H atoms have been deleted for clarity.]

As shown in Fig. 1f, the adjacent Mn^II₆ units are linked by one CH₃COO⁻, one HCOO⁻ and two IN⁻ ligands, forming a 1D chain structure. Besides, the three kinds of bridge-ligands are all parallel and reverse (Fig. 1c–f). The adjacent 1D chains are interconnected to form a 2D layer based on IN⁻ ligands only (Fig. 3a–e). The neighbouring 2D layers are further connected by IN⁻, HCOO⁻ and CH₃COO⁻ ligands, forming a 3D structure (Fig. 3f).

Fig. 3 The 1D chain structure of Mn^II₆ (a–c); the ball and stick of HIN (d); the 2D-layer of Mn^II₆ (e); the 3D structure of Mn^II₆ (f).

As shown in the Fig. 4b and S8† [Mn^II₂Mn^III(μ₃-O)(CH₃COO)₂]³⁺ unit (the first building unit; abbreviated as Mn^II₂Mn^III) consists of three Mn ions and two CH₃COO⁻ and one μ₃-O²⁻ group. As we can see in Fig. 4, the adjacent Mn^II₂Mn^III units are lined by acetic acid, forming a [Mn^II₁₂Mn^III₆(μ₃-O)₆(CH₃COO)₁₂]¹⁸⁺ (the second building unit; abbreviated as the Mn^II₁₂Mn^III₆) unit. In addition, Mn2 and Mn3 from adjacent Mn^II₂Mn^III units are linked by two CH₃COO⁻ groups to form a screwy wheel. As shown in Fig. 4d, 12CH₃COO⁻ groups evenly distribute on both sides of the wheel. The wheel of Mn^II₁₂Mn^III₆ fragments are nearly planar (Fig. 4f). As indicated in Fig. 5b, the adjacent Mn^II₁₂Mn^III₆ are interconnected only by the IN⁻ ligands, forming a porous plane. Meanwhile, the HIN ligands act as bonds furtherly linked to adjacent Mn^II₁₂Mn^III₆ wheels, resulting in a 3D structure (Fig. 5c). Besides, adjacent IN⁻ ligands are all orderly arranged in the opposite direction (Fig. 5d).

Fig. 4 The different expressional patterns of the Mn^II₁₂Mn^III₆ the coordination polymer (a) and (c–f); Ball and stick plot of Mn^II₂Mn^III unit (b).

Fig. 5 Distribution of Mn^II₁₂Mn^III₆ (a); The 2D structure of compound Mn^II₁₂Mn^III₆ (b); 3D structure of the compound Mn^II₁₂Mn^III₆ (c); 1D channel in a three-dimensional structure of compound Mn^II₁₂Mn^III₆ (d).

XPS. In order to further prove the mixed valence of compound Mn^II₁₂Mn^III₆, an XPS measurement was carried out (Fig. 6). By accurate fitting, peaks at 641.3 eV for Mn 2p_3/2 and 653.2 eV for Mn 2p_1/2 can be assigned to Mn^II ions.^57–59 The characteristic peak at 645.8 eV proves the presence of Mn^III in Mn^II₁₂Mn^III₆.^57–59 In addition, the XPS result suggests that the ratio (Mn^II : Mn^III) is 2 : 1, which is consistent with the result obtained from single-crystal X-ray diffractometry.

Fig. 6 The XPS for compound Mn^II₁₂Mn^III₆.

PXRD. PXRD of compounds Mn^II₆ and Mn^II₁₂Mn^III₆ was studied at room temperature (Fig. S12 and S13†). The experimental patterns were basically similar to simulated curves, revealing that the structures of two compounds are similar to the results of SCXRD. The main peaks for Mn^II₁₂Mn^III₆ at 2θ of 4.20°, 7.28°, 8.40° and 9.26° can be indexed to the indices of crystal face of (0 0 2), (1 1 0), (−1 1 2) and (1 1 1) reflections, respectively. For Mn^II₆, main peaks at 2θ of 8.34°, 10.52°, and 11.90° can be indexed to indices of crystal face of (−1 2 0), (0 2 0) and (0 0 1) reflections, respectively. The result of PXRD indicates that compounds Mn^II₆ and Mn^II₁₂Mn^III₆ are different Mn-based polymers, which are in agreement with SCXRD (as shown in Fig. S14†).

FT-IR spectra. The FT-IR spectra of Mn^II₆ and Mn^II₁₂Mn^III₆ were studied at room temperature in the range of ν = 4000–400 cm⁻¹ (Fig. S15 and S16†). The peaks at 3397 (Mn^II₆) and 3382 (Mn^II₁₂Mn^III₆) cm⁻¹ can be assigned to the stretching vibration of –OH from H₂O or the air. The peaks at 1612 (Mn^II₆) and 1604 (Mn^II₁₂Mn^III₆) cm⁻¹ can be due to the presence of −COO⁻. The characteristic peaks of the pyridine from the HIN are at 1550, 1400, 1342, 1218, and 1060 cm⁻¹ for Mn^II₆, and 1550, 1440, 1214, and 1014 cm⁻¹ for Mn^II₁₂Mn^III₆. The values are consistent with the related literature studies.^3,31–36

TG analysis. As shown in Fig. S17,† the TG curve of Mn^II₆ was studied. The weight loss was 4.90% (theoretical value 4.27%) from 25 to 205 °C due to the loss of two free tetrahydrofuran and one H₂O. From 205 to 800 °C, the loss was 61.74% (theoretical value 61.80%) due to the collapse of the metal framework. The TGA analysis of product Mn^II₁₂Mn^III₆ was also carried out (Fig. S18†). The TGA diagram showed two main weight losses in the curve. The first step (25–200 °C) corresponds to the release of 7.5 free water. The observed weight loss of 3.9% is similar to the theoretical values (3.3%). Then, compound Mn^II₁₂Mn^III₆ lost a mass of 36.09% in the range of 200–800 °C. It can be attributed to the thermal decomposition of metal framework.

Magnetic properties. As displayed in Fig. 7, S19 and S20,† the χ_MT–T of compounds Mn^II₆ and Mn^II₁₂Mn^III₆ was studied under 1000 Oe in the range of 1.8–300 K. At T = 300 K, the values of χ_MT were 24.87 (Mn^II₆) and 69.60 (Mn^II₁₂Mn^III₆) cm³ K mol⁻¹, which were almost consistent with the calculated values 26.25 (Mn^II₆) and 70.50 (Mn^II₁₂Mn^III₆) cm³ K mol⁻¹ (Mn^II ions, S = 5/2; Mn^III ions, S = 2).^3,18 With the temperature cooling, χ_MT of compound Mn^II₆ slowly decreased in the range of 90 K to 300 K. With cooling, χ_MT of compound Mn^II₆ rapidly decreased to 10.23 cm³ K mol⁻¹ at 9 K. However, as the temperature decreased further, χ_MT abruptly rose to 24.08 cm³ K mol⁻¹ at 2.0 K. Besides, the χ_M⁻¹vs. T was fitted based on the Curie–Weiss law (from 1.8 to 300 K) with the result C = 25.69 cm³ K mol⁻¹ and θ = −12.49 K. The negative value of θ indicates that the antiferromagnetic behaviour may exist in Mn^II₆ (Fig. S21†).¹⁸ For complex Mn^II₁₂Mn^III₆, the χ_mT decreased slowly along with the decrease in temperature, which was in good agreement with the antiferromagnetic behaviour.³ The plot of 1/χ_Mvs. T obeyed the Curie–Weiss law χ_M⁻¹ = (T − θ)/C with C = 76.57 cm³ K mol⁻¹ and θ = −37.22 K, and the negative product of θ further demonstrates antiferromagnetic interactions between adjacent Mn ions (Fig. S22†).²⁵

Fig. 7 Temperature-dependent magnetic susceptibilities for compound Mn^II₆ and Mn^II₁₂Mn^III₆.

The M–H of compounds Mn^II₆ and Mn^II₁₂Mn^III₆ was studied under T = 1.8–10 K and H = 0–7 T (Fig. S23 and S24†). Along with the increase in H, M (Mn^II₆) slowly increased, gradually saturated and reached 10.78 Nμ_B at 7 T. For Mn^II₁₂Mn^III₆, M slowly increased and reached 17.57 Nμ_B at 7 T with the increasing H.

The magnetization data of compounds Mn^II₆ and Mn^II₁₂Mn^III₆ were analysed in the range of 2–9 K, based on the Maxwell relation in equation ΔS_m(T) = ∫[∂M(T, H)/∂T]_HdH.²⁵ The compound Mn^II₆ is discussed first. The consequential maximum value of −ΔS_m is 26.27 J kg⁻¹ K⁻¹ (Mn^II₆) at approximately 2.5 K and ΔH = 7 T (Fig. 8), which was slightly smaller than the theoretical calculated value [53.17 J kg⁻¹ K⁻¹, which was obtained on R ln(2S + 1)].³³ The discrepancy could be due to the antiferromagnetic magnetic interaction among the metal ions.¹⁸ Besides, the maximum value of −ΔS_m was 37.69 J kg⁻¹ K⁻¹ (Mn^II₁₂Mn^III₆) at 2 K and ΔH = 7 T (Fig. 9), which was slightly smaller than the theoretically calculated value [62.71 J kg⁻¹ K⁻¹, which was gained by R ln(2S + 1)].^55,56 The discrepancy could be due to the antiferromagnetic magnetic interaction among metal ions.³³ The maximum values of −ΔS_m for compounds Mn^II₆ and Mn^II₁₂Mn^III₆ are large in the pure 3d- or 4f-type and 3d–4f systems (Table 2). In addition, the −ΔS_m of Mn^II₁₂Mn^III₆ is largest in the pure Mn-type compounds, except for [Mn(glc)₂(H₂O)₂]_n.³¹

Fig. 8 Curves of −ΔS_mvs. T for compound Mn^II₆.

Fig. 9 Curves of −ΔS_mvs. T for compound Mn^II₁₂Mn^III₆.

Table 2 −ΔS^max_m value for some Mn-base compounds under ΔH by the given temperature

Compound	−ΔS^maxm	ΔH (T)	Ref.
[Mn(glc)2(H2O)2]n	60.30	7.0	31
Mn ^II 12 Mn ^III 6	37.69	7.0	This work
Mn ^II 6	26.27	7.0	This work
Mn^III6Mn^II8	25.00	7.0	33
Fe^III14	20.30	7.0	51
Mn^II4	19.30	7.0	32
Mn^III6Mn^II4	17.00	7.0	33
Mn2Gd	50.10	7.0	21
Mn2Gd2	37.90	9.0	52
Gd104	46.9	7.0	53
Gd38	37.9	7.0	54
Gd18	25.9	5.0	55

---

Conclusions

In summary, two 3D complexes (Mn^II₆ and Mn^II₁₂Mn^III₆) were successfully synthesized based on solvothermal conditions. In terms of their synthesis, Mn^II₁₂Mn^III₆ was obtained by adding Gd₂O₃ and NH₄VO₃ in the synthesis of Mn^II₆, which is a valid synthesis process and may contribute to synthesizing numerous coordination polymers. Magnetic studies revealed that Mn^II₆ and Mn^II₁₂Mn^III₆ are potential magnetic materials with −ΔS_m = 26.27 and 37.69 J kg⁻¹ K⁻¹ respectively, which are much larger than the previously synthesized Mn-based coordination polymers. Moreover, the −ΔS_m value of Mn^II₁₂Mn^III₆ is almost the largest among pure Mn-type coordination polymers, making it a potential polymer in the MCE of pure 3d-type systems.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the Natural Science Foundation of China (Grant 21571103), Jiangsu Province (BK20191359), and the Major Natural Science Projects of the Jiangsu Higher Education Institution (Grant 16KJA150005).

Notes and references

1. N. E. Chakov, W. Wernsdorfer, K. A. Abboud and G. Christou, Inorg. Chem., 2004, 43, 5919–5930.
2. M. Murugesu, J. Raftery, W. Wernsdorfer, G. Christou and E. K. Brechin, Inorg. Chem., 2004, 43, 4203–4209.
3. L. Zhou, B. Zhou, S. Cao, Z. Cui, B. Qin, W. Li, X. Zhang and J. Zhang, Chem.–Eur. J., 2018, 24, 19152–19155.
4. S.-L. Li and Q. Xu, Energy Environ. Sci., 2013, 6, 1656–1683.
5. S. Liu, Y. Deng and F. Xu, Chem. Commun., 2020, 54, 6066–6069.
6. Y.-F. Li and Z.-P. Liu, J. Am. Chem. Soc., 2018, 140, 1783–1792.
7. A. I. Nguyen, D. L. M. Suess, L. E. Darago, P. H. Oyala, D. S. Levine, M. S. Ziegler, R. D. Britt and T. D. Tilley, J. Am. Chem. Soc., 2017, 139, 5579–5587.
8. T. Ghosh and G. Maayan, Angew. Chem., Int. Ed., 2019, 58, 2785–2790.
9. A. J. Tasiopoulos, A. Vinslava, W. Wernsdorfer, K. A. Abboud and G. Christou, Angew. Chem., Int. Ed., 2004, 43, 2117–2121.
10. U. S. Sadana, P. Sharma, N. C. Ortiz, D. Samal and N. Claassen, J. Plant Nutr. Soil Sci., 2005, 168, 581–589.
11. J. Liang, Z. Liu, L. Qiu, Z. Hawash, L. Meng, Z. Wu, Y. Jiang, L. K. Ono and Y. Qi, Adv. Energy Mater., 2018, 8, 1800504–1800511.
12. G. E. Kostakis, A. M. Ako and A. K. Powell, Chem. Soc. Rev., 2010, 39, 2238–2271.
13. M. Manoli, S. Alexandrou, L. Pham, G. Lorusso, W. Wernsdorfer, M. Evangelisti, G. Christou and A. J. Tasiopoulos, Angew. Chem., Int. Ed., 2016, 55, 679–684.
14. M. U. Anwar, L. N. Dawe, M. S. Alam and L. K. Thompson, Inorg. Chem., 2012, 51, 11241–11250.
15. C. J. Milios, S. Piligkosb and E. K. Brechin, Dalton Trans., 2008, 1809–1817.
16. T. Lis, Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem., 1980, 36, 2042–2046.
17. A. M. Ako, I. J. Hewitt, V. Mereacre, R. Clrac, W. Wernsdorfer, C. E. Anson and A. K. Powell, Angew. Chem., Int. Ed., 2006, 118, 5048–5051.
18. V. Mereacre, A. M. Ako, R. Clérac, W. Wernsdorfer, I. J. Hewitt, C. E. Anson and A. K. Powell, Chem.–Eur. J., 2008, 14, 3577–3584.
19. B. Zhao, H.-L. Gao, X.-Y. Chen, P. Cheng, W. Shi, D.-Z. Liao, S.-P. Yan and Z.-H. Jiang, Chem.–Eur. J., 2006, 12, 149–158.
20. T. C. Stamatatos, S. J. Teat, W. Wernsdorfer and G. Christou, Angew. Chem., Int. Ed., 2009, 48, 521–524.
21. F.-S. Guo, Y.-C. Chen, J.-L. Liu, J.-D. Leng, Z.-S. Meng, P. Vrábel, M. Orendáč and M.-L. Tong, Chem. Commun., 2012, 48, 12219–12221.
22. C. J. Milios, I. A. Gass, A. Vinslava, L. Budd, S. Parsons, W. Wernsdorfer, S. P. Perlepes, G. Christou and E. K. Brechin, Inorg. Chem., 2007, 46, 6215–6217.
23. X. Ma, D. Zhao, L.-F. Lin, S.-J. Qin, W.-X. Zheng, Y.-J. Qi, X.-X. Li and S.-T. Zheng, Inorg. Chem., 2016, 55, 11311–11315.
24. M. Riaz, R. K. Gupta, H.-F. Su, Z. Jagličić, M. Kurmoo, C.-H. Tung, D. Sun and L.-S. Zheng, Inorg. Chem., 2019, 58, 14331–14337.
25. T. C. Stamatatos, V. Nastopoulos, A. J. Tasiopoulos, E. E. Moushi, W. Wernsdorfer, G. Christou and S. P. Perlepes, Inorg. Chem., 2008, 47, 10081–10089.
26. M. Soler, W. Wernsdorfer, K. Folting, M. Pink and G. Christou, J. Am. Chem. Soc., 2004, 126, 2156–2165.
27. E. E. Moushi, C. Lampropoulos, W. Wernsdorfer, V. Nastopoulos, G. Christou and A. J. Tasiopoulos, J. Am. Chem. Soc., 2010, 132, 16146–16155.
28. M. Manoli, R. Inglis, M. J. Manos, V. Nastopoulos, W. Wernsdorfer, E. K. Brechin and A. J. Tasiopoulos, Angew. Chem., Int. Ed., 2011, 50, 4441–4444.
29. A. Vinslava, A. J. Tasiopoulos, W. Wernsdorfer, K. A. Abboud and G. Christou, Inorg. Chem., 2016, 55, 3419–3430.
30. T. Shiga, H. Nojiri and H. Oshio, Inorg. Chem., 2020, 59, 4163–4166.
31. Y.-C. Chen, F.-S. Guo, J.-L. Liu, J.-D. Leng, P. Vrábel, M. Orendáč, J. Prokleška, V. Sechovský and M.-L. Tong, Chem.–Eur. J., 2014, 20, 3029–3035.
32. J.-P. Zhao, R. Zhao, Q. Yang, B.-W. Hu, F.-C. Liu and X.-H. Bu, Dalton Trans., 2013, 42, 14509–14515.
33. M. Manoli, A. Collins, S. Parsons, A. Candini, M. Evangelisti and E. K. Brechin, J. Am. Chem. Soc., 2008, 130, 11129–11139.
34. S. Nayak, M. Evangelisti, A. K. Powell and J. Reedijk, Chem.–Eur. J., 2010, 16, 12865–12872.
35. F. Shao, J.-J. Zhuang, M.-G. Chen, N. Wang, H.-Y. Shi, J.-P. Tong, G. Luo, J. Tao and L.-S. Zheng, Dalton Trans., 2018, 47, 16850–16854.
36. N.-F. Li, Q.-F. Lin, X.-M. Luo, J.-P. Cao and Y. Xu, Inorg. Chem., 2019, 58, 10883–10889.
37. Y. Yu, X. Pan, C. Cui, X. Luo, N. Li, H. Mei and Y. Xu, Inorg. Chem., 2020, 59, 5593–5599.
38. Q. Lin, J. Li, Y. Dong, G. Zhou, Y. Song and Y. Xu, Dalton Trans., 2017, 46, 9745–9749.
39. Q. Lin, Y. Zhang, W. Cheng, Y. Liu and Y. Xu, Dalton Trans., 2017, 46, 643–646.
40. J.-L. Liu, W.-Q. Lin, Y.-C. Chen, J.-D. Leng, F.-S. Guo and M.-L. Tong, Inorg. Chem., 2013, 52, 457–463.
41. J.-B. Peng, Q.-C. Zhang, X.-J. Kong, Y.-Z. Zheng, Y.-P. Ren, L.-S. Long, R.-B. Huang, L.-S. Zheng and Z. Zheng, J. Am. Chem. Soc., 2012, 134, 3314–3317.
42. J.-B. Peng, Q.-C. Zhang, X.-J. Kong, Y.-P. Ren, L.-S. Long, R.-B. Huang, L.-S. Zheng and Z. Zheng, Angew. Chem., Int. Ed., 2011, 50, 10649–10652.
43. Y.-Z. Zheng, M. Evangelisti and R. E. P. Winpenny, Angew. Chem., Int. Ed., 2011, 123, 3776–3779.
44. W.-P. Chen, J. Singleton, L. Qin, A. Camón, L. Engelhardt, F. Luis, R. E. P. Winpenny and Y.-Z. Zheng, Nat. Commun., 2018, 9, 1–6.
45. D. I. Alexandropoulos, L. Cunha-Silva, J. Tang and T. C. Stamatatos, Dalton Trans., 2018, 47, 11934–11941.
46. J.-D. Leng, J.-L. Liu and M.-L. Tong, Chem. Commun., 2012, 48, 5286–5288.
47. L. Sun, H. Chen, C. Ma and C. Chen, Dalton Trans., 2015, 44, 20964–20971.
48. L. Wang, R. Zhao, L.-Y. Xu, T. Liu, J.-P. Zhao, S.-M. Wang and F.-C. Liu, CrystEngComm, 2014, 16, 2070–2077.
49. S. Jeong, X. Song, S. Jeong, M. Oh, X. Liu, D. Kim, D. Moon and M. S. Lah, Inorg. Chem., 2011, 50, 12133–12140.
50. X.-Y. Li, H.-F. Su, Q.-W. Li, R. Feng, H.-Y. Bai, H.-Y. Chen, J. Xu and X.-H. Bu, Angew. Chem., Int. Ed., 2019, 58, 10184–10188.
51. R. Shaw, R. H. Laye, L. F. Jones, D. M. Low, C. Talbot-Eeckelaers, Q. Wei, C. J. Milios, S. Teat, M. Helliwell, J. Raftery, M. Evangelisti, M. Affronte, D. Collison, E. K. Brechin and E. J. L. McInnes, Inorg. Chem., 2007, 46, 4968–4978.
52. T. Rajeshkumar, R. Jose, P. R. Remya and G. Rajaraman, Inorg. Chem., 2019, 58, 11927–11940.
53. J.-B. Peng, X.-J. Kong, Q.-C. Zhang, M. Orendáč, J. Prokleška, Y.-P. Ren, L.-S. Long, Z. Zheng and L.-S. Zheng, J. Am. Chem. Soc., 2014, 136, 17938–17941.
54. F.-S. Guo, Y.-C. Chen, L.-L. Mao, W.-Q. Lin, J.-D. Leng, R. Tarasenko, M. Orendáč, J. Prokleška, V. Sechovský and M.-L. Tong, Chem.–Eur. J., 2013, 19, 14876–14885.
55. K. Wang, Z.-L. Chen, H.-H. Zou, K. Hu, H.-Y. Li, Z. Zhang, W.-Y. Sun and F.-P. Liang, Chem. Commun., 2016, 52, 8297–8300.
56. X.-M. Luo, Z.-B. Hu, Q.-f. Lin, W. Cheng, J.-P. Cao, C.-H. Cui, H. Mei, Y. Song and Y. Xu, J. Am. Chem. Soc., 2018, 140, 11219–11222.
57. S. L. Xiong, J. S. Chen, X. W. Lou and H. C. Zeng, Adv. Funct. Mater., 2012, 22, 861–871.
58. X. Li, Y. Zhu, X. Zhang, J. Liang and Y. Qian, RSC Adv., 2013, 3, 10001–10006.
59. B. J. Tan, K. J. Klabunde and P. M. A. Sherwood, J. Am. Chem. Soc., 1991, 113, 855–861.

---

Footnote

† Electronic supplementary information (ESI) available: Physical measurements, crystal synthesis, additional structural pictures, XPS, PXRD, FT-IR spectra, TG analysis, magnetic properties, as well as selected bond lengths and angles. Tables S1, S2, and Fig. S1–S24. CCDC 2004735 and 2004736 for compounds Mn^II₆ and Mn^II₁₂Mn^II₆. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0ra05926a

---