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

Two three-dimensional (3D) coordination polymers, namely MnII 6(CH3COO)2(HCOO)2(IN)8(C4H8O)2(H2O) and MnIII 6MnII 12(μ3-O)6(CH3COO)12(IN)18(H2O)7.5 (abbreviated as Mn II 6 and Mn II 12 Mn III 6 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 II 6 and Mn II 12 Mn III 6 . Moreover, the values of -ΔS m (26.27 (Mn II 6 ) and 37.69 (Mn II 12 Mn III 6 ) 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. This journal is © The Royal Society of Chemistry.

Introduction

Manganese (Mn) ions owing to their variety in valency and coordination numbers make the Mn-based compounds exhibit interesting structures, such as chain-like[1] 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 MnIII8MnIV4 in 1980,[16] Mn-based compounds have played an active role in the field of magnetism.[17-21] For example, Mn (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 = 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 GdIII-based compounds displayed large magnetic entropy change (ΔSm), 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 ΔSm = 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 −ΔSm = 35.6 J kg−1 K−1.[38]

Herein, two six/eighteen-nuclear manganese coordination polymers MnII6(CH3COO)2(HCOO)2(IN)8(C4H8O)2(H2O) and MnIII6MnII12(μ3-O)6(CH3COO)12(IN)18(H2O)7.5 (abbreviated as Mn and MnMn, 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 and MnMn, and both show excellent MCE properties with −ΔSm = 26.27 (Mn) and 37.69 (MnMn) J kg−1 K−1. The −ΔSm values obtained in this work are relatively larger than those of the existing 3d-based compounds.[32,33] In 2018, [MnII3]6, with an interesting wheel structure and 18 metal MnII ions, was synthesized by Qin et al.[3] In our study, comparing MnMn with [MnII3]6, we found that (i) the valencies of Mn ions in MnMn are +2 and +3, as determined by X-ray photoelectron spectroscopy (XPS); (ii) [MnII3]6 was mainly applied to sorption; however, we deeply studied the MCE for MnMn. Moreover, among the current pure Mn-type compounds, −ΔSm (37.69 J kg−1 K−1) for MnMn 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 and MnMn 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

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 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.

Preparation of MnMn

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 MnMn 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 MnMn.

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 MnMn 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).

Summary of crystal data and structure results for compounds Mn and MnMn

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 (Å3)	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 (mm3)	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 F2	1.081	1.037
Final R indices, R1a, wR2b	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

R 1 = Σ||Fo| − |Fc||/Σ|Fo|.

wR2 = Σ[w(Fo2 − Fc2)2]/Σ[w(Fo2)2]1/2.

Structure

Single-crystal X-ray analysis was performed, which made it clear that Mn and MnMn 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 contains six MnII ions, two CH3COO− groups, two HCOO− groups and eight IN− ligands, one free H2O and two free tetrahydrofuran. Moreover, the asymmetric unit of MnMn contains twelve MnII ions, six MnIII 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 and MnMn are hexa-coordinated, displaying a near-octahedral geometry. As shown in Fig. S1,† Mn1 of Mn 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 is coordinated to six O atoms (from one HCOO− and one CH3COO− and four IN− groups). For MnMn (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 and MnMn, 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 (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 (i). (O: red, N: blue, C: black, H: white, Mn: purple for (a)–(h)).

Fig. 6

The XPS for compound MnMn.

Fig. 2

The basic building block (a) and coordination models of Mn ions (b–d) in compound MnMn. [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 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 (a–c); the ball and stick of HIN (d); the 2D-layer of Mn (e); the 3D structure of Mn (f).

As shown in the Fig. 4b and S8† [MnII2MnIII(μ3-O)(CH3COO)2]3+ unit (the first building unit; abbreviated as MnMn) consists of three Mn ions and two CH3COO− and one μ3-O2− group. As we can see in Fig. 4, the adjacent MnMn units are lined by acetic acid, forming a [MnMn(μ3-O)6(CH3COO)12]18+ (the second building unit; abbreviated as the MnMn) unit. In addition, Mn2 and Mn3 from adjacent MnMn 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 MnMn fragments are nearly planar (Fig. 4f). As indicated in Fig. 5b, the adjacent MnMn are interconnected only by the IN− ligands, forming a porous plane. Meanwhile, the HIN ligands act as bonds furtherly linked to adjacent MnMn 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 MnMn the coordination polymer (a) and (c–f); Ball and stick plot of MnMn unit (b).

Fig. 5

Distribution of MnMn (a); The 2D structure of compound MnMn (b); 3D structure of the compound MnMn (c); 1D channel in a three-dimensional structure of compound MnMn (d).

XPS

In order to further prove the mixed valence of compound MnMn, an XPS measurement was carried out (Fig. 6). By accurate fitting, peaks at 641.3 eV for Mn 2p3/2 and 653.2 eV for Mn 2p1/2 can be assigned to MnII ions.[57-59] The characteristic peak at 645.8 eV proves the presence of MnIII in MnMn.[57-59] In addition, the XPS result suggests that the ratio (MnII : MnIII) is 2 : 1, which is consistent with the result obtained from single-crystal X-ray diffractometry.

PXRD

PXRD of compounds Mn and MnMn 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 MnMn 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, 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 and MnMn 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 and MnMn were studied at room temperature in the range of ν = 4000–400 cm−1 (Fig. S15 and S16†). The peaks at 3397 (Mn) and 3382 (MnMn) cm−1 can be assigned to the stretching vibration of –OH from H2O or the air. The peaks at 1612 (Mn) and 1604 (MnMn) 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, and 1550, 1440, 1214, and 1014 cm−1 for MnMn. The values are consistent with the related literature studies.[3,31-36]

TG analysis

As shown in Fig. S17,† the TG curve of Mn 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 MnMn 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 MnMn 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 χMT–T of compounds Mn and MnMn was studied under 1000 Oe in the range of 1.8–300 K. At T = 300 K, the values of χMT were 24.87 (Mn) and 69.60 (MnMn) cm3 K mol−1, which were almost consistent with the calculated values 26.25 (Mn) and 70.50 (MnMn) cm3 K mol−1 (MnII ions, S = 5/2; MnIII ions, S = 2).[3,18] With the temperature cooling, χMT of compound Mn slowly decreased in the range of 90 K to 300 K. With cooling, χMT of compound Mn rapidly decreased to 10.23 cm3 K mol−1 at 9 K. However, as the temperature decreased further, χMT abruptly rose to 24.08 cm3 K mol−1 at 2.0 K. Besides, the χM−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 (Fig. S21†).[18] For complex MnMn, the χmT decreased slowly along with the decrease in temperature, which was in good agreement with the antiferromagnetic behaviour.[3] The plot of 1/χMvs. T obeyed the Curie–Weiss law χM−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 and MnMn.

The M–H of compounds Mn and MnMn 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) slowly increased, gradually saturated and reached 10.78 NμB at 7 T. For MnMn, M slowly increased and reached 17.57 NμB at 7 T with the increasing H.

The magnetization data of compounds Mn and MnMn were analysed in the range of 2–9 K, based on the Maxwell relation in equation ΔSm(T) = ∫[∂M(T, H)/∂T]dH.[25] The compound Mn is discussed first. The consequential maximum value of −ΔSm is 26.27 J kg−1 K−1 (Mn) 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 −ΔSm was 37.69 J kg−1 K−1 (MnMn) 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 −ΔSm for compounds Mn and MnMn are large in the pure 3d- or 4f-type and 3d–4f systems (Table 2). In addition, the −ΔSm of MnMn is largest in the pure Mn-type compounds, except for [Mn(glc)2(H2O)2].[31]

Fig. 8

Curves of −ΔSmvs. T for compound Mn.

Fig. 9

Curves of −ΔSmvs. T for compound MnMn.

−ΔSmaxm value for some Mn-base compounds under ΔH by the given temperature

Compound	−ΔSmaxm	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
MnIII6MnII8	25.00	7.0	33
FeIII14	20.30	7.0	51
MnII4	19.30	7.0	32
MnIII6MnII4	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 and MnMn) were successfully synthesized based on solvothermal conditions. In terms of their synthesis, MnMn was obtained by adding Gd2O3 and NH4VO3 in the synthesis of Mn, which is a valid synthesis process and may contribute to synthesizing numerous coordination polymers. Magnetic studies revealed that Mn and MnMn are potential magnetic materials with −ΔSm = 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 −ΔSm value of MnMn 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.