Ionothermal synthesis, magnetic transformation and hydration–dehydration properties of Co(II)-based coordination polymers

Abstract

Two low-coordinated coordination polymers, [Co(nip)₂][EMIm]₂ (1) (H₂nip = 5-nitryl-isophthalic acid, EMIm = 1-ethyl-3-methyl imidazolium) and [Co(bptc)][EMIm]₂·H₂O (2) (H₄bptc = 2,2′,4,4′-biphenyltetracarboxylic acid) were prepared under an ionic liquid medium, demonstrating that ionothermal synthesis may act as a powerful tool in the preparation of low-coordinated coordination polymers (LCCPs). Investigation on the synthetic conditions shows that a lower reaction temperature, appropriate ratio of ligand to metal ions, and hydrophobic ionic liquid are of key importance for the formation of LCCPs. Both of them feature interesting hydration–dehydration properties with reversible color changing from deep purple to pink. Solid state UV-Vis spectra measurements also indicate that the largest adsorbtion peaks are blue-shifted under hydration. Significantly, under hydration and dehydration processes, compound 2 exhibits a magnetic transformation from anti-ferromagnetism to ferrimagnetism. Essentially, this transformation arises from the formation of new propagating ways triggered by guest water.

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Introduction

Coordination polymers, because of their structural diversity¹ and controllability, are regarded as promising materials applied in molecular adsorption² and separation,³ ion exchange,⁴ catalysis,⁵ and sensor.⁶ Owing to these materials often suffering from low structural and thermal stability, their application is greatly limited. Although using metal clusters as nodes to construct coordination polymers could effectively enhance their structural and thermal stability, synthesis of low-coordinated coordination polymers is no doubt the best approach to the application of the materials, particularly in the field of catalysis, molecular recognition, molecular adsorption and selective separation and molecular sensors.⁷ Owing to LCCPs often possessing thermodynamic instability or high chemical activity, however, it is great challenge to prepare low-coordinated coordination polymers. As far as our knowledge, LCCPs are mainly obtained through two approaches: (1) using larger steric hindrance ligands to synthesize LCCPs, and (2) removing coordinated solvent molecules from coordination polymers to synthesize LCCPs. In the first case, the larger steric hindrance ligands would restrict catalysis of the Lewis-acid metal sites, while in the second case, removing the coordinated terminal molecules from the coordination polymers often results in the collapse of their frameworks.⁸

Ionothermal methodology, as a new and upcoming synthetic route, has captured increased interests in the field of crystal engineering and material science due to the specific properties of ionic liquids (ILs).⁹ By utilizing ILs as both solvents and structure-directing agents,¹⁰ ionothermal synthesis not only provides quite different networks from those obtained by hydrothermal and solvothermal synthesis,¹¹ but also effectively avoids the coordination of the water molecules or other small organic molecules that are often encountered in hydrothermal/solvothermal synthesis, and thereby pave a way to the synthesis of LCCPs. To our surprise, only one LCCPs was synthesized on the basis of ionothermal method so far.¹²

Here we report syntheses and crystal structures of two LCCPs, namely, [Co(nip)₂][EMIm]₂ (1) and [Co(bptc)][EMIm]₂·H₂O (2). Single-crystal analysis results reveal that 1 and 2 respectively exhibit 1D chain and 2D layer structure, and all the central metal ions in 1 and 2 are located in tetrahedral geometry. Investigation on the synthetic condition indicates that both the reactant ratio and reaction temperature play a key important role in the synthesis of the LCCPs.

Experimental

Materials and physical measurements

H₂nip and H₄bptc ligands are obtained commercially without further purification. The C, H, and N microanalyses were carried out with a CE instruments EA 1110 elemental analyzer. The FT-IR spectra were recorded in the range of 4000–400 cm⁻¹ with a Nicolet AVATAR FT-IR360 spectrometer. The X-ray powder diffractometry (XRPD) study was performed on Panalytical X-Pert pro diffractometer with Cu-Kα radiation. TGA curves were prepared on a SDT Q600 Thermal Analyzer. DSC were conducted on a NETZSCH DSC 200Fs. Magnetic measurements were performed by a Quantum Design MPMS superconducting quantum interference device (SQUID). Solid-state ultraviolet spectra were collected by Varian Cary 5000 with transmission and diffuse reflection modes. All hydration experiments were performed under the ambient humidity of 85% and temperature of 22–25 °C.

X-ray crystallography

Data collections were performed on an Oxford Gemini S Ultra CCD area detector at 173 K for 1 and 2 and on a Rigaku R-AXI RAPID IP diffractometer at 173 K for 3, 298 K for 4. Absorption corrections were applied by using the analytical program Tompaanalytical for 3, 4 and multiscan program. CrysAlis Red for 1 and 2. The structures were solved by direct methods, and non-hydrogen atoms were refined anisotropically by least-squares on F² using the SHELXTL program.¹³ The hydrogen atoms of organic ligands were generated geometrically (C–H, 0.96 Å; N–H, 0.90 Å). Crystal data as well as details of refinement for the compounds are summarized in Table 1. CCDC number of 1470024 to 1470027 for 1 to 4.

Table 1 Crystal data and details of data collection and refinement for compound 1–4

Compounds	1	2	3	4
Formula	C28H28N6O12Co	C28H30N4O9Co	C36H44N6O22Co3	C14H15N3O6
Mr	699.49	625.49	1089.56	320.28
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P2(1)/c	P21/c	P21/c	P2(1)/c
a/Å	14.5649(3)	10.1130(7)	7.9839(16)	7.1529(14)
b/Å	14.1256(4)	26.6767(9)	20.911(4)	15.431(3)
c/Å	15.1659(5)	13.5080(9)	15.560(5)	14.163(4)
α/deg	90	90	90	90
β/deg	103.328(3)	126.875(4)	119.16(2)	107.41(3)
γ/deg	90	90	90	90
V/Å^3	3036.16(15)	2915.2(3)	2268.5(10)	1491.6(6)
Z	4	4	2	4
Dc/g cm^−3	1.530	1.425	1.595	1.431
μ/mm^−1	0.640	0.648	1.173	0.114
Data/params	5949/424	5696/370	4449/303	2605/225
θ/deg	2.64–26.00	2.94–26.00	3.00–25.99	2.98–25.00
Obs reflns	4112	2947	3996	1402
Goof on F^2	0.988	0.936	1.061	1.086
R1[I > 2σ(I)]	0.0507	0.0844	0.0360	0.0949
wR2 (all data)	0.1402	0.2462	0.1092	0.2659

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Synthesis

Compound 1 was synthesized as follows: Co(OOCCH₃)₂·4H₂O (0.249 g) and 5-nitryl-isophthalic acid (0.160 g) in 1-ethyl-3-methylimidazolium bromide (EMIm-Br; 1.214 g) were mixed in a Teflon-lined Parr at 120 °C for about a week and then cooled to room temperature at the rate of 3 °C h⁻¹. The Modena crystals of 1 were obtained with 46.2% yield (based on H₂nip). Anal. calcd (found) for 1, C₂₈H₂₈N₆O₁₂Co (%): C, 48.08 (47.11); H, 4.03 (4.15); N, 12.01 (11.72). IR (KBr, cm⁻¹): 3427.69s, 1628.50s, 1526.16s, 1371.02s, 1345.06s, 1573.52m, 1452.20m, 1172.48m, 788.75m, 737.13m, 711.36m, 3096.98w, 1430.50w, 1195.71w, 1073.98w, 933.46w, 622.93w, 582.95w, 476.67w.

Compound 2 was synthesized as follows: Co(OOCCH₃)₂·4H₂O (0.249 g) and 2,2′,4,4′-biphenyltetracarboxylic sodium (0.209 g) in 1-ethyl-3-methylimidazolium bromide (EMIm-Br; 1.235 g) were mixed in a Teflon-lined Parr at 120 °C for about a week and then cooled to room temperature at the rate of 3 °C h⁻¹. The Modena crystals of 2 were obtained with 38.7% yield (based on Na₄bptc). Anal. calcd (found) for 2, C₂₈H₃₀N₄O₉Co (%): C, 53.72 (53.04); H, 4.80 (4.97); N, 8.95 (8.69). IR (KBr, cm⁻¹): 3428.72s, 1600.87s, 1360.44s, 1169.78s, 721.39s, 3147.93m, 3105.64m, 1470.49m, 785.76m, 623.42m, 444.17m, 1127.86w, 1005.89w, 912.20w, 828.17w.

Compound 3 was synthesized as follows: Co(OOCCH₃)₂·4H₂O (0.374 g) and 5-nitryl-isophthalic acid (0.160 g) in 1-ethyl-3-methylimidazolium bromide (EMIm-Br; 1.214 g) were mixed in a Teflon-lined Parr at 120 °C for about a week and then cooled to room temperature at the rate of 3 °C h⁻¹. The crystals of 3 were obtained in about 46% yield (based on Co(OOCCH₃)₂). Anal. calcd (found) for 3, C₃₆H₄₄N₆O₂₂Co₃ (%): C, 39.68 (40.04); H, 4.07 (3.98); N, 7.71 (7.82). IR (KBr, cm⁻¹): 1629.28s, 1565.21s, 1376.02s, 1346.92s, 3427.04m, 1535.32m, 1457.43m, 1169.95m, 735.92m, 3158.70w, 1075.10w, 787.08w, 722.22w, 684.43w, 622.27w.

Compound 4 was synthesized as follows: Co(OOCCH₃)₂·4H₂O (0.187 g) and 5-nitryl-isophthalic acid (0.211 g) in 1-ethyl-3-methylimidazolium bromide (EMIm-Br; 1.214 g) were mixed in a Teflon-lined Parr at 120 °C for about a week and then cooled to room temperature at the rate of 3 °C h⁻¹. The crystals of 4 were obtained in about 30.1% yield (based on 5-nitryl-isophthalic acid). Anal. calcd (found) for 4, C₁₄H₁₄N₃O₆ (%): C, 52.45 (51.94); H, 4.37 (4.08); N, 13.11 (12.79). IR (KBr, cm⁻¹): 3116.54w, 1625.95w, 1536.43w, 1384.65s, 1345.73w, 1182.27w, 1049.95w, 789.19w, 723.03w, 703.57w, 680.22w.

Result and discussion

Crystal structure analysis reveals that 1 crystallizes in the monoclinic space group P2₁/c, and the asymmetric unit consists of one Co(II) ion, two deprotonated nip ligands, and two EMIm ions. Each the Co(II) ion in 1 is coordinated with four monodentate carboxylates respectively from four nip ligand in a distorted tetrahedron geometry. The Co–O distances ranges from 1.940(1) to 1.949(3) Å, within the expected range reported for the Co(II) compound reported previously.¹⁴ Each independent Co(II) center bridging with adjacent ones through a pair of nip ligands generates one dimensional anionic chain along a axis as shown in Fig. 1. The EMIm ions through electrostatic force locate around the chains.

Fig. 1 1D chain structure of 1 with all the Co(II) centers in tetrahedral geometry. The EMIm cations locate around the 1D chain. Hydrogen atoms are omitted for clarity.

Single-crystal X-ray structural analysis reveals that 2 crystallizes in space group P2₁/c. There are one Co(II) ion, one deprotonated bptc ligand, two EMIm ions and one water molecule in the asymmetric unit of 2. Notably, the center Co(II) ion also adopts distorted tetrahedron geometry formed by four oxygen atoms respectively from three bptc ligands. The bond lengths of Co–O are in the range from 1.975(5) Å to 1.980(5) Å, slightly longer than those in 1. Each bptc ligand serves as a three-connected node to coordinate with three neighboring Co(II) ions through its four carboxylates in monodentate mode, generating a two-dimensional anionic network as shown in Fig. 2. The EMIm cations acting as guests and encounter ions, locates between the 2D layers. Here, it is mentioned that Co(II) ions in the H₄bptc-based compound obtained by hydrothermal condition exhibit octahedral coordination geometry.¹⁵

Fig. 2 2D anionic network of 2 with all the Co(II) centers in tetrahedral geometry. The EMIm cations locate between the 2D layers. Hydrogen atoms are omitted for clarity.

When the ratio of ligand to metal ion is lower than 0.75, compound 3 was obtained. X-ray structure analysis of a single crystal of 3 reveals that it is characterized by a monoclinic space group, P2(1)/c. The asymmetric unit consists of one Co(II) ion, one nip²⁻ ligand, one acetate and one EMIm ion. Interestingly, the Co(II) ion has a distorted octahedron coordination geometry, featuring contributions from four carboxyl oxygens from acetates and two carboxyl oxygens from nip²⁻ ligands. The corresponding bond length of Co–O are 2.0151(18)–2.278(2), slightly longer than those in 1 and 2. Each Co(II) ion connects with two adjacent ones by four acetates and forms a [Co₃(CH₃COO)₃] unit, which further generates two-dimensional anionic network by the bridge of nip²⁻ ligand, as shown in Fig. 3. Similarly, EMIm⁺ cations locates between the 2D layers.

Fig. 3 The 2D structure of compound 3.

Once the ratio of ligand to metal ion is higher than 1, an organic supramolecular 4 was obtained, as shown in Fig. 4. Each asymmetric unit include one EMIm⁺ ion and one Hnip⁻ ion. Insepcting the structure of 4, it is observed that Hnip⁻ and EMIm⁺ ions form one ion unit and further generate supramolecular by weak interaction as shown Fig. 4.

Fig. 4 The structure of compound 4.

Performing the reactions under different synthetic conditions indicates that the reactant ratio and reaction temperature significantly influence on the synthesis of LCCPs. For examples, 1 could be obtained when the ratio (ligand/metal) is in the range of 0.75 to 1. However, a six-coordinated Co(II)-based compound 3 (see PXRD of Fig. S1a†) or an organic supramolecular compound 4 as the impurity was respectively obtained when the ratio of ligand to metal ion is lower than 0.75 or higher than 1 (see PXRD of Fig. S1b†). In addition to reactant ratio, reaction temperature also plays a key contribution to the formation of LCCPs. For examples, when the reaction temperature higher than 150 °C, five-coordinated Co(II)-based compound was formed.¹⁶ Although polarity of ILs is not investigated in this work, based on Morris and his co-worker,¹² the hydrophilic ionic liquid favors to form high-coordinated compounds, while the reduction of solvent polarity favors to form LCCPs. These facts indicate that lower reaction temperature, appropriate ratio of ligand to metal ion and hydrophobic ionic liquid are of key importance for the formation of LCCPs.

Interestingly, 1 exhibits unique rehydrated and dehydrated property. As shown in Fig. 5a–c, 1 rapidly absorbs water molecules, yielding pink compounds (labeled as 1b). This pink compound 1b turns back to 1 upon heating at 65 °C for three minutes. X-ray powder diffractions study indicates that although 1b cannot remain crystallinity, it when heated at 65 °C for three minutes, 1b turns back to the original deep purple and restores its single crystalline upon removal of water molecules (Fig. 5d), demonstrating that the hydration and dehydration processes are reversible. It is mentioned that the two peaks at 2θ = 6 and 10° are not found in experimental XRD, which may be result from disorder behaviour of guest ions in the channel. Compound 2 displays the similar hydration and dehydration process to that of 1 (see Fig. S2†). However, after turning back to original deep pruple, dehydrated compound 2 (labeled as 2b) cannot keep crystalline (see Fig. S3†).

Fig. 5 (a–c) Photograph representation of the hydration and dehydration processes for 1. The deep purple crystals turn to pink upon hydration and changes back to the original color upon dehydration; (d) PXRD patterns of 1 on the process of the hydration and dehydration, together with the corresponding simulation according to single-crystal structural determinations.

The thermogravimetric analysis (TGA) is performed in the air atmosphere at the rate of 4 °C min⁻¹ for hydrated compound 1. As shown in Fig. S4,† the initial weight loss between room temperature and 62 °C for 1b is about 28.3%. Based on the residual weight of 8.26% for Co₃O₄ in 1b, the initial weight loss of 28.3% corresponds to the removal of 15 mol water molecules (calcd 27.8%) for per mole of 1b, significantly higher than that of 6 water molecules for commercial desiccant, [Co(H₂O)₆]Cl₂. More significantly, its dehydrated temperature (62 °C) is significantly lower than that of [Co(H₂O)₆]Cl₂. Owing to 1b remaining stable before 250 °C, it is reasonable to deduce that 1 is a promising desiccant. Similarly, the residual weight of 8.02% for Co₃O₄ in 2b suggests that one mole 2b contains 21 mol water molecules, consistent with the initial weight loss of 36.2% (calcd 37.8%). It is also observed that at 85 °C the weight loss of 36.2% for 2b reveals the removal of all adsorptive water. Corresponding critical temperature is also higher than that of 1b. Moreover, DSC tests of 1b and 2b were conducted in the temperature range of 0–150 °C. As shown in Fig. S5,† it is found that the first phase transition appears at 73 °C for 1b and 113 °C for 2b, which is slight larger than dehydration temperature obtained by TGA. This difference between them may be derived from system error.

The temperature dependence of the magnetic susceptibility of compounds 1, 1b, 2 and 2b were measured from 2.0 to 300 K with an applied magnetic field of 1000 Oe, as shown in Fig. 6. Before hydratation, the χ_MT values of 1 and 2 gradually decrease with the temperature decreasing and reach a minimum (1.67 for 1, 1.59 cm³ mol⁻¹ K for 2), suggesting that both of them exhibit weak antiferromagnetic properties. After hydratation, different changes regarding the plots of χ_MT vs. T appear in 1b and 2b. As the temperature decreasing, the χ_MT values of 1b still keep similar degressive trend with 1, though the plot becomes much steeper than that of 1. However, 2b displays different variation from that of 2. As the temperature decreasing, the χ_MT value of 2b gradually decreases with a minimum of 2.00 cm³ mol⁻¹ K at 14 K, and then increase abruptly until 2 K, indicating 2b features ferrimagnetic behaviour. Generally, it is obtained that after hydratation, 1 still keeps weak antiferromagnetic property, while 2 changes from antiferromagnetic property to ferrimagnetic behaviour. This differences may be explained that after hydratation, some new propagation ways in 2b are generated by aqua ligands, while water molecules of 1b only adsorb on Co(II) ions but never serving as bridges connects with adjacent Co(II) ions.

Fig. 6 Plots of χ_MT vs. T from 2 to 300 K for compounds 1, 1b, 2 and 2b.

Furthermore, solid state UV-Vis spectra measurements before and after hydratation were conducted. As shown in Fig. S6† the largest absorption peaks of 1, 1b, 2 and 2b are 587, 577, 583 and 530 nm, respectively. It is found that the blueshift phenomenons of absorption peaks occur in 1b and 2b, respectively. However, the variation of blueshift in 2 is much larger than that of 1. This difference may be due to the fact that in 2b water molecules coordinate effectively to Co(II) and thereby induces the crystal field of Co(II) ions to change from tetrahedron to octahedron, while water molecules in 1b only interact with Co(II) ions by weak interaction (such as hydrogen bond) but never essentially modify coordination geometry of Co(II) ions. It is worth noting that this result supports previous result of magnetic measurement.

Conclusions

In summary, two LCCPs were prepared under ionothermal condition, demonstrating that ionothermal synthesis may act as a powerful tool in the preparation of LCCPs due to it effectively avoiding the coordination of the water molecules or other small organic molecules to the metal centers in the formation of the coordination polymers. Investigation on the synthetic conditions shows that lower reaction temperature, appropriate ratio of ligand to metal ion and hydrophobic ionic liquid are of key importance for the formation of LCCPs. Significantly, the LCCPs exhibit unique hydration–dehydration behavior in comparison with desiccant [Co(H₂O)₆]Cl₂. Moreover, compound 2 also indicates interestingly magnetic transformation from anti-ferromagnetism to ferrimagnetism.

Acknowledgements

We thank National Natural Science Foundation of China (No. 21401166 and 21101137), Zhejiang Provincial Natural Science Foundation of China (No. LY15B010005) and startup fund (No: 101009729 and G2817101107) of Zhejiang University of Technology.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Selective bond length and angle, photograph representation of the hydration and dehydration processes for 2; solid UV-Vis spectra, TG, and PXRD curve of 1, 1b, 2 and 2b. CCDC 1470024–1470027. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra14268k

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