Solvent-dependent assemblies, structure diversities and magnetic properties of two homochiral Co(II)/Na(I) coordination polymers

Abstract

Solvent-induced synthesis of 2D and 3D homochiral Co(II)/Na(I) coordination polymers based on chiral tridentate ligand S-N-(1-hydroxymethylethyl)iminodiacetic acid (S-H₃L), formulated as [NaCo(S-L)(μ-H₂O)(H₂O)] (S-1) and [NaCo(S-L)(H₂O)] (S-2). Compound S-1 displays a 2D layered structure, i.e. further expanded into a 3D supramolecular architecture by hydrogen bond interactions, whereas S-2 features a 3D framework constructed by 1D Co(II) and Na(I) chains. In addition, magnetic study reveals the anti-ferromagnetic interaction in S-2.

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Introduction

The construction of chiral coordination polymers (Cps) has attracted significant attention not only owing to their aesthetic structures but also because of their potential applications as functional materials in the fields of nonlinear optics and ferroelectrics.¹ To date, considerable effort has been devoted to the design and synthesis of chiral Cps. However, to date, only a small number of them have been synthesized and structurally characterized.² This is because the chance of obtaining chiral Cps is very low and their preparation usually requires suitable homochiral ligands.³ Therefore, a careful selection of enantiomerically pure ligands with certain features is crucial for the synthesis of chiral Cps.

Tridentate N,O-containing ligands are a family of useful ligands in coordination chemistry due to their strong coordination ability and versatile coordination modes. To date, they have been widely used in the assembly of Cps, in particular those with mixed 3d–4f metals.⁴ For example, a large number of Cps with triethanolamine (H₃tea), nitrilotriacetic acid (H₃nta), N-(2-hydroxyethyl)iminodiacetic acid (H₃heidi) and N-(1-hydroxymethylethyl)iminodiacetic acid (H₃metheidi) have been synthesized.⁵ However, none of them are homochiral Cps because all these ligands are not enantiomerically pure. On the other hand, solvents are considered an important factor in controlling the assembly of Cps because they have effects on the control of kinetic or thermodynamic conformers, as well as the coordination modes of the ligands.⁶ For example, some solvent-dependent synthesis of Cps by changing the size, shape, component, polarity, ratio and coordination ability of the solvents has been documented.⁷ In some cases, the dimensionality of the framework from 2D to 3D can occur by varying the reaction solvents.⁷

In order to make homochiral Cps, an enantiomerically pure S-N-(1-hydroxymethylethyl)iminodiacetic acid (S-H₃L) was synthesized. By employing this ligand, two novel homochiral Co(II)/Na(I) Cps, namely [NaCo(S-L)(μ-H₂O)(H₂O)] (S-1) and [NaCo(S-L)(H₂O)] (S-2) were isolated from different solvents at room temperature. Herein, their syntheses, crystal structures and magnetic properties are discussed in detail.

Experimental

Materials and methods

All the chemicals were purchased commercially and used as received. The ligand S-H₃L was synthesized following the previous literature procedure.^5c Thermogravimetric experiments were performed using a TGA/NETZSCH STA-449C instrument heated from 30–800 °C (heating rate of 10 °C min⁻¹, nitrogen stream). Elemental analyses (C, H and N) were measured with an Elementar Vario EL III Analyzer. The powder X-ray diffraction (PXRD) patterns were recorded on crushed single crystals in the 2θ range 5°–50° using Cu-Kα radiation. The magnetic susceptibility data were collected on a Quantum Design MPMS (SQUID)-XL magnetometer.

Synthesis of [NaCo(S-L)(μ-H₂O)(H₂O)] (S-1)

A mixture of Co(NO₃)₂·6H₂O (145 mg, 0.5 mmol), NaOH (10 mg, 0.25 mmol) and S-H₃L (48 mg, 0.25 mmol) was dissolved in 20 mL of MeOH. The mixture was stirred at room temperature for another 3 h, and then the resulting red solution was filtered and the filtrate was left undisturbed at ambient temperature. After several days, red crystals of S-1 were obtained in 50% yield based on Co(NO₃)₂·6H₂O. Anal. calcd for C₇H₁₄CoNNaO₇ (S-1): C 27.47, H 4.61, N 4.58. Found: C 27.49, H 4.33, N 4.64.

Synthesis of [NaCo(S-L)(H₂O)] (S-2)

S-2 was prepared using method similar to that described for S-1 except that water was used instead of MeOH. Red crystals of S-2 were obtained in 58% yield based on Co(NO₃)₂·6H₂O. Anal. calcd for C₇H₁₁CoNNaO₆ (S-2): C 29.29, H 3.86, N 4.88. Found: C 29.12, H 4.01, N 4.83.

Crystal structure determination

Single-crystal X-ray diffraction data were collected on a Rigaku Diffractometer with a Mercury CCD area detector (Mo Kα; λ = 0.71073 Å) at room temperature. Empirical absorption corrections were applied to the data using the Crystal Clear program.⁸ The structure was solved by direct method and refined by the full-matrix least-squares on F² using the SHELXTL-97 program.⁹ Metal atoms were located from the E-maps and other non-hydrogen atoms were located in successive difference Fourier syntheses. All non-hydrogen atoms were refined anisotropically.

The organic hydrogen atoms were positioned geometrically, whereas those of the water molecules were located using the difference Fourier method and refined freely. Crystal data and other pertinent information for S-1 and S-2 are summarized in Table 1. Selected bond distances and angles of S-1 and S-2 are listed in Table S1.† Bond lengths and angles of hydrogen bonds of S-1 are listed in Table S2.†

Table 1 Crystal data and structure refinement of S-1 and S-2

Compounds	S-1	S-2
a R = Σ(‖Fo| − |Fc‖)/Σ|Fo|, wR = {Σw[(Fo^2 − Fc^2)^2]/Σw[(Fo^2)^2]}^1/2.
CCDC	1020718	1020719
Formula	C7H14CoNNaO7	C7H12CoNNaO6
Mr	306.11	288.10
Crystal system	Tetragonal	Tetragonal
Space group	P212121	P212121
a (Å)	7.008(5)	8.342(3)
b (Å)	12.151(7)	9.375(4)
c (Å)	13.928(1)	13.544(5)
α (deg)	90	90
β (deg)	90	90
γ (deg)	90	90
V (Å^3)	1186.02(1)	1059.11(7)
Z	4	4
Dc (g cm^−3)	1.714	1.807
M (mm^−1)	1.505	1.673
F(000)	628.0	588.0
GOF	1.097	1.059
R1^a	0.0310	0.0281
wR2^a	0.0853	0.0792
Flack parameter	0.0219	0.0130

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Results and discussion

Synthesis and description of crystal structures

Reactions of Co(NO₃)₂·6H₂O with NaOH and S-H₃L in the ratio of 2 : 1 : 1 in methanol or water led to two novel homochiral Co(II)/Na(I) Cps, formulated as [NaCo(S-L)(μ-H₂O)(H₂O)] (S-1) and [NaCo(S-L)(H₂O)] (S-2) (Scheme 1). They are obtained in moderate yields under the same conditions except that the solvents used are different. It has been well documented that solvents play an important role in tuning the structures of Cps. In the case of S-1 and S-2, the former obtained from methanol is a 2D layer, whereas the latter obtained from water is a 3D framework. It seems likely that water can enhance the ligand–metal interaction of S-H₃L, leading to an increase in framework connectivity.^7f

Scheme 1 The syntheses of S-1 and S-2.

Single crystal X-ray structural analysis reveals that S-1 and S-2 both crystallize in the chiral space group of P2₁2₁2₁ with the Flack parameters of 0.0219 and 0.0130, respectively (Table S1†). The asymmetric unit of S-1 contains one Co(II) ion, one Na(I) ion, one S-L³⁻ anion and two coordinated water molecules. Each Co(II) center is six-coordinated by one N atom from one S-L³⁻ ligand, three O atoms from the S-L³⁻ ligand and two O atoms from two coordinated water molecules. The Na(I) ion is three-coordinated by one μ₂-H₂O molecule and two carboxylate O atoms from two different S-L³⁻ ligands with the Na–O bond lengths varying from 2.659(3) to 2.828(4) Å (Fig. 1a). The S-L³⁻ ligand is fully deprotonated and adopts a (k¹-μ₂-k¹-k¹-k¹)-μ₃ coordination mode to connect two Co(II) and one Na(I) ion (Scheme 2A). The six-coordinated Co(II) ion adopts a CoNO5 configuration to generate a mononuclear Co(II) unit, which is expanded into a 1D chain via Na(I) and μ-H₂O bridges (Fig. 1b). These chains are further extended into a 2D layer network through the carboxylate groups of S-L³⁻ ligands and the Na(I) ions (Fig. 1c).

Fig. 1 (a) View of the coordination environment of Co(II) and Na(I) ions in S-1. Symmetry codes: A x − 1, y, z; B −x, y + 1/2, −z + 3/2; C −x, y − 1/2, −z + 3/2; D x + 1, y, z. (b) The 1D chain constructed by Co(II), Na(I) ions and S-L³⁻ ligands. (c) The 2D structure of S-1. (d) H-bonding interactions in the 3D supramolecular framework of S-1.

Scheme 2 The coordination modes of S-L³⁻ in S-1 (A) and S-2 (B).

In addition, the hydrogen bonding interactions between the O–H groups in coordinated water molecules and the oxygen atoms from uncoordinated carboxylate groups extend the 2D layers into a 3D supramolecular architecture (Fig. 1d).

The asymmetric unit of S-2 includes one Co(II) ion, one Na(I) ion, one S-L³⁻ anion and one coordinated water molecule. Each Co(II) center is six-coordinated by one N atom from one S-L³⁻ ligand, four O atoms from two different S-L³⁻ ligands and one O atom from one coordinated water molecule. The Na(I) ion is three-coordinated by three O atoms from three different S-L³⁻ ligands with the Na–O bond lengths varying from 2.625(4) to 2.901(4) Å (Fig. 2a). The S-L³⁻ ligand is fully deprotonated and adopts a (k¹-k¹-k¹-μ₂-μ₂)-μ₅ coordination mode to connect two Co(II) and three Na(I) ions through two carboxylate groups, one methoxy group and one N atom (Scheme 2B). Similar to S-1, the six-coordinated Co(II) ion in S-2 also adopts a CoNO5 configuration to generate a mononuclear Co(II) unit, which is expanded into a 1D chain by S-L³⁻ ligands (Fig. 2b). Each S-L³⁻ ligand bridges three Na(I) ions to form a 1D Na(I) chain (Fig. 2c). The combination of 1D Co(II)- and Na(I)-carboxylate chains gives rise to a more complicated 3D framework (Fig. 2d).

Fig. 2 (a) View of the coordination environment of Co(II) and Na(I) ions in S-2. Symmetry codes: A −x + 2, y − 1/2, −z + 1/2; B x − 1/2, −y + 1/2, −z; C x, y − 1, z; D −x + 2, y + 1/2, −z + 1/2; E x, y + 1, z; F x + 1/2, −y + 1/2, −z. (b) View of the 1D Co(II) chain; (c) the 1D Na(I) chain; (d) the 3D layer structure of S-2.

Solid-state circular dichroism spectra

To further confirm the homochiral nature of the products, solid-state circular dichroism (CD) spectra of S-1 and S-2 were measured in KCl pellets to observe the absolute configurations. As shown in Fig. S1,† compound S-1 exhibits a strong negative Cotton effect at 519 nm. The CD curve of S-2 is very similar to that of S-1, which has a wavelength similar to the negative Cotton effect.

X-ray diffraction and thermal stability analysis

The powder X-ray diffractions (PXRD) are performed to test the purity of S-1 and S-2 (Fig. S2†). The experimental results match well with the simulated XRD patterns, indicating the phase purity of the as-synthesized samples. Thermogravimetric analysis (TGA) measurements are conducted in the temperature range 30–800 °C under a flow of nitrogen at a heating rate of 10 °C min⁻¹ (Fig. S3†). The TGA curve of S-1 indicates that the first step from 40 to 130 °C with a weight loss of 11.99% is due to the loss of two coordinated water molecules (calcd 11.76%), and the framework starts to decompose at about 340 °C. The TGA curve of S-2 is very similar to that of S-1 with a weight loss of 6.23% from 40 to 150 °C, which is attributed to the loss of one coordinated water molecule (calcd 6.10%). Then, the framework collapses at about 340 °C.

Magnetic property

Variable-temperature magnetic susceptibility measurement is performed on the crystalline samples of S-1 and S-2 in the temperature range of 2–300 K at the field of 1000 Oe. The χ_mT vs. T plots of S-1 and S-2 exhibit a similar magnetic behavior (Fig. 3). At 300 K, the χ_mT values of S-1 and S-2 are 3.45 and 3.12 cm³ K mol⁻¹, respectively, which are greater than that expected for the spin-only value (1.88 cm³ K mol⁻¹ with S = 3/2 and g = 2.0).¹⁰ The magnetic susceptibility data for the plot of χ_m⁻¹ vs. T follow Curie–Weiss behavior (C = 3.48 cm³ mol⁻¹ and θ = −8.87 K for S-1 and C = 3.20 cm³ mol⁻¹ and θ = −10.35 K for S-2). Upon cooling, the value of χ_mT continuously decreases, reaching a minimum value of 1.83 cm³ K mol⁻¹ for S-1 and 1.72 cm³ K mol⁻¹ for S-2 at 2 K.

Fig. 3 Temperature dependence of χ_mT and χ_m⁻¹ (inset lines) values for S-1 (a) and S-2 (b).

For S-1, the nearest neighbor Co(II) atoms are separated by a long distance (–COO–Na–COO–), therefore a weak magnetic interaction between them can be expected. Hence, the gradual decrease of χ_mT upon lowering the temperature is mainly due to the zero-field splitting.

The magnetic data can be fitted according to an S = 3/2 spin state for Co(II) with a dominant zero-field splitting effect (ZFS) and a Zeeman interaction based on the spin Hamiltonian.¹¹ The best fits were obtained with g_∥ = 3.00, g_⊥ = 2.54 and |D| = 36.20 cm⁻¹. The value of |D| is comparable to that observed for similar mononuclear Co(II) complexes.¹¹

For S-2, the magnetic Co(II)-carboxylate chains (–COO–Co–COO–) are separated by Na(I) ions, thus the averaged curve of magnetic data can be fitted by the polynomial given in χ_m = Ng²β²/4κ_BT ((5 + 2.59277x + 4.00094x² − 0.23392x³)/(1 + 3.03193x + 5.58780x² + 7.92507x³)), 0.5 < κ_BT/|J| < 5.0, where χ_m = |J|/κ_BT.¹² The best fitting gives: g = 2.60, J = −2.14 cm⁻¹ and J/k_B = −3.08. Here, we do not attempt to fit the low temperature data as they are significantly perturbed by ZFS.¹²

Conclusions

In summary, two novel 2D and 3D homochiral Co(II)/Na(I) coordination polymers have been successfully synthesized and structurally characterized. S-1 displays a 2D layer structure, which is further extended into a 3D supramolecular architecture by hydrogen bonding interactions, while S-2 exhibits a 3D framework of hcb net. Magnetic susceptibility measurement reveals that S-2 exhibits antiferromagnetic interaction between the adjacent Co(II) ions. The successful preparation of S-1 and S-2 may provide a useful and promising approach to synthesize new homochiral coordination polymers.

Acknowledgements

We thank Dr X. Wu for his help in solid-state circular dichroism spectra measurements. This work was supported by the National Basic Research Program of China (973 Program, 2012CB821702), the National Natural Science Foundation of China (21233009 and 21173221) and the State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Selected bond lengths and angles, hydrogen bonds for S-1, TGA curves, CD spectra and XRD patterns. For ESI and crystallographic data in CIF or other electronic format. CCDC: 1020718 for S-1 and 1020719 for S-2. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra09263e

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