THE CONSTANT PHASE ELEMENT (CPE) ON MnO₂ ELECTRODE

1. INTRODUCTION

Recently, the design and assembly of Zn(Ⅱ) coordination polymers (CPs) have received remarkable attention because of their intriguing topological structures and potential applications in many fields^[1-4]. In order to generate novel CPs, the important step is to choose suitable organic ligands. As we know, bidentate μ₂-N, N΄ ligands bearing an appropriate auxiliary group, such as 4, 4΄-bipyridine, 1, 2-bis(4-pyridyl)-ethylene, 1, 3-bis(4-pyridyl)propane, 4-bis(2-methylimidazolyl)butane and related species, have been widely used as bridging ligands with carboxylic acids in crystal engineering^[5-9]. However, the presently known cases of CPs with carboxylic acids and bis(imidazol-1-yl)methane (bimm) or N-(4-pyridylmethyl)imidazole (pyim) organic ligand are still very rare^[10-13]. In the crystal self-assembly of CPs, these μ₂-bridging bridging ligands are expected to construct multi-dimensional compounds. Among them, the flexible bimm ligand with an alkyl spacer between two imidazole unities may coordinate with central metal ions via two imidazole-type nitrogen atoms (Scheme 1). Compared with bimm ligand, the rigid pyim ligand may coordinate with central metal ions using pyridine-type and imidazole-type nitrogen atoms, yielding intriguing structures.

Scheme1

Scheme1.  Schematic drawing of the ligands used in this work

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On the other hand, 5-nitro-1, 2, 3-benzenetricarboxylic acid (H₃nbta) containing rich coordination sites has been proved to be an excellent organic ligand, which can exhibit versatile coordination modes in the assembly of CPs^[14-16]. In this study, we have successfully prepared two auxiliary N-donor ligand-mediated Zn(Ⅱ)-containing CPs incorporating H₃nbta, {[Zn(Hnbta)(bimm)](3H₂O)}_n (1) with a 1D chain structure and {[Zn_1.5(nbta)(pyim)(H₂O)](2H₂O)}_n (2) with a 2D layer structure. Their single-crystal structures, spectral properties and thermal stabilities were investigated. Moreover, luminescence properties of compounds 1 and 2 have been studied and discussed.

2. EXPERIMENTAL

All analytical grade chemicals and solvents were purchased and used as received without further purification. The IR spectra were recorded as KBr pellets on a Nicolet Avatar-360 spectrometer in the range of 4000~400 cm⁻¹. Elemental analyses for C, H and N were carried out on a Flash 2000 elemental analyzer. Thermogravimetric analyses (TGA) were carried out on a SDTQ600 thermogravimetric analyzer. A platinum pan was used for heating the sample at a heating rate of 10 ℃/min under air atmosphere. Fluorescence measurements were recorded with a Hitachi F4500 fluorescence spectrophotometer.

2.1 Synthesis of {[Zn(Hnbta)(bimm)](3H₂O)}_n (1)

A mixture of H₃nbta (25.5 mg, 0.1 mmol), Zn(OAc)₂·2H₂O (22.0 mg, 0.1 mmol), bimm (0.029 g, 0.2 mmol), and 6 mL deionized water was sealed in a 25 mL Teflon-lined autoclave and was kept under autogenous pressure at 140 ℃ for 4 days, followed by cooling to room temperature at a rate of 5 ℃·h^-1. Block colourless crystals were collected (yield: 35% based on Zn). Elemental analysis calculated for C₁₆H₁₇N₅O₁₁Zn (%): C, 36.90; H, 3.29; N, 13.45. Found (%): C, 36.97; H, 3.25; N, 13.31. Selected IR peaks (cm^-1): 3106 (m), 1597 (m), 1560 (s), 1523 (s), 1445 (s), 1340 (s), 1276 (s), 1229 (s), 1085 (m), 1023 (m), 945 (m), 828 (m), 764 (m), 739 (m), 705 (m), 649 (m).

2.2 Synthesis of {[Zn_1.5(nbta)(pyim)(H₂O)](2H₂O)}_n (2)

An identical procedure with 2 was followed to prepare 1 except that bis(imidazol-1-yl)methane was replaced by N-(4-pyridylmethyl)imidazole (14.61 mg, 0.1 mmol). Block yellow crystals were collected (yield: 56% based on Zn). Elemental analysis calculated for C₁₇H₁₅N₄O₁₁Zn_1.5 (%): C, 37.13; H, 2.73; N, 10.19. Found (%): C, 37.16; H, 2.76; N, 10.21. Selected IR peaks (cm⁻¹): 3135 (w), 1635 (m), 1612 (m), 1568 (s), 1553 (w), 1514 (s), 1428 (s), 1347 (s), 1316 (s), 1232 (m), 1124 (m), 1062 (s), 1022 (m), 974 (m), 949 (m), 922 (m), 837 (m), 750 (m), 729 (m), 709 (m), 705 (m).

2.3 X-ray structure determination

The structures of 1 and 2 were determined by single-crystal X-ray diffraction technique. Diffraction data were collected on an Oxford Diffraction Gemini with MoKα radiation (λ = 0.71073 Å) at 293 K. The structures were solved by direct methods using the Olex2 program as an interface together with the SHELXT and SHELXL programs, in order to solve and refine the structure respectively^[17-19]. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms on water molecules were located from difference Fourier maps and were refined using a riding model. Other hydrogen atoms were placed at the calculation positions. In compounds 1 and 2, the diffused electron densities resulting from highly disordered water molecules were removed using the SQUEEZE option in PLATON. The final chemical formulas of compounds 1 and 2 were estimated from the SQUEEZE result combined with the TGA result. For compound 1: triclinic system, space group P$\overline 1$ with a = 9.3712(9), b = 9.4045(9), c = 12.9642(13) Å, α = 69.757(9)°, β = 72.145(9)°, γ = 86.916(8)º, V = 1018.60(2) ų, D_c = 1.580 g/cm³, M_r = 520.68, F(000) = 492, μ = 1.026 mm^-1, the final R = 0.0469 and wR = 0.0961, S = 1.026; for 2: monoclinic system, space group P2/n, with a = 10.2183(3), b = 13.2364(5), c = 15.0772(5) Å, β = 90.649(3)°, V = 2039.11(1) ų, D_c = 1.672 g/m³, M_r = 513.35, F(000) = 1032, μ = 1.837 mm^-1, the final R = 0.0302 and wR = 0.0820, S = 1.092. Selected bond lengths and bond angles are listed in Tables 1 and 2, respectively.

Table 1

Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for 1

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Bond	Dist.		Bond	Dist.		Bond	Dist.
Zn(1)–O(2)	1.953(2)		Zn(1)–O(4)#1	1.979(2)		Zn(1)–N(2)	2.000(3)
Zn(1)–N(5)#2	2.008(3)
Angle	(°)		Angle	(°)		Angle	(°)
O(2)−Zn(1)−O(4)#1	105.19(9)		N(2)−Zn(1)−O(2)	120.04(1)		O(2)−Zn(1)−N(5)#2	113.42(1)
N(2)−Zn(1)−O(4)#1	96.43(1)		N(5)#2−Zn(1)−O(4)#1	115.33(1)		N(2)−Zn(1)−N(5)#2	105.64(1)
Symmetry transformation: #1: 2 − x, 1 − y, –z; #2: 1 − x, 2 − y, –z

Table 2

Table 2.  Selected Bond Lengths (Å) and Bond Angles (°) for 2

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Bond	Dist.		Bond	Dist.		Bond	Dist.
Zn(1)−N(1)	1.991(2)		Zn(1)−O(1)	1.9415(2)		Zn(1)−O(6)#1	1.9899(2)
Zn(1)−O(9)	2.025(2)		Zn(2)−O(3)	2.0770(2)		Zn(2)−O(5)	2.1222(2)
Zn(2)−N(3)#3	2.156(2)
Angle	(°)		Angle	(°)		Angle	(°)
O(1)−Zn(1)−O(6)#1	105.51(8)		O(1)−Zn(1)−O(9)	100.49(9)		O(1)−Zn(1)−N(1)	123.78(9)
O(6)#1−Zn(1)−O(9)	117.89(8)		O(6)#1−Zn(1)−N(1)	104.47(9)		N(1)−Zn(1)−O(9)	105.74(1)
O(3)−Zn(2)−O(3)#2	168.91		O(3)−Zn(2)−O(5)	86.07(7)		O(3)−Zn(2)−O(5)#2	86.96(7)
O(3)−Zn(2)−N(3)#3	88.23(8)		O(3)#2−Zn(2)−N(3)#3	100.02(8)		O(3)−Zn(2)−N(3)#4	100.03(8)
O(5)#2−Zn(2)−O(5)	102.01(1)		O(5)−Zn(2)−N(3)#3	168.84(8)		O(5)−Zn(2)−N(3)#4	87.24(8)
N(3)#3−Zn(2)−N(3)#4	84.31(1)
Symmetry transformation: #1: x −1, y, z; #2: 1.5 − x, y, 1.5 −z; #3: x − 1, 2 − y, 1 − z; #4: x + 0.5, 2 − y, 0.5 + z

3. RESULTS AND DISCUSSION

Compounds 1 and 2 were obtained as block colourless crystalline materials via the reaction of H₃nbta and zinc acetate with auxiliary N-donor ligand in aqueous medium at 140 ℃ for 4 days, respectively. Compound 1 crystallizes in the triclinic P$\overline 1$ space group and exhibits a 1D chain. In the asymmetric unit, there are one independent Zn²⁺ ion, one protonated Hnbta²⁻ anion, one neutral bimm ligand and one free water molecule (Fig. 1a). The Zn(1) atom is located in a distorted tetrahedral geometry, completed by two O atoms from two different Hnbta²⁻ anions and two N atoms from two different bimm ligands. The maximum and minimum bond angles for Zn²⁺ ion are 120.04(1) and 96.43(1)°, respectively, with an average value of 109.34(2)°, which slightly deviates from the angle of 109.34° in a perfect tetrahedron. The bond lengths vary from 1.953(2) to 2.008(3) Å, which are well-matched to those observed in similar compounds^{[4, 16]}. In compound 1, a pair of oppositely arranged Hnbta²⁻ anions bind two central Zn²⁺ ions to generate a [Zn₂(Hnbta)₂] ring subunit, in which Hnbta²⁻ anion exhibits the μ₂-(μ₁-η¹: η⁰/μ₁-η¹: η⁰/μ₀-η⁰: η⁰) coordination mode. These subunits are connected by μ₂-bimm ligands to form a resultant 1D chain structure (Fig. 1b). Noted, the flexible μ₂-bimm ligand shows a relative rotation, in which the dihedral angle between two imidazole rings is 89.054° (Fig. 1c).

Figure 1

Figure 1.  (a) Coordination environment of the Zn(Ⅱ) left. Displacement ellipsoids are drawn at the 30% probability level. Hydrogen atoms are omitted for clarity. Symmetry codes: (#1): 2 − x, 1 − y, –z; (#2): 1 − x, 2 − y, –z. (b) A [Zn₂(Hnbta)₂] ring subunit. (c) 1D chain structure of compound 1

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Compound 2 crystallizes in a monoclinic space group P2/n and features a 2D layered structure. There exist one and a half crystallographically independent Zn²⁺ ions, one nbta³⁻ anion, one pyim ligand, one coordinated water molecule and two lattice water molecules in the asymmetric unit. As shown in Fig. 2a, the two Zn²⁺ ions exhibit two different coordination manners. The Zn(1) atom is also located in a distorted tetrahedral geometry, completed by two O atoms from two different nbta³⁻ anions and one imidazole N atom from one pyim ligand. The maximum and minimum bond angles for Zn²⁺ ion are 123.78(9)° and 104.47(9)°, respectively, with an average value of 109.65 °, which also slightly deviates from the angle of 109.47° in a perfect tetrahedron. The central Zn(2) atom resides at a crystallographic inversion left and assumes an octahedral coordination environment with four carboxyl atoms from three nbta^3- anions, two pyridine nitrogen atoms of two pyim ligands and one coordinated water molecule. The bond lengths of Zn–N are 1.991(2) and 2.156(2) Å, while the Zn–O bond lengths range from 1.9415(19) to 2.1221(18) Å, which fall in the normal range^{[4, 16]}. In compound 2, the nbta³⁻ anion adopts the μ₃-(μ₁-η¹: η⁰/μ₁-η¹: η⁰/μ₂-η¹: η¹) coordination mode and acts as a tridentate bridging ligand, extending the structure into an infinite 1D structure. Moreover, the 1D structure is further linked by μ₂-pyim ligand into a 2D architecture, in which the rigid μ₂-pyim ligand shows a slight rotation, and the dihedral angle between the imidazole and pyridine rings is 23.609°. Topologically, if the nbta³⁻ anion and Zn(1) atoms are simplified as three-connected nodes, the Zn(2) atoms could be considered as four-connected nodes, with the pyim ligand serving as the linear linkers. As a result, the structure of compound 2 represents an unprecedented (3, 4)-connected network with the {6².8⁴}{6².8}₄ topology (Fig. 2d).

Figure 2

Figure 2.  (a) Coordination environment of the Zn(Ⅱ) lefts. Displacement ellipsoids are drawn at the 30% probability level. Hydrogen atoms are omitted for clarity. Symmetry codes: #1: x − 1, y, z; #2: 1.5 − x, y, 1.5 – z; #3: 0.5 + x, 2 – y, 0.5 + z; #4: 1 – x, 2 – y, 1 – z. (b) 1D double structure chain constructed from nbta³⁻ anions. (c) 2D architecture of compound 2. (d) Schematic representation of the 2D (3, 4)-connected network

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To confirm the phase purity of bulk samples, the X-ray powder diffraction pattern was recorded. As seen in Fig. 3, the peak positions of experimental and simulated patterns are in good agreement with each other, demonstrating the phase purity of 1 and 2. The dissimilarities in intensity may be owing to the preferred orientation of the samples. In addition, thermal behaviors of 1 and 2 were examined by thermal gravimetric analysis (TGA) in a dry air atmosphere from 30 to 700 ℃. As shown in Fig. 4, compound 1 undergoes two steps of weight loss, with the first one of 9.66% corresponding to the removal of water molecules in the temperature range of 97~107 ℃ (calcd. 9.59%). From then on, almost no weight loss is observed until 253 ℃, beyond which the intense weight loss is attributed to the decomposition and collapse of the structure. Compound 2 also undergoes two steps of weight loss. The weight loss of 9.79% from 97 to 116 ℃ corresponds to the release of water molecules (calcd. 9.83%) and that from 273 ºC results from the decomposition and collapse of the structure.

Figure 3

Figure 3.  TGA curves for 1 and 2

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Figure 4

Figure 4.  PXRD patterns of 1 and 2 simulated from X-ray single-crystal diffraction data and experimental data

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CPs with Zn lefts usually present photoluminescent properties with potential applications such as chemical sensors and photochemistry fields^{[20, 21]}. Here, the solid-state emission spectra of 1, 2 and free ligands were explored at room temperature. As shown in Fig. 5, compound 1 shows a main peak at 471 nm with two shoulders at 453, 440 nm upon excitation at 290 nm and compound 2 shows a main peak at 420 nm with two shoulders at 452 and 469 nm under 374 nm excitation. The free H₃nbta shows a main peak at 469 nm with a shoulder at 453 nm upon excitation at 250 nm and the free bimm and pyim ligands were observed with wavelengths at 344 and 350 nm, respectively. Considering the Zn²⁺ ion is difficultly oxidized or reduced, the peaks of compounds 1 and 2 should be attributed to the transitions of Hnbta²⁻/nbta³⁻ anions because similar peaks also appear for the free H₃nbta ligand. The peaks for 1 and 2 exhibit a blue-shift with respect to the free H₃nbta, which may be tentatively assigned to the intraligand charge transfer of Hnbta²⁻/nbta³⁻ anions and/or metal-ligand coordination interactions.

Figure 5

Figure 5.  Solid-state emission spectra of 1 and 2

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4. CONCLUSION

In summary, we have successfully synthesized and characterized two new CPs based on 5-nitro-1, 2, 3-benzenetricarboxylic acid and bimm/pyim N-donor auxiliary ligands. Compound 1 is a 1D chain structure and compound 2 features a 2D network with a 4-connected sql topology. The results indicate the bimm and pyim ligands may act as additional metal linkers to mediate the structures of CPs with 5-nitro-1, 2, 3-benzenetricarboxylic acid in crystal engineering. What is more, both compounds show photoluminescence and could be good candidates for potential luminescence materials.