Anion-directed self-assembly of Cu(II) coordination compounds with tetrazole-1-acetic acid: syntheses in ionic liquids and crystal structures

Abstract

Reaction of tetrazole-1-acetic acid (1-Htza) with a Cu(II) salt in ionic liquids with different anions, [BMIM]X (BMIM = 1-butyl-3-methylimidazolium; X = Br⁻, BF₄⁻, NTf₂⁻ (NTf₂⁻ = bis((trifluoromethyl)sulfonyl)amide)), afforded five Cu(II) coordination compounds, [Cu₂(1-tza)₄]Br·H₃O·1/3H₂O (1), [Cu₂(1-tza)₄]BF₄·H₃O·H₂O (2), [Cu(μ₂-Cl)(1-tza)(1-Htza)(H₂O)]·0.5H₂O (3), [CuCl(μ₂-Cl)(1-Htza)₂(H₂O)]·H₂O (4), and [CuCl₂(1-Htza)₂]·H₂O (5). Single-crystal X-ray diffraction analyses reveal that 1–5 display various structures, and the 1-tza⁻ ligand exhibits diverse coordination modes. Compounds 1 and 2 possess higher dimensional structures (a 2-D neutral Kagomé topology network for 1 and a 3-D lvt-type topology framework for 2) with fully deprotonated 1-tza⁻ ligands. Compounds 3–5 display lower dimensional structures (1-D, 1-D and 0-D for 3, 4 and 5, respectively) with partly or fully protonated 1-Htza. The anions of ionic liquids have significant influences on the final molecular architectures, which arise from different water miscibility of ionic liquids.

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Introduction

The exploration of coordination compounds has attracted significant interest in recent decades, because of their fascinating structures¹ and potential applications in molecular magnetism,² photoluminescence,³ ferroelectric devices,⁴ ion exchange,⁵ gas storage,⁶ and catalysis.⁷ However, there are still challenges in employing traditional synthetic methods to obtain functional metal complexes with expected structures and properties. Therefore, it is quite necessary to develop new synthetic approaches in the pursuit of novel coordination compounds. Ionic liquids (ILs) can serve as reaction media instead of conventional water or organic solvents.⁸ Distinct from molecular solvents, ILs are composed entirely of ions with organic cations and inorganic (or organic) anions,⁹ and they usually possess many special properties, such as a low melting point, a wide liquid range, good thermal stabilities, suitable viscosity, high solubility for both polar and non-polar organic and inorganic substances, negligible vapour pressure and non-flammability,¹⁰ which make ILs ideal solvents widely used in the field of inorganic or inorganic–organic hybrid materials synthesis,^11,12 especially in the metal–organic frameworks (MOFs) synthesis.^13–15 In addition, ILs involved in MOFs synthesis act as charge compensating species, particularly potential templates and/or structural directing agents.^13–15 The cation and anion species of ILs can individually or cooperatively influence the resulting structures of coordination compounds.^14,15 As many examples demonstrate, IL cations are usually incorporated in the final molecular architectures, and they also have potential abilities to tune the structures of the final compounds by changing their sizes. For example, Young-Uk Kwon's group showed that varying the length of the alkyl group on the imidazolium cation has significant effects on the reaction products of Zn(NO₃)₂·6H₂O and H₃BTC.¹⁴ Meanwhile, IL anions also have the ability to direct the self-assembly of coordination compounds,¹⁵ which is different from the “templating effect” of IL cations. Moreover, the anions are not commonly contained in the final structures and their effect can be termed as the “induction effect”.^13a One example is that Morris' group has used an enantiopure anion as one component of the IL to induce homochirality in a nickel(II) complex constructed entirely using achiral building blocks, but the IL anions are not included in the product.^15a However, investigations on IL anions controlling the preparation of coordination compounds still remain less developed.

In this study, we employed [BMIM]X (BMIM = 1-butyl-3-methylimidazolium; X = Br⁻, BF₄⁻, NTf₂⁻ (NTf₂⁻ = bis((trifluorom-ethyl)sulfonyl)amide)), whose miscibility with water can be altered using different anions,¹⁶ as reaction solvents in designing Cu(II) coordination compounds. We selected bifunctional tetrazole-1-acetic acid (1-Htza) as an organic ligand, which simultaneously possesses carboxylate and tetrazolate groups. As is well known, carboxylate O atoms and tetrazolyl ring N atoms have good coordination capacities.¹⁷ Therefore, 1-Htza could exhibit versatile coordination modes (Scheme S1, ESI†) to construct transition metal,¹⁸ rare earth metal¹⁹ or heterometallic transition-rare earth metal complexes²⁰ with diverse structures from 0-D to 3-D and interesting physical performances in magnetism,^18a,b,20 photoluminescence^18d and ion exchange.^18c In this paper, five Cu-1-tza coordination compounds, [Cu₂(1-tza)₄]Br·H₃O·1/3H₂O (1), [Cu₂(1-tza)₄]BF₄·H₃O·H₂O (2), [Cu(μ₂-Cl)(1-tza)(1-Htza)(H₂O)]·0.5H₂O (3), [CuCl(μ₂-Cl)(1-Htza)₂(H₂O)]·H₂O (4), [CuCl₂(1-Htza)₂]·H₂O (5), have been successfully synthesized in [BMIM]⁺ ILs with varied anions (Scheme 1). Flexible coordination modes of 1-Htza have been found in 1–5 (Scheme 2). The effects of IL anions on the final structural assembly have been discussed. As far as we know, ILs have first been employed in the syntheses of tetrazolate–carboxylate complexes.^18–21

Scheme 1 The synthetic route of Cu-1-tza coordination compounds. Method I represents heating in air for 10 h and then standing in air for several weeks, while method II represents heating in a sealed container at the programmed temperature.

Scheme 2 Three coordination modes of the tetrazole-1-acetic acid ligand in compounds 1–5 with mode a observed in 1, b in 1, 2 and 3, c in 3, 4 and 5.

Experimental section

Materials and instruments

All chemicals were commercially available sources of analytical grade and used without further purification. The used ILs in this study were purchased from Centre for Green Chemistry and Catalysis, LICP, CAS with the grade of purity of 99%. In order to avoid absorbing moisture, the ILs were stored in a vacuum desiccator before use. The elemental analyses of C, H and N were carried out using a Vario EL III elemental analyzer. The FT-IR spectra were recorded in the 4000–400 cm⁻¹ range on a Perkin–Elmer Spectrum One Spectrometer using KBr pellets. Thermogravimetric analysis (TGA) experiments were done using a NETZSCH STA 449C Jupiter thermogravimetric analyzer in flowing nitrogen with the sample heated in an Al₂O₃ crucible at a heating rate of 5 K min⁻¹. All powder X-ray diffraction data were collected on a Rigaku Miniflex II diffractometer using Cu-Kα radiation (λ = 1.540598 Å) at 40 kV and 40 mA in the range 5.00° ≤ 2θ ≤ 65.00°.

Syntheses of 1–5

[Cu₂(1-tza)₄]Br·H₃O·1/3H₂O (1). A mixture of 1-Htza (1.5 mmol, 0.192 g), anhydrous CuCl₂ (0.5 mmol, 0.067 g) and [BMIM]Br (4.63 mmol, 1 g) in a glass beaker was heated at 100 °C in air for 10 hours and gave rise to a dark purple thick solution, the pH value of which is ca. 4, and then kept in air at room temperature for one week. Blue plate crystals of 1 suitable for X-ray analyses were obtained, washed with ethanol and dried in air. Yield: 32% (based on Cu) for 1. Anal. Calcd for C₃₆H₄₇Br₃Cu₆N₄₈O₂₈: C, 19.47; H, 2.13; N, 30.27%. Found: C, 19.72; H, 2.19; N, 30.47%. IR (KBr pellet, cm⁻¹): 3479 m br, 3120 m, 3070 m, 3006 m, 2965 m. 2876 w, 1673 s, 1574 w, 1511 m, 1422 s, 1375 s, 1294 s, 1175 s, 1099 s, 1010 m, 955 w, 885 w, 807 m, 705 s, 584 w, 522 w.

[Cu₂(1-tza)₄]BF₄·H₃O·H₂O (2). The procedure was the same as that for 1 except that [BMIM]Br was replaced by [BMIM]BF₄ (1 mL). The pH value of the reaction mixture is ca. 3. Blue prismatic crystals of 2 suitable for X-ray analyses were obtained, washed with acetone and dried in air. Yield: 40% (based on Cu) for 2. Anal. Calcd for C₁₂H₁₇BCu₂F₄N₁₆O₁₀: C, 18.94; H, 2.52; N, 29.44%. Found: C, 19.67; H, 2.78; N, 30.10%. IR (KBr pellet, cm⁻¹): 3411 m br, 3167 m, 3016 w, 2962 w, 1725 m, 1638 s, 1512 m, 1390 s, 1298 s, 1157 m, 1067 s, 966 w, 930 w, 800 m, 701 s, 589 w, 527 w.

[Cu(μ₂-Cl)(1-tza)(1-Htza)(H₂O)]·0.5H₂O (3) and [CuCl(μ₂-Cl)(1-Htza)₂(H₂O)]·H₂O (4). The procedure was the same as that for 1 with [BMIM]NTf₂ (1 mL) in place of [BMIM]Br. The pH value of the reaction mixture is ca. 3. Light blue prismatic crystals of 3 and blue block crystals of 4 suitable for X-ray analyses, which can be manually separated, were obtained. Compound 3 was washed with ethanol and dried in air, yield: 61% (based on Cu). Anal. Calcd for C₁₂H₂₀Cl₂Cu₂N₁₆O₁₁: C, 18.91; H, 2.64; N, 29.40%. Found: C, 19.62; H, 2.79; N, 29.15%. IR (KBr pellet, cm⁻¹): 3854 w, 3745 w, 3480 m br, 3118 m, 2995 w, 2948 w, 2379 w, 1731 m, 1650 s, 1508 w, 1391 s, 1305 w, 1186 m, 1098 m, 1024 m, 899 w, 799 w, 701 w. The yield of 4 is too low, so no other physical property test except single crystal X-ray diffraction and IR spectrum can be carried. IR of 4 (KBr pellet, cm⁻¹): 3422 m br, 3118 s, 2998 m, 2948 w, 2352 w, 1733 s, 1499 w, 1387 m, 1232 w, 1186 s, 1097 s, 1013 s, 939 w, 893 m, 800 w, 684 w, 568 w.

[CuCl₂(1-Htza)₂]·H₂O (5). A mixture of 1-Htza (1.5 mmol, 0.192 g), anhydrous CuCl₂ (0.5 mmol, 0.067 g) and [BMIM]NTf₂ (1 mL) was placed in a 25 mL Teflon-lined stainless-steel autoclave, and then heated at 100 °C for 3 days and cooled to room temperature at a rate of 2.5 °C h⁻¹. Blue lamellate crystals of 5 suitable for X-ray analyses were obtained. Similar to 4, the low yield of 5 restricts its physical property measurements except through single crystal X-ray diffraction and IR spectroscopy. IR (KBr pellet, cm⁻¹): 3588 w, 3444 w br, 3118 s, 2998 m, 2949 w, 1734 s, 1621 m, 1500 m, 1450 w, 1382 s, 1187 s, 1098 m, 1012 m, 905 w, 799 w, 681 m, 569 w.

Single crystal structure determination

Single crystals of 1–5 suitable for X-ray analyses were sticked to a fiberglass. Data collections were performed on a Rigaku Saturn-724 CCD diffractometer for 3 at 293 K, and a Rigaku Saturn-70 CCD diffractometer for the remaining compounds at 293 K. All diffractometers were equipped with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The intensity data sets were collected using the ω scan technique and reduced using CrystalClear software.²² The structures were solved by direct methods using the SHELXTL (version 5) crystallographic software package²³ and refined by full-matrix least-squares refinement on F². Non-hydrogen atoms were located by using difference Fourier maps and subjected to anisotropic refinement. The hydrogen atoms of water molecules except those belonging to disordered ones were located in difference Fourier syntheses and refined with O–H distances restrained to a target value of 0.85 Å and U_iso(H) = 1.5U_eq(O), and other hydrogen atoms were added according to theoretical models. Pertinent crystal data and structure refinement results for 1–5 are summarized in Table 1, and selected bonds lengths and angles are listed in Table S1 (ESI†).

Table 1 Crystallographic data and structural refinements for 1–5

	1	2	3	4	5
a R 1 = ∑(Fo − Fc)/∑Fo. b wR2 = [∑w(Fo^2 − Fc^2)^2/∑w(Fo^2)^2]^1/2.
Formula	C12H15.67BrCu2N16O9.33	C12H19BCu2F4N16O10	C6H10ClCuN8O5.5	C6H12Cl2CuN8O6	C6H10Cl2CuN8O5
M r (g mol^−1)	740.41	761.32	381.22	426.66	408.66
Space group	R3̄	I41/a	P1̄	P212121	Cc
a/Å	16.5929(16)	24.074(4)	6.537(2)	8.8662(6)	26.882(16)
b/Å	16.5929(16)	24.074(4)	9.314(3)	10.1720(6)	6.802(3)
c/Å	33.137(5)	5.1021(16)	11.282(4)	16.6340(11)	7.981(7)
α/°	90	90	108.282(3)	90	90
β/°	90	90	99.043(4)	90	103.38(3)
γ/°	120	90	98.410(2)	90	90
V/Å^3	7901.1(16)	2957.0(12)	630.0(4)	1500.17(17)	1419.7(16)
Z	9	4	2	4	4
D c/g cm^−3	1.400	1.710	2.010	1.889	1.912
μ/mm^−1	2.406	1.537	1.990	1.857	1.953
F(000)	3305	1528	384	860	820
Flack				0.040(18)	0.00(4)
Total reflns	19 763	8629	6171	12 030	4818
Unique reflns	3255	1289	2854	3409	2250
R int	0.0535	0.0582	0.0253	0.0532	0.0382
GOF	1.024	1.095	1.078	1.005	1.010
R 1[I > 2σ(I)]^a	0.0496	0.0666	0.0299	0.0392	0.0334
wR2(all data)^b	0.1779	0.1875	0.0958	0.0809	0.0864
CCDC	942747	942748	942949	942750	942751

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Results and discussion

Syntheses and thermal stability

In order to investigate the effects of IL anions on the formation of Cu-1-tza coordination compounds, we chose anhydrous CuCl₂ and 1-Htza as starting materials in [BMIM]X ILs (X = Br⁻, BF₄⁻, NTf₂⁻) with different water miscibility (Scheme 1). Compounds 1–4 were synthesized under the same reaction conditions. Compounds 1 and 2, obtained from [BMIM]Br and [BMIM]BF₄, respectively, show higher dimensional structures (2-D for 1, and 3-D for 2, respectively) with both carboxylate and tetrazolate groups of the fully deprotonated 1-tza⁻ ligand participating in coordination (Scheme 2), while both 3 and 4 achieved from [BMIM]NTf₂ exhibit a lower dimensional structure (1-D) with partially or fully protonated 1-Htza. Only the tetrazolate group of protonated 1-Htza takes part in metal complexation. The main reason is that water miscibility of ILs is distinct from each other with anions being altered. A well-known function of IL anions is their ability to control the amount of water present in ILs. [BMIM]Br and [BMIM]BF₄ are hygroscopic and can absorb a certain quantity of water when exposed to air, while [BMIM]NTf₂ is hydrophobic and only permits little water in it.^16a For this reason, the reaction mixture of 1, 2, 3 and 4 would absorb different amounts of water from air mainly determined by the hygroscopic property of the [BMIM]⁺ ILs. As an active proton acceptor, water can dramatically increase the degree of dissociation of an acid in a IL.^16c Therefore, 1-Htza in hydrophilic [BMIM]Br or [BMIM]BF₄ is more easily deprotonated than in hydrophobic [BMIM]NTf₂. Deprotonated 1-tza⁻ comprises more coordination sites and exhibits more diverse connectivity modes than protonated 1-Htza, and tends to facilitate higher dimensional products emerging. Moreover, protonated water involved in 1 and 2 also confirms the role of water miscibility of ILs. On the other hand, a closed environment may permit less amount of water as the reaction progresses. Compound 5 was synthesized in a sealed Teflon-lined stainless-steel autoclave, and only protonated 1-Htza and neutral water exist in 5, which further denotes the importance of water in affecting the degree of dissociation of the 1-Htza ligand. Thermogravimetric analysis (TGA) curves (Fig. S1, ESI†) show that 1 is stable up to 146 °C. Further heating led to a rapid weight loss arising from the decomposition of the organic ligand. For 2, the weight loss of 4.35% before 181 °C occurs in accordance with the release of the lattice water molecule (calcd: 4.64%), and then the framework sharply breaks down above 181 °C. The weight loss of 7.21% from 68 to 135 °C in 3 corresponds to the release of a half of lattice water molecule and one coordination water molecule (calcd: 7.09%). With continuous heating, the organic ligand was decomposed. The experimental PXRD patterns of 1–3 agree well with the simulated ones based on the single-crystal X-ray data (Fig. S2, ESI†), which implies that 1–3 are in a pure phase.

Structural descriptions of 1–5

[Cu₂(tza)₄]Br·H₃O·1/3H₂O (1). Compound 1 crystallizes in the trigonal space group R3̄ and features a 2-D neutral framework constructed by 1-tza⁻ ligands and centrosymmetric [Cu–(μ₂-O)₂–Cu] units. In each asymmetric unit of 1, there is one-third of the Br1 atom on a threefold axis and one-sixth of the Br2 atom at the site with bar-3 symmetry, one-third of the water oxygen O1W on a threefold axis is assigned 0.5-occupancy and one other water oxygen O2W in a general position is also assigned 0.5-occupancy. The unique Cu1 atom and the two 1-tza ligands (labeled as L_I and L_II) are in general positions with unit-occupancy. The L_I ligand exhibits a μ₃-κN14:κO11:κO11 coordination mode (mode a in Scheme 2), while the L_II ligand adopts a μ₂-κN24:κO21 coordination style (mode b in Scheme 2). Every Cu(II) atom is five-coordinated by two tetrazolate nitrogen atoms (N14, N24) and three carboxylate oxygen atoms (O21D, O11E, O11F) from two symmetry-related L_I and one symmetry-related L_II ligands (Fig. 1a). The pentacoordinated Cu(II) atom displays a distorted square pyramidal geometry with τ₅ = 0.11 (τ₅ = (β − α)/60, where α and β are the two biggest bond angles around the Cu(II) center; τ₅ = 0 for an ideal square pyramid, and τ₅ = 1 for an ideal trigonal bipyramid).²⁴ The N14, N24, O21D and O11F atoms constitute the equatorial plane, and O11E occupies the apical position. As the Jahn–Teller effect of Cu²⁺, the axial Cu–O bond length of 2.425(3) Å is significantly longer than those of Cu–N/O in the equatorial positions ranging from 1.934(2) to 2.025(3) Å. Interestingly, two Cu(II) atoms are doubly bridged by carboxylate O11 atoms to give rise to an ideal parallelogram centrosymmetric [Cu–(μ₂-O)₂–Cu] unit with a Cu(II)⋯Cu(II) distance of 3.4803(11) Å and Cu1–O11–Cu1 angle of 105.06(10)°. The paratactic L_I and L_II ligands, acting as double linkers, bridge [Cu–(μ₂-O)₂–Cu] units to generate a 2-D network parallel to the ab plane (Fig. 1b). The shortest Cu(II)⋯Cu(II) distance between adjacent dinuclear [Cu–(μ₂-O)₂–Cu] units is equal to 7.7070(11) Å. If each “double-linker” is considered as a bridge, and each [Cu–(μ₂-O)₂–Cu] unit is regarded as a four-connected node, compound 1 features a perfect 2-D Kagomé topology (Fig. 1c), which comprises a planar array of equilateral triangles and regular hexagons. Every triangle is corner-shared with three other neighboring triangles and edge-shared with three adjacent hexagons, which is also the same for every hexagon in the Kagomé network.²⁵ Although the 1-tza-based complexes have been studied extensively,^18–20 to the best of our knowledge, this kind of 2-D Kagomé array has been first observed in the reported 1-tza-based coordination compounds, which may be owing to two different kinds of coordination modes of 1-tza⁻ ligands coexisting in the framework of 1. Furthermore, 2-D layers stack over each other in an –ABC–ABC– fashion (viewed along the a-axis in Fig. 1d) to generate open channels viewed along the c-axis (Fig. 1e). Bromine anions and lattice water molecules as guests are located in these voids. However, there is still approximately 32% void volume calculated using PLATON software.²⁶

Fig. 1 Compound 1: (a) The coordination environment around the Cu(II) atom and the coordination mode of the 1-tza⁻ ligand. Symmetry codes: A: y − 1, −x + y, −z + 2; B: −y + 1, x − y + 1, z; C: −y + 2, x − y + 2, z; D: −x + y, −x + 2, z; E: x − y + 1, x + 1, −z + 2; F: −x + y, −x + 1, z. (b) A 2-D network constructed by the linkage of [Cu–(μ₂-O)₂–Cu] units and 1-tza⁻ ligands as “double-bridges”. (c) The 2-D Kagomé topology network of 1. (d) Three neighbouring layers viewed along the a-axis. (e) 2-D layers stack over each other in an –ABC–ABC– fashion to form open channels viewed along the c-axis. Hydrogen atoms are omitted for clarity.

[Cu₂(1-tza)₄]BF₄·H₃O·H₂O (2). When the reaction medium was changed to [BMIM]BF₄, compound 2 with a 3-D framework was produced, which crystallizes in the tetragonal space group I4₁/a. There is a half Cu(II) atom, one 1-tza⁻ ligand, one quarter of a BF₄⁻ anion, one quarter of a protonated lattice water cation and one quarter of a lattice water molecule in an asymmetric unit of 2 (Fig. 2a). Each Cu1 atom lies on an inversion centre and every B1 atom lies at the site with bar-4 symmetry. The Cu1 atom is surrounded by two tetrazolate nitrogen atoms(N4A, N4B) and two carboxylate oxygen atoms(O2, O2C), which belong to four different symmetry-related 1-tza⁻ ligands. Different from 1, each Cu(II) center adopts a square planar geometry with τ₄ = 0 (τ₄ = [360 − (α + β)]/141, where α and β are the two biggest bond angles around the Cu(II) center; τ₄ = 0 for a perfect square planar geometry, and τ₄ = 1 for a perfect tetrahedron).²⁴ Each 1-tza⁻ ligand displays a μ₂-κN4:κO2 coordination mode (mode b in Scheme 2) and bridges two Cu(II) atoms. The linkage of Cu(II) atoms and 1-tza⁻ ligands creates a 3-D porous network (Fig. 2b). The framework of 2 and its lvt-type topology is the same as that of the compound previously reported in the literature, except that BF₄⁻ anions instead of ClO₄⁻ anions are located in the 1-D channels of the 3-D framework.^18f

Fig. 2 (a) The coordination environment around the Cu(II) atom and coordination mode of the 1-tza⁻ ligand in 2. Symmetry codes: A: 0.25 − y, −0.75 + x, 0.25 − z; B: 0.75 + y, 0.75 − x, −0.25 + z; C: 1 − x, −y, −z; D: 0.75 − y, −0.75 + x, 0.25 + z. (b) The 3-D porous network with guest molecules in the 1-D channels. Hydrogen atoms are omitted for clarity.

[Cu(μ₂-Cl)(1-tza)(1-Htza)(H₂O)]·0.5H₂O (3). When [BMIM]NTf₂ as hydrophobic ionic liquid instead of hydrophilic [BMIM]Br and [BMIM]BF₄ was employed in the Cu(II) salt and 1-Htza reaction systems, compound 3, accompanying a very small amount of 4, was generated. Compound 3 crystallizes in the triclinic space group P1̄ and features a 1-D chain structure. The asymmetric unit of 3 consists of one Cu(II) center, one protonated 1-Htza ligand (labeled as L_I), one deprotonate 1-tza⁻ ligand (labeled as L_II), one Cl⁻ ion, one coordinated water molecule and half a lattice water molecule. The coordination geometry around the Cu(II) atom can be described as a distorted octahedron. As illustrated in Fig. 3a, two nitrogen atoms (N14, N24) from two different 1-Htza ligands, one oxygen atom (O1W) and a μ₂-bridging chloride ion (Cl1) constitute a equatorial plane, while the axial positions are filled by a carboxylate O21B atom and another symmetry-related chloride ion (Cl1A) with the axial angle Cl1A–Cu1–O21B of 176.95(5)°, deviating the ideal angle of 180°. The axial Cu–Cl/O bond lengths (Cu1–Cl1A (2.8204(10) Å) and Cu1–O21B (2.3949(19) Å)) are significantly longer than those in the equatorial plane (Cu1–Cl1 (2.2961(8) Å) and Cu1–O1W (2.0300(19) Å)), which may be owing to Jahn–Teller distortion of the Cu(II) ion. However, the Cu–N bond lengths are in the normal range with 2.011(2) and 1.997(2) Å for Cu1–N14 and Cu1–N24, respectively. Each Cl⁻ ion shows a μ₂-bridging mode connecting two Cu(II) atoms into a centrosymmetric parallelogram unit [Cu–(μ₂-Cl)₂–Cu] with a Cu(II)⋯Cu(II) distance of 3.7041(11) Å and a Cu1–Cl1–Cu1A angle of 92.18(3)°. Then, two symmetry-related L_II ligands with a μ₂-κN4:κO1 coordination mode (mode b in Scheme 2) acting as double linkers bridge [Cu–(μ₂-Cl)₂–Cu] units into a 1-D chain along the b direction (Fig. 3b). As carboxylate in the L_I ligand is protonated and the L_I ligand displays a monodentate terminal coordination mode via its N4 atom (mode c in Scheme 2), the structure of 3 cannot be further extended. However, three kinds of O–H⋯O hydrogen bonds (Table S2, ESI†) connect the 1-D chain into a 3-D supramolecular framework (Fig. S3, ESI†).

Fig. 3 (a) The coordination environment around the Cu(II) atom and the 1-Htza ligand in 3. Symmetry codes: A: −x, 1 − y, 1 − z; B: −x, −y, 1 − z. (b) View of the 1-D infinite chain of 3 along the a-axis. Hydrogen atoms are omitted for clarity.

[CuCl(μ₂-Cl)(1-Htza)₂(H₂O)]·H₂O (4). Compound 4 crystallizes in the chiral orthorhombic space group P2₁2₁2₁ and presents a 1-D zigzag chain structure. The asymmetric unit is composed of one Cu(II) atom, two protonated 1-Htza ligands, two Cl⁻ ions with different coordination modes, one coordinated water molecule and one lattice water molecule. In 4, the coordination geometry of the Cu(II) atom is regarded as a distorted octahedron with two nitrogen atoms (N14, N24) of two different 1-Htza ligands, one oxygen atom (O1W) of coordination water and one chloride ion (Cl1) in a monodentate terminal mode lying in the equatorial plane. The axial sites are occupied by two symmetry-related μ₂-chloride ions (Cl2, Cl2A) with the axial angle Cl2–Cu1–Cl2A of 177.50 (2)° (Fig. 4a). The axial Cu–Cl bond lengths are 2.8796(7) Å (Cu1–Cl2) and 2.7475(7) Å (Cu–Cl2A), respectively, which are significantly longer than that (2.2955(7) Å) in the equatorial plane. However, all Cu–N/O distances are in the normal range of 1.9998(19)–2.0062(19) Å. The adjacent Cu(II) ions are linked by Cl2 ions with a μ₂-bridged mode into an infinite 1-D zigzag chain with a Cu(II)⋯Cu(II) shortest distance of 5.0883(6) Å (Fig. 4b). Lattice water molecules are involved in hydrogen bonds (Table S2, ESI†), which stabilize the crystal structure and lead to the formation of a 2-D supramolecular network (Fig. S4, ESI†).

Fig. 4 (a) The coordination environment around the Cu(II) atom and the 1-Htza ligand in 4. Symmetry codes: A: −x, 0.5 + y, 0.5 − z; B: −x, −0.5 + y, 0.5 − z. (b) The 1-D zigzag chain viewed along the a-axis. Hydrogen atoms are omitted for clarity.

[CuCl₂(1-Htza)₂]·H₂O (5). Compound 5 crystallizes in the acentric monoclinic space group Cc and features an isolated neutral structure. Each asymmetric unit is composed of one crystallographically independent Cu atom, two 1-Htza ligands, two Cl⁻ ions and one lattice water molecule. Every Cu(II) center is four-coordinated with two nitrogen atoms from two different 1-Htza ligands and two Cl⁻ ions to form a quadrangle [CuN₂Cl₂] coordination sphere with τ₄ = 0.03 (Fig. 5). Both Cu–N and Cu–Cl bond lengths are in the normal range of 1.988(3)–1.998(3) Å and 2.2833(15)–2.2893(15) Å, respectively. All 1-Htza ligands in 5 are completely protonated in a monodentate terminal mode via its N4 atom (mode c in Scheme 2). Extensive hydrogen bonds from lattice water molecules and carboxylate group (Table S2, ESI†) connect the isolated units into a 3-D supramolecular network (Fig. S5, ESI†).

Fig. 5 The coordination environment around the Cu(II) atom and the 1-Htza ligand in 5. Hydrogen atoms are omitted for clarity.

Structural discussions of 1–5

As discussed above, compounds 1–5, obtained in [BMIM]X ILs (X = Br⁻, BF₄⁻, NTf₂⁻), possess distinct structures, and the 1-Htza ligand exhibits diverse connectivity modes (summarized in Scheme 2). Mode a is observed in 1, b in 1, 2 and 3, c in 3, 4 and 5. Modes b and c have been reported in previous literatures.^18–20 However, mode a is first observed in 1-Htza-based coordination compounds. It is well known that Cu(II) ions usually display various coordination numbers, and four/five/six-coordinated Cu(II) centers have been found in 1–5. The different coordination modes of 1-Htza linkers and varied coordination numbers of Cu²⁺ ions offer the possibility of forming coordination compounds with different structures. Notably, the tetrazolate groups of 1-Htza ligands in 1–5 connect Cu(II) atoms with the N4-position atom, probably due to the N4-site having minimum steric hindrance compared with N2/N3-atoms. This fact has been observed in many other 1-Htza-based transition metal compounds.¹⁸ On the other hand, different guest molecules are located in 1–5. Although all the title compounds possess neutral structures, protonated water molecules and a corresponding amount of anions (Br⁻, BF₄⁻) mainly from ILs are located in voids of 1 and 2, respectively, while only neutral water molecules are incorporated into 3–5. This shows that water molecules, as active proton acceptors, can effectively capture protons from 1-Htza in the medium of hygroscopic [BMIM]Br or [BMIM]BF₄.

Conclusion

In summary, we have first employed ionic liquids with different water miscibility, [BMIM]X (X = Br⁻, BF₄⁻, NTf₂⁻), as reaction media for synthesizing five new Cu-1-tza coordination compounds. Flexible coordination modes of the 1-Htza ligand and various coordination environments of Cu(II) atoms afford an opportunity to construct 1-Htza-based Cu(II) coordination compounds with different structures. Water molecules can act as active proton acceptors and ionic liquids have significant impacts on the deprotonation degree of 1-Htza with anions being altered. [BMIM]Br and [BMIM]BF₄ are hygroscopic, and fully deprotonated 1-tza⁻ exists in 1 and 2. However, [BMIM]NTf₂ is hydrophobic, and partly or fully protonated 1-Htza exists in 3–5.

Acknowledgements

This work was financially supported by 973 Program (2011CBA00505) and National Nature Science Foundation of China (21201099 and 21371170).

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Footnote

† Electronic supplementary information (ESI) available: X-ray crystallographic files in CIF format for 1–5, selected bond lengths and angles, hydrogen-bonds, coordination modes of 1-Htza, thermochemical properties, PXRD patterns, additional structural plots and FT-IR spectra. CCDC 942747–942751. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3nj01198d

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