Bis([mu]-5-car­boxy­benzene-1,3-di­carb­oxyl­ato-[kappa]²O¹:O³)bis­[(2,2'-bi-1H-imidazole-[kappa]²N³,N^3')zinc]

The title compound, [Zn₂(C₉H₄O₆)₂(C₆H₆N₄)₂], consists of two Zn^II ions, two 5-carb­oxy­benzene-1,3-dicarboxyl­ate (Hbtc^2-) dianions and two 2,2'-bi-1H-imidazole (bimz) mol­ecules. The Zn^II centre is coordinated by two carboxyl­ate O atoms from two Hbtc^2- ligands and by two imidazole N atoms of a bimz ligand, in a distorted tetra­hedral coordination geometry. Two neighbouring Zn^II ions are bridged by a pair of Hbtc^2- ligands, forming a discrete binuclear [Zn₂(Hbtc)₂(bimz)₂] structure lying across an inversion centre. Hydrogen bonds between carboxyl H atoms and carboxyl­ate O atoms and between imidazole H atoms and carboxyl­ate O atoms link the binuclear units. These binuclear units are further extended into a three-dimensional supramolecular structure through extensive O-H...O and N-H...O hydrogen bonds. Moreover, the three-dimensional nature of the crystal packing is reinforced by the [pi]-[pi] stacking. The title compound exhibits photoluminescence in the solid state, with an emission maximum at 415 nm.

S1. Comment

Crystal engineering and supramolecular chemistry have been the most active areas of materials research in recent years, owing to the intriguing structural topologies examined and their potential applications in host–guest chemistry and catalysis (Biradha & Zaworotko, 1998; Sun et al., 2003). The key to the successful construction of supramolecular architectures is the control and manipulation of coordination bonds and noncovalent interactions by carefully selecting the coordination geometry of the metal centres and the organic ligands containing appropriate functional groups (such as polycarboxylic acids and polypyridines) (Tong et al., 1999; Dong et al., 2000). Benzene-1,3,5-tricarboxylic acid is one of the most extensively studied ligands because of its versatile coordinating modes and structural robustness (Yaghi et al., 1997; Lin et al., 2007), and new compounds based on the benzene-1,3,5-tricarboxylic acid (H₃btc) ligand are observed continually (Xu et al., 2007; Jiang et al., 2010). However, almost all reported compounds with H₃btc are two- or three-dimensional structures because it has six O donors. Only five zinc compounds based on H₃btc with discrete structures have been reported (Jo et al., 2005; Du et al., 2006; Plater et al., 2001; Chen et al., 2006; Braverman et al., 2007). In addition, all five discrete structures are mononuclear zinc compounds. We report herein the synthesis and crystal structure of the title new zinc(II) H₃btc complex, (I), with a discrete binuclear structure. To the best of our knowledge, no binuclear zinc(II) structure with H₃btc has been reported previously.

The asymmetric unit of (I) consists of one Zn^II ion, one Hbtc^2- ligand and one bimz molecule. As depicted in Fig. 1, the Zn1 ion is four-coordinated by two carboxylate O atoms [O1 and O3ⁱ; symmetry code: (i) -x, -y, -z] from two Hbtc^2- ligands and two imidazole N atoms from one bimz ligand in a tetrahedral coordination environment. The [ZnO₂N₂] tetrahedron is distorted, with the O/N—Zn—O/N bond angles varying from 82.69 (6) to 130.27 (6)° (Table 1). The bond distances involving the Zn1 ion are normal and comparable with the values in related zinc(II) complexes (Du et al., 2006; Plater et al., 2001). Each Hbtc^2- anion bridges two neighbouring Zn^II ions through its two monodentate carboxylate groups (Fig. 1). The uncoordinated carboxyl group is protonated for charge balance, as suggested by the strong absorption at 1725 cm^-1 in the IR spectrum. Two Hbtc^2- anions bridge two neighbouring Zn^II ions to form a centrosymmetric binuclear structure, as depicted in Fig. 1. The bimz ligands, coordinating an a chelating fashion, take up the linking sites of the Zn^II centre and thus preclude propagation to a higher-dimensional framework.

Looking carefully at the crystal packing it can be seen that the O5—H5···O6ⁱⁱ [symmetry code: (ii) -x + 1, -y + 2, -z; (Table 2)] hydrogen bonds give a carboxylic acid inversion dimer arrangement of R²₂(8) motif (Bernstein et al., 1995), which leads to the formation of a one-dimensional chain (Fig. 2). These chains are then linked via N—H···O hydrogen bonds involving carboxylate atom O2 and the imidazole H atom on atom N2, again giving an inversion dimer arrangement, with graph-set R²₂(16) (Fig. 2). These two hydrogen bonds, O—H···O and N—H···O, lead to the formation of the two-dimensional network. A second N—H···O hydrogen bond, involving carboxylate atom O4 and the imidazole H atom on atom N4 (Table 2), gives a second inversion dimer arrangement, graph-set R²₂(16). The latter links the two-dimensional networks to form a three-dimensional hydrogen-bonded structure (Fig. 3). In the three-dimensional crystal packing, there are π–π stacking interactions between parallel benzene rings, arranged in an offset fashion with a face-to-face distance of 3.306 (3) Å, and between imidazole rings, with a centroid-to-centroid distance of 3.799 (2) Å and a dihedral angle of 3.3 (1)°. The extensive hydrogen bonds and π–π stacking interactions are responsible for the three-dimensional supramolecular framework structure.

As mentioned above, only five zinc compounds with discrete structures have been reported to date, for example, [Zn(H₂btc)₂(tazcyde)](DMF) (tazcyde = 1,4,8,11-tetraazacyclotetradecane), with an octahedrally coordinated Zn^II centre (Jo et al., 2005), and [Zn(H₂btc)₂(L)₂] [L = 4-amino-3,5-bis(4-pyridyl)-1,2,4-triazole or 4-(4-pyridinioamino)pyridyl], which contains a tetrahedrally coordinated Zn^II centre (Du et al., 2006; Braverman et al., 2007). All of them contain a second N-containing ligand, which is similar to that in the title compound. However, these mononuclear structures are quite different from the present binuclear structure. It should be noted that the combination of a similar chelating ligand, such as 2,2'-bipyridine or pyridine-2-(1-methyl-1H-pyrazol-3-yl, with the H₃btc ligand and Zn^II ions produces the mononuclear complex [Zn(H₂btc)₂(L₂)] [L₂ = pyridine-2-(1-methyl-1H-pyrazol-3-yl or 2,2'-bipyridine; Plater et al., 2001; Chen et al., 2006], which indicates that the structures of the final products are significantly influenced by the ancillary ligands.

The title compound exhibits photoluminescence in the solid state (Fig. 4). Excitation of solid samples at 341 nm at room temperature produces a luminescence peak with a maximum at 415 nm. The emission peak is essentially the same as the photoluminescence signal of the free H₃btc ligand at 408 nm (λ_ex = 334 nm; Chen et al., 2003). This indicates that the emission band of (I) is a ligand-centred (π^* → n) transition. Compared with the emission band for the free H₃btc ligand, a slight bathochromic shift is observed in (I), which may be related to the coordination of the Zn²⁺ ions to the Hbtc^2- dianions.

Related literature

For related literature, see: Bernstein et al. (1995); Biradha & Zaworotko (1998); Braverman et al. (2007); Chen et al. (2003, 2006); Dong et al. (2000); Du et al. (2006); Jiang et al. (2010); Jo et al. (2005); Lin et al. (2007); Plater et al. (2001); Sun et al. (2003); Tong et al. (1999); Xu et al. (2007); Yaghi et al. (1997).

S2. Experimental

A mixture of Zn(NO₃)₃.6H₂O (89.2 mg, 0.3 mmol), benzene-1,3,5-tricarboxylic acid (63.1 mg, 0.3 mmol) and 2,2'-bi-1H-imidazole (40.2 mg, 0.3 mmol) in a 1:1:1 molar ratio in distilled water (14 ml) was introduced into a Parr Teflon-lined stainless steel vessel (25 ml). The vessel was sealed and heated to 413 K. The temperature was held for 4 d and then the mixture was allowed to cool naturally, giving colourless prism-shaped crystals. The colourless crystalline product was filtered off, washed with distilled water and dried at ambient temperature [yield: 63%, based on Zn(NO₃)₃.6H₂O]. Analysis, calculated for C₃₀H₂₀N₈O₁₂Zn₂: C 44.20, H 2.47, N, 13.74%; found: C 44.11, H 2.36, N 13.68%. Spectroscopic analysis: IR (KBr pellet, ν, cm^-1): 3435, 1725, 1611, 1567, 1437, 1420, 1374, 1164, 1120, 1084, 1070, 1001, 858, 783, 760, 674, 664, 625, 545, 530.

S3. Refinement

O- and N-bound H atoms were located in difference maps and other H atoms were included in calculated positions. All H atoms were treated using a riding-model approximation, with O—H = 0.82 Å and U_iso(H) = 1.5U_eq(O), and with N—H = 0.86 Å and C—H = 0.93 Å and U_iso(H) = 1.2U_eq(N,C).

Computing details

Data collection: APEX2 (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT (Bruker, 2007); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008) and DIAMOND (Brandenburg, 2005); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures

	Fig. 1. The molecular structure of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. [Symmetry code: (i) -x, -y, -z.]
	Fig. 2. A perspective view of the hydrogen-bonded two-dimensional layer in the crystal structure of (I). O—H···O and N—-H···O hydrogen bonds and π–π stacking interactions as shown as dashed lines. [Symmetry codes: (ii) -x + 1, -y + 1, -z + 1; (iii) x + 1, y, z + 1.]
	Fig. 3. A view, along the b axis, of the crystal packing of (I), showing the O—H···O and N—H···O hydrogen bonds (dashed lines).
	Fig. 4. The solid-state photoluminescence spectra of (I).

Bis(µ-5-carboxybenzene-1,3-dicarboxylato- κ²O¹:O³)bis[(2,2'-bi-1H-imidazole- κ²N³,N^3')zinc]

Crystal data

[Zn2(C9H4O6)2(C6H6N4)2]	Z = 1
Mr = 815.32	F(000) = 412
Triclinic, P1	Dx = 1.806 Mg m^−3
a = 8.0458 (4) Å	Mo Kα radiation, λ = 0.71073 Å
b = 8.0633 (4) Å	Cell parameters from 4639 reflections
c = 12.2332 (7) Å	θ = 2.6–28.3°
α = 106.526 (1)°	µ = 1.68 mm^−1
β = 97.606 (1)°	T = 296 K
γ = 93.861 (1)°	Prism, colourless
V = 749.54 (7) Å^3	0.33 × 0.27 × 0.24 mm

Data collection

Bruker APEXII CCD area-detector diffractometer	3628 independent reflections
Radiation source: fine-focus sealed tube	3336 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.013
ω scans	θmax = 28.3°, θmin = 2.6°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −10→10
Tmin = 0.606, Tmax = 0.688	k = −10→10
7095 measured reflections	l = −16→15

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.025	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.068	H-atom parameters constrained
S = 1.01	w = 1/[σ^2(Fo^2) + (0.0348P)^2 + 0.3992P] where P = (Fo^2 + 2Fc^2)/3
3628 reflections	(Δ/σ)max = 0.001
235 parameters	Δρmax = 0.30 e Å^−3
0 restraints	Δρmin = −0.34 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Zn1	0.28671 (2)	0.06839 (2)	0.265169 (16)	0.03110 (7)
O1	0.21061 (17)	0.14318 (16)	0.13195 (11)	0.0383 (3)
O2	0.40444 (18)	0.36464 (18)	0.22040 (12)	0.0441 (3)
O3	−0.1170 (2)	0.12337 (19)	−0.24198 (14)	0.0569 (4)
O4	−0.1125 (2)	0.35339 (19)	−0.30505 (12)	0.0491 (4)
O5	0.31641 (17)	0.85381 (16)	−0.10009 (11)	0.0392 (3)
H5	0.3700	0.9495	−0.0867	0.059*
O6	0.46804 (17)	0.86109 (16)	0.06782 (11)	0.0401 (3)
N1	0.28368 (19)	0.24056 (19)	0.42654 (13)	0.0340 (3)
N2	0.4432 (2)	0.3887 (2)	0.59045 (13)	0.0397 (4)
H2A	0.5293	0.4242	0.6442	0.048*
N3	0.52406 (18)	0.04750 (18)	0.33745 (11)	0.0308 (3)
N4	0.72386 (19)	0.1577 (2)	0.48840 (13)	0.0366 (3)
H4A	0.7745	0.2192	0.5556	0.044*
C1	0.24078 (19)	0.3700 (2)	0.04570 (13)	0.0247 (3)
C2	0.11812 (19)	0.2856 (2)	−0.04929 (13)	0.0255 (3)
H2	0.0674	0.1751	−0.0563	0.031*
C3	0.07123 (18)	0.36608 (19)	−0.13366 (13)	0.0236 (3)
C4	0.14709 (19)	0.5312 (2)	−0.12326 (13)	0.0243 (3)
H4	0.1144	0.5862	−0.1787	0.029*
C5	0.27222 (18)	0.61388 (19)	−0.02960 (13)	0.0239 (3)
C6	0.31842 (19)	0.5331 (2)	0.05433 (13)	0.0255 (3)
H6	0.4020	0.5888	0.1167	0.031*
C7	0.2895 (2)	0.2893 (2)	0.14010 (14)	0.0282 (3)
C8	−0.0645 (2)	0.2761 (2)	−0.23473 (14)	0.0280 (3)
C9	0.3607 (2)	0.7880 (2)	−0.01567 (14)	0.0274 (3)
C10	0.4356 (2)	0.2628 (2)	0.49025 (14)	0.0304 (3)
C11	0.2899 (3)	0.4498 (3)	0.59140 (19)	0.0502 (5)
H11	0.2583	0.5375	0.6502	0.060*
C12	0.1916 (3)	0.3585 (3)	0.49023 (19)	0.0457 (5)
H12	0.0800	0.3734	0.4677	0.055*
C13	0.5634 (2)	0.1594 (2)	0.44343 (14)	0.0292 (3)
C14	0.7920 (3)	0.0397 (3)	0.40730 (17)	0.0431 (4)
H14	0.9023	0.0113	0.4141	0.052*
C15	0.6684 (2)	−0.0284 (3)	0.31485 (16)	0.0384 (4)
H15	0.6797	−0.1128	0.2472	0.046*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.03604 (12)	0.02565 (11)	0.02517 (11)	−0.00818 (7)	−0.01337 (8)	0.00817 (7)
O1	0.0496 (7)	0.0319 (6)	0.0312 (6)	−0.0056 (5)	−0.0100 (5)	0.0155 (5)
O2	0.0520 (8)	0.0398 (7)	0.0336 (7)	−0.0057 (6)	−0.0179 (6)	0.0134 (6)
O3	0.0623 (9)	0.0379 (8)	0.0579 (9)	−0.0275 (7)	−0.0259 (8)	0.0178 (7)
O4	0.0617 (9)	0.0410 (7)	0.0336 (7)	−0.0140 (6)	−0.0246 (6)	0.0124 (6)
O5	0.0489 (7)	0.0308 (6)	0.0332 (7)	−0.0148 (5)	−0.0109 (5)	0.0146 (5)
O6	0.0446 (7)	0.0318 (6)	0.0361 (7)	−0.0173 (5)	−0.0158 (5)	0.0129 (5)
N1	0.0363 (7)	0.0301 (7)	0.0307 (7)	−0.0043 (6)	−0.0017 (6)	0.0062 (6)
N2	0.0555 (10)	0.0326 (8)	0.0229 (7)	−0.0102 (7)	−0.0011 (6)	0.0023 (6)
N3	0.0363 (7)	0.0303 (7)	0.0204 (6)	−0.0024 (6)	−0.0083 (5)	0.0060 (5)
N4	0.0369 (8)	0.0401 (8)	0.0268 (7)	−0.0065 (6)	−0.0138 (6)	0.0111 (6)
C1	0.0254 (7)	0.0258 (7)	0.0213 (7)	0.0012 (6)	−0.0007 (6)	0.0066 (6)
C2	0.0276 (7)	0.0221 (7)	0.0241 (7)	−0.0021 (5)	0.0001 (6)	0.0056 (6)
C3	0.0242 (7)	0.0233 (7)	0.0193 (7)	−0.0027 (5)	−0.0010 (5)	0.0030 (5)
C4	0.0263 (7)	0.0253 (7)	0.0201 (7)	−0.0013 (6)	0.0006 (6)	0.0069 (6)
C5	0.0241 (7)	0.0230 (7)	0.0218 (7)	−0.0033 (5)	0.0006 (5)	0.0047 (5)
C6	0.0238 (7)	0.0263 (7)	0.0219 (7)	−0.0022 (5)	−0.0029 (5)	0.0040 (6)
C7	0.0329 (8)	0.0265 (7)	0.0241 (7)	0.0040 (6)	−0.0008 (6)	0.0078 (6)
C8	0.0276 (7)	0.0270 (7)	0.0234 (7)	−0.0042 (6)	−0.0028 (6)	0.0028 (6)
C9	0.0285 (7)	0.0249 (7)	0.0259 (7)	−0.0036 (6)	−0.0003 (6)	0.0065 (6)
C10	0.0406 (9)	0.0252 (7)	0.0208 (7)	−0.0093 (6)	−0.0042 (6)	0.0064 (6)
C11	0.0689 (14)	0.0376 (10)	0.0414 (11)	0.0011 (10)	0.0183 (10)	0.0042 (9)
C12	0.0461 (11)	0.0401 (10)	0.0481 (12)	0.0032 (8)	0.0090 (9)	0.0082 (9)
C13	0.0351 (8)	0.0277 (7)	0.0208 (7)	−0.0080 (6)	−0.0082 (6)	0.0091 (6)
C14	0.0382 (10)	0.0516 (12)	0.0391 (10)	0.0068 (8)	−0.0056 (8)	0.0176 (9)
C15	0.0436 (10)	0.0403 (10)	0.0295 (9)	0.0074 (8)	−0.0033 (7)	0.0108 (7)

Geometric parameters (Å, º)

Zn1—O3^i	1.9232 (14)	N4—H4A	0.8600
Zn1—O1	1.9337 (12)	C1—C6	1.387 (2)
Zn1—N3	2.0343 (14)	C1—C2	1.394 (2)
Zn1—N1	2.0696 (15)	C1—C7	1.501 (2)
O1—C7	1.271 (2)	C2—C3	1.393 (2)
O2—C7	1.239 (2)	C2—H2	0.9300
O3—C8	1.249 (2)	C3—C4	1.391 (2)
O3—Zn1^i	1.9232 (14)	C3—C8	1.508 (2)
O4—C8	1.236 (2)	C4—C5	1.393 (2)
O5—C9	1.310 (2)	C4—H4	0.9300
O5—H5	0.8200	C5—C6	1.389 (2)
O6—C9	1.222 (2)	C5—C9	1.484 (2)
N1—C10	1.331 (2)	C6—H6	0.9300
N1—C12	1.374 (2)	C10—C13	1.443 (3)
N2—C10	1.342 (2)	C11—C12	1.358 (3)
N2—C11	1.360 (3)	C11—H11	0.9300
N2—H2A	0.8600	C12—H12	0.9300
N3—C13	1.335 (2)	C14—C15	1.359 (3)
N3—C15	1.372 (2)	C14—H14	0.9300
N4—C13	1.337 (2)	C15—H15	0.9300
N4—C14	1.367 (3)
O3^i—Zn1—O1	99.73 (6)	C6—C5—C4	120.04 (14)
O3^i—Zn1—N3	117.51 (6)	C6—C5—C9	117.82 (13)
O1—Zn1—N3	130.27 (6)	C4—C5—C9	122.14 (14)
O3^i—Zn1—N1	107.73 (7)	C1—C6—C5	120.45 (13)
O1—Zn1—N1	117.49 (6)	C1—C6—H6	119.8
N3—Zn1—N1	82.69 (6)	C5—C6—H6	119.8
C7—O1—Zn1	110.38 (10)	O2—C7—O1	123.11 (15)
C8—O3—Zn1^i	155.06 (15)	O2—C7—C1	119.52 (15)
C9—O5—H5	109.5	O1—C7—C1	117.37 (14)
C10—N1—C12	105.79 (15)	O4—C8—O3	124.78 (15)
C10—N1—Zn1	109.47 (12)	O4—C8—C3	119.38 (14)
C12—N1—Zn1	144.06 (14)	O3—C8—C3	115.81 (14)
C10—N2—C11	107.41 (16)	O6—C9—O5	123.41 (15)
C10—N2—H2A	126.3	O6—C9—C5	121.76 (14)
C11—N2—H2A	126.3	O5—C9—C5	114.82 (13)
C13—N3—C15	105.56 (14)	N1—C10—N2	110.87 (17)
C13—N3—Zn1	110.63 (12)	N1—C10—C13	118.56 (14)
C15—N3—Zn1	143.44 (12)	N2—C10—C13	130.53 (16)
C13—N4—C14	106.96 (15)	C12—C11—N2	106.97 (18)
C13—N4—H4A	126.5	C12—C11—H11	126.5
C14—N4—H4A	126.5	N2—C11—H11	126.5
C6—C1—C2	119.55 (14)	C11—C12—N1	108.98 (19)
C6—C1—C7	119.03 (13)	C11—C12—H12	125.5
C2—C1—C7	121.42 (14)	N1—C12—H12	125.5
C3—C2—C1	120.22 (14)	N3—C13—N4	111.38 (16)
C3—C2—H2	119.9	N3—C13—C10	118.26 (14)
C1—C2—H2	119.9	N4—C13—C10	130.34 (15)
C4—C3—C2	119.94 (13)	C15—C14—N4	107.08 (17)
C4—C3—C8	120.03 (13)	C15—C14—H14	126.5
C2—C3—C8	120.02 (13)	N4—C14—H14	126.5
C3—C4—C5	119.79 (14)	C14—C15—N3	109.02 (17)
C3—C4—H4	120.1	C14—C15—H15	125.5
C5—C4—H4	120.1	N3—C15—H15	125.5

Symmetry code: (i) −x, −y, −z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O5—H5···O6^ii	0.82	1.88	2.6859 (17)	167
N2—H2A···O2^iii	0.86	1.99	2.667 (2)	135
N4—H4A···O4^iv	0.86	1.82	2.6792 (19)	178

Symmetry codes: (ii) −x+1, −y+2, −z; (iii) −x+1, −y+1, −z+1; (iv) x+1, y, z+1.