Crystal structure and luminescent properties of [1-(biphenyl-4-yl)-1H-imidazole-κN 3]dichloridozinc

A new imidazole-based zinc complex, synthesized using hydrothermal methods, exhibits luminescent behaviour.


Chemical context
Metal coordination polymers constructed from organic ligands and metal cations have received attention because of their structural diversity and interesting physical and chemical properties, including adsorption, molecular separation, heterogeneous catalysis and non-linear optics (Sumida et al., 2012;Colombo et al., 2012;Henke et al., 2012). The development of such materials for various applications is reliant on the functionalities and modulations of the inorganic central atoms and the organic linkers. Materials constructed from d 10 metal ions can be promising photoactive candidates (Lan et al., 2009;Qin et al., 2014). For example, a series of zinc-and cadmium-based coordination polymers were reported to be luminescent sensors for the detection of small organic molecules (Yi et al., 2012;Wang et al., 2013). On the other hand, the choice of the organic ligands or linkers is important for the supramolecular arrangement.
Among the various organic ligands used for the construction of coordination polymers, nitrogen-donor species are dominant due to their strong affinities for binding metal atoms . In particular, imidazoles are of great ISSN 2056-9890 interest for the construction of zeolite imidazolate frameworks, which exhibit high stability and practical applications (Phan et al., 2010). By further modification of imidazole ligands, various compounds with different structural set-ups have been reported, including one-dimensional, two-dimensional and three-dimensional architectures (Kan et al., 2012). Recently, two one-dimensional imidazole-based zinc complexes were synthesized by using 1,4-di(1H-imidazol-1yl)benzene (dib), and 1,3,5-tri(1H-imidazol-1-yl)benzene (tib) as ligands (Wang et al., 2014). To obtain further effects on the final structure by modification of the substituent of the imidazoles, 1-(biphenyl-4-yl)-1H-imidazole (bpi) was chosen as ligand and reacted with Zn 2+ ions in this work, yielding the title compound ZnCl 2 (C 15 H 12 N 2 ) 2 , (I). Apart from the structure determination, its photoluminescent property is also reported.

Structural commentary
As shown in Fig. 1, the asymmetric unit of (I) consists of one zinc(II) cation, two bpi ligands and two chlorine ligands. The cation has a distorted tetrahedral coordination sphere defined by the free imidazole N atoms and two Cl atoms. The Zn-N and Zn-Cl bond lengths (Table 1) are typical for tetrahedrally coordinated Zn II . The dihedral angles between the two phenyl rings in the two bpi ligands are 37.52 (14) and 42.45 (14) , respectively, while the dihedral angles between the phenyl rings and the attached imidazole rings are 37.13 (14) and 40.05 (14) .
Zn II -based compounds with metal-organic framework structures are well-known for their luminescence properties. The photoluminescence spectrum of compound (I) in the solid state is shown in Fig. 2. On excitation at 278 nm, the emission band is centred at 350 nm. Compared to the free bpi ligand, which exhibits one main fluorescent emission band around 400 nm when excited at 271 nm, the emission band of complex (I) is about 50 nm hypochromatically shifted. Considering metal atoms with a d 10 electron configuration and the bonding interactions with the ligand, such broad emission bands may be assigned to a ligand-to-ligand charge transfer (LLCT), admixing with metal-to-ligand (MLCT) and ligand-to-metal (LMCT) charge transfers (Gong et al., 2011).

Supramolecular features
As mentioned before, the imidazole-based ligands dib and tib, featuring two and three imidazole rings, respectively, can adopt different structural dimensionalities. The bpi ligand used in this study, however, has only one available N-donor, thus preventing the formation of a polymeric structure. Nevertheless, there are weak intermolecularstacking interactions between single molecules in the crystal packing. The terminal phenyl ring and the imidazole ring of a neighbouring ligand are tilted to each other by 11.72 (17) , with a centroid-to-centroid distance of 3.751 (2) Å (Fig. 3). The molecular structure of compound (I). Displacement ellipsoids were drawn at the 30% probability level.

Figure 2
Excitation and emission spectra of compound (I) in the solid state.

Figure 3
View of the crystal structure along [010] emphasizinginteractions (dotted lines and inset).

Synthesis and crystallization
All chemicals were purchased commercially and used without further purification. A mixture of ZnCl 2 (81.6 mg, 5 mmol), bpi (130 mg, 0.6 mmol), and de-ionized water (9 ml) was loaded into a 20 ml Teflon-lined stainless steel autoclave. The autoclave was sealed and heated at 423 K for 5 d, and then cooled to room temperature by switching off the furnace. Colourless block-shaped crystals were isolated, which were filtered off and washed with de-ionized water. The final product was dried at ambient temperature (yield 75% based on zinc). Analysis calculated (wt%) for ZnCl 2 (C 15 H 12 N 2 ) 2 : C,62.47;H,4.19;N,9.71. Found: C,62.45;H,4.15;N,9.79. Elemental analyses of C, H, and N were conducted on a Perkin-Elmer 2400 elemental analyser. The photoluminescence (PL) excitation and emission spectra were recorded with an F-7000 luminescence spectrometer equipped with a xenon lamp of 450 W as an excitation light source. The photomultiplier tube voltage was 400 V, the scan speed was 1200 nm min À1 , both the excitation and the emission slit widths were 5.0 nm.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were positioned geometrically with C-H = 0.93 Å and U iso (H) = 1.2U eq (C).

[1-(Biphenyl-4-yl)-1H-imidazole-κN 3 ]dichloridozinc
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.014 Δρ max = 0.31 e Å −3 Δρ min = −0.35 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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 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 (Å 2 )
x y z U iso */U eq