Crystal structure and fluorescence properties of catena-poly[[(2,2′-bi-1H-imidazole-κ2 N,N′)cadmium]-di-μ-chlorido]

The title complex shows selective sensitivity to detecting nitrobenzene in DMF media due to the fluorescent quenching.

In the polymeric title compound, [CdCl 2 (C 6 H 6 N 4 )] n , the central Cd II atom is coordinated by four chloride ligands and two N atoms from a chelating 2,2 0 -bi-1H-imidazole molecule, leading to a distorted octahedral Cl 4 N 2 coordination set. As a result of the 2 -bridging character of the Cl ligands, chains parallel to the c axis are formed, with the chelating 2,2 0 -bi-1H-imidazole ligands decorated on both sides of the chain. The luminescence properties of the complex dispersed in dimethylformamide shows that the emission intensities are significantly quenched by nitrobenzene.

Chemical context
In recent years, great efforts have been devoted to the design and assembly of coordination polymers, not only because of the aesthetic beauty of their structures but also their potential applications in the fields of gas storage, separation, magnetism or their optical properties (Thangavelu et al., 2015;Zhao et al., 2014;Erer et al., 2015;Eddaoudi et al., 2015;O'Keeffe, 2009). The structural chemistry of transition metal halides with neutral N-donor co-ligands has been investigated thoroughly, leading to a multitude of complexes with new topologies and functionalities. Such N-donor ligands include, for example, tethering ligands such as bis(4-pyridylmethyl)piperazine (Low & LaDuca, 2015), 4,4 0 -dipyridylamine (Brown et al., 2008) or 4,4 0 -bipyridine (Lyons et al., 2008). We are also interested in conjugated terminal N-heterocyclic molecules as ligands, which can endow the resulting structures with photoluminescent properties. 2,2 0 -Bi-1H-imidazole is used as such an important terminal N-donor co-ligand, which can not only direct the structural properties with hydrogen-bonding networks, but also can be used as a suitable fragment forinteractions through the imidazole rings.
We have explored the self-assembly of CdCl 2 and 2,2 0 -bi-1H-imidazole in the presence of 2,2-dimethylsuccinic acid and obtained a new polymeric cadmium complex, [Cd(2,2 0 -bi-1Himidazole)Cl 2 ] n . Its crystal structure and luminescence sensing of solvent molecules are reported in this communication.

Structural commentary
The asymmetric unit of the title compound is shown in Fig. 1. The central Cd II atom is coordinated by four chloride ligands and two nitrogen atoms from a chelating 2,2 0 -bi-1H-imidazole ISSN 2056-9890 ligand, forming a distorted Cl 4 N 2 octahedral coordination set (Fig. 2). The Cd-Cl and Cd-N bond lengths range from 2.5271 (11)-2.8150 (14) and 2.323 (3)-2.342 (4) Å , respectively. The five-membered Cd1/N1/C1/C2/N2 chelate ring is characterized by a bite angle of 72.6 (1) . The two imidazole rings of the 2,2 0 -bi-1H-imidazole ligand are nearly parallel to each other, making a dihedral angle of 0.8 (5) . The 2bridging character of the four Cl ligands leads to the formation of a chain expanding parallel to the c axis (Fig. 2).

Supramolecular features
In the presence of the chelating 2,2 0 -bi-1H-imidazole ligands that decorate the chains on both sides, the chains are directed by weakinteractions into zipper-like double-stranded chains with centroid-to-centroid distances of 3.6538 (15) and 3.9452 (14) Å , respectively. In addition, there are intermolecular hydrogen bonds between the imidazole N atoms and coordinating Cl atoms of neighboring chains ( Table 1). Thestacking interactions together with N-HÁ Á ÁCl hydrogen-bonding interactions expand the [CdCl 4/2 ] n chains to supramolecular sheets parallel to the bc plane (Fig. 2).

Luminescence properties
Coordination polymers based on d 10 metal ions and conjugated organic ligands are promising candidates for potential photoactive materials with applications in chemical sensoring or in photochemistry. In particular, solvent-dependent quenching behaviour is of interest for the development of luminescent probes for chemical species (Liu et al., 2015). Hence the luminescence properties of the title compound in different solvent emulsions were investigated. The luminescent intensities had no distinct differences if dichloromethane, acetonitrile, ethanol, ethyl acetate or benzene were selected as dispersing agents. However, the intensity had an abrupt decrease when the powdered samples of the title compound were dispersed in nitrobenzene. When the nitrobenzene solvent was gradually and increasingly added to the standard emulsions, the fluorescence intensities of the standard emulsions gradually decreased with increasing addition of nitrobenzene (Fig. 3). The fluorescence decrease was nearly proportional to the nitrobenzene concentration and intensity ultimately was found to be negligible. The efficient quenching of nitrobenzene in this system can be ascribed to the physical interaction of the solute and solvent, which induces the elec- The asymmetric unit of the title compound, with anisotropic displacement parameters drawn at the 30% probability level.

Figure 2
The supramolecular structure showing the interactions between neighbouring chains. N-HÁ Á ÁCl hydrogen bonds are shown as dashed lines. Table 1 Hydrogen-bond geometry (Å , ). tron transfer from the excited title compound to the electrondeficient nitrobenzene (Hao et al., 2013). These results have given us the impetus to carry out more detailed investigations on the sensing behaviour of the title compound.

Database survey
A search of the Cambridge Structure Database (Version 5.35; last update May 2015; Groom et al., 2016) for related Cd-based complexes with 2,2 0 -bi-1H-imidazole gave 41 hits. In most cases, 2,2 0 -bi-1H-imidazole serves as an ancillary ligand to be incorporated in carboxylate coordination polymer systems.
[Cd(2,2 0 -bi-1H-imidazole)Br 2 ] n has a very similar composition to the title compound and also shows an arrangement of polymeric chains constructed from the bridging behaviour of the Br ligand (Hester et al., 1996); however, the space group is different (C2/c).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. C-bound H atoms were positioned geometrically and constrained using a riding-model approximation, with C-H = 0.93 Å and U iso (H) = 1.2U eq (C). H atoms attached to the N atoms were found from difference maps but constrained with N-H = 0.86 Å and U iso (H) = 1.2U eq (N).

Figure 3
Fluorescence intensity of the title complex at different nitrobenzene concentrations in DMF.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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.  (2) Geometric parameters (Å, º)