Crystal structure of di-μ-chlorido-bis[dichlorido(l-histidinium-κO)cadmium(II)]

In the bimetallic title compound, [Cd2(C6N3O2H9)2Cl6], both cadmium atoms adopt a distorted CdCl4O trigonal–bipyramidal coordination geometry.

In the title compound, [Cd 2 (C 6 H 9 N 3 O 2 ) 2 Cl 6 ], the coordination polyhedra around the Cd II cations are distorted trigonal bipyramids. Two of the chloride ions (one axial and one equatorial) are bridging to the other metal atom, leading to a CdÁ Á ÁCd separation of 3.9162 (4) Å . The O atom of the l-histidinium cation lies in an axial site. In the crystal, numerous N-HÁ Á ÁCl, N-HÁ Á ÁO, C-HÁ Á ÁO and C-HÁ Á ÁCl hydrogen bonds link the molecules into a three-dimensional network. Theoretical calculations and spectroscopic data are available as supporting information.

Chemical context
As a natural amino acid, l-histidine occurs in all organisms. It is a metal chelator in plants accumulating nickel from the soil (Krä mer et al., 1996) and a part of the copper-transport system in human blood (Deschamps et al., 2005). Considerable efforts have been made to combine amino acids with organic and inorganic matrices to produce materials having a noncentrosymmetric cell, large polarizabilities and a strong nonlinear optical coefficient (Ben Ahmed et al., 2008). As a chelating ligand, l-histidine provides up to three potential binding sites, as has been shown in complexes with nickel(II) (Sakurai et al., 1978), chromium(III) (Pennington et al., 1984), cobalt(III) (Herak et al., 1981), molybdenum(V) (Wu et al., 2005), vanadium(IV) (Islam et al., 2007) and copper(II) (Deschamps et al., 2005). In this work, we report the synthesis and structure of the title cadmium complex with l-histidine, (I). Cadmium is structurally interesting as it exhibits a number of coordination numbers and geometries such as those in [CdCl 4 ] (Boufas et al., 2009), [Cd 3 Cl 11 ] (Kurawa et al., 2008), [CdCl 6 ] n (Jarboui et al., 2011) and [CdCl 4 ] n (Loseva et al., 2010).

Structural commentary
The molecular structure of the title compound is shown in Fig. 1 In the histidinium cation, the -amino and imidazole groups are protonated and positively charged, while the carboxyl group is deprotonated and negatively charged, which confirms that cations occur in their zwitterionic forms and carry a net positive charge. As expected, the imidazol rings are almost planar with r.m.s deviations for the non-H atoms of 0.003 Å in each molecule. The imidazol group is trans to the carboxyl group and gauche to the amino N atom.
The conformation of the histidine side chain can be described by the two torsion angles, 1 and 21 (IUPAC-IUB Commission on Biochemical Nomenclature, 1970). Angle 1 , which defines the disposition of the side chain with respect to the main chain, can take values in the neighbourhood of À60, +60 or 180 , corresponding to the open conformation I (g À ), closed conformation (g + ) and open conformation II (t), respectively (Krause et al., 1991). The 21 values lie near À90 or +90 but the angle often deviates from these ideal values, as a result of interactions between the imidazole ring and other groups in the structure. In the title compound, the following values are seen: 1 = À52.9 (6); 1 0 = À52.3 (5); 21 = À72.2 (7); 21 0 = À82.5 (7) . Hence, both histidinium cations adopt the sterically favourable open conformation in (I).

Supramolecular features
The extended structure of (I) is consolidated by a number of hydrogen-bonding (N-HÁ Á ÁCl and N-HÁ Á ÁO) interactions ( Table 1). The chloride anions and oxygen atoms play an important role in accepting hydrogen bonds from the amine N atom and the N atoms of the imidazolium ring. These interactions, together with weak C-HÁ Á ÁCl and C-HÁ Á ÁO interactions, generate a three-dimensional network (Fig. 2).

Database survey
A search of the Cambridge Structural Database (Version 5.38, update May 2017; Groom et al., 2016) revealed that the geometric parameters of the title compound are similar to those found in bis(creatininium) tetrachloridocadmate(II) (Boufas et al., 2009). The imidazol group conformation of the title compound is in contrast to the bent gauche conformation found in the structure of l-HisH + ÁCl À ÁH 2 O (Donohue et al., 1956(Donohue et al., , 1964, but similar to the trans conformation observed in dl-HisH + ÁCl À Á2H 2 O (Steiner, 1996)

Synthesis and crystallization
The title compound was prepared by dissolving 1 mmol (155.16 mg) of l-histidine in 50.0 ml of water with a mixture of CdCl 2 Á2H 2 O (1 mmol) and HCl (8 mmol). The resulting mixture was capped and then heated at 353 K in a water bath for 1 h under continuous stirring and then left to slowly evaporate at room temperature. After two weeks, colourless crystals were obtained, which appear to be indefinitely stable when stored in air. Theoretical calculations and spectroscopic data are available as supporting information.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were located in difference-Fourier maps and subsequently treated as riding Acta Cryst. (2019). E75, 823-825 research communications Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
Packing diagram for (I). Red dashed lines indicate hydrogen bonds.

Figure 1
The asymmetric unit of (I), with displacement ellipsoids drawn at the 50% probability level.

Di-µ-chlorido-bis[dichlorido(L-histidinium-κO)cadmium(II)]
Crystal data Absolute structure: Flack & Bernardinelli (2000) Absolute structure parameter: 0.02 (2) 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq