(μ2-Adipato-κ4 O,O′:O′′,O′′′)bis[aqua(benzene-1,2-diamine-κ2 N,N′)chloridocadmium]: crystal structure and Hirshfeld surface analysis

In the centrosymmetric binuclear title compound, the CdII atoms are linked by a μ2-adipate dianion; the distorted octahedral geometry of the metal ion is defined by a ClN2O3 donor set.


Chemical context
In the +II oxidation state, the 4d 10 cadmium(II) cation is a favourite of researchers studying coordination polymers/ metal-organic frameworks. With the ability to readily coordinate a variety of different donor atoms, i.e. both hard and soft donors, and to adopt a range of coordination geometries, a diverse array of structures can be generated. The motivation for studying cadmium(II) compounds in this context, over and above intellectual curiosity, rests primarily with evaluating their photoluminescence properties (Lestari et al., 2014;Xue et al., 2015;Seco et al., 2017).

Structural commentary
The asymmetric unit of (I) comprises half a molecule of (I), Fig. 1, with the full molecule generated about a centre of inversion. The key feature of the structure is the tetra-coordinate mode of coordination of the adipato dianion, linking the two Cd II cations. Each carboxylate group forms equivalent Cd-O bonds, the difference in the two bonds being only 0.01 Å , Table 1. More asymmetry is found in the coordination of the benzene-1,2-diamine ligand with the Cd-N1 bond length being 0.05 Å longer than Cd-N2. This may be traced to the different trans effects exerted by the oxygen atoms in that the N1 atom is trans to the carboxylate-O1 atom [N1-  ] whereas N2 is opposite to the coordinating water molecule [N2-Cd-O1W = 149.12 (7) ]. The coordination geometry is completed by the chloride anion which, owing to the presence of two chelating ligands, occupies a position cis to the aqua group. The donor set is ClN 2 O 3 and defines a distorted octahedral geometry.
As might be expected, the four-membered chelate ring formed by the carboxylate group is strictly planar (r.m.s. deviation = 0.0009 Å ). There is a twist in the chain of the dicarboxylate ligand with the bond linking the quaternary atom to the aliphatic group being + anti-clinal, i.e. the O2-C1-C2-C3 torsion angle is 145.7 (3) but, -anti-periplanar about the central bond, i.e. C1-C2-C3-C3 i is À177.6 (3) ; symmetry code: (i) Àx, 2 À y, Àz. There is a distinct kink in the five-membered ring formed by the benzene-1,2-diamine ligand. This is readily seen in the dihedral angle of 58.57 (7) formed between the plane through the CdN 2 atoms and the benzene ring.

Supramolecular features
As summarized in Table 2 The molecular structure of (I), showing the atom-labelling scheme and displacement ellipsoids at the 70% probability level. The molecule is disposed about a centre of inversion and unlabelled atoms are related by the symmetry operation (Àx, 2 À y, À z).  Symmetry codes: (i) x; y À 1; z; (ii) x; Ày À 1 2 ; z À 1 2 ; (iii) x; Ày À 1 2 ; z À 3 2 ; (iv) x; Ày þ 1 2 ; z À 3 2 . propagating along the b-axis direction, involving the carboxylate-O1 atoms, and along the c-axis direction, involving the carboxylate-O2 atoms. Thereby, a supramolecular layer is formed parallel to (100) Given this observation, it was thought worthwhile to perform a Hirshfeld surface analysis to probe the molecular packing in more detail. The results of this analysis are discussed in the next section.

Hirshfeld surface analysis
The Hirshfeld surfaces calculated for (I) provide further insight into the supramolecular associations in the crystal; the calculations were performed according to a recent publication (Jotani et al., 2017). The presence of bright-red spots appearing near water-H atoms, H1W and H2W, and carboxylate oxygen atoms, O1 and O2, on the Hirshfeld surface mapped over d norm in Fig. 3, result from the O-HÁ Á ÁO hydrogen bonds between these atoms, A view of the Hirshfeld surface for (I) mapped over d norm in the range À0.597 to +1. 425 au.

Figure 4
A view of the Hirshfeld surface for (I) mapped over the electrostatic potential in the range À0.164 to +0.204 a.u. The red and blue regions represent negative and positive electrostatic potentials, respectively. atoms on the Hirshfeld surface mapped over the calculated electrostatic potential in Fig. 4. The immediate environment about a reference molecule within the shape-index mapped Hirshfeld surface highlighting intermolecular O-HÁ Á ÁO, N-HÁ Á ÁCl interactions and short interatomic HÁ Á ÁH contacts is illustrated in Fig. 5. The overall two-dimensional fingerprint plot, Fig. 6a, and those delineated into HÁ Á ÁH, OÁ Á ÁH/HÁ Á ÁO,ClÁ Á ÁH/HÁ Á ÁCl and CÁ Á ÁH/HÁ Á ÁC contacts (McKinnon et al., 2007) are illustrated in Fig. 6b-e, respectively. The significant contributions from interatomic OÁ Á ÁH/HÁ Á ÁO and ClÁ Á ÁH/HÁ Á ÁCl contacts to the Hirshfeld surfaces, see data in Table 3, result from the involvement of water, diamine, chloride and carboxylate residues in the intermolecular interactions. The relatively high contribution from these atoms decreases the relative importance of interatomic HÁ Á ÁH contacts, i.e. to 45.4%, to the Hirshfeld surface. The presence of a short interatomic HÁ Á ÁH contact between water-H1W and methyl-H3A, Table 4, also has an influence upon the molecular packing as shown in Fig. 5. In the fingerprint plot delineated into HÁ Á ÁH contacts, Fig. 6b, this is viewed as the distribution of points at d e + d i < sum of their van der Waals radii, i.e. 2.40 Å . Another short interatomic HÁ Á ÁH contact listed in Table 4, involving benzene-H8 atoms lying at the surfaces of the layers stacked along the a axis appear to have little impact upon the packing. The intermolecular O-HÁ Á ÁO and N-HÁ Á ÁCl hydrogen bonding are recognized as the pair of spikes at d e + d i $ 1.8 and 2.5 Å , respectively, together with green points within the distributions in Fig. 6c and d, respectively. The points related to short inter-atomic OÁ Á ÁH contact between water-O1W and methyl-H3A mentioned above are merged in the plot, Fig. 6c. It can be seen from the fingerprint plot delineated into CÁ Á ÁH/HÁ Á ÁC contacts, Fig. 6e, that although these contacts make a significant contribution of 11.2% to the dumbbell-shaped Hirshfeld surface due to the presence of benzene-C atoms, the molecular packing results in inter-atomic CÁ Á ÁH/HÁ Á ÁC separations longer than van der Waals contact distances, hence they exert a negligible effect in the crystal. The low contribution from other contacts listed in Table 3 have little effect in the structure due to their large inter-atomic separations.

Database survey
A search of the crystallographic literature (Groom et al., 2016) was undertaken in order to find closely related structures to (I). Reflecting the interest in these structures, there were nearly 50 examples with the adipato dianion. In each case, the dianion bridged two Cd II cations via chelating interactions in all but one example. Often, the dicarboxylate ligand also bridged other Cd II cations, i.e. was found to be coordinating in 3 -and 4 -modes. The most closely related structure in the literature is illustrated in Scheme 2, i.e. (II) (Che et al., 2013).
The coordination geometry for one of the independent Cd II atoms in (II), being defined by two carboxylate-O atoms, derived from a tri-anionic 2 -benzene-1,3,5-tricarboxylato ligand, two nitrogen atoms from a chelating imidazo[4,5-f]-  Table 4 Summary of short inter-atomic contacts (Å ) in (I).

Synthesis and crystallization
Benzene-1,2-diamine (0.4324 g, 4 mmol) was slowly added to an aqueous solution (15 ml) of CdCl 2 Á2H 2 O (0.4026 g, 2 mmol) resulting in a yellow solution. The mixture was stirred for about 1 h when adipic acid (0.2923 g, 2 mmol) in MeOH (10 ml) was added. The mixture then was stirred for a further 3 h. The resultant solution was reduced and left for crystallization. Brown crystals of (I) were obtained after a few weeks and analysed directly.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 5. The carbon-bound H-atoms were placed in calculated positions (C-H = 0.95-0.99 Å ) and were included in the refinement in the riding model approximation, with U iso (H) set to 1.2U eq (C). The O-bound and N-bound Hatoms were located in difference-Fourier maps but were refined with distance restraints of O-H = 0.84AE0.01 Å and N-H = 0.88AE0.01 Å , and with U iso (H) set to 1.5U eq (O) and 1.2U eq (N). The maximum and minimum residual electron density peaks of 1.15 and 0.69 e Å À3 , respectively, were located 0.90 and 0.87 Å from the Cd II cation.

(µ 2 -Adipato-κ 4 O,O′:O′′,O′′′)bis[aqua(benzene-1,2-diamine-κ 2 N,N′)chloridocadmium]
Crystal data [Cd 2 Cl 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.