Crystal structure of meso-di-μ-chlorido-bis[bis(2,2′-bipyridine)cadmium] bis(1,1,3,3-tetracyano-2-ethoxypropenide) 0.81-hydrate

In the title compound, which was prepared using a hydrothermal reaction between 2,2′-bipyridine, cadmium(II) chloride and potassium 1,1,3,3-tetracyano-2-ethoxypropenide, the complex cations are linked into sheets by C—H⋯Cl hydrogen bonds.

The hydrated title salt, [Cd 2 Cl 2 (C 10 H 8 N 2 ) 4 ](C 9 H 5 N 4 O) 2 Á0.81H 2 O, was obtained from the hydrothermal reaction between 2,2 0 -bipyridine, cadmium(II) chloride and potassium 1,1,3,3-tetracyano-2-ethoxypropenide. The binuclear cation lies across a centre of inversion in the space group P2 1 /c, with the other components in general positions. The cation has approximate, but non-crystallographic 2/m symmetry and each of the Cd II atoms is a stereogenic centre, one having the Á configuration and the other the Ã configuration. In the anion, one of the C(CN) 2 units is disordered over two sets of atomic sites having occupancies 0.75 (2) and 0.25 (2). The cations are linked by two independent C-HÁ Á ÁCl hydrogen bonds to form a sheet of R 2 2 (14) and R 4 2 (24) rings.

Chemical context
Luminescent materials based on transition metals and lanthanoids have found wide applications in lighting (Pust et al., 2014), luminescence sensing (Liu et al., 2015) and optical devices (Torres et al., 2015). Among them, d 10 metal complexes comprising zinc(II) and cadmium(II) with a variety of ligands have attracted considerable attention in recent years because of their luminescence properties (Mautner et al., 2015). Organic polynitrile ligands are versatile structural components, leading to many different architectures in zero, one, two or three dimensions, and incorporating most of the 3d transition metals (Miyazaki et al., 2003;Yuste et al., 2009;Benmansour et al., 2010;Gaamoune et al., 2010;Setifi et al., 2013;Addala et al., 2015). The versatility of such ligands is based on two main properties: firstly, the ability to act as bridges, given the linear and rigid geometry of the cyano groups, and secondly, the possibility of combining these ligands with a wide variety of co-ligands, leading to an extensive variety of coordination modes. To take advantage of this behaviour, we have been using polynitrile anions in combination with other chelating or bridging neutral coligands to explore the structural and electronic characteristics of the resulting complexes, particularly with reference to molecular materials exhibiting interesting luminescent behaviour.

Figure 2
The structure of the anion in compound (I), with displacement ellipsoids drawn at the 30% probability level. Atomic sites C51 and C61 were constrained to be identical and the major and minor components of the disordered C(CN) 2 ) unit are drawn with full and dashed lines, respectively.
One of the C(CN) 2 groups in the tcnoet À anion is disordered over two sets of atomic sites, with occupancies 0.75 (2) and 0.25 (2), which are related by a mutual rotation about the C-C bond to atom C52 (Fig. 2). The dihedral angles between the central plane (C51,C52,C53) and the major and minor components of the disordered C(CN) 2 unit are 20.3 (6) and 31.6 (15) , respectively, while the dihedral angle between the central plane and the ordered C(CN) 2 unit is 17.1 (6) , such that the rotations of two C(CN) 2 units out of the central plane are in a conrotatory sense. The dihedral angle between the planes of the major and minor disorder forms is 12.4 (17) . The C-N distances in the anion are all very similar, as are the corresponding values for the two types of C-C distances in the tetracyanopropenide portion, with their magnitudes pointing to extensive delocalization of the negative charge not only over the propenide unit but also into the cyano groups, as previously discussed (Setifi et al., 2016).
The anions are linked to this sheet by C-HÁ Á ÁN hydrogen bonds, but otherwise play no part in the supramolecular assembly.

Figure 3
Part of the crystal structure of compound (I), showing the formation of a sheet of cations parallel to (100) built from two C-HÁ Á ÁCl hydrogen bonds and containing R 2 2 (14) and R 2 4 (24) ring motifs. For the sake of clarity, the anions and water molecules, and those H atoms of the cation which are not involved in the motifs shown have been omitted. already described (Fig. 3), while the other two would combine to link these sheets into a three-dimensional framework structure.

Database survey
The structure of the tcnoet À unit has been reported in salt-like compounds, both with organic cations (Setifi, Lehchili et al., 2014;Setifi et al., 2016) and with cationic metal coordination complexes (Gaamoune et al., 2010;Setifi et al., 2013), and as a coordinating ligand. Examples have been reported recently in which the tcnoet À unit acts as both a bridging and a terminal ligand with Cu II , leading to the formation of a coordination polymer in the form of a ribbon (Addala et al., 2015), and where it acts as a 3 -bridging ligand, also with Cu II , leading to the formation of a coordination polymer sheet .
The structure of the dicadmium cation present in compound (I) appears not to have been reported previously. However, in the analogous cation [( 2 -Cl) 2 (en 2 Cd) 2 ] 2+ , characterized as its chloride salt (Nä ther & Jess, 2010), the cation again lies across a centre of inversion, here in space group P2 1 /n, with a geometry at Cd very similar to that in compound (I). The related cation [( 2 -Cl) 2 (phen 2 Cd) 2 ] 2+ has been characterized in two polytungstate salts, one of them as a 4,4 0 -bipyridine solvate. In the unsolvated salt, the cation lies across a twofold rotation axis in C2/c (Wang et al., 2011); by contrast, in the solvated salt (Wang et al., 2012), the cation is almost centrosymmetric, although examination of the atomic coordinates using PLATON (Spek, 2009) suggests that the space group may be P1 rather than the reported P1 (cf. Marsh, 1999Marsh, , 2005Marsh, , 2009). Finally, we note some neutral dicadmium complexes of type ( 2 -Cl) 2 (ClCdL) 2 , where L represents a tridentate aliphatic amine ligand, which have molecular architectures similar to that in the cation of compound (I): when L represents 2-aminoethyl-3-aminopropyl amine (Gannas et al., 1980) or cis-3,5-diaminopiperidine (Pauly et al., 2000), the complexes lie across inversion centres in space group types P2 1 /n and P2 1 /c, respectively, but when L represents bis(3-aminopropyl)amine (Gannas et al., 1980), the complex lies across a twofold rotation axis in C2/c.

Synthesis and crystallization
The salt K(tcnoet) was prepared using the published method (Middleton et al., 1958). The title compound was synthesized hydrothermally under autogenous pressure from a mixture of cadmium(II) chloride (40 mg, 0.21 mmol), 2,2 0 -bipyridine (32 mg, 0.21 mmol) and K(tcnoet) (90 mg, 0.40 mmol) in water-methanol (4:1 v/v, 20 cm 3 ). This mixture was sealed in a Teflon-lined autoclave and held at 423 K for 2 d, and then cooled to ambient temperature at a rate of 10 K h À1 (yield 47%). Colourless prisms of the title compound suitable for single-crystal X-ray diffraction were selected directly from the synthesized product.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Three low-angle reflections, (100), (011) and (102), which had been attenuated by the beam stop, were omitted from the refinement. The H atoms bonded to C atoms were located in difference maps and then treated as riding atoms in geometrically idealized positions with C-H distances of 0.93 Å (pyridine), 0.96 Å (CH 3 ) or 0.97 Å (CH 2 ) and with U iso (H) = kU eq (C) where k = 1.5 for the methyl group, which was permitted to rotate but not to tilt, and 1.2 for all other H atoms bonded to C atoms. It was apparent from an early stage that the cyano groups in one of the C(CN) 2 units of the anion, that containing atom C51, are disordered over two sets of atomic sites having unequal occupancies. For the minor disorder form, the bond lengths and the 1,3 non-bonding contacts were restrained to be the same as the corresponding distances in the major form, subject to s.u. values of 0.005 and 0.01 Å , respectively. In addition, the anisotropic displacement parameters for pairs of partial-occupancy atoms occupying essentially the same physical space were constrained to be identical. Subject to these conditions, the occupancies of the major and minor disorder forms refined to 0.75 (2) and 0.25 (2). For the partial-occupancy water molecule, the atomic coordinates of the O atom were refined with U iso (O) fixed at 0.08 Å 2 , giving a refined occupancy of 0.403 (6). A difference map provided plausible locations for two H atoms associated with this O atom but neither of these sites was within  hydrogen-bonding range of any likely acceptor and hence they were probably just artefacts of the isotropic refinement. In the final analysis of variance, there was a negative value, À0.835, of K = mean(F o 2 )/mean(F c 2 ) for the group of 1177 very weak reflections having F c /F c (max) in the range 0.000 < F c /F c (max) < 0.006.  (1,1,3,3-tetracyano-2-ethoxypropenide) (Bruker, 2009); cell refinement: APEX2 and SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure:

Crystal structure of meso-di-µ-chlorido-bis[bis(2,2′-bipyridine)cadmium] bis-
SHELXL2014 (Sheldrick, 2015); molecular graphics: PLATON (Spek, 2009); software used to prepare material for publication: SHELXL2014 and PLATON (Spek, 2009). (1,1,3,3-tetracyano-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.