Crystal structure of bis(N,N,N′,N′-tetramethylguanidinium) tetrachloridocuprate(II)

The crystal structure of bis(tetramethylguanidinium) tetrachloridocuprate(II) contains distorted tetrahedral [CuCl4]2− anions and tetramethylguanidinium cations held together through N—H⋯Cl and C—H⋯Cl hydrogen bonds.


Chemical context
The title compound belongs to the series of hybrid organicinorganic materials of general formula A 2 [MX 4 ] where A is an organic cation, M a divalent transition metal and X a halide. The copper representatives of these families have been extensively studied for their magnetic, dielectric and fluorescent properties in relation to their solid-state structures (Halvorson et al., 1990). Recent studies include examination of polymorphism in relation to electrostatic properties (Awwadi & Haddad, 2012) or thermochroism (Aldrich et al., 2016), sometimes in relation to phase transitions (Kelley et al., 2015).

Structural commentary
The asymmetric unit of (I) contains a complete N,N,N 0 ,N 0tetramethylguanidinium cation and half of a [CuCl 4 ] 2À anion held together by an N-HÁ Á ÁCl hydrogen bond (Fig. 1). In the anion, the Cu-Cl distances range from 2.2396 (4) Å to 2.2557 (4) Å . They are shorter than those usually found in tetrachloridocuprate(II) anions with a square-planar config- ISSN 2056-9890 uration (Guo et al., 2015). The distortion of the flattened tetrachloridocuprate(II) anion in (I) from the ideal tetrahedral configuration can be asserted by the values of the two trans Cl-Cu-Cl angles, 135.62 (3) and 133.31 (3) . These two angles can also be used to calculate the 4 geometry index developed by Yang et al. (2007) for complexes with coordination number four to quantify such a distortion. The 4 parameter is defined as [360 -(+)] / 141 where and are the two largest Cl-Cu-Cl angles. A 4 index value of 1 corresponds to an ideal tetrahedral configuration while a value of 0 is for a perfect square-planar configuration. Here the value obtained (0.65) indicates a 'see-saw' (bisphenoidal) configuration with point group symmetry 2.
In the organic cation, the C-N distances in the central CN 3 unit [1.332 (2), 1.335 (2) and 1.342 (2) Å ] are consistent with a partial double-bond character and a positive charge delocalization, as usually found in structures involving tetramethylguanidinium cations. The central core of the cation has an almost planar-trigonal geometry, as reflected by the values for the three N-C-N angles close to 120 and the r.m.s deviation from the least-squares plane calculated for atoms C1, N1, N2 and N3 that is only 0.0006 Å . The dimethylammonium groups are twisted by 29.38 (16) (C2, C3) and 25.08 (16) (C4, C5) with respect to this plane.

Figure 1
The structures of the molecular entities in (I), drawn with displacement parameters at the 50% probability level. The N-HÁ Á ÁCl hydrogen bond is indicated by a dashed line. [Symmetry code: (i) Àx + 1, y, Àz + 3 2 .]

Database survey
A search in the Cambridge Structural Database (Version 5.37 with two updates; Groom et al., 2016) for isolated tetrachloridocuprate(II) anions without disorder returned 342 hits for a total of 389 fragments. The configurations of these fragments were analysed using the 4 index as described above. Around 60 of these (15%) have a 4 index value less than 0.1, including 29 that have a 4 index of 0 (ideal squareplanar configuration). Only four were found to have a configuration close to the ideal tetrahedral one with a 4 index value larger than 0.9. A large number of fragments (72%) has a geometry index 4 value in the 0.6-0.8 range and feature a bisphenoidal configuration as found for (I). An analysis with the modified version of the 4 index [ 4 0 , as defined by Okuniewski et al. (2015)] gives a similar distribution with only minor variation.

Synthesis and crystallization
Yellowish-green crystals were obtained by mixing in stoichiometric amounts tetramethylguanidine with CuCl 2 Á2H 2 O in ethanol.

Special details
Experimental. X-ray crystallographic data for I were collected from a single crystal sample, which was mounted on a loop fiber. Data were collected using a Bruker Venture diffractometer equipped with a Photon 100 CMOS Detector, a Helios MX optics and a Kappa goniometer. The crystal-to-detector distance was 4.0 cm, and the data collection was carried out in 1024 x 1024 pixel mode. 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.