Crystal structure of the monoclinic phase (phase IV) of bis(tetramethylammonium) tetrachloridocuprate(II)

The crystal structure of the low-temperature monoclinic phase of (Me4N)2[CuCl4] was determined at 120 K. The asymmetric unit consists of a discrete tetrahedral [CuCl4]2− anion and two crystallographically independent tetramethylammonium cations.


Chemical context
The title compound undergoes successive phase transitions at 297, 291 and 263 K (Sugiyama et al., 1980). The room temperature phase (phase I) crystallizes in the orthorhombic space group Pmcm with Z = 4 (Morosin & Lingafelter, 1961;Clay et al., 1975). Three low-temperature phases, named phases II, III and IV in the order of decreasing temperature, show incommensurate, ferroelastic commensurate monoclinic and monoclinic structures, respectively (Sugiyama et al., 1980;Gesi & Iizumi, 1980). We allowed [(CH 3 ) 4 N]Cl, CuCl 2 and thioacetamide to react in ethanol. The expected mixed ligand complex was not crystallized but instead the title compound was obtained accidentally. The crystal structure of phase IV of the title compound was determined at 120 K and is reported herein.

Structural commentary
The asymmetric unit of the title compound consists of a discrete [CuCl 4 ] 2À anion and two crystallographically tetramethylammonium cations (Fig. 1). In the anion, the four Cl atoms are inequivalent with Cu-Cl distances ranging from 2.2313 (15) to 2.2538 (16) Å . The Cl-Cu-Cl angles vary from 98.44 (7) to 133.69 (7) , indicating a distorted tetrahedral geometry around the Cu II atom. Using Houser's 4 metric [ 4 = 360 À ( + )/141], where and are the largest angles about the metal atom (Yang et al., 2007), we obtain a value of 0.658 for phase IV and 0.792 for the orthorhombic phase I. This indicates a greater deviation from an ideal tetrahedron in phase IV compared with phase I, tending towards a 'see-saw' geometry.

Supramolecular features
In the crystal, the cations and the anions are linked via weak C-HÁ Á ÁCl hydrogen bonds (Table 1 and Fig. 2), forming a three-dimensional network.

Figure 2
A packing diagram of the title compound, viewed along the a axis, showing the C-HÁ Á ÁCl hydrogen bonds (blue dashed lines).

Bis(tetramethylammonium) tetrachloridocuprate(II)
Crystal data (C 4  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 1.16 e Å −3 Δρ min = −1.03 e Å −3 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.