Crystal structure of an unknown solvate of bis(tetra-n-butylammonium) [N,N′-(4-trifluoromethyl-1,2-phenylene)bis(oxamato)-κ4 O,N,N′,O′]nickelate(II)

In the title compound, the Ni2+ cation is coordinated by two deprotonated amido N atoms and two carboxylate O atoms, setting up a square-planar coordination environment. The cations and the anion are linked by weak intra- and intermolecular C—H⋯O and C—H⋯F hydrogen bonds.


Structural commentary
The asymmetric unit of compound (I) contains one [N(n-Bu) 4 ] + cation and half of the complex anion [Ni(topbo)] 2-ISSN 2056-9890 ( Fig. 1). The anion possesses point-group symmetry 2. This imposes orientational disorder of the CF 3 group, which lies on both sides of the twofold rotation axis with 0.5 occupancy. The anion is essentially planar (root-mean-square deviation 0.145 Å ), the highest deviation from planarity being observed for C6 [0.440 (5) Å ]. The Ni 2+ cation is coordinated by two deprotonated amido N atoms and two carboxylate O atoms, resulting in a slightly distorted square-planar coordination geometry. In agreement with related nickel compounds, the Ni-N bonds are significantly shorter than the Ni-O bonds, which is due to the stronger donicity of the amido nitrogens (Fettouhi et al., 1996;Rü ffer et al., 2007aRü ffer et al., ,b, 2008Abdulmalic et al., 2013;Milek et al., 2013). Compared to the respective nickel complex without the CF 3 group (Abdulmalic et al., 2013), compound (I) exhibits longer Ni-N and Ni-O bonds. It is instructive to note that for other complexes, the presence of electron-withdrawing substituents at the benzene moiety, e.g. Cl, NO 2 , causes a shortening of the Ni-N and Ni-O bonds (Fettouhi et al., 1996;Rü ffer et al., 2008).

Synthesis and crystallization
4-Trifluoromethyl-1,2-phenylenebis(ethyl oxamate) was prepared from ethyl oxalyl chloride and 4-trifluoromethyl-1,2phenylenediamine in analogy to Cervera et al. (1998). To a solution of 4-trifluoromethyl-1,2-phenylenediamine (0.4 g, 2.22 mmol) dissolved in tetrahydrofuran (50 ml) was added dropwise via a dropping funnel a solution of ethyl oxalyl chloride (5.05 g, 4.45 mmol) in tetrahydrofuran (25 ml) within 20 min. The resulting mixture was refluxed for 30 min at 343 K, filtrated and concentrated to about one third on a rotary evaporator. The careful addition of water resulted in the precipitation of a brown solid which was filtered off and dried in air.
To a solution of 4-trifluoromethyl-1,2-phenylenebis(ethyl oxamate) (0.4 g, 1.06 mmol) in ethanol (40 ml) was added dropwise under stirring [N(n-Bu) 4 ]OH (2.76 g, 4.25 mmol, 40 wt-% aqueous solution) in water (20 ml); the resulting mixture was refluxed for 30 min. After cooling to room temperature, an aqueous solution (20 ml) of NiCl 2 Á6H 2 O (0.25 g, 1.05 mmol) was added dropwise under stirring. The yellow solution was filtered, concentrated to a volume of 20 ml on a rotatory evaporator, and extracted with dichloromethane (100 ml). The organic layer was separated, washed with water (3 x 25 ml) dried over Na 2 SO 4 and concentrated to a volume of 10 ml. The title compound was precipitated by adding Et 2 O (100 ml). The yellow solid was filtered off, washed with Et 2 O and dried in air. Single crystals were obtained by the slow diffusion of Et 2 O into a saturated solution of the title compound in CH 2 Cl 2 /thf (1:1). The molecular components of (I) drawn with displacement ellipsoids at the 50% probability level. H atoms were omitted for clarity. Only one disordered part of the -CF 3 group is shown. [Symmetry code: (A) Àx + 2, y, Àz + 3 2 .]   Table 1 Hydrogen-bond geometry (Å , ).
The overall synthetic procedure is schematically shown in Fig. 3.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. C-bonded H atoms were placed in calculated positions and constrained to ride on their parent atoms, with U iso (H) = 1.2U eq (C) and a C-H distance of 0.93 Å for aromatic and 0.97 Å for methylene protons as well as U iso (H) = 1.5U eq (C) and a C-H distance of 0.96 Å for methyl protons.
A small region of electron density at a distance of 1.6-3.7 Å from the trifluoromethyl group indicates the presence of a disordered solvent molecule. All attempts to model a disordered tetrahydrofuran, dichloromethane or diethyl ether molecule (solvents used for crystallization) failed. Therefore, the solvent contributions have been removed using the SQUEEZE procedure in PLATON (Spek, 2015). SQUEEZE calculated a void volume of approximately 310 Å 3 occupied by 24 electrons per unit cell. Scheme representing the synthesis of compound (I).