Tetraphenylarsonium cis-bis[1,2-bis(trifluoromethyl)ethene-1,2-dithiolato]platinate(II)

In the title compound, (C24H20As)[Pt(C4F6S2)2], the cation lies on a twofold rotation axis while the anion has crystallographic inversion symmetry. The PtII ion is in a slightly distorted square-planar coordination environment. The F atoms of both unique –CF3 groups are disordered over two sets of sites, the ratios of refined occupancies being 0.677 (15):0.323 (15) and 0.640 (16):0.360 (16). The crystal structure is the first to date of a monoanionic [Pt(tfd)2]− species [tfd is 1,2-bis(trifluoromethyl)ethene-1,2-dithiolate] with a non-redox-active cation.

In the title compound, (C 24 H 20 As)[Pt(C 4 F 6 S 2 ) 2 ], the cation lies on a twofold rotation axis while the anion has crystallographic inversion symmetry. The Pt II ion is in a slightly distorted square-planar coordination environment. The F atoms of both unique -CF 3 groups are disordered over two sets of sites, the ratios of refined occupancies being 0.677 (15):0.323 (15) and 0.640 (16):0.360 (16). The crystal structure is the first to date of a monoanionic [Pt(tfd) 2 ] À species [tfd is 1,2-bis(trifluoromethyl)ethene-1,2-dithiolate] with a non-redox-active cation.
In the course of our studies on the analogous platinum complexes we have crystallographically characterized [Pt(tfd) 2 ] 0 and [Pt(tfd) 2 ] 2- (Kogut et al., 2006;Tang et al., 2009, respectively). These complexes contain square-planar platinum(II), and the tfd ligand is redox-active. While tfd is not fully reduced in the charge-neutral complex (and is only formally 1,2perfluoromethylethene-1,2-dithiolate), tfd is fully reduced in the dianion. The C-C bond in the chelate ring of the neutral complex shortens upon reduction (Tang et al., 2009).
We report here on the first crystal structure of monoanionic [Pt(tfd) 2 ]with a non-redox-active cation (Fig. 1). A report on a previous structural determination (Kasper & Interrante, 1976) of the charge-transfer complex between tetrathiofulvalene and charge-neutral [Pt(tfd) 2 ] has proposed that [Pt(tfd) 2 ] is reduced by one electron and tetrathiovulvalene is oxidized to tetrathiafulvalinium; however, in order to reliably establish the structural properties of the one-electron-reduced species, use of a non-redox-active cation is preferable.

Experimental
The tetraphenylarsonium salt of [Pt(tfd) 2 ]was synthesized using a slightly modified literature procedure. The literature procedure uses acetone/ethanol as the reductant, followed by precipitation with tetraphenylarsonium chloride, (Davison et al., 1964) to obtain the product in 36% yield. We repeated that literature reaction and obtained 24% yield. We then decided to use water in THF to achieve reduction of [Pt(tfd) 2 ], followed by addition of tetraphenylarsonium chloride. It was previously observed that water in polar solvents such as THF leads to reduction of the related nickel complex Ni[(S 2 C 2 (CF 3 ) 2 ] 2 (Harrison et al., 2006). In a 10 ml vial, 24.9 mg (0.0385 mmol) of [Pt(tfd) 2 ] were dissolved in 1.0 ml ofTHF, leading to supplementary materials sup-2 a dark greenish-blue solution. Addition of 0.3 ml of H 2 O induced a rapid colour change, to produce a brownish-yellow solution. Solid tetraphenylarsonium chloride (25.3 mg, 0.068 mmol) was added to the solution while stirring, followed by an additional 0.25 ml of THF, to produce a homogeneous solution. In order to lower the solubility of the salt again, an additional 0.4 ml of H 2 O were added, and the vial was storred at 278 K for three weeks. Crystals had not formed at that time, and crystallization was induced by scratching of the inner surface of the vial with a glass rod. After three additional weeks at 378 K, [AsPh 4 ] + [Pt(tfd) 2 ]had crystallized as dark yellow blocks. The crystals were removed from the supernatant and dried under vacuum. Yield: 26.6 mg (0.0258 mmol, 67%). Inspections of the crystals with a stereomicroscope showed them to be of excellent quality. One crystal was chosen for the single-crystal X-ray structure determination.  (Sheldrick, 2008) software were used to restrain the parameters of the disordered groups.

Refinement
During the refinement of the structure, electron density peaks were located that were believed to be highly disordered solvent molecules (possibly THF). Attempts made to model the solvent molecule were not successful. The SQUEEZE option in PLATON (Spek, 2009) indicated there was a solvent cavity of volume 130.0 Å 3 containing approximately 18 electrons. In the final cycles of refinement, this contribution to the electron density was removed from the observed data. The density, the F(000) value, the molecular weight and the formula are given without taking into account the results obtained with the SQUEEZE option PLATON (Spek, 2009). Similar treatments of disordered solvent molecules have been carried out in this manner (Stähler et al., 2001;Cox et al., 2003;Mohamed et al., 2003;Athimoolam et al., 2005). Fig. 1. A view of the molecular structure of the title compound. Displacement ellipsoids are drawn at the 30% probabilty level. For the cation and anion, the primed labels are related by the symmetry operators (-x, y, -z + 1/2) and (-x + 1/2, -y + 1/2, -z + 1), respectively.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.