Crystal structure of bis(1,1,2,2-tetramethyldiphosphane-1,2-dithione-κ2 S,S′)copper(I) tetrafluoridoborate

In the title salt, [Cu(C4H12P2S2)2]BF4, both diphosphine disulfide molecules bind to the CuI atom as chelating ligands via the S atoms, forming a monovalent cation with a slightly distorted tetrahedral coordination around the CuI atom. The ligand chelate rings are twisted in opposite directions, with one in the λ and one in the δ configuration. In the crystal, possible C—H⋯F hydrogen bonds may stabilize the orientation.


Chemical context
The title compound was one of a number of phosphine sulfide copper complexes synthesized by Devon Meek and his group (Meek & Nicpon, 1965). Early reports by Meek and coworkers and by Cotton et al. (1974a) on coordination complexes of diphosphinedisulfide ligands indicated the chelating mode for these ligands to metals such as Cu I as only one of several bonding possibilities, particularly as the chelating model involves rotation about the P-P bond from the trans conformation found in the structure of the free ligands (see, for example, Lee & Goodacre, 1971). Indeed, the tetramethyldiphosphinedisulfide ligand was shown in one case to bridge copper atoms forming a polymeric chain (Cotton et al., 1974b). Our work was initiated to verify the chelating structure that had been predicted for the present compound.
We have reported this structure previously at the 1973 winter meeting of The American Crystallographic Association. The crystal structure of the corresponding hexafluoridophosphate salt has been reported by Liu et al. (2003).

Structural commentary
In this reported structure, both diphosphine disulfide molecules bind to the Cu I atom as chelating ligands via the S atoms, forming a monovalent cation with a slightly distorted tetrahedral coordination around the Cu I (Fig. 1). Liu et al. (2003) have described the structure of the PF 6 À salt of the present cation, as well as that of the corresponding silver salt.
Selected bond lengths and angles are given in Table 1. The average Cu-S distance is 2.350 (15) Å , and distances vary by up to 0.065 Å . The chelate S-Cu-S angles are 105.69 (3) and 106.94 (5) , smaller than the other S-Cu-S angles, which vary from 109.10 (3) to 114.02 (4) and average 111.1 (10) . Ligand P S distances are more constant, with an average of 1.964 (3) Å , and the P-P distances are 2.2262 (13) and 2.2166 (14) Å . The ligand chelate rings are twisted in the and configurations for S 1 P 2 P 3 S 4 and S 5 P 6 P 7 S 8 , respectively, with torsional angles about the P-P bonds of 47.97 (6) and À56.37 (6) . The geometry of the cation, including the slight distortions from regular tetrahedral geometry at the Cu I atom, is very similar to that seen by Liu et al. (2003).
A number of recent structural papers in this journal have postulated that C-H. . . O hydrogen bonds were contributing to packing of organic structures (see, for example: Salas et al., 2011;Corfield et al., 2014). This led us to investigate the possibility that FÁ Á ÁH-C hydrogen bonds were stabilizing the orientation of the BF 4 À ion. We list six putative FÁ Á ÁH-C hydrogen bonds in Table 2, and they are represented in Fig. 2. FÁ Á ÁC distances are all less than 3.5 Å , and FÁ Á ÁH distances range from 2.45 to 2.60 Å , while angles at the H atoms are reasonably close to linear.  The molecular structure of the title compound, with displacement ellipsoids at the 50% level. The dashed line indicates a hydrogen bond.  Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx, Ày + 1, Àz; (ii) Àx + 1, Ày + 1, Àz + 1; (iii) x, y, z + 1.

Figure 2
Packing of the title complex, viewed along a direction close to the b axis, with ellipsoid outlines for the anion at 30% probability. Putative C-HÁ Á Á F hydrogen bonds from four different cations to the BF 4 À anion are shown.
containing the diphosphine disulfide ligand of the present study chelated with any metal, M, found 11 structures whose coordinates were given. Database P-P and P S distances average 2.224 (5) and 1.993 (8) Å , while the M-S-P and S-P-P angles average 102.1 (9) and 106.1 (6) , respectively. In the present compound, the P S distances average 1.965 (2) Å and the average Cu-S-P angle is 98.6 (12) , both close to values for the other copper(I) compound listed, but somewhat less than values for compounds with other metals. The geometry reflects the lack of bonding seen in the copper complexes, as indicated by the small change in P S bond length and P-s vibrational mode upon coordination to copper (Liu et al., 2003). Database torsional angles indicate no preference between and configurations.

Synthesis and crystallization
Details of the synthesis and characterization of a number of phosphine sulfides, including the title compound, are given in Meek & Nicpon (1965).

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 3. Each of 18 standard reflections was measured 18-19 times during the 114 h of data collection. No significant crystal decay was noted; indeed we recorded an overall increase in intensity of 1.6% over the entire data collection. No corrections were made. Data were collected in two shells, = 0-22.5 and = 22.5-35 .
The original data reduction deleted reflections with I < 2(I), and their details are no longer available. Near the end of the final refinements, 2217 missing weak reflections were reinserted into the data file, with F 2 values set equal to (F 2 ) found for reflections with F 2 < 3(F 2 ), averaged over ten ranges of values. The arbitrary assignment of F 2 values for these weak reflections perhaps explains the high K value noted for the weakest reflections in the final refinement, where the F cal 2 values will be near zero. The 6 7 1 reflection was omitted from the final refinements, due to evidence of a transcription error: the chart record clearly indicates a very weak reflection, while the intensity retrieved from our backup storage is very large. Further, the chart record shows that the very strong 1 0 0 reflection was truncated during the scan, and this record was also omitted.
Positions of all non-hydrogen atoms were found by superposition methods. H atoms in the eight methyl groups were constrained to idealized tetrahedral positions with C-H distances of 0.96 Å . The methyl torsional angles were refined. The U eq values for all H atoms were fixed at 1.2 times the U iso of their bonded C atoms.
Initial refinements with anisotropic temperature factors for the heavier atoms and constrained hydrogen atom parameters converged smoothly, to R 1 = 0.0443 for 4223 reflections with F 2 < 2. In case there were systematic anisotropic scaling errors in the data collection that might have affected the detailed electron density around the BF 4 À anion, the intensity data were now smoothed by a 12-parameter model with XABS2 (Parkin et al., 1995). The smoothing lowered R 1 to 0.0399, but had little effect on the electron density or on the atomic parameters: the average / was 0.9; two F atoms moved by 3. We made extensive efforts to develop and refine a disordered model for the BF 4 À anion, in light of the large U ij values for the F atoms, but were unable to find a model with improved U ij and R values. Difference Fourier syntheses phased on the cation parameters always yielded four large peaks corresponding to the current F atom positions; final difference Fourier maps did show several much smaller peaks in the vicinity of the B atom, but no tetrahedral array emerged.

Bis(1,1,2,2-tetramethyldiphosphane-1,2-dithione-κ 2 S,S′)copper(I) tetrafluoridoborate
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 0.41 e Å −3 Δρ min = −0.40 e Å −3 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 R-factors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq  (5)