μ-Oxalato-bis[bis(triphenylphosphine)copper(I)] dichloromethane disolvate1

The dinuclear molecule of the title compound, [Cu2(C2O4)(C18H15P)4]·2CH2Cl2, lies across an inversion center with a strictly planar bridging oxalate ligand coordinating two CuI ions via two pairs of O atoms. Two triphenylphosphine ligands also coordinate each symmetry-related CuI ion, resulting in a distorted tetrahedral geometry [O—Cu—O = 80.57 (5)° and P—Cu—P = 125.72 (2)°]. In the crystal, there are two dichloromethane solvent molecules for each dinuclear complex.

The dinuclear molecule of the title compound, [Cu 2 (C 2 O 4 )(C 18 H 15 P) 4 ]Á2CH 2 Cl 2 , lies across an inversion center with a strictly planar bridging oxalate ligand coordinating two Cu I ions via two pairs of O atoms. Two triphenylphosphine ligands also coordinate each symmetry-related Cu I ion, resulting in a distorted tetrahedral geometry  and P-Cu-P = 125.72 (2) ]. In the crystal, there are two dichloromethane solvent molecules for each dinuclear complex.  Nevertheless, the latter compounds have important applications in, e.g., CO capture (Doyle, 1982), CVD of metallic copper (Köhler et al., 2003;Köhler & Meyer, 2004) and CO 2 fixation (Angamuthu et al., 2010). Very few oxalato complexes of copper(I) have been structurally characterized, and those that have been studied crystallographically are organometallic species containing ligands bound to the copper(I) centers via carbon atoms (Köhler et al., 2003;Teichgräber et al., 2005). To date, no examples of copper(I) oxalate compounds containing triphenylphosphine ligands coordinated through the phosphorus atoms to the metal centers have been structurally characterized.

Related literature
The molecular structure of the title compound is shown in Fig. 1. The dinuclear complex lies across an inversion center.
In addition, the asymmetric unit contains a dichloromethane solvent. The Cu I ions are bridged by a strictly planar oxalate ligand, with two oxygen atoms coordinated to each Cu I ion. The coordination geometry at each Cu I ion is distorted tetrahedral. The bite angle involving the oxalate ligand is fairly small (80.57 (5)°), while the two phosphorus atoms from the coordinated triphenylphosphine ligands form an angle of 125.72 (2)° with each symmetry-related Cu I ion. A similar geometry is observed in the copper(I) oxalate isonitrile complexes studied previously (Teichgräber et al., 2005).

Experimental
All manipulations were carried out on a Schlenk line under nitrogen unless otherwise mentioned. Initially, bis(tetrabutylammonium) oxalate was synthesized in situ by dissolving 1 ml of 1 M tetrabutylammonium hydroxide solution (in methanol; 1 mmol) and 0.045 g anhydrous oxalic acid (0.5 mmol) in 20 ml degassed absolute ethanol. Next, 0.526 g triphenylphosphine (2 mmol) were dissolved in this solution. Separately, 0.373 g tetrakis(acetonitrile)copper(I) hexafluorophosphate (1 mmol) were dissolved in 20 ml degassed absolute ethanol to form a cloudy solution, which was added to the oxalate solution. The product was formed by the metathesis reaction as a white precipitate, washed with 3 x 5 ml ice-cold degassed absolute ethanol, dried under a nitrogen stream and finally air-dried. Colorless block crystals were grown at room temperature from dichloromethane by layering with hexane under nitrogen. The title compound has also been prepared by Díez et al. (1988) by an alternate method.

Refinement
H atoms were placed in calculated positions with C-H = 0.95Å (phenyl) or C-H = 0.96Å (solvent CH 2 ) and included in the refinement in a riding-motion approximation with U iso (H) = 1.2U eq (C).

Figure 1
The molecular structure of the title compound showing displacement ellipsoids at the 30% probability level. Neither the H atoms nor the solvent molecules are shown (Symmetry code: (a) -x, -y+1, -z+1).

µ-Oxalato-bis[bis(triphenylphosphine)copper(I)] dichloromethane disolvate
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.