Bis[bis(diphenylphosphinoyl)acetonitrile-κ2 O,O′]copper(II)

The title complex, [Cu(C26H20NO2P2)2], contains a central CuII atom surrounded by two homoleptic bidentate ligands, which form two five-membered chelate rings. The Cu atom binds to four O atoms, resulting in a four-coordinate square-planar complex. The asymmetric unit contains half of the complex, the other half being completed by inversion symmetry. The Cu—O bond lengths have similar distances, viz. 1.9153 (10) Å for the pair opposite (trans) each other and 1.9373 (10) Å for the other (trans) pair. The P—O bond lengths are 1.5250 (11) Å, indicating significant electron delocalization across the O—P—C—P—O atoms in the chelate ring, resulting in a longer P—O bond length when compared to a formal double-bond P=O character (much shorter at approximately 1.47 Å). The two intersecting O—Cu—O angles are both linear at 180°, whilst the remaining L-shaped O—Cu—O bond angles are 88.26 (5) and 91.74 (5)°. The C—C N fragment is slightly distorted from linearity at 177.44 (19)° and the C N bond length of 1.151 (2) Å indicates predominantly triple-bond character.


Comment
The chemistry of the ligand bis(diphenylphosphane)acetonitrile, (dppm-CN), was recently revived when a facile preparation thereof was reported, starting with readily available acetonitrile (Braun et al., 2007). Development of its chemistry followed thereafter, including the observed sharp increase in acidity by the replacement of a proton with a cyano group on the bridging carbon atom (Spannhoff et al., 2009, and references therein). In continuing our interest in dinuclear gold(I) complexes (Van Zyl, 2010), we recently reported the first structural investigation of a chlorogold(I) complex with the ligand dppm-CN, (Sithole et al., 2011).
A relatively straightforward method for the preparation of complex (I) would entail the reaction between Li[N≡C-C(Ph 2 P=O) 2 ], prepared as described (Braun et al., 2008), and CuCl 2 or Cu(NO 3 ) 2 (molar ratio 2:1), leading to formation of complex (I) after removal of LiCl or LiNO 3 . However, in the present study, we report complex (I) was obtained in a surprisingly different manner. Formation of (I) occurred post-synthesis during the crystallization process. An initial reaction between the well characterized complexes [Au 2 {dppm-CN} 2 ], and [Cu(CH 3 CN) 4 ][PF 6 ] (1:1 molar ratio) was performed in an attempt to coordinate the copper(I) center to the CN group. The Experimental section describes a complex formed and isolated from this reaction as a colorless, free-flowing powder and suggests no sign of metal oxidation (both Au(I) and Cu(I) are closed-shell d 10 systems and are usually colorless or pale yellow compounds, hence any sign of oxidation leading to deep color changes for either metal is readily observed). However, during subsequent crystallization procedures obvious oxidation occurred, firstly at the metal center where the precursor colorless Cu(I) species oxidized to a green-colored Cu(II) complex. Secondly, the P atoms in the anionic [dppm-CN]¯ ligand did oxidize to the corresponding oxide analogue (not uncommon in the presence of air or moisture), and the ligand transferred also from the gold(I) center to the much more oxophillic Cu(II) center whilst retaining the negative anionic charge, i.e. it did not become protonated and thus neutral in the process. No attempt was made in this study to determine the concurrent reduced species, but presumably the Au(I) species reduced to Au(0) metal. Single crystal X-ray analysis subsequently revealed the green colored material to be identified as complex (I).

Experimental
Preparation and characterization of complex obtained pre-crystallization: A Schlenk flask equipped with a magnetic stirrer bar was charged with previously prepared [Au 2 {dppm-CN} 2 ] complex (400 mg, 0.33 mmol) dissolved in dichloromethane (5 ml). A dichloromethane (8 ml) solution of [Cu(CH 3 CN) 4 ][PF 6 ] (123 mg, 0.33 mmol) was then added dropwise to the gold-containing solution at room temperature. The solution turned cloudy and was stirred for an hour, after which all dichloromethane solvent was removed under reduced pressure. Dry Et 2 O (2 x 2 ml) was added to wash the product which was further dried in vacuo for 2 hrs. The product was obtained as a free flowing off-white powder. Yield: (387 mg, 0.21 mmol) 65%; Mp: 155-160 o C (decompose); 1 H NMR (400 MHz, CDCl 3 , 298 K) δ H = 7.68-7.53 (m, 40H, Ph); 31 P NMR supplementary materials sup-2 (101 MHz, CDCl 3 , 298 K) δ P = 22.8 (s, 2P). Green single crystals were obtained by slow evaporation of a dichloromethane solution of the complex.

Refinement
All non-hydrogen atoms are refined anisotropically. H atoms were calculated by geometrical methods and refined as a riding model, with C-H distace 0.95 Å and U iso (H) = 1.2U eq (C). The crystal used for the diffraction study showed no decomposition during data collection. Fig. 1. Molecular structure of the title complex. Ellipsoids are drawn at the 50% probability level. Unlabelled atoms are related to the reference ones by the (-x -y, 1-z) symmetry transformation.

Bis[bis(diphenylphosphinoyl)acetonitrile-κ 2 O,O']copper(II)
Crystal data [Cu(C 26 H 20   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.