Tetrakis(acetonitrile)copper(I) hydrogen oxalate–oxalic acid–acetonitrile (1/0.5/0.5)

In the title compound, [Cu(CH3CN)4](C2HO4)·0.5C2H2O4·0.5CH3CN, the CuI ion is coordinated by the N atoms of four acetonitrile ligands in a slightly distorted tetrahedral environment. The oxalic acid molecule lies across an inversion center. The acetonitrile solvent molecule is disordered across an inversion center and was refined with half occupancy. In the crystal, the hydrogen oxalate anions and oxalic acid molecules are linked via O—H⋯O hydrogen bonds, forming chains along [010].

In the title compound, [Cu(CH 3 CN) 4 ](C 2 HO 4 )Á0.5C 2 H 2 O 4 Á-0.5CH 3 CN, the Cu I ion is coordinated by the N atoms of four acetonitrile ligands in a slightly distorted tetrahedral environment. The oxalic acid molecule lies across an inversion center. The acetonitrile solvent molecule is disordered across an inversion center and was refined with half occupancy. In the crystal, the hydrogen oxalate anions and oxalic acid molecules are linked via O-HÁ Á ÁO hydrogen bonds, forming chains along [010].

Comment
The tetrakis(acetonitrile)copper(I) ion, an important starting material for the synthesis of copper(I) complexes, was first synthesized as the nitrate salt (Morgan, 1923). Commonly available and easily synthesized compounds containing the [Cu(CH 3 CN) 4 ] + cation generally do contain weakly coordinating anions such as BF 4 - (Heckel, 1966) or PF 6 - (Kubas et al., 1979). The structure of tetrakis(acetonitrile)copper(I) tetrafluoroborate already appears in the literature (Jones & Crespo, 1998).
In this work, we report the synthesis and crystal structure of a compound containing the [Cu(CH 3 CN) 4 ] + cation containing a potentially coordinating HC 2 O 4anion, which does not coordinate to the metal center. This is in keeping with the known affinity of nitrile ligands for Cu I (Cotton et al., 1999), and also with the hard-soft acid-base theory (Pearson, 1968), which predicts a weak interaction between the "hard" hydrogen oxalate ligand and the "soft" Cu + ion.
In the title compound, the [Cu(CH 3 CN) 4 ] + cation adopts a slightly distorted tetrahedral geometry. The HC 2 O 4anion is not coordinated to the Cu I center and the distances of the nearest O atoms to the Cu I ion are all greater than 4.7 Å. The anion is non-planar, whereas the oxalic acid molecules are strictly planar, and reside on inversion centers. All the OH groups (in the hydrogen oxalate anion and in oxalic acid) in this structure are involved in intermolecular hydrogen bonding interactions with the carboxylate O atoms of the hydrogen oxalate anion, forming one-dimensional chains along [010]. The acetonitrile solvent molecules present in the structure are disordered, and positioned in linear channels between the [Cu(CH 3 CN) 4 ] + cations, parallel to the b axis.

Experimental
All manipulations were carried out under nitrogen. In a 10 ml round-bottom flask, 180 mg anhydrous oxalic acid (2 mmol), 0.143 g copper(I) oxide (1 mmol) and 7 ml degassed, dry acetonitrile were stirred together. All the red Cu 2 O powder dissolved in 2 min., forming a white precipitate and a clear, pale blue supernatant. After 15 min. of stirring, a copious amount of white and dark purple solids settled to the bottom of the flask. The dark purple solid was likely copper metal powder. This reaction mixture was stirred for 1 hr., then heated at 313K for 15 min., during which the white solid redissolved. Cooling to room temperature produced mm-sized, colorless, air-and moisture-sensitive, platelike crystals in 2 hrs.

Refinement
H atoms were placed in calculated positions with C-H = 0.96 Å and included in the refinement in a riding-motion approximation with U iso (H) = 1.5U eq (C). H atoms bonded to O atoms were refined independently with U iso (H) = 1.5U eq (O).

Figure 1
The molecular structure of the title compound with displacement ellipsoids shown at the 50% probability level. The symmetry complete oxalic acid molecule is shown and the acetonitrile solvent molecule is half occupancy. The dotted line indicates a hydrogen bond.

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.