trans-Carbonylchloridobis[tris(4-methoxyphenyl)phosphane-κP]rhodium(I)

The title complex, [RhCl(C21H21O3P)2(CO)], is a rhodium analogue to Vaska’s complex with para-methoxy substituents on the six phosphanyl–aryl units. Two independent molecules are present in the unit cell, with their metal atoms both located on an inversion centre. This causes the chloride and carbonyl ligands to exhibit a positional disorder in a 0.5:0.5 ratio. The two RhI atoms exhibit a distorted square-planar geometry. There are a few weak intramolecular C—H⋯X interactions (X = O, Cl). Interestingly, no significant intermolecular interactions are found between the two independent molecules.

The title complex, [RhCl(C 21 H 21 O 3 P) 2 (CO)], is a rhodium analogue to Vaska's complex with para-methoxy substituents on the six phosphanyl-aryl units. Two independent molecules are present in the unit cell, with their metal atoms both located on an inversion centre. This causes the chloride and carbonyl ligands to exhibit a positional disorder in a 0.5:0.5 ratio. The two Rh I atoms exhibit a distorted square-planar geometry.
There are a few weak intramolecular C-HÁ Á ÁX interactions (X = O, Cl). Interestingly, no significant intermolecular interactions are found between the two independent molecules.
Various 'Vaska complexes' have been synthesized, exploring different metals but especially introducing different substituents on the phosphane ligands. These modifications have an impact on the steric hindrance around the metal (Clarke et al., 2002;Wilson et al., 2002), but in the case of para-substituted triaryl phosphanes the effect is purely electronic (Monge et al., 1983;Otto et al., 1999;Meijboom et al., 2006;Burgoyne et al., 2010). Since only limited data are available on this kind of complexes, we have prepared the rhodium analogue (I), [RhCl(C 21 H 21 O 3 P) 2 (CO)], bearing relatively electron-rich tri(para-methoxyphenyl)-phosphane ligands.
Two independent half-molecules are present in the asymmetric unit of compound (I), in each case with the Rh I atoms located on inversion centres. The metal atoms display a distorted square planar geometry with the phosphane ligands located in mutual trans-positions (Fig. 1). Selected bond lenghts and angles are presented in Table 1.
The carbonyl moiety has a slightly bent geometry, with Rh-C-O angles of 173.2 (14)° and 176.8 (16)° for the two molecules, respectively. In solution infrared spectroscopy only one signal was observed for the carbonyl ligand at 1974 cm -1 .
Also in solid state infrared spectroscopy of the amorphous material, only one signal was observed at 1964 cm -1 . Only when a crystalline sample was analysed, two signals were observed at 1956 and 1973 cm -1 , showing the stretching vibrations of both the independent carbonyl ligands. In 31 P NMR the signal for the phospine ligands was observed at 24.95 ppm with a J Rh-P of 124.5 Hz, which is in line with analogous complexes.
The Rh-P bond lengths fall in the range of other, analogous rhodium Vaska complexes. In contrast, the bonds of the metal to the carbonyl and chlorido ligands are significantly influenced by the electron-donating phosphane ligands. The bond to the chlorido ligand is the longest reported for this kind of complexes bearing triaryl phosphanes. The same influence is also notably present in the bonding of the carbonyl ligand. Its bond to the rhodium atom is quite short, which indicates significant metal-to-ligand electron donation. As a consequence, the C-O bond is lengthened.
There are a few weak intramolecular C-H···X interactions (X = O, Cl), which are listed in Table 2. Interestingly, no intermolecular interactions are found between the two independent molecules.

Experimental
Compound (I) was synthesized by slowly adding 4 equivalents of tri(4-methoxyphenyl)phosphane to a dimethyl formamide solution of [RhCl(CO) 2 ] 2 (McCleverty & Wilkinson, 1990). The product was precipitated with ice water and isolated by supplementary materials sup-2 filtration. Crystallization was performed by dissolving the complex in a small amount of dichloromethane which was then carefully layered with approximately 5 volumetric equivalents of hexane. The mixture was stored in a loosely closed vessel, from which yellow crystals precipitated.

Refinement
The aromatic and methyl H atoms were placed in geometrically idealized positions (C-H = 0.93-0.98) and constrained to ride on their parent atoms with U iso (H) = 1.2U eq (C) for aromatic protons and U iso (H) = 1.5U eq (C) for methyl protons.
The disordered Cl and CO ligands were constrained to have occupancies of 0.5 at each of the two positions. The highest residual electron density was located 0.90 Å from Rh1 and was essentially meaningless. The deepest hole was located 1.00 Å from Rh1.

Special details
Experimental. The intensity data was collected on a Bruker X8 Apex II 4 K Kappa CCD diffractometer using an exposure time of 30 s/frame. A total of 1318 frames was collected with a frame width of 0.5° covering up to θ=28.00° with 98.3% completeness accomplished.
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The 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 > 2σ(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.