trans-Carbonylchloridobis[dicyclohexyl(4-isopropylphenyl)phosphane]rhodium(I) acetone monosolvate

The title rhodium Vaska-type complex, trans-[RhCl{P(C6H11)2(C6H4-4-C3H7)2}2(CO)], crystallizes with an accompanying acetone solvent molecule. The metal atom shows a distorted square-planar coordination environment with selected important geometrical parameters of Rh—P = 2.3237 (6) and 2.3253 (6) Å, Rh—Cl = 2.3724 (6) Å, Rh—C = 1.802 (2) Å, P—Rh—P = 173.42 (2)° and Cl—Rh—C = 179.13 (7)°. Effective cone angles for the two P atoms are 165 and 161°, respectively. Both isopropyl groups and the acetone molecule are disordered with occupancy values of 0.523 (5):0.477 (5), 0.554 (8):0.446 (8) and 0.735 (4):0.265 (4), respectively. The crystal packing is stabilized by weak C—H⋯O and C—H⋯Cl contacts.

Thanks to the Research Academy for Undergraduates and the University of Johannesburg for financial support.
The title compound ( Fig. 1) crystallizes in the tetragonal space group P4 3 2 1 2 (Z=8). This results in molecules lying in general positions in the unit cell and hence no packing disorder of the Cl and CO moieties is observed. The asymmetric unit has one molecule of acetone situated centrally near the rhodium complex. The metal coordination environment is distorted. This is observed most prominently for the P2-Rh1-P1 angle of 173.42 (2)° (Rh1 is displaced 0.0698 (7) Å from the plane formed by P1, C01, P2 and Cl respectively; r.m.s. deviation of fitted atoms = 0.0575 Å). The Cl-Rh1-C01 angle of 179.18 (8)° appears unaffected as the chloride and carbonyl ligands are significantly less bulky than the tertiary phosphorus ligands. The distortion could possibly be attributed to packing effects induced by the isopropyl groups.
To determine the phosphorus ligand bulkiness, an adaptation of the well known Tolman cone angle model was used (Tolman, 1977). Instead of using a CPK model, the actual geometry from the crystal structure was taken to determine an 'effective cone angle ' (Otto et al., 2001). Two different cone angles of 165° and 161° were obtained for P1 and P2 respectively, indicating some flexibility of the substituents of this group 15 ligand. The difference in cone angles could possibly be attributed to interactions with the acetone solvate or packing effects induced by the isopropyl groups. Weak C-H···Cl/O interactions stabilize the crystal structure (see Table 1).

Experimental
The synthesis of the Rh-Vaska complex was adapted from the synthesis of the trans-[IrCl(CO)(PPh 3 ) 2 ] complex (see Collman et al., 1990). [RhCl 3 .xH 2 O] (50 mg, 0.24 mmol, using the anhydrous basis for the molecular weight calculation) was dissolved in dimethylformamide (DMF) and then heated under reflux for approximately one hour. During this time the colour changed from red to yellow, signalling the formation of the [Rh(µ-Cl)(CO) 2 ] 2 dimer. The solution was then allowed to cool to room temperature and dicyclohexyl-4-isopropylphenyl phosphane (159 mg; 0.5 mmol), dissolved in DMF (3 cm 3 ) was added drop wise while stirring. This was followed by the addition of ice to the mixture and a precipitate formed. The solution was then centrifuged and the precipitate collected. This was then worked up by washing with water, extracting with dichloromethane (10 cm 3 ), and drying with MgSO 4 (ca. 4 g). The solution was then evaporated to give the Vaska complex supplementary materials sup-2 in powder form. This powder was then checked for purity (see characterization below) and single crystals suitable for data collection were obtained by slow evaporation from an acetone solution.

Refinement
All hydrogen atoms were positioned in geometrically idealized positions with C-H = 1.00 Å, 0.99 Å, 0.98 Å and 0.95 Å for methine, methylene, methyl and aromatic H atoms respectively. All hydrogen atoms were allowed to ride on their parent atoms with U iso (H) = 1.2U eq , except for methyl where U iso (H) = 1.5U eq was utilized. The initial positions of methyl hydrogen atoms were located from a Fourier difference map and refined as a fixed rotor. The structure refined to a final Flack parameter of -0.029 (17). The highest residual electron density of 0.58 e.Å -3 is 0.65 Å from C6B representing no physical meaning. Initial refinement of the data showed large displacements of isopropyl groups and the acetone solvate.
Subsequent refinement cycles involved treatment of these parts to disordered refinement procedures. Geometrical (SADI) restraints were applied to isopropyl groups bonded to the phenyl rings (C114-C1A/B and C214-C4A/B), as well as to with the default standard deviations, except for ISOR where 0.005 Å 2 was used. In each case a free variable was connected to the disordered parts and refined to add to unity. For both isopropyl moieties, the free variables refined to almost 50:50 ratio's (0.523 (5):0.477 (5) and 0.554 (8):0.446 (8) for isopropyls attached to C114 and C214 respectively). The free variable for the acetone solvate refined to a ratio of 0.735 (4):0.265 (4). The final refined model shows slightly large carbon U eq(max) /U eq(min) values, but was retained as these observations are primarily associated with the disordered parts. Fig. 1. A view of the title compound showing the numbering scheme of atoms and displacement ellipsoids (drawn at a 30% probability level). Carbon atoms for the minor components of disordered parts are cyan colored. Hydrogen atoms have been omitted for clarity.