Influence of phosphorus and oxygen donor diphosphine ligands on the reactivity of rhodium(I) carbonyl complexes

The dimeric rhodium precursor [Rh(CO)(2)Cl](2) reacts with two molar equivalent of 9,9-dimethyl-4,5-bis(diphenylphosphino)xanthene [xantphos] (a), bis(2-diphenylphosphinophenyl)ether [DPEphos] (b) and their corresponding dioxide analogues xantphos dioxide (c), DPEphos dioxide (d) to afford the mono- and dicarbonyl complexes of the type [Rh(CO)Cl(L)] (1a,1b) and [Rh(CO)(2)Cl(L)] (1c,1d) respectively, where L=a-d. The complexes la-id have been characterized by elemental analyses, IR and NMR (H-1, P-31 and C-13) spectroscopy, and the structure of the ligand d was determined by single crystal X-ray diffraction, 1a-1d undergo oxidative addition (OA) reactions with different electrophiles such as CH3I, C2H5I and I-2 to give Rh(III) complexes of the types [Rh(CO)(y)(COR)ClXL] {R = -CH3 (2a-2d), -C2H5 (3a-3d); X = 1 and y = 0, L = a, b: y = 1, L = c, d} and [Rh(CO)CH2L] (4a-4d) respectively. Kinetic data for the reactions of la-id with CH3I indicate a pseudo-first-order reaction. The catalytic activity of 1a-1d for the carbonylation of methanol to acetic acid and its ester was evaluated at different CO pressure 15,20 and 33 bar at 130 C and a higher Turn Over Number (TON) (679-1768) were obtained compared to that of the well-known commercial species [Rh(CO)(2)I-2](-) (TON = 463-1000) in each case under the similar experimental conditions. (C) 2010 Elsevier B.V. All rights reserved.