Mechanistic Study of Rhodium/xantphos-Catalyzed Methanol Carbonylation

Rhodium/iodide catalysts modified with the xantphos ligand are active for the homogeneous carbonylation of methanol to acetic acid using either pure CO or CO/H₂. Residues from catalytic reactions contain a Rh­(III) acetyl complex, [Rh­(xantphos)­(COMe)­I₂] (1), which was isolated and crystallographically characterized. The xantphos ligand in 1 adopts a “pincer” κ³-P,O,P coordination mode with the xanthene oxygen donor trans to the acetyl ligand. The same product was also synthesized under mild conditions from [Rh­(CO)₂I]₂. Iodide abstraction from 1 in the presence of donor ligands (L = MeCN, CO) gives the cationic acetyl species [Rh­(xantphos)­(COMe)­I­(L)]⁺, whereas in CH₂Cl₂ migratory CO deinsertion gives [Rh­(xantphos)­(Me)­I­(CO)]⁺ (4), which reacts with H₂ to liberate methane, as observed in catalytic reactions using syngas. A number of Rh­(I) xantphos complexes have been synthesized and characterized. Oxidative addition of methyl iodide to the cation [Rh­(xantphos)­(CO)]⁺ is very slow but can be catalyzed by addition of an iodide salt, via a mechanism involving neutral [Rh­(xantphos)­(CO)­I] (6). IR spectroscopic data and DFT calculations for 6 suggest the existence in solution of conformers with different Rh–O distances. Kinetic data and activation parameters are reported for the reaction of 6 with MeI, which proceeds by methylation of the Rh center and subsequent migratory insertion to give 1. The enhancement of nucleophilicity arising from a Rh- - -O interaction is supported by DFT calculations for the S_N2 transition state. A mechanism for catalytic methanol carbonylation based on the observed stoichiometric reaction steps is proposed. A survey of ligand conformations in xantphos complexes reveals a correlation between P–M–P bite angle and M–O distance and division into two broad categories with bite angle <120° (cis) or >143° (trans).