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/H<sub>2</sub>. Residues from catalytic reactions contain a Rh­(III) acetyl complex, [Rh­(xantphos)­(COMe)­I<sub>2</sub>] (<b>1</b>), which was isolated and crystallographically characterized. The xantphos ligand in <b>1</b> adopts a “pincer” κ<sup>3</sup>-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)<sub>2</sub>I]<sub>2</sub>. Iodide abstraction from <b>1</b> in the presence of donor ligands (L = MeCN, CO) gives the cationic acetyl species [Rh­(xantphos)­(COMe)­I­(L)]<sup>+</sup>, whereas in CH<sub>2</sub>Cl<sub>2</sub> migratory CO deinsertion gives [Rh­(xantphos)­(Me)­I­(CO)]<sup>+</sup> (<b>4</b>), which reacts with H<sub>2</sub> 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)]<sup>+</sup> is very slow but can be catalyzed by addition of an iodide salt, via a mechanism involving neutral [Rh­(xantphos)­(CO)­I] (<b>6</b>). IR spectroscopic data and DFT calculations for <b>6</b> suggest the existence in solution of conformers with different Rh–O distances. Kinetic data and activation parameters are reported for the reaction of <b>6</b> with MeI, which proceeds by methylation of the Rh center and subsequent migratory insertion to give <b>1</b>. The enhancement of nucleophilicity arising from a Rh- - -O interaction is supported by DFT calculations for the S<sub>N</sub>2 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).