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] (<b>1</b>), which was isolated and crystallographically characterized. The xantphos ligand in <b>1</b> 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 <b>1</b> 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)]⁺ (<b>4</b>), 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] (<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_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)

Bifunctional Organorhodium Solid Acid Catalysts for Methanol Carbonylation

Robust, bifunctional catalysts comprising Rh­(CO)­(Xantphos) exchanged phosphotungstic acids of general formulas [Rh­(CO)­(Xantphos)]⁺_<i>n</i>[H_3–<i>n</i>PW₁₂O₄₀]^<i>n</i>− have been synthesized over silica supports which exhibit tunable activity and selectivity toward direct vapor phase methanol carbonylation. The optimal Rh:acid ratio = 0.5, with higher rhodium concentrations increasing the selectivity to methyl acetate over dimethyl ether at the expense of lower acidity and poor activity. On-stream deactivation above 200 °C reflects Rh decomplexation and reduction to Rh metal, in conjunction with catalyst dehydration and loss of solid acidity because of undesired methyl acetate hydrolysis, but can be alleviated by water addition and lower temperature operation