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 SN2 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).