%0 Generic %A Williams, Gary L. %A Parks, Christopher M. %A Smith, C. Robert %A Adams, Harry %A Haynes, Anthony %A H. M. Meijer, Anthony J. %A Sunley, Glenn J. %A Gaemers, Sander %D 2016 %T Mechanistic Study of Rhodium/xantphos-Catalyzed Methanol Carbonylation %U https://acs.figshare.com/articles/dataset/Mechanistic_Study_of_Rhodium_xantphos_Catalyzed_Methanol_Carbonylation/2579554 %R 10.1021/om2006968.s002 %2 https://acs.figshare.com/ndownloader/files/4223875 %K Rh %K SN 2 transition state %K stoichiometric reaction steps %K xanthene oxygen donor trans %K xantphos ligand %K CH %K CO %K cationic acetyl species %K DFT calculations %K IR %X 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). %I ACS Publications