The carbonylation of methanol catalysed by [RhI(CO)(PEt3)2]; crystal and molecular structure of [RhMeI2(CO)(PEt3)2]†

Abstract

[RhCl(CO)(PEt₃)₂] catalyses the carbonylation of methanol in the presence of MeI and water at a rate 1.8 times that for [RhI₂(CO)₂]^– at 150 °C. The reaction is first order in [MeI] and zero order in p_CO. However, the phosphine complex degrades to [Rh(CO)₂I₂]^– during the course of the reaction. Stoichiometric studies show that the rate of oxidative addition of MeI to [RhI(CO)(PEt₃)₂] is 57 times faster than to [RhI₂(CO)₂]^– at 298 K and that [RhMeI₂(CO)(PEt₃)₂] can be isolated and crystallographically characterised. Combination of the methyl and carbonyl ligands to give the acyl intermediate occurs 38 times slower for [RhMeI₂(CO)(PEt₃)₂] than for [RhMeI₃(CO)₂]^– but the steady state concentration of the intermediates is different in that [Rh(COMe)I₂(PEt₃)₂] is thermodynamically less stable than [RhMeI₂(CO)(PEt₃)₂]. In CH₂Cl₂, [Rh(COMe)I₂(CO)(PEt₃)₂] reductively eliminates MeCOI. [RhI(CO)(PEt₃)₂] reacts with CO to give [RhI(CO)₂(PEt₃)₂]. Catalyst degradation occurs via [RhHI₂(CO)(PEt₃)₂], formed by oxidative addition of HI to [RhI(CO)(PEt₃)₂], which reacts further with HI to give [RhI₃(CO)(PEt₃)₂] from which [Et₃PI]⁺ reductively eliminates and is hydrolysed to give Et₃PO. In the presence of water, much less [RhI₃(CO)(PEt₃)₂] and Et₃PO are formed so the catalyst is more stable, but loss of [Et₃PMe]⁺ and [Et₃PH]⁺ from [RhMeI₂(CO)(PEt₃)₂] or [RhHI₂(CO)(PEt₃)₂], respectively, lead to catalyst deactivation. The rate determining step of the catalytic reaction in the presence of water is MeI oxidative addition to [RhI(CO)(PEt₃)₂], but in the absence of water there is evidence that it may be reductive elimination of MeCOI from [Rh(COMe)I₂(CO)(PEt₃)₂]. [RhMeI₂(CO)(PEt₃)₂] has mutually trans phosphines and the methyl group trans to I.