The carbonylation of methanol catalysed by [RhI(CO)(PEt_3)_2]; crystal and molecular structure of [RhMeI_2(CO)(PEt_3)_2]

摘要

[RhCl(CO)(PEt_3)_2] catalyses the carbonylation of methanol in the presence of MeI and water at a rate 1.8 times that for [RhI_2(CO)_2]~- at 150degC. The reaction is first order in [MeI] and zero order in P_(co). However, the phosphine complex degrades to [Rh(CO)_2I_2]~- during the course of the reaction. Stoichiometric studies show that the rate of oxidative addition of MeI to [RhI(CO)(PEt_3)_2] is 57 times faster than to [RhI_2(CO)_2]~- at 298 K and that [RhMeI_2(CO)(PEt_3)_2] can be isolated and crystallographically characterised. Combination of the methyl and carbonyl igands to give the acyl intermediate occurs 38 times slower for [RhMeI_2(CO)(PEt_3)_2] than for [RhMeI_3(CO)_2]~- but the steady state concentration of the intermediates is different in that [Rh(COMe)I_2(PEt_3)_2] is thermodynamically less stable than [RhMeI_2(CO)(PEt_3)_2. In CH_2Cl_2, [Rh(COMe)_2(CO)(PEt_3)_2] reductively eliminates MeCOl.[RhI(CO)(PEt_3)_2 reacts with Co to give [RhI(CO)_2(PEt_3)_2]. Catalyst degradation occurs via [RhHI_2(CO)(PEt_3)_2]. formed by oxidative addition of HI to [RhI(CO)(PEt_3)_2], which reacts further with HI to give [RhI_3(CO)(PEt_3)_2] from which [Et_3PI]~+ reductively eliminates and is hydrolysed to give Et_3PO. In the presence of water, much less [RhI_3(CO)(PEt_3)_2 and Et_3PO are formed so the catalyst is more stable, but loss of [Et_3PMe]~+ and [Et_3PH]~+ from [RhMeI_2(CO)(PEt_3)_2] or [RhHI_2(CO)(PEt_3)_2], respectively, lead to catalyst deactivation. The rate determining step of the catalytic reaction in the presence of eater is MeI oxidative addition to [RhI(CO)(PEt_3)_2], but in the adsence of water there is evidence that it may be reductive elimination of MeCOI from [Rh(COMe)I_2-(CO)(PEt_3)_2]. [RhMeI_2(CO)(PEt_3)_2] has mutually trans phosphines and the methyl group trans to I.