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DOI: 10.1039/A905308E (Paper) Reference Section for: J. Chem. Soc., Dalton Trans., 1999, 3771-3782

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

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Joanne Rankin, Attila C. Benyei, Andrew D. Poole and David J. Cole-Hamilton

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 pCO. 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.

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