Reactivity of Ir(III) carbonyl complexes with water: alternative by-product formation pathways in catalytic methanol carbonylation

摘要

The reactions of water with a number of iridium(III) complexes relevant to the mechanism for catalyticudmethanol carbonylation are reported. The iridium acetyl, [Ir(CO)2I3(COMe)]−, reacts with water underudmild conditions to release CO2 and CH4, rather than the expected acetic acid. Isotopic labeling andudkinetic experiments are consistent with a mechanism involving nucleophilic attack by water on a terminaludCO ligand of [Ir(CO)2I3(COMe)]− to give an (undetected) hydroxycarbonyl species. Subsequent decarboxylationudand elimination of methane gives [Ir(CO)2I2]−. Similar reactions with water are observed forud[Ir(CO)2I3Me]−, [Ir(CO)2(NCMe)I2(COMe)] and [Ir(CO)3I2Me] with the neutral complexes exhibitingudmarkedly higher rates. The results demonstrate that CO2 formation during methanol carbonylation is notudrestricted to the conventional water gas shift mechanism mediated by [Ir(CO)2I4]− or [Ir(CO)3I3], but canudarise directly from key organo-iridium(III) intermediates in the carbonylation cycle. An alternative pathwayudfor methane formation not involving the intermediacy of H2 is also suggested. A mechanism is proposedudfor the conversion MeOH + CO → CO2 + CH4, which may account for the similar rates of formation of theudtwo gaseous by-products during iridium-catalysed methanol carbonylation.