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

The reactions of water with a number of iridium(iii) complexes relevant to the mechanism for catalytic methanol carbonylation are reported. The iridium acetyl, [Ir(CO)₂I₃(COMe)]^-, reacts with water under mild conditions to release CO₂ and CH₄, rather than the expected acetic acid. Isotopic labeling and kinetic experiments are consistent with a mechanism involving nucleophilic attack by water on a terminal CO ligand of [Ir(CO)₂I₃(COMe)]^- to give an (undetected) hydroxycarbonyl species. Subsequent decarboxylation and elimination of methane gives [Ir(CO)₂I₂]^-. Similar reactions with water are observed for [Ir(CO)₂I₃Me] ^-, [Ir(CO)₂(NCMe)I₂(COMe)] and [Ir(CO) ₃I₂Me] with the neutral complexes exhibiting markedly higher rates. The results demonstrate that CO₂ formation during methanol carbonylation is not restricted to the conventional water gas shift mechanism mediated by [Ir(CO)₂I₄]^- or [Ir(CO)₃I₃], but can arise directly from key organo-iridium(iii) intermediates in the carbonylation cycle. An alternative pathway for methane formation not involving the intermediacy of H₂ is also suggested. A mechanism is proposed for the conversion MeOH + CO → CO₂ + CH₄, which may account for the similar rates of formation of the two gaseous by-products during iridium-catalysed methanol carbonylation.