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 catalytic methanol carbonylation are reported. The iridium acetyl, [Ir(CO)_2I_3(COMe)]~-, reacts with water under mild conditions to release CO_2 and CH_4, 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)_2I_3(COMe)]~- to give an (undetected) hydroxycarbonyl species. Subsequent decarboxylation and elimination of methane gives [Ir(CO)_2I_2]~-. Similar reactions with water are observed for [Ir(CO)_2I_3Me] ~-, [Ir(CO)_2(NCMe)I_2(COMe)] and [Ir(CO) _3I_2Me] with the neutral complexes exhibiting markedly higher rates. The results demonstrate that CO_2 formation during methanol carbonylation is not restricted to the conventional water gas shift mechanism mediated by [Ir(CO)_2I_4]- or [Ir(CO)_3I_3], 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 H_2 is also suggested. A mechanism is proposed for the conversion MeOH + CO → CO_2 + CH_4, which may account for the similar rates of formation of the two gaseous by-products during iridium-catalysed methanol carbonylation.