Abstract
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.
Highlights
Mechanistic cycles for iridium-catalysed methanol carbonylation generally depict the Ir(III) acetyl complex [Ir(CO)2I3(COMe)]− reacting with water to eliminate acetic acid (eqn (6)), either directly or via initial reductive elimination of acetyl iodide and subsequent hydrolysis
Since our results indicate that these CO2-forming reactions occur readily under mild conditions, an obvious question is why high selectivity to acetic acid is achieved in iridium-catalysed methanol carbonylation
This study has identified a pathway for the co-formation of CO2 and CH4 from iridium methyl and acetyl complexes that participate in iridium/iodide catalysed methanol carbonylation
Summary
The carbonylation of methanol to acetic acid represents one of the most successful industrial scale applications of organometallic catalysis by transition metal complexes and has been primarily achieved using group 9 metals in combination with iodide co-catalysts.[1,2,3,4,5,6,7,8,9,10,11,12] After the initial introduction by BASF of a cobalt-based process, higher activity and selectivity under milder conditions was identified by Monsanto for rhodium and iridium-based catalysts.[13]. These reactions involve nucleophilic attack by water on a carbonyl ligand of an iridium methyl or acetyl complex, and occur without the intermediacy of H2.
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