Modulating the Elementary Steps of Methanol Carbonylation by Bridging the Primary and Secondary Coordination Spheres

The rate of catalytic methanol carbonylation to acetic acid is typically limited by either the oxidative addition of methyl iodide or the subsequent C–C bond-forming migratory insertion step. These elementary steps have been studied independently in acetonitrile solution for iridium aminophenylphosphinite (NCOP) complexes. The modular synthesis of NCOP ligands containing a macrocyclic aza-crown ether arm enables a direct comparison of two complementary catalyst optimization strategies: synthetic modification of the phenyl backbone and noncovalent modification through cation–crown interactions with Lewis acids in the surrounding environment. The oxidative addition of methyl iodide to iridium(I) carbonyl complexes proceeds readily at room temperature to form iridium(III) methylcarbonyliodide complexes. The methyl complexes undergo migratory insertion under 1 atm CO at 70 °C to produce iridium(III) acetyl species. Synthetic tuning, by incorporation of a methoxy group into the ligand backbone, had little infl...