Identification of the Reactive cis,mer Isomer of [Ir(CO)2I3Me]-: Relation to the Mechanism of Iridium-Catalyzed Methanol Carbonylation

Thermal dissociation of CO from cis,fac-[Ir(CO)(2)I3Me](-) (1a) gives the iodide-bridged dimer [{Ir(CO)I-2(mu-I)Me}(2)](2-), which was characterized crystallographically as its PhAs+ salt. This climer reacts with CO at ambient temperature to give the acetyl complex trans,mer-[Ir(CO)(2)I-3(COMe)](-). An intermediate in this reaction is the previously unobserved cis, mer-[Ir(CO)(2)I3Me](-) (1b), which was characterized by IR and NMR spectroscopy. Carbonylation of 1b is much faster (t(1/2) similar to 1 min at 25 degrees C) than for the cis,fac isomer 1a and also faster than the neutral tricarbonyl [Ir(CO)(3)I2Me]. The observations show that the relative positioning of carbonyl ligands influences reactivity more than than their number, and that CO insertion is particularly accelerated when a CO ligand is placed trans to the migrating methyl group. DFT calculations indicate that the Ir-CO bond trans to methyl contracts significantly as 1b approaches the transition state for methyl migration, facilitating stronger pi-backbonding from Ir to the spectator CO ligand, which stabilizes the transition state. The results confirm the recently proposed CO-loss mechanism for the photochemical carbonylation of 1a and suggest that the cis,mer isomer may play a minor role in the catalytic cycle for methanol carbonylation.