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)I(3)Me](-) (1a) gives the iodide-bridged dimer [{Ir(CO)I(2)(mu-I)Me}(2)](2-), which was characterized crystallographically as its Ph(4)As(+) salt. This dimer 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)I(3)Me](-) (1b), which was characterized by IR and NMR spectroscopy. Carbonylation of 1b is much faster (t(1/2) approximately 1 min at 25 degrees C) than for the cis,fac isomer 1a and also faster than the neutral tricarbonyl [Ir(CO)(3)I(2)Me]. 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.
Haynes
Anthony
A
Department of Chemistry, University of Sheffield, Sheffield, S3 7HF, United Kingdom. a.haynes@sheffield.ac.uk
Meijer
Anthony J H M
AJ
Lyons
James R
JR
Adams
Harry
H