Photochemical and Time Resolved Spectroscopic Studies of Intermediates Relevant to Iridium-Catalyzed Methanol Carbonylation: Photoinduced CO Migratory Insertion

Photoreaction, time-resolved infrared (TRIR), and DFT studies were utilized to probe transformations between
iridium complexes with possible relevance to the mechanisms of the iridium/iodide-catalyzed methanol carbonylation
to acetic acid. Solution-phase continuous and laser flash photolysis of the tetraphenylarsonium salt of the fac-[CH3Ir(CO)2I3]- anion (1a) under excess carbon monoxide resulted in migratory insertion to give the acyl complex
ion mer,trans-[Ir(C(O)CH3)(CO)2I3]- (2a). The latter was isolated as its AsPh4+ salt, and its X-ray crystal structure
was determined. TRIR spectra indicate that several transients are generated upon flash photolysis of 1a. The
principal photoreaction is CO dissociation, and this is proposed to generate the isomeric complexes fac-[CH3Ir(CO)(Sol)I3]- (ICO(fac), Sol = solvent) and mer,trans-[CH3Ir(CO)(Sol)I3]- (ICO(mer)). ICO(fac) reacts with CO to
regenerate 1a with a second-order rate constant (kCO) ∼2.5 × 107 M-1 s-1 in ambient dichloroethane, while ICO(mer) is the apparent precursor to 2a. Kinetics studies indicate the photoinduced formation of a third intermediate
(IM), hypothesized to be the anionic acyl complex fac-[Ir(C(O)CH3)(CO)(Sol)I3]-. In the absence of added CO,
these intermediates undergo dimerization to form a mixture of isomers with the apparent formula [Ir(C(O)CH3)(CO)I3]22-.
One of these dimers was isolated as the AsPh4+ salt, and the crystal structure was determined. Addition of excess
pyridine to a solution of the dimers gave the neutral complex mer,trans-[Ir(C(O)CH3)(CO)(py)2I2], which was
characterized by FTIR, NMR, and X-ray crystallography. These transformations, especially the unprecedented
photoinduced CO insertion reaction, are discussed and interpreted in terms of the factors favoring migratory insertion
dynamics.