Mechanism ofβ-Hydrogen Elimination from Square Planar Iridium(I) Alkoxide Complexes with Labile Dative Ligands

Abstract

Mechanistic studies were conducted on β-hydrogen elimination from complexes of the general formula [Ir(CO)(PPh₃)₂(OR)], which are square planar alkoxo complexes with labile ligands. The dependence of rate, isotope effect, and alkoxide racemization on phosphine concentration revealed unusually detailed information on the reaction pathway. The alkoxo complexes were remarkably stable, including those with a variety of electronically and sterically distinct groups at the β-carbon. These complexes were much more stable than the corresponding alkyl complexes. Thermolysis of these complexes in the presence of PPh₃ yielded the iridium hydride [Ir(CO)(PPh₃)₃H] and the corresponding aldehyde or ketone with rate constants that were affected little by the groups at the β-carbon. The reactions were first order in iridium complexes. At low [PPh₃], the reaction rate was nearly zero order in PPh₃, but reactions at high [PPh₃] revealed an inverse dependence of reaction rate on PPh₃. The rate constants were similar in toluene, THF, and chlorobenzene. The y-intercept of a 1/k_obs vs [PPh₃] plot displayed a primary isotope effect, indicating that the y-intercept did not simply correspond to phosphine dissociation. These data and a dependence of alkoxide racemization on [PPh₃] showed that the elementary β-hydrogen elimination step was reversible. A mechanism involving reversible β-hydrogen elimination followed by associative displacement of the coordinated ketone or aldehyde by PPh₃ was consistent with all of our data. This mechanism stands in contrast with the pathways proposed recently for alkoxide β-hydrogen elimination involving direct elimination, protic catalysts, or binuclear mechanisms and shows that alkoxide elimination can follow pathways similar to those for β-hydrogen elimination from alkyl complexes.