Oxidative addition of carboxylic acids to trans-carbonylhalogenobis-(tertiary phosphine)iridium(I) complexes

Abstract

Complexes of the type trans-[lrX(CO)L₂](L = PEt₃,PMe₂Ph, or PPh₃; X = Cl, Br, or l) undergo rapid oxidative addition with carboxylic acids RCO₂H (R = H, Me, CF₃, Ph, or 1-naphthyl) to give iridium (III) complexes [lrXH(O₂CR)(CO)L₂] corresponding to both (formal)cis and trans addition of the carboxylic acid to the iridium(I) species. In solution these complexes undergo rapid anion exchange such that, at equilibrium, two additional hydrido-species, [lrX₂H(CO)L₂] and [lrH(O₂CR)₂(CO)L₂], are present. In all these octahedral complexes the tertiary phosphine groups are mutually trans, the hydride and carbonyl groups mutually cis, and the carboxylic unit is unidentate. The ease of formation of the different complexes depends on the nature of the carboxylic acid. The cis adduct containing chloride and having hydride and carbonyl mutually trans can be prepared by the action of carbon monoxide on complexes [lrCl(H)(O₂CR)L₂] which contain a bidentate carboxylate ligand. With weak acids, e.g. acetic, conversion of the iridium(I) into the iridium(III) species is incomplete; the exchange between free and co-ordinated acid is, however, slow on the n.m.r. time scale over the range –60 to 30 °C. The adducts formed between trans-[lrX(CO)L₂](X = Cl, Br, or I) and formic acid are smoothly converted on heating in solution into dihydrido-complexes [lrXH₂(CO)L₂] with expulsion of carbon dioxide; with L = PPh₃ or PEt₃, trihydrido-complexes [lrH₃(CO)L₂] are also formed.