Mechanism of the Migratory Insertion of Carbon Monoxide in the Indenyliron(II) System. Detectable Intermediates in an Associative Pathway

The indenyl complex (CgH7)Fe(C0)2Me reacts with phosphines (PMe3, PMe2Ph, PMePh2, PPh3) in toluene to give the products of alkyl migratory insertion (C9H7)Fe(CO)(phosphine)(COR). The kinetic investigation is in agreement with a mechanism involving a rapid preequilibrium between the iron complex and the phosphine (K), followed by rate-determining alkyl migration ( k2). The reaction rates increase with decreasing cone angle and increasing basicities of the phosphines. Both the equilibrium constants for the preequilibrium step and the rate constants for C-C coupling are obtained from analysis of kinetic data. The equilibrium constant between the iron complex and PPh3 has been measured independently by UV-visible spectroscopy. Kinetic and spectroscopic (UV-visible, IR, NMR) data are indicative of a weak interaction between the iron complex and the phosphine, in which the molecular frame of the iron complex is not perturbed.