MECHANISM OF THE MIGRATORY INSERTION OF CARBON-MONOXIDE IN THE INDENYLIRON(II) SYSTEM - DETECTABLE INTERMEDIATES IN AN ASSOCIATIVE PATHWAY

The indenyl complex (eta5-C9H7)Fe(CO)2Me reacts with phosphines (PMe3, PMe2Ph, PMePh2, PPh3) in toluene to give the products of alkyl migratory insertion (eta5-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, in the order PPh3 (k2K = 5.6 x 10(-5) M-1 s-1) < PMePh2 (k2K = 7.0 x 10(-5) M-1 s-1) < PMe2Ph (k2K = 7.4 x 10(-4) M-1 s-1) < PMe3 (9.4 x 10(-4) M-1 s-1), at 40.0-degrees-C. 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.