INTERMOLECULAR COMPLEXES AS REACTIVE SPECIES IN THE MIGRATORY CO INSERTION OF INDENYLIRON(II) ALKYL COMPLEXES

The reaction of indenyl complexes (C9H7)Fe(CO)2R (R = CHMe2, Me) with phosphines in toluene (20-60-degrees-C) gives the product of alkyl migratory insertion (C9H7)Fe(CO)(phosphine)-(COR) by a mechanism involving (i) rapid formation of a molecular complex [(eta5-C9H7)Fe(CO)2R, phosphine] and (ii) rate determining alkyl migration. The equilibrium constants for the preequilibrium step (K, M-1) and the rate constants for C-C coupling (k2, s-1) are obtained from analysis of kinetic data for phosphines of widely different steric and electronic properties. The K values are greater for R = CHMe2 than for R = Me, and increase with increasing electron withdrawing character of the para substituent of isosteric triarylphosphines [P(p-XC6H4)3]. The effects of structural and electronic changes indicate that the molecular complex [(eta5-C9H7)Fe(CO)2R, phosphine] forms through an electron donor-acceptor interaction, in which the iron complex behaves as donor and the phosphine as acceptor. Correlation of K values for triarylphosphines with the sigma(para) constants displays electronic sensitivity for precursor formation aso = 0.62 for R = CHMe2 (20-degrees-C) and rho = 0.47 for R = Me (40-degrees-C). This effect is temperature dependent and levels off at the isoequilibrium temperature (60-degrees-C) for R = Me. Experiments with trialkylphosphines (PBu3, PCy3) show that the reaction proceedes by the same mechanism as with alkylaryl- and triarylphosphines and indicate that formation of the molecular complexes is not due to aromatic pi-stacking. The alkyl migration step (k2) for R = CHMe2, Which is faster than for R = Me and independent of the nature of the phosphorus donor, involves a negligible extent of Fe-P bond formation in the transition state of the isopropyl complex. Reactions do not proceed with nitrogen donors (pyridine, triethylamine) in place of phosphines, and rates are not affected by the presence of triphenylphosphine oxide.