Density functional study of catalytic silane alcoholysis at a [Fe(Cp)(CO)(PR3)+ center

Catalytic conversion of MeOH and SiH4 to MeOSiH3 and H-2 at a [Fe(Cp)(CO)(PR3)](+) center was studied computationally at a gradient-corrected level of density functional theory (BP86). Intermediates and transition states for R'=H and Ph were characterized along the catalytic cycle, the rate-determining step of which is indicated to be H-2 dissociation from [Fe(Cp)(CO)(PR3)(H-2)](+), in accord with a mechanistic study for higher substituted substrates (Brookhart, et al. J. Mol. Catal. A 1998, 130, 107). For the model catalyst (R = H), introduction of SiMe3, NMe2, COMe, NO2, CN, or Cl substituents at the Cp ring decreases the rate-determining barrier (up to 6 kcal/mol for NMe2). A loose correlation is found between these barriers and delta(Fe-57) values computed for the correspondingly derivatized precursor models Fe(C5H4X)(CO)(PH3)Me. In the transient, free [Fe(C5H4X)(CO)(PPh3)](+) complex, there can be competition between the substituent X and a phenyl group of the PPh3 ligand for a hemilabile intramolecular coordination to iron. Modification of the phosphine is thus expected to open further possibilities for tuning the catalyst properties and activities.