The mechanism for catalytic hydrosilylation by bis(imino)pyridine iron olefin complexes supported by broken symmetry density functional theory

Density functional theory (DFT, B3LYP-D3 with implicit solvation in toluene) was used to investigate the mechanisms of olefin hydrosilylation catalyzed by PDI(Fe) (bis(imino)pyridine iron) complexes, where PDI = 2,6-(ArNCMe)₂(C₅H₃N) with Ar = 2,6-R₂-C₆H₃. We find that the rate-determining step for hydrosilylation is hydride migration from Et₃SiH onto the Fe-bound olefin to form (PDI)Fe(alkyl)(SiEt₃). This differs from the mechanism for the Pt Karstedt catalyst in that there is no prior Si-H oxidative addition onto the Fe center. (PDI)Fe(alkyl)(SiEt₃) then undergoes C-Si reductive elimination to form (PDI)Fe, which coordinates an olefin ligand to regenerate the resting state (PDI)Fe(olefin). In agreement with experimental observations, we found that anti-Markovnikov hydride migration has a 5.1 kcal mol^-1 lower activation enthalpy than Markovnikov migration. This system has an unusual anti-ferromagnetic coupling between high spin electrons on the Fe center and the unpaired spin in the pi system of the non-innocent redox-active PDI ligand. To describe this with DFT, we used the "broken-symmetry" approach to establish the ground electronic and spin state of intermediates and transition states over the proposed catalytic cycles.