A Pentacoordinate Mn(II) Precatalyst That Exhibits Notable Aldehyde and Ketone Hydrosilylation Turnover Frequencies

Heating (THF)₂MnCl₂ in the presence of the pyridine-substituted bis(imino)pyridine ligand, ^PyEtPDI, allowed preparation of the respective dihalide complex, (^PyEtPDI)MnCl₂. Reduction of this precursor using excess Na/Hg resulted in deprotonation of the chelate methyl groups to yield the bis(enamide)tris(pyridine)-supported product, (κ⁵-N,N,N,N,N-^PyEtPDEA)Mn. This complex was characterized by single-crystal X-ray diffraction and found to possess an intermediate-spin (S = 3/2) Mn(II) center by the Evans method and electron paramagnetic resonance spectroscopy. Furthermore, (κ⁵-N,N,N,N,N-^PyEtPDEA)Mn was determined to be an effective precatalyst for the hydrosilylation of aldehydes and ketones, exhibiting turnover frequencies of up to 2475 min^-1 when employed under solvent-free conditions. This optimization allowed for isolation of the respective alcohols and, in two cases, the partially reacted silyl ethers, PhSiH(OR)₂ [R = Cy and CH(Me)(ⁿBu)]. The aldehyde hydrosilylation activity observed for (κ⁵-N,N,N,N,N-^PyEtPDEA)Mn renders it one of the most efficient first-row transition metal catalysts for this transformation reported to date.