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.