A Pentacoordinate Mn(II) Precatalyst That Exhibits
Notable Aldehyde and Ketone Hydrosilylation Turnover Frequencies
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Abstract
Heating (THF)<sub>2</sub>MnCl<sub>2</sub> in the presence of the pyridine-substituted bis(imino)pyridine
ligand, <sup>PyEt</sup>PDI, allowed preparation of the respective
dihalide complex, (<sup>PyEt</sup>PDI)MnCl<sub>2</sub>. 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, (κ<sup>5</sup>-<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>-<sup>PyEt</sup>PDEA)Mn. This complex was characterized by single-crystal X-ray diffraction
and found to possess an intermediate-spin (<i>S</i> = <sup>3</sup>/<sub>2</sub>) Mn(II) center by the Evans method and electron
paramagnetic resonance spectroscopy. Furthermore, (κ<sup>5</sup>-<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>-<sup>PyEt</sup>PDEA)Mn was determined
to be an effective precatalyst for the hydrosilylation of aldehydes
and ketones, exhibiting turnover frequencies of up to 2475 min<sup>–1</sup> 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)<sub>2</sub> [R = Cy and CH(Me)(<sup>n</sup>Bu)]. The aldehyde hydrosilylation
activity observed for (κ<sup>5</sup>-<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>N</i>-<sup>PyEt</sup>PDEA)Mn renders it one of the most efficient first-row
transition metal catalysts for this transformation reported to date
