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

投稿者:

School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287, United States
Inorg. Chem., Article ASAP
DOI: 10.1021/acs.inorgchem.5b01825
Publication Date (Web): October 19, 2015
Copyright © 2015 American Chemical Society

Synopsis


Reduction of the bis(imino)pyridine-supported precursor complex, (PyEtPDI)MnCl2, results in deprotonation of both backbone methyl groups and the formation of (PyEtPDEA)Mn. This complex was found to mediate the hydrosilylation of 17 different aldehydes and ketones under mild conditions. When low loadings were employed in the absence of solvent, hydrosilylation turnover frequencies as high as 2475 min−1 were achieved. Comparison to leading first-row metal catalysts reveals (PyEtPDEA)Mn to be among the most active for carbonyl hydrosilylation.

Abstract

Abstract Image
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.
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