Interconversion and reactivity of manganese silyl, silylene, and silene complexes

Manganese disilyl hydride complexes [(dmpe)₂MnH(SiH₂R)₂] (4 : R = Ph, 4 : R = Bu) reacted with ethylene to form silene hydride complexes [(dmpe)₂MnH(RHSi[double bond, length as m-dash]CHMe)] (6 : R = Ph, 6 : R = Bu). Compounds 6 reacted with a second equivalent of ethylene to generate [(dmpe)₂MnH(REtSi[double bond, length as m-dash]CHMe)] (6 : R = Ph, 6 : R = Bu), resulting from apparent ethylene insertion into the silene Si-H bond. Furthermore, in the absence of ethylene, silene complex 6 slowly isomerized to the silylene hydride complex [(dmpe)₂MnH([double bond, length as m-dash]SiEt Bu)] (3 ). Reactions of 4 with ethylene likely proceed via low-coordinate silyl {[(dmpe)₂Mn(SiH₂R)] (2 : R = Ph, 2 : R = Bu)} or silylene hydride {[(dmpe)₂MnH([double bond, length as m-dash]SiHR)] (3 : R = Ph, 3 : R = Bu)} intermediates accessed from 4 by H₃SiR elimination. DFT calculations and high temperature NMR spectra support the accessibility of these intermediates, and reactions of 4 with isonitriles or N-heterocyclic carbenes yielded the silyl isonitrile complexes [(dmpe)₂Mn(SiH₂R)(CNR')] (7a-d: R = Ph or Bu; R' = o-xylyl or Bu), and NHC-stabilized silylene hydride complexes [(dmpe)₂MnH{[double bond, length as m-dash]SiHR(NHC)}] (8a-d: R = Ph or Bu; NHC = 1,3-diisopropylimidazolin-2-ylidene or 1,3,4,5-tetramethyl-4-imidazolin-2-ylidene), respectively, all of which were crystallographically characterized. Silyl, silylene and silene complexes in this work were accessed via reactions of [(dmpe)₂MnH(C₂H₄)] (1) with hydrosilanes, in some cases followed by ethylene. Therefore, ethylene (C₂H₄ and C₂D₄) hydrosilylation was investigated using [(dmpe)₂MnH(C₂H₄)] (1) as a pre-catalyst, resulting in stepwise conversion of primary to secondary to tertiary hydrosilanes. Various catalytically active manganese-containing species were observed during catalysis, including silylene and silene complexes, and a catalytic cycle is proposed.