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

Manganese disilyl hydride complexes [(dmpe)(2)MnH(SiH(2)R)(2)] (4(Ph): R = Ph, 4(Bu): R = (n)Bu) reacted with ethylene to form silene hydride complexes [(dmpe)(2)MnH(RHSi[double bond, length as m-dash]CHMe)] (6(Ph,H): R = Ph, 6(Bu,H): R = (n)Bu). Compounds 6(R,H) reacted with a second equivalent of ethylene to generate [(dmpe)(2)MnH(REtSi[double bond, length as m-dash]CHMe)] (6(Ph,Et): R = Ph, 6(Bu,Et): R = (n)Bu), resulting from apparent ethylene insertion into the silene Si–H bond. Furthermore, in the absence of ethylene, silene complex 6(Bu,H) slowly isomerized to the silylene hydride complex [(dmpe)(2)MnH([double bond, length as m-dash]SiEt(n)Bu)] (3(Bu,Et)). Reactions of 4(R) with ethylene likely proceed via low-coordinate silyl {[(dmpe)(2)Mn(SiH(2)R)] (2(Ph): R = Ph, 2(Bu): R = (n)Bu)} or silylene hydride {[(dmpe)(2)MnH([double bond, length as m-dash]SiHR)] (3(Ph,H): R = Ph, 3(Bu,H): R = (n)Bu)} intermediates accessed from 4(R) by H(3)SiR elimination. DFT calculations and high temperature NMR spectra support the accessibility of these intermediates, and reactions of 4(R) with isonitriles or N-heterocyclic carbenes yielded the silyl isonitrile complexes [(dmpe)(2)Mn(SiH(2)R)(CNR′)] (7a–d: R = Ph or (n)Bu; R′ = o-xylyl or (t)Bu), and NHC-stabilized silylene hydride complexes [(dmpe)(2)MnH{[double bond, length as m-dash]SiHR(NHC)}] (8a–d: R = Ph or (n)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)(2)MnH(C(2)H(4))] (1) with hydrosilanes, in some cases followed by ethylene. Therefore, ethylene (C(2)H(4) and C(2)D(4)) hydrosilylation was investigated using [(dmpe)(2)MnH(C(2)H(4))] (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.