Half-Sandwich Manganese Complexes Bearing Cp Tethered N-Heterocyclic Carbene Ligands: Synthesis and Mechanistic Insights into the Catalytic Ketone Hydrosilylation

Manganese(I) complexes [(Cp(CH₂)_nNHC^Mes)Mn(CO)₂] (2a, n = 1; 2b, n = 2) bearing Cp tethered NHC ligands have been prepared in good yield from [CpMn(CO)₃] and N-mesitylimidazole as main components, using a novel synthetic strategy based on the anchoring of an imidazolium moiety to the coordinated Cp ligand, followed by an intramolecular photochemical CO substitution for the pendent NHC moiety generated in situ upon addition of a base. The catalytic performances of 2a-b in the hydrosilylation of 2-acetonaphthone with Ph₂SiH₂ were found to be essentially indifferent to the length of the linker between the Cp and the NHC moieties, and lower than the one of the nontethered [Cp(CO)₂Mn(IMes)] (1) parent complex. Still, the presence of Cp tethered NHC ligands allowed isolation and full characterization of monocarbonyl η²-silane complexes [(Cp(CH₂)_nNHC^Mes)Mn(CO)(η²-H-SiHPh₂)] (8a, n = 1; 8b, n = 2), which are generated upon photochemical reactions of 2a-b with diphenylsilane. Both of these silane complexes are inactive for the catalytic 2-acetonaphthone hydrosilylation process but were found to form catalytically active noncarbonyl species under UV irradiation, being consistent with the Ojima mechanism for ketone hydrosilylation.