Reactivity of a dearomatised pincer CoIIBr complex with PNCNHC donors: alkylation and Si–H bond activation via metal–ligand cooperation

Abstract

The double aminolysis reaction of [Co{N(SiMe₃)₂}₂] by the salt 1-(6-((dicyclohexylphosphaneyl)methyl)pyridin-2-yl)-3-(2,6-diisopropylphenyl)-1H-imidazol-3-ium bromide, which contains one phosphane, one pyridine and one imidazolium groups, of formula [o-Cy₂PCH₂(C₅H₃N)(o-C₃H₃N₂DiPP)]Br and abbreviated as (^CyPN^pyrC^im)Br, was previously shown to afford the Co(II) complex [Co(^CyP*N_aC^NHC)Br] (1) containing a dearomatised picolyl moiety in the tridentate, anionic donor ligand ^CyP*N_aC^NHC (N_a = anionic amido, P* = vinylic P donor). We now report that formation of 1 is preceded by an intermediate tentatively assignable to the 5-coordinate Co(II) complex [Co(^CyPN^pyrC^NHC){N(SiMe₃)₂}Br] (2). The reaction of 1 with LiCH₂SiMe₃ afforded the dark purple, paramagnetic [Co(^CyP*N_aC^NHC)CH₂SiMe₃] (3) with a low spin d⁷ Co^II; the electronic configurations of 1 and 3 were corroborated by EPR spectroscopy. Addition of excess (≥4 equiv.) H₂SiPh₂ to a solution of 1 gave the diamagnetic [Co(^CyP(SiHPh₂)NC^NHC)Br] (4) following Si–H activation and silylation of the ligand backbone at the α-CHP. Reduction of Co^II to Co^I by silanes is uncommon.