Synthesis, characterization, and catalytic application in aldehyde hydrosilylation of half-sandwich nickel complexes bearing (κ-C)- and hemilabile (κ-C,S)-thioether-functionalised NHC ligands.

Neutral nickel-N-heterocyclic carbene complexes, (κ-C)-[NiCpBr{R-NHC-(CH)SR'}] [Cp = η-CH; R-NHC-(CH)SR' = 1-mesityl-3-[2-(tert-butylthio)ethyl]- (1a), 1-mesityl-3-[2-(phenylthio)ethyl]- (1b), 1-benzyl-3-[2-(tert-butylthio)ethyl]- (1c), 1-benzyl-3-[2-(phenylthio)ethyl]-imidazol-2-ylidene (1d)], which bear a N-bound thioether side arm, were prepared by the reaction of nickelocene with the corresponding imidazolium bromides [R-NHC-(CH)SR'·HBr] (a-d), via conventional or microwave heating. The H NMR spectra of the benzyl-substituted species 1c and 1d showed signals for diastereotopic NCHCHS protons at room temperature. However, structural studies established the absence of coordination of the sulphur atom in the solid state, and solvent DFT calculations showed that bromide displacement by sulphur is an unfavourable process (ΔG = +13.