Bidentate bis(NHC-carboxylate) palladium and nickel complexes for the hydrosilylative reduction of carbonyl compounds

A family of 16 e^- Pd(ii) and Ni(ii) biscarbenes bearing NHC ligands decorated with different amino acids in the side chain has been synthesized. The Pd complexes were accessible through transmetalation with silver carbenes generated <i>in situ</i>, while the Ni species were obtained by deprotonation of the imidazolium salt with LDA, followed by complete ligand substitution in [NiBr₂(dme)]. The new complexes have been studied as catalysts in the reductive hydrosilylation of aldehydes and ketones with phenylsilane, followed by a hydrolysis step to form the corresponding alcohol. One of the nickel biscarbene complexes proved to be a competent catalyst, in which the presence of the carboxylate moieties appears to be beneficial. A DFT study of the electronic and steric properties of NHC-carboxylate ligands has been carried out to identify the key features of the complex that exhibits the highest catalytic activity.