Transfer hydrogenation of ketones catalysed by in situ generated nickel imino-NHC complexes

• Three new fully characterised imino-functionalised NHC ligand precursor salts were prepared. • The in situ combination of the precursors with bench-top nickel(II) salts generated corresponding NHC-Ni(II) complexes. • The complexes were used as catalysts for the transfer hydrogenation (CTH) of ketones under mild conditions. • Excellent conversions (>90%) and TON of aromatic and aliphatic ketones to corresponding alcohol products were obtained in 2-propanol. • Theoretical calculations indicated the energy barriers involved in the transformation, with the square planar Ni-H species identified as the key intermediate in the CTH process. Three new Schiff base functionalised N -heterocyclic carbene (NHC) ligand precursors: ( E )-1-(R)-3-(2-(((2-hydroxynaphthalen-1-yl) methylene) amino) ethyl)-1 H -imidazol-3-ium hexafluorophosphate(V) [R = ethyl ( 2 ), acetyl ( 3 )] and ( E )-3-(2-((2-hydroxybenzylidene) amino) ethyl)-1-propyl-1 H -benzo[d]imidazol-3-ium hexafluorophosphate(V) ( 4 ) were synthesised. Structural analysis by 1 H-NMR, 13 C-NMR, IR, MS and X-ray diffraction of a single crystal of 4 identified the compounds as constituted. The tridentate (C^N^O) ligand precursors were combined with NiCl 2 , NiCl 2 .6H 2 O, NiCl 2 (DME), NiCO 3 , and Ni(OAc) 2 to generate the corresponding in situ NHC-Ni(II) complexes, which were subsequently explored in situ and optimised as catalysts for the transfer hydrogenation of ketones in 2-propanol. Excellent conversions (> 90%) of aromatic and aliphatic ketones to corresponding alcohol products were obtained under the optimised conditions for the in situ catalyst system based on 4 :NiCl 2 at 0.05 mol% catalyst concentration.