Steric tuning of OH-functionalised N,N-manganese(I) complexes for the transfer hydrogenation of carbonyl compounds.

A new family of manganese(I) carbonyl complexes, [2-OH-6-{CMeN(2,6-R12-4-R2C6H2)}C5H3N]MnBr(CO)3 (R1 = iPr, R2 = H Mn1a, R1 = iPr, R2 = Br Mn1b, R1 = H, R2 = iPr Mn1d, R1 = H, R2 = Me Mn1e), bearing 6-(arylimino)pyridine N,N-chelating ligands that are appended with a 2-hydroxyl group, have been synthesised and evaluated as catalysts in the transfer hydrogenation (TH) of acetophenone and its derivatives. This set of complexes, differing in their steric and electronic properties of their N-aryl groups, reveal distinct catalytic performances with the least sterically hindered N-4-isopropylphenyl derivative, Mn1d, the most active allowing TON's of up to 1030. Moreover, Mn1d proved highly effective for an assortment of aryl-methyl ketones with the steric/electronic profile of the substrate having an observable effect on conversion. Mechanistic studies involving DFT calculations point towards an outer-sphere mechanism that is distinctly affected by the steric properties of the N-aryl groups of the manganese catalyst.