NHC Ligand-Based Half-Sandwich Iridium Complexes: Synthesis, Structure and Catalytic Activity in Acceptorless Dehydrogenation and Transfer Hydrogenation

A set of neutral C,C-chelate half-sandwich iridium(III) complexes have been prepared with NHC ligands that contain pendant aromatic rings as potentially chelating donor sites. The catalytic activity of such iridium complexes has been investigated for the acceptorless dehydrogenation (AD) reactions of alcohols and for the transfer hydrogenation reactions of ketones. The prepared iridium(III) complexes show excellent catalytic activity for AD reactions of a wide range of secondary alcohols, and they are also shown to be effective for the synthesis of aldehydes from primary alcohols without the observation of undesired byproducts such as esters. Additionally, these complexes are also highly efficient in transfer hydrogenation of ketones and aldehydes, which give the alcohols in good yields under mild conditions. The exact structure and bonding mode of the NHC-based iridium complexes was identified by various spectroscopies and single crystal X-ray analysis.