Title : Mechanistic understanding of the transfer hydrogenation of ketones catalysed by imino-NHC Co(III) complexes by spectroscopic and analytical methods
Abstract:
New symmetric octahedral Co complexes of tridentate imino-N-heterocyclic carbene (NHC) of the form [C^N^O(Co+3)C^N^O] were used as highly active catalysts in the catalytic transfer hydrogenation (CTH) of a range of ketones in 2-propanol as the solvent and hydrogen donor. Based on a low catalyst concentration of 0.4-mol%, significant conversions in the range of 70% to 99% were recorded at high turnover frequencies up to 1635 h-1. A mechanism to account for the steps involved in the CTH of cyclohexanone is proposed and supported by data from cyclic voltammetry, low resolution MS, UV-Vis and IR spectroscopic techniques. The UV studies displayed two strong absorption maxima at 250 and 450 nm which are assigned to the Co-O and Co-N metal to ligand charge transfer bands respectively. Monitoring these coordinate bonds indicated the hemilability of the imino (Co-NC) donor during the catalytic cycle and a clear sign that the phenoxy (Co-O) donor remained unchanged throughout the course of the reaction. Also, the IR spectra revealed that both Co-N and Co-O absorption bands remained unchanged in the region 486 and 426 cm-1 respectively. In combination with data from the MS and CV studies, a one-electron (Co3+/Co2+) based mechanism is proposed as the pathway for the CTH processes that involve cobalt hydride/alkoxide species as the intermediate products.
