Catalytic oxidation of primary aromatic alcohols using half sandwich Ir(III), Rh(III) and Ru(II) complexes: A practical and theoretical study

Abstract

The complexes [Cp*IrCl(N-(pyridin-2-ylmethylene)aniline)]PF6 (1), [Cp*RhCl(N-(pyridin-2-ylmethylene)aniline)]PF6 (2), (where Cp*=1,2,3,4,5-pentamethylcyclopentadiene) and [η6-areneRuCl(N-(pyridin-2-ylmethylene)aniline)]PF6 (3) have been synthesized and the structure and purity of these were confirmed by single crystal XRD and elemental analyses. Iridium and rhodium complexes exhibit the P21/n space group, the ruthenium complex the P21/c space group and all three complexes show the expected pseudo octahedral “piano-stool” geometry. The catalytic performance of these complexes for the dehydrogenation of primary alcohols to their respective aldehydes with different bases and solvents was investigated. The complexes of iridium and ruthenium give good conversions in different alkaline solutions. Density functional theory was applied to determine the respective MO energy levels, bond lengths, bond angles and binding energies of all the metal complexes. It was also used to study the activity, stability and intermediates of the complexes. A Gibbs free energy (ΔG) DFT calculation was carried out to help understand the reaction mechanism/catalytic cycle of the Rh complex (2). The energy barrier for oxidation of aromatic alcohols by the rhodium hydride complex is much lower (10.32kcal/mol) than the barrier for hydride transfer of the corresponding Rh benzyloxo species (15.19kcal/mol), in agreement with mechanisms proposed for related systems.