Controlling the Reactivity of Co(III)-Hydroperoxo Species by Tuning Electronic Property of TBDAP Ligand

Abstract

Metal-O2 adducts such as metal-hydroperoxo species play critical roles in metabolism mediated by Metalloenzymes. These metalloenzymes are known to have varying selectivity and reactivity depending on the electronic effect obtained from substituents in the second coordination sphere. Herein, Cl2-TBDAP and OMe2-TBDAP were synthesized as derivatives of a macrocyclic N4-type tetradentate TBDAP ligand (TBDAP = N,N′-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane). Three cobalt(III)-hydroperoxo complexes, [CoIII(Cl2-TBDAP)(O2H)(CH3CN)]2+, [CoIII(TBDAP)(O2H)(CH3CN)]2+ and [CoIII((OMe)2-TBDAP)(O2H)(CH3CN)]2+, were synthesized and characterized with various physicochemical methods, such as UV-vis, ESI-MS and resonance Raman. The electrochemical studies reveal that difference of electronic properties was derived by the substituents at the para-position of the pyridine rings of TBDAP. In the phosphine oxidation of PPh3, the reaction rates of the cobalt(III)-hydroperoxo complexes were observed to be in the order of [CoIII((OMe)2-TBDAP)(O2H)(CH3CN)]2+ > [CoIII(TBDAP)(O2H)(CH3CN)]2+ > [CoIII(Cl2-TBDAP)(O2H)(CH3CN)]2+. This difference in the reactivity may have been derived from the electronic effect of the substituents in the para-position of the pyridine rings in supporting ligands.