O–O Bond activation in H2O2 and (CH3)3C-OOH mediated by [Ni(cyclam)(CH3CN)2](ClO4)2: Different mechanisms to form the same Ni(iii) product?

Abstract

Reaction of [Ni^II(cyclam)(CH₃CN)₂](ClO₄)₂ (1) with tert-butylhydroperoxide (TBHP) or H₂O₂, in acidic media results in a formation of [Ni^III(cyclam)(CH₃CN)₂]³⁺ species (2), the nature of which is characterized by UV-vis, EPR and XPS. The formation rate of 2 is much higher when H₂O₂ is used as oxidant. In absence of acid, TBHP reacts with 1 generating the same Ni(III) species but, in contrast, no reaction is observed between H₂O₂ and 1. Addition of cis-stilbene as an oxidable substrate quenches the formation of Ni(III) for reaction with H₂O₂ only. Overall, these observations reveal a different reaction mechanism when reacting H₂O₂ with 1 than when reacting the same metal complex with TBHP, despite the fact that the final product is the same. The proposed pathways arising from these observations consist in a homolytic O–O cleavage for the reaction of 1 with TBHP in CH₃CN, but a proton assisted heterolysis for the O–O activation in H₂O₂. Density functional calculations (B3LYP and OPBE) on the thermodynamic feasibility of the two reaction processes support the proposed mechanisms, since the O–O homolysis is strongly disfavored when H₂O₂ is used as reactant.