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

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.