Reaction of [Ni(II)(cyclam)(CH(3)CN)(2)](ClO(4))(2) (1) with tert-butylhydroperoxide (TBHP) or H(2)O(2), in acidic media results in a formation of [Ni(III)(cyclam)(CH(3)CN)(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 H(2)O(2) 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 H(2)O(2) and 1. Addition of cis-stilbene as an oxidable substrate quenches the formation of Ni(III) for reaction with H(2)O(2) only. Overall, these observations reveal a different reaction mechanism when reacting H(2)O(2) 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 CH(3)CN, but a proton assisted heterolysis for the O-O activation in H(2)O(2). 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 H(2)O(2) is used as reactant.
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