Mechanism of reaction of alkyl radicals with (Ni^IIL) ²⁺ complexes in aqueous solutions

The reactions of methyl radicals, CH₃, with the macrocyclic complexes Ni^IIL^1-5 (L^1-5 = cyclam derivatives, vide infra) and Ni^IIedta in aqueous solutions were studied. Methyl radicals react with all these nickel complexes, forming intermediates with Ni^III-C σ-bonds. The L_mNi^III-CH ₃ complexes are formed in equilibria processes with relatively fast forward rate constants of k_f > 1 × 10⁸ M ^-1 s^-1 (except in the case of NiL²-trans I cyclam, where the reaction is slower). In all cases the decomposition of the transient complexes occurs via the homolytic cleavage of the metal-carbon σ-bond. When the homolysis is relatively slow, an isomerisation process of the transient is also observed with the exception of NiL², where no isomerisation was observed. The results suggest that the strength of the Ni ^III-CH₃ σ-bond is mainly affected by steric hindrance.