Cuprous complexes and dioxygen. Part X. Influence of ring size on the autoxidation of Cu(I) complexes with cis‐and trans‐N2S2 macrocyclic ligands

The autoxidation of the Cu+ complexes with a series of six 12‐, 14‐ and 16‐membered macrocycles containing the N2S2 set of donor atoms has been studied with an oxygen electrode and with the stopped‐flow technique. Inspite of the identical set of coordinating groups, the reactivity of the Cu= complexes towards O2 varies by more than 5 orders of magnitude, with rate constants between <0.1 and 2·104 M−1S−1. Simple bimolecular rate laws are obtained from initial rates of autoxidation, but successive one‐electron transfers with intermediate formation of superoxide are implied from the analysis of complete reaction curves for complexes with the 12‐membered macrocycles. The kinetic parameters are compared with the redox potentials for the corresponding CuL2+/CuL+ couples. Only a rather rough correlation is found and steric factors must in addition be responsible for the observed reactivity pattern which shows a decrease of autoxidation rate with increasing size of the macrocyclic ring. No systematic effect was observed for the influence of cis‐ vs. trans‐configuration of the donor atoms.