Electronic Structure Evaluation of an Oxidized Tris(methoxy)-Substituted Ni Salen Complex

The Ni salen complex 1 (N,N’– bis(2,3,4–tri–methoxysalicylidene)–1,2–cyclohexane–(1R,2R)–diamine Nickel(II)) was prepared containing ortho-, meta-, and para-methoxy substituted phenolate moieties. Electrochemical studies revealed that the first oxidation of 1 occurs at a similar potential to a previously reported Ni salen complex NiSaltBu,OMe employing an ortho-tBu and para-methoxy substitution pattern (Angew. Chem. Int. Ed. 2010, 49, 4989), demonstrating the counteracting effects of the methoxy substituent depending on ring location (ortho/para vs. meta). The second oxidation occurred at a much lower potential (E1/22 - E1/21 = 0.11 V) for 1, in comparison to NiSaltBu,OMe, suggesting significant localization of the ligand radical. The one-electron oxidized form was prepared and studied using a number of experimental and theoretical methods. A localized ligand radical electronic structure was confirmed by the shape and intensity of the NIR bands, which was further verified by the presence of both phenolate and phenoxyl signatures in the resonance Raman spectrum. Theoretical calculations provided insight into the degree of ligand radical localization, and the nature of the low energy bands in the vis-NIR experiment. Overall, the oxidation of 1 results in a localized ligand radical complex, with further oxidation processes easily accessible due to resonance stabilization afforded by the methoxy substituents.