Ligand radical localization in a nonsymmetric one-electron oxidized Ni(II) bis-phenoxide complex.

Abstract

The interplay of electronic structure and reactivity in transition metal complexes is an area of considerable research effort.[1, 2] The cooperative effect of redox-active ligands and metal sites in enzymatic systems,[3] and more recently in synthetic systems,[4] adds significant flexibility to catalyst function. Depending on the relative energies of the redoxactive orbitals, metal complexes with proradical ligands can exist in a limiting description as a metal-ligand radical (Mn+(L•)) or a high valent metal complex (M(n+1)+(L-)). In certain cases, subtle changes to the system through variation of the ligand field, or temperature is sufficient to shift the oxidation locus.[5, 6] Recent work in this area has focused on bis(salicylidene)diamine complexes 1-3 (Scheme 1).[6-11] The one-electron oxidized Ni derivatives exist in the ligand radical form NiII(L•-) in solution and the solid state, however the addition of exogenous ligands to NiII(L•-) in solution results in a shift in the oxidation locus to the NiIII(L2-) form.[7-10] The oxidized Cu derivative of 1 exists as the high valent metal complex in the solid state. In solution this complex exhibits temperature-dependent valence tautomerism between the ligand radical and high valent metal forms, demonstrating the nearly isoenergetic nature of the two species.[6]