Switching and redox isomerism in first-row transition metal complexes containing redox active Schiff base ligands

Abstract

The reversible redox isomerisms in first row transition metal complexes of the type ML₂ were studied. The six ML₂ complexes (M = Mn^III (1), Fe^II (2), Co^III (3), Ni^II (4), Cu^II (5) and Zn^II (6)) were synthesized with a redox active Schiff base ligand [2-(3,5-di-tert-butyl-2-hydroxyphenylamino)-4-chlorophenol] (H₃L) presenting different oxidation states from −2 to 0 (L²⁻, L⁻ and L⁰). EPR spectra and magnetic susceptibility measurements indicate the presence of complexes of the type [Mn^III(L²⁻)(L⁻)] (1) with S = 1/2, [Fe^II(L⁻)₂] (2) with S = 2, [Co^III(L²⁻)(L⁻)] (3) with S = 1/2, [Ni^II(L⁻)₂] (4) with S = 1, [Cu^II(L⁻)₂] (5) with S = 1/2 and [Zn^II(L⁻)₂] (6) with S = 0 at high temperatures. Temperature and solvents influence the electronic structures of the species and give several valence tautomers. At low temperatures these complexes present thermally induced metal-to-ligand (1, 2, 4) or ligand-to-ligand (in 5, 6) electron transfer (partial or total), resulting in compounds of the type [Mn^IV(L²⁻)₂] (1′) with S = 1/2, [Fe^III(L⁻)(L²⁻)] (2′) with S = 1, [Ni^III(L²⁻)(L⁻)] (4′) with S = 0, [Cu^II(L²⁻)(L⁰)] (5′) with S = 1/2 and [Zn^II(L²⁻)(L⁰)] (6′) with S = 1. This electron transfer is in agreement with the general trend in the redox potentials of the first row transition metal ions from Mn(II) to Zn(II), and it allows us to prepare and stabilize reversibly switchable tautomeric forms in transition metal complexes with redox-active ligands.