A combined experimental and theoretical study on a single, unsupported oxo-bridged Mn(iii,iii) dimer coordinated to two iminobenzosemiquinone π-radical anions

Abstract

Ligand H₂L^AP comprises a non-innocent 2-aminophenol unit and an innocent bis(pyridin-2-ylmethyl)amine unit. The ligand, upon reaction with an equivalent amount of Mn(ClO₄)₂·6H₂O in the presence of Et₃N under air in MeOH, provided a mono(oxo)-bridged dinuclear Mn₂ complex ({[(L^ISQ)Mn^III–O–Mn^III(L^ISQ)][(ClO₄)]₂}; 1). X-ray crystal structure analysis of complex 1 revealed that in the dicationic unit, the physical oxidation state of each Mn ion was +III and the 2-aminophenol unit of ligand H₂L^AP was in its one-electron oxidized iminobenzosemiquinone form. ¹H-NMR measurement of complex 1 confirmed that the complex acquired a diamagnetic ground state (S_t = 0). Thus, antiferromagnetic couplings among the paramagnetic centers were realized. The UV-Vis-NIR spectrum of complex 1 was consisted of ligand-to-metal charge-transfer transitions in the visible region, while ligand-to-metal and metal-to-ligand charge-transfer transitions were noticed in the near-infrared region due to the presence of iminobenzosemiquinone radical units. The cyclic voltammogram of the complex showed three one-electron oxidation waves and two one-electron reduction waves. While the first two oxidation processes were metal-based, the two successive reductions were ligand-centered. DFT-based theoretical studies confirmed the assignment.