2,2′-Azobisphenolate Ligand Bearing Electron-Donating Methoxy Substituents: Spin-Crossover Ligand Field for a Fe(III) Complex and Stabilization of Ligand-Centered Oxidation Species Due to Its Resonance Effect

To investigate the electron-donating substitution effect on spin crossover (SCO) and electrochemical behaviors for homoleptic trivalent metal complexes with 2,2′-azobisphenolate (azp) ligands, the known 2,2′-azobisphenol with 5,5′-dimethyoxy substituents (H2LOMe) was synthesized and characterized by NMR spectroscopy and single-crystal X-ray diffraction (SCXRD). The homoleptic FeIII and AlIII complexes 1-OMe and 2-OMe were prepared along with the homoleptic FeIII complex with 5,5′-dimethyl-substituted azp ligands 1-Me. The temperature dependence of magnetic susceptibility and SCXRD revealed that 1-OMe exhibits incomplete gradual SCO with the rotational motion of the ligands. Cyclic voltammograms and their simulations revealed that both the FeIII complex 1-OMe and the AlIII complex 2-OMe exhibit reversible one-electron oxidation waves, indicating that the introduction of the methoxy group enhances the stability of the oxidized species of the complexes. DFT calculations revealed that the oxidation waves for both the FeIII and AlIII complexes are ligand-centered, while the reduction wave for the FeIII complexes 1-OMe and 1-Me is metal-centered. In addition, the stability of the oxidized complex species arises from the resonance effect of the methoxy group.