Geometry-Driven Iminosemiquinone Radical to Cu(II) Electron Transfer and Stabilization of an Elusive Five-Coordinate...

The noninnocent ligand H₂L^AP(Ph) contained a bulky phenyl substituent at the ortho position to the aniline moiety. The ligand reacted with 0.5 equiv of CuCl₂·2H₂O in the presence of Et₃N under air and provided the corresponding Cu(II)-bis(imonosemiquinone) complex (1). The complex upon oxidation by a stoichiometric amount of ferrocenium hexafluorophosphate (FcPF₆) yielded the four-coordinate [Cu(II)-(imonosemiquinone)(iminoquinone)]PF₆ complex (3), while the oxidation by an equivalent amount of CuCl₂·2H₂O produced the five-coordinate Cu(I)-bis(iminoquinone)Cl complex (2). Thus, a ligand-based oxidation followed by ligand-to-metal electron-transfer was realized for the latter oxidation process. Removal of the Cl^- ion from complex 2 rendered the four-coordinate complex 4. The oxidation state of both Cu(I) and iminoquinone moieties remained unaltered upon the change in the coordination number. All the complexes were characterized by X-ray crystallography. Complexes 2, 3, and 4 were diamagnetic with an S_t = 0 ground state as evident by electron paramagnetic resonance (EPR) and ¹H NMR measurements. The UV-vis-NIR spectra of all the complexes were dominated by charge-transfer transitions. Two oxidations and two reductions waves were noticed in the cyclic voltammogram (CV) of complex 1. Complex 2 and complex 3 underwent one oxidation and three reductions. Unlike complex 3, which experienced ligand-based oxidation, in complex 2 the oxidation was metal-centered [oxidation of Cu(I)-to-Cu(II)]. UV-vis-NIR spectral changes during the fixed-potential coulometric one-electron oxidation and thereafter EPR analysis consolidated the metal-based oxidation in complex 2. Complex 2 was air stable; however, it oxidized KO₂ to oxygen molecule, and complex 1 was formed in due course as evident by UV-vis-NIR spectral changes and EPR measurements. Time dependent density functional theory calculations have been incorporated to assign the transitions that appeared in the UV-vis-NIR spectra of the complexes.