Geometry-Driven Iminosemiquinone Radical to Cu(II) Electron Transfer and Stabilization of an Elusive Five-Coordinate Cu(I) Complex: Synthesis, Characterization, and Reactivity with KO₂

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The noninnocent ligand H 2 L AP(Ph) contained a bulky phenyl substituent at the ortho position to the aniline moiety. The ligand reacted with 0.5 equiv of CuCl 2 ·2H 2 O in the presence of Et 3 N under air and provided the corresponding Cu(II)-bis(imonosemiquinone) complex ( 1 ). The complex upon oxidation by a stoichiometric amount of ferrocenium hexafluorophosphate (FcPF 6 ) yielded the four-coordinate [Cu(II)-(imonosemiquinone)(iminoquinone)]PF 6 complex ( 3 ), while the oxidation by an equivalent amount of CuCl 2 ·2H 2 O 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 1 H 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 KO 2 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.