Defining the Electronic and Geometric Structure of One-Electron Oxidized Copper−Bis-phenoxide Complexes

The geometric and electronic structure of an oxidized Cu complex ([CuSal]+; Sal = N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-(1R,2R)-diamine) with a non-innocent salen ligand has been investigated both in the solid state and in solution. Integration of information from UV−vis−NIR spectroscopy, magnetic susceptibility, electrochemistry, resonance Raman spectroscopy, X-ray crystallography, X-ray absorption spectroscopy, and density functional theory calculations provides critical insights into the nature of the localization/delocalization of the oxidation locus. In contrast to the analogous Ni derivative [NiSal]+ (Storr, T.; et al. Angew. Chem., Int. Ed. 2007, 46, 5198), which exists solely in the Ni(II) ligand-radical form, the locus of oxidation is metal-based for [CuSal]+, affording exclusively a Cu(III) species in the solid state (4−300 K). Variable-temperature solution studies suggest that [CuSal]+ exists in a reversible spin-equilibrium between a ligand-radical species [Cu(II)Sal•]+ (S = 1) and the high-valent metal form [Cu(III)Sal]+ (S = 0), indicative of nearly isoenergetic species. It is surprising that a bis-imine−bis-phenolate ligation stabilizes the Cu(III) oxidation state, and even more surprising that in solution a spin equilibrium occurs without a change in coordination number. The oxidized tetrahydrosalen analogue [CuSalred]+ (Salred = N,N′-bis(3,5-di-tert-butylhydroxybenzyl)-1,2-cyclohexane-(1R,2R)-diamine) exists as a temperature-invariant Cu(II)−ligand-radical complex in solution, demonstrating that ostensibly simple variations of the ligand structure affect the locus of oxidation in Cu−bis-phenoxide complexes.