Effect of Distortions on the Geometric and Electronic Structures of One-Electron Oxidized Vanadium(IV), Copper(II), and Cobalt(II)/(III) Salen Complexes

The ligands N,N′-bis­(3-tert-butyl-5-methoxysalicylidene)-1,2-ethanediamine and N,N′-bis­(3-tert-butyl-5-methoxysalicylidene)-1,3-propanediamine were chelated to V­(IV)O (1, 2), Cu­(II) (3, 4), Co­(II) (5), and Co­(III) (6). The X-ray crystal structures of 1–6 were solved. The vanadium center in 1–2 resides in square pyramidal geometry, with an axially bound oxo ligand, whereas the metal ion displays a tetrahedrally distorted square planar geometry in 3–5. The extent of distortion is correlated to the length of the diamine spacer: The longer the linker, the larger the tetrahedral distortions. Complex 6 is octahedral with a bidentate acetate molecule that completes the coordination sphere. All the complexes were characterized by UV–vis and EPR spectroscopies, as well as DFT calculations and electrochemistry. Complexes 1–6 exhibit a reversible one-electron oxidation wave in the range −0.11–0.26 V vs Fc⁺/Fc. The cations 1⁺ and 2⁺ were structurally characterized, showing an octahedral V­(V) ion with one oxo and one water molecule coordinated in axial positions. Their vis–NIR spectra are dominated by a band at 727 and 815 nm, respectively, which is assigned to a phenolate-to-vanadium­(V) charge transfer (CT) transition. The crystal structures of 3⁺ and 4⁺ are congruent with Cu­(II)-radical species, wherein the metal center remains four-coordinated. Both feature a Class II (Robin-Day classification scale) IVCT transition at around 1200 nm (ε > 1 mM cm^–1), indicative of partial localization of the radical. The structure of 5⁺ displays a square pyramidal cobalt ion, where the fifth (axial) coordination is occupied by a water molecule. It displays a NIR feature at 1244 nm and is described as intermediate between high spin Co­(III) and Co­(II) radical. In the presence of acetate the dimer [(5)₂(μ-OAc)]⁺ forms, which was structurally characterized and shows a blue shift and lowering in intensity of the NIR absorption band in comparison to 5⁺. Complex 6⁺ is a genuine Co­(III) radical complex, wherein the phenoxyl moiety is localized on one side of the molecule.