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

Abstract

The ligands <i>N,N</i>'-bis(3-<i>tert</i>-butyl-5-methoxysalicylidene)-1,2-ethanediamine and <i>N,N</i>'-bis(3-<i>tert</i>-butyl-5-methoxysalicylidene)-1,3-propanediamine were chelated to V(IV)═O (<b>1</b>, <b>2</b>), Cu(II) (<b>3</b>, <b>4</b>), Co(II) (<b>5</b>), and Co(III) (<b>6</b>). The X-ray crystal structures of <b>1</b>-<b>6</b> were solved. The vanadium center in <b>1</b>-<b>2</b> resides in square pyramidal geometry, with an axially bound oxo ligand, whereas the metal ion displays a tetrahedrally distorted square planar geometry in <b>3</b>-<b>5</b>. The extent of distortion is correlated to the length of the diamine spacer: The longer the linker, the larger the tetrahedral distortions. Complex <b>6</b> 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 <b>1</b>-<b>6</b> exhibit a reversible one-electron oxidation wave in the range -0.11-0.26 V vs Fc⁺/Fc. The cations <b>1</b>^<b>+</b> and <b>2</b>^<b>+</b> 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 <b>3</b>^<b>+</b> and <b>4</b>^<b>+</b> 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 <b>5</b>^<b>+</b> 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 [(<b>5</b>)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 <b>5</b>^<b>+</b>. Complex <b>6</b>^<b>+</b> is a genuine Co(III) radical complex, wherein the phenoxyl moiety is localized on one side of the molecule.