Coordination of o-benzosemiquinonate, o-iminobenzosemiquinonate and aldimine anion radicals to oxidovanadium(iv)

Abstract

The study focuses on the stabilization of organic radicals by the oxidovanadium(IV) ion and it proves to be significant in exploring the bioactivity of vanadium. In addition to the o-benzosemiquinonate and o-iminobenzosemiquinonate anion radicals, the existence of reactive aldimine anion radicals coordinated to oxidovanadium(IV) ion was detected. Radical and non-radical oxidovanadium(IV) complexes of the types [(L₁⁻)(V^IVO)(acac)] (1), [(L₂⁻)(V^IVO)(acac)] (2), [(L₁⁻)(V^IVO)(sq˙⁻)] (3), [(L₁⁻)(V^IVO)(^t-Busq˙⁻)] (4), [(L₁⁻)(V^IVO)(^NO₂isq˙⁻)] (5) and [(L₁⁻)₂(V^IVO)₂(SO₄)]·½CH₂Cl₂ (6·½CH₂Cl₂) containing redox non-innocent tridentate NNO-donor aldimines (L₁H and L₂H) as coligands are reported (acac = acetylacetonato, sq˙⁻ = o-benzosemiquinonate, ^t-Busq˙⁻ = 3,5-di-tert-butyl-o-benzosemiquinonate and ^NO₂isq˙⁻ = p-nitro-o-iminobenzosemiquinonate anion radicals). The sq˙⁻, ^t-Busq˙⁻ and ^NO₂isq˙⁻ states in complexes were established by X-ray crystallography, EPR spectroscopy and solid state cross polarization magic angle spinning (CP/MAS) ⁵¹V NMR spectroscopy, where the ⁵¹V nuclei in 3–5 were deshielded in a range from −100.3 to +608.7 ppm. The cathodic waves of 1 and 2 due to L₁⁻/L₁˙²⁻ and L₂⁻/L₂˙²⁻ redox couples are reversible. Density functional theory (DFT) calculations authenticated that 1⁻ and 2⁻ are open shell pi radical (L₁˙²⁻ and L₂˙²⁻) complexes of oxidovanadium(IV). The energies of the open shell singlet (OSS) and triplet solutions of 1⁻ and 2⁻ are lower than the corresponding closed shell singlet (CSS) solutions. In 1⁻ and 2⁻ ions, 35–39% beta spin is localized on the π_aldimine* function. The UV-vis-NIR absorption spectra of the complexes were analyzed by spectroelectrochemical measurements and time dependent (TD) DFT calculations.