Oxidovanadium Catechol Complexes: Radical versus Non-Radical States and Redox Series

A new family of oxidovanadium complexes, [(L₁^R)­(VO)­(L^R′)] (R = H, R′ = H, 1; R = H, R′ = -CMe₃, 2; R = H, R′ = Me, 3; R = -CMe₃, R′ = H, 4 and R = -CMe₃, R′ = -CMe₃, 5), incorporating tridentate L₁^RH ligands (L₁^RH = 2,4-di-R-6-{(2-(pyridin-2-yl)­hydrazono)­methyl}­phenol) and substituted catechols (L^R′H₂) was substantiated. The V–O_phenolato (cis to VO), V–O_CAT (cis to VO) and V–O_CAT (trans to VO) lengths span the ranges, 1.894(2)–1.910(2), 1.868(2)–1.887(2), and 2.120(2)–2.180(2) Å. The metrical oxidation states (MOS) of the catechols in 1–5 are fractional and vary from −1.43 to −1.60. The ⁵¹V isotropic chemical shifts of solids and solutions of 1–5 are deshielded (⁵¹V CP MAS: −19.8 to +248.6; DMSO-d₆: +173.9 to +414.55 ppm). The closed shell singlet (CSS) solutions of 1–5 are unstable due to open shell singlet (OSS) perturbations. The ground electronic states of 1–5 are defined by the resonance contribution of the catecholates (L^R′_CAT^2–) and L^R′_SQ^–• coordinated to the [VO]³⁺ and [VO]²⁺ ions. 1–5 are reversibly reducible by one electron at −(0.58–0.87) V, referenced vs ferrocenium/ferrocene, to VO²⁺ complexes, [(L₁^R–)­(VO²⁺)­(L^R′_CAT^2–)]⁻ [1–5]⁻. 1–5 display another quasi-reversible or irreversible reduction wave at −(0.80–1.32) V due to the formation of hydrazone anion radical (L₁^R2–•) complexes, [(L₁^R2–•)­(VO²⁺)­(L^R′_CAT^2–)]^2–, [1–5]^2–, with S = 1 authenticated by the unrestricted density functional theory (DFT) calculations on 1^2– and 3^2– ions. Frozen glasses electron paramagnetic resonance (EPR) spectra of [1–5]⁻ ions [e.g., for 2, g_|| = 1.948, g_⊥ = 1.979, A_|| = 164, A_⊥ = 60] affirmed that [1–5]⁻ ions are the [VO]²⁺ complexes of L^R′_CAT^2–. Spectro-electrochemical measurements and time-dependent DFT (TD DFT) calculations on 1, 3, 1^–, 3^–, and 1^2– disclosed that the near infrared (NIR) absorption bands of 1–5 at 800 nm are due to the CSS-OSS metal to ligand charge transfer which are red-shifted in the solid state and disappear in [1–5]⁻ and [1–5]^2– ions.