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

Abstract

A new family of oxidovanadium complexes, [(L₁^R)­(VO)­(L^{R^′})] (R = H, R′ = H, <b>1</b>; R = H, R′ = -CMe₃, <b>2</b>; R = H, R′ = Me, <b>3</b>; R = -CMe₃, R′ = H, <b>4</b> and R = -CMe₃, R′ = -CMe₃, <b>5</b>), incorporating tridentate L₁^RH ligands (L₁^RH = 2,4-di<i>-</i>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 <b>1</b>–<b>5</b> are fractional and vary from −1.43 to −1.60. The ⁵¹V isotropic chemical shifts of solids and solutions of <b>1</b>–<b>5</b> 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 <b>1</b>–<b>5</b> are unstable due to open shell singlet (OSS) perturbations. The ground electronic states of <b>1</b>–<b>5</b> 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. <b>1</b>–<b>5</b> 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–)]⁻ [<b>1</b>–<b>5</b>]⁻. <b>1</b>–<b>5</b> 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–, [<b>1</b>–<b>5</b>]^2–, with <i>S</i> = 1 authenticated by the unrestricted density functional theory (DFT) calculations on <b>1</b>^2– and <b>3</b>^2– ions. Frozen glasses electron paramagnetic resonance (EPR) spectra of [<b>1</b>–<b>5</b>]⁻ ions [e.g., for <b>2</b>, <i>g</i>_|| = 1.948, <i>g</i>_⊥ = 1.979, <i>A</i>_|| = 164, <i>A</i>_⊥ = 60] affirmed that [<b>1</b>–<b>5</b>]⁻ ions are the [VO]²⁺ complexes of L^R′_CAT^2–. Spectro-electrochemical measurements and time-dependent DFT (TD DFT) calculations on <b>1</b>, <b>3</b>, <b>1</b>^–, <b>3</b>^–, and <b>1</b>^2– disclosed that the near infrared (NIR) absorption bands of <b>1</b>–<b>5</b> 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 [<b>1</b>–<b>5</b>]⁻ and [<b>1</b>–<b>5</b>]^2– ions