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

A new
family of oxidovanadium complexes, [(L1R)­(VO)­(LR′)] (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 L1RH ligands (L1RH = 2,4-di-R-6-{(2-(pyridin-2-yl)­hydrazono)­methyl}­phenol)
and substituted catechols (LR′H2) was substantiated. The V–Ophenolato (cis to VO),
V–OCAT (cis to VO) and V–OCAT (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 (LR′CAT2–) and LR′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, [(L1R–)­(VO2+)­(LR′CAT2–)]− [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 (L1R2–•) complexes, [(L1R2–•)­(VO2+)­(LR′CAT2–)]2–, [1–5]2–, with S = 1 authenticated by the
unrestricted density functional theory (DFT) calculations on 12– and 32– 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 LR′CAT2–. Spectro-electrochemical measurements and time-dependent
DFT (TD DFT) calculations on 1, 3, 1–, 3–, and 12– 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.