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(R)H ligands (L1(R)H = 2,4-di-R-6-{(2-(pyridin-2-yl)hydrazono)methyl}phenol) and substituted catechols (L(R('))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 (51)V isotropic chemical shifts of solids and solutions of 1-5 are deshielded ((51)V 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 VO(2+) complexes, [(L1(R-))(VO(2+))(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-•))(VO(2+))(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.