Oxidovanadium Complexes of 2,2′-Bipyridine, 1,10 Phenanthroline, and p-Nitro-o-aminophenol – Radical versus Nonradical States

Abstract

2,2′-Bipyridine (bpy), 1,10-phenanthronline (phen) and p-nitro-o-aminophenol (^NO2LH₂) oxidovanadium complexes of the types [(L_ONO^2–)(VO)(bpy)] (1), [(L_ONO^2–)(VO)(phen)] (2) and [(L_ONO^2–)(VO)(^NO2L_ISQ^·–)][NHEt₃] (3^– NHEt₃⁺; ^NO2L_ISQ^·– = p-nitro-o-iminobenzosemiquinonate radical) were isolated with (E)-2-(3-hydroxy-3-phenyltriaz-1-en-1-yl)benzoic acid (L_ONOH₂) as a coligand. The single-crystal X-ray bond parameters and electron paramagnetic resonance (EPR) spectra authenticated that 1 and 2 are the oxidovanadium(IV) complexes of bpy and phen, whereas 3^– is the ^NO2L_ISQ^·– complex of oxidovanadium(IV) and undergoes reversible one-electron oxidation and reduction at lower potentials. Density functional theory (DFT) calculations established that the closed-shell-singlet (CSS) solution of 3^– is unstable owing to open-shell-singlet (OSS) perturbation. The CSS  OSS charge-transfer transition that appears at = 632 nm is absent for 1 and 2. Complex 3 is a hybrid of the [(L_ONO^2–)(VO²⁺)(^NO2L_IQ)] (^NO2L_IQ = p-nitro-o-iminobenzoquinone) and [(L_ONO^2–)(VO³⁺)(^NO2L_ISQ^·–)] states, whereas 3^2– is an oxidovanadium(IV) complex of the type [(L_ONO^2–)(VO²⁺)(^NO2L_AP^2–)]^2– (^NO2L_AP^2– = p-nitro-o-amidophenolato).

Oxidovanadium(IV) complexes 1–3 of 2,2′-bipyridine, 1,10-phenanthronline and the p-nitro-o-iminobenzosemiquinonate radical are reported. The 3^–  3 and 3^–  3^2– conversions are reversible owing to the formation of p-nitro-o-iminobenzoquinone and p-nitro-o-amidophenolato complexes.

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