Coordination of o-benzosemiquinonate, o-iminobenzosemiquinonate, 4,4′-di-tert-butyl-2,2′-bipyridine and 1,10-phenanthroline anion radicals to oxidovanadium(iv)

Abstract

This article reports on the stabilization of organic radical anions promoted by the oxidovanadium(IV) ion. A 3,5-di-tert-butylcatecholate (^tBucatH⁻) complex of oxidovanadium(V) of the type [(L^a_ONO²⁻)(VO³⁺)(^tBucatH⁻)] (1) was isolated using tridentate (E)-N′-((3-hydroxynaphthalen-2-yl)methylene)benzohydrazide (L^a_ONOH₂) as a coligand, whereas o-benzosemiquinonate (sq˙⁻) and p-nitro-o-iminobenzosemiquinonate (^NO2isq˙⁻) radical anion complexes of oxidovanadium(IV) of the types [(L_NNO⁻)(VO²⁺)(sq˙⁻)] (2) and [(L_NNO⁻)(VO²⁺)(^NO2isq˙⁻)] (3) were successfully isolated using (1Z,N′E)-N′-(phenyl(pyridin-2-yl)methylene)benzohydrazonic acid (L_NNOH) as a coligand. Oxidovanadium(IV) complexes of the types [(L^b_ONO²⁻)(VO²⁺)(phen)] (4) and [(L^b_ONO²⁻)(VO²⁺)(^tBubpy)] (5), which undergo reversible reduction to the 4,4′-di-tert-butyl-2,2′-bipyridine radical anion (^tBubpy˙⁻) and the 1,10-phenanthroline radical anion (phen˙⁻) to afford the coupled states [(L^b_ONO²⁻)(VO²⁺)(phen˙⁻)]⁻ (4⁻) and [(L^b_ONO²⁻)(VO²⁺)(^tBubpy˙⁻)]⁻ (5⁻), respectively, were isolated (L^b_ONOH₂ = (E)-N′-(2-hydroxybenzylidene)benzohydrazide). The molecular geometries of the complexes were confirmed by the single-crystal X-ray structure determinations of 1, 3 and 4. In 1, the V–O_phenolato length cis to VO is 1.879(2) Å and the dissimilar V–O and V–OH lengths corresponding to the ^tBucatH⁻ ligand are 1.832(2) and 2.312(2) Å, respectively. In 1, the average C–O lengths in ^tBucatH⁻ are 1.351(3) Å, whereas in 3 the average C–O and C–N lengths in ^NO2isq˙⁻ are 1.293(4) and 1.355(5) Å, respectively. In 4, the V–O_phenolato length cis to VO (1.937(3) Å) is relatively longer. The ⁵¹V NMR spectrum of 1 displays a signal at −337.2 ppm, whereas the signals for 2 and 3 are deshielded to +382.4 and +71.8 ppm, respectively. The closed-shell singlet (CSS) solutions of 3 and 5⁻ at the B3LYP/DFT level are unstable and the open-shell singlet (OSS) solutions are 0.5 and 7.3 kcal mol⁻¹ lower in energy, respectively, than the CSS solutions. In 3 and 5⁻ the alpha spin (100%) is localized on the vanadium ion, whereas the beta spin is delocalized across the aminophenol and bipyridine fragments. 2 and 3 exhibit lower-energy absorption bands at 785 and 585 nm, which are defined as CSS → OSS perturbation transitions.