Generation of the V(NO)2+ moiety from VO43– using hydroxylamine and pseudo-halide ions: synthesis and electrochemistry of [V(NO)2(L–L)2]X (L–L = 2,2′-bipyridine or 1,10-phenanthroline)

Abstract

Vanadate in aqueous alkali reacted with NH₂OH·HCL and X^–(X^–= NCS^–, N₃^– or CN^–) ions only in the presence of L–L [L–L = 1,10-phenanthroline (phen) or 2,2′-bipyridine (bipy)] ligands and at ca. 35 °C to yield [V(NO)₂(L–L)₂]X (X = NCS 1, N₃2 or CN 9, L–L = phen; X = NCS 3, N₃4 or CN 10, L–L = bipy). Anion metathesis yields [V(NO)₂(phen)₂]Y (Y = ClO₄5 or BPh₄6) and [V(NO)₂(bipy)₂]Y (Y = ClO₄7 or BPh₄8). The complexes behave as 1 : 1 electrolytes in methanol, acetonitrile and dimethylformamide and exhibit four-band electronic spectra. The solids are diamagnetic, compatible with the existence of a {V(NO)₂}⁶ moiety in all the complexes. Since the NO ligands occupy cis positions, the overall symmetry of the molecules is low, which is supported by the ¹H NMR data of 1 and 3, which show that two L–L ligands are in different magnetic environments. All the complexes exhibit two successive L–L centred single-electron reductive cyclic responses at negative potentials corresponding to E₂₉₈° of ca.–1.30 and ca.–1.50 V vs. saturated calomel electrode (SCE), typical for some d⁶ systems. At positive potentials, the phen complexes show one metal-centred oxidative cyclic response at E₂₉₈°ca. 0.60 V vs. SCE and two irreversible oxidative responses at higher potentials. The bipy complexes, however, show four consecutive irreversible oxidative peaks. In the case of the phen complex 5, the first oxidative response has been confirmed as metal centred by EPR data after exhaustive coulometry, which shows a typical ⁵¹V hyperfine (octet), as well as superhyperfine splitting (nonet) due possibly to the interaction of the unpaired electron with phen nitrogens.