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.