Alkoxo bound monooxo- and dioxovanadium(V) complexes: Synthesis, characterization, x-ray crystal structures, and solution reactivity studies

A large variety of oxovanadium(V) complexes, mononuclear VO2+ and VO3+ in addition to the dinuclear VO3+, of the structural type (VOL)(2), (VOHL)(2), VOLHQ, K(VO(2)HL), K(VO(2)H(2)L), or (salampr) (VO(2)L) {where L = Schiff base ligand possessing alkoxo group(s); HQ = 8-hydroxyquinoline; salampr = cation of reduced Schiff base derived from salicylaldehyde and 2-amino-2-methylpropan-1-ol}, bound to alkoxo, phenolate and imine groups have been synthesized in high yields and characterized by several spectral and analytical methods, including single crystal X-ray studies. While the mononuclear VO2+ complexes have been synthesized at alkaline pH, the dinuclear VO3+ complexes have been synthesized under neutral conditions using alkoxo rich Schiff base ligands. The X-ray structures indicate that the cis-dioxo complexes showed longer V-O-alkoxo bond lengths compared to the monooxo counterparts. The plot of V-O-phen bond distances of several VO3+ complexes vs the lmct showed a near linear correlation with a negative slope. The cyclic voltammograms revealed a reversible V(V)/V(IV) couple with the reduction potentials increasing to more negative ones as the number of alkoxo groups bound to V increases from 1 to 2. Moreover, the cis-dioxo VO2+ complexes are easier to reduce than their monooxo counterparts. The solution stability of these complexes was studied in the presence of added water (1:4, water:solvent), where no decomposition was observed, unlike other Schiff base complexes of V. The conversion of the dioxo complexes to their monooxo counterparts in the presence of catalytic amounts of acid is also demonstrated. The reactivity of alkoxo bound V(V) complexes is also reported. X-ray parameters are as follows. H(4)L(3): monoclinic space group, P2(1)/c; a = 10.480(3), b = 8.719(6), c = 12.954(8) Angstrom; beta = 101.67(4)degrees; V = 1126(1) Angstrom(3); Z = 4; R = 0.060, R(w) = 0.058. Complex 1: monoclinic space group, P2(1)/n; a = 12.988(1), b = 9.306(2), c = 19.730(3) Angstrom; beta = 99.94(1)degrees; V = 2348.9(7) Angstrom(3); Z = 4; R = 0.031, R(w) = 0.027. Complex 2: monoclinic space group, P2(1)/n; a = 12.282(3), b = 11.664(2), c = 12.971(4) Angstrom; beta = 97.89(2)degrees; V = 1840.5(8) Angstrom; Z = 4; R = 0.035, R(w) = 0.038. Complex 5: monoclinic space group, P2(1)/c; a = 17.274(2), b = 6.384(2), c - 16.122(2) Angstrom; beta = 116.67(1)degrees; V = 1588.7(7) Angstrom(3); Z = 4; R = 0.039, R(w) = 0.043. Complex 8: monoclinic space group, P2(1)/c; a = 11.991(1), b = 11.696(4), c = 12.564(3) Angstrom; beta = 110.47(1)degrees; V = 1650.8(8)Angstrom(3); Z = 2; R = 0.045, R(w) = 0.049.