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

Abstract

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(VO₂HL), K(VO₂H₂L), or (salampr) (VO₂L) {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. H₄L₃: monoclinic space group, P2₁/c; a = 10.480(3), b = 8.719(6), c = 12.954(8) Å; β = 101.67(4)°; V = 1126(1) ų; Z = 4; R = 0.060, R_w = 0.058. Complex 1: monoclinic space group, P2₁/n; a = 12.988(1), b = 9.306(2), c = 19.730(3) Å; β = 99.94(1)°; V = 2348.9(7) ų; Z = 4; R = 0.031, R_w = 0.027. Complex 2: monoclinic space group, P2₁/n; a = 12.282(3), b = 11.664(2), c = 12.971(4) Å; β = 97.89(2)°; V = 1840.5(8) Å; Z = 4; R = 0.035, R_w = 0.038. Complex 5: Monoclinic space group, P2₁/c; a = 17.274(2), b = 6.384(2), c = 16.122(2) Å; β = 116.67(1)°; V = 1588.7(7) ų; Z = 4; R = 0.039, R_w = 0.043. Complex 8: monoclinic space group, P2₁/c; a = 11.991(1), b = 11.696(4), c = 12.564(3) Å; β = 110.47(1)°; V = 1650.8(8) ų; Z = 2; R = 0.045, R_w = 0.049.