Synthesis, magnetism, electrochemistry and reactivity of dinuclear vanadium(III) complexes of a macrocyclic tetraimine Schiff base

Transmetallation of the tetraimine Schiff-base complex [Ba(H(2)L)(H2O)(2)][ClO4](2) with vanadium(III) trichloride, vanadyl(IV) sulfate trihydrate or vanadyl(IV) dichloride dihydrate gave [V(2)L(H2O)(4)][ClO4]Cl-2(2) 1, [V(2)L(H2O)(4)][ClO4](4) . H2O 2 and [V(2)L(H2O)(4)][SO4](2) . H2O 3, respectively {H(2)L = 1,7,14,20-tetramethyl-2,6,15,19-tetraaza[7.7](2,6)pyridinophane-1,6,14,19-tetraene-4,17-diol}. Incorporation of two vanadium(III) ions within the macrocyclic cavity also resulted from the template action of these oxovanadium(IV) salts upon 2,6-diacetylpyridine and 1,3-diaminopropan-2-ol in refluxing methanol. No evidence was obtained for any vanadium(IV)-containing species having been produced en route. Metathetical precipitation reactions of 3 with various barium salts led to the isolation of [V(2)L(H2O)(4)][X](4) . nH(2)O species (X = Cl 4, SCN 5, NO3 6, or I 7, n = 2-8). These complexes exhibit reduced magnetic moments at 295 K. Variable-temperature magnetic susceptibility studies on representative powdered samples of 1 and 5 showed weak intramolecular antiferromagnetic coupling between the vanadium(III) centres (J = -20 and -25 cm(-1), respectively) together with a smaller intermolecular contribution (8 = -2.0 and -3.7 cm(-1), respectively). Cyclic voltammetric studies on acetonitrile solutions of 2 revealed two waves attributable to sequential one-electron metal-centred reduction processes to yield [V(2)L](3+) and [V(2)L](2+) species. No evidence was found for the existence of any V-IV-containing species, through either electrochemical and/or chemical oxidation experiments.