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

Abstract

Transmetallation of the tetramine Schiff-base complex [Ba(H₂L)(H₂O)₂][ClO₄]₂ with vanadium(III) trichloride, vanadyl(IV) sulfate trihydrate or vanadyl(IV) dichloride dihydrate gave [V₂L(H₂O)₄][ClO₄]₂Cl₂1, [V₂L(H₂O)₄][ClO₄]₄·H₂O 2 and [V₂L(H₂O)₄][SO₄]₂·H₂O 3, respectively {H₂L = 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 vandium(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 [V₂L(H₂O)₄][X]₄·nH₂O species (X = Cl 4, SCN 5, NO₃6, 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 (θ=–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 [V₂L]³⁺ and [V₂L]²⁺ species. No evidence was found for the existence of any V^IV-containing species, through either electrochemical and /or chemical oxidation experiments.