Halide encapsulation by dicarboxylate oxido-vanadium cage complexes

Abstract

Compounds [Bu₄N]₂[V₈O₁₆(oda)₄⊂2Cl], 1, [Bu₄N]₂[V₈O₁₆(glut)₄⊂2Cl], 2, and [Bu₄N][V₄O₈(glut)₂⊂F], 3, (oda = oxydiacetate, O(CH₂COO)₂²⁻; glut = glutarate, CH₂(CH₂COO)₂²⁻) were obtained by a stepwise reaction of in situ prepared [Bu₄N]VO₃ with HCl (or HF for 3) and then with the dicarboxylic acid X(CH₂COOH)₂ (X = O and CH₂), under appropriate reaction conditions. Multinuclear magnetic resonance (¹H, ¹³C{¹H}, ³⁵Cl, ¹⁹F and ⁵¹V), electrochemical studies, X-ray structural determinations (single crystal and powder), thermogravimetric analyses (TGA) and Density Functional Theory (DFT) calculations were employed to characterise these polyoxovanadate complexes 1–3. They included encapsulated halide anions, two chloride ions in 1 and 2 and one fluoride ion in 3, where the shape and dimensions of the cage were governed by the halide size. The stabilizing template effect of the chloride ion towards the bowl-shaped [V₄O₈(OOCR)₄] fragment (i.e. the half part of 1 and 2), containing a crown-shaped {V₄O₈} subunit, or that of the fluoride ion towards the planar {V₄O₈} moiety in 3, was definitively demonstrated by DFT calculations. The HOMO composition of 1 prompted us to study the possible oxidation of the two encapsulated chloride ions toward a chlorine molecule. The electrochemical behaviors of 1–3 were thus investigated. However, the chlorine molecule in the model [V₈O₁₆(oda)₄⊂(Cl₂)], 6c, was not capable to stabilise the polyoxovanadate cage [V₈O₁₆(oda)₄], 4c, according to DFT calculations.