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)2 2−; glut = glutarate, CH2(CH2COO)2 2−) 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 (1 H, 13C{1 H}, 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.