Non-oxo vanadium(IV) alkoxide chemistry: solid state structures, aggregation equilibria and thermochromic behaviour in solution.

The reversible thermochromic behaviour of homoleptic [{V(OR)4}n] complexes in solution [R = Pri (product I), Bus (Bs), Nep (N) and Cy (C)] is accounted for the existence of an aggregation equilibrium involving dimeric and monomeric species in which vanadium(IV) is respectively five- and four-coordinate. Bulky R groups such as But and Pet (tert-pentoxide) prevent aggregation and therefore give rise to exclusively mononuclear compounds (Bt and Pt, respectively) that are not thermochromic. The complexes and their temperature-dependent interconversion were characterised by single crystal X-ray diffractometry, magnetic susceptibility measurements and electronic, FTIR and EPR spectroscopies in a wide temperature range. Equilibrium constants and enthalpy and entropy changes for the dimerization reactions have been determined and compared with literature data.