Synthesis, structure and properties of mononuclear oxovanadium(V) alkoxides incorporating chelated ethane-1,2-diol and propane-1,3-diol

Abstract

Dark coloured complexes of type VO(L)(Heg) and VO(L)(Hpd) have been synthesized in excellent yields by reacting bis(acetylacetonato)oxovanadium(IV) with H2L in the presence of excess ethane-1,2-diol (H2eg) and propane-1,3-diol (H2pd), respectively in acetone. Here L2− are the deprotonated forms of N-(1-hydroxyethyl)naphthaldimine (H2L1) and N-(1-hydroxy-2-methylpropyl)naphthaldimine (H2L2). Heg− and Hpd− form five- and six-membered V(O,O) chelate rings, respectively. The crystal structure of VO(L2)(Heg) is reported. The ONO-coordinating tridentate [L2]2− spans meridionally and consists of two planar segments mutually rotated along a C–N bond. The Heg− ligand forms a non-planar chelate ring in which the dimethylene bridge is in gauche conformation. The five V–O lengths are unequal, the shortest being V–O(oxo) and the longest is V–O(alcoholic) which is subjected to the trans influence of the oxo oxygen atom. In the crystal lattice, the VO(L2)(Heg) molecule forms an infinite chain helical pattern via intermolecular O⋯O hydrogen bonding. Methylene 1H resonances are systematically shifted to lower field compared to those of free H2eg and H2pd, the shift of the CH2OV resonances being particularly large. The complex multiplet 1H structure in CDCl3 is a good indication of the rigidity of the chelate ring in solution. 51V chemical shifts are found to be diagnostic of the alcohol–alkoxide chelate ring size, the shift of VO(L)(Heg) being ∼25 ppm downfield from that of VO(L)(Hpd). Low V(V)/V(IV) reduction potentials (ca. −0.30 V versus SCE) are indicative of the considerable VO3+ stabilization due to two alkoxide coordination.