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

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 (H(2)eg) and propane-19-diol (H(2)pd), 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(L-2) (Heg) is reported. The ONO-coordinating tridentate [ L-2](2-) spans meridionally and consists of two planar segments mutually popd 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( ore) 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(L-2) (Heg) molecule forms an infinite chain helical pattern via intermolecular O O hydrogen bonding. Methylene H-1 resonances are systematically shifted to lower field compared to those of free H(2)eg and H(2)pd, the shift of the CH2OV resonances being particularly large. The complex multiplet H-1 structure in CDCl3 is a good indication of the rigidity of the chelate ring in solution. V-51 chemical shifts are found to be diagnostic of the alcohol-alkoxide chelate ring size, the shift of VO(L)(Heg) being similar to 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. (C) 2000 Elsevier Science Ltd All rights reserved.