Syntheses, structure, reactivity and species recognition studies of oxo-vanadium(V) and -molybdenum(VI) complexes

Abstract

Alkoxo-rich Schiff-bases of potentially tri-, tetra- and penta-dentate binding capacity, and their sodium tetrahydroborate-reduced derivatives, have been synthesized. Their oxo-vanadium(V) and -molybdenum(VI) complexes were synthesized and characterized using several analytical and spectral techniques including multinuclear NMR spectroscopy and single-crystal X-ray diffraction studies. Eight structurally different types of complexes possessing distorted square-pyramidal, trigonal-bipyramidal and octahedral geometries have been obtained. While (VO)-O-V exhibits dimeric Structures with 2-HOC6H4CH=NC(CH2OH)(3) and 2-HOC6H4CH2-NHC(CH2OH)(3) and related ligands through the formation of a symmetric V2O2 core as a result of bridging of one of the CH2O- groups, Mo O-VI gives only mononuclear complexes even when some unbound CH2OH groups are available and the metal center is co-ordinatively unsaturated. In all the complexes the nitrogen atom from a HC=N or H2CNH group of the ligand occupies a near trans position to the M=O bond. While the Schiff-base ligands act in a tri- and tetra-dentate manner in the vanadium(V) complexes, they are only tridentate in the molybdenum(VI) complexes. Proton NMR spectra in the region of bound CH, provides a signature that helps to differentiate dinuclear from mononuclear complexes. Carbon-13 NMR co-ordination induced shifts of the bound CH, group fit well with the charge on the oxometal species and the terminal or bridging nature of the ligand. The reactivity of the vanadium(V) complexes towards bromination of the dye xylene cyanole was studied. Transmetallation reactions of several preformed metal complexes of 2-HOC6H4CH=NC(CH2OH)(3) with VO3+ were demonstrated as was selective extraction of VO3+ from a mixture of VO(acac)(2)] and MoO2(acac)(2)] using this Schiff base. The unusual selectivity and that of related derivatives for VO3+ is supported by binding constants and the solubility of the final products, and was established through a.c. conductivity measurements. The cis-MoO22+ complexes with alkoxo binding showed an average Mo-O-alk distance of 1.926 Angstrom, a value that is close to that observed in the molybdenum(VI) enzyme dmso reductase (1.92 Angstrom). Several correlations have been drawn based on the data