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 V^VO exhibits dimeric structures with 2-HOC₆H₄CHNC(CH₂OH)₃ and 2-HOC₆H₄CH₂NHC(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^VIO 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 HCN or H₂CNH group of the ligand occupies a near trans position to the MO 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 CH₂ provides a signature that helps to differentiate dinuclear from mononuclear complexes. Carbon-13 NMR co-ordination induced shifts of the bound CH₂ 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-HOC₆H₄CHNC(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 Å, a value that is close to that observed in the molybdenum(VI) enzyme dmso reductase (1.92 Å). Several correlations have been drawn based on the data.