Vanadium(V) tartrato complexes: Speciation in the H3O+(OH-)/H2VO4-/(2R,3R)-tartrate system and X-ray crystal structures of Na-4[V4O8(rac-tart)(2)]center dot 12H(2)O and (NEt4)(4)[V4O8((R,R)-tart)(2)]center dot 6H(2)O (tart = C4H2O64-)

摘要

A study of the aqueous H3O+(OH-)/H2VO4-/(2R,3R)-tartrate system has been performed at 273 K in a 1.0 mol/L Na+(Cl-) ionic medium using V-51 NMR spectroscopy. In this relatively complicated system, more than 12 different species were observed. Ligand concentration, vanadate concentration, and pH variation studies were carried out, particularly for the range of pH 5.8-8.0 and for pH 2.4. Chemical shifts, vanadium-ligand stoichiometry, and also composition and formation constants for some, but not all, species are given. Despite some reduction of vanadium(V) to vanadium(IV) in an acidic medium at pH approximate to 2.4, the stoichiometries of the principal species in solution at this pH were determined. Electrospray ionization mass spectra for some solutions were obtained and were in accordance with the conclusions drawn from the speciation studies. A series of crystalline vanadium(V) tartrato complexes M-4[V4O8(tart)(2)]center dot aq were also prepared and characterized. X-ray diffraction studies of Na-4[V4O8(rac-tart)(2)]center dot 12H(2)O (1) and (NEt4)(4)[V4O8((R,R)-tart)(2)]center dot 6H(2)O (2) revealed unique tetranuclear [V4O8(tart)(2)](4-) ions for which the {V4O4} rings have boat conformations.