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42123-G3
Studies on the Structures, Spectroscopic Properties, and Magnetic Properties of Vanadium(III) Carboxylate Complexes Formed in Aqueous Solution
The main goal of this project is to investigate the fundamental chemistry and properties of vanadium(III)/carboxylate complexes formed in aqueous solution. Compared with the higher oxidation states of vanadium, the aqueous chemistry of vanadium(III) is poorly understood. Aggregation of V(III) units in aqueous solution has been suggested in the literature; however, there is extremely limited structural evidence.
In Year 1 we structurally characterized trinuclear and tetranuclear V(III)/carboxylate complexes for acetate and related systems. We also demonstrated that these complexes retain their integrity in solution using 1H NMR spectroscopy and ES-MS. UV-vis spectroscopy measurements were also carried out.
1H NMR spectra for the V(III)/acetate and V(III)/proprionate systems showed us that upon adding 2 equiv. of these carboxylates to an acidic aqueous V(III) solution, a single complex is formed, which we labeled "Species B". It was of interest to determine (i) whether these two species are isostructural and (ii) if they are the trinuclear or tetranuclear complex. We have now determined the diffusion coefficients of these species by 1H NMR spectroscopy using pulsed field gradient spin-echo measurements (the Stejskal-Tanner experiment). If the species are isostructural for the V(III)/acetate and V(III)/proprionate systems, then
D(A)/ D(P) = M(P) / M(A)
where D(A) and D(P) are the diffusion coefficients for Species B for the V(III)/acetate and V(III)/proprionate systems, respectively, and M(P) and M(A) are the corresponding molecular weights. The diffusion coefficient data were most consistent with the structures being trimeric. The hydrodynamic radius (RH) was also calculated from the diffusion coefficients using the Stokes-Einstein law and compared with the radius of gyration (RG), which can be calculated from the X-ray structures. These values provided further support that Species B are the trimeric complexes for both systems. A comprehensive article has now been published: Inorg. Chem. 2007, 46, 1575.
We also recently submitted an article concerned with the X-ray structural characterization of [V(Gly)3]•2DMSO (gly = glycinate) and trans-[V(OH2)4Cl2]Cl•2H2O. Despite the importance of VIII in biology, only three VIII complexes of naturally occurring amino acids have been structurally characterized. To our knowledge [V(Gly)3]•2DMSO is the first structure of a vanadium complex incorporating a glycine ligand. The structure of trans-[V(OH2)4Cl2]Cl•2H2O is also of interest, given that this complex is a common by-product of aqueous reactions using VCl3 as a starting material.
Another goal of our project was to investigate the solid-state properties of V(III)/carboxylate clusters. Of special interest are the magnetic properties, since V(III) clusters can exhibit spin frustration and/or single molecule magnetic properties. In our first year efforts were hampered by the fact that in aqueous solution the clusters co-crystallize with other species; that is, pure materials can not be obtained. In our 2nd year we therefore focused on organic solvents. Procedures to crystallize trimeric V(III)/CH3OO(H) complexes are reported by Cotton et al. by reacting VCl3(THF)3 with either neat acidic acid or sodium acetate in CH2Cl2. However, our attempts using their methodology have been unsuccessful so far. 1H NMR measurements of the product solutions showed a surprising number of resonances at chemical shifts less than 10 ppm, indicating that multiple species are spontaneously forming in CH2Cl2. We have therefore recently initiated experiments using coordinating solvents such as THF, to prevent extensive polymerization and promote dissociation of the remaining chloride ligands of the VCl3(THF)3 reactant.
We have also explored the chemistry between other carboxylates and V(III) in aqueous solution using a variety of V(III) reactants. This has led to some surprises. For example, upon reacting V2(SO4)3 with thiodiacetic acid in the presence of NaCO3 (to deprotonate the ligand) and BaCO3 (to assist in the removal of sulfate as BaSO4), an eight-coordinate Ba2+ complex of S(CH3COO)2 was instead crystallized! Thiodiacetate provides 5 donor atoms for Ba2+. Structurally characterized Ba2+ complexes are rare. The aqueous solution chemistry of Ba2+ is also of interest, given that mining and dumping of industrial waste is a serious health concern in the US, due to leakage of Ba2+ into water supplies. Crystals of the same complex could also be obtained simply by reacting BaCl2 with thiodiacetate.
The graduate student, Riya Mukherjee, supported by this grant, has recently completed her PhD coursework requirements. The requested one year time extension will allow her to work further on this project, including obtaining V(III)/carboxylate complexes suitable for magnetic susceptibility measurements and speciation studies for these systems.
Budget Statement: From September 2006-August 2007 this grant was used for chemicals and supplies and to support Riya Mukherjee on a RA position from Spring 2007 onwards. The remaining grant money will be used for the same purposes.
