Vanadium
Vanadium chemistry is a relatively new area of research for our group. The coordination chemistry of vanadium is of interest for a number of reasons. Vanadium is present at 20-35 nM concentrations in seawater, making it the most abundant transition metal in the aquasphere. In Nature, vanadium occurs in a variety of oxidation states ranging from V(II) through to V(V), which includes the vanadium-dependent nitrogenases in azotobacteria (V(II)/(III)), vanadium-dependent haloperoxidases in marine algae (V(V)), amavadin in the mushroom Amanita muscaria (V(IV)), V(III) in the marine fanworm Pseudopotamilla occelata and V(III)/V(IV) in ascidians.
We are especially interested in the aqueous coordination chemistry of vanadium(III). There is no doubt that the general unsuitability of both NMR and EPR spectroscopy for the elucidation of the structures of complexes of paramagnetic vanadium(III) has severely hindered progress on the understanding and development of the coordination chemistry of vanadium(III) in aqueous solution compared with vanadium(IV) and (V). Vanadium(III) is found in some types of ascidians. Ascidians (tunicates, sea squirts) are sessile marine animals which vary from microscopic colonial forms through to individuals 8-12 inches in size. Remarkably, ascidians not only have the capability to reduce vanadates (V(V)) from seawater (H2VO4-, HVO42-, NaHVO4-) to the V(III) state, but also concentrate it up to 7 orders of magnitude (from nM concentrations in sea water up to ~1 M in their blood cells). The reason why ascidians assimilate and reduce vanadates from seawater is currently not known.
A further reason for our interest in V(III) chemistry stems from recent reports of the formation of V(III) clusters in solution with interesting structures, spectroscopic properties, and magnetic properties. V(III) clusters can exhibit spin frustration and behave as single molecule magnets, and therefore have potential applications in data storage.
Finally, V(III) complexes are also of interest with respect to their electron transfer properties and their potential to act as catalysts in industrially relevant processes and by mimicking the active sites of metalloproteins. There is also considerable interest in developing catalysts for efficient, environmentally-friendly industrial-scale syntheses in aqueous solution rather than in organic solvents.
Our first articles in this area were recently published (Inorg. Chem. 2007, 46, 1575; Inorg. Chem. 2005, 44, 5197) and are concerned with the formation of trinuclear and tetranuclear V(III)/acetate complexes in aqueous solution. Importantly, although it was proposed a number of years ago that V(III) clusters with nuclearity greater than two exist is aqueous solution, there is practically no structural data to substantiate this claim. Our structures of trinuclear and teranuclear V(III)/carboxylate clusters provide the much needed chemical precedence for the existence of polynuclear V(III) clusters in aqueous solution! The structure of one of these complexes, [V4(µ-OH)4(µ-OOCCH3)4(OH2)8]Cl4, is shown in the figure.