Hfepr studies on vanadium ( iii ) complexes as models for the active site of vanadium nitrogenase

One of the most important reactions in biology is the “fixation” (reduction) of dinitrogen (N2) to ammonia (NH3). This reaction occurs in various microorganisms and is catalyzed by the enzyme nitrogenase. Nitrogenase (N2ase) is an iron-sulfur protein that contains additional metal(s) in the active site: molybdenum (Mo-N2ase) or vanadium (V-N2ase), of which the latter enzyme is much less understood.[1] Efforts to understand V-N2ase have raised interest in low-oxidation state vanadium chemistry in a sulfur-rich ligation environment. This interest has led us to synthesize a series of vanadium(III) complexes, in which the V(III) ion is chelated by a novel phosphine ligand. This trianionic, quadridentate ligand comprises a phosphorus atom directly linked to three arylthiolate groups, which can have various substituents: none, designated as PS3; methyl, designated as PS3’; and trimethylsilyl, designated as PS3”. The remaining coordination sites are occupied by a variety of ligand(s) (L)n, such as chloride, azide (N3; molecular structure shown at left), 2,2’-bipyridine, or three hydrazine (N2H4) molecules. The overall coordination number thus ranges from 5 to 7. The formally V(III) ion is paramagnetic in all [V(PS3)L] complexes prepared.