Chemocatalytic Conversion of Dinitrogen to Ammonia Mediated by a Tungsten Complex

Whereas molybdenum dinitrogen complexes have played a major role as catalytic model systems of nitrogenase, corresponding tungsten complexes have in most cases found to be catalytically inactive. Herein we present a modified pentadentate tetrapodal (pentaPod) phosphine ligand in which two dimethylphosphine groups of the original PMe2PPh2 ligand have been replaced with phospholanes (Pln). The derived molybdenum complex [Mo(N2)(PPln2PPPh2)] generates 22 and the analogous tungsten complex [W(N2)(PPln2PPPh2)] 7 equivalents of NH3 from N2 in the presence of 180 equiv.s of SmI2/H2O, rendering the latter the first tungsten complex chemocatalytically converting N2 to NH3. In contrast, the parent tungsten complex [W(N2)(PMe2PPh2)] generates ammonia from N2 only in a slightly overstoichiometric fashion. The reasons for these reactivity differences are investigated with the help of spectroscopic and electrochemical methods.