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 were 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 pentaPod^Me (P5^Me) ligand have been replaced with phospholanes (Pln). The derived molybdenum complex [Mo(N₂)P5^Pln] generates 22 and the analogous tungsten complex [W(N₂)P5^Pln] 7 equivalents of NH₃ from N₂ in the presence of 180 equivalents of SmI₂(THF)₂/H₂O, rendering the latter the first tungsten complex chemocatalytically converting N₂ to NH₃. In contrast, the tungsten complex [W(N₂)P5^Me] 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.