Conversion of dinitrogen in its molybdenum and tungsten complexes into ammonia and possible relevance to the nitrogenase reaction

Treatment of trans-[M(N2)2(dppe)2](A)(dppe = Ph2PCH2CH2PPh2, M = Mo or W) with H2SO4 gives [M(HSO4)-(NNH2)(dppe)2][HSO4] and no ammonia or hydrazine. However, the complexes cis-[M(N2)2(PMe2Ph)4](B) and trans-[M(N2)2(PMePh2)4](C)(M = Mo or W) react with H2SO4 in methanol at 20 °C to give ammonia (ca. 1.9 NH3 per W atom and ca. 0.7 NH3 per Mo atom), together with a little hydrazine for (B; M = W) but not for (B; M = Mo). Treatment of (B; M = Mo and W) with a variety of other acids gives ammonia, but less effectively than with H2SO4. Anhydrous HBF4 also gives ammonia from (B; M = Mo or W), but (A; M = Mo or W) gives only trans-[MF(NNH2)(dppe)2][BF4]. Ammonia (1.6 NH3 per W atom) is also obtained when (B; M = W) but not (B; M = Mo) is treated with methanol alone, either at reflux or on irradiation at 20 °C for several hours. The mechanism of these reactions and their relevance to the action of nitrogenase is discussed.