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

Abstract

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.