Relevance of oxygen ligands to reduction of ligating dinitrogen

WE have reported¹ the reduction of dinitrogen, in cis-[M(N₂)₂-(PMe₂Ph)₄] (A, where M = Mo or W), to ammonia, on treatment with sulphuric acid in methanol at 20 °C. The reaction of the molybdenum complex is complete in 3–4 min but the yield (≤ 0.8 mol per Mo atom) is only about half that from the tungsten complex (≤ 1.8 mol per W atom), which requires about 0.5 h for complete reaction. We now find that ammonia is produced almost equally well, (1.7 mol per W atom), but much more slowly, from a methanol solution of A, (with M = W) either at the boil (3–4 h) or on irradiation for 30 h at 20 °C (2 × 150-W tungsten filament lamps); a boiling ethanol solution yields only 0.4 mol NH₃ per W atom. In contrast, A, (with M = Mo) gives very little ammonia (only 0.08 mol per Mo at best) in any of the above conditions; instead it evolves dinitrogen and dihydrogen.