Characterization of an intermediate in the ammonia-forming reaction of Fe(DMeOPrPE)2N2 with acid (DMeOPrPE = 1,2-[bis(dimethoxypropyl)phosphino]ethane).

The reactivity of Fe(DMeOPrPE)2N2 with water and acid was explored. (DMeOPrPE is the bidentate phosphine 1,2-[bis(dimethoxypropyl)phosphino]ethane.) The complex reacts with acid to form trans-Fe(DMeOPrPE)2(N2)H and small amounts of ammonia and hydrazine. When reacted with H2O, cis-Fe(DMeOPrPE)2(H)2 is formed. To increase the yields of ammonia and hydrazine, we investigated the effect of anion, solvent, and acid addition rate on the yields of ammonia. Of these parameters, only the properties of the anion (i.e., of the acid) had a significant impact on the yields of ammonia. The highest yields of NH3 occurred with the largest/least-coordinating anion (triflate). A short-lived purple intermediate (τ1/2 < 5 s at 23 °C) was observed in the reaction of Fe(DMeOPrPE)2N2 with triflic acid. Because the structure of this purple species could potentially provide valuable insights into the mechanism of ammonia formation, a method was developed for independently synthesizing and stabilizing the complex. Spectroscopic characterization of the purple species identified it as the paramagnetic ((DMeOPrPE)2Fe)2(μ-N2) complex (1). This purple dimer (1) exists in equilibrium with yellow, monomeric, paramagnetic Fe(DMeOPrPE)2N2 (2). The role of 1 in the formation of hydrazine and ammonia was probed by reacting 1 with acid.