Mechanisms for the Formation of NH3, N2H4, and N2H2 in the Protonation Reaction of Fe(DMeOPrPE)2N2 {DMeOPrPE = 1,2‐bis[bis(methoxypropyl)phosphino]ethane}

Abstract

AbstractResearch from the author's laboratory is reviewed on the reaction of Fe(DMeOPrPE)2N2 with acid to form ammonia and hydrazine {DMeOPrPE is the water‐soluble, bidentate phosphine 1,2‐bis[bis(methoxypropyl)phosphino]ethane}. DFT calculations suggested that a symmetric protonation mechanism was more favorable than either an asymmetric protonation mechanism or a dinuclear mechanism. Two symmetric protonation pathways were investigated by synthesizing or generating reaction intermediates. Among the various species synthesized or spectroscopically observed were hydrazine complexes, diazene complexes, and their deprotonated conjugate bases. A relatively long‐lived intermediate in the protonation reaction, identified as [((DMeOPrPE)2Fe)2(μ‐N2)]2+, hints that a dinuclear pathway might be operating, but further mechanistic investigations suggested this species is likely forming in a side‐reaction and is not involved in the formation of ammonia. The only source of electrons for the reduction of N2 in the reaction is the Fe(DMeOPrPE)2N2 complex. As discussed in this review, this feature will ultimately limit the yields of ammonia and hydrazine that can form in the reaction. The reactions pathways uncovered by this research are symmetric protonation pathways, and as such they are different from the “asymmetric” protonation pathway proposed for some Mo‐N2 complexes that protonate to form ammonia. Recent studies suggest a symmetric protonation pathway for nitrogenase (which has Fe in its active site), and it may well be a general conclusion that Fe‐N2 species produce ammonia via the symmetric protonation mechanism.