No abstract

Electronic structure calculations using density functional theory were performed on potential intermediates in the reaction of Fe(dmpe)(2)N(2) (dmpe = 1,2-bis(dimethylphosphino)ethane) with protons. Three mechanisms were investigated and compared, and the possibility of a two-electron reduction by a sacrificial Fe(dmpe)(2)N(2) complex was considered in each mechanism. A Chatt-like mechanism, involving the stepwise addition of protons to the terminal nitrogen, was found to be the least favorable. A second pathway involving dimerization of the Fe(dmpe)(2)N(2) complex, followed by the stepwise addition of protons leading to hydrazine, was found to be energetically favorable; however many of the dimeric intermediates prefer to dissociate into monomers. A third mechanism proceeding through diazene and hydrazine intermediates, formed by alternating protonation of each nitrogen atom, was found to be the most energetically favorable.

The iron phosphine complex cis-[Fe(DMeOPrPE)2(eta2-N2H4)][BPh4]2 {DMeOPrPE = 1,2-bis[bis(methoxypropyl)phosphino]ethane} was synthesized and structurally characterized. The structure exhibits the first eta2 coordination of hydrazine to iron, which may be relevant to intermediates trapped during nitrogenase turnover. The reaction of I with acid results in the formation of ammonia via a disproportionation reaction.

The reactions of the trans-Fe(DMeOPrPE)2Cl2 complex (I; DMeOPrPE = 1,2-bis(bis(methoxypropyl)phosphino)ethane) and its derivatives were studied in aqueous and nonaqueous solvents with a particular emphasis on the binding and activation of H2 and N2. The results show there are distinct differences in the reaction pathways between aqueous and nonaqueous solvents. In water, I immediately reacts to form trans-Fe(DMeOPrPE)2(H2O)Cl+. Subsequent reaction with H2 or N2 yields trans-Fe(DMeOPrPE)2(X2)Cl+ (X2=H2 or N2). In the case of H2, further reactivity occurs to ultimately give the trans-Fe(DMeOPrPE)2(H2)H+ product (III). The pathway for the reaction I --> III was spectroscopically examined: following the initial loss of chloride and replacement with H2, heterolysis of the H2 ligand occurs to form Fe(DMeOPrPE)2(H)Cl; substitution of the remaining chloride ligand by another H2 molecule then occurs to produce trans-Fe(DMeOPrPE)2(H2)H+. In the absence of H2 or N2, trans-Fe(DMeOPrPE)2(H2O)Cl+ slowly reacts in water to form Fe(DMeOPrPE)32+, II. Experiments showed that this species forms by reaction of free DMeOPrPE ligand with trans-Fe(DMeOPrPE)2(H2O)Cl+, where the free DMeOPrPE ligand comes from dissociation from the trans-Fe(DMeOPrPE)2(H2O)Cl+ complex. In nonaqueous solvents, the chloride ligand in I is not labile, and a reaction with H2 only occurs if a chloride abstracting reagent is present. Complex III is a useful synthon for the formation of other water-soluble metal hydrides. For example, the trans-[Fe(DMeOPrPE)2H(N2)]+ complex was generated in H2O by substitution of N2 for the H2 ligand in III. The trans-Fe(DHBuPE)2HCl complex (DHBuPE = 1,2-bis(bis(hydroxybutyl)phosphino)ethane, another water-solubilizing phosphine) was shown to be a viable absorbent for the separation of N2 from CH4 in a pressure swing scheme. X-ray crystallographic analysis of II is the first crystal structure report of a homoleptic tris chelate of FeII containing bidentate phosphine ligands. The structure reveals severe steric crowding at the Fe center.