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Gilbertson JD, Szymczak NK, Crossland JL, Miller WK, Lyon DK, Foxman BM, Davis J, Tyler DR: Coordination chemistry of H2 and N2 in aqueous solution. Inorg Chem. 2007 Feb 19;46(4):1205-14. Epub 2007 Jan 26.
Reactivity and mechanistic studies using trans-FeII (P2) 2X2)-type complexes (P2 = a chelating, water-solubilizing phosphine).. 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. |
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