ic061570o_si_003.cif (29.47 kB)
Coordination Chemistry of H2 and N2 in Aqueous Solution. Reactivity and Mechanistic Studies Using trans-FeII(P2)2X2-Type Complexes (P2 = a Chelating, Water-Solubilizing Phosphine)
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posted on 2007-02-19, 00:00 authored by John D. Gilbertson, Nathaniel K. Szymczak, Justin L. Crossland, Warren K. Miller, David K. Lyon, Bruce M. Foxman, Joclyn Davis, David R. TylerThe 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+ (X2H2 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.
