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)](2+) 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.

Coordination of a complete series of N2 reduction intermediates (N2H2, N2H4, and NH3) to an iron phosphine scaffold.

The series of dinitrogen reduction intermediates (N(2)H(2), N(2)H(4), and NH(3)) coordinated to the Fe(DMeOPrPE)(2)H(+)(DMeOPrPE = 1,2-[bis(dimethoxypropyl)phosphino]ethane) scaffold has been synthesized or generated. The synthesis of trans-[Fe(DMeOPrPE)(2)(NH(3))H][BPh(4)] and generation of trans-[Fe(DMeOPrPE)(2)(N(2)H(4))H][BPh(4)] were achieved by substitu tion of the dinitrogen ligand on trans-[Fe(DMeOPrPE)(2)(N(2))H][BPh(4)]. The trans-[Fe(DMeOPrPE)(2)(N(2)H(2))H](+) complex and its deprotonated conjugate base, trans-Fe(DMeOPrPE)(2)(N(2)H)H, were observed by (31)P and (1)H NMR from decomposition of trans-[Fe(DMeOPrPE)(2)(N(2)H(4))H](+) in the presence of excess hydrazine. Attempts to chemically oxidize trans-[Fe(DMeOPrPE)(2)(N(2)H(4))H](+) to trans-[Fe(DMeOPrPE)(2)(N(2)H(2))H][BPh(4)] with a variety of oxidizing agents yielded only decomposition products consistent with the intermediate formation of trans-[Fe(DMeOPrPE)(2)(N(2)H(2))H](+) prior to decomposition.

Self-assembled E(2)L(3) cryptands (E = P, As, Sb, Bi): transmetalation, homo- and heterometallic assemblies, and conformational isomerism.

A series of Group 15-containing homometallic (E(2)L(3), E = P, As, Sb, Bi) and heterometallic (AsSbL(3), AsBiL(3), PSbL(3)) supramolecular cryptands were prepared by the self-assembly of pnictogen halides with dithiolate ligand or by direct transmetalation from a heavier congener. Structural characterization by single crystal X-ray diffraction shows that the E-S bond distances and S-E-S bond angles are significantly affected by the identity of the pnictogen. (1)H NMR spectroscopy reveals that the homometallic cryptands are dynamic in solution: surprisingly one ligand "flips", perturbing the C(3) symmetry of the complex and giving a new asymmetric conformer. Density functional theory calculations were carried out on both the symmetric and the asymmetric conformations of the cryptands, and the energies were compared to those observed by NMR spectroscopy. It was found that the relative stability of the asymmetric cryptand to its symmetric conformer increases with increasing size of the Group 15 element. Finally, it is reported that if two metals are present during the self-assembly process, heterometallic cryptands form. These supramolecular cryptands are reminiscent of their organic analogues, but result from a self-assembly process rather than a stepwise synthesis. Surprisingly, they possess conformational isomerism and exhibit dynamic transmetalation in their reactivity which provides access to otherwise unattainable assemblies.

Bis{1,2-bis-[bis-(3-hydroxy-prop-yl)phosphino]ethane}dichloridoiron(II).

In the title compound, [FeCl(2)(C(14)H(32)O(4)P(2))(2)], the Fe(II) atom (site symmetry ) adopts a distorted trans-FeCl(2)P(4) octa-hedral geometry with two P,P'-bidentate ligands in the equatorial positions and two chloride ions in the axial positions. In the crystal, mol-ecules are linked by O-H⋯O and O-H⋯Cl hydrogen bonds, generating a three-dimensional network.

trans-Bis{1,2-bis-[bis-(2-methoxy-ethyl)phosphino]ethane}dichloridoiron(II).

The Fe atom in the title compound, [FeCl(2)(C(14)H(32)O(4)P(2))(2)], has a distorted octa-hedral coordination with four P atoms in equatorial positions and two Cl atoms in apical positions.

Intermediates in the reduction of N(2) to NH(3): synthesis of iron eta(2) hydrazido(1-) and diazene complexes.

Iron complexes containing hydrazido(1-) and diazene ligands were investigated as potential intermediates in the reduction of N(2) to NH(3). Generation of cis-[Fe(DMeOPrPE)(2)(eta(2)-N(2)H(3))](+) and cis-Fe(DMeOPrPE)(2)(eta(2)-N(2)H(2)) (DMeOPrPE = 1,2-bis(dimethoxypropylphosphino)ethane) was achieved by addition of base to cis-[Fe(DMeOPrPE)(2)(N(2)H(4))](2+). The hydrazine, hydrazido(1-), and diazene complexes can be interconverted by protonation/deprotonation reactions.

Aqueous coordination chemistry of H2: why is coordinated H2 inert to substitution by water in trans-Ru(P2)2(H2)H+-type complexes (P2 = a chelating phosphine)?

The reactivity of a series of trans-Ru(P(2))(2)Cl(2) complexes with H(2) was explored. The complexes reacted with H(2) via a stepwise H(2) addition/heterolysis pathway to form the trans-[Ru(P(2))(2)(H(2))H](+) dihydrogen complexes. Some of the resulting eta(2)-H(2) complexes were surprisingly inert to substitution by water, even at concentrations as high as 55 M; however, the identity of the bidentate phosphine ligand greatly influenced the lability of the coordinated eta(2)-H(2) ligand. With less electron-donating phosphine ligands, the H(2) ligand was susceptible to substitution by H(2)O, whereas with more electron-rich phosphine ligands, the H(2) ligand was inert to substitution by water. Density functional theory (DFT) calculations of the ligand substitution reactions showed that the Ru-H(2) and Ru-H(2)O complexes are very close in energy, and therefore slight changes in the donor properties of the bidentate phosphine ligand can inhibit or promote the substitution of H(2)O for H(2).

Theoretical studies of N2 reduction to ammonia in Fe(dmpe)2N2.

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.

Precursors to dinitrogen reduction: structures and reactivity of trans-[Fe(DMeOPrPE)2(eta(2)-H2)H]+ and trans-[Fe(DMeOPrPE)2(N2)H]+.

trans-[Fe(DMeOPrPE)(2)(H(2))H](+) and trans-[Fe(DMeOPrPE)(2)(N(2))H](+) (DMeOPrPE = 1,2-bis(dimethoxypropylphosphino)ethane) were synthesized and their structures determined by X-ray crystallography. These complexes are important species in a dinitrogen reduction scheme involving protonation of an iron(0) dinitrogen complex to produce ammonia. The rates of substitution of the coordinated H(2) and N(2) molecules with acetonitrile were monitored in a variety of organic solvents. The coordinated N(2) substituted approximately 6 times faster than H(2), but surprisingly the solvent had little effect on the observed rates. The results suggest that the H(2) molecule in trans-[Fe(DMeOPrPE)(2)(H(2))H](+) does not participate in hydrogen bonding to the bulk solvent, as was previously observed in the analogous Ru complex. The deprotonation of trans-[Fe(DMeOPrPE)(2)(N(2))H](+) to yield Fe(DMeOPrPE)(2)N(2) was investigated in the presence of a variety of anions, and it was found that the anion facilitates the reaction through an ion-pairing interaction in which the anion removes electron density from the hydride ligand.

(Ethane-1,2-di-yl)bis-[bis-(3-methoxy-prop-yl)methyl-phospho-nium] bis-(tetra-phenyl-borate) diethyl ether solvate.

In the course of substitution studies on the iron dihydrogen complex trans-[Fe(DMeOPrPE)(2)(H(2))H](BPh(4)) {DMeOPrPE = 1,2-bis-[bis-(methoxy-prop-yl)phosphino]ethane}, we discovered an unexpected transformation of the diphosphine ligand to a diphospho-nium dication without the use of any typical methyl-ating reagent. The P atoms in the dication of the title compound, C(20)H(46)O(4)P(2) (2+)·2C(24)H(20)B(-)·C(4)H(10)O, have a distorted tetra-hedral coordination with P-C(Me) distances of 1.791 (2) and 1.785 (2) Å. The P-C-C-P torsion angle about the central dimethyl-ene bridge is -168.3 (1)°.

Synthesis and characterization of an iron(II) eta(2)-hydrazine complex.

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.

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).

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.