Justin L. Crossland

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.

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.

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.

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).

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

The Fe atom in the title compound, [FeCl2(C14H32O4P2)2], has a distorted octahedral coordination with four P atoms in equatorial positions and two Cl atoms in apical positions.

Bis{1,2-bis­[bis­(3-hydroxy­prop­yl)phosphino]ethane}dichloridoiron(II)

In the title compound, [FeCl2(C14H32O4P2)2], the FeII atom (site symmetry ) adopts a distorted trans-FeCl2P4 octahedral geometry with two P,P′-bidentate ligands in the equatorial positions and two chloride ions in the axial positions. In the crystal, molecules are linked by O—H⋯O and O—H⋯Cl hydrogen bonds, generating a three-dimensional network.

(Ethane-1,2-di­yl)bis­[bis­(3-methoxy­prop­yl)methyl­phospho­nium] bis­(tetra­phenyl­borate) diethyl ether solvate

In the course of substitution studies on the iron dihydrogen complex trans-[Fe(DMeOPrPE)2(H2)H](BPh4) {DMeOPrPE = 1,2-bis[bis(methoxypropyl)phosphino]ethane}, we discovered an unexpected transformation of the diphosphine ligand to a diphosphonium dication without the use of any typical methylating reagent. The P atoms in the dication of the title compound, C20H46O4P2 2+·2C24H20B−·C4H10O, have a distorted tetrahedral 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 dimethylene bridge is −168.3 (1)°.