Catalytic Reduction of N_2 to NH_3 by an Fe−N_2 Complex Featuring a C‑Atom Anchor

While recent spectroscopic studies have established the presence of an interstitial carbon atom at the center of the iron–molybdenum cofactor (FeMoco) of MoFe-nitrogenase, its role is unknown. We have pursued Fe–N_2 model chemistry to explore a hypothesis whereby this C-atom (previously denoted as a light X-atom) may provide a flexible trans interaction with an Fe center to expose an Fe–N_2 binding site. In this context, we now report on Fe complexes of a new tris(phosphino)alkyl (CP^(iPr)_3) ligand featuring an axial carbon donor. It is established that the iron center in this scaffold binds dinitrogen trans to the C_(alkyl)-atom anchor in three distinct and structurally characterized oxidation states. Fe–C_(alkyl) lengthening is observed upon reduction, reflective of significant ionic character in the Fe–C_(alkyl) interaction. The anionic (CP^(iPr)_3)FeN_2^– species can be functionalized by a silyl electrophile to generate (CP^(iPr)_3)Fe–N_2SiR_3. (CP^(iPr)_3)FeN_2^– also functions as a modest catalyst for the reduction of N_2 to NH_3 when supplied with electrons and protons at −78 °C under 1 atm N_2 (4.6 equiv NH_3/Fe)

Exploring secondary-sphere interactions in Fe–N_xH_y complexes relevant to N_2 fixation

Hydrogen bonding and other types of secondary-sphere interactions are ubiquitous in metalloenzyme active sites and are critical to the transformations they mediate. Exploiting secondary sphere interactions in synthetic catalysts to study the role(s) they might play in biological systems, and to develop increasingly efficient catalysts, is an important challenge. Whereas model studies in this broad context are increasingly abundant, as yet there has been relatively little progress in the area of synthetic catalysts for nitrogen fixation that incorporate secondary sphere design elements. Herein we present our first study of Fe–NxHy complexes supported by new tris(phosphine)silyl ligands, abbreviated as [SiP^(Nme_3)] and [SiP^(iPr_2)P^(Nme)], that incorporate remote tertiary amine hydrogen-bond acceptors within a tertiary phosphine/amine 6-membered ring. These remote amine sites facilitate hydrogen-bonding interactions via a boat conformation of the 6-membered ring when certain nitrogenous substrates (e.g., NH_3 and N_2H_4) are coordinated to the apical site of a trigonal bipyramidal iron complex, and adopt a chair conformation when no H-bonding is possible (e.g., N_2). Countercation binding at the cyclic amine is also observed for anionic {Fe–N_2}− complexes. Reactivity studies in the presence of proton/electron sources show that the incorporated amine functionality leads to rapid generation of catalytically inactive Fe–H species, thereby substantiating a hydride termination pathway that we have previously proposed deactivates catalysts of the type [EP^R_3]FeN_2 (E = Si, C)

Chiral Symmetry Versus the Lattice

After mentioning some of the difficulties arising in lattice gauge theory from chiral symmetry, I discuss one of the recent attempts to resolve these issues using fermionic surface states in an extra space-time dimension. This picture can be understood in terms of end states on a simple ladder molecule.Comment: Talk at the meeting "Computer simulations studies in condensed matter physics XIV" Athens, Georgia, Feb. 19-24, 2001. 14 page

Diiron Bridged-Thiolate Complexes That Bind N_2 at the Fe^(II)Fe^(II), Fe^(II)Fe^I, and Fe^IFe^I Redox States

All known nitrogenase cofactors are rich in both sulfur and iron and are presumed capable of binding and reducing N_2. Nonetheless, synthetic examples of transition metal model complexes that bind N_2 and also feature sulfur donor ligands remain scarce. We report herein an unusual series of low-valent diiron complexes featuring thiolate and dinitrogen ligands. A new binucleating ligand scaffold is introduced that supports an Fe(μ-SAr)Fe diiron subunit that coordinates dinitrogen (N_2-Fe(μ-SAr)Fe-N_2) across at least three oxidation states (Fe^(II)Fe^(II), Fe^(II)Fe^I, and Fe^IFe^I). The (N_2-Fe(μ-SAr)Fe-N_2) system undergoes reduction of the bound N_2 to produce NH_3 (∼50% yield) and can efficiently catalyze the disproportionation of N_2H_4 to NH_3 and N_2. The present scaffold also supports dinitrogen binding concomitant with hydride as a co-ligand. Synthetic model complexes of these types are desirable to ultimately constrain hypotheses regarding Fe-mediated nitrogen fixation in synthetic and biological systems

Photoinduced Ullmann C–N Coupling: Demonstrating the Viability of a Radical Pathway

Carbon–nitrogen (C–N) bond-forming reactions of amines with aryl halides to generate arylamines (anilines), mediated by a stoichiometric copper reagent at elevated temperature (>180°C), were first described by Ullmann in 1903. In the intervening century, this and related C–N bond-forming processes have emerged as powerful tools for organic synthesis. Here, we report that Ullmann C–N coupling can be photoinduced by using a stoichiometric or a catalytic amount of copper, which enables the reaction to proceed under unusually mild conditions (room temperature or even –40°C). An array of data are consistent with a single-electron transfer mechanism, representing the most substantial experimental support to date for the viability of this pathway for Ullmann C–N couplings

Polyakov loop in chiral quark models at finite temperature

We describe how the inclusion of the gluonic Polyakov loop incorporates large gauge invariance and drastically modifies finite temperature calculations in chiral quark models after color neutral states are singled out. This generates an effective theory of quarks and Polyakov loops as basic degrees of freedom. We find a strong suppression of finite temperature effects in hadronic observables triggered by approximate triality conservation (Polyakov cooling), so that while the center symmetry breaking is exponentially small with the constituent quark mass, chiral symmetry restoration is exponentially small with the pion mass. To illustrate the point we compute some low energy observables at finite temperature and show that the finite temperature corrections to the low energy coefficients are $N_c$ suppressed due to color average of the Polyakov loop. Our analysis also shows how the phenomenology of chiral quark models at finite temperature can be made compatible with the expectations of chiral perturbation theory. The implications for the simultaneous center symmetry breaking-chiral symmetry restoration phase transition are also discussed.Comment: 24 pages, 8 ps figures. Figure and appendix added. To appear in Physical Review

Transition-Metal-Catalyzed Alkylations of Amines with Alkyl Halides: Photoinduced, Copper-Catalyzed Couplings of Carbazoles

N-alkylations of carbazoles with a variety of secondary and hindered primary alkyl iodides can be achieved by using a simple precatalyst (CuI) under mild conditions (0 °C) in the presence of a Brønsted base; at higher temperature (30 °C), secondary alkyl bromides also serve as suitable coupling partners. A Li[Cu(carbazolide)_2] complex has been crystallographically characterized, and it may serve as an intermediate in the catalytic cycle

High Energy Physics from High Performance Computing

We discuss Quantum Chromodynamics calculations using the lattice regulator. The theory of the strong force is a cornerstone of the Standard Model of particle physics. We present USQCD collaboration results obtained on Argonne National Lab's Intrepid supercomputer that deepen our understanding of these fundamental theories of Nature and provide critical support to frontier particle physics experiments and phenomenology.Comment: Proceedings of invited plenary talk given at SciDAC 2009, San Diego, June 14-18, 2009, on behalf of the USQCD collaboratio