Bridging Carbonyl and Carbyne Complexes of Weak-Field Iron: Electronic Structure and Iron-Carbon Bonding.

Carbon monoxide inhibited forms of nitrogenases have carbonyl (CO) and carbide (C) bridges, which are common in synthetic iron complexes with strong-field ligand environments but rare in iron sites with weak-field ligand environments analogous to the enzyme. Here, we explore the fundamental bonding description of bridging CO in high-spin iron systems. We describe the isolation of several diiron carbonyls and related species, and elucidate their electronic structures, magnetic coupling, and characteristic structural and vibrational parameters.

The structure of Mn(II)-bound Rubisco from Spinacia oleracea.

The rate of photosynthesis and, thus, CO fixation, is limited by the rate of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Not only does Rubisco have a relatively low catalytic rate, but it also is promiscuous regarding the metal identity in the active site of the large subunit. In Nature, Rubisco binds either Mg(II) or Mn(II), depending on the chloroplastic ratio of these metal ions; most studies performed with Rubisco have focused on Mg-bound Rubisco.

Alkaline earth metal-assisted dinitrogen activation at nickel.

Rare examples of trinuclear [Ni-N-M-N-Ni] core (M = Ca, Mg) with linear bridged dinitrogen ligands are reported in this work. The reduction of [PrNN]Ni(μ-Br)Li(thf) (1) (PrNN = 2,4-bis-(2,6-diisopropylphenylimido)pentyl) with elemental Mg or Ca in THF under an atmosphere of dinitrogen yields the complex {PrNNNi(μ-N)}M (thf) (M = Mg, complex 2 and M = Ca, complex 3). The bridging end-on (μ-N)M(thf) moiety connects the two [PrNNNi] nickelate fragments.

Tellurolate: an effective Te-atom transfer reagent to prepare the triad of group 5 metal bis(tellurides).

We show in this work how lithium tellurolate Li(X)TeCHSiMe (X = THF, = 1, 1; X = 12--4, = 2, 2), can serve as an effective Te-atom transfer reagent to all group 5 transition metal halide precursors irrespective of the oxidation state. Mononuclear and bis(telluride) complexes, namely (PNP)M(Te) (M = V; Nb, 3; Ta, 4; PNP = N[2-PPr-4-methylphenyl]), are reported herein including structural and spectroscopic data. Whereas the known complex (PNP)V(Te) can be readily prepared from the trivalent precursor (PNP)VCl, two equiv.

Accelerating σ-Bond Metathesis at Sn(II) Centers.

Molecular main-group hydride catalysts are attractive as cheap and Earth-abundant alternatives to transition-metal analogues. In the case of the latter, specific steric and electronic tuning of the metal center through ligand choice has enabled the iterative and rational development of superior catalysts. Analogously, a deeper understanding of electronic structure-activity relationships for molecular main-group hydrides should facilitate the development of superior main-group hydride catalysts.

Nitrene transfer from a sterically confined copper nitrenoid dipyrrin complex.

Despite the myriad Cu-catalyzed nitrene transfer methodologies to form new C-N bonds (, amination, aziridination), the critical reaction intermediates have largely eluded direct characterization due to their inherent reactivity. Herein, we report the synthesis of dipyrrin-supported Cu nitrenoid adducts, investigate their spectroscopic features, and probe their nitrene transfer chemistry through detailed mechanistic analyses. Treatment of the dipyrrin Cu complexes with substituted organoazides affords terminally ligated organoazide adducts with minimal activation of the azide unit as evidenced by vibrational spectroscopy and single crystal X-ray diffraction.

Outer coordination sphere influences on cofactor maturation and substrate oxidation by cytochrome P460.

Product selectivity of ammonia oxidation by ammonia-oxidizing bacteria (AOB) is tightly controlled by metalloenzymes. Hydroxylamine oxidoreductase (HAO) is responsible for the oxidation of hydroxylamine (NHOH) to nitric oxide (NO). The non-metabolic enzyme cytochrome (cyt) P460 also oxidizes NHOH, but instead produces nitrous oxide (NO).

Scrutinizing formally Ni centers through the lenses of core spectroscopy, molecular orbital theory, and valence bond theory.

Nickel K- and L-edge X-ray absorption spectra (XAS) are discussed for 16 complexes and complex ions with nickel centers spanning a range of formal oxidation states from II to IV. K-edge XAS alone is shown to be an ambiguous metric of physical oxidation state for these Ni complexes. Meanwhile, L-edge XAS reveals that the physical d-counts of the formally Ni compounds measured lie well above the d count implied by the oxidation state formalism.

Cytochrome P460 Cofactor Maturation Proceeds via Peroxide-Dependent Post-translational Modification.

Cytochrome P460s are heme enzymes that oxidize hydroxylamine to nitrous oxide. They bear specialized "heme P460" cofactors that are cross-linked to their host polypeptides by a post-translationally modified lysine residue. Wild-type cytochrome P460 may be isolated as a cross-link-deficient proenzyme following anaerobic overexpression in .

Ionothermal Synthesis of Metal-Organic Frameworks Using Low-Melting Metal Salt Precursors.

Metal-organic frameworks (MOFs) are porous, crystalline materials constructed from organic linkers and inorganic nodes with myriad potential applications in chemical separations, catalysis, and drug delivery. A major barrier to the application of MOFs is their poor scalability, as most frameworks are prepared under highly dilute solvothermal conditions using toxic organic solvents. Herein, we demonstrate that combining a range of linkers with low-melting metal halide (hydrate) salts leads directly to high-quality MOFs without added solvent.

Bonding and the role of electrostatics in driving C-C bond formation in high valent organocopper compounds.

The electronic structures and contrasting reactivity of [Cu(CF)] and [Cu(CF)(CH)] were probed using coupled cluster and valence bond calculations. The Cu-C bonds in these complexes were found to be charge shift bonds. A key finding is that electrostatics likely prevent [Cu(CF)] from accessing a productive transition state for C-C bond formation while promote one for [Cu(CF)(CH)].

Direct Reduction of NO to NO by a Mononuclear Nonheme Thiolate Ligated Iron(II) Complex via Formation of a Metastable {FeNO} Complex.

Addition of NO to a nonheme dithiolate-ligated iron(II) complex, Fe(MeTACN)(SSiMe) (), results in the generation of NO. Low-temperature spectroscopic studies reveal a metastable six-coordinate {FeNO} intermediate ( = 3/2) that was trapped at -135 °C and was characterized by low-temperature UV-vis, resonance Raman, EPR, Mössbauer, XAS, and DFT studies. Thermal decay of the {FeNO} species leads to the evolution of NO, providing a rare example of a mononuclear thiolate-ligated {FeNO} that mediates NO reduction to NO without the requirement of any exogenous electron or proton sources.

Lewis acid-assisted reduction of nitrite to nitric and nitrous oxides via the elusive nitrite radical dianion.

Reduction of nitrite anions (NO) to nitric oxide (NO), nitrous oxide (NO) and ultimately dinitrogen (N) takes place in a variety of environments, including in the soil as part of the biogeochemical nitrogen cycle and in acidified nuclear waste. Nitrite reduction typically takes place within the coordination sphere of a redox-active transition metal. Here we show that Lewis acid coordination can substantially modify the reduction potential of this polyoxoanion to allow for its reduction under non-aqueous conditions (-0.

TEMPO coordination and reactivity in group 6; pseudo-pentagonal planar (η-TEMPO)CrX (X = Cl, TEMPO).

The exposure of CrCl(THF) to 1 equiv. of TEMPO and 1 equiv. [TEMPO]Na afforded (η-O,N-TEMPO)CrCl (1, 67%); addition of [TEMPO]Na to 1 yielded (η-O,N-TEMPO)Cr(TEMPO) (2).

Electronic Structure of Ru Complexes with Electron-Rich Anilinopyridinate Ligands.

Diruthenium paddlewheel complexes supported by electron-rich anilinopyridinate (Xap) ligands were synthesized in the course of the first in-depth structural and spectroscopic interrogation of monocationic [Ru(Xap)Cl] species in the Ru oxidation state. Despite paramagnetism of the compounds, H NMR spectroscopy proved highly informative for determining the isomerism of the Ru and Ru compounds. While most compounds are found to have the polar (4,0) geometry, with all four Xap ligands in the same orientation, some synthetic procedures resulted in a mixture of (4,0) and (3,1) isomers, most notably in the case of the parent compound Ru(ap)Cl.

Iron Complexes of a Proton-Responsive SCS Pincer Ligand with a Sensitive Electronic Structure.

Sulfur/carbon/sulfur pincer ligands have an interesting combination of strong-field and weak-field donors, a coordination environment that is also present in the nitrogenase active site. Here, we explore the electronic structures of iron(II) and iron(III) complexes with such a pincer ligand, bearing a monodentate phosphine, thiolate S donor, amide N donor, ammonia, or CO. The ligand scaffold features a proton-responsive thioamide site, and the protonation state of the ligand greatly influences the reduction potential of iron in the phosphine complex.

Lithium superoxide encapsulated in a benzoquinone anion matrix.

Lithium peroxide is the crucial storage material in lithium-air batteries. Understanding the redox properties of this salt is paramount toward improving the performance of this class of batteries. Lithium peroxide, upon exposure to p-benzoquinone (-CHO) vapor, develops a deep blue color.

Reversible C-C Bond Formation, Halide Abstraction, and Electromers in Complexes of Iron Containing Redox-Noninnocent Pyridine-imine Ligands.

The exploration of pyridine-imine (PI) iron complexes that exhibit redox noninnocence (RNI) led to several interesting discoveries. The reduction of (PI)FeX species afforded disproportionation products such as (dmpPI)FeX (dmp = 2,6-Me-CH, X = Cl, Br; -X) and (dippPI)FeX (dipp = 2,6-Pr-CH, X = Cl, Br; -X), which were independently prepared by reductions of (PI)FeX in the presence of PI. The crystal structure of -Br possessed an asymmetric unit with two distinct electromers, species with different electronic GSs: a low-spin ( = 1/2) configuration derived from an intermediate-spin = 1 core antiferromagnetically (AF) coupled to an = 1/2 PI ligand, and an = 3/2 center resulting from a high-spin = 2 core AF-coupled to an = 1/2 PI ligand.

A Nonheme Mononuclear {FeNO} Complex that Produces N O in the Absence of an Exogenous Reductant.

A new nonheme iron(II) complex, Fe (Me TACN)((OSi ) O) (1), is reported. Reaction of 1 with NO gives a stable mononitrosyl complex Fe(NO)(Me TACN)((OSi ) O) (2), which was characterized by Mössbauer (δ=0.52 mm s , |ΔE |=0.

An Isolable Mononuclear Palladium(I) Amido Complex.

Mononuclear Pd(I) species are putative intermediates in Pd-catalyzed reactions, but our knowledge about them is limited due to difficulties in accessing them. Herein, we report the isolation of a Pd(I) amido complex, [(BINAP)Pd(NHAr)] (BINAP = 2,2'-bis(diphenylphosphino)-1,1'-binaphthalene, Ar = 2,6-bis(2',4',6'-triisopropylphenyl)phenyl), from the reaction of (BINAP)PdCl with LiNHAr. This Pd(I) amido species has been characterized by X-ray crystallography, electron paramagnetic resonance, and multiedge Pd X-ray absorption spectroscopy.

Dph3 Enables Aerobic Diphthamide Biosynthesis by Donating One Iron Atom to Transform a [3Fe-4S] to a [4Fe-4S] Cluster in Dph1-Dph2.

All radical -adenosylmethionine (radical-SAM) enzymes, including the noncanonical radical-SAM enzyme diphthamide biosynthetic enzyme Dph1-Dph2, require at least one [4Fe-4S](Cys) cluster for activity. It is well-known in the radical-SAM enzyme community that the [4Fe-4S](Cys) cluster is extremely air-sensitive and requires strict anaerobic conditions to reconstitute activity in vitro. Thus, how such enzymes function in vivo in the presence of oxygen in aerobic organisms is an interesting question.

The influences of carbon donor ligands on biomimetic multi-iron complexes for N reduction.

The active site clusters of nitrogenase enzymes possess the only examples of carbides in biology. These are the only biological FeS clusters that are capable of reducing N to NH , implicating the central carbon and its interaction with Fe as important in the mechanism of N reduction. This biological question motivates study of the influence of carbon donors on the electronic structure and reactivity of unsaturated, high-spin iron centers.

A Mononuclear and High-Spin Tetrahedral Ti Complex.

A high-spin, mononuclear Ti complex, [(Tp)TiCl] [Tp = hydridotris(3--butyl-5-methylpyrazol-1-yl)borate], confined to a tetrahedral ligand-field environment, has been prepared by reduction of the precursor [(Tp)TiCl] with KC. Complex [(Tp)TiCl] has a A ground state (assuming symmetry based on structural studies), established a combination of high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy, solution and solid-state magnetic studies, Ti K-edge X-ray absorption spectroscopy (XAS), and both density functional theory and ab initio (complete-active-space self-consistent-field, CASSCF) calculations. The formally and physically defined Ti complex readily binds tetrahydrofuran (THF) to form the paramagnetic adduct [(Tp)TiCl(THF)], which is impervious to N binding.

Heme P460: A (Cross) Link to Nitric Oxide.

Ammonia-oxidizing bacteria (AOB) convert ammonia (NH) to nitrite (NO) as their primary metabolism and thus provide a blueprint for the use of NH as a chemical fuel. The first energy-producing step involves the homotrimeric enzyme hydroxylamine oxidoreductase (HAO), which was originally reported to oxidize hydroxylamine (NHOH) to NO. HAO uses the heme P460 cofactor as the site of catalysis.