Activation of Strong π-Acids at [FeS] Clusters Enabled by a Noncanonical Electronic Structure.

Although Fe-S clusters are privileged metallocofactors for the reduction of N, CO, and other π-acidic substrates, their constituent metal ions─high-spin Fe and Fe─are typically not amenable to binding and activating strong π-acids. Here, we demonstrate that [FeS] clusters can overcome this limitation by adopting a noncanonical electronic structure. Specifically, we report the synthesis and characterization of a series of 3:1 site-differentiated [FeS] clusters in which the unique Fe site is bound by one of 10 electronically variable arylisocyanide ligands.

Terminal Hydride Complex of High-Spin Mn.

The iron-molybdenum cofactor of nitrogenase (FeMoco) catalyzes fixation of N via Fe hydride intermediates. Our understanding of these species has relied heavily on the characterization of well-defined 3d metal hydride complexes, which serve as putative spectroscopic models. Although the Fe ions in FeMoco, a weak-field cluster, are expected to adopt locally high-spin Fe configurations, synthetically accessible hydride complexes featuring d or d electron counts are almost exclusively low-spin.

Toward a quantitative description of solvation structure: a framework for differential solution scattering measurements.

Appreciating that the role of the solute-solvent and other outer-sphere interactions is essential for understanding chemistry and chemical dynamics in solution, experimental approaches are needed to address the structural consequences of these interactions, complementing condensed-matter simulations and coarse-grained theories. High-energy X-ray scattering (HEXS) combined with pair distribution function analysis presents the opportunity to probe these structures directly and to develop quantitative, atomistic models of molecular systems in situ in the solution phase. However, at concentrations relevant to solution-phase chemistry, the total scattering signal is dominated by the bulk solvent, prompting researchers to adopt a differential approach to eliminate this unwanted background.

Four-Coordinate Co(III) Imide with an Unusually Tilted Terminal Imido Ligand.

We report herein the synthesis and characterization of a terminal Co(III) imido complex supported by an intermediate field N,N,C heteroscorpionate. This chemistry is enabled through the development of an additional member of this ligand type featuring Ph(CH)C- substituents, one of which weakly binds and stabilizes Co in the corresponding Co(I) precursor. The Co(III) imide is low-spin with no evidence for thermal population of open-shell excited states.

Orientational analysis of atomic pair correlations in nanocrystalline indium oxide thin films.

The application of grazing-incidence total X-ray scattering (GITXS) for pair distribution function (PDF) analysis using >50 keV X-rays from synchrotron light sources has created new opportunities for structural characterization of supported thin films with high resolution. Compared with grazing-incidence wide-angle X-ray scattering, which is only useful for highly ordered materials, GITXS/PDFs expand such analysis to largely disordered or nanostructured materials by examining the atomic pair correlations dependent on the direction relative to the surface of the supporting substrate. A characterization of nanocrystalline InO-derived thin films is presented here with in-plane-isotropic and out-of-plane-anisotropic orientational ordering of the atomic structure, each synthesized using different techniques.

Facile and dynamic cleavage of every iron-sulfide bond in cuboidal iron-sulfur clusters.

Nature employs weak-field metalloclusters to support a wide range of biological processes. The most ubiquitous metalloclusters are the cuboidal Fe-S clusters, which are comprised of Fe sites with locally high-spin electronic configurations. Such configurations enhance rates of ligand exchange and imbue the clusters with a degree of structural plasticity that is increasingly thought to be functionally relevant.

Four-Coordinate Fe N and Imido Complexes Supported by a Hemilabile NNC Heteroscorpionate Ligand.

Inspired by mechanistic proposals for N reduction at the nitrogenase FeMo cofactor, we report herein a new, strongly σ-donating heteroscorpionate ligand featuring two weak-field pyrazoles and an alkyl donor. This ligand supports four-coordinate Fe(I)-N, Fe(II)-Cl, and Fe(III)-imido complexes, which we have characterized using a variety of spectroscopic and computational methods. Structural and quantum mechanical analysis reveal the nature of the Fe-C bonds to be essentially invariant between the complexes, with conversion between the (formally) low-valent Fe-N and high-valent Fe-imido complexes mediated by pyrazole hemilability.

Evidence for Low-Valent Electronic Configurations in Iron-Sulfur Clusters.

Although biological iron-sulfur (Fe-S) clusters perform some of the most difficult redox reactions in nature, they are thought to be composed exclusively of Fe and Fe ions, as well as mixed-valent pairs with average oxidation states of Fe. We herein show that Fe-S clusters formally composed of these valences can access a wider range of electronic configurations─in particular, those featuring low-valent Fe centers. We demonstrate that CO binding to a synthetic [FeS] cluster supported by -heterocyclic carbene ligands induces the generation of Fe centers via intracluster electron transfer, wherein a neighboring pair of Fe sites reduces the CO-bound site to a low-valent Fe state.

An [FeS]-Alkyl Cluster Stabilized by an Expanded Scorpionate Ligand.

Alkyl-ligated iron-sulfur clusters in the [FeS] charge state have been proposed as short-lived intermediates in a number of enzymatic reactions. To better understand the properties of these intermediates, we have prepared and characterized the first synthetic [FeS]-alkyl cluster. Isolation of this highly reactive species was made possible by the development of an expanded scorpionate ligand suited to the encapsulation of cuboidal clusters.

A Synthetic Model of Enzymatic [FeS]-Alkyl Intermediates.

Although alkyl complexes of [FeS] clusters have been invoked as intermediates in a number of enzymatic reactions, obtaining a detailed understanding of their reactivity patterns and electronic structures has been difficult owing to their transient nature. To address this challenge, we herein report the synthesis and characterization of a 3:1 site-differentiated [FeS]-alkyl cluster. Whereas [FeS] clusters typically exhibit pairwise delocalized electronic structures in which each Fe has a formal valence of 2.

Electronic Structures of an [Fe(NNR)] Redox Series: Ligand Noninnocence and Implications for Catalytic Nitrogen Fixation.

The intermediacy of metal-NNH complexes has been implicated in the catalytic cycles of several examples of transition-metal-mediated nitrogen (N) fixation. In this context, we have shown that triphosphine-supported Fe(N) complexes can be reduced and protonated at the distal N atom to yield Fe(NNH) complexes over an array of charge and oxidation states. Upon exposure to further H/e equivalents, these species either continue down a distal-type Chatt pathway to yield a terminal iron(IV) nitride or instead follow a distal-to-alternating pathway resulting in N-H bond formation at the proximal N atom.

Zerovalent Rhodium and Iridium Silatranes Featuring Two-Center, Three-Electron Polar σ Bonds.

Species with 2-center, 3-electron (2c/3e ) σ bonds are of interest owing to their fascinating electronic structures and potential for interesting reactivity patterns. Report here is the synthesis and characterization of a pair of zerovalent (d ) trigonal pyramidal Rh and Ir complexes that feature 2c/3e σ bonds to the Si atom of a tripodal tris(phosphine)silatrane ligand. X-ray diffraction, continuous wave and pulse electron paramagnetic resonance, density-functional theory calculations, and reactivity studies have been used to characterize these electronically distinctive compounds.

Design, Synthesis, and Self-Assembly of Polymers with Tailored Graft Distributions.

Grafting density and graft distribution impact the chain dimensions and physical properties of polymers. However, achieving precise control over these structural parameters presents long-standing synthetic challenges. In this report, we introduce a versatile strategy to synthesize polymers with tailored architectures via grafting-through ring-opening metathesis polymerization (ROMP).

Nitrogen Fixation via a Terminal Fe(IV) Nitride.

Terminal iron nitrides (Fe≡N) have been proposed as intermediates of (bio)catalytic nitrogen fixation, yet experimental evidence to support this hypothesis has been lacking. In particular, no prior synthetic examples of terminal Fe≡N species have been derived from N. Here we show that a nitrogen-fixing Fe-N catalyst can be protonated to form a neutral Fe(NNH) hydrazido(2-) intermediate, which, upon further protonation, heterolytically cleaves the N-N bond to release [Fe≡N] and NH.

Accelerated Oxygen Atom Transfer and C-H Bond Oxygenation by Remote Redox Changes in Fe Mn-Iodosobenzene Adducts.

We report the synthesis, characterization, and reactivity of [LFe (PhPz) OMn( PhIO)][OTf] (3: x=2; 4: x=3), where 4 is one of very few examples of iodosobenzene-metal adducts characterized by X-ray crystallography. Access to these rare heterometallic clusters enabled differentiation of the metal centers involved in oxygen atom transfer (Mn) or redox modulation (Fe). Specifically, Fe Mössbauer and X-ray absorption spectroscopy provided unique insights into how changes in oxidation state (Fe Fe Mn vs.

A Synthetic Single-Site Fe Nitrogenase: High Turnover, Freeze-Quench (57)Fe Mössbauer Data, and a Hydride Resting State.

The mechanisms of the few known molecular nitrogen-fixing systems, including nitrogenase enzymes, are of much interest but are not fully understood. We recently reported that Fe-N2 complexes of tetradentate P3(E) ligands (E = B, C) generate catalytic yields of NH3 under an atmosphere of N2 with acid and reductant at low temperatures. Here we show that these Fe catalysts are unexpectedly robust and retain activity after multiple reloadings.

Intramolecular C-H and C-F Bond Oxygenation Mediated by a Putative Terminal Oxo Species in Tetranuclear Iron Complexes.

Herein we report the intramolecular arene C-H and C-F bond oxygenation by tetranuclear iron complexes. Treatment of [LFe3(PhPz)3OFe][OTf]2 (1) or its fluorinated analog [LFe3(F2ArPz)3OFe][OTf]2 (5) with iodosobenzene results in the regioselective hydroxylation of a bridging pyrazolate ligand, converting a C-H or C-F bond into a C-O bond. The observed reactivity suggests the formation of terminal and reactive Fe-oxo intermediates.

Nitric oxide activation by distal redox modulation in tetranuclear iron nitrosyl complexes.

A series of tetranuclear iron complexes displaying a site-differentiated metal center was synthesized. Three of the metal centers are coordinated to our previously reported ligand, based on a 1,3,5-triarylbenzene motif with nitrogen and oxygen donors. The fourth (apical) iron center is coordinatively unsaturated and appended to the trinuclear core through three bridging pyrazolates and an interstitial μ4-oxide moiety.

Evaluating molecular cobalt complexes for the conversion of N2 to NH3.

Well-defined molecular catalysts for the reduction of N2 to NH3 with protons and electrons remain very rare despite decades of interest and are currently limited to systems featuring molybdenum or iron. This report details the synthesis of a molecular cobalt complex that generates superstoichiometric yields of NH3 (>200% NH3 per Co-N2 precursor) via the direct reduction of N2 with protons and electrons. While the NH3 yields reported herein are modest by comparison to those of previously described iron and molybdenum systems, they intimate that other metals are likely to be viable as molecular N2 reduction catalysts.