Isocyanide Ligation Enables Electrochemical Ammonia Formation in a Synthetic Cycle for N Fixation.

Transition-metal-mediated splitting of N to form metal nitride complexes could constitute a key step in electrocatalytic nitrogen fixation, if these nitrides can be electrochemically reduced to ammonia under mild conditions. The envisioned nitrogen fixation cycle involves several steps: N binding to form a dinuclear end-on bridging complex with appropriate electronic structure to cleave the N bridge followed by proton/electron transfer to release ammonia and bind another molecule of N. The nitride reduction and N splitting steps in this cycle have differing electronic demands that a catalyst must satisfy.

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.

Inserting Three-Coordinate Nickel into [4Fe-4S] Clusters.

Metalloenzymes can efficiently achieve the multielectron interconversion of carbon dioxide and carbon monoxide under mild conditions. Anaerobic carbon monoxide dehydrogenase (CODH) performs these reactions at the cluster, a unique nickel-iron-sulfide cluster that features an apparent three-coordinate nickel site. How nature assembles the [NiFeS]-Fe cluster is not well understood.

Opportunities for Insight into the Mechanism of Efficient CO/CO Interconversion at a Nickel-Iron Cluster in CO Dehydrogenase.

The reduction of CO with low overpotential and high selectivity is a crucial challenge in catalysis. Fortunately, natural systems have evolved enzymes that achieve this catalytic reaction very efficiently at a complex nickel-iron-sulfur cluster within carbon monoxide dehydrogenase (CODH). Extensive biochemical, crystallographic, and spectroscopic work has been done to understand the structures and mechanism involved in the catalytic cycle, which are summarized here from the perspective of mechanistic organometallic chemistry.

Fe/Thiol Cooperative Hydrogen Atom Transfer Olefin Hydrogenation: Mechanistic Insights That Inform Enantioselective Catalysis.

Asymmetric hydrogenation of activated olefins using transition metal catalysis is a powerful tool for the synthesis of complex molecules, but traditional metal catalysts have difficulty with enantioselective reduction of electron-neutral, electron-rich, and minimally functionalized olefins. Hydrogenation based on radical, metal-catalyzed hydrogen atom transfer (mHAT) mechanisms offers an outstanding opportunity to overcome these difficulties, enabling the mild reduction of these challenging olefins with selectivity that is complementary to traditional hydrogenations with H. Further, mHAT presents an opportunity for asymmetric induction through cooperative hydrogen atom transfer (cHAT) using chiral thiols.

Congested C(sp3)-rich architectures enabled by iron-catalysed conjunctive alkylation.

Catalytic cross-coupling by transition metals has revolutionized the formation of C-C bonds in organic synthesis. However, the challenge of forming multiple alkyl-alkyl bonds in crowded environments remains largely unresolved. Here, we report the regioselective functionalization of olefins with sp-hybridized organohalides and organozinc reagents using a simple (terpyridine)iron catalyst.

Post-Translational Modifications Control Phase Transitions of Tau.

The self-assembly of Tau(297-391) into filaments, which mirror the structures observed in Alzheimer's disease (AD) brains, raises questions about the role of AD-specific post-translational modifications (PTMs) in the formation of paired helical filaments (PHFs). To investigate this, we developed a synthetic approach to produce Tau(291-391) featuring N-acetyllysine, phosphoserine, phosphotyrosine, and N-glycosylation at positions commonly modified in post-mortem AD brains, thus facilitating the study of their roles in Tau pathology. Using transmission electron microscopy (TEM), cryo-electron microscopy (cryo-EM), and a range of optical microscopy techniques, we discovered that these modifications generally hinder the assembly of Tau into PHFs.

Iron(iv) alkyl complexes: electronic structure contributions to Fe-C bond homolysis and migration reactions that form N-C bonds from N.

High-valent iron alkyl complexes are rare, as they are prone to Fe-C bond homolysis. Here, we describe an unusual way to access formally iron(iv) alkyl complexes through double silylation of iron(i) alkyl dinitrogen complexes to form an NNSi group. Spectroscopically validated computations show that the disilylehydrazido(2-) ligand stabilizes the formal iron(iv) oxidation state through a strongly covalent Fe-N π-interaction, in which one π-bond fits an "inverted field" description.

Electronic Structure of Three-Coordinate Fe and Co β-Diketiminate Complexes.

The β-diketiminate supporting group, [ArNCRCHCRNAr], stabilizes low coordination number complexes. Four such complexes, where R = -butyl, Ar = 2,6-diisopropylphenyl, are studied: (nacnac)ML, where M = Fe, Co and L = Cl, CH. These are denoted , , , and and have been previously reported and structurally characterized.

Evaluating Diazene to N Interconversion at Iron-Sulfur Complexes.

Biological N reduction occurs at sulfur-rich multiiron sites, and an interesting potential pathway is concerted double reduction/ protonation of bridging N through PCET. Here, we test the feasibility of using synthetic sulfur-supported diiron complexes to mimic this pathway. Oxidative proton transfer from μ-η : η-diazene (HN=NH) is the microscopic reverse of the proposed N fixation pathway, revealing the energetics of the process.

Three-Coordinate Nickel and Metal-Metal Interactions in a Heterometallic Iron-Sulfur Cluster.

Biological multielectron reactions often are performed by metalloenzymes with heterometallic sites, such as anaerobic carbon monoxide dehydrogenase (CODH), which has a nickel-iron-sulfide cubane with a possible three-coordinate nickel site. Here, we isolate the first synthetic iron-sulfur clusters having a nickel atom with only three donors, showing that this structural feature is feasible. These have a core with two tetrahedral irons, one octahedral tungsten, and a three-coordinate nickel connected by sulfide and thiolate bridges.

Desulfurization and N Binding at an Iron Complex Derived from the C-S Activation of Benzothiophene.

Metal insertion into the C-S bonds of thiophenes is a facile route to interesting polydentate ligand scaffolds with C and S donors. Here, we describe iron-mediated C-S activation of a diphenylphosphine-functionalized benzothiophene proligand. Metalation of the proligand with "tetrakis(trimethylphosphine)iron" gives an initial five-coordinate, diamagnetic iron(II) species with two PMe ligands and a dianionic PCS pincer ligand.

Mechanism of Alkene Hydrofunctionalization by Oxidative Cobalt(salen) Catalyzed Hydrogen Atom Transfer.

Oxidative MHAT hydrofunctionalization of alkenes provides a mild cobalt-catalyzed route to forming C-N and C-O bonds. Here, we characterize relevant salen-supported cobalt complexes and their reactions with alkenes, silanes, oxidant, and solvent. These stoichiometric investigations are complemented by kinetic studies of the catalytic reaction and catalyst speciation.

Mössbauer and Nuclear Resonance Vibrational Spectroscopy Studies of Iron Species Involved in N-N Bond Cleavage.

Diketiminate-supported iron complexes are capable of cleaving the strong triple bond of N to give a tetra-iron complex with two nitrides (Rodriguez et al., , 334, 780-783). The mechanism of this reaction has been difficult to determine, but a transient green species was observed during the reaction that corresponds to a potential intermediate.

Well-Defined Iron Sites in Crystalline Carbon Nitride.

Carbon nitride materials can be hosts for transition metal sites, but Mössbauer studies on iron complexes in carbon nitrides have always shown a mixture of environments and oxidation states. Here we describe the synthesis and characterization of a crystalline carbon nitride with stoichiometric iron sites that all have the same environment. The material (formula CNHFeLiCl, abbreviated PTI/FeCl) is derived from reacting poly(triazine imide)·LiCl (PTI/LiCl) with a low-melting FeCl/KCl flux, followed by anaerobic rinsing with methanol.

Valence Delocalization and Metal-Metal Bonding in Carbon-Bridged Mixed-Valence Iron Complexes.

The carbide ligand in the iron-molybdenum cofactor (FeMoco) in nitrogenase bridges iron atoms in different oxidation states, yet it is difficult to discern its ability to mediate magnetic exchange interactions due to the structural complexity of the cofactor. Here, we describe two mixed-valent diiron complexes with C-based ketenylidene bridging ligands, and compare the carbon bridges with the more familiar sulfur bridges. The ground state of the [Fe (μ-CCO) ] complex with two carbon bridges (4) is S= , and it is valence delocalized on the Mössbauer timescale with a small thermal barrier for electron hopping that stems from the low Fe-C force constant.

Dinitrogen Binding and Functionalization from a Low-Coordinate Alkynyliron Complex.

Alkynyl complexes of low-coordinate transition metals offer a sterically open environment and interesting bonding opportunities. Here, we explore the capacity of iron(I) alkynyl complexes to bind N and isolate a N complex including its X-ray crystal structure. Silylation of the N complex gives an isolable, formally iron(IV) complex with a disilylhydrazido(2-) ligand, but natural bond orbital analysis indicates that an iron(II) formulation is preferable.

Catalytic reduction of dinitrogen to ammonia using molybdenum porphyrin complexes.

Porphyrin complexes are well-known in O and CO reduction, but their application to N reduction is less developed. Here, we show that oxo and nitrido complexes of molybdenum supported by tetramesitylporphyrin (TMP) are effective precatalysts for catalytic N reduction to ammonia, verified by N labeling studies and other control experiments. Spectroscopic and electrochemical studies illuminate some relevant thermodynamic parameters, including the N-H bond dissociation free energy of (TMP)MoNH (43 ± 2 kcal mol).

Dynamic effects on ligand field from rapid hydride motion in an iron(ii) dimer with an = 3 ground state.

Hydride complexes are important in catalysis and in iron-sulfur enzymes like nitrogenase, but the impact of hydride mobility on local iron spin states has been underexplored. We describe studies of a dimeric diiron(ii) hydride complex using X-ray and neutron crystallography, Mössbauer spectroscopy, magnetism, DFT, and calculations, which give insight into the dynamics and the electronic structure brought about by the hydrides. The two iron sites in the dimer have differing square-planar (intermediate-spin) and tetrahedral (high-spin) iron geometries, which are distinguished only by the hydride positions.