An S = 1/2 Iron Complex Featuring N2, Thiolate, and Hydride Ligands: Reductive Elimination of H2 and Relevant Thermochemical Fe-H Parameters

Synthesis and Characterization of Bridging-Diazene Diiron Half-Sandwich Complexes: The Role of Sulfur Hydrogen Bonding

We report two bridging-diazene diiron complexes [Cp*Fe(8-quinolinethiolate)]2(μ-N2H2) (1-N2H2) and [Cp*Fe(1,2-Cy2PC6H4S)]2(μ-N2H2) (2-N2H2), synthesized by the reaction of hydrazine with the corresponding thiolate-based iron half-sandwich complex, [Cp*Fe(8-quinolinethiolate)]2 (1) and Cp*Fe(1,2-Cy2PC6H4S) (2). Crystallographic analysis reveals that the thiolate sites in 1-N2H2 and 2-N2H2 can engage in N-H···S hydrogen bonding with the diazene protons. 1-N2H2 is thermally stable in both solid and solution states, allowing for one-electron oxidation to afford a cationic diazene radical complex [1-N2H2]+ at room temperature. In contrast, 2-N2H2 tends to undergo N2H2/N2 transformation, leading to the formation of a Fe(III)-H species by the loss of N2. In addition to stabilizing HN=NH species through the hydrogen bonding, the thiolate-based ligands also seem to facilitate proton-coupled electron transfer, thereby promoting N-H cleavage.

Benchmark Study of Density Functional Theory Methods in Geometry Optimization of Transition Metal-Dinitrogen Complexes

The current benchmark study is focused on determining the most precise theoretical method for optimizing the geometry of transition metal-dinitrogen complexes. To accomplish this goal, seven density functional (DF) methods from five distinct classes of density functional theory (DFT) have been selected, including B3LYP-D3(BJ), BP86-D3(BJ), PBE0-D3(BJ), ωB97X-D, M06, M06-L, and TPSSh-D3(BJ). These DFs will be utilized with the Karlsruhe basis set (def2-SVP). To carry out this benchmark study, a total of forty-two structurally diverse transition metal-dinitrogen compounds with experimentally known X-ray data have been selected from the Cambridge Crystallographic Data Centre (CCDC). Based on a comparison of the theoretical data with experimental values (X-ray) of the selected transition metal-dinitrogen compounds, statistical parameters such as root-mean-square deviation (RMSD) and N-N and M-N bond lengths are obtained to evaluate the performance of the seven chosen DFs. According to the obtained results, among all DFT methods used in the study, Minnesota functionals (M06 and M06-L) and TPSSh-D3(BJ) show good performance, with lower RMSD values. This suggests that these three methods are the most reliable for optimizing the geometry of transition metal-dinitrogen complexes. Based on the absolute errors of the N-N and M-N bond lengths relative to the X-ray data, further analysis is conducted, and it is determined that M06-L is the best functional for optimizing the geometry of transition metal-dinitrogen compounds. Additionally, the influence of using a high-level basis set (def2-TZVP) compared to def2-SVP on the calculated RMSD among the seven chosen methods is found to be negligible.

Order-order assembly transition-driven polyamines detection based on iron-sulfur complexes

Innovative modes of response can greatly push forward chemical sensing processes and subsequently improve sensing performance. Classical chemical sensing modes seldom involve the transition of a delicate molecular assembly during the response. Here, we display a sensing mode for polyamine detection based on an order-order transition of iron-sulfur complexes upon their assembly. Strong validation proves that the unique order-order transition of the assemblies is the driving force of the response, in which the polyamine captures the metal ion of the iron-sulfur complex, leading it to decompose into a metal-polyamine product, accompanied by an order-order transition of the assemblies. This mechanism makes the detection process more intuitive and selective, and remarkably improves the detection efficiency, achieving excellent polyamines specificity, second-level response, convenient visual detection, and good recyclability of the sensing system. Furthermore, this paper also provides opportunities for the further application of the iron-sulfur platform in environment-related fields.

Development of a Nickel-Catalyzed N-N Coupling for the Synthesis of Hydrazides

A nickel-catalyzed N-N cross-coupling for the synthesis of hydrazides is reported. O-Benzoylated hydroxamates were efficiently coupled with a broad range of aryl and aliphatic amines via nickel catalysis to form hydrazides in an up to 81% yield. Experimental evidence implicates the intermediacy of electrophilic Ni-stabilized acyl nitrenoids and the formation of a Ni(I) catalyst via silane-mediated reduction. This report constitutes the first example of an intermolecular N-N coupling compatible with secondary aliphatic amines.

Bimolecular Reductive Elimination of Ethane from Pyridine(diimine) Iron Methyl Complexes: Mechanism, Electronic Structure, and Entry into [2+2] Cycloaddition Catalysis

The application of bimolecular reductive elimination to the activation of iron catalysts for alkene-diene cycloaddition is described. Key to this approach was the synthesis, characterization, electronic structure determination, and ultimately solution stability of a family of pyridine(diimine) iron methyl complexes with diverse steric properties and electronic ground states. Both the aryl-substituted, (^MePDI)FeCH₃ and (^EtPDI)FeCH₃ (^RPDI = 2,6-(2,6-R₂-C₆H₃N═CMe)₂C₅H₃N), and the alkyl-substituted examples, (^CyAPDI)FeCH₃ (^CyAPDI = 2,6-(C₆H₁₁N═CMe)₂C₅H₃N), have molecular structures significantly distorted from planarity and S = 3/2 ground states. The related N-arylated derivative bearing 2,6-di-isopropyl aryl substituents, (^iPrPDI)FeCH₃, has an idealized planar geometry and exhibits spin crossover behavior from S = 1/2 to S = 3/2 states. At 23 °C under an N₂ atmosphere, both (^MePDI)FeCH₃ and (^EtPDI)FeCH₃ underwent reductive elimination of ethane to form the iron dinitrogen precatalysts, [(^MePDI)Fe(N₂)]₂(μ-N₂) and [(^EtPDI)Fe(N₂)]₂(μ-N₂), respectively, while (^iPrPDI)FeCH₃ proved inert to C-C bond formation. By contrast, addition of butadiene to all three iron methyl complexes induced ethane formation and generated the corresponding iron butadiene complexes, (^RPDI)Fe(η⁴-C₄H₆) (R = Me, Et, ⁱPr), known precatalysts for the [2+2] cycloaddition of olefins and dienes. Kinetic, crossover experiments, and structural studies were combined with magnetic measurements and Mössbauer spectroscopy to elucidate the electronic and steric features of the iron complexes that enable this unusual reductive elimination and precatalyst activation pathway. Transmetalation of methyl groups between iron centers was fast at ambient temperature and independent of steric environment or spin state, while the intermediate dimer underwent the sterically controlled rate-determining reaction with either N₂ or butadiene to access a catalytically active iron compound.

Humic acid controls cadmium stabilization during Fe(II)-induced lepidocrocite transformation

Abiotic reduction of iron (oxyhydr)oxides by aqueous Fe(II) is one of the key processes affecting the Fe cycle in soil. Lepidocrocite (Lep) occurs naturally in anaerobic, clayey, non-calcareous soils in cooler and temperate regions; however, little is known about the impacts of co-precipitated humic acid (HA) on Fe(II)-induced Lep transformation and its consequences for heavy metal immobilization. In this study, the Fe(II)-induced phase transformation of Lep-HA co-precipitates was analyzed as a function of the C/Fe ratio, and its implications for subsequent Cd(II) concentration dynamic in dissolved and solid form was further investigated. The results revealed that secondary Fe(II)-bearing magnetite commonly formed during the Fe(II)-induced transformation of Lep, which further changed the mobility and distribution of Cd(II). The co-precipitated HA resulted in a decrease in the Fe solid phase transformation as the C/Fe ratios increased. Magnetite was found to be a secondary mineral in the 0.3C/Fe ratio Lep-HA co-precipitate, while only Lep was observed at a C/Fe ratio of 1.2 using X-ray diffraction (XRD) and Mössbauer spectroscopy. Based on XRD, scanning electron microscopy (SEM), Mössbauer, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) results, newly formed magnetite may immobilize Cd(II) through surface complexes, incorporation, or structural substitution. The presence of HA was beneficial for binding Cd(II) and affected the mineralogical transformation of Lep into magnetite, which further induced the distribution of Cd(II) into the newly formed secondary minerals. These results provide insights into the behavior of Cd(II) in response to reaction between humic matter and iron (oxyhydr)oxides in anaerobic environments.

Interplay between β-Diimino and β-Diketiminato Ligands in Nickel Complexes Active in the Proton Reduction Reaction

Two Ni complexes are reported with κ⁴-P₂N₂ β-diimino (BDI) ligands with the general formula [Ni(XBDI)](BF₄)₂, where BDI is N-(2-(diphenylphosphaneyl)ethyl)-4-((2-(diphenylphosphaneyl)ethyl)imino)pent-2-en-2-amine and X indicates the substituent in the α-carbon intradiimine position, X = H for 1(BF₄)₂ and X = Ph for 2(BF₄)₂. Electrochemical analysis together with UV-vis and NMR spectroscopy in acetonitrile and dimethylformamide (DMF) indicates the conversion of the β-diimino complexes 1²⁺ and 2²⁺ to the negatively charged β-diketiminato (BDK) analogues (1-H)⁺ and (2-H)⁺ via deprotonation in DMF. Moreover, further electrochemical and spectroscopy evidence indicates that the one-electron-reduced derivatives 1⁺ and 2⁺ can also rapidly evolve to the BDK (1-H)⁺ and (2-H)⁺, respectively, via hydrogen gas evolution through a bimolecular homolytic pathway. Finally, both complexes are demonstrated to be active for the proton reduction reaction in DMF at E_app = -1.8 V vs Fc^+/0, being the active species the one-electron-reduced derivative 1-H and 2-H.

Unravelling the Mechanistic Pathway of the Hydrogen Evolution Reaction Driven by a Cobalt Catalyst

A cobalt complex bearing a κ-N₃ P₂ ligand is presented (1⁺ or Co^I (L), where L is (1E,1'E)-1,1'-(pyridine-2,6-diyl)bis(N-(3-(diphenylphosphanyl)propyl)ethan-1-imine). Complex 1⁺ is stable under air at oxidation state Co^I thanks to the π-acceptor character of the phosphine groups. Electrochemical behavior of 1⁺ reveals a two-electron Co^I /Co^III oxidation process and an additional one-electron reduction, which leads to an enhancement in the current due to hydrogen evolution reaction (HER) at E_onset =-1.6 V vs Fc/Fc⁺ . In the presence of 1 equiv of bis(trifluoromethane)sulfonimide, 1⁺ forms the cobalt hydride derivative Co^III (L)-H (2²⁺ ), which has been fully characterized. Further addition of 1 equiv of CoCp*₂ (Cp* is pentamethylcyclopentadienyl) affords the reduced Co^II (L)-H (2⁺ ) species, which rapidly forms hydrogen and regenerates the initial Co^I (L) (1⁺ ). The spectroscopic characterization of catalytic intermediates together with DFT calculations support an unusual bimolecular homolytic mechanism in the catalytic HER with 1⁺ .

A Parent Iron Amido Complex in Catalysis of Ammonia Oxidation

Parent amido complexes are crucial intermediates in ammonia-based transformations. We report a well-defined ferric ammine system [Cp*Fe(1,2-Ph₂PC₆H₄NH)(NH₃)]⁺ ([1-NH₃]⁺), which processes electrocatalytic ammonia oxidation to N₂ and H₂ at a mild potential. Through establishing elementary e^-/H⁺ conversions with the ferric ammine, a formal Fe(IV)-amido species, [1-NH₂]⁺, together with its conjugated Lewis acid, [1-NH₃]²⁺, was isolated and structurally characterized for the first time. Mechanism studies indicated that further oxidation of [1-NH₂]⁺ induces the reaction of the parent amido unit with NH₃. The formation of hydrazine is realized by the non-innocent nature of the phenylamido ligand that facilitates the concerted transfer of one proton and two electrons.

Dinitrogen binding and activation at a molybdenum-iron-sulfur cluster

The Fe-S clusters of nitrogenases carry out the life-sustaining conversion of N₂ to NH₃. Although progress continues to be made in modelling the structural features of nitrogenase cofactors, no synthetic Fe-S cluster has been shown to form a well-defined coordination complex with N₂. Here we report that embedding an [MoFe₃S₄] cluster in a protective ligand environment enables N₂ binding at Fe. The bridging [MoFe₃S₄]₂(μ-η¹:η¹-N₂) complex thus prepared features a substantially weakened N-N bond despite the relatively high formal oxidation states of the metal centres. Substitution of one of the [MoFe₃S₄] cubanes with an electropositive Ti metalloradical induces additional charge transfer to the N₂ ligand with generation of Fe-N multiple-bond character. Structural and spectroscopic analyses demonstrate that N₂ activation is accompanied by shortened Fe-S distances and charge transfer from each Fe site, including those not directly bound to N₂. These findings indicate that covalent interactions within the cluster play a critical role in N₂ binding and activation.

Hydrazine Formation via Coupling of a Nickel(III)-NH2 Radical

M(NHx ) intermediates involved in N-N bond formation are central to ammonia oxidation (AO) catalysis, an enabling technology to ultimately exploit ammonia (NH3 ) as an alternative fuel source. While homocoupling of a terminal amide species (M-NH2 ) to form hydrazine (N2 H4 ) has been proposed, well-defined examples are without precedent. Herein, we discuss the generation and electronic structure of a NiIII -NH2 species that undergoes bimolecular coupling to generate a NiII 2 (N2 H4 ) complex. This hydrazine adduct can be further oxidized to a structurally unusual Ni2 (N2 H2 ) species; this releases N2 in the presence of NH3 , thus establishing a synthetic cycle for Ni-mediated AO. Distribution of the redox load for H2 N-NH2 formation via NH2 coupling between two metal centers presents an attractive strategy for AO catalysis using Earth-abundant, late first-row metals.

Metal/Ligand Proton Tautomerism Facilitates Dinuclear H2 Reductive Elimination

Using the doubly protic bis-pyrazole-pyridine ligand (N(NN^H)₂), we have synthesized an octahedral Ir^III-H [HIr(κ³-N(NN^H)(NN^-))(CO)(^tBuPy)]⁺ ([1-MH]⁺) from an Ir^I starting material. This hydride was generated by adding sufficient electron density to the metal center such that it became the thermodynamically preferred site of protonation. It was observed via UV-vis spectroscopy that [1-MH]⁺ establishes a [^tBuPy] dependent equilibrium with a ligand protonated square-planar Ir^I [Ir(N(NN^H)₂)(CO)]⁺ ([2-LH]⁺). This example of metal/ligand proton tautomerism is unusual in that the position of the equilibrium can be controlled by the concentration of exogeneous ligand (i.e., ^tBuPy). This equilibrium was shown to be key to the reactivity of the Ir^III-H; 2 equiv of [1-MH]⁺ release H₂, converting to the Ir^II dimer [[Ir(N(NN^-)(NN^H))(CO)(^tBuPy)]₂]²⁺ ([7]²⁺) under mild conditions (observable at room temperature). Mechanistic evidence is presented to support that this dinuclear reductive elimination occurs by tautomerization of the metal hydride [1-MH]⁺ to a ligand protonated species [1-LH]⁺, from which ligand dissociation is facile, generating [2-LH]⁺. Subsequent reaction of [2-LH]⁺ with [1-MH]⁺ allows for production of H₂ and the Ir^II dimer [7]²⁺. The tautomerization between the metal-hydride and the ligand protonated species provides a low energy pathway for ligand dissociation, opening the needed coordination site. The ability to control the interconversion between a metal-hydride and a ligand-protonated congener using an exogeneous ligand introduces a new strategy for catalyst design with proton responsive ligands.

Mixed-Valent Diiron μ-Carbyne, μ-Hydride Complexes: Implications for Nitrogenase

Binding of N₂ by the FeMo-cofactor of nitrogenase is believed to occur after transfer of 4 e^- and 4 H⁺ equivalents to the active site. Although pulse EPR studies indicate the presence of two Fe-(μ-H)-Fe moieties, the structural and electronic features of this mixed valent intermediate remain poorly understood. Toward an improved understanding of this bioorganometallic cluster, we report herein that diiron μ-carbyne complex (P₆ArC)Fe₂(μ-H) can be oxidized and reduced, allowing for the first time spectral characterization of two EPR-active Fe(μ-C)(μ-H)Fe model complexes linked by a 2 e^- transfer which bear some resemblance to a pair of E_n and E_n+2 states of nitrogenase. Both species populate S = 1/2 states at low temperatures, and the influence of valence (de)localization on the spectroscopic signature of the μ-hydride ligand was evaluated by pulse EPR studies. Compared to analogous data for the {Fe₂(μ-H)}₂ state of FeMoco (E₄(4H)), the data and analysis presented herein suggest that the hydride ligands in E₄(4H) bridge isovalent (most probably Fe^III) metal centers. Although electron transfer involves metal-localized orbitals, investigations of [(P₆ArC)Fe₂(μ-H)]⁺¹ and [(P₆ArC)Fe₂(μ-H)]^-1 by pulse EPR revealed that redox chemistry induces significant changes in Fe-C covalency (-50% upon 2 e^- reduction), a conclusion further supported by X-ray absorption spectroscopy, ⁵⁷Fe Mössbauer studies, and DFT calculations. Combined, our studies demonstrate that changes in covalency buffer against the accumulation of excess charge density on the metals by partially redistributing it to the bridging carbon, thereby facilitating multielectron transformations.

Generating Potent C-H PCET Donors: Ligand-Induced Fe-to-Ring Proton Migration from a Cp*FeIII-H Complex Demonstrates a Promising Strategy

Highly reactive organometallic species that mediate reductive proton-coupled electron transfer (PCET) reactions are an exciting area for development in catalysis, where a key objective focuses on tuning the reactivity of such species. This work pursues ligand-induced activation of a stable organometallic complex toward PCET reactivity. This is studied via the conversion of a prototypical Cp*Fe^III-H species, [Fe^III(η⁵-Cp*)(dppe)H]⁺ (Cp* = C₅Me₅^-, dppe = 1,2-bis(diphenylphosphino)ethane), to a highly reactive, S = 1/2 ring-protonated endo-Cp*H-Fe relative, triggered by the addition of CO. Our assignment of the latter ring-protonated species contrasts with its previous reported formulation, which instead assigned it as a hypervalent 19-electron hydride, [Fe^III(η⁵-Cp*)(dppe)(CO)H]⁺. Herein, pulse EPR spectroscopy (^1,2H HYSCORE, ENDOR) and X-ray crystallography, with corresponding DFT studies, cement its assignment as the ring-protonated isomer, [Fe^I(endo-η⁴-Cp*H)(dppe)(CO)]⁺. A less sterically shielded and hence more reactive exo-isomer can be generated through oxidation of a stable Fe⁰(exo-η⁴-Cp*H)(dppe)(CO) precursor. Both endo- and exo-ring-protonated isomers are calculated to have an exceptionally low bond dissociation free energy (BDFE_C-H ≈ 29 kcal mol^-1 and 25 kcal mol^-1, respectively) cf. BDFE_Fe-H of 56 kcal mol^-1 for [Fe^III(η⁵-Cp*)(dppe)H]⁺. These weak C-H bonds are shown to undergo proton-coupled electron transfer (PCET) to azobenzene to generate diphenylhydrazine and the corresponding closed-shell [Fe^II(η⁵-Cp*)(dppe)CO]⁺ byproduct.

Structure and dynamics of catalytically competent but labile paramagnetic metal-hydrides: the Ti(iii)-H in homogeneous olefin polymerization

Metal hydride complexes find widespread application in catalysis and their properties are often understood on the basis of the available crystal structures. However, some catalytically relevant metal hydrides are only spontaneously formed in situ, cannot be isolated in large quantities or crystallised and their structure is therefore ill defined. One such example is the paramagnetic Ti(iii)-hydride involved in homogeneous Ziegler-Natta catalysis, formed upon activation of CpTi(iv)Cl3 with modified methylalumoxane (MMAO). In this contribution, through a combined use of electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopies we identify the nature of the ligands, their bonding interaction and the extent of the spin distribution. From the data, an atomistic and electronic model is proposed, which supports the presence of a self-assembled ion pair between a cationic terminal Ti-hydride and an aluminate anion, with a hydrodynamic radius of ca. 16 Å.

Two-State Reactivity in Iron-Catalyzed Alkene Isomerization Confers σ-Base Resistance

A low-coordinate, high spin (S = 3/2) organometallic iron(I) complex is a catalyst for the isomerization of alkenes. A combination of experimental and computational mechanistic studies supports a mechanism in which alkene isomerization occurs by the allyl mechanism. Importantly, while substrate binding occurs on the S = 3/2 surface, oxidative addition to an η¹-allyl intermediate only occurs on the S = 1/2 surface. Since this spin state change is only possible when the alkene substrate is bound, the catalyst has high immunity to typical σ-base poisons due to the antibonding interactions of the high spin state.

H2 Evolution from a Thiolate-Bound Ni(III) Hydride

Terminal Ni^III hydrides are proposed intermediates in proton reduction catalyzed by both molecular electrocatalysts and metalloenzymes, but well-defined examples of paramagnetic nickel hydride complexes are largely limited to bridging hydrides. Herein, we report the synthesis of an S = 1/2, terminally bound thiolate-Ni^III-H complex. This species and its terminal hydride ligand in particular have been thoroughly characterized by vibrational and EPR techniques, including pulse EPR studies. Corresponding DFT calculations suggest appreciable spin leakage onto the thiolate ligand. The hyperfine coupling to the terminal hydride ligand of the thiolate-Ni^III-H species is comparable to that of the hydride ligand proposed for the Ni-C hydrogenase intermediate (Ni^III-H-Fe^II). Upon warming, the featured thiolate-Ni^III-H species undergoes bimolecular reductive elimination of H₂. Associated kinetic studies are discussed and compared with a structurally related Fe^III-H species that has also recently been reported to undergo bimolecular H-H coupling.

Mononuclear Fe(I) and Fe(II) Acetylene Adducts and Their Reductive Protonation to Terminal Fe(IV) and Fe(V) Carbynes

The activity of nitrogenase enzymes, which catalyze the conversion of atmospheric dinitrogen to bioavailable ammonia, is most commonly assayed by the reduction of acetylene gas to ethylene. Despite the practical importance of acetylene as a substrate, little is known concerning its binding or activation in the iron-rich active site. "Fischer-Tropsch" type coupling of non-native C1 substrates to higher-order C≥2 products is also known for nitrogenase, though potential metal-carbon multiply bonded intermediates remain underexplored. Here we report the activation of acetylene gas at a mononuclear tris(phosphino)silyl-iron center, (SiP3)Fe, to give Fe(I) and Fe(II) side-on adducts, including S = 1/2 FeI(η2-HCCH); the latter is characterized by pulse EPR spectroscopy and DFT calculations. Reductive protonation reactions with these compounds converge at stable examples of unusual, formally iron(IV) and iron(V) carbyne complexes, as in diamagnetic (SiP3)Fe≡CCH3 and the paramagnetic cation S = 1/2 [(SiP3)Fe≡CCH3]+. Both alkylcarbyne compounds possess short Fe-C triple bonds (approximately 1.7 Å) trans to the anchoring silane. Pulse EPR experiments, X-band ENDOR and HYSCORE, reveal delocalization of the iron-based spin onto the α-carbyne nucleus in carbon p-orbitals. Furthermore, isotropic coupling of the distal β-CH3 protons with iron indicates hyperconjugation with the spin/hole character on the Fe≡CCH3 unit. The electronic structures of (SiP3)Fe≡CCH3 and [(SiP3)Fe≡CCH3]+ are discussed in comparison to previously characterized, but heterosubstituted, iron carbynes, as well as a hypothetical nitride species, (SiP3)Fe≡N. Such comparisons are germane to the consideration of formally high-valent, multiply bonded Fe≡C and/or Fe≡N intermediates in synthetic or biological catalysis by iron.

Snapshots of a Migrating H-Atom: Characterization of a Reactive Iron(III) Indenide Hydride and its Nearly Isoenergetic Ring-Protonated Iron(I) Isomer

We report the characterization of an S= 1 / 2 iron π-complex, [Fe(η⁶ -IndH)(depe)]⁺ (Ind=Indenide (C₉ H₇ ^- ), depe=1,2-bis(diethylphosphino)ethane), which results via C-H elimination from a transient Fe^III hydride, [Fe(η³ :η² -Ind)(depe)H]⁺ . Owing to weak M-H/C-H bonds, these species appear to undergo proton-coupled electron transfer (PCET) to release H₂ through bimolecular recombination. Mechanistic information, gained from stoichiometric as well as computational studies, reveal the open-shell π-arene complex to have a BDFE_C-H value of ≈50 kcal mol^-1 , roughly equal to the BDFE_Fe-H of its Fe^III -H precursor (ΔG°≈0 between them). Markedly, this reactivity differs from related Fe(η⁵ -Cp/Cp*) compounds, for which terminal Fe^III -H cations are isolable and have been structurally characterized, highlighting the effect of a benzannulated ring (indene). Overall, this study provides a structural, thermochemical, and mechanistic foundation for the characterization of indenide/indene PCET precursors and outlines a valuable approach for the differentiation of a ring- versus a metal-bound H-atom by way of continuous-wave (CW) and pulse EPR (HYSCORE) spectroscopic measurements.

Nitrogenase-Relevant Reactivity of a Synthetic Iron-Sulfur-Carbon Site

Simple synthetic compounds with only S and C donors offer a ligation environment similar to the active site of nitrogenase (FeMoco) and thus demonstrate reasonable mechanisms and geometries for N₂ binding and reduction in nature. We recently reported the first example of N₂ binding at a mononuclear iron site supported by only S and C donors. In this work, we report experiments that examine the mechanism of N₂ binding in this system. The reduction of an iron(II) tris(thiolate) complex with 1 equiv of KC₈ leads to a thermally unstable intermediate, and a combination of Mössbauer, EPR, and X-ray absorption spectroscopies identifies it as a high-spin (S = 3/2) iron(I) species that maintains coordination of all three sulfur atoms. DFT calculations suggest that this iron(I) intermediate has a pseudotetrahedral geometry that resembles the S₃C iron coordination environment of the belt iron sites in the resting state of the FeMoco. Further reduction to the iron(0) oxidation level under argon causes the dissociation of one of the thiolate donors and gives an η⁶-arene species which reacts with N₂. Thus, in this system the loss of thiolate and binding of N₂ require reduction beyond the iron(I) level to the iron(0) level. Further reduction of the iron(0)-N₂ complex gives a reactive, formally iron(-I) species. Treatment of the putative iron(-I) complex with weak acids gives low yields of ammonia and hydrazine, demonstrating that these nitrogenase products can be generated from N₂ at a synthetic Fe-S-C site. Catalytic N₂ reduction is not observed, which is attributed to protonation of the supporting ligand and degradation of the complex via ligand dissociation. Identification of the challenges in this system gives insight into the design features needed for functional biomimetic complexes.

Metal Bonding with 3d and 6d Orbitals: An EPR and ENDOR Spectroscopic Investigation of Ti3+-Al and Th3+-Al Heterobimetallic Complexes

Accessing covalent bonding interactions between actinides and ligating atoms remains a central problem in the field. Our current understanding of actinide bonding is limited because of a paucity of diverse classes of compounds and the lack of established models. We recently synthesized a thorium (Th)–aluminum (Al) heterobimetallic molecule that represents a new class of low-valent Th-containing compounds. To gain further insight into this system and actinide–metal bonding more generally, it is useful to study their underlying electronic structures. Here, we report characterization by electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) spectroscopy of two heterobimetallic compounds: (i) a Cp^tt₂ThH₃AlCTMS₃ [TMS = Si(CH₃)₃; Cp^tt = 1,3-di-tert-butylcyclopentadienyl] complex with bridging hydrides and (ii) an actinide-free Cp₂TiH₃AlCTMS₃ (Cp = cyclopentadienyl) analogue. Analyses of the hyperfine interactions between the paramagnetic trivalent metal centers and the surrounding magnetic nuclei, ¹H and ²⁷Al, yield spin distributions over both complexes. These results show that while the bridging hydrides in the two complexes have similar hyperfine couplings (a_iso = −9.7 and −10.7 MHz, respectively), the spin density on the Al ion in the Th³⁺ complex is ∼5-fold larger than that in the titanium(3+) (Ti³⁺) analogue. This suggests a direct orbital overlap between Th and Al, leading to a covalent interaction between Th and Al. Our quantitative investigation by a pulse EPR technique deepens our understanding of actinide bonding to main-group elements.

The electronic structures of Ti³⁺−Al and Th³⁺−Al heterobimetallic complexes are probed by electron−nuclear double resonance spectroscopy, revealing a much larger spin density on the Al center in the latter and the presence of a covalent Th−Al bonding interaction caused by the direct orbital overlap between Th and Al.

Catalytic hydrazine disproportionation mediated by a thiolate-bridged VFe complex

A heterobimetallic VFe complex is demonstrated to catalyse hydrazine disproportionation with yields of up to 1073 equivalents of NH₃ per catalyst, comparable to the highest turnover known for any molecular catalyst.

Methylene insertion into an Fe2S2 cluster: formation of a thiolate-bridged diiron complex containing an Fe-CH2-S moiety

Reduction of a thiolate-bridged FeIIFeIII complex leads to the cleavage of an Fe-S bond by the insertion of the methylene unit from CH2Cl2 to give a neutral FeIIFeIII complex with a novel Fe-CH2-S fragment. The structural and electrochemical differences of the alkylated and the non-alkylated Fe2S2 complexes are also examined.

The High Chemofidelity of Metal-Catalyzed Hydrogen Atom Transfer

The implementation of any chemical reaction in a structurally complex setting ( King , S. M. J. Org. Chem. 2014 , 79 , 8937 ) confronts structurally defined barriers: steric environment, functional group reactivity, product instability, and through-bond electronics. However, there are also practical barriers. Late-stage reactions conducted on small quantities of material are run inevitably at lower than optimal concentrations. Access to late-stage material limits extensive optimization. Impurities from past reactions can interfere, especially with catalytic reactions. Therefore, chemical reactions on which one can rely at the front lines of a complex synthesis campaign emerge from the crucible of total synthesis as robust, dependable, and widely applied. Trost conceptualized "chemoselectivity" as a reagent's selective reaction of one functional group or reactive site in preference to others ( Trost , B. M. Science 1983 , 219 , 245 ). Chemoselectivity and functional group tolerance can be evaluated quickly using robustness screens ( Collins , K. D. Nat. Chem. 2013 , 5 , 597 ). A reaction may also be characterized by its "chemofidelity", that is, its reliable reaction with a functional group in any molecular context. For example, ketone reduction by an electride (dissolving metal conditions) exhibits high chemofidelity but low chemoselectivity: it usually works, but many other functional groups are reduced at similar rates. Conversely, alkene coordination chemistry effected by π Lewis acids can exhibit high chemoselectivity ( Trost , B. M. Science 1983 , 219 , 245 ) but low chemofidelity: it can be highly selective for alkenes but sensitive to the substitution pattern ( Larionov , E. Chem. Commun. 2014 , 50 , 9816 ). In contrast, alkenes undergo reliable, robust, and diverse hydrogen atom transfer reactions from metal hydrides to generate carbon-centered radicals. Although there are many potential applications of this chemistry, its functional group tolerance, high rates, and ease of execution have led to its rapid deployment in complex synthesis campaigns. Its success derives from high chemofidelity, that is, its dependable reactivity in many molecular environments and with many alkene substitution patterns. Metal hydride H atom transfer (MHAT) reactions convert diverse, simple building blocks to more stereochemically and functionally dense products ( Crossley , S. W. M. Chem. Rev. 2016 , 116 , 8912 ). When hydrogen is returned to the metal, MHAT can be considered the radical equivalent of Brønsted acid catalysis-itself a broad reactivity paradigm. This Account summarizes our group's contributions to method development, reagent discovery, and mechanistic interrogation. Our earliest contribution to this area-a stepwise hydrogenation with high chemoselectivity and high chemofidelity-has found application to many problems. More recently, we reported the first examples of dual-catalytic cross-couplings that rely on the merger of MHAT cycles and nickel catalysis. With time, we anticipate that MHAT will become a staple of chemical synthesis.

Reduction and Condensation of Aldehydes by the Isolated Cofactor of Nitrogenase

Isolated nitrogenase cofactors can reduce CO, CN^-, and CO₂ to short-chain hydrocarbons in reactions driven by a strong reductant. Here, we use activity analyses and isotope labeling experiments to show that formaldehyde and acetaldehydes can be reduced as-is or reductively condensed into alkanes and alkenes by the isolated cofactor of Mo-nitrogenase in the presence of Eu^II-diethylenetriamine pentaacetate (DTPA). Further, we demonstrate that aldehydes can be condensed with CO by the isolated cofactor under the same reaction conditions, pointing to aldehyde-derived species as possible intermediates of nitrogenase-catalyzed CO reduction. Our deuterium labeling experiments suggest the formation of a cofactor-bound hydroxymethyl intermediate upon activation of the formaldehyde, as well as the release of C₂H₄ as a product upon β-hydride elimination of an acetaldehyde-derived hydroxyethyl intermediate. These findings establish the reductive condensation of aldehydes as a previously unobserved reactivity of a biogenic catalyst while at the same time shed light on the mechanism of enzymatic CO reduction and C-C bond formation, thereby providing a useful framework for further exploration of the unique reactivity and potential applications of nitrogenase-based reactions.