Author Correction: Coupling dinitrogen and hydrocarbons through aryl migration.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Coupling dinitrogen and hydrocarbons through aryl migration.

The activation of abundant molecules such as hydrocarbons and atmospheric nitrogen (N2) remains a challenge because these molecules are often inert. The formation of carbon-nitrogen bonds from N2 typically has required reactive organic precursors that are incompatible with the reducing conditions that promote N2 reactivity1, which has prevented catalysis. Here we report a diketiminate-supported iron system that sequentially activates benzene and N2 to form aniline derivatives. The key to this coupling reaction is the partial silylation of a reduced iron-dinitrogen complex, followed by migration of a benzene-derived aryl group to the nitrogen. Further reduction releases N2-derived aniline, and the resulting iron species can re-enter the cyclic pathway. Specifically, we show that an easily prepared diketiminate iron bromide complex2 mediates the one-pot conversion of several petroleum-derived arenes into the corresponding silylated aniline derivatives, by using a mixture of sodium powder, crown ether, trimethylsilyl bromide and N2 as the nitrogen source. Numerous compounds along the cyclic pathway are isolated and crystallographically characterized, and their reactivity supports a mechanism for sequential hydrocarbon activation and N2 functionalization. This strategy couples nitrogen atoms from N2 with abundant hydrocarbons, and maps a route towards future catalytic systems.

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. Electron paramagnetic resonance (EPR), Mössbauer, and superconducting quantum interference device (SQUID) data are combined with density functional theory (DFT) computations to show how the electronic structure of the cluster arises from strong magnetic coupling between the Ni, Fe, and W sites. X-ray absorption spectroscopy, together with spectroscopically validated DFT analysis, suggests that the electronic structure can be described with a formal Ni1+ atom participating in a nonpolar Ni-W σ-bond. This metal-metal bond, which minimizes spin density at Ni1+, is conserved in two cluster oxidation states. Fe-W bonding is found in all clusters, in one case stabilizing a local non-Hund state at tungsten. Based on these results, we compare different M-M interactions and speculate that other heterometallic clusters, including metalloenzyme active sites, could likewise store redox equivalents and stabilize low-valent metal centers through metal-metal bonding.

Scaffold-Oriented Asymmetric Catalysis: Conformational Modulation of Transition State Multivalency during a Catalyst-Controlled Assembly of a Pharmaceutically Relevant Atropisomer.

A new class of superbasic, bifunctional peptidyl guanidine catalysts is presented, which enables the organocatalytic, atroposelective synthesis of axially chiral quinazolinediones. Computational modeling unveiled the conformational modulation of the catalyst by a novel phenyl urea N-cap, that preorganizes the structure into the active, folded state. A previously unanticipated noncovalent interaction involving a difluoroacetamide acting as a hybrid mono- or bidentate hydrogen bond donor emerged as a decisive control element inducing atroposelectivity. These discoveries spurred from a scaffold-oriented project inspired from a fascinating investigational BTK inhibitor featuring two stable chiral axes and relies on a mechanistic framework that was foreign to the extant lexicon of asymmetric catalysis.

Chirality-matched catalyst-controlled macrocyclization reactions.

Macrocycles, formally defined as compounds that contain a ring with 12 or more atoms, continue to attract great interest due to their important applications in physical, pharmacological, and environmental sciences. In syntheses of macrocyclic compounds, promoting intramolecular over intermolecular reactions in the ring-closing step is often a key challenge. Furthermore, syntheses of macrocycles with stereogenic elements confer an additional challenge, while access to such macrocycles are of great interest. Herein, we report the remarkable effect peptide-based catalysts can have in promoting efficient macrocyclization reactions. We show that the chirality of the catalyst is essential for promoting favorable, matched transition-state relationships that favor macrocyclization of substrates with preexisting stereogenic elements; curiously, the chirality of the catalyst is essential for successful reactions, even though no new static (i.e., not "dynamic") stereogenic elements are created. Control experiments involving either achiral variants of the catalyst or the enantiomeric form of the catalyst fail to deliver the macrocycles in significant quantity in head-to-head comparisons. The generality of the phenomenon, demonstrated here with a number of substrates, stimulates analogies to enzymatic catalysts that produce naturally occurring macrocycles, presumably through related, catalyst-defined peripheral interactions with their acyclic substrates.

Mechanism of Nitrogen-Carbon Bond Formation from Iron(IV) Disilylhydrazido Intermediates during N2 Reduction.

We recently reported a reaction sequence that activates C-H bonds in simple arenes as well as the N-N triple bond in N2, delivering the aryl group to N2 to form a new N-C bond (Nature 2020, 584, 221). This enables the transformation of abundant feedstocks (arenes and N2) into N-containing organic compounds. The key N-C bond forming step occurs upon partial silylation of N2. However, the pathway through which reduction, silylation, and migration occurred was unknown. Here, we describe synthetic, structural, magnetic, spectroscopic, kinetic, and computational studies that elucidate the steps of this transformation. N2 must be silylated twice at the distal N atom before aryl migration can occur, and sequential silyl radical and silyl cation addition is a kinetically competent pathway to a formally iron(IV)-NN(SiMe3)2 intermediate that can be isolated at low temperature. Kinetic studies show its first-order conversion to the migrated product, and DFT calculations indicate a concerted transition state for migration. The electronic structure of the formally iron(IV) intermediate is examined using DFT and CASSCF calculations, which reveal contributions from iron(II) and iron(III) resonance forms with oxidized NNSi2 ligands. The depletion of electron density from the Fe-coordinated N atom makes it electrophilic enough to accept the incoming aryl group. This new pathway for the N-C bond formation offers a method for functionalizing N2 using organometallic chemistry.

Aspartyl ß-Turn-Based Dirhodium(II) Metallopeptides for Benzylic C(sp3)-H Amination: Enantioselectivity and X-ray Structural Analysis.

Amination of C(sp3)-H bonds is a powerful tool to introduce nitrogen into complex organic frameworks in a direct manner. Despite significant advances in catalyst design, full site- and enantiocontrol in complex molecular regimes remain elusive using established catalyst systems. To address these challenges, we herein describe a new class of peptide-based dirhodium(II) complexes derived from aspartic acid-containing ß-turn-forming tetramers. This highly modular system can serve as a platform for the rapid generation of new chiral dirhodium(II) catalyst libraries, as illustrated by the facile synthesis of a series of 38 catalysts. Critically, we present the first crystal structure of a dirhodium(II) tetra-aspartate complex, which unveils retention of the ß-turn conformation of the peptidyl ligand; a well-defined hydrogen-bonding network is evident, along with a near-C4 symmetry that renders the rhodium centers inequivalent. The utility of this catalyst platform is illustrated by the enantioselective amination of benzylic C(sp3)-H bonds, in which state-of-the-art levels of enantioselectivity up to 95.5:4.5 er are obtained, even for substrates that present challenges with previously reported catalyst systems. Additionally, we found these complexes to be competent catalysts for the intermolecular amination of N-alkylamides via insertion into the C(sp3)-H bond α to the amide nitrogen, yielding differentially protected 1,1-diamines. Of note, this type of insertion was also observed to occur on the amide functionalities of the catalyst itself in the absence of the substrate but did not appear to be detrimental to reaction outcomes when the substrate was present.

An Asymmetric Aromatic Finkelstein Reaction: A Platform for Remote Diarylmethane Desymmetrization.

A first-of-its-kind enantioselective aromatic Finkelstein reaction is disclosed for the remote desymmetrization of diarylmethanes. The reaction operates through a copper-catalyzed C-I bond-forming event, and high levels of enantioselectivity are achieved through the deployment of a tailored guanidinylated peptide ligand. Strategic use of transition-metal-mediated reactions enables the chemoselective modification of the aryl iodide products; thus, the synthesis of a diverse set of otherwise difficult-to-access diarylmethanes with excellent levels of selectivity is realized from a common intermediate. A mixed experimental/computational analysis of steric parameters and substrate conformations identifies the importance of remote conformational effects as a key to achieving high enantioselectivity in this desymmetrization reaction.

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 [Fe2 (µ-CCO)2 ]+ complex with two carbon bridges (4) is S= 1 / 2 ${{ 1/2 }}$ , 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. In contrast, one-electron reduction of the [Fe2 (µ-CCO)] complex with one carbon bridge (2) affords a mixed-valence species with a high-spin ground state (S= 7 / 2 ${ 7/2 }$ ), and the Fe-Fe distance contracts by 1 Å. Spectroscopic, magnetic, and computational studies of the latter reveal an Fe-Fe bonding interaction that leads to complete valence delocalization. Analysis of near-IR intervalence charge transfer transitions in 5 indicates a very large double exchange constant (B) in the range of 780-965 cm-1 . These results show that carbon bridges are extremely effective at stabilizing valence delocalized ground states in mixed-valent iron dimers.

Catalytic reduction of dinitrogen to ammonia using molybdenum porphyrin complexes.

Porphyrin complexes are well-known in O2 and CO2 reduction, but their application to N2 reduction is less developed. Here, we show that oxo and nitrido complexes of molybdenum supported by tetramesitylporphyrin (TMP) are effective precatalysts for catalytic N2 reduction to ammonia, verified by 15N2 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-1). We place these results in the context of other work on homogeneous N2 reduction catalysis.

A Guide to Tris(4-Substituted)-triphenylmethyl Radicals.

Triphenylmethyl (trityl, Ph3C•) radicals have been considered the prototypical carbon-centered radical since their discovery in 1900. Tris(4-substituted)-trityls [(4-R-Ph)3C•] have since been used in many ways due to their stability, persistence, and spectroscopic activity. Despite their widespread use, existing synthetic routes toward tris(4-substituted)-trityl radicals are not reproducible and often lead to impure materials. We report here robust syntheses of six electronically varied (4-RPh)3C•, where R = NMe2, OCH3, tBu, Ph, Cl, and CF3. The characterization reported for the radicals and related compounds includes five X-ray crystal structures, electrochemical potentials, and optical spectra. Each radical is best accessed using a stepwise approach from the trityl halide, (RPh)3CCl or (RPh)3CBr, by controllably removing the halide with subsequent 1e- reduction of the trityl cation, (RPh)3C+. These syntheses afford consistently crystalline trityl radicals of high purity for further studies.

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 N2 and isolate a N2 complex including its X-ray crystal structure. Silylation of the N2 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. The structure of this compound is similar to an earlier reported phenyl complex in which phenyl migration forms a new N-C bond, but the alkynyl group does not migrate. DFT calculations are used to test the possible reasons why the alkynyl is resistant to migration, and these show that the large Fe-C bond energy in the alkynyl complex is a factor that could contribute to the lack of migration.

Independent Tuning of the pKa or the E1/2 in a Family of Ruthenium Pyridine-Imidazole Complexes.

Two series of RuII(acac)2(py-imH) complexes have been prepared, one with changes to the acac ligands and the other with substitutions to the imidazole. The proton-coupled electron transfer (PCET) thermochemistry of the complexes has been studied in acetonitrile, revealing that the acac substitutions almost exclusively affect the redox potentials of the complex (|ΔE1/2| ≫ |ΔpKa|·0.059 V) while the changes to the imidazole primarily affect its acidity (|ΔpKa|·0.059 V ≫ |ΔE1/2|). This decoupling is supported by DFT calculations, which show that the acac substitutions primarily affect the Ru-centered t2g orbitals, while changes to the py-imH ligand primarily affect the ligand-centered π orbitals. More broadly, the decoupling stems from the physical separation of the electron and proton within the complex and highlights a clear design strategy to separately tune the redox and acid/base properties of H atom donor/acceptor molecules.

Synthesis and Surface Attachment of Molecular Re(I) Complexes Supported by Functionalized Bipyridyl Ligands.

Eleven 2,2'-bipyridine (bpy) ligands functionalized with attachment groups for covalent immobilization on silicon surfaces were prepared. Five of the ligands feature silatrane functional groups for attachment to metal oxide coatings on the silicon surfaces, while six contain either alkene or alkyne functional groups for attachment to hydrogen-terminated silicon surfaces. The bpy ligands were coordinated to Re(CO)5Cl to form complexes of the type Re(bpy)(CO)3Cl, which are related to known catalysts for CO2 reduction. Six of the new complexes were characterized using X-ray crystallography. As proof of principle, four molecular Re complexes were immobilized on either a thin layer of TiO2 on silicon or hydrogen-terminated silicon. The surface-immobilized complexes were characterized using X-ray photoelectron spectroscopy, IR spectroscopy, and cyclic voltammetry (CV) in the dark and for one representative example in the light. The CO stretching frequencies of the attached complexes were similar to those of the pure molecular complexes, but the CVs were less analogous. For two of the complexes, comparison of the electrocatalytic CO2 reduction performance showed lower CO Faradaic efficiencies for the immobilized complexes than the same complex in solution under similar conditions. In particular, a complex containing a silatrane linked to bpy with an amide linker showed poor catalytic performance and control experiments suggest that amide linkers in conjugation with a redox-active ligand are not stable under highly reducing conditions and alkyl linkers are more stable. A conclusion of this work is that understanding the behavior of molecular Re catalysts attached to semiconducting silicon is more complicated than related complexes, which have previously been immobilized on metallic electrodes.

Imine Directed Cp*RhIII -Catalyzed N-H Functionalization and Annulation with Amino Amides, Aldehydes, and Diazo Compounds.

A multicomponent annulation that proceeds by imine directed Cp*RhIII -catalyzed N-H functionalization is disclosed. The transformation affords piperazinones displaying a range of functionality and is the first example of transition metal-catalyzed multicomponent N-H functionalization. A broad range of readily available α-amino amides, including those derived from glycine, α-substituted, and α,α-disubstituted amino acids, were effective inputs and enabled the incorporation of a variety of amino acid side chains with minimal racemization. Branched and unbranched alkyl aldehydes and various stabilized diazo compounds were also efficient reactants. The piperazinone products were further modified through efficient transformations. Mechanistic studies, including X-ray crystallographic characterization of a catalytically competent five-membered rhodacycle with imine and amide nitrogen chelation, provide support for the proposed mechanism.

Electrocatalytic, Homogeneous Ammonia Oxidation in Water to Nitrate and Nitrite with a Copper Complex.

Electrocatalytic ammonia oxidation at room temperature and pressure allows energy-economical and environmentally friendly production of nitrites and nitrates. Few molecular catalysts, however, have been developed for this six- or eight-electron oxidation process. We now report [Cu(bipyalk)]+, a homogeneous electrocatalyst that realizes the title reaction in water at 94% Faradaic efficiency. The catalyst exhibits high selectivity against water oxidation in aqueous media, as [Cu(bipyalk)]+ is not competent for water oxidation.

Spin States, Bonding and Magnetism in Mixed-Valence Iron(0)-Iron(II) Complexes.

"Xenophilic" complexes offer metal-metal bonds between disparate metal sites, but the nature of the metal-metal bonding is often unclear. Here, we describe two novel complexes with unsupported Fe-Fe bonds, Lx Fe-Fp (LX = ß-aldiminate or ß-diketiminate; Fp = Fe(CO)2 Cp), that offer insight into Fe-Fe bonding. Mössbauer, magnetism, and DFT analysis indicate that the most accurate electronic structure description is LFeII ←Fe0 (CO)2 Cp, in which the Fe(CO)2 Cp is low-spin iron(0) and acts as an X-type ligand toward the high-spin iron(II) of the LFe fragment. This largely electrostatic interaction has a bond order of only 0.5. The three-coordinate high-spin iron(II) site has large zero-field splitting, and in addition its Mössbauer parameters can be used to rank the Fp- "metalloligand" as a donor; it is nearly as strong a donor as phosphides and alkyls.

Structural and Thermodynamic Effects on the Kinetics of C-H Oxidation by Multisite Proton-Coupled Electron Transfer in Fluorenyl Benzoates.

Our recent experimental and theoretical investigations have shown that fluorene C-H bonds can be activated through a mechanism in which the proton and electron are transferred from the C-H bond to a separate base and oxidant in a concerted, elementary step. This multisite proton-coupled electron transfer (MS-PCET) mechanism for C-H bond activation was shown to be facilitated by shorter proton donor-acceptor distances. With the goal of intentionally modulating this donor-acceptor distance, we have now studied C-H MS-PCET in the 3-methyl-substituted fluorenyl benzoate (2-Flr-3-Me-BzO-). This derivative was readily oxidized by ferrocenium oxidants by initial C-H MS-PCET, with rate constants that were 6-21 times larger than those for 2-Flr-BzO- with the same oxidants. Structural comparisons by X-ray crystallography and by computations showed that addition of the 3-methyl group caused the expected steric compression; however, the relevant C···O- proton donor-acceptor distance was longer, due to a twist of the carboxylate group. The structural changes induced by the 3-Me group increased the basicity of the carboxylate, weakened the C-H bond, and reduced the reorganization energy for C-H bond cleavage. Thus, the rate enhancement for 2-Flr-3-Me-BzO- was due to effects on the thermochemistry and kinetic barrier, rather than from compression of the C···O- proton donor-acceptor distance. These results highlight both the challenges of controlling molecules on the 0.1 Å length scale and the variety of parameters that affect PCET rate constants.

Control of Catalyst Isomers Using an N-Phenyl-Substituted RN(CH2CH2PiPr2)2 Pincer Ligand in CO2 Hydrogenation and Formic Acid Dehydrogenation.

A novel pincer ligand, iPrPNPhP [PhN(CH2CH2PiPr2)2], which is an analogue of the versatile MACHO ligand, iPrPNHP [HN(CH2CH2PiPr2)2], was synthesized and characterized. The ligand was coordinated to ruthenium, and a series of hydride-containing complexes were isolated and characterized by NMR and IR spectroscopies, as well as X-ray diffraction. Comparisons to previously published analogues ligated by iPrPNHP and iPrPNMeP [CH3N(CH2CH2PiPr2)2] illustrate that there are large changes in the coordination chemistry that occur when the nitrogen substituent of the pincer ligand is altered. For example, ruthenium hydrides supported by the iPrPNPhP ligand always form the syn isomer (where syn/anti refer to the relative orientation of the group on nitrogen and the hydride ligand on ruthenium), whereas complexes supported by iPrPNHP form the anti isomer and complexes supported by iPrPNMeP form a mixture of syn and anti isomers. We evaluated the impact of the nitrogen substituent of the pincer ligand in catalysis by comparing a series of iPrPNRP (R = H, Me, Ph)-ligated ruthenium hydride complexes as catalysts for formic acid dehydrogenation and carbon dioxide (CO2) hydrogenation to formate. The iPrPNPhP-ligated species is the most active for formic acid dehydrogenation, and mechanistic studies suggest that this is likely because there are kinetic advantages for catalysts that operate via the syn isomer. In CO2 hydrogenation, the iPrPNPhP-ligated species is again the most active under our optimal conditions, and we report some of the highest turnover frequencies for homogeneous catalysts. Experimental and theoretical insights into the turnover-limiting step of catalysis provide a basis for the observed trends in catalytic activity. Additionally, the stability of our complexes enabled us to detect a previously unobserved autocatalytic effect involving the base that is added to drive the reaction. Overall, by modifying the nitrogen substituent on the MACHO ligand, we have developed highly active catalysts for formic acid dehydrogenation and CO2 hydrogenation and also provided a framework for future catalyst development.

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. The N-H bond dissociation free energy, derived from the Bordwell equation, is 56 ± 2 kcal/mol. Electron paramagnetic resonance (EPR) spectroscopy and superconducting quantum interference device (SQUID) magnetometry measurements show that the iron(III) complexes with S and N as the fourth donors have an intermediate spin (S = 3/2) ground state with a large zero field splitting, and X-ray absorption spectra show a high Fe-S covalency. The Mössbauer spectrum changes drastically with the position of a nearby alkali metal cation in the iron(III) amido complex, and density functional theory calculations explain this phenomenon through a change between having the doubly occupied orbital as dz2 or dyz, as the former is more influenced by the nearby positive charge.