Evaluating Diazene to N2 Interconversion at Iron-Sulfur Complexes

Biological N2 reduction occurs at sulfur-rich multiiron sites, and an interesting potential pathway is concerted double reduction/ protonation of bridging N2 through PCET. Here, we test the feasibility of using synthetic sulfur-supported diiron complexes to mimic this pathway. Oxidative proton transfer from μ-η1 : η1-diazene (HN=NH) is the microscopic reverse of the proposed N2 fixation pathway, revealing the energetics of the process. Previously, Sellmann assigned the purple metastable product from two-electron oxidation of [{Fe2+(PPr3)L1}2(μ-η1 : η1-N2H2)] (L1=tetradentate SSSS ligand) at -78 °C as [{Fe2+(PPr3)L1}2(μ-η1 : η1-N2)]2+, which would come from double PCET from diazene to sulfur atoms of the supporting ligands. Using resonance Raman, Mössbauer, NMR, and EPR spectroscopies in conjunction with DFT calculations, we show that the product is not an N2 complex. Instead, the data are most consistent with the spectroscopically observed species being the mononuclear iron(III) diazene complex [{Fe(PPr3)L1}(η2-N2H2)]+. Calculations indicate that the proposed double PCET has a barrier that is too high for proton transfer at the reaction temperature. Also, PCET from the bridging diazene is highly exergonic as a result of the high Fe3+/2+ redox potential, indicating that the reverse N2 protonation would be too endergonic to proceed. This system establishes the "ground rules" for designing reversible N2/N2H2 interconversion through PCET, such as tuning the redox potentials of the metal sites.

Catalytic dinitrogen reduction to hydrazine and ammonia using Cr(N2)2(diphosphine)2 complexes

The synthesis, characterization of trans-[Cr(N2)2(depe)2] (1) is described. 1 and trans-[Cr(N2)2(dmpe)2] (2) catalyze the reduction of N2 to N2H4 and NH3 in THF using SmI2 and H2O or ethylene glycol as proton sources. 2 produces the highest total fixed N for a molecular Cr catalyst to date.

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.

Syntheses, Characterizations, Crystal Structures, and Protonation Reactions of Dinitrogen Chromium Complexes Supported with Triamidoamine Ligands

A novel dinitrogen-dichromium complex, [{Cr(L^Bn)}₂(μ-N₂)] (1), has been prepared from reaction of CrCl₃ with a lithiated triamidoamine ligand (Li₃L^Bn) under dinitrogen. The X-ray crystal structure analysis of 1 revealed that it is composed of two independent dimeric Cr complexes bridged by N₂ in the unit cell. The bridged N-N bond lengths (1.188(4) and 1.185(7) Å) were longer than the free dinitrogen molecule. The elongations of N-N bonds in 1 were also supported by the fact that the ν(N-N) stretching vibration at 1772 cm^-1 observed in toluene is smaller than the free N₂. Complex 1 was identified to be a 5-coordinated high spin Cr(IV) complex by Cr K-edge XANES measurement. The ¹H NMR spectrum and temperature dependent magnetic susceptibility of 1 indicated that complex 1 is in the S = 1 ground state, in which two Cr(IV) ions and unpaired electron spins of the bridging N₂^2- ligand are strongly antiferromagnetically coupled. Reaction of complex 1 with 2.3 equiv of Na or K gave chromium complexes with N₂ between the Cr ion and the respective alkali metal ion, [{CrNa(L^Bn)(N₂)(Et₂O)}₂] (2) and [{CrK(L^Bn)(N₂)}₄(Et₂O)₂] (3), respectively. Furthermore, the complexes 2 and 3 reacted with 15-crown-5 and 18-crown-6 to form the respective crown-ether adducts, [CrNa(L^Bn)(N₂)(15-crown-5)] (4) and [CrK(L^Bn)(N₂)(18-crown-6)] (5). The XANES measurements of complexes 2, 3, 4, and 5 revealed that they are high spin Cr(IV) complexes like complex 1. All complexes reacted with a reducing agent and a proton source to form NH₃ and/or N₂H₄. The yields of these products in the presence of K⁺ were higher than those in the presence of Na⁺. The electronic structures and binding properties of 1, 2, 3, 4, and 5 were evaluated and discussed based on their DFT calculations.

Catalytic Reduction of Dinitrogen into Ammonia and Hydrazine by Using Chromium Complexes Bearing PCP-Type Pincer Ligands

A series of chromium-halide, -nitride, and -dinitrogen complexes bearing carbene- and phosphine-based PCP-type pincer ligands has been newly prepared, and some of them are found to work as effective catalysts to reduce dinitrogen under atmospheric pressure, whereby up to 11.60 equiv. of ammonia and 2.52 equiv. of hydrazine (16.6 equiv. of fixed N atom) are produced based on the chromium atom. To the best of our knowledge, this is the first successful example of chromium-catalyzed conversion of dinitrogen to ammonia and hydrazine under mild reaction conditions.

Hydrogen Bonding to a Dinitrogen Complex at Room Temperature: Impacts on N2 Activation

We report an experimental and computational analysis of the effects of hydrogen bonding to a metal dinitrogen complex. A series of H-bond donors over a wide p K_a range (Δ 20) interact with the nitrogen unit of a Re^I-(N₂) complex at room temperature. Analysis by ¹⁵N NMR, IR spectroscopy, association equilibria, and DFT studies indicates that the H-bonding interaction polarizes and weakens the N-N bond. These results provide insight into the role of the secondary sphere residues in nitrogenase enzymes.

Requirements for Lewis Acid-Mediated Capture and N-N Bond Cleavage of Hydrazine at Iron

An iron complex bearing a pyridine(dicarbene) pincer was designed to probe the requirements of Lewis acid-enabled N2H4 capture and subsequent N-N bond cleavage. Appended boron Lewis acids were installed by two methods to circumvent the incompatibilities associated with Lewis acid/base quenching of free carbenes and boranes. N2H4 capture by borane Lewis acids is dependent on both the Lewis acidity and the steric profile about boron. A substitutionally inert primary coordination sphere at iron prevents an Fe-N2H4 interaction as well as N-N bond homolysis upon reduction.

Synthesis and reactivity of asymmetric Cr(i) dinitrogen complexes supported by cyclopentadienyl–phosphine ligands

Trinuclear and dinuclear Cr(i) dinitrogen complexes and mixed-valence dinuclear Cr–N₂ complex, with novel asymmetric N₂ coordination modes, are realized.

Catalytic Silylation of N2 and Synthesis of NH3 and N2H4 by Net Hydrogen Atom Transfer Reactions Using a Chromium P4 Macrocycle

We report the first discrete molecular Cr-based catalysts for the reduction of N₂. This study is focused on the reactivity of the Cr-N₂ complex, trans-[Cr(N₂)₂(P^Ph₄N^Bn₄)] (P₄Cr(N₂)₂), bearing a 16-membered tetraphosphine macrocycle. The architecture of the [16]-P^Ph₄N^Bn₄ ligand is critical to preserve the structural integrity of the catalyst. P₄Cr(N₂)₂ was found to mediate the reduction of N₂ at room temperature and 1 atm pressure by three complementary reaction pathways: (1) Cr-catalyzed reduction of N₂ to N(SiMe₃)₃ by Na and Me₃SiCl, affording up to 34 equiv N(SiMe₃)₃; (2) stoichiometric reduction of N₂ by protons and electrons (for example, the reaction of cobaltocene and collidinium triflate at room temperature afforded 1.9 equiv of NH₃, or at -78 °C afforded a mixture of NH₃ and N₂H₄); and (3) the first example of NH₃ formation from the reaction of a terminally bound N₂ ligand with a traditional H atom source, TEMPOH (2,2,6,6-tetramethylpiperidine-1-ol). We found that trans-[Cr(¹⁵N₂)₂(P^Ph₄N^Bn₄)] reacts with excess TEMPOH to afford 1.4 equiv of ¹⁵NH₃. Isotopic labeling studies using TEMPOD afforded ND₃ as the product of N₂ reduction, confirming that the H atoms are provided by TEMPOH.

Layered-Double-Hydroxide Nanosheets as Efficient Visible-Light-Driven Photocatalysts for Dinitrogen Fixation

Semiconductor photocatalysis attracts widespread interest in water splitting, CO₂ reduction, and N₂ fixation. N₂ reduction to NH₃ is essential to the chemical industry and to the Earth's nitrogen cycle. Industrially, NH₃ is synthesized by the Haber-Bosch process under extreme conditions (400-500 °C, 200-250 bar), stimulating research into the development of sustainable technologies for NH₃ production. Herein, this study demonstrates that ultrathin layered-double-hydroxide (LDH) photocatalysts, in particular CuCr-LDH nanosheets, possess remarkable photocatalytic activity for the photoreduction of N₂ to NH₃ in water at 25 °C under visible-light irradiation. The excellent activity can be attributed to the severely distorted structure and compressive strain in the LDH nanosheets, which significantly enhances N₂ chemisorption and thereby promotes NH₃ formation.

Exploring secondary-sphere interactions in Fe-N x H y complexes relevant to N2 fixation

Hydrogen bonding and other types of secondary-sphere interactions are ubiquitous in metalloenzyme active sites and are critical to the transformations they mediate. Exploiting secondary sphere interactions in synthetic catalysts to study the role(s) they might play in biological systems, and to develop increasingly efficient catalysts, is an important challenge. Whereas model studies in this broad context are increasingly abundant, as yet there has been relatively little progress in the area of synthetic catalysts for nitrogen fixation that incorporate secondary sphere design elements. Herein we present our first study of Fe-N x H y complexes supported by new tris(phosphine)silyl ligands, abbreviated as [SiPNMe3] and [SiPiPr2PNMe], that incorporate remote tertiary amine hydrogen-bond acceptors within a tertiary phosphine/amine 6-membered ring. These remote amine sites facilitate hydrogen-bonding interactions via a boat conformation of the 6-membered ring when certain nitrogenous substrates (e.g., NH3 and N2H4) are coordinated to the apical site of a trigonal bipyramidal iron complex, and adopt a chair conformation when no H-bonding is possible (e.g., N2). Countercation binding at the cyclic amine is also observed for anionic {Fe-N2}- complexes. Reactivity studies in the presence of proton/electron sources show that the incorporated amine functionality leads to rapid generation of catalytically inactive Fe-H species, thereby substantiating a hydride termination pathway that we have previously proposed deactivates catalysts of the type [EPR3]FeN2 (E = Si, C).