Isolation of a Cubane-Type Metal Sulfido Cluster with a Molecular Nitrogen Ligand

Iron-Catalyzed C-H Arylphosphorylation of Quinoxalines

A one-pot strategy for iron-catalyzed C2,3-H arylphosphorylation of electron-deficient quinoxalines with phosphines and aryl compounds is reported. The proposed method features the use of non-noble metal catalysts, the capacity of utilizing multiple aryl compounds as substrates, the simultaneous formation of C-P and C-C bonds in one pot, the simplicity of its operation, the mildness of the reaction conditions, and its compatibility with a wide range of substrates. Moreover, it offers a practical route for direct access to 2-aryl-3-phosphino N-heteroarenes, a class of potential cyclometalated C^N and N^P bidentate ligands that are difficult to prepare with existing C(sp2)-H functionalization methods.

Nitrogen reduction by the Fe sites of synthetic [Mo3S4Fe] cubes

Nitrogen (N₂) fixation by nature, which is a crucial process for the supply of bio-available forms of nitrogen, is performed by nitrogenase. This enzyme uses a unique transition-metal-sulfur-carbon cluster as its active-site co-factor ([(R-homocitrate)MoFe₇S₉C], FeMoco)^1,2, and the sulfur-surrounded iron (Fe) atoms have been postulated to capture and reduce N₂ (refs. ^3-6). Although there are a few examples of synthetic counterparts of the FeMoco, metal-sulfur cluster, which have shown binding of N₂ (refs. ^7-9), the reduction of N₂ by any synthetic metal-sulfur cluster or by the extracted form of FeMoco¹⁰ has remained elusive, despite nearly 50 years of research. Here we show that the Fe atoms in our synthetic [Mo₃S₄Fe] cubes^11,12 can capture a N₂ molecule and catalyse N₂ silylation to form N(SiMe₃)₃ under treatment with excess sodium and trimethylsilyl chloride. These results exemplify the catalytic silylation of N₂ by a synthetic metal-sulfur cluster and demonstrate the N₂-reduction capability of Fe atoms in a sulfur-rich environment, which is reminiscent of the ability of FeMoco to bind and activate N₂.

Reversible Binding of Dinitrogen on a Thiolate-Bridged Cobalt-Ruthenium Complex Supported by a Flexible Bidentate Phosphine Ligand

A well-defined thiolate-bridged cobalt-ruthenium complex is demonstrated to reversibly bind N2 by modulation of the auxiliary phosphine ligand, which is evidenced by time-dependent 1H NMR spectroscopy at different temperatures. Notably,...

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.

N2 activation on a molybdenum-titanium-sulfur cluster

The FeMo-cofactor of nitrogenase, a metal–sulfur cluster that contains eight transition metals, promotes the conversion of dinitrogen into ammonia when stored in the protein. Although various metal–sulfur clusters have been synthesized over the past decades, their use in the activation of N₂ has remained challenging, and even the FeMo-cofactor extracted from nitrogenase is not able to reduce N₂. Herein, we report the activation of N₂ by a metal–sulfur cluster that contains molybdenum and titanium. An N₂ moiety bridging two [Mo₃S₄Ti] cubes is converted into NH₃ and N₂H₄ upon treatment with Brønsted acids in the presence of a reducing agent.

Nitrogenase—whose cofactor consists of a metal–sulfur cluster—catalyzes the production of NH₃ from N₂, but designing metal–sulfur complexes capable of promoting this conversion remains challenging. Here, the authors report on the activation of N₂ by a metal–sulfur cluster containing [Mo₃S₄Ti] cubes, demonstrating NH₃ and N₂H₄ production.

Interplay between Theory and Experiment for Ammonia Synthesis Catalyzed by Transition Metal Complexes

Nitrogen fixation is an essential chemical process both biologically and industrially. Since the discovery of the first transition-metal-dinitrogen complex in 1965, a great deal of effort has been devoted to the development of artificial nitrogen fixation systems that work under mild reaction conditions. However, the transformation of chemically inert dinitrogen using homogeneous catalysts is still challenging because of the difficulty in breaking the strong triple bond of dinitrogen, and a very limited number of transition metal complexes have exhibited the catalytic activity for the direct transformation of dinitrogen into ammonia with low turnover numbers. To develop more effective nitrogen fixation systems, it is necessary to retrieve as much information as possible from the limited successful examples. Computational chemistry will provide valuable insights in the understanding of the reaction mechanisms involving unstable intermediates that are hard to isolate or characterize. We have been applying it for clarifying detailed mechanisms of dinitrogen activation and functionalization by transition metal complexes as well as for designing new catalysts for more effective nitrogen fixation. This Account summarizes recent progress in the elucidation of catalytic mechanisms of nitrogen fixation by using mono- and dinuclear molybdenum complexes, as well as cubane-type metal-sulfido clusters from a theoretical point of view. First, we briefly introduce experimental and theoretical contributions to the elucidation of the reaction mechanism of nitrogen fixation catalyzed by a mononuclear Mo-triamidoamine complex. Special attention is paid to our recent studies on Mo-catalyzed nitrogen fixation using dinitrogen-bridged dimolybdenum complexes. A possible catalytic mechanism is proposed based on theoretical and experimental investigations. The catalytic mechanism involves the formation of a monuclear molybdenum-nitride (Mo≡N) intermediate, as well as the regeneration of a dimolybdenum intermediate with the Mo-N≡N-Mo moiety. Comparison of the reactivity of di- and monomolybdenum complexes suggests that the dimolybdenum structure is essential for the catalytic activity. Synergy between the two Mo cores connected with a bridging N2 ligand is observed in the protonation of coordinated N2. Intermetallic electron transfer through the bridging N2 ligand reductively activates the coordinated N2 to be protonated. On the basis of the proposed catalytic mechanism, we used DFT calculations for rational design of dimolybdenum complexes serving as more effective catalysts for nitrogen fixation. Newly prepared dimolybdenum complexes with modified PNP-type pincer ligands exhibit greater catalytic activity than the original one.

Diiron bridged-thiolate complexes that bind N2 at the Fe(II)Fe(II), Fe(II)Fe(I), and Fe(I)Fe(I) redox states

All known nitrogenase cofactors are rich in both sulfur and iron and are presumed capable of binding and reducing N2. Nonetheless, synthetic examples of transition metal model complexes that bind N2 and also feature sulfur donor ligands remain scarce. We report herein an unusual series of low-valent diiron complexes featuring thiolate and dinitrogen ligands. A new binucleating ligand scaffold is introduced that supports an Fe(μ-SAr)Fe diiron subunit that coordinates dinitrogen (N2-Fe(μ-SAr)Fe-N2) across at least three oxidation states (Fe(II)Fe(II), Fe(II)Fe(I), and Fe(I)Fe(I)). The (N2-Fe(μ-SAr)Fe-N2) system undergoes reduction of the bound N2 to produce NH3 (∼50% yield) and can efficiently catalyze the disproportionation of N2H4 to NH3 and N2. The present scaffold also supports dinitrogen binding concomitant with hydride as a co-ligand. Synthetic model complexes of these types are desirable to ultimately constrain hypotheses regarding Fe-mediated nitrogen fixation in synthetic and biological systems.

Dinitrogen complexes of sulfur-ligated iron

We report a unique class of dinitrogen complexes of iron featuring sulfur donors in the ancillary ligand. The ligands utilized are related to the recently studied tris(phosphino)silyl ligands (2-R(2)PC(6)H(4))(3)Si (R = Ph, iPr) but have one or two phosphine arms replaced with thioether donors. Depending on the number of phosphine arms replaced, both mononuclear and dinuclear iron complexes with dinitrogen are accessible. These complexes contribute to a desirable class of model complexes that possess both dinitrogen and sulfur ligands in the immediate iron coordination sphere.

Theoretical studies of homogeneous catalysts mimicking nitrogenase

The conversion of molecular nitrogen to ammonia is a key biological and chemical process and represents one of the most challenging topics in chemistry and biology. In Nature the Mo-containing nitrogenase enzymes perform nitrogen 'fixation' via an iron molybdenum cofactor (FeMo-co) under ambient conditions. In contrast, industrially, the Haber-Bosch process reduces molecular nitrogen and hydrogen to ammonia with a heterogeneous iron catalyst under drastic conditions of temperature and pressure. This process accounts for the production of millions of tons of nitrogen compounds used for agricultural and industrial purposes, but the high temperature and pressure required result in a large energy loss, leading to several economic and environmental issues. During the last 40 years many attempts have been made to synthesize simple homogeneous catalysts that can activate dinitrogen under the same mild conditions of the nitrogenase enzymes. Several compounds, almost all containing transition metals, have been shown to bind and activate N₂ to various degrees. However, to date Mo(N₂)(HIPTN)₃N with (HIPTN)₃N= hexaisopropyl-terphenyl-triamidoamine is the only compound performing this process catalytically. In this review we describe how Density Functional Theory calculations have been of help in elucidating the reaction mechanisms of the inorganic compounds that activate or fix N₂. These studies provided important insights that rationalize and complement the experimental findings about the reaction mechanisms of known catalysts, predicting the reactivity of new potential catalysts and helping in tailoring new efficient catalytic compounds.