The functionalization of coordinated dinitrogen to form nitrogen–element bonds en route to nitrogen-containing molecules is a long-standing challenge in chemical synthesis. The strong triple bond and the nonpolarity of the N2 molecule pose thermodynamic and kinetic challenges for promoting reactivity. While heterogeneous, homogeneous, and biological catalysts are all known for catalytic nitrogen fixation to ammonia, the catalytic synthesis of more complicated nitrogen-containing organic molecules has far less precedent. The example of silyl radical additions to coordinated nitrogen to form silylamines stands as the lone example of a catalytic reaction involving N2 to form a product other than ammonia. This Review surveys the field of molecular transition metal complexes as well as recent boron examples for the formation of nitrogen–element bonds. Emphasis is placed on the coordination and activation modes of N2 in the various metal compounds from across the transition series and how these structures can rationally inform reactivity studies. Over the past few decades, the field has evolved from the addition of carbon electrophiles in a manner similar to that of protonation reactions to more organometallic-inspired reactivity, including insertions, 1,2-additions, and cycloadditions. Various N–C, N–Si, and N–B bond-forming reactions have been discovered, highlighting that the challenge for catalytic chemistry is not in the reactivity of coordinated dinitrogen but rather removal of the functionalized ligand from the coordination sphere of the metal.

Two independent synthetic routes to η2-imine titanocene complexes were developed. On one hand side, ligand exchange reactions of bis(trimethylsilyl)acetylene by (p-Tolyl)HCNPh (3) employing the Rosenthal reagent Cp2Ti{η2-C2(SiMe3)2} (1) lead to Cp2Ti{η2-(p-Tolyl)CHNPh} (5), exhibiting a titanaaziridine structure. On the other hand, the direct reductive complexation of 3 by using Cp2TiCl2 (2) and Mg as reducing agent leads also to 5, one of the rare known titanoceneaziridines without additional ligands. By using the ketimine (p-Tolyl)2CNPh (4) instead of the aldimine 3, an unexpected coordination mode was found by X-ray diffraction, exhibiting an azatitanacyclopent-4-ene structure involving one tolyl fragment. In such a way, via the reductive complexation of 4, employing 2 or Cp*TiCl3 (12), the 1-aza-2-titanacyclopent-4-ene complexes 6 and 13 are formed. Density functional calculations at the M06-2X level identify these new complexes 6 and 13 as 1-aza-2-titanacyclopent-4-enes, in agreement with an analysis based on the experimental structural parameters. A theoretical study of the bonding between the titanocene fragment and the imine ligand reveals that steric factors are more pronounced for titanaaziridines and disfavor their formation compared to azatitanacyclopentenes. This provides a rationalization for the preferred formation of titanoceneaziridines in the case of aldimine ligands and azatitanacyclopentenes when ketimines are applied. Whereas titanoceneaziridine 5 undergoes insertion reactions into the Ti–C carbon σ-bond with aldehydes, ketones, or carbodiimides to the five-membered titanacycles 20 and 21, complex 6 appears to be inert in comparable reactions.

A method for the reduction of a manganese nitride to ammonia is reported, where light-driven proton-coupled electron transfer enables the formation of weak NH bonds. Photoreduction of (saltBu)­MnVN to ammonia and a Mn­(II) complex has been accomplished using 9,10-dihydroacridine and a combination of an appropriately matched photoredox catalyst and weak Brønsted acid. Acid-reductant pairs with effective bond dissociation free energies between 35 and 46 kcal/mol exhibited high efficiencies. This light-driven method may provide a blueprint for new approaches to catalytic homogeneous ammonia synthesis under ambient conditions.

A rhodium-catalyzed method for the hydrogenation of N-heteroarenes is described. A diverse array of unsubstituted N-heteroarenes including pyridine, pyrrole, and pyrazine, traditionally challenging substrates for hydrogenation, were successfully hydrogenated using the organometallic precatalysts, [(η5-C5Me5)­Rh­(N-C)­H] (N-C = 2-phenylpyridinyl (ppy) or benzo­[h]­quinolinyl (bq)). In addition, the hydrogenation of polyaromatic N-heteroarenes exhibited uncommon chemoselectivity. Studies into catalyst activation revealed that photochemical or thermal activation of [(η5-C5Me5)­Rh­(bq)­H] induced C­(sp2)–H reductive elimination and generated the bimetallic complex, [(η5-C5Me5)­Rh­(μ2,η2-bq)­Rh­(η5-C5Me5)­H]. In the presence of H2, both of the [(η5-C5Me5)­Rh­(N-C)­H] precursors and [(η5-C5Me5)­Rh­(μ2,η2-bq)­Rh­(η5-C5Me5)­H] converted to a pentametallic rhodium hydride cluster, [(η5-C5Me5)4Rh5H7], the structure of which was established by NMR spectroscopy, X-ray diffraction, and neutron diffraction. Kinetic studies on pyridine hydrogenation were conducted with each of the isolated rhodium complexes to identify catalytically relevant species. The data are most consistent with hydrogenation catalysis prompted by an unobserved multimetallic cluster with formation of [(η5-C5Me5)4Rh5H7] serving as a deactivation pathway.

The reductive complexation of aldimines is one of the promising synthetic tools to obtain titanaaziridines by reaction of the corresponding titanium halides and magnesium as a reducing agent. The effects of the substitution pattern of the imine ligand and the nature of the titanium fragment on the degree of activation of the η 2 -coordinated N-C double bond were investigated. With Cp # TiCl 3 (Cp # = cyclopentadienyl or pentamethylcyclopentadienyl) as the precursor for the reductive complexation of the imines N-(4-methylbenzylidene)aniline (2e) and 2-methyl-1-pyrroline (7), the formation of diazatitanacyclopentanes by McMurry-like C-C coupling is found. A six-membered titanacycle 8 is obtained by a Michael-like coupling reaction of 7. By these insights, it is

The catalytic hydrogenation of a metal nitride to produce free ammonia using a rhodium hydride catalyst that promotes H 2 activation and hydrogen-atom transfer is described. The phenylimine-substituted rhodium complex (η 5 -C 5 Me 5 )Rh-( Me PhI)H ( Me PhI = N-methyl-1-phenylethan-1-imine) exhibited higher thermal stability compared to the previously reported (η 5 -C 5 Me 5 )Rh(ppy)H (ppy = 2-phenylpyridine). DFT calculations established that the two rhodium complexes have comparable Rh− H bond dissociation free energies of 51.8 kcal mol −1 for (η 5 -C 5 Me 5 )Rh( Me PhI)H and 51.1 kcal mol −1 for (η 5 -C 5 Me 5 )Rh(ppy)H. In the presence of 10 mol% of the phenylimine rhodium precatalyst and 4 atm of H 2 in THF, the manganese nitride ( tBu Salen)MnN underwent hydrogenation to liberate free ammonia with up to 6 total turnovers of NH 3 or 18 turnovers of H • transfer. The phenylpyridine analogue proved inactive for ammonia synthesis under identical conditions owing to competing deleterious hydride transfer chemistry. Subsequent studies showed that the use of a non-polar solvent such as benzene suppressed formation of the cationic rhodium product resulting from the hydride transfer and enabled catalytic ammonia synthesis by proton-coupled electron transfer.

Concepts for the thermodynamically challenging synthesis of weak N–H bonds by photoinduced proton coupled electron transfer are explored. By harvesting visible light as driving force, ammonia synthesis was achieved and mechanistically elucidated.

The steel industry is an important engine for sustainable growth, added value, and high-quality employment within the European Union. It is committed to reducing its CO2 emissions due to production by up to 50% by 2030 compared to 1990′s level by developing and upscaling the technologies required to contribute to European initiatives, such as the Circular Economy Action Plan (CEAP) and the European Green Deal (EGD). The Clean Steel Partnership (CSP, a public–private partnership), which is led by the European Steel Association (EUROFER) and the European Steel Technology Platform (ESTEP), defined technological CO2 mitigation pathways comprising carbon direct avoidance (CDA), smart carbon usage SCU), and a circular economy (CE). CE approaches ensure competitiveness through increased resource efficiency and sustainability and consist of different issues, such as the valorization of steelmaking residues (dusts, slags, sludge) for internal recycling in the steelmaking process, enhanced steel recycling (scrap use), the use of secondary carbon carriers from non-steel sectors as a reducing agent and energy source in the steelmaking process chain, and CE business models (supply chain analyses). The current paper gives an overview of different technological CE approaches as obtained in a dedicated workshop called “Resi4Future—Residue valorization in iron and steel industry: sustainable solutions for a cleaner and more competitive future Europe” that was organized by ESTEP to focus on future challenges toward the final goal of industrial deployment.