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.

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.

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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.

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