About 20 %o ft he ammonia production is used as the chemical feedstockf or nitrogen-containing chemicals. However,w hile synthetic nitrogen fixation at ambient conditions has had some groundbreaking contributions in recent years,p rogress for the direct conversion of N 2 into organic products remains limited and catalytic reactions are unknown. Herein, the rhenium-mediated synthesis of acetonitrile using dinitrogen and ethyl triflate is presented. As ynthetic cycle in three reaction steps with high individual isolated yields and recovery of the rhenium pincer starting complex is shown. The cycle comprises alkylation of an itride that arises from N 2 splitting and subsequent imido ligand centered oxidation to nitrile via a1-azavinylidene (ketimido) intermediate.Different synthetic strategies for intra-and intermolecular imido ligand oxidation and associated metal reduction were evaluated that rely on simple proton, electron, and hydrogen-atom transfer steps.
Syntheticnitrogenfixationatambientconditionsunderwenttremendous advances in the past years.[1] Several molecular catalysts for NH 3 -formation are known.[2] Tu rn-over numbers (TONs) over 60 currently define the most efficient system. [3] Similarly,c atalytic N 2 silylation with large excess of ClSiMe 3 and alkali reductants saw some remarkable progress,b ut remains mechanistically less well defined.[4] While the lions share of ammonia is used for fertilizers,around 20 %serves as feedstock for chemical synthesis,for example,ofamines,nitro compounds,o rn itriles.H owever,c atalysts for the direct introduction of N 2 into organic products remain elusive and even stoichiometric systems are scarce.P ioneering work demonstrated transformations of coordinated N 2 ,s uch as four-electron reductions to hydrazido ligands with C-electrophiles.[5] But hydrazines apparently are less-promising synthetic targets,o wing to the weak N À Ns ingle bond. More recent work evaluated pathways for E À N(E= C, Si, B) bond formation accompanied by full N 2 splitting, for example,with heterocummulenes, [6] CO, [7] silyl, [8] alkyl, [9] or boryl groups.[10]According to thermochemical arguments,n itriles are attractive targets as the formation of strong CNb onds (D 0 (HC N) = 937 kJ mol À1 )facilitates offsetting the large N 2 bond energy (941 kJ mol À1 ).[11] Cummins and co-workers reported elegant synthetic cycles for the six-electron transformation of N 2 to nitriles mediated by Mo and Nb complexes.[12] Ther outes start with initial dinitrogen splitting, [13] followed by nitride acylation with silyltriflate and acyl chloride.S ubsequent stepwise three-electron reduction requires further silyltriflate and Lewis acid (SnCl 2 or ZnCl 2 ) for oxygen removal with nitrile release in up to 38 %y ield over all five steps.[12b] In this synthetic scheme re-reduction of the (formal) catalyst is ap urely metal-centered process (Scheme 1a).This work inspired us to evaluate new ways of N 2 into nitrile conversion, which rely on intramolecular electron transfer:T he interconversion of ...
