Catalytic reduction of dinitrogen to silylamines by earth-abundant lanthanide and group 4 complexes

Dinitrogen is a challenging molecule to activate and reduce to useful products such as ammonia. The range of d-block metal complexes that can catalyze dinitrogen reduction to ammonia or tris(silyl)amines under ambient conditions has increased in recent years, and now includes more electropositive metal complexes, rather than the traditional electron-rich middle and late d-block cations used by nature and industry. Conventional f-block metal complexes do not have filled d-orbitals that would enable binding of N2. Here, arene-bridged tetraphenolates are used to form metallacyclic structures by coordination to lanthanide or group 4 cations that can bind dinitrogen in the cavity and catalyze its conversion to bis(silyl)amines in ambient conditions. The unusual double-substitution product is attributed to the steric control afforded by the metalacycle’s pocket. This is the first time that lanthanide complexes (and zirconium) have been shown to catalyze nitrogen reduction, and the reactivity of the most active, samarium, is attributed to the strong, yet accessible, reducing capacity of its divalent oxidation state, and the ability of the structure to retain group 1 metal cations in the catalyst coordination sphere. The group 4 complexes featuring small cavities are most selective for the formation of bis(silyl)amine, while the lanthanide complexes, with larger cavities, are also able to make the traditional tris(silyl)amine product from N2. A series of experimental, analytical, spectroscopic, and computational studies provide insight into the mechanism of the reductive functionalization. These results offer new catalytic applications for plentiful titanium and the more earth-abundant members of the lanthanide series that are also less toxic than many base metals used in catalysis, such as iron.