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Dinitrogen is a challenging molecule to reduce to useful products under ambient conditions. The range of d-block metal complexes that can catalyze dinitrogen reduction to ammonia or tris(silyl)amines under ambient conditions has increased recently but lacks electropositive metal complexes, such as those of the f-block, which lack filled d-orbitals that would support classical binding modes of N2. Here, metallacyclic phenolate structures with lanthanide or group 4 cations can bind dinitrogen and catalyze its conversion to bis(silyl)amines under ambient conditions. The formation of this unusual product is controlled by metallacycle sterics. The group 4 complexes featuring small cavities are most selective for bis(silyl)amine, while lanthanide complexes and the solvated uranium(IV) congener, with larger cavities, can also make a conventional tris(silyl)amine product. These results offer new catalytic applications for plentiful titanium and more earth-abundant members of the lanthanides that are also less toxic than many base metals used in catalysis.