Abstract
Intensive efforts for the transformation of dinitrogen using transition metal–dinitrogen complexes as catalysts under mild reaction conditions have been made. However, limited systems have succeeded in the catalytic formation of ammonia. Here we show that newly designed and prepared dinitrogen-bridged dimolybdenum complexes bearing N-heterocyclic carbene- and phosphine-based PCP-pincer ligands [{Mo(N2)2(PCP)}2(μ-N2)] (1) work as so far the most effective catalysts towards the formation of ammonia from dinitrogen under ambient reaction conditions, where up to 230 equiv. of ammonia are produced based on the catalyst. DFT calculations on 1 reveal that the PCP-pincer ligand serves as not only a strong σ-donor but also a π-acceptor. These electronic properties are responsible for a solid connection between the molybdenum centre and the pincer ligand, leading to the enhanced catalytic activity for nitrogen fixation.
Highlights
Intensive efforts for the transformation of dinitrogen using transition metal–dinitrogen complexes as catalysts under mild reaction conditions have been made
Despite intensive efforts for the transformation of dinitrogen gas using transition metal–dinitrogen complexes as catalysts under mild reaction conditions9–16, only a few systems have succeeded in the catalytic formation of ammonia from dinitrogen gas17–28
We found that several dinitrogen-bridged dimolybdenum complexes such as [{Mo(N2)2(PNP)}2(m-N –0.55 –0.26 Mo (N2))] (2; PNP 1⁄4 2,6-bis(di-tert-butylphosphinomethyl)pyridine)40–42 and molybdenum–nitride complexes bearing PNP-type pincer ligands43 or mer-tridentate triphosphine44 worked as more effective catalysts towards ammonia formation under ambient reaction conditions, where up to 63 equiv. of ammonia were produced based on the molybdenum atom of the catalyst
Summary
Intensive efforts for the transformation of dinitrogen using transition metal–dinitrogen complexes as catalysts under mild reaction conditions have been made. We show that newly designed and prepared dinitrogen-bridged dimolybdenum complexes bearing N-heterocyclic carbene- and phosphine-based PCP-pincer ligands [{Mo(N2)2(PCP)}2(m-N2)] (1) work as so far the most effective catalysts towards the formation of ammonia from dinitrogen under ambient reaction conditions, where up to 230 equiv. Despite intensive efforts for the transformation of dinitrogen gas using transition metal–dinitrogen complexes as catalysts under mild reaction conditions, only a few systems have succeeded in the catalytic formation of ammonia from dinitrogen gas. We found that several dinitrogen-bridged dimolybdenum complexes such as [{Mo(N2)2(PNP)}2(m-N2)] (2; PNP 1⁄4 2,6-bis(di-tert-butylphosphinomethyl)pyridine) and molybdenum–nitride complexes bearing PNP-type pincer ligands or mer-tridentate triphosphine worked as more effective catalysts towards ammonia formation under ambient reaction conditions, where up to 63 equiv. Density functional theory (DFT) calculations demonstrated that the dinitrogen-bridged dimolybdenum structure plays a vital role in the protonation of a dinitrogen ligand, where one molybdenum moiety of the dinuclear molybdenum–dinitrogen complex works as a mobile ligand to the other molybdenum moiety as an active site
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