Remarkable catalytic activity of dinitrogen-bridged dimolybdenum complexes bearing NHC-based PCP-pincer ligands toward nitrogen fixation

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.

Catalytic Nitrogen Fixation via Direct Cleavage of Nitrogen–Nitrogen Triple Bond of Molecular Dinitrogen under Ambient Reaction Conditions

“…We have now found that molybdenumiodide complexes bearing a PNP-pincer ligand have a higher catalytic activity than the so far reported molybdenumdinitrogen complexes 29,35 for ammonia formation from nitrogen gas under ambient reaction conditions, up to 830 equiv being produced based on the dinitrogen-bridged dimolybdenum complex (415 equiv of ammonia based on the molybdenum atom). This remarkable catalytic activity is realized by a novel reaction pathway, where the generation of a dinitrogen-bridged dimolybdenumiodide complex is a key point to promote direct cleavage of nitrogen nitrogen triple bond of the bridging dinitrogen ligand in the MoN¸NMo core.…”

“…As described in the present manuscript, molybdenumiodide complexes bearing a PNP-pincer ligand such as 1a and 2a have a higher catalytic activity than the so far reported molybdenum dinitrogen complexes 29,35 for ammonia formation from nitrogen gas under ambient reaction conditions, up to 830 equiv being produced based on a dinitrogen-bridged dimolybdenum complex (415 equiv of ammonia based on the molybdenum atom). If the formation of ammonia by using molybdenumiodide complexes as catalysts proceeds via the same reaction pathway, where 3 worked as a catalyst (Path A in Figure 1a), we cannot explain this remarkable catalytic activity of molybdenumiodide complexes, the higher TONs and TOFs of 1a than those of 3 and the higher selectivity of 1a than that of 3 toward the formation of ammonia prior to hydrogen gas (Table 1 and Figure 3).…”

Influence of a Metal Substrate on Small‐Molecule Activation Mediated by a Surface‐Adsorbed Complex

“…A particularly challenging case is nitrogen fixation, as it involves protonation and reduction of the most inert molecule of the universe, N 2 . Inspired by the biological process of nitrogen fixation, a number of molybdenum, iron, and other transition metal complexes have been synthesized that catalyze the conversion of N 2 to NH 3 in homogeneous solution . Besides proton donors such as HBAr F 4 [Ar F =3,5‐bis(trifluoromethyl)phenyl] and lutidinium salts, these processes require the addition of reducing agents (e.g., decamethylchromocene or KC 8 ) …”

Catalytic Conversion of Dinitrogen into Ammonia under Ambient Reaction Conditions by Using Proton Source from Water

“…To get a merit of biological nitrogen fixation, several efforts have been paid for the development of the biomimetic nitrogen fixation under ambient reaction conditions, and several catalytic systems have been evolved recently . Our group has also developed transition metal‐catalyzed reaction systems where metallocenes and substituted pyridine conjugate acids are employed as reducing agents and proton sources, respectively . Especially, the use of a dinitrogen‐bridged dimolybdenum complex bearing PCP‐type pincer ligands has succeeded in producing up to 230 equivalents of ammonia based on the catalyst .…”

“…Our group has also developed transition metal‐catalyzed reaction systems where metallocenes and substituted pyridine conjugate acids are employed as reducing agents and proton sources, respectively . Especially, the use of a dinitrogen‐bridged dimolybdenum complex bearing PCP‐type pincer ligands has succeeded in producing up to 230 equivalents of ammonia based on the catalyst . In this reaction system, metallocenes and substituted pyridine conjugate acids are not inexpensive, and are inadequate to extend these laboratory experiments to more practical scales.…”