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Unique behaviour of dinitrogen-bridged dimolybdenum complexes bearing pincer ligand towards catalytic formation of ammonia.

ABSTRACT: It is vital to design effective nitrogen fixation systems that operate under mild conditions, and to this end we recently reported an example of the catalytic formation of ammonia using a dinitrogen-bridged dimolybdenum complex bearing a pincer ligand, where up to twenty three equivalents of ammonia were produced based on the catalyst. Here we study the origin of the catalytic behaviour of the dinitrogen-bridged dimolybdenum complex bearing the pincer ligand with density functional theory calculations, based on stoichiometric and catalytic formation of ammonia from molecular dinitrogen under ambient conditions. Comparison of di- and mono-molybdenum systems shows that the dinitrogen-bridged dimolybdenum core structure plays a critical role in the protonation of the coordinated molecular dinitrogen in the catalytic cycle.

SUBMITTER: Tanaka H

PROVIDER: S-EPMC4015321 | biostudies-literature | 2014 Apr

REPOSITORIES: biostudies-literature

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Unique behaviour of dinitrogen-bridged dimolybdenum complexes bearing pincer ligand towards catalytic formation of ammonia.

Tanaka Hiromasa H Arashiba Kazuya K Kuriyama Shogo S Sasada Akira A Nakajima Kazunari K Yoshizawa Kazunari K Nishibayashi Yoshiaki Y

Nature communications 20140428

It is vital to design effective nitrogen fixation systems that operate under mild conditions, and to this end we recently reported an example of the catalytic formation of ammonia using a dinitrogen-bridged dimolybdenum complex bearing a pincer ligand, where up to twenty three equivalents of ammonia were produced based on the catalyst. Here we study the origin of the catalytic behaviour of the dinitrogen-bridged dimolybdenum complex bearing the pincer ligand with density functional theory calcul ...[more]

PMID: 24769530

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

Project description: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.

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Catalytic Dinitrogen Reduction to Ammonia at a Triamidoamine-Titanium Complex.

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Project description:A nitrogenase-inspired biomimetic chalcogel system comprising double-cubane [Mo2Fe6S8(SPh)3] and single-cubane (Fe4S4) biomimetic clusters demonstrates photocatalytic N2 fixation and conversion to NH3 in ambient temperature and pressure conditions. Replacing the Fe4S4 clusters in this system with other inert ions such as Sb(3+), Sn(4+), Zn(2+) also gave chalcogels that were photocatalytically active. Finally, molybdenum-free chalcogels containing only Fe4S4 clusters are also capable of accomplishing the N2 fixation reaction with even higher efficiency than their Mo2Fe6S8(SPh)3-containing counterparts. Our results suggest that redox-active iron-sulfide-containing materials can activate the N2 molecule upon visible light excitation, which can be reduced all of the way to NH3 using protons and sacrificial electrons in aqueous solution. Evidently, whereas the Mo2Fe6S8(SPh)3 is capable of N2 fixation, Mo itself is not necessary to carry out this process. The initial binding of N2 with chalcogels under illumination was observed with in situ diffuse-reflectance Fourier transform infrared spectroscopy (DRIFTS). (15)N2 isotope experiments confirm that the generated NH3 derives from N2 Density functional theory (DFT) electronic structure calculations suggest that the N2 binding is thermodynamically favorable only with the highly reduced active clusters. The results reported herein contribute to ongoing efforts of mimicking nitrogenase in fixing nitrogen and point to a promising path in developing catalysts for the reduction of N2 under ambient conditions.

| S-EPMC4878479 | biostudies-literature

Ambient ammonia synthesis via palladium-catalyzed electrohydrogenation of dinitrogen at low overpotential.

Project description:Electrochemical reduction of N2 to NH3 provides an alternative to the Haber-Bosch process for sustainable, distributed production of NH3 when powered by renewable electricity. However, the development of such process has been impeded by the lack of efficient electrocatalysts for N2 reduction. Here we report efficient electroreduction of N2 to NH3 on palladium nanoparticles in phosphate buffer solution under ambient conditions, which exhibits high activity and selectivity with an NH3 yield rate of ~4.5 μg mg-1Pd h-1 and a Faradaic efficiency of 8.2% at 0.1 V vs. the reversible hydrogen electrode (corresponding to a low overpotential of 56 mV), outperforming other catalysts including gold and platinum. Density functional theory calculations suggest that the unique activity of palladium originates from its balanced hydrogen evolution activity and the Grotthuss-like hydride transfer mechanism on α-palladium hydride that lowers the free energy barrier of N2 hydrogenation to *N2H, the rate-limiting step for NH3 electrosynthesis.

| S-EPMC5953946 | biostudies-literature

Synthesis, Characterization, and Comparative Theoretical Investigation of Dinitrogen-Bridged Group 6-Gold Heterobimetallic Complexes.

Project description:We have prepared and characterized a series of unprecedented group 6-group 11, N2-bridged, heterobimetallic [ML4(η1-N2)(μ-η1:η1-N2)Au(NHC)]+ complexes (M = Mo, W, L2 = diphosphine) by treatment of trans-[ML4(N2)2] with a cationic gold(I) complex [Au(NHC)]+. The adducts are very labile in solution and in the solid, especially in the case of molybdenum, and decomposition pathways are likely initiated by electron transfers from the zerovalent group 6 atom to gold. Spectroscopic and structural parameters point to the fact that the gold adducts are very similar to Lewis pairs formed out of strong main-group Lewis acids (LA) and low-valent, end-on dinitrogen complexes, with a bent M-N-N-Au motif. To verify how far the analogy goes, we computed the electronic structures of [W(depe)2(η1-N2)(μ-η1:η1-N2)AuNHC]+ (10W+) and [W(depe)2(η1-N2)(μ-η1:η1-N2)B(C6F5)3] (11W). A careful analysis of the frontier orbitals of both compounds shows that a filled orbital resulting from the combination of the π* orbital of the bridging N2 with a d orbital of the group 6 metal overlaps in 10W+ with an empty sd hybrid orbital at gold, whereas in 11W with an sp3 hybrid orbital at boron. The bent N-N-LA arrangement maximizes these interactions, providing a similar level of N2 "push-pull" activation in the two compounds. In the gold case, the HOMO-2 orbital is further delocalized to the empty carbenic p orbital, and an NBO analysis suggests an important electrostatic component in the μ-N2-[Au(NHC)]+ bond.

| S-EPMC8058778 | biostudies-literature

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