Dual Iron Sites in Activation of N2 by Iron-Sulfur Cluster Anions Fe5S2- and Fe5S3. J Phys Chem Lett 2021;12:9269-9274. [PMID: 34533969 DOI: 10.1021/acs.jpclett.1c02683]

Machine Learning for Experimental Reactivity of a Set of Metal Clusters toward C-H Activation

Understanding the mechanisms of C-H activation of alkanes is a very important research topic. The reactions of metal clusters with alkanes have been extensively studied to reveal the electronic features governing C-H activation, while the experimental cluster reactivity was qualitatively interpreted case by case in the literature. Herein, we prepared and mass-selected over 100 rhodium-based clusters (RhxVyOz- and RhxCoyOz-) to react with light alkanes, enabling the determination of reaction rate constants spanning six orders of magnitude. A satisfactory model being able to quantitatively describe the rate data in terms of multiple cluster electronic features (average electron occupancy of valence s orbitals, the minimum natural charge on the metal atom, cluster polarizability, and energy gap involved in the agostic interaction) has been constructed through a machine learning approach. This study demonstrates that the general mechanisms governing the very important process of C-H activation by diverse metal centers can be discovered by interpreting experimental data with artificial intelligence.

Nitrogen adsorption on Nb2C6H4+ cations: the important role of benzyne (ortho-C6H4)

N2 adsorption is a prerequisite for activation and transformation. Time-of-flight mass spectrometry experiments show that the Nb2C6H4+ cation, resulting from the gas-phase reaction of Nb2+ with C6H6, is more favorable for N2 adsorption than Nb+ and Nb2+ cations. Density functional theory calculations reveal the effect of the ortho-C6H4 ligand on N2 adsorption. In Nb2C6H4+, interactions between the Nb-4d and C-2p orbitals enable the Nb2+ cation to form coordination bonds with the ortho-C6H4 ligand. Although the ortho-C6H4 ligand in Nb2C6H4+ is not directly involved in the reaction, its presence increases the polarity of the cluster and brings the highest occupied molecular orbital (HOMO) closer to the lowest occupied molecular orbital (LUMO) of N2, thereby increasing the N2 adsorption energy, which effectively facilitates N2 adsorption and activation. This study provides fundamental insights into the mechanisms of N2 adsorption in "transition metal-organic ligand" systems.

Manipulating Reactivity of Ir(CH2)0-2+ Cations toward Dinitrogen at Room Temperature: A Unique Dependence on the Organic Ligand Structures

Nitrogen (N2) activation at room temperature has long been a great challenge. Therefore, the rational design of reactive species to adsorb N2, which is a prerequisite for cleavage of the strong N≡N triple bond in industrial and biological processes, is highly desirable and meaningful. Herein, the N2 adsorption process is controlled by regulating the types and numbers of organic ligands, and the organic ligands are produced through the reactions of Ir+ with methane and ethane. CH4 molecules dissociate on the Ir+ cations to form Ir(CH2)1,2+. The reaction of Ir+ with C2H6 can generate HIrC2H3+, which is different from the structure of Ir(CH2)2+ obtained from Ir+/CH4. The reactivity order of N2 adsorption is Ir(CH2)2+ > HIrC2H3+ ≫ HIrCH+ ≈ Ir+ (almost inert under similar reaction conditions), indicating that different organic ligand structures affect reactivity dramatically. The main reason for this interesting reactivity difference is that the lowest unoccupied molecular orbital (LUMO) level of Ir(CH2)2+ is much closer to the highest occupied molecular orbital (HOMO) level of N2 than those of the other three systems. This study provides new insights into the adsorption of N2 on metal-organic ligand species, in which the organic ligand dominates the reactivity, and it discovers new clues in designing effective transition metal carbine species for N2 activation.

Superior Reactivity of Molybdenum-Sulfur Cluster Anions Mo5S2- and Mo5S3- toward Dinitrogen

Inspired by the fact that Mo is a key element in biological nitrogenase, a series of gas-phase MoxSy- cluster anions are prepared and their reactivity toward N2 is investigated by the combination of mass spectrometry, photoelectron imaging spectroscopy, and density functional theory calculations. The Mo5S2- and Mo5S3- cluster anions show remarkable reactivity compared with the anionic species reported previously. The spectroscopic results in conjunction with theoretical analysis reveal that a facile cleavage of N≡N bonds takes place on Mo5S2- and Mo5S3-. The large dissociative adsorption energy of N2 and the favorable entrance channel for initial N2 approaching are proposed as two decisive factors for the superior reactivity of Mo5S2- and Mo5S3-. Besides, the modulation of S ligands on the reactivity of metal centers with N2 is proposed. The highly reactive metal-sulfur species may be obtained by the coordination of two to three sulfur atoms to bare metal clusters so that an appropriate combination of electronic structures and charge distributions can be achieved.

Activation of Dinitrogen Promoted by Adsorption of C6H6 on Fe2VC- Cluster Anions

The introduction of organic ligands is one of the effective strategies to improve the stability and reactivity of metal clusters. Herein, the enhanced reactivity of benzene-ligated cluster anions Fe₂VC(C₆H₆)^- with respect to naked Fe₂VC^- is identified. Structural characterization suggests that C₆H₆ is molecularly bound to the dual metal site in Fe₂VC(C₆H₆)^-. Mechanistic details reveal that the cleavage of N≡N is feasible in Fe₂VC(C₆H₆)^-/N₂ but hindered by an overall positive barrier in the Fe₂VC^-/N₂ system. Further analysis discloses that the ligated C₆H₆ regulates the compositions and energy levels of the active orbitals of the metal clusters. More importantly, C₆H₆ serves as an electron reservoir for the reduction of N₂ to lower the crucial energy barrier of N≡N splitting. This work demonstrates that the flexibility of C₆H₆ in terms of withdrawing and donating electrons is crucial to regulating the electronic structures of the metal cluster and enhancing the reactivity.

Spectroscopic Identification of the Dinitrogen Fixation and Activation by Metal Carbide Cluster Anions PtCn- (n = 4-6)

Nitrogen fixation is confronted with great challenges in the field of chemistry. Herein, we report that single metal carbides PtC_n^- and PtC_nN₂^- (n = 4-6) are indispensable intermediates in the process of nitrogen fixation by mass spectrometry coupled with anionic photoelectron spectroscopy, quantum chemical calculations, and simulated density-of-state spectra. The most stable isomers of these cluster anions are characterized to have linear chain structures. The fixation and activation of dinitrogen are facilitated by the charge transfer from Pt and C_n to N₂. The significance of π back-donation of the 5d orbital of the Pt atom to the antibonding π orbits of N₂ for dinitrogen fixation and activation is discussed in detail. This study not only provides a theoretical basis at the molecular level for the activation of dinitrogen by mononuclear metal carbide clusters but also provides a new paradigm for dinitrogen fixation.

An Open-Shell Superatom Cluster Ta10- with Enhanced Stability by United d-d π Bonds and d-Orbital Superatomic States

We carried out a comprehensive study on the gas-phase reactions of Ta_n^- (n = 5-27) with nitrogen using a customized reflection time-of-flight mass spectrometer coupled with a velocity map imaging apparatus (Re-TOFMS-VMI). Among the studied tantalum clusters, Ta₁₀^- exhibits prominent mass abundance indicative of its unique inertness. DFT calculation results revealed a D_4d bipyramidal prolate structure of the most stable Ta₁₀^-, which was verified by photoelectron spectroscopy experiments. The calculations also unveiled that Ta₁₀^- has the largest HOMO-LUMO gap and second-order difference of binding energy among the studied clusters. This is associated with its well-organized superatomic orbitals, which consist of both 6s and 5d orbitals of tantalum atoms, allowing for splitting of superatomic 1D and 2P orbitals and an enlarged gap between the singly occupied molecular orbital (SOMO) and unoccupied β counterpart, which brings forth stabilization energy pertaining to Jahn-Teller distortion. Also, the SOMO exhibits a united d-d π orbital pattern that embraces the central Ta₈^- moiety.

Trimetallic clusters in the sumanene bowl for dinitrogen activation

It is of great importance to find catalysts for the nitrogen reduction reaction (NRR) with high stability and reactivity. A series of M₃ clusters (M = Ti, Zr, V, and Nb) supported on sumanene (C₂₁H₁₂) were designed as potential catalysts for the NRR by taking advantage of the high reactivity of trimetallic clusters and the unique geometric and electronic properties of sumanene, a bowl-like organic molecule. Detailed mechanisms of NN bond cleavage on C₂₁H₁₂-M₃ were investigated by DFT calculations and compared with those on bare M₃ clusters. M₃ in the sumanene bowl is very stable with large binding energies, which prohibits the cohesion of M₃ into M₆. In the bowl, M₃ has a (quasi-) equilateral triangle structure with lengthened M-M bonds, which is particularly beneficial to the N₂ transfer process on Ti₃ and V₃ clusters. The N-N bond can be dissociated by both M₃ and C₂₁H₁₂-M₃ clusters without the overall energy barriers. A blurring effect is found in which some geometric and electronic properties of different metal types become similar when M₃ is supported on the substrate. Our work demonstrates that sumanene is a suitable substrate to support M₃ in the activation of N₂ with enhanced stability and maintained a high level of reactivity compared to bare M₃.

Recent Progress in Dinitrogen Activation by Gas-Phase Metal Species

Understanding the mechanisms to activate and functionalize dinitrogen (N₂) is of great importance for the rational design of nitrogen-fixation catalysts. Reactions of gas-phase species with N₂ are being actively studied to understand the bond activation and formation processes at a strictly molecular level. This Perspective provides an overview of the recent progress in combined experimental and theoretical studies on the activation and functionalization of N₂ by gas-phase metal species. New mechanistic insights into N₂ molecular adsorption, N≡N cleavage, and N-X (X = C, B, and H) formation have been introduced, in which the new reaction channels of ejecting neutral metal fragments and the coupling reactions of N₂ with other molecules are highlighted. Finally, the current challenges and outlooks of N₂ activation in the gas phase are discussed as well.

Spectroscopic Characterization of the Synergistic Mechanism of Ruthenium-Lithium Hydrides for Dinitrogen Cleavage

Elucidating the role of alkali/alkaline earth metal hydrides in dinitrogen activation remains an important and challenging goal for spectroscopic studies of bulk systems, because their spectral signatures are often masked by the collective effects. Herein, mass-selected photoelectron velocity-map imaging spectroscopic and quantum chemical calculation techniques are utilized to explore the promotion mechanism of LiH in the Ru-based catalysts toward N₂ activation. The RuHN₂^- anion is determined to be a N₂-tagged complex. In contrast, the RuHN₂(LiH)_n^- (n = 1 and 2) anions are characterized to have N≡N bond-cleaved ring structures. These observations indicate that the complexation of LiH to RuH^- significantly facilitates N≡N bond cleavage. Theoretical analyses show that the synergy between Ru and LiH efficiently lowers the energy barrier of N≡N bond cleavage. These findings clarify the pivotal roles played by the LiH species in the transition metal catalysts for N₂ activation and have important practical implications for the prospective design of high-performance catalysts via metal tuning strategies.

15 N/14N isotopic exchange in the dissociative adsorption of N2 on tantalum nitride cluster anions Ta3N3−

Adsorption and activation of dinitrogen (N2) is an indispensable process in nitrogen fixation. Metal nitride species continue to attract attention as a promising catalyst for ammonia synthesis. However, the detailed mechanisms at a molecular level between reactive nitride species and N2 remain unclear at elevated temperature, which is important to understand the temperature effect and narrow the gap between the gas phase system and condensed phase system. Herein, the 14N/15N isotopic exchange in the reaction between tantalum nitride cluster anions Ta314N3− and 15N2 leading to the regeneration of 14N2/14N15N was observed at elevated temperature (393−593 K) using mass spectrometry. With the aid of theoretical calculations, the exchange mechanism and the effect of temperature to promote the dissociation of N2 on Ta3N3− were elucidated. A comparison experiment for Ta314N4−/15N2 couple indicated that only desorption of 15N2 from Ta314N415N2− took place at elevated temperature. The different exchange behavior can be well understood by the fact that nitrogen vacancy is a requisite for the dinitrogen activation over metal nitride species. This study may shed light on understanding the role of nitrogen vacancy in nitride species for ammonia synthesis and provide clues in designing effective catalysts for nitrogen fixation.

Facile N≡N Bond Cleavage by Anionic Trimetallic Clusters V3-x Tax C4 - (x=0-3): A DFT Study

Activation of N₂ on anionic trimetallic V_3-x Ta_x C₄ ^- (x=0-3) clusters was theoretically studied employing density functional theory. For all studied clusters, initial adsorption of N₂ (end-on) on one of the metal atoms (denoted as Site 1) is transferred to an of end-on: side-on: side-on coordination on three metal atoms, prior to N₂ dissociation. The whole reaction is exothermic and has no global energy barriers, indicating that the dissociation of N₂ is facile under mild conditions. The reaction process can be divided into two processes: N₂ transfer (TRF) and N-N dissociation (DIS). For V-series clusters, which has a V atom on Site 1, the rate-determining step is DIS, while for Ta-series clusters with a Ta on Site 1, TRF may be the rate-determining step or has energy barriers similar to those of DIS. The overall energy barriers for heteronuclear V₂ TaC₄ ^- and VTa₂ C₄ ^- clusters are lower than those for homonuclear V₃ C₄ ^- and Ta₃ C₄ ^- , showing that the doping effect is beneficial for the activation and dissociation of N₂ . In particular, V-Ta₂ C₄ ^- has low energy barriers in both TRF and DIS, and it has the highest N₂ adsorption energy and a high reaction heat release. Therefore, a trimetallic heteronuclear V-series cluster, V-Ta₂ C₄ ^- , is suggested to have high reactivity to N₂ activation, and may serve as a prototype for designing related catalysts at a molecular level.