Size-dependent Reactivity of Rhodium Deuteride Cluster Anions Rh3Dn¯ (n = 0-3) toward Dinitrogen: The Prominent Role of σ Donation

Anion photoelectron velocity-map imaging using a tunable laser at a 100 kHz repetition rate

We present velocity-map imaging (VMI) of photoelectrons detached from anions using an optical parametric amplifier operating at a repetition rate as high as 100 kHz. The light source generates femtosecond (fs) laser pulses tunable from near-infrared to ultraviolet (310-2600 nm), which interact synchronously with mass-selected anion bunches. We demonstrate this technique by measuring two-dimensional projections of photoelectrons ejected from silver trimer anions, Ag3-, across a photon energy range from 2.43 to 4.00 eV (509-310 nm), with an average power of 50-300 mW. This high-repetition-rate VMI setup allows rapid data acquisition of photoelectron spectra and laboratory-frame photoelectron angular distributions of anions at various photon energies, facilitating investigation of their electronic and geometric structures. Taking advantage of the fs pulses, this approach will also enable time-resolved photoelectron imaging for tracking electronic and nuclear dynamics of anions with high efficiency.

Reactivity of Polynuclear Niobium Oxynitride Cluster Anions Nb4N5-xOx - (x=0-5) toward N2

The activation of N₂ under mild conditions remains a significant challenge in chemistry. Understanding how the composition of ligands modulates the reactivity of metal centers is pivotal for the rational design of efficient catalysts for nitrogen fixation. Herein, the reactions between polynuclear niobium oxynitride anions Nb4N5-xOx - (x=0-5) and N2 were investigated by employing mass spectrometry, photoelectron imaging spectroscopy, and theoretical calculations. The rate constants of Nb4N5-xOx -/N2 gradually decrease for x=0 to x=4, and then increase again for x=5. The sharp increase of the rate constants of Nb4O5 -/N2 corresponds to a decrease in the electron detachment energy of the Nb4O5 - cluster in the photoelectron spectroscopic experiments. Theoretical calculations suggest that the low-coordinated Nb-Nb sites in Nb4N5-xOx - (x=0-5) behaves as the active centers to bind N2 in the side-on/end-on manner. Mechanistic analysis reveals that reducing the N/O ratio leads to higher electron densities on the active Nb-Nb centers and decreased positive charge on the metal atoms, which hinders the approach of N2 to the clusters. This finding discloses fundamental insights into the impact of N/O ratio in fine-tuning the reactivity of metal centers toward N2 adsorption in related catalytic processes.

Observation of Aromatic B13(CO)n+ (n = 1-7) as Boron Carbonyl Analogs of Benzene

CO as a typical σ-donor is one of the most important ligands in chemistry, while planar B13+ is experimentally known as the most prominent magic-number boron cluster analogous to benzene. Joint gas-phase mass spectroscopy, collision-induced dissociation, and first-principles theory investigations performed herein indicate that B13+ reacts with CO successively under ambient conditions to form a series of boron carbonyl complexes B13(CO)n+ up to n = 7, presenting the largest boron carbonyl complexes observed to date with a quasi-planar B13+ core at the center coordinated by nCO ligands around it. Extensive theoretical analyses unveil both the chemisorption pathways and bonding patterns of these aromatic B13(CO)n+ monocations which, with three delocalized π bonds well-retained over the slightly wrinkled B13+ moiety, all prove to be boron carbonyl analogs of benzene tentatively named as boron carbonyl aromatics (BCAs). Their π-isovalent B12(CO)n (n = 1-6) complexes with a quasi-planar B12 coordination center are predicted to be stable neutral BCAs.

Dinitrogen Activation within Frustrated Lewis Pairs Is Promoted by Adding External Electric Fields

This study uses computational means to explore the feasibility of N2 cleavage by frustrated Lewis pair (FLPs) species. The employed FLP systems are phosphane/borane (1) and carbene/borane (2). Previous studies show that 1 and 2 react with H2 and CO2 but do not activate N2. The present study demonstrates that N2 is indeed inert, and its activation requires augmentation of the FLPs by an external tool. As we demonstrate here, FLP-mediated N2 activation can be achieved by an external electric field oriented along the reaction axis of the FLP. Additionally, the study demonstrates that FLP -N2 activation generates useful nitrogen compound, e.g., hydrazine (H2N-NH2). In summary, we conclude that FLP effectively activates N2 in tandem with oriented external electric fields (OEEFs), which play a crucial role. This FLP/OEEF combination may serve as a general activator of inert molecules.

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.

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.

Experimental and Theoretical Study of N2 Adsorption on Hydrogenated Y2C4H− and Dehydrogenated Y2C4− Cluster Anions at Room Temperature

The adsorption of atmospheric dinitrogen (N₂) on transition metal sites is an important topic in chemistry, which is regarded as the prerequisite for the activation of robust N≡N bonds in biological and industrial fields. Metal hydride bonds play an important part in the adsorption of N₂, while the role of hydrogen has not been comprehensively studied. Herein, we report the N₂ adsorption on the well-defined Y₂C₄H_0,1⁻ cluster anions under mild conditions by using mass spectrometry and density functional theory calculations. The mass spectrometry results reveal that the reactivity of N₂ adsorption on Y₂C₄H⁻ is 50 times higher than that on Y₂C₄⁻ clusters. Further analysis reveals the important role of the H atom: (1) the presence of the H atom modifies the charge distribution of the Y₂C₄H⁻ anion; (2) the approach of N₂ to Y₂C₄H⁻ is more favorable kinetically compared to that to Y₂C₄⁻; and (3) a natural charge analysis shows that two Y atoms and one Y atom are the major electron donors in the Y₂C₄⁻ and Y₂C₄H⁻ anion clusters, respectively. This work provides new clues to the rational design of TM-based catalysts by efficiently doping hydrogen atoms to modulate the reactivity towards N₂.

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.