A luminescent Ag8(DPPY)6(PhCC)6 cluster with a triangular superatomic Ag8 core.

We have synthesized single crystals of a highly stable Ag8 nanocluster protected by six ligands of diphenyl-2-phosphinic pyridine (DPPY) plus six ligands of phenylacetylene (PhCC). This Ag8(DPPY)6(PhCC)6 cluster bears a triangular superatomic Ag8 core, with the vertex and edge Ag atoms (quasi-triangle Ag6) being protected by both P and N bidentate coordination of the six DPPY ligands; meanwhile, the six PhCC ligands via µ3-C coordination form coordination on the two central Ag atoms capped on both sides of the triangle facet. Apart from the well-organized coordination of the two ligands pertaining to the balanced interactions with the Ag8 core, this Ag8 nanocluster exhibits superatomic stability with two delocalized valence electrons (1S2||1P0), assuming that the six PhCC ligands fix 6 localized electrons from the Ag atoms. Interestingly, the Ag8(DPPY)6(PhCC)6 NCs display temperature-dependent dual emissions at 330 and 535 nm under deep ultraviolet excitation. TD-DFT calculations reproduced the experimental spectrum, shedding light on the nature of excitation states and metal-ligand interactions in such a superatomic metal cluster.

Tailoring Titanium Carbide Clusters for New Materials: from Met-Cars to Carbon-Doped Superatoms.

Tailoring materials with prescribed properties and regular structures is a critical and challenging research topic. Early transition metals were found to form supermagic M8C12 metallocarbohedrenes (Met-Cars); however, stable metal carbides are not limited to this common stoichiometry. Utilizing self-developed deep-ultraviolet laser ionization mass spectrometry, here, we report a strategy to generate new titanium carbides by reacting pure Tin clusters with acetylene. Interestingly, two products corresponding to Ti17C2 and Ti19C10 exhibit superior abundances in addition to the Ti8C12 Met-Cars. Using global-minimum search, the structures of Ti17C2 and Ti19C10 are determined to be an ellipsoidal D4d and a rod-shaped D5h geometry, respectively, both with carbon-capped Ti4C moieties and superatomic features. We illustrate the electronic structures and bonding nature in these carbon-doped superatoms concerning their enhanced stability and local aromaticity, shedding light on a new class of metal-carbide nanomaterials with atomic precision.

Dehydrogenation of diborane on small Nbn+ clusters.

The reactivity of Nbn+ (1 ≤ n ≤ 21) clusters with B2H6 is studied by using a self-developed multiple-ion laminar flow tube reactor combined with a triple quadrupole mass spectrometer (MIFT-TQMS). The Nbn+ clusters were generated by a magnetron sputtering source and reacted with the B2H6 gas under fully thermalized conditions in the downstream flow tube where the reaction time was accurately controlled and adjustable. The complete and partial dehydrogenation products NbnB1-4+ and NbnB1-4H1,2,4+ were detected, indicative of the removal of H2 and likely BHx moieties. Interestingly, these NbnB1-4+ and NbnB1-4H1,2,4+ products are limited to 3 ≤ n ≤ 6, suggesting that the small Nbn+ clusters are relatively more reactive than the larger Nbn>6+ clusters under the same conditions. By varying the B2H6 gas concentrations and the reactant doses introduced into the flow tube, and by changing the reaction time, we performed a detailed analysis of the reaction dynamics in combination with the DFT-calculated thermodynamics. It is demonstrated that the lack of cooperative active sites on the Nb1+ cations accounts for the weakened dehydrogenation efficiency. Nb2+ forms partial dehydrogenation products at a faster rate. In contrast, the Nbn>6+ clusters are subject to more flexible vibrational relaxation which disperse the energy gain of B2H6-adsorption and thus are unable to overcome the energy barriers for subsequent hydrogen atom transfer and H2 release.

On the nature of Con±/0 clusters reacting with water and oxygen.

Bulk cobalt does not react with water at room temperature, but cobalt nanometals could yield corrosion at ambient conditions. Insights into the cobalt cluster reactions with water and oxygen enable us to better understand the interface reactivity of such nanometals. Here we report a comprehensive study on the gas-phase reactions of Con±/0 clusters with water and oxygen. All these Con±/0 clusters were found to react with oxygen, but only anionic cobalt clusters give rise to water dissociation whereas the cationic and neutral ones are limited to water adsorption. We elucidate the influences of charge states, bonding modes and dehydrogenation mechanism of water on typical cobalt clusters. It is unveiled that the additional electron of anionic Con- clusters is not beneficial to H2O adsorption, but allows for thermodynamics- and kinetics-favourable H atom transfer and dehydrogenation reactions. Apart from the charge effect, size effect and spin effect play a subtle role in the reaction process. The synergy of multiple metal sites in Con- clusters reduces the energy barrier of the rate-limiting step enabling hydrogen release. This finding of water dissociation on cobalt clusters put forward new connotations on the activity series of metals, providing new insights into the corrosion mechanism of cobalt nanometals.

Advances in Naked Metal Clusters for Catalysis.

The properties of sub-nano metal clusters are governed by quantum confinement and their large surface-to-bulk ratios, atomically precise compositions and geometric/electronic structures. Advances in metal clusters lead to new opportunities in diverse aspects of sciences including chemo-sensing, bio-imaging, photochemistry, and catalysis. Naked metal clusters having synergic multiple active sites and coordinative unsaturation and tunable stability/activity enable researchers to design atomically precise metal catalysts with tailored catalysis for different reactions. Here we summarize the progress of ligand-free naked metal clusters for catalytic applications. It is anticipated that this review helps to better understand the chemistry of small metal clusters and facilitates the design and development of new catalysts for potential applications.

To What Extent Do Iodomethane and Bromomethane Undergo Analogous Reactions?

Iodomethane and bromomethane (CH3I/CH3Br) are common chemicals, but their chemistry on nanometals is not fully understood. Here, we analyze the reactivity of Rhn+ (n = 3-30) clusters with halomethanes and unveil the spin effect and concentration dependence in the C-H and C-X bond activation. It is found that the reactions under halomethane-rich conditions differ from those under metal-rich conditions. Both CH3I and CH3Br undergo similar dehydrogenation on the Rhn+ clusters in the presence of small quantity reactants; however, different reactions are observed in the presence of sufficient CH3I/CH3Br, showing dominant Rh(CH3Br)x+ (x = 1-4) products but a series of RhnCxHyIz+ species (x = 1-4, y = 1-12, and z = 1-5) pertaining to H2, HI, or CH4 removal. Density functional theory calculations reveal that the dehydrogenation and demethanation of CH3Br are relatively less exothermic and will be deactivated by sufficient gas collisions if helium cooling takes away energy immediately; instead, the successive adsorption of CH3Br gives rise to a series of Rh(CH3Br)x+ species with accidental C-Br bond dissociation.

Magnetic Metal Clusters and Superatoms.

Metal clusters with tunable magnetism and chemical activity are ideal models to study magnetic order changes from microstructures to macroscopic substances, to understand the spin effect in diverse catalytic reactions, and to create information carriers of qubits in quantum computation. Precise preparation, reaction, and characterization of magnetic clusters provide a platform to understand spin-exchange interactions and geometrical/electronic structure-property relationships; thus, they are beneficial for the rational design and development of new cluster-genetic materials and spintronics microdevices. Advances in this field have discovered some high-spin magnetic clusters and superatoms, expanding the understanding of magnetism, aromaticity, cluster stability, and electron delocalization. Herein we present a perspective of the experimental and theoretical progress regarding magnetic clusters and superatoms, with the expectation of stimulating more research interest in this field.

Unusually High-Spin Fe12C12- Metallo-Carbohedrene Clusters.

Ferromagnets constructed from nanometals of atomic precision are important for innovative advances in information storage, energy conversion, and spintronic microdevices. Considerable success has been achieved in designing molecular magnets, which, however, are challenging in preparation and may suffer from drawbacks on the incompatibility of high stability and strong ferromagnetism. Utilizing a state-of-the-art self-developed mass spectrometer and a homemade laser vaporization source, we have achieved a highly efficient preparation of pure iron clusters, and here, we report the finding of a strongly ferromagnetic metal-carbon cluster, Fe12C12-, simply by reacting the Fen- clusters with acetylene in proper conditions. The unique stability of this ferromagnetic Fe12C12- cluster is rooted in a plumb-bob structure pertaining to Jahn-Teller distortion. We classify Fe12C12- as a new member of metallo-carbohedrenes and elucidate its structural stability mechanism as well as its soft-landing deposition and magnetization measurements, providing promise for the exploration of potential applications.

Rh19-: A high-spin super-octahedron cluster.

Probing atomic clusters with magic numbers is of supreme importance but challenging in cluster science. Pronounced stability of a metal cluster often arises from coincident geometric and electronic shell closures. However, transition metal clusters do not simply abide by this constraint. Here, we report the finding of a magic-number cluster Rh19- with prominent inertness in the sufficient gas-collision reactions. Photoelectron spectroscopy experiments and global-minimum structure search have determined the geometry of Rh19- to be a regular Oh­[Rh@Rh12@Rh6]- with unusual high-spin electronic configuration. The distinct stability of such a strongly magnetic cluster Rh19- consisting of a nonmagnetic element is fully unveiled on the basis of its unique bonding nature and superatomic states. The 1-nanometer-sized Oh-Rh19- cluster corresponds to a fragment of the face-centered cubic lattice of bulk rhodium but with altered magnetism and electronic property. This cluster features exceptional electron-spin state isomers confirmed in photoelectron spectra and suggests potential applications in atomically precise manufacturing involving spintronics and quantum computing.

A stable and strongly ferromagnetic Fe17O10- cluster with an accordion-like structure.

Isolated clusters are ideal systems for tailoring molecule-based magnets and investigating the evolution of magnetic order from microscopic to macroscopic regime. We have prepared pure Fen- (n = 7-31) clusters and observed their gas-collisional reactions with oxygen in a flow tube reactor. Interestingly, only the larger Fen- (n ≥ 15) clusters support the observation of O2-intake, while the smaller clusters Fen- (n = 7-14) are nearly nonreactive. What is more interesting is that Fe17O10- shows up with prominent abundance in the mass spectra indicative of its distinct inertness. In combination with DFT calculations, we unveil the stability of Fe17O10- within an interesting acordion-like structure and elucidate the spin accommodation in such a strongly ferromagnetic iron cluster oxide.

The s-p Nonhybrid Nature Causes Adaptive Superatomic States of Bismuth Clusters.

We report a joint experimental and theoretical study on the stability and reactivity of Bin + (n=5-33) clusters. The alternating odd-even effect on the reaction rates of Bin + clusters with NO is observed, and Bi7 + finds the most inertness. First-principles calculation results reveal that the lowest energy structures of Bi6-9 + exhibit quasi-spherical geometry pertaining to the jellium shell model; however, the Bin + (n≥10) clusters adopt assembly structures. The prominent stability of Bi7 + is associated with its highly symmetric structure and superatomic states with a magic number of 34e closed shell. For the first time, we demonstrate that the unique s-p nonhybrid feature in bismuth rationalizes the stability of Bi6-9 + clusters within the jellium model, by filling the 6s electrons into the superatomic orbitals (forming "s-band"). Interestingly, the stability of 18e "s-band" coincides with the compact structure for Bin + at n≤9 but assembly structures for n≥10, showing an accommodation of the s electrons to the geometric structure. The atomic p-orbitals also allow to form superatomic orbitals at higher energy levels, contributing to the preferable structures of tridentate binding units. We illustrate the s-p nonhybrid nature accommodates the structure and superatomic states of bismuth clusters.

What Determines If a Ligand Undergoes Coordination or Catalytic Activation on a Metal Cluster?

We report a joint experimental and theoretical study on the reactivity of Agn+ clusters with H2S, D2O, and NH3. Complete dehydrogenation products are observed for Agn+ reacting with H2S, but no dehydrogenation products are found for D2O or NH3 under the same reaction condition. Theoretical calculations elucidate why Agn+ clusters show different reactivities with these inorganic hydrides. NH3 shows strong coordination with Agn+, but the dehydrogenation reactions are unfavorable; in contrast, the fragile H-S bonds and stable AgnS+ products facilitate the hydrogen evolution of H2S on Agn+. We fully analyzed the metal-ligand interactions of Agn+ clusters with three molecules and illustrated the reaction dynamics and charge-transfer interactions and altered the superatomic states during the formation of cluster sulfides. We expect this study to benefit the design of stable environmentally friendly desulfurization catalysts and also the understanding of the mechanism on ligand-protected metal clusters in wet chemistry.

Nb12 +-niobespherene: a full-metal hollow-cage cluster with superatomic stability and resistance to CO attack.

Why one chemical is more stable than another is not always easy to understand. A unified answer for metal clusters has led to the establishment of the superatom concept, which rationalizes the delocalization of electrons; however, cluster stability based on superatom theory has not been confirmed unambiguously for any metal other than the s- and p-blocks of the periodic table of elements. Here, we have prepared pure niobium clusters and observed their reactions with CO under sufficient gas collision conditions. We find prominent inertness of Nb12 +, which survives CO attack. Comprehensive theoretical calculation results reveal that the inertness of Nb12 + is associated with its cage structure and well-organized superatomic orbitals, giving rise to energetic superiority among the studied clusters. It is revealed that not only the 5s but also the 4d electrons of Nb delocalize in the cluster and significantly contribute to the superatomic state, resulting in reasonable cage aromaticity. This hollow-cage cluster, which we have called a 'niobespherene', provides a clue with regard to designing new materials of all-metal aromaticity and Nb-involved catalysts free of CO poisoning.

Enhanced Stability of Rhombic Dodecahedron Nb15- with Well-Organized Superatomic States.

Well-resolved Nbn- clusters are produced and reacted with ethene and propene via a downstream flow tube reactor. Interestingly, the Nbn- clusters readily react with ethene and propene to form dehydrogenation products; however, Nb15- shows up in the mass spectra with prominent mass abundance indicating its inertness to react with olefins. For this cluster, we conduct photoelectron velocity map imaging (VMI) experiments and verify the stability of Nb15- within a highly symmetrical rhombic dodecahedron structure. Theoretical studies show that the stability of the Nb15- cluster is correlated with its superatomic nature pertaining to both geometric and electronic shell closures. Notably, the superatomic 1s orbital is dominated by the 5s electron of the central Nb atom, while the other superatomic orbitals are contributed by s-d hybridization, especially a remarkable contribution of s-dz2 hybridization. Apart from the closed shells, the highly symmetric geometry of Nb15- is associated with a regular polyhedral structure directed by all rhombus facets, embodying a magic number for body-centered dodecahedra, indicative of enhanced stability as a double magic cluster free of olefin adsorption.

A stable superatomic Cu6(SMPP)6 nanocluster with dual emission.

We have synthesized single crystals of a highly stable Cu6 nanocluster protected by six ligands of 2-mercapto-5-n-propylpyrimidine (SMPP). This Cu6(SMPP)6 cluster has a quasi-octahedral superatomic Cu6 core, with the Cu atoms being protected by both -S- and N-bidentate coordination of the SMPP ligands. Interestingly, each Cu atom is linked with an N atom, while the two neighboring Cu atoms on the same triangular facet are linked by the -S- bridge of the ligand. Single-crystal parsing results show that the altered orientation of the SMPP ligands give rise to three packing modes (named as 1, 2, and 3) of the Cu6(SMPP)6 NCs. Apart from the well-organized coordination, this Cu6(SMPP)6 nanocluster exhibits superatomic stability with a metallic core of 4 valence electrons (1S22S2||3S2), enabling to largely balance the interactions between the polynuclear core and delocalized electrons. Interestingly, the Cu6(SMPP)6 NCs display dual emissions in both ultraviolet-visible (UV-Vis) and near-infrared (NIR) regions. First-principles calculations well reproduce the experimental spectrum, shedding light on the nature of excitation states and metal-ligand interactions in the Cu6(SMPP)6 cluster.

Weak Interactions Initiate C-H and C-C Bond Dissociation of Ethane on Nbn + Clusters.

The conversion of ethane into value-added chemicals under ambient conditions has attracted much attention but the mechanisms remain not fully understood. Here we report a study on the reaction of ethane with thermalized Nbn + clusters based on a multiple-ion laminar flow tube reactor combined with a triple quadrupole mass spectrometer (MIFT-TQMS). It is found that ethane reacts with Nbn + clusters to form both products of dehydrogenation and methane-removal (odd-carbon products). Combined with density functional theory (DFT) calculations, we studied the reaction mechanisms of the C-C bond activation and C-H bond cleavage on the Nbn + clusters. It is unveiled that hydrogen atom transfer (HAT) initiates the reaction process, giving rise to the formation of Nb-C bonds and an elongated C-C distance in the HNbn + CH2 CH3 motif. Subsequent reactions allow for C-C bond activation and a competitive HAT process which is associated with CH4 removal or H2 release, resulting in the production of the observed carbides.

Catalysis of dinitrogen activation and reduction by a single Fe13 cluster and its doped systems.

Catalyzing N2 reduction to ammonia under ambient conditions is known to be significant both in the fertilizer industry and life sciences. To unveil the synergy of multiple sites, here, we have studied the catalysis of ammonia synthesis using a typical Fe13 cluster and its doped systems, Fe12X (X = V, Cr, Mn, Co, Ni, Cu, Zn, Nb, Mo, Ru, and Rh). The energetics analysis showed that center substitution (X@Fe12) was favored while doping single V, Cr, Co, and Mo atoms, whereas Mn, Ni, Cu, Zn, Nb, Ru, and Rh tended to form shell-doped structures (Fe12X). Among all the 13 clusters, Fe12Nb exhibited the lowest activation energy for N2 dissociation; moreover, in the hydrogenation process, Fe12Nb could convert N2 to ammonia efficiently. We have fully illustrated the reaction dynamics and structural chemistry essence of these diverse 13-atom systems and propose Fe12Nb as an ideal candidate for catalytic ammonia synthesis.

An integrated instrument of a tandem quadrupole mass spectrometer for cluster reaction and soft-landing deposition.

We have developed an integrated instrument system of a multiple-ion laminar flow tube (MIFT) reactor combined with a tandem quadrupole mass spectrometer (TQMS) and soft-landing deposition (SD) apparatus. A customized water-cooling magnetron sputtering (MagS) source is designed, by which we are able to attain a highly efficient preparation of metal clusters of 1-30 atoms with tunable size distributions. Following the MagS source, a laminar flow tube reactor is designed, allowing for sufficient gas-collision reactions of the as-prepared metal clusters, which is advantageous for probing magic clusters and minimizing wall effects when probing the reaction dynamics of such clusters. The customized TQMS analyzer involves a conical octupole, two linear octupoles, a quadruple ion deflector, and a 19 mm quadruple mass analyzer, allowing to decrease the pressure stepwise (from ∼5 to ∼10-9 Torr), thus ensuring high sensitivity and high resolution of the mass spectrometry analysis. In addition, we have designed a dual SD apparatus for the mass-selected deposition of clusters and their reaction products. For the whole system, abbreviated as MagS-MIFT-TQMS-SD, we have performed a detailed ions-fly simulation and quantitatively estimated the ions transfer efficiency under vacuum conditions determined by real experiments. Taking these advantages, well-resolved Pbn +, Agn +, and Nbn + clusters have been produced, allowing for meticulous studies of cluster reactions under sufficient gas-phase collisions free of electric field trapping. Also, we have tested the efficiency of the dual SD.

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 Tan- (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 D4d 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 ß 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.