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.

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.

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.

Plasma-Assisted Dinitrogen Activation on Small Cobalt Clusters: Co4 N9 + with Enhanced Stability.

We have performed a study on the accommodation of nitrogen doping toward superatomic states of transition metal clusters. By reacting cobalt clusters with N2 in the presence of plasma radiation, a large number of odd-nitrogen clusters were observed, typically Co3 N2m-1 + (m=1-5) and Co4 N2m-1 + (m=1-6) series, showing N≡N bond cleavage in the mild plasma atmosphere. Interestingly, the Co3 N7 + , Co4 N9 + , and Co5 N9 + clusters exhibit prominent mass abundances. First-principles calculation results elucidate the stability of the diverse cobalt nitride clusters and find unique stability of Co4 N9 + with a swallow-kite structure of which four coordinated N2 molecules causes a significantly enlarged HOMO-LUMO gap, while the single N atom doping gives rise to superatomic states of 1S2 1P3 ||1D0 . We reveal an efficient dinitrogen activation strategy by reacting multiple N2 molecules with cobalt clusters under a plasma atmosphere.

Gas-Phase Synthesis of Metal Olefins: Plasma-Assisted Methane Dehydrogenation and C═C Bond Formation.

Methane dehydrogenation and C-C coupling under mild conditions are very important but challenging in chemistry. Utilizing a customized time of flight mass spectrometer combined with a magnetron sputtering (MagS) cluster source, here, we have conducted a study on the reactions of methane with small silver and copper clusters simply by introducing methane in argon as the working gas for sputtering. Interestingly, a series of [M(CnH2n)]+ (M = Cu and Ag; n = 2-12) clusters were observed, indicating high-efficiency methane dehydrogenation in such a plasma-assisted chamber system. Density functional theory calculations find the lowest energy structures of the [M(CnH2n)]+ series pertaining to olefins indicative of both C-H bond activation of methane and C-C bond coupling. We analyzed the interactions involved in the [Cu(CnH2n)]+ and [Ag(CnH2n)]+ (n = 1-6) clusters and demonstrated the reaction coordinates for the "Cu+ + CH4" and "Ag+ + CH4." It is illustrated that the presence of a second methane molecule enables us to reduce the necessary energy of dehydrogenation, which concurs with the experimental observation of an absence of the metal carbine products Cu+CH2 and Ag+CH2, which are short-lived. Also, it is elucidated that the higher-lying excitation states of Cu+ and Ag+ ions enable more favorable dehydrogenation process and C═C bond formation, shedding light on the plasma assistance of the essence.

An oxygen-passivated vanadium cluster [V@V10O15]- with metal-metal coordination produced by reacting Vn- with O2.

Vanadium cluster anions are highly reactive making the preparation of pure Vn- and the observation of their reactivity extremely challenging. Herein, well-resolved anionic Vn- clusters are prepared enabling an in-depth study on their reactions with O2 in the gas phase. While pure metal clusters of a magic number are not identified due to the strong V-O bonding, interestingly an unexpected oxide V11O15- was experimentally observed in surviving O2 etching reactions. First-principles theory calculations indicate that V11O15- possesses a body-centered pentagonal prism structure (D5h, ), with the V@V10 core fully protected by 15 oxygen bridges. Such an oxygen-protected metal cluster [V@V10O15]- exhibits typical superatom orbital features pertaining to the V@V10 core which shows effective metal-metal coordination bonding. Meanwhile, the high stability of [V@V10O15]- is reinforced by the V-O-V conjugation interactions which help to maintain the structural integrity, resulting in 3D inorganic aromaticity. This finding of such an oxygen-passivated superatom cluster sheds light on the bonding nature in ligand-protected metal clusters via wet synthesis.

Reactivity of Cobalt Clusters Con±/0 with Dinitrogen: Superatom Co6+ and Superatomic Complex Co5N6.

We report a joint experimental and theoretical study on the reactions of cobalt clusters (Con±/0) with nitrogen using the customized reflection time-of-flight mass spectrometer combined with a 177.3 nm deep-ultraviolet laser. Comparing to the behaviors of neutral Con (n = 2-30) and anionic Con- clusters (n = 7-53) which are relatively inert in reacting with nitrogen in the fast-flow tube, Con+ clusters readily react with nitrogen resulting in adducts of one or multiple N2 except Co6+ which stands firm in the reaction with nitrogen. Detailed quantum chemistry calculations, including the energetics, electron occupancy, and orbital analysis, well-explained the reasonable reactivity of Con+ clusters with nitrogen and unveiled the open-shell superatomic stability of Co6+ within a highly symmetric (D3d) structure. The D3d Co6+ bears an electron configuration of a half-filled superatomic 1P orbital (i.e., 1S21P3||1D0), a large α-highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, symmetric multicenter bonds, and reasonable electron delocalization pertaining to metallic aromaticity. Topology analysis by atom-in-molecule illustrates the interactions between Con+ and N2 corresponding to covalent bonds, but the Co-N interactions in cationic Co2+N2 and Co6+N2 clusters are apparently weaker than those in the other systems. In addition, we identify a superatomic complex Co5N6+ which exhibits similar frontier orbitals as the naked Co5+ cluster, but the alpha HOMO-LUMO gap is nearly double-magnified, which is consistent with the high-abundance peak of Co5N6+ in the experimental observation. The enhanced stability of such a ligand-coordinated superatomic complex Co5N6+, along with the superatom Co6+ with aromaticity, sheds light on special and general superatoms.

Correction: A hexagonal Ni6 cluster protected by 2-phenylethanethiol for catalytic conversion of toluene to benzaldehyde.

Correction for 'A hexagonal Ni6 cluster protected by 2-phenylethanethiol for catalytic conversion of toluene to benzaldehyde' by Anthony M. S. Pembere et al., Phys. Chem. Chem. Phys., 2019, 21, 17933-17938.

Reactivity of Cobalt Clusters Con±/0 with Ammonia: Co3+ Cluster Catalysis for NH3 Dehydrogenation.

A customized reflection time-of-flight (Re-TOF) mass spectrometer combined with a 177 nm deep-ultraviolet laser has enabled us to observe well-resolved cobalt clusters Con±/0 and perform a comprehensive study of their reactivity with ammonia (NH3). The anions Con- are found to be inert, the neutrals allow the adsorption of multiple NH3 molecules, while the cationic Con+ clusters readily react with NH3 giving rise to dehydrogenation. However, incidental dehydrogenation of NH3 on Con+ is only observed for n ≥ 3. The dramatic charge- and size-dependent reactivities of Con±/0 clusters with NH3 are studied by the density functional theory (DFT)-calculation results of energetics, density of states, orbital interactions, and reaction dynamics. We illustrate the dehydrogenation from two NH3 molecules, where a significantly reduced transition-state energy barrier is found pertaining to the dimolecular co-catalysis effect. The reactivity of Co3+ with NH3 is illustrated showing effective catalysis for N-H dissociation to produce hydrogen applicable for designing ammonia fuel cells.

A hexagonal Ni6 cluster protected by 2-phenylethanethiol for catalytic conversion of toluene to benzaldehyde.

We have synthesized single crystals of a 2-phenylethanethiol-protected Ni6 nanocluster, namely Ni6(C8H9S)12, which shows a hexagonal metallic core structure and reasonable stability. Interestingly this cluster is found to be an excellent candidate for the catalytic oxidation of toluene to benzaldehyde, with 100% conversion and 91% selectivity, showing application potential as an aromatic aldehyde in industry. Using DFT calculations, we rationalize the catalytic reaction mechanism for the conversion of toluene to benzaldehyde, and demonstrate that the presence of H2O2 initiates the Ni6 cluster via a highly exothermic step to form a Ni6O* intermediate which then results in active sites for the oxidation of toluene. What is interesting is that the likely steric geometry matching of the six-ring Ni6 core, a dozen phenylethanethiol ligands and the aromatic reactant toluene could attain a synergistic effect to facilitate the production of benzaldehyde free of catalyst support.

The Doping Effect of 13-Atom Iron Clusters on Water Adsorption and O-H Bond Dissociation.

Understanding the interactions between water and Fe-based clusters is necessary to unravel the micromechanics of the surface hydrophilic property and the corrosion process of iron-related materials. Herein, a theoretical study is conducted of water adsorption and dissociation on icosahedral Fe13 and Fe12X (X = Ti, V, Cr, Mn, Co, Ni) clusters. It is found that the doping atoms have significant influence on the geometric structures, magnetic moments, and electronic states of Fe12X clusters. The center-doped clusters X@Fe12 show higher stability than the shell-doped X-Fe12; Ni@Fe12 exhibits lower activation energy for the dissociation of H2O than all the others; Ti@Fe12 strikes a weak bonding energy and high activation energy for water dissociation. Also, a water dimer finds a decreased energy barrier for O-H dissociation, and the electronic states and metal-water interactions can be altered by the support effect. This information is helpful to those working on water chemistry, anticorrosion wading devices, and high-standard potable water utilization.

Ultrafast Deep-Ultraviolet Laser Ionization Mass Spectrometry Applicable To Identify Phenylenediamine Isomers.

The application of low-fragmentation mass spectrometry to identify chemicals has been recognized to be of particular importance in chemistry, biomedicine, and materials science. Utilizing a customized all-solid-state picosecond-pulsed deep-ultraviolet (DUV) laser, here we present new advances into photoionization mass spectrometry. The DUV laser ionization mass spectrometry (DUV-LIMS) results in very clean spectra pertaining to minimized structure relaxation and fragmentation under the ultrafast ionization process. Typical DUV-LIMS applications are illustrated not only for small organic molecules but also for long-chain unsaturated hydrocarbons and clusters of benzene. The unique advantages of DUV-LIMS enable us to detect and analyze confusing organic compound mixtures, indicating promising applications. DUV-LIMS is also found to be applicable in the identification of phenylenediamine isomers. An in-depth analysis of reaction dynamics is provided showing how hydrogen-atom-transfer (HAT) initiates the distinguishable photodissociation of phenylenediamines under near-resonant excitation. In particular, ortho-phenylenediamine (OPD) finds a remarkable dehydrogenation product with comparable intensity to the molecular ion peak, which is associated with the quantum tunnelling tautomers, providing new subjects for studying intramolecular noncovalent interactions.

Direct C-C Coupling of CO2 and the Methyl Group from CH4 Activation through Facile Insertion of CO2 into Zn-CH3 σ-Bond.

Conversion of CO2 and CH4 to value-added products will contribute to alleviating the green-house gas effect but is a challenge both scientifically and practically. Stabilization of the methyl group through CH4 activation and facile CO2 insertion ensure the realization of C-C coupling. In the present study, we demonstrate the ready C-C coupling reaction on a Zn-doped ceria catalyst. The detailed mechanism of this direct C-C coupling reaction was examined based on the results from density functional theory calculations. The results show that the Zn dopant stabilizes the methyl group by forming a Zn-C bond, thus hindering subsequent dehydrogenation of CH4. CO2 can be inserted into the Zn-C bond in an activated bent configuration, with the transition state in the form of a three-centered Zn-C-C moiety and an activation barrier of 0.51 eV. The C-C coupling reaction resulted in the acetate species, which could desorb as acetic acid by combining with a surface proton. The formation of acetic acid from CO2 and CH4 is a reaction with 100% atom economy, and the implementation of the reaction on a heterogeneous catalyst is of great importance to the utilization of the greenhouse gases. We tested other possible dopants including Al, Ga, Cd, In, and Ni and found a positive correlation between the activation barrier of C-C coupling and the electronegativity of the dopant, although C-H bond activation is likely the dominant reaction on the Ni-doped ceria catalyst.

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.

Co13O8-metalloxocubes: a new class of perovskite-like neutral clusters with cubic aromaticity.

Exploring stable clusters to understand structural evolution from atoms to macroscopic matter and to construct new materials is interesting yet challenging in chemistry. Utilizing our newly developed deep-ultraviolet laser ionization mass spectrometry technique, here we observe the reactions of neutral cobalt clusters with oxygen and find a very stable cluster species of Co13O8 that dominates the mass distribution in the presence of a large flow rate of oxygen gas. The results of global-minimum structural search reveal a unique cubic structure and distinctive stability of the neutral Co13O8 cluster that forms a new class of metal oxides that we named as 'metalloxocubes'. Thermodynamics and kinetics calculations illustrate the structural evolution from icosahedral Co13 to the metalloxocube Co13O8 with decreased energy, enhanced stability and aromaticity. This class of neutral oxygen-passivated metal clusters may be an ideal candidate for genetic materials because of the cubic nature of the building blocks and the stability due to cubic aromaticity.

Plasma-Assisted Chain Reactions of Rh3+ Clusters with Dinitrogen: N≡N Bond Dissociation.

Dinitrogen activation is known as one of the most challenging subjects in chemistry. A number of well-defined metal complexes, nitrides, and clusters have been studied that show catalysis for dinitrogen activation. However, direct evidence of a complete cleavage of the N≡N triple bond at mild conditions is rather limited to date. Herein, we report a study on the dissociation of N2 on small rhodium clusters assisted by surface plasma radiation. From mass spectrometry observation, a few rhodium nitride clusters with an odd number of nitrogen atoms are produced, such as the Rh3N2m-1+ (m = 1-5) series, indicative of N≡N bond dissociation in the mild plasma atmosphere. Interestingly, Rh3N7+ is identified with outstanding mass abundance among the RhnN2m-1+ products, and its ground-state structure is in the form of Rh3N(N2)3+ by capping a nitrogen atom on the top of Rh3+ plane and hanging three N2 molecules beneath the three Rh atoms respectively, giving rise to a C3v symmetry and excellent stability. We demonstrate the catalysis of such a three-atom rhodium cluster and reveal a dinitrogen activation strategy by thermodynamics- and dynamics- favorable chain reactions of multiple N2 molecules with two rhodium clusters under plasma atmosphere.

A small bimetallic Ag3Cu2 nanocluster with dual emissions within and against Kasha's rule.

Single crystals of a small bimetallic Ag3Cu2 nanocluster protected by six ligands of 2,4-dimethylbenzene thiol are synthesized by a one-pot procedure of wet chemistry. This Ag3Cu2 nanocluster bears a trigonal bipyramid metallic core with two copper atoms located on both sides of a triangular Ag3. Interestingly, the six Cu-Ag side edges of the trigonal bipyramid are fully protected by the six ligands giving rise to reinforced stability and high chemical purity. More interestingly, this Ag3Cu2 cluster shows strong dual fluorescence emissions in both ultraviolet visible (UV-vis) and near infrared (NIR) regions. Theoretical calculations reproduce the absorption and fluorescence spectra where the NIR emission at 824 nm is assigned to the S1→ S0 transition, while the simultaneous emission in the visible band is due to the radiation of highly excited states and is against Kasha's rule.

Ligand accommodation causes the anti-centrosymmetric structure of Au13Cu4 clusters with near-infrared emission.

We synthesized an [Au13Cu4(PPh3)4(SPy)8]+ nanocluster co-capped by phosphine and thiolate ligands. Interestingly, this Au13Cu4 cluster corresponds to an anti-centrosymmetric structure with the four copper atoms coordinated to the mixed ligands on the same side of the Au13 icosahedron, which is in sharp contrast to the [Au13Cu4(PPh2Py)4(SPhtBu)8]+ and [Au13Cu2(PPh3)6(SPy)6]+ clusters which possess highly symmetric structures with well-separated Cu adatoms. Both [Au13Cu4(PPh3)4(SPy)8]+ and [Au13Cu2(PPh3)6(SPy)6]+ clusters correspond to 8 valence electron superatoms with large HOMO-LUMO gaps, respectively. The difference in structure is rooted in the nature of the mixed ligands, with the bidentate SPy binding strongly to Cu on both binding sites (-N-Cu and Au-SR-Cu) leading to the co-linking of adjacent Cu atoms, while the bidentate PPh2Py binds Cu on one site and Au on the other giving rise to a separation of the Cu atoms even in the presence of relatively higher monomer concentration. Both [Au13Cu4(PPh3)4(SPy)8]+ and [Au13Cu2(PPh3)6(SPy)6]+ display emissions in the near-IR regions. TD-DFT calculations reproduce the spectroscopic results with specified excited states, shedding light on the geometric and electronic behaviors of the ligand-protected Au13Mx clusters.