Formation of Delocalized Linear M-B-M Covalent Bonds: A Combined Experimental and Theoretical Study of BM2(CO)8+ (M = Co, Rh, Ir) Complexes.

Investigations of transition-metal boride clusters not only lead to novel structures but also provide important information about the metal-boron bonds that are critical to understanding the properties of boride materials. The geometric structures and bonding features of heteronuclear boron-containing transition metal carbonyl cluster cations BM(CO)6+ and BM2(CO)8+ (M = Co, Rh, and Ir) are studied by a combination of the infrared photodissociation spectroscopy and density functional calculations at B3LYP/def2-TZVP level. The completely coordinated BM2(CO)8+ complexes are characterized as a sandwich structure composed of two staggered M(CO)4 fragments and a boron cation, featuring a D3d symmetry and 1Eg electronic ground state as well as metal-anchored carbonyls in an end-on manner. In conjunction with theoretical calculations, multifold metal-boron-metal bonding interactions in BM2(CO)8+ complexes involving the filled d orbitals of the metals and the empty p orbitals of the boron cation were unveiled, namely, one σ-type M-B-M bond and two π-type M-B-M bonds. Accordingly, the BM2(CO)8+ complexes can be described as a linear conjugated (OC)4M═B═M(CO)4 skeleton with a formal B-M bond index of 1.5. The three delocalized d-p-d covalent bonds render compensation for the electron deficiency of the cationic boron center and endow both metal centers with the favorable 18-electron structure, thus contributing much to the overall structural stability of the BM2(CO)8+ cations. As a comparison, the saturated BRh(CO)6+ and BIr(CO)6+ complexes are determined to be a doublet Cs-symmetry structure with an unbridged (OC)2B-M(CO)4 pattern, involving a two-center σ-type (OC)2B → M(CO)4+ dative single bond along with a weak covalent B-M half bond. This work offers important insight into the structure and bonding of late transition metal boride carbonyl cluster cations.

Reactivity of cationic silver clusters with O2: a probe of interplay between clusters' geometric and electronic structures.

We explored the size-dependent reactivity of Agn+ (n = 2-22) with O2 under mild conditions and found that only a few sizes of Agn+, with even values of n = 4, 6, 12, 16, 18, and 22, are reactive. Possible structures of Agn+ (n = 2-22) were determined using a genetic algorithm with incomplete local optimizations at the DFT level, and the calculated bonding strengths of O2 on these structures are consistent with experimental observations. Analyses revealed a close relationship between the reactivity of Agn+ with O2 and its HOMO-LUMO gap: cationic silver clusters with a small HOMO-LUMO gap are reactive, which can be rationalized by the covalent character of chemical bonds between Agn+ and O2 involving their frontier orbitals. The peculiar size-dependent HOMO-LUMO gaps and reactivity with O2 correlate with the subtle interplay between the electronic configurations and geometric structures of these silver cluster cations.

Adsorption of Multiple NO Molecules on Anionic Gold Clusters: Electron Transfer and Disproportionation Enhanced by Molecular Aggregation.

The reactions of Aun- clusters with multiple nitric oxide (NO) molecules are explored at 150 K by utilizing a mini-flow-tube reactor and a time-of-flight mass spectrometer. Adsorption of multiple NO molecules is observed on most Aun-, while disproportionation reactions only occur on even-sized Aun- with n = 4, 6, 8, 20 and odd-sized ones with n = 5 and 7. Theoretical calculations reveal the geometric structures and electronic states of the products containing bimolecular and trimolecular NO units, where two NO molecules typically form dimers. Different from NO monomers that weakly interact with odd-sized Aun- and form electron-sharing covalent bonds with Au10-(D3h) and Au16-, NO dimers can extract significant charge from parent Aun-. Regarding the three NO molecules, a predilection toward condensation into trimers on even-sized Aun- is observed, while the tendency is more toward an adsorption pattern of a dimer plus a monomer on odd-sized Aun-. The NO trimers register even higher charge gain from Aun- as compared with the NO dimers, which leads to an elevated degree of activation and induces the progression of disproportionation reactions. Therefore, when considering the reaction between NO and Aun-, it appears that NO has a propensity to form dimers or trimers on Aun-. This behavior of aggregate formation substantially enhances the ability of NO to absorb negative charges from Aun- although the occurrence of disproportionate dissociation reactions is initiated only for specific sizes.

Infrared Photodissociation Spectroscopy of Mass-Selected Dinuclear Transition Metal Boride Carbonyl Cluster Cations.

The transition-metal-boron bonding interactions and geometric structures of heterodinuclear transition metal carbonyl cluster cations BM(CO)n+ (M = Co, Ni, and Cu) are studied by a combination of the infrared photodissociation spectroscopy and density functional theory calculations at the B3LYP/def2-TZVP level. The BCu(CO)5+ and BCo(CO)6+ cations are characterized as an (CO)2B-M(CO)3/4+ structure involving an σ-type (OC)2B → M(CO)3,4+ dative bonding with end-on carbonyls, while for BNi(CO)5,6+ complexes with a bridged carbonyl, a 3c-2e bond involving the 5σ electrons of the bridged carbonyl and an electron-sharing bond between the B(CO)2 fragment and the Ni(CO)2,3+ subunits were revealed. Moreover, the fundamental driving force of the exclusive existence of a bridged carbonyl group in the boron-nickel complexes has been demonstrated to stem from the desire of the B and Ni centers for the favorable 8- and 18-electron structures.

Adsorption of O2 on the Preferred -O-Au Sites of Small Gold Oxide Clusters: Charge-dependent Interaction and Activation.

Decades of research have illuminated the significant roles of gold/gold oxide clusters in small molecule catalytic oxidation. However, many fundamental questions, such as the actual sites to adsorb and activate O2 and the impact of charge, remain unanswered. Here, we have utilized an improved genetic algorithm program coupled with the DFT method to systematically search for the structures of Au1-5Ox-/+/0 (x = 1-4) and calculated binding interactions between Au1-5Ox-/+/0 (x = 1-2) and O2, aiming to determine the active sites and to elucidate the impact of different charge states in gold oxide systems. The results revealed that the reactivity of all three kinds of small gold oxide clusters toward O2 is strongly site-dependent, with clusters featuring an -O-Au site exhibiting a preference for adsorption. The charges on small gold oxide clusters significantly impact the interaction strength and the activation degree of adsorbed O2: in the case of anionic cluster, the interaction between O2 and the -O-Au sites leads to a chemical reaction involving electron transfer, thereby significantly activating O2; in neutral and cationic clusters, the adsorption of O2 on their -O-Au sites can be viewed as an electrostatic interaction. Pointedly, for cationic clusters, the highly concentrated positive charge on the Au atom of the -O-Au sites can strongly adsorb but hardly activate the adsorbed O2. These results have certain reference points for understanding the gold oxide interfaces and the improved catalytic oxidation performance of gold-based systems in the presence of atomic oxygen species.

A Coppoborylene Stabilized by Multicenter Covalent Bonding and Its Amphoteric Reactivity to CO.

A cationic copper-stabilized coppoborylene was prepared and structurally characterized via infrared photodissociation spectroscopy and density functional theory calculations. This structure exemplifies a new class of borylenes stabilized by three-center-two-electron metal-boron-metal covalent bonding interaction, displaying exceptional σ-acidity and unparalleled π-donor capability for CO activation that outperforms all of the known transition metal cations and is comparable or even superior to the documented base-trapped borylenes. Its neutral form represents a monovalent boron compound with a strongly reactive amphoteric boron center built on transition-metal-boron bonds, which inspires the design and synthesis of new members of the borylene family.

The global minimum of Ag30: a prolate spheroidal structure predicted using a genetic algorithm with incomplete local optimizations at the DFT level.

Genetic algorithms have been widely used to explore global minimum points of atomic clusters, and their incorporation with ab initio calculations (including density functional theory methods) as local optimization approaches increases their ability to accurately locate the global minimum points on complicated potential energy surfaces. However, the local optimizations using ab initio calculations significantly increase the computational cost relative to those based on empirical or semi-empirical calculations. Herein, we develop a genetic algorithm program with an incomplete local optimization strategy at the DFT level. Using several representative clusters as test examples, this program showed high efficiency in locating their global minimum points. The low-lying isomers of Ag30 were explored using this program, and the determined global minimum is a prolate spheroidal structure. The elongated spheroidal shape causes degeneracy lifting of the free electron shells, and endows Ag30 with a large HOMO-LUMO gap. The sharp increase of silver clusters' reactivity around the sizes with 30 valence electrons observed in our previous experiments could be correlated with this theoretical figure.

Single atom alloy clusters Agn-1X- (X = Cu, Au; n = 7-20) reacting with O2: Symmetry-adapted orbital model.

Single atom alloy AgCu catalysts have attracted great attention, since doping the single Cu atom introduces narrow free-atom-like Cu 3d states in the electronic structure. These peculiar electronic states can reduce the activation energies in some reactions and offer valuable guidelines for improving catalytic performance. However, the geometric tuning effect of single Cu atoms in Ag catalysts and the structure-activity relationship of AgCu catalysts remain unclear. Here, we prepared well-resolved pristine Agn - as well as single atom alloy Agn-1Cu- and Agn-1Au- (n = 7-20) clusters and investigated their reactivity with O2. We found that replacing an Ag atom in Agn - (n = 15-18) with a Cu atom significantly increases the reactivity with O2, while replacement of an Ag with an Au atom has negligible effects. The adsorption of O2 on Agn - or Agn-1Cu- clusters follows the single electron transfer mechanism, in which the cluster activity is dependent on two descriptors, the energy level of α-HOMO (strong correlation) and the α-HOMO-LUMO gap (weak correlation). Our calculation demonstrated that the cluster arrangements caused by single Cu atom alloying would affect the above activity descriptors and, therefore, regulates clusters' chemical activity. In addition, the observed reactivity of clusters in the representative sizes with n = 17-19 can also be interpreted using the symmetry-adapted orbital model. Our work provides meaningful information to understand the chemical activities of related single-atom-alloy catalysts.

Various bond interactions between NO and anionic gold clusters: a theoretical calculation.

We studied the electronic and geometrical structures of AunNO- (n = 1-20) using the B3LYP method with relatively large basis sets to understand the size-dependent reactivities of Aun- with NO in recent experiments. In most cases, the Aun- in a AunNO- maintains its original geometries, and NO bonds with one gold atom in the N-atop pattern. The theoretical adsorption energy of NO in an even-sized AunNO- is generally larger than those in its odd-sized neighbors, which is consistent with the observed even-odd oscillation in experiments. Various bond interactions are identified between Aun- and NO according to analyses on the NO bond lengths, NO stretching frequencies, charge transfer extents, and densities of states of AunNO-. For the odd-sized AunNO- in their doublet states, the bonds between Aun- and NO can be described as weak dative bonds in the small size range, or even weaker electrostatic interactions for the large ones. For the even-sized AunNO- in their triplet states (with the exception of Au10NO- and Au16NO-), the electron transfer from Aun- to NO forms a triplet NO- and a neutral Aun, which bind together mainly through electrostatic interactions. The lowest-lying structures of Au10NO- and Au16NO- are in their singlets, and the bonds between NO and their gold parts can be described as polar covalent bonds. The stabilities of these two exceptional complexes are enhanced by the closed electron shells formed on their gold parts.

Adsorption and Activation of O2 on Small Gold Oxide Clusters: the Reactivity Dominated by Site-Specific Factors.

We experimentally explored adsorption and activation of O2 on small anionic clusters AuxOy- containing one to five gold atoms and between one and three oxygen atoms using an instrument including a magnetron sputtering cluster source, a micro flow reactor running at low temperature, and a time-of-flight mass spectrometer. Some species, including AuO-, one isomer of Au2O2-, Au3O-, one isomer of Au3O3-, and Au5O2-, can adsorb an O2 molecule. We theoretically explored the structures of these active species and the inert ones appearing in the experiment by combining a structure search strategy based on the genetic algorithm and the density functional theory (DFT) calculations. Impressively, all active species observed in the experiment have a -O-Au site, in which the gold atom is a dangling or a vertex atom. Each -O-Au site can strongly adsorb one O2 with its Au atom to form a straight-line structure -O-Au-O-, and the adsorbed O2 is significantly activated by accepting one electron with one of its π2p* orbitals. With no exception, all oxygen sites and the -O-Au-Au sites in AuxOy- are inert. Analyses on the density of states (DOS) of representative species well interpret the physical origins of the activity of -O-Au and the inertness of -O-Au-Au. The observations that site-specific factors dominate the reactivity of gold oxide clusters with O2 are in contrast to what happens in the reactions of Aun- with O2, where clusters' reactivity is completely determined by their global spins and electron detachment energies. The new conclusions in this work offer a reference to understand the crucial O2 activation processes in gold-based catalysts, since various gold oxide structures are commonly observed in these systems.

Significant π bonding in coinage metal complexes OCTMCCO- from infrared photodissociation spectroscopy and theoretical calculations.

A series of coinage metal complexes in the form of TMC(CO)n - (TM = Cu, Ag, Au; n = 0-3) were generated using a laser-ablation supersonic expansion ion source in the gas phase. Mass-selected infrared photodissociation spectroscopy in conjunction with quantum chemical calculations indicated that the TMC(CO)3 - complexes contain a linear OCTMCCO- core anion. Bonding analyses suggest that the linear OCTMCCO- anions are better described as the bonding interactions between a singlet ground state TM+ metal cation and the OC/CCO2- ligands in the singlet ground state. In addition to the strong ligands to metal σ donation bonding components, the π-bonding components also contribute significantly to the metal-ligand bonds due to the synergetic effects of the CO and CCO2- ligands. The strengths of the bonding of the three metals show a V-shaped trend in which the second-row transition metal Ag exhibits the weakest interactions whereas the third-row transition metal Au shows the strongest interactions due to relativistic effects.

Exploring the Effects of a Doping Silver Atom on Anionic Gold Clusters' Reactivity with O2.

Reactivities of AgAun-1- (n = 3-10) with O2 at a low temperature were studied using an instrument combining a magnetron sputter cluster source, a microflow reactor, and a time-of-flight mass spectrometer. Their reaction products as well as size-dependent kinetic rates were nearly identical to those of corresponding Aun- (n = 3-10). Previous experiments showed that the Ag atom in AgAun-1- (n = 3-10) was fully or partially enclosed by the gold atoms. We studied the adsorption of O2 on these reported structures using the B3LYP theory with relatively large basis sets. The theoretical results indicate that the adsorption sites as well as the adsorption energies of O2 on AgAun-1- (n = 3-10) are nearly identical to those on the corresponding Aun- (n = 3-10). The O2 adsorption on a series of proposed isomers of AgAun-1- (denoted as Aun-1Ag-), in which the silver atom was on the protruding site, was explored using the same theoretical methods. The O2 tends to bond with the protruding Ag atoms, and the binding energies are apparently higher than those on the corresponding Aun- and AgAun-1-. The adsorption and activation of O2 on Aun-, AgAun-1-, and Aun-1Ag- were correlated with their global electron detachment energies (VDEs) as well as the element types of the adsorption sites. Generally, low VDE values and silver sites facilitate the O2 adsorption, and these two factors separately dominate in various cluster species. The revealed effects of a doping silver atom in small gold clusters are helpful to understand the role of the residual silver components in many nano gold catalysts.

Adsorption and reactions of NO molecules on anionic gold clusters in the size range below 1 nm: effects of clusters' global electronic properties.

We systemically studied adsorption and reactions of NO on Aun- (n≤ 80) using a mini flow-tube reactor running at 150 K. For Aun- (n≤ 11), their reactions with NO mainly formed cluster complexes containing various numbers of NO units; for Aun- (n≥ 12), most active sizes eventually formed specific complexes Aun(NO)3-. The relative rates of the reactions with the first NO were measured. Correlations between these relative rates and the adiabatic detachment energies (ADEs) of Aun- revealed the dominant effect of the clusters' spins and a more complicated electron transfer mechanism than that of reactions with O2. Au20- as well as previously reported Au4,6,8- is an exceptional size, which eventually formed the disproportionate product Au20NO2-, and all these four sizes have very low ADEs. The effects of the clusters' global electronic properties on adsorption and reactions of NO on anionic gold are helpful to understand catalytic mechanisms of gold-based catalysts in NO removal reactions.

Infrared photodissociation spectroscopic studies of ScO(H2O)n=1-3Ar+ cluster cations: solvation induced reaction of ScO+ and water.

We investigate the gaseous ScO(H2O)1-3Ar+ cations prepared by laser vaporization coupled with supersonic molecular beam using infrared photodissociation spectroscopy in the O-H stretching region. The cation structures are characterized by comparing the experimentally observed frequencies with the simulated vibration spectra. We reveal that stoichiometric ScO(H2O)Ar+ is intrinsically the hydrated oxide cation expressed as H2O-ScOAr+ hydrate rather than Sc(OH)2Ar+ dihydroxide, although the former is higher in energy by 29.5 kcal mol-1 than the latter. Interestingly, when more water molecules are introduced to the complex, we find that the stoichiometric ScO(H2O)2-3Ar+ embraces the core subunit of Sc(OH)2+. Theoretical calculations suggest that the energy barrier of hydrogen transfer plays a critical role in the isomerization from hydrated complex to dihydroxide. When more than one water molecule is involved in the complex, the hydrogen transfer becomes nearly barrierless through a six-member cyclic transition state, leading to the reduction in the energy barrier from 21.8 kcal mol-1 to 4.2 kcal mol-1. Altogether, we conclude that the solvent molecules such as water can decrease the energy barrier and thus induce the formation of hydroxy species in the hydrolysis process.

Adsorption of O2 on Anionic Gold Clusters in the 0-1 nm Size Range: An Insight into the Electron Transfer Dynamics from Kinetic Measurements.

We systematically studied the adsorption of O2 on Au n- in the size range of 0-1 nm at low temperatures and determined new active sizes with n = 22, 24, 34, and 36. The kinetic measurements more clearly showed the correlation between the reactivity of Au n- with O2 and their electronic properties: the sizes with a closed electron shell are always inert, and the sizes with an unpaired electron can chemically adsorb one O2 molecule if their adiabatic detachment energies (ADEs) are lower than a threshold around 3.5 eV. This ADE threshold dividing the active and inert Au n- is independent of the clusters' sizes, global geometries, and local adsorption sites. According to the widely accepted electron transfer mechanism, this threshold could stand for the case in which the total energy of the Au n- and an O2 roughly equals that of the spin crossover point of the potential surfaces of Au n-O2- and Au n-···O2.

Co-Adsorption of O2 and CO on Au2- : Infrared Photodissociation Spectroscopy and Theoretical Study of [Au2 O2 (CO)n ]- (n=2-6).

The co-adsorption of O2 and CO on anionic sites of gold species is considered as a crucial step in the catalytic CO oxidation on gold catalysts. In this regard, the [Au2 O2 (CO)n ]- (n=2-6) complexes were prepared by using a laser vaporization supersonic ion source and were studied by using infrared photodissociation spectroscopy in the gas phase. All the [Au2 O2 (CO)n ]- (n=2-6) complexes were characterized to have a core structure involving one CO and one O2 molecule co-adsorbed on Au2- with the other CO molecules physically tagged around. The CO stretching frequency of the [Au2 O2 (CO)]- core ion is observed around ν˜ =2032-2042 cm-1 , which is about 200 cm-1 higher than that in [Au2 (CO)2 ]- . This frequency difference and the analyses based on density functional calculations provide direct evidence for the synergy effect of the chemically adsorbed O2 and CO. The low lying structures with carbonate group were not observed experimentally because of high formation barriers. The structures and the stability (i.e., the inertness in a sense) of the co-adsorbed O2 and CO on Au2- may have relevance to the elementary reaction steps on real gold catalysts.

Infrared photodissociation spectroscopy of M(N2)n(+) (M = Y, La, Ce; n = 7-8) in the gas phase.

M(N2)n(+) (M = Y, La, Ce; n = 7-8) complexes have been studied by infrared photodissociation (IRPD) spectroscopy and density functional theory (DFT) calculations. The experimental results indicate that the N-N stretching vibrational frequencies are red-shifted from the gas-phase N2 value. The π back-donation is found to be a main contributor in these systems. IRPD spectra and DFT calculations reveal the coexistence of two isomers in the seven-coordinate M(N2)7(+) and eight-coordinate M(N2)8(+) complexes, respectively. The present studies on these metal-nitrogen complexes shed light on the interactions and coordinations toward N2 with transition and lanthanide metals.

The adsorption and activation of NO on silver clusters with sizes up to one nanometer: interactions dominated by electron transfer from silver to NO.

The adsorption and activation of NO on microsilver species provide the foundation to understand the mechanism of NO removing reactions on silver based catalysts. However, the diversiform of the geometrical structures and electronic properties of microsilver species in condensed phases has posed considerable challenges for exploring these interactions. We study the reactions of NO with bare silver clusters Agn(±) (7-69) in the gas phase using a continuous flow reactor running at low temperatures. Evidence for NO unit adsorption, the formation of (NO)2 and the reduction of NO is observed on different cluster sizes. The kinetic rates of initial NO unit adsorption are closely related to silver clusters' global electronic properties. The low electron binding energy and the unpaired electron of a silver cluster favor the adsorption and activation of NO. In particular, the clusters with one less electron than those of closing electron shells are generally inert and the sizes having one more electron outside these shells are generally quite reactive. These observations depict a general figure of interactions between NO and microsilver species, in which electron transfer from silver to NO dominates.

CO oxidation by the atomic oxygen on silver clusters: structurally dependent mechanisms generating free or chemically bonded CO2.

Atomic oxygen on silver is the crucial active species in many catalytic oxidation processes, while it is a big challenge to explore the relationship between its activity and molecular-level structures in condensed phases. We carried out kinetic measurements of the gas phase reactions between AgnO- (n = 1-8) and CO, in which the oxygen atoms were predicted to be terminal ones in AgO- and Ag2O-, in quasi-Ag-O-Ag chains for Ag3O- and Ag4O-, and on the two-fold or three-fold bridging positions in AgnO- (n = 5-8). All these oxygen species are highly reactive even at a low temperature of 150 K. AgnO- (n = 1, 2, 5-8) with terminal or bridging oxygen generate free CO2, while the quasi-chains of AgnO- (n = 3, 4) generate chemically bonded CO2 with a structural formula of Agn-CO2-Ag2- (n = 1, 2). Density functional theory calculations well interpreted all experimental observations, showing that no extra excitation energies are needed to initiate all these reactions. The structurally dependent mechanisms and the formation of chemically bonded CO2 revealed in this work help us to catch a glimpse of some important processes and intermediates on real silver catalysts.

Adsorption of O2 on anionic silver clusters: spins and electron binding energies dominate in the range up to nano sizes.

Exploring the reactivity of metal clusters is an important task in cluster science, while only a few previous studies involve the reactions of nano-sized ones. Here we report a kinetic measurement on reactions of Ag(n)(-) (n = 6-69) with O2 using a flow reactor running at 120 K. Their relative rates were obtained by fitting decay processes of parent ions at different O2 flow rates. Comparing the variations of the kinetic rates and the photodetachment energies of Ag(n)(-) (i.e. the binding energies of their excess electrons), we distinguished the separate effect of clusters' spins or their electron binding strength. This work firstly shows that reactions of O2 and Ag(n)(-) up to nano sizes are still dominated by the clusters' global electronic properties. This conclusion is conceptually important for understanding the reaction mechanisms on silver based nanocatalysts.