From the Superatom Model to a Diverse Array of Super‐Elements: A Systematic Study of Dopant Influence on the Electronic Structure of Thiolate‐Protected Gold Clusters

The electronic properties of doped thiolate‐protected gold clusters are often referred to as tunable, but their study to date, conducted at different levels of theory, does not allow a systematic evaluation of this claim. Here, using density functional theory, the applicability of the superatomic model to these clusters is critically evaluated, and related to the degree of structural distortion and electronic inhomogeneity in the differently doped clusters, with dopant atoms Pd, Pt, Cu, and Ag. The effect of electron number is systematically evaluated by varying the charge on the overall cluster, and the nominal number of delocalized electrons, employed in the superatomic model, is compared to the numbers obtained from Bader analysis of individual atomic charges. We find that the superatomic model is highly applicable to all of these clusters, and is able to predict and explain the changing electronic structure as a function of charge. However, significant perturbations of the model arise due to doping, due to distortions of the core structure of the Au₁₃[RS(AuSR)₂]₆^? cluster. In addition, analysis of the electronic structure indicates that the superatomic character is distributed further across the ligand shell in the case of the doped clusters, which may have implications for the self‐assembly of these clusters into materials. The prediction of appropriate clusters for such superatomic solids relies critically on such quantitative analysis of the tunability of the electronic structure.     

Highly Efficient Circularly Polarized Luminescence from Chiral Au13 Clusters Stabilized by Enantiopure Monodentate NHC Ligands

Chiral Au nanoclusters have promising application prospects in chiral sensing, asymmetric catalysis, and chiroptics. However, enantiopure superatomic homogold clusters with crystallographic structures emitting bright circularly polarized luminescence (CPL) remain challenging. In this study, we designed chiral N-heterocyclic carbenes (NHCs), and for the first time enantioselectively synthesized a pair of monovalent cationic superatomic Au₁₃ clusters. This new enantiomeric pair of clusters has a quasi-C₂ symmetric core and exhibited CPL with an unprecedent solution-state quantum yield (QY) of 61 % among those of the atomically precise Au nanoclusters. DFT calculations provided insights into the circular dichroism behavior, and revealed the origin of CPL from superatomic Au clusters. This work opens a new avenue for developing novel homochiral nanoclusters using chiral NHC ligands and provides fundamental understanding of the origin of the chiroptics of metal clusters.     

Structures and stabilities of 3d-transition metal-benzene organometallic clusters

In the gas phase, we have successfully synthesized organometallic clusters, M_n(benzene)_m (M=3d transition metal atoms), by using a laser vaporization method. The measurements of mass spectra and ionization energies (E_i) have revealed that the organometallic clusters can take two types of structures; layered sandwich structures (m = n + 1) and metal clusters saturatedly covered with benzenes. For early transition metals of Sc, Ti, and V, only the multiple decker sandwich structure clusters were preferentially produced, in which benzene and metal atoms are alternately piled up. For late transition metals of Co and Ni, the metal clusters saturatedly surrounded by benzenes were also produced as well as the sandwich clusters. Furthermore, the E_is of M₁(benzene)₂ (M = Sc-Ni) were systematically measured and their electronic properties will be discussed.     

Understanding and Practical Use of Ligand and Metal Exchange Reactions in Thiolate‐Protected Metal Clusters to Synthesize Controlled Metal Clusters

It is now possible to accurately synthesize thiolate (SR)‐protected gold clusters (Au_n(SR)_m) with various chemical compositions with atomic precision. The geometric structure, electronic structure, physical properties, and functions of these clusters are well known. In contrast, the ligand or metal atom exchange reactions between these clusters and other substances have not been studied extensively until recently, even though these phenomena were observed during early studies. Understanding the mechanisms of these reactions could allow desired functional metal clusters to be produced via exchange reactions. Therefore, we have studied the exchange reactions between Au_n(SR)_m and analogous clusters and other substances for the past four years. The results have enabled us to gain deep understanding of ligand exchange with respect to preferential exchange sites, acceleration means, effect on electronic structure, and intercluster exchange. We have also synthesized several new metal clusters using ligand and metal exchange reactions. In this account, we summarize our research on ligand and metal exchange reactions.     

Mackay,Anti-Mackay,Double-Mackay,Pseudo-Mackay,and Related Icosahedral Shell Clusters

Mackay introduced two important crystallographic concepts in a short paper published 40 years ago. One is the icosahedral shell structure (iss) consisting of concentric icosahedra displaying fivefold rotational symmetry. The number of atoms contained within these icosahedral shells and subshells agrees well with the magic numbers in rare gas clusters, (C₆₀) _N molecules, and some metal clusters determined by mass spectroscopy or simulated on energy considerations. The cluster of 55 atoms within the second icosahedral shell occurs frequently and has been called Mackay icosahedron, or simply MI, which occurs not only in various clusters, but also in intermetallic compounds and quasicrystals. The second concept is the hierarchic icosahedral structures caused by the presence of a stacking fault in the fcc packing of the successive triangular faces in the iss. For instance, a fault occurs after the ABC layers resulting an ABCB packing. This is, in fact, a hierarchic icosahedral structure of a core icosahedron connected to 12 outer icosahedra by vertex sharing, or an icosahedron of icosahedra (double MI. Contrary to Mackay's iss, a faulted hierarchic icosahedral shell is, in fact, a twinlike face capping of the underlying triangles; it is, therefore, called an anti-Mackay cluster. The hierarchic icosahedral structure in an Al-Mn-Pd icosahedral quasicrystal has a core of body-centered cube rather than an icosahedron and, therefore, is called a pseudo-Mackay cluster. The hierarchic icosahedral structures have been studied separately in the past in the fields of clusters, nanoparticles, intermetallic compounds, and quasicrystals, but the underlying geometry should be the same. In the following a unified geometrical analysis is presented.     

Interaction of Silver Clusters with Cholesterol Ligands

The structures of small silver clusters (Ag₂, Ag₃, Ag₁₃) and their interaction with ligands of the cholesterol series, namely, cholesterol (Ch) and thiocholesterol (TCh), are calculated by the density functional theory (DFT) methods and the second-order Møller–Plesset perturbation theory (MP2). Trends in the geometric structure and interaction energy of the silver–cholesterol ligand cluster are estimated depending on the size of the metal cluster and the replacement of cholesterol by the sulfur-containing analogue. For Ch–Ag₁₃ and TCh–Ag₁₃ systems, the geometric structure of the metallic subsystem is a slightly distorted icosahedron with the Ag–Ag bond length ranged 2.8–3.0 Å. Cholesterol is coordinated to the icosahedral facet so that the shortest Ag–O distances range from 2.3 to 2.4 Å. In the case of thiocholesterol, a configuration in which the ligand is coordinated by the edge of the icosahedron with two approximately equal distances of Ag–S (2.537 and 2.547 Å) is preferable.     

Trends in Geometric,Energetic, Electronic,and Magnetic Properties of Vanadium–Copper Clusters Cu<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>V with <Emphasis Type="Italic">n</Emphasis> = 1–12: Density Functional Calculations

A systematic quantum chemical investigation on the geometric, energetic, electronic and magnetic properties of vanadium-copper nanoalloy clusters (n = 1–12) is performed by using BPW91/LanL2DZ calculations. The calculated results show that the structural evolution of Cu _n V clusters favors a compact and icosahedral growth pattern and V atom favors occupying the most highly coordinated position. Energetic properties show that doping of one V atom contributes to strengthening the stability of the copper clusters with the growth of the clusters. The stacking mode of clusters apparently has a more important effect on the clusters stability than the electronic structure. However, electronic structures have some contribution to the stability of Cu _n V clusters as well. The electronic properties of Cu _n V are analyzed through vertical ionization potential (VIP), vertical electron affinity (VEA) and chemical hardness (η). The magnetism calculations show that when doping V atom in copper clusters, the cluster system generate a very large magnetic moment and its contribution mainly comes from the 3d orbital of doping-V atom.     

D6h‐Au42 Isomer: A Golden Aromatic Toroid Involving Superatomic π‐Orbitals that Follow the Hückel (4n+2)π rule

Recently, it has been shown that the superatom concept is intimately connected to relevant tools of great chemical significance, such as the Lewis structure model and the VSEPR theory, which has been employed to understand hybridized and dimeric‐like molecules. This suggests a potential rational construction of superatomic clusters mimicking more complex structures. Here, we extend another well‐employed concept to the superatomic clusters, to construct a novel Au₄₂ isomer with resemblance to cyclic aromatic molecules. It is shown that the Hückel (4n+2)π rule is ready to be applied, predicting aromatic behavior latterly supported by the favorable evaluation of the induced shielding cone formation. The D₆h isomer of Au₄₂ described here exhibits inherent characteristics mimicking aromatic hydrocarbon rings, displaying π‐superatomic orbitals and related properties. This new cluster is the first member of the superatomic clusters family to exhibit an aromatic π‐electron system.     

Semiempirical and ab-initio calculations of semiconductor clusters: variation of structure and symmetry with size for different compounds

Semiempirical (by extended Hückel method) and ab initio RHF SCF calculations are used for the wide range of cluster structures M_xX_y, where M = Cd,Ag; X = S,I: semiempirical - up to M₂₀X₃₅, and ab initio - for small clusters less than ten atoms. Variation of electronic structure with size for the fragments with tetrahedral coordination (bulklike sphalerite structures) and for some clusters of the lower symmetry allows to predict their possible geometries which are compared with experimental data. The chemical bonding factor (the chemical nature of bounded atoms, coordination number for metal and non-metal atoms, hybridization, etc) is of more importance in properties of the clusters than the familiar quantum confinement effect of semiconductor clusters (like CdS, CdSe, PbS, etc. ). The essential difference in regularities of small cluster formation is analysed for CdS- and AgI- based structures.     

Theoretical study of stepwise chlorinated aluminide clusters Al<Subscript>13</Subscript>Cl<Stack><Subscript><Emphasis Type="Italic">n</Emphasis></Subscript><Superscript>−</Superscript></Stack> (<Emphasis Type="Italic">n</Emphasis> = 1–9)

The electronic and geometric structures, energy stabilities, normal mode frequencies, and spin density distributions (in radicals) of different stepwise-chlorinated aluminum clusters Al₁₃Cl _n ^? (n = 1–9) are calculated within the B3LYP approximation of the density functional theory using 6-31G* and 6-311+G* basis sets. The results are compared with analogous computation data on hydrides Al₁₃H _n ^? (n = 1–12) obtained at the same level. The general qualitative pattern for related series of hydrides, chlorides, and iodides (as well as fluorides and bromides) turns out to be similar in many respects. For all Al₁₃X _n ^? clusters with different electronegative substituents X, there is a set of a considerable number of low-lying closely spaced inner isomers (with a centered icosahedral cage), marquee isomers, and outer isomers (capped). The effects found by calculations in centered icosahedral isomers—localization of spin density on the trans-Al* atom in radical anions and its associated trans addition rule for an even substituent and the zigzag (odd-even) dependence of the energies D _n (X) of successive addition of substituents X to the metal cage on n described in the framework of the molecular model of the valence states of the Al ₁₃ ^? superatom—should also be shared by many Al₁₃X _n ^? series with different X’s. The differences between hydrides Al₁₃H _n ^? and chlorides Al₁₃Cl _n ^? of the same type are quantitative. For the hydrides, inner isomers are preferable in the first half of the series (n = 1–6); and in the second half (n = 7–12), outer isomers are more favorable. For the chlorides, icosahedral isomers are preferable only at the very beginning of the series. In the other cases, nonicosahedral structures are most favorable, for which the situation becomes very complicated due to the large number of position isomers and the aforementioned simple rules found for centered icosahedral structures are fulfilled to a considerably less extent or not at all.     

General properties of the electronic structure of alkali metal clusters and Ia-IIa mixed clusters

The results of the systematic ab-initio CI investigation of neutral and charged Li _n , Na _n , BeLi _k and MgNa _k clusters are summarized and analyzed. The general characteristic features of the electronic structure are pointed out:a) The participation of the atomic orbitals, which are empty in Ia and IIa metal atoms, allows for a higher valency of these atoms in clusters.b) Jahn-Teller and pseudo-Jahn-Teller effects strongly influence the electronic and geometric structure of clusters.c) Deformations of cluster geometry can lead to biradicaloid structures with higher spin multiplicity in their ground states.d) The peculiarities of the electronic structures of clusters can be deduced from the presence of many “surface” atoms. The theoretical results agree with experimental data presently available and they are useful for interpretation of the experimental findings.     

Symmetry and ferromagnetic clusters

Isomers of pure Fe₁₃ and icosahedral Fe₁₂X clusters are studied using the all-electron linear-combination-of-Gaussian-type-orbital (LCGTO) local-density-functional (LDF) methods that allow the spin and geometry of the cluster to be determined self-consistently. The Fe₁₃ ground state is icosahedral. The icosahedral cluster also has the greatest magnetic moment because of increased symmetry-required orbital degeneracy for electrons of different spins. The central atom of the icosahedral iron cluster has been varied to optimize the spin of the cluster keeping the oribital contribution to the magnetic moment quenched. Varying the central atom under this constraint can alter the magnetic moment by more than 20%. Similar studies have begun on 55-atom icosahedral iron clusters.     

Theoretical Study of Structural Symmetry in Ternary Clusters

The geometrical symmetry presents an intriguing theoretical problem in many kinds of clusters. The diversity of geometrical structures is associated with cluster sizes, different model functions and potential parameters, and ternary clusters are investigated to study the relationship between geometrical symmetry and homotopic symmetry. Ternary Lennard-Jones model potential is studied with different parameters, and the putative global minimum structures of A₁₃B₃₀C₁₂ clusters are optimized using an adaptive immune optimization algorithm. The results show that there mainly exist five geometrical symmetry structures, i.e., Mackay icosahedral, fivefold partial Mackay icosahedral, sixfold pancake, partial double Mackay icosahedral, and amorphous structures. Furthermore, the number of bonds is used to distinguish the geometrical symmetry. The importance of geometrical symmetry and homotopic symmetry determined by potential parameters is discussed. It was found that in the optimization it is more important to generate geometrical symmetry than to optimize homotopic symmetry.     

Structure and bonding in [M6X8] units of nonmetallic transition metal cluster compounds

A simple quantum mechanical method, based on the Wolfsberg-Helmholtz approximation, has been applied to some nonmetallic4d and5d transition metal cluster compounds containing M₆X₈ units. Our aim was to calculate bonding energies ofM₆ clusters as a function of electronic configuration and symmetry. Energy sequences of clusters containing elements adjacent to each other in the periodic table are discussed, in particular, niobium and molybdenum as well as tungsten and rhenium. Compared with experimentally well-characterized compounds, the computional results show good qualitative agreement. So the occurrence ofM₆ clusters withO_h or lower symmetry can be explained by electronic effects.     

Stabilizing heteroatom-centered 16-vertex group 11 tetrahedral architectures: Bonding and structural considerations toward versatile endohedral species

Density functional theory (DFT) calculations were carried out on a series of clusters made of a centered tetrahedral 16-atom superatomic cage having 20 or 18 jellium electrons (je) and structurally related to [Au₂₀], namely [X@M₁₆] (M = group 11; X = group 2, 4, 12, 14 element). Such species provide further information of how two different electron counts offer a more preferred endohedral situation for specific group elements. Calculations show that the encapsulated atom provides supplementary orbitals to stabilize the bonding M₁₆ MO's. Different favored electron counts are found depending on the nature of the encapsulated atom, as observed by the formation of 20-je species when encapsulating a group 14 element and 18-je species when encapsulating a group 2 element. In addition, the capabilities to enable reactive sites along the cage structure are found via the formation of σ holes at the coinage-metal edges, as shown by their electrostatic potential surface. Such naked species, which constitute an interesting addition to libraries of examples as small models for doped M(111) surfaces of fcc metals, reveal that different superatomic electronic configurations can favor the encapsulation of certain group elements. These results can guide further design of endohedral species.     

Chlorine-passivated superatom Al<Subscript>37</Subscript> clusters for nonlinear optics

Utilizing a facile top-down synthetic procedure, here we report the finding of a chlorine-passivated Al₃₇ superatom cluster. It is demonstrated that the presence of electrophilic groups, severing as protecting ligands, alters the valence electron count of the metallic core and stabilize the as-prepared aluminum clusters especially when even-numbered chlorine atoms are located at equilibrium positions. Following the discussion regarding their reasonable stabilities, we illustrate the feasible reaction pathways in forming such chlorine-passivated Al₃₇ superatom clusters which bear delocalized superatomic orbitals with five valence 3P⁵ electrons shifting to the chlorine ligands indicative of a closed electron shell 2F¹⁴ of the metal core. The successful synthesis of such chlorine-protected aluminum clusters evidences the compatibility of general theory of cluster chemistry in both gas phase and wet chemistry. Such simple-ligand-protected aluminum clusters exhibit reverse-saturated-absorption (RSA) nonlinear optical property pertaining to electronic transitions within the discrete energy states of cluster materials.     

First‐principles study of molecular hydrogen dissociation on doped Al12X (X = B,Al, C,Si, P,Mg, and Ca) clusters

Inspired by the concept of superatom via substitutionally doping an Al₁₃ magic cluster, we investigated the H₂ molecule dissociation on the doped icosahedral Al₁₂X (X = B, Al, C, Si, P, Mg, and Ca) clusters by means of density functional theory. The computed reaction energies and activation barriers show that the concept of superatom is still valid for the catalysis behavior of doped metal clusters. The hydrogen dissociation behavior on metal clusters characterized by the activation barrier and reaction energy can be tuned by controllable doping. Thus, doped Al₁₂X clusters might serve as highly efficient and low‐cost catalysts for hydrogen dissociation. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009     

Electronic Structures of Perovskite Oxides of Transition Metals of the Type LaMO3 (M = Ti-Ni) as Revealed by MSXα Investigations

Systematic investigations of the electronic structures of MO^9-₆ (M = Ti-Ni) clusters, as in the LaMO₃ type perovskite oxides, have been carried out by employing the multiple-scattering Xα (MSXα) method. The crystal-field splitting of the metal d level is found to increase, while the oxygen-to-metal charge-transfer energy decreases across the transition metal series. Systematic trends are also seen in the mixing (covalency) between the metal d and the oxygen 2p orbitals in the series.     

High-Spin and Reactive Fe13 Cluster with Exposed Metal Sites

Atomically defined large metal clusters have applications in new reaction development and preparation of materials with tailored properties. Expanding the synthetic toolbox for reactive high nuclearity metal complexes, we report a new class of Fe clusters, Tp*₄W₄Fe₁₃S₁₂ , displaying a Fe₁₃ core with M−M bonds that has precedent only in main group and late metal chemistry. M₁₃ clusters with closed shell electron configurations can show significant stability and have been classified as superatoms. In contrast, Tp*₄W₄Fe₁₃S₁₂ displays a large spin ground state of S=13. This compound performs small molecule activations involving the transfer of up to 12 electrons resulting in significant cluster rearrangements.