Wei- Li

Observation of an all-boron fullerene

After the discovery of fullerene-C60, it took almost two decades for the possibility of boron-based fullerene structures to be considered. So far, there has been no experimental evidence for these nanostructures, in spite of the progress made in theoretical investigations of their structure and bonding. Here we report the observation, by photoelectron spectroscopy, of an all-boron fullerene-like cage cluster at B40(-) with an extremely low electron-binding energy. Theoretical calculations show that this arises from a cage structure with a large energy gap, but that a quasi-planar isomer of B40(-) with two adjacent hexagonal holes is slightly more stable than the fullerene structure. In contrast, for neutral B40 the fullerene-like cage is calculated to be the most stable structure. The surface of the all-boron fullerene, bonded uniformly via delocalized σ and π bonds, is not perfectly smooth and exhibits unusual heptagonal faces, in contrast to C60 fullerene.

Planar hexagonal B 36 as a potential basis for extended single-atom layer boron sheets

Boron is carbons neighbour in the periodic table and has similar valence orbitals. However, boron cannot form graphene-like structures with a honeycomb hexagonal framework because of its electron deficiency. Computational studies suggest that extended boron sheets with partially filled hexagonal holes are stable; however, there has been no experimental evidence for such atom-thin boron nanostructures. Here, we show experimentally and theoretically that B36 is a highly stable quasiplanar boron cluster with a central hexagonal hole, providing the first experimental evidence that single-atom layer boron sheets with hexagonal vacancies are potentially viable. Photoelectron spectroscopy of B36(-) reveals a relatively simple spectrum, suggesting a symmetric cluster. Global minimum searches for B36(-) lead to a quasiplanar structure with a central hexagonal hole. Neutral B36 is the smallest boron cluster to have sixfold symmetry and a perfect hexagonal vacancy, and it can be viewed as a potential basis for extended two-dimensional boron sheets.

The B35 Cluster with a Double-Hexagonal Vacancy: A New and More Flexible Structural Motif for Borophene

Elemental boron is electron-deficient and cannot form graphene-like structures. Instead, triangular boron lattices with hexagonal vacancies have been predicted to be stable. A recent experimental and computational study showed that the B36 cluster has a planar C6v structure with a central hexagonal hole, providing the first experimental evidence for the viability of atom-thin boron sheets with hexagonal vacancies, dubbed borophene. Here we report a boron cluster with a double-hexagonal vacancy as a new and more flexible structural motif for borophene. Photoelectron spectrum of B35(-) displays a simple pattern with certain similarity to that of B36(-). Global minimum searches find that both B35(-) and B35 possess planar hexagonal structures, similar to that of B36, except a missing interior B atom that creates a double-hexagonal vacancy. The closed-shell B35(-) is found to exhibit triple π aromaticity with 11 delocalized π bonds, analogous to benzo(g,h,i)perylene (C22H12). The B35 cluster can be used to build atom-thin boron sheets with various hexagonal hole densities, providing further experimental evidence for the viability of borophene.

Experimental and Theoretical Evidence of an Axially Chiral Borospherene

Chirality plays an important role in chemistry, biology, and materials science. The recent discovery of the B40(-/0) borospherenes marks the onset of a class of boron-based nanostructures. Here we report the observation of axially chiral borospherene in the B(39)(-) nanocluster on the bases of photoelectron spectroscopy, global minimum searches, and electronic structure calculations. Extensive structural searches in combination with density functional and CCSD(T) calculations show that B(39)(-) has a C3 cage global minimum with a close-lying C2 cage isomer. Both the C3 and C2 B(39)(-) cages are chiral with degenerate enantiomers. The C3 global minimum consists of three hexagons and three heptagons around the vertical C3 axis. The C2 isomer is built on two hexagons on the top and at the bottom of the cage with four heptagons around the waist. Both the C3 and C2 axially chiral isomers of B(39)(-) are present in the experiment and contribute to the observed photoelectron spectrum. The chiral borospherenes also exhibit three-dimensional aromaticity, featuring σ and π double delocalization for all valence electrons. Molecular dynamics simulations reveal that these chiral B(39)(-) cages are structurally fluxional above room temperature, compared to the highly robust D(2d)B40 borospherene. The current findings add chiral members to the borospherene family and indicate the structural diversity of boron-based nanomaterials.

B22- and B23-: all-boron analogues of anthracene and phenanthrene.

Clusters of boron atoms exhibit intriguing size-dependent structures and chemical bonding that are different from bulk boron and may lead to new boron-based nanostructures. We report a combined photoelectron spectroscopic and ab initio study of the 22- and 23-atom boron clusters. The joint experimental and theoretical investigation shows that B(22)(-) and B(23)(-) possess quasi-planar and planar structures, respectively. The quasi-planar B(22)(-) consists of fourteen peripheral atoms and eight interior atoms in a slightly buckled triangular lattice. Chemical bonding analyses of the closed-shell B(22)(2-) species reveal seven delocalized π orbitals, which are similar to those in anthracene. B(23)(-) is a perfectly planar and heart-shaped cluster with a pentagonal cavity and a π-bonding pattern similar to that in phenanthrene. Thus, B(22)(-) and B(23)(-), the largest negatively charged boron clusters that have been characterized experimentally to date, can be viewed as all-boron analogues of anthracene and phenanthrene, respectively. The current work shows not only that boron clusters are planar at very large sizes but also that they continue to yield surprises and novel chemical bonding analogous to specific polycyclic aromatic hydrocarbons.

Transition-Metal-Centered Monocyclic Boron Wheel Clusters (M©Bn): A New Class of Aromatic Borometallic Compounds

Atomic clusters have intermediate properties between that of individual atoms and bulk solids, which provide fertile ground for the discovery of new molecules and novel chemical bonding. In addition, the study of small clusters can help researchers design better nanosystems with specific physical and chemical properties. From recent experimental and computational studies, we know that small boron clusters possess planar structures stabilized by electron delocalization both in the σ and π frameworks. An interesting boron cluster is B(9)(-), which has a D(8h) molecular wheel structure with a single boron atom in the center of a B(8) ring. This ring in the D(8h)-B(9)(-) cluster is connected by eight classical two-center, two-electron bonds. In contrast, the clusters central boron atom is bonded to the peripheral ring through three delocalized σ and three delocalized π bonds. This bonding structure gives the molecular wheel double aromaticity and high electronic stability. The unprecedented structure and bonding pattern in B(9)(-) and other planar boron clusters have inspired the designs of similar molecular wheel-type structures. But these mimics instead substitute a heteroatom for the central boron. Through recent experiments in cluster beams, chemists have demonstrated that transition metals can be doped into the center of the planar boron clusters. These new metal-centered monocyclic boron rings have variable ring sizes, M©B(n) and M©B(n)(-) with n = 8-10. Using size-selected anion photoelectron spectroscopy and ab initio calculations, researchers have characterized these novel borometallic molecules. Chemists have proposed a design principle based on σ and π double aromaticity for electronically stable borometallic cluster compounds, featuring a highly coordinated transition metal atom centered inside monocyclic boron rings. The central metal atom is coordinatively unsaturated in the direction perpendicular to the molecular plane. Thus, chemists may design appropriate ligands to synthesize the molecular wheels in the bulk. In this Account, we discuss these recent experimental and theoretical advances of this new class of aromatic borometallic compounds, which contain a highly coordinated central transition metal atom inside a monocyclic boron ring. Through these examples, we show that atomic clusters can facilitate the discovery of new structures, new chemical bonding, and possibly new nanostructures with specific, advantageous properties.

A photoelectron spectroscopy and ab initio study of B21−: Negatively charged boron clusters continue to be planar at 21

The structures and chemical bonding of the B(21)(-) cluster have been investigated by a combined photoelectron spectroscopy and ab initio study. The photoelectron spectrum at 193 nm revealed a very high adiabatic electron binding energy of 4.38 eV for B(21)(-) and a congested spectral pattern. Extensive global minimum searches were conducted using two different methods, followed by high-level calculations of the low-lying isomers. The global minimum of B(21)(-) was found to be a quasiplanar structure with the next low-lying planar isomer only 1.9 kcal/mol higher in energy at the CCSD(T)/6-311-G* level of theory. The calculated vertical detachment energies for the two isomers were found to be in good agreement with the experimental spectrum, suggesting that they were both present experimentally and contributed to the observed spectrum. Chemical bonding analyses showed that both isomers consist of a 14-atom periphery, which is bonded by classical two-center two-electron bonds, and seven interior atoms in the planar structures. A localized two-center two-electron bond is found in the interior of the two planar isomers, in addition to delocalized multi-center σ and π bonds. The structures and the delocalized bonding of the two lowest lying isomers of B(21)(-) were found to be similar to those in the two lowest energy isomers in B(19)(-).

Aromatic Metal-Centered Monocyclic Boron Rings: Co©B8− and Ru©B9−†

Bulk boron, which is characterized by 3D cage-like structural features, is a refractory material. 2] However, 3D cage structures were suggested to be unstable for small boron clusters, and planar or quasi-planar structures have been proposed instead. Experimental studies combined with high-level calculations have shown that small boron cluster ions are planar up to at least B20 , whereas Bn + ions have been found to be planar up to n = 16. The chemical bonding in the planar boron clusters has been found to be quite remarkable; in addition to the strong and localized bonding in the circumferences, there are two types of delocalized bonding—the in-plane s and the out-of-plane p bonding, each of which follows the (4N + 2) H ckel rule for aromaticity. In particular, systems with six s and six p electrons (N = 1) are doubly aromatic, and give rise to highly symmetric planar clusters, such as B8 2 and B9 , which each contain a central B atom and a B7 and B8 monocyclic ring, respectively. In the D7h B8 2 and D8h B9 molecular wheels, each B atom in the circumference contributes two electrons to the B–B peripheral covalent bonds and one electron to the delocalized bonds, whereas the central B atom contributes all its valence electrons to the delocalized bonds. These novel bonding situations suggest that other atoms with appropriate numbers of valence electrons and sizes may be able to replace the central boron atom to produce M Bn-type clusters. [12]

Observation of the Highest Coordination Number in Planar Species: Decacoordinated Ta©B10− and Nb©B10− Anions

Coordination number is one of the most fundamental characteristics of molecular structures. Molecules with high coordination numbers often violate the octet and the 18 electron rules and push the boundary of our understanding of chemical bonding and structures. We have been searching for the highest possible coordination number in a planar species with equal distances between the central atom and all peripheral atoms. To successfully design planar chemical species with such high coordination one must take into account both mechanical and electronic factors. The mechanical factor requires the right size of the central atom to fit into the cavity of a monocyclic ring. The electronic factor requires the right number of valence electrons to achieve electronic stability of the high-symmetry structure. Boron is known to form highly symmetric planar structures owing to its ability to participate simultaneously in localized and delocalized bonding. The planar boron clusters consist of a peripheral ring featuring strong two-center-two-electron (2c-2e) B–B s bonds and one or more central atoms bonded to the outer ring through delocalized s and p bonds. The starting point for the present work is that the bare eight-atom and nine-atom planar boron clusters were found to reach coordination number seven in the D7h B8 neutral or B8 2 as a part of the LiB8 cluster 3] or eight in the D8h B9 molecular wheel. The CB6 2 , C3B4, and CB7 wheel-type structures with hexaand heptacoordinated carbon atom were first considered computationally by Schleyer and co-workers. The high symmetry hypercoordinated structures were found to be local minima because they “fulfill both the electronic and geometrical requirements for good bonding”. 9] In particular, Schleyer and co-workers pointed out that the wheel structures are p aromatic with 6 p electrons. In joint photoelectron spectroscopy (PES) and theoretical studies it was shown that carbon occupies the peripheral position in such clusters rather than the center, because C is more electronegative than B and thus prefers to participate in localized 2c-2e s bonding, which is possible only at the circumference of the wheel structures. 11] A series of planar wheel-type boron rings with a main group atom in the center and coordination numbers 6– 10 have been probed theoretically. So far the joint PES and ab initio studies of aluminum-doped boron clusters showed that the aluminum atom avoids the central position in the AlB6 , AlB7 , AlB8 , AlB9 , AlB10 , and AlB11 systems. Recently, a transition-metal-doped boron cluster, Ru B9 , with the highest coordination number known to date was reported. We developed a chemical bonding model, which allows the design of planar molecules with high coordination numbers. According to the model, 2n electrons in the M Bn species form n 2c-2e peripheral B–B s bonds. The remaining valence electrons form two types of delocalized bonding, in-plane s and out-of-plane p bonding, and therefore, should satisfy the (4N + 2) H ckel rule separately for s and p aromaticity to attain highly symmetric structures with high electronic stability. In pure wheel-type boron clusters each B atom in the circumference contributes two electrons to the B–B peripheral covalent bonds and one electron to the delocalized bonds, whereas the central B atom contributes all its valence electrons to the delocalized bonds. Thus, out of 26 valence electrons in B8 2 or 28 in B9 , 14 or 16 valence electrons form peripheral covalent 2c-2e s bonds, leaving six s and six p electrons (N = 1 for the 4N + 2 rule) for double (s and p) aromaticity. However, pure planar boron clusters cannot go beyond coordination number eight because of the mechanical factor (the small size of the central boron atom). For example, the B10 cluster does not contain a ninecoordinated boron atom, because the boron atom is too small to fit in the central position of a B9 ring. [2] Since the central atom participates only in delocalized bonding, atoms more electronegative than boron such as carbon avoid the central position. 11,19] Transition-metal atoms, on the other hand, are well-suited for the central position in M Bn species. To satisfy the peripheral B B bonding and the s and p H ckel aromaticity for N = 1, the electronic requirement for the central atom in high-symmetry species, such as M Bn k , is x = 12 n k, where x is the valence of the transition-metal atom M. Ru B9 satisfies the formula and is the first example of an [*] T. R. Galeev, Prof. Dr. A. I. Boldyrev Department of Chemistry and Biochemistry Utah State University, Logan, UT 84322 (USA) E-mail: a.i.boldyrev@usu.edu Homepage: http://www.chem.usu.edu/~boldyrev/

[B30]−: A Quasiplanar Chiral Boron Cluster†

Chirality is vital in chemistry. Its importance in atomic clusters has been recognized since the discovery of the first chiral fullerene, the D2 symmetric C76. A number of gold clusters have been found to be chiral, raising the possibility to use them as asymmetric catalysts. The discovery of clusters with enantiomeric structures is essential to design new chiral materials with tailored chemical and physical properties. Herein we report the first inherently chiral boron cluster of [B30](-) in a joint photoelectron spectroscopy and theoretical study. The most stable structure of [B30](-) is found to be quasiplanar with a hexagonal hole. Interestingly, a pair of enantiomers arising from different positions of the hexagonal hole are found to be degenerate in our global minimum searches and both should co-exist experimentally because they have identical electronic structures and give rise to identical simulated photoelectron spectra.