Search

Size-selected negatively-charged boron clusters (B n - ) have been found to be planar or quasi-planar in a wide size range. Even though cage structures emerged as the global minimum at B 39 - , the global minimum of B 40 - was in fact planar. Only in the neutral form did the B 40 borospherene become the global minimum. How the structures of larger boron clusters evolve is of immense interest. Here we report the observation of a bilayer B 48 - cluster using photoelectron spectroscopy and first-principles calculations. The photoelectron spectra of B 48 - exhibit two well-resolved features at low binding energies, which are used as electronic signatures to compare with theoretical calculations. Global minimum searches and theoretical calculations indicate that both the B 48 - anion and the B 48 neutral possess a bilayer-type structure with D 2h symmetry. The simulated spectrum of the D 2h B 48 - agrees well with the experimental spectral features, confirming the bilayer global minimum structure. The bilayer B 48 -/0 clusters are found to be highly stable with strong interlayer covalent bonding, revealing a new structural type for size-selected boron clusters. The current study shows the structural diversity of boron nanoclusters and provides experimental evidence for the viability of bilayer borophenes.

Since the discovery of the B 40 borospherene, research interests have been directed to the structural evolution of even larger boron clusters. An interesting question concerns if the borospherene cages persist in larger boron clusters like the fullerenes. Here we report a photoelectron spectroscopy (PES) and computational study on the structures and bonding of B 41 - and B 42 - , the largest boron clusters characterized experimentally thus far. The PE spectra of both clusters display broad and complicated features, suggesting the existence of multiple low-lying isomers. Global minimum searches for B 41 - reveal three low-lying isomers (I-III), which are all related to the planar B 40 - structure. Isomer II (C s , 1 A') possessing a double hexagonal vacancy is found to agree well with the experiment, while isomers I (C s , 3 A'') and III (C s , 1 A') both with a single hexagonal vacancy are also present as minor isomers in the experiment. The potential landscape of B 42 - is found to be much more complicated with numerous low-lying isomers (VII-XII). The quasi-planar structure VIII (C 1 , 2 A) containing a double hexagonal vacancy is found to make major contributions to the observed PE spectrum of B 42 - , while the other low-lying isomers may also be present to give rise to a complicated spectral pattern. Chemical bonding analyses show isomer II of B 41 - (C s , 1 A') and isomer VIII of B 42 - (C 1 , 2 A) are π aromatic, analogous to that in the polycyclic aromatic hydrocarbon C 27 H 13 + (C 2v , 1 A 1 ). Borospherene cage isomers are also found for both B 41 - and B 42 - in the global minimum searches, but they are much higher energy isomers.

Chirality plays an important role in nature. Nanoclusters can also exhibit chiral properties. We report herein a joint experimental and theoretical investigation on the geometric and electronic structures of B31- and B32- clusters, using photoelectron spectroscopy in combination with first-principles calculations. Two degenerate quasi-planar chiral C1 enantiomers (I and II, 1A) with a central hexagonal vacancy are identified as the global minima of B31-. For B32-, two degenerate boat-like quasi-planar chiral C2 structures (VI and VII, 2A) with a central hexagonal vacancy are also found as the global minima, with a low-lying chair-like Ci B32- (VIII, 2Au) also present in the experiment as a minor isomer. The chiral conversions in quasi-planar B31- and B32- clusters are investigated and relatively low barriers are found due to the high flexibility of these monolayer clusters, which feature multiple delocalized σ and π bonds over buckled molecular surfaces.

Inspired by recent observations of the highest coordination numbers of CN = 10 in planar wheel-type complexes in D10h Ta@B10- and CN = 20 in double-ring tubular species in D10d Ta@B20- and theoretical prediction of the smallest endohedral metalloborospherene D2 Ta@B22- (1) with CN = 22, we present herein the possibility of larger endohedral metalloborospherenes C2 Ta@B23 (2), C2 Ta@B24+ (3), C2v Ta@B24- (4), C1 Ta@B25 (5), D2d Ta@B26+ (6), C2 Ta@B272+ (7), and C2 Ta@B283+ (8) based on extensive first-principles theory investigations. These cage-like Ta@Bqn complexes with B6 pentagonal or B7 hexagonal pyramids on their surface turn out to be the global minima of the systems with CN = 23, 24, 24, 25, 26, 27, and 28, respectively, unveiling the highest coordination number of CN = 28 in spherical environments known in chemistry. Detailed bonding analyses show that 1-8 as superatoms conform to the 18-electron configuration with a universal σ + π double delocalization bonding pattern. They are effectively stabilized via spd-π coordination interactions between the Ta center and ηn-Bn ligand which match both geometrically and electronically. Such complexes may serve as embryos of novel metal-boride nanomaterials.

Boron clusters have been found to exhibit a variety of interesting electronic, structural, and bonding properties. Of particular interest are the recent discoveries of the 2D hexagonal B 36 -/0 which led to the concept of borophenes and the 3D fullerene-like B 40 -/0 which marked the onset of borospherene chemistry. Here, we present a joint photoelectron spectroscopic and first-principles study of B 37 - and B 38 - , which are in the transition size range between the 2D borophene-type clusters and the 3D borospherenes. These two clusters are found to possess highly stable 2D global-minimum structures consisting of a double-hexagonal vacancy. Detailed bonding analyses reveal that both B 37 - and B 38 - are all-boron analogues of coronene (C 24 H 12 ) with a unique delocalized π system, featuring dual π aromaticity. These clusters with double hexagonal vacancies can be viewed as molecular motifs for the χ3-borophene which is the most stable form of borophenes recently synthesized on an Ag(111) substrate.

Planar boron clusters B 11 - , B 13 + , B 15 + , and B 19 - have been introduced recently as molecular Wankel motors or tank treads. Here we present a universal mechanism for these dynamically fluxional clusters; that is, they are molecular rotors with inner wheels that rotate almost freely in pseudo-rotating outer bearings, analogous to rotating molecules trapped in pseudo-rotating cages. This mechanism has significant quantum mechanical consequences: the global-minimum structures of the clusters have C 2v symmetry, whereas the wheels rotating in pseudo-rotating bearings generate rosette-type shapes with D 9h , D 10h , D 11h , and D 13h symmetries. The related rotational/pseudo-rotational energies appear with characteristic band structures, effecting the dynamics.