Sea-shell-like B31+ and B32: two new axially chiral members of the borospherene family

Since the discovery of the cage-like borospherenes D2d B40−/0 and the first axially chiral borospherenes C3/C2 B39−, a series of fullerene-like boron clusters in different charge states have been reported in theory. Based on extensive global minimum searches and first-principles theory calculations, we present herein two new axially chiral members C2 B31+ (I) and C2 B32 (VI) to the borospherene family. B31+ (I) features two equivalent heptagons on the top and one octagon at the bottom on the cage surface, while B32 (VI) possesses two equivalent heptagons on top and two equivalent heptagons at the bottom. Detailed bonding analyses show that both sea-shell-like B31+ (I) and B32 (VI) follow the universal σ + π double delocalization bonding pattern of the borospherene family, with ten delocalized π bonds over a σ skeleton, rendering spherical aromaticity to the systems. Extensive molecular dynamics simulations show that these novel borospherenes are kinetically stable below 1000 K. The IR, Raman, and UV-vis spectra of B31+ (I) and B32 (VI) are computationally simulated to facilitate their future experimental characterizations.


Introduction
As the lighter neighbour of carbon in the periodic table, boron is a typical electron-decient element which shares with carbon the rare ability to form stable covalently bonded molecular frameworks with multicentre-two-electron bonds (mc-2e bonds) in both polyhedral molecules and bulk allotropes. 1,2 Persistent joint photoelectron spectroscopy (PES) experimental and rst-principles theory investigations by Lai-Sheng Wang and co-workers in the past two decades on size-selected negatively-charged boron clusters B n À (n ¼ 3-42) have revealed a rich landscape for boron nanoclusters from planar or quasi-planar (2D) structures (n ¼ 3-38, 41, and 42) to cage-like borospherenes (n ¼ 39, 40). [3][4][5][6][7][8] The rst all-boron fullerenes D 2d B 40 , dubbed borospherenes, were discovered in 2014, marking the onset of borospherene chemistry. 5 The spherically aromatic borospherene D 2d B 40 is found to be composed of twelve interwoven boron double chains with two hexagons at the top and bottom and four heptagons on the waist. It features a unique bonding pattern of s + p double delocalization, with twelve delocalized p bonds spherically distributed over a s skeleton. The axially chiral B 39 À appears to be the only boron cluster monoanion observed in experiments to date which has a cage-like global minimum (GM). 4 The spherically aromatic borospherene family has been expanded by our group at rstprinciples theory level to the cage-like B n q series (n ¼ 36-42, q ¼ n À 40) in different charge states which are all composed of twelve interwoven double chains with a s + p double delocalization bonding pattern. 4,5,[9][10][11][12] Two lowest-lying cage-like C s B 39 + isomers in the same bonding pattern were also predicted in theory. 13 25 with vibrational frequencies checked to make sure all the low-lying isomers obtained were true minima. All these calculations were implemented using the Gaussian 16 program. 26 To obtain more accurate relative energies, the top ve lowest-lying isomers of and B 32 (VI) at 500 K, 700 K, and 1000 K for 30 ps (Fig. S2, ESI †) using the CP2K soware, 35 with the GTH-PBE pseudopotential and the DZVP-MOLOPT-SR-GTH basis set adopted.

Structures and stabilities
As shown in the congurational energy spectrum of B 31 + at the Fig. 2a    bottom (Fig. S1b, ESI †). It is worth noticing that the quasiplanar C s B 32 (VIII) predicted by Nguyen et al. 36 and the double-ring tubular D 16d B 32 (IX) proposed by Zhao's group 37 in Fig. 2b turned out to be the third and fourth lowest-lying isomers of neutral B 32 lying 0.25 eV and 0.32 eV higher in energy than our C 2 GM (VI) at CCSD(T), respectively (Fig. 2b). B 32 (VI) thus has the lowest energy in all the structures obtained to date for neutral B 32 . The much-concerned quasi-planar C 2 B 32 (X) with a B 6 hexagon at the center which corresponds to the experimentally observed third isomer of C 2 B 32 À (ref. 22) appears to be 0.33 eV less stable than B 32 (VI) at CCSD(T) (Fig. 2b). Extensive molecular dynamics (MD) simulations were performed on B 31 + (I) and B 32 (VI) to check the dynamical stabilities of these axially chiral borospherenes. As shown in Fig. S2 (ESI), † both B 31 + (I) and B 32 (VI) appear to be dynamically stable between 500-1000 K. The calculated average root-mean-squaredeviations (RMSD) and maximum bond length deviations (MAXD) of B 31 + (I) are RMSD ¼ 0.07, 0.11, and 0.13 A and MAXD ¼ 0.28, 0.46 and 0.58 A at 500 K, 700 K, and 1000 K, respectively. The corresponding values of B 32 (VI) turn out to be RMSD ¼ 0.07, 0.08 and 0.10 A and MAXD ¼ 0.21, 0.27 and 0.36 A at 500 K, 700 K, and 1000 K, respectively. No high energy isomers are observed in these MD simulation processes.

Bonding pattern analyses
The high thermodynamic and dynamic stabilities of these axially chiral borospherenes originate from their unique electronic structures and bonding patterns. We choose to use the widely used AdNDP approach developed by Boldyrev and coworkers to analyse both the localized and delocalized bonding interactions in these novel species. 29 (Fig. 3a). B 32 (VI) possesses a similar bonding pattern with B 31 + (I) (Fig. 3b). It contains 2Â2c-2e s bonds, 32Â3c-2e s bonds, and 4Â4c-2e s bonds on the cage surface. There exist 10 delocalized p bonds spherically distributed over the s-skeleton, including 6Â4c-2e p bonds and 4Â5c-2e p bonds with ON ¼ 1.78-1.90 |e|, in an overall symmetry of C 2 . Both B 31 + (I) and B 32 (VI) thus possess 10 delocalized p bonds over a s-skeleton and follow the universal s + p double delocalization bonding pattern of the borospherene family. 4,5,[9][10][11][12][13][14][15][16] Detailed bonding analyses further indicate that C s B 32 (VII), the second lowestlying isomer of B 32 , also matches the s + p double delocalization bonding pattern, with 10 delocalized p bonds over a s skeleton (Fig. S3,   Paper RSC Advances C s B 39 + , C 3 B 39 À , C 2 B 39 À , D 2d B 40 , C 1 B 41 + , and C 2 B 42 2+ in different charge states possessing 12 delocalized p bonds, respectively. These borospherenes all appear to be spherically aromatic with the negative calculated NICS values of NICS ¼ À21 to À43 ppm. It is these delocalized p bonds that help to maintain the cage-like structures of the borospherene family and render spherical aromaticity to the systems.

Spectral simulations
Infrared photodissociation (IR-PD) spectra in combination with rst-principles theory calculations have proven to be an effective approach in characterizing novel clusters. 38 (Fig. 4). The strong UV-vis peaks originate from electronic excitations from the deep inner shells to the high-lying unoccupied molecular orbitals of the systems, while the weak absorption bands above 500 nm are attributed to electronic excitations from the occupied frontier orbitals (HOMO and HOMOÀ1) to the unoccupied frontier orbitals (LUMO, LUMO+1, and LUMO+2).

Summary
We have performed in this work an extensive rst-principles theory investigation on sea-shell-like C 2 B 31 + (I) and C 2 B 32 (VI), presenting two new axially chiral members to the borospherene family. These novel borospherenes follow the universal s + p double delocalization bonding pattern of the borospherene family, with 10 delocalized p bonds over an s skeleton on the cage surface, rendering spherical aromaticity to these borospherene species. B 31 + (I) may be characterized in gas-phases IR-PD spectral measurements, while B 32 (VI) may be detected in matrix isolation infrared spectroscopy. 40 More investigations on cage-like B n +/0 clusters are currently in progresses to further expand the borospherene family and enrich borospherene chemistry.

Conflicts of interest
There are no conicts to declare.