B₃₃^– and B₃₄^–: Aromatic Planar Boron Clusters with a Hexagonal Vacancy

Abstract

Systematic experimental and theoretical studies have shown that anionic boron clusters (B n -) possess planar or quasi-planar (2D) structures in a wide range of cluster size. The 2D structures consist of B 3 triangles often decorated with tetragonal and pentagonal defects. As n increases, hexagonal vacancies appear to be a key structural feature, which underlies the stability of borophenes. The correlation of the defects with cluster size is important to understand the stability and structural evolution of boron clusters. Here we report an investigation of the structures and chemical bonding of B 33 -and B 34 -using photoelectron spectroscopy (PES) and density functional

Fluxional Bonds in Planar B19−, Tubular Ta@B20−, and Cage‐Like B39−

“…This observation provides spectroscopic evidence for the pseudo‐rotation of the B 6 pentagonal unit inside B 13 outer bearing, suggesting the existence of fluxional bonds in B 19 − , similar to the situation observed in B 13 + in IR‐PD measurements . B 19 − is the first planar boron monoanion with fluxional bonds observed in PES measurements …”

“…They could have been buried in the measured broad PES features of B 39 − , again suggesting that M 1 and M 2 may coexist as minor isomers with the two lowest‐lying C 3 B 39 − and C 2 B 39 − in PES experiments. Fluxional B 39 − is the only monoanion boron cluster observed to date which possesses a cage‐like ground‐state structure . Fluxional bonds help to stabilize cage‐like B 39 − monoanions, similar to the situations in planar molecular Wankel motors B 11 − , B 13 + , B 15 + , and B 19 − .…”

“…Detailed bonding analyses were performed using the adaptive natural density partitioning (AdNDP) method which recovers both the localized and delocalized bonding elements of the concerned systems. AdNDP has been successfully applied to a wide range of boron and boron‐based nanoclusters . The reaction rate constant k from a true minimum to its transition state neighbor was calculated using Eyring standard transition state theory …”

“…Boron has a strong propensity to form delocalized multi‐center‐two‐electron (mc‐2e) bonds in both bulk allotropes and polyhedral molecules due to its prototypical electron‐deficiency . Persistent joint photoelectron spectroscopy (PES) and first‐principles theory investigations on small monoanionic boron clusters in the past decade have unveiled a rich landscape from planar or quasi‐planar B n −/0 ( n = 3–30, 33–38) to cage‐like borospherenes D 2d B 40 −/0 and C 3 /C 2 B 39 − which are all characterized with delocalized multicenter σ and π bonds . Delocalized bonds have also been reported in metal‐doped boron clusters.…”

Fluxional bonds in quasi‐planar and half‐sandwich (M = K, Rb, and Cs)

“…Fluxional bonds also exist in other planar boron nanoclusters including C 2v B 11 − , C 2v , and C 2v , which all exhibit fluxional behaviors with extremely low‐energy barriers on the potential energy surfaces. Persistent joint photoelectron spectroscopy (PES) and first‐principles theory investigations in the past decade have revealed a rich landscape for small boron nanoclusters from planar or quasi‐planar molecular sheets ( n = 3–30, 33–38) to cage‐like borospherenes ( n = 39, 40) which are all characterized with delocalized multicenter σ‐ and π‐bonds (mc‐2e σ‐ or π‐bonds, m ≥ 3) . Delocalized multicenter bonds also exist in transition‐metal‐doped boron nanoclusters .…”