Concentric Inner 2π/6σ and Outer 10π/14σ Aromaticity Underlies the Dynamic Structural Fluxionality of Planar B₁₉^– Wankel Motor Cluster

Abstract

Planar C 2v B19 – global-minimum (GM) cluster is known as a molecular Wankel motor, featuring unique chemical bonding and structural fluxionality. While the geometry, bonding, and molecular dynamics of the cluster are documented in the literature, it remains warranted to fully understand its bonding nature and unravel the mechanism behind the structural dynamics. We shall offer herein an updated bonding model on the bases of canonical molecular orbital (CMO) analysis and adaptive natural density partitioning (AdNDP), further aided by natural bond orbital (NBO) analysis and orbital composition calculations. The computational data indicate that the B19 – cluster has inner 2π/6σ and outer 10π/14σ concentric 4-fold π/σ aromaticity. Being spatially isolated from each other, the inner B6 disk supports 2π and 6σ subsystems, whereas the outer B18 double-ring ribbon has 10π and 14σ subsystems. All 4-fold π/σ subsystems are intrinsically delocalized and conform to the (4n + 2) Hückel rule for aromaticity. The change of Wiberg bond index (WBI) from GM to transition-state (TS) for radial B–B links is minimal and uniform, which offers a semiquantitative measure of structural dynamics and underlies the low energy barrier.

Aromaticity of Singlet and Triplet Boron Disk-like Clusters: A Test for Electron Counting Aromaticity Rules

“…This finding can be rationalized using the AdNDP bond analysis which showed that for all disk-like clusters having n out B atoms along the outermost ring, there are exactly 2n out σ 2c−2e B−B bonds. 27,29 The population of the other subshell can also be predicted by the simple counting rules which have been previously formulated. 26 In the following discussion, the studied boron clusters were grouped based on similarities in their electronic and geometrical characteristics.…”

“…Using the adaptive natural density partitioning (AdNDP) method, 53 Yan et al 27 showed that 26 σ electrons contribute to the bonding along the outer B 13 ring, 10 σ electrons are delocalized inside the pentagonal B 6 unit, and the rest 10 σ electrons are delocalized between the inner and outer rings. Recently, using the AdNDP and canonical MO (CMO) analyses, Li et al 29 proposed a somewhat different bonding situation in B 19 − , claiming that there are 6 σ electrons delocalized inside the central B 6 unit and 14 σ electrons which are delocalized between the inner and outer rings.…”

“…Understanding the electronic structure and chemical bonding in boron clusters has become a very challenging task. − The aromaticity has been widely employed to rationalize the structure, reactivity, and stability of boron clusters. − The studies of electronic properties of boron-based complexes have further evolved the aromaticity concept. Boron clusters can exhibit aromaticity, antiaromaticity, disk, double, or even conflicting aromaticity. ,, The aromaticity concept is traditionally related to organic molecules, and several simple electron counting rules have been established, which can instantly provide molecular aromaticity characterization. , The well-known Hückel rule states that the 4 n + 2 monocyclic annulenes exhibit aromaticity, while 4 n ones exhibit antiaromaticity .…”

Spatial and Electronic Structures of BeB8 and MgB8: How far Does the Analogy Go?

“…[38][39][40][41][42] Many boron-based clusters exhibit two orthogo-nal sets of orbitals, which can cause double, or even conflicting aromaticity. [39,40,[43][44][45][46][47] Another unique characteristic of boron clusters is the disk aromaticity which can be found in planar and quasi-planar pure boron clusters. [15,48,49] There are several electron counting rules that can be used to predict and rationalize the aromaticity and antiaromaticity of a given system.…”

“…Aromatic character of boron clusters has been much studied in the past [38–42] . Many boron‐based clusters exhibit two orthogonal sets of orbitals, which can cause double, or even conflicting aromaticity [39,40,43–47] . Another unique characteristic of boron clusters is the disk aromaticity which can be found in planar and quasi‐planar pure boron clusters [15,48,49] .…”