Zinc-Group Element-Stabilized Boron Analogues of Cyclopropenyl Cation Featuring π Aromaticity

Abstract

Planar boron clusters often display π-bonding patterns analogous to those found in aromatic hydrocarbons, making them intriguing inorganic counterparts of classical hydrocarbon systems. Herein, we report zinc-group element-stabilized boron analogues of the cyclopropenyl cation (C₃H₃⁺), formulated as M₃B₃⁺ (M = Zn, Cd, and Hg). Global minimum searches reveal that these clusters adopt highly symmetric D_3h structures with a ¹A₁′ electronic ground state and exhibit pronounced dynamic stability. Chemical bonding analyses demonstrate that M₃B₃⁺ reproduces the fundamental bonding motif of the cyclopropenyl cation, consisting of covalent M-B interactions and a three-center two-electron (3c–2e) π bond delocalized over the B₃ core. The resulting π-electron system satisfies Hückel’s (4n + 2) rule, giving rise to π aromaticity, as supported by magnetic aromaticity indices. Notably, in M₃B₃⁺ (M = Zn and Cd), the vertical T₁ excited states exhibit σ-antiaromaticity, highlighting an unusual aromaticity reversal upon electronic excitation.