Photoelectron spectroscopy of lithium and gold alloyed boron oxide clusters: charge transfer complexes, covalent gold, hyperhalogen, and dual three-center four-electron hyperbonds

Abstract

We report on the structural and electronic properties and chemical bonding in a series of lithium and gold alloyed boron oxide clusters: B₂O₃⁻, LiB₂O₃⁻, AuB₂O₃⁻, and LiAuB₂O₃⁻. The clusters have been produced by laser vaporization and characterized using photoelectron spectroscopy, in combination with the Coalescence Kick and Basin Hopping global-minimum searches and density-functional theory and molecular orbital theory calculations. Electron affinities of B₂O₃, LiB₂O₃, AuB₂O₃, and LiAuB₂O₃ neutral clusters are measured to be 1.45 ± 0.08, 4.25 ± 0.08, 6.05 ± 0.08, and 2.40 ± 0.08 eV, respectively. The experimental and computational data allow the cluster structures to be established for the anions as well as their neutrals. While B₂O₃⁻ (C_2v) is bent, the three alloy clusters, LiB₂O₃⁻ (C_∞v), AuB₂O₃⁻ (C_s), and LiAuB₂O₃⁻ (C_∞v), adopt linear or quasi-linear geometries with a metal center inserted between BO and OBO subunits, featuring charge transfer complexes, covalent gold, hyperhalogen, and dual three-center four-electron (3c-4e) π hyperbonds. The current results suggest the possibility of altering and fine-tuning the properties of boron oxides via alloying, which may lead to markedly different electronic structures and chemical reactivities. The LiB₂O₃ cluster belongs to the class of oxidizing agents called superhalogens, whereas AuB₂O₃ is a hyperhalogen species. Dual 3c-4e π hyperbonds represent a critical bonding element in boron oxides and are considered to be the root of delocalized bonding and aromaticity therein.