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

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− (C2v) is bent, the three alloy clusters, LiB2O3− (C∞v), AuB2O3− (Cs), 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.