Gold chloride clusters with Au(iii) and Au(i) probed by FT-ICR mass spectrometry and MP2 theory

Abstract

Microsolvated clusters of gold chloride are probed by electrospray ionization mass spectrometry (ESI-MS) and scalar relativistic electronic structure calculations. Electrospray ionization of aqueous AuCl₃ leads to mononuclear clusters of types [AuCl₂]⁺(H₂O)_n (n = 0–4), [AuOHCl]⁺(H₂O)_n (n = 0–1) and [AuCl₂]⁺(HCl)₂(H₂O)_n (n = 0–4). In addition, strong ion signals due to dinuclear [Au₂Cl_5−xOH_x]⁺(H₂O)_n (x = 0–1) are present in ESI mass spectra of aqueous AuCl₃, with the abundance of individual dinuclear species controlled by the concentration-dependent variation of the precursor complexes [AuCl_2−xOH_x]⁺(H₂O)_n and AuCl₃. Equilibrium structures, energies and thermodynamic properties of mono- and dinuclear gold clusters have been predicted using MP2 and CCSD(T) theory, and these data have been applied to examine the influence of microsolvation on cluster stability. Specifically, results from CCSD(T) calculations indicate that non-covalently bound ion-neutral complexes Au⁺(Cl₂)(H₂O)_n, with formal Au(I), are the dominant forms of mononuclear gold with n = 0–2, while higher hydrates (n > 2) are covalently bound [AuCl₂]⁺(H₂O)_n complexes in which gold exists as Au(III). MP2 calculations show that the lowest energy structure of dinuclear gold is an ion-molecule cluster [Au₂Cl(Cl₂)₂]⁺ consisting of a single-bridged digold-chloronium ion bound end-on to two dichlorine ligands, with two higher energy isomers, single-bridged [Au₂Cl₃(Cl₂)]⁺ and double-bridged [Au₂Cl₅]⁺ clusters. Finally, Au⋯Au interactions in the singly-bridged clusters [Au₂Cl(Cl₂)₂]⁺(H₂O)_n and [Au₂Cl₃(Cl₂)]⁺(H₂O)_n are examined employing a wide range of computational tools, including natural bond order (NBO) analysis and localized orbital locator (LOL) profiles.