Towards determining molecular structure with ESI-MS backed by computational methods: structures of subnanoclusters of Pd and Cu chlorides, ion dynamics in vacuum, and challenges to the methodology

Abstract

Many practically relevant inorganic solution systems have complex compositions with tens or hundreds of distinct species. Here, we present an approach to analyzing ESI-MS spectra combining a set of scripts for peak assignment and a quantum chemical methodology for the determination of the structure of selected ions. We selected solutions of CuCl, PdCl₂, and the CuCl–PdCl₂ mixture as models of popular precatalysts in cross-coupling reactions and the Wacker process that can form “cocktail”-type systems. The spectra exhibited a great number of signals of mono- and bimetallic oligomeric chloride subnanoclusters. Few oligometallic ions had core–shell structures, according to the computations; the structure of most ions was completely unsymmetric, with bridging Cl⁻ ligands supporting the oligomeric structures. Born-Oppenheimer molecular dynamics showed that some ions were structurally flexible under the selected conditions. Many considered ions exhibited rich configurational and conformational isomerism. The activation (polarization) of the N₂ molecule (from the drying gas used during electrospray ionization) by some ions was determined by the analysis of electron density distributions. For the first time, we describe a flexible approach for semiautomatic analysis of highly complex mass-spectra of organometallic systems in solution with the possibility of revealing molecular structures.