Fragmentation chemistry of DMSO complexes of metal dications

Recent developments in ion sources have enabled generation and mass-spectrometric investigation of multiply charged metal cations coordinated with various ligands. These ions exhibit rich dissociation chemistry, including electron transfer, proton transfer, and ligand cleavage. Dimethyl sulfoxide (DMSO) is the preeminent aprotic solvent. While complexes of two dipositive metal ions with DMSO were observed previously, the fragmentation of these species has remained essentially unknown. Here we present an extensive MS/MS investigation of DMSO complexes of doubly charged divalent metal cations. We determined the minimum sizes at which dications were stable against charge reduction as well as critical sizes above which no electron or proton transfer occurs. For intermediate sizes, low-energy fragmentation pathways were elucidated in detail. Both proton and electron transfers were observed, depending on the metal. We found diverse ligand cleavage channels, the most intense being the sequential scission of one or both carbon-sulfur bonds in one or several DMSO molecules with competitive eliminations of CH3 and CH4 neutrals. This behavior is completely different from that exhibited by acetone complexes of metal dications, or from that of acetonitrile complexes, in which ligand cleavage is always accompanied by charge reduction. Scission of a sulfur-oxygen bond in DMSO forming mostly ligated metal oxide cations is less intense; this is always associated with charge reduction.