DMSO Complexes of Trivalent Metal Ions:  First Microsolvated Trications Outside of Group 3

The advent of electrospray ionization source opened the door to generation of multiply charged metal ions complexed with organic molecules. A significant amount of work on ligated dications has appeared over the past decade. In contrast, only several microsolvated tripositive ions have been reported, involving solely the few rare earths with the lowest third ionization energies (IEs) of all elements (<23 eV). Here trications of numerous trivalent metals outside of group 3 are shown to coordinate dimethyl sulfoxide (DMSO), an eminent aprotic solvent. These include both main group elements (Al, Ga, In, Bi) and transition metals (V, Fe, Cr) with the third IE up to 31 eV, which is 22 eV above the IE of DMSO. Fragmentation of M³⁺(DMSO)_n for these metals (plus La, Yb, and Sc) has been characterized in detail using collision-induced dissociation (CID). A rich, highly element specific dissociation chemistry is observed, including the homolytic C−S cleavage in (+3) charge state and various charge-reduction processes, such as dissociative electron and proton transfer and heterolytic SO cleavage with and without a concomitant proton transfer. Characteristic sizes for the charge reduction in M³⁺(DMSO)_n and M²⁺(DMSO)_n have been measured as a function of the relevant elemental IE. These reveal no intrinsic gap between the stabilities of dication and trication complexes, once the IE is adjusted for. This, in particular, suggests that even microsolvated tetracations may exist.