Fortuitous Ion–Molecule Reaction Enables Enumeration of Metal–Hydrogen Bonds Present in Gaseous Ions

Upon mass selection and ion activation under mass spectrometric conditions, gaseous formate adducts of many metal formates undergo decarboxylation and form product ions that bear metal–hydrogen bonds. Fortuitously, we noted that negative-ion spectra of several such formate adducts showed many peaks that could not be rationalized by the conventional fragmentation pathways attributed to the precursor ion. Subsequent experimentation proved that these enigmatic peaks are due to an ion–molecule reaction that takes place between traces of adventitious water vapor in the collision gas and the in situ formed product anions bearing metal–hydrogen bonds, generated by the fragmentation of the formate adducts. Results show that metal–hydrogen bonds of the group 2 elements are particularly susceptible to this reaction. For example, in the product-ion spectrum of [Sr­(η²-O₂CH)₃]⁻, the peak at m/z 91 for SrH₃^– was accompanied by three peaks at higher m/z ratios. These peaks, at m/z 107, 123, and 139, represented SrH₂(OH)₁^–, SrH₁(OH)₂^–, and Sr­(OH)₃^–, respectively. These satellite peaks, which were separated by 16 m/z units, were attributed to adducts formed due to the high affinity of gas-phase anions bearing metal–hydrogen bonds to water. Although undesired, these peaks are diagnostically useful to determine the number of metal–hydrogen bonds present in a precursor ion. Even though the peaks were less pronounced, analogous reactions were noted from the adducts of the group 1 elements as well. Moreover, Gibbs free energy values computed for the interaction of [H-Mg­(η²-O₂CH)₂]⁻ with water to form [HO-Mg­(η²-OCOH)₂]⁻ and H₂ indicated that this is an exergonic reaction.