Synthesis of gas-phase, group II metal hydride cations using preparative tandem mass spectrometry

Interest in Main Group metal hydrides remains high because of their potential use as hydrogen storage materials and as green catalysts. However, the high reactivity of metal hydrides along with the solvent effects and complex equilibria of condensed phase studies makes study of their intrinsic chemistry a challenge. In principle, the challenge can be overcome by using ion preparation methods such as collision-induced dissociation to synthesize species within the relatively inert gas-phase environment of ion-trapping mass spectrometers. In this study, preparative (ion trap) tandem mass spectrometry (PTMS n ) was used to generate a series of binary, gas-phase metal hydride cations. In particular, we show that the relatively inert environment of a quadrupole ion trap permits (a) the synthesis of discrete, group II metal hydride cations, [MetH] + (Met = Ca, Sr, Ba) and (b) allows for monitoring of intrinsic reactions with neutral species such as H 2 O and CH 2 Cl 2 . When exposed to H 2 O, the [MetH] + cations undergo a hydrolysis reaction to form the respective cationic, metal hydroxides. When allowed to react with CH 2 Cl 2 , two reaction pathways are observed, one leading to formation of [Met(CHCl 2 )] + (with elimination of neutral H 2 ) and the other producing [MetCl] + (by elimination of CH 3 Cl). The reactions to generate the [MetH] + cations, and the ion-molecule reactions with H 2 O and CH 2 Cl 2 , were modeled using density functional theory (DFT) and 2nd – order Moller-Plesset (MP2) calculations to provide optimized structures of reactants, intermediates, products and transition states. In general, the computed enthalpies and free energies for the reactions are in good qualitative agreement with the experimental outcomes. • Preparative tandem mass spectrometry (PTMS n ) is used to generate gas-phase metal hydride cations by collision-induced dissociation of [Met(O 2 C-H)(H 2 O) 2 ] + (Met = Ca, Sr, Ba). • [MetH] + cations hydrolyze to metal hydroxides. With CH 2 Cl 2 , they yield [Met(CHCl 2 )] + via H 2 loss or [MetCl] + via CH 3 Cl elimination. • In general, computed enthalpies and free energies (DFT and MP2) for the reactions are in good qualitative agreement with the experimental outcomes.