Decomposition of Halogenated Molybdenum Sulfide Dianions [Mo₃S₇X₆]^2- (X = Cl, Br, I)

Molybdenum sulfides are considered a promising and inexpensive alternative to platinum as a catalyst for the hydrogen evolution reaction. In this study, we perform collision-induced dissociation experiments in the gas phase with the halogenated molybdenum sulfides [Mo₃S₇Cl₆]^2-, [Mo₃S₇Br₆]^2-, and [Mo₃S₇I₆]^2-. We show that the first fragmentation step for all three dianions is charge separation via loss of a halide ion. As a second step, further halogen loss competes with the dissociation of a disulfur molecule, whereas the former becomes energetically more favorable and the latter becomes less favorable from chlorine via bromine to iodine. We show that the leaving S₂ group is composed of sulfur atoms from two bridging groups. These decomposition pathways differ drastically from the pure [Mo₃S₁₃]^2- clusters. The obtained insight into preferred dissociation pathways of molybdenum sulfides illustrate possible reaction pathways during the activation of these substances in a catalytic environment.