Collision-induced gas-phase reactions of perhalogenated closo-dodecaborate clusters – a comparative study

The gas phase reactivity of perhalogenated closo-dodecaborate clusters [B12X12]2− (X = F, Cl, Br, I) with N-tetraalkylated ammonium counter ions was investigated by electrospray ionization ion trap mass spectrometry (ESI-IT-MS). Collisions with the background gases introduced a broad variety of gas phase reactions. This study represents the first experimental approach to a new class of boron-rich boron clusters that are not accessible in the condensed phase. The anionic ion pair [B12X12 + N(CnH2n+1)4]− is generally found as the ion of highest mass. Its reaction sequence starts with an alkyl transfer from the ammonium ion to the dodecaborate cluster. Subsequently, the alkylated intermediate [B12X12 + CnH2n+1]− decomposes to give very reactive ions of the general formula [B12X11]−. These ions possess a free boron vertex and immediately bind to the residual gases N2 and H2O in the ion trap by formation of the corresponding adducts [B12X11 + N2]− and [B12X11 + H2O]−. Subsequent fragmentations of the water adduct repetitively substitute halogen atoms by hydroxyl groups. The fragmentation process of the free anion [B12X12]2− depends on the applied excitation energy and on the halogen substituent X. A radical dehalogenation of the B12 unit is observed for X = I, whereas for X = Cl or F the loss of small molecules (mainly BX3) dominates. The different reaction behavior is explained by the different electron affinity of the halogens and the strength of the boron–halogen-bonds. Surprisingly, isolation of the fragment ion [B12I9]− in the ion trap yields the highly stable [B24I18]2− dianion. This observation suggests a reaction between two negative ions in the gas phase.