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

Abstract

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(C_nH_2n+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 + C_nH_2n+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.