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

Abstract

The gas phase reactivity of perhalogenated closo-dodecaborate clusters [B(12)X(12)](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 [B(12)X(12) + N(C(n)H(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 [B(12)X(12) + C(n)H(2n+1)](-) decomposes to give very reactive ions of the general formula [B(12)X(11)](-). These ions possess a free boron vertex and immediately bind to the residual gases N(2) and H(2)O in the ion trap by formation of the corresponding adducts [B(12)X(11) + N(2)](-) and [B(12)X(11) + H(2)O](-). Subsequent fragmentations of the water adduct repetitively substitute halogen atoms by hydroxyl groups. The fragmentation process of the free anion [B(12)X(12)](2-) depends on the applied excitation energy and on the halogen substituent X. A radical dehalogenation of the B(12) unit is observed for X = I, whereas for X = Cl or F the loss of small molecules (mainly BX(3)) 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 [B(12)I(9)](-) in the ion trap yields the highly stable [B(24)I(18)](2-) dianion. This observation suggests a reaction between two negative ions in the gas phase.

From an Icosahedron to a Plane: Flattening Dodecaiodo‐dodecaborate by Successive Stripping of Iodine

“…For full details, see ref. 8. Bottom: Icosahedral and planar structures, showing the B atom numbering used.…”

“…Recently, we prepared B 12 ions with any number of iodine substituents from 12 to one, by fragmenting B 12 I 12 2− through electrospray ionization–ion‐trap mass spectrometry (ESI‐MS) 8. At the end of the fragmentation path, B 12 I 1 − loses a neutral B 12 unit, thus leaving behind I − (Figures 1 and 2).…”

“…At the end of the fragmentation path, B 12 I 1 − loses a neutral B 12 unit, thus leaving behind I − (Figures 1 and 2). 8…”

The effects of halogen elements on the opening of an icosahedral B12 framework

“…B12X12 2− ) is often studied by the electrospray ionization mass spectrometry (ESI-MS). 18 The ions produced by ESI-MS often can capture internal energy in the range of 100 -200 kJ mol −1 , which corresponds to temperature of several thousands Kelvin. Therefore, we computed the Gibbs free energies, ΔG, for both open-and close-B12X7 − at 300, 1500, and 5000 K, through the harmonic approximations for the minimum structures on the potential energy surface at 0 K. The Gibbs free energies were computed by ΔG = Gico -Gope = ΔU + RT -TΔS (U is the internal energy, T is the temperature, R is the gas constant and S is the entropy), and the data are given in Table 1.…”

“…Previously, a complete series of partly iodinated B12In x− (with n = 1 to 12, x = 1 or 2) was successfully generated in the gas-phase and was studied by mass spectroscopy. 18 Together with theoretical calculations, it was shown that the most stable structures for n ≥ 8 remain the icosahedral B12 cages. The planar structures are preferred for n ≤ 4.…”