Mathias Keßler

Synthesis, Crystal Structure, and Reactivity of the Strong Methylating Agent Me2B12Cl12

The efficiency of methylating reagents strongly depends on the weakly coordinating properties of the anion. The introduction of carborane anions [CHB11R5X6] (R = Me, Cl; X = Cl, Br) and the synthesis of the methylating agents Me(CHB11Me5X6) (X = Cl, Br) by Reed was a recent breakthrough. The replacement of triflate anions by the more weakly coordinating carborane anions [CHB11R5X6] (R = Me, Cl; X = Cl, Br) significantly increased the methylating power. Me(CHB11Me5X6) (X = Cl, Br) methylates benzene and converts alkanes into the corresponding alkyl cations with concomitant formation of methane. Very recently, perhalogenated dodecaborate cluster anions [B12X12] 2 (X = F, Cl) came to attention as weakly coordinating dianions. Improved syntheses for [B12F12] 2 [3b] and [B12Cl12] 2 [4a] have been developed and make these dianions available on a large scale. They have been applied to stabilize unusual dications and the first diprotic superacid H2B12Cl12. [4c] These anions are thus of great interest as weakly coordinating dianions for methylating agents and stabilization of the resulting cations. We therefore attempted to methylate the perchlorinated dodecaborate cluster [B12Cl12] 2 and explore its properties. In a well-known reaction, methyl fluoride was treated with the Lewis acid AsF5 in liquid sulfur dioxide at temperatures below 30 8C to give [MeOSO][AsF6] [Eq. (1)], which can be subsequently used to methylate very weak donor molecules. Treatment of [MeOSO][AsF6], prepared in situ, with M2[B12Cl12] (M = Li, Na, K) in liquid sulfur dioxide at 70 8C yielded methylated [B12Cl12] [Eq. (2)].

On the Oxidation of the Three‐Dimensional Aromatics [B12X12]2− (X=F, Cl, Br, I)

The perhalogenated closo-dodecaborate dianions [B12 X12 ](2-) (X=H, F, Cl, Br, I) are three-dimensional counterparts to the two-dimensional aromatics C6 X6 (X=H, F, Cl, Br, I). Whereas oxidation of the parent compounds [B12 H12 ](2-) and benzene does not lead to isolable radicals, the perhalogenated analogues can be oxidized by chemical or electrochemical methods to give stable radicals. The chemical oxidation of the closo-dodecaborate dianions [B12 X12 ](2-) with the strong oxidizer AsF5 in liquid sulfur dioxide (lSO2 ) yielded the corresponding radical anions [B12 X12 ](⋅-) (X=F, Cl, Br). The presence of radical ions was proven by EPR and UV/Vis spectroscopy and supported by quantum chemical calculations. Use of an excess amount of the oxidizing agent allowed the synthesis of the neutral perhalogenated hypercloso-boranes B12 X12 (X=Cl, Br). These compounds were characterized by single-crystal X-ray diffraction of dark blue B12 Cl12 and [Na(SO2 )6 ][B12 Br12 ]⋅B12 Br12 . Sublimation of the crude reaction products that contained B12 X12 (X=Cl, Br) resulted in pure dark blue B12 Cl12 or decomposition to red B9 Br9 , respectively. The energetics of the oxidation processes in the gas phase were calculated by DFT methods at the PBE0/def2-TZVPP level of theory. They revealed the trend of increasing ionization potentials of the [B12 X12 ](2-) dianions by going from fluorine to bromine as halogen substituent. The oxidation of all [B12 X12 ](2-) dianions was also studied in the gas phase by mass spectrometry in an ion trap. The electrochemical oxidation of the closo-dodecaborate dianions [B12 X12 ](2-) (X=F, Cl, Br, I) by cyclic and Osteryoung square-wave voltammetry in liquid sulfur dioxide or acetonitrile showed very good agreement with quantum chemical calculations in the gas phase. For [B12 X12 ](2-) (X=F, Cl, Br) the first and second oxidation processes are detected. Whereas the first process is quasi-reversible (with oxidation potentials in the range between +1.68 and +2.29 V (lSO2 , versus ferrocene/ferrocenium (Fc(0/+) ))), the second process is irreversible (with oxidation potentials ranging from +2.63 to +2.71 V (lSO2 , versus Fc(0/+) )). [B12 I12 ](2-) showed a complex oxidation behavior in cyclic voltammetry experiments, presumably owing to decomposition of the cluster anion under release of iodide, which also explains the failure to isolate the respective radical by chemical oxidation.

Protic Anions [H(B12X12)]− (X=F, Cl, Br, I) That Act as Brønsted Acids in the Gas Phase

The acidity of protic cations and neutral molecules has been studied extensively in the gas phase, and the gas-phase acidity has been established previously as a very useful measure of the intrinsic acidity of neutral and cationic compounds. However, no data for any anionic acids were available prior to this study. The protic anions [H(B12X12)](-) (X = F, Cl, Br, I) are expected to be the most acidic anions known to date. Therefore, they were investigated in this study with respect to their ability to protonate neutral molecules in the gas phase by using a combination of mass spectrometry and quantum-chemical calculations. For the first time it was shown that in the gas phase protic anions are also able to protonate neutral molecules and thus act as Brønsted acids. According to theoretical calculations, [H(B12I12)](-) is the most acidic gas-phase anion, whereas in actual protonation experiments [H(B12Cl12)](-) is the most potent gas-phase acidic anion for the protonation of neutral molecules. This discrepancy is explained by ion pairing and kinetic effects.

The First Step of the Oxidation of Elemental Sulfur: Crystal Structure of the Homopolyatomic Sulfur Radical Cation [S8].+

The oxidation of elemental sulfur in superacidic solutions and melts is one of the oldest topics in inorganic main group chemistry. Thus far, only three homopolyatomic sulfur cations ([S4 ]2+ , [S8 ]2+ , and [S19 ]2+ ) have been characterized crystallographically although ESR investigations have given evidence for the presence of at least two additional homopolyatomic sulfur radical cations in solution. Herein, the crystal structure of the hitherto unknown homopolyatomic sulfur radical cation [S8 ].+ is presented. The radical cation [S8 ].+ represents the first step of the oxidation of the S8 molecule present in elemental sulfur. It has a structure similar to the known structure of [S8 ]2+ , but the transannular sulfur⋅⋅⋅sulfur contact is significantly elongated. Quantum-chemical calculations help in understanding its structure and support its presence in solution as a stable compound. The existence of [S8 ].+ is also in accord with previous ESR investigations.