Jonas Warneke

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.

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

It has been shown by electrospray ionization-ion-trap mass spectrometry that B(12)I(12)(2-) converts to an intact B(12) cluster as a result of successive stripping of single iodine radicals or ions. Herein, the structure and stability of all intermediate B(12)I(n)(-) species (n=11 to 1) determined by means of first-principles calculations are reported. The initial predominant loss of an iodine radical occurs most probably via the triplet state of B(12)I(12)(2-), and the reaction path for loss of an iodide ion from the singlet state crosses that from the triplet state. Experimentally, the boron clusters resulting from B(12)I(12)(2-) through loss of either iodide or iodine occur at the same excitation energy in the ion trap. It is shown that the icosahedral B(12) unit commonly observed in dodecaborate compounds is destabilized while losing iodine. The boron framework opens to nonicosahedral structures with five to seven iodine atoms left. The temperature of the ions has a considerable influence on the relative stability near the opening of the clusters. The most stable structures with five to seven iodine atoms are neither planar nor icosahedral.

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.

Evidence for an intrinsic binding force between dodecaborate dianions and receptors with hydrophobic binding pockets

A gas phase binding study revealed strong intrinsic intermolecular interactions between dianionic halogenated closo-dodecaborates [B12X12](2-) and several neutral organic receptors. Oxidation of a tetrathiafulvalene host allowed switching between two host-guest binding modes in a supramolecular complex. Complexes of β-cyclodextrin with [B12F12](2-) show remarkable stability in the gas phase and were successfully tested as carriers for the delivery of boron clusters into cancer cells.

1-Naphthylamine functionalized Pt nanoparticles: electrochemical activity and redox chemistry occurring on one surface

We present the preparation and electrochemical application of Pt nanoparticles (Pt NPs) functionalized with 1-naphthylamine. Under electrochemical conditions, Pt surface bound 1-naphthylamine (NA) can be reversibly switched (oxidized and reduced), while simultaneously electrocatalytic reactions (e.g. CO oxidation) can proceed on the Pt surface. While the redox activity of the ligand is established immediately after functionalization, the functionalized NPs have to be stored as a colloidal dispersion in tetrahydrofuran (THF) prior to deposition onto the support material in order to induce their catalytic activity. We interpret this catalytic activation due to partial desorption of ligands from the particle surface induced by storing the particles in THF. However, the experimental results do not indicate a loss of ligands from the ligand shell, but evidence that the ligands form oligomers when kept as colloids in THF. As a result the catalytic surface becomes partially available while the redox activity of the ligands is maintained.

Electron-Induced Hydration of an Alkene: Alternative Reaction Pathways†

Electron-induced reactions in condensed mixtures of ethylene and water lead to the synthesis of ethanol, as shown by post-irradiation thermal desorption spectrometry (TDS). Interestingly, this synthesis is not only induced by soft electron impact ionization similar to a previously observed electron-induced hydroamination but also, at low electron energy, by electron attachment to ethylene and a subsequent acid/base reaction with water.

XPS study of thermal and electron-induced decomposition of Ni and Co acetylacetonate thin films for metal deposition

Optimizing thin metal film deposition techniques from metal-organic precursors such as atomic layer deposition, chemical vapor deposition (CVD), or electron beam-induced deposition (EBID) with the help of surface science analysis tools in ultrahigh vacuum requires a contamination-free precursor delivery technique, especially in the case of the less volatile precursors. For this purpose, the preparation of layers of undecomposed Ni(acac)2 and Co(acac)2 was tried via pulsed spray evaporation of a liquid solution of the precursors in ethanol into a flow of nitrogen on a CVD reactor. Solvent-free layers of intact precursor molecules were obtained when the substrate was held at a temperature of 115 °C. A qualitative comparison of thermally initiated and electron-induced precursor decomposition and metal center reduction was carried out. All deposited films were analyzed with respect to chemical composition quasi in situ by x-ray photoelectron spectroscopy. Thermally initiated decomposition yielded higher metal...

Electron-driven and thermal chemistry during water-assisted purification of platinum nanomaterials generated by electron beam induced deposition

Focused electron beam induced deposition (FEBID) is a versatile tool for the direct-write fabrication of nanostructures on surfaces. However, FEBID nanostructures are usually highly contaminated by carbon originating from the precursor used in the process. Recently, it was shown that platinum nanostructures produced by FEBID can be efficiently purified by electron irradiation in the presence of water. If such processes can be transferred to FEBID deposits produced from other carbon-containing precursors, a new general approach to the generation of pure metallic nanostructures could be implemented. Therefore this study aims to understand the chemical reactions that are fundamental to the water-assisted purification of platinum FEBID deposits generated from trimethyl(methylcyclopentadienyl)platinum(IV) (MeCpPtMe3). The experiments performed under ultrahigh vacuum conditions apply a combination of different desorption experiments coupled with mass spectrometry to analyse reaction products. Electron-stimulated desorption monitors species that leave the surface during electron exposure while post-irradiation thermal desorption spectrometry reveals products that evolve during subsequent thermal treatment. In addition, desorption of volatile products was also observed when a deposit produced by electron exposure was subsequently brought into contact with water. The results distinguish between contributions of thermal chemistry, direct chemistry between water and the deposit, and electron-induced reactions that all contribute to the purification process. We discuss reaction kinetics for the main volatile products CO and CH4 to obtain mechanistic information. The results provide novel insights into the chemistry that occurs during purification of FEBID nanostructures with implications also for the stability of the carbonaceous matrix of nanogranular FEBID materials under humid conditions.

Identification of a neutral loss from precursor ions with nearly Gaussian‐shaped isotopic patterns via inverted isotopic patterns of product ions

RATIONALE Direct atomic composition determination of ions with very broad Gaussian-shaped isotopic patterns is challenging because no monoisotopic peak is available for high accurate mass determination and no characteristic shapes in isotopic patterns are visible. METHODS Isolation and fragmentation of the ions corresponding to one peak (one nominal mass) in the center of the broad Gaussian-shaped isotopic pattern lead to a mass spectrum with the product ion signal showing the inverted full isotopic profile of the neutral fragment. RESULTS We have introduced a convenient method for the fast and straightforward identification of a neutral loss for molecular ions with broad isotopic patterns. The theoretical considerations underlying this method are explained and its practical limitations are considered. The benefits of this method are exemplified by guiding a reader through the analysis of a complex mixture of bridged carborate clusters, compounds with very broad isotopic patterns. CONCLUSIONS The presented method can be efficiently used for the determination of atomic compositions of compounds with broad isotopic patterns by their fragmentation using mass spectrometry. This method should significantly facilitate the mass spectrometric analysis of compounds containing several atoms with broad isotopic distributions, such as Ge, Sn, Mo, Ru and Hg, and, thus, can considerably broaden the use of mass spectrometry as an analytical method in inorganic and organometallic chemistry. Copyright