Strong oxidizing agents often contain fluorine and are not compatible with substrates forming strong element-fluorine bonds. Therefore, there is need for new strong oxidizers. Cyclic voltammetric measurements on the weakly coordinating anion [Me NB Cl ] in liquid SO revealed an oxidation wave at +2.59 V versus Fc . The oxidation of Na[Me NB Cl ] with AsF in liquid sulfur dioxide gives the boron-cluster-based radical Me NB Cl as a dark blue solid in quantitative yield. The radical was characterized by vibrational, NMR, and EPR spectroscopy and by its crystal structure. To explore the potential of the radical Me NB Cl to act as a strong oxidizing agent, it was reacted with different compounds having high ionization energies. Thianthrene (ionization energy 7.8 eV) and hexabromobenzene (8.8 eV) were oxidized to the corresponding cations. The reaction with elemental iodine (9.3 eV) gave the [I ] cation. These reactive cations are stabilized in the solid state by the weakly coordinating anion [Me NB Cl ] , which was formed as a reduced product in the oxidation reactions.
While hydrogen bridging is very common in boron chemistry, halogen bridging is rather rare. The simplest halogen-bridged boron compounds are the [B2X7](-) anions (X = F, Cl, Br, I), of which only [B2F7](-) has been reported to exist experimentally. In this paper a detailed theoretical and synthetic study on the [B2X7](-) anions is presented. The structures of [B2X7](-) anions have been calculated at the MP2/def2-TZVPP level of theory, and their local minima have been shown to be of C2 symmetry in all cases. The bonding situation varies significantly between the different anions. While in [B2F7](-) the bonding is mainly governed by electrostatics, the charge is almost equally distributed over all atoms in [B2I7](-) and additional weak iodine···iodine interactions are observed. This was shown by an atoms in molecules (AIM) analysis. The thermodynamic stability of the [B2X7](-) anions was estimated in all phases (gas, solution, and solid state) based on quantum-chemical calculations and estimations of the lattice enthalpies using a volume-based approach. In the gas phase the formation of [B2X7](-) anions from [BX4](-) and BX3 is favored in accord with the high Lewis acidity of the BX3 molecules. In solution and in the solid state only [B2F7](-) is stable against dissociation. The other three anions are borderline cases, which might be detectable under favorable conditions. However, experimental attempts to identify [B2X7](-) (X = Cl, Br, I) anions in solution by (11)B NMR spectroscopy and to prepare stable [PNP][B2X7] salts failed.
The partly halogenated and N-alkylated closo-dodecaborates [BClHN(propyl)] and [BBrHNR] (R = ethyl-pentyl) were prepared by alkylation of [BHNH] and subsequent halogenation with elemental chlorine or N-bromosuccinimide. Simple metathesis reactions yielded the [HNMe], [Cmim], [NBu], and Na salts, which were characterized by heteronuclear NMR and IR spectroscopy as well as electrospray ionization mass spectrometry. The crystal structures of the salts [HNMe][BBrHN(ethyl)]·CHCN, [HNMe][BBrHN(propyl)], Na[BBrHN(butyl)], and [HNMe][BClHN(propyl)]·CHCN were determined by single-crystal X-ray diffraction. The [Cmim] salts are thermally stable to temperatures higher than 300 °C. The melting points are between 57 and 80 °C, which classify the [Cmim] salts of [BClHN(propyl)] and [BBrHNR] (R = propyl-pentyl) as ionic liquids. The anions are oxidized only at potentials higher than 2 V versus Fc as determined by cyclic voltammetry. The solubility of the sodium salts in CHCl solution was determined by NMR spectroscopy. With the increasing length of the alkyl chain attached to the ammonio group the solubility is significantly enhanced. A solubility up to 125 mmol/L for Na[BBrHN(pentyl)] in dichloromethane was determined. In addition, the trialkylation of the perchlorinated anion [BClNH] was investigated in detail. A Hofmann elimination was observed to occur at higher temperatures, when alkyl groups with β-hydrogen atoms were introduced. Organic substituents without β-hydrogen atoms gave more stable compounds; however, trialkylation proved to be difficult presumably due to steric hindrance. The crystal structure of the byproduct [PPh][BClN(propargyl)] was determined.
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