Investigation of the Reactivity of Polyhedral Borane Anions With Carbon-Based Nucleophiles and Electrophiles
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The substitution chemistry of the [trans-B20H18]2- ion was first investigated by Hawthorne and coworkers The nucleophilic attack of the [trans-B20H18]2- ion by a hydroxide ion, alkoxide ion, thiolate ion, and amine ion have been reported in the literature. To date, there have been no reports in the literature of nucleophilic attack of the [trans-B20H18]2- ion with carbon nucleophiles. Previous investigations of the interaction of the [trans-B20H18]2- ion with carbon nucleophiles have not been completely characterized. Initial characterization using X-ray diffraction has been complicated due to difficulty in obtaining singles crystals of the reduced derivatives. Single crystals of the oxidized derivatives, [B20H17X]2-, are more easily acquired. However, when characterized using X-ray diffraction, unexpected results were obtained, presumably as a result of the xviii harsh reaction conditions of the oxidation reaction. Therefore, an investigation of alternative oxidizing agents is warranted. As a result, the overall goal of the research project is to create a series of [B20H17X]4- ions and [B20H17X]2- ions where X is a carbon-based nucleophile. Specific aims for the research project are:
- Investigate the reaction of the [trans-B20H18]2- ion with a variety of carbon nucleophiles,
- Investigate the oxidation of the resulting [B20H17X]4- ions to form a series of [B20H17X]2- ions, restoring the three-center two-electron bond, and
- Characterize the [B20H17X]2- ions by single crystal X-ray diffraction.
A series of reactions were completed which yielded reduced, substituted derivatives of the [B20H18]4- ion where the substituent is a carbon-based nucleophile. Carbon nucleophiles investigated included the acetylide ion, the methyl anion, the acetonitrile anion, and the cyanide ion. Due to the difficulty of growing single crystals, suitable for characterization by X-ray crystallography, the resulting products were oxidized with an organic oxidizing agent, p-benzoquinone, which allowed the react accomplished under significantly milder conditions than the previously used oxidizing agent, ferric chloride. The location of the substituent appears to be a result of the hardness or the softness of the nucleophile utilized. As a result of the research, a new theory regarding the factors that control the location of the substituent has been proposed. Further characterization of several products, using X-ray diffraction, will be needed to provide supporting evidence.
The oxidation of the reduced product in the presence of an electrophile was investigated using several electrophiles in order to investigate the potential to selectively substitute the resulting product. Reaction conditions necessary for the formation of the products have been determined. Preliminary results indicate that the substituted [trans-B20H18]4- ion may be successfully oxidized to a substituted [trans-B20H18]2- ion. The data also suggest that the reactions yield a bridge complex instead of a doubly substituted product as hypothesized. However, final verification by X-ray diffraction needs to be completed.