Selective two-electron reductive defluorination of 1-Me-CB(11)F(11-) and related anions: synthesis and characterization of salts of 1-Me-12-R-CB(11)F(10-) and related anions
Date
2008
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Abstract
In this dissertation, a method to selectively defluorinate and functionalize the antipodal B-F bond in 1-R-CB11F11-anion is described. The defluorination reaction mechanism using sodium naphthalenide to form an intermediate anion was investigated by 19F{ 11B} NMR, cyclic voltammetry, and computational analysis. The stoichiometric amount of sodium naphthalenide to reduce 1-Me-CB11F11 - was determined to be two. 19F{11B} NMR of the intermediate did not show a peak due to the antipodal fluorine atom, which suggested that the B-F bond is broken. Cyclic voltammetry of 1-Me-CB 11F11- showed a reversible one-electron reduction. In presence of excess Na+, the reduction occurred as an irreversible two-electron process, but at the same potential as the one-electron process. The addition of excess 18-crown-6 restored the reversible one-electron process. This suggested that Na+ is probably incorporated in the structure of the intermediate. Based on these results, two structures were proposed, one with B···F···Na+ linkage the other with B···Na+···F - linkage at the antipodal position. The intermediate did not show electron-exchange between the intermediate and the starting material, 1-Me-CB 11F11-. This suggested that the structure with B···F···Na+ is unlikely. Computational analysis of the proposed structures predicted that the latter structure is 55 kJ/mol more stable than the former. From these results, the structure of the intermediate was suggested to be (1-Me-CB 11F11···Na+···F -)2-. Using various electrophiles, several derivatives of 1-Me-12-R-CB 11F10-. For example, when methyl iodide was added to the intermediate solution, the major product was 1,12-Me2-CB 11F10-. An isomer, 1-Me-7-X-CB11F 10- was formed as one of the byproducts, however, the isomeric ratio of the 12-isomer and the 7-isomer was 9:1. X-ray crystal structures of four new compounds NMe4(1,12-Me 2-CB11F10), Cs(1-Me-12-I-CB11 F10), Cs(1-Me-12-SiPh3-CB11F10), and Ag2(1-Me-12-SiPh3-CB11F 10)2·C6H6 were obtained. The characteristics of each structure are described. One of them, Ag2(1-Me-12-SiPh 3-CB11F10)2·C6H 6, contained two Ag+ with different coordination spheres. One of them was the first example of tetrahediral Ag(arene)4 +. In order to investigate the effect of substitution on the B12 vertex of 1-Me-CB11F11- on the coordinating and ion-pairing abilities of the anions, DFT calculations were performed with 1-Me-CB11F11-, 1,12-Me2-CB 11F10-, and 1-Me-12-H-CB11F 10-. The calculations predicted that both 1,12-Me 2-CB11F10- and 1-Me-12-H-CB 11F10- had slightly less ion-pairing ability than 1-Me-CB11F11-. Especially, significant change was observed on the B 12-X direction on the ion-pairing ability. The solution conductivities of N(n-Bu)4(1-Me-CB 11F11) and N(n-Bu)4(1,12-Me 2-CB11F10) also showed that N(n-Bu) 4(1,12-Me2-CB11F10) is 16% more conductive than N(n-Bu)4(1-Me-CB11F11). The DFT-predicted relative energies of the 12-, 7-, and 2-isomers of Fe(Cp)(CO) 2(1-Me-CB11F11), Fe(Cp)(CO)2(1,12-Me 2-CB11F10), and Fe(Cp)(CO)2(1-Me-12-H-CB 11F10) were also calculated. For Fe(Cp)(CO)2(1-Me-CB 11F10), the 12-isomer was predicted to be the most stable. This suggests that the B12-vertex is the most strongly coordinating. For Fe(Cp)(CO) 2(1,12-Me2CB11F10), the strongest coordinating site is on the B7-F7 direction, however, the difference between the 12- and 7-isomers was only 1.8 kJ/mol. For Fe(Cp)(CO)2(1-Me-12-H-CB 11F10), 12-isomer was the most stable, and 7- and 2- isomers were predicted to have much higher energies than the 12-isomer.
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carboranes
defluorination
sodium naphthalenide
two-electron reduction
weakly coordinating anions
inorganic chemistry