Owing to the favored Coulombic attraction between the ammonium group and
anion which stabilizes the B-F/B-CN bond against heterolysis, cationic borane [25]+ has
great affinity toward anions than its neutral analog, and is capable of capturing fluoride
or cyanide from water under bi-phasic conditions. By placing the fluorophilic silyl
group adjacent to an electrophilic carbocation, a novel fluoride sensor [45]+ was
obtained. Sensing occurs via a fluoride induced methyl migration from the silicon to
adjacent electrophilic methylium center which is unprecedented. As a result of its
strong fluoride affinity, [45]+ is able to react with KF in aqueous media at pH 7.0.
The electrochemistry study of these cationic Lewis boranes reveals that the
cationic character of these boranes serves to decrease their reduction potential and
increase the stability of the resulting radicals. In this part of the research, we have
prepared a cationic borane [27]+, which features two reversible reduction waves at -0.86
and -1.56 (vs. Fc/Fc+) corresponding to the formation of stable neutral and anionic
derivatives. The one-electron reduction of [27]+ leads to the formation of a boron
containing neutral radical featuring an unusual boron-carbon one-electron π bond. Further reduction of 27• results in the formation of the borataalkene derivative [27]-,
which features a formal B=C double bond. The structural changes accompanying the
stepwise population of the B-C π-bond are also determined, and this sequential
population of B-C π-bonding orbital is also supported by theoretical computations.
In order to understand the impact of the cationic nature of these boranes on their
oxidative power, three novel cationic boranes ([34]+, [35]2+, and [36]3+) have been
synthesized and their oxidative power were examined via cyclic voltammetry. The CV
data of these compounds shows that the reduction potential of these triarylboranes is
linearly proportional to the number of the pendant cationic substituents. Substitution of
a mesityl group by an ArN+ group leads to an increase of the reduction potential by 260
mV.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-2672 |
| Date | 15 May 2009 |
| Creators | Chiu, Ching-Wen |
| Contributors | Gabbai, Francois P |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | electronic, application/pdf, born digital |
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