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Description:
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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 . |