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.

The recognition and capture of fluoride, cyanide and azide anions is attracting great deal of attention due to the negative effects of these anions on the environment and on human health. One of common methods used for the recognition and capture of these anions is based on triarylboranes, the Lewis acidity of which can be enhanced via variation the steric and electronic properties of the boron substituents. This dissertation is dedicated to the synthesis of novel boron-based anion receptors that, for the most part, feature an onium group bound to one of the aryl substituents. The presence of this group is shown to increase the anion affinity of the boron center via Coulombic effects. Another interesting effect is observed when the onium group is juxtaposed with the boron atom. This is for example the case of naphthalene-based compounds bearing a dimesitylboryl moiety at one of the peri-position and a sulfonium or telluronium unit at the other peri position. Fluoride anion complexation studies with these sulfonium or telluronium boranes, show that the boron-bound fluoride anion is further stabilized by formation of a B-F->Te/S bridge involving a lp(F)->sigma*(Te/S-C) donor acceptor interaction. Some of the sulfonium boranes investigated have been shown to efficiently capture fluoride anions from wet methanolic solutions. The resulting fluoride/sulfonium borane adducts can be triggered to release a "naked" fluoride equivalent in organic solution and thus show promise as new reagents for nucleophilic fluorination chemistry. Interestingly, the telluronium systems show a greater fluoride anion affinity than their sulfonium analogs. This increase is assigned to the greater spatial and energetic accessibility of the sigma* orbital on the tellurium atom which favors the formation of a strong B-F->Te interaction. This dissertation is concluded by an investigation of the Lewis acidic properties of B(C6Cl5)3. This borane, which has been reported to be non-Lewis acidic by other researchers, is found by us to bind fluoride, azide and cyanide anions in dichloromethane with large binding constants. This borane is also reactive toward neutral Lewis bases, such as p-dimethylaminopyridine, in organic solvents.

Although fluoride is used at low concentrations in drinking water as a means of promoting dental health, it poses a danger at high exposure levels where it can lead to skeletal fluorosis or other adverse effects. Cyanide is notoriously toxic, and its large scale use in industrial processes warrants the need for close monitoring to remain aware of potential contamination of water sources and other environmental resources. Based on these considerations, it is critical to continue to develop improved methods of monitoring fluoride and cyanide concentrations in water. However, molecular recognition of these anions in water poses considerable challenges. For fluoride, this is due largely to its high hydration enthalpy (Ho = -504 kJ mol-1), which drastically reduces its reactivity in water. Additionally, the strong basicity of cyanide (pKa of (HCN) = 9.3) may obscure its detection in neutral water due to protonation. In addition to achieving detection of these anions in water, it is most desirable to have information of the detection event relayed in the form of a positive, rather than negative, response (i.e., turn-on vs turn-off). The general strategy of appending cationic groups to triarylboranes imparts beneficial Coulombic, inductive, and sometimes chelate effects that have allowed a number of these Lewis acidic receptors to sense fluoride and cyanide in aqueous environments. With the goal of developing new triarylborane-based receptors that show enhanced affinities for these anions, as well as turn-on responses to detection, a series of pyridinium boranes were synthesized and studied. Having recognized that the inherent Lewis acidity of antimony(V) species might be exploited for anion sensing, we also describe initial studies on the ability of tetraorganostibonium ions (R4Sb+) and cationic transition metal-triarylstibine complexes (R3SbM+) to complex fluoride. Finally, the electropositivity of antimony and its ability to form stable compounds in both the +3 and +5 oxidation states have led us to begin investigations into the bonding and redox reactivity of novel metal stibine/stiborane complexes.