Fluoride Ion Recognition by Chelating and Cationic Boranes

Abstract

Because of the ubiquity of fluoride ions and their potential toxicity at high doses, researchers would like to design receptors that selectively detect this anion. Fluoride is found in drinking water, toothpaste, and osteoporosis drugs. In addition, fluoride ions also can be detected as an indicator of uranium enrichment (via hydrolysis of UF(6)) or of the chemical warfare agent sarin, which releases the ion upon hydrolysis. However, because of its high hydration enthalpy, the fluoride anion is one of the most challenging targets for anion recognition. Among the various recognition strategies that are available, researchers have focused a great deal of attention on Lewis acidic boron compounds. These molecules typically interact with fluoride anions to form the corresponding fluoroborate species. In the case of simple triarylboranes, the fluoroborates are formed in organic solvents but not in water. To overcome this limitation, this Account examines various methods we have pursued to increase the fluoride-binding properties of boron-based receptors. We first considered the use of bifunctional boranes, which chelate the fluoride anion, such as 1,8-diborylnaphthalenes or heteronuclear 1-boryl-8-mercurio-naphthalenes. In these molecules, the neighboring Lewis acidic atoms can cooperatively interact with the anionic guest. Although the fluoride binding constants of the bifunctional compounds exceed those of neutral monofunctional boranes by several orders of magnitude, the incompatibility of these systems with aqueous media limits their utility. More recently, we have examined simple triarylboranes whose ligands are decorated by cationic ammonium or phosphonium groups. These cationic groups increase the electrophilic character of these boranes, and unlike their neutral analogs, they are able to complex fluoride in aqueous media. We have also considered cationic boranes, which form chelate complexes with fluoride anions. Our work demonstrates that Coulombic and chelate effects are additive and can be combined to boost the anion affinity of Lewis acidic hosts. The boron compounds that we have investigated present a set of photophysical and electrochemical properties that can serve to signal the fluoride-binding event. We can also apply this approach to cyanide complexation and are continuing our investigations in that area.

Chromogenic and fluorogenic chemosensors and reagents for anions. A comprehensive review of the year 2009

“…F -binding with the boron atom leads to a modulation of the electronic properties of the chromophore resulting in changes in colour and/or fluorescence emission. 123 In these studies it was observed that the introduction of cationic boranes favours the binding of F -in aqueous solutions due to the presence of coulombic attractive interactions. anions.…”

Formulation of Catechin Hydrate Nanocapsule and Study of its Bioavailability

“…Cationic borane is known as a potent chelator for fluoride [3]. It is now important to explore the therapeutic role of boranes and antioxidant catechin hydrate either alone or in combination; delivered through nanocapsule to treat fluoride induced oxidative damage in rats.…”

“…Cationic boranes form chelate complexes with fluoride anions [3]. Catechin hydrate, a polyphenolic flavonol antioxidant exhibits its properties against many diseases including ischemic neurodegeneration, atherosclerosis, stomach cancer etc.…”

The Methylene-bridged Dialuminium Compound R2Al-CH2-AlR2 [R=CH(SiMe3)2] as an Effective Chelating Lewis Acid – Adducts with Halides, Perchlorate and Isopropylamide

“…NMR spectra were recorded in C 6 [10] and LiC 6 H 2 (2,4,6-i-Pr) 3 [63] were obtained according to literature procedures. Commercially available LiCl (anhydrous), LiBr (anhydrous), KClO 4 and H 2 N(i-Pr) were used without further purification.…”