Fluoride anion recognition

Takeuchi, M., T. Shioya, and T.M. Swager. 2001. Allosteric fluoride anion recognition by a doubly strapped porphyrin. Angew Chem Int Ed Engl 40 3372—3375. 

In the area of ion sensing, cation recognition by electrodes containing functionalized redox-active polymers has been an area of considerable interest. Fabre and co-workers have reported the development of a boronate-functionalized polypyrrole as a fluoride anion-responsive electroactive polymer film. The electropolymerizable polypyrrole precursor (11) (Fig. 11) was synthesized by the hydroboration reaction of l-(phenylsulfonyl)-3-vinylpyrrole with diisopinocampheylborane followed by treatment with pinacol and the deprotection of the pyrrole ring.33 The same methodology was utilized for the production of several electropolymerizable aromatic compounds (of pyrrole (12) (Fig. 11), thiophene (13 and 14) (Fig. 11), and aniline) bearing boronic acid and boronate substituents as precursors of fluoride- and/or chloride-responsive conjugated polymer.34... 

There are a limited number of fluorescent sensors for anion recognition. An outstanding example is the diprotonated form of hexadecyltetramethylsapphyrin (A-7) that contains a pentaaza macrocydic core (Figure 10.31) the selectivity for fluoride ion was indeed found to be very high in methanol (stability constant of the complex 105) with respect to chloride and bromide (stability constants < 102). Such selectivity can be explained by the fact that F (ionic radius 1.19 A) can be accommodated within the sapphyrin cavity to form a 1 1 complex with the anion in the plane of the sapphyrin, whereas Cl and Br are too big (ionic radii 1.67 and 1.82 A, respectively) and form out-of-plane ion-paired complexes. A two-fold enhancement of the fluorescent intensity is observed upon addition of fluoride. Such enhancement can be explained by the fact that the presence of F reduces the quenching due to coupling of the inner protons with the solvent. 

Probably the most precious occurrence of anion recognition is that related to size exclusion selectivity. This takes place when the receptor, providing for instance a spheroidal cavity, includes only spherical anions of radius less than or equal to a definite value. In this context, the smallest anion, fluoride, has offered vast opportunities. 

Recently, Shinkai and co-workers (85) reported a ferroceneboronic acid receptor (26). This receptor showed selective electrochemical recognition of fluoride ions over other halides, the redox potential for the ferrocene unit being perturbed on the addition of F . There is, however, only one orbital overlap interaction provided by this host and it is likely the selectivity arises solely due to the hardness of the fluoride anion. 

Castellano et al.221 reported the formation of a luminescence lifetime-based sensor for cyanide and other counterions using Ru11 diimines possessing MLCT excited states with the anion recognition capabilities of 2,3-di(l//-2-pyrrolyl)quinoxaline (DPQ). Using time-resolved photoluminescence decay, its viability as a lifetime-based sensor for anions has been tested. There were significant changes to the UV-vis and steady-state emission properties after the addition of several ions (e.g., fluoride, cyanide, and phosphate). 

Lehaire, M.-L. Scopelliti, R. Piotrowski, H. Severin, K. Selective recognition of fluoride anion using a Li -metallacrown complex. Angew. Chem. Int. Ed. 2002, 41, 1419-1422. 

Takeuchi. M. Shioya, T. Swager, T.M. Allosteric fluoride 43. anion recognition by doubly strapped porphyrin. Angew. 

Mizuno T, Wei WH, Eller LR, Sessler JL (2002) Phenanthroline complexes bearing fused dipyrrolylquinoxaline anion recognition sites efficient fluoride anion receptors. J Am Chem Soc 124(7) 1134-1135. doi 10.1021/ja017298t... 

As a seminal contribution to anion recognition, in 1997, Crabtree and co-workers reported the synthesis and anion-binding properties of isophthalamide-linked bidentate amide-based receptors 4a,b [21]. Crystal stmcture of 4a Br showed 1 1 complexation with Br (Fig. 5.2). In the stmcmre, the bromide was held with two H-bonds by the amide NH groups at N Br distances of 3.436 and 3.634 A, and N-H—Br angles of 116 and 172 . The isophthalamide spacer could be replaced with a 2,6-diamidopyridine unit to give 4b, which was selective for fluoride anion in CD2CI2 ( Ta = 2.4 x 10 M ) [22]. 

The macrocychc hexaimine stmcture of Figure 19a forms a homodinuclear cryptate with Cu(I) (122), whereas crown ether boron receptors (Fig. 19b) have been appHed for the simultaneous and selective recognition of complementary cation—anion species such as potassium and fluoride (123) or ammonium and alkoxide ions (124) to yield a heterodinuclear complex (120). 

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