American Chemical Society
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Synthesis and Properties of a Cationic Bidentate Lewis Acid

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posted on 2007-10-01, 00:00 authored by Min Hyung Lee, François P. Gabbaï
As part of our efforts to increase the fluoride affinity of bidentate Lewis acids, we have set out to determine if the F- anion chelation occurring in such systems can be complemented by favorable Coulombic attractions. To explore this idea, the neutral B/Hg bidentate Lewis acid 1-{Mes2B}-8-{(2,6-Me2-4-Me2NC6H2)Hg}C10H6 (3) and its cationic analogue [1-{Mes2B}-8-{(2,6-Me2-4-Me3NC6H2)Hg}C10H6]+ ([4]+) have been synthesized and studied. Compound 3 as well as the triflate salt of [4]+ react with [S(NMe2)3][Me3SiF2] to afford the corresponding fluoride complexes [32-F]- and [42-F]. Spectroscopic and structural studies confirm that the F- anion bridges the two Lewis acidic centers in both [32-F]- and [42-F]. UV−vis titration experiments carried out in tetrahydrofuran/water (9/1, v/v) mixtures indicate that the fluoride binding constants of 3 and [4]+ are clearly differentiated and are equal to 1.3 (±0.1) × 102 M-1 and 6.2 (±0.2) × 104 M-1, respectively. The enhanced fluoride binding constant of [4]+, when compared to 3, confirms that the chelate effect occurring in these types of fluoride receptors can be combined with favorable Coulombic attractions to strengthen the host−guest interaction. Cation [4]+ remains highly selective for F- over other environmentally abundant anions including Cl-, Br-, NO3-, H2PO4-, and HSO4- and shows only a weak response to OAc-. Finally, the addition of an aqueous solution of Al3+ to a solution containing [42-F] leads to complete regeneration of [4]+, showing that F- binding is reversible.