Synthesis and properties of a cationic bidentate lewis acid

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-Me-2-4-Me2NC6H2)Hg}C10H6 (3) and its cationic analogue [1-{MeS2B}-8-{(2,6-Me-2-4-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 [3-mu(2)-F](-) and [4-mu(2)-F]. Spectroscopic and structural studies confirm that the F- anion bridges the two Lewis acidic centers in both [3-mu(2)-F](-) and [4-mu(2)-F]. UV-vis fitration 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) X 10(2) M-1 and 6.2 (+/- 0.2) x 10(4) 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 [4-mu(2)-F] leads to complete regeneration of [4](+), showing that F- binding is reversible.