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-{Mes(2)B}-8-{(2,6-Me(2)-4-Me(2)NC(6)H(2))Hg}C(10)H(6) (3) and its cationic analogue [1-{Mes(2)B}-8-{(2,6-Me2-4-Me(3)NC(6)H(2))Hg}C(10)H(6)]+ ([4]+) have been synthesized and studied. Compound 3 as well as the triflate salt of [4]+ react with [S(NMe(2))3][Me(3)SiF(2)] to afford the corresponding fluoride complexes [3-micro(2)-F]- and [4-micro(2)-F]. Spectroscopic and structural studies confirm that the F- anion bridges the two Lewis acidic centers in both [3-micro(2)-F]- and [4-micro(2)-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) 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-, NO(3)(-), H(2)PO(4)(-), and HSO(4)(-) and shows only a weak response to OAc(-). Finally, the addition of an aqueous solution of Al3+ to a solution containing [4-micro(2)-F] leads to complete regeneration of [4]+, showing that F(-) binding is reversible.