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-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
[3-μ2-F]- and [4-μ2-F]. Spectroscopic and structural studies confirm that the F- anion bridges the two Lewis acidic
centers in both [3-μ2-F]- and [4-μ2-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 [4-μ2-F] leads
to complete regeneration of [4]+, showing that F- binding is reversible.