Anion Chelation via Double Chalcogen Bonding: The Case of a Bis-telluronium Dication and Its Application in Electrophilic Catalysis via Metal-Chloride Bond Activation.
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Telluronium cations have long been known to engage their counteranions via secondary interactions. Yet, this property has rarely been exploited for anion binding. Motivated by such an application, we have now synthesized a bis-telluronium dication (2+) that was obtained as a tetrafluoroborate salt by reaction of 2,7-di-tert-butyl-9,9-dimethylxanthene-4,5-diboronic acid with phenoxatellurine difluoride and BF3OEt2. As confirmed by the formation of Te-(-BF4)-Te bridges in the structure of [BF4]2, 2+ functions as a bidentate Lewis acid toward anions. [BF4]2 has also been converted into the more exposed [BArF24]2 ([BArF24]- = [B(3,5-(CF3)2C6H3)4]-). The latter, which readily ionizes Ph3CCl, displays a chloride anion binding constant that exceeds that of a monofunctional model compound by almost 4 orders of magnitude. The unique properties of this new bis-telluronium dication are further highlighted by its ability to activate Ph3PAuCl and cis-(Ph3P)2PtCl2, leading to catalytic systems highly active in the cycloisomerization of propargylamide or enyne substrates.