An exclusive fluoride receptor: fluoride-induced proton transfer to a quinoline-based thiourea

A new quinoline-based tripodal thiourea has been synthesized, which exclusively binds fluoride anion in DMSO, showing no affinity for other anions including chloride, bromide, iodide, perchlorate, nitrate, and hydrogen sulfate. As investigated by H-1 NMR, the receptor forms both 1:1 and 1:2 complexes yielding binding constants of 2.32(3) (in log beta(1)) and 4.39(4) (in log beta(2)), respectively. The quinoline groups are protonated by fluoride-induced proton transfer from the solution to the host molecule. The 1:2 binding is due to the interactions of one fluoride with NH binding sites of urea sites and another fluoride with secondary +NH binding sites within the tripodal pocket. The formation of both 1:1 and 1:2 complexes has been confirmed by theoretical calculations based on density functional theory (DFT). (C) 2014 Elsevier Ltd. All rights reserved.