Structural and Optical Studies of Fluoride ion binding using N-heteroaromatic based ligands.

Abstract

8-hydroxyquinoline and imidazole, two important N-heteroaromatic systems, have a strong affinity towards various anions via their acidic OH or NH protons. Three receptor ligands, 5-(1H-benzo[d]imidazol-2-yl)quinolin-8-ol (1), 5-(benzo[d]thiazol-2-yl)quinolin-8-ol (2), and 4-(1H-benzo[d]imidazol-2-yl)benzene-1,3-diol (3) were synthesized and their fluoride (F–) ion binding properties were investigated. These ligands can selectively attach F– ion, and their respective F– complexes 1-TBAF, 2-TBAF, and 3-TBAF (TBAF = tetrabutylammonium fluoride) were characterized using single crystal X-ray analysis, NMR, UV-Vis, Hirshfeld surface (HS) analysis and computational studies Their solid-state structural analysis revealed that in each case, the F− ion is strongly bound within the receptor molecule scaffold by NH, OH, and aryl-CH hydrogen bonding interactions, and in the case of 1-TBAF and 3-TBAF, the F− binding is further strengthened by inclusion of water molecules, forming fluoride-water cluster complexes. 1H NMR exhibited the disappearance of NH and OH proton signals, upon adding TBAF, indicating the NH and OH protons as the primary site for F– ion coordination. UV-vis absorbance spectra studied their deep sensing limits towards F– ion. 1 and 2 both showed good responsive behavior starting from 0.5 µM of F– ion, though obvious changes occurred from F– concentration of 180 and 12 µM, respectively. HS analysis quantified the non-covalent interatomic contacts between the atoms and Fluoride, and found that 1-TBAF contributed a maximum of 19.2 % of all atoms···F contacts to the HS. Density functional theory studies were done on 1-TBAF, 2-TBAF, and 3-TBAF crystal systems, and the band structure and total density of states were evaluated on 1-TBAF–3-TBAF. The optical response evaluation of 2-TBAF was also investigated.