title: Hydrogen and halogen bonding in a concerted act of anion recognition: F⁻ induced atmospheric CO₂ uptake by an iodophenyl functionalized simple urea receptor.
abstract: Two simple urea based para-halo substituted [Iodo (L1) and Bromo (L2)] acyclic receptors have been extensively studied as a receptor for various anions. Receptors L1 efficiently uptake atmospheric CO2 and stabilize as air-stable crystals of HCO3(-) dimer (complex 1a) in the presence of n-tetrabutylammonium (n-TBA) fluoride through the simultaneous formation of hydrogen and halogen bonding, yielding a tetrahedrally surrounded non-covalent coordinated complex. However, receptor L2, in the presence of n-TBA salt of F(-), has been found to form a complex with the octahedral SiF6(2-) anion, where the coordination environment of the anion is merely governed by multiple N-H···F (anion) interactions. The fluoride induces an uptake of aerial CO2 only for L1, which is due to the unique ability of L1 to simultaneously form both hydrogen and halogen bonds with an anionic guest. The most decisive evidence supporting the ability of L1 to form a halogen bond is obtained via crystallizing the acetate complex of both the receptors. The receptor L1 stabilizes the acetate anion via both H-bonding and halogen bonding interactions, while the receptor L2 only forms H-bonding interactions with acetate anion. The solution-state anion binding properties of L1 and L2 have been investigated by qualitative and quantitative (1)H NMR titration experiments with halides and oxyanions in DMSO-d6. Both the receptors showed strong solution-state binding with F(-), HCO3(-) and CH3COO(-), as observed in the solid-state, whereas both of them have been found to be less interactive with other anions such as Cl(-), Br(-), I(-), NO3(-), HSO4(-), and H2PO4(-).
authors.affiliation: Department of Chemistry, Indian Institute of Technology, Guwahati, Assam 781 039, India. gdas@iitg.ernet.in.
authors.forename: R
authors.initials: R
authors.lastname: Chutia
authors.forename: G
authors.initials: G
authors.lastname: Das