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 SiF₆²⁻ 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 ¹H 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⁻.