Effect of substitution on halide/hydrated halide binding: a case study of neutral bis-urea receptors

Four neutral dipodal receptors have been purposefully designed and synthesized by varying the substituents from a phenyl to naphthyl moiety to investigate the anion coordinating activities of bis-urea receptors. Crystal structure investigation of the complexes correctly validates the fact that the receptors L-1-L-4 form halide as well as hydrated halide complexes. Most interestingly in complex 2a, the phenyl based neutral bis-urea receptor L-2 binds with two chloride ions via participation of N-HCl- interaction from the adjacent urea group and aromatic C-H center dot center dot center dot Cl- interaction along with the construction of a halide-water-halide bridged architecture supported by another identical receptor moiety. Meanwhile, another phenyl-based bis-urea receptor L-1 binds to one bromide ion (1b) which is coordinated with two water molecules (from the lattice) forming a suitable 2:1 anion-receptor interaction. In all cases, the coordinated -NH protons from adjacent urea moieties play an important role in terms of coordination, and are oriented in a trans fashion with respect to the core substituent. The transformation from the phenyl to naphthyl ring shows a clear distinction in halide binding which is very much consistent with the size as well as hydrophobic nature of the substituent.