The Squaramide versus Urea Contest for Anion Recognition

Abstract

The interaction of a neutral squaramide-based receptor, equipped with two 4-nitrophenyl substituents (R_sq), with halides and oxoanions has been studied in MeCN. UV/Vis and ¹H NMR spectroscopy titration experiments clearly indicated the formation of 1:1 hydrogen bonding [R_sq⋅⋅⋅X]⁺ complexes with all the investigated anions. X-ray diffraction studies on the chloride and bromide complex salts confirmed the 1:1 stoichiometry and indicated the establishment of bifurcated hydrogen-bond interactions between the squaramide-based receptor and the halide anion that involved both 1) amide NH and 2) aryl proximate CH fragments, for a total of four bonds. Probably due to the contribution of CH fragments, complexes of R_sq with halides are 1 to 2 orders of magnitude more stable than the corresponding ones with the analogous urea-based receptor that contains two 4-nitrophenyl substituents (R_ur). In the case of oxoanions, R_sq forms complexes, the stability of which decreases with the decreasing basicity of the anion (H₂PO₄⁻>NO₂⁻≈HSO₄⁻>NO₃⁻), and is comparable to that of complexes of the urea-based receptor R_ur. Such a behaviour is ascribed to the predominance of different contributions: electrostatic interaction for halides, acid-to-base ‘frozen’ proton transfer for oxoanions. Finally, with the strongly basic anions F⁻ and CH₃COO⁻, R_sq first gives genuine hydrogen-bond complexes of 1:1 stoichiometry; then, upon addition of a second anion equivalent, it undergoes deprotonation of one NH fragment, with the simultaneous formation of the dianion hydrogen-bond complexes, [HF₂]⁻ and [CH₃COOH⋅⋅⋅CH₃COO]⁻, respectively. In the case of the urea-based derivative R_ur, deprotonation takes place with fluoride but not with acetate. The apparently higher Brønsted acidity of R_sq with respect to R_ur reflects the capability of the squaramide receptor to delocalise the negative charge formed on NH deprotonation over the cyclobutene-1,2-dione ring and the entire molecular framework.