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 1 H 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 (H 2 PO 4 − >NO 2 − ≈HSO 4 − >NO 3 − ), 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 CH 3 COO − , 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, [HF 2 ] − and [CH 3 COOH⋅⋅⋅CH 3 COO] − , 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.