1,3-bis(4-nitrophenyl)urea (1) interacts through hydrogen bonding with a variety of oxoanions in an MeCN solution to give bright yellow 1:1 complexes, whose stability decreases with the decreasing basicity of the anion (CH3COO- > C6H5COO- > H2PO4- > NO2- > HSO4- > NO3-). The [Bu4N][1.CH3COO] complex salt has been isolated as a crystalline solid and its molecular structure determined, showing the formation of a discrete adduct held together by two N-H...O hydrogen bonds of moderate strength. On the other hand, the F- ion first establishes a hydrogen-bonding interaction with 1 to give the most stable 1:1 complex, and then on addition of a second equivalent, induces urea deprotonation, due to the formation of HF2-. The orange-red deprotonated urea solution uptakes carbon dioxide from air to give the tetrabutylammonium salt of the hydrogencarbonate H-bond complex, [Bu4N][1.HCO3], whose crystal and molecular structures have been determined.

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) (-) approximately 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)COOHCH(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.

The urea-based receptor 1 (1-(7-nitrobenzo[1,2,5]oxadiazol-4-yl)-3-(4-nitrophenyl)urea, L--H), interacts with X- ions in MeCN, according to two consecutive steps: 1) formation of a hydrogen-bond complex [L--H...X]-; 2) deprotonation of L--H to give L- and [HX2]-, as shown by spectrophotometric and 1H NMR titration experiments. Step 2) takes place with more basic anions (fluoride, carboxylates, dihydrogenphosphate), while less basic anions (Cl-, NO2-, NO3-) do not induce proton transfer. On crystallisation from a solution containing L--H and excess Bu4NF, the tetrabutylammonium salt of the deprotonated urea derivative (Bu4N[L]) was isolated and its crystal and molecular structure determined.

A cyclam-like macrocycle with an integrated push-pull chromophore selectively detects Cu2+ inclusion through both orange-to-yellow colour change and quenching of the green fluorescence.