Positional Isomeric Effect in Nitrophenyl Functionalized Tripodal Urea Receptors toward Binding and Encapsulation of Anions

A set of three positional isomers of nitro-phenyl functionalized tris(urea) scaffold has been extensively studied as the anion coordinating host. The para-isomer, L-1, has been structurally authenticated to self-assemble into a dimeric (pseudo)molecular capsule in the presence of oxoanions of varied dimensionality (carbonate and terephthalate anions). Whereas the meta-isomer, L-2, has been found to self-assemble into a dimeric molecular capsule in presence of inorganic oxoanions such as hydrogen phosphate and adopts a flat trigonal planar like geometry in the presence of the terephthalate anion where each receptor side arm is in interaction with a carboxylate group of the anion. However, successful crystallization of the ortho-isomer, L-3, in the presence of different oxoanions was not fruitful presumably due to the steric effect provided by the nitro group at the ortho-position, which hinders the facile inclusion and coordination of an anion due to electrostatic factor, as confirmed by 2D NOESY NMR analysis of the free receptor. Qualitative and quantitative H-1 NMR experimental results showed that the ortho-isomer binds with oxoanions rather feebly as reflected from the apparent binding constant values of HCO3- and H2PO42- as compared to the meta- and para-isomers which can coordinate to these anions very strongly via encapsulation, as confirmed by 2D NOESY NMR analysis.