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, L1, 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, L2, 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, L3, 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 1H 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.