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A set of three positional isomers of nitro-phenyl functionalizedtris(urea) scaffold has been extensively studied as the anion coordinatinghost. The para-isomer, L1, has been structurally authenticated to self-assemble intoa dimeric (pseudo)molecular capsule in the presence of oxoanions ofvaried dimensionality (carbonate and terephthalate anions). Whereasthe meta-isomer, L2, has been found to self-assemble into a dimeric molecularcapsule in presence of inorganic oxoanions such as hydrogen phosphateand adopts a flat trigonal planar like geometry in the presence ofthe terephthalate anion where each receptor side arm is in interactionwith a carboxylate group of the anion. However, successful crystallizationof the ortho-isomer, L3, in the presence of different oxoanions was not fruitful presumablydue to the steric effect provided by the nitro group at the ortho-position, which hinders the facile inclusion and coordinationof an anion due to electrostatic factor, as confirmed by 2D NOESYNMR 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 theapparent binding constant values of HCO3–and H2PO42–as comparedto the meta- and para-isomers whichcan coordinate to these anions very strongly via encapsulation, asconfirmed by 2D NOESY NMR analysis. [ABSTRACT FROM AUTHOR]

Tris[4-(nitrophenyloxy)ethyl]amine (L), a simple tripodal podand, may be reproducibly crystallized in either of three polymorphic forms. An understanding of the structural features associated with polymorphism in L has been made in terms of morphology, single-crystal structure, powder X-ray diffraction, and differential scanning calorimetric measurements. Polymorphs I, II, and III crystallize in three different crystal systemstriclinic (P1̅), monoclinic (P21/n), and orthorhombic (Pna2(1)), respectivelyand single-crystal X-ray analysis revealed conformational differences in the arrangement of the flexible arms when crystallized from solutions under different growth conditions. The conformational difference results principally from the bridge-head amino torsion τamino (C−N1amino−C−C) relative to the p-nitrophenol fragment. The different polymorphs display a range of different 3D hydrogen bonding arrangements, constructed from an intermolecular C−H···O hydrogen bonding array. The supramolecular self-assembly features in the three forms are guided by the control methylene C−H···Namino dimeric interaction in form I, the π···π interaction in form II, and a weak C−H··· π interaction in form III that differentiate the overall packing in the three forms. Three conformational polymorphs of a tripodal podand are obtained by solvent mediated crystallization. Conformational differences arise due to the differences in the torsion in three arms in all three polymorphs. Polymorph I has a C−H···Namino dimeric interaction, polymorph II has π···π and nitro−nitro interactions, and polymorph III involves a C−H··· π interaction, which differentiate the overall packing in the three forms. [ABSTRACT FROM AUTHOR]