Conformational Polymorphism of a Simple Tripodal Podand Bearing Nitro Functionality

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.