Hexagonal Host Framework of sym-Aryloxytriazines Stabilised by Weak Intermolecular Interactions

ABSTRACT

2,4,6-Tris(4-halophenoxy)-1,3,5-triazine 1 is a convenient C₃ starting material for the self-assembly of hexagonal open frameworks mediated via the halogen…halogen trimer synthon and the π-stacked Piedfort Unit (PU). We examine in this paper crystal structures of 2,4,6-tris(2-iodo-3-pyridyloxy)-1,3,5-triazine 2 , 2,4,6-tris(3-iodophenoxy)-1,3,5-triazine 3 , 2,4,6-tris(6-methyl-3-pyridyloxy)-1,3,5-triazine 4 , and 2,4,6-tris[4-(4′-bromophenyl) phenoxy)]-1,3,5-triazine 5 . Triazine 2 forms isostructural 2:1 host⋅guest adducts (guest = mesitylene, collidine) in the rhombohedral space group R3¯ such that the host architecture is stabilised by the C_3i-PU and a helix of C─H…N interactions. The crystal structure of 3 is different from its chloro/bromo derivatives signifying the importance of the more polarisable I atom compared to Cl, Br. Pairs of C─H…O and C─H…N hydrogen bonds and C_3i-PU sustain the columnar structure of 3 (space group R3¯). The PU has pseudo trigonal symmetry in picolinoxy triazine 4 (space group P2₁/n). In contrast to the phenyl derivatives, the extended aryl arms in biphenyl 5 do not adopt a trigonal conformation: two biphenyl groups are oriented parallel that participate in Br…Br and Br…π interactions. We note that 1 and 2 readily form hexagonal host lattices for guest inclusion, while 3 , 4 , and 5 crystallise in solvent-free form. Thermal measurements (TGA, DSC) indicate that guest release occurs at a higher temperature in the cage type host⋅guest clathrates compared to the channel inclusion compounds for the same solvent. Statistics from the Cambridge Structural Database using CSD Symmetry show that the phenoxytriazine scaffold is unique among the trigonal molecules for the carry-over of symmetry relation from molecule to crystal. The ease of predicting crystal packing and space group in this family of compounds ( 1 , 2 ) makes them good candidates for the crystal engineering of host frameworks.

Keywords:

• DSC
• halogen…halogen trimer
• hexagonal crystal strctures
• Piedfort units
• sym-aryloxytriazunes
• TGA

Acknowledgments

An thanks the Department of Science and Technology for research funding (SR/S5/OC-02/2002). BKS and SA thank the CSIR, and RB thanks the UGC for fellowship support. The UPE programme of UGC supports UoH. TCWM thanks the Hong Kong Research Grants Council Earmarked Grant CUHK 402003.

Notes

^a See Scheme 2 for abbreviated names of host and guest.