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Three-Dimensional Hexagonal Structures from a Novel Self-Complementary Molecular Building Block

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journal contribution
posted on 22.04.2000, 00:00 by C. V. Krishnamohan Sharma, Abraham Clearfield
Nitrilotri(methylphosphonic acid), 1H6, exists as a zwitter ion in its pure form and reveals a complex three-dimensional structure with no predictable structural patterns. However, when acid 1H6 is reacted with 1,7-phenanthroline, 2, 1,10-phenanthroline, 3, acridine, 6, and tripropylamine, 8, in a 1:1 molar ratio, monodeprotonation of 1H6 triggers a prototypal self-complementary 3D hexagonal architecture in complexes 9 (1H5·H·2H2O), 10 (1H5·3H), 11 (1·5H)·(5)0.5, and 12 (1·8H). The networks are stabilized by very strong ionic hydrogen bonds and offer a viable new strategy for the design of stable and predictable 3D open organic networks. However, monodeprotonation of acid 1H6 with quinoline, 6, does not lead to the expected open hexagonal structure in 13 (1·6H) as quinolinium cations cannot effectively fill the void space if open hexagonal open structures were formed. The unique three-dimensional connectivity of these open networks does not permit interweaving, and template effects play a critical role in the formation of open structures. The double deprotonation of acid 1H6 with amines 2, 4,7-phenanthroline, 3, and diisopropylamine, 7, leads to the formation of complexes 14 (1H4·(2H)2·2H2O), 15 (1H4·(4H)2·5H2O), and 16 (1H4·7H) with one-dimensional chain structures only. Crystal structures of 916 together indicate the importance of the ionic hydrogen bond donor:acceptor ratio in determining 3D hexagonal network formation. Further, the proton transfer from acid 1H6 to phenanthrolines influences the absorption properties of 9, but has no effect on 10. 1,7-Phenanthroline (mp 80 °C) changes its color from pale yellow to deep orange-red upon protonation and exhibits stability up to 325 °C. These results bring out a new concept, that we may be able to control absorption properties and heat stability of pigments/colorants by encapsulating different structural forms into a rigid three-dimensional host matrix by optimizing hydrogen bond properties.