Picking Partners

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Science  09 Nov 2007:
Vol. 318, Issue 5852, pp. 889
DOI: 10.1126/science.318.5852.889b

The evolution of strategies for synthesizing complex organic molecules relied in large part on a deepening understanding of the relative reaction rates of often subtly differing functional groups. Rational assembly of larger, supramolecular structures, which are held together by an array of noncovalent interactions, is now proceeding along a similar course, with predictive principles emerging from careful systematic studies. In this vein, Aakeröy et al. have explored the competition between hydrogen and halogen bonding in the cocrystallization of several small aromatic building blocks. One crystallizing partner contained two nitrogen sites (pyridine and benzimidazole), each of which could potentially act as an acceptor toward either a polarized hydrogen or a halogen. The donor partners each bore an oxime (CH=NOH) group, which could interact through either the C-H or the O-H site, as well as a fluorine, bromine, or iodine substituent poised for halogen bonding. Crystallography revealed that in all three cocrystals, the benzimidazole N formed an H bond with the oxime OH. In the F- and Br-substituted systems, the pyridine N attracted the C-H group; only in the iodo case did a halogen bond form instead. — JSY

J. Am. Chem. Soc. 129, 10.1021/ja073201c (2007).

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