Recognition of anions and neutral guests by dicationic pyridine-2,6- dicarboxamide receptors

Alejandro Dorazco-González, Herbert Höpfl, Felipe Medrano, Anatoly K. Yatsimirsky

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(Figure Presented) Dicationic N-methylated at pyridyl or quinolyl moieties derivatives of three isomers of N,N' -bis(pyridyl)pyridine-2,6-dicarboxamide (o-, m-, and p-1) and of N,N'-bis(3-quinolyl)pyridine-2,6- dicarboxamide (4) strongly bind anions in MeCN (log AMn the range 3.5-6.5) with pronounced selectivity for Cl- and also bind neutral urea and amide guests with log Kin the range 1.1-2.8. Crystal structures of the trifiate salts of m-1, p-1, and 4 show that amide NH and pyridinium o-CH groups are directed inside the receptor cleft with their four protons forming a circle of radius ca. 2.35 A optimal for inclusion of CF. The binding of anions to these protons is evident from the crystal structure of a mixed triflate/chloride salt of P-1, calculated (DFT/B3LYP 6-31G**) structures of 1:1 complexes of all receptors with Cl-, and results of 1H NMR titrations. In the crystal structure of o-l pyridinium N-Me+ groups are directed inside the receptor cleft impeding the anion complexation, but calculations demonstrate that simple rotation of pyridinium rings in opposite directions by ca. 30° creates a cavity to which the Cl- ion can fit forming 4 hydrogen bonds to amide NH and aliphatic CH groups of N-Me+. The results of 1H NMR titrations confirm this type of binding in solution. Anions quench the intense fluorescence of 4, which allows their fluorescent sensing in the μM range. A new methodology for determination of anion binding constants to strongly acidic receptors by inhibitory effects of anions on the receptor deprotonation by an external base has been developed. High affinity and selectivity of anion complexation by dicationic pyridine-2,6-dicarboxamides is attributed to the rigid preorganized structure of receptors, the high acidity of NH and CH groups, and the electrostatic charge effect.

Original languageEnglish
Pages (from-to)2259-2273
Number of pages15
JournalJournal of Organic Chemistry
Issue number7
StatePublished - 2 Apr 2010


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