abstract
A hexaamine cage with pyridyl spacers was synthesized in good yield by a [2+3] Schiff-base condensation followed by sodium borohydride reduction. The protonation constants of the receptor as well as its association constants with Cl(-), NO(3)(-), AcO(-), ClO(4)(-), SO(4)(2-), H(2)PO(4)(-) and H(2)AsO(4)(-) were determined by potentiometry at 298.2 +/- 0.1 K in H(2)O/MeOH (50:50 v/v) and at ionic strength 0.10 +/- 0.01 M in KTsO. These studies revealed that although dihydrogen phosphate is less charged than sulfate, it is still appreciably bound by the receptor at low pH, suggesting that the pyridyl nitrogen is accepting hydrogen bonds from dihydrogen phosphate. It is also shown that dihydrogen phosphate is capable of effectively competing with sulfate for the receptor at higher pH, being selective for hydrogen phosphate at pH about 7.0. (31)P NMR experiments supported these findings. The fact that the receptor shows such a marked preference for hydrogen phosphate based mainly in its hydrogen bond accepting/donating ability in a highly competitive medium such as water/methanol mixed solvent is quite remarkable. Single-crystal X-ray diffraction determinations of anion associations between H(6)Pyr(6+) receptor and nitrate, sulfate, and phosphate are consistent with the existence of [(H(6)Pyr)(NO(3))(3)(H(2)O)(3)](3+), [(H(6)pyr)(SO(4))(2)(H(2)O)(4)](2+), and [(H(6)pyr)(HPO(4))(2)(HPO(4))(2)(H(2)PO(4))-(H(2)O)(2)](+) cations. One nitrate anion is embedded into the H(6)Pyr(6+) cage of the first supermolecule whereas in the second and third ones the anions are located in the periphery of the macrobicycle.
keywords
ANION COORDINATION CHEMISTRY; SULFATE-BINDING PROTEIN; POLYAMMONIUM RECEPTORS; AQUEOUS-SOLUTION; HYDROGEN-BONDS; MACROBICYCLIC RECEPTOR; MOLECULAR RECOGNITION; BICYCLIC CYCLOPHANE; THERMODYNAMICS; EQUILIBRIA
subject category
Chemistry
authors
Mateus, P; Delgado, R; Brandao, P; Felix, V
our authors
Paula Cristina Ferreira da Silva Brandão
Assistant Researcher
Vítor Félix
Associate Professor with Aggregationacknowledgements
m The authors acknowledge FCT, with coparticipation of the European Community funds FEDER, for Financial support under projects PTDC/QUI/56569/2004 and PTDC/QUI/68582/2006. The NMR spectrometers are part of the National NMR Network and were acquired with funds from FCT and FEDER. We also acknowledge M. C. Almeida for providing elemental analysis and ESI-MS data from the Elemental Analysis and Mass Spectrometry Service at the ITQB. Pedro Mateus thanks FCT for grant SFRH/BD/36159/2007.