Polyaza Cryptand Receptor Selective for Dihydrogen Phosphate

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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-, NO3-, AcO-, ClO4-, SO42-, H2PO4- and H2AsO4- were determined by potentiometry at 298.2 +/- 0.1 K in H2O/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. P-31 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)(NO3)(3)(H2O)(3)](3+), [(H(6)pyr)(SO4)(2)(H2O)(4)](2+), and [(H(6)pyr)(HPO4)(2)(HPO4)(2)(H2PO4)-(H2O)(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.