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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 32, Number 3
BKCSDE 32(3)
March 20, 2011 

Formation of a Unique 1:2 Calcium-Calixquinone Complex in Aqueous Media
Sun Kil Kang, One Sun Lee, Suk Kyu Chang, Doo Soo Chung, Hasuck Kim, Taek Dong Chung*
Calix[4]arenemonoquinone-triacid, Calcium ion, NMR titration, Binding constant, Carlo simulation
We report the complexation behavior of calix[4]arenemonoquinone-triacid (CTAQ), which is an electroactive and water-soluble receptor for calcium ion. UV-visible and NMR spectroscopic studies revealed that CTAQ in aqueous media forms 1:2 as well as 1:1 (metal ion:CTAQ) stoichiometric complexes with Ca2+, Sr2+, and Ba2+ ions. The nonlinear fitting of titration curves based on UV-visible absorption spectra showed that the binding constants of CTAQ for Ca2+ ion are 4 (± 2) × 106 M–1 for 1:1 and 1.4 (± 0.5) × 1011 M–2 for 1:2 complex. NMR conformational studies and the titration curves corroborate that the Ca2+:CTAQ complex in aqueous solution is not present in the form of merely 1:1 one, being consistent with UV-visible spectrophotometric results. The Monte Carlo simulation supports the presence of a stable conformer of 1:2 complexes in which a Ca2+ ion is interposed between two CTAQs at the global minimum. This is the first model of 1:2 stoichiometric complex of calix[4]arene and alkaline earth ions in aqueous media.
793 - 799
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