Formation of a Unique 1:2 Calcium-Calixquinone Complex in Aqueous Media

Abstract

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) x 10(6) M-1 for 1:1 and 1.4 (+/- 0.5) x 10(11) 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.