Hexaazamacrocycle containing pyridine and its dicopper complex as receptors for dicarboxylate anions


The host-guest binding interactions of the hexaazamacrocycle [26]py(2)N(4), in its tetraprotonated form H-4[26]py(2)N(4)(4+) as well as in its dicopper(II) complex [Cu-2([26]py(2)N(4))(H2O)(4)](4+), with dicarboxylate anions of different stereoelectronic requirements, such as oxalate (ox(2-)), malonate (mal(2-)), succinate (suc(2-)), fumarate (fu(2-)) and maleate (ma(2-)), were evaluated. The association constants were determined using potentiometric methods in aqueous solution, at 298.0 K and 0.10 mol.dm(-3) KCl. These values for the tetraprotonated ditopic receptor with the dicarboxylate anions revealed that the main species in solution corresponds to the formation of {H-4[26]py(2)N(4)(A))21 (pH approximate to 4-9), A being the substrate anion. The values determined are not especially high, but the receptor exhibits selectivity for the malonate anion. The study of the cascade complexes revealed several species in solution, involving mononuclear and dinuclear complexes, mainly protonated and hydrolysed species, as well as the expected complexes [Cu-2([26]py(2)N(4))(A)(H2O)(2+) or [Cu-2([26]py(2)N(4))(A)(2)(H2O)(y)]. Ox(2-) and mal(2-) form cascade complexes with only one anion, which will necessarily bridge the two copper atoms because of the symmetrical arrangement of the dinuclear complex. The two other studied anions, suc(2-) and ma(2-), form species involving two substrate anions, although species with only one suc(2-) anion were also found. UV/Vis and EPR spectroscopy have shown that the dicopper complex can operate as a sensor to detect and quantitatively determine oxalate spectrophotometrically because of the red shift of the maximum of the visible band observed by addition of ox(2-) to an aqueous solution of the dinuclear copper complex. However the selectivity of [Cu-2([26]py(2)N(4))(H2O)(4)](4+) as a receptor for ox(2-) in the studied series is not sufficiently high to detect ox(2-) spectrophotometrically in the presence of the other anions. Molecular dynamics simulations indicated that the H-4[26]py(2)N(4)(4+) receptor provides a large and flexible cavity to accommodate the studied anions. Molecular recognition is based in electrostatic interactions rather than in multiple hydrogen-bonding interactions acting cooperatively. By contrast, the [Cu-2([26]py(2)N(4))](4+) receptor has a well-shaped cavity with adequate size to uptake these anions as bridging ligands with formation of four Cu-O bonds. The ox(2-) anion is encapsulated within the cascade complex while the remaining anions are located above the N-6 macrocyclic plane, suggesting a selective coordination behaviour of this receptor. In spite of our molecular simulation being carried out in gas phase, the modelling results are consistent with the solution studies. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)



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Li, F; Delgado, R; Felix, V

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