Anion binding by a binuclear Cu(II) polyamine macrocyclic complex
Introduction
The involvement of metal ions in anion binding processes is giving an important contribution to the development of the emerging area of anion coordination, in which metal free receptors and metallated receptors are defining two contiguous sub-areas of investigation.1, 2, 3 The activity of metal ions in promoting and/or enhancing anion binding takes place via different mechanisms which may involve: (i) direct coordination, (ii) propagation of their Lewis acidity to a neighbouring binding site, (iii) structuring effect on the receptor. A few examples4, 5, 6, 7 of these binding modes are shown in Figure 1; many more can be found in the literature.8 Direct binding of anions to metal centres is a typical example of coordination chemistry. The novelty of anion receptors consists in offering to the anion a well-defined coordination environment in which the anion can be selectively coordinated. In this respect binuclear macrocyclic complexes may be efficient anion receptors, as illustrated in Figure 1a and b.4, 5 A nice example of enhanced anion binding ability determined by propagation of the Lewis acidity of coordinated metal cations is given by the cleft system in Figure 1c.6 The presence of the two metal cations increases the acidity of the amide NH protons strengthening the hydrogen bond interaction with the anion. As a matter of fact, the receptor containing Ru was found to bind anions more weakly than those containing Os, which display a greater Lewis acidity.
Figure 1d depicts a significative example of the structuring function that metal ions may have in determining the shape of the anion receptors. In this particular case, coordination of a K+ ion leads to the formation of an intramolecular sandwich-like complex that reduces the size of the anion hosting cavity favouring the selective binding of Cl− over H2PO4−.7
In this paper, we report the crystal structure of the binuclear [(Cu2L)Cl]⊃Cl(H2O)(ClO4)2 complex with the macrocyclic ligand L (1,4,7,10,19,22,25,28-octaaza-13,16,31,34-tetraoxa-cyclohexatriacontane), in which both direct and indirect activation of anion binding promoted by Cu2+, as well as the structuring effect of this metal cation on anion binding, synergistically participate in the assembly of a polymeric inclusion compound in the solid state.
Section snippets
Results and discussion
L is a ditopic ligand forming a large number of very stable, mono- and bi-nuclear Cu2+ complexes in aqueous solution. The stability constants determined for such species 0.1 mol dm−3 Me4NCl aqueous solution at 298.1 K are reported in Table 1. According to its ditopic nature, L displays very similar equilibrium constants for the binding of both first and second metal ion, which are lodged in identical ligand moieties separated by long ethereal spacers. Similarly, one metal ion can be bound at
Materials
The ligand L was synthesised as already reported.14 Crystals of {[(Cu2L)Cl]⊃Cl(H2O)(ClO4)}2 suitable for X-ray analysis were obtained by slow evaporation, at room temperature, of an aqueous solution containing L and 2 equivalents of CuCl2 after addition of NaClO4 in excess. The complex gave satisfactory elemental analysis.
Potentiometric measurements
All potentiometric measurements were performed in 0.1 mol dm−3 Me4NCl aqueous solution at 298.1 K. The apparatus and the procedures adopted were described previously.15 Ligand
Acknowledgements
Financial support from the Italian MURST (COFIN2000) is gratefully acknowledged.
References (17)
Coord. Chem. Rew
(2000)- et al.
Inorg. Chim. Acta
(1996) - Bianchi, A.; Bowman-James, C.; Garcia-España, E., Eds. Supramolecular Chemistry of Anions; Wiley-VCH: New York,...
- et al.
Angew. Chem. Int. Ed.
(2001) - et al.
Chem. Commun.
(1998) - et al.
Inorg. Chem.
(1999) - et al.
Chem. Commun.
(1997) - et al.
Chem. Commun.
(1998)