Mono- and bi-nuclear copper(II) complexes with polyazacyclophane receptors containing two different binding sites

Abstract

The macrobicyclic compound 1,4,7,-trimethyl-19,22,28,31-tetraoxa-1,4,7,14,23-pentaaza[9.25]-p-cyclophane (L¹) has been prepared. It comprises a N₃ and a cyclic N₂O₄ subunit, connected by p-phenylene spacers. Protonation of L¹ (and L², in which the N₂O₄ unit is replaced by a N₅ moiety) has been studied by means of potentiometric measurements. The compounds L¹ and L² bind up to five and six protons in aqueous solution above pH 2, respectively. Proton and ¹³C NMR spectra at different pH values allow the determination of the stepwise protonation sites. Considering the [H₄L¹]⁴⁺ species, the acidic protons are located on the benzylic nitrogens. Copper(II) co-ordination by L¹ and L² has been potentiometrically studied (298.1 K, 0.1 mol dm³ NMe₄Cl aqueous solution): L¹ forms only a mononuclear complex, while L² gives both mono- and bi-nuclear species in aqueous solution. In the [CuL¹]²⁺ complex the metal is co-ordinated by the three tertiary nitrogens of the N₃ unit, while the N₂O₄ moiety shows a high tendency to protonation. These solution data are confirmed by the crystal structure of [Cu(HL¹)Cl(H₂O)][ClO₄]₂. In the [Cu(HL¹)Cl(H₂O)]²⁺ cation the metal is co-ordinated by the three amine groups of the triaza moiety and by a chloride anion, in a rather unusual square-planar geometry. A water molecule is encapsulated by the N₂O₄ moiety, held by a hydrogen-bond network. In the mononuclear [CuL²]²⁺ complex the Cu^II is preferentially lodged inside the pentaaza moiety. Such a complex can add a further copper(II) ion, which is co-ordinated by the N₃ binding unit.