Mono- and dinuclear CuII and ZnII complexes of cyclen-based Bis(macrocycles) containing two aminoalkyl pendant arms of different lengths

Abstract : The basicity and coordination properties towards CuII and ZnII of the bis(macrocycles) L1, L2 and L3 have been investigated by means of potentiometric, 1H NMR and UV/Vis spectroscopic titrations in aqueous solutions. The synthesis of L1 and L3 is also described. The three ligands are composed of two [12]aneN4 units separated by a p-phenylene spacer and differ in the length of the aminoalkyl pendant arms linked to each macrocyclic unit. L1-L3 form mono- and dinuclear complexes in aqueous solutions; in the dinuclear species each metal ion is coordinated by one of the two identical [12]aneN4 ligand moieties, as shown by the crystal structures of the complexes [Cu2L1]Cl 4·8H2O, [Zn2L2](ClO4) 4 and [Zn2L3](ClO4)4·H 2O. In all structures the metal ion is penta-coordinate, and is bound to the four nitrogen donors of the cyclic unit and to the amine group of the side arm. The stability of both the [ML]2+ and [M 2L]4+ complexes in aqueous solution decreases in the order L1 > L2 > L3. At the same time, both the [Cu2L] 4+ and [Zn2L]4+ complexes show a different ability in proton binding among the three ligands, with the [M 2L1]4+ complexes displaying the highest basicity. These results are explained in terms of the decreasing number of nitrogen donors involved in CuII or ZnII binding on passing from L1 to L3; in other words, while in the L1 dinuclear complexes each metal ion is coordinated to the four amine groups of a [12]aneN4 moiety and to the amine group of the side arm, in the L3 ones the metal cations are bound only to the four donor atoms of a cyclic moiety, the aminobutyl group not being coordinated. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.