New terpyridine-containing macrocycle for the assembly of dimeric Zn(II) and Cu(II) complexes coupled by bridging hydroxide anions and π-stacking interactions

The synthesis of the new terpyridine-containing macrocycle 2,5,8,11,14-pentaaza[15](6,6″)cyclo(2,2′:6′,2″) -terpyridinophane (L) is reported. The ligand contains a pentaamine chain linking the 6,6″ positions of a terpyridine unit. A potentiometric, ¹H NMR, UV-vis spectrophotometric and fluorescence emission study on the acid-base properties of L in aqueous solutions shows that the first four protonation steps occur on the polyamine chain, whereas the terpyridine nitrogens are involved in proton binding only at strongly acidic pH values. L can form both mono- and dinuclear Cu(II), Zn(II), Cd(II), and Pb(II) complexes in aqueous solution. The crystal structures of the Zn(II) and Cd(II) complexes {[ZnLH]₂(μ-OH)}(ClO₄)₅ (6) and {[CdLH] ₂(μ-Br)}(ClO₄)₅·4H₂O (7) show that two mononuclear [MLH]³⁺ units are coupled by a bridging anion (OH^- in 6 and Br^- in 7) and π-stacking interactions between the terpyridine moieties. A potentiometric and spectrophotometric study shows that in the case of Cu(II) and Zn(II) the dimeric assemblies are also formed in aqueous solution containing the ligand and the metals in a 1:1 molar ratio. Protonation of the complexes or the addition of a second metal ion leads to the disruption of the dimers due to the increased electrostatic repulsions between the two monomeric units.