Polynuclear Complexes: Two Amino-Phenol Macrocycles Spaced by Several Linear Polyamines: Synthesis, Binding Properties, and Crystal Structure

The synthesis and characterization of the new polytopic ligands 1,14-bis(3,6,9-triaza-15-hydroxybicyclo[9.3.1]pentadeca-11,13,115-trien-6-yl)-3,6,9,12-tetraazatetradecane L1, 1,15-bis(3,6,9-triaza-15-hydroxybicyclo[9.3.1]pentadeca-11,13,115-trien-6-yl)-3,6,10,13-tetraazapentadecane L2, and 1,16-bis(3,6,9-triaza-15-hydroxybicyclo[9.3.1]pentadeca-11,13,115-trien-6-yl)-3,7,10,14-tetraazahexadecane L3, containing two equal amino-phenol macrocycles spaced by several linear tetraamines, are reported. The basicity and coordination behavior toward the Cu(II) ion were potentiometrically determined in aqueous solution at 298.1 K. All the ligands show similar acid-base properties behaving as octaprotic bases in the examined pH range (pH = 2-12). The acid protons of L1-L3 cannot be removed under the experimental conditions used; thus, the main deprotonated species obtainable in aqueous solution are the neutral ligands, having amphionic character as demonstrated by UV-vis experiments. These species are able to form mono-, di-, and trinuclear Cu(II) complexes having stoichiometry [CuL]2+, [Cu2L]4+, and [Cu3L]6+, respectively, that can lose one or two protons giving rise to [CuH-1L]+, [Cu2H-2L]2+, and [Cu3H-2L]4+. Depending on the used ligand to metal molar ratio, the mono-, di-, or trinuclear species prevail over the others in solution. Both di- and trinuclear complexes are able to add secondary ligands (such as OH-), and in some cases two Cu(II) can cooperate to stabilize themby coordinating the guest in a bridged conformation. The structure of the [Cu2L3]4+ cation was resolved by X-ray analysis of the {[Cu2L3](ClO4)4 3 3H2O}2 3 H2O crystalline complex. It shows that each Cu(II) is penta-coordinated by one phenolate oxygen, two amine functions, belonging to one macrocyclic unit, and two amine functions of the spacer; in this species the distance between the two Cu(II) is about 5.3 Å.