Coordination Chemistry of a New Co-facial Binucleating Macropolycycle Derived from 1,4,7-Triazacyclononane

We have prepared and characterized a new phenol-based compartmental ligand (H 2L) incorporating 1,4,7-triazacyclononane ([9]aneN 3), and we have investigated its coordination behavior with Cu II, Zn II, Cd III, and Pb II. The protonation constants of the ligand and the thermodynamic stabilities of the 1:1 and 2:1 (metal/ligand) complexes with these metal ions have been investigated by means of potentiometric measurements in aqueous solutions. The mononuclear [M(L)] complexes show remarkably high stability suggesting that, along with the large number of nitrogen donors available for metal binding, deprotonated phenolic functions are also involved in binding the metal ion. The mononuclear complexes [M(L)] show a marked tendency to add a second metal ion to afford binuclear species. The formation of complexes [M 2(H 2L)] 4+ occurs at neutral or slightly acidic pH and is generally followed by metal-assisted deprotonation of the phenolic groups to give [M 2(HL)] 3+ and [M 2(L)] 2+ in weakly basic solutions. The complexation properties of H 2L have also been investigated in the solid state. Crystals suitable for X-ray structural analysis were obtained for the binuclear complexes [Cu 2(L)](BF 4) 2·1/2MeCN (1), [Zn 2(HL)] (ClO 4) 3·1/2MeCN (2), and [Pb 2(L)](ClO 4) 2·2MeCN (4). In 1 and 2, the phenolate O-donors do not bridge the two metal centers, which are, therefore, segregated each within an N 5O-donor compartment. However, in the case of the binuclear complex [Pb 2(L)](ClO 4) 2·2MeCN (4), the two Pb II centers are bridged by the phenolate oxygen atoms with each metal ion sited within an N 5O 2-donor compartment of L 2-, with a Pb⋯Pb distance of 3.9427(5) Å.