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

We have prepared and characterized a new phenol-based compartmental ligand (H2L) incorporating 1,4,7-triazacyclononane ([9]aneN3), and we have investigated its coordination behavior with CuII, ZnII, CdII, and PbII. 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 [M2(H2L)]4+ occurs at neutral or slightly acidic pH and is generally followed
by metal-assisted deprotonation of the phenolic groups to give [M2(HL)]3+ and [M2(L)]2+ in weakly basic solutions.
The complexation properties of H2L have also been investigated in the solid state. Crystals suitable for X-ray
structural analysis were obtained for the binuclear complexes [Cu2(L)](BF4)2·1/2MeCN (1), [Zn2(HL)](ClO4)3·1/2MeCN
(2), and [Pb2(L)](ClO4)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 N5O-donor compartment. However, in the case of the binuclear complex
[Pb2(L)](ClO4)2·2MeCN (4), the two PbII centers are bridged by the phenolate oxygen atoms with each metal ion
sited within an N5O2-donor compartment of L2-, with a Pb···Pb distance of 3.9427(5) Å.