Coordination properties of a new hexaazamacrocycle containing thiophene units as pendant arms

The synthesis of the macrocyclic ligand 1,4,7,13-tetramethyl-10,16-bis(o-thiophenylmethyl)-1,4,7,10,13,16-hexaazacyclooctadecane (L2) is reported. This compound presents a hexaaza macrocyclic backbone bearing two thiophenylmethyl side arms. Ligand protonation and Cu(II), Zn(II), Cd(II) and Pb(II) coordination were studied by means of potentiometric measurements. In the metal complexes all nitrogens seem to be involved in metal coordination, while the sulfur atoms of the heteroaromatic subunits do not participate in metal binding, as confirmed by the crystal structure of [PbL2Cl]·BPh4. In the [PbL2Cl]+ cation the metal is enveloped by the macrocyclic ligand, coordinated by the six nitrogens and a chloride anion with a distorted monocapped trigonal prismatic geometry. Although L2 forms complexes of lower stability with respect to other N-functionalized hexaazamacrocycles, this ligand displays selective coordination of the larger Cd(II) and Pb(II) over Zn(II). All the metal complexes under investigation are readily extractable from aqueous solutions to organic solvents. Therefore, L2 is a promising selective extracting agent for metal cations.