Complexation properties of a new macrocyclic polyaminic ligand (L) containing amidic pendant arms: crystal structure of [PbL](ClO4)2

Synthesis and characterisation of the new macrocyclic ligand 1,7-dimethyl-4,10-di(methylcarbamoylmethy)-1,4,7,10-tetraazacyclododecane (L) are reported. The ligand, based on cyclen (1,4,7,10-tetraazacyclododecane), has been functionalised by the insertion of two methyl groups and two amidic pendant arms linked to the amine nitrogens. The interaction of L with H⁺, Na(I), Ca(II), Cu(II), Zn(II), Pb(II), and Gd(III) ions has been studied by potentiometric titrations, microcalorimetric and ¹H NMR measurements in 0.1 mol dm⁻³ Me₄NCl aqueous solution at 298.1±0.1 K. The thermodynamic data suggest that the N₄ moiety is the binding site for Cu(II) and Zn(II), while in the case of Pb(II) also the pendant arms are coordinated to the metal ion. The crystal structure of PbL](ClO₄)₂ (space group P2₁/a, a=12.883(2) Å, b=12.259(3) Å, c=17.275(5) Å, β=108.65(2)°, V=2585.0(11) ų, Z=4, R=0.0660, R_W ²=0.1467) shows the metal ion hexa-coordinated by the four nitrogen atoms of the cyclic tetra-amine and by the two amidic oxygens of the pendant arms.