Synthesis and coordination properties of new macrocyclic ligands: equilibrium studies and crystal structures

Abstract

The synthesis and characterization of two new macrocyclic ligands, the bis-macrocyclic compound 2,6-bis(1,4,13-triaza-7,10-dioxacyclopentadec-1-ylmethyl)phenol (L) and 38-methoxy-1,4,13,16,19,28-hexaaza-7,10,22,25-tetraoxatricyclo[14.14.7.1^32,36]octatriconta-32,34,Δ^36,38-triene (L1) are reported. Equilibrium studies of basicity and coordination properties toward metal ions such as Cu(II), Zn(II), Cd(II) and Pb(II) were performed for ligand L by potentiometric measurements in aqueous solution (298.1 ± 0.1 K, I = 0.15 mol dm⁻³). L behaves as a hexaprotic base (logK₁ = 10.93, logK₂ = 9.70, logK₃ = 8.79, logK₄ = 8.05, logK₅ = 6.83, logK₆ = 2.55). All metal ions form stable mono- and dinuclear complexes: logK_MLH−1 = 25.61 for Cu(II), 15.37 for Zn(II), 12.58 for Cd(II) and 13.79 for Pb(II); logK_M2LH−1 = 31.61 for Cu(II), 23.38 for Zn(II), 24.49 for Cd(II) and 23.68 for Pb(II). All these dinuclear species show a great tendency to add the OH⁻ group: the equilibrium constant for the addition reaction was found to be logK_M2LH−1OH = 4.77 for Cu(II), 5.66 for Zn(II), 2.8 for Cd(II) and 3.18 for Pb(II). In the case of Ni(II), kinetic inertness prevents the possibility of solution studies. The dinuclear solid adducts [Ni₂H₋₁L(N₃)₃]·EtOH and [Cu₂H₋₁L(N₃)](ClO₄)₂ were characterized by X-ray analysis.