Synthesis of cryptand-like calix[4]arene derivatives and their binding abilities towards transition metal ions

Abstract

25, 26, 27, -N, N', N" –tris (2-ethoxy)benzyl)ethylenetetraamine-p-tert-butylcalix[4]arene, 6a and 25, 26, 27,-N, N', N" –tris( (4-ethoxy)benzyl) ethylenetetraamine-p-tert-butylcalix[4]arene, 6b, have been synthesized. Ammonium derivatives of 6a and 6b were used for the investigation of proton affinity and binding properties towards transition metal ions including Co²⁺, Cu²⁺ and Zn²⁺ by means of potentiometric titration. The titrations were carried out in 1.00x10⁻² M Bu₄NCF₃SO₃ in methanol at 25℃. Ligands 6a and 6b exhibited 4 protonation constants in the pH range of 2-11.5 that revealed to proton affinity of both secondary and tertiary amine groups in the tris (2-aminoethyl) amine (tren) moiety. First two protonation constants of ligand 6a were higher than that of 6b while the third and fourth proton affinity in 6b were higher than 6a. Both 6a and 6b were able to form 1:1 complexes with Cu²⁺ ion as well as Co²⁺. However, in case of Zn²⁺ ions, both mononuclear and dinuclear complexes were formed. The complexation ability of both 6a and 6b varied as follows: Co²⁺<<Cu²⁺ >> Zn²⁺. The stability of metal complexes for 6a and 6b may be affected from the geometry of each metal ion and a size of the binding unit in their molecules. Hydroxo species were found in monomuclear complexes of Zn²⁺ with 6a and Co²⁺ with 6b in alkaline solutions.