Bifunctional Cyclam-Based Ligands with Phosphorus Acid Pendant Moieties for Radiocopper Separation – Thermodynamic and Kinetic Studies

Two novel macrocyclic ligands based on trans-substituted cyclam with N-methyl and N-(4-aminobenzyl) groups as well as with two methylphosphinic (H₂L1) or methylphosphonic (H₄L2) acid pendant arms were synthesised and investigated in solutions. The ligands form stable complexes with transition metal ions. Both ligands show a high thermodynamic selectivity for divalent copper over nickel(II) and zinc(II) (K(CuL) is higher than K(Ni/ZnL) by about 7 orders of magnitude). Complexation is significantly faster for the phosphonate ligand H₄L2 probably due to stronger coordination ability of more basic phosphonate groups, which efficiently bind the metal ion in the out-of-cage complex and helps its in-cage binding. The rate of complexation of Cu(II) by the phosphinate ligand H₂L1 is comparable to that of cyclam itself and it derivatives with non-coordinating substituents. Acid-assisted decomplexation of the copper(II) complexes is relatively fast (½ 44 and 42 s in 1 M aq. HCl at 25 °C for H₂L1 and H₄L2, respectively). Combination of the properties is convenient for selective copper removal/purification. Thus, the title ligands were employed in preparation of ion-selective resins for radiocopper(II) separation. Glycidyl-methacrylate copolymer beads were modified with the ligands through diazotation reaction. The separation ability of the modified polymers was tested with cold copper(II) or non-carrier-added (NCA) ⁶⁴Cu in the presence of a high excess of both nickel(II) and zinc(II). The experiments exhibited high overall separation efficiency leading to 60–70 % recovery of radiocopper with high selectivity over the other metal ions originally present in 900-times molar excess. The results showed the concept of chelating resins with properly tuned selectivity of complexing moieties can be employed for radiocopper separation.