Cyclam (1,4,8,11-tetraazacyclotetradecane) with one methylphosphonate pendant arm: a new ligand for selective copper(II) binding
Abstract
Preparation of new ligand, study of complexing properties with transition metal ions. The new ligand, [(1,4,8,11-tetraazacyclotetradecan-1-yl) methyl] phosphonic acid (H(2)te1P, H2L), was synthesized and its complexing properties towards selected metal ions were studied potentiometrically.
The ligand forms a very stable complex with copper(II) (log β(CuL) = 27.34), with a high selectivity over binding of other metal ions (i.e. log β(ZnL) = 21.03). The crystal structures of the free ligand (in its protonated form with bromide as counter-ion) and two copper( II) complexes ( obtained by crystallization at various pH) were determined.
The free ligand adopts the common conformation for such macrocycles with the protonated nitrogen atoms in the corners of a virtual rectangle. In the trans-Br, O-[Cu(Br)(Hte1P)].
H2O species, the central metal ion is surrounded by four in plane nitrogen atoms, one oxygen atom of the pendant moiety in the apical position and a bromide anion positioned trans to the oxygen atom, forming a distorted octahedral coordination sphere. In the compound [Cu(H2te1P)][Cu(Hte1P)]Br3 . 6 H2O, obtained from a highly acidic solution, the bromide anions are placed further away from the copper(II) ion and the coordination environment (N4O) is thus square-pyramidal.
In both structures, the protons are associated with non-coordinated phosphonate oxygen atoms.