Complexes of tetraazacycles bearing methylphosphinic/phosphonic acid pendant arms with copper(II), zinc(II) and lanthanides(III). A comparison with their acetic acid analogues

A comparison of complexing properties of cyclen and cyclam derivatives containing acetic acid pendant arms on one hand and their methylphosphonic or methylphosphinic acid analogues on the other is made mainly with complexes of copper and lanthanides due to their applications in medicine. The stability constant values of Cu(II) complexes are determined mainly by the basicity of amine groups. The influence of other possible effects, such as the macrocyclic effect in tetraazacycle in contrast to linear amines or the number of additional donor atoms in or outside of pendant arms and their basicity, seems to be very small. The stability constants for Gd(III), in addition to the basicity of amines, are also influenced by the basicity of the pendants and their number. The values of cyclam derivatives are lower than those of cyclen and it corresponds to the ring size effect as was found for zinc(II) complexes. In contrast to Cu(II), the Gd(III) stability constants of the phosphonic acid ligands are also lower than those with H4dota derivatives. A comparison of co-ordination polyhedra of the carboxylic and phosphonic or phosphinic derivatives shows similar motifs that are more determined by the macrocyclic effect than by the kind of donor groups in pendant arms. The differences between the polyhedra observed result from longer C–P(O) bond in the phosphorus derivatives than that C–C(O) in the acetate pendants. Consequently, the lanthanide(III) complexes with phosphorus acid derivatives are more sterically hindered; oxygen atoms in the O4 plane are close to one another and there is insufficient room for co-ordination of a water molecule, which is crucial in MRI applications.