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 cont aining acetic acid pendant arms on one hand and their methylphosphonic or m ethylphosphinic acid analogues on the other is made mainly with complexes o f copper and lanthanides due to their applications in medicine. The stabili ty constant values of Cu(II) complexes are determined mainly by the basicit y of amine groups. The influence of other possible effects, such as the mac rocyclic 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 penda nts 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 t he phosphonic acid ligands are also lower than those with H(4)dota derivati ves. A comparison of co-ordination polyhedra of the carboxylic and phosphon ic 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) b ond in the phosphorus derivatives than that CC(O)in the acetate pendants. C onsequently, the lanthanide(III) complexes with phosphorus acid derivatives are more sterically hindered; oxygen atoms in the O-4 plane are close to o ne another and there is insufficient room for co-ordination of a water mole cule. which is crucial in MRI applications. (C) 2001 Elsevier Science B.V. All rights reserved.