Metal Complexes with Macrocyclic Ligands. XI. Ring size effect on the complexation rates with transition metal ions

✍ Scribed by Alex P. Leugger; Liselotte Hertli; Thomas A. Kaden

• Publisher: John Wiley and Sons
• Year: 1978
• Tongue: German
• Weight: 509 KB
• Volume: 61
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.19780610703

✦ Synopsis

Abstract

The 12‐16 membered tetraazamacrocycles 1‐6 were synthesized, their protonation constants and complexation kinetics measured at 25° and I = 0.50. The results of Table 1 Show that p__K__ is strongly influenced by the ring size whereas p__K__ and p__K__ are relatively insensitive to it. This can be understood in terms of electrostatic interactions of the positive charges when located on adjacent amino groups.

The kinetics of complex formation between the macrocyclic ligands and several transition metal ions have been studied by pH‐stat and stopped‐flow techniques and the results have been analyzed as bimolecular reactions between the metal ion and the different protonated species of the ligands. The rate constants, given in Table 2, show that the macrocycles react less rapidly than analogous open chain amines. However, for a given protonated species of the ligand the rate of complexation follows the order Cu^2+^ > Zn^2+^ > Co2^+^ > Ni^2+^ which parallels the sequence of their water exchange rates. For the diprotonated tetraamines LH reacting with Cu^2+^ the slower rates seem to be mainly a consequence of electrostatic interactions, since a correlation between log__k__ and p__K__ exists. For LH^+^, however, the complexation rates of a metal ion with the different macrocycles are all in one order of magnitude and do not depend in a regular way on the ring size or the basicity of the ligand. It is therefore suggested that in this case other factors such as unfavourable preequilibria must be considered as important.