Premium
Metal Complexes with Macrocyclic Ligands. XI . Ring size effect on the complexation rates with transition metal ions
Author(s) -
Leugger Alex P.,
Hertli Liselotte,
Kaden Thomas A.
Publication year - 1978
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19780610703
Subject(s) - chemistry , protonation , ring size , metal , reaction rate constant , ligand (biochemistry) , ring (chemistry) , kinetics , metal ions in aqueous solution , ion , transition metal , crystallography , inorganic chemistry , stereochemistry , computational chemistry , organic chemistry , catalysis , biochemistry , physics , receptor , quantum mechanics
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 3 His strongly influenced by the ring size whereas p K 2 Hand p K 1 Hare 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 2 2reacting with Cu 2+ the slower rates seem to be mainly a consequence of electrostatic interactions, since a correlation between log k LH 2Cuand p K 3 Hexists. 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.