Formation kinetics of pendant arm polyamine macrocycles with copper(II)

The complexation reaction between Cu(ii) and a series of cyclam analogue ma crocycles with dangling primary amine groups was investigated in aqueous so lution. The PH dependence of the reaction was measured between PH 0 and 5.5 covering a range of different protonation stages of the ligands. The kinet ics was observed by stopped-flow measurements followed at many wavelengths. An initial second order reaction results in an intermediate which isomeris es to a stable complex. The observed rate constants for the initial step va ried between 1.5 x 10(-2) M-1 s(-1) at PH 0 and 6.3 x 10(5) M-1 s(-1) at PH 5.3; for the secondary step the rates varied between 2 x 10(-4) s(-1) at P H I and 24 s(-1) at PH 5.3. The PH dependence was successfully analysed in terms of the different reactivities for differently protonated ligands for the first step and differently protonated complexes for the second step. Li gands with unprotonated dangling primary amines react with rate constants o f 10(6)-10(7) M-1 s(-1), the rates drop dramatically upon protonation of th e arms to ca. 10-10(2) M-1 s(-1) which are very similar to the rates of the parent cyclam ligand of equivalent protonation. The PH dependence of the s econdary isomerisation reaction is explained in similar ways: protonated, d angling arms inhibit the isomerisation reaction completely.