Synthesis, crystal structure and formation kinetics of zinc(II) and copper(II) complexes with 5,12-dimethyl-1,4,8,11-tetraazacyclotetradecane

The complex [Zn(L)Cl][ClO4](1) (L = 5,12-dimethyl-1,4,8,11-tetraazacyclotetradecane) has been prepared and characterized by X-ray crystallography. The structure of 1 consists ofisolated [Zn(L)C1](+) cations and ClO4- counter anions. The coordination of the zinc atom is a distorted square pyramid with four nitrogens of the macrocycle occupying the basal sites [Zn-N-av = 2.121(9) Angstrom] and a terminal chloride ligand at the apical position with a Zn-Cl distance of 2.275(3) Angstrom. The complexation reactions of Zn2+ and Cu2+ with L have been studied by using stopped-flow spectrophotometry. The measurements were made at 25.0+/-0.1 degrees C in an aqueous solution of 0.10 M (NaClO4) ionic strength. The formation reaction takes place by rapid formation of an intermediate complex (MH(2)L(2-*)), which proceeds to the final product in the rate-determining step. The stability constants (logK(MH2L2-*)) and specific base-catalysed rate constants (k(OH)) of intermediate complexes have been determined from the kinetic data. In the pH range examined, the monoprotonated (HL(3-)) form of L is the kinetically active species despite its low concentration. (C) 1997 Elsevier Science Ltd.