Acid-catalysed dissociation of the copper(II) and lead(II) complexes of macrocyclic diazapolyoxa-N,N'-diacetic acids

Abstract

The dissociation rate constants of the complexes of Cu2+ and Pb2+ with 1,4,10,13-tetraoxa-7,16-diazacyclo-octadecane-N,N'-diacetic acid (H2L2) and the copper(II) complex of 1,4,10-trioxa-7,13-diazacyclopentadecane-N,N'-diacetic acid (H2L1) have been determined at 25.0 degrees C in aqueous media (I = 0.20 mol dm(-3), LiClO4). In the presence of an excess of strong acid, 1.3-100 mmol dm(-3) HClO4, the dissociation reactions follow the rate law -d[ML](T)/dt = (k(d) + K(MHL)k(H1)[H+] + K(MHL)k(H2)[H+](2))/(1 + K-ML[H+])[ML](T) where [ML](T) = [ML] + [MHL], k(H1), and k(H2) are the respective rate constants for the specified reactions and MHL is the protonated metal-ligand complex. The values of k(Hi) (i = 1 or 2) decrease according to the sequence [PbL2] much greater than [CuL2] > [CuL1]. The formation rate constants for M2+ reacting with (HL2)(-) and H2L2 are (2.3-4.2) x 10(8) and (2.2-17) x 10(2) dm(3) mol(-1), respectively. The relatively low values for the diprotonated species are attributed to an unfavourable equilibrium concentration of the reactive species -O2CCH2N(CH2CH2OCH2CH2OCH2CH2)(2)N+HCH2CO2H. The dissociation of [PbL2], but neither of the copper complexes, is subject to general acid catalysis (HA = MeCO2H and ClCH2CO2H). The exchange reaction of [PbL2] with Cu2+ in buffered (MeCO2H-MeCO2-) mildly acidic media shows a first-order dependence on [H+], [MeCO2H] and [Cu2+].