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 Cu 2+ and Pb 2+ with 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-N,N ′-diacetic acid (H 2 L 2 ) and the copper(II) complex of 1,4,10-trioxa-7,13-diazacyclopentadecane-N,N′ -diacetic acid (H 2 L 1 ) have been determined at 25.0 °C in aqueous media (I = 0.20 mol dm -3 , LiClO 4 ). In the presence of an excess of strong acid, 1.3–100 mmol dm -3 HClO 4 , 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 [PbL 2 ] [CuL 2 ] > [CuL 1 ]. The formation rate constants for M 2+ reacting with (HL 2 ) - and H 2 L 2 are (2.3–4.2) × 10 8 and (2.2–17) × 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 - O 2 CCH 2 N(CH 2 CH 2 OCH 2 CH 2 OCH 2 CH 2 ) 2 N + HCH 2 CO 2 H. The dissociation of [PbL 2 ], but neither of the copper complexes, is subject to general acid catalysis (HA = MeCO 2 H and ClCH 2 CO 2 H). The exchange reaction of [PbL 2 ] with Cu 2+ in buffered (MeCO 2 H–MeCO 2 - ) mildly acidic media shows a first-order dependence on [H + ], [MeCO 2 H] and [Cu 2+ ].