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⁺ ].