Comparative stability constants for metal ions with tetraazamacrocycles of various ring sizes carrying a single amine or carboxylate pendant group Yakup Baran Department of Chemistry, The University of Karaelmas, 67100 Zonguldak, Turkey Anne Bayada, Geoffrey A. Lawrance*, Peter G. Lye, Marcel Maeder and Eric N. Wilkes Discipline of Chemistry, School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia Received 15 August 2002; accepted 09 September 2002 Abstract Acid dissociation constants for a series of tetraazamonocycloalkanes of {13}–{15} membered ring sizes, as well as for a related ‘reinforced’ {15R} tetraazabicycloalkane, each with one primary amine or carboxylate C-pendant, are reported. Further, formation constants with the metal ions Zn II , Cd II , Pb II , Hg II , Mn II and Co II have been determined. The ligands differ in their selection of metal ions, reflected in the formation constants for 1:1 complexation (K ML ). Differences arise in part from the increasing ring size or rigidity and also from the availability of potential pendant O-donors versus N-donors in stepping from amine to carboxylate pendant ligands, implying a role for the pendants in binding the metal ions. Introduction A range of macrocycles with pendant groups attached directly to a carbon of the macrocyclic ring have been developed via metal-directed Mannich-type chemistry over recent years in a number of studies [1, 2]. These ligands offer an enhanced donor set capacity through the presence of the pendant groups, usually primary amines or carboxylates, which have been shown to participate in binding to both inert and labile metal ions in a range of solid state structures [1–6]. More impor- tantly, detailed analysis of the kinetics of complexation of copper(II) in solution have produced strong evidence for a role for the pendants in the mechanism of complexation, and imply that the pendant groups are also involved in coordination to labile metal ions in solution as well as the solid state [7]. Consequently, it was anticipated that this should be reflected in the stability constants with different metal ions. To date, only a limited number of the C-pendant macrocyclic systems have been examined in terms of stability constants [8–10]. Here, we report the results of an extended study of ligands (1)–(8). Experimental Syntheses of the ligands (1)–(8) via metal-directed Mannich-type reactions have been reported. These were prepared as previously described [2–6, 11, 12], and recrystallised to yield samples of high spectroscopic and analytical purity. Potentiometric titrations were carried out essentially as described earlier [13], using a Metrohm 665 auto- mated burette and an IBM clone computer fitted with a Fylde Scientific pH card and connected to a Metrohm combined glass electrode. All measurements were fully automated under the control of the computer. Titrations were performed under nitrogen at 25.0 ± 0.1 °C in constant ionic strength (I ¼ 0.5 M, KCl) aqueous solutions. Solutions of ligand (6 · 10 )4 M) alone and in the presence of 0.9 or 0.5 equivalents of metal ion were titrated with at least 83 increments (of 4 · 10 )6 L ) of 0.4 M NaOH. The solutions contained six equivalents of acid, sufficient to protonate the number of basic sites present and provide one equivalent of excess acid, to ensure titrations commenced in the acidic region. Equilibrium constants were calculated from potentiometric data with a TUR- BO BASIC version of the program TITFIT [14]. For each system, each titration was repeated at least three times, with good reproducibility (±0.1 log units) in the determined constants. Results and discussion Polyamines and polyaminoacids of a wide range of topologies have been the subject of detailed examination of their stability constants with metal ions, and this work has been well reviewed [15, 16]. The series of pendant-arm tetraazamacrocycles of differing ring size * Author for correspondence Transition Metal Chemistry 28: 460–463, 2003. 460 Ó 2003 Kluwer Academic Publishers. Printed in the Netherlands.