Synthesis and metal ion complexation behavior of polycarboxylate 18-crown-6 ethers derived from tartaric acid

Abstract

. Can. J. Chem. 66, 1097 (1988). The metal ion complexation behavior of four 18-crown-6 ethers derived from (+)-and meso-tartaric acid is examined. Preparations of a meso-crown ether diacid and of a crown ether hexacid from three units of (+)-tartaric acid are described. Acidity constants and stability constants for complexation of metal cations in aqueous solution were determined by potentiometric titration. The complexes are substantially stabilized by favourable electrostatic interactions and are of similar stability to complexes of cryptands and EDTA. The complexation behavior of the series can be rationalized in terms of electrostatic interactions, direct coordination of the cations by at least one carboxylate from the crown ether periphery, and rigidification of the ligands as the anionic charge increases. Distributions of charge influence the relative stability of isomeric complexes. Highly charged polycarboxylate crown ethers are effective, but relatively unselective, cation complexing agents for a range of cations. The complexes are stable to pH 3 and the ligands can be used as simultaneous pH and metal ion buffers. The underlying theme in the complexation chemistry of 18C6 prefer an ideal D3d symmetry (1 8). Thus, both RR1 8C6A2 macrocycles, is the control of complex formation by control of and 18C6A4 are incapable of symmetrically interacting with a ligand structure through synthesis ( l , 2 ) . From this perspective, bound cation, either by direct contact or by indirect coulombic crown ethers derived from (+)-tartaric acid possess a number of attraction without some distortions of the macrocycle from appealing features as frameworks for the construction of specific complexing agents. Firstly, the carboxyl groups provide an easy synthetic entry to a wide range of derivatives (3-7). Secondly, the tartaric acid units show a strong conformational preference for an anti disposition of the carboxylates and other carboxylate derived groups (8-12). This limits overall conformational flexibility (8,11,12) and leads to well-defined conformations in which the carboxylate derived groups lie axial to the plane of the macrocycle (8-12). Lateral interactions between substituent groups and bound substrates are thus favored (3,4,13,14). Finally, the presence of charged groups on the macrocyclic periphery leads to markedly enhanced cation complexation relative to uncharged ligands of similar structure (3,8,12). This is primarily a result of a favorable electrostatic interaction between the anionic ligand and the complexed cation (5,8).Additional effects may be important. The carboxylates could act as additional ligating sites in a fashion similar to the neutral donor arms of lariat ethers (15,16). Evidence from infrared and nuclear magnetic resonance spectroscopy (12), from electron spin resonance spectroscopy (1 I), and from analysis of stability constant data (12,17) suggests that direct and cooperative carboxylate-cation interactions occur, especially with small cations in solvents of moderate polarity. Such contac...

Polycarboxylate diaza crown ethers derived from R,R-(+)-tartaric acid: synthesis and complexation of metal ions

“…The 13C NMR spectra of 2.2A4 showed a pH dependence whereby "extra" resonances appeared at intermediate pH (3)(4)(5)(6). At the extremes of pH (2 or 12), the expected four line spectra were observed for the ligand (plus signals for the counterion).…”

“…The errors introduced are minimized by the use of a constant ionic strength, low ligand concentration, and corrections to the calibration of the pH electrode (below). Stepwise formation constants as would apply to the association of a partly protonated ligand with a metal ion can be derived from the PImh for the system (4,6).…”

“…1 (4)(5)(6). The tartarate derived units impart rigidity to the ligands, leading to well defined conformations, and the carboxylates, when ionized, provide a strong electrostatic field which greatly enhances complex stability for alkali metal and alkaline earth cations.…”

“…The tartarate derived units impart rigidity to the ligands, leading to well defined conformations, and the carboxylates, when ionized, provide a strong electrostatic field which greatly enhances complex stability for alkali metal and alkaline earth cations. Despite the favorable Coulombic interactions, binding of heavy metal cations occurs only to a limited extent due to a mismatch of the hard oxygen donor set and the softer guest cations (6). An obvious ligand modification which might lead to stabilized crown ether type complexes of heavier cations would be the substitution of some of the donor atoms for N or S. We have previously reported dithia crown ethers bearing four carboxylates (7); the purpose of this report is to describe the chemistry of the diaza polycarboxylate crown ethers illustrated in the lower part of Fig.…”

“…J. Chem. Downloaded from www.nrcresearchpress.com by 54.191.190 (4)(5)(6), or N-methylene acetic acid (D,) .…”