No abstract

ASHOK ANANTANARAYAN and THOMAS M. FYLES. Can. J. Chem. 68, 1338 (1990.The synthesis and complexation properties of polycarboxylate diaza crown ethers based on R,R-(+)-tartaric acid are described. Cesium carbonate mediated macrocyclization of a bis-tosylamide precursor with a bis-tosylate precursor provided the protected crown ethers. Photochemical deprotection of the tosylamides and hydrolysis of the carboxamides yielded dicarboxylic and tetracarboxylic acid derivatives of 1, lO-diaza-18-crown-6. N-Methylenecarboxylate (N-acetate) derivatives were prepared by N-alkylation with bromoacetic acid. The synthetic and purification procedures developed provide samples of the ligands in a metal-free form. Acidity and stability constants for complexation of alkali metal, alkaline earth, late-and post-transition metal cations were determined by potentiometric titration. The ligands form complexes which show enhancement of stability by charge-charge and chelate interaction with the carboxylates. In comparison with crown ether polycarboxylates these aza crown ethers showed a selectivity for softer metal ions relative to alkali and alkaline earth metal ions. N-Methylenecarboxylate chelate ligands were found to bind almost all types of metal ions, due to a highly co-operative array of charge-charge, chelate and crown ether interactions. Metal ion recognition by synthetic and naturally occurring ligands is a complex process involving a cooperative match of the complementary properties of the host and the guest. A key factor in the design of new ligands with unique complexation behaviors is the choice of ligand donor sites (1). Within a class of ligands, such as the crown ethers or cryptands, clear changes in selectivity of cation binding are provoked by substitution of the oxygen donors of a parent ring system for nitrogen or sulfur donor sites (2). Such substitutions alter the primary character of the donor (Lewis basicity) as well as the geometrical and conformational properties of the ligand (3). Thus in a system involving highly cooperative binding interactions, donor atom substitution can profoundly influence binding selectivity.Our interest in ligand design has centered on crown ethers derived from R,R-(+)-tartaric acid: the di-, tetra-, and hexaacids illustrated at the top of Fig. 1 (4-6). 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. 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 th...

No abstract

No abstract

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.