Crown ethers polycarboxylate

The third class of ligands we mention here is that of polyaza-macrocycles, crown ethers, and calixarenes bearing polycarboxylate arms.293 294 299-301... 

Polycarboxylate crown ethers such as (205) are suitable ligands for potentiometric studies of mixed-metal complexes of Al3+ and alkali or alkaline-earth cations.303 A similar (+)-18-crown-6-tetracarboxylic acid, chemically immobilized on a chiral stationary phase (CSP), can selectively recognize both enantiomers of some analytes.304 Calixarene polycarboxylates such as (206) and (207) are useful ligands toward alkali-305,306 and also transition-metal ions,307 308 with applications in... 

Polycarboxylate Crown Ethers Synthesis, Complexation, Applications... 

Abstract. Crown ethers derived from tartaric acid present a number of interesting features as receptor frameworks and offer a possibility of enhanced metal cation binding due to favorable electrostatic interactions. The synthesis of polycarboxylate crown ethers from tartaric acid is achieved by simple Williamson ether synthesis using thallous ethoxide or sodium hydride as base. Stability constants for the complexation of alkali metal and alkaline earth cations were determined by potentiometric titration. Complexation is dominated by electrostatic interactions but cooperative coordination of the cation by both the crown ether and a carboxylate group is essential to complex stability. Complexes are stable to pH 3 and the ligands can be used as simultaneous proton and metal ion buffers. The low extractibility of the complexes was applied in a membrane transport system which is a formal model of primary active transport. 

The central reaction is, of course, the Williamson ether synthesis. Early reports on the preparation of tartaric acid ethers [11], suggested that the base thallous ethoxide, (TlOEt), was essential to avoid epimerization of the chiral centers. The first syntheses thus utilized this base in dimethyformamide (DMF), and oligo-ethylenglycol diiodides for the preparation of di- and tetra-carboxylate crown ethers [4, 12]. More recently, we found that by strict control of stoichiometry, sodium hydride could be used successfully to displace tosylate without loss of chiral integrity [5]. Scheme 1 shows a recent synthesis of an 18-crown-6 hexaacid from three units of (H-)tartaric acid [13]. This route illustrates all the key features in the syntheses of polycarboxylate crown ethers. 

The complexes of the polycarboxylate crown ethers of Figure 1 are substantially more stable than those of their parent, 18-crown-6. The stability constants of Table I, in fact, fall into the range usually associated with cryptands 2.2.1 and 2.2.2 [19] and EDTA [20]. However, relative to these ligands, the polycarboxylate crown ethers appear as relatively indiscriminant cation complexing agents. Very little molecular recognition based on ion size difference is occurring. Nonetheless, there are a number of features of the complexation of cations by polycarboxylate crown ethers which have lead to some simple but unique applications. 

Polycarboxylate crown ethers bind cations over a wide range of pH. Although the protonated complexes tend to be less stable than the complexes of the fully deprotonated ligand, nonetheless cation binding does occur, even in acidic solution (pH 3). This is in sharp contrast to the cryptands and EDTA, in which protonated complexes are vastly less stable or do not form. Furthermore, the first pK s of these ligands are much more basic [19, 20], thus cation complexation even at pH 7 cannot be easily achieved (some very stable EDTA complexes, with Ca for example, can form into weakly acidic solution (pH 6)). 

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