Ligands, cation-binding

Changes in the Inter- and intramolecular interaction and in the structure of carbanion pairs on adding cation-binding ligands (ethers, crown ethers, poly-amines) is briefly reviewed. 

The complexity of anionic polymerization arises from the existence of a variety of species in the type of solvents often used in these reactions free ions, different kinds of ion pair, triple ions, ion pair dimers, etc. Cation-binding ligands, added to catalyze the polymerization or to modify the product structure, further increase the number of species ... 

Although the first all-sulfur macrocycles were prepared many years ago " the first systematic study of such compounds was initiated by Busch and his coworkers , who were interested in the cation binding properties of such ligands. A sequential synthesis was utilized to produce 1,4,8,11-tetrathiacyclotetradecane [tetrathia-14-crown-4 (70)] . In the first step, 1,3-propanedithiol is metallated using sodium and alkylated with 2-chloroethanol. The diol was then treated with thiourea to form the dimercapto-dithioether compound 9. The latter was once again metallated with sodium and allowed to react with 1,3-dibromopropane. The yield of 70 in the ring closure step, conducted at high dilution in absolute ethanol, was 7.5% after recrystallization. The entire sequence is illustrated in Eq. (6.8) . ... 

Figure 1. Plot of sodium cation binding in anhydrous methanol (at 25jp.l °C) vs. the total number of oxygen atoms present in each ligand.

Polarity. Cation complexing ligands are of the endopolarophilic type, with their polar binding sites turned inside towards the cation. The outer surface is then formed by the lipophilic hydrocarbon groups. 

Figure 20. Schematic drawings of conformational changes upon cation binding by some sophisticated ligands and the slopes (a) and intercepts (7A5b) of AH-TAS plots as measures of conformational change and desolvation.

In the case of the sodium ion complex of the smaller benzo[15]crown-5 macrocycle, the cation binds to all five oxygen atoms but lies 0.75 A above the plane of the macrocycle. With the larger potassium ion, benzo[15]crown-5 forms a 2 1 complex in which the cation sits at the center of a sandwich formed by two macrocycles. The [12]crown-4 ligand with only four coordinating oxygens in each ring forms a 2 1 sandwich complex with sodium (B-78MI52102). 

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