Abstract
The formation of 1 : 1 and 2 : 1 complexes of crown ethers and cryptands with K + was studied by means of potentiometric and calorimetric titrations in acetonitrile and propylene carbonate solutions. With the exception of the diaza crown ether (21) all twelve-and fifteen-membered monocyclic ligands examined are able to form 2 : 1 complexes. Structural changes of the ligand (21) during complex formation are mainly responsible for this observation. The complexes formed with the cryptands (222) and (221) are more stable compared to the monocyclic ligands. The smallest cryptand (211) is not able to encapsulate the potassium ion completely. Therefore, the formation of 2 : 1 complexes in both solvents are measured. The reaction enthalpies indicate that the macrobicyclic structure of the ligand is important during the formation of the 1 : 1 complexes. During the formation of the 2 : 1 complexes the cryptand (211) behaves like the diaza crown ether (21). Therefore, the conclusion is that the formation of 2 : 1 complexes with cryptands is only observable in a given solvent if a reaction with the corresponding diaza crown ether is measured.
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