Crown ether styryl dyes 17. Influence of the length of the N-substituent in betaines of chromogenic 15-crown-5-ethers on complexation with cations of alkaline-earth metals

The stoichiometry of complexation of crown ether styryl dyes with Mg2+, Ca2+, and Ba2+ ions and the dependence of the stability constants of these complexes on the length of theN-sulfoalkyl substituent were investigated. Introduction of a terminal sulfo group into theN-ethyl substituent had but a small effect on the stability constant for the complexes with 1 : I stoichiometry. Increase in the length of theN-substituent by one or two methylene groups resulted in a jumpwise rise of this constant. The effect observed was attributed to the formation of the intramolecular ion pair. The dimerization constant for Mg2+ complexes increased dramatically when passing from the sulfopropylN-substituent to the sulfobutyl one. The increase in the constant results from the decrease in steric strains in the dimeric complex.