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 the N-sulfoalkyl substituent were investigated. Introduction of a terminal sulfo group into the N-ethyl substituent had but a small effect on the stability constant for the complexes with 1:1 stoichiometry. Increase in the length of the N-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 sulfopropyl N-substituent to the sulfobutyl. one. The increase in the constant results from the decrease in steric strains in the dimeric complex.