14. Metal Complexes with Macrocyclic Ligands. Part XXXI. Protonation Studies and Complexation Properties of Tetraazamacrocyclic Methylenephosphonates with Earth-Alkali Ions

Article abstract of DOI:10.1002/hlca.19900730115

The three ligands 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylenephosphonic acid) (1), 1,4,7,11-tetraazacyclotridecane-1,4,7,11-tetrakis(methylenephosphonic acid) (2), and 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetrakis(methylenephosphonic acid) (3) have been synthesized by condensation of the corresponding macrocycles with formaldehyde and phosphorous acid.The protonation and stability constants with the earth-alkali ions have been determined at 25 deg C and I=0.1 m (Me4N(NO3)) by potentiometric titrations.Because of the high values of the first two protonation constants, 1H-NMR measurements were necessary to determine them.Titrations in different supporting electrolytes (NaNO3, KNO3, RbNO3, CsNO3, and Me4N(NO3)) show that their choice is of paramount importance, as the above ligands can form complexes with alkali-metal ions.The potentiometric results for the earth-alkali ions show that beside mononuclear complexes of different degrees of protonation (, n=0-4), also binuclear species are formed (, m=0-2).It is interesting that 1 with the smallest macrocyclic ring has the greatest tendency to form binuclear complexes, which are so stable that they partially prevent the formation of the corresponding mononuclear species.For, , , and , the stability sequence is Mg²⁺ < Ca²⁺ > Sr²⁺ > Ba²⁺, whereas for , , and , the stability steadily decreases from Mg²⁺ to Ba²⁺.