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Metal Complexes with Macrocyclic Ligands. Part XXXI . Protonation studies and complexation properties of tetraazamacrocyclic methylenephosphonates with earth‐alkali ions
Author(s) -
Delgado Rita,
Siegfried Liselotte C.,
Kaden Thomas A.
Publication year - 1990
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19900730115
Subject(s) - protonation , potentiometric titration , chemistry , alkali metal , titration , stability constants of complexes , formaldehyde , inorganic chemistry , alkaline earth metal , metal , ion , metal ions in aqueous solution , chemical stability , medicinal chemistry , organic chemistry
Abstract 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‐1,4,8,1 1‐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° and I = 0.1 M (Me 4 )N(NO 3 ) by potentiometric titrations. Because of the high values of the first two protonation constants, 1 H‐NMR measurements were necessary to determine them. Titrations in different supporting electrolytes (NaNO 3 , KNO 3 , RbNO 3 , CsNO 3 , and Me 4 N(NO 3 )) 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 ([MLH n ], n = 0–4), also binuclear species are formed ([M 2 LH m ], 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 [ML], [MLH], [M 2 L], and [M 2 LH], the stability sequence is Mg 2+ < Ca 2+ > Sr 2+ > Ba 2+ , whereas for [MLH 2 ], [MLH 3 ], and [MLH 4 ], the stability steadily decreases from Mg 2+ to Ba 2+ .