The effect of N-alkyl substitution on binding of polycarboxylic anions by linear polyammonium cations

The interaction between three N-alkyl-substituted polyamines [1,1,4,4-tetra methylethylenediamine, 1,1,4,7,7,-pentamethyldiethylenetriamine and 1,1,4,7 ,10,10-hexamethyltriethylenetetramine; general formula C3nNnH(7n+2)] With f our polycarboxylic ligands (malonate, citrate, 1,2,3-propanetricarboxylate, and 1,2,3,4-butanetetracarboxylate) has been studied potentiometrically in aqueous solution at 25 degreesC. For all the systems, the species ALH(r) ( r = 1, 2 ... n + m - 1; n and m are the maximum degrees of protonation for the amine A and the carboxylic ligand L, respectively) are formed. The stab ility of these species is quite high (in particular, that of species with r = n and r = II + 1) and is a linear function of the charges involved in th e formation reaction. The effect of N-alkyl substitution is quite small: co mparison with the analogous unsubstituted polyamines C(2n-2) NnH(5n-2)-n sh ows only a small decrease in stability. On average we have -DeltaG degrees/ n = 5.9 kJ-mol(-1) for N-alkyl-substituted amines and 6.6 kJ-mol(-1) for u nsubstituted ones (n denoting the number of possible salt bridges). Other l inear charge-stability relationships are considered.