BINDING OF CARBOXYLIC LIGANDS BY PROTONATED AMINES

The formation and stability of some mixed proton complexes A(p)L(q)H(r ) (A = triethylenetetramine, tetraethylenepentamine or pentaethylenehe xamine; L = acetate, malonate, citrate, propane-1,2,3-tricarboxylate o r butane-1,2,3,4-tetracarboxylate), was studied in aqueous solution at T = 25 degrees C. It was been found, for all the systems, the formati on of different species with p = 1, q = 1...3 and r = 1...(n + m - 1) (n and m are the maximum degrees of protonation for the two ligands). In addition, also the following species were found: for A = tetraethyl enepentamine and L = malonate, AL(2)H(r) (r = 4,5,6); for A = pentaeth ylenehexamine and L = acetate, AL(2)H(r) (r = 5,6) and AL(3)H(6); for A = pentaethylenehexamine and L = malonate, AL(2)H(7). The stability o f these species is generally quite high and follows the trends triethy lenetetramine < tetraethylenepentamine < pentaethylenehexamine, and ac etate < malonate < citrate approximate to propane-1,2,3-tricarboxylate < butane-1,2,3,4-tetracarboxylate. Some proton-amine-carboxylic ligan d complexes, previously reported, follow a similar trend. The charges of the Ligands involved in the complexation reactions play a fundament al role in the stability of the species, and some linear relationships are reported.