Binding of carboxylic ligands by protonated amines

Abstract

The formation and stability of some mixed proton complexes A_pL_qH_r(A = triethylenetetramine, tetraethylenepentamine or pentaethylenehexamine; L = acetate, malonate, citrate, propane-1,2,3-tricarboxylate or butane-1,2,3,4-tetracarboxylate), was studied in aqueous solution at T= 25 °C. It was been found, for all the systems, the formation 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 = tetraethylenepentamine and L = malonate, AL₂H_r(r= 4,5,6); for A = pentaethylenehexamine and L = acetate, AL₂H_r(r= 5,6) and AL₃H₆; for A = pentaethylenehexamine and L = malonate, AL₂H₇. The stability of these species is generally quite high and follows the trends triethylenetetramine < tetraethylenepentamine < pentaethylenehexamine, and acetate < malonate < citrate ≈ propane-1,2,3-tricarboxylate < butane-1,2,3,4-tetracarboxylate. Some proton–amine–carboxylic ligand complexes, previously reported, follow a similar trend. The charges of the ligands involved in the complexation reactions play a fundamental role in the stability of the species, and some linear relationships are reported.