Binding of benzene-1,2,3,4,5,6-hexacarboxylate by polyammonium cations

The interaction between mellitate (benzene-1,2,3,4,5,6-hexacarboxylate) and diethylenetriamine and spermidine was studied by potentiometric and calorimetric techniques in aqueous solution at t = 25°C, moreover by diffractometric technique in the solid state. Thermodynamic parameters of several A_pLH_q species with p = 1, q = 2…5, for both systems, were calculated; in addition the following species were found: A₂LH₆, for A = dien; ALH₆, for A = spd. The stabilities of these species are quite high; for example formation constants regarding the ALH₃³⁻ species are: logK⁰ = 5.7 for A = dien and logK⁰ = 10.1 for A = spd. Thermodynamic parameters are generally function of charges involved in the formation reaction and some linear relationships were found. The speciation profiles show that the formation percentages, for the mononuclear species in particular, are high, in a wide pH range. The sequestering ability of mlt towards polyamines was analysed, also considering the already published data regarding ethylenediamine. By considering the formation constants at various ionic strength and the enthalpy values of triamine–mellitate species, it was possible to evaluate the dependence of sequestering ability on ionic strength, temperature and pH. Solid state investigation of two cocrystals, made by mellitate trianion and triammonium cations, provides information on the formation of supramolecular anionic layers. Comparison with the stability of mellitate–alkaline earth metals species was made.