Binding of inorganic and organic polyanions by protonated open chain polyamines in aqueous solution

The stability constants of complexes between protonated open chain polyamines (with pyridine and imidazole as well) and organic or inorganic polyanions (16 amines, 16 anions, 87 systems, 435 complexes), have been collected and compared. The main characteristic of these systems (both with organic and inorganic polyanions) is the high number of species that can be formed, ie., m+n-1 species, with m = maximum value of i in AH(i)(j+) and n = maximum value of j in HjL(z-j-). All the data showed that the stability of these complexes is strongly dependent on the charges involved in the formation reaction. Less important, but significant, factors affecting the stability of these complexes are the distance between the charges and the number of aminogroups in the polyamines, and the difference between the charges of the reactants. Moreover, it has been found that inorganic polyanions form more stable mixed complexes with respect to those of organic polyanions. A general equation, log K = f(z(cat), z(an), z) (z(cat) = charge of protonated amine, z(an) = charge of anion, z = \z(cat) + z(an)\), has been found for proton-amine-inorganic anions and for proton-amine-organic anions complexes: log K = -2.3 + 1.42 z(cat) + bxz(an) - 0.45 z (b = 1.44(5) and 1.09(5) for inorganic and organic anions, respectively), with a mean deviation of similar to 0.5 log units. Though this deviation is quite great, the predictive value, when analysing multicomponent systems, may be very important. A simple linear free energy relationship gives -Delta G degrees = 7.5 +/- 0.3 and 6.4 +/- 0.4 kJ mol(-1) n(-1) for inorganic and organic anions, respectively (n = number of possible salt bridges). T Delta S degrees also depends an charges and, for some systems, T Delta S degrees = 5.5 x z(cat) x z(an) J mol(-1) has been found. Moreover, spectroscopic evidence (UV/CD) for the formation of these complexes is reported. These species are generally not very stable (if compared with transition metal complexes), but they are not negligible in a correct modelling and speciation of ionic multicomponent solutions.