Interactions between metal cations and the ionophore lasalocid. Part 8.—Enthalpies and entropies of formation of 1 : 1 and 2 : 1 complexes of lasalocid anion and salicylate with some divalent transition-metal cations in methanol

Abstract

Complexation in methanol of Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺ and Zn²⁺ cations by lasalocid and salicylic acid has been studied using reaction calorimetry. Formation of the two successive complexes AM⁺ and A₂M of the acid ligand, AH, with cations M²⁺ has previously been shown to occur, and was confirmed here. Enthalpies of formation of the complexes AM⁺ and A₂M were simultaneously determined using a novel calorimetric method. The corresponding entropies could then be obtained from the previously recorded Gibbs functions. Compared with the other cations, Fe²⁺ shows very specific behaviour corresponding to stronger interactions and more rigid structures. For a given cation there is no large difference between analogous thermodynamic functions obtained for lasalocid and for the model species salicylic acid. It is suggested that in the lasalocid anion the salicylate group is the main coordination site in methanol for the cations studied here.