Thermodynamic and spectroscopic study of the binding of dimethyltin(IV) by citrate at 25°C

Abstract

Thermodynamic (potentiometric and calorimetric) and spectroscopic (1H NMR, 119Sn M¨ossbauer) studieswere performed in aqueous solution in order to characterize the interaction of dimethyltin(IV) cation with citrate ligand. Six species {(CH3)2Sn(cit)−; [(CH3)2Sn]2(cit)2 2−; (CH3 )2Sn(cit)H0; (CH3)2Sn(cit)OH2−; [(CH3)2Sn]2(cit)OH0; [(CH3)2Sn]2(cit)(OH)2 −} were found. All the species formed in this system are quite stable and formation percentages are fairly high. For example, at pH = 7.5 and C(CH3)2Sn = Ccit = 10 mmol l−1, % for [(CH3)2Sn]2(cit)(OH)2 − and (CH3)2Sn(cit)OH2− species reaches 65%. Overall thermodynamic parameters obtained show that the main contribution to stability is entropic in nature. Thermodynamic parameters were discussed in comparison with a simple tricarboxylate ligand (1,2,3-propanetricarboxylate). Two empirical relationships were derived from thermodynamic formation parameters. Spectroscopic results fully confirm the speciation model defined potentiometrically and showthe mononuclear species to have an eq-(CH3)2Tbp structurewith different arrangements around the metal, while for [(CH3)2Sn]2(cit)(OH)2 − there are two different Sn(IV) environments, namely trans-(CH3)2 octahedral and cis-(CH3)2 Tbp.