The reversible formation of tight ion pairs within platinum(II) complexes - A study of thermodynamic parameters governing noncovalent interactions

Compounds like [(HR2DTO)Pt(H2R2DTO)][Cl] (DTO = dithiooxamidate; R = ethyl or benzyl) can be involved in acid– base reactions in chloroform, playing the role of either the acid or the base partner. When the complexes react with pyridines, neutral complexes like [(HR2DTO)2Pt] are formed. Such processes are characterized by small enthalpy changes, so they are substantially entropy-driven. We explained this as a consequence of the solvent released when HCl is transferred from tight-ion-pair platinum compounds to pyridines. When [(HR2DTO)Pt(H2R2DTO)][Cl] species react with HCl, complexes like [(H2R2DTO)Pt(H2R2DTO)][Cl]2 are formed, where a new weak hydrogen-bonding-based interaction of the type N···H–Cl is alpy changes show values typical of a strong interaction (R = ethyl, ∆H° = –146.3 Å} 9.7 kJ mol–1; R = benzyl, ∆H° = –67.7 Å} 2.0 kJ mol–1), apparently too large for hydrogen bonding. Also, the entropy lowering is too large to be attributable to a simple association process (R = ethyl, ∆H° = –398 Å} 32 J K–1 mol–1; R = benzyl, ∆H° = –154 Å} 7 J K–1 mol–1). Both large enthalpy and entropy lowering have been attributed to the role played by the solvent in the acid–base processes. In the present systems, the solvent strongly affects not only the entropy factor, but also the enthalpy factor. Such conclusions can be generalized to give useful information for controlling similar noncovalent, electrostatic interactions