Equilibrium studies,synthesis and characterisation of zwitterionic buffer (HEPES and HEPPS) dimethyltin(IV) complexes

The interaction of the dimethyltin(IV) dication (DMT) with some selected buffers “Good’s buffer”, such as HEPES and HEPPS (LH₂⁺) were investigated using a potentiometric technique. The stepwise formation constants of the complexes formed in solution were calculated using the non-linear least-square program MINIQUAD-75. The concentration distribution of the various complex species was evaluated as a function of pH. The effect of dioxane as a solvent on the protonation constants of ligands and the formation constants of dimethyltin(IV) complexes were discussed. The thermodynamic parameters ΔH° and ΔS° calculated from the temperature dependence of the equilibrium constants were investigated. The bonding sites of the dimethyltin(IV) complexes in solid state with HEPES and HEPPS were characterised by means of elemental analyses and FTIR. The NMR (¹H, ¹³C) spectra of the DMT-HEPES complex indicate a strongly distorted octahedral geometry around the tin atom. Thermal analyses (TGA and DTA) are also discussed. Thermodynamic parameters, such as activation energy (E_a), pre-exponential factor (A), entropy of activation (ΔS^*) and free energy of activation (ΔG^*) have been calculated for each step, employing the integral method of Coats and Redfern. The reaction enthalpy (ΔH) is obtained from DTA data.