The solution chemistry of organotin compounds. Part 2. Equilibrium and thermodynamic studies of complex formation between dimethyltin dichloride and picolines

Abstract

The donor–acceptor complexes formed between Me₂SnCl₂ and 2-, 3-, and 4-picolines have been investigated by analysing the concentration and temperature dependence of ¹H, ¹³C, and ¹¹⁹Sn n.m.r. parameters. Detailed explanations of the data are based on the chemical equilibria which occur. It is concluded that two types of complex, 1:1 and 1:2 Me₂SnCl₂–picoline, coexist in solution. The equilibrium constants (K₁ and K₂) and the complex formation shifts (Δ_AB and Δ_AB2) were determined at several temperatures, and the thermodynamic parameters (ΔH°_AB, ΔH°_AB2, ΔS°_AB, and ΔS°_AB2) derived. Entropy terms are shown to be responsible for the small free energy change in case of 2-picoline. Enthalpy terms are discussed in relation to the basicity of picolines. Me₂SnCl₂ was found to be partly hydrolysed by moisture producing 1,3-dichloro-1,1,3,3-tetramethyldistannoxane, and all sample preparations were done in dry boxes to prevent this. ¹H and ¹¹⁹Sn n.m.r. data of this distannoxane are also assigned.