The 1∶1 and 2∶1 complexes of diethyl ether with tin tetrachloride and their stability, studied by 119Sn NMR spectroscopy

Abstract

A variable-temperature ¹¹⁹Sn NMR study of mixtures of tin tetrachloride and diethyl ether dissolved in dry dichloromethane has evidenced the existence of two complexes. The first (B, δ −600), predominates at a 5∶1 Et₂O-to-SnCl₄ molar ratio; it corresponds to the compound (Et₂O)₂·SnCl₄ (2∶1 complex.) The other, A, more labile and exchanging with free tin chloride in solution, predominates at a 2∶1 molar ratio of reactants. The signal for (A + SnCl₄) splits into separate resonances, for SnCl₄ (δ −163) and for A (δ −579), at temperatures below 205 K. Complex A was identified as Et₂O–SnCl₄ (1∶1 complex). The concentrations of the components at various temperatures were determined by the integration of spectra and by chemical shift interpolation, and the two equilibrium constants, K_a and K_b, were calculated. The formation of a complex with one ether molecule had ΔH° −6.1 ± 1.6 kcal mol⁻¹ and ΔS° −20.2 ± 5.3 cal mol⁻¹ K⁻¹, whereas for the second step, ΔH° and ΔS° were −8.5 ± 0.6 kcal mol⁻¹ and −26.6 ± 1.6 cal mol⁻¹ K⁻¹, respectively. Thus, the enthalpy of complex formation in both steps is not much lower than for the reaction of Et₂O with BF₃. The lability of the complexes of SnCl₄ is caused by the unfavorable entropy of reaction.