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, Ka and Kb, 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.