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

A variable-temperature Sn-119 NMR study of mixtures of tin tetrachloride an d diethyl ether dissolved in dry dichloromethane has evidenced the existenc e of two complexes. The first (B, delta -600), predominates at a 5:1 Et2O-t o-SnCl4 molar ratio; it corresponds to the compound (Et2O)(2). SnCl4 (2:1 c omplex.) 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 (delta -163) an d for A (delta -579), at temperatures below 205 K. Complex A was identified as Et2O-SnCl4 (1:1 complex). The concentrations of the components at vario us temperatures were determined by the integration of spectra and by chemic al shift interpolation, and the two equilibrium constants, K-a and K-b, wer e calculated. The formation of a complex with one ether molecule had DeltaH degrees -6.1 +/- 1.6 kcal mol(-1) and DeltaS degrees -20.2 +/- 5.3 cal mol (-1) K-1, whereas for the second step, DeltaH degrees and DeltaS degrees we re -8.5 +/- 0.6 kcal mol(-1) and -26.6 +/- 1.6 cal mol(-1) K-1, respectivel y. 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 Sn Cl4 is caused by the unfavorable entropy of reaction.