29Si chemical shifts temperature dependence

FIGURE 21. Temperature dependence of the 29Si chemical shift for 125a in CDCI3. Reproduced from Reference 171 by permission of Hiithig Fachverlage... 

Evidence that 63 is pentacoordinate also in solution is provided by the typically pentacoordinate 29Si chemical shift, and by its highly characteristic temperature dependence, as follows [5, ppm (T, K)] —41.4 (330), —39.6 (300), —37.2 (275), —35.6 (243).41 The inverse relationship is the evidence for the coordination of the chloride to silicon (Si <— Cl—), rather than O —Si coordination (which was reported to have a regular dependence of 5(29Si) on temperature, i.e., downfield shift as the temperature increases).34 However, the present results do not permit the distinction of a bridged complex in solution from one in which a different chloride is attached to each silicon. 

Solvent and Temperature Dependence of 29Si Chemical Shifts for 66b... 

Equilibrium constants were obtained for each temperature from the temperature-dependent 29Si chemical shifts, leading to the reaction enthalpy and entropy of ionization for 66b —6.3 kcal mol-1 and —21 cal mol-1 deg-1, respectively. These values are in agreement with the reversible... 

The ionization process was initially evident from a dramatic temperature dependence of the 29Si NMR chemical shifts (Fig. 19) upon cooling of a solution of 31, 32, or 34 in CD2C12 a rapid downfield shift of the silicon resonance was found, followed by its disappearance in the baseline and emergence of two signals at low temperatures, near —70 and —130 ppm, corresponding, respectively, to ionic 48-50 and covalent 31, 32, or 34 (Table XIV). 

Like the mononuclear chelates,66 binuclear 55a,c,j (but not 55g ) undergo reversible ionization which is enhanced at low temperature, as is evident from the temperature-dependent 29Si NMR spectra (Fig. 24, Table XVIII). From the lowfield chemical shift already at 300 K (Table XVIII), relative to the mononuclear analogs 31a,c,j (Table XIV), it was concluded that significant ionization takes place at ambient temperature, to a much greater extent than in 31.69... 

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