Entropies of Organolithium Aggregation Based on Measured Microsolvation Numbers
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- 作者：Rudolf Knorr ; Thomas Menke ; Kathrin Ferchland
- 刊名：Organometallics
- 出版年：2013
- 出版时间：January 28, 2013
- 年：2013
- 卷：32
- 期：2
- 页码：468-472
- 全文大小：267K
- 年卷期：v.32,no.2(January 28, 2013)
- ISSN：1520-6041
文摘
The recent measurement (J. Am. Chem. Soc.2008, 130, 14179鈥?4188) of the microsolvation numbers of monodentate, nonchelating ethereal donor ligands coordinating to the monomers and dimers of two sterically shielded 鈺怌(aryl)鈥揕i compounds permits the determination of well-founded dimerization enthalpies (螖H0) and entropies (螖S0) from properly formulated equilibrium constants, which must include the concentrations of the free donor ligands. The monomers are found to dimerize endothermically (螖H0 > 0) in [D8]toluene solution in the presence of the donor tBuOMe or THF, but only slightly exothermically (螖H0 = 鈭?.5 kcal per mol of dimer) with the donor Et2O. The dimerization entropies 螖S0 (in cal mol鈥? K鈥?) with the respective equivalents of released donor ligands are 7.2 and 11.0 (with 2 equiv of tBuOMe in the two cases), 6.1 (with 2 Et2O), and 34.1 (with 4 THF). It is shown that the improper omission of microsolvation from the equilibrium constant (a usual practice when the ligand numbers are not known) can lead to 鈥渃ontaminated鈥?aggregation entropies 螖S蠄, which may deviate considerably from the 鈥渢rue鈥?entropies 螖S0. A method is provided for estimating the required microsolvation numbers from 13C/Li NMR coupling constants 1JC,Li for less congested organolithium types whose coordinated and free donor ligands cannot be distinguished by NMR integration.