Solvent and Pressure Effects on the Motions of Encapsulated Guests: Tuning the Flexibility of a Supramolecular Host

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• 作者：Jeffrey S. Mugridge ; Achim Zahl ; Rudi van Eldik ; Robert G. Bergman ; Kenneth N. Raymond
• 刊名：The Journal of the American Chemical Society
• 出版年：2013
• 出版时间：March 20, 2013
• 年：2013
• 卷：135
• 期：11
• 页码：4299-4306
• 全文大小：443K
• 年卷期：v.135,no.11(March 20, 2013)
• ISSN：1520-5126

文摘

The supramolecular host assembly [Ga₄L₆]^12- [1; L = 1,5-bis(2,3-dihydroxybenzamido)naphthalene] contains a flexible, hydrophobic interior cavity that can encapsulate cationic guest molecules and catalyze a variety of chemical transformations. The Ar鈥揅H₂ bond rotational barrier for encapsulated ortho-substituted benzyl phosphonium guest molecules is sensitive to the size and shape of the host interior space. Here we examine how changes in bulk solvent (water, methanol, or DMF) or applied pressure (up to 150 MPa) affect the rotational dynamics of encapsulated benzyl phosphonium guests, as a way to probe changes in host cavity size or flexibility. When host 1 is dissolved in organic solvents with large solvent internal pressures (鈭?i>U/鈭?i>V)_T, we find that the free energy barrier to Ar鈥揅H₂ bond rotation increases by 1鈥? kcal/mol, compared with that in aqueous solution. Likewise, when external pressure is applied to the host鈥揼uest complex in solution, the bond rotational rates for the encapsulated guests decrease. The magnitude of these rate changes and the volumes of activation obtained using either solvent internal pressure or applied external pressure are very similar. NOE distance measurements reveal shorter average host鈥揼uest distances (0.3 脜) in organic versus aqueous solution. These experiments demonstrate that increasing solvent internal pressure or applied external pressure reduces the host cavity size or flexibility, resulting in more restricted motions for encapsulated guest molecules. Changing bulk solvent or external pressure might therefore be used to tune the physical properties or reactivity of guest molecules encapsulated in a flexible supramolecular host.