Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 132, Issue 45, Pages 16256-16264Publisher
AMER CHEMICAL SOC
DOI: 10.1021/ja107656g
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Funding
- NSF [CHE-0233882, CHE-0840505]
- Office of Science, Office of Basic Energy Sciences
- Division of Chemical Sciences, Geosciences, and Biosciences of the U.S. Department of Energy at LBNL [DE-AC02-05CH11231]
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The supramolecular host assembly [Ga4L6](12-) (1; L = 1,5-bis[2,3-dihydroxybenzamido]naphthalene) encapsulates cationic guest molecules within its hydrophobic cavity and catalyzes a variety of chemical transformations within its confined interior space. Despite the well-defined structure, the host ligand framework and interior cavity are very flexible and 1 can accommodate a wide range of guest shapes and sizes. These observations raise questions about the steric effects of confinement within 1 and how encapsulation fundamentally changes the motions of guest molecules. Here we examine the motional dynamics (guest bond rotation and tumbling) of encapsulated guest molecules to probe the steric consequences of encapsulation within host 1. Encapsulation is found to increase the Ph-CH2 bond rotational barrier for ortho-substituted benzyl phosphonium guest molecules by 3 to 6 kcal/mol, and the barrier is found to depend on both guest size and shape. The tumbling dynamics of guests encapsulated in 1 were also investigated, and here it was found that longer, more prolate-shaped guest molecules tumble more slowly in the host cavity than larger but more spherical guest molecules. The prolate guests reduce the host symmetry from T to C-1 in solution at low temperatures, and the distortion of the host framework that is in part responsible for this symmetry reduction is observed directly in the solid state. Analysis of guest motional dynamics is a powerful method for interrogating host structure and fundamental host-guest interactions.
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