A 3-fold “Butterfly Valve” in Command of the Encapsulation’s Kinetic Stability. Molecular Baskets at Work
2008-11-12T00:00:00Z (GMT) by
Molecular basket 1, composed of a semirigid tris-norbornadiene framework and three revolving pyridine-based gates at the rim, has been built to “dynamically” enclose space and as such regulate molecular encapsulation. The gates were shown to fold via intramolecular hydrogen bonding and thereby form a C3v symmetrical receptor: the 1H NMR resonance for the amide N−H protons of the pyridine gates appeared downfield (δ = 10.98 ppm), and the N−H vibrational stretch (IR) was observed at 3176 cm−1. Accordingly, density functional theory (DFT, B3LYP) investigations revealed for the closed conformers of 1 to be energetically the most stable and dominant. The gearing of the pyridine “gates”, about their axis, led to the interconversion of two dynamic enantiomers 1A and 1B comprising the clockwise and counterclockwise seam of intramolecular hydrogen bonds. Dynamic 1H NMR spectroscopic measurements and line-shape simulations suggested that the energy barrier of 10.0 kcal/mol (ΔG⧧A/B, 298 K) is required for the 1A/B interconversion, when CCl4 occupies the cavity of 1. Likewise, the activation free energy for CCl4 departing the basket was found to be 13.1 kcal/mol (ΔG⧧, 298 K), whereas the thermodynamic stability of 1:CCl4 complex was −2.7 kcal/mol (ΔG°, 298 K). In view of that, CCl4 (but also (CH3)3CBr) was proposed to escape from, and a molecule of solvent to enter, the basket when the gates rotate about their axis: the exit of CCl4 requires the activation energy of 12.7 kcal/mol (ΔG⧧A/B + ΔG°), similar to the experimentally found 13.1 kcal/mol (ΔG⧧).