Conformational behavior of pyrazine-bridged and mixed-bridged cavitands: a general model for solvent effects on thermal "vase-kite" switching

The controllable switching of suitably bridged resorcin4]arene cavitands between a "vase" conformation, with a cavity capable of guest inclusion, and a "kite" conformation, featuring an extended flattened surface, provides the basis for ongoing developments of dynamic molecular receptors, sensors, and molecular machines. This paper describes the synthesis, X-ray crystallographic characterization, and NMR analysis of the "vase-kite" switching behavior of a fully pyrazine-bridged cavitand and five other mixed-bridged quinoxaline-bridged cavitands with one methylene, phosphonate, or phosphate bridge. The pyrazine-bridged resorcin4]arene cavitand displayed an unexpectedly high preference for the kite conformation in nonpolar solvents, relative to the quinoxaline-bridged analogue. This observation led to extensive solvent-dependent switching studies that provide a detailed picture of how solvent affects the thermal vase-kite equilibration. As for any thermodynamic process in the liquid phase, the conformational equilibrium is affected by how the solvent stabilizes the two individual states. Suitably sized solvents (benzene and derivatives) solvate the cavity of the vase form and reduce the propensity for the vase-to-kite transition. Correspondingly, the kite geometry becomes preferred in bulky solvents such as mesitylene, incapable of penetrating the vase cavity. As proposed earlier by Cram, the kite form is preferred at low temperatures due to the more favorable enthalpy of solvation of the enlarged surface. Furthermore, the kite conformation is more preferred in solvents with substantial hydrogen-bonding acidity: weak hydrogen-bonding interactions between the mildly basic quinoxaline and pyrazine nitrogen atoms and solvent molecules are more efficient in the open kite than in the closed vase form. Vase-to-kite conversion is entirely absent in dipolar aprotic solvents lacking any H-bonding acidity. Thermal vase-kite switching requires fully quinoxaline- or pyrazine-bridged cavitands, whereas pH-controlled switching is also applicable to systems incorporating only two or three such bridges.