Equilibrium Isotope Effects on Noncovalent Interactions in a Supramolecular Host–Guest System

The self-assembled supramolecular complex [Ga<sub>4</sub>L<sub>6</sub>]<sup>12‑</sup> (<b>1</b>; L = 1,5-bis­[2,3-dihydroxybenzamido]­naphthalene) can act as a molecular host in aqueous solution and bind cationic guest molecules to its highly charged exterior surface or within its hydrophobic interior cavity. The distinct internal cavity of host <b>1</b> modifies the physical properties and reactivity of bound guest molecules and can be used to catalyze a variety of chemical transformations. Noncovalent host–guest interactions in large part control guest binding, molecular recognition and the chemical reactivity of bound guests. Herein we examine equilibrium isotope effects (EIEs) on both exterior and interior guest binding to host <b>1</b> and use these effects to probe the details of noncovalent host–guest interactions. For both interior and exterior binding of a benzylphosphonium guest in aqueous solution, protiated guests are found to bind more strongly to host <b>1</b> (<i>K</i><sub>H</sub>/<i>K</i><sub>D</sub> > 1) and the preferred association of protiated guests is driven by enthalpy and opposed by entropy. Deuteration of guest methyl and benzyl C–H bonds results in a larger EIE than deuteration of guest aromatic C–H bonds. The observed EIEs can be well explained by considering changes in guest vibrational force constants and zero-point energies. DFT calculations further confirm the origins of these EIEs and suggest that changes in low-frequency guest C–H/D vibrational motions (bends, wags, etc.) are primarily responsible for the observed EIEs.