Rotational Dynamics of an Amphidynamic Zirconium Metal-Organic Framework Determined by Dielectric Spectroscopy

A zirconium metal-organic framework with a difluorophenylene rotator bearing a permanent electric dipole of ∼3.2 D was synthesized, and its rotational motion was analyzed by temperature- and frequency-dependent broadband dielectric spectroscopy. While solid-state NMR confirms fast rotation qualitatively, the dissipation factors measured between 113 and 153 K suggested an activation energy E_a = 2.6 kcal/mol, but deviations from a single Debye relaxation suggested a dynamic process that cannot be accounted for by a well-defined potential with a single activation barrier. The dynamic heterogeneity of the dipolar rotor was confirmed by analysis in terms of a Cole-Cole relaxation, which suggested a mean barrier of ∼1.9 kcal/mol, with a heterogeneity that decreases as temperature increases. Based on the single-crystal structure, we propose that the kinetic heterogeneity results from a temperature-dependent potential where rotation motion is mediated by the escape of the rotator from an energy well created by a double Ph-H···F-Ph hydrogen bond.