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 Ea = 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.