Rotational Dynamics of Diazabicyclo[2.2.2]octane in Isomorphous Halogen-Bonded Co-crystals: Entropic and Enthalpic Effects

Based
on rotational dynamics measurements carried out with isomorphic
co-crystals formed by halogen-bonding (XB) between tritylacetylene
halides (TrX) and diazabicyclo[2.2.2]­octane (dabco), we were able
to distinguish the sources of the enthalpic and entropic components
in the rotational free energy barrier. We describe the formation of
the 1:1 co-crystals (TrX···N­(R)3N) obtained
from 1 equiv of dabco and 1 equiv of either TrI or TrBr, respectively,
to give 4a and 4b instead of the potential
2:1 complexes. The co-crystals were prepared by solvent evaporation
and mechanochemical synthesis. No co-crystal with TrCl was obtained,
reflecting the weaker nature of the TrCl···NR3 interaction. Single-crystal X-ray diffraction confirmed structures
that resemble a spinning top on a tripod and revealed that the two
XB co-crystals are isomorphous, with slightly different C–X···NR3 (X = I, Br) distances and packing interactions. Quadrupolar-echo 2H NMR experiments with 2H-labeled samples showed
that fast rotation of dabco in these co-crystals follows a six-fold
potential energy surface with three lowest energy minima. Variable-temperature 1H NMR spin–lattice relaxation (VT 1H T1) data revealed rotational dynamics with indistinguishable
pre-exponential factors and small but distinguishable activation energies.
The activation energy of 4b (Ea = 0.71 kcal mol–1) is the lowest reported in the
field of amphidynamic crystals. Using the Eyring equation, we established
that their activation entropy for rotation is small but negative (ΔS⧧ = −3.0 cal mol–1 K–1), while there is almost a 2-fold difference
in activation enthalpies, with 4a having a higher barrier
(ΔH⧧ = 0.95 kcal mol–1) than 4b (ΔH⧧ = 0.54 kcal mol–1). Analysis of
the rotator cavity in the two co-crystals revealed subtle differences
in steric interactions that account for their different activation
energies.