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 halogenbonding (XB) between tritylacetylene halides (TrX) and diazabicyclo[2.2.2]octane (dabco), we were able to distinguish the sources of the enthalpic and entropie 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-echo²H NMR experiments with²H-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 (E_a = 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.