Catalano, L.; Pérez-Estrada, S.; Wang, H.-H.; Ayitou, A. J.-L.; Khan, S. I.; Terraneo, G.; Metrangolo, P.; Brown, S.; Garcia-Garibay, M. A. ?Rotational Dynamics of Diazabicyclo[2.2.2]octane in Isomorphous Halogen-Bonded Co-crystals: Entropic and Enthalpic Effects? J. Am. Chem. Soc. 2017, 139, 843-848, DOI: 10.1021/jacs.6b10780.
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 (TrXN(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 TrClNR3 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?XNR3 (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 (?S? = ?3.0 cal mol?1 K?1), while there is almost a 2-fold difference in activation enthalpies, with 4a having a higher barrier (?H? = 0.95 kcal mol?1) than 4b (?H? = 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.