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

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• 作者：Luca CatalanoSalvador Perez-Estrada ; Hsin-Hua Wang ; Anoklase J.-L. Ayitou ; Saeed I. Khan ; Giancarlo Terraneo ; Pierangelo Metrangolo ; Stuart Brown ; Miguel A. Garcia-Garibay
• 刊名：Journal of the American Chemical Society
• 出版年：2017
• 出版时间：January 18, 2017
• 年：2017
• 卷：139
• 期：2
• 页码：843-848
• 全文大小：412K
• ISSN：1520-5126

文摘

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 (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.