title: Rotational Dynamics of Diazabicyclo[2.2.2]octane in Isomorphous Halogen-Bonded Co-crystals: Entropic and Enthalpic Effects.
abstract: 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···NR3interaction. 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-temperature1H NMR spin-lattice relaxation (VT1H 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-1K-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.
authors.affiliation: Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095, United States.|Laboratory of Nanostructured Fluorinated Materials (NFMLab), Department of Chemistry, Materials, and Chemical Engineering "Giulio Natta", Politecnico di Milano , via L. Mancinelli 7, 20131 Milano, Italy.
authors.forename: Luca
authors.identifier: 0000-0002-1003-6512
authors.initials: L
authors.lastname: Catalano
authors.affiliation: Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095, United States.
authors.forename: Salvador
authors.initials: S
authors.lastname: Perez-Estrada
authors.affiliation: Department of Physics and Astronomy, University of California , Los Angeles, California 90095, United States.
authors.forename: Hsin-Hua
authors.initials: HH
authors.lastname: Wang
authors.affiliation: Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095, United States.
authors.forename: Anoklase J-L
authors.initials: AJ
authors.lastname: Ayitou
authors.affiliation: Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095, United States.
authors.forename: Saeed I
authors.initials: SI
authors.lastname: Khan
authors.affiliation: Laboratory of Nanostructured Fluorinated Materials (NFMLab), Department of Chemistry, Materials, and Chemical Engineering "Giulio Natta", Politecnico di Milano , via L. Mancinelli 7, 20131 Milano, Italy.
authors.forename: Giancarlo
authors.initials: G
authors.lastname: Terraneo
authors.affiliation: Laboratory of Nanostructured Fluorinated Materials (NFMLab), Department of Chemistry, Materials, and Chemical Engineering "Giulio Natta", Politecnico di Milano , via L. Mancinelli 7, 20131 Milano, Italy.|VTT Technical Research Centre of Finland Ltd. , Biologinkuja 7, FI-02044 Espoo, Finland.
authors.forename: Pierangelo
authors.initials: P
authors.lastname: Metrangolo
authors.affiliation: Department of Physics and Astronomy, University of California , Los Angeles, California 90095, United States.
authors.forename: Stuart
authors.initials: S
authors.lastname: Brown
authors.affiliation: Department of Chemistry and Biochemistry, University of California , Los Angeles, California 90095, United States.
authors.forename: Miguel A
authors.identifier: 0000-0002-6268-1943
authors.initials: MA
authors.lastname: Garcia-Garibay