Anna Olejniczak

Supramolecular reaction between pressure-frozen acetonitrile phases alpha and beta.

The balance of weak CH...N bonds involving the H 3C- and -CN groups has been related to the structural rearrangement between centrosymmetric and polar acetonitrile structures. The linear highly polar molecules arrange antiparallel in phase alpha below a freezing temperature of 225 K/0.1 MPa and above a freezing pressure of 0.38(5) GPa/296 K and in a polar mode in phase beta below 206 K/0.1 MPa and at pressures higher than 0.63(5) GPa/296 K. The alpha <--> beta phase transition has been considered as a supramolecular reaction between 2-dimensional and 3-dimensional hydrogen-bonding networks, the latter favoring the polar association. Acetonitrile has been in situ pressure-frozen, and its structure has been determined at room temperature by single-crystal X-ray diffraction at 0.57(5), 0.63(5), and 1.50(5)GPa. The crystallization pressure at 296 K has been determined as 0.38(5) GPa both by ruby fluorescence in a diamond anvil-cell and by the compressibility measurement in a cylinder-and-piston device. Acetonitrile mixed with methanol trimerized at 1.60 GPa and 473 K yielding 4-amino-2,6-dimethylpyrimidine: it was in situ crystallized, and the structure of a single crystal, recovered at ambient conditions, was determined.

Competing hydrogen-bonding patterns and phase transitions of 1,2-diaminoethane at varied temperature and pressure

1,2-diaminoethane has been in-situ pressure- and temperature-frozen; apart from two known low-temperature phases, Ialpha and II, three new phases, Ibeta, Igamma and III, have been observed and their structures determined by X-ray diffraction. The measurements at 0.1 MPa were carried out at 274, 243 and 224 K, and 296 K measurements were made at 0.15 GPa (phase Ialpha), at 0.3 and 1.1 GPa (phase Ibeta), at 1.5 GPa (phase Igamma), and at 0.2, 0.3 and 0.5 GPa (phase III). All these phases are monoclinic, space group P2(1)/c, but the unit-cell dimension of phases Ialpha and III are very different at 296 K: aIalpha=5.078 (5), bIalpha=7.204 (8), cIalpha=5.528 (20) A, betaIalpha=115.2 (2) degrees at 0.15 GPa, and aIII=5.10 (3), bIII=5.212 (2), cIII=7.262 (12) A, betaIII=111.6 (4) degrees at 0.2 GPa, respectively; in both phases Z=2. An ambient-pressure low-temperature phase II has been observed below 189 K. Discontinuities in the unit-cell dimensions and in the N...N distance mark the isostructural transition between phases Ialpha and Ibeta at 0.2 GPa, which can be attributed to a damping process of the NH2 group rotations. In phase Igamma the unit-cell parameter a doubles and Z increases to 4. The molecule has inversion symmetry in all the structures determined. 1,2-Diaminoethane can be considered as a simple structural ice analogue, but with NH...N hydrogen bonds and with the H-atom donors (four in one molecule) in excess over H-atom acceptors (two per molecule). Thus, the transformations of 1,2-diaminoethane phases involving the conformational dynamics affect the hydrogen-bonding geometry and molecular association in the crystal. The 1,2-diaminoethane:1,2-dihydroxyethane mixture has been separated by pressure-freezing, and a solid 1,2-diaminoethane crystal in liquid 1,2-dihyroxyethane has been obtained.

Pressure induced transformations of 1,4-diazabicyclo[2.2.2]octane (dabco) hydroiodide: diprotonation of dabco, its N-methylation and co-crystallization with methanol

The non-solvated crystals of 1,4-diazabicyclo[2.2.2]octane hydroiodide (dabcoHI, [C6H13N2]+I−) obtained at normal conditions are hexagonal and exhibit giant dielectric response and anisotropic relaxer properties. The isochoric crystallization of dabcoHI at 1.80 GPa yields non-solvated dabcoHI crystal and a [C6H14N2]2+2I−:3CH3OH solvate (dabco2HI:3CH3OH) solvate, while pressure exceeding 2.30 GPa triggers the synthesis of N-methyl-1,4-diazabicyclo[2.2.2]octanium iodide, which co-crystallized with methanol (dabcoCH3I:2CH3OH, [C7H15N2]+I−:2CH3OH). Structures of both solvates were determined by single-crystal X-ray diffraction. Compared to non-solvated dabcoHI governed by NH+⋯N hydrogen bonds, in the two methane solvates investigated there are NH⋯O, N⋯HO, and OH⋯I hydrogen bonds, which illustrates the preference to replace the NH+⋯N hydrogen bonds with those involving oxygen atoms in high pressure.

Halogen⋯halogen contra C–H⋯halogen interactions

Pressure affects the competition between C–H⋯X hydrogen bonds and X⋯X halogen⋯halogen interactions. In bromomethane, CH3Br, pressure changes the molecular arrangement of the two solid-state phases of this compound: low-pressure phase α is dominated by halogen⋯halogen interactions, whereas above 1.5 GPa the β phase is governed by C–H⋯halogen bonds. The CH3Br phase α is isostructural with solid CH3I of orthorhombic space group Pnma, while CH3Br phase β is polar, isostructural with CH3Cl and CH3CN crystals, of orthorhombic space group Cmc21. The crystal structures of CH3Cl (b.p. = 249.1 K) and CH3Br (b.p. = 276.7 K) have been determined by high pressure single-crystal X-ray diffraction up to 4.38 GPa and 2.85 GPa, respectively. In CH3Br, pressure of 1.5 GPa enforces the close packing and opposite electrostatic-potential matching between molecular surfaces in contact. The interweaved C–H⋯X bonded diamondoid networks of β-CH3X are similar to those of acetonitrile, H2O ice VII and solidified X2 halogens. The phase diagrams of CH3Br and CH3Cl have been constructed.

Weak intermolecular interactions and molecular aggregation in isostructural dihaloperfluoroethanes

A series of 1,2-dihalotetrafluoroethanes X(CF2)2X (X = Br, I) has been in-situ pressure crystallized in a diamond-anvil cell (DAC), their structures determined by single crystal X-ray diffraction and rationalized by weak intermolecular interactions. For isoelectronic and isostructural 1,2-dibromotetrafluoroethane (BrCF2CF2Br), 1,2-diiodotetrafluoroethane (ICF2CF2I) and 1-bromo-2-iodotetrafluoroethane (BrCF2CF2I), the crystals are monoclinic, space groupP21/n, with the midpoint of the C–C bond located at the centre of inversion. The freezing pressures of these compounds have been determined to be 0.80 GPa, 0.30(5) GPa and 0.10(5) GPa for BrCF2CF2Br, BrCF2CF2I and CF2ICF2I, respectively. In the structure of ICF2CF2I, the –CF2–CF2– moiety is orientationally disordered at 0.16 GPa, but it becomes ordered at 0.86 GPa; in the BrCF2CF2I crystal the Br and I atoms are substitutionally disordered; and the BrCF2CF2Br structure is completely ordered. The formation of isostructural crystals by these compounds and different types of molecular disorder can be rationalized by the intermolecular interactions at varied thermodynamical conditions. The phenomenon of “self-healing” has been observed, in which crystal defects, formed during initial crystallization in the DAC, repaired themselves during the annealing process, which greatly facilitates the formation of single crystals. Pressure considerably affects the crystal morphology of ICF2CF2I, where the elimination of –CF2–CF2– disorder coincides with the appearance of dendritic forms of the crystal phases.

Halogen⋯oxygen aggregation and disorder modes in pressure frozen XCF2CF2X : 1,4-dioxane (X = Br,I) complexes

Formation of cocrystals and halogen⋯O interactions was studied in the structures of 1,4-dioxane with 1,2-diiodo-, 1-bromo-2-iodo- and 1,2-dibromoperfluoroethanes. Mixtures pressure-frozen in a diamond-anvil cell have been investigated. Structures of cocrystals of 1,2-diiodoperfluoroethane:1,4-dioxane at 0.30(5) GPa/296(2) K and of 1-bromo-2-iodoperfluoroethane:1,4-dioxane at 0.62(5) GPa/296 K were determined by single-crystal X-ray diffraction. Also the single-crystal of 1,4-dioxane separated from its mixture with 1,2-dibromoperfluoroethane, which remained liquid, was investigated at 0.42 GPa/296 K. The cocrystal of ICF2CF2I:C4H8O2 and the C4H8O2 crystals freeze into the same phases as those obtained by cooling; the BrCF2CF2I:C4H8O2 cocrystal was not synthesized before. In all these cocrystals the structures are dominated by halogen⋯O interactions. In the structure of ICF2CF2I:C4H8O2 the –CF2–CF2– moiety is disordered about the I⋯I molecular axis and the C4H8O2 molecule rotates about the O⋯O axis, too; in BrCF2CF2I:C4H8O2 the Br and I atoms are disordered at the same position with half occupancy, but the C4H8O2 molecule is ordered. The pressure-freezing of 1,4-dioxane from the BrCF2CF2Br:C4H8O2 mixture yielded ordered phase II of 1,4-dioxane previously obtained from the neat compound by cooling. Spectacular kinetic crystallization of ICF2CF2I:C4H8O2 in the form of hexagonal snowflakes has been observed in the diamond-anvil cell.

Remote halogen switch of amine hydrophilicity

Bromide and iodide anions switch hydrogen-bonding patterns in otherwise isostructural dimethanol solvates N-methyl-1,4-diazabicyclo[2.2.2]octanium bromide (dabcoCH3Br·2CH3OH) and analogous iodide (dabcoCH3I·2CH3OH), both synthesized in the high-pressure version of the Menshutkin reaction at 1.2 and 2.4 GPa, respectively. The magnitudes of the high pressure triggering these reactions correspond to identical molecular volumes of both solvates.

Fluorinated enamines of nucleobases as precursors of nucleoside analogues. Synthesis, spectroscopic and structural studies

The 1H and 19F NMR spectra of N-α-fluoro-β-trifluoromethylenamines and isostructural N-β-fluoro-β-trifluoromethylenamines of nucleobases dissolved in CDCl3 and DMSO-d6 have shown distinct differences associated with the conformational conversion between the Z and E stereoisomers. In the E stereoisomer the tetrafluoropropenyl group is rotated relative to the heteroring plane, whereas the Z stereoisomer assumes the most planar structure. The flat conformation of the Z stereoisomer is stabilised by internal hydrogen bonding between Cα–H and the carbonyl oxygen (pyrimidinic bases) or the endocyclic nitrogen (purinic bases). Large variations in the magnitudes of the chemical shifts of Cβ–H, Cα–F, C6–H and C8–H were observed, i.e. the chemical shift increased with increasing polarity and ability of DMSO-d6 to establish intermolecular hydrogen bonds in competition with intramolecular hydrogen bonds. The (Z)-N4-benzoyl-N1-(1,3,3,3-tetrafluoroprop-1-enyl)cytosine crystals undergo a phase transition at 230 K induced by the tetrafluoropropenyl substituent reorientations. Between 300 and 230 K the molecules are present in two conformations, and below 230 K the molecules gradually assume five conformations, remaining in a stable equilibrium with intermolecular forces, evidenced by single-crystal X-ray diffraction.

Reverse sequence of transitions in prototypic relaxor 1,4-diazabicyclo[2.2.2]octane

Two phase transitions consecutively reduce the crystal symmetry with rising temperature in 1,4-diazabicyclo[2.2.2]octane hydrobromide, [C6H13N2]+·Br−, dabcoHBr. Low-temperature prototypic hexagonal phase III, space group Pm2, Z = 1 above 458 K transforms into orientational states of orthorhombic phase II, space group Cmc21, Z = 4 and above 471 K to orthorhombic phase I, space group Pca21, Z = 8. The reverse transitions have been attributed to entropy and enthalpy contributions of the proton disordering, disproportionation of dabcoH+ cations into neutral dabco molecules and dabcoH2+2 dications at nanodomain walls, opposite propeller versus planar dabco conformations and conversions between NH+⋯N and NH+⋯Br− hydrogen bonds in dabcoHBr phase III, the NH+⋯N bonded relaxor material. All of the structural-disorder features in the NH+⋯N bonded aggregates are essential for the short-range polarization and giant dielectric response in dabcoHBr phase III. Despite the first-order type of the phase transitions, the lattice dimensions of phases I and II remain related to the prototypical relaxor phase III.

High pressure used for producing a new solvate of 1,4-diazabicyclo[2.2.2]octane hydroiodide

High pressure efficiently induces polymorphic transformations and various solvates of 1,4-diazabicyclo[2.2.2]octane hydroiodide (dabcoHI). However, it has defied the formation of solvates with methanol. In this article we report on the isochoric crystallization method of producing dabcoHI from methanol:ethanol:water solution above 3 GPa leading to the mixed solvate with methanol, albeit with water as well. A single crystal of dabcoHI·H2O·CH3OH has been grown in situ in a diamond-anvil cell, and its structure determined by X-ray diffraction. In its structure the NH+⋯N hydrogen bonds, which link the dabco cations into chains in all 10 unsolvated dabcoHI polymorphs, are replaced by H-bonds NH+⋯OH⋯N analogous to those in two high-pressure polymorphs of monohydrate dabcoHI·H2O. The pressure-induced solvation of dabcoHI has been linked to the dimension of voids in the neat compound, and the systematic pressure effect destabilizing the NH+⋯N bonds has been demonstrated for the series of dabcoHI polymorphs and solvates.