Philippe Négrier

Polymorphism of even saturated carboxylic acids from n-decanoic to n-eicosanoic acid

The polymorphism of normal saturated even carboxylic acids from n-decanoic to n-eicosanoic acid is discussed. Seven crystal modifications, including polymorphs and polytypes, were identified and fully characterized by the combination of calorimetric measurements (DSC) at atmospheric and high pressures, X-ray powder diffraction, FT-IR spectroscopy and scanning electron microscopy (SEM). All seven crystal forms, including polymorphs and polytypes, are observed at room temperature. Forms A2 and Asuper are triclinic, form C is monoclinic and forms E and B show both a monoclinic and an orthorhombic polytype. The triclinic modifications A2 and Asuper predominate for acids up to n-tetradecanoic acid (C14H27O2H). The orthorhombic and the monoclinic forms predominate for acids from n-hexadecanoic (C16H31O2H) up to n-eicosanoic acid (C20H39O2H). When the temperature is increased, all the crystal modifications transform irreversibly to the C form. In the first part of this paper, cell parameters for the different forms are given, the observed temperatures and enthalpies of the transitions are reported and the stability of the different forms is discussed. In the second part, we state the main contribution of each technique for the identification and interpretation of the polymorphism of even numbered carboxylic acids.

Site-Selective Synthesis of Janus-type Metal-Organic Framework Composites†

Herein, bipolar electrochemistry is applied in a straightforward way to the site-selective in situ synthesis of metal-organic framework (MOF) structures, which have attracted tremendous interest in recent years because of their significant application potential, ranging from sensing to gas storage and catalysis. The novelty of the presented work is that the deposit can be intentionally confined to a defined area of a substrate without using masks or templates. The intrinsic site-selectivity of bipolar electrochemistry makes it a method of choice to generate, in a highly controlled way, hybrid particles that may have different functionalities combined on the same particle. The wireless nature of electrodeposition allows the potential for mass production of such Janus-type objects.

Spin transition with a large thermal hysteresis near room temperature in a water solvate of an iron(III) thiosemicarbazone complex

The magnetic properties of the monohydrated ferric complex Li[Fe(5Brthsa)2]·H2O (H2-5Brthsa = 5-bromosalicylaldehyde thiosemicarbazone) have been investigated by SQUID and Mossbauer measurements. The S = 1/2 ↔ S = 5/2 spin transition of the ferric ion is accompanied by a quite broad hysteresis (ΔT = 39 K) centred around 313 K. The spin states involved are characterised by the quadrupole splittings ΔEQ(2T2) = 2.584 ± 0.002 mm s−1 and ΔEQ(6A1) = 0.338 ± 0.006 mm s−1 and the isomer shifts δIS (2T2) =+0.262 ± 0.001 mm s−1 and δIS (6A1) =+0.294 ± 0.002 mm s−1 at 77 and 360 K, respectively. A powder X-ray diffraction study at various temperatures demonstrates the occurrence of a crystallographic first-order phase transition of the lattice coupled to the spin conversion. The enthalpy and entropy variations associated with the transition have been estimated from DSC measurements at ΔH = 5.7 ± 0.5 kJ mol−1 and ΔS = 18 ± 2 J mol−1 K−1. The existence of a crystallographic first-order phase transition associated to the spin crossover is consistent with the cooperative character of the process. This phase transformation might originate from the modification of the extended hydrogen-bond network.

Melting behaviour in the n-alkanol family. Enthalpy-entropy compensation

The melting behaviour was studied in ten systems: C15OH–C16OH, C16OH–C17OH, C17OH–C18OH, C18OH–C19OH, C19OH–C20OH with Δn = 1 (difference in chain length), C15OH–C17OH, C16OH–C18OH, C17OH–C19OH, C18OH–C20OH with Δn = 2, and C16OH–C20OH with Δn = 4. The phase that melts is either the monoclinic R′IV(C2/m, Z = 4) or the hexagonal R′II(Rm, Z = 6) rotator form. One of the most important issues in the melting of these systems is that when the two original compounds of the system are isostructural, the phase diagram does not always show total miscibility. In the systems studied here, only the C15OH–C16OH, C18OH–C19OH and C19OH-C20OH systems show total miscibility. In the other systems in which the two original compounds are isostructural, miscibility is partial, as in the systems where the two original compounds are not isostructural. In this family, as in other families of mixed crystals, there is an excess enthalpy-entropy compensation. This compensation has a temperature dimension, and is called the compensation temperature (θ) of the family and/or subfamily. In the case of the R′II and R′IV rotator forms of the n-alkanols family its value is 362 K. This value is in line with the trend show by a large group of organic and inorganic mixed crystalline materials.

Lidocaine/L-menthol binary system: cocrystallization versus solid-state immiscibility.

We present the synthesis, structure determination, and thermodynamic properties of a never reported cocrystal prepared with lidocaine and L-menthol. The temperature-composition phase diagram of the lidocaine/L-menthol binary system was achieved using differential scanning calorimetry and X-ray diffraction experiments. The present study demonstrates that the only way to perform a phase equilibrium survey of the lidocaine/L-menthol system is to prepare the binary mixtures from the cocrystal, an equimolar stoichiometric compound of L-menthol and lidocaine. We describe a process that is crucial to elaborate pharmaceutical agents that remain in their thermodynamical stable state throughout their preparation, manufacture, and storage for effective use.

The crystallographic phase transition for a ferric thiosemicarbazone spin crossover complex studied by X-ray powder diffraction

The crystal structure of a spin-transition compound, namely the thiosemicarbazone ferric complex Li[Fe(5BrThsa)2]·H2O, was solved from powder X-ray diffraction data at temperatures where the high-spin (373 K) and low-spin (150 K) phases prevail. The methodology is based on traditional approaches (direct methods) combined with direct space strategy. Both phases crystallise in the monoclinic system P21/c. At 373 K, the characteristics of the [FeN2O2S2] coordination core are consistent with those reported for high-spin iron(III) thiosemicarbazone complexes: a distorted coordination polyhedron and non-equivalent metal–ligand bond lengths. When the temperature is reduced to 150 K, a decrease of the β angle from ca. 101° (373 K) to ca. 90° (150 K) is the only major modification of the cell parameters. The low-spin molecular structure reveals significant differences in bond lengths and bond angles compared to the high-spin structure. Finally, an extended hydrogen-bond network is implicated in the cooperative phase transition, as supported by strong intermolecular contacts between the ferric complexes and the water molecules and the crystallographic phase transition is associated with pronounced lattice reorganization.

Degree of miscibility between non-isomorphous plastic phases: binary system NPG (neopentyl glycol)-TRIS[tris(hydroxymethyl)aminomethane]

The experimental phase diagram between the plastic crystals neopentyl glycol (NPG) and tris(hydroxymethyl)aminomethane (TRIS) has been established. The importance of the use of complementary techniques with different timescales in order to obtain the most realistic phase diagram is underlined. The miscibility is discussed on the basis of the geometric conditions (size and shape of molecules and unit-cell dimensions) and of the role of the intermolecular interactions in the structure of the mixed crystals. Binary phase diagrams formed by related, previously reported neopentane derivatives are also analysed in the same manner.

First experimental demonstration of crossed isodimorphism: (CH3)3CCl + CCl4 melting phase diagram

The melting phase diagram of the two-component system 2-methyl-2-chloro-propane ((CH3)3CCl) and carbon tetrachloride (CCl4) has been determined by combining X-ray powder diffraction and thermal analysis. The isomorphism relation between the orientationally disordered (OD) stable face-centered cubic (FCC) phase of (CH3)3CCl and the metastable FCC phase of CCl4 has been demonstrated throughout the continuous evolution of the lattice parameters and the existence of the two-phase equilibrium [FCC + L] for the whole range of composition, despite the monotropy of the FCC phase for the CCl4 component. This equilibrium interferes with a rhombohedral plus liquid ([R + L]) equilibrium giving rise to a peritectic invariant. A thermodynamic analysis, in terms of the crossed isodimorphism, has also been performed in order to obtain the excess properties of the FCC and R OD phases.

Polymorphism in 2-X-Adamantane Derivatives (X = Cl, Br)

The polymorphism of two 2-X-adamantane derivatives, X = Cl, X = Br, has been studied by X-ray powder diffraction and normal- and high-pressure (up to 300 MPa) differential scanning calorimetry. 2-Br-adamantane displays a low-temperature orthorhombic phase (space group P212121, Z = 4) and a high-temperature plastic phase (Fm3̅m, Z = 4) from 277.9 ± 1.0 K to the melting point at 413.4 ± 1.0 K. 2-Cl-adamantane presents a richer polymorphic behavior through the temperature range studied. At low temperature it displays a triclinic phase (P1̅, Z = 2), which transforms to a monoclinic phase (C2/c, Z = 8) at 224.4 ± 1.0 K, both phases being ordered. Two high-temperature orientationally disordered are found for this compound, one hexagonal (P63/mcm, Z = 6) at ca. 241 K and the highest one, cubic (Fm3̅m, Z = 4), being stable from 244 ± 1.0 K up to the melting point at 467.5 ± 1.0 K. No additional phase appears due to the increase in pressure within the studied range. The intermolecular interactions are found to be weak, especially for the 2-Br-adamantane compound for which the Br···Br as well as C-Br···H distances are larger than the addition of the van der Waals radii, thus confirming the availability of this compound for building up diamondoid blocks.

Polymorphism of CBr2Cl2

The polymorphism of dibromodichloromehtane (CBr2Cl2) has been studied by means of a wide set of experimental techniques as a function of temperature and pressure. From the p–V–T diagram and the derived p–T diagram the volume variations at the transition points have been calculated and compared with those obtained by means of X-ray and neutron powder diffraction. By combining the experimental techniques, it has been demonstrated the existence of a high-pressure orientationally disordered rhombohedral phase (aR ≈ 14.6 ± 0.3 A and α ≈ 89.2 ± 0.2°). The existence of a glass transition within the monoclinic (C2/c, Z = 32) low-temperature ordered phase associated with the freezing of exchange positions between Cl and Br atoms is analyzed considering the asymmetry of the intermolecular interactions by means of the study of the thermal expansion tensor. The change of the derivative of the aspherism index, previously reported as a possible “fingerprint” for this kind of glass transitions, is found.