Nucleophilic Substitution Reactions in the [B3H8]- Anion in the Presence of Lewis Acids

As a result of our study on the interaction between the octahydrotriborate anion with nucleophiles (Nu = THF, Ph₃P, Ph₂P-(CH₂)₂-PPh₂ (dppe), Ph₃As, Et₃N, PhNH₂, C₅H₅N, CH₃CN, Ph₂CHCN)) in the presence of a wide range of Lewis acids (Ti(IV), Hf(IV), Zr(IV), Al, Cu(I), Zn, Mn(II), Co(II) halides and iodine), a number of substituted derivatives of the octahydrotriborate anion [B₃H₇Nu] are obtained. It is found that the use of TiCl₄, AlCl₃, ZrCl₄, HfCl₄, CuCl and iodine leads to the highest product yields. In this case, it is most likely that the reaction proceeds through the formation of an intermediate [B₃H₇-HMXn_x], which was detected by NMR spectroscopy. The structures of [Ph₃P·B₃H₇] and [PhNH₂·B₃H₇] were determined by X-ray diffraction.

Use of a miniature diamond-anvil cell in a joint X-ray and neutron high-pressure study on copper sulfate pentahydrate

Single-crystal X-ray and neutron diffraction data are usually collected using separate samples. This is a disadvantage when the sample is studied at high pressure because it is very difficult to achieve exactly the same pressure in two separate experiments, especially if the neutron data are collected using Laue methods where precise absolute values of the unit-cell dimensions cannot be measured to check how close the pressures are. In this study, diffraction data have been collected under the same conditions on the same sample of copper(II) sulfate pentahydrate, using a conventional laboratory diffractometer and source for the X-ray measurements and the Koala single-crystal Laue diffractometer at the ANSTO facility for the neutron measurements. The sample, of dimensions 0.40 × 0.22 × 0.20 mm3 and held at a pressure of 0.71 GPa, was contained in a miniature Merrill-Bassett diamond-anvil cell. The highly penetrating diffracted neutron beams passing through the metal body of the miniature cell as well as through the diamonds yielded data suitable for structure refinement, and compensated for the low completeness of the X-ray measurements, which was only 24% on account of the triclinic symmetry of the sample and the shading of reciprocal space by the cell. The two data-sets were combined in a single 'XN' structure refinement in which all atoms, including H atoms, were refined with anisotropic displacement parameters. The precision of the structural parameters was improved by a factor of up to 50% in the XN refinement compared with refinements using the X-ray or neutron data separately.

Ionic liquid and lysine co-assisted synthesis of the highly dispersed Ni/SAPO-11 catalyst

The synthesis of highly-dispersed non-noble based catalysts and its application on hydrogenation /dehydrogenation still remains challenging. Therefore, it is of great importance to protect metal from aggregation in both calcination...

Pressure-induced superelastic behaviour of isonicotinamide

Dynamic organic crystals have come to the fore as potential lightweight alternatives to inorganic actuators providing high weight-to-force ratios. We have observed pressure-induced superelastic behaviour in Form I of isonicotinamide. The reversible single-crystal to single-crystal transformation exhibited by the system is an important component for functioning actuators. Crucially, our observations have enabled us to propose a mechanism for the molecular movement supported by Pixel energy calculations, that may pave the way for the future design and development of functioning dynamic crystals.

How can fluorine directly and indirectly affect the hydrogen bonding in molecular systems? - A case study for monofluoroanilines

Hydrogen bonds (HBs) directly engaging fluorine has been extensively studied, but the indirect effect of fluorine on adjacent donors and acceptors is poorly understood and still difficult to predict. The indirect and direct effect of the fluorination of aniline on HB patterns observed in monofluoroanilines was studied via experimental (vibrational spectroscopy and crystal structure analysis) and theoretical (ab initio molecular dynamics and electrostatic surface potential) methods. It was found that a fluorine substituent decreases the strength and frequency of N-H⋯N HBs and, at the same time, increases the acidity of CH protons, enhancing the competitiveness of weaker interactions. Additionally, the position of fluorine in the aromatic ring strongly affects the C-F bond length, and a direct intramolecular N-H⋯F HB causes an increase in the N-H bond stability. We also provide a methodology to identify and separate individual HBs concerning the type of donor or acceptor from the ab initio molecular dynamics trajectories.

Substituent effects of nitro group in cyclic compounds

Numerous studies on nitro group properties are associated with its high electron-withdrawing ability, by means of both resonance and inductive effect. The substituent effect of the nitro group may be well described using either traditional substituent constants or characteristics based on quantum chemistry, i.e., cSAR, SESE, and pEDA/sEDA models. Interestingly, the cSAR descriptor allows to describe the electron-attracting properties of the nitro group regardless of the position and the type of system. Analysis of classical and reverse substituent effects of the nitro group in various systems indicates strong pi-electron interactions with electron-donating substituents due to the resonance effect. This significantly affects the pi-electron delocalization of the aromatic ring decreasing the aromatic character, evidenced clearly by HOMA values. Use of the pEDA/sEDA model allows to measure the population of electrons transferred from the ring to the nitro group.

High-pressure polymorphism in pyridine

Single crystals of the high-pressure phases II and III of pyridine have been obtained by in situ crystallization at 1.09 and 1.69 GPa, revealing the crystal structure of phase III for the first time using X-ray diffraction. Phase II crystallizes in P212121 with Z' = 1 and phase III in P41212 with Z' = ½. Neutron powder diffraction experiments using pyridine-d5 establish approximate equations of state of both phases. The space group and unit-cell dimensions of phase III are similar to the structures of other simple compounds with C 2v molecular symmetry, and the phase becomes stable at high pressure because it is topologically close-packed, resulting in a lower molar volume than the topologically body-centred cubic phase II. Phases II and III have been observed previously by Raman spectroscopy, but have been mis-identified or inconsistently named. Raman spectra collected on the same samples as used in the X-ray experiments establish the vibrational characteristics of both phases unambiguously. The pyridine molecules interact in both phases through CH⋯π and CH⋯N interactions. The nature of individual contacts is preserved through the phase transition between phases III and II, which occurs on decompression. A combination of rigid-body symmetry mode analysis and density functional theory calculations enables the soft vibrational lattice mode which governs the transformation to be identified.

N–H⋯O hydrogen bonding to the alkoxy oxygen of a carboxylic ester group: crystal structures of methyl 2,6-diaminobenzoate and its derivatives

Molecules with hydrogen bonds from amino groups to both oxygens of a carboxylic ester are described, and other examples of hydrogen bonding to an ester's alkoxy oxygen atom are highlighted.

Pressure-Induced Polymorphism of Caprolactam: A Neutron Diffraction Study

Caprolactam, a precursor to nylon-6 has been investigated as part of our studies into the polymerization of materials at high pressure. Single-crystal X-ray and neutron powder diffraction data have been used to explore the high-pressure phase behavior of caprolactam; two new high pressure solid forms were observed. The transition between each of the forms requires a substantial rearrangement of the molecules and we observe that the kinetic barrier to the conversion can aid retention of phases beyond their region of stability. Form II of caprolactam shows a small pressure region of stability between 0.5 GPa and 0.9 GPa with Form III being stable from 0.9 GPa to 5.4 GPa. The two high-pressure forms have a catemeric hydrogen-bonding pattern compared with the dimer interaction observed in ambient pressure Form I. The interaction between the chains has a marked effect on the directions of maximal compressibility in the structure. Neither of the high-pressure forms can be recovered to ambient pressure and there is no evidence of any polymerization occurring.

The Effect of Pressure on Halogen Bonding in 4-Iodobenzonitrile

The crystal structure of 4-iodobenzonitrile, which is monoclinic (space group I2/a) under ambient conditions, contains chains of molecules linked through C≡N···I halogen-bonds. The chains interact through CH···I, CH···N and π-stacking contacts. The crystal structure remains in the same phase up to 5.0 GPa, the b axis compressing by 3.3%, and the a and c axes by 12.3 and 10.9 %. Since the chains are exactly aligned with the crystallographic b axis these data characterise the compressibility of the I···N interaction relative to the inter-chain interactions, and indicate that the halogen bond is the most robust intermolecular interaction in the structure, shortening from 3.168(4) at ambient pressure to 2.840(1) Å at 5.0 GPa. The π∙∙∙π contacts are most sensitive to pressure, and in one case the perpendicular stacking distance shortens from 3.6420(8) to 3.139(4) Å. Packing energy calculations (PIXEL) indicate that the π∙∙∙π interactions have been distorted into a destabilising region of their potentials at 5.0 GPa. The structure undergoes a transition to a triclinic ( P 1 ¯ ) phase at 5.5 GPa. Over the course of the transition, the initially colourless and transparent crystal darkens on account of formation of microscopic cracks. The resistance drops by 10% and the optical transmittance drops by almost two orders of magnitude. The I···N bond increases in length to 2.928(10) Å and become less linear [<C-I∙∙∙N = 166.2(5)°]; the energy stabilises by 2.5 kJ mol^-1 and the mixed C-I/I..N stretching frequency observed by Raman spectroscopy increases from 249 to 252 cm^-1. The driving force of the transition is shown to be relief of strain built-up in the π∙∙∙π interactions rather than minimisation of the molar volume. The triclinic phase persists up to 8.1 GPa.

Antisolvent addition at extreme conditions

Antisolvent addition at high pressure (0.8 GPa) allows crystallization and recovery to ambient pressures of metastable form II paracetamol.

A liquid crucible model for aggregation of phenylacetylene in the gas phase

Structural transformation from a π-stacked dimer to an aromatic C–H⋯π trimer and a tetramer.

The role of H-bond in the high-pressure chemistry of model molecules

Pressure is an extraordinary tool to modify direction and strength of intermolecular interactions with important consequences on the chemical stability of molecular materials. The decrease of the distance among nearest neighbour molecules can give rise to reactive configurations reflecting the crystal arrangement and leading to association processes. In this context, the role of the H-bonds is very peculiar because their usual strengthening with rising pressure does not necessarily configure a decrease of the reaction activation energy but, on the contrary, can give rise to an anomalous stability of the system. In spite of this central role, the mechanisms by which a chemical reaction is favoured or prevented by H-bonding under high pressure conditions is a poorly explored field. Here we review a few studies where the chemical behaviour of simple molecular systems under static compression was related to the H-bonding evolution with pressure. These results are able to clarify a wealth of changes of the chemical and physical properties caused by the strengthening with pressure of the H-bonding network and provide additional tools to understand the mechanisms of high-pressure reactivity, a mandatory step to make these synthetic methods of potential interest for applicative purposes.

One-dimensional diamondoid polyaniline-like nanothreads from compressed crystal aniline

One-dimensional diamondoid polyaniline-like nanothreads combine the outstanding mechanical properties of carbon nanotubes with the versatility of NH₂ groups.

Low-dimensional nanomaterials such as highly ordered polyaniline (PANI) have attracted considerable interest due to their expected extraordinary electronic and optoelectronic properties. In spite of several attempts, the attainment of atomically well-ordered PANI is a long-standing challenge. Pressure-induced polymerization of aromatic molecules in the crystal phase has been demonstrated as a practicable route for the synthesis of highly ordered polymers but this approach has never been tested to produce PANI. Here we show the synthesis of diamondoid polyaniline-like nanothreads at 33 GPa and 550 K by compressing aniline in crystal phase-II. Infrared spectroscopy, transmission electron microscopy, X-ray diffraction data, and DFT calculations support the formation of this totally new polyaniline-like nanothread. The NH₂-enriched carbon nanothread combines the outstanding mechanical properties of carbon nanotubes with the versatility of NH₂ groups decorating the exterior of the nanothreads representing potential active sites for doping and as linkers for molecules with biological interest and inorganic nanostructures. The synergy of all of these properties emphasizes the strong potential of this material to be applied in a broad range of areas, from chemistry to materials engineering.

Acceptor properties of amino groups in aminobenzene crystals: study from the energetic viewpoint

The role of the N–H⋯N hydrogen bonds in the organization of the crystals of the aniline and diaminobenzenes has been studied.

Investigation of Methacrylic Acid at High Pressure Using Neutron Diffraction

This article shows that pressure can be a low-intensity route to the synthesis of polymethacrylic acid. The exploration of perdeuterated methacrylic acid at high pressure using neutron diffraction reveals that methacrylic acid exhibits two polymorphic phase transformations at relatively low pressures. The first is observed at 0.39 GPa, where both phases were observed simultaneously and confirm our previous observations. This transition is followed by a second transition at 1.2 GPa to a new polymorph that is characterized for the first time. On increasing pressure, the diffraction pattern of phase III deteriorates significantly. On decompression phase III persists to 0.54 GPa before transformation to the ambient pressure phase. There is significant loss of signal after decompression, signifying that there has been a loss of material through polymerization. The orientation of the molecules in phase III provides insight into the possible polymerization reaction.

New insights into the phase diagram of a magnetic perovskite, LaCo₁/₃Mn₂/₃O₃

We report the crystal structure of the orthorhombic perovskite LaCo1/3Mn2/3O3 as determined by neutron diffraction from 5-300 K. A high-temperature x-ray diffraction study is also reported from 290-900 K. At temperatures above 570 K, LaCo1/3Mn2/3O3 transforms to a rhombohedral structure with space group R3̄c. This rhombohedral phase is also observed in the material at high pressure and the crystal structure has been determined by in situ neutron diffraction at 4.7 GPa. Finally, the ferromagnetic behaviour has been determined by magnetometry and the magnetic structure has been determined using low temperature neutron diffraction at ambient pressure.

Investigation of acrylic acid at high pressure using neutron diffraction

This article details the exploration of perdeuterated acrylic acid at high pressure using neutron diffraction. The structural changes that occur in acrylic acid-d₄ are followed via diffraction and rationalized using the Pixel method. Acrylic acid undergoes a reconstructive phase transition to a new phase at ∼0.8 GPa and remains molecular to 7.2 GPa before polymerizing on decompression to ambient pressure. The resulting product is analyzed via Raman and FT-IR spectroscopy and differential scanning calorimetry and found to possess a different molecular structure compared with polymers produced via traditional routes.

Competition between hydrogen bonding and dispersion interactions in the crystal structures of the primary amines

In the short chain amines H-bonding dominates crystal packing, but dispersion wins-out for the long chain compounds. The cross-over point occurs between butyl and pentylamine, where interactions are finely balanced.

A high-pressure single-crystal to single-crystal phase transition in DL-alaninium semi-oxalate monohydrate with switching-over hydrogen bonds

A single-crystal to single-crystal transition in DL-alaninium semi-oxalate monohydrate at a pressure between 1.5 and 2.4 GPa was studied by single-crystal X-ray diffraction and Raman spectroscopy. This is the first example of a single-crystal diffraction study of a high-pressure phase transition in a crystalline amino acid salt hydrate. Selected hydrogen bonds switch over and become bifurcated, whereas the others are compressed continuously. The transition is accompanied by pronounced discontinuities in the cell parameters and volume versus pressure, although no radical changes in the molecular packing are induced. Although, in contrast to DL-alanine, in the crystal structure of the salt there are short O-H···O hydrogen bonds, the structure of the salt is more compressible. At the same time, the structure of DL-alanine does not undergo pressure-induced phase transitions, whereas the structure of DL-alaninium semi-oxalate monohydrate does, and at a relatively low pressure. The anisotropy of lattice strain for the low-pressure phase differs from that on cooling at ambient pressure; interestingly, the anisotropy of the pressure-induced compression of the high-pressure phase is quite similar to the lattice strain of the low-pressure phase on cooling.