The curious case of proton migration under pressure in the malonic acid and 4,4'-bipyridine cocrystal

In the search for new active pharmaceutical ingredients, the precise control of the chemistry of cocrystals becomes essential. One crucial step within this chemistry is proton migration between cocrystal coformers to form a salt, usually anticipated by the empirical ΔpKa rule. Due to the effective role it plays in modifying intermolecular distances and interactions, pressure adds a new dimension to the ΔpKa rule. Still, this variable has been scarcely applied to induce proton-transfer reactions within these systems. In our study, high-pressure X-ray diffraction and Raman spectroscopy experiments, supported by DFT calculations, reveal modifications to the protonation states of the 4,4'-bipyridine (BIPY) and malonic acid (MA) cocrystal (BIPYMA) that allow the conversion of the cocrystal phase into ionic salt polymorphs. On compression, neutral BIPYMA and monoprotonated (BIPYH+MA-) species coexist up to 3.1 GPa, where a phase transition to a structure of P21/c symmetry occurs, induced by a double proton-transfer reaction forming BIPYH22+MA2-. The low-pressure C2/c phase is recovered at 2.4 GPa on decompression, leading to a 0.7 GPa hysteresis pressure range. This is one of a few studies on proton transfer in multicomponent crystals that shows how susceptible the interconversion between differently charged species is to even slight pressure changes, and how the proton transfer can be a triggering factor leading to changes in the crystal symmetry. These new data, coupled with information from previous reports on proton-transfer reactions between coformers, extend the applicability of the ΔpKa rule incorporating the pressure required to induce salt formation.

High-pressure preference for reduced water content in porous zinc aspartate hydrates

The zinc aspartate (ZnAsp2) complex, a common dietary supplement, preferentially crystallizes as the dihydrate (ZnAsp2·2H2O) from aqueous solution. Under normal conditions the dihydrate easily transforms into the sesquihydrate (ZnAsp2·1.5H2O). The dihydrate crystal structure is triclinic, space group P1, and the sesquihydrate is monoclinic, space group C2/c. However, their structures are closely related and similarly consist of zinc aspartate ribbons parallel to pores accommodating water molecules. These porous structures can breathe water molecules in and out depending on the temperature and air humidity. High pressure above 50 MPa favours the sesquihydrate, as shown by recrystallizations under pressure and compressibility measured by single-crystal X-ray diffraction up to 4 GPa. This preference is explained by the reduced volume of the sesquihydrate and water compressed separately, compared with the dihydrate. The sesquihydrate undergoes an isostructural phase transition when the voids collapse at 0.8 GPa, whereas no phase transitions occur in the dihydrate, because its pores are supported by increased water content.

Lab in a DAC - high-pressure crystal chemistry in a diamond-anvil cell

The diamond-anvil cell (DAC) was invented 60 years ago, ushering in a new era for material sciences, extending research into the dimension of pressure. Most structural determinations and chemical research have been conducted at ambient pressure, i.e. the atmospheric pressure on Earth. However, modern experimental techniques are capable of generating pressure and temperature higher than those at the centre of Earth. Such extreme conditions can be used for obtaining unprecedented chemical compounds, but, most importantly, all fundamental phenomena can be viewed and understood from a broader perspective. This knowledge, in turn, is necessary for designing new generations of materials and applications, for example in the pharmaceutical industry or for obtaining super-hard materials. The high-pressure chambers in the DAC are already used for a considerable variety of experiments, such as chemical reactions, crystallizations, measurements of electric, dielectric and magnetic properties, transformations of biological materials as well as experiments on living tissue. Undoubtedly, more applications involving elevated pressure will follow. High-pressure methods become increasingly attractive, because they can reduce the sample volume and compress the intermolecular contacts to values unattainable by other methods, many times stronger than at low temperature. The compressed materials reveal new information about intermolecular interactions and new phases of single- and multi-component compounds can be obtained. At the same time, high-pressure techniques, and particularly those of X-ray diffraction using the DAC, have been considerably improved and many innovative developments implemented. Increasingly more equipment of in-house laboratories, as well as the instrumentation of beamlines at synchrotrons and thermal neutron sources are dedicated to high-pressure research.

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

Above 3.1 GPa, the solvate with water and methanol is formed, which cannot be obtained at normal pressure.

The role of fluids in high-pressure polymorphism of drugs: different behaviour of β-chlorpropamide in different inert gas and liquid media

Compression of β-chlorpropamide gives different phases depending on the choice of non-dissolving pressure-transmitting fluid (paraffin, neon and helium).

Structure prediction of the solid forms of methanol: an ab initio random structure searching approach

Liquid methanol and methanol clusters have been comprehensively studied to reveal their local structure and hydrogen bond networks.

Supramolecular interactions in the solid state

In the last few decades, supramolecular chemistry has been at the forefront of chemical research, with the aim of understanding chemistry beyond the covalent bond. Since the long-range periodicity in crystals is a product of the directionally specific short-range intermolecular interactions that are responsible for molecular assembly, analysis of crystalline solids provides a primary means to investigate intermolecular interactions and recognition phenomena. This article discusses some areas of contemporary research involving supramolecular interactions in the solid state. The topics covered are: (1) an overview and historical review of halogen bonding; (2) exploring non-ambient conditions to investigate intermolecular interactions in crystals; (3) the role of intermolecular interactions in morphotropy, being the link between isostructurality and polymorphism; (4) strategic realisation of kinetic coordination polymers by exploiting multi-interactive linker molecules. The discussion touches upon many of the prerequisites for controlled preparation and characterization of crystalline materials.

High-pressure crystallography of periodic and aperiodic crystals

More than five decades have passed since the first single-crystal X-ray diffraction experiments at high pressure were performed. These studies were applied historically to geochemical processes occurring in the Earth and other planets, but high-pressure crystallography has spread across different fields of science including chemistry, physics, biology, materials science and pharmacy. With each passing year, high-pressure studies have become more precise and comprehensive because of the development of instrumentation and software, and the systems investigated have also become more complicated. Starting with crystals of simple minerals and inorganic compounds, the interests of researchers have shifted to complicated metal-organic frameworks, aperiodic crystals and quasicrystals, molecular crystals, and even proteins and viruses. Inspired by contributions to the microsymposium 'High-Pressure Crystallography of Periodic and Aperiodic Crystals' presented at the 23rd IUCr Congress and General Assembly, the authors have tried to summarize certain recent results of single-crystal studies of molecular and aperiodic structures under high pressure. While the selected contributions do not cover the whole spectrum of high-pressure research, they demonstrate the broad diversity of novel and fascinating results and may awaken the reader's interest in this topic.

Halocuprate(i) zigzag chain structures with N-methylated DABCO cations – bright metal-centered luminescence and thermally activated color shifts

Two halocuprate(i) chain structures were synthesized and studied with respect to their emission properties that are governed by two interacting chain segments.

Spectral and structural studies of dimethylphenyl betaine hydrate

Hydrates of betaines can be divided into four groups depending on the interactions of their water molecules with the carboxylate group. Dimethylphenyl betaine crystallizes as monohydrate (1), in which water molecules mediate in hydrogen bonds between the carboxylate groups. The water molecules are H-bonded only to one oxygen atom of the dimethylphenyl betaine molecules and link them into a chain via two O(1W)-H⋯O hydrogen bonds of the lengths 2.779(2) and 2.846(2)Å. The structures of monomer (2) and dimer (4) hydrates in vacuum, and the structure of monomer (3) in an aqueous environment have been optimized by the B3LYP/6-311++G(d,p) approach and the geometrical results have been compared with the X-ray diffraction data of 1. The calculated IR frequencies for the optimized structure have been used for the assignments of FTIR bands, the broad absorption band in the range 3415-3230 cm(-1) has been assigned to the O(1w)-H⋯O hydrogen bonds. The correlations between the experimental (1)H and (13)C NMR chemical shifts (δexp) of 1 in D2O and the magnetic isotropic shielding constants (σcalc) calculated by the GIAO/B3LYP/6-311G++(d,p) approach, using the screening solvation model (COSMO), δexp =a+b σcalc, for optimized molecule 3 in water solution are linear and well reproduce the experimental chemical shifts.

Crystal structure of 1,4-diazoniabicyclo[2.2.2]octane diiodide monohydrate

1,4-diazoniabicyclo[2.2.2]octane diiodide monohydrate was synthesized by protonation of 1,4-diazabicyclo[2.2.2]octane (DABCO) with an excess of concentrated hydroiodic acid in water. It forms colourless rod-like crystals. At room temperature an orthorhombic unit cell, space group Cmc21, a = 7.781(2) Å, b = 13.010(3) Å, c = 11.610 (2) Å, V = 1175.3(4) Å3, Z = 4, is observed. Both nitrogen atoms bear a proton. The hydrate water ämolecule within the compound is statistically disordered on a site besides a crystallographic mirror plane at room temperature. DSC measurements indicate a phase transition at about –10 °C. A single crystal X-ray diffraction measurement at 123 K reveals a primitive unit cell, space group Pca21, a = 12.8835(2) Å, b = 7.6819(1) Å, c = 11.4392(2) Å, V = 1132.16(3) Å3, Z = 4. The water molecule with its hydrogen atoms is well ordered at this temperature. Two almost linear hydrogen bonds O—H···I are formed. The formation of hydrogen bonds is also detected by IR-spectroscopy with O—H stretching frequencies at -v = 3403 cm–1 and -v = 3354 cm–1, respectively.