Exploring mechanochemistry of pharmaceutical cocrystals: effect of incident angle on molecular mixing during simulated indentations of two organic solids.

The solid-state reaction of the active pharmaceutical ingredient theophylline with citric acid is a well-established example of a mechanochemical reaction, leading to a model pharmaceutical cocrystal. Here, classical force field molecular dynamics was employed to investigate the molecular mixing and structural distortion that take place on the mechanically driven indentation of a citric acid nanoparticle on a slab of crystalline theophylline. Through non-equilibrium molecular dynamics simulations, a 6 nm diameter nanoparticle of citric acid was introduced onto an open (001) surface of a theophylline crystal, varying both the angle of incidence of the nanoparticle between 15° and 90° and the indentation speed between 1 m s-1 and 16 m s-1. This theoretical study enabled the evaluation of how these two parameters promote molecular mixing and overall structural deformation upon the mechanical contraction of theophylline and citric acid, both of which are important parameters underlying mechanochemical cocrystallisation. The results show that the angle of incidence plays a key role in the molecular transfer ability between the two species and in the structural disruption of the initially spherical nanoparticles. Changing the indentation speed, however, did not lead to a discernible trend in molecular mixing, highlighting the importance of the incident angle in mechanochemical events in the context of supramolecular chemistry, such as the disruption of the crystal structure and molecular transfer between molecular crystals.

Enzymatic depolymerization of highly crystalline polyethylene terephthalate enabled in moist-solid reaction mixtures.

Less than 9% of the plastic produced is recycled after use, contributing to the global plastic pollution problem. While polyethylene terephthalate (PET) is one of the most common plastics, its thermomechanical recycling generates a material of lesser quality. Enzymes are highly selective, renewable catalysts active at mild temperatures; however, they lack activity toward the more crystalline forms of PET commonly found in consumer plastics, requiring the energy-expensive melt-amorphization step of PET before enzymatic depolymerization. We report here that, when used in moist-solid reaction mixtures instead of the typical dilute aqueous solutions or slurries, the cutinase from Humicola insolens can directly depolymerize amorphous and crystalline regions of PET equally, without any pretreatment, with a 13-fold higher space-time yield and a 15-fold higher enzyme efficiency than reported in prior studies with high-crystallinity material. Further, this process shows a 26-fold selectivity for terephthalic acid over other hydrolysis products.

Utilizing "Latent" Carbon: Repositioning Hydrogen-Bonded Synthons and Assemblies via Halogen Bonding to π-Systems.

Molecules bearing carboxylic acid, amide, and hydroxyl groups are ubiquitous in crystal engineering, where robust hydrogen-bonded synthons centred on these functionalities enable reliable crystal structure design. We now show that halogen bonding to the carbon π-system of such molecules, traditionally ignored in crystal engineering, permits the recognition and directional assembly of the resulting hydrogen-bonded structural subunits, leaving the archetypal hydrogen-bonded ring, ladder, and chain homosynthons intact, but repositioned in space. When applied to heteromolecular synthons, this enables rearranging more complex hydrogen-bonded motifs and the evolution of binary cocrystals into ternary ones through "latent" carbon-based recognition sites, demonstrating a rational approach to build higher-order solid-state supramolecular assemblies.

Mechanochemical Synthesis of Boroxine-linked Covalent Organic Frameworks.

We report a rapid, room-temperature mechanochemical synthesis of 2- and 3-dimensional boroxine covalent organic frameworks (COFs), enabled by using trimethylboroxine as a dehydrating additive to overcome the hydrolytic sensitivity of boroxine-based COFs. The resulting COFs display high porosity and crystallinity, with COF-102 being the first example of a mechanochemically prepared 3D COF, exhibiting a surface area of ca. 2,500 m2 g-1. Mechanochemistry enabled a >20-fold reduction in solvent use and ~100-fold reduction in reaction time compared with solvothermal methods, providing target COFs quantitatively with no additional work-up besides vacuum drying. Real-time Raman spectroscopy permitted the first quantitative kinetic analysis of COF mechanosynthesis, while transferring the reaction design to Resonant Acoustic Mixing (RAM) enabled synthesis of multi-gram amounts of the target COFs (tested up to 10 g).

Experimentally Validated Ab Initio Crystal Structure Prediction of Novel Metal-Organic Framework Materials.

First-principles crystal structure prediction (CSP) is the most powerful approach for materials discovery, enabling the prediction and evaluation of properties of new solid phases based only on a diagram of their underlying components. Here, we present the first CSP-based discovery of metal-organic frameworks (MOFs), offering a broader alternative to conventional techniques, which rely on geometry, intuition, and experimental screening. Phase landscapes were calculated for three systems involving flexible Cu(II) nodes, which could adopt a potentially limitless number of network topologies and are not amenable to conventional MOF design. The CSP procedure was validated experimentally through the synthesis of materials whose structures perfectly matched those found among the lowest-energy calculated structures and whose relevant properties, such as combustion energies, could immediately be evaluated from CSP-derived structures.

Three-in-One: Dye-Volatile Cocrystals Exhibiting Intensity-Dependent Photochromic, Photomechanical, and Photocarving Response.

Cocrystallization of a cis-azobenzene dye with volatile molecules, such as pyrazine and dioxane, leads to materials that exhibit at least three different light-intensity-dependent responses upon irradiation with low-power visible light. The halogen-bond-driven assembly of the dye cis-(p-iodoperfluorophenyl)azobenzene with volatile halogen bond acceptors produces cocrystals whose light-induced behavior varies significantly depending on the intensity of the light applied. Low-intensity (<1 mW·cm-2) light irradiation leads to a color change associated with low levels of cis → trans isomerization. Irradiation at higher intensities (150 mW·mm-2) produces photomechanical bending, caused by more extensive isomerization of the dye. At still higher irradiation intensities (2.25 W·mm-2) the cocrystals undergo cold photocarving; i.e., they can be cut and written on with micrometer precision using laser light without a major thermal effect. Real-time Raman spectroscopy shows that this novel photochemical behavior differs from what would be expected from thermal energy input alone. Overall, this work introduces a rational blueprint, based on supramolecular chemistry in the solid state, for new types of crystalline light-responsive materials, which not only respond to being exposed to light but also change their response based on the light intensity.

Toward Mechanistic Understanding of Mechanochemical Reactions Using Real-Time In Situ Monitoring.

The past two decades have witnessed a rapid emergence of interest in mechanochemistry-chemical and materials reactivity achieved or sustained by the action of mechanical force-which has led to application of mechanochemistry to almost all areas of modern chemical and materials synthesis: from organic, inorganic, and organometallic chemistry to enzymatic reactions, formation of metal-organic frameworks, hybrid perovskites, and nanoparticle-based materials. The recent success of mechanochemistry by ball milling has also raised questions about the underlying mechanisms and has led to the realization that the rational development and effective harnessing of mechanochemical reactivity for cleaner and more efficient chemical manufacturing will critically depend on establishing a mechanistic understanding of these reactions. Despite their long history, the development of such a knowledge framework for mechanochemical reactions is still incomplete. This is in part due to the, until recently, unsurmountable challenge of directly observing transformations taking place in a rapidly oscillating or rotating milling vessel, with the sample being under the continuous impact of milling media. A transformative change in mechanistic studies of milling reactions was recently introduced through the first two methodologies for real-time in situ monitoring based on synchrotron powder X-ray diffraction and Raman spectroscopy. Introduced in 2013 and 2014, the two new techniques have inspired a period of tremendous method development, resulting also in new techniques for mechanistic mechanochemical studies that are based on temperature and/or pressure monitoring, extended X-ray fine structure (EXAFS), and, latest, nuclear magnetic resonance (NMR) spectroscopy. The new technologies available for real-time monitoring have now inspired the development of experimental strategies and advanced data analysis approaches for the identification and quantification of short-lived reaction intermediates, the development of new mechanistic models, as well as the emergence of more complex monitoring methodologies based on two or three simultaneous monitoring approaches. The use of these new opportunities has, in less than a decade, enabled the first real-time observations of mechanochemical reaction kinetics and the first studies of how the presence of additives, or other means of modifying the mechanochemical reaction, influence reaction rates and pathways. These studies have revealed multistep reaction mechanisms, enabled the identification of autocatalysis, as well as identified molecules and materials that have previously not been known or have even been considered not possible to synthesize through conventional approaches. Mechanistic studies through in situ powder X-ray diffraction (PXRD) and Raman spectroscopy have highlighted the formation of supramolecular complexes (for example, cocrystals) as critical intermediates in organic and metal-organic synthesis and have also been combined with isotope labeling strategies to provide a deeper insight into mechanochemical reaction mechanisms and atomic and molecular dynamics under milling conditions. This Account provides an overview of this exciting, rapidly evolving field by presenting the development and concepts behind the new methodologies for real-time in situ monitoring of mechanochemical reactions, outlining key advances in mechanistic understanding of mechanochemistry, and presenting selected studies important for pushing forward the boundaries of measurement techniques, data analysis, and mapping of reaction mechanisms.

Polymorphs and solid solutions: materials with new luminescent properties obtained through mechanochemical transformation of dicyanoaurate(I) salts.

We report the use of mechano- and thermochemical methods to create new solid-state luminescent materials from well-known inorganic salts, potassium dicyanoaurate(I) KAu(CN)2, and potassium dicyanocuprate(I) KCu(CN)2. In particular, manual grinding or ball milling of commercial samples of KAu(CN)2 led to the formation of a novel polymorph of the salt, herein termed m-KAu(CN)2, evident by a significant change in color of the fluorescence emission of the solid material from orange to violet. The formation of m-KAu(CN)2 is reversible upon addition of small amounts of solvents, and powder X-ray diffraction analysis indicates that the structure of m-KAu(CN)2 might be related to that of pristine KAu(CN)2 through a change in ordering of Au(CN)2- ions in a layered structure. Thermal treatment of KAu(CN)2 led to the discovery of another polymorph of this well-known salt, herein termed t-KAu(CN)2, making KAu(CN)2 a rare example of a system in which thermochemical and mechanochemical treatments lead to the formation of different, in each case previously not reported, polymorphic forms. The thermally-induced transformation from KAu(CN)2 to t-KAu(CN)2 takes place around 250 °C and proceeds in a crystal-to-crystal fashion, which enabled the preliminary structural characterisation through single crystal X-ray diffraction, revealing the retention of the layered structure and a change in ordering of Au(CN)2- ions. Milling of the simple salt KAu(CN)2 in the presence of equimolar amounts or less of its copper(I)-based analogue coordination polymer KCu(CN)2 leads to the formation of a series of solid solution materials, isostructural to m-KAu(CN)2 and with visible fluorescence emission distinct from KCu(CN)2 or any herein investigated forms of KAu(CN)2.

Direct observation by high resolution transmission electron microscopy of gold(III) particle transformation during aging reduction reaction.

We use a gold nanoparticle synthesis as a model system to study the morphological and compositional changes in gold(III) precursor particles, while reduction is taking place during aging after mechanical activation. Scanning transmission electron microscopy coupled with a high-angle annular dark field detector revealed the nanoscale changes in particle morphology, while electron energy loss spectroscopy mapped the changes in the chemical landscape during the reduction process. Tracking a specific region of interest on the sample grid allowed for comparisons to be made of the same particles across a two day monitoring period. High-angle annular dark field images permitted the visualization of particle size reduction of the gold salt while electron energy loss spectroscopy captured the surprising mobility of the lighter chlorine and sodium ions in a solid matrix during the reduction process. This system offers unique insight into precursor particle reactivity in the solid phase, which is relevant for many mechanochemical and aging-based reactions.

The "η-sweet-spot" (ηmax) in liquid-assisted mechanochemistry: polymorph control and the role of a liquid additive as either a catalyst or an inhibitor in resonant acoustic mixing (RAM).

Resonant acoustic mixing (RAM) offers a simple, efficient route for mechanochemical synthesis in the absence of milling media or bulk solvents. Here, we show the use of RAM to conduct the copper-catalysed coupling of sulfonamides and carbodiimides. This coupling was previously reported to take place only by mechanochemical ball milling, while in conventional solution environments it is not efficient, or does not take place at all. The results demonstrate RAM as a suitable methodology to conduct reactions previously accessed only by ball milling and provide a detailed, systematic overview of how the amount of liquid additive, measured by the ratio of liquid volume to weight of reactants (η, in µL mg-1), can affect the course of a mechanochemical reaction and the polymorphic composition of its product. Switching from ball milling to RAM allowed for the discovery of a new polymorph of the model sulfonylguanidine obtained by catalytic coupling of di(cyclohexyl)carbodiimide (DCC) and p-toluenesulfonamide, and the ability to control reaction temperature in RAM enabled in situ control of the polymorphic behaviour of this nascent product. We show that the reaction conversion for a given reaction time does not change monotonically but, instead, achieves a maximum for a well-defined η-value. This "η-sweet-spot" of conversion is herein designated ηmax. The herein explored reactions demonstrate sensitivity to η on the order of 0.01 µL mg-1, which corresponds to an amount of liquid additive below 5 mol% compared to the reactants, and is at least one to two orders of magnitude lower than the η-value typically considered in the design of liquid-assisted ball milling mechanochemical reactions. Such sensitivity suggests that strategies to optimise liquid-assisted mechanochemical reactions should systematically evaluate η-values at increments of 0.01 µL mg-1, or even finer. At η-values other than ηmax the reaction conversion drops off, demonstrating that the same liquid additive can act either as a catalyst or an inhibitor of a mechanochemical reaction, depending on the amount.

In situ monitoring of mechanochemical MOF formation by NMR relaxation time correlation.

In this paper, we present a new approach to monitoring mechanochemical transformations, based on a magnetic resonance (MR) method in which relaxation time correlation maps are used to track the formation of the popular metal-organic framework (MOF) materials Zn-MOF-74 and ZIF-8. The two-dimensional (2D) relaxation correlation measurement employed yields a spectrum which visually and analytically identifies different 1H environments in the sample of interest. The measurement is well-suited to analyzing solid mixtures, and liquids, in complex systems. Application in this work to monitoring MOF formation shows changes in signal amplitudes, and their MR lifetime coordinates, within the 2D plots as the reaction progresses, confirming reaction completion. This new measurement provides a simple way to analyse solid-state reactions without dissolution, and there is a logical pathway to benchtop measurement with a new generation of permanent magnet-based MR instruments. The methodology described permits measurement in an MR compatible milling container, which may be directly transferred from the shaker assembly to the MR magnet for in situ measurement of the entire reaction mixture.

Shaking Things from the Ground-Up: A Systematic Overview of the Mechanochemistry of Hard and High-Melting Inorganic Materials.

We provide a systematic overview of the mechanochemical reactions of inorganic solids, notably simple binary compounds, such as oxides, nitrides, carbides, sulphides, phosphides, hydrides, borides, borane derivatives, and related systems. Whereas the solid state has been traditionally considered to be of little synthetic value by the broader community of synthetic chemists, the solid-state community, and in particular researchers focusing on the reactions of inorganic materials, have thrived in building a rich and dynamic research field based on mechanically-driven transformations of inorganic substances typically seen as inert and high-melting. This review provides an insight into the chemical richness of such mechanochemical reactions and, at the same time, offers their tentative categorisation based on transformation type, resulting in seven distinct groupings: (i) the formation of adducts, (ii) the reactions of dehydration; (iii) oxidation-reduction (redox) reactions; (iv) metathesis (or exchange) reactions; (v) doping and structural rearrangements, including reactions involving the reaction vessel (the milling jar); (vi) acid-base reactions, and (vii) other, mixed type reactions. At the same time, we offer a parallel description of inorganic mechanochemical reactions depending on the reaction conditions, as those that: (i) take place under mild conditions (e.g., manual grinding using a mortar and a pestle); (ii) proceed gradually under mechanical milling; (iii) are self-sustained and initiated by mechanical milling, i.e., mechanically induced self-propagating reactions (MSRs); and (iv) proceed only via harsh grinding and are a result of chemical reactivity under strongly non-equilibrium conditions. By elaborating on typical examples and general principles in the mechanochemistry of hard and high-melting substances, this review provides a suitable complement to the existing literature, focusing on the properties and mechanochemical reactions of inorganic solids, such as nanomaterials and catalysts.

Low-Dimensional Magnetism in Multivariate Copper/Zinc MOF-74 Materials Formed via Different Mechanochemical Methods.

MOF-74 is an archetypal magnetic metal-organic framework (MOF) family, with metal nodes bridged by 2,5-dioxido-1,4-benzenedicarboxylic acid (H4dobdc) and arranged into one of the simplest representations of the 1D Ising magnetic model. Recently, a novel mechano-synthetic approach opened a pathway toward a series of bimetallic multivariate (1:1) M1M2-MOF-74 materials, with the uniform distribution of metal cations in the oxometallic chains, offering a unique opportunity to investigate low-dimensional magnetism in these heterometallic MOFs. We explore here how different mechanochemical procedures affect the interaction between the metal nodes of the model system of three multivariate copper(II)/zinc(II)-MOF-74 materials, two of which were obtained through a template-controlled procedure, and the third one was obtained by recently developed mechanical MOF-alloying combined with subsequent accelerated aging. While the three Cu/Zn-MOF-74 products have almost identical powder X-ray diffraction (PXRD) diffractograms and Fourier transform infrared spectra, they differ significantly in their magnetic properties, as revealed through detailed magnetization and X-band and multifrequency high-field electron spin resonance measurements. The magnetic results of the three multivariate Cu/Zn-MOF-74s were compared to the properties of the monometallic Cu-MOF-74, which shows antiferromagnetic intrachain and weaker ferromagnetic interchain interactions. Energy-dispersive X-ray spectroscopy/scanning electron microscopy and solid-state nuclear magnetic resonance spectroscopy helped rationalize the observed differences in magnetization, and in situ synchrotron PXRD monitoring of template-controlled MOF formation revealed different reaction pathways when using the zinc or copper intermediates, involving even the fleeting occurrence of a rare MOF-74 polymorph.

Mechanochemical methods for the transfer of electrons and exchange of ions: inorganic reactivity from nanoparticles to organometallics.

Mechanochemistry by milling, grinding, extrusion or other types of shear and mechanical agitation has shown novel reactivity for a wide range of reactions, not seen in traditional solution-based environments. While the area has been extensively investigated and reviewed in the context of organic and solid-state supramolecular chemistry, less attention has been given to the recent advances in the context of inorganic transformations. Here we provide a perspective of inorganic mechanochemical reactions, focusing on transformations that are based on transfer of charged species: exchange of ions and electrons (redox reactions). These types of mechanochemical transformations typically lead to the formation of new nanoparticles and organometallic complexes. Herein, we provide an overview of mechanochemical reactivity that complements the recent developments in organic synthesis and catalysis.

SpeedMixing: Rapid Tribochemical Synthesis and Discovery of Pharmaceutical Cocrystals without Milling or Grinding Media.

We present SpeedMixing, a rapid blending technology, as an approach for fast mechanosynthesis and discovery of model pharmaceutical cocrystals through rapid spinning in the absence of bulk solvents and milling/grinding media. Syntheses of pharmaceutical cocrystals based on the active pharmaceutical ingredients (APIs) carbamazepine, dihydrocarbamazepine, and nicotinamide demonstrate SpeedMixing as a method for rapid, scalable, as well as controllable and selective synthesis of cocrystals, cocrystal polymorphs and stoichiomorphs, including the discovery of an unexpected methanol solvate of the archetypal cocrystal of carbamazepine and saccharin, which has eluded extensive screens over 20 years.

Metal-Catalyzed Organic Reactions by Resonant Acoustic Mixing.

We demonstrate catalytic organic synthesis by Resonant Acoustic Mixing (RAM): a mechanochemical methodology that does not require bulk solvent or milling media. Using as model reactions ruthenium-catalyzed ring-closing metathesis and copper-catalyzed sulfonamide-isocyanate coupling, RAM mechanosynthesis is shown to be faster, operationally simpler than conventional ball-milling, while also providing the first example of a mechanochemical strategy for ruthenium-catalyzed ene-yne metathesis. Reactions by RAM are readily and directly scaled-up without any significant changes in reaction conditions, as shown by the straightforward 200-fold scaling-up of the synthesis of the antidiabetic drug Tolbutamide, from hundreds of milligrams directly to 30 grams.

Mechanochemical bottom-up synthesis of phosphorus-linked, heptazine-based carbon nitrides using sodium phosphide.

Herein, we present the bottom-up, mechanochemical synthesis of phosphorus-bridged heptazine-based carbon nitrides (g-h-PCN). The structure of these materials was determined through a combination of powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), 31P magic angle spinning nuclear magnetic resonance (MAS NMR), density functional theory (DFT) and electron energy loss spectroscopy (EELS). Compared to traditional furnace-based techniques, the presented method utilizes milder conditions, as well as shorter reaction times. Both samples of g-h-PCN directly after milling and aging and after an hour of annealing at 300 °C (g-h-PCN300) show a reduction in photoluminescent recombination, as well as a nearly two-time increase in photocurrent under broad spectrum irradiation, which are appealing properties for photocatalysis.

Real-Time Observation of "Soft" Magic-Size Clusters during Hydrolysis of the Model Metallodrug Bismuth Disalicylate.

Colloidal bismuth therapeutics have been used for hundreds of years, yet remain mysterious. Here we report an X-ray pair distribution function (PDF) study of the solvolysis of bismuth disalicylate, a model for the metallodrug bismuth subsalicylate (Pepto-Bismol). This reveals catalysis by traces of water, followed by multistep cluster growth. The ratio of the two major species, {Bi9O7} and {Bi38O44}, depends on exposure to air, time, and the solvent. The solution-phase cluster structures are of significantly higher symmetry in comparison to solid-state analogues, with reduced off-center Bi3+ displacements. This explains why such "magic-size" clusters can be both stable enough to crystallize and sufficiently labile for further growth.

After 200 Years: The Structure of Bleach and Characterization of Hypohalite Ions by Single-Crystal X-Ray Diffraction*.

We report the first X-ray single crystal structures of hypochlorite (ClO- ) and hypobromite (BrO- ) salts, including hydrated sodium hypochlorite, a staple of the chlorine industry and ubiquitous bleaching and disinfection agent for almost 200 years. The structures, supported by variable-temperature Raman spectroscopy on individual crystals and periodic density-functional theory (DFT) calculations, provide insight into solid-state geometry and supramolecular chemistry of hypohalite ions.

Linker Substituents Control the Thermodynamic Stability in Metal-Organic Frameworks.

We report the first systematic experimental and theoretical study of the relationship between the linker functionalization and the thermodynamic stability of metal-organic frameworks (MOFs) using a model set of eight isostructural zeolitic imidazolate frameworks (ZIFs) based on 2-substituted imidazolate linkers. The frameworks exhibit a significant (30 kJ·mol-1) variation in the enthalpy of formation depending on the choice of substituent, which is accompanied by only a small change in molar volume. These energetics were readily reproduced by density functional theory (DFT) calculations. We show that these variations in the enthalpy of MOF formation are in linear correlation to the readily accessible properties of the linker substituent, such as the Hammett σ-constant or electrostatic surface potential. These results provide the first quantifiable relationship between the MOF thermodynamics and the linker structure, suggesting a route to design and tune MOF stability.