A brief and systematic overview of recent advances in understanding the mechanism of mechanochemical cocrystallization at macroscopic (bulk phase transformations) and microscopic levels (molecular ...recognition) is given. The review particularly addresses neat and liquid-assisted grinding approaches to cocrystal formation.
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The past decade has seen a reawakening of solid-state approaches to chemical synthesis, driven by the search for new, cleaner synthetic methodologies. Mechanochemistry, i.e., chemical transformations ...initiated or sustained by mechanical force, has been advancing particularly rapidly, from a laboratory curiosity to a widely applicable technique that not only enables a cleaner route to chemical transformations but offers completely new opportunities in making and screening for molecules and materials. This Outlook provides a brief overview of the recent achievements and opportunities created by mechanochemistry, including access to materials, molecular targets, and synthetic strategies that are hard or even impossible to access by conventional means.
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Conspectus 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.
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We provide a brief overview of the first techniques for direct, real-time observation of mechanochemical reactions by milling. Whereas mechanisms and kinetics of solid-state reactions induced by ...temperature or pressure have been extensively investigated, transformations of materials under continuous impact in a milling assembly remain largely unexplored and based on ex situ studies. The recent introduction and development of techniques for in situ monitoring of milling reactions by synchrotron X-ray powder diffraction and Raman spectroscopy has enabled the first direct insight into milling mechanochemistry, opening a new area for studies of chemical reactivity. So far, these techniques have revealed rapid, multistep reaction mechanisms and metastable intermediates that are impossible or difficult to observe or isolate in solution and have highlighted shortcomings of ex situ mechanistic studies. These pioneering advances also highlight the low level of mechanistic understanding and future challenges in developing a clear mechanistic picture of physicochemical transformations by milling.
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Unusually long thermal half-lives of perhalogenated cis-azobenzenes enabled their structural characterization and the first evidence of a crystal-to-crystal cis → trans azobenzene isomerization. ...Irradiation with visible light transforms a perhalogenated cis-azobenzene single crystal into a polycrystalline aggregate of its trans-isomer in a photomechanical transformation that involves a significant, controllable, and thermally irreversible change of crystal shape. This is the first demonstration of permanent photomechanical modification of crystal shape in an azobenzene.
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Mechanochemistry by ball milling is garnering significant attention as an efficient and versatile means of chemical synthesis which minimizes the need for solvents and reduces the amount of energy ...required to conduct many molecular transformations. Understanding milling reactions often requires monitoring the reaction of solid forms, with little or ideally no disruption of the milling process. Herein, we provide a broad but succinct summary of how different ex situ and the more recently developed in situ techniques have been applied to monitor mechanochemical reactions, revealing reaction pathways and the mechanisms driving different solid-state molecular and materials transformations by milling. The rapidly evolving field of monitoring milling reactions has already revolutionized our understanding of mechanochemical reactions, revealing complex self-assembled phases as intermediates in catalytic and other types of synthesis through ball milling. The multitude of recently reported techniques for investigating ball milling reactions, many of which are touched upon in this summary, promise to dramatically increase the pace of mechanochemical reaction development and the understanding of solid-state chemistry.
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We describe the development of a mechanochemical approach for Ru-catalyzed olefin metathesis, including cross-metathesis and ring-closing metathesis. The method uses commercially available catalysts ...to achieve high-yielding, rapid, room-temperature metathesis of solid or liquid olefins on a multigram scale using either no or only a catalytic amount of a liquid.
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Phosphorylated polymers are versatile materials for a broad range of applications from flame-retardant coatings to bioactive scaffolds. Traditionally, they are synthesized in solution using corrosive ...concentrated phosphoric acid and energy-intensive drying techniques. In the past decade, mechanochemistry has proven to be a valuable tool for green chemists to conduct new transformations, with minimal waste, often solvent-free. This work presents the phosphorylation of cellulose nanocrystals, poly(ethylene glycol), poly(vinyl alcohol), poly(vinyl chloride), and lignin through mechanochemical processes with phosphorus pentoxide to produce reproducible phosphorylation for potential flame-retardant applications. Through 31P magic angle spinning (MAS) NMR, loadings of up to 3300 mmol/kg were determined for cellulose nanocrystals, far superior to loadings in solution around 1600 mmol/kg, and loadings of up to 4375 mmol/kg were obtained for synthetic polymers such as poly(vinyl alcohol).
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Selective catalytic reduction of CO2 to methanol has tremendous importance in the chemical industry. It mitigates two critical issues in the modern society, the overwhelming climate change and the ...dependence on fossil fuels. The most used catalysts are currently based on mixed copper and zinc phases, where the high surface of active copper species is a critical factor for the catalyst performance. Motivated by the recent breakthrough in the controllable synthesis of bimetallic MOF-74 materials by ball milling, we targeted to study the potential of ZnCu-MOF-74 for catalytic CO2 reduction. Here, we tested whether the nanosized channels decorated with readily accessible and homogeneously distributed Zn and Cu metal sites would be advantageous for the catalytic CO2 reduction. Unlike the inactive monometallic Cu-MOF-74, ZnCu-MOF-74 shows moderate catalytic activity and selectivity for the methanol synthesis. Interestingly, the postsynthetic mechanochemical treatment of desolvated ZnCu-MOF-74 resulted in amorphization and a significant increase in both the activity and selectivity of the catalyst despite the destruction of the well-ordered and porous MOF-74 architecture. The results emphasize the importance of defects for the MOF catalytic activity and the potential of amorphous MOFs to be considered as heterogeneous catalysts. Scanning electron microscopy (SEM), X-ray powder diffraction (XRD) and 13C magic angle-spinning nuclear magnetic resonance (MAS NMR) were applied to establish quantitative structure–reactivity relationships. The apparent activation energy of rate reaction kinetics has indicated different pathway mechanisms, primarily through reverse water–gas shift (RWGS). Prolonged time on stream productivity, stability and deactivation were assessed, analysing the robustness or degradation of metal–organic framework nanomaterials. Scalable MOF production processes are making the latter more appealing within emerging industrial decarbonisation, in particular for carbon capture and utilisation (CCU) or hydrogen carrier storage. Acknowledging scale, the costs of fabrication are paramount.
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The implementation of environmentally friendly, solvent-free mechanochemical synthesis in an undergraduate chemistry teaching laboratory is described. As a model reaction, the experiment addresses ...the catalytic mechanochemical synthesis of a small library of sulfonylureas, including a known active pharmaceutical ingredient, tolbutamide. The experiment introduces students to mechanochemistry, and the use of automated ball milling as a solvent-free synthetic methodology with a choice of two instruments: a mixer mill and a planetary ball mill. The isolation and purification of final products is achieved simply by washing with water and characterization is performed through powder X-ray diffraction. As milling reactions under wet and dry conditions lead to the formation of two diverse polymorphs of tolbutamide, this experiment also provides an opportunity to discuss topics relevant to solid-state pharmaceutical materials science, such as crystal packing of molecules and polymorphism, and it is an illustration of the emergent field of medicinal mechanochemistry in an undergraduate teaching laboratory.
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