Spiers Memorial Lecture: Mechanochemistry, tribochemistry, mechanical alloying - retrospect, achievements and challenges

The paper presents a view on the achievements, challenges and prospects of mechanochemistry. The extensive reference list can serve as a good entry point to a plethora of mechanochemical literature.

Time-Resolved In Situ Monitoring of Mechanochemical Reactions

Mechanochemical transformations offer environmentally benign synthesis routes, whilst enhancing both the speed and selectivity of reactions. In this regard, mechanochemistry promises to transform the way in which chemistry is done in both academia and industry but is greatly hindered by a current lack of mechanistic understanding. The continued development and use of time-resolved in situ (TRIS) approaches to monitor mechanochemical reactions provides a new dimension to elucidate these fascinating transformations. We here discuss recent trends in method development that have pushed the boundaries of mechanochemical research. New features of mechanochemical reactions obtained by TRIS techniques are subsequently discussed, which sheds light on how different TRIS approaches have been used. Emphasis is placed on the strength of combining complementary techniques. Finally, we outline our views on the potential of TRIS methods in mechanochemical research, towards establishing a new, environmentally benign paradigm in the chemical sciences.

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.

Using Desmotropes, Cocrystals, and Salts to Manipulate Reactivity in Mechanochemical Organic Reactions

Performing reactions in the solid state offers the largely unexplored possibility of influencing reactivity by manipulating the solid form of the starting reactants. In this work, we explore the use of various solid forms of barbituric acid and its effect on reaction paths and kinetics in a Knoevenagel condensation reaction with vanillin. Modifications of barbituric acid included the use of its desmotrope, a cocrystal, and a salt as the starting reactant. Comparing these reactions with the reaction starting from the commercial keto tautomer of barbituric acid, we find that the reaction kinetics could be accelerated or decelerated, together with a change in the reaction mechanism. Exploring solid forms of reactants can be used as general methodology for manipulating mechanochemical reactivity, further highlighting the benefits of conducting reactions in the solid state, because many of the modifications of solids become unavailable upon dissolution.

Tribochemistry, Mechanical Alloying, Mechanochemistry: What is in a Name?

Over the decades, the application of mechanical force to influence chemical reactions has been called by various names: mechanochemistry, tribochemistry, mechanical alloying, to name but a few. The evolution of these terms has largely mirrored the understanding of the field. But what is meant by these terms, why have they evolved, and does it really matter how a process is called? Which parameters should be defined to describe unambiguously the experimental conditions such that others can reproduce the results, or to allow a meaningful comparison between processes explored under different conditions? Can the information on the process be encoded in a clear, concise, and self-explanatory way? We address these questions in this Opinion contribution, which we hope will spark timely and constructive discussion across the international mechanochemical community.

Mechanochemistry: A Green Approach in the Preparation of Pharmaceutical Cocrystals

Mechanochemistry is considered an alternative attractive greener approach to prepare diverse molecular compounds and has become an important synthetic tool in different fields (e.g., physics, chemistry, and material science) since is considered an ecofriendly procedure that can be carried out under solvent free conditions or in the presence of minimal quantities of solvent (catalytic amounts). Being able to substitute, in many cases, classical solution reactions often requiring significant amounts of solvents. These sustainable methods have had an enormous impact on a great variety of chemistry fields, including catalysis, organic synthesis, metal complexes formation, preparation of multicomponent pharmaceutical solid forms, etc. In this sense, we are interested in highlighting the advantages of mechanochemical methods on the obtaining of pharmaceutical cocrystals. Hence, in this review, we describe and discuss the relevance of mechanochemical procedures in the formation of multicomponent solid forms focusing on pharmaceutical cocrystals. Additionally, at the end of this paper, we collect a chronological survey of the most representative scientific papers reporting the mechanochemical synthesis of cocrystals.

Isotope Labeling Reveals Fast Atomic and Molecular Exchange in Mechanochemical Milling Reactions

Using tandem in situ monitoring and isotope-labeled solids, we reveal that mechanochemical ball-milling overcomes inherently slow solid-state diffusion through continuous comminution and growth of milled particles. This process occurs with or without a net chemical reaction and also occurs between solids and liquid additives that can be practically used for highly efficient deuterium labeling of solids. The presented findings reveal a fundamental aspect of milling reactions and also delineate a methodology that should be considered in the study of mechanochemical reaction mechanisms.

Ball size or ball mass – what matters in organic mechanochemical synthesis?

The effects of milling ball mass, size and material are isolated for a model mechanochemical co-crystallisation.

A salt or a co-crystal – when crystallization protocol matters

By controlling nucleation and growth through choice of crystallization conditions, the stable co-crystal or metastable salt can be reproducibly obtained in accordance with Ostwald's rule of stages and the concept of ‘disappearing polymorphs’.

Ball-free mechanochemistry: in situ real-time monitoring of pharmaceutical co-crystal formation by resonant acoustic mixing

Co-crystal formation by resonant acoustic mixing was followed for the first time by in situ real-time X-ray powder diffraction.

Challenges of Mechanochemistry: Is In Situ Real-Time Quantitative Phase Analysis Always Reliable? A Case Study of Organic Salt Formation

Mechanochemical methods offer unprecedented academic and industrial opportunities for solvent-free synthesis of novel materials. The need to study mechanochemical mechanisms is growing, and has led to the development of real-time in situ X-ray powder diffraction techniques (RI-XRPD). However, despite the power of RI-XRPD methods, there remain immense challenges. In the present contribution, many of these challenges are highlighted, and their effect on the interpretation of RI-XRPD data considered. A novel data processing technique is introduced for RI-XRPD, through which the solvent-free mechanochemical synthesis of an organic salt is followed as a case study. These are compared to ex situ studies, where notable differences are observed. The process is monitored over a range of milling frequencies, and a nonlinear correlation between milling parameters and reaction rate is observed. Kinetic analysis of RI-XRPD allows, for the first time, observation of a mechanistic shift over the course of mechanical treatment, resulting from time evolving conditions within the mechanoreactor.

Tandem In Situ Monitoring for Quantitative Assessment of Mechanochemical Reactions Involving Structurally Unknown Phases

We report herein quantitative in situ monitoring by simultaneous PXRD and Raman spectroscopy of the mechanochemical reaction between benzoic acid and nicotinamide, affording a rich polymorphic system with four new cocrystal polymorphs, multiple phase transformations, and a variety of reaction pathways. After observing polymorphs by in situ monitoring, we were able to isolate and characterize three of the four polymorphs, most of which are not accessible from solution. Relative stabilities among the isolated polymorphs at ambient conditions were established by slurry experiments. Using two complementary methods for in situ monitoring enabled quantitative assessment and kinetic analysis of each studied mechanochemical reaction, even when involving unknown crystal structures, and short-lived intermediates. In situ Raman monitoring was introduced here also as a standalone laboratory technique for quantitative assessment of mechanochemical reactions and understanding of mechanochemical reactivity. Our results provide an important step toward a complete and high-throughput quantitative approach to mechanochemical reaction kinetics and mechanisms, necessary for the development of the mechanistic framework of milling reactions.

Screening for new pharmaceutical solid forms using mechanochemistry: A practical guide

Within the pharmaceutical industry, and elsewhere, the screening for new solid forms is a mandatory exercise for both existing and new chemical entities. This contribution focuses on mechanochemistry as a versatile approach for discovering new and alternative solid forms. Whilst a series of recently published extensive reviews exist which focus on mechanistic aspects and potential areas of development, in this review we focus on particular practical aspects of mechanochemistry in order to allow full optimisation of the approach in searches for new solid forms including polymorphs, salts and cocrystals as well as their solvated/hydrated analogues. As a consequence of the apparent experimental simplicity of the method (compared to more traditional protocols e.g. solvent-based methods), the high efficiency and range of conditions available in a mechanochemical screen, mechanochemistry should not be considered simply as an alternative method when other screening methods are not successful, but rather as a key strategy in any fully effective solid form screen providing reduced effort and time as well as the potential of requiring reduced amounts of material.

In Situ Investigation of a Self-Accelerated Cocrystal Formation by Grinding Pyrazinamide with Oxalic Acid

A new cocrystal of pyrazinamide with oxalic acid was prepared mechanochemically and characterized by PXRD, Raman spectroscopy, solid-state NMR spectroscopy, DTA-TG, and SEM. Based on powder X-ray diffraction data the structure was solved. The formation pathway of the reaction was studied in situ using combined synchrotron PXRD and Raman spectroscopy. Using oxalic acid dihydrate the initially neat grinding turned into a rapid self-accelerated liquid-assisted grinding process by the release of crystallization water. Under these conditions, the cocrystal was formed directly within two minutes.

Structure-forming units of amino acid maleates. Case study ofL-valinium hydrogen maleate

A new salt of L-valinium hydrogen maleate was used as an example to study structure-forming units in amino acid maleates. This compound was crystallized, its structure solved from single-crystal X-ray diffraction data, and the phase purity of the bulk powder sample confirmed by X-ray powder diffraction and FT–IR spectra. The stability of the new salt was analyzed using density functional theory andPIXELcalculations with focus on theC22(12) structure-forming crystallographic motif. This motif was of particular interest as it is common for almost all maleates. The exceptionally high ability of maleic acid to form salts with various amino acids was rationalized.

Mechanochemical reactions studied by in situ Raman spectroscopy: base catalysis in liquid-assisted grinding

Monitoring byin situRaman spectroscopy of a mechanochemical substitution reaction on a carbonyl group reveals base catalysis akin to catalysis in solution.

Bis(paracetamol) pyridine – a new elusive paracetamol solvate: from modeling the phase diagram to successful single-crystal growth and structure–property relations

In addition to the synthon approach, it is equally important to consider phase diagrams when searching for practical methods of crystallising multi-component crystals, either as single crystals or as powders.