On the Mechanism of Cocrystal Mechanochemical Reaction via Low Melting Eutectic: A Time-Resolved In Situ Monitoring Investigation

Mechanochemistry: Unravelling the Impact of Metal Leaching in Organic Synthesis

Solvent-free techniques have gained considerable attention in recent years due to their environmental advantages and potential to enable chemical reactivities beyond the reach of traditional solution-based methods. Mechanochemistry has emerged as a groundbreaking approach to drive sustainable chemical processes. Despite its promise, some challenges still need to be explored, including the overlooked issue of material leaching during grinding, a phenomenon in which components from milling media or reaction vessels, such as stainless steel, unintentionally alter reaction outcomes. This study investigates the role of metal leaching in reducing arylnitrosamines by using a poorly soluble solid reagent, thiourea dioxide (TDO), focusing on stainless steel vessels. By comparing conventional mechanochemical methods with innovative solvent-free vibratory techniques, we assess the extent of metal contamination and its impact on reaction efficiency. These findings provide new insights into how material leaching influences chemical processes and offer valuable guidance for optimizing these forward-looking and green methodologies.

Salt or Cocrystal Puzzle Solved by Mechanochemistry: The Role of Solvent in the Pamoic Acid Case Study

Many drugs are nowadays marketed as salts. Cocrystallization is also emerging as a convenient tool to modify in vivo activity of pharmacologically active compounds. Given the marked difference in physicochemical properties between salts and cocrystals, the possibility of obtaining crystalline systems composed of the same building blocks in their neutral or charged form is desirable. Pamoic acid (PAM) is widely used in pharmaceutical formulation as pamoate salt, and we here propose a unique synthetic strategy to obtain a cocrystal of PAM rather than a salt by tuning mechanochemical conditions. Our findings have been corroborated by means of computational analyses, relating to the noncovalent interactions in the crystal structure with the formation of the different crystalline forms. A detailed analysis of the structure containing PAM in its neutral, anionic or dianionic form present in the Cambridge Structural Database (CSD) helped generalizing our results.

Kinetics of the mechanochemical transformations in the "glycine - oxalic acid dihydrate" system revisited: The role of water

Introduction: Kinetics of the mechanochemical transformations in the "glycine-oxalic acid dihydrate" system were revisited, in order to compare the results obtained for ball milling of the same reactants in different ball-milling devices.Methods: The results obtained in a commercial vibrational mill NARVA Vibrator DDR-GM9458 (ex situ study, this work) were compared with the previously published studies: ex situ in a home-made restricted-impact device and in situ in a Retsch MM400 vibrational mill.Results: We studied the effect of various factors on the mechanochemical transformations in this system under different conditions, such as the air humidity, the effect of the frequency of mechanical pulses on the existence of the induction period, the effect of the starting glycine polymorph on the duration of the induction period in case of a high-frequency vibrational ball milling, or the formation of G2O and GO as two competing products, the former dominating at the early stage of treatment as a "kinetic", faster crystallizing phase, and the latter formed as the only final thermodynamically stable product after a prolonged treatment.Discussion: The abovementioned results were interpreted consistently considering the possibility that water released from oxalic acid crystal hydrate may have a significant effect on the mechanochemical transformations, even though it does not enter crystal structures of bis-glycinium oxalate (G2O) and glycinium semioxalate (GO) products.

Mechanochemical Transformations of Pharmaceutical Cocrystals: Polymorphs and Coformer Exchange

Transformations of solid samples under solvent-free or minimal solvent conditions set the future trend and define a modern strategy for the production of new materials. Of the various technologies tested in recent years, the mechanochemical approach seems to be the most promising for economic and ecological reasons. The aim of this review article is to present the current state of art in solid state research on binary systems, which have found numerous applications in the pharmaceutical and materials science industries. This article is divided into three sections. In the first part, we describe the new equipment improvements, which include the innovative application of thermo-mechanochemistry, sono-mechanochemistry, photo-mechanochemistry, electro-mechanochemistry, as well as resonant acoustic mixing (RAM), and transformation under high-speed sample spinning ("SpeedMixing"). A brief description of techniques dedicated to ex-situ and in-situ studies of progress and the mechanism of solid matter transformation (PXRD, FTIR, Raman and NMR spectroscopy) is presented. In the second section, we discuss the problem of cocrystal polymorphism highlighting the issue related with correlation between mechanochemical parameters (time, temperature, energy, molar ratio, solvent used as a liquid assistant, surface energy, crystal size, crystal shape) and preference for the formation of requested polymorph. The last part is devoted to the description of the processes of coformer exchange in binary systems forced by mechanical and/or thermal stimuli. The influence of the thermodynamic factor on the selection of the best-suited partner for the formation of a two-component stable structure is presented.

On the physical processes of mechanochemically induced transformations in molecular solids

Initiating or sustaining physical and chemical transformations with mechanical force - mechanochemistry - provides an opportunity for more sustainable chemical processes, and access to new chemical reactivity. These transformations, however, do not always adhere to 'conventional' chemical wisdom, making them difficult to design and rationalise. This challenge is exacerbated by the fact that not all mechanochemical transformations are equal, with mechanical force playing a different role in different types of processes. In this review we discuss some of the different roles mechanical force can play in mechanochemical transformations, set primarily against the author's own research. We classify mechanochemical reactions broadly as those (1) where mechanical energy is for mixing, (2) where mechanical energy alters the stability of the reagent, and (3) where mechanical energy directly excites the solid. Finally, we demonstrate how - while useful - these classifications have fuzzy boundaries and concepts from across them are needed to understand many mechanochemical reactions.

A New Look at the Mechanism of Cocrystal Formation and Coformers Exchange in Processes Forced by Mechanical and/or Thermal Stimuli - ex situ and in situ Studies of Low-Melting Eutectic Mixtures

Three different devices: ball mill, hot stage melting, and magic angle spinning (MAS) NMR rotor were used for the preparation of ethenzamide (ET) cocrystals with glutaric acid (GLU), ethylmalonic acid (EMA) and maleic acid (MAL) as coformers. In each case, well-defined binary systems (ET:EMA, ET:GLU, ET:MAL) were obtained. The common features of the two solvent free methods of cocrystal formation (grinding, melting) are presented on the basis of arguments obtained by solid state NMR spectroscopy. Thermal analysis (Differential Scanning Calorimetry) proved that the eutectic phase arises over a wide range of molar ratios of components for each of the binary systems. NMR techniques, supported by theoretical calculations, allowed to provide details about the pathway of the reaction mechanism with atomic accuracy. It was found that the formation of ET cocrystals is a complex process that requires five steps. Each step has been recognized and described. Variable temperature 1D and 2D MAS NMR experiments allowed to track physicochemical processes taking place in a molten state. Moreover, it was found that in a multicomponent mixture consisting of all four components, ET, EMA, GLU, and MAL, ET in the molten phase behaves as a specific selector choosing only one partner to form binary cocrystals according to energy preferences. The process of exchange of coformers in binary systems during grinding, melting, and NMR measurements is described. The stabilization energies (Estab ) and molecular electrostatic potential (MEP) maps computed for the cocrystals under discussion and their individual components rationalize the selection rules and explain the relationships between individual species.

Structural diversity of cocrystals formed from acridine and two isomers of hydroxybenzaldehyde: 3-hydroxybenzaldehyde and 4-hydroxybenzaldehyde

Cocrystals formed from acridine and two isomers of hydroxybenzaldehyde: 3-hydroxybenzaldehyde (1) and 4-hydroxybenzaldehyde (2) were synthesized and structurally characterized. Single-crystal X-ray diffraction measurements show that compound 1 crystallizes in the triclinic P1̄ space group, whereas compound 2 crystallizes in the monoclinic P2₁/n space group. In the crystals of title compounds, the molecules interact via O-H⋯N and C-H⋯O hydrogen bonds, and C-H⋯π and π-π interactions. DCS/TG measurements indicate that compound 1 melts at a lower temperature than the separate cocrystal coformers, whereas compound 2 melts at a higher temperature than acridine but at a lower temperature than 4-hydroxybenzaldehyde. The FTIR measurements reveal that the band attributed to the stretching vibrations of the hydroxyl group of hydroxybenzaldehyde disappeared, but several bands appeared in the range of 3000-2000 cm^-1.

Accurate extrinsic and intrinsic peak broadening modelling for time-resolved in situ ball milling reactions via synchrotron powder X-ray diffraction

The debate on the mechanisms which underpin mechanochemical reactions via ball mill grinding is still open. Our ability to accurately measure the microstructural (crystal size and microstrain) evolution of materials under milling conditions as well as their phase composition as a function of time is key to the in-depth understanding of the kinetics and driving forces of mechanochemical transformations. Furthermore, all ball milling reactions end with a steady state or milling equilibrium - represented by a specific phase composition and relative microstructure - that does not change as long as the milling conditions are maintained. The use of a standard sample is essential to determine the instrumental contribution to the X-ray powder diffraction (XRPD) peak broadening for time-resolved in situ (TRIS) monitoring of mechanochemical reactions under in operando conditions. Using TRIS-XRPD on a ball milling setup, coupled with low-energy synchrotron radiation, we investigated different data acquisition and analysis strategies on a silicon standard powder. The diffraction geometry and the microstructural evolution of the standard itself have been studied to model the instrumental contribution to XRPD peak broadening throughout the grinding activity. Previously proposed functions are here challenged and further developed. Importantly, we show that minor drifts of the jar position do not affect the instrumental resolution function significantly. We here report and discuss the results of such investigations and their application to TRIS-XRPD datasets of inorganic and organic ball mill grinding reactions.

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.