A Case Study on the Desired Selectivity in Solid-State Mechano- and Slow-Chemistry, Melt, and Solution Methodologies

Mechanochemistry for Organic and Inorganic Synthesis

In recent years, mechanochemistry has become an innovative and sustainable alternative to traditional solvent-based synthesis. Mechanochemistry rapidly expanded across a wide range of chemistry fields, including diverse organic compounds and active pharmaceutical ingredients, coordination compounds, organometallic complexes, main group frameworks, and technologically relevant materials. This Review aims to highlight recent advancements and accomplishments in mechanochemistry, underscoring its potential as a viable and eco-friendly alternative to conventional solution-based methods in the field of synthetic chemistry.

Uncovering Factors Controlling Reactivity of Metal-TEMPO Reaction Systems in the Solid State and Solution

Nitroxides find application in various areas of chemistry, and a more in-depth understanding of factors controlling their reactivity with metal complexes is warranted to promote further developments. Here, we report on the effect of the metal centre Lewis acidity on both the distribution of the O- and N-centered spin density in 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and turning TEMPO from the O- to N-radical mode scavenger in metal-TEMPO systems. We use Et(Cl)Zn/TEMPO model reaction system with tuneable reactivity in the solid state and solution. Among various products, a unique Lewis acid-base adduct of Cl2Zn with the N-ethylated TEMPO was isolated and structurally characterised, and the so-called solid-state 'slow chemistry' reaction led to a higher yield of the N-alkylated product. The revealed structure-activity/selectivity correlations are exceptional yet are entirely rationalised by the mechanistic underpinning supported by theoretical calculations of studied model systems. This work lays a foundation and mechanistic blueprint for future metal/nitroxide systems exploration.

Fresh Impetus in the Chemistry of Calcium Peroxides

Redox-inactive metal ions are essential in modulating the reactivity of various oxygen-containing metal complexes and metalloenzymes, including photosystem II (PSII). The heart of this unique membrane-protein complex comprises the Mn4CaO5 cluster, in which the Ca2+ ion acts as a critical cofactor in the splitting of water in PSII. However, there is still a lack of studies involving Ca-based reactive oxygen species (ROS) systems, and the exact nature of the interaction between the Ca2+ center and ROS in PSII still generates intense debate. Here, harnessing a novel Ca-TEMPO complex supported by the β-diketiminate ligand to control the activation of O2, we report the isolation and structural characterization of hitherto elusive Ca peroxides, a homometallic Ca hydroperoxide and a heterometallic Ca/K peroxide. Our studies indicate that the presence of K+ cations is a key factor controlling the outcome of the oxygenation reaction of the model Ca-TEMPO complex. Combining experimental observations with computational investigations, we also propose a mechanistic rationalization for the reaction outcomes. The designed approach demonstrates metal-TEMPO complexes as a versatile platform for O2 activation and advances the understanding of Ca/ROS systems.

Solid-state mechanochemistry for the rapid and efficient synthesis of tris-cyclometalated iridium(iii) complexes

Tris-cyclometalated iridium(iii) complexes have received widespread attention as attractive prospective materials for e.g., organic light-emitting diodes (OLEDs), photoredox catalysts, and bioimaging probes. However, their preparation usually requires prolonged reaction times, significant amounts of high-boiling solvents, multistep synthesis, and inert-gas-line techniques. Unfortunately, these requirements represent major drawbacks from both a production-cost and an environmental perspective. Herein, we show that a two-step mechanochemical protocol using ball milling enables the rapid and efficient synthesis of various tris-cyclometalated iridium(iii) complexes from relatively cheap iridium(iii) chloride hydrate without the use of significant amounts of organic solvent in air. Notably, a direct one-pot procedure is also demonstrated. The present solid-state approach can be expected to inspire the development of cost-effective and timely production methods for these valuable iridium-based complexes, as well as the discovery of new phosphorescent materials, sensors, and catalysts.

Synthesis without solvent: consequences for mechanochemical reactivity

Solvents are so nearly omnipresent in synthetic chemistry that a classic question for their use has been: "What is the best solvent for this reaction?" The increasing use of mechanochemical approaches to synthesis-by grinding, milling, extrusion, or other means-and usually with no, or only limited, amounts of solvent, has raised an alternative question for the synthetic chemist: "What happens if there is no solvent?" This review focuses on a three-part answer to that question: when there is little change ("solvent-optional" reactions); when solvent needs to be present in some form, even if only in the amounts provided by liquid-assisted (LAG) or solvate-assisted grinding; and those cases in which mechanochemistry allows access to compounds that cannot be obtained from solution-based routes. The emphasis here is on inorganic and organometallic systems, including selected examples of mechanosynthesis and mechanocatalysis. Issues of mechanochemical depictions and the adequacy of LAG descriptions are also reviewed.

In situ study of Au nanoparticle formation in a mechanochemical-aging-based method

As we strive to perform chemical transformations in a more sustainable fashion, enabling solid-state reactions through mechanochemistry has emerged as a highly successful approach. Due to the wide-ranging applications of gold nanoparticles (AuNPs), mechanochemical strategies have already been employed for their synthesis. However, the underlying processes surrounding gold salt reduction, nucleation and growth of AuNPs in the solid state are yet to be understood. Here, we present a mechanically activated aging synthesis of AuNPs, through a solid-state Turkevich reaction. Solid reactants are only briefly exposed to input of mechanical energy before being aged statically over a period of six weeks at different temperatures. This system offers an excellent opportunity for an in situ analysis of both reduction and nanoparticle formation processes. During the aging period, the reaction was monitored using a combination of X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy, powder X-ray diffraction and transmission electron microscopy, to gain meaningful insights into the mechanisms of solid-state formation of gold nanoparticles. The acquired data allowed for the establishment of the first kinetic model for solid-state nanoparticle formation.

Maleic Acid as a Co-Former for Pharmaceutically Active GABA Derivatives: Mechanochemistry or Solvent Crystallization?

In this study, we compare the mechanochemical and classical solvent crystallization methods for forming maleates of GABA and its pharmaceutically active derivatives: Pregabalin, Gabapentin, Phenibut, and Baclofen. Common characterization techniques, like powder and single crystal X-ray diffraction, IR-spectroscopy, differential scanning calorimetry, thermogravimetric analysis and ¹H-NMR spectroscopy, are used for the evaluation of structural and physicochemical properties. Our work shows that maleate formation is possible with all investigated target compounds. Large increases in solubility can be achieved, especially for Pregabalin, where up to twentyfold higher solubility in its maleate compared to the pure form can be reached. We furthermore compare the mechanochemical and solvent crystallization regarding quickness, reliability of phase production, and overall product quality. A synthetic route is shown to have an impact on certain properties such as melting point or solubility of the same obtained products, e.g., for Gabapentin and Pregabalin, or lead to the formation of hydrates vs. anhydrous forms. For the GABA and Baclofen maleates, the method of crystallization is not important, and similarly, good results can be obtained by either route. In contrast, Phenibut maleate cannot be obtained pure and single-phase by either method. Our work aims to elucidate promising candidates for the multicomponent crystal formation of blockbuster GABA pharmaceuticals and highlight the usefulness of mechanochemical production routes.

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.

Mechanochemistry: New Tools to Navigate the Uncharted Territory of "Impossible" Reactions

Mechanochemical transformations have made chemists enter unknown territories, forcing a different chemistry perspective. While questioning or revisiting familiar concepts belonging to solution chemistry, mechanochemistry has broken new ground, especially in the panorama of organic synthesis. Not only does it foster new "thinking outside the box", but it also has opened new reaction paths, allowing to overcome the weaknesses of traditional chemistry exactly where the use of well-established solution-based methodologies rules out progress. In this Review, the reader is introduced to an intriguing research subject not yet fully explored and waiting for improved understanding. Indeed, the study is mainly focused on organic transformations that, although impossible in solution, become possible under mechanochemical processing conditions, simultaneously entailing innovation and expanding the chemical space.

Mechanochemical vs Wet Approach for Directing CO2 Capture toward Various Carbonate and Bicarbonate Networks

The distinct research areas related to CO₂ capture and mechanochemistry are both highly attractive in the context of green chemistry. However, merger of these two areas, i.e., mechanochemical CO₂ capture, is still in an early stage of development. Here, the application of biguanidine as an active species for CO₂ capture is investigated using both solution-based and liquid-assisted mechanochemical approaches, which lead to a variety of biguanidinium carbonate and bicarbonate hydrogen-bonded networks. We demonstrate that in solution, the formation of the carbonate vs bicarbonate networks can be directed by the organic solvent, while, remarkably, in the liquid-assisted mechanochemical synthesis employing the same solvents as additives, the selectivity in network formation is inversed. In general, our findings support the view of mechanochemistry not only as a sustainable alternative but rather as a complementary strategy to solution-based synthesis.