Solvate-Assisted Grinding: Metal Solvates as Solvent Sources in Mechanochemically Driven Organometallic Reactions

Mechanochemical Synthesis of Chromium(III) Complexes Containing Bidentate PN and Tridentate P-NH-P and P-NH-P' Ligands

Chromium(III) complexes bearing bidentate {NH2(CH2)2PPh2: PN, (S,S)-[NH2(CHPh)2PPh2]: P'N} and tridentate [Ph2P(CH2)2N(H)(CH2)2PPh2: P-NH-P, (S,S)-(iPr)2PCH2CH2N(H)CH(Ph)CH(Ph)PPh2: P-NH-P'] ligands have been synthesized using a mechanochemical approach. The complexes {cis-[Cr(PN)Cl2]Cl (1), cis-[Cr(P'N)Cl2]Cl (2), mer-Cr(P-NH-P)Cl3 (3), and mer-Cr(P-NH-P')Cl3 (4)} were obtained in high yield (95-97%) via the grinding of the respective ligands andthe solid Cr(III) ion precursor [CrCl3(THF)3] with the aid of a pestle and mortar, followed by recrystallization in acetonitrile. The isolated complexes are high spin. A single-crystal X-ray diffraction study of 2 revealed a cationic chromium complex with two P'N ligands in a cis configuration with P' trans to P' with chloride as the counteranion. The X-ray study of 4 shows a neutral Cr(III) complex with the P-NH-P' ligand in a mer configuration. The difference in molecular structures and bulkiness of the ligands influence the electronic, magnetic, and electrochemical properties of the complexes as exhibited by the bathochromic shifts in the electronic absorption peaks of the complexes and the relative increase in the magnetic moment of 3 (4.19 μβ) and 4 (4.15 μβ) above the spin only value (3.88 μβ) for a d3 electronic configuration. Complexes 1-4 were found to be inactive in the hydrogenation of an aldimine [(E)-1-(4-fluorophenyl)-N-phenylmethanimine] under a variety of activating conditions. The addition of magnesium and trimethylsilyl chloride in THF did cause hydrogenation at room temperature, but this occurred even in the absence of the chromium complex. The hydrogen in the amine product came from the THF solvent in this novel reaction, as determined by deuterium incorporation into the product when deuterated THF was used.

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.

Fast and scalable solvent-free access to Lappert's heavier tetrylenes E{N(SiMe3)2}2 (E = Ge, Sn, Pb) and ECl{N(SiMe3)2} (E = Ge, Sn)

Iconic Lappert's heavier tetrylenes E{N(SiMe3)2}2 (E = Ge (1), Sn (2), Pb (3)) have been efficiently prepared from GeCl2·(1,4-dioxane), SnCl2 or PbCl2 and Li{N(SiMe3)2} via a completely solvent-free one-pot mechanochemical route followed by sublimation. This fast, high-yielding and scalable approach (2 has been prepared in a 100 mmol scale), which involves a small environmental footprint, represents a remarkable improvement over any synthetic route reported over the last five decades, being a so far rare example of the use of mechanochemistry in the realm of main group chemistry. This solventless route has been successfully extended to the preparation of other heavier tetrylenes, such as ECl{N(SiMe3)2} (E = Ge (4), Sn (5)).

Acceleration of Diels-Alder reactions by mechanical distortion

Challenges in quantifying how force affects bond formation have hindered the widespread adoption of mechanochemistry. We used parallel tip-based methods to determine reaction rates, activation energies, and activation volumes of force-accelerated [4+2] Diels-Alder cycloadditions between surface-immobilized anthracene and four dienophiles that differ in electronic and steric demand. The rate dependences on pressure were unexpectedly strong, and substantial differences were observed between the dienophiles. Multiscale modeling demonstrated that in proximity to a surface, mechanochemical trajectories ensued that were distinct from those observed solvothermally or under hydrostatic pressure. These results provide a framework for anticipating how experimental geometry, molecular confinement, and directed force contribute to mechanochemical kinetics.

Mechanochemical Syntheses of Ln(hfac)3(H2O)x (Ln = La-Sm, Tb): Isolation of 10-, 9-, and 8-Coordinate Ln(hfac)n Complexes

Volatile lanthanide coordination complexes are critical to the generation of new optical and magnetic materials. One of the most common precursors for preparing volatile lanthanide complexes is the hydrate with the general formula Ln(hfac)₃(H₂O)_x (x = 3 for La-Nd, x = 2 for Sm) (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato). We have investigated the synthesis of Ln(hfac)₃(H₂O)_x using more environmentally sustainable mechanochemical approaches. Characterization of the products using Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, elemental analysis, and powder X-ray diffraction shows substantial differences in product distribution between methods. The mechanochemical synthesis of the hydrate complexes leads to a variety of coordination compounds including the expected hydrate product, the known retro-Claisen impurity, and hydrated protonated Hhfac ligand depending on the technique employed. Surprisingly, 10-coordinate complexes of the form Na₂Ln(hfac)₅·3H₂O for Ln = La-Nd were also isolated from reactions using a mortar and pestle. The electrostatic bonding of lanthanide coordination complexes is a challenge for obtaining reproducible reactions and clean products. The reproducibility issues are most acute for the large, early lanthanides whereas for the mid to late lanthanides, reproducibility in terms of product distribution and yield is less of an issue because of their smaller size and greater charge to radius ratio. Ball milling increases reproducibility in terms of generating the desired Ln(hfac)₃(H₂O)_x along with hydrated Hhfac (tetraol) and free Hhfac products. The results illustrate the dynamic behavior of lanthanide complexes in solution and the solid state as well as the structural diversity available to the early lanthanides.

Mechanochemical reactions to prepare Ln(hfac)₃(H₂O)_x (Ln = La-Sm, Tb) complexes are used to illustrate the highly variable coordination chemistry of the early lanthanides compared to the mid-lanthanides. Using either a mortar and pestle or ball mill results in unexpected products and different product distributions. An open mortar and pestle can yield 10-coordinate pentakis-hfac complexes of La, Ce, Pr, and Nd, whereas ball milling consistently results in Ln(hfac)₃(H₂O)_x complexes with volatile Hhfac and hydrated Hhfac·2H₂O (tetraol) that can be removed by vacuum.

Assembly of Uranyl Peroxides from Ball Milled Solids

Mechanochemistry enables transformations of highly insoluble materials such as uranium dioxide or the mineral studtite [(UO₂)(O₂)(H₂O)₂]·(H₂O)₂ into uranyl triperoxide compounds that can subsequently assemble into hydroxide-bridged uranyl peroxide dimers in the presence of lithium hydroxide. Dissolution of these solids in water yields uranyl peroxide nanoclusters including U₂₄, Li₂₄[(UO₂)(O₂)(OH)]₂₄. Insoluble uranium solids can transform into highly soluble uranyl peroxide phases in the solid state with miniscule quantities of water. Such reactions are potentially applicable to uranium processing in the front and back end of the nuclear fuel cycle.

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.

Mechanochemical Grignard Reactions with Gaseous CO2 and Sodium Methyl Carbonate

A one-pot three-step protocol for the preparation of Grignard reagents from organobromides in a ball mill and their subsequent reactions with gaseous carbon dioxide (CO 2 ) or sodium methyl carbonate providing aryl and alkyl carboxylic acids in up to 82% yield is reported. Noteworthy are the short reaction times and the significantly reduced solvent amounts [2.0 equiv. for liquid assisted grinding (LAG) conditions]. Unexpectedly, aryl bromides with methoxy substituents lead to symmetric ketones as major products.