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.

Machine-Learning-Assisted Discovery of Mechanosynthesized Lead-Free Metal Halide Perovskites for the Oxidative Photocatalytic Cleavage of Alkenes

Lead-free metal halide perovskites can potentially be air- and water-stable photocatalysts for organic synthesis, but there are limited studies on them for this application. Separately, machine learning (ML), a critical subfield of artificial intelligence, has played a pivotal role in identifying correlations and formulating predictions based on extensive datasets. Herein, an iterative workflow by incorporating high-throughput experimental data with ML to discover new lead-free metal halide perovskite photocatalysts for the aerobic oxidation of styrene is described. Through six rounds of ML optimization guided by SHapley Additive exPlanations (SHAP) analysis, BA2CsAg0.95Na0.05BiBr7 as a photocatalyst that afforded an 80% yield of benzoic acid under the standard conditions is identified, which is a 13-fold improvement compared to the 6% with when using Cs2AgBiBr6 as the initial photocatalyst benchmark that is started. BA2CsAg0.95Na0.05BiBr7 can tolerate various functional groups with 22 styrene derivatives, highlighting the generality of the photocatalytic properties demonstrated. Radical scavenging studies and density functional theory calculations revealed that the formation of the reactive oxygen species superoxide and singlet oxygen in the presence of BA2CsAg0.95Na0.05BiBr7 are critical for photocatalysis.

Rigid PN cages as 3-dimensional building blocks for crystalline or amorphous networked materials

Three-dimensional covalent connectors are valuable synthons for accessing crystalline or amorphous networks. Currently, fused polycyclic alkanes are employed as connectors in this context. We debut phosphorus-nitrogen (PN) cages as new 3-dimensional (3-D) inorganic connectors that yield crystalline and amorphous networks, including examples with gas porosity. We show that the high tunability of PN cages accelerates network diversification and the presence of a responsive 31P NMR spectroscopic handle provides structural insight. Collectively, this work unlocks a new and convenient 3-D synthon for reticular chemistry.

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)).

Tinkering with Mechanochemical Tools for Scale Up

Mechanochemistry provides an environmentally benign platform to develop more sustainable chemical processes by limiting raw materials, energy use, and waste generation while using physically smaller equipment. A continuously growing research community has steadily showcased examples of beneficial mechanochemistry applications at both the laboratory and the preparative scale. In contrast to solution-based chemistry, mechanochemical processes have not yet been standardized, and thus scaling up is still a nascent discipline. The purpose of this Minireview is to highlight similarities, differences and challenges of the various approaches that have been successfully applied for a range of chemical applications at various scales. We hope to provide a discussion starting point for those interested in further developing mechanochemical processes for commercial use and/or industrialisation.

Main Group Molecular Switches with Swivel Bifurcated to Trifurcated Hydrogen Bond Mode of Action

Artificial molecular machines have captured the full attention of the scientific community since Jean-Pierre Sauvage, Fraser Stoddart, and Ben Feringa were awarded the 2016 Nobel Prize in Chemistry. The past and current developments in molecular machinery (rotaxanes, rotors, and switches) primarily rely on organic-based compounds as molecular building blocks for their assembly and future development. In contrast, the main group chemical space has not been traditionally part of the molecular machine domain. The oxidation states and valency ranges within the p-block provide a tremendous wealth of structures with various chemical properties. Such chemical diversity─when implemented in molecular machines─could become a transformative force in the field. Within this context, we have rationally designed a series of NH-bridged acyclic dimeric cyclodiphosphazane species, [(μ-NH){PE(μ-N^tBu)₂PE(NH^tBu)}₂] (E = O and S), bis-P^V₂N₂, displaying bimodal bifurcated R²₁(8) and trifurcated R³₁(8,8) hydrogen bonding motifs. The reported species reversibly switch their topological arrangement in the presence and absence of anions. Our results underscore these species as versatile building blocks for molecular machines and switches, as well as supramolecular chemistry and crystal engineering based on cyclophosphazane frameworks.

Pre-arranged building block approach for the orthogonal synthesis of an unfolded tetrameric organic-inorganic phosphazane macrocycle

Inorganic macrocycles remain challenging synthetic targets due to the limited number of strategies reported for their syntheses. Among these species, large fully inorganic cyclodiphosphazane macrocycles have been experimentally and theoretically highlighted as promising candidates for supramolecular chemistry. In contrast, their hybrid organic-inorganic counterparts are lagging behind due to the lack of synthetic routes capable of controlling the size and topological arrangement (i.e., folded vs unfolded) of the target macrocycle, rendering the synthesis of differently sized macrocycles a tedious screening process. Herein, we report-as a proof-of-concept-the combination of pre-arranged building blocks and a two-step synthetic route to rationally enable access a large unfolded tetrameric macrocycle, which is not accessible via conventional synthetic strategies. The obtained macrocycle hybrid cyclodiphosphazane macrocycle, cis-[μ-P(μ-NtBu)]2(μ-p-OC6H4C(O)O)]4[μ-P(μ-NtBu)]2 (4), displays an unfolded open-face cavity area of 110.1 Å2. Preliminary theoretical host-guest studies with the dication [MeNC5H4]22+ suggest compound 4 as a viable candidate for the synthesis of hybrid proto-rotaxanes species based on phosphazane building blocks.

Sustainable Production of Reduced Phosphorus Compounds: Mechanochemical Hydride Phosphorylation Using Condensed Phosphates as a Route to Phosphite

In pursuit of a more sustainable production of phosphorous acid (H₃PO₃), a versatile chemical with phosphorus in the +3 oxidation state, we herein report that condensed phosphates can be employed to phosphorylate hydride reagents under solvent-free mechanochemical conditions to furnish phosphite (HPO₃ ^2-). Using potassium hydride as the hydride source, sodium trimetaphosphate (Na₃P₃O₉), triphosphate (Na₅P₃O₁₀), pyrophosphate (Na₄P₂O₇), fluorophosphate (Na₂PO₃F), and polyphosphate ("(NaPO₃) _n ") engendered phosphite in optimized yields of 44, 58, 44, 84, and 55% based on total P content, respectively. Formation of overreduced products including hypophosphite (H₂PO₂ ^-) was identified as a competing process, and mechanistic investigations revealed that hydride attack on in-situ-generated phosphorylated phosphite species is a potent pathway for overreduction. The phosphite generated from our method was easily isolated in the form of barium phosphite, a useful intermediate for production of phosphorous acid. This method circumvents the need to pass through white phosphorus (P₄) as a high-energy intermediate and mitigates involvement of environmentally hazardous chemicals. A bioproduced polyphosphate was found to be a viable starting material for the production of phosphite. These results demonstrate the possibility of accessing reduced phosphorus compounds in a more sustainable manner and, more importantly, a means to close the modern phosphorus cycle.

Mechanosynthesis of Higher-Order Cocrystals: Tuning Order, Functionality and Size in Cocrystal Design*

The ability to rationally design and predictably construct crystalline solids has been the hallmark of crystal engineering research. To date, numerous examples of multicomponent crystals comprising organic molecules have been reported. However, the crystal engineering of cocrystals comprising both organic and inorganic chemical units is still poorly understood and mostly unexplored. Here, we report a new diverse set of higher-order cocrystals (HOCs) based on the structurally versatile-yet largely unexplored-phosph(V/V)azane heterosynthon building block. The novel ternary and quaternary cocrystals reported are held together by synergistic and orthogonal intermolecular interactions. Notably, the HOCs can be readily obtained either via sequential or one-pot mechanochemical methods. Computational modelling methods reveal that the HOCs are thermodynamically driven to form and that their mechanical properties strongly depend on the composition and intermolecular forces in the crystal, offering untapped potential for optimizing material properties.

Mechanochemical Rearrangements

Molecular rearrangements are a powerful tool for constructing complex structures in an atom- and step-economic manner, translating multistep transformations into an intrinsically more sustainable process. Mechanochemical molecular rearrangements become an even more appealing eco-friendly synthetic approach, especially for preparing active pharmaceutical ingredients (APIs) and natural products. Still in their infancy, rearrangements promoted by mechanochemistry represent a promising approach for chemists to merge molecular diversity and green chemistry perspectives toward more selective and efficient syntheses with a reduced environmental footprint.

Sustainability Assessment of Mechanochemistry by Using the Twelve Principles of Green Chemistry

In recent years, mechanochemistry has been growing into a widely accepted alternative for chemical synthesis. In addition to their efficiency and practicality, mechanochemical reactions are also recognized for their sustainability. The association between mechanochemistry and Green Chemistry often originates from the solvent-free nature of most mechanochemical protocols, which can reduce waste production. However, mechanochemistry satisfies more than one of the Principles of Green Chemistry. In this Review we will present a series of examples that will clearly illustrate how mechanochemistry can significantly contribute to the fulfillment of Green Chemistry in a more holistic manner.

Mechanochemical Metathesis between AgNO3 and NaX (X = Cl, Br, I) and Ag2XNO3 Double-Salt Formation

Here we describe real-time, in situ monitoring of mechanochemical solid-state metathesis between silver nitrate and the entire series of sodium halides, on the basis of tandem powder X-ray diffraction and Raman spectroscopy monitoring. The mechanistic monitoring reveals that reactions of AgNO₃ with NaX (X = Cl, Br, I) differ in reaction paths, with only the reaction with NaBr providing the NaNO₃ and AgX products directly. The reaction with NaI revealed the presence of a novel, short-lived intermediate phase, while the reaction with NaCl progressed the slowest through the well-defined Ag₂ClNO₃ intermediate double salt. While the corresponding iodide and bromide double salts were not observed as intermediates, all three are readily prepared as pure compounds by milling equimolar mixtures of AgX and AgNO₃. The in situ observation of reactive intermediates in these simple metathesis reactions reveals a surprising resemblance of reactions involving purely ionic components to those of molecular organic solids and cocrystals. This study demonstrates the potential of in situ reaction monitoring for mechanochemical reactions of ionic compounds as well as completes the application of these techniques to all major compound classes.

Completely Solvent-free Protocols to Access Phase-Pure, Metastable Metal Halide Perovskites and Functional Photodetectors from the Precursor Salts

Mechanochemistry is a green, solid-state, re-emerging synthetic technique that can rapidly form complex molecules and materials without exogenous heat or solvent(s). Herein, we report the application of solvent-free mechanochemical ball milling for the synthesis of metal halide perovskites, to overcome problems with solution-based syntheses. We prepared phase-pure, air-sensitive CsSnX3 (X = I, Br, Cl) and its mixed halide perovskites by mechanochemistry for the first time by reactions between cesium and tin(II) halides. Notably, we report the sole examples where metastable, high-temperature phases like cubic CsSnCl3, cubic CsPbI3, and trigonal FAPbI3 were accessible at ambient temperatures and pressures without post-synthetic processing. The perovskites can be prepared up to "kilogram scales." Lead-free, all-inorganic photodetector devices were fabricated using the mechanosynthesized CsSnBr1.5Cl1.5 under solvent-free conditions and showed 10-fold differences between on-off currents. We highlight an essentially solvent-free, general approach to synthesize metastable compounds and fabricate photodetectors from commercially available precursors.

Catechol and 1,2,4,5-tetrahydroxybenzene functionalized cyclodiphosphazane ligands: synthesis, structural studies, and transition metal complexes

This paper describes the syntheses of two novel cyclodiphosphazane derivatives and their coordination chemistry with Cu^I, Ru^II, Rh^I, Pd^II and Au^I is also described.

Main group mechanochemistry: from curiosity to established protocols

In the last few decades, mechanochemistry has become rapidly established as a powerful tool enabling environmentally-benign and sustainable chemical syntheses. Not only have these techniques been demonstrated as viable alternatives to traditional solution-based syntheses, but they have also received attention for their ability to enable new reactivity and "unlocking" novel compounds inaccessible by conventional methods. Reflecting the rising popularity of mechanochemistry, many excellent reviews highlighting its benefits have recently been published. Whilst the scope of most of these focuses on organic chemistry, transition-metal catalysis, porous framework materials, coordination compounds and supramolecular synthesis, few have addressed the use of mechanochemical ball milling for the synthesis of compounds containing s- and p-block elements. This tutorial review turns the spotlight towards mechanochemical research in the field of inorganic main group chemistry, highlighting significant advantages that solid-state inorganic reactions often possess, and the potential for these to drive the development of greener methodologies within the modern main group arena.

Synthesis of Unique Phosphazane Macrocycles via Steric Activation of C-N Bonds

Herein we describe that oxidation reactions of the dimeric cyclophosphazanes, [{P(μ-NR)}₂(μ-NR)]₂, R = ^tBu (1), to produce a series of diagonally dioxidized products P₄(μ-N ^tBu)₆E₂ [E = O (2), S (3), and Se (4)] and tetraoxidized frameworks. The latter display an unexpected C-N bond activation and cleavage to produce a series of novel phosphazane macrocyclic arrangements containing newly formed N-H bonds. Macromolecules P₄(μ-N ^tBu)₄(μ-NH)₂O₄ (5) and P₄(μ-N ^tBu)₃(μ-NH)₃E₄, E = S (6) and Se (7), dicleaved and tricleaved products, respectively, are rare examples of dimeric macrocycles containing NH bridging groups. Our theoretical and experimental studies illustrate that the extent to which these C-N bonds are cleaved can be controlled by modification of steric parameters in their synthesis, by adjusting either the steric bulk of the substituents in the parent framework or the size of the chalcogen element introduced during the oxidation process. Our findings represent new synthetic pathways for the synthesis of otherwise-elusive macrocycle arrangements within the phosphazane family.

cis-Cyclodiphosph(v/v)azanes as highly stable and robust main group supramolecular building blocks

Bench-top stable cis-cyclodiphosph(v/v)azanes are demonstrated to form robust R21(8) bifurcated hydrogen-bonds and PSe⋯Br halogen bonds. This work highlights the potential of cyclodiphosph(v/v)azane building blocks in creating new supramolecular assemblies.