Highly Photostable and Fluorescent Microporous Solids Prepared via Solid-State Entrapment of Boron Dipyrromethene Dyes in a Nascent Metal-Organic Framework.

We report a strategy to synthesize highly emissive, photostable, microporous materials by solid-state entrapment of boron dipyrromethene (BODIPY) fluorophores in a metal-organic framework. Solvent-free mechanochemistry or accelerated aging enabled quantitative capture and dispersal of the PM605 dye within the ZIF-8 framework starting from inexpensive, commercial materials. While the design of emissive BODIPY solids is normally challenged by quenching in a densely packed environment, herein reported PM605@ZIF-8 materials show excellent emissive properties and to the best of our knowledge an unprecedented ∼10-fold enhancement of BODIPY photostability. Time-resolved and steady-state fluorescence studies of PM605@ZIF-8 show that interchromophore interactions are minimal at low dye loadings, but at higher ones lead to through-pore energy transfer between chromophores and to aggregate species.

Mechanochemical Catalytic Transfer Hydrogenation of Aromatic Nitro Derivatives.

Mechanochemical ball milling catalytic transfer hydrogenation (CTH) of aromatic nitro compounds using readily available and cheap ammonium formate as the hydrogen source is demonstrated as a simple, facile and clean approach for the synthesis of substituted anilines and selected pharmaceutically relevant compounds. The scope of mechanochemical CTH is broad, as the reduction conditions tolerate various functionalities, for example nitro, amino, hydroxy, carbonyl, amide, urea, amino acid and heterocyclic. The presented methodology was also successfully integrated with other types of chemical reactions previously carried out mechanochemically, such as amide bond formation by coupling amines with acyl chlorides or anhydrides and click-type coupling reactions between amines and iso(thio)cyanates. In this way, we showed that active pharmaceutical ingredients Procainamide and Paracetamol could be synthesized from the respective nitro-precursors on milligram and gram scale in excellent isolated yields.

Mechanochemical synthesis of thioureas, ureas and guanidines.

In this review, the recent progress in the synthesis of ureas, thioureas and guanidines by solid-state mechanochemical ball milling is highlighted. While the literature is abundant on their preparation in conventional solution environment, it was not until the advent of solvent-free manual grinding using a mortar and pestle and automated ball milling that new synthetic opportunities have opened. The mechanochemical approach not only has enabled the quantitative synthesis of (thio)ureas and guanidines without using bulk solvents and the generation of byproducts, but it has also been established as a means to develop "click-type" chemistry for these classes of compounds and the concept of small molecule desymmetrization. Moreover, mechanochemistry has been demonstrated as an effective tool in reaction discovery, with emphasis on the reactivity differences in solution and in the solid state. These three classes of organic compounds share some structural features which are reflected in their physical and chemical properties, important for application as organocatalysts and sensors. On the other hand, the specific and unique nature of each of these functionalities render (thio)ureas and guanidines as the key constituents of pharmaceuticals and other biologically active compounds.

Mechanochemical ruthenium-catalyzed olefin metathesis.

We describe the development of a mechanochemical approach for Ru-catalyzed olefin metathesis, including cross-metathesis and ring-closing metathesis. The method uses commercially available catalysts to achieve high-yielding, rapid, room-temperature metathesis of solid or liquid olefins on a multigram scale using either no or only a catalytic amount of a liquid.

Trapping Reactive Intermediates by Mechanochemistry: Elusive Aryl N-Thiocarbamoylbenzotriazoles as Bench-Stable Reagents.

Monitoring of mechanochemical thiocarbamoylation by in situ Raman spectroscopy revealed the formation of aryl N-thiocarbamoylbenzotriazoles, reactive intermediates deemed unisolable in solution. The first-time isolation and structural characterization of these elusive molecules demonstrates the ability of mechanochemistry to access otherwise unobtainable intermediates and offers a new range of masked isothiocyanate reagents.

Development of C-N coupling using mechanochemistry: catalytic coupling of arylsulfonamides and carbodiimides.

Reported herein is the mechanochemical synthesis of sulfonyl guanidines, a family of molecules which are relevant as pharmaceuticals and herbicides, by direct coupling of sulfonamides and aromatic or aliphatic carbodiimides. Attempts to conduct the coupling in solution have either failed or given very low conversions, thus demonstrating mechanochemistry as the necessary component for the discovery of this synthetic strategy.

Laboratory real-time and in situ monitoring of mechanochemical milling reactions by Raman spectroscopy.

Mechanistic understanding of mechanochemical reactions is sparse and has been acquired mostly by stepwise ex situ analysis. We describe herein an unprecedented laboratory technique to monitor the course of mechanochemical transformations at the molecular level in situ and in real time by using Raman spectroscopy. The technique, in which translucent milling vessels are used that enable the collection of a Raman scattering signal from the sample as it is being milled, was validated on mechanochemical reactions to form coordination polymers and organic cocrystals. The technique enabled the assessment of the reaction dynamics and course under different reaction conditions as well as, for the first time, direct insight into the behavior of liquid additives during liquid-assisted grinding.

Real-time in situ powder X-ray diffraction monitoring of mechanochemical synthesis of pharmaceutical cocrystals.

Looking in: The penetrating power of high-energy X-rays provides a means to monitor in situ and in real time the course of ball-milling reactions of organic pharmaceutical solids by detecting crystalline phases and assessing the evolution of their particle sizes. Upon switching from neat grinding to liquid-assisted grinding, cocrystal formation is enabled or tremendously accelerated, while the reaction mechanism alters its course.

Click mechanochemistry: quantitative synthesis of "ready to use" chiral organocatalysts by efficient two-fold thiourea coupling to vicinal diamines.

Mechanochemical methods of neat grinding and liquid-assisted grinding have been applied to the synthesis of mono- and bis(thiourea)s by using the click coupling of aromatic and aliphatic diamines with aromatic isothiocyanates. The ability to modify the reaction conditions allowed the optimization of each reaction, leading to the quantitative formation of chiral bis(thiourea)s with known uses as organocatalysts or anion sensors. Quantitative reaction yields, combined with the fact that mechanochemical reaction conditions avoid the use of bulk solvents, enabled solution-based purification methods (such as chromatography or recrystallization) to be completely avoided. Importantly, by using selected model reactions, we also show that the described mechanochemical reaction procedures can be readily scaled up to at least the one-gram scale. In that way, mechanochemical synthesis provides a facile method to fully transform valuable enantiomerically pure reagents into useful products that can immediately be applied in their designed purpose. This was demonstrated by using some of the mechanochemically prepared reagents as organocatalysts in a model Morita-Baylis-Hillman reaction and as cyanide ion sensors in organic solvents. The use of electronically and sterically hindered ortho-phenylenediamine revealed that mechanochemical reaction conditions can be readily optimized to form either the 1:1 or the 1:2 click-coupling product, demonstrating that reaction stoichiometry can be more efficiently controlled under these conditions than in solution-based syntheses. In this way, it was shown that excellent stoichiometric control by mechanochemistry, previously established for mechanochemical syntheses of cocrystals and coordination polymers, can also be achieved in the context of covalent-bond formation.

Highly Efficient Solid-State Hydrolysis of Waste Polyethylene Terephthalate by Mechanochemical Milling and Vapor-Assisted Aging.

Despite significant methodological and technological advancements in chemical recycling of synthetic polymers, an efficient and quantitative conversion of post-consumer polyethylene terephthalate (PET) into terephthalic acid (TPA) under ambient conditions of temperature and pressure still remains a challenge. In this respect, the application of mechanochemistry and multiple advantages offered by solid-state ball milling and vapor-assisted aging have remained insufficiently explored. To further expand their potential, the implementation of organic solvent-free milling as a superior methodology for successful alkaline depolymerization of waste PET (e. g., bottles and textile) into TPA monomer in near-quantitative yields was reported herein. The solid-state alkaline PET hydrolysis was also shown to proceed in excellent yields under aging conditions in humid environment or in the presence of alcohol vapors. Moreover, the performance of mechanochemical ball milling and aging in the gram-scale depolymerization of PET into TPA was demonstrated.

Mechanochemically-assisted solid-state photocatalysis (MASSPC).

In this proof-of-concept study, we demonstrate the first successful transfer of transition metal-free photocatalysis from the conventional solution environment into the solid state, which is enabled by simultaneous irradiation with visible light and mechanochemical ball milling.

In situ X-ray diffraction monitoring of a mechanochemical reaction reveals a unique topology metal-organic framework.

Chemical and physical transformations by milling are attracting enormous interest for their ability to access new materials and clean reactivity, and are central to a number of core industries, from mineral processing to pharmaceutical manufacturing. While continuous mechanical stress during milling is thought to create an environment supporting nonconventional reactivity and exotic intermediates, such speculations have remained without proof. Here we use in situ, real-time powder X-ray diffraction monitoring to discover and capture a metastable, novel-topology intermediate of a mechanochemical transformation. Monitoring the mechanochemical synthesis of an archetypal metal-organic framework ZIF-8 by in situ powder X-ray diffraction reveals unexpected amorphization, and on further milling recrystallization into a non-porous material via a metastable intermediate based on a previously unreported topology, herein named katsenite (kat). The discovery of this phase and topology provides direct evidence that milling transformations can involve short-lived, structurally unusual phases not yet accessed by conventional chemistry.

Mechanosynthesis of pharmaceutically relevant sulfonyl-(thio)ureas.

We demonstrate the first application of mechanochemistry to conduct the synthesis of sulfonyl-(thio)ureas, including known anti-diabetic drugs tolbutamide, chlorpropamide and glibenclamide, in good to excellent isolated yields by either stoichiometric base-assisted or copper-catalysed coupling of sulfonamides and iso(thio)cyanates.

Quantitative in situ and real-time monitoring of mechanochemical reactions.

An experimental technique for in situ and real-time monitoring of mechanochemical reactions in a shaker ball mill was recently described, which utilises highly penetrating X-ray radiation available at the ID15B beamline of the European Synchrotron Radiation Facility. Herein, we describe the first attempts to perform such reaction monitoring in a quantitative fashion, by introducing an internal X-ray diffraction standard. The use of silicon as an internal standard resolved the issue with variations of the amount of the sample in the X-ray beam due to the non-uniform distribution of the sample in the reaction jar and allowed, via Rietveld analysis, the first quantitative estimate of the amorphous phase content in a mechanochemical reaction as it is being milled. We also highlight problems associated with the non-ideal mixing of the reaction mixture.

One-pot mechanosynthesis of aromatic amides and dipeptides from carboxylic acids and amines.

Environmentally friendly one-pot synthesis of amides, bis-amides and dipeptides by mechanochemical carbodiimide-mediated coupling of carboxylic acids and amines is described; high reaction yields and simple aqueous work-up allow for the clean, practical and fast preparation of a variety of compounds containing the amide bond from readily accessible reagents.

Desymmetrisation of aromatic diamines and synthesis of non-symmetrical thiourea derivatives by click-mechanochemistry.

ortho- and para-Phenylenediamines were desymmetrised and quantitatively transformed into mono- and bis-(thio)ureas or mixed thiourea-ureas through a one-pot mechanochemical click reaction sequence; mechanochemical desymmetrisation proceeds quantitatively without excess reagents and allows the controlled extension of a molecular structure by combining normally competing reactions.

Towards an environmentally-friendly laboratory: dimensionality and reactivity in the mechanosynthesis of metal-organic compounds.

We present a proof-of-principle study of an environmentally-friendly approach to laboratory research, in which the synthesis and structural characterisation of metal-organic complexes and frameworks are achieved without using bulk solvents; our study addresses the use of heteroditopic ligands for manipulating the dimensionality of metal-organic materials and describes how kinetic obstacles in such mechanosynthesis can be overcome.