Search

Crown ethers were discovered in 1967 and since then, the interest in their properties and applications increased. Crown ethers have specific molecular shapes and planar conformation, but the flexibility of the ring can change depending on the interactions with guest ions or molecules. The cation encapsulation in the 18-crown-6 ether (18c6) cavity is well-known in solution chemistry, however, the supramolecular recognition may also be accomplished in the solid state. Our goal was to explore the complexation of 18c6 with potassium salts in a ball mill and test the reactivity of the formed [18c6K]OH complex. The successful complexation was evidenced by powder X-ray diffraction (PXRD) and IR spectroscopy. The Cambridge Structural Database served as a tool for searching the previously reported crystal structures and matching them with experimentally observed ones, while a new crystal structure of a monohydrate complex of the 18c6 and the KCl ion pair was solved and refined against laboratory PXRD data. It was found that the initial squashed conformation of 18c6 molecules in the pure solid opens with milling. The catalytic properties of [18c6K]OH complex were tested in two organic reactions, the Henry reaction and the nucleophilic aromatic substitution reaction. The results indicate the potential of using crown ether complexes in solid-state catalysis where their use is still underexplored.

The ability to predict mechanochemical reaction pathways and kinetics based on the mechanical energy input as well as the characteristics of the targeted chemical reactivity is required for efficient experimental planning. While we have recently established linearity between energy input and reaction progress in a purely mechanochemically activated reaction, we have here tested the application of the classical solution- based Hammett correlation in mechanochemical Schiff base formation from p-nitrobenzaldehyde and differently substituted anilines. Tracking chemical transformations in real time using in situ Raman spectroscopy enables the examination of the connection between chemical, rheological and kinetic factors. It was shown that the properties of reaction material such as hardness and density affect the elasticity of the collisions in a ball mill and the different course of the reaction. The linear dependence of the reaction probability and the Hammett constants derived from the solution indicates that the current models borrowed from physical-organic chemistry can be exploited to further the understanding of milling reactions.

Solution-based physical organic chemistry was marked by the development of Hammett correlation, which further served as a major tool for deciphering the mechanisms of organic reactions in solution. If applicable to mechanochemical reactions, it will provide support for understanding the reaction mechanisms and expansion of this important synthetic methodology. As a model system to test the validity of the Hammett equation under mechanochemical conditions, we focused on imine formation from an aromatic aldehyde and anilines. Using in situ Raman reaction monitoring and kinetic analysis, we find a linear relationship between the reaction probability and Hammett constants indicating that this classical concept of chemical reactivity is valid for reactions conducted by ball milling and could prove useful for optimization of mechanochemical reactions. The Hammett plot showed that the imine formation is sensitive to the nucleophilicity of the aniline partner with the reaction probability increasing with stronger electron donor substituents, as expected. We further explore reaction mechanism of imine formation on a catalysed reaction, in a contrast to the otherwise facile imine preparation by neat grinding.

Cucurbit[n]uril macrocycles (CB[n]) represents are well-established hosts for the encapsulation of guests in the aqueous medium, often with extreme binding affinities up to 7.2 x 1017 M−1. The Cucurbit[n]uril family has been shown to greatly enhance the solubility of hydrophobic substrates in aqueous medium upon encapsulation.[1-3] Solvent-free mechanochemical synthesis (ball milling) has been shown to be a useful method for the preparation of inclusion complexes with cucurbit[7]uril and adamantane and phenyl derivates as model molecules of guests.[4] Here we report on the mechanochemical synthesis of several phenylenediamine derivatives inclusion complexes with Cucurbit[n]uril. The formation of the inclusion complex was carried out by milling. Successful formation of individual inclusions was in situ monitoring of grinding reactions by Raman spectroscopy which enabled direct insight into the formation of the Host-Guest complex and the mechanochemistry of grinding.[5]. Host-guest complexation in the solid-state was studied and characterized by X-ray powder diffraction, FTIR, NMR spectroscopy, and differential scanning calorimetry.

Solid-state synthesis by mechanochemistry serves as an alternative to solution synthesis, but for a wider application within the framework of green chemistry, detailed investigations about reaction mechanisms at the molecular level are crucial [1]. One of current disadvantages of mechanochemistry is insufficient understanding of the influence of milling parameters on energy transfer during ball milling. In this paper, simple one-step trimerization of nickel(II) dibenzoylmethane was used as a model system to systematically examine by in situ Raman spectroscopy the effect of the milling frequency, and the material and mass of milling balls on the mechanochemical oligomerization reaction rate (Figure 1). By varying milling parameters, we have modified energy input required for the chemical transformation leading to different reaction profiles. Based on numerical simulations of the milling processes, a linear relationship was established between the reaction rate and the energy dose absorbed by the sample. Consequently, the energy dose increases with the frequency of impacts, mass and the average ball velocity, but decreases with the hardness of milling media to the minimum energy needed to initiate chemical conversion.

Acid-mediated hydrogen–deuterium exchange reactions (H/D exchange) are among the oldest methods known for labeling of aromatic compounds. Nevertheless, site-selective high-degree deuteration of small molecules is still a complex procedure even though labelled substrates are in high demand. Activation of the aromatic C–H bond by metal centers leads to cyclometalated complexes that in acidic environments can be converted back to the starting ligand. If a deuterated acid is used, deuterium can be introduced to the ligand at the site of the metalation. Recently our group prepared and studied a series of mono- and dicyclopalladated azobenzene complexes (Scheme 1). Experiments involving amino acid cysteine as a side ligand showed that this reaction leads to isolation of the azobenzene ligand. If a deuterated cysteine (Cys4D) is used, site-specific deuteration at ortho position to the azo group of the ligand occurs. Here we present the mechanistic DFT study of cleavage of the cyclopalladated complex by cysteine and formation of the ortho-deuterated azobenzene. Reaction involves formation of the cysteine cyclopalladated complex followed by a transfer of the deuteron from uncoordinated cysteine to the ortho-carbon atom.

Bromination reactions represent one of the most useful transformations in organic chemistry as the reaction delivers products with new C–Br bonds that can be further functionalized. Traditionally, bromination of arenes is conducted in solution using molecular bromine, a corrosive and toxic reagent. In this presentation a solvent-free mechanochemical alternative to carry out the bromination of arenes in a ball mill will be described. Key for the mechanochemical protocol was the use of zeolites as recyclable catalysts which enabled the halogenation to proceed using various solid brominating agents. Posterior transfer of the ball milling approach into a continuous mechanochemical protocol using a twin- screw extruder was also accomplished. This new approach features green chemistry aspects of mechanochemistry, and complements other mechanochemical bromination methodologies.

Cucurbit[n]urils (n = 5-10) are rigid host molecules with hydrophobic cavities that can bind guests in 1:1, 2:1, or 1:2 complexes and have been used previously to bind hydrophobic guests in water often with extreme binding affinities up to 7.2 x 1017 M−1 .[1-3] Solvent-free mechanochemical synthesis (ball milling) has been shown to be a useful method for the preparation of inclusion complexes with cucurbit[7]uril and adamantane and phenyl derivates as model molecules of guests.[4] Therefore, we researched various phenylenediamine derivatives and ferrocene inclusion complexes with Cucurbit[n]uril. Studies included mechanochemical formation of inclusion complex in vibrational mill. In situ monitoring of grinding reactions by Raman spectroscopy enabled direct insight into the successful formation of the HostGuest complex and the mechanochemistry of grinding.[5]. Host-guest complexation in the solidstate was studied and characterized by X-ray powder diffraction, FTIR, NMR spectroscopy, and differential scanning calorimetry.

Primjena mehanokemijskih postupaka na industrijskoj skali ograničena je radi nepotpunog razumijevanja mehanizama mehanokemijskih reakcija. U ovom radu istraživan je utjecaj frekvencije mljevenja, materijala i mase kuglica na mehanokemijsku oligomerizaciju niklovog(II) dibenzoilmetanata primjenom Ramanove spektroskopije in situ. Pripravljeni spojevi karakterizirani su rendgenskom difrakcijom, XRD, i termogravimetrijskom analizom. Izabrani modelni sustav jednostupanjska je reakcija trimerizacije čija se brzina povećava porastom frekvencije mljevenja, korištenjem većeg promjera kuglica za mljevenje kao i materijala veće gustoće (ahat < cirkonijev dioksid < nehrđajući čelik < volframov karbid). Praćeni parametri bitno utječu na prijenos energije te reaktivnost sustava što je i potvrđeno kinetičkim reakcijskim profilima i numeričkim simulacijama. Dobiveni rezultati od velike su važnosti za optimizaciju mehanokemijskih procesa kao i njihovo dublje razumijevanje.

Zeolites are microporous crystalline materials broadly used as ion-exchangers, adsorbents and catalysts, among others. They are comprised of TO4 tetrahedra which assemble into a supramolecular network with well-defined channels and cavities.1 Today there are 253 structural types of zeolites with different properties and applications.2 Zeolites are usually synthesized over a few days or weeks by solvothermal methods.1 This kind of process is very inefficient in terms of time and resources. In an attempt to develop eco-friendlier, cheaper and more rapid methods of zeolite synthesis, we have employed thermally controlled mechanochemistry ; enabling mechanochemical reactions to be conducted at temperatures up to 250 °C.3 We have found that by milling zeolite Y with different bases (sodium, potassium and cesium hydroxide) with and without water at 110 °C yields new products in only a few minutes. Combined with in situ PXRD measurements collected at the Deutsches Elektronen-Synchrotron (DESY), we have identified the intermediate and final products along with the time of their formation. This is to our knowledge the first successful interzeolite conversion carried out by mechanochemical means.

The collected set of results clearly illustrates the potential of mechanochemical treatment for carrying out interzeolite conversion reactions in a rather short period of time and with significantly reduced water content with respect to conventionally performed reactions of this type. Evidently, this makes the ball milling procedure more ecologically sound than the classic method of interzeolite conversion. Further, the phase composition and the properties of the formed material arise as an outcome of the subtle interplay of kinetic and thermodynamic factors. They ensue as a consequence of numerous events (condensation, (de)polymerization, (de)protonation, etc.) taking place concurrently in the synthesis mixture. It is expected that the findings reported here will provide guidelines for designing and conducting further ball milling experiments to achieve even more environment-friendly process of interzeolite conversion.

Mechanochemistry has the promising perspective to transcend traditional solution synthesis by supporting the concepts of green chemistry. However, a deeper understanding of the fundamental principles of mechanochemical processes require also the knowledge about energy transfer during milling, which is essential for optimisation of mechanochemistry in the industrial context and transferability of mechanochemical processing accross different milling technologies.1 Here, by using in situ Raman spectroscopy, we report a systematic investigation how milling parameters, milling frequency as well as size and mass of a milling ball, influence the kinetics of a purely mechanochemical oligomerization of nickel(II) dibenzoylmethanate. A detailed kinetic analysis accompanied by advanced numerical simulations revealed a linear correlation between the kinetic rate constant and the impact energy needed for the chemical transformation.

Halogenated arenes are important precursors in cross-coupling reactions and components of numerous organic and natural compounds, pharmaceuticals, new materials, etc. Although ball milling methods are widely used for the synthesis of various classes of compounds, their application in the synthesis of halogenated arenes is still scarce. In this work, we present a catalyst- and additive- free protocol for the halogenation of 4, 4’- functionalized electron-rich azobenzenes in a ball mill under NG conditions, using N-halosuccinimides as a halogen source. These transformations resulted in the products halogenated in ortho- position to the electron-donating groups, while the reactions of azobenzenes containing dimethylamino group as substituent with NIS led to imidation products. The protocols described above provide a mild and environmentally friendly approach for the halogenation of Csp2–H bonds and the imidation of Csp3–H bonds of azobenzenes with electron-donating substituents by electrophilic activation using N-halosuccinimides.

Zeoliti su mikroporozni kristalni materijali koji se sastoje od TO4 tetraedara koji grade supramolekulske mreže s dobro definiranim kanalima i šupljinama. Danas su poznata 253 različita strukturna tipa zeolita. Našli su široku primjenu kao ionski izmjenjivači, adsorbensi i katalizatori, zbog čega su vrlo značajni materijali u industriji. Klasične metode priprave zeolita obično uključuju korištenje solvotermalnih metoda i uvjeta koje traju nekoliko dana ili čak tjedana i troše puno resursa i vremena.[1] S ciljem da se pronađu jeftinije i brže metode sinteze zeolita, koristili smo termički kontroliranu mehanokemiju – novo razvijenu tehniku koja omogućuje izvođenje mehanokemijskih reakcija pri povišenim temperaturama do 250 °C. [2] U ovom smo radu pokazali kako se mljevenjem zeolita Y s različitim bazama (kalijevim, natrijevim i cezijevim hidroksidom), s i bez vode, pri 110 °C produkt može dobiti u samo nekoliko minuta, dok solvotemalnim metodama takav produkt ne nastaje niti nakon dva dana. Uz pomoć in situ PXRD mjerenja napravljenih na Deutsches Elektronen- Synchrotronu (DESY), identificirali smo intermedijerne faze, finalne produkte te vremena njihovog nastajanja, dajući mogućnost provođenja selektivnih reakcija interzeolitne konverzije zeolita Y kako bi se priredili drugi zeoliti.