Mechanochemical grinding reactions

Mechanochemical grinding reactions are known for providing a pathway to products that are not accessible by any other procedures due to the essentially solid reaction environment. In the context of solid-state tautomerism, the mechanochemistry has recently almost become an unavoidable technique, yielding new tautomeric forms as in the case of the keto form of thiobarbituric acid [25] or the enol form of barbituric acid [78]. Milling was also used for facilitating the solid-state transformation between the solid tautomeric forms, such as the mechanochemical conversion of keto-amino tautomer of 6-[(3-hydroxy-pyridin-2-ylamino)-methylene]-cyclohexa-2,4-dienone to more stable enol form. Scheme 13.10 [45]. These examples will be discussed in more detail in the latter sections. 

Required Schiff base substrates were also prepared in solvent-free ball-mill conditions by mechanochemical transimination reaction. Grinding of an equimolar amount of the methyl ester of glycine hydrochloride and benzophenone imine for 3 h exclusively produced Schiff base (Scheme 2.60). Aqueous washing eliminated ammonium chloride, and the pure product was obtained without recrystallization in nearly quantitative yield. The reaction time was reduced to 3h compared with one night for the synthesis in solution. 

As with the hydrothermal method, solid-solid grinding reactions, or mechanochemical reactions as they are also called, is a method with long tradition in solid state chemistry that has recently been applied also to molecular chemistry [56,57]. This method has several advantages ... 

As early as in the fourth century BC, native cinnabar was rubbed with vinegar in a copper mortar with a copper pestle yielding the liquid metal, that is, mercury [19]. This is probably the most ancient documented chemical recipe and describes a mechanochemical reaction, a chemical reaction that is induced by the direct absorption of mechanical energy [20]. A small amount of hquid accelerates or even enables mechanochemical reactions between solids. Mechanical treatment provides energy to break chemical bonds, or facihtates grinding reactions, which proceed due to an increase of the contact area between the sohd reactants. 

The possibility to form a porous material through mechanochemical grinding was first demonstrated in the acid-base reaction between copper acetate and Hina, which led to the construction of the open copper(II) isonicotinate framework Cu(ina)2. Neat grinding resulted in the quantitative formation of the porous framework, with the acetic acid and water byproducts most likely included in the pores (Figure 11a). The same approach was subsequently used to construct the industrially relevant open framework HKUST-1 (pore diameter about 9 A) by... 

Note Shearing, stretching, and grinding are typical methods for the mechanochemical generation of reactive sites, usually macroradicals, in polymer chains that undergo mechanochemical reactions. 

Both solid-solid and solid-gas types of reactions lead from solid reactants to a solid product without the use of solvents. Solvent-less processes, however, are not necessarily solid-state processes. Indeed, it has been argued [8d,e] that many solid-state syntheses cannot be regarded as bona fide solid-solid reactions because they occur with the intermediary of a liquid phase, such as a eutectic phase or a melt, or may require destruction of the crystals prior to reaction. This latter situation is often observed, for instance, in the case of reactions activated by co-grinding, since the heat generated in the course of the mechanochemical process can induce local melting at the interface between the different crystals, or when kneading, i.e. grinding in the presence of small amounts of solvent, takes place (vide infra). 

Certain solid phases, on the other hand, cannot be obtained (even as microcrystalline powders) by crystallization experiments, but instead can be generated only by other types of preparation procedure. Some types of preparation processes commonly (or in some cases inherently) yield microcrystaUine products, including (1) preparation of materials directly from solid-state chemical reactions (see Sect. 6.6), (2) preparation of materials by solid-state desolvation processes (see Sect. 6.4), (3) preparation of materials by solid-state grinding (mechanochemical) processes (see Sect. 6.2), and (4) preparation of materials directly by rapid precipitation from solution (as opposed to crystallization) (see Sect. 6.7). Again, structure determination from powder XRD data may represent the only opportunity for determining the structural properties of new solid phases obtained by such processes. 

To enhance reactivity in ceramic reactions, intimate mixtures of small particles are preferred. Vigorous grinding, however, does not only reduce the particle size. Using high energy mills, considerable local pressures and temperatures may be achieved. So-called mechanochemical reactions are very rare with fluorides ... 

Mechanochemical reactions are carried ont by means of mechanical activation in high energetic apparatus. Grinding devices with high load rates are used for this purpose, such as planetary, vibratory centrifugal, shock-reflective and ring roll mills, attritors, etc. 

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