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.

Mechanochemistry provides a rapid, efficient route to metal−organic framework Zn-MOF-74 directly from a metal oxide and without bulk solvent. In situ synchrotron X-ray diffraction monitoring of the reaction course reveals two new phases and an unusual stepwise process in which a close-packed intermediate reacts to form the open framework. The reaction can be performed on a gram scale to yield a highly porous material after activation.M etal−organic frameworks (MOFs) 1 are advanced materials with applications ranging from storage and separation of fuel gases, 2 CO 2 sequestration, 3 and degradation of nerve agents 4 to fuel cells, 5 catalysis, 6 drug delivery 7 and light harvesting. 8 Commercialization of MOFs has highlighted unique synthetic challenges, 9 often involving solvothermal conditions and soluble reagents which, while common in a laboratory, are intractable in large-scale manufacturing due to issues of cost, toxicity, and explosive (nitrates) or corrosive (chlorides) nature. 9,10 It was recently demonstrated that liquid-catalyzed mechanochemistry 11 (e.g., liquid-assisted grinding, LAG) permits facile, room-temperature transformation of safer metal oxide, carbonate, or hydroxide reactants into MOFs, resulting in cleaner, more atom-efficient processes that avoid external bases and production of mineral acids or their salts as byproducts. 12,13 Indeed, MOFs can now be manufactured mechanochemically on a large scale by extrusion. 14 However, scope of mechanochemistry for making currently relevant MOFs remains modest, limited to HKUST-1 and ZIF-8. 15 We now describe the development and mechanistic investigation of a mechanochemical milling approach to Zn-MOF-74, 16 a member of the popular M-MOF-74 (CPO-27) family of materials, 17−21 from stoichiometric ZnO and 2,5-dihydroxyterephthalic acid (H 4 dhta) (Figure 1). By using the very recently introduced technique for real-time in situ X-ray powder diffraction (XRPD) monitoring, 22,23 we reveal a previously not seen mechanism of mechanochemical MOF synthesis, where the formation of a low-density metal−organic structure proceeds via a close-packed reaction intermediate.Without included guests, Zn-MOF-74 has the composition Zn 2 (H 2 O) 2 (dhta), consisting of Zn 2+ coordinated by H 4 dhta anions and water. We attempted the synthesis of Zn-MOF-74 on 1.1 mmol scale (∼400 mg, see SI) by milling ZnO and H 4 dhta in 2:1 stoichiometric ratio, using 250 μL of water as the grinding liquid. 24 The liquid-to-solid ratio (η) 25 of 0.625 μL/mg was selected based on our previous experience in LAG mechanosynthesis of open MOFs. 13a,15a In situ experiments were done at the European Synchrotron Radiation Facility (ESRF) beamline ID15B using X-rays of 0.142 Å wavelength and also at a new measurement site at the Deutsches Elektronen-Synchroton (DESY) beamline P02.1, which provided improved signal-tonoise ratio and higher resolution data by using 0.207 Å radiation. 22,23 Milling was conducted using a modified Retsch mill operating at 30 Hz, in a 14 mL poly(methy...

We provide the first combined experimental and theoretical evaluation of how differences in ligand structure and framework topology affect the relative stabilities of isocompositional (i.e., true polymorph) metal-organic frameworks (MOFs). We used solution calorimetry and periodic DFT calculations to analyze the thermodynamics of two families of topologically distinct polymorphs of zinc zeolitic imidazolate frameworks (ZIFs) based on 2-methyl- and 2-ethylimidazolate linkers, demonstrating a correlation between measured thermodynamic stability and density, and a pronounced effect of the ligand substituent on their stability. The results show that mechanochemical syntheses and transformations of ZIFs are consistent with Ostwald's rule of stages and proceed toward thermodynamically increasingly stable, more dense phases.

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.

We report the first systematic experimental and theoretical study of the relationship between the linker functionalization and the thermodynamic stability of metal–organic frameworks (MOFs) using a model set of eight isostructural zeolitic imidazolate frameworks (ZIFs) based on 2-substituted imidazolate linkers. The frameworks exhibit a significant (30 kJ·mol–1) variation in the enthalpy of formation depending on the choice of substituent, which is accompanied by only a small change in molar volume. These energetics were readily reproduced by density functional theory (DFT) calculations. We show that these variations in the enthalpy of MOF formation are in linear correlation to the readily accessible properties of the linker substituent, such as the Hammett σ-constant or electrostatic surface potential. These results provide the first quantifiable relationship between the MOF thermodynamics and the linker structure, suggesting a route to design and tune MOF stability.