Thermal Reactivity in Metal Organic Materials (MOMs): From Single-Crystal-to-Single-Crystal Reactions and Beyond

Kinetic trapping of 2,4,6-tris(4-pyridyl)benzene and ZnI2 into M12L8 poly-[n]-catenanes using solution and solid-state processes

Here, we show that in a supramolecular system with more than 20 building blocks forming large icosahedral M₁₂L₈ metal-organic cages (MOCs), using the instant synthesis method, it is possible to kinetically trap and control the formation of interlocking M₁₂L₈ nanocages, giving rare M₁₂L₈ TPB-ZnI₂ poly-[n]-catenane. The catenanes are obtained in a one-pot reaction, selectively as amorphous (a1) or crystalline states, as demonstrated by powder X-ray diffraction (powder XRD), thermogravimetric (TG) analysis and ¹H NMR. The 300 K M₁₂L₈ poly-[n]-catenane single crystal X-ray diffraction (SC-XRD) structure including nitrobenzene (1) indicates strong guest binding with the large M₁₂L₈ cage (i.e., internal volume ca. 2600 ų), allowing its structural resolution. Conversely, slow self-assembly (5 days) leads to a mixture of the M₁₂L₈ poly-[n]-catenane and a new TPB-ZnI₂ (2) coordination polymer (i.e., thermodynamic product), as revealed by SC-XRD. The neat grinding solid-state synthesis also yields amorphous M₁₂L₈ poly-[n]-catenane (a1'), but not coordination polymers, selectively in 15 min. The dynamic behavior of the M₁₂L₈ poly-[n]-catenanes demonstrated by the amorphous-to-crystalline transformation upon the uptake of ortho-, meta- and para-xylenes shows the potential of M₁₂L₈ poly-[n]-catenanes as functional materials in molecular separation. Finally, combining SC-XRD of 1 and DFT calculations specific for the solid-state, the role of the guests in the stability of the 1D chains of M₁₂L₈ nanocages is reported. Energy interactions such as interaction energies (E), lattice energies (E*), host-guest energies (E_host-guest) and guest-guest energies (E_guest-guest) were analysed considering the X-ray structure with and without the nitrobenzene guest. Not only the synthetic control achieved in the synthesis of the M₁₂L₈ MOCs but also their dynamic behavior either in the crystalline or amorphous phase are sufficient to raise scientific interest in areas ranging from fundamental to applied sides of chemistry and material sciences.

ZIF-L to ZIF-8 Transformation: Morphology and Structure Controls

The control of the structure, shape, and components of metal-organic frameworks, in which metal ions and organic ligands coordinate to form crystalline nanopore structures, plays an important role in the use of many electrochemical applications, such as energy storage, high-performance photovoltaic devices, and supercapacitors. In this study, systematic controls of synthesis variables were performed to control the morphology of ZIF-8 during the ZIF-L-to-ZIF-8 transformation of ZIF-L, which has the same building block as ZIF-8 but forms a two-dimensional structure. Furthermore, additional precursors or surfactants (Zn2+, 2mIm, and CTAB) were introduced during the transition to determine whether the alteration could be regulated. Lastly, the partial substitution insertion of a new organic precursor, 2abIm, during the ZIF-L-to-ZIF-8 transformation of ZIF-L was achieved, and modulation of the adsorption and pore characteristics (suppression of gate-opening properties of ZIF-8) has been confirmed.

Dehydrohalogenation reactions in second-sphere coordination complexes

The latest advances of solid-state dehydrohalogenation and halogenation reactions of hydrogen bonded halometallate salts from the second sphere coordination perspective are reported. Since the second sphere englobes many different materials, our focus has been limited to outer sphere adducts where protonated organic cations act as outer sphere hydrogen bond donors and transition metal anions act as first sphere hydrogen bond acceptors. This is our attempt to analyze dehydrohalogenation/hydrohalogenation reactions viewed as transformations from the second sphere coordination to first sphere coordination of a complex and vice versa. The examples describe a unique solid-state chemistry and reactivity in outer sphere adducts where C-H, N-H and M-X chemical bonds are cleaved and new M-N and H-X bonds are formed (where M = Cu, Zn, Co, Pt, Pd, Hg and X = Cl, Br). The transformations are induced by external stimuli, mainly by mechanochemical and thermal methods. Different reactivities have been observed depending on the lability of the transition metals, the position of the reacting functional groups in the cations and the relative position of organic cations and metal anions. The reverse hydrohalogenation reactions (i.e., from the first sphere coordination to second sphere coordination) via the gas-solid chemisorption process occur even if the materials are non-porous implying a rather dynamic behaviour of these materials. Moreover, due to the implicit changes in the coordination sphere of transition metal ions, dehydrohalogenation/halogenation reactions allow structure-function correlation to be established, for instance involving optical, sensing and magnetic aspects.

Mechanochemistry: A Green Approach in the Preparation of Pharmaceutical Cocrystals

Mechanochemistry is considered an alternative attractive greener approach to prepare diverse molecular compounds and has become an important synthetic tool in different fields (e.g., physics, chemistry, and material science) since is considered an ecofriendly procedure that can be carried out under solvent free conditions or in the presence of minimal quantities of solvent (catalytic amounts). Being able to substitute, in many cases, classical solution reactions often requiring significant amounts of solvents. These sustainable methods have had an enormous impact on a great variety of chemistry fields, including catalysis, organic synthesis, metal complexes formation, preparation of multicomponent pharmaceutical solid forms, etc. In this sense, we are interested in highlighting the advantages of mechanochemical methods on the obtaining of pharmaceutical cocrystals. Hence, in this review, we describe and discuss the relevance of mechanochemical procedures in the formation of multicomponent solid forms focusing on pharmaceutical cocrystals. Additionally, at the end of this paper, we collect a chronological survey of the most representative scientific papers reporting the mechanochemical synthesis of cocrystals.

Structural elucidation of microcrystalline MOFs from powder X-ray diffraction

Ab initio powder XRD structure solution and MOFs.