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In this work, we report a spiral gas–solid two-phase flow (S-GSF) method based on spiral jet mills for the continuous mechanochemical preparation of urea co-crystals for the first time. Urea–adipic acid (UAA), urea–catechol (URCAT), urea–salicylic acid (USA) and urea–gypsum (URCASU) co-crystals were prepared to demonstrate the feasibility of this approach. The prepared products were characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, differential scanning calorimetry, and thermogravimetric analysis. The results showed that powdered products of the urea co-crystals can be obtained continuously by using this method without the addition of any solvent in the preparation process. Moreover, the UAA co-crystal prepared by S-GSF is in single polycrystalline form I, while form II, which appears in milling and ball milling, is not observed. The transcrystallization behavior among different polymorphs of the UAA co-crystal was studied and their thermodynamic stability was investigated by theoretical calculations. The results indicated that form I is a thermodynamically stable polymorph, and S-GSF is selective for the formation of form I. This phenomenon can be attributed to the fact that S-GSF provides strong mechanical action, as well as a low-temperature reaction environment, which distinguishes it from existing mechanochemical methods.

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Reactions of a photostable bidentate ligand trans-5-styrylpyrimidine (5-Spym) with various silver(I) salts resulted in the formation of coordination polymers (CPs) of composition {[Ag(5-Spym)(H2O)](NO3)} (1), {[Ag(5-Spym)(H2O)](BF4)} (2), {[Ag(5-Spym)](SbF6)} (3), {[Ag(5-Spym)(CF3SO3)]} (4), and {[Ag(5-Spym)(CF3CO2)]} (5). The first four CPs (1–4) were found to be one-dimensional (1D) zigzag coordination polymers, where 5-Spym ligands from the neighbouring chains assembled in an infinite parallel arrangement in a head-to-tail fashion. These CPs exhibited [2 + 2] photocycloaddition reaction in the solid state upon exposure to sunlight. On the other hand, the structure of 5 was determined to be a two-dimensional (2D) coordination polymer, and it was found to be photostable. In these series of compounds, the coordination number of Ag(I) varied from 2 to 4, and the coordination geometries were observed to be linear, T-shaped, trigonal, square planar and distorted tetrahedral. By varying the counteranions in the silver(I) salts, various coordination numbers and geometries, and thus structural diversity, and various solid-state photo-reactivities were observed. The scope of mechanochemistry in accessing these CPs is also discussed. Various types of intermolecular interactions observed in this series of compounds have been verified by analyses of their Hirshfeld surfaces.

A microwave-assisted hydrothermal solution synthesis methodology is used to prepare 3D hierarchical flowery TiO2 microspheres quickly and without calcination. The X-ray diffraction analysis showed that the microspheres are crystallized in the anatase form. Scanning electron microscopy showed that the anatase 3D hierarchical flowery TiO2 microspheres comprised many nm-thick petal-like features evenly distributed in each microsphere. The use of microwaves (for only 10 min at 200 W) acted as the additional source of energy that resulted in the accelerated growth of flowery TiO2 microspheres that otherwise would require long durations to form under only hydrothermal conditions. The microspheres exhibited a large specific surface area of 87.58 m2 g−1 and unique pore characteristics, making them suitable for photocatalysis. In this context, the measured photodegradation efficiency of the 3D hierarchical flowery anatase-TiO2 microspheres in degrading methylene blue dye was as high as 98%.

Bimolecular complexes [ML2(MeOH)2][ML2(EtOH)2], where M is CoII, NiII or CoII0.5NiII0.5 and L is a deprotonated enaminoketone 4-(3′,3,3′-trifluoro-2′-oxopropylidene)-2,2,5,5-tetramethyl-3-imidazolidin-1-oxyl, were found to be isolated in the solid phase from a solution containing MeOH and EtOH in the ratio 1 : 10. Their solids have a layered-polymer structure due to hydrogen bonds between the OH-groups of the coordinated ROH molecules and nitroxide groups of the neighboring [ML2(ROH)2] molecules. The peculiar crystal structures of [ML2(MeOH)2][ML2(EtOH)2] have ordered alternation of methanol- and ethanol-containing supramolecular layers. Detailed crystallochemical analysis showed that during the formation of [ML2(MeOH)2][ML2(EtOH)2] crystals, the MeOH containing layer is significantly deformed in comparison with the [ML2(MeOH)2] structure. This indicates that the ethanol-containing layer determines the crystal formation, while the {ML2(MeOH)2} layer “adjusts” to it. Magnetochemical study of the bimolecular complexes [ML2(MeOH)2][ML2(EtOH)2] revealed magnetic ordering at low temperatures.

Two new metal(II) complexes [M(dmv)2(py)2]·3H2O {M = Ni (1), Zn (2)} were self-assembled from a metal(II) chloride salt, N,N′-dimethylvioluric acid (Hdmv), and pyridine (py). They were isolated as stable crystalline solids and characterized, including by single crystal X-ray diffraction. Structures 1 and 2 are composed of discrete [M(dmv)2(py)2] blocks in which the six-coordinate metal(II) atoms show a slightly distorted {MN4O2} octahedral geometry. The adjacent [M(dmv)2(py)2] units are hydrogen bonded with an involvement of disordered water clusters into 1D H-bonded chains. DFT calculations were used to analyse the unconventional anti-parallel CO⋯CO interaction observed between the monomeric units in 1 and 2. Complex 1 also refers to an elongated tetragonal bipyramid for which a positive zero-field splitting is confirmed by magnetic studies. The complex matches the magnetostructural D-correlation for hexacoordinate Ni(II) complexes with moderate structural and magnetic anisotropy with Dstr = +7.45 pm, and Dmag = +5.11 cm−1. Solution state NMR spectroscopy reveals a slow isomerization process related to the bidentate-N,O and bidentate-O,O coordination of dimethylviolurate in 2, with the barrier ranging from 76.4 to 77.9 kJ mol−1. The 1H NMR spectra confirm the presence of three water molecules which exhibit significant chemical shift dependence on temperature indicating strong binding to the complex in solution phase.

A new polymorf of Schiff base (N-(4′-methoxybenzylidene)-4-aminobenzoic acid) and five of its distinct salts (chloride, bromide, nitrate, hydrosulfate and dihydrophosphate) were designed and synthesized. Detailed examinations were conducted to determine the structure and spectral properties of these compounds, employing methods like X-ray diffraction, as well as IR, UV-Vis and fluorescence spectroscopy. Interestingly, all the synthesized Schiff base salts displayed fluorescence when in solid state. However, the intensity of this fluorescence was found to differ, depending on the type of anion present within the structure. This result provides evidence that a strategy similar to supramolecular complexation can effectively be used to adjust the emission wavelength and control the intensity of fluorescence in the resulting solid materials.