Studying the mechanisms of racemic resolution of the model compound N-(3-nitrobenzoyl)aspartic acid by formation and fractional crystallization of brucinium diastereomeric salts, we found that depending on the environment (solvents) the recognition leads to effective or ineffective racemic resolution. During successful racemic resolution, bis(brucinium)-N-(3-nitrobenzoyl)-L-aspartate hydrate methanol solvate precipitates as the first fraction. In turn, double salts and solid solution are formed in the fruitless racemic resolution. Formation of the double salt results from recognition of centrosymmetric anionic dimers on the common brucinium corrugated layer. The common brucinium corrugated layer together with its solvation sphere participates in recognition of the anions, leading to formation of the solid solution. Changing the solvent system according to its dielectric constant, more dispersive or more directional interactions gain importance. The racemic resolution and formation of bis(brucinium)-N-(3-nitrobenzoyl)-L-aspartate hydrate methanol solvate are related to three-point recognition of the anions by brucinium self-assembly.

Molecular−ionic crystals, which are built of organic cations and discrete anionic components with AMX 4 stoichiometry, easily induce structural order−disorder transitions. These phenomena can generate interesting ferroic properties. Herein, we have synthesized and characterized a novel AMX 4 -type crystal, (C 8 H 20 N)-[BF 4 ] (1). Thermal analysis, (DSC, TGA) indicates one structural phase transition (PT) at 257/259 K (cooling/heating, I ↔ II). The structure of 1 has been solved at two temperatures, 120 and 298 K, with single-crystal X-ray diffraction. The high-temperature phase I (1-HT) shows a tetragonal structure with the space group I4/mmm, whereas the low-temperature phase II (1-LT) is monoclinic with the space group P2 1 /a. The PT belongs, thus, to the improper ferroelastic species 4/mmmF2/m with a 4-fold multiplication of the unit cell. All four possible ferroelastic domains of phase II have been identified in polarized light microscopy. The monoclinic deformation has been shown to give rise to a characteristic parquet-like texture preserving integrity of 1. The molecular mechanism of PT involving changes in the cationic and anionic dynamics has been postulated based on measurements of the 1 H and 19 F NMR spin−lattice relaxation times, T 1 , and second-moment, M 2 , and of the dielectric responses. The ac and dc conductivity measurements have been used to determine the transport properties of the charge carriers in 1. The intrinsic features of molecular−ionic crystals, such as the dynamics of molecular motions, structural instability with a lattice-symmetry change, formation of domain patterns, etc., provide a crystal with unique ferroelastic properties. This type of crystal may be useful as an functional material.

The binuclear coordination compound of copper­(I) tetrafluoroborate with the ligand N-[3-(4-(chlorophenyl)-2-propen-1-ylidene]-4H-1,2,4-triazol-4-amine (pClcinatrz) was investigated in this work. A structural analysis suggests that the photochemical [2 + 2] dimerization can lead to the formation of a tetranuclear or a one- or two-dimensional coordination compound. Among the possible products, the first arose. The photoinduced changes in the crystal were observed by X-ray diffraction methods and by absorption and emission spectroscopy. The compound before and after photochemical reaction exhibits blue emission in the solid state at 77 K. The emission was assigned to metal to ligand charge transfer (MLCT) excited states and was compared to the π*–n emission of the free ligand.