This collection highlights the diversity and quality of research conducted by Latin American scientists, and emphasizes the interdisciplinary nature of crystallography, highlighting its connections with other research fields and presenting innovative and thought-provoking studies.
The synthesis and characterization of six novel coumarin derivatives containing O and S atoms are described here, namely, ethyl 2-oxo-2H-chromene-3-carboxylate, C12H10O4 (S1a), ethyl 2-sulfanylidene-2H-chromene-3-carboxylate, C12H10O3S (S2a), ethyl 2-sulfanylidene-2H-chromene-3-carbothioate, C12H10O2S2 (S3a), ethyl 8-methoxy-2-oxo-2H-chromene-3-carboxylate, C13H12O5 (S1b), ethyl 8-methoxy-2-sulfanylidene-2H-chromene-3-carboxylate, C13H12O4S (S2b), and ethyl 8-methoxy-2-sulfanylidene-2H-chromene-3-carbothioate, C13H12O3S2 (S3b). Compounds S1a/b were synthesized in good yields following the Knoevenagel condensation method. The thiocarbonyl analogues of these compounds, S2a/b and S3a/b, were obtained using Lawesson's reagent as a thionating compound. The structures of S2a/b and S3a/b were confirmed using FT-IR, 1H and 13C NMR, and UV-Vis spectroscopy, and single-crystal X-ray diffraction. Hirshfeld surface and energy framework analyses show that stacked π-π ring interactions occur for all the structures obtained here, and various hydrogen-bond interactions link the stacks to form three-dimensional energy frameworks.
Electron counting helped realize the resolution revolution in single-particle cryoEM and is now accelerating the determination of MicroED structures. Its advantages are best demonstrated by new direct electron detectors capable of fast (kilohertz) event-based electron counting (EBEC). This strategy minimizes the inaccuracies introduced by coincidence loss (CL) and promises rapid determination of accurate structures. We used the Direct Electron Apollo camera to leverage EBEC technology for MicroED data collection. Given its ability to count single electrons, the Apollo collects high-quality MicroED data from organic small-molecule crystals illuminated with incident electron beam flux densities as low as 0.01-0.045 e-/Å2/s. Under even the lowest flux density (0.01 e-/Å2/s) condition, fast EBEC data produced ab initio structures of a salen ligand (268 Da) and biotin (244 Da). Each structure was determined from a 100° wedge of data collected from a single crystal in as few as 50 s, with a delivered fluence of only ∼0.5 e-/Å2. Fast EBEC data collected with a fluence of 2.25 or 3.33 e-/Å2 also facilitated a 1.5 Å structure of thiostrepton (1665 Da). While refinement of these structures appeared unaffected by CL, a CL adjustment applied to EBEC data further improved the distribution of intensities measured from the salen ligand and biotin crystals. However, CL adjustment only marginally improved the refinement of their corresponding structures, signaling the already high counting accuracy of detectors with counting rates in the kilohertz range. Overall, by delivering low-dose structure-worthy data, fast EBEC collection strategies open new possibilities for high-throughput MicroED.
Synthesis experiments were conducted in the ternary Rb2O-CaO-SiO2 system, resulting in the formation of a hitherto unknown compound with the composition Rb2Ca2Si2O7, i.e. dirubidium dicalcium pyrosilicate. Single crystals of sufficient size and quality were recovered from a starting mixture with an Rb2O:CaO:SiO2 molar ratio of 2:1:3. The educts were confined in a lid-covered platinum crucible and gradually cooled from 1050 °C at a rate of 0.3 °C min-1 to 800 °C before being finally quenched in air to ambient conditions. The crystal structure was investigated at -80 and 15 °C from single-crystal X-ray diffraction data, with structure determination performed using direct methods. The compound was found to be of orthorhombic symmetry, belonging to the space group Pmmn (No. 59), with a = 5.7363 (6), b = 13.8532 (12), c = 9.9330 (10) Å, V = 789.34 (13) Å3 and Z = 4 (at 15 °C). The final refinement calculations at ambient temperature converged at R1 = 0.030 and wR2 = 0.076 for 773 observed reflections with I > 2σ(I). The silicate anion is based on pyrosilicate units of composition [Si2O7]6- with point-group symmetry m (Cs). Charge compensation is achieved by the incorporation of rubidium and calcium cations distributed among a total of five independent sites within the asymmetric unit. Two of the nontetrahedrally coordinated cation sites (M4 and M5) are exclusively occupied by calcium cations, which are surrounded by six O atoms in the form of octahedra or trigonal prisms, respectively. The rubidium cations on the M1-M3 sites show more complex coordination environments. The M2 site, for example, is characterized by a tricapped trigonal prism polyhedron. Notably, the M3 site exhibits a 50% population of Ca2+ and Rb+, respectively. The compound shows closer structural resemblances with K2Ca2Si2O7 and can be derived from a hexagonal aristotype with space-group symmetry P63/mmc by displacements of the atoms. The corresponding distortion modes can be classified by certain irreducible representations of the high-symmetry parent phase. Structural investigations were completed by determining the thermal expansion tensor for the temperature interval between -80 and 15 °C.
The potential anticancer drug for leukemia, butane-1,4-diyl bis(2,2,2-trifluoroethane-1-sulfonate) (BFS), C8H12F6O6S2, a fluorine analogue of busulfan, was synthesized and analyzed for the first time using various experimental and theoretical techniques, including NMR spectroscopy, single-crystal X-ray diffraction, thermochemistry and computational methods. Crystallographic analysis revealed that BFS crystallizes in the monoclinic space group P21/n, with its structure stabilized by covalent bonding, torsional flexibility and hydrogen bonding. Efficient packing and symmetry interactions balance density and stability. Theoretical calculations, using molecular mechanics, semi-empirical methods and density functional theory (DFT), with functionals such as M06-2X-D3, M08-HX-D3, CAM-B3LYP-D3 and B97D3, confirmed that BFS adopts a compact geometry stabilized by intramolecular hydrogen bonds, consistent with spectroscopic measurements. Gravimetric analysis was used to estimate the solubility of BFS in water at room temperature.
Organic amine crown ether supramolecular compounds, based on crystal engineering design, have already made significant research progress in functional devices such as ferroelectrics, ferroelastics and nonlinear optical materials, especially those involving anilinium cations, which have attracted widespread attention. In comparison, benzylammonium cations have been less studied and most counter-ions are inorganic metal salt anions, crystallizing in centrosymmetric space groups without nonlinear optical (NLO) response. By changing the anion, we have obtained two types of crown ether inclusion compounds, namely, benzylammonium bis(methanesulfonyl)azanide-1,4,7,10,13,16-hexaoxacyclooctadecane (1/1), C7H10N+·C2H6NO4S2-·C12H24O6 or [(BA)(18-crown-6)][DMSA] [BA = benzylammonium and DMSA = bis(methanesulfonyl)azanide] and benzylammonium (methanesulfonyl)(trifluoromethylsulfonyl)azanide-1,4,7,10,13,16-hexaoxacyclooctadecane (1/1), C7H10N+·C2H3F3NO4S2-·C12H24O6 or [(BA)(18-crown-6)][TfNMs] [TfNMs = (methylsulfonyl)(trifluoromethylsulfonyl)azanide]. Both compounds crystallize in the polar chiral space group P21 at 100 K and exhibit a clear second harmonic generation (SHG) active characteristic, showing potential for use in quadratic nonlinear optical fields. Moreover, this work is the first to report crown-ether-based compounds using TfNMs as the counter-ion, enriching the choice of counter-anion for crown-ether-based compounds.
The first X-ray crystal structure is reported for [dihydrobis(pyrazol-1-yl)borato-κ2N,N']tris(tetrahydrofuran-κO)magnesium(II) tetrabenzylborate, [Mg(C6H8BN4)(C4H8O)3][B(C7H7)4)], (I), which was obtained by reacting crude [K(C6H8BN4)] with an equimolar amount of (PhCH2)MgCl in tetrahydrofuran at room temperature and was isolated as a side product. The crystal structure analysis reveals the existence of a pronounced boat-shaped central Mg(N-N)2B moiety with a weak pseudo-axial B-H...Mg interaction. The existence of a novel counter-ion, [B(C7H7)4)]-, has been demonstrated by the first evidence of reactivity between KBH4 and (PhCH2)MgCl (in a 1:4 molar ratio) to give a polymeric potassium tetrabenzylborate, poly[[μ4-dihydrobis(pyrazol-1-yl)borato-κ2N,N']potassium(I)], [K{B(C7H7)4}]n, (II), with predominant (η6-C6H5)...K, and (η3-C6H5)...K interactions. Density functional theory (DFT) calculations were conducted to determine the energy, the frontier molecular orbitals (HOMO and LUMO) and the molecular electrostatic potential (MEP) in order to explain the stability of the structures in the experimental results for complexes (I) and (II).
Two crystal structures of bis(2,3,5,6-tetramethylphenyl)phosphine, C20H27P, are reported constituting the first recorded case of polymorphism in a secondary phosphine (R2PH). The two structures differ in their conformation and, as a result, the steric hindrance experienced at the phosphorus centre is observed to be dependent on the packing environment. Each polymorph exhibits a distinct supramolecular structure; in polymorph I the molecules are arranged in columns in two directions, whereas polymorph II forms layers. There is a distinct lack of significant intermolecular interactions in either form, with the exception of some weak Me...π interactions observed in polymorph II. These interactions are likely the cause of the variation in the C-P-C angles observed between the two structures.
In this study, we present a new N-derivative of L-phenylalanine with 2-naphthaldehyde (PN), obtained by the Schiff base formation procedure and its subsequent reduction. This compound was crystallized as a zwitterion {2-[(naphthalen-2-ylmethyl)azaniumyl]-3-phenylpropanoate, C20H19NO2}, as an anion in a sodium salt (catena-poly[[diaquasodium(I)-di-μ-aqua] 2-[(naphthalen-2-ylmethyl)amino]-3-phenylpropanoate monohydrate], {[Na(H2O)4](C20H18NO2)·H2O}n), as a cation in a chloride salt [(1-carboxy-2-phenylethyl)(naphthalen-2-ylmethyl)azanium chloride acetic acid monosolvate, C20H20NO2+·Cl-·CH3COOH], and additionally acting as a ligand in the pentacoordinated zinc compound aquabis{2-[(naphthalen-2-ylmethyl)amino]-3-phenylpropanoato-κO}zinc(II), [Zn(C20H18NO2)2(H2O)] or [Zn(PN)2(H2O)], denoted (PN-Zn), with the amino acid derivative in its carboxylate form. Interestingly, both enantiomers of the zinc complex co-exist within the crystalline structure, one constructed by the ligands with the L (or S) configuration and the other with the ligands having the D (or R) configuration, represented as L,L-PN-Zn and D,D-PN-Zn, respectively. Also, in the structure of the zwitterion, the racemate L,D is observed. These results imply that chirality inversion of the amino acid derivative synthesized from enantiomerically pure L-phenylalanine is taking place, a phenomenon known as oscillatory transenantiomerization. The analysis of these crystal structures reveals that they are primarily stabilized through electrostatic interactions assisted by hydrogen bonds. An interesting finding is that the conformation of PN varies along this family: it is unfolded in the zwitterionic and cationic forms, and folded in the anionic form. To evaluate such conformational differences, we propose the use of a dimensionless Shape Factor quantity defined as the Structural Aspect Ratio (SAR), computed from the geometrical features of the parallelepiped that tightly encloses a conformer constructed by rigid spheres. This parameter provides a simple but useful tool to distinguish conformational differences, providing insights that complement traditional structural analyses. The study of the structural features, conformational diversity, chirality and supramolecular properties of these compounds is also supported by density functional theory (DFT) calculations.
The article by Guzmán-Hernández & Jancik [(2024). Acta Cryst. C80, 766-774] is an excellent example of how QC-QCT (quantum crystallography-quantum chemical topology) methodology can extract structural information from a crystal.
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