Identifying and characterizing metal-binding sites (MBS) within macromolecular structures is imperative for elucidating their biological functions. CheckMyMetal (CMM) is a web based tool that facilitates the interactive valid­ation of MBS in structures determined through X-ray crystallography and cryo-electron microscopy (cryo-EM). Recent updates to CMM have significantly enhanced its capability to efficiently handle large datasets generated from cryo-EM structural analyses. In this study, we address various challenges inherent in validating MBS within both X-ray and cryo-EM structures. Specifically, we examine the difficulties associated with accurately identifying metals and modeling their coordination environments by considering the ongoing reproducibility challenges in structural biology and the critical importance of well annotated, high-quality experimental data. CMM employs a sophisticated framework of rules rooted in the valence bond theory for MBS validation. We explore how CMM validation parameters correlate with the resolution of experimentally derived structures of macromolecules and their complexes. Additionally, we showcase the practical utility of CMM by analyzing a representative cryo-EM structure. Through a comprehensive examination of experimental data, we demonstrate the capability of CMM to advance MBS characterization and identify potential instances of metal misassignment.

The TIR (Toll/interleukin-1 receptor) domain represents a vital structural element shared by proteins with roles in immunity signalling pathways across phyla (from humans and plants to bacteria). Decades of research have finally led to identifying the key features of the molecular basis of signalling by these domains, including the formation of open-ended (filamentous) assemblies (responsible for the signalling by cooperative assembly formation mechanism, SCAF) and enzymatic activities involving the cleavage of nucleotides. We present a historical perspective of the research that led to this understanding, highlighting the roles that different structural methods played in this process: X-ray crystallography (including serial crystallography), microED (micro-crystal electron diffraction), NMR (nuclear magnetic resonance) spectroscopy and cryo-EM (cryogenic electron microscopy) involving helical reconstruction and single-particle analysis. This perspective emphasizes the complementarity of different structural approaches.

3D electron diffraction (3DED) is increasingly employed to determine molec­ular and crystal structures from micro-crystals. Indomethacin is a well known, marketed, small-molecule non-steroidal anti-inflammatory drug with eight known polymorphic forms, of which four structures have been elucidated to date. Using 3DED, we determined the structure of a new ninth polymorph, σ, found within an amorphous solid dispersion, a product formulation sometimes used for active pharmaceutical ingredients with poor aqueous solubility. Subsequently, we found that σ indomethacin can be produced from direct solvent evaporation using di­chloro­methane. These results demonstrate the relevance of 3DED within drug development to directly probe product formulations.

X-ray and neutron crystallography, as well as cryogenic electron microscopy (cryo-EM), are the most common methods to obtain atomic structures of biological macromolecules. A feature they all have in common is that, at typical resolutions, the experimental data need to be supplemented by empirical restraints, ensuring that the final structure is chemically reasonable. The restraints are accurate for amino acids and nucleic acids, but often less accurate for substrates, inhibitors, small-molecule ligands and metal sites, for which experimental data are scarce or empirical potentials are harder to formulate. This can be solved using quantum mechanical calculations for a small but interesting part of the structure. Such an approach, called quantum refinement, has been shown to improve structures locally, allow the determination of the protonation and oxidation states of ligands and metals, and discriminate between different interpretations of the structure. Here, we present a new implementation of quantum refinement interfacing the widely used structure-refinement software Phenix and the freely available quantum mechanical software ORCA. Through application to manganese superoxide dismutase and V- and Fe-nitro­genase, we show that the approach works effectively for X-ray and neutron crystal structures, that old results can be reproduced and structural discrimination can be performed. We discuss how the weight factor between the experimental data and the empirical restraints should be selected and how quantum mechanical quality measures such as strain energies should be calculated. We also present an application of quantum refinement to cryo-EM data for particulate methane monooxygenase and show that this may be the method of choice for metal sites in such structures because no accurate empirical restraints are currently available for metals.

Three solid solutions of [CH3NH3]CoxNi1−x(HCOO)3, with x = 0.25 (1), x = 0.50 (2) and x = 0.75 (3), were synthesized and their nuclear structures and magnetic properties were characterized using single-crystal neutron diffraction and magnetization measurements. At room temperature, all three compounds crystallize in the Pnma orthorhombic space group, akin to the cobalt and nickel end series members. On cooling, each compound undergoes a distinct series of structural transitions to modulated structures. Compound 1 exhibits a phase transition to a modulated structure analogous to the pure Ni compound [Cañadillas-Delgado, L., Mazzuca, L., Fabelo, O., Rodríguez-Carvajal, J. & Petricek, V. (2020). Inorg. Chem. 59, 17896–17905], whereas compound 3 maintains the behaviour observed in the pure Co compound reported previously [Canadillas-Delgado, L., Mazzuca, L., Fabelo, O., Rodriguez-Velamazan, J. A. & Rodriguez-Carvajal, J. (2019). IUCrJ, 6, 105–115], although in both cases the temperatures at which the phase transitions occur differ slightly from the pure phases. Monochromatic neutron diffraction measurements showed that the structural evolution of 2 diverges from that of either parent compound, with competing hydrogen bond interactions that drive the modulation throughout the series, producing a unique sequence of phases. It involves two modulated phases below 96 (3) and 59 (3) K, with different q vectors, similar to the pure Co compound (with modulated phases below 128 and 96 K); however, it maintains the modulated phase below magnetic order [at 22.5 (7) K], resembling the pure Ni compound (which presents magnetic order below 34 K), resulting in an improper modulated magnetic structure. Despite these large-scale structural changes, magnetometry data reveal that the bulk magnetic properties of these solid solutions form a linear continuum between the end members. Notably, doping of the metal site in these solid solutions allows for tuning of bulk magnetic properties, including magnetic ordering temperature, transition temperatures and the nature of nuclear phase transitions, through adjustment of metal ratios.

A significant part of the present and future of optoelectronic devices lies on thin multilayer heterostructures. Their optical properties depend strongly on strain, being essential to the knowledge of the stress level to optimize the growth process. Here the structural and microstructural characteristics of sub-micron a-ZnO epilayers (12 to 770 nm) grown on r-sapphire by metal–organic chemical vapour deposition are studied. Morphological and structural studies have been made using scanning electron microscopy and high-resolution X-ray diffraction. Plastic unit-cell distortion and corresponding strain have been determined as a function of film thickness. A critical thickness has been observed as separating the non-elastic/elastic states with an experimental value of 150–200 nm. This behaviour has been confirmed from ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy measurements. An equation that gives the balance of strains is proposed as an interesting method to experimentally determine this critical thickness. It is concluded that in the thinnest films an elongation of the Zn—O bond takes place and that the plastic strained ZnO films relax through nucleation of misfit dislocations, which is a consequence of three-dimensional surface morphology.

Traditional X-ray methods are extensively applied to commercial cement samples in order to determine their physical and chemical properties. Powder patterns are routinely used to quantify the composition of these phase mixtures, but structure determination becomes difficult because of reflection overlapping caused by the high number of different crystal structures. The fast-growing 3D electron diffraction technique and its related automated acquisition protocols arise as a potentially very interesting tool for the cement industry, since they enable the fast and systematic acquisition of diffraction data from individual particles. In this context, electron diffraction has been used in the investigation of the different crystalline phases present in various commercial clinkers for cement. Automated data collection procedures and subsequent data processing have enabled the structural characterization of the different crystal structures from which the α'H polymorph of Ca2SiO4 (belite) exhibited satellite reflections. Its average crystal structure has been known since 1971 and satellite reflections have been reported previously, yet the modulation was never fully described by means of the superspace formalism. Here, the incommensurately modulated structure is solved and refined using harmonic and crenel functions in the superspace group Pnma(α00)0ss, showing the potential of 3D electron diffraction for systematic crystallographic characterizations of cement. A full description of the different belite polymorphs is provided considering this modulated structure.

Quantum crystallography is an emerging research field of science that has its origin in the early days of quantum physics and modern crystallography when it was almost immediately envisaged that X-ray radiation could be somehow exploited to determine the electron distribution of atoms and molecules. Today it can be seen as a composite research area at the intersection of crystallography, quantum chemistry, solid-state physics, applied mathematics and computer science, with the goal of investigating quantum problems, phenomena and features of the crystalline state. In this article, the state-of-the-art of quantum crystallography will be described by presenting developments and applications of novel techniques that have been introduced in the last 15 years. The focus will be on advances in the framework of multipole model strategies, wavefunction-/density matrix-based approaches and quantum chemical topological techniques. Finally, possible future improvements and expansions in the field will be discussed, also considering new emerging experimental and computational technologies.

A few real case examples are presented on how to report magnetic structures, with precise step-by-step explanations, following the guidelines of the IUCr Commission on Magnetic Structures [Perez-Mato et al. (2024). Acta Cryst. B80, 219–234]. Four examples have been chosen, illustrating different types of single-k magnetic orders, from the basic case to more complex ones, including odd-harmonics, and one multi-k order. In addition to acquainting researchers with the process of communicating commensurate magnetic structures, these examples also aim to clarify important concepts, which are used throughout the guidelines, such as the transformation to a standard setting of a magnetic space group.

Synthesis experiments were conducted in the quaternary system K2O–Na2O–CaO–SiO2, resulting in the formation of a previously unknown compound with the composition K0.72Na1.71Ca5.79Si6O19. Single crystals of sufficient size and quality were recovered from a starting mixture with a K2O:Na2O:CaO:SiO2 molar ratio of 1.5:0.5:2:3. The mixture was confined in a closed platinum tube and slowly cooled from 1150°C at a rate of 0.1°C min−1 to 700°C before being finally quenched in air. The structure has tetragonal symmetry and belongs to space group P4122 (No. 91), with a = 7.3659 (2), c = 32.2318 (18) Å, V = 1748.78 (12) Å3, and Z = 4. The silicate anion consists of highly puckered, unbranched six-membered oligomers with the composition [Si6O19] and point group symmetry 2 (C2). Although several thousands of natural and synthetic oxosilicates have been structurally characterized, this compound is the first representative of a catena-hexasilicate anion, to the best of our knowledge. Structural investigations were completed using Raman spectroscopy. The spectroscopic data was interpreted and the bands were assigned to certain vibrational species with the support of density functional theory at the HSEsol level of theory. To determine the stability properties of the novel oligosilicate compared to those of the chemically and structurally similar cyclo­silicate combeite, we calculated the electronegativity of the respective structures using the electronegativity equalization method. The results showed that the molecular electronegativity of the cyclo­silicate was significantly higher than that of the oligostructure due to the different connectivities of the oxygen atoms within the molecular units.

An exhaustive search for missing symmetry was performed for 223 076 entries in the ICSD (2023-2 release). Approximately 0.65% of them can be described with higher symmetry than reported. Out of the identified noncentrosymmetric entries, ∼74% can be described by centrosymmetric space groups; this has implications for compatible physical properties. It is proposed that the information on the correct space group is included in the ICSD.

The reaction of cadmium bromide tetra­hydrate with 3-amino­pyrazole (3-apz) in ethano­lic solution leads to tautomerization of the ligand and the formation of crystals of the title compound, catena-poly[[di­bromido­cadmium(II)]-bis­(μ-3-amino-1H-pyrazole)-κ2N3:N2;κ2N2:N3], [CdBr2(C3H5N3)2]n or [CdBr2(3-apz)2]n. Its asymmetric unit consists of a half of a Cd2+ cation, a bromide anion and a 3-apz mol­ecule. The Cd2+ cations are coordinated by two bromide anions and two 3-apz ligands, generating trans-CdN4Br2 octa­hedra, which are linked into chains by pairs of the bridging ligands. In the crystal, the ligand mol­ecules and bromide anions of neighboring chains are linked through inter­chain hydrogen bonds into a two-dimensional network. The inter­molecular contacts were qu­anti­fied using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing the relative qu­anti­tative contributions of the weak inter­molecular contacts.

In the title compound, C15H14N2O2·H2O, the 1H-pyrrole ring makes a dihedral angle of 59.95 (13)° with the phenyl ring. In the crystal, the mol­ecules are connected by C—H⋯O hydrogen bonds into layers parallel to the (020) plane, while two mol­ecules are connected to the water mol­ecule by two N—H⋯O hydrogen bonds and one mol­ecule by an O—H⋯O hydrogen bond. C—H⋯π and π–π inter­actions further link the mol­ecules into chains extending in the [overline{1}01] direction and stabilize the mol­ecular packing. According to a Hirshfeld surface study, H⋯H (49.4%), C⋯H/H⋯C (23.2%) and O⋯H/H⋯O (20.0%) inter­actions are the most significant contributors to the crystal packing.

The title mol­ecule, C29H44N8O, adopts a conformation resembling a two-bladed fan with the octyl chains largely in fully extended conformations. In the crystal, C—H⋯O hydrogen bonds form chains of mol­ecules extending along the b-axis direction, which are linked by weak C—H⋯N hydrogen bonds and C—H⋯π inter­actions to generate a three-dimensional network. A Hirshfeld surface analysis of the crystal structure indicates that the most important contributions for the crystal packing are from H⋯H (68.3%), H⋯N/N⋯H (15.7%) and H⋯C/C⋯H (10.4%) inter­actions.

The reaction between the (R,S)-fixolide 4-methyl­thio­semicarbazone and PdII chloride yielded the title compound, [Pd(C20H30N3S)2]·C2H6O {common name: trans-bis­[(R,S)-fixolide 4-methyl­thio­semicarbazonato-κ2N2S]palladium(II) ethanol monosolvate}. The asymmetric unit of the title compound consists of one bis-thio­semicarbazonato PdII complex and one ethanol solvent mol­ecule. The thio­semicarbazononato ligands act as metal chelators with a trans configuration in a distorted square-planar geometry. A C—H⋯S intra­molecular inter­action, with graph-set motif S(6), is observed and the coordination sphere resembles a hydrogen-bonded macrocyclic environment. Additionally, one C—H⋯Pd anagostic inter­action can be suggested. Each ligand is disordered over the aliphatic ring, which adopts a half-chair conformation, and two methyl groups [s.o.f. = 0.624 (2):0.376 (2)]. The disorder includes the chiral carbon atoms and, remarkably, one ligand has the (R)-isomer with the highest s.o.f. value atoms, while the other one shows the opposite, the atoms with the highest s.o.f. value are associated with the (S)-isomer. The N—N—C(=S)—N fragments of the ligands are approximately planar, with the maximum deviations from the mean plane through the selected atoms being 0.0567 (1) and −0.0307 (8) Å (r.m.s.d. = 0.0403 and 0.0269 Å) and the dihedral angle with the respective aromatic rings amount to 46.68 (5) and 50.66 (4)°. In the crystal, the complexes are linked via pairs of N—H⋯S inter­actions, with graph-set motif R22(8), into centrosymmetric dimers. The dimers are further connected by centrosymmetric pairs of ethanol mol­ecules, building mono-periodic hydrogen-bonded ribbons along [011]. The Hirshfeld surface analysis indicates that the major contributions for the crystal cohesion are [atoms with highest/lowest s.o.f.s considered separately]: H⋯H (81.6/82.0%), H⋯C/C⋯H (6.5/6.4%), H⋯N/N⋯H (5.2/5.0%) and H⋯S/S⋯H (5.0/4.9%).

In the title mol­ecule, C7H7N3O, the pyrimidine ring is essentially planar, with the propynyl group rotated out of this plane by 15.31 (4)°. In the crystal, a tri-periodic network is formed by N—H⋯O, N—H⋯N and C—H⋯O hydrogen-bonding and slipped π–π stacking inter­actions, leading to narrow channels extending parallel to the c axis. Hirshfeld surface analysis of the crystal structure reveals that the most important contributions for the crystal packing are from H⋯H (36.2%), H⋯C/C⋯H (20.9%), H⋯O/O⋯H (17.8%) and H⋯N/N⋯H (12.2%) inter­actions, showing that hydrogen-bonding and van der Waals inter­actions are the dominant inter­actions in the crystal packing. Evaluation of the electrostatic, dispersion and total energy frameworks indicates that the stabilization is dominated by the electrostatic energy contributions. The mol­ecular structure optimized by density functional theory (DFT) calculations at the B3LYP/6–311 G(d,p) level is compared with the experimentally determined structure in the solid state. The HOMO–LUMO behaviour was also elucidated to determine the energy gap.

The mol­ecular and crystal structure of a discrete [Ni8(μ4-OH)6(μ-4-Rpz)12]2− (R = H; pz = pyrazolate anion, C3H3N2−) cluster with an unprecedented, perfectly cubic arrangement of its eight Ni centers is reported, along with its lower-symmetry alkyl-functionalized (R = methyl and n-oct­yl) derivatives. Crystals of the latter two were obtained with two identical counter-ions (Bu4N+), whereas the crystal of the complex with the parent pyrazole ligand has one Me4N+ and one Bu4N+ counter-ion. The methyl derivative incorporates 1,2-di­chloro­ethane solvent mol­ecules in its crystal structure, whereas the other two are solvent-free. The compounds are tetra­butyl­aza­nium tetra­methyl­aza­nium hexa-μ4-hydroxido-dodeca-μ2-pyrazolato-hexa­hedro-octa­nickel, (C16H36N)(C4H12N)[Ni8(C3H3N2)12(OH)6] or (Bu4N)(Me4N)[Ni8(μ4-OH)6(μ-pz)12] (1), bis­(tetra­butyl­aza­nium) hexa-μ4-hydroxido-dodeca-μ2-(4-methyl­pyrazolato)-hexa­hedro-octa­nickel 1,2-di­chloro­ethane 7.196-solvate, (C16H36N)2[Ni8(C4H5N2)12(OH)6]·7.196C2H4Cl2 or (Bu4N)2[Ni8(μ4-OH)6(μ-4-Mepz)12]·7.196(ClCH2CH2Cl) (2), and bis­(tetra­butyl­aza­nium) hexa-μ4-hydroxido-dodeca-μ2-(4-octylpyrazolato)-hexa­hedro-octa­nickel, (C16H36N)2[Ni8(C11H19N2)12(OH)6] or (Bu4N)2[Ni8(μ4-OH)6(μ-4-nOctpz)12] (3). All counter-ions are disordered (with the exception of one Bu4N+ in 3). Some of the octyl chains of 3 (the crystal is twinned by non-merohedry) are also disordered. Various structural features are discussed and contrasted with those of other known [Ni8(μ4-OH)6(μ-4-Rpz)12]2− complexes, including extended three-dimensional metal–organic frameworks. In all three structures, the Ni8 units are lined up in columns.

The title compound, C15H15F3N2O3S, crystallizes in the monoclinic system, space group I2/a, with Z = 8. As expected, the nine-membered heterobicyclic system is virtually planar and the cyclo­hexyl group adopts a chair conformation. There is structural evidence for intra­molecular N—S⋯O chalcogen bonding between the benziso­thia­zolinone S atom and one O atom of the nitro group, approximately aligned along the extension of the covalent N—S bond [N—S⋯O = 162.7 (1)°]. In the crystal, the mol­ecules form centrosymmetric dimers through C—H⋯O weak hydrogen bonding between a C—H group of the electron-deficient benzene ring and the benzo­thia­zolinone carbonyl O atom with an R22(10) motif. In contrast to the previously described N-acyl 7-nitro-5-(tri­fluoro­meth­yl)benzo[d]iso­thia­zol-3(2H)-ones, the title N-cyclo­hexyl­methyl analogue does not inhibit growth of Mycobacterium aurum and Mycobacterium smegmatis in vitro.

In the title compound, C19H20Cl2N2O, the seven-membered 1,4-diazepane ring adopts a chair conformation while the 4-chloro­phenyl substituents adopt equatorial orientations. The chloro­phenyl ring at position 7 is disordered over two positions [site occupancies 0.480 (16):0.520 (16)]. The dihedral angle between the two benzene rings is 63.0 (4)°. The methyl groups at position 3 have an axial and an equatorial orientation. The compound exists as a dimer exhibiting inter­molecular N—H⋯O hydrogen bonding with R22(8) graph-set motifs. The crystal structure is further stabilized by C—H⋯O hydrogen bonds together with two C—Cl⋯π (ring) inter­actions. The geometry was optimized by DFT using the B3LYP/6–31 G(d,p) level basis set. In addition, the HOMO and LUMO energies, chemical reactivity parameters and mol­ecular electrostatic potential were calculated at the same level of theory. Hirshfeld surface analysis indicated that the most important contributions to the crystal packing are from H⋯H (45.6%), Cl⋯H/H⋯Cl (23.8%), H⋯C/C⋯H (12.6%), H⋯O/O⋯H (8.7%) and C⋯Cl/Cl⋯C (7.1%) inter­actions. Analysis of the inter­action energies showed that the dispersion energy is greater than the electrostatic energy. A crystal void volume of 237.16 Å3 is observed. A mol­ecular docking study with the human oestrogen receptor 3ERT protein revealed good docking with a score of −8.9 kcal mol−1.

Two new copper dimers, namely, bis­(dimethyl sulfoxide)­tetra­kis­(μ-pyrene-1-carboxyl­ato)dicopper(Cu—Cu), [Cu2(C17H9O2)4(C2H6OS)2] or [Cu2(pyr-COO−)4(DMSO)2] (1), and bis­(di­methyl­formamide)­tetra­kis­(μ-pyrene-1-carboxyl­ato)dicopper(Cu—Cu), [Cu2(C17H9O2)4(C3H7NO)2] or [Cu2(pyr-COO−)4(DMF)2] (2) (pyr = pyrene), were synthesized from the reaction of pyrene-1-carb­oxy­lic acid, copper(II) nitrate and tri­ethyl­amine from solvents DMSO and DMF, respectively. While 1 crystallized in the space group Poverline{1}, the crystal structure of 2 is in space group P21/n. The Cu atoms have octa­hedral geometries, with four oxygen atoms from carboxyl­ate pyrene ligands occupying the equatorial positions, a solvent mol­ecule coordinating at one of the axial positions, and a Cu⋯Cu contact in the opposite position. The packing in the crystal structures exhibits π–π stacking inter­actions and short contacts through the solvent mol­ecules. The Hirshfeld surfaces and two-dimensional fingerprint plots were generated for both compounds to better understand the inter­molecular inter­actions and the contribution of heteroatoms from the solvent ligands to the crystal packing. In addition, a Cu2+/Cu1+ quasi-reversible redox process was identified for compound 2 using cyclic voltammetry that accounts for a diffusion-controlled electron-donation process to the Cu dimer.

The title compound, C20H16N2O2, is composed of two monosubstituted benzene rings and one benzimidazole unit. The benzimidazole moiety subtends dihedral angles of 46.16 (7) and 77.45 (8)° with the benzene rings, which themselves form a dihedral angle of 54.34 (9)°. The crystal structure features O—H⋯N and O—H⋯O hydrogen-bonding inter­actions, which together lead to the formation of two-dimensional hydrogen-bonded layers parallel to the (101) plane. In addition, π–π inter­actions also contribute to the crystal cohesion. Hirshfeld surface analysis indicates that the most significant contacts in the crystal packing are: H⋯H (47.5%), O⋯H/H⋯O (12.4%), N⋯H/H⋯N (6.1%), C⋯H/H⋯C (27.6%) and C⋯C (4.6%).

The title compound, [RuCl2(C33H43N3O)], is an example of a new generation of N,N-dialkyl ruthenium catalysts with an N—Ru coordination bond as part of a six-membered chelate ring. The Ru atom has an Addison τ parameter of 0.244, which indicates a geometry inter­mediate between square-based pyramidal and trigonal–bipyramidal. The complex shows the usual trans arrangement of the two chlorides, with Ru—Cl bond lengths of 2.3515 (8) and 2.379 (7) Å, and a Cl—Ru—Cl angle of 158.02 (3)°. One of the chlorine atoms and the atoms of the 2-meth­oxy-N-methyl-N-[(2-methyl­phen­yl)meth­yl]ethane-1-amine group of the title complex display disorder over two positions in a 0.889 (2): 0.111 (2) ratio.

The title complex, [ZnCl(C12H15N3O2)2][ZnCl3(CH3CN)], was synthesized and its structure was fully characterized through single-crystal X-ray diffraction analysis. The complex crystallizes in the ortho­rhom­bic system, space group Pbca (61), with a central zinc atom coordinating one chlorine atom and two pyrrolidinyl-4-meth­oxy­phenyl azoformamide ligands in a bidentate manner, utilizing both the nitro­gen and oxygen atoms in a 1,3-heterodiene (N=N—C=O) motif for coordinative bonding, yielding an overall positively (+1) charged complex. The complex is accompanied by a [(CH3CN)ZnCl3]− counter-ion. The crystal data show that the harder oxygen atoms in the heterodiene zinc chelate form bonding inter­actions with distances of 2.002 (3) and 2.012 (3) Å, while nitro­gen atoms are coordinated by the central zinc cation with bond lengths of 2.207 (3) and 2.211 (3) Å. To gain further insight into the inter­molecular inter­actions within the crystal, Hirshfeld surface analysis was performed, along with the calculation of two-dimensional fingerprint plots. This analysis revealed that H⋯H (39.9%), Cl⋯H/H⋯Cl (28.2%) and C⋯H/H⋯C (7.2%) inter­actions are dominant. This unique crystal structure sheds light on arrangement and bonding inter­actions with azo­formamide ligands, and their unique qualities over similar semicarbazone and azo­thio­formamide structures.

In the structure of the title compound, C19H19N3O5S·C4H8O2, the two independent dioxane mol­ecules each display inversion symmetry. The pyrazole ring is approximately parallel to the aromatic ring of the oxy-ethanone group and approximately perpendicular to the tolyl ring of the sulfonyl substituent. An extensive system of classical and `weak' hydrogen bonds connects the residues to form a layer structure parallel to (201), within which dimeric subunits are conspicuous; neighbouring layers are connected by classical hydrogen bonds to dioxanes and by `weak' hydrogen bonds from Htol­yl donors.

This paper summarizes brief perspectives on the historic process of establishing an African Crystallographic Association (AfCA) and includes representative references. It covers activities within four arbitrarily selected, approximate time slots, i.e., 1890s–1999, 2000–2013, 2014–2019 and 2020–2023. A genuine attempt is made to include appropriate role players, organizations and accompanying events within these periods. It concludes with the official admission of AfCA as the fifth Regional Associate of the IUCr at the 26th Congress and General Assembly of the IUCr in Melbourne, Australia in 2023.

The title compound, [Cu(HL)2(H2O)2] or [Cu(C4H4N3O2)2(H2O)2], is a mononuclear octa­hedral CuII complex based on 5-methyl-1H-1,2,4-triazole-3-carb­oxy­lic acid (H2L). [Cu(HL)2(H2O)2] was synthesized by reaction of H2L with copper(II) nitrate hexa­hydrate (2:1 stoichiometric ratio) in water under ambient conditions to produce clear light-blue crystals. The central Cu atom exhibits an N2O4 coordination environment in an elongated octa­hedral geometry provided by two bidentate HL− anions in the equatorial plane and two water mol­ecules in the axial positions. Hirshfeld surface analysis revealed that the most important contributions to the surface contacts are from H⋯O/O⋯H (33.1%), H⋯H (29.5%) and H⋯N/N⋯H (19.3%) inter­actions.

The title compound, [Co(NCS)2(C6H7NO)]n or Co(NCS)2(2-methyl­pyridine N-oxide), was prepared by the reaction of Co(NCS)2 and 2-methyl­pyridine N-oxide in methanol. All crystals obtained by this procedure show reticular pseudo-merohedric twinning, but after recrystallization, one crystal was found that had a minor component with only a very few overlapping reflections. The asymmetric unit consists of one CoII cation, two thio­cyanate anions and one 2-methyl­pyridine N-oxide coligand in general positions. The CoII cations are octa­hedrally coordinated by two O-bonding 2-methyl­pyridine N-oxide ligands, as well as two S- and two N-bonding thio­cyanate anions, and are connected via μ-1,3(N,S)-bridging thio­cyanate anions into chains that are linked by μ-1,1(O,O) bridging coligands into layers. No pronounced directional inter­molecular inter­actions are observed between the layers. The 2-methyl­pyridine coligand is disordered over two orientations and was refined using a split model with restraints. Powder X-ray diffraction (PXRD) indicates that a pure sample was obtained and IR spectroscopy confirms that bridging thio­cyanate anions are present. Thermogravimetry and differential thermoanalysis (TG-DTA) shows one poorly resolved mass loss in the TG curve that is accompanied by an exothermic and an endothermic signal in the DTA curve, which indicate the decomposition of the 2-methyl­pyridine N-oxide coligands.

In the title compound, C25H25NO7S, the mol­ecular conformation is stabilized by intra­molecular O—H⋯O and N—H⋯O hydrogen bonds, which form S(6) and S(8) ring motifs, respectively. The mol­ecules are bent at the S atom with a C—SO2—NH—C torsion angle of −70.86 (11)°. In the crystal, mol­ecules are linked by C—H⋯O and N—H⋯O hydrogen bonds, forming mol­ecular layers parallel to the (100) plane. C—H⋯π inter­actions are observed between these layers.

In the title compound, C20H18N2S, the asymmetric unit comprises two similar mol­ecules (A and B). In mol­ecule A, the central thio­phene ring makes dihedral angles of 89.96 (12) and 57.39 (13)° with the 1H-pyrrole rings, which are bent at 83.22 (14)° relative to each other, and makes an angle of 85.98 (11)° with the phenyl ring. In mol­ecule B, the corresponding dihedral angles are 89.49 (13), 54.64 (12)°, 83.62 (14)° and 85.67 (11)°, respectively. In the crystal, mol­ecular pairs are bonded to each other by N—H⋯N inter­actions. N—H⋯π and C—H⋯π inter­actions further connect the mol­ecules, forming a three-dimensional network. A Hirshfeld surface analysis indicates that H⋯H (57.1% for mol­ecule A; 57.3% for mol­ecule B), C⋯H/H⋯C (30.7% for mol­ecules A and B) and S⋯H/H⋯S (6.2% for mol­ecule A; 6.4% for mol­ecule B) inter­actions are the most important contributors to the crystal packing.

In the title compound, C18H22O7, two hexane rings and an oxane ring are fused together. The two hexane rings tend toward a distorted boat conformation, while the tetra­hydro­furan and di­hydro­furan rings adopt envelope conformations. The oxane ring is puckered. The crystal structure features C—H⋯O hydrogen bonds, which link the mol­ecules into a three-dimensional network. According to a Hirshfeld surface study, H⋯H (60.3%) and O⋯H/H⋯O (35.3%) inter­actions are the most significant contributors to the crystal packing.

At room temperature, the title salt, C7H10NO+·Cl−, is ortho­rhom­bic, space group Pbca with Z' = 1, as previously reported [Zhao (2009). Acta Cryst. E65, o2378]. Between 250 and 200 K, there is a solid-state phase transition to a twinned monoclinic P21/c structure with Z' = 2. We report the high temperature structure at 250 K and the low-temperature structure at 100 K. In the low-temperature structure, the –NH3 hydrogen atoms are ordered and this group has a different orientation in each independent mol­ecule, in keeping with optimizing N—H⋯Cl hydrogen bonding, some of which are bifurcated: these hydrogen bonds have N⋯Cl distances in the range 3.1201 (8)–3.4047 (8) Å. In the single cation of the high-temperature structure, the NH hydrogen atoms are disordered into the average of the two low-temperature positions and the N⋯Cl hydrogen bond distances are in the range 3.1570 (15)–3.3323 (18) Å. At both temperatures, the meth­oxy group is nearly coplanar with the rest of the mol­ecule, with the C—C—O—C torsion angles being −7.0 (2)° at 250 K and −6.94 (12) and −9.35 (12)° at 100 K. In the extended ortho­rhom­bic structure, (001) hydrogen-bonded sheets occur; in the monoclinic structure, the sheets propagate in the (010) plane.

In the title compound, C20H18O4, the dihedral angle between the 2H-chromen-2-one ring system and the phenyl ring is 89.12 (5)°. In the crystal, the mol­ecules are connected through C—H⋯O hydrogen bonds to generate [010] double chains that are reinforced by weak aromatic π–π stacking inter­actions. The unit-cell packing can be described as a tilted herringbone motif. The H⋯H, H⋯O/O⋯H, H⋯C/C⋯H and C⋯C contacts contribute 46.7, 24.2, 16.7 and 7.6%, respectively, to its Hirshfeld surface.

The title compound, C10H11N5O2S2, consists of an unexpected tautomer with a protonated nitro­gen atom in the triazine ring and a formal exocyclic double bond C=N to the sulfonamide moiety. The ring angles at the unsubstituted nitro­gen atoms are narrow, at 115.57 (12) and 115.19 (12)°, respectively, whereas the angle at the carbon atom between these N atoms is very wide, 127.97 (13)°. The inter­planar angle between the two rings is 79.56 (5)°. The mol­ecules are linked by three classical hydrogen bonds, forming a ribbon structure. There are also unusual linkages involving three short contacts (< 3 Å) from a sulfonamide oxygen atom to the C—NH—C part of a triazine ring.

In the title compound, C10H8N4O3·C3H7NO, the asymmetric unit contains two crystallographically independent mol­ecules A and B, each of which has one DMF solvate mol­ecule. Mol­ecules A and B both feature intra­molecular N—H⋯O hydrogen bonds, forming S(6) ring motifs and consolidating the mol­ecular configuration. In the crystal, N—H⋯O and O—H⋯O hydrogen bonds connect mol­ecules A and B, forming R22(8) ring motifs. Weak C—H⋯O inter­actions link the mol­ecules, forming layers parallel to the (overline{2}12) plane. The DMF solvent mol­ecules are also connected to the main mol­ecules (A and B) by N—H⋯O hydrogen bonds. π–π stacking inter­actions [centroid-to-centroid distance = 3.8702 (17) Å] between the layers also increase the stability of the mol­ecular structure in the third dimension. According to the Hirshfeld surface study, O⋯H/H⋯O inter­actions are the most significant contributors to the crystal packing (27.5% for mol­ecule A and 25.1% for mol­ecule B).

The synthesis and crystallographic analysis of the title compound, C9H9N3O2S, are reported. The compound crystallizes in the monoclinic space group P21/c, revealing characteristic bond lengths and angles typical of thio­semicarbazone groups. The supra­molecular organization primarily arises from hydrogen bonding and π–π stacking inter­actions, leading to distinctive dimeric formations.

The asymmetric unit of the title compound, catena-poly[[[aqua­bis­(pyridine-κN)cadmium(II)]-μ2-4,4'-(1H-1,2,4-triazole-3,5-di­yl)dibenzoato-κ4O,O':O'',O'''] 4.5-hydrate], {[Cd(C16H9N3O4)(C5H5N)2(H2O)]·4.5H2O}n or {[Cd(bct)(py)2(H2O)]·4.5H2O}n (I), consists of a Cd2+ cation coordinated to one bct2– carboxyl­ate dianion, two mol­ecules of pyridine and a water mol­ecule as well as four and a half water mol­ecules of crystallization. The metal ion in I possesses a penta­gonal–bipyramidal environment with the four O atoms of the two bidentately coordinated carboxyl­ate groups and the N atom of a pyridine mol­ecule forming the O4N equatorial plane, while the N atom of another pyridine ligand and the O atom of the water mol­ecule occupy the axial positions. The bct2– bridging ligand connects two metal ions via its carb­oxy­lic groups, resulting in the formation of a parallel linear polymeric chain running along the [1overline{1}1] direction. The coordinated water mol­ecule of one chain forms a strong O—H⋯O hydrogen bond with the carboxyl­ate O atom of a neighboring chain, leading to the formation of double chains with a closest distance of 5.425 (7) Å between the cadmium ions belonging to different chains. Aromatic π–π stacking inter­actions between the benzene fragments of the anions as well as between the coordinated pyridine mol­ecules belonging to different chains results in the formation of sheets oriented parallel to the (overline{1}01) plane. As a result of hydrogen-bonding inter­actions involving the water mol­ecules of crystallization, the sheets are joined together in a three-dimensional network.

The title compound, C16H11N2OF, is a member of the azo dye family. The dihedral angle subtended by the benzene ring and the naphthalene ring system measures 18.75 (7)°, indicating that the compound is not perfectly planar. An intra­molecular N—H⋯O hydrogen bond occurs between the imino and carbonyl groups. In the crystal, the mol­ecules are linked into inversion dimers by C—H⋯O inter­actions. Aromatic π–π stacking between the naphthalene ring systems lead to the formation of chains along [001]. A Hirshfeld surface analysis was undertaken to investigate and qu­antify the inter­molecular inter­actions. In addition, energy frameworks were used to examine the cooperative effect of these inter­molecular inter­actions across the crystal, showing dispersion energy to be the most influential factor in the crystal organization of the compound.

The title compound, N1,N2-di­methyl­ethane­dihydrazide, C4H10N4O2, was obtained by the methyl­ation of oxalyl dihydrazide protected with phthalimide. The mol­ecule is essentially non-planar with a dihedral angle between the two planar hydrazide fragments of 86.5 (2)°. This geometry contributes to the formation of a multi-contact three-dimensional supra­molecular network via C—H⋯O, N—H⋯O and N—H⋯N hydrogen bonds.

Reaction of CoBr2 with 2-methyl­pyridine N-oxide in n-butanol leads to the formation of the title compound, [CoBr2(C6H7NO)2] or [CoBr2(2-methyl­pyridine N-oxide)2]. Its asymmetric unit consists of one CoII cation as well as two bromide anions and two 2-methyl­pyridine N-oxide coligands in general positions. The CoII cations are tetra­hedrally coordinated by two bromide anions and two 2-methyl­pyridine N-oxides, forming discrete complexes. In the crystal structure, these complexes are linked predominantly by weak C–H⋯Br hydrogen bonding into chains that propagate along the crystallographic a-axis. Powder X-ray diffraction (PXRD) measurements indicate that a pure phase was obtained. Thermoanalytical investigations prove that the title compound melts before decomposition; before melting, a further endothermic signal of unknown origin was observed that does not correspond to a phase transition.

We report the synthesis and structural characterization of three crystalline borylated ortho-silylaryl tri­fluoro­methane­sulfonates: 5-(4,4,5,5-tetra­methyl-1,3,2-dioxaborolan-2-yl)-2-(tri­methyl­sil­yl)phenyl tri­fluoro­methane­sulfonate, C16H24BF3O5SSi (1a), 4-(4,4,5,5-tetra­methyl-1,3,2-dioxaborolan-2-yl)-2-(tri­methyl­sil­yl)phenyl tri­fluoro­methane­sulfonate, C16H24BF3O5SSi (1b), and 2-methyl-4-(4,4,5,5-tetra­methyl-1,3,2-dioxaborolan-2-yl)-6-(tri­methyl­silyl)phen­yl tri­fluoro­methane­sulfonate, C17H26BF3O5SSi (2), which are versatile aryne precursors. For all three compounds, the heteroatom substituents are almost coplanar with the central aromatic moiety. C—heteroatom bonding metrics are unexceptional and fall withing the typical range of C—B, C—Si, and C—O single bonds. Despite numerous electronegative sites, only weak inter­molecular inter­actions are observed in the solid state.

The reaction of Co(NCS)2 with 4-methyl­pyridine N-oxide (C6H7NO) leads to the formation of two compounds, namely, tetra­kis­(4-methyl­pyridine N-oxide-κO)bis­(thio­cyanato-κN)cobalt(II), [Co(NCS)2(C6H7NO)4] (1), and tris­(4-methyl­pyridine N-oxide-κO)bis­(thio­cyanato-κN)cobalt(II), [Co(NCS)2(C6H7NO)3] (2). The asymmetric unit of 1 consists of one CoII cation located on a centre of inversion, as well as one thio­cyanate anion and two 4-methyl­pyridine N-oxide coligands in general positions. The CoII cations are octa­hedrally coordinated by two terminal N-bonding thio­cyanate anions in trans positions and four 4-methyl­pyridine N-oxide ligands. In the extended structure, these complexes are linked by C—H⋯O and C—H⋯S inter­actions. In compound 2, two crystallographically independent complexes are present, which occupy general positions. In each of these complexes, the CoII cations are coordinated in a trigonal–bipyramidal manner by two terminal N-bonding thio­cyanate anions in axial positions and by three 4-methyl­pyridine N-oxide ligands in equatorial positions. In the crystal, these complex mol­ecules are linked by C—H⋯S inter­actions. For compound 2, a nonmerohedral twin refinement was performed. Powder X-ray diffraction (PXRD) reveals that 2 was nearly obtained as a pure phase, which is not possible for compound 1. Differential thermoanalysis and thermogravimetry data (DTA–TG) show that compound 2 start to decompose at about 518 K.

In the title benzyl­ideneaniline Schiff base, C18H22N2O, the aromatic rings are inclined to each other by 46.01 (6)°, while the Car—N= C—Car torsion angle is 176.9 (1)°. In the crystal, the only identifiable directional inter­action is a weak C—H⋯π hydrogen bond, which generates inversion dimers that stack along the a-axis direction.

The (S)-(+)-1-(4-bromo­phen­yl)-N-[(4-methoxyphen­yl)methyl­idene]ethyl­amine ligand, C16H16BrNO, (I), was synthesized through the reaction of 4-meth­oxy­anisaldehyde with (S)-(−)-1-(4-bromo­phen­yl)ethyl­amine. It crystallizes in the ortho­rhom­bic space group P212121 belonging to the Sohncke group, featuring a single mol­ecule in the asymmetric unit. The refinement converged successfully, achieving an R factor of 0.0508. The PdII com­plex bis­{(S)-(+)-1-(4-bromo­phen­yl)-N-[(4-methoxyphen­yl)methyl­idene]ethyl­amine-κN}di­chlorido­pal­ladium(II), [PdCl2(C16H16BrNO)2], (II), crystallizes in the monoclinic space group P21 belonging to the Sohncke group, with two mol­ecules in the asymmetric unit. The central atom is tetra­coordinated by two N atoms and two Cl atoms, resulting in a square-planar configuration. The imine moieties exhibit a trans configuration around the PdII centre, with average Cl—Pd—N angles of approximately 89.95 and 90°. The average distances within the palladium com­plex for the two mol­ecules are ∼2.031 Å for Pd—N and ∼2.309 Å for Pd—Cl.

The crystal structures and Hirshfeld surface analyses of three similar azo compounds are reported. Methyl 4-{2,2-di­chloro-1-[(E)-phenyl­diazen­yl]ethen­yl}benzoate, C16H12Cl2N2O2, (I), and methyl 4-{2,2-di­chloro-1-[(E)-(4-methyl­phen­yl)diazen­yl]ethen­yl}benzoate, C17H14Cl2N2O2, (II), crystallize in the space group P21/c with Z = 4, and methyl 4-{2,2-di­chloro-1-[(E)-(3,4-di­methyl­phen­yl)diazen­yl]ethen­yl}benzoate, C18H16Cl2N2O2, (III), in the space group Poverline{1} with Z = 2. In the crystal of (I), mol­ecules are linked by C—H⋯N hydrogen bonds, forming chains with C(6) motifs parallel to the b axis. Short inter­molecular Cl⋯O contacts of 2.8421 (16) Å and weak van der Waals inter­actions between these chains stabilize the crystal structure. In (II), mol­ecules are linked by C—H⋯O hydrogen bonds and C—Cl⋯π inter­actions, forming layers parallel to (010). Weak van der Waals inter­actions between these layers consolidate the mol­ecular packing. In (III), mol­ecules are linked by C—H⋯π and C—Cl⋯π inter­actions forming chains parallel to [011]. Furthermore, these chains are connected by C—Cl⋯π inter­actions parallel to the a axis, forming (0overline{1}1) layers. The stability of the mol­ecular packing is ensured by van der Waals forces between these layers.

The title compound, C16H17N3O3, is racemic as it crystallizes in a centrosymmetric space group (Poverline{1}), although the trans disposition of substituents about the central C—C bond is established. The five- and six-membered rings are oriented at a dihedral angle of 75.88 (8)°. In the crystal, N—H⋯N hydrogen bonds form chains of mol­ecules extending along the c-axis direction that are connected by inversion-related pairs of O—H⋯N into ribbons. The ribbons are linked by C—H⋯π(ring) inter­actions, forming layers parallel to the ab plane. A Hirshfeld surface analysis indicates that the most important contributions for the crystal packing are from H⋯H (45.9%), H⋯N/N⋯H (23.3%), H⋯C/C⋯H (16.2%) and H⋯O/O⋯H (12.3%) inter­actions. Hydrogen bonding and van der Waals inter­actions are the dominant inter­actions in the crystal packing. The volume of the crystal voids and the percentage of free space were calculated to be 100.94 Å3 and 13.20%, showing that there is no large cavity in the crystal packing. Evaluation of the electrostatic, dispersion and total energy frameworks indicates that the stabilization is dominated by the electrostatic energy contributions in the title compound. Moreover, the DFT-optimized structure at the B3LYP/6–311 G(d,p) level is compared with the experimentally determined mol­ecular structure in the solid state. The HOMO–LUMO behaviour was elucidated to determine the energy gap.

The in­do­line portion of the title mol­ecule, C16H13NO2, is planar. In the crystal, a layer structure is generated by C—H⋯O hydrogen bonds and C—H⋯π(ring), π-stacking and C=O⋯π(ring) inter­actions. The Hirshfeld surface analysis of the crystal structure indicates that the most important contributions for the crystal packing are from H⋯H (43.0%), H⋯C/C⋯H (25.0%) and H⋯O/O⋯H (22.8%) inter­actions. Hydrogen bonding and van der Waals inter­actions are the dominant inter­actions in the crystal packing. The volume of the crystal voids and the percentage of free space were calculated to be 120.52 Å3 and 9.64%, respectively, showing that there is no large cavity in the crystal packing. Evaluation of the electrostatic, dispersion and total energy frameworks indicate that the stabilization is dominated by the dispersion energy contributions in the title compound. Moreover, the DFT-optimized structure at the B3LYP/6-311G(d,p) level is compared with the experimentally determined mol­ecular structure in the solid state.

The title compound, C27H20N4O3S, crystallizes in the monoclinic system, space group P21/n, with Z = 4. The global shape of the mol­ecule is determined by the orientation of the substituents on the central 4H-1,2,4-triazole ring. The nitro­phenyl ring, phenyl ring, and naphthalene ring system are oriented at dihedral angles of 82.95 (17), 77.14 (18) and 89.46 (15)°, respectively, with respect to the triazole ring. The crystal packing features chain formation in the b-axis direction by S⋯O inter­actions. A Hirshfeld surface analysis indicates that the highest contributions to surface contacts arise from contacts in which H atoms are involved.

In the crystal of the title compound, C6H9ClN2O, mol­ecular pairs form dimers with an R22(8) motif through N—H⋯O hydrogen bonds. These dimers are connect into ribbons parallel to the (100) plane with R44(10) motifs by N—H⋯O hydrogen bonds along the c-axis direction. In addition, π–π [centroid-to-centroid distance = 3.4635 (9) Å] and C—Cl⋯π inter­actions between the ribbons form layers parallel to the (100) plane. The three-dimensional consolidation of the crystal structure is also ensured by Cl⋯H and Cl⋯Cl inter­actions between these layers. According to a Hirshfeld surface study, H⋯H (43.3%), Cl⋯H/H⋯Cl (22.1%) and O⋯H/H⋯O (18.7%) inter­actions are the most significant contributors to the crystal packing.

A new lanthanide coordination polymer, poly[[tri­aqua­bis­(μ4-phthalato)(μ3-pyridine-2,5-di­carboxyl­ato)dipraseodymium] monohydrate], {[Pr2(C7H3NO4)2(C8H4O4)(H2O)3]·H2O}n or {[Pr2(phth)2(pydc)(H2O)3]·H2O}n, (pydc2− = pyridine-2,5-di­carboxyl­ate and phth2− = phthalate) was synthesized and characterized, revealing the structure to be an assembly of di-periodic {Pr2(pydc)(phth)2(H2O)3}n layers. Each layer is built up by edge-sharing {Pr2N2O14} and {Pr2O16} dimers, which are connected through a new coordin­ation mode of pydc2− and phth2−. These layers are stabilized by inter­nal hydrogen bonds and π–π inter­actions. In addition, a three-dimensional supra­molecular framework is built by inter­layer hydrogen-bonding inter­actions involving the non-coordinated water mol­ecule. Thermogravimetric analysis shows that the title compound is thermally stable up to 400°C.

The title mol­ecule, [Fe2(C5H5)2(C23H17ClN2)]·C3H7NO, is twisted end to end and the central N/C/N unit is disordered. In the crystal, several C—H⋯π(ring) inter­actions lead to the formation of layers, which are connected by further C—H⋯π(ring) inter­actions. A Hirshfeld surface analysis of the crystal structure indicates that the most important contributions for the crystal packing are from H⋯H (60.2%) and H⋯C/C⋯H (27.0%) inter­actions. Hydrogen bonding, C—H⋯π(ring) inter­actions and van der Waals inter­actions dominate the crystal packing.