Mythical compounds

There is a need for validation methods to ensure the quality and consistency of reported data, but a recent article by Raymond and Girolami [Acta Cryst. (2023), C79, https://doi.org/10.1107/S2053229623007088] will alert chemists, crystallographers, referees, and editors to not always trust the crystal structure but also to ensure that the chemistry is consistent with previous chemistry.

Pathological crystal structures

Although analysing crystallographic results with available software tools can catch many types of errors, others can be detected only by combining knowledge of both crystallography and chemistry. We discuss several such examples from the published literature, and for each of them we identify what lessons they teach us.

Crystal structure and synthesis of the bis­(anthracene)dicuprate dianion as the dipotassium salt, [K(tetra­hydro­furan)₂]₂[{Cu(9,10-η²-anthracene)}₂], the first anionic arene com­plex of copper

An unprecedented example of an anionic (arene)copper com­plex has been prepared and structurally characterized in the salt [K(THF)₂]₂[Cu₂(anthracene)₂] (THF is tetra­hydro­furan), which con­tains the first homoleptic anthracene com­plex of a d- or f-block metal bound only to the central ring of anthracene.

Perovskite-related structures of Ba₂YAlO₅ and the β and α phases of Ba₆Y₂Al₄O₁₅ containing AlO₄ tetra­hedra

Single crystals of Ba₂YAlO₅ and of the α and β phases of Ba₆Y₂Al₄O₁₅ suitable for X-ray structural analysis were obtained via grain growth of polycrystalline samples prepared by solid-state reactions. In each of these com­pounds, BO₆ octa­hedra in ABO₃ perovskite-type structures were replaced by AlO₄ tetra­hedra and YO₆ octa­hedra. Polycrystalline samples in which some Y atoms were replaced with Eu exhibited orange–red luminescence.

Crystallographic, spectroscopic and thermal studies of 1-(4-bromo­phen­yl)-5-(2,5-di­methyl-1H-pyrrol-1-yl)-3-methyl-1H-pyrazole

A new pyrrole–pyrazole hybrid, synthesized through a citric acid-catalyzed Paal–Knorr reaction, presents a crystal structure controlled by C—H⋯π and dispersion forces. Despite the weak inter­actions, the solid shows thermal stability.

Proton tautomerism and stereoisomerism in 5-[(di­methyl­amino)­methyl­idene]-4-[3/4-(tri­fluoromethyl­phen­yl)amino]-1,3-thia­zol-2(5H)-ones: synthesis, crystal structure and spectroscopic studies

5-[(Di­methyl­amino)­methyl­idene]-4-{[3-(tri­fluoro­meth­yl)phen­yl]amino}-1,3-thia­zol-2(5H)-one and the [4-(tri­fluoro­meth­yl)phen­yl]amino derivative, with the tri­fluoro­methyl group at the arene ring in the meta and para positions, were synthesized to study the structural changes occurring on prototropic tautomerism of the amidine system. Both isomers exist in the liquid and solid phases in the amino tautomeric form.