Guest editors Shinya Hayami, Stephen M. Holmes and Malcolm A. Halcrow introduce this Journal of Materials Chemistry C themed issue on spin-state switches in molecular materials chemistry.

Changing mechanisms: More than half of the known classes of copper oxidase contain a chemically modified amino acid within their active site. Recent experiments by Klinman et al. on the biogenesis of one of these, topaquinone (TPQ; see scheme), have shown that this occurs by a different mechanism from that observed for the aqueous aerobic synthesis of hydroxybenzoquinones in vitro.

Kleiner Wirt für kleine Gäste: Die direkte Kupplung von Cyclotrithiophenolen führt zu einem neuartigen Cryptophantyp mit labilen Disulfidbrücken, bei dem es sich um das kleinste bisher beschriebene Cryptophan handelt. Das neue Cryptophan (siehe Struktur; S gelb, C grau, H hellgrau) vermag in Lösung Methan oder Stickstoff zu binden.

Review: 82 refs.

Four bis[2‐{pyrazol‐1‐yl}‐6‐{pyrazol‐3‐yl}pyridine] ligands have been synthesized, with butane‐1,4‐diyl (L1), pyrid‐2,6‐diyl (L2), benzene‐1,2‐dimethylenyl (L3) and propane‐1,3‐diyl (L4) linkers between the tridentate metal‐binding domains. L1 and L2 form [Fe2(μ−L)2]X4 (X−=BF4− or ClO4−) helicate complexes when treated with the appropriate iron(II) precursor. Solvate crystals of [Fe2(μ−L1)2][BF4]4 exhibit three different helicate conformations, which differ in the torsions of their butanediyl linker groups. The solvates exhibit gradual thermal spin‐crossover, with examples of stepwise switching and partial spin‐crossover to a low‐temperature mixed‐spin form. Salts of [Fe2(μ−L2)2]4+ are high‐spin, which reflects their highly twisted iron coordination geometry. The composition and dynamics of assembly structures formed by iron(II) with L1−L3 vary with the ligand linker group, by mass spectrometry and 1H NMR spectroscopy. Gas‐phase DFT calculations imply the butanediyl linker conformation in [Fe2(μ−L1)2]4+ influences its spin state properties, but show anomalies attributed to intramolecular electrostatic repulsion between the iron atoms.