Temperature induced reversible structural and magnetic changes in a crystal of tetrachlorosemiquinone anion radical

Abstract

The anion radicals with nonaromatic p-systems, such as semiquinones, are interesting in the design of functional materials. Magnetic properties of solvates of alkali salts of tetrachlorosemiquinone (chloranil) radical anion are tuned by crystal engineering using different solvents (2-butanone and acetonitrile) and/or cations (potassium and ammonium) in crystals. The structural and magnetic characteristics of two salts were studied by variable-temperature single crystal X-ray diffraction and magnetic susceptibility measurements. DFT and CAS-MP2 calculations were used to correlate magnetic and structural properties of radical anions. The solid-state structure of potassium tetrachlorosemiquinone 2-butanone solvate, influenced by temperature, switches between a paramagnetic-like monomeric radical form at high temperature (200 K) and diamagnetic dimeric one at low temperature (100 K). The reversible transition is controlled by solvent and cation types which affect intrastack distance and magnetic properties.

Partially Covalent Two-Electron/Multicentric Bonding between Semiquinone Radicals

“…8a) shows a good matching between electron-rich and electron-poor areas of two radicals in the intra dimer: the closest contacts (2.86 Å) are between the electron-rich oxygen and the electron-depleted carbonyl C atoms. Therefore, we can conclude that the "longitudinal" offset [37] of 2.06 Å which is commonly observed in stacked semiquinones, [24,28,30,34,36,37] occurs because of electrostatic reasons (minimization of repulsion between the two charged anions), rather than steric ones. The short distance between the radicals is due to covalent 2e/mc bonding.…”

“…Such a concave shape has been observed in dimers for a number of other organic radicals. [45][46][47][48] In the antiferromagnetic structures, [29,30] the radicals are essentially planar within the experimental error, or with minor chair-like distortion (τ < 1.5 °). (Table S14 in the Supporting Information, Fig.…”

Iodide···π Interactions of Perhalogenated Quinoid Rings in Co-crystals with Organic Bases

“…While preparing solid-state semiquinone systems, 17,[19][20][21][22] we serendipitously obtained a series of co-crystals of tetrachloro-and tetrabromoquinone (Cl4Q and Br4Q, respectively, Scheme 1) with iodide salts of organic cations (mostly derivatives of N-methylpyridinium, Scheme 2), in which close contacts between the quinoid ring and the iodide anion occur almost without exception.…”

Chemical Crystallography before X‐ray Diffraction

“…However, modern cutting-edge research in solid-state chemistry and physics challenges this view: today crystals are designed for many special purposes. Materials science is concerned with the design of crystals with exceptional electrical [130] or magnetic properties [131] (so-called functional materials), which often involve reversible phase transitions; [132] solid-state phase transitions are a sub-field of research on their own and include study of reaction mechanism; [133] many dynamic phenomena such as proton transfer, [134] electron transfer, [135] and magnetic exchange interactions [136] are most conveniently studied in crystals; there are many chemical reactions that take place in single crystals, some of them reversible; finally, template-directed solid-state synthesis takes place in specifically designed crystals. [137] There is no unequivocal definition of life; we can only describe it.…”