X-ray charge density was determined and analyzed for two polymorphs of the Nmethylpyridinium salt of the tetrachlorosemiquinone radical anion and its analogous closed-shell relatives, tetrachloroquinone (chloranil) and tetrachlorohydroquinone. The study, which was combined with calculations of electron delocalization, electrostatic potentials, and aromaticity, presents details of electronic structure of the semiquinoid ring. This comparative study reveals that the negative charge is delocalized over the entire semiquinone radical, and that the chlorine substituents play a crucial role in its stabilization through induction effect. In general, the semiquinoid ring has partially delocalized π-electrons and is approximately halfway between a quinoid and an aromatic ring. In the orthorhombic polymorph with stacks of equidistant radicals electron density between the rings of almost 0.05 e Å-3 and four (3,-1) saddle points between the contiguous rings were found. In the diamagnetic triclinic polymorph, comprising strongly bound radical dimers (with significant covalent character-'pancake bond'), maximum electron density between the rings exceeds 0.095 e Å-3 and multiple (3,-1) critical points are found. However, only negligible electron density is observed between the dimers. Thus, in the radical anion stacks spin coupling, along with dispersive and polarization effects, defines interplanar distance and magnetic behaviour, whereas intermolecular electrostatic potential determines the ring offset.

A study of strong halogen bonding within three series of halogen-bonded complexes, derived from seven para-substituted pyridine derivatives and three N-halosuccinimides (iodo, bromo and chloro), has been undertaken with the aid of single-crystal diffraction, solution complexation and computational methods. The halogen bond was compared with the hydrogen bond in an equivalent series based on succinimide. The halogen-bond energies are in the range -60 to -20 kJ mol and change regularly with pyridine basicity and the Lewis acidity of the halogen. The halogen-bond energies correlate linearly with the product of charges on the contact atoms, which indicates a predominantly electrostatic interaction. The binding enthalpies in solution are around 19 kJ mol less negative due to solvation effects. The optimised geometries of the complexes in the gas phase are comparable to those of the solid-state structures, and the effects of the supramolecular surroundings in the latter are discussed. The bond energies for the hydrogen-bonded series are intermediate between the halogen-bond energies of iodine and bromine, although there are specific differences in the geometries of the halogen- and hydrogen-bonded complexes.

The use of a dodecanuclear zirconium acetate cluster as a precursor enables the rapid, clean mechanochemical synthesis of high-microporosity metal-organic frameworks NU-901 and UiO-67, with surface areas up to 2250 m2 g-1. Real-time X-ray diffraction monitoring reveals that mechanochemical reactions involving the conventional hexanuclear zirconium methacrylate precursor are hindered by the formation of an inert intermediate, which does not appear when using the dodecanuclear acetate cluster as a reactant.

During the last 50 years a vast number of structural studies on Schiff bases derived from hydroxyaryl aldehydes and their coordination compounds have been undertaken. In this review we present a systematic and brief overview of the most important discoveries and achievements accomplished in this field. The occurrence of Schiff bases (and their complexes) derived from nine most commonly used hydroxyaryl aldehydes in the Cambridge Structural Database (CSD) has been investigated. Keto-enol tautomerism and intramolecular hydrogen bonding in salicylaldimines and naphthaldimines is discussed, with a comment of the most common errors in determining the correct tautomeric form. Also, the interrelationship of crystal packing and the substituents on the Schiff base is studied. Finally, we give a short overview of conformational differences between free Schiff bases and Schiff bases bonded to metal ions as well as some structural characteristics of Schiff base metal complexes.

Two salts of tetrachloro-(chloranil, Cl 4 Q) and tetrabromosemiquinone (bromanil, Br 4 Q) radical anions with an organic cation Nmethylpyridinium (N-MePy), displaying a variety of tunable electrical and magnetic properties, are characterized. These systems comprise π-stacks of equidistant semiquinone radical anions, without Peierls deformation and diamagnetic spin coupling. Both are stable up to 150 and 120 °C in air, respectively, and are semiconductors with conductivities of 10 −6 −10 −7 (Ω cm) −1 in the single crystals. The room-temperature concomitant crystallization of two polymorphs with different magnetic properties (antiferromagnetic and Peierls-distorted diamagnetic) has been observed for Cl 4 Q•N-MePy. The semiquinone radical organic salts represent a novel class of potential (multi)functional materials with tunable properties.

The crystal structure of [4-damp]) [Cl Q] (4-damp=4-dimethylamino-N-methylpyridinium, Cl Q=tetrachloroquinone) salt is built up from slipped columnar stacks of quinoid rings composed of closely bound trimers with the intra-trimer separation distance of 2.84 Å and total charge of -2 whereas the inter-trimer distance is 3.59 Å. The individual rings exhibit partial negative charges that are distributed unevenly among the three Cl Qs in the trimer. The strong interactions within a trimer (Cl Q) have a partially covalent character with two-electron/multicentered bonding, that is extended over three rings, plausibly termed as "pancake bonding". The electron pairing within this multicentre bond leads to the fact that the crystals are diamagnetic and act as insulators. The studies of the structure and nature of bonding are based on X-ray charge density analysis and density functional theory.

The structural parameters and semiconductivity of crystals with stacked 5,6-dichloro-2,3-dicyanosemiquinone (DDQ) radicals were studied for a series of nine salts of DDQ with substituted N-ethyl- and N-methylpyridinium cations.

Over the past several decades, halogen bonding has become a standard ‘tool’ in supramolecular chemistry and particularly in crystal engineering. Traditionally, the study of halogen bonding has primarily been focused...