Pancake-bonding of semiquinone radicals under variable temperature and pressure conditions

The effects of temperature (100-370 K) and pressure (0-6 GPa) on the non-localized two-electron multicentric covalent bonds (`pancake bonding') in closely bound radical dimers were studied using single-crystal X-ray diffraction on a 4-cyano-N-methylpyridinium salt of 5,6-dichloro-2,3-dicyanosemiquinone radical anion (DDQ) as the sample compound. On cooling, the anisotropic structural compression was accompanied by continuous changes in molecular stacking; the discontinuities in the changes in volume and b and c cell parameters suggest that a phase transition occurs between 210 and 240 K. At a pressure of 2.55 GPa, distances between radical dimers shortened to 2.9 Å, which corresponds to distances observed in extended π-bonded polymers. Increasing pressure further to 6 GPa reduced the interplanar separation of the radicals to 2.75 Å. This may indicate that the covalent component of the interaction significantly increased, in accordance with the results of DFT calculations reported elsewhere [Molčanov et al. (2019), Cryst. Growth Des. 19, 391-402].

Towards understanding π-stacking interactions between non-aromatic rings

The first systematic study of π interactions between non-aromatic rings, based on the authors' own results from an experimental X-ray charge-density analysis assisted by quantum chemical calculations, is presented. The landmark (non-aromatic) examples include quinoid rings, planar radicals and metal-chelate rings. The results can be summarized as: (i) non-aromatic planar polyenic rings can be stacked, (ii) interactions are more pronounced between systems or rings with little or no π-electron delocalization (e.g. quinones) than those involving delocalized systems (e.g. aromatics), and (iii) the main component of the interaction is electrostatic/multipolar between closed-shell rings, whereas (iv) interactions between radicals involve a significant covalent contribution (multicentric bonding). Thus, stacking covers a wide range of interactions and energies, ranging from weak dispersion to unlocalized two-electron multicentric covalent bonding ('pancake bonding'), allowing a face-to-face stacking arrangement in some chemical species (quinone anions). The predominant interaction in a particular stacked system modulates the physical properties and defines a strategy for crystal engineering of functional materials.

Influence of organic cations on the stacking of semiquinone radical anions

A series of salts of tetrachloro- and tetrabromosemiquinone radical anions reveal four types of stacks: 1) pancake bonded dimers, 2) pancake-bonded trimers, 3) equidistant radicals and 4) a novel type of equidistant stacks of partially charged radicals.

Pancake Bonding in π-Stacked Trimers in a Salt of Tetrachloroquinone Anion

The crystal structure of [4-damp])₂ [Cl₄ Q]₃ (4-damp=4-dimethylamino-N-methylpyridinium, Cl₄ 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 Cl₄ Qs in the trimer. The strong interactions within a trimer (Cl₄ Q)₃^2- 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.

Probing semiconductivity in crystals of stable semiquinone radicals: organic salts of 5,6-dichloro-2,3-dicyanosemiquinone (DDQ) radical anions

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.