engleski

Strong π•••π interactions of dianionic quinoid rings: alkali salts of 2, 5-dihydroxybenzoquinone

A first study of π•••π interactions between quinoid rings revealed that they stack unlike aromatic rings: they form face-to-face stacks without offset. Centroid•••centroid distances of about 3.30 Å are much shorter than in aromatic stacks, 0.3 Å shorter than sum of van der Waals radii and are actually the shortest ever observed. The novel type of interaction may play an important role in supramolecular synthesis and crystal engineering. Due to a specific distribituon of electron density (quinoid rings are not aromatic and their π-electrons are not delocalised), repulsion of electron-rich π-systems is minimal, and is easily outperformed by σ-π attractions. All previously described quinoid stacks, found in alkali salts of chloranilic acid, comprised neutral or mono anionic rings ; in dianionic rings repulsion of π-systems cannot be avoided, and doubly charged rings stack with larger centroid distances (> 3.6 Ǻ) and offset (> 1.7 Ǻ). Thus, stacks of quinoid dianions resemble aromatic ones. In a series of alkali salts of 2, 5-dihydroxyquinone, however, dianionic quinoid rings stack without offset. Substitution of chlolrine atom with a hydrogen plays a crucial role: bulky, electron-rich chlorine atoms in contiguous rings repulse other atoms (chlorines and oxygens), destabilising the stacks. Small electron-poor hydrogen atoms cause little electrostatic repulsion and steric hindrance, maximising σ-π attraction between C–H and C–O bonds in contigous rings. Therefore, dianionic rings stack without offset. However, due to strong electrostatic repulsion of two negative charges, centroid distances are slightly longer than the sum of van der Waals radii.

π•••π interactions; stacking; dihyroxyquinone; crystallography

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