engleski

A crystallographer's cabinet of curiosities: unusual π-hole interactions

π-hole is an area of positive electrostatic potential in a π-electron system, which most often occurs in electron-poor conjugated rings and some functional groups such as a carbonyl. It can form attractive interactions with electron-rich groups (anions, lone pairs or electron-rich π-systems), and such interactions are known as π-hole interactions. This is a relatively little-studied type of intermolecular interactions, so their nature is still a disputable issue. The main component of a π-hole interaction is usually electrostatic, but a small degree of charge transfer may also be present. Therefore, particular types of π-hole interactions are anion···π, lone pair···π (lp···π) and n→π*, the latter one denoting a charge transfer from a lone pair (i.e. a non-bonding orbital) of an electron-rich group to a π antibonding orbital of the electron-poor π-system. The most commonly occurring π-hole interactions involve carbonyl moieties and electron-depleted aromatic rings as 'acceptors', however, our recent works reveal that electron-poor quinoid rings and semiquinone radicals have much more pronounced π-holes, and are therefore much stronger 'acceptors' of π-hole interactions. In co-crystals of iodide salts with perhalogenquinones the common motive is a sandwich-like I-···quinone···I- moiety with close contacts between the iodide anion and carbon skeleton of the quinoid ring. Distances between the iodide and the ring centroid range between 3.60 and 3.85 Å, which is slightly shorter than sum of van der Waals radii for C and I. Black colour of the crystals indicates a considerable degree of charge transfer from the iodide to the quinoid ring (the neutral quinones are yellow). A most unusual case is lithium 2, 5-dihydroxyquinonate DMSO solvate (Li2DHQ·DMSO) which we recently prepared: it involves a close contact between a sulphur atom and the ring centroid of 3.46 Å, and the closest S···C distance is only 3.25 Å. This is the first instance of a) a non aromatic ring and b) a (di)anionic ring forming a lp···π contact. This unprecedented interaction has been studied by X-ray charge density, to gain more insight into its nature.

pi-hole ; crystal engineering ; charge density

Hrvatsko-francuska bilaterala, COGITO