Unprecedented [small pi][small pi] interaction between an aromatic ring and a pseudo-aromatic ring formed through intramolecular H-bonding in a bidentate Schiff base ligand: crystal structure and DFT calculations

A combination of a single crystal X-ray diffraction study and density functional theory calculations has been applied to a bidentate Schiff base compound to elucidate different cooperative non-covalent interactions involved in the stabilization of the keto form over the enol one in the solid state. The single crystal X-ray structure reveals a remarkable supramolecular assembly of the keto form through a cyclic hydrogen bonded dimeric motif. The most interesting feature in the supramolecular assembly is the formation of a 'dimer of dimer' motif by [small pi][small pi], CH[small pi] and NO/OO interactions in which the [small pi][small pi] interaction involving the aromatic phenyl ring and the intramolecularly hydrogen bonded pseudo-aromatic ring of the keto form lying just above or below the phenyl ring of the other dimer seems to be unprecidented. The optimized geometry of the hydrogen bonded dimeric motif of the keto form of the organic molecule has been obtained by DFT calculations and agrees very well with that found within the crystalline state. The X-ray crystallographic geometry of the 'dimer of dimer' has also been computed, which shows that in the HOMO, the [small pi] electrons are localized in the phenyl rings away from each other, while in the LUMO, there is a strong [small pi]-[small pi] interaction between the phenyl ring of one dimer with the pseudo-aromatic ring of another dimer with an energy estimated to be 7.95 kJ mol-1. Therefore, on HOMO [rightward arrow] LUMO excitation there is localization of [small pi] electrons in the central part of the complex moiety which plays a stabilizing role of the dimer of dimer motif in the solid state.