Inter- and intramolecular C-H⋯O bonding in the anions of 1,3-indandione derivatives

Derivatives of a series of 1,3-indandione (1) self-condensation products (2-4) and their anions (2a-4a) were investigated by NMR and UV/Vis spectroscopy and quantum mechanical calculations. These compounds exhibit a number of unusual features in their NMR spectra. Parent compound 1 is nearly not ionized in DMSO, whereas 3 is ionized completely. In contrast, derivatives 2 and 4 undergo partial ionization in DMSO and interaction between the neutral and anionic species affords the anionic intermolecular complexes, which exhibit both temperature- and concentration-dependent NMR spectra. The NMR spectra of pure salts 2a and 3a are concentration independent, but temperature dependent. Charge derealization in anions 2a and 3a gives rise to restricted rotation over the partial double bonds connecting the indane fragments. Energies of barriers to rotation were estimated from the temperature dependence of the ¹³C NMR spectra. The intramolecular aromatic C-H⋯O hydrogen bonding found previously in 2-4 is also observed in corresponding anions 2a-4a.