Crystal structure, Hirshfeld surface analysis and computational study of a rhodamine B–salicylaldehyde Schiff base derivative

Abstract

The molecular structure of the title compound {systematic name: 3′,6′-bis(diethylamino)-2-[(2-hydroxybenzylidene)amino]spiro[isoindoline-1,9′-xanthen]-3-one}, C35H36N4O3 or RbSa, can be seen as being composed of two parts sharing a central quaternary carbon atom. Both the xanthene and isoindole moieties are nearly planar: 14 atoms in the former moiety show an r.m.s. deviation of 0.0411 Å and eleven atoms in the latter moiety show an r.m.s. deviation of 0.0545 Å. These two planes are almost perpendicular to each other, the angle between the mean planes being 87.71 (2)°. The title compound appears to be in its enol form. The corresponding H atom was located and freely refined at a distance of 1.02 (3) Å from the O atom and 1.72 (2) Å from the N atom. The strong intramolecular hydrogen bond O—H...N bridging the hydroxyl group and its neighboring nitrogen atom forms an S(6) graph-set motif. Apart from the intramolecular O—H...N hydrogen bond, C—H...O interactions are observed between two neighbouring RbSa molecules related by an inversion center. The C—O donor–acceptor distance is 3.474 (2) Å. Moreover, C—H...π interactions are observed between the C—H bond of one of the ethyl groups and the centroid of the benzene ring of the isoindole moiety. The C...centroid distance is 3.8191 (15) Å. No π–π interactions are observed in the crystal structure as the shortest distance between ring centroids is more than 4 Å. A Hirshfeld surface analysis of the crystal structure indicates that the most important contributions for the crystal packing are from H...H, C...H/H...C, O...H/H...O and N...H/H...N interactions. DFT calculations at the CAM-B3LYP/6–31 G(d) level were carried out to gain a better understanding of the relative energies and the tautomerization process between two possible conformers (keto and enol), as well as the transition state of the title compound.