Substituent effects in nitro derivatives of carbazoles investigated by comparison of low-temperature crystallographic studies with density functional theory (DFT) calculations

Abstract

The crystal structure of 9H-carbazole, C12H9N, (I), has been redetermined at low temperature for use as a reference structure in a comparative study with the structures of 1-nitro-9H-carbazole, C12H8N2O2, (II), and 9-nitrocarbazole, C12H8N2O2, (III). The molecule of (I) has crystallographically imposed mirror symmetry (Z' = 0.5). All three solid-state structures are slightly nonplanar, the dihedral angles between the planes of the arene and pyrrole rings ranging from 0.40 (7)° in (III) to 1.82 (18)° in (II). Nevertheless, a density functional theory (DFT) study predicts completely planar conformations for the isolated molecules. To estimate the influence of nitro-group substitution on aromaticity, the HOMA (harmonic oscillator model of aromaticity) descriptor of π-electron delocalization has been calculated in each case. The HOMA indices for the isolated and solid-state molecules are relatively consistent and decrease in value for aromatic rings that are substituted with a π-electron-withdrawing nitro group. Substitution of the arene ring influences the π-electron delocalization in the ring only weakly, showing strong resistance to a perturbation of its geometry, contrary to what is observed for nitro substitution of the five-membered heterocyclic pyrrole ring. In (II), the molecules are arranged in near-planar dimers connected to each other by strong N-H···O hydrogen bonds that stack parallel to the crystallographic b axis. A similar stacking arrangement is observed in (III), although here the stacked structure is formed by stand-alone molecules.

Crystal structure of dimethyl 9H-carbazole-2,7-dicarboxylate

“…1. The bond distances and angles are similar to those observed in some carbazole derivatives (Clarke & Spink, 1969;Gajda et al, 2014,) and a structurally characterized carbazole dicarboxylic acid (Weseliński et al, 2014). The carbazole unit is nearly planar with a mean deviation from planarity of 0.037 Å.…”

“…For the synthesis of the title compound, see: Olkhovik et al (2008). For the crystal structures of some carbazoles, see: Clarke & Spink (1969); Gajda et al (2014). For the structure of 9H-carbazole-3,6-dicarboxylic acid, see: Weseliń ski et al (2014).…”

Characterization and Biological Potency of Mono- to Tetra-Halogenated Carbazoles

“…In this case, deshielding contributions from the ring current of this phenyl ring and a steric interaction with H-5 may be the major factors. The closeness of approach of the hydrogens (and of the phenyl ring) is revealed by X-ray structural studies of carbazole. , It is well-known that where molecular geometry forces compression between hydrogen atoms, appreciable downfield shifts result. This deshielding has been ascribed to the migration of electron density away from the interacting protons .…”

A Palladium-Catalyzed Ullmann Cross-Coupling/Reductive Cyclization Route to the Carbazole Natural Products 3-Methyl-9H-carbazole, Glycoborine, Glycozoline, Clauszoline K, Mukonine, and Karapinchamine A

“…These data can be obtained free of charge via , by e-mailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax: + 44 1223 336033). Single-crystal X-ray analyses of compound 1 have been reported previously …”