X-ray Structure of N-(4-Nitrophenacyl)-4,5-diazafluoren-9-onium Bromide

Abstract

Helicity may be induced via intramolecular strain. This type of distortion has usually been associated with systems containing five or more ortho-condensed aromatic rings.1 Recently, helical distortion was detected by NMR spectroscopy and X-ray analysis in novel tetracyclic moieties synthesized from 1,10-phenanthrolinium and 4,5-diazafluorenonium salts.2 Also, in the NMR spectra of N-phenacyl 1,10-phenanthrolinium bromides, the methylenic protons appear as a broad singlet, suggesting a strained stereostructure due to an intramolecular hydrogen bond near the coalescence temperature, which was eventually proved by X-ray analysis.3 However, in the case of 4,5-diazafluorenonium salts, the methylenic signal appears as a sharp singlet, indicating no intramolecular strain, or less strain than in the case of N-(4-chlorophenacyl)phenanthrolinium bromides.3 Furthermore, preliminary molecular modeling with various methods, such as MM2, MM3, PM5 and MNDOd, resulted in a planar structure for the 4,5-diazafluorenonium moiety. In order to verify these deductions, the representative compound 1 (Fig. 1) was subjected to X-ray analysis. Compound 1 was prepared according to a reported method 4 from 4,5-diazafluoren-9-one and 2-bromo-4′-nitroacetophenone. Suitable crystals for X-ray crystallography were obtained by recrystallization from acetic acid.A single crystal measuring 0.15×0.08×0.15 mm was selected for intensity data collection, mounted on a diffractometer and cooled in a continuous stream of nitrogen vapor at 113±2 K. Structure solution by direct methods revealed that a molecule of acetic acid co-crystallizes with the formula unit 1, yielding a solvate. All hydrogen atoms were first located in difference Fourier maps and subsequently added at idealized positions with Uiso set at 1.2-times those of their parent atoms. A full-matrix The quaternization of a nitrogen atom of the 4,5-diazafluoren-9-one molecule, manifested in the title cation, introduces significant asymmetry, reflected in differences in the geometries of the two pyridyl moieties. In constrast to related, previously reported tetracyclic systems that display a high degree of helical distortion, steric congestion in the title cation is minimal, rendering the 4,5-diazafluoren-9-onium moiety virtually planar. This is consistent with predictions based on NMR spectroscopy.

Azahelicenes and other similar tri and tetracyclic helical molecules

“…Again, both salts and target compounds show helical distortion, even though it is significantly reduced by the presence of a smaller central five membered ring instead of the benzene ring in pyrrolophenanthrolines 38. [41][42][43] Br Scheme 27…”