Overcrowded 1,8-diazafluorenylidene-chalcoxanthenes. Introducing nitrogens at the fjord regions of bistricyclic aromatic enes

Abstract

The effects of introducing nitrogen atoms in the fjord regions and chalcogen bridges on the conformations of overcrowded bistricyclic aromatic enes (1, X ≠ Y) (BAEs) were studied. 9-(9′H-1′,8′-Diazafluoren-9′-ylidene)-9H-thioxanthene (12), 9-(9H-1′, 8′-diazafluoren-9′-ylidene)-9H-selenoxanthene (13), 9-(9′H-1′,8′-diazafluoren-9′-ylidene)-9H-telluroxanthene (14), 9-(9′H-1′,8′-fluoren-9-ylidene)-9H-xanthene (15) and 9-(9′H-1′,8′-fluoren-9′-ylidene)-9H-fluorene (16) were synthesized by two-fold extrusion coupling reactions of 1,8-diaza-9H-fluoren-9-one (19)/chalcoxanthenthiones (24–27) (or /9H-fluorene-9-thione (30)). The 1′,8′-diazafluoren-9-ylidene-chalcoxanthenes (11) were compared with the respective fluoren-9-ylidene-chalcoxanthenes (10). The structures of 12–16 were established by ¹H, ¹³C, ⁷⁷Se, and ¹²⁵Te NMR spectroscopies. The crystal and molecular structures of 12–14 were determined by X-ray analysis. The yellow molecules of 12–14 adopted mono-folded conformations with folding dihedrals in the chalocoxanthylidene moieties of 62.7° (12), 62.4° (13) and 59.9° (14). The folding dihedrals in the respective 1′,8′-diazafluorenylidene moieties were very small, ca. 2°, compared with 10.2/8.0° in (9′H-fluoren-9′-ylidene)-9H-selenoxanthene (7). A 5° pure twist of C⁹C^9′ in 14 is noted. The degrees of overcrowding in the fjord regions of 12–14 (intramolecular non-bonding distances) were relatively small. The degrees of pyramidalization of C⁹ and C^9′ were 17.0/3.0° (12), 17.4/2.4° (13) and 2.2/2.2° (14). These high values in 12 and 13 stem from the resistance of the 1,8-diazafluorenylidene moiety to fold and from the limits in the degrees of folding of the thioxanthylidene and selenoxanthylidene moieties (due to shorter S¹⁰–C^4a/S¹⁰–C^10a and Se¹⁰–C^4a/Se¹⁰–C^10a bonds, as compared with the respective Te–C bonds in 14). The molecules of 15 and 16 adopt twisted conformations, a conclusion drawn from the ¹H NMR chemical shifts of the fjord regions protons (H¹ and H⁸) at 8.70 (15) and 9.00 ppm (16) and from their colors and UV/VIS spectra: 15 is purple (λ_max = 521 nm) and 16 is orange–red. A comparison of the NMR spectra of 11 and 10 (Δδ = δ(11) – δ(10)) showed substantial downfield shifts of 0.56–0.62 ppm of the fjord regions protons of twisted 15 and 16: Δδ (C⁹) were negative (upfield): −4.0 (12), −3.7 (13), −3.4 (14), −7.1 (15), −5.0 ppm (16), while Δδ (C^9′) were positive (downfield) = +6.8 (12), +6.5 (13), +5.8 (14), +11.7 (15), +7.7 ppm (16). In 15, Δδ (C⁹) – Δδ (C^9′) = +18.8 ppm, attributed to a push–pull character and significant contributions of zwitterionic structures in the twisted conformation. The ⁷⁷Se and ¹²⁵Te NMR signals of 13 and 14 were shifted upfield relative to the respective fluorenylidene-chalcoxanthene derivatives: Δδ⁷⁷Se = 17.2 ppm and Δδ¹²⁵Te = 22.0 ppm. The presence of the nitrogen atoms (N^1′ and N^8′) in 13 and 14 causes shielding of the selenium and tellurium nuclei.