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

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 C9C9′ 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 C9′ 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–C4a/S10–C10a and Se10–C4a/Se10–C10a 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 (λmax = 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 (C9′) were positive (downfield) = +6.8 (12), +6.5 (13), +5.8 (14), +11.7 (15), +7.7 ppm (16). In 15, Δδ (C9) – Δδ (C9′) = +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 (N1′ and N8′) in 13 and 14 causes shielding of the selenium and tellurium nuclei.