Tungsten-mediated synthesis of triazafluorenes

Reaction of 2-pyridinecarboxaldehyde with aliphatic 1,2-diamines in aqueous/ethanolic solution mediated by [W(CN)4O2]4− affords the cationic disubstituted triazafluorenes in good yields. Single-crystal X-ray analysis of 4,9-bis(2-pyridyl)-4a,9a-dihydro-3H,9H-3,8a,9a-triazafluorenium hexafluorophosphate salt 1 reveals the formation of a novel heterocyclic tricycle, triaza analogue of fluorene. The identity and purity of the compounds have been verified with elemental analysis as well as IR, 1H and 13C NMR, UV–Vis, Raman and cyclic voltammetry measurements. Aliphatic 1,3- and 1,4-diamines as well as cycloaliphatic and aromatic ones are not reactive. The proposed pathway for the tungsten-mediated formation of triazafluorenes is discussed.