First Isocyanoazulene and Its Homoleptic Complexes

An efficient synthesis of remarkably stable 6-isocyanoazulene (CN⁶Az), the first nonbenzenoid organic isocyanide, is described. Its superb π-accepting potential as a ligand was demonstrated through cyclic voltammetry studies on the binary complexes [Cr(CN⁶Az)₆]^0/+. The paramagnetic shift patterns for [Cr(CN⁶Az)₆]⁺, the only azulenic π-system studied by paramagnetic NMR, suggest that Cr(dπ) → CN⁶Az(pπ*) electron delocalization involves both rings of the azulenyl substituents in [Cr(CN⁶Az)₆]⁺. This conclusion is supported by density functional theory calculations on the complex [Cr(CN⁶Az)₆][BF₄].