5-Amino-3-imino-1,2,6,7-tetracyano-3H-pyrrolizine: characterization of the solvent-free solid phase and its sensing properties

The 5-amino-3-imino-1,2,6,7-tetracyano-3H-pyrrolizine (LH, C11H3N7), previously characterized as the 2:1 1-chloronaphthalene adduct, has been further investigated as a solvent-free solid phase. Strong intermolecular interactions take place in this phase, as revealed by the optical spectra of evaporated LH thin films (lambda(max) = 615 and 570 nm) compared to the optical spectrum of LH in solution (lambda(max) = 580 nm). C-13 NMR spectra also support the occurrence of intermolecular attractive CN group interactions in the solid state. X-Ray diffraction patterns indicate that the controlled sublimation process of LH (T-subl = 200 degrees C, 10(-6) mmHg) leads to films composed of highly oriented crystallites, with two main sets of diffracting planes parallel to the film surface. The refractive index of LH as an evaporated thin film has also been determined in the 400-800 nm spectral range (n = 1-2). LH interacts with ammonia and/or water in the gas phase. In the first case the acid-base reaction (LH + NH3 reversible arrow L'. NH4+) occurs. The resulting L' anion (L' = C11H2N7-) is the 2-(5-amino-3,4-dicyano-2H-pyrrol-2-ylidene)-1,1,2-tricyanoethanide (A, lambda(max) = 525 nm) or the isomer 1,2,6,7-tetracyano-3,5-dihydro-3,5-diiminopyrrolizinide (B, lambda(max) = 680 nm, depending on the relative amount of water to ammonia in the gas phase. This reaction is driven by the hydrogen bonding of NH4+ to B and/or to water. In the second case a fast proton scrambling occurs.