Generation of quintet dinitrenes by the γ-radiolysis of crystalline 2,4,6-triazido-3,5-dicyanopyridine

Abstract

High-spin nitrenes are promising organic magnetic materials for molecular electronics.1 These compounds are usually obtained by low-temperature solid-phase photolysis of aromatic azides, the aromatic ring of which is protected by appropriate substituents from undesirable intramolecular reactions with nitrenic centres.2 Unfortunately, many substituents (e.g., CF3 or CN4) in aryl nitrenes do not protect them from photochemical rearrangements and this substantially cuts down the potential of photochemical synthesis of high-spin nitrenes. Thus, for instance, our previous attempts5 to obtain high-spin nitrenes 4, 5 and 6 by the photolysis of triazide 1 were unsuccessful. In order to develop a method for generating photochemically unstable high-spin nitrenes, the γ-radiolysis of crystalline triazide 1 at 77 K was studied. Although 2,4,6-triazidopyridines are high-energy compounds and readily decompose upon the action of UV light, X-ray, temperature or pressure, triazide 1† showed unexpectedly high stability toward γ-rays. Only a very prolonged γ-irradiation of crystalline 1 in a vacuum‡ at 77 K led to the appearance of two EPR signals at 3030 and 3346 G [Figure 1(a)], which are characteristic of quintet 2,4and 2,6dinitrenopyridines, respec-tively.6 Similar EPR spectra with two characteristic signals of quintet 2,4-dinitreno-6-azido3,5-dichloropyridine at 3009 G and of 2,6-dinitreno-4-azido3,5-dichloropyridine at 3345 G have been recorded during the photolysis of 2,4,6-triazido-3,5-dichloropyridine 7 in frozen (77 K) solutions in 2-methyl-tetrahydrofuran.7 Owing to the presence of chlorine atoms in the 3and 5-positions of the pyridine ring, high-spin nitrenes were photochemically stable and available by photochemical synthesis.5 The successful generation of quintet dinitrenes 4 and 5 from triazide 1 shows that light-sensitive high-spin nitrenes can be readily prepared by low-temperature γ-radiolysis of crystalline azides. This method is insensitive to transparency of the solid phase and allows the generation of high-spin nitrenes in the whole volume of a crystalline sample but not only on its surface. Our kinetic studies of the initial stages of the γ-radiolysis of crystalline triazide 1 showed that radiation yields of quintet dinitrenes 4 and 5 are equal to 0.200 and 0.015 per 100 eV of the energy applied, respectively. Note that quintet dinitrenes 4 and 5 were also obtained in nearly the same radiation yields (0.130 and 0.017) on γ-irradiation of crystalline 1 in air‡ at 77 K. This indicates that generation of high-spin nitrenes in azide crystals by γ-radiolysis is insensitive to air. According to statistical considerations, quintet dinitrenes 4 to 5 should be formed in equal quantities. The much higher radiation yield of quintet 4 indicates that α-azido groups of triazide 1 on comparison with its γ-azido group are much more sensitive toward the action of γ-irradiation. The preferential decomposition of the α-azido groups has been observed in the γ