Triplet States of Tetrazoles, Nitrenes, and Carbenes from Matrix Photolysis of Tetrazoles, and Phenylcyanamide as a Source of Phenylnitrene

Photolysis of 1- and 5-aryltetrazoles at 5–10 K using a 266 nm laser immediately generates their triplet excited states, which are characterized by their electron spin resonance (ESR) spectra with zero-field splitting parameters D = 0.12–0.13 cm^–1 and E = 0.002–0.008 cm^–1. Further photolysis of all of the aryltetrazoles affords arylnitrenes (D ≅ 1 cm^–1), and in the case of 5-aryltetrazoles also arylcarbenes (D ≅ 0.5 cm^–1). The formation of arylnitrenes from 5-aryltetrazoles, where no aryl–N bond is present, is explained by the photochemical rearrangement of initially formed nitrile imines ArCN⁺N^–R to carbodiimides. The monosubstituted carbodiimide PhNCNH isomerizes to phenylcyanamide, PhNH–CN, and photolysis of the latter causes rapid elimination of HCN and formation of phenylnitrene. When N-methyl groups are present in the tetrazoles, methylnitrene, CH₃–N, is formed too. In the case of 5-phenyltetrazole, additional hydrogen shift and fragmentation afford cyano- and isocyanonitrenes, NCN and CNN.