Matrix isolation photolysis study of tetrazolo[1,5-b]pyridazine

Abstract

Photolysis (254 nm) of tetrazolo[1,5-b]pyridazine (TP) in 2-methyltetrahydrofuran at 7 K does not produce electron spin resonance absorptions that are usually characteristic of a triplet nitrene. The UV-vis spectrum formed upon photolysis of TP in 2-methyltetrahydrofuran at 77 K is also consistent with the lack of formation of triplet 3-pyridazylnitrene. Photolysis (254 nm) of tetrazolo[1,5-b]pyridazine (TP), i.e., 3-azidopyridazine in an Ar matrix at 16 K produces an infrared spectrum consistent with the formation of Z-cis-1-cyano-3-diazopropene, 1,3,7-triazacyclohepta-1,2,4,6-tetraene, and 3-cyanocyclopropene whose spectra were adequately simulated by density functional theory with a 6-31G* basis set. There is no spectral evidence to support the formation of triplet 3-pyridazylnitrene. Cyclization of the singlet pyridazylnitrene is observed in minute amounts to ultimately produce the ring-expanded 1,3,7-triazacyclohepta-1,2,4,6-tetraene (ν_NCN = 1981 cm⁻¹). This observation indicates that the earlier vibrational assignment to 1,3,7-triazacyclohepta-1,2,4,6-tetraene in the 2-pyrimidylnitrene system (ν_NCN = 2045 cm⁻¹) is in error, and should be attributed to the 1972 cm⁻¹ vibrational absorption in the previously recorded spectrum.