Triazoloazine–Diazomethylazine Valence Isomerization. [1,2,3]Triazolo­[1,5‑a]­pyridines and 2‑Diazo­methyl­pyridines

2-Diazomethylpyridines 1D and 6D, the valence isomers of [1,2,3]­triazolo­[1,5-a]­pyridines 1T and 6T, have been observed directly at ∼2080 cm–1 by a combination of mild flash vacuum pyrolysis (FVP) at 200–600 °C with low temperature IR spectroscopy. Calculations confirm a ca. 17 kcal/mol barrier for the formation of 2-diazo­methyl­pyridine 1D from [1,2,3]­triazolo­[1,5-a]­pyridine 1T, the diazo compound lying ca. 5 kcal/mol above the triazole. In the higher temperature range (400–600 °C) 2-diazomethyl­pyridine 1D eliminates N2 with formation of 2-pyridyl­carbene 2 and rearrangement to 1-cyano­cyclo­penta­diene 4. 2-Diazo­methyl­pyridine 1D undergoes 1,3-dipolar cycloaddition with tetra­cyano­ethylene (TCNE) at 20–90 °C to yield 3-(2-pyridyl)­cyclo­propane­tetra­carbo­nitrile 11 and 3-(tricyano­vinyl)-[1,2,3]­triazolo­[1,5-a]­pyridine 13T via unobserved pyrazolines 10 and 12. FVP of triazole 13T affords an IR absorption at 2080 cm–1 ascribed to the corresponding diazo compound 13D.