Spatial Structure and Nontrivial Stereodynamics of Tricyclic Perhydro-1,2,4,5-Tetrazines

Thermolysis of 6-aryl-1,5-diazabicyclo[3.1.0]hexanes in methanol leads to the respective dimers, 5,11-diaryltetrahydro-1H,7Hdipyrazolo[1,2-a:1',2'-d]tetrazines. X-ray structural analysis of these compounds reveals the perhydrotetrazine ring adopting a chair conformation with the equatorially fused five-membered rings and with the equatorial orientation of the aryl substituents. A hindered rotation of the aryl substituents around the C–Ar bonds was observed by low-temperature NMR. Another NMR-observed process that takes place at low temperature is a reversible isomerization of the major eeee conformer into a slightly less stable aeee isomer. Density functional theory calculations (B3LYP/6-31+G(d)) on the conformational transformations of the parent tricyclic perhydrotetrazine and its 5,11-diphenyl derivative are in good agreement with the experimental findings and reveal that a conformation with the orthogonal orientation of the planes of the aryl substituents in respect to the average plane of the six-membered ring has the lowest energy.