Synthesis and ESR of a nitroxide biradical of approximate D 2 h symmetry

A nitroxide biradical 2,6‐dioxy‐2,6‐diaza‐1,1,3,3,5,5,7,7‐octamethyldicyclopentano[ b;e ]pyridine (1) and the corresponding monoradical (2‐oxy‐6‐hydroxy) were prepared starting from 2,2,5,5‐tetramethyl‐3‐oxopyrrolidine by successive condensation with formaldehyde (KOH) or DMF (K in THF), NH 4 OAc and oxidation (Na 2 WO 4 ‐H 2 O 2 ). In ethanol, biradical 1 shows a quintet (| J | ≫ | a N | = 14.6 G). In frozen ethanol solution, the experimental spectra (X‐band at 120 K and 240 GHz at 7, 15 and 25 K) can be computer simulated with the following parameters: D = −64 G; E = 1 G; g xx = 2.0093; g yy = 2.0064; g zz = 2.0020; A xx = A yy = 8 G; A zz = 32.2 G. From the temperature dependence of the high‐field spectra, D is found to be negative.