Photolysis and Oxidation of Azidophenyl-Substituted Radicals: Delocalization in Heteroatom-Based Radicals

2-(4-Azidophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (14), 2-(4-azidophenyl)benzimidazole-1-oxide-3-oxyl (16), 2-(4-azidophenyl)-1,2,6-triphenylverdazyl (19), 2-(3-azidophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (21), and (3-azidophenyl)-N-tert-butyl-N-aminoxyl (25) were photolyzed in frozen solution to give S = 3/2 state ESR spectra of the
corresponding nitrenophenyl radicals with the following zero-field splitting parameters:  |D/hc| =
0.277 cm-1, |E/hc| ≤ 0.002 cm-1 (7 from 14); |D/hc| = 0.256 cm-1, |E/hc| ≤ 0.002 cm-1 (8 from 16);
|D/hc| = 0.288 cm-1, |E/hc| ≤ 0.002 cm-1 (9 from 19); |D/hc| = 0.352 cm-1, |E/hc| = 0.006 cm-1 (10
from 21); |D/hc| = 0.336 cm-1, |E/hc| = 0.004 cm-1 (11 from 25). UB3LYP/6-31G* computations
and ESR spectroscopic analyses suggest that these are nitreno radicals, even para-linked systems
with possible quinonoidal resonance forms. Neat samples of azidophenyl radicals 14 and 21 showed
bulk paramagnetic behavior, consistent with the lack of close contacts in their crystal structures.
Efforts to make photolabile coordination complexes of 14 and 21 with paramagnetic transition metal
ions were unsuccessful:  Cu(ClO4)2·6H2O instead oxidized them to the corresponding diamagnetic
nitrosonium perchlorate salts.