Preparation, Structural Characterization, and Thermochemistry of an Isolable 4‑Arylphenoxyl Radical

The
preparation and full characterization of the 4-(nitrophenyl)­phenoxyl
radical, 2,6-di-tbutyl-4-(4′-nitrophenyl)
phenoxyl radical (tBu2NPArO•) is described. This is a rare example of an isolable
and crystallographically characterized phenoxyl radical and is the
only example in which the parent phenol is also crystallographically
well-defined. Analysis of EPR spectra indicates some spin delocalization
onto the secondary aromatic ring and nitro group. Equilibrium studies
show that the corresponding phenol has an O–H bond dissociation
free energy (BDFE) of 77.8 ± 0.5 kcal mol–1 in MeCN (77.5 ± 0.5 kcal mol–1 in toluene).
This value is higher than related isolated phenoxyl radicals, making
this a useful reagent for hydrogen atom transfer (HAT) studies. Additional
thermochemical and spectroscopic parameters are also discussed.