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.