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

The preparation and full characterization of the 4-(nitrophenyl)phenoxyl radical, 2,6-diibutyl-4-(4'-nitrophenyl) phenoxyl radical (Bu2NPArO center dot) is described. This is a rare example of an isolable and crystallographically III 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 0 H bond dissociation free energy (BDFE) of 77.8 +/- 0.5 kcal mo1-1 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.