A C-C Bonded Phenoxyl Radical Dimer with a Zero Bond Dissociation Free Energy

The 2,6-di-tert-butyl-4-methoxyphenoxyl radical is shown to dimerize in solution and in the solid state. The X-ray crystal structure of the dimer, the first for a para-coupled phenoxyl radical, revealed a bond length of 1.6055(23) angstrom for the C4-C4a bond. This is significantly longer than typical C-C bonds. Solution equilibrium studies using both optical and IR spectroscopies showed that the kg for dissociation is 1.3 +/- 0.2 M at 20 degrees C, indicating a C-C bond dissociation free energy of -0.15 +/- 0.1 kcal mol(-1). Van't Hoff analysis gave an exceptionally small bond dissociation enthalpy (BDE) of 6.1 +/- 0.5 kcal mol(-1). To our knowledge, this is the smallest BDE measured for a C-C bond. This very weak bond shows a large deviation from the correlation of C-C bond lengths and strengths, but the computed force constant follows Badger's rule.