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) Å 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 Keq for dissociation is
1.3 ± 0.2 M at 20 °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.