Dissociation of the phenanthroimidazole dimer - A highly delocalized radical - Gomberg revisited

Results of a calculational study at the B3LYP/6-3/G* level concerning the dimerization of the phenanthroimidazole radical are reported herein. The optimized minimum energies of six dimer structures corresponding to C-C, C-N, and N-N bonding have been obtained. The lowest energy dimer is found to be that resulting from N-N bonding (12). However, this does not correspond to an X-ray structure study of the experimentally isolated dimer (13), which reveals C-N bonding, as is also deduced from an NMR investigation. The variance between the experimental structure and the calculational lowest energy one is examined on the basis of steric interactions: virtually retained planarity, and hence, full conjugation in the two phenanthroimidazole halves in 12, vs. twisting and lack of coplanarity in the other dimer structures. Importantly, spin density calculations reveal a negligible spin density on the nitrogens, rendering dimer 12 formation through N-N bonding highly improbable. A bond dissociation energy of 16.5 kcal/mol (1 cal = 4.184 J) is calculated for the experimental dimer 13, which is by far the lowest C-N bond energy recorded so far in the literature. As well, this value is of the same order of magnitude as the calculated C-C BDE in hexaphenylethane, which had been postulated by Gomberg in 1900 as the molecule formed on dimerization of the triphenymethyl radical, but so far not observed experimentally.