Fluxional σ-bonds of 2,5,8-tri-tert-butyl-1,3-diazaphenalenyl dimers: Stepwise [3,3], [5,5] and [7,7] sigmatropic rearrangements via π-dimer intermediates

Inspired by experimental evidence of the thermally accessible π-dimer of the title compound, DAzPh (7), we propose that the σ-dimer (8) can undergo a variety of σ-bond shifts representing very unusual multi-faceted fluxional bonding between two neutral π-radicals. In this paper, we present a theoretical study of the sigmatropic rearrangement of the DAzPh σ-dimers. Out of the six σ-bonded tautomers three are competitive: a degenerate pair resulting from a [5,5] sigmatropic rearrangement and a non-degenerate product of a [3,3] sigmatropic rearrangement with barriers of 10.21 kcal mol^-1 and 10.00 kcal mol^-1, respectively. Both of these rearrangements occur stepwise through a π-dimer intermediate (9), which is 1.33 kcal mol^-1 higher in energy than the σ-dimer (8). These data are consistent with optical and paramagnetic susceptibility experiments and offer a natural interpretation for the unusual C-C contact distance of 2.153 Å obtained by X-ray diffraction by Morita et al. Another new σ-dimer (15) with a different dipole-dipole stacking pattern is predicted, the energy of which is very close to that of 8, and is likely to be isolable under suitable conditions. The new σ-dimer (15) is expected to undergo stepwise [7,7] sigmatropic rearrangement. Thus we observed a complete spectrum of sigmatropic rearrangement reactions in these DAzPh dimers. The π-dimers 4, 9 and 17 show decreasing order of SOMO-SOMO splittings consistent with the UV-vis absorbance. The calculated paramagnetism is in good agreement with experiments providing further evidence for the presented interpretation of fluxional bonding in the DAzPh σ-dimers.