White phosphorus degradation with a NacNac aluminum carbene analogue: the biradical reaction mechanism.

The title compound Al-NacNac is isolobal to the imidazol-2-ylidene (NHC); the latter is considered as a nucleophilic carbene. However, the title compound is different from a typical carbene, as aluminum is a heavier group 13 element with a predominant inert s orbital. Its singlet ground state is a poor Lewis donor (acceptor) toward white phosphorus, but its corresponding lowest energy triplet state forms a strong Al-P bond with (opened) white phosphorus. The reaction of Al-NacNac with white phosphorus proceeds in two steps: after the addition of a first carbene analogue, a second one is added, resulting in a transient biradicaloid species. This undergoes facile subsequent rearrangement, and a final ring closure reaction leads to the observed product with a bicyclobutane moiety. It is determined by intramolecular bond formation of two phosphorus centered radicals. Finally, a structure with a large singlet-triplet energy separation is formed. An analogy to the noninnocent ligand character as well as the exciplex view of the monoadduct of white phosphorus with the Al-NacNac system is drawn.