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

Oxidative Addition of π‑Bonds and σ‑Bonds to an Al(I) Center: The Second-Order Carbene Property of the AlNacNac Compound

The oxidative addition of π- and σ-bonds is studied by means of quantum chemical investigations at a MCSCF and density functional level of sophistication. The title compound (AlNacNac) induces first-order strong donor–acceptor abilities in the triplet state, giving rise to biradicaloid adducts. At second-order, it reveals carbene character. The energy barriers for the 1,2-addition reactions are fairly small, resulting from an oxidative addition, which differs from the classical 1,2-addition reaction of a carbene to an olefin. For the splitting of σ-bonds (H–X) the energy barriers are largely driven by the strengths of the H–X bonds. The metal Al increases continuously its oxidation state from the educt over the transition state to the product. This implies that in the latter complexes the metal is positive and the olefin overall negative in charge. Ethylene itself does not form a stable adduct; it is still in equilibrium with AlNacNac plus ethylene. However, electron releasing substituents stabilize the addition product. The stabilities of various three-membered ring systems are evaluated. Hydrogen splitting possesses a relatively large barrier

On the Stability of Perfluoroalkyl-Substituted Singlet Carbenes: A Coupled-Cluster Quantum Chemical Study

A series of trifluoromethyl-substituted carbenes R–C(:)–CF₃ (R = NMe₂, OMe, F, PMe₂, P­(NMe₂)₂, P­(N­(Pr-i)₂)₂, SMe, Cl); (dimethylamino)­(perfluoroalkyl)­carbenes Me₂N–C­(:)–R (R = CF₃, C₂F₅, <i>n</i>-C₃F₇, <i>i</i>-C₃F₇, and <i>t</i>-C₄F₉) and symmetrically substituted carbenes R–C(:)–R (R = NMe₂, OMe, F, PMe₂, SMe, Cl) have been investigated by means of quantum chemistry methods. Different levels of approximation were used, including the CCSD­(T) approach also known in quantum chemistry as the “golden standard”, in combination with three different basis sets (TZVP, cc-pVDZ, cc-pVTZ). Relative stabilities of carbenes have been estimated using the differences between the singlet and triplet ground state energies (Δ<i>E</i>_ST) and energies of the hydrogenation reaction for the singlet and triplet ground states of the carbenes. The latter seem to correlate better with stability of carbenes than the Δ<i>E</i>_ST values. The ¹³C NMR chemical shifts of the methylidene carbon indicate the more high-field chemical shift values in the known, isolable carbenes compared to the unstable ones. This is the first report on the expected chemical shifts in the highly unstable singlet carbenes. Using these criteria, some carbene structures from the studied series (as, for instance, Me₂N–C­(:)–CF₃, Me₂N–C­(:)–C₃F₇-<i>i</i>) are proposed as good candidates for the experimental preparation