On the acceptor properties of a Al-Nacnac carbene analogue, a density functional investigation

The Al-Nacnac compound is isolobal to the imidazol-2-ylidene (NHC) and forms donor complexes with phosphines and NHCs. These structural types were studied by density functional calculations. They reveal a strong biradical character, since donor addition to the Al-Nacnac reduces the singlet stability of the free carbene-analogue. This facilitates further biradical reactions, such as a subsequent hydrogen shift to the final corresponding tetravalent aluminium product. The latter is determined by maximum singlet stability, i.e. a large singlet-triplet energy separation. Relevance to the analogues Wittig ylids is drawn; these are constituted formally by addition of a phosphine to a carbene, and are well-known as stable synthons in organic chemistry.