Scandium Phosphonioketene: Synthesis, Bonding and Reactivity

Reaction of a scandium phosphoniomethylidene with carbon monoxide provides the first scandium phosphonioketene (1). X-ray diffraction analysis shows that the complex has a very short Sc-C bond (2.138(2) Å), and DFT calculations indicate that this unusual short bond length is due to the significant contribution of ionic coulomb interaction between carbon and scandium and the η² -O,C coordination fashion. Complex 1 is thermally stable, albeit shows high reactivity towards a series of unsaturated substrates, including carbon dioxide, ketone, imine, nitrile and isocyanide. In the reaction with tert-butyl isocyanide, not only an insertion of tert-butyl isocyanide into the Sc-C bond occur, but also a C-H activation on the phenyl ring. DFT calculations show that the reactivity of 1 operated by nucleophilic properties, and therefore the reaction mechanism favors the nucleophilic attack to isocyanide as a rate-determining step, followed by the stepwise C-H activation through an interesting C-H deprotonation.