Europium(II) Ynolate Complex: Synthesis, Bonding, and Reactivity

Anionic ketenes [RCCO]^- exhibit two resonance forms: ketenyl anions and ynolate anions. While ketenyl complexes have been isolated and characterized, crystalline ynolate complexes remain elusive. Herein, we report the first lanthanide ynolate complex, namely, the europium ynolate complex. A σ-bond metathesis reaction of Me₃Si(H)CN₂ with a europium(II) hydrido or alkyl complex yields a europium(II) nitrilimine, which reacts with CO to provide a europium(II) ynolate complex. Single-crystal X-ray diffraction and DFT calculations confirm the dominance of the ynolate resonance form, showing two orthogonal π orbitals between the C-C unit and a significant negative charge (-0.83) on the oxygen atom. Reactivity studies demonstrate C-centered trapping products with trimethylsilyl trifluoromethanesulfonate, benzophenone, isothiocyanate, and carbodiimide. Mechanistic DFT analysis of the carbodiimide reaction reveals a gradual shift from ynolate to ketenyl anion, enhancing reactivity for [2 + 2] cyclization.