Cyclopentadienone-like Behavior of Fluorenone and 4,5-Diazafluoren-9-one

The aromatic compounds fluorenone (1) and 4,5-diazafluoren-9-one (2) behave like cyclopentadienones in their reaction with [Cp*Co(C₂H₄)₂] (3a), affording the sandwich-type complexes [Cp*Co(η⁴-1)] (4) and [Cp*Co(η⁴-2)] (5), respectively, whose structures have been determined by X-ray diffraction. Both complexes show double-bond fixation in their η²-coordinated six-membered rings and belong to a new structure type of mononuclear organometallics. The cobalt-coordinated ligands 1 and 2 present in 4 and 5, respectively, may be replaced by two molecules of carbon monoxide. In accord with their cyclopentadienone-like nature, they are nucleophilic and undergo O-acylation with acetyl chloride, affording cobaltocenium species.