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

The aromatic compounds fluorenone (<b>1</b>) and 4,5-diazafluoren-9-one (<b>2</b>) behave like cyclopentadienones in their reaction with [Cp*Co(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub>] (<b>3a</b>), affording the sandwich-type complexes [Cp*Co(η<sup>4</sup>-<b>1</b>)] (<b>4</b>) and [Cp*Co(η<sup>4</sup>-<b>2</b>)] (<b>5</b>), respectively, whose structures have been determined by X-ray diffraction. Both complexes show double-bond fixation in their η<sup>2</sup>-coordinated six-membered rings and belong to a new structure type of mononuclear organometallics. The cobalt-coordinated ligands <b>1</b> and <b>2</b> present in <b>4</b> and <b>5</b>, 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.