Fluorene Complexes of Group 9 Metals: Fluorene Effect and Application for Reductive Amination

The η⁶-fluorene cyclopentadienyl complexes [(η⁵-C₅R₅)­M­(η⁶-fluorene)]­(SbF₆)₂ (1: M = Co, R = Me; 2a: M = Rh, R = H; 2b: M = Ir, R = H) were synthesized by the iodide abstraction from [(η⁵-C₅R₅)­MI₂]_n with AgSbF₆ in the presence of fluorene. The procedure is also suitable for the synthesis of the indenyl derivatives [(η⁵-indenyl)­M­(η⁶-fluorene)]­(SbF₆)₂ (3a: M = Rh; 3b: M = Ir) starting from [(η⁵-indenyl)­MI₂]_n. The structures of [1]­(SbF₆)₂ and [2b]­(SbF₆)₂ were determined by X-ray diffraction. The rhodium complex [2a]­(SbF₆)₂ readily undergoes the replacement of the fluorene ligand by mesitylene, being more reactive than the benzene derivative [CpRh­(η⁶-C₆H₆)]­(SbF₆)₂. According to experimental data and DFT calculations, the fluorene elimination proceeds via a cascade of η⁶ → η⁵ → η¹ haptotropic rearrangements. The complex [2b]­(SbF₆)₂ (at 1 mol % loading) effectively catalyzes the reductive amination reaction between aldehydes (or ketones) and primary aromatic (or secondary aliphatic) amines in the presence of carbon monoxide, giving the corresponding secondary and tertiary amines in the range of yields 59–91%. This protocol uses water as a solvent.