Indenyl Rhodium Complexes with Arene Ligands: Synthesis and Application for Reductive Amination

An efficient protocol for synthesis of indenyl rhodium complexes with arene ligands has been developed. The hexafluoroantimonate salts [(η5-indenyl)­Rh­(arene)]­(SbF6)2 (arene = benzene (2a), o-xylene (2b), mesitylene (2c), durene (2d), hexamethylbenzene (2e), and [2.2]­paracyclophane (2g)) were obtained by iodide abstraction from [(η5-indenyl)­RhI2]n (1) with AgSbF6 in the presence of benzene and its derivatives. The procedure is also suitable for the synthesis of the dirhodium arene complex [(μ-η:η′-1,3-dimesitylpropane)­{Rh­(η5-indenyl)}2]­(SbF6)4 (3) starting from 1,3-dimesitylpropane. The structures of [2e]­(SbF6)2, [2g]­(SbF6)2, and [3]­(SbF6)4 were determined by X-ray diffraction. The last species has a sterically unfavorable conformation, in which the bridgehead carbon atoms of the indenyl ligand are arranged close to the propane linker between two mesitylene moieties. Experimental and DFT calculation data revealed that the benzene ligand in 2a is more labile than that in the related cyclopentadienyl complexes [(C5R5)­Rh­(C6H6)]2+. Complex 2c effectively catalyzes the reductive amination reaction between aldehydes and primary (or secondary) amines in the presence of carbon monoxide, giving the corresponding secondary and tertiary amines in very high yields (80–99%). This protocol is the most active in water.