Indenyl Rhodium Complexes with Arene Ligands: Synthesis
and Application for Reductive Amination
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Abstract
An
efficient protocol for synthesis of indenyl rhodium complexes
with arene ligands has been developed. The hexafluoroantimonate salts
[(η<sup>5</sup>-indenyl)Rh(arene)](SbF<sub>6</sub>)<sub>2</sub> (arene = benzene (<b>2a</b>), <i>o</i>-xylene (<b>2b</b>), mesitylene (<b>2c</b>), durene (<b>2d</b>), hexamethylbenzene (<b>2e</b>), and [2.2]paracyclophane (<b>2g</b>)) were obtained by iodide abstraction from [(η<sup>5</sup>-indenyl)RhI<sub>2</sub>]<sub><i>n</i></sub> (<b>1</b>) with AgSbF<sub>6</sub> 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(η<sup>5</sup>-indenyl)}<sub>2</sub>](SbF<sub>6</sub>)<sub>4</sub> (<b>3</b>) starting from 1,3-dimesitylpropane. The structures of [<b>2e</b>](SbF<sub>6</sub>)<sub>2</sub>, [<b>2g</b>](SbF<sub>6</sub>)<sub>2</sub>, and [<b>3</b>](SbF<sub>6</sub>)<sub>4</sub> 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 <b>2a</b> is more labile than that
in the related cyclopentadienyl complexes [(C<sub>5</sub>R<sub>5</sub>)Rh(C<sub>6</sub>H<sub>6</sub>)]<sup>2+</sup>. Complex <b>2c</b> 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
