Binuclear cyclopentadienylrhodium halides and cyanides in the +2 to +3 oxidation states

The pentamethylcyclopentadienylrhodium chlorides (η5-Me5C5)2Rh2(μ-Cl)2 and (η5-Me5C5)2Rh2(μ-Cl)2Cl2 have been synthesized, structurally characterized by X-ray crystallography, and shown to be useful precursors for the synthesis of a variety of organorhodium derivatives, including intermediates for catalytic reactions. We now report density functional theory studies on related Cp2Rh2Xn (X = Cl, F, CN; n = 2, 3, 4) derivatives of the unsubstituted cyclopentadienyl ligand. The lowest energy structures of such species are found to be Cp2Rh2(μ-X)2Xn−2 with terminal Cp ligands and two bridging X ligands. Such predicted structures for Cp2Rh2Cl2 and Cp2Rh2Cl4 are close to the experimental structures for the permethylated derivatives (η5-Me5C5)2Rh2Cln (n = 2, 4). For Cp2Rh2Cl2 the relatively small singlet–triplet separation may relate to the experimentally observed high reactivity of the Rh–Rh bond in (η5-Me5C5)2Rh2(μ-Cl)2. For the rhodium(III) derivatives Cp2Rh2X4 (X = Cl, F, CN), the doubly bridged structures with trans configurations of the two Rh–X bonds to the terminal X groups have lower energies than the corresponding cis isomers. The lowest energy Cp2Rh2(μ-CN)2(CN)n−2 structures have the bridging cyano groups bonded to one rhodium atom through the carbon atom and to the other rhodium atom through the nitrogen atom. Such structures with the bridging cyano groups oriented in the same or opposite directions have similar energies. The Rh(II,III) mixed valence Cp2Rh2X3 structures are predicted to be disfavored relative to disproportionation into Cp2Rh2X2 + Cp2Rh2X4.

The lowest energy Cp2Rh2Xn (X = Cl, F, CN; n = 2, 3, 4) structures have terminal Cp ligands and two bridging X ligands. For Cp2Rh2(μ-X)2X2 the structures with trans configurations of the two Rh–X bonds to the terminal X groups have lower energies than the corresponding cis isomers. The Rh(II,III) mixed valence Cp2Rh2X3 structures are disfavored relative to Cp2Rh2X2 + Cp2Rh2X4.Figure optionsDownload as PowerPoint slideHighlights
► The favored Cp2Rh2Xn (X = Cl, F, CN; n = 2, 3, 4) structures have terminal Cp ligands and two bridging X ligands.
► For Cp2Rh2(μ-X)2X2 the trans isomers are favored over the corresponding cis isomers.
► The Cp2Rh2X3 structures are disfavored relative to Cp2Rh2X2 + Cp2Rh2X4.