Coexistence of Metallocene Cations and Anions

We report the synthesis and structural characterization of the rhodocene anion [Rh(C5(CH3)5)(C5(CF3)5)]− [1]− as the [Co(C5(CH3)5)2]+ salt, representing an unprecedented coexistence of metallocene cations and anions in different oxidation states. [1]− was synthesized by the reduction of the rhodocenium cation [Rh(C5(CH3)5)(C5(CF3)5)][BF4] [1]+[BF4]− with two equivalents of decamethylcobaltocene [Co(C5(CH3)5)2], since the strongly electron-withdrawing effect of the [C5(CF3)5)]− ligand shifts the first and second reduction potentials of [1]+ to moderate values of −0.90 V and −1.46 V vs Fc+/Fc. The respective salt [Co(C5(CH3)5)2]+[1]− was characterized by single crystal X-ray diffraction (XRD), also providing the first example of an isolated and structurally characterized 4d metallocene anion. Whereas the Rh(III) cation has two η5- bound Cp ligands, the perfluorinated Cp* ligand is only η3-bound in the Rh(I) anion in order to obey the 18 electron rule. The reduction of the rhodocenium center is also accompanied by a significant shift of the 103Rh NMR signal from −9308 ppm [1]+ to −6895 ppm [1]− (referenced to Rh(acac)3), reflecting the change in coordination geometry.