Coexistence of Metallocene Cations and Anions

Abstract

We report the synthesis and structural characterization of the rhodocene anion [Rh(C5Me5)(C5(CF3)5)]− [1]− as the [Co(C5Me5)2]+ salt, representing an unprecedented coexistence of metallocene cations and anions in different oxidation states. [Co(C5Me5)2]+ [1]− was synthesized by the reduction of the rhodocenium cation [Rh(C5Me5)(C5(CF3)5)][BF4] ([1]+[BF4]−) with two equivalents of decamethylcobaltocene [Co(C5Me5)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 and −1.46 V vs FcH0/+. [Co(C5Me5)2]+[1]− was characterized by single-crystal X-ray diffraction (XRD), providing the first example of a structurally characterized 4d metallocene anion. Whereas the RhIII cation has two η5-bound Cp ligands, the perfluorinated Cp* ligand is only η3-bound in the RhI anion in order to obey the 18-electron rule. The reduction of the rhodocenium center is accompanied by a significant shift of the 103Rh NMR signal from −9308 ppm of [1]+ to −6895 ppm of [1]− (referenced to Rh(acetylacetonate)3).