Introducing the Perfluorinated Cp* Ligand to Coordination Chemistry

Abstract

The perfluorinated Cp* anion [C5(CF3)5]− is the extremely electron deficient counterpart of the well-studied Cp* ligand [C5(CH3)5]−. Although its first synthesis took place as early as 1980, the perfluorinated Cp* remained a synthetic dead end for decades, due to its low reactivity. Here, the preparation and full characterization of its first coordination complex [Rh(COD)(C5(CF3)5)] (COD = 1,5-cyclooctadiene) is presented. This allowed for extraordinary insights on the bonding situation between a metal and an electron-poor Cp ligand and revealed [C5(CF3)5]− as the weakest bound Cp ligand known.

The weakly bonding character of the perfluorinated Cp* is demonstrated by the quantitative and even reversible substitution by toluene to the cationic [Rh(COD)(PhMe)][C5(CF3)5] complex. Also the metallocenes [M(C5H5)(C5(CF3)5)] (M = Fe, Ru) are studied with respect to an unprecedented substitution lability of the perfluorinated Cp* ligand. Their photolysis in MeCN not only yields [M(C5H5)(MeCN)3][C5(CF3)5], but also reveals a unique photo/thermoswitchability by the back-reaction to the corresponding ruthenocene.

The low nucleophilicity and high oxidative resistance of [C5(CF3)5]− is further demonstrated by the coexistence with electrophilic and oxidizing cations and its introduction into the group of weakly coordinating carbanions (WCCAs). The prepared salts comprise hydride-accepting [(C6H5)3C]+, valuable Ag(I) reagents, oxidizing [Fe(C5H5)2]+ or [N(p-C6H4Br)3]+ and Brønsted acidic [H(m,m-NC5H3F2)2]+.

The preparation of the first complete series of coinage metal Cp coordination compounds with [M(C5(CF3)5)(PtBu3)] (M = Cu, Ag, Au) demonstrates the ability of the perfluorinated Cp* to stabilize metal centers by its extreme oxidative resistance. The binding modes between metal and ligand range from η3 to η1 and illustrate the coordinative versatility of [C5(CF3)5]−.

The impact of the perfluorinated Cp* ligand on the redox chemistry of metal complexes is demonstrated by the synthesis of the extremely electron deficient ferrocene [Fe(C5H5)(C5(CF3)5)]. Oxidation potentials of E1/2 = +1.35 V (vs. Fc/Fc+) represent the highest reported values obtained for any ferrocene. The corresponding stable and storable ferrocenium [Fe(C5H5)(C5(CF3)5)][AsF6] is the strongest organometallic oxidant to date and even capable of C-H activation.

Finally, the rhodocenium salt [Rh(C5(CH3)5)(C5(CF3)5)][BF4] is prepared. The surprisingly high reduction potentials E1/2 = −0.90 and −1.46 V (vs. Fc/Fc+) combined with the coordinative flexibility of [C5(CF3)5]− allow for the twofold reduction by decamethylcobaltocene [Co(C5(CH3)5)2]. The reported [Co(C5(CH3)5)2][Rh(C5(CH3)5)(C5(CF3)5)] not only represents the first structurally characterized 4d metallocene anion, but also shows an unprecedented coexistence with a metallocene cation.