“Heavy Fluorous” Cyclopentadienes and Cyclopentadienyl Complexes with Three to Five Ponytails: Facile Syntheses from Polybromocyclopentadienyl Complexes, Phase Properties, and Electronic Effects

Substitution Lability of the Perfluorinated Cp* Ligand in [Rh(COD)(C5(CF3)5)] Towards Triphenylpnictogens EPh3 (E=N, P, As, Sb, Bi)

Triphenylpnictogens EPh3 (E=N, P, As, Sb, Bi) are able to displace the perfluorinated Cp* ligand in [Rh(COD)(C5(CF3)5)] (COD=1,5-cyclooctadiene) in up to quantitative yield. The resulting ionic products contain [C5(CF3)5]- as uncoordinated counter anion. The cations feature [Rh(COD)]+ fragments, coordinated by one (N, Bi), two (P, As) or three (Sb) triphenylpnictogen moieties. Whereas coordination via the pnictogen is observed for P, As and Sb, π-coordination of the aryl rings is observed for N and Bi.

Synthesis and Coordination Chemistry of Fluorinated Cyclopentadienyl Ligands

The incorporation of fluorinated substituents in cyclopentadienyl anions leads to distinct changes in the properties of the corresponding metal complexes, e.g., with regards to electrochemical properties or metal–ligand bonding energies. This review summarizes the synthetic challenges of the preparation and coordination of fluorinated cyclopentadienyl anions.

1 Introduction

2 Fluorinated Cyclopentadienyls

3 Cyclopentadienyls with One CF3 Group

4 Cyclopentadienyls with Four CF3 Groups

5 Cyclopentadienyls with Five CF3 Groups

6 Cyclopentadienyls with ‘Fluorous Ponytails’

7 Cyclopentadienyls with C6F5 Groups

8 Trends in Interaction Energies

9 Conclusion

Introducing the Perfluorinated Cp* Ligand into Coordination Chemistry

The reaction of AgBF4 and [Rh(COD)Cl]2 (COD=1,5-cyclooctadiene) in presence of [NEt4 ][C5 (CF3 )5 ] afforded the fluorocarbon soluble complex [Rh(COD)(C5 (CF3 )5 )] by salt metathesis. This complex represents the first example for a successful coordination of the weakly basic [C5 (CF3 )5 ]- ligand, since its first synthesis in 1980. In addition to [Rh(COD)(C5 (CF3 )5 )] also the byproduct [Rh(COD)(C5 (CF3 )4 H)] was isolated and fully characterized. Accompanying DFT studies showed that the interaction energy of the [C5 (CF3 )5 ]- ligand towards the 12-electron fragment [Rh(COD)]+ is ≈70 kcal mol-1 lower in comparison to [C5 (CH3 )5 ]- due to reduced electrostatic interactions and weaker π-donor properties of the ligand. The quantitative but reversible substitution of the [C5 (CF3 )5 ]- ligand by toluene, converting it into a weakly coordinating anion, experimentally proved the extraordinary weak bonding interaction.

Synthesis and characterization of 1,2,3,4,5-pentafluoroferrocene

Starting from ferrocene, pentafluoroferrocene [Fe(C5F5)(C5H5)] can be prepared in five steps via a one-pot lithiation-electrophilic fluorination strategy. Pentafluoroferrocene was characterized by multinuclear NMR and IR spectroscopy, by cyclovoltammetry as well as X-ray (solid) and electron diffraction (gas) and the experimental results compared with DFT calculations.

Dehydration of (Perfluoroalkyl)tetramethylcyclopentenols

(Perfluoroalkyl) tetramethylcyclopentenols (alkyl = n-butyl, n-hexyl, n-octyl) were dehydrated to a complex mixture of endo, endo-(perfluoroalkyl) tetramethyl-cyclopentadienes and their endo-, exo-isomers. It was found in preliminary screening experiments that the best reagent for this transformation, giving an 89% yield of isomeric product mixture, was P₂O₅ in benzene at 80-90 °C. Products were characterized on the basis of their mass spectra and retention time information, and some peaks in the mass spectra were identified from their molecular fragments. Structures were assigned to the three most abundant products of (perfluorohexyl)tetramethylcyclopentenol dehydration. Formal dehydration kinetics showed a second order reaction in benzene but zeroth order with induction period in chlorobenzene, suggesting mass transfer limitations in the more polar chlorobenzene. Some of the products were formed by consecutive isomerization of the others, as shown by the kinetic analysis.