Introducing the Perfluorinated Cp* Ligand into Coordination Chemistry

Abstract 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)4H)] 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.

2 spectra are device-internal calibrated relative to the resonance signal of tetramethylsilane, by the unified chemical shift scale. The given multiplicities are phenomenological, thus the actual appearance of the signals is stated and not the theoretically expected one. The following abbreviations were used and analogously combined to designate multiplicities: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), mc (centrosymmetric multiplet). For centrosymmetric multiplets the center and for non-symmetric multiplets the interval is stated. Evaluation of spectra was performed with Mestrelab Research MNova 7. [3] Infrared (IR) spectroscopy IR spectroscopy was measured on a FT (Fourier transformation) Nicolet iS10 or iS50 IRspectrometer. The sample was directly measured by ATR (attenuated total reflection) technique.
Characteristic absorptions are given in wavenumbers ṽ [cm 1 ] and intensities are stated as vs (very strong), s (strong), m (medium) and w (weak).

High resolution mass spectroscopy (HRMS)
HRMS was recorded using a Varian MAT 711 spectrometer by electron impact ionization (EI) or electrospray ionization (ESI) at the department of mass spectroscopy at the Freie Universität Berlin. A detailed listing of fragmentation is dispensed, instead the molecular ion peak or a characteristic fragment peak is stated.

X-ray diffraction (XRD)
X-Ray data were collected on a BRUKER D8 Venture system. Data were collected at 100(2) K using graphite monochromated Mo Kα radiation (λα = 0.71073 Å). The strategy for the data collection was evaluated by using the Smart software. The data were collected by the standard "ψ-ω scan techniques" and were scaled and reduced using Saint+software. The structures were solved by using Olex2, [4] the structure was solved with the XT [5] structure solution program using Intrinsic Phasing and refined with the XL refinement package [6,7] using Least Squares minimization. Part instructions were used to deal with disorder. Bond length and angles were measured with Diamond Crystal and Molecular Structure Visualization Version 4.6.2. [8] Drawings were generated with POV-Ray. [9] 3

1,2,3,4-tetrakis(trifluoromethyl)cyclopentadienide and tetraethylammonium 1,2,3,4,5-pentakis(trifluoromethyl)cyclopentadienide
In a dried 1000 mL pressure flask anhydrous KF (60 g, 1.0 mol, 11 equiv.) was placed in anhydrous and degassed sulfolane (190 mL) under an atmosphere of argon. Anhydrous and degassed 18-crown-6 (8.7 g, 33 mmol, 0.33 equiv.) and hexachlorobuta-1,3-diene (15 mL, 96 mmol, 1.0 equiv.) were added at rt. The resulting reaction mixture was carefully shaken and cooled to196 °C in high vacuum. The properly closed pressure flask was slowly warmed to 190 °C and stirred at this temperature for 3 d. Then the resulting black suspension was cooled to rt and the volatiles were removed in high vacuum. The remaining mixture was filtrated under an atmosphere of argon and the residue was extracted with anhydrous MeCN (3 × 40 mL). The filtrate was warmed to 40 °C and all MeCN was removed in high vacuum, while stirring. The resulting solution was put under high vacuum and H2SO4 (conc., 200 mL, 3.6 mol, 38 equiv.) was added dropwise at rt over a period of 3 h, while stirring and continuously collecting the volatiles in a cold trap of 196 °C. After complete addition, the mixture remained for additional 2 h in high vacuum.
The cold trap was put under argon and slowly warmed to 0 °C, giving a pale yellow liquid. Then CH2Cl2 (20 mL) and a solution of [NEt4][OH] (35% in water, 10 mL, 24 mmol, 0.25 equiv.) were added and the reaction mixture was stirred for 15 min at rt, giving a deep red solution. The aqueous layer was separated and extracted with CH2Cl2 (4 × 20 mL). The combined organic layers were dried over MgSO4, filtrated and the solvent was removed under reduced pressure. The remaining solid was suspended in Et2O (~5 mL) and recrystallized twice from CH2Cl2 (~10 mL) by slowly cooling to 20 °C. The crystalline residue was decanted and washed with Et2O (2 × 5 mL). The analytical data are consistent with those reported in literature. [16,17] η 2 ,η 2 -(Cycloocta-1,5-diene)-η 5