Electrochemical Studies of Organometallic Complexes with Tetra-n-butylammonium Tetrakis[3,5-bis(trifluoromethyl)phenyl]borate as the Electrolyte. X-ray Crystal Structure of [C₅(CF₃)(CH₃)₄]Fe(C₅H₅)

The tetra-n-butylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TBA+TFPB^-) electrolyte/methylene chloride solvent system improves the electrochemical reversibility of pentamethylcyclopentadienyl (Cp*) ruthenocenes, Cp*RuCp' (Cp' = fluorenyl, indenyl, cyclopentadienyl (Cp), acetylcyclopentadienyl, pentachlorocyclopentadienyl), as indicated by i_pc/i_p,a ratios of 0.79-1.0 as determined by cyclic voltammetry. The quasi-reversible potentials (E°') of the Cp*RuCp' complexes and the complete series of group 9 Cp₂M and Cp*₂M complexes (M = Fe, Ru, Os) are also reported in TBA⁺TFPB^_/CH₂Cl₂. In addition, a study of the E°' values of group 9 complexes containing the (trifluoromethyl)tetramethylcyclopentadienyl (Cp^‡) ligand indicate that the Cp^‡ complexes are slightly (0.06-0.08 V per Cp^‡) more difficult to oxidize than the cyclopentadienyl (Cp) derivatives. The structure of [C₅(CF₃)(CH₃)₄]Fe(C₅H₅) was determined at -101 °C by a single-crystal X-ray diffraction study. The structure shows eclipsed Cp^‡ and Cp rings, and the iron to Cp^‡ centroid distance (1.643 Å) is slightly shorter than the iron to Cp centroid distance (1.651 Å); otherwise, no extreme differences in the coordination of the Cp^‡ and Cp rings are noted. An infrared spectroelectrochemistry study of trans-[Cp^‡Fe(CO)₂]₂ shows that it is electrochemically oxidized to trans-[Cp^‡Fe(CO)₂]₂⁺ in TBA⁺TFPB-/CH₂C1₂.