Pentamethylcyclopentadienyl chromium complexes in ethylene polymerization and oxidation reactions

Organometallic chromium complexes containing Cp*(η5-C ₅Me₅) as an ancillary ligand, and in various oxidation states such as +III, +V, and +VI have been synthesized. Cp*CrIII(THF)(CH ₂SiMe₃)(OSiPh₃), prepared by protonation of Cp*Cr(THF)(CH ₂SiMe₃)₂ with Ph₃SiOH, is a structural model for the Union Carbide catalyst in which the siloxide ligand serves as a proxy for the silica surface to which the chromium is attached. This complex showed catalytic activity towards ethylene at ambient conditions. Attempts to synthesize a terminal chromium alkylidene have been made. Reactions of Cp*Cr(THF)(CH₂SiMe₃)₂ or metallacycle Cp*(THF)Cr(CH ₂)₂SiMe₂ with 1,2-bis(dimethylphosphino)ethane (dmpe) led to the formation of dinuclear chromium alkyls [Cp*Cr(CH₂SiMe ₃)₂]₂(μ-η1:η 1-dmpe) and [Cp*Cr(CH₂)₂SiMe₂] ₂(μ-η1:η1-dmpe), respectively. On the other hand a reaction of Cp*(THF)Cr(CH₂)₂SiMe ₂ with 2,2′-bipyridine (bipy) gave the unusual mononuclear Cp*Cr(η3-C₁₄H₁₈N ₂Si) in which the bipy-ligand was functionalized. Single electron oxidation of the latter complex with ferricenium hexafluorophosphate ([Cp ₂Fe]PF₆) yielded dinuclear dication [Cp*₂Cr ₂(μ-η3:η3-C₂₈H ₃₆N₄Si₂)][PF₆]₂, resulting from oxidative ligand coupling. Electrochemical experiments showed this reaction to be reversible and supported an EC mechanism.; Cp*CrVI(O)₂(CH₂SiMe₃) was obtained from the oxidation of Cp*Cr(O)(CH₂SiMe₃) ₂ with H₂O₂. It was capable of oxidizing PPh ₃ to OPPh₃ and formed dinuclear [Cp*Cr(O)(CH₂SiMe ₃)]₂(μ-O). Another chromium oxo complex, namely Cp*Cr V(O)Cl₂, was prepared by the oxidation of [Cp*Cr(μ-Cl)Cl ₂]₂ with dioxygen. Its reactivity with oxidizable substrates has been investigated. It was capable of transferring an oxygen atom to easily oxidizable molecules such as PPh₃, and AsPh₃, It could also activate weak X-H bonds (X = C, N) and dehydrogenate selected organic molecules, giving H₂O as a by-product. Some of these reactions could be carried out catalytically in the presence of excess dioxygen. However, Cp*Cr(O)Cl₂ did not react with less electron rich substrates such as olefins, amines, CO, or common hydrocarbons. Calculations using density functional theory (DFT) suggested that the failure to expoxidize olefins was due to a high kinetic barrier. Following guidance provided by the calculations, the system was adjusted by substitution of Cp for Cp*. The greater electrophilicity of the Cp derivative was proved by its ability to transfer an oxygen atom to organic substrates that did not react with the Cp* analog.