A Mechanism for the Photochemical Conversion of FpSi₂Me₅ to FpSiMe₃ (Fp = η⁵-C₅H₅)Fe(CO)₂). Infrared Evidence for an Intermediate Iron-Silylene Complex

The photochemistry of the complex FpSi₂Me₅ (Fp = (η⁵-C₅H₅)Fe(CO)₂) is compared to that of FpSiMe₃ using a variety of IR spectroscopic techniques. UV photolysis of FpSi₂Me₅ in Ar, N₂, or CO matrices leads to CO loss and formation of CpFe(CO)Si₂Me₅ (Cp = (η⁵-C₅H₅)). The equivalent product is not observed on identical treatment of FpSiMe₃, but, in N₂ matrices, small quantities of CpFe(CO)(N₂)SiMe₃ are generated. Fast time-resolved IR spectroscopy is used to identify primary CO loss photoproducts on flash photolysis of both FpSi₂Me₅ and FpSiMe₃ in room temperature n-heptane solution. Under these conditions, CpFe(CO)SiMe₃ reacts rapidly with CO to regenerate FpSiMe₃ or with PPh₃ to give CpFe(CO)-(PPh₃)SiMe₃. By contrast, CpFe(CO)Si₂Me₅ is unreactive toward CO or PPh₃ and undergoes an intramolecular decay to give a thermally stable secondary product, X. X is shown to undergo a secondary photochemical reaction with a variety of ligands to give monosilyl products, CpFe(CO)(L)SiMe₃ (L = CO, PPh₃, C₂H₄, or N₂). A species with a ν(Si-H) IR band is also produced in each of these reactions. For the reactions of FpSi₂Me₅ and FpSiMe₃ with N₂, liquid xenon was employed as a cryogenic solvent to stabilize CpFe(CO)(N₂)SiMe₃. A mechanism is proposed in which decay of CpFe(CO)Si₂Me₅ occurs by intramolecular oxidative addition of the Si-Si bond to the Fe center, giving a thermally stable silyl(silylene) complex, CpFe(CO)(=SiMe₂)SiMe₃ (X). Ejection of the SiMe₂ fragment from X occurs as a second photochemical step in the deoligomerization. It is suggested that isomerization of the SiMe₂ moiety occurs under photochemical conditions, giving a species with Si-H bond(s) and explaining why free silylene fragments have not been trapped in previous studies.