A MECHANISM FOR THE PHOTOCHEMICAL CONVERSION OF (ETA-C5H5)FE(CO)2SI2ME5 TO (ETA-C5H5)FE(CO2)2SIME3 - INFRARED EVIDENCE FOR AN INTERMEDIATE IRON SILYLENE COMPLEX

The photochemistry of the complex FpSi2Me5 (Fp = (eta-5-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 = (eta-5-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 nu-(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.