Photochemical Reactions of (η5-Pentamethylcyclopentadienyl)dicarbonyliron-Alkyl and -Silyl Complexes: Reversible Ethylene Insertion into an Iron-Silicon Bond and Implications for the Mechanism of Transition-Metal-Catalyzed Hydrosilation of Alkenes

Article abstract of DOI:10.1021/ja00272a035

Near-UV irradiation of (η⁵-C5Me5)Fe(CO)2SiMe3 in a methylcyclohexane matrix at ca. 77 K results in dissociative loss of CO, as evidenced by the appearance of an absorption at 2132 cm^-1 due to free CO in the IR spectrum.The 366-nm quantum yield for CO loss from (η⁵-C5Me5)Fe(CO)2SiMe3 at 298 K is shown to be at least 0.22 +/- 0.03 mol/einstein as measured by the photosubstitution yield for formation of (η⁵-C5Me5)Fe(CO)(SiMe3)PPh3 in methylcyclohexane containing ca. 0.05 M PPh3.Photolysis of (η⁵-C5Me5)Fe(CO)2SiMe3 in the presence of HSiR'3 (R' = Me, Et in alkane solution, or pure HSiEt3) results in the loss of CO and the oxidative addition of HSiR'3 to form trans-(η⁵-C5Me5)Fe(CO)(SiMe3)(SiR'3)H which has been characterized by IR and 1H NMR.The 366-nm quantum yield for this reaction is 0.20 +/- 0.02 mol/einstein.Photolysis of (η⁵-C5Me5)Fe(CO)2SiMe3 in C2H4 saturated alkane solution results in the formation of (η⁵-C5Me5)Fe(CO)(C2H4)SiMe3, as evidenced by the growth of a single band in the IR spectrum and the 13C NMR when 99percent 13C-enriched C2H4 is used.The C2H4 then inserts reversibly into the Fe-Si bond.In the absence of added 2e^- donor ligands the C2H4 insertion product can undergo β-hydrogen transfer to form (η⁵-C5Me5)Fe(CO)2H and Me3Si(CH=CH2).In the presence of CO (η⁵-C5Me5)Fe(CO)2CH2CH2SiMe3 is formed. (η⁵-C5Me5)Fe(CO)2CH2CH2SiMe3 has been isolated and characterized.Near-UV photolysis of (η⁵-C5Me5)Fe(CO)2CH2CH2SiMe3 in an alkane solution at 298 K results in both β-hydrogen and β-SiMe3 transfer.This is evidenced by the appearance of both Me3Si(CH=CH2) and C2H4 in the 1H NMR.The ratio of β-hydrogen transfer to β-SiMe3 transfer is ca. 2:1.Photolysis of (η⁵-C5Me5)Fe(CO)2R (R = Me, Et) in an alkane solution containing HSiR'3 results in the formation of RH and trans-(η⁵-C5Me5)Fe(CO)(SiR'3)2H.The 366-nm quantum yield for this process, 0.7 +/- 0.1, is consistent with CO loss as the primary step.Preliminary evidence based on the growth of a visible absorption in the UV-vis spectrum is given for the formation of (η⁵-C5Me5)Fe(CO)(SiMe3)(Me)H at 173 K by photolysis of (η⁵-C5Me5)Fe(CO)2Me in the presence of HSiMe3.The results reported here provide examples of all the reactions postulated to occur in the hydrosilation of alkenes by a mechanism which involves olefin insertion into a M-Si bond as a key step.