GAS-PHASE PHOTODISSOCIATION OF (ETA-5-CYCLOPENTADIENYL)FE(CO)2R AND (ETA-5-INDENYL)FE(CO)2R IN A MOLECULAR-BEAM - COMPETITIVE LOSS OF ALKYL RADICAL AND ALKENE DETECTED BY VACUUM-ULTRAVIOLET IONIZATION AND TIME-OF-FLIGHT MASS-SPECTROMETRY

Molecular beam photofragmentation measurements on (cyclopentadienyl)(alkyl)iron compounds directly determine the relative importance of Fe-alkyl bond homolysis and beta-hydride elimination. The UV photodissociation of CpFe(CO)2CH2CH3 (1, Cp = eta5-cyclopentadienyl), CpFe(CO)2CH2CH2CH3 (6), and InFe(CO)2CH2CH2CH3 (7, In = eta5-indenyl) leads to extensive fragmentation under collision-free conditions, and each of the products carries away little translational energy. Fragmentation of CpFe(CO)2CH2CH3 (1) Produces CpFeH, CpFe, CpH, Cp, Fe, and .CH2CH3, as detected by vacuum-UV ionization and time-of-flight mass spectrometry. The CpFeH, CpFe, and .CH2CH3 fragments come from absorption of one UV photon, while CpH, Cp, and Fe come from two-photon dissociation. The presence of CpFeH suggests beta-hydride elimination, but the large ionization potential of the accompanying alkene product (CH2=CH2) prevents its detection. Similarly, the ionization potential of CO is too large for VUV ionization in these experiments. Fragmentation of CpFe(CO)2CH2CH2CH3 (6) produces CpFeH, CpFe, CpH, Cp, Fe, .CH2-CH2CH3, and the alkene product CH2=CHCH3, which is observable by VUV ionization. Thus, homolysis of the Fe-alkyl bond (which produces propyl radical) and beta-hydride elimination (which produces propene) are competing processes in the gas phase. We estimate a ratio of propyl radical to propene of 1.5 +/- 0.8. Photofragmentation of the indenyl complex (eta5-In)Fe(CO)2CH2CH2CH3 (7) yields a ratio of propyl radical to propene of 1.1 +/- 0.6. The similarity of the ratios obtained from the eta5-Cyclopentadienyl complex 6 and the eta5-indenyl complex 7 suggests that ring slippage is unimportant in the fragmentation process.