Photochemistry of Alkyldicarbonyl(η⁵-cyclopentadienyl)iron and -ruthenium. Ligand Substitution and Alkene Elimination via Photogenerated 16-Valence-Electron Intermediates

Photoreaction of (η5-C5H5)M(CO)₂R (M = Fe, Ru; R = CH₃, C₂H₆, n-C₅H₁₁) and (η⁵-C₅Me₅)Fe(CO)₂C₂H₅ involves dissociative loss of CO as the primary photoprocess following near-uv excitation. The quantum yield for photosubstitution of (η5-C5H5)M(CO)₂CH₃ is independent of the incoming ligand or its concentration (above 10⁻³ M); the quantum yield is 0.4 ± 0.05 for M = Ru and 0.7 ± 0.03 for M = Fe. All of the complexes for R having β-hydrogens yield the analogous (η⁵-C₅R′₅)M(CO)₂H (R′ = H, Me) complexes upon near-uv irradiation at 298 K in fluid, alkane solution. For the (η5-C5H5)Fe(CO)₂C₂H₅ complex the photochemical formation of the hydride occurs even at 77 K in alkane matrices, and there are no infrared detectable intermediates at an extent conversion of <5%. Irradiation of (η5-C5H5)Ru(CO)₂C₂H₅ yields photoreaction at 77 K, initially giving infrared spectral changes consistent with the formation of (η5-C5H5)Ru(CO)(C₂H₄)(H) followed by spectral changes consistent with the formation of the (η5-C5H5)Ru(CO)₂H. Likewise in rigid (paraffin) media at 298 K, initial spectral changes accompanying near-uv irradiation of (η5-C5H5)Ru-(CO)₂C₂H₅ are consistent with the formation of the ethylene-hydride complex. At 298 K, high concentrations of PPh₃ suppress the formation of iron-hydride products from irradiation of (η5-C5H5)Fe(CO)₂C₂H₅ in a manner consistent with the intermediacy of (η5-C5H5)Fe(CO)C₂H₅ from the dissociative loss of CO from the starting dicarbonyl. For the (η5-C5H5)Ru(CO)₂CH₃, irradiation at 77 K in an alkane matrix yields infrared spectral changes consistent with the formation of (η5-C5H5)Ru(CO)CH₃, providing direct evidence for the photogeneration of a 16-valence-electron intermediate. The rate of β-hydrogen transfer for the Fe species created by light is estimated to be >3 X 10² s⁻¹ at 298 K, and 77 K data suggest that the activation energy for β-hydrogen transfer from photogenerated intermediates is less than ∼6 kcal/mol for both the Fe and Ru complexes.