Spectroscopy, photochemistry, and photoinitiation behavior of group 8 metallocenes
Morgan, Matthew James
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The electronic properties of benzoyl-substituted ferrocenes are influenced by the mixing of appreciable metal-to-ligand charge transfer (MLCT) character into the low-energy excited states of these complexes. Visible light irradiation of benzoyl-substituted ferrocenes in a strongly coordinating solvent causes metal-ring cleavage that produces the benzoylcyclopentadienide anion and a half-sandwich iron (II) cationic complex. The quantum efficiency of photoinduced metal-ring bond cleavage in 1,1’-dibenzoylferrocene (DFc) remains reasonably constant over a range of excitation wavelengths that encompass the two low-energy absorption bands of the complex. This behavior suggests that the initially populated Franck-Condon excited state undergoes very rapid electronic and vibrational relaxation to yield a common thermally equilibrated excited (thexi) state of DFc, from which reaction occurs. Increasing solution temperature does not influence the excited state reactivity. Spectral similarities exist between the benzoyl-substituted ferrocenes and the silicon-bridged ferrocenophanes. Spectroscopic, photochemical, and photoinitiation studies are reported for Fe(-C5H4)2(SiR2) (FcSiR2, where R = Me or Ph). The spectral behavior of a FcSiR2 is mainly caused by the ring tilt and bond angle distortions inherent in the complex rather than the presence of significant MLCT character in the low-energy excited states of the complex. The silicon-bridged ferrocenophanes are very photoreactive in room-temperature methanol with disappearance quantum yields that approach unity. This photochemistry is wavelength dependent as evidenced by the almost 50% decrease in the quantum yield upon irradiation of a FcSiR2 in methanol at 313 nm. Lowering the temperature of solutions of FcSiR2 to 5oC increases the photoreaction rate. Solvent also influences the photoreaction rate as evidenced by the sharp decrease in the quantum yield upon switching to hexane. Mechanistic studies reveal that the photochemical process occurs via metal-ring cleavage, substantiated by the identification of the free ligand species, R2(-C5H4)2Si (R2SiCp2, where R = Me or Ph).