Efficient photodissociation of anions from benzoyl-functionalized ferrocene complexes

Spectroscopic and photochemical studies of several benzoyl-functionalized ferrocene complexes in nonaqueous solvents are reported. Bands observed above 300 nm in the electronic absorption spectrum of the unsubstituted complex, Fe(eta(5)-C5H5)(2), and assigned to ligand field transitions shift to longer wavelengths and intensify upon introduction of a benzoyl group into one or both cyclopentadienide rings. Such behavior suggests that these transitions have acquired some charge-transfer character. Visible-light (546 nm) irradiation of 1,1'-dibenzoylferrocene, III, dissolved in CH3CN, CH3OH, or ethyl alpha-cyanopropionate causes ring-metal cleavage to produce the benzoylcyclopentadienide ion, C6H5C(O)C5H4-, and the corresponding half-sandwich cationic complex, Fe[(eta(5)SC(5)H(4))C(O)C6H5](S)(3)+ (S is solvent). The disappearance quantum yield, phi(dis), for III is 0.45 in CH3OH and 0.28 in ethyl alpha-cyanopropionate and is unaffected by the presence of dissolved O-2, added H2O (10 000 ppm), or added methanesulfonic acid (30 ppm). 1,1'-Dibenzoylferrocenes containing substitutents on both phenyl rings undergo photoinduced ring-metal cleavage in CH3OH with phi(dis), values very similar to that of III, while monobenzoyl-ferrocenes are appreciably less photoreactive. A mechanism that accommodates the photochemical behavior of benzoyl-functionalized ferrocene complexes is discussed. In addition, a previous suggestion concerning the role of III in the photoinitiated anionic polymerization of an alpha-cyanoacrylate monomer is reconsidered in light of the present study.