Photochemistry of η⁴-cyclopentadiene Iron Tricarbonyl Complexes: Transfer of The 5-endo Substituent To The Iron Center Following Dissociative Loss of Carbon Monoxide

Near-UV irradiation of the complexes (η⁴-C5H₅R¹)Fe(CO)3 (R¹= exo~H, Ia; exo-CH₂Ph (exo-Bz), Ib) and (η⁴-C₅Me₄R¹R²)Fe(CO)₃(R¹= exo-Me, R²= endo-Me, Ha; R¹= exo-Me, R²= endo-H, Ilb; R¹= exo-Bz; R²= endo-Me, IIe) where R¹and R²are on the doubly allylic carbon, C-5, in an exo and endo position, respectively, results in the dissociative loss of CO. For all complexes the prompt photoproduct is a coordinatively unsaturated, 16e (η⁴-cyclopentadiene)Fe(CO)₂that can be detected by IR at low temperature, — 77 K. Irradiation (λ> 420 nm) of the 16e species at low temperature (—77 K) results in transfer of the 5-endo substituent to form (η⁵-cyclopentadienyl)Fe(CO)₂R²(R²= H, Me) products. Transfer of the 5-endo substituent can also occur thermally upon warmup, but the transfer of the 5-endo group is not very competitive with back-reaction of the 16e species and CO to regenerate the starting (η⁴-cyclo~ pentadiene)Fe(CO)₃complexes. The 366-nm quantum yield (at 10-⁷einstein/min) for CO loss at 298 K in alkane solution is —0.1; the formation quantum yields for the (η⁵“Cyclopentadienyl)Fe(CO)₂H species are also —0.1, but the formation quantum yields for the (η⁵-cyclopentadienyl)Fe(CO)₂Me species are <10^-2. All results are consistent with thermal or light-activated transfer of the 5-endo substituent (H or Me) following light-induced loss of CO from the parent η⁴-cyclopentadiene)Fe(CO)₃. The thermal rate of the transfer of a 5-endo-Me vs. a 5-endo-H is much slower on the basis of the thermal rates of reactions of (η⁴-C₅Me₅R²)Fe(CO)2(l-pentene) (R²= endo-H or endo-Me) which give (η⁵-C₅Me₅)Fe(CO)₂R².