Photochemical Ring Slippage of Bis(pentafluorophenyl)titanocene: Reaction kinetics and matrix isolation of the primary photoproduct

Laser flash photolysis and matrix‐isolation techniques were applied to elucidate the photochemistry of the orange complex bis(pentafluorophenyl)titanocene ( = bis(η 5 ‐cyclopentadienyl)bis(pentafluorophenyl)titanium‐ (IV), Cp 2 Ti IV {C 6 F 5 } 2 , Cp = η 5 ‐C 5 H 5 , 1 ) which is used as a polymerization photo‐initiator. The primary photo‐reaction of 1 is the formation of a highly reactive blue isomer X with unit quantum yield. In carefully dried and degassed benzene solution, the photoisomer X rearranges to starting material 1 with a first‐order reaction, k = 5 · 10 3 s −1 at room temperature. X is highly reactive towards H 2 O, MeOH, acetone, MeCN, MeNO 2 , butane‐1,4‐diyl diacrylate, 2,2,6,6‐tetramethylpiperidine N ‐oxide, CO, O 2 , and N 2 ; absolute bimolecular rate constants range from 10 6 to 10 9 M −1 · s −1 . The primary photoisomerization 1→X is tentatively ascribed to a cyclopentadienyl ring slippage from η 5 to a lower hapticity, a process that opens up coordinative unsaturation.