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(η⁵-cyclopentadienyl)bis(pentafluorophenyl)titanium- (IV), Cp₂Ti^IV{C₆F₅}₂, Cp = η⁵-C₅H₅, 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 · 10³ s^?1 at room temperature. X is highly reactive towards H₂O, MeOH, acetone, MeCN, MeNO₂, butane-1,4-diyl diacrylate, 2,2,6,6-tetramethylpiperidine N-oxide, CO, O₂, and N₂; absolute bimolecular rate constants range from 10⁶ to 10⁹ M ^?1 · s^?1. The primary photoisomerization 1→X is tentatively ascribed to a cyclopentadienyl ring slippage from η⁵ to a lower hapticity, a process that opens up coordinative unsaturation.