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), Cp2TiIV{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 109M−1 · s−1. The primary photoisomerization 1X is tentatively ascribed to a cyclopentadienyl ring slippage from η5 to a lower hapticity, a process that opens up coordinative unsaturation.

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