Effect of Arene Methylation on Photochemical Arene Replacement Reactions of [(η⁵-C5(CH₃)5)M(η⁶-arene)]⁺ (M = Fe, Ru) Complexes

The quantum yields (ϕ) for the photochemical arene release from [Cp*M(η⁶-arene)] ⁺ complexes (Cp* = η⁵-pentamethylcyclo-pentadienyl) have been measured for M = Fe(II) and Ru(II) in acetonitrile solutions at room temperature. (Fe: arene = toluene (ϕ = 0.007), hexamethylbenzene (ϕ = 0.00002). Ru: arene = benzene (ϕ = 0.19), mesitylene (ϕ = 0.025), hexamethylbenzene (ϕ= 0.0019).) For both M = Fe and Ru, permethylation of the cyclopentadienyl ligand decreases the arene release quantum yields as compared to those of the analogous cyclopentadienyl compounds. For M = Fe, the decreases are factors of 113 and 20 500, respectively, for arene = toluene and hexamethylbenzene. The very large decrease observed for the (pentamethylcyclo-pentadienyl)(hexamethylbenzene)iron complex is consistent with an important steric blocking effect of the five pentamethyl-cyclopentadienyl methyl groups. For the three (pentamethylcyclopentadienyl)ruthenium compounds studied, decreases are more modest relative to the cyclopentadienyl compounds (1.8, 3.4, and 7.4 for M = Ru, where arene = benzene, mesitylene, and hexamethylbenzene). In this case, a linear correlation persists between log (ϕ/(1-ϕ)) and σ_p, the Hammett parameter for methyl substituents. The linearity of the log (ϕ/(1-ϕ)) vs. σ_p plot indicates that steric effects are not present to hinder the participation of acetonitrile in the transition state of the highly methylated Ru complexes studied. The increase in the Hammett ρ parameter from +1.38 to +2.05 upon cyclopentadienyl methylation of the Ru complexes suggests a slight increase in negative charge occurs at the arene in the arene release transition state of the pentamethylcyclopentadienyl complexes relative to the cyclopentadienyl complexes.