Effect of Arene Substituents and Temperature on the Arene Replacement Reactions of [(η⁵-C₅H₅)Fe(η⁶-arene)]+ and [(η⁵-C₅H₅)Ru(η⁶-arene)]+

The photochemical removal of the arene from [(rj₅-C₅H₅)M(N;6-arene)]⁺ has been studied for complexes of Fe(II) and Ru(II): (Fe, arene = p-dichlorobenzene, benzene, toluene, toluene-8, p-xylene, mesitylene, durene, pentamethylbenzene, hexamethylbenzene, hexaethylbenzene, and tri-Zm-butylbenzene; Ru, p-dichlorobenzene, benzene, toluene, mesitylene, pentamethylbenzene, hexamethylbenzene, and tri-ZerZ-butylbenzene). The photoactive state in these complexes has been identified as the distorted a3E, ligand field excited state. A linear correlation exists between log (0/(1-0)) and, the Hammett parameter for chloro-and methylarene substituted complexes (up to five substituents for Fe and six substituents for Ru). The Hammett p parameter is ⁺1.03 and ⁺0.53 for for the Fe complexes in CH₂CI₂ and CH₃CN solution, respectively; for the Ru complexes in CH3CN solution p = +1.38. These data indicate a small amount of negative charge builds up at the arene in the transition state for the arene release reaction in these systems. Deviations from the linear correlation of log (0/(1-0)) with were found for the sterically hindered [(jj₅-C₅H₅)Fe(j₆-hexamethylbenzene)]+, [(t75-C₅H₅)-Fe(p₆-hexaethylbenzene)], and [(p₅-C₅H₅)Fe(7)6-l, 3,5-tri-ZerZ-butylbenzene)]+ complexes. These deviations result from the steric interactions which hinder the participation of anions and solvent molecules in the transition state. The temperature dependence of photochemical arene release from [(rj5-C₅H₅)M(7)6-arene)]X (M = Fe, arene = p-xylene, hexamethylbenzene, X^-1 = BF^4-1, solvent = CH₂CI₂, CH₃CN; M = Ru, arene = benzene, X' = PF6, solvent = CH3CN) has been investigated. Plots of log (0/(1-0)) vs. 1/T are linear in all cases. From the slopes of these lines the difference in the apparent activation energies (Eap-Ea nr) between the product-forming step and the nonradiative decay of the a3E, excited state are 1.0-3.6 kcal/mol. These studies indicate that the M-arene bond is nearly broken in the a3Et excited state. Medium effects on the most sterically hindered Fe complexes investigated ([(7i5-C5H5)Fe(7;6-hexamethylbenzene)]+ and [(7j5-C₅H₅)Fe(?)6-hexaethylbenzene)]“1”) show that a limiting value of about 0.0007 occurs for the quantum yield of arene release for [(tj5-C5H5)Fe(r;6-hexaethylbenzene)]+ in CH₂CI₂ solutions with AsF^6- present. This quantum yield represents an upper limit of the dissociative arene release for the [(7j5-C₅H₅)Fe(?76-arene)]+ systems.