Photochemical Generation of a Reactive Transition-Metal Fragment. Photochemically Induced Arene Replacement Reactions of the Cyclopentadienyl(p-xylene)Iron(II) Ion

Visible-light irradiation of FeCp(p-xyl)+ (Cp = cyclopentadienyl, p-xyl = p-xylene) in organic solvents in the presence of suitable ligands leads to the formation of products in which the three coordination sites occupied by p-xylene have been replaced to give complexes of the form FeCpL₃ ⁺ (L3 = (p-CNPhCH₃)₃, (CO)₃, hexamethylbenzene, and triphos), which have been isolated and characterized as PF₆ ^- or BF₄ ^- salts. The quantum yields for the formation of these products are rather large. For example, irradiation (436 nm) of the LF bands of FeCp(p-xyl)⁺ in methylene chloride in the presence of 0.05 M triphos (bis(2-(diphenylphosphino)ethyl)phenylphosphine) yields FeCp(triphos)⁺ with θ = 0.57 ± 0.06. The course of the photochemical reaction was different in methylene chloride solution with 1, 10-phenanthroline added or in aqueous 0.1 NH₂SO₄ solution. The 436-nm irradiation of FeCp(p-xyl)⁺ in methylene chloride in the presence of 0.1 M 1, 10-phenanthroline leads to the formation of Fe(phen)32⁺ with a quantum yield of 0.58 α 0.06. Similarly, 436-nm-irradiation of FeCp(p-xyl)⁺ in 0.1 N aqueous H₂SO₄ solution leads to the formation of Fe2⁺(aq) with a quantum yield of 0.81 α 0.08. The possible mechanisms for these reactions are tentatively discussed in terms of replacement of p-xylene to produce a reactive transition-metal fragment containing the FeCp⁺ unit, which undergoes addition of incoming ligand L to form complexes of the form FeCpL₃ ⁺ or FeL_n2⁺.