Hexa-Fe(III) Carboxylate Complexes Facilitate Aerobic Hydrocarbon Oxidative Functionalization: Rh Catalyzed Oxidative Coupling of Benzene and Ethylene to Form Styrene

Fe(II) carboxylates react with dioxygen and carboxylic acid to form Fe₆(μ–OH)₂(μ₃–O)₂(μ–X)₁₂(HX)₂ (X = acetate or pivalate), which is an active oxidant for Rh-catalyzed arene alkenylation. Heating (150–200 °C) the catalyst precursor [(η²–C₂H₄)₂Rh(μ–OAc)]₂ with ethylene, benzene, Fe(II) carboxylate, and dioxygen yields styrene >30-fold faster than the reaction with dioxygen in the absence of the Fe(II) carboxylate additive. It is also demonstrated that Fe₆(μ–OH)₂(μ₃–O)₂(μ–X)₁₂(HX)₂ is an active oxidant under anaerobic conditions, and the reduced material can be reoxidized to Fe₆(μ–OH)₂(μ₃–O)₂(μ–X)₁₂(HX)₂ by dioxygen. At optimized conditions, a turnover frequency of ∼0.2 s^–1 is achieved. Unlike analogous reactions with Cu(II) carboxylate oxidants, which undergo stoichiometric Cu(II)-mediated production of phenyl esters (e.g., phenyl acetate) as side products at temperatures ≥150 °C, no phenyl ester side product is observed when Fe carboxylate additives are used. Kinetic isotope effect experiments using C₆H₆ and C₆D₆ give k_H/k_D = 3.5(3), while the use of protio or monodeutero pivalic acid reveals a small KIE with k_H/k_D = 1.19(2). First-order dependencies on Fe(II) carboxylate and dioxygen concentration are observed in addition to complicated kinetic dependencies on the concentration of carboxylic acid and ethylene, both of which inhibit the reaction rate at a high concentration. Mechanistic studies are consistent with irreversible benzene C–H activation, ethylene insertion into the formed Rh–Ph bond, β–hydride elimination, and reaction of Rh–H with Fe₆(μ–OH)₂(μ₃–O)₂(μ–X)₁₂(HX)₂ to regenerate a Rh-carboxylate complex.