Mechanistic Studies of Styrene Production from Benzene and Ethylene Using [(η²-C₂H₄)₂Rh(μ-OAc)]₂ as Catalyst Precursor: Identification of a Bis-Rh^I Mono-Cu^(II) Complex As the Catalyst

We report a combined experimental and computational study focused on the mechanism of oxidative conversion of benzene and ethylene to styrene using [(η²-C₂H₄)₂Rh(μ-OAc)]₂ as the catalyst precursor in the presence of Cu(OPiv)₂ (OPiv = pivalate). Using [(η²-C₂H₄)₂Rh(μ-OAc)]₂ as the catalyst precursor, ∼411 turnovers of styrene are observed after 1 h, giving an apparent turnover frequency of ∼0.11 s⁻¹ (calculated assuming the binuclear structure is maintained in the active catalyst). We identify the catalyst resting state to be [(η²-C₂H₄)₂Rh^I(μ-OPiv)₂]₂(μ-Cu), which is a heterotrinuclear molecular complex in which a central Cu^(II) atom bridges two Rh moieties. At high Rh concentration in the presence of Cu(OPiv)₂ and pivalic acid (HOPiv), the trinuclear complex [(η²-C₂H₄)₂Rh^I(μ-OPiv)₂]₂(μ-Cu) converts to the binuclear Rh(II) complex [(HOPiv)Rh^(II)(μ-OPiv)₂]₂, which has been identified by ¹H NMR spectroscopy and single crystal X-ray diffraction. The binuclear Rh(II) [(HOPiv)Rh^(II)(μ-OPiv)₂]₂ is not a catalyst for styrene production, but under catalytic conditions [(HOPiv)Rh^(II)(μ-OPiv)₂]₂ can be partially converted to the active catalyst, the Rh–Cu–Rh complex [(η²-C₂H₄)₂Rh^I(μ-OPiv)₂]₂(μ-Cu), following an induction period of ∼6 h. Using quantum chemical calculations, we sampled possible mononuclear and binuclear Rh species, finding that the binuclear Rh(II) [(HOPiv)Rh^(II)(μ-OPiv)₂]₂ paddle-wheel is a low energy global minimum, which is consistent with experimental observations that [(HOPiv)Rh^(II)(μ-OPiv)₂]₂ is not a catalyst for styrene formation. Further, we investigated the mechanism of styrene production starting from [(η²-C₂H₄)₂Rh^I(μ-OAc)₂]₂(μ-Cu), [(η²-C₂H₄)₂Rh(μ-OAc)]₂, and (η²-C₂H₄)₂Rh(κ²-OAc). For all reaction pathways studied, the predicted activation barriers for styrene formation from [(η²-C₂H₄)₂Rh(μ-OAc)]₂ and (η²-C₂H₄)₂Rh(κ²-OAc) are too high compared to experimental kinetics. In contrast, the overall activation barrier for styrene formation predicted by DFT from the Rh–Cu–Rh complex [(η²-C₂H₄)₂Rh^I(μ-OPiv)₂]₂(μ-Cu) is in agreement with experimentally determined rates of catalysis. Based on these results, we conclude that incorporation of Cu(II) into the active Rh–Cu–Rh catalyst reduces the activation barrier for benzene C–H activation, O–H reductive elimination, and ethylene insertion into the Rh–Ph bond.