Mechanistic Studies Of Styrene Production From Benzene And Ethylene Using [(Eta(2)-C2h4)(2)Rh(Mu-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 [(eta(2)-C2H4)(2)Rh(mu-OAc)](2) as the catalyst precursor in the presence of Cu(OPiv)(2) (OPiv = pivalate). Using [(eta(2)-C2H4)(2)Rh(mu-OAc)](2) as the catalyst precursor, similar to 411 turnovers of styrene are observed after 1 h, giving an apparent turnover frequency of similar to 0.11 s(-1) (calculated assuming the binuclear structure is maintained in the active catalyst). We identify the catalyst resting state to be [(eta(2)-C2H4)(2)Rh-I(mu-OPiv)(2)](2)(mu-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 [(eta(2)-C2H4)(2)Rh-I(mu-OPiv)(2)](2)(mu-Cu) converts to the binuclear Rh(II) complex [(HOPiv)Rh-II(mu-OPiv)(2)](2), which has been identified by H-1 NMR spectroscopy and single crystal X-ray diffraction. The binuclear Rh(II) [(HOPiv)Rh-II(mu-OPiv)(2)](2) is not a catalyst for styrene production, but under catalytic conditions [(HOPiv)Rh-II(mu-OPiv)(2)](2) can be partially converted to the active catalyst, the Rh-Cu-Rh complex [(eta(2)-C2H4)(2)Rh-I(mu-OPiv)(2)](2)(mu-Cu), following an induction period of similar to 6 h. Using quantum chemical calculations, we sampled possible mononuclear and binuclear Rh species, finding that the binuclear Rh(II) [(HOPiv)Rh-II(mu-OPiv)(2)](2) paddle-wheel is a low energy global minimum, which is consistent with experimental observations that [(HOPiv)Rh-II(mu-OPiv)(2)](2) is not a catalyst for styrene formation. Further, we investigated the mechanism of styrene production starting from [(eta(2)-C2H4)(2)Rh-I(mu-OAc)(2)](2)(mu-Cu), [(eta(2)-C2H4)(2)Rh(mu-OAc)](2), and (eta(2) -C2H4)(2)Rh(kappa(2)-OAc). For all reaction pathways studied, the predicted activation barriers for styrene formation from [(eta(2)-C2H4)(2)Rh(mu-OAc)](2) and (eta(2) -C2H4)(2)Rh(kappa(2)-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 [(eta(2)-C2H4)(2)Rh-I(mu-OPiv)(2)](2)(mu-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.