Carbon Dioxide Activation by a Uranium(III) Complex Derived from a Chelating Bis(aryloxide) Ligand

The new dianionic ligand C₆H₄{<i>p</i>-C­(CH₃)₂C₆H₂Me₂O^–}₂ (=<i>p</i>-Me₂bp), featuring two aryloxide donors and a central arene ring, has been synthesized and used to prepare the mixed-ligand U­(III) compound [U­(Cp*)­(<i>p</i>-Me₂bp)], which exhibits an η⁶ interaction with the uranium center. Reductive activation of CO₂ was investigated using [U­(Cp*)­(<i>p</i>-Me₂bp)] in supercritical CO₂, which gave a dinuclear uranium carbonate complex, {U­(Cp*)­(<i>p</i>-Me₂bp)}₂(μ-η¹:η²-CO₃), cleanly and selectively. Reactivity studies in conventional solvents using lower pressures of CO₂ showed the formation of a rare U­(IV) oxalate complex, {U­(Cp*)­(<i>p</i>-Me₂bp)}₂(μ-η²:η²-C₂O₂), alongside {U­(Cp*)­(<i>p</i>-Me₂bp)}₂(μ-η¹:η²-CO₃). The relative ratio of the last two products is temperature dependent: at low temperatures (−78 °C) oxalate formation is favored, while at room temperature the carbonate is the dominant product. The U­(IV) iodide [U­(Cp*)­(<i>p</i>-Me₂bp)­I] was also synthesized and used as part of an electrochemical study, the results of which showed that [U­(Cp*)­(<i>p</i>-Me₂bp)] has a U^IV/U^III redox couple of −2.18 V vs FeCp₂^+/0 as well as a possible electrochemically accessible U^III/U^II reduction process at −2.56 V vs FeCp₂<sup>+/0