Controlling selectivity in the reductive activation of CO₂ by mixed sandwich uranium(III) complexes

The synthesis and molecular structures of a range of uranium(III) mixed sandwich complexes of the type [U(η⁸-C₈H₆(1,4-SiMe₃)₂)(η⁵-Cp^Me₄R)] (R = Me, Et, ⁱPr, ^tBu) and their reactivity towards CO₂ are reported. The nature of the R group on the cyclopentadienyl ring in the former has a significant effect on the outcome of CO₂ activation: when R = Me, the products are the bridging oxo complex {U[η⁸-C₈H₆(1,4-SiMe₃)₂](η⁵-Cp^Me₅)}₂(μ-O) and the bridging oxalate complex {U[η⁸-C₈H₆(1,4-SiMe₃)₂](η⁵-Cp^Me₅)}₂(μ-η²:η²-C₂O₄); for R = Et or ⁱPr, bridging carbonate {U[η⁸-C₈H₆(1,4-SiMe₃)₂](η⁵-Cp^Me₄R)}₂(μ-η¹:η²-CO₃) and bridging oxalate complexes {U[η⁸-C₈H₆(1,4-SiMe₃)₂](η⁵-Cp^Me₄R)}₂(μ-η²:η²-C₂O₄) are formed in both cases; and when R = ^tBu the sole product is the bridging carbonate complex {U[η⁸-C₈H₆(1,4-SiMe₃)₂](η⁵-Cp^Me₄tBu)}₂(μ-η¹:η²-CO₃). Electrochemical studies on both the uranium(III) complexes and the dimeric uranium(IV) CO₂ reduction products have been carried out and all exhibit quasi reversible redox processes; in particular, the similarities in the U(III)/U(IV) redox couples suggest that the selectivity in the outcome of CO₂ reductive activation by these complexes is steric in origin rather than electronic. The latter conclusion is supported by a detailed computational DFT study on the potential mechanistic pathways for reduction of CO₂ by this system.