Reactivity and Cross-Reactivity in Redox Activation of the Uranyl Ion

Abstract

The properties of the uranyl dication (UO22+) are governed in many respects by its redox chemistry, and functionalizing the strong U–O bonds often requires reduction from U(VI) to U(V) with solution-phase multicomponent reaction chemistry that involves both strong reductants and electrophiles. Here, we report the patterns of reactivity and cross-reactivity displayed by a model system in which oxo-activating conditions have been tested that involve either i) electrochemical or chemical reduction, or ii) coordinating or non-coordinating solvents. In acetonitrile (CH3CN), a complex of the uranyl(V) monocation [UO2+] can be formed reliably through treatment of the U(V) species with tris(pentafluorophenyl)borane (BCF), but it also results in a mixture of products arising in part due to direct electron transfer to BCF. In dichloromethane (CH2Cl2), attempts to chemically reduce and functionalize U(VI) revealed undesired cross-reactivity that precludes reduction of U(VI); Cp*2Co can undergo cross-reactions with both CH2Cl2 and BCF, evidently by electron transfer from Co(II) followed by further reactions. Despite the cross reactivity in CH2Cl2, one uranium-containing product could be crystallized, and was characterized by solid-state X-ray diffraction analysis. This complex features a trinuclear, formally [UV,UIV,UV] core, but contains only four of the total of six oxo equivalents that were expected on the basis of the U(VI) starting material, confirming that electrophilic reactivity can proceed in this system upon reduction. Along this line, computational studies have been used to establish the baseline electronic properties of the U(V) and U(IV) centers in the trinuclear product, as well as gain insight to structural changes induced by the reduction of this compound. The documentation of the reactivity and cross-reactivity patterns here are essential to guide design of improved multicomponent systems for actinide processing.