Two-Electron Redox Reactivity of Thorium Supported by Redox-Active Tripodal Frameworks

The high stability of the + IVoxidation state limits thorium redox reactivity. Here we report the synthesis and the redox reactivity of two Th(IV) complexes supported by the arene-tethered tris(siloxide) tripodal ligands [(KOSiR₂ Ar)₃ -arene)]. The two-electron reduction of these Th(IV) complexes generates the doubly reduced [KTh((OSi(O^t Bu)₂ Ar)₃ -arene)(THF)₂ ] (2^OtBu ) and [K(2.2.2-cryptand)][Th((OSiPh₂ Ar)₃ -arene)(THF)₂ ](2^Ph -crypt) where the formal oxidation state of Th is +II. Structural and computational studies indicate that the reduction occurred at the arene anchor of the ligand. The robust tripodal frameworks store in the arene anchor two electrons that become available at the metal center for the two-electron reduction of a broad range of substrates (N₂ O, COT, CHT, Ph₂ N₂ , Ph₃ PS and O₂ ) while retaining the ligand framework. This work shows that arene-tethered tris(siloxide) tripodal ligands allow implementation of two-electron redox chemistry at the thorium center while retaining the ligand framework unchanged.