Cation-mediated Conversion of the State of Charge in Uranium Arene Inverted-sandwich Complexes

Two new arene inverted-sandwich complexes of uranium supported by siloxide ancillary ligands [K{U(OSi(OtBu)3)3}2(μ-η(6):η(6)-C7H8)] (3) and [K2{U(OSi(OtBu)3)3}2(μ-η(6):η(6)-C7H8)] (4) were synthesized by the reduction of the parent arene-bridged complex [{U(OSi(OtBu)3)3}2(μ-η(6):η(6)-C7H8)] (2) with stoichiometric amounts of KC8 yielding a rare family of inverted-sandwich complexes in three states of charge. The structural data and computational studies of the electronic structure are in agreement with the presence of high-valent uranium centers bridged by a reduced tetra-anionic toluene with the best formulation being U(V)-(arene(4-))-U(V), KU(IV)-(arene(4-))-U(V), and K2U(IV)-(arene(4-))-U(IV) for complexes 2, 3, and 4 respectively. The potassium cations in complexes 3 and 4 are coordinated to the siloxide ligands both in the solid state and in solution. The addition of KOTf (OTf=triflate) to the neutral compound 2 promotes its disproportionation to yield complexes 3 and 4 (depending on the stoichiometry) and the U(IV) mononuclear complex [U(OSi(OtBu)3)3(OTf)(thf)2] (5). This unprecedented reactivity demonstrates the key role of potassium for the stability of these complexes.