UC Berkeley

Two-electron reduction of the amidate-supported U(III) mono(arene) complex U(TDA)₃ (2) with KC₈ yields the anionic bis(arene) complex [K[2.2.2]cryptand][U(TDA)₂] (3) (TDA = N-(2,6-di-isopropylphenyl)pivalamido). EPR spectroscopy, magnetic susceptibility measurements, and calculations using DFT as well as multireference CASSCF methods all provide strong evidence that the electronic structure of 3 is best represented as a 5f⁴ U(II) metal center bound to a monoreduced arene ligand. Reactivity studies show 3 reacts as a U(I) synthon by behaving as a two-electron reductant toward I₂ to form the dinuclear U(III)-U(III) triiodide species [K[2.2.2]cryptand][(UI(TDA)₂)₂(μ-I)] (6) and as a three-electron reductant toward cycloheptatriene (CHT) to form the U(IV) complex [K[2.2.2]cryptand][U(η⁷-C₇H₇)(TDA)₂(THF)] (7). The reaction of 3 with cyclooctatetraene (COT) generates a mixture of the U(III) anion [K[2.2.2]cryptand][U(TDA)₄] (1-crypt) and U(COT)₂, while the addition of COT to complex 2 instead yields the dinuclear U(IV)-U(IV) inverse sandwich complex [U(TDA)₃]₂(μ-η⁸:η³-C₈H₈) (8). Two-electron reduction of the homoleptic Th(IV) amidate complex Th(TDA)₄ (4) with KC₈ gives the mono(arene) complex [K[2.2.2]cryptand][Th(TDA)₃(THF)] (5). The C-C bond lengths and torsion angles in the bound arene of 5 suggest a direduced arene bound to a Th(IV) metal center; this conclusion is supported by DFT calculations.