A Uranium(II) Arene Complex That Acts as a Uranium(I) Synthon

Abstract

Two-electron reduction of the amidate-supported U(III) mono(arene) complex U(TDA)₃ (<b>2</b>) with KC₈ yields the anionic bis(arene) complex [K[2.2.2]cryptand][U(TDA)₂] (<b>3</b>) (TDA = <i>N</i>-(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 <b>3</b> is best represented as a 5f⁴ U(II) metal center bound to a monoreduced arene ligand. Reactivity studies show <b>3</b> 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)] (<b>6</b>) 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)] (<b>7</b>). The reaction of <b>3</b> with cyclooctatetraene (COT) generates a mixture of the U(III) anion [K[2.2.2]cryptand][U(TDA)₄] (<b>1-crypt</b>) and U(COT)₂, while the addition of COT to complex <b>2</b> instead yields the dinuclear U(IV)-U(IV) inverse sandwich complex [U(TDA)₃]₂(μ-η⁸:η³-C₈H₈) (<b>8</b>). Two-electron reduction of the homoleptic Th(IV) amidate complex Th(TDA)₄ (<b>4</b>) with KC₈ gives the mono(arene) complex [K[2.2.2]cryptand][Th(TDA)₃(THF)] (<b>5</b>). The C-C bond lengths and torsion angles in the bound arene of <b>5</b> suggest a direduced arene bound to a Th(IV) metal center; this conclusion is supported by DFT calculations.