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

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[2.2.2cryptand]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[2.2.2cryptand](UI(TDA)2)2(μ-I) (6) and as a three-electron reductant toward cycloheptatriene (CHT) to form the U(IV) complex K[2.2.2cryptand]U(η7-C7H7)(TDA)2(THF) (7). The reaction of 3 with cyclooctatetraene (COT) generates a mixture of the U(III) anion K[2.2.2cryptand]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)32(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[2.2.2cryptand]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.