Identification of Oxidation State +1 in a Molecular Uranium Complex

Abstract

The concept of oxidation state plays a fundamentally important role in defining the chemistry of the elements. In the f block of the periodic table, well-known oxidation states in compounds of the lanthanides include 0, +2, +3 and +4, and oxidation states for the actinides range from +7 to +2. Oxidation state +1 is conspicuous by its absence from the f-block elements. Here we show that the uranium(II) metallocene [U(η⁵-C₅^<i>i</i>Pr₅)₂] and the uranium(III) metallocene [IU(η⁵-C₅^<i>i</i>Pr₅)₂] can be reduced by potassium graphite in the presence of 2.2.2-cryptand to the uranium(I) metallocene [U(η⁵-C₅^<i>i</i>Pr₅)₂]^- (<b>1</b>) (C₅^<i>i</i>Pr₅ = pentaisopropylcyclopentadienyl) as the salt of [K(2.2.2-cryptand)]⁺. An X-ray crystallographic study revealed that <b>1</b> has a bent metallocene structure, and theoretical studies and magnetic measurements confirmed that the electronic ground state of uranium(I) adopts a 5f³(7s/6d_<i>z</i>²)¹(6d_{<i>x</i>²-<i>y</i>²}/6d_<i>xy</i>)¹ configuration. The metal-ligand bonding in <b>1</b> consists of contributions from uranium 5f, 6d, and 7s orbitals, with the 6d orbitals engaging in weak but non-negligible covalent interactions. Identification of the oxidation state +1 for uranium expands the range of isolable oxidation states for the f-block elements and potentially signposts a synthetic route to this elusive species for other actinides and the lanthanides.