Identification of oxidation state +1 in a molecular uranium complex

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 Pr5)2] and the uranium­(III) metallocene [IU­(η5-C5 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 Pr5)2]− (1) (C5 i Pr5 = pentaisopropylcyclopentadienyl) as the salt of [K­(2.2.2-cryptand)]+. An X-ray crystallographic study revealed that 1 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 z 2 )1(6d x 2–y 2 /6d xy )1 configuration. The metal–ligand bonding in 1 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