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(η 5 -C 5 i Pr 5 ) 2 ] and the uranium(III) metallocene [IU(η 5 -C 5 i Pr 5 ) 2 ] can be reduced by potassium graphite in the presence of 2.2.2-cryptand to the uranium(I) metallocene [U(η 5 -C 5 i Pr 5 ) 2 ] - ( 1 ) (C 5 i Pr 5 = 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 5f 3 (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.