Theoretical investigation of U(I) arene complexes: is the elusive monovalent oxidation state accessible?

Commonly accepted uranium oxidation states in molecular complexes are III+ to VI+. Recently, this has been extended experimentally by the synthesis of seminal U(II) complexes, Y center dot[U(Cp')(3)] and Y center dot[URE] (Y = K+(2.2.2-cryptand), Cp' = C5H4SiMe3, H3RE = ((ArOH)-Ar-Ad,Me)(3)mesitylene and Ad = adamantyl). Relativistic density functional theory has been applied to explore whether the uranium oxidation state (+I) is possibly accessible. Calculations show that the U() complex of a heterocalix[4]arene (H2L) is energetically stable. It features a 5f(5)-dominated electronic configuration, four delta(U-Ar)-bond orbitals and a moderate U-II/U-I reduction potential. Its stability and structural/bonding/energetic properties were corroborated by comparisons with theoretically designed complexes [U-I(Cp)(3)](2-) and [(URE)-R-I](2-).