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

Abstract

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·[U(Cp′)₃] and Y·[UR^E] (Y = K⁺(2.2.2-cryptand), Cp′ = C₅H₄SiMe₃, H₃R^E = (^Ad,MeArOH)₃mesitylene 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(I) complex of a heterocalix[4]arene (H₂L) is energetically stable. It features a 5f⁵-dominated electronic configuration, four δ(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 [U^IR^E]²⁻.