Abstract
Neptunium complexes in the formal oxidation states II, III, and IV supported by cyclopentadienyl ligands are explored, and significant differences between Np and U highlighted as a result. A series of neptunium(iii) cyclopentadienyl (Cp) complexes [Np(Cp)(3)], its bis-acetonitrile adduct [Np(Cp)(3)(NCMe)(2)], and its KCp adduct K[Np(Cp)(4)] and [Np(Cp')(3)] (Cp' = C(5)H(4)SiMe(3)) have been made and characterised providing the first single crystal X-ray analyses of Np(III) Cp complexes. In all NpCp(3) derivatives there are three Cp rings in η(5)-coordination around the Np(III) centre; additionally in [Np(Cp)(3)] and K[Np(Cp)(4)] one Cp ring establishes a μ-η(1)-interaction to one C atom of a neighbouring Np(Cp)(3) unit. The solid state structure of K[Np(Cp)(4)] is unique in containing two different types of metal-Cp coordination geometries in the same crystal. Np(III)(Cp)(4) units are found exhibiting four units of η(5)-coordinated Cp rings like in the known complex [Np(IV)(Cp)(4)], the structure of which is now reported. A detailed comparison of the structures gives evidence for the change of ionic radii of ca. -8 pm associated with change in oxidation state between Np(III) and Np(IV). The rich redox chemistry associated with the syntheses is augmented by the reduction of [Np(Cp')(3)] by KC(8) in the presence of 2.2.2-cryptand to afford a neptunium(ii) complex that is thermally unstable above -10 °C like the U(II) and Th(II) complexes K(2.2.2-cryptand)[Th/U(Cp')(3)]. Together, these spontaneous and controlled redox reactions of organo-neptunium complexes, along with information from structural characterisation, show the relevance of organometallic Np chemistry to understanding fundamental structure and bonding in the minor actinides.