Reduction chemistry of neptunium cyclopentadienide complexes: from structure to understanding

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' = C5H4SiMe3) have been made and characterised providing the first single crystal X-ray analyses of Np-III Cp complexes. In all NpCp3 derivatives there are three Cp rings in eta(5)-coordination around the Np-III centre; additionally in [Np(Cp)(3)] and K[Np(Cp)(4)] one Cp ring establishes a mu-eta(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 eta(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 KC8 in the presence of 2.2.2-cryptand to afford a neptunium(II) complex that is thermally unstable above -10 degrees 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.