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Reduction chemistry of neptunium cyclopentadienide complexes: from structure to understanding.

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' = C5H4SiMe3) have been made and characterised providing the first single crystal X-ray analyses of NpIII Cp complexes. In all NpCp3 derivatives there are three Cp rings in ?5-coordination around the NpIII centre; additionally in [Np(Cp)3] and K[Np(Cp)4] one Cp ring establishes a ?-?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. NpIII(Cp)4 units are found exhibiting four units of ?5-coordinated Cp rings like in the known complex [NpIV(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 NpIII and NpIV. 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 °C like the UII and ThII 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.

SUBMITTER: Dutkiewicz MS

PROVIDER: S-EPMC5431675 | biostudies-literature | 2017 Apr

REPOSITORIES: biostudies-literature

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Reduction chemistry of neptunium cyclopentadienide complexes: from structure to understanding.

Dutkiewicz Michał S MS Apostolidis Christos C Walter Olaf O Arnold Polly L PL

Chemical science 20170130 4

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 t ...[more]

PMID: 28553487

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Project description:There is significant interest in ligands that can stabilize actinide ions in oxidation states that can be exploited to chemically differentiate 5f and 4f elements. Applications range from developing large-scale actinide separation strategies for nuclear industry processing to carrying out analytical studies that support environmental monitoring and remediation efforts. Here, we report syntheses and characterization of Np(iv), Pu(iv) and Am(iii) complexes with N-tert-butyl-N-(pyridin-2-yl)hydroxylaminato, [2-( t BuNO)py]-(interchangeable hereafter with [( t BuNO)py]-), a ligand which was previously found to impart remarkable stability to cerium in the +4 oxidation state. An[( t BuNO)py]4 (An = Pu, 1; Np, 2) have been synthesized, characterized by X-ray diffraction, X-ray absorption, 1H NMR and UV-vis-NIR spectroscopies, and cyclic voltammetry, along with computational modeling and analysis. In the case of Pu, oxidation of Pu(iii) to Pu(iv) was observed upon complexation with the [( t BuNO)py]- ligand. The Pu complex 1 and Np complex 2 were also isolated directly from Pu(iv) and Np(iv) precursors. Electrochemical measurements indicate that a Pu(iii) species can be accessed upon one-electron reduction of 1 with a large negative reduction potential (E 1/2 = -2.26 V vs. Fc+/0). Applying oxidation potentials to 1 and 2 resulted in ligand-centered electron transfer reactions, which is different from the previously reported redox chemistry of UIV[( t BuNO)py]4 that revealed a stable U(v) product. Treatment of an anhydrous Am(iii) precursor with the [( t BuNO)py]- ligand did not result in oxidation to Am(iv). Instead, the dimeric complex [AmIII(μ2-( t BuNO)py)(( t BuNO)py)2]2 (3) was isolated. Complex 3 is a rare example of a structurally characterized non-aqueous Am-containing molecular complex prepared using inert atmosphere techniques. Predicted redox potentials from density functional theory calculations show a trivalent accessibility trend of U(iii) < Np(iii) < Pu(iii) and that the higher oxidation states of actinides (i.e., +5 for Np and Pu and +4 for Am) are not stabilized by [2-( t BuNO)py]-, in good agreement with experimental observations.

Project description:Recent investigations into the effects of dimensional reduction on halide double perovskites have revealed an intriguing change in band structure when the three-dimensional (3D) perovskite is reduced to a two-dimensional (2D) perovskite with inorganic sheets of monolayer thickness (n = 1). The indirect bandgap of 3D Cs2AgBiBr6 becomes direct in the n = 1 perovskite whereas the direct bandgap of 3D Cs2AgTlBr6 becomes indirect at the n = 1 limit. Here, we apply a linear combination of atomic orbitals approach to uncover the orbital basis for this bandgap symmetry transition with dimensional reduction. We adapt our previously established method for predicting band structures of 3D double perovskites for application to their 2D congeners, emphasizing new considerations required for the 2D lattice. In particular, we consider the inequivalence of the terminal and bridging halides and the consequences of applying translational symmetry only along two dimensions. The valence and conduction bands of the layered perovskites can be derived from symmetry adapted linear combinations of halide p orbitals propagated across the 2D lattice. The dispersion of each band is then determined by the bonding and antibonding interactions of the metal and halide orbitals, thus affording predictions of the essential features of the band structure. We demonstrate this analysis for 2D Ag-Bi and Ag-Tl perovskites with sheets of mono- and bilayer thickness, establishing a detailed understanding of their band structures, which enables us to identify the key factors that drive the bandgap symmetry transitions observed at the n = 1 limit. Importantly, these insights also allow us to make the general prediction that direct → indirect or indirect → direct bandgap transitions in the monolayer limit are most likely in double perovskite compositions that involve participation of metal d orbitals at the band edges or that have no metal-orbital contributions to the valence band, laying the groundwork for the targeted realization of this phenomenon.

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Reduction chemistry of neptunium cyclopentadienide complexes: from structure to understanding.

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Reduction chemistry of neptunium cyclopentadienide complexes: from structure to understanding.

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