PCET-driven Reactivity of Neptunyl(VI) Yields Oxo-bridged Np(V) and Np(IV) Species

A reaction sequence based upon the principles of proton-coupled electron transfer (PCET) has been used to access two unconventional oxo-deficient polynuclear complexes of neptunium (Np). The complexes featuring mono-μ2-oxo motifs assemble under ambient atmosphere upon dissolution of neptunyl(VI) diacetate dihydrate (NpO2(OAc)2(H2O)2·HOAc) in methanol followed by addition of a supporting pentadentate ligand (LNM); one complex is a mixed-valent [NpV,NpIV,NpV ] trimer with two bridging μ2-oxos and the other is a [NpV,NpV ] dimer featuring a single μ2-oxo. The outer Np centers are also capped with terminal oxo ligands. Spectroscopic and spectrokinetic findings show that intermediate [NpV O2(OAc)]n species form prior to metal chelation by LNM; electrolysis experiments demonstrate that production of Np(V) gives rise to asynchronous proton transfer that does not occur otherwise (in the Np(VI) state) as well as condensation (loss of H2O) and formation of the polynuclear complexes. The oxo-deficient nature of these products is attributable to the reduction/condensation reaction sequence of PCET. Consequently, PCET reactivity appears poised to complement more established techniques for interconverting actinide oxidation states, a prospect with considerable applications in fuel recycling for low-carbon nuclear energy

Observations Regarding the Synthesis and Redox Chemistry of Heterobimetallic Uranyl Complexes Containing Group 10 Metals

Literature reports have demonstrated that Schiff-base-type ligands can serve as robust platforms for the synthesis of heterobimetallic complexes containing transition metals and the uranyl dication (UO22+). However, efforts have not advanced to include either synthesis of complexes containing second- or third-row transition metals or measurement of the redox properties of the corresponding heterobimetallic complexes, despite the significance of actinide redox in studies of nuclear fuel reprocessing and separations. Here, metalloligands denoted [Ni], [Pd], and [Pt] that contain the corresponding Group 10 metals have been prepared and a synthetic strategy to access species incorporating the uranyl ion (UO22+) has been explored, toward the goal of understanding how the secondary metals could tune uranium-centered redox chemistry. The synthesis and redox characterization of the bimetallic complex [Ni,UO2] was achieved, and factors that appear to govern extension of the chosen synthetic strategy to complexes with Pd and Pt are reported here. Infrared and solid-state structural data from X-ray diffraction analysis of the metalloligands [Pd] and [Pt] show that the metal centers in these complexes adopt the expected square planar geometries, while the structure of the bimetallic [Ni,UO2] reveals that the uranyl moiety influences the coordination environment of Ni(II), including inducement of a puckering of the ligand backbone of the complex in which the phenyl rings fold around the nickel-containing core in an umbrella-shaped fashion. Cyclic voltammetric data collected on the heterobimetallic complexes of both Ni(II) and Pd(II) provide evidence for uranium-centered redox cycling, as well as for the accessibility of other reductions that could be associated with Ni(II) or the organic ligand backbone. Taken together, these results highlight the unique redox behaviors that can be observed in multimetallic systems and design concepts that could be useful for accessing tunable multimetallic complexes containing the uranyl dication

Synthesis, Isolation, and Study of a Heterobimetallic Uranyl Crown-Ether Complex

Although crown ethers can selectively bind many metal cations, little is known regarding the properties of crown ether complexes of the uranyl dication, UO22+. Here, the synthesis and characterization of an isolable complex in which the uranyl dication is bound in an 18-crown-6-like moiety are reported. A tailored macrocyclic complex featuring an accessory Pt(II) center was used to drive capture of UO22+ by the crown, as demonstrated by results from single-crystal X-ray diffraction analysis. The U(V) oxidation state becomes accessible at a quite positive potential (E1/2) of –0.18 V vs. Fc+/0 upon complexation, representing the most positive UVI/UV potential yet reported for the UO2n+ core moiety. Joint computational studies show that the electronic structure of the U(V) form results in significant weakening of U–Ooxo bonding despite the quite positive reduction potential at which this species can be accessed, underscoring that crown-ligated uranyl species could demonstrate unique reactivity under only modestly reducing conditions

Evidence for Ligand- and Metal-Centered Reduction in Polypyridyl Dicarboxylate Complexes of Ru(II) and U(VI)

Polypyridyl dicarboxylates have been established as oxidatively robust ligands capable of effectively binding heavy metals, but the reductive electrochemical properties of complexes supported by these ligands have not been explored to date. Here, the redox properties of Ru(II) and uranyl(VI) (UO22+) complexes of 2,2′-bipyridyl-6,6′-dicarboxylate (bdc), 2,2′:6′,2″-terpyridyl-6,6″-dicarboxylate (tdc), and 4′-phenyl-2,2′:6′,2″-terpyridyl-6,6″-dicarboxylate (Phtdc) have been investigated, revealing that these ligands can enable both ligand- and metal-centered reductions. In control ruthenium complexes, electrochemical and spectroelectrochemical data supported by theoretical findings from density functional theory suggest electron density in the reduced forms primarily resides on the ligands. In bdc complexes of uranyl, electrochemical data and theoretical findings support the involvement of both ligand- and metal-centered reductive behavior. This “non-innocent” redox chemistry, along with support for the assertion that these ligands bind large metals effectively, suggests that polypyridyl dicarboxylates could be useful in new schemes for reductive activation of challenging metal-containing species. The observation of ligand-centered reduction events is also in agreement with the recognized “non-innocent” redox activity of related 2,2′-bipyridyl systems that lack appended carboxylate functionalities

Crystal structure of [μ-1κ2C1,C4:2(1,2,3,4-η)-1,2,3,4-tetraphenylbuta-1,3-diene-1,4-diyl]bis(tricarbonylosmium)(Os—Os)

In the title complex C34H20O6Os2 or (μ-η4-C4Ph4)Os2(CO)6, one Os atom is part of a metallacyclopentadiene ring, while the second Os atom is π-bonded to the organic portion of this ring. The distance of 2.7494 (2) Å between the two Os atoms is typical of an Os—Os single bond. Three carbonyl ligands are attached to each Os atom and these six carbonyls adopt an eclipsed conformation. There are no bridging or semibridging CO groups. Two carbonyl ligands and all four phenyl groups are disordered over two slightly different positions for which each atom in the minor components is displaced less than 1 Å from the corresponding atom in the major components. The refined occupancies of the major components of the carbonyl ligands are 0.568 (16) and 0.625 (13), while those for the phenyl rings are 0.50 (3), 0.510 (12), 0.519 (18), and 0.568 (12)