Preparation of Neptunyl and Plutonyl Acetates To Access Nonaqueous Transuranium Coordination Chemistry.

Uranyl diacetate dihydrate is a useful reagent for the preparation of uranyl (UO22+) coordination complexes, as it is a well-defined stoichiometric compound featuring moderately basic acetates that can facilitate protonolysis reactivity, unlike other anions commonly used in synthetic actinide chemistry such as halides or nitrate. Despite these attractive features, analogous neptunium (Np) and plutonium (Pu) compounds are unknown to date. Here, a modular synthetic route is reported for accessing stoichiometric neptunyl(VI) and plutonyl(VI) diacetate compounds that can serve as starting materials for transuranic coordination chemistry. The new NpO22+ and PuO22+ complexes, as well as a corresponding molecular UO22+ complex, are isomorphous in the solid state, and in solution show similar solubility properties that facilitate their use in synthesis. In both solid and solution state, the +VI oxidation state (O.S.) is maintained, as demonstrated by vibrational and optical spectroscopy, confirming that acetate anions stabilize the oxidizing, high-valent +VI states of Np and Pu as they do for the more stable U(VI). All three acetate salts readily react with a model diprotic ligand, affording incorporation of U(VI), Np(VI), and Pu(VI) cores into molecular coordination compounds that occurs concomitantly with elimination of acetic acid; the new complexes are high-valent, yet overall charge neutral, facilitating entry into nonaqueous chemistry by rational synthesis. Computational studies reveal that the dianionic ligand framework assists in stabilizing the +VI O.S. via donation to the 5f shells of the actinides, highlighting the potential usefulness of protonolysis reactivity toward preparation of stabilized high-valent transuranic species.

Synthesis, Isolation, and Study of Heterobimetallic Uranyl Crown Ether Complexes.

Although crown ethers can selectively bind many metal cations, little is known regarding the solution properties of crown ether complexes of the uranyl dication, UO22+. Here, the synthesis and characterization of isolable complexes in which the uranyl dication is bound in an 18-crown-6-like moiety are reported. A tailored macrocyclic ligand, templated with a Pt(II) center, captures UO22+ in the crown moiety, 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. Isolation and characterization of the U(V) form of the crown complex are also reported here; there are no prior reports of reduced uranyl crown ether complexes, but U(V) is clearly stabilized by crown chelation. 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 Uranium(VI/V) Redox Supported by 2,2'-Bipyridyl-6,6'-dicarboxylate.

The 2,2'-bipyridyl-6,6'-dicarboxylate ligand (bdc) has been shown in prior work to effectively capture the uranyl(VI) ion, UO22+, from aqueous solutions. However, the redox properties of the uranyl complex of this ligand have not been addressed despite the relevance of uranium-centered reduction to the nuclear fuel cycle and the presence of a bipyridyl core in bdc, a motif long recognized for its ability to support redox chemistry. Here, the bdc complex of UO22+ (1-UO2) has been synthetically prepared and isolated under nonaqueous conditions for the study of its reductive chemical and electrochemical behavior. Spectrochemical titration data collected using decamethylcobaltocene (Cp*2Co) as the reductant demonstrate that 1e- reduction of 1-UO2 is accessible, and companion near-infrared and infrared spectroscopic data, along with theoretical findings from density functional theory, provide evidence that supports the accessibility of the U(V) oxidation state. Data obtained for control ruthenium complexes of bdc and related polypyridyl dicarboxylate ligands provide a counterpoint to these findings; ligand-centered reduction of bdc in these control compounds occurs at potentials more negative than those measured for reduction of 1-UO2, further supporting the generation of uranium(V) in 1-UO2. Taken together, these results underscore the usefulness of bdc as a ligand for actinyl ions and suggest that it could be useful for further studies of the reductive activation of these unique species.

Asymmetric diosmium sawhorse complexes.

Three asymmetric diosmium(I) carbonyl sawhorse complexes have been prepared by microwave heating. One of these complexes is of the type Os2(µ-O2CR)(µ-O2CR')(CO)4L2, with two different bridging carboxylate ligands, while the other two complexes are of the type Os2(µ-O2CR)2(CO)5L, with one axial CO ligand and one axial phosphane ligand. The mixed carboxylate complex Os2(µ-acetate)(µ-propionate)(CO)4[P(p-tolyl)3]2, (1), was prepared by heating Os3(CO)12 with a mixture of acetic and propionic acids, isolating Os2(µ-acetate)(µ-propionate)(CO)6, and then replacing two CO ligands with two phosphane ligands. This is the first example of an Os2 sawhorse complex with two different carboxylate bridges. The syntheses of Os2(µ-acetate)2(CO)5[P(p-tolyl)3], (3), and Os2(µ-propionate)2(CO)5[P(p-tolyl)3], (6), involved the reaction of Os3(CO)12 with the appropriate carboxylic acid to initially produce Os2(µ-carboxylate)2(CO)6, followed by treatment with refluxing tetrahydrofuran (THF) to form Os2(µ-carboxylate)2(CO)5(THF), and finally addition of tri-p-tolylphosphane to replace the THF ligand with the P(p-tolyl)3 ligand. Neutral complexes of the type Os2(µ-O2CR)2(CO)5L had not previously been subjected to X-ray crystallographic analysis. The more symmetrical disubstituted complexes, i.e. Os2(µ-formate)2(CO)4[P(p-tolyl)3]2, (8), Os2(µ-acetate)2(CO)4[P(p-tolyl)3]2, (4), and Os2(µ-propionate)2(CO)4[P(p-tolyl)3]2, (7), as well as the previously reported symmetrical unsubstituted complexes Os2(µ-acetate)2(CO)6, (2), and Os2(µ-propionate)2(CO)6, (5), were also prepared in order to examine the influence of axial ligand substitution on the Os-Os bond distance in these sawhorse molecules. Eight crystal structures have been determined and studied, namely µ-acetato-1κO:2κO'-µ-propanoato-1κO:2κO'-bis[tris(4-methylphenyl)phosphane]-1κP,2κP'-bis(dicarbonylosmium)(Os-Os) dichloromethane monosolvate, [Os2(C2H3O2)(C3H5O2)(C21H21P)2(CO)4]·CH2Cl2, (1), bis(µ-acetato-1κO:2κO')bis(tricarbonylosmium)(Os-Os), [Os2(C2H3O2)2(CO)6], (2) (redetermined structure), bis(µ-acetato-1κO:2κO')pentacarbonyl-1κ2C,2κ3C-[tris(4-methylphenyl)phosphane-1κP]diosmium(Os-Os), [Os2(C2H3O2)2(C21H21P)(CO)5], (3), bis(µ-acetato-1κO:2κO')bis[tris(4-methylphenyl)phosphane]-1κP,2κP-bis(dicarbonylosmium)(Os-Os) p-xylene sesquisolvate, [Os2(C2H3O2)2(C21H21P)2(CO)4]·1.5C8H10, (4), bis(µ-propanoato-1κO:2κO')bis(tricarbonylosmium)(Os-Os), [Os2(C3H5O2)2(CO)6], (5), pentacarbonyl-1κ2C,2κ3C-bis(µ-propanoato-1κO:2κO')[tris(4-methylphenyl)phosphane-1κP]diosmium(Os-Os), [Os2(C3H5O2)2(C21H21P)(CO)5], (6), bis(µ-propanoato-1κO:2κO')bis[tris(4-methylphenyl)phosphane]-1κP,2κP-bis(dicarbonylosmium)(Os-Os) dichloromethane monosolvate, [Os2(C3H5O2)2(C21H21P)2(CO)4]·CH2Cl2, (7), and bis(µ-formato-1κO:2κO')bis[tris(4-methylphenyl)phosphane]-1κP,2κP-bis(dicarbonylosmium)(Os-Os), [Os2(CHO2)2(C21H21P)2(CO)4], (8).

Crystal structure of [µ-1κ2C1,C4:2(1,2,3,4-η)-1,2,3,4-tetra-phenyl-buta-1,3-diene-1,4-di-yl]bis-(tri-carbonyl-osmium)(Os-Os).

In the title complex C34H20O6Os2 or (µ-η4-C4Ph4)Os2(CO)6, one Os atom is part of a metalla-cyclo-penta-diene ring, while the second Os atom is π-bonded to the organic portion of this ring. The distance of 2.7494 (2) Šbetween 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 Šfrom the corresponding atom in the major components. The refined occupancies of the major com-ponents 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).