Reductive silylation of the uranyl ion with Ph3SiOTf

Homoleptic Acetylacetonate (acac) and β-Ketoiminate (acnac) Complexes of Uranium

Transmetalation of potassium salts of differently substituted acetylacetonate (acac) and β-ketoiminate (acnac) with [U(I)₃(dioxane)_1.5] and [U(I)₄(dioxane)₂] resulted in the formation of homoleptic, octahedral complexes [U(^tBuacnac^Ph)₃] (with ^tBuacnac^Ph = 2,2,6,6-tetramethyl-5-(phenylimino)heptan-3-onate) in the oxidation states +III and +IV and the homoleptic, square prismatic complexes [U^IV(^Meacnac^Ph)₄] (with ^Meacnac^Ph = 4-(phenylimino)pentan-2-onate) and the homoleptic, square antiprismatic complexes [U(^tBuacac)₄] [with acac = 2,2,6,6-tetramethyl-3,5-heptanedionate (^tBuacac), 2,2,6,6-tetramethyl,4-methyl-3,5-heptanedionate (^tBuac^Meac), and 2,2,6,6-tetramethyl-4-phenyl-3,5-heptanedionate (^tBuac^Phac)] in oxidation states +III, +IV, and +V. Oxidation of [U^III(^tBuacnac^Ph)₃] (1) with AgOTf yielded [U^IV(^tBuacnac^Ph)₃][OTf] (2), which was fully characterized by single-crystal X-ray diffraction analysis, a combination of ultraviolet/visible/near-infrared, nuclear magnetic resonance, and infrared spectroscopies, and solid-state superconducting quantum interference device magnetization studies. Complexation of the sterically less encumbering ligand derivative ^Meacnac^Ph provided access to the tetravalent, square antiprismatic complex [U^IV(^Meacnac^Ph)₄] (3). Cyclovoltammetric analysis of the square antiprismatic [U^IV(^tBuacac)₄] (4), [U^IV(^tBuac^Meac)₄] (5), and [U^IV(^tBuac^Phac)₄] (6) revealed reversible anodic and cathodic waves, attributable to the U(III/IV) and U(IV/V) redox couples, both being chemically accessible, as tested in the case of 5. The corresponding U(III) and U(V) compounds, [K(2.2.2-cryptand)][U^III(^tBuac^Meac)₄] (7) and [U^V(^tBuac^Meac)₄][SbF₆] (8), were synthesized accordingly. Unfortunately, reduced 7 proved to be too reactive for isolation and could only be detected by electron paramagnetic resonance spectroscopy. Notably, electrochemical studies on homoleptic uranium(IV) complexes with differently derivatized (R) ac^Rac ligands (R = H, Me, or Ph) feature large electrochemical windows of up to 2.91 V, measured between the uranium(III) and the uranium(V) species, in addition to high stability toward repeated potential scans.

Coordination of Uranyl to the Redox-Active Calix[4]pyrrole Ligand

Reaction of [Li(THF)]₄[L] (L = Me₈-calix[4]pyrrole]) with 0.5 equiv of [U^VIO₂Cl₂(THF)₂]₂ results in formation of the oxidized calix[4]pyrrole product, [Li(THF)]₂[L^Δ] (1), concomitant with formation of reduced uranium oxide byproducts. Complex 1 can also be generated by reaction of [Li(THF)]₄[L] with 1 equiv of I₂. We hypothesize that formation of 1 proceeds via formation of a highly oxidizing cis-uranyl intermediate, [Li]₂[cis-U^VIO₂(calix[4]pyrrole)]. To test this hypothesis, we explored the reaction of 1 with either 0.5 equiv of [U^VIO₂Cl₂(THF)₂]₂ or 1 equiv of [U^VIO₂(OTf)₂(THF)₃], which affords the isostructural uranyl complexes, [Li(THF)][U^VIO₂(L^Δ)Cl(THF)] (2) and [Li(THF)][U^VIO₂(L^Δ)(OTf)(THF)] (3), respectively. In the solid state, 2 and 3 feature unprecedented uranyl-η⁵-pyrrole interactions, making them rare examples of uranyl organometallic complexes. In addition, 2 and 3 exhibit some of the smallest O-U-O angles reported to date (2: 162.0(7) and 162.7(7)°; 3: 164.5(5)°). Importantly, the O-U-O bending observed in these complexes suggests that the oxidation of [Li(THF)]₄[L] does indeed occur via an unobserved cis-uranyl intermediate.

Site-selective halogenation of mixed-valent vanadium oxide clusters

Here, we expand on the synthesis and characterization of chloride-functionalized polyoxovanadate-alkoxide (POV-alkoxide) clusters, to include the halogenation of mixed-valent vanadium oxide assemblies.

Differential uranyl(v) oxo-group bonding between the uranium and metal cations from groups 1, 2, 4, and 12; a high energy resolution X-ray absorption, computational, and synthetic study

Uranyl Pacman takes them all: the bonding of s- and d-block cations to uranyl is compared by experiment, spectroscopy and theory.

The uranyl(vi) ‘Pacman’ complex [(UO₂)(py)(H₂L)] A (L = polypyrrolic Schiff-base macrocycle) is reduced by Cp₂Ti(η²-Me₃SiC Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CSiMe₃) and [Cp₂TiCl]₂ to oxo-titanated uranyl(v) complexes [(py)(Cp₂Ti^IIIOUO)(py)(H₂L)] 1 and [(ClCp₂Ti^IVOUO)(py)(H₂L)] 2. Combination of Zr^II and Zr^IV synthons with A yields the first Zr^IV–uranyl(v) complex, [(ClCp₂ZrOUO)(py)(H₂L)] 3. Similarly, combinations of Ae⁰ and Ae^II synthons (Ae = alkaline earth) afford the mono-oxo metalated uranyl(v) complexes [(py)₂(ClMgOUO)(py)(H₂L)] 4, [(py)₂(thf)₂(ICaOUO)(py) (H₂L)] 5; the zinc complexes [(py)₂(XZnOUO)(py)(H₂L)] (X = Cl 6, I 7) are formed in a similar manner. In contrast, the direct reactions of Rb or Cs metal with A generate the first mono-rubidiated and mono-caesiated uranyl(v) complexes; monomeric [(py)₃(RbOUO)(py)(H₂L)] 8 and hexameric [(MOUO)(py)(H₂L)]₆ (M = Rb 8b or Cs 9). In these uranyl(v) complexes, the pyrrole N–H atoms show strengthened hydrogen-bonding interactions with the endo-oxos, classified computationally as moderate-strength hydrogen bonds. Computational DFT MO (density functional theory molecular orbital) and EDA (energy decomposition analysis), uranium M₄ edge HR-XANES (High Energy Resolution X-ray Absorption Near Edge Structure) and 3d4f RIXS (Resonant Inelastic X-ray Scattering) have been used (the latter two for the first time for uranyl(v) in 7 (ZnI)) to compare the covalent character in the U^V–O and O–M bonds and show the 5f orbitals in uranyl(vi) complex A are unexpectedly more delocalised than in the uranyl(v) 7 (ZnI) complex. The O_exo–Zn bonds have a larger covalent contribution compared to the Mg–O_exo/Ca–O_exo bonds, and more covalency is found in the U–O_exo bond in 7 (ZnI), in agreement with the calculations.

Uranyl Functionalization Mediated by Redox-Active Ligands: Generation of O-C Bonds via Acylation

A series of uranyl compounds with the redox-active iminoquinone ligand have been synthesized, and their electronic structures elucidated using multinuclear NMR, EPR, electronic absorption spectroscopies, SQUID magnetometry, and X-ray crystallography. Characterization and analysis of the iminoquinone (iq⁰) complex, (^dippiq)UO₂(OTf)₂THF (1-iq), the iminosemiquinone (isq^1-) complex, (^dippisq)₂UO₂THF (2-isq), and the amidophenolate (ap^2-) complex, [(^dippap)₂UO₂THF][K(18-crown-6)(THF)₂]₂(3-ap crown) show that reduction events are ligand-based, with the uranium center remaining in the hexavalent state. Reactivity of 2-isq with B-chlorocatecholborane or pivaloyl chloride leads to U-O_uranyl bond scission and reduction of U(VI) to U(IV) concomitant with ligand oxidation along with organic byproducts. ¹⁸O isotopic labeling experiments along with IR spectroscopy, mass spectrometry, and multinuclear NMR spectroscopy confirm that the organic byproducts contain oxygen atoms which originate from U-O_uranyl bond activation.

Double uranium oxo cations derived from uranyl by borane or silane reduction

A new type of double uranium oxo cation [O-U-O-U-O]4+ is prepared by selective oxygen-atom abstraction from macrocyclic uranyl complexes using either boranes or silanes. A significant degree of multiple U[double bond, length as m-dash]O bonding is evident throughout the U2O3 core, but either trans-,cis- or trans-,trans-OUOUO motifs can be isolated as boron- or silicon-capped oxo complexes. Further controlled deoxygenation of the borylated system is also possible.

Axially Symmetric U-O-Ln- and U-O-U-Containing Molecules from the Control of Uranyl Reduction with Simple f-Block Halides

The reduction of U^VI uranyl halides or amides with simple Ln^II or U^III salts forms highly symmetric, linear, oxo-bridged trinuclear U^V /Ln^III /U^V , Ln^III /U^IV /Ln^III , and U^IV /U^IV /U^IV complexes or linear Ln^III /U^V polymers depending on the stoichiometry and solvent. The reactions can be tuned to give the products of one- or two-electron uranyl reduction. The reactivity and magnetism of these compounds are discussed in the context of using a series of strongly oxo-coupled homo- and heterometallic poly(f-block) chains to better understand fundamental electronic structure in the f-block.

Unusual intramolecular CH⋯O hydrogen bonding interaction between a sterically bulky amide and uranyl oxygen

An unusual intramolecular CH⋯O hydrogen bonding interaction between a sterically bulky amide and uranyl oxygen is found to selectively extract uranyl.

Inner-sphere vs. outer-sphere reduction of uranyl supported by a redox-active, donor-expanded dipyrrin

The uranyl(vi) complex UO₂Cl(L) of the redox-active, acyclic diimino-dipyrrin anion, L^- is reported and its reaction with inner- and outer-sphere reductants studied. Voltammetric, EPR-spectroscopic and X-ray crystallographic studies show that chemical reduction by the outer-sphere reagent CoCp₂ initially reduces the ligand to a dipyrrin radical, and imply that a second equivalent of CoCp₂ reduces the U(vi) centre to form U(v). Cyclic voltammetry indicates that further outer-sphere reduction to form the putative U(iv) trianion only occurs at strongly cathodic potentials. The initial reduction of the dipyrrin ligand is supported by emission spectra, X-ray crystallography, and DFT; the latter also shows that these outer-sphere reactions are exergonic and proceed through sequential, one-electron steps. Reduction by the inner-sphere reductant [TiCp₂Cl]₂ is also likely to result in ligand reduction in the first instance but, in contrast to the outer-sphere case, reduction of the uranium centre becomes much more favoured, allowing the formation of a crystallographically characterised, doubly-titanated U(iv) complex. In the case of inner-sphere reduction only, ligand-to-metal electron-transfer is thermodynamically driven by coordination of Lewis-acidic Ti(iv) to the uranyl oxo, and is energetically preferable over the disproportionation of U(v). Overall, the involvement of the redox-active dipyrrin ligand in the reduction chemistry of UO₂Cl(L) is inherent to both inner- and outer-sphere reduction mechanisms, providing a new route to accessing a variety of U(vi), U(v), and U(iv) complexes.

Subtle Interactions and Electron Transfer between U(III) , Np(III) , or Pu(III) and Uranyl Mediated by the Oxo Group

A dramatic difference in the ability of the reducing An(III) center in AnCp3 (An=U, Np, Pu; Cp=C5 H5 ) to oxo-bind and reduce the uranyl(VI) dication in the complex [(UO2 )(THF)(H2 L)] (L="Pacman" Schiff-base polypyrrolic macrocycle), is found and explained. These are the first selective functionalizations of the uranyl oxo by another actinide cation. At-first contradictory electronic structural data are explained by combining theory and experiment. Complete one-electron transfer from Cp3 U forms the U(IV) -uranyl(V) compound that behaves as a U(V) -localized single molecule magnet below 4 K. The extent of reduction by the Cp3 Np group upon oxo-coordination is much less, with a Np(III) -uranyl(VI) dative bond assigned. Solution NMR and NIR spectroscopy suggest Np(IV) U(V) but single-crystal X-ray diffraction and SQUID magnetometry suggest a Np(III) -U(VI) assignment. DFT-calculated Hirshfeld charge and spin density analyses suggest half an electron has transferred, and these explain the strongly shifted NMR spectra by spin density contributions at the hydrogen nuclei. The Pu(III) -U(VI) interaction is too weak to be observed in THF solvent, in agreement with calculated predictions.

Reductive silylation of Cp*UO2((Mes)PDI(Me)) promoted by Lewis bases

Functionalization of the uranyl moiety (UO2(2+)) in Cp*UO2((Mes)PDI(Me)) (1-PDI) ((Mes)PDI(Me) = 2,6-((Mes)N=CMe)2C5H3N; Mes = 2,4,6-triphenylmethyl), which bears a reduced, monoanionic pyridine(diimine) ligand, is reported. Silylating reagents, R3Si-X (R = Me, X = Cl, I, OTf, SPh; R = Ph, X = Cl), effectively add across the strong O=U=O bonds in the presence of the Lewis base, OPPh3, generating products of the form (R3SiO)2UX2(OPPh3)2 (R = Me, X = I (2-OPPh3), Cl (3-OPPh3), SPh (5-OPPh3), OTf (6-OPPh3); R = Ph, X = Cl (4-OPPh3)). During this transformation, reduction to uranium(iv) occurs with loss of (Cp*)2 and (Mes)PDI(Me), each of which acts as a one-electron source. In the reaction, the Lewis base serves to activate the silyl halide, generating a more electrophilic silyl group, as determined by (29)Si NMR spectroscopy, that undergoes facile transfer to the oxo groups. Complete U-O bond scission was accomplished by treating the uranium(iv) disiloxide compounds with additional silylating reagent, forming the family (Ph3PO)2UX4. All compounds were characterized by (1)H NMR, infrared, and electronic absorption spectroscopies. X-ray crystallographic characterization was used to elucidate the structures of 2-OPPh3, 4-OPPh3, 5-OPPh3, and 6-OPPh3.

Oxo Ligand Substitution in a Cationic Uranyl Complex: Synergistic Interaction of an Electrophile and a Reductant

Reaction of [U(VI)O2(dppmo)2(OTf)][OTf] (dppmo = Ph2P(O)CH2P(O)Ph2) with 4 equiv of Ph3SiOTf and 2 equiv of Cp2Co generates the U(IV) complex U(IV)(OTf)4(dppmo)2 (1), as a yellow-green crystalline solid in 83% yield, along with Ph3SiOSiPh3 and [Cp2Co][OTf]. This reaction proceeds via a U(IV) silyloxide intermediate, [U(IV)(OSiPh3)(dppmo)2(OTf)2][OTf] (2), which we have isolated and structurally characterized. Similarly, reaction of [U(VI)O2(TPPO)4][OTf]2 (TPPO = Ph3PO) with 6 equiv of Me3SiOTf and 2 equiv of Cp2Co generates the U(IV) complex, [Cp2Co][U(IV)(OTf)5(TPPO)2] (3), as a yellow-green crystalline solid in 76% yield, concomitant with formation of Me3SiOSiMe3, [Ph3POSiMe3][OTf], and [Cp2Co][OTf]. Complexes 1 and 3 have been fully characterized, including analysis by X-ray crystallography. The conversion of [U(VI)O2(dppmo)2(OTf)][OTf] and [U(VI)O2(TPPO)4][OTf]2 to complexes 1 and 3, respectively, represents rare examples of well-defined uranyl oxo ligand substitution.

Control of oxo-group functionalization and reduction of the uranyl ion

Uranyl complexes of a large, compartmental N8-macrocycle adopt a rigid, "Pacman" geometry that stabilizes the U(V) oxidation state and promotes chemistry at a single uranyl oxo-group. We present here new and straightforward routes to singly reduced and oxo-silylated uranyl Pacman complexes and propose mechanisms that account for the product formation, and the byproduct distributions that are formed using alternative reagents. Uranyl(VI) Pacman complexes in which one oxo-group is functionalized by a single metal cation are activated toward single-electron reduction. As such, the addition of a second equivalent of a Lewis acidic metal complex such as MgN″2 (N″ = N(SiMe3)2) forms a uranyl(V) complex in which both oxo-groups are Mg functionalized as a result of Mg-N bond homolysis. In contrast, reactions with the less Lewis acidic complex [Zn(N″)Cl] favor the formation of weaker U-O-Zn dative interactions, leading to reductive silylation of the uranyl oxo-group in preference to metalation. Spectroscopic, crystallographic, and computational analysis of these reactions and of oxo-metalated products isolated by other routes have allowed us to propose mechanisms that account for pathways to metalation or silylation of the exo-oxo-group.