Uranium(III) Redox Chemistry Assisted by a Hemilabile Bis(phenolate) Cyclam Ligand: Uranium–Nitrogen Multiple Bond Formation Comprising a <i>trans</i>-{RNU(VI)NR}²⁺ Complex

A new monoiodide U­(III) complex anchored on a hexadentate dianionic 1,4,8,11-tetraazacyclotetradecane-based bis­(phenolate) ligand, [U­(κ⁶-{(^tBu2ArO)₂Me₂-cyclam})­I] (<b>1</b>), was synthesized from the reaction of [UI₃(THF)₄] (THF = tetrahydrofuran) and the respective potassium salt K₂(^tBu2ArO)₂Me₂-cyclam and structurally characterized. Reactivity of <b>1</b> toward one-, two-, and four-electron oxidants was studied to explore the reductive chemistry of this new U­(III) complex. Complex <b>1</b> reacts with one-electron oxidizers, such as iodine and TlBPh₄, to form the seven-coordinate cationic uranium­(IV) complexes [U­(κ⁶-{(^tBu2ArO)₂Me₂-cyclam})­I]­[X] (X = I (<b>2-I</b>), BPh₄ (<b>2-BPh</b>_<b>4</b>)). The new uranium­(III) complex reacts with inorganic azides to yield the pseudohalide uranium­(IV) complex [U­(κ⁶-{(^tBu2ArO)₂Me₂-cyclam})­(N₃)₂] (<b>4</b>) and the nitride-bridged diuranium­(IV/IV) complex [(κ⁴-{(^tBu2ArO)₂Me₂-cyclam})­(N₃)­U­(μ-N)­U­(κ⁵-{(^tBu2ArO)₂Me₂-cyclam})] (<b>5</b>). Two equivalents of [U­(κ⁶-{(^tBu2ArO)₂Me₂-cyclam})­I] (<b>1</b>) effect the four-electron reduction of 1 equiv of PhNNPh to form the bis­(imido) complex [U­(κ⁴-{(^tBu2ArO)₂Me₂-cyclam})­(NPh)₂] (<b>6</b>) and the U­(IV) species <b>2-I</b>. Moreover, the hemilability of the hexadentate ancillary ligand (^tBu2ArO)₂Me₂-cyclam^2– allows to perform the reductive cleavage of azobenzene with an unprecedented formation of a <i>trans</i>-bis­(imido) complex. The complexes were characterized by NMR spectroscopy, and all the new uranium complexes were structurally authenticated by single-crystal X-ray diffraction

Oxidation of Actinyl(V) Complexes by the Addition of Nitrogen Dioxide Is Revealed via the Replacement of Acetate by Nitrite

The gas-phase complexes AnO₂(CH₃CO₂)₂^– are actinyl­(V) cores, An^VO₂⁺ (An = U, Np, Pu), coordinated by two acetate anion ligands. Whereas the addition of O₂ to U^VO₂(CH₃CO₂)₂^– exothermically produces the superoxide complex U^VIO₂(O₂)­(CH₃CO₂)₂^–, this oxidation does not occur for Np^VO₂(CH₃CO₂)₂^– or Pu^VO₂(CH₃CO₂)₂^– because of the higher reduction potentials for Np^V and Pu^V. It is demonstrated that NO₂ is a more effective electron-withdrawing oxidant than O₂, with the result that all three An^VO₂(CH₃CO₂)₂^– exothermically react with NO₂ to form nitrite complexes, An^VIO₂(CH₃CO₂)₂(NO₂)⁻. The assignment of the NO₂^– anion ligand in these complexes, resulting in oxidation from An^V to An^VI, is substantiated by the replacement of the acetate ligands in AnO₂(CH₃CO₂)₂(NO₂)⁻ and AnO₂(CH₃CO₂)₃^– by nitrites, to produce the tris­(nitrite) complexes AnO₂(NO₂)₃^–. The key chemistry of oxidation of An^V to An^VI by the addition of neutral NO₂ is established by the substitution of acetate by nitrite. The replacement of acetate ligands by NO₂^– is attributed to a metathesis reaction with nitrous acid to produce acetic acid and nitrite

On the Origins of Faster Oxo Exchange for Uranyl(V) versus Plutonyl(V)

Activation of uranyl­(V) oxo bonds in the gas phase is demonstrated by reaction of U¹⁶O₂⁺ with H₂¹⁸O to produce U¹⁶O¹⁸O⁺ and U¹⁸O₂⁺. In contrast, neptunyl­(V) and plutonyl­(V) are comparatively inert toward exchange. Computed potential energy profiles (PEPs) reveal a lower yl oxo exchange transition state for uranyl­(V)/water as compared with neptunyl­(V)/water and plutonyl­(V)/water. A correspondence between oxo exchange rates in gas phase and acid solutions is apparent; the contrasting oxo exchange rates of UO₂⁺ and PuO₂⁺ are considered in the context of covalent bonding in actinyls. Hydroxo exchange of U¹⁶O₂(¹⁶OH)⁺ with H₂¹⁸O to give U¹⁶O₂(¹⁸OH)⁺ proceeded much faster than oxo exchange, in accord with a lower computed transition state for OH exchange. The PEP for the addition of H₂O to UO₂⁺ suggests that both UO₂⁺·(H₂O) and UO­(OH)₂⁺ should be considered as potential products

Synthesis and Properties of Uranium Sulfide Cations. An Evaluation of the Stability of Thiouranyl, {SUS}²⁺

Atomic uranium cations, U⁺ and U²⁺, reacted with the facile sulfur-atom donor OCS to produce several monopositive and dipositive uranium sulfide species containing up to four sulfur atoms. Sequential abstraction of two sulfur atoms by U²⁺ resulted in US₂²⁺; density functional theory computations indicate that the ground-state structure for this species is side-on η²-S₂ triangular US₂²⁺, with the linear thiouranyl isomer, {SU^VIS}²⁺, some 171 kJ mol^–1 higher in energy. The result that the linear thiouranyl structure is a local minimum at a moderate energy suggests that it should be feasible to stabilize this moiety in molecular compounds

Gas-Phase Reactions of Molecular Oxygen with Uranyl(V) Anionic ComplexesSynthesis and Characterization of New Superoxides of Uranyl(VI)

Gas-phase complexes of uranyl­(V) ligated to anions X^– (X = F, Cl, Br, I, OH, NO₃, ClO₄, HCO₂, CH₃CO₂, CF₃CO₂, CH₃COS, NCS, N₃), [UO₂X₂]⁻, were produced by electrospray ionization and reacted with O₂ in a quadrupole ion trap mass spectrometer to form uranyl­(VI) anionic complexes, [UO₂X₂(O₂)]⁻, comprising a superoxo ligand. The comparative rates for the oxidation reactions were measured, ranging from relatively fast [UO₂(OH)₂]⁻ to slow [UO₂I₂]⁻. The reaction rates of [UO₂X₂]⁻ ions containing polyatomic ligands were significantly faster than those containing the monatomic halogens, which can be attributed to the greater number of vibrational degrees of freedom in the polyatomic ligands to dissipate the energy of the initial O₂-association complexes. The effect of the basicity of the X^– ligands was also apparent in the relative rates for O₂ addition, with a general correlation between increasing ligand basicity and O₂-addition efficiency for polyatomic ligands. Collision-induced dissociation of the superoxo complexes showed in all cases loss of O₂ to form the [UO₂X₂]⁻ anions, indicating weaker binding of the O₂^– ligand compared to the X^– ligands. Density functional theory computations of the structures and energetics of selected species are in accord with the experimental observations

Gas-Phase Uranyl, Neptunyl, and Plutonyl: Hydration and Oxidation Studied by Experiment and Theory

The following monopositive actinyl ions were produced by electrospray ionization of aqueous solutions of An^VIO₂(ClO₄)₂ (An = U, Np, Pu): U^VO₂⁺, Np^VO₂⁺, Pu^VO₂⁺, U^VIO₂(OH)⁺, and Pu^VIO₂(OH)⁺; abundances of the actinyl ions reflect the relative stabilities of the An­(VI) and An­(V) oxidation states. Gas-phase reactions with water in an ion trap revealed that water addition terminates at AnO₂⁺·(H₂O)₄ (An = U, Np, Pu) and AnO₂(OH)⁺·(H₂O)₃ (An = U, Pu), each with four equatorial ligands. These terminal hydrates evidently correspond to the maximum inner-sphere water coordination in the gas phase, as substantiated by density functional theory (DFT) computations of the hydrate structures and energetics. Measured hydration rates for the AnO₂(OH)⁺ were substantially faster than for the AnO₂⁺, reflecting additional vibrational degrees of freedom in the hydroxide ions for stabilization of hot adducts. Dioxygen addition resulted in UO₂⁺(O₂)­(H₂O)_<i>n</i> (<i>n</i> = 2, 3), whereas O₂ addition was not observed for NpO₂⁺ or PuO₂⁺ hydrates. DFT suggests that two-electron three-centered bonds form between UO₂⁺ and O₂, but not between NpO₂⁺ and O₂. As formation of the UO₂⁺–O₂ bonds formally corresponds to the oxidation of U­(V) to U­(VI), the absence of this bonding with NpO₂⁺ can be considered a manifestation of the lower relative stability of Np­(VI)

Dissociation of Gas-Phase Bimetallic Clusters as a Probe of Charge Densities: The Effective Charge of Uranyl

Complementary experimental and computational methods for evaluating relative charge densities of metal cations in gas-phase clusters are presented. Collision-induced dissociation (CID) and/or density functional theory computations were performed on anion clusters of composition MM′A_{(<i>m+n+</i>1)}^–, where the two metal ions have formal charge states M^<i>m+</i> and M′^<i>n+</i> and A is an anion, NO₃^–, Cl^–, or F^– in this work. Results for alkaline earth and lanthanide metal ions reveal that cluster CID generally preferentially produces MA_(<i>m+</i>1)^– and neutral M′A_<i>n</i> if the surface charge density of M is greater than that of M′: the metal ion with the higher charge density takes the extra anion. Computed dissociation energies corroborate that dissociation occurs via the lowest energy process. CID of clusters in which one of the two metal ions is uranyl, UO₂²⁺, shows that the effective charge density of U in uranyl is greater than that of alkaline earths and comparable to that of the late trivalent lanthanides; this is in accord with previous solution results for uranyl, from which an effective charge of 3.2+ was derived

Diamine Bis(phenolate) as Supporting Ligands in Organoactinide(IV) Chemistry. Synthesis, Structural Characterization, and Reactivity of Stable Dialkyl Derivatives

The homoleptic compounds [U­(salan-R₂)₂] (R = Me (<b>1</b>), ^tBu (<b>2</b>)) were prepared in high yield by salt-metathesis reactions between UI₄(L)₂ (L = Et₂O, PhCN) and 2 equiv of [K₂(salan-R₂)] in THF. In contrast, the reaction of the tetradentate ligands salan-R₂ with UI₃(THF)₄ leads to disproportionation of the metal and to mixtures of U­(IV) [U­(salan-R₂)₂] and [U­(salan-R₂)­I₂] complexes, depending on the ligand to M ratio. The reaction of K₂salan-Me₂ ligand with U­(IV) iodide and chloride salts always leads to mixtures of the homoleptic bis-ligand complex [U­(salan-Me₂)₂] and heteroleptic complexes [U­(salan-Me₂)­X₂] in different organic solvents. The structure of the heteroleptic complex [U­(salan-Me₂)­I₂(CH₃CN)] (<b>4</b>) was determined by X-ray studies. Heteroleptic U­(IV) and Th­(IV) chloride complexes were obtained in good yield using the bulky salan-^tBu₂ ligand. The new complexes [U­(salan-^tBu₂)­Cl₂(bipy)] (<b>5</b>) and [Th­(salan-^tBu₂)­Cl₂(bipy)] (<b>8</b>) were crystallographically characterized. The salan-^tBu₂ halide complexes of U­(IV) and Th­(IV) revealed good precursors for the synthesis of stable dialkyl complexes. The six-coordinated alkyl complexes [Th­(salan-^tBu₂)­(CH₂SiMe₃)₂] (<b>9</b>) and [U­(salan-^tBu₂)­(CH₂SiMe₃)₂] (<b>10</b>) were prepared by addition of LiCH₂SiMe₃ to the chloride precursor in toluene, and their solution and solid-state structures (for <b>9</b>) were determined by NMR and X-ray studies. These complexes are stable for days at room temperature. Preliminary reactivity studies show that CO₂ inserts into the An–C bond to afford a mixture of carboxylate products. In the presence of traces of LiCl, crystals of the dimeric insertion product [Th₂Cl­(salan-^tBu₂)₂(μ-η¹:η¹-O₂CCH₂SiMe₃)₂(μ-η¹:η²-O₂CCH₂SiMe₃)] (<b>11</b>) were isolated. The structure shows that CO₂ insertion occurs in both alkyl groups and that the resulting carboxylate is easily displaced by a chloride anion

Thorium and Uranium Carbide Cluster Cations in the Gas Phase: Similarities and Differences between Thorium and Uranium

Laser ionization of AnC₄ alloys (An = Th, U) yielded gas-phase molecular thorium and uranium carbide cluster cations of composition An_<i>m</i>C_<i>n</i>⁺, with <i>m</i> = 1, <i>n</i> = 2–14, and <i>m</i> = 2, <i>n</i> = 3–18, as detected by Fourier transform ion-cyclotron-resonance mass spectrometry. In the case of thorium, Th_<i>m</i>C_<i>n</i>⁺ cluster ions with <i>m</i> = 3–13 and <i>n</i> = 5–30 were also produced, with an intriguing high intensity of Th₁₃C_<i>n</i>⁺ cations. The AnC₁₃⁺ ions also exhibited an unexpectedly high abundance, in contrast to the gradual decrease in the intensity of other AnC_<i>n</i>⁺ ions with increasing values of <i>n</i>. High abundances of AnC₂⁺ and AnC₄⁺ ions are consistent with enhanced stability due to strong metal–C₂ bonds. Among the most abundant bimetallic ions was Th₂C₃⁺ for thorium; in contrast, U₂C₄⁺ was the most intense bimetallic for uranium, with essentially no U₂C₃⁺ appearing. Density functional theory computations were performed to illuminate this distinction between thorium and uranium. The computational results revealed structural and energetic disparities for the An₂C₃⁺ and An₂C₄⁺ cluster ions, which elucidate the observed differing abundances of the bimetallic carbide ions. Particularly noteworthy is that the Th atoms are essentially equivalent in Th₂C₃⁺, whereas there is a large asymmetry between the U atoms in U₂C₃⁺

Magnetic Properties of the Layered Lanthanide Hydroxide Series Y_<i>x</i>Dy_8‑x(OH)₂₀Cl₄·6H₂O: From Single Ion Magnets to 2D and 3D Interaction Effects

The magnetic properties of layered dysprosium hydroxides, both diluted in the diamagnetic yttrium analogous matrix (LYH:0.04Dy), and intercalated with 2,6-naphthalene dicarboxylate anions (LDyH-2,6-NDC), were studied and compared with the recently reported undiluted compound (LDyH = Dy₈(OH)₂₀Cl₄·6H₂O). The Y diluted compound reveals a single-molecule magnet (SMM) behavior of single Dy ions, with two distinct slow relaxation processes of the magnetization at low temperatures associated with the two main types of Dy sites, 8- and 9-fold coordinated. Only one relaxation process is observed in both undiluted LDyH and intercalated compounds as a consequence of dominant ferromagnetic Dy–Dy interactions, both intra- and interlayer. Semiempirical calculations using a radial effect charge (REC) model for the crystal field splitting of the Dy levels are used to explain data in terms of contributions from the different Dy sites. The dominant ferromagnetic interactions are explained in terms of orientations of easy magnetization axes obtained by REC calculations together with the sign of the superexchange expected from the Dy–O–Dy angles