15 research outputs found
Gas-Phase Energetics of Actinide Oxides: An Assessment of Neutral and Cationic Monoxides and Dioxides from Thorium to Curium â
Uranium(III) Redox Chemistry Assisted by a Hemilabile Bis(phenolate) Cyclam Ligand: UraniumâNitrogen Multiple Bond Formation Comprising a <i>trans</i>-{RNî»U(VI)î»NR}<sup>2+</sup> Complex
A new
monoiodide UÂ(III) complex anchored on a hexadentate dianionic
1,4,8,11-tetraazacyclotetradecane-based bisÂ(phenolate) ligand, [UÂ(Îș<sup>6</sup>-{(<sup>tBu2</sup>ArO)<sub>2</sub>Me<sub>2</sub>-cyclam})ÂI]
(<b>1</b>), was synthesized from the reaction of [UI<sub>3</sub>(THF)<sub>4</sub>] (THF = tetrahydrofuran) and the respective potassium
salt K<sub>2</sub>(<sup>tBu2</sup>ArO)<sub>2</sub>Me<sub>2</sub>-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 UÂ(III) complex. Complex <b>1</b> reacts with one-electron oxidizers, such as iodine and TlBPh<sub>4</sub>, to form the seven-coordinate cationic uraniumÂ(IV) complexes
[UÂ(Îș<sup>6</sup>-{(<sup>tBu2</sup>ArO)<sub>2</sub>Me<sub>2</sub>-cyclam})ÂI]Â[X] (X = I (<b>2-I</b>), BPh<sub>4</sub> (<b>2-BPh</b><sub><b>4</b></sub>)). The new uraniumÂ(III) complex
reacts with inorganic azides to yield the pseudohalide uraniumÂ(IV)
complex [UÂ(Îș<sup>6</sup>-{(<sup>tBu2</sup>ArO)<sub>2</sub>Me<sub>2</sub>-cyclam})Â(N<sub>3</sub>)<sub>2</sub>] (<b>4</b>) and
the nitride-bridged diuraniumÂ(IV/IV) complex [(Îș<sup>4</sup>-{(<sup>tBu2</sup>ArO)<sub>2</sub>Me<sub>2</sub>-cyclam})Â(N<sub>3</sub>)ÂUÂ(ÎŒ-N)ÂUÂ(Îș<sup>5</sup>-{(<sup>tBu2</sup>ArO)<sub>2</sub>Me<sub>2</sub>-cyclam})] (<b>5</b>). Two equivalents of [UÂ(Îș<sup>6</sup>-{(<sup>tBu2</sup>ArO)<sub>2</sub>Me<sub>2</sub>-cyclam})ÂI]
(<b>1</b>) effect the four-electron reduction of 1 equiv of
PhNî»NPh to form the bisÂ(imido) complex [UÂ(Îș<sup>4</sup>-{(<sup>tBu2</sup>ArO)<sub>2</sub>Me<sub>2</sub>-cyclam})Â(NPh)<sub>2</sub>] (<b>6</b>) and the UÂ(IV) species <b>2-I</b>.
Moreover, the hemilability of the hexadentate ancillary ligand (<sup>tBu2</sup>ArO)<sub>2</sub>Me<sub>2</sub>-cyclam<sup>2â</sup> allows to perform the reductive cleavage of azobenzene with an unprecedented
formation of a <i>trans</i>-bisÂ(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 AnO<sub>2</sub>(CH<sub>3</sub>CO<sub>2</sub>)<sub>2</sub><sup>â</sup> are actinylÂ(V) cores, An<sup>V</sup>O<sub>2</sub><sup>+</sup> (An = U, Np, Pu), coordinated by two acetate
anion ligands. Whereas the addition of O<sub>2</sub> to U<sup>V</sup>O<sub>2</sub>(CH<sub>3</sub>CO<sub>2</sub>)<sub>2</sub><sup>â</sup> exothermically produces the superoxide complex U<sup>VI</sup>O<sub>2</sub>(O<sub>2</sub>)Â(CH<sub>3</sub>CO<sub>2</sub>)<sub>2</sub><sup>â</sup>, this oxidation does not occur for Np<sup>V</sup>O<sub>2</sub>(CH<sub>3</sub>CO<sub>2</sub>)<sub>2</sub><sup>â</sup> or Pu<sup>V</sup>O<sub>2</sub>(CH<sub>3</sub>CO<sub>2</sub>)<sub>2</sub><sup>â</sup> because of the higher reduction potentials
for Np<sup>V</sup> and Pu<sup>V</sup>. It is demonstrated that NO<sub>2</sub> is a more effective electron-withdrawing oxidant than O<sub>2</sub>, with the result that all three An<sup>V</sup>O<sub>2</sub>(CH<sub>3</sub>CO<sub>2</sub>)<sub>2</sub><sup>â</sup> exothermically
react with NO<sub>2</sub> to form nitrite complexes, An<sup>VI</sup>O<sub>2</sub>(CH<sub>3</sub>CO<sub>2</sub>)<sub>2</sub>(NO<sub>2</sub>)<sup>â</sup>. The assignment of the NO<sub>2</sub><sup>â</sup> anion ligand in these complexes, resulting in oxidation from An<sup>V</sup> to An<sup>VI</sup>, is substantiated by the replacement of
the acetate ligands in AnO<sub>2</sub>(CH<sub>3</sub>CO<sub>2</sub>)<sub>2</sub>(NO<sub>2</sub>)<sup>â</sup> and AnO<sub>2</sub>(CH<sub>3</sub>CO<sub>2</sub>)<sub>3</sub><sup>â</sup> by
nitrites, to produce the trisÂ(nitrite) complexes AnO<sub>2</sub>(NO<sub>2</sub>)<sub>3</sub><sup>â</sup>. The key chemistry of oxidation
of An<sup>V</sup> to An<sup>VI</sup> by the addition of neutral NO<sub>2</sub> is established by the substitution of acetate by nitrite.
The replacement of acetate ligands by NO<sub>2</sub><sup>â</sup> 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 uranylÂ(V) oxo bonds in the gas phase is
demonstrated
by reaction of U<sup>16</sup>O<sub>2</sub><sup>+</sup> with H<sub>2</sub><sup>18</sup>O to produce U<sup>16</sup>O<sup>18</sup>O<sup>+</sup> and U<sup>18</sup>O<sub>2</sub><sup>+</sup>. In contrast,
neptunylÂ(V) and plutonylÂ(V) are comparatively inert toward exchange.
Computed potential energy profiles (PEPs) reveal a lower yl oxo exchange
transition state for uranylÂ(V)/water as compared with neptunylÂ(V)/water
and plutonylÂ(V)/water. A correspondence between oxo exchange rates
in gas phase and acid solutions is apparent; the contrasting oxo exchange
rates of UO<sub>2</sub><sup>+</sup> and PuO<sub>2</sub><sup>+</sup> are considered in the context of covalent bonding in actinyls. Hydroxo
exchange of U<sup>16</sup>O<sub>2</sub>(<sup>16</sup>OH)<sup>+</sup> with H<sub>2</sub><sup>18</sup>O to give U<sup>16</sup>O<sub>2</sub>(<sup>18</sup>OH)<sup>+</sup> proceeded much faster than oxo exchange,
in accord with a lower computed transition state for OH exchange.
The PEP for the addition of H<sub>2</sub>O to UO<sub>2</sub><sup>+</sup> suggests that both UO<sub>2</sub><sup>+</sup>·(H<sub>2</sub>O) and UOÂ(OH)<sub>2</sub><sup>+</sup> should be considered as potential
products
Diamine bis (phenolate) as supporting ligands in organoactinide (iv) chemistry. Synthesis, structural characterization, and reactivity of stable dialkyl derivatives
International audienceThe homoleptic compounds [U(salan-R2)2] (R = Me (1), tBu (2)) 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)] (4) 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)] (5) and [Th(salan-tBu2)Cl2(bipy)] (8) 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] (9) and [U(salan-tBu2)(CH2SiMe3)2] (10) were prepared by addition of LiCH2SiMe3 to the chloride precursor in toluene, and their solution and solid-state structures (for 9) 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 AnâC 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(ÎŒ-η1:η1-O2CCH2SiMe3)2(ÎŒ-η1:η2-O2CCH2SiMe3)] (11) 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
Synthesis and Properties of Uranium Sulfide Cations. An Evaluation of the Stability of Thiouranyl, {Sî»Uî»S}<sup>2+</sup>
Atomic uranium cations, U<sup>+</sup> and U<sup>2+</sup>, 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 U<sup>2+</sup> resulted in US<sub>2</sub><sup>2+</sup>; density functional theory computations indicate that the ground-state
structure for this species is side-on η<sup>2</sup>-S<sub>2</sub> triangular US<sub>2</sub><sup>2+</sup>, with the linear thiouranyl
isomer, {Sî»U<sup>VI</sup>î»S}<sup>2+</sup>, some 171
kJ mol<sup>â1</sup> 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 ComplexesîžSynthesis and Characterization of New Superoxides of Uranyl(VI)
Gas-phase
complexes of uranylÂ(V) ligated to anions X<sup>â</sup> (X =
F, Cl, Br, I, OH, NO<sub>3</sub>, ClO<sub>4</sub>, HCO<sub>2</sub>, CH<sub>3</sub>CO<sub>2</sub>, CF<sub>3</sub>CO<sub>2</sub>, CH<sub>3</sub>COS, NCS, N<sub>3</sub>), [UO<sub>2</sub>X<sub>2</sub>]<sup>â</sup>, were produced by electrospray ionization and
reacted with O<sub>2</sub> in a quadrupole ion trap mass spectrometer
to form uranylÂ(VI) anionic complexes, [UO<sub>2</sub>X<sub>2</sub>(O<sub>2</sub>)]<sup>â</sup>, comprising a superoxo ligand.
The comparative rates for the oxidation reactions were measured, ranging
from relatively fast [UO<sub>2</sub>(OH)<sub>2</sub>]<sup>â</sup> to slow [UO<sub>2</sub>I<sub>2</sub>]<sup>â</sup>. The reaction
rates of [UO<sub>2</sub>X<sub>2</sub>]<sup>â</sup> 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 O<sub>2</sub>-association complexes. The
effect of the basicity of the X<sup>â</sup> ligands was also
apparent in the relative rates for O<sub>2</sub> addition, with a
general correlation between increasing ligand basicity and O<sub>2</sub>-addition efficiency for polyatomic ligands. Collision-induced dissociation
of the superoxo complexes showed in all cases loss of O<sub>2</sub> to form the [UO<sub>2</sub>X<sub>2</sub>]<sup>â</sup> anions,
indicating weaker binding of the O<sub>2</sub><sup>â</sup> ligand
compared to the X<sup>â</sup> 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<sup>VI</sup>O<sub>2</sub>(ClO<sub>4</sub>)<sub>2</sub> (An = U, Np, Pu): U<sup>V</sup>O<sub>2</sub><sup>+</sup>, Np<sup>V</sup>O<sub>2</sub><sup>+</sup>, Pu<sup>V</sup>O<sub>2</sub><sup>+</sup>, U<sup>VI</sup>O<sub>2</sub>(OH)<sup>+</sup>, and Pu<sup>VI</sup>O<sub>2</sub>(OH)<sup>+</sup>; abundances of
the actinyl ions reflect the relative stabilities of the AnÂ(VI) and
AnÂ(V) oxidation states. Gas-phase reactions with water in an ion trap
revealed that water addition terminates at AnO<sub>2</sub><sup>+</sup>·(H<sub>2</sub>O)<sub>4</sub> (An = U, Np, Pu) and AnO<sub>2</sub>(OH)<sup>+</sup>·(H<sub>2</sub>O)<sub>3</sub> (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 AnO<sub>2</sub>(OH)<sup>+</sup> were substantially faster
than for the AnO<sub>2</sub><sup>+</sup>, reflecting additional vibrational
degrees of freedom in the hydroxide ions for stabilization of hot
adducts. Dioxygen addition resulted in UO<sub>2</sub><sup>+</sup>(O<sub>2</sub>)Â(H<sub>2</sub>O)<sub><i>n</i></sub> (<i>n</i> = 2, 3), whereas O<sub>2</sub> addition was not observed for NpO<sub>2</sub><sup>+</sup> or PuO<sub>2</sub><sup>+</sup> hydrates. DFT
suggests that two-electron three-centered bonds form between UO<sub>2</sub><sup>+</sup> and O<sub>2</sub>, but not between NpO<sub>2</sub><sup>+</sup> and O<sub>2</sub>. As formation of the UO<sub>2</sub><sup>+</sup>âO<sub>2</sub> bonds formally corresponds to the
oxidation of UÂ(V) to UÂ(VI), the absence of this bonding with NpO<sub>2</sub><sup>+</sup> can be considered a manifestation of the lower
relative stability of NpÂ(VI)