David D. Schnaars

Bonding Trends Traversing the Tetravalent Actinide Series: Synthesis, Structural, and Computational Analysis of AnIV(Aracnac)4 Complexes (An = Th, U, Np, Pu; Aracnac = ArNC(Ph)CHC(Ph)O; Ar = 3,5-tBu2C6H3)

A series of tetravalent An(IV) complexes with a bis-phenyl β-ketoiminate N,O donor ligand has been synthesized with the aim of identifying bonding trends and changes across the actinide series. The neutral molecules are homoleptic with the formula An((Ar)acnac)(4) (An = Th (1), U (2), Np (3), Pu (4); (Ar)acnac = ArNC(Ph)CHC(Ph)O; Ar = 3,5-(t)Bu(2)C(6)H(3)) and were synthesized through salt metathesis reactions with actinide chloride precursors. NMR and electronic absorption spectroscopy confirm the purity of all four new compounds and demonstrate stability in both solution and the solid state. The Th, U, and Pu complexes were structurally elucidated by single-crystal X-ray diffraction and shown to be isostructural in space group C2/c. Analysis of the bond lengths reveals shortening of the An-O and An-N distances arising from the actinide contraction upon moving from 1 to 2. The shortening is more pronounced upon moving from 2 to 4, and the steric constraints of the tetrakis complexes appear to prevent the enhanced U-O versus Pu-O orbital interactions previously observed in the comparison of UI(2)((Ar)acnac)(2) and PuI(2)((Ar)acnac)(2) bis-complexes. Computational analysis of models for 1, 2, and 4 (1a, 2a, and 4a, respectively) concludes that both the An-O and the An-N bonds are predominantly ionic for all three molecules, with the An-O bonds being slightly more covalent. Molecular orbital energy level diagrams indicate the largest 5f-ligand orbital mixing for 4a (Pu), but spatial overlap considerations do not lead to the conclusion that this implies significantly greater covalency in the Pu-ligand bonding. QTAIM bond critical point data suggest that both U-O/U-N and Pu-O/Pu-N are marginally more covalent than the Th analogues.

Reduction of Pentavalent Uranyl to U(IV) Facilitated by Oxo Functionalization

Addition of 2 equiv of B(C(6)F(5))(3) to [Cp*(2)Co][U(V)O(2)((Ar)acnac)(2)] (1) [(Ar)acnac = ArNC(Ph)CHC(Ph)O; Ar = 3,5-(t)Bu(2)C(6)H(3)] results in the formation of [Cp*(2)Co][U(V){OB(C(6)F(5))(3)}(2)((Ar)acnac)(2)] (2) in good yield. Reduction of 2 with 1 equiv of Cp*(2)Co generates [Cp*(2)Co](2)[U(IV){OB(C(6)F(5))(3)}(2)((Ar)acnac)(2)] (3), also in good yield. This reaction is chemically reversible, as shown by the reaction of 3 with AgOTf, which regenerates 2. Interestingly, addition of only 1 equiv of B(C(6)F(5))(3) to 1 does not produce the monofunctionalized U(V) complex. Instead, the products of disproportionation, namely, 3 and U(VI)O(2)((Ar)acnac)(2), are observed in a 1:1 ratio.

Borane-Mediated Silylation of a Metal–Oxo Ligand

The addition of 1 equiv of HSiPh(3) to UO(2)((Ar)acnac)(2) ((Ar)acnac = ArNC(Ph)CHC(Ph)O; Ar = 3,5-(t)Bu(2)C(6)H(3)), in the presence of 1 equiv of B(C(6)F(5))(3), results in the formation of U(OSiPh(3))(OB{C(6)F(5)}(3))((Ar)acnac)(2) (1), via silylation of an oxo ligand and reduction of the uranium center. The addition of 1 equiv of Cp(2)Co to 1 results in a reduction to uranium(IV) and the formation of [Cp(2)Co][U(OSiPh(3))(OB{C(6)F(5)}(3))((Ar)acnac)(2)] (2) in 78% yield. Complexes 1 and 2 have been characterized by X-ray crystallography, while the solution-phase redox properties of 1 have been measured with cyclic voltammetry.

Structural and vibrational properties of U(VI)O2Cl4(2-) and Pu(VI)O2Cl4(2-) complexes.

In actinide chemistry, it has been shown that equatorial ligands bound to the metal centers of actinyl ions have a strong influence on the chemistry and therefore the electronic structure of the O═An═O moiety. While this influence has received a significant amount of attention, considerably less research has been done to investigate how the identity of the actinide metal itself (U, Np, Pu, Am) affects the actinyl stretching frequencies. Herein, we present the structural and spectroscopic characterization of six actinyl tetrachloride compounds (M2AnO2Cl4: M = Rb, Cs, Me4N; An = U, Pu) as well as the stretching and interactive force constants of the actinyl moiety in each species. Our results show a decrease in the stretching force constant and a weakening of the An═O bond when traversing the actinides from uranyl to plutonyl, which is interesting because the solid state molecular structures show a slight contraction of the An═O bond length when uranium is replaced with plutonium. Additionally, the interaction force constants for both the uranyl and plutonyl compounds were found to be negative, which corresponds to a reduction of the force constant for the symmetric stretching mode.

Silylation of the Uranyl Ion Using B(C6F5)3-Activated Et3SiH

Addition of 2 equiv of HSiEt(3) to UO(2)((Ar)acnac)(2) ((Ar)acnac = ArNC(Ph)CHC(Ph)O, Ar = 3,5-(t)Bu(2)C(6)H(3)) in the presence of 1 equiv of B(C(6)F(5))(3) results in formation of the U(V) bis(silyloxide) complex [U(OSiEt(3))(2)((Ar)acnac)(2)][HB(C(6)F(5))(3)] (1) in 80% yield. Also produced in the reaction, as a minor product, is U(OSiEt(3))(OB{C(6)F(5)}(3))((Ar)acnac)(2) (2). Interestingly, thermolysis of 1 at 85 °C for 24 h also results in formation of 2, concomitant with production of Et(3)SiH. Addition of 1 equiv of Cp(2)Co to 1 results in formation of U(OSiEt(3))(2)((Ar)acnac)(2) (3) and [Cp(2)Co][HB(C(6)F(5))(3)] (4), which can be isolated in 61% and 71% yields, respectively. Complexes 1-3 have been characterized by X-ray crystallography, while the solution-phase redox properties of 1 have been measured with cyclic voltammetry.

Supramolecular Interactions in PuO2Cl42– and PuCl62– Complexes with Protonated Pyridines: Synthesis, Crystal Structures, and Raman Spectroscopy

The synthesis, crystal structures, and Raman spectra of seven plutonium chloride compounds are presented. The materials are based upon Pu(VI)O2Cl4(2-) and Pu(IV)Cl6(2-) anions that are charge balanced by protonated pyridinium cations. The single crystal X-ray structures show a variety of donor-acceptor interactions between the plutonium perhalo anions and the cationic pyridine groups. Complementary Raman spectra show that these interactions can be probed through the symmetric vibrational mode of the plutonyl moiety. Unlike previously reported studies in similar uranyl(VI) systems, the facile redox chemistry of plutonium in aqueous solution has demonstrated the feasibility of using not only the An(VI)O2Cl4(2-) anion with approximate D4h symmetry but also the approximately Oh An(IV)Cl6(2-) anion in order to manipulate both the structure and dimensionality of such hybrid materials.

Differences in actinide metal–ligand orbital interactions: comparison of U(IV) and Pu(IV) β-ketoiminate N,O donor complexes

Syntheses and characterization of UCl(2)((Ar)acnac)(2), UI(2)((Ar)acnac)(2), and PuI(2)((Ar)acnac)(2) are reported ((Ar)acnac denotes a bis-phenyl β-ketoiminate ligand where Ar = 3,5-(t)Bu(2)C(6)H(3)). Structural analyses and computations show significant metal-ligand orbital interaction differences in U(IV) vs. Pu(IV) bonding.

Reactivity of UH3 with mild oxidants

Addition of 2 equiv of I2 to a stirring suspension of UH3 in Et2O results in vigorous gas evolution and the formation of UI4(OEt2)2 (1), which can be isolated in good yields as an air- and moisture-sensitive brick-red powder. Addition of 3 equiv of AgBr to UH3 in DME produces UBr3(DME)2 (2), while addition of 4 equiv of AgX to UH3 in DME-CH2Cl2 provides UX4(DME)2 (X = Br, 3; Cl, 4). Similarly, the reaction of 4 equiv of AgOTf with UH3 in neat DME generates U(OTf)4(DME)2 (5). Each of these reactions proceeds with the evolution of hydrogen. Complex can also be generated by reaction of 4 equiv of Me3SiI with UCl4 in Et2O. All complexes were fully characterized, including analysis by X-ray crystallography.

Reactivity of UI4(OEt2)2 with phenols: probing the chemistry of the U–I bond

Solutions of UI4(OEt2)2 in Et2O were found to deposit orange crystals of [H(OEt2)2][UI5(OEt2)] (1) upon standing at room temperature. The proton in the cation of 1 most likely originates from the surface of the glass vial in which the solution was stored. Reactions of UI4(OEt2)2 with 1 equiv. of ArOH in toluene, followed by addition of THF, provides UI3(OAr)(THF)x (Ar = Ph, x = 3, 2; Ar = 2,6-Ph2C6H3, x = 2, 3). UI4(OEt2)2 also reacts with 2 equiv. of ArOH (Ar = Ph, 4-tBuC6H4, 2,6-Me2C6H3, C6F5) in toluene, followed by addition of THF, to generate UI2(OC6H5)2(THF)3 (4), UI2(O-4-tBuC6H4)2(THF)3 (5), UI2(O-2,6-Me2C6H3)2(THF)3 (6) and UI2(OC6F5)2(THF)3 (7), in moderate yields. Complete conversion to the products requires the use of a dynamic vacuum to remove the HI generated upon addition of the phenol.

Lattice Solvent and Crystal Phase Effects on the Vibrational Spectra of UO2Cl42

We present the structural and spectroscopic characterization of six uranyl tetrachloride compounds along with a quantified analysis showing the influence of both the crystallographic phase and the lattice solvent upon the vibrational properties of the uranyl moiety. From the uranyl symmetric and asymmetric stretching frequencies we use a valence bond potential model to calculate the stretching and interaction force constants of the uranyl moiety in each compound. Quantifying these second-sphere influences provides insight into the vibrational properties, and indirectly the electronic structure, of the uranyl ion in its ground state. These data provide a better guide for assessing the validity of future comparisons with respect to bond strength, length, and electronic properties among series of actinyl compounds where non-actinide variables may be at play.