Small-Scale Metal-Based Syntheses of Lanthanide Iodide, Amide, and Cyclopentadienyl Complexes as Analogues for Transuranic Reactions

Small-scale reactions of the Pu analogues La, Ce, and Nd have been explored in order to optimize reaction conditions for milligram scale reactions of radioactive plutonium starting from the metal. Oxidation of these lanthanide metals with iodine in ether and pyridine has been studied, and LnI₃(Et₂O)_<i>x</i> (<b>1-Ln</b>; <i>x</i> = 0.75–1.9) and LnI₃(py)₄ (<b>2-Ln</b>; py = pyridine, NC₅H₅) have been synthesized on scales ranging from 15 mg to 2 g. The THF adducts LnI₃(THF)₄ (<b>3-Ln</b>) were synthesized by dissolving <b>1-Ln</b> in THF. The viability of these small-scale samples as starting materials for amide and cyclopentadienyl f-element complexes was tested by reacting KN­(SiMe₃)₂, KCp′ (Cp′ = C₅H₄SiMe₃), KCp′′ (Cp′′ = C₅H₃(SiMe₃)₂-1,3), and KC₅Me₄H with <b>1-Ln</b> generated in situ. These reactions produced Ln­[N­(SiMe₃)₂]₃ (<b>4-Ln</b>), Cp′₃Ln (<b>5-Ln</b>), Cp″₃Ln (<b>6-Ln</b>), and (C₅Me₄H)₃Ln (<b>7-Ln</b>), respectively. Small-scale samples of Cp′₃Ce (<b>5-Ce</b>) and Cp′₃Nd (<b>5-Nd</b>) were reduced with potassium graphite (KC₈) in the presence of 2.2.2-cryptand to check the viability of generating the crystallographically characterizable Ln²⁺ complexes [K­(2.2.2-cryptand)]­[Cp′₃Ln] (<b>8-Ln</b>; Ln = Ce, Nd)

Probing Ni[S₂PR₂]₂ Electronic Structure to Generate Insight Relevant to Minor Actinide Extraction Chemistry

A method to evaluate the electronic structure of minor actinide extractants is described. A series of compounds containing effective and ineffective actinide extractants (dithiophosphinates, S₂PR₂^–) bound to a common transition metal ion (Ni²⁺) was analyzed by structural, spectroscopic, and theoretical methods. By using a single transition metal that provides structurally similar compounds, the metal contributions to bonding are essentially held constant so that subtle electronic variations associated with the extracting ligand can be probed using UV-vis spectroscopy. By comparison, it is difficult to obtain similar information using analogous techniques with minor actinide and lanthanide complexes. Here, we demonstrate that this approach, supplemented with ground state and time-dependent density functional theory, provides insight for understanding why high separation factors are reported for the extractant HS₂P­(<i>o</i>-CF₃C₆H₄)₂, while lower values are reported and anticipated for other HS₂PR₂ derivatives (R = C₆H₅, <i>p</i>-CF₃C₆H₄, <i>m</i>-CF₃C₆H₄). The implications of these results for correlating electronic structure with the selectivity of HS₂PR₂ extractants are discussed

Influence of Pyrazolate vs <i>N</i>‑Heterocyclic Carbene Ligands on the Slow Magnetic Relaxation of Homoleptic Trischelate Lanthanide(III) and Uranium(III) Complexes

Two isostructural series of trigonal prismatic complexes, M­(Bp^Me)₃ and M­(Bc^Me)₃ (M = Y, Tb, Dy, Ho, Er, U; [Bp^Me]⁻ = dihydrobis­(methypyrazolyl)­borate; [Bc^Me]⁻ = dihydrobis­(methylimidazolyl)­borate) are synthesized and fully characterized to examine the influence of ligand donor strength on slow magnetic relaxation. Investigation of the dynamic magnetic properties reveals that the oblate electron density distributions of the Tb³⁺, Dy³⁺, and U³⁺ metal ions within the axial ligand field lead to slow relaxation upon application of a small dc magnetic field. Significantly, the magnetization relaxation is orders of magnitude slower for the <i>N</i>-heterocyclic carbene complexes, M­(Bc^Me)₃, than for the isomeric pyrazolate complexes, M­(Bp^Me)₃. Further, investigation of magnetically dilute samples containing 11–14 mol % of Tb³⁺, Dy³⁺, or U³⁺ within the corresponding Y³⁺ complex matrix reveals thermally activated relaxation is favored for the M­(Bc^Me)₃ complexes, even when dipolar interactions are largely absent. Notably, the dilute species U­(Bc^Me)₃ exhibits <i>U</i>_eff ≈ 33 cm^–1, representing the highest barrier yet observed for a U³⁺ molecule demonstrating slow relaxation. Additional analysis through lanthanide XANES, X-band EPR, and ¹H NMR spectroscopies provides evidence that the origin of the slower relaxation derives from the greater magnetic anisotropy enforced within the strongly donating <i>N-</i>heterocyclic carbene coordination sphere. These results show that, like molecular symmetry, ligand-donating ability is a variable that can be controlled to the advantage of the synthetic chemist in the design of single-molecule magnets with enhanced relaxation barriers

Advancing Understanding of the +4 Metal Extractant Thenoyltrifluoroacetonate (TTA^–); Synthesis and Structure of M^IVTTA₄ (M^IV = Zr, Hf, Ce, Th, U, Np, Pu) and M^III(TTA)₄^– (M^III = Ce, Nd, Sm, Yb)

Thenoyltrifluoroacetone (HTTA)-based extractions represent popular methods for separating <i>micro</i>scopic amounts of transuranic actinides (i.e., Np and Pu) from <i>macro</i>scopic actinide matrixes (e.g. bulk uranium). It is well-established that this procedure enables +4 actinides to be selectively removed from +3, + 5, and +6 f-elements. However, even highly skilled and well-trained researchers find this process complicated and (at times) unpredictable. It is difficult to improve the HTTA extractionor find alternativesbecause little is understood about why this separation works. Even the identities of the extracted species are unknown. In addressing this knowledge gap, we report here advances in fundamental understanding of the HTTA-based extraction. This effort included comparatively evaluating HTTA complexation with +4 and +3 metals (M^IV = Zr, Hf, Ce, Th, U, Np, and Pu vs M^III = Ce, Nd, Sm, and Yb). We observed +4 metals formed neutral complexes of the general formula M^IV(TTA)₄. Meanwhile, +3 metals formed anionic M^III(TTA)₄^– species. Characterization of these M­(TTA)₄^<i>x</i>– (<i>x</i> = 0, 1) compounds by UV–vis–NIR, IR, ¹H and ¹⁹F NMR, single-crystal X-ray diffraction, and X-ray absorption spectroscopy (both near-edge and extended fine structure) was critical for determining that Np^IV(TTA)₄ and Pu^IV(TTA)₄ were the primary species extracted by HTTA. Furthermore, this information lays the foundation to begin developing and understanding of why the HTTA extraction works so well. The data suggest that the solubility differences between M^IV(TTA)₄ and M^III(TTA)₄^– are likely a major contributor to the selectivity of HTTA extractions for +4 cations over +3 metals. Moreover, these results will enable future studies focused on explaining HTTA extractions preference for +4 cations, which increases from Np ^IV to Pu^IV, Hf^IV, and Zr^IV

Vanadium Bisimide Bonding Investigated by X‑ray Crystallography, ⁵¹V and ¹³C Nuclear Magnetic Resonance Spectroscopy, and V L_3,2-Edge X‑ray Absorption Near-Edge Structure Spectroscopy

Syntheses of neutral halide and aryl vanadium bisimides are described. Treatment of VCl₂(N<i>t</i>Bu)­[NTMS­(N^<i>t</i>Bu)], <b>2</b>, with PMe₃, PEt₃, PMe₂Ph, or pyridine gave vanadium bisimides via TMSCl elimination in good yield: VCl­(PMe₃)₂(N^<i>t</i>Bu)₂ <b>3</b>, VCl­(PEt₃)₂(N^<i>t</i>Bu)₂ <b>4</b>, VCl­(PMe₂Ph)₂(N^<i>t</i>Bu)₂ <b>5</b>, and VCl­(Py)₂(N^<i>t</i>Bu)₂ <b>6</b>. The halide series (Cl–I) was synthesized by use of TMSBr and TMSI to give VBr­(PMe₃)₂(N^<i>t</i>Bu)₂ <b>7</b> and VI­(PMe₃)₂(N^<i>t</i>Bu)₂ <b>8</b>. The phenyl derivative was obtained by reaction of <b>3</b> with MgPh₂ to give VPh­(PMe₃)₂(N^<i>t</i>Bu)₂ <b>9</b>. These neutral complexes are compared to the previously reported cationic bisimides [V­(PMe₃)₃(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄] <b>10</b>, [V­(PEt₃)₂(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄] <b>11</b>, and [V­(DMAP)­(PEt₃)₂(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄] <b>12</b> (DMAP = dimethylaminopyridine, PFTB = perfluoro-<i>tert</i>-butoxide). Characterization of the complexes by X-ray diffraction, ¹³C NMR, ⁵¹V NMR, and V L_3,2-edge X-ray absorption near-edge structure (XANES) spectroscopy provides a description of the electronic structure in comparison to group 6 bisimides and the bent metallocene analogues. The electronic structure is dominated by π bonding to the imides, and localization of electron density at the nitrogen atoms of the imides is dictated by the cone angle and donating ability of the axial neutral supporting ligands. This phenomenon is clearly seen in the sensitivity of ⁵¹V NMR shift, ¹³C NMR Δδ_αβ, and L₃-edge energy to the nature of the supporting phosphine ligand, which defines the parameters for designing cationic group 5 bisimides that would be capable of breaking stronger σ bonds. Conversely, all three methods show little dependence on the variable equatorial halide ligand. Furthermore, this analysis allows for quantification of the electronic differences between vanadium bisimides and the structurally analogous mixed Cp/imide system CpV­(N^<i>t</i>Bu)­X₂ (Cp = C₅H₅^1–)

Vanadium Bisimide Bonding Investigated by X‑ray Crystallography, ⁵¹V and ¹³C Nuclear Magnetic Resonance Spectroscopy, and V L_3,2-Edge X‑ray Absorption Near-Edge Structure Spectroscopy

Syntheses of neutral halide and aryl vanadium bisimides are described. Treatment of VCl₂(N<i>t</i>Bu)­[NTMS­(N^<i>t</i>Bu)], <b>2</b>, with PMe₃, PEt₃, PMe₂Ph, or pyridine gave vanadium bisimides via TMSCl elimination in good yield: VCl­(PMe₃)₂(N^<i>t</i>Bu)₂ <b>3</b>, VCl­(PEt₃)₂(N^<i>t</i>Bu)₂ <b>4</b>, VCl­(PMe₂Ph)₂(N^<i>t</i>Bu)₂ <b>5</b>, and VCl­(Py)₂(N^<i>t</i>Bu)₂ <b>6</b>. The halide series (Cl–I) was synthesized by use of TMSBr and TMSI to give VBr­(PMe₃)₂(N^<i>t</i>Bu)₂ <b>7</b> and VI­(PMe₃)₂(N^<i>t</i>Bu)₂ <b>8</b>. The phenyl derivative was obtained by reaction of <b>3</b> with MgPh₂ to give VPh­(PMe₃)₂(N^<i>t</i>Bu)₂ <b>9</b>. These neutral complexes are compared to the previously reported cationic bisimides [V­(PMe₃)₃(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄] <b>10</b>, [V­(PEt₃)₂(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄] <b>11</b>, and [V­(DMAP)­(PEt₃)₂(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄] <b>12</b> (DMAP = dimethylaminopyridine, PFTB = perfluoro-<i>tert</i>-butoxide). Characterization of the complexes by X-ray diffraction, ¹³C NMR, ⁵¹V NMR, and V L_3,2-edge X-ray absorption near-edge structure (XANES) spectroscopy provides a description of the electronic structure in comparison to group 6 bisimides and the bent metallocene analogues. The electronic structure is dominated by π bonding to the imides, and localization of electron density at the nitrogen atoms of the imides is dictated by the cone angle and donating ability of the axial neutral supporting ligands. This phenomenon is clearly seen in the sensitivity of ⁵¹V NMR shift, ¹³C NMR Δδ_αβ, and L₃-edge energy to the nature of the supporting phosphine ligand, which defines the parameters for designing cationic group 5 bisimides that would be capable of breaking stronger σ bonds. Conversely, all three methods show little dependence on the variable equatorial halide ligand. Furthermore, this analysis allows for quantification of the electronic differences between vanadium bisimides and the structurally analogous mixed Cp/imide system CpV­(N^<i>t</i>Bu)­X₂ (Cp = C₅H₅^1–)

Carbon K‑Edge X‑ray Absorption Spectroscopy and Time-Dependent Density Functional Theory Examination of Metal–Carbon Bonding in Metallocene Dichlorides

Metal–carbon covalence in (C₅H₅)₂MCl₂ (M = Ti, Zr, Hf) has been evaluated using carbon K-edge X-ray absorption spectroscopy (XAS) as well as ground-state and time-dependent hybrid density functional theory (DFT and TDDFT). Differences in orbital mixing were determined experimentally using transmission XAS of thin crystalline material with a scanning transmission X-ray microscope (STXM). Moving down the periodic table (Ti to Hf) has a marked effect on the experimental transition intensities associated with the low-lying antibonding 1<i>a</i>₁* and 1<i>b</i>₂* orbitals. The peak intensities, which are directly related to the M–(C₅H₅) orbital mixing coefficients, increase from 0.08(1) and 0.26(3) for (C₅H₅)₂TiCl₂ to 0.31(3) and 0.75(8) for (C₅H₅)₂ZrCl₂, and finally to 0.54(5) and 0.83(8) for (C₅H₅)₂HfCl₂. The experimental trend toward increased peak intensity for transitions associated with 1<i>a</i>₁* and 1<i>b</i>₂* orbitals agrees with the calculated TDDFT oscillator strengths [0.10 and 0.21, (C₅H₅)₂TiCl₂; 0.21 and 0.73, (C₅H₅)₂ZrCl₂; 0.35 and 0.69, (C₅H₅)₂HfCl₂] and with the amount of C 2p character obtained from the Mulliken populations for the antibonding 1<i>a</i>₁* and 1<i>b</i>₂* orbitals [8.2 and 23.4%, (C₅H₅)₂TiCl₂; 15.3 and 39.7%, (C₅H₅)₂ZrCl₂; 20.1 and 50.9%, (C₅H₅)₂HfCl₂]. The excellent agreement between experiment, theory, and recent Cl K-edge XAS and DFT measurements shows that C 2p orbital mixing is enhanced for the diffuse Hf (5d) and Zr (4d) atomic orbitals in relation to the more localized Ti (3d) orbitals. These results provide insight into how changes in M–Cl orbital mixing within the metallocene wedge are correlated with periodic trends in covalent bonding between the metal and the cyclopentadienide ancillary ligands

Synthesis and Structure of (Ph₄P)₂MCl₆ (M = Ti, Zr, Hf, Th, U, Np, Pu)

High-purity syntheses are reported for a series of first, second, and third row transition metal and actinide hexahalide compounds with equivalent, noncoordinating countercations: (Ph₄P)₂TiF₆ (<b>1</b>) and (Ph₄P)₂MCl₆ (M = Ti, Zr, Hf, Th, U, Np, Pu; <b>2</b>–<b>8</b>). While a reaction between MCl₄ (M = Zr, Hf, U) and 2 equiv of Ph₄PCl provided <b>3</b>, <b>4</b>, and <b>6</b>, syntheses for <b>1</b>, <b>2</b>, <b>5</b>, <b>7</b>, and <b>8</b> required multistep procedures. For example, a cation exchange reaction with Ph₄PCl and (NH₄)₂TiF₆ produced <b>1</b>, which was used in a subsequent anion exchange reaction with Me₃SiCl to synthesize <b>2</b>. For <b>5</b>, <b>7</b>, and <b>8</b>, synthetic routes starting with aqueous actinide precursors were developed that circumvented any need for anhydrous Th, Np, or Pu starting materials. The solid-state geometries, bond distances and angles for isolated ThCl₆^2–, NpCl₆^2–, and PuCl₆^2– anions with noncoordinating counter cations were determined for the first time in the X-ray crystal structures of <b>5</b>, <b>7</b>, and <b>8</b>. Solution phase and solid-state diffuse reflectance spectra were also used to characterize <b>7</b> and <b>8</b>. Transition metal MCl₆^2– anions showed the anticipated increase in M–Cl bond distances when changing from M = Ti to Zr, and then a decrease from Zr to Hf. The M–Cl bond distances also decreased from M = Th to U, Np, and Pu. Ionic radii can be used to predict average M–Cl bond distances with reasonable accuracy, which supports a principally ionic model of bonding for each of the (Ph₄P)₂MCl₆ complexes

Vanadium Bisimide Bonding Investigated by X‑ray Crystallography, ⁵¹V and ¹³C Nuclear Magnetic Resonance Spectroscopy, and V L_3,2-Edge X‑ray Absorption Near-Edge Structure Spectroscopy

Syntheses of neutral halide and aryl vanadium bisimides are described. Treatment of VCl₂(N<i>t</i>Bu)­[NTMS­(N^<i>t</i>Bu)], <b>2</b>, with PMe₃, PEt₃, PMe₂Ph, or pyridine gave vanadium bisimides via TMSCl elimination in good yield: VCl­(PMe₃)₂(N^<i>t</i>Bu)₂ <b>3</b>, VCl­(PEt₃)₂(N^<i>t</i>Bu)₂ <b>4</b>, VCl­(PMe₂Ph)₂(N^<i>t</i>Bu)₂ <b>5</b>, and VCl­(Py)₂(N^<i>t</i>Bu)₂ <b>6</b>. The halide series (Cl–I) was synthesized by use of TMSBr and TMSI to give VBr­(PMe₃)₂(N^<i>t</i>Bu)₂ <b>7</b> and VI­(PMe₃)₂(N^<i>t</i>Bu)₂ <b>8</b>. The phenyl derivative was obtained by reaction of <b>3</b> with MgPh₂ to give VPh­(PMe₃)₂(N^<i>t</i>Bu)₂ <b>9</b>. These neutral complexes are compared to the previously reported cationic bisimides [V­(PMe₃)₃(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄] <b>10</b>, [V­(PEt₃)₂(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄] <b>11</b>, and [V­(DMAP)­(PEt₃)₂(N^<i>t</i>Bu)₂]­[Al­(PFTB)₄] <b>12</b> (DMAP = dimethylaminopyridine, PFTB = perfluoro-<i>tert</i>-butoxide). Characterization of the complexes by X-ray diffraction, ¹³C NMR, ⁵¹V NMR, and V L_3,2-edge X-ray absorption near-edge structure (XANES) spectroscopy provides a description of the electronic structure in comparison to group 6 bisimides and the bent metallocene analogues. The electronic structure is dominated by π bonding to the imides, and localization of electron density at the nitrogen atoms of the imides is dictated by the cone angle and donating ability of the axial neutral supporting ligands. This phenomenon is clearly seen in the sensitivity of ⁵¹V NMR shift, ¹³C NMR Δδ_αβ, and L₃-edge energy to the nature of the supporting phosphine ligand, which defines the parameters for designing cationic group 5 bisimides that would be capable of breaking stronger σ bonds. Conversely, all three methods show little dependence on the variable equatorial halide ligand. Furthermore, this analysis allows for quantification of the electronic differences between vanadium bisimides and the structurally analogous mixed Cp/imide system CpV­(N^<i>t</i>Bu)­X₂ (Cp = C₅H₅^1–)

Diniobium Inverted Sandwich Complexes with μ‑η⁶:η⁶‑Arene Ligands: Synthesis, Kinetics of Formation, and Electronic Structure

Monometallic niobium arene complexes [Nb­(BDI)­(N^<i>t</i>Bu)­(R-C₆H₅)] (<b>2a</b>: R = H and <b>2b</b>: R = Me, BDI = <i>N</i>,<i>N</i>′-diisopropylbenzene-β-diketiminate) were synthesized and found to undergo slow conversion into the diniobium inverted arene sandwich complexes [[(BDI)­Nb­(N^<i>t</i>Bu)]₂(μ-RC₆H₅)] (<b>7a</b>: R = H and <b>7b</b>: R = Me) in solution. The kinetics of this reaction were followed by ¹H NMR spectroscopy and are in agreement with a dissociative mechanism. Compounds <b>7a</b>-<b>b</b> showed a lack of reactivity toward small molecules, even at elevated temperatures, which is unusual in the chemistry of inverted sandwich complexes. However, protonation of the BDI ligands occurred readily on treatment with [H­(OEt₂)]­[B­(C₆F₅)₄], resulting in the monoprotonated cationic inverted sandwich complex <b>8</b> [[(BDI^#)­Nb­(N^<i>t</i>Bu)]­[(BDI)­Nb­(N^<i>t</i>Bu)]­(μ-C₆H₅)]­[B­(C₆F₅)₄] and the dicationic complex <b>9</b> [[(BDI^#)­Nb­(N^<i>t</i>Bu)]₂(μ-RC₆H₅)]­[B­(C₆F₅)₄]₂ (BDI^# = (ArNC­(Me))₂CH₂). NMR, UV–vis, and X-ray absorption near-edge structure (XANES) spectroscopies were used to characterize this unique series of diamagnetic molecules as a means of determining how best to describe the Nb–arene interactions. The X-ray crystal structures, UV–vis spectra, arene ¹H NMR chemical shifts, and large <i>J</i>_CH coupling constants provide evidence for donation of electron density from the Nb d-orbitals into the antibonding π system of the arene ligands. However, Nb L_3,2-edge XANES spectra and the lack of sp³ hybridization of the arene carbons indicate that the Nb → arene donation is not accompanied by an increase in Nb formal oxidation state and suggests that 4d² electronic configurations are appropriate to describe the Nb atoms in all four complexes