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National Laboratory, Los Alamos, New Mexico 87545,
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National Laboratory, Los Alamos, New Mexico 87545,
National Laboratory, Los Alamos, New Mexico 87545,
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Abstract
A series of reactions using sodium bis(mesitoyl)phosphide (MesBAP−) with the actinide starting material AnI3(thf)4 (An = U, Np, Pu) as well as the analogous reaction with CeI3(thf)4 was performed. Similar U and Np An(BAP)4 (An = U, Np) products were obtained in the +4-oxidation state. The thorium homologue was synthesized using ThI4(dme)2 to generate Th(BAP)4, which was employed as a diamagnetic and predominantly redox inert metal center for comparison. The resulting product isolated from the analogous reaction with PuI3(thf)4 was [Na(thf)][Pu(BAP)4], where the metal coordinated four ligands and retained the +3-oxidation state. This result is distinct from that obtained from the reaction with CeI3(thf)4, where the Ce3+ product was Ce(BAP)3(thf)2. The compounds were isolated and characterized by X-ray diffraction, ultraviolet−visible (UV−vis)-nIR and NMR spectroscopies, and cyclic voltammetry. The synthetic results reveal how different redox stabilities across the 5f series can result in divergent reactivity of Pu. Electrochemical experiments produced redox features that suggest the potential to form reduced complexes supported by the MesBAP ligand framework.
Product Variation in Reactions of MI3(thf)4 with Bis(Mesitoyl)Phosphide Across the M = U, Np, Pu Series Stephanie H. Carpenter,† Margaret R. Jones,† Daniel J. Lussier, Andrew J. Gaunt, Jesse Murillo,* and Aaron M. Tondreau* Cite This: https://doi.org/10.1021/acs.inorgchem.4c04433 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information ABSTRACT: A series of reactions using sodium bis(mesitoyl)phosphide (MesBAP−) with the actinide starting material AnI3(thf)4 (An = U, Np, Pu) as well as the analogous reaction with CeI3(thf)4 was performed. Similar U and Np An(MesBAP)4 (An = U, Np) products were obtained in the +4-oxidation state. The thorium homologue was synthesized using ThI4(dme)2 to generate Th(MesBAP)4, which was employed as a diamagnetic and predominantly redox inert metal center for comparison. The resulting product isolated from the analogous reaction with PuI3(thf)4 was [Na(thf)][Pu(MesBAP)4], where the metal coordinated four ligands and retained the +3-oxidation state. This result is distinct from that obtained from the reaction with CeI3(thf)4, where the Ce3+ product was Ce(MesBAP)3(thf)2. The compounds were isolated and characterized by X-ray diffraction, ultraviolet−visible (UV−vis)-nIR and NMR spectroscopies, and cyclic voltammetry. The synthetic results reveal how different redox stabilities across the 5f series can result in divergent reactivity of Pu. Electrochemical experiments produced redox features that suggest the potential to form reduced complexes supported by the MesBAP ligand framework. ■ INTRODUCTION Exploration of chemical systems that traverse the 5f series and examine the unique physical properties of uranium, neptunium, and plutonium can provide strong insight into fundamental differences between these metals. For air- and moisture-free systems, few studies span U → Pu, but with the continuously growing use of materials for energy and nuclear deterrence, that number is likely to grow.1−10 Owing to differences in the physical properties of the actinide series, such as III/IV redox potentials, Th (+3.7 V) > U (+0.553 V) > Np (−0.219 V) > Pu (−0.97 V),11−13 and An-O computed diatomic bond strengths, Th (877 kJ/mol) > U (755 kJ/mol) > Np (731 kJ/mol) > Pu (656 kJ/mol),14 these elements are expected to display variety in their reactivity. A precursor to an established photoinitiator,15−20 bis- acylphosphide (BAP) ligands are bidentate, anionic P-based ligands analogous to acac ligands (acac = β-diketonate), a motif more commonly employed in actinide chemistry.21−26 Along with the spectroscopic advantages of the incorporated phosphorus atom, our prior studies with sodium bis(mesitoyl)phosphide (MesBAP−) highlight varied reactivity among different metals, namely uranium, yttrium, and europium2+/3+. This suggests unique behavior dependent upon both the metal identity and steric profile of the arene derivative installed on the BAP manifold.27−29 The prior uranium study demon- strated the reactivity of UI3(dioxane)1.5 with MesBAP− in tetrahydrofuran (THF) to isolate and characterize U(MesBAP)4. 27 Alternatively, the more sterically encumbered trippBAP− ligand (tripp = 2,4,6-triisopropylphenyl) facilitated the isolation of the formally U(III) species, U(trippBAP)3, and spectroscopic results suggested that significant electron density resides on the ligand framework. Notable, though, was the control that the additional steric demand of trippBAP− afforded the synthesis, enabling the isolation of the six-coordinate U(trippBAP)3. Further studies of lanthanide complexes supported by MesBAP have been reported by the Demir group. Synthesis of the heteroleptic chloride-bridged dimers of the type M[{MesBAP}2(thf)(μ-Cl)]2 (M = Y, Gd, Dy), was achieved along with the determination of a weak antiferromagnetic exchange between the metal centers that was supported by broken symmetry density functional theory (DFT) calculations.30 Additionally, the Meyer group demonstrated the Received: October 17, 2024 Revised: February 26, 2025 Accepted: March 12, 2025
Articlepubs.acs.org/IC
© XXXX The Authors. Published by American Chemical Society A https://doi.org/10.1021/acs.inorgchem.4c04433 Inorg. Chem. XXXX, XXX, XXX−XXX This article is licensed under CC-BY-NC-ND 4.0 D ow nl oa de d vi a 27 .3 4. 65 .1 11 o n A pr il 8, 2 02 5 at 0 2: 06 :3 4 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s. chemical reduction of U(MesBAP)4 to isolate the corresponding U(III) species, [K][U(MesBAP)4], further highlighting the ability of the BAP ligand motif to stabilize low-valent actinide complexes.31 Computational investigations described the potential for redox noninnocence with BAP ligands, providing computational support for our observed spectroscopic results with U(trippBAP)3, which suggested the presence of ligandbased electron density rather than a 5f3 U(3+) metal ion configuration.32 Further investigation of the necessary conditions for redox noninnocence is of continued interest to our group. The simplicity of the ligand architecture coupled with the resultant highly crystalline products holds potential success for the study of the coordination chemistry of MesBAP− with transuranium elements. Installation of the ligand along a homologous series of actinide starting materials may also provide distinct product distributions given the differences observed for III/IV redox couples in the actinide series. This report intends to further explore the unanticipated reactivity observed with MesBAP− and UI3(dioxane)1.5 by adapting the chemistry to neptunium and plutonium starting materials and assessing the reaction products. To further highlight differences in reactivity between lanthanide and actinide metals, actinide synthetic results are compared to those arising from homologous cerium starting materials. For the synthetic reactions described herein, comparable reaction conditions were maintained across the suite of compounds by reaction of three equivalents of MesBAP− with MI3(thf)4 starting materials (M = An = U, Np, Pu; M = Ln = Ce)33−36 (Scheme 1). Additionally, the homoleptic thorium complex Th(MesBAP)4 was pursued through the reaction of ThI4(dme)4 and four equivalents of MesBAP−, serving a crucial role as the sole diamagnetic complex obtained, as well as being the metal with the lowest inherent proclivity for III/IV redox chemistry. The series of products were analyzed via 1H and 31P{1H} NMR, ultraviolet−visible (UV−vis)-nIR, and IR spectroscopy, and additionally contextualized through comparison of their electrochemical behavior. ■ EXPERIMENTAL SECTION General Considerations. Caution! Depleted uranium (primary isotope 238U, t1/2 = 4.47 × 109 years) is a weak α-emitter. Neptunium (237Np, t1/2 = 2.144 × 106 years) is an α-emitter and its 233Pa daughter is a β- and γ-emitter, where both isotopes present serious health threats. 239Pu (the primary isotope of the material used in this study) of nominal weapons grade composition is an α-emitting nucleus with a half-life of 24,110 years and presents serious health risks. Natural thorium (primary isotope 232Th, t1/2 = 1.40 x 1010 years) is an α-emitter that contains radioactive daughters, including Ra and Ac, where all elements present serious health risks. Only persons trained to handle such material should perform work and only in an adequately prepared laboratory setting. All manipulations of plutonium and neptunium chemistry were conducted in a radiation laboratory equipped with high-ef f iciency particulate air (HEPA) f iltered hoods and in a negative pressure glovebox with a purif ied helium atmosphere. Additional safeguards used to monitor radiation levels include, but are not limited to, continuous air monitoring, hand-held radiation monitoring devices, and hand, foot, and full-body contamination monitoring. The work presented was carried out at Los Alamos National Laboratory. All air- and moisture-sensitive manipulations using Ce, U, and Th were carried out using standard Schlenk techniques or in an MBraun drybox containing a purified argon atmosphere maintained at <0.1 ppm of O2. All transuranium complexes were synthesized in a negative pressure MBraun LabMaster glovebox configured for safe containment of transuranium isotopes including HEPA-filter outlets and the antechamber plumbed directly into a HEPA-filtered fume hood. The glovebox atmosphere was maintained with a standalone Vacuum Atmosphere GenesisTM oxygen and moisture removal system, and to facilitate air- and moisture-sensitive chemistry the suitability of the atmosphere was verified using a dilute toluene solution of [Ti(Cp)2(μ-Cl)]2. 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer operating at 400.132, 100.693, and 161.978 MHz, respectively. All 1H and 13C chemical shifts are reported relative to SiMe4. 31P chemical shifts are reported relative to the referenced 1H NMR spectra. CHN analyses were conducted by Midwest Microlabs, 6212 N. Shadeland Ave., Ste. 110, Indianapolis, IN 46250. UV−vis-nIR spectra were collected at RT on a Cary 5000 UV−vis-nIR spectrophotometer from Agilent Technologies. IR analyses were performed using a Bruker α II compact FT-IR spectrometer. THF and DME were dried over Na/K, filtered through dry, neutral alumina, and stored on 4 Å molecular sieves prior to use. Anhydrous solvents used for transuranium work were purchased and stored for several weeks over activated 4 Å molecular sieves before use. Diethyl ether, toluene, and C6D6 were degassed and dried over 4 Å molecular sieves prior to use. C6D6 was purchased from Cambridge Isotope Laboratories. ThI4(dme)2, 33 UI3(thf)4, 34 NpI3(thf)4, 35 PuI3(thf)4, 35 and CeI3(thf)4, 36 were made according to published procedures. Na(MesBAP) was synthesized as performed previously.27 Single crystals suitable for X-ray diffraction of U, Th, and Ce were coated with Krytox in a drybox, placed on a nylon loop. Np and Pu crystals were coated in either NVH oil or paratone-N oil and then mounted in a 0.5 mm thin-walled quartz capillary which was sealed with wax at both ends inside the drybox and then coated with acrylic Scheme 1. (Left) Synthesis of Np(MesBAP)4 and [Na(thf)][Pu(MesBAP)4], Both of Which were Performed Analogously to the Preparation of U(MesBAP)4, is Summarized and Highlights the Structural Differences of the U/Np Products Versus that of Pu. (Right) Synthesis of the Complexes Ce(MesBAP)3(thf)2 and Th(MesBAP)4 https://doi.org/10.1021/acs.inorgchem.4c04433 Inorg. Chem. XXXX, XXX, XXX−XXX B in a fume hood to provide shatter-resilience. The prepared crystals were then transferred to the goniometer head of a Bruker AXS D8 Quest diffractometer equipped with a graphite-monochromatized molybdenum Kα X-ray tube (λ = 0.71073 Å) and a CMOS detector, or to a Bruker D8 Quest using a molybdenum Kα X-ray tube (λ = 0.71073 Å) IμS 3.0 Microfocus source X-ray generator.37 A hemisphere routine was used for data collection and determination of lattice constants. The space group was identified, and the data was processed using the Bruker SAINT+ program and corrected for absorption using SADABS.38,39 The structures were solved using SHELXTL completed by subsequent Fourier synthesis and refined by full-matrix least-squares procedures.40,41 Olex2 software was used as the graphical interface.42,43 Crystallographic data for all structures is available from the Cambridge Structural Database. Cyclic voltammetry (CV) experiments for all compounds reported were performed on a Bio-Logic SP50 potentiostat. A 3-electrode cell was used, composed of an Au disk electrode (2 mm diameter), a Pt wire counter electrode (1 mm thickness), and a Ag/AgCl pseudoreference electrode made from Ag wire (0.5 mm thickness) dipped in concentrated FeCl3 to afford a surface layer of AgCl, which was then washed with deionized water and acetone, and dried in vacuo before use. No membrane or frit was used to isolate the working electrode in the cell. Electrodes were stored in an inert atmosphere glovebox and cleaned using THF/Toluene/ether washes before and after measurements of each compound were made. A small volume (3 mL), high-recovery, V-vial was used for experimentation. Cell resistances were <500 Ω, and the open circuit potential was checked before each measurement, ensuring ΔV/s < 50 mV. No ohmic compensation (iR) was applied to the measurements or the treatment of data. Between each run, the solution was manually agitated, and the working electrode surface was wiped clean before beginning the next measurement. All potentials are reported versus the [Cp2Fe]0/+ couple, referenced as an internal standard. Solutions utilized in the electrochemical studies were all run with [nPr4N][BArF24] as the supporting electrolyte in THF solution, which was synthesized as previously described44 and recrystallized from a DCM/ Et2O/n-hexane solution followed by grinding of the crystals and drying in vacuo for 48 h. All potentials are reported versus the [Cp2Fe]0/+ couple. Small quantities (approximately equal molar amount relative to the analyte) of [Cp2Fe] were added to the electrochemical cell immediately after the last scan of the analyte was made. The resulting observed [Cp2Fe]0/+ couple was used as an internal standard (scanned at 100 mV/s in the same scan direction used for the experiment) to reference measured values. Synthetic Descriptions. Preparation of Np(MesBAP)4. In a 20 mL glass scintillation vial with a PTFE-coated stirrer bar, NpI3(thf)4 (24.9 mg, 27.6 μmol) was added along with 28.6 mg (79.4 μmol, 3 equiv) of NaMesBAP as solids. Approximately 3 mL of THF was added to this mixture and the solution became dark green with red highlights, which was stirred for 1.5 h at room temperature. Volatiles were removed to give a tacky dark (hunter green/red) tacky solid, which was dissolved in 1 mL of toluene. The reaction solution was then filtered through a glass filter disk, and the vial and frit were rinsed with an additional 1 mL of toluene to give a dark filtrate. The filtrate was concentrated down to approximately 1.0 mL and then moved into a 4 mL vial, which was then placed in a larger 20 mL vial filled with ∼5 mL of pentane for vapor diffusion. This was left sealed at room temperature overnight. After 16 h, a crop of dark crystals formed in the small vial. After 48 h, these crystals were harvested to give 16.6 mg of Np(MesBAP)4 (10.7 μmol, 39% crystalline yield). 1H NMR (C6D6, 23 °C, ppm): δ 6.00 (s, 2H, Ar C−H), 1.72 (s, 3H, p-Ar CH3), 1.08 (s, 6H, o-Ar CH3); 31P{1H} NMR (C6D6, 23 °C): δ 40.0 ppm. Preparation of [Na(thf)][Pu(MesBAP)4]. In a 20 mL glass scintillation vial with a PTFE-coated stirrer bar, PuI3(thf)4 (10.0 mg, 11.1 μmol) was added along with 12.0 mg (33.3 μmol, 3 equiv) of Na(MesBAP) as solids. THF (∼2 mL) was added, forming an orange suspension, which was stirred for 1.5 h at room temperature, at that time volatiles were removed to give a dark tacky solid. This material was dissolved in toluene, and the orange suspension was then filtered through a glass filter disk to give a green filtrate. An additional 1 mL of toluene was used to rinse the vial and frit. The filtrate was concentrated down to approximately 1.5 mL and then moved into a 4 mL vial, which was then placed in a larger 20 mL vial filled with ∼5 mL of pentane to facilitate vapor diffusion. This was left sealed at room temperate in under inert atmosphere overnight. After 16 h, a crop of dark crystals formed in the small vial. After 48 h, these crystals were harvested to give 4.2 mg of material (2.38 μmol, 21% crystalline yield). The crystals formed in an overlapping, polycrystalline, morphology but careful fracturing produced a specimen sufficient for single-crystal X-ray diffraction (SC-XRD) analysis. Analysis of the data gave the determination of the molecular structure as [Na(thf)][Pu(MesBAP)4]. 1H NMR (C6D6, 23 °C, ppm): δ 6.35−6.40 (br m), 3.5 (br. s, THF), 2.75- 1.75 (br. m), 1.5−1 (br. m); 31P{1H} NMR (C6D6, 23 °C, ppm): δ 84.8, 79.5. Preparation of Ce(MesBAP)3(thf)2. In a 20 mL glass scintillation vial with a PTFE-coated stirrer bar, CeI3 (35 mg, 67.2 μmol) was stirred with THF for 1 h to generate the known THF adduct, and the THF was then removed under reduced pressure. To the remaining, white solid was added 73 mg (201 μmol, 3 equiv) of Na(MesBAP) as solids, followed by ∼3 mL of THF. The reaction was allowed to stir for 1.5 h at room temperature. The reaction was filtered through a glass frit and the volatiles were removed under reduced pressure. Attempts to crystallize the orange material from toluene resulted in the formation of orange powder. To the orange solid was added a drop of THF, and dissolution of the material was accomplished with ∼0.15 mL of diethyl ether, followed by the use of a pentane vapor diffusion to generate orange crystalline material. After this time, the mother liquor was decanted and the crystals were washed with cold pentane prior to drying under vacuum (45% crystalline yield). Analysis for Ce(MesBAP)3(thf)2. Calc.: C = 64.80%, H = 6.56%, N = 0%. Anal.: C = 64.81%, H = 6.89%, N = 0%. 1H NMR (C6D6, 23 °C, ppm): δ 6.74 (s, 12H) 3.06 (s, 36H), 2.07 (s, 18H), −1.36 (s, 8H), −2.37 (s, 8H); 31P{1H} NMR (C6D6, 23 °C): δ 108.9 ppm. Preparation of Th(MesBAP)4. A 20 mL glass scintillation vial was charged with ThI4(dme)2 (25 mg, 0.027 mmol) and Na(MesBAP) (43 mg, 0.109 mmol), which were then taken up in 2 mL THF and allowed to stir for 30 min. The resulting solution was concentrated in vacuo, and the resulting yellow solid was washed with diethyl ether to remove any excess starting material. The remaining solid was extracted into toluene and filtered through Celite to remove sodium iodide. The toluene supernatant was recrystallized by layering with pentane and placed in the freezer to yield the product as bright yellow crystals, 30 mg, 73%. 1H NMR (THF-d8, 23 °C, ppm): δ 6.73 (s, 4H), 2.27 (s, 6H), 2.04 (s, 12H); 31P{1H} NMR (THF-d8, 25 °C): δ 107.0 ppm; 13C{1H} NMR (THF-d8, 23 °C, ppm): δ 239.8 (d, J = 93.7 Hz, P-C�O), 140.4 (d, J = 30.5 Hz, ipso-CMes), 140.1, 137.9 (d, J = 2.8 Hz, p- CMes), 133.9, 128.4, 127.9, 20.1, 19.4(d, J = 2.7 Hz, o-CH3). Synthesis and Characterization. Np(MesBAP)4. The addition of three equivalents of MesBAP− to NpI3(thf)4 resulted in the formation of a dark green/orange solution, which produced dark, well-shaped crystals out of toluene. Dilute solutions of the crystals were red-orange in appearance (Figures S1 and S13). Solution NMR of the product, performed in deuterated benzene (C6D6), indicated the formation of the desired complex, Np(MesBAP)4. The 1H NMR spectrum consisted of three resonances integrating to the expected 2:3:6 ratio (found at δ 6.00, 1.72, and 1.08 ppm, respectively) attributed to the protons of the mesityl fragment. The 31P{1H} NMR spectrum is also consistent with the generation of this species, where the observed resonance was a sharp singlet centered at δ −40.0 ppm; this is a substantial shift from the resonance observed for either U(MesBAP)4 (δ 57.7 ppm) or the free ligand (∼δ 81 ppm). Absorption experiments furnished spectra that were consistent with the Np4+ ion, with f → f and f → d transitions between 700 and 1050 nm,45,46 along with the intense π → π* transition attributed to the ligand at 419 nm (Figure S13).27,47 Interpretation of these data led to the assignment as the homoleptic tetrakis (bis(dimesitoyl)phosphide)neptunium compound, Np(MesBAP)4. Spontaneous oxidation to the +4-oxidation state is attributed to stabilization by the electron-rich BAP ligand − results that mirror those arising from the homologous reaction with uranium. Disproportionation reactions of the actinide elements are well https://doi.org/10.1021/acs.inorgchem.4c04433 Inorg. Chem. XXXX, XXX, XXX−XXX C established in nonaqueous media, which likely are responsible for the observed unanticipated products.48 Confirmation of molecular assignment was provided by singlecrystal X-ray diffraction (SC-XRD) experiments performed on the isolated crystals (SI 3.3). The crystal data was solved in the triclinic P1 space group, which differed from the unit cell found for U(MesBAP)4. The asymmetric unit cell comprised one molecule of Np(MesBAP)4 with two half molecules of disordered toluene on either side of the complex that were masked in the final solution. Both Λ and Δ enantiomers are present in the grown unit cell to maintain unit cell achirality. The Np center was found as an eight-coordinate square antiprism that is distorted due to the constraints of the chelate (Figure 1). The twist angle of the two planes formed by the oxygen atoms of the BAP ligands in Np(MesBAP)4 was found to be 35.5°, with a plane−plane distance of 2.487(1) Å. This compares with the reported structural values found in U(MesBAP)4 of 35.2° and 2.472(2) Å. [Na(thf)][Pu(MesBAP)4] and Ce(MesBAP)3(thf)2. The reaction with Pu was performed under identical conditions, where three equivalents of MesBAP− and PuI3(thf)4 were allowed to react in THF. Following workup and crystallization from toluene, low yields (21%) of dark crystals were harvested from the green solution and analyzed. Unlike the analogous Np- and U-based complexes, the NMR spectroscopy was not adequate to assign a product. The 31P{1H} NMR spectrum in C6D6 consists of 2 weak, broad resonances at δ 79 and δ 83 ppm that overlapped and integrated roughly to 1:3 and were tentatively assigned to the chemically distinct MesBAP ligands present in the solid-state structure, which could be observed if sodium exchange is slow on the NMR time scale in C6D6. The 1H NMR spectrum displayed broad resonances in approximately the characteristic shift range expected for protons of mesityl groups; however, chemically inequivalent sites convoluted the signals. The UV−vis-nIR spectrum was also ambiguous as the common transitions for either Pu3+ or Pu4+ were masked by intense absorbances that overwhelmed the highenergy portion of the spectrum. Broad and weak absorption bands assigned at 760, 890, 1026, and 1100 nm were observed that are consistent with a PuIII ion.11,49−51 The π → π* transition of the coordinated ligand was found at 390 nm. Analysis of SC-XRD data allowed for the structural assignment as the Na salt of tetrakis(bis(dimesitoyl)phosphide)plutonium, [Na(thf)][Pu(MesBAP)4], [Pu(MesBAP)4]−. The structure of [Pu(MesBAP)4]− was solved in the monoclinic space group P21/n. The asymmetric unit cell contained one molecule of the title complex along with one solvent molecule of toluene. The Na+ cation shared three BAP ligands with Pu and was capped by one molecule of THF that was disordered over two positions. The fourth BAP was bound solely to the Pu center, with one of the carbonyl O atoms coordinated nearly trans (Na−Pu−O angle = 172.34(9)°) to the Na (Figure 2). The coordination geometry of the Pu is best described as a dodecahedron with molecular distortion induced by the asymmetric interaction of the Na+ ion. The results with Pu and MesBAP− were https://doi.org/10.1021/acs.inorgchem.4c04433 Inorg. Chem. XXXX, XXX, XXX−XXX D reproduced 3 times and are distinct from other structurally established acac-type complexes of Pu, which are reported in the Pu4+ oxidation state;52−54 however, those syntheses and characterization were performed in air. Additionally, this result on Pu is analogous to the recent report of [K][U(MesBAP)4] by Meyer, 31 which employed KC8 with U(MesBAP)4 to generate a related uranium complex, highlighting the differences in redox behavior between Pu3+ and U3+. The An-O bond lengths observed in the solid-state structures of U(MesBAP)4, Np(MesBAP)4, and [Pu(MesBAP)4]− were compared (Table S3). Between the uranium and neptunium congeners, some evidence of actinide contraction is observed, but several of the An-O bonds are within error of the ESD (ESD = estimated standard deviation) of one another. However, An-O bonds were found averaged at 2.34 Å (U) vs 2.32 Å (Np). These distances are consistent with the previously reported homoleptic acac-derivatives of Np4+.52,55 Data analysis of SC-XRD experiments performed on [Pu(MesBAP)4]− shows that the Pu−O bond lengths are consistently elongated over the Np and U complexes, averaging over 2.4 Å. Additionally, the Pu−O bonded oxygen atoms that also coordinate the sodium ion are further elongated to roughly 2.5 Å due to the additional interaction. These elongated Pu−O bond distances further support the +3-oxidation state for [Pu(MesBAP)4]−, as the average Pu bond distances in the previously established Pu(TTA)4 (TTA = Thenoyltrifluoroacetonate, an acac derivative) are found at 2.32 Å.52 This change in Pu−O bond distance is consistent with increased ionic radius of a Pu3+ center and increased electrostatic repulsion between the π-electrons of the oxygen and the Pu3+ center. Ce is generally considered a surrogate for Pu due to its similar size,55,56 observable Ce3+/4+ oxidation potential (−1.74 V for Ce vs −0.86 V for Pu),57,58 and itinerant behavior of the f-electrons in materials chemistry.59,60 Here, the similarities between Pu and Ce in the synthesis of the BAP-coordinated complexes were probed, and the analogous CeI3(thf)4 starting material was subjected to comparable reaction conditions. Following workup and recrystallization, the data acquired on the bright orange crystals led to the assignment as Ce(MesBAP)3(thf)2 (Figure 3). Analysis of the material by 1H NMR spectroscopy showed resonances consistent with the ligand mesityl groups, with additional broad resonances at δ −1.38 and δ −2.37 ppm attributed to bound THF; the resonances integrate roughly to two equivalents of THF relative to six mesityl groups. The 31P{1H} NMR spectrum is marked by a single resonance centered at δ 108.9 ppm. Solution UV−vis measurements were performed on Ce(MesBAP)3(thf)2 (Figure S15), and the intense orange coloration was manifest in an ε value of ∼15,000 M−1 cm−1 at 377 nm. This intensity is consistent, although lower, than those previously observed for BAP-supported Eu3+ species, which we measured at over 28,000 M−1 cm−1 with wavelength maxima between 395 and 420 nm.28,29 The Ce−O bond distances are consistent with previously structurally characterized acac-supported Ce3+ analogues. Both molecules present in the asymmetric unit cell of Ce(MesBAP)3(thf)2 display average Ce−O bond lengths to the BAP ligands of 2.42 Å, with the distances to the coordinated THF molecules observed roughly 0.18 Å longer. Known structural analogues are supported by three hexafluoroacetylacetonate ligands with either glyme or diglyme to complete the coordination sphere, with the Ce−O bond lengths averaging 2.44 and 2.47 Å, respectively.61,62 Presentation of additional M−O bond lengths and related linear trends based on known values of metal ionic radii can be found starting on page S24. While the +3-oxidation state is maintained in both Pu and Ce products, the product arising from the reaction with CeI3(thf)4 is different than that isolated from the Pu reaction. Thus, the reason for Pu forming a salt complex despite identical reaction conditions to Ce, and the similar ionic radii of Ce3+/Pu3+, is not obvious. Admittedly, the Pu reaction is performed in a different drybox than the Ce, with small differences in workup conditions as Ce(MesBAP)3(thf)2 required slightly different crystallization conditions, which could affect the product outcome. The low yield of crystalline Pu material may also indicate the presence of more soluble species in solution, but analysis of crude mixtures was not pursued given the additional risks associated with handling plutonium. Electrochemical Experiments and Th(MesBAP)4 as an Electrochemical Benchmark. The series of complexes obtained following the synthetic effort were characterized using electrochemical cyclic voltammetry. Electrochemistry of air- and moisture-free transuranium complexes continues to provide unique insight into the behavior of these less common actinides.63−71 The pursuit of electrochemistry and the assignments of the resultant peaks was supported through the synthesis of Th(MesBAP)4. Thorium is established to be redox innocent under most conditions, maintaining a +4-oxidation state. This property has been employed previously to establish ligand-based redox events. Here, we wanted to establish a redox couple for (MesBAP−) on a metal, as the electrochemical data for Na-bound (MesBAP−) was consistent with compound decomposition as described previously.31 The reduction waves associated with Th(MesBAP)4 were therefore envisioned to likely arise from ligand centered events. Synthesis of Th(MesBAP)4 was achieved from ThI4(dme)2 as a starting material by reaction with four equivalents of MesBAP− in THF (Figure 4). Bright yellow crystals were isolated in moderate yields (75%), and the data acquired was consistent with the assignment of the product as Th(MesBAP)4. The 1H NMR spectrum consists solely of characteristic mesityl resonances from the chemically equivalent BAP ligands; similarly, the expected resonances ascribed to the mesityl groups are observed in the 13C{1H} NMR. The doublet resonance observed in the 13C{1H} NMR spectrum attributed to the carbonyl carbon atoms of the ligand displayed only slight downfield shifts relative to that of MesBAP− (acquired in THF-d8, ∼ δ 240 vs δ 232 ppm), with minimal difference in J-coupling value from 89.9 to 93.6 Hz, demonstrating a mild change to the P−C bond double bond character imparted by coordination to the Th center (Figure 4). However, significant differences in chemical shift values of metalchelated BAP ligands represent a hallmark of the 31P{1H} NMR spectra, with a singlet appearing at δ 107 ppm. Solution UV−vis measurements were performed on Th(MesBAP)4 (Figure S16), and the intense yellow coloration of the solution exhibited an ε value of over 50,000 M−1 cm−1 at 410 nm. This value is similar to those previously observed for BAP-supported Eu2+ species, which we measured at over 40,000 M−1 cm−1. These are consistent with chargetransfer intensities, and as noted above, energetically similar to other BAP-supported systems that we have studied, suggesting a ligandbased π → π* transition.28,29 Collection and analysis of SC-XRD data of bright yellow blocks confirmed the assignment of the product as Th(MesBAP)4. The data was solved in the monoclinic space group C2/c, with the asymmetric https://doi.org/10.1021/acs.inorgchem.4c04433 Inorg. Chem. XXXX, XXX, XXX−XXX E unit cell comprising 1/2 of the full molecule and with one molecule of toluene as lattice solvent. Both Λ and Δ enantiomers were observed in the unit cell, which is expected for an achiral space group. The twist angle, as described in Figure 1, of ligand oxygen atoms about the thorium center was found to be 33.4° with a plane−plane distance of 2.499(4) Å. Increased plane−plane distance is observed for Th(MesBAP)4 relative to the analogous U(MesBAP)4 (2.472(1) Å) and Np(MesBAP)4 (2.487(1) Å), consistent with contraction of the ionic radius characteristically seen in the actinide series. The more acute twist angle of Th compared to U (35.2°) and Np (35.5°) structures can be explained by both the relative increase in radius as well as differences in lattice solvent. A summary of select metrical parameters of this series can be found in Table S3. Cyclic voltammetry (CV) experiments were performed on THF solutions of the actinide series Th(MesBAP)4, U(MesBAP)4, Np(MesBAP)4 and [Pu(MesBAP)4]−, as well as the Ce(MesBAP)3(thf)2 complex. The neutral thorium, uranium, neptunium, and cerium complexes share similar electrochemical features (Figure 5). However, the “ate” plutonium complex displayed quite different behavior in the CV experiments, having no discernible reversable redox features. The resultant voltammograms for Ce(MesBAP)3(thf)2, Th(MesBAP)4, U(MesBAP)4 and Np(MesBAP)4 show two distinct redox events labeled I and II (Figure 5). The redox event II appears as a quasi- reversible feature centered at −2.44, −1.69 and −1.39 V vs Fc+/0, respectively, for the cerium, uranium, and neptunium complexes. These values are within the Ce(III/II) and U/Np(IV/III) redox couples reported in the literature for related coordination complexes in nonaqueous environments.46,66−68,72−75 However, the corresponding feature for the Th complex, centered at −1.96 V vs Fc+/0, is far more positive than reported Th(IV/III) couples (ranging from −2.76 to −3.48 V), which suggests that this feature may be primarily of ligand origin,76−79 but further experiments would be needed to confirm this. A recent theoretical study of U(MesBAP)4 and its reduced counterpart [U(MesBAP)4]− investigates the capacity for the BAP ligand framework to accept electron density from the metal under appropriate coordination conditions.32 The related work of Meyer reported the electrochemical characteristics of U(MesBAP)4 independently, with the dominant redox features being similar between their and our experiments.31 As discussed in this previous work, the U-metal center is likely the locus for the observed redox event in the CV of U(MesBAP)4, 27,31,32 which would suggest that the Th and U reduction processes diverge, giving U(III)(BAP)40 and Th(IV)(BAP)4−1, though additional experimental evidence is required to explore the potential ligand reduction for Th. The similarity between uranium and neptunium also suggests that the neptunium-based https://doi.org/10.1021/acs.inorgchem.4c04433 Inorg. Chem. XXXX, XXX, XXX−XXX F anionic species [Np(MesBAP)4]− would bear a Np3+ center akin to the uranium congener, an interesting application of the BAP motif and line of experimentation that we continue to pursue. A second redox event observed for the reported complexes is also noted (feature “I” in Figure 5). Due to its proximity to the edge of the solvent window, assignments of the peak potential values are more difficult to make with precision. However, we calculated the E1/2 values of these quasi-reversible features to be −2.86, −2.50, and −2.36 V vs Fc+/0 for the Ce, U, and Np complexes, respectively. For the Th complex only a single, irreversible redox feature is seen at −2.51 V, which disappears when the scan is conducted in the opposite direction (Figure S27). Again, given the similarities in these voltammograms, we tentatively assign these redox events to ligandbased events. Interestingly, a trend can be observed in the redox potentials of both features “I” and “II” which seems to suggest a relationship between the coordinated metal species and the location of the events, indicating a stabilization effect based on metal ion character. Electrochemical experiments performed on [Pu(MesBAP)4]− showed four electron transport events, localized at −2.21, −1.73, −0.02, and 0.20 V which could not be conclusively assigned (Figure 5). The CV of [Pu(MesBAP)4]− contains the most anodically shifted features (−0.02 and 0.20 V vs Fc+/0) of any of the reported complexes in this work, although U(MesBAP)4 displays a weak irreversible wave at 0.20 V vs Fc+/0 as well. Qualitatively, the close spacing of these redox waves may be evidence of the presence of closely related ionic/ neutral species of the Pu complex assessed during the CV experiment. Given the “ate” formulation for the Pu complex, we propose that the loss or exchange of the sodium ion of [Na(thf)][Pu(MesBAP)4] upon introduction of the supporting electrolyte leads to undesired reactions or decomposition. Indeed, during the collection of cyclic voltametric data, colorless precipitate was observed forming in the electrochemical cell upon the application of electrical potential. Unfortunately, we were unable to synthesize and isolate the neutral Pu complex [Pu(MesBAP)3] or its Pu4+ counterpart [Pu(MesBAP)4], either of which would serve as valuable benchmarks in understanding the electrochemical behavior of [Na(thf)][Pu(MesBAP)4]. ■ SUMMARY AND CONCLUSIONS The synthetic results of this study highlight the unique behavior of plutonium, with the isolated product distinct from those arising from analogous reactions of uranium, neptunium, and cerium. In the case of uranium and neptunium, the homoleptic U/NpIV(MesBAP)4 species were isolated. Plutonium generated a product that maintained the +3-oxidation state while incorporating four MesBAP− ligands as the “ate” complex [Pu(MesBAP)4]−. The reactivity of cerium produced anticipated results and allowed for the isolation of Ce(MesBAP)3(thf)2. The thorium analog, Th(MesBAP)4, was synthesized starting from ThI4(dme)2 and was used as a diamagnetic reference compound where the metal center is largely redox inert. The phosphorus atom of the BAP ligand demonstrated utility as a handle for 31P{1H} NMR spectroscopy. Comparison of Th(MesBAP)4 with the ligand precursor [Na][(MesBAP)] showed a shift in the 31P{1H} NMR resonance to δ 107.0 ppm in THF-d8 for Th(MesBAP)4 from ∼δ 81 ppm of the free ligand. There is also little difference in the observed JPC coupling values of the 13C{1H} NMR spectrum, between [Na][(MesBAP)] (JPC = 89.5 Hz) and Th(MesBAP)4 (JPC = 93.7 Hz), which should vary with significant changes to the nature of P−C bonding of the ligand. For the paramagnetic complexes Ce(MesBAP)3(thf)2 and Np(MesBAP)4, the 31P{1H} NMR resonances shift dramatically. While the known U(MesBAP)4 31P{1H} NMR resonance was established at δ 57.7 ppm, the same resonance for Ce(MesBAP)3(thf)2 and Np(MesBAP)4 are found at δ 108.9 and δ −40.0 ppm, respectively. Unfortunately, analysis of [Pu(MesBAP)4]− is not as clear. As an “ate” complex, the 31P{1H} and 1H NMR spectra are complex and not readily assigned. [Pu(MesBAP)4]− was synthesized in triplicate and the product obtained in each case was identical, but additional experiments would be needed to fully characterize the complex and understand the Pu product distributions. The results from the electrochemical experiments are consistent with the expected redox trends of cerium, uranium, and neptunium and follow the expected pattern based on stabilization of the trivalent species. However, each voltammogram showed clean and reversible first redox waves as well as a second feature that suggests the possibility for further chemical reduction. The first redox feature of Th(MesBAP)4 was significantly shifted from established ThIV/III redox couples and may suggest a ligand centered event, while those of Np and U are found within the IV/III couple of both metals and would likely give rise to the An3+ complexes. Unfortunately, the electrochemical data for [Pu(MesBAP)4]− was complicated due to the “ate” nature of the product and expected ion exchange in the analyte solution. The simplicity of the BAP manifold, in conjunction with the spectroscopic and physical properties of the products, has proven effective for investigating actinide and transuranium chemistry. Performing homologous reactions from U → Pu has allowed for a reactivity study that finds differentiation between products of plutonium and the lighter actinides. The formation of the anionic plutonium complex is of potential interest for separations, as charged species could be separated from neutral lanthanide and actinide congeners. The intense coloration of BAP-supported complexes is also being pursued as a method to resolve metal identities within mixtures. We also continue to explore the MesBAP− motif in the synthesis of low-oxidation state complexes. ■ ASSOCIATED CONTENT *sı Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c04433. Electronic Supporting Information contains the experimental descriptions; photographs of crystals and product solutions; UV−vis-nIR characterization spectra; and crystallographic data including refinement parameters and metrical data for all complexes (PDF)
Accession Codes
Deposition Numbers 2303353−2303355, 2327999, and 2382277 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via the joint Cambridge Crystallographic Data Centre (CCDC) and Fachinformationszentrum Karlsruhe Access Structures service. ■ AUTHOR INFORMATION
Corresponding Authors
Jesse Murillo − Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States; Email: jmurillo@lanl.gov Aaron M. Tondreau − Chemistry Division, Los Alamos
National Laboratory, Los Alamos, New Mexico 87545,
United States; orcid.org/0000-0003-0440-5497; Email: tondreau_a@lanl.gov https://doi.org/10.1021/acs.inorgchem.4c04433 Inorg. Chem. XXXX, XXX, XXX−XXX G
Authors
Stephanie H. Carpenter − Chemistry Division, Los Alamos
National Laboratory, Los Alamos, New Mexico 87545,
United States; orcid.org/0000-0002-1293-3786 Margaret R. Jones − Chemistry Division, Los Alamos
National Laboratory, Los Alamos, New Mexico 87545,
United States; orcid.org/0000-0002-0540-6312 Daniel J. Lussier − Chemistry Division, Los Alamos United States Andrew J. Gaunt − Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States; orcid.org/0000-0001-9679-6020 Complete contact information is available at: https://pubs.acs.org/10.1021/acs.inorgchem.4c04433
Author Contributions
†S.H.C. and M.R.J. contributed equally to this work.
Notes
The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was conceived and executed at Los Alamos National Laboratory. Los Alamos National Laboratory is operated by Triad National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy (Contract No. 89233218CNA000001). This work was funded by the U.S Department of Energy, Office of Science, Office of Basic Energy Sciences, Heavy Element Chemistry Program (2020LANLE372, DE-AC52-06NA25396) and we are grateful for their continued support. AMT would like to thank Sarah K. Tondreau for assistance with organization and editing of this manuscript. S.H.C. and J.M. are grateful for additional postdoctoral support provided by the Glenn T. Seaborg Institute of Los Alamos National Laboratory. ■ REFERENCES (1) Su, J.; Cheisson, T.; McSkimming, A.; Goodwin, C. A. P.; DiMucci, I. M.; Albrecht-Schönzart, T.; Scott, B. L.; Batista, E. R.; Gaunt, A. J.; Kozimor, S. 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