Kevin D. John

Actinide Redox-Active Ligand Complexes: Reversible Intramolecular Electron-Transfer in U(dpp-BIAN)2/U(dpp-BIAN)2(THF)

Actinide complexes of the redox-active ligand (dpp-BIAN)(2-) (dpp-BIAN = 1,2-bis(2,6-diisopropylphenylimino)acenaphthylene), U(dpp-BIAN)(2) (1), U(dpp-BIAN)(2)(THF) (1-THF), and Th(dpp-BIAN)(2)(THF) (2-THF), have been prepared. Solid-state magnetic and single-crystal X-ray data for complex 1 indicate a ground-state U(IV)-pi*(4) configuration, whereas a (dpp-BIAN)(2-)-to-uranium electron transfer occurs for 1-THF, resulting in a U(III)-pi*(3) ground configuration. The solid-state magnetic data also indicate that interconversion between the two forms of the complex is possible, limited only by the ability of tetrahydrofuran (THF) vapor to penetrate the solid upon cooling of the sample. In contrast to those in the solid state, spectroscopic data acquired in THF indicate only the presence of the U(IV)-pi*(4) form for 1-THF in solution, evidenced by electronic absorption spectra and by measurement of the solution magnetic moment in THF-d(8) using the Evans method. Also reported is the electrochemistry of the complexes collected in CH(2)Cl(2), CF(3)C(6)H(5), and THF. As expected from the solution spectroscopic data, only small differences are observed in half-wave potentials of ligand-based processes in the presence of THF, consistent with the solution U(IV)-pi*(4) configuration of the complexes in all cases. Density functional theory calculations were undertaken for complexes 1 and 1-THF to determine if intrinsic energetic or structural factors underlie the observed charge-transfer process. While the calculated optimized geometries agree well with experimental results, it was not possible to arrive at a convergent solution for 1-THF in the U(III)-pi*(3) configuration. However, perturbations in the orbital energies in 1 versus 1-THF for the U(IV)-pi*(4) configuration do point to a diminished highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap in 1-THF, consistent with the solid-state magnetic data. These results represent the first example of a stable and well-defined, reversible intramolecular electron transfer in an actinide complex with redox-active ligands.

1,4-dicyanobenzene as a scaffold for the preparation of bimetallic actinide complexes exhibiting metal-metal communication.

Reaction of two equivalents of [(C5Me4Et)2U(CH3)(Cl)] (6) or [(C5Me5)2Th(CH3)(Br)] (7) with 1,4-dicyanobenzene leads to the formation of the novel 1,4-phenylenediketimide-bridged bimetallic organoactinide complexes [((C5Me4Et)2(Cl)U)(2)(mu-(N=C(CH3)-C6H4-(CH3)C=N))] (8) and [((C5Me5)2(Br)Th)2(mu-(N=C(CH3)-C6H4- (CH3)C==N))] (9), respectively. These complexes were structurally characterized by single-crystal X-ray diffraction and NMR spectroscopy. Metal-metal interactions in these isovalent bimetallic systems were assessed by means of cyclic voltammetry, UV-visible/NIR absorption spectroscopy, and variable-temperature magnetic susceptibility. Although evidence for magnetic coupling between metal centers in the bimetallic U IV/U IV (5f2-5f2) complex is ambiguous, the complex displays appreciable electronic communication between the metal centers through the pi system of the dianionic diketimide bridging ligand, as judged by voltammetry. The transition intensities of the f-f bands for the bimetallic U IV/U IV system decrease substantially compared to the related monometallic ketimide chloride complex, [(C5Me5)2U(Cl)(-N=C(CH3)-(3,4,5-F(3)-C6H2))] (11). Also reported herein are new synthetic routes to the actinide starting materials [(C5Me4Et)(2)U(CH3)(Cl)] (6) and [(C5Me5)2Th(CH3)(Br)] (7) in addition to the syntheses and structures of the monometallic uranium complexes [(C5Me4Et)2UCl2] (3), [(C5Me4Et)2U(CH3)2] (4), [(C5Me4Et)2U(-N==C(CH3)-C6H4-C==N)2] (10), and 11.

Application of ion exchange and extraction chromatography to the separation of actinium from proton-irradiated thorium metal for analytical purposes

Actinium-225 (t1/2=9.92d) is an α-emitting radionuclide with nuclear properties well-suited for use in targeted alpha therapy (TAT), a powerful treatment method for malignant tumors. Actinium-225 can also be utilized as a generator for (213)Bi (t1/2 45.6 min), which is another valuable candidate for TAT. Actinium-225 can be produced via proton irradiation of thorium metal; however, long-lived (227)Ac (t1/2=21.8a, 99% β(-), 1% α) is co-produced during this process and will impact the quality of the final product. Thus, accurate assays are needed to determine the (225)Ac/(227)Ac ratio, which is dependent on beam energy, irradiation time and target design. Accurate actinium assays, in turn, require efficient separation of actinium isotopes from both the Th matrix and highly radioactive activation by-products, especially radiolanthanides formed from proton-induced fission. In this study, we introduce a novel, selective chromatographic technique for the recovery and purification of actinium isotopes from irradiated Th matrices. A two-step sequence of cation exchange and extraction chromatography was implemented. Radiolanthanides were quantitatively removed from Ac, and no non-Ac radionuclidic impurities were detected in the final Ac fraction. An (225)Ac spike added prior to separation was recovered at ≥ 98%, and Ac decontamination from Th was found to be ≥ 10(6). The purified actinium fraction allowed for highly accurate (227)Ac determination at analytical scales, i.e., at (227)Ac activities of 1-100 kBq (27 nCi to 2.7 μCi).

Proton-induced cross sections relevant to production of 225Ac and 223Ra in natural thorium targets below 200 MeV

Cross sections for (223,)(225)Ra, (225)Ac and (227)Th production by the proton bombardment of natural thorium targets were measured at proton energies below 200 MeV. Our measurements are in good agreement with previously published data and offer a complete excitation function for (223,)(225)Ra in the energy range above 90 MeV. Comparison of theoretical predictions with the experimental data shows reasonable-to-good agreement. Results indicate that accelerator-based production of (225)Ac and (223)Ra below 200 MeV is a viable production method.

225Ac and 223Ra production via 800 MeV proton irradiation of natural thorium targets

Cross sections for the formation of (225,227)Ac, (223,225)Ra, and (227)Th via the proton bombardment of natural thorium targets were measured at a nominal proton energy of 800 MeV. No earlier experimental cross section data for the production of (223,225)Ra, (227)Ac and (227)Th by this method were found in the literature. A comparison of theoretical predictions with the experimental data shows agreement within a factor of two. Results indicate that accelerator-based production of (225)Ac and (223)Ra is a viable production method.

Toward new paradigms in mixed-valency: ytterbocene–terpyridine charge-transfer complexes

(C5Me5)2Yb x OEt2 reacts with terpyridine and tetrapyridinylpyrazine to afford new mixed-valent systems.

Proton irradiation parameters and chemical separation procedure for the bulk production of high-specific-activity 186gRe using WO3 targets

Abstract Rhenium-186g (T1/2= 89.2 h) is a β− emitter suitable for therapeutic applications. Current production methods rely on reactor production via 185Re(n,γ) which results in low specific activities, thereby limiting its use. Production by p,d activation of enriched 186W results in a 186gRe product with a higher specific activity, allowing it to be used for targeted therapy with limited receptors. A test target consisting of pressed, sintered natWO3 was proton irradiated at Los Alamos (LANL-IPF) to evaluate product yield and impurities, irradiation parameters and wet chemical Re recovery for proof-of-concept for bulk production of 186gRe. We demonstrated isolation of 186gRe in 97% yield from irradiated natWO3 targets within 12 h of end of bombardment (EOB) via an alkaline dissolution followed by anion exchange. The recovery process has potential for automation, and WO3 can be easily recycled for recurrent irradiations. A 186gRe batch yield of 42.7 ± 2.2 μCi/μAh or 439 ± 23 MBq/C was obtained after 24 h in an 18.5 μA proton beam. The target entrance energy was determined to be 15.6 MeV. The specific activity of 186gRe at EOB was measured to be 1.9 kCi (70.3 TBq) mmol−1, which agrees well with the result of a previous 185,186mRe co-production EMPIRE and TALYS modeling study assuming similar conditions. Utilizing enriched 186WO3, we anticipate that a proton beam of 250 μA for 24 h will provide batch yields of 256 mCi (9.5 GBq) of 186gRe at EOB with specific activities even higher than 1.9 kCi (70.3 TBq) mmol−1, suitable for therapy applications.

Monomeric f-element chemistry with sterically encumbered allyl ligands

AbstractA new class of allyl-lanthanide salts of the type [K(thf) 4 ][(C 3 H 3 (SiMe 3 ) 2 ) 3 LnI] (Ln / Ce, Pr, Nd, Gd, Tb, Dy, Er) have beenprepared and isolated by reaction of three equivalents of the 1,3-bis(trimethylsilyl)allyl anion with LnI 3 . The neutral complex[C 3 H 3 (SiMe 3 ) 2 ] 3 Nd(thf) has been isolated from the reaction of the triflate complex Nd(O 3 SCF 3 ) 3 with three equivalents of the 1,3-bis(trimethylsilyl)allyl anion. These complexes have been structurally characterized using single crystal X-ray diffraction.# 2003 Elsevier B.V. All rights reserved. Keywords: Allyl; Lanthanide; Tetrahydrofuran; Complex; Crystal structure; Bulley ligands 1. IntroductionSeveral examples of homoleptic and pseudo-homo-leptic allyl-lanthanide complexes have recently beenprepared and shown to be catalytically active [1 / 10].For instance, Nd(allyl) 3 (solv) X has been employed as acatalyst for stereospecific butadiene polymerization [4].However, the allyl moiety undergoes insertion, becom-ing incorporated into the growing chain, and the fate ofthe catalyst thereafter is not well understood. Our hopeis to gain better control of the activity of thesecomplexes by protecting the allyl moiety using bulkyend groups that will hinder insertion. Extremely bulkycyclopentadienyl ligands have been used effectively toimprove the stability of base-free metallocenes of thedivalent lanthanides [11

Ac, La, and Ce radioimpurities in 225Ac produced in 40–200 MeV proton irradiations of thorium

Abstract Accelerator production of 225Ac addresses the global supply deficiency currently inhibiting clinical trials from establishing 225Acs therapeutic utility, provided that the accelerator product is of sufficient radionuclidic purity for patient use. Two proton activation experiments utilizing the stacked foil technique between 40 and 200 MeV were employed to study the likely co-formation of radionuclides expected to be especially challenging to separate from 225Ac. Foils were assayed by nondestructive γ-spectroscopy and by α-spectroscopy of chemically processed target material. Nuclear formation cross sections for the radionuclides 226Ac and 227Ac as well as lower lanthanide radioisotopes 139Ce, 141Ce, 143Ce, and 140La whose elemental ionic radii closely match that of actinium were measured and are reported. The predictions of the latest MCNP6 event generators are compared with measured data, as they permit estimation of the formation rates of other radionuclides whose decay emissions are not clearly discerned in the complex spectra collected from 232Th(p,x) fission product mixtures.

Cross sections from proton irradiation of thorium at 800 MeV

Nuclear formation cross sections are reported for 65 nuclides produced from 800-MeV proton irradiation of thorium foils. These data are useful as benchmarks for computational predictions in the ongoing process of theoretical code development and also to the design of spallation-based radioisotope production currently being considered for multiple radiotherapeutic pharmaceutical agents. Measured data are compared with the predictions of three MCNP6 event generators and used to evaluate the potential for 800-MeV productions of radioisotopes of interest for medical radiotherapy. In only a few instances code predictions are discrepant from measured values by more than a factor of two, demonstrating satisfactory predictive power across a large mass range. Similarly, agreement between measurements presented here and those previously reported is good, lending credibility to predictions of target yields and radioimpurities for high-energy accelerator-produced radionuclides.