Eva R. Birnbaum

Metalloporphyrin-catalyzed homogeneous oxidation in supercritical carbon dioxide

We report results from a study of the reactivity of the halogenated porphyrins tetrakis(pentafluorophenyl)porphyrinato iron(III) chloride [Fe(TFPP)Cl] and β-octabromo-tetrakis(pentafluorophenyl)porphyrinato iron(III) chloride [Fe(TFPPBr8)Cl] with dioxygen and cyclohexene in supercritical carbon dioxide. A lower limit for the solubility of the iron porphyrins in sc CO2 was determined. Both halogenated metalloporphyrins were active catalysts for oxidation of cyclohexene to epoxide and allylic oxidation products in sc CO2. In 12 h at 80°C, up to 350 and 580 turnovers were observed for Fe(TFPP)Cl and Fe(TFPPBr8)Cl, respectively. We have also explored several organic solvent reactions at high temperature and pressure to benchmark relative activity and selectivity. Activity is higher in organic solvent, but accompanied by substantial oxidation of, or reaction with the solvent. Selectivity for epoxidation with Fe(TFPPBr8)Cl is higher in sc CO2 than in organic solvents, with up to 34% cyclohexene oxide produced.

On the mechanism of catalytic alkene oxidation by molecular oxygen and halogenated iron porphyrins

Abstract The halogenated porphyrin, 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(pentafluorophenyl)porphyriato-iron(III) chloride, [Fe(TFPPBr8)Cl], catalyzes the oxidation of cyclohexene in the presence of molecular oxygen or iodosobenzene. With PhIO, 77% epoxide is observed, consistent with a mechanism involving a high-valent metal-oxo species. With dioxygen, however, allylic alcohol and ketone are observed, suggesting a different mechanism. The relatively high activity of the [Fe(TFPPBr 8 )Cl] O 2 system suggests that the reaction involves the formation and decomposition of alkyl peroxides.

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.

How do electronegative substituents make metal complexes better catalysts for the oxidation of hydrocarbons by dioxygen

Modeling studies support a radical-chain autoxidation mechanism for the aerobic oxidation of isobutane catalyzed by halogenated porphyrin iron complexes. A key role of the electronegative halogen substituents is to increase the FeIII / IIredox potential and thereby accelerate oxidation of the intermediate t-butyl hydroperoxide by (porph)FeIII. The electronic structures of electronegatively substituted salen iron complexes have been characterized by several techniques, and related to changes in catalytic activity for oxidation of cyclohexene. The crystal and molecular structure of [Fe((NO2)4salen)(H2O)Cl] is reported.

Selective Anion Binding from Water Using Soluble Polymers

We investigated water-soluble dendrimers and polymers as ligands for selective anion binding. PAMAM Starburst dendrimers and polyethylenimine have been chemically modified to incorporate new ligands or hydrogen bonding groups at the primary amines. These molecules, along with the unmodified parent compounds, bind arsenate, chromate, and phosphate, even in the presence of competing anions such as chloride. Such systems could potentially be employed for remediation of contaminated water and soils using well-established ultrafiltration technology. Selectivity is achieved through bringing together moieties with a combination of electrostatic and hydrogen bonding capabilities. Total binding capacity for several metallooxyanions has been determined, as well as relative ion selectivity.

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.

Catalysis of aerobic olefin oxidation by a ruthenium perhaloporphyrin complex

Abstract The perhalogenated porphyrin ruthenium complex (TFPPCl 8 )Ru(CO) (TFPPCl 8 = octachlorotetrakis(pentafluorophenyl)porphyrin) catalyzes aerobic oxidation of olefins at room temperature. Cyclohexene is oxidized primarily at the allylic position, and styrene primarily to benzaldehyde, indicating a radical autoxidation mechanism. Reactions are enhanced by visible light. Reaction with m -chloroperbenzoic acid converts the ruthenium complex to (TFPPCl 8 )Ru(O) 2 , but such oxo complexes do not appear to participate in catalytic aerobic oxidation.

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.

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.