Advances in heavy alkaline earth chemistry provide insight into complexation of weakly polarizing Ra2+, Ba2+, and Sr2+ cations.

Numerous technologies-with catalytic, therapeutic, and diagnostic applications-would benefit from improved chelation strategies for heavy alkaline earth elements: Ra2+, Ba2+, and Sr2+. Unfortunately, chelating these metals is challenging because of their large size and weak polarizing power. We found 18-crown-6-tetracarboxylic acid (H4COCO) bound Ra2+, Ba2+, and Sr2+ to form M(HxCOCO)x-2. Upon isolating radioactive 223Ra from its parent radionuclides (227Ac and 227Th), 223Ra2+ reacted with the fully deprotonated COCO4- chelator to generate Ra(COCO)2-(aq) (log KRa(COCO)2- = 5.97 ± 0.01), a rare example of a molecular radium complex. Comparative analyses with Sr2+ and Ba2+ congeners informed on what attributes engendered success in heavy alkaline earth complexation. Chelators with high negative charge [-4 for Ra(COCO)2-(aq)] and many donor atoms [≥11 in Ra(COCO)2-(aq)] provided a framework for stable complex formation. These conditions achieved steric saturation and overcame the weak polarization powers associated with these large dicationic metals.

Oxidizing Americium(III) with Sodium Bismuthate in Acidic Aqueous Solutions.

Historic perspectives describing f-elements as being redox "inactive" are fading. Researchers continue to discover new oxidation states that are not as inaccessible as once assumed for actinides and lanthanides. Inspired by those contributions, we studied americium(III) oxidation in aqueous media under air using NaBiO3(s). We identified selective oxidation of Am3+(aq) to AmO22+(aq) or AmO21+(aq) could be achieved by changing the aqueous matrix identity. AmO22+(aq) formed in H3PO4(aq) (1 M) and AmO21+(aq) formed in dilute HCl(aq) (0.1 M). These americyl products were stable for weeks in solution. Also included is a method to recover 243Am from the americium and bismuth mixtures generated during these studies.

Advancing the Am Extractant Design through the Interplay among Planarity, Preorganization, and Substitution Effects.

Advancing the field of chemical separations is important for nearly every area of science and technology. Some of the most challenging separations are associated with the americium ion Am(III) for its extraction in the nuclear fuel cycle, 241Am production for industrial usage, and environmental cleanup efforts. Herein, we study a series of extractants, using first-principle calculations, to identify the electronic properties that preferentially influence Am(III) binding in separations. As the most used extractant family and because it affords a high degree of functionalization, the polypyridyl family of extractants is chosen to study the effects of the planarity of the structure, preorganization of coordinating atoms, and substitution of various functional groups. The actinyl ions are used as a structurally simplified surrogate model to quickly screen the most promising candidates that can separate these metal ions. The down-selected extractants are then tested for the Am(III)/Eu(III) system. Our results show that π interactions, especially those between the central terpyridine ring and Am(III), play a crucial role in separation. Adding an electron-donating group onto the terpyridine backbone increases the binding energies to Am(III) and stabilizes Am-terpyridine coordination. Increasing the planarity of the extractant increases the binding strength as well, although this effect is found to be rather weak. Preorganizing the coordinating atoms of an extractant to their binding configuration as in the bound metal complex speeds up the binding process and significantly improves the kinetics of the separation process. This conclusion is validated by the synthesized 1,2-dihydrodipyrido[4,3-b;5,6-b]acridine (13) extractant, a preorganized derivative of the terpyridine extractant, which we experimentally showed was four times more effective than terpyridine at separating Am3+ from Eu3+ (SFAm/Eu ∼ 23 ± 1).

Author Correction: Leveraging excited-state coherence for synthetic control of ultrafast dynamics.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Leveraging excited-state coherence for synthetic control of ultrafast dynamics.

Design-specific control over excited-state dynamics is necessary for any application seeking to convert light into chemical potential. Such control is especially desirable in iron(II)-based chromophores, which are an Earth-abundant option for a wide range of photo-induced electron-transfer applications including solar energy conversion1 and catalysis2. However, the sub-200-femtosecond lifetimes of the redox-active metal-to-ligand charge transfer (MLCT) excited states typically encountered in these compounds have largely precluded their widespread use3. Here we show that the MLCT lifetime of an iron(II) complex can be manipulated using information from excited-state quantum coherences as a guide to implementing synthetic modifications that can disrupt the reaction coordinate associated with MLCT decay. We developed a structurally tunable molecular platform in which vibronic coherences-that is, coherences reflecting a coupling of vibrational and electronic degrees of freedom-were observed in ultrafast time-resolved absorption measurements after MLCT excitation of the molecule. Following visualization of the vibrational modes associated with these coherences, we synthetically modified an iron(II) chromophore to interfere with these specific atomic motions. The redesigned compound exhibits a MLCT lifetime that is more than a factor of 20 longer than that of the parent compound, indicating that the structural modification at least partially decoupled these degrees of freedom from the population dynamics associated with the electronic-state evolution of the system. These results demonstrate that using excited-state coherence data may be used to tailor ultrafast excited-state dynamics through targeted synthetic design.

Insights into the excited state dynamics of Fe(ii) polypyridyl complexes from variable-temperature ultrafast spectroscopy.

In an effort to better define the nature of the nuclear coordinate associated with excited state dynamics in first-row transition metal-based chromophores, variable-temperature ultrafast time-resolved absorption spectroscopy has been used to determine activation parameters associated with ground state recovery dynamics in a series of low-spin Fe(ii) polypyridyl complexes. Our results establish that high-spin (5T2) to low-spin (1A1) conversion in complexes of the form [Fe(4,4'-di-R-2,2'-bpy')3]2+ (R = H, CH3, or tert-butyl) is characterized by a small but nevertheless non-zero barrier in the range of 300-350 cm-1 in fluid CH3CN solution, a value that more than doubles to ∼750 cm-1 for [Fe(terpy)2]2+ (terpy = 2,2':6',2''-terpyridine). The data were analyzed in the context of semi-classical Marcus theory. Changes in the ratio of the electronic coupling to reorganization energy (specifically, H ab 4/λ) reveal an approximately two-fold difference between the [Fe(bpy')3]2+ complexes (∼1/30) and [Fe(terpy)2]2+ (∼1/14), suggesting a change in the nature of the nuclear coordinate associated with ground state recovery between these two types of complexes. These experimentally-determined ratios, along with estimates for the 5T2/1A1 energy gap, yield electronic coupling values between these two states for the [Fe(bpy')3]2+ series and [Fe(terpy)2]2+ of 4.3 ± 0.3 cm-1 and 6 ± 1 cm-1, respectively, values that are qualitatively consistent with the second-order nature of high-spin/low-spin coupling in a d6 ion. In addition to providing useful quantitative information on these prototypical Fe(ii) complexes, these results underscore the utility of variable-temperature spectroscopic measurements for characterizing ultrafast excited state dynamics in this class of compounds.