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.

Ratiometric Turn-On Fluorophore Displacement Ensembles for Nitroaromatic Explosives Detection.

There is a recognized need in the area of explosives detection for fluorescence-based sensing systems that are capable of not only producing a turn-on response but also generating a distinctive spectral signature for a given analyte. Here, we report several supramolecular ensembles displaying efficient fluorophore displacement that give rise to an increase in fluorescence intensity upon exposure to various nitroaromatic compounds. The synthetic supramolecular constructs in question consist of a tetrathiafulvalene (TTF)-based pyrrolic macrocycle, benzo-TTF-calix[4]pyrrole (Bz-TTF-C4P), and fluorescent dyes, monomeric or dimeric naphthalenediimide (NDI) and perylenediimide (PDI) derivatives, as well as chloride or hexafluorophosphate (PF6-) salts of rhodamine 6G (Rh-6G). In chloroform solution, these assemblies exist in the form of discrete supramolecular complexes or oligomeric aggregates depending on the specific dye combinations in question. Each ensemble was tested as a potential explosive-responsive fluorescence indicator displacement assay (FIDA) by challenging it with a series of di- and trinitroaromatic compounds and examining the change in fluorescence spectral characteristics. Upon addition of nitroaromatic compounds (NACs), either a "turn-on" or a "turn-off" fluorescent response was observed depending on the nature of the constituent fluorophore and, where applicable, the counteranion. The FIDAs based on the PDI derivatives were found to display not only a ratiometric fluorescence enhancement but also analyte-dependent spectral changes when treated with NACs. The NAC-induced fluorescence spectral response of each ensemble was rationalized on the basis of various solution-phase spectroscopic studies, as well as single-crystal X-ray diffraction analyses.

Porphyrinoid f-Element Complexes.

Porphyrin and related pyrrole-containing macrocycles, collectively porphyrinoids, are versatile ligands that allow access to a multitude of coordination modes. Judicious modification of the porphyrin core as well as the pendant substituents has extended the coordination chemistry of porphyrinoids to include systems that are able to stabilize f-block element complexes with possible utility. This review focuses on our group's efforts to prepare expanded porphyrin and porphyrinogen ligands that can serve as tools to study and apply f-element metal coordination chemistry: it covers the background of the topic, selected syntheses, and application of these species in the chemical and medical sciences.

Condition-Dependent Coordination and Peroxidase Activity of Hemin-Aß Complexes.

The peroxidase activity of hemin-peptide complexes remains a potential factor in oxidative damage relevant to neurodegeneration. Here, we present the effect of temperature, ionic strength, and pH relevant to pathophysiological conditions on the dynamic equilibrium between high-spin and low-spin hemin-Aß40 constructs. This influence on peroxidase activity was also demonstrated using 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) and dopamine (DA) oxidation rate analyses with increasing ratios of Aß16 and Aß40 (up to 100 equivalents). Interaction and reactivity studies of aggregated Aß40-hemin revealed enhanced peroxidase activity versus hemin alone. Comparison of the results obtained using Aß16 and Aß40 amyloid beta peptides revealed marked differences and provide insight into the potential effects of hemin-Aß on neurological disease progression.

Synthesis and Characterization of a Binuclear Copper(II)-dipyriamethyrin Complex: [Cu2(dipyriamethyrin)(µ2-1,1-acetato)2].

The reaction between dipyriamethyrin and copper(II) acetate [Cu(OAc)2] afforded what is, to our knowledge, the first transition metal-dipyriamethyrin complex. Molecular and electronic characterization of this binuclear Cu(II) complex via EPR, UV-vis, and single crystal X-ray diffraction analysis revealed marked differences between the present constructs and previously reported binuclear copper(II) hexaphyrin species. UV-vis titration analyses provided evidence for a homotropic positive allosteric effect, wherein the binuclear species is formed without significant intermediacy of a monomeric complex.

Tetrathiafulvalene-calix[4]pyrrole: a versatile synthetic receptor for electron-deficient planar and spherical guests.

The first tetrakis-tetrathiafulvalene-calix[4]pyrrole (TTF-C[4]P) was reported in 2004. Early on it and related π-extended TTF-C[4]Ps were found to function as both anion receptors and as hosts for planar electron deficient neutral guests, including nitroaromatic explosives. Anion binding was found to occur with a 1 : 1 binding stoichiometry and to stabilise the cone C[4]P conformation, whereas planar electron deficient guests were bound in a cooperative 1 : 2 fashion to the 1,3-alternate conformer. Addition of strongly complexing anions was found to trigger release of the electron deficient guests concurrent with a conformational change to the cone form. Subsequent studies led to the discovery of anion-induced complexation with C60, and the finding that the resulting complexes would support fast photoinduced electron transfer events. Synthetic advances then led to the preparation of nonsymmetric TTF-C[4]Ps where a single moiety organises the receptor in either the 1,3-alternate conformation or the partial cone conformation, thus modifying both selectivity and sensitivity. TTF-C[4]P-based stimulus responsive systems, that rely on anions and cations as controlling inputs, have also been developed and studied in recent years. This review provides a summary of TTF-C[4]P-related chemistry.

Impact of Vertex Functionalization on Flexibility of Porous Organic Cages.

Efficient carbon capture requires engineered porous systems that selectively capture CO2 and have low energy regeneration pathways. Porous liquids (PLs), solvent-based systems containing permanent porosity through the incorporation of a porous host, increase the CO2 adsorption capacity. A proposed mechanism of PL regeneration is the application of isostatic pressure in which the dissolved nanoporous host is compressed to alter the stability of gases in the internal pore. This regeneration mechanism relies on the flexibility of the porous host, which can be evaluated through molecular simulations. Here, the flexibility of porous organic cages (POCs) as representative porous hosts was evaluated, during which pore windows decreased by 10-40% at 6 GPa. POCs with sterically smaller functional groups, such as the 1,2-ethane in the CC1 POC resulted in greater imine cage flexibility relative to those with sterically larger functional groups, such as the cyclohexane in the CC3 POC that protected the imine cage from the application of pressure. Structural changes in the POC also caused CO2 adsorption to be thermodynamically unfavorable beginning at ∼2.2 GPa in the CC1 POC, ∼1.1 GPa in the CC3 POC, and ∼1.0 GPa in the CC13 POC, indicating that the CO2 would be expelled from the POC at or above these pressures. Energy barriers for CO2 desorption from inside the POC varied based on the geometry of the pore window and all the POCs had at least one pore window with a sufficiently low energy barrier to allow for CO2 desorption under ambient temperatures. The results identified that flexibility of the CC1, CC3, or CC13 POCs under compression can result in the expulsion of captured gas molecules.

PuCl3{CoCp[OP(OEt)2]3}: transuranic elements entering the field of heterometallic molecular chemistry.

Recent advances enabled the discovery of heterometallic molecules for many metals: main group, d-block, lanthanides, and some actinides (U, Th). These complexes have at least two different metals joined by bridging ligands or by direct metal-metal bonding interactions. They are attractive because they can enable chemical cooperativity between metals from different parts of the periodic table. Some heterometallics provide access to unique reactivity and others exhibit physical properties that cannot be accessed by homometallic species. We envisioned that transuranic heterometallics might similarly enable new transuranic chemistry, though synthetic routes to such compounds have yet to be developed. Reported here is the first synthesis of a molecular transuranic complex that contains plutonium (Pu) and cobalt (Co). Our analyses of PuCl3{CoCp[OP(OEt)2]3} showed Pu(iv) and Co(iii) were present and suggested that the Pu(iv) oxidation state was stabilized by the electron donating phosphite ligands. This synthetic method - and the demonstration that Pu(iv) can be stabilized in a heterobimetallic molecular setting - provides a foundation for further exploration of transuranic multimetallic chemistry.

Photochemical separation of plutonium from uranium.

Plutonium-based technologies would benefit if chemical hazards for purifying plutonium were reduced. One critical processing step where improvements could be impactful is the adjustment of plutonium oxidation-states during separations. This transformation often requires addition of redox agents. Unfortunately, many of the redox agents used previously cannot be used today because their properties are deemed incompatible with modern day processing facilities and waste stream safety requirements. We demonstrated herein that photochemistry can be used as an alternative to those chemical agents. We observed that (1) Pu4+ → Pu3+ and UO22+ → U4+ photoreduction proceeded in HCl(aq) and HNO3(aq) and (2) photogenerated Pu3+(aq) and U4+(aq) could be separated using anion exchange chromatography (high yield, >90%; good separation factor, 322).

Synthesis, solid-state, solution, and theoretical characterization of an "in-cage" scandium-NOTA complex.

Developing chelators that strongly and selectively bind rare-earth elements (Sc, Y, La, and lanthanides) represents a longstanding fundamental challenge in inorganic chemistry. Solving these challenges is becoming more important because of increasing use of rare-earth elements in numerous technologies, ranging from paramagnets to luminescent materials. Within this context, we interrogated the complexation chemistry of the scandium(III) (Sc3+) trication with the hexadentate 1,4,7-triazacyclononane-1,4,7-triacetic acid (H3NOTA) chelator. This H3NOTA chelator is often regarded as an underperformer for complexing Sc3+. A common assumption is that metalation does not fully encapsulate Sc3+ within the NOTA3- macrocycle, leaving Sc3+ on the periphery of the chelate and susceptible to demetalation. Herein, we developed a synthetic approach that contradicted those assumptions. We confirmed that our procedure forced Sc3+ into the NOTA3- binding pocket by using single crystal X-ray diffraction to determine the Na[Sc(NOTA)(OOCCH3)] structure. Density functional theory (DFT) and 45Sc nuclear magnetic resonance (NMR) spectroscopy showed Sc3+ encapsulation was retained when the crystals were dissolved. Solution-phase and DFT studies revealed that [Sc(NOTA)(OOCCH3)]1- could accommodate an additional H2O capping ligand. Thermodynamic properties associated with the Sc-OOCCH3 and Sc-H2O capping ligand interactions demonstrated that these capping ligands occupied critical roles in stabilizing the [Sc(NOTA)] chelation complex.

Reduced texaphyrin: A ratiometric optical sensor for heavy metals in aqueous solution.

We report here a water-soluble metal cation sensor system based on the as-prepared or reduced form of an expanded porphyrin, texaphyrin. Upon metal complexation, a change in the redox state of the ligand occurs that is accompanied by a color change from red to green. Although long employed for synthesis in organic media, we have now found that this complexation-driven redox behavior may be used to achieve the naked eye detectable colorimetric sensing of several number of less-common metal ions in aqueous media. Exposure to In(III), Hg(II), Cd(II), Mn(II), Bi(III), Co(II), and Pb(II) cations leads to a colorimetric response within 10 min. This process is selective for Hg(II) under conditions of competitive analysis. Furthermore, among the subset of response-producing cations, In(III) proved unique in giving rise to a ratiometric change in the ligand-based fluorescence features, including an overall increase in intensity. The cation selectivity observed in aqueous media stands in contrast to what is seen in organic solvents, where a wide range of texaphyrin metal complexes may be prepared. The formation of metal cation complexes under the present aqueous conditions was confirmed by reversed phase high-performance liquid chromatography, ultra-violet-visible absorption and fluorescence spectroscopies, and high-resolution mass spectrometry.

Amethyrin-type expanded porphyrins that display anti-aromatic character upon protonation.

The use of protonation to switch nonaromatic expanded porphyrins to their corresponding anti-aromatic forms has not been widely explored. Here, we show that free-base pyriamethyrin and dipyriamethyrin display nonaromatic character, as inferred from NMR spectroscopic analyses, their optical properties, and theoretical calculations. Addition of two protons extends the π - conjugation of these amethyrin analogues and yields formally anti-aromatic systems.

Metallotexaphyrins as MRI-Active Catalytic Antioxidants for Neurodegenerative Disease: A Study on Alzheimer's Disease.

The complex etiology of neurodegeneration continues to stifle efforts to develop effective therapeutics. New agents elucidating key pathways causing neurodegeneration might serve to increase our understanding and potentially lead to improved treatments. Here, we demonstrate that a water-soluble manganese(II) texaphyrin (MMn) is a suitable magnetic resonance imaging (MRI) contrast agent for detecting larger amyloid beta constructs. The imaging potential of MMn was inferred on the basis of in vitro studies and in vivo detection in Alzheimer's disease C. elegans models via MRI and ICP-MS. In vitro antioxidant- and cellular-based assays provide support for the notion that this porphyrin analog shows promise as a therapeutic agent able to mitigate the oxidative and nitrative toxic effects considered causal in neurodegeneration. The present report marks the first elaboration of an MRI-active metalloantioxidant that confers diagnostic and therapeutic benefit in Alzheimer's disease models without conjugation of a radioisotope, targeting moiety, or therapeutic payload.

UO2 2+-mediated ring contraction of pyrihexaphyrin: synthesis of a contracted expanded porphyrin-uranyl complex.

A new mixed hexaphyrin, pyrihexaphyrin (0.1.0.0.1.0) (1), was prepared via an acid catalyzed cyclization between 5,5'-(pyridine-2,6-diyl)bis(pyrrole-2-carbaldehyde) (2) and terpyrrole (3). This expanded porphyrin undergoes a ring contraction upon metallation with uranyl silylamide [UO2[N(SiMe3)2]2] under anaerobic conditions followed by purification over basic aluminum oxide exposed to air. The uranyl-contracted pyrihexaphyrin (0.0.0.0.1.0) complex (4) produced as a result contains a unique structural architecture and possesses a formally 22 π-electron globally aromatic periphery, as inferred from NMR spectroscopy, single crystal X-ray diffraction, and computational analyses. Support for the proposed contraction mechanism came from experimental data and DFT calculations. Proton NMR and mass spectroscopic analysis provided the first insight into expanded porphyrin-mediated activation of the uranyl dication (UO2 2+).

Correction: UO2 2+-mediated ring contraction of pyrihexaphyrin: synthesis of a contracted expanded porphyrin-uranyl complex.

[This corrects the article DOI: 10.1039/C9SC01593K.].