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Poe, Todd N.

Publications and source records attributed to Poe, Todd N..

11 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Altering the spectroscopy, electronic structure, and bonding of organometallic curium(III) upon coordination of 4,4'–bipyridine

Structural and electronic characterization of (Cp' 3 Cm) 2 (μ–4,4'–bpy) (Cp' = trimethylsilylcyclopentadienyl, 4,4'–bpy = 4,4'–bipyridine) is reported and provides a rare example of curium–carbon bonding. Cp' 3 Cm displays unexpectedly low energy emission that is quenched upon coordination by 4,4'–bipyridine. Electronic structure calculations on Cp' 3 Cm and (Cp' 3 Cm) 2 (μ–4,4'–bpy) rule out significant differences in the emissive state, rendering 4,4'–bipyridine as the primary quenching agent. Comparisons of (Cp' 3 Cm) 2 (μ–4,4'–bpy) with its samarium and gadolinium analogues reveal atypical bonding patterns and electronic features that offer insights into bonding between carbon with f-block metal ions. Here we show the structural characterization of a curium–carbon bond, in addition to the unique electronic properties never before observed in a curium compound.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Isolation of a californium(II) crown–ether complex

The actinides, from californium to nobelium (Z = 98–102), are known to have an accessible +2 oxidation state. Understanding the origin of this chemical behaviour requires characterizing Cf II materials, but investigations are hampered by the fact that they have remained dificult to isolate. This partly arises from the intrinsic challenges of manipulating this unstable element, as well as a lack of suitable reductants that do not reduce Cf III to Cf°. Here we show that a Cf II crown–ether complex, Cf(18-crown-6)I 2 , can be prepared using an Al/Hg amalgam as a reductant. Spectroscopic evidence shows that Cf III can be quantitatively reduced to Cf II , and rapid radiolytic re-oxidation in solution yields co-crystallized mixtures of Cf II and Cf III complexes without the Al/Hg amalgam. Furthermore, quantum-chemical calculations show that the Cf– ligand interactions are highly ionic and that 5f/6d mixing is absent, resulting in weak 5f→5f transitions and an absorption spectrum dominated by 5f→6d transitions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Two Neptunium(III) Mellitate Coordination Polymers: Completing the Series Np–Cf of Trans-Uranic An(III) Mellitates

For this work, two neptunium(III) mellitates, 237 Np 2 (mell)(H 2 O) 9 ·1.5H 2 O (Np-1α) and 237 Np 2 (mell)(H 2 O) 8 ·2H 2 O (Np-1β), have been synthesized from 237 NpCl 4 (dme) 2 by reduction with KC 8 and subsequent reaction with an aqueous solution of mellitic acid (H 6 mell). Characterization by single-crystal X-ray crystallography and UV–vis–NIR spectroscopy confirms that the neptunium is in its +3 oxidation state and both polymorphs are isostructural to the previously reported plutonium mellitates. Of the two morphologies, Np-1α is indefinitely stable in air, while Np-1β slowly oxidizes over several months. This is due to the change in the energy of the metal-ligand charge-transfer absorption exhibited by these compounds attributed to differing numbers of carboxylate bonds to Np(III), where in Np-1β the energy is low enough to result in spontaneous oxidation.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Cyclopentadienyl coordination induces unexpected ionic Am-N bonding in an americium bipyridyl complex

Variations in bonding between trivalent lanthanides and actinides is critical for reprocessing spent nuclear fuel. The ability to tune bonding and the coordination environment in these trivalent systems is a key factor in identifying a solution for separating lanthanides and actinides. Coordination of 4,4'-bipyridine (4,4'-bpy) and trimethylsilylcyclopentadienide (Cp') to americium introduces unexpectedly ionic Am-N bonding character and unique spectroscopic properties. Here we report the structural characterization of (Cp' 3 Am) 2 (μ -4,4'-bpy) and its lanthanide analogue, (Cp' 3 Nd) 2 (μ - 4,4'-bpy), by single-crystal X-ray diffraction. Spectroscopic techniques in both solid and solution phase are performed in conjunction with theoretical calculations to probe the effects the unique coordination environment has on the electronic structure.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

Structural and Spectroscopic Analysis of Ln(II) 18-crown-6 and Benzo-18-crown-6 Complexes (Ln = Sm, Eu, Yb)

Three Ln 2+ 18-crown-6 complexes of the formula Ln(18-crown-6)I 2 (Ln = Sm, Eu, Yb) were isolated from the explicit synthesis of the corresponding LnI 2 salts with 18-crown-6 and tetrabutylammonium tetraphenylborate in organic media under air-free conditions. Each metal complex forms a distorted hexagonal bipyramidal geometry and crystallizes in the monoclinic space group P2 1 /n. Comparatively, crystallization of Ln(benzo-18-crown-6)I 2 (Ln = Sm, Eu, Yb) from the reaction of LnI2 with tetrabutylammonium tetraphenylborate and benzo-18-crown-6 in THF/ethanol under similarly air-free conditions yields two polymorphs. The first form, α, crystallizes in the monoclinic space group P2 1 /c (or the nonstandard setting P2 1 /n for α-Yb); whereas the second polymorph, β, crystallizes in P$\bar{1}$. While the geometries of the molecules only vary slightly, the molecular packing and intramolecular contacts are quite different. In the structure of β, π–π interactions between the benzo- moieties of adjacent molecules are observed, whereas these interactions are absent in α. Despite the similarities in these classically 4f n+1 lanthanide systems, the complexes display distinct spectroscopic features in their respective absorption and photoluminescence spectra. Broadband 5d → 4f photoluminescence was observed for the Sm and Eu compounds in the NIR region and UV–visible region, respectively. None of the three Yb compounds exhibit photoluminescence UV–visible-NIR region; however, a unique photooxidation event was observed resulting in characteristic Yb(III) 4f → 4f transitions in the NIR region of the absorption spectra of these compounds. Finally, these findings are discussed along with structural comparisons of the 18-crown-6 and benzo-18-crown-6 compounds as well as other reported Ln(II) crown complexes in the literature.

crystals↗
DOE OSTI2021

Understanding the Stabilization and Tunability of Divalent Europium 2.2.2B Cryptates

Lanthanides such as europium with more accessible divalent states are useful for studying redox stability afforded by macrocyclic organic ligands. Substituted cryptands, such as 2.2.2B cryptand, that increase the oxidative stability of divalent europium also provide coordination environments that support synthetic alterations of Eu(II) cryptate complexes. Two single crystal structures were obtained containing nine-coordinate Eu(II) 2.2.2B cryptate complexes that differ by a single coordination site, the occupation of which is dictated by changes in reaction conditions. A crystal structure containing a [Eu(2.2.2B)Cl] + complex is obtained from a methanol–THF solvent mixture, while a methanol–acetonitrile solvent mixture affords a [Eu(2.2.2B)(CH 3 OH)] 2+ complex. While both crystals exhibit the typical blue emission observed in most Eu(II) containing compounds as a result of 4f 6 5d 1 to 4f 7 transitions, computational results show that the substitution of a Cl– anion in the place of a methanol molecule causes mixing of the 5d excited states in the Eu(II) 2.2.2B cryptate complex. Additionally, magnetism studies reveal the identity of the capping ligand in the Eu(II) 2.2.2B cryptate complex may also lead to exchange between Eu(II) metal centers facilitated by π-stacking interactions within the structure, slightly altering the anticipated magnetic moment. Finally, the synthetic control present in these systems makes them interesting candidates for studying less stable divalent lanthanides and the effects of precise modifications of the electronic structures of low valent lanthanide elements.

Anions↗
DOE OSTI2020

A Single Small-Scale Plutonium Redox Reaction System Yields Three Crystallographically-Characterizable Organoplutonium Complexes

Here, an approach to obtaining substantial amounts of data from a hazardous starting material that can only be obtained and handled in small quantities is demonstrated by the investigation of a single small-scale reaction of cyclooctatetraene, C 8 H 8 , with a solution obtained from the reduction of Cp' 3 Pu (Cp' = C 5 H 4 SiMe 3 ) with potassium graphite. This one reaction coupled with oxidation of a product has provided single-crystal X-ray structural data on three organoplutonium compounds as well as information on redox chemistry thereby demonstrating an efficient route to new reactivity and structural information on this highly radioactive element. The crystal structures were obtained from the reduction of C 8 H 8 by a putative Pu(II) complex, (Cp' 3 Pu II ) 1– , generated in situ, to form the Pu(III) cyclooctatetraenide complex, [K(crypt)][(C 8 H 8 ) 2 Pu III ], 1-Pu , and the tetra(cyclopentadienyl) Pu(III) complex, [K(crypt)][Cp' 4 Pu III ], 2-Pu . Oxidation of the sample of 1-Pu with Ag(I) afforded a third organoplutonium complex that has been structurally characterized for the first time, (C 8 H 8 ) 2 Pu IV , 3-Pu . Complexes 1-Pu and 3-Pu contain Pu sandwiched between parallel (C 8 H 8 ) 2– rings. The (Cp' 4 Pu III ) – anion in 2-Pu features three η 5 -Cp' rings and one η 1 -Cp' ring, which is a rare example of a formal Pu–C η 1 -bond. In addition, this study addresses the challenge of small-scale synthesis imparted by radiological and material availability of transuranium isotopes, in particular that of pure metal samples. A route to an anhydrous Pu(III) starting material from the more readily available Pu IV O 2 was developed to facilitate reproducible syntheses and allow complete spectroscopic analysis of 1-Pu and 2-Pu . Pu IV O 2 was converted to Pu III Br 3 (DME) 2 (DME = CH 3 OCH 2 CH 2 OCH 3 ) and subsequently Pu III Br 3 (THF) x , which was used to independently synthesize 1-Pu , 2-Pu , and 3-Pu .

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2020

Pressure-Induced Spectroscopic Changes in a Californium 1D Material Are Twice as Large as Found in the Holmium Analog

Here, in this study, the synthesis, characterization, and pressure response of a 1D californium mellitate (mellitate = 1,2,3,4,5,6-benzenehexacarboxylate) coordination polymer, Cf 2 (mell)(H 2 O) 10 ·4H 2 O (Cf-1), are reported. The Cf–O lengths within the crystal structure are compared to its gadolinium (Gd-1) and holmium (Ho-1) analogs as well. These data show that the average Cf–O bond distance is slightly longer than the average Gd–O bond, consistent with trends in effective ionic radii. UV–vis-NIR absorption spectra as a function of pressure were collected using diamond-anvil techniques for both Cf-1 and Ho-1. These experiments show that the Cf(III) f → f transitions have a stronger dependence on pressure than that of the holmium analog. In the former case, the shift is nearly linear with applied pressure and averages 6.6 cm –1 /GPa, whereas in the latter, it is <3 cm –1 /GPa.

absorption spectroscopy↗
DOE OSTI2020

Trivalent f-Element Squarates, Squarate-Oxalates, and Cationic Materials, and the Determination of the Nine-Coordinate Ionic Radius of Cf(III)

The synthesis, structure, and solid-state UV–vis–NIR spectroscopy of four new f-element squarates, M 2 (C 4 O 4 ) 3 (H 2 O) 4 (M = Eu, Am, Cf) and Sm(C 4 O 4 )(C 4 O 3 OH)(H 2 O) 2 ·0.5H 2 O, four new cationic lanthanide squarate chlorides, [M 4 (C 4 O 4 ) 5 (H 2 O) 12 ]Cl 2 ·5H 2 O (M = Eu, Dy, Ho Er), and two new actinide squarate oxalates, M 2 (C 4 O 4 ) 2 (C 2 O 4 )(H 2 O) 4 (M = Am, Cf), are presented. All of the metal centers are trivalent. Single-crystal X-ray diffraction analysis reveals that M 2 (C 4 O 4 ) 3 (H 2 O) 4 and Sm(C 4 O 4 )(C 4 O 3 OH)(H 2 O) 2 ·0.5H 2 O have a two-dimensional sheet structure constructed from MO 7 (H 2 O) 2 monocapped square-antiprismatic (coordination number (CN) = 9) metal centers and SmO 6 (H 2 O) 2 square-antiprismatic (CN = 8) metal centers, respectively, whereas M 2 (C 4 O 4 ) 2 (C 2 O 4 )(H 2 O) 4 have a three-dimensional (3D) structure constructed from MO 7 (H 2 O) 2 monocapped square-antiprismatic (CN = 9) metal centers. Additionally, the cationic framework materials [M 4 (C 4 O 4 ) 5 (H 2 O) 12 ]Cl 2 ·5H 2 O have a 3D structure constructed from two crystallographically unique MO 5 (H 2 O) 3 square-antiprismatic (CN = 8) metal centers. In these structures, the squarate ligands bind to the metal centers with varying coordination modes and denticities. The results of this study provide another example of the nonparallel chemistry between the lanthanides and transplutonium elements. From the crystallographic data for the isotypic series M 2 (C 4 O 4 ) 3 (H 2 O) 4 (M = La–Nd, Sm, Eu) and the linear regression fit to a plot of the unit cell volume as a function of the cube of the ionic radius, the nine-coordinate ionic radius of Cf 3+ was determined to be 1.127 ± 0.003 Å. Lastly, computational analysis of the americium and californium complexes M 2 (C 4 O 4 ) 3 (H 2 O) 4 and M 2 (C 4 O 4 ) 2 (C 2 O 4 )(H 2 O) 4 reveals three important attributes: (i) the 5f orbitals are nonbonding in all cases, with the bonding differences occurring with the empty 6d orbitals; (ii) the Cf complexes exhibit more covalent character than their Am counterparts; and (iii) there is more covalent character in the squarate-oxalate complexes than in the squarate complexes.

Crystallography↗