Probing the Subtle Differences between Promethium and Curium

doi:10.21203/rs.3.rs-4788524/v1

Curium (²⁴⁴Cm) from the plutonium (²³⁸Pu) waste stream at Oak Ridge National Laboratory is coextracted with promethium (¹⁴⁷Pm) because of similar chemical and physical properties. To gain insight for improving this separation, their fundamental properties are experimentally and computationally probed in a 2,2’:6’,2”-terpyridine crystal system. Analysis of the isostructural compounds via single crystal X-ray diffraction and quantum theory of atoms in molecules (QTAIM) revealed that although promethium (¹⁴⁷Pm) and ²⁴⁴Cm share the same predicted VI-coordinate ionic radius, they have minute structural differences. Although the differences between Pm and Cm are subtle in this crystal system, insights for potentially optimizing the separation between ¹⁴⁷Pm and ²⁴⁴Cm are gained. Furthermore, this study develops the chemistries of two rare elements in the solid state and experimentally portrays promethium’s often omitted position within the lanthanide series.

Physical sciences/Chemistry/Coordination chemistry

Physical sciences/Chemistry/Inorganic chemistry/Solid-state chemistry

Physical sciences/Chemistry/Inorganic chemistry/Chemical bonding

Since its discovery in 1945, promethium (Pm) remains the least studied element among the lanthanide series because of its scarcity and radiolytic nature. Previous investigations on milligram samples of Pm were performed decades ago using its most abundant isotope, ¹⁴⁷Pm. Many of these earlier studies examined the spectroscopic nature of simple Pm compounds such as Pm halide salts and oxides.^1-5 Compounds of Pm typically are pale pink or purple in color, which is close to that of its neighboring lanthanide, neodymium (Nd).⁶ Unsurprisingly, like other lanthanide elements, Pm possesses Laporte- forbidden f-f transitions in the visible region that gives it a unique spectroscopic signature. The two prominent absorption peaks of Pm appear at 548 nm and 568 nm and correspond to the ⁵G₄ ← ⁵I₄ and ⁵G₂ ← ⁵I₄ transitions, respectively. Like the spectroscopic studies, structural experiments involving Pm have been relegated to simple halide salts and oxides. ^7-9 Interestingly, one study examined the phase change of metallic Pm under pressure. The results of this study showed that the double hexagonal close-packed (dhcp) structure of metallic Pm switched to the face-centered cubic (fcc) structure type at 10 GPa, and Pm is the last metal in the lanthanide series that undergoes this phase transition with pressure.¹⁰ Another isolated structure examined the more complex organometallic structure, PmCp₃ (Cp = cyclopentadiene). However, structural knowledge of this compound was measured by powder X-ray diffraction and only identified the unit cell parameters of the Pm organometallic compound.¹¹

Today, the major application of Pm is measuring the thickness of thin films by utilizing the β-decay property of ¹⁴⁷Pm.¹² However, ¹⁴⁷Pm has recently regained interest because of its potential use in lightweight betavoltaic batteries. With a half-life of 2.62 years, ¹⁴⁷Pm batteries could have a potential lifespan of ~five years, and ¹⁴⁷Pm has been previously demonstrated to work as a battery in pacemakers.¹³ Unfortunately, the two primary pathways for making ¹⁴⁷Pm are challenging. The first, and perhaps least efficient of the two methods, includes the irradiation of ¹⁴⁶Nd(¹⁴⁶Nd[n,ɣ]¹⁴⁷Nd(t_1/2=10.98 d, β-). This method necessitates a challenging adjacent lanthanide purification of ¹⁴⁷Pm from large quantities of Nd along with long irradiation times to produce micrograms of ¹⁴⁷Pm.^14,15 The second, more feasible pathway, comes from mining the waste stream of irradiated neptunium (Np) in the production of heat source plutonium-238 (HSPu) at Oak Ridge National Laboratory (ORNL).¹⁶ This pathway, however, does not come without its challenges. Isolation of ¹⁴⁷Pm from ²³⁸Pu waste streams takes place at the shared hot cell processing facility of the Radiochemical Engineering and Development Center (REDC) at ORNL. Specifically, these hot cells are shared with the ²⁵²californium (²⁵²Cf) production program which utilizes curium (Cm) for their target material. Because of the shared processing equipment, ²⁴⁴Cm is often present and coextracted with the ¹⁴⁷Pm product even after significant processing. The existence of ²⁴⁴Cm in the ¹⁴⁷Pm material is problematic due to its neutron activity and fissile behavior. Currently, multiple solvent extractions of the ¹⁴⁷Pm material are required to remove the trace amounts of ²⁴⁴Cm.
The coextraction of Cm with Pm in processing at ORNL suggests that the two elements have very similar chemistries. Both elements share the same dominant oxidation state (+3) and same predicted ionic radii (+3, VI-coordinate = 0.97 Å) thereby complicating the separation.¹⁷ While there have been several studies examining the separation between Eu and Am,^18-21 far less comparison studies exists between Cm and Pm. Recently, an extended X-ray absorption fine structure (EXAFS) experiment has been performed on a Pm complex in nitric acid media to provide information on the bonding of Pm in solution.²² The results obtained were demonstrated with the ligand bipyrrolidine diglycolamide and measured Pm—O bond distances of 2.476(16) Å, but the general chemistry of Pm remains vastly unexplored. To further our expansion of Pm and Cm chemistry and aid in improving the Pm/Cm separation, we prepared iso structural compounds of Pm and Cm using the well characterized ligand, 2,2’:6’,2”-terpyridine (Terpy). The results provided solid-state bond distances of the Pm³⁺ ion through single crystal X-ray diffraction (scXRD). The Pm and Cm compounds are characterized by spectroscopic and computational techniques to further examine the fundamental differences between the two and provide guidance in the Pm/Cm separation. Additionally, the work within also provides one of the few examples of how Pm resides within the lanthanide series, which is often omitted because of Pm’s scarce and radiolytic nature.

Synthesis and Crystallography. Isostructural complexes to those reported in literature¹⁹ of the formula, [M(Terpy)₂(NO₃)₂][M(Terpy)(NO₃)₄]·2MeCN (M = Ce—Er, Cm), were obtained by mixing one equivalent of the metal nitrate salt with three equivalents of Terpy in acetonitrile (MeCN). Additionally, to minimize radioactive exposure, ²⁴⁸Cm was used in place of ²⁴⁴Cm. To obtain crystals suitable for scXRD, caution must be taken to avoid preparing a solution that is too concentrated, otherwise immediate precipitation occurs. Formation of crystals suitable for scXRD form between 2-48 hours. In the case of Pm, a prickly pear colored solution (Figure 1) develops in which pale pinkish-purple crystals form within two hours. However, these crystals quickly turn brown after a short period (~24 hours) from radiolytic damage. All the metal compounds crystallized in the triclinic space group P-1. There are two distinct metal sites within the crystal structure. One site is a 10-coordinate metal site bound by two-tridentate Terpy ligands and two-bidentate nitrate anions to form a [M(Terpy)₂(NO₃)₂]⁺¹ (2:1 MTerpy) cationic species in the asymmetric unit. The second site consists of a 10-coordinate metal center bound by one-tridentate Terpy ligand and four nitrate anions. Three nitrates coordinate in a bidentate manner and one nitrate coordinates in a monodentate fashion to form a [M(Terpy)(NO₃)₄]^-1(1:1 MTerpy) anionic species. The outer-sphere crystal environment is completed by two co-crystallized MeCN solvent molecules. Formulation of a nine-coordinate, tris-Terpy chelate system in which the metal site is completely saturated by Terpy ligands was not obtainable using the metal nitrate salts under these reaction conditions.

For PmTerpy, this structural system provides one of the first examples of scXRD Pm bond distances with an organic chelator. As expected, the bond distances for Pm lie between those observed for Nd and Sm. The average Pm—N bond distance in the cationic 2:1 PmTerpy species was observed to be 2.612(16) Å. This lines up nicely within the lanthanide series between NdTerpy [Nd—N_avg = 2.621(17)] and SmTerpy [Sm—N_avg=2.599(17) Å]. Likewise, this expected trend was observed in the anionic 1:1 PmTerpy species within the crystal structure with an average Pm—N bond distances of 2.576(9) Å which lies in-between the average Nd_(Terpy)—N bond distance of 2.580(3) Å and Sm_(Terpy)—N bond distance of 2.556(8) Å.
Given the generally accepted concept that 5f elements have a stronger preference for N-donor ligands than 4f elements, it was hypothesized that the Cm—N bond distances would be shorter than Pm although the two elements have the same predicted ionic radius. This hypothesis was supported in this Terpy system in which the average Cm_(Terpy)—N bonds in the 2:1 cationic and 1:1 anionic metal site were 2.601(16) Å and 2.551(3) Å, respectively.

Spectroscopy. The solid-state UV-Vis absorption spectrum of PmTerpy was measured alongside NdTerpy, SmTerpy, and CmTerpy (Figure 2). The absorption spectrum of NdTerpy was as expected with the major absorption bands occurring at 527 nm (⁴G_7/2), 579 nm (⁴G_5/2), 740nm (⁴F_7/2), and 795 (⁴F_5/2) nm. Likewise, the major absorption peak at 404 nm (⁴P_3/2) is observed in SmTerpy, although this peak occurs alongside an intra-ligand transition most likely due to a p ® p* transition in the Terpy ligand. Like SmTerpy, CmTerpy has absorption peaks at higher energies, 381 nm, 385 nm, and 401 nm which matches with absorption spectra of other solid-state Cm compounds in literature.^23-25 Additionally, the emission spectrum of CmTerpy was measured because Cm³⁺is known to have a characteristic emission peak from Laporte-forbidden f-f transitions. This peak is located at 614 nm (inset of Figure 3) and matches closely with values reported for this singular peak in literature.^23-25

Although the major peaks of the Pm³⁺ ion are observed in the solid-state absorption spectra, there are some features worth noting. At times, the color of lanthanide compounds can be attributed to their ligand environment, but their color often comes from their major absorption bands. For example, Nd compounds are often portrayed to be light purple in color because of the intense absorption bands between 508 nm and 575 nm. The most intense bands of Pm³⁺ occur in a similar wavelength range to neodymium and would thus be expected to share a similar color to that of Nd³⁺or Er³⁺. In the PmTerpy absorption spectra, the dominant peaks are located at 548 nm and 568 nm. These peaks lie between the major peaks observed for NdTerpy. However, while the color of the solution and crystals were initially prickly pear, the color of the PmTerpy crystals quickly turned brown (Figure 2), likely from radiation damage in the organic containing compound. The expected Terpy based transition at 360 nm is significantly broadened and red-shifted to ~ 420 nm which can also be attributed to the radioactive nature of Pm. This effect has been observed in the solid-state absorbance of compounds containing berkelium (Bk), which is also a high specific activity β-emitter with a short half-life (²⁴⁹Bk t_1/2 = 330 d).²⁶ Like Sm, Eu, Tb, and Dy, PmTerpy also has luminescence peaks in the visible spectrum. However, these peaks are not observable due to the broad band observed at higher energy wavelengths. The PmTerpy compound also has a rich NIR emission spectrum with intense peaks at 797 nm, 828 nm, 872 nm, 903 nm, and 934 nm. All these peaks except the 872 nm peak agree with those computationally predicted using the ligand-field density functional theory (LFDFT) as shown in Figure 3. This is likely due to emission from a higher-lying emissive state as reported elsewhere for Pm³⁺ doped into a LaCl₃ chloride matrix.²⁷ Our calculations show that the multiplet structure of a Pm chloride complex and our Terpy system should not be significantly different and similar emission should be expected (Supplementary Figure 1).

Bonding. The structures of the 2:1 and 1:1 complexes were obtained computationally from the crystal structures (see Methods Computational details) with errors < 2% in the M-N bond lengths (Supplementary Table 1). These structures were used to interrogate and compare the nature of the PmTerpy bonds to those of the Nd, Sm, and Cm analogs for the 1:1 and 2:1 complexes. The QTAIM formalism has proven sufficient to this aim, where the concentration of the electron density, energy densities, and delocalization indices have been the most popular metrics to shed light on the nature of the metal-ligand interaction. Table 1 summarizes the M-N_Terpy average values for the selected QTAIM metrics. The accumulation of electron density is similar in all M-N bonds with Cm-N being slightly increased compared to the lanthanides. A similar trend is observed for the

Table 1. Averaged QTAIM metrics for the M-N bonds in the 1:1 and 2:1 complexes. Accumulation of electron density, ρ(r), is given in e^- Å^-3; kinetic (G), potential (V), and total energy (H) densities in kJ mol^-1 Å^-3. All these metrics including the delocalization indices, 𝛿(r), were calculated at the bond critical point.

	Bond	ρ(r)	G(r)	V(r)	H(r)	𝛿(r)
MTerpy 1:1	Nd-N	0.2831	634.5	-704.2	-69.7	0.2506
	Pm-N	0.2853	658.1	-720.5	-62.4	0.2533
	Sm-N	0.2799	641.8	-700.5	-58.7	0.2467
	Cm-N	0.3196	771.3	-859.2	-87.9	0.2740
MTerpy 2:1	Nd-N	0.2955	669.2	-749.7	-80.5	0.2613
	Pm-N	0.2805	642.9	-702.3	-59.4	0.2454
	Sm-N	0.2810	640.8	-702.3	-61.5	0.2560
	Cm-N	0.3085	738.8	-815.6	-76.8	0.2646

delocalization index. The negative values of the total energy density suggest that there is a non-negligible covalent interaction between the metal and Terpy ligands for all complexes. Though the overall metrics suggest a similar bonding pattern for all complexes, slight differences indicate an increased degree of covalency in Cm. However, it is noteworthy that the Nd-N bonds in the 2:1 show a more negative H(r) value (-80.5 kJ mol^-1 Å^-3) compared to that of Cm (-76.8 kJ mol^-1 Å^-3).
If we contrast these results with energy decomposition analyses, a similar behavior is observed (Table 2). For example, Cm shows a slight increase in orbital (2-5 kcal/mol) and total interaction energies (3-4 kcal/mol) compared to the rest of the lanthanide complexes. Though the electrostatic component remains the most significant in magnitude, it is canceled out by the high Pauli repulsion frequently seen with neutral ligand fragments. This highlights the role of orbital interactions in these complexes.

Table 2. Energy decomposition analysis (kcal/mol) of the 1:1 metal complexes.

	Nd	Pm	Sm	Cm
Pauli	106.3	109.1	105.6	124.3
Electrostatic	-105.2	-108.7	-105.6	-121.4
Orbital	-38.9	-40.9	-38.7	-43.3
Solvent	-90.8	-89.3	-89.9	-92.6
Total	-126.1	-127.3	-126.1	-130.5

Examining both Pm and Cm in this isostructural series reveals the true difficulty in separating the two elements. When conditions, ligands, or extractants are identical, like in processing, the two elements possess very similar chemistries. Many structural and chemical factors that make separations between elements feasible are absent in this situation. The structural differences between Pm and Cm in the crystal system are miniscule. The two elements share a similar bonding behavior, even at higher coordination numbers. Additionally, both elements’ oxidation and reduction potentials to either the +2 or +4 state are not feasible in either ambient nor aqueous processing conditions. While it was expected for Cm to have somewhat shorter bonds to the nitrogen containing Terpy ligand, the differences are not significant. In this crystal system, Cm bond lengths align closely to Sm (Figure 4). Therefore, purification of Cm from the Pm product is essentially in the realms of an adjacent lanthanide separation, which are extremely tedious. Oftentimes, computations may reveal significant differences in bonding as observed in separation studies between Eu and Am.^18-21 However, only minute differences in bonding are shown by the computational calculations within this Terpy system. The lack of substantial difference in bonding, and similar bond lengths to Pm and Sm, could possibly be attributed to the ligand system itself and the reluctance of Cm³⁺ (5f⁷) to engage its 5f shell in covalent interaction.

The major differences between PmTerpy and CmTerpy arise in their spectroscopy. Of the actinides, Cm could be considered the most lanthanide-like because its half-filled 5f orbital makes obtaining other oxidation states challenging. This relegates Cm to almost solely existing in the +3-oxidation state in compounds other than oxides. Furthermore, Cm possesses a sharp luminescence spectrum in the visible range like europium (Eu), terbium (Tb), Sm, and dysprosium (Dy). Likewise, Pm also has luminescence bands in the visible region of the spectrum. The luminescence peaks in the visible region appear at higher energies, and these observed luminescence features could potentially be used in aiding their separation from each other by spectroscopically monitoring their elution from resins in a column purification.

In summary, the complicated separation of ¹⁴⁷Pm from actinides and other fission products is critical for the long-term success of ¹⁴⁷Pm production. During the extraction of ¹⁴⁷Pm, ²⁴⁴Cm is coextracted and necessitates multiple solvent extractions to remove trace amounts of the ²⁴⁴Cm neutron source from the ¹⁴⁷Pm product. To shed further light on this purification, we prepared single crystals of both ¹⁴⁷Pm and Cm in an isostructural lanthanide series with Terpy. Our results further demonstrate the similarity between Pm and Cm while also signalizing the subtle differences between the two elements. In the analyzed crystal structures, though Pm and Cm have the same predicted VI-coordinate ionic radius, Cm bond distances are slightly shorter due to slightly enhanced bonding with the nitrogen atoms of the Terpy ligand. These Cm bond distances align more so with Sm, essentially deeming the separation between Pm and Cm like an adjacent lanthanide separation in this Terpy system. These results were further supported by QTAIM and energy decomposition calculations. The most observable difference between Pm and Cm in the Terpy structures arises from their luminescence, which could potentially be used as indicators on a chromatographic separation. This work is one of the few current studies that experimentally positions Pm within the lanthanide series and demonstrates the necessity for further study in the chemistry of these two rare elements.

Synthesis of Crystal Structures

Caution! ¹⁴⁷Pm and ²⁴⁸Cm are radioactive in nature. ¹⁴⁷Pm (t_1/2 = 2.62y) has a high specific activity (928.4 Ci/g), and experiments with both elements were performed under the supervision of radiological control technicians (RCTs) in laboratories equipped with high efficiency particulate air (HEPA) filters. ¹⁴⁷Pm and ²⁴⁸Cm were received at REDC through the National Isotope Development Center (NIDC). All reagents and solvents were used as received from the manufacturer.

The preparation of crystals suitable for single crystal X-ray Diffraction (scXRD) were prepared by mixing 1 equivalent of the metal nitrate salt with 3 equivalents of 2,2’:6’,2”-terpyridine (Terpy) (Sigma, 98%) in a 0.5 -1.0 mL of acetonitrile (MeCN). For forming PmTerpy crystals, the hydrated promethium (Pm) nitrate salt (2 Ci ¹⁴⁷Pm³⁺, 2.1 mg, 0.014 mmol) was carefully mixed with Terpy (9.8 mg, 0.042 mmol) in 0.5 mL of MeCN. Crystals suitable for scXRD formed after ~ two hours in a glovebox. Similarly, single crystals on CmTerpy were prepared by mixing the hydrated metal nitrate salt (2.0 mg ²⁴⁸Cm³⁺, 0.008 mmol) with Terpy (7.4 mg, 0.032 mmol) in 0.5 mL of MeCN. Crystals suitable for scXRD formed overnight.

Single Crystal X-ray Diffraction

X-ray diffraction measurements were performed on a Bruker D8 Venture diffractometer equipped with an Iµs 3.0 molybdenum X-ray source (λ = 0.71073 Å). APEX4 software was used for data collection and unit cell determination.²⁸ The crystal structures were determined using SHELXL or SHELXT software within the OLEX2 graphical user interface (GUI) software.^29–31 Hydrogen atoms bound to carbon were geometrically added at calculated positions. For the radioactive crystals, PmTerpy and CmTerpy, crystals were coated in immersion oil (Type NVH) and then subsequently coated in quick-setting epoxy on a Mitegen Cryoloop. A plastic sheath was then epoxied to the Mitegen Cryoloop base for further radiological protection.³² This sheath prevents collection of structures at cold temperatures due to icing. Therefore, collections were performed at room temperature. Crystallographic data is located in Supplementary Table 2.

Single Crystal Spectroscopy

Single crystals of MTerpy were placed on a microscope slide and contained in Type NVH immersion oil. Absorbance and emission spectra were collected on a CRAIC Technologies 2030PV Pro™ Microspectrophotometer. Emission spectra were collected by exciting with a single wavelength (365 nm or 420 nm).

Computations

Density Functional Theory (DFT) calculations were performed to further characterize the complexes synthesized. Fully optimized structures were obtained using the meta-Generalized Gradient Approximation (meta-GGA) functional TPSS including Grimme’s D3 dispersion corrections along with the Slater-type polarized triple-ζ (STO-TZP). Scalar relativistic effects were included using the ZORA Hamiltonian and implicit solvation effects via the COSMO approximation simulating the complex in acetonitrile. The crystal structure was used as a starting point for the geometry optimization step. The bonding situation was then interrogated through single point calculations on the optimized geometries where Quantum Theory of Atom in Molecules (QTAIM) metrics and energy decomposition analyses (EDA) were performed. These single point calculations were carried out using the hybrid-GGA functional PBE0 to reduce the over delocalization of the electron density on the complexes. The interaction energies of the complexes were calculated the considering metal nitrate fragment interacting with one and two Terpy ligands for the 1:1 and 2:1 complexes, respectively.

To aid the interpretation of the promethium absorption and emission spectra, the ligand-field DFT (LFDFT) approximation³³ was considered using the same level of theory as in the single point calculations. Within LFDFT, the multiplet structure for an fⁿ electron configuration is calculated using the Kohn-Sham orbitals obtained from an average of configuration (AOC) calculation. This allows for cost-efficient multiconfigurational calculations recovering static and varying degrees of dynamic correlation depending on the DFT functional of choice.

All calculations were performed in the ADF module within AMS2024.^34,35

Acknowledgments: The authors would like to acknowledge the staff of the REDC processing facility, the promethium production campaign team, and the radiological protection program at ORNL for their assistance in handling, monitoring, and preparation of samples. The authors would also like to thank Nikki Thiele for use of laboratory resources in preparing the radioactive samples.

Funding: This research is supported by the U. S. Department of Energy Isotope Program, managed by the Office of Isotope R&D and Production. TBV and ANG acknowledge the support of startup funds and Michigan State University.

Accession Codes: CCDC depositions 23663233-23663296 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Author Information:
Author Affiliations
Department of Chemistry, Michigan State University; East Lansing, MI, 48824, United States

Trenton B. Vogt, Alyssa N. Gaiser

Facility for Rare Isotope Beams, Michigan State University; East Lansing, MI, 48824, United States
Trenton B. Vogt, Alyssa N. Gaiser

Radioisotope Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee

Megan E. Simms, Connor J. Parker, April J. Miller, Laetitia H. Delmau, Samantha K. Cary, Cristian Celis-Barros, Frankie D. White

Nuclear Energy and Fuel Cycle Division, Oak Ridge National Laboratory; Oak Ridge, TN, 37830, United States

Richard T. Mayes

Contributions
T.B.V, M.E.S, C.J.P, S.K.C, F.D.W executed the synthesis sample preparations, and characterizations. A.J.M., L.H.D., R.T.M performed the isotope processing and dispensing. C.C.B. performed computations. A.N.G, F.D.W conceptualized the project.

Correspondence
Cristian Celis-Barros, [email protected]
Frankie D. White, [email protected]

Ethics Declarations: The authors declare that they have no competing interests.

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PmCmTerpyV1SI.docx
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checkcif files NCHEM-24071824
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cif files NCHEM-24071824

Probing the Subtle Differences between Promethium and Curium

Status:

Version 1

Abstract

Figures

Main

Results and Discussion

Discussion

Conclusion

Methods

Synthesis of Crystal Structures

Single Crystal X-ray Diffraction

Single Crystal Spectroscopy

Computations

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1