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. 2026 Mar 2;65(10):5299–5305. doi: 10.1021/acs.inorgchem.5c04674

Structural Analysis of Curium, Promethium, and Early Lanthanides Using 2,2′:6′,2″-Terpyridine in the Presence of Acid

Trenton B Vogt †,‡, Megan E Simms §, Alyssa N Gaiser †,‡,*, Frankie D White §,*
PMCID: PMC12997161  PMID: 41771045

Abstract

Trivalent lanthanides and actinides are difficult to separate, often relying on subtle differences in bonding between the 4f– and 5f–orbitals. However, trivalent Cm poses additional complexity owing to its half-filled 5f7 shell. This configuration causes Cm to have very similar properties to several of the early lanthanides, more specifically, similarly sized Pm. In this study, similarities were probed through isolating an isostructural series between the early lanthanides (La–Eu) and Cm with the oligoamine 2,2′:6′2″-terpyridine in the presence of acid. Single-crystal X-ray diffraction studies highlighted the similarities of Pm and Cm under these conditions and further demonstrated the difficulty in separating these elements. Despite the similarities, differences exist between interactions of Pm and Cm with terpyridine in the solid state. Interestingly, Cm appears to be more similar to Sm than to Pm. Furthermore, this study continues to probe the fundamental chemistry of Pm and Cm with respect to slight differences that can be leveraged in their separation from each other.

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Introduction

The use of the oligopyridine 2,2′:6′,2″-terpyridine (Terpy) is ubiquitous across the periodic table. First isolated in 1932, the nearly planar tridentate cavity of the ligand, in addition to its interesting photochemical and photophysical properties, led to Terpy and its derivatives being used in catalysis, supramolecular structure construction, and ion recognition. − More recently, Terpy and other N-donor ligands have been the subject of ongoing investigations for potential use in the separation of trivalent lanthanides (Ln) and actinides (An), specifically in the context of spent and used nuclear fuel (SNF and UNF) reprocessing. , Separations of f-elements involving irradiated material typically employ a solvent extraction process due to the scalability of the technique and the presence of acidic solutions commonly used in the initial dissolution of the irradiated material. Within this process, the selective separation of similarly sized Ln and An ions is challenging and typically requires leveraging the slightly more diffuse 5f-orbitals of the An ions to participate in partially covalent interactions, whereas Ln–ligand interactions are almost exclusively electrostatic in nature. − N-donor ligands are an attractive option for this application because they are softer Lewis bases than traditional O-donor amide, phosphate, or phosphine oxide functionalities used in the DIAMEX, PUREX, or TRUEX processes, respectively. These softer N-donors have a greater affinity for the softer Lewis acid An cations under the basic principles of traditional hard–soft acid–base (HSAB) theory and should allow for a greater separation from harder Ln cations. −

Initial studies highlighted the effectiveness of N-containing ligands in solvent extraction processes; however, the identity of the extracted complexes was often unknown. , Furthermore, several studies identified the ability of oligoamines and other N-containing extractant molecules to exhibit multiple protonation states in the presence of low acid concentrations, adding complexity to the solvent extraction process. − In one case, the presence of 2-bromodecanoic acid synergistically interacted with Terpy to selectively extract Am­(III) over Eu­(III) with a separation factor (SFAm/Eu) of 7. Further investigations using Sm identified the presence of a doubly protonated Terpy moiety ([H2Terpy]2+) in solution during the complexation of the metal cation, resulting in the complex [H2Terpy­(NO3)]­[Sm­(Terpy)­(NO3)4] being isolated in the solid state. To date, direct comparisons of this system with An to probe the origin of selectivity for Am­(III) over Eu­(III) in the solid state are scarce. Previously, a similar study investigated the selective complexation of U­(III) using Terpy with the nearly identical-sized Ce­(III) (SFU/Ce = 3.0) and Nd­(III) (SFU/Nd = 2.5). In the solid state, a significantly shorter average U–N(Terpy) bond length was indicative of a stronger metal–ligand interaction attributed to the involvement of π back-bonding in the U complex that is absent in the 4f metal complexes. Investigations into solid-state bond lengths have additionally been used to identify deviations from purely electrostatic bonding models for U­(III) in isostructural complexes between U­(III), La­(III), and Ce­(III), leading to a shorter U–N(Terpy) bond length and manifesting as a higher separation factor for U­(III). While these examples provide a clear comparison between similarly sized 4f and 5f ions, the early actinides are known for their ability to participate in bonding with a significant degree of covalency, whereas later An­(III) (Am–Cf) bonding tends to remain largely electrostatic in the absence of high pressure. ,

This similarity in bonding between 4f and later 5f metals is demonstrated by the separation of Cm from Pm. This separation currently requires several solvent extraction steps to remove even trace amounts of Cm during the isolation of pure 147Pm. Pm and Cm share much of the same chemistry owing to predominantly existing as trivalent ions in solution and possessing identical ionic radii (+III, VI-coordinate = 0.97 Å). The difficulty in their separation is compounded when considering Cm­(III), which is redox-inactive and resists forming partially covalent interactions due to the stability of the half-filled 5f7 shell, causing it to act very similarly to Ln­(III) ions in solution. To develop effective separation techniques, we must probe the fundamental coordination environment of Pm­(III) and Cm­(III) for slight differences under the same conditions. This study aimed to act as a continuation of a previous study in the presence of acid to better simulate the conditions of solvent extraction. The results of this study present solid-state bond distances obtained from single-crystal X-ray diffraction (scXRD), which allowed for a direct comparison of Pm­(III) and Cm­(III) in an isostructural system. Single crystals of several early lanthanides (La - Eu) were also isolated to develop the presence of Pm in the Ln series, which is often omitted due to its radioactivity. Fundamental differences between Pm and Cm were identified through further characterization using spectroscopic techniques.

Results and Discussion

Synthesis and Crystallography

A series of seven new complexes of the formula [(H2Terpy)­(NO3)]­[M­(Terpy)­(NO3)4] (M = La–Pm, Eu, Cm; MH 2 Terpy) were prepared in a modified fashion from that previously seen in the literature for Sm. To better model the typical conditions of f-block solvent extraction, HNO3 was used to isolate all compounds instead of the 2-bromodecanoic acid used in the previous study. To continue developing the position of Pm among adjacent Ln elements, previously isolated SmH 2 Terpy was also isolated using this modified procedure. Crystals suitable for scXRD were grown overnight from an acetonitrile solution containing a 1:1 molar ratio of the M­(III) ion and Terpy that was pretreated with 1.5 equiv of dilute HNO3 (1 M). The crystals grew as large blocks in the monoclinic P2 1 /n space group. The metal site is 11-coordinate and consists of one tridentate Terpy ligand and four bidentate NO3 – anions, forming an anionic complex (Figure ). The cocrystallized cation is made up of a [H2Terpy]2+ cation coordinating a single O of an NO3 – anion through H-bond interactions. This doubly protonated Terpy moiety has been seen several times in solid-state investigations with the Ln and An elements. ,,− Interestingly, in all but one investigation, the doubly protonated [H2Terpy]2+ cation coordinates to counterions present in the metal starting material (Cl–, − NO3 – , − ) or the solvent used for crystallization (H2O, ,− MeCN). Alternatively, in one case, the doubly protonated ligand instead complexes with an f-block metal site ([Eu­(NO3)5]2–) through H-bond interactions with the terminal O of a coordinated NO3 –.

1.

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Crystal structure of [(H2Terpy)­(NO3)]­[147Pm­(Terpy)­(NO3)4] (PmH 2 Terpy) shown at 50% thermal ellipsoid probability. H atoms bonded to C atoms have been removed for clarity. H-bonding interactions in the [(H2Terpy)­(NO3)]+ moiety are denoted with a red dotted line. Gray = C, white = H, blue = N, red = O, pink = Pm.

The metal site of these MH 2 Terpy compounds is similar to the [M­(Terpy)­(NO3)4]− (M = Nd–Sm, Cm; MTerpy) anion published previously, differing only in the denticity of the equatorial NO3 – anion (Figure ). This difference in denticity causes a significantly longer average M–N(Terpy) bond length in the MH 2 Terpy compounds when compared with MTerpy compounds, which is consistent with the difference of approximately 0.06 Å between 10- and 11-coordinate Ln–Terpy complexes. This increased denticity leads to a greater steric interaction between coordinated NO3 – groups when considering the range of M–O(NO3) bond lengths as the Ln series is traversed (Figure S3). This range is the greatest in EuH 2 Terpy, where the longest M–O(NO3) bond is 0.2658(16) Å longer than the shortest M–O bond. Interestingly, the longest M–O(NO3) bond is seen in the equatorial NO3 – trans to the central N of the Terpy ligand for each complex presented here. Additionally, when considering the difference in steric interaction between Pm and Cm, due to both ions sharing the same ionic radii, they would be expected to have a similar range of M–O(NO3) bonds; however, in PmH 2 Terpy, that range is 0.2239(39) Å, whereas in CmH 2 Terpy, it is significantly longer with a range of 0.2420(60) Å. Despite the great variability in the M–O distances, there is very little distortion in the Terpy ligand across the series (Figure S4). When considering only the nonradioactive MH 2 Terpy compounds, the Terpy ligand coordinated to the metal site is nearly planar with the two N–C–C–N dihedral angles ranging from 5.71° to 6.2° and from 4.73 to 5.27°. There is a greater deviation from planarity in the [H2Terpy]2+ cation with N–C–C–N dihedral angles ranging from −3.61° to −3.82° and from 13.2° to 13.49° when again considering only the stable MH 2 Terpy compounds. This range of Terpy dihedral angles fit the literature values for [H2Terpy­(NO3)]­[Sm­(Terpy)­(NO3)4] well; however, in all cases, either PmH 2 Terpy, CmH 2 Terpy, or both have statistically significantly different dihedral angles. In the [H2Terpy]2+ cation, similar or less distortion was seen in the dihedral angles of Pm (−3.9(3)°, 11.0(3)°) and Cm (−3.1(4)°, 11.9(4)°) compounds than in those of the stable Ln compounds. Most of the dihedral angles of the coordinated Terpy ligand for Pm(6.3(3)°, 3.8(3)°) and Cm(6.8(4)°, 5.2(4)°) are also outside of the range defined by the stable Ln compounds. These differences could be due to slight radiolytic degradation occurring in the crystals of PmH 2 Terpy and CmH 2 Terpy that is absent in the stable Ln analogs.

2.

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(a) PmH 2 Terpy metal site presented in this study and (b) [Pm­(Terpy)­(NO3)4]− (PmTerpy) anion published previously. Hydrogen atoms have been removed for clarity. Gray = C, blue = N, red = O, and pink = Pm. (c) Average M–N(Terpy) bond lengths in the MH 2 Terpy metal site (black) and the MTerpy anion (red) (M = Nd–Sm, Cm). (d) The M–N(Terpy) bond lengths to the central N atom (Nc, black) in Terpy and the average between the two distal N atoms (Nd, red) in MH 2 Terpy (M = Nd–Sm, Cm) compounds. Standard deviations in the bond lengths are displayed as error bars.

To develop the presence of Pm in the Ln series, the adjacent elements, Nd and Sm, are discussed in detail in addition to Cm. The average M–N(Terpy) bond distances for NdH 2 Terpy and SmH 2 Terpy were 2.648(7) Å and 2.623(13) Å, respectively (Figure ). In the case of PmH 2 Terpy, the average distance was 2.644(9) Å. This distance unsurprisingly falls between those observed in NdH 2 Terpy and SmH 2 Terpy. In comparing this interaction with CmH 2 Terpy, the softer 5f-metal cation was expected to display significantly stronger bonding to the moderately soft Terpy ligand over the harder 4f-metal cation with the same predicted ionic radius as seen in the previous investigation between Pm and Cm (Figure ). Despite this, the average M–N bond distance for CmH 2 Terpy was 2.625(12) Å, which is not statistically significantly different from that of PmH 2 Terpy when considering error in the measurement, which is reflective of the difficult Pm/Cm separation.

When the bond lengths between M­(III) ions and the two distal N atoms (Nd) are considered separately from the central N atom (Nc), a trend develops across the Ln series for this system. For the early MH 2 Terpy (M = La, Ce) compounds, Nc > Nd; however, this trend flips at PrH 2 Terpy, where Nd > Nc and continues to diverge as the series is traversed (Figure S5). When considering the differences in the interaction between Terpy and Pm or Cm, both Nd and Nc are significantly longer for Pm than for Cm (Figure ). Additionally, when considering the adjacent Ln elements to Pm, both Nd and Nc are more similar between Pm and Nd, whereas both values are significantly shorter in SmH 2 Terpy. Furthermore, a difference in bonding is seen when the out-of-plane distance between the central pyridine ring of Terpy and the metal center. The early Ln­(III) ions are too large to sit in the plane of this central pyridine ring of Terpy, whereas the later Ln­(III) ions are smaller and able to better interact in the same plane of this central pyridine (Figure S6). For PmH 2 Terpy, this out-of-plane distance is 0.534(15) Å, whereas in CmH 2 Terpy, this distance is only 0.500(17) Å. Thus, the interaction between Cm­(III) and Terpy is more similar to that of the smaller Ln­(III) ions in SmH 2 Terpy [0.506(16) Å] and EuH 2 Terpy [0.499(17) Å]. Conversely, the out-of-plane interaction between Pm­(III) and Terpy is more similar to the larger Pr­(III) ion in PrH 2 Terpy [0.540(15) Å] and the Nd­(III) ion in NdH 2 Terpy [0.527(17) Å].

Spectroscopy

Solid-state ultraviolet–visible (UV–vis) spectra were collected for all of the MH 2 Terpy compounds (Figure S1). Each spectrum is dominated by the allowed intraligand π–π* transition of Terpy, with the Laporte-forbidden metal f–f transitions occurring at lower energies and lower intensities. The spectra for PmH 2 Terpy and CmH 2 Terpy are shown in Figure . The spectra for NdH 2 Terpy and SmH 2 Terpy are also included to provide stable analogs to Pm and Cm.

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(a) Reaction between Terpy in the presence of HNO3 and 147Pm­(III) formed a faintly pink solution. (b) Faintly yellow CmH 2 Terpy crystals growing out of a pink solution formed during the reaction between 248Cm­(III) and Terpy in the presence of HNO3. (c) Solid-state absorption spectrum of MH 2 Terpy (M = Nd–Sm, Cm) compounds. Inlaid are pictures of the crystals of each compound with a 50 μm scale for reference.

The spectrum of NdH 2 Terpy appeared as expected with the major Nd-based absorption bands occurring at 527, 579, 740, and 795 nm, giving rise to the slight purple color of the crystal (Figure ). The f–f transition with the largest intensity here occurs at 579 nm and is one of the more absorbing transitions within the Ln elements and provides a qualitative assessment of the purity of Pm. , While Nd has several absorption peaks in the same range as Pm (500–600 nm), none of these peaks are centered closer than 9 nm from the center of any Pm­(III)-based absorption peaks in PmH 2 Terpy. The most intense of these peaks occurs at 568 nm, with less intense peaks surrounding this peak at 546, 548, and 588 nm, which match well with the literature and match the previously reported PmTerpy structure. ,− These peaks give rise to the pink color seen in the reaction solution (Figure ). These peaks are also well resolved from the Sm­(III)-based absorption peak of SmH 2 Terpy appearing at 404 nm. ,

Interestingly, during the reaction to form CmH 2 Terpy, immediately upon addition of hydrated Cm­(NO3)3, the solution became faintly pink (Figure ). This was not seen for any of the Ln elements other than in PmH 2 Terpy where the pink color originates from the Pm­(III)-based f–f transitions. Upon crystallization of CmH 2 Terpy, this pink color is lost, and the crystals instead appear faintly yellow. In contrast to Pm­(III), Cm­(III) f–f transitions are higher in energy with the most intense peaks appearing at 378, 384, and 400 nm, and two less intense transitions occur at 437 and 456 nm, which are in good agreement with previous studies. − Cm­(III) also has a characteristic red-orange emission peak that occurs at 614 nm (Figure S2), which was confirmed when exciting with 365 nm light and was in good agreement with the literature. −

Despite the pink crystallization solution, the crystals of PmH 2 Terpy appeared brown when viewed under a microscope. The crystals of CmH 2 Terpy also appeared to show some yellowing when compared to the clarity seen in the stable Ln analogs. Previously, M–Terpy compounds incorporating high-activity β– emitting isotopes (147Pm, 249Bk) have seen similar discoloration, typically accompanied by a broadening of Terpy-based π–π* transition and a bathochromic shift attributed to the radiolytic degradation of Terpy. , Here, these crystals appear to have undergone some degradation, leading to the color changes; however, the high-energy region of the PmH 2 Terpy absorption spectrum matches those of the stable Ln analogs in NdH 2 Terpy and SmH 2 Terpy.

Conclusions

Currently, several solvent extraction steps are required to remove the trace Cm contaminant present during the production of 147Pm at the US Department of Energy’s Oak Ridge National Laboratory (ORNL), whereas several other Ln–An separations have been achieved by leveraging the ability of An elements to form partially covalent interactions, due to the half-filled 5f 7 shell of Cm, the difference in the bonding between Pm and Cm with the moderately soft N-donor Terpy remains small. Despite this, in the solid state, there are now multiple examples of significant differences between Pm and Cm when using a softer donor ligand. Thus, it appears that with careful ligand consideration, a separation between Pm and Cm can be rooted in the basic principles of HSAB theory despite the stability of the 5f7 shell of Cm. Further studies investigating Pm and Cm are needed to corroborate these trends and determine the contributing factors leading to the slight differences presented here. In developing the often-omitted position of Pm among the Ln series, the M–Terpy interaction between Pm and Nd is very similar with respect to M–Nd, M–Nc, out-of-plane distance, and average M–N(Terpy) distance. Conversely, there is a significant difference between Pm and Sm with respect to M–Nd and M–Nc, albeit small.

The synthesis of the reported compounds under mildly acidic conditions provided insight into the multitude of conditions that can impact the effectiveness of f-block separations, such as consumption of the extractant by protons. Furthermore, the data presented here continue to support the concept of shorter bond distances in An elements when compared with Ln elements of identical ionic radii using softer donor ligands, which also continues to support the theory that 5f elements form bonds with more partial covalent character than the 4f elements. Despite the 5f7 electronic configuration of Cm­(III), this trend continues when compared to that of Pm­(III), and the data presented here provide another example of how Pm and Cm fit within f-element trends.

Experimental Section

Caution! 248Cm (t 1/2 = 3.48 × 105 years) and 147Pm (t 1/2 = 2.62 years) are radioactive in nature and represent a serious health hazard. Additionally, 147Pm represents a potential health risk due to its β– emission and high specific activity (928.4 Ci/g). All experiments involving these elements were conducted either in fume hoods or in negative-pressure gloveboxes, each equipped with high-efficiency particulate air (HEPA) filters under the supervision of licensed radiological control technicians. 147Pm and 248Cm (99.4% purity) were received at the ORNL Radiochemical Engineering Development Center (REDC) as hydrated nitrate salts. All reagents were used as received from the manufacturer. No attempt was made to exclude air or moisture from the reactions.

Pm Purification

Due to the decay of 147Pm with a half-life of 2.62 years to 147Sm (t 1/2 = 1.073 × 1011 years), it is imperative to perform a purification before being used. This purification was achieved using a solvent ionic-liquid-impregnated (SILI) resin with N,N-dioctyldiglycolamic acid (DODGAA) as the extractant, which was previously used for the nearly baseline separation of the stable light Ln elements. The load solution containing 147Pm was prepared by dissolving all solids received in 6 M HNO3 before transferring to a 0.8 cm × 4 cm Bio-Rad Poly-Prep chromatography column containing a 2 mL bed volume (BV) of the SILI resin pretreated with 6 M HNO3. The first 5 BVs of 6 M HNO3 added to the column after the load solution showed no activity and were thus discarded. The next 3 BVs were collected and sampled for activity at the Transuranic Analytical Laboratory at REDC using liquid scintillation counting. The ratio of 147Pm/147Sm was obtained by sending a sample to the Radioactive Material Analytical Laboratory at ORNL where liquid chromatography–mass spectrometry was used. The fraction used for this work was determined to have a purity of >99% (±0.1%) when compared with any Sm contaminants. The next 4 BVs of 6 M HNO3 were collected as one fraction, sampled, and determined to also have a purity of >99% (±0.1%). To avoid any significant Sm contamination, the remaining activity was stripped with 3 M HNO3 until the elution fractions no longer contained detectable levels of activity (10 BVs).

Synthesis

(H2Terpy)­(NO3)­[M­(Terpy)­(NO3)4] (M = La–Nd, Sm, Eu)

The synthesis of each lanthanide was performed by mixing 192 μL of a 25 mM solution of 2,2′:6′2″-terpyridine (1.1 mg, 4.8 μmol) in acetonitrile with 7.2 μL of a 1 M HNO3 solution (1.5 equiv). To this mixture was layered 48 μL of a 0.1 M solution of the respective hydrated lanthanide nitrate salt (4.8 μmol) in acetonitrile on top. Crystals suitable for scXRD grew in several hours or overnight as large blocks.

(H2Terpy)­(NO3)­[M­(Terpy)­(NO3)4] (M = Pm, Cm)

The synthesis for Pm and Cm was performed identically to that described previously apart from the scale. Both were synthesized using 0.5 mg of the respective M­(III) metal cation; thus, the reagents were scaled to use the equivalent number of moles on this scale. In PmH 2 Terpy, pink, block crystals grew within 6 h but quickly turned brown likely due to radiation damage. Similarly, in CmH 2 Terpy, faint yellow crystals grew overnight but quickly gained a brown tint.

Crystallographic Studies

X-ray diffraction studies were conducted on a Bruker D8 Venture instrument equipped with an Iμs 3.0 Mo source (λ = 0.71073 Å). Apex4 software was used for data collection and unit cell determination. Crystal structures were determined using the SHELX software within the OLEX2 graphical user interface. − Single crystals for Ln elements (excluding Pm) were coated in immersion oil before being mounted on a MiTeGen Cryoloop. Due to the radioactive nature of Pm and Cm, a single crystal of each respective compound was first isolated in immersion oil before being mounted to a MiTeGen Cryoloop using quick-setting epoxy to reduce the possibility of contamination. Once the epoxy had set, the MiTeGen loop was surveyed for external contamination before being placed into the scXRD instrument. PmH 2 Terpy data were collected at room temperature because of a cryostream malfunction at the time of data collection. All other crystal data were collected at 100 K. Non-H atoms were identified in the Fourier difference map and refined anisotropically. H atoms bound to C atoms were added at calculated positions and not refined further. H atoms bound to an N atom were located in the Fourier difference map and refined freely for all compounds other than CmH 2 Terpy, in which all H atoms were added at calculated positions.

Single-Crystal Spectroscopy

Solid-state UV–vis data were collected on a CRAIC Technologies 2030PV Pro microspectrophotometer. Single crystals of MH 2 Terpy were placed in Type NVH immersion oil on a glass microscope slide, and data were collected from 300 to 900 nm. The exposure time was autooptimized by the Craic Lambdafire software. The emission spectrum for CmH 2 Terpy was collected by exciting at a single wavelength (365 nm).

Supplementary Material

ic5c04674_si_001.pdf (495.9KB, pdf)

Acknowledgments

This research was supported by the U S Department of Energy Isotope Program, managed by the Office of Isotope R&D and Production (F.D.W.). The authors also acknowledge the support of start-up funds and Michigan State University (A.N.G.). The authors would like to acknowledge the staff of the REDC processing facility, the Pm 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 Dr. Nikki Thiele for the use of laboratory resources in preparing the radioactive samples.

Glossary

Abbreviations

BV

bed volume

DODGAA

N,N-dioctyldiglycolamic acid

HEPA

high-efficiency particulate air

HSAB

hard–soft acid–base

ORNL

Oak Ridge National Laboratory

REDC

Radiochemical Engineering Development Center

scXRD

single-crystal X-ray diffraction

SILI

solvent ionic-liquid-impregnated

Terpy

2,2′:6′,2″-terpyridine

UV–vis

ultraviolet–visible

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.5c04674.

  • Spectroscopic measurements, additional bond length data, and crystallographic tables (PDF)

T.B.V and M.E.S carried out synthesis and characterization. T.B.V., F.D.W., and A.N.G. conceptualized the project. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

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