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Article

Neptunyl(VI) Nitrate Coordination Polymer with Bis(2-pyrrolidone) Linkers Highlighting Crystallographic Analogy and Solubility Difference in Actinyl(VI) Nitrates

by
Tomoyuki Takeyama
1,
Juliane März
2,
Ryoma Ono
1,
Satoru Tsushima
2,3 and
Koichiro Takao
1,*
1
Laboratory for Zero-Carbon Energy, Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1 N1-32, O-okayama, Meguro-ku, Tokyo 152-8550, Japan
2
Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstraße 400, 01328 Dresden, Germany
3
International Research Frontiers Initiative (IRFI), Institute of Innovative Research, Tokyo Institute of Technology, 2-12-1 N1-32, O-okayama, Meguro-ku, Tokyo 152-8550, Japan
*
Author to whom correspondence should be addressed.
Inorganics 2023, 11(3), 104; https://doi.org/10.3390/inorganics11030104
Submission received: 6 February 2023 / Revised: 25 February 2023 / Accepted: 27 February 2023 / Published: 1 March 2023
(This article belongs to the Special Issue Inorganics: 10th Anniversary)
Browse Figures
Versions Notes

Abstract

:
NpO2(NO3)2 units are connected by bis(2-pyrrolidone) linker molecules with the trans-1,4-cyclohexyl bridging part (L1) to form a one-dimensional coordination polymer, [NpO2(NO3)2(L1)]n. Molecular and crystal structures of this compound are nearly identical to that of the UO22+ analogue, while its aqueous solubility is greatly enhanced, probably owing to weaker thermodynamic stability of the complexation in NpO22+ compared with that in UO22+.

Graphical Abstract

1. Introduction

In nuclear fuel recycling, spent nuclear fuels are dissolved in HNO3(aq), and then subjected to separation and purification of fissile materials such as U and Pu. This process is called reprocessing, and constitutes one of the most important steps in the nuclear fuel cycle [1]. While the plutonium uranium redox extraction (PUREX) process, based on solvent extraction, is well-known as a standard approach for this purpose, a variety of other reprocessing methods based on wet chemical and pyrochemical bases have also been proposed. Our group is also developing a new precipitation-based reprocessing called nuclear fuel materials selective precipitation, NUMAP [2], where N-alkylated 2-pyrrolidone derivatives (NRPs, Figure 1) are employed for efficient and simultaneous precipitation of U and Pu from HNO3(aq) [3,4]. In the NUMAP approach, Pu will be deliberately recovered together with U for the sake of securing nuclear proliferation resistivity. This is in strong contrast to PUREX, which has been developed to isolate Pu for nuclear weapons. Similar precipitation-based reprocessing concepts are also proposed by other groups [5,6].
Understanding the coordination chemistry of early actinides with NO3− as well as with selected ligands like NRPs is highly essential to establish foundations of chemical separation in any wet chemical reprocessing processes. The early actinides such as U, Np, and Pu exhibit rich redox chemistry, with common oxidation states varying between III and VI [7]. In penta- and hexavalent states, actinyl ions (AnO2m+; An = U, Np, Pu; m = 1, 2) are formed, where the equatorial plane is offered for additional coordination of extra ligands, as exemplified by AnO2(NO3)2(NRP)2 in Figure 1. By exploiting such uniqueness in coordination chemistry, we extended the molecular design of NRPs to double-headed ones, DHNRPs (Figure 1), to form a much less-soluble [UO2(NO3)2(DHNRP)]n coordination polymer to further improve the recovery efficiency of UO22+ precipitate [8,9,10].
In the reprocessing process, Pu is intended to be recovered together with UO22+, whereas Np should be removed from them. In the first phase of NUMAP development, hydrophobic NRP-like NCP (Figure 1) has been preferably employed to achieve high recovery yields of UO22+ and PuO22+ [4,11,12,13,14]. Therefore, we are now strongly motivated to explore how NpO22+ and PuO22+, the heavier analogues of UO22+, react with DHNRPs in HNO3(aq). While the stability of hexavalent states of Np and Pu are generally decreasing, more common oxidation numbers for them are V and IV, respectively. Nevertheless, we are still interested in determining the coordination behavior of the AnO22+ family with DHNRPs, through which chemical analogy may be observed. Accordingly, it is valuable to first gain knowledge about what happens on NpO22+, which is only allowed to be handled in a dedicated facility, in order to judge whether or not it is worth studying PuO22+ in this direction. Herein, we have assessed the structural chemistry of an NpO22+ coordination polymer with NO3− and DHNRPs (L1, rac-L2), as well as its solubility, in 3 M HNO3(aq), typically used in the reprocessing process.

2. Results and Discussion

A 3 M HNO3 solution dissolving 100 mM NpO22+ (25 μL) was layered with a blank 3 M HNO3(aq) (50 μL) and that containing 200 mM L1 (12.5 μL) from bottom to top in a glass tube (ϕ5 mm) inserted into a 2 mL glass vial, where NpO22+:L1 = 1:1. This layered mixture was stored in a silent place overnight in a glovebox dedicated for Np experiments. Through natural diffusion of the components in the solution phase, yellowish-brown crystals suitable for X-ray crystallography deposited, as shown in Figure 2. A γ-ray spectrometry of the supernatant allowed to know that this crystalline deposit contained Np and that its yield was 44% based on the initial loading of NpO22+.
Molecular and crystal structures of this compound were determined by single crystal X-ray analysis at 293 K. As a result, this compound shown in Figure 3 was found to consist of NpO2(NO3)2 units connected by L1 linker molecules to form an infinite one-dimensional (1D) coordination polymer, [NpO2(NO3)2(L1)]n, which should be formed through the following reaction in the HNO3 solution.
n NpO22+ + 2n NO3− + n L1 = [NpO2(NO3)2(L1)]n(cr),
While we have mentioned above that Np exhibits a wide variety of oxidation numbers from III to VI, or even to VII, as one of unique trends of early actinide elements [7,15,16], its hexavalent oxidation state remained unchanged in the current system.
Crystallographic data of [NpO2(NO3)2(L1)]n are summarized in Table 1. This coordination polymer crystallized in triclinic P−1 with a = 6.0172(2) Å, b = 7.6661(2) Å, c = 11.0985(3) Å, α = 75.9330(10)°, β = 84.488(2)°, and γ = 76.353(2)°. A volume of its unit cell (V) was 482.18(2) Å3. As shown in this table, all of the crystallographic data of the current compound are almost identical to those of its UO22+ analogue, [UO2(NO3)2(L1)]n (triclinic P–1, a = 5.8962(4) Å, b = 7.6330(6) Å, c = 11.0279(9) Å, α = 76.841(5)°, β = 84.480(6)°, γ = 76.229(5)°, and V = 468.93(6) Å3), we reported previously [9]. Both UO22+ and NpO22+ form isomorphous crystalline phases of [AnO2(NO3)2(L1)]n because of the chemical similarity of AnO22+ ions.
As shown in Figure 3, the central Np atom is surrounded by two axial O atoms of NpO22+ (Oax) and by six equatorial O atoms of two bidentate NO3− ions (ONO3) and two monodentate 2-pyrrolidone moieties of the different L1 molecules (OL) to afford a hexagonal bipyramidal coordination polyhedron. The asymmetric unit comprises only one Np, one Oax, one NO3−, and half of L1, which is expanded by an inversion center (symmetry operation: (i) 1 − x, 1 − y, −z) to complete a whole structure of the [NpO2(NO3)2(L1)] monomer. As a result, these sets of ligands are located at trans-positions, which is rather ordinarily found in actinyl(VI) nitrates, as reviewed elsewhere [17,18].
Selected structural parameters of [NpO2(NO3)2(L1)]n were summarized in Table 2 together with those of [UO2(NO3)2(L1)]n [9]. The Np≡Oax bond length (Np(1)−O(1)) is 1.744(3) Å, while longer distances can be found in Np−ONO3 (Np−O(2), Np−O(3): mean 2.56 Å) and Np−OL (Np−O(5): 2.341(2) Å) interactions formed in the equatorial plane. The Oax≡Np≡Oax bond angle in NpO22+ is exactly 180° owing to its centrosymmetric structure in this compound. These structural data are common to those observed in NpO2(NO3)2(OPPh3)2 (Np≡Oax: 1.739(10) Å, Np−ONO3: 2.53 Å (mean), Np−OOPPh3: 2.363(8) Å), which is a trans-dinitrato NpO22+ complex with two monodentate ligands solely known so far [19]. There are several related structures in which the equatorial coordination is constrained in a cis-geometry by additional bidentate ligands, such as 2,2′-bipyridyl (bpy) and BrCH2COO−, or by dimerization through two μ-OH− bridges [20,21,22]. Regardless of such cis-trans isomerization, the Np≡Oax distance of the current compound (1.739(10) Å) is not very different from the reported cis-structures (1.73–1.76 Å). In contrast, it seems to be difficult to directly compare the Np−ONO3 interactions in [NpO2(NO3)2(L1)]n in the trans-geometry (mean: 2.53 Å) to those in the known cis-complexes owing to the wider variety in Np−ONO3 lengths of the latter series from 2.47 Å to 2.56 Å.
As shown in Table 2, [NpO2(NO3)2(L1)]n is isostructural with [UO2(NO3)2(L1)]n in terms of molecular structure as well, where the contribution of the actinide contraction is not clearly observed. Charushnikova et al. reported a molecular structure of [N(CH3)4][NpVO2(NO3)2(H2O)2], where Np is pentavalent [23]. Although the coordination geometry around its Np5+ center was also hexagonal bipyramidal in a similar manner to the current compound, the bond distances of this Np(V) complex (Np≡Oax: 1.823(1) Å, Np−ONO3: 2.62 Å (mean), Np−OH2O: 2.473(1) Å) are critically different from those in Table 2. The significantly shorter interactions around the Np center of [NpO2(NO3)2(L1)]n reported here indicate that its hexavalence is maintained.
Each crystal lattice of this compound contains a [NpO2(NO3)2(L1)] monomer, which polymerizes along the c-axis (Figure 3, Figure 4 and Figure S1). In the directions of a- and b-axes, the [NpO2(NO3)2(L1)]n coordination polymers stack each other through C−H···O interactions between the independent 1D chains (D⋯A: C⋯Oax = 3.46–3.55 Å, C⋯ONO3 = 3.28–3.63 Å, C⋯OL = 3.61–3.65 Å; D−H⋯A: C−H⋯Oax = 2.58–2.60 Å, C−H ⋯ONO3 = 2.42–2.70 Å, C−H⋯OL = 2.68–2.69 Å). These packing trends are also common to those of [UO2(NO3)2(L1)]n [9].
Previously, we defined a mean volume of one C atom of an alkyl group (R, Figure 1) of NRPs as a compactness parameter (CP, Å3) as follows [12].
CP = (V/Z − V0/Z0)/2NC,
where V and Z are the cell volume and the number of UO2(NO3)2(NRP)2 units in a crystal lattice, respectively. The subscript “0” indicates those of UO2(NO3)2(2-pyrrolidone)2, where NRP with R = H (Figure 1) is employed. NC is the number of C atoms in R. CP can be compared as a measure of packing efficiency of UO2(NO3)2(NRP)2, and is an important factor to govern the solubility of UO22+ in HNO3(aq), i.e., recovery yield of U in our NUMAP reprocessing for the nuclear fuel recycling. Thereafter, this concept was expanded to the coordination polymers of [UO2(NO3)2(DHNRP)]n, where half of the number of C atoms in R’ of DHNRP (Figure 1) was taken as NC owing to its stoichiometric ratio [2,9]. Based on the structural similarity with the UO22+ analogue discussed above, CP of [NpO2(NO3)2(L1)]n was calculated to be 16.4 Å3 (V0/Z0 = 383.9 Å3 at 293 K) [9]. This is slightly greater than that of [UO2(NO3)2(L1)]n (CP = 15.6 Å3), but still much smaller than those of UO2(NO3)2(NRP)2 complexes (CP = 20.4 − 30.3 Å3). Therefore, the higher packing efficiency of NpO2(NO3)2 units is still maintained in [NpO2(NO3)2(L1)]n.
Up to now, we have not found any significant differences arising from variation in the hexavalent actinide center of [AnO2(NO3)2(L1)]n from the viewpoint of structural chemistry. Seemingly, such a similarity might allow us to anticipate co-crystallization of any AnO22+ ions from HNO3(aq) after loading L1 to form the heterometallic 1D coordination polymer. Nevertheless, we should reject this expectation, because the recovery yield of NpO22+ from 3 M HNO3(aq) was only 44% after loading an equimolar amount of L1 at 293 K, as described above. This means that the solubility of [NpO2(NO3)2(L1)]n is 16 mM under this condition, which is one order of magnitude higher than that of [UO2(NO3)2(L1)]n (2.48 mM) [9]. It is rather close to that of UO2(NO3)2(NRP)2 (R = cyclohyexyl, 17.6 mM, CP = 21.4 Å3) [2]. Here, we found a uniqueness of NpO22+.
As mentioned above, the structural features cannot explain this gap. We wonder whether the difference in solubility of [AnO2(NO3)2(L1)]n between An = U and Np could be ascribed to the thermodynamic stability of the complexation around AnO22+. Indeed, stability constants (log β) of AnO22+ complexes in several ligand systems (CO32−, CH3COO−, F−) sequentially decrease with an increase in the atomic number of the center metal, as summarized in Table S1 [16,24]. We are also aware that such a trend is not always clear for other weaker ligands (Cl−, SO42−), and that log β of NO3− is only available for UO22+ despite the highest relevance to the current work. Although it is still difficult to know exactly why the solubility of [NpO2(NO3)2(L1)]n is unexpectedly higher than the UO22+ analogue, this knowledge provides an important implication to consider the recovery of PuO22+ using L1 from HNO3(aq) in our NUMAP reprocessing for nuclear fuel recycling [2].
Using rac-L2 (see Figure 1), we previously confirmed that [UO2(NO3)2(rac-L2)]n deposited from 3 M HNO3(aq) [9]. Therefore, we examined what happens in a mixture of NpO22+ with rac-L2 as follows. The aqueous solution with 3 M HNO3 and 100 mM NpO22+ (25 μL) was placed at the bottom of the ϕ5 mm glass tube, followed by subsequent loading of a blank 3 M HNO3(aq) (50 μL) and that dissolving 200 mM rac-L2 (12.5 μL), where NpO22+: rac-L2 = 1:1. Finally, nothing deposited even after several days passed. However, this result is not very surprising, because we have already known that rac-L2 affords a more soluble coordination polymer of [UO2(NO3)2(rac-L2)]n (14.8 mM) compared with [UO2(NO3)2(L1)]n (2.48 mM) [9]. Accordingly, the solubility of the NpO22+ analogue with rac-L2 would also be higher than that of L1 described above (16 mM). When the sample solution was fully mixed, the total concentration of NpO22+ was 28.6 mM. While there are no reasons to exclude the possibility of formation of the [NpO2(NO3)2(rac-L2)]n coordination polymer in a similar manner to [UO2(NO3)2(rac-L2)]n, its solubility in 3 M HNO3(aq) would be higher than 28.6 mM, the total concentration of NpO22+ loaded to the reaction mixture of the current experimental runs.

3. Conclusions

In this paper, we have succeeded in the synthesis and structure determination of the NpO22+ coordination polymer, [NpO2(NO3)2(L1)]n. While its molecular and crystal structures are nearly identical to those of the UO22+ analogue, a significant difference in aqueous solubility was observed between them. At this moment, we suppose that it is related to the thermodynamic stability of the formed complexes. Although there are only two cases of UO22+ and NpO22+, such a trend would be extendable to PuO22+. If so, the solubility of [PuO2(NO3)2(L1)]n should be too high to recover PuO22+ efficiently from the HNO3 feed solution dissolving the spent nuclear fuels. Furthermore, deposition of [NpO2(NO3)2(rac-L2)]n was unsuccessful, most probably because of its higher solubility compared with that of [NpO2(NO3)2(L1)]n. This trend in solubility would to be predicted to be enhanced with an increase in the atomic number. Therefore, recovery of Pu as PuO22+ using DHNRP will be less expectable.
In contrast, we have recently succeeded in the preparation of An4+ compounds with L1 or rac-L2 from HNO3(aq), where [An(NO3)6]2− (An = Th, U, Np) forms a sparingly soluble salt with anhydrous H+ countercations polymerized through hydrogen bonds with these DHNRPs [10,25,26]. The solubility of (HDHNRP)2[An(NO3)6] is only several millimolars, regardless of the difference in An4+ tested so far, where there is little periodic trend, unlike that of AnO22+ found in this work. In conclusion, it would be better to anticipate the recovery of Pu in its tetravalent form, Pu4+, which is the most common oxidation state of Pu in the feed solution after dissolution of the spent nuclear fuels in HNO3(aq). Accordingly, our next task is to examine whether or not Pu4+ indeed forms (HDHNRP)2[Pu(NO3)6] with solubility low enough for its recovery in nuclear fuel recycling.

4. Experimental

4.1. Materials

Caution! 237Np is a radioactive isotope emitting alpha radiation (specific activity: 2.63×107 Bq g–1 with T1/2 = 2.14 × 106 years). It has to be handled in dedicated facilities with appropriate equipment for radioactive materials to avoid health risks caused by radiation exposure. All of the operations in handling Np were performed in a dedicated glove box in the control area of HZDR. Furthermore, HNO3 used in this work should be used under great caution for chemical safety because of its strong acidity and oxidizing nature.
All reagents used were of reagent grade and used as received, if not specified. The L1 and rac-L2 ligands were synthesized as described below [9]. Neptunyl(VI) nitrate hydrate (NpVIO2(NO3)2·nH2O, n ~ 5) was prepared by dissolving NpO2 (29.8 mg, 0.111 mmol) in conc. HNO3 (aq.). This solution was refluxed for two days and concentrated to near to dryness on a hot plate. The final residue of NpVIO2(NO3)2·nH2O was dissolved in 1.11 mL of 3 M HNO3 (aq.) to prepare 100 mM NpVIO2(NO3)2 with 3 M HNO3 stock solution. The concentration of NpO22+ was determined by the γ-ray spectrometry.

4.2. Synthesis of L1

In a 200 mL round bottom flask, trans-1,4-cyclohexanediamine (1.225 g, 10.73 mmol, TCI), K2CO3 (6.10 g, 44.2 mmol, Wako), and THF (80 mL, Wako) were loaded. Under cooling on an ice bath and vigorous stirring, 4-chlorobutyryl chloride (2.45 mL, 3.09 g, 21.90 mmol, TCI) was slowly added through a dropping funnel. This mixture was stirred at 0 °C for 2 h, and then further reacted at room temperature overnight. After removal of the solvent by evaporation under reduced pressure, water (70 mL) was loaded to the residue, followed by sonication and stirring at room temperature. The colorless insoluble solid was recovered by filtration, washed with H2O several times, and dried under vacuum to obtain N,N’-trans-cyclohexane-1,4-diylbis(4-chlorobutanamide) (2.50 g, 72% yield). 1H NMR (δ/ppm vs. TMS, CDCl3, 399.78 MHz) 5.47 (NH, bs, 2H), 3.76 (N−CHax, br, 2H), 3.58 (CH2Cl, t, 4H), 2.33 (CH2, t, 4H), 2.11 (CH2, quintet, 4H), 2.01 (CHeq, d, 4H), 1.27 (CHax, td, 4H). 13C NMR (δ/ppm vs. TMS, CDCl3, 100.53 MHz) 171.03, 47.71, 44.67, 33.50, 31.79, 28.22.
This precursor (2.42 g, 7.44 mmol) was loaded in a 200 mL round bottom flask together with CH2Cl2 (70 mL), followed by cooling on the ice bath. To this solution, potassium tert-butoxide (1.85 g, 16.5 mmol) dissolved in THF (40 mL) was slowly added through a dropping funnel. The reaction mixture was stirred at 0 °C overnight. After passing through a Celite pad, the filtrate was concentrated by a rotary evaporator. Adding hexane (40 mL) to the residual oil under sonication allowed to form a colorless solid, which was recovered by suction filtration and dried under vacuum to obtain L1 (1.54 g) in 83% yield. 1H NMR (δ/ppm vs. TMS, CDCl3, 399.78 MHz) 3.97 (N−CHax, br, 2H), 3.33 (CH2, t, 4H), 2.39 (CH2, t, 4H), 2.00 (CH2, quintet, 4H), 1.77 (CHeq, d, 4H), 1.60 (CHax, td, 4H). 13C NMR (δ/ppm vs. TMS, CDCl3, 100.53 MHz) 171.03, 47.71, 44.67, 33.50, 31.79, 28.22.

4.3. Synthesis of rac-L2

In a 500 mL round bottom flask, trans-1,2-cyclohexanediamine (5.08 g, 44.4 mmol, TCI), triethylamine (8.99 g, 88.8 mmol, Kanto), and THF (200 mL, Wako) were loaded. Under cooling on an ice bath and vigorous stirring, 4-chlorobutyryl chloride (9.95 mL, 12.5 g, 88.9 mmol, TCI) was slowly added through a dropping funnel. This mixture was stirred at 0 °C for 2 h, and then further reacted at room temperature overnight. After filtration of the reaction mixture, the obtained solid was dispersed in H2O (150 mL). The insoluble colorless solid was recovered by filtration, washed with H2O several times, and dried under vacuum to obtain N,N’-trans-cyclohexane-1,2-diylbis(4-chlorobutanamide) (6.58 g, 46% yield). The filtrate after the reaction was concentrated by the rotary evaporator. To the residue, CH2Cl2 (130 mL) and 2 M HCl aq (10 mL) were loaded, followed by vigorous stirring of this mixture. The organic layer was separated from the aqueous phase and mixed with K2CO3 and MgSO4. The separated supernatant was concentrated and triturated with cold hexane (100 mL) in the ice bath. The deposited colorless solid was recovered by filtration, washed with cold hexane, and dried under vacuum to additionally obtain N,N’-trans-cyclohexane-1,2-diylbis(4-chlorobutanamide) (3.54 g, 25% yield). 1H NMR revealed that both compounds prepared here were identical to each other. The total yield was 71%. 1H NMR (δ/ppm vs. TMS, CDCl3, 399.78 MHz) 5.58 (NH, bd, 2H), 3.94 (N−CHax, br, 2H), 3.62 (CH2Cl, t, 4H), 2.36 (CH2, t, 4H), 2.12 (CH2, quintet, 4H), 1.77 (CHeq, m, 4H), 1.57 (CHax, m, 4H). 13C NMR (δ/ppm vs. TMS, CDCl3, 100.53 MHz) 170.98, 45.78, 44.69, 33.48, 28.37, 28.16.
This precursor (3.54 g, 11.0 mmol) was loaded in a 200 mL round bottom flask together with THF (110 mL), followed by cooling in the ice bath. To this solution, potassium tert-butoxide (2.57 g, 22.9 mmol) dissolved in THF (30 mL) was slowly added through a dropping funnel. The reaction mixture was stirred at 0 °C for 1 h and further reacted at room temperature for 6 h. After passing through a Celite pad, the filtrate was concentrated by a rotary evaporator. The residue was mixed with CH2Cl2 (80 mL) and 1 M HCl aq (10 mL), followed by stirring for several minutes. The separated organic layer was mixed with K2CO3 and MgSO4. After removal of the solid residues, the filtrate was evaporated under reduced pressure to dryness to obtain a colorless microcrystalline compound of rac-L2 (2.46 g) at a 90% yield. 1H NMR (δ/ppm vs. TMS, CDCl3, 399.78 MHz) 4.04 (N−CHax, br, 2H), 3.51 (CH2, t, 4H), 2.39 (CH2, t, 4H), 2.03 (CH2, quintet, 4H), 1.87 (CHeq, m, 4H), 1.730 (CHax, m, 4H). 13C NMR (δ/ppm vs. TMS, CDCl3, 100.53 MHz) 175.02, 48.02, 45.73, 31.42, 27.18, 18.42.

4.4. Synthesis of [NpVIO2(NO3)2(L1)]n

A stock solution of 100 mM NpVIO22+ in 3 M HNO3 (25 μL), a blank 3 M HNO3(aq) (50 μL), and a stock solution of 200 mM L1 in 3 M HNO3(aq) were layered from bottom to top of a glass tube (ca. ϕ5 mm diameter) in a stepwise manner, followed by standing in a silent place in the glovebox at 293 K. After one day passed, brown block crystals of [NpVIO2(NO3)2(L1)]n were obtained at 44% yield, which was estimated by the following procedure. The supernatant (1 μL) was loaded into a ca. ϕ10 mm glass vial and subjected to γ-ray spectrometry, performed at VKTA Dresden. The γ-ray emission from the sample was counted for 25 min real time plus 0.71% dead time.

4.5. Crystallographic Analysis

The X-ray diffraction data of the well-shaped single crystal of [NpVIO2(NO3)2(L1)]n were collected by the Bruker D8 VENTURE diffractometer equipped with hybrid pixel array detector and graphite monochromated Mo Kα radiation (λ = 0.71073 Å). A selected single crystal of the sample was mounted on a MiTeGen Dual Thickness MicroMounts with mineral oil. Intensity data were collected by taking oscillation photographs. Reflection data were integrated with the Bruker SAINT software package. Absorption correction was applied using the strong absorber option of SADAS22. The structures were solved by the direct method and refined anisotropically for non-hydrogen atoms by full-matrix least-squares calculations with the SHELX program suite [27]. Each refinement was continued until all shifts were smaller than one-third of the standard deviations of the parameters involved. Hydrogen atoms were located at the calculated positions. All hydrogen atoms were constrained to an ideal geometry with C–H = 0.95 Å. The thermal parameters of all hydrogen atoms were related to those of their parent atoms by U(H) = 1.2Ueq(C). All calculations were performed using the Olex2 crystallographic software program package [28]. The crystallographic data of [NpO2(NO3)2(L1)]n are summarized in Table 1 together with those of [UO2(NO3)2(L1)]n, which we reported previously [9].

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/inorganics11030104/s1. Figure S1: Packing diagram of [NpO2(NO3)2(L1)]n; Table S1: Logarithmic stability constants (log β) of typical actinyl(VI) complexes (AnO2(L)nm+; An = U, Np, Pu; L = CO32−, CH3COO−, F−, Cl−, NO3−, SO42−), in aqueous systems at 298 K and zero ionic strength; and references cited therein.

Author Contributions

T.T.: Conceptualization, Data curation, Formal analysis, Investigation, Funding acquisition, Project administration, Writing—original draft. J.M.: Conceptualization, Methodology, Resources, Formal analysis, Investigation, Writing—review and editing. R.O.: Conceptualization, Investigation, Funding acquisition, Project administration, Writing—review and editing. S.T.: Conceptualization, Investigation, Funding acquisition, Project administration, Writing—review and editing. K.T.: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Project administration, Supervision, Writing—original draft, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This work was partly supported by JSPS KAKENHI (No. JP20KK0119 to S.T., JP21J11942 to T.T., JP22J23423 to R.O.) and JSPS Overseas Challenge Program for Young Researchers (No. JP202280007 to T.T.).

Data Availability Statement

CCDC 2208277 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif (accessed on 24 February 2023), or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336033.

Acknowledgments

We thank Stephan Weiß (HZDR) for their technical assistance related to γ-ray spectrometry.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Inorganics 11 00104 g001 550
Figure 1. Schematic structures of NRPs, DHNRPs, and their actinyl(VI) nitrate complexes.
Figure 1. Schematic structures of NRPs, DHNRPs, and their actinyl(VI) nitrate complexes.
Inorganics 11 00104 g001
Inorganics 11 00104 g002 550
Figure 2. Photomicrograph of deposited crystals of [NpO2(NO3)2(L1)]n.
Figure 2. Photomicrograph of deposited crystals of [NpO2(NO3)2(L1)]n.
Inorganics 11 00104 g002
Inorganics 11 00104 g003 550
Figure 3. ORTEP view of [NpO2(NO3)2(L1)]n at the 50% probability level. Crystallographic data on [NpO2(NO3)2(L1)]n: CCDC 2208277, C14H22N4Np1O10, Fw = 643.35, 0.22 × 0.09 × 0.04 mm3, triclinic, P–1, a = 6.0172(2) Å, b = 7.6661(2) Å, c = 11.0985(3) Å, α = 75.9330(10)°, β = 84.488(2)°, γ = 76.353(2)°, V = 482.18(2) Å3, Z = 1, T = 293 K, Dcalcd = 2.216 g/cm3, μ(Mo Ka) = 5.450 mm−1, GOF = 1.173, R1(I > 2σ) = 0.0284, wR2(all) = 0.0512.
Figure 3. ORTEP view of [NpO2(NO3)2(L1)]n at the 50% probability level. Crystallographic data on [NpO2(NO3)2(L1)]n: CCDC 2208277, C14H22N4Np1O10, Fw = 643.35, 0.22 × 0.09 × 0.04 mm3, triclinic, P–1, a = 6.0172(2) Å, b = 7.6661(2) Å, c = 11.0985(3) Å, α = 75.9330(10)°, β = 84.488(2)°, γ = 76.353(2)°, V = 482.18(2) Å3, Z = 1, T = 293 K, Dcalcd = 2.216 g/cm3, μ(Mo Ka) = 5.450 mm−1, GOF = 1.173, R1(I > 2σ) = 0.0284, wR2(all) = 0.0512.
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Inorganics 11 00104 g004 550
Figure 4. C−H···O interactions occurring in the crystal structure of [NpO2(NO3)2(L1)]n. Although more interactions can be found there, only some of them are drawn with dotted lines here for clarity. Color codes, Np: yellow, O: red, N: purple, C: grey, H: white.
Figure 4. C−H···O interactions occurring in the crystal structure of [NpO2(NO3)2(L1)]n. Although more interactions can be found there, only some of them are drawn with dotted lines here for clarity. Color codes, Np: yellow, O: red, N: purple, C: grey, H: white.
Inorganics 11 00104 g004
Table
Table 1. Crystallographic data of [AnO2(NO3)2(L1)]n (An = Np, U).
Table 1. Crystallographic data of [AnO2(NO3)2(L1)]n (An = Np, U).
[NpO2(NO3)2(L1)]n[UO2(NO3)2(L1)]n
formulaC14H22N4NpO10C14H22N4O10U
FW643.35644.38
systemtriclinictriclinic
space groupP−1 (#2)P−1 (#2)
crystal size/mm30.22 × 0.09 × 0.040.400 × 0.300 × 0.200
a/Å6.0172(2)5.8962(4)
b/Å7.6661(2)7.6330(6)
c/Å11.0985(3)11.0279(9)
α/°75.9330(10)76.841(5)
β/°84.488(2)84.480(6)
γ/°76.353(2)76.229(5)
V/Å3482.18(2)468.93(6)
Z11
T/K29393
Dcalc/g⋅cm−32.2162.282
μ(Mo Kα)/mm−15.4508.720
R1 (I > 2σ)0.02840.0174
wR2 (all)0.05120.0393
GOF1.1731.042
CCDC No.22082771573161
ref.this work[9]
Table
Table 2. Selected structural parameters of [AnO2(NO3)2(L1)]n.
Table 2. Selected structural parameters of [AnO2(NO3)2(L1)]n.
An = Np aAn = U b
bond distance (Å)
An−OaxNp−O(1)  1.744(3)1.769(2)
An−ONO3Np−O(2)  2.551(3)2.546(2)
Np−O(3)  2.562(3)2.550(2)
An−OLNp−O(5)  2.341(2)2.362(2)
Ccarb = OLC(1)−O(5) 1.254(4)1.260(4)
Ccarb−NamideC(1)−N(2) 1.325(4)1.329(4)
bond angles (°)
Oax≡An≡OaxO(1)−Np(1)−O(1)i 180180
An−OL = CNp(1)−O(5)−C(1) 138.3(2)134.4(2)
tilt angles from AnO22+ equatorial plane (°)
NO3−13.812.3
2-pyrrolidone43.046.6
a This work at 293 K. Atomic notations follow Figure 3. Symmetry operation: (i) 1 − x, 1 − y, −z. b [9] at 93 K.
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Takeyama, T.; März, J.; Ono, R.; Tsushima, S.; Takao, K. Neptunyl(VI) Nitrate Coordination Polymer with Bis(2-pyrrolidone) Linkers Highlighting Crystallographic Analogy and Solubility Difference in Actinyl(VI) Nitrates. Inorganics 2023, 11, 104. https://doi.org/10.3390/inorganics11030104

AMA Style

Takeyama T, März J, Ono R, Tsushima S, Takao K. Neptunyl(VI) Nitrate Coordination Polymer with Bis(2-pyrrolidone) Linkers Highlighting Crystallographic Analogy and Solubility Difference in Actinyl(VI) Nitrates. Inorganics. 2023; 11(3):104. https://doi.org/10.3390/inorganics11030104

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Takeyama, Tomoyuki, Juliane März, Ryoma Ono, Satoru Tsushima, and Koichiro Takao. 2023. "Neptunyl(VI) Nitrate Coordination Polymer with Bis(2-pyrrolidone) Linkers Highlighting Crystallographic Analogy and Solubility Difference in Actinyl(VI) Nitrates" Inorganics 11, no. 3: 104. https://doi.org/10.3390/inorganics11030104

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