Uncovering Heavy Actinide Covalency: Implications for Minor Actinide

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Uncovering Heavy Actinide Covalency: Implications for Minor Actinide Partitioning Aditi Chandrasekar† and Tapan K. Ghanty*,‡,§ †

Homi Bhabha National Institute, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu 603102, India Theoretical Chemistry Section, Chemistry Group, Bhabha Atomic Research Centre, Mumbai 400 085, India § Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai 400 094, India ‡

Inorg. Chem. Downloaded from pubs.acs.org by WEBSTER UNIV on 03/01/19. For personal use only.

S Supporting Information *

ABSTRACT: Across the actinide period, the stability of the trivalent oxidation state predominates in the heavy actinides, making their chemical nature close to that of rare earth elements. The resemblance in their chemistry poses difficulties in separating heavy actinides from lanthanides, which is a vital separation in the minor actinide partitioning process. Actinide contraction has conventionally implied electrostatic actinide-ligand interactions among the heavy actinides. The present study challenges this conventional understanding and reveals increasing covalency in the actinide-ligand bond across Am to Cf. Complexes of Am, Cm, Bk, and Cf have been examined for their electronic structure with a focus on the nature of their interactions with different ligands within the framework of density functional theory, where the relativistic effects have been incorporated by using zero-order regular approximation and spin−orbit coupling. The choice of ligands selected for this study facilitates the effect of the donor atom as well as denticity to be accounted for. Hence, heavy actinide complexes of the N- and O-donor ligand dipicolinic acid, S and O mixed donor ligands of the Cyanex type, and an octadentate ligand N,N,N′N′-tetrakis[(6-carboxypyridin-2-yl)methyl]ethylenediamine have been optimized and evaluated. In each case energy decomposition analysis has been used to explicitly decompose the metal−ligand interaction energy into components which have then been analyzed. Irrespective of the hard−soft characteristics of donor atoms or the denticity of the ligands, steadily increased covalency has been observed across Am to Cf. Inspection of the ligand highest energy occupied molecular orbitals and metal orbitals sheds light on the origin of the unexpected covalency. An overall increase in bonding and also the orbital contribution along the Am−Cf series is clearly due to the enhancement in covalency, which is complementary to the orbital degeneracy induced covalency proposed very recently by Batista and co-workers.

1. INTRODUCTION Spent nuclear fuel contains a variety of isotopes, of which the unburnt fuel materials uranium and plutonium are recovered and recycled.1−3 In the remaining elemental mixture which is disposed as nuclear waste, lanthanides form the largest proportion.4 Neutron capture by uranium and plutonium in the reactor produce heavier actinides such as Am and Cm, which are classified as minor actinides. These actinides, predominantly existing in the trivalent state, along with some of their daughter products, are long-lived alpha emitters, making the nuclear waste more hazardous as well as increasing the waste storage time. Partitioning of minor actinides from the lanthanides is vital both in terms of the safe storage of nuclear waste, as well as for the transmutation of minor actinides to reduce their radiotoxicity.5−7 Many of the lanthanides are neutron poisons, and their presence does not allow effective transmutation of the actinide element by neutron bombardment.8,9 Though lanthanide−actinide separation is one of the important steps in any nuclear establishment, it is well-known that this separation is very difficult because of the very similar chemical behaviors of trivalent lanthanides and the corresponding actinides, and hence determination of electronic © XXXX American Chemical Society

structures of lanthanide and actinide compounds and understanding their chemistry with various ligands are very important. Over the years, there have been a number of reports on the lanthanides;10 however, the situation is complicated in the case of actinides because of several experimental and theoretical challenges. Nevertheless, in recent years there has been considerable interest to investigate compounds and complexes containing heavy actinides11−18 though the investigations on heavy actinide compounds had started a few decades back.19,20 With the advancement in experimental techniques and theoretical methodologies, now it has been possible not only to prepare various complexes of heavy actinides, but to characterize them through various means including electronic structure determinations theoretically. One of the most important inferences from some of these reports11−13 has been the capability of heavier actinides such as californium and berkelium to participate in more pronounced covalent bonding in their borates and dipicolinates. This observation is counterintuitive since across the Received: December 3, 2018

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DOI: 10.1021/acs.inorgchem.8b03358 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

3018,41,42 with two S donor sites, Cyanex 27243,44 with two oxygen donor sites and Cyanex 30244,45 with mixed S and O donor sites have been investigated and proved successful for the separation of the minor actinides.42 The ability of Cyanex 301 to separate Am3+ from lanthanides has been attributed to the increased covalency shown by Am. Nevertheless, solvent has also been shown to play an important role in the separation of Am3+ from Eu3+.42,43 In the recent past Dolg and co-workers have emphasized the effect of solvation of the trivalent lanthanide and actinide ions on the separation behavior by Cyanex 301 ligand and provided in-depth insights by considering the hydrated complexes of the metal ions in the process of selective extraction of Am3+ over Eu3+.43 It has been concluded that the gas phase dissociation energy trends are not adequate and effect of solvent is required to explain the selectivity of Cyanex 301 toward Am3+ over Eu3+. Though the trivalent oxidation state is predominant, very recently, heavy actinide covalency has been theoretically investigated in their higher pentavalent oxidation state,17 and another recent report investigates complexes of heavy actinides in their trivalent and tetravalent states by extended X-ray absorption fine structure (EXAFS) and density functional theory (DFT).46 Apart from EXAFS, in recent years, ligand Kedge X-ray absorption spectroscopy (XAS) in conjunction with DFT has been found to be highly successful in determining the extent of metal−ligand covalency. For instance, Kozimor and co-workers have shown that americium-chlorine covalency in AmCl63− is clearly due to a larger Am(III) 5f-orbital mixing than that of 4f-orbital mixing for Eu(III).31 Metal−ligand covalency has also been probed for many other systems by using the ligand K-edge XAS technique.47−51 The covalency in the actinide ligand bond has been ingeniously addressed by Formanuik et al. via the superhyperfine coupling of actinide unpaired electrons with spin active ligand nuclei, using electron paramagnetic resonance (EPR) spectroscopy.14 Furthermore, an octadentate ligand N,N,N′N′-tetrakis[(6-carboxypyridin-2-yl)methyl]ethylenediamine (TPAEN) has been investigated experimentally and found to complex Am(III) and Cm(III) with higher stability constants compared to the lanthanides.52,53 From previous experimental and theoretical reports, it is likely that covalency driven preferential bonding to actinides over lanthanides by certain ligands is a key marker in choosing extractants for minor actinide partitioning. Though a considerable amount of work has been done on the separation of minor actinides (Am, Cm) from lanthanides, there are only few reports11−13,18,46 in the literature involving complexation of heavy actinides, particularly, Bk and Cf, because of their radiological toxicity and close chemical similarities with heavy lanthanides. Another issue that arises with heavy actinides is their nuclear instability, and this consequently limits the experimental probing of chemical and physical properties. This limitation creates a gap between the experimentalists and theoreticians. Moreover, theoretical elucidation of the nature of chemical bonding in heavy actinide complexes through energy decomposition analysis (EDA)54 has never been reported in the literature. It is worth mentioning here that EDA has been highly successful in the elucidation of nature of bonding in various chemical systems including metal−ligand complexes.55−60 Therefore, in the present study, Am3+, Cm3+, Bk3+, and Cf3+ complexes have been evaluated for their geometry, structure,

actinide series, elements should show more ionic bonding because of the harder nature of the late actinides. Very recently, the origin of such enhanced covalent bonding in the heavier actinides has been elucidated by Batista and coworkers.15 In this work, apart from experimental studies, extensive theoretical calculations have been done on the actinide metal atoms, their complexes, and also the bare ligand dianion (dipicolinate). Subsequent theoretical analyses suggest that the origin of enhanced covalency in heavier actinides in their dipicolinate complexes is due to the near degeneracy of the 5f orbitals of the heavier actinides with the ligand molecular orbitals. On the other hand, for the earlier actinides like americium and curium the metal atom energy levels are associated with higher orbital energy as compared to that of the ligand orbitals, which resulted in weak interaction between the metal−ligand orbitals. Thus, the covalency in heavier actinides has nothing to do with the spatial orbital overlap originated covalency; rather, degeneracy of metal and ligand orbitals plays an important role here. In the recent past, while investigating the actinide dioxide systems (AnO2, An = Th− Es) computationally, Martin and co-workers21 have also shown that the degeneracy of actinide 5f orbitals and oxygen 2p orbitals leads to significant orbital mixing and covalency in the intermediate region of the actinide series (PuO2−CmO2). Similarly, a significant orbital interaction has been found in the (C5Me5)2[iPrNC(Me)NiPr]An complexes with An as trivalent Pu and Am due to a near degeneracy between the An 5f orbitals and the cyclopentadienyl π-orbitals.22 Very recently, the energy-degeneracy driven covalency has also been demonstrated for the tetravalent actinide chlorides AnCl62− (An = Th, U, Np, Pu), where significant mixing between Cl 3p and An 5f and 6d orbitals has been found.23 The early actinides can be separated from trivalent lanthanide elements by exploiting their characteristic to exist in multiple oxidation states.24−26 However, along the actinide period the 5f orbital tends to become increasingly localized making the trans-plutonium actinides very lanthanide-like in their chemistry.27 The predominant trivalent oxidation state of heavier actinides, coupled with the similarity in ionic radii with the lanthanides, places higher demands on ligand specificity for their separation.28−30 Nevertheless, a saving grace is the relative diffusivity of the 5f orbitals compared to 4f, which in turn enables more covalent interactions of actinides with soft donor ligands compared to those seen in the equivalent lanthanide complexes.31,32 Several ligands with N28 and S donor sites have been explored both experimentally and theoretically for this purpose including 1,10-phenanthroline derivatives with mixed donor atoms.33,34 The concept of intraligand synergism, by invoking the hard−soft acid−base theory, provided an understanding that mixed donor atoms increase the specificity of ligands.35,36 The inclusion of a softdonor such as nitrogen to an existing oxygen donor ligand made the ligand as a whole a better actinide binder. Rather than the lanthanide binding, the softer actinide bound more strongly with the harder oxygen atom in the ligand, enabling better minor actinide recovery. The concept of intraligand synergism, where harder oxygen binds with softer actinides in a stronger way, is counterintuitive from the viewpoint of hard− soft−acid−base principle; however, the proposed theoretical concept has been validated experimentally by a number of researchers.36−39 It has also been found that the Cyanex class of compounds have displayed good separation factors for Am(III) and Cm(III) over the lanthanide elements.40 Cyanex B


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3. RESULTS AND DISCUSSION 3.1. Cyanex Ligands. The heavy actinide ion (Am3+, Cm3+, Bk3+, and Cf3+) complexes with the bidentate Cyanex 272, Cyanex 302, and Cyanex 301 ligands have been optimized and analyzed for their geometry (Table 1). As a representative

and interaction energy to understand various chemical interactions that exist in the metal−ligand bonding and particularly the change in the extent of covalency while moving along the series in the complexes. Geometry optimization followed by EDA has been carried out for these actinides in their complexes with dipicolinic acid (DPA), Cyanex 301, 302, and 272 and TPAEN ligands. Choice of the TPAEN ligand is also motivated by the fact that in a single ligand there are eight donor sites, making the structure of the complex more rigid. Moreover, choice of ligands in the present work has been made in such a way so that the effect of denticity on the trends in the variations of different energy components can be accomplished in a systematic way, starting from bidentate Cyanex ligands, tridentate DPA, and octadentate TPAEN. The choice of ligands selected for this study facilitates the effect of different donor atoms and denticities to be taken into account as these parameters change for the different complexes. Calculations have been done both in the gas and the solvent phases. The effect of spin−orbit coupling has also been considered. It remains to be seen whether the results obtained from the EDA are able to provide important insights on the increase in the extent of covalency along the series as continues to be reported in the literature from time to time.

Table 1. Computed Structural Parameters (Å) of Actinide Cyanex Complexes Using the PBE/TZ2P Method bond lengths Cyanex 301 actinide Am Cm Bk Cf

M−S 2.829 2.818 2.797 2.794

Cyanex 302 M−O 2.338 2.327 2.306 2.308

M−S 2.909 2.902 2.881 2.866

Cyanex 272 M−O 2.408 2.403 2.381 2.373

of other actinide structures which have similar geometry, Figure 1 shows the optimized structures for Cf complexes with

2. COMPUTATIONAL METHODS Geometries of the bare ligands and the actinide complexes have been optimized using the ADF 2016 program.54,61 The truncated form of the Cyanex ligands has been employed where the 2,4,4-trimethylpentyl group has been replaced by a methyl group. All calculations have been carried out by employing PBE functional in conjunction with TZ2P Slater type orbital (STO) basis sets.62 Electrons in the 6s, 6p, 6d, 7f sub shells in the actinides, 2s, 2p sub shells for carbon, nitrogen, and oxygen and 3s, 3p sub shells for phosphorus and sulfur have been explicitly treated. All the optimized structures have been confirmed to be energy minima by running analytical frequency calculations.63−65 Solvent effect has been incorporated using the COSMO approach66 with water as the solvent (epsilon = 78.39). The atomic radius values used in the COSMO model are 1.30, 2.00, 1.83, 1.72, and 2.16 Å for H, C, N, O, and S, respectively. The radius for all the actinide atoms was taken as 2.223 Å.15 Relativistic effects have been incorporated by the ZORA approximation as implemented in ADF.67−69 Energy decomposition analysis has been performed in which the interaction energy between the ligands and the metal has been decomposed into individual components. This interaction energy ΔEint can be broken down into components, viz. ΔEint = ΔEelec + ΔE Pauli + ΔEorb

Figure 1. Optimized geometries of Cf complexes with Cyanex 272, Cyanex 302, and Cyanex 301 ligands.

Cyanex 272, Cyanex 302, and Cyanex 301 ligands. The binding site changes in the case of each ligand. Cyanex 301 [bis(2,4,4trimethylpentyl) dithiophosphinic acid] binds via two sulfur atoms, Cyanex 302 [bis(2,4,4-trimethylpentyl)thiophosphinic acid] binds via one sulfur and one oxygen atom, and Cyanex 272 [bis (2,4,4-trimethylpentyl)phosphinic acid] binds via two oxygen atoms. The actinides form charge neutral 1:3 complexes with the Cyanex extractants as reported earlier for the Cyanex 301 complexes with trivalent lanthanides and actinide.40 Here it may be noted that experimental evidence shows that the Cyanex 301 ligand forms neutral complexes of the type ML3, where the ligand binds with the metal ion in a bidentate fashion; however, stoichiometries are different for the Cyanex 272 and the Cyanex 302 complexes because of the intermolecular hydrogen bonding in Cyanex 272 dimer and both monodentate and bidentate mode of coordination in the case of Cyanex 302 complexes.70−73 Nevertheless, here our objective is to investigate the different energy components of various complexes using different ligands in a unified and systematic manner; consequently, in this work we have considered ML3 type of stoichiometry for all the Cyanex series of ligands. The metal−oxygen bond lengths are shorter than those between metal−sulfur. For metal−oxygen as well as metal−sulfur bonds, the actinide−extractant bond lengths decrease monotonically from Am to Cf complexes. The shrinking ionic radii from Am through Cf are one of the causes for the shortening in bond length. Figure 2 shows the EDA of heavy actinide complexes with Cyanex 272, Cyanex 302, and Cyanex 301 ligands as computed using eq 1. The variation in interaction components viz. Pauli repulsion, electrostatic interaction, and orbital interaction are

(1)

where ΔEelec and ΔEPauli denote the attractive electrostatic interaction energy and the Pauli repulsive energy, respectively, between the fragments, and ΔEorb is the stabilizing orbital interaction term. ΔEorb includes the electron-pair bonding, charge transfer (HOMO−LUMO interactions) and the polarization term (empty/occupied mixing of orbitals belonging to one fragment due to the presence of the other). Therefore, this orbital component in the decomposition is a measure of the covalency in the interaction.54 For the bond energy decomposition calculations the open shell configuration of the central metal ion (6, 7, 6, and 5 unpaired electrons for Am, Cm, Bk, and Cf, respectively) has been defined for the explicitly treated electrons which are not considered in the frozen core. In complexes consisting of more than one kind of ligand, the different types of ligands have been considered as different fragments during the analysis. The effect of solvent (dielectric constant value of 78.39 corresponding to the aqueous phase) has been considered during the energy calculations for the fragment as well as the complex in the EDA. C


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Figure 2. EDA of heavy actinide complexes with Cyanex 272, Cyanex 302, and Cyanex 301 ligands. The different interaction components are plotted from Am−Cf. It is to be noted that the Pauli repulsion is repulsive (positive sign), and the electrostatic and the orbital interactions are attractive (negative sign) in nature.

shown from Am−Cf. The results indicate that on moving toward the heavy actinides the electrostatic interaction decreases. The orbital interaction is more and suggests more overlap and hence extensive covalent interactions. On changing the binding site from oxygen to sulfur, there is increased covalency viz. orbital overlap and less electrovalence. The increased covalency is a reflection of the higher polarizability of sulfur as an electron donor due to its large size compared to oxygen.74 The same factor makes the electrostatic interaction the highest in the Cyanex 272 complexes and least in the Cyanex 301 complexes for all the actinides. Oxygen being a harder electron donor makes the actinide−oxygen interaction more electrostatic than the metal−sulfur interaction. Here it is important to note that the calculated values of various energy components agree well with the trends obtained by Dolg and co-workers while modeling the separation of trivalent lanthanides from the valence isoelectronic actinides.41,43 Moreover, it is interesting to find that the calculated energy components in the gas phase itself is able to provide insight related to the preference of Cyanex 301 toward Am3+ as compared to the affinity of Cyanex 272 toward the same metal ion, as reported by Dolg and co-workers while calculating the changes of the Gibbs free energies in the liquid−liquid extraction reactions for the extraction of Am3+ and Eu3+ with Cyanex 301 and Cyanex 272 ligands. 3.2. Dipicolinic Acid (DPA) Ligands. Next we have considered the complexes of trivalent heavy actinide ions with DPA. In these complexes there is a flexibility to vary the number of DPA ligands bound to the metal as the complexes are formed and stabilized in the aqueous phase. Hence, the actinide coordination is completed by complexing water molecules. Figure 3 shows the optimized geometries of Am complexes with DPA in the ratio 1:1, 1:2, and 1:3. The complex with water in the absence of DPA is also shown. The other heavy actinide complexes have similar orientations and are hence not shown in the figure. The DPA ligand donates electron density to the trivalent actinide through two carboxylate oxygens and an aromatic nitrogen, making it a tricoordinate ligand with a charge of −2. Accordingly the net charges on the DPA complexes with triply charged actinide ions are +3, + 1, −1, and −3 for the water complex, 1:1, 1:2, and 1:3 DPA complexes, respectively. The metal−extractant bond lengths are shown in Table 2 where M is the metal. M−O bond lengths are shorter than the M−N bond lengths in all the structures. In general, on moving from Am to Cf, the M−O bond lengths are shortened with the

Figure 3. Optimized geometries of metal-water and 1:1, 1:2, and 1:3 DPA complexes of Am(III).

Table 2. Computed Structural Parameters (Å) of Actinide− DPA Complexes Using PBE/TZ2P Method bond lengths actinide Am Cm Bk Cf

M(DPA)(H2O)5

M(DPA)2(H2O)2

M−O 2.296 2.304 2.288 2.272

M−O 2.395 2.392 2.373 2.378

M−N 2.494 2.444 2.437 2.442

M−N 2.539 2.544 2.508 2.522

M(DPA)3 M−O 2.522 2.521 2.492 2.495

M−N 2.632 2.626 2.598 2.602

exception of Cm complexes where there is an increase in the bond length. Unlike M−O bond lengths, the M−N bond lengths tend to decrease from Am to Bk, and show a marginal increase at Cf. In the case of a given actinide, as the number of DPA ligands in the complex increases, the M−O as well as the M−N bond lengths increase. As more DPA ligands are available for electron donation to the cationic metal center, the electron donation from the ligands is distributed. Consequently, the contribution from each DPA ligand is reduced, in turn increasing the bond lengths. The gas phase EDA of the heavy actinide−DPA complexes with 1:1, 1:2, and 1:3 metal/DPA ratios are shown in Figure 4. The complex with water and no DPA ligand is also shown for comparison. On moving from Am to Cf there is a decrease in the electrostatic interaction component and an increase in the orbital overlap. This shows an increase in covalency in the actinide−DPA interaction. As the number of DPA ligands in the complex is increased from none to 3, the orbital overlap and hence covalency are systematically decreased. The increase D


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Figure 4. Gas phase EDA of actinide−DPA complexes for Am,Cm, Bk, and Cf. It is to be noted that the Pauli repulsion is repulsive (positive sign), and the electrostatic and the orbital interactions are attractive (negative sign) in nature.

Figure 5. Solvent corrected EDA of actinide−DPA complexes for Am,Cm, Bk, and Cf. It is to be noted that the Pauli repulsion is repulsive (positive sign), and the electrostatic and the orbital interactions are attractive (negative sign) in nature.

in the bond distances from 1:1 to 1:3 DPA consequently leads to less overlap between the ligand and metal orbitals. EDA has been performed in the solvent phase for all the metal−DPA systems (Figure 5) with water as the solvent. The trends observed in the gas phase are retained in the solvent phase as well. In the M(H2O)8 complexes there is a significant decrease in the electrostatic interaction energies from the gas to the solvent phase. As the solvent medium is water, which has high dielectric constant, the electrostatic interactions between the polar metal complex and solvent increase. This decreases the intracomplex electrostatic interactions. 3.3. N,N,N′N′-Tetrakis[(6-carboxypyridin-2-yl)methyl]ethylenediamine (TPAEN) Ligand. We have focused to another ligand, viz. TPAEN, and investigated the complexation of the heavy actinides with this ligand. Figure 6 shows the optimized geometries of the heavy actinide complexes with TPAEN octadentate ligand. The ligand donates electrons to the triply charged actinide through four carboxylate oxygens and four nitrogen atoms as shown. The net charge on the complexes is −1. Table 3 shows the M−O and M−N bond distances between the actinide and ligand atoms at the binding site. From Am to Cf the M−O and M−N bond distances generally decrease. In the cases of both Cyanex and TPAEN complexes with actinides there is an overall decrease in the average bond length from Am to Cf complexes. The decrease in bond length is more than the contraction of the ionic radii which is 0.025 Å.75 From Tables 1 and 3 it can be seen that the decrease in the average bond length is 0.035 Å in Cyanex 301, 0.043 Å (M−S bond), 0.030 Å (M−O bond) in Cyanex 302 complexes, and 0.035 Å in Cyanex 272 complexes. In the case of the TPAEN ligand the decreases in

Figure 6. Optimized geometries of Am3+, Cm3+, Bk3+, and Cf3+ complexes with the octa-coordinate TPAEN ligand.

Table 3. Computed Bond Lengths (Å) of Actinide−TPAEN Complexes Obtained Using the PBE/TZ2P Method bond lengths M(TPAEN) actinide Am Cm Bk Cf

M−O 2.352 2.362 2.333 2.338

M−N 2.872 2.847 2.840 2.828

average bond length from Am to Cf complexes are 0.044 and 0.014 Å for the M−N and M−O bonds, respectively. A closer E


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component has been provided in the Supporting Information in tabular form (Table S1). The gas and solvent phase EDA of the heavy actinide− TPAEN complexes are shown in Figure 8. In both cases, on moving from Am to Cf there is a decrease in the electrostatic interaction component and an increase in the orbital overlap. It is interesting to note that a similar trend is followed in the gas phase as well as in the presence of solvent. For all the ligands studied, the effect of spin−orbit coupling has been investigated (Figures S1−S3). The trends of lowering in the electrostatic interaction and increase in the orbital interaction toward the heavy actinides are followed even in the presence of spin−orbit effects. The contribution of the orbital component is lower for Cm, and the contribution of the electrostatic component is marginally higher than the expected trend line in the case of all the ligands. This behavior could be due to the half-filled f orbitals in the Cm3+ ion which is not present in the other three ions studied. The effect of spin− orbit coupling on the EDA components has also been investigated. This lowering in the orbital contribution in the case of Cm is less pronounced when spin−orbit effects are introduced. 3.4. Ligand and Actinide Energy Levels. All the ligands investigated above, with varied binding site atoms and denticities, show that the covalency in the metal−ligand bonds increases toward the heavy actinide. This is completely unexpected as the actinides become harder acids across the period and are hence much more inclined to electrostatic interactions with anionic ligands. The basis for the covalency in heavy actinide complexes has been further probed by undertaking an energy level analysis, where the valence orbitals of the actinides Am−Cf are compared with the HOMO of the ligands. Very recently, Batista and co-workers have performed a similar kind of actinide and ligand energy level analysis for the complexes of Am−Cf with the DPA ligand15 for the trivalent oxidation state of the actinides and also shown for the trivalent and tetravalent oxidation states using the hydroxypyridinone ligand.46 Figure 9 shows the energy level diagram for the heavy actinides, DPA and TPAEN ligands in the solvent phase. The energy difference between the 5f energy level of Cf and the HOMO of DPA and TPAEN are schematically indicated. The energy difference between the ligand (DPA and TPAEN) HOMO and actinide energy levels is the maximum for Am and decreases monotonically. The energy difference between the 5f orbitals of Cf and the ligands is the least compared to the other

inspection of Table 3 reveals that most of the bond distances are decreased in going from Am to Cf, except the increase in the M−O bond distance in going from Am to Cm. This trend is somewhat similar to the monotonic decreasing trend in the experimentally observed lattice parameter for AnO2 (An = Pa− Es), which shows a deviation in the case of the CmO2 system. This deviation has been attributed to the near half-filled like configuration in the Cm−Bk region.21 Very recently, an unusual trend in the variation of thermodynamic properties has also been observed in the Cm−Bk region in their complexes with the hydroxypyridinone ligand.76 The additional decrease in bond length is reflected in the complexation energy trends as depicted in Figure 7. From Am

Figure 7. Total bonding energy for the heavy actinides with the Cyanex 301 Cyanex 302, Cyanex 272, TPAEN, and DPA ligands.

to Cf the complexes are more stabilized for all the ligands investigated. The increasing complexation energy across the actinide series has been experimentally shown for the DPA ligand15 where the experimentally determined stability constants for the 1:3 metal−DPA complexes follow an increasing trend. The most stable bonding energies are observed for the DPA ligand, followed by the TPAEN and then the Cyanex ligands in the order 272, 302, and 301 respectively. The higher bonding energies are present for ligands with oxygen donor atom and less for softer donors like N and S.74 The shortening in bond lengths over and above the shrinking of the ionic radii across Am−Cf is hence a consequence of enhanced covalency as predicted by the energy decomposition results. The total energy of each

Figure 8. EDA of actinide−TPAEN complexes for Am,Cm, Bk, and Cf in the gas phase and solvent phase. It is to be noted that the Pauli repulsion is repulsive (positive sign), and the electrostatic and the orbital interactions are attractive (negative sign) in nature. F


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closeness in chemistry between minor actinides and lanthanides, several ligands require screening for identifying those specific to the actinides. Considering these challenges, a DFT platform to study minor actinide specific ligands is a wise alternative. In our work, deeper investigations into the interaction energy have been elaborated and proved successful in predicting the increase in the covalency along the heavy actinide series, though it is counterintuitive from the viewpoint of actinide contraction. The results presented here create a new direction for the search and evaluation of minor actinide specific extractants which are of immense utility in their partitioning from high-active nuclear waste streams.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 9. Energy level diagram for the heavy actinides, DPA, and TPAEN ligands in the solvent phase. The energy difference between the Cf 5f energy level and the HOMO of DPA and TPAEN are schematically indicated.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03358. Energy decomposition analysis plots for the heavy actinide complexes with Cyanex 272, Cyanex 302, Cyanex 301, DPA, and TPAEN ligands after incorporating spin−orbit coupling (Figures S1−S3). Total energy of each component has been provided in Table S1 (PDF)

actinides. Though the actinides contract across the period and are believed to participate in more electrostatic interactions with anionic ligands, they show greater covalency.

4. CONCLUSION The separation of minor actinides from lanthanides is one of the vital stages in a nuclear fuel cycle. A vast number of extractants have been explored in order to achieve this challenging separation. Heavy actinides become harder acids due to the contraction of the 5f orbitals, making their interactions with anionic ligands more electrostatic. Coupled with this, their dominant trivalent chemistry makes minor actinide partitioning all the more cryptic. Determination of electronic structures of lanthanide and actinide compounds and understanding their complexation chemistry with various ligands are welcome avenues toward screening for the right extractants. The present work explicitly decomposed the actinide−ligand interaction into components viz. electrostatic, orbital overlap, and Pauli repulsion. Ligands of different denticities and binding site atoms have been judiciously chosen. Counterintuitive to the popular belief, our results uncovered decreased electrostatic interaction between ligand and actinide across Am to Cf and increased orbital interactions within the framework of EDA scheme, shown for the first time. These findings established greater covalency in the heavy actinide−ligand interaction. Further, observations on the ligand and metal energy levels clearly showed that from Am to Cf the actinide energy level was increasingly closer to the ligand HOMO. Total bonding energy has also been computed for all the actinide−ligand systems. The overall bonding is increased along the series, and also the orbital contribution. It is clearly due to the enhancement in covalency, which is complementary to the orbital degeneracy induced covalency proposed by Batista and co-workers.15,46 The results reported in the present work along with the recent experimental reports12,13 in conjunction with the energy degeneracy driven covalency effect for the heavier actinides15,46 warrant a rethinking of the decades old proposition in designing minor actinide selective ligands for use in the back-end nuclear fuel cycle. In fact, Batista and co-workers have already advocated for an amendment to the original Seaborg actinide hypothesis.15 Experimental investigations using minor actinides involve great care due to their radiotoxicity. Moreover, due to the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +91-22-25595089. ORCID

Tapan K. Ghanty: 0000-0001-7434-3389 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.C. thanks HBNI for the fellowship, acknowledges computing time on NEHA and IVY clusters at IGCAR. T.K.G. would like to thank the Computer Division, BARC, for the supercomputing facilities. It is a pleasure to thank Dr. N. Sivaraman and Dr. P. D. Naik for their support.

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