Complexation and Extraction of Trivalent Actinides and Lanthanides

Review pubs.acs.org/CR

Complexation and Extraction of Trivalent Actinides and Lanthanides by Triazinylpyridine N‑Donor Ligands Petra J. Panak*,†,‡ and Andreas Geist*,† †

Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal (INE), P.O. Box 3640, 76021 Karlsruhe, Germany Ruprecht-Karls-Universität Heidelberg, Physikalisch Chemisches Institut (PCI), Im Neuenheimer Feld 253, 69120 Heidelberg, Germany

‡

3.3.3. Speciation Studies by UV/Vis Spectrophotometry 3.3.4. Speciation Studies by ESI-MS 3.3.5. Speciation Studies by NMR 3.3.6. Thermodynamic Data of An(III) and Ln(III) Complexation by BTBP 4. Water-Soluble BTP and BTBP 4.1. SO 3 -Ph-BTP As a Masking Agent for Actinides(III) 4.1.1. Extraction 4.1.2. Application in a GANEX Process 4.2. Complexation of Cm(III) and Eu(III) with SO3Ph-BTP 4.3. SO3-Ph-BTBP 5. Conclusions and Outlook 6. Appendix: List of Compounds Author Information Corresponding Author Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 2. Lipophilic Bistriazinylpyridines (BTP) 2.1. Extraction and Process Tests Using BTP 2.1.1. n-Pr-BTP 2.1.2. i-Pr-BTP 2.1.3. CyMe4-BTP 2.1.4. CA-BTP 2.2. Chemical and Radiation Stability of BTP 2.3. Complexation of An(III) and Ln(III) with BTP 2.3.1. Structures of An(III)- and Ln(III)-BTP complexes 2.3.2. Speciation Studies by TRLFS 2.3.3. Speciation Studies by UV/vis Spectrophotometry 2.3.4. Speciation Studies by ESI-MS 2.3.5. Speciation Studies by NMR 2.3.6. Thermodynamic Data of An(III) and Ln(III) Complexation by BTP 3. Lipophilic Bistriazinyl Bipyridines (BTBP) 3.1. Extraction and Process Tests Using BTBP 3.1.1. C5-BTBP 3.1.2. CyMe4-BTBP 3.1.3. 1c-SANEX Process 3.1.4. GANEX Process 3.1.5. Improving Kinetics in BTBP Systems 3.2. Chemical and Radiation Stability of BTBP 3.2.1. C5-BTBP 3.2.2. CyMe4-BTBP 3.3. Complexation of An(III) and Ln(III) with BTBP 3.3.1. Structures of An(III)- and Ln(III)-BTBP Complexes 3.3.2. Speciation Studies by TRLFS

© 2013 American Chemical Society

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1. INTRODUCTION The transuranium elements (TRU) neptunium, plutonium, americium, and curium are produced in nuclear fuel during reactor operation from neutron capture by uranium-235/238 and consecutive nuclear reactions. Of the TRU, plutonium and americium dominate the long-term radiotoxicity and heat load of used nuclear fuels. Separating and recycling them as nuclear fuel (the so-called Partitioning and Transmutation strategy) could have a beneficial impact for the design of a safe final repository for highly radioactive nuclear waste.1−3 The required separation could be achieved by solvent extraction. Solvent extraction is a versatile technique for separating ionic solutes. A solvent extraction process in its most simple case consists of two steps. In the first step (extraction), the organic phase (an extracting agent dissolved in an appropriate diluent) is contacted with the aqueous phase. The solute to be extracted from the aqueous phase reacts with the extracting agent, forming a complex that is soluble in the organic phase. After phase separation, the solute is released in a second step

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Special Issue: 2013 Nuclear Chemistry Received: August 16, 2012 Published: January 29, 2013 1199

dx.doi.org/10.1021/cr3003399 | Chem. Rev. 2013, 113, 1199−1236

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(stripping) from the organic phase to another aqueous phase having a chemical composition different from that of the initial aqueous phase (e.g., different acidity or ionic strength). The organic phase usually is recycled after a dedicated treatment (e.g., washing with acidic and/or basic solutions) to remove the remaining impurities and degradation products. Solvent extraction is widely applied from analytical separations in the submilliliter range up to large industrialscale processes such as copper purification.4 Solvent extraction is also used in the nuclear fuel cycle for separating uranium from ore leach solutions5 and for separating uranium and plutonium from irradiated nuclear fuel via the PUREX process.6−8 Worldwide, new solvent extraction processes for the separation of TRU from used nuclear fuels are under development.9 The most elaborate process schemes are based on an improved PUREX process to separate uranium, neptunium, and plutonium. In a following process (such as TRUEX, TRPO, or DIAMEX9), americium, curium, and the “fission lanthanides” (yttrium and lanthanum thru dysprosium) are extracted from the PUREX raffinate solution (a solution containing americium and curium along with all the fission and corrosion products in approximately 3−4 mol/L HNO3). Finally, americium and curium (which are present as trivalent cations) need to be separated from the fission lanthanides in a SANEX (selective actinide extraction9) processsome of the fission lanthanides are strong neutron absorbers, requiring separation from the actinides to be recycled. Trivalent actinides and lanthanides have chemically similar behavior; lanthanides(III) are sometimes used as nonradioactive surrogates when studying actinides(III). Both are predominantly trivalent in solution; they have overlapping ionic radii and are “hard” ions, preferably coordinating to ligands with hard donor atoms such as oxygen. In consequence, common extracting agents (which coordinate via oxygen atoms) are not capable of separating actinides(III) from lanthanides(III). Rather, actinides are extracted similarly to lanthanides(III) of corresponding ionic radius. This is well illustrated by studies from Peppard et al. published more than 50 years ago.10 In the early 1980s, Musikas et al. reported on the preferential complexation of actinides(III) over lanthanides(III) by azide or ortho-phenanthroline11 and on the preferential extraction of Am(III) over Eu(III) by TPTz (see Appendix).12 This triggered work in many institutions to find nitrogen (and also sulfur13−23) donor ligands useful as selective extracting agents for trivalent actinides. Whereas sulfur donor compounds have always been second choice because of waste-management issues, a vast number of bis-, ter-, and quadridentate heterocyclic nitrogen donor ligands have been synthesized and tested as extracting agents with potential selectivity for actinides(III) over lanthanides(III). The lion’s share of this work has been performed in a series of European research programs.24−29 Unfortunately, almost all of these compounds were not able to extract actinides(III) from solutions with an acidity greater than 0.1 mol/L. Furthermore, a lipophilic anion source such as a 2-bromocarboxylic acid (HA) had to be added to the organic phase to form extractable complexes according to a cation-exchange mechanism, M3 +aq + 3HA org + nLorg ⇌ MLnA3org + 3H+aq

Figure 1. General molecular structures of BTP (left) and BTBP (right).

left)32 are able to extract Am(III) (representing actinides(III)) with high selectivity over Eu(III) (representing lanthanides(III)) directly from solutions containing up to molar nitric acid by solvation of the metal nitrates (eq 2). This makes them highly suitable because hydrometallurgical processing of used nuclear fuels involves nitric acid aqueous phases. M3 +aq + 3NO3−aq + 3BTPorg ⇌ M(BTP)3 (NO3)3org

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These first-generation BTPs, however, suffered from low stability against nitric acid and radiation.33 After identifying the degradation mechanism, a new stable BTP, 2,6-bis(5,5,8,8tetramethyl-5,6,7,8-tetrahydro-benzo-1,2,4-triazin-3-yl)pyridine (CyMe4-BTP, Figure 2 left)34 was developed at Reading

Figure 2. CyMe4-BTP (left) and CyMe4-BTBP (right).

University, U.K. Unfortunately, this molecule extracts actinides(III) too efficiently, resulting in problems during stripping (i.e., releasing the extracted solute from the organic phase by reversing the reaction shown in eq 2), and has too slow extraction kinetics. A BTP not suffering from these drawbacks was not found until several years later, when bis-2,6-(5,6,7,8tetrahydro-5,9,9-trimethyl-5,8-methano-benzo-1,2,4-triazin-3yl)pyridine (CA-BTP, Figure 3) was synthesized at Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal (INE).35 BTPs are discussed in detail in section 2.

Figure 3. CA-BTP.

Evolving from the BTP, Reading University, U.K., synthesized 6,6′-bis(5,6-dialkyl-1,2,4-triazin-3-yl)-2,2′-bipyridines (BTBPs, Figure 1 right).36,37 Of the BTBPs, 6,6′-bis(5,5,8,8tetramethyl-5,6,7,8-tetrahydrobenzo-1,2,4-triazin-3-yl)-2,2′-bipyridine (CyMe4-BTBP, Figure 2 right) is the current European reference molecule for the development of actinide(III) separation processes.38 Because of its favorable extraction properties, several successful continuous lab-scale process tests were carried out with CyMe4-BTBP.39−42 BTBPs are discussed in detail in section 3. Recently, water-soluble BTP and BTBP were synthesized and tested for their use as selective hydrophilic masking agents for actinides(III).43−46 These show promise for the development of

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It was in this context that Kolarik et al.30,31 showed that 2,6bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridines (BTPs, Figure 1 1200


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hexyl,67 heptyl,67 octyl,67 phenyl,30,34 p-tolyl,30 and p-tertbutylphenyl.30 A few BTPs substituted in the 4-position of the pyridine ring were also synthesized (Appendix): MeO-BTP and Cl-BTP.89 Furthermore, several annulated BTPs were synthesized: CyMe4-BTP (Figure 2 left),64,83,90 BzCyMe4-BTP (Appendix),90 and CA-BTP (Figure 3).35

separation processes based on the selective stripping of actinides from an organic phase loaded with actinides(III) and lanthanides(III). These compounds are discussed in section 4. Several related reviews cover the heterocyclic nitrogen donor ligands’ synthesis47 and nonactinide coordination chemistry,47 protonation and actinide(III) and lanthanide(III) complexation,48 and evolution as selective actinide(III) extracting agents.49−51 The latter three give a good description of the evolution from 2,2′:6′,2″-terpyridine ligands up to BTP and BTBP. The European development of separation processes for trivalent actinides was also reviewed.52 A further review9 provides a very good overview of worldwide development toward actinide(III) separation, including but not limited to Ndonor extracting agents. So why publish another review on nitrogen donor ligands? We present a review with a focus on the two classes of ligands most promising for an application as agents for separating actinides(III) from lanthanides(III), namely, BTP and BTBP. Whereas previous reviews mainly address historic development, synthesis, and extraction properties, this paper additionally aims at giving a comprehensive review of the work performed on the more fundamental aspects related to actinide separations, such as structures of actinide and lanthanide complexes and speciation studies. A good amount of work has been carried out to understand the unique properties of BT(B)P and to understand the reasons for their high selectivity for actinides(III) over lanthanides(III). This is important because more than 30 years of research and development have not offered the “perfect” nitrogen donor separating agent; only a purposeful search for improved compounds will successfully achieve this. Covering the period from 1999 (the year the first paper on the use of BTP for separating Am(III) from Eu(III) was published), this review is more than just a compilation of the literature; some lessons learned will be discussed and future studies to better understand such systems will be suggested.

2.1. Extraction and Process Tests Using BTP

Not all of the BTPs synthesized proved to be useful as selective extracting agents for actinides(III); e.g., some of them have low solubility in organic diluents applicable for solvent extraction or other drawbacks such as slow kinetics (as compared, e.g., to extraction systems used in the PUREX or DIAMEX processes). Whereas they do not dissolve sufficiently in kerosene (which is the preferred diluent in industrial solvent extraction processes), n-Pr-BTP, i-Pr-BTP, CyMe4-BTP, and CA-BTP have sufficient solubility in more polar diluents (such as kerosene/1-octanol mixtures or 1-octanol). Their extraction behavior has been studied in more detail. To this, distribution ratios of metal ions are determined. The distribution ratio D is the ratio of the solute’s (i.e., the metal ion extracted) concentration in the organic phase over its concentration in the aqueous phase after extraction; DM = [M]org/[M]aq. 2.1.1. n-Pr-BTP. In 1999, Kolarik et al.30 reported that lipophilic BTPs are able to extract Am(III) nitrate from nitrate solutions containing up to 0.9 mol/L HNO3 with high selectivity over Eu(III). For example, a solution of 33 mmol/ L 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (n-Pr-BTP, Figure 1 left, R = n-propyl) in kerosene/2-ethylhexanol (because n-Pr-BTP’s solubility in pure kerosene is too low) extracted Am(III) with a distribution ratio of 23 from an aqueous phase consisting of 0.9 mol/L HNO3 + 1 mol/L NH4NO3 (cf. Figure 4). The selectivity over Eu(III) was

2. LIPOPHILIC BISTRIAZINYLPYRIDINES (BTP) The first synthesis of tridentate 2,6-bis(1,2,4-triazin-3-yl)pyridine ligands is described by Case.32 As shown in Scheme 1, the BTP ligands are synthesized by condensation reaction of Scheme 1. Synthesis of 2,6-Bis(1,2,4-triazin-3-yl)pyridine Ligands32

Figure 4. Extraction of Am(III) and Eu(III) into BTP solution, with distribution ratios as a function of BTP concentration. Organic phase, i-Bu-BTP (open symbols) or n-Pr-BTP (solid symbols) in kerosene/2ethylhexanol (80:20 vol.). Aqueous phase, 241Am(III) + 152Eu(III) in 0.90 mol/L HNO3 + 1.0 mol/L NH4NO3. Reprinted with permission from 30. Copyright 1999 Taylor & Francis Ltd.

SFAm(III)/Eu(III) = DAm(III)/DEu(III) = 130. Considering that up to then no other N-donor extracting agent was capable of selectively extracting Am(III) from solutions with an acidity higher than ≈0.1 mol/L, this was regarded as a breakthrough in the field of actinide(III)/lanthanide(III) separation. In a more detailed study,31 the distribution of n-Pr-BTP between kerosene/2-ethylhexanol and nitric acid solutions was studied and protonation and self-association constants were

pyridine-2,6-dicarbohydrazonamide with a 1,2-diketone.30,31,50 In this manner, BTP compounds with various alkyl groups were prepared and tested for their complexation properties and their ability to extract and separate trivalent actinides from lanthanides (Figure 1 left): R = H,53,54 methyl,30,34,54−67 ethyl,30,34,55,63,65−70 n-propyl30,31,33,56−60,62,65−67,70−82 i-propyl,31,33,58−60,65,79,83−87 n-butyl,30,67,88 i-butyl,30,55 pentyl,67 1201


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derived. n-Pr-BTP has a tendency to self-association in a kerosene/alcohol diluent that becomes less pronounced with increasing nitric acid concentration. Unfortunately, the authors did not take into account the coextraction of nitric acid by 2ethylhexanol,91 meaning that the protonation of BTP may be overestimated. From slope analysis of the extraction of Am(III) and Eu(III) from HNO3 + NH4NO3 solutions into solutions of n-Pr-BTP in kerosene/2-ethylhexanol, the following conclusions were drawn: The slight curvature of the slopes of log DAm(III) or log DEu(III) versus [BTP] is explained by self-association. When plotting log DAm(III) or log DEu(III) versus [BTP]monomeric (as calculated with the self-association constants), slopes of 2.9 were found, meaning that the trivalent metal ions are extracted as 1:3 complexes, [M(BTP)3]3+. From the slopes of log(DAm(III)/γAm(III)) or log(DEm(III)/γEm(III)) versus nitrate activity being close to four, it was concluded that Am(III) and Eu(III) are extracted from acidic nitrate solution according to

Figure 6. Extraction of actinides(III) (circles), lanthanides(III) (triangles), and yttrium(III) (diamond or square) into n-Pr-BTP solution. Organic phase, 15 mmol/L (filled symbols) or 40 mmol/L (open symbols) of n-Pr-BTP in kerosene/1-octanol (70:30 vol.). Aqueous phase, 0.1 mol/L HNO3 + 1.9 mol/L NH4NO3 (filled symbols) or 1 mol/L HNO3 (open symbols). Data are from 92.

M3 +aq + H+aq + 4NO3−aq + 3BTPorg ⇌ M(BTP)3 (NO3)3 HNO3org

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(CEA) to run a countercurrent separation process test in a 16stage laboratory-scale mixer-settler unit.26,80,81,93 The feed was a highly radioactive solution consisting of 126 mg/L of 241Am(III), 1 mg/L of 244Cm(III), and inactive metal ions (Fe(III), Y(III), Ru(III), Pd(II), and La(III)−Gd(III)) in 1 mol/L HNO3; the organic phase was a solution of 40 mmol/L n-PrBTP in kerosene/1-octanol (70:30 vol.); the stripping phase was 0.05 mol/L HNO3. The results were promising; 2 mol/L HNO3. This prevents the application of CA-BTP for processes running at higher nitric acid concentrations. Otherwise, CA-BTP has extraction properties similar to those of n-Pr-BTP. A solution of 50 mmol/L CA-BTP in kerosene/1-octanol (70:30 vol.) extracts Am(III) from 1 mol/L

2.1.2. i-Pr-BTP. The isopropyl analogue, 2,6-bis(5,6diisopropyl-1,2,4-triazin-3-yl)pyridine (i-Pr-BTP, Figure 1 left, R = isopropyl), showed higher distribution ratios for the extraction of Am(III) than n-Pr-BTP. However, i-Pr-BTP was found to have peculiar extraction kinetics: whereas Am(III) was extracted rather quickly, the kinetics of Eu(III) extraction were much slower. Thus, the selectivity decreases with time: at equilibrium, i-Pr-BTP has very low selectivity for Am(III) over Eu(III), SFAm(III)/Eu(III) ≈ 431,33 (see Figure 8 (filled symbols, solid line)). Interestingly, with an aqueous solution containing lanthanides(III) and other metal ions in g/L concentration (rather than only trace concentrations of 241Am(III) and 152 Eu(III) radionuclides), the reverse was observed, with a selectivity of SFAm(III)/Eu(III) ≈ 20033 (see Figure 8 (open symbols, dashed line)). It was not commented whether this difference is due to the presence of metal ions in high concentrations or due to the changed diluent (1-octanol versus kerosene/1-octanol). Owing to its better stability compared to n-Pr-BTP,33 the CEA designed a countercurrent lab-scale separation process with a solvent consisting of 10 mmol/L i-Pr-BTP + 0.5 mol/L DMDOHEMA (N,N′-dimethyl-N,N′-dioctyl-2-(2hexyloxyethyl)malonamide (see Appendix), added as a phase transfer catalyst to accelerate kinetics) in 1-octanol.94 A highly radioactive feed solution prepared from irradiated nuclear fuel was used. Two process tests were run in a 32-stage centrifugal contactor setup, one test without and one test with solvent recycling. In the test without solvent recycling, > 99.9% 1203


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a significant color change was observed after 1 day of phase contact due to oxidative degradation.30 The attempt to increase stability by replacing the alkyl groups by an aromatic substituent such as phenyl or p-tolyl was not successful because of the very low solubility of these BTP compounds.30 p-tertButylphenyl-BTP dissolved well enough, but the replacement of the alkyl by aryl groups strongly affected the extraction properties and the selectivity of the ligand. Very low distribution ratios for An(III) and Ln(III) between 0.1 and 0.2 and a SF of 2 were found.30 To further enhance stability in contact with nitric acid, CyMe4-BTP has been designed for An(III)/Ln(III) separation. The alkyl groups were replaced by cyclohexyl moieties substituted with methyl groups in the benzylic positions to avoid oxidative degradation. The stability toward nitric acid has been tested successfully; CyMe4-BTP was not degraded by boiling in 3 mol/L HNO3.27,34 To overcome the drawbacks of CyMe4-BTP (too high Am(III) distribution ratios, slow kinetics), a new bis-2,6(5,6,7,8-tetrahydro-5,9,9-trimethyl-5,8-methano-1,2,4-benzotriazin-3-yl)pyridine (CA-BTP) ligand has been synthesized (Figure 3). Regarding the design of stable ligands, the following had to be considered: oxidative degradation of BTP or BTBP can proceed along two pathways. In the first pathway the triazine ring decay is induced by oxidation at the benzylic positions, forming ketones with subsequent loss of the acylic side chain. This is a known pathway for the degradation of acyl functions of N-heterocycles in the presence of nucleophiles with formation of zwitterionic heterocyclic species. This degradation process requires primary or secondary carbons in the benzylic positions. The decay of the triazine ring can also be induced by double-bond formation between the benzylic carbon and the adjacent carbon atom in the triazine ring. This may happen if a secondary or tertiary carbon is present at one of the benzylic positions. In the case of quaternary carbons, oxidative degradation according to the pathways mentioned above should be prevented, as is the case for CyMe4-BTP. CABTP carries tertiary and quaternary carbon atoms in the benzylic positions, thus effectively preventing ketone formation. As double bonds do not form at the bridgehead of a small bridged-ring system (Bredt’s rule100), a decay of the triazine ring induced by double-bond formation is also prevented. Indeed, CA-BTP showed good stability: a CA-BTP solution was kept in contact with a solution of Am(III) and Eu(III) in 1 mol/L HNO3 for more than 100 days; Am(III) and Eu(III) distribution ratios did not change.35 Radiolytic degradation of BTP ligands has been studied for various alkylated BTP ligands. Nilsson et al.69 investigated the radiolysis of Et-BTP in 1-hexanol as a function of the dose rate (up to 20 kG). They found a decrease of the americium(III) distribution ratios with increasing absorbed dose and proposed a mechanism initiated by direct radiolysis of the alcoholic solvent. Addition of nitrobenzene to 1-hexanol solutions of EtBTP inhibited the radiolytic degradation. This was not solely due to the aromatic ring because tert-butyl benzene did not stabilize the BTP molecule but was attributed to scavenging of solvated electrons and α-hydroxy alkyl radicals by nitrobenzene.69,98 These results indicated that solvated electrons and α-hydroxy alkyl radicals might be important intermediates of BTP radiolysis. The radiolysis products of nitrobenzene did not seem to have any significant effect on the extraction. However, the authors mentioned that, if 2,4-dinitrophenol was formed during gamma irradiation of nitrobenzene, there might

Figure 9. Comparison of Am(III) and Eu(III) extraction kinetics by CA-BTP (filled symbols) and CyMe4-BTBP (open symbols), with distribution ratios as a function of contacting time. Aqueous phase, 241 Am(III) + 152Eu(III) in 1 mol/L HNO3. Organic phase, 10 mmol/L CA-BTP in kerosene/1-octanol (70:30 vol.) or 10 mmol/L CyMe4BTBP in 1-octanol. Shaking speed, 300/min. Reprinted with permission from ref 35. Copyright 2011 Taylor & Francis Ltd.

HNO3 with DAm(III) = 10 and SFAm(III)/Eu(III) ≈ 100. Cm(III) is slightly better extracted than Am(III) (SFCm(III)/Am(III) = 1.5). The lanthanide extraction profile is almost identical to that for n-Pr-BTP (cf. Figure 6).35 It is concluded that the extraction mechanism is similar to that reported for n-Pr-BTP, cf. eq 2. 2.2. Chemical and Radiation Stability of BTP

Stability against nitric acid and radiolysis is an important requirement of extracting agents with regard to an application in a sustainable industrial process. Despite the radiation stability of a number of compounds with a single triazine functional group,97 first-generation BTPs have been found to be sensitive to oxidative and radiolytic degradation.98 Therefore, a great deal of research effort has been expended in improving their stability for nuclear solvent extraction applications. These investigations resulted in a series of structural modifications toward derivates with improved stability. In this context it is important to know the relevant dose rates to which a SANEX solvent would be subjected when used in an industrial process. To address this, some calculations were performed, showing that dose rates vary from 29 Gy/h (UO2 fuel with a rather low burn-up of 35 GWd/tHM) to 14.3 kGy/h (20% Pu, 2% Am, 2% Cm fast reactor fuel, 150 GWd/tHM burn-up).99 n-Alkylated BTPs are known to undergo rapid oxidative degradation in contact with nitric acid, induced by the abstraction of hydrogen from the benzylic position of the triazine rings (see Figure 10). Subsequently alcohols and ketones, which finally induce the loss of alkyl chains and the decay of the triazine scaffold, are formed.33 Branched alkyl moieties improve the stability versus oxidative degradation, as observed for i-Pr-BTP.33 An increased stability was also observed for i-Bu-BTP compared to n-Bu-BTP, where

Figure 10. Benzylic positions in BTP. 1204


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Table 2. Average M(III)−N Bond Lengths R for [M(BTP)3]3+ Solid-State Compounds; rCN=9 Are M(III) Ionic Radii for a Coordination Number of 9102,103

be a possibility that 2,4,6-trinitrophenol, i.e., picric acid, was also existent, which may form highly explosive substances with metal ions.101 Analogously to the chemical stability, the radiation stability of alkylated BTP ligands was also expected to be enhanced by the introduction of cyclohexyl moieties substituted with methyl groups (CyMe4-BTP). Investigations of Hudson et al.34 on the radiolysis of i-Pr-BTP and CyMe4-BTP in 1-octanol at an absorbed dose of 100 kGy displayed a 80% decomposition for both ligands. Again, the addition of the radical-scavenger nitrobenzene led to reduced decomposition (15%) under equal conditions, indicating that radical intermediates were responsible for the decomposition of BTP ligands. A significant improvement of radiolytic stability was observed for BzCyMe4BTP (Appendix) in 1-octanol, showing no measurable decomposition at an absorbed dose of 100 kGy.34 Thus, it appears that the additional annulated ring added significant extra resistance to radiolysis, both in terms of direct and indirect radiolysis. These results were supported by quantum chemical calculations on the density functional theory (DFT) level.34

ligand

metal salt

R [pm]

rCN=9 [pm]

R − rCN=9 [pm]

reference

Me-BTP Me-BTP Me-BTP n-Pr-BTP n-Pr-BTP n-Pr-BTP n-Pr-BTP CyMe4BTP n-Pr-BTP

La(OTf)3 Ce(OTf)3 CeI3 CeI3 Sm(NO3)3 Tm(NO3)3 Yb(NO3)3 Y(NO3)3

264 262 262 261 258 250 247 248

122 120 120 120 113 105 104 108

142 142 142 141 145 145 143 140

57 57 56 56 74 74 74 34

UI3

254

121

133

56

those of the heavier lanthanides. Average Ce(III)−N bond lengths are 262 (Me-BTP) and 261 pm (n-Pr-BTP). Different alkyl moieties on the triazines did not have a significant influence on the structures.56 Also lanthanum(III) and cerium(III) triflate formed isostructural 1:3 complexes with Me-BTP. Average M(III)−N bond lengths are 264 (M = La) and 262 pm (M = Ce).57 These studies showed that different lanthanides(III) and BTP form isostructural 1:3 complexes, [M(BTP)3]3+, regardless of the counterion and the BTP used. An exception were the complexes formed from nitrates of lighter lanthanides(III) (La, Pr, and Nd) where nitrate anions displaced BTP ligands. Because BTPs are intended for separating actinides(III) from lanthanides(III), there is a strong interest in the structures of the respective actinide(III) complexes. An isostructural 1:3 complex also formed with U(III) and n-Pr-BTP, having an average U(III)−N bond length of 254 pm.56 This represents a significantly shortened bond length compared to Ce(III), which is remarkable considering the similar ionic radii of U(III) and Ce(III); see Table 2. This could be an indication for a higher degree of covalence in the U(III)−N bond compared to the Ln(III)−N bond in the BTP complexes and was considered the first structural explanation for their selectivity. In conclusion, BTPs are able to form 1:3 complexes with U(III) and lanthanides with nine nitrogens from three BTPs coordinating the metal ion in a tricapped trigonal prismatic configuration. No anions or solvent molecules are found in the first coordination sphere. Bond lengths follow the lanthanide(III) cation radii, but a significantly shorter bond length is found in the 1:3 U(III)-BTP complex. 2.3.1.2. Structures in Solution. The solid-state structures gave rise to new questions: Are the solid-state structures the same as those relevant for liquid−liquid extraction (i.e., structures in solution)? Is U(III) a good surrogate for the actinides(III) of interest (which are Am(III) and Cm(III))? To answer these questions, the solution structures of Cm(III)- and Eu(III)-BTP complexes prepared by extracting the M(III) nitrates from nitrate solution into a solution of n-PrBTP in kerosene/1-octanol (70:30 vol.) were investigated by extended X-ray absorption fine-structure (EXAFS) spectroscopy.75 This method provides valuable information on the coordination sphere of the Ln(III) and An(III) complexes formed during the extraction process. The study confirmed the presence of 1:3 complexes, [M(n-Pr-BTP)3]3+ in solution, which agrees with solvent extraction31,73,81 and crystallography studies.56,57,74 Taking into account the uncertainty in bond length determination by EXAFS being ±2 pm,104 the Cm(III)−

2.3. Complexation of An(III) and Ln(III) with BTP

The previous sections have shown that BTPs are promising extracting agents for separating actinides(III) from lanthanides(III). A thorough understanding on a molecular level of how these ligands interact with actinides(III) and lanthanides(III) and what are the driving forces behind their selectivity requires studying their complexation behavior. Addressing these questions, structures and speciation of the complexes formed between actinides(III) or lanthanides(III) and BTBs were investigated. A deeper understanding of the processes involved is useful for optimizing ligand structures on the one hand and for designing successful separation processes on the other hand. 2.3.1. Structures of An(III)- and Ln(III)-BTP complexes. Liquid−liquid distribution studies on BTP extracting agents indicated that actinides(III) and lanthanides(III) are extracted as 1:3 complexes, [M(BTP)3]3+, according to eq 2. However, these studies did not yield any information on the coordination mode or structures of these complexes. They were soon followed by crystallography studies and by structural studies in solution. 2.3.1.1. Solid-State Structures. The addition of lanthanide(III) nitrates (Sm, Tm, and Yb) to a solution of n-Pr-BTP in ethanol in a [M]/[L] ratio of 1:1 resulted in the crystallization of 1:3 complexes, [M(n-Pr-BTP)3]3+.74 The metal ion was coordinated by the pyridine nitrogen and by the two triazinyl nitrogens in 2-position of each BTP molecule. No further ligands were found in the first coordination sphere, making for a 9-coordinate complex of approximate C3 symmetry. Average M(III)−N bond lengths are 258 (Sm), 250 (Tm), and 247 pm (Yb); see Table 2. The bond lengths follow the lanthanide contraction. No 1:3 complexes were observed when the nitrates of lighter lanthanides(III) (La, Pr, and Nd) were reacted with solutions of BTP in alcohols; rather, various complexes with [M]/[L] ratios of 2:1, 1:1, 2:2, and 1:2 were found, with nitrate anions completing the coordination spheres.53,55 On the other hand, 1:3 complexes did form with the light lanthanides(III) if a weakly coordinating anion is used rather than nitrate.56,57 Diffusion of pentane into pyridine solutions containing cerium(III) iodide and different BTPs resulted in the formation of 1:3 [M(BTP)3]3+ complexes isostructural with 1205


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N bond length of 257 pm did not significantly differ from that found for the Eu(III)-BTP complex. Thus, the study concluded that no significant structural difference exists between the Cm(III)-BTP and Eu(III)-BTP complexes in solution. In a continuation of the above study, the n-Pr-BTP complexes of Pu(III), Am(III), Sm(III), Gd(III), Dy(III), Ho(III), Tm(III), and Lu(III) (prepared by extracting the M(III) nitrates from nitrate solution into a solution of n-PrBTP in kerosene/1-octanol (70:30 vol.)) and of U(III) (prepared from UI3(THF)4 in pyridine) were investigated by EXAFS.72,77,92 In all cases, the complex structures in solution are [M(n-Pr-BTP)3]3+ with the M(III)−N bond lengths R given in Table 3. Average bond lengths in the lanthanide(III) Table 3. Average M(III)−N Bond Lengths R for [M(BTP)3]3+ Compounds in Solution As Determined by EXAFS; rCN=9 Are M(III) Ionic Radii for a Coordination Number of 9;102,103 Complexes Prepared by Extraction from Nitrate Solution into n-Pr-BTP Dissolved in Kerosene/1Octanol (70:30 vol.) (Except U(III); Prepared by Mixing UI3(THF)4 and n-Pr-BTP in Pyridine) ligand

metal salt

R [pm]

rCN=9 [pm]

R − rCN=9 [pm]

reference

n-Pr-BTP n-Pr-BTP n-Pr-BTP n-Pr-BTP n-Pr-BTP n-Pr-BTP n-Pr-BTP n-Pr-BTP n-Pr-BTP n-Pr-BTP n-Pr-BTP

Sm(NO3)3 Eu(NO3)3 Gd(NO3)3 Dy(NO3)3 Ho(NO3)3 Tm(NO3)3 Lu(NO3)3 UI3 Pu(NO3)3 Am(NO3)3 Cm(NO3)3

260 256 255 256 256 254 252 257 256 256 257

113 112 111 108 107 105 103 121 118 116 115

147 144 144 148 149 149 149 136 138 140 142

92 75 72 92 92 92 72 72 77 72 75

Figure 11. Average M(III)−N bond lengths R in [An(BTP)3]3+ and [Ln(BTP)3]3+ complexes determined by X-ray crystallography (open symbols) and by EXAFS in solution (solid symbols) as a function of M3+ radius rCN=9. Lines indicate the result from regression analysis. Reprinted with permission from ref 92. Copyright 2013 Elsevier.

and Eu(III) complexes with various inorganic ligands and has been successfully applied for the determination of stability constants and thermodynamic data in aqueous solution.106−118 Speciation of lanthanide(III) and actinide(III) complexes with organic ligands is expected to be more complex than that of inorganic complexes because of intramolecular energy transfer processes from organic molecules to the metal ion and vice versa, which results in either an increase or quenching of the fluorescence process.119,120 From different spectroscopic parameters, including features and positions of excitation and emission bands, as well as fluorescence lifetimes, information on the number and type of coordinating ligands is obtained.121−124 Furthermore, the high sensitivity of this method allows speciation in the submicromolar concentration range. Eu(III) and Cm(III) are used as model elements for trivalent lanthanides/actinides because of their excellent fluorescence properties. Details on the absorption/fluorescence processes of Eu(III) and Cm(III) are given in refs 125 and 106, respectively. 2.3.2.1. Eu(III) Complexation with Alkylated BTP Ligands. First speciation studies on the complexation of Eu(III) with iPr-BTP were performed by Colette et al.60 The fluorescence spectra were recorded in the wavelength range from 570 to 720 nm, displaying emission bands of the 5D0 → 7F1, 7F2, 7F3, and 7 F4 transitions. Prior to complexation with i-Pr-BTP, the fluorescence spectra of Eu(III) in various MeOH/H2O mixtures had been measured. With increasing methanol content the fluorescence intensity and the fluorescence lifetime increased. The 5D0 → 7F2 transition is a hypersensitive transition, which means that the intensity of this band strongly depends on the coordination sphere of the Eu(III). Changes in the inner coordination sphere are reflected by an increased or decreased ratio of the 5D0 → 7F1 and 5D0 → 7F2 emission bands. In the case of increasing amounts of methanol, a decrease of the 7F2 /7F1 transition ratio was observed, indicating a pronounced complexation of Eu(III) with methanol. However, fluorescence lifetime measurements showed that, for MeOH/H2O mixtures containing up to 50% methanol in volume, the coordination of methanol molecules was negligible. A similar effect was observed for nitrate anions. Although nitrate is considered as a weak ligand in aqueous solution, the fluorescence lifetime of Eu(III) in MeOH/H2O (1:1 vol.)

complexes follow the lanthanide contraction, as already observed in the solid state. In contrast to this, average bond lengths of 256−257 pm were found in the actinide(III) complexes, regardless of the cation radius decreasing from 121 (U(III)) to 115 pm (Cm(III)). This structural difference between the [An(BTP)3]3+ and [Ln(BTP)3]3+ complexes (i.e., invariance versus dependence of M(III)−N bond length on ionic radius) is also reproduced by quantum chemical calculations treating the 5f electrons explicitly.77,105 The results from the solid-state and solution actinide and lanthanide [M(BTP)3]3+ complex structures are compared in Figure 11.92 Generally, average M(III)−N bond lengths are longer in the solution structures than in the solid state by ≈3 pm [U(III), 3 pm; Sm(III), 2 pm; Tm(III), 4 pm], showing relaxation from the crystal structure packing. Whereas the bond lengths follow the lanthanide contraction in both the solid-state and solution structures, this is not the case for the actinide(III) complexes. The R − rCN=9 values for the An(III) are shorter than those for the Ln(III) (Table 3), which was interpreted as an indication of a generally higher ionic bonding character for the lanthanide(III) complexes compared to the actinide(III) complexes studied.92 2.3.2. Speciation Studies by TRLFS. Various efforts have been made to study the coordination chemistry of Cm(III) and Eu(III) with BTP ligands in aqueous60,76 and organic65,72,75,87 solutions using time-resolved laser fluorescence spectroscopy (TRLFS). TRLFS is a versatile tool for speciation of Cm(III) 1206


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Figure 12. (a) Fluorescence spectra of Eu(III) in water/methanol (1:1 vol.) at increasing n-Pr-BTP concentration. [Eu(III)]ini = 2.14 × 10−5 mol/L. (b) Distribution of Eu(III) complex species in water/methanol (1:1 vol.) as a function of n-Pr-BTP concentration. [Eu(III)]ini = 2.14 × 10−5 mol/L. Reprinted with permission from ref 76. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

increased from 110 μs (perchlorate medium) to 125 μs in the presence of nitrate due to the formation of Eu(III)-nitrate inner-sphere complexes. The fluorescence spectrum of Eu(III) in presence of an excess of i-Pr-BTP revealed the formation of one single Eu(III)BTP complex. The fluorescence lifetime of this species was 2700 ± 150 μs. According to ref 124 this lifetime corresponds to the formation of the [Eu(i-Pr-BTP)3]3+ complex, proving the complete replacement of the nine inner-sphere water molecules of the Eu(III) aquo ion by three tridentate N-donor ligands. The formation of Eu-BTP complexes with a lower number of BTP ligands, i.e., [Eu(i-Pr-BTP)]3+ and [Eu(i-Pr-BTP)2]3+, was not observed in MeOH/H2O (1:1 vol.) medium.60 Similar results were obtained by Miguirditchian et al.61 for Me-BTP in methanol/water (1:1 vol.). Although the spectrum of Eu(III) (at [Eu3+] = [Me-BTP] = 10−4 mol/L) was dominated by the emission spectrum of the Eu(III)-solvent complex, the formation of the [Eu(Me-BTP)3]3+ complex with a fluorescence lifetime of 1900 μs was confirmed by the fluorescence spectrum recorded after a delay time of 800 μs. A short lifetime component (τ = 191 μs) observed during fluorescence lifetime measurements gave the first hints on the existence of a 1:1 [Eu(Me-BTP)]3+ species. A further study on the complexation of Cm(III) and Eu(III) with n-Pr-BTP in water/methanol (1:1 vol.) mixture was performed by Trumm et al.76 The Eu(III) fluorescence spectra resulting from the 5D0 → 7F1 and 5D0 → 7F2 transitions as a function of the n-Pr-BTP concentration are shown in Figure 12. The emission spectrum of the solvated Eu(III) species ([Eu(solv.)]3+) in water/methanol (1:1 vol.) showed broad emission bands with emission maxima at 593.4 nm (5D0 → 7F1) and 617.4 nm (5D0 → 7F2). The transition ratio 7F1/7F2 of the solvated species was 2.88 and thus lower than 3.74 for the aquo ion with nine water molecules in the inner coordination sphere, indicating a decrease in symmetry. In contrast to the results of Colette et al.,60 this difference in the intensity ratio was caused by a significant impact of methanol ligands on the coordination of Eu(III) in the MeOH/H2O (1:1 vol.) medium. This was also reflected in a longer fluorescence lifetime (118 μs) compared to the aquo ion (110 μs). With increasing amount of n-Pr-BTP, a new complex species was formed, displaying a distinct splitting of the 5D0 → 7F1 emission band with maxima at 592.3 and 597.1 nm and a narrow 5D0 → 7F2 transition band at 618.5 nm.

The 7F1/7F2 transition ratio for this species was 0.86. Furthermore, the fluorescence intensity increased by a factor of FI = 80 compared to that of [Eu(solv.)]3+. Although the resolution of the spectra was higher than that in Colette et al.,60 the comparison clearly showed that in both cases an identical Eu-BTP species was formed. The fluorescence lifetime τ of this species was determined to be 1727 μs, which is shorter than the fluorescence lifetime reported earlier,60 but still proving the absence of quenching molecules (water and alcohol) in the inner coordination sphere. Therefore, these results also confirmed the exclusive formation of [Eu(n-Pr-BTP)3]3+ in water/methanol (1:1 vol.). Speciation of [Eu(solv)]3+ and [Eu(n-Pr-BTP)3]3+ (species 3) as a function of n-Pr-BTP concentration is shown in Figure 12. The speciation of Eu-BTP complexes in kerosene/1-octanol (70:30 vol.) differs significantly from that in water/methanol. As described in ref 75, the fluorescence spectrum of Eu(III) resulting from the 5D0 → 7F1 and 5D0 → 7F2 transitions changed upon addition of increasing concentrations of n-PrBTP, indicating that at least two different [Eu(n-Pr-BTP)n]3+ (n = 1−3) species were formed for BTP concentrations ranging from 10−4 to 5 × 10−3 mol/L. The formation of different species with increasing n-Pr-BTP concentration was also confirmed by changes of the transition ratio (7F2/7F1). Whereas the Eu3+ aquo ion displayed a transition ratio 7F2/7F1 < 0.6, the ratio increased to 1.16 for [n-Pr-BTP] = 10−4 mol/L and decreased again to 1.06 ([n-Pr-BTP] = 5 × 10−3 mol/L) with increasing ligand concentration. At high ligand concentrations the fluorescence spectrum in kerosene/1-octanol (70:30 vol.)) was identical to the [Eu(i-Pr-BTP)3]3+ 60 and [Eu(n-PrBTP)3]3+ 76 fluorescence spectra measured in methanol/water (1:1 vol.), indicating that Eu(n-Pr-BTP)33+ was exclusively formed under these conditions. For the first time, a lower Eu-n-Pr-BTP complex was identified by its fluorescence lifetime:72 the fluorescence lifetime of Eu(III) in kerosene/1-octanol (70:30 vol.) was 382 μs, which is significantly higher than 110 μs in aqueous medium. This is due to the presence of nitrate ions in the inner coordination sphere and the reduced quenching of the 5D0 level of Eu(III) by 1-octanol molecules compared to coordinated H2O in the inner coordination sphere. Eu(III) in the presence of increasing amounts of BTP showed a biexponential fluorescence intensity decay with short-lived (524 ± 5 μs) 1207


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Figure 13. (a) Fluorescence spectrum of Eu(III) at 5 × 10−4 mol/L n-Pr-BTP and the spectra of the pure components [Eu(n-Pr-BTP)]3+ and [Eu(nPr-BTP)3]3+, taking into account the respective fractions. This figure was published in ref 72. Copyright 2007 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved. (b) 1/τ versus nBTP for Eu(III)-Et-BTP in methanol (solid line)65 and Eu(III)-n-Pr-BTP in kerosene/1octanol (70:30 vol.) (dashed line).72 Reprinted with permission from ref 65. Copyright 2011 Oldenbourg Wissenschaftsverlag GmbH.

and long-lived (2437 ± 22 μs) lifetime components. The fluorescence lifetime τ of Eu(III) is determined by radiative and nonradiative decay processes of the excited state. In aqueous solution nonradiative decay is predominantly caused by energy transfer from the Eu(III) ion excited state to vibrational states (OH vibrations) of coordinated water molecules. Hence, the replacement of water molecules in the inner coordination sphere with complexing ligands leads to an increase in fluorescence lifetime due to a reduced nonradiative decay. An empirical linear correlation has been developed by Kimura and Choppin relating the number of inner-sphere water molecules of Eu(III) and the fluorescence decay rate.122 A similar method was applied for the determination of the number of BTP molecules coordinating Eu(III). 72 The following linear correlation between the number of BTP molecules in the inner coordination sphere of Eu(III) and the fluorescence decay rate (Kobs = 1/τ) was derived: n·BTP = − 4.08·Kobs + 10.67

decay data in the presence of Et-BTP also showed two lifetime components: a long-lived component of 2560 ± 150 μs, which was attributed to the [Eu(Et-BTP)3]3+ complex, and a shortlived component of 890 ± 60 μs. Using a similar linear correlation as described in ref 72, the fluorescence lifetimes of the [Eu(Et-BTP)n]3+ (n = 1−3) complexes were determined to be 500 μs for n = 1, 890 μs for n = 2, and 2560 μs for n = 3. Therefore, the species with the short fluorescence lifetime was clearly identified as the [Eu(Et-BTP)2]3+ complex. The fluorescence lifetimes of the [Eu(Et-BTP)n]3+ (n = 1−3) complexes in methanol65 are in excellent agreement with those in kerosene/1-octanol (70:30 vol.).72 Plots of 1/τ versus nBTP for Eu(III)-ethyl-BTP in methanol (solid line)65 and Eu(III)-npropyl-BTP in kerosene/1-octanol (70:30 vol.)72 (dashed line) are shown in Figure 13b. Because of the significant differences in the speciation of Eu(III)-BTP complexes in alcoholic and aqueous solution, a systematic study on the complexation of Eu(III) with i-Pr-BTP in 1-octanol with various contents of water was performed by Vu.132 As the quantity of water in 1-octanol was shown to be rather high (e.g., ≈4 × 10−2 mol/L in commercially available 1octanol and ≈2 mol/L after contact with an aqueous phase), the water molecules as strong complexing ligands for Eu(III) were expected to have a considerable impact on the speciation of Eu-BTP complexes. Eu(III) nitrate complexation by i-PrBTP in 1-octanol with two different water concentrations (4 × 10−2 and 2 mol/L) was studied for [i-Pr-BTP] = 0−8 mmol/L. Changes in the speciation were followed by the 7F1/7F2 transition ratio increasing from 0.2 to 1.3 with increasing ligand concentration for a water content of 4 × 10−2 mol/L. In the presence of 2 mol/L water, a 7F1/7F2 transition ratio > 0.4 was observed for lower ligand concentrations, whereas for [i-PrBTP] ≥ 3 mmol/L, the course of the 7F1/7F2 transition ratio increasing up to 1.3 is identical to that of the lower water concentration. These results showed that variable water contents in alcohol-based solvents have a strong influence on the formation of [Eu(n-Pr-BTP)n]3+ (n = 1−3) complexes, in particular at lower ligand concentrations where the 1:1 and 1:2 Eu-BTP complexes are the prevailing species. At [i-Pr-BTP] ≥ 3 mmol/L the 1:3 [Eu(n-Pr-BTP)3]3+ complex was preferentially formed and the impact of the water content in the diluent was negligible. Under these conditions the emission spectrum

(4)

Using this equation, the fluorescence lifetimes of the [Eu(nPr-BTP)n]3+ (n = 1−3) complexes were calculated to be 532 μs for n = 1, 873 μs for n = 2, and 2437 μs for n = 3, and the EuBTP species with the short-lived and long-lived lifetime components were attributed to the [Eu(n-Pr-BTP)]3+ and the [Eu(n-Pr-BTP)3]3+ complexes, respectively. The fluorescence spectra of both species are shown in Figure 13a. Similar studies on the Eu(III) complexation with BTP have been performed by Rawat et al.65 using Et-BTP in methanol. The authors also reported a significant change in the transition ratio 7F2/7F1 with increasing BTP concentration. The 7F2/7F1 ratio of Eu(NO3)3 in methanol was determined to be 3.32. At low ligand concentration (10−3 mol/L), the value decreased to 1.49. A further decrease to 1.27 was observed for higher ligand concentration (8 × 10−3 mol/L). Although the total numbers of the 7F2/7F1 ratios differed slightly from those in kerosene/1octanol (70:30 vol.), the trend was almost identical. Fluorescence lifetime measurements also confirmed that upon addition of increasing amounts of BTP various [Eu(EtBTP)n]3+ complexes are formed. The lifetime of the 5D0 state of Eu(III) in methanol was found to be 360 μs, which is in very good agreement with the fluorescence lifetime of Eu(III) in kerosene/1-octanol (70:30 vol.) of 382 μs. The fluorescence 1208


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Figure 14. (a) Normalized fluorescence spectra of Cm(III) in water/methanol (1:1 vol.) at increasing n-Pr-BTP concentration. [Cm(III)]ini = 1.82 × 10−7 mol/L. (b) Distribution of Cm(III) complex species as a function of the n-Pr-BTP concentration. Reprinted with permission from ref 76. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The fluorescence lifetime of Cm(III) in kerosene/1-octanol (70:30 vol.) was 394 μs, whereas the fluorescence lifetime decreased to 313 ± 8 μs upon formation of the [Cm(n-PrBTP)3]3+ complex. This lifetime is significantly shorter than the calculated radiative lifetime of 1250 ± 80 μs126 expected for Cm(III) with 9-fold coordination by three tridentate ligands, replacing all nine coordinated solvent molecules. The observed decrease of the fluorescence lifetime of the [Cm(n-Pr-BTP)3]3+ complex results from an intramolecular energy transfer process from the Cm(III) excited state to a low-lying triplet state of the organic ligand. Such energy transfer processes can proceed in both directions, either from the excited state of the metal ion to the organic ligand or vice versa, resulting in either an increase of the fluorescence intensity or a decrease of the fluorescence lifetime, respectively, with increasing number of organic ligands in the inner coordination sphere. For a number of Cm(III) complexes with organic ligands, including BTPs and BTBPs, both effects occur simultaneously.76,127 Therefore, the determination of coordinated BTP molecules from the fluorescence lifetime, as was performed for [Eu(n-Pr-BTP)n]3+ (n = 1−3) complexes,72,65 cannot be applied to [Cm(n-Pr-BTP)n]3+ (n = 1−3). Similar quenching processes were observed for [Cm(III)(ClPh)2PSSH-synergist] complexes (see Appendix).128 The lifetimes (221 μs for [Cm(ClPh)2PSSH-TBP], 267 μs for [Cm(ClPh)2PSSH-TOPO], and 318 μs for [Cm(ClPh)2PSSHT2EHP]) were also significantly shorter than the theoretical lifetime of 1250 ± 80 μs.126 A further study on the complexation of Cm(III) with n-PrBTP was performed in methanol/water (1:1 vol.), aiming at a comparison of the complexation properties of Eu(III) (cf. section 2.3.2.1) and Cm(III) toward n-Pr-BTP under the same conditions.76 Furthermore, it should be clarified whether the selectivity of BTP is confined to the organic phase. This information is of particular relevance for the development of innovative separation processes based on selective complexation of An(III) in the aqueous phase (cf. section 4). The evolution of the Cm(III) fluorescence spectra in water/ methanol (1:1 vol.) resulting from the 6D′7/2 → 8S′7/2 transition at increasing concentration of n-Pr-BTP is shown in Figure 14a. The [Cm(solv.)]3+ complex displays a broad emission band at 594.3 nm. In comparison to the Cm-aquo ion with nine water molecules in the inner coordination sphere, the emission band was shifted 0.6 nm to higher wavelength and the

and the fluorescence lifetime (2200 μs) were in good agreement with those of [Eu(n-Pr-BTP)3]3+ in kerosene/1octanol (70:30 vol.),75,72 [Eu(i-Pr-BTP)3]3+ in methanol/ water,60 and [Eu(Et-BTP)3]3+ in methanol medium.65 2.3.2.2. Cm(III) Complexation with Alkylated BTP Ligands. Because of its excellent fluorescence properties, Cm(III) is used as representative to study complexation reactions of trivalent actinides in solution. The fluorescence process of Cm(III) is characterized by a high quantum yield and a long fluorescence lifetime in the μs−ms range. Therefore, the sensitivity of TRLFS for Cm(III) is extremely high. The limit of detection has been determined by Wimmer et al.111 to be 5.5 × 10−12 mol/L (signal-to-noise ratio = 1) in 0.1 mol/L HClO4. To derive speciation information a Cm(III) concentration of 10−9−10−7 mol/L is used. Contrary to Eu(III), the emission bands of Cm(III) complexes with organic and inorganic ligands exhibit strong bathochromic shifts of up to 20 nm, facilitating the detection and quantitative determination of individual Cm(III) species even in complex mixtures.106 The first results on the complexation of Cm(III) with n-PrBTP in kerosene/1-octanol (70:30 vol.) were reported by Denecke et al.75 In the ligand concentration range of 10−6−5 × 10−5 mol/L, one species was formed exclusively, which was identified to be the 1:3 [Cm(n-Pr-BTP)3]3+ complex. The fluorescence spectrum of [Cm(n-Pr-BTP)3]3+ displayed an emission band at 613.0 nm with a distinct shoulder at the blue flank of the spectrum. In comparison to that of the Cm3+ aquo ion (593.7 nm),106 the spectrum of [Cm(n-Pr-BTP)3]3+ is shifted 19.3 nm to higher wavelength. This enormous bathochromic shift is the result of a strong ligand field splitting due to a 9-fold coordination of Cm(III) by three tridentate BTP ligands. The formation of 1:1 [Cm(n-Pr-BTP)]3+ and 1:2 [Cm(n-Pr-BTP)2]3+ complexes was not observed under these conditions. Although a quantitative determination of the Eu(III)- and Cm(III)-n-Pr-BTP complexes was not performed at that time, the comparison with the results on Eu(III) with nPr-BTP in kerosene/1-octanol (70:30 vol.) clearly showed that [Cm(n-Pr-BTP)3]3+ is formed at much lower ligand concentration, in accordance with the higher affinity of n-Pr-BTP toward An(III) and consistent with its selectivity in liquid− liquid extraction.31 In contrast to Eu(III), no linear correlation between the number of BTP molecules in the inner coordination sphere of Cm(III) and the fluorescence decay rate (Kobs = 1/τ) (see section 2.3.2.1 and ref 72) was observed. 1209


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Figure 15. (a) Spectral variation of Et-BTP (10−4 mol/L) in the course of titration with Eu(III) solution (3.8 × 10−4 mol/L) in methanol at I = 0.01 mol/L (TPAN). (b) Variation in the absorbance of Et-BTP at 335 nm and speciation of [Eu(Et-BTP)n]3+ (n = 1−3) complexes. Reprinted with permission from ref 65. Copyright 2011 Oldenbourg Wissenschaftsverlag GmbH.

fluorescence lifetime increased from 65 μs106 (of the aquo complex) to 71 μs due to the coordination of methanol instead of water. Details on the solvation of Cm(III) in various binary aqueous solvent mixtures (including methanol/water) obtained by TRLFS are presented in ref 129. In the presence of increasing n-Pr-BTP concentration, two Cm-BTP complex species were formed simultaneously, displaying emission maxima at 602.0 and 613.2 nm, respectively.76 The first species, identified to be the 1:1 [Cm(n-Pr-BTP)]3+ complex, was solely observed for ligand concentrations < 10−5 mol/L with a maximum fraction of 9.1%. At higher ligand concentrations the 1:3 [Cm(n-Pr-BTP)3]3+ complex (emission maximum at 613.2 nm) prevailed. The distribution of the Cm(III) complex species in water/methanol (1:1 vol.) as a function of the n-Pr-BTP concentration is shown in Figure 14b. Comparison with the species distribution of the [Eu(n-Pr-BTP)n]3+ (n = 1−3) complexes (Figure 12b76) shows that much lower ligand concentrations are required to form the 1:3 [Cm(n-Pr-BTP)3]3+ species, which was also reflected by a difference in the stability constants of Cm(III) and Eu(III) of >2 orders of magnitude (cf. section 2.3.6). The formation of the 1:3 [Cm(n-Pr-BTP)3]3+ complex was confirmed by slope analysis. The double logarithmic plot of the spectroscopically determined concentration ratios ([Cm(n-PrBTP)3]3+/[Cm(solv.)]3+) as a function of the n-Pr-BTP concentration provided a linear correlation with a slope of 2.99. The fluorescence spectrum of the 1:3 complex is identical to the spectrum in kerosene/1-octanol (70:30 vol.).75 The fluorescence lifetime was determined to be 345 μs, which is in good agreement with the value of 313 ± 8 μs of [Cm(n-PrBTP)3]3+ reported by Denecke et al.72 It is also significantly shorter than the expected lifetime of 1250 ± 80 μs for a 9-fold coordinated Cm(III) species.126 This confirmed that the fluorescence decay of [Cm(n-Pr-BTP)3]3+ in methanol/water (1:1 vol.) is also mainly determined by an intramolecular energy transfer between the Cm(III) and the BTP ligands, resulting in a strong decrease of the fluorescence lifetime. As already mentioned,72 the determination of the number of complexing BTP ligands from the fluorescence decay constants according to eq 4 is not possible. A similar behavior was observed for Cm-BTBP complex species in methanol/water (1:1 vol.)127 (cf. section 3.3.2). On the other hand, [Cm(n-PrBTP)3]3+ exhibited a significantly higher fluorescence intensity compared to the solvated Cm(III) ion (FI ([Cm(n-Pr-

BTP)3]3+/[Cm(solv.)]3+) = 17), indicating that energy is also transferred from the organic ligands to the metal ion at the same time. This process is known as “sensitized fluorescence emission” and requires a high absorptivity of the aromatic ligands at the wavelength of 396.6 nm used for excitation of Cm(III) and an energy overlapping between triplet states of the ligand and excited levels of Cm(III), resulting in a high energy transfer yield. More details on the mechanism of this process obtained for Cm(III) complexes with aromatic ligands are given in ref 120. The relative fluorescence intensity of [Eu(n-PrBTP)3]3+ (FI ([Eu(n-Pr-BTP)3]3+/[Eu(solv.)]3+) = 80)76 is even higher than that of the respective Cm(III) species, indicating a significantly stronger energy transfer to the metal ion. Energy transfer processes in the opposite direction leading to a decrease in the fluorescence lifetimes were not observed for Eu-BTP complexesneither in organic nor aqueous solvents (cf. section 2.3.2.1). 2.3.3. Speciation Studies by UV/vis Spectrophotometry. Complementary to TRLFS, UV/vis spectrophotometry is commonly used to identify and quantify Ln(III) and An(III) complexes with N-donor ligands. This method not only provided valuable information on different species that were formed at various conditions but also was used to determine stability constants and thermodynamic data of the complex formation (ΔH, ΔS, and ΔG) (cf. section 2.3.6).61,65,77,78 Two different approaches were used: In the case of lanthanides, which have very low absorption coefficients for f−f transitions ( 2 and the only one present for n ≥ 3. For n < 3, also the 1:2 complex was found.56 When Me-BTP was added to a solution containing both U(III) and Ce(III), only the U(III) 1:3 complex formed for n ≤ 3; Ce(III) 1:2 and 1:3 complexes were detected for n > 3, showing a preferential complexation of U(III).56 Relaxation titrations were performed with Gd(III) and MeBTP or CyMe4-BTP.34 By measuring the relaxation rate 1/T1 per mmol Gd(III) as a function of ligand-to-metal ion concentration ratio, the stoichiometry of the final complex can be determined. Coordination of BTPs to the Gd(III) ions replaces solvent molecules; these relax slower when in the bulk solution. The relaxivity decreases until a plateau is reached. This plateau was reached for a ligand-to-metal ion concentration ratio of 3:1 both with Me-BTP and CyMe4-BTP, confirming the formation of 1:3 [Gd(BTP)3]3+ complexes. The total relaxivity decrease in formation of the [Gd(CyMe4BTP)3]3+ complex corresponds to the displacement of 8−9 acetonitrile molecules. A lower relaxivity plateau was found for CyMe4-BTP compared to Me-BTP, indicating that the bulkiness of the CyMe4 groups limits solvation. 2.3.6. Thermodynamic Data of An(III) and Ln(III) Complexation by BTP. A great deal of effort has been expended in determining stability constants of lanthanide and actinide complexes with BTP ligands (Tables 4 and 5). The methods used were time-resolved laser fluorescence spectroscopy (TRLFS),60,76,87 UV/vis/NIR spectrophotometry,61,65,78 and electrospray ionization mass spectrometry (ESI-MS).58,59 The thermodynamic data, such as ΔH, ΔS, and ΔG, were determined by temperature-dependent TRLFS, UV/vis, and micro-calorimetry. As already discussed above, the speciation of [Ln(BTP)n]3+ and [An(BTP)n]3+ (n = 1−3) complexes depends strongly on the diluent. Therefore, the stability constants obtained for different solvents or solvent mixtures are conditional stability constants and can differ by several orders of magnitude. In the following only stability constants determined in the same solvent are compared. The stability constants of the 1:3 complexes are of particular interest, as the 1:3 complexes are the species formed in biphasic extraction systems. 2.3.6.1. Stability Constants. The stability constant of the [Eu(i-Pr-BTP)3]3+ complex in aqueous solution [methanol/ water (1:1 vol.)] was determined by TRLFS in the conditional pH range from 2.8−4.6. The value of log β3 = 14.3 ± 0.660 did not show any systematic pH dependence in the pH region considered. A slightly lower constant of log β3 = 13.4 ± 0.6 was obtained by Vu for [Eu(i-Pr-BTP)3]3+ in 1-octanol (c(H2O) = 0.04 mol/ L), also by the use of TRLFS.87 Increasing the nitrate concentration and the amount of water in the solvent led to a decrease in the stability constant to log β3 = 12.4 ± 0.4 and 9.8 ± 0.4, respectively. In contrast to the results of Colette et al.

excludes itself from the solvent due to the alkyl chains at the triazinyl rings. Similar observations were described130 when the 1:3 complex of Cm(III) with n-Pr-BTP in H2O/i-PrOH (1:1 vol.) was treated with increasing concentrations of HClO4. The speciation was followed by TRLFS. The [Cm(n-Pr-BTP)3]3+ complex remained stable even at pH values below 0.5; no competition between complexation and protonation was observed. In the case of Eu(III)-Me-BTP complexation, a contrarious trend was observed.58 The species distribution of the [Eu(Me-BTP)n]3+ (n = 1−3) complexes shifted to lower complexes when increasing the pH from 2.8 to 4.6. This was explained by the authors by the less hydrophobic character of the Me-BTP ligand exterior increasing the stability of the 1:1 complexes via hydrogen bonds with solvent molecules. Because of the results of ref 58, i-Pr-BTP was used to extend the speciation studies with ESI-MS to the complete lanthanide series.59 The experiments were performed in methanol/water (1:1 vol.) in a conditional pH range of 2.8−4.6. In good agreement with the results obtained for Eu(III), the Ln(III) complexation with i-Pr-BTP led to the exclusive formation of 1:3 complexes with the whole lanthanide series. In a further study, the complexation of n-Pr-BTP with lanthanides was investigated in different alcoholic solvents by nanoelectrospray mass spectrometry.71 The previous measurements of Eu(III) in the water/methanol system58 were repeated to prove the low invasiveness of the nano-ESI approach. In agreement with earlier observations,58 the exclusive formation of 1:3 complexes [Eu(n-Pr-BTP)3(NO3)]2+ and [Eu(n-PrBTP)3]3+ was confirmed, even at substoichiometric n-Pr-BTP concentrations. The speciation changed significantly when the chain length of the alcohols was increased from methanol, via 2propanol and 1-hexanol, to 1-octanol. Whereas the 1:3 complexes [Eu(n-Pr-BTP)3(NO3)]2+ and [Eu(n-Pr-BTP)3]3+ were the dominating species in methanol, increasing amounts of the 1:2 complexes [Eu(n-Pr-BTP)2(NO3)2]+ and [Eu(n-PrBTP)2(NO3)]2+ were formed in 2-propanol, 1-hexanol, and 1octanol. In 1-octanol even 3.1% of the 1:1 complex [Eu(n-PrBTP)(NO3)]2+ was observed. These results confirmed a strong influence of the diluent on the speciation of the Eu(III)-n-PrBTP complexes. Further speciation studies on selected lanthanides with n-Pr-BTP were performed in 1-octanol at [L]/[M] ratios of 2:1 and 10:1. Besides monomeric species [Eu(n-Pr-BTP)n(NO3)m](3−m)+ (n = 1−3; m = 1−2), dimeric species (i.e., [Ln2(n-Pr-BTP)4(NO3)4]2+ and [Ln2(n-PrBTP)3(NO3)4]2+) were observed. The abundance of the dimers decreased with decreasing ionic radius. For both ligand concentrations the fraction of 1:3 complexes continuously increased from La(III) to Lu(III). Even with an excess of ligand, the formation of the 1:3 complex was not favored for La(III), Pr(III), and Eu(III). The relative amount of 1:3 complexes detected for different lanthanides(III) shows a qualitative agreement with their extractability (cf. Figure 6). 2.3.5. Speciation Studies by NMR. The formation of 1:3 complexes between actinides(III) or lanthanides(III) and BTP in solution was also confirmed by 1H-NMR titrations. Titration of a solution of 20 mmol/L UI3 in pyridine with Me-BTP showed the exclusive formation of the 1:3 complex [U(Me1213


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in methanol/water (1:1 vol.),60 both 1:2 and 1:3 complexes were formed under these conditions. The 1:2 complexes were found to contain nitrate ions in the inner coordination sphere, favoring their formation in organic solvents. In the absence of competing nitrate anions, no 1:2 complexes formed in 1octanol solution and the stability constant of the [Eu(i-PrBTP)3]3+ complex increased to log β3 = 13.9 ± 0.4. A comparative study on the complexation behavior of different alkylated BTPs (Me-BTP, n-Pr-BTP, and i-Pr-BTP) toward Eu(III) by ESI-MS revealed that the stability constant of the [Eu(n-Pr-BTP)3]3+ complex (log β3 = 12.0 ± 0.5) in methanol/water (1:1 vol.) is by 2 orders of magnitude lower than that of [Eu(i-Pr-BTP)3]3+ (log β3 = 14.0 ± 0.6).58 These two values are in excellent agreement with the Eu(III) stability constants obtained by TRLFS for n-Pr-BTP (log β3 = 11.9 ± 0.2)76 and i-Pr-BTP (log β3 = 14.3 ± 0.6).60 In the case of MeBTP, no 1:3 complexes were formed. The stability constants of the [Eu(Me-BTP)]3+ and the [Eu(Me-BTP)2]3+ complexes were log β1 = 2.9 ± 0.2 and log β2 = 6.3 ± 0.1, respectively. The log β1 value for Eu-Me-BTP is in excellent agreement with the one determined in methanol/water (75:25 vol.) by UV/vis.61 Because of its prominent complexation properties, i-Pr-BTP was used to determine stability constants of 1:3 complexes of the entire lanthanide series in methanol/water (1:1 vol.).59 The data presented in Table 4 show that log β3 of the 1:3 lanthanide(III) complexes increased almost regularly from La(III) (log β3 = 11.7 ± 0.1) to Lu(III) (log β3 = 16.7 ± 0.8). Such an increase in the stability of lanthanide complexes throughout the series is in agreement with an electrostatic bonding model in which the formation constants increase with the effective nuclear charge of the lanthanide ion. In addition to stability constants of [Ln(BTP)n]3+ (n = 1−3) complexes in aqueous alcoholic solvents, log βn values were also obtained in methanol (I = 0.01 mol/L Pr4NNO3 or Et4NNO3) by UV/vis spectrophotometry.65,78 In the case of La(III), only the 1:1 complex was observed. This was attributed to the higher ionic radius of La3+ and the rather small nitrogen cavity of BTP as predicted by theoretical calculations.54 The log β1 of [La(EtBTP)]3+65 was found to be equal to that reported for [La(n-PrBTP)]3+,78 whereas the log β2 and log β3 values for Eu(III) differed by 0.5 orders of magnitude (Et-BTP: log β2 = 8.94 ± 0.05, log β3 = 13.61 ± 0.1; n-Pr-BTP: log β2 = 9.5 ± 0.03, log β3 = 14.2 ± 0.2). As shown in Table 4, the lighter lanthanides (except La(III)) formed 1:2 and 1:3 complexes with Et-BTP, whereas in the case of the heavier ones, only 1:2 complexes were observed.65 This was explained by a decrease of the coordination number from 9 to 8 due to the decrease in ionic radius within the lanthanide series. The log β2 values of the [Ln(n-Pr-BTP)2]3+ complexes increased from Nd(III) to Tb(III); then there was a decrease at Ho(III), and the values increased again from Tm(III) to Lu(III). This “S”-shaped behavior of the log β2 values corresponds to the increase in ionic potential and the decrease in coordination number near Tb(III). A similar trend in the stability constants along the lanthanide series was also observed for other N-donor ligands.131 These results are in contrast to the course of the log β3 values observed for lanthanide complexes (except Pm(III)) with i-PrBTP in methanol/water (1:1 vol.), where a monotonic increase of the stability constants from La(III) to Lu(III) occurred.59 Figure 17 compares the stability constants to distribution data for the extraction of lanthanides(III). Considering the uncertainties in the stability constants, the two profiles are in

Figure 17. Comparison of conditional stability constants log β3 for the complexation of lanthanides(III) with i-Pr-BTP in H2O/methanol (circles; data from ref 59/Table 4) with distribution ratios for the extraction of lanthanides(III) from 0.1 mol/L HNO3 + 1.9 mol/L NH4NO3 into a solution of n-Pr-BTP in kerosene/1-octanol (70:30 vol.) (triangles; data from 92, cf. Figure 6).

good agreement. This indicates that the extraction selectivity is governed by differences in the stability constants and agrees with conclusions in ref 71. The same was also observed for BTP’s selectivity for actinides(III) over lanthanides(III); see below. A graphical summary depicting the variation of Eu(III) log βn (n = 2 and 3) values for BTP ligands with varying number of Catoms in the alkyl chain for different solvent media (methanol/ water and methanol) was presented by Rawat at al.65 (see Figure 18). This figure illustrates that the log βn (n = 2 and 3)

Figure 18. Variation of log βn (n = 2 and 3) of Eu(III)-BTP complexes with alkyl moiety R. Filled symbols, methanol solution; open symbols, H2O/methanol solution. Triangles, log β2; circles, log β3. Figure according to ref 65 with data from Table 4.

values are ≈2 orders of magnitude higher in pure methanol solution than those in water−methanol. This was explained by a lower energy required for desolvating the metal ion in pure methanol compared to water−methanol, where the Ln(III) ion is predominantly coordinated with water molecules. Regardless of the medium, the log β2 and log β3 values slightly increased with alkyl chain length, which was attributed to hydrophobic interactions. Furthermore, the increase of the log β2 and log β3 is more distinct in methanol than in water/methanol, proving again the dominant role of metal ion solvation in aqueous medium. The difference in the stability constants of [Eu(n-PrBTP)3]3+ and [Eu(i-Pr-BTP)3]3+ complexes of 2 orders of 1214


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Table 6. Thermodynamic Data (ΔH, ΔS, and ΔG) of the Complex Formation of Ln(III)/An(III) BTP Complexes; Values in Italics Represent 1:1 and 1:2 Complexes ligand i-Pr-BTP

n-Pr-BTP Et-BTP

i-Pr-BTP Me-BTP

M3+ Eu

Eu Cm Eu ML2 ML3 Eu ML: La Eu Lu Am

ΔH (kJ/mol)

ΔS (J/(mol·K))

−29 ± 3 (pH 2.8) −32 ± 3 (pH 4.6) −26.4 ± 1.8 −36.5 ± 4.7

173 ± 10 (pH 2.8) 164 ± 10 (pH 4.6) 138 ± 7 148 ± 17

−47 ± 2 −73 ± 2 −105 ± 5

13 ± 6 17 ± 6

2.1 −6.8 3.2

45 34 59

ΔG (20 °C) (kJ/mol) −79.7 (pH 2.8)

solvent methanol/H2O (1:1 vol.), 6.2 × 10 nitrate

method −4

−2.2 × 10

−3

M

ref

TRLFS

60

methanol/H2O (1:1 vol.), 1.85 × 10−4 − 2.48 × 10−4 M nitrate

TRLFS

76

methanol, 0.01 M nitrate

microcalorimetry

65

1-octanol, nitrate methanol/H2O (75:25 vol.)

microcalorimetry UV/vis

132 61

−80.1 (pH 4.6) −66.8 ± 2.8 −79.9 ± 7.5 −51.0 −77.7

−12.6 −16.6 −15.4 −23.4

−32 ± 3 kJ/mol and ΔS = 164 ± 10 J/(mol·K) at pH 4.6. No pH dependency was observed in the pH region studied. These results show that the complexation reaction is both enthalpyand entropy-driven, which is in contrast to the complex formation of lanthanides/actinides with inorganic ligands in aqueous solution133 where ΔH is generally positive. The exothermic enthalpy for BTP complexes is due to multidentate coordination of the BTP ligands allowing for an effective compensation of the dehydration enthalpy of the cation. Nevertheless, the enthalpy variation associated to Eu(III) complexation by three i-Pr-BTP determined in methanol/water (1:1 vol.) by TRLFS60 was less exothermic than the one measured in 1-octanol using microcalorimetry (ΔH = −105 ± 5 kJ/mol),132 indicating that the desolvation enthalpy is significantly lower in 1-octanol than in water. Thermodynamic data on the Eu(III) complexes with Et-BTP in methanol (also determined by microcalorimetry) confirmed these findings.65 The highly exothermic nature of the complexation reaction suggested a strong interaction between Et-BTP and the Eu(III) ion. The entropy term was very small compared to the enthalpy term and ΔG of the complexation reaction was dominated by the enthalpy term. This confirmed the minor solvation of Eu(III) in methanol compared to aqueous medium, resulting in a smaller increase of the entropy due to the release of solvent molecules from the inner coordination sphere. Further TRLFS investigations in the temperature range of 10−50 °C have been performed to determine thermodynamic data of the 1:3 Cm(III)- and Eu(III)-n-Pr-BTP complexes in methanol/water (1:1 vol.).76 For both metal ions, negative enthalpy changes were observed (ΔHCm(III) = −36.5 ± 4.7 kJ/ mol; ΔHEu(III) = −26.4 ± 1.8 kJ/mol). By comparison, ΔH was 10.1 kJ/mol more negative for the complexation of Cm(III) than for that of Eu(III). The entropy change was found to be ΔS(Cm(III)) = 148 J/(mol·K) and ΔS(Eu(III)) = 138 J/ (mol·K), resulting in a difference ΔΔS = −10 J/(mol·K), which was within the error range. This led to the conclusion that the complexation mechanism is comparable for both metal ions. The difference in ΔG (20 °C) determined from the Gibbs− Helmholtz equation was ΔΔG (20 °C) = −13.1 kJ/mol and resulted mainly from the difference in ΔH. This is in good agreement with ΔΔG (20 °C) = −RT ln[β3(Cm(III))/ β3(Eu(III))] = −14.0 kJ/mol derived from the spectroscopic titration experiments (see Tables 4 and 5), which underlined the accuracy of the obtained data.

magnitude is also explained by hydrophobic interactions. In the case of the 1:1 complexes, the hydrophobic interactions are assumed to be negligible as the stability of the complex is determined mainly by electrostatic interaction. Consequently, equal stability constants for [La(Et-BTP)]3+ and [La(n-PrBTP)]3+ were obtained.65,78 The stability constants of the [Cm(n-Pr-BTP)3]3+ and [Eu(n-Pr-BTP)3]3+ complexes in methanol/water (1:1 vol.) were determined by TRLFS.76 This comparative study showed that the stability constant of Cm(III) (log β3 = 14.4 ± 0.2) is 2.5 orders of magnitude higher than that of the respective Eu(III) complex (log β3 = 11.9 ± 0.2). From these stability constants a separation factor SFCm(III)/Eu(III) = β3(Cm(III))/ β3(Eu(III)) = 320 was calculated. This value is in good agreement with SFCm(III)/Eu(III) = 230 from extraction experiments.71,92 Furthermore, TRLFS titrations were also performed at varied nitrate concentrations of (2.48−17.36) × 10−4 mol/L (Cm(III)) and (1.85−7.92) × 10−4 mol/L (Eu(III)) to verify a possible impact of nitrate on the stability constants of the 1:3 complexes. The results clearly showed that the stability constants were not affected by increasing amounts of nitrate in the nitrate concentration range studied. The stability constant of [Am(Me-BTP)3]3+ in methanol/ water (75:25 vol.) (log β3 = 9.9) presented in ref 61 was determined by UV/vis/NIR spectrophotometry (cf. Table 5). In comparison to the results of 1:3 lanthanide-BTP complexes listed in Table 4 and in comparison to the value of [Cm(n-PrBTP)3]3+, this value seems to be quite low. This might be due to the fact that only small fractions of [Am(Me-BTP)3]3+ species were formed (in the case of Eu(III), no 1:3 complexes were observed with Me-BTP; see Table 4), which might have caused difficulties in the evaluation of the Am(III) absorption spectra. 2.3.6.2. Thermodynamic Data. The thermodynamic data (ΔH, ΔS, and ΔG; cf. Table 6) for the complexation reaction of trivalent lanthanides/actinides with various BTP ligands were determined by microcalorimetry65,132 and temperature-dependent TRLFS60,76 or UV/vis.61 In ref 60 the enthalpic and entropic data of the complexation of Eu(III) with i-Pr-BTP in methanol/water (1:1 vol.) were derived by the temperature dependency of the stability constants in the temperature range of 11−77 °C. The enthalpy and entropy variations associated to the [Eu(i-Pr-BTP)3]3+ complex formation were ΔH = −29 ± 3 kJ/mol and ΔS = 173 ± 10 J/(mol·K) at pH 2.8 and ΔH = 1215


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Scheme 2. Synthesis of 6,6′-Bis(1,2,4-triazin-3-yl)-2,2′-bipyridine Ligands50

solution of 68 mmol/L C2-BTBP in 1,1,2,2-tetrachloroethane extracts with DAm(III) = 650, DEu(III) = 4.1, and SFAm(III)/Eu(III) = 160 from 1 mol/L HNO3. However, C2-BTBP’s solubility in more appropriate diluents such as kerosene−alcohol mixtures or lipophilic alcohols was too low, which is why BTBPs with longer alkyl moieties were tested. 3.1.1. C5-BTBP. Of the n-alkylated BTBPs tested, C5-BTBP (Figure 1 right, R = n-pentyl) is the one studied in most detail regarding selective Am(III) extraction and stability (cf. section 3.2.1). The extraction of Am(III), Ln(III), and several d-block metal ions from nitric acid into solutions of C5-BTBP was studied by Foreman et al.37 Figure 19 shows the extraction of

The comparison of the thermodynamic data of [Eu(n-PrBTP)3]3+76 and [Eu(i-Pr-BTP)3]3+60 revealed an interesting aspect. The difference in ΔG derived by temperaturedependent TRLFS measurements was determined to be 79.7 kJ/mol (i-Pr-BTP) − 66.8 kJ/mol (n-Pr-BTP) = 12.9 kJ/mol. Calculation of ΔΔG from the ratio of the stability constants of the respective 1:3 complexes obtained by titration studies (see Table 4) according to ΔΔG = −RT ln 1014.2/1011.9 = 12.9 kJ/ mol provided an identical value. Therefore, the difference in the stability constants of 2.3 orders of magnitude was confirmed by a more negative free enthalpy for i-Pr-BTP, with the difference of 12.9 kJ/mol in ΔG matching perfectly the difference in the stability constants of the 1:3 complexes. This again verified the accuracy of the thermodynamic data presented in refs 76 and 60.

3. LIPOPHILIC BISTRIAZINYL BIPYRIDINES (BTBP) Following the success of the BTP molecules, new compounds were synthesized at Reading University, U.K., with two things in mind: they should have a donor pattern similar to those of the BTP, and they should be quadridentate. The reasoning behind the latter was that such a molecule would form 1:2 complexes rather than the 1:3 complexes formed by BTP. In consequence, when the extracting agent degrades due to radiolysis, its decrease in concentration would have a smaller effect on distribution ratios in the case of a 1:2 complex than in the case of a 1:3 complex. The synthesis of alkylated 6,6′bis(1,2,4-triazin-3-yl)-2,2′-bipyridines (BTBPs, see Figure 1 right) in 2005 was a logical step.36 As shown in some of the studies discussed below, BTBPs extract actinides(III) and lanthanides(III) from nitric acid by solvation of the metal nitrates in the form of 1:2 complexes, M3 +aq + 3NO3−aq + 2BTBPorg ⇌ M(BTBP)2 (NO3)3org

Figure 19. Am(III) and Eu(III) extraction into C5-BTBP (filled symbols)37 or n-Pr-BTP solution (open symbols),92 with distribution ratios (DAm(III), circles; DEu(III), triangles) as a function of nitric acid concentration. Organic phase, 10 mmol/L C5-BTBP or n-Pr-BTP in kerosene/1-octanol (70:30 vol.). Aqueous phase, 241Am(III) + 152Eu(III) (spike concentrations) in HNO3.

(5)

Am(III) and Eu(III) from nitric acid of varied concentration into C5-BTBP solution. Similar to BTP, BTBPs act as solvating extracting agents, with distribution ratios increasing with nitrate concentration. Stripping is feasible at low nitrate (or nitric acid) concentration. The selectivity for Am(III) over Eu(III) is in the range of SFAm(III)/Eu(III) ≈ 200. The figure illustrates an interesting difference between BTP and BTBP: whereas with n-Pr-BTP distribution ratios start decreasing beyond 1 mol/L HNO3, this is not the case with C5BTBP (cf. also Figure 21, showing that extraction increases up to 2 mol/L HNO3 with CyMe4-BTBP). Because the decreasing

The BTBP ligands are synthesized as shown in Scheme 2. After oxidation of 2,2′-bipyridine with hydrogen peroxide in acetic acid and a subsequent Reissert-Henze reaction of the resulting bis-N-oxide, the corresponding dinitrile is obtained. The addition of hydrazine to the dinitrile generates the dicarbohydrazonamide, which is condensed with a 1,2-diketone to form the alkylated BTBP.50 3.1. Extraction and Process Tests Using BTBP

The first BTBP synthesized, C2-BTBP (Figure 1 right, R = ethyl), proved to extract Am(III) selectively over Eu(III).36 A 1216


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Furthermore, Nilsson et al.134 studied in detail the extraction of several actinides(III) and all lanthanides(III) (except Pm and Tm) into C5-BTBP in cyclohexanone. Slope analysis of the distribution data clearly showed that Am(III), Cm(III), Cf(III), and Eu(III) are extracted as 1:2 complexes, as already found for extraction into C5-BTBP dissolved in kerosene/1-octanol (70:30 vol.).37 Distribution ratios are significantly higher with cyclohexanone compared to kerosene/1-octanol,37 but the selectivity pattern is unaffected; see Figure 20. Extraction was found to proceed reasonably fast with cyclohexanone (with equilibrium for actinides(III) and Eu(III) being reached after ≈5 min of vigorous shaking). However, increasingly slower kinetics were observed for the heavier lanthanides(III): whereas for La(III)−Dy(III) similar distribution ratios were observed after 1 and 60 min shaking, Ho(III)−Lu(III) distribution ratios increased significantly between 1 and 60 min. The studies mentioned pointed out the chemical and radiolytic instability of C5-BTBP, as to be expected from the presence of benzylic protons at the triazinyl rings. Consequently, its stability was investigated in depth (cf. section 3.2.1). 3.1.2. CyMe4-BTBP. Prompted by the excellent stability of CyMe4-BTP (cf. section 2.2), the analogous CyMe4-BTBP (Figure 2 right) was synthesized at Reading University, U.K. Indeed, a solution of 10 mmol/L CyMe4-BTBP + 0.25 mol/L DMDOHEMA in 1-octanol in contact with 1 mol/L nitric acid for 60 days did not degrade.136 Consequently, the extraction of actinides(III) and lanthanides(III) into solutions of CyMe4-BTBP in 1-octanol was studied in detail.38 Solutions of CyMe4-BTBP in kerosene/ 1-octanol or 1-octanol extracted Am(III) from 1 mol/L nitric acid selectively over Eu(III) (SFAm(III)/Eu(III) ≈ 150), but extraction kinetics were found to be too slow for a possible process application. Adding DMDOHEMA accelerated kinetics, and an optimized solvent composition of 10 mmol/L CyMe4-BTBP + 0.25 mol/L DMDOHEMA in 1-octanol was selected for further testing and development. The addition of the unselective extracting agent slightly decreased selectivity to SFAm(III)/Eu(III) ≈ 120, but nevertheless a good separation of Am(III) + Cm(III) from all lanthanides(III) was possible (the smallest separation factor was SFCm(III)/Dy(III) ≈ 30). As shown in Figure 21, Am(III) and Cm(III) are extracted (D > 1) from nitric acid >0.5 mol/L, while stripping into dilute nitric acid should be possible. Unfortunately, stripping into dilute nitric acid was not efficient: on the one hand nitric acid coextracted by 1-octanol91 in the extraction step was released during stripping, and on the other hand stripping was simply slow. This could be overcome by replacing dilute nitric acid by a glycolate solution; fast and efficient stripping (DAm(III) ≈ 10−3) was achieved. In good agreement with studies using C5-BTBP, CyMe4-BTBP (with or without DMDOHEMA) was shown to extract Am(III) as the 1:2 complex, Am(BTBP)2(NO3)3. In the absence of DMDOHEMA, Eu(III) is also extracted as the 1:2 complex. In the presence of DMDOHEMA, slope analysis was not applicable for Eu(III) due to significant Eu(III) coextraction by DMDOHEMA. Despite its promising properties, CyMe4-BTBP’s limited solubility in 1-octanol-based diluents of 10−20 mmol/L38 is a drawback. Furthermore, the loading capacity is rather low; a solvent composed of 15 mmol/L CyMe4-BTBP + 0.25 mol/L DMDOHEMA in 1-octanol has a loading capacity of ≈1 mmol/L Am(III).99 A precipitate forms upon extraction of higher concentrations.

distribution ratios are explained by metal nitrates and nitric acid competing for BTP,73 one may assume that BTBP extracts nitric acid to a lesser extent, indicating that it is less prone to protonation. Unfortunately, no experimental data comparing BTP and BTBP protonation under comparable conditions are available; calculations do not yield unambiguous results. Despite its rather slow extraction kinetics (equilibrium was reached after 3.5 h), C5-BTBP showed promising properties for An(III)/Ln(III) separation. A solution of 15 mmol/L C5BTBP in kerosene/1-octanol (70:30 vol.) extracted Am(III) with good selectivity over all Ln(III) (Ln = La−Lu; Pm and Tm were not measured) + Y(III) from 1 mol/L nitric acid (Figure 20). The lighter Ln(III) were less extracted than the

Figure 20. Extraction of actinides(III) (circles), lanthanides(III) (triangles), and yttrium(III) (diamond) into C5-BTBP. Pm(III) and Tm(III) were not measured. Organic phase, 15 mmol/L C5-BTBP in kerosene/1-octanol (70:30 vol.) (open symbols)37 or 5 mmol/L C5BTBP in cyclohexanone (filled symbols).134 Aqueous phase, 1 mol/L HNO3 (open symbols) or 0.01 mol/L HNO3 + 0.99 mol/L NaNO3 (filled symbols).

heavier ones, with best extraction observed for Er(III)−Yb(III). Y(III) was extracted similar to Nd(III).37 Extraction of lanthanides by C5-BTBP is comparable to n-Pr-BTP (Figure 6), with some small differences: with the latter, Y(III) is extracted similar to Sm(III); the decrease in distribution ratios for the heaviest Ln(III) is slightly more pronounced; and the separation factors between neighboring Ln(III) are generally slightly larger with BTP. C5-BTBP’s selectivity for Am(III) over Eu(III) is SFAm(III)/Eu(III) ≈ 200, compared to 130 for n-PrBTP.30 The influence of C5-BTBP concentration on Am(III) and Ln(III) distribution ratios indicates extraction of 1:2 complexes, M(BTBP)2(NO3)3.37 Nilsson et al.135 studied the extraction of Am(III) and Eu(III) by C5-BTBP using different diluents. A tremendous influence of the diluent was observed; when extracting Am(III) from 0.01 mol/L HNO3 + 0.99 mol/L NaNO3 into solutions of 5 mmol/L C5-BTBP, DAm(III) varied by almost 7 orders of magnitude (from ≈103 to ≈2 × 10−4), decreasing in the order nitrobenzene > cyclohexanone > 1,1,2,2-tetrachloroethane > benzaldehyde > 1-decanol > anisole > trichloroethylene > tertbutylbenzene. Also, DAm(III) was found to decrease with increasing length of the alkyl moiety in different 1-alkanols (hexanol−decanol), showing distribution ratios to increase largely with the dielectric constant of the diluent. Depending on the diluent, the formation of both 1:1 and 1:2 complexes was concluded. 1217


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positions of the pyridines deteriorated extraction kinetics.137 Also, changing the CyMe4 side-groups to 5-ring O- or Sheterocycles (Cy5-O-Me4-BTBP, Cy5-S-Me4-BTBP, see Appendix) had an adverse effect both on solubility and on extraction performance. Distribution ratios of DAm(III) < 1 and selectivities of SFAm(III)/Eu(III) < 50 were found for the extraction of Am(III) and Eu(III) from 0.1−4 mol/L HNO3 into solutions of Cy5-OMe4-BTBP or Cy5-S-Me4-BTBP in different diluents.138 These values are inferior to those found for CyMe4-BTBP. As a recent variation of the CyMe4-BTBP molecule, CyMe4BTPhen (see Appendix) was synthesized at Reading University, U.K.139 It was reasoned that a BTBP ligand cis-locked along the bipyridine bond might have advantages with respect to extraction thermodynamics and kinetics. As shown in Figure 23, CyMe4-BTPhen is a much stronger extracting agent;

Figure 21. Extraction of Am(III), Cm(III), and Eu(III) by CyMe4BTBP + DMDOHEMA in 1-octanol. Distribution ratios as a function of initial nitric acid concentration. Organic phase, 10 mmol/L CyMe4BTBP + 0.25 mol/L DMDOHEMA in 1-octanol. Aqueous phase, 241 Am(III) + 244Cm(III) + 152Eu(III) + lanthanides(III) (La−Lu, 10 mg/L each) + Y(III) in HNO3. Data are from ref 38.

Since previous work pointed out a low solubility of CyMe4BTBP in 1-octanol or kerosene/1-octanol diluents,38 the solubility of CyMe4-BTBP (and other BTBPs) was determined in a variety of diluents.96 CyMe4-BTBP’s solubility was determined to 8 mmol/L (1-octanol), 10 mmol/L (1-hexanol), 17 mmol/L (cyclohexanone), and 50 mmol/L (nitrobenzene) at 293 K. t-Bu-CyMe4-BTBP (i.e., CyMe4-BTBP with a tertbutyl moiety added in 4-position, see Appendix) has a largely increased solubility of 0.3 mol/L (cyclohexanone, 293 K). Although dissolution enthalpy is more positive for t-Bu-CyMe4BTBP (124 kJ/mol) than for CyMe4-BTBP (39 kJ/mol) (values for cyclohexanone), this is overcompensated by a huge gain in entropy (98 J/(mol·K) versus 410 J/(mol·K)), accounting for an increasingly better solubility of t-BuCyMe4-BTBP compared to CyMe4-BTBP with increasing temperature (cf. Figure 22). This study also points out that both solubility and distribution ratios increase with the diluent’s dielectric constant. The same influence of diluent polarity on distribution ratios for the extraction of Am(III) into C5-BTBP solutions is described in an earlier study.135 Further attempts at improving the solubility of CyMe4-BTBP by adding methyl or tert-butyl moieties on the 4- and 4′-

Figure 23. Am(III) and Eu(III) extraction into CyMe4-BTPhen (filled symbols)139 or CyMe4-BTBP solution (open symbols),38 with distribution ratios (DAm(III), circles; DEu(III), triangles) as a function of nitric acid concentration. Organic phase, 10 mmol/L CyMe4BTPhen in 1-octanol or 10 mmol/L CyMe4-BTBP + 0.25 mol/L DMDOHEMA in 1-octanol. Aqueous phase, 241Am(III) + 152Eu(III) (spike concentrations) in HNO3.

Am(III) distribution ratios increase several orders of magnitude compared to CyMe4-BTBP. Also, selectivity is slightly better; SFAm(III)/Eu(III) ≈ 250 versus ≈150 for CyMe4-BTBP.38 Finally, CyMe4-BTPhen has better extraction kinetics than CyMe4BTBP; a phase-transfer catalyst is claimed to be unnecessary to achieve reasonably fast kinetics. Unfortunately, distribution ratios are too high when using 1-octanol as solvent; with DAm(III) = 17 at 1 mmol/L HNO3, efficient stripping is not possible. This problem was solved by replacing 1-octanol with 1-octanol−toluene mixtures; at 1 mmol/L HNO3, DAm(III) < 1 for solvents containing ≥40% (vol.) toluene.139 Despite some drawbacks such as its limited solubility in 1octanol-based diluents and its slow kinetics, CyMe4-BTBP was selected as the European reference molecule for the development of a SANEX process for the separation of Am(III) and Cm(III) from the “fission lanthanides” (Y, La−Dy). Consequently, lab-scale demonstration process tests were developed around two solvents, (a) 15 mmol/L CyMe4-BTBP + 5 mmol/L TODGA in 1-octanol39,42 or (b) 15 mmol/L CyMe4BTBP + 0.25 mol/L DMDOHEMA in 1-octanol.40,140 In 2008, a spiked test was performed at Forschungszentrum Jülich, Germany,39,42 in a 16-stage centrifugal contactor setup, using a solvent of 15 mmol/L CyMe4-BTBP + 5 mmol/L TODGA in 1-octanol. The feed solution contained 241Am(III) and 244Cm(III) (2.5 MBq/L each) and 1.9 g/L lanthanides(III)

Figure 22. Free enthalpy of dissolution (lines, calculated from ΔH and ΔS values) and solubility (symbols) of t-Bu-CyMe4-BTBP and CyMe4BTBP in cyclohexanone as a function of temperature. Data are from ref 96. 1218


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Extraction of Ni(II)149 and Pd(II)150 by CyMe4-BTBP has been described earlier. The coextraction of Pd(II) could be suppressed by the addition of a water-soluble masking agent such as bimet ((2S,2′S)-4,4′-(ethane-1,2-diylbis(sulfanediyl))bis(2-aminobutanoic acid), cf. Appendix). TBP was later replaced by DEHBA (N,N-di(2-ethylhexyl)-nbutanamide).151,152 This solvent component is advantageous with respect to secondary waste treatment. Compared to the solvent containing TBP, small differences were observed regarding the extraction of some fission products, which, however, do not disfavor the solvent containing DEHBA. 3.1.5. Improving Kinetics in BTBP Systems. Early work on i-Pr-BTP33 (cf. section 2.1.2) confirmed that adding a second extracting agent accelerates kinetics. The same was found with CyMe4-BTBP when DMDOHEMA was added.38 A similar acceleration was observed with TODGA.42 Finally, also N-donor compounds such as 3,3′-bis-1,2,4-triazines or 3pyridin-2-yl-1,2,4-triazines were found to accelerate the kinetics of Am(III) and Eu(III) extraction into solutions of C2-BTBP in 1,1,2,2-tetrachloroethane.153 In this case, the accelerating effect was ascribed to the partial aqueous solubility of the auxiliary agents. This explanation, however, does not hold for the very lipophilic DMDOHEMA and TODGA molecules. They may act as phase-transfer catalysts, but they also cause smaller droplets in shaking tube experiments, increasing the specific interfacial area. To clearly distinguish these effects, extraction and stripping kinetics for the system Am(III)−NO3−/CyMe4BTBP−TODGA−diluent were studied in a quantitative way using a stirred cell.154 With an organic phase consisting of CyMe4-BTBP in 1-octanol, Am(III) extraction rate increased first order with both CyMe4-BTBP concentration and interfacial area. In contrast, with an organic phase consisting of CyMe4-BTBP + TODGA in 1-octanol, Am(III) extraction rate was practically independent of CyMe4-BTBP concentration. However, Am(III) extraction rate increased first order with both TODGA concentration and interfacial area. TODGA had a positive kinetic effect; e.g., Am(III) extraction rate increased by ≈1 order of magnitude when 5 mmol/L TODGA was added to the organic phase. TODGA also accelerated stripping into dilute HNO3 by ≈1 order of magnitude. In conclusion, TODGA does accelerate the rate of the chemical reaction, which is located at the interface in both the absence and the presence of TODGA. Replacing the diluent 1-octanol by cyclohexanone had a positive kinetic effect, as observed earlier.139,148,155 Cyclohexanone seems to be acting as a phase-transfer catalyst, similar to TODGA or DMDOHEMA.

in 1.3 mol/L HNO3. Approximately 99.9% Am(III) and 99.7% Cm(III) with ≈0.3% of the lanthanides were recovered in the product solution. However, owing to slow kinetics, comparatively low flow rates of 10 mL/h had to be used. Furthermore, in 2008 a similar test was performed at the ITU.40,140,141 Solvent was a solution of 15 mmol/L CyMe4BTBP + 0.25 mol/L DMDOHEMA in 1-octanol. An actual highly radioactive feed solution was used in this test, containing ≈80 mg/L Am(III), 30 mg/L Cm(III), and 1.4 g/L Ln(III) in 2 mol/L HNO3. As with the spiked test, flow rates were 10 mL/h. Only 0.01% Am(III) and 0.09% Cm(III) remained in the raffinate; 10 kGy, alpha irradiation resulted in a more pronounced decrease in DAm(III) (which was, however, due to more oxidative degradation from a longer contact time). The same degradation products were identified as already described for gamma irradiation.158 Finally, the radiolytic and oxdiative stability of C5-BTBP in cyclohexanone in the presence of an aqueous phase was studied.160 Stability versus oxidative degradation was tested by contacting solutions of C5-BTBP in cyclohexanone with water or with 0.01 mol/L HNO3 + 0.99 mol/L NaNO3; radiolytic stability was tested on (a) pure organic phases; (b) organic phases pre-equilibrated with 0.01 mol/L HNO3 + 0.99 mol/L NaNO3; and (c) organic phases in contact with 0.01 mol/L HNO3 + 0.99 mol/L NaNO3. The results were evaluated by determining DAm(III) and DEu(III) and by measuring C5-BTBP and degradation product concentrations using HPLC and LCMS. After 330 h in contact with water, 2 mmol/L of initially 5 mmol/L C5-BTBP in cyclohexanone remained; the same decrease was observed after 200 h when in contact with 0.01 mol/L HNO3 + 0.99 mol/L NaNO3. This reflected C5-BTBP's very low chemical stability. Whereas the C5-BTBP concentration decreased from 5 to 3.5 mmol/L after a dose of 50 kGy when only the solvent was irradiated, 30 kGy was sufficient to almost completely destroy C5-BTBP when the pre-equilibrated solvent or the solvent in contact with aqueous phase was irradiated. The growth of degradation products was quite similar for the pre-equilibrated solvent and the solvent in contact with aqueous phase. The conclusion from these studies is that C5-BTBP degradation is initiated by an oxidative attack of the pentyl moieties, inducing the loss of alkyl chains and the decay of the triazine scaffold, as already observed with n-alkylBTP (cf. section 2.2). 3.2.2. CyMe4-BTBP. Despite the facts that CyMe4-BTBP is the current European reference molecule for SANEX process development and that C5-BTBP is of no practical use due to its instability, less work has been devoted to studying the stability of CyMe4-BTBP. CyMe4-BTBP was shown to have excellent

stability in contact with nitric acid; when a solution of 10 mmol/L CyMe4-BTBP + 0.25 mol/L DMDOHEMA in 1octanol was kept in contact with 1 mol/L nitric acid for 60 days, no changes in DAm(III) and DEu(III) were observed.38,136 Also, t-Bu-CyMe4-BTBP (see Appendix) dissolved in cyclohexanone did not show any degradation (as from changes in DAm(III) and DEu(III)) when in contact with 1 mol/L HNO3 for 230 days.161 Low gamma doses (20 kGy) did not have an adverse effect on the extraction properties of a solvent consisting of CyMe4BTBP in cyclohexanone or 1-hexanol. Except for a slight increase of DAm(III), distribution ratios of Am(III) and Eu(III) remained constant.156 A solution of t-Bu-CyMe4-BTBP in cyclohexanone was gamma-irradiated at different dose rates (15 Gy/h and 1.2 kGy/ h), and distribution ratios were determined for the extraction of Am(III) and Eu(III) from 1 mol/L HNO3.161 Whereas an irradiation of up to 100 kGy with the low dose rate did not affect DAm(III) and DEu(III), irradiation with the same dose at a rate of 1.2 kGy/h resulted in a decrease of DAm(III) and DEu(III) to ≈50%. A possible explanation for this difference was given considering the fact that the irradiation with the lower dose rate took longer. More oxygen could have entered the sample, which could have a protective effect by oxidizing HNO2 to HNO3. The stability of a GANEX solvent consisting of CyMe4BTBP + TBP in cyclohexanone was studied.162 No change in DAm(III) and DEu(III) was observed in extraction experiments when a solution of 10 mmol/L CyMe4-BTBP + 1.1 mol/L TBP in cyclohexanone was kept in contact with 4 mol/L HNO3 for 54 days. However, a significant decrease of CyMe4-BTBP concentration was detected by HPLC, indicating that the degradation product(s) have similar properties as the extracting agent. Following gamma radiolysis up to 200 kGy, 67% of the initial CyMe4-BTBP concentration was detected by HPLC. When extracting with the irradiated solvent, DAm(III) and DEu(III) decreased to ≈50% of the initial values. Considering that Am(III) and Eu(III) are extracted as 1:2 complexes, these two results are in good agreement. However, they are in contrast to the above results from acid contact, which showed a decrease in CyMe4-BTBP concentration but no decrease in distribution ratios. When the solvent was irradiated in contact with 4 mol/L HNO3, DAm(III) and DEu(III) remained constant up to a dose of 200 kGy, demonstrating a protective effect of the aqueous phase, as described by Mincher et al.98 Furthermore, a study on the radiolytic stability of the solvent used in a hot SANEX process test40 was performed.99 A solvent composed of 15 mmol/L CyMe4-BTBP + 0.25 mol/L DMDOHEMA in 1-octanol was loaded with different concentrations of 241Am(III) and 244Cm(III) by extraction from 1 mol/L HNO3 to study alpha radiolysis with dose rates of approximately 50 Gy/h, 200 Gy/h, and 1 kGy/h. For gamma radiolysis, the solvent in contact with 1 mol/L HNO3 spiked with 241Am(III) was centered in a holder containing four fuel rods of used UO2 fuel with a burnup of ≈50 GWd/tHM, resulting in a dose rate of ≈220 Gy/h. Whereas the alpha radiolysis experiment with a dose rate of 50 Gy/h did not yield conclusive results, no significant decrease in DAm(III) was observed whether irradiating with 0.2 or with 1 kGy/h up to a dose of ≈120 kGy. After an alpha dose of 375 kGy at 1 kGy/h, DAm(III) decreased from initially 12 to a value of 7. Gamma irradiation was performed up to a dose of 1.2 MGy after which DAm(III) dropped to ≈2. The decrease in BTBP concentration 1220


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was calculated from the decrease in DAm(III), taking into account that Am(III) is extracted as a 1:2 complex. From this, degradation rate constants of 7.9 × 10−4 kGy−1 (alpha radiolysis) and 1.1 × 10−3 kGy−1 (gamma radiolysis) were calculated, i.e., the degradation rate is 40% higher for gamma radiation than for alpha radiation.

agreement with an average bond length of 257.5 pm in the analogous 1:2 CyMe4-BTBP complex.163 3.3.2. Speciation Studies by TRLFS. The first speciation studies on the complexation of Cm(III) and Eu(III) with t-BuC2-BTBP (cf. Appendix) in water/2-propanol (1:1 vol.) have been performed by Trumm et al.127 The aim of this comparative work was to pinpoint differences in the thermodynamic stability of Cm(III) and Eu(III) complexes with a BTBP. The evolution of the Cm(III) fluorescence spectra resulting from the 6D′7/2 → 8S′7/2 transition in the presence of an increasing amount of t-Bu-C2-BTBP is shown in Figure 25a. The spectrum of Cm(III) in water/2-propanol (1:1 vol.) displays a broad emission band at 594.3 nm, which is shifted by 0.6 nm bathochromically compared to the Cm(III) aquo ion (593.7 nm).106,109 This is due to a partial exchange of water molecules of the inner coordination sphere by 2propanol, which is also expressed in a slightly longer fluorescence lifetime of 68 μs (compared to 65 μs of the aquo complex109,106). More details on the solvation of Cm(III) in water/alcohol mixtures are given in ref 129. The formation of Cm-nitrate inner-sphere complexes was not expected under these conditions.112 With increasing t-Bu-C2-BTBP concentration the emission band shifted to 618.9 nm, indicating a larger ligand field splitting caused by stronger complexation of the metal ion due to the formation of a Cm-BTBP complex species. This species was identified to be the [Cm(t-Bu-C2-BTBP)2]3+ complex. The composition of the complexed species was determined by slope analysis, yielding a slope of 2.04 ± 0.05 for Cm(III).127 The species distribution of the [Cm(t-Bu-C2-BTBP)n]3+ (n = 0 and 2; species 1 in Figure 25) complexes as a function of the t-BuC2-BTBP concentration is shown in Figure 25b. The Eu(III) fluorescence spectra resulting from the 5D0 → 7 F1 and 5D0 → 7F2 transitions as a function of the t-Bu-C2BTBP concentration are shown in Figure 26a. In contrast to Cm(III), changes in the Eu(III) coordination sphere cause only small shifts of the fluorescence bands in the range of several nm. However, because the 5D0 →7F2 transition is hypersensitive, changes in the ligand field of Eu(III) are reflected by variation of the transition ratio (7F1/7F2). The emission spectrum of the solvated Eu(III) species ([Eu(solv.)]3+) in water/2-propanol (1:1 vol.) shows two broad bands at 593.4 and 617.4 nm with a transition ratio 7F1/7F2 of 3.16. A new species formed upon addition of ligand, diplaying a distinct splitting of the emission bands (Figure 26a) and a transition ratio of 0.31. Simultaneously, the fluorescence intensity increased significantly, resulting in a FI factor of the complexed species of 61 compared to the solvated Eu(III). Analogously to Cm(III), the number of coordinated t-Bu-C2-BTBP ligands was determined by slope analysis. The slope of 2.05 ± 0.06 obtained for Eu(III) confirmed the formation of the 1:2 [Eu(tBu-C2-BTBP)2]3+ species. The species distribution of the [Eu(t-Bu-C2-BTBP)n]3+ (n = 0 and 2; species 2 in Figure 26) complexes as a function of the t-Bu-C2-BTBP concentration is shown in Figure 26b. Comparison of the complexation of Eu(III) and Cm(III) with t-Bu-C2-BTBP showed that in the case of Eu(III) a much higher ligand concentration is required to form the 1:2 [M(t-Bu-C2-BTBP)2]3+ species. This is also reflected by the difference in the stability constants of Cm(III) and Eu(III) (cf. section 3.3.6). To determine the coordination number (8, 9, or 10) of the [M(t-Bu-C2-BTBP)2]3+ complexes and the nature of additional coordinating ligands, fluorescence lifetime measurements of the

3.3. Complexation of An(III) and Ln(III) with BTBP

3.3.1. Structures of An(III)- and Ln(III)-BTBP Complexes. Despite the fact that BTBPs extract An(III) and Ln(III) as 1:2 complexes, most crystal structures are from 1:1 complexes, except for one Eu(III)-BTBP 1:2 complex. No crystal structure of an An(III) complex is available to date. Also, no structures of An(III)-BTBP or Ln(III)-BTBP complexes in solution have been reported. Reacting a solution of C2-BTBP in dichloromethane with an excess of europium nitrate in acetonitrile resulted in a 1:1 complex, [Eu(C2-BTBP)(NO3)3].36 The Eu(III) ion is coordinated by four BTBP nitrogens and by three bidentate nitrate anions. The Eu(III)−N bond lengths are 260 pm. The same 1:1 complexes, [Ln(C2-BTBP)(NO3)3], were found for Ln = Y, La, Ce, Pr, Nd, Sm, Dy, Ho, and Er. However, 1:1 complexes [Ln(C2-BTBP)(NO3)2(H2O)]+ were identified for Ln = Yb and Lu.136 The lanthanide ion is 10coordinate in the former and 9-coordinate in the latter, reflecting the different ionic radii. A graphic representation of the average Ln(III)−N bond lengths (Figure 24) shows that these follow the lanthanide ion contraction.

Figure 24. Average Ln(III)−N bond lengths, R (filled symbols), and average Ln(III)−N bond lengths corrected for Ln(III) ion radius, R − rCN=9 (open symbols) for 1:1 Ln(III)-C2-BTBP complexes. Data are from ref 136.

A solution of CyMe4-BTBP in dichloromethane was added to a solution of europium nitrate in acetonitrile.163 Slow evaporation yielded crystals containing the 1:2 complex, [Eu(CyMe4-BTBP)2(NO3)]2+. In this complex, one nitrate coordinates in a bidentate fashion with an average Eu(III)−O bond length of 256 pm. The average Eu(III)−N bond length is 257.5 pm. Recrystallization in dichloromethane/acetonitrile transformed the 1:2 complex into a 1:1 complex, [Eu(CyMe4BTBP)(NO3)3]. The average Eu(III)−N bond length of 253.8 pm is significantly shorter than the value of 260 pm reported for the 1:1 complex, [Eu(C2-BTBP)(NO3)3].36 By reaction of CyMe4-BTPhen (cf. Appendix) with Eu(NO3)3, crystals of a 1:2 complex, which was identified as [Eu(CyMe4-BTPhen)2(NO3)]2+, were obtained.139 The average Eu(III)−N bond length is 258.5 pm, which is in good 1221


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Figure 25. (a) Normalized fluorescence spectra and (b) species distribution of Cm(III) in water/2-propanol (1:1 vol.) as a function of the t-Bu-C2BTBP concentration ([Cm(III)]ini = 1.82 × 10−7 mol/L). Lines are calculated with log K = 11.1, cf. Table 7.127 Reproduced with permission from ref 127. Copyright 2010 The Royal Society of Chemistry.

Figure 26. (a) Fluorescence spectra and (b) species distribution of Eu(III) in water/2-propanol (1:1 vol.) as a function of the t-Bu-C2-BTBP concentration. [Eu(III)]ini = 2.14 × 10−5 mol/L. Lines are calculated with log K = 9.0, cf. Table 7.127 Reproduced with permission from ref 127. Copyright 2010 The Royal Society of Chemistry.

≈40 mmol/L H2O) had fluorescence lifetimes of 1835−1905 μs. In contrast to the results of Trumm et al.,127 the author proposed a complex [Eu(BTBP)2]3+ having 8-fold coordination without nitrate in the inner coordination sphere. This was concluded from the low intensity of the 5D0 → 7F0 transition, indicating a symmetrical environment around Eu(III). The corresponding Cm(III)-BTBP complexes exhibited shorter fluorescence lifetimes in the range of 201−301 μs.127 Similar to Cm(III)-BTP complexes (cf. section 2.3.2.2), a strong quenching process by intramolecular energy transfer between the metal ion and the organic ligand was observed. Although the determination of the number of complexing BTBP ligands from the fluorescence decay constants was not possible, the relative fluorescence lifetimes of the various Cm(III) complexes prepared from Cm(NO3)3 or Cm(ClO4)3 in water/2-propanol or 2-propanol followed the trend observed for Eu(III). Therefore, it was concluded that 9-fold coordinated [M(t-Bu-C2-BTBP)2(X)]3+ complexes are formed for both Cm(III) and Eu(III), with an additional water (in aqueous solution) or nitrate (alcoholic solution) in the inner coordination sphere. Further studies on the stoichiometry of Eu(III) complexes with different BTBPs (CyMe4-BTBP, C5-BTBP, and t-Bu-C2BTBP) in 1-octanol solution were performed by Steppert et al.163 The influence of nitrate on the composition of the various BTBP complexes was investigated by using Eu(NO3)3 or

Cm(III) and Eu(III) 1:2 complexes in various diluents (2propanol and water/2-propanol (1:1 vol.)) and in the presence of different anions (perchlorate and nitrate) were performed.127 The fluorescence lifetime of the 1:2 complex prepared from Eu(ClO4)3 in water/2-propanol (1:1 vol.) was 775 μs, indicating the presence of 0.8 water molecules in the inner coordination sphere.121 The 1:2 complex prepared from Eu(NO3)3 exhibited an almost identical fluorescence lifetime of 769 μs, leading to the conclusion that in the presence of water a 9-fold coordinated complex species of the composition [Eu(t-Bu-C2-BTBP)2(H2O)]3+ was formed. The fluorescence lifetime of the 1:2 complex (prepared from Eu(NO3)3) in 2propanol was significantly longer (2048 μs). Thus, it was reasonably assumed that no quenching water or 2-propanol molecules are coordinated to Eu(III) and the ninth coordination site is occupied by nitrate. The formation of an 8-fold coordinated complex species in the absence of water was excluded as the emission spectra of the 1:2 Eu(III)-t-Bu-C2BTBP complexes showed no significant differences in 2propanol and water/2-propanol (1:1 vol.). This was also found for Eu(III)-BTBP (C5-BTBP or CyMe4BTBP) 1:2 complexes prepared from Eu(NO3)3 in 1-octanol solution.87 [Eu(BTBP)2(H2O)0.5]3+ complexes were identified in water-saturated 1-octanol (containing ≈2 mol/L H2O), as concluded from their fluorescence lifetimes of 1220−1250 μs. The 1:2 complexes prepared in “dry” 1-octanol (containing 1222


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Figure 27. (left) Spectral variations corresponding to the complexation of Yb(III) with C5-BTBP ([L] = 10−5 mol/L; 0 ≤ R = [M]/[L] ≤ 2.2). I = 10−2 mol/L Et4NNO3 in methanol at 25 °C. (right) Distribution curves of all the species present in the Yb(III)-C5-BTBP system in methanol at 25 °C ([L] = 10−5 mol/L). Reprinted with permission from ref 78. Copyright 2010 American Chemical Society.

1:1 Eu(III)-BTBP complex was observed for CyMe4-BTBP and t-Bu-C2-BTBP with fluorescence lifetimes of 1200 and 1700 μs, respectively, two different 1:1 species were identified in the case of C5-BTBP (τ = 900 and 1200 μs). The 1:2 complexes of all BTBP ligands studied displayed an identical lifetime of 2000 μs, confirming the absence of solvent molecules in the inner coordination sphere in the presence of nitrate. According to their fluorescence lifetimes, the emission bands were attributed to 1:1 and 1:2 complexes with the following stoichiometry: [Eu(L)(NO3)x(1-octanol)0−1](3−x)+ (x = 1−2) and [Eu(L)2(NO3)y](3−y)+ (y = 0−1). Further information on the inner-sphere coordination of nitrate was obtained by vibronic side-band spectroscopy (VSBS). These side bands have their origin in the coupling of electronic transitions and vibrational modes of the ligands (called vibronic coupling), leading to Stokes-shifted side peaks of the corresponding electronic transition. Free NO3− (trigonal planar) has D3h symmetry. If the nitrate is coordinated in a bidentate fashion, the symmetry is lowered (C2v and lower), resulting in a splitting of the ν4(E′) vibration band (725 cm−1) to ν5(A) and ν3(A) with 710 and 740 cm−1, respectively, for C1 symmetry.165 In163 VSBs correlated to the ν5 and ν3 vibrational transitions of coordinated nitrate were used as they do not overlap with the electronic transitions in the fluorescence spectra. According to the positions of ν5 and ν3 in comparison to the VSB spectrum of the reference compound [Eu(C2-BTBP)(NO3)3]·MeCN at 20 K, the formation of the 1:1 complex [Eu(C5-BTBP)(NO3)3] was confirmed. The VSB spectra of the 1:2 Eu-BTBP complexes with the three different BTBP ligands differed significantly. In addition to a broad band at 685 cm−1 assigned to ligand vibrational modes, a weaker band appeared at 720 cm−1 for t-Bu-C2-BTBP and C5-BTBP but not for CyMe4BTBP, indicating the presence of nitrate in the inner coordination sphere of the 1:2 complexes with the sterically less demanding t-Bu-C2-BTBP and C5-BTBP ligands. In the case of the bulkier CyMe4-BTBP, the probability for a nitrate ion to enter the first coordination shell is significantly decreased. Furthermore, the VSB spectra of the solution complexes of [Eu(C5-BTBP)2(NO3)]2+ and [Eu(t-Bu-C2BTBP)2(NO3)]2+ clearly showed that the nitrate is coordinated in a monodentate fashion.

Eu(ClO4)3. In the perchlorate system the emission band resulting from the 5D0 → 7F0 transition of Eu(III) is located at 579.0 nm, displaying a fluorescence lifetime of 180 μs. This is in good agreement with ref 87, confirming the formation of a [Eu(1-octanol)7(H2O)]3+ solvent species. In the presence of increasing concentrations of CyMe4-BTBP, C5-BTBP, or t-BuC2-BTBP, 1:1 and 1:2 Eu(III)-BTBP complexes are formed with emission maxima at 579.6 and 581.2 nm, respectively. The fluorescence lifetime of the 1:1 complexes of 340 μs was identical for all different ligands studied and was attributed to a complex with the composition [Eu(BTBP)(1-octanol)1−2(H2O)]3+. The lifetimes of the 1:2 complexes increased with increasing bulkiness of the alkyl moieties in the order t-BuC2-BTBP (τ = 1020 μs), C5-BTBP (τ = 1400 μs), and CyMe4BTBP (τ = 2000 μs). These results showed that the difference is due to steric effects of the different ligands. Whereas the bulkier CyMe4-BTBP ligand prevents additional solvent molecules from entering the inner coordination sphere of Eu(III), the shorter fluorescence lifetimes of the 1:2 complexes with the less demanding t-Bu-C2-BTBP and C5-BTBP ligands were caused by quenching contributions of coordinated solvent molecules. This was confirmed for t-Bu-C2-BTBP by the use of O-deuterated 1-octanol (saturated with D2O). As OD vibrations do not quench the fluorescence of Eu(III),164 the lifetime increased from 1020 to 2000 μs upon deuteration. In addition to the speciation obtained from the D0 → 7F0 emission bands, the splitting and relative intensity of the 5D0 → 7F1 and 5 D0 → 7F2 transitions gave further information on the local symmetry of the complexes. The 5D0 → 7F1−2 spectra of Eu(III) with the different BTBP ligands were similar, but not identical, displaying 3- and 4-fold splitting, respectively. Suggesting tetragonal symmetry (square-antiprismatic coordination) with D2d or S4 point groups, 2- and 3-fold splitting of the 5D0 → 7F1 and 5D0 → 7F2 transitions was expected. However, the actual splitting of these transitions indicated a slight deviation from this tetragonal symmetry, confirming the interaction of solvent molecules. In the presence of nitrate, the speciation of Eu(III)-BTBP complexes was more complicated. The 5D0 → 7F0 emission band of Eu(III)-nitrate in 1-octanol was determined to be at 579.8 nm with an emission lifetime of 430 μs, corresponding to a [Eu(NO3)3(1-octanol)2(H2O)] species. Whereas one single 1223


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Table 7. Conditional Stability Constants of Ln(III)-BTBP and An(III)-BTBP Complexes ligand

M3+

t-Bu-C2-BTBP

Eu Cm Eu Cm Eu La Nd Eu Gd Er Yb Eu La Eu Yb

C5-BTBP CyMe4-BTBP C5-BTBP

C5-BTBP CyMe4-BTBP

1:1

4.5 ± 0.2 4.4 4.8 5.0 5.7 4.15 7.4 8.0 5.6 4.4 6.5 5.9

± ± ± ± ± ± ± ± ± ± ±

0.3 0.3 0.1 0.3 0.06 0.3 0.2 0.6 0.2 0.2 0.1

1:2

solvent

9.0 ± 0.15 11.1 ± 0.15 9.4 ± 0.4 10.8 ± 0.6 9.2 ± 0.3 10.0 ± 0.3 10.8 ± 0.1 11.3 ± 0.2 10.2 ± 0.3 13.4 ± 0.2 13.9 ± 0.4 10.5 ± 0.6 8.8 ± 0.1 11.9 ± 0.5

2-propanol/H2O (1:1 vol.), 1.85 × 10−4 − 1.74 × 10−3 M nitrate, 20 °C

TRLFS

method

127

ref

1-octanol/H2O (0.04 M), nitrate

TRLFS

87

1-octanol/H2O (0.04 M), nitrate methanol, 0.01 M nitrate, 25 °C

TRLFS UV/vis

87 78

methanol methanol, 0.01 M nitrate, 25 °C

microcalorimetry UV/vis

78 78

ESI-TOF-MS.163 In this study the BTBP complexes were formed in a monophasic 1-octanol solution (i.e., not saturated with water). In contrast to the results obtained by extraction,166 the 1:1 complex Eu(C5-BTBP)(NO3)2(1-octanol)+ was the dominating species at substoichiometric ligand to metal ion ratio ([L]/[M] = 0.5); only a small fraction of the 1:2 complex Eu(C5-BTBP)2(NO3)2+ was detected. At higher ligand concentration ([L]/[M] = 2) the 1:2 complexes Eu(C5BTBP)2(NO3)2+ and Eu(C5-BTBP)2(NO3)2+ were the prevailing species. In addition, a dimeric species [Eu(C5-BTBP)(NO3)3] [Eu(C5-BTBP)2(NO3)]2+ emerged at higher ligand concentrations. Furthermore, it was found that the 1:2 complex Eu(BTBP)2(NO3)2+ did not attach water molecules during the ESI process, indicating that this species is more hydrophobic than the other species. This might be evidence that the nitrate is effectively shielded by the moieties of the BTBP ligand. 3.3.5. Speciation Studies by NMR. 1H NMR titrations of lanthanum nitrate with C5-BTBP were performed in a 38:62 mixture of deuterated chloroform and acetonitrile.136 It was found that, for metal ion-to-ligand concentration ratios n < 0.5, 1:2 La−C5−BTBP complexes were formed. By the use of NMR, it was not possible to distinguish between [La(C5BTBP) 2 ] 3 + , [La(C5-BTBP) 2 (H 2 O)] 3 + , and [La(C5BTBP)2(NO3)]2+ species. For metal ion-to-ligand concentration ratios > 0.5, the 1:1 [La(C5-BTBP)(NO3)3] species prevailed. Relaxation titrations of Gd(III) were performed with CyMe4BTBP and Me-BTBP similar to that with Gd(III) and CyMe4BTP and Me-BTP34 (cf. section 2.3.5). Because of the replacement of solvent molecules upon complexation, the relaxivity decreases until a plateau is reached when a complex is fully formed. For both BTBP ligands, the formation of 1:2 complexes was confirmed. The relaxivity plateau of the 1:2 [Gd(CyMe4-BTBP)2]3+ complex was significantly higher than that of 1:3 [Gd(CyMe4-BTP)3]3+, indicating a better solvation for the BTBP complex. According to the difference between the plateaus of 0.17 s−1 mmol−1, it was suggested that one acetonitrile molecule remained solvated in the 1:2 [Gd(CyMe4BTBP)2]3+ complex. Replacing the CyMe4 substituents of BTP and BTBP by methyl groups led to relaxivity plateaus that were systematically higher. It thus seemed that the bulkiness of the substituents is an important factor that affects solvation. 3.3.6. Thermodynamic Data of An(III) and Ln(III) Complexation by BTBP. In contrast to BTP, stability

3.3.3. Speciation Studies by UV/Vis Spectrophotometry. Speciation studies by UV/vis spectrophotometry on the formation of lanthanide BTBP complexes in methanol using C5-BTBP and CyMe4-BTBP were performed by HubscherBruder et al.78 Complexation was followed by spectrophotometric titrations of the ligands against the metal ions. The spectral variation (involving several isosbestic points) of the ligand absorption spectrum as a result of Yb(III) complex formation is presented in Figure 27a. Analysis of the spectrophotometric data showed the formation of 1:1 and 1:2 complexes with C5-BTBP for all lanthanides studied. The species distribution of the Yb(III)-C5-BTBP complexes as a function of the metal ion concentration is shown in Figure 27b. In the case of CyMe4-BTBP, two species (1:1 and 1:2 complexes) were formed with La(III) and Eu(III) whereas Yb(III) formed only a 1:1 [Yb(CyMe4-BTBP)]3+ complex.78 3.3.4. Speciation Studies by ESI-MS. Additional speciation studies on Am(III) and Eu(III) BTBP complexes have been performed by electrospray ionization mass spectrometry (ESI-MS).166 Aim of this study was to compare the influence of diluents and side groups of the extracting agents on complex formation. Three different diluents, nitrobenzene, 1-octanol, and cyclohexanone, and two extracting agents, C5-BTBP and CyMe4-BTBP, were used. The Am(III) and Eu(III) complexes were prepared by extraction with BTBP from 1 mol/L nitric acid, meaning that the organic phases were water saturated. The results revealed that for both extracting agents, regardless of the diluent, the following 1:2 complexes were formed: Am(BTBP)23+, Am(BTBP)2(NO3)2+, Eu(BTBP)23+, and Eu(BTBP) 2 (NO 3 ) 2+ . To compare structures and to get information on the stability of these species fragmentation spectra (MS2) in the gas phase and energy resolved mass spectra with cone voltage variation were recorded. No 1:1 complexes were observed under these conditions. The absence of M(BTBP)2(NO3)2+ and the fact that M(BTBP)2(NO3)2+ was the main ion in the mass spectra suggested that one nitrate is bound to the metal ions. Therefore, the M(BTBP)2(NO3)3 species observed in liquid−liquid extraction experiments was interpreted as a 1:2 complex with one nitrate in the inner coordination sphere and with the other nitrate anions located in the outer sphere. This is in good agreement with the spectroscopic results presented by Steppert et al.163 Further speciation studies on the complexation of Eu(NO3)3·6 H2O with C5-BTBP were performed using Nano1224


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Table 8. Thermodynamic Data (ΔH, ΔS, and ΔG) of the Complex Formation of Cm(III)- and Eu(III)-BTBP Complexes M3+

complex

ΔH (kJ/mol)

ΔS (J/ (mol·K))

ΔG (kJ/mol)

t-Bu-C2BTBP

Eu

1:2

−23.0 ± 3.8

99.2 ± 12.3

−52.0 ± 5.2

2-propanol/H2O (1:1 vol.), 1.85 × 10−4 − 2.48 × 10−4 M nitrate, 20 °C

TRLFS

127

C5-BTBP

Cm Eu Eu

1:2 1:1 1:2

−34.7 ± 1.6 −12 ± 2 −28 ± 1

93.4 ± 5.5 67 ± 17 107 ± 13

−62.1 ± 2.6 −32 ± 3 −60 ± 3

methanol, nitrate, 25 °C

microcalorimetry

78

ligand

solvent

method

ref

by microcalorimetry (log β1 = 5.6 ± 0.6) agree well, stability constants for the 1:2 Eu(III)-C5-BTBP complex differ by almost 1 order of magnitude (log β2 = 11.3 ± 0.2 versus log β2 = 10.5 ± 0.6; cf. Table 7). The stability constants of the 1:1 and 1:2 Ln(III)-CyMe4BTBP complexes also increased from La(III) to Eu(III), whereas the value of the 1:1 Yb(III)-CyMe4-BTBP species was significantly lower than that of Eu(III). The formation of a 1:2 Yb(III)-CyMe4-BTBP complex was not observed. Comparing complex stabilities for C5-BTBP and CyMe4-BTBP, no clear trend is observed. This is in contrast to results of alkylated BTPs (cf. section 2.3.6), where a difference in the stability constants of [Eu(n-Pr-BTP)3]3+ and [Eu(i-Pr-BTP)3]3+ complexes of 2 orders of magnitude occurred.60,76 The comparison of the spectroscopically determined stability constants of the 1:2 Eu(III)-BTBP species in different diluents78,87,127 showed that the log β2 value is 2 orders of magnitude higher in pure methanol medium than that in water/ 2-propanol (1:1 vol.). This is in excellent agreement with respective results on BTP ligands65 (cf. section 2.3.6) and can also be explained by a lower desolvation energy required for the metal ion in pure alcohol medium compared to water−alcohol mixtures, where the Ln(III) ion is coordinated mainly by water molecules. The thermodynamic data (ΔH, ΔS, and ΔG) of the complexation reaction of Ln(III) and An(III) with alkylated BTBPs determined in 2-propanol/H2O and methanol by TRLFS127 and microcalorimetry,78 respectively, are summarized in Table 8. The enthalpic and entropic data of the complexation of Eu(III) with t-Bu-C2-BTBP in 2-propanol/water (1:1 vol.) were derived by the temperature dependency of the stability constants in the temperature range of 10−50 °C. The enthalpy and entropy variations associated to the [M(t-Bu-C2BTBP)2(H2O)]3+ complex formation were ΔH = −23.0 ± 3.8 kJ/mol and ΔS = 99.2 ± 12.3 J/(mol·K) (M = Eu(III)) and ΔH = −34.7 ± 1.6 kJ/mol and ΔS = 93.4 ± 5.5 J/(mol·K) (M = Cm(III)).127 Negative enthalpy changes were found for the complexation of both metal ions; thus, the reaction is exothermic in both cases. ΔH is 11.7 kJ/mol more negative for the complexation of Cm(III) than for that of Eu(III). The difference in the entropy change of Cm(III) and Eu(III) was 5.8 J/(mol·K), which is within the error range of the method, indicating an identical complexation mechanism for both metal ions. According to the Gibbs−Helmholtz equation, ΔG(20 °C) values of −52.0 ± 5.2 kJ/mol (Eu(III)) and −62.1 ± 2.6 kJ/ mol (Cm(III)) were derived. Thus, the observed difference in ΔG (20 °C) of 10.1 kJ/mol resulted mainly from the difference in ΔH and was in good agreement with ΔΔG (20 °C) = −RT ln[β2(Cm(III))/β2(Eu(III))] = −11.8 kJ/mol derived from the spectroscopic titration experiments (cf. Table 7), confirming the accuracy of the obtained data. Furthermore, the complexation of Eu(III) with C5-BTBP was studied in methanol using microcalorimetry.78 In agree-

constants on the complexation of Ln(III) and An(III) with BTBP are scarce. The methods used were time-resolved laser fluorescence spectroscopy (TRLFS),87,127 UV/vis spectrophotometry,78 and microcalorimetry.78 As already discussed for An(III)- and Ln(III)-BTP complexes, the stability constants depend strongly on the diluent. In a comparative study, the stability constants of the 1:2 complexes of Cm(III) and Eu(III) with t-Bu-C2-BTBP in water/2-propanol (1:1 vol.) were determined by TRLFS.127 This study revealed that the stability constant of Cm(III) (log β2 = 11.1 ± 0.15) is 2 orders of magnitude higher than that of the respective Eu(III) complex (log β2 = 9.0 ± 0.15) (cf. Table 7). The ratio of the stability constants corresponds to a calculated separation factor SFCm(III)/Eu(III) = β2(Cm(III))/ β2(Eu(III)) = 125. This value is in excellent agreement with SFCm(III)/Eu(III) = 120 obtained from biphasic extraction experiments (calculated from SFAm(III)/Eu(III) = 20037 and SFAm(III)/Cm(III) = 1.6538). Furthermore, TRLFS titrations at increasing nitrate concentrations of (2.48−17.36) × 10−4 mol/ L (Cm(III)) and (1.85−7.92) × 10−4 mol/L (Eu(III)) clearly showed that the stability constants were not affected by increasing amounts of nitrate in the nitrate concentration range studied. Whereas the formation of 1:1 Cm(III) and Eu(III) complexes with BTBP is suppressed in the presence of water,127 stability constants of 1:1 complexes of Eu(III) with C5-BTBP (log β1 = 4.5 ± 0.2) and CyMe4-BTBP (log β1 = 4.4 ± 0.3) were determined by Vu in 1-octanol solution (containing 0.04 mol/L H2O) by TRLFS.87 In the presence of a considerable amount of water, the stability constants of the 1:2 Eu(III)-C5-BTBP (log β2 = 9.4 ± 0.4) and Eu(III)-CyMe4BTBP (log β2 = 9.2 ± 0.3) complexes in 1-octanol are in excellent agreement with that of Eu(III) with t-Bu-C2-BTBP in water/2-propanol (1:1 vol.) (log β2 = 9.0 ± 0.15).127 The comparison of the stability constants of Cm(III) and Eu(III) complexes with C5-BTBP shows that the stability constant of Cm(III) (log β2 = 10.8 ± 0.6) is 1.5 orders of magnitude higher than that of the respective Eu(III) complex (log β2 = 9.4 ± 0.4). Considering the given error ranges, these results are also in excellent agreement with ref 127, confirming the correlation of the difference in stability constants of trivalent lanthanides and actinides with BTBP and their selectivity observed in liquid−liquid extraction studies. Further stability constants of the 1:1 and 1:2 complexes of various Ln(III) with C5-BTBP and CyMe4-BTBP in methanol were determined by UV/vis spectrophotometry.78 In the case of C5-BTBP, the stability constants increased in the lanthanide series from La(III) to Yb(III) for both 1:1 and 1:2 complexes. The decrease of the stability constants of Gd(III) reported in this work78 was interpreted by Rawat et al.65 by a potential change in the stoichiometry of the complexes due to a decrease in the coordination number from 9 to 8 within the lanthanide series. Although the stability constants for the 1:1 Eu(III)-C5BTBP complex determined by UV/vis (log β1 = 5.7 ± 0.3) and 1225


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ment with ref 127, the formation of the 1:2 Eu(III)-BTBP complex is both enthalpically and entropically driven (ΔH = −28 ± 1 kJ/mol; ΔS = 107 ± 13 J/(mol·K)). In contrast to the formation of the 1:3 Eu(III)-BTP complexes where the reaction enthalpy became more negative and the entropy decreased significantly when the diluent was changed from methanol/ water (1:1 vol.) to pure methanol, only a slight decrease of ΔH was observed for the 1:2 Eu-BTBP complex in methanol compared to 2-propanol/water (1:1 vol.), whereas the entropy remained constant within the error range. However, ΔG is 8 kJ/mol more negative in the alcoholic medium, corresponding to an increase of the stability constant by a factor of 26. This result does not agree with an increase of the stability constants of almost 2 orders of magnitude (cf. Table 7) but might be explained by the discrepancy of the stability constants obtained by UV/vis and microcalorimetry given in ref 78.

Figure 29. Effect of SO3-Ph-BTP on the extraction of Am(III) and Eu(III) into TODGA solution. Organic phase, 0.2 mol/L TODGA + 5% vol. 1-octanol in kerosene. Aqueous phase, 241Am(III) + 152Eu(III) (1 kBq/mL each) in HNO3, with (filled symbols, solid lines) or without (open symbols, dashed lines) 9 mmol/L SO3-Ph-BTP. A/O = 1, T = (293 ± 0.5) K. Reprinted with permission from ref 45. Copyright 2012 Taylor & Francis Ltd.

4. WATER-SOLUBLE BTP AND BTBP Selectivity in liquid−liquid extraction processes is usually achieved by a selective lipophilic extracting agent. An alternative way is using water-soluble masking agents, which selectively complex solutes to prevent their extraction. The TALSPEAK (trivalent actinide−lanthanide separation by phosphorus reagent extraction from aqueous komplexes) process167−169 is a long-known example: actinides(III) are complexed in the aqueous phase by polyaminocarboxylates such as DTPA (diethylenetriaminepentaacetic acid, cf. Appendix) to prevent their extraction into a solvent composed of HDEHP (bis(2-ethylhexyl)phosphate) in a diluent, while lanthanides(III) are extracted. Without the complexing agent HDEHP would coextract actinides(III).10 i-SANEX (innovative or inverse SANEX) processes based on this principle have been revisited in Europe.170,171 Because BTP and BTBP retain their selectivity in an aqueous phase,76,127 it was obvious to test hydrophilic BTPs or BTBPs as masking agents selective for actinides(III). Thus, hydrophilic BTP and BTBP were synthesized by sulfonation172 of phenyl-BTP and phenylBTBP (Figure 1, R = phenyl) and tested for selective complexation of actinides(III).43−45

product of TODGA’s selectivity (SFEu(III)/Am(III) ≈ 7) and the selectivity of BTP, which is typically SFAm(III)/Eu(III) ≈ 150.31,71 Slope analysis of the extraction data implied that the complex formed with Am(III) or Eu(III) and SO3-Ph-BTP is a 1:2 complex.45 However, hydrophobic BTPs are known to form 1:3 complexes. Also, spectroscopic studies on the complexation of Cm(III) and Eu(III) with SO3-Ph-BTP identified the formation of 1:3 complexes.46,173 As shown in ref 45, the system SO3-Ph-BTP/TODGA is efficient for an An(III)/Ln(III) group separation. With an aqueous phase containing 18 mmol/L SO3-Ph-BTP, Am(III) and Cm(III) distribution ratios were 1 for a range of nitric acid concentrations of 0.2 mol/L < [HNO3] < 0.8 mol/L; cf. Figure 30. On the basis of these data and additional kinetic measurements in a single-stage centrifugal contactor, a flowsheet for a continuous i-SANEX process was developed.174,175 This

4.1. SO3-Ph-BTP As a Masking Agent for Actinides(III)

4.1.1. Extraction. As shown in Figure 29,45 TODGA extracts Am(III) and Eu(III) from HNO3 solution with high distribution ratios (open symbols). Adding 9 mmol/L SO3-PhBTP (Figure 28) to the aqueous phase significantly suppresses

Figure 28. SO3-Ph-BTP. Note that although the position of the substitution is not specified, recent (unpublished) NMR results indicate meta substitution.

Am(III) extraction, whereas it has only a small influence on Eu(III) extraction. For 0.1 mol/L < [HNO3] < 0.6 mol/L, DAm(III) < 1 and DEu(III) > 1. The separation factor SFEu(III)/Am(III) is in the range of 250−1000 (as compared to SFEu(III)/Am(III) ≈ 7 without SO3-Ph-BTP, according to the selectivity of TODGA). With increasing SO3-Ph-BTP concentration, SFEu(III)/Am(III) approaches a value of ≈1200. This separation factor is the

Figure 30. Effect of SO3-Ph-BTP on the extraction of An(III) and Ln(III) into TODGA solution. Organic phase, 0.2 mol/L TODGA + 5% vol. 1-octanol in kerosene. Aqueous phase, 18 mmol/L SO3-PhBTP + 241Am(III) + 244Cm(III) (1 kBq/mL each) + (Y(III) + Ln(III), 20 mg/L each) in HNO3. A/O = 1, T = (293 ± 0.5) K. Reprinted with permission from ref 45. Copyright 2012 Taylor & Francis Ltd. 1226


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Figure 31. (a) Normalized Cm(III) fluorescence spectra with increasing concentrations of SO3-Ph-BTP in water at pH = 3.0. [Cm(III)]ini = 1.2 × 10−7 mol/L, [SO3-Ph-BTP] = 0−9.93 × 10−4 mol/L; (b) Cm(III) species distribution as a function of the SO3-Ph-BTP concentration in water at pH = 3.0. Symbols, experimental data. Lines, calculated with log β1 = 5.4, log β2 = 9.3, log β3 = 12.2. Reproduced with permission from ref 46. Copyright 2012 The Royal Society of Chemistry.

Figure 32. (a) Normalized Eu(III) fluorescence spectra with increasing concentrations of SO3-Ph-BTP in water at pH = 3.0. [Eu(III)]ini = 1.4 × 10−5 mol/L, [SO3-Ph-BTP] = 0−1.27 × 10−3 mol/L. (b) Eu(III) species distribution as a function of the SO3-Ph-BTP concentration in water at pH = 3.0. Symbols, experimental data. Lines, calculated with log β1 = 5.2, log β2 = 8.0, and log β3 = 10.2. Reproduced with permission from ref 46. Copyright 2012 The Royal Society of Chemistry.

flowsheet predicts that a process using TODGA as extracting agent and SO3-Ph-BTP as a selective stripping agent for Am(III) and Cm(III) in a 32-stage centrifugal contactor setup would recover >99.9% of Am(III) and Cm(III) from a PUREX raffinate with only small amounts of impurities. 4.1.2. Application in a GANEX Process. SO3-Ph-BTP is also used in the development of a GANEX second-cycle process led by the National Nuclear Laboratory, U.K. The concept of this process is to coextract TRU and lanthanides(III) into a solvent composed of 0.2 mol/L TODGA + 0.5 mol/L DMDOHEMA in kerosene, followed by a selective TRU stripping using a solution of SO3-Ph-BTP + AHA (acetohydroxamic acid, cf. Appendix) in HNO3.176−178 A flowsheet was calculated, and lab-scale continuous countercurrent tests have been performed recently; the results are not yet available.

spectra resulting from the 6D′7/2 → 8S′7/2 transition in the presence of increasing amounts of SO3-Ph-BTP is shown in Figure 31a. With increasing SO3-Ph-BTP concentration, emission bands at 593.8, 602.1, 610.0, and 617.2 nm occurred stepwisely. These bands were assigned to [Cm(SO3-PhBTP)n(H2O)9−3n] complex species with n = 0−3. The Cm(III) species distribution as a function of the SO3-Ph-BTP concentration is shown in Figure 31b. The Cm(III) complex formation was also investigated in presence of 0.5 M HNO3.173 In contrast to the complexation in H2O (pH 3), the intermediate Cm(III) complex species, namely, the 1:1 [Cm(SO3-Ph-BTP)] and the 1:2 [Cm(SO3-PhBTP)2] complexes, were almost completely suppressed. Moreover, the stability constant of the 1:3 [Cm(SO3-PhBTP)3] complex decreased. This was explained by the competitive influence of the nitrate with respect to Cm(III) and a possible protonation of the SO3-Ph-BTP molecules, reducing the complexation properties of the ligand. On the basis of these preliminary studies, further investigations on the influence of nitrate and ligand protonation on the complexation

4.2. Complexation of Cm(III) and Eu(III) with SO3-Ph-BTP

Speciation studies on the complexation of Cm(III)- and Eu(III)-perchlorate with SO3-Ph-BTP in water at pH = 3.0 were performed by time-resolved laser fluorescence spectroscopy (TRLFS).46 The evolution of the Cm(III) fluorescence 1227


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extraction ligands to form 1:3 complexes is an important requirement for a selective separation of An(III) from Ln(III) in a biphasic extraction process. This was also confirmed by a recent study on 1:3 Cm(III) and Eu(III) complexes with C5BPP, displaying a separation factor of SFAm(III)/Eu(III) ≈ 100.180 The enthalpy and entropy of the M(III)-SO3-Ph-BTP complex formation was determined by the temperature dependency of the conditional stability constants in the temperature range T = 10−60 °C. ΔH, ΔS, and ΔG were obtained for each complexation step. The thermodynamic data of the formation of the [M(SO3-Ph-BTP)n] (n = 1−3; M = Cm(III), Eu(III)) complexes are listed in Table 10. Positive

and extraction properties at process-relevant conditions are highly required. The evolution of the fluorescence spectra of Eu(III) resulting from the 5D0 → 7F1 and the 5D0 → 7F2 transitions as a function of the SO3-Ph-BTP concentration in water at pH 3 is presented in Figure 32a.46 The emission spectrum of the highly symmetric [Eu(H2O)9]3+ complex (D3h) exhibited a 7F2/7F1 ratio of 0.55, which is in good agreement with the literature value of 0.5.179 Upon formation of the asymmetric [Eu(SO3-Ph-BTP)(H2O)6] complex, the intensity ratio increased to 2.75. Further coordination with SO3-Ph-BTP ligands resulted again in a decrease, with values of 1.94 for the 1:2 and 1.03 for the 1:3 Eu(III)-SO3-Ph-BTP species, indicating an increase of the Eu(III) local symmetry. Incidentally, the value obtained for [Eu(SO3-Ph-BTP)3] is in excellent agreement with that of 1.06 for [Eu(n-Pr-BTP)3]3+ in 1-octanol.75 The fluorescence intensity factors (FI) of the Eu(III)-SO3-Ph-BTP complexes are exceptionally high in comparison to Cm(III), resulting in significantly decreased species concentrations, in particular for the 1:3 [Eu(SO3-Ph-BTP)3] complex (cf. Figure 32b). In addition, the excited-state lifetimes of the Cm(III) and Eu(III) [M(SO3-Ph-BTP)3] complex species were determined to 201.6 ± 2 μs and 1926.5 ± 10 μs, respectively. The fluorescence lifetime of the 1:3 [Cm(SO3-Ph-BTP)3] complex was significantly shorter than the expected lifetime of 1250 ± 80 μs126 for a 9-fold coordinated Cm(III) species. This decrease is caused by an intramolecular energy-transfer process from the metal ion to the organic ligand (cf. section 2.3.2). In contrast to Cm(III), no quenching of the fluorescence emission via intramolecular energy transfer was observed for Eu(III). According to ref 121, the fluorescence lifetime of the [Eu(SO3Ph-BTP)3] complex corresponds to a 9-fold coordination with N-donor ligands, confirming the absence of quenching water molecules in the inner coordination sphere. These results are in very good accordance to the results obtained for the 1:3 [M(nPr-BTP)3]3+ (M = Cm(III); Eu(III)) complexes in water/ methanol (1:1 vol.)76 and kerosene/1-octanol (70:30 vol.)72 (cf. section 2.3.2). The conditional stability constants log βn (n = 1−3) of the stepwise formation of the 1:1, 1:2, and 1:3 Cm(III)/Eu(III)SO3-Ph-BTP complexes are summarized in Table 9. The

Table 10. Thermodynamic Data of the Formation of the [M(SO3-Ph-BTP)n] (n = 1−3) Complex Species in H2O (pH = 3.0)

Cm(III)

Eu(III)

Cm(III)-SO3-Ph-BTP

Eu(III)-SO3-Ph-BTP

log βn

log βn

Δlog βn

1 2 3

5.4 ± 0.1 9.3 ± 0.2 12.2 ± 0.3

5.2 ± 0.1 8.0 ± 0.2 10.2 ± 0.3

0.2 1.3 2.0

ΔS (J/ (mol·K))

ΔG (25 °C) (kJ/ mol)

5.9 ± 0.7 −8.2 ± 2.1 −22.0 ± 3.5 17.7 ± 1.3 4.5 ± 2.8 −12.3 ± 4.0

125.9 ± 2.4 156.2 ± 7.1 165.6 ± 11.8 154.8 ± 3.7 169.0 ± 7.7 156.5 ± 10.4

−31.7 ± 1.7 −55.0 ± 4.6 −71.6 ± 7.5 −28.5 ± 2.7 −45.9 ± 5.4 −58.9 ± 7.6

enthalpy changes of 5.9 ± 0.7 and 17.7 ± 1.3 kJ/mol were obtained for the formation of the [M(SO3-Ph-BTP)] species of Cm(III) and Eu(III), respectively. Thus, the first complexation step is an endothermic process. Because of the large entropy changes of 125.9 ± 2.4 J/(mol·K) (Cm(III)) and 154.8 ± 3.7 J/(mol·K) (Eu(III)), the complexation reaction was entropydriven and exergonic. The following complexation steps revealed negative values of ΔH for both metal ions, whereas the contribution of the entropy change decreased with increasing number of coordinated SO3-Ph-BTP ligands. Regarding the separation of trivalent actinides from lanthanides, the thermodynamic data of the formation of the 1:3 [M(SO3Ph-BTP)3] complexes are of particular interest. As shown in Table 10, both reactions are entropy- and enthalpy-driven. Whereas the difference in ΔS was within the error range, the reaction enthalpy of the 1:3 Cm-SO3-Ph-BTP complex was 9.7 kJ/mol more negative than that of Eu(III). Thus, the difference in ΔG (−58.9 ± 7.6 kJ/mol for Eu(III) and −71.6 ± 7.5 kJ/ mol for Cm(III)) of 12.7 kJ/mol resulted mainly from the difference in ΔH and agreed very well with ΔΔG(25 °C) = −RT ln(β3,Cm(III)/β3,Eu(III)) = −11.4 kJ/mol obtained from the titration studies. It is also in good accordance with ΔΔG(25 °C) of −13.1 kJ/mol observed for [Cm(n-Pr-BTP)3]3+ and [Eu(n-Pr-BTP)3]3+ in methanol/water (1:1 vol.).76 As the difference in ΔG (corresponding to the difference in the stability constants of the 1:3 complexes) is a measure of the selectivity, the results on SO3-Ph-BTP confirmed that the hydrophilization of BTP ligands does not affect their selectivity (cf. Table 11). Although the selectivity of BTP ligands is not decisively affected by the solvent, the stability constants and the thermodynamic data are strongly dependent on the diluent. The thermodynamic data of the formation of the 1:3 M(III)BTP complexes with SO3-Ph-BTP in H2O, n-Pr-BTP in H2O:MeOH (1:1 vol.), and Et-BTP in MeOH are compared in Table 11. As already discussed in section 2.3.6, the log β3 values of Ln(III)-BTP complexes increase with decreasing

Table 9. Conditional Stability Constants of the [M(SO3-PhBTP)n(H2O)9−3n] (n = 1−3) Complexes in H2O (pH = 3.0)46 n

1 2 3 1 2 3

ΔH (kJ/mol)

stability constant of the 1:3 complex of Cm(III) (log β3 = 12.2 ± 0.3) is 2 orders of magnitude higher than that of the respective Eu(III) complex (log β3 = 10.2 ± 0.3). This difference corresponds to a separation factor of SFCm(III)/Eu(III) = β3,Cm(III)/β3,Eu(III) = 100, which is in agreement with SO3-PhBTP’s separation factor of SFAm(III)/Eu(III) ≈ 150.45 Comparison of the stability constants of the 1:1 and 1:2 Cm(III) or Eu(III) complexes revealed that the high difference in the stability constants corresponding to the distinct selectivity for An(III) over Ln(III) is only achieved if the metal ion is completely coordinated by N-donor atoms. Therefore, the ability of 1228


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Table 11. Thermodynamic Data of the Formation of the 1:3 M(III) Complexes with SO3-Ph-BTP in H2O, n-Pr-BTP in H2O:MeOH (1:1 vol.), and Et-BTP in MeOH ligand (solvent) SO3-Ph-BTP (H2O)

n-Pr-BTP (H2O:MeOH 1:1 vol.)

Et-BTP (MeOH)

metal ion

ΔH [kJ/mol]

ΔS [J/(K·mol)]

Cm(III) Eu(III) Δ Cm(III) Eu(III) Δ Eu(III)

−22.0 ± 3.5 −12.3 ± 4.0 9.7 −36.5 ± 4.7 −26.4 ± 1.8 10.1 −73 ± 2

165.6 ± 11.8 156.5 ± 10.4 9.1 148 ± 17 138 ± 7 10 17 ± 6

−71.6 −58.9 12.7 −79.9 −66.8 13.1 −77.7

± 7.5 ± 7.6 ± 7.5 ± 2.8

log β3

ref

12.2 10.2 2.0 14.4 11.9 2.5 13.6

46 46 46 76 76 76 65

the entropy ΔSd of the desolvation step is significantly higher in aqueous solution than in alcololic medium, resulting in less negative ΔH values and higher ΔS values for the overall complexation reaction. Compared to H2O, the coordination of MeOH is less pronounced and the desolvation energy required for the first desolvation step decreases. This is accompanied by a distinct decrease of the desolvation entropy. Therefore, the overall values of the enthalpy ΔH and the entropy ΔS decrease with decreasing content of water.

content of water in the diluent, e.g., log β3(Eu(III)) = 10.2 (H2O), 11.9 (H2O/MeOH (1:1 vol.)), and 13.6 (MeOH). The stability constants of the [Cm(BTP)3] species follow the same trend. The value in H2O:MeOH (1:1 vol.)76 is 2 orders of magnitude higher than that in pure H 2 O medium. 46 Unfortunately, no data on trivalent actinides are available in methanol. As shown in Table 11, the enthalpy becomes more negative whereas the entropy decreases significantly with decreasing content of water. This results in slightly more negative values of ΔG, corresponding to the observed increase of log β3. Considering the complexation reaction as a two-step mechanism including the desolvation of the metal ion and the solvated ligand:

4.3. SO3-Ph-BTBP

Except for a few data points on the extraction of Am(III) and Eu(III) in a SO3-Ph-BTBP/TODGA system,43 no complete study on the complexation properties of SO3-Ph-BTBP has been published. As shown in Table 12, addition of 9 mmol/L

[M(Solv)n ]3 + + [L(Solv)p ]x −

Table 12. Extraction of Am(III) and Eu(III) into TODGA Solution, Influence of SO3-Ph-BTBP on Distribution Ratios and Separation Factor; Organic Phase, 0.2 mol/L TODGA + 5% vol. 1-Octanol in Kerosene; Aqueous Phase, 241Am(III) + 152 Eu(III) in 0.5 mol/L HNO343

⇌ [M(Solv)m ]3 + + [L(Solv)q ]x − + (n − m + p − q)Solv

ΔG [kJ/mol]

(6)

and the subsequent complexation reaction according to [M(Solv)m ]3 + + [L(Solv)q ]x − ⇌ [ML(Solv)m + q ](3 − x) + (7)

the following conclusions regarding ΔH and ΔS of the complexation process can be drawn: • The enthalpy ΔHd and the entropy ΔSd of the desolvation step are both positive, indicating that the desolvation of the metal ion and the ligand is entropydriven.

[SO3-Ph-BTBP]

DAm(III)

DEu(III)

SFEu(III)/m(III)

0 9 mmol/La

120 0.33

900 150

7.5 450

a

Taking into account the molecular weight of the tetrasodium salt, which was not the case in ref 43.

SO3-Ph-BTBP suppresses the extraction of Am(III) from 0.5 mol/L HNO3 into 0.2 mol/L TODGA from DAm(III) = 120 to DAm(III) = 0.33. The extraction of Eu(III) is less affected; DEu(III) decreases from 900 to 150. Accordingly, selectivity increases from SFEu(III)/Am(III) = 7.5 to SFEu(III)/Am(III) = 450. Comparison with Figure 29 shows that the extraction performance of SO3Ph-BTBP is similar to that of SO3-Ph-BTP.

• The enthalpy ΔHc and the entropy ΔSc of the complexation step are both negative. This step is enthalpy-driven. For lanthanide and actinide complexes with inorganic ligands ΔHd ≫ ΔHc, resulting in a positive ΔH of the complex formation.133 In contrast to that, multidentate N-donor ligands show significantly stronger metal ion− ligand interactions. Thus, the contribution of ΔHc exceeds ΔHd, and the complexation reaction is observed to be exothermic. Negative ΔH values were obtained for all 1:3 An(III)-BTP and Ln(III)-BTP complexes, independent of the solvent. Furthermore, the contribution of ΔSc compared to ΔSd is negligible. Whereas the second step mainly depends on the nature/ charge of the metal ion and the ligand, the desolvation step is strongly affected by solvent molecules. In the pure aqueous solution, well-ordered solvation spheres with a high symmetry are formed for the highly charged metal ions, e.g., D3h for [M(H2O)9]3+,181 but also for the 4-fold negatively charged SO3-Ph-BTP. Accordingly, the change in the enthalpy ΔHd and

5. CONCLUSIONS AND OUTLOOK The present review compiles results and data on the complexation and extraction of actinides(III) and lanthanides-

Figure 33. SO3-Ph-BTBP. Note that although the position of the substitution is not specified, recent (unpublished) NMR results indicate meta substitution. 1229


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recently.130,180,182,183 We are convinced that understanding the role of the anion is of major importance for designing improved extracting agents. Comparison of the stability constants implies that the high selectivity is only achieved if the metal ions are completely coordinated by N-donor ligands, as is the case in the 1:3 BTP and 1:2 BTBP complexes. The ability of ligands to form fully coordinated complexes with high stability constants is an important requirement for a high selectivity. Further investigations are required to support this statement. The selectivity observed in liquid−liquid extraction experiments correlates directly to differences in the respective stability constants of the 1:3 (BTP) or 1:2 (BTBP) complexes. This is true for both the actininide(III)/lanthanide(III) selectivity and the intra-lanthanide selectivity. Because of this correlation, TRLFS is a versatile tool in ligand design to provide direct feedback on the complexation and thus extraction properties of new ligand classes. For example, if the Cm(III) and Eu(III) formation constants are found to be favorable but the solubility is poor, synthesizing a more lipophilic ligand seems worthwhile. An interesting observation is made when comparing the extraction and complexation data of BTPs with those of BTBPs: whereas alkylation of BTPs has a tremendous influence on their complexation and extraction properties (e.g., n-Pr-BTP versus i-Pr-BTP or CyMe4-BTP), only a small effect is observed for BTBPs (e.g., C5-BTBP versus CyMe4-BTBP). This could be due to different sterical demands in the 1:3 BTP versus 1:2 BTBP complexes. Quantum-chemical calculations could help to explain this observation. Finally, we observe that there is a substantial lack of studies with actinides(III), which is understandable because working with actinides requires a suitable infrastructure. However, there are several institutions capable of performing such studies. We hope that this review triggers a renewed interest and new studies will be performed in this fascinating field!

(III) by BTPs and BTBPs. Both ligand families have been shown to be highly efficient extracting agents for the selective extraction of actinides(III) from nitric acid solutions. Although the first representatives suffered from low stability, later compounds such as CyMe4-BTBP, CyMe4-BTPhen, and CABTP are promising compounds for the development of separation processes. Several successful lab-scale process tests, some of them with highly radioactive feed solutions, were performed using CyMe4-BTBP. The recently developed watersoluble BTP and BTBP open the door to improved separation processes. Despite the huge progress made over the last 13 years, an application in an industrial process may require further optimization. Systematic improvement, however, should be based on a comprehensive understanding of the driving forces on a molecular level. To address this, a number of fundamental studies have been performed in recent years. Structures, stoichiometries, and stabilities of actinide(III) and lanthanide(III) complexes and thermodynamic data of the complex formation were determined by various analytical methods. Some important conclusions are drawn from these fundamental studies: compared to many other similar Ndonor ligands, BT(B)Ps form strong complexes with actinides(III), which may explain their unique properties as N-donor extracting agents for actinides(III). Their selectivity for actinides(III) over lanthanides(III) is due to differences in the stability of the respective complexes. The final complexes (1:3 for BTP; 1:2 for BTBP) identified in solution and in the solid state represent the complexes that are extracted from an aqueous into an organic phase. This demonstrates the strong link between the fundamental (complexation) and the more applied (extraction) studies. Analysis and comparison of the data presented in this review lead to following important conclusions: Of the many N-donor extracting agents developed over several decades, only BTP and BTBP are capable of selectively extracting actinide(III) nitrates from nitric acid solutions, while the other compounds require a more lipohilic anion to form extractable complexes. There is a clear lack of studies addressing this issue. The first studies on structurally modified BTPs (all requiring a lipophilic anion) have been performed

6. APPENDIX: LIST OF COMPOUNDS The following is a list of compounds referenced in this manuscript.

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Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal (INE), P.O. Box 3640, 76021 Karlsruhe, Germany. Phone: +49-721-608-26249. E-mail: andreas.geist@ kit.edu.

AUTHOR INFORMATION Corresponding Author

*Petra J. Panak, Ruprecht-Karls-Universität Heidelberg, Physikalisch Chemisches Institut (PCI), Im Neuenheimer Feld 253, 69120 Heidelberg, Germany, and Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal (INE), P.O. Box 3640, 76021 Karlsruhe, Germany. Phone: +49-721608-24469. E-mail: [email protected]. Andreas Geist,

Notes

The authors declare no competing financial interest. 1232


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Biographies

ACKNOWLEDGMENTS We acknowledge financial support from the German Federal Ministry of Education and Research (BMBF) (contract numbers 02NUK012A and 02NUK012D), the foundation of Energie Baden-Württemberg (EnBW), and the European Commission (projects ACSEPT−Contract No. FP7-CP-2007211 267 and ACTINET-I3). REFERENCES (1) Magill, J.; Berthou, V.; Haas, D.; Galy, J.; Schenkel, R.; Wiese, H. W.; Heusener, G.; Tommasi, J.; Youinou, G. Nucl. Energy 2003, 42, 263. (2) Potential Benefits and Impacts of Advanced Nuclear Fuel Cycles with Actinide Partitioning and Transmutation. NEA No. 6894; OECD, Nuclear Energy Agency (NEA): Paris, 2011. (3) Salvatores, M.; Palmiotti, G. Prog. Part. Nucl. Phys. 2011, 66, 144. (4) Robinson, T.; Moats, M.; Davenport, W.; Karcas, G.; Demetrio, S.; Domic, E. In Proc. International Solvent Extraction Conference (ISEC 2008), Tucson, AZ, September 15−19, 2008; Moyer, B. A., Ed. (5) Uranium Extraction Technology; International Atomic Energy Agency: Vienna, Austria, 1993. (6) Lantham, W. B.; Runion, T. C. PUREX Process for Plutonium and Uranium Recovery; USAEC report ORNL-479; Oak Ridge National Laboratory: Oak Ridge, TN, 1949. (7) Warf, J. C. J. Am. Chem. Soc. 1949, 71, 3257. (8) Schulz, W. W.; Navratil, J. D. Science and Technology of Tributyl Phosphate, Vol. III; CRC Press: Boca Raton, FL, 1990. (9) Hill, C. In Ion Exchange and Solvent Extraction; Moyer, B. A., Ed.; CRC Press: Boca Raton/London/NY, 2010; Vol. 19. (10) Peppard, D. F.; Mason, G. W.; Maier, J. L.; Driscoll, W. J. J. Inorg. Nucl. Chem. 1957, 4, 334. (11) Musikas, C., Le Marois, G., Fitoussi, R., Cuillerdier, C., Navratil, J. D., Schulz, W. W., Eds.; ACS symposium series 117; American Chemical Society: Washington, DC, 1980. (12) Musikas, C.; Vitorge, P.; Pattee, D. In Proc. Internat. Solvent Extr. Conf. (ISEC 1983), Denver, CO, August 26−September 2, 1983. (13) Jarvinen, G. D.; Barrans, R. E.; Schroeder, N. C.; Wade, K. L.; Jones, M. M.; Smith, B. F.; Mills, J. L.; Howard, G.; Freiser, H.; Muralidharan, S. In Separation of f Elements; Nash, K. L., Choppin, G. R., Eds.; Plenum Press: New York, 1995. (14) Zhu, Y. J.; Chen, J.; Jiao, R. Z. Solvent Extr. Ion Exch. 1996, 14, 61. (15) Zhu, Y. J.; Chen, J. F.; Choppin, G. R. Solvent Extr. Ion Exch. 1996, 14, 543. (16) Chen, J.; Jiao, R. Z.; Zhu, Y. J. Solvent Extr. Ion Exch. 1996, 14, 555. (17) Modolo, G.; Odoj, R. J. Radioanal. Nucl. Chem. 1998, 228, 83. (18) Modolo, G.; Odoj, R. J. Alloys Compd. 1998, 271, 248. (19) Modolo, G.; Odoj, R. Solvent Extr. Ion Exch. 1999, 17, 33. (20) Modolo, G.; Nabet, S. Solvent Extr. Ion Exch. 2005, 23, 359. (21) Modolo, G.; Kluxen, P.; Geist, A. Radiochim. Acta 2010, 98, 193. (22) Klaehn, J. R.; Peterman, D. R.; Harrup, M. K.; Tillotson, R. D.; Luther, T. A.; Law, J. D.; Daniels, L. M. Inorg. Chim. Acta 2008, 361, 2522. (23) Peterman, D. R.; Greenhalgh, M. R.; Tillotson, R. D.; Klaehn, J. R.; Harrup, M. K.; Luther, T. A.; Law, J. D. Sep. Sci. Technol. 2010, 45, 1711. (24) Kolarik, Z.; Schuler, R.; Müllich, U. Partition of High-Level Radioactive Waste; EUR 16958; European Commission: Luxembourg, 1996. (25) Madic, C.; Hudson, M. J. High-Level Liquid Waste Partitioning by Means of Completely Incinerable Extractants; EUR 18038; European Commission: Luxembourg, 1998. (26) Madic, C.; Hudson, M. J.; Liljenzin, J.-O.; Glatz, J.-P.; Nannicini, R.; Facchini, A.; Kolarik, Z.; Odoj, R. New Partitioning Techniques for Minor Actinides, Final Report; EUR 19149; European Commission: Luxembourg, 2000.

Petra J. Panak obtained her Ph.D. degree in 1996 from the Technische Universität München, Germany (Munich Technical University). After postdoctoral positions at the Institute of Radiochemistry, Helmholtz Center Dresden-Rossendorf (Dresden, Germany), and at the Glenn T. Seaborg Center, Lawrence Berkeley National Laboratory (Berkeley, California, U.S.A.), she started her work as a staff scientist at the Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, where she built up a research group on “Coordination chemistry of actinides”. She obtained her Habilitation from the University of Heidelberg in 2006. Since 2008, she has an endowed professorship on radiochemistry at the University of Heidelberg (funded by Energie Baden-Württemberg, EnBW). She was awarded the Fritz-Strassmann-Preis of the German Chemical Society (GDCh) in 2005 and was elected as a member of the steering committee of the nuclear chemistry division of the German Chemical Society (GDCh) in 2010. Her research interests are focused on coordination chemistry and thermodynamics of actinides and lanthanides with inorganic, organic (in particular “partioning relevant” N-donor ligands), and bioligands.

Andreas Geist studied chemistry and obtained his Dr. rer. nat. from Technische Universität München, Germany, in 1997. From 1997, he has been working as a staff scientist at the Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, where he is group leader for the project “separation of actinides from highly radioactive wastes”. He is a member of the German ProcessNet (formerly DECHEMA) Liquid−Liquid Extraction Committee. His research focuses on the separation of actinides by solvent extraction, covering various aspects such as fundamental studies, extracting agent design and assessment, and process development. 1233


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(27) Madic, C.; Testard, F.; Hudson, M. J.; Liljenzin, J.-O.; Christiansen, B.; Ferrando, M.; Facchini, A.; Geist, A.; Modolo, G.; Espartero, A. G.; De Mendoza, J. PARTNEWNew Solvent Extraction Processes for Minor Actinides, Final Report; CEA-R-6066; Commissariat à l’Énergie Atomique: France, 2004. (28) Madic, C.; Ouvrier, N. Radiochim. Acta 2008, 96, 183. (29) Bourg, S.; Hill, C.; Caravaca, C.; Rhodes, C.; Ekberg, C.; Taylor, R.; Geist, A.; Modolo, G.; Cassayre, L.; Malmbeck, R.; Harrison, M.; de Angelis, G.; Espartero, A.; Bouvet, S.; Ouvrier, N. Nucl. Eng. Des. 2011, 241, 3427. (30) Kolarik, Z.; Müllich, U.; Gassner, F. Solvent Extr. Ion Exch. 1999, 17, 23. (31) Kolarik, Z.; Müllich, U.; Gassner, F. Solvent Extr. Ion Exch. 1999, 17, 1155. (32) Case, F. H. J. Heterocycl. Chem. 1971, 8, 1043. (33) Hill, C.; Guillaneux, D.; Berthon, L.; Madic, C. J. Nucl. Sci. Technol. 2002, Suppl. 3, 309. (34) Hudson, M. J.; Boucher, C. E.; Braekers, D.; Desreux, J. F.; Drew, M. G. B.; Foreman, M. R. S.; Harwood, L. M.; Hill, C.; Madic, C.; Marken, F.; Youngs, T. G. A. New J. Chem. 2006, 30, 1171. (35) Trumm, S.; Geist, A.; Panak, P. J.; Fanghänel, T. Solvent Extr. Ion Exch. 2011, 29, 213. (36) Drew, M. G. B.; Foreman, M. R. S. J.; Hill, C.; Hudson, M. J.; Madic, C. Inorg. Chem. Commun. 2005, 8, 239. (37) Foreman, M. R. S. J.; Hudson, M. J.; Geist, A.; Madic, C.; Weigl, M. Solvent Extr. Ion Exch. 2005, 23, 645. (38) Geist, A.; Hill, C.; Modolo, G.; Foreman, M. R. S. J.; Weigl, M.; Gompper, K.; Hudson, M. J.; Madic, C. Solvent Extr. Ion Exch. 2006, 24, 463. (39) Modolo, G.; Sypula, M.; Geist, A.; Hill, C.; Sorel, C.; Malmbeck, R.; Magnusson, D.; Foreman, M. R. S. In Actinide and Fission Product Partitioning and Transmutation; 10th Information Exchange Meeting, Mito, Japan, October 6−10, 2008; OECD-NEA: Issy-les-Moulineaux, France, 2010; NEA No. 6420. (40) Magnusson, D.; Christiansen, B.; Foreman, M. R. S.; Geist, A.; Glatz, J. P.; Malmbeck, R.; Modolo, G.; Serrano-Purroy, D.; Sorel, C. Solvent Extr. Ion Exch. 2009, 27, 97. (41) Wilden, A.; Schreinemachers, C.; Sadowski, F.; Gülland, S.; Sypula, M.; Modolo, G.; Magnusson, D.; Geist, A.; Lewis, F. W.; Harwood, L. M.; Hudson, M. J. In Nuclear Fuel Cycle Conference, Manchester, U.K., April 23−25, 2012. (42) Modolo, G.; Wilden, A.; Daniels, H.; Geist, A.; Magnusson, D.; Malmbeck, R. Radiochim. Acta 2013, 101, 1. (43) Müllich, U.; Geist, A.; Zevaco, T. Patent EP 2 377 861 A1, 2011. (44) Geist, A.; Müllich, U.; Modolo, G.; Wilden, A. In Proc. Internat. Solvent Extr. Conf. (ISEC 2011), Santiago, Chile, October 3−7, 2011. (45) Geist, A.; Müllich, U.; Magnusson, D.; Kaden, P.; Modolo, G.; Wilden, A.; Zevaco, T. Solvent Extr. Ion Exch. 2012, 30, 433. (46) Ruff, C. M.; Müllich, U.; Geist, A.; Panak, P. J. Dalton Trans. 2012, 41, 14594. (47) Halcrow, M. A. Coord. Chem. Rev. 2005, 249, 2880. (48) Kolarik, Z. Chem. Rev. 2008, 108, 4208. (49) Ekberg, C.; Fermvik, A.; Retegan, T.; Skarnemark, G.; Foreman, M. R. S.; Hudson, M. J.; Englund, S.; Nilsson, M. Radiochim. Acta 2008, 96, 225. (50) Lewis, F. W.; Hudson, M. J.; Harwood, L. M. Synlett 2011, 2609. (51) Hudson, M. J.; Harwood, L. M.; Laventine, D. M.; Lewis, F. W. Inorg. Chem. 2012, DOI: 10.1021/ic3008848. (52) Modolo, G.; Wilden, A.; Geist, A.; Magnusson, D.; Malmbeck, R. Radiochim. Acta 2012, 100, 715. (53) Boucher, C.; Drew, M. G. B.; Giddings, P.; Harwood, L. M.; Hudson, M. J.; Iveson, P. B.; Madic, C. Inorg. Chem. Commun. 2002, 5, 596. (54) Ionova, G.; Rabbe, C.; Guillaumont, R.; Ionov, S.; Madic, C.; Krupa, J. C.; Guillaneux, D. New J. Chem. 2002, 26, 234. (55) Drew, M. G. B.; Guillaneux, D.; Hudson, M. J.; Iveson, P. B.; Madic, C. Inorg. Chem. Commun. 2001, 4, 462. (56) Iveson, P. B.; Riviere, C.; Guillaneux, D.; Nierlich, M.; Thuery, P.; Ephritikhine, M.; Madic, C. Chem. Commun. 2001, 1512.

(57) Berthet, J. C.; Miquel, Y.; Iveson, P. B.; Nierlich, M.; Thuery, P.; Madic, C.; Ephritikhine, M. J. Chem. Soc., Dalton Trans. 2002, 3265. (58) Colette, S.; Amekraz, B.; Madic, C.; Berthon, L.; Cote, G.; Moulin, C. Inorg. Chem. 2002, 41, 7031. (59) Colette, S.; Amekraz, B.; Madic, C.; Berthon, L.; Cote, G.; Moulin, C. Inorg. Chem. 2003, 42, 2215. (60) Colette, S.; Amekraz, B.; Madic, C.; Berthon, L.; Cote, G.; Moulin, C. Inorg. Chem. 2004, 43, 6745. (61) Miguirditchian, M.; Guillaneux, D.; François, N.; Airvault, S.; Ducros, S.; Thauvin, D. Nucl. Sci. Eng. 2006, 153, 223. (62) Berthet, J. C.; Thuery, P.; Dognon, J. P.; Guillaneux, D.; Ephritikhine, M. Inorg. Chem. 2008, 47, 6850. (63) Drew, M. G. B.; Foreman, M. R. S.; Geist, A.; Hudson, M. J.; Marken, F.; Norman, V.; Weigl, M. Polyhedron 2006, 25, 888. (64) Benay, G.; Schurhammer, R.; Desaphy, J.; Wipff, G. New J. Chem. 2011, 35, 184. (65) Rawat, N.; Bhattacharyya, A.; Ghosh, S. K.; Gadly, T.; Tomar, B. S. Radiochim. Acta 2011, 99, 705. (66) Wei, Y.; Sabharwal, K. N.; Kumagai, M.; Asakura, T.; Uchiyama, G.; Fujine, S. J. Nucl. Sci. Technol. 2000, 37, 1108. (67) Wei, Y.; Hoshi, H.; Kumagai, M.; Asakura, T.; Uchiyama, G. J. Nucl. Sci. Technol. 2002, Suppl. 3, 761. (68) Bhattacharyya, A.; Mohapatra, P. K.; Roy, A.; Gadly, T.; Ghosh, S. K.; Manchanda, V. K. Hydrometallurgy 2009, 99, 18. (69) Nilsson, M.; Andersson, S.; Ekberg, C.; Foreman, M. R. S.; Hudson, M. J.; Skarnemark, G. Radiochim. Acta 2006, 94, 103. (70) Kolarik, Z. Solvent Extr. Ion Exch. 2003, 21, 381. (71) Steppert, M.; Walther, C.; Geist, A.; Fanghänel, T. New J. Chem. 2009, 33, 2437. (72) Denecke, M. A.; Panak, P. J.; Burdet, F.; Weigl, M.; Geist, A.; Klenze, R.; Mazzanti, M.; Gompper, K. C. R. Chim. 2007, 10, 872. (73) Weigl, M.; Geist, A.; Müllich, U.; Gompper, K. Solvent Extr. Ion Exch. 2006, 24, 845. (74) Drew, M. G. B.; Guillaneux, D.; Hudson, M. J.; Iveson, P. B.; Russell, M. L.; Madic, C. Inorg. Chem. Commun. 2001, 4, 12. (75) Denecke, M. A.; Rossberg, A.; Panak, P. J.; Weigl, M.; Schimmelpfennig, B.; Geist, A. Inorg. Chem. 2005, 44, 8418. (76) Trumm, S.; Panak, P. J.; Geist, A.; Fanghänel, T. Eur. J. Inorg. Chem. 2010, 3022. (77) Banik, N. L.; Schimmelpfennig, B.; Marquardt, C. M.; Brendebach, B.; Geist, A.; Denecke, M. A. Dalton Trans. 2010, 39, 5117. (78) Hubscher-Bruder, V.; Haddaoui, J.; Bouhroum, S.; Arnaud-Neu, F. Inorg. Chem. 2010, 49, 1363. (79) Charbonnel, M. C.; Flandin, J. L.; Giroux, S.; Presson, M. T.; Madic, C.; Morel, J. P. In Proc. Internat. Solvent Extr. Conf. (ISEC 2002), Cape Town, South Africa, March 18−21, 2002. (80) Hill, C.; Heres, X.; Calor, J.-N.; Guillaneux, D.; Mauborgne, B.; Rat, B.; Rivalier, P.; Baron, P. In Proc. Internat. Conf. Future Nuclear Systems (GLOBAL 1999), Jackson Hole, WY, August 29−September 3, 1999. (81) Hill, C.; Guillaneux, D.; Hérès, X.; Boubals, N.; Ramain, L. In Proc. Internat. Conf. ATALANTE 2000 (Scientific Research on the Backend of the Nuclear Fuel Cycle for the 21st Century), Avignon, France, October 24−26, 2000. (82) Kolarik, Z.; Rais, J. Solvent Extr. Ion Exch. 2002, 20, 227. (83) Benay, G.; Schurhammer, R.; Wipff, G. Phys. Chem. Chem. Phys. 2010, 12, 11089. (84) Chevrot, G.; Schurhammer, R.; Wipff, G. Phys. Chem. Chem. Phys. 2007, 9, 1991. (85) Chevrot, G.; Schurhammer, R.; Wipff, G. Phys. Chem. Chem. Phys. 2007, 9, 5928. (86) Hill, C.; Desreux, J. F.; Ekberg, C.; Espartero, A. G.; Galetta, M.; Modolo, G.; Geist, A.; Selucky, P.; Narbutt, J.; Madic, C. Radiochim. Acta 2008, 96, 259. (87) Vu, T. H. Étude par spectrométrie de fluorescence de la solvatation et de la complexation des ions Eu(III) en milieu octanol et à l’interface avec l’eau., Ph.D. thesis; CEA-R-6229; Commissariat à l’énergie atomique: Saclay, France, 2009. 1234


Chemical Reviews

Review

(88) Wei, Y.; Hoshi, H.; Kumagai, M.; Asakura, T.; Morita, Y. J. Alloys Compd. 2004, 374, 447. (89) Trumm, S.; Wipff, G.; Geist, A.; Panak, P. J.; Fanghänel, T. Radiochim. Acta 2011, 99, 13. (90) Hudson, M. J.; Foreman, M. R. S.; Hill, C.; Huet, N.; Madic, C. Solvent Extr. Ion Exch. 2003, 21, 637. (91) Geist, A. Solvent Extr. Ion Exch. 2010, 28, 596. (92) Banik, N. L.; Denecke, M. A.; Geist, A.; Modolo, G.; Panak, P. J.; Rothe, J. Inorg. Chem. Commun. 2013, DOI: 10.1016/ j.inoche.2012.11.032. (93) Rat, B.; Hérès, X. In Proc. Internat. Conf. ATALANTE 2000 (Scientific Research on the Back-end of the Nuclear Fuel Cycle for the 21st Century), Avignon, France, October 24−26, 2000. (94) Hill, C.; Berthon, C.; Guillaneux, D.; Dancausse, J. P.; Madic, C. In Proc. Internat. Conf. ATALANTE 2004 (Advances for Future Nuclear Fuel Cycles), Avignon, France, 2004. (95) Hill, C.; Berthon, L.; Madic, C. In Proc. Internat. Conf. GLOBAL 2005 (Nuclear Energy Systems for Future Generation and Global Sustainability), Tsukuba, Japan, October 9−13, 2005. (96) Ekberg, C.; Aneheim, E.; Fermvik, A.; Foreman, M.; LofstromEngdahl, E.; Retegan, T.; Spendlikova, I. J. Chem. Eng. Data 2010, 55, 5133. (97) Lanças, F. M.; Queiroz, M. E. C. J. Radioanal. Nucl. Chem. Lett. 1995, 199, 395. (98) Mincher, B. J.; Modolo, G.; Mezyk, S. P. Solvent Extr. Ion Exch. 2010, 28, 415. (99) Magnusson, D.; Christiansen, B.; Malmbeck, R.; Glatz, J. P. Radiochim. Acta 2009, 97, 497. (100) Marchs̀ Advanced Organic Chemistry: Reactions, Mechanisms, and Structure; Smith, M. B., March, J., Eds.; John Wiley & Sons: Hoboken, NJ, 2007. (101) Cechova, S.; Macasek, F. J. Radioanal. Nucl. Chem. Art. 1991, 149, 281. (102) Shannon, R. D. Acta Crystallogr. 1976, A32, 751. (103) David, F. J. Less-Common Met. 1986, 121, 27. (104) Bunker, G. Introduction to XAFSA Practical Guide to X-ray Absorption Fine Structure Spectroscopy; Cambridge University Press: 2010. (105) Guillaumont, D. J. Phys. Chem. A 2004, 108, 6893. (106) Edelstein, N. M.; Klenze, R.; Fanghänel, T.; Hubert, S. Coord. Chem. Rev. 2006, 250, 948. (107) Kim, J. I.; Klenze, R.; Wimmer, H. Eur. J. Solid State Inorg. Cherm. 1991, 28, 347. (108) Kim, J. I.; Wimmer, H.; Klenze, R. Radiochim. Acta 1991, 54, 35. (109) Klenze, R.; Kim, J. I.; Wimmer, H. Radiochim. Acta 1991, 52− 53, 97. (110) Wimmer, H.; Kim, J. I.; Klenze, R. Radiochim. Acta 1992, 58− 59, 165. (111) Wimmer, H.; Klenze, R.; Kim, J. I. Radiochim. Acta 1992, 56, 79. (112) Skerencak, A.; Panak, P. J.; Hauser, W.; Neck, V.; Klenze, R.; Lindqvist-Reis, P.; Fanghänel, T. Radiochim. Acta 2009, 97, 385. (113) Kim, M. A.; Panak, P. J.; Breban, D. C.; Priemyshev, A.; Yun, J. I.; Mansel, A.; Kim, J. I. Colloids Surf., A 2007, 296, 206. (114) Stumpf, T.; Fanghänel, T.; Grenthe, I. J. Chem. Soc., Dalton Trans. 2002, 3799. (115) Fanghänel, T.; Kim, J. I.; Paviet, P.; Klenze, R.; Hauser, W. Radiochim. Acta 1994, 66−7, 81. (116) Paviet, P.; Fanghänel, T.; Klenze, R.; Kim, J. I. Radiochim. Acta 1996, 74, 99. (117) Fanghänel, T.; Weger, H. T.; Konnecke, T.; Neck, V.; PavietHartmann, P.; Steinle, E.; Kim, J. I. Radiochim. Acta 1998, 82, 47. (118) Aas, W.; Steinle, E.; Fanghänel, T.; Kim, J. I. Radiochim. Acta 1999, 84, 85. (119) Klenze, R.; Panak, P.; Kim, J. I. J. Alloys Compd. 1998, 271, 746. (120) Panak, P.; Klenze, R.; Kim, J. I.; Wimmer, H. J. Alloys Compd. 1995, 225, 261.

(121) Horrocks, W. D.; Sudnick, D. R. J. Am. Chem. Soc. 1979, 101, 334. (122) Kimura, T.; Choppin, G. R. J. Alloys Compd. 1994, 213, 313. (123) Kimura, T.; Kato, Y. J. Alloys Compd. 1998, 278, 92. (124) Kimura, T.; Kato, Y. J. Alloys Compd. 1998, 275, 806. (125) Lanthanide probes in life, chemical and earth science, theory and practice; Bünzli, J.-C. G., Choppin, G. R., Eds.; Elsevier: Amsterdam, 1989. (126) Beitz, J. V. Handbook on the Physics and Chemistry of Rare Earths; 1994. (127) Trumm, S.; Lieser, G.; Foreman, M. R. S.; Panak, P. J.; Geist, A.; Fanghänel, T. Dalton Trans. 2010, 39, 923. (128) Weigl, M.; Denecke, M. A.; Panak, P. J.; Geist, A.; Gompper, K. Dalton Trans. 2005, 1281. (129) Trumm, S.; Lieser, G.; Panak, P. J. Radiochim. Acta 2011, 99, 783. (130) Beele, B. B.; Müllich, U.; Schwörer, F.; Geist, A.; Panak, P. J. Proc. Chem. 2012, 7, 146. (131) Miguirditchian, M.; Guillaneux, D.; Guillaumont, D.; Moisy, P.; Madic, C.; Jensen, M. P.; Nash, K. L. Inorg. Chem. 2005, 44, 1404. (132) Vu, T.-H.; Charbonnel, M.-C.; Boubals, N.; Couston, L.; Arnaud, F. In Proc. Int. Conf. ATALANTE 2008 (Nuclear Fuel Cycles for a Sustainable Future), Montpellier, France, May 19−23, 2008. (133) Rao, L. Chem. Soc. Rev. 2007, 36, 881. (134) Nilsson, M.; Ekberg, C.; Foreman, M. R. S.; Hudson, M.; Liljenzin, J. O.; Modolo, G.; Skarnemark, G. Solvent Extr. Ion Exch. 2006, 24, 823. (135) Nilsson, M.; Andersson, S.; Drouet, F.; Ekberg, C.; Foreman, M. R. S.; Hudson, M.; Liljenzin, J. O.; Magnusson, D.; Skarnemark, G. Solvent Extr. Ion Exch. 2006, 24, 299. (136) Foreman, M. R. S.; Hudson, M. J.; Drew, M. G. B.; Hill, C.; Madic, C. Dalton Trans. 2006, 1645. (137) Lewis, F. W.; Harwood, L. M.; Hudson, M. J.; Distler, P.; John, J.; Stamberg, K.; Núñez, A.; Galán, H.; Espartero, A. G. Eur. J. Org. Chem. 2012, 2012, 1509. (138) Harwood, L. M.; Lewis, F. W.; Hudson, M. J.; John, J.; Distler, P. Solvent Extr. Ion Exch. 2011, 29, 551. (139) Lewis, F. W.; Harwood, L. M.; Hudson, M. J.; Drew, M. G. B.; Desreux, J. F.; Vidick, G.; Bouslimani, N.; Modolo, G.; Wilden, A.; Sypula, M.; Vu, T. H.; Simonin, J. P. J. Am. Chem. Soc. 2011, 133, 13093. (140) Magnusson, D.; Christiansen, B.; Glatz, J. P.; Malmbeck, R.; Modolo, G.; Serrano-Purroy, D.; Sorel, C. Radiochim. Acta 2009, 97, 155. (141) Magnusson, D.; Malmbeck, R. Solvent Extr. Ion Exch. 2012, 30, 115. (142) Wilden, A.; Schreinemachers, C.; Sypula, M.; Modolo, G. Solvent Extr. Ion Exch. 2011, 29, 190. (143) Magnusson, D.; Geist, A.; Wilden, A.; Modolo, G. Solvent Extr. Ion Exch. 2013, 31, 1. (144) Adnet, J.-M.; Miguirditchian, M.; Hill, C.; Hérès, X.; Lecomte, M.; Masson, M.; Brossard, P.; Baron, P. In Proc. Internat. Conf. GLOBAL 2005 (Nuclear Energy Systems for Future Generation and Global Sustainability), Tsukuba, Japan, October 9−13, 2005. (145) Miguirditchian, M.; Sorel, C.; Camès, B.; Bisel, I.; Baron, P.; Espinoux, D.; Calor, J.-N.; Viallesoubranne, C.; Lorrain, B.; Masson, M. In Proc. Internat. Conf. GLOBAL 2009 (The Nuclear Fuel Cycle: Sustainable Options & Industrial Perspectives), Paris, France, September 6−11, 2009. (146) Miguirditchian, M.; Roussel, H.; Chareyre, L.; Baron, P.; Espinoux, D.; Calor, J.-N.; Viallesoubranne, C.; Lorrain, B.; Masson, M. In Proc. Internat. Conf. GLOBAL 2009 (The Nuclear Fuel Cycle: Sustainable Options & Industrial Perspectives), Paris, France, September 6−11, 2009. (147) Retegan, T.; Ekberg, C.; Dubois, I.; Fermvik, A.; Skarnemark, G.; Wass, T. J. Solvent Extr. Ion Exch. 2007, 25, 417. (148) Aneheim, E.; Ekberg, C.; Fermvik, A.; Foreman, M. R. S.; Retegan, T.; Skarnemark, G. Solvent Extr. Ion Exch. 2010, 28, 437. 1235


Chemical Reviews

Review

(178) Bell, K.; Carpentier, C.; Carrott, M.; Geist, A.; Gregson, C.; Hérès, X.; Magnusson, D.; Malmbeck, R.; McLachlan, F.; Modolo, G.; Müllich, U.; Sypula, M.; Taylor, R.; Wilden, A. Proc. Chem. 2012, 7, 392. (179) Bünzli, J. C. G.; Yersin, J. R. Inorg. Chem. 1979, 18, 605. (180) Bremer, A.; Ruff, C. M.; Girnt, D.; Müllich, U.; Rothe, J.; Roesky, P. W.; Panak, P. J.; Karpov, A.; Müller, T. J. J.; Denecke, M. A.; Geist, A. Inorg. Chem. 2012, 51, 5199. (181) Skanthakumar, S.; Antonio, M. R.; Wilson, R. E.; Soderholm, L. Inorg. Chem. 2007, 46, 3485. (182) Girnt, D.; Roesky, P. W.; Geist, A.; Ruff, C. M.; Panak, P. J.; Denecke, M. A. Inorg. Chem. 2010, 49, 9627. (183) Bremer, A.; Geist, A.; Panak, P. J. Dalton Trans. 2012, 41, 7582.

(149) Ekberg, C.; Dubois, I.; Fermvik, A.; Retegan, T.; Skarnemark, G.; Drew, M. G. B.; Foreman, M. R. S.; Hudson, M. J. Solvent Extr. Ion Exch. 2007, 25, 603. (150) Aneheim, E.; Ekberg, C.; Foreman, M. R. S. Hydrometallurgy 2012, 115, 71. (151) Aneheim, E.; Ekberg, C.; Mabile, N. In Proc. Internat. Solvent Extr. Conf. (ISEC 2011), Santiago, Chile, October 3−7, 2011. (152) Aneheim, E.; Ekberg, C.; Foreman, M. R. S.; LofstromEngdahl, E.; Mabile, N. Sep. Sci. Technol. 2012, 47, 663. (153) Narbutt, J.; Krejzler, J. Radiochim. Acta 2008, 96, 219. (154) Geist, A.; Magnusson, D.; Müllich, U. In Actinide and Fission Product Partitioning and Transmutation, 12th Information Exchange Meeting, OECD-NEA, Prague, Czech Republic, September 24−27, 2012. (155) Retegan, T.; Ekberg, C.; Fermvik, A.; Foreman, M. R. S.; Skarnemark, G. In Proc. International Solvent Extraction Conference (ISEC 2008), Tucson, AZ, September 15−19, 2008; Moyer, B. A., Ed. (156) Retegan, T.; Ekberg, C.; Englund, S.; Fermvik, A.; Foreman, M. R. S.; Skarnemark, G. Radiochim. Acta 2007, 95, 637. (157) Fermvik, A.; Berthon, L.; Ekberg, C.; Englund, S.; Retegan, T.; Zorz, N. Dalton Trans. 2009, 6421. (158) Fermvik, A.; Gruner, B.; Kvicalova, M.; Ekberg, C. Radiochim. Acta 2011, 99, 113. (159) Fermvik, A. J. Radioanal. Nucl. Chem. 2011, 289, 811. (160) Fermvik, A.; Aneheim, E.; Gruner, B.; Hajkova, Z.; Kvicalova, M.; Ekberg, C. Radiochim. Acta 2012, 100, 273. (161) Fermvik, A.; Ekberg, C.; Englund, S.; Foreman, M. R. S. J.; Modolo, G.; Retegan, T.; Skarnemark, G. Radiochim. Acta 2009, 97, 319. (162) Aneheim, E.; Ekberg, C.; Fermvik, A.; Foreman, M. R. S.; Gruner, B.; Hajkova, Z.; Kvicalova, M. Solvent Extr. Ion Exch. 2011, 29, 157. (163) Steppert, M.; Cisarova, I.; Fanghänel, T.; Geist, A.; LindqvistReis, P.; Panak, P.; Stepnicka, P.; Trumm, S.; Walther, C. Inorg. Chem. 2012, 51, 591. (164) Kimura, T.; Nagaishi, R.; Kato, Y.; Yoshida, Z. Radiochim. Acta 2001, 89, 125. (165) Tsaryuk, V.; Zolin, V.; Puntus, L.; Savchenko, V.; Legendziewicz, J.; Sokolnicki, J.; Szostak, R. J. Alloys Compd. 2000, 300, 184. (166) Retegan, T.; Berthon, L.; Ekberg, C.; Fermvik, A.; Skarnemark, G.; Zorz, N. Solvent Extr. Ion Exch. 2009, 27, 663. (167) Weaver, B.; Kappelmann, F. A. TALSPEAK: A New Method of Separating Americium and Curium from the Lanthanides by Extraction from an Aqueous Solution of an Aminopolyacetic Acid Complex with a Monoacidic Organophosphate or Phosphonate; USAEC report ORNL-3559; Oak Ridge National Laboratory: Oak Ridge, TN, 1964. (168) Persson, G.; Svantesson, I.; Wingefors, S.; Liljenzin, J.-O. Solvent Extr. Ion Exch. 1984, 2, 89. (169) Nilsson, M.; Nash, K. L. Solvent Extr. Ion Exch. 2007, 25, 665. (170) Hérès, X.; Baron, P.; Hill, C.; Ameil, E.; Martinez, I.; Rivalier, P. In Proc. Int. Conf. ATALANTE 2008 (Nuclear Fuel Cycles for a Sustainable Future), Montpellier, France, May 19−23, 2008. (171) Sypula, M.; Wilden, A.; Schreinemachers, C.; Malmbeck, R.; Geist, A.; Taylor, R.; Modolo, G. Solvent Extr. Ion Exch. 2012, 30, 748. (172) Traister, G. L.; Schilt, A. A. Anal. Chem. 1976, 48, 1216. (173) Ruff, C. M.; Müllich, U.; Geist, A.; Panak, P. J. ATW, Int. J. Nucl. Power 2011, 56, 489. (174) Magnusson, D.; Geist, A.; Malmbeck, R.; Modolo, G.; Wilden, A. Proc. Chem. 2012, 7, 245. (175) Wilden, A.; Modolo, G.; Sypula, M.; Geist, A.; Magnusson, D. Proc. Chem. 2012, 7, 418. (176) Bell, K.; Geist, A.; McLachlan, F.; Modolo, G.; Taylor, R.; Wilden, A. Proc. Chem. 2012, 7, 152. (177) Bell, K.; Carpentier, C.; Carrott, M.; Geist, A.; Gregson, C.; Heres, X.; Magnusson, D.; Malmbeck, R.; Modolo, G.; Müllich, U.; Sypula, M.; Taylor, R.; Wilden, A. In Nuclear Fuel Cycle Conference, Manchester, U.K., April 23−25, 2012. 1236


Complexation and Extraction of Trivalent Actinides and Lanthanides

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