Lanthanide speciation in potential SANEX and GANEX actinide

16 Pages·2013·3.94 MB·English

Lanthanide speciation in potential SANEX
and GANEX actinide/ 2 lanthanide
separations using Tetra-N-Donor
extractants
Article
Accepted Version
Whittaker, D. M., Griffiths, T. L., Helliwell, M., Swinburne, A.
N., Natrajan, L. S., Lewis, F., Harwood, L., Parry, S. A. and
Sharrad, C. A. (2013) Lanthanide speciation in potential
SANEX and GANEX actinide/ 2 lanthanide separations using
Tetra-N-Donor extractants. Inorganic Chemistry, 52 (7). pp.
3429-3444. ISSN 0020-1669 doi:
https://doi.org/10.1021/ic301599y Available at
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Lanthanide Speciation in Potential SANEX and GANEX Actinide/
1
Lanthanide Separations Using Tetra-N-Donor Extractants
2
Daniel M. Whittaker,*,† Tamara L. Griffiths,† Madeleine Helliwell,‡ Adam N. Swinburne,†
3
Louise S. Natrajan,† Frank W. Lewis,§ Laurence M. Harwood,§ Stephen A. Parry,∥
4
and Clint A. Sharrad*,†,#,⊥
5
†
6 CentreforRadiochemistryResearch,SchoolofChemistry,TheUniversityofManchester,OxfordRoad,ManchesterM139PL,U.K.
‡
7 School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
§
8 Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, U.K.
∥
9 Harwell Science and Innovation Campus, Diamond Light Source Ltd., Diamond House, Didcot, Oxfordshire OX11 0DE, U.K.
#
10 School of Chemical Engineering and Analytical Science, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
⊥
11 Research Centre for Radwaste and Decommissioning, Dalton Nuclear Institute, The University of Manchester, Oxford Road,
12 Manchester M13 9PL, U.K.
*
13 S Supporting Information
14 ABSTRACT: Lanthanide(III) complexes with N-donor ex-
15 tractants, which exhibit the potential for the separation of
16 minor actinides from lanthanides in the management of spent
17 nuclear fuel, have been directly synthesized and characterized
18 inbothsolutionandsolidstates.CrystalstructuresofthePr3+,
19 Eu3+,Tb3+,andYb3+complexesof6,6′-bis(5,5,8,8-tetramethyl-
20 5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline
21 (CyMe4-BTPhen) and the Pr3+, Eu3+, and Tb3+ complexes of
22 2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotria-
23 zin-3-yl)-2,2′-bypyridine (CyMe4-BTBP) were obtained. The
24 majority of these structures displayed coordination of two of
25 thetetra-N-donorligandstoeachLn3+ion,evenwheninsomecasesthecomplexationswereperformedwithequimolaramounts
26 of lanthanide and N-donor ligand. The structures showed that generally the lighter lanthanides had their coordination spheres
27 completed by a bidentate nitrate ion, giving a 2+ charged complex cation, whereas the structures of the heavier lanthanides
28 displayed tricationic complex species with a single water molecule completing their coordination environments. Electronic
29 absorptionspectroscopictitrationsshowedformationofthe1:2Ln3+/LN4‑donorspecies(Ln=Pr3+,Eu3+,Tb3+)inmethanolwhen
30 the N-donor ligand was in excess. When the Ln3+ ion was in excess, evidence for formation of a 1:1 Ln3+/LN4‑donor complex
31 species was observed. Luminescent lifetime studies of mixtures of Eu3+ with excess CyMe4-BTBP and CyMe4-BTPhen in
32 methanol indicated that the nitrate-coordinated species is dominant in solution. X-ray absorption spectra of Eu3+ and Tb3+
33 species, formed by extraction from an acidic aqueous phase into an organic solution consisting of excess N-donor extractant in
34 purecyclohexanoneor30%tri-n-butylphosphate(TBP)incyclohexanone,wereobtained.ThepresenceofTBPintheorganic
35 phase did not alter lanthanide speciation. Extended X-ray absorption fine structure data from these spectra were fitted using
36 chemical models established by crystallography and solution spectroscopy and showed the dominant lanthanide species in the
37 bulk organic phase was a 1:2 Ln3+/LN‑donor species.
■
38 INTRODUCTION Uranium Reduction EXtraction), which is a biphasic solvent 49
extraction process whereby {UO }2+ and Pu4+, from SNF
39 The reprocessing of irradiated spent nuclear fuel (SNF) has 2 50
40 been performed since the 1940s, with the initial motivation to dissolved in nitric acid (3−4 M), are extracted into an organic 51
41 isolate plutonium for military purposes but more recently with phase containing tri-n-butyl phosphate (TBP; Figure 1) in a 52f1
42 the purpose to separate and recover both uranium and hydrocarbon diluent (e.g., n-dodecane or odorless kero- 53
43 plutonium in order to maximize the resources available to sene).1−4 The uranium and plutonium are transferred into 54
44 generate civil nuclear energy.1,2 Reprocessing can also reduce
45 the volume of nuclear waste generated with high levels of
Special Issue: Inorganic Chemistry Related to Nuclear Energy
46 radioactivity due to the presence of long-lived radionuclides.1,2
47 This separation is most commonly performed by PUREX Received: July23, 2012
48 (Plutonium URanium EXtraction, also known as Plutonium
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Figure 1. Structures ofTBP (far left), BTP(center left), CyMe-BTBP (centerright), andCyMe-BTPhen (far right).
4 4
55 the organic phase by forming charge-neutral complexes with Uranic EXtraction) process, where the addition of octyl- 110
56 TBP(i.e.,[UO2(TBP)2(NO3)2]and[Pu(TBP)2(NO3)4]).1,3−5 (phenyl)-N,N-diisobutylcarboylmethylphosphineoxide to the 111
57 The plutonium, after reduction to Pu3+, and uranium are then organic phase in the core PUREX process allows Am3+ and 112
58 back-extracted into an aqueous phase for reuse. The aqueous Cm3+ to be extracted alongside {UO2}2+ and Pu4+, leaving the 113
59 phase remaining after the initial separation, known as highly lanthanide ions and other fission products in the aqueous 114
60 active raffinate (HAR), contains over 99.9% of the fission phase.14,15 115
61 products (e.g., lanthanide isotopes, 137Cs, 90Sr, 99Tc) and the The SANEX (Selective ActiNide EXtraction) solvent 116
62 minoractinideactivationproducts(neptunium,americium,and extraction process8,9 aims to separate the minor actinides 117
63 curium)withdecontaminationfactorsof106−108achievedbya americium and curium from the lanthanide fission products 118
64 multistage separation process.1 The long-term management of remaining after plutonium and uranium removal by PUREX 119
65 HAR, after conversion into an appropriate wasteform, can be and fission product separation (except the lanthanides) by 120
66 extremely problematic, in part due to the presence of DIAMEX (DIAMide EXtraction)16 using only carbon-, hydro- 121
67 americium and curium, which are highly radioactive and have gen-, oxygen-, and nitrogen-containing compounds as extrac- 122
68 very long half-lives (up to 105 years).1,2 tants, diluents, or phase modifiers. A class of molecules that 123
69 Considerable efforts have been made recently to develop showed early promise for the selective extraction of An3+ over 124
70 advanced separation methodologies in order to maximize fuel Ln3+ in a SANEX process were the tridentate 2,6-bis(5,6- 125
71 resources and reduce the impact of nuclear waste while dialkyl-1,2,4-triazin-3-yl)pyridines (BTPs; Figure 1).7,17 How- 126
72 providing a proliferation-resistant fuel cycle (i.e., no pure ever,manyoftheseextractantmoleculessufferedproblemsthat 127
73 plutonium is isolated).1,2,4,6−11 This forms part of the precluded them from use in plant-scale extractions including 128
74 “PartitioningandTransmutation”strategy,whereitisproposed poor stability, slow extraction kinetics, and inefficient back- 129
75 that all of the actinides in SNF, including the minor actinides, extractionduetohighAnIIIaffinities.7Furtherdevelopmentsin 130
76 canbeseparatedandrecycledasnuclearfuel.Anotheroptionis the use of triazinyl-based N-donor extractants for actinide/ 131
77 to “burn” the separated actinides, which will also result in lanthanide separations have led to the tetradentate ligand 2,9- 132
78 conversion to short-lived fission product nuclides but without bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3- 133
79 nuclear energy production for public consumption. This yl)-2,2′-bipyridine (CyMe4-BTBP; Figure 1), which exhibits 134
80 provides the added benefit of converting most of the long- significant potential for use in SANEX separations, with 135
81 lived actinides in SNF to shorter-lived fission product nuclides separation factors for Am3+ over Eu3+ found to be 136
82 compared to current spent fuel management options. As a ∼150.7,18,19TheCyMe4-BTBPextractant hasbeensuccessfully 137
83 result, the “Partitioning and Transmutation” strategy can tested for the extraction of genuine actinide/lanthanide feed 138
84 significantly reduce the time it takes for SNF to decay to through a 16-stage centrifugal contactor setup with excellent 139
85 radioactivity levels of natural uranium and therefore the recoveries for americium and curium (>99.9%) but has been 140
86 necessary design lifetime of any nuclear waste repository.7−11 shown to undergo radiolytic degradation at doses that will be 141
87 One of the major separation challenges that need to be encounteredatthehighminoractinideloadingsobtainedinthe 142
88 overcome for this strategy to be successful is the separation of reprocessing of, for example, fast reactor fuels.19 The kinetics 143
89 americium and curium from the lanthanide fission products. for actinide extraction with CyMe4-BTBP are still relatively 144
90 This is because the high neutron absorption cross sections of slow, so the addition of a phase-transfer catalyst is necessary 145
91 some of the lanthanide ions present in SNF both decrease the [e.g., N,N′-dimethyl-N,N′-dioctylethylethoxymalonamide 146
92 flux in a reactor and create more activation products, thereby (DMDOHEMA)] if this extractant is to be used for large- 147
93 makingtransmutationalessattractiveoptionifthelanthanides scale partitioning.19 In an attempt to improve the kinetics of 148
94 cannot be separated from the actinides.10 Achieving this extraction with these tetradentate N-donor extractants, greater 149
95 separation is extremely difficult because of the chemical conformational rigidity was enforced in the ligand backbone 150
96 similarities between americium, curium, and the lanthanides, withthesynthesisof2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahy- 151
97 which all most commonly exist in the III+ oxidation state in dro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline (CyMe4- 152
98 solution.12Consequently,organicmoleculesthatcanselectively BTPhen; Figure 1).20 This rigid ligand displays very high 153
99 extract actinides, in particular Am3+ and Cm3+, over the Ln3+ separation factors for Am3+ over Eu3+ (up to 400) with 154
100 ions are of great interest, as is evident by the number of significantly faster kinetics of extraction compared to those 155
101 different ligand systems and processes that have been found for CyMe4-BTBP, thereby eliminating the need for a 156
102 developed by various groups in the field of partition- phase-transfer catalyst.20 These high separation factors even at 157
103 ing.2,4,6−10,12−22 Examples include the TALSPEAK (Trivalent low acidities for the aqueous phase may prove problematic 158
104 Actinide Lanthanide Separation by Phosphorus reagent duringback-extractions,7butthe useofalternative diluents has 159
105 Extraction from Aqueous Komplexes) process, which uses shown that efficient back-extractions may be achievable when 160
106 diethylenetriaminepentaacetic acid in a lactic acid solution to using the CyMe4-BTPhen extractant.20 161
107 hold back Am3+ and Cm3+ in the aqueous phase while the An alternative concept being considered in Europe for the 162
108 lanthanideionsareextracted intotheorganic phase containing recovery of actinides from SNF is the GANEX (Group 163
109 di(2-ethylhexyl)phosphoric acid,13,14 and the TRUEX (TRans- ActiNide EXtraction) process, which is proposed to consist 164
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165 oftwocycles.16,21,22Mostoftheuraniumisremovedinthefirst Table 1. List of Synthesized Complexes
166 cycle, while the second cycle recovers all of the remaining
compound
167 actinides, mainly the transuranics neptunium through curium, formula number
116689 cfiossnicounrrpernotdlyucitns fovaurnydinignospxiednattifounel,sitnactelusdi(nIgII−thVeI)lanftrhoamnidtehse. [Pr(CyMe4-BTPhen)2(NO3)](NO3)2·10H2O 1
[Pr(CyMe-BTPhen)(NO)] 2
170 The GANEX process is aimed for generation IV nuclear fuel [Pr(NO4)]·1.63EtO2H·0.735HO
3 5 2
171 cycles, where plutonium is likely to exist in higher [Eu(CyMe-BTPhen)(HO)](NO)·9HO 3
4 2 2 3 3 2
172 concentrations during partitioning processes compared to [Tb(CyMe-BTPhen)(HO)](NO)·9HO 4
173 those found in the processing of SNF in current cycles.21 [Yb(CyMe4-BTPhen)2(H2O)](NO3)3·9H2O 5
4 2 2 3 3 2
174 ThemajornoveltywithGANEXcomparedtomostothermore [Pr(CyMe-BTBP)(NO)](NO)·4EtOH·HO 6
4 2 3 3 2 2
175 technologically mature separation processes is that the [Pr(CyMe-BTBP)(NO)] [Pr(NO)](NO)·6CHCN 7
4 2 3 2 3 6 3 3
176 plutonium is routed with the minor actinides rather than with [Eu(CyMe-BTBP)(NO)](NO)·4EtOH·2HO 8
177 themajorityoftheuranium.TheseparationofAm3+andCm3+ [Eu(CyMe4-BTBP)(2NO3)]·tolue3n2e 2 9
4 3 3
178 from the lanthanide ions in a SANEX process is already [Tb(CyMe-BTBP)(HO)](NO)·4EtOH 10
4 2 2 3 3
179 considered extremely challenging, so performing the same
180 separation in addition to partitioning neptunium, plutonium, miscibility of the solvent mixture. In all examples, yellow 227
181 and any remaining uranium from all of the fission products in crystals were obtained. Elemental analysis, single-crystal XRD 228
182 the secondstage ofthe GANEXprocess is evenmore difficult. (see the Solid-State Structure section), and electrospray 229
183 A single extractant in the organic phase is unlikely to achieve ionization mass spectrometry (ESI-MS, positive ion) indicated 230
184 the group separation of multiple actinides in variable oxidation that, in the majority ofcases, complex cations ofstoichiometry 231
185 stateswithappropriateefficiencies.Consequently,theperform- 1:2 Ln3+/CyMe4-BTPhen with nitrate counterions were 232
186 ance of multiple extractants in the organic phase, typically obtained even though the syntheses were conducted with 233
118878 aelxrpelaodryedesftoarbliussheedinfroamGoAtNheErXseppraorcaetisosn.16p,2r1o,2c2esAsesn,uhmasbebreeonf eoqnulyimeoxlcaerptaimonouwntass ofofuLnnd(NduOri3n)g3 atnhde sCyynMthee4s-iBsToPfhtehne. TPrh3e+ 223345
118990 dpioffteernentitalextirnaccltuandtincgomNbi,nNa,tNio′n,Ns ′h-atveetraboecetnylsdhiogwlyncotloamhiadvee coofmaple1x:2ofPCry/MCey4M-BeT4-PBhTePnh,ewnherceomthpelemxajocratpiorondubcuttcownsitihsteda 223367
119912 (wTitOhDTGBAP,;aunsdedCiynMDe4I-ABMTBEPX)wwithithTBDPM.1D6,2O1,2H2EMA, TODGA [cPryrs(tNalOliz3a)t5i]o2n−ocfotuhnistemrioixntuprerelseednttopeisroclaattiioonniocfuansitm.aTllhaeminoiutinatl 223389
193 TheN-donorextractantsCyMe4-BTPhenandCyMe4-BTBP ofthiscationicspecieswithonlynitratepresentascounterions, 240
194 have already demonstrated potential as extractants for as determined by XRD (see the Solid-State Structure section). 241
119956 pmairntoitrionaicntginidSeNsFfrmomixtutrhees,lainnthpaanritdiceus.l7a,r18−th20e Hseopwareavteior,nthoef Tsphheersetruisctucroaml dpeletteerdmibnyatiaonssinsghloewntihtraattetheanLionn3+fcoorortdhienaPtiro3n+ 224423
197 mode of action of these ligands with these metal ions in complexes (1 and 2), while for the Eu3+, Tb3+, and Yb3+ 244
198 extraction conditions has not been definitively established. complexes (3−5), a single molecule of water completes the 245
199 Here, we have produced numerous Ln3+ complexes across the coordination sphere (see the Solid-State Structure section). 246
200 lanthanide series with both CyMe4-BTPhen and CyMe4-BTBP However, ESI-MS spectrometry of all the studied Ln3+ 247
201 ligands using a direct synthetic approach. These complexes complexes with CyMe4-BTPhen from a methanol (MeOH) 248
202 have been fully characterized in both solution and solid states solution indicates that a nitrate ion is coordinated, and there 249
203 using multiple techniques including electronic absorption wasnoevidencetosuggestthatawatermoleculewaspresentin 250
204 spectroscopy, luminescence spectroscopy, and single-crystal the coordination sphere. 251
205 X-ray diffraction (XRD). We have then used X-ray absorption The synthesis of Ln3+ complexes (Ln = Pr, Eu, Tb) of 252
206 spectroscopy (XAS) to probe the lanthanide (europium and CyMe4-BTBP (see Table 1 for the list) was also attempted by 253
207 terbium) species, which have been extracted into the organic addingaDCMsolutionoftheligandto0.5equivofLn(NO3)3 254
208 phase using conditions similar to those proposed for SANEX in MeOH. The powder obtained upon evaporation of the 255
209 andGANEXseparationprocessesthatuseCyMe4-BTPhenand reactionmixturewasbestcrystallizedbyslowevaporationfrom 256
210 CyMe4-BTBP. The extended X-ray absorption fine structure a 1:1:1:1 by volume mixture of toluene, isopropyl alcohol, 257
211 (EXAFS) of the Ln LIII-edge XAS spectra obtained from each ethanol, and DCM. The alcohols dissolve the complexes 258
212 ofthesesystemshasbeenfittedtostructuralmodelsestablished reasonablywell,whiletheuseoftolueneandDCMreducesthe 259
213 by characterization of the directly synthesized Ln3+ complexes solubility of the complexes, assists in controlling the rate of 260
214 with these N-donor extractants, thus providing definitive evaporation, and provides reasonable miscibility in these 261
215 evidence for Ln3+ speciation in the bulk organic phase during solvent mixtures. Characterization of the bulk crystallized 262
216 e■xtraction processes. materialobtainedfromalloftheattemptedLn3+complexations 263
RESULTS AND DISCUSSION of CyMe4-BTBP indicated that a mixture of products was 264
217 present, which is likely to be due to the formation of products 265
218 Synthesis. LnIII complexes of the extractant CyMe4- with different combinations of Ln3+/CyMe4-BTBP ratios and 266
t1 219 BTPhen (see Table 1 for the list) were readily synthesized by anionic molecular ions (i.e., NO3−, [Ln(NO3)6]3−, [Ln- 267
220 theadditionofLn(NO3)3(Ln=Pr,Eu,Tb,Yb)inacetonitrile (NO3)5]2−). However, the selection of individual crystals 268
221 to1molequivofCyMe4-BTPhenindichloromethane(DCM). obtained from these reactions was able to afford the structural 269
222 The reaction solution was allowed to evaporate to dryness, determination of a number of products by XRD. The vast 270
223 leaving a powder that could be crystallized from a mixture of majority of these structures indicated complex cations of 1:2 271
224 CH3CN,DCM,andethanolinavolumeratioof∼2:2:1,where Ln3+/CyMe4-BTBP stoichiometry (6−8 and 10) with nitrates 272
225 CH3CNreadilydissolvesthecomplex,DCMactstoreducethe (6−8 and 10) and metallonitrates (7) present as counterions. 273
226 solubilityofthecomplexinsolution,andethanolimprovesthe ThefirststructuresofLn-BTBPcomplexes tobeisolatedwere 274
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275 withtheligand6,6′-bis(5,6-diethyl-1,2,4-triazin-3-yl)-2,2′-bipyr-
276 idine (C2-BTBP), and these had a single C2-BTBP molecule
277 coordinated to the Ln3+ ion.23 It was noted that in solution
278 both 1:1 and 1:2 Ln3+/C2-BTBP complexes were observed.23
279 Morerecently,crystalsof[Eu(CyMe4-BTBP)2(NO3)]2+witha
280 metallonitrate counterion and the charge-neutral species
281 [Eu(CyMe4-BTBP)(NO3)3] were isolated by slow evaporation
282 from a mixture of DCM and CH3CN.24 Our attempts to form
283 the Eu3+ complex of CyMe4-BTBP produced a 1:1 Eu3+/
284 CyMe4-BTBP molecular species with a toluene molecule
285 present as a solvent of crystallization (9) in addition to the
286 1:2 Eu3+/CyMe4-BTBP complex cation containing species but
287 with only nitrate counterions present in the lattice. The Pr3+
288 and Eu3+ complexes isolated in the solid state (6−9) have one
289 ormorenitrateionscompletingthecoordinationsphere,while
290 onlytheTb3+complexofCyMe4-BTBPhasawatermoleculein Figure 2. UV−visible absorption spectroscopic titration of CyMe-
291 itscoordinationenvironment.TheESI-MSspectraofallofthe 4
BTPhen with Pr(NO) in MeOH (initial conditions, [CyMe-
292 CyMe4-BTBP complexes obtained from MeOH indicated that BTPhen] = 2.0 × 10−353M, volume = 2.0 mL; titrant condition4s,
293 the only intact molecular species present was [Ln(CyMe4- [Pr(NO)]= 4.0 × 10−4 M).
294 BTBP)2(NO3)]2+. The ESI-MS spectra of the CyMe4-BTPhen 3 3
295 complexes provide comparable results and are in agreement is present in solution are most likely explained by an 337
296 with similar ESI-MS studies previously performed onextracted equilibrium being established between 1:1 and 1:2 Ln3+/ 338
229978 sthoelut1io:2n:s1oLfnE3+u/3C+ywMiteh4-BBTTBBPP/eNxtOra3c−tacnotsm.2p5leTxhiiss sduogmgeinstasntthiant CfoyrmMew4-itBhTiPnchreenassinpgecaiedsd,itwiohnesreofmLonr(eN1O:13)3c.oSmimplielaxribsehlikaveiloyrtios 333490
299 solution, while other compositions were only present in observed for the titrations of CyMe4-BTBP with Ln(NO3)3 341
300 solution in minor quantities, if at all. (see Figure 3 for Eu3+ and Supporting Information). 342f3
301 Solution Spectroscopy. The UV−visible absorption
302 spectra of complexes 2−4, isolated in a pure bulk form,
303 dissolved in MeOH are dominated by charge-transfer
304 transitionsintheUVregionofthespectra(seetheSupporting
305 Information). These transitions are most likely due to π−π*
306 transitions from the aromatic nature of the CyMe4-BTPhen
307 ligand. A clear difference in the spectral profile is observed
308 between the free CyMe4-BTPhen ligand and Ln3+ complexes,
309 indicating that the electronic structure of the CyMe4-BTPhen
310 molecule is perturbed upon LnIII coordination. Essentially no
311 differenceisobservedbetweenthespectroscopicprofilesfor2−
312 4, indicating that there is little or no influence by the type of
313 coordinating lanthanide ion on the electronic structure of the
314 CyMe4-BTPhen ligand. The limited solubility of these
315 complexes in most common solvents precluded the study of
316 thetypicallyweaklyabsorbingf−ftransitionsofthelanthanides
317 in 1-cm-path-length cells. Figure 3. UV−visible absorption spectroscopic titration of CyMe4-
331189 lanTthitarnatidioenisonosf,PCry3+M,Ee4u-3B+,TaPnhdenTba3n+dinCMyeMOeH4-BwTeBrePpewrfitohrmtehde B×2.T01B×0P−14w0Mi−t5h).MEu,(vNolOum3)e3i=n2M.0emOLH;(tiitnriatniatlccoonnddiittiioonnss,,[[ECuy(MNeO4-3B)T3]B=P]4.=0
320 to study the lanthanide speciation behavior of these extractant
321 molecules, in particular the equilibrium between 1:1 and 1:2
322 Ln3+/LN4‑donor species. The titrations of CyMe4-BTPhen with Ainbtesonrspittyiownithmathxieminaitaiatl2a2d8ditainondo2f8L9nn(NmOs3h)a3rupplytode0c.r5eaesqeuiivn. 334434
323 eachofthelanthanidesstudiedshowthatthereisessentiallyno Twoabsorptionmaximaareseentoemergeat334and346nm 345
324 differenceinthetitrationprofileswithdifferentlanthanideions with the initial addition of Ln(NO3)3. Further additions of 346
f2 325 (see Figure 2 for Pr3+ and Supporting Information). Sharp Ln(NO3)3 also result in a subtle decrease in the absorption 347
326 decreasesintheintensityoftheabsorptionmaximaforfreeCy- intensity for most of the spectrum. Therefore, it can be 348
327 Me4-BTPhenat261and295nmwiththeadditionofupto0.5 deduced that the 1:2 Ln3+/CyMe4-BTPhen−CyMe4-BTBP 349
328 equiv of Ln(NO3)3 are observed. The absorption maximum at complex is probably most favored to form, but the 1:1 species 350
329 261nmalsoshiftsto∼266nmwiththeadditionofLn(NO3)3. canbeforcedtoforminsolutionwithexcessLn3+ionpresent. 351
330 Isosbestic points are observed at 229 and 279 nm. Further Similar results have been previously observed for Ln3+ 352
331 additionsofLn(NO3)3,upto3equiv,resultinasubtledecrease complexation behavior with analogous BTBP ligands.26 353
332 in the absorption intensity for most of the spectrum but with The overall stability constants for both 1:1 and 1:2 Ln3+/ 354
333 no changes in the shape of the spectral profile. This indicates CyMe4-BTPhen−CyMe4-BTBP species were determined by 355
334 that the 1:2 Ln3+/CyMe4-BTPhen complex forms with the fittingtheappropriatespectrophotometrictitrationdata(Table 356t2
335 initial addition of Ln(NO3)3, as expected.23,26 The subtle 2).Thesefitsconfirmthattheformation ofbothMLandML2 357t2
336 changesinthespectrawhenmorethan0.5equivofLn(NO3)3 (where L is the N-donor ligand) species does occur over the 358
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Table 2. Fitted Metal−Ligand Overall Stability Constants and water in the coordination sphere of these lanthanide 401
Determined from UV−Visible Spectroscopic Data Using species, as has been performed previously to investigate the 402
Hyperquad28 (I = 0 M in MeOH; T = 25 °C) coordination behavior of other extractant molecules.30 403
ExcitationandemissionspectraoftheEu3+andTb3+complexes 404
overallstabilityconstant(logβ )
ML withCyMe4-BTPhenandCyMe4-BTBParedisplayedinFigure 405f4
N-donor standard standard 4 and in the Supporting Information. Excitation into the 406f4
ligand Ln3+ logβ deviationa logβ deviationa σb
11 12
CyMe- Pr3+ 4.7 0.5 11.8 0.1 0.0032
4
BTPhen
Eu3+ 7.9 0.5 15.6 1.0 0.0028
Tb3+ 8.1 0.5 13.2 0.5 0.011
CyMe- La3+ 4.4c 0.2c 8.8c 0.1c
4
BTBP
Pr3+ 10.9 0.7 18.9 1.1 0.0042
Eu3+ 9.5 0.6 16.9 1.1 0.0067
6.5c 0.2c 11.9c 0.5c
Tb3+ 8.8 0.2 15.9 0.4 0.0037
Yb3+ 5.9c 0.1c
aStandarddeviationsdeterminedbythefittingprocess.bGoodness-of-
fit parameter. cReference 27 (I = 0.01 M EtNNO; T = 25 °C; in
4 3
MeOH; determined by UV−visible absorptionspectroscopy).
359 conditions used in these titrations, as has been observed
360 previously in similar titrations of CyMe4-BTBP with La3+ and
361 Eu3+.27 The speciation plots corresponding to the titrations Figure 4. Emission (following excitation at 320 nm), excitation
362 withCyMe4-BTBP(seetheSupportingInformation)showthe (monitoring emission at 616 nm), and absorption spectra of
363 initial emergence of the ML2 species when less than 0.5 mol [Eu(CyMe4-BTPhen)2(X)]n+ in MeOH (X = H2O/NO3−; n = 3
364 equiv of lanthanide is present (relative to L), with further and 2).
365 additionsoflanthanideshowingtheincreasingformationofthe
366 MLspecies.Themagnitudeofthelanthanidestabilityconstants
336678 ftohretmheidC-laynMthea4n-BidTeBsPwistphelcoiewseirnsdtiacbaitleitsytchoengsrtaenattessotbatffiainnietdy ffoorr iTnbtr3a+ligcaonmdpalebxseosrpptiroondubcaenddsch(a2r8a0c−te3ri3s0ticnmf-c)enotfetrhede Eeum3+issaionnd 440078
369 the lanthanides at either end of the series, which is in spectra with resolvable bands due to the 5D0 to 7FJ and 5D4 to 409
370 agreement with previous work and the corresponding 7FJ (J = 0−6) transitions, respectively. The emission spectrum 410
337712 danisdtrDibMutiDonOHraEtMiosAfionrtoLnn-o3+ctaenxotrla.1c2t,2io7nTsheussipnegciaCtiyoMnep4l-oBtTsBfoPr oalflotwheedEΔu3J+=co2mtpralenxseitsioanre, wdhoimchiniastehdypbeyrsethnesiteivleecttroic-tdhieposiltee- 441112
373 the CyMe4-BTPhen titrations (see the Supporting Informa- symmetry. The absence of a hyperfine structure in this band 413
337745 tBioTnB)Pi.nFdoicratPer3b+e,htahveio1r:2diLffner/eCnytMfroe4m-BtThaPthoebnsseprveecdiesfoirsCfayvMoree4d- itnhdeiceaxtpesertihmaetnthtaelctoimmeplesxceaslee.3x1istTahseaesmingislseioenmipssriovfielesspefcoierstohne 441145
376 to form, compared to the 1:1 M/L species even at relatively Eu3+complexesaresimilartothoseobservedwithotherBTBP 416
377 high metal concentrations due to a highly positive cooperative ligands, but in our examples, the splitting of the 5D0 to 7F2 417
378 effect for the formation of the ML2 species. However, this transition at ∼617 nm upon complexation with the N-donor 418
379 strongcooperative effectdiminishessubstantially withprogress ligands is not resolved, which has been observed previously in 419
380 along the lanthanide series where the ML species is some examples.24,32 The respective excitation spectra recorded 420
381 predominantly favored for Tb3+ even at reasonably low metal at the emission maxima (545 nm for Tb3+ and 616 nm for 421
382 concentrations. The stability constants for 1:1 Ln/CyMe4- Eu3+) display ligand-centered absorption bands that overlap 422
383 BTPhen increases as the lanthanide series is traversed. The well with the absorption spectra, indicating that sensitized 423
384 differences observed between the lanthanide stability behaviors emission is occurring in all of the systems under study. 424
385 for complexes of CyMe4-BTBP and CyMe4-BTPhen are most In order to assess the inner coordination sphere of the 425
386 likely due to the lack of flexibility in the BTPhen backbone, complexes, lifetime data were recorded in MeOH and MeOH- 426
387 resulting in the greater likelihood of a mismatch between the d4 following 320 nm excitation (e.g., see Figure 5) and the 427f5
338889 laasntthheanliadnethioanniidcerasdeiruiessainsdtrtahveerCseydM. e4-BTPhen binding cavity ninugmtboeHr oofrrcoocokr’sdienqauteadtioMne(OeqH1m)3o3lecules determined accord- 442289
390 Theabsorptionspectroscopicprofilesshowedlittledifference ⎡⎛ ⎞ ⎛ ⎞⎤
391 betweenthelightandheavylanthanides,butXRDstudies(see q = A⎢⎜ 1 ⎟ − ⎜⎜ 1 ⎟⎟⎥
392 the Solid-State Structure section) indicate that the heavy bound MeOH ⎣⎢⎝τ ⎠ ⎝τ ⎠⎦⎥
393 lanthanides in the 1:2 Ln3+/CyMe4-BTPhen−CyMe4-BTBP MeOH CD3OD (1) 430
394 complexes prefer to have their coordination sphere completed whereAisaproportionalityconstant;A=2.1msforEu3+and 431
395 by water, whereas the lighter lanthanide complexes generally A = 8.4 ms for Tb3+. 432
396 prefer to have nitrate in their coordination environment, a For solutions of Eu3+ and CyMe4-BTPhen in a 2:1 molar 433
397 consequence of the lanthanide contraction. This is commonly ratio, this gave a q value of 0.3; an identical q value was 434
398 observed in a series of lanthanide complexes of a given obtained for the analogous complex with CyMe4-BTBP of 0.3 435
399 multidentate ligand.29 Luminescence studies were therefore (Table 3). This strongly suggests that the first coordination 436t3
400 undertaken in an attempt to assess the involvement of nitrate sphere of the complexes is completed by ligation of nitrate 437
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Solid-State Structure. Single-crystal XRD studies of 466
complexes of Tb3+, Eu3+, and Pr3+ with ligands CyMe4-BTBP 467
and CyMe4-BTPhen were obtained (1−4 and 6−10, 468
respectively). The complex of Yb3+ with CyMe4-BTPhen was 469
also studied (5). Complexes 3−5 are isostructural crystallizing 470
intheorthorhombicspacegroupFdd2.Plotsofthesestructures 471
are displayed in Figures 6−11 (complexes 1, 3, 6, and 8−10) 472f6(cid:1)f11
Figure 5. Time-resolved emission spectrum of [Eu(CyMe -
4
BTPhen)(X)]n+ in MeOH following excitation at 320 nm (X =
2
HO/NO−; n= 3 and2).
2 3
Table3.Photophysical PropertiesofSolutionsofLn(NO )
3a3
withTetra-N-DonorLigandsina1:2MolarRatioat298K
complex λ (nm) τ (ms) τ (ms) q
em MeOH MeOD MeOH
[Eu(BTBP)(X)]n+ 617 1.94 2.61 0.3
2
[Eu(BTPhen)(X)]n+ 617 1.49 1.87 0.3
2 Figure 6. ORTEP plot of the complex cation of 1, with
aAlllifetimeswererecordedbyTCSPCat320nmexcitationusinga5 crystallographic numbering (H atoms omitted). Probability ellipsoids
W xenon flashlamp and are subject to a ±10% error. Identical data of 50% displayed.
withinerrorwereobtainedfor1:3and1:5solutionsofEu3+/LN4‑donor
and the crystalline complexes 3 and 8. andtheSupportingInformation(complexes2,4,5,and7)with 473
crystal data given in Tables 4 and 5. In the vast majority of 474t4t5
cases (1−8 and 10), two of the N-donor ligands (either 475
438 anions rather than exchangeable solvent molecules, and there CyMe4-BTBPorCyMe4-BTPhen)werefoundtocoordinateto 476
443490 mocacyupbyeinagmtihniosrscpoeocrideisntahtiaotnexsisittes,wfoirthtehietsheerEwua3t+ercoormMpleeOxeHs. tahneotmheertalligcaenndte(rwoactceurpoyrinngitfroauter)coococrudpinyaintigonascitaevsityeabchet,wweietnh 447778
441 Becausetheemissivequantumyieldofasolvatedspecieswould
442 bemuchlower,thecontributiontotheinitialemissionintensity
443 willbelow,perhapsprecludingobservationofasecondspecies
444 in solution, and/or the rate of solvent and nitrate anion
445 exchange ismuch faster than the luminescence time scale,so a
446 nonintegervalueofqisdetermined.Similardatawereobtained
447 for1:3and1:5molarratiosofEu3+withbothN4-donorligands
448 and the isolated complexes 3 and 8, suggesting that the 1:2
449 Ln3+/LN4‑donor complex is the only emissive species formed
450 under these conditions.
451 InthecaseoftheTb3+complexesofbothligands,excitation
452 intotheligandabsorptionbandsresultedincomparativelyweak
453 emission spectra. This is unsurprising given the estimated
454 tripletenergiesoftheligandsandthehigh-energyemissive5D4
455 excited state and suggests that back-energy transfer from the
456 Tb3+ excited-state manifold to the ligand triplet state is a
457 competitive nonradiative decay process.34 This is corroborated
458 bythefactthattheradiativelifetimesfortheTb3+emissionare
459 extremely short; the kinetic traces could be satisfactorily fitted
460 with two exponential functions, giving lifetime values of
461 approximately 18 and 6 μs (for solutions of BTBP in
462 MeOH). Moreover, the kinetic traces recorded without a
463 time gate and delay additionally exhibit a short-lived Figure 7. ORTEP plot of the complex cation of 3, with
464 component of nanosecond order, which we attribute to crystallographic numbering (H atoms omitted). Probability ellipsoids
465 ligand-centered emission. of 50% displayed.
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Tb3+, and Yb3+ such that only water can access this binding 495
cavityinthesesolid-statesystems.However,previousworkhas 496
shown that the 1:2 complex of Eu3+/CyMe4-BTPhen can be 497
obtainedwithanitrateioncompletingthecoordinationsphere 498
in the solid state where MeOH was used as the reaction 499
solvent,20 thus indicating that the position of the equilibrium 500
between bound nitrate and bound water in these Ln3+ 501
complexes may be influenced by the choice of solvent. The 502
nitrate-coordinated complexes form 2+ charged complex 503
cations,whilethewater-coordinatedcomplexesformtricationic 504
complex cations, where charge balance is achieved with 505
nonbinding nitrate anions in the crystal lattice (1 and 3−5) 506
or with an anionic metallonitrato species (2). The previously 507
obtained [Eu(CyMe4-BTPhen)2(NO3)]2+ solid-state complex 508
was alsocharge-balanced with apentanitratolanthanide anionic 509
species.20 510
All of the M−N bond lengths in the CyMe4-BTPhen- 511
containing structures decrease as the lanthanide series is 512
traversed from left to right (Table 6), as expected due to the 513t6
Figure 8. ORTEP plot of the complex cation of 6, with lanthanide contraction. In all cases, the lanthanide ion sits 514
crystallographic numbering (H atoms omitted). Probability ellipsoids outside of the plane of the N-donor ligand cavity. The out-of- 515
of 50%displayed. plane displacement of the Ln3+ ion from the average plane 516
defined by the four coordinating N atoms for each N-donor 517
ligand follows a trend similar to that of the bond lengths by 518
decreasing across the lanthanide series: ∼0.80/0.71, 0.77/0.62, 519
0.56, 0.55, and 0.51 Å for species 1−5, respectively. The 520
average M−Ntriazinyl bonds lengths are consistently longer than 521
those for the M−NPhen bonds in the Eu3+, Tb3+, and Yb3+ 522
complexes (3−5). This may imply that a greater degree of 523
interactionexistsbetweentheLn3+ionandthephenanthroline 524
N-donor atoms than that with the triazinyl N-donor atoms. 525
However,thesamecannotbesaidforthestructuresofthePr3+ 526
complexes obtained (1 and 2), where in some instances the 527
M−NPhen bond lengths are, in fact, longer than the M−Ntrazinyl 528
bond distances. The previously obtained structure of [Eu- 529
(CyMe4-BTPhen)2(NO3)]2+ shows little difference between 530
theEu−NtriazinylandEu−NPhenbonddistances.20Therefore,itis 531
mostlikelythetriazinylgroupsthatarerestrainedtobefurther 532
away from the Ln3+ center relative to the phenanthroline 533
backboneastheLncenterapproachestheplaneoftheCyMe4- 534
BTPhen binding cavity, as this is only evident for the latter 535
lanthanides.TheLn−Owaterbonddistancesalsodecreaseasthe 536
Figure 9. ORTEP plot of the complex cation of 8, with lanthanide series is traversed from left to right because of 537
crystallographic numbering (H atoms omitted). Probability ellipsoids lanthanide contraction (Table 6). The Pr−Onitrat́e bond 538
of 50%displayed. distances for 1 and 2́ [2.592(7) and 2.544(7) Å for 1; 539
2.581(5) and 2.605(5) Å for 2] are t́ypical for Pr3+ complexes 540
479 the two bound N-donor ligands, giving a distorted capped with coordinated nitrates (2.5−2.8 Å).23,35,36 541
480 square-antiprismaticgeometryabouttheLn3+center.Thisleads Where CyMe4-BTBP is the ligand, both 1:1 (9) and 1:2 542
481 to a total coordination number of 9 for water-coordinated Ln3+/CyMe4-BTBP(6−8and10)coordinationstructureswere 543
482 complexes (3−5 and 10) and 10 for the bidentate nitrate- isolated.StructuresofmetalcomplexeswithCyMe4-BTBPhave 544
483 coordinated complexes (1, 2, and 6−8). only been previously obtained for Eu3+, U4+, and {UO2}2+.36,37 545
448845 LNF4‑odornotrhecoLonr3d+incoatmiopnlexsteosicwhiitohmCeytMriees4-hBaTvPehbeene,nonilsyol1a:t2edLna3n+d/ PBwrToerBvkiP,o,uissuoslsainttuegddiaesstpruroecfptuathrreaetsioccnoomnsspiimsletiixlnaagrtiotonof tothhfeatEsuad3me+secwri1ibt:eh2daCninydMt1he:4i1s- 555444678
448867 sfotruuncdtutroalloyccchuapryacttheerizreedmaininitnhge csooloidrdisntaatteio.nThsietensitinratteheioPnr3i+s Ereus3p+e/cCtiyvMelye)4.-BHToBwPevecro,mthpelseexesstrufcotuunredse(xshtriubcittudrieffsere8ntacnrdyst9al, 554590
488 complexesisolated,whileasinglewatermoleculecompletesthe forms due to either different counterions or alternate solvent 551
489 coordination sphere for the CyMe4-BTPhen complexes of the molecules of crystallization present in the lattices.24 Further 552
490 heavier Ln3+ ions investigated in this study (Eu3+, Tb3+, and structural information has been obtained for Ln3+ complexes 553
491 Yb3+ in 3−5). This is likely to be due to a combined effect of with C2-BTBP, where only 1:1 Ln/C2-BTBP complexes were 554
492 the lanthanide contraction and the structural rigidity of the isolated, essentially for the entire lanthanide series.23 The 555
493 CyMe4-BTPhen ligand sterically hindering the remaining remaining coordination sites were occupied by three nitrate 556
494 coordination sites in the more contracted structures of Eu3+, anions to give charge-neutral species.23 The structure of the 557
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Figure10.ORTEPplotofthecomplexmoleculeof9,withcrystallographicnumbering(Hatomsomitted).Probabilityellipsoidsof50%displayed.
C2-BTBP-containing structures.23 The Ln−Onitrate bond 579
lengths also clearly decrease across the series, demonstrating 580
lanthanide contraction again. The 1:2 Ln3+/CyMe4-BTBP 581
complexes bear further similarity to those of CyMe4-BTPhen 582
with the Ln3+ ion located outside of the average plane of the 583
tetra-N-donor cavity and this displacement following the same 584
trend as that of the bond lengths, decreasing across the series: 585
∼0.73/0.78, 0.72/0.76, 0.69, and 0.56 Å for 6−8 and 10, 586
respectively.However,the1:1Eu3+/CyMe4-BTBPcomplex(9) 587
does effectively sit in the plane average plane of the ́four N- 588
donor atoms (out-of-plane displacement ∼0 Å). The 589
coordination bond lengths and motifs observed in the 590
structures of the 1:1 and 1:2 Eu3+/CyMe4-BTBP complexes 591
(8 and 9) are similar to those observed for the structures 592
obtained previously for the same complex molecules but in 593
different crystal forms.24 There is little difference observed in 594
the Ln−N bond lengths between the 1:1 and 1:2 Ln3+/BTBP 595
Figure 11. ORTEP plot of the complex cation of 10, with complex molecular species obtained here and elsewhere,23 596
crystallographic numbering (H atoms omitted). Probability ellipsoids
suggesting that if there are indeed any cooperative or 597
of 50%displayed. destructive effects for 1:2 Ln3+/Cy-Me4-BTBP binding over 598
the 1:1 Ln3+/Cy-Me4-BTBP complex, they do not significantly 599
558 Eu3+ complex, 9, is analogous to the Ln3+ complexes of C2- altertheN-donorcoordinationenvironment.Incontrasttothe 600
555690 BchTaBrgPe. bFaolarnctehewcaastiaocnhiicevLedn3+eitchoemr pwleitxhesexotrfaClaytMticee4-BniTtrBaPte, CbeytMweee4n-BTthPehMen−sNtrubipcytuarneds,tMhe−reNitsrianzinoylcbleoanrldyliednegntthifisabfoler tarlelnodf 660012
561 anions (6, 8, and 10) or in combination with a the CyMe4-BTBP complexes. This suggests that the greater 603
562 hexanitratometallo anion (7). The two crystalline forms flexibility of the bipyridyl group, relative to the phenanthroline 604
556634 ooffbtearisnefudrthfreorminsthigehtcoinmtopltehxeateioqnuiloibfriEuum3+bewtiwtheeCny1M:1e4a-nBdT1B:P2 gbripoyurpid,yalllNowastommisniwmhaelndcisotoinrcdtiinoantedbettoweaeLnn3th+eiotnr.iazinyl and 660056
565 Ln/BTBP−BTPhen complex stoichiometries. Although it may XASofLanthanide-ExtractedSpecies.XASspectrawere 607
566 bepossibleforbothofthesestoichiometriestobeisolated,the obtained for Eu3+ and Tb3+ species formed by extraction from 608
567 vast majority of the structural evidence indicates that the an acidic aqueous phase into an organic phase containing an 609
568 lanthanides preferentially coordinate to two of these tetra N- excess of either CyMe4-BTBP or CyMe4-BTPhen in cyclo- 610
569 donor ligands from this class of extractant molecules. In hexanone as a guide for speciation in a potential SANEX 611
570 contrasttotheCyMe4-BTPhenstructures, metal-boundnitrate process.StudieswerealsoperformedforpotentialGANEX-like 612
571 ions are observed with all CyMe4-BTBP species except Tb3+. systems where the organic phase also included 30% TBP. XAS 613
572 This is presumably due to the greater flexibility afforded from spectra were obtained for the crystallographically characterized 614
573 the bipyridine, compared to the “locked” phenanthroline, solids [Eu(CyMe4-BTPhen)2(H2O)]3+ (3) and [Tb(CyMe4- 615
574 permitting the sterically larger bidentate nitrate anion, relative BTPhen)2(H2O)]3+(4)forcomparative purposes.Thespectra 616
575 to water, to bind to the Ln3+ center. obtained show little difference between the extracted species 617
576 ForallofthecomplexesofCyMe4-BTBP(6−10),theLn−N with or without the presence of TBP (Figures 11 and 12 and 618f12
t7 577 bond distances (Table 7) decrease as the lanthanide series is the Supporting Information). This indicates that the presence 619
578 traversedfromlefttoright,similartotheCyMe4-BTPhen-and ofTBPdoesnotinfluencelanthanidespeciationwhenusedina 620
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