FULL PAPER

DOI:10.1002/ejic.201402596

An Insight into the Complexation of Pyrazine-Functionalized Calix[4]arenes with Am3+ and Eu3+ –Solvent Extraction and Luminescence Studies inRoom-Temperature Ionic Liquids

Seraj A. Ansari,[a] Prasanta K. Mohapatra,*[a] Arijit Sengupta,[a]

Nicolai I. Nikishkin,[b] Jurriaan Huskens,[b] and Willem Verboom[b]

Keywords: Complexation / Calixarenes / Extractants / Actinides / Luminescence

The complexation behaviour of six pyrazine-functionalizedcalix[4]arenes, containing substituents such as carbamoyl di-octyl (L-1, L-2), diisopropyl phosphonate (L-3, L-4) and di-phenyl phosphoryl (L-5, L-6), with Am3+ and Eu3+ ions wasinvestigated by solvent extraction as well as by luminescencespectroscopy (only for Eu3+) in room-temperature ionic li-quids (RTILs), [Cnmim][NTf2] (1-alkyl-3-methylimidazolium,n = 4, 6, and 8). The spectacular enhancement in the extrac-tion values of the trivalent metal ions in RTILs, as compared

1. Introduction

Although nuclear energy is believed to be one of themost suited alternatives to the energy produced from fossilfuels, its public acceptance rests on the development of safedisposal technologies for the highly hazardous radioactivewaste emanating from various operations. The currentlyevolving radioactive-waste remediation strategy involvesseparation of trivalent lanthanides from trivalent actinidesby using N-donor ligands such as BTP (bis-triazinyl pyr-idine), BTBP (bis-triazinyl bipyridyl), etc.[1] Recently, an-other class of N-donor ligands , the pyrazine ligands, havebeen used for the separation of trivalent lanthanides fromtrivalent actinide ions with encouraging results.[2] It wasthought of interest to graft the N-donor pyrazine function-ality onto a calix[4]arene scaffold, and we have recently re-ported the synthesis and actinide–lanthanide extraction aswell as separation behavior of several such ligands.[3] How-ever, the extraction behavior and the meager separation fac-tors were not very encouraging apart from cases wherechlorinated cobalt dicarbollide was used as an auxiliary li-

[a] Radiochemistry Division, Bhabha Atomic Research Centre,Trombay, Mumbai 400085, IndiaE-mail: mpatra@barc.gov.inwww.barc.gov.in

[b] Laboratory of Molecular Nanofabrication, MESA+ Institutefor Nanotechnology, University of Twente,P. O. Box 217, 7500 AE Enschede, The NetherlandsSupporting information for this article is available on theWWW under http://dx.doi.org/10.1002/ejic.201402596.

Eur. J. Inorg. Chem. 2014, 5689–5697 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim5689

to those reported in a molecular diluent from 3 M HNO3, wasrather unusual and was further investigated by luminescencestudies. Luminescence studies involving Eu3+ and performedwith L-4 and L-6 confirmed the 1:1 stoichiometry of the ex-tracted species (Eu3+ L) with logβ values of 4.42�0.51 and5.18�0.38 for L-4 and L-6, respectively. Judd–Ofelt param-eters, which give information about the nature of bonding aswell as the coordinating environment, were computed fromthe emission spectra of the Eu3+ L-4 and Eu3+ L-6 complexes.

gand. It was of interest to enhance the extraction efficiencyof these ligands by using another suitable diluent.

Room-temperature ionic liquids (RTIL) enhance themetal-ion extractability manifold and have been used to ex-tract actinides and lanthanides with a variety of ligandssuch as TBP (tributyl phosphate),[4] TTA (thenoyltrifluoro-acetone),[5] CMPO (carbamoyl methyl phosphine oxide),[6]

malonamides,[7] and diglycolamides[8] from near neutral toacidic feed solutions. RTILs are widely studied as alterna-tive diluents because of their several advantages, such aslow volatility, amazing ability to dissolve organic and inor-ganic compounds, and their tunability.[9] Very limited re-ports are available on the extraction of metal ions with N-donor ligands in any RTIL. We have recently carried out aseries of studies on the evaluation of RTILs as diluents fordiglycolamide-functionalized calix[4]arenes, and the resultswere highly promising for possible application in the sepa-ration of actinide ions from actual radioactive wastefeeds.[10] Therefore, it is of interest to investigate the separa-tion behavior of the pyrazine-functionalized calix[4]arenesin RTILs for the extraction of actinide ions from acidic feedsolutions.

The present study is the first ever report on the complex-ation behavior of trivalent lanthanide and actinide ionswith several pyrazine-based calix[4]arene ligands, and theresults were obtained through extraction studies and spec-troscopic investigations in room-temperature ionic liquids.The luminescence spectroscopic data were used for the cal-culation of Judd–Offelt (J–O) parameters, which in turn

www.eurjic.org FULL PAPER

were used for obtaining structural information about thecomplexes. In the absence of single-crystal X-ray diffractiondata (because of inability of obtaining single crystals withthese bulky ligands), the structural information obtainedfrom J–O parameters may be quite useful.

2. Results and Discussion

2.1 Solvent Extraction Studies

A series of functionalized pyrazine-based calix[4]areneextractants L-1 to L-6 (Figure 1) were evaluated for the sep-aration of actinide and lanthanide ions under acidic feedconditions similar to those prevailing in nuclear waste (3 m

HNO3). Though n-dodecane is a preferred solvent for asolvent extraction system, these bulky ligands show a poorsolubility in paraffinic solvents. Our previous study in-volved their solutions in nitrobenzene.[3] However, in viewof the toxic nature of this diluent and also because of therelatively poor extractability of the metal ions, it wasnecessary to evaluate alternative diluents. Therefore, weevaluated the RTILs [Cnmim][NTf2] (n = 4, 6, 8) as alterna-tive diluents in the present work.

The distribution data of AmIII and EuIII obtained withthe ligands L-1 to L-6 from 3 m HNO3 are summarized inTable 1. The number and the nature of the substituents onthe calix[4]arene platform have a significant effect on thedistribution ratios. Obviously, the tetrasubstituted calix[4]-arene-based ligands L-2, L-4, and L-6 show a higher extrac-tion ability than the corresponding disubstituted ones L-1,L-3, and L-5. This is possibly due to the availability of ahigher number of donor atoms in the ligands that containfour pendant arms, which, if coordinated, may result in thecomplete dehydration of the inner coordination sphere ofthe metal ions. These complexes are likely to be more lipo-philic compared to the complexes with the ligands contain-ing two pendant arms, which may contain some inner-sphere water molecules. The extraction behavior of the dif-ferently substituted ligands follows the order: diphenylphosphoryl � diisopropyl phosphonate � carbamoyl di-octyl. The extraction of AmIII and EuIII by both ligands L-1 and L-2 was hardly appreciable. Similarly, the ligands L-3 and L-5 exhibit only moderate extraction ability. On theother hand, the tetrafunctionalized calix[4]arene-based li-gands L-4 and L-6, show appreciable extraction of the tri-valent f elements. The extraction with L-6 was even oneorder of magnitude higher than that with L-4. This featureis correlated with the higher electron density on the coordi-nating O atom of the diphenyl phosphoryl groups com-pared to that of the O atom of the diisopropyl phosphonatemoieties. It is worth mentioning that the distribution ratiosof AmIII and EuIII with these ligands are orders of magni-tude higher in RTIL medium than in the nitrobenzene me-dium reported earlier.[3] Furthermore, the possible partici-pation of N atom in the complexation imparts selectivityfor Am3+ extraction (because of soft–soft interactions) overEu3+ extraction. The remarkable enhancement in the metal-ion extraction is rather unusual, particularly at a moderate

Eur. J. Inorg. Chem. 2014, 5689–5697 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim5690

acid concentration of 3 m HNO3, where even strong extract-ants for trivalent actinides and lanthanides, such as DGA(diglycolamide) functionalized calix[4]arenas have lower Dvalues.[9c] Monitoring the L-4 and L-6 distribution ratiosshows that they decrease with increasing alkyl chain lengthof the RTILs from n-butyl via n-hexyl to n-octyl(Table 1).[10a] However, the separation factor (S.F.) betweenthe two metal ions (Am3+/Eu3+) was modest (�2.3) in allthree RTILs.

2.1.1 Effect of Aqueous Phase Acidity

The effect of a varying nitric acid concentration in therange 0.01–6.00 m on the extraction of AmIII ions was inves-tigated with L-4 and L-6 in the three RTILs. Figure 2 showsthat the extraction of AmIII ions decreases with increasingnitric acid concentration of the feed in [C4mim][NTf2] and[C6mim][NTf2], which points to an ion-exchange mecha-nism for the extraction. On the other hand, in the case of[C8mim][NTf2], the extraction of AmIII ions increases withincreasing nitric acid concentration, which points to a sol-vation mechanism in case of both ligands. The ion-exchangeextraction equilibrium in [C4mim][NTf2] medium can bewritten as Equation (1).

Am3+(aq.) + xL(IL) + 3[C4mim+](IL) = Am(L)x

3+(IL) + 3[C4mim+](aq.)

(1)

Here, the species with the subscripts (aq.) and (IL) refer tothe aqueous and ionic-liquid phases, respectively. Thoughthe ion-exchange mechanism is responsible for very highextraction of metal ions by the ionic liquids, it also involvespartitioning of large fractions of the ionic liquids into theaqueous phase, which limits their acceptability as alterna-tive diluents. In case of molecular diluents, the extractionof the metal ions usually proceeds through a solvationmechanism (particularly for neutral donor ligands). Themajor difference in the ion-exchange and solvation mecha-nism is seen in the metal-ion extraction profiles, that is, adecrease in metal-ion extraction with increasing nitric acidconcentration is seen for the ion-exchange mechanism,whereas an opposite trend is observed with the solvationmechanism. In case of RTILs as the diluents, the solvationmechanism has also been reported, especially with longeralkyl chains, as in [C8mim][NTf2] and [C10mim][NTf2].[11]

The equilibrium for the solvation mechanism can be repre-sented as Equation (2).

Am3+(aq.) + 3NO3

–(aq.) + xL(IL) = Am(NO3)3·xL(IL) (2)

This is in line with reports in the literature, where anion-exchange mechanism was found for RTILs having smallalkyl chains, and a solvation mechanism was found forRTILs with long alkyl chains.[11]

The distribution ratios of other metal ions, which are im-portant with respect to nuclear waste management, werealso measured with L-4 and L-6, and the results are sum-marized in Table 2. Interestingly, the extraction of AmIII

ions was significantly higher than that of PuIV ions withboth ligands. On the other hand, hexavalent uranyl ionswere not extracted, which suggests that the ligands can be

www.eurjic.org FULL PAPER

Figure 1. Structural formulae of the pyrazine-functionalized calix[4]arene ligands.

used for the selective recovery of trivalent actinides fromthe bulk of uranium. Both ligands hardly show any extrac-tion of fission product elements such as CsI and SrII. The

Eur. J. Inorg. Chem. 2014, 5689–5697 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim5691

extraction of AmIII ions was only slightly higher than thatof EuIII ions in both cases. One may expect a preferentialcomplexation of AmIII over EuIII with pyrazine ligands be-

www.eurjic.org FULL PAPER

Table 1. Distribution ratios D of AmIII and EuIII obtained with L-1–L-6 along with the separation factors (S.F. = DAm/DEu); ligandconcentration: 1.0 �10–3 m in [Cnmim][NTf2] (n = 4, 6, and 8); aqueous phase: 3 m HNO3.

Ligand [C4mim][NTf2] [C6mim][NTf2] [C8mim][NTf2] Nitrobenzene[a],[b]

AmIII EuIII S.F. AmIII EuIII S.F. AmIII EuIII S.F. AmIII EuIII S.F.

L-1 0.001 0.001 1.00 0.002 0.001 2.00 0.001 0.001 1.00 0.001 0.001 1.00L-2 0.004 0.003 1.33 0.009 0.004 2.25 0.008 0.004 2.00 0.001 0.001 1.00L-3 0.16 0.13 1.23 0.06 0.003 20.0 0.03 0.03 1.00 0.01 0.01 1.00L-4 62.1 43.2 1.44 15.04 8.84 1.70 7.24 4.59 1.58 0.05 0.03 1.67L-5 2.19 1.2 1.83 2.05 1.27 1.61 1.07 0.88 1.22 0.19 0.17 1.12L-6 850 375 2.27 395 239 1.65 241 204 1.18 13.5 14 0.96

[a] Data taken from ref.[3] [b] Ligand concentrations: 5.0 �10–3 m.

Figure 2. Effect of HNO3 concentration on the extraction of AmIII

ions by (a) L-4 and (b) L-6. Ligand concentration: 1 mmol/L in[Cnmim][NTf2] (n = 4, 6 and 8).

cause of the presence of soft N donor sites in the ligands(soft–soft interaction). However, from the results, it is evi-dent that the presence of two coordinating O atoms maypartially suppress the soft-soft interaction with the N atom,which results in S.F. values close to 1 in many cases.

Eur. J. Inorg. Chem. 2014, 5689–5697 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim5692

Table 2. Distribution of different metal ions by L-4 and L-6; ligandconcentration: 1 mmol/L in [Cnmim][NTf2] (n = 4, 6 and 8);aqueous phase: 3 m HNO3.

DM at 3 m HNO3

[C4mim][NTf2] [C6mim][NTf2] [C8mim][NTf2]

L-4 L-6 L-4 L-6 L-4 L-6

AmIII 62.1 464 15.04 279 7.24 241EuIII 43.2 376 8.84 239 4.6 204PuIV 2.7 159 3.05 107 2.91 105UVI 0.03 0.35 0.02 0.27 0.03 0.22SrII 0.02 0.003 0.02 0.002 0.01 0.001CsI 0.01 0.002 0.01 0.003 0.01 0.003

2.1.2 Nature of the Extracted Species

The stoichiometry of the extracted AmIII species was de-termined at 1 m HNO3 for both ligands in the three RTILsby slope analysis. The distribution coefficients of AmIII

were measured at varying ligand concentrations between0.01 and 0.1 mmol/L. The plot of log DAm against the li-gand concentration yields slopes of approximately 1 forboth ligands (Figure 3). The results indicate that thestoichiometry of the predominantly extracted species is 1:1(metal/ligand) in all three RTILs. This observation was fur-ther corroborated by fluorescence studies with Eu3+ ions,which are considered as a surrogate for Am3+ ions (videinfra).

2.2 Luminescence Spectroscopic Studies

Eu3+ ions exhibit well characterized emission spectra inthe visible region because of transitions identified as5D0 � 7F2 (617 nm), 5D0 � 7F1 (592 nm), and 5D0 � 7F4

(690 nm).[12] Upon complexation, the intensity of the Eu3+

luminescence in the resulting metal–ligand complexeschanges relative to that in the aquo ion because of changesin symmetry. Figure 4 shows the luminescence spectra ofEu3+ ions, which were obtained upon the titration of metalions with ligand in a 5:1 acetonitrile/water mixture. Charac-teristic peaks were observed at 591, 617, and 692 nm, andthey correspond to the de-excitation of the 5D0 level to the7F1, 7F2, and 7F4 levels of the Eu3+ ion, respectively.

Solvent extraction studies with the ligands L-4 and L-6showed that the extracted AmIII complex predominantly

www.eurjic.org FULL PAPER

Figure 3. Study of the effect of ligand concentration on the D valueof AmIII ion extraction to determine the stoichiometry of the ex-tracted species. Ligands: (a) L-4 and (b) L-6; aqueous phase: 1 mHNO3.

has a 1:1 (metal/ligand) stoichiometry. In order to establishthe stoichiometry of the extracted species and their stabilityconstants, single-phase fluorescence studies were performedwith EuIII ions in a 5:1 acetonitrile/water mixture. For thedetermination of the stability constants of the M–L com-plexes, the method of linear combination of the lumines-cence spectra was followed. The complexation equilibria forEu3+ and L can be represented as follows [Equation (3)].

(3)

Here, β is the conditional stability constant and can begiven as Equation (4).

(4)

Eur. J. Inorg. Chem. 2014, 5689–5697 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim5693

The above equation can be solved with Equation (5).

(5)

Figure 4. Luminescence spectra of solutions containing 10–4 mol/L (a) Eu(NO3)3, (b) Eu3+ L-4 complex, and (c) Eu3+ L-6 complex.Solvent: acetonitrile/water (5:1) in aqueous nitrate medium at pH3; excitation wavelength: 230 nm.

Here, [Eu]complex is the europium ion complexed with li-gand, [Eu·Ln]3+. For a given europium species, the lumines-cence intensity at a fixed wavelength is proportional to itsconcentration in solution. When two molar equivalents ofpyrazine ligand are added, no uncomplexed europium ionsremain in solution, so the only europium species present insolution is the complex [Eu·Ln]3+. The luminescence spec-trum obtained for a given ligand-to-metal ratio can be ad-justed with a linear combination of the spectra of uncom-plexed Eu3+ ions and of a complexed ones, and these twospectra can be obtained experimentally. The coefficients ofthe linear combination allow to determine the relative pro-portions of the two species, [Eu]free and [Eu]complex, presentin solution at thermodynamic equilibrium. If the amountof ligand initially added is known, it is possible to calculatethe concentration of the remaining free ligand, and then theconditional stability constant of [Eu·Ln]3+ can be deter-mined. Figure 5 shows a graphical representation of Equa-tion (5). A linear regression fit of the data yielded a slopevalue of 1, confirming the formation of a 1:1 metal–ligandcomplex for both ligands. The calculated logβ values were4.42� 0.51 and 5.18�0.38 for L-4 (Figure 5, a) and L-6(Figure 5, b), respectively. The higher log β value in case ofL-6 points to a better complexation of the ligand with EuIII

ions, which is reflected in higher distribution ratios. Theplot shown in Figure 5 (a) appeared to be slightly deviatingfrom the straight-line fit at higher as well as at lower con-centrations of free ligand. This trend was obtained becauseof the wider range of free-ligand concentrations used in thecase of L-4 as compared to L-6 (Figure 5, b). Attempts tofit the data points in two distinct sets of lines yielded slope

www.eurjic.org FULL PAPER

values of 0.89�0.13 (excluding high-concentration datapoints) and 1.57 �0.13 (excluding low-concentration datapoints). Though the lower slope value conformed to 1:1(M:L) complexed species, the higher slope value is possiblydue to the presence of both 1:1 and 1:2 species.

Figure 5. Ratio of complexed Eu3+ to free Eu3+ ions plotted againstthe Concentration of free ligand for the calculation of the stabilityconstants. Ligands: (a) L-4 and (b) L-6; diluent: acetonitrile/water(5:1) in aqueous nitrate medium at pH 3; temperature: 25 °C.

The lifetime of the excited Eu3+ ions was determinedfrom the luminescence decay curves. Typical lifetime spectraare depicted in Figure 6, where the decay profiles forEu3+ L-4 (Figure 6, a) and Eu3+ L-6 (Figure 6, b) are pre-sented. The decay curves of all Eu3+ complexes follow amono-exponential pattern, which indicates the presence ofsingle species. The empirical formula[13] NH2O = (1.06/τ) –0.19 was used to calculate the number of inner coordinationsphere water molecules (NH2O). When used in the empiricalformula, the lifetime data indicated no water molecules inthe EuIII complex of both ligands. Figure 4 shows that thereare considerable differences in the emission profiles of Eu3+

ions in aqueous medium and in complexed form. There are

Eur. J. Inorg. Chem. 2014, 5689–5697 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim5694

a maximum of five sets of emission lines in the spectra, andthey were assigned to the appropriate transitions in theEu3+ ion. The relatively high intensity of the 5D0 � 7F2

transition in the Eu3+ complexes of L-4 and L-6 comparedto the aquo complex of the Eu3+ ion reveals that the localenvironment around the Eu3+ ion becomes asymmetricupon complexation with respect to the inversion symmetry.This can be quantified by the asymmetry factor determinedwith Equation (6).[14]

A = I(5D0 � 7F2)/I(5D0 � 7F1) (6)

Figure 6. Decay curves of (a) the Eu3+ L-4 complex and (b) theEu3+ L-6 complex. Solvent: acetonitrile/water (5:1); temperature:25 °C; excitation wavelength: 230 nm.

Here, A is the asymmetry factor, and I(5D0 � 7F2) andI(5D0 � 7F1) are the intensities of the 5D0 � 7F2 and the5D0 � 7F1 transitions, respectively. It may be mentionedhere that because 5D0 � 7F1 is a magnetic-dipole transition,the intensity of the corresponding peak is independent ofthe ligand field. On the other hand, the intensity of the5D0 � 7F2 transition (electric-dipole transition) is hypersen-sitive to the ligand field. In the present case, the asymmetryfactor A of the Eu3+ L-6 complex was slightly higher thanthat of the Eu3+ L-4 complex. The emission scheme of Eu3+

is well known, and the splitting patterns of each transition

www.eurjic.org FULL PAPER

can be correlated with the local symmetry around the Eu3+

ion.[15] We tried to get an idea on the local symmetry ofthe metal ion in the complex from the splitting patterns(Supporting Information). Though C4v symmetry seemsprobable from these calculations, further studies are re-quired to conclusively prove this.

2.2.1 Calculation of Judd–Ofelt Intensity Parameters

The intensity of observed transitions in rare-earth ionsin different hosts is often parameterized on the basis ofthree Judd–Ofelt parameters, that is, Ωλ (λ = 2, 4, 6),[16] asgiven by Equation (7).[17]

f = [8π2mcv/3h(2J + 1)] [(n2 + 2)2/9n] Σλ Ωλ(ψ||Uλ||ψ�J�)2 (7)

The Eu3+ ion poses serious difficulties in the applicationof J–O analysis. Only a few transitions, namely, those to5D1, 5D2, and 5L6 from the 7F0 and 7F1 ground state com-ponents, are observed with a good intensity. The transitionto the 5D3 state from the ground state has a very poor inten-sity. The reduced matrix elements ||Uλ||2 (λ = 2, 4, 6) for theinduced electric-dipole transitions, which are necessary toevaluate the J–O parameters, are non-zero for only a fewtransitions and hence provide only a few equations for theleast-square fitting. Also, among the three reduced matrixelements ||Uλ||2 (λ = 2, 4, 6), ||U4||2 is zero for all the ob-served absorption transitions.

The emission bands arising from the 5D0 � 7FJ (J = 2, 4,and 6) transitions of Eu3+ are electric-dipole-allowed. Thespontaneous emission probability for the electric-dipoletransition (5D0 � 7F2), termed as Aed, from ψ (initial mani-fold) to ψ� (terminal manifold) may be given by the follow-ing Equation (8).[18]

Aed = 64π4e2ν3 n(n2 + 2)2/[3h(2J + 1)] 9) Σ Ωλ |�ψ||Uλ||ψ��|2 (8)

Here, ν is the frequency of the transition (in cm–1), whereash, e, and n represent the Planck constant, the electroncharge, and the refractive index, respectively. The square ofthe matrix elements of the tensor operator term|�ψ||Uλ||ψ��|2 is assumed to be independent of the host ma-trix. The spontaneous emission probability of the magnetic-dipole transition (5D0 � 7F1), which is termed hereafter asAmd is given by Equation (9).[18]

Amd = 64π4ν3n3Smd/3h(2J + 1) (9)

Here, J is the total angular momentum of the excited stateand Smd is the magnetic-dipole line strength, which is me-dium independent and is considered a constant term. FromEquation (7), it may be easy to understand that the inten-sities of the transitions are decided by the |�ψ||Uλ||ψ��|2 val-ues. Therefore, the intensity ratios of the 5D0 � 7F2,4,6 andthe 5D0 � 7F1 transitions may yield the Ωλ values throughthe following Equation (10).[18]

√IJ(ν)dν/√Imd(ν)dν = AJ/Amd = 64π4e2ν3/3h(2J + 1) � n(n2 + 2)2/9Amd Ωλ |�ψ||Uλ||ψ��|2) (10)

Out of the possible Ωλ values, only the Ω2 and Ω4 param-eters can be calculated, whereas the intensity parameter Ω6

Eur. J. Inorg. Chem. 2014, 5689–5697 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim5695

(concerning the 5D0 � 7F6 emission) could not be deter-mine. The results are given in Table 3. These parameters,also known as the Judd–Ofelt intensity parameters, reflectthe local structure and bonding around the rare-earth ionof interest (Eu3+). It is well known that Ω2 gives an ideaabout the extent of covalency between the metal ion andthe coordinating atoms of the ligand. This parameter canalso give information about the asymmetry of the Eu3+ ionin a particular environment.

Table 3. J–O constants and other photophysical parameters of theEu3+ complexes of L-4 and L-6.

System Eu3+ L-4 Eu3+ L-6

Ω2 [cm2] 3.85E-20 4.11E-20Ω4 [cm2] 3.29E-20 3.05E-20τR [s] 5.20E-3 5.96E-3τNR [s] 1.66E-3 2.74E-3H 2.42E-1 3.15E-1A 2.36 2.51Lifetime [ms] 1.260 1.875NH2O 0 0Nligand 1 1β2 4.34E-1 5.26E-1β4 1.85E-1 1.97E-1β6 1.84E-1 2.09E-1Amd 3.52E+1 3.52E+1A2ed 8.34E+1 8.84E+1A4ed 3.56E+1 3.30E+1

The Ω2 value of the Eu3+ L-6 system is larger than thatof the Eu3+ L-4 complex, which indicates an enhanced co-valence and a strengthening of the Eu–O bond, that is, ahigh bond energy and a short bond length. Apparently, inL-6, the planar phenyl group causes less steric crowdingaround the ligating moieties (phosphoryl group) than thesecondary isopropoxy groups. As a result, with L-6 themetal ion is more tightly bound than with L-4. Ω4, whichis a measure of the long range ordering, is also larger forthe Eu3+ complex of L-4 than for the complex of L-6.

2.2.2 Radiative Properties of Eu3+–DGA Complexes

Several radiative properties, such as the total radiativetransition probabilities, [A(ψJ, ψ�J�)], branching ratios (βn),and excited state lifetimes of the Eu3+ ion in the complexedspecies can also be calculated from the J–O parameters.Equation (11) can be used for calculating A(ψJ, ψ�J�) forthe transition ψJ �ψ�J�.[18]

A(ψJ,ψ�J�) = Aed + Amd (11)

Furthermore, AT(ψJ) (the total transition probability in-cluding the radiative and non-radiative components) can berelated with τrad (predicted radiative lifetime of the excitedstate) and β by Equations (12) and (13).

τrad(ψJ) = 1/AT (ψJ) (12)

β(ψJ) = A(ψJ, ψ�J�)/AT(ψJ) (13)

The luminescence lifetime (τ) of the Eu3+ L-6 complex islarger than that of the L-4 complex, which points to ad-ditional non-radiative pathways for the decay of the excitedEu3+ ion with L-4, where a phenyl moiety is substituted by

www.eurjic.org FULL PAPER

an isopropoxy group. A similar trend was also observed forthe corresponding quantum efficiency values (η). Thebranching ratio (βn; n = 2, 4, 6) follows the trendβ2 � β4 ≈β6, which reveals that Eu3+ resides in an asymmet-ric environment in case of both complexes Eu3+ L-4 andEu3+ L-6. The magnetic- and electric-dipole transitionprobabilities behave in the fashion of A2ed � A4ed ≈Amd.The radiative and non-radioactive lifetimes, the branchingratios of different transitions, the quantum efficiency, theprobability of the electric-dipole and magnetic-dipole tran-sitions, etc. are summarized in Table 3.

3. Conclusions

Several calix[4]arene-based ligands functionalized withtwo or four pyrazine pendent arms containing different sub-stituents, such as carbamoyl dioctyl, diisopropyl phos-phonate, and diphenyl phosphoryl groups, were evaluatedfor the extraction of actinide and lanthanide ions in room-temperature ionic liquids [Cnmim][NTf2] (n = 4, 6, 8). Sol-vent extraction studies revealed that the calix[4]arenes func-tionalized with four pyrazine pendants substituted with di-isopropyl phosphonate (L-4) or diphenyl phosphoryl (L-6)groups show appreciable extraction of trivalent actinide andlanthanide ions with a very low ligand concentration(1 mmol/L). The extraction is an order of magnitude higherin RTIL media than in nitrobenzene. The extraction ofmetal ions followed an ion-exchange mechanism withRTILs containing shorter alkyl chains ([C4mim][NTf2] and[C6mim][NTf2]) and a solvation mechanism with the RTILcontaining a longer alkyl chain ([C8mim][NTf2]). Slopeanalyses suggested that of 1:1 AmIII L species in all threeRTILs, which was subsequently confirmed by luminescencestudies involving analogous Eu3+ (used as a surrogate forAm3+ ions) complexes. The logβ values were calculated tobe 4.42� 0.51 and 5.18�0.38 for L-4 and L-6, respectively.From the splitting patterns of Eu3+ emission lines, the sym-metry in the Eu3+ complexes was determined to be C4v forboth the ligands.

4. Experimental Section4.1 Materials: A series of functionalized pyrazine-based p-tert-but-ylcalix[4]arene ligands, bis- and tetrakis-dimides (L-1 and L-2), bis-and tetrakis-isopropyl phosphinate (L-3 and L-4), and bis- andtetrakis-diphenylphosphine oxides (L-5 and L-6), were synthesizedas reported previously (Figure 1).[3] The purity of the ligands wasensured by 1H NMR spectroscopy. The room-temperature ionicliquids 1-butyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)-imide ([C4mim][NTf2]), 1-hexyl-3-methylimidazolium-bis(trifluoro-methylsulfonyl)imide ([C6mim][NTf2]), and 1-octyl-3-methylimid-azolium-bis(trifluoromethylsulfonyl)imide ([C8mim][NTf2]) wereprocured from IoliTec, Germany. The radiotracers 241Am,152,154Eu, 233U, and 239Pu were used from the laboratory stock aftertheir radiochemical purities were confirmed by alpha as well as bygamma spectrometric analysis, which was performed by using sili-con surface barrier and high-purity-germanium (HPGe) detectors,respectively. Other radionuclides, 85,89Sr and 137Cs, were procured

Eur. J. Inorg. Chem. 2014, 5689–5697 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim5696

from the Board of Radiation and Isotope Technology (BRIT),Mumbai, India, and their radiochemical purities were ascertainedby using a HPGe detector prior to their use.

4.2 Distribution Measurements: The distribution ratio of the metalions was measured by equilibrating equal volumes of the organicand aqueous (containing the required radiotracers) phases in astoppered test tube at room temperature. After equilibration, thetwo phases were centrifuged, and the organic and aqueous phaseswere separated for analysis. The assay of 233U and 239Pu was car-ried out by using a liquid scintillation counter employing a toluene-based scintillator, which contained 0.7% (w/v) PPO (2,5-di-phenyloxazole), 0.03 % (w/v) POPOP {1,4-bis[2-(5-phenyloxazoyl)]-benzene}, and 10% (v/v) HDEHP [bis(2-ethylhexyl)phosphoricacid]. The assays of gamma-emitting radionuclides such as 241Am,152,154Eu, 137Cs, and 85,89Sr were carried out by employing a well-type NaI(Tl) gamma scintillation counter coupled with a multi-channel analyzer. The distribution ratio of metal ions (DM) wasobtained as the concentration of the metal ions in the organicphase (in terms of counts per unit time) divided by that in an equalvolume of the aqueous phase. All experiments were performed atleast in triplicate and the obtained data were within the error limitsof �5%.

4.3 Luminescence Measurements: Photoluminescence excitationspectra of Eu3+ ions were recorded with an Edinburgh F-900fluorescence spectrometer with a Xe lamp as the excitation source,M-300 monochromators, and a Peltier-cooled photomultiplier tubeas detector. The acquisition and analysis of the data were carriedout with F-900 software supplied by Edinburgh Analytical Instru-ments, UK. The excitation wavelength was fixed at 230 nm,whereas the emission spectra were recorded in the range 550–750 nm. The luminescence decay curves were fitted to an ex-ponential function to obtain the lifetime/decay rates of the excitedstates. The reproducibility of the lifetimes of the excited states waswithin �3 μs.

Supporting Information (see footnote on the first page of this arti-cle): Splitting patterns of different transitions of the Eu3+-L-4 andEu3+-L-6 complexes.

Acknowledgments

The authors (S. A. A., P. K. M. and A. S.) thank Dr. A. Goswami,head of the Radiochemistry Division, BARC, for his constant en-couragement.

[1] a) F. W. Lewis, M. J. Hudson, L. M. Harwood, Synlett 2011,2609; b) M. J. Hudson, L. M. Harwood, D. M. Laventine, F. W.Lewis, Inorg. Chem. 2013, 52, 3414.

[2] N. I. Nikishkin, J. Huskens, J. Assenmacher, A. Wilden, G. Mo-dolo, W. Verboom, Org. Biomol. Chem. 2012, 10, 5443.

[3] N. I. Nikishkin, J. Huskens, S. A. Ansari, P. K. Mohapatra, W.Verboom, New J. Chem. 2013, 37, 391.

[4] a) M. S. Murali, N. Bonville, G. R. Choppin, Solvent Extr. IonExch. 2010, 28, 495; b) I. Billard, A. Ouadi, E. Jobin, J. Cham-pion, C. Gaillard, S. Georg, Solvent Extr. Ion Exch. 2011, 29,577.

[5] L. N. Puntus, K. J. Schenk, J. C. G. Bunzli, Eur. J. Inorg. Chem.2005, 4739.

[6] a) A. E. Visser, M. P. Jensen, I. Laszak, K. L. Nash, G. R.Choppin, R. D. Rogers, Inorg. Chem. 2003, 42, 2197; b) K.Nakashima, F. Kubota, T. Maruyama, M. Goto, Ind. Eng.Chem. Res. 2005, 44, 4368; c) G. A. Pribylova, J. Radioanal.Nucl. Chem. 2011, 288, 693.

[7] a) M. Bonnaffé-Moity, A. Ouadi, V. Mazan, S. Miroshnich-enko, D. Ternova, S. Georg, M. Sypula, C. Gaillard, I. Billard,

www.eurjic.org FULL PAPER

Dalton Trans. 2012, 41, 7526; b) A. B. Patil, P. Pathak, V. S.Shinde, S. V. Godbole, P. K. Mohapatra, Dalton Trans. 2013,42, 1519.

[8] a) Y. Shen, X. Tan, L. Wang, W. Wu, Sep. Purif. Technol. 2011,78, 298; b) S. Panja, P. K. Mohapatra, S. C. Tripathi, P. M.Gandhi, P. Janardan, Sep. Purif. Technol. 2012, 96, 289–295.

[9] a) X. Sun, H. Luo, S. Dai, Chem. Rev. 2012, 112, 2100; b) K.Binnemans, Chem. Rev. 2007, 107, 2593; c) P. R. Vasudeva Rao,K. A. Venkatesan, A. Rout, T. G. Srinivasan, K. Nagarajan,Sep. Sci. Technol. 2012, 47, 204; d) M. L. Dietz, J. A. Dzielawa,I. Laszak, B. A. Young, M. P. Jensen, Green Chem. 2003, 5,682; e) A. E. Visser, R. P. Swatloski, W. M. Reichert, R. May-ton, S. Sheff, A. Wierzbicki, J. H. Davis, R. D. Rogers, Chem.Commun. 2001, 135.

[10] a) M. Iqbal, P. K. Mohapatra, S. A. Ansari, J. Huskens, W.Verboom, Tetrahedron 2012, 68, 7840; b) P. K. Mohapatra, M.Iqbal, D. R. Raut, W. Verboom, J. Huskens, S. V. Godbole,Dalton Trans. 2012, 41, 360; c) P. K. Mohapatra, A. Sengupta,M. Iqbal, J. Huskens, W. Verboom, Inorg. Chem. 2013, 52,2533; d) D. R. Raut, P. K. Mohapatra, S. A. Ansari, S. V. God-bole, M. Iqbal, D. Manna, T. K. Ghanty, J. Huskens, W. Ver-boom, RSC Adv. 2013, 3, 9296; e) P. K. Mohapatra, A. Seng-upta, M. Iqbal, J. Huskens, S. V. Godbole, W. Verboom, DaltonTrans. 2013, 42, 8558.

[11] V. A. Cocalia, M. P. Jensen, J. D. Holbrey, S. K. Spear, D. C.Stepinski, R. D. Rogers, Dalton Trans. 2005, 1966.

[12] a) P. A. Tanner, Y. Y. Yeung, L. Ning, J. Phys. Chem. A 2013,117, 2771; b) M. H. V. Werts, R. T. F. Jukes, J. W. Verhoeven,Phys. Chem. Chem. Phys. 2002, 4, 1542.

Eur. J. Inorg. Chem. 2014, 5689–5697 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim5697

[13] a) W. D. Horrocks, D. R. Sudnick, Acc. Chem. Res. 1981, 14,384; b) W. D. Horrocks, D. R. Sudnick, J. Am. Chem. Soc.1979, 101, 334; c) T. Kimura, Y. Kato, J. Alloys Compd. 1998,278, 92.

[14] D. Janczewski, D. N. Reinhoudt, W. Verboom, E. Malinowska,M. Pietrzak, C. Hill, C. Allignole, New J. Chem. 2007, 31, 109.

[15] C. Görller-Walrand and K. Binnemans, in: Handbook on thePhysics and Chemistry of Rare Earths (Eds.: K. Gscheidner Jr.,L. Eyring), Elsevier, Amsterdam, 1996; vol. 23, p. 121–283.

[16] a) B. R. Judd, Phys. Rev. 1962, 127, 750; b) G. S. Ofelt, J.Chem. Phys. 1962, 37, 511.

[17] A. Kumar, D. K. Rai, S. B. Rai, Spectrochim. Acta Part A 2002,58, 2115.

[18] a) O. L. Malta, H. F. Britob, J. F. S. Menezesb, F. R. Gongal-vese Silva, S. Alves Jr., F. S. Farias Jr., A. V. M. de Andrade, J.Lumin. 1997, 75, 255; b) G. F. de Sá, O. L. Malta, C.de Mello Donegá, A. M. Simas, R. L. Longo, P. A. Santa-Cruz, E. F. da Silva Jr., Coord. Chem. Rev. 2000, 196, 165; c)M. Kumar, T. K. Seshagiri, S. V. Godbole, Phys. B 2013, 410,141; d) W. Shipeng, Z. Xiaoping, H. Shui, Z. Liqun, L. Li, J.Rare Earths 2008, 26, 787; e) D. Zhao, X. Qiao, X. Fan, M.Wang, Phys. B 2007, 395, 10; f) A. Sengupta, S. V. Godbole,P. K. Mohapatra, M. Iqbal, J. Huskens, W. Verboom, J. Lumin.2014, 148, 174.

Received: June 26, 2014Published Online: October 10, 2014