Hydrophilic Clicked 2,6-Bis-Triazolyl-pyridines Endowed with High Actinide Selectivity and Radiochemical Stability: Toward a Closed Nuclear Fuel Cycle

Article abstract of DOI:10.1021/jacs.6b03106

There is still an evident need for selective and stable ligands able to separate actinide(III) from lanthanide(III) metal ions in view of the treatment of the accumulated radioactive waste and of the recycling of minor actinides. We have herein demonstrated that hydrophilic 2,6-bis-Triazolyl-pyridines are able to strip all actinides in all the different oxidation states from a diglycolamide-containing kerosene solution into an acidic aqueous phase. The ascertained high actinide selectivity, efficiency, extraction kinetics, and chemical/radiolytic stability spotlight this hydrophilic class of ligands as exceptional candidates for advanced separation processes fundamental for closing the nuclear fuel cycle and solving the environmental issues related to the management of existing nuclear waste.

Full text of DOI:10.1021/jacs.6b03106

Communication
pubs.acs.org/JACS
Hydrophilic Clicked 2,6-Bis-triazolyl-pyridines Endowed with High
Actinide Selectivity and Radiochemical Stability: Toward a Closed
Nuclear Fuel Cycle
Elena Macerata,*,§ Eros Mossini,§ Stefano Scaravaggi,§ Mario Mariani,§ Andrea Mele,¶,∥ Walter Panzeri,¶,∥
Nathalie Boubals,† Laurence Berthon,† Marie-Christine Charbonnel,† Francesco Sansone,‡
Arturo Arduini,‡ and Alessandro Casnati*,‡
§Nuclear Engineering Division, Department of Energy, Politecnico di Milano, P.zza L. da Vinci 32, I-20133 Milano, Italy
¶Department of Chemistry, Materials and Chemical Engineering “G. Natta”, Politecnico di Milano, Piazza L. da Vinci, 32, I-20133
Milano, Italy
∥CNR-ICRM, Via L. Mancinelli, 7, I-20131 Milano, Italy
†Nuclear Energy Division, RadioChemistry & Processes Department, CEA, 30207 Bagnols-sur-Cez
̀
e, France
‡Dipartimento di Chimica, Universita
̀
di Parma, Parco Area delle Scienze 17/a, 43124 Parma, Italy
S
* Supporting Information
proposed that are mainly based on the combined coextraction of
An and Ln from the PUREX (Plutonium URanium EXtraction
process) raffinate,6 followed by the subsequent An/Ln
separation, necessary to successfully implement the An trans-
mutation. The European strategy,7 initially proposed few
decades ago, aims at coextracting An(III) and Ln(III) in an
organic phase with the DIAMEX (DIAMide EXtraction) and
DIAMEX-like processes by using malonamides or the highly
efficient N,N,N′,N′-tetraoctyl diglycolamide (TODGA, see
Figure S9).8 Furthermore, several processes were proposed for
the selective An separation from Ln, such as the SANEX
(Selective ActiNide EXtraction) process.9 Enormous progresses
were made in the latest years in the design of selective An ligands.
Several soft heterocyclic N-donor ligands showed important An/
Ln selectivities.10 However, due to the basicity of nitrogen atoms
involved in cation coordination, only a few of them are able to
operate under highly acidic conditions. Among these ligands,
BTPs (bis-triazinylpyridine) and BTBPs (bis-triazinyl bipyr-
idine) are characterized by a remarkably interesting selectivity11
but also by low chemical stability under the typical harsh
conditions and, sometimes, by slow kinetics. Recently, it was
proposed to combine the unselective An−Ln extraction by
TODGA with the selective back-extraction (stripping) into an
aqueous layer of the trivalent An cations through the so-called i-
SANEX (innovative SANEX) process or, even better, of all the
actinides in all their possible oxidation states via the GANEX
(Group ActiNides EXtraction) process.12 However, despite of
the different ligands proposed for stripping purposes,10,12,13 none
of them gather all the required characteristics to implement an
industrial process, which are (i) water solubility, (ii) ability to
complex hard metal ions such as An, (iii) An/Ln selectivity, (iv)
hydrolytic and radiolytic stability, (v) high loading capacity, (vi)
fast kinetics of (de)complexation, and (vii) complete inciner-
ability (contain only C, H, O, N atoms).
ABSTRACT: There is still an evident need for selective
and stable ligands able to separate actinide(III) from
lanthanide(III) metal ions in view of the treatment of the
accumulated radioactive waste and of the recycling of
minor actinides. We have herein demonstrated that
hydrophilic 2,6-bis-triazolyl-pyridines are able to strip all
actinides in all the different oxidation states from a
diglycolamide-containing kerosene solution into an acidic
aqueous phase. The ascertained high actinide selectivity,
efficiency, extraction kinetics, and chemical/radiolytic
stability spotlight this hydrophilic class of ligands as
exceptional candidates for advanced separation processes
fundamental for closing the nuclear fuel cycle and solving
the environmental issues related to the management of
existing nuclear waste.
he search for efficient and robust ligands for the selective
Tcomplexation and extraction of actinide (An) cations in the
presence of large amounts of lanthanide (Ln) metal ions and
other fission products is a topic of high priority not only for a
sustainable development and public acceptance of nuclear energy
but also for the treatment of the existing radioactive wastes.1
Whatever decision the governments will take on the future of
nuclear energy, we have the duty versus future generations to
ensure a proper and safer management of all the radioactive
wastes produced in the latest 75 years for energy and weapon
production.2 The option of a closed nuclear fuel cycle,3 that is
recycling of the reusable components of nuclear waste as new
fuel, would allow a significant reduction of the waste volume and
a more convenient exploitation of natural resources.4 Moreover,
it would also reduce the long-term radiotoxicity and decay heat of
the final nuclear wastes. In the last decades great efforts have been
devoted to this challenging issue and the Partitioning and
Transmutation strategy is being considered worldwide a
conceivable way to achieve this goal.1a,5 Within this approach,
several multicycle advanced separation processes have been
Received: March 24, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.6b03106
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Journal of the American Chemical Society
Communication
Scheme 1. Synthesis of Ligands 1−3 Used in the Present Study
Table 1. DM and SF Values for the Stripping of Am3+ from a TODGA-Based Organic Phase into a PyTri-Based Aqueous Solution
blank: no ligand
0.1 M ligand 1
0.15 M ligand 2
0.15 M ligand 3
a
stripping phase
element
[HNO3] = 0.25 M
[HNO3] = 0.25 M
[HNO3] = 0.25 M
[HNO3] = 0.25 M
b
b
b
b
DM
% in org
DM
% in org
DM
% in org
DM
% in org
241Am
152Eu
La
16.3
113
1.29
94.2
99.1
56.3
0.17
24.5
14.5
96.1
57.3
0.27
27.0
21.2
96.4
51.5
0.77
44
37.7
97.8
52.8
1.34
144.35
7.88
1.06
99.96
3.92
1.12
57.14
1.45
c
SFEu/Am
6.93
0.08
c
SFLa/Am
a
b
Organic phase in the stripping tests: 0.2 M TODGA in kerosene/1-octanol (95/5 v/v) loaded with i-SANEX feed. DM is defined as the ratio
c
between the concentration/activity of the metal (M) in organic phase over that in aqueous phase. SFLn/An is defined as the ratio between DLn and
DAn.
We herein propose the use of the pyridine-2,6-bis(1H-1,2,3-
triazol-4-yl) (PyTri, Scheme 1) as a proper N3 chelating moiety
for the An/Ln separation, which comprises all the requested
characteristics. Although this “clicked” binding motif has been
intensively studied in the latest 15 years as terdentate ligating
units both in supramolecular and coordination chemistry,14a to
the best of our knowledge, there are no examples in the literature
concerning the use of hydrophilic ligands thereof derived for
actinide coordination or in the An/Ln separation. Only very few
reports deal with Lanthanide-PyTri complexes in the solid state
or in aprotic solvents also as potential bioimaging agents.14b The
recent publication by Kiefer et al. on a lipophilic 2,6-bis[1-(p-
tolyl)-1H-1,2,3-triazol-4-yl]pyridine showing an interesting
selectivity in binding Cm(III) over Eu(III) in CH3CN but
unable to extract Am and Eu into an organic phase, prompted us
to disclose our results on the class of hydrophilic PyTri
ligands.14c We synthesized a series of ligands 1−3 (Scheme 1)
based on the PyTri chelating motif and characterized by the
presence, at their periphery, of an increasing number of OH
groups to ensure solubility in aqueous solutions. Ligands 114b
and 2 were straightforwardly prepared by clicking 2,6-diethynyl-
pyridine (4) with the azides 5 and 6, respectively, under classical
CuAAC conditions. Ligand 3 was, however, obtained after a
similar cycloaddition reaction but starting from the tris-
acetylated azide 7,15 followed by Zemplen deprotection by
MeONa in MeOH and acidification. Ligands 1−3, characterized
by NMR and ESI-MS techniques, are soluble up to 0.15−0.20 M
in water and diluted nitric acid solutions (see SI for details).
Moreover, UV−vis measurements showed that the loss of ligands
1−3 from the acidic layer to the organic phase (kerosene/1-
octanol 95/5 v/v, both in the absence and in the presence of 0.2
M TODGA) was negligible within the experimental error. The
potentiality of this class of ligands in the selective An extraction
was verified by performing liquid−liquid extraction tests and
measuring the DM values of Eu(III), Am(III), Cm(III), and
Pu(IV). Two different organic phases were obtained by
contacting a 0.2 M TODGA solution in kerosene/1-octanol
95/5 v/v with an equal volume of 3 M nitric acid feeds containing
only trivalent An (i-SANEX feed) or tri- and tetravalent An
(GANEX feed), in addition to inactive Y and Ln (from La to Gd)
(see Tables S1). The TODGA-based solutions were able to
coextract 99.9% of An and Ln present in the aqueous phases as
determined by ICP-MS, α, and γ spectrometry. Subsequently,
these TODGA-based loaded organic solutions were contacted
with the acidic stripping phases containing ligands 1−3. HNO3
concentration was monitored after each step (see SI). As shown
in Table 1, the results of a blank experiment (0.25 M HNO3
stripping solution with no hydrophilic ligand) indicate that, apart
from the lighter La ion, only a small percentage of the other
cations is back extracted in the acidic aqueous phase. Less than
1% of Eu and 6% of Am are released under these conditions, in
agreement with the already known poor Ln/An selectivity of
TODGA.8b The introduction of the hydrophilic PyTri ligands in
the aqueous layer considerably increases the Am extraction in
water by competing with TODGA. This results in a decrease of
the DM values by nearly 2 orders of magnitude, while those of Eu
are decreased to a much lower extent. In the case of 1, for
instance, the Am concentration in the organic phase is reduced
from 94% to 14%, while the Eu concentration only from 99% to
96%.
All the ligands 1−3 show an impressive selectivity for Am since
the Eu over Am separation factors (SFEu/Am) increase from 7, the
typical value for TODGA observed in the blank tests, to 144 for
ligand 1, 100 for 2, and 57 for 3 (Table 1). This indicates that a
clear separation between Eu, bound to TODGA in the organic
phase, and Am, better coordinated to PyTri ligands in the
B
DOI: 10.1021/jacs.6b03106
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Communication
aqueous layer than to TODGA in the organic phase, is taking
place. The observed Am/La selectivity is consistently lower than
the Am/Eu selectivity and results from a known inability of
TODGA to tightly bind lighter Ln ions rather than from an
intrinsic affinity of PyTri ligands for La(III) ion. The DLa values,
in fact, remain practically constant in the absence or presence of
the hydrophilic ligands. However, the Am/La selectivity
displayed is certainly high enough to implement an effective
separation process. The same ligands were also tested in the
stripping conditions simulating the GANEX process (Figure 1
Although the annual doses to which extractants are exposed may
change consistently depending on fuel type and burn-up, a value
of 100 kGy is often considered as a realistic average annual
absorbed dose.16 For these reasons stripping experiments were
carried out with 1 and 2 solutions irradiated up to 200 kGy with a
60Co source (2.5 kGy/h) and fresh TODGA-based organic
phases loaded from i-SANEX feed (for details see SI).
For ligand 1 the distribution coefficients (Figure 2) and the
SFEu/Am values remain almost constant in the whole absorbed
Figure 2. Distribution coefficients for ligand 1 stripping solutions as a
function of the absorbed dose. Organic phase: 0.2 M TODGA in
kerosene/1-octanol (95/5 v/v) loaded with i-SANEX feed. Aqueous
phase: irradiated 0.08 M of ligand 1, [HNO3] = 0.44 M (2.5 kGy/h).
Figure 1. DM values for the stripping of Am3+, Cm3+, and Pu4+ from a
TODGA-based organic phase into a PyTri-based aqueous solution, in
comparison with the blank experiment.
and Table S2). The data nicely point out the ability of these
ligands to coextract An in different oxidation states, even if with
different extraction efficiencies. Remarkably, these preliminary
tests outlined that ligands 1 and 2 have a quite similar behavior
with DAm(III), DCm(III), and DPu(IV) close to 0.1, while DEu is around
14. Both the ligands showed a SFEu/Am around 240 and a
percentage of Am recovery close to 95% in one step. Ligand 2
seems to be slightly more selective for Pu (SFEu/Pu = 280)
compared to ligand 1 (SFEu/Pu = 180). Despite of its higher water
solubility, ligand 3 always shows lower selectivity for An both in i-
SANEX and GANEX conditions, indicating that the presence of
the hard amide carbonyl groups might interfere with the
separation process (Table S3). For this reason, further studies
were limited to ligands 1 and 2.
An important aspect to be considered in nuclear reprocessing
involving high radiation fields is the kinetic issue. The cation
complexation equilibria should take place with fast kinetics in
order to ensure that extraction reaches the equilibrium within the
very short residence time of the phases in the centrifugal
contactors, devices typically used in industrial applications. For
this aim, the organic phases loaded with an i-SANEX feed and the
PyTri-based stripping phases were shaken with a benchtop
shaker for different time periods ranging from 5 to 60 min. The
DM values of both ligands 1 and 2 reach their maximum within 5
min indicating that the equilibrium is achieved very fast and that
there are no kinetics limitations (see SI). A further demanding
requirement to be satisfied for a successful ligand is its chemical
stability under the harsh process conditions. Ligands in fact
should be hydrolytically stable at the low pH conditions and at
the high temperatures arising, as well as radiolytically resistant
under the intense radiation fields, mainly due to the α-emitters
present in the radioactive solutions to be decontaminated.
Aggressive hydroxyl radicals are formed under these conditions
so that a strong and radical resistant organic structure is required.
dose range considered, consistently with a high radiolytic stability
of this ligand. A slight decrease of DM values was observed for
both Am and Eu in the case of ligand 2 for absorbed doses higher
than 100 kGy (Figure S10). The radiolytic stability of 1 solutions
was further studied by means of several analytical techniques.
HPLC-DAD analyses showed that the ligand concentration
decreases by 4% and 9% in samples irradiated at 100 kGy (2.5
and 0.14 kGy/h) and at 200 kGy (2.5 kGy/h), respectively, as
also confirmed by direct UV−vis measurements at λ = 300 nm.
Possible structures of the main radiolytic byproducts observed in
the HPLC chromatograms (see SI) were proposed on the basis
of ESI-MS and tandem MS analyses. The proposed degradation
paths entail with the oxidation of the alkyl lateral chains by
reaction with radical species (presumably OH radicals) coming
from diluent radiolysis, while the chelating PyTri structure seems
to be unaffected, in complete agreement with the retention of the
extracting capability after irradiation. In view of using such
stripping solutions in multicycle processes, a viable recycling step
was devised. 241Am and 152Eu spiked batch experiments showed
that the cations complexed by PyTri ligands could be removed by
re-extraction with a fresh TODGA-based solution in the amount
of 97% for Am and 100% for Eu in a single stage (see SI). In order
to shed a light on the reason for the observed selectivity, the
stoichiometry and stability of the metal−ligand complexes were
studied. Higher stoichiometry was observed for Am−1
complexes than for Eu−1 complexes, as shown by liquid−liquid
extraction tests at different ligand concentrations (see Figure
S12) as well as by ESI-MS experiments in homogeneous
solutions (see SI, Section 2.7). Stability constants for the
formation of Am(III), Pu(IV), and Eu(III) complexes with
ligands 1 and 2 were determined by UV−vis titrations. The
spectra were fitted with the 1:1 (and 1:2) complexes giving the
apparent logβ′1:1 (and logβ′1:2) values (Table 2). For ligand 1 the
stability constant for Pu(IV) is 2 orders of magnitude higher than
C
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Journal of the American Chemical Society
Communication
Table 2. Apparent Stability Constants (logβ′metal:ligand) for
Am(III), Pu(IV), and Eu(III) Complexes with Ligands 1 and 2
Obtained by UV−vis Titration in Methanol/Water (75/25 v/
v) at 25 °C
REFERENCES
■
(1) (a) Magill, J.; Berthou, V.; Haas, D.; Galy, J.; Schenkel, R.; Wiese,
H. W.; Heusener, G.; Tommasi, J.; Youinou, G. Nucl. Energy 2003, 42,
263−277. (b) Baisden, P. A.; Choppin, G. R. Nuclear Waste
Management and the Nuclear Fuel Cycle. In Radiochemistry and Nuclear
Chemistry; Nagyl, S., Ed.; EOLSS Publishers: Oxford, U.K., 2007
(c) Reprocessing and Recycling of Spent Nuclear Fuel; Woodhead
Publishing: Oxford, 2015.
(2) Brook, B. W.; Alonso, A.; Meneley, D. A.; Misak, J.; Blees, T.; van
Erp, J. B. Sust. Mater. Techn. 2014, 1−2, 8−16.
(3) Lake, J. A.; Bennett, R. G.; Kotek, J. F. Sci. Am. 2002, 286, 72−81.
(4) Salvatores, M.; Palmiotti, G. Prog. Part. Nucl. Phys. 2011, 66, 144−
166.
(5) (a) Nash, K. L.; Madic, C.; Mathur, J. N.; Lacquement, J. In The
Chemistry of the Actinide and Transactinide Elements; Morss, L. R.,
Edelstein, N. M., Fuger, J., Eds.; Springer: Dordrecht, The Netherlands,
2006; Vol. 4, Chapter 24, pp 2622−2798. (b) Nash, K. L.; Lumetta, G. J.;
Clark, S. B.; Friese, J. I. Significance of the Nuclear Fuel Cycle in the 21st
Century. In Separations for the Nuclear Fuel Cycle in the 21st Century;
ACS Symposium Series;; Lumetta, G. J., Nash, K. L., Clark, S. B., Friese,
J. I., Eds.; ACS:Washington, DC, 2006; Vol. 933, pp 3−20.
(6) Science and Technology of Tributyl Phosphate; CRC Press Inc.: Boca
Raton, FL, 1990; Vol. III.
(7) (a) 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, 3427−3435. (b) Modolo, G.; Geist, A.; Miguirditchian, M. Minor
Actinide Separations in the Reprocessing of Spent Nuclear Fuels:
Recent Advances in Europe. In Reprocessing and Recycling of Spent
Nuclear Fuel; Woodhead Publishing: Oxford, 2015; pp 245−287.
(8) (a) Modolo, G.; Vijgen, H.; Serrano-Purroy, D.; Christiansen, B.;
Malmbeck, R.; Sorel, C.; Baron, P. Sep. Sci. Technol. 2007, 42, 439−452.
(b) Ansari, S. A.; Pathak, P.; Mohapatra, P. K.; Manchanda, V. K. Chem.
Rev. 2012, 112, 1751−1772. (c) Galan, H.; Zarzana, C. A.; Wilden, A.;
Nunez, A.; Schmidt, H.; Egberink, R. J. M.; Leoncini, A.; Cobos, J.;
Verboom, W.; Modolo, G.; Groenewold, G. S.; Mincher, B. J. Dalton
Trans. 2015, 44, 18049−18056.
(9) Madic, C.; Hudson, M. J.; Liljenzin, J. O.; Glatz, J. P.; Nannicini, R.;
Facchini, A.; Kolarik, Z.; Odoj, R. Prog. Nucl. Energy 2002, 40, 523−526.
(10) Hudson, M. J.; Harwood, L. M.; Laventine, D. M.; Lewis, F. W.
Inorg. Chem. 2013, 52, 3414−3428.
(11) Panak, P. J.; Geist, A. Chem. Rev. 2013, 113, 1199−1236.
(12) Geist, A.; Mullich, U.; Magnusson, D.; Kaden, P.; Modolo, G.;
Wilden, A.; Zevaco, T. Solvent Extr. Ion Exch. 2012, 30, 433−444.
(13) Kolarik, Z. Chem. Rev. 2008, 108, 4208−4252.
(14) (a) Byrne, J. P.; Kitchen, J. A.; Gunnlaugsson, T. Chem. Soc. Rev.
2014, 43, 5302−5325. (b) Li, Y.; Huffman, J. C.; Flood, A. H. Chem.
Commun. 2007, 2692−2694. (c) Kiefer, C.; Wagner, A.; Beele, B. B.;
Geist, A.; Panak, P. J.; Roesky, P. W. Inorg. Chem. 2015, 54, 7301−7308.
(15) Scaravaggi, S.; Macerata, E.; Galletta, M.; Mossini, E.; Casnati, A.;
Anselmi, M.; Sansone, F.; Mariani, M. J. Radioanal. Nucl. Chem. 2015,
303, 1811−1820.
(16) (a) Fermvik, A.; Berthon, L.; Ekberg, C.; Englund, S.; Retegan, T.;
Zorz, N. Dalton Trans. 2009, 6421−6430. (b) Magnusson, D.;
Christiansen, B.; Malmbeck, R.; Glatz, J. Radiochim. Acta 2009, 97,
497−502. (c) Mincher, B. J.; Modolo, G.; Mezyk, S. P. Solvent Extr. Ion
Exch. 2009, 27, 1−25. (d) Fermvik, A. J. Radioanal. Nucl. Chem. 2011,
289, 811−817.
metal ion
ligand
Logβ′1:1
Logβ′1:2
counterion
Cl−
a
Am(III)
1
1
2
1
1
2
3.2 0.3
3.5 0.3
3.1 0.3
4.3 0.3
2.4 0.1
3.0 0.1
6.2 0.3
6.8 0.3
5.5 0.3
b
−
Am(III)
NO3
a
Am(III)
Cl−
c
−
Pu(IV)
NO3
Eu(III)
Cl−
Cl−
Eu(III)
a
b
pH = 4. In 0.44 M HNO3; the Am logβ′ values take into account the
c
ligand protonation (pKa = 2.1). With addition of HNO3 (∼0.6 M to
avoid the hydrolysis of Pu); the Pu logβ′1:1 values take into account
the ligand protonation (pKa = 2.1).
that of Eu(III). The higher affinity of ligands 1 and 2 for Am
compared to Eu also results in the formation of 1:2 complexes as
also supported by ESI-MS data (Figure SI13) and stripping tests
with high cL/cM ratios (Figure S12). Interestingly, the preference
of ligands 1 and 2 for An over Ln is consistent with the selectivity
for An found in extraction tests.
In summary, we have demonstrated the possibility to
successfully use hydrophilic PyTri ligands for the selective
stripping of Actinide ions having different oxidation states into a
water solution. The competitive TODGA-PyTri system used
takes advantage not only of a lower affinity of TODGA for
actinides but also of a higher affinity of the clicked triazole
derivatives for Am(III) and Pu(IV), as demonstrated by the
stability constants in homogeneous methanol/water solutions.
We believe that this extracting system, when applied to the
treatment of radioactive waste, will lead to paramount break-
through in the development of advanced separation processes
toward the closure of the nuclear fuel cycle.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b03106.
■
S
Synthetic procedures and spectroscopic data, experimen-
tal details of the extraction experiments, complexation
studies, radiolytic stability tests, and stripping phase
recycling (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*elena.macerata@polimi.it
*casnati@unipr.it
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by EU-FP7 ACSEPT (Grant no.
211267) and SACSESS (Grant no. 323282) projects. We thank
the Centro Interdipartimentale Misure “G. Casnati” of Parma
University for the NMR and MS facilities. We also thank
ACTINET and TALISMAN projects for funding Stefano
Scaravaggi (five months) and Eros Mossini (three months) for
abroad research periods in CEA Marcoule.
D
DOI: 10.1021/jacs.6b03106
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX