Utilizing electronic effects in the modulation of BTPhen ligands with respect to the partitioning of minor actinides from lanthanides

Article abstract of DOI:10.1039/c3cc45126g

Effects of bromine substitution at the 5 and 5,6-positions of the 1,10-phenanthroline nucleus of BTPhen ligand on their extraction properties for Ln(iii) and An(iii) cations have been studied. Compared to C5-BTPhen, electronic modulation in BrC5-BTPhen and Br₂C5-BTPhen enabled these ligands to be fine-tuned in order to enhance the separation selectivity of Am(iii) from Eu(iii). The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc45126g

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Utilizing electronic effects in the modulation of
BTPhen ligands with respect to the partitioning of
minor actinides from lanthanides†
Cite this: Chem. Commun., 2013,
49, 8534
Received 8th July 2013,
Ashfaq Afsar,a Dominic M. Laventine,a Laurence M. Harwood,*a Michael J. Hudsona
and Andreas Geistb
Accepted 7th August 2013
DOI: 10.1039/c3cc45126g
www.rsc.org/chemcomm
Effects of bromine substitution at the 5 and 5,6-positions of the
1,10-phenanthroline nucleus of BTPhen ligand on their extraction
properties for Ln(III) and An(III) cations have been studied. Compared to
C5-BTPhen, electronic modulation in BrC5-BTPhen and Br2C5-BTPhen
enabled these ligands to be fine-tuned in order to enhance the
separation selectivity of Am(III) from Eu(III).
The spent nuclear fuel produced by a typical light water reactor
consists mainly (>98.5%) of uranium and short-lived fission products
which do not pose a long term hazard.1–3 However, approximately
1 wt% of the spent fuel is composed of plutonium and minor
actinides (Am, Cm, Np), which are highly radiotoxic.1–3 The PUREX
(Plutonium and URanium EXtraction) process is used to remove Pu
and U from spent fuel, so that it can be reused as mixed oxide fuel.4,5
Fig. 1 Structures of CyMe4–BTP, CyMe4–BTBP, CyMe4–BTPhen and C5-BTPhen.
During the development of BTPhens, the aliphatic side
However, the remaining high level liquid waste (PUREX raffinate) chains attached to the traizine units of the ligand were varied
still contains the minor actinides that account for the major to increase the solubility in the organic phase or provide a better
long-term radiotoxicity of used nuclear fuel despite comprising resistance towards radiolysis. One such BTPhen ligand was tetra
less than 0.1% by weight.
pentyl-BTPhen (Fig. 1), denoted henceforth as C5-BTPhen.13
The work described in this communication primarily focuses on
Separation of the minor actinides from the lanthanides and
other fission products is a key step in the partitioning and trans- the extraction abilities of this ligand and investigates the influence
mutation scenario for reprocessing of used nuclear fuel, and has of subtle electronic effects upon ligand selectivity for the minor
been achieved using nitrogen-bearing ligands; for instance the 2,6- actinides over the lanthanides. Although both the lanthanides and
di(1,2,4-triazine-3-yl)pyridines (BTPs)6 and 6,60-bis(1,2,4-triazin-3-yl)- actinides are f-block elements, the 4f orbitals in the lanthanides do
2,20-bipyridines (BTBPs)7–9 (see Fig. 1). In a recent development, the not extend past the 5d orbitals, whereas the 5f orbitals in actinides
2,20-bipyridine moiety was replaced by a 1,10-phenanthroline moiety do extend further than the 6d orbitals.14,15 This means that,
(BTPhen).1–3,10,11 The effect of this new modification on the ligand although possessing very similar properties, Ln(III)–ligand inter-
extraction properties was startling. Extractions with solutions of actions are considered to be more ionic and less covalent than
CyMe4–BTPhen (DAm r 1000) were about 2 orders of magnitude An(III)–ligand interactions.14,15 Thus it is proposed that ‘‘softer’’
more efficient than those of CyMe4–BTBP (DAm r 10), and the ligands bind more effectively to actinides.14,15
resulting SF were in the range 68–400. More importantly, the
The 5,6-double bond in phenanthroline can be readily
kinetics of extraction were significantly faster, such that full functionalized and we decided to explore the electronic effects
equilibrium was reached within 15 min without the need for of substituents at this position on the ligating selectivity in the
addition of a phase transfer agent.1,10,12
BTPhens. 5-Bromo-2,9-dimethyl-1,10-phenonthroline 2 was obtained
following the literature procedure (Scheme 1).16 In addition,
when excess bromine was used, bromination of 2,9-dimethyl-
1,10-phenanthroline 1 yielded the novel 5,6-dibromo-2,9-dimethyl-
1,10-phenanthroline 3.
a School of Chemistry, University of Reading, Whiteknights, Reading,
Berkshire RG6 6AD, UK. E-mail: l.m.harwood@reading.ac.uk
b Institute for Nuclear Waste Disposal (INE), Karlsruhe Institute of Technology
(KIT), 76021 Karlsruhe, Germany
The corresponding BTPhen ligands 7b and 7c were synthesized
from 2 and 3 as previously described1,13 with the modification that
† Electronic supplementary information (ESI) available: Experimental details and
characterization of compounds. See DOI: 10.1039/c3cc45126g
c
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Scheme 1 Bromination of 2,9-dimethyl-1,10-phenanthroline 1.
Fig. 3 Graph showing C5-BTPhen 7a composition at increasing La equivalents.
Scheme 2 Synthesis of C5-BTPhen ligands.
the final step was performed with dodecane-6,7-dione17 in
dioxane at reflux, rather than in THF at reflux (Scheme 2).
NMR spectroscopic titrations of ligands with diamagnetic
metal salts were used to determine the stoichiometries of the
complexes in solution.18–21 NMR titrations of the C5-BTPhen 7a
with a number of trivalent lanthanide (lanthanum and lutetium)
salts were thus performed in deuterated acetonitrile (CD3CN).
La(III) (ionic radius = 116 pm)15 and Lu(III) (ionic radius =
98 pm)15 were chosen to give the greatest difference in of the
Ln(III) radius. The results of the titrations indicated that the
ligand initially formed 2 : 1 ligand–metal complexes in solution,
since conversion to a new species was complete after addition
of 0.5 equivalents of metal salt for all lanthanides tested.
Subsequent formation of 1 : 1 complexes was observed upon
addition of excess metal salt (Fig. 2–5), although in all cases,
the majority of the BTPhen remained bound in 2 : 1 complexes
even in the presence of 3 equivalents of Ln(III).
Fig. 4 Stacked 1H NMR spectra (8.0–9.3 ppm) of C5-BTPhen 7a (0.01 M) titrated
with Lu(NO3)3 in CD3CN.
Fig. 5 Graph showing C5-BTPhen 7a composition at increasing Lu equivalents.
In the case of the lanthanum complexes, formation of the
1 : 1 complex occurred to a greater degree for a given metal
concentration, compared to the corresponding lutetium
complex. These results suggest that the relative stabilities of
2 : 1 complexes compared to the 1 : 1 complexes are greater for
lutetium than for the lanthanum complexes. It has been
postulated that the 2 : 1 complex is the extracting species owing
to the metal centre being completely enclosed by the hydrophobic
shell formed by the two ligands.
Ligands 7a, 7b and 7c were also evaluated for their ability to
extract Am(III) selectively in the presence of Eu(III) from aqueous
nitric acid solutions into 1-octanol. The distribution factor for
Am(III) and Eu(III) (DAm and DEu) and the separation factor for
americium over europium (SFAm/Eu) for 7a in 1-octanol as a
Fig. 2 Stacked 1H NMR spectra (8.0–9.3 ppm) of C5-BTPhen 7a (0.01 M) titrated
with La(NO3)3ꢀ6H2O in CD3CN.
c
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Am(III) was observed (DAm = 100) (Fig. 8) to those found in 7a and
7b but, the D value for Eu(III) was approximately one order of
magnitude lower than that of 7a, and the resulting separation
factor (SFAm/Eu = ca. 800 at 4 M HNO3) was superior to those
observed for 7a and 7b.
In conclusion, we have reported the synthesis, lanthanide specia-
tion and Am(III)–Eu(III) solvent extraction properties of C5-BTPhen
ligands with bromine substitution at the 5 or 5,6-positions of the
1,10-phenanthroline moiety. The presence of bromine did not affect
the capacity of the ligands to extract Am(III) but decreased the
extraction of Eu(III) from 4 M HNO3 with the result that the SFAm/Eu
ratios were substantially greater than is the case with C5-BTPhen.
The Br atoms inductively withdraw electron density from the rings
and we conclude that this reduces the electron donating capacity of
the phenanthroline nitrogens making the ligand less effective for
complexing with lanthanides.
Fig. 6 Extraction of Am(III) and Eu(III) by C5-BTPhen 7a in 1-octanol as a function
of nitric acid concentration.
The authors would like to acknowledge the EPSRC, MBase
and Actinet-I3 for financial support. Use of the Chemical
Analysis Facility (CAF) at the University of Reading is gratefully
acknowledged.
Notes and references
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by 7c in 1-octanol were subsequently investigated and compared
with the corresponding data for 7a and 7b. A similar D value for
c
8536 Chem. Commun., 2013, 49, 8534--8536
This journal is The Royal Society of Chemistry 2013