exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

global electron number of the redox process is increased
because of either the regeneration of the
initial uranyl species (DISP) or the reduction of a
second species formed by the chemical reaction
(ECE).
Fig. 2: cyclic voltammograms of 0.01 M [UO 2 Cl 4 ] 2-
with x eq. Cl - in [BuMeIm][Tf 2 N] at 25°C at a GC
electrode at 25 mV.s -1 .
Effect of the IL cation: redox behavior of tetrahalouranyl
complex in [MeBu 3 N][Tf 2 N]
Fig. 3 presents the voltammograms of the tetrahalo
uranyl complexes [UO 2 X 4 ] 2- with 10 eq. of halide
ions in both ILs. In [MeBu 3 N][Tf 2 N], the reduction
peaks of [UO 2 Cl 4 ] 2– and [UO 2 Br 4 ] 2– are observed
respectively at -1.74 V and -1.56 V. In both ILs, the
reduction potential of the bromo complex is higher
than for the chloro analog. This indicates that
[UO 2 Br 4 ] 2– is less stable than [UO 2 Cl 4 ] 2– . We can
also observe for each complex that the reduction
potential is more than 300 mV less negative in
[MeBu 3 N][Tf 2 N] than in [BuMeIm][Tf 2 N]. This can
be account to a difference of solvation as we have
already demonstrated that [BuMeIm] + interacts
more strongly than [MeBu 3 N] + by H-bonding. We
have also shown that the intermediate species
[U V O 2 X 4 ] 3– is less stable in [MeBu 3 N][Tf 2 N] than in
[BuMeIm][Tf 2 N].
0 eq Cl - 5 eq Cl - 10 eq Cl -
0.025 V.s -1 - 0.21 0.52
0.05 V.s -1 - 0.32 0.63
0.1 V.s -1 0.14 0.37 0.71
0.2 V.s -1 0.20 0.42 0.74
0.5 V.s -1 0.27 0.51 0.76
1 V.s -1 0.33 0.54 0.82
2 V.s -1 0.35 0.62 0.84
Table 1: I pa1 /I pc1 measured for [UO 2 Cl 4 ] 2- (0.01M)
with 0, 5 and 10 eq of Cl – in [BuMeIm][Tf 2 N] as a
function of the potential scan rate at 25°C.
With a large excess of Cl - , the chemical reaction is
no more observed by voltammetry. Our measurements
have shown that in these conditions,
[U VI O 2 Cl 4 ] 2- is reduced via a quasireversible
monoelectronic transfer to form [U V O 2 Cl 4 ] 3- .
Effect of the uranyl ligand: electrochemical behavior
of [UO 2 Br 4 ] 2– in [BuMeIm][Tf 2 N]
If the switching cathodic potential is -2.7 V, the cyclic
voltammogram presents a single reduction
peak at -1.24 V and two oxidation peaks at -0.3
and 0.3 V, this latter corresponding to a redissolution
peak. Then, the reduction of the tetrabromouranyl
complex leads also to the formation of a
deposit at the electrode surface. In the potential
region [-1.6 V ; -0.1 V], when Br - are added to the
solution, a new anodic peak appears around -1.18
V. When the Br - concentration increases, the intensity
of this new peak increases and the intensity of
the anodic peaks at higher potentials decreases.
Moreover, at a done Br – concentration, the ratio I pa
(-1.18 V)/I pc (-1.24 V) increases with the potential
scan rate. Nevertheless, for the same halide
amount and the same potential scan rate, this ratio
is lower in the case of the bromo complex than for
its chloro analog. This suggests that the intermediate
species [U V O 2 Br 4 ] 3- is less stable than
[U V O 2 Cl 4 ] 3- .
Fig. 3: Voltammograms of 0.01 M [UO 2 Cl 4 ] 2– with
10 eq of Cl - (red curves) and 0.01 M [UO 2 Br 4 ] 2–
with 10 eq of Br - in [BuMeIm][Tf 2 N] at 25°C (solid
curves) and in [MeBu 3 N][Tf 2 N] at 60°C (dashed
curves) at a GC electrode (0.07 cm 2 ) at 0.1 V.s -1 .
Controlled potential electrolysis
Quantitative electrolysis of the tetrahalouranyl
complexes in both ionic liquids give a deposit at a
platinum cathode. XRD analysis suggests that this
solid phase is composed of amorphous oxide or
hydroxide uranium(IV). Moreover, the voltammetric
analysis of the solution at the end of the electrolysis
indicates that 4 equivalents of halide ions are
produced per eq. of reduced uranyl complex.
Suggested mechanism
The above results allowed us to propose the two
following mechanisms for the reduction of the tetrahalouranyl
complexes in ILs.
DISP mechanism:
[U VI O 2 X 4 ] 2- + e - [U V O 2 X 4 ] 3-
2 [U V O 2 X 4 ] 3- [U IV ] + [U VI O 2 X 4 ] 2- + 4 X -
ECE mechanism:
[U VI O 2 X 4 ] 2- + e - [U V O 2 X 4 ] 3-
[U V O 2 X 4 ] 3- [U V O 2 X (4-x) ] x-3 + x X -
[U V O 2 X (4-x) ] x-3 + e - [U IV ] + (4-x) X -
In both mechanisms, [U IV ] corresponds to oxide or
hydroxide species.
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