Experimental and theoretical approaches to redox innocence of ligands in uranyl complexes: What is formal oxidation state of uranium in reductant of uranyl(VI)?

Article abstract of DOI:10.1021/ic5006314

Redox behavior of [UO₂(gha)DMSO]^-/UO ₂(gha)DMSO couple (gha = glyoxal bis(2-hydroxanil)ate, DMSO = dimethyl sulfoxide) in DMSO solution was investigated by cyclic voltammetry and UV-vis-NIR spectroelectrochemical technique, as well as density functional theory (DFT) calculations. [UO₂(gha)DMSO]^- was found to be formed via one-electron reduction of UO₂(gha)DMSO without any successive reactions. The observed absorption spectrum of [UO ₂(gha)DMSO]^-, however, has clearly different characteristics from those of uranyl(V) complexes reported so far. Detailed analysis of molecular orbitals and spin density of the redox couple showed that the gha^2- ligand in UO₂(gha)DMSO is reduced to gha ^?3- to give [UO₂(gha)DMSO]^- and the formal oxidation state of U remains unchanged from +6. In contrast, the additional DFT calculations confirmed that the redox reaction certainly occurs at the U center in other uranyl(V/VI) redox couples we found previously. The noninnocence of the Schiff base ligand in the [UO₂(gha)DMSO]^-/UO ₂(gha)DMSO redox couple is due to the lower energy level of LUMO in this ligand relative to those of U 5f orbitals. This is the first example of the noninnocent ligand system in the coordination chemistry of uranyl(VI).

Full text of DOI:10.1021/ic5006314

Article
pubs.acs.org/IC
Experimental and Theoretical Approaches to Redox Innocence of
Ligands in Uranyl Complexes: What Is Formal Oxidation State of
Uranium in Reductant of Uranyl(VI)?
Koichiro Takao,*,† Satoru Tsushima,‡ Toshinari Ogura,† Taro Tsubomura,§ and Yasuhisa Ikeda†
†Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, 152-8550 Tokyo, Japan
̅
‡Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, P.O. Box 51 01 19, 01314 Dresden, Germany
§Department of Materials and Life Science, Seikei University, 3-3-1 Kichijoji-kitamachi, Musashino-shi, 180-8633 Tokyo, Japan
S
* Supporting Information
ABSTRACT: Redox behavior of [UO2(gha)DMSO]−/
UO2(gha)DMSO couple (gha = glyoxal bis(2-hydroxanil)ate,
DMSO = dimethyl sulfoxide) in DMSO solution was
investigated by cyclic voltammetry and UV−vis−NIR
spectroelectrochemical technique, as well as density functional
theory (DFT) calculations. [UO2(gha)DMSO]− was found to
be formed via one-electron reduction of UO2(gha)DMSO
without any successive reactions. The observed absorption
spectrum of [UO2(gha)DMSO]−, however, has clearly differ-
ent characteristics from those of uranyl(V) complexes reported so far. Detailed analysis of molecular orbitals and spin density of
the redox couple showed that the gha2− ligand in UO2(gha)DMSO is reduced to gha•3− to give [UO2(gha)DMSO]− and the
formal oxidation state of U remains unchanged from +6. In contrast, the additional DFT calculations confirmed that the redox
reaction certainly occurs at the U center in other uranyl(V/VI) redox couples we found previously. The noninnocence of the Schiff
base ligand in the [UO2(gha)DMSO]−/UO2(gha)DMSO redox couple is due to the lower energy level of LUMO in this ligand
relative to those of U 5f orbitals. This is the first example of the noninnocent ligand system in the coordination chemistry of
uranyl(VI).
([UVO2(salophen)DMSO]−, Chart 1),6 the coordination
1. INTRODUCTION
chemistry of uranyl(V) has been explored in the past
It is well-known that uranium, neptunium, and plutonium in
decade.7−11 As a result, various aspects including photophysical
the early actinide series show rich redox chemistry in contrast
properties, molecular structures, and reactivity of uranyl(V)
to the lanthanide elements. Their oxidation state may vary from
III to VI (or even II and VII under some specific conditions).1
complexes were clarified. Most works started from uranyl(VI)
compounds as parent species to prepare uranyl(V) through
The variety of the oxidation numbers facilitates mutual
electrochemical or chemical one-electron reduction. All the
separation of these actinide elements, and their recovery from
a mixture with other elements like fission products.2 This is the
most fundamental and important basis of reprocessing methods
former works thought that ligands surrounding U are redox-
inactive or innocent as designated by C. K. Jørgensen.12 This
could be reasonable in a traditional sense, because the
for spent nuclear fuels. The rich redox chemistry of these
nonbonding 5fδu and 5fϕu orbitals are lying at the lowest
actinides and related coordination behavior are also highly
energy among vacant orbitals,13 and therefore the electron
relevant for understanding of their environmental mobility in
geological disposal of radioactive wastes.3
injected into a uranyl(VI) species should be placed to one of
them. In contrast, unoccupied molecular orbitals arising from
At V and VI oxidation states, U, Np, and Pu generally form
actinyl ions, [MO2]n+ (n = 1, 2), in solutions.1 These species
ligands should have antibonding character, which gives them
higher energy than the nonbonding ones like 5fδu and 5fϕu.
Therefore, ligands just play a spectator role in the redox
reactions.
Recently, it was clarified for various transition metal
complexes of which ligands are not always innocent toward
the redox reaction, but are sometimes also redox-active, namely,
noninnocent. The latest findings and its attractive aspects in
exhibit unique coordination behavior arising from their linear
[OMO]2+ geometries, that is, additional ligands can
interact with the metal centers only in the equatorial planes
of the actinyl ions. The uranyl(V) is usually very unstable
because of its sensitivity toward oxidation and disproportiona-
tion.1,4 Therefore, its chemistry had been rather exotic and not
well understood previously. After the unexpected isolation of
tetrakis(triphenylphosphine oxide)dioxouranium(V) triflate by
Berthet et al.5 and our finding of a uranyl(V) complex stabilized
Received: March 19, 2014
by an auxiliary tetradentate Schiff base ligand
Published: May 21, 2014
© 2014 American Chemical Society
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reaction mixture. To this solution was added dropwise
UO2(DMSO)5(ClO4)2 (0.158 g) dissolved in hot methanol (2 mL),
followed by vigorous stirring for 3 h at room temperature in the dark.
The supernatant was removed by syringe through a septum cap, and
the residue was washed with the degassed methanol (3 × 3 mL). The
deep purple residue in the flask was dried under suction at room
temperature. Yield: 0.070 g (68%). Characterization for this
compound has been performed by using 1H NMR and IR
spectroscopy. 1H NMR (400 MHz, DMSO-d6, δ/ppm vs TMS):
9.45 (s, 2H, −NCH−), 7.95 (dd, 2H, Ph−H), 7.51 (td, 2H, Ph−H),
6.9−6.8 (m, 4H, Ph−H), 2.58 (s, 6H, OS(CH3)2). IR (KBr pellet,
ν/cm−1): 950 (OUO asymmetric stretching), 995 (SO
stretching), 1584 (CN stretching).
Chart 1. Schematic Structures of Uranyl(VI) Complexes
2.3. Methods. Cyclic voltammetry (CV) measurements were
performed at 298 K under a dry argon atmosphere using BAS
ALS660B. A three-electrode system consisted of a Pt disk working
electrode (electrode surface area: 0.020 cm2), a Pt wire counter
electrode, and an Ag/Ag+ reference electrode (BAS RE-5B, 0.1 M
TBAClO4 + 0.01 M AgNO3/CH3CN). A ferrocene/ferrocenium ion
redox couple (Fc/Fc+) was taken as the internal reference redox
system.20 Dissolved oxygen gas in the sample solutions was removed
by passing argon gas through for at least 20 min prior to starting
experiments. To compensate the iR drops, the cyclic voltammograms
of the blank solution (0.1 M TBAPF6/DMSO) were also recorded
under the same potential sweep rate and potential range, and
subtracted from those of UO2(gha)DMSO in DMSO.
catalytic chemistry are summarized in a Forum of Inorganic
Chemistry14 and several comprehensive reviews.15 In this
manner, metal and ligand(s) can synergistically cooperate,
and ligands can also play a much more prominent role in the
elementary bond activation steps in a catalytic cycle. The
noninnocence of ligands would be found also in the coordination
chemistry of the actinides. However, such an issue has not been
dealt with in detail so far, while there are only a few discussions
on low-valent organoactinides16 as well as on octavalent
plutonium.17 This situation should arise from the fact that
experimental evidence indicating the noninnocent behavior of
ligands in an actinide complex has been poorly obtained.
In this article, we present the first example of a redox
noninnocent ligand, glyoxal bis(2-hydroxanil)ate (gha2−), in its
uranyl complex, UO2(gha)DMSO (Chart 1). This chemistry
was studied by both experimental and theoretical approaches
using electro- and spectroelectrochemical techniques and
quantum chemical (DFT) calculations. Other uranyl(V/VI)
redox couples we found formerly (Chart 1)6,7 were also
investigated by the same computational method to confirm that
(1) their ligands are really redox innocent and (2) the formal
oxidation states of U in the reductants are really +5.
UV−vis spectroelectrochemical measurements for UO2(gha)DMSO
in DMSO were performed with a SHIMADZU UV-3150 spectropho-
tometer equipped with an optically transparent thin layer electrode
(OTTLE) cell.21 Its optical path length was 2.1 × 10−2 cm. The three-
electrode system was the same as that in the CV experiment with a
replacement of the working electrode by a Pt gauze (80 mesh). The
potential on OTTLE was controlled by BAS ALS660B. The
absorption spectrum at each potential was recorded after equilibrium
of the electrochemical reaction, which completed within 2 min. The
sample solution in the OTTLE cell was deoxygenated by passing dry
argon gas through at least 20 min prior to starting the experiment.
From a CH2Cl2/ethanol solution dissolving UO2(gha)DMSO,
purple-black crystals of a desolvated dinuclear species, [UO2(gha)]2,
deposited. A single crystal X-ray diffraction experiment for this
compound was performed using the following procedure. The single
crystal was mounted in a cryo-loop together with paraffin oil and
placed in a low-temperature nitrogen gas stream at 213 K. Intensity
data were collected using an imaging plate area detector in a Rigaku
RAXIS RAPID diffractometer with graphite-monochromated Mo Kα
radiation (λ = 0.71075 Å). The structure of [UO2(gha)]2 was solved
by the SIR92 direct method22 and expanded using Fourier techniques.
All non-hydrogen atoms were anisotropically refined using SHELXL-
97.23 Each hydrogen atom included in the structure was refined as
riding on its parent carbon atom with Uiso(H) = 1.2Ueq(C). The final
cycle of full-matrix least-squares refinement on F2 was based on
observed reflections and parameters, and converged with unweighted
and weighted agreement factors, R and wR. All computations were
performed using the CrystalStructure crystallographic software
package.24 Crystallographic data for [UO2(gha)]2: C28H20N4O8U;
Mw = 1016.55; 0.456 × 0.432 × 0.400 mm3; orthorhombic; Pbca (61);
a = 14.4373(5), b = 12.5919(4), c = 14.7778(5) Å; V = 2686.5(2) Å3;
Z = 4; ρcalcd = 2.513 Mg·m−3; μ = 12.105 mm−1; T = 213 K; 24726
2. EXPERIMENTAL AND COMPUTATIONAL DETAILS
2.1. Materials. Pentakis(dimethyl sulfoxide)dioxouranium(VI)
perchlorate, [UO2(DMSO)5](ClO4)2, was prepared by replacing
N,N-dimethylacetamide (DMA) with dimethyl sulfoxide (DMSO) in
a procedure to obtain the similar DMA solvate reported by Bowen et
al.18 The DMSO solvent (Kanto Chemical Co., Ind.) used in the
electro- and spectroelectrochemical experiments was dried over 4A
molecular sieves (Wako). Tetra-n-butylammonium hexafluorophos-
phate (TBAPF6, Wako) was used without further purification. All
other chemicals were of reagent grade and used as received.
2.2. Preparation of UO2(gha)DMSO.19 Opaque white powder of
glyoxal bis(2-hydroxanil) (H2gha) was prepared by condensation
between o-aminophenol (0.781 g) and glyoxal (40 wt % aqueous 0.513
g) in degassed and warmed ethanol (9 mL) under argon atmosphere,
followed by filtration, washing with n-pentane, and drying under
suction. Yield: 0.625 g (74%). 1H NMR (400 MHz, CDCl3, δ/ppm vs
TMS): 6.90−6.65 (m, 8H, Ph−H), 5.33 (s, 2H, NCH), 4.89 (bs,
2H, OH).
reflections; 3070 independent; Rint = 0.0824; R = 0.0338 ([F2
>
2σ(F2)]); wR = 0.0723 (all data); GOF = 1.068; Θ = 3.10 to 27.47°;
Δρmax = 1.530 e−·Å−3; Δρmin = −1.630 e−·Å−3. The ORTEP drawing is
shown in Figure S1 in the Supporting Information.
2.4. DFT Calculations. [UO2(gha)DMSO]−/UO2(gha)DMSO,
[ U O 2 ( s a l o p h e n ) D M S O ] − / U O 2 ( s a l o p h e n ) D M S O ,
[UO2(dbm)2DMSO]−/UO2(dbm)2DMSO, and [UO2(saldien)]−/
UO2(saldien) were tested. All calculations were performed using the
Gaussian 09 (Rev. C.01) program25 employing the DFT method with
Becke’s three-parameter hybrid functional and Lee−Yang−Parr’s
gradient-corrected correlation functional (B3LYP).26 For uranium,
the effective core potential and basis set were provided by Stuttgart
In a Schlenk flask filled with argon, H2gha (0.045 g) was dissolved
in methanol (5 mL) degassed by purging dry argon gas for 10 min
prior to use. Three droplets of pyridine were also loaded in this
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RSC ECP.27 The most diffuse basis functions on uranium with the
exponent 0.005 (all s, p, d, and f type functions) were omitted as in
previous studies.28 The 6-31G(d,p) basis sets were used for other
elements (C, H, N, O, S). Atomic coordinates of each complex were
obtained from the reported crystal structure of the same or a similar
compound7b,29 and optimized to be energy minima through
vibrational frequency analysis where no imaginary frequency was
found to be present. Twenty singlet excited states of UO2(gha)DMSO
were included in the time-dependent (TD) DFT calculations. Spin−
orbit effect and the basis set superposition error corrections were
neglected.
vs Fc/Fc+, respectively. In each run, the formal potential (E°′ =
(Epc + Epa)/2) is constant at −1.194 V vs Fc/Fc+ within 1 mV
error regardless of different potential sweep rate (v), implying
that the reduction and oxidation observed here are coupled
with each other. The peak potential separation (ΔEp = |Epc
−
Epa|) is almost constant at 0.070 V at v = 0.050 to 0.150 V·s−1,
while it slightly increases at the faster v. This shows shift of the
redox reaction from an electrochemically reversible system to
the quasireversible one. From the relationship between the
31
current value at Epc (ipc) and v1/2
,
the diffusion coefficient
(DO) of UO2(gha)DMSO in this system was estimated as 2.8 ×
10−7 cm2·s−1 at 298 K.
3. RESULTS AND DISCUSSION
To determine the electron stoichiometry (n) in the reduction
of UO2(gha)DMSO (Figure 1), a UV−vis spectroelectrochem-
ical experiment was performed. The absorption spectra of the
DMSO solution dissolving UO2(gha)DMSO (4.0 × 10−4 M)
and 0.1 M TBAPF6 were recorded at different potentials
stepwise varied from −1.075 to −1.313 V vs Fc/Fc+. Figure 2
3.1. Characterization and Electro- and Spectroelec-
trochemistry of UO2(gha)DMSO. The parent uranyl(VI)
complex, UO2(gha)DMSO, was obtained from a reaction
between H2gha and UO2(DMSO)5(ClO4)2 in the degassed
methanol with moderate yield. Although this compound has
already been reported by Cattalini et al.,19 it is worthwhile to
present some comments concerning its coordination chemistry.
The structure determination of the single crystal deposited
from the CH2Cl2/ethanol solution revealed that a dimeric
complex, [UO2(gha)]2, was formed. A similar dimerization in
the chlorinated noncoordinating solvents has also been
observed in other uranyl(VI) complexes with the tetradentate
29b
Schiff base ligand, salophen2−. The H NMR spectrum of a
DMSO-d6 solution dissolving the product indicated the C2v
symmetry of the UO2(gha) moiety. Furthermore, the NMR
signal at 2.58 ppm arises from free DMSO which has been
involved in the solid compound. This solvent molecule seems
to be replaced by DMSO-d6 and released to the bulk.30 The
peak integral clearly demonstrates that the stoichiometry of
DMSO to UO2(gha) is 1:1. Additionally, the IR spectrum of
the solid compound shows the characteristic peaks of OU
O asymmetric stretching (ν3, 950 cm−1), SO stretching (995
cm−1), and CN stretching (1584 cm−1). The ν3 frequency
disagrees with 890 cm−1 reported by Cattalini et al.19 Our
assignment should be more reliable, because it is supported by
the DFT calculation discussed later.
1
Figure 2. UV−vis absorption spectra of DMSO solution dissolving
UO2(gha)DMSO (4.0 × 10−4 M) and TBAPF6 (0.1 M) recorded at
different potentials from −1.075 to −1.313 V vs Fc/Fc+ together with
that measured prior to the potential application.
displays the obtained spectra together with that of the initial
solution prior to starting this experiment. As the potential on
the working electrode is negatively polarized, absorbance
increases at around 370, 440, 520, and 770 nm, while that
decreases at around 580 nm. Isosbestic points were observed at
406, 538, and 678 nm, indicating that only the redox
equilibrium of UO2(gha)DMSO takes place in this system.
Using absorbance at 517 nm, the concentration ratio (CO/CR)
of the oxidant (i.e., UO2(gha)DMSO) to the reductant at each
potential was calculated. The relationship between CO/CR and
E should follow the Nernstian equation, eq 1.
Cyclic voltammograms of UO2(gha)DMSO (3.24 × 10−3
mol·dm−3 = M) in DMSO containing 0.1 M TBAPF6 are
shown in Figure 1. The electrochemical data from this figure
are listed in Table S1 (Supporting Information). Cathodic and
anodic peaks were observed at −1.23 (Epc) and −1.16 V (Epa)
E = E°′ + (RT/nF)ln(CO/CR)
(1)
where E°′, R, T, and F are the formal potential of the redox
reaction of interest, the gas constant (8.314 J·mol−1·K−1), the
absolute temperature (here 298 K), and the Faraday constant
(96485 C·mol−1), respectively. In the ln(CO/CR)−E plot
(Figure S2, Supporting Information), the slope and intercept
of the best fit line of eq 1 to the data points are 0.028 and
−1.195, respectively. Consequently, the electron stoichiometry
n has been evaluated as 0.92, which reveals that UO2(gha)-
DMSO undergoes the following one-electron reduction in this
system.
UO2(gha)DMSO + e− = [UO2(gha)DMSO]−
(2)
The intercept (−1.195 V vs Fc/Fc+) corresponds to E°′ of the
[UO2(gha)DMSO]−/UO2(gha)DMSO redox couple, which is
in agreement with that observed in the CV experiments.
Figure 1. Cyclic voltammograms of UO2(gha)DMSO (3.24 × 10−3
M) in DMSO containing 0.1 M TBAPF6. Initial scan direction:
cathodic.
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The lowest E, −1.313 V vs Fc/Fc+, is lower than E°′ by 0.118
V, which is enough negative to give 99% mole fraction of the
reductant in accordance with eq 1. Therefore, the absorption
spectrum recorded at this potential is assigned to that of
[UO2(gha)DMSO]− in DMSO. The molar absorption spectra
of UO2(gha)DMSO and [UO2(gha)DMSO]− in DMSO are
compared in Figure 3. In our former investigations,6,7 it has
Figure 4. Optimized structure of UO2(gha)DMSO. Light blue, U;
yellow, S; red, O; blue, N; gray, C; white, H.
Figure 3. Molar absorption spectra of UO2(gha)DMSO (black) and
[UO2(gha)DMSO]− (red) in DMSO. Noises at 1700−1900 nm arise
from the solvent.
Table 1. Selected Bond Distances (R/Å) in Optimized
Structures of UO2(gha)DMSO and [UO2(gha)DMSO]−
Together with Those of UO2(gha)OH2 and [UO2(gha)]2 in
Crystalline States
been clarified that the electrochemical reduction of a uranyl-
(VI) complex usually affords the corresponding uranyl(V)
species. Although the formed uranyl(V) always suffers from its
decomposition governed by oxidation and disproportionation
(2U(V) → U(IV) + U(VI)), a uranyl(V) complex stabilized by
strong auxiliary ligands generally exhibits characteristic
absorption bands at 650, 750, 900, 1450, and 1900 nm arising
from electric-dipole forbidden f−f transition in 5f1 config-
uration and charge transfer from the axial oxygen atoms to
U5+.7b,c,32 In contrast, the absorption spectrum of [UO2(gha)-
DMSO]− in the current system (Figure 3) is clearly different
from those of the uranyl(V) complexes reported so far.7b,c
There are two peaks and one shoulder at 870, 770, and 700 nm
with significantly larger molar absorptivities (ε, 5000−8100
M−1·cm−1), while no specific bands and shoulders are lying in
the range from 900 to 1900 nm. If U5+ occurs in
[UO2(gha)DMSO]−, another characteristic peak should be
observed at 1400−1600 nm. Although an additional absorption
band has been actually observed at 1940 nm (ε = 1100 M−1·
cm−1), it is difficult to find the similarity of the absorption
spectrum of [UO2(gha)DMSO]− with those of the other
uranyl(V) complexes. Therefore, the formal oxidation state of
U in [UO2(gha)DMSO]− is unlikely to be +5 despite the one-
electron reduction of UO2(gha)DMSO. Our next interest is
which part of UO2(gha)DMSO is reduced through the
electrochemical reaction affording [UO2(gha)DMSO]−. How-
ever, it is not a straightforward task to experimentally identify
the location of the unpaired electron in the reductant.
Therefore, we decided to have support from the quantum
chemical calculations.
UO2(gha)
UO2(gha)
DMSO
[UO2(gha)
DMSO]−
ΔR
a
b
OH2
[UO2(gha)]2
(mean)
U−Oax
U−Oeq
1.77
1.77
2.33
2.36
2.42
1.762(5)
1.758(5)
2.253(5)
1.784
1.779
2.295
2.345
2.446
1.790
1.802
2.293
2.344
2.545
+0.015
−0.002
c
2.443(5)
c
U−
2.425(5)
+0.099
Osolv
U−N
2.54
2.56
1.28
2.567(6)
2.539(6)
1.282(9)
2.604
2.609
1.302
2.536
2.537
1.340
−0.070
N−
+0.038
C20,21
1.31
1.44
1.300(9)
1.44(1)
1.303
1.437
1.341
1.396
C20−
−0.041
C21
a
b
X-ray crystallographically determined by Bandoli et al.29a This work
c
(Figure S1, Supporting Information). Interatomic distances between
U and bridging phenolic oxygen atom.
3.2. DFT Calculations for UO2(gha)DMSO and
[UO2(gha)DMSO]− and Related Uranyl(V/VI) Complexes.
3.2.1. UO2(gha)DMSO and [UO2(gha)DMSO]−. First, the
structure optimization of UO2(gha)DMSO was performed.
The optimized structure is shown in Figure 4. The selected
bond distances are summarized in Table 1 together with those
solvent molecules may give different distances. Although the
computed U−N distances show larger deviations from the
crystal structure by 0.06 Å, such a difference should be still
acceptable. The same calculations were also performed for the
reductant, [UO2(gha)DMSO]−, resulting in atomic geometries
with no imaginary frequencies (Figure S4, Supporting
Information) which are very similar to those of the oxidant.
The optimized structure of [UO2(gha)DMSO]− should also
be compared with the actual one experimentally evidenced.
However, our attempts to crystallize this species (e.g., vapor
diffusion of ether) were unsuccessful to date. The important
29a
of crystalline UO2(gha)OH2 and [UO2(gha)]2 (Figure S1,
Supporting Information). Full metrics in these complexes are
compared in Figure S3 (Supporting Information). As a result,
the bond distances in the optimized structure are in good
agreement with those of the experimental data within 0.02 Å for
all U−O distances except for U−Osolv. The difference in U−
Osolv (ca. 0.03 Å) is not very surprising, because the different
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point would be choice of a countercation. Formerly, potassium
ion embedded in a crown ether was employed to crystallize the
reduced uranyl complexes with −1 charge by other research
group.10 In these cases, the axial oxygen atom of the uranyl
moiety also makes a coordination bond with K+. This effect is,
however, unfavorable, because it will complicate comparison
and interpretation of the calculated structures present alone.
The other choice is the quaternary ammonium ion, which has
already been introduced in the current system as a supporting
electrolyte. However, it is not simple to recover the millimolar
reductant from the submolar electrolyte matrix in little volatile
DMSO (bp 189 °C). The structural information on
[UO2(gha)DMSO]− in DMSO could be in situ obtained by
X-ray absorption spectroscopy as we have done formerly.7e
The variation in the interatomic distances (ΔR) of
UO2(gha)DMSO and [UO2(gha)DMSO]− through the
reduction is relatively significant in U−N (−0.07 Å) and U−
Osolv (+0.10 Å), while those of U−Oeq (0.00 Å) and UOax
(+0.01 Å) are very small or virtually negligible. In former
studies, it was clarified that the UOax distances are
lengthened about 0.05−0.10 Å by reducing U6+ to U5+,7e,33
whereas the atomic geometries of multidentate ligand(s) in the
equatorial plane are not affected greatly. Therefore, the current
computed structures of UO2(gha)DMSO and [UO2(gha)-
DMSO]− suggest that the unpaired electron in [UO2(gha)-
DMSO]− would not localize at U, but somewhere on the
ligands in the equatorial plane.
Table 2. Calculated Spin Densities on Non-Hydrogen Atoms
of [UO2(gha)DMSO]− Together with Atomic Notations
atom
spin density
atom
spin density
atom
spin density
U1
O2
O3
O4
O5
O6
N7
N8
C9
−0.0141
0.0087
0.0087
0.0145
0.0164
0.0000
0.2438
0.2528
0.0578
C10
C11
C12
C13
C14
C15
C16
C17
0.1088
−0.0391
0.0931
C18
C19
C20
C21
C22
S23
0.1079
−0.0708
0.1260
−0.0169
0.0600
0.1122
−0.0729
0.0004
−0.0176
0.0924
C24
C28
0.0000
−0.0406
0.0001
N− moiety in [UO2(gha)DMSO]−. Therefore, the formal
oxidation state of U remains unchanged at +6 even after the
reduction. This conclusion is corroborated by very similar
natural charges35 on U in the different redox states, which have
been estimated as 1.439 for UO2(gha)DMSO and 1.411 for
[UO2(gha)DMSO]−.
Molecular orbital (MO) distribution also provides us helpful
information to visualize where the unpaired electron is present
in the molecule of interest. In a usual one-electron reduction
reaction, the electron should be injected into the lowest
unoccupied MO (LUMO) of a molecule with a closed
electronic configuration. This MO is no longer LUMO in the
reductant, but gets to be called the singly occupied MO
(SOMO). Therefore, our DFT calculation results should be
examined about the distributions of LUMO of the oxidant and
SOMO of the reductant which are depicted in Figures 5 and S5
(Supporting Information), respectively. It can be found that the
two MOs are almost identical to each other in their spatial
distribution. It should be emphasized that the contributions of
atomic orbitals (AOs) of U are very small (2.4% in the oxidant,
The frequency calculation results also lead us to the same
conclusion. It is well-known that the uranyl ion has a linear
OUO geometry which shows characteristic symmetric
(ν1) and asymmetric (ν3) stretching vibrations in Raman and
IR spectra, respectively. In the previous studies, large red shifts
of the ν1 and ν3 frequencies (Δν1 ≈ −60 cm−1, Δν3 ≈ −130
cm−1) were observed in the reduction from U6+ to U5+.7a,d,34
Additionally, the Schiff base compounds usually exhibit a strong
IR absorption arising from CN stretching (νCN) in the
azomethine groups (−NCH−) at 1500−1650 cm−1, which is
not largely affected by the difference in the oxidation state of U.
In our DFT calculations, the ν1 and ν3 frequencies of
[UO2(gha)DMSO]− are 845.19 and 928.40 cm−1, respectively.
These values are smaller than those in UO2(gha)DMSO (ν1,
872.33 cm−1; ν3, 959.72 cm−1), but the differences are only 27
and 30 cm−1 for ν1 and ν3, respectively, which are not
suggestive of difference in the oxidation state of U in these
complexes. The ν3 frequency was experimentally observed at
950 cm−1 in the IR spectrum, demonstrating the validity of the
frequency calculation. In summary, these results suggest that
the oxidation state of U seems not to be largely affected by the
reduction from UO2(gha)DMSO to [UO2(gha)DMSO]−. The
calculated frequencies of νCN and SO stretching in the
coordinated DMSO (νSO) in the oxidant are 1639.59 and
1026.18 cm−1, respectively, whereas similar values have also
been obtained in the reductant (νCN, 1615.92 cm−1; νSO
,
1035.26 cm−1).
For further discussion about location of the unpaired
electron in [UO2(gha)DMSO]−, the calculated spin density
distribution is summarized in Table 2. As a result, it was
clarified that the total fraction of the spin density in the
−N(7)C(20)−C(21)N(8)− moiety is 0.735. Most of the
remaining fractions are dispersed into the C atoms of the
phenyl groups. In contrast, the spin density on U is close to
zero. Consequently, the unpaired electron which gives the
electronic spin S = 1/2 is mainly localized on the −NC−C
Figure 5. Distribution of lowest unoccupied molecular orbital in
UO2(gha)DMSO. The isovalue of the surface is 0.02 au.
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1.8% in the reductant) in both cases, while those of AOs in
atoms constructing the gha2−/3− ligand are exclusively
predominant. These MOs should have a character of a π*
orbital of the ligand. Therefore, there is little chance for the
unpaired electron to stay in U.
In conclusion, all the findings from the experimental UV−
vis−NIR absorption spectroscopy and the theoretical DFT
calculations indicate that the formal oxidation state of U
remains unchanged at +6 even in the reduction from
UO2(gha)DMSO to [UO2(gha)DMSO]−. In addition, the
DFT calculations gave a suggestion that the unpaired electron
in [UO2(gha)DMSO]− is mainly located at −NC−CN−
moiety of the Schiff base ligand. This means that the character
of gha2− is redox noninnocent in this system. Another ligand in
the equatorial plane, DMSO, seems to be innocent from all the
viewpoints in the DFT calculations.
3.2.2. Related Uranyl(V/VI) Complexes. At this stage, we
wondered if the formal oxidation states of U in the uranyl(V)
complexes we reported earlier are truly +5. Therefore, we
performed DFT calculations for the uranyl(VI) complexes
[UO2 (salophen)DMSO, UO2 (dbm)2 DMSO, and
UO2(saldien); see Chart 1], all of which were experimentally
suggested to give the corresponding uranyl(V) species.
The structure optimization for each uranyl(VI) complex was
performed, followed by frequency calculation. The optimized
structures of these uranyl(VI) complexes are shown in Figure
S6 (Supporting Information), and the selected bond distances
and frequencies are summarized in Tables S2−4 (Supporting
Information) together with those obtained experimentally. The
calculated bond distances agree with those from X-ray
crystallography within 0.06, 0.05, and 0.10 Å for
UO2(salophen)DMSO, UO2(dbm)2DMSO, and UO2(saldien),
respectively. The calculated ν3 frequency of each complex is
also in good agreement with that experimentally observed by
taking the scaling factor 0.95 into account. These results on
each uranyl(VI) species suggest that its optimized structure was
successfully obtained. In all the complexes, disagreements of the
U−N distances (ΔR = 0.05−0.10 Å) tend to be somewhat
larger than those of U−Oeq (ΔR = 0.00−0.08 Å). However, no
improvements were observed even in the use of larger basis sets
like 6-311G(d,p) for nonmetallic elements.
Figure 6. Distribution of lowest unoccupied molecular orbitals in
UO2(salophen)DMSO (top), UO2(dbm)2DMSO (middle), and
UO2(saldien) (bottom). The isovalue of the surface is 0.02 au.
The main concern on this section is the shapes of LUMOs in
UO2(salophen)DMSO, UO2(dbm)2DMSO, and UO2(saldien).
The LUMOs of these complexes were drawn on their
molecular structures in Figure 6. Although some contributions
from the auxiliary ligands (salophen2−, dbm−, saldien2−) to
LUMOs can be found, the main component in each system is
obviously a 5fδu orbital of U. Therefore, the additional electron
in the reduction of these uranyl(VI) complexes would be
inserted in U 5f orbitals shown in Figure 6, i.e., the reduction
product is uranyl(V).
We have also tried the calculations for structure optimization
and frequency analyses of the uranyl(V) complexes,
[UO2(salophen)DMSO]−, [UO2(dbm)2DMSO]−, and
[UO2(saldien)]−. Although the shapes of SOMOs of these
uranyl(V) complexes (Figure S7, Supporting Information) are
somewhat different from LUMOs of the corresponding
uranyl(VI) in Figure 6, the main contributor to SOMO in
each uranyl(V) complex is undoubtedly U 5fδu orbital in a
similar manner to LUMOs of uranyl(VI). Furthermore, the
calculated spin densities in U of [UO2(salophen)DMSO]−,
[UO2(dbm)2DMSO]−, and [UO2(saldien)]− were 1.077,
0.967, and 1.034, respectively. Consequently, the DFT
calculations for [UO2(salophen)DMSO]−/UO2(salophen)-
DMSO, [UO2(dbm)2DMSO]−/UO2(dbm)2DMSO, and
[UO2(saldien)]−/UO2(saldien) revealed that salophen2−,
dbm−, saldien2−, and DMSO in these systems are innocent,
and that the formal oxidation states of U in their reductants are
certainly +5. Therefore, these reductants show the characteristic
properties of uranyl(V) different from [UO2(gha)DMSO]−.
The optimized structures of [UO2(salophen)DMSO]−,
[UO2(dbm)2DMSO]−, and [UO2(saldien)]− are actually not
always in harmony with those found in our former EXAFS
experiments7e,29b as shown in Tables S2−S4 (Supporting
Information). In the current DFT calculations, no solvent
effects were taken into account, while the EXAFS data were
obtained from the solution samples. This difference might be
responsible for the disagreement found in the atomic
coordinates and the interatomic distances.10f,36 The bonding
interaction between U and the donor atoms in the equatorial
plane may be affected by the solvation more largely than those
of U−Oax, because the former is actually weaker than the latter.
As a matter of fact, the deviation in the interatomic distance is
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much more remarkable in the equatorial plane of [UO2]+
compared with the axial direction. Nevertheless, we find
systematic trends in the structure modification upon reduction
from U(VI) to U(V). For instance, the ν1 and ν3 frequencies
were predicted to decrease with the reduction in all the
systems. Such a phenomenon was experimentally demonstrated
by the vibrational spectroscopy.7a,d Another general tendency
found in these structures is lengthening of the U−Oax bond by
0.04−0.05 Å, being in line with the experimental evidence
obtained by EXAFS and X-ray crystallography.5,7e,9a,b,10d,f
It should be interesting to examine why gha2− in UO2(gha)-
DMSO is noninnocent and why the others studied here are
innocent. The difference in energy levels of LUMOs may give a
hint to answer to these questions. Figure 7 shows the energy
contribution to the absorption band lowest in energy is
attributable to the HOMO−LUMO transition which signifi-
cantly has a character of π−π* in gha2−. Such a transition is
never found in the other uranyl(VI) complexes studied here,
because there are no π* orbitals lying under the U 5f orbitals.
In conclusion, the innocence of ligand(s) in the uranyl
complexes is determined by the energy level of LUMO in the
ligand compared with those of U 5f orbitals.
4. CONCLUSION
In this paper, we studied the detailed redox chemistry of
UO2(gha)DMSO by means of electro- and spectroelectro-
chemical techniques. Our initial motivation was only to find a
new uranyl(V) complex using gha2− ligand to expand the
coordination chemistry of this unstable and exotic oxidation
state, whereas it was also known that gha2− bridging two Ru
centers could show the noninnocent behavior.37 The one-
electron reduction of the parent UO2(gha)DMSO in the
DMSO solution was experimentally evidenced, while the
observed characteristics in the absorption spectrum of the
generated [UO2(gha)DMSO]− turned out to be much different
from those of uranyl(V) species known so far. Therefore, we
had doubt whether the oxidation state of U in [UO2(gha)-
DMSO]− is certainly +5 or not. This argument was reasonably
concluded by the DFT calculations. The unpaired electron in
[UO2(gha)DMSO]− was found to be exclusively localized on
the tetradentate Schiff base ligand, namely, gha•3− anion radical.
In contrast, there was no evidence to suggest the reduction of
uranium, demonstrating that its oxidation state does not change
from +6 regardless of the redox reaction between UO2(gha)-
DMSO and [UO2(gha)DMSO]−. In other uranyl(V/VI)
couples we reported previously, the situation is much different.
The unpaired electron is exclusively localized on U atoms in
[UO2(salophen)DMSO]−, [UO2(dbm)2DMSO]−, and
[UO2(saldien)]−, suggesting that the formal oxidation state of
U in these complexes is +5. This is in line with their common
characteristics of the absorption spectra. The disagreement in
the optical properties of [UO2(gha)DMSO]− with uranyl(V)
complexes is fully addressed in terms of difference in the redox
innocence of the equatorial auxiliary ligand(s). To the best of
our knowledge, [UO2(gha)DMSO]−/UO2(gha)DMSO cur-
rently studied is the first example of the redox noninnocent
ligand system in the coordination chemistry of uranyl(VI). In
recent research activities, some actinide complexes are
frequently subjected to catalysis of organic syntheses. However,
such use is extensively limited to low-valent organoactinides,
which are always very unstable under ambient condition and,
therefore, must be handled with great care and proficiency. In
contrast, uranyl(VI) is most accessible in the various oxidation
states of uranium, which is also most available in the actinide
elements. If some catalytic activity arising from the noninnocent
ligand system of uranyl(VI) is established, it may lead to one of
efficient and sophisticated use of natural and depleted uranium
resources.
Figure 7. Calculated MO energy diagrams of uranyl(VI) complexes.
diagrams of MOs in the uranyl(VI) complexes extracted from
the DFT calculation results. In UO2(salophen)DMSO,
UO2(dbm)2DMSO, and UO2(saldien), MOs predominantly
consisting of the U 5f orbitals are lying among the LUMOs.
Although such a tendency is also observed in UO2(gha)DMSO,
another unoccupied MO is present at the lower position in
energy than U 5f orbitals. This MO has already been shown in
Figure 5. The π* character of LUMO in UO2(gha)DMSO was
evidenced by its close similarity with that of gha2− (Figure S8,
Supporting Information), which resulted from the single point
energy calculation of gha2− under the same computation level,
basis sets, and atomic geometries as UO2(gha)DMSO except
for removal of [UO2]2+ and DMSO. Actually, LUMO+1 in
UO2(gha)DMSO consists almost purely of U 5fδu orbital as
shown in Figure S9 (Supporting Information). The narrower
HOMO−LUMO gap can be related to the higher E°′ of
[UO2(gha)DMSO]−/UO2(gha)DMSO (−1.195 V vs Fc/Fc+)
than those of other redox couples studied here (E°′ = −1.550 V
for [UO2(salophen)DMSO]−/UO2(salophen)DMSO, −1.362
V for [UO2(dbm)2DMSO]−/UO2(dbm)2DMSO, −1.584 V for
[UO2(saldien)]−/UO2(saldien)).6,7b,c The characteristic con-
tribution of the π* orbital in UO2(gha)DMSO can also be
confirmed experimentally. The solid UO2(gha)DMSO and its
DMSO solution are significantly colored in purple, while the
others are yellow or orange. As a matter of fact, UO2(gha)-
DMSO exhibits the unique absorption band and shoulders at
500−600 nm as shown in Figure 2. The TD DFT calculation
excellently reproduces the experimental UV−vis absorption
spectrum and provides the assignment for each electronic
transition (Figure S10, Supporting Information). The largest
ASSOCIATED CONTENT
* Supporting Information
■
S
ORTEP drawing of [UO2(gha)]2, Nernstian plot for Figure 2,
full metrics in uranyl−gha complexes, optimized structures and
MO distributions of uranyl(V/VI) complexes and gha2−, molar
absorption spectrum of UO2(gha)DMSO in DMSO together
with absorption peak positions predicted by TD DFT
calculation, electrochemical data of UO2(gha)DMSO in Figure
5778
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Inorganic Chemistry
Article
Am. Chem. Soc. 2010, 132, 495−508. (g) Mougel, V.; Biswas, B.;
Pecaut, J.; Mazzanti, M. Chem. Commun. 2010, 46, 8648−8650.
(h) Mougel, V.; Horeglad, P.; Nocton, G.; Pecaut, J.; Mazzanti, M.
Chem.Eur. J. 2010, 16, 14365−14377. (i) Mougel, V.; Pecaut, J.;
Mazzanti, M. Chem. Commun. 2012, 48, 868−870.
1, and experimental and calculated selected bond distances and
characteristic frequencies of uranyl(V/VI) complexes. CIF file
for UO2(gha)DMSO. This material is available free of charge
via the Internet at http://pubs.acs.org.
(11) (a) Graves, C. R.; Kiplinger, J. L. Chem. Commun. 2009, 3831−
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ktakao@nr.titech.ac.jp.
Notes
■
(12) Jørgensen, C. K. Coord. Chem. Rev. 1966, 1, 164−178.
(13) Matsika, S.; Zhang, Z.; Brozell, S. R.; Blaudeau, J. P.; Wang, Q.;
Pitzer, R. M. J. Phys. Chem. A 2001, 105, 3825−3828.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was partly supported by the research grant provided
by Seikei University.
■
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