The Effect of Guest Metal Ions on the Reduction Potentials of Uranium(VI) Complexes: Experimental and Theoretical Investigations

Article abstract of DOI:10.1002/chem.201904253

Two mononuclear uranyl complexes, [UO₂L¹] (1) and [UO₂L²]?0.5 CH₃CN?0.25 CH₃OH (2), have been synthesized from two multidentate N₃O₄ donor ligands, N,N′-bis(5-methoxys

Full text of DOI:10.1002/chem.201904253

A Journal of
Accepted Article
Title: The effect of guest metal ions on the reduction potentials
of uranium(VI) complexes: Experimental and theoretical
investigations
Authors: Tanmoy Kumar Ghosh, Prithwish Mahapatra, Michael G. B.
Drew, Antonio Franconetti, Antonio Frontera, and Ashutosh
Ghosh
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The effect of guest metal ions on the reduction potentials of uranium(VI)
complexes: Experimental and theoretical investigations
Tanmoy Kumar Ghosh,[a] Prithwish Mahapatra,[a] Michael G. B. Drew,[b]
Antonio Franconetti,[c] Antonio Frontera,[c]* Ashutosh Ghosh[a]*
aDepartment of Chemistry, University College of Science, University of Calcutta, 92, A.P.C.
Road, Kolkata 700 009, India, E-mail address: ghosh_59@yahoo.com
bSchool of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG6
6AD, United Kingdom
cDepartament de Química, Universitat de les Illes Balears, Crta de Valldemossa km 7.5,
07122 Palma de Mallorca (Baleares), SPAIN, E-mail: toni.frontera@uib.es
Abstract
Two mononuclear uranyl complexes, [UO2L1] (1) and [UO2L2]·CH3CN·CH3OH (2) have
been synthesized from two multidentate N3O4 donor ligands, N,N'-bis(5-
methoxysalicylidene)diethylenetriamine
(H2L1)
and
N,N'-bis(3-
methoxysalicylidene)diethylenetriamine (H2L2) and structurally characterized. Both
complexes showed a reversible U(VI)/U(V) couple at −1.571 and −1.519 V respectively, in
cyclic voltammetry. The reduction potential of the U(VI)/U(V) couple shifted towards more
positive potential on addition of Li+, Na+, K+ and Ag+ metal ions to the acetonitrile solution
of complex 2 and the resulting potential was correlated with the Lewis acidity of the metal
ions and was also justified by theoretical DFT calculations. No such shift in reduction
potential was observed for complex 1. All four bimetallic products, [UO2L2Li0.5](ClO4)0.5 (3),
[UO2L2Na(ClO4)]2 (4), [UO2L2Ag(NO3)(H2O)] (5) and [(UO2L2)2K(H2O)2]PF6 (6), formed
on addition of the Li+, Na+, Ag+ and K+ metal ions, respectively, to acetonitrile solution of
complex 2, were isolated in solid state and structurally characterized by single crystal X-ray
diffraction. In all the species, the inner N3O2 donor set of the ligand encompasses the
equatorial plane of uranyl ion and the outer open compartment with O2O'2 donor sites hosts
the second metal ion.
1
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Introduction
The redox behavior of transition metal complexes is extremely important for their reactivity
and efficiency towards various catalytic oxidation/reduction, polymerization and coupling
reactions.[1-4] It has been found that in many cases, the presence of a specific redox-inactive
metal ion is crucial for the functioning of various naturally occurring enzymes.[5-7] For
example, the Ca2+ ion is vital for oxo-bridged tetra-manganese based enzyme in Photosystem
II (PS II) which oxidizes water to generate molecular oxygen.[8-11] The Sr2+ substituted PS II
can also provide oxygen evolution, but on substitution by several other cations, like Cd2+, its
oxygen evolving activity is lost, indicating that not only the charge but also Lewis acidity of
metal ions may be a key factor for its reactivity.[12-16] Inspired by the observation in natural
enzymes and the utility of redox-inactive metal ions, several groups have synthesized
bimetallic Ni(II), Co(II), Fe(II/III), Mn(II/III) complexes with various Lewis acidic metal
ions, like Li+, Na+, K+, Ca2+, Sr2+ etc.[12][17-20] The electrochemical properties of these
complexes are widely studied and a relationship between the Lewis acidity of redox-inactive
metal ions and the reduction potential of corresponding hetro-bimetallic complex is
established.[17-20] The difference in electron withdrawing ability of these redox-inactive metal
ions leads to the modification in ligand field around the redox-active metal ions which would
result in the considerable amendment in electronic structure as well as shifting of redox
potential. Till date, these studies are performed by using bimetallic transition metal
complexes, but our objective is to extend these studies to actinide metals such as uranium.
The interest of coordination complexes of uranium in current research is growing due
to their significant photoluminescent, photocatalytic, electrochemical, optical and molecular
magnetic properties.[21-27] The studies of these properties of uranium are helpful for waste
management in nuclear reactors to prevent water and soil pollution. Uranium forms stable
complex with multidentate organic ligands, a fact which is exploited in the detection and
separation of uranium from nuclear slag, ground and seawater.[28-31] It shows multiple
oxidation states, varying from +III to +VI but in aerobic condition, the common oxidation
state is +VI which is stabilized as the uranyl dication (UO22+).[32-39] A convenient method for
synthesis of U(V) compound is to reduce the corresponding U(VI) species
electrochemically.[40-43] Several complexes of U(V) with N2O2 donor salen type Schiff bases
have been prepared by this method. In these complexes, the N2O2 donor set along with a
solvent molecule form the pentagonal basal plane around U with two axial oxygen
2
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atoms.[42,43] Such U(VI) compounds usually show a reversible redox cycle at a potential
around –1.6 V (vs. Fc+/Fc) which is susceptible to the donor atoms of coordinated ligands and
the solvent environment.[43]
Diamines such as 1,2-ethanediamine or 1,3-propanediamine readily undergo 1:2
condensation with o-vaniline to produce a group of bi-compartmental ligands which have
been used extensively for the synthesis of phenoxido bridged heterometallic complexes.[44-45]
The inner compartment of such a ligand hosts a 3d metal ion through the coordination of
N2O2 donor set of atoms whereas the open O2O'2 compartment accommodates various
oxophilic cations including lanthanides.[44-47] Such ligands are known to coordinate to uranyl
ion through the N2O2 donor set but do not behave as a compartmental ligand presumably due
to the bigger size and preference for penta coordination around the basal plane of uranium.
On the other hand, the Schiff base formed by the 1:2 condensation of diethylenetriamine and
o-vaniline is expected to be well-suited to host a uranyl ion in its inner N3O2 donor
compartment[41] while the outer O2O'2 donor set remains suitably positioned to coordinate to a
second metal ion.
In
the present
work,
we
have synthesized
(H2L1)
two
ligands,
and
N,N'-bis(5-
N,N'-bis(3-
methoxysalicylidene)diethylenetriamine
methoxysalicylidene)diethylenetriamine (H2L2) and their mononuclear U(VI) complexes
[UO2L1] (1) and [UO2L2]·CH3CN·CH3OH (2). The electrochemical properties of these
complexes are studied by cyclic voltammetric measurements. A substantial shift of the
reduction potential of U(VI)/U(V) couple is observed upon addition of Li+, Na+, K+ and Ag+
metal ions to the acetonotrile solution of complex 2 and these shifts have been rationalized
considering the Lewis acidity of the respective metal ions and also by theoretical DFT
calculations. The bimetallic complexes, [UO2L2Li0.5](ClO4)0.5 (3), [UO2L2Na(ClO4)]2 (4),
[(UO2L2)2K(H2O)2]PF6 (6), and [UO2L2Ag(NO3)(H2O)] (5) have also been synthesized by
reacting complex 2 with Li+, Na+, K+ and Ag+ metal ions, respectively and characterized by
single crystal X-ray diffraction. The existence of these species in solution phase is confirmed
by ESI-mass spectral analyses. It is to be mentioned here that this is the first study where the
shift of the reduction potential of U(VI)/U(V) couple by the influence of a second metal ion is
investigated.
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Results and discussion
Syntheses of the complexes
The polydentate Schiff base ligands, H2L1 and H2L2 were prepared by 1:2 condensations of
diethylenetriamine and 5-methoxysalicylaldehyde and o-vanillin, respectively in methanol.
These ligands (H2L1 and H2L2) were reacted with the UO2(NO3)2·6H2O in 1:1 molar ratio at
room temperature to form new mononuclear complexes [UO2L1] (1) and
[UO2L2]·CH3CN·CH3OH (2), respectively. When complex 2 was reacted with
Li(ClO4)·6H2O, Na(ClO4)2, AgNO3 and KPF6 in 1:1 molar ratio at room temperature in
acetonitrile
solution,
hetero-bimetallic
complexes,
[UO2L2Li0.5](ClO4)0.5
(3),
[UO2L2Na(ClO4)]2 (4), [UO2L2Ag(NO3)(H2O)] (5) and [(UO2L2)2K(H2O)2]PF6 (6),
respectively were separated out as crystalline products. On the other hand, the same reaction
with complex 1 did not yield any such bimetallic complex; instead complex 1 itself separated
out on slow evaporation of the solvent. The syntheses of complexes 1–6 are depicted in
Scheme 1.
Scheme 1: Syntheses of complexes 1−6.
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IR spectra of complexes
In the IR spectra of all complexes 1−6, there is a strong and sharp band at around 1630 cm−1
due to the stretching of azomethine ν(C=N) group of the Schiff base ligand (Figures S1−S6,
Supporting Information) and at around 890 cm−1 for asymmetric stretching of uranyl (UO2)
group. Another strong and sharp band is observed in the IR spectra of all these complexes in
the range of 3186−3248 cm−1 for the stretching of N−H group in the Schiff base ligand. Here,
the N−H stretching in all the complexes is in lower frequency than that in free ligand (3343
cm−1) which indicate the possibility of H-bonding. A strong band at around 1080 cm−1 for
complexes 3 and 4 indicates the presence of the perchlorate anion. In the IR spectra of
complex 5, the presence of the nitrate anion is evident from a strong sharp band at 1384 cm−1.
The characteristic IR band for hexafluorophosphate anion in complex 6 is observed at around
845 cm−1.
Electronic spectroscopy
Electronic spectra of all complexes 1−6, were recorded in acetonitrile solvent (Figures S7,
Supporting Information). The spectrum of [UO2L1] exhibits one band at 368 nm in
acetonitrile whereas that of [UO2L2] shows two bands at 410 and 264 nm. These bands are
due to ligand to metal charge transfer transitions. On the other hand, complexes 3−6 show
two absorption bands at 395, 267 nm for complex 3; 400, 266 nm for complex 4; 400, 267
nm for complex 5 and 397, 268 nm for complex 6 for ligand to metal charge transfer.
ESI-Mass Spectrometric Study
ESI-mass spectra of all complexes 1−6 were recorded in acetonitrile solution, which are
shown in Figures S8−S13, Supporting Information. In these spectra, the base peaks are
observed at m/z = 640.45 (calcd 640.22) for [UO2L1 + H]+, 662.21 (calcd 662.20) for [UO2L2
+ Na]+, 646.23 (calcd 646.23) for [UO2L2 + Li]+, 662.20 (calcd 662.20) for [UO2L2 + Na]+,
746.12 (calcd 746.11) for [UO2L2 + Ag]+ and 678.58 (calcd 678.17) for [UO2L2 + K]+ for
complexes 1−6, respectively. In case of complexes 1 and 2, the base peak appeared due to H+
and Na+ adducts of these complexes, respectively. Whereas, in complexes 3−6 the base peaks
correspond only to the respective bimetallic species.
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1H NMR Study
1H NMR spectra of all complexes 1−6 and complex 2 + Li+ were recorded in CD3CN solution
(Figures S14−S20, Supporting Information). In these spectra, slight downfield shift for the
signals correspond to the aromatic protons of the ligand was observed for all the bimetallic
complexes 3−6 compared to that of complex 2. These results are expected as the
incorporation of Lewis acidic metal ions leads to the withdrawal of the electron density from
the [UO2L2] and thus enhances the deshielding of aromatic protons resulting in a downfield
shift. The 1H NMR spectra of complex 3 show doublet of doublets for two aromatic protons,
similar to that in complex 2, except that these peaks are shifted to the downfield region.
However, on the addition of eqiumolar amount of Li+ ion to complex 2, these doublets shift
even more downfield with less pronounced splitting (Figure S21 in Supporting Information)
like those of complexes 4 and 6.
Description of structures
Figure 1.The structure of 1 which has crystallographic mirror symmetry with ellipsoids
shown at 30% probability.
The structure of 1, UO2L1 which has crystallographic mirror symmetry is shown in Figure 1
together with the atomic numbering scheme in the coordination sphere.
In 1, the metal atom is seven-coordinate with a pentagonal bipyramidal environment being
bonded to two terminal oxygen atoms in axial positions and the five donor atoms of the
ligand in the equatorial plane. The two terminal oxygen atoms are 1.748(6), 1.768(6) Å from
the metal with a O-U-O angle of 177.2(3)o while in the equatorial plane, unique distances are
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U-N 2.561(5), 2.579(7) Å and U-O 2.210(4) Å. These five atoms show a r.m.s. deviation of
0.133 Å with the metal atom 0.035(3) Å from the plane. However the ligand is severely bent
and the two aromatic rings intersect at an angle of 63.1(1)o. Selected bond lengths and bond
angles of complex 1 are given in Table S1 (Supporting Information). The unique nitrogen
atom N22 forms a weak intermolecular hydrogen bond to a terminal oxygen O2$1 ($1 =
x+½, y, ½-z) with dimensions shown in Table S4 (Supporting Information).
Figure 2. The structure of 2A with ellipsoids at 30% probability. The structure of 2B is
similar though there is no solvent molecule in the cavity. The methanol solvent molecule is
refined with 50% occupancy. Dotted line shows the hydrogen bonding.
In complex 2 there are two molecules of UO2L2 in the asymmetric unit together with solvent
molecules of MeCN and MeOH, the latter being refined with 50% occupancy. The two
molecules show approximate mirror symmetry though not crystallographically imposed. The
two UO2L2 molecules, named A, shown in Figure 2, and B have equivalent structures. The
difference between L2 with the -OMe in an ortho position in 2 compared to L1 with the -OMe
in a para position in 1 facilitates the formation of a cavity adjacent to the uranyl atom though
there is no significant interaction between the -OMe groups and the uranium. However unlike
structures, 3, 4 and 5 the cavity is not filled with an additional metal. In molecule A there is a
solvent methanol molecule refined with 50% occupancy which forms a hydrogen bond O1-
H…O10 (dimensions in Table S4 (Supporting Information) but this solvent molecule remains
well above the plane of the cavity. There is no such solvent interaction with molecule B. The
geometry around the uranium atoms in molecules A and B is very similar to that found in 1.
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The r.m.s. deviations of the five donor atoms in the equatorial planes are 0.031, 0.103 Å with
the metal 0.037(5), 0.010(4) Å respectively from the plane. However, there are significant
differences in the conformation of the ligand in A and B. For example, the two aromatic rings
intersect at 19.2(7)o in A and 50.5(4)o in B and in A, O10 and O34 are 0.72(2), 0.02(2) Å and
in B 0.24(2), 0.40(2) Å from the equatorial plane. Selected bond lengths and bond angles of
complex 2 are given in Table S2 (Supporting Information). In both A and B, N22 forms an
intermolecular donor hydrogen bond to a terminal oxygen atom, details in Table S4
(Supporting Information).
Figure 3.The structure of 3 which contains crystallographic mirror symmetry with ellipsoids
at 30% probability.
The structure of 3 which has crystallographic mirror symmetry is shown in Figure 3. The
complex contains UO2L2 with the same basic structure as in 2, but in this structure the cavity
is occupied by a lithium ion. The charge is balanced by a disordered perchlorate anion. As the
chlorine was in a special position, the maximum possible amount of Li(ClO4) per uranium
complex was 0.5 and it was decided to refine with this value although reduced occupancy is
feasible. In addition the perchlorate oxygen atoms were disordered over several positions.
The geometry of the uranium coordination sphere was similar to that found in 1 and 2. In the
pentagonal bipyramid the five donor atoms in the equatorial plane show a r.m.s. deviation of
0.071 Å with the uranium atom 0.052(8) Å from the plane. The two aromatic rings intersect
at an angle of 47.1(7)o. The Li ion forms bonds to O10 and O11 and the symmetry
equivalents but O10 is disordered over two positions so it is difficult to assess the exact
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coordination geometry around the Li ion. Selected bond lengths and bond angles of complex
3 are given in Table S1 (Supporting Information). As in other structures, N22 forms a
intermolecular donor hydrogen bond to a terminal oxygen atom, O1 (3/2-x, ½-y, z+1/2)
details in Table S4 (Supporting Information).
.
Figure 4. The dimeric structure (UO2L2NaClO4)2 of complex 4 with ellipsoids at 30%
probability.
The structure of 4 which contains no crystallographic symmetry is shown in Figure 4. Here
the basic structure is the same as that of 3 but with a sodium ion in the cavity. The geometry
of the UO2L2 moiety is similar to that found in the other structures. The atoms in the
equatorial plane show a r.m.s. of 0.045 Å with the metal 0.006(4) Å from the plane. However
in this structure the two aromatic planes are closely planar with an intersecting angle of
3.4(9)o. This is clearly a consequence of the strong binding to the sodium ion in the cavity
and the formation of a dimer. The four oxygen atoms of the ligand O10, O11, O33, O34 are
bonded to the sodium at distances of 2.299(10), 2.449(9), 2.431(9), 2.303(10) Å with a r.m.s.
deviation of 0.062 Å from a plane from which the sodium deviates by 0.115(6) Å. In addition
to these bonds the sodium is bonded to two oxygen atoms of the perchlorate O41 and O43 at
2.50(2), 2.70(2) Å and to a terminal oxygen atom O1 (1-x, 2-y, 2-z) at 2.568(10) Å. This
latter bond creates a dimeric structure as shown in Figure 4. Selected bond lengths and bond
angles of complex 4 are given in Table S3 (Supporting Information). As in the other
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structures N22 forms an intermolecular donor hydrogen bond, but in this case to a perchlorate
oxygen atom, details in Table S4 (Supporting Information).
Figure 5. The structure of 5 with ellipsoids at 30% probability.
In the structure of 5, which has no imposed symmetry, the metal in the cavity is silver but it is
too large to fit readily into the cavity (shown in Figure 5). As a result the ligand deviates far
more from planarity than in complex 4. The r.m.s. deviation of the five donor atoms in the
equatorial plane is 0.120 Å and the uranium atom is 0.034(7) Å from the plane. The two
aromatic planes intersect at 52.9(7)o. For the silver coordination sphere, the r.m.s. deviation
of the four oxygen atoms in the ligand is 0.034 Å with the metal is 1.417(8) Å from the plane.
The silver atom is also bonded to a water molecule O3 at 2.352(15) Å and two oxygen atoms
of a nitrate ligand O(42) at 2.60(2) Å and O43 at 2.65(2) Å. If the nitrate is considered as one
bonding site, then the geometry around the silver atom is best described as a trigonal prism.
Selected bond lengths and bond angles of complex 5 are given in Table S3 (Supporting
Information). In this structure N22 forms an intermolecular hydrogen bond to a nitrate
oxygen atom while the water molecule O3 forms hydrogen bonds to a terminal oxygen atom
O1, details in Table S4 (Supporting Information).
We could not establish the structure of 6 in controvertibly from the X-ray data due to disorder
but a likely structure containing a potassium ion on a center of symmetry sandwiched
between two UO2L2 moieties was established and is shown as (Figure S22, Supporting
Information).
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Electrochemistry
Cyclic voltammograms (CVs) of complexes 1 and 2 were recorded in acetonitrile with
respect to a ferrocene/ferrocenium (Fc0/+) electrode at different scan rates. For complexes 1
and 2, the peaks are observed at Ec= −1.604 and −1.560 V on forward cathodic scan for
reduction process at electrode surface, whereas on reverse scan, anodic peaks are observed at
Ea= −1.538 and −1.478 V, respectively at 100 mVs−1 scan rate. The peak separation values
(∆Epc = Ecathodic−Eanodic) are −0.066 and −0.082 V with the E1/2 values of −1.571 and −1.519 V
for complexes 1 and 2, respectively (Figure 6). The reduction processes for both 1 and 2 are
nearly reversible, and the appearance of peaks are due to the reduction of U(VI) to U(V) in
the uranyl complexes. Another very small peak is observed for both complexes 1 and 2 at
higher scan rates prior to the reduction potential of U(VI/V) couple. For complex 2 which is
possibly due to the presence of little amount of impurity (Na+ ion or other impurity) from the
reference Ag/AgCl electrode. But for complex 1, a small peak appear may be for other
reasons as it does not form complex with Na+. It is to be noted that this feature is less
pronounced for 1 compared to 2. Such small feature for the reduction of other U(VI)
complexes has also been observed by other group and has been ascribed to the presence of
small “unknown impuritiy”.[26] After the addition of equimolecular redox inactive mono-
positive metal ions in the solution of complex 2, the cyclic voltammograms of mixtures are
recorded. In these cyclic voltammograms the corresponding reduction potentials of U(VI) are
observed at more positive potential than that of complex 2. The peaks are observed at Ec =
−1.328, −1.391 and −1.436 V on forward scan after the addition of Li+, Na+ and K+,
respectively. On reverse scan the corresponding anodic peaks are appeared at Ea = −1.242,
−1.261 and −1.341 V. Peak separation values (∆Epc) are −0.086, −0.130 and −0.095 V with
the E1/2 values of −1.285, −1.326 and −1.388 V for the mixture of complex 2 with Li+, Na+
and K+, respectively as shown in Figure 7 and Figures S23−S25 (Supporting Information). In
these complexes a considerable shift in the reduction potential of U(VI/V) couple is observed.
The shifts of E1/2 values with respect to complex 2 are −0.234, −0.193 and −0.131 V for Li+,
Na+ and K+, respectively Table S5 (Supporting Information). On the contrary, the cyclic
voltammograms of complex 2 show two peaks on forward scan at Ec= −1.361 and −1.541 V
after the addition of redox active Ag+ metal ion and on reverse scan the anodic peaks appear
at Ea = −1.287 and −1.472 V. Here the peak separation values (∆Epc) are −0.074 and −0.069
V with the E1/2 values of −1.324 and −1.507 V, respectively. Electrochemical data of all the
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complexes are listed in Table S6 (Supporting Information). The peak at E1/2 = −1.507 V is
due to the U(VI/V) reduction couple, which is closer to the E1/2value of complex 2 as shown
in Figure 7 and S26 (Supporting Information). This peak indicates that some [UO2L2] species
are free from silver in solution, whereas the other peak at E1/2 = −1.324 V is due to the
reduction of U(VI) to U(V) in the bimetallic Ag-U complex. The Ag+ ion starts to reduce at a
potential of −0.01 V and therefore some of Ag+ ions get reduced during the potential scan
producing the free [UO2L2] species as shown in S27 (Supporting Information). The higher
cathodic current of the Ag-U complex compared to the others indicates that this reduction
process is continuing during the scanning process. In the mixture of complex 2 and second
metal ions (Li+, Na+, and K+) the shift in reduction potential of U(VI/V) couple towards the
lower potential range as shown in Figure 8, indicates the presence of cation coordinated
[UO2L2] species in the solution, which is also confirmed from ESI-mass spectrometry. The
differential pulse voltammograms of the complex 2 and a mixture of complex 2 with the
guest metal ions Li+, Na+ or K+ also show the similar change of potential as shown in Figure
8. In these hetero-bimetallic complexes the anodic shift in reduction potential of the U(VI)
can be correlated with the Lewis acidity of the cations. The variation in reduction potential of
U(VI/V) couple with the Lewis acidity of the cations is shown in Figure 9. The plot of E1/2
values of U(VI/V) couple with the pKa values of Li+, Na+ and K+ metal ions shows a linear
relationship, in which the reduction potential of uranium decreases with the decrease in pKa
values of the metal ions.[48,49] The electronic structure of the complexes as well as the redox
potential of U(VI) is changed by the addition of guest metal ions to the neutral metalloligand
(complex 2). Therefore, all the complexes are energetically characterized for better
understanding about the shift in reduction potential of the U(VI) in these complexes. The
electrochemical measurements in the acetonitrile solution of complexes 4−6 show very
similar results as described above by the addition of equimolecular mono-positive metal ions
for Na+, Ag+ and K+, respectively to the solution of complex 2. However, complex 3 shows
an additional peak at −1.528 V (Figure S28 in Supporting Information) which is not observed
when equimolecular Li+ ion is added to the solution of complex 2. The position of this
additional peak indicates the presence of free [UO2L2] species in the solution confirming that
the occupancy of Li+ ion is less than one as observed in the X-ray structure.
Although there are several reports of Schiff base complexes with an appended crown
functionality and their wide utility towards electro-redox, we have shown here for the first
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time that a common bi-compartmental Schiff base ligand can also be very useful to study the
effect of a second metal ion on the reduction potential of the U(VI)/U(V) couple.
Figure 6. Cyclic voltammograms of complexes 1 (left) and 2 (right) which are recorded in
acetronitrile (0.1 M TBAP) with a glassy carbon working electrode at different scan rate.
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Figure 7. Cyclic voltammograms of UO2L2 + M+ [M+ = Li+(3), Na+(4), Ag+(5), K+(6)] which
are recorded in acetonitrile (0.1 M TBAP) with a glassy carbon working electrode at 100
mVs−1 scan rate.
Figure 8. Cyclic voltammograms of UO2L2 (2) and UO2L2 + M+ [M+ = Li+ (3), Na+ (4), K+
(6)] (left) which are recorded in actronitrile (0.1 M TBAP) at 100 mVs−1 scan rate and DPV
of UO2L2 (2) and UO2L2 + M+ [M+ = Li+ (3), Na+ (4), K+ (6)] (right) with a glassy carbon
working electrode at 20 mVs−1 scan rate.
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Figure 9. Dependence of half wave potentials (E1/2) of the bimetallic complexes
([(UO2L2)M]n+) vs. pKa of M(aqua)n+ ions as measure of their Lewis acidity.
Density Functional Theory (DFT) Studies.
The full geometries of the [MLUO2]+ series of complexes were optimized using DFT M06-
2X/def2-TZVP level of theory without including counter-anion in the calculations. For
uranium we have used the def2-SVP basis set, as implemented in TURBOMOLE. The
optimized complexes are shown in Figure 10. The computed bond distances are in good
agreement with those obtained in the X-ray data. The deviation of the bond distances for the
Schiff-base ligand is less than 0.01 Å. There is a greater discrepancy (∼0.2 Å) in the
coordination distances, which are systematically longer in the computed structure. This is
likely due to packing effects that are not considered in the calculations in addition to the
absence of the counter ion, specially the coordinated nitrate anion and water molecule in the
Ag complex that is responsible for the large discrepancy in coordination bond with the -OMe
group (difference 0.2 Å). Moreover, in the K complex the discrepancies are likely due to the
dimeric nature of the system in the solid state.
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Figure 10. Optimized geometries of UO2-M(I) complexes (M = Li, Na, K, and Ag).
Distances in Å, values in black correspond to the theoretical and in red to the experimental
ones (X-ray, mean distances).
In Table 1 we summarize the energy of the LUMO in M = Li, Na, K and Ag complexes. It is
interesting to note that the reduction potential and LUMO energies exhibit a parallel
behavior. The LUMO energies and plots have been obtained by performing energy
calculations on the optimized M06-2X/def2-TZVP geometries at the M06-2X/SDD level of
theory using Gaussian-16 program. In fact, we have found a strong linear correlation between
the LUMO energy and the reduction potential (Ec), as shown in Figure 11 (R2 = 0.9986),
which gives reliability to the theoretical method and also suggests that these bimetallic
complexes are monomeric in solution. The representations of the LUMOs are provided in
Figure 12 and S29 (Supporting Information) for all complexes included in the regression plot
along with the LUMO energies. In all cases the LUMO is composed by the fzx –zy
2
2 atomic
orbital at U with some contribution of the antibonding π orbital of the C=N bond.
Table 1. Experimental reduction potential, theoretical LUMO energy and Mulliken charge at
the metal (q) corresponding to the optimized geometries shown in Figure. 10.
Complex
UO2+K
Ec U(VI/V) (mV)
-1.436
Elumo (eV)
-3.5806
-3.6494
-3.7356
-3.6883
qM (e)
0.92
0.83
0.62
0.64
UO2+Na
UO2+ Li
UO2+Ag
-1.391
-1.328
-1.361
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Figure 11. Correlation plot of ELUMO vs Ec for Li(I), Na(I), K(I) and Ag(I) complexes.
Figure 12. Plots of the LUMOs (0.04 a.u. isosurface) for Li, Na, K and Ag complexes.
In Table 1 we also summarize the electron charge of the cationic guest in the complexes.
Interestingly the amount of positive charge at the guest metal also correlates with the
experimental reduction potential. That is, the highest energetic LUMO and lowest value of
reduction potential (–1436 mV) corresponds to K+–complex that presents the largest atomic
charge at the K+ (0.92 e). Similarly, the lowest LUMO and highest value of reduction
potential (–1328 mV) corresponds to the Li+–complex that exhibits the lowest atomic charge
(0.62 e). Therefore, the charge transference from the complex to the cation is larger in Li
(around 0.48 e) than in K (only 0.08 e). This charge transfer is also related with the pKa
correlation described above in Figure 9.
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We have also optimized the complexes upon the one-electron reduction and computed the
spin density plot, that is shown in Figure 13 for M = Li, Na, K and Ag complexes. The plot is
very similar in all cases with the unpaired electron located on the U metal center. Some
negative spin is delocalized onto the O atoms of the UO2 moiety. The density plots obtained
for the reduced species are similar to the shape of the LUMOs in the cationic compounds
(represented in Figure 12).
Figure 13. Spin density plots for the reduced species [U(V)O2-M(I)] using the 0.001
isodensity.
Conclusions
We have shown here that the Schiff base ligands derived from the di-condensation of
diethylenetriamine with p-vaniline (H2L1) or o-vaniline (H2L2) readily form complexes with
U(VI) (1 and 2 respectively) coordinating to the pentagonal basal plane of the uranyl ion
through the N3O2 donor set of atoms. Both the complexes showed reversible U(VI)/U(V)
couple on cyclic voltammetry in acetonitrile solution but a significant shift in reduction
potential was only observed for 2 on addition of Li+, Na+, K+ and Ag+ to the solution. This
shift in reduction potential is due to the inductive effect of the Lewis acidic metal ions, which
pull the electron density from the uranyl centre to make it easily reducible from U(VI) to
U(V). The correlation between Lewis acidic characters of metal ions (pKa) with the reduction
potentials of U(VI)/U(V) couple showed a linear relationship, and the DFT studies showed a
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good agreement in this regard. The structural analyses revealed that in 2, two phenoxido
oxygen and two methoxy oxygen atoms are in close proximity to form a cavity. In the Na-
adduct, the planarity and strong bonding of these four oxygen atoms indicate that the size of
the cavity fit well to accommodate a sodium ion but too big for lithium ion as 50% of the
cavity remained unoccupied in the Li-adduct. On the other hand, the Ag+ and K+ reside well
above the mean plane of the four oxygen atoms indicating that these ions are too big
compared to the size of the cavity. The other side of K+ is so much exposed that four oxygen
atoms of another unit are attached to form an 1:2 adduct. Complex 1 neither showed any shift
of potential of U(VI)/U(V) couple nor any adduct was formed on addition of the same metal
ions indicating the importance of the cavity for shift of the potential and formation of the
adduct.
Experimental Section
Starting materials
All chemicals and solvents were of reagent grade and were commercially available. They
were use without any further purification.
Synthesis of Schiff base ligands
H2L1 = N,N'-bis(5-methoxysalicylidene)diethylenetriamine) and H2L2 = N,N'-bis(3-
methoxysalicylidene)diethylenetriamine)
The Schiff base ligand H2L1 was prepared by following method; 10 mmol (1.521 g) of 5-
methoxysalicylaldehyde was mixed with 5 mmol (0.540 mL) of diethylenetriamine in
methanol (25 mL). The resulting solution was refluxed for 3 h and then directly used for
synthesis of complex 1.
The Schiff base ligand H2L2 was synthesized in our laboratory by following the methods
reported earlier.[50,51]
Synthesis of the complex [UO2L1] (1)
A methanolic solution of Schiff base ligand H2L1 (0.5 mmol, 20 mL) was added to the
acetronitrile solution (10 mL) of UO2(NO3)2·6H2O (0.250 g, 0.5 mmol), followed by
triethylamine (1 mmol, 0.14 mL) with constant stirring. The resulting solution was stirred for
30 min and then kept in a desiccator. Red coloured single crystals of X-ray quality started to
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appear inside the beaker, after 3-4 days which were separated by filtration after 6 days and
dried in air.
Yield: 0.207 g (65%) C20H23N3O6U (639.44) Calculated C, 37.57; H, 3.63; N, 6.57; Found
C, 37.51; H, 3.55; N, 6.63; IR: ν(N–H) = 3248 cm–1, ν(C=N) = 1632 cm–1, ν(UO2) asymmetric
stretching) = 882 cm–1. ESI-MS (positive ion mode, CH3CN) calcd. m/z = 640.22; found at
1
m/z = 640.45, which are assigned as [UO2L1 +H]+. H NMR (400 MHz, CD3CN, ppm): δ
9.37 (s, 2H), δ 7.21 (dd, 2H), δ 7.06 (d, 2H), δ 6.91 (d, 2H), δ 4.64 (t, 2H), δ 4.52 (m, 2H), δ
4.14 (m, 2H), δ 3.76 (s, 6H), δ 3.52 (m, 2H).
Synthesis of complex [UO2L2]·CH3CN·CH3OH (2)
Complex 2 was synthesized by following the same procedure as for complex 1, using Schiff
base ligand H2L2 (0.185 g, 0.5 mmol) insted of H2L1. Red coloured single crystals of X-ray
quality appeared inside the beaker, which was separated out by filtration after 4-5 days.
Yield: 0.227 g (68%) C21.25H25.50N3.5O6.25U (667.98). Calculated C, 38.21; H, 3.85; N, 7.34;
Found C, 38.28; H, 3.77; N, 7.39; ; IR: ν(N–H) = 3214 cm–1, ν(C=N) = 1625 cm–1, ν(UO2
asymmetric stretching) = 883 cm–1. ESI-MS (positive ion mode, CH3CN) calcd. m/z =
662.20; found at m/z = 662.21, which are assigned as [UO2L2 +Na]+. 1H NMR (400 MHz,
CD3CN, ppm): δ 9.41 (s, 2H), δ 7.22 (dd, 2H), δ 7.14 (dd, 2H), δ 6.63 (t, 2H), δ 4.64 (m, 2H),
δ 4.53 (m, 2H), δ 4.15 (m, 2H), δ 4.07 (s, 6H), δ 3.51 (m, 2H).
Synthesis of the complex [UO2L2Li0.5](ClO4)0.5 (3), [UO2L2Na(ClO4)]2 (4),
[UO2L2AgNO3(H2O)] (5) and [(UO2L2)2K(H2O)2]PF6 (6). All the complexes 3–6 were
prepared in a similar manner, by dissolving the complex 2 [UO2L2] (0.334 g, 0.5 mmol) in
acetronitrile solution (20 mL) and then solid Li(ClO4)·6H2O (0.0532 g, 0.5 mmol) for 3,
NaClO4 (0.0611 g, 0.5 mmol) for complex 4, AgNO3 (0.085 g, 0.5 mmol) for complex 5,
KPF6 (0.046 g, 0.25 mmol) for 6 was added to it. The resulting solution was stirring for 15
min. For each case, diffraction quality red single crystals were obtained after a few days on
slow diffusion of diethyl ether into the solution.
Complex 3. Yield 0.213 g (62%) C20H23Cl0.5Li0.5N3O8U (688.65). Calculated C, 34.68; H,
3.35; N, 6.07; Found C, 34.72; H, 3.33; N, 6.14; IR: ν(N–H) = 3197 cm–1, ν(C=N) = 1625 cm–1,
ν(UO2 asymmetric stretching) = 879 cm–1, ν(ClO4) = 1086 cm–1. ESI-MS (positive ion mode,
CH3CN) calcd. m/z = 646.23; found at m/z = 646.23, which are assigned as [UO2L2 +Li]+. 1H
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NMR (400 MHz, CD3CN, ppm): δ 9.40 (d, 2H), δ 7.27 (dd, 2H), δ 7.17 (dd, 2H), δ 6.69 (t,
2H), δ 4.65 (t, 2H), δ 4.55 (m, 2H), δ 4.20 (m, 2H), δ 4.10 (s, 6H), δ 3.54 (m, 2H).
Complex 4. Yield 0.228 g (60%) C20H23ClN3NaO10U (760.87). Calculated C, 31.53; H, 3.04;
N, 5.52; Found C, 31.59; H, 2.96; N, 5.56; IR: ν(N–H) = 3241 cm–1, ν(C=N) = 1631 cm–1, ν(UO2
asymmetric stretching) = 898 cm–1, ν(ClO4) = 1089 cm–1. ESI-MS (positive ion mode, CH3CN)
calcd. m/z = 640.22 and 662.20; found at m/z = 640.22 and 662.20 which are assigned as
[UO2L2 + H]+ and [UO2L2 + Na]+ respectively. 1H NMR (400 MHz, CD3CN, ppm): δ 9.41 (s,
2H), δ 7.24 (d, 2H), δ 7.15 (d, 2H), δ 6.65 (t, 2H), δ 4.64 (t, 2H), δ 4.54 (m, 2H), δ 4.16 (m,
2H), δ 4.08 (s, 6H), δ 3.52 (m, 2H).
Complex 5. Yield 0.252 g (61%) C20H25AgN4O10U (827.34). Calculated C, 29.03; H, 3.05;
N, 6.77; Found C, 28.95; H, 3.01; N, 6.82; IR: ν(N–H) = 3186 cm–1, ν(C=N) = 1627 cm–1, ν(UO2
asymmetric stretching) = 892 cm–1, ν(NO3) = 1384 cm–1. ESI-MS (positive ion mode, CH3CN)
1
calcd. m/z = 746.11; found at m/z = 746.12, which are assigned as [UO2L2 +Ag]+. H NMR
(400 MHz, CD3CN, ppm): δ 9.41 (s, 2H), δ 7.22 (dd, 2H), δ 7.14 (dd, 2H), δ 6.63 (t, 2H), δ
4.64 (t, 2H), δ 4.53 (m, 2H), δ 4.16 (m, 2H), δ 4.07 (s, 6H), δ 3.51 (m, 2H).
Complex 6. Yield 0.225 g (60%) C40H50F6KN6O14PU2 (1498.67). Calculated C, 32.05; H,
3.36; N, 5.61; Found C, 32.09; H, 3.31; N, 5.65; IR: ν(N–H) = 3233 cm–1, ν(C=N) = 1629 cm–1,
ν(UO2 asymmetric stretching) = 872 cm–1, ν(PF6) = 845 cm–1. ESI-MS (positive ion mode,
CH3CN) calcd m/z = 678.17 and 640.22; found at m/z, 678.58 and 640.59 which are assigned
as [UO2L2 + K]+ and [UO2L2 +H]+ respectively. 1H NMR (400 MHz, CD3CN, ppm): δ 9.45
(s, 2H), δ 7.19 (d, 2H), δ 7.17 (d, 2H), δ 6.67 (t, 2H), δ 4.66 (m, 2H), δ 4.56 (m, 2H), δ 4.23
(m, 2H), δ 3.96 (s, 6H), δ 3.56 (m, 2H).
Physical measurements
Elemental analyses (C, H and N) were performed using a Perkin–Elmer 2400 series II CHN
analyzer. IR spectra in KBr pellets (4000–400 cm−1) were recorded using a Perkin-Elmer RXI
FTIR spectrophotometer. The electronic absorption spectra (500–250 nm) in acetronitrile
solution were collected in a Hitachi U-3501 spectrophotometer. ESI mass spectra were
recorded on a WATERS Xevo G2-S QTof mass spectrometer in HRMS grade acetronitrile.
1H NMR spectra were recorded in CD3CN solvent for complexes 1−6 in a Bruker 400 MHz
NMR instrument, where tetramethylsilane was used as an internal standard.
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Electrochemical measurements
The electrochemical studies were performed using a Basi-Epsilon C3 Cell instrument at a
scan rate of 50–400 mVs–1 within the potential range of 0 to –1.60 V vs. Ag/AgCl. Cyclic
voltammograms were carried out using 0.1 MTBAP as supporting electrolyte and 1.0×10–3 M
of complexes in acetronitrile solution which are deoxygenated by argon purging. The
working electrode was a glassy carbon disk (0.32 cm2) which was polished with alumina
solution, washed with absolute acetone and accetronitrile, and air dried. The reference
electrode was Ag/AgCl, with platinum as counter electrode. The potentials are given with
respect to ferrocene/ferrocenium couple. All experiments were performed in standard
electrochemical cells at 25°C.
X-ray Crystallographic data collection and refinement
Data for all complexes 1–6 were recorded on a Bruker-AXSSMART APEXII diffractometer
equipped with a graphite monochromatorand Mo Kα (λ = 0.71073 Å) radiation. The crystals
were positioned at 60 mm from the CCD. 360 frames were measured with a counting time of
10 s. The structures were solved using direct methods with the Shelxs97 program[52]. The
non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms
bonded to carbon were included in geometric positions and given thermal parameters
equivalent to 1.2 times those of the atom to which they were attached. The hydrogen atoms
bonded to oxygen and nitrogen were located in difference Fourier maps and refined with
distance constraints. The structures were refined using Shelxl16-6[53] on F2. In 3, the lithium
ion and perchlorate anion were given occupancies of 50%. The perchlorate anion was
disordered four-fold axis and was refined with distance constraints. Details of the
crystallographic data for complexes 1–5 summarized in Table 2. CCDC 1953884–1953888
contain the supplementary crystallographic data of complexes 1–5, respectively for this paper
and CCDC 1953891 is for complex 6 which is given in Supporting Information.
Theoretical methods
The full geometries of the [MLUO2]+ series of complexes were optimized using DFT M06-
2X/def2-TZVP level of theory[54] without including counter-anion in the calculations. For
uranium we have used the def2-SVP basis set, as implemented in TURBOMOLE v. 7.0
program[55], which has been used for the optimizations. The minimum nature of the
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complexes was verified using frequency calculations. The orbital energies and plots were
computed using the Gaussian-09 program[56] using the same functional and the SDD basis
set[57]
.
Acknowledgements
T. K. G. is thankful to the University Grants Commission (UGC), New Delhi, for Senior
Research Fellowship [Sr. no. 2061510191, Ref. no. 21/06/2015 (I) EU-V, dated 15/12/2015].
We thank USIC, The University of Burdwan, West Bengal, India, for providing the single-
crystal X-ray diffractometer facility.
Keywords: U(VI) complexes • electrochemical study • bi-metallic complexes • theoretical
investigations • crystal structure
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27
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10.1002/chem.201904253
Chemistry - A European Journal
Table 2: Crystallographic data and structure refinement of complexes 1−5.
Compound
1
2
3
4
5
Chemical formula
C20H23N3O6U
C21.25H25.50N3.5O6.25 C20H23Cl0.5Li0.5
C20H23ClN3Na C20H25AgN4O10U
O10U
U
N3O8U
692.64
Tetragonal
I4mcm
20.8865(13)
20.8865(13)
10.7830(7)
90
Formula weight
Crystal system
Space group
a(Å)
639.44
Orthorhombic
Pnma
667.98
Monoclinic
P21
761.88
orthorhombic
Pccn
827.34
Orthorhombic
Pbca
10.0615(6)
24.6632(16)
8.3400(5)
90
10.3620(6)
11.2175(6)
20.1552(11)
91.618(2)
2341.8(2)
4
20.281(6)
21.469(6)
11.018(3)
90
19.354(2)
12.1934(14)
20.284(2)
90
b (Å)
c (Å)
β (ᵒ)
V (Å3)
2096.6(2)
4
4704.0(7)
8
4797(2)
8
4768.8(10)
8
Z
ρcalc(g cm-3)
T (K)
2.052
3.256
1.945
2.107
2.296
293(2)
296(2)
1.895
298(2)
7.003
293(2)
6.958
297(2)
7.639
µ (Mo Kα) (mm-1)
F(000)
7.885
1216
1278
2624
2904
3136
Reflections collected 26442
20807
38726
31542
4386
44510
Independent
reflections
1898
6926
2163
4345
Reflections with I > 1789
6355
1894
3360
2918
2σ(I)
Final
[I>2σ(I)]R1 , wR2
indices 0.0296, 0.0657
0.0302, 0.0782
0.0303, 0.0735
0.0679,0.1491 0.0954, 0.1639
a
b
R1[a],wR2[b] [all data]
0.0320, 0.0668
1.285
0.0346, 0.0805
1.044
0.0394, 0.0810
1.136
0.0877,0.1601 0.1544,0.1863
GOF c
1.124
1.165
Residual
Density, e/Å-3
electron 1.797,
1.010, -1.256
1.107,-0.965
3.555,-1.627
2.988,-3.392
-2.881
aR1 = ∑||Fo|−|Fc||/∑|Fo|,bwR2 (Fo ) = [∑[w(Fo − Fc2)2/∑w Fo ] and cGOF = [∑[w(Fo − Fc2)2/(Nobs– Nparams)]
2
2
4 ½
2
½
28
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10.1002/chem.201904253
Chemistry - A European Journal
Table of Contents Only
The effect of guest metal ions on the reduction potentials of uranium(VI)
complexes: Experimental and theoretical investigations
Tanmoy Kumar Ghosh,[a] Prithwish Mahapatra,[a] Michael G. B. Drew,[b]
Antonio Franconetti,[c] Antonio Frontera,[c]* Ashutosh Ghosh[a]*
A substantial shift of the reduction potential of U(VI)/U(V) couple in presence of guest metal
ions. The DFT studies showed a good agreement with experimental results.
29
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