The synthesis and spectroscopic characterization of an aromatic uranium amidoxime complex

Article abstract of DOI:10.1016/j.ica.2014.06.023

A tridentate aromatic amidoxime ligand and its corresponding uranyl complex have been synthesized. The complex was rigorously characterized with single-crystal X-ray diffraction, solid-state NMR, infrared absorption, and Raman spectroscopy. Unlike previous amidoxime complexes, the crystal structure confirms the complexation of only a single tridentate amidoxime ligand to the uranium, and solid state NMR confirms uranyl binding in the bulk sample. The Raman spectrum of the complex is dominated by the strong UO vibration which exhibits a significant shift from the UO vibration in unbound uranyl. Density functional theory (DFT) aided in calculations of vibrational modes as well as providing insight into the electronic structure of this uranyl complex.

Full text of DOI:10.1016/j.ica.2014.06.023

Inorganica Chimica Acta 421 (2014) 374–379
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
The synthesis and spectroscopic characterization of an aromatic uranium
amidoxime complex
a
a
a
a
a
Karl J. Bernstein , Chi-Linh Do-Thanh , Deborah A. Penchoff , S. Alan Cramer , Christopher R. Murdock ,
a
b
a,
⇑
a,
⇑
Zheng Lu , Robert J. Harrison , Jon P. Camden , David M. Jenkins
a
Department of Chemistry, University of Tennessee, Knoxville, TN 37996, United States
Institute for Advanced Computational Science, Stony Brook University, Stony Brook, NY 11794, United States
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 May 2014
Received in revised form 12 June 2014
Accepted 14 June 2014
Available online 5 July 2014
A tridentate aromatic amidoxime ligand and its corresponding uranyl complex have been synthesized.
The complex was rigorously characterized with single-crystal X-ray diffraction, solid-state NMR, infrared
absorption, and Raman spectroscopy. Unlike previous amidoxime complexes, the crystal structure con-
firms the complexation of only a single tridentate amidoxime ligand to the uranium, and solid state
NMR confirms uranyl binding in the bulk sample. The Raman spectrum of the complex is dominated
by the strong U@O vibration which exhibits a significant shift from the U@O vibration in unbound uranyl.
Density functional theory (DFT) aided in calculations of vibrational modes as well as providing insight
into the electronic structure of this uranyl complex.
Keywords:
Uranium
Raman
Theoretical calculation
Solid state NMR
Infrared
Ó 2014 Elsevier B.V. All rights reserved.
1
. Introduction
Aqueous uranyl chemistry is experiencing a renaissance due to
the normal amidoximes [10]. Recent advances in understanding
the binding of polydentate amidoximes to uranyl by Hay [14]
and Rao [15–17] provided insight into the structural motifs that
are possible when employing these ambidentate ligands.
While simple amidoximes have been well characterized by a
variety of spectroscopic techniques, their cyclic counterparts have
received far less attention [18]. To our knowledge, detailed spec-
troscopic measurements, such as NMR, IR, and Raman, have not
been collected on any of the cyclic amidoxime uranyl complexes.
Vibrational spectroscopy is of particular interest since these initial
measurements would be necessary to determine if these systems
could be viable in the field of nuclear forensics.
In this manuscript, we describe the synthesis of a tridentate
amidoxime ligand that complexes with uranyl in a 1:1 fashion.
The uranyl complex was structurally characterized by single crys-
tal X-ray diffraction. In addition to the structural characterization,
additional solid state spectroscopic methods such as NMR, IR, and
Raman were employed. The Raman spectroscopy suggests that
conjugated ligands like this one could lead to novel detection
methods for actinides.
two distinct, but related, applications: extraction of uranium from
seawater and nuclear forensics [1–4]. Due to the very low concen-
tration of uranium for each application, sorption methods are con-
sidered the only viable option as a result of financial and
environmental considerations [5,6]. The oceans contain the uranyl
ion at an uniform concentration of 3.3 ppb (ppb) [7,8]; this ubiq-
uity of uranium in nature and its destructive potential as a weapon
underpin the necessity of nuclear forensics. This field is concerned
with who has harnessed uranium’s deadly capabilities and it has
typically relied heavily on lab-based mass spectrometry for analy-
sis [2,9].
Uranyl extraction from seawater and spectroscopic detection
for nuclear forensics could benefit from the development of highly
selective uranyl binding ligands. Several complexing agents for
sorptive methods of uranyl extraction have been investigated,
and evaluated based on selectivity, robustness, loading capacity,
and rate of uptake [5,10–13]. After extensive methodology evalua-
tion, amidoxime functionalized polymers were selected as optimal
[
5]. Further investigation of the uranyl complexing polymers
2
. Experimental
revealed the presence of cyclic, polydentate amidoximes alongside
2.1. Materials
⇑
Corresponding authors.
E-mail addresses: jcamden@ion.chem.utk.edu (J.P. Camden), jenkins@ion.chem.
The compound acenaphtho[1,2-c][1,2,5]thiadiazole 8,8-dioxide
utk.edu (D.M. Jenkins).
(1) was prepared via a modification of the published procedure
http://dx.doi.org/10.1016/j.ica.2014.06.023
0
020-1693/Ó 2014 Elsevier B.V. All rights reserved.

K.J. Bernstein et al. / Inorganica Chimica Acta 421 (2014) 374–379
375
of Qian [19]. Compound 1 was also prepared by Wright [20]. 1,8-
2.4. Vibrational spectroscopy
dicyanonaphthalene (2) was synthesized in a manner similar to
Ege’s pyrolysis approach (Ege provided no characterization of the
product except melting point) [21]. 1,8-naphthalimide dioxime
Excitation was provided by a HeNe laser (HNL100L, Thorlabs)
with a laser line of 632.8 nm and a power of 0.6 mW at the objec-
tive. An inverted microscope (Ti-U, Nikon Eclipse) with a 10Â, 0.30
NA dry objective (Nikon) in the 180° configuration was used in
conjunction with Rayleigh filter and dichroic (Semrock) to collect
the Raman scattering. Spectra were recorded with a 300 cm focal
(3) was prepared via adaptation of the published procedures of
Forrester [22]. Our detailed synthetic procedures and analytical
data are described below. All other reagents were purchased from
commercial vendors and used without further purifications.
length spectrometer (Acton, Princeton Instruments) containing a
À1
1
200 groove mm holographic grating, and a 10
l
m slit with an
2
.2. Methods and instrumentation
attached liquid-nitrogen-cooled deep-depletion charge-coupled
device (CCD, Spec10, Princeton Instruments). Spectra were cali-
brated with cyclohexane. Infrared spectra were collected on a
Thermo Scientific Nicolet iS10 with a Smart iTR accessory for atten-
uated total reflectance. Raman and IR spectra were both obtained
from the crystalline solid.
1H and 13C{ H} solution spectra were recorded at ambient tem-
1
1
perature on a Varian VNMRS 500 MHz narrow-bore system. H,
1
3
1
C{ H}, HSQC, and HMBC NMR chemical shifts were referenced
to the residual solvent. Solid 13C CP MAS NMR samples were
recorded on a Varian Inova 400 MHz spectrometer and referenced
to an external adamantane sample. 13C CP MAS with interrupted
decoupling (to suppress protonated carbons) was applied using a
spin rate of 6 kHz with the dipolar dephasing turned on for
2.5. Computational methodology
Geometry optimizations and vibrational frequencies were
obtained with the NWCHEM [25] package using density functional
theory (DFT), and the B3LYP functional [26–28]. The Stuttgart
RSC 1997 effective core potential and its corresponding basis set
was used for uranium [29] (with the most diffuse S, P, D, and F
functions removed, i.e., those with exponent 0.005, since they per-
tain only to the neutral metal atom and cause numerical problems
in molecules), and the 6-311++g⁄⁄ basis set was used for oxygen,
nitrogen, carbon, and hydrogen atoms [30]. The ECP accounts for
relativistic effects and replaces 60 electrons of uranium with a
pseudopotential. Basis sets were obtained from the EMSL database
[31,32]. This computational protocol has been established as pro-
ducing reliable geometries and energetics for uranyl complexes
[33]. Geometry optimizations were performed with tight conver-
gence settings. Frequency calculations were performed to obtain
thermochemical corrections, and to verify that the structures
1
00 ls. All mass spectrometry analyses were conducted at the
Mass Spectrometry Center located in the Department of Chemistry
at the University of Tennessee. The DART analyses were performed
using a JEOL AccuTOF-D time-of flight (TOF) mass spectrometer
with a DART (direct analysis in real time) ionization source from
JEOL USA, Inc. (Peabody, MA). The ESI/MS analyses were performed
using a QSTAR Elite quadrupole time-of-flight (QTOF) mass spec-
trometer with an electrospray ionization source from AB Sciex
(
Concord, Ontario, Canada). Sample solutions of complexes for
mass spectrometry were prepared in acetonitrile, tetrahydrofuran,
or DMSO (for uranium complex, 4, only). Carbon, hydrogen, and
nitrogen analyses were obtained from Atlantic Microlab, Norcross,
GA.
2.3. X-ray structure determination
reflect local minima. Volumetric data was generated with NWCHEM,
modeled with Visual Molecular Dynamics (VMD) [34] and plotted
with Persistence of Vision Raytracer (Pov-Ray) [35].
Data was collected on a Bruker SMART APEX II three circle dif-
fractometer equipped with a CCD area detector and operated at
800 W power (45 kV, 40 mA) to generate Mo K radiation
k = 0.71073 Å). The incident X-ray beam was focused and mono-
1
(
a
2.6. Synthesis of acenaphtho[1,2-c][1,2,5]thiadiazole 8,8-dioxide, 1
chromated using Bruker Excalibur focusing optics. Single crystals
were mounted on nylon CryoLoops (Hampton Research) with Par-
atone-N (Hampton Research) and frozen at À173 °C. Initial scans of
each specimen were taken to obtain preliminary unit cell parame-
ters and to assess the mosaicity (i.e. breadth of spots between
frames) of the crystal to select the required frame width for data
collection. For all cases frame widths of 0.5° were judged to be
appropriate and full hemispheres of data were collected using
Sulfamide (15.8 g, 161 mmol) and p-toluenesulfonic acid mono-
hydrate (1.36 g, 7.14 mmol) were added to acenaphthenequinone
(10.0 g, 53.8 mmol) in absolute ethanol (400 mL), and the mixture
was heated at 80 °C for 3 days. After cooling to room temperature,
the solvent was removed by rotary evaporation. The residue was
then extracted with methylene chloride (2 Â 350 mL) from water
(350 mL), and a light brown precipitate formed above the organic
layer. The precipitate was collected, and the combined organic lay-
ers were dried with magnesium sulfate, filtered, and concentrated.
The concentrate was combined with the precipitate and recrystal-
lized from boiling pyridine (200 mL) to give the product as a light
the BRUKER APEX2 software suite to carry out overlapping
u and x
scans at detector setting of 2h = 28°. Following data collection,
reflections were sampled from all regions of the Ewald sphere to
re-determine unit cell parameters for data integration. Following
exhaustive review of collected frames the resolution of the dataset
was judged, and, if necessary, regions of the frames where no
coherent scattering was observed were removed from consider-
ation for data integration using the BRUKER SAINTPLUS program [23].
Data was integrated using a narrow frame algorithm and was sub-
sequently corrected for absorption. Absorption corrections were
performed for both samples using the SADABS program [23]. Space
group determination and tests for merohedral twinning were car-
ried out using XPREP [23]. In all cases, the highest possible space
group was chosen. Final models were refined anisotropically (with
the exception of H atoms). The Addsym subroutine of PLATON [24]
was utilized to assure that no additional symmetry could be
applied to the models.
brown powder (7.35 g, 56.4% yield).
1
6
H NMR (DMSO-d , 499.74 MHz): d 8.49 (d, J = 7.8 Hz, 2H), 8.45
1
3
6
(d, J = 7.8 Hz, 2H), 8.01 (t, J = 7.7 Hz, 2H). C NMR (DMSO-d ,
125.67 MHz): d 166.24, 148.57, 133.26, 130.77, 129.71, 126.80,
123.13. IR (neat): 1630, 1581, 1485, 1415, 1355, 1170, 1091,
À1
1070, 1038, 986, 942, 920, 831, 776, 738, 694, 663 cm . HR-
+
DART/MS (m/z): [M+H] 243.0219 (found); C12
7
H N
O
2 2
S 243.0228
(calcd).
2.7. Synthesis of 1,8-dicyanonaphthalene, 2
Acenaphtho[1,2-c][1,2,5]thiadiazole 8,8-dioxide (1) (2.04 g,
8.41 mmol) was pyrolyzed under vacuum (150 mTorr) at 180 °C
in a water-cooled sublimation apparatus overnight. The resulting

3
76
K.J. Bernstein et al. / Inorganica Chimica Acta 421 (2014) 374–379
solids were collected and resublimed under vacuum (150 mTorr) at
vacuum (150 mTorr) at 180 °C which caused the loss of sulfur diox-
ide and formation of 1,8-dicyanonaphthalene, 2. The loss of sulfur
dioxide to form aromatic dinitriles has previously been reported by
Ege and Hay [21,36]. Following the method of Forrester, 1,8-dicy-
anonaphthalene reacted with hydroxylamine in a refluxing ethanol
and water mixture to form 1,8-naphthalimide dioxime, 3, (Np-
1
9
80 °C overnight to give the product as a pure yellow solid (1.36 g,
0.6% yield).
1
3
H NMR (CDCl , 499.74 MHz): d 8.18 (d, J = 8.4 Hz, 2H), 8.13 (d,
13
J = 8.4 Hz, 2H), 7.68 (t, J = 7.6 Hz, 2H).
3
C NMR (CDCl ,
1
1
1
7
C
25.67 MHz): d 138.00, 134.68, 133.55, 128.99, 126.84, 116.81,
09.05. IR (neat): 3061, 2227, 2215, 1586, 1571, 1512, 1372,
360, 1230, 1218, 1174, 1111, 1092, 990, 970, 943, 828, 779,
3
CAO-H ) [22]. The ligand (3) can be purified by crystallization from
absolute ethanol and must be stored under nitrogen to prevent
À1
+
1
61, 694, 666 cm . HR-DART/MS (m/z): [M+H] 179.0605 (found);
179.0609 (calcd).
slow decomposition. Compounds 1–3 were characterized by
H
1
3
12
7
H N
2
and C NMR as well as HR-DART/MS and IR.
The uranyl complex, (Np-CAO-H )U(O) (NO
2
2
3
)(CH
3
OH), 4, was
prepared by mixing 3 and uranyl nitrate hexahydrate in methanol
Scheme 1). Red crystals of 4 that were suitable for single crystal
2
.8. Synthesis of 1,8-naphthalimide dioxime, 3, (Np-CAO-H
3
)
(
diffraction were formed in 45% yield after 2 days. Single crystal dif-
fraction demonstrated that the uranyl center is coordinated to a
single cyclic dioxime ligand, a bidentate nitrate group, and a meth-
anol in the equatorial plane. This result is in contrast to the uncon-
jugated cyclic imide dioxime ligand prepared by Rao wherein two
equivalents bind to the metal center. Even addition of excess 3 still
yielded 4 and not the bis tridentate amidoxime uranyl product
Hydroxylamine hydrochloride (0.982 g, 14.1 mmol) and potas-
sium hydroxide (0.793 g, 14.1 mmol) were added to 1,8-dicyano-
naphthalene (0.504 g, 2.83 mmol) in water (18 mL) and ethanol
(
6 mL). The reaction mixture was stirred at 80 °C overnight and
then cooled to room temperature, filtered, and washed with cold
ethanol. The solid residue was recrystallized from boiling absolute
ethanol (50 mL) and dried under vacuum for 1 h to give the prod-
uct as yellow needles (0.351 g, 54.6% yield). The product was
stored under nitrogen to prevent oxidation that occurs over a 4 d
(
Np-CAO-H
2 2 2
) U(O) .
X-ray diffraction shows that the geometry about the uranium
atom can best be described as hexagonal bipyramidal (Fig. 1)
[37–39]. Each atom that is ligated to the uranium has a respec-
tively ligated atom that is nearly trans to it. The two oxo ligands
are bound in the axial positions and show only a mild distortion
of the O AUAO bond angle of 177.1°. The UAO bond distances
period to form the dark red-brown 1,8-naphthalimide.
1
6
H NMR (DMSO-d , 499.74 MHz): d 11.05 (s, 2H), 8.96 (s, 1H),
8
2
1
1
.14 (d, J = 7.0 Hz, 2H), 8.06 (d, J = 7.9 Hz, 2H), 7.63 (t, J = 7.6 Hz,
1
3
6
H). C NMR (DMSO-d , 125.67 MHz): d 141.02, 132.40, 129.42,
13
1
2
26.40, 125.66, 121.90, 120.59. C SSNMR (Adamantane standard,
00.53 MHz): d 144.65, 130.84, 127.05, 125.19, 120.97. 13C SSNMR
are asymmetric (UAO , 1.781(6); UAO , 1.767(6)) and are well
1
2
within the range of similar uranyl complexes [40–42]. Like Rao’s
glutarimidedioxime uranyl complex, central nitrogen (N4) is
deprotonated and a proton shift occurs on the oxime groups creat-
ing an extended aromatic structure [15]. The UAN4 bond distance
for 4 is 2.514(1) Å, which is slightly shorter than the UAN bond dis-
tance (2.563(3) Å) found in Rao’s glutarimidedioxime uranyl com-
plex [15]. This chelating imide dioxime is a rare example of
tridentate binding to uranyl with this class of ligands [15], and con-
trasts to the binding motif of amidoximates to uranyl [33].
(
interrupted decoupling, Adamantane standard, 100.53 MHz): d
1
1
8
44.82, 131.16, 125.33, 120.95. IR (neat): 3407, 3384, 3058, 2813,
642, 1606, 1469, 1433, 1387, 1332, 1217, 1165, 1085, 1003, 957,
À1
98, 872, 827, 794, 764, 719, 685, 667, 641, 610 cm . HR-DART/
+
MS (m/z): [M+H] 228.0773 (found); C12
H
10
N
3
O
2
228.0773 (calcd).
OH), 4
A solution of 1,8-naphthalimide dioxime (0.0386 g, 0.170 mmol)
2
2 2 3 3
.9. Synthesis of (Np-CAO-H )U(O) (NO )(CH
in methanol (19 mL) was added to a solution of uranyl nitrate hexa-
hydrate (0.0873 g, 0.173 mmol) in methanol (19 mL). The complex
was formed and crystallized by slow evaporation from the methanol
solution after 2 d. The remaining solution was decanted, and the red
crystals were washed once with methanol before air drying
3
.2. Spectroscopic characterization of uranyl complex
Although a few similar uranyl imide dioxime complexes have
been prepared previously by Rao and others, almost no detailed
spectroscopic work has been undertaken outside of UV–Vis
measurements [15–17]. Detailed NMR spectroscopy has not been
(
0.0453 g, 45.2% yield).
1
3
C SSNMR (Adamantane, 100.53): d 151.92, 132.52, 127.97,
1
3
1
1
1
1
14.68.
C
SSNMR (interrupted decoupling, Adamantane,
00.53): d 151.73, 132.03, 125.32, 114.24. IR (neat): 3307, 2915,
627, 1596, 1512, 1370, 1341, 1287, 1238, 1223, 1170, 1108,
055, 1040, 1008, 910, 836, 808, 796, 770, 750, 705, 688 cm
À1
.
+
HR-ESI/MS (m/z): [MÀNO
3
ÀCH
3
OH+2DMSO] 652.1269, [MÀNO3-
+
+
ÀCH
3
OH+DMSO] 574.1144, [MÀNO
3
ÀCH
3
OH] 496.1042 (found);
+
+
[
MÀNO
3
ÀCH OH+2DMSO] 652.1301, [MÀNO
74.1162, [MÀNO
U: C, 26.45; H, 2.05; N, 9.49. Found: C, 25.32; H, 1.71;
3
3
ÀCH
3
OH+DMSO]
+
5
C
3
ÀCH
3
OH] 496.1023 (calcd). Anal. Calc. for
13
12 4 8
H N O
N, 9.62%.
3
. Results and discussion
3.1. Synthesis of ligand and uranyl complex
We exploited a three-step strategy to prepare the conjugated
cyclic imide dioxime ligand. We synthesized acenaphtho[1,2-
c][1,2,5]thiadiazole 8,8-dioxide, 1, via an acid-catalyzed condensa-
tion reaction with sulfamide in refluxing ethanol that was modified
from Qian (Scheme 1) [19]. Following a modification of Ege’s syn-
thesis [21], compound 1 was subjected to pyrolysis under dynamic
Scheme 1. Synthesis of cyclic imide dioxime, 3, and its uranyl complex, 4.

K.J. Bernstein et al. / Inorganica Chimica Acta 421 (2014) 374–379
377
Fig. 1. Crystal structure of (Np-CAO-H
tridentate binding of cyclic amidoxime ligand. Green, blue, red, and gray ellipsoids
50% probability) represent U, N, O, and C, respectively. White spheres represent H.
2 2 3 3
)U(O) (NO )(CH OH) (4) that shows
(
Selected bond distances (Å) and angles (°) are as follows: UAO
1
, 1.781(6); UAO
, 2.572(6); UAO
, 170.1(2); O AUAO
2
,
8
,
6
,
1
2
1
.767(6); UAO
.462(7); UAN
76.8(2); O AUAN
3
, 2.398(6); UAO
, 2.514(7); O AUAO
, 169.0(2).
4
, 2.420(6); UAO
5
, 2.589(6); UAO
6
4
1
2
, 177.1(3); O
3
AUAO
8
4
5
4
collected for uranyl imide dioxime complexes and provides an
effective method for determination of uranyl complexation. One
area that would also be of potential importance for nuclear foren-
sics is a detailed understanding of the vibrational spectroscopy
associated with binding ligands such as imide dioximes and the
specific actinides targeted.
Since uranyl complex 4 has very low solubility in common
organic solvents, we employed solid state 1 C CP MAS NMR. To
unambiguously determine the peak positions for each carbon,
3
1
13
Fig. 2. (A) The 13C CP MAS NMR of 3 with interrupted decoupling exhibiting the
resonances of the four quaternary carbons. (B) The C CP MAS NMR of 4 with
solution state H and C NMR, along with HSQC and HMBC spec-
tra, were collected for ligand 3 in DMSO-d (SI Figs. S1 and S2).
13
6
13
interrupted decoupling revealing the shifted resonances of the four quaternary
These solution spectra were compared to a C CP MAS NMR spec-
trum of 3 (SI Fig. S3), but the width of the peaks in the 13C CP MAS
NMR spectrum made assignment of each individual carbon signal
problematic.
⁄
carbons in the ligand. Denote spinning sidebands.
À1
À1
To simplify the assignment of the peak positions, in particular
the imine carbon as it is the closest in proximity to the bound ura-
nyl in 4, we turned to interrupted decoupling NMR. Interrupted
decoupling serves to suppress protonated carbons, therefore
removing a majority of the aromatic carbons and allowing for
assignment of the individual carbon atoms of interest [43,44].
When the protonated carbons were suppressed, four clearly
resolved resonances at 121.0, 125.3, 131.2, and 144.8 ppm were
observed (Fig. 2A). An interrupted decoupling NMR was then col-
lected for complex 4 as standard 13C CP MAS NMR again led to
broad, unassignable peaks (SI Fig. 3B) and showed four resonances
at 114.7, 128.0, 132.5, and 151.9 ppm. The imine carbon (denoted
1642 cm , and
ligand–UO complex 4, it exhibits a combination of stretches from
the ligand, UO
mNAO stretch at 957 cm . Upon mixing the
2
2
, and NO
3
, moieties. Of special note is the shifting of
À1
peaks in the uranyl region (ꢀ1000–800 cm ) as shown in Fig. 3.
2
All of the characteristic vibrations from the UO and ligand 3 in this
region have shifted from their original positions.
The Raman spectrum of the uranyl salt is dominated by the
À1
m
U@O,s stretch at 867 cm while the
m
U@O,as stretch is virtually
indistinguishable from noise. The Raman spectrum of the ligand
3 exhibits a small fluorescence background even after purification
by recrystallization, although this is not unexpected from a conju-
gated system. The Raman spectrum is characterized by an intense
À1
À1
‘
‘a’’ in Fig. 2) resonance shifts significantly downfield by 7.1 ppm as
mode at 1380 cm and a weaker feature at 511 cm . Upon bind-
ing, the large polarizability of the UO group results in the spectra
of the complex 4 being dominated by the
as mentioned previously[46,48,49], the
the carbons are deshielded compared to the free ligand, providing
evidence that the ligand is now bound [38]. Solid state 13C CP MAS
NMR has rarely been employed on uranyl complexes and, to our
knowledge, interrupted decoupling 13C CP MAS NMR has never
been performed [42]. This solid state spectroscopic method is an
effective technique that demonstrates that the ligand is bound to
the uranyl in the bulk material.
While NMR may not be effective for nuclear forensics due to
lack of portability, vibrational spectroscopy of uranyl complexes
may be more insightful for field-based tests. The vibrational spec-
2
m
U@O,s stretch. However,
U@O,as stretch at 918 cm
À1
m
is forbidden in the Raman spectrum and is practically unnotice-
able. In the Raman spectrum the U@O,s stretch is free of competing
m
ligand vibrations, has a very strong intensity and is distinct. Thus
the frequency of the stretch can serve as a clear indicator for the
complexation of uranyl.
Due to the expansive, vibration free region around the
m
U@O,s
stretch it is likely that the symmetric stretch from other actinides
would also be clearly visible with this ligand system. Many theo-
retical and experimental studies have demonstrated the sequential
troscopy of uranyl species, UO
2
(NO
3
)
2
Á6H
2
O in particular, have
been previously characterized [45–48,9], and our results are in
agreement with these previous studies. The IR spectrum of the ura-
nyl nitrate hexahydrate is dominated by the intense and broad
trend of the mAn@O,s and mAn@O,as stretching frequencies of uranium,
neptunium, and plutonium oxycation moities [50–53]. We believe
the sparseness of the uranyl region in the Raman spectra coupled
with the sequential nature of the actinide oxycations vibrational
modes presents an excellent opportunity to use ligand 3 as a detec-
tion sensor for the associated hexavalent actinide cations.
-1
asymmetric uranyl (
m
U
@O,as) stretch at 935 cm , while a weak
À1
symmetric uranyl (
m
U
@O,s) stretch is visible at 867 cm . The ligand
3
exhibits characteristic imide and oxime vibrations with the m
OAH
À1
À1
stretch at 3407 cm
,
m
NAH stretch at 3384 cm
, mN@C stretch at

3
78
K.J. Bernstein et al. / Inorganica Chimica Acta 421 (2014) 374–379
À1
but clearly shows the
8
m
U@O,s stretch at 867 cm that is shifted to
À1
42 cm in the complex. This stretch in the complex 4 is free from
significant competing vibrations down to 800 cm
10 cm which allows for it to be quickly and easily identified
À1
and up to
À1
9
unlike either of the uranyl modes in the IR. Overall, the shifting
of the frequencies in the uranyl region can be explained by the
expansion of the conjugated system in the ligand to include the
uranyl moiety. Further experimental evidence is presented in the
bond lengths between uranium and the oxo ligands. The uranyl
group in the uranyl nitrate has bond lengths of 1.770 and
1
.749 Å while the group in the complex (4) has bond lengths of
1
.781 and 1.767 Å, respectively [54]. The inclusion of the uranyl
moiety in the conjugated system leads to loss of electron density
between the uranium and oxo ligands. This weakens the bonds,
leading to elongated bond lengths.
3.3. DFT studies of uranyl complex
DFT calculations of the uranyl complex were performed to gain
a greater understanding of the electronic structure of the system.
Electronic orbital modeling of uranyl systems has been discussed
previously [55–63]. The geometry-optimized structure was com-
pared with the crystal structure and generally found to be in good
agreement given the comparison of gas and crystal phase results
(
e.g., all UAO distances agreed within 0.08 Å except to the metha-
nol oxygen which was 0.13 Å shorter in the computation). Upon
binding, one significant change in the ligand geometry was a short-
ening by approximately ꢀ0.09 Å of the O3AN2 and N1AO4 dis-
tances in the optimized structure of
4 compared to the
experimentally determined structure. This bond shortening sug-
gests delocalized electron density on the OANACANACANAO
ligand backbone, which could substantiate the notion of a highly
conjugated system. These results were consistent with Rao’s find-
ing for a uranyl complexed to glutarimidedioxime [15]. In addition,
the ligand–U bond lengths were all within 0.10 Å of the experi-
mentally determined structures except for the bound methanol
(
O8), which was 0.13 Å further from uranium in the calculated
structure. Overall, the accurate reproduction of the experimental
complex gives us confidence that our electronic structure methods
are valid.
Fig. 3. The uranyl region (U(O)
bottom). The spectra of ligand 3, complex 4, and uranyl nitrate hexahydrate are
shown from top to bottom. The positions, left to right, of the antisymmetric (
and symmetric ( ) U@O stretches are labeled for uranyl and the complex while
competing vibrations are labeled for the ligand.
2
) of the vibrational spectra, both IR (top) and Raman
(
as
m )
m
s
Density functional theory (DFT) calculations of vibrational
modes aided in the understanding of changes between the individ-
ual molecular systems and the combined complex 4. In particular it
À1
was noted that the
m
U@O,s stretch at 867 cm is barely discernible
in the IR of the uranyl complex. Once bound, the
mU@O,s band is
À1
blue-shifted to 842 cm and overwhelmed by an intense ligand
À1
À1
3
vibration at 835 cm . The mU@O,as stretch at 935 cm is clearly
visible in the uranyl complex, though upon binding the uranyl
mode is again blue-shifted and a competing vibration from the
À1
ligand 3 at 905 cm again overwhelms and merges with the ura-
nyl vibration. In both cases the U@O stretch in the IR spectrum is
obscured either partially or completely by modes in the ligand 3.
Fig. 4. (A) LUMO of (Np-CAO-H
C) Orbital splitting diagram of complex 4 obtained from DFT calculations. All
calculations used NWChem/B3LYP/6-311++g⁄⁄.
2 2 3 3
)U(O) (NO )(CH OH) (4). (B) HOMO of complex 4.
(
The Raman spectrum of the uranyl salt lacks the
mU@O,as stretch

K.J. Bernstein et al. / Inorganica Chimica Acta 421 (2014) 374–379
379
The molecular orbitals around the HOMO and LUMO are, natu-
rally, of special interest. When examining the highest occupied
molecular orbital (HOMO) (Fig. 4b) and adjacent lower orbitals,
[5] H.J. Schenk, L. Astheimer, E.G. Witte, K. Schwochau, Sep. Sci. Technol. 17 (1982)
293.
1
[
6] J. Kim, C. Tsouris, R.T. Mayes, Y. Oyola, T. Saito, C.J. Janke, S. Dai, E. Schneider, D.
Sachde, Sep. Sci. Technol. 48 (2013) 367.
the majority of contributions come from ligand-based
(
(
with a small contribution (10%) from an f-orbital. This result is in
contrast with the LUMO+1 to LUMO+4 orbitals that show exclu-
sively f-orbital character (Fig. S6). Finally, we note that the complex
p
-orbitals
Fig. S5). In the case of the lowest unoccupied molecular orbital
LUMO) (Fig. 4a), most of the electron density is also ligand based,
[7] R.V. Davies, J. Kennedy, K.M. Hill, R.W. McIlroy, R. Spence, Nature 203 (1964)
110.
1
[
[
8] M.J. Tamada, Jpn. Inst. Energy 88 (2009) 249.
9] D. Bhandari, S.M. Wells, S.T. Retterer, M. Sepaniak, J. Anal. Chem. 81 (2009)
8061.
[
[
10] L. Astheimer, H.J. Schenk, E.G. Witte, K. Schwochau, Sep. Sci. Technol. 18 (1983)
07.
11] A. Zhang, G. Uchiyama, T. Asakura, Adsorp. Sci. Technol. 21 (2003) 761.
3
4
6
has a reasonably large HOMO–LUMO gap of approximately
1.24 kcal/mol.
[12] A. Zhang, T. Asakura, G. Uchiyama, React. Funct. Polym. 57 (2003) 67.
[
[
[
13] A. Zhang, G. Uchiyama, T. Asakura, Sep. Sci. Technol. 2003 (1829) 38.
14] B.P. Hay, O. Clement, G. Sandrone, D.A. Dixon, Inorg. Chem. 37 (1998) 5887.
15] G.X. Tian, S.J. Teat, Z.Y. Zhang, L.F. Rao, Dalton Trans. 41 (2012) 11579.
4
. Conclusion
[16] X. Sun, G. Tian, C. Xu, L. Rao, S. Vukovic, S.O. Kang, B.P. Hay, Dalton Trans. 43
2014) 551.
(
[
[
17] G.X. Tian, S.J. Teat, L.F. Rao, Dalton Trans. 42 (2013) 5690.
18] C.L. Bell, C.N.V. Nambury, L. Bauer, J. Org. Chem. 29 (1964) 2873.
In summary, we present a comprehensive synthetic, spectro-
scopic, and theoretical characterization of an aromatic tridentate
amidoxime uranyl complex. The X-ray structure of the complex
demonstrates that unlike Rao’s tridentate amidoxime complex, only
one equivalent of this aromatic ligand binds to the uranyl ion. The
solid state NMR spectrum of the complex is, to our knowledge, the
first NMR taken of a combined amidoxime-uranium complex and
demonstrates complexation of the bulk material. DFT afforded
insight into the electronic structure of this uranyl complex as well
as assisted in assigning the vibrational modes. While the U@O vibra-
tions were obscured in the IR spectrum by nearby vibrations, they
were easily observed in the Raman spectrum and these signals in
the Raman spectra could potentially serve as a spectroscopic marker
for uranium detection. The thorough evaluation of this complex’s
vibrational spectra is a critical first step in the development of cyclic
polydentate amidoximes for nuclear forensics and may also aid
in situ investigations of extraction of uranyl from seawater.
[19] X.H. Qian, Y. Xiao, Tetrahedron Lett. 43 (2002) 2991.
[
[
[
20] J.B. Wright, J. Org. Chem. 1964 (1905) 29.
21] G. Ege, E. Beisiege, Synthesis (1974) 22.
22] A.R. Forrester, H. Irikawa, R.H. Thomson, S.O. Woo, T.J. King, J. Chem. Soc.,
Perkin Trans. 1 (1981) 1712.
[
[
23] Bruker, SAINT-PLUS, Bruker AXS Inc., M., Wisconsin, USA, 2007.
24] A.L. Spek, PLATON, A Multipurpose Crystallographic Tool, Utrecht University,
Utrecht, The Netherlands, 2005.
[25] M. Valiev, E.J. Bylaska, N. Govind, K. Kowalski, T.P. Straatsma, H.J.J. Van Dam, D.
Wang, J. Nieplocha, E. Apra, T.L. Windus, W. de Jong, Comput. Phys. Commun.
181 (2010) 1477.
[
26] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[27] A.D. Becke, Phys. Rev. A 38 (1988) 3098.
[28] C.T. Lee, W.T. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.
[
[
[
29] M. Dolg, H. Stoll, H. Preuss, R.M. Pitzer, J. Phys. Chem. 97 (1993) 5852.
30] R. Krishnan, J.S. Binkley, R. Seeger, J.A. Pople, J. Chem. Phys. 72 (1980) 650.
31] D. Feller, J. Comput. Chem. 17 (1996) 1571.
[32] K.L. Schuchardt, B.T. Didier, T. Elsethagen, L.S. Sun, V. Gurumoorthi, J. Chase, J.
Li, T.L. Windus, J. Chem. Inf. Model. 47 (2007) 1045.
[
33] S. Vukovic, L.A. Watson, S.O. Kang, R. Custelcean, B.P. Hay, Inorg. Chem. 51
2012) 3855.
(
[
[
34] W. Humphrey, A. Dalke, K. Schulten, J. Mol. Graph. 14 (1996) 33.
35] Persistence of Vision (TM) Raytracer, version 3.7.0, Persistence of Vision Pty.
Ltd., Williamstown, Victoria, Australia, 2004.
Acknowledgments
[
36] C.D. Grant, S.O. Kang, B.P. Hay, J. Org. Chem. 78 (2013) 7735.
C.D. and D.M.J. were supported by the Fuel Resources Campaign
in the Fuel Cycle Research and Development Program, Office of
Nuclear Energy, U.S. Department of Energy (DOE). K.J.B. and J.P.C.
were supported by the U.S. National Science Foundation under
Grant CHE-1150687. D.P. and R.J.H. were supported by the National
Science Foundation under grant OCI-0904972. The DFT calcula-
tions in this manuscript were supported by the National Science
Foundation under Grant numbers 0711134, 0933959, 1041709,
and 1041710 and the University of Tennessee through the use of
the Kraken computing resource at the National Institute for Com-
putational Sciences (http://www.nics.tennessee.edu). In addition,
DFT results in this manuscript are based in part upon work per-
formed on the computer system at the Institute for Advanced Com-
putational Science, SUNY Stony Brook with support from New York
State and the IACS endowment fund.
[37] L.L. Liang, Y.G. Cai, N.S. Weng, R.L. Zhang, J.S. Zhao, J.F. Wang, H.L. Wu, Inorg.
Chem. Commun. 12 (2009) 86.
[
[
38] S.B. Deb, S. Kannan, M.G.B. Drew, J. Coord. Chem. 63 (2010) 3620.
39] P.S. Barber, S.P. Kelley, R.D. Rogers, RSC Adv. 2 (2012) 8526.
[40] G.X. Tian, L.F. Rao, S.J. Teat, G.K. Liu, Chem. Eur. J. 15 (2009) 4172.
[
[
41] S. Kannan, M.A. Moody, C.L. Barnes, P.B. Duval, Inorg. Chem. 47 (2008) 4691.
42] J. Jiang, M.J. Sarsfield, J.C. Renshaw, F.R. Livens, D. Collison, J.M. Charnock, M.
Helliwell, H. Eccles, Inorg. Chem. 41 (2002) 2799.
[43] D.W. Duff, G.E. Maciel, Macromolecules 23 (1990) 4367.
[44] M.V. Luzgin, A.V. Toktarev, V.N. Parmon, A.G. Stepanov, J. Phys. Chem. C 117
(
2013) 22867.
[
[
45] C. Nguyen Trung, G.M. Begun, D.A. Palmer, Inorg. Chem. 31 (1992) 5280.
46] K. Ohwada, Spectrochim. Acta, Part A 35 (1979) 1283.
[47] S.P. Pasilis, A. Blumenfeld, Inorg. Chem. 50 (2011) 8302.
[
[
[
48] J.I. Bullock, J. Chem. Soc. A (1969) 781.
49] A.A. Zazhogin, H.D. Lutz, A.I. Komyak, J. Mol. Struct. 482–483 (1999) 189.
50] C. Madic, G.M. Begun, D.E. Hobart, R.L. Hahn, Inorg. Chem. 1984 (1914) 23.
[51] C. Madic, D.E. Hobart, G.M. Begun, Inorg. Chem. 22 (1983) 1494.
[
[
[
52] P.J. Hay, R.L. Martin, G. Schreckenbach, J. Phys. Chem. A 104 (2000) 6259.
53] The Chemistry of the Actinide and Transactinide Elements, 4th ed., 2011.
54] J.C. Taylor, M.H. Mueller, Acta Crystallogr. 19 (1965) 536.
Appendix A. Supplementary data
[55] J. Su, P.D. Dau, Y.H. Qiu, H.T. Liu, C.F. Xu, D.L. Huang, L.S. Wang, J. Li, Inorg.
Chem. 52 (2013) 6617.
[
56] J.C. Berthet, P. Thuery, J.P. Dognon, D. Guillaneux, M. Ephritikhine, Inorg. Chem.
7 (2008) 6850.
[57] P.D. Dau, J. Su, H.T. Liu, J.B. Liu, D.L. Huang, J. Li, L.S. Wang, Chem. Sci. 3 (2012)
137.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2014.06.023.
4
1
[
[
[
58] J. Su, K. Zhang, W.H.E. Schwarz, J. Li, Inorg. Chem. 50 (2011) 2082.
59] M. Straka, P. Hrobarik, M. Kaupp, J. Am. Chem. Soc. 127 (2005) 2591.
60] G. Nocton, P. Horeglad, V. Vetere, J. Pecaut, L. Dubois, P. Maldivi, N.M.
Edelstein, M. Mazzanti, J. Am. Chem. Soc. 132 (2010) 495.
61] C.W. Abney, S.B. Liu, W.B. Lin, J. Phys. Chem. A 117 (2013) 11558.
62] G.M. Lombardo, A.L. Thompson, F.P. Ballistreri, A. Pappalardo, G.T. Sfrazzetto,
G.A. Tomaselli, R.M. Toscano, F. Punzo, Dalton Trans. 2012 (1951) 41.
63] P.F. Weck, E. Kim, B. Masci, P. Thuery, K.R. Czerwinski, Inorg. Chem. 49 (2010)
References
[
[
1] K. Mayer, M. Wallenius, Z. Varga, Chem. Rev. 113 (2012) 884.
2] M.J. Kristo, S.J. Tumey, Nucl. Instrum. Methods Phys. Res., Sect. B 294 (2013)
[
[
6
56.
3] H. Sodaye, S. Nisan, C. Poletiko, S. Prabhakar, P.K. Tewari, Desalination 235
2009) 9.
4] P.S. Barber, S.P. Kelley, C.S. Griggs, S. Wallace, R.D. Rogers, Green Chem. 2014
1828) 16.
[
[
(
[
1465.
(