Crystal growth, structural characterization, and magnetic properties of new uranium(IV) containing mixed metal oxalates: Na2U 2M(C2O4)6(H2O) 4 (M = Mn2+, Fe2+</su

Article abstract of DOI:10.1021/ic3026733

A series of new mixed-metal oxalates containing U⁴⁺ and divalent transition metal cations, Na₂U₂M(C₂O ₄)₆(H₂O)₄ (M = Mn²⁺, Fe²⁺, Co²⁺,

Full text of DOI:10.1021/ic3026733

Article
pubs.acs.org/IC
Crystal Growth, Structural Characterization, and Magnetic Properties
of New Uranium(IV) Containing Mixed Metal Oxalates:
2
+
2+
2+
2+
Na U M(C O ) (H O) (M = Mn , Fe , Co , and Zn )
2
2
2 4 6
2
4
†
†
‡
,†
Jeongho Yeon, Mark D. Smith, Athena S. Sefat, and Hans-Conrad zur Loye*
†Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States
‡Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
*
S Supporting Information
4+
ABSTRACT: A series of new mixed-metal oxalates containing U and divalent
2
+
2+
2+
transition metal cations, Na U M(C O ) (H O) (M = Mn , Fe , Co , and
2
2
2
4 6
2
4
2+
Zn ), were synthesized via a hydrothermal route and structurally characterized by
single crystal X-ray diffraction. All of the materials are triclinic, with space group P1
The three-dimensional structure of these isostructural uranates consists of oxalate
̅
.
4
+
bridged UO10 and MO polyhedra. The U cation is surrounded by five oxalate
6
2
+
ligands, while the M cations are bonded to two oxalate ligands and four water
molecules. The magnetic susceptibility data of these mixed metal oxalates were
measured as a function of temperature and result in a value of the effective magnetic
4
+
moment of 3.50 μ for U cation in the Zn member, while the total effective
B
2
+
2+
2+
moment of the Mn , Fe , and Co members are 6.01, 5.46, and 5.06 μB,
respectively. For all materials, negative Weiss constants were observed revealing that
the materials exhibited local antiferromagnetic interactions. The U4+ cation exhibits
a singlet ground state at low temperature. The materials were further characterized by infrared, UV−vis reflectance spectroscopy,
and thermal analysis.
4+
7−14
INTRODUCTION
extended structures containing U .
In particular the
■
versatile germanate and silicate structures were targeted, as
The chemistry of uranium in extended structures has been
extensively investigated due to its importance for long-term
nuclear waste storage and for the development of fuel rod
they are able to accommodate the large U4+ cation.
9,10,12
These
6
+
hydrothermal reaction conditions are known to reduce U to
the +5 and +4 oxidation states, eliminating the need to start
1
−5
assemblies.
The majority of these efforts has focused on the
4+
with a U8 reagent. However, this can lead to mixed valent
most commonly found uranium species, the uranyl cation,
UO22 , which is usually observed in octahedral, pentagonal
bipyramidal, or hexagonal bipyramidal coordination environ-
ments, and which typically exhibits two short and four or more
longer bonds. Noticeably fewer investigations have focused on
,9,11,13
4+
+
products.
Of additional interest, clearly, are U based
structures that contain a second metal that also has unpaired
electrons, for example, a first row transitions element, to
investigate potential magnetic interactions between the
uranium-f and transition metal-d electrons.
4
+
the chemistry of U , perhaps in part due to the requirement
that it has to be reacted under inert or reducing rather than
To create U4+ containing extended structures, we pursued
the preparation of mixed metal oxalates using uranium oxalate
6
+
ambient conditions that can be used for U chemistry. The
coordination environment of U is quite different from that of
15
4
+
hexahydrate, U(C O ) (H O) , a convenient starting material
2 4 2 2 6
6
+
6+
that contains uranium in the +4 oxidation state. The +4
oxidation state can easily be maintained during synthesis, as the
oxalate group creates a mild reducing solution environment at
slightly elevated temperatures. Furthermore, it is well-known
that the oxalate anion plays an important role in the separation
and purification of actinide elements in general, due to its very
prevalent chelating ability, where the oxalate group can be
either terminally bidentate or act as a bridging tetradentate
U , due to its significantly larger size (ionic radii, CN = 8; U
0.86 Å, U4 = 1.00 Å), which results in larger coordination
+
6
=
environments, such as UO , UO , or UO polyhedra.
8
9
10
4
+
Consequently, U containing materials tend to exhibit
extended structures quite different from those observed for
related U6+ containing systems.
6
+
and U4+
Another distinct difference between the U
4
+
2
chemistry is that U (f ) contains two unpaired f-electrons
16
6+
ligand. This extensive chelating ability has led to a large
collection of oxalate based framework structures containing a
that give rise to magnetic behavior, which is absent in the U
f ) species. To investigate the magnetic properties of U
0
4+
(
containing materials, several groups have explored the
4
+
preparation of U containing oxides and have used high
temperature, high pressure hydrothermal methods to prepare
Received: December 5, 2012
Published: February 1, 2013
©
2013 American Chemical Society
2199
dx.doi.org/10.1021/ic3026733 | Inorg. Chem. 2013, 52, 2199−2207

Inorganic Chemistry
Article
Figure 1. Photographic images of single crystals of (a) Na U Mn(C O ) (H O) , (b) Na U Fe(C O ) (H O) , (c) Na U Co(C O ) (H O) , and
2
2
2
4
6
2
4
2
2
2
4
6
2
4
2
2
2
4
6
2
4
(
d) Na U Zn(C O ) (H O) . The crystals are approximately 0.6 mm in length.
2 2 2 4 6 2 4
wide variety of metal cations, including molecular struc-
CoCl
·6H O (J.T. Baker, 99+%), ZnCl (Alfa Aesar, 98+%), and
2 2 2
1
7,18
19,20
21,22
H C O ·2H O (Alfa Aesar, 98%) were used as received. Warning!
tures,
1-D chain structures,
sheet structures,
as
2
2
4
2
2
3,24
Although the uranium precursors used contain depleted uranium, standard
safety measures for handling radioactive substance should be followed.
Synthesis. Single crystals of the reported materials were grown via
well as extended three-dimensional structures,
depending
on the specific structure directing role of the oxalate groups in
the material. In the case of uranium oxalates, a significant
a hydrothermal route using U(C O ) (H O) as a U(IV) precursor.
6
+
2
4
2
2
6
number of investigations have focused on U containing
U(C O ) (H O) was synthesized using UO (NO ) ·6H O, Na S O ,
4
+
2
4
2
2
6
2
3
2
2
2
2
7
materials, while only a very small number of U containing
oxalate structures with unique crystallographic sites have been
H C O ·2H O, and HCl. A 2 mmol portion of UO (NO ) ·6H O was
2
2
4
2
2
3
2
2
dissolved in dilute HCl (2 mL conc HCl and 18 mL of water) and
warmed to 80 °C. A 4 mmol sample of Na S O and 1 mL of conc
HCl were added to the solution while stirring. The mixture was kept at
80 °C until the UO (NO ·6H O dissolved and was finally filtered to
give a clear green solution. To this, a solution of H C O ·2H O (3.6
2
3
reported. These include (NH ) U (C O ) (H O) , U-
4
2
2
2
4
5
2
0.7
2
2
7
1
5
25
(
C O ) (H O) , α- and β-K U(C O ) (H O) , α- and β-
2
4 2
2
6
4
2
4 4
2
4
2
4
6
27
U(C O ) (H O) ,
(
UF (C O ) (H O),
K UMn-
2
)
3
2
2
2
4
2
2
2
3
2
4
0.5
2
2
2
16
C O ) (H O) ,
2
(C(NH ) ) U(C O ) (H O) ,
2
2
4
2
4
4
2
9
2
3
4
2
4
4
2
2
28
29
mmol in 5 mL of water) was slowly added with stirring, causing a
precipitate to form. The mixture was stirred for another 30 min, and
the solid precipitate was filtered, washed with water and acetone, and
dried at room temperature. The identity and phase purity of the
U(C O ) (H O) product was confirmed by powder X-ray diffraction
K U Mg (C O ) (H O) , and Ba U(C O ) (H O) . Of
2
2
2
2
4 7
2
11
2
2
4 4
2
8
these, K UMn(C O ) (H O) is the only mixed-metal system
2
2
4 4
2
9
2
+
that includes a paramagnetic element (Mn ) other than the
cation.
In this article, we report on the synthesis, characterization,
4
+
U
2
4
2
2
6
(see Figure S1).
4
+
and magnetic properties of a new series of U containing
2+
2+
For the preparation of Na U M(C O ) (H O) (M= Mn , Fe ,
and Co ), 0.4 mmol of U(C
mmol of H C O ·2H O, and 6 mL of H O were combined with 0.2
mmol of MnCl ·4H O, FeCl ·4H O, and CoCl ·6H O, respectively.
2
2
2
4
6
2
4
2+
2+
mixed metal oxalates, Na U M(C O ) (H O) (M = Mn ,
2
2
2
4 6
2
4
2
O
4
)
2
(H
2
O) , 0.6 mmol of Na CO , 5
6 2 3
2
+
2+
2+
Fe , Co , and Zn ), representing rare examples of mixed-
metal uranium oxalates containing two different magnetic
cations.
2 2 4 2 2
2
2
2
2
2
2
For the preparation of Na
U
2
Zn(C
O
4
)
(H O) , 0.4 mmol of
2
2
6
2 4
U(C O ) (H O) , 0.8 mmol of Na CO , 5 mmol of H C O ·2H O,
2
4
2
2
6
2
3
2
2
4
2
and 6 mL of H O were combined with 0.2 mmol of ZnCl .
2
2
EXPERIMENTAL SECTION
■
The respective solutions were placed into 23 mL Teflon−lined
Reagents. U(C O ) (H O) was prepared from
autoclaves. The autoclaves were closed, heated to 150 °C at a rate of 5
2
4
2
2
6
−
1
UO (NO ) (H O) (Fisher, ACS grade), H C O ·2H O (Alfa Aesar,
°C m , held for 2 days, and cooled to room temperature at a rate of 6
1
2
3
2
2
6
2
2
4
2
−
9
8%), and Na S O (Alfa Aesar, 85+%). Na CO (Alfa Aesar, 99.5%),
°C h . The mother liquor was decanted from the single crystal
products, which were isolated by filtration and washed with distilled
2
2
7
2
3
MnCl ·4H O (Alfa Aesar, 99%), FeCl ·4H O (Alfa Aesar, 98+%),
2
2
2
2
2
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Inorganic Chemistry
Table 1. Crystallographic Data for Na U M(C O ) (H O) (M = Mn , Fe , Co , and Zn )
Article
2
+
2+
2+
2+
2
2
2
4 6
2
4
Na U Mn(C O ) (H O)
Na U Fe(C O ) (H O)
Na U Co(C O ) (H O)
Na U Zn(C O ) (H O)
4
2
2
2
4
6
2
4
2
2
2
4
6
2
4
2
2
2
4
6
2
4
2
2
2
4
6
2
fw
1177.16
triclinic
1178.07
triclinic
1181.15
triclinic
1187.59
triclinic
cryst syst
space group
a (Å)
P1
̅
(No. 2)
P1
̅
(No. 2)
P1
̅
(No. 2)
̅
P1 (No. 2)
6.4556(5)
7.9032(6)
12.7572(9)
75.295(1)
85.231(1)
68.820(1)
587.00(8)
1
6.4504(7)
7.8759(8)
6.4419(1)
7.8595(1)
12.7780(2)
75.735(1)
85.286(1)
68.464(1)
6.4418(2)
7.8671(2)
12.8053(4)
75.622(1)
85.185(1)
68.435(1)
584.60(3)
1
b (Å)
c (Å)
12.7802(13)
75.622(2)
85.269(2)
68.674(2)
585.85(11)
1
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
583.206(15)
1
3
density (Mg/m )
abs coeff (mm 1)
cryst size (mm3)
θ range
3.330
3.339
3.363
3.373
−
14.464
14.573
14.728
15.009
0.08 × 0.05 × 0.04
1.65−30.11
99.2%
0.08 × 0.06 × 0.05
1.64−30.07
99.3%
0.36 × 0.08 × 0.02
1.64−35.25
99.2%
0.20 × 0.06 × 0.02
1.64−28.29
99.9%
completeness to θmax
R (int)
0.0600
0.0580
0.0509
0.0498
2
GOF (F )
1.020
0.974
1.018
1.045
a
R(F)
0.0318
0.0300
0.0320
0.0242
2
o
b
R (F )
w
0.0647
0.0584
0.0686
0.0540
a
R(F) = Σ||F | − |F ||/Σ|F |. R (F ) = [Σw(F − F ) /Σw(F ) ]1/2.
b
2
2
2
2
2 2
o
c
o
w
o
o
c
o
2
+
2+
2+
2+
Table 2. Selected Interatomic Distances (Å) for Na U M(C O ) (H O) (M = Mn , Fe , Co , and Zn )
2
2
2
4 6
2
4
Na U Mn(C O ) (H O)
Na U Fe(C O ) (H O)
Na U Co(C O ) (H O)
Na U Zn(C O ) (H O)
2 2 2 4 6 2 4
2
2
2
4
6
2
4
2
2
2
4
6
2
4
2
2
2
4
6
2
4
U(1)−O(6)
U(1)−O(5)
U(1)−O(9)
U(1)−O(11)
U(1)−O(12)
U(1)−O(10)
U(1)−O(1)
U(1)−O(4)
U(1)−O(2)
U(1)−O(3)
M(1)−O(7)
M(1)−O(7)
M(1)−O(14)
M(1)−O(14)
M(1)−O(13)
M(1)−O(13)
2.346(4)
2.392(4)
2.441(4)
2.455(4)
2.463(4)
2.468(4)
2.487(4)
2.513(4)
2.519(4)
2.531(4)
2.163(4)
2.163(4)
2.203(5)
2.204(5)
2.239(4)
2.239(4)
2.348(4)
2.395(4)
2.439(4)
2.458(4)
2.475(4)
2.473(4)
2.496(4)
2.511(4)
2.513(4)
2.528(4)
2.132(4)
2.132(4)
2.133(5)
2.134(5)
2.181(5)
2.181(5)
2.346(3)
2.400(3)
2.440(3)
2.449(3)
2.474(3)
2.465(3)
2.487(3)
2.505(3)
2.513(3)
2.520(3)
2.096(3)
2.096(3)
2.089(4)
2.089(4)
2.149(3)
2.149(3)
2.352(3)
2.387(3)
2.445(3)
2.455(3)
2.473(4)
2.472(3)
2.492(3)
2.511(3)
2.515(3)
2.518(3)
2.065(3)
2.065(3)
2.112(4)
2.112(4)
2.181(4)
2.181(4)
water and acetone. In all cases the reaction yielded a single phase
product consisting of light green plate crystals in approximate 90%
yield based on U(C O ) (H O) . The addition of an excess of oxalic
acid dihydrate is necessary to obtain high yield and high quality
crystals (see Figure 1).
anions, and half each of two more oxalate ions located on inversion
centers. The Mn, Fe, Co, and Zn atoms are also located on inversion
centers. All non-hydrogen atoms were refined with anisotropic
displacement parameters. After locating and refining all atoms
anisotropically, the calculated electron density difference maps showed
maxima corresponding to reasonable positions for water hydrogen
atoms in the vicinity of oxygen atoms O13 and O14. These positions
gave physically sensible water molecule and hydrogen bonding
geometries. For refinement, O−H distances were restrained to
d(O−H) = 0.84(2) Å, and H−H distances were restrained to be
similar. All hydrogen atoms were assigned a common isotropic
displacement parameter. Crystallographic data and selected intera-
tomic distances are listed in Tables 1 and 2, respectively.
Powder X-ray Diffraction. Powder X-ray diffraction data were
collected on a Rigaku D/Max-2100 powder X-ray diffractometer using
Cu Kα radiation. The step-scan covered the angular range 5−70° 2θ in
steps of 0.04°. No impurities were observed, and the calculated and
experimental PXRD patterns are in excellent agreement (see Figure
S2).
2
4
2
2
6
Single Crystal X-ray Diffraction. X-ray diffraction intensity data
from plate crystals were measured at room temperature on a Bruker
3
0
SMART APEX diffractometer (Mo Kα radiation, λ = 0.710 73 Å).
3
0
The raw area detector data frames were processed with SAINT+. An
absorption correction based on the redundancy of equivalent
reflections was applied to the data with SADABS. The reported
unit cell parameters were determined by least-squares refinement of a
large array of reflections taken from each data set (Table 1). Difference2
Fourier calculations and full-matrix least-squares refinement against F
were performed with SHELXTL.
The four reported materials are isostructural and crystallize in the
triclinic space group P1 (No. 2), which was confirmed by the
successful solution and refinement of the structures. The asymmetric
unit consists of one uranium atom, one sodium atom, one divalent
transition metal atom, two water molecules, two complete oxalate
30
31
̅
2
201
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Inorganic Chemistry
Article
Infrared Spectroscopy. IR spectra were recorded on a Perkin
−1
Elmer, Spectrum 100, FT-IR spectrometer in the 650−4000 cm
range.
UV−Vis Diffuse Reflectance Spectroscopy. Diffuse reflectance
spectra of polycrystalline powder samples of the reported materials
were obtained using a Perkin-Elmer Lambda 35 UV−vis scanning
spectrophotometer equipped with an integrating sphere in the range
2
00−900 nm.
Thermal Analyses. Thermogravimetric analyses were carried out
on a TA Instruments SDT Q600 simultaneous DTA-TGA by heating
the single crystals at a rate of 10 °C/min under flowing N and 5% H /
2
2
N gas up to a temperature of 900 °C.
2
Magnetic Property Measurements. The dc magnetization was
measured as a function of temperature using a Quantum Design
MPMS SQUID magnetometer. The polycrystalline samples were
placed in a gelatin capsule, secured by a small amount of epoxy. For a
typical temperature sweep experiment, the sample was cooled to 5 K
under zero-field cooled (zfc) conditions, and data were collected by
heating the sample from 5 to 350 K in an applied field of 1000 Oe.
The sample was then cooled in the applied field (fc) to 5 K while data
were collected.
RESULTS AND DISCUSSION
■
Synthesis. Hydrothermal methods are often used to
4+
generate U in situ, enabling the researcher to start with a
6+
uranium source containing uranium is in its highest, U ,
oxidation state. It is difficult to control the redox reaction,
however, and hence mixed valent uranium containing products
are often obtained. To eliminate the need for such a redox
4
+
process, we synthesized the U containing oxalate, U-
C O ) (H O) , in our laboratory using the method by
(
2
4 2
2
6
2
5
Favas. In addition, to maintain the uranium oxidation state
during the reaction, we introduce an excess of oxalic acid to the
4+
reaction mixture. The oxalic acid stabilizes both the U and the
divalent transition metal cations in solution, creates a low pH
environment, and is incorporated into the product. In the
absence of the excess oxalic acid, the reactions generate
extremely low yield, only poor quality crystals, and an
unidentified powder impurity. In the presence of an excess
oxalic acid, however, high quality single crystals of Na U M-
Figure 2. Polyhedral structure representation of Na U M-
2
2
2+
2+
2+
2+
(
C O ) (H O) (M = Mn , Fe , Co , and Zn ) along the (a) a-
2 4 6 2 4
and (b) b-axis. The three-dimensional network consists of UO10 and
MO polyhedra bridged by oxalate groups. For clarity, the H atoms of
6
2+
water molecules attached to the M cations are omitted.
2
2
2
+
2+
2+
2+
(
C O ) (H O) (M = Mn , Fe , Co , and Zn ), as shown in
2 4 6 2 4
Figure 1, were obtained in essentially quantitative yield.
cations are located in a 5-fold coordination environment with
four oxygen atoms from four oxalates and the other oxygen
atom from one water molecule. The Na−O bond distances
range from 2.485(5) to 2.515(4) Å. The local coordination
environment of each cation is shown in Figure 4. Bond valence
Structures. All members of the Na U M(C O ) (H O)
2
2
2
4 6
2
4
2+
2+
2+
2+
(
M = Mn , Fe , Co , and Zn ) series crystallize in the
triclinic space group P1. The materials are isostructural and
exhibit a three-dimensional structure consisting of UO and
̅
10
2
+
2+
2+
2+
32,33
2+
+
MO (M = Mn , Fe , Co , and Zn ) polyhedra bridged by
sum calculations
1.94−1.97 for M , and 3.91−3.94 for U , which are all in
good agreement with the expected values, as well as confirming
resulted in values of 0.98−1.03 for Na ,
6
2‑
4+
oxalate, C O , ligands. As shown in Figures 2 and 3, the layers
2
4
4
‑
of U (C O ) in the ab-plane are connected by MO6
2
2
4 6
4+
octahedra along the c-axis resulting in a three-dimensional
network. The Na atoms reside in the channels that run along
the a-axis. Ten oxygen atoms from five oxalate ligands surround
the +4 oxidation state of U .
There are three unique oxalate anions, of which two act as
bidentate ligands and one as a terdentate ligand. The former
4
+
4+
the crystallographically unique U cations with U−O bond
connect the U cations via a side-by-side bonding scheme,
4+
distances ranging from 2.346(4) to 2.531(4) Å. Of these five
which is typically found in U containing oxalate compounds
4
+
oxalate ligands, four are further linked to four additional U
and which creates an average U−U separation of 6.400 Å. The
2
+
4+
cations and one is bonded to one M (M = Mn, Fe, Co, and
latter oxalate group links U and a divalent cation, leaving one
4+
Zn) cation. The coordination environment of the U cations,
terminal oxygen atom that is not bonded to another cation. The
cation connectivity is shown in Figure 5.
4
+
UO , is consistent with previously reported U containing
10
oxalates, and the U−O bond lengths are consistent with the
The reported materials exhibit several interesting structural
features that result from the connectivity of the different cation
polyhedra. For example, the layers located in the ab-plane
contain U4+ cations that are bridged by the tetradentate
bridging oxalate groups as shown in Figure 5. These layers are
1
6,21,25,28
average bond length of 2.479 Å for known materials.
The unique divalent cations are found in a nearly regular
octahedron with the M−O bond distances ranging from
2.065(3) to 2.239(4) Å. In this octahedron, four of the oxygen
ligands are water molecules, while the other two belong to two
oxalates that connect to both M and U cations. The sodium
further connected through MO polyhedra along the c-axis.
One of the interesting structural features is the existence of
6
2
+
4+
2
202
dx.doi.org/10.1021/ic3026733 | Inorg. Chem. 2013, 52, 2199−2207

Inorganic Chemistry
Article
three-membered rings consisting of three uranium cations and
three oxalate groups that run along the a- and b-axes. Of the
known U4+ containing oxalates, only two other compounds,
1
5,26
U(C O ) (H O) (x = 2 and 6),
exhibit the layer-type
2
4 2
2
x
structure and contain UO10 and UO polyhedra, respectively.
8
As stated earlier, the high coordination number is character-
istic of actinide elements in low oxidation states. Studies based
on the known hydration numbers in actinides suggest that the
4+
U
cation can possess a 10-fold coordination environment,
while the other tetravalent actinide cations can only have a
3
4−36
coordination number of up to nine.
As shown in Figure 4,
the U4+ cation is in fact observed in UO10 polyhedra that are
surrounded by five oxalate ligands. This structural motif is also
2
0
found in other oxalates such as (NH ) U (C O ) (H O)
4
2
2
2
4 5
2
0.7
2
5
and α- and β-K U(C O ) (H O) . This coordination
4
2
4
4
2
4
environment is potentially better defined as a sphenocorona
rather than as a bicapped square pyramid.
The oxalate ligands bridge the UO10 and MO polyhedra
6
resulting in a three-dimensional network. In general, the oxalate
ligand exhibits two bonding modes, one as a bidentate
terminating and one as a tetradentate bridging ligand. As
expected, the former is found in low-dimensional struc-
3
7,38
tures,
while the latter is observed in high-dimensional
2
6,27
structures.
There are three unique oxalate groups in the
reported materials. As shown in Figure 5, the two oxalate
groups linking the U4+ cations act as tetradentate bridging
4
+
2+
ligands, while the other oxalate ligand between the U and M
cations behaves as a terdentate ligand. In the known uranium
oxalates it appears that the terdentate oxalate ligand motif is
2
0
only found in (NH ) UO (C O ) , which is a molecular
4
2
2
2
4 2
6+
compound containing the U cation. Na U M(C O ) (H O)
2
2
2
4 6
2
4
2+
2+
2+
2+
4+
(
M = Mn , Fe , Co , and Zn ) is thus the first U
containing structural series to exhibit the bridging terdentate
oxalate motif in an extended structure. It is expected that this
bonding mode influences the C−O bond distances between
bridging and terminal oxalate ligands. As seen in Table S5, the
average C(3)−O(8) bond distance (1.218 Å) in a terminal
position of the terdentate oxalate ligand is in fact slightly
shorter than the average C−O distance (1.256 Å) in
tetradentate bridging oxalate ligands.
Figure 3. Polyhedral diagram of layers composed of the U4+ cations
2+
2+
and oxalate anions for Na U M(C O ) (H O) (M = Mn , Fe ,
2
2
2
4
6
2
4
Infrared Spectroscopy. The infrared spectra for the
2+
2+
Co , and Zn ) along the (a) a- and (b) b-axis are shown. For clarity,
the M2+ cations and water molecules are omitted.
reported materials are very similar between 650 and 4000
−1
cm due to the fact that they are isostructural. The bands
−1
observed in the region 3200−3600 cm are attributable to O−
2
+
2+
2+
2+
4+
Figure 4. Local coordination environments of Na U M(C O ) (H O) (M = Mn , Fe , Co , and Zn ). The U cation is surrounded by five
2
2
2
4
6
2
4
oxalate ligands (a), the M2 cations are bonded to four water molecules and two oxalate groups (b), and the Na cation is linked to one water
+
+
molecule and four oxalate groups (c).
2
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Inorganic Chemistry
Article
Figure 5. Connectivity between the U cation and M2+ (M = Mn , Fe , Co , and Zn ) cations. All four oxygen atoms in the oxalate ligand are
4
+
2+
2+
2+
2+
bonded to two U4 cations in part a, whereas in part b three oxygen atoms in the oxalate ligand are linked to the U and M cations leaving one
oxygen atom in a terminal position. For the sake of clarity, the H atoms around the M cation are omitted.
+
4+
2+
2+
H vibrations in the water molecules. The bands found around
cations are present, the transitions are not clearly seen because
600 cm− can be attributed to asymmetric stretching of C−O
1
1
the d−d transitions are weak and broader than the f−f
−
1
vibrations, whereas the bands observed in 1200−1450 cm can
transitions. The absorption bands above 400 nm are f−f
4+
be attributed to the symmetric stretching of C−O vibrations.
transitions from the U cations, which include transitions from
−1
3
1
3
The bands below 1000 cm can be assigned to C−C, C−O−
O, or metal−oxygen vibrations. The assignments are consistent
the ground state of H
4
to the excited states such as I
6 1
, P ,
1
1
3
3
3
G , D , P , H , H , etc., based on the energy level diagram
4
2
0
6
4
3
9,40
4+
41−43
with previously reported data.
Figure S3.
The IR spectra are given in
for the U cation. 4
The optical data therefore confirms the
+
presence of the U cation. The band gaps estimated by the
onset of the absorption edge resulted in values of approximate
UV−Vis Diffuse Reflectance Spectroscopy. UV−vis
diffuse reflectance data were measured on ground crystals of
2+
2+
3
.1 eV for Na U M(C O ) (H O) (M = Mn , Co , and
2 2 2 4 6 2 4
2+
2
+
Zn ), and 2.5 eV for Na U Fe(C O ) (H O) , respectively,
all materials of the series, Na U M(C O ) (H O) (M = Mn ,
2
2
2
4 6
2
4
2
2
2
4 6
2
4
2+
2+
2+
suggesting that they are semiconducting materials.
Fe , Co , and Zn ). As seen in Figure 6, all spectra are similar
except the iron containing uranate, where the broader
absorption band is observed around 450 nm. For all spectra,
it can be expected that the absorption bands are mainly
attributed to the f−f transitions in the U cation. It is likely
that although d−d transitions in the divalent transition metal
Thermal Analyses. The thermal behavior of the materials
was investigated using thermogravimetric analysis (TGA) over
the temperature range of RT to 1000 °C under nitrogen flow.
TGA data are shown in Figures S4 and S5. The first observed
weight loss starts at approximately 200 °C due to the loss of
waters of hydration. This weight loss continues as the
temperature is increased to 900 °C and includes the
decomposition of the oxalate groups. The experimental weight
losses of 42.5%, 43.1%, 43.9%, and 46.4% are in good
agreement with the calculated weight losses of 42.8%, 42.8%,
4
+
4
2.7%, and 42.5%, for Mn, Fe, Co, and Zn containing uranates,
respectively. The final residues, based on a powder X-ray
4
4
45
46
diffraction analysis, are NaUO3, UO2, and MnO in the
44
45
case of Na U Mn(C O ) (H O) ; NaUO , UO2, and an
2
2
2
4 6
2
4
3
unidentified phases in the case of Na U Fe(C O ) (H O) and
2
2
2
4 6
2
4
4
4
47
Na U Co(C O ) (H O) ; and NaUO , Na U O , and
ZnO in the case of Na U Zn(C O ) (H O) . For all
compounds, the residues contain U and U containing
products, indicating that a redox reaction occurred during the
decomposition process. An additional thermal analysis using 5%
H gas was performed on Na U Zn(C O ) (H O) to see if the
2
2
2
4
6
2
4
3
2
2
7
4
8
2
2
2
4
6
2
4
5+
6+
2
2
2
2
4 6
2
4
outcome under hydrogen was different from what was observed
using nitrogen. It turned out that the overall result is almost the
same, as shown in Figure S4; however, the ZnO residue was no
longer evident, as it was presumably reduced to the metal.
Magnetic Properties. The Na U M(C O ) (H O) (M =
Figure 6. UV−vis diffuse reflectance spectra for Na U Zn-
2
2
2
4 6
2
4
2
2
2+
2+
2+
2+
2+
2+
2+
2+
(
C O ) (H O) (M = Mn , Fe , Co , and Zn ).
Mn , Fe , Co , and Zn ) system is convenient for studying
2
4
6
2
4
2
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Inorganic Chemistry
Article
the magnetic properties of U4+ and its interactions with
thermally accessible at the low temperature. Figure 8 shows
4
+
2
magnetic divalent transition elements. The U (f ) cation has
two unpaired f-electrons that will contribute to the magnetic
temperature dependence of the χ T data. The χ T value drops
m
m
2
+
5
2+
6
properties, and the transition metal cations Mn (d ), Fe (d ),
2+
7
and Co (d ) have d-electrons that also will play a role; only
zinc is nonmagnetic in this series. This allows us to investigate
the contribution to the magnetic susceptibility of only uranium
in Na U Zn(C O ) (H O) , and the combined contributions
2
2
2
4 6
2
4
of uranium and manganese, cobalt and iron in Na U M-
2
2
2
+
2+
2+
(
C O ) (H O) (M = Mn , Fe , and Co ), respectively.
2 4 6 2 4
The temperature dependence of the magnetic susceptibility
for Na U Zn(C O ) (H O) , reflecting only the contribution
2
2
2
4 6
2
4
4+
of U , in an applied field of 1000 Oe is shown in Figure 7. No
Figure 8. Plot of χ T vs temperature for Na U Zn(C O ) (H O) .
m
2
2
2
4
6
2
4
Data were collected in an applied field of 1000 Oe.
off gradually with decreasing temperature, indicating the
existence of antiferromagnetic interactions, which are consistent
with the negative Weiss constant of −126.4 K from the curve
fitting in the high temperature region. Below around 100 K,
however, the values of χ T decrease rapidly and tend toward
m
−
1
zero with a minimum value of 0.082 emu K mol (μeff = 0.81
μB) at 5 K. This is likely due to the presence of a zero-field
splitting and contribution of a singlet ground state of the
Figure 7. Temperature dependence of the molar magnetic
susceptibility, χ , of Na U Zn(C O ) (H O) measured in an applied
4+
U cation at low temperature, which is also observed in several
m
2
2
2
4
6
2
4
known materials containing the U4+ cation.
14,49−52
field of 1000 Oe. Inset shows the inverse susceptibility vs temperature
plot with fit to the Curie−Weiss law.
The temperature dependence of the magnetic susceptibility
2+
2+
2+
data for Na U M(C O ) (H O) (M = Mn , Fe , and Co )
2
2
2
4 6
2
4
in an applied field of 1000 Oe are shown in Figures 9−11. The
differences are observed between the zero field cooled (zfc) and
field cooled (fc) data. The inverse susceptibility versus
temperature plot is shown in the inset. The susceptibility
data were fit to the Curie−Weiss law, χ = C/(T − θ), in the
linear temperature range 200−350 K, where C is the Curie
constant and θ is the paramagnetic Weiss constant. The
constants extracted from the curve fitting are summarized in
magnetic properties of the three materials are similar to each
other, but quite different from that of Na U Zn(C O ) (H O) ,
2
2
2
4 6
2
4
4
+
Table 3. On the basis of the fit, the U cation gives a magnetic
moment of 3.50 μ , in good agreement with the expected value
B
of 3.58 μ , calculated using the Russell−Saunders coupling
B
4
3
scheme (g = / and J = 4) for a H ground state. As seen in
J
5
4
Figure 7, the magnetization increases gradually with decreasing
temperature, and temperature independent behavior is found
below around 5 K indicating that excited states are not
Table 3. Constants Extracted from the Magnetic
2
+
Susceptibility Data for Na U M(C O ) (H O) (M = Mn ,
2
2
2
4 6
2
4
2+
2+
2+
a
Fe , Co , and Zn )
C (emu K mol−1)
Θ (K)
μ /μ
eff
B
Na U Mn(C O ) (H O)
4
7.68
6.90
6.37
3.04
−30.1
−26.7
−51.8
−126.4
6.01
5.46
5.06
3.50
2
2
2
4
6
2
Na U Fe(C O ) (H O)
4
2
2
2
4
6
2
Na U Co(C O ) (H O)
4
2
2
2
4
6
2
Na U Zn(C O ) (H O)
4
Figure 9. Temperature dependence of the molar magnetic
susceptibility, χ , of Na U Mn(C O ) (H O) measured in an applied
field of 1000 Oe. Inset shows the inverse susceptibility vs temperature
2
2
2
4
6
2
m
2
2
2
4
6
2
4
a
C, Θ, and μ represent the Curie constant, the Weiss constant, and
eff
effective magnetic moment, respectively.
plot with fit to the Curie−Weiss law.
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Inorganic Chemistry
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The χ T versus T data are shown in Figure 12. For all
m
materials, the χ T values decrease gradually with decreasing
m
Figure 10. Temperature dependence of the molar magnetic
susceptibility, χ , of Na U Fe(C O ) (H O) measured in an applied
m
2
2
2
4
6
2
4
field of 1000 Oe. Inset shows the inverse susceptibility vs temperature
plot with fit to the Curie−Weiss law.
Figure 12. Plot of χ T vs temperature for Na U M(C O ) (H O)
4
m
2
2
2
4
6
2
2+
2+
2+
(M = Mn , Fe , and Co ). Data were collected in an applied field of
1
000 Oe.
temperature. This indicates that the presence of antiferromag-
netic interactions, which are consistent with the negative Weiss
constants of −30.1, −26.7, and −51.8 K for the Mn, Fe, and Co
containing uranates, respectively. The high temperature values
2
+
of χ T result from the combined contributions of the M and
m
4+
U cations. At 5 K, however, the χ T values are 4.56 (6.05 μ ),
.92(4.85 μ ), and 1.84 (3.85 μ ) emu K mol , which are close
m
B
−1
2
B B
to the expected spin-only values of 4.36 (5.92 μ ), 2.98 (4.90
B
−1
2+
2+
2+
μ ), and 1.86 (3.87 μ ) emu K mol for Mn , Fe , and Co ,
B
B
respectively. This is consistent with the χ T data for
m
Na U Zn(C O ) (H O) shown in Figure 8, where the
2
2
2
4
6
2
4
4+
negligible magnetic contributions of the U cation is found
at 5 K. For this reason, the contribution from the divalent
cations dominates the magnetic moments at low temperature.
4
+
This is consistent with the local ground state of the U
magnetic center at low temperatures being a singlet state. For
all of the reported materials, although local antiferromagnetic
interactions were expected on the basis of the negative Weiss
constants, no long-range transitions were observed down to 5
K. This is not unexpected because the magnetic cations, U
and divalent cations, are bridged by oxalate groups, which
Figure 11. Temperature dependence of the molar magnetic
susceptibility, χ , of Na U Co(C O ) (H O) measured in an applied
field of 1000 Oe. Inset shows the inverse susceptibility vs temperature
plot with fit to the Curie−Weiss law.
m
2
2
2
4
6
2
4
4
+
2
+
2+
attributable to the presence of the magnetic Mn , Fe , and
2+
Co cations in the structure. As seen in Figures 9−11, the
magnetizations increase gradually as the temperature decreases;
however, the magnetizations increase rapidly below around 50
K. The values obtained from the curve fitting to the Curie−
Weiss law in the high temperature range (200 K < T < 350 K)
are summarized in Table 3, where the effective magnetic
moments are close to the usually observed values of 5.9, 5.1−
results in relatively long metal−metal separations of over 5 and
4+
2+
4+
4+
6 Å for U −M and U −U , respectively.
CONCLUSIONS
■
We have successfully synthesized and characterized new
reduced uranium oxalates, Na U M(C O ) (H O) (M =
2
2
2
4 6
2
4
2+
2+
2+
2+
Mn , Fe , Co , and Zn ). The isostructural materials exhibit
a three-dimensional crystal structure consisting of UO10 and
2
+
2+
2+
5
.5, and 4.1−5.2 μ for Mn , Fe , and Co , respectively. In
B
4+
this case, the theoretical magnetic moment of 3.58 μ for U
MO polyhedra bridged by oxalate groups. The magnetic
B
6
4+
was used to obtain the experimental magnetic moments of the
divalent cations. Conversely, the magnetic moment of the U4+
cation can be calculated from the Na U Mn(C O ) (H O)
4
behavior of the U cation was investigated in Na U Zn-
2 2
(C O ) (H O) and found to exhibit temperature independent
2
4 6
2
4
behavior at low temperature. The effective magnetic moment of
2
2
2
4 6
2
2
+
4+
data when using a moment of μ = 5.92 μ for Mn , resulting
in a value of 3.65 μ for U , which is slightly larger than the
3.50 μB for the U cation was obtained from the high
eff
B
4
+
temperature data. At low temperatures the magnetic moment
B
4
+
expected value of (3.58 μ ) and the observed value of (3.50 μ )
tended toward zero, consistent with the ground state of the U
B
B
in the Zn containing uranate.
cation being a singlet state. For the other materials, the
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206
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Inorganic Chemistry
Article
magnetization increased rapidly at low temperature, and
exhibited local antiferromagnetic interactions.
(27) Wang, C.-M.; Liao, C.-H.; Chen, P.-L.; Lii, K.-H. Inorg. Chem.
2
006, 45, 1436.
(28) Imaz, I.; Bravic, G.; Sutter, J.-P. Chem. Commun. 2005, 993.
(
29) Spirlet, M. R.; Rebizant, J.; Kanellakopulos, B.; Dornberger, E. J.
ASSOCIATED CONTENT
Supporting Information
X-ray data in CIF format, powder XRD patterns, IR and UV−
vis spectra, TGA diagrams, and atomic coordinates and
equivalent isotropic displacement parameters. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
Less-Common Met. 1986, 122, 205.
30) SMART Version 5.625, SAINT+ Version 6.45, and SADABS
*
S
(
Version 2.05; Bruker Analytical X-ray Systems, Inc.: Madison, WI,
2001.
(31) SHELXTL Version 6.14; Bruker Analytical X-ray Systems, Inc.:
Madison, WI, 2000.
(32) Brese, N. E.; O’Keeffe, M. Acta Crystallogr. 1991, B47, 192.
(33) Brown, I. D.; Altermatt, D. Acta Crystallogr. 1985, B41, 244.
(34) Torapava, N.; Persson, I.; Eriksson, L.; Lundberg, D. Inorg.
AUTHOR INFORMATION
Corresponding Author
E-mail: zurloye@mailbox.sc.edu.
■
*
Chem. 2009, 48, 11712.
35) Ikeda-Ohno, A.; Hennig, C.; Rossberg, A.; Funke, H.; Scheinost,
A. C.; Bernhard, G.; Yaita, T. Inorg. Chem. 2008, 47, 8294.
36) Moll, H.; Denecke, M. A.; Jalilehvand, F.; Sandstroem, M.;
Grenthe, I. Inorg. Chem. 1999, 38, 1795.
37) Mikhailov, Y. N.; Gorbunova, Y. E.; Shishkina, O. V.;
(
Notes
(
The authors declare no competing financial interest.
(
ACKNOWLEDGMENTS
■
Serezhkina, L. B.; Serezhkin, V. N. Zh. Neorg. Khim. 2000, 45, 1825.
(38) Elovskikh, N. N.; Rumyantseva, K. T. Zh. Neorg. Khim. 1962, 7,
2639.
(39) Trpkovska, M.; Soptrajanov, B.; Pejov, L. Glas. Hem. Tehnol.
Maked. 2002, 21, 111.
Research supported by the U.S. Department of Energy, Office
of Basic Energy Sciences, Division of Materials Sciences and
Engineering under Award DE-SC0008664.
(
6
(
40) Scott, K. L.; Wieghardt, K.; Sykes, A. G. Inorg. Chem. 1973, 12,
55.
41) Binnemans, K.; Couwenberg, I.; De, L. H.; Gorller-Walrand, C.;
Adam, J. L. J. Alloys Compd. 1999, 285, 105.
42) Carnall, W. T.; Liu, G. K.; Williams, C. W.; Reid, M. F. J. Chem.
Phys. 1991, 95, 7194.
REFERENCES
■
(
2
1) Yamaji, A.; Nakano, Y.; Uchikawa, S.; Okubo, T. Nucl. Technol.
012, 179, 309.
(
(
2) Kim, K.-T. J. Nucl. Mater. 2010, 404, 128.
3) Oji, L. N.; Martin, K. B.; Stallings, M. E.; Duff, M. C. Nucl.
(
Technol. 2006, 154, 237.
(
(
(43) Krupa, J. C. Inorg. Chim. Acta 1987, 139, 223.
4) Jackson, J. M.; Burns, P. C. Can. Mineral. 2001, 39, 187.
5) Burns, P. C.; Olson, R. A.; Finch, R. J.; Hanchar, J. M.; Thibault,
(44) Chippindale, A. M.; Dickens, P. G.; Harrison, W. T. A. J. Solid
State Chem. 1989, 78, 256.
45) Macdonald, J. E.; Clausen, K.; Garrard, B.; Hackett, M. A.;
Y. J. Nucl. Mater. 2000, 278, 290.
6) Shannon, R. D.; Prewitt, C. T. Acta Crystallogr., Sect. B 1969, 25,
25.
(
(
9
(
(
(
Hayes, W.; Osborn, R.; Schnabel, P.; Hutchings, M. T. High Temp.−
High Pressures 1985, 17, 27.
7) Liu, H.-K.; Chang, W.-J.; Lii, K.-H. Inorg. Chem. 2012, 50, 11773.
8) Diwu, J.; Albrecht-Schmitt, T. E. Inorg. Chem. 2012, 51, 4432.
9) Chen, C.-L.; Nguyen, Q. B.; Chen, C.-S.; Lii, K.-H. Inorg. Chem.
(46) Sasaki, S.; Fujino, K.; Takeuchi, Y. Proc. Jpn. Acad., Ser. B 1979,
5
5, 43.
(47) Gasperin, M. J. Less-Common Met. 1986, 119, 83.
2
012, 51, 7463.
(
(
48) Lucas, L. N. D. Proc. Phys. Soc., London 1951, 64A, 943.
49) Siladke, N. A.; Meihaus, K. R.; Ziller, J. W.; Fang, M.; Furche, F.;
(
(
10) Nguyen, Q.-B.; Lii, K.-H. Inorg. Chem. 2011, 50, 9936.
11) Nguyen, Q. B.; Liu, H.-K.; Chang, W.-J.; Lii, K.-H. Inorg. Chem.
Long, J. R.; Evans, W. J. J. Am. Chem. Soc. 2011, 134, 1243.
50) Nocton, G.; Pecaut, J.; Mazzanati, M. Angew. Chem., Int. Ed.
008, 47, 3040.
51) Schelter, E. J.; Morris, D. E.; Scott, B. L.; Thompson, J. D.;
Kiplinger, J. L. Inorg. Chem. 2007, 46, 5528.
52) Evans, W. J.; Miller, K. A.; Ziller, J. W.; Greaves, J. Inorg. Chem.
007, 46, 8008.
2
011, 50, 4241.
(
2
(
(
(
(
12) Liu, H.-K.; Lii, K.-H. Inorg. Chem. 2011, 50, 5870.
13) Lin, C.-H.; Lii, K.-H. Angew. Chem., Int. Ed. 2008, 47, 8711.
14) Lai, Y.-L.; Chiang, R.-K.; Lii, K.-H.; Wang, S.-L. Chem. Mater.
2
008, 20, 523.
15) Grinbergs, A.; Petrzhak, G. I.; Lozhkina, G. S.; Karago, L. V.
Radiokhimiya 1972, 14, 397.
16) Andreev, G.; Budantseva, N.; Fedoseev, A.; Moisy, P. Inorg.
Chem. 2011, 50, 11481.
17) Serezhkina, L. B.; Peresypkina, E. V.; Neklyudova, N. A.;
Virovets, A. V. Crystallogr. Rep. 2010, 55, 769.
(
2
(
(
(
(
18) Alcock, N. W. J. Chem. Soc., Dalton Trans. 1973, 1616.
(
19) Artem’eva, M. Y.; Vologzhanina, A. V.; Dolgushin, F. M.;
Antipin, M. Y.; Serezhkina, L. B.; Serezhkin, V. N. Zh. Neorg. Khim.
2
004, 49, 2068.
(
20) Alcock, N. W. J. Chem. Soc., Dalton Trans. 1973, 1610.
(
21) Duvieubourg-Garela, L.; Vigier, N.; Abraham, F.; Grandjean, S.
J. Solid State Chem. 2008, 181, 1899.
22) Art’emeva, M. Y.; Mikhailov, Y. N.; Gorbunova, Y. E.;
Serezhkina, L. B.; Serezhkin, V. N. Zh. Neorg. Khim. 2003, 48, 1470.
23) Chapelet-Arab, B.; Nowogrocki, G.; Abraham, F.; Grandjean, S.
J. Solid State Chem. 2005, 178, 3046.
24) Moertl, K. P.; Sutter, J.-P.; Golhen, S.; Ouahab, L.; Kahn, O.
Inorg. Chem. 2000, 39, 1626.
25) Favas, M. C.; Kepert, D. L.; Patrick, J. M.; White, A. H. J. Chem.
Soc., Dalton Trans. 1983, 571.
26) Clavier, N.; Hingant, N.; Rivenet, M.; Obbade, S.; Dacheux, N.;
Barre, N.; Abraham, F. Inorg. Chem. 2010, 49, 1921.
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