Structural characterization of environmentally relevant ternary uranyl citrate complexes present in aqueous solutions and solid state materials

Article abstract of DOI:10.1039/c4dt02442g

Organic acids are important metal chelators in environmental systems and tend to form soluble complexes in aqueous solutions, ultimately influencing the transport and bioavailability of contaminants in surface and subsurface waters. This is particularly true for the formation of uranyl citrate complexes, which have been utilized in advanced photo- and bioremediation strategies for soils contaminated with nuclear materials. Given the complexity of environmental systems, the formation of ternary or heterometallic uranyl species in aqueous solutions are also expected, particularly with Al(iii) and Fe(iii) cations. These ternary forms are reported to be more stable in aqueous solutions, potentially enhancing contaminant mobility and uptake by organisms, but the exact coordination geometries of these soluble molecular complexes have not been elucidated. To provide insight into the nature of these species, we have developed a series of geochemical model compounds ([(UO₂)₂Al₂(C₆H₄O₇)₄]^6- (U₂Al₂), [(UO₂)₂Fe₂(C₆H₄O₇)₄]^6- (U₂Fe₂-1) and [(UO₂)₂Fe₂(C₆H₄O₇)₄(H₂O)₂]^6- (U₂Fe₂-2) and [(UO₂)₂Fe₄(OH)₄(C₆H₄O₇)₄]^8- (U₂Fe₄)) that were characterized by single-crystal X-ray diffraction and vibrational spectroscopy. Mass spectroscopy was then employed to compare the model compounds to species present in aqueous solutions to provide an enhanced understanding of the ternary uranyl citrate complexes that could be relevant in natural systems.

Full text of DOI:10.1039/c4dt02442g

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For submission to Dalton Transactions
Structural characterization of environmentally relevant ternary
uranyl citrate complexes present in aqueous solutions and solid state
materials.
Madeline Basile, Daniel K. Unruh, Erin Flores, Adam Johns, and Tori Z. Forbes*
Department of Chemistry, University of Iowa, Iowa City, IA 52242, USA
*
corresponding author; Email: toriꢀforbes@uiowa.edu
Abstract
Organic acids are important metal chelators in environmental systems and tend to form soluble
complexes in aqueous solutions, ultimately influencing the transport and bioavailability of
contaminants in surface and subsurface waters. This is particularly true for the formation of
uranyl citrate complexes, which have been utilized in advanced photoꢀ and bioremediation
strategies for soils contaminated with nuclear materials. Given the complexity of environmental
systems, the formation of ternary or heterometallic uranyl species in aqueous solutions are also
expected, particularly with Al(III) and Fe(III) cations. These ternary forms are reported to be
more stable in aqueous solutions, potentially enhancing contaminant mobility and uptake by
organisms, but the exact coordination geometries of these soluble molecular complexes have not
been elucidated. To provide insight into the nature of these species, we have developed a series
6
ꢀ
6ꢀ
of geochemical model compounds ([(UO ) Al (C H O ) ] (U Al ), [(UO ) Fe (C H O ) ]
2
2
2
6
4
7 4
2
2
2 2
2
6
4
7 4
6
ꢀ
8ꢀ
(
U Fe ꢀ1) and [(UO ) Fe (C H O ) (H O) ] (U Fe ꢀ2) and [(UO ) Fe (OH) (C H O ) ]
2 2 2 2 2 6 4 7 4 2 2 2 2 2 2 4 4 6 4 7 4
(
U Fe )) that were characterized by singleꢀcrystal Xꢀray diffraction and vibrational spectroscopy.
2
4
Mass spectroscopy was then employed to compare the model compounds to species present in
aqueous solutions to provide an enhanced understanding of the ternary uranyl citrate complexes
that could be relevant in natural systems.
1

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Introduction
3ꢀ
Citrate (C H O(COO) ) is a naturally occurring polycarboxylate anion that plays an
5
5
3
important role in the mobility and bioavailability of metals in aqueous environmental systems. A
significant amount of the citrate found in natural systems is formed through biomass decay or
1
produced by the plant roots or microorganisms. Extrusion of citrate by plant roots increases
solubility and enhances uptake of essential trace elements, such as Fe(III) or P (in the molecular
form of orthophosphate), but can be also be a strategy of ameliorating heavy metal and Al
2
ꢀ4
toxicity. Citrate concentrations in bulk soil is estimated to be <50uM, but can be concentrated
3
ꢀ5
in the rhizosphere at levels between 5ꢀ50 ꢁmol/g soil. Due to the ability of the citrate ligand to
solubilize a wide range of metal cations and oxyanions, it has also been utilized as a soil
6
, 7
amendment to mobilize contaminants for “pumpꢀandꢀtreat” remediation strategies.
Hexavalent uranium is one such radioactive, heavy metal contaminant that can be
8
impacted by the presence of citrate in subsurface environments. Uranium is a naturally
9
, 10
occurring radioactive element that is primarily utilized as a fuel source for nuclear power.
The mining and milling of uranium and the historical production of transuranic materials for
11, 12
nuclear weapons comprises the major sources for environmental contamination.
Under
aerobic conditions, uranium exists in the hexavalent state and forms two strong bonds to O atoms
2
+ 13
to create a nearly linearꢀdioxo cation, U(VI)O .
This uranyl cation can form carbonate
2
complexes in solution, undergo hydrolysis to form polynuclear species, or precipitate as uranyl
mineral phases depending on the aqueous environmental conditions. Under slightly acidic
conditions, citrate will complex to the uranyl cation through the equatorial plane in a polydentate
1
4, 15
fashion, forming a mononuclear species.
As the pH increases, hydrolysis of the metal center
1
4, 16, 17
can take place, forming dimers, trimers, and other polynuclear species.
The presence of
2

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2+
citrate in the environment has been linked to enhanced mobility of U(VI)O2 as Eps et al, have
2
+
observed more than a 365ꢀfold increase in the solubility of U(VI)O2 when citric acid was added
1
8
as an amendment.
In addition, the inclusion of citrate during the phytoremediation of
2
+
19
U(VI)O2 from contaminated soils increased the accumulation and uptake by a factor of 14.
These previously reported studies suggest that the presence of the citrate ligand will have a
2
+
profound effect on the bioavailability and mobility of UO2 in subsurface environments.
While the formation of uranyl citrate species in subsurface environments is relatively
well understood, the presence of other metals in soil systems adds an additional level of
1
4, 16, 17
complexity to the system.
Trivalent iron and aluminum are abundant in soils, occurring as
solid bulk and nanomineral phases, more mobile colloids, or solubilized organic chelates, such as
5
ꢀ
4ꢀ
those observed for citrate (cit) complexes. Mononuclear forms, including [M(cit) ] , [M(Hcit) ]
2
2
3
ꢀ
,
and [M(Hcit) ] (M = Al(III), Fe(III)), have been observed in slightly acidic pH regions,
2
yꢀ
20ꢀ22
whereas polynuclear complexes ([M OH (cit) ] ) form under more neutral conditions.
2
x
2
Given the ability of citrate to bind to an array of metal cations, ternary or heterometallic species
2
+
are also likely to form and 1:1:2 U(VI)O :Fe(III):citrate complexes have been observed
2
23
previously in aqueous systems. Formation of ternary molecular species could impact the long
2+
term stability of U(VI)O ꢀbearing mineral due to higher solubility of the resultant phases and
2
several previous studies have reported enhanced mobility and stability in mixedꢀmetal systems.
2
3ꢀ26
Other studies hypothesize that contaminant mobility will decrease when excess citrate binds
to the surface of Fe(III) and Al(III) oxide mineral surfaces, providing enhanced adsorption and
2
6, 27
2+
retention.
While the fate and transport of U(VI)O /(Fe(III), Al(III))/citrate complexes in
2
environmental systems is unclear, the previously reported data demonstrates the importance of
2
6, 27
these ternary phases.
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Despite the impact of these species in solution, only a handful of studies investigating
2
+
ternary U(VI)O :(Fe(III), Al(III)):citrate complexes have been previously reported in the
2
2
8, 29
literature and very few contain the structural characteristics of the resultant phases.
Developing a complete picture of the ternary species present in solution is difficult given the
complexity of the system and the multitude of possible polynuclear phases that are difficult to
discern using potentiometric and spectroscopic approaches. We are interested in providing a
deeper understanding of the formation of ternary phases for actinides in aqueous conditions
through the structural characterization of geochemical model compounds. These models are
created through the crystallization of the molecular species into an extended 3ꢀD lattice through
charge balancing cations, which allows for the structural characteristics of the species to be
determined using singleꢀcrystal Xꢀray diffraction. This solid compound can then be chemically
characterized by a variety of techniques, which provides foundational knowledge that can be
applied to aqueous solution studies. Herein, we present the synthesis and chemical
6
ꢀ
characterization of three novel ternary molecular species: [(UO ) Al (C H O ) ] (U Al ),
2
2
2
6
4
7 4
2
2
6
ꢀ
8ꢀ
[
(UO ) Fe (C H O ) ] (U Fe ), and [(UO ) Fe (OH) (C H O ) ] (U Fe ). These solid phases
2 2 2 6 4 7 4 2 2 2 2 4 4 6 4 7 4 2 4
were compared to previously reported speciation studies to provide additional insight into the
formation of ternary phases in environmentally relevant aqueous solutions.
Results and Discussion
Structural Descriptions
6
ꢀ
[
(UO ) Al (C H O ) ] (U Al ).
2 2 2 6 4 7 4 2 2
2
+
A ternary molecule with a U(VI)O :Al(III):citrate ratio of 2:2:4 is observed in the
2
U Al species (Fig. 1). The average bond length between the U(VI) metal center and the axial O
2
2
4

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2
+
atoms was 1.767(4) Å, which is typically observed for the near linear uranyl U(VI)O2 cation.
Six additional equatorial bonds with distances ranging from 2.341(5) – 2.581(5) Å result in the
formation of a hexagonal bipyramidal coordination geometry about the U(VI) cation. Each
Al(III) cation is octahedrally coordinated by six ligands with distances ranging from 1.821(5) to
1.905(3) Å. The citrate molecule contains three carboxylate functional groups and one central
hydroxyl group that allows the chelator to form up to four bonds with a metal center. In the case
of the U Al species, two citrate molecules bridge one uranyl polyhedron and one Al(III)
2
2
octahedron through the central hydroxyl functional group. In addition, each of the citrate
molecules bond to the Al(III) cation through two carboxylate functional groups, leaving one arm
2
+
available to complex to the U(VI) atom, resulting in the 1:1:2 U(VI)O :Al(III):Cit species. Two
2
2
+
of these building units selfꢀassemble to form the 2:2:4 species, with the central U(VI)O2 dimer
created through bridging O atoms of with the carboxylate functional groups and the two Al
octahedra decorating the exterior of the molecule. Overall, the cluster is negatively charged with
6
ꢀ
a molecular formula of [(UO ) Al (C H O ) ] .
2
2
2
6
4
7 4
6
ꢀ
The [(UO ) Al (C H O ) ] species can be crystallized into two separate compounds,
2
2
2
6
4
7 4
U Al ꢀ1 and U Al ꢀ2, due to the presence of different charge balancing cations within the
2
2
2
2
crystalline lattice that lead to changes in molecular packing and unit cell dimensions. For U Al ꢀ
2
2
1
, the negative charge on the cluster is balanced by three piperazinium cations, while the
presence of two strontium and one ethylenediamonium cations results in a different crystalline
packing for U Al ꢀ2. Additional interstitial water molecules participate in hydrogen bonding
2
2
interactions and are located throughout the crystalline lattice void spaces in both compounds.
The amount of water present in the crystalline lattice was verified by TGA, leading to the overall
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chemical
formulas
of
[(C H N ) ][(UO ) Al (C H O ) ](H O)
8
and
4
12 2 3
2 2
2
6
4
7 4
2
[
Sr (C H N )][(UO ) Al (C H O ) ](H O) for U Al ꢀ1 and U Al ꢀ2, respectively.
2
2
10
2
2 2
2
6
4
7 4
2
13
2
2
2
2
6
ꢀ
6ꢀ
[
(UO ) Fe (C H O ) ] and [(UO ) Fe (C H O ) (H O) ] (U Fe ꢀ1 and U Fe ꢀ2)
2 2 2 6 4 7 4 2 2 2 6 4 7 4 2 2 2 2 2 2
2
+
The heterometallic cluster found in U Fe ꢀ1 is isostructural to the U(VI)O /Al(III)
2
2
2
tetramer, but U Fe ꢀ2 exhibits a slight variation in the chelation of Fe(III) metal center by the
2
2
citrate molecule (Fig. 2). Two central U(VI) hexagonal bipyramids are again observed with
average UꢀO and UꢀO (ax = axial, eq = equatorial) bond lengths of 1.764(4) and 2.478(4) Å,
ax
eq
respectively. Each of the two Fe(III) octahedra are bonded to the exterior of the uranyl dimer
through a bridging hydroxo group, with FeꢀO bond lengths ranging from 1.935(4) to 2.058(4) Å.
While chelation of the U(VI) polyhedra by the citrate ligand is identical to the ternary
2
+
U(VI)O /Al(III) citrate species in both complexes, a slight variation is observed for the Fe(III)
2
metal centers. Tetradentate chelation of the Fe(III) cation in U Fe ꢀ1 occurs via two of the citrate
2
2
carboxylate O atoms, one shared O atom between the two metal cations, and an O atom
associated with the uranyl cation. In the case of U Fe ꢀ2, tridentate chelation is observed
2
2
because one arm of the citrate ligand is not complexed to the metal center, leading to the
presence of an additional water molecule bound to the Fe(III) cation. In both species, the overall
6
ꢀ
charge on the cluster is negative six, but the molecular formula is [(UO ) Fe (C H O ) ] and
2
2
2
6
4
7 4
6
ꢀ
[
(UO ) Fe (C H O ) (H O) ] for U Fe ꢀ1 and U Fe ꢀ2, respectively.
2 2 2 6 4 7 4 2 2 2 2 2 2
Differences in the crystalline packing for U Fe ꢀ1 and U Fe ꢀ2 can also be observed due
2
2
2
2
to the identity of the charge balancing cations present in the reaction vial. The negative six
charge associated with the cluster in U Fe ꢀ2 is balanced by three doubly deprotonated
2
2
piperazinium cations, while the U Fe ꢀ1 cluster is balanced by two piperazinium cations and a
2
2
hydrated magnesium cation. Water molecules again occupy the interstitial regions of the
6

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compounds and partake in hydrogen bonding interactions between the core cluster units, which
results in the crystallization of the neutral [Mg(H O) (C H N ) ][(UO ) Fe (C H O ) ](H O)
7
2
4
4
12 2 2
2 2
2
6
6
7 4
2
and [(C H N ) ][(UO ) Fe (C H O ) (H O) ](H O) for U Fe ꢀ1 and U Fe ꢀ2, respectively.
4
12 2 3
2 2
2
6
4
7 4
2
2
2
2
2
2
2
2
8
ꢀ
[
(UO ) Fe (C H O ) O (OH) ] (U Fe )
2 2 4 6 4 7 4 2 4 2 4
A slightly larger ternary complex with a U:Fe:citrate ratio of 2:4:4 is observed in the
U Fe compound (Fig. 3). Each uranyl cation is surrounded by five ligands, resulting in a
2
4
pentagonal bipyramidal geometry with average U(VI)ꢀO and U(VI)ꢀO bonds of 1.791(5) and
ax
eq
2.359(5) Å, respectively. The Fe(III) cation is coordinated by six ligands in an octahedral
coordination with bond distances ranging from 1.855(5) to 2.119(5) Å. Two ꢁ ꢀOH bridged
2
Fe(III) dimers are further bonded through ꢁ ꢀO atoms, forming a tetrameric Fe unit that
3
4
comprises the core of the U Fe ternary species. Each uranyl polyhedra is bonded to the central
2
4
molecular unit via the ꢁ ꢀO atom and through the bridging hydroxyl functional group of the
3
2
+
citrate ligand to create a U:Fe ratio of 2:4. Four citrate molecules bridge the U(VI)O2 and
Fe(III) polyhedra, completing the inner sphere coordination shell for both metals. The molecular
cluster of the U Fe compound has a negative eight charge that is balanced out by four
2
4
ethylenediammonium cations. These ancillary cations plus fourteen water molecules per cluster,
are located within the interstitial regions and aid in linking the U Fe clusters into a 3ꢀD
2
4
crystalline lattice. The overall formula for the neutral, heterometallic compound U Fe is
2
4
[
(C H N ) ][(UO ) Fe (C H O ) O (OH) ](H O) .
2 10 2 4 2 2 4 6 4 7 4 2 4 2 14
Vibrational Spectroscopy
Fourier transform infrared spectroscopy of the U Fe , U Fe , and U Al compounds
2
4
2
2
2
2
confirms the citrate complexation of the metal center and the presence of solvated and ligated
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ꢀ1
water (Fig. 4). All spectra contain a strong vibrational mode at 925 cm that corresponds to the
3
0
ꢀ1
ꢀ1
ν3 asymmetric stretch of the uranyl cation. Bands present at 1390 cm and 1600 cm correlate
ꢀ
to the asymmetric and symmetric stretching modes associated with the –COO functional group
1
4, 31
of the citrate molecule, respectively.
Several OꢀH stretching vibrations from free solvent
molecules, ligated water groups, and the bridging hydroxyl groups were observed as a broad
ꢀ1 30
peak ranging from 2900 to 3650 cm . The bending mode associated with the solvent water
ꢀ1
molecules can also be observed at approximately 1640 cm , but overlaps with the asymmetric
ꢀ
32
stretching band of the –COO functional group.
Additional weak peaks related to CꢀH
ꢀ1
vibrational modes are present between 400ꢀ1600 cm .
The ν symmetry stretching vibration associated with the uranyl cation dominates the
1
Raman spectra for all three complexes and can be utilized to provide some information regarding
ꢀ1
bond strength (Fig. 5). This band is observed at 835 cm for both U Al and U Fe and is red
2
2
2
2
shifted compared to the aqueous pentaaqua uranyl species that was previously reported at 870
ꢀ1
ꢀ1
cm . A decrease of 30ꢀ60 cm from the solvated uranyl cations is common for species that are
3
3,
complexed by electron donating ligands, such as carboxylate and hydroxyl functional groups.
3
4
ꢀ1
The ν band is observed at 813 cm for U Fe complex and the additional red shift may be
1
2
4
explained by the presence of the bridging ꢁ ꢀO in the molecule that will provide additional
3
electron donation and elongation of the uranyl bond. Due to the observed changes in the Raman
spectra for the U Fe and the U Fe species, the position of the ν symmetry stretch may be
2
2
2
4
1
utilized to identify ternary species in solution.
Mass Spectrometry
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Electrospray ionization mass spectrometry (ESIꢀMS) verifies the presence of stable
anionic species in solution and can be related to the ternary species characterized in the solid
state by singleꢀcrystal Xꢀray diffraction. The soft ionization of an electrospray source has the
ability to transfer material into the gas phase relatively intact which makes ESIꢀMS a desirable
1
4, 35, 36
technique for the characterization of aqueous ionic species with high molecular weights.
Fragmentation of the molecular species occurs quite readily for these complexes; therefore, it is
important to investigate the anionic species associated with U Fe , U Fe , and U Al compounds
2
4
2
2
2
2
that can be observed with ESIꢀMS.
To investigate the stability of the ternary species under a variety of aqueous conditions,
the solid material was redissolved in water with the pH adjusted between 4 and 10 with a
NH OH buffer. This provides a solution that contains just one molecular cluster instead of a
4
range of polynuclear species, which provides exact information on the fragmentation that occurs
during the ESIꢀMS measurement. These initial studies provide the basis for understanding
environmentally relevant solution that contain a multitude of molecular species, charge balancing
cation/anions, or other organic matter. In all cases, the 2:2:4 U:Fe, Al:Cit or the 2:4:4 U:Fe:Cit
molecule were not observed as the major species. Instead, the species that was detected
2
ꢀ
corresponded to [UO –(Fe,Al)ꢀ 2Cit + H] with an m/z = 351 for U Fe and U Fe and 337 for
2
2
2
2
4
U Al . This 1:1:2 U:(Fe, Al):Cit anionic species is dominant at pH values greater than 6, with
2
2
only a handful of minor fragments observed with relative abundance <15. The molecular species
associated with these minor peaks were not identified, but suggests additional fragmentation to
mononuclear and binuclear species. Full spectra of the solutions at pH 8ꢀ9 are available in the
supporting information section and are representative of all solutions.
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Relationships to aqueous solutions
The objective of the current study is to structurally characterize ternary molecular species
to use as geochemical model compounds and provide additional chemical characterization to
serve as foundational knowledge for the identification of these species in aqueous conditions.
An important point regarding crystallization is that the species isolated in the solidꢀstate may not
represent the major species in solution, but instead is preferentially crystallized based on the
identity of the charge balancing cation or other spectator ions present in solution. Due to this
fact, it is necessary to compare the data collected on the molecular species present in the solidꢀ
state lattice to studies performed on aqueous systems. As a slight extension, structural
information for the first and second coordination sphere of the metals can also be useful for the
adsorption of contaminants on mineral surfaces. In the current section, we compare the
structural details of the U Al , U Fe , and U Fe complexes to previously reported data on
2
2
2
2
2
4
related systems.
Under acidic conditions, U(VI) speciation is dominated by the pentaaquauranyl complex,
2
+
[
(UO )(H O) ] , but upon increasing pH hydrolysis of the metal center can occur, which results
2 2 5
3
7
in the formation of larger polynuclear molecules. Hydrolysis can occur via two pathways,
olation and oxolation, where the former generally occurs by the elimination of a water molecule
3
8, 39
and the formation of hydroxo bridges.
Oxolation ensues when a hydroxo or oxohydroxo
complex is initially formed and water molecule is not available as a leaving group, resulting in
3
8, 39
condensation that proceeds via deprotonation of a hydroxo group to form a oxo bridge.
Hydrolysis of the uranyl cation in aqueous solution generally occurs when the pH > 3 with an
2
+
40ꢀ42
olation reaction resulting in the formation of the dimeric [(UO ) (ꢁ ꢀOH) (H O) ] species.
2
2
2
2
2
6
This species is never observed as the sole soluble complex because it is in thermodynamic
1
0

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2
+
+
40
equilibrium with monomeric U(VI)O2 and (U(VI)O )(OH) forms.
Increasing alkalinity
2
2+
results in the creation of trimeric species that have previously been identified as [(UO ) (OH) ]
2
3
4
+
40, 42
and [(UO ) (OH) ] by potentiometric studies.
Several uranyl trimer have been crystallized
2
3
5
4
3ꢀ46
into the solid phase and structurally characterized by singleꢀcrystal Xꢀray diffraction.
In
nearꢀneutral aqueous solutions, the trimeric species that is generally observed contains a central
ꢁ3
ꢀO and three ꢁ ꢀOH bridges and this form has been reported as the most thermodynamically
2
4
7
stable phase based upon DFT studies.
Addition of complexing agents, such as citrate, can influence the speciation and structural
4
6, 48
characteristics of the resulting hydrolysis products.
In binary systems, uranyl citrate dimers
begin to coalesce at a pH of approximately three and the proposed structure based on NMR
studies suggests that the uranyl moieties are bridged by the αꢀhydroxy groups on two separate
2
+
14, 17, 49, 50
citrate ligands result in a molecule with a U(VI)O :citrate ratio of 2:2.
Given the steric
2
hindrance, it is postulated that only two of the carboxylate functional groups associated with a
49
citrate molecule are chelated to the metal centers, leaving one arm uncomplexed.
This
arrangement of the citrate would result in a square bipyramidal coordination about the metal
center, although it is possible that an additional water molecules could be present to form the
more widely observed pentagonal bipyramidal geometry. Under slightly acidic to neutral
conditions, a trimeric species is expected as the dominate form, but two different species with
2+
variations in the U(VI)O :citrate ratio (3:3 and 3:2) have been previously observed in
2
14
solution.
The structural characteristics of the 3:3 U:cit species have yet to be determined
through any means, although it is postulated that a ring structure exists for the 3:2 U:cit
1
7
molecule.
1
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Fe(III) and Al(III) are also susceptible to hydrolysis in aqueous conditions and can form
large polyncuclear species with cubaneꢀtype structures, such as the, Fe , Fe , Fe , Fe , Fe and
7
9
11
13
17,
5
1ꢀ55
Fe molecular complexes.
Hydrolysis of the Fe(III) cation also takes place through olation
1
9
3
9
reactions with the formation of ꢁ ꢀOH bridging the metal centers. Under acidic conditions and
2
56
in the presence of citrate, dimeric Fe(III) can form with Fe:citrate ratios of 2:2 or 2:3. In both
species, the Fe(III) metal centers are bridged through the hydroxyl functional groups of the
citrate, which is similar to that observed for the uranyl dimeric complex. Trimeric Fe(III)
citrates have also been reported with a “voided cubane” core that link together through the
5
7
carboxylate end members, forming a nonairon (III) citrate complex or the “ferric tripleꢀdecker”.
Aluminum citrate speciation is very similar to Fe(III), however no large polynuclear species have
2
1, 58
been structurally characterized.
2
+
Formation of heterometallic or ternary species containing U(VI)O2 and Fe(III) or
Al(III) hydrolysis products are likely to occur in environmental systems, given the ability of the
2
+
citrate ligand to complex all three metal cations. An Al(III)ꢀ U(VI)O2 complex has previously
been detected using amperometric and spectrophotometric techniques that indicated the presence
of a central U atom was bonded to two citrate molecules and an Al(III) cation is likely bound to
2
3
2+
O atoms associated with the uranyl polyhedra.
Ternary Fe(III)/U(VI)O2 species have
previously been reported by several studies and initial characterization of the complex suggested
a binuclear iron core coordinated in a bidentate fashion to two complexed uranyl polyhedra
2
4, 28, 29
through the central carboxylate functional group of the citrate.
Additional structural studies conducted on solutions and solid phases with
2
+
28
U(VI)O :Fe(III):Cit ratio of 1:1:2 was previously performed by Dodge and Francis using
2
Extended Xꢀray Absorption Fine Structure (EXAFS) Spectroscopy. Bond lengths based upon
1
2

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2
8
singleꢀcrystal Xꢀray diffraction of the compounds reported herein and the reported EXAFS data
for the ternary ironꢀuranyl citrate complexes are summarized in Table 1. Distinct differences in
the bond lengths are observed between the two structural characterization techniques, suggesting
the formation of very different complexes. EXAFS data of the solution phase indicated that a
Fe(III) dimer was the central core of the molecular species, which is opposite to that observed in
Fe U . A tetrameric iron core is observed for the Fe U species, but there are differences in the
2
2
4
2
atomꢀatom distances between the metal centers. The EXAFS data provides Fe(III)ꢀFe(III)
distances of 3.74 and 3.61 Å for the solid phase and 15mM solution, respectively, whereas the Xꢀ
ray diffraction data observed distances at 3.073 Å for the ꢁ ꢀOH bridged Fe(III) dimers and
2
3
.602 Å and for the ꢁ ꢀO bridging mode. A peak was observed in the Fourier Transform of the
3
EXAFS data at 3.02 Å, but it was assigned to the Fe(III)ꢀC distance. Additional discrepancies
between the data includes an unexplained distance at 2.72 Å from the Fe(III) cation in the
+
EXAFS spectra. Both the solution and the solid phase material contained Na cations due to the
2
8
addition of NaOH and thus Dodge and Francis assigned this peak to a FeꢀNa distance. The
2
+
+
U(VI)O :Fe(III):citrate species presented in this work do not contain additional Na cations,
2
thus this distance could not be confirmed.
The importance of understanding the structural details of these complexes is highlighted
2+
by studies that indicate the ternary 1:1:2 U(IV)O :Fe(III):Cit are resistant to biodegradation in
2
aqueous solutions. Photoꢀ and biodegradation of these soluble complexes has been identified as
a potential pathway to the environmental remediation of uranium thus it is important to
2
9
understand the impact of ternary complexes present in solution. Dodge and Francis previously
reported that these mixed metal species are completely recalcitrant to biodegradation by
2
+
Pseudomons fluorescens, whereas the 1:1 Fe(III):Cit and U(VI)O :Cit complexes readily
2
1
3

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5
9
degrade. Additional investigations by Francis et al. on a range of metal citrate complexes
suggested that the complexation of the alcohol group to the metal center results in the resistance
to degradation. The complex formed in U Fe , as well as those observed in U Al and U Fe ,
2
2
2
2
2
4
contain complexed alcohol groups, suggesting that these species will also exhibit a resistance to
biodegradation by microbes.
Conclusions
2
+
Four ternary U(VI)O :(Fe(III),Al(III)):Cit complexes were structurally and chemically
2
characterized to provide an enhanced understanding of the formation of molecular species in
heterometallic aqueous systems. While discrepancies exist between the singleꢀcrystal Xꢀray
diffraction data and the results from the EXAFS data, the ability to compare atomic distances, is
crucial for providing structural understanding of the species observed in aqueous systems. In
addition, spectroscopic studies of solid state materials provide an ideal model complex that can
be utilized to predict the expected vibrational bands in aqueous solution and can aid in the
identification of the molecular fractionation that are observed in mass spectrometry. Further
2
+
characterization of molecular heterometallic U(VI)O /(Fe(III), Al(III)) structures is important
2
for providing a complete picture of speciation in aqueous solutions and leading to advancements
in remediation strategies and transport modeling of actinides in environmental systems.
Experimental Section
Synthesis. UO (NO ) ·6H O was purchased from Flynn Scientific Inc., Fe(NO ) ·9H O from
2
3 2
2
3 3
2
Alfa Aesar, Al(NO ) ·9H O from Acros Organic, and C H O ·H O from J.T. Baker. All
3
2
2
6
8
7
2
chemicals were reagent grade and used without further purification.
1
4

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[
(C H N ) ][(UO ) Al (C H O ) ](H O) : U Al ꢀ1 was obtained by the combination of 2 mL
4 12 2 3 2 2 2 6 4 7 4 2 8 2 2
of 0.2 M aqueous uranyl nitrate hexahydrate (0.4 mmol), 2 mL of 0.2 M aqueous aluminum
nitrate nonahydrate (0.4 mmol) and 4 mL of 0.2 M aqueous citric acid monohydrate (0.8 mmol)
in a 20 mL glass scintillation vial. The pH of the resulting solution was increased to 4.1 with 0.8
M aqueous piperazine hexahydrate and U Al ꢀ1 was crystallized out by a liquidꢀliquid diffusion
2
2
method with acetonitrile (1:2 solution:solvent) in low (>5%) yield.
[
Sr (C H N )][(UO ) Al (C H O ) ](H O) : U Al ꢀ2 was obtained by the combination of 2
2 2 10 2 2 2 2 6 4 7 4 2 13 2 2
mL of 0.2 M aqueous uranyl nitrate hexahydrate (0.4 mmol), 2 mL of 0.2 M aqueous aluminum
nitrate nonahydrate (0.4 mmol) and 4 mL of 0.2 M aqueous citric acid monohydrate (0.8 mmol)
in a 20 mL glass scintillation vial. The pH of the resulting solution was increased to 4.0 with 20
v/v% aqueous ethylenediamine and then 0.235 mL of 0.2 M aqueous Sr(NO ) (11.1 ꢁmol) was
3
2
incorporated into the reaction solution. U Al ꢀ2 was crystallized out by liquidꢀliquid method
2
2
diffusion with acetonitrile (1:2 solution:solvent) in 72% yield.
[
(Mg(H O) )(C H N ) ][(UO ) Fe (C H O ) ](H O) : U Fe ꢀ1 was obtained by the
2 4 4 12 2 2 2 2 2 6 4 7 4 2 7 2 2
combination of 2 mL of 0.2 M aqueous uranyl nitrate hexahydrate (0.4 mmol), 2 mL of 0.2 M
aqueous ferric nitrate nonahydrate (0.4 mmol) and 4 mL of 0.2 M aqueous citric acid
monohydrate (0.8 mmol) in a 20 mL glass scintillation vial. The pH of the resulting solution was
increased to 4.0 with 0.8 M aqueous piperazine hexahydrate and then 0.235 mL of 0.2M aqueous
Mg(NO ) ·6H O (11.1 ꢁmol) was incorporated into the reaction solution. U Fe ꢀ1 was
3
2
2
2
2
crystallized out by liquidꢀliquid diffusion with acetonitrile (1:1 solution:solvent) in 80% yield.
1
5

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[
(C H N ) ][(UO ) Fe (C H O ) (H O) ](H O) : U Fe ꢀ2 was obtained by the combination of
4 12 2 3 2 2 2 6 4 7 4 2 2 2 2 2 2
2
mL of 0.2 M aqueous uranyl nitrate hexahydrate (0.4 mmol), 2 mL of 0.2 M aqueous ferric
nitrate nonahydrate (0.4 mmol) and 4 mL of 0.2 M aqueous citric acid monohydrate (0.8 mmol)
in a 20 mL glass scintillation vial. The pH of the resulting solution was increased to 4.0 with 0.8
M aqueous piperazine hexahydrate and U Fe ꢀ2 was crystallized out by liquidꢀliquid diffusion
2
2
with acetonitrile (1:1 solution:solvent) in low (>5%) yield.
[
(C H N ) ][(UO ) Fe (C H O ) O (OH) ](H O) : U Fe was obtained by the combination of
2 10 2 4 2 2 4 6 4 7 4 2 4 2 14 2 4
2
mL of 0.2 M aqueous uranyl nitrate hexahydrate (0.4 mmol), 2 mL of 0.2 M aqueous ferric
nitrate nonahydrate (0.4 mmol) and 4 mL of 0.2 M aqueous citric acid monohydrate (0.8 mmol)
in a 20 mL glass scintillation vial. The pH of the resulting solution was increased to 7.0 with 20
v/v% aqueous ethylenediamine and U Fe was crystallized out by liquidꢀliquid diffusion with
2
4
tetrahydrofuran (1:4 solution:solvent) in 86% yields.
SingleꢀCrystal Xꢀray Diffractometry. The crystalline data acquisition for all five complexes
were collected on a Nonius Kappa CCD singleꢀcrystal Xꢀray diffractometer equipped with
graphiteꢀmonochromated Mo Kα radiation (λ = 0.7107 Å) and a lowꢀtemperature cryostat set at
6
0
1
00 K. The Nonius COLLECT software acquired the data and the Bruker APEX II software
performed the Lorentzian, polarization, data integration and background correction on the
6
1
collected data.
A semiꢀempirical correction for crystal absorption was completed using the
6
1
SADABS program within the APEX II software and all crystal structures were determined by
2
direct methods and refined on the basis of F for all unique data using the Bruker SHELXTL
6
2
version 5.01 software. The U, Al, Fe, Sr, and Mg atoms were determined in the intial structure
1
6

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solution and the O, C, and N atoms were identified in the difference Fourier maps calculated
following refinement of the partial structure models. Hydrogen atoms associated with the
organic componense were constrained using a riding model. When ordered water molecules
were present in the interstitial regions, H atoms were identified electron density and placed using
the DFIX restraints. H atoms were not placed on disordered water molecules. Selected data
collection parameters and crystallographic information are provided in Table 2. Crystallographic
information files (CIFs) for each compound are available free of charge at the Cambridge
Crystallographic Data Centre upon providing reference numbers 1018644ꢀ1018648.
Thermal Gravimetric Analysis. A TA Instruments TGA Q500 was utilized to parse the
thermal stability of the compound and quantify the hydration states and overall water content.
This was achieved by loading approximately 15ꢀ20 mg of each sample into an alumina pan and
heating under a stream of air from 25ꢀ1000 °C increasing with a ramp rate of 2 °C/min. The full
TGA data can be found in the supporting information section.
Vibrational Spectroscopy. The presence of solvated water and ligated citrate complexation of
the metal center was confirmed with two vibrational spectroscopy techniques. Each solid
compound was pressed into a translucent pellet with KBr as the binder and data was collected on
ꢀ1
a Nicolet FTIR Spectrometer from 500ꢀ4000 cm . High Resolution Raman Spectroscopy of a
single crystal samples was performed on a Nicolet Almega XR High Performance Dispersive
Spectrometer equipped with a 785 nm excitation laser.
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7

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Mass Spectroscopy. A ThermoElectron LCQ Deca Quadrupole IonꢀTrap Mass Spectrometer
equipped with an ESI source was used to observe and characterize the existence of the clusters’
anionic core in solution. The spectrometer was operated in the negative ion mode and analyzed
ꢀ1
the samples via direct infusion at a flow rate of 20 uL min into the ESI source with a
Hamiltonian 250 uL glass syringe. Resultant spectra are an average of approximately 50 scan,
collected with a count time of 0.2 s/scan, and nitrogen was employed as the nebulizing gas.
Programmable ESI parameters were set as follows: capillary voltage, ꢀ16 V; capillary
temperature, 110 °C; ionization spray voltage, 3.5 kV; tube lens offset voltage, 20 V. Crystals
were dissolved in aqueous solutions, and fragmentation of the anionic species was investigated
by varying the pH of the ammonium hydroxide buffer between 4.1 and 10.0 pH.
Acknowledgments
We would like to acknowledge support from the University of Iowa Center for Global and
Regional Environmental research. In addition, we thank the UI Central Microscopy Research
Facility for use of the Raman microspectrometer, Dr. Lynne Tesh in the UI Mass Spectrometry
Facility,
and
Dr.
Edward
Gillan
for
use
of
the
FTIR
spectrometer.
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8

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2
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TABLES
Table 1. Average atomꢀatom distances obtained from singleꢀcrystal Xꢀray diffraction for the ternary clusters
8
ꢀ
6ꢀ
(
[
[(UO ) Fe (OH) (C H O ) ]
(U Fe ),
2
[(UO ) Fe (C H O ) ]
(U Fe ꢀ1
)and
2
2
4
4
6
4
7 4
2
4
2 2
2
6
6ꢀ
4
7 4
2
2
6ꢀ
(UO ) Fe (C H O ) (H O) ] (U Fe ꢀ2) and [(UO ) Al (C H O ) ] (U Al ꢀ1 and U Al ꢀ2)) compared to
2
2
2
6
4
7 4
2
2
2
2 2
2
6
4
7 4
2
2
2
2
2
8
the previously reported values determined by EXAFS spectroscopy by Dodge and Francis for a solid
and solution phase with U:Fe:cit ratio of 1:1:2.
Average AtomꢀAtom Distances
U Fe4
2
U Fe ꢀ1
U Fe ꢀ2
U Al ꢀ1
U Al ꢀ2
2 2
2
2
2
2
2
2
SingleꢀCrystal Xꢀray Diffraction
MetalꢀO Distance (Å)
2.014
1.79(5)
2.359
1.985
1.767(4)
2.477
1.995
1.762(4)
2.479
1.876
1.772(3)
2.475
1.874
1.763(5)
2.459
UꢀOax Distance (Å)
UꢀOeq Distances (Å)
2
.74(1)
2.78(2)
2.99(3)
3.322(1)
2.67(2)
2.86(4)
3.26(2)
2.66(2)
2.85(4)
3.26(4)
2
3
.859,
.081
MetalꢀC Distance (Å)
2.96(8)
3
.33(4)
ꢀ
3
3
3
3
.037.
MetalꢀMetal Distance (Å)
MetalꢀU Distance (Å)
ꢀ
ꢀ
ꢀ
.602
.422
.390
3.55(4)
3.53(1)
3.46(1)
3.45(2)
Aq
15 mM)
EXAFS (Dodge & Francis, 2003)
Solid
(
MetalꢀO Distance (Å)
UꢀOax Distance (Å)
UꢀOeq Distances (Å)
Feꢀ Na Distance (Å)
MetalꢀC Distance (Å)
FeꢀFe Distance (Å)
1.99
1.77
2.36
2.72
3.02
3.74
1.98
1.77
2.37
2.64
3.03
3.61
2
2

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Table 2. Selected crystallographic information for (C H N ) [(UO ) Al (C H O ) ](H O) (U Al ꢀ1),
4
12 2 3
2 2
2
6
4
7 4
2
8
2
2
Sr (C H N )[(UO ) Al (C H O ) ](H O)
(U Al ꢀ2) (Mg(H O) )(C H N ) [(UO ) Fe (C H O ) ]
2
2
10
2
2 2
2
6
4
7 4
2
13
2
2
2
4
4
12 2 2
2 2
2
6
4
7 4
(
(
H O)7
(U Fe ꢀ1),
(C H N ) [(UO ) Fe (C H O ) (H O) ](H O)
(U Fe
ꢀ2),
and
2
2
2
4
12 2 3
2 2
2
6
4
7 4
2
2
2
2
2
2
C H N ) [(UO ) Fe (C H O ) O (OH) ](H O) (U Fe ).
2
10 2 4
2 2
4
6
4
7 4
2
4
2
14
2
4
U Al ꢀ1
U Al ꢀ2
U Fe ꢀ1
U Fe ꢀ2
U Fe4
2
2
2
2
2
2
2
2
2
ꢀ
1
FW (g mol )
Space group
a (Å)
1754.98
P2 /n
1791.75
P-1
1792.84
1740.66
2112.53
P-1
P-1
P-1
1
9.4451(10)
30.530(3)
9.7188(11)
90
8.7930(8)
10.6713(9)
14.5554(13)
78.605(3)
85.842(3)
86.569(3)
1333.9(2)
1
9.2106(6)
9.8799(7)
8.1422(13)
11.376(2)
14.798(3)
84.551(6)
89.290(6)
77.814(6)
1333.7(4)
1
11.2951(8)
11.6508(9)
14.0541(11)
77.664(3)
71.186(3)
66.255(2)
1595.0(2)
1
b (Å)
c (Å)
15.1866(10)
84.120(2)
88.101(2)
83.115(2)
1364.49(16)
1
α (°)
β (°)
95.311(3)
90
γ (°)
3
V (Å )
2790.4(5)
2
Z
ꢀ
3
ρcalc (mg m )
2.089
2.231
2.182
2.167
2.199
ꢀ
1
ꢁ
(mm )
5.944
8.192
6.568
6.697
6.071
F (000)
Crystal Size
1712
0.120 x 0.060
x 0.010
842
0.060 x 0.060
x 0.040
864
0.120 x 0.080
x 0.060
842
0.200 x 0.040
x 0.005
1036
0.180 x 0.170
x 0.020
(
mm)
Theta Range
°)
2.544 to
27.463
1.430 to
25.379
1.348 to
27.105
2.900 to
25.097
1.538 to
25.014
(
ꢀ
12 < h < 12
39 < k < 39
12 < l < 12
5 350/6364
ꢀ10 < h < 10
ꢀ11 < h < 11
ꢀ9 < h < 9
ꢀ13 < h < 13
Data
Collected
ꢀ
ꢀ12 < k < 12
ꢀ17 < l < 17
ꢀ12 < k < 12
ꢀ19 < l < 19
ꢀ13 < k < 13
ꢀ17 < l < 17
ꢀ12 < k < 13
ꢀ16 < l < 16
ꢀ
Reflections
Collected/
Unique
5
20 450/4864
23 483/6002
23 431/4725
18 604/5575
[
Rint = 0.0550] [Rint = 0.0470] [Rint = 0.0245] [Rint = 0.0660] [Rint = 0.0550]
2
GOF of F
1.055
1.08
1.067
1.013
1.102
Final R
Indices
I > 2σ(I)]
R Indices
all data)
R1 = 0.0252
R1 = 0.0417
R1 = 0.0346
R1 = 0.0353
R1 = 0.0385
wR = 0.0480
wR = 0.1104
wR = 0.0795
wR = 0.0644
wR = 0.0948
2
2
2
2
2
[
R1 = 0.0396
R1 = 0.0502
R1 = 0.0429
R1 = 0.0580
R1 = 0.0453
(
wR = 0.0524
wR = 0.1166
wR = 0.0836
wR = 0.0715
wR = 0.0981
2
2
2
2
2
2
3

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FIGURE CAPTIONS
Figure 1. Polyhedral units of U Al representing the heterometallic molecular core composed of
2
2
two central uranyl cations and two exterior aluminum cations linked via tetradentate chelation of
four citrate ligands. The yellow and blue polyhedra correspond to the U(VI) and Al(III) cations
and the black and red spheres are C and O atoms, respectively.
Figure 2. Polyhedral representations of U Fe ꢀ1 (a) and U Fe ꢀ2 (b) are composed of two
2
2
2
2
central uranyl hexagonal bipyramids and two exterior ferric octahedral chelated by four citrate
molecules. The yellow and brown polyhedra denote the U(VI) and Fe(III) cations and the black
and red spheres are C and O atoms, respectively. Major differences between the two species are
highlighted by the green circle, showing additional water molecules complexed to the Fe(III)
polyhedra and the unbound carboxylate arm of the citrate molecule in U Fe ꢀ2.
2
2
2
+
Figure 3. U Fe contains four central Fe(III) cations and two outer UO
cations that are
tetradentate coordination to form the
2
4
2
chelated by four citrate ligands in
a
8
ꢀ
2+
[
(UO ) Fe (C H O ) O (OH) ] cluster. The yellow and brown polyhedra denote the U(VI)O
2 2 4 6 4 7 4 2 4 2
and Fe(III) cations and the black and red spheres are C and O atoms, respectively.
Figure 4. The infrared spectra for U Fe , U Fe , and U Al indicate the presence of ligated and
2
4
2
2
2
2
solvated water constituents in the crystalline material and confirm the citrate ligand is complexed
to the metal center.
Figure 5. The Raman spectra for U Fe , U Fe , and U Al reveals the red shifted ν symmetric
2
4
2
2
2
2
1
stretch associated with the uranyl moiety that corresponds to complexation of the citrate
molecule and formation of polynuclear ternary species.
2
4

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TOC graphic
Structural characterization of U(VI):(Al(III), Fe(III)):citrate complexes provides insight into the
formation of polynuclear clusters present in environmentallyꢀrelevant aqueous solutions.
2
5

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1
53x129mm (96 x 96 DPI)

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27x143mm (96 x 96 DPI)

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75x144mm (96 x 96 DPI)