Excision of uranium oxide chains and ribbons in the novel one-dimensional uranyl iodates K2[(UO2)3 (IO3)4O2] and Ba[(UO2)2(IO3)2O2] (H2O)

Article abstract of DOI:10.1021/ic010342l

The alkali metal and alkaline-earth metal uranyl iodates K₂[(UO₂)₃ (IO₃)₄O₂] and Ba[(UO₂)₂ (IO₃)₂O₂](H₂O) have been prepared from the hydrothermal reactions of KCl or BaCl₂ with UO₃ and I₂O₅ at 425 and 180 °C, respectively. While K₂[(UO₂)₃ (IO₃)₄O₂] can be synthesized under both mild and supercritical conditions, the yield increases from <5% to 73% as the temperature is raised from 180 to 425 °C. Ba[(UO₂)₂(IO₃)₂O₂ (H₂O), however, has only been isolated from reactions performed in the mild temperature regime. Thermal measurements (DSC) indicate that K₂[(UO₂)₃ (IO₃)₄O₂] is more stable than Ba[(UO₂)₂ (IO₃)₂O₂](H₂O) and that both compounds decompose through thermal disproportionation at 579 and 575 °C, respectively. The difference in the thermal behavior of these compounds provides a basis for the divergence of their preparation temperatures. The structure of K₂[(UO₂)₃(IO₃) ₄O₂] is composed of _∞¹[(UO₂)₃ (IO₃)₄O₂]^2- chains built from the edge-sharing UO₇ pentagonal bipyramids and UO₆ octahedra. Ba[(UO₂)₂(IO₃)₂](H₂O) consists of one-dimensional _∞¹[(UO₂)₂ (IO₃)₂O₂]^2- ribbons formed from the edge sharing of distorted UO₇ pentagonal bipyramids. In both compounds the iodate groups occur in both bridging and monodentate binding modes and further serve to terminate the edges of the uranium oxide chains. The K⁺ or Ba²⁺ cations separate the chains or ribbons in these compounds forming bonds with terminal oxygen atoms from the iodate ligands. Crystallographic data: K₂[(UO₂)₃(IO₃) ₄O₂], triclinic, space group P1, a = 7.0372(5) A, b = 7.7727(5) A, c = 8.9851(6) A, α = 93.386(1)^c, β = 105.668(1)°, γ = 91.339(1)°, Z = 1; Ba[(UO₂)₂(IO₃)₂O₂] (H2_O), monoclinic, space group P2₁/c, a = 8.062(4) A, b = 6.940(3) A, c = 21.67(1), β=98.05(1)°, Z = 4.

Full text of DOI:10.1021/ic010342l

Inorg. Chem. 2001, 40, 3959-3963
3959
Excision of Uranium Oxide Chains and Ribbons in the Novel One-Dimensional Uranyl
Iodates K2[(UO2)3(IO3)4O2] and Ba[(UO2)2(IO3)2O2](H2O)
Amanda C. Bean,† Michael Ruf,‡ and Thomas E. Albrecht-Schmitt*,†
Department of Chemistry, Auburn University, Auburn, Alabama 36849, and Bruker AXS,
5465 East Cheryl Parkway, Madison, Wisconsin 53711
ReceiVed March 28, 2001
The alkali metal and alkaline-earth metal uranyl iodates K2[(UO2)3(IO3)4O2] and Ba[(UO2)2(IO3)2O2](H2O) have
been prepared from the hydrothermal reactions of KCl or BaCl2 with UO3 and I2O5 at 425 and 180 °C, respectively.
While K2[(UO2)3(IO3)4O2] can be synthesized under both mild and supercritical conditions, the yield increases
from <5% to 73% as the temperature is raised from 180 to 425 °C. Ba[(UO2)2(IO3)2O2](H2O), however, has only
been isolated from reactions performed in the mild temperature regime. Thermal measurements (DSC) indicate
that K2[(UO2)3(IO3)4O2] is more stable than Ba[(UO2)2(IO3)2O2](H2O) and that both compounds decompose through
thermal disproportionation at 579 and 575 °C, respectively. The difference in the thermal behavior of these
compounds provides a basis for the divergence of their preparation temperatures. The structure of K2[(UO2)3-
1
(IO3)4O2] is composed of [(UO2)3(IO3)4O2]2- chains built from the edge-sharing UO7 pentagonal bipyramids
∞
1
∞
and UO6 octahedra. Ba[(UO2)2(IO3)2O2](H2O) consists of one-dimensional [(UO2)2(IO3)2O2]2- ribbons formed
from the edge sharing of distorted UO7 pentagonal bipyramids. In both compounds the iodate groups occur in
both bridging and monodentate binding modes and further serve to terminate the edges of the uranium oxide
chains. The K+ or Ba2+ cations separate the chains or ribbons in these compounds forming bonds with terminal
oxygen atoms from the iodate ligands. Crystallographic data: K2[(UO2)3(IO3)4O2], triclinic, space group P1h, a )
7.0372(5) Å, b ) 7.7727(5) Å, c ) 8.9851(6) Å, R ) 93.386(1)°, â ) 105.668(1)°, γ ) 91.339(1)°, Z ) 1;
Ba[(UO2)2(IO3)2O2](H2O), monoclinic, space group P21/c, a ) 8.062(4) Å, b ) 6.940(3) Å, c ) 21.67(1), â)
98.05(1)°, Z ) 4.
Introduction
and pH, the range of chemical and physical properties of these
compounds can be significantly expanded through the use of
Contemporary studies on uranium compounds have focused
primarily on the preparation of inorganic/organic hybrids under
mild hydrothermal conditions. These investigations have af-
forded a large series of low-dimensional and open-framework
fluorides and oxyfluorides,1-10 phosphates,11 and molybdates.12
While the chemistry of uranium under hydrothermal conditions
is greatly influenced by choice of template, reaction temperature,
polydentate ligands with variable binding modes such as
tellurites,13-17 germanates,18-23 and silicates.24-26 These and
other ligands can further impart chemical, structural, and
physical properties not obtainable with redox inactive and
structurally inflexible ligands.
Using these aforementioned concepts, we have begun the
exploration of uranyl iodates, for which there are very few well-
established examples. While uranyl iodates have been reported
in the past, the characterization of these compounds has been
† Auburn University.
‡ Bruker AXS.
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10.1021/ic010342l CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/30/2001

3960 Inorganic Chemistry, Vol. 40, No. 16, 2001
Bean et al.
highly limited.27,28 In the presence of oxidizing iodate ligands,
uranium has thus far only been isolated in the hexavalent
oxidation state, which generally occurs in the form of a linear
uranyl, UO22+, unit.26,29,30 The uranyl cation, however, shows
substantial structural diversity as up to six additional ligands
can be incorporated into the equatorial plane. This gives rise to
six-coordinate [UO2X4]n- octahedra, seven-coordinate [UO2X5]n-
pentagonal bipyramids, and eight-coordinate [UO2X6]n- hex-
agonal bipyramids.24-26 Some uranium-bearing minerals contain
multiple types of uranyl polyhedra giving rise to stunning levels
Experimental Section
Syntheses. UO3 (99.8%, Alfa-Aesar), I2O5 (98%, Alfa-Aesar), KCl
(Ultrapure, Alfa-Aesar), and BaCl2 (99.999%, Alfa-Aesar) were used
as received. Distilled and Millipore filtered water was used in all
reactions. The resistance of the water was 18.2 MΩ. Quartz tubes were
rinsed with distilled and Millipore filtered water and dried in an oven
prior to use. CAUTION: While the UO3 contains depleted U, standard
precautions for handling radioactiVe materials should be followed.
Extreme care should always be taken when scoring and opening sealed
tubes from hydrothermal reactions since these are typically under
pressure. These tubes should always be frozen in liquid nitrogen and
scored while wearing thick gloVes and a full-face shield. SEM/EDX
analyses were performed using a JEOL 840/Link Isis instrument. Ba,
I, and K percentages were calibrated against standards. Typical results
are within 3% of actual ratios. IR spectra were collected on a Nicolet
5PC FT-IR spectrometer from KBr pellets.
of structural complexity as found in wo¨lsendorfite, Pb6.16Ba0.36
-
[(UO2)14O19(OH)4](H2O)12.31 We have established that the
hydrothermal chemistry of even the simple uranyl iodates UO2-
(IO3)2 and UO2(IO3)2(H2O) is actually highly complex and that
the structures of these compounds are unique to this system.28,32
K2[(UO2)3(IO3)4O2]. UO3 (62 mg, 0.22 mmol), I2O5 (72 mg, 0.22
mmol), and KCl (16 mg, 0.22 mmol) were loaded in a quartz tube and
mixed by shaking the tube. Water (0.6 mL) was then added to the solids,
which represents a fill level of approximately 45%. The tube was then
frozen in liquid nitrogen, evacuated, and sealed. The tube was then
placed in a Leco Tem-Press 27-mL autoclave, and the remainder of
the volume of the autoclave was filled with water. The autoclave was
sealed, placed in a vertical tube furnace, and heated at 10 °C/min to
425 °C. After 72 h the furnace was cooled over 24 h to 23 °C. The
tube was subsequently removed from the autoclave and frozen in liquid
nitrogen to allow for safe opening. The solid product consisted of golden
needles of K2[(UO2)3(IO3)4O2] and elemental iodine. The solids were
washed with methanol (removing iodine) and allowed to dry; yield,
85 mg (73% yield based on U). EDX analysis for K2[(UO2)3(IO3)4O2]
provided a K:U:I ratio of 2:3:4. IR (KBr, cm-1): ν(UdO) 896 (s);
ν(UdO, U-O, and IdO) 862 (s), 829 (s), 794 (s), 787 (s), 742 (s),
719 (s), 691 (s, b), 501 (s, b), 443 (m).
Ba[(UO2)2(IO3)2O2](H2O). UO3 (286 mg, 1 mmol), I2O5 (334 mg,
1 mmol), and BaCl2 (244 mg, 1 mmol) were loaded in a 23-mL PTFE-
lined autoclave. Water (1.5 mL) was then added to the solids. The
autoclave was sealed, placed in a box furnace, and heated to 180 °C.
After 72 h the furnace was cooled at 9 °C/h to 23 °C. The product
consisted of a pale yellow solution over golden needles of Ba[(UO2)2-
(IO3)2O2](H2O), yellow truncated tetragonal bipyramids of UO2(IO3)2-
(H2O), and iodine. The mother liquor was decanted from the crystals,
which were then washed with water and methanol (removing iodine)
and allowed to dry; yield, 323 mg (60% yield based on U). EDX
analysis for Ba[(UO2)2(IO3)2O2](H2O) provided a Ba:U:I ratio of 1:2:
2. IR (KBr, cm-1): ν(UdO) 904 (s, sh); ν(UdO, U-O, and IdO)
862 (s), 831 (s), 790 (s), 775 (s), 713 (s), 570, (m, sh) 524 (m), 455
(s).
Crystallographic Studies. Golden crystals of K2[(UO2)3(IO3)4O2]
and Ba[(UO2)2(IO3)2O2](H2O) with the dimensions of 0.65 mm × 0.03
mm × 0.03 mm and 0.235 mm × 0.023 mm × 0.018 mm, respectively,
were mounted on glass fibers and aligned on a Bruker SMART APEX
CCD X-ray diffractometer. Intensity measurements were performed
using graphite-monochromated Mo KR radiation from a sealed tube
and a monocapillary. SMART was used for preliminary determination
of the cell constants and data collection control. For K2[(UO2)3(IO3)4O2],
the intensities of reflections of a sphere were collected by a combination
of 6 sets of exposures (frames). Each set had a different φ angle for
the crystal, and each exposure covered a range of 0.3° in ω. A total of
3600 frames were collected with an exposure time per frame of 6 s.
Crystals of Ba[(UO2)2(IO3)2O2](H2O) proved to be twins, and
RLATT was used to separate reflections from different domains of the
twinned sample for indexing with SMART. GEMINI determined the
twin law as a 2° rotation around reciprocal [010]. The intensities of
reflections of a sphere were collected by a combination of 3 sets of
exposures (frames). Each set had a different φ angle for the crystal,
and each exposure covered a range of 0.3° in ω. A total of 1800 frames
were collected with an exposure time per frame of 30 s.
Iodates in general have a propensity for crystallizing in non-
centrosymmetric space groups, and therefore the possibility
exists for preparing uranyl iodates with atypical physical
properties.33-35 Our interest in these compounds, however, is
based on the presence of two redox active centers with strong
oxidizing abilities and a stereochemically active lone pair,
making these compounds applicable to the development of
selective oxidation catalysts.36-38 Furthermore, the stabilities of
uranyl iodates may play an important role in the fate of 129I in
nuclear fuel waste.39 Therefore, the chemistry of uranyl iodates
is of importance as it relates to the long-term storage of nuclear
reactor waste, especially if seepage into groundwater occurs.24,39-42
Herein we report the hydrothermal syntheses, structural char-
acterization, vibrational spectroscopy, and thermal behavior of
two one-dimensional uranyl iodates with unprecedented struc-
tures, K2[(UO2)3(IO3)4O2] and Ba[(UO2)2(IO3)2O2](H2O). These
compounds are further illustrations of the striking differences
between mild and supercritical hydrothermal reaction condi-
tions.43,44
(27) (a) Tsivadze, A. Y.; Krot, N. N.; Muchnik, B. I. Proc. Moscow Symp.
Chem. Transuranium Elem. 1976, 89-94. (b) Tsivadze, A. Y.;
Muchnik, B. I. Krot, N. N. Zh. Neorg. Khim. 1972, 17, 3324.
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Elements; Katz, J. J., Seaborg, G. T., Morss, J. R., Eds.; Chapman
and Hall: London, 1986.
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(31) Burns, P. C. Am. Mineral. 1999, 84, 1661.
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13, 1266.
(33) Bergman, J. G.; Boyd, G. D.; Ashkin, A.; Kurtz, S. K. J. Appl. Phys.
1969, 40, 2860.
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1973, 7, 186. (b) Abrahams, S. C.; Sherwood, R. C.; Bernstein, J. L.;
Nassau, K. J. Solid State Chem. 1973, 7, 205.
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1996, 384, 341-343.
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83, 1263.
(38) Gates, B. C. Catalytic Chemistry; Wiley: New York, 1992; Chapter
6.
(39) Burns, P. C. Personal communication.
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147, 390.
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critical Fluids; Jessop, P. G., Leitner, W., Eds.; Wiley-VCH: New
York, 1999; pp 213-241.
For both K2[(UO2)3(IO3)4O2] and Ba[(UO2)2(IO3)2O2](H2O), deter-
mination of integral intensities and global cell refinement were
performed with the Bruker SAINT (v 6.02) software package using a

Novel One-Dimensional Uranyl Iodates
Inorganic Chemistry, Vol. 40, No. 16, 2001 3961
Table 1. Crystallographic Data for K2[(UO2)3(IO3)4O2] and
Ba[(UO2)2(IO3)2O2](H2O)
In contrast to the need for supercritical conditions in the
synthesis of K2[(UO2)3(IO3)4O2] in reasonable yield, the reaction
of BaCl2 with UO3 and I2O5 produces Ba[(UO2)2(IO3)2O2](H2O)
in 60% yield only under relatively mild hydrothermal conditions,
reaction 2. UO2(IO3)2(H2O) is also isolated from this reaction.
Ba[(UO2)2(IO3)2O2](H2O) can also be prepared in quartz tubes
with a reaction temperature of 180 °C. In the syntheses of both
K2[(UO2)3(IO3)4O2] and Ba[(UO2)2(IO3)2O2](H2O) small amounts
of I2 are produced, most likely through thermal disproportion-
ation of IO3-. Washing the product mixtures with methanol
rapidly removes I2 so that it does not interfere in subsequent
characterization. In the absence of water, solid-state reactions
involving UO3 and I2O5 produce ill-defined product mixtures
in the form of powders with the majority of the I2O5 having
undergone thermal decomposition or disproportionation.
formula
formula mass (amu) 1619.89
space group
a (Å)
b (Å)
K2[(UO2)3(IO3)4O2] Ba[(UO2)2(IO3)2O2](H2O)
1077.22
P21/c (No. 14)
8.062(4)
6.940(3)
21.67(1)
90
P1h (No. 2)
7.0372(5)
7.7727(5)
8.9851(6)
93.386(1)
105.668(1)
91.339(1)
471.98(6)
1
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
98.05(1)
90
1200.2(9)
4
T (°C)
λ (Å)
-100
25
0.710 73
5.950
353.70
0.0591
0.710 73
5.699
F
calcd (g cm-3
)
µ(Mo KR) (cm-1
)
327.49
0.0303
R(F) for
Fo2 > 2σ(Fo )a
2
Rw(Fo )b
0.0751
0.1118
BaCl2 + 2UO3 + I2O5 + 2H2O 180 °C, 3 days8
2
H2O
2
2
a R(F) ) ∑||Fo| - |Fc||/∑|Fo|. b Rw(Fo ) ) [∑[w(Fo - Fc2)2]/
Ba[(UO2)2(IO3)2O2](H2O) + 2HCl (2)
4
∑wFo ]1/2
.
The choice of utilizing chloride salts as sources of alkali and
alkaline-earth metal cations is based on our observations that,
in hydrothermal reactions of U(VI), chloride is easily exchanged
with harder ligands such as oxo donors and fluoride. Further-
more, the chloride anions can serve as mineralizing agents aiding
both in the dissolution of UO3 and in the formation of single
crystals. EDX measurements establish that chloride is not
incorporated into these compounds.
narrow-frame integration algorithm. A semiempirical absorption cor-
rection was applied based on the intensities of symmetry-related
reflections measured at different angular settings using SADABS.45 The
program suite SHELXTL (v 5.1) was used for space group determi-
nation (XPREP), structure solution (XS), and refinement (XL).46
Uranium, barium, and iodine atoms dominate the X-ray scattering in
Ba[(UO2)2(IO3)2O2](H2O), making the determination of hydrogen
positions for the water molecule unreliable, and they were not included
in the structural model. The final refinement included displacement
parameters for all atoms and a secondary extinction parameter. Some
crystallographic details are listed in Table 1 for K2[(UO2)3(IO3)4O2]
and Ba[(UO2)2(IO3)2O2](H2O); additional details can be found in the
Supporting Information.
Thermal Behavior. The thermal behavior of uranium-bearing
minerals and compounds has been reviewed extensively by
Cˇ ejka.47 Differential scanning calorimetry (DSC) and thermo-
gravimetric analysis (TGA) measurements have been reported
for UO2(IO3)2 and UO2(IO3)2(H2O) by our group.32 DSC
measurements indicate that K2[(UO2)3(IO3)4O2] undergoes a
small endothermic transformation at 491 °C and decomposes
at 579 °C. Decomposition probably occurs through thermal
disproportionation of iodate as we have noted similar thermal
disproportionation temperatures for UO2(IO3)2 and UO2(IO3)2-
(H2O).32 Unfortunately, we have been unable to isolate new
products from hydrothermal reactions of KCl with UO3 and I2O5
above 500 °C. Ba[(UO2)2(IO3)2O2](H2O) shows lower thermal
stability with respect to K2[(UO2)3(IO3)4O2] not only through
the expected loss of water at low temperatures (210 °C) but
also through a series of low-temperature, small endotherms at
304, 358, 420, and 491 °C prior to a large endotherm at 553 °C
and thermal disproportionation at 575 °C. While the details of
the thermal transformations of these compounds prior to
decomposition are not understood, these substantial differences
probably account for the variations in the preparation temper-
atures of these compounds and indicate the importance of
exploring hydrothermal syntheses in multiple temperature
regimes. Thermograms for K2[(UO2)3(IO3)4O2] and Ba[(UO2)2-
(IO3)2O2](H2O) are given in the Supporting Information.
Structures. K2[(UO2)3(IO3)4O2]. The structure of K2[(UO2)3-
Differential Scanning Calorimetry Measurements. Thermal data
for K2[(UO2)3(IO3)4O2] and Ba[(UO2)2(IO3)2O2](H2O) were collected
using a TA Instruments model 2920 differential scanning calorimeter
(DSC). Samples (20 mg) were encapsulated in aluminum pans and
heated at 10 °C/min from 25 to 600 °C under a nitrogen atmosphere.
Results and Discussion
Syntheses. The reaction of UO3 with I2O5 in the presence of
KCl in aqueous media at 425 °C for 72 h results in the formation
of K2[(UO2)3(IO3)4O2] in 73% yield, reaction 1. While K2-
[(UO2)3(IO3)4O2] can be prepared under both mild and super-
critical conditions, the yield increases from <5% to 73% as
the temperature is raised from 180 to 425 °C. Furthermore, the
crystal size improves from microcrystals of poor quality to high-
quality single crystals as long as 3 mm. In addition, we have
explored varying starting material ratios and inevitably found
that K2[(UO2)3(IO3)4O2] is the only potassium-containing
product formed. Reactions with decreased potassium content
also produce UO2(IO3)2 and UO2(IO3)2(H2O).32 At low tem-
peratures, UO2(IO3)2(H2O) becomes the dominant product of
these reactions.
(IO3)4O2] consists of infinite one-dimensional 1 [(UO2)3-
2KCl + 3UO3 + 2I2O5 + H2O 425 °C, 3 days8
∞
(IO3)4O2]2- chains running down the a-axis. These chains are
constructed from dimers of edge-sharing UO7 pentagonal
bipyramids that are linked together with distorted UO6 octahedra.
This creates a zigzagging uranium oxide chain as depicted in
Figure 1a. A polyhedral representation of this structure is shown
in Figure 1b. This is the first observation of this type of
H2O
K2(UO2)3(IO3)4O2 + 2HCl (1)
(45) SADABS. Program for absorption correction using SMART CCD based
on the method of Blessing: Blessing, R. H. Acta Crystallogr. 1995,
A51, 33-38.
(46) Sheldrick, G. M. SHELXTL PC, Version 5.0, An Integrated System
for SolVing, Refining, and Displaying Crystal Structures from Dif-
fraction Data; Siemens Analytical X-Ray Instruments, Inc.: Madison,
WI, 1994.
(47) Cˇ ejka, J. In Uranium: Mineralogy, Geochemistry and the EnViron-
ment; Burns, P. C., Finch, R., Eds.; Mineralogical Society of
America: Washington, DC, 1999; Chapter 1.

3962 Inorganic Chemistry, Vol. 40, No. 16, 2001
Bean et al.
Table 2. Selected Bond Distances (Å) for K2[(UO2)3(IO3)4O2]
U(1)-O(1)
2.409(3)
I(2)-O(4)
I(2)-O(5)
I(2)-O(6)
I(1)‚‚‚O(4)
K(1)-O(3)
K(1)-O(4)
K(1)-O(5)
K(1)-O(7)
K(1)-O(8)
K(1)-O(8)′
K(1)-O(9)
K(1)-O(10)
1.808(3)
1.791(3)
1.818(3)
2.397(3)
2.694(4)
2.957(3)
2.701(3)
3.149(4)
2.789(4)
2.811(4)
2.783(4)
2.785(4)
U(1)-O(2)
2.359(3)
U(1)-O(6)
2.398(3)
U(1)-O(7)
2.251(3)
U(1)-O(7)′
2.262(3)
U(1)-O(8) (UdO)
U(1)-O(9) (UdO)
U(2)-O(1)
1.796(4)
1.796(4)
2.451(3) (×2)
2.217(3) (×2)
1.791(4) (×2)
1.856(3)
U(2)-O(7)
U(2)-O(10) (UdO)
I(1)-O(1)
I(1)-O(2)
I(1)-O(3)
1.805(3)
1.778(4)
Å for U-O bonds in the equatorial plane. U-O bond lengths
for the distorted UO6 octahedra vary from 1.791(4) Å for
UdO bonds of the uranyl unit to 2.217(3)-2.451(3) Å for U-O
bonds in the equatorial plane. The longest U-O bonds for both
types of polyhedra are created with the µ3-O atom of the
bridging iodate. Iodate ligands in UO2(IO3)2 also contain oxygen
atoms that adopt µ3 binding modes.32 However, the iodate
groups in UO2(IO3)2 chelate the uranium centers. As O(1) is
the µ3 atom, the IdO bond distance of 1.856(3) Å is lengthened
with respect to the other two IdO bond distances of 1.778(4)
and 1.805(3) Å. IdO bond lengths for the monodentate iodate
are 1.791(3), 1.808(3), and 1.818(3) Å and show far less vari-
ation than the bridging iodate group. In addition, there are close
I‚‚‚O contacts of 2.397(3) Å between the terminal oxygen atoms
from the monodentate iodate ligand and the neighboring iodine
atom from the iodate that bridges uranium centers. These types
of contacts are common in iodate structures and have also been
observed in both UO2(IO3)2 and UO2(IO3)2(H2O).32 An impor-
tant difference between K2(UO2)3(IO3)4O2 and the two afore-
mentioned compounds is that interchain and interlayer I‚‚‚O
interactions have been replaced by K-O(IO3-) ionic bonds.
K-O bond distances range from 2.694(4) to 3.149(4) Å.
Selected bond lengths are given in Table 2. Bond valence sum
calculations provide values of 6.140 and 5.841 for U(1) and
U(2), respectively.48,49 Parameters for seven- and six-coordinate
U(VI) from Burns et al. were used in this calculation.24
Ba[(UO2)2(IO3)2O2](H2O). The structure of Ba[(UO2)2-
(IO3)2O2](H2O) consists of edge-sharing distorted pentagonal
bipyramidal UO7 units that form one-dimensional uranium oxide
ribbons whose edges are terminated by iodate ligands as shown
in Figure 3a. These ribbons run down the b-axis and are
separated by the Ba2+ cations and water molecules. This com-
pound also displays a new structural motif for one-dimensional
hexavalent uranium compounds. As in K2[(UO2)3(IO3)4O2], there
are two crystallographically unique uranium centers. However,
in this structure both uranium atoms are seven-coordinate, highly
distorted pentagonal bipyramids. The pentagonal bipyramids
containing U(1) share two edges with neighboring polyhedra,
and these units line the edges of the ribbons, whereas the
pentagonal bipyramids containing U(2) are on the interior of
the ribbons and share four edges with neighboring polyhedra
as shown in Figure 3b. The iodate ligands adopt both mono-
dentate and bridging binding modes and terminate the edges of
the ribbon as found in K2[(UO2)3(IO3)4O2]. The Ba2+ cations
are contained within distorted tricapped trigonal prisms where
two rectangular faces and one triangular face are capped (Figure
4). Again, the longest distance is to the oxygen atom capping
the triangular face (3.145(11) Å).
1
∞
Figure 1. (a) Infinite one-dimensional [(UO2)3(IO3)4O2]2- chains
constructed from dimers of edge-sharing UO7 pentagonal bipyramids
that are linked together with distorted UO6 octahedra running down
the a-axis in K2[(UO2)3(IO3)4O2]. (b) A polyhedral representation of
the one-dimensional chains in K2[(UO2)3(IO3)4O2] with UO7 pentagonal
bipyramids shown in orange and UO6 octahedra in yellow.
Figure 2. Bicapped trigonal prismatic coordination geometry of the
K+ cations in K2[(UO2)3(IO3)4O2].
connectivity in one-dimensional uranyl-containing structures.25,26
The K+ cations separate the chains from one another and are
coordinated by terminal oxygen atoms from iodate ligands. The
iodate ligands adopt both bridging and monodentate binding
modes. The bridging iodates link the dimers of edge-sharing
UO7 pentagonal bipyramids and form two opposite corners of
the distorted UO6 octahedra. The monodentate iodates are solely
bound to the UO7 polyhedra. Both types of iodates terminate
the edges of these one-dimensional chains. The remaining oxide
ligands are µ3-O atoms and are corners mutually shared by the
dimers of edge-sharing UO7 pentagonal bipyramids and the UO6
octahedra. The K+ cations are coordinated by eight oxygen
atoms from the iodate ligands in a bicapped trigonal prismatic
geometry as illustrated in Figure 2. The capping of the prism
occurs on one rectangular face and on one of the ends of the
trigonal prism. However, the latter capping atom lies 3.149(4)
Å away from the potassium center, which is approximately 0.2
Å farther than the other oxygen atoms.
The UdO bond lengths of 1.793(11) and 1.821(9) Å for U(1)
and 1.823(10) and 1.835(10) Å for U(2) are within expected
The U-O bond lengths for the UO7 units range from 1.796-
(4) Å for UdO bonds of the uranyl unit to 2.251(3)-2.409(3)
(48) Brown, I. D.; Altermatt, D. Acta Crystallogr. 1985, B41, 244.
(49) Brese, N. E.; O’Keeffe, M. Acta Crystallogr. 1991, B47, 192.

Novel One-Dimensional Uranyl Iodates
Inorganic Chemistry, Vol. 40, No. 16, 2001 3963
Table 3. Selected Bond Distances (Å) for Ba[(UO2)2(IO3)2O2](H2O)
U(1)-O(1)
2.461(9)
2.533(9)
2.309(10)
1.821(9)
1.793(11)
2.225(9)
2.153(9)
2.663(9)
2.389(10)
2.278(9)
1.823(10)
1.835(10)
2.252(8)
2.263(9)
1.808(9)
I(1)-O(2)
1.775(9)
U(1)-O(4)
I(1)-O(3)
1.798(10)
1.810(10)
1.797(10)
1.768(10)
2.794(10)
2.739(10)
2.735(10)
2.807(9)
2.848(10)
3.146(10)
2.776(11)
2.851(10)
2.66(2)
U(1)-O(5)
I(2)-O(4)
U(1)-O(7) (UdO)
U(1)-O(8) (UdO)
U(1)-O(9)
I(2)-O(5)
I(2)-O(6)
Ba(1)-O(2)
Ba(1)-O(3)
Ba(1)-O(6)
Ba(1)-O(7)
Ba(1)-O(8)
Ba(1)-O(9)
Ba(1)-O(10)
Ba(1)-O(11)
Ba(1)-O(13)
U(1)-O(12)
U(2)-O(4)
U(2)-O(9)
U(2)-O(9)′
U(2)-O(10)
U(2)-O(11)
U(2)-O(12)
U(2)-O(12)′
I(1)-O(1)
Conclusions
While the topologies of the one-dimensional uranium oxide
chains and ribbons in K2[(UO2)3(IO3)4O2] and Ba[(UO2)2-
(IO3)2O2](H2O) have not been previously observed in other
compounds, both of these structural motifs can be excised from
three-dimensional and two-dimensional layered uranium oxide
compounds.24-26 For instance, the structure of K2(UO2)3O3
contain chains of edge-sharing UO7 pentagonal bipyramids that
are linked together by UO6 octahedra.50 The one-dimensional
zigzagging chains in K2[(UO2)3(IO3)4O2] can be derived from
this structure. Likewise, protasite, Ba[(UO2)3O3(OH)2](H2O)3,
contains sheets built from edge-sharing UO7 pentagonal bipyra-
mids that can be cleaved to yield the ribbons found in
Ba[(UO2)2(IO3)2O2](H2O).51 In fact, the mineral billietite, Ba-
[(UO2)3O2(OH)3]2(H2O)4, possesses both of these sheet topolo-
gies.51
1
∞
Figure 3. (a) One-dimensional [(UO2)2(IO3)2O2]2- ribbons in the
structure of Ba[(UO2)2(IO3)2O2](H2O) consisting of edge-sharing
distorted pentagonal bipyramidal UO7 units. (b) A polyhedral repre-
sentation of the one-dimensional ribbons built from edge-sharing
distorted pentagonal bipyramids in Ba[(UO2)2(IO3)2O2](H2O).
The trigonal pyramidal, iodate anions in K2[(UO2)3(IO3)4O2]
and Ba[(UO2)2(IO3)2O2](H2O) play a unique role in this
structural chemistry in that they appear to terminate the edges
of the one-dimensional architectures. By comparison, when
tetrahedral anions such as sulfate are employed, these units stitch
one-dimensional chains and ribbons together to produce two-
dimensional sheets as found in zippeite, K[(UO2)2(SO4)(OH)3]-
(H2O), which contains ribbons of UO7 pentagonal bipyramids
linked to one another on both edges by sulfate to produce two-
dimensional sheets.52 Iodate and other trigonal pyramidal anions
such as selenite and tellurite can, however, bridge between
uranyl units to yield layers as found in UO2(IO3)2(H2O),32 Ba-
[(UO2)3(SeO3)2O2](H2O)3,53 and UO2(TeO3).54 It will be of
interest to discover whether iodate continues to terminate chain
edges in other alkali metal, alkaline-earth metal, and transition
metal uranyl iodates, which is a topic of current efforts in our
group.
Figure 4. Ba2+ cations in Ba[(UO2)2(IO3)2O2](H2O) in a distorted
tricapped trigonal prismatic environment where two rectangular faces
and one triangular face are capped.
ranges for uranyl moieties. The equatorial bond lengths, which
range from 2.153(8) to 2.533(9) Å for U(1) and 2.252(8) to
2.663(9) Å for U(2), show substantial variation in bond distances
beyond what is normally observed. The basis for this distortion
is again found with the µ3-O atom from an iodate ligand with
the longest U-O bonds of 2.533(9) and 2.663(9) Å being with
this oxygen atom. The shortened U-O bonds are found with
the µ3-oxo ligand within the ribbons. The IdO bond distances
range from 1.768(10) to 1.810(10) Å. As in K2[(UO2)3(IO3)4O2],
there are I‚‚‚O contacts; however, these are approximately 0.3
Å longer than those in K2[(UO2)3(IO3)4O2] with a distance of
2.667(9) Å between the terminal oxygen atoms from the
monodentate iodate ligand and the neighboring iodine atom from
the iodate that bridges uranium centers. Ba-O contacts occur
from 2.66(2) to 3.145(11) Å, with the shortest bond occurring
with the bound water molecule. Bond valence sum calculations
provide values of 6.127 and 5.813 for U(1) and U(2), respec-
tively.48,49 Parameters for seven-coordinate U(VI) from Burns
et al. were used in this calculation.24 Selected bond lengths are
given in Table 3 for Ba[(UO2)2(IO3)2O2](H2O).
Acknowledgment. This work was supported by NASA
(Alabama Space Grant Consortium) and Auburn University.
Supporting Information Available: DSC thermograms and Figures
S1 and S2 depicting portions of the K2[(UO2)3(IO3)4O2] and Ba[(UO2)2-
(IO3)2O2](H2O) structures. X-ray crystallographic files for K2[(UO2)3-
(IO3)4O2] and Ba[(UO2)2(IO3)2O2](H2O) in CIF format. This material
is available free of charge via the Internet at http://pubs.acs.org.
IC010342L
(50) Saine, M. C. J. Less-Common Met. 1989, 154, 361.
(51) Pagoaga, M. K.; Appleman, D. E.; Stewart, J. M. Am. Mineral. 1987,
72, 1230.
(52) Vochten, R.; Van Haverbeke, L.; Van Springel, K.; Blaton, N.; Peeters,
O. M. Can. Mineral. 1995, 33, 1091.
(53) Cooper, M. A.; Hawthorne, F. C. Can. Mineral. 1995, 33, 1103.
(54) Meunier, G.; Galy, J. Acta Crystallogr. 1973, B29, 1251.