Polynuclear cation-cation complexes of pentavalent uranyl: Relating stability and magnetic properties to structure

Article abstract of DOI:10.1021/ja804766r

Reaction of {[UO₂Py₅][μ-KI₂Py ₂]}_n (1) with 2 equiv of potassium dibenzoylmethanate (Kdbm) in pyridine or acetonitrile affords, respectively, the corresponding tetranuclear complexes of pentavalent uranyl {[UO₂(dbm) ₂]₂[μ-K(Py)₂]₂[μ₈- K(Py)]}₂I₂·Py₂ (2) (in 70% yield) and {[UO₂(dbm)₂]₂[μ-K(MeCN)₂] [μ₈-K]}₂ (3) (in 40% yield) in which four UO ₂⁺ are mutually coordinated (T-shaped "cation- cation" interaction). The X-ray structures of 2 and 3 show also the presence of, respectively, six and four potassium cations involved in UO ₂⁺...K⁺ interactions. Reaction of 2 with an excess of 18-crown-6 (18C6) affords the dimeric complex [UO ₂(dbm)₂K(18C6)]₂ (4) presenting a diamond-shaped interaction between two UO₂⁺ groups, in 45% yield. ¹H and PFGSTE diffusion NMR spectroscopy of 2 and 3 in pyridine show unambiguously the presence of UO₂⁺... UO₂⁺ and UO₂⁺...K⁺ interactions (tetrametallic species) in solution, which leads to a rapid (7 days) disproportionation of pentavalent uranyl to afford [U(dbm)₄] and [UO₂(dbm)₂] species. The UO₂ ⁺...K⁺ interaction plays an important synergistic role in the stabilization of the UO₂⁺...UO ₂⁺ interactions. Accordingly, the lower affinity of (K(18C6))⁺ for the uranyl(V) oxygen in complex 4 results in a lower number of coordinated K⁺ and therefore in a weakened UO ₂⁺...UO₂⁺ interaction. The UO₂⁺...UO₂⁺ interactions is completely disrupted in dmso or in the presence of Kdbm, preventing disproportionation of pentavalent uranyl. Solid-state variable-temperature magnetic susceptibility studies showed the unambiguous presence of antiferromagnetic coupling between the two oxo-bridged uranium centers of complex 4, with the appearance of a maximum in χ versus T at ～5 K. The different behavior of the tetrameric complex 3, which probably involves a magnetic coupling occurring at lower temperature, can be ascribed to the different geometric arrangement of the interacting uranyl(V) groups.

Full text of DOI:10.1021/ja804766r

Published on Web 11/17/2008
Polynuclear Cation-Cation Complexes of Pentavalent Uranyl:
Relating Stability and Magnetic Properties to Structure
Gre´gory Nocton, Pawel Horeglad, Jacques Pe´caut, and Marinella Mazzanti*
Laboratoire de Reconnaissance Ionique et Chimie de Coordination, SerVice de Chimie
Inorganique et Biologique (UMR-E 3 CEA-UJF), INAC, CEA-Grenoble,
38054 Grenoble Cedex 09, France
Received June 22, 2008; E-mail: marinella.mazzanti@cea.fr
Abstract: Reaction of {[UO2Py5][µ-KI2Py2]}n (1) with 2 equiv of potassium dibenzoylmethanate (Kdbm) in
pyridine or acetonitrile affords, respectively, the corresponding tetranuclear complexes of pentavalent uranyl
{[UO2(dbm)2]2[µ-K(Py)2]2[µ8-K(Py)]}2I2 ·Py2 (2) (in 70% yield) and {[UO2(dbm)2]2[µ-K(MeCN)2][µ8-K]}2 (3) (in
40% yield) in which four UO2+ are mutually coordinated (T-shaped “cation-cation” interaction). The X-ray
structures of 2 and 3 show also the presence of, respectively, six and four potassium cations involved in
UO2+ · · ·K+ interactions. Reaction of 2 with an excess of 18-crown-6 (18C6) affords the dimeric complex
[UO2(dbm)2K(18C6)]2 (4) presenting a diamond-shaped interaction between two UO2+ groups, in 45% yield.
1H and PFGSTE diffusion NMR spectroscopy of 2 and 3 in pyridine show unambiguously the presence of
UO2+ · · ·UO2+ and UO2+ · · ·K+ interactions (tetrametallic species) in solution, which leads to a rapid (7 days)
disproportionation of pentavalent uranyl to afford [U(dbm)4] and [UO2(dbm)2] species. The UO2+ · · ·K+
+
interaction plays an important synergistic role in the stabilization of the UO2+ · · ·UO2 interactions.
Accordingly, the lower affinity of (K(18C6))+ for the uranyl(V) oxygen in complex 4 results in a lower number
of coordinated K+ and therefore in a weakened UO2+ · · ·UO2+ interaction. The UO2+ · · ·UO2+ interactions is
completely disrupted in dmso or in the presence of Kdbm, preventing disproportionation of pentavalent
uranyl. Solid-state variable-temperature magnetic susceptibility studies showed the unambiguous presence
of antiferromagnetic coupling between the two oxo-bridged uranium centers of complex 4, with the
appearance of a maximum in ꢀ versus T at ∼5 K. The different behavior of the tetrameric complex 3,
which probably involves a magnetic coupling occurring at lower temperature, can be ascribed to the different
geometric arrangement of the interacting uranyl(V) groups.
Introduction
interactions between neptunium ions, giving rise to ferromag-
netic (or more rarely to antiferromagnetic) ordering through a
The chemistry of actinyl cations (AnO2+/2+) plays a crucial
role both in nuclear technology and in the environmental
mobility of actinides.1,2 In particular, the mutual coordination
of one actinyl oxo atom as an equatorial ligand to the actinide
center of an adjacent group, also known as a “cation-cation”
interaction3 (CCI), plays a key role in the solid-state and solution
chemistry of these species. While the oxo groups of the
uranyl(VI) cation (UO22+) are rarely involved in the coordination
of other cations,4-6 CCIs are frequently found in the solid-state
structures of (NpO2+),7 probably as a result of the higher
negative charge of the oxo groups in the pentavalent neptunyl.
CCIs play a critical role in the enhancement of magnetic
superexchange pathway.8-10 CCIs have also been reported to
play an important role in the rapid aqueous disproportionation
of pentavalent uranyl UO2+ to U(IV) and uranyl(VI) species.11,12
Recent theoretical studies13 suggest that the disproportionation
proceeds via the formation of dimeric CCI complexes, but
unambiguous experimental evidence for the presence of such
species in solution and of their implication in the dispropor-
tionation is lacking.14
Except at low pH (2-2.5)2 and in concentrated carbonate
media, UO2 is highly unstable in water.15-17 However,
+
pentavalent uranyl is an important intermediate in biomediated
(1) May, I.; Copping, R.; Cornet, S. M.; Talbot-Eeckelears, C. E.; Gaunt,
A. J.; John, G. H.; Redmond, M. R.; Sharrad, C. A.; Sutton, A. D.;
Collison, D.; Fox, O. D.; Jones, C. J.; Sarsfield, M. J.; Taylor, R. J. J.
Alloys Compd. 2007, 444, 383–386.
(8) Forbes, T. Z.; Burns, P. C.; Soderholm, L.; Skanthakumar, S. Chem.
Mater. 2006, 18, 1643–1649.
(9) Almond, P. M.; Skanthakumar, S.; Soderholm, L.; Burns, P. C. Chem.
Mater. 2007, 19, 280–285.
(2) Edelstein, N. M.; Lander, G. H. The Chemistry of the Actinide and
Transactinide Elements; Springer: Dordrecht, 2006.
(3) Sullivan, J. C.; Zielen, A. J.; Hindman, J. C. J. Am. Chem. Soc. 1961,
83, 3373–3378.
(10) Forbes, T. Z.; Burns, P. C.; Skanthakumar, S.; Soderholm, L. J. Am.
Chem. Soc. 2007, 129, 2760–2761.
(11) Ekstrom, A. Inorg. Chem. 1974, 13, 2237–2241.
(12) Newton, T. W.; Baker, F. B. Inorg. Chem. 1965, 4, 1166–1170.
(13) Steele, H.; Taylor, R. J. Inorg. Chem. 2007, 46, 6311–6318.
(14) Szabo, Z.; Toraishi, T.; Vallet, V.; Grenthe, I. Coord. Chem. ReV.
2006, 250, 784–815.
(4) Kubatko, K.-A.; Burns, P. C. Inorg. Chem. 2006, 45, 10277–10281.
(5) Sarsfield, M. J.; Helliwell, M. J. Am. Chem. Soc. 2004, 126, 1036–
1037.
(6) Sullens, T. A.; Jensen, R. A.; Shvareva, T. Y.; Albrecht-Schmitt, T. E.
J. Am. Chem. Soc. 2004, 126, 2676–2677.
(15) Ikeda, A.; Hennig, C.; Tsushima, S.; Takao, K.; Ikeda, Y.; Scheinost,
A. C.; Bernhard, G. Inorg. Chem. 2007, 46, 4212–4219.
(7) Krot, N. N.; Grigoriev, M. S. Russ. Chem. ReV. 2004, 73, 89–100.
9
10.1021/ja804766r CCC: $40.75
2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 16633–16645 16633

A R T I C L E S
Nocton et al.
reduction of stable hexavalent uranyl ions,18,19 making its
chemistry highly relevant for the understanding of uranium
mobility in the environment and for the development of
the development of actinide-based functional materials.2,31,32
However, among the few reported polymetallic uranium
complexes33,34 for which the magnetic properties have been
investigated, in only one case have unambiguous f1-f1 magnetic
coupling between uranium centers been observed.35 Notably in
the dimeric U(V) imido complex [(MeC5H4)3U2][µ-1,4-N2C6H4]
antiferromagnetic coupling between the U(V) centers is mediated
by the imido linkage.
+
remediation strategies. Moreover, stable UO2 complexes are
ideal candidates for the development of photocatalysts and for
active materials for efficient electric power storage due to the
reversibility of the electron-transfer reaction UO22+ · · ·UO2+ and
to the possibility of photochemically generating highly reducing
UO2 intermediates.20,21 While spectroscopic studies of elec-
+
We recently showed that the reaction of 1 with potassium
dibenzoylmethanate (Kdbm) in pyridine allows isolation of
+
trochemically prepared UO2 compounds suggested higher
+
stability of UO2+ in organic anhydrous media,22,23 only recently
have a few UO2+ complexes been isolated and crystallographi-
cally characterized.24-27 Among these, the stable pyridine
solvate of the pentavalent uranyl iodide {[UO2Py5][µ-KI2Py2]}n,
1,28 (I) is an excellent precursor for the study of the coordination
chemistry of pentavalent uranyl.29,30 Some of the isolated
complexes show CCIs with alkali or transition metal cations,
but no UO2+ · · ·UO2+ interactions were demonstrated so far for
any of these compounds in the solid state or in solution.
crystals of the first example of a UO2 complex having a
T-shaped UO2+ · · ·UO2 interaction (IIa).29 Here we describe
+
the synthetic procedure allowing the isolation in a pure form
of two new cation-cation complexes of dibenzoylmethanate
(dbm-) with different nuclearity presenting T-shaped (IIa) or
diamond-shaped (IIb) (dinuclear) CCIs together with their
structural characterization and stability studies under different
conditions. Magnetic susceptibility measurements allow us to
demonstrate the first unambiguous example of magnetic cou-
pling between uranium centers mediated by an oxo group in
molecules.
Results and Discussion
Synthesis, Crystal, and Molecular Structure. The polymeric
pentavalent uranyl iodide {[UO2Py5][µ-KI2Py2]}n (1) was
prepared in 75% yield via the oxidation of U(III) with a mixture
of water and PyNO followed by addition of potassium iodide.28
Compound 1 reacts rapidly with O2 or pyridine-N-oxide in
pyridine to form the U(VI) adduct [UO2I2(Py)3], whereas the
reaction with water proceeds more slowly (is completed after 1
week) and leads to unidentified products. Complex 1 is soluble
in dmso and acetonitrile where it rapidly decomposes but is
stable enough for coordination chemistry studies. A higher
stability associated with a lower solubility is observed in
pyridine and thf. Accordingly compound 1 is an excellent
starting material for the development of the coordination
chemistry of pentavalent uranyl in different solvents.30
Beside the crucial information that the structural and reactivity
studies of cation-cation complexes can provide in the under-
standing of the disproportionation mechanism and the develop-
ment of new synthetic strategies for the preparation of solution
+
stable UO2 complexes, CCIs also provide a route to the
assembly of multimetallic uranium complexes. The design of
multimetallic molecular actinide complexes with magnetic
exchange interactions between actinide centers is of high current
interest as models for the understanding of the electronic
structure of actinide materials and because they could lead to
The reaction of 1 with 2 equiv of Kdbm salt (dbm-
)
dibenzoylmethanate) in pyridine or in acetonitrile leads to the
respective solvate bis-ligand complexes in 75% (pyridine) and
40% (acetonitrile) yield (Scheme 1). The crystal structure of
the pyridine solvate {[UO2(dbm)2]2[µ-K(Py)2]2[µ8-K(Py)]}2I2 ·
Py2 (2) has been reported in a preliminary communication.29
Crystalsoftheacetonitrilesolvate{[UO2(dbm)2]2[µ-K(MeCN)2][µ8-
K]}2 (3) were obtained by letting stand a concentrated (24 mM)
solution of the complex 3 in acetonitrile at room temperature.
(16) Docrat, T. I.; Mosselmans, J. F. W.; Charnock, J. M.; Whiteley, M. W.;
Collison, D.; Livens, F. R.; Jones, C.; Edmiston, M. J. Inorg. Chem.
1999, 38, 1879–1882.
(17) Clark, D. L.; Hobart, D. E.; Neu, M. P. Chem. ReV. 1995, 95, 25–48.
(18) Ilton, E. S.; Haiduc, A.; Cahill, C. L.; Felmy, A. R. Inorg. Chem.
2005, 44, 2986–2988.
+
The tetrameric UO2 complexes of dbm- can be isolated
(19) Renshaw, J. C.; Butchins, L. J. C.; Livens, F. R.; May, I.; Charnock,
J. M.; Lloyd, J. R. EnViron. Sci. Technol. 2005, 39, 5657–5660.
(20) Shirasaki, K.; Yamamura, T.; Shiokawa, Y. J. Alloys Compd. 2006,
408, 1296–1301.
analytically pure from pyridine by diffusion of diisopropylether
(over 1 h), while longer crystallization times in pyridine can
result in contamination by disproportionation products. Complex
2 slowly disproportionates in pyridine solution to form the
[UIV(dbm)4] and [UVIO2(dbm)2] complexes. However, the time
scale of the disproportionation (hours) allows the isolation and
characterization of 2 by NMR. The disproportionation occurs
(21) Bakac, A.; Espenson, J. H. Inorg. Chem. 1995, 34, 1730–1735.
(22) Mizuoka, K.; Tsushima, S.; Hasegawa, M.; Hoshi, T.; Ikeda, Y. Inorg.
Chem. 2005, 44, 6211–6218.
(23) Kim, S. Y.; Asakura, T.; Morita, Y.; Uchiyama, G.; Ikeda, Y.
Radiochim. Acta 2005, 93, 75–81.
(24) Berthet, J.-C.; Nierlich, M.; Ephritikhine, M. Angew. Chem., Int. Ed.
2003, 42, 1952–1954.
(25) Berthet, J. C.; Siffredi, G.; Thuery, P.; Ephritikhine, M. Chem.
Commun. 2006, 3184–3186.
(31) Schelter, E. J.; Wu, R. L.; Scott, B. L.; Thompson, J. D.; Morris, D. E.;
Kiplinger, J. L. Angew. Chem., Int. Ed. 2008, 47, 2993–2996.
(32) Curro, N. J.; Caldwell, T.; Bauer, E. D.; Morales, L. A.; Graf, M. J.;
Bang, Y.; Balatsky, A. V.; Thompson, J. D.; Sarrao, J. L. Nature 2005,
434, 622–625.
(26) Hayton, T. W.; Wu, G. J. Am. Chem. Soc. 2008, 130, 2005–2014.
(27) Arnold, P. L.; Patel, D.; Wilson, C.; Love, J. B. Nature 2008, 451,
315–U313.
(28) Natrajan, L.; Burdet, F.; Pecaut, J.; Mazzanti, M. J. Am. Chem. Soc.
2006, 128, 7152–7153.
(33) Ephritikhine, M. Dalton Trans. 2006, 2501–2516.
(34) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Science 2005, 309, 1835–
1838.
(29) Burdet, F.; Pecaut, J.; Mazzanti, M. J. Am. Chem. Soc. 2006, 128,
16512–16513.
(30) Horeglad, P.; Nocton, G.; Pe´caut, J.; Mazzanti, M. Manuscript in
preparation.
(35) Rosen, R. K.; Andersen, R. A.; Edelstein, N. M. J. Am. Chem. Soc.
1990, 112, 4588–4590.
9
16634 J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008

Polynuclear Cation-Cation Complexes of Uranyl(V)
A R T I C L E S
Scheme 1
Table 1. Selected Structural Parameters for 2 and 3 (#1 ) -x +
1, -y + 2, -z + 1 for 2 and #1 ) -x + 3/2, -y + 3/2, -z for 3;
Distances in Å, Angles in deg)
structural parameters
2
3
U(1)-O(14)
1.830(10)
1.924(10)
2.346(8)
2.45(1)
1.934(8)
1.812(9)
2.37(1)
2.43(2)
2.58(1)
3.613(1)
2.9(3)
2.86(0)
2.82(6)
2.905(15)
3.32(9)
2.68(1)
3.10(1)
2.8(1)
2.88(2)
4.278(6)
179.0(4)
90.7(4)
88.4(3)
178.9(4)
92.1(4)
88.5(3)
173.8(5)
1.808(10)
1.916(10)
2.400(9)
2.45(2)
1.924(9)
1.830(10)
2.41(1)
2.44(4)
2.59(1)
3.39(1)
2.9(2)
U(1)-O(11)
U(1)-O(13)#1
mean U(1)-Odbm
U(2)-O(13)
U(2)-O(12)
U(2)-O(11)
mean U(2)-Odbm
K(1)-O(12)
K(1)-O(11)
mean K(1)-Odbm
K(1)-NPy orACN
Mean K(2)-Odbm
K(2)-Npy
mean K(2)-Oyl
K(3)-O(14)#1
K(3)-O(13)
mean K(3)-Odbm
K(3)-Npy
mean U-U
O(14)-U(1)-O(11)
O(14)-U(1)-O(13)#1
O(11)-U(1)-O(13)#1
O(12)-U(2)-O(13)
O(12)-U(2)-O(11)
O(13)-U(2)-O(11)
U(1)-O(13)-U(2#1)
2.82(2)
2.78(3)
3.04(10)
4.318(6)
179.3(5)
95.8(4)
84.8(4)
177.2(5)
92.0(4)
85.4(4)
171.6(5)
faster in acetonitrile, but the lower solubility of 3 with respect
to the U(IV) and UVIO22+ species allows the isolation of 3 in a
pure form in relatively high yield.
ORTEP views of the cation {[UO2(dbm)2]2[µ-K(Py)2]2[µ8-
K(Py)}2+ in 229 and of 3 are presented in Figures 1-3 together
with schematic representations of the structure, and selected
bond distances and angles are given in Table 1. Structures of
both 2 and 3 consist of centrosymmetric tetramers where two
inequivalent uranyl(V) groups are coordinated to each other in
a monodentate fashion to form a square plane. In both structures
each UO2+ coordinates two adjacent uranyl(V) groups resulting
(2.58(1)-2.68(1) Å) bond distances are found for potassium
ions binding uranyl(V) oxygens not involved in CCI as
compared to those found for potassium ions binding uranyl(V)
oxygens involved in a CCI (3.10(1)-3.39(1) Å). In both
+
in a T-shaped CCI (two UO2 groups arranged perpendicular
to each other).
+
complexes the identity of the UO2 groups is not lost in the
In 2 and 3 each UO2+ is also coordinated by the four oxygens
of two bidentate dbm- ligands resulting in the presence of four
seven-coordinate uranium ions with a pentagonal bipyramidal
geometry with uranyl(V) oxygen atoms in the axial positions.
The main difference between the structures of 2 and 3 is the
number of potassium ions involved in an additional CCI with
formation of the cation-cation complex; however, the CCIs
result nevertheless in a significant lengthening of the involved
UdO bonds (1.916(10) -1.924(9) Å), while the value of the
U-Oyl distances (1.808(10) and 1.830(10) Å) for the oxo groups
not involved in UO2+ · · ·UO2+ interactions are similar to those
found in complex 1 (1.836(2) and 1.834(2) Å)28 and in the two
other reported mononuclear pentavalent uranyl complexes
(ranging from 1.810(4) to 1.828(4) Å).24,26 This desymmetri-
zation, which results in a mean difference between the two UdO
bonds of 0.11 Å, is the consequence of the strong electrostatic
+
the UO2 oxygens. In 2 eight uranyl(V) oxygens interact with
six potassium (three crystallographically inequivalent) ions
arranged in an octahedral fashion around the plane formed by
the four uranyl(V) ions with four potassium ions forming a
square plane that includes the plane of the uranyl(V) ions and
two potassium ions located below and above this plane (at 2.60
Å). The potassium ions K(1) and K(3), in the plane, are
coordinated by two bridging dbm- oxygens, one uranyl(V)
oxygen, and two pyridine nitrogens. Another uranyl(V) oxygen
from an adjacent [UO2(dbm)2]- complex is located at a longer
distance (3.613 and 3.10(1) Å for K(1) and K(3), respectively).
The two potassium ions in apical positions are nine-coordinated
by four bridging dbm- oxygens connecting four different
uranyl(V) complexes, by four uranyl(V) oxygens, and by one
pyridine nitrogen. In 3, only four bridging potassium ions (two
crystallographically inequivalent) are involved in a CCI with
the uranyl(V) oxygens resulting in a neutral molecule, whereas
in the structure of complex 2 two iodide anions balance the
charge due to the additional potassium ions. In 3, only two
crystallographically equivalent potassium ions are found in the
plane formed by the four interacting uranyl(V) groups. The
coordination environment of the potassium ions is very similar
in the two structures, beside the absence of solvent coordinated
to the apical potassium ion in 3. In both complexes shorter K-O
+
interaction between the two UO2 cations. A similar desym-
metrization of the uranyl(V) fragment has also been recently
observed in dinuclear complexes presenting a UO2+ · · ·M2+ (M
) Fe, Zn) interaction.27
The difference in the number of potassium ions coordinated
+
to the UO2 oxygens in 2 and 3 leads only to some small but
significant modification of the metric parameters in the square
+
plane defined by the UO2+ · · ·UO2 interaction as highlighted
in Figure 4. The main difference in the structural parameters is
the longer distance between the uranyl(V) oxygens and the
+
uranium centers of the interacting UO2 groups in complex 3
(2.400(9) Å) with respect to complex 2 (2.346(8) Å), which
+
suggest the presence of a weaker UO2+ · · ·UO2 interaction in
3, underlying the important role that the presence of coordinating
cations can play in the stabilization of these interactions.
Moreover shorter average K(2)-Oyl (Oyl ) uranyl(V) oxygen)
distances are found in 3 (3.04(10) Å) with respect to 2 (3.32(9)
Å); this indicates that the removal of two potassium ions is
somewhat compensated for by a stronger K-O interaction of
the remaining potassium ions.
9
J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008 16635

A R T I C L E S
Nocton et al.
Figure 1. ORTEP views of the cation {[UO2(dbm)2]2[µ-K(Py)2]2[µ8-K(Py)]}2+ of (left) 2 and (right) 3. The structures are represented along the axis formed
by the two potassium ions that are in apical positions with respect to the plane of the uranium atoms (H, the carbons of the phenyl groups of dbm- ligands
and all coordinated solvents molecules were omitted for clarity, thermal ellipsoid are at 30% level, C atoms are represented in gray, O in red, K in purple
and U in green). (Symmetry transformations used to generate equivalent atoms A ) -x + 1, -y + 2, -z + 1 for 2 and A ) -x + 3/2, -y + 3/2, -z for
3.)
Figure 2. ORTEP view of 2 (left) and schematic representations of the structure (right). In the ORTEP view the structure is represented along the axis
formed by the two potassium ions that are in apical positions with respect to the plane of the uranium atoms (H were omitted for clarity, thermal ellipsoid
are at 30% level, C atoms are represented in gray, O in red, K in purple, N in blue and U in green).
Figure 3. ORTEP view of 3 (left) and schematic representation of the structure (right). In the ORTEP view the structure is represented along the axis
formed by the two potassium ions that are in apical positions with respect to the plane of the uranium atoms (H were omitted for clarity, thermal ellipsoid
are at 30% level, C atoms are represented in gray, O in red, K in purple, N in blue, and U in green).
+
The coordination of the potassium counterions to the UO2
oxygens in the tetrameric solid-state structures of 2 and 3 could
play an important electronic or/and structural role in promoting
the CCI in solution and therefore could influence the stability
toward disproportionation. In order to evaluate these effects we
have investigated the reaction of 2 with 18-crown-6 (18C6).
We have taken advantage of the affinity of (18C6) for potassium
to remove potassium from the structure and to reduce the acidity
of eventual remaining bound potassium cations. The addition
of 3 equiv of (18C6) to a 2:1 reaction mixture of Kdbm and 1
in pyridine resulted in a rapid color change from blue to green
and analytically pure [UO2(dbm)2K(18C6)]2, 4 was isolated in
45% yield in thf. Crystals of 4 suitable for X-ray diffraction
were grown from a pyridine/hexane solution. 4 was found to
crystallize as a centrosymmetric dimer in which two [UO2-
(dbm)2]- units are assembled through a diamond shaped CCIs
(U-O-U angle ) 101.7°) consisting of two mutually coordi-
nated monodentate UO2+ groups. To our knowledge, only one
example of this type of dimeric CCI has been previously
ascribed to a NpO2 complex.7 The complex formula and the
+
9
16636 J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008

Polynuclear Cation-Cation Complexes of Uranyl(V)
A R T I C L E S
Figure 5. ORTEP view of [UO2(dbm)2K(18C6)]2 (4) (thermal ellipsoid
are at 30% level, C are represented in gray, O in red, K in purple, and U
in green) Selected distances (Å) and angles (deg): U(1)-O(1U), 1.850(4);
U(1)-O(2U), 1.941(4); U(1)-(2U)A, 2.384(4); U(1)-O(22), 2.410(4);
U(1)-O(42), 2.422(4); U(1)-O(41), 2.435(4); U(1)-O(21), 2.442(4);
O(1U)-K(1), 2.760(5); O(41)-K(1), 3.125(5); mean K-O18-C-6, 2.93(5);
O(1U)-U(1)-O(2U), 175.80(18); O(1U)-U(1)-O(2U)A, 101.68(16);
U(1)-O(1)-K(1), 110.11(18). (Symmetry transformations used to generate
equivalent atoms A: -x, -y, -z).
positions, four oxygen atoms of two bidentate dbm- ligands,
+
and one bridging oxo group of the adjacent UO2 in the
equatorial plane. The dimeric structure results in a significant
distortion of the coordination geometry from the ideal pentago-
nal bipyramidal geometry with the bridging oxo ligands located
0.995 Å below the mean plane formed by the other equatorial
ligands. Similarly to what is observed in complexes 2 and 3,
the CCI in the case of 4 results in a desymmetrization of the
+
UO2 groups with a long (1.941(4) Å) and a shorter (1.850(4)
Å) U-O distance. The U-O bond for the uranyl(V) oxygen
coordinated to potassium (1.850(4) Å) is slightly longer with
respect to monomeric complexes reported in the literature (U-O
distances in the range ) 1.810(4)-1.828(4) Å)24,26 that do not
present binding cations, despite the weak coordinating ability
of [K(18C6)]+. The difference between the two UdO bonds is
smaller (0.09 Å) than one observed in the tetramers 2 and 3
(0.11 Å), suggesting a weaker UO2+ · · ·UO2+ interaction in the
dimeric structure. However the difference remains much larger
than the average difference observed in cation-cation complexes
of pentavalent neptunium (0.04 Å), suggesting that even in this
geometry a stronger interaction occurs in the uranium species.
Solution Structure. Although CCIs have been well character-
ized in the solid state for neptunyl(V) compounds, detailed
solution-phase structural data have proved elusive despite the
numerous studies that have used IR,37 UV-vis,38 Raman,37
EXAFS,38 or HEXS39 spectroscopies. In this work we used for
the first time NMR spectroscopy to unambiguously verify the
presence of CCIs in solution.
Figure 4. Details of the solid-state structures of 2, 3, and 4 showing the
+
interacting UO2+ · · ·UO2 with associated distances (thermal ellipsoid are
at 30% level, 20x atoms are represented in red and U atoms in green).
value of the UdO distances (1.941(4) and 1.850(4) Å) are in
agreement with the presence of two pentavalent uranyl com-
plexes. The structure of [UO2(dbm)2K(18C6)]2 is shown in
Figure 5.
+
Each UO2 is also involved in a CCI with a (18C6) bound
potassium ion through the uranyl(V) oxygen not involved in
The solution structure of the complexes 2, 3, and 4 in different
solvent conditions was studied by UV-vis, ESI-MS, and 1-D
the UO2+ · · ·UO2 interaction. The value of the UdO-K
+
distance of 2.760(5) Å falls in the range of UdO-K distances
(2.658(6)-3.230(6) Å) reported for complexes of hexavalent
uranyl forming a CCIs with an [K(18C6)]+36 and is significantly
longer than the shorter UdO-K distance (2.58 Å) found in
the tetramers 2 and 3. The latter suggests that the interaction of
the [K(18C6)]+ cation with the pentavalent uranyl oxygen is
significantly weaker than the one of K+ alone. The U atoms in
4 are seven-coordinate, by two trans oxo groups in axial
(36) Thuery, P.; Masci, B.; Takimoto, M.; Yamato, T. Inorg. Chem.
Commun. 2007, 10, 795–799.
(37) Guillaume, B.; Begun, G. M.; Hahn, R. L. Inorg. Chem. 1982, 21,
1159–1166.
(38) Stout, B. E.; Choppin, G. R.; Nectoux, F.; Pages, M. Radiochim. Acta
1993, 61, 65–67.
(39) Skanthakumar, S.; Antonio, M. R.; Soderholm, L. Inorg. Chem. 2008,
47, 4591–4595.
9
J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008 16637

A R T I C L E S
Nocton et al.
Table 2. Diffusion Coefficient Measured at 400 MHz and 298 K, Calculated Hydrodynamic Radius, and Solution Molecular Weight for
Complexes 2 (M ) 4303.62 g mol-1), 3 (M ) 3391.91 g mol-1), and 4 (M ) 2039.34 g mol-1
)
solvent
compound
D [m2 s-1
]
rsph [Å] Msol [g mol-1] found
expected formula
dmso η ) 1.987 mPa s (298 K)
UVIO2(dbm)2(dmso) (M ) 794.7 g mol-1
)
1.64(2) × 10-10
6.7
7.0
2
2
1.56(3) × 10-10
940 ( 75
[UO2(dbm)2(dmso)]K
pyridine η ) 0.879 mPa s (298 K)
2.21(2) × 10-10 11.3
4045 ( 250a {[UO2(dbm)2]4[µ-K6(Py)x]}2+
UIV(dbm)4 (M ) 1130 g mol-1
)
3.52(3) × 10-10
2.62(3) × 10-10
3.68(3) × 10-10
3.65(5) × 10-10
3.05(2) × 10-10
2.13(9) × 10-10
2.32(2) × 10-10
3.20(5) × 10-10
7.1
9.5
6.7
6.8
c
c
c
c
3
2740 ( 260
992 ( 80
1010 ( 110
[UO2(dbm)2]4[µ-K4(Py)x]
[UO2(dbm)2(Py)]K
[UO2(dbm)2K(18C6)]
b
4
pyridine/dmso 100:5
UIV(dbm)4 (M ) 1130 g mol-1
)
2 A
2 B
2 C
3317 ( 440
2567 ( 120
978 ( 90
{[UO2(dbm)2]4[µ-K6(Py)x]}2+
[UO2(dbm)2]4[µ-K4(Py)x]
[UO2(dbm)2(S)x]K
b
a Reference 29. The molecular weight of K(18C6) (M ) 303.2 g mol-1) was found to be 678 g mol-1. This indicates the presence of an exchange
between free and uranyl(V) bound K(18C6). c The determination of rsph was not possible because the information on the specific viscosity of the
pyridine/dmso 100:5 mixture was not available. A, B and C: the dissolution of 2 in the Pyridine/dmso 100:5 mixture allowed us to measure the
diffusion coefficient of three different sets of signals, which were respectively attributed to 2, 3, and the monomer [UO2(dbm)2]K. The calculated value
of the spherical hydrodynamic radius (11.3 Å for 2 and 9.5 Å for 3 in pyridine) are in a good agreement with the value estimated from the crystal
structure (10.8 Å for 2 and 10.0 for 3).
and 2-D COSY, NOESY and PFGSTE diffusion NMR spec-
troscopy. All studies were performed immediately after dis-
solution of the isolated compounds to avoid the presence of
the decomposition products which can be detected after few
hours in pyridine or thf solution.
monomeric species. The two C4h symmetric species were
assigned, by 1-D and PFGSTE NMR experiments and by
comparison of the spectrum of the isolated complex 3, in
pyridine solution, to the two T-shaped tetrameric assemblies
{[UO2(dbm)2]4[µ-Kx(Py)y]}n+ (x ) 4, n ) 0 minor species and
x ) 6, n ) 2 major species) containing a different number of
potassium cations. The presence, in solution, of the tetrameric
assembly was also confirmed by ESI-mass spectroscopy (ESI/
MS m/z ) 1549 {[UO2(dbm)2]4K6}2+).
The translational diffusion coefficient (D) of the different
species in pyridine and dmso was determined relative to a
reference compound by NMR experiments (using a pulsed-field
gradient stimulated echo (PFGSTE) sequence). Assuming that
the studied species can be roughly modeled by spheres pos-
sessing a hydrodynamic radius rsph (rsph ) kBT/6πηD) where η
(Pa s-1) is the viscosity of the medium and kB (m2 kg-2 K-1) is
Boltzmann’s constant. Their autodiffusion coefficients D are
related to their molecular weight for compounds with similar
partial specific volumes.40 The determination of translational
diffusion coefficients for shape-similar complexes is thus an
efficient tool for deducing the molecular mass of unknown
species in solution when a similar reference compound is
measured under the same conditions (eq 1).41 If an exchange
process is occurring in the experiment time-scale between
different solution species, only an average molecular weight
can be obtained. The calculated values of spherical hydrody-
namic radius and molecular weight with respect to a chosen
reference of 2, 3, and 4 in pyridine solution and of 2 in dmso
are presented in Table 2.
Dissolution of 2 in thf leads to a suspension. After filtration
of the separated KI salt, the 1D NMR shows only the signals
assigned to complex [UO2(dbm)2]4[µ-K4(thf)x]. Complex 3 once
isolated is very insoluble in acetonitrile but soluble in pyridine
and thf. The proton NMR spectrum of 1.6 mM thf solutions of
complex 3 (Figure 7) recorded immediately after dissolution
shows the presence of only one rigid C4h symmetric solution
species, in agreement with the presence in solution of the
polymetallic assembly [UO2(dbm)2]4[µ-K4(thf)x] and with the
retention of the solid-state structure. A similar 1D NMR
spectrum is observed when complex 3 is prepared in situ from
the reaction of a 2:1 mixture of K(dbm) and 1 (12.8 mM) in
CD3CN (Figure S11, Supporting Information). The proton NMR
spectrum of a 1.6 mM pyridine solution of 3 (Figure S12,
Supporting Information) shows the presence of two sets of
signals in the ratio 100:17, in agreement with the presence of
the tetrameric C4h symmetric solution species [UO2(dbm)2]4[µ-
K4(Py)x] and the monomeric C2V symmetric species [UO2-
(dbm)2(py)x]K. The NMR experiment do not demonstrate that
the coordination of the potassium cation to the uranyl(V) oxygen
occurs in the monomeric species. The assignment of these
species was confirmed by PFGSTE diffusion NMR spectros-
D1
D2
M2
3
)
(1)
M
ꢀ
1
The proton NMR spectrum of 2 in a 3.3 mM pyridine solution
(Figure 6) recorded immediately after dissolution shows the
presence of three sets of signals in the ratio 12:15:100 in
agreement with the presence of one C2V symmetric species and
two rigid C4h symmetric solution species.
+
copy. These results show unambiguously that the UO2+ · · ·UO2
T-shaped interaction leading to a tetrameric assembly is retained
completely in acetonitrile and thf, whereas the partial disruption
of the tetrameric assembly to yield a monomeric species occurs
in pyridine. Furthermore, the unambiguous presence of two
different tetrameric solution species (corresponding to the
isolated tetramers 2 and 3) in pyridine shows that the
UO2+ · · ·K+ cation-cation interaction is maintained in solution.
The C2V species were assigned by both 1-D and COSY NMR
to a monomeric [UO2(dbm)2(Py)]K complex. PFGSTE diffusion
NMR experiments (after addition of 5% dmso) confirmed this
assignment but do not demonstrate that the coordination of the
potassium cation to the uranyl(V) oxygen occurs in this
The evolution of the proton NMR spectrum after the addi-
tion of increasing amounts of dmso to a 3.4 mM pyridine solution
of 2 (Figure S13, Supporting Information) shows an increase of
the signals assigned to the tetrameric species [UO2(dbm)2]4[µ-
K4(Py)x] and of those assigned to the monomeric complex
(40) Waldeck, A. R.; Kuchel, P. W.; Lennon, A. J.; Chapman, B. E. Prog.
Nucl. Magn. Reson. Spectrosc. 1997, 30, 39–68.
(41) Terazzi, E.; Torelli, S.; Bernardinelli, G.; Rivera, J. P.; Benech, J. M.;
Bourgogne, C.; Donnio, B.; Guillon, D.; Imbert, D.; Bunzli, J. C. G.;
Pinto, A.; Jeannerat, D.; Piguet, C. J. Am. Chem. Soc. 2005, 127, 888–
903.
9
16638 J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008

Polynuclear Cation-Cation Complexes of Uranyl(V)
A R T I C L E S
Figure 6. 1H NMR spectrum at 400 MHz and at 298 K of a 3.3 mM solution of 2 in pyridine-d5 solution. The seven signals of the major species
{[UO2(dbm)2]4[µ-K6(Py)x]}2+ are labeled with blue triangles (red dots label the signals assigned to [UO2(dbm)2]4[µ-K4(Py)x] complex and green squares the
signals assigned to the monomeric [UO2(dbm)2(py)x]K complex) (* ) solvents and impurities).
Figure 7. 1H NMR spectrum at 400 MHz and at 298 K of 3 in thf-d8
solution (1.6 mM) (* ) thf).
Figure 9. 1H NMR spectrum at 400 MHz and at 298 K of 4 in a 7.2 mM
pyridine solution.
resulting in the complete disruption of the tetrameric structure.
However the disruption of the K+ · · ·UO2+ interaction could also
play an important role.
Figure 8. 1H NMR spectrum at 400 MHz and 298 K of 3 (1.6 mM) in
dmso-d6.
To further evaluate the influence of the coordinated potassium
ion on the solution structure and on the presence of CCI in
solution, increasing amounts of (18C6) were added to a 14.9
mM pyridine solution of 2 (Figure S14, Supporting Information).
The proton NMR spectra at increasing (18C6):K ratios show
the successive increase of the signals assigned to the tetrameric
species [UO2(dbm)2]4[µ-K4(Py)x] and of those assigned to
complex 4 accompanied by a decrease of the signals assigned
to the tetrameric {[UO2(dbm)2]4[µ-K6(Py)x]}2+ complex. At a
(18C6):K ratio of 1:1 only the signals of complex 4 are
observed.
accompanied by a decrease of the signals assigned to the
tetrameric {[UO2(dbm)2]4[µ-K6(Py)x]}2+ complex. These results
show that the presence of increasing amounts of dmso, which
can effectively compete with the uranyl(V) oxygens in the
coordination of K+ and UO2+, results in the disruption of the
K+ · · ·UO2+ and of the UO2+ · · ·UO2+ CCI to yield a monomeric
species.
Solutions of 2 and 3 in deuterated dmso-d6 show the same
proton NMR spectra (Figure 8) with only one set of signals in
agreement with the presence of the monomeric species
[UO2(dbm)2(dmso)x]K in solution. The presence of the mono-
meric complex was confirmed by PFGSTE diffusion NMR
performed on complex 3, but do not demonstrate that the
coordination of the potassium cation to the uranyl(V) oxygen
occurs in this monomeric species. The strongly coordinating
dmso ligand effectively competes with the uranyl(V) oxygen
for the fifth coordination site at the uranyl(V) metal centers,
The 1H NMR spectrum of 4 in pyridine-d5 (Figure 9) shows
a single set of signals for the dbm protons, indicating the
presence of C2V symmetric species in solution. The PFGSTE
diffusion NMR allowed calculation of an average molecular
weight (M ) 1010 g mol-1) intermediate between the molecular
weight of a monomeric and a dimeric species. This value is
also intermediate between the molecular weight of monomeric
species complexed or not by (18C6). In this case the assignment
9
J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008 16639

A R T I C L E S
Nocton et al.
dbm- oxygen atoms already bound to the uranyl(V) moiety also
contributes to increase the Lewis acidity of the uranium center
by decreasing the basicity of the ligand. Accordingly the
multiple binding of potassium to the uranyl(V) and dbm-
oxygen is probably crucial to the stability of the final tetrameric
cation-cation assembly.
Scheme 2
Stability Studies. CCIs have been proposed to play an
important role in the disproportionation of the UO2+ into U(IV)
2+
.
11,12 In the previous section we have shown that only
and UO2
rigid tetrameric CCI complexes are present in thf solution, while
partial or complete dissociation occurs to yield monomeric
species in pyridine and in dmso, respectively. Dissociation of
the tetrameric complexes is also observed on the NMR time
scale in pyridine in the presence of (18C6), although
UO2+ · · ·UO2+ interactions can clearly occur under these condi-
tions. In order to assess the effect of CCIs on the stability of
these pentavalent uranyl species, stability studies were performed
by proton NMR spectroscopy under different conditions.
The proton NMR spectra of pyridine solutions of complex 2
were recorded at different times (0-7 days) (Figure 10).
Decomposition starts after a few hours and is complete after 7
days. The presence of the disproportionation product [U(dbm)4]
was unambiguously characterized by NMR and by X-ray
diffraction on isolated crystals. This product accounts for around
of the solution species remains ambiguous and could involve
any or all of the equilibriums described in Scheme 2.
The ESI-MS mass spectroscopy shows only the molecular
peak corresponding to {[UO2(dbm)2][K(18C6)]2}+ (m/z )
1322). The UV-vis spectrum of 4 in pyridine is very similar
to the spectrum of 2 in dmso and differs significantly from those
of the tetrameric complex in pyridine or thf (see Figures S1
and S2, Supporting Information). These results seem to indicate
that monomeric species are predominant in solutions of 4 in
+
25% of the starting amount of UO2 complex. The other
identified species is [UO2(dbm)2] (∼10%). Additional signals
corresponding to dbm protons of unidentified species are found
in the diamagnetic region. The isolated complex 3 has a very
similar behavior in pyridine solution (Scheme 3).
+
pyridine, although UO2+ · · ·UO2 interaction can occur.
These results can be interpreted in terms of a lower affinity
of the [K(18C6)]+ cation, with respect to uncomplexed K+, for
The proton NMR of thf solutions of complex 3 were also
recorded at different times (0-25 days; Figure S15, Supporting
Information). The decomposition proceeds similarly to what is
observed for 2 and 3 in pyridine, but is slower. It starts after
∼20 h, but 30% of the starting compound is still present after
7 days and decomposition is complete only after 25 days. The
presence of the disproportionation product [U(dbm)4] was
unambiguously characterized. This product accounts for ∼30%
+
the UO2 oxygen atoms which leads to a reduced number of
coordinated potassium ions in the presence of (18C6). Although
the UO2+ · · ·UO2+ interaction can still occur in the presence of
(18C6) as evidenced by the isolation of the dimer 4 in the solid
state, the partial removal of coordinated potassium ion leads to
less strong CCIs in pyridine solution. Proton NMR studies show
that the addition of potassium iodide (1.8 equiv) to a solution
of 4 in pyridine leads again to formation of the tetrametallic
complexes. This result provides additional compelling evidence
that the potassium ion stabilizes the tetrameric versus the
dimeric/monomeric forms. The effects of potassium on the
+
of the starting amount of UO2 complex. The other identified
species include some Kdbm and [UO2(dbm)2] (∼15%). Addition
of [UO2(dbm)2] to the decomposed mixture allowed identifica-
tion of the presence of the [UO2(dbm)2] species as a decomposi-
tion product and led to a significant change in the signals of the
[U(dbm)4] complex, which become narrower. This suggests the
presence of an exchange interaction between the decomposition
products.
+
formation of UO2+ · · ·UO2 interactions are probably of both
+
structural and electronic nature. Potassium binding to the UO2
oxygens not involved in CCI increases the Lewis acidity and
therefore the electrophilic character of the U center, favoring
the binding of a uranyl(V) oxygen in a UO2+ · · ·UO2+ CCI. The
weaker interaction of the potassium ions with the uranyl(V)
oxygens involved in the CCI should have a minimal effect on
the basicity of the uranyl oxygens and therefore on their ability
to act as ligands in CCI. In contrast potassium binding to the
Addition of water to the solution of 3 in thf results in a much
faster decomposition with the total disappearance of the NMR
2+
signals assigned to the UO2 species observed after 20 h. In
this case, the decomposition also results in the formation of
[U(dbm)4] (∼30% of the starting UO2+ after 20 h) (Scheme 4).
Faster decomposition in the presence of water is in agreement
with the disproportionation mechanism proposed by quantum
chemical studies, which involves a protonation step of the
bridging “yl” oxygen of the uranyl(V) cation-cation complex
by water.13
The proton NMR of the complex 4 in pyridine at different
times shows a somewhat different behavior. Decomposition also
starts after ∼20 h and ∼40% of the starting compound is
decomposed after 3 days, but afterward the decomposition
process slows down and after 2 weeks ∼45% of the starting
[UO2 (dbm)2]- complex is still present (Scheme 5).
V
Figure 10. 1H NMR spectrum at 400 MHz and 298 K of 2 in pyridine-d5
after 7 days.
Furthermore the most important identified decomposition
product is the potassium salt of the free dbm- ligand with only
9
16640 J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008

Polynuclear Cation-Cation Complexes of Uranyl(V)
A R T I C L E S
Scheme 3
Scheme 4
Scheme 5
species, suggesting its involvement in an exchange with the
decomposition products. Addition of an excess of the
[UO2VI(dbm)2] complex to the final mixture of disproportion-
ation products leads to an important broadening of the signals
V
of [UO2 (dbm)2]- and of [UO2VI(dbm)2], in agreement with the
presence of [UO2VI(dbm)2] among the decomposition products
(Figure 11).
As discussed in the previous section, the complexes 2, 3, and
4 dissociate completely in dmso to yield the monomeric
complexes
[UO2(dbm)2(dmso)x]K
or
Scheme 6
[UO2(dbm)2(dmso)x]K(18C6), which are fully stable (Schemes
7 and 8). The proton NMR shows no decomposition up to 1
month. The addition of 1000 equiv of H2O to a dmso solution
of [UO2(dbm)2(dmso)x]K results after 2 days in the presence of
very small amounts of disproportionation products (3% U(IV)
and 3% U(VI), showing that the presence of water promotes to
some extent the disproportionation. The high stability of the
monomeric pentavalent uranyl complex of dbm- in dmso can
be interpreted in terms of its strong coordination to the uranium
center. The dmso coordination probably prevents the
UO2+ · · ·UO2 interaction by competing with the uranyl(V)
+
very small amounts of [U(dbm)4] formed after 7 days (7% of
V
oxygen for the available binding site in the equatorial plane of
the uranyl(V) moiety. While the retention of a rigid polymetallic
arrangement in pyridine leads to complete decomposition of
complexes 2 and 3, the monomeric [UO2(dbm)2(dmso)x]K is
fully stable. The observed stability of [UO2(dbm)2(dmso)x]K is
in agreement with the electrochemical studies of Ikeda and co-
workers, who observed that [UO2VI(dbm)2(dmso)] can be quasi-
initial [UO2 (dbm)2]- complex).
This suggests that the decomposition process proceeds
according to a different mechanism with respect to the tetrameric
species 2 and 3, which probably involves ligand loss from a
dimeric cation-cation intermediate associated with the electron-
transfer process. The interaction of the resulting dbm- with
unreacted [UO2 (dbm)2]- leads to its stabilization and is
V
reversibly reduced in dmso to
a
stable and pure
responsible for slowing down the decomposition process. This
was confirmed by the high stability of a 1:1 solution of 4 and
[K(18C6)(dbm)] in pyridine, for which no traces of decomposi-
tion were observed after several weeks (Scheme 6).
[UO2(dbm)2(dmso)]K complex without any successive reac-
tion.22 The UV-vis solution spectrum of 2 in DMSO (Figure
S2) is comparable to the one reported by Ikeda22 for the
electrochemically generated [UO2(dbm)2(dmso)]- complex,
while a significantly different shape is found for the UV-vis
solution spectrum of 2 and 3 in pyridine (Figure S1, Supporting
Information). The presence of CCI is probably responsible for
the observed differences, but the transitions due to CCI are
difficult to trace.
1H 2D NOESY spectroscopy confirmed the presence of a
rapid exchange between 4 and [K(18C6)(dbm)] in pyridine. The
V
interaction of [K(18C6)(dbm)] with [UO2 (dbm)2]- probably
competes with the CCI preventing disproportionation. While
the presence of the signals corresponding to hexavalent uranyl
could not be identified in the spectrum of final decomposition
products, the proton NMR spectra of 4 at different times shows
an increasing broadening of the signals of the pentavalent uranyl
In conclusion, these results provide strong evidence that the
formation of inner-sphere CC (UO2+ · · ·UO2+) complexes is
9
J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008 16641

A R T I C L E S
Nocton et al.
Figure 11. 1H NMR spectra of a 7.2 mM solution of 4 in pyridine-d5 at 400 MHz and 298 K (a) after 28 days and (b) after 28 days and addition of
[UO2(dbm)2Py] (signals for dbm protons of [UO2(dbm)2K(18C6)]2 (green squares); [Kdbm(18C6)] (blue dots); [U(dbm)4] (red triangles)).
Scheme 7
Scheme 8
compounds of actinides remain very limited.2,42,43 The 5f1 ions
are particularly interesting for the interpretation of magnetic data
because electron repulsion is absent. However the scarcity of
reported U(V) complexes has also limited the number of
magnetic studies on this oxidation state,44-46 and only one
example of unambiguous antiferromagnetic coupling between
two U(V) ions covalently linked by bridging imido ligands has
been described.35 Temperature-dependent magnetic susceptibil-
ity data were collected for the isolated analytically pure
microcrystalline 1, 3, and 4 from 2 to 300 K and are shown in
Figures 12 and 13, respectively. Samples of 1 display effective
magnetic moments that vary with the temperature, reaching an
effective magnetic moment of 2.57 µB per uranium at 300 K,
which is very close to the theoretical value calculated for the
free 5f1 ion in the L-S coupling scheme (µeff ) 2.54 µB)47 thus
suggesting the presence of well localized 5f electrons. The plot
of the magnetic ꢀ and ꢀT data versus T of 1 (Figure 12) show
involved in the dispropotionation of pentavalent uranyl in
organic solvents. Furthermore we have showed that counterion
+
binding can stabilize the UO2+ · · ·UO2 interaction, resulting
in fast decomposition. Conditions such as the presence of
coordinating solvent or anions that prevent the formation of CCI
complexes in solution stop the decomposition of pentavalent
uranyl.
Both electronic and steric factors probably contribute to the
instability of the complexes of pentavalent uranyl. Preliminary
studies on diketonate ligands presenting groups with different
steric hindrance show that decomposition occurs rapidly in all
cases in pyridine or thf with these bidentate ligands, but the
bulkier groups lead to faster disproportionation rates. Bidentate
ligands are well suited to isolate polymetallic intermediates, but
a subtle balance of the steric hindrances minimizing ligand
displacement and providing sufficient stability to cation-cation
intermediates is crucial for the isolation of CC complexes.
These results also suggest that the stabilization of mono-
nuclear pentavalent uranyl complexes suitable for reactivity
studies could prove difficult with bidentate ligands and that
ligands with higher denticity are likely to provide more stable
species.
(42) Clark, D. L.; Hecker, S. S.; Jarvinen, G. D.; Neu, M. P. The Chemistry
of the Actinide and Transactinide Elements; Springer: Dordrecht, 2006.
(43) Schelter, E. J.; Morris, D. E.; Scott, B. L.; Thompson, J. D.; Kiplinger,
J. L. Inorg. Chem. 2007, 46, 5528–5536.
(44) Selbin, J.; Ortego, J. D. Chem. ReV. 1969, 69, 657–671.
(45) Graves, C. R.; Yang, P.; Kozimor, S. A.; Vaughn, A. E.; Clark, D. L.;
Conradson, S. D.; Schelter, E. J.; Scott, B. L.; Thompson, J. D.; Hay,
P. J.; Morris, D. E.; Kiplinger, J. L. J. Am. Chem. Soc. 2008, 130,
5272–5285.
Magnetic Susceptibility. The rich magnetic properties of 5f
elements make them very attractive for the design of actinide-
based functional materials. However, the fundamental under-
standing of the molecular parameters governing magnetic
exchange interactions and electron delocalization in molecular
(46) Castro-Rodriguez, I.; Olsen, K.; Gantzel, P.; Meyer, K. J. Am. Chem.
Soc. 2003, 125, 4565–4571.
(47) Boudreaux, E. A.; Mulay, L. N. Theory and Applications of Molecular
Paramagnetism; Wiley: New York, 1976; p 512.
9
16642 J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008

Polynuclear Cation-Cation Complexes of Uranyl(V)
A R T I C L E S
Figure 12. Temperature-dependent magnetic susceptibility data for 1 in
the range of 2-300 K. At 300 K we calculated a µeff of 2.57 µB per uranium
(ꢀdia ) -5.34 × 10-4 emu mol-1, m ) 31.41 mg, M ) 1116.63 g mol-1).
two distinct temperature regions with different slopes: a high
temperature regime between 50 and 300 K, which is linear with
a large slope, and a lower temperature regime between 2 and
∼50K where the ꢀT product decreases rapidly. Such temperature
dependency has been observed in a few isolated imido com-
plexes of U(V) and has been interpreted in terms of the presence
of an isolated crystal field ground state (only contribution to
the magnetization at low temperature) and of an excited crystal
field state that becomes populated at high temperatures, therefore
contributing to the magnetization.2,35,45 The temperature de-
pendency plots of ꢀ and ꢀT of both polymetallic complexes 3
and 4 show a similar behavior in the high temperature region,
but they reach an effective magnetic moment per uranium at
300 K (µeff ) 1.64 µB for 3 and µeff ) 1.69 µB for 4), much
lower than the one found for 1 and the theoretical value
calculated for the free 5f1 ion. Similar low values of the effective
magnetic moment at high temperature have been reported for
simple coordination complexes of the halides,44 for mononuclear
imido complexes,2 and for polyoxoclusters of U(V) with a U6O13
core.48
The presence of an antiferromagnetic coupling at room
temperature was proposed to explain the low effective magnetic
moment in this polyoxocluster (1.8 µB), while the low magnetic
moment observed in mononuclear compounds has been inter-
preted in terms of a covalent character of the metal-ligand
interaction that results in a reduction of the orbital magnetism,46
although this interpretation appears controversial.45To elucidate
if the low effective magnetic moment of the tetranuclear
complex 3 and of the dinuclear complex 4 could arise from
antiferromagnetic interactions at room temperature, we have
compared the magnetic moment measured in the solid state with
the one measured in solution by the Evans method.49 The value
of the magnetic moment of 3 measured in pyridine solution (1.6
µB at 298 K) where it retains its polynuclear structure is very
similar to the value found in the solid state and to the value
measured in dmso solution where the complex is monomeric
(1.7 µB). A similar value was measured for complex 4 in
pyridine solution (1.5 µB). These results rule out the presence
of an antiferromagnetic coupling in complexes 3 and 4 at room
temperature. In the context of the very limited literature on
Figure 13. Temperature-dependent magnetic susceptibility data for 3 (blue
circles) and 4 (red circles) in the range of 2-300 K. A µeff of 1.64 µB per
uranium at 300 K was calculated for 3 (ꢀdia ) -1.53 × 10-3 emu mol-1
,
m ) 5.80 mg, M ) 3220.82 g mol-1). A µeff of 1.69 µB per uranium at 300
K was calculated for 4 (ꢀdia ) -9.33 × 10-4 emu mol-1, m ) 15.00 mg,
M ) 2039.52 g mol-1).
magnetic studies of uranium(V) compounds, a possible inter-
pretation of the reduced magnetic moment measured at room
temperature for 3 and 4 could be the presence of a covalent
contribution to the metal-ligand interaction in complexes 3 and
4. However a recent report on a series of analogous imido halide
complexes of U(V) show that simple conclusions on the extent
of covalency are difficult to draw from room temperature
magnetic moments.45
The magnetic behavior of 4 at very low temperature differs
significantly from the behavior observed for 1 and shows
unambiguously the presence of antiferromagnetic coupling
between the two oxo-bridged U(V) ions (Figure 13) with the
appearance of a maximum in ꢀ versus T at ∼5 K. Magnetic
coupling is likely to occur through a superexchange pathway
mediated by the uranyl(V) oxygens involved in the diamond-
shaped CCI. Magnetic studies on neptunyl frameworks have
shown that CCIs (resulting in Np-Np distance of about 4.1 Å)
can provide a superexchange pathway that leads to magnetic
ordering.8 While the maximum in ꢀ versus T appeared at higher
temperature (∼20 K) in the only other example of reported
5f1-5f1 magnetic coupling,35 this is the first example of
magnetic coupling between uranium ions mediated by oxo
ligands in an isolated molecule. The ꢀT data versus T of 3 show
two different regimes with a breaking point at 25K as observed
for 4. In 3 however, the ꢀ data increase with decreasing the
temperature until 4 K, where the magnetic susceptibility
probably achieves a maximum. However, the lack of information
at lower temperatures makes it difficult to ascertain if magnetic
(48) Duval, P. B.; Burns, C. J.; Clark, D. L.; Morris, D. E.; Scott, B. L.;
Thompson, J. D.; Werkema, E. L.; Jia, L.; Andersen, R. A. Angew.
Chem., Int. Ed. 2001, 40, 3357–3361.
(49) Evans, D. F. J. Am. Chem. Soc. 1959, 2003–2005.
9
J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008 16643

A R T I C L E S
Nocton et al.
coupling occurs for the tetrameric complex 3 with a maximum
at lower temperature than for complex 4. The complex shows
a linear field dependence of the magnetization at 6 K.
dimer). These results provide the first example of magnetic
coupling between uranium ions via uranyl(V) oxo bridges and
open broad perspectives for the use of cation-cation complexes
as a new class of polymetallic uranium compounds for the study
of magnetic interactions and intermetallic communication in
actinides and for the development of new functional actinides
materials. Work in both directions is in progress.
The different behavior of complexes 3 and 4 can be related
to the very different geometric arrangement of the interacting
+
UO2 groups (see Figure 4), which also leads to significantly
shorter U-U distance in complex 4 (3.462 Å in 4 vs 4.318(6)
Å average U-U distance in 3). These results suggest that CCI
complexes could prove particularly well suited for establishing
a magnetostructural correlation in uranium chemistry.
Experimental Section
General. All manipulations were carried out under an inert argon
atmosphere using Schlenk techniques and a MBraun glovebox
equipped with a purifier unit. The water and oxygen levels were
always kept at less than 1 ppm. The solvents were purchased from
Aldrich in their anhydrous form, conditioned under argon, and
vacuum-distilled from K/benzophenone (pyridine, thf, hexane, and
diisopropylether) or CaH2 (acetonitrile). Commercial anhydrous
dmso-d6 was further dried over molecular sieves preliminarily
heated at 200°. Depleted uranium turnings were purchased from
the “Socie´te´ Industrielle du Combustible Nucle´aire” of Annecy
(France). Dibenzoylmethane (dbmH), pyridine N-oxide, and 18-
crown-6 (18C6) were purchased from Aldrich and dried under
vacuum at 50 °C for 7 days. [UI3(thf)4]53 and [UI4(PhCN)4]54 were
synthesized as previously described. Elemental analyses were
performed under argon by Analytische Laboratorien GMBH at
Lindlar, Germany.
Extensive studies have been performed on oxo-bridged
d-block metal complexes in order to establish magnetostructural
correlations.50 A fit to experimental data for a large number of
oxide-bridged diiron(III) complexes indicated that the magnetic
interaction decreases when M-O distance increases or when
the Fe-O-Fe angle increases.51 However magnetostructural
correlations between complexes of different nuclearity is less
straightforward even for these extensively studied systems.52
Conclusions
We have described synthetic procedures that allow for the
isolation of cation-cation complexes of pentavalent uranyl. The
structural characterization of these new cation-cation com-
plexes, tetrameric and dimeric, of pentavalent uranyl with the
dbm- ligand, has allowed the unambiguous identification of
[UO2(dbm)2Py] was synthesized as previously described.55 1H
NMR (pyridine-d5, 298 K, 400 MHz): 8.64 (s, 8H); 7.63 (s, 12H);
+
the presence of UO2+ · · ·UO2 CCIs in solution. Furthermore
1
7.37 (s, 2H). H NMR (thf-d8, 298 K, 400 MHz): 8.57 (s, 8H);
the presented results highlight the importance of the presence
7.62 (s, 12H); 7.40 (s, 2H). [U(dbm)4]56 was prepared in situ by
+
of a cation binding the uranyl(V) oxygen (K+ · · ·UO2 CCIs)
1
reacting 1 equiv of [UI4(PhCN)4] and 4 equiv of Kdbm, H NMR
in the stabilization of these interactions. NMR studies have been
used for the first time to unambiguously identify CCIs in solution
and to relate the stability of pentavalent uranyl with the
formation of cation-cation complexes. The decomposition of
pentavalent uranyl was not observed in conditions that block
CCIs, whereas fast disproportionation of pentavalent uranyl to
form U(IV) and U(VI) species and other unidentified products
is observed for cation-cation complexes. The presence of
coordinating solvents or anions can prevent the CCIs by
competing with the “yl” oxygen in the coordination of the
uranium center, showing that an appropriately chosen ligand
[U(dbm)4] (thf-d8, 298 K, 400 MHz): 14.17 (s, 4H); 7.50 (s, 8H);
4.97 (s, 16H); 2.56(s, 16H). 1H NMR [U(dbm)4]: (pyridine-d5, 298
K, 400 MHz): 14.25 (s, 4H); 7.59 (s, 8H); 5.37 (s, 16H); 3.95(s,
16H). 1H NMR [U(dbm)4]: (dmso-d6, 298 K, 400 MHz): 15.54 (s,
4H); 6.23 (s, 8H); 5.81 (s, 16H); 4.86(s, 16H).
FTIR spectra were recorded with a Perkin-Elmer 1600 Series
FTIR spectrophotometer. UV-vis measurements were carried out
with a Varian Cary 50 Probe spectrophotometer in quartz cells
(optical path lengths: 1 mm and 1 cm) adapted with J. Young valves.
1H NMR spectra were recorded on Varian UNITY and MER-
CURY 400 MHz and Bruker 200 MHz spectrometers. NMR
chemical shifts are reported in ppm with solvent as internal
reference.
Static magnetic properties were measured using a Quantum
Design SQUID MPMS-XL 5.0 susceptometer; ultra-low field
capability (0.05 G for the 5 T magnets; continuous low temperature
control/temperature sweep mode (CLTC), sweep rate 0.001-10
K/min.
The samples were pressed under argon into a Kel-F or an
aluminum container, which was then sealed under vacuum in a 5
mm Suprasil quartz tube. Contribution to the magnetization from
the Kel-F or the aluminum container and tube were measured
independently and subtracted from the total measured signal.
Diamagnetic corrections were made using Pascal’s constants.
Mass spectra were obtained with a Finnigan LCQ-ion trap
equipped with an electrospray source in a 1:5 pyridine/acetonitrile
mixture that was prepared and filtered on microporous filters in
the glovebox and maintained under argon until injection in the
spectrometer. The experimental isotopic profile was compared in
each case to the theoretical one (see Supporting Information).
+
set can lead to highly stable UO2 complex. The presence of
water was found to accelerate significantly the decomposition
when CCIs could be formed, while a smaller effect is observed
on monomeric species, supporting the importance of CCIs in
the disproportionation of pentavalent uranyl in water.
Despite their low solution stability, an appropriate choice of
synthetic conditions allows the isolation of the tetrameric
{[UO2(dbm)2]2[µ-K(MeCN)2][µ8-K]}2 and the dimeric [UO2-
(dbm)2K(18C6)]2 in a pure form, making possible the study of
their magnetic properties. The measured temperature-dependent
magnetic susceptibility highlights the presence of unambiguous
antiferromagnetic coupling between the two uranium centers
of the [UO2(dbm)2K(18C6)]2 dimer with appearance of a
maximum in ꢀ versus T at ∼5 K. The coupling probably occurs
via a superexchange pathway through the bridging oxygen atoms
involved in the diamond-shaped CCI. The different behavior
of the tetrameric complex, which probably involves a magnetic
coupling occurring at lower temperature, can be ascribed to the
different geometric arrangement of the interacting uranyl(V)
groups (T-shaped in the tetramer versus diamond-shaped in the
(53) Avens, L. R.; Bott, S. G.; Clark, D. L.; Sattelberger, A. P.; Watkin,
J. G.; Zwick, B. D. Inorg. Chem. 1994, 33, 2248–2256.
(54) Enriquez, A. E.; Scott, B. L.; Neu, M. P. Inorg. Chem. 2005, 44, 7403–
7413.
(50) Kahn, O. Molecular Magnetism; Wiley-WCH: New York, 1993.
(51) Weihe, H.; Gudel, H. U. J. Am. Chem. Soc. 1997, 119, 6539–6543.
(52) Canada-Vilalta, C.; O’Brien, T. A.; Brechin, E. K.; Pink, M.; Davidson,
E. R.; Christou, G. Inorg. Chem. 2004, 43, 5505–5521.
(55) Alcock, N. W.; Flanders, D. J.; Pennington, M.; Brown, D. Acta
Crystallogr., Sect. C: Cryst. Struct. Commun. 1987, 43, 1476–1480.
(56) Kepert, D. L.; Patrick, J. M.; White, A. H. J. Chem. Soc., Dalton
Trans. 1983, 567–570.
9
16644 J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008

Polynuclear Cation-Cation Complexes of Uranyl(V)
A R T I C L E S
Synthesis of Kdbm. A solution of dbmH (200 mg, 0.89 mmol,
1 equiv) in thf (3 mL) was added to a suspension of KH (39.2 mg,
0.98 mmol, 1.1 equiv) in thf (1 mL). The mixture was stirred for
1 h until the gas clearing was complete, resulting in a yellow
solution. After removal of the excess of KH by filtration, 4 mL of
diisopropylether was slowly added to the resulting pale yellow
filtrate. The Kdbm was precipitated out of the solution as a pale
yellow solid, which was filtered, washed with a small amount of
thf, and dried under vacuum (208 mg, 88% yield). 1H NMR
(pyridine-d5, 298 K, 400 MHz): 8.64 (m, 6H, Hpara,meta-Ph); 7.62
(3 mL) and dried under vacuum (61.6 mg, 45%) ESI/MS m/z )
1322 {[UO2(dbm)2][K(18C6)]2}+ Anal. Calcd for C42H46UO12K:
1
C, 49.46; H, 4.55; N, 0.00. Found: C, 49.32; H, 4.75; N, 0.33. H
NMR (pyridine-d5, 298 K, 400 MHz): 6.82 (br, t. 4H, Hpara-Ph);
5.03 (br dd, 8H, Hmeta-Ph); 4.87 (br 24H, -CH2O-); 2.52 (br D, 8H,
1
H
ortho-Ph); -0.28 (br, 2H, CO-CH-CO). H NMR of 4 (pyridine-
d5, 333 K, 400 MHz): 6.82 (br, 4H, Hpara-Ph); 5.32 (br dd, 8H,
meta-Ph); 4.76 (br 24H, -CH2O-); 2.91 (br D, 8H, Hortho-Ph); 0.32
H
(br, 2H, CO-CH-CO).
Stability Studies. The stability of 2 and 4 was examined in
pyridine-d5, and the stability of 3 and of a 1:1 mixture of 3 and
water was studied in thf-d8. In each case the concentration equalled
1
(m, 4H, Hortho-Ph); 7.34 (s, 1H, CO-CH-CO). H NMR (dmso-d6,
298 K, 200 MHz): 7.79 (m, 4H, Hortho-Ph); 7.32 (m, 6H, Hpara,meta
-
1
1
6.7 mM U. The samples were monitored in time by H NMR at
Ph); 6.20 (s, 1H, CO-CH-CO). H NMR (pyridine-d5 + (18C6)
400 MHz and 298 K. The NMR spectra were recorded after 80,
220, 460, 1560, 3000, 4400, 10080, 20160 min (for 4), 80, 220,
460, 1560, 3000, 10080 min (for 2), from 100 and 37500 min (for
3) and from 10 to 1220 min for the 1:1 mixture of 3 with water.
Amount (%) of each attributed species was estimated from
integration and given in function of time (see Figures S15-S17,
Supporting Information).
(1equiv), 298 K, 400 MHz): 8.37 (d, J ) 6.4 Hz, 4H, Hortho-Ph);
7,48 (t, J ) 6.4 Hz, 4H, Hmeta-Ph); 7,39 (t, J ) 6.4 Hz, 2H, Hpara
-
Ph); 6.93 (s, 1H, CO-CH-CO).
Syntheses of Pentavalent Uranyl Complexes. The synthesis
of {[UO2Py5][µ-KI2Py2]}n (1) was performed in a gram scale
according to the previously described procedure.28 The higher scale
and the use of a longer crystallization time (3 weeks) led to a
significant increase of the final yield (75%). The synthesis of
{[UO2(dbm)2]2[µ-K(Py)2]2[µ8-K(Py)]}2I2 ·Py2 (2) was performed on
a 50 mg scale according to the previously described procedure (70%
yield).29 Anal. Calcd for {[UO2(dbm)2]2[µ-K(Py)2]2[µ8-K(Py)}2I2 ·
Py2 ·4iPr2O·4KI: C 41.98, H 3.52, N 2.52. Found: C 42.15, H 3.70,
N 2.62.
X-ray Crystallography. All diffraction data were taken using
a Bruker SMART CCD area detector three-circle diffractometer
(Mo KR radiation, graphite monochromator, λ ) 0.71073 Å). To
prevent oxidation, the crystals were coated with a light hydrocarbon
oil and quickly transferred to a stream of cold nitrogen on the
diffractometer. The cell parameters were obtained with intensities
detected on three batches of 15 frames with a 180 s and 10 s
exposure time for, respectively, 3 and 4. The crystal-detector
distance was 5 cm. The data were collected for 0.3° increments in
ω with a 120 s exposure time for 3 and 180 s for 4. A full
hemisphere of data was collected for each complex. At the end of
data collection, the first 50 frames were recollected to establish
that crystal decay had not taken place during the collection. Unique
intensities with I > 10σ(I) detected on all frames using the Bruker
SMART program57 were used to refine the values of the cell
parameters. The substantial redundancy in data allows empirical
absorption corrections to be applied using multiple measurements
of equivalent reflections with the SADABS Bruker program.57
Space groups were determined from systematic absences, and they
were confirmed by the successful solution of the structure (Table
1). Complete information on crystal data and data collection
parameters is given in Supporting Information. The structures were
solved by direct methods using the SHELXTL 5.03 package.58 For
complexes 4, all atoms, including hydrogen atoms, were found by
difference Fourier syntheses. All non-hydrogen atoms were aniso-
tropically refined on F2, and hydrogen atoms were isotropically
refined. For complexes 3 hydrogen atoms were included in
calculated positions with isotropical thermal coefficients.
Synthesis of {[UO2(dbm)2]2[µ-K(MeCN)2][µ8-K]}2 (3). An
acetonitrile suspension (0.4 mL) of Kdbm (40.2 mg, 153 µmol, 2
equiv) was added to an acetonitrile (0.4 mL) suspension of 1 (85.7
mg, 76.7 µmol, 1 equiv). A deep green solution was formed, which
was then stirred for 15 min and filtered to remove a white solid.
The deep green filtrate was allowed to stand at room temperature
for 10 h to allow crystallization. Small blue crystals were obtained,
filtered, washed with acetonitrile (0.5 mL), and dried (9.8 mg). The
evaporation of the resulting filtrate allowed the isolation, after
standing 24 h at room temperature, of a second crop of crystals,
which were thoroughly washed with acetonitrile (2 mL) to remove
traces of decomposition product ([U(dbm)4]); 15.4 mg of analyti-
1
cally pure compound was obtained (total yield 42%). H NMR
(pyridine-d5, 400 MHz, 298 K): δ 7.46 (t, J ) 7.8 Hz, 8H); 6.40
(t, J ) 7.0 Hz, 16H); 6.16 (t, J ) 7.5 Hz, 8H); 5.66 (d, J ) 7.0 Hz,
16H); 4.89 (t, J ) 6.9 Hz, 16H); 3.60 (d, J ) 6.5 Hz, 16H); 3.60
1
(s, 4H). H NMR (thf-d8, 400 MHz, 298 K): δ 7.38 (s, 8H); 6.35
(s, 16H); 6.02 (s, 8H); 5.31 (s, 16H); 4.56 (s, 16H); 2.87 (s, 16H);
1.95 (s, 4H). 1H NMR (CD3CN, 400 MHz, 298 K): δ 7.47 (s, 8H);
7.01 (s, 16H); 6.13 (s, 16H); 5.95 (s, 8H); 4.99 (s, 16H); 4.72 (s,
1
16H); 3.61 (s, 4H). H NMR of 3 (thf-d8, 323 K, 400 MHz): δ
7.32 (dd, J ) 6.4, 8.6 Hz, 4H); 6.38 (dd, J ) 6.4, 8.6 Hz, 8H);
6.02 ((dd, J ) 6.4, 8.6 Hz, 4H); 5.38 (d, J ) 8.6 Hz, 8H); 4.69
(dd, J ) 6.4, 8.6 Hz, 8H); 3.19 (d, J ) 6.4 Hz, 8H); 2.20 (s, 4H).
ESI/MSm/z)1549{[UO2(dbm)2]4K6}2+.Anal.Calcdfor{[UO2(dbm)2]2[µ-
K(MeCN)][µ8-K]}20.7KI: C 46.24, H2.94, N 0.87. Found: C 46.31,
H 3.54, N 0.91.
Acknowledgment. This work was supported by the Commis-
sariat a` l’Energie Atomique, Direction de l’Energie Nucle´aire and
by the “Actinet” European network. We thank Fabien Burdet for
preliminary work, Pierre Alain Bayle for his help with the NMR
experiments, Jean-Franc¸ois Jacquot for the magnetic measurements,
and Colette LeBrun for the help with the ESI-MS experiments.
Synthesis of [UO2(dbm)2K(18C6)]2 (4). A solution of Kdbm
(70.0 mg, 268 µmol, 2 equiv) in pyridine (2 mL) was added to a
suspension (150 mg, 134 µmol, 1 eq U) of 1 in pyridine (1 mL).
The mixture was stirred for 30 min until 1 completely dissolved to
give a blue solution, which was subsequently filtered. The pyridine
was removed under vacuum to give a blue solid, which was
dissolved in thf (5 mL) to afford a blue solution and a white
precipitate (KI). The precipitate was removed by filtration. A
solution (1 mL) of 18-crown-6 (106.5 mg, 3 equiv) in thf was added
to the filtrate, and the resulting solution was vigorously stirred for
30 s and then allowed to stand at room temperature for 15 min to
afford a blue microcrystalline solid. The solid was filtered, washed
three times with thf (3 mL) and dried under vacuum. Finally the
solid was dissolved in pyridine (0.5 mL), and hexane (8 mL) was
added to the resulting solution to yield 4 as a blue-green solid.
The obtained blue-green solid was washed three times with hexane
Supporting Information Available: Crystallographic details
for compounds 3 and 4 as CIF files; UV-vis spectra of 2, 3,
and 4; NMR spectra and IR spectra of 2 and 3; description of
the pulse sequence, equations, and experimental conditions used
for the determination of the diffusion coefficients. This material
is available free of charge via the Internet at http://pubs.acs.org.
JA804766R
(57) SMART: Software Package for Use with the SMART Diffractometer;
Bruker: Madison, WI, 1995.
(58) Sheldrick, G. M. SHELXTL, 6.14; University of Go¨ttingen: Go¨ttingen,
Germany, 2006.
9
J. AM. CHEM. SOC. VOL. 130, NO. 49, 2008 16645