Synthesis, structure, and bonding of stable complexes of pentavalent uranyl

Article abstract of DOI:10.1021/ja9037164

Stable complexes of pentavalent uranyl [UO₂(salan- ^tBu₂)(py)K]_n (3), [UO₂(salan- ^tBu₂)(py)K(18C6)] (4), and [UO₂(salophen- ^tBu₂)(thf)]K(thf)₂}_n (8) have been synthesized from the reaction of the complex {[UO₂py ₅][KI₂py₂]}_n (1) with the bulky amine-phenolate ligand potassium salt K₂(salan-^tBu ₂) or the Schiff base ligand potassium salt K₂(salophen- ^tBu₂) in pyridine. They were characterized by NMR, IR, elemental analysis, single crystal X-ray diffraction, UV-vis spectroscopy, cyclic voltammetry, low-temperature EPR, and variable-temperature magnetic susceptibility. X-ray diffraction shows that 3 and 8 are polymeric and 4 is monomeric. Crystals of the monomeric complex [U^VO₂(salan- ^tBu₂)(py)][Cp*₂Co], 6, were also isolated from the reduction of [U^VIO₂(salan-^tBu ₂)(py)], 5, with Cp*2Co. Addition of crown ether to 1 afforded the highly soluble pyridine stable species [UO₂py₅]I ?py (2). The measured redox potentials E_1/2 (U ^VI/U^V) are significantly different for 2 (-0.91 and -0.46 V) in comparison with 3, 4, 5, 7 and 9 (in the range -1.65 to -1.82 V). Temperature-dependent magnetic susceptibility data are reported for 4 and 7 and give μ_eff of 2.20 and 2.23 μ_B at 300 K respectively, which is compared with a μ_eff of 2.6(1) μ_B (300 K) for 2. Complexes 1 and 2 are EPR silent (4 K) while a rhombic EPR signal (g _x = 1.98; g_y = 1.25; g_z = 0.74 (at 4 K) was measured for 4. The magnetic and the EPR data can be qualitatively analyzed with a simple crystal field model where the f electron has a nonbonding character. However, the temperature dependence of the magnetic susceptibility data suggests that one or more excited states are relatively low-lying. DFT studies show unambiguously the presence of a significant covalent contribution to the metal-ligand interaction in these complexes leading to a significant lowering of the π_u*. The presence of a back-bonding interaction is likely to play a role in the observed solution stability of the [UO ₂(salan-^tBu₂)(py)K] and [UO ₂(salophen-^tBu₂)( py)K] complexes with respect to disproportionation and hydrolysis.

Full text of DOI:10.1021/ja9037164

Published on Web 12/15/2009
Synthesis, Structure, and Bonding of Stable Complexes of
Pentavalent Uranyl
Gre´gory Nocton,† Pawel Horeglad,† Valentina Vetere,†,‡ Jacques Pe´caut,†
Lionel Dubois,† Pascale Maldivi,† Norman M. Edelstein,§ and Marinella Mazzanti*,†
CEA, INAC, SCIB, Laboratoire de Reconnaissance Ionique et Chimie de Coordination,
CEA-Grenoble, 38054 GRENOBLE, Cedex 09, France, UJF, LCIB, UMR-E 3 CEA-UJF,
Grenoble I, 38041 Grenoble cedex 9, France, UniVersite´ de Toulouse, Laboratoire de Chimie et
Physique Quantique, UMR5626, 118 route de Narbonne, F-31062, Toulouse Cedex, France, and
Lawrence Berkeley National Laboratory, Berkeley, California 94720-8175
Received May 7, 2009; E-mail: marinella.mazzanti@cea.fr
Abstract: Stable complexes of pentavalent uranyl [UO2(salan-tBu2)(py)K]n (3), [UO2(salan-tBu2)(py)K(18C6)]
(4), and [UO2(salophen-tBu2)(thf)]K(thf)2}n (8) have been synthesized from the reaction of the complex
{[UO2py5][KI2py2]}n (1) with the bulky amine-phenolate ligand potassium salt K2(salan-tBu2) or the Schiff
base ligand potassium salt K2(salophen-tBu2) in pyridine. They were characterized by NMR, IR, elemental
analysis, single crystal X-ray diffraction, UV-vis spectroscopy, cyclic voltammetry, low-temperature EPR,
and variable-temperature magnetic susceptibility. X-ray diffraction shows that 3 and 8 are polymeric and
4 is monomeric. Crystals of the monomeric complex [UVO2(salan-tBu2)(py)][Cp*2Co], 6, were also isolated
from the reduction of [UVIO2(salan-tBu2)(py)], 5, with Cp*2Co. Addition of crown ether to 1 afforded the
highly soluble pyridine stable species [UO2py5]I·py (2). The measured redox potentials E1/2 (UVI/UV) are
significantly different for 2 (-0.91 and -0.46 V) in comparison with 3, 4, 5, 7 and 9 (in the range -1.65 to
-1.82 V). Temperature-dependent magnetic susceptibility data are reported for 4 and 7 and give µeff of
2.20 and 2.23 µB at 300 K respectively, which is compared with a µeff of 2.6(1) µB (300 K) for 2. Complexes
1 and 2 are EPR silent (4 K) while a rhombic EPR signal (gx ) 1.98; gy ) 1.25; gz ) 0.74 (at 4 K) was
measured for 4. The magnetic and the EPR data can be qualitatively analyzed with a simple crystal field
model where the f electron has a nonbonding character. However, the temperature dependence of the
magnetic susceptibility data suggests that one or more excited states are relatively low-lying. DFT studies
show unambiguously the presence of a significant covalent contribution to the metal-ligand interaction in
these complexes leading to a significant lowering of the πu*. The presence of a back-bonding interaction
is likely to play a role in the observed solution stability of the [UO2(salan-tBu2)(py)K] and [UO2(salophen-
tBu2)(py)K] complexes with respect to disproportionation and hydrolysis.
Introduction
So far, the reactivity of the few isolated compounds in this class
remain almost unexplored.7 Aside from its high fundamental
Molecular uranium chemistry has undergone significant
expansion in the last few decades.1-5 One of the most exciting
recent new entries into this area is pentavalent uranyl. Due to
their low stability, compounds of pentavalent uranyl remain rare
and only in the past few years have some isolable systems been
reported. An overview of the synthesis and characterization of
these systems has been recently covered in two review articles.6,7
interest, the chemistry of pentavalent uranyl has important
environmental implications. Notably, the UO2+ species has been
identified as a key intermediate in the anaerobic bacterial8,9 or
mineral mediated10 reduction of highly soluble hexavalent uranyl
species to insoluble U(IV) compounds. Since this process
reduces the mobility of uranium in the environment, it is highly
relevant for the speciation of uranium in the environment and
for the development of remediation strategies.11 Accordingly,
† CEA-Grenoble and CEA-UJF.
+
a better knowledge of the chemical properties of UO2 will
‡ Universite´ de Toulouse.
certainly contribute to the general understanding of the bio-
geochemical reduction of uranium in the environment. More-
§ Lawrence Berkeley National Laboratory.
(1) Bart, S. C.; Meyer, K. Organomet. Coord. Chem. Actinides 2008, 127,
119–176.
+
over, UO2 provides a reasonable and practical model for the
(2) Fox, A. R.; Bart, S. C.; Meyer, K.; Cummins, C. C. Nature 2008,
455, 341–349.
+
more highly radioactive NpO2 cation due to their identical
(3) Sessler, J. L.; Melfi, P. J.; Pantos, G. D. Coord. Chem. ReV. 2006,
250, 816–843.
(8) Lovley, D. R.; Phillips, E. J. P.; Gorby, Y. A.; Landa, E. R. Nature
1991, 350, 413–416.
(4) Korobkov, I.; Sandro, G. Prog. Inorg. Chem. 2005, 54, 321–349.
(5) Ephritikhine, M. Dalton Trans. 2006, 2501–2516.
(6) Graves, C. R.; Kiplinger, J. L. Chem. Commun. 2009, 3831–3853.
(7) Arnold, P. L.; Love, J. B.; Patel, D. Coord. Chem. ReV. 2009, 253,
1973–1978. Mougel, V.; Horeglad, P.; Nocton, G.; Pe´caut, J.;
Mazzanti, M. Angew. Chem., Int. Ed. 2009, 48, 8477–8480.
(9) 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.
(10) Ilton, E. S.; Haiduc, A.; Cahill, C. L.; Felmy, A. R. Inorg. Chem.
2005, 44, 2986–2988.
(11) Morris, D. E. Inorg. Chem. 2002, 41, 3542–3547.
9
10.1021/ja9037164  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 495–508 495

A R T I C L E S
Nocton et al.
Scheme 1. Reaction of 1 with K2(salan-tBu2) and K2(salophen-tBu2)
+
charge. The low charge of the NpO2 renders problematic its
separation in nuclear fuel reprocessing.12 The design of suitable
+
ligands for the selective complexation and extraction of AnO2
species requires a better understanding of the molecular
parameters governing the bonding and reactivity of these species.
+
Though kinetic studies on aqueous UO2 go a long way
back,13,14 the low stability of this species in aqueous solutions
+
has prevented the isolation of UO2 compounds. Pentavalent
uranyl can be stabilized in concentrated carbonate media,15-17
but otherwise it readily disproportionates to U(IV) and ura-
nyl(VI) species.12 Only the use of anhydrous and anaerobic
+
conditions allowed the electrochemical production of pure UO2
complexes in solution with Schiff base or diketonate ligands,
but these complexes were never isolated using this route.12,18-20
After a first report of the crystal structure of a serendipitously
obtained cationic complex ([UO2(OPPh3)4]+),21 only in the last
three years have several complexes of pentavalent uranyl been
reproducibly synthesized using different synthetic procedures.22-31
However, most of these complexes show a limited solution
stability and undergo disproportionation which is likely to
involve the formation of a dimeric complex through the mutual
binding of two uranyl(V) groups (commonly known as
cation-cation interaction).32 Our group has reported the only
example of such intermediates, which demonstrated a sufficient
stability to allow their isolation.28 Full stability of pentavalent
uranyl complexes with respect to disproportionation in solution
has been obtained by Arnold and co-workers27 in tetrahydrofuran
solution, in the presence of a transition metal cation by using
bulky macrocyclic ligands which disfavors dimer formation.
Aside from this system, a remarkable solution stability has also
been observed for the simple pyridine solvate {[UO2py5]-
[KI2py2]}n (1)22 and analogous complexes containing the
[UO2py5]+ unit, in spite of the absence of bulky ligands that
can provide steric protection along the equatorial plane.29
Moreover, in these complexes, the high Lewis basicity of the
uranyl oxygen often results in the presence of cations coordi-
nated to the UO2 group. The little experimental information
+
available on the reported uranyl(V) compounds suggests that
electronic factors and cation binding should play an important
role in the stabilization of pentavalent uranyl. We have recently
shown in a preliminary communication that tetradentate bulky
amine-phenolate ligands can stabilize pentavalent uranyl in
pyridinesolutionandallowsthesynthesisofthestable[UO2(salan-
tBu2)(py)K]n polymer (Scheme 1).30 Here we report the synthesis
and structure of a new highly stable complex of pentavalent
uranyl supported by the bulky Schiff base ligand salophen-
2-
tBu2
.
These complexes were synthesized from the previously
reported iodide derivative {[UO2py5][KI2py2]}n (1)22 according
to Scheme 1. The synthesis and structure of mononuclear
analogues of the salan-tBu2 complex will also be described. The
structural, magnetic, redox and spectroscopic properties, and
the solution stability with respect to disproportionation and
ligand dissociation of these complexes, have been investigated
in detail. The experimental study has been paired with a
qualitative analysis of the magnetic data and DFT studies of
the electronic structure and bonding. The presented results
provide important new information on the molecular parameters
allowing the stabilization of pentavalent uranyl species, which
include cation binding and steric and electronic ligand effects.
(12) Morss, L. R.; Edelstein, N. M.; Fuger, J. The Chemistry of the Actinide
and Transactinide Elements; Springer: Dordrecht, 2006.
(13) Newton, T. W.; Baker, F. B. Inorg. Chem. 1965, 4, 1166–1170.
(14) Ekstrom, A. Inorg. Chem. 1974, 13, 2237–2241.
(15) Ikeda, A.; Hennig, C.; Tsushima, S.; Takao, K.; Ikeda, Y.; Scheinost,
A. C.; Bernhard, G. Inorg. Chem. 2007, 46, 4212–4219.
(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.
Results and Discussion
(17) Clark, D. L.; Hobart, D. E.; Neu, M. P. Chem. ReV. 1995, 95, 25–48.
(18) Mizuoka, K.; Tsushima, S.; Hasegawa, M.; Hoshi, T.; Ikeda, Y. Inorg.
Chem. 2005, 44, 6211–6218.
Synthesis, Molecular Structure and Reactivity. The pentava-
lent uranyl iodide {[UO2py5][KI2py2]}n (1), previously re-
ported,22 is stable in pyridine where it is however only slightly
soluble. Although its solubility is sufficient for coordination
chemistry studies,28,30 its low solubility prevents the study of
its solution properties. The reaction of 1 with the K2salan-tBu2
(H2salan-tBu2 ) N,N′-bis(2-hydroxybenzyl-3,5-di-tert-butyl)-1,2-
dimethylaminomethane) in pyridine led to the isolation of the
stable complex [UO2(salan-tBu2)(py)K]n (3). We have recently
communicated the crystal structure of this complex.30 The
presence of K+ · · ·UO2+ cation-cation interactions in complexes
1 and 3 results in the formation of polymeric structures. Only
a few mononuclear compounds of pentavalent uranyl have been
reported, and all have been prepared from the reduction of the
hexavalent analogue.25,26,29 Here we show that the mononuclear
compounds 2 and 4 can be isolated by reacting the polymeric
complexes 1 and 3 with 18C6 (Scheme 2). 18C6 has a strong
affinity for the potassium cation, and the resulting (K18C6)+
complex is expected to bind less strongly to the uranyl oxygen.
(19) Mizuoka, K.; Ikeda, Y. Radiochim. Acta 2004, 92, 631–635.
(20) Kim, S. Y.; Tomiyasu, H.; Ikeda, Y. J. Nucl. Sci. Technol. 2002, 39,
160–165.
(21) Berthet, J.-C.; Nierlich, M.; Ephritikhine, M. Angew. Chem., Int. Ed.
2003, 42, 1952–1954.
(22) Natrajan, L.; Burdet, F.; Pecaut, J.; Mazzanti, M. J. Am. Chem. Soc.
2006, 128, 7152–7153.
(23) Burdet, F.; Pecaut, J.; Mazzanti, M. J. Am. Chem. Soc. 2006, 128,
16512–16513.
(24) Berthet, J. C.; Siffredi, G.; Thuery, P.; Ephritikhine, M. Chem.
Commun. 2006, 3184–3186.
(25) Hayton, T. W.; Wu, G. J. Am. Chem. Soc. 2008, 130, 2005–2014.
(26) Hayton, T. W.; Wu, G. Inorg. Chem. 2008, 47, 7415–7423.
(27) Arnold, P. L.; Patel, D.; Wilson, C.; Love, J. B. Nature 2008, 451,
315–318.
(28) Nocton, G.; Horeglad, P.; Pecaut, J.; Mazzanti, M. J. Am. Chem. Soc.
2008, 130, 16633–16645.
(29) Berthet, J. C.; Siffredi, G.; Thuery, P.; Ephritikhine, M. Dalton Trans.
2009, 3478–3494.
(30) Horeglad, P.; Nocton, G.; Filinchuck, Y.; Pe´caut, J.; Mazzanti, M.
Chem. Commun. 2009, 1843–1845.
(31) Hayton, T. W.; Wu, G. Inorg. Chem. 2009, 48, 3065–3072.
(32) Steele, H.; Taylor, R. J. Inorg. Chem. 2007, 46, 6311–6318.
9
496 J. AM. CHEM. SOC. VOL. 132, NO. 2, 2010

Stable Complexes of Pentavalent Uranyl
A R T I C L E S
Scheme 2. Reaction of 1 and 3 with 18C6 Crown Ether in Pyridine
In compounds 3, 4 and 6, the uranium atom is seven
coordinated with regular pentagonal bipyramidal coordination
geometry. The four donor atoms of the salan-tBu22- (O2N2) and
the nitrogen of coordinated pyridine form a pentagonal plane
(with a mean deviation of 0.07 Å in 3, 0.10 Å in 4 and 0.16 Å´
in 6). The ligand wraps around the metal with the two methyl
groups in a trans position taking a chiral pseudo-C2 symmetry
for all three complexes. Both enantiomers are found in the
structure leading to a racemic space group. The axial positions
are occupied by two oxo ligands with UdO distances between
1.819 and 1.868 Å (mean UdO ) 1.825(8) Å for 3, 1.859(11)
Å for 4 and 1.856(14) Å for 6), which all show a significant
lengthening of the UdO bonds in comparison with the corre-
sponding UO2 complex [UO2(salan-tBu2)(py)] (UdO )
2+
1.764(4) and 1.790(3) Å).30 The angle O-U-O in the UO2+ is
in the range 177.38-178.9°. The equatorial positions are
occupied by the two oxygen atoms (mean U-O distance: 2.35(4)
Å in 3, 2.339(10) Å in 4 and 2.380(12) Å in 6) and the two
nitrogen atoms (mean U-N: 2.703(18) Å in 3, 2.705(11) Å in
4 and 2.772(12) Å in 6) of salan-tBu22- ligand, and the nitrogen
atom of the pyridine (U-Npyridine 2.618(16) Å in 3, 2.629(3) Å
in 4 and 2.700(11) Å in 6). A significant lengthening of the
UdO bond distances is found in 4 and 6 with respect to 3.
Surprisingly longer UdO bond distances are found in the
presence of the weakly coordinating (K18C6)+ cation or in
Previous studies show that the addition of 18C6 to a potassium
bound tetrameric complex of pentavalent uranyl results in the
disruption of the polynuclear structure.33
The addition of 18C6 to a suspension of 1 in pyridine leads
to the complete dissolution of the complex. The stability of a
1:1 solution of 1 and 18C6 in pyridine was investigated by
UV-visible spectroscopy over a period of one month, and no
decomposition was observed. This result confirms the remark-
+
able effect of the pyridine ligand in stabilizing the UO2 even
absence of coordinating cation. This is associated, at least in 6
in the absence of coordinated cation. X-ray quality crystals of
the complex [UO2py5]I·py (2) were isolated by slow diffusion
of hexane into a pyridine solution containing complex 1 and
crown ether in a U:18C6 1:1 ratio. The molecular structure of
the complex was confirmed by X-ray diffraction, which shows
the presence of isolated [UO2(Py)5]+ cation and I- anion. Dur-
ing the preparation of this manuscript, crystals of complex 2
have also been obtained by reduction of the hexavalent iodide
[UO2py5I2] with TlC5H5 and the crystal structure of 2 has been
reported.29
2-
with a significant lengthening of the distance U-salan-tBu2
,
suggesting that the absence of coordinated potassium has a
significant effect on the interaction of the uranyl group with
the ligands in the equatorial plane (see computational and
magnetic data). In all of the compounds, the U-Npyridine distance
is shorter than the U-Nsalan distances with differences ranging
from 0.076 to 0.085 Å.30 The proton NMR spectrum of pyridine
solution of 3, 4 and 6 are very similar and all show the presence
of two sets of signals. One set of signals (major species) was
assigned to the presence in solution of monomeric rigid C2
symmetric enantiomers (λ and δ), consistent with the ligand
conformation in the solid state structure, while the second set
of signals indicate the presence of a minor Cs symmetric isomer
(see Figure S1 in Supporting Information). The ratios of C2:Cs
at room temperature vary only slightly for the three complexes
(from 100:6 for 3,30 to 100:9 for 4 and 100:14 for 6). The ratio
of C2:Cs is temperature dependent (100:11 for 3 at 333 K),30
suggesting the presence of an equilibrium among the different
conformers in solution. All three salan-tBu2 complexes 3, 4 and
6 show the same high stability with respect to the dispropor-
tionation reaction in pyridine and dmso solution (up to 30 days).
However, a lower stability is observed for complex 3 compared
to 4 in thf with partial disproportionation occurring for 3 already
after 14 days (only 33% of pentavalent uranyl remaining) and
full stability of 4 up to 30 days. Partial decomposition was
observed also for complex 4 in toluene (only 67% of the
pentavalent uranyl complex is left after 14 days).
The X-ray diffraction studies of the mononuclear complex
+
[UO2(salan-tBu2)(py)K(18C6)] (4) show that a K+ · · ·UO2
interaction is still present for one uranyl oxygen but does not
lead to the formation of a polymeric structure. This is probably
the result of the steric crowding and the lower coordinating
power of the K(18C6)+. To investigate the effect of the
coordinating cation on the stability and redox properties of the
uranyl complex, we have also used a different synthetic approach
to the preparation of pentavalent uranyl that involves the
reduction of the hexavalent analogue by an appropriate reducing
agent. The reduction of [UO2(salan-tBu2)(py)] (5) with an excess
of Cp*2Co led to the isolation of X-ray quality crystals of the
pentavalent complex [UO2(salan-tBu2)(py)][Cp*2Co] (6) (Scheme
3).
However, this synthetic method does not allow the quantita-
tive isolation of the pentavalent complex in a pure form because
of the difficulty in separating the excess of cobaltocene necessary
for the completeness of the reduction. The addition of potassium
iodide to a solution of 6 in pyridine affords complex 3,
confirming the presence of pentavalent uranyl.
The crystal structures of complexes 4 and 6 were determined
by X-ray diffraction studies. ORTEP views of 4 and 6 are
presented in Figures 1 and 2, respectively, while selected bond
distances and angles for 3, 4 and 6 are given in Table 1.
To prepare complexes of pentavalent uranyl with increased
stability toward disproportionation in all organic solvent and
with higher stability toward ligand dissociation in the presence
of water, we have turned to Schiff base ligands.
The formation of a [UO2(salophen)(dmso)]- (H2salophen )
N,N′-phenylene-bis-salicylideneimine complex by electrochemi-
cal reduction of the hexavalent analogue in dmso had been
(33) Nocton, G.; Horeglad, P.; Pe´caut, J.; Mazzanti, M. J. Am. Chem. Soc.
2008, 130, 16633–16645.
9
J. AM. CHEM. SOC. VOL. 132, NO. 2, 2010 497

A R T I C L E S
Nocton et al.
Scheme 3
previously reported,34 but the complex had never been isolated
following this route. Our attempts to isolate the [U(salophen-
)(py)K] complex from the reaction of 1 with salophen2- in
pyridine led to the formation of a mixture of UO2+, UO22+ and
U(IV) products as a result of the very rapid decomposition of
the pentavalent uranyl complex. To synthesize and characterize
stable uranyl(V) complexes with a Schiff base ligand, we
decided to investigate the reaction of 1 with the bulky Schiff
important effect of the presence of bulky groups in preventing
the disproportionation reaction probably by preventing the
formation of the cation-cation intermediate complex. However,
2-
a difference in stability was found for the salophen-tBu2
2-
complex as compared to the salan-tBu2 one in some solvents
in spite of the similar steric bulk. This suggests that electronic
parameters should play an important role in the stability of these
complexes. Important differences were also found in the
reactivity of these complexes with stoichiometric amounts of
water. The reactivity of 3, 4 and 7 with 1-1000 equiv of water
was investigated by NMR spectroscopy. Compound 3 reacts
slowly with water in a 1:1 ratio leading to release of the free
H2salan-tBu2. The reaction proceeds much faster in the presence
of 10 equiv of water (few hours) with complete disappearance
of the pentavalent species after 24 h. Moreover the reaction of
7 with water shows that no free ligand dissociation or metal
oxidation occurs for this complex in the presence of 10 equiv
of water up to 30 days and that a very large excess of water is
needed before ligand dissociation or metal oxidation occurs.
2-
base ligand salophen-tBu2 (H2salophen-tBu2 ) N,N′-phe-
nylene-bis-(3,5-di-tert-butylsalicylideneimine). The use of the
bulkier ligand salophen-tBu22- affords the complex [UO2(salophen-
tBu2)(py)K] (7), which shows a full solution stability (up to one
month) in pyridine (Scheme 4).
The proton NMR of complex 7 in pyridine solution shows
paramagnetically shifted signals in agreement with the presence
of a C2V symmetric solution species and indicating the presence
of UO2+. X-ray quality crystals were obtained from a thf solu-
tion of 7. The X-ray diffraction analysis revealed the presence
of the polymeric complex {[UO2(salophen-tBu2)(thf)]K(thf)2}n,
8. An ORTEP view of the [UO2(salophen-tBu2)(thf)]K(thf)2 unit
is shown in Figure 3 while a view of the coordination polymer
8 is shown in Figure 4. In 8, the uranium atom is seven
coordinated with regular pentagonal bipyramidal coordination
geometry. The four donor atoms of the salophen-tBu22- (O2N2)
and the oxygen of thf form a pentagonal plane (with a mean
deviation of 0.0308 Å). The ligand wraps around the uranium
adopting a Cs saddle shaped conformation. The axial positions
are occupied by two oxo ligands with UdO distances of
1.8534(17) and 1.8491(18) Å, falling in the range of the UdO
distances found in 3, 4 and 6. The U-Ophenolate distances (mean
value 2.39(4)Å) are longer and U-N distances (mean 2.59(2)
Å) are shorter in comparison with 3, 4 and 6 complexes. In
complex 8, both oxo groups are involved in a CCI with the
2-
These results suggest that the ligand salophen-tBu2 provides
a higher stability of the pentavalent complex with respect to
ligand dissociation and metal oxidation as compared to the salan-
2-
tBu2 . Finally, the addition of 1000 equiv of water to complex
7 leads to oxidation to the hexavalent analogue, but release of
free ligand is not observed, suggesting a high stability of this
complex with respect to ligand dissociation.
To understand the parameters leading to the stabilization of
pentavalent uranyl, we have investigated in detail the spectro-
scopic and magnetic properties of these complexes and per-
formed a computational study of the electronic structure.
Electronic Structure. The structures of the electronic ground
states of the complexes [UO2py5]+, [UO2(salan-tBu2)(py)K],
[UO2(salan-tBu2)(py)]-, [UO2(salan)(py)K] and [UO2(salophen-
Me2)(thf)K] were calculated using density functional theory
(DFT) approaches. All of the studied structures have been fully
optimized in gas phase and in two solvents used in the
+
K(thf)2 ion linking the uranium complexes into a 1D chain
with a mean K-O distance of 2.66(4) Å and a U · · ·K· · ·U angle
of 139.08(2)°. The K(thf)2+ ion also coordinates the phenolate
oxygen of one [UO2(Salophen-tBu2]- unit (K-O distance of
2.8778(18) Å). The distance between neighboring U ions is 7.63
Å and the U· · ·K· · ·U angle is 139.08°. This results in a smaller
separation between U ions than in the polymeric complex
{[UO2py5][KI2py2]}n (1) (9.35 Å) but larger in comparison with
[UO2(salan-tBu2)(py)K]n (3) (6.63 Å). The presence of a
pentavalent uranyl leads to a significant elongation of the mean
U-O(salophen-tBu2) (2.267(7)Å in U(VI) and 2.386(25) Å in
U(V)) and mean U-N(salophen-tBu2) (2.544(5) in U(VI) and
2.598(13) Å in U(V)) distances and of the UdO(oxo) bond
lengths (1.782(6) Å in U(VI) and 1.842(7) Å in U(V)) with
respect to the previously reported [UVIO2(salophen)(dmf)]
complex.15
Complex 7 shows full stability in all investigated organic
solvents pyridine, dmso, and toluene. These results confirm the
Figure 1. ORTEP view of 4 with thermal ellipsoids at the 30% probability
level (hydrogen atoms were omitted for clarity) and schematic representation
of the structure.
(34) Mizuoka, K.; Kim, S. Y.; Hasegawa, M.; Hoshi, T.; Uchiyama, G.;
Ikeda, Y. Inorg. Chem. 2003, 42, 1031–1038.
9
498 J. AM. CHEM. SOC. VOL. 132, NO. 2, 2010

Stable Complexes of Pentavalent Uranyl
A R T I C L E S
Figure 2. ORTEP view of 6 with thermal ellipsoids at the 30% probability level (hydrogen atoms were omitted for clarity) and schematic representation
of the structure.
Table 1. Selected Structural Parameters for 3, 4, 6 and 8 (Distances Are Given in Å, Angles in deg.)
t
structural parameters
[UO2(salan-tBu2)(py)K]n, 3
[UO2(salan-tBu2)(py)K(18C6)], 4
[UO2(salan-tBu2)(py)][Cp*2Co], 6
{[UO2(salophen- Bu2)(thf)]K(thf)2}n, 8
U-O(1U)
U-O(2U)
U-O(1)
1.819(12)
1.830(12)
2.323(11)
2.379(11)
2.715(16)
2.690(14)
2.618(16)
-
1.868(2)
1.853(2)
2.347(3)
2.332(3)
2.696(3)
2.714(3)
2.629(3)
-
1.846(9)
1.866(9)
2.388(10)
2.371(10)
2.773(12)
2.771(12)
2.700(11)
-
1.8529(18)
1.8497(18)
2.4115(18)
2.3602(18)
2.611(2)
U-O(2)
U-N(1)
U-N(2)
U-N(3)*
U-O(3)*
K(1)-O(1U)
O(1)-U-O(2)
2.586(2)
-
2.5673(18)
2.6303(19)
177.14(8)
2.663(13)
178.9(6)
2.651(2)
177.38(11)
-
178.7(4)
* Coordinated molecule of solvent (pyridine or thf).
Scheme 4
experiments (i.e., pyridine and toluene). In Table 2 we have
reported selected computed distances and angles and the
corresponding experimental data obtained by X-ray diffraction.
Since no significant differences were found between the
optimized structural parameters of gas phase and solvated
structures, in Table 2 we report only the geometrical parameters
obtained in pyridine solvent together with the experimental mean
distances (in italics in Table 2), while toluene and gas results
are given in the Supporting Information.
It should be underlined that the theoretical structures contain-
ing K+ are somewhat different from the experimental crystal-
lographic structures. For instance, the experimental structures
of [UO2(salan-tBu2)(py)K] and {[UO2(salophen-tBu2)(thf)]-
K(thf)2} are polymeric, so that each uranyl oxygen binds one
Figure 3. ORTEP view of the coordination environment of the uranyl group
in 8 with thermal ellipsoids at the 30% probability level (hydrogen atoms
and disorder on thf molecule were omitted for clarity).
Figure 4. Mercury view of the coordination polymer 8 along the c
axis.
9
J. AM. CHEM. SOC. VOL. 132, NO. 2, 2010 499

A R T I C L E S
Nocton et al.
Table 2. Geometrical Parameters Optimized at the PBE/TZP Level of Theory with Pyridine As Solventa
U-O2u
U-O1u
O2u -U-O1u
U-O1
U-O2
U-N1-2
U-L
+
UO2
1.80
179.5
179.7
178.2 (178.7)
178.0 (178.9)
178.3
[UO2py5]+
1.84 (1.839)
1.86 (1.856)
1.83 (1.825)
1.84
1.84 (1.839)
2.61 (2.614)
2.59 (2.700)
2.57 (2.618)
2.56
[UO2(salan-tBu2)(py)]-
[UO2(salan-tBu2)(py)K]
[UO2(salan)(py)K]
2.33 (2.38)
2.25 (2.351)
2.32
2.33 (2.38)
2.39 (2.351)
2.39
2.77 (2.772)
2.74 (2.698)
2.80
1.89
1.89
1.87
[UO2(salophen-Me2)(thf)K]
1.84(1.851)
178.7
2.30(2.39)
2.32
2.58(2.586)
2.57(2.567)
a In italic, experimental mean distances. The numbering of atoms is taken from Figures 1-3. L represents N(pyridine) for the five complexes
containing pyridine and O(THF) for the last one.
A very different picture is obtained for the salan-tBu2
complexes. As we can see from Figure 5, the MO diagram of
the complex [UO2(salan-tBu2)(py)K] in pyridine solvent exhibits
the “degenerate HOMOs” set (-2 eV) higher in energy with
respect to the [UO2(py)5]+ set (-5.8 eV). The unpaired alpha
electron is also in this case located in two “degenerate HOMOs”
orbitals each occupied by a half electron. These orbitals result
from the combination of one f (δ or π) orbital localized on the
metal center with a π* orbital of the pyridine. A significant
pyridine π* component (35%-40%) is observed, with a slight
variation of this contribution depending on the optimization
conditions (gas or solvent). The LUMO orbital is also a δ-type
and the first φ-type orbital is the LUMO+1, respectively, at
0.18 and 0.24 eV higher with respect to the HOMOs. Moreover,
the HOMO-1 orbital is not, as found for the [UO2py5]+ system,
the nonbonding σ orbital of the UO2+ (which is in this case the
HOMO-9). The four pairs of unrestricted orbitals located below
the HOMO level (identified by a red box on Figure 5) are
directly localized on the salan ligand (π-system of the phenolate
groups). The presence of a back-donation interaction between
the f orbitals of U and the π* orbital of the pyridine, which is
not present in the [UO2py5]+ species, is consistent with the slight
shortening (0.04-0.05 Å) of the bond distance between the
uranium and the pyridine ligand in the salan complexes
compared to those of the [UO2py5]+ complex. Although a
comparison of such structurally different compounds is not
straightforward, it should be noted that the steric parameters
and the charge of the metal ions in the two complexes should
lead to shorter U-Npy distances in the cationic [UO2py5]+
complex.
2-
potassium ion. In our theoretical model, we have used a model
structure where just one cation is linked to one of the uranyl
oxygens. This leads to a greater dissymmetry in our structures,
because the two oxygen atoms in the uranyl unit (U-O) are
not equivalent, as for the U-O distances involving the ligand
oxygen atoms. As can be seen in Table 2, except for these
distances, there is good overall agreement (within 0.01 Å and
1°) between the calculated and experimental structures.
To investigate the nature of the bonding between pentavalent
uranyl and the various ligands described in this work and to
gain more insight into the origins of the stability of these
complexes, we have performed an analysis of the Kohn-Sham
molecular orbitals (MO).
The electronic structure of the [UO2py5]+ cation arises from
an essentially electrostatic interaction between the pyridine
+
ligands acting as σ-donors and the UO2 cation. The MO
diagram in pyridine solvent is depicted in Figure 5. The
HOMO-1 is a uranyl nonbonding σ MO. Much lower in energy
are found the orbitals centered on the ligands. The unpaired
alpha electron issuing from the U(V) (5f1) electron doublet
configuration is described by an almost degenerate set of two
MOs being each occupied by half an electron. They are pure f
orbitals, one being of δ-type and the other of φ-type, by
reference to the labels in uranyl linear symmetry (a representa-
tion of the δ, φ and π in the uranyl linear symmetry is given in
Figure S26, Supporting Information). For the sake of simplicity,
this set of two orbitals will be called “degenerate HOMOs” in
the following descriptions. The first empty orbital, the LUMO,
is a π type f orbital localized on the uranium and is only 0.038
eV higher in energy. Such a picture is consistent with previous
multiconfigurational studies.35,36 For instance, in the case of
2-
The strong π-donor character of the salan-tBu2 ligand
[UO2(CO3)3]5-, the ground state σ2 δu was found only 403 cm-1
(described mainly by a bonding combination of π orbitals of
the O atoms with the uranyl MOs), combined with the
π-acceptor character of the pyridine ligand thus results in a
synergistic effect which stabilizes efficiently the whole edifice.
The important effect of this synergistic interaction is supported
by the experimental data showing a higher stability of the salan-
tBu22- complex in pyridine with respect to other non π-acceptor
solvents such as thf.
u
(0.049 eV) lower in energy than the first excited state σ2 φu.
u
For the isolated system UO2+, the ground state calculated
from multiconfigurational CASSCF studies is 88%σ2 φu
u
+12%σ2 δu.37 In the monodeterminantal DFT scheme, the only
u
way to take into account the almost degenerate nature of the
σ2 φu and σ2 δu configurations is to allow a fractional occupation
u
u
of the f(δ) and f(φ) orbitals. The energy of these “degenerated
HOMOs” obtained from the gas phase calculation and from the
calculation in pyridine solvent are respectively -0.5 eV and
-5.8 eV. It must be underlined that in both cases the unpaired
electron is in a bound state. Nevertheless, the effect of the
solvent, here described by a dielectric polarizable medium, on
the energy of the MO is very strong for charged species (anions
or cations), and all the levels are shifted down to more negative
values with respect to the corresponding gas phase MO.
The participation of 5f orbitals to covalent metal-ligand
interactions in U(V) species has been previously reported for
organometallic complexes.38 Notably, DFT studies of urani-
um(V) imido halide complexes indicated that both U-halide and
UdN bonds exhibit covalency with significant participation of
the ligand orbitals to the HOMO which contains the f1 electron.38
Another effect of the interaction of the salan-tBu2 ligand
2-
with the uranyl group is a slight decrease of the UdO bond
strength with respect to the pure UO2+ fragment. The computed
(35) Ruiperez, F.; Danilo, C.; Real, F.; Flament, J. P.; Vallet, V.; Wahlgren,
U. J. Phys. Chem. A 2009, 113, 1420–1428.
(36) Gagliardi, L.; Grenthe, I.; Roos, B. O. Inorg. Chem. 2001, 40, 2976–
2978.
(38) 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.
(37) Gagliardi, L.; Roos, B. O.; Malmqvist, P. A.; Dyke, J. M. J. Phys.
Chem. A 2001, 105, 10602–10606.
9
500 J. AM. CHEM. SOC. VOL. 132, NO. 2, 2010

Stable Complexes of Pentavalent Uranyl
A R T I C L E S
Figure 5. MO diagram of [UO2py5]+ [UO2(salan-tBu2)(py)K] and [UO2(salophen-Me2)(thf)K] complexes. For each complex, the HOMOs (a, c e) and the
σ nonbonding uranyl MO are represented (b, d and f). The φ, δ, σ and π labels on the graph refer to the symmetry of the f orbitals of the linear uranyl unit.
U-O distances for the isolated uranyl units in the gas phase
are 1.77 Å while in the salan-tBu2 complexes they are between
1.83 and 1.84 Å. Similar distances are found for the [UO2py5]+
system. These results are consistent with previous computational
studies which have afforded a U-O bond length of 1.742 Å at
MRCI level for the isolated UO2+ and 1.898 Å (B3LYP-CPCM)
for the U-O distance in the [UO2(CO3)3]5- system.35
The presence of the potassium ion leads to some variation in
the bond lengths with respect to the anionic structure [UO2(salan-
tBu2)(py)]-. Moreover, the potassium ion has an important effect
on the energy of the “degenerate HOMOs” set, which we have
further investigated. Notably, according to the Koopman’s
theorem, the energy of the HOMO allows a good estimation of
the absolute value of the ionization potential (true in DFT if an
exact echange/correlation potential has been used).39 The
optimization of the gas phase anionic structure results in a
positive orbital energy of around 1 eV for the degenerate
HOMOs. This suggests that the unpaired electron is not bonded
to the molecule and that uranyl(V) can easily oxidize to U(VI).
In the presence of K+, all of the levels are shifted down and
the “degenerate HOMOs” have energies of around -1.6 eV.
When the K+ ion is placed at the position of the Co ion in the
Cp*2Co structure (thus with a K-O1u distance of 6.8 Å), the
energy of the “degenerate HOMOs” increases (-0.5 eV). In
solvent, we observe the same trend (i.e., increase of the HOMO
(39) Koch, W.; Holthausen, M. C. A Chemist’s Guide to Density Functional
Theory; Wiley VCH: Weinheim, 2000.
9
J. AM. CHEM. SOC. VOL. 132, NO. 2, 2010 501

A R T I C L E S
Nocton et al.
energy with the increase of the K-O1u distance), but the effect
is much weaker. Indeed, as we discussed previously, the
dielectric medium stabilizes the charged system, so that the
energy of the “degenerate HOMOs” is -1.7 eV for the anionic
form and -2 eV with K+. By varying the position of the K+
ion to the one corresponding to Co in the Cp*2Co compound,
we obtained an energy of -1.8 eV.
the salophen-tBu22- (-1.65 V vs Fc+/Fc) complex with respect
2-
to the salan-tBu2 one (-1.74 V vs Fc+/Fc). This description
is also in agreement with the calculated and experimental bond
distances (Table 2), which show a shortening in the U-Nligand
bonds (calcd ) 2.58 Å, experimental ) 2.59 Å) with respect
2-
to the salan-tBu2 one (calcd ) 2.74 Å, experimental ) 2.69
Å). Thus, in this case the two synergistic interactions (π donation
from the phenolate oxygen atoms combined with a π-back-
bonding interaction with the CdN double bonds) are localized
The cyclic voltammetry (see electrochemistry section and
Table 4) of the [UO2(salan-tBu2)(py)K]n, 3, [UO2(salan-
tBu2)(py)K(18C6)], 4, and [UO2(salan-tBu2)(py)], 5 complexes
show that in all cases the same U(VI) species are obtained in
the oxidation process. So, in this particular case, we can directly
2-
on the salophen-Me2 ligand. This is in agreement with the
observed stability of the salophen-tBu2 complex in all solvents
and with higher resistance to hydrolysis.
+
correlate the redox potential of the UO22+/UO2 couple in all
A fragment analysis was performed to compute the energy
of the bonding between the uranyl dioxo unit and the ligand
(see Figure S13, Supporting Information), which shows that in
complexes (from -1.82 to -1.74 V vs Fc+/Fc) to the computed
ionization energy of the unpaired electron in the UO2+species.
We can observe that when the effect of the cation is reduced
by the presence of bound 18C6, the redox potential of the
2-
the gas phase the salophen-Me2 is -1.2 eV more strongly
bound to the uranyl than the salan-tBu22-. In pyridine, the
difference is -0.8 eV. This should results in a stronger stability
of [UO2(salophen-Me2)K] compared to [UO2(salan-tBu2)(py)K]
with respect to the ligand dissociation reaction.
+
UO22+/UO2 couple (-1.81 V vs Fc+/Fc) is very close to the
one found in the absence of potassium (-1.82 V vs Fc+/Fc for
complex 5). The reduction potential increases to -1.74 V vs
Fc+/Fc when the K+ is bounded directly to the structure. This
is consistent with the increase of the ionization potential in going
from structure 5 to 3. These results clearly point to a significant
electronic effect of the potassium counterion in the stabilization
of the unpaired electron which leads to an increase of the redox
The qualitative assignment of the magnetic data discussed
below is clearly pointing to δ, φ as the ground state. It should
be noted that in our DFT studies we have used a scalar
relativistic approach at 0 K. This approach does not include
spin-orbit effects or the probable populations of nearly occupied
excited states at higher temperature. Therefore, since the sets
of δ, φ and π type of f orbitals are close in energy, the
description of the exact ordering of these orbitals would need
more sophisticated multiconfigurational and spin-orbit ap-
proaches. Nevertheless, the latter effects will not change the
geometry or the bonding properties of the uranyl complexes
that we have obtained by DFT. What should be retained is that
the energy of the π orbital in the salophen-tBu22- and the salan-
potential of the UO22+/UO2 couple.
We have previously reported that the dianionic salan-Me2
+
2-
(salan-Me2H2 ) N,N′-bis(2-hydroxybenzyl-3,5-dimethyl)-1,2-
dimethylaminomethane) ligand in pyridine leads to the formation
+
of the UO2 complex of salan-Me2, which rapidly dispropor-
tionates to U(IV) and to UO22+ species.30 The different reactivity
observed in pyridine solution for the pentavalent uranyl
2-
2-
complexes of salan-Me2 and salan-tBu2 was interpreted in
terms of the importance of the steric profile of the tert-butyl
groups in preventing disproportionation.30
2-
tBu2 complexes is significantly lower than in the [UO2py5]+
as a result of the presence of significant back-bonding interaction.
To evaluate the effect of the substituents on the phenolate
ring on the electronic structure of the resulting pentavalent
uranyl complex, we have also studied the structure of [UO2(salan-
H2)(py)K]. From a geometrical point of view, there are no
significant variations in the bond lengths (see Table 2).
Moreover, the electronic structure is very similar to the one
Electronic Spectroscopy. The electronic absorption spectra
of the complexes 2-7 and 9 have been measured in pyridine
solution at room temperature and the data are reported in Table
3. The window of measure was limited to 300-1500 nm
because of the pyridine absorption. The assignment of the
electronic absorption spectra is not obvious due to the scarcity
of experimental and theoretical data in the literature. Spectral
studies were reported in dmso for the electrochemically
produced mononuclear complexes [UO2(salophen)(dmso)]-
(with bands at 640, 740, 860, 1470, 1890 nm), [UO2(dbm)2-
(dmso)]- (with bands at 650, 750, 900, 1400, 1875 nm) and
for [UO2(CO3)2]5- (with bands at 760, 990, 1140, 1600, and
1800 nm) in D2O.18,34,40 In these compounds the spectral
features are very similar to each other in spite of the presence
of different ligands coordinated on the equatorial plane and
therefore the electronic transitions were assigned to electronic
transitions in the UO2+ core. The results of theoretical calcula-
tions recently performed on these complexes lead the authors
to the same conclusion that the observed transitions correspond
t
found for the [UO2(salan-tBu2)(py)K] complex. As for the Bu
structure, the “degenerate HOMOs” consists in two orbitals, with
eigenvalue of -2 eV, either a f(δ) orbital or f(π), each one
combined with a significant pyridine component of π*. Thus, the
ligand substituents, H or tBu, have no influence on the electronic
structure, suggesting that the higher stability of the [UO2(salan-
tBu2)(py)K] complex in pyridine with respect to the dispropor-
tionation reaction is only due to sterical parameters.
Finally, we have analyzed the electronic structure of the Schiff
base complex [UO2(salophen-Me2)(thf)K]. The main optimized
geometrical parameters are collected in Table 2 and the MO
diagram is depicted in Figure 5. The HOMO describing the
unpaired alpha electron is no longer degenerate. The HOMO is
a combination of an f(δ) orbital on U and a π* orbital (50%)
on the NdC Schiff base of the salophen-Me2 ligand (Figure 5).
The empty orbitals show the following ordering: the LUMO is
again a f(δ) (0.17 eV higher than the HOMO), the LUMO+1
is the first φ-type orbital (+0.32 eV), and the f(π) is the
LUMO+2 (+0.36 eV). The energy of the HOMO (-2.7 eV in
gas phase, -2.87 eV in toluene and -3.11 eV in pyridine) is
to transitions in the UO2 core.35
+
The spectra of complexes 5 and 9 are consistent with the
presence of a hexavalent uranyl and show characteristic bands
that could be assigned to LMCT transition and πf π* of the
ligand. In the pentavalent uranyl complexes, 1-4, 6 and 7, the
bands occurring at wavelengths higher than 500 nm are
2-
lower than the one of the salan-tBu2 ligand (see Figure 5).
(40) Mizuguchi, K.; Park, Y. Y.; Tomiyasu, H.; Ikeda, Y. J. Nucl. Sci.
Technol. 1993, 30, 542–548.
This is consistent with the higher redox potential measured for
9
502 J. AM. CHEM. SOC. VOL. 132, NO. 2, 2010

Stable Complexes of Pentavalent Uranyl
A R T I C L E S
Table 3. UV-Visible, NIR Data for Complexes 2-7 and 9 in Pyridine Solution
U(VI)O22+ complexes
U(V)O2+ complexes
complex
λ (nm)
ε (cm-1 M-1
)
complex
λ (nm)
ε (cm-1 M-1
)
[UO2(salan-tBu2)(py)], 5
390
480
357
432
3800
3170
32000
17400
{[UO2py5][KI2py2]}n,
[UO2py5]I·py, 2
1
456
456
610
741
504
676
850
513
676
505
423
632
726
853
1431
905
1050
110
63
100
25
5
167
75
[UO2(salophen-tBu2)(py)], 9
[UO2(salan-tBu2)(py)K]n,
3
[UO2(salan-tBu2)(py)K(18C6)], 4
[UO2(salan-tBu2)(py)][Cp*2Co], 6
[UO2(salophen-tBu2)(py)K], 7
20000
200
314
363
170
consistent with f-f transitions and with the presence of a 5f1
configuration while the bands below 500 nm can be assigned
to LMCT transition or to πf π* of the ligand. The πf π*
Table 4. Summary of Redox Potential Data for Complexes 2-5, 7
and 9 in ∼0.1 M [NBu4][PF6]/Pyridine Solution at Room
Temperaturea
2-
transition of the salophen-tBu2 ligand in equatorial position
complex
E1/2 (UVI/UV) V)
∆Ep,100mV/s (V)
is located for 7 and 9 in the range 423-432 nm. The absorption
spectrum of the complex 7 shows the expected bands assigned
to f-f transitions and the observed ε values and peak position
are comparable to those previously describes by Ikeda for the
[UO2(salophen)(dmso)]- complex in dmso solution.18 The
absorption spectra of the complexes 3 and 4 are very similar
with close peak positions (∼505 nm, ∼675 nm) and very similar
ε values in agreement with the expected presence of analogous
structures in solution. The band at ∼850 nm could not be
observed for 4 because of the very small ε. These bands were
assigned to Laporte forbidden f-f transitions. The spectrum of
6 presents an additional band at ∼505 nm but the ε value was
difficult to determinate because of the presence of several species
in solution which contribute to the global absorbance. However,
[UO2py5]I·py, 2
-0.91, -0.46
-1.74
[UO2(salan-tBu2)(py)K]n,
3
0.13
0.14
0.18
0.13
0.09
[UO2(salan-tBu2)(py)K(18C6)], 4
[UO2(salan-tBu2)(py)], 5
-1.81
-1.82
[UO2(salophen-tBu2)(py)K], 7
[UO2(salophen-tBu2)(py)], 9
-1.65
-1.67
a Potential values are given vs the Fc+/Fc couple.
+
the shape of the spectrum confirms the presence of UO2
complex as the major product in pyridine solution. Interestingly,
the ε values for the complexes 3 and 4 are smaller than those
of 7 in spite of the similar symmetry of the coordination
geometry of the solution species (pentagonal bipyramid). This
could be explained by a strong overlap of the metal and the
ligand orbitals in the case of 7 which partially allows the
transition. This is in agreement with the computed electronic
structure of 7. The spectra of 1 and 2 show a band at 456 nm
which is difficult to assign because the ε value is higher than
those interpreted as Laporte forbidden f-f transitions but could
be interpreted as a charge-transfer band. The two other bands
observed in the case of 2 (610 and 741 nm) can be assigned as
f-f transitions. Theoretical computation will be needed to
Figure 6. Room temperature cyclic voltamogramm for 3 and 5 in pyridine
at 100 mV/s (0.1 M [NBu4][PF6] as supporting electrolyte).
in the range of the UO22+/UO2+ potential measured in anhydrous
solutions (-1.3 V/-1.82 V, vs Fc+/Fc).20,41 To evaluate the
influence of a coordinated countercation, in this case the
potassium ion, the redox properties of 5 (E1/2 ) -1.82 V vs
Fc+/Fc) were also studied in the same conditions. The small
but significant differences in term of potential (Figure 6) between
the two complexes indicates that the potassium does have an
influence even in the presence of a large excess of the [NBu4]+
cation. These results indicate that the coordination of the
potassium in pyridine solution results in the stabilization of the
uranyl(V) complex with respect to the oxidation reaction. This
compares well with the NMR experiments and DFT calculations.
No significant stabilizing effect was found in the presence of
the less coordinating cation (K18C6)+.
confirm this assignment.
2+
Electrochemistry. The values of the redox potentials UO2
/
+
UO2 couple provide a direct measure of the stability of the
isolated pentavalent complexes with respect to the oxidation
reaction to the hexavalent analogue. Therefore the influence of
different parameters as the structure of the supporting ligand
or the presence of coordinating cations on the stability of the
+
UO2 species can be directly probed by cyclic voltammetry
experiments. The solution redox properties of the complexes
2-5, 7 and 9 have been studied by cyclic voltammetry in
pyridine solution and the respective values of the E1/2 (V) of
the couple vs the couple Fc+/Fc are listed in Table 4.
The cyclic voltammogram of the complex 3 reveals a nearly
(41) Hayton, T. W.; Boncella, J. M.; Scott, B. L.; Batista, E. R.; Hay, P. J.
reversible redox feature at E1/2 ) -1.74 V vs Fc+/Fc. This is
J. Am. Chem. Soc. 2006, 128, 10549–10559.
9
J. AM. CHEM. SOC. VOL. 132, NO. 2, 2010 503

A R T I C L E S
Nocton et al.
examples27,28 which include our report of complexes presenting
+
an unambiguous UO2+ · · ·UO2 interaction. The solid state
temperature-dependent magnetic susceptibility of complex 1 had
been also described28 (see Figure S22, Supporting Information)
and was found to be very similar to that of the dinuclear
macrocyclic complex presenting a UO2+ · · · Zn interaction
reported by P. Arnold et al.27 Temperature-dependent magnetic
susceptibility data were collected for the isolated complexes 4
and 7 from 5 to 300K and are given in Figure S22, Supporting
Information. Samples of 4 and 7 display effective magnetic
moments that vary with temperature reaching an effective
magnetic moment of respectively 2.20 µB and 2.23 µB at 300
K. The effective moments in pyridine solution (at 300K) were
also measured for 4 and 7 by the Evans method50 and were
found to be in agreement with those measured in the solid state
(i.e., respectively µeff ) 2.25(10) µB and µeff ) 2.14(10) µB). The
very low solubility of 1 in pyridine renders it impossible to mea-
sure the effective moment by the same method. However, the
effective moment in solution was measured for the soluble complex
2 and was found to be µeff ) 2.6(1) µB. This is in agreement with
the value measured for 1 in the solid state (µeff ) 2.57 µB).33 While
the theoretical effective magnetic moment calculated for a free 5f1
ion in the L-S coupling scheme is µeff ) 2.54 µB51 the range of
the effective magnetic moment values reported for U(V) complexes
is large (1.42-2.57 µB)27,38,45-47,49,52-55 indicating that the
coordination sphere has an important influence on the
magnetic moment. Important differences are observed be-
tween the magnetic data of complexes 4 and 7 and those of
complex 1 and 2. Notably complex 1 in the solid state and
complex 2 in solution show higher values of the effective
magnetic moment at room temperature (µeff ) 2.57 µB and
µeff ) 2.6(1) µB, respectively) which are very close to the
theoretical value for an isolated J ) 5/2 multiplet.
Figure 7. Room temperature cyclic voltammogram for 2 in pyridine at 10
mV/s (0.1 M [NBu4][PF6] as supporting electrolyte, * impurity).
The cyclic voltammograms of the salophen-tBu22- complexes
7 and 9 reveal reversible redox features at similar E1/2 values
(-1.65 and -1.67 V vs Fc+/Fc respectively) without significant
effect of the potassium coordination being observed for this
system. These values are lower than those measured for the
complex [UO2(salophen)(dmso)] (-1.55 V vs Fc+/Fc) in
dmso.34 The difference can be attributed to the difference in
the coordinated solvent (pyridine or dmso) or/and to the presence
of tert-butyl groups. The ∆E1/2 difference between the salophen-
tBu22- and the salan-tBu22- complexes (0.15 V) unambiguously
+
indicates a higher stability of the UO2 complex of salophen-
tBu2 with respect to the oxidation reaction.
2-
The cyclic voltammogram of the complex 2 (Figure 7) is more
complicated and reveals two redox features at Ep,a ) -0.93 V
vs Fc+/Fc and Ep,a ) -0.46 V vs Fc+/Fc. The presence of two
+
features suggests the presence of two UO2 species in the
Plots of the magnetic µeff data vs T (Figure S22, Supporting
Information) decreases for complexes 4 and 7 with decreasing
temperature as it was observed for 1.28 However, a significant
difference is observed in the low-temperature susceptibilities
(<50 K). The results of the DFT calculation indicate that for
both complexes 4 and 7, the HOMO (SOMO) is an f-π*
admixture whereas for 1 the SOMO is a pure f orbital. This
might indicate that the value of the effective moment is
influenced by the nature of the SOMO.
EPR measurements can provide important information on the
nature of metal-ligand bonding but only a very limited number
of EPR data have been reported for U(V) compounds.48,49,54,55,56
In addition, several uranium(V)imido complexes have been
reported to be EPR silent due to their µ ) (3/2 crystal-field
ground state.45,47,52,54 Gourier and co-workers48 concluded from
a detailed comparative EPR study of the electronic structure of
pyridine solution. The ∆Ep are respectively 0.16 and 0.23 V at
10 mV/s and increase rapidly with increasing the scan speed
suggesting a slow kinetics of the electron transfer which are
usually associated with structural rearrangements.11 In this case,
the presence of an exchange in pyridine solution between species
with or without coordinated iodide could be responsible for the
observed behavior. The values of the UO22+/UO2+ potential were
found to be much higher than those measured for the complexes
3, 4 and 7 in agreement with the DFT calculation. This
difference in redox potential is likely to play a role in the
observed stability in pyridine solution of 1 and 2, which do not
present bulky groups preventing the disproportionation through
a cation-cation complex formation.
Magnetic Properties. The fundamental understanding of the
molecular parameters governing magnetic exchange interactions
and electron delocalization in molecular compounds of actinides
remain very limited.42-44 The 5f1 ions are particularly useful
for the interpretation of magnetic data because electron repulsion
is absent. However the scarcity of reported U(V) complexes
has limited the number of magnetic studies on this oxidation
state to simple salts and few organometallic complexes.38,42,45-49
(47) Castro-Rodriguez, I.; Olsen, K.; Gantzel, P.; Meyer, K. J. Am. Chem.
Soc. 2003, 125, 4565–4571.
(48) Gourier, D.; Caurant, D.; Berthet, J. C.; Boisson, C.; Ephritikhine,
M. Inorg. Chem. 1997, 36, 5931–5936.
(49) Bart, S. C.; Anthon, C.; Heinemann, F. W.; Bill, E.; Edelstein, N. M.;
Meyer, K. J. Am. Chem. Soc. 2008, 130, 12536–12546.
(50) Evans, D. F. J. Am. Chem. Soc. 1959, 2003–2005.
(51) Boudreaux, E. A.; Mulay, L. N. Theory and Applications of Molecular
Paramagnetism; Wiley: New York, 1976.
+
The magnetic data for UO2 complexes are limited to three
(42) Edelstein, N. M.; Lander, G. H. Magnetic properties; Springer:
Dordrecht, 2006.
(52) Brennan, J. G.; Andersen, R. A. J. Am. Chem. Soc. 1985, 107, 514–
516.
(43) Clark, D. L.; Hecker, S. S.; Jarvinen, G. D.; Neu, M. P. The Chemistry
of the Actinide and Transactinide Elements; Springer: Dordrecht, 2006.
(44) Schelter, E. J.; Morris, D. E.; Scott, B. L.; Thompson, J. D.; Kiplinger,
J. L. Inorg. Chem. 2007, 46, 5528–5536.
(53) Castro-Rodriguez, I.; Nakai, H.; Meyer, K. Angew. Chem., Int. Ed.
2006, 45, 2389–2392.
(54) Meyer, K.; Mindiola, D. J.; Baker, T. A.; Davis, W. M.; Cummins,
C. C. Angew. Chem., Int. Ed. 2000, 39, 3063–3066.
(55) Gourier, D.; Caurant, D.; Arliguie, T.; Ephritikhine, M. J. Am. Chem.
Soc. 1998, 120, 6084–6092.
(45) Rosen, R. K.; Andersen, R. A.; Edelstein, N. M. J. Am. Chem. Soc.
1990, 112, 4588–4590.
(46) Selbin, J.; Ortego, J. D. Chem. ReV. 1969, 69, 657–671.
9
504 J. AM. CHEM. SOC. VOL. 132, NO. 2, 2010

Stable Complexes of Pentavalent Uranyl
A R T I C L E S
Table 5. Summary of Magnetic Data
complex
µeff (300K)
µeff (6K)
EPR data (4K)
{[UO2py5][KI2py2]}n,
1
2.57
0.80
-
No signal in the solid state No signal in dilute solution (5 × 10-4M)
[UO2py5]I·py, 2
2.6(1)b
No signal
[UO2(salan-tBu2)(py)K]n,
3
2.35b
-
gx ) 1.94; gy ) 0.98; gz ) 0.75
gx ) 1.98; gy ) 1.25; gz ) 0.74
Very broad signal (no fit possible)
g ) 2.2 (5K in frozen methyl THF sol.)
[UO2(salan-tBu2)(py)K(18C6)], 4
[UO2(salophen-tBu2)(py)K], 7
[UO(OSiMe3)(thf)Zn2I2(L)] (ref.27
2.20a 2.25(10)b
2.23a 2.14(10)b
2.38a
1.05
0.72
)
a Solid state. b Solution.
organouranium(V) amide and alkoxide compounds that in amide
complexes the 5f orbital is essentially non bonding but a
covalent character appears for the interaction of 5f orbitals with
oxygen ligands. To obtain more details of the electronic structure
of the complexes 1, 3, 4 and 7, X-band EPR spectroscopy was
performed at low temperature. The results of EPR measurements
are summarized in Table 5 and spectra are shown in the
Supporting Information.
+
Recent theoretical calculations for the gas phase UO2 ion
including spin-orbit coupling show that the six lowest energy
states are derived from the 5fδu, 5fφu, and 5fπu orbitals, well-
separated from the next higher lying state corresponding to the
first oxygen to uranium charge transfer state.31 Further calcula-
tions (including spin-orbit coupling) for the [UO2(CO3)3]5-
complex indicate that the first four states in the complex also
correspond to states that come from the nonbonding 5fδ and
5fφ orbitals in the UO2+ free ion. Interestingly, these calculations
show a significant decrease in the energy of the πu* orbital in
the complex compared to the free gas phase UO2+ ion, (19049
cm-1) to 7467 cm-1 (complex, gas phase) or 7444 cm-1
(complex, solution), attributed in this paper to much lower
oxygen 2p character from the yl-oxygen resulting in a less
antibonding π* orbital. Our calculations discussed earlier suggest
extensive π bonding between the pyridine molecule and
Figure 8. Schematic energy level diagram of the 5f orbitals for the
uranyl(V) free ion. It is assumed the axial crystal field is the strongest
interaction. Column A shows the relative energies of 5f orbitals. The 5fσ
antibonding orbital is not shown as it is shifted to much higher energy.
Column B gives the relative energies when spin-orbit coupling is included.
2-
uranyl(V) orbitals in the salan-tBu2 complex.
As discussed above, the four lowest states are derived from
the nonbonding 5f φ and δ orbitals and it is these states which
determine the measured magnetic properties of the uranyl(V)
complexes. Inclusion of the π* orbital within a few hundred
cm-1 of the ground state would not markedly affect the model
calculation that follows. Thus crystal field theory can be utilized
to calculate, at least qualitatively, the magnetic properties of
uranyl(V) complexes for the temperature range from 0 to 300
2
The levels in B are each doubly degenerate, a linear combination of F5/2
2
and F7/2 basis states, and labeled by the Jz quantum number.
ion in the equatorial plane is at least C2, which means that there
are B22,B24,B44,B26,B64,B66 terms in the crystal field potential. As-
suming the δ and φ states are relatively close, then only the
fourth order terms, B44 and B46, could mix the Jz states shown in
column B in Figure 8, to lead to a nonzero value for g⊥. In the,
Supporting Information we present a qualitative calculation for
the g values vs the relative energies of the δ and φ states.
Qualitatively, the EPR data for 3, 4, and 7 can be ap-
proximately fit with this simple model when g| ≈ 2, g⊥ ≈ 1.3
(assuming g| ) |g1|, and g⊥ ≈ (|g2 + g3|)/2) and (ꢀδ - ꢀφ) )
-1435 cm-1, see, Supporting Information). It is interesting to
note that the EPR data for [UO(OSiMe3)(thf)Zn2I2(L)]27 can
also be qualitatively fit with this model if we assume their
measured g value of 2.20 is g1 (or g|), and that g2 and g3 are
too broad to be determined. The temperature dependence of the
magnetic susceptibility data also suggest that one or more
excited states are relatively low-lying, that is within a few
K, corresponding to an energy range of 0 to ∼200 cm-1
.
Equatorial coordination of the uranyl(V) ion introduces an
equatorial crystal field which could mix the Jz ) (5/2 and Jz
) (3/2 states (see Figure 8). The UO2+ ion in 1 and 2 has five
pyridine molecules surrounding the U ion in the equatorial plane
and the five nearest neighbor nitrogen atoms are in the equatorial
plane. Assuming that these nearest neighbor nitrogen atoms are
the major contribution to the equatorial crystal field potential
the symmetry of this potential is D5h. When spin-orbit coupling
is included the low-lying free ion δ and φ states are mixed and
the “good” quantum number in the SLJJz representation is (
Jz. This is shown schematically in Figure 8. For |g⊥| > 0, the
doubly degenerate ground state wave function must contain a
mixture of Jz values that differ by (1. In an equatorial field of
D5h symmetry this mixture of Jz states is not possible so g⊥ )
hundred cm-1
.
Conclusions
2
0. Since the intensity of the EPR signal is proportional to g⊥ ,
there should be no signal observed for 1 and 2.
For compounds 3, 4, and 7 listed in Table 5, again considering
only the nearest neighbors, the site symmetry at the uranium
In this work, we have presented the synthesis, magnetic and
redox properties, reactivity and the DFT analysis of the
electronic structure and bonding for a series of stable complexes
of pentavalent uranyl containing ligands with very different
steric and electronic properties. The cationic complex [UO2py5]+
(56) Edelstein, N.; Brown, D.; Whittaker, B. Inorg. Chem. 1974, 13, 563.
9
J. AM. CHEM. SOC. VOL. 132, NO. 2, 2010 505

A R T I C L E S
Nocton et al.
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 preliminary heated
at 200 °C. Depleted uranium turnings were purchased from the
“Socie´te´ Industrielle du Combustible Nucle´aire” of Annecy (France).
Pyridine N-oxide and 18-crown-6 (18C6) were purchased from
Aldrich and dried under vacuum at 50 °C for 7 days. Elemental
analyses were performed under argon by Analytische Laboratorien
GMBH at Lindlar, Germany. FTIR spectra were recorded with a
Perkin-Elmer 1600 Series FTIR spectrophotometer. UV-visible
measurements were carried out with a Varian Cary 50 Probe
spectrophotometer and a Lambda 9 Perkin Elemer spectrophotom-
eter 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 MERCURY 400 MHz and Bruker 200 MHz spec-
trometers. NMR chemical shifts are reported in ppm with solvent
as internal reference.
Methods. 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
has in the equatorial plane five weakly bound pyridine ligands
acting as σ-donors only, with the f unpaired electron localized
in a isolated metal orbital. The high stability of this complex is
probably the result of the high redox potential that makes the
oxidation of this species to the hexavalent analogue in pyridine
solution difficult. The high stability of this species in pyridine
disfavors the disproportionation reaction even in the absence
of bulky ligands. The weakly associated pyridine ligands can
be easily replaced by π-donor ligands such as the amino-
phenolate salan ligands and the Schiff-base salophen ligands.
However, these ligands are not able to stabilize pentavalent
uranyl with respect to the disproportionation reaction in the
absence of bulky substituents which can disfavor the formation
of dimeric intermediates. As a result, the reaction of
{[UO2py5][KI2py2]}n with salophen2- or with salan-Me22- leads
to unstable species which rapidly disproportionate to form U(IV)
2+
and UO2 species. Conversely the highly stable complexes
[UO2(salan-tBu2)(py)K] and [UO2(salophen-tBu2)(py)K] can be
prepared in pyridine solution. The magnetic and the EPR data
can be qualitatively analyzed with a simple crystal field model
where the f electron has a nonbonding character. However, the
DFT studies show unambiguously the presence of a significant
covalent contribution to the metal-ligand interaction in these
complexes leading to a significant lowering of the πu*. These
two results are not contradictory because the presence of π*
orbital within a few hundred cm-1 of the ground state would
not markedly affect the model calculation used to analyze the
magnetic data. Moreover, the temperature dependence of the
magnetic susceptibility data also suggests that one or more
excited states are relatively low-lying, that is within a few
hundred cm-1. The presence of the back-bonding interaction is
likely to play a role in the stabilization of pentavalent uranyl
both with respect to the disproportionation reaction and to the
hydrolysis reaction. Notably, in spite of the presence of ligands
with similar steric hindrance, the stability of the salan-tBu2
complex with respect to the disproportionation is reduced in
solvents other than pyridine, but the salophen-tBu2 complex is
fully stable in all organic solvents. Moreover, the [UO2(salophen-
tBu2)(py)K] complex shows a higher stability than [UO2(salan-
tBu2)(py)K] with respect to the oxidation reaction as evidenced
by its higher redox potential and a much higher stability with
respect to the reaction of ligand dissociation promoted by
stoichiometric amounts of water. Finally, cations which can
coordinate the yl-oxygens of the pentavalent uranyl moiety play
also an important role on the overall electronic structure and
stability of the complex. This is clearly shown by the DFT
studies and by the measured redox potential which is signifi-
cantly decreased for the salan-tBu2 complexes in the absence
of the coordinating potassium cation. These results provide an
unprecedented complete description of the structure and bonding
in stable pentavalent uranyl complexes. The overall picture
indicates that complexes of pentavalent uranyl fully stable in
organic solution and resistant to hydrolysis can be prepared by
a careful tuning of both steric and electronic properties of the
supporting ligand coupled to an appropriate choice of counter-
ions. These results should promote the expansion of the
coordination chemistry of pentavalent uranyl and suggest that
a careful ligand design might also lead to higher stability with
respect to hydrolysis.
constants. At 300 K, we calculated a µeff of 2.20 µB for 4 (ꢀdia
)
-7.68 × 10-4 emu/mol, m ) 25.81 mg, M ) 2037.08 g/mol). At
300 K, we calculated a µeff of 2.23 µB for 7 (ꢀdia ) -5.46 × 10-4
emu/mol, m ) 15.60 mg, M ) 1147.78 g/mol).
Cyclic voltammetry experiments were performed using a PAR
273 potentiostat. All experiments were performed in a glovebox.
The working electrode consisted of a platinum disk embedded in
PTFE (1 mm diameter or 2.5 mm diameter), a Pt counterelectrode,
and an Ag/AgCl reference electrode. Solutions employed during
cyclic voltammetry studies were typically 2.5 mM in the uranium
complex and 0.1 M in [Bu4N][PF6]. All potentials are reported
versus the [Cp2Fe]0/+ couple.
The EPR measurements were performed in quartz tubes with
J.Young valves. EPR spectra at X-band were recorded with an EMX
Bruker spectrometer fitted with an OXFORD Instrument ESR900
cryostat. All spectra were recorded under unsaturated conditions
with the following set of parameters: T ) 4K, P ) 3.17 µW,
amplitude modulation 9G, frequency modulation 100 kHz.
Computational Methods. The theoretical study of uranium
complexes has been performed with the ADF2007 DFT code.57-59
A GGA functional (namely PBE) has been chosen combined with
Triple-ꢁ Slater type orbital (STO) valence basis sets. The core
density was generated for all atoms from four-components
Dirac-Slater calculations and then kept frozen in molecular
computations. The valence space includes the 6s, 6p, 6d, 5f, 7s
orbitals plus one 7p polarization function for U and 2s, 2p orbitals
plus 3d polarization function for K, N, C and O. The corresponding
auxiliary sets of STO functions were used for all the atoms to fit
the molecular density and to generate the Coulomb and exchange
potentials. The scalar relativistic corrections were included in the
valence space via the ZORA formalism. All calculations have been
performed in an unrestricted formalism for the ground doublet state
of U(V). Solvent effects have been computed with a Polarizable
(57) Velde, G. T.; Bickelhaupt, F. M.; Baerends, E. J.; Guerra, C. F.; Van
Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001,
22, 931–967.
Experimental Section
(58) Boerrigter, P. M.; Velde, G. T.; Baerends, E. J. Int. J. Quantum Chem.
1988, 33, 87–113.
General Procedures. All manipulations were carried out under
an inert argon atmosphere using Schlenk techniques and an MBraun
(59) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 41–51.
9
506 J. AM. CHEM. SOC. VOL. 132, NO. 2, 2010

Stable Complexes of Pentavalent Uranyl
A R T I C L E S
Continuum Model using the COSMO routine in ADF and molecular
orbital pictures obtained by the ADF graphical user interface. The
description of the M-ligand bonding was achieved by the symmetry
fragment-based construction of KS molecular orbitals (MO) in ADF:
the final MO’s are built as linear combinations of symmetrized-
fragment orbitals (SFOs) in the molecular symmetry.
tBu2)(py)][Cp*2Co] (6) (orange). However, all of the attempts to
separate the two compounds in a larger synthesis failed due to their
similar solubility properties in pyridine. 1H NMR (Pyridine-d5; 298
K; 200 MHz): -12.81 (br, 2H); -10.17 (br, 2H); -7.08 (s, 3H,
-N(CH3)-); -6.18 (br, 2H, -NCH2-Ph); -4.56 (s, 18H, -tBu);
0.11 (s, 18H, -tBu); 2.76 (s, 2H, -CH-aromatic); 4.55 (s, 2H,
-CH-aromatic); 9.28 (br, 2H, -NCH2-Ph).
The NMR spectrum of a solution of 5 in deuterated pyridine
recorded after addition of 2 equiv of Cp*2Co still shows much
broader signals than those found for [UO2(salan-tBu2)(py) K]n. The
addition of [UO2(salan-tBu2)(py)] to this reaction mixture does not
result in the presence of additional signals but only in the further
broadening of the U(V) peaks and shift of the signals. These results
point to the presence of U(VI) and U(V) exchanging species even
in the presence of an excess of Cp*2Co and suggests that the two
redox systems Cp*2Co+/Cp*2Co and UO22+/UO2+ are in equilibrium
in pyridine solution.
Syntheses. [UI3(thf)4]60 and [UI4(PhCN)4]61 were synthesized
as previously described. The synthesis of the pentavalent uranyl
complexes {[UO2py5][KI2py2]}n, (1) and of [UO2(salan-tBu2)-
(py)(K)]n ·2KI (3)22,33 and of the hexavalent uranyl complexes
[UO2(salan-tBu2)(py)] (5), [UO2(salophen)(py)] (10) were performed
according to the previously described procedure.22,33
Isolation of [UO2py5]I.Py (2). {[UO2py5][KI2py2]}n (30.0 mg,
0.027 mmol, 1 equiv) was added to a stirred solution of 18-crown-6
(18C6) (7.8 mg, 0.030 mmol, 1.1 equiv) in pyridine (0.6 mL). The
resulting red solution was filtered. Then, iPr2O (8 mL) was slowly
added, resulting in the immediate formation of a rust colored solid.
i
The solid was subsequently washed 3 times with 3 mL of Pr2O
The addition of potassium iodide leads to the formation of 3.
and dried under vacuum (20 mg). Anal. calcd for [UO2py3]-
I.K(18C6)I, C27H39N3O8I2KU: C, 30.46; H, 3.69; N, 3.95. Found:
C, 29.11; H, 3.75; N, 3.80.
Dissolution of 6 in toluene results in its complete reoxidation to
the hexavalent complex 5 due to the solvent influence on the redox
potential and to the presence of the redox active counterion.
Synthesis of [UO2(salophen-tBu2)(py)K] (7). A solution of 69.0
mg of K2(salophen-tBu2) (0.112 mmol, 1 equiv) in pyridine (3.0
mL) was added to 124.9 mg (0.112 mmol) of {[UO2py5][KI2py2]}n
resulting, within minutes, in a dark-green solution. The solution
was evaporated under vacuum to the volume of 0.5 mL. The
addition of iPr2O (10 mL) to the resulting solution afforded a green
The presence of KI(18C6) salts, which are very difficult to
separate, does not allow to rule out the presence of traces of
polymeric complex in the bulk compound, but the absence of
starting polymer was confirmed by a comparison of the IR spectra.
Red crystal (blocks) of 2 suitable for X-ray diffraction studies
were obtained by slow diffusion over a 3 weeks period of hexane
into a 2 × 10-2 M pyridine solution of {[UO2py5][KI2py2]}n.
Synthesis of [UO2(salan-tBu2)(py)(K18C6)] (4). A solution of
18-crown-6 (18C6) (105 mg, 0.400 mmol, 3.3 equiv) in toluene (4
mL) 104.0 mg (0.120 mmol, 1 equiv) was rapidly added to a freshly
prepared (to avoid decomposition) pink suspension of [UO2(salan-
tBu2)(py)K]n ·2KI in toluene (2 mL), resulting in a violet solution.
After 5 min of stirring, a violet solid formed, which was filtered,
washed 3 times with 3 mL of toluene and dried under vacuum (104
mg, 55% yield) Anal. calcd for [UO2(salan-tBu2)(py)(K18C6)].
2K(18C6)I, C75H131UN3O22K3I2: C, 44.24; H, 6.49; N, 2.06. Found:
C, 44.24; H, 6.45; N, 2.05.
i
solid that was filtered, washed 3 times with 4 mL Pr2O and dried
1
under vacuum. Yield: 95.0 mg (74%). H NMR (Pyridine-d5; 298
K; 400 MHz): -2.26 (s, 9H, -tBu), 0.58 (s, 9H, -tBu), 3.17 (br.
1H, -NPh-), 4.55 (s, 1H, -CH-aromatic); 6.27 (br. 1H, -NPh-); 7.34
(s, 1H, -CH-aromatic); 7.69 (s, 1H, -CHdN-). Anal. Calcd for
[UO2(salophen-tBu2)(py)K]·1.33KI C41H51UN3O4K2.33 1.33
: C, 42.86;
I
H, 4.44; N, 3.66. Found: C, 42.86; H, 4.56; N, 3.57.
Crystals of complex {[UO2(salophen-tBu2)(thf)]K(thf)2}n, 8 were
obtained by recrystallization of complex 7 from thf solution (6.9
mM) over 3 days.
Synthesis of [UO2(salophen-tBu2)(py)] (9). A light yellow
pyridine solution (2 mL) of UO2(NO3)3, 5H2O (66.9 mg, 0.124
mmol, 1eq.) was added to a pyridine solution (2 mL) of H2salophen-
tBu2 (49.4 mg, 0.124 mmol, 1eq.). The resulting red solution was
stirred for 2 h at room temperature. The solvent was evaporated in
Vacuum (1 mL) and hexane was slowly layered on the top. After
16 h (overnight) at -18 °C a red powder formed and was filtrated
to yield 38 mg (35%) of [UO2(salophen-tBu2)(py)]. 1H NMR
(Pyridine-d5; 298 K; 400 MHz): 9.79 (s, 2H); 8.02 (d, 2H, J ) 9.6
Hz); 7.83 (d, 2H, J ) 9.4 Hz); 7.54 (m, 2H); 7.37 (m, 2H); 1.50
(s, 18H); 1.48 (s, 18H). UV: 432 nm (ε ) 17676 cm-1 mol-1), 357
nm (ε ) 31703 cm-1 mol-1). Anal. calcd for C41H51O4N3U: C,
55.46; H, 5.79; N, 4.73. Found: C, 55.46; H, 5.99; N, 5.19.)
Stability Studies. The stability of 3, 4, 6 and 7 was investigated
in pyridine-d5. For each sample, the concentration was about several
mM (4 - 4.72 mM, 7 - 6.90 mM, 3 - 7.24 mM) and the samples
The amount of cocrystallized K(18C6)I can be reduced if the
reaction of [UO2(salan-tBu2)(py)K]n ·2KI with excess crown ether
is carried out in pyridine. The complex [UO2(salan-tBu2)(py)-
(K18C6)].K(18C6)I can be isolated by addition of n.hexane to
pyridine followed by recrystallization in toluene.
1H NMR (Pyridine-d5; 298 K; 400 MHz): Two sets of signals
corresponding to isomers C2 and Cs in the ratio 100:9 were found.
First set (I): -12.62 (br, 2H, -NCH2-); -10.09 (br, 2H,
-NCH2-); -8.35 (s, 3H, -N(CH3)-); -6.39 (br, 2H, -NCH2-Ph);
-4.74 (s, 18H, -tBu); 0.18 (s, 18H, -tBu); 3.06 (s, 2H,
-CH-aromatic); 4.62 (s, 2H, -CH-aromatic); 5.70 (s, 2H, -CH2O-)
11.49 (br, 2H, -NCH2-Ph). Second set (II): -12.89 (br, 2H,
-NCH2-); -8.46 (br, 2H, -NCH2-Ph) -6.52 (s, 3H, -N(CH3)-);
-5.57 (br, 2H, -NCH2-); -3.42 (s, 18H, -tBu); 0.24 (s, 18H,
-tBu); 2.82 (s, 2H, -CH-aromatic); 4.91 (s, 2H, -CH-aromatic); 6.00
(br, 2H, -NCH2-Ph).
1
were monitored in time by H NMR at 400 MHz and 298 K. The
1
Violet crystals suitable for X-ray diffraction were obtained by
letting stand at room temperature a solution of 3 reacted with 18C6
(3.2 mg, 1.1 equiv per K+) in 0.7 toluene-d8.
stability in presence of water was also studied by H NMR at 400
MHz and 298K in 1:X (X, 1-1000) mixture of the complex and
water in pyridine.
Isolation of [UO2(salan-tBu2)(py)][Cp*2Co] (6). Cp*2Co (24.8
mg, 1.5 equiv) was added to a red pyridine solution (2 mL) of
[UO2(salan-tBu2)(py)] (43.8 mg, 1 equiv). The resulting red
suspension was stirred for 16 h at room temperature. The resulting
clear orange solution was filtrated and the solvent was evaporated
to 0.5 mL. Then, the solution was let standing for 16 h at -20 °C.
Two different types of X-ray quality crystals formed. X-ray
diffraction confirmed that one of the crystal products was Cp*2Co
(deep violet) and the other was found to be the complex [UO2(salan-
X-Ray Crystallography. Diffraction data were taken using a
Oxford-Diffraction XCallibur S Kappa geometry diffractometer
(Mo KR radiation, graphite monochromator, λ ) 0.71073 Å).
To prevent evaporation of cocrystallized solvent molecules the
crystals were coated with light hydrocarbon oil and the data were
collected at 150 K. The cell parameters were obtained with
intensities detected on three batches of 5 frames. The crystal-
detector distance was 4.5 cm. For three settings of Φ and θ,
196 narrow data were collected for 1° increments in ω with a
20 s exposure time for 2, 60 s for 4, 120 s for 6 and 30 s for 8.
Unique intensities detected on all frames using the Oxford-
diffraction Red program were used to refine the values of the
(60) Avens, L. R.; Bott, S. G.; Clark, D. L.; Sattelberger, A. P.; Watkin,
J. G.; Zwick, B. D. Inorg. Chem. 1994, 33, 2248–2256.
9
J. AM. CHEM. SOC. VOL. 132, NO. 2, 2010 507

A R T I C L E S
Nocton et al.
Table 6. Crystallographic Data for Complexes, 4, 6 and 8
4·2(Toluene)
6·4.5C5H5
8
Formula
C65 H99 K N3 O10 U
C81.50 H111.50 Co N7.5 O4 U
0.15 × 0.04 × 0.03
Triclinic
C52 H78 K N2 O8 U
0.34 × 0.12 × 0.07
Monoclinic
P21/c
5223.89(9)
17.46080(19)
21.55872(18)
15.06929(15)
90
Crystal size (mm)
cryst system
space group
volume (Å3)
a (Å)
0.1 × 0.1 × 0.05
Monoclinic
j
C2/c
13297.5(4)
31.4372(6)
20.4126(4)
20.8384(3)
90
96.0642(19)
90
P1
4044.7(11)
11.850(2)
13.9279(12)
26.375(5)
82.170(10)
84.019(14)
69.999(12)
2
b (Å)
c (Å)
R (deg)
ꢂ (deg)
112.9413(12)
90
4
γ (deg)
Z
8
formula weight (g/mol)
1359.60
1.358
2.559
5608
1557.24
1.279
2.255
1608
1136.29
1.445
3.239
2316
density (calcd) (g cm-3
absorption coefficient (mm-1
F(000)
)
)
temp (K)
150.(2)
150(2)
150(2)
total no. reflections
unique reflections
Final R indices [I > 2σ(I)]
34513
15349
65088
13293 [0.0286]
R1 ) 0.0320, wR2 ) 0.0695
2.440 and -0.534
0.919
9270 [0.0605]
R1 ) 0.0756, wR2 ) 0.1745
4.440 and -0.932
1.003
12734 [0.0299]
R1 ) 0.0257, wR2 ) 0.0532
1.172 and -0.529
0.882
Largest diff. peak and hole (e.A-3
GOF
)
cell parameters. The substantial redundancy in data allows
empirical absorption corrections to be applied using multiple
measurements of equivalent reflections with the ABSPACK
Oxford-diffraction program. Space groups were determined from
systematic absences, and they were confirmed by the successful
solution of the structure (Table 6). The structures were solved
by direct methods using the SHELXTL 6.14 package and for
all structures all atoms, including hydrogen atoms, were found
by difference Fourier syntheses. All non-hydrogen atoms were
anisotropically refined on F in 2 and 6 and 8. For 4 disordered
tert-butyl C29b, C30b and C31b (occupancy factor at 0.13) has
been refined isotropically. For 2 and 6, hydrogen atoms were
fixed in ideal position. For 4, hydrogen atoms were refined
isotropically except for toluene solvents and disordered tert-
butyl for which the hydrogen atoms have been constrained. For
8, Hydrogen atoms were refined isotropically except for three
thf molecules and the disordered tert-butyl for which the
hydrogen atoms have been constrained.
Acknowledgment. This work was supported by the Commis-
sariat a` l’Energie Atomique, Direction de l’Energie Nucle´aire. We
thank Pierre Alain Bayle for his help with the NMR experiments
and Jean-Franc¸ois Jacquot for the magnetic measurements.
Supporting Information Available: Crystallographic details
for compounds 4, 6 and 8 (as CIF files), UV-visible spectra of
1, 2, 4, 6-7 and 9, NMR spectra and IR spectra of 4, 5, 7 and
9, electrochemistry data for compounds 3-5 and 7-9, EPR
data for 4 and experimental description of the reactivity with
water. Computational data. This material is available free of
charge via the Internet at http://pubs.acs.org.
(61) Enriquez, A. E.; Scott, B. L.; Neu, M. P. Inorg. Chem. 2005, 44, 7403–
7413.
JA9037164
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508 J. AM. CHEM. SOC. VOL. 132, NO. 2, 2010