Exploring the uranyl organometallic chemistry: From single to double uranium-carbon bonds

Article abstract of DOI:10.1021/ja201276h

Uranyl organometallic complexes featuring uranium(VI)-carbon single and double bonds have been obtained from uranyl UO₂X₂ precursors by avoiding reduction of the metal center. X-ray diffraction and density functional theory analyses

Full text of DOI:10.1021/ja201276h

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Exploring the Uranyl Organometallic Chemistry: From Single to
Double UraniumꢀCarbon Bonds
Jean-Christophe Tourneux,† Jean-Claude Berthet,*,† Thibault Cantat,*,† Pierre Thuꢀery,†
Nicolas Mꢀezailles,*,‡ and Michel Ephritikhine*,†
†CEA, IRAMIS, SIS2M, CNRS UMR 3299, CEA/Saclay, 91191 Gif-sur-Yvette, France
‡Laboratoire “Hꢀetꢀeroꢀelꢀements et Coordination”, CNRS UMR 7653, Ecole Polytechnique, 91128 Palaiseau Cꢀedex, France
S Supporting Information
b
Reasoning that stable uranyl organometallic complexes would
ABSTRACT: Uranyl organometallic complexes featuring
uranium(VI)ꢀcarbon single and double bonds have been
obtained from uranyl UO2X2 precursors by avoiding reduction
of the metal center. X-ray diffraction and density functional
theory analyses of these complexes showed that the UꢀC and
UdC bonds are polarized toward the nucleophilic carbon.
require the use of alkyllithium compounds with high oxidation
potentials, we investigated the coordination chemistry of bis-
(thiophosphinoyl)methane anions with uranyl(VI) salts. Both
the [CH(Ph2PS)2]ꢀ monoanion ([SCHS]ꢀ) and the [C(Ph2-
PS)2]2ꢀ dianion ([SCS]2ꢀ) are available, and importantly, the
geminal dianion and its nitrogen counterparts, bis(iminophos-
phorano)methanediides, have proven useful for the synthesis of a
new type of metal carbene complex wherein the four electrons of
the formal MdC double bond are provided by the sole carbenic
precursor.10,11
2þ
ompounds containing the hexavalent uranyl ion UO2
Addition of [UO2(OTf)2]4b to 1 molar equiv of Li[SCHS] in
diethyl ether immediately gave a red solution that deposited red
crystals of [UO2(SCHS)(OTf)(OEt2)] (1); after filtration and
successive washings with toluene and Et2O, pure 1 was obtained
in 65% yield. A similar reaction of [UO2(OTf)2] and 2 molar equiv
of Li[SCHS] in tetrahydrofuran (THF) afforded red crystals of the
bis(methanide) complex [UO2(SCHS)2] THF (2 THF), which
C
have flourished considerably during the last years, enlarging
a very active domain of coordination chemistry. The most notable
advances, which have been described as nothing else than a
revolution,1 concern uranyl-based supramolecular assemblies and
materials with unprecedented shapes, topologies, and potential
applications, such as uranyl nanotubules2 and nanospheres,3 which
have been built using organic templates and other metal cations
under hydrothermal conditions. Other major progress is related to
the synthesis of new precursors and their use in anhydrous organic
solvents,4 leading to the discovery of structural and chemical features5
that have changed generally accepted ideas concerning the coordina-
3
3
were isolated in 85% yield after the usual workup. Complex 2 was
alternatively prepared in 64% yield from a 1:2 mixture of [UO2I2-
(THF)3]4c and K[SCHS] in THF. The comproportionation reac-
tion of [UO2(OTf)2] and 2 in THF led to the quantitative
formation of [UO2(SCHS)(OTf)(THF)], whichwasalsoobtained
by dissolving 1 in THF (Scheme 1).
2þ
tion geometry and stability of the UO2 ion. In this context, it is
noteworthy that the synthesis of uranyl compounds with metalꢀ
carbon bonds remains a challenging goal, and only a handful of such
complexes can be identified among the thousands of uranyl deriva-
tives that have been structurally characterized. The first attempts at
their preparation date back to the 19th century with reactions of
[UO2Cl2] and Frankland or Grignard reagents,6 and much later
attempts employed organolithium reagents;7 however, they consis-
tently failed to access uranyl organometallic species. The reasons
behind this behavior are now better understood and mainly arise
from the reduction of UVI to UV by the alkyl ligand.5d Thus, only two
uranyl complexes with alkyl or π-organometallic ligands have been
observed to date: the cyclopentadienyl compound [NEt4]2[UO2-
(C5Me5)(CN)3]8 and the bis(iminophosphoranyl) complex [UO2-
(H-bipm)Cl]2 and its derivatives [H2-bipm = H2C(Ph2PNSi-
Me3)2].9 Interestingly, the cyclopentadienyl uranyl complex was
not synthesized by treatment of a [UO2X2] precursor with the
[C5Me5]ꢀ anion, a reaction that instead provides a convenient route
to the corresponding pentavalent [UO2X] derivative,5d but was iso-
lated from the reaction of the linear metallocene [NEt4]3[U-
(C5Me5)2(CN)5] and pyridine N-oxide.8 In view of the general
utility of alkyl complexes in the synthesis of functional derivatives,
there is a large gap in uranium coordination chemistry to fill.
The crystal structure of 1 is shown in Figure 1. The complex
adopts the classical pentagonal bipyramidal configuration, with the
equatorial plane defined by the U, C1, S1, S2, O3, and O6 atoms
[root-mean-square (rms) deviation 0.095 Å] perpendicular to the
linear UO2 moiety. The displacement of the central carbon C1 from
the plane containing the metal and the other four coordinating
atoms in the equatorial plane, 0.327(13) Å, is much smaller than the
value of 0.88 Å observed in [UO2(H-bipm)Cl]2 and its derivatives,9
a difference that likely reflects the smaller steric hindrance of the
SCHS ligand in comparison with H-bipm. The almost planar
UꢀSꢀPꢀCꢀPꢀS core in 1 contrasts with the boat conformation
adopted by the H-bipm ligands in the aforementioned uranyl com-
pounds and by the SCHS ligand in [U(SCS)(SCHS)(NEt2)],11c
the only other uranium compound with this ligand. The UꢀC1 dis-
tance of 2.647(12) Å is smaller than those measured in the H-bipm
uranyl compounds, which vary from 2.691(8) to 2.707(3) Å,9 and
that of 2.819(5) Å found for the SCHS ligand of the uranium(IV)
complex.11c The average UꢀS distances of the SCHS ligand in
Received: February 16, 2011
Published: March 31, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja201276h J. Am. Chem. Soc. 2011, 133, 6162–6165
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Scheme 1. Synthesis of Uranyl Organometallic
Complexes 1ꢀ3
Figure 2. X-ray structure of 3 with 50% probability ellipsoids.
The successful stabilization of the uranyl(VI) alkyl complexes
1 and 2 prompted us to investigate the coordination chemistry of
the more reducing dianionic [SCS]2ꢀ species as an entry to the
first uranyl carbene complexes. Treatment of an equimolar
mixture of [UO2(OTf)2] and CH2(Ph2PS)2 with an excess of
LiNEt2 in pyridine afforded the uranyl carbene complex
[UO2(SCS)(py)2] (3). After the reaction mixture was stirred
for 12 h at 20 °C and then evaporated to dryness, 3 was extracted
with toluene and isolated as an orange powder in 95% yield.
Complex 3 was alternatively obtained in almost quantitative yield
from a 1:1 mixture of [UO2(OTf)2(py)3] and Li2SCS in toluene;
the LiOTf byproduct was discarded by filtration, and 3 was
recovered after solvent evaporation. Finally, the protonolysis
reaction of the uranyl amide [UO2{N(SiMe3)2}2(THF)2]13 with
CH2(Ph2PS)2 in pyridine also led to the formation of 3; the
solution was heated at 60 °C for 6 h and then evaporated to
dryness, after which crystallization of the residue from pyridine/
pentane gave dark-orange crystals of 3 together with dark-brown
Figure 1. X-ray structures of (left) 1 and (right) 2 with 50% probability
ellipsoids. Phenyl rings have been omitted for clarity.
crystals of 3 0.5py (50% yield).
the uranium(IV) and uranyl complexes are identical, with values of
2.889(9) and 2.882(6) Å, respectively.
3
The crystal structure of 3 is shown in Figure 2. The equatorial
plane of the pentagonal bipyramidal complex defined by the U, S, N
and C1 atoms also contains the P atoms, with an rms deviation of
0.076 Å. While the UꢀSꢀPꢀCꢀPꢀS core of the SCS ligand is
While a few compounds of Ca2þ, Ba2þ, Sm2þ, Eu2þ, and Nd3þ
with two bis(iminophosphorano)methanide ligands have been
crystallographically characterized,12 2 is the first bis(methanide)
complex of uranium(VI). Its crystal structure is shown in Figure 1.
The two SCHS ligands are very distinct. One is similar to that in 1,
having an almost planar conformation with the central C1 atom in
the equatorial plane of the UO2 fragment, which also contains the U
and four S atoms (rms deviation 0.063 Å); the UꢀC1 distance is
equal to 2.656(3) Å. The other SCHS ligand adopts a boat confor-
mation and a bidentate ligation mode, as the UꢀC26 distance of
3.863(3) Å is too large to define a direct UꢀC interaction. There-
fore, complex 2 exhibits a zwitterionic structure with cationic U and
anionic C26 separation. The distinct charge distributions within the
two SCHS ligands and their distinct coordination modes induce
variations in the interatomic distances and SꢀUꢀS angles; in
particular, the UꢀS1 and UꢀS2 bonds [av 2.914(10) Å] are longer
than the UꢀS3 and UꢀS4 bonds [av 2.824(8) Å], whereas the
S1ꢀUꢀS2 angle of 128.13(2)° is much larger than S3ꢀUꢀS4
[86.59(3)°]. The 1H and 13C NMR spectra of 2 in THF-d8 at 25 °C
indicated complete equivalence of the two SCHS ligands on the
NMR time scale. A single triplet corresponding to the methine
protons was visible at δ 2.60; decoalescence of this signal and
broadening of the phenyl resonances were observed at ꢀ107 °C,
but the slow-limit spectrum could not be attained. These results can
be explained by an intramolecular dynamic process leading to
interchange of the tri- and bidentate SCHS ligands in solution,
reflecting the hemilabile nature of the methanide carbonꢀuranium
interaction.12c
slightly less planar in 3 0.5py than in 3 (rms deviation of 0.147 vs
3
0.062 Å), the distances and angles are quite identical in both
structures. The UꢀC distances of 2.430(6) Å in 3 and 2.420(3)
Å in 3 0.5py are 0.2 Å smaller than those of the methanide SCHS
3
ligands of 1 and 2 and also that of the NHC ligand of [UO2Cl2-
(IMes)2] (IMes =1,3-dimesitylimidazole-2-ylidene).9 However, this
distance is 0.1 Å larger than those measured in the UIV carbenes
[U(SCS)(BH4)2(THF)2]11a and [U(C5H5)2U(SCS)],11b and the
UꢀS distances, with an average value of 2.872(7) Å for both 3 and
3 0.5py, are quite identical to those found in these same complexes,
3
while the ionic radius of the UO22þ ion is ∼0.2 Å smaller than that
of U4þ.14 These features suggest that the multiple bond character of
the UꢀC bond is less pronounced in 3 than in the UIV compounds.
The deviation from linearity of the UO2 fragment, which displays
angles of 171.8(2)° in 3 and 168.5(1)° in 3 0.5py, is among the
3
largest ever measured, whereas the UdO bond lengths remain
unexceptional.8,15 This severe bending likely results from repulsion
between the negatively charged oxo groups and the electron-rich
ligands, as previously observed in [NEt4]2[UO2(C5Me5)(CN)3].8
The peculiar structural parameters of 3(i.e., the relatively long UꢀC
bond and the bent UO2 unit) were subjected to density functional
theory (DFT) analysis.
Because of the paucity of uranyl organometallic species, a
relativistic DFT analysis of model complexes was performed to
gain further insights into the electronic nature of the UꢀC and
UdC bonds in complexes 1ꢀ3 (see the Supporting Information
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COMMUNICATION
[SCS]2ꢀ carbenic precursor, 0.64 electron is transferred to the
uranium atom. Overall, the formation of the UdC π bond upon
conversion of 1 and 2 to 3 is clearly reflected by the change in the
uraniumꢀcarbon Wiberg bond order, which increases from 0.43 in
alkyl complexes 1 and 2 to 0.91 in 3.
Interestingly, the UdC double bond in 3 remains polarized
toward the nucleophilic carbon, as illustrated by the reaction of 3
with benzophenone to give the alkene Ph2CdC(Ph2PS)2.11a
The UdC bond polarization in 3 is even more marked than in
U
IV complexes.11 This result is surprising in view of the greater
electrophilicity of UVI relative to UIV, which was expected to
maximize the UdC interaction and therefore electron donation
from the [SCS]2ꢀ ligand to uranium. However, inspection of the
UdO Wiberg bond indexes and qO charges in 1, 2, and 3 showed
that the uraniumꢀoxo interactions are only slightly perturbed by
the formation of the UdC double bond. In other words, among
the three double bonds present in the uranium coordination
sphere (one UdC and two UdO bonds), the metal ion
preferentially stabilizes the oxo ligands, leaving a nucleophilic
carbene center. Interestingly, in the absence of oxo ligands, the
expected shortening and decrease in the polarized character of
the UdC bond were observed upon oxidation of UIV to UV in
[UCl3(bipm)]ꢀ.11d
In summary, the use of the anions [SCHS]ꢀ and [SCS]2ꢀ
has allowed us to access rare uranyl organometallic species,
especially the first uranyl carbene complex, by avoiding reduction
of the metal ion. Both uraniumꢀcarbon single and double bonds
were stabilized, pushing back the limits of stable uranyl(VI)
complexes with reducing ligands. Reactivity studies to introduce
unusual functional groups in the coordination sphere of uranyl
and isolate the pentavalent organouranyl derivatives are now
under scrutiny.
Figure 3. (a) Selected KohnꢀSham orbitals of 3. (b) Results of NBO
analysis [charges (q) and Wiberg bond indices (n)] of 1, 2, and 3.
for computational details). The optimized geometries compared
well with those observed experimentally by X-ray diffraction. For
example, in 3, the UdC bond distance was calculated as 2.41 Å [vs
2.430(6) Å experimentally], and the bending of the OdUdO
backbone was well-reproduced [165.8° calcd vs 171.81(18)° exptl].
Inspection of the KohnꢀSham orbitals of 1, 2, and 3 revealed that
the highest occupied molecular orbitals (HOMOs) describe the
interaction between the uranium(VI) ion and the SCS and SCHS
ligands, whereas the MOs for the UdO “yl” interactions are lower in
energy. The HOMOꢀ3 orbital in 3 describes the UdC σ bond,
while the π bond was found to be the HOMO of the complex
(Figure 3). Importantly, these two MOs exhibit small antibonding
interactions between the uranium ion and the oxo atoms, accounting
for the bending of the OdUdO backbone. Natural bond orbital
(NBO) analysis of 1ꢀ3 was then undertaken in order to avoid the
overestimation of covalency usually encountered in Mulliken popu-
lation analysis (Figure 3). Alkyl complexes 1 and 2 exhibited only a
single UꢀC bond described by a polarized NBO resulting from the
donation of the carbon lone pair of the SCHSꢀ ligand (∼86%, with
12% 2s character and 88% 2p character) to a vacant metal orbital
(∼14%) having 30% 6d character and 25% 5f character. In contrast,
the UdC interaction in 3 is properly described by two distinct
NBOs, as expected from prior studies on UIV and ZrIV complexes.11
Donation from a carbon sp2.30 lone pair (80.6%) to a uranium hybrid
orbital (19.4%) having 24.0% 6d and 30.2% 5f character forms the
UdC σ bond. On the other hand, the UdC π bond is made of a
carbon pure 2p orbital (87.3%) and a metal hybrid orbital (12.7%)
having 23.3% 6d and 68.1% 5f character. These data clearly support
the formation of a polarized UdC bond and show the contribution
of the uranium 5f orbitals in establishing the double bond. As a result,
the four electrons of the UdC bond in 3 are polarized as follows:
while 3.36 electrons remain located on the carbon atom of the
’ ASSOCIATED CONTENT
S
Supporting Information.
X-ray crystallographic data
b
(CIF) and experimental and computational details. This material
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
jean-claude.berthet@cea.fr; thibault.cantat@cea.fr; nicolas.mezailles@
polytechnique.edu; michel.ephritikhine@cea.fr
’ ACKNOWLEDGMENT
This work was supported by CEA, CNRS, and the Ecole
Polytechnique. We thank CINES for the allowance of computer
time (Project c2011086494) and Direction de l’Energie
Nuclꢀeaire (RBPCH) of the CEA for financial support.
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