A direct route to bis(imido)uranium(V) halides via metathesis of uranium tetrachloride

Article abstract of DOI:10.1021/ja2108432

Metathesis reactions between uranium tetrachloride and lithium 2,6-diisopropylphenylamide in the presence of 4,4′-dialkyl-2,2′- bipyridyl (R₂bpy; R = Me, ^tBu) or triphenylphosphine oxide (tppo) appear to generate bis(imido)uranium(IV) in situ. These extremely reactive complexes abstract chloride from dichloromethane to generate U(NDipp)₂Cl(R₂bpy)₂ or U(NDipp) ₂Cl(tppo)₃ (Dipp = 2,6-ⁱPr₂C ₆H₃). The preparation of the bromide and iodide analogues U(NDipp)₂X(R₂bpy)₂ was achieved by addition of CH₂X₂ (X = Br, I) to the uranium(IV) solutions. The uranium(V) halides were characterized by X-ray crystallography and found to exhibit linear N-U-N units and short U-N bonds. Electrochemical measurements were made on the chloride bipyridine species, which reacts readily with iodine or ferrocenium to generate bis(imido)uranium(VI) cations.

Full text of DOI:10.1021/ja2108432

Communication
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A Direct Route to Bis(imido)uranium(V) Halides via Metathesis of
Uranium Tetrachloride
Robert E. Jilek,† Liam P. Spencer,† Richard A. Lewis,‡ Brian L. Scott,† Trevor W. Hayton,‡
and James M. Boncella*,†
†MPA Division, Los Alamos National Laboratory, MS J514, Los Alamos, New Mexico 87545, United States
‡Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States
S
* Supporting Information
tetrachloride and may be generated via a bis(imido)uranium-
(IV) intermediate.
ABSTRACT: Metathesis reactions between uranium
tetrachloride and lithium 2,6-diisopropylphenylamide in
the presence of 4,4′-dialkyl-2,2′-bipyridyl (R2bpy; R = Me,
tBu) or triphenylphosphine oxide (tppo) appear to
generate bis(imido)uranium(IV) in situ. These extremely
reactive complexes abstract chloride from dichloromethane
to generate U(NDipp)2Cl(R2bpy)2 or U(NDipp)2Cl-
(tppo)3 (Dipp = 2,6-iPr2C6H3). The preparation of the
bromide and iodide analogues U(NDipp)2X(R2bpy)2 was
achieved by addition of CH2X2 (X = Br, I) to the
uranium(IV) solutions. The uranium(V) halides were
characterized by X-ray crystallography and found to exhibit
linear N−U−N units and short U−N bonds. Electro-
chemical measurements were made on the chloride
bipyridine species, which reacts readily with iodine or
ferrocenium to generate bis(imido)uranium(VI) cations.
We recently reported the synthesis of [U(μ-NDipp)-
Cl2(THF)2]2 by metathesis of UCl4 with 2 equiv of
LiNHDipp.2f We have since attempted reactions of UCl4 with
4 equiv of LiNHDipp and have been unable to identify the
major products. This prompted us to explore analogous
metathesis reactions with Lewis base adducts of UCl4.
Surprisingly, the reaction of UCl4(Me2bpy)2 with 4 equiv of
LiNHDipp does not yield a bis(imido)uranium(IV) species but
rather provides single crystals of U(NDipp)2Cl(Me2bpy)2 (1a),
the first monomeric bis(imido)uranium(V) complex (Scheme
1).
Scheme 1
t has been nearly 30 years since the first imidouranium
I
complexes were discovered,1 and recently, the chemistry of
this functional unit has garnered much attention among
actinide scientists. The primary reason for this interest is the
opportunity to probe the fundamental differences in bonding
and reactivity between the transition-metal series and actinide
elements (i.e., whether the f orbitals significantly alter the M
NR reactivity). Much of the research in this area has focused on
mono(imido)uranium(IV)1a,2 and -uranium(V)1b,3 compounds,
with bis(imido) species almost exclusively limited to uranium-
(VI).2c,4 The discovery of U(NR)2I2(THF)x, which contains a
Initially, we believed that adventitious CH2Cl2 was the cause
of this oxidation. To test this hypothesis, we developed a
rational synthesis of a series of halide complexes wherein
UCl4(Me2bpy)2 is treated with 4 equiv of LiNHDipp in THF
followed by addition of 0.5 equiv of CH2X2. This procedure
generates U(NDipp)2X(Me2bpy)2 [X = Cl (1a), Br (2a), I
(3a)] in yields of 74−80% (Scheme 1). The imido ligands in
1a−3a may be generated either intramolecularly via α-
hydrogen abstraction or externally with a Lewis base serving
as a proton shuttle between Dipp-amides. Given the reaction
conditions under which these compounds are generated, either
mechanism is possible. The formation of the bromide and
iodide complexes 2 and 3 from UCl4 suggests that halogen
atom abstraction from dihalomethane by a U(IV) intermediate
that does not contain Cl− leads to the formation of the U(V)
products. The identity of this intermediate, while unknown, is
either a bis(imido)uranium(IV), imidobis(amido)uranium(IV),
2+
U(NR)2 unit that is isostructural with UO22+, allowed for
direct comparisons of these isoelectronic molecules. The results
2+
demonstrated that the bonding in U(NR)2 is more covalent
in nature than in uranyl.4c Pentavalent UO2 species are much
+
less common because of their propensity to disproportionate;
however, there have been a number of recent reports
concerning the synthesis of this important ion.5 Unfortunately,
there is only a single reported example of a bis(imido)uranium-
(V) compound, namely, [U(μ-NtBu)(NtBu)I(tBu2bpy)]2
(R2bpy = 4,4′-dialkyl-2,2′-bipyridyl),6 and because of the
bridging nature of the imido ligand, the UN bond is
significantly lengthened. Thus, comparisons with monomeric
+
UO2 are not appropriate. We now report the syntheses of a
series of monomeric bis(imido)uranium(V) complexes, U-
t
(NDipp)2X(R2bpy)2 (X = Cl, Br, I; R = Me, Bu; Dipp =
Received: November 17, 2011
2,6-iPr2C6H3), which are readily prepared from uranium
© XXXX American Chemical Society
A
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Communication
or tetrakis(amido)uranium(IV) complex, though the last of
these is very unlikely since the mono(imido) complex, [U(μ-
NDipp)Cl2(THF)2]2, is formed in the reaction of UCl4 with 2
equiv of LiNHDipp.2f
crystal X-ray crystallography (Figure 2). The cation in 4 is
structurally similar to 2b, but the U−Nimido bond lengths of
The proton NMR spectra of 1a−3a are nearly identical and
contain resonances that are significantly broadened, which is
expected for paramagnetic uranium(V) compounds.6,7 Chem-
ical shifts at 5.6, 13.4, and 25.4 ppm are assigned to the Dipp
imido ligands, while a broad region centered around −2.0 ppm
arises from coordinated Me2bpy.8
Compounds 1a−3a were found to be isostructural by single-
crystal X-ray crystallography, although the structures were badly
disordered. For this reason, a similar procedure was used to
t
prepare the Bu2bpy analogues 1b−3b, which provided higher-
quality single crystals.9 Complexes 1b−3b are mononuclear
with distorted pentagonal-bipyramidal geometries about
uranium [the solid-state molecular structures are shown in
Figure 1 and Figures S9 and S10 in the Supporting Information
Figure 2. Solid-state molecular structure of 4 with thermal ellipsoids
−
drawn at the 50% probability level; the I3 ion has been omitted for
clarity. The molecule shown is one of two independent molecules in
the asymmetric unit. Selected bond lengths (Å) and angles (deg):
U1−N1 = 1.856(8), U1−Br1 = 2.787(2), N1−U1−N1A = 177.4(5).
1.869(7) and 1.856(8) Å are more than 0.1 Å shorter in the
U(VI) compound. These U−N bond lengths are similar to
those observed for other bis(imido)uranium(VI) complex-
es.4c,14 Treatment of 4 with 3 equiv of decamethylcobaltacene
1
in THF-d8 results in rapid formation of 2a, as observed by H
NMR spectroscopy, demonstrating the chemical reversibility of
the U(V)/U(VI) redox couple.
The reaction of 1a with excess CoCp*2 [E1/2 = −1.94 V vs
Fc/Fc+]15 was also performed to determine the possibility of
isolating a more reduced species. In THF-d8, a reaction
occurred over 10−12 days wherein 1a was slowly replaced by a
new and as yet unidentified paramagnetic complex. The
reaction rate was not consistent with a straightforward outer-
sphere electron transfer, despite the large reduction potential
difference between CoCp*2 and 1a (vide infra), suggesting that
this is a more complicated transformation. Experiments to
elucidate the nature of this new species are ongoing.
We also investigated the solution-phase redox properties of
complex 1a by cyclic voltammetry (CV). In CH2Cl2, 1a exhibits
two quasi-reversible redox features at −1.06 and −1.22 V vs Fc/
Fc+ in its cyclic voltammogram (see the SI). We have assigned
the feature at −1.06 V as a U(V)/U(VI) oxidation and
tentatively assigned the feature at −1.22 V as a bipyridine-based
reduction. Importantly, the CV data for 1a represent the first
electrochemical parameters determined for the bis(imido)
series of complexes. The U(V)/U(VI) redox potential of 1a
is much more negative than that observed for the parent uranyl
ion UO22+(aq) (−0.35 V vs Fc/Fc+),12 and even that for the
mono(imido)uranium(V) complex U(NSiMe3)(N-
[SiMe3]2)3 (−0.41 V vs Fc/Fc+),13 but it is comparable to
that observed for [UO2(OH)4]2− (−1.17 V vs Fc/Fc+),12 which
contains four strongly electron-donating hydroxide ligands and
a dianionic charge. Overall, these data demonstrate the
electron-rich nature of the uranium center in 1a and
corroborate the strong π-donating character of the imido
substituents in this system.4c
Figure 1. Solid-state molecular structure of 1b with thermal ellipsoids
drawn at the 50% probability level. Selected bond lengths (Å) and
angles (deg): U1−N1 = 1.977(4), U1−N2 = 1.980(4), U1−N3 =
2.683(4), U1−N4 = 2.594(4), U1−N5 = 2.595(4), U1−N6 =
2.659(4), U1−Cl1 = 2.729(2), N1−U1−N2 = 166.02(16), N1−U1−
Cl1 = 94.93(12), N2−U1−Cl1 = 97.84(12).
(SI)]. The coordinated bipyridine ligands contain C−C bond
t
lengths consistent with neutral Bu2bpy ligands rather than
radical anions.10 The imido ligands occupy the axial positions
with nearly linear N−U−N angles of 166.02(16), 165.9(2), and
174.9(2)°, respectively. The U−Nimido bond lengths are almost
identical throughout the series, and for 1b, the values are
1.977(4) and 1.980(4) Å. These bond lengths are longer than
the terminal imido distances in [U(μ-NtBu)(NtBu)I(tBu2bpy)]2
[UNavg = 1.898(7) Å].6 They are also significantly longer
than the UO distances in [UO2(tppo)4]OTf [1.819(6) Å]5b
and UO2(Ar2nacnac)(Ph2MePO)2 [1.819(6) Å],5e even con-
sidering the difference in atomic radii.11 The bond lengths
indicate stronger bonding in the UO2+ complexes, although the
steric bulk of the Dipp groups almost certainly causes a
lengthening of the UN bonds in 1b−3b.
The chemical oxidation of 1a and 2a was carried out with
iodine or ferrocenium. Proton NMR spectroscopy confirmed
that these reactions result in clean conversion to [U-
(NDipp)2X(Me2bpy)2]+ (X = Cl, Br). The triiodide salt,
[U(NDipp)2Br(Me2bpy)2]I3 (4), was characterized by single-
B
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Journal of the American Chemical Society
Communication
(2) (a) Burns, C. J.; Smith, W. H.; Huffman, J. C.; Sattelberger, A. P.
J. Am. Chem. Soc. 1990, 112, 3237−3239. (b) Arney, D. S. J.; Burns, C.
J. J. Am. Chem. Soc. 1993, 115, 9840−9841. (c) Arney, D. S. J.; Burns,
C. J. J. Am. Chem. Soc. 1995, 117, 9448−9460. (d) Stewart, J. L.;
Andersen, R. A. New J. Chem. 1995, 19, 587−595. (e) Diaconescu, P.
L.; Arnold, P. L.; Baker, T. A.; Mindiola, D. J.; Cummins, C. C. J. Am.
Chem. Soc. 2000, 122, 6108−6109. (f) Jilek, R. E.; Spencer, L. P.;
Kuiper, D. L.; Scott, B. L.; Williams, U. J.; Kikkawa, J. M.; Schelter, E.
J.; Boncella, J. M. Inorg. Chem. 2011, 50, 4235−4237.
(3) (a) Zalkin, A.; Brennan, J. G.; Andersen, R. A. Acta Crystallogr.,
Sect. C 1988, 44, 1553−1554. (b) Bart, S. C.; Anthon, C.; Heinemann,
F. W.; Bill, E.; Edelstein, N. M.; Meyer, K. J. Am. Chem. Soc. 2008, 130,
12536−12546. (c) 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.
For further comparison, we attempted to prepare bis(imido)-
uranium(V) complexes containing redox-inactive ligands. Thus,
the reaction of UCl4 with 3 equiv of triphenylphosphine oxide
(tppo) and 4 equiv of LiNHDipp in the presence of CH2Cl2
provided U(NDipp)2Cl(tppo)3 (5). The successful isolation of
5 from UCl4 shows that the presence of a potentially redox-
active ligand such as bpy is not necessary for the oxidation to
U(V) and is consistent with the reaction occurring from a
U(IV) intermediate. While 5 has been structurally characterized
(Figure S12), we have been unable to separate this compound
from the U(VI) byproduct U(Ndipp)2Cl2(tppo)2, which forms
regardless of the amount of dichloromethane present. Thus, it
appears that further oxidation of the bis(imido)uranium(V)
complex via halogen abstraction is a more facile reaction when
tppo is coordinated. Qualitatively, this indicates that 5 is even
more electron-rich than its bipyridine analogues 1−3.16
We have presented facile syntheses of a series of bis(imido)-
uranium(V) complexes, U(NDipp)2X(R2bpy)2 (X = Cl, Br, I; R
= Me, tBu) and U(NDipp)2Cl(tppo)3, from uranium
tetrachloride and lithium 2,6-diisopropylphenylamide. These
reactions proceed via salt metathesis and subsequent oxidation
via halogen atom abstraction, resulting in the first monomeric
bis(imido)uranium(V) species. Most notably, the products
appear to be generated via extremely reactive intermediates.
While these intermediates remain uncharacterized, their
reactivity is consistent with bis(imido)uranium(IV) com-
pounds, which present an intriguing target for future synthetic
studies. We have reported the first electrochemical data on
bis(imido)uranium complexes, which indicate highly electron-
rich uranium(V) centers. Importantly, the ease with which
these molecules are prepared from tetravalent precursors
suggests that this methodology could be applied to transuranic
compounds, which would greatly enhance our understanding of
AnN multiple bonding.
(4) (a) Arney, D. S. J.; Burns, C. J.; Smith, D. C. J. Am. Chem. Soc.
1992, 114, 10068−10069. (b) Warner, B. P.; Scott, B. L.; Burns, C. J.
Angew. Chem., Int. Ed. 1998, 37, 959−960. (c) Hayton, T. W.;
Boncella, J. M.; Scott, B. L.; Palmer, P. D.; Batista, E. R.; Hay, P. J.
Science 2005, 310, 1941−1943.
(5) (a) Arnold, P. L.; Patel, D.; Wilson, C.; Love, J. B. Nature 2008,
451, 315−318. (b) Berthet, J.; Nierlich, M.; Ephritikhine, M. Angew.
Chem., Int. Ed. 2003, 42, 1952−1954. (c) Berthet, J.; Siffredi, G.;
Thuery, P.; Ephritikhine, M. Dalton Trans. 2009, 3478−3494.
(d) Mougel, V.; Horeglad, P.; Nocton, G.; Pecaut, J.; Mazzanti, M.
Angew. Chem., Int. Ed. 2009, 48, 8477−8480. (e) Hayton, T. W.; Wu,
G. J. Am. Chem. Soc. 2008, 130, 2005−2014. (f) Arnold, P. L.; Hollis,
E.; White, F. J.; Magnani, N.; Caciuffo, R.; Love, J. B. Angew. Chem.,
Int. Ed. 2011, 50, 887−890. (g) Graves, C. R.; Kiplinger, J. L. Chem.
Commun. 2009, 3831−3853. (h) Arnold, P. L.; Love, J. B.; Patel, D.
Coord. Chem. Rev. 2009, 253, 1973−1978.
(6) Spencer, L. P.; Schelter, E. J.; Yang, P.; Gdula, R. L.; Scott, B. L.;
Thompson, J. D.; Kiplinger, J. L.; Batista, E. R.; Boncella, J. M. Angew.
Chem. 2009, 121, 3853−3856.
(7) Eller, P. G.; Vergamini, P. J. Inorg. Chem. 1983, 22, 3184−3189.
(8) Regardless of the Lewis base, the chemical shifts at 5.6, 13.4, and
25.4 ppm remained constant for U(NDipp)2X(L)y compounds. Our
assignments are based on this fact as well as on NMR integrations.
(9) Compounds 1a−3a are quite soluble in most organic solvents,
and for this reason they are difficult to isolate in appreciable yields.
(10) (a) Wiley, R. O.; Von Dreele, R. B.; Brown, T. M. Inorg. Chem.
1980, 19, 3351−3356. (b) Kraft, S. J.; Fanwick, P. E.; Bart, S. C. Inorg.
Chem. 2010, 49, 1103−1110. The average C−Cbridge distance is
1.486(9) Å, which is statistically identical to the corresponding
distance in free 2,2′-bpy [1.490(3) Å]. This distance is much shorter in
complexes containing coordinated bpy radical anions.
ASSOCIATED CONTENT
* Supporting Information
■
S
Complete details of the preparation and characterization of 1−
4, including X-ray crystallographic details for 1b−3b, 4, and 5
(CIF), and electrochemical data for 2. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
boncella@lanl.gov
(11) Emsley, J. The Elements, 3rd ed.; Oxford University Press:
Oxford, U.K., 1998.
(12) Morris, D. E. Inorg. Chem. 2002, 41, 3542−3547.
(13) Burns, C. J.; Smith, W. H.; Huffman, J. C.; Sattelberger, A. P. J.
Am. Chem. Soc. 1990, 112, 3237−3239.
(14) (a) Hayton, T. W.; Boncella, J. M.; Scott, B. L.; Batista, E. R.;
Hay, P. J. J. Am. Chem. Soc. 2006, 128, 10549−10559. (b) Spencer, L.
P.; Yang, P.; Batista, E. R.; Scott, B. L.; Boncella, J. M. Inorg. Chem.
2009, 48, 11615−11623.
(15) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877−910.
(16) We observed no formation of U(VI) during the preparation of
compounds 1−3, even in the presence of excess dihalomethane.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
R.E.J. thanks the Seaborg Institute (Los Alamos National
Laboratory) for fellowships to support his work. We also thank
the LANL LDRD Program for funding portions of this work.
T.W.H. thanks the Heavy Element Program of the DOE Office
of Basic Energy Sciences (BES) for support of the electro-
chemical experiments. J.M.B. and R.E.J. also thank the DOE
BES Heavy Element Progam at LANL for supporting the work
on the chemical oxidation of the U(V) complexes.
REFERENCES
■
(1) (a) Cramer, R. E.; Panchanatheswaran, K.; Gilje, J. W. J. Am.
Chem. Soc. 1984, 106, 1853−1854. (b) Brennan, J. G.; Andersen, R. A.
J. Am. Chem. Soc. 1985, 107, 514−516.
C
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