Low-valent uranium iodides: Straightforward solution syntheses of UI 3 and UI4 etherates

Article abstract of DOI:10.1021/ic801138e

Uranium turnings react with elemental iodine in diethyl ether at room temperature, with sonication and/or stirring, over a period of days to afford UI₃, UI₄(OE_t2)₂, or UI ₄(OBuⁿ₂) depending on the stoichiometry or ether solvent. This is the first room temperature, and thus safe and convenient, synthesis of UI₃.

Full text of DOI:10.1021/ic801138e

Inorg. Chem. 2008, 47, 8577-8579
Low-Valent Uranium Iodides: Straightforward Solution Syntheses of UI3
and UI4 Etherates
Christopher D. Carmichael, Natalie A. Jones, and Polly L. Arnold*
School of Chemistry, UniVersity of Edinburgh, West Mains Road, Edinburgh, Scotland EH9 3JJ
Received June 20, 2008
Uranium turnings react with elemental iodine in diethyl ether at
room temperature, with sonication and/or stirring, over a period of
days to afford UI3, UI4(OEt2)2, or UI4(OBun2) depending on the
stoichiometry or ether solvent. This is the first room temperature,
and thus safe and convenient, synthesis of UI3.
imparts solubility to the metal center, the presence of THF can
restrict chemistry. For example, uranium(III) is known to ring
open THF,8 and the synthesis of UI3(THF)4 is performed below
10 °C to minimize the formation of ring-opened products.5 The
use of unsolvated UI3 has permitted the synthesis of complexes
unobtainable in the presence of strong donor solvents. For
example, Cloke and co-workers have utilized UI3 in the
synthesis of an unsolvated pentalene complex that activates
dinitrogen9 and a trimetallic system that activates diethyl ether.10
The Evans group has used UI3 in developing new syntheses
for various tetramethylcyclopentadienyluranium halides.11
This is in contrast to the coordination chemistry of ura-
nium(IV), where the solvent-free chloride, UCl4, is a commonly
used starting material. The tetravalent iodide, UI4, is rarely
utilized; its synthesis is laborious, and it is unstable at room
temperature, decomposing to UI3 and I2.12 Recently, however,
synthetically useful preparations of UI4 supported by nitrile
ligands have been developed, including the complexes
UI4(NCMe)4 and UI4(NCPh)4.13 Trivalent plutonium is more
stable than trivalent uranium, and a solution synthesis of
PuI3(OEt2)x, formed as a pale-blue powder from an excess of
plutonium metal and I2 in diethyl ether, was recently reported
and used in the synthesis of Pu[N(SiMe3)2]3.14
An understanding of the subtle differences in bonding
between actinide(III) and lanthanide(III) complexes is key to
their effective separation in nuclear waste, and low-oxidation-
state uranium chemistry provides important model systems for
this. The idea of exploiting covalency differences between the
5f and 4f metals in actinide(III)/lanthanide(III) separation has
been explored both theoretically1 and synthetically2 and has
been discussed in a recent review.3 Fundamental to the
continuing synthetic exploration of this chemistry is the avail-
ability of appropriate low-valent uranium compounds.
Low-valent uranium(III) chemistry is typically accessed via
the metal or the tetrahydrofuran (THF) adduct of the uranium
iodide, UI3(THF)4.4,5 The synthetic utility of the solvated iodide
has been considerably greater than that of the chloride
UCl3(THF)x, which is made in situ and is a poorly understood
material.6 Readily synthesized from a THF solution of amal-
gamated uranium turnings and iodine, UI3(THF)4 and related
solvent adducts are at the center of an ever-expanding literature
of low-valent uranium chemistry.3,7 However, while the THF
Historically, UI3 has been synthesized by the reduction of
UI4 with zinc metal in sealed silica vessels at about 600 °C.15,16
Recently, Cloke optimized Corbett’s solid-state metal iodide
synthesis,17 demonstrating that UI3 can be synthesized from
* To whom correspondence should be addressed. E-mail: polly.arnold@
ed.ac.uk.
(8) Boisson, C.; Berthet, J.-C.; Lance, M.; Nierlich, M.; Ephritikhine, M.
Chem. Commun. 1996, 2129. Evans, W. J.; Kozimor, S. A.; Ziller,
J. W. J. Am. Chem. Soc. 2003, 125, 14264.
(9) Cloke, F. G. N.; Hitchcock, P. B. J. Am. Chem. Soc. 2002, 124, 9352.
(10) Larch, C. P.; Cloke, F. G. N.; Hitchcock, P. B. Chem. Commun. 2008,
82.
(11) Evans, W. J.; Kozimor, S. A.; Ziller, J. W.; Fagin, A. A.; Bochkarev,
M. N. Inorg. Chem. 2005, 44, 3993.
(12) Bagnall, K. W.; Brown, D.; Jones, P. J.; du Preez, J. G. H. J. Chem.
Soc. 1965, 350.
(1) Choppin, G. R. J. Alloys Compd. 2002, 344, 55. Mazzanti, M.;
Wietzke, R.; Pecaut, J.; Latour, J.-M.; Maldivi, P.; Remy, M. Inorg.
Chem. 2002, 41, 2389. Ingram, K. I. M.; Kaltsoyannis, N.; Gaunt,
A. J.; Neu, M. P. J. Alloys Compd. 2007, 444-445, 369.
(2) Karmazin, L.; Mazzanti, M.; Pecaut, J. Chem. Commun. 2002, 654.
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(3) Ephritikhine, M. Dalton Trans. 2006, 2501.
(4) Clark, D. L.; Sattelberger, A. P.; Bott, S. G.; Vrtis, R. N. Inorg. Chem.
1989, 28, 1771.
(13) (a) Berthet, J. C.; Thuery, P.; Ephritikhine, M. Inorg. Chem. 2005,
44, 1142. (b) Enriquez, A. E.; Scott, B. L.; Neu, M. P. Inorg. Chem.
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J. G.; Zwick, B. D. Inorg. Chem. 1994, 33, 2248.
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Anderson, R. A. Inorg. Chem. 1979, 18, 1507. Van der Sluys, W. G.;
Burns, C. J.; Sattelberger, A. P. Organometallics 1989, 8, 855.
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Inorg. Chem. 2008, 47, 26.
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10.1021/ic801138e CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 19, 2008 8577
Published on Web 09/03/2008

COMMUNICATION
uranium and HgI2 at 320 °C in a sealed glass ampule.9 Evans
and co-workers have demonstrated a mercury-free synthesis by
controlled reaction of the elements at elevated temperature.11
Although these reactions produce UI3 in excellent yields, they
involve the heating of uranium turnings to red heat11 or utilize
mercury9 and are thus potentially dangerous or have undesirable
environmental implications, respectively. We report herein
simple, direct solution syntheses of the unsolvated uranium(III)
iodide, UI3, and the new solvates UI4(OEt2)2 and UI4(OBun2)
by reaction of the elements at room temperature mediated by
diethyl ether or dibutyl ether.
U + 1.5I2 Et O8 UI3
(1)
2
1
Figure 1. Displacement ellipsoid drawing of 2. Ellipsoids are drawn at 50%
probability. Hydrogen atoms are omitted for clarity. Selected distances (Å) and
angles (deg): U-O, 2.366(8); U-I, 2.9614(6); O(1)-U-I(1), 84.68(1);
O(1)-U-I(1′), 95.32(1); I(1)-U-I(1′), 89.36(3); I(1)-U-I(1′′), 169.36(2);
I(1)-U-I(1′′′), 91.63(3).
Adduct-free UI3 is synthesized according to eq 1. The
addition of diethyl ether to an ampule containing cleaned
uranium turnings, less than 1.5 equiv of iodine, and a large ovoid
stirbar produced an immediate brown color of dissolved iodine.
The heterogeneous mixture was vigorously stirred, with oc-
casional sonication (but see below), over 48 h, during which
time the solution became dark red and the majority of the
uranium reacted. Occasional sonication, coupled with vigorous
stirring, was continued for 5 days, and a finely divided, purple-
black solid was deposited. The precipitation of the solid was
accompanied by fading of the red solution to a pale purple and
consumption of the remaining uranium. The mixture was
filtered, and the residue was washed several times with diethyl
ether and then dried under vacuum to yield the product, UI3
(1), as a dark-purple-black solid in essentially quantitative yield
(96%). An electronic absorption spectrum of the 800-1400 nm
region, obtained by dissolving 1 in THF, is in excellent
agreement with previously obtained spectra of UI3(THF)4
(Figure 1 in the Supporting Information).5
The material as isolated contains small quantities of impurities
that, like the UI3, are almost completely insoluble in noncoor-
dinating solvents, but its purity is sufficient to allow its use in
further synthetic chemistry. Analytical values for uranium and
iodine were approximately 3% below calculated values,11,15 and
a CHN analysis indicates that a small amount of carbon and
hydrogen is present. Exhaustive extraction of 1 with the
coordinating solvents THF, pyridine, and 1,2-dimethoxyethane
(DME) produces the solvated complexes UI3(THF)4, UI3(py)4,
and UI3(DME)2 in excellent yields, leaving an insoluble black
solid residue (1-2% by weight). The identity of this material
remains unknown, however, given recent work on reductive
cleavage of ether by uranium(III);10 the material is assumed to
be carbon-rich products arising from a small amount of diethyl
ether decomposition and possibly residual metallic impurities
from the surface of the turnings.
ing a red solid after 48 h (eq 2). Concentration of the solution
to approximately 5 mL volume, filtration, and drying of the
solid under vacuum yield the solvate UI4(OEt2)2 (2) as a
microcrystalline red solid in 77% yield. Uranium and iodine
analyses are in agreement with the expected values. Complex
2 is sensitive to loss of diethyl ether if heated above ambient
temperature, turning from red to black. In solution, 2 displays
two broad and paramagnetically shifted singlets at -10.45 and
-22.54 ppm in the 1H NMR spectrum, attributable to the methyl
and methylene protons of diethyl ether, respectively. The
electronic absorption spectrum obtained in diethyl ether shows
several weak and broad absorptions typical of six-coordinate
uranium(IV) species (Figure 1 in the Supporting Information).18
Evans’ method yields a value of 2.15 µB. This value is
3
significantly lower than that calculated for a H4 ground state
of free U4+ (3.58 µB), but the magnetic moments reported for
normal uranium(IV) coordination complexes are invariably
lower and usually in the range 2.5-2.8 µB.19
Concentration of a diethyl ether solution of 2 yields dark-
red, octahedral crystals of X-ray quality. The structure of 2 is
a distorted octahedron (Figure 1), with trans-bound diethyl ether
molecules. Complex 2 crystallizes in the high-symmetry tet-
ragonal space group I41/acd, resulting in crystallographically
equivalent U-I and U-O bond distances. The U-I bond
distance [2.9614(6) Å] is at the short end of the range found in
similar uranium(IV) iodide complexes UI4(py)3 [2.9558(4)-
3.0438(4) Å],13a UI4(NCPh)4 [3.027(1) Å],13b and UI4[Od
C(NMe2)2]2 [2.9986(3)-3.027(3) Å]18 presumably because of
the weaker donor strength of diethyl ether. Coordination of
diethyl ether to uranium complexes is rare, with only two
structurally characterized examples reported in the literature,
the borohydride complex U(BH4)4(OEt2)20 and the oxygen-
bridged dimer U2Cl2O2(Cp*py)2(OEt2)221 [Cp*py ) 1,2,3,4-
tetramethyl-5-(2-pyridyl)cyclopentadienide]; the U-O bond
distance of 2 falls in between the reported distances. A distortion
U + 2I2 Et O8 UI4(Et2O)2
(2)
2
2
We postulated that the red solution observed during the
reaction to form 1 is a soluble uranium tetraiodide, which
subsequently reacted with more uranium turnings to form
insoluble UI3. An independent reaction of uranium turnings with
2 equiv of I2 in diethyl ether, encouraged by vigorous stirring
and occasional sonication, affords a bright-red solution contain-
(18) Preez, J. G. H.; Zeelie, B.; Casellato, U.; Graziani, R. Inorg. Chim.
Acta 1987, 129, 289.
(19) Lam, O. P.; Anthon, C.; Heinemann, F. W.; Connor, J. M.; Meyer,
K. J. Am. Chem. Soc. 2008, 130, 6567.
(20) Rietz, R. R.; Zalkin, A.; Templeton, D. H.; Edelstein, N. M.;
Templeton, L. K. Inorg. Chem. 1978, 17, 653.
8578 Inorganic Chemistry, Vol. 47, No. 19, 2008

COMMUNICATION
of the iodide ligands from the equatorial plane by approximately
5° is also observed. A similar distortion has been noted in the
dimethylurea complex mentioned above.18
solvated triiodides.23 However, uranium and thorium were found
to be unreactive under similar conditions.23 The reaction of
uranium turnings with a diethyl ether solution of 1,2-diiodoet-
hane did not produce any evidence of reaction, even under
prolonged heating to 40 °C and sonication of the reaction
mixture.
The preparation of the uranium turnings is important to the
synthesis of 1, with the highest yields achieved utilizing uranium
turnings cleaned according to standard procedures5 but not
amalgamated with mercury. Reactions using amalgamated
turnings, or reactions carried out with the addition of a small
amount (1-2 mg) of HgI2 to cleaned turnings, lead to
significantly lower isolated yields of 1, with correspondingly
larger amounts of 2 remaining in the filtrate. The reason for
this exaggerated effect is not known, although it is possible that
an amalgamated surface retards the reduction of uranium(IV)
to uranium(III). The direct synthesis of 2 appears to be
uninfluenced by the presence of mercury.
In order to demonstrate the utility of these solution-based
syntheses, 1 and 2 have been used to prepare other uranium(III)
and uranium(IV) starting materials. For example, the uranium
tris(silylamide), U[N(SiMe3)2]3, can be efficiently synthesized
from UI3(THF)4 and Na[N(SiMe)3] in THF in 82% yield (on a
19 g scale).5 The addition of toluene to a mixture of 1 and
K[N(SiMe3)2] on a small scale afforded a dark-red-purple
solution from which U[N(SiMe3)2]3 was isolated in 63% yield,
lower mainly because of the small scale of the reaction.
Likewise, U(Cp)3I24 can be made in 71% yield from 2 and
NaCp.
In conclusion, we have developed the first room-temperature
procedure to synthesize the important uranium(III) precursor
UI3, using simple solution techniques, and demonstrated its
utility in the synthesis of other uranium(III) starting materials.
Using the same procedure, we have also synthesized the
uranium(IV) solvate UI4(OEt2)2 and, in dibutyl ether,
UI4(OBun2). This demonstrates that thermally stable tetravalent
uranium iodides are also readily accessible in the presence of
bases with protons of very low acidity and should prove to be
valuable reagents in low-oxidation-state uranium chemistry.
To confirm that 2 can be an intermediate in the synthesis of
1, uranium turnings were treated with 2, according to eq 3. After
stirring for only a few hours, a small amount of dark precipitate
was noted. The reaction was stirred with occasional sonication
for 5 days, during which time the red color of the solution faded,
and a dark solid was deposited. After 5 days, 1 was isolated,
as described above, in 93% yield. Uranium and iodine analyses
were in agreement with those obtained for the samples above.
3UI4(Et2O)2 + U Et O8 4UI3
(3)
2
2
1
This result suggests that, during the synthesis of 1, iodine
quickly oxidizes the uranium metal to uranium(IV), which is
then followed by a slower reduction to uranium(III) by the
remaining uranium metal. The intermediate nature of 2 can be
further reinforced by stopping a reaction prior to completion,
when the solution remains dark red. For example, a reaction
filtered after 48 h yielded a small amount of 1 (25% by U),
while a significant amount of 2 was isolated from the filtrate
(50% by U).
It must be noted that the reactions to synthesize 1 and 2 are
very dependent on the efficacy of agitation. Reactions that utilize
either vigorous stirring or periodic sonication, but not both, can
take up to 20 days to reach completion. It is thought that a
combination of sonication and stirring helps to maintain a clean
reaction surface better than either alone. Elevation of the reaction
temperature was also investigated as a means to reduce reaction
times. Reactions performed in refluxing diethyl ether rapidly
form 2, in the form of a dark-red solution; however, no further
reaction was observed with the remaining uranium turnings,
even upon extended heating (10 days at 40 °C). Once cooled
to ambient temperature, however, the formation of 1 was
complete within 5 days. It is not known why a heated solution
of 2 does not react with uranium; perhaps the elevated
temperature hinders the presentation of a clean metal surface
for reaction.
Reactions performed at room temperature in dibutyl ether
produce an insoluble green solid after several days that has been
formulated as UI4(OBun2) (3) based on elemental analyses. 1H
NMR experiments using a known internal standard (1,3,5-tri-
tert-butylbenzene) confirm the presence of one dibutyl ether
per uranium center. However, because of its insolubility in
noncoordinating solvents, we were unable to grow single
crystals suitable to determine the solid-state structure. No further
reactivity of 3 with uranium was observed in dibutyl ether, even
with prolonged sonication, presumably because of its insolubil-
ity.
Acknowledgment. We gratefully acknowledge financial
support from the Royal Society (Canada/USA Research Fel-
lowship to CDC), the Alexander von Humboldt Foundation,
and the U.K. EPSRC. We also thank the EPSRC National
Crystallography Service for the data collection of 2.
Supporting Information Available: Experimental details for
complexes 1-3 and crystallographic data for 2 in CIF format. This
material is available free of charge via the Internet at http://pubs.acs.org.
The atomic coordinates for this structure have been deposited with
the Cambridge Crystallographic Data Centre as CCDC 691375. The
coordinates can be obtained, on request, from the Director, Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
U.K.
Samarium and ytterbium diiodides can be prepared by the
reaction of the metal with THF solutions of 1,2-diiodoethane,22
and similar reactions with plutonium and neptunium form the
IC801138E
(21) Le Borgne, T.; Theury, P.; Ephritikhine, M. Acta Crystallogr. 2002,
C58, m8.
(22) Watson, P. L.; Tulip, T. H.; Williams, I. Organometallics 1990, 9,
1999.
(23) Karraker, D. G. Inorg. Chim. Acta 1987, 139, 189.
(24) Sung-Yu, N. K.; Hsu, F. F.; Chang, C. C.; Her, G. G.; Chang, C. T.
Inorg. Chem. 1981, 20, 2727. Fischer, R. D.; von Ammon, B.;
Kanellakopoulos, B. J. Organomet. Chem. 1970, 25, 123.
Inorganic Chemistry, Vol. 47, No. 19, 2008 8579