Halide Effects in the Synthesis of Mixed Uranium(IV) Aryloxide-Halide Compounds

Article abstract of DOI:10.1021/ic9803309

Several uranium(IV) aryloxide and mixed aryloxide-halide compounds of the formula UX_4-z(OAr)_z (X = Cl, Br, I; OAr = 2,6-di-tert-butylphenoxide; z = 2, 3, 4) have been prepared by the reaction of KOAr with the appropriate uranium tetrahalide in tetrahydrofuran (THF). The complex UI₄(CH₃CN)₄ reacts with 2 equiv of KOAr in THF to yield the compound I₂U(OAr)₂ (1). The utility of this reaction is limited by the instability of UI₄(CH₃CN)₄ in THF, however, and isolated yields of the THF adduct of 1 do not exceed 50%. In contrast, UBr₄(CH₃CN)₄ is stable in THF solution and reacts with 2 equiv of KOAr to yield the dibromide, Br₂U(OAr)₂-(THF) (4·THF), in 77% yield. Under identical reaction conditions, UCl₄ reacts to yield the complex [K(THF)₄]-[UCl₃(OAr)₂] (5) in 68% yield. The uranium center in compound 5 is coordinated by two trans aryloxide ligands and three chloride ligands in a trigonal bipyramidal arrangement. Two chloride ligands of each unit are also coordinated to a potassium ion, forming an infinite chain in the lattice. Compounds 1 and 5 will further react with 1 equiv of KOAr to yield the compounds XU(OAr)₃ (2, X = I; 6, X = Cl). Both UI₄(CH₃CN)₄ and UCl₄ will react with a 4.2 equiv of KOAr to yield the previously characterized compound U(OAr)₄ (3). The stability of the mixed aryloxide-halide uranium complexes toward ligand redistribution reactions has been investigated.

Full text of DOI:10.1021/ic9803309

4040
Inorg. Chem. 1998, 37, 4040-4045
Halide Effects in the Synthesis of Mixed Uranium(IV) Aryloxide-Halide Compounds
Steven D. McKee,1a Carol J. Burns,*,1b and Larry R. Avens1a
The Chemical Science and Technology Division and The Nuclear Material Technology Division,
Los Alamos National Laboratory, Los Alamos, New Mexico 87545.
ReceiVed March 24, 1998
Several uranium(IV) aryloxide and mixed aryloxide-halide compounds of the formula UX4-z(OAr)z (X ) Cl,
Br, I; OAr ) 2,6-di-tert-butylphenoxide; z ) 2, 3, 4) have been prepared by the reaction of KOAr with the
appropriate uranium tetrahalide in tetrahydrofuran (THF). The complex UI4(CH3CN)4 reacts with 2 equiv of
KOAr in THF to yield the compound I2U(OAr)2 (1). The utility of this reaction is limited by the instability of
UI4(CH3CN)4 in THF, however, and isolated yields of the THF adduct of 1 do not exceed 50%. In contrast,
UBr4(CH3CN)4 is stable in THF solution and reacts with 2 equiv of KOAr to yield the dibromide, Br2U(OAr)2-
(THF) (4‚THF), in 77% yield. Under identical reaction conditions, UCl4 reacts to yield the complex [K(THF)4]-
[UCl3(OAr)2] (5) in 68% yield. The uranium center in compound 5 is coordinated by two trans aryloxide ligands
and three chloride ligands in a trigonal bipyramidal arrangement. Two chloride ligands of each unit are also
coordinated to a potassium ion, forming an infinite chain in the lattice. Compounds 1 and 5 will further react
with 1 equiv of KOAr to yield the compounds XU(OAr)3 (2, X ) I; 6, X ) Cl). Both UI4(CH3CN)4 and UCl4
will react with a 4.2 equiv of KOAr to yield the previously characterized compound U(OAr)4 (3). The stability
of the mixed aryloxide-halide uranium complexes toward ligand redistribution reactions has been investigated.
Introduction
to that observed for better known uranium coordination environ-
ments. The one-electron oxidation chemistry of the compound
U(OAr)3 (OAr ) 2,6-di-tert-butylphenoxide) has been inves-
tigated and been shown to be analogous to that of the uranium-
The coordination and organometallic chemistry of uranium
is marked by the accessibility of a multiplicity of oxidation
states.2 This variability in oxidation states permits multiple
electron-transfer chemistry, which can be of use in effecting
chemical transformations at the metal center, provided one
employs ancillary ligands which are tolerant of changes in the
oxidation state of the uranium. An example of such a ligand
set is the bis(pentamethylcyclopentadienyl) framework, which
is known to stabilize uranium complexes in all oxidation states
from +3 to +6.3-5 It is desirable to identify other such redox-
tolerant ligand sets, to permit synthetic variations of both the
electronic and steric constraints at the metal center. One
possible candidate ancillary ligand is the 2,6-disubstituted
aryloxide ligand. This ligand has been successfully introduced
at both U(III) and U(IV) to generate neutral homoleptic
complexes.6 Further, the reactivity of these complexes is similar
7
(III) compounds Cp3U and U[N(SiMe3)2]3 (e.g., U(OAr)3 is
oxidized by a suitable halide source to yield the uranium(IV)
compounds of the formula XU(OAr)3, where X ) F, Cl, Br,
I).8
To further develop the chemistry of aryloxide groups as
ancillary ligands in organouranium chemistry, it is necessary
to investigate alternative routes for the preparation of mixed
aryloxide-halide compounds of uranium(IV). The preparations
of the aryloxide compounds U(OAr′)4 (OAr′ ) 2,6-diisopro-
pylphenoxide), [Li(THF)4][U(OAr′)5], and ClU(OAr′′)3 (OAr′′
) 2,4,6-tri-tert-butylphenoxide) have been reported by Lappert
and co-workers from UCl4 and the lithium salt of the aryloxide,6a,9
but this work has yet to be extended to other uranium halides
or aryloxide salts.
There are several factors to be probed in developing prepara-
tive routes to mixed aryloxide-halide complexes. The first of
these is the comparative utility of the heavier halides of uranium
as reagents. Given the difficulty of reported syntheses of UCl4,10
it is of significant interest to examine the applicability of other,
more readily prepared tetravalent uranium halide reagents.
Another goal of these investigations is to develop general routes
for the preparation of the compounds X2U(OAr)2 (X ) Cl, Br,
I). These compounds have an ancillary ligand set most directly
analogous to the bis(pentamethylcyclopentadienyl) framework
which has been so successfully employed in uranium organo-
(1) (a) NMT-6, Mail Stop E510, Los Alamos. (b) CST-18, Mail Stop
J514, Los Alamos.
(2) Weigel, F. In The Chemistry of the Actinide Elements; J. J. Katz, G.
T. Seaborg, L. R. Morss, Eds.; Chapman and Hall: New York, 1986.
(3) (a) Manriquez, J. M.; Fagan, P. J.; Marks, T. J. J. Am. Chem. Soc.
1978, 100, 3939. (b) Manriquez, J. M.; Fagan, P. J.; Marks, T. J. J.
Am. Chem. Soc. 1979, 101, 5075. (c) Fagan, P. J.; Manriquez, J. M.;
Maata, E. A.; Seyam, A. M.; Marks, T. J. J. Am. Chem. Soc. 1981,
103, 6650. (d) Fagan, P. J.; Manriquez, J. M.; Marks, T. J.
Organometallics 1982, 1, 170.
(4) Arney, D. S. J.; Burns, C. J. J. Am. Chem. Soc. 1993, 115, 9840.
(5) (a) Arney, D. S. J.; Burns, C. J.; Smith, D. C. J. Am. Chem. Soc.
1992, 114, 10068. (b) Arney, D. S. J.; Burns, C. J. J. Am. Chem. Soc.
1995, 117, 9448.
(6) (a) Blake, P. C.; Lappert, M. F.; Taylor, R. G.; Atwood, J. L.; Zhang,
H. Inorg. Chim. Acta 1987, 139, 13. (b) Van Der Sluys, W. G.; Burns,
C. J.; Huffman, J. C.; Sattelberger, A. P. J. Am. Chem. Soc. 1988,
110, 5924. (c) Van Der Sluys, W. G.; Sattelberger, A. P.; Streib, W.
E.; Huffman, J. C. Polyhedron 1989, 8, 1247. (d) Berg, J. M.; Clark,
D. L.; Huffman, J. C.; Morris, D. E.; Sattelberger, A. P.; Streib, W.
E.; Van Der Sluys, W. G.; Watkins, J. G. J. Am. Chem. Soc. 1992,
114, 10811.
(7) (a) Stewart, J. L. Ph.D. Dissertation, University of California, Berkeley,
CA 1988. (b) Brennan, J. G. Ph.D. Dissertation, University of
California, Berkeley, CA 1987.
(8) Avens, L. R.; Barnhart, D.; Burns, C. J.; McKee, S. D.; Smith, W. H.
Inorg. Chem. 1994, 33, 4245.
(9) Hitchcock, P. B.; Lappert, M. F.; Singh, A.; Taylor, R. G.; Brown, D.
J. Chem. Soc., Chem. Commun. 1983, 561.
(10) Sherill, H. J.; Durret, D. G.; Selbin, J. Inorg. Synth. 1974, 5, 243.
S0020-1669(98)00330-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/15/1998

Mixed Uranium(IV) Aryloxide-Halide Compounds
Inorganic Chemistry, Vol. 37, No. 16, 1998 4041
U(O-2,6-t-Bu2C6H3)2Br2(THF) (4). A solution of 0.068 g (2.78 ×
10-4 mol) of KOAr in 10 mL of THF was added to a stirred solution
of 0.100 g (1.39 × 10-4 mol) of UBr4(CH3CN)4 in 20 mL of THF
dropwise over a 10 min period. The solution was stirred for ca. 8 h,
by which time the solution was opaque due to the formation of a
precipitate. The THF solvent was removed under reduced pressure,
and the residue was extracted with 10 mL of a 2:1 toluene/hexane
solution and filtered through Celite. This solution was again dried to
yield a yellow solid. The solid was redissolved in 5 mL of hexane
with a small amount of THF added and the solution was cooled to
-40 °C, producing a golden yellow solid. Filtration yielded 0.094 g
of solid Br2U(OAr)2(THF), 4‚THF (77%). The compound could be
desolvated under dynamic vacuum to yield Br2U(OAr)2 (4).
metallic chemistry. The complex I2U(OAr)2 has been prepared
by direct oxidation of U(OAr)3,8 but the chloride and bromide
complexes cannot be prepared by analogous routes. Finally, it
is important to assess the susceptibility of the aryloxide
framework to ligand redistribution, a process documented for
sterically unsaturated uranium cyclopentadienide compounds.11
In this contribution, we report on these alternative routes for
the synthesis of mixed aryloxide-halide compounds, and on
the stability of mixed aryloxide-halide compounds of uranium
with regard to ligand redistribution.
Experimental Section
Data for 4. IR (cm-1 in Nujol): 1582 (vw), 1406 (s), 1364 (sh),
1306 (w), 1266 (m), 1219 (sh), 1206 (m), 1186 (vs), 1118 (s), 1092
(sh), 1034 (vw), 1000 (m), 958 (vw), 926 (w), 881 (m), 862 (vs), 830
(sh), 820 (vs), 794 (s), 749 (vs), 723 (m), 667 (vs), 631 (w), 548 (w),
451 (w). NMR: (C6D6) 28.5 (4 H), 20.1 (2 H), -6.2 (36 H) ppm.
Anal. Calcd for UBr2O2C28H42: C, 41.6; H, 5.24. Found: C, 41.15;
H, 5.20.
Data for 4‚THF. 1H NMR: (C6D6) 28.5 (s, 4 H), 20.1 (s, 2 H), 13.0
(br s, 4 H), 6.5 (br s, 4 H), -6.2 (s, 36 H) ppm. Anal. Calcd for
UBr2O3C32H50: C, 44.46; H, 5.83. Found: C, 44.72; H, 5.76.
[K(THF)4][UCl3(O-2,6-t-Bu2C6H3)2] (5). A solution of 0.258 g
(1.06 × 10-3 mol) of KOAr in 10 mL of THF was added to a stirred
solution of 0.200 g (5.27 × 10-4 mol) of UCl4 in 20 mL of THF
dropwise over a 10 min period. The solution was permitted to stir for
15 h. The mixture was filtered through Celite, and the filtrate was
reduced to less than 10 mL in volume and layered with hexane. The
mixture was cooled to -40 °C. After several days, dull yellow needles
were observed. The solid was isolated, washed with hexane, and dried
under vacuum. This yielded 0.385 g [K(THF)4][UCl3(OAr)2], 5, in
68% yield. Although stable upon isolation, solid 5 is susceptible to
loss of THF over time.
All operations were performed using standard Schlenk techniques
under UHP grade argon or in a Vacuum Atmospheres drybox under
helium. Tetrahydrofuran (THF), diethyl ether (Et2O), hexane, dimethox-
yethane (dme), and toluene were dried and distilled under nitrogen from
either sodium benzophenone or Na-K alloy. Benzene-d6 was dried
over CaH2, vacuum transferred, and freeze-pump-thawed three times
before use. THF-d8 was dried over Na-K/benzophenone, vacuum
transferred, and freeze-pump-thawed three times before use.
The uranium(IV) halides UI4(CH3CN)4, UBr4(CH3CN)4, and UCl4
were prepared by literature methods.10,12 Unless otherwise indicated,
we will designate 2,6-di-tert-butylphenoxide as OAr throughout this
paper. The compounds XU(OAr)3 (X ) Cl, Br, I), I2U(OAr)2, and
U(OAr)4 were independently prepared by alternate methods for direct
spectroscopic characterization by infrared and 1H NMR.6,8 Potassium
2,6-di-tert-butylphenoxide (KOAr) was synthesized from 2,6-di-tert-
butylphenol and potassium hydride.13
1H NMR spectra (250.13 MHz) were measured on a Bruker AF250
MHz spectrometer, with the chemical shifts (in ppm) reported relative
to the protio impurity of the deuterated solvent. All spectra were
recorded at 298 K unless indicated otherwise. Line widths were
relatively sharp for all compounds (3-15 Hz full width at half-height),
unless otherwise noted. Infrared spectra were obtained on a Bio-Rad
FTS-40 infrared spectrometer in Nujol on KBr plates. Elemental
analysis were performed in our laboratories on a Perkin-Elmer 2400
CHN analyzer. The samples were prepared and sealed in aluminum
capsules in the drybox prior to combustion.
Synthesis and Characterization. U(O-2,6-t-Bu2C6H3)2I2(THF) (1‚
THF). A solution containing 0.108 g (4.42 × 10-4 mol) of KOAr in
20 mL of THF was added dropwise to a stirred solution of 0.200 g
(2.20 × 10-4 mol) of UI4(CH3CN)4 in 25 mL of THF over a 10 min
period. The solution was stirred for ca. 20 min, by which time the
reaction mixture was opaque. The solvent was removed from the
reaction mixture in vacuo. The resulting brown oil was redissolved in
a solution of 10 mL of toluene and 1 mL of hexane and filtered through
Celite. The filtrate was concentrated to dryness to produce an orange
solid; this was redissolved in a minimum volume of THF, layered with
hexane, and cooled to -40 °C. After several days an orange precipitate
was collected by filtration. Additional batches were collected from
the mother liquor by layering with more hexane. The combined isolated
yield of I2U(OAr)2(THF), 1‚THF, from all batches was 50% (0.11 g).
The compound could be desolvated under dynamic vacuum to yield
I2U(OAr)2 (1).
U(O-2,6-t-Bu2C6H3)3I (2). A solution of 0.040 g (1.64 × 10-4 mol)
of KOAr in 10 mL of THF was added to a stirred solution of 0.150 g
(1.66 × 10-4 mol) of 1 in 20 mL of THF dropwise over a 10 min
period. A precipitate was observed to form and the solution became
more yellow. The solution was permitted to stir for ca. 2 h, at which
time the THF solvent was removed in vacuo. The resulting residue
was redissolved with 20 mL of hexane, permitted to stir for a period
of time, and finally filtered through Celite. The filtrate was concentrated
under reduced pressure and cooled to -40 °C. Pure IU(OAr)3, 2, was
isolated by filtration in 88% yield (0.14 g).
IR (cm-1 in Nujol): 1582 (vw), 1403 (s), 1370 (sh), 1308 (w), 1265
(m), 1211 (s), 1189 (s), 1122 (s), 1056 (m), 1039 (w), 1016 (w), 1005
(w), 920 (w), 881 (w), 859 (vs), 821 (s), 796 (m), 749 (s), 727 (w),
661 (s), 560 (sh), 549 (w), 460 (sh), 449 (m). 1H NMR: (C6D6) 30.6
(s, 2 H), 21.8 (s, 1 H), 10.0 (br s, THF), 2.7 (br s, THF), -3.8 (s, 18
H) ppm. Anal. Calcd for KUCl3O6C44H74: C, 48.82; H, 6.89.
Found: C, 48.23; H, 7.17.
U(O-2,6-t-Bu2C6H3)3Cl (6). A solution of 0.023 g (9.41 × 10-5
mol) of KOAr in 10 mL of THF was added to a stirred solution of
0.100 g (9.24 × 10-5 mol) of 5 in 20 mL of THF dropwise over a 10
min period. The solution was permitted to stir for 4 h, at which time
the solution was concentrated to dryness. The resulting residue was
extracted with 20 mL of hexane. The mixture was permitted to stir
and was then filtered through Celite. The solvent was removed in vacuo
to obtain a yellow solid. Recrystallization from a concentrated Et2O
solution at -40 °C yielded 0.072 g of ClU(OAr)3, 6, in 88% yield.
U(O-2,6-t-Bu2C6H3)4 (3). Synthesis from 2. A solution of 0.027
g (1.10 × 10-4 mol) of KOAr in 10 mL of THF was added to a stirred
solution of 0.100 g (1.02 × 10-4 mol) of 2 in 20 mL of THF dropwise
over a 10 min period. A precipitate was observed to form as the
solution became darker. The solution was permitted to stir for 4 h, at
which time the solvent was removed under reduced pressure. The
resulting residue was dissolved in toluene and filtered through Celite.
The solvent was removed in vacuo to yield a golden solid; recrystal-
lization from hexane at -40 °C yielded 0.092 g of U(OAr)4, 3 (85%).
Synthesis from UI4(CH3CN)4. A solution of 0.124 g (5.07 × 10-4
mol) of KOAr in 10 mL THF was added to a stirred solution of 0.110
g (1.21 × 10-4 mol) of UI4(CH3CN)4 in 20 mL of THF dropwise over
a 10 min period. The solution was permitted to stir for 15 h, at which
time the solvent was removed by vacuum to yield a tacky oil. The oil
was redissolved in toluene, filtered through Celite, and concentrated
to dryness. The residue was redissolved in a minimum of hexane, and
the solution was cooled to -40 °C. After several days, compound 3
was isolated by filtration in 54% yield (0.069 g).
(11) Ernst, R. D.; Kennelly, W. J.; Day, C. S.; Day, V. W.; Marks, T. J. J.
Am. Chem. Soc. 1979, 101, 2656.
(12) du Preez, J. G. H.; Zeelie, B. Inorg. Chim. Acta 1986, 118, L25.
(13) Geerts, R. L.; Huffman, J. C.; Caulton, K. C. Inorg. Chem. 1986, 25,
1803.
Synthesis from 6. A solution containing 0.027 g (1.10 × 10-4 mol)
of KOAr in 10 mL of THF was added to a stirred solution of 0.100 g
(1.12 × 10-4 mol) of 6 in 20 mL of THF dropwise over a 10 min

4042 Inorganic Chemistry, Vol. 37, No. 16, 1998
McKee et al.
Table 1. Crystallographic Parameters for K(THF)4[UCl3(OAr)2] (5)
Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for
[K(THF)4][UCl3(OAr)2] (5)
empirical formula
space group
temp (°C)
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
C44H74Cl3KO6U
P1h
Distances
-70
U(1)-Cl(1)
U(1)-Cl(2)
U(1)-O(1)
K(1)-Cl(1)
K(1)-Cl(3A)
K(1)-O(3)
K(1)-O(5)
O(1)-C(1)
2.667(3)
2.595(3)
2.122(6)
3.174(4)
3.213(4)
2.694(7)
2.684(9)
1.366(12)
11.007(2)
12.538(3)
19.704(4)
75.33(3)
83.59(3)
70.33(3)
2576.1(9)
2
U(1)-Cl(3)
U(1)-O(2)
2.636(3)
2.123(6)
K(1)-O(4)
K(1)-O(6)
O(2)-C(15)
2.687(11)
2.709(8)
1.388(9)
Z
F
calc (g/cm3)
1.452
Angles
Cl(1)-U(1)-Cl(2)
Cl(2)-U(1)-Cl(3)
Cl(1)-U(1)-O(1)
Cl(3)-U(1)-O(1)
Cl(2)-U(1)-O(2)
O(1)-U(1)-O(2)
Cl(1)-K(1)-O(3)
O(3)-K(1)-O(4)
O(3)-K(1)-O(5)
Cl(1)-K(1)-O(6)
O(4)-K(1)-O(6)
134.4(1) Cl(1)-U(1)-Cl(3)
93.0(1)
132.6(1)
radiation (λ (Å))
Mo KR (0.710 73)
1082.51
3.56
3.0-45.0° (ω-2θ)
4.25
fw
µ (mm-1
)
83.5(2) Cl(2)-U(1)-O(1)
96.2(2) Cl(1)-U(1)-O(2)
95.6(2) Cl(3)-U(1)-O(2)
167.0(3)
80.0(2) Cl(1)-K(1)-O(4)
90.3(3) Cl(1)-K(1)-O(5)
82.1(2) O(4)-K(1)-O(5)
103.1(3) O(3)-K(1)-O(6)
84.1(3) O(5)-K(1)-O(6)
93.4(2)
83.6(2)
92.7(2)
range (scan type)
Ra
Rw
a
6.11
79.5(3)
101.2(2)
172.3(3)
172.9(3)
103.0(3)
79.1(2)
a R ) ∑|Fo - Fc|/∑Fo; Rw ) ∑ w|F -F |/ wF .
x
x
o
c
o
period. The yellow solution was permitted to stir for 4 h, at which
time the solvent was removed by vacuum. The resulting residue was
extracted with toluene, and the solution was filtered through Celite.
The filtrate was concentrated to dryness to yield a golden solid.
Recrystallization from hexane at -40 °C yielded 0.089 g of U(OAr)4,
3 in 76% yield.
Synthesis from UCl4. A solution of 0.270 g (1.10 × 10-3 mol) of
KOAr dissolved in 20 mL of THF was added to a stirred solution of
0.100 g (2.63 × 10-4 mol) of UCl4 in 30 mL THF dropwise over a 10
min period. The solution was permitted to stir for 12 h, at which time
the solvent was removed by vacuum to yield a tacky residue. The
residue was extracted with toluene, and the solution was filtered through
Celite. The filtrate was concentrated to dryness; recrystallization from
hexane at -40 °C yielded 3 in 91% yield (0.25 g).
Ligand Redistribution Studies. In a representative procedure, a 5
mm NMR tube attached to a J. Young valve was loaded with 0.010 g
(1.10 × 10-5 mol) of UI4(CH3CN)4 and 0.011 g (1.04 × 10-5 mol) of
U(OAr)4. The solids were dissolved in a sufficient quantity of THF-
d8 or C6D6 (approximately 2 mL). The valve was closed, and the
mixture was monitored by 1H NMR spectroscopy. These experiments
were carried out at 25 and 60 °C.
Cl(1) -K(1)-Cl(3A) 153.9(1) Cl(1)-K(1)- Cl(3A)
U(1)-Cl(1)-K(1)
156.7(1) U(1)-Cl(3)-K(1A) 130.4(1)
bromide, and iodide precursors. The only reported well-
characterized tetravalent uranium iodide complex available for
study is UI4(CH3CN)4.12 This complex was expected to yield
clean products in metathesis reactions by analogy with the
known chemistry of the trivalent analogue UI3(THF)3.14 The
tetravalent iodide is isolated as an orange solid from the reaction
of uranium metal and elemental iodine in acetonitrile and is
freely soluble in THF. It has limited thermal stability, however,
and will decompose in THF solvent to yield a haloalkoxide
complex arising from the ring opening of tetrahydrofuran.15 In
the solid state, UI4(CH3CN)4 appears to decompose over several
months in an inert atmosphere drybox, presumably by the loss
of I2. The instability of UI4(CH3CN)4 in THF contrasts with
the stability of the related thorium complex ThI4(THF)4, which
has been successfully employed as a reagent in methathesis
reactions in THF.16
Crystal Structure Determination of K(THF)4[UCl3(OAr)2]. Crys-
tallization of 5 was accomplished by slow diffusion of hexane into a
concentrated THF solution of the compound at -40 °C. The crystals
were manipulated in mineral oil under an argon stream. A portion of
a yellow-brown needle of dimensions 0.30 mm × 0.32 mm × 0.40
mm was selected, mounted on a glass fiber with Apiezon “H” grease,
and transferred to the goniostat cooled to -70 °C. Data were collected
on the Enraf-Nonius CAD4 diffractometer with graphite monochro-
mated Mo KR radiation (λ ) 0.710 73 Å). Cell constants and an
orientation matrix were obtained by least-squares refinement from 25
reflections. The cell was determined to be triclinic. The data were
collected in ω-2θ scan mode. A series of high ø (above 80°)
reflections were scanned to provide the basis for an empirical absorption
correction. No crystal decay was evident during data collection.
The solution of the data was accomplished by direct methods in the
space group P1h. All remaining non-hydrogen atoms were located in
succeeding difference Fourier maps and were refined anisotropically.
The final residuals for the full-matrix least-squares refinement were R
) 4.25 and Rw ) 6.11, and the GOF ) 1.30 based on 496 refined
parameters. All calculations were carried out using the SHELXTL
PLUS software provided by Siemens Analytical. Crystallographic
parameters, bond distances, and bond angles for 5 are provided in Tables
1 and 2.
Addition of 2 equiv of KOAr to a THF solution of UI4(CH3-
CN)4 yields the compound I2U(OAr)2(THF), 1‚THF, in 50%
yield. The yield of 1‚THF by this route is lower than that
previously reported for the reaction of U(OAr)3 with carbon
tetraiodide.8 The reaction is also not as selective, and repeated
recrystallizations of the product are necessary to generate
analytically pure 1‚THF. It is possible that impurities are
introduced which are products of the ring opening of THF,
although the yield of the desired product is still moderate even
when freshly prepared UI4(CH3CN)4 is employed. Attempts
to carry out the synthesis of 1 in other solvents have not been
successful.
Mixed aryloxide-iodide complexes are more effective re-
agents in subsequent metathesis reactions, and the compound
I2U(OAr)2 will react with 1 equiv of KOAr in THF to yield the
trisaryloxide compound IU(OAr)3,8 2, in 88% yield. It is
important that the KOAr solution be added to the I2U(OAr)2
(14) (a) Clark, D. L.; Sattelberger, A. P.; Bott, S. G.; Vrtis, R. N. Inorg.
Chem. 1989, 28, 1771. (b) Schake, A. R.; Avens, L. R.; Burns, C. J.;
Clark, D. L.; Sattelberger, A. P.; Smith, W. H. Organometallics 1993,
12, 1497. (c) The reaction of UI3(THF)4 with 3 equiv of KOAr yields
(THF)U(OAr)3 by 1H NMR. This compound has been previously
prepared by an alternative route in ref 6a.
Results and Discussion
(15) Avens, L. R.; Barnhart, D.; Burns, C. J.; McKee, S. D. Inorg. Chem.
1996, 35, 537.
(16) Clark, D. L.; Frankcom, T. M.; Miller, M. M.; Watkin, J. G. Inorg.
Chem. 1992, 31, 1628.
In the course of developing synthetic routes to a series of
mixed uranium aryloxide-halide compounds, we have exam-
ined the metathesis reactions of tetravalent uranium chloride,

Mixed Uranium(IV) Aryloxide-Halide Compounds
Inorganic Chemistry, Vol. 37, No. 16, 1998 4043
Scheme 1. Synthesis of Uranium Iodide-Aryloxide
Compounds
Scheme 2. Synthesis of Uranium Chloride-Aryloxide
Compounds
Figure 1. ORTEP drawing of [K(THF)4][UCl3(OAr)2] (5) with the
atomic numbering scheme. The hydrogen atoms are omitted for clarity.
rather the “ate” complex [K(THF)4][UCl3(OAr)2], 5, isolated
as yellow-brown needles in 68% yield. The infrared spectrum
of 5 is remarkably similar to the spectrum of compounds 1 and
4, with the added appearance of weak stretches at 1056, 1016,
and 1005 cm-1 assigned to coordinated THF ligands.17 The
solid is soluble in THF, benzene, and toluene, and the 1H NMR
in C6D6 reveals only one set of aryloxide resonances. The
single-crystal X-ray structure determination of 5 has been
undertaken in order to determine the structural features associ-
ated with “ate” complexation (Figure 1).
solution slowly in order to avoid formation of U(OAr)4, 3.
Attempts to prepare 2 from UI4(CH3CN)4 and 3 equiv of KOAr
have resulted in low yields of the desired product. This
difference in the effectiveness of UI4(CH3CN)4 and mixed
aryloxide-iodide complexes as precursors in metathesis reac-
tions is again demonstrated in the synthesis of U(OAr)4, 3. This
compound can be prepared by two routes as depicted in Scheme
1. The first is the addition of KOAr to 2, which generates 3 in
85% yield. An alternative route for the preparation of 3 utilizes
the reaction of UI4(CH3CN)4 with a slight excess (4.2 equiv)
of KOAr. The isolated yield of the latter route is only 54%.
Unlike UI4(CH3CN)4, UBr4(CH3CN)4 is stable in THF
solvent; there is no evidence of reduction or solvent activation
in solutions maintained for 1-2 days. The synthesis of the
mixed bromide-aryloxide complex Br2U(OAr)2(THF), 4‚THF,
is readily accomplished in 77% yield from UBr4(CH3CN)4 (eq
1). The 1H NMR does not indicate the formation of any other
bromide-aryloxide compounds, including a possible “ate”
complex (vide infra), or the trisaryloxide BrU(OAr)3.8
The uranium atom lies in the center of an approximate trigonal
bipyramid of oxygen and chlorine atoms, with the aryloxide
groups oriented trans in the axial positions. The O(1)-U-
O(2) angle is slightly bent at 167.0(3)°. The overall geometry
for 5 is similar to that very briefly reported by Lappert et al.
for the structurally characterized compound [U(OAr′′)2Cl2(µ-
Cl)Li(THF)3] (OAr′′ ) 2,4,6-tri-tert-butylphenoxide).9b This
compound is may be considered also to have a trigonal
bypyramidal arrangments of ligands about the uranium center,
but with a cis disposition of the aryloxide ligands (the O-U-O
angle is reported to be 158°). For comparison, the five-
coordinate uranium(IV) compound I2U(OAr)2(THF)8 also has
a trigonal bypyramidal arrangment of ligands about the uranium
center, but with a cis disposition of the aryloxide ligands. The
uranium-oxygen (aryloxide) distances in 5 are identical (U-
O(1) ) 2.122(6) Å, U-O(2) ) 2.123(6) Å). These distances
are within the range observed for a variety of four- and five-
coordinate uranium(IV) aryloxide complexes (2.07-2.19 Å).6,8,9
The U-O-C(ipso) angles of the aryloxide ligands (U-O(1)-
C(1); 154.7(5)°, and U-O(2)-C(15); 153.2(7)°) are similar to
that in the tetrakisaryloxide complex U(O-2,6-t-Bu2C6H3)4
(154.0(6)°), and more bent than is commonly found in other
actinide aryloxide complexes6,8,9 except in those cases where
“ate” complexation involves an interaction beween the aryloxide
oxygen and the cation.18
UBr4(CH3CN)4 + 2KOAr THF8 Br2U(OAr)2 + 2KBr (1)
The dibromide, Br2U(OAr)2, 4, is very similar to 1 in its
physical and chemical properties. Like 1, compound 4 can be
isolated from solution as a stable THF adduct.8 Solid isolated
from the THF/hexane recrystallization analyzes as the adduct
Br2U(OAr)2‚THF. The 1H NMR of this material is identical to
4, except for the presence of broad resonances assigned to
coordinated THF (observed at 13.0 and 6.5 ppm in C6D6).
These resonances sharpen upon cooling the sample. Addition-
ally, the aryloxide resonances show little deviation in chemical
shift between the THF-free compound 4 and 4‚THF. Upon
exposure to dynamic vacuum, the THF-coordinated material
readily loses THF to generate the base-free dibromide 4.
By comparison, the use of UCl4 in related metathesis reactions
is well documented. Lappert and co-workers prepared a variety
of substituted aryloxide complexes of the U(IV) by reaction of
the tetrachloride with the lithium salts of the aryloxide anion.9
We have examined a series of metathesis reactions with the
potassium salt of 2,6-di-tert-butylphenoxide analogous to those
carried out with UI4(CH3CN)4 in order to examine the product
compositions and yields (Scheme 2).
The three chloride atoms lie in the equitorial plane of the
pentagonal bipyramid. The U-Cl distances are not equivalent
(U-Cl(1) ) 2.667(4) Å, U-Cl(2) ) 2.595(3) Å, and U-Cl(3)
) 2.636(3) Å. The lengthening of the U-Cl(1) and U-Cl(3)
distances is attributed to the interaction with the potassium (vide
infra). In UCl4(THF)3, the average U-Cl distance is 2.595(2)
(17) (a) Deacon, G. B., Tuong, T. D. Polyhedron 1988, 7, 249. (b) Lewis,
J.; Miller, J. R.; Richards, R. L.; Thompson, A. J. Chem. Soc. 1965,
5850.
(18) Barnhart, D. M.; Burns, C. J.; Sauer, N. N.; Watkin, J. G. Inorg. Chem.
1995, 34, 4079.
In contrast to the reactions of UX4(CH3CN)4 (X ) Br, I),
addition of 2 equiv of KOAr to a THF solution of UCl4 does
not yield the neutral bis(aryloxide) dichloride complex, but

4044 Inorganic Chemistry, Vol. 37, No. 16, 1998
McKee et al.
Compound 5 reacts with 1 equiv of KOAr in THF to yield
the trisaryloxide compound ClU(OAr)3, 6, in 88% yield. It is
again important that the KOAr solution be added slowly to the
[K(THF)4][UCl3(OAr)2] solution in order to avoid formation
of U(OAr)4, 3, which is observed to be an impurity in the
reaction product.
The tetrakisaryloxide U(OAr)4, 3, can also be prepared by
two routes. The first is the addition of KOAr to ClU(OAr)3, 6,
which yields U(OAr)4 in 76% isolated yield. The alternative
route utilizes reaction of UCl4 with a slight excess KOAr. The
isolated yield of the latter route is 91% isolated yield.
Ligand Redistribution Studies. An additional factor to
explore in evaluating the use of aryloxide ligands as ancillary
ligands in uranium organometallic chemistry is the susceptibility
of this ligand environment to engage in redistribution reactions
(i.e. halide-aryloxide exchange resulting in the formation of
an equilibrium mixture of compounds U(OAr)xXy). Steric bulk
in the ancillary ligand set can be used to inhibit these reactions,
however. For instance, the bis(pentamethylcyclopentadienyl)-
actinide framework has been employed extensively to stabilize
dihalide complexes of uranium and thorium against the ligand
redistribution problems which plague the unsubstituted cyclo-
pentadienyl analogues.10 It is unclear from previous work with
uranium whether 2,6-disubstituted aryloxide ligands are suf-
ficiently bulky to inhibit these reactions. The ability of bis-
(aryloxide) uranium diiodide and dibromide to form base adducts
with THF, and for the dichloride to form a five-coordinate “ate”
complex demonstrates that this framework is certainly less
sterically saturated than (C5Me5)2UCl2. Aryloxide ligand
redistribution has been reported to occur in certain reactions;
for example, the oxidation of U(OAr)3 with O2 yielded the
uranium(IV) compound U(OAr)4.8 The isolation and charac-
terization of the bis(aryloxide) compounds 1 and 3, however,
suggests that these compounds are not readily susceptible to
ligand exchange.
Figure 2. Packing diagram for [K(THF)4][UCl3(OAr)2] (5) illustrating
the infinite chain produced by the bridging potassium atoms.
Å.19 The Cl-U-Cl angles in 5 deviate from the ideal 120°
expected for the equatorial groups of a trigonal bipyramidal
molecule. While angles Cl(1)-U-Cl(2) and Cl(1)-U-Cl(3)
are 134.4(1) and 132.6(1)°, respectively, the Cl(2)-U-Cl(3)
angle is significantly smaller at 93.0(1)°. This angle is similar
to the 98.3(1)° found for the I-U-I angle observed in
I2U(OAr)2(THF).8
While the compound [U(OAr′′)2Cl2(µ-Cl)Li(THF)3] (OAr′′
) 2,4,6-tri-tert-butylphenoxide)9b exists in isolated ion pairs in
the solid state, the packing diagram for 5 shows that the
compound forms an infinite chain in the solid state through the
interactions between the potassium and the chloride atoms
(Figure 2). The potassium atom lies in a distorted octahedral
geometry, with axial chloride groups that connect two different
uranium centers, forming a repeating section in the chain. The
Cl(1)-K-Cl(3a) angle is 153.9(1)°, and the K-Cl(1) distance
is 3.174(4) Å. The octahedral coordination sphere about
potassium is completed by four THF molecules. The K-O(THF)
distances vary from 2.684(9) to 2.709(8) Å, and the O-K-O
angles are found to vary from 82.1(2) to 103.3(3)°. Oligomer-
ization via bridging chloride has been observed in the compound
[(C5Me5)2CeCl2K(THF)]n.20 However, the potassium atom in
this compound is five coordinate, with a single THF occupying
the coordination sphere (K-O(THF) ) 2.70(1) Å). The K-Cl
distances for [(C5Me5)2CeCl2K(THF)]n vary from 3.077(3) to
3.157(3) Å. Clark and co-workers have recently shown that a
number of lanthanide aryloxide compounds (aryloxide ) 2,6-
diisopropylphenoxide) possess solid-state structures where the
potassium atom interacts with the aryl rings or the aryloxide
oxygen atoms.21 This possible interaction is clearly not
observed in 5. The other five-coordinate potassium “ate”
complex of interest is the uranium(III) compound {[K(THF)2]2-
[U(NHAr′)5]}‚THF (Ar′ ) 2,6-diisopropylphenyl), where the
solid-state structure indicates η6-π interactions between the
arene ring and the potassium.22
To gauge the lability of the aryloxide group in these uranium
halide-aryloxide compounds, a series of experiments have been
conducted to look for possible redistribution. Initial experiments
were carried out using mixtures of UCl4 and U(OAr)4, UBr4(CH3-
CN)4 and U(OAr)4, and UI4(CH3CN)4 and U(OAr)4 (eq 2).
These experiments were performed in THF-d8. For the homo-
UX4 + U(OAr)
(X ) Cl, Br, I)
4 f
XU(OAr)3 + X2U(OAr)2 + X3U(OAr) (2)
leptic uranium(IV) halides UX4 (X ) Cl, Br, I), there is no
evidence of aryloxide scrambling with U(OAr)4 to form mixed
products UXy(OAr)4-y (X ) Cl, Br, I; y ) 1-4),23 either at
room temperature or elevated temperatures (prolonged heating
at 60 °C eventually results in decomposition of the samples).
A similar experiment was performed using the mixtures Br2U-
(OAr)2/U(OAr)4 and I2U(OAr)2/U(OAr)4 in either C6D6 of THF-
d8 (eq 3). There does not appear to be any reaction between
X2U(OAr)
2 + U(OAr)4 f
(X ) Br, I)
XU(OAr)3 + X2U(OAr)2 + X3U(OAr) (3)
(19) Van Der Sluys, W. G.; Berg, J. M.; Barnhart, D.; Sauer, N. N. Inorg.
Chim. Acta 1993, 204, 251.
(20) Evans, W. J.; Olofson, J. M.; Zhang, H.; Atwood, J. L. Organometallics
1988, 7, 629.
the dihalide complexes and uranium tetrakisaryloxide, although
the monohalide species may be formed by other means. At
(21) Clark, D. L.; Huffman, J. C.; Watkin, J. G. Inorg. Chem. 1992, 31,
1554.
(22) Nelson, J. E.; Clark, D. L.; Burns, C. J.; Sattelberger, A. P. Inorg.
Chem. 1992, 31, 1973.
(23) Preliminary crossover experiments between UI3(THF)4 and U(OAr)3
in THF-d8 also show that there is no crossover reaction to form
compounds of the formula UI3-x(OAr)x.

Mixed Uranium(IV) Aryloxide-Halide Compounds
Inorganic Chemistry, Vol. 37, No. 16, 1998 4045
room temperature, there is no evidence by NMR for the
formation of IU(OAr)3 (or BrU(OAr)3) over a 24 h period.
However, IU(OAr)3 and BrU(OAr)3 are formed when these
experiments are carried out at elevated temperatures due to the
thermal instability of the dihalide (eq 4).
UBr4(CH3CN)4 with 2 equiv of potassium aryloxide generates
the neutral complex Br2U(OAr)2, while the analogous reaction
with UCl4 yields only the “ate” complex [K(THF)4][UCl3-
(OAr)2]. This distinction may not be significant in subsequent
reactivity, however; preliminary experiments suggest that
compound 5 serves as a “Cl2U(OAr)2” synthetic equivalent in
both metathesis reactions and in reactions with strongly
coordinating Lewis bases.
I2U(OAr)2 60 °C8 IU(OAr)3
(4)
We have also examined the susceptibility of these complexes
to ligand redistribution. Neither aryloxide nor halide exchange
is observed. It is conceivable that ligand redistribution in all
mixed uranium aryloxide-halide is inhibited to some degree
by the steric bulk of the 2,6-di-tert-butyl phenoxide ligand. This
contention is not supported, however, by the observation that
redistribution does not occur even in complexes that are
coordinatively unsaturated. In related high-valent uranium
aryloxide chemistry, we have seen a similar lack of redistribution
chemistry, even though structures have been isolated which
demonstrate that bulky aryloxide groups can bridge metal
centers.25 We are currently exploring the role of steric hindrance
with smaller aryloxide groups. If in fact there is an electronic
origin to the kinetic stability of these species toward redistribu-
tion, the aryloxide supporting ligand sets investigated here could
be attractive substitutes to carbocyclic ligands in organouranium
chemistry.
The diiodide decomposes most readily; conversion of Br2U-
(OAr)2 to BrU(OAr)3 is significantly slower. The presence of
U(OAr)4 does not alter the rate of formation of the monohalide
products. Other mixed iodide-aryloxide complexes should be
formed in the redistribution reaction in eq 4, but they are not
1
observed by H NMR in solution. Prolonged heating at the
elevated temperature leads to complete sample decomposition.
In an effort to explore the possibility of halide exchange, an
experiment was conducted in which compounds 1 and 4 were
allowed to react at room temperature (eq 5). By NMR, there
I2U(OAr)2 + Br2U(OAr)2 f IBrU(OAr)2
(5)
is no evidence to support any formation of the mixed halide
IBrU(OAr)2. The absence of detectable halide exchange
contrasts with the observation that this process occurs readily
in UX4L2 compounds.24
Conclusions
Acknowledgment. We thank Dr. Paul Hurlburt for the
supply of UBr4(CH3CN)4. We acknowledge the financial support
of the U.S. Department of Energy, Office of Basic Energy
Sciences, Chemical Sciences Division.
We have examined the utility of uranium(IV) chloride,
bromide, and iodide halides in metathesis reactions for the
production of mixed halide-aryloxide complexes. The choice
of halide precursor plays a role both in the nature and the yield
of the isolated product. While I2U(OAr)2 can be prepared from
UI4(CH3CN)4, the thermal stability problems of the halide in
THF complicate the use of the tetraiodide as an efficient starting
material. The use of either UBr4(CH3CN)4 and UCl4 in
metathesis reactions leads to the clean formation of mixed
halide-aryloxide products, but the product identity can be a
function of the starting material. For instance, reaction of
Supporting Information Available: Tables listing the complete
sets of positional and equivalent isotropic thermal parameters of the
non-hydrogen atoms, bond lengths and angles, hydrogen coordinates
and isotropic thermal parameters, and anisotropic thermal parameters
for compound 5 are available (19 pages). Ordering information is given
on any current masthead page.
IC9803309
(24) Avens, L. R.; Burns, C. J.; McKee, S. D. Unpublished results.
(25) Wilkerson, M. P.; Scott, B. L.; Burns, C. J. Manuscript in preparation.