Ligand metalation in the reactivity of a tetravalent uranium amides

Article abstract of DOI:10.1021/ic902567m

The reactions of organolithium reagents with the tetravalent UCl ₂[N(SiMe₃)₂]₂ and its DME solvate have been examined. Treatment of both compounds with methyl-lithium in diethyl ether resulted in one electron re

Full text of DOI:10.1021/ic902567m

Inorg. Chem. 2010, 49, 3409–3418 3409
DOI: 10.1021/ic902567m
Ligand Metalation in the Reactivity of a Tetravalent Uranium Amides
Ilia Korobkov and Sandro Gambarotta*
Department of Chemistry, University of Ottawa, Ottawa, ON K1N 6N5, Canada
Received December 23, 2009
The reactions of organolithium reagents with the tetravalent UCl2[N(SiMe3)2]2 and its DME solvate have been
examined. Treatment of both compounds with methyl-lithium in diethyl ether resulted in one electron reduction of the
metal center and γ-deprotonation of one of the ligands. The dimeric {U[(μ-CH2-SiMe2)N(SiMe3)]2[μ-Li(DME)]}2 (1)
was isolated from the reaction mixture regardless of the amount of MeLi employed. The employment of LiCH2SiMe3 in
DME led instead to multiple γ-deprotonation events at the same carbon atom with formation of the trimetallic {U[(μ-
CH2-SiMe2)N(SiMe3)][N(SiMe3)2]}2{U[(μ3-C-SiMe2)N(SiMe3)][N(SiMe3)2]}{μ-OMe} (2) cluster centered on a
fully deprotonated carbon atom. Crystallographic analysis revealed the presence of μ-OCH3 units in the cluster as
generated by DME solvent cleavage. A similar reaction carried out in the absence of DME led to the isolation of a
closely related trimetallic {U[μ-(CH2-SiMe2)N(SiMe3)][N(SiMe3)2]}2{U[(μ3-C-SiMe2)N(SiMe3)][(μ-CH2-SiMe2)-
N(SiMe3)]} (3). One additional γ-deprotonated fragment replacing the bridging methoxy group of 2 was present in this
case. The presence of a fully deprotonated carbon atom bridging three metal centers and of one silicon atom was
confirmed by both X-ray structures and NMR data. An attempt to reduce UCl2[N(SiMe3)2]2 with KC8 in a coordinating
solvent resulted in ligand scrambling with the formation of two products. The first is a trimeric U(III) cluster formul-
ated as {U-μ-Cl[N(SiMe3)2][DME]}2{U-μ-Cl[N(SiMe3)2]2}{μ3-Cl}2 (4). The second was U[N(SiMe3)2]3. A similar
reduction reaction carried out in noncoordinating toluene resulted instead in an attack on the ligand affording the
dimeric {U-μ-Cl[N(SiMe3)2]2[dN(SiMe3)]}2 (5). Alkylation of UCl2[N(SiMe3)2]2 with n-butyl-lithium in hexane
surprisingly yielded the pentavalent U[(μ-CH2-SiMe2)N(SiMe3)]2[N(SiMe3)2] (6). The acquisition of one additional
ligand during the reaction hinted at the presence of other products in the reaction mixture.
Introduction
transformations reported so far in the literature are dinitro-
gen activation/reduction1 and cleavage,2 depolymerization of
poly silanoles with cleavage of the Si-O bond,2 CO oligo-
merization,3 unusual coordination of small molecules,4 in-
cluding a unique case of sp3-C-H bond coordination,4a and
oxidative elimination of H2.5
It is today commonly agreed that low valent uranium has
the potential for providing a rich and diversified chemical
reactivity of the caliber of that observed for some div-
alent lanthanides. Among the most exciting and promising
As usual, ligand features are central to the performance of
the metal and to the stabilization of the low-oxidation state.
First, it should be noticed that authentic oxidation states
lower than þ3 are unknown for uranium. Only in the
presence of π-systems have a few reduced species (or low-
valent synthons) been prepared and characterized.6-8 The
particular nature of the ligand donor atom also seems to be of
primary importance. The reactivity of low-valent uranium
becomes particularly enhanced in the presence of nitrogen
donor atoms as indicated by the few cases of dinitrogen
reversible fixation1 and cleavage.2 The first well-documented
reduction affording a highly reduced uranium complex with
*To whom correspondence should be addressed. E-mail: Sandro.
Gambarotta@uottawa.ca.
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Kozimor, S. A.; Ziller, J. W. J. Am. Chem. Soc. 2003, 125, 14264.
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r
2010 American Chemical Society
Published on Web 03/04/2010
pubs.acs.org/IC

3410 Inorganic Chemistry, Vol. 49, No. 7, 2010
Korobkov and Gambarotta
an “inverted-sandwich” structure was obtained by using
bulky amido ligands.6,7 Furthermore, bulky amido ligands
may be effectively used also for the purpose of obtaining
solvent-free compounds given that the presence of solvent in
the coordination sphere often leads to fragmentation pro-
cesses interfering with the reactivity of the metal center.9
This background prompted us to revisit the reduction
chemistry of uranium supported by the bis-silazanate anion.
This ligand has been successfully used by Andersen et al. in
his pioneering work in the chemistry of the tetravalent state.10
In the field of f-block element chemistry, it is known to afford
low coordinate complexes with excellent yields for several
metals.11 Low coordination numbers, the absence of coordi-
nated solvent molecules, and high solubility in hydrocarbons
and aromatic solvents in addition to high ligand basicity,
altogether, make these derivatives excellent precursors for
reactivity studies.1d,7b,11,12 A recurrent feature discovered so
far for this ligand system is its direct involvement in the
organometallics chemistry of the metal center (γ-metalation).
In the chemistry of uranium, products resulting from γ-
metalation were utilized for a variety of organometallic
transformations including nucleophilic behavior13 and inser-
tion reactions14 and for discovering examples of unusual
reactivity.14a,c Furthermore, the lability of the Si-N bond, in
combination with the availability of electronsprovidedby the
reduced state, allows formation of bridging or terminally
bonded imido groups6b,13a,14b,15 via elimination of one of the
two Me3Si residues.16 As far as the trivalent state is con-
cerned, U[N(SiMe3)2]3 is known but does not display unusual
reactivity. This behavior is certainly not typical for a U(III)
metal center which, when combined to N-donor atoms,
characteristically shows extreme reactivity.1b,c,3,4,17 On the
other hand, the large bulk generated by the three equatorial
ligands makes the metal inaccessible to reagents other than
protons.
In this work, we have studied the reduction chemistry of
tetravalent uranium silazanate derivatives aiming at explor-
ing the possibility of preparing lower-valent complexes for
reactivity purposes. As correctly observed by Andersen et al.,
we also observed that this chemistry is indeed dominated by
γ-metalation. Herein, we report how different extents of γ-
deprotonation may lead to cluster formation. It was also
observed that attempts to lower the oxidation state with
strong reductants resulted instead in reoxidation of the metal
center at the expense of the ligand possibly through dispro-
portionative pathways.
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Experimental Section
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All operations were performed under a nitrogen atmo-
sphere with rigorous exclusion of oxygen and water using
standard Schlenk and glovebox techniques. Hexane and
toluenesolventswerepurifiedbypassing through Al2O3 filled
columns and deoxygenated prior to use by several vacuum/
nitrogen purges. DME was dried over LiAlH4 and distilled
under N2 prior to utilization. Benzene-d6 was obtained from
“C/D/N isotopes”, dried over Na/K alloy, distilled, and
stored over molecular sieves (4A). n-Butyl-lithium solution
(2.5 M) in hexane and methyl-litium solution (1.6 M) in
diethyl ether were purchased from Aldrich and used as re-
ceived. LiCH2(SiMe3) was prepared according to a modified
literature procedure18 using boiling cyclohexane as a solvent.
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Article
Inorganic Chemistry, Vol. 49, No. 7, 2010 3411
KC8 was prepared according to a literature procedure.19
Complexes {UCl2[N(SiMe3)2]2}2 and UCl2[N(SiMe3)2]2[C4-
H10O2] were synthesizedaccording toliterature procedures.20
Samples for magnetic susceptibility measurements were car-
ried out at room temperature using a Gouy balance (Johnson
Matthey). Magnetic moments were calculated by following
standard methods, and corrections for underlying diamag-
netism were applied to the data.21 NMR spectra were re-
corded at 293 K on a Varian Inova 500 MHz spectrometer.
Chemical shifts were referenced to internal solvent reso-
nances and reported in parts per million relatively to Me4Si.
Elemental analyses were performed on a Perkin-Elmer 2400
CHN analyzer. Data for X-ray single crystal structure deter-
mination were collected with a Bruker diffractometer
equipped with a 1K SMART CCD area detector. Chemical
degradation experiments were performed in a sealed vessel
connected to a high vacuum line, by condensing a stoichio-
metric amount of gaseous HCl over the sample kept at liquid
N2 temperatures. After reaching room temperature, the vo-
latiles were pumped off and collected with a Toepler pump
and analyzed using a GC with a molecular sieves column and
TC detector.
74%). Elem. Anal. for U3C37H106N6OSi12 found (calculated):
C, 25.97 (26.10); H, 6.14 (6.28); N, 4.90 (4.94). 1H NMR (500 MHz,
benzene-d6, 20 °C): δ 29.36 (27H, N-Si(CH3)3), 9.85 (3H,
O-CH3), 7.19 (54H, N[Si-(CH3)3]2), -0.13 (broad, 18H, Si-
(CH3)2), -46.90 (4H, Si-CH2-U). IR (Nujol mull, cm-1): 1589
(sh), 1261 (s), 1162 (s), 968 (vs), 926 (vs), 850 (vs), 769 (s), 710
(m), 670 (m), 639 (w), 561 (sh), 476 (sh), 443 (m), 415 (m). μeff
1.79 μB.
=
Preparation of {U[μ-(CH2-SiMe2)N(SiMe3)][N(SiMe3)2]}2-
{U[(μ3-C-SiMe2)N(SiMe3)][(μ-CH2-Si Me2)N(SiMe3)]} (3).
A suspension of UCl2[N(SiMe3)2]2 (0.45 g, 0.7 mmol) in hexane
(20 mL) was combined with a solution of LiCH2SiMe3 (0.14 g,
1.5 mmol) in the same solvent (5 mL). An instant color change
from pale green to dark brown was observed upon addition.
After 6 h of stirring at room temperature, the solvent was
removed in vacuo. The resulting dark-brown powdery material
was extracted with hexane (30 mL). The volume of the extract
was reduced to 7 mL and the solution allowed to stand at -37 °C
for 4 days. Dark brown prisms of 3 were formed, which were
separated, washed with of cold hexane (5 mL, -37 °C), and
dried under a N2 atmosphere (0.26 g, 0.16 mmol, 67%). Elem.
Anal. for U3C36H102N6Si12 found( calculated): C, 25.57 (25.89);
H, 6.07 (6.16); N, 4.99 (5.03). 1H NMR (500 MHz, benzene-d6,
20 °C): δ 24.11 (36H, N-Si(CH3)3), 7.22 (36H, N[Si-(CH3)3]2),
-13.48 (18H, Si-(CH3)2), -16.85 (6H, Si--(CH3)2), -43.10
(6H, Si-CH2-U). IR (Nujol mull, cm-1): 1627 (sh),1259 (vs),
1093 (vs), 1022 (vs), 800 (br, vs). μeff = 1.65 μB.
Preparation of {U[(μ-CH2-SiMe2)N(SiMe3)]2[μ-Li(DME)]}2
(1). A solution of methyl-lithium (9 mL of 1.6 M, 14.4 mmol)
in diethyl ether was slowly added to a cooled solution (-37 °C)
of UCl2[N(SiMe3)2]2(DME) (3.40 g, 4.72 mmol) in DME (20 mL).
The reaction mixture was allowed to warm up to room tem-
perature under vigorous stirring. The color changed from the
initial pale-green to intense brown-red while a colorless insolu-
ble material separated. After 2 additional hours of stirring at
room temperature, the solvent was removed in vacuo and the
resulting orange solid suspended in hexane (35 mL). The
insoluble materials were eliminated by centrifugation and the
volume reduced (11 mL) by evaporation in vacuo. The solution
was allowed to stand at -37 °C for 48 h, after which dark red-
orange prisms were formed. After washing with cold hexane
(-37 °C, 3 mL) and drying for 30 min under N2, crystalline 1 was
obtained (2.87 g, 4.12 mmol, 87%). Upon further drying, dark
red-orange crystals of 1 deteriorated with the formation of
yellow-orange powdery material. Elem Anal. for UC16H44Li-
N2O2Si4 found (calculated): C, 29.31 (29.39); H, 6.63 (6.78); N,
4.23 (4.28). 1H NMR (500 MHz, benzene-d6, 20 °C): δ 3.30 (6H,
DME, CH3-O), 3.12 (4H, DME, -CH2-O), -13.10 (12H, Si-
(CH3)2), -28.57 (18H, Si-(CH3)3), -47.02 (4H, Si-CH2-U).
IR (Nujol, KBr, cm-1): 1602 (sh), 1482 (sh), 1453 (s), 1246 (m),
980 (vs), 822 (vs), 675 (w), 632 (m), 614 (m), 479 (m), 434 (w).
Preparation of {U-μ-Cl[N(SiMe3)2][DME]}2{U-μ-Cl[N-
(SiMe3)2]2}{μ3-Cl}2 (4). A solution of UCl2[N(SiMe3)2]2[DME]
(1.60 g, 2.2 mmol) in DME (25 mL) was treated with freshly
prepared KC8 (0.61 g, 4.5 mmol). The mixture was stirred at
room temperature for 3 days, during which the color slowly
changed to dark red-purple. When no further color change
could be detected, the solvent was removed in vacuo and the
residual dark-red solid was extracted with boiling toluene (15 mL).
The volume of the dark-red solution was reduced to 5 mL and
allowed to stand at room temperature for 48 h. Two different
crystalline materials (dark-red blocks and purple needles) sepa-
rated in the mixture. After washing with hexanes (40 mL) at
room temperature, the needle-shaped crystals completely dis-
appeared. Recrystallization of the remaining solid material from
hot toluene led to the isolation of dark-red blocks of 4 (0.69 g,
0.4 mmol, 52%). Elem. Anal. for U3C39H100Cl5N4O4Si8 found
(calculated): C, 25.87 (25.95); H, 5.47 (5.58); N, 3.09 (3.10). IR
(Nujol mull, cm-1): 1604 (sh), 1459 (s), 1444 (vs), 1247 (br vs),
1086 (m), 1014 (m), 908 (vs), 879 (sp), 840 (vs, b), 782 (s), 758 (s),
694 (vs), 669 (vb). The hexanes extracted were concentrated (10 mL)
and allowed to stand at -37 °C, affording crystals of the second
μeff = 2.53 μB.
Preparation of {U[(μ-CH2-SiMe2)N(SiMe3)][N(SiMe3)2]}2-
{U[(μ3-C-SiMe2)N(SiMe3)][N(SiMe3)2]}{μ-OMe} (2). A solu-
tion of UCl2[N(SiMe3)2]2(DME) (0.764 g, 1.0 mmol) in hexane
(10 mL) was combined with a solution of LiCH2SiMe3 (0.20 g,
2.2 mmol) in the same solvent (5 mL). An instant color change
from pale green to dark orange, accompanied by the formation
of pale insoluble material, was observed upon mixing. Stirring
was continued at room temperature for an additional 2 h, and
the insoluble material was eliminated by centrifugation. The
resulting solution was concentrated to about 5 mL and allowed
to stand at -37 °C. After three days, dark orange crystals were
formed, which were separated, washed with cold hexane (3 mL,
-37 °C), and dried under a N2 atmosphere (0.44 g, 0.26 mmol,
compound. A database search of the crystallographic cell para-
22
meters identified this second product as U[N(SiMe3)2]3.
1.47 μB.
μ
=
eff
Preparation of {U-μ-Cl[N(SiMe3)2]2[dN(SiMe3]}2 (5). A sus-
pension of UCl2[N(SiMe3)2]2 (0.26 g, 0.4 mmol) in 20 mL of
toluene was treated with freshly prepared KC8 (0.17 g, 1.2 mmol).
The reaction mixture was stirred at room temperature for 7
days, during which, the color steadily changed from pale green
to deep brown-red. The solvent was then removed in vacuo and
the residual dark brown-red solid extracted with hexanes (35 mL).
The volume of hexane extracts was reduced in vacuo to 10 mL
and allowed to stand at -37 °C. After 3 days, a mixture of
crystals with two different shapes was obtained. Fractional
crystallization from toluene/hexane mixtures afforded dark
brown-red prisms of 5 (0.12 g, 0.1 mmol, 42%). Storage of the
mother liquor at -37 °C for 3 additional days afforded complex
6 in crystalline form (0.05 g, 0.04 mmol, 18%). Elem. Anal. for
(18) Connolly, J. W.; Urry, G. Inorg. Chem. 1963, 2, 645.
(19) Schwindt, M. A.; Lejon, T.; Hegedus, L. B. Organometallics 1990, 9,
2814.
(20) McCullough, L. G.; Turner, H. W.; Andersen, R. A.; Zalkin, A.;
Templeton, D. H. Inorg. Chem. 1981, 20, 2869.
(21) (a) Mabbs, M. B.; Machin, D. Magnetism and Transition Metal
Complexes; Chapman and Hall: London, 1973. (b) Foese, G.; Gorter, C. J.; Smits,
L. J. Constantes Selectionnes, Diamagnetisme, Paramagnetisme, Relaxation
Paramagnetique; Masson: Paris, 1957.
(22) Preparation and characterization: Andersen, R. A. Inorg. Chem.
1979, 18, 1507. Structural data: Stewart, J. L.; Andersen, R. A. Polyhedron, 1998,
17, 953.

3412 Inorganic Chemistry, Vol. 49, No. 7, 2010
Korobkov and Gambarotta
U2C30H90Cl2N6Si10 found (calculated): C, 26.40 (26.44); H, 6.47
(6.66); N, 6.15 (6.17). IR (Nujol mull, cm-1): 1578 (sh), 1259 (e),
1160 (e), 973 (vs), 919 (vs), 843 (vs), 777 (s), 715 (m), 675 (m), 644
(w), 556 (sh), 470 (sh), 440 (m), 415 (m). μeff = 2.48 μB.
Table 1. Crystal Data and Data Collection Parameters of Complexes 1, 2, and 3
1
2
3
formula
U2C38H102
-
U3C37H106
N6OSi12
1702.45
triclinic
P1
11.923(3)
13.947(4)
22.855(6)
79.887(6)
75.655(5)
67.832(5)
3395.7(15)
2
-
U3C36H102
N6Si12
1670.41
triclinic
P1
14.826(3)
15.141(3)
15.561(3)
75.030(3)
89.431(4)
86.129(4)
3366.8(12)
2
-
Li2N4O4Si8
1393.90
monoclinic
P21/n
14.862(3)
11.980(2)
18.250(3)
90
97.579(4)
90
3220.8(10)
2
1.437
Preparation of U[(μ-CH2-SiMe2)N(SiMe3)]2[N(SiMe3)2] (6).
A suspension of UCl2[N(SiMe3)2]2 (0.500 g, 0.79 mmol) in cold
hexane (-37 °C, 15 mL) was treated with a cooled solution of n-
BuLi in the same solvent (0.65 mL, 2.5 M, 1.63 mmol). The
reaction mixture was allowed to warm to room temperature
while being vigorously stirred. The color of the solution changed
to dark purple-red and a visible amount of insoluble material
appeared. After stirring for 4 h at room temperature, the solvent
was removed in vacuo and the resulting solid residue extracted
with 30 mL of boiling hexane. The volume of the extract was
reduced to 10 mL, and the solution was allowed to be left
standing at -37 °C. Dark-red prisms of 6 were formed in 3
days, which were washed with cold hexanes (3 mL) and dried
under a N2 atmosphere (0.183 g, 0.26 mmol, 33%). Elem. Anal.
for UC18H52N3Si6 found (calculated): C, 30.07 (30.15); H, 7.17
(7.31); N, 5.79 (5.86). IR (Nujol mull, cm-1): 1585 (sh), 1249 (s),
1152 (s), 960 (vs), 930 (vs), 845 (vs), 759 (s), 722 (m), 676 (m), 631
(w), 551 (sh), 444 (w), 425 (w). μeff = 1.48 μB.
fw
cryst syst
space group
˚
a, A
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
volume, A
Z
3
˚
F
calc g/cm3
μ, mm-1
F(000)
1.665
7.378
1644
1.58-21.96
1.648
7.438
1608
1.35-28.50
5.203
1384
1.66-20.81
T range
(deg)
limiting
indices
(h, k, l)
reflns
(14, 0 to þ11,
0 to þ18
(12, (14,
0 to þ23
(19, ( 19,
0 to þ20
3309/3309
8068/8068
23433/14059
collected/
unique
Rint
GOF
R1 (obs/all)
wR2 (obs/all)
X-Ray Crystallography
0.0000
0.647
0.0476/0.0868
0.0897/0.0961
0.0000
0.934
0.0668/0.1664
0.0797/0.0985
0.0218
1.013
0.0285/0.0399
0.0659/0.0706
For all the compounds, the results presented are the best of
several trials. The crystals were mounted on thin glass fibers
using paraffin oil and cooled to the data collection tempera-
ture. Crystals in general scattered very poorly and displayed
an additional tendency to rapidly decay under the X-ray
beam. Therefore, data collection times have been necessarily
short. In turn, this resulted in poor scattering at a wide angle
and occasionally in poor crystal solution parameters. None-
theless, in every case, the quality of the structures was
sufficient to establish the connectivity. Data were collected
on a Bruker-AXS SMART 1k CCD diffractometer. Data for
all the compounds were collected with a sequence of 0.3° ω
scans at 0, 120, and 240° in j. Initial unit cell parameters were
determined from 60 data frames collected at the different
sections of the Ewald sphere. Semiempirical absorption
corrections based on equivalent reflections were applied.23
Systematic absences in the diffraction data set and unit-cell
parameters were consistent with monoclinic P21/n for 1 and
5; triclinic P1 for 2, 3, and 4; and orthorhombic Pnma for 6.
Solutions in centrosymmetric space groups for all of the
compounds yielded chemically reasonable and computation-
ally stable results of refinement. The structures were solved
by direct methods, completed with difference Fourier synthe-
sis, and refined with full-matrix least-squares procedures
based on F2. In all the structures, the compound molecules
were located in general positions. The structures of com-
plexes 1 and 5 represent dimeric formations with two mon-
omeric units related by an inversion center symmetry
operator. The structure of complex 6 displays a 50:50
disorder of the U moiety with two disordered units related
by a mirror plane symmetry operator. Carbon atoms of a
cocrystallized hexane solvent molecule in 1 were refined
isotropicaly due to partial occupancy coupled with signifi-
cant thermal motion disorder and in order to maintain an
optimal data to parameters ratio. In all the structures, all
non-hydrogen atoms, with the exceptions mentioned above,
were refined with anisotropic displacement coefficients. All
hydrogen atoms were treated as idealized contributions. All
4
5
6
formula
U3C39H100
Cl5N4O4Si8
1805.29
triclinic
-
U2C30H90
Cl2N6Si10
1362.94
monoclinic
P21/n
11.910(3)
14.303(3)
17.715(4)
90
99.568(3)
90
2975.8(11)
2
1.521
5.751
1340
1.84-28.90
(13, ( 18,
( 22
-
UC18H52
N3Si6
717.20
orthorhombic
Pnma
18.607(3)
8.440(1)
21.682(3)
90
90
90
3405.2(8)
4
1.399
4.988
1428
2.17-21.96
(19, ( 8,
( 22
-
fw
cryst syst
space group
P1
˚
a, A
15.1810(17)
15.6704(18)
15.9987(18)
78.675(2)
87.335(2)
66.676(2)
3424.8(7)
2
1.751
7.445
1734
1.75-21.96
(16, ( 17,
0 to þ17
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
volume, A
Z
3
˚
F
calc g/cm3
μ, mm-1
F(000)
T range (deg)
limiting
indices
(h, k, l)
reflns
collected/unique
Rint
GOF
R1 (obs/all)
wR2 (obs/all)
30603/9777
16720/6814
23771/2383
0.2162
1.034
0.0597
1.053
0.0939
1.031
0.0631/0.1145 0.0475/0.0959 0.0448/0.0844
0.1354/0.1467 0.0797/0.0907 0.0752/0.0848
scattering factors are contained in several versions of the
SHELXTL program library, version 6.12.24 Crystallographic
data for all structures are reported in Table 1. Selected bond
distancesand anglesare given in the Supporting Information.
Results and Discussion
As previously observed by Andersen et al.,10 attempts to
alkylate the tetravalent uranium tris-amido-chloride complex
UCl[N(SiMe3)2]3 inevitably afforded γ-deprotonation of the
hexamethyldisilazane ligand. By using the uranium precursor
UCl2[N(SiMe3)2]2(DME), we have now obtained multiple
(23) Blessing, R. Acta Crystallogr. 1995, A51, 33.
(24) Sheldrick, G. M. Bruker AXS: Madison, WI, 2001.

Article
Inorganic Chemistry, Vol. 49, No. 7, 2010 3413
Scheme 1
Scheme 2
deprotonation and even reduction to the trivalent state
depending on the nature of the organo-lithium employed.
For example, low temperature reaction with two equivalents
of methyl lithium gave the trivalent {U[(μ-CH2-SiMe2)N-
(SiMe3)]2[μ-Li(DME)]}2 (1) in near to quantitative yield
(Scheme 1). Complex 1 does not contain terminal U-Me
functions but had both silazanate groups deprotonated at
one methyl group forming methylenes. The presence of the
lithium countercation coordinated to the two methylenes
implies reduction of the metal center to the trivalent state.
The reaction was accompanied by formation of methane
(observed in the reaction mixtures of experiments performed
in sealed NMR tubes). The 1H NMR spectrum of 1 was in
agreement with the formulation as yielded by the crystal
structure showing the expected three broad singlets at -13.1,
-28.6, and -47.0 ppm with a relative intensity of 6:9:2 and
respectively assigned to the Si-(Me)2, N-SiMe3, and
-CH2- groups. The resonances of the coordinated DME
were located at 3.12 ppm and 3.30 ppm. Degradation of 1 in a
Toepler pump apparatus did not yield hydrogen gas, thus
excluding the possibility for the complex to be a hydride
derivative.
groups and also the intervention of a third equivalent of
alkyl-lithium, exclusively acting as a reducing agent. The
C-H σ-bond metathesis of silazanate groups by several
reagents is widely precedented in actinide chemistry.15 At-
tempts to prevent reduction by using a lower amount of MeLi
simply decreased the yield of 1.
A very similar reaction under comparable conditions was
carried out in hexane with [(trimethylsilyl) methyl]lithium,
affording the trimeric tetravalent uranium cluster {U[(μ-
CH2-SiMe2)N(SiMe3)][N(SiMe3)2]}2{U[(μ3-C-SiMe2)N-
(SiMe3)][N(SiMe3)2]}{μ-OMe} (2) (Scheme 1). The trinuc-
lear structure is composed by three “U[N(SiMe3)2]” units
each bearing another silazanate with different extents of
deprotonation. Two units displayed the same single depro-
tonation and formation of methylene. The third instead has
undergone triple deprotonation with formation of a carbyne
capping the triangulo structure. The crystal structure revealed
the unexpected presence of one bridging methoxy group,
obviously arising from DME cleavage. The 1H NMR spec-
trum was in agreement with the formulation showing five
broadened singlets in the expected 27:3:54:18:4 ratio. The
intense resonance at 29.36 ppm integrating for 27H is
assigned to the Me groups of the γ-deprotonated amido
ligands. The second peak with the relative intensity of 3H
The formation of 1 implies two C-H σ-bond metathetic
events of two silazanate Me groups by two incoming Me

3414 Inorganic Chemistry, Vol. 49, No. 7, 2010
Korobkov and Gambarotta
Scheme 3
Scheme 4
situated at 9.85 ppm is assigned to the bridging methoxy
group. The most intense peak of the spectrum positioned at
7.19 ppm (54H) is unequivocally assigned to the 18 Me
groups of three unaltered N(SiMe3)2 ligands. Two partly
overlapping broad signals centered at -0.13 ppm are as-
signed to the six Si(Me)2 units and account for 18 protons.
Finally, one remaining signal at -46.90 ppm is attributed to
the four protons of the bridging methylene groups. No signs
of possible dynamic behavior have been observed at variable
temperatures.
At first glance, the formation of 2 seems to be the result of a
complex transformation. Instead, it can be easily rationalized
by assuming the intermediate formation of three identical
[N(SiMe3)2][μ-CH2-Si(Me)2-N(SiMe3)]UR units. Two al-
kyls from two units metatesized the same -CH2- group of a
third unit giving rise to the carbyne. The third U-R group
attacked a DME solvent molecule forming the -OMe group
(Scheme 2). Accordingly, the presence of MeOCH2CH2CH2-
SiMe3 in the reaction mixture was detected in the GC-MS of
the mother liquor. No reduction of the metal centers occurred
in this case as a probable result of the lesser reducing power of
the particular lithium-alkyl employed in this case.
To prevent the interference of DME in the reactivity, a
reaction under identical conditions was carried out by using
the dimeric and solvent-free {U[μ-Cl]Cl[N(SiMe3)2]2}2 as a
starting material. Similar to the transformation described
above and despite the poor solubility in hexane of this
starting uranium complex, the reaction also occurred very
rapidly upon mixing, affording complete solubilization and
color change. Black crystals of {U[μ-(CH2-SiMe2)N(SiMe3)]-
[N(SiMe3)2]}2{U[μ3-(C-SiMe2)N(SiMe3)] [(μ-CH2-SiMe2)
N(SiMe3)]} (3) were obtained after a suitable workup
(Scheme 3).
Complex 3 is also trimeric with an arrangement of the
ligands closely reminiscent of 2. The main difference consists
of the absence of the MeO unit. A similarly triply deproto-
nated tricapping group also is the salient feature of this
structure. This formulation is not only indicated by the
geometrical parameters provided by the crystal structure
1
but also by the H NMR spectrum. The higher molecular
symmetry results in a simpler spectrum consisting of five
resonances in the ratio 6:6:3:1:1. The first resonance is
observed at 24.11 ppm and is assigned to the protons of four
N-SiMe3 fragments of γ-deprotonated ligands. The second
peak of the same intensity at 7.22 ppm is assigned to four
SiMe3 groups, which belongs to unaltered hexamethyldisila-
zanate groups. A magnetically different environment for
Si(Me)2 protons of bridging versus tricapped fragments
results in the appearance of two peaks at -13.48 ppm and
-16.85 ppm with a relative intensity of 3:1. Finally, the last
resonance positioned at -43.10 ppm could be assigned to the
six protons of three bridging -CH2- fragments.
The formation of 3 may be rationalized with the same
arguments illustrated in Scheme 2, the only difference being
the attack of the third U-R alkyl function of the third unit to
the other Si-CH3 moiety, thus affording the third μ-CH2-
unit.
The spontaneous reduction observed during the forma-
tion of 1 encouraged us to attempt a more systematic
exploration of the reduction chemistry of uranium to
probe the ability of the silazanate ligand system to stabilize
more reduced species. Reduction of UCl2[N(SiMe3)2]2-
[DME] with KC8 in DME proceeds rather slowly, with
the reaction being completed after 3 days of stirring at
room temperature. The reaction progress was indicated by
both the color change of the reaction mixture and the

Article
Inorganic Chemistry, Vol. 49, No. 7, 2010 3415
Scheme 5
disappearance of the characteristic coloration of potas-
sium graphite. After workup and precipitation, the dark
red-purple {U-μ-Cl[N(SiMe3)2][DME]}2{U-μ-Cl[N(SiMe3)2]2}-
{μ3-Cl}2 (4) was isolated as a crystalline solid (Scheme 4).
The crystal structure revealed a triangular cluster where
three uranium atoms are bridged by five chlorides. Two
metal centers bear only one silazanate, while the third has
retained two. The oxidation state is clearly trivalent, and
the 1H NMR spectrum did not provide any additional
information due to very large line broadening and over-
lapping. Degradation of 4 in a Toepler pump apparatus did
not give hydrogen gas, thus excluding the possibility of the
complex being a hydride derivative.
presence of hydrogen bonded to the N atom. Unfortu-
nately, the NMR was uninformative due to severe line
broadening and overlapping, but the low magnetic mo-
ment [μeff = 2.18 μB per dimetallic unit] was in the expec-
ted range.
Complex 6 instead appears to be the result of the acquisi-
tion of one additional silazanate, γ-metalation of two methyl
groups, and two-electron oxidation. Even in this case, the
NMR spectrum was uninformative, but the magnetism [μeff
=
1.48 μβ] was in agreement with the electronic configuration of
a monomeric pentavalent uranium complex.
The formation of both compounds containing uranium in
the rather unusual pentavalent state and from a reduction
carried out with a strong reducing reagent such as KC8 is
surprising. It can be explained only by assuming the forma-
tion of transient reduced species followed by reoxidation at
the expense of the silazanate ligand system. In addition, the
acquisition of the third ligand possibly indicates the occur-
rence of a disproportionative pathway. In the case of complex
5, the reoxidation resulted in the formal elimination of one
-SiMe3 group with formation of the imido function. The
formation of 6 instead implies a formal elimination of H2
from U[N(SiMe3)2]3, although the reaction must follow a
different pathway, given that U[N(SiMe3)2]3 is a stable
compound. However, no hydrogen gas was detected when
experiments were carried out in a sealed vessel connected to a
Toepler pump. While a rationalization for the formation of 5
is rather straightforward, the formation of 6 is more puzzling.
The same complex may also be generated upon treatment of
the same starting material with two equivalents of n-BuLi,
the use of which as both alkylating and reducing agent is
commonknowledge intransition metalchemistry. In analogy
with the formation of 1, we reasoned that alkylation of {U[μ-
Cl]Cl[N(SiMe3)2]2}2 with two equivalent of n-BuLi may
cause the double γ-metalation of the two silazanate ligands.
The oxidation to the pentavalent state and the acquisition of
the third silazanate may instead be achieved via dispro-
portionation of an intermediate trivalent species. The iden-
tities of the byproducts of these reactions remain so far
unknown.
The only noticeable feature of this reaction is the ligand
dissociation, as partly occurred during the reduction. A
variable amount of the tris-silazanate uranium complex
accompanied this reaction, thus indicating that ligand scram-
bling is an important aspect of this transformation.
The presence of DME coordinated to two of the three
uranium atoms of the triangulo cluster might have a quench-
ing power in the reactivity of the trivalent uranium centers,
and thus, we have attempted an identical reduction of the
DME-free tetravalent precursor {U[μ-Cl]Cl[N(SiMe3)2]2}2
with K/graphite in toluene. The reaction took almost seven
days to be completed, possibly as a result of the very low
solubility of both reagents in toluene. The progress of the
reaction was indicated by the disappearance of the characteristic
tint of KC8 as well as the increasing coloration of the mother
liquor. Workup of the reaction mixture, followed by frac-
tional crystallization, afforded two separate compounds
{U-μ-Cl[N(SiMe3)2]2[dN(SiMe3)]}2 (5) and U[(μ-CH2-
SiMe2)N(SiMe3)]2[N(SiMe3)2] (6), both containing the me-
tal center in its pentavalent state (Scheme 5).
The formation of 5 implies acquisition of one additional
silazanate and cleavage of one of them to form a termin-
ally bonded imido group with the dinuclear core as-
sembled via a residual bridging chlorine atom. The
imido group forms the expected short bond distance with
the metal but deviates significantly from linearity (160.5°).
The IR spectrum, however, neatly excluded the possible

3416 Inorganic Chemistry, Vol. 49, No. 7, 2010
Korobkov and Gambarotta
pare well with those of other actinide silazanate com-
plexes.15
Complex 2. The crystal structure of 2 reveals a tri-
nuclear cluster consisting of three uranium metal centers,
each bearing one intact and one deprotonated silazanate
ligand. One methoxide group bridging two of the three
uranium centers completes the structure. The other three
silazanate groups are deprotonated to different extents.
Two are singly deprotonated, each bridging one pair of
uranium atoms with the methylene groups. The third
appears to be triply deprotonated with the carbyne group
bridging the three uranium atoms. Each of the uranium
atoms is located at the center of a trigonal bipyramid. One
of the axial positions of each bipyramid is occupied by the
shared μ3-carbyne carbon atom [U(1)-C(31)=2.362(19)
Figure 1. Thermal ellipsoid diagrams of 1 with thermal ellipsoid drawn
at the 50% probability level. Hydrogen atoms, methyl carbon atoms of
SiMe3 units, and thermal ellipsoids of some carbon atoms have been
omitted for clarity. Symmetry operator: -1, 0, 0/0, -1, 0/0, 0, -1.
˚
˚
˚
A, U(2)-C(31)=2.284(19) A, U(3)-C(31)=2.415(6) A]
(Figure 2).
One of the three uranium metals has the nitrogen donor
atom of the unaltered hexamethyldisilazane in the second
˚
axial position [U(3)-N(5) = 2.313(16) A, N(5)-U(3)-
C(31)=171.8(6)°]. Instead both U(1) and U(2) bear in the
second axial position a nitrogen donor atom of the amido-
˚
methylene ligands [U(1)-N(2) = 2.240(15) A, U(2)-
˚
N(4)=2.231(15) A, C(31)-U(1)-N(2)=147.6(5)°, C(31)-
U(2)-N(4) = 145.2(5)°]. Both atoms also have one of
their equatorial positions occupied by the nitrogen of one
intact hexamethyldisilazane unit [U(1)-N(1)=2.289(15)
˚
˚
A, N(1)-U(1)-N(2)=100.8(5)°, U(2)-N(3) = 2.323(15)
A, N(3)-U(2)-N(4)=103.6(6)°]. Their second equatorial
positions are occupied by the oxygen atom of the bridging
˚
methoxy group [U(1)-O(1)=2.288(12) A, U(2)-O(1) =
˚
2.313(13)A, O(1)-U(1)-N(1)=126.4(5)°, O(1)-U(1)-
N(2) = 97.7(5)°, O(1)-U(2)-N(3) = 124.9(5)°, O(1)-
U(2)-N(4) = 90.3(5)°, U(1)-O(1)-U(2) = 107.0(5)°],
while the third sites are occupied by the bridging methy-
lene groups of the two methylene-pentamethyldisilazanate
Figure 2. Thermal ellipsoid diagrams of 2 with thermal ellipsoid drawn
at the 50% probability level. Hydrogen atoms, methyl carbon atoms of
some SiMe3 units, and thermal ellipsoids of some carbon atoms have been
omitted for clarity.
˚
ligands [U(1)-C(12) = 2.62(2) A, C(12)-U(1)-N(1) =
128.9(6)°, C(12)-U(1)-N(2) = 69.0(6)°, U(2)-C(24) =
Crystal Structure Descriptions
˚
2.62(2) A, C(24)-U(2)-N(3)=129.4(6)°, C(24)-U(2)-
Complex 1. The crystal structure of 1 comprises a sym-
metry generated dimer formed by two uranium and two
lithium centers surrounded by four deprotonated silaza-
nate ligands. The sharing by uranium of one of its μ-CH2
groups with the second uranium center of an identical
unit and with one lithium cation assembles the polyme-
tallic structure. Each uranium atom resides in the center
of a distorted trigonal bipyramidal environment. One
axial position of the bipyramid is occupied by the nitro-
gen atom of a methylene-pentamethydisilazane frag-
N(4) = 70.9(6)°]. In turn, the U(3) center has the two
above methylene groups occupying the two equatorial
˚
positions [U(3)-C(12)=2.57(2) A, U(3)-C(24)=2.66(2)
˚
A, [U(1)-C(12)-U(3) = 90.9(6)°, U(2)-C(24)-U(3) =
90.1(6)°; C(12)-U(3)-C(24) = 94.8(7)°, C(12)-U(3)-
N(5)=101.5(6)°, C(24)-U(3)-N(5)=92.5(6)°], the last
being occupied by the nitrogen atom of the carbyne-amido
˚
ligand [U(3)-N(6) = 2.261(15) A, N(6)-U(3)-N(5) =
107.1(6)°, N(6)-U(3)-C(12) = 119.8(6)°]. Even in this
case, the U-C and U-N distances compare well with
those of other actinide silazanate complexes.15
˚
ment [U(1)-N(2) = 2.296(10) A, N(2)-U(1)-C(1) =
161.1(4)°], whose methylene group located in the equa-
Complex 3. Complex 3 also is a trimetallic cluster with a
structure reminiscent of 2, being composed by three
silazanate units and three additional silazanates deproto-
nated to different extents. The main difference with 2
consists of the replacement of the μ-OMe group with a μ-
methylene moiety. As in 2, each U metal center lies in the
center of a trigonal bipyramid with one common axial
position occupied by the tetrahedral carbyne carbon
atom C(31) μ3-bonded to the three uranium metal centers
˚
torial position [U(1)-C(7) = 2.518(13) A, C(7)-U(1)-
C(1) = 90.9(4)°] bridges one lithium center. The other
axial position is occupied by the methylene of the second
deprotonated silazanate ligand which bridges the second
uranium and the lithium atom. The corresponding nitro-
gen occupies the second equatorial position [U(1)-C(1)=
˚
˚
2.362(12) A, U(1)-N(1)=2.302(10) A, C(7)-U(1)-N(1)=
111.7(4)°, C(1)-U(1)-N(1) = 79.0(3)°] (Figure 1). The
last equatorial position is occupied by the bridging met-
hylene of the symmetry generated unit [U(1)-C(1A) =
˚
˚
[U(1)-C(31) = 2.361(4) A, U(2)-C(31) = 2.352(5) A,
˚
U(3)-C(31)=2.339(4) A] (Figure 3).
Each U metal center has the nitrogen donor atom from
˚
2.372(10)A, C(1A)-U(1)-C(7)=131.9(4)°, C(1A)-U(1)-
C(1) = 80.4(4)°]. The uranium metric parameters com-
each of three γ-metalated-amido units in its second axial

Article
Inorganic Chemistry, Vol. 49, No. 7, 2010 3417
Figure 4. Partial thermal ellipsoid diagrams of 4 with thermal ellipsoid
drawn at the 50% probability level. Hydrogen atoms, methyl carbon
atoms of some SiMe3 units, and thermal ellipsoids for all carbon atoms
have been omitted for clarity.
Figure 3. Partial thermal ellipsoid diagrams of 3 with thermal ellipsoid
drawn at the 50% probability level. Hydrogen atoms, methyl carbon
atoms of some SiMe3 units, and thermal ellipsoids of some carbon atoms
have been omitted for clarity.
˚
position [U(1)-N(2)=2.286(4) A, U(2)-N(4)=2.276(4)
˚
˚
A, U(3)-N(5) = 2.275(4) A, N(2)-U(1)-C(31) =
144.23(16)°,N(4)-U(2)-C(31)=141.06(15)°,N(5)-U(3)-
C(31)=153.21(15)°]. The three methylene atoms bridge a
˚
pair of uranium centers each [U(1)-C(10)=2.658(6) A,
˚
˚
C(10)-U(2) = 2.557(6) A, U(2)-C(22) = 2.677(5) A,
˚
˚
C(22)-U(3) = 2.602(5) A, U(3)-C(28) = 2.634(5) A,
˚
C(28)-U(1) = 2.553(5) A] occupying equatorial posi-
tions of two neighboring U metal centers [C(10)-U(1)-
C(31) = 78.71(18)°, C(10)-U(1)-C(28) = 105.29(17)°,
C(22)-U(2)-C(31) = 81.08(15)°, C(10)-U(2)-C(22) =
112.60(17)°, C(28)-U(3)-C(31) = 84.05(15)°, C(22)-
U(3)-C(28) = 124.81(16)°] and assembling the trimeric
unit [U(1)-C(10)-U(2)=93.06(18)°, U(2)-C(22)-U(3)=
89.15(19)°, U(3)-C(28)-U(1) = 89.03(18)°]. The last
equatorial positions in the coordination polyhedra of
two uranium metal centers are occupied by two nitrogen
donor atoms of two unaltered hexamethyldisilazane
Figure 5. Partial thermal ellipsoid diagrams of 5 with thermal ellipsoids
drawn at the 50% probability level. Hydrogen atoms and thermal
ellipsoids for all carbon atoms have been omitted for clarity. Symmetry
operator: -1, 0, 0/0, -1, 0/0, 0, -1.
four coordination sites occupied by the four bridging
chlorine atoms, two by the oxygen of one coordinated
˚
DME [U(1)-O(1)=2.593(13) A, U(1)-O(2)=2.592(12)
˚
˚
ligands [U(1)-N(1) = 2.303(4) A, N(1)-U(1)-C(31) =
A, O(1)-U(1)-Cl(4) = 71.5(3)°, O(2)-U(1)-Cl(4) =
106.97(15)°, N(1)-U(1)-C(10)=147.6(2)°, U(2)-N(3) =
112.5(3)°, O(1)-U(1)-Cl(2) = 84.3(3)°, O(2)-U(1)-
Cl(1) = 72.0(3)°], and the last by the nitrogen atom of
˚
2.315(4) A, N(3)-U(2)-C(31) =113.12(15)°, N(3)-U(2)-
˚
C(22)=129.25(15)°]. The last equatorial position of the
third uranium is occupied by the nitrogen atom of the
same carbyne-pentamethyldisilazane unit [U(3)-N(6) =
one intact silazanate group [U(1)-N(1) = 2.298(14) A,
˚
N(1)-U(1)-Cl(4)=163.9(3)° U(2)-N(4)=2.304(14) A,
N(4)-U(2)-Cl(4) = 96.7(3)°]. The third U atom is loca-
ted instead in the center of a distorted octahedron with
four coordination sites occupied by the bridging chlorine
atoms and two nitrogen atoms of two intact silazanate
located in the two remaining cis positions [N(2)-U(3)-
N(2)=104.10(12)°] (Figure 4).
˚
2.223(4) A, N(6)-U(3)-C(31) = 75.97(15)°, N(6)-U(3)-
C(28)=121.32(16)°].
Complex 4. The structure of 4 is also trimeric but with
the three uranium metal centers bridged by five chlorine
atoms. Three are each bridging one pair of metals forming
˚
a rather planar U3Cl3 core [U(1)-Cl(1) = 2.841(5) A,
Complex 5. Complex 5 is dimeric and consists of two
identical uranium-containing units bridged by two chlor-
ine atoms. The coordination geometry of uranium is
distorted trigonal bipyramidal with the axial positions oc-
˚
˚
U(1)-Cl(2)=2.809(5) A, U(2)-Cl(1)=2.843(4) A, U(2)-
˚
˚
Cl(3)=2.812(4) A, U(3)-Cl(3)=2.875(5) A, U(3)-Cl(2)=
2.891(5) A, Cl(1)-U(1)-Cl(4) = 72.45(12)°, Cl(2)-
˚
˚
U(1)-Cl(4)=73.57(13)°, Cl(1)-U(1)-Cl(5)=75.52(13)°,
Cl(2)-U(1)-Cl(5)=76.39(13)°], while the other two are
placed in apical positions on the two opposite sides of the
U3Cl3 plane, each being μ3-bonded to the three metals
cupied by one bridging chloride [U(1)-Cl(1)=2.829(2) A,
Cl(1)-U(1)-Cl(1A) = 72.79(6)°, U(1)-Cl(1)-U(1A) =
107.21(6)°] and the second by the imido nitrogen atom
˚
[U(1)-N(3) =2.081(5) A, N(3)-U(1)-Cl(1A) =154.87(13)°]
(Figure 5).
˚
˚
[U(1)-Cl(4)=3.103(5) A, U(1)-Cl(5)=2.885(4) A, Cl(4)-
˚
U(1)-Cl(5) = 68.51(12)°, U(2)-Cl(4) = 2.879(4) A,
U(2)-Cl(5)=3.068(5) A, Cl(4)-U(2)-Cl(5)=69.09(12)°,
U(3)-Cl(4)=3.074(5) A, U(3)-Cl(5)=3.065(4) A, Cl(4)-
U(3)-Cl(5) = 66.71(12)°]. The three metals reside in
different coordination environments. Two have the same
heavily distorted pentagonal bipyramidal geometry with
The two equatorial positions are occupied by the
nitrogen atoms of two hexamethyldisalazanate ligands
˚
˚
˚
˚
[U(1)-N(1)=2.237(5) A, U(1)-N(2)=2.249(5), N(1)-
U(1)-Cl(1)=111.87(14)°, N(2)-U(1)-Cl(1)=126.54(13)°]
and by the second bridging chlorine atom [U(1)-Cl(1A)=
2.847(2) A]. The terminally bonded imido group deviates
˚

3418 Inorganic Chemistry, Vol. 49, No. 7, 2010
Korobkov and Gambarotta
˚
[U(1)-N(2) = 2.241(7) A] and axial positions [U(1)-
C(7)=2.830(18) A] (Figure 6). The last equatorial posi-
˚
tion around the U metal center is occupied by the nitrogen
donor atom of an unaltered hexamethyldisalazanate
˚
ligand [U(1)-N(1) = 2.240(7) A, N(1)-U(1)-N(3) =
113.2(2)°, N(1)-U(1)-C(13)=143.2(4)°].
Conclusions
Alkylation of DME-solvate and unsolvate forms of U[N-
(SiMe3)2]2Cl2 under different reduction conditions resulted in
the formation of several γ-deprotonated products with multi-
ple deprotonations having occurred in two cases at the same
carbon atom. We have observed that either reduction of
the metal center or solvent cleavage may accompany the
transformation depending on the reaction conditions. Alky-
lation with n-butyl lithium yielded a surprising pentavalent
compound. The formation under reducing conditions of a
compound with the metal center in an oxidation state higher
than in the starting material was accompanied by ligand
migration. This possibly hints at the occurrence of a dis-
proportionation of a transient highly reactive low-valent
intermediate in addition to its reoxidation at the expense of
the ligand.
Figure 6. Partial thermal ellipsoid diagrams of 6 with thermal ellipsoids
drawn at the 50% probability level. Hydrogen atoms and methyl carbon
atoms of the SiMe3 units have been omitted for clarity.
from linearity as a possible result of the steric hindrance
[U(1)-N(3)-Si(5) = 160.8(2)°] and forms the expected
˚
short U-N distance [U(1)-N(3)=2.081(5) A ].
Complex 6. Complex 6 is monomeric and consists of a
single uranium atom located in the center of a distorted
trigonal bipyramid, as determined by the carbon and
nitrogen atoms of two methylene-pentamethyldisila-
zane ligands and the nitrogen of one intact silazanate
[C(13)-U(1)-N(3) = 65.7(3)°, C(7)-U(1)-N(3) =
149.2(4)°, N(3)-U(1)-N(2)=114.3(2)°, C(13)-U(1)-N-
(2)=91.3(4)°]. The first methylene-pentamethyldisilazane
ligand occupies one axial position with the nitrogen atom
Acknowledgment. This work was supported by the
Natural Science and Engineering Council of Canada
(NSERC).
Supporting Information Available: Crystal data and structure
refinement, atomic coordinates and equivalent isotropic dis-
placement parameters, bond lengths and angles, anisotropic
displacement parameters, hydrogen coordinates and isotropic
displacement parameters, and torsion angles for complexes
1-6. This material is available free of charge via the Internet
at http://pubs.acs.org.
˚
[U(1)-N(3) = 2.258(6) A] and one equatorial with the
˚
methylene group [U(1)-C(13)=2.985(19) A]. The second
has the same arrangement except that the nitrogen and
methylene are respectively located on the equatorial