Tuning uranium-nitrogen multiple bond formation with ancillary siloxide ligands

Article abstract of DOI:10.1021/ja405815b

The new homoleptic ate U(III) siloxide [K(18c6)][U(OSi(O ^tBu)₃)₄] 2 was prepared in 69% yield by reduction of [U(OSi(O^tBu)₃)₄] 3 with KC ₈. The reaction of the neutral U(III) sil

Full text of DOI:10.1021/ja405815b

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Article
Tuning Uranium-Nitrogen Multiple Bond
Formation with Ancillary Siloxide Ligands.
Clement Camp, Jacques Pecaut, and Marinella Mazzanti
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja405815b • Publication Date (Web): 19 Jul 2013
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Tuning Uranium-Nitrogen Multiple Bond Formation with Ancil-
lary Siloxide Ligands.
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Clément Campa, Jacques Pécauta and Marinella Mazzanti*a
aLaboratoire de Reconnaissance Ionique et Chimie de Coordination, SCIB, UMRꢀE3 CEAꢀUJF, INAC, CEAꢀ
Grenoble, 17 rue des Martyrs, Fꢀ38054 Grenoble Cedex 09
Supporting Information Placeholder
which is relevant to the problem of spent nuclear fuel
ABSTRACT: The new homoleptic “ate” U(III) siloxide
reprocessing.19ꢀ23 Interest in uranium nitrides is also
arising from the possibility of using uranium nitride as
an alternative nuclear fuel in generationꢀIV power
reactors because of its higher melting point and enꢀ
hanced thermal conductivity compared to the currentꢀ
ly used uranium oxides.24,25 A classic route to metalꢀ
imides and metalꢀnitrides involves the reaction of
organic and inorganic azides with reducing metal
complexes.2,26 In uranium chemistry several U(VI) cisꢀ
bis(imido) and terminal U(V) mono(imido) complexꢀ
es4ꢀ6,12,18,26ꢀ35 have been obtained from U(IV) and
U(III) precursors respectively in a twoꢀelectron transꢀ
fer with organic azides. In contrast U(VI) and U(V)
bis(imido) complexes presenting the imido groups in a
trans configurations have proven elusive using the
azide route, but few examples of these compounds
have been recently reported using alternative synthetꢀ
ic procedures.16,36ꢀ39
[K(18c6)][U(OSi(OtBu)3)4], 2 has been prepared in 69%
yield by reduction of [U(OSi(OtBu)3)4], 3 with KC8. The
reaction of the neutral U(III) siloxide complex
[U(OSi(OtBu)3)2(ꢁꢀOSi(OtBu)3)]2, 1 with adamantyl azide
leads to the isolation of the dinuclear U(VI) imido comꢀ
plex [U2(NAd)4(OSi(OtBu)3)4], 4. The Xꢀray crystal strucꢀ
ture shows the presence of a “cationꢀcation interaction”
between the two [U(NAd)2]2+ groups. In contrast the
reactions of 2 with the trimethylsilyl and adamantyl azꢀ
ides
afford
the
U(V)
imido
complexes
and
[K(18c6)][U(NSiMe3)(OSi(OtBu)3)4]
5-TMS
[K(18c6)][U(NAd)(OSi(OtBu)3)4] 5-Ad pure in 48 % and
66 % yield respectively. The reaction of 2 with CsN3 in
THF at ꢀ40°C yields a mixture of products from which
the azido U(IV) complex [K(18c6)][U(N3)(OSi(OtBu)3)4],
7 and the the ꢁꢀnitrido diꢀuranium(V) complex [KU(ꢁꢀ
N)(OSi(OtBu)3)]2, 8 were isolated. The crystal structure of
8 shows the presence of a rare U2N2 core with two nitrido
atoms bridging two uranium centers in a diamondꢀ
shaped geometry. In contrast, the reaction of 1 with CsN3
Fewer uranium nitrides have been reported compared
to imido complexes.34,41ꢀ47 Besides two examples of
uranium nitrides isolated from the dinitrogen reducꢀ
tion by a U(III) complex,40,41 they have been obtained
from the reaction of low valent uranium with inorganꢀ
ic azides.
affords
the
diuranium(IV)
complex
Cs{(ꢂꢀ
N)[U(OSi(OtBu)3)3]2}, 9 presenting a nitrido ligand
bridging two uranium and one cesium cations. These
results show the importance of the coordination enviꢀ
ronment in the outcome of the reaction of U(III) with
azides.
Depending on the ligand environment this reaction
has led to the synthesis of terminal boraneꢀcapped
nitrides,42 dinuclear bridging nitrides,43,44 large azꢀ
ido/nitrido clusters,45ꢀ47 transient mononuclear nitride
promoting intramolecular CꢀH activation,48 or stable
terminal nitrides.33,49
Introduction :
Nitrido and imido complexes have been the focus of
numerous studies in transition metal chemistry due to
their important implication in nitrogen fixation, atom
and group transfer reactions, and catalysis.1,2 Since the
first reports in the 80’s of uraniumꢀimido complexes3,4
there has been also an increased interest for actinide
compounds containing metalꢀnitrogen multiple bondꢀ
ing.5ꢀ9 The focus of actinide scientists on such comꢀ
pounds has been driven by the possibility of promotꢀ
ing novel reactivity and catalytic transformations as a
result of the larger size of actinides and of the inꢀ
volvement of f orbitals in bonding.10ꢀ12 Moreover actiꢀ
nide imido and nitrido complexes are particularly
attractive species for gaining a better understanding of
the nature of bonding in molecular actinide species13ꢀ18
Differing steric and electronic environments at the
uranium center have a critical impact on the forꢀ
mation of nitrido and imido complexes, on their geꢀ
ometry, stability and reactivity. For example the optiꢀ
mal tuning of the steric and electronic environment
has been the key to the recent isolation of the first
uranium complexes containing a terminal nitride.33,49
However, only a very limited number of studies have
explored the effect of different steric environments in
similar ligand systems.12,18,50
We have recently shown that bulky siloxide ligands,
due to their σ+π donating ability and their multiple
binding modes,51,52 provide an attractive alternative
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coordination environment for the stabilization of
highly reactive trivalent uranium complexes.
in 3, and lie in the usual range of what is observed in
other U(IV) alkoxide and siloxide compounds.53,59ꢀ62
The structures of complexes 3 and 3-py highlight the
ability of the OSi(OtBu)3 ligand to adapt its coordinaꢀ
tion mode to its environment, allowing the stabilizaꢀ
tion of the lowꢀcoordinate uranium in both coordinatꢀ
ing and non coordinating solvents.
Chart 1. U(III) siloxide complexes [U(OSi(OtBu)3)3]2,
1 and [K(18c6)][U(OSi(OtBu)3)4] 2.
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Here we investigate the reactivity with organic and
inorganic azides of two U(III) systems presenting a
different number of alkoxy(siloxide) ligands, the neuꢀ
tral trisꢀsiloxide dimer [U(OSi(OtBu)3)3]2,153 and the
new
monomeric
“ate”
complex
[K(18c6)][U(OSi(OtBu)3)4] 2 (Chart 1). It should be
noted that studies on uranium “ate”ꢀsalts are
rare28,34,40,54ꢀ58 mostly due to the general belief that
such compounds will result in a saturated coordinaꢀ
tion sphere and limited reactivity. Here we show that
both the neutral and the “ate” complexes 1 and 2 react
with organic and inorganic azides. The difference in
steric demand and charge results in a very different
outcome of the reactivity for the two U(III) siloxide
species providing new insight into the formation of
U=N multiple bonding and affording new imido and
nitride complexes of uranium.
Figure 1. Ellipsoid plot for [U(OSi(OtBu)3)4(py)2] 3-py
crystallized from pyridine; probability 50%. Disorder, hyꢀ
drogen atoms and methyl groups are omitted for clarity.
Bond distances (Å): U1ꢀOavg 2.177(12), U1ꢀN1 2.630(3).
(O1A = O1 ꢀx+1,y,ꢀz+3/2)
Results and discussion:
Mononuclear U(IV) and U(III) tetra-siloxide
complexes.
Scheme 1. Synthesis of [K(18c6)][U(OSi(OtBu)3)4], 2.
The reaction of [UI4(OEt2)2] with 4 equiv of the potasꢀ
sium salt of trisꢀtertiobutoxysilanol KOSi(OtBu)3 in
THF affords, after recrystallization from hexane and
drying in vacuum, the uranium(IV) homoleptic comꢀ
plex [U(OSi(OtBu)3)4] 3, in 88% yield (Scheme 1). This
complex was first isolated in our group from the ligꢀ
and redistribution of a silanol carbonate U(IV) comꢀ
plex.53 This compound is fully soluble in polar solvents
(THF, pyridine) and in hydrocarbon solvents (hexane,
toluene). In the solid state structure of 3 one siloxide
ligand adopts a bidentate coordination mode, while
the three other siloxide ligands are coordinated in a
terminal monodentate fashion (see supporting inforꢀ
mation). A single proton resonance is observed in the
1H NMR spectrum of 3 in deuterated nꢀhexane or
toluene solutions suggesting that the coordination of
the OtBu group is labile in solution or involved in fast
exchange.
The reduction of complex 3 with potassium graphite
in THF performed in the presence of 18c6 crown ether
affords
the
uranium(III)
“ate”
complex
[K(18c6)][U(OSi(OtBu)3)4] 2, which was isolated pure
in 69% yield (Scheme 1). The Xꢀray diffraction analyꢀ
sis shows the presence of an ionꢀpair structure comꢀ
posed of a K(18c6) cation and of a [U(OSi(OtBu)3)4]
anion (Figure 2). The uranium ion is coordinated by
four terminal siloxide groups with a tetrahedral geomꢀ
etry. The measured UꢀO bond lengths of the terminal
siloxide (mean UꢀOsiloxide = 2.228(17) Å) are longer
than in complex 3 (mean UꢀOsiloxide = 2.13(4) Å) and
are in the range of typical UꢀO distances in uraniꢀ
um(III) siloxide and alkoxide complexes.53,59,61 The
difference in the coordination mode adopted by one of
the siloxide ligand in the U(III) complex 2, with reꢀ
spect to the homoleptic U(IV) complex 3 (monodenꢀ
Single crystals obtained from saturated pyridine soluꢀ
tions of [U(OSi(OtBu)3)4] were analyzed by Xꢀray difꢀ
fraction and revealed that this homoleptic structure is
not retained in coordinating solvent. The crystal strucꢀ
ture
of
the
green
pyridine
adduct
[U(OSi(OtBu)3)4(py)2] 3-py, set out in Figure 1, shows
that the uranium center is hexacoordinated with a
distorted octahedral geometry by four terminal monoꢀ
dentate silanol ligands and two pyridine molecules
bound in a cis fashion. The UꢀOꢀ bond distances (UꢀOꢀ
2.177(12) Å) are slightly longer than what is found
avg
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tate versus bidentate) is probably the result of the
has been little explored compared to their neutral
counterparts.
difference in electron density. The U(III) cation is
larger in size than the U(IV) and thus it should favour
higher coordination numbers but the higher electronꢀ
rich character of trivalent uranium might disfavour
the coordination of the butoxide group.
The “ate”ꢀsalt complex 2 and the previously reported
neutral
analogue
complex
[U(OSi(OtBu)3)2(ꢁꢀ
OSi(OtBu)3)]2, 1, provide interesting systems for invesꢀ
tigating the effect of charge and steric bulk in the reacꢀ
tivity of U(III) complexes. The current interest in niꢀ
trido and imido complexes of highꢀvalent uranium
prompted us to investigate the reactivity of these siꢀ
loxide complexes with organic and inorganic azides.
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Reactivity with organic azides.
The reaction of the previously reported neutral U(III)
siloxide complex [U(OSi(OtBu)3)2(ꢁꢀOSi(OtBu)3)]2, 153
with adamantyl azide (one equivalent per uranium)
(Scheme 2) leads to the immediate formation of the
U(IV) complex [U(OSi(OtBu)3)4], 3, and of the dinuꢀ
clear
bisꢀimido
complex
of
uranium(VI)
[U2(NAd)4(OSi(OtBu)3)4], 4, in ratio 2:1 (See H NMR
1
in supporting information).
Scheme 2. Reaction of [U(OSi(OtBu)3)2(ꢁꢀOSi(OtBu)3)]2,
1 with adamantyl azide.
Figure 2. Ellipsoid plot for [K(18c6)][U(OSi(OtBu)3)4] 2
crystallized from toluene; probability 50%. Hydrogen atꢀ
oms, methyl groups and solvent molecules are omitted for
clarity. Bond distance (Å): U1ꢀOavg 2.228(17).
The room temperature magnetic moment (2.63 ꢂβ)
measured using the Evans method63 for a THF soluꢀ
tion of 2 falls in the range of the values reported for
other U(III) coordination compounds.31,64ꢀ66 This valꢀ
ue is lower than the theoretical value (3.62 ꢁB) calcuꢀ
lated for a 5f3 ion with a full spinꢀorbit coupling as
often observed in trivalent uranium complexes.67
The proton NMR of 2 in THF and toluene solution
shows the presence of one signal for the siloxide proꢀ
tons and one signal for the 18c6 protons in agreement
with the presence of S4 symmetric solution species.
The UVꢀvisible absorption spectrum of 2 recorded
from THF solution (see supporting information) disꢀ
plays a large band centered at λ = 366 nm (ε = 3420
L.cmꢀ1.molꢀ1). This absorption, which is responsible of
the orange color of the complex, is attributed to a
5f3ꢀ5f26d1 transition, a classical feature of U(III)
complexes.68ꢀ70 A series of less intense (ε < 150 L.cmꢀ
1.molꢀ1) sharp resonances that we assign to 5fꢀ5f
transitions are spread over the entire recorded range
above 500 nm. In comparison, the spectrum recorded
for the complex 3-THF (see supporting information)
exhibits much weaker (ε < 50 L.cmꢀ1.molꢀ1) 5fꢀ5f
transitions, with a maximum centered at λ = 608 nm
as usually observed for U(IV) complexes.31
Complex 4 crystallizes from hexane solution in the
monoclinic space group P21/n, and its solidꢀstate moꢀ
lecular structure is represented in Figure 3. The dinuꢀ
clear complex contains a cationꢀcation interaction72
between two [U(NAd)2]2+ units (U1ꢀN3ꢀU2 angle =
108.4(5)°). The two [U(OSi(OtBu)3)2(NAd)2] moieties
are also held together by a bridging siloxide with a
U…U distance of 3.7687(7) Å. Both [U(NAd)2]2+ motifs
are nearly linear (N1ꢀU1ꢀN2 angle = 173.6(5)° ; N3ꢀ
U2ꢀN4 angle = 169.2(5)°) and can be seen as nitrogen
2+
analogues of the UO2 moiety. The U=N imido bond
distances for the terminal imido groups are short
(U=Navg = 1.89(5) Å), in agreement with a multiple
uraniumꢀnitrogen bonding, and fall in the range of
those reported for U(VI) imido complexes.16,27,28,35ꢀ
37,39,73ꢀ76
The U=N bond distance of the imido group
(U2ꢀN3 = 1.957(10) Å) involved in the cationꢀcation
interaction is, as expected, longer than those of the
terminal imido groups but significantly shorter than
the U1ꢀN3 bond length (2.661(12) Å), in agreement
with the formulation proposed. The arrangement of
the two imido groups is not planar (See figure 3b)
(with N4U2N3U1 and N1U1N2N2 planes almost perꢀ
pendicular at 85.23 °). This differs from the previously
reported cationꢀcation complexes of pentavalent uraꢀ
nyl which all show a Tꢀshaped or diamond shaped
Anionic U(III) complexes are less common than their
neutral analogues and mostly limited to cyclopentadiꢀ
enyl and cyclooctatetraenyl derivatives.34 Other examꢀ
ples of “ate”ꢀsalt complexes of U(III) have been reꢀ
ported with calix[4]tetrapyrrole and bulky silylamido
ligands.34,40,71 Moreover, the reactivity of “ate” comꢀ
plexes of U(III) supported by electron donor ligands
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80,81
coplanar arrangement.38,77ꢀ79
The coordination
;= 1.957(10), U1ꢀN3 = 2.661(12), U1ꢀN3ꢀU2 =108.4(5), N1ꢀ
U1ꢀN2 = 173.6(5), N3ꢀU2ꢀN4 =169.2(5); (bottom) detail of
the bisꢀimido cationꢀcation core in 4.
geometry of U2 is best described as a distorted trigoꢀ
nal bipyramid with three OSi(OtBu)3 ligands occupyꢀ
ing the equatorial plane. U1 is hexacoordinated in a
highly distorted octahedral geometry by two adamanꢀ
tyl imidos in trans position, one terminal monodenꢀ
tate siloxy ligand, one bridging bidentate siloxy ligand
and one bridging imido ligand.
The formation of the complexes 3 and 4 from the reꢀ
action of 1 with adamantly azide could be the result of
the disproportionation of an unstable U(V) bis(imido)
intermediate. While the disproportionation reaction of
uranyl(V) to yield uranyl(VI) and U(IV) species in
aqueous or organic media86,89ꢀ93 has been the subject
of high interest in past and more recent years, to the
best of our knowledge there are no previous reports of
the disproportionation of uranyl(V) imido complexes.
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While cisꢀbis(imido) complexes of U(VI) have been
known since 1992,27,28,35,73 82
only in 2005 the first
,
example of stable U(VI) transꢀbis(imido) complexes,
[U(NR)2I2(THF)2], were prepared from the reaction of
uranium metal or U(III) with iodine and alkylꢀ or arꢀ
ylamines.37,74 A few other examples of U(VI) transꢀ
bis(imido) complexes in different ligand environments
have been prepared in the last few years16,36,39,75,76
which have shown interesting reactivity.74,83,84 Comꢀ
pound 4 provides a new example of uranium(VI)
transꢀbis(imido) complex and the first one showing a
cationꢀcation interaction between the two imido
groups. Examples of dinuclear bisꢀimido complexes
are rare in uranium chemistry38,85 and they all present
a diamondꢀshaped geometry. Cationꢀcation interacꢀ
tion between uranyl groups is found in several recentꢀ
ly reported uranyl(V) complexes,77,78,86 but only rarely
observed in uranyl(VI) complexes.86ꢀ88
However, one example of the formation of a U(VI)
imido complex and of a U(IV) complex from the reacꢀ
tion of U(III) with organic azides has been reported.35
The mechanism proposed by the authors is based on
previously reported reactivity of U(V) imides with
U(III)94 and involves the comproportionation of the
U(V) intermediate [(C5Me5)2UCl(N=R)] with the
U(III) starting complex {Na[(C5Me5)2UCl2]} to afford
the imido [(C5Me5)2U(N=R)] and the bisꢀchloride
[(C5Me5)2UCl2] U(IV) complexes. Further reaction of
the U(IV) imido with azide yields the U(VI) cisꢀ
bis(imido) product. A similar mechanism could also
be invoked for the formation of 3 and 4 from 1 as alꢀ
ternative to the disproportionation route. (see supꢀ
porting for the scheme of the two possible mechaꢀ
nisms). Further work will be directed to further invesꢀ
tigate this reactivity.
We have also studied the reaction of the “ate” salt
complex 2 with organic azides in order to investigate
how the increased steric bulk and the difference in
charge and redox potential influence the outcome of
the reaction with respect to complex 1.
The reaction of complex 2 with trimethylsilyl and
adamantyl azides in toluene proceeds quickly at room
temperature with a colour change from orange to dark
brown to afford the uranium(V) imido complexes,
respectively [K(18c6)][U(NSiMe3)(OSi(OtBu)3)4] 5-
TMS and [K(18c6)][U(NAd)(OSi(OtBu)3)4] 5-Ad
which were isolated pure in 48 % and 66 % yield reꢀ
spectively (Scheme 3). 5-TMS and 5-Ad provide new
examples of stable U(V) mono(imido) complexes.
Complex 2 activates organic azides but the resulting
U(V) imido complexes do not undergo disproportionaꢀ
tion as observed in the case of complex 1 (Scheme 2).
The increased steric bulk and the anionic charge of the
5-TMS and 5-Ad complexes compared to a neutral
trisꢀsiloxide imido complex probably disfavour a biꢀ
molecular mechanism, and thereby prevent the disꢀ
proportionation.
Scheme 3. Reaction of [K(18c6)][U(OSi(OtBu)3)4] 2
with organic azides.
Figure 3. (top) Ellipsoid plot for [U2(NAd)4(OSi(OtBu)3)4] 4
crystallized from hexane; probability 50%. Hydrogen atoms
and methyl groups are omitted for clarity. Bond distances
(Å) and angles (°): UꢀOavg 2.20(2), U=Navg 1.937(7), U2ꢀN3
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ligands in the equatorial plane (average UꢀOeq bond
distance 2.173(9) Å for 5-TMS and 2.21(2) Å for 5-
Ad). This is expected as the axial position is less steriꢀ
cally constrained than the equatorial ones, allowing a
closer approach of the sterically hindered tris(tertꢀ
butoxy)siloxide axial ligand. Additionally this could be
further amplified by an inverse trans influence interꢀ
action which typically occurs in highꢀvalent U(V) and
U(VI) systems bearing multiply bonded imido and oxo
ligands.18,95,96
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These anionic heteroleptic complexes are highly soluꢀ
ble in toluene and are stable in solution for several
days. The H NMR spectra of both compounds are in
1
agreement with a C3ꢀsymmetry for the complexes in
solution with respectively one and four proton resoꢀ
nances for the SiMe3 and the adamantyl groups. The
UVꢀvisible spectra of both complexes (see supporting
information) display a broad unresolved band in the
UVꢀvisible region attributed to charge transfer transiꢀ
tions. In addition, a weaker Laporteꢀforbidden fꢀf
transition is observed around 910 nm. The value of the
magnetic moment per uranium, measured at 298 K
using the Evans method,63 for a toluene solution of 5-
TMS (2.12 ꢂB) and 5-Ad (2.34 ꢂB) is close to the calꢀ
culated effective magnetic moment at room temperaꢀ
ture for a U(V) 5f1 complex (2.54 ꢂB) and is similar to
the value found in other U(V) imido complexes.26,31
Figure 4. Ellipsoid plot for of the anion in
[K(18c6)][U(NAd)(OSi(OtBu)3)4] 5-Ad crystallized from
hexane; probability 50%. [K(18c6)] counter cation, hydroꢀ
gen atoms, methyl groups and solvent molecules are omitꢀ
ted for clarity. Bond distances (Å): U1ꢀOavg 2.20(2), U1ꢀN1
1.937(7).
Structural data for both compounds were obtained by
Xꢀray diffraction analysis. In both complexes, the uraꢀ
nium cation is pentacoordinated in a distorted trigoꢀ
nal bipyramid geometry by three monodentate siꢀ
loxyde ligands in the equatorial plane and one monoꢀ
dentate siloxyde ligand and a trimethylsilyimido or an
adamantylimido ligand, in 5-TMS and 5-Ad respecꢀ
tively, in axial position. The short UꢀN bond distances,
respectively of 1.889(11) Å for 5-TMS and 1.937(7) Å
for 5-Ad, lie in the lower side of the range of U=N
distances (1.89 to 2.12 Å) in previously reported U(V)
imido complexes.4,7,17,18,26,29,31,36,38 The UꢀNꢀR (R =
SiMe3, Ad) angles (163.4(5)° and 172.3(5)°) are close
to linearity. The short U=N bond distance and the
linear UꢀNꢀR angle are structural features characterisꢀ
tic of a strong pi interaction between the two donating
lone pairs of the nitrogen atom and the fꢀorbitals of
the uranium cation and are indicative of the multiple
bonding between the imido moiety and the uranium
centre.5,6
Figure 5. Ellipsoid plot for of the anion in
[K(18c6)][U(NSiMe3)(OSi(OtBu)3)4] 5-TMS crystallized
from toluene; probability 50%. [K(18c6)] counter cation,
hydrogen atoms, disordered atoms, methyl groups and
solvent molecules are omitted for clarity. Bond distances
(Å): U1ꢀOavg 2.16(2), U1ꢀN1 1.889(11).
The UꢀO bond distances in 5-TMS (UꢀOavg = 2.16(2)
Å) and 5-Ad (UꢀOavg = 2.20(2) Å) are in the same
range than the ones in 2 (UꢀOavg = 2.228(17) Å). In
both complexes, the UꢀO bond distances for the siloxꢀ
ide group opposite to the amido group (2.130(7) Å for
5-TMS and 2.180(6) Å for 5-Ad) are slightly shorter
than the UꢀO distances observed for the three siloxide
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ligand and the azido ligand are found in a cisꢀ
arrangement (N61ꢀU1ꢀN1 angle: 80.93(13)°).
The isolation of an U(IV) azidoꢀamido complex from
the reaction of U(III) with an organic azide is unprecꢀ
edented and is probably the result of the high reactiviꢀ
ty of the nucleophilic imido complex 5-TMS with
TMSꢀN3 which should lead to new interesting imidoꢀ
group transfer chemistry.97
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The different reactivity observed with TMSꢀN3 comꢀ
pared to that with AdꢀN3 and leading to the formation
of 6 could arise from the higher stability of the silicon
based trimethylsily radical compared to the carbon
based adamantyl radical in a possible radical trans-
fer pathway.12
The redox properties of 5-TMS and 5-Ad in THF
solution were investigated by electrochemistry. The
cyclovoltammetric curves recorded for the U(V) imꢀ
idos in THF solution (see supporting information)
show that it is possible to oxidize them to U(VI) imꢀ
idos. The U(VI)/U(V) process is reversible, and occurs
at E1/2 = ꢀ0.7 V and ꢀ1.0 V vs the Fc+/Fc couple respecꢀ
tively for 5-TMS and 5-Ad. The +0.3 V shift in potenꢀ
tial indicates that the NꢀAd ligand favours the +VI
oxidation state due to the higher electronꢀdonor charꢀ
acter of the adamantyl group. An irreversible reducꢀ
tion process is observed at Epc = ꢀ2.9 V and ꢀ3.2 V vs
the Fc+/Fc couple respectively for 5-TMS and 5-Ad,
and is attributed to the reduction to U(IV). The sepaꢀ
ration between uranium oxidation and reduction proꢀ
cesses in both systems is similar (ꢃE = 2.2 V).
Figure 6. Spaceꢀfilling representations of the anion in
[K(18c6)][U(NSiMe3)(OSi(OtBu)3)4] 5-TMS crystallized
from toluene. [K(18c6)] counter cation, hydrogen atoms
and solvent molecules are omitted for clarity.
A probable reason for the limited number of isolated
U(V) imido compounds7 is that, depending on the
supporting ligands and on the azide substituent, the
reaction of U(III) complexes with organic azides can
follow alternative pathways leading to multiple prodꢀ
ucts.12,18,26,31,32
To our knowledge, electrochemical studies of U(V)
imidos is limited to the complexes of the general forꢀ
mula [U(Cp)2(NAr)(X)] (Ar = 2,4,6ꢀtBu3ꢀC6H2, 2,6ꢀ
iPr2ꢀC6H3 ; X = F, Cl, Br, I) reported by Kiplinger et
al.17 These species exhibit much higher potential
(range : 0.11 V to ꢀ0.19 V) for the U(VI)/U(V) couple.
The lower value for the oxidation of the U(V) imido
observed with the siloxide ligands vs the Cp and halide
ones is in agreement with the stronger electronꢀ
donating character of the siloxide ligands and with the
anionic character of the complexes 5-TMS and 5-Ad.
The observed U(VI)/U(V) redox potential compares
well with those reported for homoleptic U(V) ate comꢀ
plexes bearing highly electronꢀrich ligands [UL6]ꢀ (L =
OtBu, CH2SiMe3, NC5H10, N=CtBuPh)54ꢀ56,75,98 ranging
from ꢀ1.12 V, to ꢀ1.52 V vs the Fc+/Fc couple.
Figure 7. Ellipsoid plot for of the anion in [U(N3ꢀ
[K(18c6)U(N3)(N(SiMe3)2(OSi(OtBu)3)3],
6
crystallized
from toluene; probability 50%. Hydrogen atoms, methyl
groups from OSiOtBu ligands and solvent molecules are
omitted for clarity. Bond distances (Å): U1ꢀOavg 2.146(8),
U1ꢀN61 2.330(4), U1ꢀN1 2.409(4), N1ꢀN2 1.184(5), N2ꢀN3
1.157(6).
Notably, the imido complex 5-TMS is isolated in low
These preliminary studies suggest that these monoꢀ
meric anionic U(V) imido compounds provide convenꢀ
ient attractive precursors for exploring the chemistry
of high valent uranium.
1
yield (48%) and the H NMR spectrum of the reaction
mixture of 2 with trimethylsilyl azide shows the presꢀ
ence of several byꢀproducts. After separation of pure
5-TMS from the reaction mixture, few crystals of the
The observed high reactivity of the “ate” complex 2
with organic azides prompted us to investigate the
possibility of obtaining nitrido complexes in this novel
ligand environment from the reaction of 2 with inorꢀ
ganic azides.
azidoꢀamido
complex
[K(18c6)U(N3)(N(SiMe3)2(OSi(OtBu)3)3] 6 were obꢀ
tained from the concentrated mother liquor. The solꢀ
idꢀstate structure (Figure 7) shows that the uranium
cation in 6 is pentacoordinated in a distorted trigonal
bipyramid fashion by one azido ligand in apical posiꢀ
tion, three monodentate siloxy ligands and one silylꢀ
amido ligand in the equatorial plane. The silylamido
Nitride formation from the reaction of U(III)
siloxides with CsN3.
The reaction of [K(18c6)][U(OSi(OtBu)3)4] 2 with ceꢀ
sium azide in THF at ꢀ40°C proceeded in 24 hours to
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afford a complex mixture of compounds. From this
dinitrogen
(ꢂꢀη2:η2ꢀN2)
bridged
diuranium
mixture two types of crystals, pale blue/green and
brown diamondꢀshaped plates, formed at ꢀ40°C. Xꢀray
diffraction studies show the presence of the U(IV)
azido complex [K(18c6)U(N3)(OSi(OtBu)3)4], 7 (see
supporting information) and of the di ꢁꢀnitrido diuꢀ
ranium(V) complex [KU(ꢁꢀN)2(OSi(OtBu)3)]2 8.
complexes60,99,100 but with metrical parameters comꢀ
patible with the presence of a bridging diazenido ligꢀ
and.
Scheme 4. Reaction of [K(18c6)][U(OSi(OtBu)3)4] 2
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with cesium azide.
Figure 8. Ellipsoid plot for [KU(ꢂꢀN)(OSi(OtBu)3)3]2
8
crystallized from toluene; probability 50%. Hydrogen atꢀ
oms, methyl groups and solvent molecules are omitted for
clarity. Bond distances (Å) and angles (deg): U1ꢀOavg
2.23(3), U1ꢀN1 2.022(5), U1ꢀN1A 2.101(6), U1ꢀU1A
3.2960(6), N1ꢀN1A 2.479(8), U1ꢀN1ꢀU1A 106.1(2).
The structure of 8 is shown in Figure 8. The molecule
has a crystallographically imposed symmetry center in
between the two uranium cations. The most interestꢀ
ing feature in the structure of 8 is the presence of two
nitrido atoms bridging two uranium centers in a diaꢀ
mondꢀshaped geometry. Only one example of diaꢀ
mondꢀshaped U2N2 nitride has been previously reꢀ
ported for
a diuranium mixed valent complex
U(V)/U(IV) isolated from dinitrogen reduction.40
However, complex 8 is the first example of a U2N2
core obtained from the reaction of U(III) with azides.
The overall neutral charge of the complex is consistent
with a formal +V oxidation state for each uranium
cation, coordinated to six monoanionic siloxide ligꢀ
ands and two trianionic nitrido ligands in a distorted
square pyramid geometry. Bond valence sum calculaꢀ
tions are in agreement with a U(V) oxidation state (see
supporting). The U2N2 core in 8 is planar, with a
N1…N1A separation of 2.479(8) Å ruling out the presꢀ
ence of a chemical bond between the two nitrogen
atoms. The U…U separation is 3.2960(6) Å. The short
UꢀN bond distances (2.101(6) and 2.023(5) Å) indicate
a multiple bond character and are similar to the values
of UꢀN bond distances found in other nitride bridged
uranium compounds.40,46 All the metrical parameters
including the UꢀNꢀU angles (106.1(2)° in 8 in the U2N2
core are similar to those found in the U2N2 core of the
Figure 9. U2(ꢁꢀN)2 core view and metric parameters from
the crystal structure of 8.
The formation of this unprecedented U(V)/U(V) comꢀ
plex is likely to occur trough a highly reactive U(V)
terminal nitrido intermediate supported by the
crowded environment provided by the four siloxides
by elimination of N2 from a U(III) azido intermediate.
The highly charged tetrasiloxide terminal nitride will
then afford the dinitrido complex trough the loss of
one siloxide ligand. An alternative intermediate inꢀ
volving a diuranium complex with a bridging azide
ligand is less probable. Attempts to prepare complex 8
pure in significant amounts failed, preventing further
characterization of this complex. Attempts to trap a
transient terminal nitride were also unsuccessful. This
is in line with the high reactivity demonstrated by the
U(IV)/U(V)
complex
supported
by
a
caꢀ
lix[4]tetrapyrrole ligand40 although a shorter U…U
distance is found in 8 (3.2960(6) Å in 8 and 3.355(6)
Å
in
[{K(dme)(calix[4]tetrapyrrole)U}2(µꢀNK)2]ꢀ
[K(dme)4]40) as expected given the higher oxidation
state of the uraniums in 8. A similar U2N2 diamondꢀ
shaped core is also found in three reported example of
elusive
uranium
nitride
intermediate
[UN{N(SiMe3)2}(C5Me5)2] which engages in intramoꢀ
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lecular CꢀH bond activation yielding the U(IV) amido
complex (C5Me5)(C5Me4CH2NH)U[N(SiMe3)2].48
ands results in a disymmetric structure. While U1 is
coordinated by one terminal siloxide and two siloxides
A
recently isolated terminal U(V) nitride also shows an
extreme reactivity and its isolation and solution stabilꢀ
ity are very dependent on the experimental condiꢀ
tions.49 The high reactivity of the complex 2 with
TMSCl, pyridine or [B(C6F6)]3 prevents their use for
trapping a possible terminal nitrido intermediate.2,42,49
bridging the U and Cs centers, U2 is coordinated by a
terminal siloxide ligand, a siloxide ligand bridging the
U and Cs centers and a third one acting as a bidentate
Oꢀ/OtBu ligand. Thus U1 is tetracoordinated in a
pseudoꢀtetrahedral geometry while U2 is pentacoorꢀ
dinated in a distorted square pyramidal environment.
The mean value for the UꢀOꢀ bond distances (UꢀOꢀavg
=
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Since in the final complex 8 only three coordinated
siloxide ligands are bound to the uranium center, we
have also investigated the reactivity of the trisꢀsiloxide
U(III) complex previously reported.
2.19(3) Å) are in the range of those found in other
U(IV)ꢀsiloxide complexes. This, together with bond
valence sum calculations and variableꢀtemperature
magnetic moment measurements (see supporting
information) confirm the U(IV)ꢀN3ꢀꢀU(IV) formulation
for the complex. This compound slowly decomposes in
THF solution at r.t. over 1 week.
Scheme 5. Synthesis of the dinuclear uraniꢀ
um(IV)/uranium(IV) nitride Cs{(ꢂꢀN)[U(OSi(OtBu)3)3]2}
9.
The reaction of [U(OSi(OtBu)3)2(ꢁꢀOSi(OtBu)3)]2, 1,
with cesium azide was performed in THF at ꢀ40°C.
The reaction proceeds in 24 hours to afford the comꢀ
plex Cs{(ꢂꢀN)[U(OSi(OtBu)3)3]2} 9 in 47 % yield
(Scheme 5). This compound is soluble in THF, toluene
and hexane, and is crystallized upon cooling a saturatꢀ
ed toluene solution to ꢀ40°C. The crystal structure
determined by Xꢀray studies is presented in Figure 10.
It consists of a heterotrinuclear (U,U,Cs) complex.
Two uranium(IV) cations are held together by a bridgꢀ
ing nitrido N3ꢀ ligand in a nearly linear fashion (UꢀNꢀ
U angle : 170.2(3)°) and with short UꢀN nitride bond
distances (U1ꢀN1 2.058(5) Å, U2ꢀN1 2.079(5) Å) in
agreement with the presence of a multiple U=N bond.
These values are close to those found for the two other
reported uranium nitrides containing the linear
U=N=U fragment (U=N distance ranging from 2.05 to
2.09 Å and UꢀNꢀU angle ranging from 160 to 175
°).43,46
Figure 10. Ellipsoid plot for Cs{(ꢂꢀN)[U(OSi(OtBu)3)3]2} 9,
crystallized from a saturated hexane solution ; probability
50%, Hydrogen atoms and methyl groups are omitted for
clarity. Bond distances (Å) and angles (deg): U1ꢀN1
2.058(5), U2ꢀN1 2.079(5), U1ꢀOꢀavg 2.183(14), U2ꢀOtBu
2.713(4), U2ꢀOꢀavg 2.19(3).
The formation of this linear nitride could involve a
mononuclear nitride or an azido bridged dinuclear
complex as intermediates. The later intermediate is
supported by the outcome of the reaction of the U(III)
complex 1 with the isoelectronic CS2 molecule which
affords the dinuclear complex U(IV)ꢀ(CS2)2ꢀꢀU(IV)
complex.53 The different outcome of the reaction with
CsN3 for the complexes 1 and 2 clearly underlines the
importance of the coordination environment for the
control of the reactivity and for determining the strucꢀ
ture of the final product.
Uranium nitride complexes remain rare and complex
9 is only the second example of a dinuclear uranium
nitride complex featuring a linear U(IV)ꢀNꢀU(IV) moꢀ
tif. The closely related dinuclear anionic U(IV)ꢀNꢀ
U(IV) complex [(ꢁꢀN)(U(N[tꢀBu]Ar)3)2]Na has been
previously obtained from the reaction of the trisꢀ
amido complex {(THF)U(N[tꢀBu]Ar)3} (Ar = 3,5ꢀ
Me2C6H3) with sodium azide.43 The main difference
between the structure of the later anionic nitride comꢀ
plex and that of complex 9 is the neutral character of
complex 9 and its heterometallic structure. Notably, a
cesium cation is held in the structure through the coꢀ
ordination of three siloxy ligands which act as bridgꢀ
ing bidentate ꢁꢀη2 ligands. This cesium cation lies at
the exact apical position of the nitride ligand (U1ꢀN1ꢀ
Cs1 angle: 86.6(1)°), pointing at the 2pz filled orbital of
the N3ꢀ ligand, with a CsꢀN distance of 3.393(4) Å. The
coordination of the alkaline cation by the siloxide ligꢀ
The isolation of the bisꢀnitride U(V) dimer suggests
that it might be possible to find conditions leading to
mononuclear nitrides using siloxides as supporting
ligands. Moreover the synthesis of the heterotrimetalꢀ
lic complex 9 provides a nice precursor for heteropolꢀ
ymetallic nitrido systems. Future studies will be diꢀ
rected to investigate the reaction mechanism and the
bonding of these systems and to identify the condiꢀ
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tions for the isolation of mononuclear and polynuclear
nitride species.
ter in quartz cells (optical path lengths: 1 mm)
adapted with Young valves.
Concluding Remarks.
Mass spectra were acquired on a LXQꢀlinear ion
trap (Thermo Scientific, San Jose, CA, USA), equipped
with an electrospray source in a THF solution which
was prepared and filtered on microporous filters in
the gloveꢀbox and maintained under argon until injecꢀ
tion in the spectrometer. Electrospray full scan specꢀ
tra, in the range of m/z 50 – 3000 amu, were obtained
by infusion through fused silica tubing at 2 – 10 ꢀL
min–1. The LXQ calibration (m/z 50ꢀ2000) was
achieved according to the standard calibration proceꢀ
dure from the manufacturer (mixture of cafꢀ
feine/MRFA and Ultramark 1621). The LXQ calibraꢀ
tion (m/z 2000ꢀ4000) was performed with ES tuning
mix (Agilent). The temperature of the heated capillary
of the LXQ was set to the range of 180 – 220 °C, the
ion spray voltage was in the range of 1 – 3 kV with an
injection time of 5 ꢀ 100 ms. The experimental isotopic
profile was compared in each case to the theoretical
one.
A new stable homoleptic “ate” U(III) tetraꢀsiloxide
complex has been prepared by reduction of its U(IV)
analogue. The reaction of this complex and of the neuꢀ
tral U(III) trisꢀsiloxide analogue [U(OSi(OtBu)3)2(ꢁꢀ
OSi(OtBu)3)]2 with organic and inorganic azides leads
to the formation of different products. Notably, while
the reaction of the U(III) tetrakisꢀsiloxide with organic
azides affords new examples of stable U(V) imido
complexes, the same reaction with the trisꢀsiloxide
U(III) complex leads to the formation of a dimeric
bis(imido) complex of U(VI) via a highly reactive U(V)
intermediate. This complex presents an unprecedentꢀ
ed cationꢀcation interaction of the two transꢀimido
groups. The different outcome of the reaction of the
U(III) trisꢀ and tetrakisꢀ siloxide complexes with orꢀ
ganic azides shows the important role of the steric
environment in stabilizing U(V) imido species. A very
different outcome is also observed in the reaction of
the U(III) tris and tetrakisꢀ siloxide complexes with
CsN3. A clean reaction occurs with the U(III) trisꢀ
siloxide complex leading to a rare heterometallic niꢀ
trido bridged diuranium(IV) complex. In contrast the
reaction of the U(III) tetraꢀsiloxide leads to a complex
mixture of products from which we isolated a diꢀ
nitrido bridged diꢀuranium(V) complex which is likely
to form trough a terminal nitride intermediate.
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Starting materials. Unless otherwise noted, reaꢀ
gents were purchased from commercial suppliers and
used without further purification. The solvents were
purchased from Aldrich or Eurisotop (deuterated solꢀ
vents) in their anhydrous form, conditioned under
argon and vacuum distilled from K/benzophenone
(toluene, hexane, pyridine and THF). All reagents
were dried under highꢀvacuum for 7 days prior to use.
Syntheses were performed using glassꢀcovered stirring
bars. Depleted uranium turnings were purchased from
the "Société Industrielle du Combustible Nucléaire" of
Annecy (France). (tBuO)3SiOH was purchased from
These results show that the siloxide ligands provide a
versatile tool to modulate the steric and electronic
environment of lowꢀvalent uranium and to prepare
new mononuclear and polynuclear species containing
U=N multiple bonds. Future studies will be directed to
investigated the nature of bonding and the reactivity
of these systems.
ABCR chemicals and sublimed prior to use.
53
[UI4(OEt2)2],101
[U(OSi(OtBu)3)2(ꢁꢀOSi(OtBu)3)]2,
were prepared according to the published procedures.
(tBuO)3SiOK was prepared upon reaction of
(tBuO)3SiOH with one equivalent of KH in THF.
Caution: Depleted uranium (primary isotope 238U) is
a weak αꢀemitter (4.197 MeV) with a halfꢀlife of
4.47×109 years. Manipulations and reactions should
be carried out in monitored fume hoods or in an inert
EXPERIMENTAL SECTION
General Considerations. Unless otherwise noted,
all manipulations were carried out at ambient temperꢀ
ature under an inert argon atmosphere using Schlenk
techniques and an MBraun glovebox equipped with a
purifier unit. The water and oxygen levels were always
kept at less than 1 ppm. Glassware was dried overꢀ
night at 130°C before use.
atmosphere glovebox in
a radiation laboratory
equipped with αꢀ and βꢀcounting equipment.
Caution: Because of their potentially explosive charꢀ
acter, all reactions involving azido compounds should
be carried out with care, in small quantities and using
appropriate protection.
1H and 13C NMR spectroscopy experiments were
carried out using NMR tubes adapted with J. Young
Synthesis of [U(OSi(OtBu)3)4] 3.
1
valves. H NMR spectra were recorded on Bruker 200
MHz and Varian Mercury 400 MHz spectrometers. 13
C
A solution of [UI4(OEt2)2] (227.5 mg, 0.25 mmol, 1
equiv.) in THF (2 mL) was added to a solution of
KOSi(OtBu)3 (308.0 mg, 1.02 mmol, 4 equiv.) in THF
(2 mL). The reaction mixture was stirred at room
temperature for 4 hours. The resulting light blue susꢀ
pension was filtered to remove KI and the green filꢀ
trate was evaporated to dryness to give a microcrystalꢀ
line pale green solid. This solid was dissolved twice in
3 mL of hexane, triturated, and evaporated to dryness.
The resulting solid was dissolved in 1 mL hexane and
NMR spectra were recorded on Bruker 50 MHz specꢀ
trometer. NMR chemical shifts are reported in ppm
with solvent as internal reference.
Elemental analyses were performed under argon
by Analytische Laboratorien GMBH at Lindlar, Gerꢀ
many.
UV-visible spectroscopy measurements were carꢀ
ried out with a Varian Cary 50 Probe spectrophotomeꢀ
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cooled to ꢀ40°C. After 12 hours, blue crystals were
brown solution was added a 5 mL toluene solution of
recovered and dried in vacuo for 4 hours giving
[U(OSi(OtBu)3)4], 3 as a light lilac powder (289.0 mg,
0.22 mmol, 88 % yield). Single crystals suitable for Xꢀ
ray diffraction analysis were obtained upon cooling a
concentrated toluene solution of [U(OSi(OtBu)3)4] to ꢀ
40°C. Recrystallization of this complex in pyridine
AdN3 (21.3 mg, 0.120 mmol, 2 equiv.) and the reaction
mixture was vigorously stirred for 4 hours at room
temperature. A strong N2 bubbling was observed. The
resulting dark brown solution was taken to dryness
and the residue was extracted with 0.5 mL hexane,
filtered and the filtrate was cooled to ꢀ40°C. Crystals
were grown from this solution in 2 days. The crystals
were collected and dried in vacuo to give 4 as a brown
crystalline solid (22.2 mg, 0.010 mmol, yield of 35 %
calculated taking account of the reaction stoechiomeꢀ
try). The presence of small amounts (∼ 6%) of the seꢀ
cond reaction product (3) in this solid prevented the
obtention of a satisfactory elemental analysis. Brown
crystals of 4 suitable for XRD were grown similarly.
4
5
6
7
8
9
afforded
green
single
crystals
of
[U(OSi(OtBu)3)4(py)2], 3-Py.
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1H NMR (200 MHz, hexaneꢀd14, 298 K): δ = 0.98 (s,
108H, CH3).
1H NMR (200 MHz, tolueneꢀd8, 298 K): δ = 1.06 (s,
108H, CH3).
1H NMR (200 MHz, THFꢀd8, 298 K): δ = 1.9 (br s,
108H, CH3).
1H NMR (200 MHz, pyꢀd5, 298 K): δ = 1.90 (s, 54H,
CH3) ; 1.90 (s, 54H, CH3).
1H NMR (200 MHz, pyꢀd5, 263 K): δ = 9.7 (br s, 54H,
CH3) ; ꢀ2.7 (br s, 54H, CH3).
1H NMR (200 MHz, pyꢀd5, 353 K): δ = 3.1 (br s, 108H,
CH3).
13C NMR (50 MHz, hexaneꢀd14, 298 K): δ = 28.99 (s,
CH3), 68.33 (s, OC(CH3)3).
13C NMR (50 MHz, tolueneꢀd8, 298 K): δ = 31.29 (s,
CH3), 67.74 (s, OC(CH3)3).
1H NMR (200 MHz, tolueneꢀd8, 298 K): δ 2.29 (brs,
3H, Hadamantyl), 1.91 (brs, 9H, Hadamantyl), 1.83 (brs,
108H, OC(CH3)3), 1.54 (brs, 3H, Hadamantyl), 1.23 (brs,
6H, Hadamantyl), 1.12 (brs, 18H, Hadamantyl), 0.67 (brs,
18H, Hadamantyl).
ESꢀMS: m/z= 2141.7 [M+OH]ꢀ.
Synthesis
of
[K(18c6)][U(NSiMe3)(OSi(OtBu)3)4] 5-TMS.
100 ꢂL of a 0.94 M THF solution of trimethylsilyl azꢀ
ide (0.094 mmol, 1 equiv.) were added to a cold (ꢀ
Anal. Calcd for [U(OSi(OtBu)3)4]: C48H108O16Si4U: C
44.6; H 8.4; N 0.0; Found: C 44.3; H 8.3; N <0.2.
ESꢀMS: m/z= 1313.5 [M+Na]+; 1329.5 [M+K]+.
40°C)
2
mL
THF
solution
of
[K(18c6)][U(OSi(OtBu)3)4] (150.0 mg; 0.094mmol, 1
equiv.). An immediate color change from dark orange
to brown was observed together with N2 evolution.
The resulting solution was stirred for 30 minutes beꢀ
fore the solvent was removed in vacuo. The resulting
brown oil was extracted with 2 mL toluene, filtered
and the filtrate was stored 48 hours at ꢀ40° C to afford
Synthesis of [K(18c6)][U(OSi(OtBu)3)4] 2.
KC8 (25.3 mg; 0.187 mmol, 1.5 eq) was added to a cold
(ꢀ 40°C) solution of [U(OSi(OtBu)3)4] (161.0 mg, 0.125
mmol, 1 equiv.) and 18ꢀcrownꢀ6 (32.9 mg, 0.125
mmol, 1 equiv.) in THF (2 mL). The resulting dark
orange suspension was stirred for 1 hour at ꢀ40° C.
The suspension was centrifuged, the supernatant was
collected and the solvent was removed in vacuo. The
resulting solid was washed with 1 mL hexane and 6 x 1
mL toluene and dried in vacuo to give
[K(18c6)][U(OSi(OtBu)3)4], 2 as an orange solid in
69% yield (138 mg, 0.087 mmol). Recrystallisation of
this orange solid in toluene produced Xꢀray quality
crystals of 2.
[K(18c6)][U(NSiMe3)(OSi(OtBu)3)4], 5-TMS as
a
brown crystalline solid in 48% yield (75.5 mg, 0.045
mmol). Similar procedure was followed to produce
brown triangular shaped single crystals suitable for Xꢀ
ray diffraction.
1H NMR (400 MHz, tolueneꢀd8, 298 K): δ 10.98 (s,
9H, SiMe3), 3.51 (s, 24H, 18c6), ꢀ0.25 (brs, 108H,
OC(CH3)3).
Anal. Calcd for [K(18c6)][U(NSiMe3)(OSi(OtBu)3)4]:
C63H141O22Si4NKU: C 45.0; H 8.5; N 0.8; Found: 44.5;
H 8.2; N 0.9.
1H NMR (400 MHz, tolueneꢀd8, 298 K): δ 3.40 (s,
24H, 18c6), ꢀ0.15 (brs, 108H, OC(CH3)3).
1H NMR (400 MHz, THFꢀd8, 298 K): δ 3.64 (s, 24H,
18c6), 1.18 (brs, 108H, OC(CH3)3).
ESꢀMS: Due to the extreme reactivity of this comꢀ
pound towards dioxygen, it was not possible to acꢀ
quire a mass spectrum for this compound. Indeed,
even when taking all the usual precautions, the only
compound observed in the spectra was the uranyl(VI)
trisꢀsiloxy [UO2(OSi(OtBu)3)3]ꢀ complex (m/z=
1059.5).
Anal.
Calcd
for
[K(18c6)][U(OSi(OtBu)3)4]:
C60H132O22Si4KU: C 45.2; H 8.3; N 0.00; Found: C
45.0; H 8.3; N <0.1.
ESꢀMS: m/z= 1290.4 [U(OSi(OtBu)3)4]ꢀ.
The remaining pale brown filtrate was cooled down to
ꢀ40°C. This afforded pale blue single crystals of
[K(18c6)][U(N3)(N(SiMe3)2(OSi(OtBu)3)3], 6 suitable
for Xꢀray diffraction.
Reaction of 1 with AdN3, isolation of 4.
A
vial was charged with [U(OSi(OtBu)3)2(ꢁꢀ
OSi(OtBu)3)]2 (123.5 mg, 0.060 mmol, 1 equiv.) and
the compound was dissolved in 5 mL toluene. To this
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8
9
The reaction of 2 with TMSꢀN3 was also performed in
toluene but the outcome of the reaction was similar to
the one observed in THF.
to ꢀ40°C. This cold solution was added onto cold CsN3
(10.7 mg, 0.061 mmol, 1 equiv.) and the reaction mixꢀ
ture was vigorously stirred with a glassꢀcoated stir bar
for 24 hours at ꢀ40°C. The resulting brownish red
solution was taken to dryness and the residue was
extracted with 2 mL hexane, filtered and the filtrate
was cooled to ꢀ40°C. Crystals were grown from this
solution in 10 days. The crystals were collected and
dried in vacuo to give 9 as a pure crystalline solid
(63.6 mg, 0.029 mmol, 47 %). Brownꢀred crystals of 9
suitable for XRD were grown by slow evaporation of a
hexane solution of 9.
Synthesis of [K(18c6)][U(NAd)(OSi(OtBu)3)4]
5-Ad.
A toluene solution (2 mL) of adamantylazide (11.1 mg,
0.063 mmol, 1 equiv.) was added to a 4 mL toluene
suspension of [K(18c6)][U(OSi(OtBu)3)4] (100.0 mg;
0.063 mmol, 1 equiv.). Immediately, the orange susꢀ
pension turned dark brown, all the solid solubilized
and a strong N2 bubbling was observed. The resulting
solution was stirred for 2 hours before filtration. The
filtrate was taken to dryness and extracted with 2 mL
hexane. The brown suspension was filtered and the
brown filtrate was cooled to ꢀ40°C for 24 hours to
afford [K(18c6)][U(NAd)(OSi(OtBu)3)4], 5-Ad as a
brown crystalline solid (72.1 mg, 0.041 mmol, 66 %).
Single crystals suitable for Xꢀray diffraction were
grown by slow evaporation of a hexane solution of the
complex. This complex is highly soluble in toluene.
1H NMR (400 MHz, tolueneꢀd8, 298 K): δ 21.83 (brs,
6H, Hadamantyl), 14.93 (s, 3H, Hadamantyl), 10.61 (d, 3H,
Hadamantyl), 8.45 (d, 3H, Hadamantyl), 3.48 (s, 24H, 18c6),
ꢀ0.73 (brs, 108H, OC(CH3)3).
Anal. Calcd for [K(18c6)][U(NAd)(OSi(OtBu)3)4]:
C70H147O22N1Si4K1U1: C 48.2; H 8.5; N 0.8; Found C
48.0; H 8.3; N 0.9.
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ESꢀMS: m/z= 1957.8 ([9 ꢀ Si(OtBu)3 + 2H]+), 1027.6
([U(OSi(OtBu)3)3]+).
1H NMR (200 MHz, tolueneꢀd8, 298 K): δ = ꢀ1.16 (s,
108H, CH3), ꢀ2.49 (s, 54H, CH3).
Anal. Calcd for 2: C72H162CsNO24Si6U2: C 39.2; H 7.4;
N 0.6; Found C 38.9; H 7.5; N, 0.8.
Electrochemical Methods. Cyclic voltammetry
data were carried out at room temperature in an arꢀ
gonꢀfilled glovebox described above. Data were colꢀ
lected using a Biologic SPꢀ300 potentiostat connected
to a personal computer. All samples were 2 mM in
complex with 0.1 M [Bu4N][PF6] supporting electroꢀ
lyte in THF solution. The experiments were carried
out with a vitrous carbon working electrode, a platiꢀ
num wire counter electrode and an Ag/AgCl reference
electrode. Potential calibration was performed at the
end of each data collection cycle using the ferroꢀ
cene/ferrocenium [(C5H5)2Fe]+/0 couple as an internal
standard.
ESꢀMS: Due to the extreme reactivity of this comꢀ
pound towards dioxygen, it was not possible to acꢀ
quire a mass spectrum for this compound. Indeed,
even when taking all the usual precautions, the only
compound observed in the spectra was the uranyl(VI)
trisꢀsiloxy [UO2(OSi(OtBu)3)3]ꢀ complex (m/z=
1059.5).
X-ray Crystallography. Diffraction data were taken
using an OxfordꢀDiffraction XCallibur S kappa geomeꢀ
try diffractometer (MoꢀKα radiation, graphite monoꢀ
chromator, λ = 0.71073 Å). To prevent evaporation of
coꢀcrystallised solvent molecules the crystals were
coated with light hydrocarbon oil and the data were
collected at 150 K. The cell parameters were obtained
with intensities detected on three batches of 5 frames.
The crystalꢀdetector distance was 4.5 cm. The number
of settings and frames has been established taking in
consideration the Laue symmetry of the cell by CrysAꢀ
lisPro Oxfordꢀdiffraction software.102 289 for 3, 662
for 3-py, 141 for 2, 252 for 4, 384 for 5-Ad, 221 for 5-
TMS, 509 for 6, 492 for 7, 444 for 8 and 1019 for 9
narrow data were collected for 1° increments in ω with
a 2 s exposure time for 3, 20 s for 3-py, 300 s for 2,
150 s for 4, 98 s for 5-Ad, 90 s for 5-TMS, 15 s for 6,
4 s for 7, 110 s for 8 and 10 s for 9. Unique intensities
detected on all frames using the Oxfordꢀdiffraction
Red program were used to refine the values of the cell
parameters. The substantial redundancy in data alꢀ
lows empirical absorption corrections to be applied
using multiple measurements of equivalent reflections
with the ABSPACK Oxfordꢀdiffraction program.102
Space groups were determined from systematic abꢀ
sences, and they were confirmed by the successful
Reaction of [K(18-c-6)][U(OSi(OtBu)3)4] with
CsN3.
A vial was charged with [K(18c6)][U(OSi(OtBu)3)4]
(50.0 mg, 0.031 mmol, 1 equiv.) and the compound
was dissolved in 2 mL THF at ꢀ40°C. This solution was
added onto CsN3 (5.5 mg, 0.031 mmol, 1 equiv.) and
the reaction mixture was vigorously stirred with a
glassꢀcoated stir bar for 24 hours at ꢀ40°C. The resultꢀ
ing brownish red solution was taken to dryness and
the residue was extracted with 2 mL toluene, filtered
and the filtrate was cooled at ꢀ40°C. Two types of crysꢀ
tals suitable for XRD were grown from this solution :
pale
blue
rectangular
parallelepipeds
of
[K(18c6)][U(N3)(OSi(OtBu)3)4] 7 and brown rhombusꢀ
faced parallelepipeds of [KU(ꢂꢀN)2(OSi(OtBu)3)3]2, 8.
1H NMR (400 MHz, tolueneꢀd8, 298 K) of crystals of 8
shows only one peak at ꢀ1.76 ppm.
Synthesis of Cs{(µ-N)[U(OSi(OtBu)3)3]2} 9.
A
vial was charged with [U(OSi(OtBu)3)2(ꢁꢀ
OSi(OtBu)3)]2 (124.9 mg, 0.061 mmol, 1 equiv.) and
the compound was dissolved in 2 mL THF and cooled
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solution of the structure. The structures were solved
py where hydrogen atoms from the pyridine and the
by direct methods using the SHELXTL 6.14
package.103 All nonꢀhydrogen atoms were found by
difference Fourier syntheses and refined on F2. Hyꢀ
drogen atoms were fixed in ideal position except for 3-
Si1 silanol moieties were found by Fourier synthesis
and refined. Full crystallographic details are given in
Tables 1 and 2.
8
9
A. C.; Scott, B. L.; Shuh, D. K.; Wilkerson, M. P. J. Am.
Chem. Soc. 2013, 135, 2279ꢀ2290.
ASSOCIATED CONTENT
Supporting Information. H NMR, ESI/MS and
UVꢀvis spectra, magnetic and electrochemistry data,
crystallographic and structural details, Xꢀray crysꢀ
tallographic data and files in CIF format. This mateꢀ
rial is available free of charge via the Internet at
http://pubs.acs.org.
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(16)
Spencer, L. P.; Yang, P.; Scott, B. L.;
Batista, E. R.; Boncella, J. M. Inorg. Chem. 2009, 48,
2693ꢀ2700.
(17)
Graves, C. R.; Yang, P.; Kozimor, S.
A.; Vaughn, A. E.; Clark, D. L.; Conradson, S. D.;
Schelter, E. J.; Scott, B. L.; Thompson, J. D.; Hay, P. J.;
Morris, D. E.; Kiplinger, J. L. J. Am. Chem. Soc. 2008,
130, 5272ꢀ5285.
AUTHOR INFORMATION
(18)
Lam, O. P.; Franke, S. M.; Nakai,
H.; Heinemann, F. W.; Hieringer, W.; Meyer, K. Inorg.
Chem. 2012, 51, 6190ꢀ6199.
Corresponding Author
* marinella.mazzanti@cea.fr
(19)
Wietzke, R.; Mazzanti, M.; Latour,
J.ꢀM.; Pecaut, J.; Cordier, P.ꢀY.; Madic, C. Inorg. Chem.
1998, 37, 6690ꢀ6697.
Author Contributions
All authors have given approval to the final version
of the manuscript.
(20)
Mazzanti, M.; Wietzke, R.; Pécaut,
J.; Latour, J.ꢀM.; Maldivi, P.; Remy, M. Inorg. Chem.
2002, 41, 2389ꢀ2399.
ACKNOWLEDGMENT
We thank Colette Lebrun for her help with mass
spectroscopy characterizations. We acknowledge
(21)
Denecke, M. A.; Panak, P. J.;
Burdet, F.; Weigl, M.; Geist, A.; Klenze, R.; Mazzanti,
M.; Gompper, K. C. R. Chim. 2007, 10, 872ꢀ882.
(22)
Hudson, M. J.; Harwood, L. M.;
support from the Commissariat
à
l′Energie
Laventine, D. M.; Lewis, F. W. Inorg. Chem. 2013, 52,
3414ꢀ3428.
Atomique, Direction de l′Energie Nucléaire, RBPCH
program and from the “Agence Nationale de la Reꢀ
cherche”, (ANRꢀ10ꢀBLANꢀ0729).
(23)
Panak, P. J.; Geist, A. Chem. Rev.
2013, 113, 1199ꢀ1236.
(24)
Streit, M.; Ingold, F. J. Eur. Ceram.
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