U(IV) and U(V) azide complexes supported by amide or aryloxide ligands

Article abstract of DOI:10.1039/b909879h

The uranium(iv) azides [Li(THF)₃]₂[U(OAr) ₄(N₃)₂], Ar = 2,6-Me₂C ₆H₃, and {[Na(THF)₄][U[N(SiMe₃) ₂]₃(N₃)₂

Full text of DOI:10.1039/b909879h

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U(IV) and U(V) azide complexes supported by amide or aryloxide ligands†
Skye Fortier, Guang Wu and Trevor W. Hayton*
Received 19th May 2009, Accepted 13th July 2009
First published as an Advance Article on the web 24th July 2009
DOI: 10.1039/b909879h
The uranium(IV) azides [Li(THF)3]2[U(OAr)4(N3)2], Ar =
2,6-Me2C6H3, and {[Na(THF)4][U[N(SiMe3)2]3(N3)2]}x have
been synthesised and structurally characterised. Oxidation
of these complexes affords [Li(THF)3][U(OAr)5(N3)] and
U[N(SiMe3)2]3(N3)2, which are the first azides of U(V).
Metal azides are useful precursors for the synthesis of imido1–4
and nitrido5,6 complexes. This reactivity is particularly attractive
for use in actinide chemistry as it offers a means of accessing
metal–ligand multiple bonds. Probing the degree of covalency in
actinide–ligand interactions is an active area of research,7–11 and
complexes possessing multiple bonds are useful for elucidating
the extent of 6d- and 5f-orbital participation in bonding. Fur-
thermore, azides offer a possible route to uranium nitride (UN),
a potential fuel for Generation-IV nuclear power reactors.12–14
However, only a few uranium nitride complexes are known.15–19
Of these, [(C5Me5)2U(m-N)U(m-N3)(C5Me5)2]4,15 [(C5Me5)U(m-
I)2]3(m3-N),16 {[Cs(MeCN)3][U4(m4-N)(m-N3)8(MeCN)8I6]}x,17 and
t
19
≡
(C6F5)3BN U[N( Bu)(3,5-Me2C6H3)]3 were synthesised from
azide precursors, illustrating the utility of azides to generate
uranium nitrido complexes. To this end, we have synthesised a
series of uranium azide complexes, including the first reported
azides of uranium(V).
Fig. 1 ORTEP diagram of [Li(THF)3]2[U(OAr)4(N3)2] (1) with 50%
probability ellipsoids. Asterisks indicate symmetry related atoms. Selected
◦
˚
bond lengths (A) and angles ( ): U1–N1 = 2.442(6), U1–O1 = 2.191(4),
U1–O2 = 2.169(4), N1–N2 = 1.181(8), N2–N3 = 1.173(8), Li1–N3 =
2.02(1), U–N1–N2 = 141.9(4), U–O1–C1 = 175.6(3), U1–O2–C9 =
168.7(4), N1–N2–N3 = 177.6(7), O1–U1–O2 = 90.1(1), N1–U1–O1 =
91.1(2), N1–U1–O2 = 90.2(7).
Alcoholysis of [Li(THF)]2[U(OtBu)6] with 6 equiv. ArOH (Ar =
2,6-Me2C6H3) in the presence of 2 equiv. Me3SiN3 generates the
U(IV) bis(azide), [Li(THF)3]2[U(OAr)4(N3)2] (1), in moderate yield
(eqn (1)).20 The reaction is likely driven by the formation of strong
Si–O bonds, presumably yielding either Me3SiOtBu or Me3SiOAr
range of other structurally characterised uranium azides, which
display U–Nazide bond distances and U–Na–Nb bond angles of
1
as by-products. The H NMR spectrum of 1 in C6D6 consists
◦
15,17,21–25
˚
of three broad peaks at 8.86, 12.26, and 14.12 ppm in a 3 : 1 : 2
ratio, respectively, assignable to the aryloxide ligands. A very broad
resonance is also observed at -3.75 ppm, assignable to coordinated
THF.
Recrystallisation of 1 from hexanes/Et2O provides colourless
material suitable for X-ray crystallographic analysis.‡ Complex 1
crystallises in the monoclinic space group P21/n,
2.219(6)–2.564(12) A and 121.5(5)–178.6(7) , respectively.
The azide moiety of 1 is linear (N1–N2–N3 = 177.6(7)◦) and
˚
it exhibits symmetrical N–N distances (N1–N2 = 1.181(8) A,
˚
N2–N3 = 1.173(8) A). These equivalent N–N distances are
suggestive of a largely ionic U–Nazide interaction,26–28 and they differ
from those observed in the uranium heptaazide [Bu4N][U(N3)7]
22
˚
˚
(av. Na–Nb = 1.20 A; Nb–Ng = 1.10 A), which may possess
U–Nazide interactions with larger covalent character. The Li–Nazide
˚
distance (Li–N3 = 2.02(1) A) is comparable to other Li–Nazide
(1)
bond lengths.29,30 Lewis acid coordination to the azide moiety
could potentially induce nitrogen elimination,31 but in complex
1 coordination of Li+ to the N3 unit does not give rise to this
-
and its solid-state molecular structure is shown in Fig. 1. The
octahedral uranium centre is coordinated by four aryloxide ligands
and two trans-azido ligands. Complex 1 exhibits a U–Nazide bond
mode of reactivity.
We have also synthesised an amide-supported U(IV) azide
complex. Addition of excess NaN3 to a THF solution of
UCl[N(SiMe)3]332 produces a pink suspension after 24 h. Filtration
of the mixture and recrystallisation from hexanes/THF affords
{[Na(THF)4][U[N(SiMe3)2]3(N3)2]}x (2) as pink crystals in 54%
˚
length of U1–N1 = 2.442(6) A and a U–Na–Nb bond angle
of U1–N1–N2 = 141.9(4)◦. These parameters fall within the
Department of Chemistry and Biochemistry, University of California, Santa
Barbara, Santa Barbara, CA 93106. E-mail: hayton@chem.ucsb.edu
† Electronic supplementary information (ESI) available: Experimental
details. CCDC reference numbers 733176–733179. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/b909879h
1
yield (Scheme 1). The H NMR spectrum of 2 in C6D6/CD2Cl2
exhibits a broad resonance at -4.25 ppm, corresponding to the
methyl protons of the amide ligand.
352 | Dalton Trans., 2010, 39, 352–354
This journal is
The Royal Society of Chemistry 2010
©

Complex 3 crystallises in the orthorhombic space group P212121,
and its solid-state molecular structure is shown in Fig. 3. This
species exhibits an octahedral uranium centre ligated by one
azide and five aryloxide ligands. The U–Nazide distance (U1–N1 =
˚
2.318(8) A) is shorter than that observed in 1, consistent with a
smaller U5+ ionic radius. The U–Na–Nb bond angle of 3 (U–N1–
N2 = 129.4(6)◦) is also considerably smaller than that observed
in 1. Oxidation of 1 to 3 does not affect the parameters of the
azide moiety. The N–N distances of the azide ligand in 3 remain
Scheme 1
˚
˚
symmetrical (N1–N2 = 1.17(1) A, N2–N3 = 1.15(1) A) with a
In the solid-state, 2 crystallizes in the monoclinic space group
C2/c as a 1D coordination polymer (Fig 2). Each uranium
centre is trigonal bipyramidal and coordinated by three equatorial
amido ligands and two axial azide ligands. The repeating unit is
linked by a sodium cation which bridges two azide ligands. The
linear N–N–N bond angle (N1–N2–N3 = 176.7(9)◦). The Li–Nazide
˚
distance (Li1–N3 = 2.02(2) A) is also similar to that in 1.
˚
U–Nazide distance (U1–N1 = 2.337(6) A) is notably shorter than
that of 1 but comparable to other UIV–Nazide bond lengths.15,17,21–24
Complex 2 also exhibits a large U–Na–Nb bond angle (U1–N1–
N2 = 163.9(5)◦). The metrical parameters of the azide ligands
˚
˚
in 2 (N1–N2 = 1.175(7) A, N2–N3 = 1.146(9) A; N1–N2–N3 =
176.7(8)◦) are comparable to those found in 1, and the Na–Nazide
˚
distance (Na–N3 = 2.408(8) A) is consistent with other reported
Na–Nazide bond lengths.33,34
(2)
Fig. 3 ORTEP diagram of [Li(THF)3][U(OAr)5(N3)] (3) with 50%
◦
˚
probability ellipsoids. Selected bond lengths (A) and angles ( ): U1–N1 =
2.318(8), U1–O1 = 2.092(7), U1–O2 = 2.118(5), U1–O3 = 2.109(7),
U1–O4 = 2.122(5), U1–O5 = 2.098(6), N1–N2 = 1.17(1), N2–N3 =
1.15(1), Li1–N3 = 2.02(2), U–N1–N2 = 129.4(6), U–O1–C1 = 170.4(6),
U1–O2–C9 = 168.3(8), U1–O3–C17 = 168.5(6), U1–O4–C25 = 164.1(6),
U1–O5–C33 = 175.0(6), N1–N2–N3 = 176.7(9), N1–U1–O2 = 83.8(3),
O1–U1–O2 = 95.9(3), O2–U1–O3 = 88.8(2), O2–U1–O4 = 167.7(3),
O2–U1–O5 = 92.8(2).
The 1H NMR spectrum of 3 in C6D6 is consistent with its solid-
state molecular structure. The spectrum exhibits three peaks at
3.13, 5.64, and 8.32 ppm in a 3 : 1 : 2 ratio, respectively, assignable
to the equatorial aryloxide ligands. A second set of resonances,
also in a 3 : 1 : 2 ratio, is observed at 2.11, 5.33, and 7.68 ppm,
respectively, assignable to the axial aryloxide ligand. The relative
ratio between the these two peak sets is 4 : 1.
Fig. 2 ORTEP diagram of {[Na(THF)4][U[N(SiMe3)2]3(N3)2]}x (2) with
◦
˚
50% probability ellipsoids. Selected bond lengths (A) and angles ( ):
U1–N1 = 2.337(6), U1–N4 = 2.274(4), U1–N5 = 2.272(7), N1–N2 =
1.175(7), N2–N3 = 1.146(9), Na–N3 = 2.408(8), U1–N1–N2 = 163.9(5),
N1–N2–N3 = 176.7(8), N4–U1–N5 = 118.5(1), N1–U1–U4 = 91.0(2),
N1–U1–N5 = 90.6(1).
We have also investigated the oxidation of complex 2. Addition
of AgOTf to an Et2O suspension of 2 results in the rapid
formation of a deep-red solution, but no obvious gas evolution
(Scheme 1). Recrystallization from hexanes yields dark red crystals
It has recently been demonstrated that oxidation of a metal
azide can elicit nitride formation.35 Following this strategy, a
solution of 1 in THF was treated with excess AgCl (eqn (2)).
The reaction mixture immediately turns deep red, concomitant
1
in good yield, and characterisation of this material by H NMR
spectroscopy and X-ray crystallography indicates the formation
of U[N(SiMe3)2]3(N3)2 (4). Complex 4 is thermally stable and we
see no evidence that it undergoes disproportionation.
1
with gas evolution. As indicated by H NMR spectroscopy, the
reaction generates a mixture of products, however we were able
to isolate a few red crystals in low yield. Characterisation of this
material by X-ray crystallography reveals the formation of a U(V)
azide complex [Li(THF)3][U(OAr)5(N3)] (3), the result of a 1e-
oxidation of 1 and exchange of an azido group for an aryloxide.
In the solid-state, 4 exhibits a trigonal bipyramidal coordination
geometry (Fig. 4). As anticipated, the U–Nazide bond length
˚
(U1–N1 = 2.226(3) A) is shorter than that observed in 2, while the
U–Na–Nb bond angle in 4 (U1–N1–N2 = 175.5(2)◦) approaches
linearity. Unlike complexes 1–3, the N–N distances in 4 are no
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 352–354 | 353
©

range 1.42 < q < 27.10, of which 11 085 were unique. GOF = 0.940, R1 =
0.0538 [for 7010 reflections with I > 2s(I)] and wR2 = 0.1255 (for all
data). Crystal data for 4·C6H14: C24H68N9Si6U, M = 889.44, monoclinic,
˚
˚
˚
˚
space group C2/c, a = 18.767(1) A, b = 14.6497(8) A, c = 16.8791(9) A,
◦
3
˚
b = 111.295(2) , V = 4323.7(4) A , Z = 4, T = 150(2) K, l = 0.71073 A,
Rint = 0.0540; a total of 17 620 reflections collected in the range 1.81 < q <
26.73, of which 4513 were unique. GOF = 1.011, R1 = 0.0243 [for 4016
reflections with I > 2s(I)] and wR2 = 0.0581 (for all data).
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Fig. 4 ORTEP diagram of U[N(SiMe3)2]3(N3)2·C6H14 (4·C6H14) with 50%
◦
˚
probability ellipsoids. Selected bond lengths (A) and angles ( ): U1–N1 =
2.226(3), U1–N4 = 2.166(2), U1–N5 = 2.168(3), N1–N2 = 1.190(4),
N2–N3 = 1.136(5), U1–N1–N2 = 175.5(2), N1–N2–N3 = 179.5(4),
N4–U1–N5 = 116.61(6), N1–U1–N4 = 90.55(9), N1–U1–N5 = 89.70(7).
˚
˚
longer equivalent (N1–N2 = 1.190(4) A, N2–N3 = 1.136(5) A),
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1838.
indicative of slight activation of the azide moiety.
In addition to exploring their oxidation, we have also probed
the reduction chemistry of our azide complexes. U3+ is a powerful
reducing agent36 and is capable of reducing azides.17 However,
addition of UIII[N(SiMe3)2]337 to 4 does not induce N2 elimination,
instead its results in conproportionation, affording the previously
prepared U(IV)
16 W. J. Evans, K. A. Miller, J. W. Ziller and J. Greaves, Inorg. Chem.,
2007, 46, 8008–8018.
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20 Tractable material can only be isolated by performing the azide and
aryloxide exchange simultaneously. If either reagent is omitted, only
complex product mixtures are generated.
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Andersen, Organometallics, 2005, 24, 4251–4264.
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(3)
azide (N3)U[N(SiMe3)2]3 (5) (eqn (3)).38 Likewise, addition of
Na/Hg amalgam to a THF solution of 2 fails to induce N2
elimination. In fact, no reaction is observed between these two
reagents.
In summary, we have synthesized and structurally characterised
a series of U(IV) and U(V) azide complexes supported by aryloxide
and amide ligands. These complexes exhibit unique redox chem-
istry which we are continuing to investigate, with the intent of
producing nitrido complexes of uranium.
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‡ Crystal data for 1: C56H84Li2N6O10U, M = 1253.20, monoclinic, space
˚
3
˚
˚
group P21/n, a = 11.4965(8) A, b = 22.525(2) A, c = 12.6040(9) A, b =
◦
˚
˚
114.922(2) , V = 2959.9(4) A , Z = 2, T = 150(2) K, l = 0.71073 A,
Rint = 0.1679; a total of 23 433 reflections collected in the range 2.15 < q <
27.10, of which 6184 were unique. GOF = 0.904, R1 = 0.0438 [for 3802
reflections with I > 2s(I)] and wR2 = 0.1214 (for all data). Crystal data
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32 H. W. Turner, R. A. Andersen, A. Zalkin and D. H. Templeton, Inorg.
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˚
˚
˚
˚
a = 19.425(1) A, b = 11.8336(8) A, c = 25.319(2) A, b = 104.369(2) ,
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3
˚
V = 5637.8(7) A , Z = 4, T = 150(2) K, l = 0.71073 A, Rint = 0.1600; a
total of 23 319 reflections collected in the range 2.03 < q < 26.37, of which
5716 were unique. GOF = 1.011, R1 = 0.0483 [for 4793 reflections with I >
2s(I)] and wR2 = 0.1209 (for all data). Crystal data for 3: C52H69LiN3O8U,
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M = 1109.07, orthorhombic, space group P212121, a = 12.525(1) A, b =
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˚
˚
˚
20.098(2) A, c = 20.536(2) A, V = 5170(1) A , Z = 4, T = 150(2) K,
˚
l = 0.71073 A, Rint = 0.1611; a total of 42 047 reflections collected in the
354 | Dalton Trans., 2010, 39, 352–354
This journal is
The Royal Society of Chemistry 2010
©