Synthesis and Structure of Nitride-Bridged Uranium(III) Complexes

Article abstract of DOI:10.1021/jacs.5b12620

The reduction of the nitride-bridged diuranium(IV) complex Cs[{U(OSi(O^tBu)₃)₃}₂(-N)] affords the first example of a uranium nitride complex containing uranium in the +III oxidation state. Two nitride-bridged complexes containing the heterometallic fragments Cs₂[UIII?-N?-UIV] and Cs₃[UIII?-N?-UIII] have been crystallographically characterized. The presence of two or three Cs⁺ cations binding the nitride group is key for the isolation of these complexes. In spite of the fact that the nitride group is multiply bound to two uranium and two or three Cs⁺ cations, these complexes transfer the nitride group to CS₂ to afford SCN^- and uranium(IV) disulfide.

Full text of DOI:10.1021/jacs.5b12620

Communication
pubs.acs.org/JACS
Synthesis and Structure of Nitride-Bridged Uranium(III) Complexes
Lucile Chatelain, Rosario Scopelliti, and Marinella Mazzanti*
́ ́ ́
Institut des Sciences et Ingenierie Chimiques, Ecole Polytechnique Federale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
S
* Supporting Information
heterometallic structure. The ability of the OSi(OtBu)3 ligand
to bind to Cs+, thus stabilizing highly charged species,
ABSTRACT: The reduction of the nitride-bridged
diuranium(IV) complex Cs[{U(OSi(OtBu)3)3}2(μ-N)]
affords the first example of a uranium nitride complex
containing uranium in the +III oxidation state. Two
nitride-bridged complex‐e‐s‐ containing the heterometallic
motiva‐t‐e‐d us to explore the possibility of stabilizing the
‐‐‐
UNU fragment in highly reduced uranium species.
The reduction of complex 1 with 1 equiv or a large excess of
Cs0 in tetrahydrofuran (THF) at −40 °C under argon allowed
the synthesis and characterization of the U(III)/U(IV) complex
Cs2[{U(OSi(OtBu)3)3}2(μ-N)] (2) in 67% yield and the
U(III)/U(III) complex Cs3[{U(OSi(OtBu)3)3}2(μ-N)] (3) in
77% yield, respectively (Scheme 1). The solid-state molecular
‐‐‐
‐‐‐ ‐‐‐
fragments Cs2[UIIINUIV] and Cs3[UIIINUIII
]
have been crystallographically characterized. The presence
of two or three Cs+ cations binding the nitride group is key
for the isolation of these complexes. In spite of the fact
that the nitride group is multiply bound to two uranium
and two or three Cs+ cations, these complexes transfer the
nitride group to CS2 to afford SCN− and uranium(IV)
disulfide.
Scheme 1. Synthesis of Cs2[{U(OSi(OtBu)3)3}2(μ-N)] (2)
and Cs3[{U(OSi(OtBu)3)3}2(μ-N)] (3)
olecular uranium nitrides are attractive synthetic targets
Mbecause of their potential as precursors to ceramic
materials or as efficient molecular catalysts.1 Notably, uranium-
(III) mononitride, UN,2 a solid that is difficult to synthesize
and solubilize, has been proposed for alternative nuclear fuels3
and as an effective catalyst in dinitrogen reduction to
ammonia.4 Moreover, molecular nitride complexes are also
important to gain a better understanding of f orbital implication
in multiple bonding and covalency in actinide−ligand bonds.5
Uranium nitride chemistry remains much less developed than
the d-block counterparts. In recent years several molecular
complexes of uranium have been prepared that contain nitride
groups bridging two or more uranium ions6 or terminal nitride
groups.7 Most of these complexes contain uranium in its +IV
oxidation state, with a few systems containing U(V) and U(VI).
In spite of their relevance in materials science and catalysis and
the anticipated attractive reactivity of uranium(III) nitrides, no
molecular uranium(III) nitride complex has been isolated in
solution or in the solid state. The isolation of molecular
uranium(III) nitrides is essential for investigating the reactivity
of the UIII−nitride bond, which in turn will lead to convenient
routes to nitride materials and the design of molecular catalysts.
Here we report the first examples of nitride-bridged complexes
containing uranium in the +III oxidation state.
structures of complexes 2 and 3 were determined by single-
crystal X-ray diffraction (Figures 1 and 2). In both complexes 2
and 3, each uranium ion is coordinated by a nitride group and
three siloxide oxygens with a pseudotetrahedral geometry. The
two U−N distances in complex 3 are equivalent as a result of
the twofold crystallographic axis passing through one Cs and
the nitride ion. In complex 2 the two U−N distances are similar
(Table 1), suggesting the presence of nonlocalized charge.
In all of the complexes 1−3, the Cs+ cations are bound to the
bridging nitride and to the siloxide oxygens. In complex 2, two
Recently we reported the synthesis and molecular structure
of the dinuclear uranium(IV)/uranium(IV) nitride Cs[{U(OSi-
(OtBu)3)3}2(μ-N)] (1). This complex remains a rare
example6f,h‐‐o‐ f a dinuclear uranium nitride complex featuring a
Cs+ cations bind the nitride in an almost linear way (Cs−N−Cs
‐‐‐ ‐‐‐
angle =161.8(4)°) with the Cs−N−Cs and the UNU
fragments located in the same plane and perpendicular to each
other. In complex 3, the three Cs+ cations bind the nitride to
‐‐‐
linear UIVNUIV fragment (U−N−U angle = 170.2(3)°)
form an irregular triangle located in a plane perpendicular to
‐‐‐ ‐‐‐
the UNU fragment (Cs−N−Cs angles: 119.1(4),
and short U−N distances (U1−N1, 2.058(5) Å; U2−N1,
2.079(5) Å) indicative of U−N multiple bonds. Moreover, in
complex 1 the Cs+ cation binds the bridging nitride and six
oxygen atoms from the siloxide ligands, affording a unique
Received: December 3, 2015
© XXXX American Chemical Society
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DOI: 10.1021/jacs.5b12620
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
value of the U−N bond distance in the nitride core increases by
‐‐‐ ‐‐‐
about 0.08 Å in the fully reduced Cs3[UIIINUIII]system
‐‐‐ ‐‐‐
compared with the Cs[UIVNUIV] unit, probably as a result
of the presence of additional electrons at the uranium center.
Such an increase is similar to the increase in the average U−O
bond length (0.09 Å), which can be related to the difference
between the ionic radii of U(III) and U(IV) (0.135 Å). A
smaller variation (0.03 Å) was observed by Cummins and co-
workers in the successive oxidation of a linear
U(IV)NU(IV) fragment supported by amide ligands to
U(V)NU(V).6f The larger variation observed in the
successive reduction of the siloxide complex 1 is at least partly
due to the presence of an increasing number of Cs+ cations
Figure 1. Crystallographic structure of Cs2[{U(OSi(OtBu)3)3}2(μ-N)]
(2) crystallized from a saturated THF solution. The ellipsoid
probability is 50%, and hydrogen atoms, methyl groups, and solvent
molecules have been omitted for clarity. Atoms: C (gray), O (red), Si
(light yellow), N (light blue), Cs (purple), and U (green).
binding the nitride group a‐n‐d‐ thus polarizing and reducing the
‐‐‐
electron density on the UNU fragment. Lengthening of
U−N bonds upon alkali-metal ion coordination to the N atom
has been observed in dinuclear U(IV) imido complexes.7d
Complex 2 can be prepared analytically pure and stored in
the solid state under argon at −40 °C for several weeks, but it is
very reactive and can only be handled in solution at −40 °C.
Complex 3 can be obtained analytically pure but decomposes
very quickly both in the solid state and in THF solution at −40
°C, yielding mixtures of complexes 2 and 3 and free siloxide
ligand. The extremely high reactivities of complexes 2 and 3 are
in agreement with the absence in the literature of any molecular
nitride compounds containing uranium in the +III oxidation
state. The presence of the multidentate siloxide groups capable
of binding the Cs+ cation is key to the isolation of complex 3.
Notably, the reduction of 1 with an excess of Cs0 in the
presence of crown ether 18C6 leads to intractable reaction
mixtures containing the free ligand as the only NMR-detectable
species. This indicates that when the Cs+ cation is removed by
the crown ether from the coordination pocket formed by the
Figure 2. Crystallographic structure of Cs3[{U(OSi(OtBu)3)3}2(μ-N)]
(3) crystallized from a saturated THF solution. The ellipsoid
probability is 50%, and hydrogen atoms, methyl groups, and disorder
on Cs2 have been omitted for clarity. Atoms: C (gray), O (red), Si
(light yellow), N (light blue), Cs (purple), and U (green).
siloxide l‐i‐g‐ands in 1, it becomes impossible to isolate the
‐‐‐
[
UIIINUIII
]
3− species from the reduction of 1. The proton
NMR spectra of complexes 2 and 3 in THF solution show the
presence of only one signal for the six siloxide ligands, in
agreement with the presence of symmetry-related siloxides. In
the case of complex 2, this can interpreted in terms of the
fluxionality of the bound Cs cation. Proton NMR studies
showed that the addition of crown ether to complex 2 in THF
results in the removal of the bound Cs+, leading to a significant
decrease in the stability. In contrast, the addition of crown ether
to complex 3 in THF does not lead to Cs removal.
108.9(3), and 132.0(7)°). The Cs−N distances are longer than
those found in an imido-bridged U(IV) complex (mean Cs−N
= 3.075(10) Å).7d
‐‐‐ ‐‐‐
Complexes 2 and 3 display a linear UNU motif with
U−N−U angles comparable to that found in complex 1 (Table
1). The U−N distances in complexes 2 and 3 fall in the range
2.081−2.1495 Å and are longer than those found in 16g and
previously reported U(IV)/U(V) nitrides containing the linear
UNU motif (2.012(16)−2.090(8) Å).6a,f,h These distances
remain much shorter than U(III)−N single-bond distances
(e.g., U−Ncyanate = 2.456(7) Å,8 U−Ndinitrogen = 2.401(8)−
2.423(8) Å,9 and U−Namide = 2.320(4) Å in U[N(SiMe3)2]310).
Longer U−N distances were also found in a U(IV) cluster with
a U4(μ4-N) core (2.271(3)−2.399(5) Å).6d This points to the
presence of UIII−N multiple bonding in 2 and 3. The mean
Significant changes were also observed in the cyclic
voltammogram of 1 when the electrochemistry was carried in
the presence of 18C6. The cyclic voltammogram of complex 1
measured in THF (see the Supporting Information) shows two
irreversible electrochemical events at −2.34 and −0.92 V (vs
[Cp2Fe]0/+), corresponding to reduction and oxidation of the
complex. The irreversibility of these redox events is probably
Table 1. Comparative Structural Parameters of Complexes 1−3
‐‐‐ ‐‐‐
‐‐‐ ‐‐‐
‐‐‐ ‐‐‐
[UIVNUIV] (1)
[UIIINUIV] (2)
[
UIIINUIII] (3)
U1−N (Å)
U2−N (Å)
U−Oavg (Å)
Cs1−N (Å)
Cs2−N (Å)
U−N−U (deg)
2.058(5)
2.079(5)
2.19(3)
3.393(4)
−
2.099(12)
2.081(12)
2.243(25)
3.276(12)
3.635(12)
169.1(7)
2.1495(12)
2.282(24)
3.348(8)
3.22(2)
170.2(3)
174.2(11)
B
DOI: 10.1021/jacs.5b12620
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
due to the important rearrangement of the coordination sphere
during the redox processes. After removal of Cs+, the reduction
wave is shifted to lower potential (Epc = −2.43 V), indicating
that the reduction of complex 1 is more difficult in the absence
of coordinated Cs+.
Complexes 2 and 3 provide the first examples of isolated
molecular nitride complexes containing uranium in the +III
oxidation state. These systems are expected to show high
reactivity with a wide range of substrates because of the low
oxidation state of uranium.11 Previous reactivity studies of
nitride-bridged uranium compounds are limited to a single
example in which the UIVNUIV fragment reacts as a
masked metallonitrene with NaCN.6f
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b12620.
■
S
Experimental procedures and spectral data (PDF)
Crystallographic data for 4 (CIF)
Crystallographic data for 3 (CIF)
Crystallographic data for 2·2THF (CIF)
AUTHOR INFORMATION
Corresponding Author
*marinella.mazzanti@epfl.ch
■
Preliminary reactivity studies carried out with CS2 showed
that complexes 2 and 3 can transfer the nitride group to
electrophilic substrates in spite of the fact that the nitride group
is located in a protective pocket provided by the siloxide ligands
and the multimetallic binding by two U and three Cs cations
(see figures in the Supporting Information). The reactivity of
complexes 2 and 3 with CS2 is in agreement with a nucleophilic
character of the nitride. Notably, the addition of 13CS2 at −40
°C in THF to the bridging nitride led to the isolation of the
disulfide-bridged diuranium(IV) complex (Cs(THF))2[{U-
(OSi(OtBu)3)3}2(μ-S)2] (4) in 25% yield (Scheme 2), which
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Swiss National Science
Foundation and by the Ecole Polytechnique Fed
■
́
er
́
ale de
Lausanne (EPFL). We thank Euro Solari for carrying out the
elemental analyses and for technical support. We thank Marta
Falcone for some preliminary experiments.
REFERENCES
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DOI: 10.1021/jacs.5b12620
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