Facile CO Cleavage by a Multimetallic CsU2Nitride Complex

Article abstract of DOI:10.1002/anie.201606796

Uranium nitrides are important materials with potential for application as fuels for nuclear power generation, and as highly active catalysts. Molecular nitride compounds could provide important insight into the nature of the uranium–nitride bond, but currently little is known about their reactivity. In this study, we found that a complex containing a nitride bridging two uranium centers and a cesium cation readily cleaved the C≡O bond (one of the strongest bonds in nature) under ambient conditions. The product formed has a [CsU₂(μ-CN)(μ-O)] core, thus indicating that the three cations cooperate to cleave CO. Moreover, the addition of MeOTf to the nitride complex led to an exceptional valence disproportionation of the CsU^IV–N–U^IVcore to yield CsU^III(OTf) and [MeN=U^V] fragments. The important role of multimetallic cooperativity in both reactions is illustrated by the computed reaction mechanisms.

Full text of DOI:10.1002/anie.201606796

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Communications
Chemie
International Edition: DOI: 10.1002/anie.201606796
German Edition: DOI: 10.1002/ange.201606796
CO Cleavage
Facile CO Cleavage by a Multimetallic CsU2 Nitride Complex
Marta Falcone, Christos E. Kefalidis, Rosario Scopelliti, Laurent Maron,* and
Marinella Mazzanti*
Abstract: Uranium nitrides are important materials with
potential for application as fuels for nuclear power generation,
and as highly active catalysts. Molecular nitride compounds
could provide important insight into the nature of the
uranium–nitride bond, but currently little is known about
their reactivity. In this study, we found that a complex
containing a nitride bridging two uranium centers and
terminal monometallic nitrido complexes and only one
example of nitride-bridged dimetallic complex[5f] effect nitro-
gen-atom transfer to CO to afford cyanate, which in some
cases is spontaneously extruded.[5d] Reductive carbonylation
of monometallic UV and UVI nitrides by CO to afford the
isocyanate ligand has also been reported recently.[5e] How-
ever, the complete cleavage of CO by a nitride complex has
not been reported so far.
ꢀ
a cesium cation readily cleaved the C O bond (one of the
strongest bonds in nature) under ambient conditions. The
product formed has a [CsU2(m-CN)(m-O)] core, thus indicating
that the three cations cooperate to cleave CO. Moreover, the
addition of MeOTf to the nitride complex led to an exceptional
valence disproportionation of the CsUIV–N–UIV core to yield
Several examples of CO cleavage by metal complexes
have been reported,[6] and these reactions often yield metal–
carbide complexes and oxo clusters. The binding[7] and the
reduction[8] of CO by uranium(III) complexes has been
demonstrated. Several examples of CO reductive homologa-
tion effected by uranium(III) systems with the formation of
deltate,[9] squarate,[9b,10] or ethynediolate dianions[9b,11] have
also been identified. However, uranium compounds that
effect the deoxygenation of CO have not yet been described.
We recently demonstrated the unusually high nucleophilic
character of the nitride-bridged diuranium complex [Cs{[U-
(OSi(OtBu)3)3]2(m-N)}] (1),[2d,12] which reacted under ambient
III
V
=
CsU (OTf) and [MeN U ] fragments. The important role of
multimetallic cooperativity in both reactions is illustrated by
the computed reaction mechanisms.
U
ranium nitrides are of great interest because of their
potential application in both stoichiometric and catalytic
transformations and in materials science and engineering.[1]
The recent discovery and characterization of stable mono-
nuclear and dinuclear uranium nitride complexes[2] has now
rendered the investigation of the reactivity of these species
accessible.
À
conditions with CO2 and CS2 to form a C N bond and yield
cyanate, thiocyanate, and unprecedented dicarbamate spe-
cies. Herein we show that the unusually high nucleophilic
character of 1, and the multimetallic cooperativity of the CsU2
core, lead to complete cleavage of the CO triple bond to yield
a cyanide. The cyanide ligand in 1 is readily transferred to
electrophiles, such as MeOTf and Me3SiI, thus yielding
organic nitriles. We also demonstrate that the direct methyl-
ation of the nitride complex 1 leads to disproportionation of
the uranium cation: completely unprecedented reactivity in
nitride chemistry.
Carbon monoxide is an inexpensive and readily available
C1 feedstock used in industry for the production of a wide
variety of chemicals, such as methanol, acetic acid, phosgene,
and hydrocarbons.[3] A key step in Fischer–Tropsch hydro-
carbon production from CO and H2 is the cleavage of the CO
triple bond, which is the strongest chemical bond (dissociation
energy at 298 K: 1076 kJmolÀ1). This process requires the use
of heterogeneous transition-metal catalysts at elevated tem-
peratures.[4] The cleavage of the CO bond under mild
conditions is an important fundamental challenge in the
search for new routes for the production of functionalized
organic molecules from CO.
The addition of stoichiometric or excess amounts of CO or
13CO (1 equiv) to the diuranium(IV) complex [Cs{[U(OSi-
(OtBu)3)3]2(m-N)}] (1) in toluene at room temperature led to
an immediate color change of the solution from brown to light
blue (Scheme 1). The proton NMR spectrum recorded at
258C immediately after the reaction showed the disappear-
ance of the peak assigned to complex 1 and the appearance of
a single new signal at À1.34 ppm. Blue crystals of the complex
[Cs{[U(OSi(OtBu)3)3]2(m-CN)(m-O)}] (2) were isolated in
60% yield at À408C. Proton NMR studies showed that
The direct addition of CO to a metal nitride is a rare event
observed only for highly nucleophilic nitride complexes of d-
block transition metals, such as V, Fe, and Hf.[5] Notably, a few
[*] M. Falcone, R. Scopelliti, Prof. M. Mazzanti
Institut des Sciences et Ingꢀnierie Chimiques
Ecole Polytechnique Fꢀdꢀrale de Lausanne (EPFL)
1015 Lausanne (Switzerland)
E-mail: marinella.mazzanti@epfl.ch
Dr. C. E. Kefalidis, Prof. L. Maron
LPCNO,CNRS & INSA, UPS, Universitꢀ de Toulouse
135 Avenue de Rangueil, 31077 Toulouse (France)
E-mail: laurent.maron@irsamc.ups-tlse.fr
Scheme 1. CO cleavage reaction effected by complex 1 under ambient
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201606796.
conditions to afford the complex [Cs{[U(OSi(OtBu)3)3]2(m-CN)(m-O)}]
(2).
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complex 2 was stable in toluene at room temperature for
several hours. The 13C NMR spectrum of complex 2 showed
the presence of a signal at 866 ppm, which was assigned to the
13CN ligand. Traces of decomposition, evident from a new
NMR peak assigned to the [U(OSi(OtBu)3)4] complex,
appeared only after three days, probably as a result of
ligand scrambling. The solid-state structure of 2 (Figure 1)
Scheme 2. Reactions of 2 with the electrophiles MeOTf and Me3SiI to
afford MeCN and Me3SiCN. OTf=trifluoromethanesulfonate.
one new major signal at À1 ppm. This signal was assigned to
an intermediate oxo iodide species “[Cs{[U(OSi(OtBu)3)3]2-
(m-O)(I)}]”. This species decomposes rapidly and after 48 h
only a peak assigned to [U(OSi(OtBu)3)4] was observed. The
formation of a stable m-oxo iodide complex and Me3Si13CN
was observed in the reaction of the dihafnium oxo cyanide
complex [{(h5-C5Me4H)2Hf(NCO)}(m-O){(h5-C5Me4H)2Hf-
(CN)}] with Me3SiI.[14] The formation of [U(OSi(OtBu)3)4]
from the intermediate iodide is the result of ligand scram-
bling. Ligand scrambling leading to the formation of bis(oxo)
complexes and unidentified products was previously observed
for the diuranium(IV) oxo- and cyanate-bridged complex
“[Cs{[U(OSi(OtBu)3)3]2(m-NCO)(m-O)}]”.[12] The reaction of
2 with MeOTf proceeded more slowly, and after 24 h at room
temperature a large amount of starting material was still
present. The 1H NMR spectrum in [D8]toluene after 24 h
showed the presence of the signal assigned to [U(OSi-
(OtBu)3)4] and a broad signal at 0.63 ppm assigned to
CH313CN. The formation of CH313CN was confirmed by the
presence of a signal at 113 ppm in the 13C NMR spectrum. The
cyanide-alkylation reaction was complete after overnight
heating at 558C. NMR studies did not show evidence of
products arising from the alkylation of the bridging oxo
ligand. Overall, the successive addition of CO and MeOTf (or
Me3SiI) to complex 1 resulted in the conversion of CO into
organic nitrile compounds.
For comparison, we also investigated the reactivity of the
nitride complex 1 with MeOTf. The reaction of 1 with MeOTf
(1 equiv) at room temperature resulted in a slow (24 h) color
change of the solution from brown to green and a mixture of
complexes 3 and 4 (Scheme 3). Storage of the solution at
À408C resulted in the isolation of light-blue crystals of the
bis(imido) complex [{UV(OSi(OtBu)3)3}2(m-NMe)2], (4).
Attempts to isolate the two products in pure form from this
reaction were not successful. The reaction was faster when
1 was treated with 2 equivalents of MeOTf, but the same
mixture of complexes 3 and 4 was obtained, as indicated by
the proton NMR spectrum. However, in this case storage of
Figure 1. Molecular structure of [Cs{[U(OSi(OtBu)3)3]2(m-CN)(m-O)}]
(2). Thermal ellipsoids are depicted at 50% probability. Hydrogen
atoms and methyl groups were omitted for clarity. Selected bond
distances [ꢁ]: U1–O105 2.124(4), U1–N1 2.576(6), N1–C113 1.155(9),
U2–O105 2.144(4), U2–C113 2.610(7).
shows the presence of a diuranium(IV) complex in which two
[U(OSi(OtBu)3)3] fragments are bridged by one oxo ligand
and one cyanide group. This structure indicates that the highly
nucleophilic character of the bridging nitride promotes the
ready cleavage and deoxygenation of carbon monoxide to
ꢀ
afford a N C triple bond and a bridging oxo group. The
À
resulting cyanide ligand adopts a bridging mode with U C
À
(2.610(7) ꢀ) and U N (2.576(6) ꢀ) bond distances compara-
ble to those reported for cyanide-bridged diuranium(IV)
complexes (2.549(8)–2.583(2) ꢀ).[13] The N C bond distance
À
(1.155(9) ꢀ) compares well to that found in cyanide-bridged
diuranium(IV) complexes (1.177(4) ꢀ).[13b]
The outcome of the reaction of 1 with CO is very different
from that recently reported for the reaction of mononuclear
uranium nitride complexes with CO, which led to the
reductive carbonylation of UV and UVI, thus affording UIII–
OCN and UIV–OCN complexes, respectively.[5e] This remark-
able difference in reactivity shows the importance of multi-
metallic cooperativity in CO scission.
To confirm the presence of a cyanide ligand in complex 2
and to investigate the possibility of transferring the cyanide
ligand to organic substrates, we treated complex 2 with Me3SiI
and with MeOTf (Scheme 2). The reaction of [Cs{[U(OSi-
(OtBu)3)3]2(m-13CN)(m-O)}] with Me3SiI (1 equiv) at room
temperature in benzene led to an immediate colour change
and afforded a white suspension. The 13C NMR spectrum of
the reaction mixture showed the disappearance of the peak
assigned to the uranium-bound 13CN group and the appear-
ance of a new signal at 126.3 ppm, which was assigned to the
cyanide group of Me3Si13CN.[14] After the addition of Me3SiI,
proton NMR studies showed the immediate disappearance of
the peak assigned to complex 2 and the appearance of only
Scheme 3. Methylation reaction of complex 1 with MeOTf to form
complexes 3 and 4.
2
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the reaction mixture at À408C for 24 h resulted in the
isolation of green-blue crystals of the complex [Cs2{[U(OSi-
(OtBu)3)3]2(m-OTf)2}]·2(C7H8) (3·2(C7H8)) in 60% yield. The
crystallization of complex 3 is probably favored by the
presence of excess MeOTf in the solution. Subsequent cooling
of the mother liquor afforded a few crystals of complex 4.
Owing to its low stability, the isolation of complex 4 in
sufficient amounts for further characterization was not
possible. The low stability of the UV imido complex 4 is
consistent with the observed instability of previously reported
“[UV(OSi(OtBu)3)3}(NR)]” (R = adamantyl) intermediate,
which that could not be isolated because it rapidly dispro-
portionated to afford [UIV(OSi(OtBu)3)4] and [UVI2(NR)4-
(OSi(OtBu)3)4].[2d]
Figure 3. Thermal-ellipsoid drawing (50% probability) for [{U(OSi-
(OtBu)3)3}2(m-NMe)2] (4a; only one complex is displayed). Hydrogen
atoms and methyl groups were omitted for clarity. Selected bond
distances [ꢁ]: U2–N2 2.309(6), U2–N2A 2.309(6), U2–U2A 3.8489(6),
N2–C113 1.441(14); A=Àx+1,Ày+2,Àz+1.
The solid-state structure of 3 (Figure 2) consists of
a diuranium(III) complex in which two [U(OSi(OtBu)3)3]
fragments are bridged by two triflate anions, each one binding
the two uranium centres and a cesium cation in a m3:k2-
reported[17] but did not lead to valence disproportionation.
In contrast, the alkylation of 1 promotes the disproportiona-
tion of the metal center, thus leading to the transformation of
two UIV cations into a UV and a UIII complex. This reactivity is
exceptional, since UIV complexes are generally much more
stable than their UIII and UV analogues, which are usually
more prone to disproportionation reactions.[18] For example,
the diuranium(IV) imido-bridged complex [(MeC5H)4U2-
(m-NPh)2] was obtained from the comproportionation reac-
tion of the imido complex [(MeC5H)3UV NPh] with the
tris(cyclopentadienyl) complex [(MeC5H5)3UIII].[19] The reac-
tivity of complex 1 can be explained by the presence of
asymmetric bonding in the UIV–N–UIV core, which may be
formulated as a mixed-valence UV–N–UIII complex, but this
formulation is not supported by either the optimized geom-
etry or the metrical parameters in complex 1 (see the
Supporting Information).[2d]
The unique reactivity of complex 1 towards CO and
MeOTf was further explored by examining plausible reaction
pathways through calculations at the DFT level of theory (see
the Supporting Information for computational details).[20] Full
systems without any simplifications or geometrical constraints
were considered in the calculations.[21] In the computed
profile for the reaction of 1 with MeOTf (see Figure S21 in
the Supporting Information), the first step involves the
coordination of MeOTf to complex 1 to yield a mixed-valence
intermediate, int-B, which is enthalpically more stable than
the charge-delocalized species, int-A. Interestingly, no tran-
sition state for direct methyl transfer from MeOTf to the
nitride (in an outer-sphere-type mechanism) could be located,
as every attempt led to either int-A or int-B. The Cs+ ion plays
a crucial role in holding the fragments together in the
1
À
(O,O):h -O fashion. The mean U Osiloxide bond length is
Figure 2. Thermal-ellipsoid drawing (50% probability) for [Cs2{[U(OSi-
(OtBu)3)3]2(m-OTf)2}] (3). Hydrogen atoms, methyl groups, and inter-
stitial solvent molecules were omitted for clarity. Selected bond
distances [ꢁ]: U1–O61 2.537(3), U1–O62 2.479(3), U1–O63 2.323(3),
S1–O61 1.504(3), S1–O62 1.492(3), S1–O63 1.462(3), U1–U1A 5.6356-
(1); A=Àx+1,Ày+2,Àz+1.
similar (2.165(3) ꢀ) to the U Osiloxide distance found in UIV
À
complex 1 (U–Oavg: 2.19(3) ꢀ) and in the diuranium(III)
tris(siloxide) complex [{U(OSi(OtBu)3)2(m-OSi(OtBu)3)}2]
(2.193(4) ꢀ).[15] The U–U distance is longer (5.6356(1) ꢀ)
than that found in the [{U(OSi(OtBu)3)2(m-OSi(OtBu)3)}2]
complex (3.9862(2) ꢀ).
The solid-state structure of 4 was determined by X-ray
crystallography (Figure 3). The unit cell consists of two
independent diuranium(V) complexes, 4a and 4b, in which
two [U(OSi(OtBu)3)3] fragments are bridged by two methyl-
imido ligands. The mean U–Osiloxide bond length (2.110(5) ꢀ)
is slightly shorter than that found in the UV terminal imido
complex [K(18c6)][U(NSiMe3)(OSi(OtBu)3)4] (U1–Oavg
:
2.16(2) ꢀ).[2d] The average U–U distance in 4 is 3.8612(6) ꢀ.
The U–N distances in the two complexes are slightly different
(2.309(6) ꢀ in 4a and 2.319(6) ꢀ in 4b). These values can be
compared to those found in the rare examples of previously
reported imido-bridged diuranium(V) complexes (2.387(5)–
2.078(5) ꢀ).[16] The formation of imide complexes from the
alkylation of nucleophilic bridging nitrides has been
Figure 4. 2D and 3D representation of DFT-computed ts-BC.
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Figure 5. DFT-computed enthalpy profile for the reaction of CO with complex 1 to afford complex 2 at room temperature (the values are given in
kcalmolÀ1). The inset shows a 3D representation of the active core of ts-G2.
dinuclear mixed-valence complex, but also in stabilizing the
anionic, monomeric formally UV nitride molecular part. From
int-B, the system can reach the transition state ts-BC
(Figure 4), in which the methyl group is transferred from
the triflate moiety to terminal nitride. This reaction can be
better described as methyl transfer from the oxygen atom to
the nitrogen atom at the Cs atom rather than at the uranium
center (see Figure 4). The uranium atoms appear to act solely
as Lewis acid centers to activate the nitride and methyl triflate
ligands. This process leads to the formation of complex int-C,
a mixed-valence bimetallic complex containing two fragments
(a UIII–sulfonate–Cs moiety and a UV imido moiety), that
further dimerizes to yield the two homoleptic UIII and UV
experimental complexes, 3 and 4, respectively.
scission from carbon monoxide) takes place with an activation
barrier of 15.1 kcalmolÀ1.
The high nucleophilic reactivity of the nitride ligand in the
heterodimetallic complex 1 led to the first example of
complete cleavage of CO by a nitride complex under ambient
conditions. CO cleavage afforded a new complex, 2, contain-
ing two UIV cations bridged by an oxo and a cyanide group.
This complex can transfer the cyanide group to electrophiles,
À
thus leading to N C bond formation and the release of
organic molecules, such as CH3CN or Me3SiCN. An excep-
tional outcome was also observed for the reaction of the UIV
center with electrophilic MeOTf, which led to unprecedented
valence disproportionation of UIV to UIII and UV with
concomitant formation of a UV imido complex. DFT studies
support the presence of a multimetallic cooperative effect in
the complete cleavage of CO and in the methyl transfer to the
uranium-bound nitride. The results show novel reactivity of
highly nucleophilic metal nitrides, which might provide new
routes to the use of the readily available substrate CO in the
synthesis of valuable chemicals.
A computational mechanistic investigation of the reaction
between complex 1 and CO was also carried out (Figure 5).
The closest, unique analogue of such exceptional reactivity is
the high-temperature (1108C) cleavage of CO by a diazenido
dihafnium complex to yield a m-oxo/m-cyanido species in
a reaction that proceeds through the formation of a stable
oxamidide intermediate.[14] Computational and experimental
studies suggest that this stable oxamidide species forms via an
unstable nitride intermediate.[22] However, the uranium
nitride reactivity appears to be significantly different
(Figure 5). First the CO molecule binds one uranium center
with an activation barrier of 8.4 kcalmolÀ1, thus yielding an
unstable CO adduct (endothermic formation by 8.1 kcal
molÀ1). Subsequently, the CO molecule undergoes barrierless
migratory insertion into the uranium–nitride bond. The
insertion yields int-E, which is a h2(C,N):k2(O,N) bimetallic
complex. This intermediate int-E then undergoes an isomer-
ization process assisted by the Cs cation to yield the
isoenergetic but reactive isomer int-G (see the Supporting
Information for the full isomerization mechanistic part). The
geometry of complex int-G is quite interesting, as the cyanate
ligand exhibits h1-N coordination with one uranium center
and h2(C,O) coordination with the other uranium center
(h1,h2 is a classical coordination mode for bimetallic uranium
complexes of triatomic molecules, such as CO2[15]). From int-
Acknowledgements
We acknowledge support from the Swiss National Science
Foundation and from the Ecole Polytechnique Fꢁdꢁrale de
Lausanne (EPFL). We thank Euro Solari for carrying out the
elemental analyses.
Keywords: carbon monoxide cleavage · cooperativity · nitrides ·
uranium · uranium disproportionation
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Received: July 13, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
CO Cleavage
M. Falcone, C. E. Kefalidis, R. Scopelliti,
L. Maron,* M. Mazzanti*
&&&& — &&&&
Facile CO Cleavage by a Multimetallic
CsU2 Nitride Complex
The power of cooperation: A heteropoly-
metallic uranium nitride with a CsUIV–N–
UIV core effected complete cleavage of the
ligand, which could be alkylated readily to
afford organic nitriles. The important role
of multimetallic cooperativity in these
reactions was identified by computation
of the reaction mechanisms.
ꢀ
strong C O bond of carbon monoxide
under ambient conditions (see scheme).
The reaction led to the formation of a CNÀ
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!