CO2 conversion to isocyanate via multiple N-Si bond cleavage at a bulky uranium(III) complex

Article abstract of DOI:10.1039/c5cc06707c

The reaction of the sterically saturated uranium(iii) tetrasilylamido complex [K(18c6)][U(N(SiMe₃)₂)₄] with CO₂ leads to CO₂ insertion into the U-N bond affording the stable U(iv) isocyanate complex [K(18c6)][U(N(SiMe₃)₂)₃(NCO)₂]_n that was crystallographically characterized. DFT studies indicate that the reaction involves the [2+2] cyclo-addition of a double bond of OCO to the U-N(SiMe₃)₂ bond and proceeds to the final product through multiple silyl migration steps.

Full text of DOI:10.1039/c5cc06707c

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CO conversion to isocyanate via multiple N–Si
2
bond cleavage at a bulky uranium(III) complex†
Cite this:DOI: 10.1039/c5cc06707c
ab
abc
d
ab
Clement Camp, Lucile Chatelain,
Christos E. Kefalidis, Jacques Pecaut,
´
´
d
c
Received 10th August 2015,
Accepted 27th August 2015
Laurent Maron* and Marinella Mazzanti*
DOI: 10.1039/c5cc06707c
www.rsc.org/chemcomm
The reaction of the sterically saturated uranium(III) tetrasilylamido
complex [K(18c6)][U(N(SiMe ] with CO leads to CO
into the U–N bond affording the stable U(IV) isocyanate complex [U(N(SiMe ) ) ], 1, leads to CO insertion into the U–N bond
Herein we show that the reaction of CO with the sterically
2
)
3 2
)
4
2
2
insertion saturated uranium(III) tetrasilylamido complex [K(18c6)]-
3
2 4
2
[
K(18c6)][U(N(SiMe
terized. DFT studies indicate that the reaction involves the [2+2] cyclo- [K(18c6)][U(N(SiMe
addition of a double bond of OQCO to the U–N(SiMe
bond and combination with reactivity studies to investigate the mechanism
proceeds to the final product through multiple silyl migration steps.
3
)
2
)
3
(NCO)
2
]
n
that was crystallographically charac- and to the formation of the stable U(IV) isocyanate complex
3
)
2
)
3
(NCO) 2. DFT studies were used in
2 n
]
3 2
)
leading to the formation of complex 2. Complex 2 provides a rare
example of cyanate formation at a uranium center. To date
The reactivity of uranium(III) with small molecules such as CO , CO or there have been only three examples of uranium-mediated
2
ꢀ
N has been attracting increasing interest in recent years due to the OCN formation and they involve the reaction of CO with a
2
1
,2
13
14
15
ability of uranium to promote unusual transformations. Bulky nitride, imido or nitrosyl complex.
amides have been successfully used in uranium chemistry, as innocent Complex 1 was prepared from [U(N(SiMe
ancillary ligands, as alternatives to the ubiquitous cyclopentadienyl to the previously reported procedure. The reaction of 1 with
1
6
3
)
2
)
4
]
according
1
7
3
4
systems. In particular the simple neutral [U(N(SiMe
3 2 3
) ) ] complex has 2.5 equivalents of carbon dioxide proceeded slowly (completed
provided a versatile precursor and has demonstrated interesting after 48 hours) at room temperature affording after workup
5
6
reactivity including arene reduction and functionalization, CO and recrystallization the U(IV) bis-cyanate complex [K(18c6)]-
7
8
activation, and nitride formation. In contrast, the ability of the [U(N(SiMe ) ) (NCO) ] , 2 as a pale pink microcrystalline solid
3
2 3
2 n
U–N bonds to undergo insertion reactions has been much less in 48% yield (Scheme 1). The stoichiometry of this reaction
1c
explored compared to U–C s-bonds. Only a few examples of requires the presence of additional uranium compounds and
insertion of CO into U(III)–NR and U(IV)–NR bonds leading to reduced by-products (U(III) has been oxidized to U(IV)) that have
the formation of U(III) and U(IV) carbamates have been reported. not been isolated. THF solutions of 2 are stable for at least 48h
2
2
2
9
10
1
2
Examples of the insertion of CO into metal–silylamide bonds at room temperature. The H NMR spectrum for 2 recorded
11
12
have been reported for main group, d-block and f-block metals
but are much rarer than the insertion of CO
into N-alkylamide resonance at ꢀ10.9 ppm corresponding to the three equivalent
bonds. In particular, [U (N(SiMe ) ) ] was reported to react with {N(SiMe ) } moieties together with a resonance at 4.7 ppm for
from deuterated THF at room temperature displays a broad
2
III
3
2 3
3 2
CO to give OQCQNSiMe and a second product identified as the the potassium crown ether counter cation.
2
3
12a
tetravalent uranium silanolate [U(OSiMe ) ].
The X-ray crystal structure of 2 shows the presence of a 1D
coordination polymer (Fig. 1). The uranium environment is
trigonal bipyramidal with three silylamido ligands at equatorial
3
4
a
b
Univ. Grenoble Alpes, INAC-SCIB, F-38000 Grenoble, France
CEA, INAC-SCIB, Reconnaissance Ionique et Chimie de Coordination,
1
7 Rue des Martyrs, F-38000 Grenoble, France
c
Institut des Sciences et Ing ´e nierie Chimiques, Ecole Polytechnique F ´e d ´e rale de
Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
E-mail: marinella.mazzanti@epfl.ch
d
LPCNO, CNRS & INSA, UPS, Universit ´e de Toulouse, 135 Avenue de Rangueil,
F-31077 Toulouse, France
†
Electronic supplementary information (ESI) available: Synthetic details, full
1
13
crystallographic data, H and C NMR spectra, ESI/MS spectra and computa-
tional data are included. CCDC 1417064. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c5cc06707c
Scheme 1
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ꢀ
positions and two NCO anions at apical positions. A K(18c6) could be assigned to the quaternary carbon of a carbamate or an
cation bridges two NCO ligands from two distinct uranium isocyanate intermediate. No evident color change was observed
13
complexes. The U1–Namido bond distances fall in the range during the reaction. Additional C NMR signals are also observed
13
of what was observed in the U(IV) silylamido species. Notably in the 5.6–1.8 ppm region. Further addition of 1 equivalent of CO2
the structure of 2 is closely related to the isoelectronic azide leads to a slow color change of the reaction mixture from dark
1
3
species [Na(THF)
4
][U{N(SiMe
3 2 3 3 2
) } (N ) ] reported by Hayton and purple to light pink. C NMR monitoring of the reaction showed a
5
e
13
coworkers featuring similar geometry and uranium-ligand slow evolution with time with the disappearance of the C NMR
bond distances. The three atoms XQCQX units are disordered signal at 307 ppm and the appearance of the C NMR signal at
across the mirror plane of the P21/m space group and refine 492.4 ppm assigned to the isocyanate complex 2. All the C NMR
0
13
13
equally well when N or O or a mixture of both are used at the X resonances in the 5.6–1.8 ppm region remained present in the final
0
13
and X positions. Thus, the X-ray data do not allow us to
C NMR spectrum but with increased intensity. In particular, two
1
discriminate between a cyanate (coordination through O) and an signals at 0.07 ppm in the H NMR spectrum and at 1.84 ppm in the
13
isocyanate (coordination through N) ligand. Similar disorder issues
were observed in the [{Me Al(m-OSiMe Mg(THF) (m-OCN)}
complex and in the dimeric U(IV) complex [U(Z-C
C NMR spectrum were assigned to the hexamethyldisiloxane
(SiMe O by-product. Signals for SiMe NCO were not observed.
)- The H NMR spectrum of the final reaction mixture showed
However, in most previously reported U(IV) com- the resonances assigned to complex 2 and additional broad
2
3
)
2
2
3
]
3
1
)
2
3
11c
i
8 6 3 2
H {Si Pr -1,4}
15
(
2
Z-Cp*)(NCO)] .
18
plexes the NCO ligand is N-bound and DFT calculations are in shifted resonances assigned to unidentified U(IV) products.
agreement with an N-bound coordination (see below and ref. 13). Mass spectrometry studies showed the presence of [U (OSiMe ) ]
2 3 9
Therefore the structure was refined with N-bound OCN ligands and [U (NCO)(OSiMe ) ] species (see ESI†).
ꢀ
ꢀ
2
3 8
(Fig. 1). The two U–NNCO bond distances at 2.337(3) and 2.338(4) Å
In order to shed light on the mechanistic aspects of this peculiar
are in the range of those found in the few uranium isocyanate reactivity outcome, DFT investigations using the B3PW91 func-
1
3
18b
19
complexes reported (2.338(3), 2.389(6) and 2.336(5) Å ).
tional were performed. The system of choice is the full anionic
ꢀ
1
ꢀ
The absorption band at 2201 cm in the IR spectrum of 2 was system [U(N(SiMe
assigned to the asymmetric stretching mode nNCO. This value is account. The anionic [U(IV)–CO
similar to those found in the few terminal or bridging U(IV) from the electronic reduction of CO
and U(III) isocyanate complexes reported (2199–2122 cm ).
All the spectroscopic and analytical data (see ESI†) support reduction). The first step of the mechanism involves the [2+2]
the assignment of the three atoms in 2 as NCO ligands. Notably cyclo-addition of a double bond from the OQCO radical anion to
the quaternary carbon (after reaction with CO ) resonance at the U–N(SiMe ) bond, as it is shown in Fig. 2. The activation
d = 492.4 ppm in the C NMR spectrum, the microanalytical barrier for this process is found to be relatively small (14.8 kcal
3
)
2
)
4
] , where the counter-cation is not taken into
13
ꢁ
ꢀ
2
]
intermediate Int1 is formed
18
15
2
that occurs at the coordina-
13
ꢀ1
tion to the initial U(III) complex (so-called coordination-induced
20
ꢁ
1
3
2
3 2
1
3
ꢀ
1
data, and the parent ion in the mass spectrum for the anion mol ). In particular, in the transition state the amide group that is
ꢀ
[U(N(SiMe
3
)
2
)
3
(NCO)
2
]
(m/z = 802.4) are in agreement with the involved in the insertion step is considerably far from the uranium
assigned formula.
center, at a non-bonding distance (dU–N = 3.63 Å). The nature of this
1
13
13
2
H and C NMR studies of the reaction of 1 with CO were late-transition state is most probably due to the directionality of
performed. After addition of 1 equivalent of CO the reaction leads the occupied molecular orbital of the nitrogen with respect to the
to the slow disappearance of the H signals assigned to complex 1 electrophilic carbon of the carbon dioxide molecule. Moreover, the
with completion after 48 hours. The C NMR spectrum of the final steric hindrance of the ligand environment may also play an
reaction mixture shows the presence of a signal at 307 ppm that important role in the non-bonding situation between the nitrogen
and the uranium, providing a logical explanation for the outcome
13
2
1
13
of this step. In particular, the IRC calculation did not converge into
Fig. 1 Solid-state molecular structure of [K(18c6)][U(N(SiMe
crystallized from THF/hexane. Hydrogen atoms and disorder are omitted
for clarity. Uranium (green), potassium (purple), silicon (yellow), nitrogen
blue), oxygen (red) and carbon (grey) atoms are represented with 30%
3 2 3 2 n
) ) (NCO) ]
2
(
probability ellipsoids. Selected bond distances [Å] and angles [deg]:
U1–N1 2.2679(2), U1–N2 2.2696(2), U1–O/N42 = 2.3358(1), U1–O/N41 =
Fig. 2 Part of the energy profile that leads to the formation of the
isocyanate complex 2.
0
0
2
.3370(1), X42–C42–X 42 = 171.6(1), X41–C41–X 41 = 175.5(1).
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Fig. 3 Part of the energy profile that leads to the formation of the
isocyanate complex 2.
Fig. 4 Part of the energy profile that leads to the formation of the
isocyanate complex 2.
an O,N-bound carbamate intermediate, but instead into an their nitrogen atoms while the oxygen atoms bind the potassium
O,O-bound one (Int2), with a subsequent significant drop in enthalpy atoms of the K(18c6) cation.
ꢀ
1
energy (43.0 kcal mol with respect to the CO reduced adduct).
Besides, since the experimental reactivity is found to be
From the ligated dianionic O,O-bound carbamate the reac- influenced by the number of CO equivalents reacted with U(III),
tion can proceed through a sequence of two successive silyl a second CO molecule can undergo a nucleophilic attack at the
migration steps (Fig. 2 and 3).
carbon of the Z -cyanate group in intermediate Int6 (Fig. 4).
2
2
2
2
More specifically, the first step refers to the migration of the This will result in the formation of an oxalate-like complex,
trimethylsilyl group from the nitrogen atom to one oxygen of the Int9. Such reactivity is reminiscent of the oxalate formation in U(III)
21
carbamate group, surmounting a moderate activation energy barrier chemistry. Two molecules of Int9 can then disproportionate in
ꢀ
1
2ꢀ
of 14.4 kcal mol . From the ensuing intermediate, Int3, rotation order to give the bis-cyanate product Int8 and a CO
around the Osiloxide–C bond of the silyl group is required for the Si (Int10). Also in this route, an important stabilization energy is
complex
2
ꢀ
1
atom to approach the coordinated oxygen atom, for the second found (almost 130 kcal mol ). Attempts to compute a reaction
migration to occur. This isomerization is an almost thermo-neutral pathway involving the insertion of CO into a U(III) species were not
process, with enthalpy energy difference of only 4.3 kcal mol in successful but showed that such a pathway is higher in energy.
favor of Int3. The following migration of the silyl group is more
In conclusion the bulky uranium(III) tetrasilylamido complex
energy demanding than the previous one, with an accessible activa- [K(18c6)][U(N(SiMe ] reacts with CO to afford a rare example of
a U(IV) isocyanate complex. DFT computational studies suggest that
2
ꢀ
1
)
)
3
2
4
2
ꢀ
1
‡
tion energy barrier of 26.9 kcal mol in terms of DH .
In the following intermediate, Int5, a kind of cyanate frag- the reaction proceeds through carbon dioxide reduction followed
2
ment has been formed, now developing an Z -N,O-type coordi- by the [2+2] cyclo-addition of the carbonyl double bond of the
nation with the uranium atom. This fragment can easily expel a reduced carbon dioxide to the U–N(SiMe
3 2
) bond and multiple silyl
siloxane molecule, which is observed experimentally, through migration. The reactivity of this bulky ‘‘ate’’ complex differs from
1
2a
an almost barrierless energy micro-step, via TS5–6. The resulting that reported for the neutral analogue [U(N(SiMe
3
)
2
)
3
]
highlight-
2
ꢀ
intermediate, Int6, is formally described as an OCN complex ing the importance of the coordination environment for controlling
of U(IV). At this point, one can envision two different potential the CO
conversion at the uranium center.
2
paths, the first corresponding to the isomerization of the
This work was supported by the CEA, the Swiss National
cyanate group to give a terminal bonded isocyanate complex Science Foundation, and by the EPFL. We thank O. Cooper for
or to the nucleophilic attack by a second CO molecule on the recording some analytical data. LM is member of the Institut
2
2ꢀ
carbon atom of the OCN moiety (Fig. 4). The first possibility Universitaire de France. Humboldt Foundation and Calmip are
results in an important stabilization energy of 21.4 kcal mol with also acknowledged.
respect to the Z -bound one. Then a putative free NCO radical
ꢀ1
2
(
originating from a second uranium complex) can coordinate the Notes and references
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O-bound one, (see Int8 in the ESI†), by almost 10 kcal mol . This
observation suggests that most probably the two OCN groups in
the X-ray structure of complex 2 are bound to the uranium through
1
2
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