CO2 conversion to phenyl isocyanates by uranium(vi) bis(imido) complexes

Article abstract of DOI:10.1039/c9cc07411b

Uranium(vi) trans-bis(imido) complexes [U(κ⁴-{(^tBu₂ArO)₂Me₂-cyclam})(NPh)(NPh^R)] react with CO₂ to eliminate phenyl isocyanates and afford uranium(vi) trans-[OUNR]²⁺ complexes, including [U(κ⁴-{(^tBu₂ArO)₂Me₂-cyclam})(NPh)(O)] that was crystallographically characterized. DFT studies indicate that the reaction proceeds by endergonic formation of a cycloaddition intermediate; the secondary reaction to form a dioxo uranyl complex is both thermodynamically and kinetically hindered.

Full text of DOI:10.1039/c9cc07411b

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CO conversion to phenyl isocyanates
2
by uranium(VI) bis(imido) complexes†‡
Cite this: Chem. Commun., 2020,
6, 431
5
ab
c
ab
Leonor Maria,
*
Nuno A. G. Bandeira, * Joaquim Marçalo,
a
d
Received 20th September 2019,
Accepted 4th December 2019
Isabel C. Santos
and John K. Gibson
DOI: 10.1039/c9cc07411b
rsc.li/chemcomm
4
tBu
2
Uranium(VI) trans-bis(imido) complexes [U(j -{( ArO)
2
Me
2
-cyclam})- Activation of CO by hexavalent uranium imido complexes has
2
R
(
NPh)(NPh )] react with CO
and afford uranium(VI) trans-[OQUQNR] complexes, including complexes is rare. In such a rare case, the terminal nitride
2
to eliminate phenyl isocyanates not yet been achieved, and in general CO activation by U(VI)
2
2
+
20
4
tBu
2
TIPS
[
U(j -{(
2 2
2
ArO) Me -cyclam})(NPh)(O)] that was crystallographi- [U(Tren )(N)] reacted with CO to yield a U(VI) oxo-cyanate
cally characterized. DFT studies indicate that the reaction proceeds complex by bond metathesis and cleavage of a CQO bond, with
V
TIPS
20
by endergonic formation of a cycloaddition intermediate; the rapid decomposition of the product to [U (Tren )(O)].
secondary reaction to form a dioxo uranyl complex is both thermo-
dynamically and kinetically hindered.
Several structurally characterized uranium(VI) imido com-
2
1–23
plexes have recently been reported,
of the UQNR bonds remain scarce, particularly for U(VI)
but reactivity studies
2
4
2
+
Uranium complexes have emerged as attractive candidates for trans-bis(imido) complexes. Such trans-[RNQUQNR] systems
1
–3
activation of the very stable CQO bonds of carbon dioxide,
notably are isoelectronic with thermodynamically stable uranyl,
2
+
25
which must occur during CO functionalization. U(III) with {OQUQO} , but exhibit greater bond covalency. It has been
2
2
6
appropriate supporting ligands can effectively reduce carbon proposed that the inverse trans-influence (ITI) of the two
dioxide, giving rise to various products that include an end-on- uranium–nitrogen multiple bonds stabilizes and reduces
ꢀ
À
,6
4
5,6
2+
bound CO
2
U(IV) complex, uranium oxo species, carbon reactivity of the {RNQUQNR} moiety. Accordingly, Boncella
5
7,8
9
10
t
2 2 3 2
monoxide, carbonates, oxalates, and isocyanate. It was and co-workers showed that treating [U (Q N Bu) I (OPPh ) ]
also demonstrated that U(IV) complexes with suitable ancillary with PhNQCQO does not result in a uranium oxo complex
t
ligands and functionalities can facilitate CO insertion into U–E but rather an imido transfer takes place to form [U (Q N Bu)-
2
1
1–14
27
bonds (E = C, N, O, S).
U(V) terminal oxo complexes [{( ArO)
upon treatment of U(V) mono-imidos with CO
2+2] cycloaddition with elimination of isocyanates.
Meyer and co-workers showed that (Q NPh)I (OPPh ) ]. We here demonstrate that U(VI) trans-
2 3 2
R
3
tacn}UQO] are formed bis(imido) complexes supported by a bis(phenolate) cyclam
2
+
2
, probably via ligand react with CO
2
to produce stable trans-[OQUQNR]
Similar with elimination of aryl isocyanate.
2
transformations have been observed with high oxidation state Based on previous results for azobenzene activation with
1
3,15
[
CO
16–19
6
tBu
2
early transition metals, such as titanium imido complexes.
U(III) complex [U(k -{( ArO)
2
Me
2
-cyclam})I] (1), with formation
of [U(k -{( ArO) Me -cyclam})(NPh) ] (2), the initial focus
4
tBu
2
28
2
2
2
a
2
studies was on the synthesis of the new U(VI) bis(imido)
Centro de Ci ˆe ncias e Tecnologias Nucleares (C TN), Instituto Superior T ´e cnico,
Universidade de Lisboa, 2695-066 Bobadela, Portugal.
4
tBu
2
[
U(k -{( ArO) Me -cyclam})(NPh)(NTol)] (3), with a goal of asses-
2 2
E-mail: leonorm@ctn.tecnico.ulisboa.pt
sing reactivity with CO
TolNNPh) with 2 equiv. of U(III) complex 1 in toluene at room
temperature resulted in four-electron cleavage of the NQN double
2
. The reaction of 4-methylazobenzene
b
c
Centro de Qu ´ı mica Estrutural (CQE), Instituto Superior T ´e cnico,
Universidade de Lisboa, 2695-066 Bobadela, Portugal
(
BioISI and Centro de Qu ´ı mica e Bioqui ´ı mica, Faculdade de Ci ˆe ncias, Universidade
de Lisboa, 1749-016 Lisboa, Portugal. E-mail: nuno.bandeira@ciencias.ulisboa.pt
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley,
CA 94720, USA
4
tBu
2
bond, affording the soluble bis(imido) [U(k -{( ArO)
2 2
Me -
d
cyclam})(NPh)(NTol)] (3) and the insoluble U(IV) compound
6
tBu
2
[U(k -{( ArO) Me -cyclam})I][I], in a 1 : 1 ratio (Scheme 1).
2
2
†
A data set collection of computational results is available in the ioChem-BD
Quality single crystals for X-ray diffraction were obtained by
slow evaporation of a n-hexane/thf solution for two weeks.
Refinement of the XRD data confirmed the expected U(VI)
complex, with the phenyl imido and p-methyl phenyl imido
ligands in a trans arrangement (N5–U1–N6 = 173.7(4)1) with an
38
repository and can be accessed via https://doi.org/10.19061/iochem-bd-6-21.
Electronic supplementary information (ESI) available: Computational and
experimental details, NMR spectra, and detailed X-ray data. CCDC 1954718 and
954719. For ESI and crystallographic data in CIF or other electronic format see
‡
1
DOI: 10.1039/c9cc07411b
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Scheme 2 Synthesis of uranium(VI) oxo-imido complex 4.
Scheme 1 Synthesis of uranium(VI) bis(imido) complexes 2 and 3.
t
two centered at 1.57 ppm and overlapping with a Bu resonance of
13
overall distorted octahedral geometry (Fig. S2 in ESI‡). Struc- 4. C NMR spectroscopy confirmed formation of the two U(VI)
tural parameters of 3, including short U–Nimido distances of mono-oxo imido complexes, with the appearance of two pairs of
tBu
2
1
.909(6) and 1.911(7) Å, are similar to those in the symmetric carbon resonances for the C–O phenolates of {( ArO) Me -
2 2
2
8
2À
bis(imido) complex 2 (1.895(2) and 1.907(2) Å) (structural cyclam} (4: 166.75 and 166.56 ppm; 5: 166.81 and 166.69 ppm).
tBu
2
parameters are in ESI‡ Table S2).
2 2 2
The reaction of [U{( ArO) Me -cyclam}(NPh) ] (2) with an
The H NMR spectrum of 3 is characteristic of a diamagnetic excess of CO in toluene resulted in a dark cherry red solid after
compound, consistent with the asymmetric structure identified evaporation of the solvent (Scheme 2). H NMR of the crude
in the solid state. H and C NMR (see Fig. S3 and S4 in ESI‡) solid was consistent with formation of 4 and a minor side
1
2
1
1
13
2
9
revealed two sets of resonances for the phenolate arms of product, perhaps from dimerization of phenyl isocyanate.
tBu
2
2À
{(
ArO) Me -cyclam} and two distinct sets of resonances Recrystallization of the crude solid from hexane provided single
2 2
for the phenyl imido and p-methyl phenyl imido ligands.
Frozen brown solutions of 2 and 3 in benzene-d
exposed to an excess of CO (ca. 30 equiv.). Upon warming to
room temperature, the colour of the solutions changed slowly about the uranium center, with the {( ArO) Me -cyclam}
crystals; X-ray diffraction confirmed the molecular structure as oxo
were imido complex [U(k -{( ArO) Me -cyclam})(O)(NPh)] (4) (Fig. 1).
2 2
4
tBu
2
6
2
Like complexes 2 and 3, 4 exhibits an octahedral geometry
tBu
2
2À
2
2
4
from brown to dark cherry red. Following the reaction by ligand adopting a k -N O coordination mode, and the oxo and
2
2
1
H NMR revealed that bis(imido) U(VI) complex 2 was comple- phenyl imido are trans-oriented. The geometric parameters of
tely consumed within 7 hours, with ingrowth of new ortho, the {RNQUQO} core (U1–O3 1.787(3) Å, U1–N5 1.879(3) Å,
meta, and para phenyl proton resonances at 5.19, 6.90 and O3–U1–N5, 176.4(1)1) are comparable to those of previously
2
+
iPr
2
30
5
.75 ppm, and aromatic resonances between 6.86 and 6.58 ppm, reported [U (Q NPh )(O)Cl (tppo) ] (U–O 1.778(2), U–N 1.847(3)),
2 2
consistent with formation of a new uranium phenyl imido while the U–O(aryloxide) and U–N(cyclam) bond distances are
complex and elimination of phenyl isocyanate (Fig. S6 and S7 similar to those of 2 and 3.
2
+
in ESI‡).
Crystallographically characterized trans-[OQUQNR] com-
1
The H NMR spectrum of the new complex exhibited inte- pounds are scarce, mostly prepared by oxygen atom transfer
tBu
2
2À
grated intensities for the {( ArO)
imido protons of 1 : 1, and revealed loss of the C
the bis(phenolate) cyclam ligand. The benzylic protons gave [U (Q N Bu) I (THF) ] with water reagent B(C F ) ÁH O also led to
2 2
Me -cyclam} and phenyl to U(IV) monoimido complexes or reductive cleavage of nitrite by
3
0–32
2
symmetry of a U(V) imido complex.
Treatment of U(VI) trans-bis(imido)
t
2
2
2
6
5 3
t
2
33
rise to two AB systems with J coupling of 12.1 and 12.3 Hz, formation of a uranyl-like complex, [U(O) (Q N Bu)I (THF) ].
AB
2
2
t
and the four Bu phenolate substituents appeared as 4 singlets,
We have shown here the first activation of CO by a U(VI)
imido complex, specifically a trans-bis(imido), with formation
2
tBu
2
2À
indicating that the two phenolate arms of {( ArO)
have different chemical environments. In addition, two resonances
for the NCH protons were observed at 2.01 and 1.99 ppm and the
phenyl imido protons showed only one set of resonances, which
2
Me
2
-cyclam}
3
13
indicates free rotation about the N–Cipso bond. The C NMR data
1
(Fig. S8–S10 in ESI‡) corroborated the H NMR results, consistent
4
tBu
2
with formation of U(VI) oxo imido complex [U(k -{( ArO) Me -
2
2
cyclam})(NPh)(O)] (4).
An NMR tube-scale reaction of [U(k -{( ArO)
NPh)(NTol)] (3) with CO showed that the bis(imido) complex
is quantitatively converted to a mixture of mono-oxo imido
4
tBu
2
2 2
Me -cyclam})-
(
2
4
tBu
2
4
tBu
2
[
U(k -{( ArO)
2 2 2
Me -cyclam})(NPh)(O)] (4) and [U(k -{( ArO) -
Me -cyclam})(NTol)(O)] (5), in approximately 1 : 1 ratio, along with
2
the corresponding aryl isocyanates (Fig. S13 and S14 in ESI‡).
1
Although most H NMR resonances of the two uranium com-
pounds overlap, it was possible to differentiate resonances that
match the phenyl imido ligand of 4 from those of the methyl
t
Fig. 1 Molecular structure of [U(k -{(tBu2ArO)
4
Me
phenyl imido ligand of 5 (see Fig. S14 in ESI‡). Two of the Bu
2
2
-cyclam})(NPh)(O)] (4)
phenolate resonances of 5 are at 2.08 and 1.86 ppm, with the other with 50% probability thermal ellipsoids.
432 | Chem. Commun., 2020, 56, 431--434
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of a terminal uranium oxo bond. Selective formation of trans-
VI
2+
[
OQU QNR] complexes 4 and 5 by multiple bond metathesis
is presumably driven by a thermodynamic preference for
UQO bond formation with release of aryl isocyanate. Complex
4
tBu
2
[
U(k -{( ArO) Me -cyclam})(NPh)(O)] (4) did not exhibit reactivity
2 2
2 6
upon CO exposure in benzene-d , possibly reflecting enhanced
stability of complex 4 due to a greater inverse trans influence upon
26
UQO bond formation.
To elucidate mechanistic aspects of the observed reactivity,
3
4
the ADF program was used with the rev-PBE-D3 density
3
5–37
functional
to compute the CO2 cycloaddition/isocyanate
extrusion reaction pathway (see ESI‡ for Computational
details). The starting reference state A (energy ꢁ 0) is the two
reactants infinitely separated (Fig. 2). van der Waals interaction
Fig. 4 Free energy profile for the reaction of [U(k -{(tBu2ArO)
cyclam})(NPh)(O)] (4) with CO .
2
4
Me
-
2
2
between the reactants yields association complex B, which is
À1
followed by transition state, TS
B
at 103.3 kJ mol above A. The
À1
À1
CO
2
insertion into a uranium imido site yields intermediate C a substantial barrier of 116 kJ mol above A and 140 kJ mol
À1
at 59.5 kJ mol above A. Cycloaddition via TS
B
is a donor– above B, consistent with the observed slow kinetics at 25 1C. The
À1
acceptor process, as revealed by the frontier orbital in Fig. 3. free energy of 26.2 kJ mol to dissociate complex 4 (steps D–E) is
À1
A second transition state (TS ) corresponds to cleavage of mostly due to the electronic contribution (DE = +31.2 kJ mol ).
C
isocyanate intermediate C and leads to formation of the uranium The interaction in D is presumably dominated by dispersion
imido mono-oxo complex 4 and phenyl isocyanate (D). TS
C
presents bonding between the uranium complex and phenyl isocyanate.
Formation of the oxo-imido complex 4 in D is a thermo-
dynamic minimum, which evidently hampers subsequent addi-
tion of a second CO
2 2
. The reaction profile for addition of CO to
4
is shown in Fig. 4. Two aspects clearly differ from the profile
in Fig. 2: formation of metallacycle intermediate H is more
À1
endergonic than for C (DDG1 = +24.9 kJ mol ); and both
G H
transition states TS and TS are higher energy compared with
TS
TS
B
and TS
is 8.7 kJ mol
C
. In particular, rate determining transition state
À1
H
higher energy than TS
C
. Notably, the
À1
exergonicity of step I (À53.4 kJ mol ) is not as favourable as
À1
for step D in Fig. 2 (À81.5 kJ mol ). Overall, it is apparent that
2
CO addition to complex 4 is both thermodynamically and
kinetically hindered vis- a` -vis CO2 addition to bis(imido)
complex 2, consistent with observation of only the first process.
1
H NMR spectra were collected during reaction of 2 with CO
excess at
2
Fig. 2 Free energy profile for the reaction of [U(k -{(tBu2ArO)
4
Me
2
2
-
under pseudo-first-order conditions of a large CO
2
2 2
cyclam})(NPh) ] (2) with CO .
constant concentration. A plot of the ln(molar fraction) of
reagent as a function of time yields a pseudo-first-order rate.
The rate was obtained for different temperatures such that
using the Arrhenius equation and plotting ln kobs versus 1/T
À1
provided a reaction activation energy E = 102 Æ 12 kJ mol
a
(
see ESI‡ for details). This E is somewhat smaller than the
a
À1
computed value of 140 kJ mol in Fig. 2, which presumably
reflects limitations of the computational and/or experimental
methodologies (e.g. sample homogeneity during NMR acquisi-
tion). The effect of the higher pressure used in the experiments
should be insignificant.
In conclusion, U(VI) trans-bis(imido) complexes supported by
a bis(phenolate) cyclam ligand react with excess CO to afford
2
2
+
4
tBu
2
the trans-[OQUQNAr] species [U(k -{( ArO) Me -cyclam})-
2
2
(NAr)(O)]. DFT computational studies suggest that the reaction
proceeds via endergonic formation of a [2+2] cycloaddition
intermediate, with subsequent extrusion of phenyl isocyanate
and formation of the U(VI) oxo-imido computed to be exergonic.
Fig. 3 A clipped detail of the HOMOÀ5 of TS
B
showing the U(5f) - O and
2
N - C donor–acceptor interactions with the incoming CO molecule
establishing new bonds.
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These reactions are unprecedented examples of activation and 13 S. C. Bart, C. Anthon, F. W. Heinemann, E. Bill, N. M. Edelstein and
K. Meyer, J. Am. Chem. Soc., 2008, 130, 12536–12546.
4 C. Lescop, T. Arliguie, M. Lance, M. Nierlich and M. Ephritikhine,
J. Organomet. Chem., 1999, 580, 137–144.
cleavage of CO
2
mediated by uranium(VI) imido complexes.
1
We acknowledge the financial support from the Fundaç ˜a o
para a Ci ˆe ncia e a Tecnologia through projects UID/MULTI/ 15 A.-C. Schmidt, F. W. Heinemann, L. Maron and K. Meyer, Inorg.
Chem., 2014, 53, 13142–13153.
6 C. L. Boyd, E. Clot, A. E. Guiducci and P. Mountford, Organometal-
lics, 2005, 24, 2347–2367.
0
2
2
0612/2019, UID/MULTI/04046/2019 and UID/MULTI/04349/
019, through the post doc fellowships SFRH/BPD/101840/
1
014 and SFRH/BPD/110419/2015 to LM and NAGB, respectively, 17 U. J. Kilgore, F. Basuli, J. C. Huffman and D. J. Mindiola, Inorg.
Chem., 2006, 45, 487–489.
and through RNEM – Portuguese Mass Spectrometry Network,
ref. LISBOA-01-0145-FEDER-022125, also supported by the Lisboa
1
8 A. E. Guiducci, C. L. Boyd, E. Clot and P. Mountford, Dalton Trans.,
2009, 5960–5979.
Regional Operational Programme (Lisboa2020), under the PT2020 19 J. C. Anderson and R. B. Moreno, Org. Biomol. Chem., 2012, 10,
1
334–1338.
Partnership Agreement, through the European Regional Develop-
ment Fund (ERDF). JKG was supported by U.S. Department of
2
0 P. A. Cleaves, C. E. Kefalidis, B. M. Gardner, F. Tuna, E. J. L. McInnes,
W. Lewis, L. Maron and S. T. Liddle, Chem. – Eur. J., 2017, 23, 2950–2959.
Energy, Basic Energy Sciences, Chemical Sciences, Geosciences, 21 T. W. Hayton, Dalton Trans., 2010, 39, 1145–1158.
2
2
2 T. W. Hayton, Chem. Commun., 2013, 49, 2956–2973.
3 M. A. Boreen and J. Arnold, in The Heaviest Metals: Science and
Techonology of the Actinides and Beyond, ed. W. J. Evans and T.P.
Hanusa, Wiley, 2019, pp. 105–122.
4 C. J. Tatebe, K. E. Gettys and S. C. Bart, in Handbook on the Physics
and Chemistry of Rare Earths, ed. J.-C. G. B u¨ nzli and V. K. Pecharsky,
Elsevier, 2018, vol. 54, pp. 1–42.
5 T. W. Hayton, J. M. Boncella, B. L. Scott, P. D. Palmer, E. R. Batista
and P. J. Hay, Science, 2005, 310, 1941–1943.
6 R. G. Denning, J. Phys. Chem. A, 2007, 111, 4125–4143.
7 L. P. Spencer, P. Yang, B. L. Scott, E. R. Batista and J. M. Boncella,
J. Am. Chem. Soc., 2008, 130, 2930–2931.
and Biosciences, Heavy Element Chemistry, at LBNL under Con-
tract DE-AC02-05CH1123. The authors thank Dr M. C. Oliveira for
the HR-MS analysis.
2
Conflicts of interest
2
There are no conflicts to declare.
2
2
Notes and references
2
8 L. Maria, I. C. Santos, V. R. Sousa and J. Marçalo, Inorg. Chem., 2015,
1
2
3
4
B. M. Gardner and S. T. Liddle, Eur. J. Inorg. Chem., 2013, 3753–3770.
54, 9115–9126.
H. S. La Pierre and K. Meyer, Prog. Inorg. Chem., 2014, 58, 303–415. 29 A. R. Katritzky, T.-B. Huang and M. V. Voronkov, J. Org. Chem., 2001,
P. L. Arnold and Z. R. Turner, Nat. Rev. Chem., 2017, 1, 2–16. 66, 1043–1045.
I. Castro-Rodriguez, H. Nakai, L. N. Zakharov, A. L. Rheingold and 30 R. E. Jilek, N. C. Tomson, R. L. Shook, B. L. Scott and J. M. Boncella,
K. Meyer, Science, 2004, 305, 1757–1759. Inorg. Chem., 2014, 53, 9818–9826.
I. Castro-Rodriguez and K. Meyer, J. Am. Chem. Soc., 2005, 127, 31 E. Lu, O. J. Cooper, J. McMaster, F. Tuna, E. J. L. McInnes, W. Lewis,
1242–11243. A. J. Blake and S. T. Liddle, Angew. Chem., Int. Ed., 2014, 53, 6696–6700.
O. Cooper, C. Camp, J. P ´e caut, C. E. Kefalidis, L. Maron, S. Gambarelli 32 K. C. Mullane, A. J. Lewis, H. Yin, P. J. Carroll and E. J. Schelter,
and M. Mazzanti, J. Am. Chem. Soc., 2014, 136, 6716–6723. Inorg. Chem., 2014, 53, 9129–9139.
O. T. Summerscales, A. S. P. Frey, F. G. N. Cloke and P. B. Hitchcock, 33 T. W. Hayton, J. M. Boncella, B. L. Scott and E. R. Batista, J. Am.
Chem. Commun., 2008, 198–200. Chem. Soc., 2006, 128, 12622–12623.
A.-C. Schmidt, A. V. Nizovtsev, A. Scheurer, F. W. Heinemann and 34 G. te Velde, F. M. Bickelhaupt, E. J. Baerends, C. Fonseca Guerra,
5
6
7
8
9
1
K. Meyer, Chem. Commun., 2012, 48, 8634–8636.
N. Tsoureas, L. Castro, A. F. R. Kilpatrick, F. G. N. Cloke and
L. Maron, Chem. Sci., 2014, 5, 3777–3788.
S. J. A. van Gisbergen, J. G. Snijders and T. Ziegler, J. Comput. Chem.,
2001, 22, 931–967.
35 Y. Zhang and W. Yang, Phys. Rev. Lett., 1998, 80, 890.
1
0 C. Camp, L. Chatelain, C. E. Kefalidis, J. P ´e caut, L. Maron and 36 J. P. Perdew, K. Burke and M. Ernzerhof, Phys. Rev. Lett., 1996, 77,
M. Mazzanti, Chem. Commun., 2015, 51, 15454–15457. 3865–3868.
1 K. G. Moloy and T. J. Marks, Inorg. Chim. Acta, 1985, 110, 127–131. 37 L. Goerigk, H. Kruse and S. Grimme, ChemPhysChem, 2011, 12, 3421–3433.
2 W. J. Evans, J. R. Walensky and J. W. Ziller, Organometallics, 2010, 38 M. A´ lvarez-Moreno, C. de Graaf, N. L o´ pez, F. Maseras, J. M. Poblet
1
1
2
9, 945–950.
and C. Bo, J. Chem. Inf. Model., 2015, 55, 95–103.
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