C?H Bond Trifluoromethylation of Arenes Enabled by a Robust, High-Valent Nickel(IV) Complex

Article abstract of DOI:10.1002/anie.201706237

The robust, high-valent Ni^IV complex [(Py)₂Ni^IVF₂(CF₃)₂] (Py=pyridine) was synthesized and fully characterized by NMR spectroscopy, X-ray diffraction, and elemental analysis. It reacts with aromatic compounds at 25 °C to form the corresponding benzotrifluorides in nearly quantitative yield. The monomeric and dimeric Ni^IIICF₃ complexes 2?Py and 2 were identified as key intermediates, and their structures were unambiguously determined by EPR spectroscopy and X-ray diffraction. Preliminary kinetic studies in combination with the isolation of reaction intermediates confirmed that the C?H bond-breaking/C?CF₃ bond-forming sequence can occur both at Ni^IVCF₃ and Ni^IIICF₃ centers.

Full text of DOI:10.1002/anie.201706237

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Accepted Article
Title: C-H Bond Trifluoromethylation of Arenes Enabled by a Robust,
High-Valent NiIV-Complex.
Authors: Florian D'Accriscio, Pilar Borja, Nathalie Saffon-Merceron,
Marie Fustier-Boutignon, Nicolas Mézailles, and Noel Nebra
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Angewandte Chemie International Edition
10.1002/anie.201706237
COMMUNICATION
C–H Bond Trifluoromethylation of Arenes Enabled by a Robust,
IV
High-Valent Ni -Complex.
[
a]†
[a]†
[b]
[a]
Florian D’Accriscio,
Pilar Borja,
Nathalie Saffon-Merceron, Marie Fustier-Boutignon, Nicolas
[
a]
[a]
Mézailles, * Noel Nebra *
Dedication ((optional))
IV
IV
Abstract: The robust, high-valent Ni complex [(Py)
2
Ni F
2
(CF
3 2
) ] 3,
species was demonstrated under mild heating to allow for the C–
C, C–Heteroatom and C–CF couplings. However outstanding
these achievements are, the trifluoromethylation of arenes is
restricted to the intramolecular version. Moreover strongly
donating polydentate ligand scaffolds, and prior functionalization
of the aromatic ring, is required for this chemistry to proceed.
is reported. The structure of 3 was fully characterized by NMR, XRD
and EA. 3 reacts with aromatics at 25°C to form the corresponding
benzotrifluorides in nearly quantitative yield. Monomeric and dimeric
3
III
.
3
Ni CF complexes 2Py and 2 were identified as key intermediates,
and their structures were unambiguously assigned by EPR and XRD.
Preliminary kinetic study coupled to experimental isolation of reaction
intermediates proves that the C–H bond breaking/C–CF
3
bond
IV
III
forming sequence can occur both with the Ni CF and Ni CF centers.
3
3
Since the 2000s, much effort has been devoted to the ideal,
environmentally friendly transformation of aromatics through a
[
1]
direct C–H bond activation. Over the last few years, Ni
complexes have emerged as excellent catalysts for the C–H bond
[
2]
functionalization yet the intrinsic properties of first row metals
i.e. weak metal-ligand interactions and high propensity to
undergo SET processes) often precludes precise mechanism
(
[
3]
elucidation. Several works on C–H bond functionalization have
recently been reported suggesting the crucial participation of
IV
[4]
high-valent Ni -intermediates. However, in light of the low-
IV
[5]
stability and/or accessibility of Ni -species, whether or not such
Ni -intermediates are involved in a C–H bond activation event
remains unclear, and little information is available about Ni -
involvement in C–C and C–Heteroatom bond formations.
Among all kinds of C–C bond forming reactions, the highly
IV
[
6]
IV
[
7]
[
8]
desirable C–CF
elimination of Ar–CF
be unfeasible, as shown by Vicic
3
coupling remains a challenge. In fact, reductive
II
3
from well-defined ArNi CF
3
was proven to
[
9a]
[9b]
IV
and Grushin.
By analogy
Scheme 1. (a) Isolable, coupling-competent Ni CF3 organometallics. (b) First
[10,11]
IV
with the chemistry of high-valent Pd and Cu organometallics,
different scenarios and new mechanistic pathways involving high-
C–H bond functionalization of aromatics from an isolated, well-defined Ni .
valent Ni species were later envisaged to allow easier C–CF
3
[
12]
IV
bond formation. Since 2015, the first Ni CF
3
organometallics (I-
We present here the synthesis and full characterization of
IV
IV
IV, Scheme 1a) have been synthesized by Sanford and co-
2 2 3 2 3
the high-valent [(Py) Ni F (CF ) ] complex (3), the first Ni CF -
[
13]
workers,
using scorpionate-type ligands to stabilize the high-
complex which does not need to be stabilized by sophisticated
ancillary ligand. The reactivity of this robust complex toward inert
Aryl–H was studied, and the Ni -mediated C–H bond
IV
IV
valent Ni center. The coupling-competent nature of these ArNi
IV
C
functionalization of aromatics has been demonstrated for the first
time. Preliminary kinetic studies were performed and showed two
successive regimes. The first, involving the Ni -complex 3, is fast
†
†
[
[
a]
b]
Dr. F. D’Accriscio, Dr. P. Borja, Dr. M. Fustier-Boutignon, Dr. N.
Mézailles, Dr. N. Nebra
IV
Laboratoire Hétérochimie Fondamentale et Appliquée
Université Paul Sabatier, CNRS
18 Route de Narbonne, 31062 Toulouse (France)
E-mail: mezailles@chimie.ups-tlse.fr, nebra-muniz@chimie.ups-tlse.fr
Dr. N. Saffon-Merceron
Institut de Chimie de Toulouse ICT-FR2599
Université Paul Sabatier, CNRS
and leads to partial Ar–CF
3
formation and new high-valent
3
“Ni (CF )(F)” species. The second, independent regime, involving
III
1
III
the “Ni (CF
3
)(F)” species is much slower, and leads to the nearly
II
quantitative Ar–CF
3
formation together with Ni species. Both
III
IV
3 3
“Ni (CF )(F)” and “Ni (CF )(F)” complexes are competent to
3
1062 Toulouse Cedex (France)
cleave C–H bonds and perform the aromatic trifluoromethylation.
†
Both authors contributed equally to this work.
II
Our strategy relied on the oxidation of [(Py)
precursor 1 with a formal source of “F ”, i.e. XeF . After careful
optimization, the best conditions were obtained using 2.0 equiv.
of XeF in 1,2-dichlorobenzene (DCB; see ESI). After 1.5h stirring
at –10°C the reaction medium turned to intense yellow color.
2 3 2
Ni (CF ) ]
The supplementary materials contain experimental and spectral
details. Metrical parameters for the structures of 1, 2, 2 Py and 3 are
available free of charge from the Cambridge Crystallographic Data
Center under references number CCDC-1540793, CCDC-1540795,
CCDC-1540796 and CCDC-1540797, respectively.
2
2
.
2
19
F
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COMMUNICATION
NMR analysis of the reaction medium confirmed the formation of
coordination mode with the three pyridines placed in meridional
complex 3, as the exclusive diamagnetic species displaying two
position, and both CF
fashion. Final proof for the stepwise Ni CF
3
-groups bonded to the Ni-center in a cis
2
II
III
IV
signals that resonate at –26.1 (triplet, JF,F = 16.3 Hz, 2 CF
3
) and
3
to Ni CF
3
to Ni CF
3
2
.
–418.3 (septuplet, JF,F = 16.3 Hz, 2F) ppm. These chemical shifts, oxidation sequence was illustrated by reacting the isolated 2/2 Py
IV
along with this multiplicity pattern, indicates the presence of two
chemically equivalent CF and fluoride ligands bonded to a single
Ni-center, thus providing further evidence for the efficient
2 3
mixture and 1 equiv. of XeF at –10°C. The Ni CF complex 3
was then formed in high yield (80%; step iii in Scheme 2).
3
IV
3
oxidation of 1 into the Ni CF complex 3. The synthesis of 3 can
be easily scaled up and its isolation as a yellow powder was
accomplished in excellent yield (1 gram scale, 86%; Scheme 2).
The structure of 3 was unequivocally confirmed by XRD of single
crystals. Accordingly (Figure 1a), the Ni-center adopts a distorted
octahedral geometry having a cis,cis,trans configuration with
mutually trans fluorides displaying a deviation from linearity
(
176.09(6)°). The Ni–C bond distances (1.960(2) and 1.969(2) Å)
IV
[13]
are in the range for the rare known Ni CF
3
complexes,
with
short Ni–N bond distances (2.058(2) and 2.036(2) Å) and
remarkably short Ni–F bond distances (1.783(2) and 1.788(2) Å).
The appearance of a deep red-violet color within minutes at
–
10°C during the oxidation process and its subsequent fading to
III
yellow suggested the intermediacy of Ni CF
3
species. This
Scheme 2. Direct vs stepwise synthesis of 3. Reaction conditions: i) 2 equiv. of
observation encouraged us to perform the one electron oxidation
2 2
XeF in chlorobenzene at –40°C; ii) 1 equiv. of XeF in chlorobenzene at –40°C;
III
to Ni complex (step ii in Scheme 2). As apparent by EPR, a
and iii) 1 equiv. of XeF2 in DCB at –10°C.
III
.
mixture of two Ni CF
3
complexes, 2 and 2 Py (vide infra), were
was used (see Figure 1b). The
/2 Py mixture was isolated as a deep-violet powder. Violet single
obtained when 1 equiv. of XeF
2
.
2
crystals of complex 2 were obtained by slow diffusion of pentane
.
into a concentrated solution of the mixture of 2 and 2 Py in
dichloromethane (Figure 1c). XRD analysis shows that complex 2
crystallizes as a dimer bearing two distinct Ni-centers that are
linked by two bridging fluoride ligands. Interestingly, the
coordination environment around each Ni-unit is different, thus
providing a highly unusual, dissymmetric structure for 2. One of
the Ni-centers exhibits an octahedral (Oh) environment, whereas
the second is in square pyramidal (SP) environment. The two Ni
centers are linked by two bridging fluorine atoms, and possess
two CF
3
ligands (in ax. eq. positions for Ni in pseudo Oh, and eq.
III
III
eq. for Ni in SP). Ni –Ni dinuclear complexes featuring bridging
halogens have been reported recently.
[
14]
III
These Ni dimers are
diamagnetic (singlet spin state) or paramagnetic (triplet spin
state) depending on the magnitude of the antiferromagnetic
III
coupling between both Ni centers. DFT calculations fully support
2
to be paramagnetic, as the triplet dimer is calculated 43.5
kcal/mol lower than the corresponding singlet, in agreement with
the fact that complex 2 is NMR-silent but seen by EPR. Therefore
the bridging fluoride ligands do not favor antiferromagnetic
III
Figure 1. Ellipsoid drawing (50% probability) for the molecular structure of 3 (a),
coupling between the two Ni centers in 2. The frozen solution
.
.
2
(c) and 2 Py (d). (b) EPR spectrum (blue line) for the mixture of 2 and 2 Py in
EPR spectrum (Figure 1b) of complex 2 presents superhyperfine
3:1 PrCN:DCB at 120 K and the simulated spectrum (red line) using the
coupling with a single fluorine atom (A
dissociation of dimer 2 to form a monomer of type [L
L = Py, PrCN; x = 1–3) because of the coordinating nature of the
F
= 220 G), thus indicating
following parameters: 2: g
x
= 2.237; g
y z N F
= 2.172; g = 2.018 (A = 18.4 G, A =
III
.
x
3 2
Ni F(CF ) ]
220 G); 2 Py: g = 2.232; g = 2.166; g = 2.018 (A = 19.6 G).
x
y
z
2N
(
solvent used for EPR measurement. We thus postulated that
IV
additional Py would favor the formation of monomeric species.
Despite its unique structure, Ni -complex 3 is air stable for
days in the solid state and can be stored in glove-box for months
without decomposition. The high stability of 3 contrasts deeply
with its reactivity towards aromatics. Indeed, when dissolved in
III
Excess of Py was added to the crude mixture of Ni CF
3
complexes resulting in the concomitant formation of a unique
III
.
[15]
Ni CF
3
complex 2 Py (see EPR; Figure S12).
Cooling the
chlorobenzene/diethyl ether/pyridine reaction mixture at –30°C
resulted in the formation of blueish single crystals of 2 Py. XRD
1,2-dichlorobenzene,
near
quantitative
formation
of
.
dichlorobenzotrifluorides (94%, mixture 1.0:1.4 of isomers; see
Figure 2a) is obtained at room temperature within 3 days, as
analysis reveals the formation of an unprecedented monomeric
III
19
Ni CF
3
complex bearing three pyridines, two CF
3
-groups and a
shown by F NMR. To the best of our knowledge, the direct
terminal fluoride ligand (Figure 1d). The Ni-atom shows an Oh
aromatic trifluoromethylation of DCB promoted by 3 represents
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Angewandte Chemie International Edition
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COMMUNICATION
the first C–H bond functionalization event mediated by a well-
necessarily formed. Fast ligand redistribution then occurs to form
IV
II
II
defined, isolated Ni
organometallic. This evolution is
equimolar amounts of [(Py)
2 2 2 3 2
Ni F ] and [(Py) Ni (CF ) ]. This
accompanied by two subsequent changes in color which
suggested the involvement of Ni species during the reaction.
proposal is supported by recent observations by Sanford and co-
III
[13b]
II
workers
bearing a unique CF
ligand metathesis leading to [Ni (CF
who showed that in situ generated Ni CF
3
complexes
This prompted us to follow the kinetics of the transformation by
3
-ligand are highly prone to undergo fast
1
9
II
F NMR, which revealed a mechanism featuring two distinct
regimes (Figure S4b). First, 3 quickly reacts with DCB to produce
the corresponding Ar–CF coupled product in ca. 60% after 2h at
3
)
2
]
complexes. The
II
[(Py)
complex 3 in a comproportionation reaction to form 2/2 Py.
2 3 2
Ni (CF ) ] complex 1 instantaneously reacts with starting
.
[16]
3
room temperature (Figure 2b) via the C–H bond
Despite the above demonstrated coupling-competent nature
of 3 for the innate trifluoromethylation of aromatics, the precise
mechanism was still not understood at this point. Two alternatives
activation/functionalization sequence. Concomitantly, complete
1
9
loss of NiCF
3
signals was seen in F NMR, pointing the formation
of paramagnetic species. It is to be noted that it is the fastest C–
were envisaged. The first one involves a C–H bond activation at
IV
CF
3
coupling from any known NiCF
3
complex. At this time, the
] complex 1 starts growing
the Ni , with concomitant elimination of HF, followed by C–CF
3
II
signal corresponding to [(Py)
2
Ni (CF
3
)
2
bond formation in the coordination sphere. The second one
IV
slowly but steadily. Its formation parallels the evolution of Ar–CF
content. At the end of the reaction, 1 was formed in more than
5% and Ar–CF in 94% yields.
With no doubt, Ni CF
3
3
involves the formation of a •CF radical from Ni , trapped by the
Ar–H, followed by the formal elimination of HF. The first
mechanism was evaluated by DFT calculations, but a low energy
process consistent with the experimental kinetics could not be
found. In parallel, the radical mechanism was experimentally
tested. A first radical trap experiment was performed for the 3-
promoted trifluoromethylation of DCB in presence of two equiv. of
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) under strictly
otherwise identical conditions. The quantitative formation of
3
3
III
3
species are responsible for the
second part of the transformation, which shows that not only Ni
but also Ni complexes are able to promote the desirable C–H
IV
III
3
breaking/C–CF coupling sequence of events. EPR spectra were
recorded during the coupling process and not only confirmed the
III
.
formation of Ni -intermediates, but also revealed that 2 and 2 Py
1
9
are these species (Figure 2c). We thus carried out the reaction
3
TEMPO–CF was evidenced by F NMR monitoring (δ –56.5
.
starting from an isolated mixture of 2/2 Py. Most satisfying, the
ppm), and definitively confirmed by DCI-HRMS analysis of the
reaction mixture (Figure S8c). A second experiment was carried
out using two equiv. of N-tert-butyl--phenylnitrone (PBN). After
24 hours, only 20% of trifluoromethylation of DCB was observed
3
rate observed for the Ar–CF bond creation in the second part of
the kinetics was nicely reproduced using this mixture, thereby
III
proving their involvement. The formation of these Ni -species can
be rationalized in the following manner. According to the global
in addition to the product of •CF
(detected by EPR,
3
radical addition on the nitrone
Figure S9).
II
mass balance, during the first sequence [(Py)
2
Ni F(CF
3
)] is
Figure 2. (a) Innate trifluoromethylation of 1,2-dichlorobenzene mediated by 3. (b) Reaction evolution at 25°C for the first 4 hours using 1-fluoroheptane as an
.
internal standard. (c) EPR monitoring of the reaction evolution at 25°C: evidence for in situ formation and consumption of
2 and 2 Py.
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Angewandte Chemie International Edition
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COMMUNICATION
The involvement of •CF
from the Ni -species 3 is thus demonstrated, which had not
been unambiguously illustrated in the past. It is to be noted that
Lancaster and co-worker proposed very recently that the Ni CF
3
radicals in the Ar–CF
3
coupling
[6]
Sanford and co-workers reported the intramolecular C–H bond
IV
IV
activation of aromatic rings to yield cyclometalated ArNi complexes
under oxidative conditions. E. Chong, J. W. Kampf, A. Ariafard, A. J.
Canty, M. S. Sanford, J. Am. Chem. Soc. 2017, 139, 6058–6061.
a) G. E. Martinez, C. Ocampo, Y. J. Park, A. R. Fout, J. Am. Chem. Soc.
IV
3
[7]
complexes of type III reported by Sanford may be a new case
2016, 138, 4290–4293; b) J. W. Schultz, K. Fuchigami, B. Zheng, N. P.
[
17]
study of “inverted” ligand field, and that III would be prone to
Rath, L. M. Mirica, J. Am. Chem. Soc. 2016, 138, 12928–12934; c) M.
B. Watson, N. P. Rath, L. M. Mirica, J. Am. Chem. Soc. 2017, 139, 35–
38.
[
18]
eliminate •CF
3
radicals.
In conclusion, we have devised a facile route of access to
IV
IV
a unique Ni complex 3, namely [(Py)
2
Ni F
2
(CF
3
)
2
]. Remarkably,
[8]
For reviews about aromatic trifluoromethylation and perfluoroalkylation,
see: a) O. A. Tomashenko, V. V. Grushin, Chem. Rev. 2011, 111,
4475–4521; T. Liang, C. N. Neumann, T. Ritter, Angew. Chem., Int. Ed.
3
is stable in solid form for months despite the absence of highly
IV
donor, polydentate ancillary ligands. The Ni complex allows
direct Ar–CF bond formation in nearly quantitative yields from
inert Ar–H derivatives.
to experimental isolation of reaction intermediates proved for the
first time that the C–H bond breaking/C–CF bond forming
2013, 52, 8214–8264; c) C. Alonso, E. Martinez de Marigorta, G.
3
Rubiales, F. Palacios, Chem. Rev. 2015, 115, 1847–1935.
[
19]
The preliminary kinetic study coupled
[9]
a) G. G. Dubinina, W. W. Brennessel, J. L. Miller, D. A. Vicic,
Organometallics 2008, 27, 3933–3938; b) J. Jover, F. M. Miloserdov, J.
Benet-Buchholz, V. V. Grushin, F. Maseras, Organometallics 2014, 33,
3
IV
III
sequence can occur from well-defined Ni and Ni centers albeit
with distinct reaction rates, the former being much faster than
the latter. We believe that the accessibility of complex 3 will
open new avenues for research in Ni -chemistry directed toward
aromatic trifluoromethylation and C–H bond functionalizations.
6531–6543.
IV
[10] For related examples of well-defined, coupling-competent Pd CF
3
organometallics, see: a) Y. Ye, N. D. Ball, J. W. Kampf, M. S. Sanford,
J. Am. Chem. Soc. 2010, 132, 14682–14687; b) D. C. Powers, E. Lee,
A. Ariafard, M. S. Sanford, B. F. Yates, A. J. Canty, T. Ritter, J. Am.
Chem. Soc. 2012, 134, 12002–12009; c) N. D. Ball, J. W. Kampf, M. S.
Sanford, J. Am. Chem. Soc. 2010, 132, 2878–2879; d) N. D. Ball, B.
Gary, Y. Ye, M. S. Sanford, J. Am. Chem. Soc. 2011, 133, 7577–7584.
[11] a) P. Sehnal, R. J. K. Taylor, I. J. S. Fairlamb, Chem. Rev. 2010, 110,
IV
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[
12] a) C.-P. Zhang, H. Wang, A. Klein, C. Biewer, K. Stirnat, Y. Yamaguchi,
0016-01), CNRS and Université de Toulouse for financial
L. Xu, V. Gomez-Benitez, D. A. Vicic, J. Am. Chem. Soc. 2013, 135,
support. We are grateful to CalMip (CNRS, Toulouse, France)
for access to calculation facilities. L. Rechignat is acknowledged
for EPR spectroscopy analyses and simulation of 2 and 2 Py.
8
141–8144; b) F. Tang, N. P. Rath, L. M. Mirica, Chem. Commun. 2015,
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Kampf, A. J. Canty, M. S. Sanford, J. Am. Chem. Soc. 2016, 138,
.
16105–16111.
III/IV
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Keywords: High-valent species • Ni
organometallics •
Fluorine • Aromatic trifluoromethylation • Mechanism elucidation
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1]
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[
2]
3]
J. Yamaguchi, K. Muto, K. Itami, Top. Curr. Chem. (Z) 2016, 374, 55,
doi: 10.1007/s41061-016-0053-z.
.
[15] The absence of superhyperfine coupling with the fluorine atom in 2 Py
[
a) X. Hu, Chem. Sci. 2011, 2, 1867–1886; b) S. Z. Tasker, E. A.
Standley, T. F. Jamison, Nature 2014, 509, 299–309; c) V. P. Ananikov,
ACS Catal. 2015, 5, 1964–1971.
may be due to fluorine dissociation in presence of coordinating ligands
III
(Py or PrCN). For similar behavior in Ni F complexes, see: a) W. Zhou,
S. Zheng, J. W. Schultz, N. P. Rath, L. M. Mirica, J. Am. Chem. Soc.
2016, 138, 5777–5780; b) W. Zhou, M. B. Watson, S. Zheng, N. P.
Rath, L. M. Mirica, Dalton Trans 2016, 45, 15886–15893; c) H. Lee, J.
Börgel, T. Ritter, Angew. Chem.; Int. Ed. 2017, 56, 6966–6969; Angew.
Chem. 2017, 129, 7070–7073.
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Yamaguchi, N. Chatani, Angew. Chem., Int. Ed. 2016, 55, 3162–3165;
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[16] As further proof of the mechanism, when complexes 1 and 3 were
.
[
a) U. Kölle, F. Khouzami, H. Lueken, Chem. Ber. 1982, 115, 1178–
mixed, the mixture 2/2 Py was seen by EPR.
1
196; b) J. L. Robbins, N. Edelstein, B. Spencer, J. C. Smart, J. Am.
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Lancaster, J. Am. Chem. Soc. 2016, 138, 1922–1931; b) R. Hoffmann,
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Chem. Rev. 2016, 116, 8173–8192.
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IV
[19] As preliminary extension of this work, the Ni CF
3
complex 3 was found
to be highly efficient for the innate trifluoromethylation of pyridine to
yield Py–CF in 91% after 2 days at room temperature (ratio of o,m,p-
3
regioisomers equal to 5.9:3.7:1; Figure S10). The scope of the aromatic
trifluoromethylation is currently under investigation in our laboratories.
2
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60–261; Angew. Chem. 2009, 121, 266–267; i) R. Mitra, K.-R
2
Pörschke, Angew. Chem., Int. Ed. 2015, 54, 7488–7490; Angew. Chem.
015, 127, 7596–7598.
2
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Angewandte Chemie International Edition
10.1002/anie.201706237
COMMUNICATION
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COMMUNICATION
A very robust, high-valent
Florian D’Accriscio, Pilar Borja, Nathalie
Saffon-Merceron, Marie Fustier-
Boutignon, Nicolas Mézailles,* Noel
Nebra*
IV
[
(Py)
2
Ni F
2
(CF
3
)
2
] complex is isolated
IV
in a gram-scale. This unique Ni CF
3
species reacts with aromatics to yield
benzotrifluorides in >90% yield. The
mechanism of the reaction has been
comprehensively investigated and
Page No. – Page No.
C–H Bond Trifluoromethylation of
proved the involvement of
Arenes Enabled by a Robust, High-
III
unprecedented Ni CF
intermediates.
3
key
IV
Valent Ni -Complex.
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