C(sp2)-CF3Reductive Elimination from Well-Defined Argentate(III) Complexes [nBu4N][Ag(Ar)(CF3)3]

Article abstract of DOI:10.1021/acs.organomet.1c00195

The preparation of a series of well-defined, shelf-stable square planar aryl(tris(trifluoromethyl))argentate(III) complexes [nBu4N]+[Ag(Ar)(CF3)3]- and their C(sp2)-CF3 bond-forming reductive elimination are described. Mechanistic studies of the C(sp2)-CF3 reductive elimination from these complexes, including kinetic studies, effect of temperature and solvent, and DFT calculations, indicate that the C(sp2)-CF3 bond-forming process occurred via a concerted reductive elimination pathway.

Full text of DOI:10.1021/acs.organomet.1c00195

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C(sp2)‑CF3 Reductive Elimination from Well-Defined Argentate(III)
Complexes [nBu4N][Ag(Ar)(CF3)3]
Zehai Lu, Shihan Liu, Yu Lan,* Xuebing Leng, and Qilong Shen*
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ABSTRACT: The preparation of a series of well-defined, shelf-stable
square planar aryl(tris(trifluoromethyl))argentate(III) complexes
[nBu4N]+[Ag(Ar)(CF3)3]− and their C(sp2)-CF3 bond-forming
reductive elimination are described. Mechanistic studies of the
C(sp2)-CF3 reductive elimination from these complexes, including
kinetic studies, effect of temperature and solvent, and DFT
calculations, indicate that the C(sp2)-CF3 bond-forming process
occurred via a concerted reductive elimination pathway.
Thus, isolation of stable Ag(III) complexes and studying
their carbon−carbon forming reductive-eliminations remain a
grand unsolved challenge. Herein, we report, for the first time,
the preparation of a series of well-defined, shelf-stable square
planar anionic aryl(tris(trifluoromethyl))argentate(III) com-
plexes [nBu4N]+[Ag(Ar)(CF3)3]−. Thermolysis of these
complexes in CDCl3 at 85 °C generated trifluoromethylarenes
in 85−99% yields, and mechanistic studies indicate that the
C(sp2)-CF3 bond-forming process from these complexes
occurred via a concerted reductive elimination pathway.
INTRODUCTION
■
It is well-known that the transition-metal-mediated or
-catalyzed process was composed by a series of elemental
reactions, and a good knowledge of these elemental reactions is
highly important for us to gain fundamental understanding of
the whole catalytic process and consequently to improve the
catalyst’s performance, including turnover numbers and
turnover frequencies, or to develop new transition-metal-
mediated or -catalyzed reactions.1 Silver, unlike its congeners
copper2 and gold3 or its neighbors nickel,4 palladium,5 and
platinium,6 is perhaps one of the least understood late
transition metals. Specifically, recently, several silver-mediated
or catalytic carbon−carbon7 or carbon−heteroatom8 bond-
forming cross-coupling reactions were reported. Yet, the
mechanisms of these coupling reactions remain elusive. In
particular, elemental reactions of the putative intermediate
silver(III) complexes in these coupling reactions were almost
completely unexplored. One major problem that halts the
investigation of organometallic chemistry of silver(III)
complexes is the thermal instability of silver(III) complexes.
With few exceptions,9 silver(III) complexes are kinetically
unstable and typically undergo spontaneous decomposition at
ambient conditions. For instance, in 2013, Wang and co-
workers described silver-mediated Sandmeyer-type trifluor-
omethylation of aryl diazonium salts. Mechanistic studies
suggested that the reaction proceeds via a trifluoromethlated
silver(III) intermediate. Nevertheless, the putative intermedi-
ate was not isolated and characterized.10 Remarkably, in 2014,
Ribas and co-workers reported the preparation of a stable
triazamacrocyclic chelating ligand coordinated silver(III)
complex and its stoichiometric reactions with a variety of
nucleophiles, such as sulfonamide, carboxylic acid, phenol and
thiol, cyanide, malonate, aryl boronic acid, and halides, even
though the key intermediates [aryl−silver(III)−nucleophile]
that underwent reductive elimination were not isolated.11
RESULTS AND DISCUSSION
■
Preparation of [nBu4N]+[Ag(Ar)(CF3)3]− 3a−e. Com-
plexes [nBu4N]+[Ag(Ar)(CF3)3]− 3a−e were synthesized in
three steps, as shown in Figure 1. Treatment of AgF with 5.0
equiv of TMSCF3 in the presence of 4.0 equiv of KF in
anhydrous CH3CN at −40 °C for 2 h, followed by oxidation of
the in situ generated AgCF3 with PhI(OCOCF3)2 and then
cation exchange with nBu4NBr generated [nBu4N]+[Ag-
(CF3)4]− 112 in 60% yield (Figure 1, eq 1). Subsequent
treatment of complex 1 with HBF4·Et2O, followed by the
addition of 1,10-phenanthroline afforded an isolable complex
[(Phen)Ag(CF3)3] 2 in 78% yield (Figure 1, eq 2), and its
structure was further established by X-ray diffraction of its
single crystals (Figure 2).13 Treatment of complex 2 with aryl
pinacol boronates in the presence 1.0 equiv of Cs2CO3 at 60
°C for 1.5 h and then further cation exchange with nBu4NBr
gave stable anionic organosilver(III) complexes [nBu4N]+[Ag-
Received: March 30, 2021
Published: May 17, 2021
https://doi.org/10.1021/acs.organomet.1c00195
© 2021 American Chemical Society
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represent the first isolated and characterized silver(III)
complexes bearing both an aryl group and a trifluoromethyl
group.
Thermolysis of Complexes [nBu4N]+[Ag(Ar)(CF3)3]−
3a−e. As shown in Table 1, heating of [nBu4N]+[Ag(Ar)-
Table 1. Reductive Elimination of Organosilver(III)
Complexes [nBu4N]+[Ag(Ar)(CF3)3]− 3a−e
time
(h)
yield
(%)
rate
t1/2
entry complex
R
(× 10−4 s−1
)
(min)
Figure 1. Preparation of anionic organosilver(III) complexes
[nBu4N]+[Ag(Ar)(CF3)3]− 3a−e.
1
2
3
4
5
3a
3b
3c
3d
3e
OMe
Me
H
1
3
3
9
12
95
91
85
94
88
27.08
5.05
4.60
1.70
1.05
4.3
22.9
25.1
68.1
61.6
F
CF3
(CF3)3]− 3a−e in CDCl3 at 85 °C for 0.5−10 h generated the
trifluoromethylated arenes 4a−e in 85−99% yields and the
corresponding organosilver(I) complex [nBu4N]+[Ag(CF3)2]−
1
5 in 95% yields, which was fully characterized by H and 19F
NMR and elemental analysis that matched the previously
reported data.14 The structure of complex 5 was further
confirmed by X-ray diffraction of its single crystals (see the
Supporting Information for details). X-ray data showed that
the bond angle for C1−Ag−C2 in complex 5 is 179.68°,
suggesting a linear configuration. In addition, the bond lengths
for Ag−C1 and Ag−C2 are identical to be 2.101(4) Å. In the
solid state, [nBu4N]+[Ag(CF3)2]− was not sensitive to
moisture, but it was rather sensitive to air. In the solution, it
is also quite stable because no decomposition was detected for
a solution of [nBu4N]+[Ag(CF3)2]− in DMF, CH3CN, THF,
or dioxane for at least 3 h at 60 °C.
Figure 2. X-ray structure of complex [(Phen)Ag(CF3)3] 2.
(Ar)(CF3)3]− 3a−e in 60−89% yields (Figure 1, eq 3).
Complexes 3a−e were fully characterized by conventional
spectroscopic and microanalytical methods. For example, the
19F NMR spectrum of complex 3d displayed two sets of
doublets at −29.17 (d, J = 43.2 Hz) and −33.47 (d, J = 36.8
Hz) ppm with a ratio of 2:1, respectively, indicating the
symmetric structure of the complex.
The structure of complex 3d was further ambiguously
confirmed by the X-ray diffraction analyses of its single crystals
(Figure 3). The sum of the bond angles around the silver metal
center is 359°, which suggests that Ag(III) complexes adopt a
square planar geometry. These air-, moisture-, or light-
insensitive complexes could be stored on the shelf for 3
months without decomposition. Notably, complexes 3a−e
Mechanistic Study. The high yields in the C(sp2)-CF3
bond-forming processes from complexes [nBu4N]+[Ag(Ar)-
(CF3)3]− 3a−e promoted us to study these processes in detail.
As shown in Figure 4, presumptively, reductive elimination
from the Ag(III) complex [nBu4N]+[Ag(Ar)(CF3)3]− pro-
ceeds via five different pathways. In pathway A, the Ag−
C(CF3) bond in complex [nBu4N]+[Ag(Ar)(CF3)3]− under-
goes a homolytic cleavage to give a trifluoromethyl radical and
[AgII(Ar)(CF3)2]−, which then reductively eliminates to give
Ar−CF3 and [Ag0(CF3)]−. Oxidation of this species by the
trifluoromethyl radical generates [Ag(CF3)2]−. Alternatively, in
pathway B, heterolytic cleavage of the Ag−C(CF3) bond of
complex [nBu4N]+[Ag(Ar)(CF3)3]− forms a trifluoromethyl
anion and neutral intermediate [Ag(Ar)(CF3)2], which further
reductively eliminates to give Ar−CF3 and neutral AgCF3.
Recombination of the trifluoromethyl anion with AgCF3
affords [Ag(CF3)2]−. Likewise, the Ag−Ar bond of complex
[nBu4N]+[Ag(Ar)(CF3)3]− may also undergo heterolytic
cleavage to generate an aryl anion and the neutral intermediate
[Ag(CF3)3]. Nucleophilic attack of the aryl anion toward the
trifluoromethyl group in [Ag(CF3)3] gives Ar−CF3 (pathway
C in Figure 4). Pathway D involves the association of the
solvent to generate a five-coordinate intermediate, which then
undergoes reductive elimination to give Ar−CF3. Finally,
complex [nBu4N]+[Ag(Ar)(CF3)3]− may proceed through a
concerted reductive elimination process via a three-membered
Figure 3. X-ray structure of complex 3d. ORTEP drawing at 30%
probability; hydrogen atoms are omitted for clarity. Selected bond
lengths (Å) and bond angles (degrees): Ag(1)−C(1), 2.079(12);
Ag(1)−C(2), 2.104(10); Ag(1)−C(3), 2.069(8); Ag(1)−C(4),
2.048(7); C(1)−Ag(1)−C(2), 92.6(5); C(3)−Ag(1)−C(2),
90.6(5); C(4)−Ag(1)−C(1), 87.5(4); C(4)−Ag(1)−C(3), 89.2(3).
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clearly rule out the radical pathway A for reductive elimination
from complex 3b.
Second, to probe if the reductive elimination from complex
3b occurs via a ligand dissociation pathway, we studied the
reductive elimination of complex 3b at 85 °C in the presence
of 5.0 equiv of an “ate”-type nucleophile lithium phenyl n-butyl
pinacol boronate (Figure 6, eq 5). In a separate experiment, we
determined that transferring a phenyl group from the boronate
to complex [(Phen)Ag(CF3)3] 2 occurred readily at room
temperature to generate ionic Ag(III) complex [Ag(Ar)-
(CF3)3]− in 32% yield in CDCl3 and 80% yield in DMSO,
respectively (Figure 6, eq 6). Therefore, if the reaction
proceeds via a reversible ligand dissociation, a neutral three-
coordinated Ag(III) intermediate [Ag(Ar)(CF3)2] 8 or
[Ag(CF3)3] 8′ would form. A fast transfer of the phenyl
group from phenyl borate to coordinationally unsaturated
intermediate 8 or 8′, followed by reductive elimination from
these complexes, would then produce 2-fluoro-4-methyl-
benezene 4b or trifluoromethylbenzene. Experimentally, the
formation of 2-fluoro-4-methyl-benezene 4b or trifluorome-
thylbenzene was not observed as determined by 19F NMR and
GC/MS spectroscopies, indicating that the dissociation
pathway B for the reductive elimination from complex 3b is
unlikely.
To gain more support to disfavor the ligand dissociation
pathway, we studied the kinetics of the reductive elimination
from complex 3b, and it was found that the reaction followed
first-order kinetics. Eyring analysis of the reaction rate
constants at various temperature versus 1/T gave activation
parameters of ΔH⧧ = 27.81 1.97 kcal/mol and ΔS⧧ = 3.34
0.37 e.u. (Figure 7A). A small entropy change is against a rate-
determining ligand dissociation pathway, which is consistent
with our experiment result. Furthermore, DFT calculations
showed that the barrier for dissociation of a trifluoromethyl
anion from complex 3b to generate a neutral three-coordinated
silver(III) species 8 is 44.6 kcal/mol, suggesting that
dissociating a trifluoromethyl anion from complex 3b is a
rather difficult process (Figure 5). Thus, dissociation pathway
B or C for the Ar−CF3 bond-forming reductive elimination
from complex 3b was disfavored.
Figure 4. Possible pathways for reductive elimination from
organosilver(III) complexes [nBu4N]+[Ag(Ar)(CF3)3]− 3b.
transition state to give Ar−CF3 and [Ag(CF3)2]− (pathway E
in Figure 4).
First, to determine whether thermal decomposition of
complexes 3a−e proceeds via radical pathway A, we
investigated the reaction [nBu4N]+[Ag(2-fluoro-4-MeC6H4)-
(CF3)3]− 3b in the absence or presence of a radical scavenger
or single-electron transfer (SET) inhibitor. It was found that
reactions in the presence of 1.0 equiv of TEMPO or 1,4-
dinitrobenzene occurred after 3.0 h at 85 °C to give compound
4b in 92% and 89%, respectively (eq 4). These results showed
Third, to evaluate whether solvent was associated in the
C(sp2)-CF3 reductive elimination of complexes 3a−e, we
studied the reductive elimination of complex 3b in five
different solvents with different polarities and coordinating
abilities (CDCl3, ClCH2CH2Cl, DMF, DMSO, or PhCN)
(Figure 7b). Studies showed that reactions in all five solvents
occurred in full conversion after 1.0−12 h at 85 °C to give
compound 4b in 85−95% yields, respectively. If the reductive
elimination reaction occurs via a rate-limiting solvent
association pathway, reactions in coordinating solvents should
be faster than those in noncoordinating solvents. As shown in
Figure 7B, our kinetic studies of these reactions revealed that
reductive elimination in less polar solvents CDCl3 or
ClCH2CH2Cl (kobs = 5.05 × 10−4 s−1 and 4.19 × 10−4 s−1)
were faster than those in more polar solvent DMF or DMSO
(kobs = 2.36 × 10−4 s−1 and 2.96 × 10−4 s−1). In addition,
reaction in coordinated solvent PhCN (kobs = 1.97 × 10−4 s−1)
was slower than that in noncoordinating solvents such as
CDCl3 or ClCH2CH2Cl. These results render the solvent
association pathway D unlikely.
that there is no significant difference for reactions in the
presence or absence of the radical or SET inhibitors, suggesting
that the free radical is unlikely to be involved in the reductive
elimination from complex 3b.
To gain more evidence for the noninvolvement of free
radical in this process, we monitored the thermolysis process of
complex 3b by periodically drawing an aliquot of the reaction
mixture and then measured by electron paramagnetic
resonance (EPR) spectroscopy. Not surprisingly, no signal
related to radical was observed. In addition, DFT calculation
showed that with a bond strength of 35.5 kcal/mol, the Ag−
CF3 bond in complex 3b is relatively strong and difficult to
undergo homolytic cleavage. Even if the homolytic cleavage of
the Ag−CF3 bond in complex 3b occurs, the new formed
intermediate [AgII(Ar)(CF3)2]− 6 needs to overcome an
additional barrier of 11.9 kcal/mol to form a three-member
ring transition state 7-ts that leads to reductive elimination
(Figure 5). Thus, the overall activation free energy barrier
(ΔG‡ = 47.4 kcal/mol) for pathway A is much higher than that
of the concerted bond-forming pathway D (ΔG⧧ = 26.0 kcal/
mol) (Figure 5). These experimental and theoretical results
Mechanistic Proposal for Reductive Elimination from
Complex 3. The above experimental results, including
reactions in the radical or SET inhibitors, EPR studies, effect
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Figure 5. Calculated activation free energies for the reductive elimination of [nBu4N]+[Ag(Ar)(CF3)3]− and optimized structures for the transition
states 7-ts, 9-ts, and 10-ts are shown. Selected bond distances [Å] are provided.
Figure 6. Reaction of [nBu4N]+[Ag(2-fluoro-4-MeC6H4)(CF3)3]− 3b the presence of 5.0 equiv of an “ate”-type nucleophile lithium phenyl n-butyl
pinacol boronate.
of the temperature and solvents, and DFT calculation suggest,
that Ar−CF3 bond-forming reductive elimination from
complex [nBu4N]+[Ag(Ar)(CF3)3]− 3b proceed through a
concerted pathway via a three-membered transition state 10-ts,
which is pathway E in Figure 4. In addition, the calculated
enthalpy (ΔH⧧ = 26.4 kcal/mol) is only slightly less than the
experimental enthalpy (ΔH⧧ = 27.81 1.97 kcal/mol), which
supports that our rationale on the mechanism of the reductive
elimination from complex 3b is reasonable. It is worth noting
that reductive elimination from an analogous “ate”-type formal
Cu(III) complex [nBu4N][Cu(ArF)(CF3)3] also occurred via a
similar synergistic bond-breaking and bond-formation pathway.
Yet, reductive elimination from “ate”-type formal Cu(III)
complex [nBu4N]+[Cu(ArF)(CF3)3]− is much faster than that
from [nBu4N]+[Cu(ArF)(CF3)3]− because [nBu4N]+[Cu(Ar)-
(CF3)3]− bearing an aryl group instead of a polyfluoroaryl
group reductively eliminates quantitatively at room temper-
ature.15 This phenomenon is in line with the general trend in
organometallic chemistry that the first row transition metal
complexes reductively eliminate faster than second row metal
complexes.16
CONCLUSION
■
The isolation, characterization, and C(sp2)-CF3 reductive
elimination of shelf-stable square planar aryl(tris-
(trifluoromethyl))argentate(III) complexes [nBu4N]+[Ag(Ar)-
(CF3)3]− 3 were described. Detailed mechanistic studies,
including reactions in the presence of a radical or SET
inhibitor, EPR studies, effect of the temperature and solvents,
and DFT calculations, suggest that Ar−CF3 bond-forming
reductive elimination from complex 3 proceeds through a
concerted pathway via a three-membered transition state.
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University, Zhengzhou 450001, P. R. China; Email: lanyu@
zzu.edu.cn
Qilong Shen − Key Laboratory of Organofluorine Chemistry,
Shanghai Institute of Organic Chemistry, University of
Chinese Academy Sciences, Chinese Academy of Sciences,
Shanghai 200032, P. R. China; orcid.org/0000-0001-
5622-153X; Email: shenql@sioc.ac.cn
Authors
Zehai Lu − Key Laboratory of Organofluorine Chemistry,
Shanghai Institute of Organic Chemistry, University of
Chinese Academy Sciences, Chinese Academy of Sciences,
Shanghai 200032, P. R. China; orcid.org/0000-0001-
6114-9456
Shihan Liu − College of Chemistry and Molecular Engineering,
Zhengzhou University, Zhengzhou 450001, P. R. China
Xuebing Leng − Key Laboratory of Organofluorine Chemistry,
Shanghai Institute of Organic Chemistry, University of
Chinese Academy Sciences, Chinese Academy of Sciences,
Shanghai 200032, P. R. China; orcid.org/0000-0003-
3291-8695
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00195
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the financial support from
National Natural Science Foundation of China (21625206,
21632009, and 21822303).
Figure 7. (A) Eyring plot of reductive elimination of [nBu4N]+[Ag-
(Ar)(CF3)3]− 3b in CDCl3. (B) Effect of solvent polarity and
coordinating property on the reductive elimination reactions from
complex [nBu4N]+[Ag(Ar)(CF3)3]− 3b.
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Considering the privileged role of the trifluoromethyl group in
the field of medicinal chemistry in the search of lead
compounds for new discovery, the elucidation of the
mechanism for C(sp2)-CF3 bond-forming reductive elimina-
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00195.
■
sı
1H and 19F NMR of complexes 1, 2, 3a−e, and 5 (PDF)
X-ray structures of complexes 2, 3d, and 5 (XYZ)
Accession Codes
CCDC 1581352−1581353 and 1582389 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
Yu Lan − School of Chemistry and Chemical Engineering,
Chongqing University, Chongqing 400030, P. R. China;
College of Chemistry and Molecular Engineering, Zhengzhou
■
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