Labile Cu(I) catalyst/spectator Cu(II) species in copper-catalyzed C-C coupling reaction: Operando IR, in situ XANES/EXAFS evidence and kinetic investigations

Article abstract of DOI:10.1021/ja310111p

Insights toward the Cu-catalyzed C-C coupling reaction were investigated through operando IR and in situ X-ray absorption near-edge structure/extended X-ray absorption fine structure. It was found that the Cu(I) complex formed from the reaction of CuI with β-diketone nucleophile was liable under the cross-coupling conditions, which is usually considered as active catalytic species. This labile Cu(I) complex could rapidly disproportionate to the spectator Cu(II) and Cu(0) species under the reaction conditions, which was an off-cycle process. In this copper-catalyzed C-C coupling reaction, β-diketone might act both as the substrate and the ligand.

Full text of DOI:10.1021/ja310111p

Article
pubs.acs.org/JACS
Labile Cu(I) Catalyst/Spectator Cu(II) Species in Copper-Catalyzed C−
C Coupling Reaction: Operando IR, in Situ XANES/EXAFS Evidence
and Kinetic Investigations
Chuan He,† Guanghui Zhang,†,‡ Jie Ke,† Heng Zhang,† Jeffrey T. Miller,‡ Arthur J. Kropf,‡
and Aiwen Lei*,†,‡
†College of Chemistry, Molecular Sciences, Wuhan University, Wuhan, 430072, People's Republic of China
‡Chemical Science, Engineering Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, Illinois 60439, United States
S
* Supporting Information
ABSTRACT: Insights toward the Cu-catalyzed C−C cou-
pling reaction were investigated through operando IR and in
situ X-ray absorption near-edge structure/extended X-ray
absorption fine structure. It was found that the Cu(I) complex
formed from the reaction of CuI with β-diketone nucleophile
was liable under the cross-coupling conditions, which is usually
considered as active catalytic species. This labile Cu(I)
complex could rapidly disproportionate to the spectator
Cu(II) and Cu(0) species under the reaction conditions,
which was an off-cycle process. In this copper-catalyzed C−C coupling reaction, β-diketone might act both as the substrate and
the ligand.
Scheme 1. Active Copper Catalyst in C−N and C−C
INTRODUCTION
■
Coupling Reactions
Since the pioneering and remarkable work reported by
Ullmann and Goldberg in the last century, the copper-mediated
C−C and C−heteroatom bond forming reactions have been
well developed and extensively applied in academic and
industrial areas.1 However, there is still a general lack of
knowledge about the mechanisms due to the easy accessibility
of copper to multiple oxidation states and limited stability of
compounds with C−Cu or N−Cu bonds.1a−c,f In these Cu-
driven coupling processes, especially for C−heteroatom bond
formations, the starting copper salts or copper complexes are
usually used in various oxidation states including Cu(0), Cu(I),
and Cu(II). It seems that almost any copper salt could be used
as the precursor for the coupling reactions with similar
behavior.1h,2 Revealing the real active catalyst species must be
the primary and fundamental task for the mechanistic
investigations.
Early studies indicate that Cu(I) species might be the active
catalyst in the copper-catalyzed C−heteroatom coupling
reactions.2,3 Recently, some insights toward copper-catalyzed
arylation for C−N bond formation were discussed.3c−e,g,i,m,o
Copper(I) amidate complexes of chelating 1,2-diamine ligands
were proposed as the probable intermediates in the copper-
catalyzed amidations (Scheme 1). Notably, Cu(0) precursor
has been demonstrated to be oxidized by aryl halides to afford
the active Cu(I) complex in the Ullmann-type reactions by
using electrochemical techniques.3k Meanwhile, Cu(II) pre-
cursor could be reduced by alcohols or amines to generate
active Cu(I) species in the reaction monitored by UV−vis and
NMR spectroscopy.3j Besides the C−heteroatom couplings,
copper-catalyzed arylation of C nucleophiles has also been used
as one of the important C−C bond forming reactions.1c,d,f,4
Some significant and efficient copper-catalyzed Hurtley-type
arylations of β-dicarbonyl compounds have been reported in
recent years.3h,5 However, scarce examples were reported
regarding the active catalyst and reaction mechanism (Scheme
1).6 Herein, we communicate our direct observation of a
complex formation between CuI and a β-diketone nucleophile
monitored by operando IR and in situ X-ray absorption near-
edge structure (XANES)/extended X-ray absorption fine
structure (EXAFS).7 We also identified that such a Cu(I)
complex is very labile, which would rapidly disproportionate
Received: October 17, 2012
© XXXX American Chemical Society
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into Cu(0) and Cu(acac)2 as an off-cycle process in the
reaction. Both experimental results and computational
calculations indicated that the Cu(I) β-diketonate complex
might be the active catalyst in the arylation reactions.
RESULTS AND DISCUSSION
■
Copper-Catalyzed C−C Bond Forming Reaction. Due
to the interest in the mechanism of copper-catalyzed C−C
couplings, our initial effort was focused on the arylation of a C
nucleophile using aryl iodides and β-diketones. In the early
studies, we found that the direct arylation product was not easy
to obtain under the typical conditions.5g Under anhydrous
conditions, aryl iodide 1a could react with acetylacetone 2a
affording the direct arylation product 3a. The results are shown
in Figure 1. Using 10 mol % CuI as the catalyst precursor, the
Figure 2. (a) Overall three-dimensional Fourier transform IR (3D-
FTIR) profile of the stepwise stoichiometric reaction. (b)
Amplification of the 3D-FTIR profile.
Figure 1. Kinetic profiles by employing different amounts of copper
catalyst loadings. Reactions were monitored by operando IR. The
yields of 3a were determined by gas chromatography with naphthalene
as the internal standard.
consumed, and a peak of K(acac) was accumulated. Then,
addition of a stoichiometric amount of CuI to the mixture
resulted in the consumption of K(acac) immediately and the
formation of a new peak. At first glance, we deduced this new
peak as the [Cu+(acac)Ln] complex. However, the shape of the
new peak was unusual (black frame in Figure 2a). Meanwhile,
its ConcIRT spectrum9 even looks like the spectrum of
Cu(II)(acac)2.
After careful reinvestigation of this new peak, we found that
its ConcIRT spectrum was broader in the 1606−1552 cm−1
region than the authentic Cu(II)(acac)2. Amplification of this
area disclosed very important information. As shown in Figure
2b, the peak of K(acac) at 1616 cm−1 was fully consumed
within 10 s after the addition of CuI. Meanwhile, a peak at 1594
cm−1 was immediately accumulated to its maximum value also
within 10 s. It is noteworthy that the peak at 1594 cm−1 was
then decreasing rapidly, and another new peak at 1582 cm−1
was formed. In addition, we also clearly observed the formation
of precipitate in the reaction vessel that is speculated as Cu
metal.
Further data analysis with the iC IR software revealed more
clear kinetic information, and the results are shown in Figure 3.
The kinetic profiles of ConcIRT represent the concentration
versus time, and the “component” represents a single
compound. Three components were produced and detected
from the stoichiometric reaction between 2a, K3PO4, and CuI.
Component 1 immediately decreased as soon as the addition of
CuI. We could assign component 1 as the K(acac) complex I
(1616 and 1511 cm−1, see the Supporting Information, Figure
S1, also confirmed by 13C NMR, Supporting Information,
Figure S9). During the consumption of K(acac)(component 1),
reaction provided the C−C cross-coupling product 3a in 48%
yield. To improve the reaction yields, we initially tried to
employ higher catalyst loading. The reaction using 30 mol %
CuI gave 49% yield, and the reaction of 50 mol % CuI afforded
50% yield. When even 100 mol % CuI was employed, the
reaction yield was only 56%, which is quite strange and
unanticipated. No obvious improvement has been achieved
when more copper catalyst is employed. To gain more detailed
information about this C−C bond forming reaction, kinetic
experiments monitored by operando IR were carried out. As
shown in Figure 1, it is interesting that the reactions not only
have similar yields but also behave with similar kinetic features.
The rate of this transformation was not accelerated by
increasing the CuI catalyst. These results indicate that most
of the CuI, which was added as the catalyst, is a spectator. An
unknown copper species, which originated from CuI in the
reaction solution, is the catalytically active species, and its
concentration would not vary significantly with increasing the
amount of CuI.
Stepwise Stoichiometric Reaction between Acetyla-
cetone 2a, K3PO4, and CuI. To elucidate the nature of the
copper catalyst in this C−C bond forming reaction, the
stepwise stoichiometric reactions between acetylacetone 2a,
K3PO4, and CuI were monitored by using operando IR.8 The
reaction vessel was first charged with 2a in DMSO, followed by
sequential addition of K3PO4 and CuI. As shown in Figure 2a,
after the addition of K3PO4, the broad peak of 2a was
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Figure 3. (a) Kinetic profiles of the stoichiometric reaction between
K(acac) and CuI. (b) Amplification of IR profile. (c) The ConcIRT
spectra of components 2 and 3.
one could observe the rapid formation of component 2. It is
interesting to note that component 2 seemed to be an
intermediate, which decreased readily. Meanwhile, a new
component, 3, was formed simultaneously (Figure 3a). By
comparing the ConcIRT spectrum with the authentic sample,
component 3 was assigned to be Cu(II)(acac)2 complex III
(Figure 3c, 1582 and 1523 cm−1, see the Supporting
Information, Figure S4).
As shown in Figure 3b, when CuI was introduced, the
consumption of component 1 only generated component 2.
However, without further addition of reactants, the con-
sumption of component 2 could occur, and it generated
component 3, which we preliminary assigned as Cu(II)(acac)2
and some Cu-metal-like precipitate. That is to say, the
compound represented by component 2 could disproportionate
to Cu(0) and Cu(II)(acac)2.10 Thus, it is reasonable to figure
out that component 2 might be the [Cu+(acac)Ln] complex II
(Figure 3c, 1594 and 1511 cm−1). In a word, Cu(I) complex II,
which rapidly formed from the reaction of K(acac) and CuI,
was labile (Scheme 2).
Figure 4. (a) Normalized Cu XANES spectra. Line 1: the solution of
the reaction of 2a and K3PO4 with CuI. Line 2: the solution of
authentic Cu(acac)2. Line 3: the precipitate of the reaction of 2a and
K3PO4 with CuI. Line 4: Cu foil. Line 5: the solution of the reaction of
2a, K3PO4, and CuI with PPh3. (b) k2-weighted R-space EXAFS
spectra of the solution of the reaction of 2a and K3PO4 with CuI (2.8
< k < 10.8 Å−1 and 0.9 < R < 1.8 Å). c) k2-weighted R-space EXAFS
spectra of the precipitate of the reaction of 2a and K3PO4 with CuI
(2.7 < k < 12.1 Å−1 and 1.6 < R < 2.7 Å).
In addition, the EXAFS data were also collected for the same
reaction. The k2-weighted R-space spectra and a fit of the
reaction solution are shown in Figure 4b. In the reaction
solution, the Cu species was observed as four-oxygen bonded at
a distance of 1.92 Å, which is identical to that of the DMF
solution of Cu(acac)2. Meanwhile, the k2-weighted R-space
spectra of the reaction precipitate and the fit are shown in
Figure 4c. From the first shell fits, a Cu−Cu bond distance of
2.55 Å was determined. The coordination number of 10.5
indicates that the average particle size is about 6−7 nm.11 The
EXAFS also provides direct evidence that the reaction of 2a and
K3PO4 with CuI resulted in the formation of Cu(acac)2 and
metallic Cu.
Scheme 2. Stoichiometric Reaction between K(acac) and
CuI
Complex II, which is speculated as [Cu+(acac)Ln], is labile,
and it is difficult to directly elucidate its structure even under
mild conditions. The employment of extra ligand in the
reaction might affect the disproportionation of the labile
complex II and provide some structural information. Thus,
further experiment using PPh3 as the extra ligand was carried
out. As shown in Figure 5a, when PPh3 and CuI were
introduced, complex I K(acac) (component A) was consumed
rapidly, while a new component B was generated. Component
B has nearly the same ConcIRT spectrum with complex II
[Cu+(acac)Ln] in the region 1500−1620 cm−1 (see the
Supporting Information, Figure S6) and was assigned as
Furthermore, in situ X-ray absorption experiments were
carried out toward the resulted solution and precipitate of the
reaction of acetylacetone 2a and K3PO4 with CuI in DMF. As
shown in Figure 4a, the Cu K-edge XANES spectrum of the
reaction solution (line 1) with an edge energy of 8983.7 eV
confirms the generation of Cu(II) species. Meanwhile, the edge
energy (8979.0 eV) and features of the reaction precipitate
(line 3) was characteristic of metallic Cu (line 4). The XANES
provides direct evidence that the reaction of 2a and K3PO4 with
CuI resulted in the formation of Cu(acac)2 and metallic Cu.
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To further assess the potential structure of the complex II
[Cu+(acac)Ln] species, we computed the relative free energy of
several possible forms of Cu(I) and Cu(II) species in the
reaction solution (see the Supporting Information, Schemes S1
and S2). The tetrahedral-structured ate complex containing two
acetylacetonate molecules most probably exists in the reaction.
The density functional theory (DFT)-calculated IR absorbances
of [Cu(I)(acac)2]− and Cu(II)(acac)2 were nicely in
accordance with the experimental results in Figure 3c (Figure
6). The peaks in the 1620−1550 cm−1 region were the ν(C
Figure 5. (a) Kinetic profiles of the reaction between 2a, K3PO4, CuI,
and PPh3. Reaction was monitored by operando IR. (b) k2-weighted R-
space EXAFS spectra of Cu(acac)2 (2.8 < k < 10.8) and the reaction of
2a, K3PO4, and CuI with PPh3 (2.5 < k < 11.2).
complex IV [Cu(I)(acac)(PPh3)n]. Kinetic profiles revealed
that complex IV did not turn to Cu(II)(acac)2 as quick as
complex II under the same conditions (Figure 5a). These
results provided further evidence that we had directly observed
the labile [Cu+(acac)Ln] species by using operando IR, and the
IR absorbances at 1594 and 1511 cm−1 were its parts of the IR
spectroscopic spectrum.
Figure 6. DFT calculation of the potential structure of complex II
[Cu+(acac)Ln] species and the IR absorbances of Cu(I) complex II
and Cu(II) complex III.
Moreover, the reaction of 2a, K3PO4, and CuI in the
presence of PPh3 was also investigated by the hard X-ray
absorption experiment. As shown in Figure 4a, the edge energy
of the solution with PPh3 (line 5) is shifted to 8980.4 eV and
has a pre-edge feature that is common for Cu+ compounds. In
addition, it is clearly different from the reaction solution
without PPh3 that is identical to that of Cu(acac)2 (Figure 5b).
The magnitude peak for the solution with PPh3 is shifted to a
longer distance, which is consistent with both Cu−O and Cu−
P scatters. The Cu−P bond distance is longer than that of Cu−
O. In addition, the different bond distance, phase, and
amplitude result in interference of the two scatters leading to
smaller peaks. The fits indicate there are two Cu−O bonds at
2.03 Å, that is, longer than that in Cu(acac)2 and two Cu−P
bonds at 2.21 Å in the Cu coordination sphere. In addition,
there is clearly no Cu−I bonds in the solution complex. In the
presence of PPh3, Cu does not disproportionate due to the
PPh3 coordination. The in situ XANES/EXAFS experiments
revealed that the structure of CuI with 2a, K3PO4, and PPh3 in
DMF is Cu(acac)(PPh3)2.
O) stretching modes for the two copper β-diketonate
complexes, while the peaks in the 1530−1460 cm−1 region
were the ν(CC) stretching modes.
Stoichiometric Reactions Involving Aryl Iodide 1a. To
gain further insight of the Cu-catalyzed C−C coupling reaction,
the reaction of 2a (1.0 mmol), K3PO4 (1.5 mmol), CuI (0.5
mmol), and ethyl 4-iodobenzoate 1a (0.5 mmol) was
investigated using operando IR. The kinetic profiles were
compiled in Figure 7. The generation of the arylation product
Figure 7. Kinetic profiles of the reaction of 2a, K3PO4, CuI, and 1a.
If this short-life [Cu+(acac)Ln] species is the active catalyst in
the C−C coupling reaction, the facile disproportionation will
definitely cut down the reaction efficiency. Because PPh3 can
increase the stability of the liable [Cu+(acac)Ln] complex II, it
may be beneficial for the catalytic C−C coupling reaction.
Unfortunately, the yields were even lower by employing PPh3
in the reaction. This result indicated that increasing the stability
of the intermediate catalyst would also decrease the reactivity in
the catalytic reaction. Actually, some other common ligands
were tested in the reactions. However, almost all those ligands
suppressed the reaction more or less (see the Supporting
Information, Table S1).
3a (orange curve) paralleled the consumption of 1a (violet
curve) (see the Supporting Information, Figures S7 and S8)
and Cu(II)(acac)2. This again indicated that, even under this
condition, the formation of Cu(II)(acac)2 was facile. However,
the direct stoichiometric reaction between Cu(II)(acac)2 and
1a had also been examined (eq 1); no product 3a was observed
under the standard conditions.
From Figure 7, the mixture of 2a, K3PO4, CuI, and 1a would
immediately generate Cu(II)(acac)2, and no [Cu(I)(acac)2]−
complex II is observed in the IR spectrum. The slow reaction,
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Cu(0) and Cu(II) species. Then, ArX could oxidatively add to
Cu(I) complex II to generate a Cu(III) intermediate V. The
resulting species underwent reductive elimination that would
finally produce the corresponding product. Interestingly, in this
copper-catalyzed transformation, acetylacetone acted both as
the substrate and the ligand.
CONCLUSION
■
In conclusion, (1) we have directly observed and evidenced the
labile complex of Cu(I) and a C nucleophile by operando IR
and in situ XANES/EXAFS. (2) Kinetic studies and computa-
tional calculations revealed that this labile β-diketone Cu(I)
species might be the active catalyst for the copper-catalyzed C−
C coupling reaction. The labile Cu(I) species could rapidly
disproportionate to the spectator Cu(II) and Cu(0) species in
the reaction, which was an off-cycle process. (3) PPh3 could
stabilize the labile Cu(I) β-diketonate species. However, it also
inhibited the reactivity of the C−C coupling reaction. (4) In
this copper-catalyzed arylation, β-diketone might act both as
the substrate and the ligand. We believe that these kinetic
investigations provide a useful insight into the nature of
catalytic reaction, which will be helpful for the understanding of
the mechanism and for the synthetic applications.
which led to the C−C coupling product 3a, was different from
eq 1. All these results indicated that the real catalytic species in
the reaction of 2a, K3PO4, CuI, and 1a was only a trace amount,
although 10, 30, 50, or even 100 mol % of the catalyst precursor
was added.
To elucidate whether Cu(0) is the active catalyst for the
transformation, a Hg(0)-poisoning experiment by adding excess
Hg(0) was carried out. Generally, Hg(0) strongly poisons
metal(0) catalysts by amalgamating the metal or adsorbing on
its surface, which would obviously suppress the catalytic
reaction.12 As shown in (eq 2), 30 equiv of Hg(0) (300
equiv vs Cu catalyst) had no effect on the reaction. These
experiments suggest that Cu(0) might not be the active catalyst.
Discussion and Proposed Mechanism. The investiga-
tions of these stoichiometric reactions inspired us to rationalize
the “abnormal” reaction yields of different CuI loading and the
kinetic observation in Figure 1. Although different amounts of
CuI were employed, only the minority Cu(I) complex II might
play a role as the actor catalyst. It is possible that there is an off-
cycle process in this copper-catalyzed reaction (Scheme 3) that,
no matter how much copper was used, the reaction rate and
yield would not vary too much (Figure 1).
ASSOCIATED CONTENT
* Supporting Information
■
S
General information, experimental details and characterization,
and computational calculation details. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
aiwenlei@whu.edu.cn
■
a
Scheme 3. Putative Mechanism of the Catalytic Cycle
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the “973” Project from the MOST
of China (2011CB808600) and the National Natural Science
Foundation of China (21025206, 20832003, 20972118,
20972119). The authors also thank the support from “the
Fundamental Research Funds for the Central Universities”,
Program for New Century Excellent Talents in University
(NCET) and Program for Changjiang Scholars and Innovative
Research Team in University (IRT1030). Use of the Advanced
Photon Source was supported by the U.S. Department of
Energy, Office of Basic Energy Sciences, under contract no. DE-
AC02-06CH11357. MRCAT operations are supported by the
Department of Energy and the MRCAT member institutions.
Partial funding for J.T.M. was provided by Chemical Sciences,
Geosciences and Biosciences Division, U.S. Department of
Energy, under contract no. DE-AC0-06CH11357. This work
was also funded by the Chemical Sciences and Engineering
Division at Argonne National Laboratory. In addition, we thank
Prof. Mei-Xiang Wang (Tsinghua University) for providing
Cu(III) sample as gift.
a
TS-OA = transition state of oxidative addition. TS-RE = transition
state of reductive elimination.
To assess the proposed reaction pathway for the activation of
aryl iodide, we also computed the energy barrier for oxidative
addition of ArI to Cu(I) complex II. The free energy of
activation for oxidative addition of ArI to the Cu(I) complex to
form the Cu(III) intermediate was calculated to be 27.9 kcal
mol−1 (see the Supporting Information, Figure S10). This
computational result indicated that the Cu(III) species lie at
energy that is accessible under this conditions.6 Accordingly,
the putative mechanism was shown in Scheme 3. The reaction
was initiated by nucleophilic substitution to form labile Cu(I)
complex II, which would easily disproportionate to the off-cycle
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(8) Usually, β-diketone compounds were employed as ligands in
copper-catalyzed reactions,3i for which the nucleophilic substitution
might occur first to form the complex of copper and nucleophile.
(9) Relative concentration profiles are calculated by ConcIRT LIVE
for products, intermediates, and starting materials. ConcIRT LIVE is
especially valuable for trending chemical species that have overlapping
peaks. ConcIRT LIVE uses a type of mathematical algorithm known as
curve-resolution. Curve-resolution algorithms have the capability to
group wavenumber values that change absorbance intensity in the
same manner. For each group, ConcIRT LIVE calculates the
F
dx.doi.org/10.1021/ja310111p | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX