Homolytic cleavage of the O-Cu(II) bond: XAFS and EPR spectroscopy evidence for one electron reduction of Cu(II) to Cu(i)

Article abstract of DOI:10.1039/c6cc01413e

The investigation into the active copper(i) catalysts from copper(ii) precursors has become a fundamental and important task in copper catalysis. In this work, we demonstrate that the ^tBuO^- anion serves not only as a base but also as a mediator to promote the reduction of Cu(ii) to Cu(i) in copper catalysis. XAFS and EPR spectroscopy evidence the [Cu(O^tBu)₃]^- ate complex as the key intermediate which undergoes homolytic-cleavage of the O-Cu(ii) bond generating [Cu(O^tBu)₂]^- ate complex.

Full text of DOI:10.1039/c6cc01413e

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: H. Yi, G. Zhang, J.
Xin, Y. Deng, J. T. Miller, A. J. Kropf, E. E. Bunel, X. Qi, Y. Lan, J. Lee and A. Lei, Chem. Commun., 2016,
DOI: 10.1039/C6CC01413E.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/chemcomm

Page 1 of 5
ChemComm
View Article Online
DOI: 10.1039/C6CC01413E
Journal Name
RSCPublishing
COMMUNICATION
Homolytic cleavage of O-Cu(II) bond: XAFS and
EPR spectroscopy evidence for one electron reduction
of Cu(II) to Cu(I)
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2014,
Accepted 00th January 2014
a,§
a,§
a
a
b,e
Hong Yi, Guanghui Zhang, Jie Xin, Yi Deng, Jeffrey T. Miller, Arthur J.
b
b
c
c
d
a,b,f
Kropf, Emilio E. Bunel, Xiaotian Qi, Yu Lan, JyhꢀFu Lee, Aiwen Lei*
DOI: 10.1039/x0xx00000x
www.rsc.org/
t
ꢀ
The investigation into the active copper(I) catalysts from on this reduction process. Recently, the role of BuO anion as the
copper(II) precursors has become
a fundamental and electron donor has been uncovered and used in homolytic aromatic
7
important task in copper catalysis. In this work, we substitution reactions. This baseꢀpromoted homolytic aromatic
t
-
demonstrate that the BuO anion not only serves as base but substitution (BHAS) provides a new horizon for synthetic chemistry
also the mediate to promote reduction of Cu(II) to Cu(I) in in the preparation of biaryl products. The mechanistic research
copper catalysis. XAFS and EPR spectroscopy evidence revealed that the tert-butoxide anion worked not only as a base but
t
-
8
t
ꢀ
[
Cu(O Bu) ] ate complex as the key intermediate which also as a singleꢀelectron donor (Scheme 1a). Although BuO anion
3
t
ꢀ
undergoes homolytic-cleavage of O-Cu(II) bond generating in BHAS reactions has been well demonstrated, the role of BuO
t
-
[
Cu(O Bu) ] ate complex.
anion serving as the electron donor in copperꢀcatalyzed coupling
reactions has been less explored.
2
Copperꢀmediated coupling reactions have been powerful tools in
organic chemistry for the synthesis of biologically and
1
pharmaceutically useful compounds. Despite the wide usage of
copperꢀmediated C−C and C−heteroatom bond forming reactions,
understanding of the mechanism in copper chemistry still remains
2
limited. As can be seen from previous reports, Cu(I) and Cu(II) salts
and complexes are universally utilized as catalyst precursors, and
3
Cu(I) species are generally found to be the active catalysts, even
t
ꢀ
Scheme 1. (a) The single electron transfer between BuO anion and ligand in
when Cu(II) precursors are used. Only a few literatures are focused
t
ꢀ
4
homolytic aromatic substitution reactions, (b) BuO anion as an mediate to
on the transformation of precursors to active intermediates.
promote the reduction of Cu(II) to Cu(I).
Therefore, the investigation into the active copper(I) catalysts from
copper(II) precursors and the real active catalytic species became a
fundamental and important task.
Therefore, elucidation the role of tert-butoxide anion in copper
catalysis will shed light on the synthetic chemistry. In this work, we
demonstrate that the [Cu(O Bu) ] ate complex could be generated
from the reduction of Cu(II) precursor in presence of NaO Bu
Scheme 1b). Xꢀray absorption fine structure (XAFS) and electron
paramagnetic resonance (EPR) spectroscopic evidence was provided
for the formation of [Cu(O Bu) ] ate complex which undergoes
homolyticꢀcleavage of OꢀCu(II) bond to generate [Cu(O Bu) ] ate
t
ꢀ
t
t
The BuO anion (from BuONa, BuOK etc) has been widely used
t
ꢀ
2
5
as the base in copperꢀmediated coupling reactions. However, in
t
recent reported copperꢀcatalyzed CꢀH functionalization reactions
(
t
using NaO Bu as the base, both Cu(I) and Cu(II) catalyst precursors
6
were able to facilitate the transformations. It was proposed that the
t
ꢀ
3
t
ꢀ
BuO could act as an electron donor to reduce the Cu(II) species to
t
ꢀ
2
the active Cu(I) complex. But no solid evidence has been provided
complex.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 1

ChemComm
Page 2 of 5
View Article Online
COMMUNICATION
DOI: 10.1039/C6CC01413E
Journal Name
t
Our initial effort was focused on the reaction of excess of BuONa hyperfine parameters were Ax= 37 G, Ay = 25 G, and Az = 197 G,
with CuCl2 under nitrogen atmosphere at roomꢀtemperature. As indicating that the new species is mononuclear Cu(II) with nearly
10
shown in the EPR spectra (Figure 1a), an EPR silent species was axial symmetry and weakꢀfield oxygen ligands.
t
formed after adding 10 equivalent NaO Bu into the CuCl solution.
2
This observation prompted us to further probe the reaction process.
1
1
1
.4
.2
.0
As shown in the Xꢀray absorption near edge spectra (XANES,
Figure 1b), the obtained copper species has an edge energy of 8980.9
eV, which is consistent with the edge energy of Cu(I) complex. The
intense peak at about 8982 eV of Cu(I) typically suggests a linear
twoꢀcoordination environment. The extended Xꢀray absorption fine
structure (EXAFS) fitting results are consistent with the above
XANES analysis. As shown in Table 1, the obtained Cu (I) species
was determined to be twoꢀoxygen bonded at an average distance of
0.8
0.6
0
.4
.2
0
t
ꢀ
0.0
1
.83 Å suggesting the formation of an ate complex [Cu(O Bu) ] , in
2
t
ꢀ
8970
8980
8990
9000
9010
which two BuO coordinate to copper forming a linear Cu(I) ate
Energy (eV)
complex, similar to the structure of copper(I) fluorinated alkoxide
9
complex. These results revealed that the CuCl can be reduced to
2
t
Cu(I) ate complex in the presence of excess NaO Bu.
2800
3000
2500
3000
3500
Field / G
2
Figure 2. XANES spectra of CuCl standard sample, the reaction
o
2500
3000
Field / G
3500
4000
t
between CuCl
2
and NaO Bu in DMF at ꢀ50 C. (b) Xꢀband EPR spectral
t o
(
160K) of the reaction between CuCl and NaO Bu in DMF at ꢀ50 C.
2
Fitting of the Rꢀspace EXAFS spectrum revealed that the new Cu(II)
species was threeꢀcoordinate with an average bond distance of 1.86
Å, and no chloride was observed in the first coordination sphere
(Table 1). Recently the paramagnetic trigonalꢀplanar copper(II) ate
complexes using potassium alkoxide salt KOC(CH )(CF ) have
1
0
0
0
.2
.8
.4
3
3 2
11
been evidenced by Doerrer and coꢀworkers. Therefore, we assigned
II
t
ꢀ
this new Cu(II) species in our system as [Cu (O Bu) ] ate complex.
3
a
Table 1. Summary of EXAFS first shell fitting results
.0
8
edge or preꢀ
coordination
.97
8.98
8.99
9.00
9.01
OS
CN
d (Å)
edge energy
atom
Energy (keV)
(
eV)
t
Figure 1.
(a
) EPR spectra of the reaction between CuCl
BuONa (0.5 mmol) in 5.0 mL DMF; black line: CuCl , red line: reaction
system. ( ) XANES spectra of CuCl solid standard, CuCl solid standard
(0.2 mmol) and BuONa (2.0 mmol) in 2.0
2
(0.05 mmol) and
2
[Cu(O Bu) ]Na
8
(
980.9
edeg)
t
b
+1
+2
O
2.2
3.0
1.83
1.86
2
(DMF solution)
b
2
t
t
and reaction between CuCl
mL DMF.
2
[
Cu(O Bu)
3
]Na
8
977.2
c
O
(
DMF solution)
(
preꢀedge)
To gain more insights into this reduction process, we monitored
t
o
a
b
the reaction between CuCl and NaO Bu at ꢀ50 C. The operando IR
OS: oxidation state; CN: coordination number; d: bond distance. Rꢀspace
ꢀ1
2
spectra and fitting results (3.08 < k < 11.80 Å and 1.08 < R < 1.83 Å),
was also used to monitor the reaction process and generation of a
new species was observed immediately upon addition of excess
t
2
EXAFS spectra of reactions between 10 eq of BuONa with CuCl in DMF.
c
II
t
Fitting results of the RꢀSpace EXAFS spectrum of [Cu (O Bu)
3
]Na (FT: 2.51
t
o
BuONa to CuCl in DMF at ꢀ50 C (Fig. S3). The XANES spectrum
ꢀ1
2
ꢀ 11.55 Å , fitting range: 1.00 ꢀ 1.79 Å
gave a preꢀedge at 8977.5 eV, close to the 8977.2 eV value of CuCl2
in DMF confirming that the valence state of formed species is Cu(II)
t
Based on above results, reaction of CuCl with excess BuONa
2
II
t
ꢀ
o
I
t
ꢀ
(Fig. 2). Xꢀband EPR analysis of the new copper species at 160 K gave [Cu (O Bu) ] at ꢀ50 C, but [Cu (O Bu) ] at room
3
2
gave a set of axial signals with gx = gy = 2.001 and gz =2.242. The temperature. In situ XANES experiment was carried out to test
2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 3 of 5
Journal Name
ChemComm
View Article Online
COMMUNIC01AT13ION
DOI: 10.1039/C6CC 4 E
II
t
ꢀ
the hypothesis that [Cu (O Bu) ] was the key intermediate in the
3
II
t
ꢀ
reduction process. The [Cu (O Bu) ] was firstly prepared
between CuCl and BuONa at ꢀ50 C (Figure 3a, red line). As the
3
t
o
2
temperature increases, the formation of a Cu(I) compound with
edge energy of 8980.9 eV concomitant with consumption of
II
t
ꢀ
compound [Cu (O Bu) ] , indicating that the newly formed
3
ꢀ
Scheme 2. The proposed mechanism of reduction process between CuCl
NaO Bu
2
and
I
t
t
compound [Cu (O Bu) ] was the reduction product of
Cu (O Bu) ] . In addition, we also used inꢀsitu to probe this
3
2
II
t
ꢀ
[
process and the formation of tBuOH was well observed,
In conclusion, we have observed and evidenced that the reduction
t
providing further evidence for the reduction (Figure 3b). EPR of Cu(II) precursors into Cu(I) species promoted by NaO Bu
spectroscopy was also used to study the solvent effect on this utilizing XAFS and EPR spectroscopy. XAFS and EPR spectroscopy
t
ꢀ
process. We found that the solvents such as DMF, DMA or NMP also evidenced [Cu(O Bu)
can promote this reduction process. However, no reduction which undergoes hemolytic cleavage of OꢀCu(II) bond generating
] ate complex as the key intermediate
3
t
ꢀ
occurs when performing in dioxane, THF or toluene.
[Cu(O Bu)
]
2
ate complex. We believe these spectroscopic
investigations provide useful insight into the origin and structure of
copper species in solution, which would be helpful for the
understanding of the mechanism and future design of catalysts.
1
1
0
0
0
0
0
.2
.0
.8
.6
.4
.2
This work was supported by the 973 Program (2012CB725302),
the National Natural Science Foundation of China (21390400,
21520102003, 21272180 and 21302148), the Hubei Province
Natural Science Foundation of China (2013CFA081), the Research
Fund for the Doctoral Program of Higher Education of China
(20120141130002), and the Ministry of Science and Technology of
.0
8
China (2012YQ120060). The Program of Introducing Talents of
Discipline to Universities of China (111 Program) is also appreciated.
Use of the Advanced Photon Source was supported by the U. S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences, under Contract No. DEꢀAC02ꢀ06CH11357. MRCAT
operations are supported by the Department of Energy and the
MRCAT member institutions. This work was also funded by the
Chemical Sciences and Engineering Division, Argonne National
Laboratory. We also thank all the team members at the beamline
960
8970
8980
8990
9000
9010
Energy (eV)
1
7C1 of the National Synchrotron Radiation Research Center
(NSRRC) in Taiwan.Notes and references
a
College of Chemistry and Molecular Sciences, the Institute for Advanced
Studies, Wuhan University, Wuhan, Hubei 430072, People’s Republic of
China.
0
500
1000 1500 2000 2500 3000
t (s)
b
Chemical Sciences and Engineering Division, Argonne National
Laboratory, 9700 South Cass Ave., Argonne, Illinois 60439, United States.
c
Figure 3.
of BuOH by in-situ IR.
(
a
) XANES spectra of the reduction process. (
b
) The detection
School of Chemistry and Chemical Engineering, Chongqing University,
t
Chongqing 400030, People’s Republic of China.
d
National Synchrotron Radiation Research Center, Hsinchu 30076,
Based on the EPR, XAFS and in-situ IR results, we propose the
mechanism of reduction as follows: [Cu (O Bu) ] ate complexis
quickly generated between the reaction of CuCl and excess NaO Bu.
II
t
ꢀ
Taiwan
3
e
School of Chemical Engineering, Purdue University, 480 Stadium Mall
t
2
Dr., West Lafayette, Indiana 47907, United States.
II
t
ꢀ
f
Then [Cu (O Bu) ] goes through a homolyticꢀcleavage of OꢀCu(II) State Key Laboratory of Organometallic Chemistry, Shanghai Institute of
3
I
t
ꢀ
Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road,
bond and yields the linear Cu(I) ate complex [Cu (O Bu) ] and a
tertꢀbutoxyl radical. Density functional theory (DFT) calculation was
employed to study the reduction of [Cu (O Bu) ] to provide further
support. The reaction free energy is calculated to be 9.6 kcal/mol
2
Shanghai 200032, P. R. China
§
ꢀ
Hong Yi and Guanghui Zhang contributed equally
Electronic Supplementary Information (ESI) available: Experimental
II
t
3
†
procedure, detailed IR, EPR, and XANES/EXAFS results; detailed
endothermic, which indicates the reduction is feasible. The formed information on DFT calculation.. See DOI: 10.1039/c000000x/
tertꢀbutoxyl radical can abstract one hydrogen from the solvent to
generate BuOH.
t
1. G. Evano, N. Blanchard and M. Toumi, Chem. Rev., 2008, 108, 3054.
2
. (a) E. Sperotto, G. P. M. van Klink, G. van Koten and J. G. de Vries,
Dalton. Trans., 2010, 39, 10338; (b) F. Monnier and M. Taillefer,
Angew. Chem. Int. Ed., 2009, 48, 6954; (c) S. V. Ley and A. W.
Thomas, Angew. Chem. Int. Ed., 2003, 42, 5400; (d) J. Hassan, M.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 3

ChemComm
Page 4 of 5
View Article Online
COMMUNICATION
DOI: 10.1039/C6CC01413E
Journal Name
Sévignon, C. Gozzi, E. Schulz and M. Lemaire, Chem. Rev., 2002, 102
359.
. (a) S.ꢀL. Zhang, L. Liu, Y. Fu and Q.ꢀX. Guo, Organometallics, 2007,
, 4546; (b) A. Ouali, M. Taillefer, J.ꢀF. Spindler and A. Jutand,
,
1
3
2
6
Organometallics, 2007, 26, 65; (c) M. Mansour, R. Giacovazzi, A.
Ouali, M. Taillefer and A. Jutand, Chem. Commun., 2008, 6051; (d) G.
Lefèvre, G. Franc, A. Tlili, C. Adamo, M. Taillefer, I. Ciofini and A.
Jutand, Organometallics, 2012, 31, 7694; (e) R. Giri and J. F. Hartwig,
J. Am. Chem. Soc., 2010, 132, 15860.
4
5
. (a) G. Zhang, H. Yi, G. Zhang, Y. Deng, R. Bai, H. Zhang, J. T. Miller,
A. J. Kropf, E. E. Bunel and A. Lei, J. Am. Chem. Soc., 2014, 136, 924;
(
b) G. Franc and A. Jutand, Dalton. Trans., 2010, 39, 7873.
. (a) L. Zhang, J. Cheng, B. Carry and Z. Hou, J. Am. Chem. Soc., 2012,
34, 14314; (b) Y. Wei, H. Zhao, J. Kan, W. Su and M. Hong, J. Am.
1
Chem. Soc., 2010, 132, 2522; (c) H. Inomata, K. Ogata, S.ꢀi. Fukuzawa
and Z. Hou, Org. Lett., 2012, 14, 3986; (d) H. Q. Do and O. Daugulis,
J. Am. Chem. Soc., 2009, 131, 17052; (e) H. Q. Do and O. Daugulis, J.
Am. Chem. Soc., 2007, 129, 12404; (f) L. Chu and F. L. Qing, J. Am.
Chem. Soc., 2012, 134, 1298; (g) C. Dey, E. Larionov and E. P. Kundig,
Org. Biomol. Chem., 2013, 11, 6734; (h) C. Dey and E. P. Kundig,
Chem. Commun., 2012, 48, 3064.
6
7
. (a) N. Matsuda, K. Hirano, T. Satoh and M. Miura, Org. Lett., 2011, 13
,
2
860; (b) Q. Liu, P. Wu, Y. Yang, Z. Zeng, J. Liu, H. Yi and A. Lei,
Angew. Chem. Int. Ed., 2012, 51, 4666.
. (a) G.ꢀP. Yong, W.ꢀL. She, Y.ꢀM. Zhang and Y.ꢀZ. Li, Chem.
Commun., 2011, 47, 11766; (b) S. Yanagisawa, K. Ueda, T. Taniguchi
and K. Itami, Org. Lett., 2008, 10, 4673; (c) Y. Wu, S. M. Wong, F.
Mao, T. L. Chan and F. Y. Kwong, Org. Lett., 2012, 14, 5306; (d) C.ꢀL.
Sun, Y.ꢀF. Gu, W.ꢀP. Huang and Z.ꢀJ. Shi, Chem. Commun., 2011, 47
,
9
813; (e) C. L. Sun, H. Li, D. G. Yu, M. Yu, X. Zhou, X. Y. Lu, K.
Huang, S. F. Zheng, B. J. Li and Z. J. Shi, Nat Chem, 2010, 2, 1044; (f)
E. Shirakawa, K.ꢀi. Itoh, T. Higashino and T. Hayashi, J. Am. Chem.
Soc., 2010, 132, 15537; (g) Y. Qiu, Y. Liu, K. Yang, W. Hong, Z. Li, Z.
Wang, Z. Yao and S. Jiang, Org. Lett., 2011, 13, 3556; (h) W. Liu, H.
Cao, H. Zhang, H. Zhang, K. H. Chung, C. He, H. Wang, F. Y. Kwong
and A. Lei, J. Am. Chem. Soc., 2010, 132, 16737.
8
. (a) S. Zhou, G. M. Anderson, B. Mondal, E. Doni, V. Ironmonger, M.
Kranz, T. Tuttle and J. A. Murphy, Chem. Sci., 2014,
A. Jutand and A. Lei, Chem. Commun., 2015, 51, 545; (c) A. Studer
and D. P. Curran, Nat. Chem., 2014, , 765; (d) A. Studer and D. P.
5, 476; (b) H. Yi,
6
Curran, Angew. Chem. Int. Ed., 2011, 50, 5018.
9
1
1
. J. S. Lum, L. Tahsini, J. A. Golen, C. Moore, A. L. Rheingold and L. H.
Doerrer, Chem. Eur. J., 2013, 19, 6374.
0.A. E. King, B. L. Ryland, T. C. Brunold, S. S. Stahl, Organometallics
012, 31, 7948.
,
2
1. S. F. Hannigan, J. S. Lum, J. W. Bacon, C. Moore, J. A. Golen, A. L.
Rheingold and L. H. Doerrer, Organometallics, 2013, 32, 3429.
4
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 5 of 5
ChemComm
View Article Online
DOI: 10.1039/C6CC01413E
Author Contribution Statement
Aiwen Lei conceived the project. Hong Yi, Guanghui Zhang and Yi Deng performed the EXAFS experiments,
analyzed the data, and discussed the results. Hong Yi and Jie Xin performed the EPR and IR experiments. Jeffrey T.
Miller, Arthur J. Kropf, Emilio E. Bunel and Jyh-Fu Lee helped to analyse the XAFS data and improve the quality
of the paper. Xiaotian Qi and Yu Lan performed the DFT caclulations in this article. Hong Yi, Guanghui Zhang and
Aiwen Lei wrote the paper. Hong Yi and Guanghui Zhang wrote the Supplementary Information and contributed
other related materials. Hong Yi and Guanghui Zhang contributed equally to this work.