Iron carbonyl cluster-incorporated Cu(i) NHC complexes in homocoupling of arylboronic acids: An effective [TeFe3(CO)9]2- ligand

Article abstract of DOI:10.1039/c5dt03242c

A new type of TeFe₃(CO)₉-incorporated dicopper NHC complex was obtained directly from one-pot reactions. By the introduction of the cluster anion [TeFe₃(CO)₉]^2- and NHCs as the ligands, these di-Cu(i)-based complexes exhibited pronounced catalytic activities toward the homocoupling of arylboronic acids with low Cu loadings and high yields (up to 98%).

Full text of DOI:10.1039/c5dt03242c

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Iron carbonyl cluster-incorporated Cu(I) NHC
complexes in homocoupling of arylboronic
Cite this: Dalton Trans., 2015, 44,
16675
acids: an effective [TeFe3(CO)9]2− ligand†
Received 22nd August 2015,
Accepted 25th August 2015
Chien-Nan Lin,a Chung-Yi Huang,a Chia-Chi Yu,a Yen-Ming Chen,a Wei-Ming Ke,a
Guan-Jung Wang,a Gon-Ann Lee*b and Minghuey Shieh*a
DOI: 10.1039/c5dt03242c
www.rsc.org/dalton
A new type of TeFe3(CO)9-incorporated dicopper NHC complex inexpensive Cu-catalysts without the need for additives.8a,b,9,13
was obtained directly from one-pot reactions. By the introduction The mechanism for Cu-catalyzed homocoupling reactions of aryl-
of the cluster anion [TeFe3(CO)9]2− and NHCs as the ligands, these boronic acids is not yet obvious, but, a plausible mechanism
di-Cu(I)-based complexes exhibited pronounced catalytic activities that involves the formation of an unstable Cu(III) state under
toward the homocoupling of arylboronic acids with low Cu load- an atmosphere of O2 has been proposed.8a,13a–c With our inter-
ings and high yields (up to 98%).
est in anionic iron carbonyl chalcogenides,2c we attempted to
introduce the electron-donating and bulky cluster anion
[TeFe3(CO)9]2– (ref. 14) or/and NHCs as the ligands for the cata-
lytic center Cu(I) to study the ligand effect. Herein, we report
our design of a series of TeFe3(CO)9-incorporated mono-
or bidentate-NHC di-Cu(I) complexes, forming a new type
of ternary Te–Fe–Cu NHC carbonyl complex, such as
[(μ4-Te)Fe3(CO)9Cu2(Me2Im)2] (1), [(μ3-Te)Fe3(CO)9Cu2-
(MeImCH2ImMe)] (2), [(μ4-Te)Fe3(CO)9Cu2(MeIm(CH2)2ImMe)]
(3), and [(μ4-Te)Fe3(CO)9Cu2(MeIm(CH2)3ImMe)] (4), which
were first used as Cu(NHC)-containing catalysts in the homo-
coupling of arylboronic acids with low Cu loadings (0.5 or
1.0 mol%) and high turnovers per copper (54–196). These were
the highest values compared with other reported homo-
couplings of arylboronic acids (TONs per copper <50) when
using homogeneous Cu-based catalysts, and the value of 196
was the highest among metal-based cases.8b,9–13
Heterometallic clusters, comprising two or more different tran-
sition metals, are of great interest because of their special pro-
perties1 and applications in magnetism,1,2 nanoscience,1,2b,3
and catalysis.1,4 Recently, the study of N-heterocyclic carbenes
(NHCs) as ancillary ligands in organometallic catalysis has
become a flourishing field of research.5 On the other hand,
many bulky metal carbonyl clusters with π-accepting CO
groups are known as electron reservoirs2b,c,6 and are expected
to act as potential ligands to modulate electronic structures of
their resultant catalysts. Although quite a few metal carbonyl
NHC clusters have been reported,7 only some heterometallic
ones were characterized, presumably due to the lack of sys-
tematic syntheses.7d–h Until now, transition metal carbonyl
NHC clusters are yet to be used as catalysts for organic
reactions.
Symmetrical biaryls are important organic compounds for
synthetic and medicinal chemistry.8 A convenient synthetic
route to symmetrical biaryls was via the Suzuki homocoupling
reaction.9 Despite the fact that some precious metal(Pd,9,10
Rh,9,11 Au9,12)-mediated Suzuki homocoupling reactions have
been reported with good catalytic activities, additional addi-
tives or ligands are usually required in these catalytic systems.
Recently, an alternative route was achieved by the use of
A one-pot synthesis of the NHC-incorporated TeFe3Cu2-
based complex [(μ4-Te)Fe3(CO)9Cu2(Me2Im)2] (1) was achieved
from the reaction of [(μ3-Te)Fe3(CO)9Cu2(MeCN)2]15 and 1,3-di-
methylimidazolium iodide (Me2Im·HI) along with KOtBu in a
1 : 2 : 2 ratio in THF at room temperature (Scheme 1). As shown
in Fig. 1, complex 1 consisted of a distorted TeFe3(CO)9Cu tri-
gonal-bipyramidal core with a TeFe2 ring in the equatorial
plane, in which the Fe2Cu triangular plane was further capped
aDepartment of Chemistry, National Taiwan Normal University, 88, Sec. 4, Tingchow
Road, Taipei 11677, Taiwan, Republic of China. E-mail: mshieh@ntnu.edu.tw
bDepartment of Chemistry, Fu Jen Catholic University, 510, Zhongzheng Road,
Xinzhuang, New Taipei 24205, Taiwan, Republic of China.
E-mail: 016850@mail.fju.edu.tw
†Electronic supplementary information (ESI) available: Experimental details,
computational details, spectroscopic data, and detailed experimental procedure
for the catalytic reactions. CCDC 1011560–1011563. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c5dt03242c
Scheme 1 One-pot synthesis of the monodentate-NHC complex 1.
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Fig. 1 ORTEP of 1 at 30% probability; hydrogen atoms are omitted for
clarity.
Scheme 2 The synthesis of bidentate-NHC complexes 2–4.
[PF6]2,18 was synthesized and its catalysis toward homo-
coupling of 4-bromophenylboronic acid was performed as well.
As shown in Table 1, [Et4N]2[TeFe3(CO)9] (entry 1) exhibited
no activity toward 4-bromophenylboronic acid. However, when
[Cu(MeCN)4][BF4]19 (entry 2) was employed, 47% yield of 4,4′-
dibromobiphenyl was obtained, revealing that Cu was essential
for the catalytic performance. Interestingly, [Cu2(MeIm-
(CH2)2ImMe)2][PF6]2 and [TeFe3(CO)9Cu2(MeCN)2] (entries 3
and 4) exhibited excellent catalytic yields (85%, 8 h; 85%, 3 h),
respectively, which showed that bidentate-NHC and
[TeFe3(CO)9]2− were good ancillary ligands for these di-Cu-
based catalysts. Although having similar TONs, [TeFe3(CO)9-
by another copper atom with the two Cu atoms covalently
bonded (2.5949(9) Å). Each of the two copper atoms co-
ordinated with a Me2Im ligand with an average Cu–Ccarbene
bond of 1.933(5) Å which was within the range of the reported
Cu–Ccarbene bonds (ca. 1.80–2.20 Å) in Cu(I)–NHC complexes.16
[TeFe3(CO)9Cu2(MeCN)2] was slightly air-sensitive, but complex
1 could survive in air for 10 days, which was confirmed by
powder X-ray diffraction (PXRD) (Fig. S1, ESI†), indicating that
the NHC ligand could help stabilize the TeFe3(CO)9Cu2 core.
Further, a similar methodology was also applied to the con-
struction of the bidentate-NHC complexes, [(μ3-Te)Fe3(CO)9-
Cu2(MeImCH2ImMe)] (2), [(μ4-Te)Fe3(CO)9Cu2(MeIm(CH2)2-
ImMe)] (3), and [(μ4-Te)Fe3(CO)9Cu2(MeIm(CH2)3ImMe)] (4), by
the treatment of [(μ3-Te)Fe3(CO)9Cu2(MeCN)2] with diimid-
azolium salts, 1,1′-dimethyl-3,3′-methylene-diimidazolium
diiodide (MeImCH2ImMe·H2I2), 1,1′-dimethyl-3,3′-ethylene-
diimidazolium dibromide (MeIm(CH2)2ImMe·H2Br2), and
1,1′-dimethyl-3,3′-propylene-diimidazolium diiodide (MeIm-
(CH2)3ImMe·H2I2), and KOtBu in a 1 : 1 : 2 ratio at room temp-
erature in 11–21% yields. The yields of 2–4 could be improved
to 40–51% if [TeFe3(CO)9Cu2(MeCN)2] was treated with MeIm
(CH2)nImMe carbenes (n = 1–3) in situ from the deprotonation
reaction of diimidazolium salts with KOtBu in THF
(Scheme 2). Similar to 1, complexes 2–4 each possessed a
TeFe3(CO)9Cu2 core with the Cu–Cu bond further bridged with
MeIm(CH2)nImMe (n = 1–3) (Fig. S2–S4, ESI†). According to
the data available at the Cambridge Crystallographic Data
Centre,17 complexes 1–4 represented the first examples of
NHC-containing mixed-metal carbonyl chalcogenide complexes.
To study the catalytic activity of [TeFe3(CO)9Cu2(MeCN)2]
and 1–4 toward arylboronic acids, 4-bromophenylboronic acid
was chosen as a model substrate. All reactions were monitored
by TLC to ensure complete conversion of the starting
materials. The reaction conditions were optimized to include
the use of 1.0 mol% of Cu loading, methanol as the solvent,
room temperature, an atmosphere of O2, and without any
additives such as bases, ligands, or other oxidants (Table 1
and Table S1, ESI†). For comparison, the di-MeIm(CH2)2-
ImMe-bridged dicopper(I) complex, [Cu2(MeIm(CH2)2ImMe)2]-
Cu2(MeCN)2] had higher turnover frequency (TOF = 28.3 h−1
)
than that for [Cu2(MeIm(CH2)2ImMe)2][PF6]2 (10.6 h−1), indi-
cating that [TeFe3(CO)9]2− functioned as a better ligand than
the bidentate-NHC ligand (MeIm(CH2)2ImMe) in this catalytic
reaction. When both NHC and [TeFe3(CO)9]2− were introduced,
the catalytic efficiencies (∼44.0 h−1) of catalysts 1–4 (entries
5–8) were significantly higher than [Cu2(MeIm(CH2)2ImMe)2]-
[PF6]2 (10.6 h−1) (entry 3), but only slightly better than [TeFe3-
(CO)9Cu2(MeCN)2] (28.3 h−1) (entry 4), showing the importance
of the TeFe3 core effect. In addition, these catalysts with the
mono- or bidentate-NHC attached to the TeFe3(CO)9Cu2 core
exhibited comparable efficiencies (∼44.0 h−1) (entries 5–8),
demonstrating again that the catalyst activities were associated
with the TeFe3 core. As the Cu loading of catalysts 1–4 was
lowered from 1.0 to 0.5 mol%, the yields remained almost the
same but with increased TONs and decreased TOFs (entries
5–8 and 9–12). These results indicated that these TeFe3(CO)9-
incorporated dicopper complexes 1–4 exhibited similar and
pronounced catalytic activities which were better than other
conventional catalysts such as Au, Fe, Pd, and Cu in homo-
coupling of 4-bromophenylboronic acid, which required higher
temperature, additional base, external additives, longer reac-
tion time, and higher loading of the catalyst (Table S2, ESI†).
The homocoupling mechanism of [TeFe3(CO)9Cu2(MeCN)2]
and 1–4 was proposed to occur via a Cu(I)/Cu(III) catalytic
cycle,8a,13a in which these catalysts underwent transmetalation
with arylboronic acid under aerobic conditions, which involved
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Table 1 Homocoupling of 4-bromophenylboronic acid with different catalystsa
Entry
Catalyst
Cu (mol%)
Time (h)
Yieldb,c (%)
TONd
TOFe (h−1
)
1
2
3
4
5
6
7
8
[Et4N]2[TeFe3(CO)9]
[Cu(MeCN)4][BF4]
[Cu2(MeIm(CH2)2ImMe)]2[PF6]2
[TeFe3(CO)9Cu2(MeCN)2]
[TeFe3(CO)9Cu2(Me2Im)2] (1)
[TeFe3(CO)9Cu2(MeImCH2ImMe)] (2)
[TeFe3(CO)9Cu2(MeIm(CH2)2ImMe)] (3)
[TeFe3(CO)9Cu2(MeIm(CH2)3ImMe)] (4)
[TeFe3(CO)9Cu2(Me2Im)2] (1)
[TeFe3(CO)9Cu2(MeImCH2ImMe)] (2)
[TeFe3(CO)9Cu2(MeIm(CH2)2ImMe)] (3)
[TeFe3(CO)9Cu2(MeIm(CH2)3ImMe)] (4)
0f
12
60
8
3
2
2
2
2
12
12
12
12
0
47
85
85
88
87
88
88
82
84
80
78
—
47
85
85
88
87
88
88
164
168
160
156
—
0.8
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.5
0.5
0.5
10.6
28.3
44.0
43.5
44.0
44.0
13.7
14.0
13.3
13.0
9
10
11
12
a Reaction conditions: 4-bromophenylboronic acid (1.0 mmol), MeOH (3.0 mL), 25 °C, O2 (1 atm, balloon). b The isolated yield as an average of
three runs. c All reactions were monitored by TLC. d Turnover number per copper for the biaryl product. e TON per hour. f 3.0 mol% of Fe loading.
weak Cu–Cu bond cleavages (Wiberg bond indices, oxidation nearly paralleled the increased catalyst activities
0.028–0.035, Table S3, ESI†) to make the Cu atoms more ([TeFe3(CO)9Cu2(MeCN)2] and 1–4 (0.015–0.134 V; 85–88%,
spatially open toward arylboronic acid, accompanied with the 28.3–44.0 h−1); [Cu2(MeIm(CH2)2ImMe)2][PF6]2 (0.399 V; 85%,
oxidation of the TeFe3Cu2 core, followed by reductive elimin- 10.6 h−1); [Cu(MeCN)4][BF4] (1.004 V; 47%, 0.8 h−1)) (Table S4,
ation to release the biaryl products (Fig. S5, ESI†).
ESI†), which substantiated that the oxidation of Cu(I) coupled
In order to understand the oxidative behavior of [TeFe3- with the TeFe3(CO)9 fragment played a key role in the catalytic
(CO)9Cu2(MeCN)2] and 1–4, differential pulse voltammetry mechanism of these TeFe3(CO)9-incorporated copper complexes.
(DPV) was measured and each showed two one-electron quasi-
To further explore the scope and generality of the present
reversible oxidations between 0.015 and 0.286 V vs. SCE (Fig. 2, protocol under the optimized conditions, the homocoupling
and Table S4, ESI†), which was assigned to the oxidation of of arylboronic acids by catalyst 1 was examined and summar-
the TeFe3Cu2 core, according to the HOMOs in DFT calcu- ized in Table 2. Our studies showed that the homocoupling of
lations (Fig. S6, ESI†). For comparison, the DPVs for [Cu- ortho-bromophenylboronic acid gave the corresponding biaryls
(MeCN)4][BF4] and [Cu2(MeIm(CH2)2ImMe)2][PF6]2 were also in lower yields (54%, entry 1) than those (68%, entry 2; 88%,
studied. It was interesting to find that the ease of their first entry 3) with meta and para substituents because of the
notable steric hindrance. Also, the meta- and para-substituted
arylboronic acids bearing a stronger electron-withdrawing
Table 2 Substrate scope of arylboronic acid using catalyst 1a
Entry
R
Cu (mol%)
Time (h)
Yieldb,c (%)
1
2
3
4
5
6
7
ortho-Br
meta-Br
para-Br
meta-NO2
para-NO2
meta-NO2
para-OMe
1.0
1.0
1.0
1.0
1.0
0.5
1.0
2
2
2
24
24
38
2
54
68
88
97
99d
98
70
a Reaction conditions: arylboronic acid (1.0 mmol), MeOH (3.0 mL),
25 °C, O2 (1 atm, balloon). b Isolated yield as an average of three runs.
c All reactions were monitored by TLC. d An addition–elimination side
product 4-nitrophenol was isolated in 21% yield due to the presence of
Fig. 2 DPVs in MeCN for [TeFe3(CO)9Cu2(MeCN)2] and 1–4. Conditions:
electrolyte, 0.1 M Bu4NClO4; working electrode, glassy carbon; scan H2O in the reaction and the major product 4,4′-dinitrobiphenyl was
rate, 100 mV s−1. Potentials are vs. SCE. The inset shows the first one-
electron quasi-reversible oxidation between 0.00 and 0.20 V vs. SCE.
obtained in 78% yield. Therefore, the yield of 4,4′-dinitrobiphenyl was
calculated as 99% (78%/79%).
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nitro group gave the desired products in excellent yields (97%,
entry 4; 99%, entry 5) which were higher than that (70%, entry
7) for phenylboronic acid with an electron-donating methoxy
group or those (68%, entry 2; 88%, entry 3) with a weak
electron-withdrawing bromo group. A Hammett plot20,21 of
log(KArX/KArBr) (X = Br, NO2, OMe) versus σ is shown in Fig. S7
(ESI†). The observed positive value (0.19) of ρ indicated that
the reaction rate was accelerated by the electron-withdrawing
substituents of arylboronic acid. Even when the Cu loading of
catalyst 1 was decreased from 1.0 to 0.5 mol% in catalyzing
3-nitrophenylboronic acid, an excellent yield (98%, entry 6)
was maintained with turnovers up to 196.
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In conclusion, we have discovered a facile route to a new
type of TeFe3(CO)9-incorporated mono- or bidentate-NHC Cu(I)
complexes 1–4 which could effectively catalyze the homo-
coupling of arylboronic acids under an atmosphere of O2 at room
temperature with high turnovers per copper (54–196). These
catalyst efficiencies were significantly enhanced by the partici-
pation of the electron-donating and bulky cluster anion
[TeFe3(CO)9]2− as an inorganic ligand due to the ease of oxi-
dation of the TeFe3Cu2 core. Further investigation will focus
on the use of other modified anionic carbonyl chalcogenide
clusters as inorganic ligands in order to fine-tune the elec-
tronic and steric factors of the resultant Cu catalysts for the
efficient homocoupling of various arylboronic acids.
This work was supported by the Ministry of Science and
Technology of Taiwan (Grant No. 101-2113-M-003-005-MY3 to
M. Shieh). We are also grateful to the National Center for
High-Performance Computing where the Gaussian package
and computer time were provided. Our gratitude also goes to
the Academic Paper Editing Clinic, NTNU.
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