A tetranuclear CuII2DyIII2 coordination cluster as a Suzuki (C-C) coupling reaction promoter

Article abstract of DOI:10.1039/C8DT04371J

The air-stable tetranuclear coordination cluster [CuII2DyIII2L₄(NO₃)₂(CH₃CN)₂]·2(CH₃CN), which can be obtained in high yield, promotes the Suzuki coupling reaction of phenylboronic acid with substituted aryl halides under environmentally benign conditions.

Full text of DOI:10.1039/C8DT04371J

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A tetranuclear Cu2IIDyI2II coordination cluster as a
Suzuki (C–C) coupling reaction promoter†
Cite this: Dalton Trans., 2018, 47,
17202
Prashant Kumar,a Kieran Griffiths,a Christopher E. Anson, b Annie K. Powell
*
b,c
Received 2nd November 2018,
Accepted 15th November 2018
a
and George E. Kostakis
*
DOI: 10.1039/c8dt04371j
rsc.li/dalton
The
air-stable
tetranuclear
coordination
cluster easily functionalized. The initial reports involved noble and
[CuI2IDyI2IIL4(NO3)2(CH3CN)2]·2(CH3CN), which can be obtained in expensive Pd-based catalysts; however several methodologies,
high yield, promotes the Suzuki coupling reaction of phenyl- based on Earth-abundant, low cost, first row transition
boronic acid with substituted aryl halides under environmentally elements such as Fe,19 Co,20 Ni21 and Cu22 have been identi-
benign conditions.
fied. In the case of Cu promoted catalysis,23,24 the high
loading required to achieve the C–C bond-formation in high
yields is considered a significant drawback.
Long term research is required to achieve synthetic control in
combining 3d and 4f centres with organic ligands as well as to
understand the influence of each centre on the studied pro-
perties; however this methodology has yielded entities with
exceptional magnetic1 or luminescent behaviour.2 The devel-
opment of Earth-abundant metal-based catalytic method-
ologies to replace the existing practices that use noble and rare
elements is a key target in modern science.3 In this direction,
3d/4f in situ4–6 or well-characterised coordination clusters have
recently emerged as sustainable catalytic platforms with advan-
tages of ease of synthesis, good abundance (in spite of belong-
ing to the so-called rare earths) and low cost.7 Our group
has established the efficacy of a series of tetranuclear coordi-
nation clusters [M2IILn2IIIL4(X)2(solv)2] (M = Ni, Co, Cu, Zn, X =
Cl, NO3, Ln = Y, Dy, Gd) for domino-electrocyclization and
Lewis acid reactions,8–11 whereas other groups have, success-
fully, used them in redox,12–14 co-polymerization15 and carbon
dioxide fixation reactions.16,17
In our quest to investigate and expand the applicability of
the [MI2ILn2IIIL4] 3d/4f catalytic species further and given
the successful use of CuII based compounds in the
Suzuki–Miyaura reaction, we synthesised the compound
[CuI2IDyI2IIL4(NO3)2(CH3CN)2]·2(CH3CN), 1·2(CH3CN), where
H2L is the Schiff base ligand (E)-2-(2-hydroxy-3-methoxybenzy-
lidene-amino)phenol. Compound 1 is synthesized in two high-
yielding steps (overall yield: 93%), under ambient conditions,
using commercially available starting materials, from the reac-
tion of Cu(NO3)2·3H2O, Dy(NO3)3·6H2O, H2L and Et3N in
MeCN (20 mL) in a molar ratio of 2 : 4 : 4 : 9. After mixing, the
resulting solution was filtered and greenish crystals of 1·2
(CH3CN) were obtained in 3 days from the mother liquor and
collected by filtration, washed with Et2O, and dried in air.
Compound 1 crystallizes in the orthorhombic space group
Pbca. Its core topology is of the well-known tetranuclear defect
dicubane or the butterfly type (Fig. 1). The CuII ions are in the
central “body” positions and the DyIII ions are at the “wingtip”
positions with a separation of 3.4105(9) Å, whereas the Cu–Cu
distance is 3.411 Å. The coordination environment of both the
metals is almost saturated; five out of six and six out of eight
coordination sites of the CuII and DyIII centres, respectively,
are occupied by the ligand. The final coordination site on the
CuII ion is occupied by a solvent MeCN molecule and the
remaining two sites on the DyIII ion are occupied by two
Metal-promoted Suzuki–Miyaura coupling includes organo-
boron derivatives and carbon electrophiles and represents a
valuable procedure for C–C bond formation (Scheme 1).18 This
synthetic protocol uses commercially available boron reac-
tants, which are relatively stable and non-toxic and can be
aDepartment of Chemistry, School of Life Sciences, University of Sussex,
Brighton BN19QJ, UK. E-mail: G.Kostakis@sussex.ac.uk
bInstitute of Inorganic Chemistry, Karlsruhe Institute of Technology,
76131 Karlsruhe, Germany. E-mail: Annie.powell@kit.edu
cInstitute of Nanotechnology, Karlsruhe Institute of Technology, Postfach 3640,
76021 Karlsruhe, Germany
†Electronic supplementary information (ESI) available: Synthesis, crystallo-
graphic details, catalyst screening, and NMR data. CCDC 1860013 for 1. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8dt04371j
Scheme 1
reaction.
A
representative example of the Suzuki C–C coupling
17202 | Dalton Trans., 2018, 47, 17202–17205
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TLC. Two experiments in the absence of compound 1 were per-
formed as blank to exclude the release of Pd nanoparticles
from the magnetic stirrer. After 24 h, compound 1 showed
higher activity in the DME : water mixture (1 : 1 ratio) with 89%
yield of the desired 1,1′-biphenyl product (Table 1, entry 4).
The reaction was performed in other solvents such as water,
ethanol, DME (dimethoxyethane), DMF (dimethylformamide),
DMF : water (1 : 1), THF (tetrahydrofuran), and THF : water
(1 : 1); however the corresponding compound was obtained in
lower yield. Therefore, the DME : water mixture (1 : 1) was
chosen as the solvent for the subsequent experiments.
Furthermore, the effect of a base on the reaction perform-
ance was studied using different bases such as K2CO3, KOH,
Na2CO3, NaOH, Et3N, Cs2CO3 and KOtBu in a mixed DME and
water solvent (Fig. S2†). We identified that K2CO3 is the desir-
able base for the reaction with higher product yield for this
reaction. In the absence of a catalyst, the coupling product is
formed in very low yield (>2%) using K2CO3 as the base.
Therefore, we may envisage that the presence of the base is
essential to activate boronic acid via enhancing the polariz-
ation of the organic ligand, thus facilitating transmetallation.
The next step was to use different loadings of the catalyst. As
shown in Table 2, the initial experiment of 5.0 mol% is the
optimum condition.
Then, we investigated the influence of temperature on the
coupling reaction between phenylboronic acid and iodoben-
zene in a range of temperatures (0 °C–120 °C). Control experi-
ments showed that 1 can show good conversion at 80 °C but is
inactive at temperatures lower than 40 °C (Fig. S3†). At 100 and
120 °C, the catalyst affords the corresponding compound in
lower yield; therefore we concluded that the temperature of
80 °C was the optimum temperature. Moreover, experiments in
the presence of Cu salts under similar catalytic conditions
yielded the expected product with significantly higher loadings
when compared to those with 1. For example, various Cu(II)
salts (Table S1†) show up to 33% conversion, whereas the use
of Dy(NO3)3 does not promote the organic transformation. The
use of Cu(I) salts yields the corresponding product in 40%
yield, which is slightly higher than that for the Cu(II) salts,
Fig. 1 The molecular structure of compound 1 in the crystal. Color
codes: CuII (dark blue), DyIII (cyan), oxygen (red), nitrogen (light blue),
and carbon (grey). H-atoms are omitted for clarity.
oxygen atoms from a chelating nitrate anion. This compound
is isoskeletal to the previously reported tetranuclear com-
pounds and more specifically to [CuI2IYI2IIL4(NO3)2(DMF)2],
which was found to be an efficient cooperative catalyst in the
Michael addition reaction.8 Thermal studies under a N2 atmo-
sphere (Fig. S1†) identify a partial loss of the solvent molecules
(9.83%) and that compound 1 is stable up to 250 °C. At this
point the framework collapses to yield the corresponding
CuI2IDyI2IIO5 residue (calculated: 31.20%, found: 31.01%). To
further confirm the identity of compound 1 in solution we
recorded a UV-Vis spectrum in CH3CN. This revealed a broad
peak (500 to 700 nm), characteristic of CuII in this coordi-
nation environment (Fig. S2†). Preliminary magnetic studies
identify the paramagnetic character of compound 1 and that
the Cu centres are in oxidation state II.
With the catalyst in hand, we chose phenylboronic acid and
iodobenzene as model substrates and attempted to identify
environmentally friendly solvents25 to perform the reaction
(Table 1). The completion of the reaction was monitored by
Table 1 Influence of solventsa
Table 2 Optimization experiments for the coupling reaction of phenyl
boronic acid and iodobenzene catalysed by compound 1a
Entry
Solvent
Yieldb
1
2
3
4
5
6
7
8
Water
Ethanol
DME
DME : water(1 : 1)
DMF
DMF : water(1 : 1)
THF
THF : water(1 : 1)
10
12
46
89
66
79
36
48
Entry
Catalyst (mol%)
Yieldb
1
2
3
4
5
1
2
5
10
20
48
56
89
72
69
a Iodobenzene (1.0 mmol), phenylboronic acid (1.1 mmol), PPh3 a Iodobenzene (1.0 mmol), phenylboronic acid (1.1mmol), PPh3
(2.0 mol%), K2CO3 (5.0 mmol), Cu2Dy2 (5.0 mol%), 80 °C. b Isolated (2.0 mmol), K2CO3 (5.0 mmol), DME : water (1 : 1), 80 °C. b Isolated
yield.
yield.
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Table 4 Reaction of 2-bromopyridine with different phenylboronic
acidsa
while the in situ combination of Cu(NO3)2/Dy(NO3)3/H2L gave a
poor outcome (entry 7, Table S1†), showcasing the advantage
of the use of the well-characterised material.
With the optimal reaction conditions in hand, we then
extended the scope of the reaction and performed the coupling
reactions between different aryl halides and phenylboronic
acid. As a first step, we investigated the influence of the substi-
tution on the aromatic ring of the aryl halide. The reactions
with substitution in the ortho and para positions of the aryl
halide (–NO2) yielded the corresponding products in moderate
to good yields. The electron-withdrawing group (–NO2) of the
aryl halide enhanced the rate of cross-coupling. For example,
the nitro (–NO2) group at the para position of the aromatic aryl
halide furnished the desired product 4ac in 76% yield
(Table 3) and the electron-donating group (–CH3) at the para
position of the aryl halide produced the corresponding
product 4ad in 54% yield (Table 3). Furthermore, the reaction
of sterically hindered halides with phenylboronic acid afforded
4ba in 38% yield (Table 3). The slight increase in the yield of
4ac when compared with that of 4ab may be attributed to the
influence of the strong electron-withdrawing group (NO2) that
results in the reductive halogenation of the aryl halides.
a Aryl halide (1.0 mmol), phenylboronic acid (1.1 mmol), PPh3
(2.0 mol%), K2CO3 (5.0 mmol), Cu2Dy2 (5.0 mol%), 80 °C. b Isolated
yield.
Furthermore, we explored different substituted aryl boronic
acids and nitrogen-based heteroaryl halides (2-bromopyridine)
using 1 as a precatalyst under the same reaction conditions
and the results are summarized in Table 4. Reactions with
both electron-rich and electron-poor aryl boronic acids pro-
ceeded smoothly. A quantitative yield of 2-phenylpyridine was
obtained in the presence of 5.0 mol% of catalyst 1 at 80 °C in
8 h (Table 4, 7aa). Electron withdrawing and electron donating
groups at the ortho/para position afforded the corresponding
products in good yield as compared to that of the meta-posi-
tion (Table 4).
Finally, for an initial understanding of the catalyst’s role we
also tested this coupling reaction with other topologically equi-
valent catalysts, varying the 3d part i.e. CoI2IDy2, Ni2IIDy2 and
ZnI2IDy2.8,26,27 However, all the reactions failed to produce the
desired product 4aa as compared to 1, thus identifying the
suitability and superiority of 1 as the catalyst for the Suzuki
Coupling reaction under these reaction conditions and justify-
ing our selection.
Table 3 Reaction of phenylboronic acid with different aryl halidesa
Conclusions
Extending our studies in exploring the use of 3d/4f coordi-
nation clusters as organic transformation promoters, in this
work we report, for the first time, the cross-coupling activity of
compound 1. The very promising results highlight the richness
of the reactivity of the 3d/4f chemistry and its potential appli-
cations. The catalytic conversion proceeds under environmen-
tally benign conditions and thus the current methodology has
a sustainable character. More experiments are required to fully
understand the role of 1 in the present organic transformation
and these experiments are under investigation. The present
results open new research avenues for the chemistry of Cu/4f
coordination clusters.
a Aryl halide (1.0 mmol), phenylboronic acid (1.1 mmol), PPh3
(2.0 mol%), K2CO3 (5.0 mmol), Cu2Dy2 (5.0 mol%), 80 °C. b Isolated
yield.
Conflicts of interest
There are no conflicts to declare.
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