Monodispersed and stable nano copper(0) from copper- aluminium hydrotalcite: Importance in C-C couplings of deactivated aryl chlorides

Article abstract of DOI:10.1002/adsc.201201007

A simple one-step approach for the preparation of highly monodispersed nano copper(0) stabilized on alumina [Cu(0)/Al₂O₃] by thermal reduction of copper-aluminium hydrotalcite (Cu-Al HT) under a hydrogen atmosphere is described. The transformation of Cu-Al HT to Cu(0)/Al ₂O₃ occurrs via dehydroxylation of divalent and trivalent metal hydroxides and decarboxylation of carbonate anions present in the interlayers of hydrotalcite, as confirmed by XPS, XANES, XRD and TEM analysis. Cu(0)/Al₂O₃ nano composites were used as an efficient catalyst in the C-C coupling of deactivated aryl chlorides. The high efficiency and reusability exhibited by Cu(0)/Al₂O₃ outline its potential as an alternative over traditional noble metal-based catalysts in C-C coupling reactions. Copyright

Full text of DOI:10.1002/adsc.201201007

FULL PAPERS
DOI: 10.1002/adsc.201201007
Monodispersed and Stable Nano Copper(0) from Copper-
À
Aluminium Hydrotalcite: Importance in C C Couplings of
Deactivated Aryl Chlorides
Racha Arundhathi,a,c Dandu Damodara,a Kakita Veera Mohan,b
Mannepalli Lakshmi Kantam,a and Pravin R. Likhara,*
a
Inorganic and Physical Chemistry Division, CSIR – Indian Institute of Chemical Technology, Hyderabad – 500076, India
Fax : (+91)-40-2716-0921; phone: (+91)-40-2719-3510; e-mail: plikhar@iict.res.in
Center for Nuclear Magnetic Resonance Division, CSIR – Indian Institute of Chemical Technology, Hyderabad – 500076,
b
India
c
Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1–3
Machikaneyama, Toyonaka, Osaka 560-8531, Japan
Received: November 14, 2012; Revised: January 17, 2013; Published online: February 22, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201201007.
Abstract: A simple one-step approach for the prepa- nano composites were used as an efficient catalyst in
À
ration of highly monodispersed nano copper(0) stabi- the C C coupling of deactivated aryl chlorides. The
lized on alumina [Cu(0)/Al2O3] by thermal reduction high efficiency and reusability exhibited by Cu(0)/
of copper-aluminium hydrotalcite (Cu-Al HT) under Al2O3 outline its potential as an alternative over tra-
À
a hydrogen atmosphere is described. The transforma- ditional noble metal-based catalysts in C C coupling
tion of Cu-Al HT to Cu(0)/Al2O3 occurrs via dehy- reactions.
droxylation of divalent and trivalent metal hydrox-
ides and decarboxylation of carbonate anions present
in the interlayers of hydrotalcite, as confirmed by Keywords: alkenes; alkynes; aryl chlorides; boronic
À
XPS, XANES, XRD and TEM analysis. Cu(0)/Al2O3 acids; C C couplings; nano copper
Introduction
tempts for developing stable and monodispersed
copper nanoparticles using inexpensive precursors
Heterogeneous catalysis at the nanoparticle surface is still remains a challenge and focus area of intense re-
currently a subject of investigation in the catalysis search.
community because of the attractive physical and
For the past few years, our group has been involved
chemical properties as compared with the bulk mate- in the synthesis and exploration of metal hydrotalcites
rials.[1,2] The synthesis of metal nanoparticulates with in various organic transformations.[11] The general for-
controllable size, shape and surface properties is vital mula of metal hydrotalcites is as follows:
in exploring the catalytic activity in various organic [M(II)1Àx MACHTNUTGRNEUNG
(III)x (OH)2]nÀ AnÀx/n yH2O, where M(II)
transformations.[3,4] In this context, copper nanocol- and M
ACHTNUTRGNE(NUG III) are divalent and trivalent cations such as
loids[5] and nanoclusters[6] have recently been tested in Cu2+, Mg2+2Àand Al3+, respectively, and OHÀ, ClÀ,
À
the C C coupling and other cross-coupling reac- NO3 , CO3 etc.[12] can be the compensating anion.
À
tions,[7] fundamental processes of paramount impor- Cu-Al hydrotalcite (Cu-Al HT), in particular, is a sub-
tance in organic synthesis, that are currently dominat- ject of investigation in the cross-coupling reactions.
ed by the use of gold, silver, platinum and palladi- With limitations in the synthesis of stable nanoparti-
um.[8] However, the propensity of surface oxidation of culate Cu with controllable size, and high surface
metallic Cu is a key issue in synthesizing the stable area, we envisaged Cu-Al HT as the appropriate pre-
Cu nanoparticles.[9] To date, only limited success has cursor for the synthesis of Cu(0) nanoparticles.[13]
been achieved in synthesizing stable Cu nanoparti-
cles.[10] In general, the synthesized Cu nanoparticles
(less than 10 nm) exhibit limited size monodispersity
and/or are susceptible to oxidation. Consequently, at-
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Results and Discussion
firmed with auger spectral analysis (see the Support-
ing Information).[15] The XPS results of Cu and Al
clearly show that, under the reaction conditions,
copper is selectively reduced to elemental copper
Preparation of Cu(0)/Al2O3 from Cu-Al HT
The precursor, Cu-Al HT, was prepared by the co- whereas the oxidation state of aluminum remains the
precipitation method reported in the literature.[11,12] In same (aluminum hydroxide transformed into its
the synthesis of copper nanoparticles, Cu-Al HT simple oxide through a dehydration process). Thus at
(500 mg) was placed in a closed furnace equipped elevated temperature and in the presence of hydro-
with an inlet and outlet for gas. The furnace was evac- gen, the Cu-Al HT undergoes dehydroxylation and
uated and refilled with nitrogen followed by introduc- decarboxylation via H2O and CO2 evolution produc-
tion of molecular hydrogen (H2) with a flow rate ing Cu(0)/Al2O3.
1 mL per minute. The furnace was slowly heated to
The XRD patterns of Cu(0)/Al2O3 showed that all
1908C in the presence of a hydrogen flow for 2 h. The the peak positions for the copper nanoparticles are
resultant black-colored powder Cu(0)/Al2O3 was al- consistent and comparable with metallic copper and
lowed to cool to room temperature under a nitrogen signify the polycrystalline nature of Cu(0)/Al2O3
atmosphere. Cu(0)/Al2O3 was collected from the (Figure 2). The X-ray reflections are indexed on the
holder and stored in a dry desiccator.[14] Copper nano- basis of the fcc structure of copper with a space group
particles on alumina were characterized and con- of Fm3m (ASTM card no. 4-0836, X-ray diffraction
firmed by XPS, EXAFS-XANES, XRD, and TEM files for inorganic materials). The calculated lattice
analyses.
constant, a=3.6147 ꢁ, is in good agreement with the
literature value (3.6150 ꢁ). The diffraction peaks at
2q=43.5, 50.6, and 74.3 can be indexed as the [111],
[200], and [220] planes of copper with cubic symme-
try.[16]
Characterization of Cu(0)/Al2O3
The X-ray photoelectron spectrum (XPS) showed
Transmission electron microscopic (TEM) images
that the Cu 2p core level binding energies of Cu-Al confirmed the monodispersity of Cu(0) nanoparticles
HT were at 934.71 and 954.38 eV for the +2 oxidation reduced from Cu-Al HT precursor consisting of
state while peaks observed in the H2 reduced sample spherical nanoparticles with a narrow size disrtibu-
were at 932.45 and 952.21 eV, corresponding to ele-
mental Cu(0) (Figure 1). The absence of two satellite
peaks at 962.63 eV and 942.98 eV strongly suggests
the selective formation of Cu(0) metal as further con-
Figure 1. [A] XPS spectra of Cu-Al HT and [B] Cu(0)/
Al2O3.
Figure 2. [A] XRD patterns of Cu-Al HT and [B] Cu(0)/
Al2O3.
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Monodispersed and Stable Nano Copper(0) from Copper-Aluminium Hydrotalcite
examined in the Heck reaction of 4-chlorotoluene
with styrene using Cu(0)/Al2O3 (25 mg, 0.97 mol%
relative to aryl chloride) which yielded 1-methyl-4-
styrylbenzene in 88% yield. In order to examine the
efficiency of Cu(0)/Al2O3, several other copper-based
catalysts were reduced and used in combination with
different bases and solvents (see the Supporting Infor-
mation for screening of various Cu catalysts, bases
and solvents) for their catalytical activity.
Among the different bases screened, K2CO3 had
a pronounced effect on product yield and an 88%
yield of coupled product was obtained when N,N-di-
methylformamide (DMF) solvent was used at 1108C
(Table 2). It was seen that the coupling of less reac-
tive aryl chlorides such as 4-chlorotoluene and 4-
chloroanisole with styrene and 4-methylstyrene, pro-
ceed smoothly and good to excellent yields were ob-
tained under the optimized experimental conditions.
Similarly, we have also examined the catalytic activity
of Cu(0)/Al2O3 in the couplings of various aryl chlor-
ides with phenylboronic acids (Suzuki); tributyl-
Figure 3. Fourier transform (FT) Cu K-edge spectra of the
reduced Cu-Al HT (Cu(0)/Al2O3) and as synthesized Cu-Al
HT.
Table 1. Structural parameters for the Cu(0)/Al2O3 and ref-
erence samples.
Sample
Scattering atom CN
R [ꢁ] dF/DW
AHCTUNGTRENNUNG
À
Cu(0)/Al2O3 Cu Cu
8.9
2
12
6
1.901
12
2.450 À1.127/0.065
st
À
Cu2O
Cu O (1 )
1.859
3.104
3.599
and electron-deficient aryl chlorides were allowed to
couple with phenylboronic acid, excellent yields of bi-
phenyls were obtained except in the case of coupling
between phenylboronic acid and chlorobenzene.
Various aryl chlorides were used with 1.5 equiva-
À
Cu Cu
nd
À
Cu O (2 )
À
CuO
Cu foil
Cu O
À
Cu Cu
2.860
lents of tributylACTHNUTRGNEUNG(phenyl)tin for the Stille-type coupling
tion. The histogram showed an average particle diam- in the presence of catalytic amount of Cu(0)/Al2O3
À
eter of 7.6 nm with a standard deviation of 1.0 nm and selectively gave the C C coupled biaryls in excel-
and relative standard deviation of 0.294 nm (see the lent yields. The reaction of chlorobenezene with
Supporting Information for TEM images). The parti- tributyl
ACHTUNGTRENNUNG
cle size, 7.6 nm calculated using Scherrerꢂs equation yield of biphenyl after 15 h.
was in agreement with the TEM analysis.
The formation of Cu(0)/Al2O3 was further investi- and aliphatic alkynes afforded excellent yields of the
gated by EXAFS-XANES analysis (Figure 3). The coupled products at room temperature, irrespective of
atomic scale analysis of Cu(0) nanoparticles using Cu the electronic nature of the substituent (generally de-
K-edge EXAFS showed that the position of the FT activating group) present on the aromatic ring of
À
peak derived from the Cu Cu shell for Cu(0) appears chlorobenzene (Table 3). The catalytic activity of
at 2.45 ꢁ and is identical with the peak derived from Cu(0)/Al2O3 was also investigated in the Sonogashira
Cu metal foil (Table 1).
coupling of 4-methoxychlorobenzene with an unsatu-
Although there is no change observed in the oxida- rated alcohol (propargyl alcohol) and excellent yields
tion state of Al in the XPS study before and after of the coupled product was observed.
thermal reaction, the chemical shift of Al changes in
13Al NMR and suggests the deformity in the HT
frame-work[17] (see 13Al NMR in the Supporting Infor- Reusability of the Catalyst
mation and its tranformation into Al2O3).
The extent of Cu leaching and the change in the
chemical or electronic state of Cu from the spent
À
Catalytic Activity of Cu(0)/Al2O3 in C C Coupling
Reactions
Cu(0)/Al2O3 catalyst after the Heck and Sonogashira
coupling reactions were studied by AAS and XPS
analysis. The true heterogeneity of the catalyst was
The catalytic activity of Cu(0) nanoparticles was in- examined with the experimental evidence that when
À
vestigated in a number of industrially important C C the reaction was discontinued during the first half of
coupling reactions. Initially, the catalytic activity was its reaction time (progress of reaction was monitored
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Table 2. Cross-coupling reactions of aryl chlorides.[a]
[a]
Reaction conditions: aryl chloride (1 mmol), olefin (1.2 mmol, for Heck coupling), boronic acid
(1.2 mmol, for Suzuki coupling), tributyl(phenyl)tin (1.2 mmol, for Stille-type coupling), Cu(0)/Al2O3
(25 mg, Cu: 0.97 mol%), K2CO3 (1.5 mmol), DMF (2 mL), 1108C.
AHCTUNGTRENNUNG
by GC), the hot filtrate (solid catalyst was removed leaching from Cu(0)/Al2O3 catalyst was found. The
by filtration under hot conditions) was stirred for the ICP/AES and XPS analyses of recovered catalyst also
next half reaction time, and no further coupling prod- suggest that the Cu content is unchanged as well as
uct was observed, which clearly suggests that the reac- there being no changes in the chemical or electronic
tion does not proceed when devoid of the active cata- state of elemental Cu which rule out the possible for-
lyst. Similarly, the filtrate from the above experimen- mation of CuAl2O4 species during coupling reactions
tation was tested by the AAS technique and no Cu (see the Supporting Information for AAS and XPS
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Monodispersed and Stable Nano Copper(0) from Copper-Aluminium Hydrotalcite
Table 3. Sonogashira coupling reaction of aryl chlorides.[a]
[a]
Reaction conditions: aryl chloride (1 mmol), alkyne (1.2 mmol) Cu(0)/Al2O3 (25 mg, Cu:
0.97 mol%), K2CO3 (1.5 mmol), DMF (2 mL) at room temperature.
study). Advantageously, Cu(0)/Al2O3 can be used for Experimental Section
À
a number of important C C coupling reactions, there-
by eliminating the use of expensive conventional pal- General Procedure for Heck, Suzuki and Stille
ladium-based catalysts, organic ligands, the need for Coupling Reactions
very high temperatures and high catalyst loadings.
A 25-mL round-bottomed flask equipped with a magnetic
Moreover, the catalyst can be recovered and reused
stir bar was charged with aryl chloride (1.0 mmol), coupling
in all reactions for several cycles (see the Supporting
partner (1.2 mmol), K2CO3 (1.5 mmol), Cu(0)/Al2O3 (25 mg,
Information for recovery and reusability study for all
Cu:0.97 mol%) and DMF (2 mL) under an N2 atmosphere.
The reaction mixture was stirred at 1108C for the required
period of time. After completion of the reaction, as judged
by TLC, the reaction mixture was diluted with ethyl acetate
and centrifuged to separate the catalyst. The catalyst was
washed several times with ethyl acetate to extract all organic
compounds. The reaction mixture was concentrated to
a small volume and purified by silica gel chromatography
À
C C coupling reactions). The size and oxidation state
of the Cu(0) nanoparticles of the reused Cu(0)/Al2O3
catalyst remained unchanged as shown in the TEM
and XPS analyses (see the Supporting Information
for TEM and XPS of reused Cu(0)/Al2O3, respective-
ly).
À
column to afford the corresponding C C coupled product.
All the products were characterized by 1H and 13C NMR
and their mass spectral data were compared with those of
the literature reports.
Conclusions
We have developed a simple and efficient method for
the preparation of monodispersed and highly stable
Cu nanoparticles from Cu aluminium hydrotalcite
General Procedure for Sonogashira Coupling
Reactions
À
precursor for industrially important catalytic C C
In a Schlenck tube dried under vacuum and filled with N2
were placed aryl chloride (1.0 mmol), phenylacetylene
(1.2 mmol), K2CO3 (1.5 mmol), Cu(0)/Al2O3 (25 mg, Cu:
0.97 mol%) and DMF (2 mL). The reaction mixture was
stirred at room temperature. After completion of the reac-
tion, as judged by TLC, the reaction mixture was diluted
with diethyl ether. The catalyst was removed by centrifuging
the reaction mixture and the organic layer was washed with
water and dried over anhydrous Na2SO4. The organic layer
was concentrated under vacuum and subjected to flash
coupling reactions (Heck, Suzuki, Sonogashira and
Stille-type coupling). The efficiency and stability of
the Cu(0)/Al2O3 catalyst has been demonstrated by
activating unreactive aryl chlorides and studying the
recovery and reusability properties for coupling reac-
tions. Thus, Cu(0)/Al2O3 constitutes a unique catalytic
system and opens up an attractive alternative to noble
À
metal-based catalysts for the formation of C C bond.
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column chromatography using silica gel 60–120 to afford the
corresponding C C coupled product.
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À
Acknowledgements
RA and DD thank CSIR and UGC for their fellowships. We
also thank Prof. B. Viswanathan, Head, NCCR, IIT-Madras
and Dr. Lingaiah, I&PC Division, IICT Hyderabad for sci-
entific discussions.
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