Magnetically recyclable magnetite-palladium (Nanocat-Fe-Pd) nanocatalyst for the Buchwald-Hartwig reaction

Article abstract of DOI:10.1039/c4gc00558a

The immobilization of Pd on the magnetite surface afforded Nanocat-Fe-Pd using inexpensive precursors and its catalytic role in the Buchwald-Hartwig reaction for arylation of amines and amides was investigated; C-N bond formation was achieved in moderate to excellent yields and the catalyst could be separated and recycled up to five cycles by magnetic decantation without a significant loss in yield. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00558a

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Magnetically recyclable magnetite–palladium
(Nanocat-Fe–Pd) nanocatalyst for the
Buchwald–Hartwig reaction†
Cite this: DOI: 10.1039/c4gc00558a
Sofia Sá,a Manoj B. Gawande,*a,b Alexandre Velhinho,c João Pedro Veiga,c
Nenad Bundaleski,d João Trigueiro,d Alexander Tolstogouzov,d
Orlando M. N. D. Teodoro,d Radek Zboril,b Rajender S. Varma*e and
Paula S. Branco*a
Received 30th March 2014,
Accepted 17th April 2014
The immobilization of Pd on the magnetite surface afforded Nanocat-Fe–Pd using inexpensive precursors
and its catalytic role in the Buchwald–Hartwig reaction for arylation of amines and amides was investi-
gated; C–N bond formation was achieved in moderate to excellent yields and the catalyst could be separ-
ated and recycled up to five cycles by magnetic decantation without a significant loss in yield.
DOI: 10.1039/c4gc00558a
www.rsc.org/greenchem
the catalytic systems cannot overcome the difficulty of catalyst
separation from the final product which involves cumbersome
Introduction
Reagents and complexes containing transition metals are fun- work-up, thus reducing their scope. Heterogeneous catalysis is
damental in modern organic synthesis because they allow well suited for addressing this problem as it has the advantage
apparently impossible reactions to occur. The palladium- of a straightforward recovery and reuse of the catalyst from the
catalyzed cross-coupling of (hetero)aryl (pseudo)halides and reaction mixture by simple filtration or decantation; recent
amines including amides, imides, or sulfonamides, known as developments to separate such catalysts magnetically, by apply-
the Buchwald–Hartwig cross-coupling reaction, has emerged ing an external magnet, has highlighted the use of magnetic
as a highly effective C–N bond-forming protocol that is nanoparticles (MNPs) as the ideal solution.
employed on industrial scales in a diverse array of products,
Palladium immobilization on solid supports has been
such as pharmaceuticals and agrochemicals.1 Being a metal applied mainly for catalytic hydrogenations and for C–C cross
catalyzed process, the recovery of the costly noble metal coupling type reactions,3 the typical solid supports being
species is essential and also the production of arylamines carbon, metal oxides,4b,5 molecular sieves,6 carbon meso-
uncontaminated by metal species needs to be certified since porous materials,7 and polymeric materials.8 Heterogeneous
metal contamination is strictly regulated by the pharma- palladium catalyst systems based on silica supports are among
ceutical industry.2 The advantages of homogeneous catalysts the most widely used due to the excellent stability (chemical
such as high selectivity, better yield, and easy optimization of and thermal), good accessibility and porosity, and efficient
anchorage of organic functionalities.9 Nevertheless, the Buch-
wald–Hartwig reactions using silica-supported Pd have rarely
aREQUIMTE, Department of Chemistry, Faculdade de Ciências e Tecnologia,
been explored and without success.10 Yet, Uozumi and co-
Universidade Nova de Lisboa, 2829-516 Caparica, Lisbon, Portugal.
workers11 achieved a Buchwald–Hartwig amination in water
E-mail: paula.branco@fct.unl.pt; Fax: +351 212948550; Tel: +351 212948300
bRegional Centre of Advanced Technologies and Materials, Faculty of Science,
Department of Physical Chemistry, Palacky University, Šlechtitelů 11,
783 71 Olomouc, Czech Republic. E-mail: manoj.gawande@upol.cz
cCENIMAT/I3N Departmento de Ciência dos Materiais, Faculdade de Ciências e
Tecnologia, FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
dCentre for Physics and Technological Research (CeFITec), Department of Physics,
Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516
Caparica, Portugal
under heterogeneous conditions by using an immobilized
palladium complex, coordinated with an amphiphilic PS-PEG
resin-supported di(tert-butyl)phosphine ligand. Although
higher yields of triarylamines were obtained the protocol
involves very harsh conditions, such as refluxing in a KOH
(20 M) solution. The immobilization of metals and organic
ligands over the surface of magnetic nanoparticles is a popular
alternative to bridge the gap between heterogeneous and
homogeneous catalysis and has emerged as an environ-
mentally benign chemical alternative to conventional methodo-
logies.12 The major advantages are the combination of well-
eSustainable Technology Division, National Risk Management Research Laboratory,
US Environmental Protection Agency, 26 West Martin Luther King Drive, MS 443,
Cincinnati, Ohio 45268, USA. E-mail: Varma.Rajender@epa.gov
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c4gc00558a
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emission gun scanning electron microscopy and electron dis-
persive spectrometry (FEG-SEM-EDS), and X-ray photoelectron
spectroscopy (XPS).
Although the sample’s weight percentage of Pd was deter-
mined to be 9.89% by ICP-AES analysis, there are no traces of
a crystalline Pd phase in the XRD spectrum of Nanocat-Fe–Pd
(see ESI†) presumably due to the very small size of Pd clusters
deposited on magnetite23 and/or its amorphous nature.
A comparison between unmodified Fe3O4 and synthesized Pd-
enriched MNPs clearly shows overlapping of both spectra in
the frame of the experimental error, with all XRD lines being
attributed to Fe3O4. The high crystallinity is evidenced by the
observed peaks’ sharpness. The crystallite size of the Nanocat-
Fe–Pd MNPs determined using the Debye–Scherrer equation,
on the basis of the (311) line, was found to be 29.1 nm, in
agreement with the result obtained from the TEM which
shows a size distribution between 30 and 40 nm.
In the XPS survey spectrum of the Nanocat-Fe–Pd the main
lines of oxygen, iron and palladium are clearly visible. The XPS
spectrum of the Pd 3d line is shown in Fig. 1. Since the shapes
of Pd 3d5/2 and Pd 3d3/2 lines clearly indicate presence of at
least two different chemical phases, each line of the Pd 3d
doublet was fitted to two contributions (peaks A and B for
Pd 3d5/2 and peaks C and D for Pd 3d3/2). Peaks A and C are
assumed to correspond to the 1st chemical phase, while the
peaks B and D correspond to the 2nd phase. Appropriate con-
straints for the peak intensities, characteristic of d lines, were
adopted: SA : SC = SB : SD = 3 : 2. Additionally, we consider the
peak widths corresponding to the same line to be equal.
Pseudo-Voigt profile GL (30) was used for all peaks and the
background was assumed to be of Shirley type.
Scheme 1 Buchwald–Hartwig C–N cross coupling reaction using
Nanocat-Fe–Pd.
known procedures for catalyst heterogenization with tech-
niques for magnetic separation. A variety of catalytic systems
involving metal and organic ligand supported MNPs have been
devised and employed in synthetic organic reactions such as
nano-magnetite dopa-supported Pd,13 magnetite–cysteine,14
magnetite–nickel,15 magnetite–molybdenum16 and related
catalytic systems.17 Recently, magnetic Fe3O4 NPs coated with
APTES free NH2 were found effective in binding the in situ
formed Pd nanoparticles;18 the catalyst showed high catalytic
activity in the reduction of nitroaromatic compounds. A newer
magnetic composite catalyst has been prepared by immobiliz-
ing a Pd(0) complex onto a Mg–Al-layered double hydroxide
shell supported on a magnetite core.19 Both catalysts displayed
excellent performance for the Suzuki and Heck coupling reac-
tions under ligand-free conditions. As a continuation of our
efforts to develop greener protocols using nanocatalysis,20–22
we have developed a new, simple, and efficient protocol for
functionalization of magnetite with Pd and its application to the
Buchwald–Hartwig C–N cross coupling reaction thus enabling
the preparation of di- and tri-phenylamines (1a–j) (Scheme 1).
This protocol avoids several multistep processes of post synthetic
functionalization and the use of linkers between the metal and
magnetite, thus rendering the present protocol cost effective.
The expected positions of the Pd 3d5/2 line for Pd metal24
and PdO,25 respectively, are marked by arrows. Apparently, Pd
is present at the surface of the nanocatalyst in two main
phases i.e. as PdO and in its metallic form (oxidation state of
Pd(0)). However, since the expected Pd 3d5/2 line position for
Pd(OH)2 differs by only 0.1 eV with respect to that of PdO,26
the presence of palladium hydroxide cannot be excluded. Both
Results and discussion
Nanocat-Fe–Pd MNPs were prepared by a simple wet impreg-
nation method using PdCl2 as the palladium source, followed
by chemical reduction (Scheme 2). We believe that due to the
weak van der Waals forces and hydrogen bonding the Pd
species are strongly attached to the surface of magnetite. The
as synthesized nanocatalyst was well characterized by X-ray
diffraction (XRD), inductive coupled plasma-atomic emission
spectroscopy (ICP-AES), transmission electron microscopy
(TEM), secondary ion mass spectrometry (SIMS), field-
Fig. 1 XPS spectrum of the Pd 3d line. The expected line positions in
Scheme 2 Synthesis of Nanocat-Fe–Pd MNPs.
the case of metallic Pd and PdO are marked by arrows.
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PdO and Pd(OH)2 may have been generated independently in
the hydrolysis of PdCl2 under the basic conditions used. As a
matter of fact, it is suggested23a that palladium is initially de-
posited from a basic solution in the form of Pd(OH)2 colloids,
which are then transformed into PdO upon drying. In any
case, from the relative intensities of the corresponding contri-
butions to the Pd 3d line, 60% of all Pd is bound as PdO (and/
or Pd(OH)2), while the rest (i.e. 40%) is in the zero-valent state,
Pd(0).
The analysis by SIMS shows the characteristic peaks of Fe+
(with isotope patterns in the range of m/z 54–58), FeO+, FeH+
+
and FeH2 . The near-surface regions of the sample exhibited
the presence of hydrogen, carbon and hydrocarbons, and
some common electro-positive contaminants like Na+, K+ and
Ca+. The intensities of these ions were found to be rather
great; however, it should be taken into account that the posi-
tive secondary ion yields of alkaline and alkaline-earth metals
are very high as compared to other elements. Ga+ peaks are
due to analytic primary ions implanted into the samples
during analysis, and some small content of In+ ions originates
from the substrate onto which the nanocatalyst was supported.
Chlorine, a very electro-negative element, was detected which
is related to the production process.
Fig. 3 TOF-SIMS spectra of Nanocat-Fe–Pd MNPs (after 30
sputtering).
s of
equiaxial particles are visible, almost entirely detached from
each other. The corresponding EDS profile exhibits a weak Pd
peak, which is consistent with the ICP-AES results.
The as synthesized and well-characterized Nanocat-Fe–Pd
catalyst was explored for the coupling reaction of iodobenzene
with aniline using Xantphos as a ligand and cesium carbonate
as a base. The ideal reaction conditions were found using
25 mg of Fe3O4–Pd MNPs (0.023 mmoles of Pd or 8.5 mol% in
relation to aniline) to be 0.27 mmol of aniline, 0.54 mmol of
iodobenzene, 0.54 mmol of a base and 0.0134 mmol (4.9 mol%)
of Xantphos. The reaction proceeded cleanly in dioxane
during 24 h providing the coupling product, diphenylamine
(1a), in 98% yield (Table 1, entry 1). When the reaction was
performed with PPh3 as a ligand the yield decreased slightly
and the time of reaction increased substantially (entry 2). No
reaction was observed in the absence of Xantphos which
means that a ligand is essential to stabilize the Pd species over
the surface of magnetite; the literature documents that Xant-
phos is the most suitable ligand for the Buchwald–Hartwig
reaction.29 After optimization of the reaction conditions, the
catalytic activity of Fe3O4–Pd MNPs was explored with other
amine/amide substrates (Table 1).
Fig. 2 shows the surface mass spectrum, and Fig. 3 displays
the mass spectrum after 30 s of sputtering by defocusing the
Ga+ beam, i.e. after removal of a few surface layers. Using the
computer code DECO27 for identification and decomposition
of these data, we estimated that the intensity of Pd has
decreased 3 times after the sputtering (with normalization to
Fe) with approximate calculations for the concentration of Pd
on the surface (Fig. 3) using the relative sensitivity factors.
According to Wilson,28 the sensitivity of SIMS analysis to Fe is
5–6 times bigger than its sensitivity to Pd. It is interesting that
+
the normalized intensity of Fe2 ions increased 6 fold after
sputtering.
The TEM and SEM images of Fe3O4 and Nanocat-Fe–Pd
MNPs are depicted in Fig. 4a–e; particles are observed in nano
range sizes (20–35 nm). The clustering tendencies formerly
observed (Fig. 4a and b) notwithstanding, small groups of
As expected, iodobenzene gave a better result (3 fold better
than bromobenzene, Table 1, entry 1 vs. 4) which reflects the
ease of oxidative addition of the Pd(0) complex30 to the C–I
bond. With bromobenzene the yield of the reaction could only
be improved with a different amine to aryl halide ratio, 1 : 1,
instead of 1 : 2 (Table 1, entry 5 vs. 4). Exceptional results were
obtained with more electron deficient halides such as p-nitro-
bromobenzene that allowed the synthesis of p-nitro-N-phenyl-
aniline (1b) (entry 6) in quantitative yield. Electron donating
substituents on the amine substrate gave a better result than
when present as substituents in the aryl halide (entry 8 vs. 7);
p-methoxy-N-phenylaniline (1c) was obtained in 78% vs. 23%
yield. Predictably, electron withdrawing groups on the amine
substrate lowered the yield of the reaction product (entry 10 vs.
8); p-nitro-N-phenylaniline (1b) was obtained only in 45%
yield.
Fig. 2 TOF-SIMS spectra of Nanocat-Fe–Pd MNPs (surface).
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Fig. 4 SEM images of Nanocat-Fe–Pd (a) and (b); (c) EDS profile of Nanocat-Fe–Pd; (d) TEM image of Fe3O4 and (e) TEM image of Nanocat-Fe–Pd.
A moderate yield was obtained for p-bromo-N-phenylaniline
The arylation of diphenylamine with iodobenzene led to
(1e) (66%) ascribable to the formation of lateral products (not the triphenylamine (1h) in 46% yield (entry 20). Using p-nitro-
identified) probably arising from other parallel reactions due bromobenzene, the yield of p-nitro-N,N-diphenylaniline (1i)
to the reactivity of the C–Br bond in the presence of a Pd cata- was also low in yield (47%, entry 21). All the attempts to
lyst. The method proved to be of general applicability, as the improve the yield using excess of aryl halide or increasing the
primary amides, acetamide, gave acetanilide (1f) in almost temperature by changing the solvent were unsuccessful
quantitative yield (entry 12). However, for the arylation of the (entry 22). Less nucleophilic and basic substrates such as imid-
secondary amides, acetanilide, only highly reactive substrates azole or phenol were unreactive under these reaction con-
such as p-nitrobromobenzene (entry 15 vs. 13) were able to ditions (entries 18 and 19); the adsorption of phenol at the
perform the arylation to p-(4-nitrophenyl)-N-phenylacetamide surface of magnetite cannot be ruled out. Aliphatic amines
(1g) in very good yield (96%). This excellent result could only such as cyclohexylamine and tert-butylamine gave no reaction
be obtained in the presence of dry dioxane. At the reaction product (entries 23 and 24); after 24 h, iodobenzene was still
temperature, only a small amount of water is sufficient to present but no amine was observed. We cannot exclude here
hydrolyse the amide, and only 1b was observed in moderate the fact that basic amines are good ligands for palladium and,
yield (entry 14 vs. 15). Less reactive amides such as p-tolueno- consequently, being trapped in the catalyst renders it inactive.
sulfonamide afforded no product either with iodobenzene or
p-nitrobromobenzene (entries 16 and 17).
To prove that the reaction is heterogeneous, a standard
leaching experiment was conducted by the hot filtration
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Table 1 Results of C–N coupling reactions between arylhalides and nitrogenated substrates over Nanocat-Fe–Pda
Entry
Amine (1)
Aryl halide
Time (h)
Product (2)
Yield (%)
1
2
3
PhNH2
PhNH2
PhNH2
PhI
PhI
PhI
24
68
24
PhNHPh (1a)
PhNHPh (1a)
98
74b
No reactionc
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
PhNH2
PhNH2
PhNH2
PhNH2
PhBr
PhBr
pNO2C6H4Br
pMeOC6H4I
PhI
PhI
PhI
PhI
PhI
PhI
pNO2C6H4Br
pNO2C6H4Br
PhI
pNO2C6H4Br
PhI
PhI
268
96
24
144
48
120
120
21
PhNHPh (1a)
PhNHPh (1a)
30
74d
PhNHC6H4-NO2p (1b)
PhNHC6H4-OMep (1c)
PhNHC6H4-OMep (1c)
PhNHC6H4-NO2m (1d)
PhNHC6H4-NO2p (1b)
PhNHC6H4-Br (1e)
CH3CONHPh (1f)
100
23
78
56
45
66
97
No reaction
43
96
No reaction
No reaction
No reaction
No reaction
46
pMeOC6H4NH2
mNO2C6H4NH2
pNO2C6H4NH2
pBrC6Hr4NH2
CH3CONH2
PhNHCOCH3
PhNHCOCH3
PhNHCOCH3
TsSO2NH2
TsSO2NH2
Imidazole
Phenol
Ph2NH
Ph2NH
Ph2NH
tert-Butylamine
Cyclohexylamine
49
168
24
PhNHC6H4-NO2p (1b)
PhNCOCH3C6H4-NO2p (1g)e
No reaction
No reaction
No reaction
24
No reaction
PhI
pNO2C6H4Br
PhI
PhI
PhI
144
144
93
24
24
Ph3N (1h)
Ph2NC6H4-NO2p (1i)
Ph3N (1h)
47
39f
No reaction
No reaction
No reaction
No reaction
a Reaction conditions: aniline (0.27 mmol), Xantphos (0.0134 mmol; 4.9 mol%), Cs2CO3 (0.54 mmol), aryl halide (0.54 mmol), Fe3O4–Pd (25 mg;
0.023 mmoles of Pd or 8.5 mol% in relation to aniline), dioxane (2.5 mL), reflux. b PPh3 (0.0134 mmol) as a ligand. c No Xantphos. d Aniline
(0.54 mmol). e Dry dioxane. f PhI (3 equivalents) and DMF at reflux. The physical data obtained for all the products fully agree with those reported
in the literature.31
method. The reaction between aniline and iodobenzene was
allowed to proceed for 20 minutes in the presence of Fe3O4–
Conclusions
Pd, Xantphos and Cs2CO3 at reflux in dioxane (standard con- In summary, we have developed an efficient and viable hetero-
ditions, Table 1). After decanting the solution, the hot filtered geneous magnetically separable Nanocat-Fe–Pd nanocatalyst
reaction mixture was then stirred without a catalyst for 24 h; for the Buchwald–Hartwig reaction which could be prepared
no formation of diphenylamine was observed even after 24 h, from inexpensive precursors and without resorting to cumber-
which indicates that no homogeneous catalysis was involved. some linkers or ligands; various amines and amides are con-
To rule out any ambiguity due to shorter duration verted to corresponding products in good to excellent yield.
(20 minutes), the same hot filtration test was repeated for 24 h Due to sturdy interaction between Pd and ferrite MNPs, the
under the same conditions. After decanting the solution, the catalyst could be reused and recycled without any significant
hot filtered reaction mixture was then subjected to ICP ana- loss in catalytic activity and the negligible Pd leaching observed
lysis. The Pd content found was negligible (0.00054 mmoles), was not responsible for the results. The salient feature of the
of the total amount of palladium present. Independent homo- catalyst is its robustness and magnetically recoverable nature
geneous reactions performed with exactly this small amount which avoids the necessity for a traditional filtration process.
of palladium using PdCl2 were unsuccessful. No formation
of diphenlamine (1a) was observed, which means that no
homogeneous catalyst could be involved. Also, the reusability
of the catalyst was tested on the reaction for the preparation of
diphenylamine (1a) starting with aniline and iodobenzene;
Experimental
Preparation of Fe3O4 MNPs32
after five consecutive cycles only a slight loss of activity was
observed as the yields from all the experiments were above
90%.
This result is further supported by the XPS analysis of the
recovered catalyst. From XPS data (SI), no changes in the palla-
dium species were observed; the percentage of Pd(0) and PdO
(Pd(OH)2) was almost the same. This clearly indicates that
after the recyclability study, catalyst integrity remained struc-
turally intact.
FeCl3·6H2O (5.4 g) and urea (3.6 g) were dissolved in water
(200 mL) at 85 to 90 °C for 2 h. The solution turned brown. To
the resulting reaction mixture, cooled to room temperature,
was added FeSO4·7H2O (2.8 g) and then 0.1 M NaOH until
pH 10. The molar ratio Fe(III) to Fe(II) in the above system was
nearly 2.0. The obtained hydroxides were treated by ultrasound
in the sealed flask at 30 to 35 °C for 30 min. After ageing for
5 h, the obtained black powder (Fe3O4) was washed, and dried
under vacuum.
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Preparation of Nanocat-Fe–Pd
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Magnetite nanoparticles Fe3O4 (2 g), palladium chloride (to get
10 wt% of Pd on magnetite) and KCl (0.5 g) were stirred at
room temperature in aqueous solution (50 mL) for 1 h. After
impregnation, the suspension was adjusted to pH 12 by
adding sodium hydroxide (1.0 M) and further stirred for 20 h.
The solid was washed with distilled water (5 × 10 mL). The
resulting Nanocat-Fe–Pd particles were sonicated for 10 min,
washed with distilled water and subsequently with ethanol,
and dried under vacuum at 60 °C for 24 h. The Pd content was
determined by ICP-AES and it was found to be 9.89%.
Experimental procedure for the Buchwald–Hartwig reaction
catalyzed by Nanocat-Fe–Pd
Amine (0.27 mmol), Xantphos (0.0134 mmol, 4.9 mol%
in relation to aniline), Cs2CO3 (0.54 mmol), aryl halide
(0.54 mmol), Fe3O4–Pd (25 mg; 0.023 mmoles of Pd or 8.5 mol%
in relation to aniline) were stirred in dioxane at reflux for an
appropriated time After completion of the reaction (followed
by thin layer chromatography), the catalyst was separated mag-
netically, washed with ethyl acetate and the organic extracts
evaporated in a vacuum. The crude product was purified by
column chromatography on silica gel using n-hexane and ethyl
acetate as the eluents.
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Acknowledgements
This work has been supported by Fundação para a Ciência e a
Tecnologia through grants PEst-C/EQB/LA0006/2011, PEst-
C/CTM/LA0025/2011 and PEst-OE/FIS/UI0068/2011, as well as
the research fellowship (SFRH/BPD/64934/2009). The author
gratefully acknowledges the support of the Operational
Program Research and Development for Innovations – Euro-
pean Regional Development Fund (project CZ.1.05/2.1.00/
03.0058), the Operational Program Education for Competitive-
ness – European Social Fund (project CZ.1.07/2.3.00/30.0041)
of the Ministry of Education, Youth and Sports and by the
Technology Agency of the Czech Republic – “Competence
Centers” (project no. TE01020218).
13 V. Polshettiwar and R. S. Varma, Org. Biomol. Chem., 2009,
7, 37–40.
14 (a) M. B. Gawande, A. Velhinho, I. D. Nogueira,
C. A. A. Ghumman, O. M. N. D. Teodoro and P. S. Branco,
RSC Adv., 2012, 2, 6144–6149; (b) M. B. Gawande, A. Rathi,
I. D. Nogueira, C. A. A. Ghumman, N. Bundaleski,
O. M. N. D. Teodoro and P. S. Branco, ChemPlusChem,
2012, 77, 865–871.
15 M. B. Gawande, A. K. Rathi, P. S. Branco, I. D. Nogueira,
A. Velhinho, J. J. Shrikhande, U. U. Indulkar, R. V. Jayaram,
C. A. A. Ghumman, N. Bundaleski and O. M. N. D.
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