Monodentate palladium(0)-[60]fullerene complexes of diphosphine ligands as efficient and sustainable nanocatalysts for the Mizoroki-Heck coupling reaction of aryl chlorides

Article abstract of DOI:10.1039/c7nj00741h

Two mononuclear coordination complexes of fullerene[60] with Pd(dba)₂ (dba = dibenzylideneacetone), [(η²-C₆₀)Pd(Ph₂P(CH₂)₂PPh₂C(H)C(O)R)₂] (R = C₁₀H₇ (1), C₆H₄Cl (2)), have been prepared using a simple procedure to explore new directions in palladium catalysis and nanocarbon chemistry. The palladium(0)-[60]fullerene complexes incorporating unsymmetrical phosphorus ylides have been characterized by ¹H, ¹³C and ³¹P NMR spectroscopic methods and other conventional techniques such as IR, TGA, SEM, ICP-OES, EDX and TEM analysis. Attributed to the enhanced dispersity and uniform size of the Pd nanoparticles with phosphine-functionalized fullerenes, the prepared catalysts exhibited comparable catalytic activity to those reported for the Mizoroki-Heck coupling reaction of aryl chlorides. Because of their abundance and low cost, aryl chlorides are the most desirable substrates from an industrial point of view in Csp²-Csp² coupling reactions. Furthermore, overall bonding modes in the palladafullerenes of bifunctional diphosphine-based ligands were investigated by spectroscopic analysis and theoretical calculations. DFT studies of geometry-optimized monodentate and bidentate structures for 1 were calculated at the B3LYP/LANL2MB level of theory to understand the origin of the observed coordination modes.

Full text of DOI:10.1039/c7nj00741h

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Monodentate palladium(0)ꢀ[60]fullerene complexes of diphosphine ligands as
efficient and sustainable nanocatalysts for MizorokiꢀHeck coupling reaction of
aryl chlorides
Seyyed Javad Sabouncheia*, Marjan Hosseinzadeha, Morteza Zarepourꢀjevinanib
and Bahram Ghanbarib
a Faculty of Chemistry, BuꢀAli Sina University, Hamedan,65174, Iran
b Department of Chemistry, Sharif University of Technology, PO Box 11155ꢀ3516, Tehran, Iran
Abstract
Two mononuclear coordination complexes of fullerene[60] with Pd(dba)2 (dba=
dibenzylideneacetone), [(η2ꢀC60)Pd(Ph2P(CH2)2PPh2=C(H)C(O)R)2] (R= C10H7 (1), C6H4Cl(2)),
have been prepared through a simple procedure to explore new directions in palladium catalysis
and nanocarbon chemistry. The palladium(0)ꢀ[60]fullerene complexes incorporating
unsymmetrical phosphorus ylides have been characterized by 1H, 13C and 31P NMR
spectroscopic methods and other conventional techniques such as IR, TGA, SEM, ICPꢀOES,
EDX and TEM analysis. Attributed to the enhanced dispersity and uniform size of Pd
nanoparticles with phosphineꢀfunctionalized fullerenes, the prepared catalysts exhibited
comparable catalytic activity to those reported for MizorokiꢀHeck coupling reaction of aryl
chlorides. Because of their abundance and low cost, aryl chlorides are the most desirable
substrates from an industrial point of view in Csp2ꢀCsp2 coupling reactions. Furthermore, overall
bonding modes in the palladafullerenes of bifunctional diphosphineꢀbased ligands were
investigated by spectroscopic analysis and theoretical calculations. DFT studies of geometryꢀ
optimized monodentate and bidentate structures for 1 were calculated at the B3LYP/LANL2MB
level of theory to understand the origin of the observed coordination modes.
* Corresponding author. E–mail address: jsabounchei@yahoo.co.uk (S.J. Sabounchei).
1

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Keywords: Fullerene; Phosphorus ylide; Palladium nanocatalyst; Heterogeneous; Heck reaction.
Introduction
The discovery of fullerenes in 1985 by Kroto, Curl and Smalley1 opened up new vistas in
nanoscience and marked the beginning of a new field of chemical research. Transition metalꢀ
fullerene systems are currently challenging topics of interest for their modern chemistry,
physicochemical properties, and outstanding biological activities.2,3 In addition, the synthesis of
such fullereneꢀcontaining complexes and related carbon nanostructures offers the potential to
exploit the chemical reactivity, redox and electron acceptor characteristics, magnetic and optical
activity and novel structural features.4,5 Although there are many precedents for the preparation
of metallofullerenes,6ꢀ8 only few studies on the palladiumꢀfullerene complexes containing
phosphorus ligands have been reported so far. As the versatile functionalized diphosphines such
as αꢀketo stabilized phosphorus ylides can be coordinated to C60 and palladium core through the
free PPh2 group and/or formally negatively charged carbanion,9 they are interesting for
subsequent coordination chemistry studies (Scheme 1).
Scheme 1. Monoꢀ and bidentate palladium(0)ꢀ[60]fullerenes containing dppe (1,2ꢀ
bis(diphenylphosphino)ethane) ligands.
2

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During the last three decades, different palladiumꢀfullerene complexes have been developed
extensively as efficient catalysts for a variety of reactions due to their unique nanostructures and
higher surface areas than traditional Pdꢀbased catalysts.10ꢀ14 Sulman and Leszczynski first
investigated the catalytic activity of η2ꢀC60Pd(PPh3)2 complex in the selective hydrogenation of
acetylenic alcohols.10,12,14 Recently, the use of palladium catalysts supported on functionalized
fullerenes (C60) as potential nanomediators in the formation of CꢀC bonds has been also
reported.15 The successful generation of such Pd–phosphine catalysts based on stabilized
nanocarbons would provide a powerful tool for advanced organic synthesis.
The results from the studies mentioned above demonstrate clearly the possibility of using the
palladafullerenes as nanocatalysts in the MizorokiꢀHeck crossꢀcoupling reaction which is one of
the fundamental palladium(0)ꢀcatalyzed CꢀC bondꢀforming reactions. Although there are many
examples of catalytic systems for the Heck reaction,16ꢀ18 the application of highꢀperformance Pd
nanoparticles as accessible, robust, and recyclable heterogeneous catalysts specially for aryl
chlorides due to their low yield and long reaction times is still a challenge.19ꢀ21 Since aryl
chlorides are relatively abundant and inexpensive, they are the most desirable substrates from an
industrial point of view in Csp2ꢀCsp2 cross coupling reactions.
As a part of our research program aiming at the investigation of fullereneꢀbased carbon
nanostructures,9 herein, we report the synthesis, characterization and application of new
palladium(0)ꢀ[60]fullerene complexes systematically as heterogeneous nanocatalysts in the
MizorokiꢀHeck coupling reaction of unreactive aryl chlorides. Because of their electronꢀrich
phosphine ligands and special nanostructures, they might be ideal candidates for the
development of a unique class of heterogeneous catalysts. Furthermore, we turned our attention
to a detailed study of the overall bonding modes in the palladafullerenes incorporating
3

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phosphorus ylides of the type Ph2P(CH2)2PPh2=C(H)C(O)R (R= C10H7 (Y1), C6H4Cl (Y2)) acting
in a terminal monodentate Pꢀdonor and/or a bidentate chelate P,Cꢀdonor mode, by NMR
spectroscopy and DFT calculations.
Experimental
General considerations
All manipulations were carried out under dry nitrogen using standard Schlenk techniques unless
otherwise noted. Toluene and chloroform were used as reagent grade and dried over
Na/benzophenone and P2O5, respectively. C60 (+99.5%), Pd(dba)2 and all other chemicals were
purchased from commercial vendors and used without further purification. NMR spectra were
recorded at the ambient temperature on Bruker Avance 400 MHz and 250 MHz instruments in
CDCl3 or DMSOꢀd6 as solvents. Fourier transform IR spectra were recorded on a Shimadzu 435ꢀ
Uꢀ04 spectrophotometer as KBr pellets, in the 200ꢀ4000 cmꢀ1 region.
Synthesis of phosphorus ylides
These compounds were prepared using a modified literature procedure.22 A solution of triethyl
amine (0.5 mL) in toluene (1.0 mL) was added dropwise to a stirring solution of
[Ph2P(CH2)2PPh2CH2C(O)C10H7]Br (323 mg, 0.5 mmol) in toluene (3.0 mL). The reaction
mixture was stirred for an additional 20 min and then filtered off. The filtrate was concentrated
to half the original volume and petroleum ether (10 mL) was added, resulting in formation of a
yellow precipitate. The residue was collected by centrifugation and washed with additional
petroleum ether (3 × 10 mL). The precipitate was dried in vacuo to afford a bright yellow
powder. Ph2P(CH2)2PPh2=C(H)C(O)C10H7 (Y1); Yield: 246 mg (87%), M.p. 142ꢀ144 °C.
Selected IR absorption in KBr (cmꢀ1): 1577 (C=O). 31P NMR (162.15 MHz, CDCl3) δP (ppm) ꢀ
4

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3
3
1
12.93 (d, PPh2, JPꢀP = 47.99 Hz), 16.88 (d, PCH, JPꢀP = 47.99 Hz). H NMR (400.62 MHz,
CDCl3) δH (ppm) 2.81 (br, 2H, CH2), 2.24 (br, 2H, CH2), 4.30 (br, 1H, PCH), 7.23–8.05 (m,
13
27H, Har). C NMR (100.72 MHz, CDCl3) δC (ppm) 20.63 (d, PCH, 1JP–C = 71.31 Hz), 126.57–
142.09 (m, Car), 184.85 (s, CO). The 1H and 31PNMR spectral data of
Ph2PCH2PPh2=C(H)C(O)C6H4Cl (Y2) matched those reported in the literature.22
Synthesis of [(η2ꢀC60)Pd(Ph2P(CH2)2PPh2=C(H)C(O)C10H7)2] (1)
A solution of Pd(dba)2 (0.029 g, 0.05 mmol) in toluene (3 mL) was added to a C60 (0.036 g, 0.05
mmol) toluene solution being stirred, resulting in a black suspension by the end of the addition.
A solution of phosphorus ylide (0.1 mmol) in toluene was then injected slowly into the resulting
suspension. The reaction mixture was stirred for an additional 2h and consequently 30 mL of
hexane was layered over the solution. Afterwards, the solid residue was triturated with diethyl
ether twice to obtain the corresponding palladium(0)ꢀ[60]fullerene compound as a dark green
solid. Yield: 70 mg (72%), M.p. > 300 °C. Selected IR absorption in KBr (cmꢀ1): 517 (C60), 1573
3
(C=O). 31P NMR (101.25 MHz, CDCl3) δP (ppm) 16.48 (d, PPh2, JPꢀP = 43.53 Hz), 17.52 (d,
3
1
PCH, JPꢀP = 44.55 Hz). H NMR (250.13 MHz, CDCl3) δH (ppm) 2.85 (m, 8H, CH2), 4.23 (d,
2
2H, PCH, JP–H = 22.51), 7.14–8.41 (m, 54H, Har). 13C NMR (62.90 MHz, CDCl3) δC (ppm)
21.26 (m, CH2), 47.45 (d, PCH, 1JP–C = 105.67 Hz), 124.85ꢀ134.16 (m, Car), 141.56ꢀ145.75 (m,
C60), 185.12 (s, CO).
Synthesis of [(η2ꢀC60)Pd(Ph2P(CH2)2PPh2=C(H)C(O)C6H4Cl)2] (2)
Yield: 64 mg (67%), M.p. > 300 °C. Selected IR absorption in KBr (cmꢀ1): 522 (C60), 1578
3
(C=O). 31P NMR (101.25 MHz, CDCl3) δP (ppm) 16.53 (d, PPh2, JPꢀP = 43.53 Hz), 17.35 (d,
3
1
PCH, JPꢀP = 43.53 Hz). H NMR (250.13 MHz, CDCl3) δH (ppm) 2.65 (m, 8H, CH2), 4.08 (t,
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2
2H, PCH, JP–H = 22.51), 7.13–7.86 (m, 48H, Har). 13C NMR (62.90 MHz, CDCl3) δC (ppm)
21.64 (m, CH2), 47.17 (d, PCH, 1JP–C = 108.82 Hz), 128.46ꢀ133.35 (m, Car), 142.23ꢀ144.88 (m,
C60), 184.09 (s, CO).
General procedure for MizorokiꢀHeck reaction. In the presence of air, a vial was charged with
aryl chloride (1 mmol), styrene or ethyl acrylate (1.5 mmol), K2CO3 (1.5 mmol) and Pd catalyst
(0.001 mol%) in methanol (2 ml). The reaction mixture was stirred for the required period of
time at 60 ̊C to complete the reaction, as monitored by TLC. After being cooled to ambient
temperature, the mixture was extracted with nꢀhexane (15 ml). The organic layer was then dried
1
over CaCl2 and evaporated under the reduced pressure to give a crude product analyzed by H
and 13C NMR spectroscopy.
Results and discussion
Synthesis and characterization
A toluene solution of phosphorus ylides Y1 and Y2 reacted with [C60Pdx], which is achieved by
inꢀsitu reaction of C60 and Pd(dba)2, afforded new monodentate Pꢀcoordinated complexes [(η2ꢀ
C60)Pd(Y1)2] (1) and [(η2ꢀC60)Pd(Y2)2] (2) (Scheme 2). Other possible side products such as P, Cꢀ
coordinated [(η2ꢀC60)Pd(κ2ꢀY1)] and [(η2ꢀC60)Pd(κ2ꢀY2)] complexes have not been observed in
the reactions, as evidenced by spectroscopic analysis. The palladiumꢀfullerene complexes can be
precipitated as a result of slow diffusion of hexane into the obtained toluene solution. In contrast
to phosphorus ylides incorporating dppm (1,2ꢀBis(diphenylphosphino)methane) ligands, which is
able to form five membered P, Cꢀcoordinated complexes with fullerenes, the formation of six
membered P, Cꢀcoordinated complexes of dppe (1,2ꢀBis(diphenylphosphino)ethane) ones was
not favorable.9 This may be due to the ring strain effect in sixꢀmembered complexes of [(η2ꢀ
6

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C60)Pd(κ2ꢀY)] type on one hand and as a result of further attack of free PPh2 of diphosphine
ligands on the other hand, leading to formation of Pꢀcoordinated complexes [(η2ꢀC60)Pd(Y)2].
Based on the known chemistry of palladium(0)ꢀ[60]fullerene complexes,23ꢀ27 an implicit
mechanism for the formation of these complexes is proposed (Scheme 2).
bidentate chelate P,Cꢀdonor mode
was not observed
monodentate Pꢀcoordinated complexes
as the major Products
Scheme 2. Proposed pathway for the synthesis of complexes 1 and 2.
1
13
The structure of products was fully corroborated by Fourier transform infrared (FTꢀIR), H, C
and 31P NMR spectroscopy. Further conventional techniques such as scanning electron
microscopy (SEM), inductively coupled plasma optical emission spectroscopy (ICPꢀOES),
transmission electron microscopy (TEM), thermogravimetry (TGA) and energyꢀdispersive Xꢀray
spectroscopy (EDX) were also used for the characterization of the new nanocatalyst [(η2ꢀ
C60)Pd(Y1)2] (1).
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According to the scheme 2, the reaction of polymeric complex C60Pdn with either the phosphorus
ylides Y1 or Y2 leads to the η1ꢀPꢀcoordinated palladium(0)ꢀfullerene complexes 1 and 2,
1
13
respectively, without any evidence of byꢀproduct formation. In contrast to the H and C NMR
31
signals, the P resonances are usually broad and sensitive to even relatively small variations in
the chemical environment around the phosphorus atoms of the complexes. The 31P{1H} NMR
spectrum of complex 1 features two doublets at 16.48 and 17.52 ppm, shifted downfield from
that of Y1 (ꢀ12.93 and 16.88 ppm) due to the coordination of the ylides to palladium through the
31
free phosphine groups. Similarly, the P signal splitting of two phosphorus atoms at 16.53 and
17.35 ppm suggests that the coordination of the ylides has occurred in the complex 2. The
smaller downfield shifts for the both of PPh2 and PCH phosphorus atoms in AB pattern
compared to those of the AX system in bidentate phosphorus based complexes,9,22,28 indicate the
η1ꢀPꢀcoordinated mode of phosphorus ylides in the palladium(0)ꢀfullerene complexes 1 and 2.
The 1HNMR spectra of the complexes 1 and 2 in CDCl3 each exhibit one broad peak due to the
one Pꢀbound CH group at 4.23 and 4.08 ppm, respectively, revealing the Csp2ꢀP bond in these
complexes. Furthermore, no significant changes in resonance values were observed for the ylidic
protons after complexation, which is in agreement with the Pꢀcoordination character of the ylides
in these complexes. The presence of the carbonyl group in the Pꢀchelated palladium(0)
complexes 1 and 2 is also confirmed by 13C{1H} NMR spectroscopy displaying signals at 185.12
and 184.09 ppm, respectively, which remained unaffected compared to the parent ylides Y1 and
Y2 (184.85 and 183.91 ppm, respectively). The IR spectra of complexes 1 and 2 both
demonstrate a sharp ν (C=O) band at 1573 and 1578 cmꢀ1, respectively. These unchanged
absorption bands compared to the ν (C=O) stretching bands of their precursors Y1 (1577 cmꢀ1)
and Y2 (1579 cmꢀ1) suggest that the coordination of the ylides occurs only through the PPh2
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groups. Furthermore, the IR spectra of both complexes display four active bands of C60 around
522, 579, 1184 and 1435 cm−1. Therefore, the spectroscopic data lend support to the
monodentate η1ꢀPꢀcoordination in complexes 1 and 2 of their overall bonding modes.
For investigating the size and surface morphology of the prepared catalysts, SEM images of C60
and palladium(0)ꢀ[60]fullerene complex 1 were obtained as shown in Figure 1. The particles
were so strongly aggregated that discrete particles were impossible to find and average particle
size was difficult to obtain. However, the agglomerated particles were dispersed in SEM image
of complex 1 having similar or close crystallographic orientations. As depicted in Figure 1, the
catalyst particle sizes which labelled in the SEM image are in the range 43.3ꢀ 81.0 nm.
b
a
Fig. 1. SEM photographs of (a) C60, and (b) Palladium(0)ꢀ[60]fullerene complex 1.
TEM study provides further insights into the morphology and nanometer scale of the prepared
catalyst 1. The results revealed that the palladium particles were homogeneously deposited on
the surfaces of the supporting modifiedꢀfullerenes. Furthermore, the presence of spherical Pd
nanoparticles is clearly distinguished as dark spots in Figure 2, with a mean particle diameter of
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approximately 22.05 nm. TEM images show that the fullereneꢀfunctionalized palladium
nanoparticles possess almost spherical morphology with relatively good monodispersity.
Fig. 2. TEM images of palladium(0)ꢀ[60]fullerene complex 1.
The EDX measurements evidence the presence of all individual elements as expected. Elemental
spectrum determined from the EDX analysis is shown in Figure 3, which demonstrates the
presence of palladium, phosphorus, oxygen and carbon in the palladium nanostructure 1. Also, to
obtain an insight into the accurate amount of Pd in prepared catalysts, the ICPꢀOES analysis has
been applied. According to the ICP analysis, the palladium content in complexes 1 and 2 has
been determined 5.24 and 5.30 wt%, respectively.
Fig. 3. EDX analysis of palladium(0)ꢀ[60]fullerene complex 1.
10

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In order to investigate the thermal stability of the new nanocatalyst 1, TG analysis was conducted
in an inert atmosphere. As shown in Figure 4 (a), for C60, the light mass loss (3.7 wt%) below
250 ºC can be attributed to the evaporation of absorbed solvents such as water. A thermal
decomposition starting after 380 ºC, with a weight loss of about 96%, can be assigned to carbon
cage and amorphous carbon decompose in fullerene. For C60ꢀPd(0) complex 1 (Figure 4 (b)), the
heating of the compound from 300 ºC to 480 ºC resulted in 30.7% weight loss, mainly due to the
removal of phosphorusꢀcontaining functional ligand. The second weight loss occurs at
temperature range of 500ꢀ660 ºC which is related to the decomposition of fullerene (C60).
Therefore, the amount of palladium on the surface of the supporting modifiedꢀfullerene is
estimated to be 25.1% from the TG analysis.
Fig. 4. TGA spectra of (a) C60, and (b) Palladium(0)ꢀ[60]fullerene complex 1.
MizorokiꢀHeck coupling reaction
Following on initial work,9 further improvement in catalytic system design triggered the use of
new palladafullerene complexes for the Mizoroki−Heck reaction. The reaction of economical
aryl chlorides and olefines such as styrene and electronꢀdeficient ethyl acrylate is presented for
catalytic performance evaluation of the Pd catalysts. Some starting materials of the optimized
catalyst systems, mentioned in Table 1, are reported as a model.
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Table 1. Selected experimental results a.
cat. loading
[mol %]
Yield
[%]b
90
Entry
1
Olefin
Aryl chloride
Catalyst
1
2
0.01
3a
4a
94
95
93
1
2
2
3
0.01
3a
3a
3b
3b
3b
3a
3a
3a
3b
3b
3b
4b
4c
4a
4b
4c
4a
4b
4c
4a
4b
4c
92
93
1
2
0.01
90
93
1
2
4
0.01
91
91
1
2
5
0.01
88
91
1
2
6
0.01
93
92
1
2
7
0.001
0.001
0.001
0.001
0.001
0.001
92
90
1
2
8
91
89
1
2
9
89
90
1
2
10
11
12
86
88
1
2
85
88
1
2
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a
Reaction conditions for Mizoroki−Heck coupling reaction:
3 (1.5 mmol), 4 (1 mmol),
K2CO3 (1.5 mmol), methanol (2 mL), 60 ºC and catalysts
b yield of isolated product (
).
1 and 2.
5
Indeed,
our
initial
optimized
conditions
involved
[(η2ꢀ
C60)Pd(Ph2P(CH2)2PPh2=C(H)C(O)C10H7)2] (1) as the palladium nanocatalyst and K2CO3 as
base. However, the difference between bases was less pronounced under the optimized
conditions, and thus, K2CO3 was chosen as inexpensive and readily available inorganic base
(Table 2). Subsequently, the effect of different solvent systems was also investigated in the
reaction. The reactions were conducted using methanol which provides the heterogeneous
condition for the methanolꢀinsoluble catalyst, with respect to the high catalyst recovery, product
isolation, and recycling. It is inferred that the fullereneꢀfunctionalized palladium nanoparticles as
nanocatalysts display elevated catalytic properties compared to the classical catalysts. Under the
same conditions, similar catalytic activities were observed using 0.001 mol% of [(η2ꢀ
C60)Pd(Ph2P(CH2)2PPh2=C(H)C(O)C6H4Cl)2] (2) with high efficiency for the reactions described
above (Table 1).
Table 2. Optimization of base and solvent for Heck crossꢀcoupling reaction of 1ꢀchloroꢀ4ꢀ
methoxybenzene with styrene a.
Entry
Base
Solvent
Temp (°C)
Time (h)
Isolated yield (%)
1
2
Na2CO3
K2CO3
methanol
methanol
60
60
10
3
82
91
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3
4
5
6
7
Cs2CO3
NaOAc
K2CO3
K2CO3
K2CO3
methanol
60
60
2
10
4
90
77
90
86
78
methanol
DMF
120
140
100
NMP
4
dioxane
9
a
Reaction conditions: 1ꢀchloroꢀ4ꢀmethoxybenzene (1 mmol), Styrene (1.5 mmol), base (1.5
mmol) and 2 ml solvent.
It is demonstrated that switch from palladium(II)ꢀ to palladium(0)ꢀcomplexes for Csp2ꢀCsp2 bondꢀ
forming reactions allows the use of recoverable catalysts for several times, while retaining high
productivity and stability (Figure 5). Neither of heterogeneous nanocatalysts showed
considerable loss of activity after five cycles. Leaching experiments were performed to examine
the stability of the Pd nanoparticles. After the first run, ICP analysis showed that the amount of
palladium leached out into the solvent was only 1.16 ppm for 1 and 1.20 ppm for 2. There were
no Pd leachings detected for these catalysts after the first and second recycles. Besides, after
three hours for both runs, removed were the catalysts from the solutions and the reactions were
continued for three hours, while the conversion of styrene to chlorobenzene remained unchanged
implying that there were no contributions from palladium leached into reaction solutions. The
concentrations of palladium residue in the solutions were found to be 1.80 and 2.08 ppm for
catalyst 1 and 1.52 and 2.01 ppm for catalyst 2 after the fourth and fifth runs, respectively. Based
on the ICP analysis, the slight loss of reactivity was ascribed to metal leaching over the
successive reuses.
As shown in Figure 6, the structure of the reused nanocatalyst was investigated via IR spectra,
confirming the truly heterogeneous nature of the catalyst. Although the aryl chlorides bearing
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electronꢀwithdrawing groups react more slowly than electronꢀdonating ones, the conversion of
both aryl chlorides represent the desired products in high yields.
100
92
Cat. 1
Cat. 2
90
88
87
86
90
80
70
60
50
40
30
20
10
0
85
85
81
77
75
Fresh
1st recycle
2nd recycle
3rd recycle
4th recycle
Run of experiments
Fig. 5. Recycling of the catalysts 1and 2 in the Heck reaction of chlorobenzene with styrene in
methanol at 60 ºC.
a
b
c
Fig. 6. FTIR spectra of (a) Y1, (b) fresh and (c) used Palladium(0)ꢀ[60]fullerene catalyst 1.
Further decrease in catalyst loading down to 0.001 mol% and also increasing in the scale of
reaction by a factor of 10 without significant difference in catalytic performance, make them
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promising for efficient upscaling and advanced applications. Another clear advantage of using
29
the prepared palladium(0)ꢀ[60]fullerenes instead of corresponding palladium(II) complexes as
catalysts for the Heck reactions is the fact that the latter require reduction of palladium(II) by
oxidation of a phosphorus atom as a preactivation step to generate the Pd(0) active species.30
These results may confirm the validation of our initial choice to concentrate on the modification
of the Pd(II)ꢀbased catalysts substituent.
Theoretical studies
As a continuation of our previous work,9 DFT computational studies were carried out to explore
the nature of metalꢀligand bonds in (η2ꢀC60)Pd(Y1) and (η2ꢀC60)Pd(Y1)2 complexes in more
details. So, it used DFT calculations at B3LYP/6ꢀ31G*/LANL2MB level of theory using the
Gaussian 09 program for optimization of the complexes.31 The optimized structures of (η2ꢀ
C60)Pd(Y1) and (η2ꢀC60)Pd(Y1)2 complexes are shown in Figure 7. Also in Supporting
Information the Cartesian atomic coordinates of both compounds are available. It is notable that
in these complexes the Pd atom can be coordinated to two different types of C‒C bonds in the
C60 structure; the bonds at the junction of two hexagons, [6,6]ꢀbonds, and those between one
pentagon and one hexagon [5,6]ꢀbonds. In this work, the same as our previous work and based
on experimental observations for similar complexes,9, 32 we have assumed the Pd atom to be only
attached to a [6,6]ꢀdouble bond.
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(η2ꢀC60)Pd(Y1)
(η2ꢀC60)Pd(Y1)2
Fig. 7. The optimized structures of (η2ꢀC60)Pd(Y1) and (η2ꢀC60)Pd(Y1)2 complexes calculated by
B3LYPꢀ6ꢀ31G*/LANL2MB basis set.
Based on DFT calculations at BP86ꢀD3/TZP level of theory, Energy decomposition analysis
(EDA) was performed to study the theory values of the palladium‒C60 and palladium‒Y1 bond
energies in (η2ꢀC60)Pd(Y1) and (η2ꢀC60)Pd(Y1)2 complexes. The EDA decomposes the interaction
energy between two considered fragments A and B into four terms which can be interpreted in a
chemically meaningful way. The four terms are including the quasiclassical electrostatic
interaction between the frozen charges of the fragments (∆Eelec); the exchange (Pauli) repulsion
between electrons possessing the same spin (∆EPauli); the orbital interaction term (∆Eorb) and the
dispersion contributions (∆Edisp).
The EDA results are obtained by DFT calculation and collected in Table 3. The interaction
energy values, ꢁEint, between (η2ꢀC60)Pd and phosphorus ylide (Y1) fragments are somewhat
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larger than those between (η2ꢀC60) and Pd‒phosphorus ylide (Y1) fragments. As can be seen, the
percentage contribution of ꢁEelec term in the series of studied complexes is in the range of 52ꢀ
57% for (η2ꢀC60)PdꢀY and 53ꢀ54% for (η2ꢀC60)ꢀPdY bond. Indeed, in these complexes and for
both (η2ꢀC60)‒PdY and (η2ꢀC60) Pd‒Y bonds the contribution of ∆Eelec term in total interaction
energy is larger than ∆Eorb term.
Table 3. Energy decomposition analysis (EDA) data [kcal/mol] at BP86ꢀD3/TZP//B3LYPꢀ6ꢀ31G*/
LANL2MB level of theory.
Complexes
Fragments
∆Eint
∆EPauli
129.25
189.13
∆Eelec
∆Eorb
∆Edisp
(η2ꢀC60)Pd...(Y1)
(η2ꢀC60)...Pd(Y1)
ꢀ86.31
ꢀ69.87
ꢀ123.09(57%)
ꢀ140.14(54%)
ꢀ71.42(33%)
ꢀ98.55(38%)
ꢀ21.05(10%)
ꢀ20.31(8%)
(η2ꢀC60)Pd(Y1)
(η2ꢀC60)Pd...(Y1)2
(η2ꢀC60)...Pd(Y1)2
ꢀ93.55
ꢀ78.43
90.23
ꢀ96.38(52%)
ꢀ151.13(53%)
ꢀ52.90(29%)
ꢀ34.50(19%)
(η2ꢀC60)Pd(Y1)2
206.86
ꢀ109.57(38%) ꢀ24.59(9%)
In accordance with the spectroscopic results, it is demonstrated that the ylide (Y1) acted in a
terminal monodentate Pꢀdonor mode to form Pꢀcoordinated complex. The calculated values for
interaction energy are consonant with the above observation. As can be seen in Table 3, the value
of interaction energy between (η2ꢀC60)Pd and Y1 in (η2ꢀC60)Pd(Y1)2 complex (ꢀ93.55 kcal/mol) is
larger than that between (η2ꢀC60)Pd and Y1 in (η2ꢀC60)Pd(Y1) (ꢀ86.31 kcal/mol). Thus, two Pꢀ
coordinated Y1 molecules in (η2ꢀC60)Pd(Y1)2 complex have a larger interaction with (η2ꢀC60)Pd
than one P, Cꢀcoordinated Y1 molecule. In another word, the Pꢀcomplexation with C60 or linear
product seems to be more probable and more stronger than bidentate P, Cꢀchelation with C60.
Additionally, more experiments were done to confirm the above results and the following
reaction was studied by theoretical calculations.
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(η2ꢀC60)Pd(Y1) + Y1 → (η2ꢀC60)Pd(Y1)2
ꢁE = ꢀ9.16 kcal/mol
As is clearly shown, the formation of Pꢀcoordinated (η2ꢀC60)Pd(Y)2 complex by addition of ylide
Y1 to P, Cꢀcoordinated (η2ꢀC60)Pd(Y1) complex with negative value for differences energy, ꢁE,
is exothermic which can be done spontaneously. These results suggest that the (η2ꢀC60)Pd(Y1)2
complex with two ligands behave as monodentate Pꢀdonors highly supported by the data of
NMR spectroscopy.
The visualization of the electron distribution in various orbitals at different electronic states, e.g.
HOMO, HOMOꢀ1, LUMO, and LUMO+1 for (η2ꢀC60)Pd(Y1) and (η2ꢀC60)Pd(Y1)2 complexes
was also evaluated by DFT calculations (Figure 8). It is intuitive that the HOMOs distribution of
the (η2ꢀC60)Pd(Y1) and (η2ꢀC60)Pd(Y1)2 complexes, as expected, is essentially situated on
palladiumꢀylide moiety while LUMO and LUMO+1 are located on the C60 entity as an acceptor
molecule. Fullerenes, as excellent πꢀacceptor molecules, differ from other acceptor molecules
with wellꢀknown features in terms of their larger size, spherical shape, high symmetry, unique
electronic structure, and polarizability. In donorꢀacceptor dyad systems with C60, like the current
study, fullerenes act as acceptor moieties.
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Fig. 8. HOMOs and LUMOs of (η2ꢀC60)Pd(Y) (top) and (η2ꢀC60)Pd(Y)2 (bottom) complexes in
various electronic states, e.g., (a) HOMO, (b) HOMOꢀ1, (c) LUMO, (d) LUMO+1 done by
DFT/B3LYP/6ꢀ31G* calculations.
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Conclusion
The present study combines the organoꢀfullerene chemistry with the phosphineꢀbased Pd
catalytic systems to report the new palladium(0)ꢀ[60]fullerene complexes as efficient
nanocatalysts for MizorokiꢀHeck crossꢀcoupling reaction. The high activity and enhanced
stability of the catalysts can be attributed to the nature of palladiumꢀbased nanoparticles and the
bulky diphosphine ligands. On the other hand, a simple preparation procedure of the reusable
nanocatalysts and the high catalytic performance for unreactive aryl chlorides with low catalyst
loading were recognized as their most influential factors. Based on the spectroscopic results, the
exclusive formation of Pꢀcoordinated (η2ꢀC60)Pd(Y)2 complexes was achieved using appropriate
phosphonium ylides. DFT calculations also confirmed that the terminal monodentate Pꢀdonor
mode of applied phosphonium ylides is more stable than the bidentate chelate P,Cꢀdonor mode in
complexes 1 and 2. This promising field will go on further to elucidate the steric and electronic
factors for the binding energy of bifunctional diphosphineꢀbased ligands on metallofullerene
surfaces that influence their catalytic activity in other types of coupling reactions.
Acknowledgements
Funding of our research from the BuꢀAli Sina University is gratefully acknowledged.
Notes and references
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Monodentate palladium(0)-[60]fullerene complexes of diphosphine ligands as
efficient and sustainable nanocatalysts for Mizoroki-Heck coupling reaction of
aryl chlorides
Seyyed Javad Sabouncheia*, Marjan Hosseinzadeha, Morteza Zarepour-jevinanib
and Bahram Ghanbarib
a Faculty of Chemistry, Bu-Ali Sina University, Hamedan,65174, Iran
b Department of Chemistry, Sharif University of Technology, PO Box 11155-3516, Tehran,Iran
R
O
Ph P
PPh
Ph
P
P
Ph
O
R
Heterogeneous Palladium Nanoparticle Catalysts
High Levels of Yield (up to 93%)
The present article combines fullerene chemistry with phosphine-based Pd catalytic systems to
report the new palladium(0)-[60]fullerene complexes as heterogeneous nanocatalysts for carbon–
carbon bond transformations of unreactive aryl chlorides.
* Corresponding author. E–mail address: jsabounchei@yahoo.co.uk (S.J. Sabounchei).