Dinuclear macrocyclic palladium complexes having pincer coordinating groups and their catalytic properties in Mizoroki-Heck reactions

Article abstract of DOI:10.1016/j.jorganchem.2011.08.024

An improved synthetic method of palladium(II) dinuclear macrocyclic complexes have been described. Each of the two isomers of the complexes [Pd ₂LBr₂] has a macrocyclic ligand L in which two 2,6-bis(diaminomethyl)phenyl units coordinate to the Pd(II) centers with N, C, N donor atoms. Substitution of the bromo ligands of one of the isomer of the complexes with acetonitrile ligands affords a new dinuclear complex. Catalytic activities of these complexes were studied for the Mizoroki-Heck type reactions of iodobenzene and styrene. High turnover number up to 30,000 was achieved using one of the isomer of the complexes.

Full text of DOI:10.1016/j.jorganchem.2011.08.024

Journal of Organometallic Chemistry 696 (2011) 3657e3661
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Dinuclear macrocyclic palladium complexes having pincer coordinating groups
and their catalytic properties in MizorokieHeck reactions
1
2
*
Taro Tsubomura , Mutsumi Chiba, Shogo Nagai, Mariko Ishihira, Kenji Matsumoto , Toshiaki Tsukuda
Department of Materials and Life Science, Seikei University, Kichijoji-kitamachi, Musashino, Tokyo 180-8633, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 May 2011
Received in revised form
An improved synthetic method of palladium(II) dinuclear macrocyclic complexes have been described.
Each of the two isomers of the complexes [Pd LBr ] has a macrocyclic ligand L in which two
,6-bis(diaminomethyl)phenyl units coordinate to the Pd(II) centers with N, C, N donor atoms. Substi-
tution of the bromo ligands of one of the isomer of the complexes with acetonitrile ligands affords a new
dinuclear complex. Catalytic activities of these complexes were studied for the MizorokieHeck type
reactions of iodobenzene and styrene. High turnover number up to 30,000 was achieved using one of the
isomer of the complexes.
2
2
2
1
August 2011
Accepted 18 August 2011
Keywords:
Macrocyclic complexes
Pincer complexes
Dinuclear complexes
MizorokieHeck reaction
Catalysis
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
chloroformeether in 10% yield. The low-yield of the compound
prevented us to do further study for catalysis etc. Recently, we
A large number of ECE pincer complexes (E ¼ donor atoms or
groups such as NR , PR , SR, etc) have been reported due to the
found a new and easy method to obtain both isomers with
improved yields. In this paper, the improved method, the substi-
tution reaction of the bromo ligand, and MizorokieHeck-type
catalytic reactions mediated by the complexes are presented.
2
2
potential applications in diverse fields such as catalysis, polymer
and dendrimer chemistry, and pharmaceuticals [1e4]. In these
studies, palladium complexes have been widely explored because
of their activity in many organic reactions [5]. A number of mech-
anistic studies have been carried out, and there has been contro-
versy on this topic [6].
2. Results and discussion
In 2001, we reported the synthesis and the structure of macro-
cyclic palladium complexes bearing two pincer ligating groups [7].
The complex has two isomers; U-type (Pd-U) and Z-type (Pd-Z),
which are shown in Scheme 1. Overall conformations of the two
isomers are different from each other, and the configurations
around the coordinated nitrogen atoms are also different for the
two isomers. In the original paper, two isomers were obtained by
two distinct methods. Pd-Z was prepared by lithiation of the
2.1. Synthesis and characterization of Pd-U and Pd-Z
After the publication of the synthetic work of the two isomers of
the macrocyclic complexes, we studied more convenient and
effective route to synthesize the complexes. Recently, we found that
1) the reaction of the dibromo ligand precursor, LBr , and
2
“Pd(dba) ” affords a mixture of the two isomers and 2) the two
2
isomers show clearly different solubility in ethanol. Pd-U is far
dibromo precursor (LBr
the lithium complex with [PdBr
<5%). On the other hand, Pd-U was directly obtained by the
reaction of LBr with “Pd(dba) ”. Pure Pd-U was obtained from
2
, see Scheme 1) followed by the reaction of
more soluble in ethanol at r.t. than Pd-Z, so that the two isomers
can be easily separated from the mixture of the two isomers. The
products in the reaction depend on the solvent used in the reaction.
2
(cod)], but the yield was very low
(
2
2
The reaction in chloroform and work-up with ethanol gave Pd-Z in
1
5
3% yield. The signals due to Pd-U were not detected by H NMR in
*
Corresponding author. Tel.: þ81 422 37 3752; fax: þ81 422 37 3871.
the filtrate after isolation of Pd-Z when the reaction was performed
in chloroform. On the other hand, both the Pd-Z and Pd-U were
isolated using THF as the reaction solvent. Separation of the isomers
using ethanol gave the Z and U isomers in 10% and 31%,
respectively.
E-mail address: tsubomura@st.seikei.ac.jp (T. Tsubomura).
1
Present address: Department of Chemistry, Faculty of Science, Kochi University,
2
-5-1 Akebono-cho, Kochi 780-8520, Japan.
Present address: Faculty of Education Human Sciences, University of Yamana-
2
shi, 4-4-37 Takeda, Kofu, Yamanashi 400-8510, Japan.
0
022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.08.024

3658
T. Tsubomura et al. / Journal of Organometallic Chemistry 696 (2011) 3657e3661
Fig. 1. The structure of Pd-Z2 showing the numbering scheme of the atoms. Thermal
ellipsoids are drawn at 50% probability level.
2 3 2
Pd L(CH CN)
]2 cation, two hexafluorophosphate anions, and
þ
[
two benzene molecules of solvation. The complex cation has
a crystallographic center of inversion at the central position of the
complex, as found in the crystal structure of the Pd-Z complex. N1
and N2 are asymmetric nitrogen atoms, however, the complex
cation itself shows an achiral meso-type structure. For the coordi-
nation of the macrocyclic ligand, the average PdeN (pincer ligand)
bond length for Pd-Z2 (2.11 Å) is somewhat shorter than that of Pd-
Z and Pd-U but the differences are very small (see Table 1). PdeC1
(1.924(4) Å) length is slightly longer than the other two complexes.
The two previous complexes have bromo ligands located at the
trans position of the C atoms, which may be the main reason of the
shorter PdeC bonds (1.924(4) Å). PdeN (acetonitrile) bond length
(2.149(4) Å) is somewhat longer than those in the previously re-
ported similar N^C^N pincer Pd(II) complexes with an acetonitrile
Scheme 1. Preparation of the macrocyclic complexes.
As reported in the previous paper [7], the two isomers can be
1
clearly characterized by H NMR. Aromatic protons of Pd-Z are
observed in more low-field region than those of Pd-U. Recently we
found that Pd-U slowly changes to Pd-Z in chloroform. In the
chloroform-d solution of Pd-U, the NMR signals gradually dimin-
ished, and those of Pd-Z appeared. After 50 days kept at room
temperature, the signals of both complexes were found with about
ꢀ
1
8
:1 ratio. At 45 C, Pd-U isomerized almost completely to Pd-Z after
days (see Fig. S1(a) in supporting information). On the other hand
ꢀ
the NMR spectrum of Pd-Z showed no change even at 45 C in 8
days (Fig. S1(b)). The result demonstrates that Pd-Z is more ther-
modynamically stable than the Pd-U complex. Obviously, the
synthesis of Pd-U in THF is kinetically controlled.
ꢀ
ligand [9]. The bond angle, C1-Pd-N3, is almost linear (176.9(2) ).
2.3. Catalytic activity
2.2. Synthesis and structure of the acetonitrile complex Pd-Z2
In order to examine the catalytic activity of the complexes,
MizorokieHeck type reaction was studied (Scheme 2). The reaction
of iodobenzene (2 mmol) and styrene (3 mmol) were investigated.
The results under various reaction conditions are shown in Table 2.
The reaction products are trans-stilbene and 1,1-diphenylethylene
Considering that an acetonitrile ligand is generally more labile
than a halogeno ligand [8], we thought that the macrocycle
complex bearing acetonitrile ligands may be more suitable as
a catalyst. Therefore substitution of the bromo ligands of the
macrocyclic complex with acetonitrile ligands has been studied.
Reaction of Pd-Z with silver hexafluorophosphate in acetonitrile
(gem-stilbene), and the ratio of the two products was almost
always constant, ca. 6:1 (Fig. 2). It should be noted that oxygen-free
condition is not necessary for the reaction although dry solvent is
necessary to drive the reaction (see run 1 and 2).
Almost no difference has been found in the catalytic activities of
the Pd-U and Pd-Z complexes (compare runs 2 vs. 3, 5 vs. 6 and 7 vs.
gave the bis(acetonitrile)-substituted complex, [Pd
PF , Pd-Z2, with a moderate yield. In the aromatic region in the
H NMR spectrum, a triplet at 7.05 ppm and a doublet at 6.85 ppm
2 3 2
L(CH CN) ]
(
6 2
)
1
are observed, which is similar to Pd-Z. One more characteristic
feature of the NMR signals of these macrocyclic complexes is the
presence of AB doublet of doublets at ca. 4 ppm, which is assigned
to the inequivalent methylene protons adjacent to the aromatic
groups. The difference in the chemical shifts between the two
doublets for Pd-U and Pd-Z are 0.10 and 0.37 ppm [7], respectively.
For Pd-Z2, the difference is 0.23 ppm, which is in the middle of the
value of Pd-Z and Pd-U.
8). One possible interpretation of the result was that, in the heated
Table 1
Comparison of the structural parameters of palladium macrocycles.
2þ
[
Pd
2.109(4),
.116(4)
1.924(4)
2.149(4) X ¼ N
2
L(CH
3
CN)
2
]
2 2 2 2
, (Pd-Z2) [Pd LBr ] (Pd-Z) [Pd LBr ] (Pd-U)
PdeN
2.131(4),
2.104(6),
2
2.122(4)
2.136(6)
The crystal structure of the Pd-Z2, [Pd
2
L(CH
3
CN)
2
](PF
6
)
2
$
PdeC
PdeX
1.919(5)
1.914(6)
(
C
6
6
H )
2
, was clarified by X-ray crystallography. The structure of the
2.569(1) X ¼ Br
2.562(2) X ¼ Br
N1ePdeN2 163.6(2)
C1ePdeX
176.9(2) X ¼ N
162.5(2)
164.8(2)
complex cation is shown in Fig. 1 and the important structural
parameters are listed in Table 1. The triclinic unit cell contains one
173.7(2) X ¼ Br
173.9(2) X ¼ Br

T. Tsubomura et al. / Journal of Organometallic Chemistry 696 (2011) 3657e3661
3659
Table 2
1
00
The MizorokieHeck-reaction of iodobenzene and styrene using dinuclear palladium
complexes as catalysts.
trans-stilbene
a
1,1-diphenylethylene
Run Catalyst (mol%) Temp/ C Time/h Trans Gem Yield Tonb
ꢀ
Solvent
0
DMFb
89 DMF
93 DMF
8
6
4
2
0
0
0
0
0
total
1
2
3
4
5
6
7
8
9
1
1
1
Z (1)
Z (1)
U (1)
Z (1)
Z (0.5)
U (0.5)
Z (0.1)
U (0.1)
Z2 (0.1)
Z (0.01)
Z2 (0.01)
Z (0.001)
130
130
130
150
130
130
150
150
150
150
150
165
5
5
5
4
5
5
4
4
5
0
0
0
89
93
85
90
88
90
93
61
94
77
81
72
77
76
75
75
51
78
12
12
13
13
12
15
18
10
16
85 DMF
180 DMF
178 DMF
900 DMF
930 DMF
610 DMF
9400 DMF
DMF
0
1
2
22
24
24
Trace Trace
24 5.5
30
30,000 DMA
a
Conditions; iodobenzene (2 mmol), styrene (3 mmol), palladium catalyst, and
0
1
2
3
4
5
tributylamine (6 mmol) dissolved in the dried solvent (10 mL) unless otherwise
noted. See Experimental section for details.
time / h
b
Non-dried solvent was used.
1
00
0.1mol% Pd-Z
0.1mol% Pd-U
0.1mol% Pd-Z2
possible reaction is the exchange of the bromide ligands with the
solvent molecules.
80
60
40
20
0
ꢀ
At the catalyst load of 0.1 mol %, higher temperature, 150 C, was
necessary to complete the reaction in tolerable reaction time (run 7
ꢀ
and 8). At elevated temperature (165 C), high turn-over-number,
30,000, was obtained (run 12). Although some papers reported
that acetonitrile complexes have good catalytic activities [10], the
activity of Pd-Z2 is considerably low (run 9 and 11).
MizorokieHeck reaction mediated by the Pd(II) pincer type
complexes have been extensively studied. Some papers pointed out
that colloidal palladium(0) particles released from the pincer
complexes at elevated temperature act as real catalyst [6,11,12].
Meanwhile, DFT calculations show that Pd(II)ePd(IV) cycle without
decomposing the complexes is a viable mechanistic alternative to
a palladium nanoparticle catalyzed reaction [13]. We have no
information on the reaction mechanism of the catalysis mediated
by our pincer dinuclear complexes; however it should be pointed
out that decomposition of pincer complexes under the applied
reaction conditions starts with the coordination of amine to
palladium by opening one of the side arms [14]. In our complexes,
palladium atoms are in the macrocyclic ligands, so it is reasonably
accepted that the side arm of the pincer coordinating moiety are
more resistant to opening than normal pincer ligands. Other thing
to be considered is that the acetonitrile complex has considerably
low catalytic activity; the bromo ligands possibly contribute to
stabilize the þ4 state of the palladium atom.
0
1
2
3
4
5
time / h
Fig. 2. The yields vs. time plot for the MizorokieHeck reaction. Upper, Catalyst Pd-Z
ꢀ
ꢀ
(
1 mol%), 130 C; lower, Catalyst Pd-Z, Pd-U and Pd-Z2 (0.01 mol%), 150 C, total
yield of the products is shown.
solution, Pd-U rapidly isomerizes to Pd-Z as stated above. In order
to investigate the Pd species in DMF solution, the change in the
NMR spectra has been studied. Pd-Z in DMF-d7 solvent showed
a well-resolved NMR spectrum similar to that in chloroform-
d (Fig. S2(a)). However, Pd-U gave broad signals in DMF-d7, the
chemical shift values of which were similar to those of Pd-U
recorded in chloroform-d (see Fig. S2(b)). After heating in DMF-
ꢀ
d7 at 100 C for 1 h, the NMR of Pd-Z showed no change, but that
3. Conclusion
of Pd-U indicated a considerable low-field shift of the aromatic
protons, which suggested that isomerization from Pd-U to Pd-Z
occurred. The chemical shift values of Pd-U after heating were
similar to those of Pd-Z, but the spectrum yet showed broad signals
as before the heating. Although the detailed picture of the behavior
of the complexes in DMF solution is not clear, the results show that
Pd-U is more unstable than Pd-Z in DMF, and some reactions should
occur for Pd-U besides the isomerization reaction. One of the
We found the improved synthetic methods of the previously
reported two isomers, Pd-U and Pd-Z, of the macrocyclic dinuclear
palladium(II) complexes. The bromo ligands of the dinuclear
macrocyclic palladium(II) complexes can be substituted with
acetonitrile ligands and the Z-type structure was found for the
acetonitrile complex by X-ray. The catalytic activities of the
complexes toward the MizorokieHeck reaction of iodobenzene and
styrene have been studied, and very high turnover numbers were
recorded for the reaction.
Pd(II) catalyst
4. Experimental
I
+
+
All reagents were purchased from commercial suppliers and
used without further purification. Acetonitrile and THF were
trans
gem
dried over molecular sieves 3A and 5A, respectively, for overnight.
Scheme 2. Mizoroki-Heck reaction studied in this work.
2,6-Bis(bromomethyl)bromobenzene
was
prepared
by

3660
T. Tsubomura et al. / Journal of Organometallic Chemistry 696 (2011) 3657e3661
bromination of 2-bromo-m-xylene with N-bromo-succinimide
15] “Pd(dba) ”, Pd (dba) $dba, was synthesized by a literature
method [16]. NMR spectra were obtained with a JEOL L-400 or
catalyst. The mixture was heated on an oil bath. The products were
analyzed with a gas chromatograph (Shimadzu model GC-1700)
equipped with a CP-Select 624 CB capillary column (0.32 mm id,
30 m) and an FID detector.
[
2
2
3
a
D
-500 spectrometer.
4.1. Macrocyclic ligand precursor LBr
2
6 6 2
4.5. Crystal structure determination of [Pd L(CH CN) ](PF ) $(C H )
2
3
2
6 2
1
.5
g
(4.4 mmol) of 2,6-Bis(bromomethyl)bromobenzene,
All measurements were made on a Rigaku Saturn CCD area
detector with graphite monochromated MoK radiation. A color-
0
0
2
.54 mL (4.4 mmol) of N,N -dimethyl-1,3 propanediamine, and
.84 g (8.8 mmol) of cesium carbonate were successively added to
acetonitrile (1 L). The mixture was stirred for 7 days at room
temperature. The precipitates were filtered off, and then the filtrate
was concentrated under reduced pressure by a rotary evaporator to
give a pale yellow solid. To the solid was added toluene with
a
3
less crystal of the dimensions 0.2 ꢁ 0.2 ꢁ 0.2 mm was used. The
ꢀ
data were collected at a temperature of ꢂ150 ꢃ 1 C to a maximum
ꢀ
2q value of 54.9 . A total of 1800 oscillation images were collected.
The crystal-to-detector distance was 55.01 mm. Of the 10,259
reflections that were collected, 4770 were unique (Rint ¼ 0.031);
equivalent reflections were merged. Data were collected and pro-
ꢀ
heating at 70 C until all solid was dissolved. After the solution was
left in a refrigerator overnight, the compound was obtained as
colorless needles were obtained by filtration (yield, 16%).
cessed using CrystalClear [17]. The linear absorption coefficient,
m
,
ꢂ1
for MoK
a
radiation is 9.294 cm . A numerical absorption correc-
tion was applied which resulted in transmission factors ranging
from 0.711 to 0.830. The data were corrected for Lorentz and
polarization effects.
4
.2. [Pd
2
LBr
2
] (Pd-U and Pd-Z)
(0.20 g) and “Pd
Pd-Z: LBr
2
2
(dba)
2
” (0.32 g) were added to 25 mL
The structure was solved by a direct method [18] and expanded
using Fourier techniques. Some non-hydrogen atoms were refined
anisotropically, while the rest were refined isotropically. Some
hydrogen atoms were refined isotropically and the rest were refined
using the riding model. The final cycle of full-matrix least-squares
ꢀ
of chloroform, and then the mixture was stirred for 24 h on a 35 C
water bath under argon atmosphere. After the reaction, the mixture
was filtered and the filtrate was concentrated to dryness in a rotary
evaporator. Ethanol was added to the residue, and the Z-form of the
complex was obtained by filtration. (144 mg, 53%).
2
refinement on F was based on 4770 observed reflections and 318
Pd-U: The above reaction was performed in 25 mL of THF instead
variable parameters and converged (largest parameter shift was
0.09 times its esd). All calculations were performed using the
CrystalStructure [19] crystallographic software package using
SHELXL-97.
of chloroform. The mixture obtained after the reaction (24 h at
ꢀ
3
(
1
5
C) was concentrated to a half volume and filtered. Ethanol
30 mL) was added to the yellow-brown residue and stirred for
0 min. Filtration of the mixture gave a brownish yellow solid. The
filtrate of the ethanol solution was evaporated to dryness, and
0 mL of toluene was added and stirred for 5 min, then U-form of
the complex was obtained by filtration as a pale yellow powder
31%). The brownish yellow solid was recrystallized from chloro-
Crystal Data; [Pd L(CH CN) ](PF ) $(C H ) : C H N Pd(PF )
2
3
2
6 2
6
6 2
15 22
3
6
(C H ) (fw: 572.85), a ¼ 10.5615 (2), b ¼ 10.8288(1), c ¼ 12.1443
6
6
ꢀ
2
(8) Å,
a
¼ 65.433 (15),
b
¼ 65.698 (15),
g
¼ 75.691 (17) , V ¼ 1146.4
3
3
(2) Å , Space Group ¼ P-1 (#2), Z ¼ 2, Dcalc ¼ 1.549 g/cm ,
ꢂ1
ꢀ
(
m
(MoK
a
) ¼ 9.42 cm , MoK
a
(
l
¼ 0.71070 Å), 2
q
max
¼ 54.9 No. of
form (20 mL) to give the Z form of the complex (10%). NMR chemical
shift values of Pd-U and Pd-Z coincide with the reported data [7].
Reflections total: 10,259, Unique: 4770 (R
Variables
¼ 0.031), No. of
int
¼
242, R1 (I
>
2.00
s
(I))
¼
0.0474, R (All
reflections) ¼ 0.0534, wR (All reflections) ¼ 0.143, GOF ¼ 1.058,
2
4
2
.3. [Pd L(CH
3
CN)
2
](PF
6
)
2
(Pd-Z2)
Max. shift/error in the final cycle ¼ 0.001, Max. and min. electron
3
density in the final Diff. Map ¼ 2.01 and ꢂ0.95 e/Å
A
dichloromethane solution (10 mL) containing 65 mg
(
0.26 mmol) of AgPF was added dropwise to an acetonitrile solu-
6
Acknowledgment
tion of the Z-form of the Pd complex (51 mg, 0.065 mmol) in the
dark. The mixture was stirred for 3 h, and the pale yellow
suspension was filtered. The filtrate was concentrated to dryness in
an evaporator. 30 mL of acetonitrile was added to the residue and
the solution was again concentrated to dryness. The residue was
then washed with 10 mL of chloroform and then with 30 mL of
acetonitrile. Light-brown colored powders of the complex (38 mg,
This study has been in part supported by a grant of “Strategic
Research Base Development Program for Private Universities” from
the Ministry of Education, Culture, Sports, Science & Technology in
Japan.
Appendix. Supplementary material
5
7%) were obtained. 1H NMR (400 MHz, CD
3
CN), 7.05t (2H,
J ¼ 7.4 Hz, 4-CH), 6.85d (4H, J ¼ 7.4 Hz, 3- and 5-CH), 4.30d (4H,
CCDC 824070 contain crystallographic data for Pd-U2. These
data can be obtained free of charge from The Cambridge Crys-
tallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
J ¼ 15.7 Hz, AreCHH), 4.07 (4H, J ¼ 15.7 Hz, AreCHH), 3.37m (4H,
3 2 2 2
NeCHH), 2.93s (12H, CH ), 2.84m (4H, CH CH CH ), 2.66m (4H,
NeCHH). Anal.; C, 35.07; H, 4.71; N, 8.18. Calcd. for
Pd (1/2CHCl , 1051.2): C, 34.85; H, 4.27; N, 7.99.
C
H
30 44
F
12
N
P
6 2
2
3
Crystals for X-ray analysis were grown from acetonitrile-benzene
solution to give colorless crystals.
Appendix. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2011.08.024.
4.4. Catalysis
DMF and DMA of the dried synthetic grade were used as the
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added to the flask followed by the tributylamine (6 mmol) and the
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