2-Phenylimidazole-PdCl2 and 2-phenylimidazoline-PdCl2 complexes: Single-crystal and powder X-ray diffractometry,1H NMR spectra, and comparison of catalytic activities in coupling reactions

Article abstract of DOI:10.1021/om800230y

Single-crystal X-ray diffractometry and powder X-ray diffractometry of the complexes dichlorobis(2-phenyl-1H-imidazole)palladium(II) (1) and dichlorobis(2-phenyl-4,5-dihydro-1H-imidazole)palladium(II) (2) and ¹H NMR structure elucidation in DMF-d₇ solution are discussed. In the case of complex 1, we found that changing the solvent for recrystallization from a mixture of DMF and toluene (method A) to only DMF (method B) afforded complexes 2 and 2′, having crystallographically different structures. ¹H NMR studies indicated that the spectra of the solid samples 2 and 2′ are the same in DMF-d₇. This indicated that the Pd-N bond rotates easily in solution, whereas complexes 2 and 2′ coexist through strong packing in the solid state. The catalytic activities of Pd complexes 1 and 2 in coupling reactions such as the Mizoroki-Heck reaction and Suzuki-Miyaura coupling will also be disclosed.

Full text of DOI:10.1021/om800230y

3748
Organometallics 2008, 27, 3748–3752
2-Phenylimidazole-PdCl2 and 2-Phenylimidazoline-PdCl2
1
Complexes: Single-Crystal and Powder X-ray Diffractometry, H
NMR Spectra, and Comparison of Catalytic Activities in Coupling
Reactions†
Kenjiro Kawamura, Satoshi Haneda, Zhibin Gan, Kazuo Eda, and Masahiko Hayashi*
Department of Chemistry, Graduate School of Science, Kobe UniVersity, Nada, Kobe 657-8501, Japan
ReceiVed March 11, 2008
Single-crystal X-ray diffractometry and powder X-ray diffractometry of the complexes dichlorobis(2-
phenyl-1H-imidazole)palladium(II) (1) and dichlorobis(2-phenyl-4,5-dihydro-1H-imidazole)palladium(II)
1
(2) and H NMR structure elucidation in DMF-d7 solution are discussed. In the case of complex 1, we
found that changing the solvent for recrystallization from a mixture of DMF and toluene (method A) to
only DMF (method B) afforded complexes 2 and 2′, having crystallographically different structures. 1H
NMR studies indicated that the spectra of the solid samples 2 and 2′ are the same in DMF-d7. This
indicated that the Pd-N bond rotates easily in solution, whereas complexes 2 and 2′ coexist through
strong packing in the solid state. The catalytic activities of Pd complexes 1 and 2 in coupling reactions
such as the Mizoroki-Heck reaction and Suzuki-Miyaura coupling will also be disclosed.
have reported on diphosphinoazine-Pd(II) complexes that
feature a novel tridentate ligand for Mizoroki-Heck reactions
with high TON and TOF (h-1).6 There have been various
attempts to improve the Mizoroki-Heck reaction by activating
aryl chloride using Pd compounds such as (CH3CN)2-
PdCl2-PPh4Cl,7 Pd(OAc)2 with excess P(OEt)3,8 and hetero-
geneous Pd/C9 as catalysts.10
On the other hand, nitrogen-based ligands, such as pyridine
or imidazole derivatives, have attracted much interest among
organic chemists because of their low toxicity and high stability
compared to phosphine-based ligands. However, there are few
reports of highly catalytic reactive complexes bearing nitrogen
ligands, although there are many reports of nitrogen ligands.11,12
We are interested in the possibility of using imidazoles and 4,5-
Introduction
Palladium complexes play crucial roles as homogeneous
catalysts in a variety of coupling reactions in organic synthesis.1
Recently, well-designed ligands have contributed significantly
to improving catalytic activity. Most of them are phosphine-
based ligands, because increasing bulkiness and introducing
electron-donating functions increase the reactivity of aryl
chlorides, enabling their use as substrates.2 The development
of palladacyclic catalysts3 and N-heterocyclic carbene palladium
catalysts4 should be mentioned as well. With regard to other
types of ligands in coupling reactions, Dai and co-workers have
focused on the development of amide-based phosphines as the
5
ˇ
P,O-ligands for coupling reactions. Cerma´k and co-workers
ˇ
* To whom correspondence should be addressed. Tel.: +81-803-5687.
Fax: +81-803-5688. E-mail: mhayashi@kobe-u.ac.jp.
† This paper is dedicated to Prof. Dr. Ryoji Noyori on the occation of
his 70th birthday.
(6) Vcˇela´k, J.; StorchJ.; Czako´ova´, M.; Cerma´k, J. J. Mol. Catal. A:
Chem. 2004, 222, 121. For a tridentate nitrogen ligand-palladium complex
catalyzed Mizoroki-Heck reaction: Mazet, C.; Gade, L. H. Eur. J. Inorg.
Chem. 2003, 1161.
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1993, 58, 7421. (b) Asselt, R.; Elsevier, C. J. Tetrahedron 1994, 50, 323.
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Braussaud, N.; Skelton, B. W.; White, A. H. J. Organomet. Chem. 2000,
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Schareina, T.; Kempe, R.; Wurst, K.; Buchmeiser, M. R. J. Organomet.
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10.1021/om800230y CCC: $40.75
2008 American Chemical Society
Publication on Web 07/16/2008

Pd 2–Phenylimidazole and 2–Phenylimidazoline Complexes
Organometallics, Vol. 27, No. 15, 2008 3749
Scheme 1. ORTEP Drawings of Palladium Complexes 1, 2, and 2′ (50% Thermal Ellipsoids)
dihydroimidazoles (imidazolines) as ligands because they are
structurally simple, readily available, and inexpensive, and they
allow for facile introduction of various substituents into their
framework.
with phenylimidazole in methanol at 20 °C for 3 days.15 Our
present method for the preparation of 1 and 2 is much simpler.
The complexes 2-phenylimidazole-PdCl2 (1) and 2-phenyl-
imidazoline-PdCl2 (2) were easily prepared by mixing 1 equiv
of PdCl2 and 2 equiv of 2-phenylimidazole and 2-phenylimi-
dazoline, respectively, in DMF at 50-70 °C for 1-2 h.
Complexes 1 and 2 were both recrystallized from a mixture of
DMF and toluene (method A) and from only DMF (method
B).
Herein, we report on the crystal structure analyses of the
complexes dichlorobis(2-phenyl-1H-imidazole)palladium(II) (2-
phenylimidazole-PdCl2) (1) and dichlorobis(2-phenyl-4,5-di-
hydro-1H-imidazole)palladium(II) (2-phenylimidazoline-PdCl2)
(2) using single-crystal X-ray diffractometry (XRD) and powder
1
XRD and a structure analysis in solution (DMF-d7) using H
Single-Crystal X-ray Diffractometry (XRD) Study. Na-
varro-Ranninger et al. reported only the NMR chemical shifts
of complex 1, and an X-ray analysis was not reported. As far
as we know, we are the first to report the XRD result of 1 and
the synthesis and characterization of 2 by XRD and NMR
spectroscopy. Scheme 1 summarizes the single-crystal XRD
structures of 2-phenylimidazole-PdCl2 (1) and 2-phenyl-
imidazoline-PdCl2 (2) obtained by methods A and B. It was
found that Pd complex 1 alone was produced by both methods
A and B. On the other hand, when method B was used, complex
2, having two Cl atoms and the phenyl moieties of 2-phenylimi-
dazole coordinating to Pd in a trans fashion, was produced
together with complex 2′, which had the two phenyl groups cis
to each other. In contrast, method A yielded only the trans
isomer 2 in the solid state. Powder XRD patterns disclosed in
Figure 1 also confirmed these observations.
NMR spectroscopy. We also discuss their catalytic activities in
the Mizoroki-Heck reaction13 and the Suzuki-Miyaura cou-
pling reaction.14
Results and Discussion
Synthesis of 2-Phenylimidazole-PdCl2 (1) and 2-Phenyl-
imidazoline-PdCl2 (2) Complexes. Navarro-Ranninger et al.
reported that complex 1 was synthesized by reacting Li2PdCl4
(12) Monodentate nitrogen ligand-palladium complex catalyzed
Mizoroki-Heck reactions: (a) Gossage, R. A.; Jenkins, H. A.; Yadav, P. N.
Tetrahedron Lett. 2004, 45, 7689. (b) Eisnor, C. R.; Gossage, R. A.; Yadav,
P. N. Tetrahedron 2006, 62, 3395. (c) Haneda, S.; Ueba, C.; Eda, K.;
Hayashi, M. AdV. Synth. Catal. 2007, 349, 833.
(13) (a) Mizoroki, T.; Mori, K.; Ozaki, A. Bull. Chem. Soc. Jpn. 1971,
44, 581. (b) Heck, R. F.; Nolley, J. P. J. Org. Chem. 1972, 37, 2320.
(14) (a) Suzuki, A. Pure Appl. Chem. 1985, 57, 1749. (b) Suzuki, A.
Pure Appl. Chem. 1991, 63, 419. (c) Suzuki, A. Pure Appl. Chem. 1994,
66, 213. (d) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457.
(15) Navarro-Ranninger, C.; Zamora, F.; Lopez-Solera, L.; Monge, A.;
Masaguer, J. R. J. Organomet. Chem. 1996, 506, 149.
(16) Izumi, F.; Ikeda, T. Mater. Sci. Forum 2000, 321, 198.

3750 Organometallics, Vol. 27, No. 15, 2008
Kawamura et al.
Figure 1. Powder X-ray diffraction patterns of (I) the 1-phenylimidazole-PdCl2 complex prepared by method A, (II) the
1-phenylimidazole-PdCl2 complex prepared by method B, (III) the 1-phenylimidazoline-PdCl2 complex prepared by method A, and (IV)
the 1-phenylimidazoline-PdCl2 complex prepared by method B: (a) measured patterns; (b) simulation of powder XRD data using RIETAN-
2000.16
Powder XRD Study. All of the X-ray diffraction patterns
I-IV in Figure 1 include both the actual measurements (a) and
simulation patterns calculated by RIETAN-200016 using the data
collected by single-crystal XRD (b). Parts I and II of Figure 1
show powder XRD patterns of Pd complex 1 prepared by
methods A and B, respectively. The same patterns were
observed, as the complexes prepared by methods A and B had
the same stereochemical and crystallographical structure. Part
III of Figure 1 shows powder XRD patterns of complex 2
prepared by method A, while part IV shows the patterns of a
mixture of complexes 2 and 2′ obtained by method B. In the
case of part IV (PdCl2-2-phenylimidazoline, method B), when
we obtained the single crystal by slow evaporation of the solvent,
we observed the formation of 2′ by single-crystal XRD;
however, this Pd complex 2′ should have conformational
polymorphism (complex 2). Actually, each colored peak in part
IVa of Figure 1 is assigned to either complex 2 or 2′. In
simulated patterns (part IVb of Figure 1), the pattern shown in
red is for complex 2, while that in blue is for complex 2′. These
results confirmed that the reaction of 2 equiv of 2-phenylimi-
dazoline with 1 equiv of PdCl2 gives a mixture of trans and cis
(with regard to the phenyl group in 2-phenylimidazoline)
complexes in the solid state.
Mizoroki-Heck Reaction Catalyzed by Pd Complexes
1 and 2. To compare the catalytic activities of complexes 1
and 2 in the Mizoroki-Heck reaction, we examined the reaction
of 4-bromotoluene and tert-butyl acrylate in the presence of
0.1 mol % Pd catalyst in DMF at 120 °C.
de Vries mentioned the role of Pd species in the Mizoroki-
Heck reaction under high temperature, and he proposed the
mechanism of ligand dissociation to produce quasi-naked Pd
species under these conditions.17 Considering his proposal, it
is interesting to investigate whether there is a difference in
catalytic activity between Pd complexes 1 and 2. Actually, as
shown in Table 1 and Figure 3, we observed a difference in
catalytic activity. That is, Pd complex 2 exhibited higher
catalytic activity than Pd complex 1. In the case of the
2-phenylimidazoline-PdCl2 complex 2, the precipitation of Pd
black occurred partially during the coupling reaction. On the
other hand, in the case of the 2-phenylimidazole-PdCl2 complex
1, Pd black was not formed at all under the same conditions.
This observation suggested that the 2-phenylimidazoline ligand
dissociated more easily than the 2-phenylimidazole ligand.
However, we do not think Pd black worked as a catalyst.
Therefore, the above observation, that the partial formation of
Pd black in the case of Pd complex 2 and no formation of Pd
black in the case of Pd complex 1, should suggest that the ease
of dissociation of the 2-phenylimidazoline ligand promoted the
formation of the more reactive unsaturated Pd species. A more
detailed elucidation still remains to be solved in the future.
The reason for the induction period of catalysts 1 and 2 as shown
in Figure 3 is also not clear at present. However, we believe
1H NMR Analysis of Pd Complexes 1 and 2. Figure 2
shows the 1H NMR spectra of 2-phenylimidazole, 2-phenylimi-
dazoline, and their PdCl2 complexes. Analysis of the spectra of
the PdCl2 complexes in DMF-d7 revealed that the solid sample
2′ gave the same spectrum as that of 2. This indicates that the
Pd-N bond rotates easily in solution, whereas complexes 2 and
2′ are formed in the solid state as a result of crystal packing.
(17) de Vries, J. G. Dalton Trans. 2006, 421.

Pd 2–Phenylimidazole and 2–Phenylimidazoline Complexes
Organometallics, Vol. 27, No. 15, 2008 3751
1
Figure 2. H NMR spectra of 2-phenylimidazole, 2-phenylimidazoline, and their PdCl2 complexes 1 and 2.
Table 1. Mizoroki-Heck Reaction Catalyzed by 1 and 2
Scheme 2. Suzuki-Miyaura Coupling Catalyzed by Pd Com-
plexes 1 and 2
yield of Mizoroki-Heck product/%a
entry
time/h
1
2
1
2
3
4
5
6
3
6
9
12
18
24
3
12
73
89
97
96
96
11
11
18
88
96
reactivity than 1 (Scheme 2); that is, the reaction of 4-bromo-
toluene with phenylboronic acid in the presence of 0.1 mol %
of Pd complex 2 in DMF at 120 °C for 12 h gave the coupling
product in 78% yield, whereas use of Pd complex 1 under the
same conditions afforded the product in 61% yield.
In conclusion, we determined the structures in the solid state
of the complexes 2-phenylimidazole-PdCl2 (1) and 2-phenyl-
imidazoline-PdCl2 (2) by single-crystal X-ray diffractometry
and powder X-ray diffractometry and the structures in DMF-d7
solution by 1H NMR spectroscopy. Pd complex 2 was found to
exhibit higher reactivity than complex 1 in the Mizoroki-Heck
reaction and the Suzuki-Miyaura coupling reaction.
a Yields described in this table indicate isolated yields averaged from
two or three experiments.
Experimental Section
Synthesis of Dichlorobis(2-phenyl-1H-imidazole)palladium(II)
(1). To a suspension of PdCl2 (177.3 mg, 1.0 mmol) in DMF (5
mL) was added 2-phenylimidazole (288.4 mg, 2.0 mmol) under an
argon atmosphere. The mixture was stirred at 50 °C for 2 h, until
a clear orange-yellow solution was formed. After dilution of the
resultant solution with DMF (10 mL), toluene (40 mL) was added
to precipitate the Pd complex. The Pd complex was isolated as a
light yellow powder by filtration, washed with toluene, and dried
in air. Yield: 438.0 mg (93%). Anal. Calcd for C18H16N4Cl2Pd: C,
46.47; H, 3.46; N, 12.03. Found: C, 46.47; H, 3.53; N, 12.06.
A single crystal of complex 1 ((2-phenylimidazole)2PdCl2 ·
2DMF) suitable for XRD analysis was obtained by slow diffusion
of toluene (2 mL) into a solution containing 60.0 mg of (2-
phenylimidazole)2PdCl2 powder in DMF (5 mL) (method A). Yield:
Figure 3. Relationship between reaction time and yield of
Mizoroki-Heck reaction product.
the difference in the rate of ligand dissociation to produce the
real catalytic species causes the different induction periods
between Pd catalysts 1 and 2.
Suzuki-Miyaura Coupling Catalyzed by Pd Complexes
1and2.Pdcomplexes1and2alsocatalyzedtheSuzuki-Miyaura
coupling reaction. Here as well, complex 2 exhibited higher

3752 Organometallics, Vol. 27, No. 15, 2008
Kawamura et al.
Table 2. Crystal Data and Collection Parameters for Complexes 1, 2, and 2′
1
2
2′
formula
formula wt
T (K)
C24H30N6O2Cl2Pd
611.86
193(2)
C18H20N4Cl2Pd
469.68
296(2)
C18H20N4Cl2Pd
469.68
193(2)
radiation
cryst syst
space group
unit cell dimens (Å)
a (Å)
Mo-K R (λ ) 0.71073 Å)
Mo-K R (λ ) 0.71073 Å)
Mo-K R (λ ) 0.71073 Å)
monoclinic
monoclinic
monoclinic
P21/c
P21/c
P21/c
10.6460(15)
7.745(3)
10.5073(15)
b (Å)
9.0631(13)
12.647(4)
13.1943(18)
c (Å)
14.374(2)
11.215(3)
14.060(2)
ꢀ (deg)
105.509(2)
1336.4(3)
120.353(17)
947.9(6)
105.395(2)
1879.3(5)
V (Å3)
Z
2
2
4
Dcalcd (Mg/m3)
F(000)
1.520
624
0.927
1.646
472
1.269
1.660
944
1.280
µ(Mo KR) (mm-1
cryst size (mm3)
θ range (deg)
)
0.28 × 0.19 × 0.14
0.25 × 0.19 × 0.18
0.30 × 0.25 × 0.08
1.99-26.55
2.65-27.13
2.01-26.55
index ranges
-13 e h e 7, -10 e k e 10,
-18 e l e 7
-9 e h e 9, -14 e k e 15,
-14 e l e 8
-11 e h e 13, -16 e k e 16,
-17 e l e 11
no. of rflns measd
total
6639
5139
9426
unique
2463
1917
3505
Rint
0.0466
0.0232
0.0499
structure soln
refinement
no. of variables
GOF
R1
wR2
direct methods
full-matrix least squares on F2
162
115
306
1.024
0.0287
0.0788
1.052
0.0254
0.0723
1.025
0.0309
0.0780
31.7 mg (41%). Mp: >300 °C. IR (KBr): νmax (cm-1) 3184, 3158,
(400 Hz, DMF-d7): δ 8.67 (d, 2H, J ) 7.6 Hz), 7.96 (s, 1H), 7.65
(t, 1H, J ) 7.2 Hz), 7.57 (dd, 2H, J ) 7.6, 7.2 Hz), 4.03 (t, 2H, J
) 10.4 Hz), 3.63 (t, 2H, J ) 10.4 Hz). Anal. Calcd for
C18H20N4Cl2Pd: C, 46.03; H, 4.29; N, 11.93. Found: C, 45.99; H,
4.27; N, 11.88.
X-ray Crystallography. X-ray data for 1, 2, and 2′ were
collected on a Bruker Smart 1000 CCD diffractometer. An empirical
absorption correction was applied using the SADABS program. The
structure was solved by direct methods and refined by full-matrix
least-squares calculations on F2 using the SHELXL-97 program
package.18 Crystal data and details of the data collection and
structure refinement are summarized in Table 2.
1
3117, 1661, 1568, 1506, 1390, 1108, 777, 718, 708, 694, 664. H
NMR (400 Hz, DMF-d7): δ 13.27 (s, 1H), 8.89 (d, 2H, J ) 7.6
Hz), 7.67 (dd, 2H, J ) 6.4, 7.6 Hz), 7.60 (t, 1H, J ) 6.4 Hz), 7.48
(d, 2H, J ) 1.2 Hz). Anal. Calcd for C24H30N4O2Cl2Pd: C, 47.11;
H, 4.94; N, 13.74. Found: C, 46.99; H, 4.95; N, 13.61.
A single crystal of complex 1 ((2-phenylimidazole)2PdCl2 ·
2DMF) suitable for XRD analysis was obtained directly by slow
evaporation of the resultant solution under vacuum (method B).
Synthesis of Dichlorobis(2-phenyl-4,5-dihydro-1H-imidazole)-
palladium(II) (2). To a suspension of PdCl2 (177.3 mg, 1.0 mmol)
in DMF (3 mL) was added 2-phenylimidazoline (292.4 mg, 2.0
mmol) under an argon atmosphere. The mixture was stirred at 70
°C for 1 h, until a clear orange solution was formed.
A single crystal of complex 2 suitable for XRD analysis was
obtained by slow diffusion of toluene into the resultant solution
(method A). After dilution of the resultant solution with DMF (10
mL), toluene (40 mL) was carefully added to the DMF solution of
the complex. After 3 days, orange crystalline 2 was obtained by
filtration and dried in air. Yield: 329.0 mg (70%). Mp: 239 °C dec.
IR (KBr): νmax (cm-1) 3267, 3242, 2963, 2879, 1616, 1598, 1578,
1511, 1485, 1469, 1274, 1185, 1147, 776, 701. 1H NMR (400 Hz,
DMF-d7): δ 8.67 (d, 2H, J ) 7.6 Hz), 7.96 (s, 1H), 7.65 (t, 1H, J
) 7.2 Hz), 7.57 (dd, 2H, J ) 7.6, 7.2 Hz), 4.03 (t, 2H, J ) 10.4
Hz), 3.63 (t, 2H, J ) 10.4 Hz). Anal. Calcd for C18H20N4Cl2Pd: C,
46.03; H, 4.29; N, 11.93. Found: C, 45.45; H, 4.41; N, 11.86.
A single crystal of complex 2′ suitable for X-ray diffraction
analysis was directly obtained by slow evaporation of the resultant
solution under vacuum (method B), whereas the orange crystal 2
coexisted. This mixture of complexes 2 and 2′ was also used for
the sample for powder XRD analysis. The mixture of 2 and 2′ was
filtered, and then a yellow-orange crystal of 2′ was isolated and
dried in air. Mp: 230 °C dec. IR (KBr): νmax (cm-1) 3271, 2875,
1610, 1601, 1577, 1512, 1472, 1270, 1187, 775, 698. 1H NMR
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research on Priority Areas “Advanced
Molecular Transformations of Carbon Resources” and Grant
No. B17340020 from the Ministry of Education, Culture,
Sports, Science and Technology of Japan.
Supporting Information Available: Text and figures giving
experimental procedures and compound characterization data and
CIF files giving X-ray crystal data for 1, 2, and 2′. This material is
available free of charge via the Internet at http://pubs.acs.org. CCDC
660480 for complex 1, 675645 for complex 2, and 660479 for
complex 2′ contain supplementary crystallographic data for this
paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data-request/cif.
OM800230Y
(18) Sheldrick, G. M. SHELXL-97, Programs for the Refinement Crystal
Structure Analysis; University of Go¨ttingen, Go¨ttingen, Germany, 1997.