Synthesis, characterization, and crystal structures of three cis/trans pyridine-cyclopalladated ferrocenylimine complexes, and their catalysis in suzuki reactions

Article abstract of DOI:10.1071/CH07014

Three new pyridine?cyclopalladated ferrocenylimine complexes 2a?c have been easily prepared and characterized by elemental analysis, ESI-MS, ¹H NMR, and IR spectra. Their detailed structures are determined by single-crystal X-ray analysis. Palladacycle 2a is found to be a cis complex in the solid state, while 2b and 2c are trans complexes. These complexes were found to be efficient for the Suzuki reaction of aryl bromides with phenylboronic acid. Typically, using 0.2 mol% of 2c in the presence of 1.5 equivalents of K₂CO ₃ as base in toluene at 100°C provided the coupled products in good to excellent yields. CSIRO 2007.

Full text of DOI:10.1071/CH07014

Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2007, 60, 190–195
www.publish.csiro.au/journals/ajc
Synthesis, Characterization, and Crystal Structures of Three
cis/trans Pyridine–Cyclopalladated Ferrocenylimine
Complexes, and Their Catalysis in Suzuki Reactions
Chen Xu,A Jun-Fang Gong,A,B Yan-Hui Zhang,A Yu Zhu,A and Yang-Jie WuA,B
ADepartment of Chemistry, Henan Key Laboratory of Chemical Biology and Organic Chemistry,
Key Laboratory of Applied Chemistry of Henan Universities, Zhengzhou University,
Zhengzhou 450052, China.
BCorresponding authors. Email: wyj@zzu.edu.cn; gongjf@zzu.edu.cn
Three new pyridine–cyclopalladated ferrocenylimine complexes 2a–c have been easily prepared and characterized by
elemental analysis, ESI-MS, 1H NMR, and IR spectra. Their detailed structures are determined by single-crystal X-ray
analysis. Palladacycle 2a is found to be a cis complex in the solid state, while 2b and 2c are trans complexes. These
complexes were found to be efficient for the Suzuki reaction of aryl bromides with phenylboronic acid. Typically, using
0.2 mol% of 2c in the presence of 1.5 equivalents of K2CO3 as base in toluene at 100◦C provided the coupled products
in good to excellent yields.
Manuscript received: 19 January 2007.
Final version: 22 February 2007.
Introduction
R1
Pd
R1
R1
Pd
The palladium-catalyzed Suzuki reaction is a versatile method
for the formation of C–C bonds in organic synthesis,[1–3] and
gives access to a wide range of biaryl compounds. For industrial
applications it is necessary to develop highly active catalysts,
which allow the use of low catalyst loadings and inexpen-
sive but usually less reactive starting materials.[4] Recently,
great progress has been made by using palladium complexes
with bulky, electron-rich phosphine ligands.[5–7] In addition,
cyclopalladated compounds (palladacycles) also provide effi-
cient catalytic systems for the Suzuki coupling.[8–10] Indolese
and coworkers[11] and Bedford et al.[12] have demonstrated
that isolated or in-situ formed secondary or tertiary phosphine
(e.g., HP(But)2, HPCy2 or PCy3, PBut ) adducts of dimeric
phosphorus- or nitrogen-containing palla3dacycles exhibit excel-
lent activity at very low catalyst loadings when aryl chlorides,
both activated and unactivated, are used as substrates in the
Suzuki reaction. We have also found that tricyclohexylphos-
phine (PCy3)–cyclopalladated ferrocenylimine complexes are
efficient catalysts for the coupling of aryl chlorides.[13,14] How-
ever, the high price usually associated with bulky phosphines
along with difficulties to remove their decomposition byprod-
ucts have encouraged researchers to explore alternatives for
phosphines. In this regard, the N-heterocyclic carbene (NHC)
palladacycle complex developed by Nolan and coworkers[15,16]
has been found to be able to couple unactivated aryl chlorides
with arylboronic acids at room temperature in short reaction
times although the catalyst loadings are relatively high (2 mol%).
During the past decade we have systematically studied
the cyclopalladation of Schiff base-type ferrocenylimines[17–19]
and some of the obtained palladacycles have been suc-
cessfully used in Suzuki coupling.[13,14,20–22] To further
explore the applications of cyclopalladated ferrocenylimines in
N
R2
N
R2
N
R2
py
Pd Cl
ϩ
Fe
Fe
Fe
CH2Cl2, r.t.
Py
Cl
2
Cl
Py
1a–c
2a
2b, c
R1 ϭ CH3; R2 ϭ p-CH3-C6H4 (a)
R1 ϭ CH3; R2 ϭ p-Br-C6H4 (b)
R1 ϭ H;
R2 ϭ Cy (c)
Scheme 1.
palladium-catalyzed coupling reactions, we prepared three
new pyridine–cyclopalladated ferrocenylimine complexes 2a–c
(Scheme 1) and examined their activity in the Suzuki reaction.
Interestingly, complex 2a is found to adopt a cis configura-
tion of the coordinated pyridine (py) to the imino nitrogen in
the solid state. To our knowledge, there has been no report
concerning the cis structure of monomeric imine and amine-
based palladacycles.[12–14,17,19,21,23–26] The crystal structures of
these phosphine-free palladacycles and their catalysis in Suzuki
reactions are presented below.
Results and Discussion
Synthesis and Characterization of Complexes 2a–c
The chloride-bridged palladacyclic dimers 1a–c were prepared
according to published procedures.[17,21] Each of them was
treated with 1.1 equivalents of pyridine per palladium in CH2Cl2
at room temperature for 30 min to afford the corresponding
adducts 2a–c as red solids in 78–89% yield. All the new com-
plexes are air- and moisture-stable both in the solid state and in
© CSIRO 2007
10.1071/CH07014
0004-9425/07/030190

Three cis/trans Pyridine–Cyclopalladated Ferrocenylimine Complexes
191
solution. They are very soluble in chloroform, dichloromethane,
and acetone, but insoluble in petroleum spirits and n-hexane.
Complexes 2a–c were characterized by elemental analysis,
IR and 1H NMR spectroscopy, and electrospray ionization mass
spectrometry (ESI-MS) analysis. The IR spectra of the free
ferrocenylimines show an intense sharp band in the range 1620–
1640 cm−1. For complexes 2a–c, this band shifts to lower energy
(1547–1586 cm−1), which indicates that the imino nitrogen atom
is coordinated to palladium through its lone pair.[13,17] The 1H
NMR spectrum of complex 2c is consistent with the proposed
structure and indicates the presence of only one isomer in solu-
tion. The trans geometry of this isomer is confirmed by X-ray
crystallographic analysis (see below). In contrast, the 1H NMR
spectra of 2a–b clearly show that they are a mixture of cis and
trans isomers in solution. Comparison with the 1H NMR spec-
troscopic data for complex 2c suggests that the major signals
are attributable to the trans isomer. For example, the doublet at
δ 9.03 and two triplets at δ 7.84 and 7.42 are assigned to the
signals of pyridine protons in 2c. The corresponding signals of
the major isomer in 2a and 2b appear at δ 9.03, 7.82, 7.39 and
δ 9.02, 7.83, 7.41, respectively.
respectively. The bicyclic system formed by the palladacy-
cle and the C5H3 moiety is approximately coplanar (dihedral
angles of 2.9◦, 2.5◦, and 2.2◦ for complexes 2a–c, respec-
tively). In each structure, the two Cp rings are almost parallel
with dihedral angles of 5.8◦, 4.0◦, and 1.4◦, respectively, for
2a–c. In all cases the C(6)–Pd(1)–N(1) bond angle is around
80◦, which is essentially identical to those in the correspond-
ing PPh3 or PCy3–cyclopalladated ferrocenylimines.[13,14,21,27]
The Pd(1)–C(6) [1.971(4) Å], Pd(1)–N(1) [2.065(3) Å], and
Pd(1)–N(2) [2.050(3) Å] bond lengths of trans complex 2c
aresimilartothose of related trans palladacycles with pyridine
[1.890(5)–2.004(5) Å, 1.981(4)–2.082(2) Å, and 2.030(5)–
2.043(2) Å].[25,26] The Pd(1)–C(6) [2.205(5) Å], Pd(1)–N(1)
[2.329(4) Å], and Pd(1)–N(2) [2.436(4) Å] bond lengths of cis
complex 2a are obviously longer than the above corresponding
Crystal Structures
The crystal structures of complexes 2a–c were determined to
confirm the details. All the crystals were obtained by recrys-
tallization from CH2Cl2/petroleum spirits at room temperature.
The molecules are shown in Figs 1–3 (displacement ellipsoids
are drawn at the 30% probability level). Selected bond lengths
and angles are listed in Table 1.
As can be seen, the molecules of 2b·CH2Cl2 and 2c adopt
a trans configuration of the coordinated pyridine to the imino
nitrogen. To the best of our knowledge, the crystal structures
of all monomeric imine and amine-based palladacycles indicate
that, in the solid state, the trans isomer crystallizes preferentially.
Surprisingly, the single-crystal X-ray analysis of complex 2a
reveals a cis configuration in the solid state.
The Pd atom in each complex is in a slightly distorted
square-planar environment bonded to the nitrogen atom of
the py, the chlorine atom, and the C6 and nitrogen atoms of
the ferrocenylimine. The deviations of the Pd atoms from the
planes are 0.0298, 0.0161, and 0.0224 Å for complexes 2a–c,
Fig. 2. Molecular structure of complex 2b·CH2Cl2. CH2Cl2 and hydrogen
atoms are omitted for clarity.
Fig. 1. Molecular structure of complex 2a. Hydrogen atoms are omitted for clarity.

192
C. Xu et al.
values, which is attributed to the steric demand of the cis struc-
ture. The trans effect of the metalated carbanion may also be
responsible for the longer Pd(1)–N(2) bond length of 2a. For
trans 2b·CH2Cl2, the bond lengths around Pd are unexpectedly
long.ThePd(1)–C(6)andPd(1)–N(1)bondlengthsof 2a arealso
evidently longer than those of the corresponding trans PCy3 and
PPh3–cyclopalladatedferrocenylimines[2.019(6), 1.998(3), and
2.170(5), 2.136(3) Å],[14,27] while the Pd(1)–C(6) and Pd(1)–
N(1) bond lengths of 2c are slightly shorter than those of the
corresponding PCy3–cyclopalladated ferrocenylimine [1.998(3)
and 2.150(3) Å].[13]
Fig. 4 shows that the crystal structure of 2c is different
from those of complexes 2a, 2b·CH2Cl2, and the corresponding
PCy3–cyclopalladated ferrocenylimine.[13] In complex 2c, the
chlorine atom not only forms a hydrogen bond with the adjacent
C–H group of py (Cl1S · · · H21N = Cl1N · · · H21T = 2.816 Å),
but also forms a hydrogen bond with the adjacent C–H group of
the Cp ring (Cl1S · · · H4T = Cl1N · · · H4O = 2.902 Å), which
are responsible for the construction of the one-dimensional
network structure of 2c.
Suzuki Coupling Reaction
The coupling of 4-bromotoluene with phenylboronic acid was
chosen as a model reaction to evaluate the effectiveness of the
new cyclopalladated complexes 2a–c in a Suzuki reaction. The
reaction was performed under a nitrogen atmosphere in anhy-
drous toluene in the presence of K2CO3 as base at 100◦C for
12 h. The palladacycles 2a and 2b had similar activity and pro-
duced the coupled products in 84 and 82% yields with a catalyst
loading of 0.2 mol%. Palladacycle 2c was found to be the most
active among the three catalysts: it gave the product in excel-
lent isolated yield (93.4%) under the same reaction conditions
(Table 2, entries 1–3).
Fig. 3. Molecular structure of complex 2c. Hydrogen atoms are omitted
for clarity.
Table 1. Selected bond lengths (Å) and angles [deg.] for 2a–c
In the following experiments, the Suzuki coupling reac-
tions of a variety of electronically and structurally diverse aryl
bromides with phenylboronic acid were investigated by using
complex 2c as catalyst under the same reaction conditions. Sim-
ilar to the result of 4-bromotoluene, excellent yields (97–98%)
were also obtained in the case of other electron-neutral aryl bro-
mides such as bromobenzene and 1-bromonaphthalene (Table 2,
entries 4, 5). For the very electron-rich 4-bromoanisole, the yield
decreased slightly but was still very high (90%, entry 6). ortho-
Substituents were tolerated and even the very sterically hindered
2-bromo-m-xylene provided the biaryl product in 75.8% isolated
yield (entries 7–9). For electron-deficient aryl bromides such
as 3-bromonitrobenzene and 4-bromobenzonitrile, the catalyst
Bonds and angles
2a
2b·CH2Cl2
2c
Pd(1)–C(6)
Pd(1)–N(1)
Pd(1)–N(2)
Pd(1)–Cl(1)
2.205(5)
2.329(4)
2.436(4)
2.5929(16)
1.465(6)
80.31(18)
176.72(18)
96.56(15)
92.44(15)
90.73(12)
172.35(11)
2.211(7)
2.321(6)
2.272(6)
2.706(2)
1.460(9)
80.9(2)
1.971(4)
2.065(3)
2.050(3)
2.4022(12)
1.285(5)
81.43(15)
93.84(16)
174.75(12)
176.97(13)
89.16(10)
95.59(9)
N(1)–C(11)
C(6)–Pd(1)–N(1)
C(6)–Pd(1)–N(2)
N(2)–Pd(1)–N(1)
C(6)–Pd(1)–Cl(1)
N(2)–Pd(1)–Cl(1)
N(1)–Pd(1)–Cl(1)
89.8(3)
170.7(2)
178.12(18)
91.23(19)
98.04(16)
Fig. 4. One-dimensional network structure of complex 2c formed by hydrogen bonds. Non-hydrogen bonding H atoms are omitted for
clarity.

Three cis/trans Pyridine–Cyclopalladated Ferrocenylimine Complexes
193
loadings could be lowered to 0.1 mol% without loss of activity
(entries 10, 11). Coupling of heteroaryl bromides with phenyl-
boronic acid with this catalyst system was also studied. 3-
Bromopyridine was found to be an efficient coupling partner in
this system that gave 92.9% isolated yield. The result was much
better than that of 2-bromopyridine (entries 12, 13). Finally,
with bromothiophenes moderate yields can be obtained under
the same conditions (entries 14, 15).
catalysts for the Suzuki reaction of a variety of aryl bromides
with phenylboronic acid.
Experimental
General
All other chemicals were used as purchased. Solvents were dried
and freshly distilled before use. Suzuki reactions were carried
out under a dry nitrogen atmosphere using standard Schlenk
techniques. Melting points were measured using a WC-1 micro-
scopic apparatus and were uncorrected. Elemental analyses were
determined with aThermo Flash EA 1112 elemental analyzer. IR
spectra were collected on a BrukerVECTOR22 spectrophotome-
ter as KBr pellets. 1H NMR spectra were recorded on a Bruker
DPX-400 spectrometer in CDCl3 with TMS as an internal stan-
dard. Mass spectra were measured on a LC-MSD-Trap-XCT
instrument.
Conclusions
The three new phosphine-free palladacycles 2a–c were eas-
ily synthesized and structurally characterized. Single-crystal
X-ray analysis confirmed the cis structure of palladacycle 2a
as well as the trans structures of palladacycles 2b and 2c in
the solid state. These complexes were found to be efficient
General Method for the Synthesis of Pyridine–
Cyclopalladated Ferrocenylimines 2a–c
Table 2. Suzuki coupling of aryl bromides with phenylboronic acid
catalyzed by 2a–c
A solution of chloride-bridged palladacyclic dimer 1a–c
(0.1 mmol) and pyridine (0.22 mmol) in dichloromethane
(10 mL) was stirred at room temperature for 30 min.The product
was separated by passing through a short silica gel column with
ethyl acetate as eluent. The first band was collected to afford
complex 2a–c after evaporation of the solvent.
Entry
Aryl halide
Catalyst
[mol%]
Product
Yield
[%]
1
2
3
4
p-MeC6H4Br
p-MeC6H4Br
p-MeC6H4Br
PhBr
2a (0.2)
2b (0.2)
2c (0.2)
2c (0.2)
p-MePh–Ph
p-MePh–Ph
p-MePh–Ph
Ph–Ph
84.5
82.1
93.4
97.4
[PdCl{[(η5-C5H5)]Fe[(η5-C5H3)C(CH3)=NC6H4-
4-CH3]}(py)] 2a
Br
5
2c (0.2)
98.0
Yield 86.5%. mp 214◦C (dec.). Found: C 53.3, H 4.2, N 5.3.
Calc. for C24H23ClFeN2Pd: C 53.7, H 4.3, N 5.2%. νmax
(KBr)/cm−1 3088 w, 2923 w, 1598 m, 1548 s, 1470 s, 1443 s,
1321 m, 1232 m, 1105 m, 1062 w, 1015 w, 815 m. The complex
exists as a mixture of cis- and trans-isomers in CDCl3 solution,
trans/cis ≈ 1.5:1. δH (400 MHz, CDCl3, trans-isomer) 9.03 (2H,
d, J 5.0, pyH), 7.82 (1H, t, J 7.2, pyH), 7.39 (2H, t, J 6.4, pyH),
7.18 (2H, d, J 7.6, C6H4), 6.98 (2H, m, C6H4), 4.50 (1H, s,
C5H3), 4.33 (5H, s, C5H5), 4.31 (1H, s, C5H3), 3.92 (1H, s,
C5H3), 2.35 (3H, s, CH3), 2.02 (3H, s, CH3). δH (400 MHz,
CDCl3, cis-isomer) 8.25 (2H, d, J 4.4, pyH), 7.49 (1H, t, J 7.2,
pyH), 6.98 (2H, m, overlapped with C6H4 of trans-isomer, pyH),
6.85 (2H, m, C6H4), 6.62 (2H, br s, C6H4), 5.12 (1H, s, C5H3),
4.47 (1H, s, C5H3), 4.45 (1H, s, C5H3), 4.40 (5H, s, C5H5), 2.20
(3H, s, CH3), 2.02 (3H, s, overlapped with CH3 of trans-isomer,
CH3). m/z (ESI+) 501.0 (M+ − Cl).
6
7
p-MeOC6H4Br
2c (0.2)
2c (0.2)
p-MeOPh–Ph
90.3
89.9
Br
Me
Me
8
9
o-MeC6H4Br
2c (0.2)
2c (0.2)
o-MePh–Ph
84.5
75.8
Me
2,6-Me2Ph–Ph
Br
Me
NO2
[PdCl{[(η5-C5H5)]Fe[(η5-C5H3)C(CH3)=NC6H4-
10
11
2c (0.1)
2c (0.1)
m-NO2Ph–Ph
p-NCPh–Ph
98.5
99.8
Br
4-Br]}(py)] 2b
Yield 89.3%. mp 239◦C (dec.). Found: C 41.7, H 3.4, N 4.1.
Calc. for C24H22BrCl3FeN2Pd: C 42.0, H 3.2, N 4.1%. νmax
(KBr)/cm−1 3058 w, 2371 w, 1601 w, 1547 s, 1469 s, 1443 m,
1230 m, 1102 w, 1065 m, 1004 m, 813 m. The complex exists
as a mixture of cis- and trans-isomers in CDCl3 solution,
trans/cis ≈ 2.7:1. δH (400 MHz, CDCl3, trans-isomer) 9.02 (2H,
d, J 5.1, pyH), 7.83 (1H, t, J 7.6, pyH), 7.50 (2H, d, J 8.2, C6H4),
7.41 (2H, t, J 6.7, pyH), 6.97 (2H, m, C6H4), 4.53 (1H, d, J 2.1,
C5H3), 4.35 (1H, s, C5H3), 4.33 (5H, s, C5H5), 3.97 (1H, d,
J 1.8, C5H3), 2.03 (3H, s, CH3). δH (400 MHz, CDCl3, cis-
isomer) 8.28 (2H, d, J 4.7, pyH), 7.55 (1H, m, pyH), 7.19 (2H,
d, J 7.9, C6H4), 7.06 (2H, t, J 6.4, pyH), 6.64 (2H, br s, C6H4),
5.16 (1H, s, C5H3), 4.49 (2H, s, C5H3), 4.40 (5H, s, C5H5), 2.03
(3H, s, overlapped with CH3 of trans-isomer, CH3). m/z (ESI+)
566.8 (M+ − Cl).
p-NC–C6H4Br
12
2c (0.2)
61.9
N
Br
Br
N
Ph
Ph
13
14
2c (0.2)
2c (0.2)
92.9
68.3
N
S
N
S
Br
Br
Ph
Ph
15
2c (0.2)
62.1
S
S

194
C. Xu et al.
Table 3. Crystallographic data and structure refinement for complexes 2a–c
Parameter
2a
2b·CH2Cl2
2c
Empirical formula
Formula weight
Crystal system
Space group
Crystal size [mm3]
a [Å]
b [Å]
c [Å]
α [deg.]
C24H23ClFeN2Pd
537.14
Monoclinic
C2/c
0.20 × 0.18 × 0.17
19.956(4)
17.662(4)
17.497(4)
90
C24H22BrCl3FeN2Pd
686.95
Triclinic
C22H25ClFeN2Pd
515.14
Monoclinic
P21/c
0.20 × 0.18 × 0.17
13.381(3)
15.466(3)
10.272(2)
90
P1
0.20 × 0.18 × 0.16
11.439(2)
13.171(3)
13.955(3)
80.48(3)
β [deg.]
92.23(3)
90
6162(2)
66.43(3)
73.39(3)
1843.7(6)
1.237
105.12(3)
90
2052.2(7)
1.667
γ [deg.]
V [Å3]
Dc [g cm−3
]
1.158
Z
8
2
4
GOF (on F2)
1.093
1.061
1.041
Data/restraints/parameters
R1, wR2 [I > 2σ(I)]A
5442/0/262
0.0577, 0.1218
5411/0/289
0.0699, 0.1637
3550/0/245
0.0347, 0.0925
ꢀ
ꢀ
ꢀ
ꢀ
AR1 =
||Fo| − Fc||/ |Fo|; wR2 = [ (F2o − Fc2)2/ w(Fo2)2]1/2
.
[PdCl{[(η5-C5H5)]Fe[(η5C5H3)CH=NCy]}(py)] 2c
Details of the crystal structure determination of 2a–c are
summarized in Table 3.
Yield 78.5%. mp 238◦C (dec.). Found: C 51.1, H 4.8, N
5.5. Calc. for C22H25ClFeN2Pd: C 51.3, H 4.9, N 5.4%. νmax
(KBr)/cm−1 2925 s, 2851 m, 2372 w, 1586 s, 1446 s, 1405 w,
1321 w, 1293 w, 1236 w, 1102 w, 1066 m, 819 m. δH (400 MHz,
CDCl3) 9.03 (2H, d, J 5.0, pyH), 7.92 (1H, s, CH=N), 7.84 (1H,
t, J 7.6, pyH), 7.42 (2H, t, J 7.0, pyH), 4.44 (1H, d, J 2.2, C5H3),
4.25 (5H, s, C5H5), 4.21 (1H, t, J 2.2, C5H3), 4.06 (1H, m,
N-CHC5H10), 3.84 (1H, d, J 2.0, C5H3), 2.42 (1H, m, NCy),
2.10 (1H, m, NCy), 1.83 (2H, m, NCy), 1.72 (1H, m, NCy),
1.15–1.56 (5H, m, NCy). m/z (ESI+) 479.0 (M+ − Cl).
Acknowledgments
We are grateful to the National Natural Science Foundation of China
(20472074), the Innovation Fund for Outstanding Scholar of Henan Province
(0621001100), and the Natural Science Foundation of Henan Province
(0611012100) for financial support of this work.
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Suzuki Reactions
A prescribed amount of catalyst, aryl bromide (0.5 mmol),
phenylboronic acid (0.6 mmol), K2CO3 (0.75 mmol), and
toluene (2.0 mL) were placed in a Schlenk tube under nitro-
gen. The reaction mixture was then heated at 100◦C for 12 h
(time not optimized), and then allowed to cool. The reaction
mixture was extracted three times with dichloromethane, and
then the combined organic layers were washed with water, dried
(MgSO4), and evaporated to dryness. The products were iso-
lated by flash chromatography on silica gel. All the coupled
1
products are known compounds and were characterized by H
NMR spectroscopy.
[8] For reviews on palladacycles in catalysis, see:
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