Synthesis, crystal structures and catalytic activity of three cyclopalladated 6-bromo-2-ferrocenylquinoline complexes with N-heterocyclic carbenes (NHCs) and triphenylphosphine

Article abstract of DOI:10.1016/j.ica.2014.07.037

Three cyclopalladated 6-bromo-2-ferrocenylquinoline complexes with NHCs and PPh₃ 1-3 have been synthesized and characterized by elemental analysis, IR, NMR and single-crystal X-ray diffraction. Their application to Suzuki coupling of phenylboro

Full text of DOI:10.1016/j.ica.2014.07.037

Inorganica Chimica Acta 423 (2014) 11–15
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, crystal structures and catalytic activity of three
cyclopalladated 6-bromo-2-ferrocenylquinoline complexes
with N-heterocyclic carbenes (NHCs) and triphenylphosphine
b
a
a
Chen Xu a, , Hong-Mei Li , Zhi-Qiang Wang , Wei-Jun Fu
⇑
a College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang, Henan 471022, China
b Department of Life Science, Luoyang Normal University, Luoyang, Henan 471022, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 May 2014
Received in revised form 11 July 2014
Accepted 12 July 2014
Available online 30 July 2014
Three cyclopalladated 6-bromo-2-ferrocenylquinoline complexes with NHCs and PPh3 1–3 have been
synthesized and characterized by elemental analysis, IR, NMR and single-crystal X-ray diffraction. Their
application to Suzuki coupling of phenylboronic acid containing hydroxymethyl was also investigated. An
efficient 3/Cu cocatalyzed oxidation/Suzuki coupling of aryl chlorides with phenylboronic acids contain-
ing hydroxymethyl in air has been developed.
Ó 2014 Elsevier B.V. All rights reserved.
Keywords:
Cyclopalladated complex
N-Heterocyclic carbene
Oxidation
Suzuki coupling
1. Introduction
for the synthesis of biarylaldehydes from aryl chlorides and arylbo-
ronic acids containing hydroxymethyl.
Cyclopalladated complexes have attracted a great deal of atten-
tion due to their high activity as precatalysts in coupling reactions
[1]. In general, adducts of palladacycles combine the stability
induced by the presence of a palladacycle framework with phos-
phine ligands or NHCs, and were far more active than the corre-
sponding dimeric palladacycles. For example, Buchwald [2], Nolan
[3] groups have reported the use of dialkylbiaryl phosphine-palla-
dacycles and NHC-palladacycles as highly efficient catalysts for
Suzuki reaction of unactivated aryl chlorides. In addition, palla-
dium-catalyzed alcohol oxidation to carbonyl compounds is one
of the most fundamental and widely utilized transformations in
organic synthesis [4]. Great recent progress has been obtained for
the palladium-catalyzed oxidation by using NHC ligands or nitrogen
donor ligands reported by Sigman and co-workers [5]. Recently, we
have also developed NHC-palladacycles/Cu cocatalyzed oxidation/
Suzuki reaction for the synthesis of biarylaldehydes from chloro-
phenylmethanol and arylboronic acids [6]. As a continuation of
our interest in the synthesis and application of cyclopalladated fer-
rocene complexes [7], we prepared three adducts of palladacycle 1–
3 (Scheme 1) and examined their catalytic activity in Suzuki
reaction and described 3/Cu(OAc)2-cocatalyzed one-pot reaction
2. Experimental
2.1. General procedures
Solvents were dried and freshly distilled prior to use. All other
chemicals were commercially available except for the 6-bromo-
2-ferrocenylquinoline which was prepared according to the pub-
lished procedure [8]. Elemental analyses were determined with a
Carlo Erba 1160 Elemental Analyzer. IR spectra were collected on
a Bruker VECTOR22 spectrophotometer in KBr pellets. NMR spectra
were recorded on a Bruker DPX-400 spectrometer in CDCl3 with
TMS as an internal standard. Crystallographic data for 1 and 2, 3
were measured on a Bruker SMART APEX-II CCD diffractometer
and a Xcalibur, Eos, Gemini diffractometer, respectively. The data
were corrected for Lorentz polarization factors as well as for
absorption. Structures were solved by direct methods and refined
by full-matrix least-squares methods on F2 with the SHELX-97 pro-
gram. All non-hydrogen atoms were refined anisotropically.
2.2. The synthesis of cyclopalladated complex 1
⇑
A mixture of 6-bromo-2-ferrocenylquinoline (1 mmol), Li2PdCl4
(1 mmol) and NaOAc (1.1 mmol) in 10 mL of dry methanol was
Corresponding author.
E-mail address: xubohan@163.com (C. Xu).
http://dx.doi.org/10.1016/j.ica.2014.07.037
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

12
C. Xu et al. / Inorganica Chimica Acta 423 (2014) 11–15
1
L = PPh3
L =
N
N
N
Br
Li2PdCl4
NaOAC
N
Br
L
2
Fe
Fe
Pd
Cl
N
N
L =
L
3
Scheme 1. Synthesis of 1–3.
stirred for 12 h at rt. The formed red solid (yield: 92%) were col-
lected by filtration, and can be assigned as a chloride-bridged pal-
ladacyclic dimer [6,7]. Without further purification, the above
dimer was treated with PPh3 (1.1 mmol) in CH2Cl2 at rt for 1 h.
The product 1 was separated by passing through a short silica gel
column with CH2Cl2 as eluent. Red solid, yield: 93%. Anal. Calc.
for C37H28BrClFeNPPd: C, 55.88; H, 3.55; N, 1.76. Found: C, 55.99;
H, 3.41; N, 1.93%. IR (KBr, cmÀ1): 2918, 1595, 1506, 1462, 1372,
1260, 1187, 1068, 1018, 918, 806,774. 1H NMR (400 MHz, CDCl3):
d 9.69 (s, 1H, ArH), 7.73–7.99 (m, 9H, ArH), 7.41–7.43 (m, 10H,
ArH), 4.68 (s, 1H, C5H3), 4.15 (s, 1H, C5H3), 3.74 (s, 5H, C5H5),
3.67 (s, 1H, C5H3).
Fig. 1. Molecular structure of complex 1. CH2Cl2 and H atoms are omitted for
clarity. Selected bond lengths (Å) and angles (°): Pd(1)–C(1) 1.977(3), Pd(1)–P(1)
2.2361(9), Pd(1)–N(1) 2.193(3), Pd(1)–Cl(1) 2.4233(9), and C(1)–Pd(1)–N(1)
80.95(13), C(1)–Pd(1)–P(1) 93.55(11), N(1)–Pd(1)–Cl(1) 97.51(8), P(1)–Pd(1)–Cl(1)
94.65(3).
2.3. General procedure for the synthesis of cyclopalladated complexes
2 and 3
ArH), 7.05 (d, 1H, ArH), 2.41 (s, 3H, CH3). 13C NMR (100 MHz,
CDCl3): 196.7, 157.6, 149.6, 147.9, 139.7, 129.0, 128.8, 127.1,
123.3, 121.7, 21.4.
2-(Pyrazin-2-yl)benzaldehyde (4n). Anal. Calc. for C11H8N2O: C,
71.73; H, 4.38; N, 15.21. Found: C, 71.88; H, 4.26; N, 15.34%. IR
(KBr, cmÀ1): 3063, 1687, 1656, 1446, 1348, 1295, 1194, 1102,
1027, 837, 738. 1H NMR (400 MHz, CDCl3): d 9.94 (s, 1H, CHO),
8.31 (s, 1H, ArH), 7.92 (d, 1H, ArH), 7.65–7.72 (m, 5H, ArH). 13C
NMR (100 MHz, CDCl3): 198.7, 140.1, 138.7, 138.2, 134.4, 131.2.
A Schlenk tube was charged with the above dimer (0.5 mmol),
the corresponding imidazolium salt (1.2 mmol) and KtOBu
(2.0 mmol) under nitrogen. Dry THF was added by a cannula and
the solution was stirred for 3 h at rt. The product was separated
by passing through a short silica gel column with CH2Cl2 as eluent,
the second band was collected and afforded the corresponding
NHC-palladacycles 2 and 3. (2): Red solid, yield 89%. Anal. Calc.
for C40H37BrClFeN3Pd: C, 57.37; H, 4.45; N, 5.02. Found: C, 57.52;
H, 4.26; N, 5.15%. IR (KBr, cmÀ1): 2916, 1592, 1548, 1480, 1402,
1321, 1103, 1018, 920, 731. 1H NMR (400 MHz, CDCl3): d 9.36
(s, 1H, ArH), 7.79 (d, 1H, ArH), 7.66 (s, 1H, ArH), 7.59 (m, 1H,
ArH), 7.07–7.22 (m, 5H, ArH), 6.85 (s, 1H, ArH), 6.71 (s, 1H, ArH),
4.59 (s, 1H, C5H3), 4.20 (s, 1H, C5H3), 3.97 (s, 1H, C5H3), 3.43 (s,
5H, C5H5), 2.91 (s, 3H, CH3), 2.52 (s, 3H, CH3), 2.35 (s, 6H, CH3),
2.17 (s, 3H, CH3), 1.92 (s, 3H, CH3). (3): Red solid, yield 88%. Anal.
Calc. for C46H49BrClFeN3Pd: C, 59.95; H, 5.36; N, 4.56. Found: C,
60.12; H, 5.18; N, 4.73%. IR (KBr, cmÀ1): 2957, 1589, 1547, 1462,
1361, 1329, 1259, 1103, 1040, 920, 819, 810, 699. 1H NMR
(400 MHz, CDCl3): d 9.51 (s, 1H, ArH), 7.79 (d, 1H, ArH), 7.68 (s,
1H, ArH), 7.47–7.55 (m, 4H, ArH), 7.18–7.26 (m, 5H, ArH), 7.15
(d, 1H, ArH), 4.95–5.02 (m, 1H, CH), 4.56 (s, 1H, C5H3), 4.25
(s, 1H, C5H3), 3.89 (s, 1H, C5H3), 3.27 (m, 6H, C5H5+CH), 2.87–
2.98 (m, 2H, CH), 1.84 (d, 3H, CH3), 1.61 (d, 3H, CH3), 1.52 (d, 3H,
CH3), 1.45 (d, 3H, CH3), 1.25 (d, 3H, CH3), 1.08 (d, 3H, CH3), 0.83
(s, 3H, CH3), 0.62 (s, 3H, CH3).
3. Results and discussion
3.1. Synthesis and structures of complexes 1–3
The synthetic route for three adducts of palladacycle 1–3 is
shown in Scheme 1. These complexes have been easily prepared
from the bridge-splitting reaction and characterized by elemental
2.4. General procedure for the one-pot oxidation/Suzuki coupling
A 10 mL round-bottom flask was charged with the prescribed
amount of catalyst Pd/Cu, aryl chlorides (0.5 mmol), phenylboronic
acids containing hydroxymethyl (0.75 mmol), Cs2CO3 (1.0 mmol)
and dioxane (5 mL) in air. The reaction mixture was then placed
in an oil bath and heated at 110 °C for 24 h. After removal of the
solvent, the resulting residue was purified by flash chromatogra-
phy on silica gel using CH2Cl2 as eluent. The products 4a–k, and
4m are known compounds [6,9] except for 4l and 4n.
2-(4-Methylpyridin-2-yl)benzaldehyde (4l). Anal. Calc. for
Fig. 2. Molecular structure of complex 2. CH2Cl2 and H atoms are omitted for
clarity. Selected bond lengths (Å) and angles (°): Pd(1)–C(6) 1.975(4), Pd(1)–C(20)
1.990(4), Pd(1)–N(1) 2.190(3), Pd(1)–Cl(1) 2.438(5), and C(6)–Pd(1)–N(1)
80.66(14), C(6)–Pd(1)–C(20) 92.77(15), N(1)–Pd(1)–Cl(1) 96.90(14), C(20)–Pd(1)–
Cl(1) 89.09(15).
C
13H11NO: C, 79.16; H, 5.62; N, 7.10. Found: C, 79.38; H, 5.43; N,
7.26%. IR (KBr, cmÀ1): 3056, 1681, 1603, 1556, 1446, 1440, 1295,
1071, 1029, 825, 734. 1H NMR (400 MHz, CDCl3): d 9.91 (s, 1H,
CHO), 7.97 (d, 2H, ArH), 7.55 (s, 1H, ArH), 7.38–7.48. (m, 3H,

C. Xu et al. / Inorganica Chimica Acta 423 (2014) 11–15
13
3.2. Pd/Cu cocatalyzed oxidation/Suzuki reaction
The palladium-catalyzed Suzuki reaction is an extremely pow-
erful approach to generate biaryl compounds. Generally, Suzuki
reaction of aryl halides with arylboronic acids containing hydroxy-
methyl produces the corresponding biarylalcohols [7b,11].
Recently, we have found NHC-palladacycles/Cu cocatalyzed reac-
tion involving oxidation and Suzuki reaction of aryl chlorides con-
taining hydroxymethyl [6]. One possible reaction mechanism is via
Pd/Cu cocatalyzed alcohol oxidation and Pd catalyzed Suzuki reac-
tion. In order to extend the scope of this method, we performed the
coupling of 4-hydroxymethylphenylboronic acid with chloroben-
zene to evaluate the effectiveness of the new palladacycles 1–3
(Table 1, entries 1–3). 3 was the most efficient among these palla-
dacycles (entry 3), however, 1 was inactive under the same reac-
tion conditions (Cs2CO3, dioxane, air). Additionally, the relative
catalyst [Pd(bpp)(IMes)Cl] [6] in comparison with 3 showed
slightly lower activity for the above reaction, producing the cou-
pled product in 71% yield. The coupling reactions of 4-hydroxy-
methyl-phenylboronic acid with a variety of electronically and
structurally diverse aryl chlorides were investigated (entries 4–
7). Electron-withdrawing substrates reacted to give the
corresponding products 4d and 4e, the yields (93% and 91%) are
higher than the yields of electron-donating substrates containing
–CH3 and –OCH3 groups. This protocol was also successfully
applied to synthesis of 2-biarylcarboxaldehydes via oxidation and
Suzuki reaction of 2-hydroxymethylphenylboronic acid (entries
8–10). An obvious decrease in the yield (51%) of meta-substitution
product 4g was observed (entry 9).
Fig. 3. Molecular structure of complex 3. CH2Cl2 and H atoms are omitted for
clarity. Selected bond lengths (Å) and angles (°): Pd(1)–C(9) 1.983(4), Pd(1)–C(20)
1.993(4), Pd(1)–N(1) 2.151(3), Pd(1)–Cl(1) 2.4201(10), and C(9)–Pd(1)–N(1)
80.19(14), C(9)–Pd(1)–C(20) 95.22(15), N(1)–Pd(1)–Cl(1) 95.11(9), C(20)–Pd(1)–
Cl(1) 89.19(10).
analysis, IR and 1H NMR. Their crystals were obtained by recrystal-
lization from CH2Cl2–petroleum ether solution at rt. The molecules
are shown in Fig. 1–3. CH2Cl2 molecule in 1 is disordered, bromine
atom and CH2Cl2 molecule in 2 are also disordered. The molecule of
1–3 adopts a trans configuration of the coordinated PPh3 and NHCs
to the quinoline nitrogen. The bicyclic system formed by the palla-
dacycle and the C5H3 moiety is approximately coplanar (dihedral
angles of 9.1, 10.8, and 14.4o for 1–3, respectively). The Pd–Ccarb
[1.990(4) and 1.993(4) Å] bond lengths of complexes 2 and 3 are
similar to those of related carbene adducts [6,7c,10], while they
are evidently shorter than Pd–P bond length of 1 [2.2361(9) Å].
The Suzuki reactions of heteroarylboronic acids serve as impor-
tant building blocks for the construction of biologically active com-
pounds [12]. However, the Suzuki reactions of 2-pyridyl boronic
Table 1
Reaction of aryl chlorides with phenylboronic acid containing hydroxymethyl catalyzed by Pd/Cu(OAc)2.a
OH
CHO
or
R
R
or
Cat Pd/Cu
2-N-
2-N-
+
Cl
(HO)2B
HetAr
HetAr
Entry
1c
Aryl halide
Cl
Product
Yield(%)b
0
4a
4a
4a
4b
CHO
CHO
CHO
2d
3
75
86
83
Cl
Cl
4
Cl
CHO
5
6
4c
80
93
H3CO
CHO
Cl
H3CO
4d
O
C
O
C
CHO
Cl
O2N
7
8
4e
4f
91
72
O2N
Cl
CHO
Cl
OHC
(continued on next page)

14
C. Xu et al. / Inorganica Chimica Acta 423 (2014) 11–15
Table 1 (continued)
Entry
Aryl halide
Product
Yield(%)b
51
9
4g
4h
Cl
OHC
10
O
84
O
C
Cl
C
OHC
11
12
4i
4j
88
79
Cl
Cl
CHO
N
OHC
N
N
N
13
14
4k
4l
82
71
CHO
Cl
N
N
OHC
Cl
N
N
N
N
15
16
4m
4n
90
78
N
N
N
CHO
Cl
Cl
N
OHC
N
N
a
Reaction conditions: aryl chloride (0.5 mmol), phenylboronic acid containing hydroxymethyl (0.75 mmol), 3/Cu(OAc)2 (1/5 mol%), Cs2CO3 (1.0 mmol), dioxane (5 mL),
110 °C, 24 h, air atmosphere.
b
Isolated yield.
Catalyst 1 (1 mol%).
Catalyst 2 (1 mol%).
c
d
acids have been relatively less reported [13]. The difficulty may be
attributed to its instability [13b,c]. So, the coupling of 2-N-hetero-
aryl halides with arylboronic acid provided a valuable complement
the preparation of the corresponding 2-aryl-N-heterocycles. In the
following experiments, the Suzuki coupling of a variety of 2-N-het-
eroaryl chlorides with phenylboronic acids containing hydroxy-
methyl was investigated under the same reaction conditions.
Similar to the results of chlorobenzene, good yields (79–88%) were
obtained in the case of 2-chloropyridine (entries 11 and 12). For 2-
chloro-4-methylpyridine, the yields of the coupled products 4k
and 4l could be reached 71–82% (entries 13 and 14). Finally, the
couplings of 2-chloropyrazine with phenylboronic acids containing
hydroxymethyl also gave good yields (entries 15 and 16).
Young Teachers in Henan Provincial Institutions of Higher Educa-
tion of China (2013GGJS-151), the tackle key problem of science
and technology Project of Henan Province (122102310452) and
the Science Foundation of Henan Education Department
(14A150049).
Appendix A. Supplementary material
CCDC 1001676–1001678 contain the supplementary crystallo-
graphic data for complexes 1–3, respectively. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at http://dx.doi.org/10.1016/j.ica.2014.07.037.
4. Conclusions
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NHC- palladacycle/Cu cocatalyzed oxidation and Suzuki reaction
for the synthesis of biarylaldehydes from phenylboronic acids con-
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