α-Hydroxyimine palladium complexes: Synthesis, molecular structure, and their activities towards the Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1016/j.jorganchem.2013.01.018

An activity-promoting strategy for phosphine-free catalytic systems is presented in this study. The strategy is based on a series of non-conjugated N,O ligands with bulky substituents, [Ar-NC(R)-(R)C(CH₃)OH] (L1, R = Acenaphthyl, Ar = 2,6-diiso

Full text of DOI:10.1016/j.jorganchem.2013.01.018

Journal of Organometallic Chemistry 729 (2013) 95e102
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Journal of Organometallic Chemistry
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a-Hydroxyimine palladium complexes: Synthesis, molecular structure, and their
activities towards the SuzukieMiyaura cross-coupling reaction
*
*
Xiao Tang, Ying-Tang Huang, Huan Liu, Rui-Zhi Liu, Dong-Sheng Shen , Ning Liu, Feng-Shou Liu
School of Chemistry and Chemical Engineering, Guangdong Pharmaceutical University, Zhongshan, Guangdong 528458, China
a r t i c l e i n f o
a b s t r a c t
Article history:
An activity-promoting strategy for phosphine-free catalytic systems is presented in this study. The
strategy is based on a series of non-conjugated N,O ligands with bulky substituents, [AreN]C(R)e(R)
C(CH3)OH] (L1, R ¼ Acenaphthyl, Ar ¼ 2,6-diisopropylphenyl; L2, R ¼ Ph, Ar ¼ 2,6-dimethylphenyl;
L3, R ¼ Ph, Ar ¼ 2,6-diisopropylphenyl). The reaction of PdCl2 with 1 equiv of the ligand L1e3 in
methanol affords the four-coordinate palladium complex with the general formula LPdCl2 (1e3). The
molecular structures of L3, as well as the palladium complexes 1 and 3, were established by single-crystal
X-ray diffraction studies. The application of these palladium complexes as precatalysts was examined for
the SuzukieMiyaura cross-coupling of a range of aryl bromides with arylboronic acids. 3, which in-
corporates bulky substituents, was shown to be effective in the cross-coupling reactions at 0.01 mol%
palladium loading, affording the corresponding biaryls in high yields.
Received 10 December 2012
Received in revised form
19 January 2013
Accepted 22 January 2013
Keywords:
a
-Hydroxyimine palladium complex
SuzukieMiyaura reaction
Aryl bromide
Arylboronic acid
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
of a strong palladiumeimine bond that helps to prevent catalyst
decomposition [26]. Moreover, other attractive features are that
Biaryls are widely found in the synthesis of natural products,
pharmaceuticals, and functional polymer materials [1e3]. There-
fore, it is important to develop general methods to construct such
compounds. One such path is the palladium-catalysed Suzukie
Miyaura reaction, which involves the cross-coupling of an aryl
halide with an arylboronic acid to form the respective biaryl
structure [4e21]. In the past few decades, there have been
numerous efforts to develop efficient phosphine-based catalysts for
this transformation [16,18]. The key feature of these catalysts lies in
the sterically bulky and electron-rich ligands that have great facility
in the catalytic cycle. As a result, they exhibit outstanding perfor-
mance in the synthesis of biaryls, even using sterically hindered
substrates under mild conditions [22e25]. However, most phos-
phines are air sensitive, toxic, and therefore difficult to handle.
The development of stable and environmentally friendly
phosphine-free ligands for the preparation of biaryls has been
intensely investigated in recent years [14,15,26e40]. Among the
ligands investigated, the N,O-based ligands have exhibited good
activity for the SuzukieMiyaura cross-coupling, which is often
these phosphine-free ligands are easily accessible synthetically and
that they have rather flexible architectures. These properties allow
for the steric and electronic effects to be tuned to a large extent,
giving the possibility of screening these compounds for their cat-
alytic properties. For example, salicylaldimine (I) (Fig. 1) palladium
complexes exhibited fine catalytic activities [26e30]. In addition, b-
ketoamine (II) and quinoline-8-carboxylate (III) (Fig. 1) demon-
strated broad applicability and efficiency towards a wide range of
aryl bromide substrates under mild and simple reaction conditions
[26,32,33]. Despite this, very few examples of these catalysts were
running at a low (ꢀ0.01 mol%) palladium loadings. Thus, the design
of new and readily available phosphine-free catalyst, which can
significantly lower the catalyst loading and improve the reactivity
with broad substrate generality, is still in high demand.
Previous investigations using calculations and kinetic mea-
surements suggested that the strong s-donor ability of the ligands
would accelerate the oxidative addition, which is believed to be the
rate determining step in the catalytic cycle [38e42]. It is reasonable
that the discrimination between the phosphine-free and
phosphine-based ligands can be primarily attributed to electronic
factors. To enhance the activities of the N,O ligands, some ancillary
ligand such as triphenylphosphine was added, which provides
more electron donation towards the metal centre. As a result,
highly active catalysts were obtained [43e45]. In view of the N,O
ligands reported, conjugated molecular structures were found in
attributed to their s-donating ability that allows for the formation
* Corresponding authors. Fax: þ86 0760 88207928.
E-mail addresses: sds8@163.com (D.-S. Shen), fengshou2004@126.com
(F.-S. Liu).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.01.018

96
X. Tang et al. / Journal of Organometallic Chemistry 729 (2013) 95e102
2.2. Syntheses and characterization
2.2.1. Synthesis of 2,6-(iPr)2C6H3eN]C(acenaphthyl)e
C(acenaphthyl)(Me)-OH (L1)
2,6-(iPr)2C6H3eN]C(acenaphthyl)eC(acenaphthyl)]O
(1a,
5 mmol) was dissolved in 10 ml toluene under a nitrogen atmo-
sphere, and trimethylaluminum (7 ml, 1.0 M) was added slowly
through a syringe at room temperature, and then the reaction was
heated to reflux for 4 h. When having reached the determined time,
the solution was cooled to 0 ꢂC, and the reaction mixture was
carefully hydrolysed with 5% aqueous NaOH solution. The organic
product was extracted with ethyl acetate, dried over MgSO4, and
evaporated the solvent. The desired product obtained as yellow
solid. The crude material was crystallized from ethanol as light
yellow crystal in 92% yield. Non enantioselective donation was
observed with the compound, which suggested a completely rac-
emic ligand. Moreover, isomers were detected by NMR in 1:1 ratio.
Fig. 1. N,O-Bidentate ligands.
most cases to generally reduce the electron donation ability. We
surmised that non-conjugated ligands, combined with steric sub-
stitutions, can not only furnish adequate electron donation to the
metal centre, to promote the oxidative addition process, but also
favour the reductive elimination. Therefore, a further improvement
in catalytic activity could be achieved without the addition of any
ancillary ligand. In this regard, we have become interested in the
1H NMR (300 MHz, CDCl3),
d
(ppm): Isomer 1: 1H NMR: (CDCl3,
300 MHz), (ppm): 7.86e7.62 (m, 4H, AreH), 7.29e7.20 (m, 4H,
d
AreH), 6.51e6.49 (m, 1H, AreH), 3.13 (s, 1H, OH), 2.98 (m, 2H,
CH(CH3)2), 1.90 (s, 3H, CH3), 1.25 (d, J ¼ 6.9 Hz, 6H, CH3), 1.15 (d,
J ¼ 6.9 Hz, 6H, CH3). 13C NMR: (CDCl3, 75 MHz),
d (ppm): 174.34,
146.01, 142.68, 138.43, 136.24, 135.65, 130.92, 129.37, 128.89, 128.41,
127.85, 124.91, 124.10, 123.51, 119.49, 78.69, 28.41, 27.61, 23.43.
non-conjugated a-hydroxyimine ligand (IV) (Fig. 1) where its steric
Isomer 2: 1H NMR: (CDCl3, 300 MHz),
d (ppm): 7.86e7.62 (m, 4H,
and electronic properties can be easily modified. The arylimine
moiety is anticipated to be perpendicular to the coordination plane.
While the substituents on the backbone of the ligand are directed
away from the metal centre, it would hinder the rotation of the
arylimine moiety, and thus provide stability to the catalytic centre
in further catalytic steps. Herein, we report the synthesis of the
ligands and palladium complexes and their structural characteri-
sation and describe the catalytic properties of these precatalysts for
the SuzukieMiyaura cross-coupling reaction.
AreH), 7.29e7.20 (m, 4H, AreH), 6.51e6.49 (m, 1H, AreH), 3.13 (s,
1H, OH), 2.98 (m, 2H, CH(CH3)2), 1.90 (s, 3H, CH3), 1.02 (d, J ¼ 6.9 Hz,
6H, CH3), 0.83 (d, J ¼ 6.9 Hz, 6H, CH3). 13C NMR: (CDCl3, 75 MHz),
d
(ppm): 174.34, 146.01, 142.68, 138.43, 136.24, 135.65, 130.92,
129.37, 128.89, 128.41, 127.85, 124.91, 124.10, 123.16, 119.49, 78.69,
27.94, 27.61, 23.09. Elemental analysis calculated for C25H27NO: C,
83.99; H, 7.61; N, 3.92. Found: C, 83.91; H, 7.56; N, 3.87.
2.2.2. Synthesis of 2,6-(CH3)2C6H3eN]C(Ph)eC(Ph)(Me)-OH (L2)
Following the above procedure, L2 was isolated as white crystal
in 87% yield. Isomers were detected by NMR in 1:1 ratio. Isomer 1:
2. Experimental section
1H NMR: (CDCl3, 300 MHz),
d (ppm): 7.48e7.46 (m, 2H, AreH),
2.1. Physical measurements and materials
7.38e7.30 (m, 3H, AreH), 7.13e7.08 (m, 1H, AreH), 7.00e6.89 (m,
3H, AreH), 6.78e6.74 (m, 2H, AreH), 6.46e6.44 (m, 2H, AreH), 2.22
2,6-Dimethylaniline and 2,6-diisopropylaniline were purchased
from Aldrich Chemical and were distilled under reduced pressure
before being used. TMA (1 M, hexane) was purchased from Aldrich
Chemical. Acenaphthenequinone and benzil were purchased from
Alfa Aesar Chemical and used as received. Toluene was refluxed
over metallic sodium for 24 h before being used. 2,6-(i-Pr)2C6H3e
N]C(An)eC(An)]O [46], and 2,6-(i-Pr)2C6H3eN]C(Ph)eC(Ph)]
O [47], were prepared according to literature procedures.
The NMR data of ligands and biaryls were obtained on a Varian
Mercury-Plus 300 MHz spectrometer at ambient temperature, us-
ing CDCl3 as solvent and referenced versus TMS as standard. The
NMR data of palladium complexes were obtained on a Varian
Mercury-Plus 300 MHz spectrometer, using DMSO-d6 as solvent.
Elemental analyses were determined with a Vario EL Series Ele-
mental Analyser from Elementar. The X-ray diffraction data of
(s, 3H, CH3), 1.81 (s, 6H, CH3). 13C NMR: (75 MHz),
d (ppm): 174.47
(C]N), 145.55, 142.80, 134.43, 128.59, 128.04, 127.84, 127.57, 127.23,
126.90, 126.52, 125.39, 123.42, 76.68, 25.89, 18.66. Isomer 2: 1H
NMR: (CDCl3, 300 MHz),
d (ppm): 7.48e7.46 (m, 2H, AreH), 7.38e
7.30 (m, 3H, AreH), 7.13e7.08 (m, 1H, AreH), 7.00e6.89 (m, 3H,
AreH), 6.78e6.74 (m, 2H, AreH), 6.46e6.44 (m, 2H, AreH), 2.22 (s,
3H, CH3), 1.80 (s, 6H, CH3). 13C NMR: (75 MHz),
d (ppm): 174.47 (C]
N), 145.55, 142.80, 134.43, 128.59, 128.04, 127.74, 127.57, 127.23,
126.90, 126.42, 125.39, 123.42, 76.68, 25.89, 18.48. Elemental anal-
ysis calculated for C23H23NO: C, 83.85; H, 7.04; N, 4.25. Found: C,
83.73; H, 7.01; N, 4.16.
2.2.3. Synthesis of 2,6-(iPr)2C6H3eN]C(Ph)eC(Ph)(Me)-OH (L3)
Following the above procedure, L3 was isolated as white crystal
in 95% yield. Isomers were detected by NMR in 1:1 ratio. Isomer 1:
single crystals were obtained with the
u
ꢁ 2
q
scan mode on
1H NMR: (CDCl3, 300 MHz),
d (ppm): 7.51e7.36 (m, 4H, AreH),
a
Bruker SMART 1000 CCD diffractometer with graphite-
7.13e7.08 (m, 1H, AreH), 7.02e6.74 (m, 6H, AreH), 6.45e6.42 (m,
2H, AreH), 3.05 (m, 2H, CH(CH3)2), 1.82 (s, 3H, CH3), 1.54 (br, 1H,
OH),1.30 (d, J ¼ 3 Hz, 6H, CH3),1.12 (d, J ¼ 6.9 Hz, 6H, CH3). 13C NMR:
ꢀ
monochromated Mo K
a
radiation (
l
¼ 0.71073 A) at 173 K. The
structure was solved using direct methods, and further refinement
with full-matrix least squares on F2 was obtained with the SHELXTL
program package. All non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms were introduced in calculated positions
with the displacement factors of the host carbon atoms. TEM ob-
servations were performed on a TEM (JEM100CX, Japan) with an
accelerating voltage of 100 kV. A drop of solution was deposited
onto a carbon coated copper grid, dried at room temperature.
(CDCl3, 75 MHz),
d (ppm): 174.34, 146.01, 142.68, 138.43, 136.24,
135.65, 130.92, 129.37, 128.89, 128.41, 127.85, 124.91, 124.10, 123.51,
119.49, 78.69, 28.41, 27.61, 23.43. Isomer 2: 1H NMR: (CDCl3,
300 MHz), d (ppm): 7.51e7.36 (m, 4H, AreH), 7.13e7.08 (m, 1H, Are
H), 7.02e6.74 (m, 6H, AreH), 6.45e6.42 (m, 2H, AreH), 2.53 (m, 2H,
CH(CH3)2), 1.82 (s, 3H, CH3), 1.54 (br, 1H, OH), 1.27 (d, J ¼ 3 Hz, 6H,
CH3), 0.71 (d, J ¼ 6.9 Hz, 6H, CH3). 13C NMR: (CDCl3, 75 MHz),

X. Tang et al. / Journal of Organometallic Chemistry 729 (2013) 95e102
97
Scheme 1. The synthetic route of N,O ligands and palladium complexes.
d
(ppm): 174.34, 146.01, 142.68, 138.43, 136.24, 135.65, 130.92,
6H, CH(CH3)2), 0.71 (d, J ¼ 6.9 Hz, 6H, CH(CH3)2). 13C NMR (75 MHz,
129.37, 128.89, 128.41, 127.85, 124.91, 124.10, 123.16, 119.49, 78.69,
27.94, 27.61, 23.09. Elemental analysis calculated for C27H31NO: C,
84.11; H, 8.10; N, 3.63. Found: C, 84.07; H, 8.02; N, 3.60.
DMSO-d6): d (ppm) 173.05, 144.30, 135.12, 134.26, 127.51, 126.80,
126.48, 125.22, 122.90, 122.16, 121.81, 77.52, 29.41, 27.10, 23.75,
21.21. Elemental analysis calculated for C27H31Cl2NOPd: C, 57.61; H,
5.55; N, 2.49. Found: C, 57.42; H, 5.61; N, 2.43.
2.2.4. Synthesis of [2,6-(iPr)2C6H3eN]C(acenaphthyl)e
C(acenaphthyl)(Me)-OH]PdCl2 (1)
2.3. General procedure for SuzukieMiyaura reaction
L1 (1.0 mmol) and palladium dichloride (1.0 mmol) were com-
bined in 10 ml methanol at room temperature. After the reaction
was heated to reflux overnight, the methanol was removed under
reduced pressure. The residue was washed with 2 ꢃ 20 ml of
hexane to afford compound 1 as brown solid in 92% yield. The red
crystal suitable for X-ray diffraction was prepared in methanol at
room temperature. Isomers were detected by NMR in 1:1 ratio. 1H
In a round bottle, palladium complexes (0.01 mol% mmol), aryl
halides (1.0 mmol), arylboronic acid (1.2 mmol), base (2.0 mmol) and
4 ml of solvent were added with a magnetic stir bar. The reaction
mixturewascarried out atthe described temperaturefor the required
time, and then the solvent was removed under reduced pressure. The
residual was diluted with Et2O (5 ml), followed by extraction twice
(2 ꢃ 5 ml) with Et2O. The organic layer was dried with anhydrous
MgSO4, filtered and evaporated under vacuum. The crude products
were purified by silica-gel column chromatography using petroleum
ethereethyl acetate (20/1) as an eluent, and the isolated yield was
then calculated based on the feeding of the aryl halide. The isolated
corresponding products were characterized by 1H NMR and 13C NMR.
NMR (300 MHz, DMSO-d6):
d (ppm) Isomer 1: 7.97e6.33 (m, 9H,
AreH), 2.96 (m, 2H, CH(CH3)2), 1.76 (s, 3H, CH3), 1.14 (d, J ¼ 6.9 Hz,
6H, CH(CH3)2), 1.10 (d, J ¼ 6.9 Hz, 6H, CH(CH3)2). 13C NMR (75 MHz,
DMSO-d6):
d (ppm) 173.60, 146.67, 137.67, 135.30, 131.99, 130.65,
129.75, 129.06, 128.75, 128.21, 124.84, 123.93, 123.27, 121.60, 119.60,
119.80, 77.57, 27.82, 23.51, 22.99. Isomer 2: 1H NMR (300 MHz,
DMSO-d6):
d (ppm) 7.97e6.33 (m, 9H, AreH), 2.81 (m, 2H,
CH(CH3)2), 1.76 (s, 3H, CH3), 0.88 (d, J ¼ 6.9 Hz, 6H, CH(CH3)2), 0.85
(d, J ¼ 6.9 Hz, 6H, CH(CH3)2). 13C NMR (75 MHz, DMSO-d6):
d (ppm)
3. Results and discussion
173.60, 144.84, 137.67, 135.14, 131.99, 130.65, 129.75, 128.94, 128.66,
127.85, 124.65, 123.54, 122.59, 121.60, 119.60, 76.41, 27.68, 23.30,
22.83. Elemental analysis calculated for C25H27Cl2NOPd: C, 56.14; H,
5.09; N, 2.62. Found: C, 56.03; H, 5.14; N, 2.56.
3.1. Synthesis and characterisation of
compounds
a-hydroxyimine ligand
The
a-hydroxyimine compound has been studied previously
and has been used as a cell transformation reagent [48]. There are
several synthetic routes that have been explored [49e53]. One
2.2.5. Synthesis of [2,6-(CH3)2C6H3eN]C(Ph)eC(Ph)(Me)-OH]
PdCl2 (2)
Following the above procedure, 2 was isolated in 94% yield.
Isomers were detected by NMR in 1:1 ratio. 1H NMR (300 MHz,
DMSO-d6):
d
(ppm) Isomer 1: 7.54e6.63 (m,13H, AreH), 2.02 (s, 3H,
(ppm)
CH3), 1.88 (s, 6H, CH3). 13C NMR (75 MHz, DMSO-d6):
d
174.74, 147.65, 136.56, 132.72, 130.06, 128.97, 128.53, 127.97, 127.49,
127.06, 125.72, 124.62, 122.98, 80.46, 30.04, 18.64. Isomer 2: 1H
NMR (300 MHz, DMSO-d6):
d
(ppm) 7.54e6.63 (m, 13H, AreH), 2.02
(ppm)
(s, 3H, CH3), 1.86 (s, 6H, CH3). 13C NMR (75 MHz, DMSO-d6):
d
174.74, 145.49, 135.15, 130.70, 129.60, 128.60, 128.44, 127.92, 127.37,
126.23, 125.36, 124.62, 122.98, 78.13, 29.44, 18.60. Elemental anal-
ysis calculated for C23H23Cl2NOPd: C, 54.51; H, 4.57; N, 2.76. Found:
C, 54.35; H, 4.60; N, 2.71.
2.2.6. Synthesis of [2,6-(iPr)2C6H3eN]C(Ph)eC(Ph)(Me)-OH]
PdCl2 (3)
Following the above procedure, 3 was isolated in 95% yield.
Isomers were detected by NMR in 1:1 ratio. 1H NMR (300 MHz,
DMSO-d6):
d (ppm) Isomer 1: 7.56e6.59 (m, 13H, AreH), 2.99 (m,
2H, CH(CH3)2), 1.90 (s, 3H, CH3), 1.15 (d, J ¼ 6.9 Hz, 6H, CH(CH3)2),
1.08 (d, J ¼ 6.9 Hz, 6H, CH(CH3)2). 13C NMR (75 MHz, DMSO-d6):
d
(ppm) 173.05, 145.06, 135.47, 134.26, 127.51, 126.80, 126.62,
Fig. 2. Molecular structure of L3 depicted with 30% thermal ellipsoids. Hydrogen
atoms have been omitted for clarity. Selected bond distances (Å) and angles (ꢂ): N(1)-
C(19) 1.268(2), N(1)-C(2) 1.430(2), O(1)-C(26) 1.419(12), C(19)-C(18) 1.495(2), C(19)-
C(26) 1.546 (2), C(26)-C(25) 1.533(2), C(26)-C(27) 1.524(2).
125.22, 122.90, 122.16, 121.81, 77.52, 29.41, 27.54, 23.94, 21.39. 1H
NMR (300 MHz, DMSO-d6): (ppm) Isomer 2: 7.56e6.59 (m, 13H,
AreH), 2.53 (m, 2H, CH(CH3)2), 1.90 (s, 3H, CH3), 1.01 (d, J ¼ 6.9 Hz,
d

98
X. Tang et al. / Journal of Organometallic Chemistry 729 (2013) 95e102
benzoin, while refluxing toluene or in the presence of molecular
sieves, was unsuccessful. This result may have been caused by the
steric hindrance of the -hydroxy ketone and the lower nucleo-
a
philicity of the bulky substituted aniline. To obtain the desired
ligands with bulky substituents, we devised a synthetic approach
based on the selective reductive addition of
an organoaluminum reagent [52,53]. As illustrated in Scheme 1,
the -imino ketones were first treated with trimethylaluminum to
a-imino ketones with
a
form the coordinated complexes in situ, and a chemoselective
addition to C]O by methyl group occurred subsequently. As
a result,
high yields.
The structures of the ligands obtained were confirmed by their
a-hydroxyimine ligands were obtained by hydrolysis in
NMR spectra. The chemical shifts from 76.68 to 78.69 ppm in the
13C NMR represent the signals for the CeOH of the ligands, and the
chemical shifts from 174.32 to 174.47 ppm in the downfield region
are the corresponding peaks for the carbons to the nitrogen of the
imine moiety. For compounds L1e3, there are multiple peaks in the
range of 0.71e2.22 ppm, which are attributed to the methyl and
isopropyl signals in 1H NMR. However, no enantioenriched ligands
were obtained through the CeC bond formation. The result can be
explained by the diastereoselective addition of the alkylaluminum
Fig. 3. Molecular structure of 1$2H2O. Hydrogen atoms and two uncoordinated water
molecules have been omitted for clarity. Selected bond distances(Å) and angles (ꢂ):
Pd(1)-O(1) 2.070(4), Pd(1)-N(1) 2.049(5), Pd(1)-Cl(1) 2.240(2), Pd(1)-Cl(2) 2.275(2),
O(1)-C(24) 1.428(7), N(1)-C(13) 1.289(7), N(1)-Pd(1)-O(1) 80.87(17), N(1)-Pd(1)-Cl(1)
94.33(14), O(1)-Pd(1)-Cl(1) 175.11(12), N(1)-Pd(1)-Cl(2) 173.70(14), O(1)-Pd(1)-Cl(2)
92.96(12), Cl(1)-Pd(1)-Cl(2) 91.85(7).
to the a-imino ketones in the absence of a chiral catalyst [54].
The recrystallisation of L3, by the slow evaporation of a meth-
anol solution, gave crystals suitable for single crystal X-ray dif-
fraction. The molecular structure of L3 is shown in Fig. 2, along with
selected bond lengths. The arylimine moiety presents the E ge-
ometry in the solid state. The structure clearly shows that the ter-
tiary carbon is indeed located on the hydroxy group attached to
O(1), rather than that of N(1), which further proves that the
reductive addition is carried out at the ketone position. Although
the structure of C(26) is sp3 hybridised, the N(1)eC(19)eC(26)e
O(1) dihedral is nearly planar, as shown by a torsion angle of
5.7(2)ꢂ. The aniline group attached to N(1) is almost perpendicular
to the above plane, with a dihedral angle of 89.5ꢂ. Within the crystal
such synthesis is through a samarium(II) iodide-mediated three-
component coupling of organic halides, isocyanides, and carbonyl
compounds [49]. Another route is by the reaction of imi-
doyllithium with aldehydes, followed by hydrolysis with water
[50,51]. However, the synthesis of
a-hydroxyimine compounds
containing both bulky backbones and imine moieties were less
favoured under the above methods, but the direct condensation
was considered to be feasible. Our initial experiments showed that
the condensation between 2,6-diisopropylphenyl aniline and
Table 1
Crystallographic data for complexes.
L3
1$2H2O
3$2CH3OH
Formula
C27H31NO
385.53
Colourless, block
0.15 ꢃ 0.13 ꢃ 0.11
Monoclinic
C2/c
C25H27Cl2NO3Pd
566.78
Red, block
0.42 ꢃ 0.33 ꢃ 0.16
Monoclinic
P2(1)/c
293(2)
11.479
16.607
16.315
C29H39Cl2NO3Pd
626.91
Red, block
0.44 ꢃ 0.41 ꢃ 0.14
Orthorhombic
Pbca
Formula weight
Crystal colour, form
Crystal size (mm)
Crystal system
Space group
T (K)
173(2)
173(2)
ꢀ
a (A)
24.065(5)
10.044(2)
18.698(4)
90
100.220(4)
90
14.689(2)
18.340(3)
21.992(3)
90
90
90
ꢀ
b (A)
ꢀ
c (A)
a
b
g
(deg)
(deg)
(deg)
90
120.37
90
2683.3
3
ꢀ
V (A )
4447.7(15)
1.151
5924.6(15)
1.406
Dc (g cmꢁ3
)
1.403
q
range (deg)
1.72e27.06
ꢁ26 ꢀ h ꢀ 30,
ꢁ12 ꢀ k ꢀ 12,
ꢁ10 ꢀ l ꢀ 23
1664
1.90e27.84
ꢁ14 ꢀ h ꢀ 14,
ꢁ17 ꢀ k ꢀ 21,
ꢁ20 ꢀ l ꢀ 21
1152
1.85e27.09
ꢁ13 ꢀ h ꢀ 18,
ꢁ16 ꢀ k ꢀ 23,
ꢁ22 ꢀ l ꢀ 28
2592
Limiting indices
F(000)
Z
m
8
4
8
(mmꢁ1
)
0.069
0.915
0.836
Reflections collected/unique
R(int)
10,980/4749
0.0221
15,065/6059
0.0602
28,939/6509
0.0524
Data/restraints/parameters
Goodness of fit
4749/0/268
1.025
0.0425/0.1024
0.0646/0.1151
0.287 and ꢁ0.194
6059/1/297
1.021
0.0584/0.1269
0.1453/0.1640
1.112 and ꢁ0.873
6509/0/347
0.990
0.0405/0.0881
0.0796/0.1066
1.184 and ꢁ0.402
R1/wR2 [I > 2
R1/wR2 (all data)
Largest diff. peak and hole (e$A
s
(I)]a
ꢀꢁ3
)
P
P
P
P
a
R1
¼
jjFoj ꢁ jFcjj/ jFoj, wR2 ¼ [ w(F2o ꢁ F2c)2/ w(Fo2)2]1/2
.

X. Tang et al. / Journal of Organometallic Chemistry 729 (2013) 95e102
99
palladium dichloride in methanol. All of the palladium complexes
were soluble in polar organic solvents such as methanol, ethanol,
DMF, and DMSO, whereas they are only sparingly soluble in CH2Cl2
and CHCl3. These complexes were confirmed via NMR and ele-
mental analysis. The NMR analyses of these complexes also showed
the presence of two isomers (1:1 in ratio), which was confirmed the
racemic mixtures, as similar to that of the ligands.
To further confirm the structures of these complexes, single
crystals of 1 suitable for X-ray crystallographic analysis were grown
from a methanol solution. ORTEP diagrams are given in Fig. 3 along
with selected bond lengths and bond angles. The data collection
and structure refinement parameters are summarised in Table 1.
Complex 1 in the solid state cocrystallises with two molecules of
water, which do not interact with the metal centre; therefore, they
are omitted for clarity. As seen in Fig. 3, 1 contains one chelating
ligand and two chloride anions around the palladium metal centre.
The bond lengths of Pd(1)eN(1), Pd(1)eO(1), Pd(1)eCl(1), Pd(1)e
ꢀ
Cl(2) are 2.049(5), 2.070(4), 2.240(2), 2.275(2) A, respectively. The
palladium complex adopts a slightly distorted square-planar coor-
dination geometry (the sum of the angles around the palladium
atom is 360.0(3)ꢂ), while the arylimine moiety is nearly perpen-
dicular to the chelate ring (the dihedral angle is 89.1ꢂ), which is
believed to be due from the steric demand of the isopropyl sub-
stituents on the phenyl rings.
Similarly, the sterically bulky complex 3 also exists in a square-
planar coordination environment (Fig. 4). In comparison with the
planar backbone (derived from acenaphthenequinone) of 1, the
diphenyl backbone of 3 is tilted towards the chelate ring, with the
dihedral angle being 88.5ꢂ for the ring bonded to C(19) and 51.9ꢂ for
the ring bonded to C(26). Therefore, additional axial steric bulk
behind the metal centre is introduced, which could inhibit the
rotation of the aniline group around the NeCanilne bond. The
dihedral angle between the aniline moiety and the chelate ring is
89.2ꢂ, indicating that there is a large steric effect on the backbone,
which is confirmed by the increasing torsion angle. Moreover, as an
electron-rich group, the diphenyl group would further enhance the
interaction of the N, O coordinated atoms that are bonded to metal
Fig. 4. Molecular structure of 3$2CH3OH. Hydrogen atoms and two uncoordinated
methanol molecules have been omitted for clarity. Selected bond distances(Å) and
angles (ꢂ): Pd(1)-O(1) 2.015(2), Pd(1)-N(1) 2.029(3), Pd(1)-Cl(1) 2.257(1), Pd(1)-Cl(2)
2.272(9), O(1)-C(26) 1.450(4), C(19)-N(1) 1.283(4), O(1)-Pd(1)-N(1) 79.64(11), O(1)-
Pd(1)-Cl(1), 175.57(8), N(1)-Pd(1)-Cl(1) 96.77(8), O(1)-Pd(1)-Cl(2) 93.39(7), N(1)-
Pd(1)-Cl(2) 173.01(9), Cl(1)-Pd(1)-Cl(2), 90.22(4).
lattice, a short interaction between the OeH hydrogen and the
ꢀ
nitrogen atom is observed, with a distance of 2.025 A (N(1)eH).
3.2. Synthesis and characterisation of
(II) complexes
a-hydroxyimine palladium
The
a-hydroxyimine palladium complexes were readily
obtained from the reaction of the corresponding ligands with
Table 2
Palladium complexes screening and optimization of reaction conditions on SuzukieMiyaura cross-coupling reaction.a
Entry
Complex
Base
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
PdCl2
K2CO3
K2CO3
K2CO3
K2CO3
K3PO4
CH3COONa
KF
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
EtOH/H2O
Toluene/H2O
Dioxane/H2O
THF/H2O
22
78
82
93
85
80
83
21
98
17
40
60
39
95
97
91
1
2
3
3
3
3
3
3
3
3
3
3
3
3
3
NEt3
9
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
10
11
12
13
14
15
16
Ether/H2O
DMA/H2O
DMF/H2O
Acetone/H2O
a
Reaction conditions: 1-bromonaphthalene (1 mmol), phenylboronic acid (1.2 mmol), base (2.0 mmol), organic solvent/H2O ¼ 2 ml/2 ml; under aerobic atmosphere at
room temperature for 2 h.
b
Isolated yields.

100
X. Tang et al. / Journal of Organometallic Chemistry 729 (2013) 95e102
centre. The Pd(1)eCl(1) and Pd(1)eCl(2) bond lengths are 2.257(1)
and 2.275(2) A, respectively. The bond lengths are typical of a PdeCl
temperature (entry 1, Table 2). However, when adding the a-
ꢀ
hydroxyimine palladium complexes, the yield of product increased
dramatically into the range of 78e93% (entries 2e4, Table 2), thus
indicating the validity of the N,O-based catalysts.
bond and are similar to that of 1. However, the Pd(1)eN(1) and
ꢀ
Pd(1)eO(1) bond lengths are 2.029(3) and 2.015(2) A, respectively,
which are much shorter than that of 1 and also of the six-
membered b-ketoamine palladium complex [43], suggesting that
The catalytic activities of 1e3 were then evaluated for the
SuzukieMiyaura cross-coupling reaction. In an effort to understand
how these complexes could promote the coupling reaction most
efficiently, different steric properties of imine moieties and back-
bones (derived from diketones) were examined under similar reac-
tion conditions. The precatalysts with bulky backbones showed
higher activities than their analogues. For instance, with an ace-
naphthyl group, 1 gave a yield of 78% (entry 2, Table 2), whereas 3
further increased the yield to 93% (entry 4, Table 2). On account of
bearing the same aniline substitution between 1 and 3, the observed
differences in catalytic properties can be ascribed to both the steric
hindrance and electron-donating properties of the backbone groups.
Although 1 contains a large acenaphthyl group, it exists in a planar
geometry which lies far away from the palladium centre, and thus
has less of an effect on protecting the metal centre. Comparatively, as
depicted in the X-ray structural analysis, 3 bears diphenyl groups on
the backbone and thus could exhibit a certain steric hindrance on the
catalytic centre. Moreover, as an electron-rich group, the diphenyl
group could further enhance the electronic donation to metal centre.
Similar to previous reports[55,56], the bulkier2,6-diisopropylphenyl
there is a stronger interaction between the palladium metal centre
and the coordinated atoms. Such modulation of the steric and
electronic density of the ligand backbone would subsequently in-
fluence the environment of the palladium centre and thus influence
the catalytic performance of our target complexes.
3.3. SuzukieMiyaura cross-coupling promoted palladium
complexes
In the initial study of the performance of the a-hydroxyimine
palladium complexes, we examined the coupling of 1-
bromonaphthalene and phenylboronic acid under aerobic condi-
tions in the presence of K2CO3 as a model reaction for the Suzukie
Miyaura cross-coupling, and the results are presented in Table 2.
The obtained data clearly demonstrate that the palladium com-
plexes are effective catalyst precursors. For comparison, in the
absence of the N,O ligand, only a 22% yield of 1-phenylnaphthalene
was obtained in the presence of 0.01 mol% of PdCl2 at room
Table 3
SuzukieMiyaura cross-coupling reaction of aryl halides with arylboronic acids under aerobic conditions.a
Entry
X
R1
R2
Pd (mol%)
T (ꢂC)
t (hr)
Yieldb (%)
1
2
3
4
5
6
7
8
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
4-NO2
4-Cl
4-CHO
4-CH3OC
H
H
H
H
H
H
H
H
H
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.05
0.05
0.05
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
60
60
60
60
60
60
60
60
60
60
60
60
60
60
0.1
0.5
2
2
2
2
2
2
4
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
6
6
6
99
97
98
95
96
92
93
98
89
98
96
97
95
91
93
96
78
94
90
87
86
95
84
92
90
93
94
91
ed
ed
12
4-CH3
4-CH3O
1-Naphthyl
2-NH2
9c
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
H
H
2-CH3
4-CHO
4-CH3OC
4-NO2
4-CH3O
1-Naphthyl
4-tBu
4-CH3
4-CH3
4-CH3O
1-Naphthyl
4-CH3OC
4-tBu
4-CH3
4-CH3
4-CH3
4-CH3
4-CH3
4-CH3
2-CH3
2-CH3
2-CH3
2-CH3
2-CH3
2-CH3
2-CH3
4-Cl
4-Cl
4-Cl
4-Cl
4-Cl
H
4-CHO
4-CH3OC
4-CHO
4-NO2
4-tBu
4-CH3
4-CH3O
4-CH3
4-CHO
Cl
Cl
H
H
a
Reaction conditions: aryl halides (1.0 mmol), arylboronic acids (1.2 mmol), Na2CO3 (2.0 mmol), EtOH:H2O ¼ 2 ml:2 ml, under aerobic atmosphere.
b
c
Isolated yield.
Under nitrogen atmosphere.
Not determined.
d

X. Tang et al. / Journal of Organometallic Chemistry 729 (2013) 95e102
101
substituent on the imine moieties (3) showed higher activities than
their analogues with a 2,6-dimethylphenyl substituent (2). For
example, with a 2,6-dimethylphenyl imine moiety, 2 gave a yield of
82% (entry 2, Table 2), which was much less effective than that of 3.
These results can be ascribedto the increase in steric hindrance of the
substituents, leading to an increase in the rate of the elimination
process [57].
To further examine the catalytic activity of 3, which was found
to be a better precatalyst, the reaction conditions were varied using
different bases and solvents. As illustrated in Table 2, in an ethanol/
water mixture, the use of Na2CO3 resulted in near completion,
giving 98% yield of the product; other bases such as K3PO4,
CH3COONa, KF, and NEt3 produced lower yields (entries 5e9,
respectively). Moreover, the screening of solvents revealed that
DMA/water and DMF/water mixtures were quite effective (entries
14 and 15, Table 2). Considering that the ethanol/water mixture is
readily available and relatively inexpensive, it was chosen for fur-
ther study.
Using the available optimal conditions, the scope of the Suzukie
Miyaura cross-coupling reaction was investigated. High catalytic
efficiencies were observed for the coupling of a variety of aryl
bromides that incorporated various deactivating and activating
groups on the arylboronic acid (Table 3). For instance, the reaction
between 4-bromoanisole and phenylboronic acid can be carried out
using 0.01 mol% of 3 as the precatalyst in 2 h at room temperature
to give 93% yield of the product (entry 7, Table 3). The cross-
coupling reaction proceeded rapidly even when increasing the
ortho steric effect of the aryl bromides (entries 9 and 10, Table 3)
and while the TON (Turnover Numbers) were as high as ca 104.
Encouraged by these results we turned our attention to the cross-
coupling with arylphenylboronic acids. Substitution at the para-
position of the arylphenylboronic acid with electron-donating
groups such as 4-methyl showed little effect on the yields of the
product (entries 11e16, Table 3). However, an ortho methyl and para
chloro group on the arylphenylboronic acid resulted in a drop of
activity. For example, the product obtained from the coupling of 2-
methylphenylboronic acid with 4-methylphenyl bromide resulted
in 78% yield (entry 17, Table 3). To our delight, excellent yields could
be further increased when the experiments were performed at an
elevated reaction temperature of 60 ꢂC (Entries 18e28, Table 3). The
required higher reaction temperature could be ascribed to the
steric and electron-deficient arylboronic acid, which hinders the
rate of the transmetalation process in the cross-coupling. However,
the catalytic system was less effective with substrates such as aryl
chlorides, even at a high catalyst loading of 0.05 mol% (entries 29e
31, Table 3).
Fig. 5. TEM images of palladium nanoparticles in EtOHeH2O. Reaction conditions:
4-methylphenyl bromide (1 mmol), 2-methylphenylboronic acid (1.2 mmol),
3
(0.01 mol %), Na2CO3 (1 mmol), 60 ꢂC, under aerobic atmosphere.
nitrogen and oxygen atoms, the diphenyl backbone, and the non-
conjugated nature of the ligand enables the facile insertion of the
metal centre into the carbonehalogen bond, thus enhancing the
catalytic activity.
Moreover, the morphology and particle size of the catalyst also
played important role on catalytic performance [62e64]. Suzukie
Miyaura reaction protocols have been shown to form palladium
nanoparticles possessing high catalytic activity [65,66]. Examina-
tion of the cross-coupling between 4-methylphenyl bromide and 2-
methylphenylboronic acid was carried out in EtOH/H2O cosolvent.
At a temperature of 60 ꢂC and 2 h after the reaction, the trans-
mission electron microscopy (TEM) image of the catalyst displayed
that a size between 20 and 30 nm palladium nanoparticles are
formed under the experimental conditions (Fig. 5), which indicated
a further promotion of the activity of the N,O-based catalyst.
It is important to note that the results obtained using
a-
hydroxyimine palladium complexes prove their superiority relative
to the quinoline-8-carboxylate complexes [34], and can even
compare with the most efficient reported phosphine-free ligands
[58]. The carboxylated complexes, with an acetate anion, were
directly bonded to the palladium leading to an acceleration of the
oxidative addition [59e61]. However, a bis-chelated coordination
mode, in which two N,O ligands bond to one palladium centre,
would effectively restrain the transmetalation. Comparatively, the
4. Conclusions
In this work, the non-conjugated a-hydroxyimine ligands and
their palladium complexes (1e3) with different backbones and
arylimines were developed. Structures of all of the palladium
complexes were characterised by NMR and EA. Moreover, 1 and 3
were characterised by single crystal X-ray diffraction. These palla-
dium complexes were tested in the SuzukieMiyaura cross-coupling
reaction, showing that they exhibited high TON, ca 104, in the
coupling of various aryl bromides and arylboronic acids. These in-
vestigations highlight the significance of bulky steric groups on the
employed N,O ligand backbones, as well as the non-conjugated
nature of the ligand, for the stabilisation of the catalytic in-
termediates and the enhancement of catalytic activity. Fur-
thermore, this study gave insight into the structureereactivity
relationship of the cross-coupling reaction with phosphine-free
palladium catalysts, especially when considering the low catalyst
loading levels.
non-conjugated a-hydroxyimine palladium complexes synthesised
in this study were confirmed to exist in the mono-chelated mode.
Most importantly, the NePdeO bond angles are ca 80ꢂ in these
five-membered ring complexes, which are much smaller than that
of the palladium complexes bearing a six-membered chelating ring
(IeIII, ca 95ꢂ). The smaller bond angle will further provide suffi-
cient space for the transmetalation process. Therefore, bulky
backbones and arylimines can greatly benefit the shielding of the
metal centre, both for the protection of the formation of Pd(0) and
to further stabilise Pd(2þ). The combination of electron-rich

102
X. Tang et al. / Journal of Organometallic Chemistry 729 (2013) 95e102
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