Novel bis(azole) pincer palladium complexes: Synthesis, structures and applications in Mizoroki-Heck reactions

Article abstract of DOI:10.1039/c0dt01414a

Two novel NCN-pincer complex precursors bearing frameworks of 2,6-bis(oxazol-4-yl)benzene (A) and 2-(thiazol-4-yl)-6-(oxazol-4-yl)benzene (B) were synthesized. Palladations of A and B afforded two new bis(azole) pincer complexes, [(A-κ³NCN)PdBr] (1) and [(B-κ³NCN) PdBr] (2). Both complexes were fully characterized by NMR, MS, DSC-TGA and single-crystal X-ray diffraction analysis. Complex 1 crystallizes in a noncentrosymmetric orthorhombic space group Cmc2₁ (No. 36, Z = 4). Complex 2 crystallizes in a centrosymmetric monoclinic space group P2 ₁/n (No. 14, Z = 4). Despite the similarity in their chemical formulas, the structures of the two complexes are subtly different: they are built up of two-dimensional supramolecular layers with identical topology, but stacked in different sequences, i.e., the layers in complex 1 are stacked in an AAAA-type fashion, while those in complex 2 are stacked in an alternating AA^-1AA^-1 sequence (A denotes a layer; A^-1 stands for A's inversion symmetry equivalent). In addition, the complexes showed good catalytic activity toward Mizoroki-Heck reactions. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0dt01414a

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PAPER
Novel bis(azole) pincer palladium complexes: synthesis, structures and
applications in Mizoroki–Heck reactions†
Qun-Li Luo,* Jian-Ping Tan, Zhi-Fu Li, Yue Qin, Lin Ma and Dong-Rong Xiao*
Received 19th October 2010, Accepted 26th January 2011
DOI: 10.1039/c0dt01414a
Two novel NCN-pincer complex precursors bearing frameworks of 2,6-bis(oxazol-4-yl)benzene (A) and
2-(thiazol-4-yl)-6-(oxazol-4-yl)benzene (B) were synthesized. Palladations of A and B afforded two new
3
3
bis(azole) pincer complexes, [(A-k NCN)PdBr] (1) and [(B-k NCN)PdBr] (2). Both complexes were
fully characterized by NMR, MS, DSC-TGA and single-crystal X-ray diffraction analysis. Complex 1
crystallizes in a noncentrosymmetric orthorhombic space group Cmc21 (No. 36, Z = 4). Complex 2
crystallizes in a centrosymmetric monoclinic space group P21/n (No. 14, Z = 4). Despite the similarity
in their chemical formulas, the structures of the two complexes are subtly different: they are built up of
two-dimensional supramolecular layers with identical topology, but stacked in different sequences, i.e.,
the layers in complex 1 are stacked in an AAAA-type fashion, while those in complex 2 are stacked in an
alternating AA-1AA-1 sequence (A denotes a layer; A-1 stands for A’s inversion symmetry equivalent).
In addition, the complexes showed good catalytic activity toward Mizoroki–Heck reactions.
previous research,20 a novel palladium pincer complex derived
Introduction
from 1,3-bis(oxazol-2-yl)benzene showed good catalytic activity
toward cross-coupling reactions. We herein report the synthesis,
structure and property characterizations of two novel NCN-pincer
palladium complexes, in which the N-donor atoms are from
oxazol-4-yl and thiazol-4-yl moieties.
Owing to high stability, feasible structure modifications, and
remarkable catalytic activities, complexes with pincer-type1 triden-
tate ligands have attracted intensive interest since the pioneering
work of Shaw2 and van Koten.3 Various complexes containing
C, N, P, S or Se donor atoms were successively found.4–11
Due to the fact that palladium complexes can catalyze multiple
transformations,12–14 many efforts have focused on preparing their
ligands or precursors with novel structural features, such as sym-
metrical, asymmetrical or unsymmetrical pincer backbones.15–16
Although the synthesis and properties of several NCN-pincer
palladium complexes containing two units of amines, imines,
pyridines, oxazolines or other N-containing heterocycles were
well documented,15–18 their applications as catalysts to cross-
coupling reactions were, in our opinion, studied very little. To
the best of our knowledge, no more than seven examples of
palladium complexes bearing two heterocycle groups as the source
of both N-donor atoms were applied to Mizoroki–Heck reactions.
Among them, the pendant groups basically belong to five types
of heterocycle, i.e. cyclic aminal,17a pyridine,8,18b,19b imidazoline,16e
pyrazole19a and 1,2,4-triazole,18d whereas oxazolyl or thiazolyl are
seldom introduced into the pincer ligand as a donor group. In
Results and discussion
Synthesis of proligands
In order to obtain synthesis routes that were as concise as possible,
we chose a-bromoketone C as a key intermediate to construct
proligand A and B (Scheme 1). The synthesis of a-bromoketone
C started from 2-bromo-m-xylene (E), as summarized in Scheme
S1.† 2-Bromo-m-xylene (E) was oxidized with KMnO4 to yield
2-bromoisophthalic acid, and subsequently treated with SOCl2
to obtain 2-bromoisophthaloyl chloride (D).21 Acid chloride D
was converted to a-bromoketone C by means of a diazomethane
reaction.
Using microwave heating for 15 min, bis(oxazole) A was
synthesized in good isolated yield (85%) from the toluene solution
of a-haloketone C, with a large excess of acetamide (15 times the
theoretical amount). Comparatively, only 40% yield was obtained
via traditional heating for 3 days in a sealed tube even under
the optimized condition. When the mixtures containing half the
above-mentioned amountof acetamidewereheatedin amicrowave
oven for 30 min, the semi-cycliziation product F was isolated in an
acceptable yield (42%). The cyclization of F formed proligand B
in 76% yield.
College of Chemistry and Chemical Engineering, Southwest Univer-
sity, Chongqing, 400715, China. E-mail: qlluo@swu.edu.cn, xiaodr98@
swu.edu.cn; Fax: +86-23-6825 4000; Tel: +86-23-68252360
† Electronic supplementary information (ESI) available: The additional
figures and tables including 3D supramolecular structures of 1, TG–DTG–
DSC thermograms of 1 and 2, NMR spectra data of A, B, 1 and 2, and
X-ray crystallographic files for 1 and 2 in CIF format. CCDC reference
numbers 785152 and 785153. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0dt01414a
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Scheme 1 Retrosynthetic analysis of proligand A and B.
The NMR and mass spectra of A and B are found to be in
agreement with their molecular structures. The singlet signals of
the 1H NMR spectra at 8.2 ppm and 7.3 ppm are characteristic for
the protons at the 5-position of oxazolyl and thiazolyl, respectively.
The corresponding signals of the 13C NMR spectra are presented
at 137 ppm and 117 ppm, respectively. Additionally, the structures
are supported by mass spectra. ESI-MS of A and B demonstrate
the base peak at m/z = 319.0 and 335.0 for molecular cation [M +
H]+, respectively.
which influence the nature of the electronic situation of the azolyl
rings and the benzene ring. By comparing the H NMR spectra
1
of the complexes and their proligands, the chemical shifts of CH
protons on aromatic rings decrease, ranging from 0.2 ppm to 0.7
ppm, whereas those of the methyl protons adjacent to the donor
nitrogen atoms increase by ca 0.5 ppm. Moreover, some changes
in the spin–spin coupling patterns are observed. In proligand A,
three protons on the phenyl appear as a clear AX2 spin system,
whereas in complex 1, they alter into a pseudo-A3 spin system and
resonate at 7.1 ppm as a singlet. Similar alternation of the spin
systems between B and 2 are present. In the comparison of the 13
C
Synthesis and characterization of palladium (II) complexes
NMR spectra of the complexes and their proligands, the chemical
shifts of the Cipso increase by more than 35 ppm, and those of the
quaternary carbons next to the donor N increase by 7–10 ppm,
while the resonances of the CH carbons at a meta-position to Cipso
and at a meta-position to the donor N decrease by 7–10 ppm in
the complexes.
Crystals of complexes 1 and 2 suitable for X-ray diffraction
analysis were grown by slow diffusion of EtOAc and hexane into
a DMF solution of each complex. An atom numbering diagram
of the fundamental unit for 1 and 2 is shown in Fig. 1. Selected
crystallographic data are listed in Table 1 and 2.
The strategy of oxidative addition3b with Pd(dba)2 (dba =
dibenzylideneacetone)22 was applied in the synthesis of [(A-
3
3
k NCN)PdBr] (1) and [(B-k NCN)PdBr] (2), as shown in Scheme
2. Both palladations of proligand A and B smoothly generated
more than 85% yield of complex 1 and 2 in a refluxing benzene
solution. Complex 1 is poorly soluble in common solvents (less
than 5 mg mL-1 solvent) such as MeOH, MeCN, DMSO, acetone,
chloroform, etc., less soluble in hydrocarbon, and more soluble in
DMF. Heating can increase its solubility. Complex 2 is also poorly
soluble in common solvents but slightly better than 1.
Complex 1 crystallizes in a noncentrosymmetric space group
Cmc21, an ultra-rare example of planar pincer complexes without
any chiral elements,8a despite the fact that complex 1 is a polar
molecule and can be used as a potential building block to construct
a noncentrosymmetric structure. There is one crystallographically
independent Pd atom in the crystal structure of 1 (shown in
Fig. 1). Each Pd is primarily coordinated by one bromine,
one carbon and two nitrogen atoms from the pincer ligand
to furnish a distorted square-planar coordination. As a result,
C–Pd–N bond angles decrease to 79.96(14)◦ and 79.89(18)◦
compared with the 90◦ angle between cis-ligands in an ideal
square planar geometry. The Cipso–Pd–Br angle (179.27(11)◦) is
The formations of Pd (II) complex 1 and 2 are supported by
positive mode ESI mass spectrometry. Differing from that of the
proligands, the bromine of the complexes is easily dissociated
under MS ionization conditions. Therefore, the peaks of the
complexes’ molecular cation are hardly observed. Instead, the
peaks of fragment cation are present. The ESI-MS of 1 shows
a base peak at m/z = 736.8 for the ion–molecule complex [2M +
CO2 + H - Br2]+, and that of 2 at m/z = 771.0 for the ion–molecule
complex fragment cation [2M + H + 2MeOH - 2HBr - CH4]+.
NMR spectra changes (shown in Table S1†) between the
complexes and their proligands are attributed to the C–Pd s-
bonding and the coordination of N-donor atoms in the complexes
Scheme 2 Synthesis of complex 1 and 2.
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Fig. 1 Molecular structures of 1 (a) and 2 (b) with ellipsoids shown at 50% thermal probability. Hydrogen atoms are omitted for clarity.
Table 1 Crystal data and structure refinement for compounds 1 and 2
the C–H ◊ ◊ ◊ Br hydrogen bonds link discrete molecules into
one-dimensional (1D) supramolecular chains along the c-axis,
and the adjacent chains are linked into two-dimensional (2D)
supramolecular layers parallel to the bc plane by van der Waals
interactions between discrete molecules in the adjacent chains.
Secondly, the close contact distance of the aromatic rings between
Complex
1
2
Formula
fw
C14H11BrN2O2Pd
425.56
293(2)
0.71073
Orthorhombic
Cmc21
6.7355(13)
11.269(2)
18.178(4)
90
90
90
1379.7(5)
4
2.049
C14H11BrN2OPdS
441.62
293(2)
0.71073
Monoclinic
P21/n
10.610(2)
11.703(2)
12.051(2)
90
106.59(3)
90
1434.1(5)
4
2.045
4.220
T (K)
˚
l (A)
˚
adjacent 2D layers is 3.45 A, thus, the 2D supramolecular arrays
Crystal system
Space group
are further extended by p–p stacking interactions into a three-
dimensional (3D) noncentrosymmetric supramolecular network
along the a-axis. Due to the lack of inversion symmetry and
the centering of the unit cell, all layers stack in an entirely
parallel AAAA sequence (Fig. 3 and Fig. S1†), resulting in the
noncentrosymmetric structure.
Differing from 1, complex 2 crystallizes in a centrosymmetric
space group P21/n, in which an asymmetric unit contains one
palladium atom and one pincer ligand (Fig. 1). The crystallograph-
ically independent Pd atom exhibits a distorted square-planar
geometry, being coordinated by one bromine (Pd–Br 2.5411(9)
˚
a (A)
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
3
˚
V (A )
Z
Dc (g cm-3)
m (mm-1)
4.242
5015/1760
0.0280
Reflns collected/unique
R(int)
10463/3437
0.0320
1.012
0.0421, 0.1185
0.0654, 0.1285
GOF on F2
0.831
˚
˚
R1a, wR2 [I > 2s(I)]
0.0188, 0.0392
0.0202, 0.0395
0.028(7)
b
A), one carbon (Pd–C 1.946(5) A) and two nitrogen atoms (Pd–N
˚
R1, wR2 (all data)
2.076(4) and 2.089(4) A). The lengths of the Pd–C and Pd–N bonds
Flack parameter
˚
are longer than those of complex 1 by 0.007–0.009 A. The distorted
arrangement leads to the inner C–Pd–N bond angles of 80.2(2)◦
and 80.09(19)◦, somewhat less than the expected 90◦ between
cis-ligands in an ideal square planar geometry. The bromine is
slightly puckered from the coordination plane with an angle of
8.97◦ between Pd–Br bond and the N(1)–Cipso–N(2) plane.
The topology of 2D supramolecular layers in 2 is identical to
that of 1. The major difference between the two structures is in
the way of stacking (Fig. 2 and Fig. 4). Because of the inversion
symmetry, the stacking of the 2D layers in 2 is in an alternating
antiparallel sequence and can be denoted as AA-1AA-1 (A-1 stands
for the inversion image of A).24
2
a R1 = RꢀF0| - |FCꢀ/R|F0|; b wR2 = R[w(F0 - FC2)2]/R[w(F02)2]1/2
almost linear, whereas the N(1)–Pd–N(2) angle (159.85(16)◦) is
˚
bent. The Pd–C bond distance (1.937(4) A) is within the range
found in related bis(oxazolinyl)phenyl (Phebox) structures (1.928–
˚
1.940 A), but shorter than that of the [2,6-bis(oxazol-2-yl)phenyl-
3
20
˚
˚
k NCN]palladium(II) complex (1.959(2) A) by 0.022 A, and the
Pd–N bond lengths (2.069(3) and 2.080(2) A) are slightly longer
˚
than those of the palladium complexes derived from Phebox and
20,23
˚
˚
Bulk-level molecular electronic/photonic properties of solid
materials require the absence of a center of symmetry. The rational
synthesis of noncentrosymmetric solids remains a challenge.25a
Normally, polar molecules tend to set up an antiparallel molecular
arrangement more readily, especially in the absence of more
energetic intermolecular interactions such as H-bonding, and their
bis(oxazol-2-yl)phenyl (2.040–2.076 A) by 0.004–0.040 A.
Complex 1 lies exactly on bc crystallographic mirror plane and
contains two types of supramolecular interactions, namely C–
H ◊ ◊ ◊ Br hydrogen bonding and aromatic p–p stacking interactions
(shown in Fig. 2 and Fig. 3). Firstly, the C(3) ◊ ◊ ◊ Br(1) distance is
˚
4.03 A, indicating a weak C–H ◊ ◊ ◊ Br hydrogen bond. Therefore,
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◦
˚
Table 2 Selected bond lengths [A] and angles [ ] for 1 and 2
1
Pd(1)–C(10)
Pd(1)–Br(1)
O(2)–C(13)
O(2)–C(12)
C(10)–Pd(1)–N(1)
C(10)–Pd(1)–N(2)
N(1)–Pd(1)–N(2)
C(2)–N(1)–Pd(1)
C(4)–N(1)–Pd(1)
C(9)–C(10)–Pd(1)
C(2)–O(1)–C(3)
C(13)–N(2)–C(11)
1.937(4)
2.5425(8)
1.329(6)
1.373(7)
79.96(14)
79.89(18)
159.85(16)
141.7(3)
113.2(3)
119.9(3)
106.7(3)
105.6(4)
Pd(1)–N(2)
O(1)–C(2)
N(1)–C(2)
N(1)–C(4)
C(10)–Pd(1)–Br(1)
N(1)–Pd(1)–Br(1)
N(2)–Pd(1)–Br(1)
C(13)–N(2)–Pd(1)
C(11)–N(2)–Pd(1)
C(5)–C(10)–Pd(1)
C(13)–O(2)–C(12)
C(2)–N(1)–C(4)
2.080(2)
1.343(5)
1.320(5)
1.423(5)
179.27(11)
100.77(9)
99.38(14)
141.2(4)
113.3(3)
120.1(3)
106.9(4)
105.1(3)
Pd(1)–N(1)
O(1)–C(3)
N(2)–C(13)
N(2)–C(11)
2.069(3)
1.370(6)
1.300(6)
1.408(6)
2
Pd(1)–C(10)
Pd(1)–N(1)
N(1)–C(4)
N(2)–C(11)
C(10)–Pd(1)–N(1)
C(10)–Pd(1)–N(2)
N(1)–Pd(1)–N(2)
C(2)–N(1)–Pd(1)
C(4)–N(1)–Pd(1)
C(5)–C(10)–Pd(1)
C(12)–S(1)–C(13)
C(3)–O(1)–C(2)
1.946(5)
2.076(4)
1.411(8)
1.406(6)
80.2(2)
80.09(19)
160.09(18)
137.9(5)
113.2(4)
119.0(4)
94.5(3)
Pd(1)–N(2)
S(1)–C(12)
N(1)–C(2)
O(1)–C(3)
C(10)–Pd(1)–Br(1)
N(1)–Pd(1)–Br(1)
N(2)–Pd(1)–Br(1)
C(13)–N(2)–Pd(1)
C(11)–N(2)–Pd(1)
C(9)–C(10)–Pd(1)
C(2)–N(1)–C(4)
C(13)–N(2)–C(11)
2.089(4)
1.541(6)
1.299(7)
1.561(9)
172.77(13)
100.07(14)
99.84(11)
137.7(4)
112.4(3)
118.8(4)
108.9(5)
109.9(4)
Pd(1)–Br(1)
S(1)–C(13)
N(2)–C(13)
O(1)–C(2)
2.5411(9)
1.602(6)
1.291(7)
1.547(7)
95.3(4)
Fig. 2 One-dimension supramolecular chains of compound 1 (up) and 2(down).
crystallization may be disrupted by dipolar aggregation.25b In that
sense, a molecule without strong intermolecular interactions and a
chiral element like 1 is inclined to crystallize in a centrosymmetric
space group. Furthermore, complex 2 is a planar chiral compound
in which four different groups in the coordination plane are around
the Pd centre. Any coordination or interaction along a vector
perpendicular to the coordination plane will lead to stereogenity
and asymmetry in complex 2, resulting in supramolecular assem-
blies. Actually, 2 crystallizes in a centrosymmetric space group,
whereas 1 in a noncentrosymmetric one that was expected for 2
rather than for 1. In a comparison of the structures of 1 and 2,
it is found that coplanarity of the planar pincer complex may
have an important impact on the 3D packing arrangement of the
compounds. When the atoms of the compound arrange in perfect
coplanarity, 2D supramolecular layers stack into a 3D network
in an entirely parallel way, and the compound crystallizes in a
noncentrosymmetric space group, while in poor coplanarity, the
2D layers stack into a 3D network in an alternating antiparallel
way, and the compound crystallizes in a centrosymmetric space
group.
DSC-TGA was carried out on the complexes, and the results
are shown in Table 3 and Fig. S2.† The DTG and DSC
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Fig. 3 Two-dimension and three-dimension arrangements of compound 1.
in DMSO-d6. On the one hand, comparing the 1H NMR spectra
at 20 ◦C and 70 ◦C, the only difference is a slight change of
chemical shift of the proton signals (Fig. S3†). On the other
hand, after heating the solution at a relatively high temperature
for several hours, and placing this solution aside for dozens of
days without any special care, and then heating at an elevated
Table 3 TG-DSC phenomenological data of the complexes under nitro-
gen atmosphere
TG
DSC
Temp.
Observed
mass loss
Peak
Complex
range/◦C
temp./◦C
Exo/endo
1
temperature, there was no perceptible impurity shown in the H
1
r.t.–300
300–500
500–950
r.t.–280
280–455
455–980
0.31%
45.09%
9.21%
0.15%
34.80%
15.45%
—
341
—
—
309
—
—
Endo
—
—
Endo
—
NMR spectra (Fig. S4†). These results indicate that the complex
neither noticeably decomposes nor transforms its coordinating
pattern during heating and after longtime heating and storage
as well as continuously reheating at high temperature in DMSO
solution.
2
traces show that the decomposition process of both complexes
is endothermal and completed in a single step. The initial
decomposition temperatures of both complexes are higher than
280 ◦C. For complex 1, the temperature is even higher than 300 ◦C.
These temperatures are higher than those of normal palladium
bidentate complexes,26 although to the best of our knowledge
no data of thermal analysis concerning pincer palladium com-
plexes are available to date. For example, di-m-hydroxo-bis[(2-
Catalysis
With the complexes in hand, their catalytic behaviors for
Mizoroki–Heck reactions27 were evaluated under standard con-
ditions. Reports to date of pincer palladacycles as powerful
catalysts for Heck reactions mainly focus on phosphorus-, sulfur-,
nitrogen-, and selenium- containing structures, including N-
heterocyclic carbene (NHCs) compounds.4–11,15,16 Most of them
show good catalytic activity toward aryl iodides, and less activity
toward chloride and deactivated bromide substrates.4 Therefore,
aryl bromide as the halide substrate was selected to test at first.
As shown in Table 4, both complexes can be used with aryl
bromide, and complex 1 is slightly more active than 2 under
the given conditions (entry 1–4). The activity test was focused
hereafter on complex 1. The long-term storing hardly affected
the complex’s activity (entry 1 vs. 5), nor the aerobic atmosphere
(entry 1 vs. 6). The additive of 20 mol% tetrabutylammonium
bromide (TBAB) did not improve the yield (entry 1 vs. 7).
The tests of various halides outline the substrate scope (entry
8–29). With a loading of 1 mol% [Pd], the catalyst is highly
active toward deactivated aryl iodide and activated aryl bromide
within several hours under relatively mild conditions, but shows
◦
2
(2-pyridyl)phenyl-k NC1]palladium(II) decomposed at 195 C,26a
Pd2(dmba)2Cl2(m-bpe) [dmba = N,N-dimethylbenzylamine, bpe =
◦
trans-1,2-bis(4-pyridyl)ethylene]◦ at 148 C,26b Pd(8-qnS)2 (8-
qnSH = 8-quinolinethiol) at 230 C,26c and trans-[Pd(ampy)2](sac)2
(ampy = 2-aminomethylpyridine, sac = saccharinate) at 245 ◦C.26d
Based on the thermogravimetry data, the thermal decomposi-
tion pathway of the complexes is proposed in Scheme S2.† Ac-
cording to the proposed pathway, heat initiates the decomposition
of the complexes at the phenyl part of the ligands, and the Pd atom
finally holds heteroatoms such as Br, N, O and/or S in the residue.
After the hydrocarbon parts are removed from the ligands, the
organic complexes transform into inorganic forms.
In the solid state, the thermostability of the complex in the
solution state was evaluated through NMR spectra of complex 1
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Fig. 4 Two-dimensional and three-dimensional arrangements of compound 2.
weak activity toward unactivated aryl bromide. The performance
is equivalent to that of N-heterocyclic carbene complexes9d,15f
and bis(pyridyl) pincer complexes.8b Furthermore, the orders
of magnitude of the catalyst’s turnover number (TON) and
turnover frequency (TOF) reached as high as 105 and 104 h-1,
respectively, with good isolated yields (entry 18, 19). The values are
are stacked into the 3D supramolecular structures in different
manners, resulting in a noncentrosymmetric space group for 1
and a centrosymmetric space group for 2.
equivalent to those of bis(pyridyl)-,8a bis(amino)-,17a bis(imino)-
Experimental
18a
,
and bis(phenylthiomethyl)-10c pincer complexes. Therefore,
General remarks
the complex is efficient under standard Mizoroki–Heck reaction
conditions, and it is not necessary to exclude oxygen from the
reaction mixtures or store the catalyst with special care.
Unless otherwise noted, all manipulations were performed with-
out taking precautions to exclude air and moisture. Commercially
available reagents were used as received. Solvents for reactions
were dried with standard procedures and stored with Schlenk
flasks over molecular sieves under N2 if not noted otherwise. All
solvents for chromatographic separations were distilled before use.
Microwave reactions were conducted with a Galanz WP800TL23-
K3 microwave oven. 1H and 13C NMR spectra were recorded on
a Bruker av 300 spectrometer normally at 293 K unless otherwise
noted and the chemical shifts (d) were internally referenced by
the residual solvent signals relative to tetramethylsilane (1H, 13C).
ESI mass spectra were measured using an Agilent technologies
6110 Quadrupole LC/MS spectrometer or Shimadzu LCMS-
2020 spectrometer. Elemental analyses were performed on an
Element AR Vario EL elemental analyzer at the Department
of Analytical Chemistry, Shanghai Institute of Materia Medica
(SIMM), Chinese Academy of Sciences (CAS). Differential Scan-
ning Calorimetry and Thermogravimetry analysis (DSC-TGA)
Conclusions
Two new NCN pincer proligands containing oxazol-4-yl and
thiazol-4-yl groups were synthesized. Palladation of proligands
3
led to more than 85% yield of the k NCN pincer complex 1
and 2, respectively. The complexes have been fully characterized
by NMR, MS, DSC-TGA and single-crystal X-ray diffraction
analysis. They show good thermostablility in both solid and
solution forms, and are resistant to deterioration under long-
term storage. The catalytic investigations show that they are highly
active to Mizoroki–Heck reactions of deactivated aryl iodides and
activated aryl bromides under relatively mild reaction conditions
with short reaction times even under an aerobic atmosphere,
whereas deactivated substrates of aryl bromides gave moderate to
poor results. Meanwhile, the reaction procedures and the catalyst
are easy to handle.
◦
was undertaken at 10 C min-1 on TA SDT Q600 instrument
The remarkable aspect of the crystal chemistry of 1 and 2
is that, despite the similarity in the chemical formulas and the
identical topology of the 2D supramolecular layers, the layers
(sample mass 3–5 mg, under nitrogen atmosphere at the flow rate
of 100 mL min-1). Traces are shown in Fig. S1† and data are
summarized in Table 3.
3606 | Dalton Trans., 2011, 40, 3601–3609
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Table 4 Mizoroki–Heck coupling reactionsa catalyzed by complex 1 and 2
Entry
R
X
Y
[Pd]/mol%
Temp/◦C
Time/h
Yield (isolated)/%
1
2
3
4-Ac
4-Ac
4-Ac
4-Ac
4-Ac
4-Ac
4-Ac
H
4-Me
4-OMe
4-OMe
4-OMe
4-OMe
4-OMe
4-Me
H
H
H
H
H
4-CN
4-CN
4-CN
H
4-Me
4-OMe
4-CN
H
Br
Br
Br
Br
Br
Br
Br
I
I
I
I
I
I
I
I
I
I
I
I
I
Br
Br
Br
Br
Br
Br
Br
Br
Br
–CO2Me
–CO2Me
–Ph
1(1%)
120
120
160
160
120
120
120
120
120
120
120
120
120
160
160
160
160
160
160
160
120
120
120
120
120
120
160
160
160
7
7
7
7
7
7
7
7
7
7
7
15
26
5
5
5
91
76
84
75
85
88
84
2(1%)
1(1%)
4
–Ph
2(1%)
5b
6c
–CO2Me
–CO2Me
–CO2Me
–CO2Me
–CO2Me
–CO2Me
–CO2Me
–CO2Me
–CO2Me
–Ph
–Ph
–Ph
–Ph
–Ph
1(1%)
1(1%)
7d
8
1(1%)
1(1%)
>99e
>99e
>99e
94
9
1(1%)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
1(1%)
1(0.1%)
1(0.001%)
1(0.0005%)
1(1%)
84
25 (49)f
>99g
>99g
>99g
94
1(1%)
1(1%)
1(0.1%)
1(0.001%)
1(0.0001%)
Blank
5
20
40
40
7
7
25
7
7
7
7
20
20
80
78
0
95
–Ph
–Ph
–CO2Me
–CO2Me
–CO2Me
–CO2Me
–CO2Me
–CO2Me
–Ph
1(1%)
1(0.1%)
1(0.01%)
1(1%)
76
13 (78)f
34
1(1%)
25
18
86
25
1(1%)
1(1%)
–Ph
–Ph
1(1%)
4-Me
1(1%)
17
a Reaction conditions: 1 mmol of aryl halide, 1.5 mmol of substituted olefin, 5 mL of NMP, 3 mmol of KOAc for methyl acrylate, 1.5 mmol DIPEA for
styrene. b Reaction performed after catalyst had been stored for 3 months without any special care. c Reaction carried out under aerobic atmosphere.
d With addition of 20 mol% of [N(n-C4H9)4]Br. e The obtained product was just from simple extraction and whose mass was equivalent to calculated
amount while the NMR spectrum showed almost no impurity. f Numbers in parentheses are percentage of recovered aryl bromide. g TLC monitoring
showed aryl iodide was totally converted into product without perceptible impurity within 5 h.
Synthesis of proligands
8.20 (s, 2H, Hc), 7.85 (d, 3J = 7.8 Hz, 2H, Hf), 7.44 (t, 3J = 7.7 Hz,
1
1H, Hg), 2.54 (s, 3H, Ha). 13C { H} NMR (75 MHz, CDCl3): d =
160.5 (Cb), 138.6 (Cd), 137.9 (Cc), 133.4 (Ce), 130.2 (Cf), 127.4
(Cg), 120.7 (Ch), 13.8 (Ca). ESI-MS: m/z (%) = 318.9 (98) [M]+,
320.9 (100) [M + 2]+, 341.0 (48) [M + Na]+, 342.9 (47) [M + 2 +
Na]+. Anal. Calc. for C14H11BrN2O2·0.3H2O: C, 51.81; H, 3.60; N,
8.63. Found: C, 51.81; H, 3.40; N, 8.42%.
2,6-Bis(2-methyloxazol-4-yl)bromobenzene (A). A 50 mL seal-
able polytetrafluoroethylene reactor was charged with a-
bromoketone C21b,c (160 mg, 0.4 mmol), acetamide (708 mg,
˚
12 mmol), 5 mL of toluene, and a small amount of 4 A molecular
sieves. After the reactor was sealed, the reaction mixture was
heated with a microwave oven for 15 min and cooled to room
temperature. The reaction mixture was then diluted with 20 mL of
EtOAc, followed by extraction with EtOAc (3 ¥ 20 mL each). The
combined organic phase was washed with water and saturated
aqueous NaCl, dried over anhydrous Na2SO4 and then filtered
and concentrated. The crude product was purified by flash column
chromatography (silica, hexanes : EtOAc 20 : 1 as eluent) to afford
2,6-bis(2-methyloxazol-4-yl◦)-bromobenzene (109 mg, 85% yield)
as a white solid. mp 75–76 C. 1H NMR (300 MHz, CDCl3): d =
2-(2-Methylthiazole-4-yl)-6-(2-methyloxazole-4-yl)bromobenze-
ne (B). The procedure of synthesizing the semi-cyclization
product F was identical to that of A except for using 304 mg
of acetamide (6 mmol) and prolonging the heating time to 30 min.
The crude product was purified by column chromatography to
afford F (61 mg, 42% yield) as a white solid. 1H NMR (300 MHz,
3
CDCl3): d = 8.25 (s, 1H, oxazolyl-H), 8.07 (d, J = 7.8 Hz, 1H,
3
3
Phenyl-H), 7.47 (t, J = 7.7 Hz, 1H, Phenyl-H), 7.27 (d, J =
7.8 Hz, 1H, Phenyl-H), 4.46 (s, 2H, CH2), 2.54 (s, 3H, CH3).
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1
(s, 3H, Ha). 13C { H} NMR (75 MHz, CDCl3): d = 174.5 (Cb¢),
167.3 (Cb), 161.6 (Cd¢), 157.1 (Ch), 147.2 (Cd), 137.3 (Ce¢), 134.3
(Ce), 130.2 (Cc), 125.5 (Cg), 120.7 (Cff¢), 22.1 (Ca¢), 15.3 (Ca).
MS (ESI-MS): m/z (%) = 345.0 (90) [M - Br - CH4]+, 727.1
(30) [2 M + H + H2O - Br2 - CH4]+, 771.0 (100) [2 M + H +
2MeOH - 2HBr - CH4]+, 945.3 (17)[2 M + H2O + CO2]+. Anal.
Calc. for C14H11BrN2OPdS·0.05dba (dba = dibenzylideneacetone):
C, 39.34; H, 2.60; N, 6.18. Found: C, 39.41; H, 2.86; N, 6.04%.
Thiacetamide (90 mg, 1.2 mmol) and F (130 mg, 0.36 mmol) were
dissolved in 2 mL of DMF. After being stirred for 24 h at room
temperature, the resultant mixture was diluted with 5 mL of H2O,
and then extracted with EtOAc (three portions of 10 mL each). The
combined organic phase was consequently washed with water and
brine, dried over anhydrous MgSO4 and then purified by column
chromatography (silica, hexanes : EtOAc 4 : 1 as eluent) to◦afford
1
B (85 mg, 76% yield) as a light yellow solid. mp 48–49 C. H
3
NMR (300 MHz, CDCl3): d = 8.26 (s, 1H, Hc), 7.93 (dd, J =
7.5 Hz, 4J = 1.5 Hz, 1H, Hf), 7.52 (dd, 3J = 8.1 Hz, 4J = 1.8 Hz,
Mizoroki–Heck catalysis
3
1H, Hf¢), 7.42 (t, J = 7.5 Hz, 1H, Hg), 7.36 (s, 1H, Hc¢), 2.79
1
(s, 3H, Ha¢), 2.54 (s, 3H, Ha). 13C { H} NMR (75 MHz, CDCl3):
In a typical run, a reaction tube with a condenser was charged with
a mixture of substituted olefin (1.5 mmol), aryl halide (1.0 mmol),
base (3 mmol of KOAc for methyl acrylate, 1.5 mmol DIPEA
for styrene), NMP (5.0 mL) and catalyst (indicated amount).
The mixture was heated at the indicated temperature (typically,
at 120 10 ◦C for methyl acrylate, at 160 10 ◦C for styrene) for
several hours (typically, 7 h for methyl acrylate, 5 h for styrene).
After being cooled to the ambient temperature, it was diluted with
10 mL of distilled water, and extracted with EtOAc (three portions
of 15 mL each). The combined organic phase was washed with
distilled water and saturated aqueous NaCl, dried over anhydrous
MgSO4 and then filtered and concentrated. The crude product was
purified by column chromatography (silica, hexane or hexane–
EtOAc mixture as eluent). The isolated products were confirmed
via 1H NMR and 13C NMR.
d = 164.8 (Cb¢), 160.5 (Cb), 154.0 (Cd¢), 138.4 (Cd), 137.5 (Ce¢),
137.0 (Cc), 133.4 (Ce), 130.9 (Cf¢), 130.4 (Cf), 127.2 (Cg), 121.6
(Ch), 19.2 (Ca¢), 13.8 (Ca). ESI-MS: m/z (%) = 335.0 (100) [M +
H]+, 337.0 (92) [M + 2 + H]+. Anal. Calc. for C14H11BrN2OS: C,
50.16; H, 3.31; N, 8.36. Found: C, 50.19; H, 3.25; N, 8.31%.
Synthesis of complexes
Under an Ar atmosphere, a 25 mL Schlenk flask was charged with
0.3 mmol of proligand, 173 mg of Pd(dba)2 (0.3 mmol), and 15 mL
of dry benzene. The reaction mixture was heated to reflux for 2
h, then cooled to room temperature and stirred for further 2 h.
The resultant mixture was directly transferred on to a diatomite
column and eluted firstly with hexane to remove dba and then
with mixed solvent (CHCl3 : MeOH = 3: 1). The collected target
complex was crystallized from DMF–CHCl3–MeOH. Crystals of
complexes suitable for X-ray diffraction analysis were grown by
slow diffusion of EtOAc and hexane into a DMF solution of the
corresponding complex.
X-Ray crystal structure determinations
Suitable single crystals with dimensions of 0.34 ¥ 0.19 ¥ 0.08 mm
for 1 and 0.32 ¥ 0.18 ¥ 0.11 mm for 2 were glued on a glass fiber.
Diffraction intensity data were collected on a Bruker Apex CCD
diffractometer with graphite-monochromated Mo–Ka radiation
˚
(l = 0.71073 A) at 293 K. Absorption corrections were applied
using the multiscan technique. The structures were solved by the
direct method and refined by the full-matrix least-squares method
on F2 using the SHELXL-97 software.28 All of the non-hydrogen
atoms were refined anisotropically. The organic hydrogen atoms
were generated geometrically. During the refinement, a restrained
refinement comment “ISOR” is used to restrain the Non-H atoms
with the ADP problems, these atoms are as follows: Br1, C1, C14,
S1, O1A, S1A, O1 in 2. The S1 and O1 sites in 2 are mutually
equally disordered, each with 50% occupancy. The crystal data
and structure refinement of compounds 1 and 2 are summarized
in Table 1. Selected bond lengths and angles for 1 and 2 are listed
in Table 2.
[2,6-Bis(2-methyloxazol-4-yl)phenyl-j3-NCN]bromopalladium-
(II) (1). Pale yellow granules, 87% yield; mp > 215 ◦C. 1H NMR
(300 MHz, CDCl3): d = 7.70 (s, 2H, Hc), 7.13 (s, 3H, Hf & Hg),
1
3.01 (s, 3H, Ha). 13C { H} NMR (75 MHz, CDCl3): d = 167.0
(Cb), 158.5 (Ch), 147.6 (Cd), 134.4 (Ce), 130.2 (Cc), 127.2 (Cg),
120.7 (Cf), 15.3 (Ca). ESI-MS: m/z (%) = 344.9 (27) [M - Br]+,
736.8 (100) [2M + CO2 + H–Br2]+, 820.8 (4)[2M - Br + CH3OH +
H2O]+. Anal. Calc. for C14H11BrN2O2Pd·0.15DMF (DMF = N,N-
dimethylformamide): C, 39.76; H, 2.79; N, 6.90. Found: C, 39.58;
H, 2.66; N, 7.14%.
CCDC reference numbers: 785152 for 1 and 785153 for 2
Acknowledgements
We gratefully acknowledge financial support from NSFC
(20971105), SRF for ROCS, SEM ([2007]1108), the Natural
Science Foundation of Chongqing Science & Technology Com-
mission (CSTC 2006BB4026), the Fundamental Research Funds
for the Central Universities (XDJK2009C093, XDJK2009B014),
the Science and Technology Foundation of Southwest University
(SWUB2006001), and National Innovation Experiment Program
for University Students (091063504).
2-(2-Methylthiazole-4-yl)-6-(2-methyloxazole-4-yl)phenyl-j3-
NCN]bromopalladium(II) (2). yellow granules, 86% yield; mp >
215 C. H NMR (300 MHz, CDCl3): d = 7.70 (s, 1H, Hc), 7.18
(s, 1H, Hc¢), 7.20–7.10 (m, 3H, Hff¢ & Hg), 3.24 (s, 3H, Ha¢), 3.02
◦
1
3608 | Dalton Trans., 2011, 40, 3601–3609
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The Royal Society of Chemistry 2011
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