Tetra- And Dinuclear Palladium Complexes Based on a Ligand of 2,8-Di-2-pyridinylanthyridine: Preparation, Characterization, and Catalytic Activity

Article abstract of DOI:10.1021/acs.organomet.1c00226

Complexation of L [L = 5-phenyl-2,8-di-2-pyridinyl-anthyridine] with [Pd(CH3CN)4](BF4)2 and [Pd(CH3CN)3Cl](BF4) in a molar ratio of 1:2 rendered the corresponding dinuclear complexes [Pd2L (CH3CN)4](BF4)4 (1) and [Pd2L (CH3CN)2Cl2](BF4)2 (2), respectively. However, treatment of L with (COD)PdCl2 followed by anion exchange yielded a tetranuclear complex [Pd4L3Cl4](PF6)4(4a). Structures of these complexes are characterized by both spectroscopy and X-ray crystallography. Interconversion of these three complexes was studied via the manipulation of stoichiometric ratio of ligand to metal precursor. The catalytic activity of these complexes for carbonylative Suzuki-Miyaura cross-coupling was investigated. Complex 2 shows an excellent catalytic activity on the reaction of aryl iodide with arylboronic acid in the presence of atmospheric pressure of CO to give the corresponding benzophenones.

Full text of DOI:10.1021/acs.organomet.1c00226

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Article
Tetra- and Dinuclear Palladium Complexes Based on a Ligand of 2,8-
Di-2-pyridinylanthyridine: Preparation, Characterization, and
Catalytic Activity
Shih-Chieh Aaron Lin,† Bo-Kai Su,† Yi-Hung Liu, Shie-Ming Peng, and Shiuh-Tzung Liu*
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ABSTRACT: Complexation of L [L = 5-phenyl-2,8-di-2-pyridinyl-anthyridine]
with [Pd(CH3CN)4](BF4)2 and [Pd(CH3CN)3Cl](BF4) in a molar ratio of 1:2
rendered the corresponding dinuclear complexes [Pd2L (CH3CN)4](BF4)4 (1)
and [Pd2L (CH3CN)2Cl2](BF4)2 (2), respectively. However, treatment of L with
(COD)PdCl2 followed by anion exchange yielded a tetranuclear complex
[Pd4L3Cl4](PF6)4(4a). Structures of these complexes are characterized by both
spectroscopy and X-ray crystallography. Interconversion of these three complexes
was studied via the manipulation of stoichiometric ratio of ligand to metal
precursor. The catalytic activity of these complexes for carbonylative Suzuki−
Miyaura cross-coupling was investigated. Complex 2 shows an excellent catalytic
activity on the reaction of aryl iodide with arylboronic acid in the presence of atmospheric pressure of CO to give the corresponding
benzophenones.
ligand 5-phenyl-2,8-bis(6′-bipyridinyl)-1,9,10-anthyridine (L,
Scheme 1), which is capable of accommodating two metal
ions.8 Coordination of this ligand with metal ions afforded a
set of dimetallic complexes including ruthenium(II), nickel(II),
and copper(II), in which the distances between the metal ions
are ca. 5 Å.8 We are also delighted to learn that these dinuclear
metal complexes show a possible synergistic effect in various
catalytic reactions.7 Herein, we report the preparation of
dipalladium complexes associated with L and the interconver-
sion of di- and tetra-nuclear complexes. Furthermore, the
catalytic activity of these complexes on the carbonylative
Suzuki−Miyaura cross-coupling is revealed.
INTRODUCTION
■
One of the current research interests in homogeneous catalysis
is the investigation of di- or polymetallic complexes as catalysts,
because these species may prompt cooperative effects, thus
improving the selectivity and possibly imparting a new
activity.1 Owing to the remarkable activity of palladium ions
in catalysis, the study of dipalladium-catalyzed reaction
becomes an interesting task and receives much attention.
Quite a few dipalladium complexes are known to show the
catalytic activities in cross-coupling,2,3 olefin polymerization,4
C−H activation/addition,5 hydroxycarbonylation,6 and reduc-
tion.7 An important feature of these dinuclear catalysts is that
the metal ions are accommodated by a designed ligand,
allowing them to be arranged in a close proximity.
In our previous works, we found that dipalladium complex
[Pd2(bpnp)(μ-OH)(CF3CO2) 2](CF3CO2) (Scheme 1) is
active for catalytic reduction of nitroarene under atmospheric
pressure of hydrogen at 50 °C.7a The mechanistic study
indicates that the cooperation of the two palladium centers,
which are separated by 3.1696(7) Å, is responsible for the
highly activity of the catalyst. We have also synthesized another
RESULTS AND DISCUSSION
■
Preparation and Characterization of Complexes. As
shown in Scheme 2, treatment of L with [Pd(CH3CN)4]-
(BF4)2 in acetonitrile at 50 °C afforded bis-palladium(II)
complex 1 in 92% yield. Initial evidence that two metal ions
were coordinated within the ligand came from 1H NMR
spectrometric studies. The signals for the protons on the
anthyridine moiety in complex 1 appear as two sets of doublets
at 8.81 and 8.46 ppm, indicating the coordination of two metal
Scheme 1. Structures of Ligands
Received: April 8, 2021
Published: June 30, 2021
https://doi.org/10.1021/acs.organomet.1c00226
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Scheme 2. Preparation of Dipalladium Complexes
Table 1. Selected Bond Distances (Å) and Bond Angles
(deg)
complex 1
1.983(5)
2.014(5)
complex 2
2.001(3)
2.058(3)
N1−Pd1
N2−Pd1
N4−Pd(n)
N5−Pd(n)
Pd1−X
2.006(5) (n = 1)
1.990(4) (n = 1)
2.006(5) (X = N4)
2.022(3) (n = 2)
1.996(3) (n = 2)
2.2802(10) (X = Cl1)
2.2841(10)
Pd2−Cl2
Pd1−Y
1.990(4) (Y = N5)
80.5(2)
1.996(3) (Y = N6)
80.52(13)
80.31(13)
N1−Pd1−N2
N4−Pd2−N5
Cl1−Pd1−N1
93.88(10)
Å. It is noticed that the anthyridine ligand does not maintain
the whole structure in-plane owing to the steric congestion of
ligands associated with the two metal ions within the “pocket”.
The dihedral angle of N1−C5−C6−N2 is 0.8°, suggesting a
planar arrangement of chelating ring. However, the calculated
dihedral angle between two planes defined by N1−Pd1−N2
and N1A−Pd1A−N2A is 35.13° in complex 1, clearly
indicating that two chelating rings are not seated in the same
plane as shown in Figure 2 from the side view. The distance of
N5···N5A is separated by 3.50 Å, which is due to the steric
relief of two coordinating acetonitrile ligands.
ions within the ligand to make a symmetrical nature. ESI-MS
analysis also provides the support of a dinuclear notion. Peaks
at m/z 225.3299 (Calcd 225.3264 for [Pd2L (H2O)2(OH)]3+)
and m/z 231.3230 (Calcd 231.3299 for [Pd2L
(H2O)3(OH)]3+) were observed, as the acetonitrile ligands
were replaced by water molecules.
Similarly, dipalladium complex 2 was obtained by reaction of
1
L with [Pd(MeCN)3Cl](BF4). H NMR signals of 2 is quite
similar to those for 1. ESI-MS spectrum of 2 shows a peak at
m/z 356.4513, which is consistent with the formula of [2 −
2(CH3CN) − 2(BF4) + H2O]2+, suggesting a dinuclear
palladium structure. Nevertheless, the solid-state structures of
both complexes 1 and 2 were determined by single-crystal X-
ray diffraction analysis. Their ORTEP plots are shown in
Figure 1, and the relevant structural parameters are
summarized in Table 1.
The molecular structure of 1 consists of an anthyridine
ligand L hosting two palladium centers, which complete their
coordination sphere with two acetonitrile ligands (Figure 1A).
At the metal ions, the coordination geometry is a distorted
square planar form. The average Pd−N bond length is about
2.0 Å, and the through space distance of Pd···Pd is 5.0303(7)
Figure 2. Side view of cationic portion of 1.
Figure 1. ORTEP plots of cationic portion of 1 (A) and 2 (B) (30% probability level, label of some aromatic carbons are omitted for clarity).
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The solid-state structure of complex 2 (Figure 1B) is
essentially similar to that of 1 except for the chloride ligand
associated to the metal centers. Again, the two chelating ring
within the complex is not coplanar as evidenced by 41.99° of
the dihedral angle between two planes defined by N1−Pd1−
N2 and N4−Pd2−N5. The distance between N6 and N7 is
3.48 Å, quite similar to that in 1. These observations essential
resembling to those in complex 1. The Pd···Pd distance in 2 is
5.2098(4) Å, slightly longer than that in 1. It is worth
mentioning that both chloride ligands in 2 are seated trans to
the anthyridine nitrogen and are oriented outward from the
“pocket” of the ligand.
Next, we investigated the complexation of L with of
Pd(COD)Cl2 and hoped to obtain dipalladium chloride
complex 3. However, treatment of L with Pd(COD)Cl2 did
not yield expected complex 3. Instead, this complexation gave
palladium complex 4a as a yellow solid upon anion metathesis
with KPF6 (Scheme 3). The 1H NMR spectrum of 4a shows a
Figure 3. ORTEP plot of cationic portion of 4 (30% probability level,
label of some aromatic carbons are omitted for clarity).
Scheme 3. Preparation of Tetranuclar Palladium Complexes
bridging anthyridine ligand L (red color) and a chloride. The
Pd−N bond lengths are in the range of 1.99−2.06 Å, typical
distance for such bonding. The values of Pd···Pd distances
within the molecule are Pd1···Pd2 [5.4976(10) Å], Pd2···Pd3
[4.0535(9) Å], Pd3···Pd4 [5.5584(9)Å], and Pd1···Pd4
11.4656(10) Å], which exclude any interaction between
these metal ions. All bite angles of N−Pd−N′ in chelating
rings are about 80°, which is similar to those of bipyridine−
metal complexes. All other bond distances and bond angles are
in the normal range except the angles of N2−Pd1−N6
[100.6(3)°], N4−Pd2−N7 [102.8(3)°], N12−Pd4−N10
[101.6(3)°], and N14−Pd3−N9 [97.9(3)°] due to the
geometrical constraint of the ligand in such coordination
modes.
As shown in the crystal structure, the cationic portion of 4a
presents a symmetry axis passing through the atoms of C38
and N8, having C2 symmetry. This is consistent with its H
1
complicated splitting pattern, which suggests a species that is
not dipalladium. Crystals of the complex were obtained upon
diffusion of ether vapor into an acetone solution of the
complex. The structure obtained by X-ray crystallography
technique is presented in Figure 3, clearly revealing the
structure containing four palladium atoms supported by three
anthyridine ligands.
NMR signals in solution (Figure 4). One is expected to
observe three chemical-nonequivalent α-pyridinyl hydrogens in
4a (as illustrated in three different colors, Scheme 3) in its 1H
NMR spectrum. Indeed, three sets of doublets appear at δ
9.43, 9.01, and 8.69 with an integration ratio of 2:2:2. Overall,
the spectroscopic data obtained in solution are in agreement
with the solid-state structure.
Interconversion of Di- and Tetranuclear Palladium
Species. With the structural background of 4a, the tetranuclear
species 4b can be prepared directly by treatment of L with
[Pd(MeCN)3Cl](BF4) stoichiometrically. Furthermore, the
interconversion of di- into tetra-nuclear species was inves-
tigated by manipulation of the stoichiometric ratio of ligand
versus metal ions. Substitution of acetonitrile ligands in 1 by
chloride to give 2 was achieved by the addition of two
equivalent of tetrabutylammonium chloride (Scheme 4, step
a). On the other hand, abstraction chlorides from 2 by
treatment of AgBF4 gave 1 accordingly (Scheme 4, step b).
Conversion of dimetallic species 2 into 4b required the
addition of chloride ligand and free ligand L (Scheme 4, step
c). Furthermore, tetra-metallic species 4b could be converted
into 2 by addition of two equivalent of [Pd(MeCN)3Cl](BF4)
to a solution of 4b in acetonitrile (step d). Finally, Addition of
excess of silver salts and palladium ions to a solution of 4b in
In the structure of 4a, L acts as both chelating and bridging
ligands to stabilize the tetra-palladium framework. Accordingly,
this tetra-nuclear species is formed via the coordination of four
nitrogen donors from the bridging anthyridine ligand (red
color in Scheme 3) to four palladium centers of two
“[Pd2LCl]” moieties, respectively, i.e., four nitrogen atoms of
the bridging anthyridine ligand act as a monodentate to each
palladium ion individually. In order to have such a
coordination mode, the pyridinyl ring of the bridging
anthyridine ligand is bisected with the plane of anthyridine
ring, as evidenced by the dihedral angles N6−C32−C33−N7
and N9−C43−C44−N10 being 115.0(8) and 121.3(8)°,
respectively (Table 2). The bipyridinyl moiety on the other
two anthyridine ligands acts a bidentate donor to chelate to the
metal center (green and blue color in structure 4 (Scheme 3).
All palladium atoms exhibit slightly distorted square planar
environments resulting from their coordination to the
chelating bipyridinyl moiety of L, a nitrogen donor of the
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Table 2. Selected Bond Distances (Å), Bond Angles (deg), and Dihedral Angles (deg)
Pd1−N1
Pd1−N2
Pd1−N6
Pd1−Cl1
Pd2−N5
Pd2−N4
Pd2−N7
Pd2−Cl2
N1−Pd1−N2
N1−Pd1−N6
N2−Pd1−N6
N4−Pd2−N5
N4−Pd2−N7
N5−Pd2−N7
N2−Pd1−Cl1
N4−Pd1−Cl2
N1−C5−C6−N2
N1−C5−C6−N2
N6−C32−C33−N7
1.999(7)
2.024(7)
2.064(7)
2.284(2)
2.006(7)
2.058(7)
2.066(6)
2.280(2)
80.2(3)
179.3(3)
100.6(3)
80.4(3)
102.8(3)
173.7(3)
170.3(2)
170.5(2)
10.2(12)
11.6(12)
115.0 (8)
Pd3−N15
Pd3−N14
Pd3−N9
Pd3−Cl3
Pd4−N11
Pd4−N12
Pd4−N10
Pd4−Cl4
N14−Pd3−N15
N14−Pd3−N9
N15−Pd3−N9
N11−Pd4−N12
N11−Pd4−N10
N12−Pd4−N10
N14−Pd3−Cl3
N12−Pd4−Cl4
N14−C70−C71−N15
N11−C59−C60−N12
N9−C43−C44−N10
2.000(7)
2.051(7)
2.029(7)
2.285(2)
2.012(7)
2.054(7)
2.020(7)
2.295(2)
80.7(3)
97.9(3)
177.6(3)
82.0(3)
175.3(3)
101.6(3)
169.13(19)
172.3(2)
12.9(12)
0.4(12)
121.3(8)
1
Figure 4. H NMR spectrum of 4a (in CD3CN).
phenyl iodide with 4-tolueneboronic acid catalyzed by complex
2 (eq 1).
Scheme 4. Interconversion of Di- and Tetra-palladium
Complexes
Initial tests investigated the effect of the CO source on the
model reaction. It is known that the carbon monoxide source
dramatically affects the efficiency of carbonylation.9,10 Quite
often, the carbonylative coupling reactions perform in the
presence of high pressure of CO or metal carbonyls. By using
complex 2 as the catalyst, the carbonylative coupling reaction
of PhI with tolueneboronic acid in a mixed solvent (CH3CN/
H2O) proceeded smoothly to give I in good yields, either
under atmospheric pressure of CO or with the use of a
stoichiometric amount of Mo(CO)6, but showed poor
selectivity for Cr(CO)6 or W(CO)6 (Table 3, entries 1−4).
Considering the easy purification, we decided to carry out this
reaction under the atmospheric pressure of CO for further
investigation.
a
Conditions: 1 (0.025 mmol), Bu4NCl (0.05 mmol), 50 °C.
b
Conditions: 2 (0.025 mmol), AgBF4 (0.1 mmol), 50 °C.
c
Conditions: 2 (0.025 mmol), Bu4NCl (0.025 mmol), L (0.01
d
mmol), RT. Conditions: 4b (0.008 mmol), Pd(MeCN)3(Cl)(BF4)
(0.017 mmol), 50 °C. Conditions: 4b (0.008 mmol), AgBF4 (0.035
mmol), Pd(MeCN)4(BF4)2, (0.017 mmol), 50 °C.
e
acetonitrile rendered the bimetallic complex 1 directly (step e).
Scheme 4 summarizes the interconversion among these
complexes.
Catalytic Activity of Palladium Complexes. After prepara-
tion and characterization, the catalytic activity of these
complexes was examined in the carbonylative Suzuki−Miyaura
coupling reaction of aryl iodides with arylboronic acids.9 In
order to ascertain the optimal reaction conditions, different
reaction parameters were screened with the model reaction of
By using water as the solvent, the reaction reached the
highest conversion but less selectivity (entry 5). However, we
found that running these reactions in the presence of moisture
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a
Table 3. Optimization of Carbonylative Coupling of PhI with p-MeC6H4B(OH)2
c
product (%)
b
entry
CO source
base
solvent
conv. (%)
I
II
d
1
Mo(CO)6
Cr(CO)6
W(CO)6
CO
CO
CO
CO
CO
CO
CO
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K2CO3
K2CO3
K2CO3
K2CO3
CH3CN/H2O (4:6)
CH3CN/H2O (4:6)
CH3CN/H2O (4:6)
CH3CN/H2O (4:6)
H2O
ClCH2CH2Cl
toluene
xylene
EtOH
dioxane
xylene
xylene
75
76
60
75
92
52
45
60
60
20
51
80
100
70
trace
12
10
trace
27
18
trace
9
d
d
d
d
2
3
4
5
6
7
8
9
56
36
72
63
32
42
47
35
trace
44
75
95
93
17
10
11
12
13
14
trace
trace
trace
CO
CO
CO
CO
e
e f
,
xylene
xylene
e fg
, ,
100
a
Reaction conditions: C6H5I (0.5 mmol), p-MeC6H4B(OH)2 (0.75 mmol), 2 (2.5 × 10−3 mmol), and base (1.5 mmol) in solvent (1 mL) at 50 °C
b
c
d
e
f
for 16 h. Metal carbonyls (0.25 mmol); CO (1 atm). Based on NMR integration. p-MeC6H4OH as the side product Using 1.5 mol % of 2. At
80 °C. Mercury (0.3 g) was added at the beginning of the reaction.
g
gave a fair amount of phenol as the side product (entries 1−5).
In order to avoid the production of phenol, we re-investigated
this reaction in various anhydrous organic solvents (entries 6−
11). It appears that toluene or xylene is a good choice for a
better selectivity, predominantly giving I (entries 7 and 8).
Further screens with bases in xylene, it seems that the use of
K2CO3 gave a better selectivity than that of K3PO4, i.e., less
amount of Suzuki coupling product (entry 11). Furthermore,
running the reaction with a higher loading of 2 (1.5 mol %)
improved the yield dramatically (entry 12). Finally, raising the
reaction temperature to 80 °C rendered carbonylative coupling
product I in 95% yield (entry 13). A mercury test was applied
to this catalysis by loading of Hg(0) at the beginning of the
reaction, and the production of I remains similar (entry 14),
indicating a homogeneous catalytic system.
Next, we tested other palladium precursors to observe the
effect of the bimetallic complexes on the reaction (Table 4).
Bimetallic complexes 1 and 2 were shown to be efficient
catalysts for this process in comparison to the monometallic
complexes. We note that complex 4b is a poor catalyst (entry
3). Presumably, the stable ligand coordination around the
metal center inhibits the catalytic process, suggesting that
dissociation of a labile ligand in complexes 1 and 2 allows the
substrate to bind toward the palladium center more effectively.
Monopalladium complexes as catalyst precursors exhibited
some activity, but it was not as good as complex 2 in term of
conversion and selectivity (entries 4−11). In monopalladium
catalysis, a fair amount of Suzuki coupling product II was
obtained. The use of Pd2(dba)3 as the catalyst gave a fair
conversion, but the catalysis might proceed through palladium
clusters as evidenced by the deposition of black metal during
the reaction (entry 12). Comparison of catalytic activity of
some mononuclear complexes and 2 at 80 °C was studied, and
it showed that complex 2 shows the best activity at a higher
temperature.
Table 4. Activity of Various Complexes in Carbonylative
Suzuki−Miyaura Coupling
entry
metal complex (mol %)
conv. yield of I yield of II
1
2
3
4
5
6
7
8
9
complex 2 (1.5 mol %)
complex 1 (1.5 mol %)
complex 4b (1.5 mol %)
Pd(MeCN)4(BF4)2 (3.0 mol %)
Pd(OAc)2 (3.0 mol %)
Pd(dppe)Cl2 (3.0 mol %)
[PdCl(ppy)]2 (3.0 mol %)
(bpy)PdCl2 (3.0 mol %)
80%
75%
19%
15%
60%
56%
68%
20%
10%
75%
58%
4%
12%
32%
35%
46%
14%
7%
8%
11%
trace
21%
4%
13%
trace
trace
(bpy)Pd(MeCN)2(BF4)2
(3.0 mol %)
10
11
12
(t-Bubpy)PdCl2 (3.0 mol %)
(MeCN)2PdCl2 (3.0 mol %)
Pd2(dba)3 (1.5 mol %)
complex 2 (1.5 mol %)
Pd(OAc)2 (3.0 mol %)
25%
70%
61%
100%
84%
89%
90%
15%
43%
43%
95%
25%
48%
48%
8%
18%
16%
-
58%
36%
40%
b
13
b
14
15
b
Pd(dppe)Cl2 (3.0 mol %)
[PdCl(ppy)]2 (3.0 mol %)
b
16
a
Reaction conditions: C6H5I (0.5 mmol), p-MeC6H4B(OH)2 (0.75
mmol), Pd complex, and K2CO3 (1.5 mmol) in xylene (1 mL) under
CO (1 atm) at 50 °C for 16 h. NMR yields are based on 1,3,5-
b
trimethoxybenzene as the internal standard. At 80 °C; dppe =
Ph2PCH2CH2PPh2; ppy = 2-phenylpyridine; bpy = 2,2′-bipyridine; t-
Bubpy = 2,2′-bis(4-tBu-pyridine)
along the reaction course, and the results are shown in Figure
5. Regardless of activated or deactivated substrates, complex 2
shows a higher activity in the carbonylative coupling reaction
as compared to that of [(bpy)PdCl2]. As shown in the Figure
5, within 30 min, the yields of p-(MeCO)C6H4COPh and p-
(MeO)C6H4COPh catalyzed by 2 reach 73 and 32%,
respectively, whereas the yields of these two products from
the catalysis of [(bpy)PdCl2] are 14 and 7%, respectively.
Clearly, the presence of an adjacent palladium ion in dimetallic
system 2 plays an important role to achieve the enhancements
of yields in production of I, indicating a synergistic or
cooperative effect in the catalysis via complex 2. This outcome
suggests that complex 2 does benefit from the cooperative
effect during the reaction. However, attempts to identify the
For a further comparison of activities between di- and
monometallic species, we monitored the reaction course of the
coupling reaction catalyzed by [(bpy)PdCl2] and 2 (Scheme
5). Thus, product distributions from the reaction of a mixture
of p-CH3COC6H4I and p-MeOC6H4I with a sufficient amount
of PhB(OH)2 catalyzed by 2 or [(bpy)PdCl2] were analyzed
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Scheme 5. Competition of Carbonylative Coupling Reaction Catalyzed by 2 and (bpy)PdCl2
Furthermore, we tested various substituted aryl iodides and
arylboronic acids as substrates to demonstrate the scope and
limitation of the catalytic system (Table 5). As listed in the
Table 5, under the optimized reaction conditions, aryl iodide
reacted with a wide range of substituted arylboronic acids
(entries 1−9) to give the corresponding ketones in high to
excellent yields except p-hydroxyphenylboronic acid (entry 4).
However, it was observed that the electronic nature of
substituents in aryl iodides does not have a significant
influence on the yields with the reaction of p-methylphenyl-
boronic acids (entries 10−19). Various substituted substrates
were also suitable for the coupling reaction catalyzed by 2 to
render the corresponding benzophenones (entries 20−25).
CONCLUSIONS
Figure 5. Carbonylation of a mixture of p-(MeCO)C6H4I and p-
(MeO)C6H4I with C6H5B(OH)2 catalyzed by 2 (red line) and
[(bpy)PdCl2] (blue line) under the optimized conditions.
■
In this work, di- and tetranuclear palladium complexes hosted
by anthyridine-based ligand L have been prepared and
structurally characterized. Notably, interconversion among
these complexes could be achieved by manipulation of
stoichiometry of ligand and metal ions. Furthermore, dinuclear
palladium complexes appear to be excellent precatalysts for
carbonylative Suzuki−Miyaura coupling of aryl iodide and
arylboronic acid in the presence of atmospheric pressure of
carbon monoxide, but not tetranuclear species 4. From the
activity analysis of various palladium species, complex 2 does
show an excellent catalytic activity, indicating that this likely
has a synergistic or cooperative effect operating in the catalytic
system. In addition, various substrates aryl iodides and
arylboronic acids were suitable for this catalytic coupling
reaction to prepare various benzophenones. However, the
mechanistic detail of this catalysis catalyzed by the dinuclear
complex remains unclear in current stage.
possible intermediates catalyzed by 2 were unsuccessful.
Fortunately, we found that the absorption spectrum of the
solution from the catalytic reaction showed a peak at λmax
420 nm, suggesting the existence of the dipalladium framework
of the complex during the catalysis.
∼
Since the palladium centers in 2 are separated by 5 Å, it is
quite unlikely to have a metal−metal bond interaction between
them. Thus, it is not possible to have a catalytic active species
or an intermediate involving a metal−metal bond or
interaction for the acceleration of binuclear catalysis, which
is different from the well-known Pd(I) dimers investigated by
Schoenebeck.3 We believe that the insertion and coupling step
occurs at one of the palladium centers, whereas the other metal
ion is responsible for assistance to accelerate the reaction. One
possibility is that the dissociation of iodide ligand from the
oxidative adduct is facilitated by the bridging coordination
toward the adjacent palladium ion (Scheme 6), which might
promote the insertion of CO.11 Furthermore, upon insertion of
the carbonyl ligand, the coordinated unsaturated species could
be stabilized by the recoordination of iodide. In other words,
the adjacent palladium ion in complex 2 serves to stabilize the
catalytic intermediates that enhance the catalytic efficiency.
Running the reaction in the presence of AgBF4 under the
optimal conditions (Table 3, entry 13) gave a lower conversion
and a different product distribution (Scheme 7), implying that
the existence of iodide in the reaction medium might play a
role in acceleration of the catalysis. Again, none of the
proposed intermediates was identified.
EXPERIMENTAL SECTION
■
General Information. Chemicals and solvents were analytical-
grade and were used without further purification. Nuclear magnetic
resonance (NMR) spectra were recorded on a 400 MHz
spectrometer. Chemical shifts are given in parts per million (ppm)
relative to Me4Si for 1H and 13C NMR. Ligand L was prepared
according to the reported methods.8d
Preparation of [Pd2L(MeCN)4](BF4)4 (1). A mixture of Pd-
(MeCN)2Cl2 (25.3 mg, 0.098 mmol) and AgBF4 (38.0 mg, 0.20
mmol) in acetonitrile (1 mL) was stirred at 80 °C under nitrogen
atmosphere for 3 h. Upon cooling, L (20.1 mg, 0.049 mmol) was
added to the above solution. The resulting mixture was stirred at 50
°C for 20 h. The reaction mixture was filtrated to remove silver salt,
and the filtrate was added to ether to give yellow precipitate. The
Scheme 6. Metal Ion Assists the Dissociation of Iodide and Stabilization of Intermediates
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Scheme 7. Carbonylation of PhI with p-MeC6H4B(OH)2 in the Presence of AgBF4
103) nm. Anal. Calcd for C31H23B2Cl2F8N7Pd2·CH3CN: C, 39.96; H,
2.64; N, 11.30. Found: C, 39.62; H, 2.36; N, 10.96.
a
Table 5. Scope of Catalytic Carbonylative Coupling
Preparation of [Pd4L3Cl4](PF6)4 (4a). A round-bottomed flask
loaded with a mixture of L (50.2 mg, 0.12 mmol), Pd(COD)Cl2 (73.1
mg, 0.25 mmol), and KPF6 (112.3 mg, 0.61 mmol) in a solution of
dichloromethane and acetone (7.5 mL) was flashed with nitrogen.
The resulting mixture was stirred at room temperature for 18 h. Ether
(5 mL) was added to yield a yellow precipitate, which was collected
and washed with water, ether, and dichloromethane. After drying
under reduced pressure, the desired complex was obtained as a bright
yellow solid (70.3 mg, 73%). Recrystallization from acetone/ether
entry
R1
R2
isolated yield
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
96%
93%
88%
0%
p-Me
p-MeO
p-HO
o-MeO
o-CHO
o-F
3,5-(CF3)2
2,6-(MeO)2
p-Me
p-Me
p-Me
p-Me
p-Me
1
gave a yellow crystal suitable for X-ray crystallographic analysis. H
70%
62%
70%
84%
69%
86%
85%
73%
64%
75%
70%
74%
70%
80%
64%
75%
58%
61%
42%
78%
75%
NMR (400 MHz, CD3CN): δ = 10.09 (d, J = 8.8 Hz, 2H), 9.91 (d, J
= 7.4 Hz, 2H), 9.43 (d, J = 5.6 Hz, 2H), 9.03 (d, J = 5.6 Hz, 2H), 8.81
(d, J = 8.8 Hz, 2H), 8.70−8.61 (m, 4H), 8.58−8.50 (m, 2H), 8.51−
8.31 (m, 10H), 8.25 (d, J = 8.8 Hz, 2H), 8.11−8.03 (m, 2H), 7.95−
7.71 (m, 11H), 7.63−7.55 (m, 2H), 7.50 (d, J = 6.8 Hz, 4H), 7.40−
7.33 (m, 2H), 6.96 (d, J = 7.4 Hz, 2H). 13C NMR (100 MHz,
CD3CN): δ = 166.0, 165.4, 164.2, 157.0, 156.3, 156.0, 155.7, 155.6,
155.5, 154.0, 153.5, 153.4, 153.3, 152.4, 145.0, 143.9, 143.0, 142.6,
141.7, 140.2, 131.4, 131.3, 131.1, 130.9, 130.4, 130.2, 129.6, 129.3,
128.7, 128.2, 127.8, 127.7, 127.3, 125.5, 123.0, 122.8, 120.7, 120.0.
UV−vis (MeOH) λmax (ε): 208 (44.5 × 103), 260 (23.2 × 103), 295
(13.1 × 103), 408 (14.9 × 103), 428 (15.6 × 103) nm. Anal. Calcd for
C81H51Cl4F24N15P4Pd4·[(CH3CH2)2O][(CH3)2CO]: C, 42.04; H,
2.69; N, 8.36. Found: C, 42.36; H, 2.38; N, 8.12.
9
H
p-Me
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
p-MeO
p-MeCO
p-CN
p-Cl
o-Me
m-Me
p-Me
p-Me
p-Me
p-Me
o-MeO
1-naphthyl
9-phenanthrenyl
p-MeO
p-MeO
p-MeCO
o-F
Preparation of [Pd4L3Cl4](BF4)4 (4b). A mixture of L (9.7 mg,
0.024 mmol) and [Pd(MeCN)3Cl](BF4) (10.8 mg, 0.031 mmol) in
acetonitrile (1 mL) was stirred at room temperature under nitrogen
atmosphere for 20 h. Ether (2 mL) was added to give a yellow solid.
Filtration and washing with ether yielded the desired complex as a
p-Me
o-F
2,6-(MeO)2
o-F
p-MeO
o-F
1
yellow powder (15.1 mg, 90%). H NMR (400 MHz, CD3CN): δ =
10.09 (d, J = 8.8 Hz, 2H), 9.92 (d, J = 8.0 Hz, 2H), 9.43 (d, J = 5.8
Hz, 2H), 9.03 (d, J = 5.6 Hz, 2H), 8.81 (d, J = 8.8 Hz, 2H), 8.70−
8.61 (m, 4H), 8.56 (d, J = 9.0 Hz, 2H), 8.51−8.36 (m, 10H), 8.28 (d,
J = 9.2 Hz, 2H), 8.13−8.07 (m, 2H), 7.95−7.88 (m, 3H), 7.86−7.76
(m, 8H), 7.63−7.56 (m, 2H), 7.52−7.47 (m, 4H), 7.41−7.35 (m,
2H), 6.97 (d, J = 7.1 Hz, 2H). 13C NMR (100 MHz, CD3CN): δ =
167.2, 166.6, 165.5, 158.2, 157.5, 157.1, 156.8, 155.1, 154.7, 154.6,
154.5, 153.6, 146.2, 145.1, 144.4, 143.8, 142.9, 141.4, 132.6, 132.5,
132.3, 132.26, 132.25, 132.1, 131.6, 131.4, 130.8, 130.51, 130.50,
129.9, 129.4, 129.0, 128.5, 126.7, 124.2, 124.1, 124.0, 122.0, 121.2.
UV−vis (MeOH) λmax (ε): 206 (36.6 × 103), 258 (18.6 × 103), 296
(11.0 × 103), 408 (12.7 × 103), 428 (13.0 × 103) nm. Anal. Calcd for
C81H51B4Cl4F16N15Pd4: C, 45.27; H, 2.39; N, 9.78. Found: C, 45.56;
H, 2.02; N, 9.76.
Catalysis−Carbonylative Suzuki−Miyaura Coupling Reac-
tion. A mixture of aryl iodide (0.5 mmol), arylboronic acid (0.75
mmol), K2CO3 (1.5 mmol), and complex 2 (1.5 × 10−3 mmol) in
xylene (1 mL) was placed in a reaction tube, which was evacuated and
refilled with CO. The reaction vessel was heated at 80 °C for a period
of time as indicated in Table 4. After the reaction, the mixture was
diluted with H2O and extracted with CH2Cl2 (2 mL × 2). The
extractions were combined, dried over anhydrous MgSO4. The filtrate
was concentrated and purified by column chromatography on silica
gel to give the desired products. The spectral data of the organic
products are essentially identical to those reported ones. Spectral data
of all catalysis products are given in the Supporting Information.
Crystallography. Crystals suitable for X-ray determination were
obtained for 1, [2·(CH3CN)], and [4a (Et2O)(Me2CO)]. The
structure was solved using the SHELXS-97program12 and refined
using the SHELXL-97program13 by full-matrix least-squares on F2
values. All crystal data and other parameters are collected in the
p-CN
p-CN
p-MeO
a
Reaction conditions: ArI (0.5 mmol), ArB(OH)2 (0.75 mmol),
complex 2 (7.5 × 10−3 mmol), and K2CO3 (1.5 mmol) in xylene (1
mL) under CO (1 atm) at 80 °C overnight.
yellow solid was collected and washed with ether to yield the desired
complex as an orange-yellow powder (51.0 mg, 92%). 1H NMR (400
MHz, CD3CN) δ = 8.81 (d, J = 8.6 Hz, 2H), 8.74 (m, 2H), 8.65 (m,
2H), 8.51 (m, 2H), 8.46 (d, J = 8.6 Hz, 2H), 7.93 (m, 2H), 7.84−7.74
(m, 3H), 7.61 (m, 2H), 1.96 (s, 12H, CH3CN). 13C NMR (100 MHz,
CD3CN) δ = 168.5, 158.1, 155.4, 154.3, 153.6, 147.7, 145.2, 144.1,
132.3, 132.2, 131.9, 130.5, 130.0, 124.6, 122.0, 118.6, 2.02. UV−vis
(MeOH) λmax (ε): 206 (50.7 × 103), 254 (30.0 × 103), 293 (18.2 ×
103), 407 (15.6 × 103), 428 (17.6 × 103) nm. Anal. Calcd for
C35H29B4F16N9Pd2·(H2O)2: C, 35.88; H, 2.84; N, 10.76. Found: C,
35.52; H, 2.51; N, 10.59.
Prepartion of [Pd2L(MeCN)2Cl2](BF4)2 (2). A mixture of L (10.3
mg, 0.025 mmol) and Pd(MeCN)3(Cl)(BF4) (18.5 mg, 0.053 mmol)
in a 10 mL reaction tube was flashed with nitrogen. Degassed
acetonitrile (1 mL) was syringed into the mixture, and the resulting
solution was stirred at 50 °C for 20 h. Upon addition of ether, the
desired complex was precipitated as an orange-yellow powder (19.8
1
mg, 91%). H NMR (400 MHz, CD3CN) δ = 9.34 (d, J = 6.9 Hz,
2H), 8.82 (d, J = 9.0 Hz, 2H), 8.62 (m, 2H), 8.49 (m, 4H), 7.92 (m,
2H), 7.86 (m, 3H), 7.64 (m, 2H), 1.96 (s, 6H, CH3CN). 13C NMR
(100 MHz, CD3CN) δ = 166.8, 158.8, 157.7, 154.3, 153.8, 145.8,
144.0, 132.5, 132.2, 132.0, 130.8, 130.5, 129.3, 125.9, 124.4, 121.7,
1.5. UV−vis (MeOH) λmax (ε): 205 (47.0 × 103), 221 (39.8 × 103),
258 (24.2 × 103), 295 (14.6 × 103), 407 (13.3 × 103), 427 (14.0 ×
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Supporting Information. CCDC numbers for these complexes are
2054364 (1), 2054364 (2), and 2054366 (4a). All crystallographic
data of complexes are deposited in the Supporting Information.
spectroscopy. J. Organomet. Chem. 2020, 923, 121409. (b) Correa-
Ayala, E.; Campos-Alvarado, C.; Chavez, D.; Hernandez-Ortega, S.;
Morales-Morales, D.; Miranda-Soto, V.; Parra-Hake, M. Dipalladium-
(I) complexes of ortho- and para-functionalized 1,3-bis(aryl)triazenide
ligands: Synthesis, structure and Catalytic activity. Inorg. Chim. Acta
2019, 490, 130−138. (c) Li, X.; Zhang, G.; Chao, M.; Cao, C.; Pang,
G.; Shi, Y. Synthesis and catalytic activity of chiral linker-bridged bis-
N-heterocyclic carbene dipalladium complexes. J. Chem. Res. 2018, 42,
320−325. (d) Ghorbani-Choghamarani, A.; Naghipour, A.; Babaee,
H.; Notash, B. Synthesis, crystal structure study and high efficient
Catalytic activity of di-μ-bromo-trans-dibromobis[(benzyl)(4-
methylphenyl)(phenyl)phosphine] dipalladium(II) in Suzuki-Miyaura
and Heck-Mizoroki C-C coupling reactions. Polyhedron 2016, 119,
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00226.
■
sı
Crystal data of complexes 1, [2·(CH3CN)], and [4a
(Et2O)(i-PrOH)]. Spectral data for catalysis products
(PDF)
Accession Codes
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CCDC 2054364−2054366 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
Shiuh-Tzung Liu − Department of Chemistry, National
Taiwan University, Taipei 10617, Taiwan; orcid.org/
0000-0002-0544-006X; Email: stliu@ntu.edu.tw
■
Authors
Shih-Chieh Aaron Lin − Department of Chemistry, National
Taiwan University, Taipei 10617, Taiwan
Bo-Kai Su − Department of Chemistry, National Taiwan
University, Taipei 10617, Taiwan
Yi-Hung Liu − Department of Chemistry, National Taiwan
University, Taipei 10617, Taiwan
Shie-Ming Peng − Department of Chemistry, National
Taiwan University, Taipei 10617, Taiwan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00226
Author Contributions
†S.C.A.L. and B.K.S. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Ministry of Science and Technology, Taiwan for
financial support (MOST109-2113-M-002-010-MY2). We also
thank Instrumentation Center (NTU), Ministry of Science and
Technology Taiwan for the assistance in X-ray crystallography.
The mass spectrometry technical research services from NTU
Consortia of Key Technologies for mass measurement is
acknowledged.
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