Hetero-Bimetallic Complexes Based on an Anthyridine Ligand Preparation and Catalytic Activity

Article abstract of DOI:10.1021/acs.organomet.9b00682

Complexation of anthyridine-based ligand L with [(η⁶-cymene)RuCl₂]₂, [Cp*RhCl₂]₂, and [Cp*IrCl₂]₂ yielded a mononuclear complex: [(N,N-L)Ru(η⁶-cymene)Cl]Cl (1), [(N,N-L)Rh(Cp*)Cl]Cl (2), and [(N,N-L)Ir(Cp*)Cl]Cl (3), respectively [L = 5-phenyl-2,8-di-2-pyridinylanthyridine]. Upon treatment with (CH₃CN)PdCl₂, complexes 1-3 underwent o-metalation to yield heterobimetallic complexes Ru-Pd, Rh-Pd, and Ir-Pd, respectively. Complexes were all characterized by spectroscopic method, and some are further confirmed by X-ray crystallography. Complex Ru-Pd exhibits catalytic activities for the tandem reactions of Suzuki-Miyaura coupling/transfer hydrogenation of p-bromoacetophenone with phenylboronic acid in isopropanol, whereas Ir-Pd shows a moderate activity. However, complex Rh-Pd does not behave the same way. Furthermore, catalytic activity of these heterobimetallic complexes toward debromination/transfer hydrogenation of p-bromoacetophenonewas also investigated. The catalytic pathways of these processes were studied and discussed. This study reveals the base used in the reactions plays an important role in the reaction pathway.

Full text of DOI:10.1021/acs.organomet.9b00682

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Hetero-Bimetallic Complexes Based on an Anthyridine Ligand
Preparation and Catalytic Activity
Shih-Chieh Aaron Lin, Yi-Hung Liu, Shie-Ming Peng, and Shiuh-Tzung Liu*
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
S
* Supporting Information
ABSTRACT: Complexation of anthyridine-based ligand L with [(η6-
cymene)RuCl2]2, [Cp*RhCl2]2, and [Cp*IrCl2]2 yielded a mononuclear
complex: [(N,N-L)Ru(η6-cymene)Cl]Cl (1), [(N,N-L)Rh(Cp*)Cl]Cl (2),
and [(N,N-L)Ir(Cp*)Cl]Cl (3), respectively [L = 5-phenyl-2,8-di-2-
pyridinylanthyridine]. Upon treatment with (CH3CN)PdCl2, complexes
1−3 underwent o-metalation to yield heterobimetallic complexes Ru−Pd,
Rh−Pd, and Ir−Pd, respectively. Complexes were all characterized by
spectroscopic method, and some are further confirmed by X-ray
crystallography. Complex Ru−Pd exhibits catalytic activities for the tandem
reactions of Suzuki−Miyaura coupling/transfer hydrogenation of p-
bromoacetophenone with phenylboronic acid in isopropanol, whereas Ir−
Pd shows a moderate activity. However, complex Rh−Pd does not behave the same way. Furthermore, catalytic activity of these
heterobimetallic complexes toward debromination/transfer hydrogenation of p-bromoacetophenonewas also investigated. The
catalytic pathways of these processes were studied and discussed. This study reveals the base used in the reactions plays an
important role in the reaction pathway.
exhibit excellent catalytic activity on various organic trans-
formations. In these studies, we also demonstrated that the
catalytic activity is benefited by the synergistic behavior
between the metal ions. Owing to the interest in tandem
reactions, we decided to extend our study to the preparation of
heterobimetallic complexes based on ligand L and the potential
catalytic activity in tandem reactions.
INTRODUCTION
■
The use of bimetallic complexes as homogeneous catalysts to
increase reactivity or create new activity via a “cooperative”
interaction between metal ions or substrates has received much
attention in recent years.1 Among this context, heterobimetallic
complexes are of significant interest in the area of catalytic
tandem processes due to the presence of two different metal
ions for carrying out two (more) possible different trans-
formations, offering a desired product via multiple trans-
formations in a one-pot fashion.2−5 In order to reach the
mentioned activity, it is important to have the metal ions
accommodated in a rigid scaffold with a spatial distance
allowing the communication of metal ions.2
Over the past decade, we have reported the preparation of
homobimetallic complexes based on 5-phenyl-2,8-bis(2-pyr-
idinyl)-1,9,10-anthyridine (L) and 5-phenyl-2,8-bis(6′-bipyr-
idinyl)-1,9,10-anthyridine (L′) ligands (Scheme 1) with the
metal ions of nickel, copper, and ruthenium.6 The metal ions
are separated by about 5 Å, which is suitable for the
cooperative interaction.6,7 Indeed, these bimetallic complexes
RESULTS AND DISCUSSION
■
Preparation and Characterization of Heterobimetal-
lic Complex Ru−Pd. Complexation of L with an equimolar
amount of [Ru(η6-cymene)Cl2]2 in a refluxing acetonitrile
solution gave the mononuclear ruthenium complex as orange-
1
red solid in 81% yield (Scheme 2). H NMR spectrum of 1
reveals that the anthyridine protons of the ligand L are split
into four sets of doublets, clearly matching the unsymmetrical
nature of the proposed structure. The aromatic protons of η6-
cymene moieties appear as four sets of doublets and the methyl
protons of the isopropyl group exhibit as two sets of doublets,
which can be ascribed to the hindered rotation of the cymene
ring in the complex. ESI-HRMS of 1 in CH3CN matrix shows
a peak at m/z = 682.1318, which is consistent with the formula
of [1 − Cl] + as a mononuclear species. However, the detailed
structure of 1 is further revealed by single-crystal X-ray
crystallographic analysis.
Scheme 1. Anthyridine-Based Polydentates
Treatment of 1 with (CH3CN)2PdCl2 in DMF readily
provided the dinuclear complex Ru−Pd as orange-red solid.
Received: October 12, 2019
© XXXX American Chemical Society
A
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Scheme 2. Preparation of Bimetallic Complex Ru−Pd
The complex was purified by crystallization and characterized
by NMR and Mass spectroscopy. To our surprise, the
coordination of Pd toward L is not in a bidentate N∧N
mode but rather a C∧N mode as evidenced by NMR
1
spectroscopic analysis. The H NMR spectrum of Ru−Pd
shows the disappearance of the anthyridine protons at C7,
whereas 13C NMR spectrum exhibits a downfield shift at 172.9
ppm, which is attributed to the Pd−C moiety, i.e., the
coordination of carbon ligand to the metal center. Apparently,
coordination of the ligand to the palladium ion readily causes
the C−H activation to yield the palladacycle, not a simple N∧N
coordination. ESI-HRMS of Ru−Pd in MeOH matrix shows a
peak at m/z = 821.9981, which is in agreement with the
+
formula of [(Ru−Pd) − Cl] .
Figure 1. ORTEP plot of cationic portion of 1 (30% probability level,
labels of some aromatic carbons are omitted for clarity).
The measurement of conductivity of Ru−Pd in DMF gave
the values of ∼0 Ω−1 cm2 mol−1, indicating the neutral
property of the complex in solution. The Ru(II) center of Ru−
Pd retains its cationic nature, similar to that in complex 1. The
Pd(II) center is surrounded by a carbon ligand, two chlorides,
and a pyridinyl-nitrogen donor, exhibiting an anionic form of
palladate. Overall, this molecule shows a zwitterionic complex,
consistent with the conductivity measurement.
In addition to the spectroscopic characterization, the
detailed structural formulations of 1 and Ru−Pd were
confirmed by single-crystal X-ray studies. ORTEP plots of
the cationic portion of 1 and Ru−Pd are displayed in Figures 1
and 2, respectively, whereas selected bond lengths and angles
are listed in Tables 1 and 2. The coordination geometry
features of Ru centers in a three-legged piano-stool arrange-
ment in both 1 and Ru−Pd are similar to those found in
related complexes, such as [(η6-benzene)RuCl(N∧N)]Cl. The
p-cymene ring exhibits the common η6 coordination mode and
occupies three facial positions in the metal environment.6a The
Ru−Carene bond lengths exhibit slightly dissimilar distances, but
these are in the normal range (average 2.195 and 2.212 Å for 1
and Ru−Pd, respectively). The chloride ligand occupies an
additional coordination site with Ru−Cl bond distances in
2.407(2) and 2.390(2) Å for 1 and Ru−Pd, respectively.
For the Pd(II) environment in Ru−Pd is approximately
square planar with the “Pd(C∧N)” moiety and two chloride
ligands around the metal center, which makes the anionic
palladium center. The narrow bite angle of N(5)−Pd(1)−
Figure 2. ORTEP plot of Ru−Pd (30% probability level).
C(15) [82.72(19)°] is comparable to those found in other
orthopalladated complexes.8 The Pd(1)−Cl(3) bond trans to
C(15) [2.404(1) Å] is significantly longer than the other
Pd(1)−Cl(2) bond [2.299(1) Å], owing to the trans influence
of carbon donor versus nitrogen donor.
Preparation and Characterization of Complexes Ir−
Pd and Rh−Pd. A similar strategy was adopted to prepare
complexes of Rh−Pd and Ir−Pd (Scheme 3). Complexation
of L with [Cp*MCl2]2 (M = Rh and Ir) gave the
corresponding mono nuclear complexes 2 and 3, respectively.
These monometallic complexes were used as precursors for the
B
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Table 1. Selected Bond Distances (Å) and Angles (deg) of
Complex 1
Ru(1)−N(1)
Ru(1)−N(2)
Ru(1)−Cl(1)
Ru(1)−C(29)
Ru(1)−C(30)
Ru(1)−C(31)
Ru(1)−C(32)
Ru(1)−C(33)
Ru(1)−C(34)
2.066(7)
2.104(7)
2.407(2)
2.215(10)
2.186(9)
2.180(9)
2.212(8)
2.166(9)
2.211(9)
N(2)−Ru(1)−N(1)
Cl(1)−Ru(1)−N(2)
Cl(1)−Ru(1)−N(1)
N(2)−Ru(1)−C(29)
N(2)−Ru(1)−C(30)
N(1)−Ru(1)−C(29)
N(1)−Ru(1)−C(30)
76.8(3)
84.59(19)
85.52(19)
143.2(4)
108.9(3)
139.1(4)
170.6(3)
Figure 3. ORTEP plot of Ir−Pd (30% probability level).
Table 2. Selected Bond Distances (Å) and Angles (deg) of
Ru−Pd, Rh−Pd, and Ir−Pd
Ru−Pd
Rh−Pd
Ir−Pd
there is no significant deviation of the structural parameters
around the palladium centers except the angle of Cl(2)−
Pd(1)−Cl(3), presumably due to the crystal packing.
complex
M(1)−N(1)
M(1)−N(2)
M(1)−Cl(1)
Pd(1)−C(15)
Pd(1)−N(5)
Pd(1)−Cl(2)
Pd(1)−Cl(3)
N(2)−M(1)−N(1)
Cl(1)−M(1)−N(2)
Cl(1)−M(1)−N(1)
Cl(2)−Pd(1)−Cl(3)
C(15)−Pd(1)−Cl(2)
C(15)−Pd(1)−Cl(3)
N(5)−Pd(1)−C(15)
(M = Ru)
(M = Rh)
(M = Ir)
2.083(4)
2.114(4)
2.3992(13)
1.978(5)
2.043(4)
2.299(1)
2.404(1)
76.20(15)
83.27(11)
86.10(11)
91.49(5)
92.33(16)
176.17(16)
82.72(19)
2.086(8)
2.126(7)
2.397(2)
1.969(10)
2.045(8)
2.311(3)
2.400(3)
75.8(3)
87.20(19)
84.9(2)
90.92(9)
92.0(3)
2.073(5)
2.121(5)
2.4019(18)
1.975(6)
2.039(5)
2.3095(16)
2.4077(17)
76.2(2)
87.57(15)
86.54(18)
92.75(6)
92.45(18)
173.9(2)
81.8(2)
Catalysis. Similar to the reported works in tandem
reactions of Suzuki−Miyaura coupling followed by transfer
hydrogenation catalyzed by a heterobimetallic Ru(II)−Pd(II)
complex,3a,5b,e we expected that the newly prepared complexes
in this work should have a similar activity. Accordingly, the use
of complex Ru−Pd as the catalyst was investigated in coupling
of p-bromoacetophenone with phenylboronic acid in the
presence of base leading to p-phenylacetophenone (I) and 1-
(4-biphenyl)ethanol (II)(Table 3).
By screening of various bases, it appears that the
concentration of sodium hydroxide at 1.6 M is the best choice
to reach the final product II in 97% yield (Table 3, entry 4). It
is noticed that the reaction gave exclusively the coupling
product when Cs2CO3 was used as the base, i.e., the carbonyl
functionality remains intact (entry 1). With t-butoxide as the
base in the reaction, a mixture of various products were
obtained, including the direct reduction and debromination/
transfer hydrogenation of p-bromoacetophenone to yield 1-(p-
bromophenyl)ethanol and 1-phenylethanol, respectively. Car-
rying out the reaction at temperatures of 70 or 80 °C is
suitable (entry 6) but incomplete at 60 °C (entry 7). We note
that the amount of base used in this catalysis appears to be
critical for the product distribution (entries 8−11). With a
lower concentration of NaOH in the reaction, the coupling
reaction did not proceed; instead, 1-phenylethanol (debromi-
nation product, 22%) and 1-(p-bromophenyl)ethanol (reduc-
tion of keto function, 18%) were observed (entry 8). At the
concentration of [NaOH] = 0.8 M, the reaction cleanly yielded
the Suzuki−Miyaura coupling product within 2.5 h (entry 9).
However, production of II (coupling/transfer hydrogenation
product) increases as the loaded amount of NaOH increases
177.1(3)
81.8(4)
preparation of heterobimetallic complexes. Thus, reactions of 2
and 3 with an equimolar amount of (CH3CN)2PdCl2 in DMF
readily afforded desired bimetallic complexes Rh−Pd and Ir−
Pd in excellent yields, respectively. All complexes including 2,
3, Rh−Pd, and Ir−Pd were characterized by NMR and mass
spectroscopy methods. In addition, single crystals of 2, Rh−
Pd, and Ir−Pd were obtained for X-ray diffraction analyses to
confirm their structural details.
The structures of complexes Rh−Pd and Ir−Pd are
isomorphous, similar to that of Ru−Pd. The complex Ir−Pd,
as typical, crystallizes in the monoclinic space group C2/c. The
chelations of ligand L with two metal ions are favored by the
stable five-member ring of metallocycles, as shown by the
ORTEP diagram in Figure 3. Crystallographic details of Rh−
Pd and 2 are included in the Supporting Information. Selected
bond distances and bond angles of these complexes are
summarized in Table 2 for comparison. As shown in the table,
Scheme 3. Preparation of Heterobimetallic Complexes Rh−Pd and Ir−Pd
C
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a,b
Table 3. Suzuki Coupling/Transfer Hydrogenation of p-Bromoacetophenone
yield (%)
c
entry
1
2
cat. (mol %)
base
temp.
time
conv.
I
II
Ru−Pd (1)
Ru−Pd (1)
Ru−Pd (1)
Ru−Pd (1)
Ru−Pd (0.5)
Ru−Pd (1)
Ru−Pd (1)
Ru−Pd (1)
Ru−Pd (1.5)
Ru−Pd (1.5)
Ru−Pd (1.5)
Cs2CO3 (1.2 M)
KOtBu (1.2 M)
NaOH (1.2 M)
NaOH (1.6 M)
NaOH (1.6 M)
NaOH (1.6 M)
NaOH (1.6 M)
NaOH (0.08 M)
NaOH (0.8 M)
NaOH (1.6 M)
NaOH (2.4 M)
NaOH (1.6 M)
80 °C
80 °C
80 °C
80 °C
80 °C
70 °C
60 °C
80 °C
80 °C
80 °C
80 °C
80 °C
20 h
20 h
20 h
20 h
20 h
20 h
20 h
2.5 h
2.5 h
2.5 h
2.5 h
20 h
100%
80%
95%
16%
5%
d
18%
93%
97%
80%
96%
78%
3
4
5
6
7
100%
100%
100%
100%
100%
40%
100%
100%
100%
56%
15%
17%
e
8
9
100%
30%
6%
10
11
12
57%
74%
f
a
Reaction conditions: mixture of 4-bromoacetophenone (0.4 mmol), phenylboronic acid (0.6 mmol), base, and Ru−Pd in i-PrOH (1 mL) for 20
b
c
d
h. NMR yields based on the average of two runs. Based on the amount of p-bromoacetophenone. Accompanied with 1-(p-bromophenyl)ethanol
e
(24%) and 1-phenylethanol (14%) as the side product. 1-(p-Bromophenyl)ethanol (18%) and 1-phenylethanol (22%) as the observed products.
f
1-(p-Bromophenyl)ethanol (56%) was obtained.
(entries 10−11), which is expected for an Oppenauer-type
reaction.
Scheme 4. Pathways of Tandem Reactions of p-
Bromoacetophenone with PhB(OH)2
Monitoring the reaction of p-bromoacetophenone with
phenylboronic acid catalyzed by Ru−Pd was studied to learn
more insight into the reaction pathway (Figure 4). It appears
ment, we found that the reaction of A with PhB(OH)2
catalyzed by Ru−Pd under the optimized conditions rendered
coupling product II in 43% yield after 20 h. Obviously, the rate
of the coupling reaction of PhB(OH)2 with p-bromoaceto-
phenone is much faster than that with p-(bromophenyl)ethan-
1-ol (A), presumably due to the electronic effect of the
substrate, as expected. This observation is consistent with
other reported work.2,5
We also examined the activity of Ru−Pd on debromination/
transfer hydrogenation of 4-bromoacetophenone, i.e., in the
absence of phenylboronic acid. Optimization of this reaction
catalyzed by Ru−Pd is summarized in Table 4. Indeed, three
possible products A−C are expected in this catalysis. Similar to
the above coupling reaction, debromination/transfer hydro-
genation is also sensitive the base used. The larger the amount
of NaOH used, the higher the yield of debromination/transfer
hydrogenation obtained (Table 4, entries 1−4). However,
debromination proceeded cleanly to render acetophenone (B),
when Cs2CO3 was adopted to be the base in the reaction
(entry 6), similar to the above coupling reaction (Table 3,
entry 1), indicating that the presence of Cs2CO3 in the
reaction does not favor the carbonyl reduction. As the catalyst
loading increased up to 1.5 mol %, production of C reached
100% (Table 4, entry 8).
Figure 4. Reaction kinetics for tandem Suzuki coupling/transfer
hydrogenation of p-bromoacetophenone. Reaction conditions: p-
bromoacetophenone (0.4 mmol), PhB(OH)2 (0.6 mmol), NaOH
(1.6 mmol), and Ru−Pd (1 mol %) in i-PrOH (1 mL) at 80 °C.
that complex Ru−Pd is quite active for the coupling reaction.
There is no significant induction period observed for this
catalysis. Within 5 min, p-bromoacetophenone was fully
consumed and converted into p-phenylacetophenone I
(91%) and p-(bromophenyl)ethan-1-ol A (9%). Subsequently,
initial product I was slowly reduced to give II, whereas
Suzuki−Miyaura coupling of A with phenylboronic acid
occurred slowly to yield II (Scheme 4). This observation
clearly suggests the major pathway of this tandem reactions via
coupling followed by transfer hydrogenation, which is
consistent with the other studies.2a,d,e,5e In a parallel experi-
With observation of the different product distribution, we
tried to understand more about the mechanism of this catalytic
process. Thus, studying the kinetics of the resultant reaction
profile for the reduction of p-bromoacetophenone catalyzed by
D
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a
Table 4. Debromination and Transfer Hydrogenation of 4-Bromoacetophenone
b
yield
entry
cat. (mol %)
base
conv.
A
B
C
c
1
Ru−Pd (1)
Ru−Pd (1.5)
Ru−Pd (1)
Ru−Pd (1)
Ru−Pd (1)
Ru−Pd (1)
Ru−Pd (1)
Ru−Pd (1.5)
Ru−Pd (1.5)
Ru−Pd (1.5)
Ru−Pd (1.5)
NaOH (0.08 M)
NaOH (0.2 M)
NaOH (0.4 M)
NaOH (0.8 M)
NaOH (1.6 M)
Cs2CO3(0.8 M)
KOtBu (0.8 M)
NaOH (0.8 M)
NaOH (0.8 M)
K2CO3(0.8 M)
Na2CO3(0.8 M)
40%
78%
94%
100%
100%
89%
18%
36%
66%
19%
23%
22%
30%
2
3
4
5
6
7
8
5%
21%
77%
73%
82%
99%
61%
10%
19%
100%
89%
100%
100%
15%
d
9
10
11
13%
7%
trace
10%
a
b
Reaction conditions: a mixture of 4-bromoacetophenone (0.4 mmol), base, and catalyst in i-PrOH (1 mL) for 20 h at 80 °C. NMR yields based
c
d
on the average of two runs. 2.5 h. 15 h.
Ru−Pd under the optimized conditions was performed
(Figure 5). Again, the catalyst is quite active for the catalysis,
debromination takes place in the first step.2 We believe that the
base plays a key role in leading the reaction pathway. As shown
in Table 4, at a lower concentration of NaOH, both
debromination and hydrogen-transfer reduction are compet-
itive pathways (Table 4, entries 1−2). Also, a complete
debromination of p-bromoacetophenone took place when
Cs2CO3 was used as the base. Apparently, the hydrogen-
transfer reduction is favored under the condition of a higher
concentration of NaOH.
In order to realize more about the sequence of this relay
catalysis, we monitored the reactions of a mixture of p-
(bromophenyl)ethan-1-ol A and acetophenone B under the
catalytic conditions [NaOH] = 0.4 and 0.8 M, respectively.
These results are shown in Figure 6. Obviously, the hydrogen
transfer reduction of acetophenone proceeded much faster
than the debromination under the NaOH concentration at 0.4
M (Figure 6, blue line). However, by doubling the
concentration of NaOH in the same reaction, both hydro-
gen-transfer reduction of acetophenone and debromination of
p-(bromophenyl)ethan-1-ol leading to C proceeded compet-
itively (Figure 6, red line). Furthermore, the completion of
reaction is achieved within 5 h. Obviously the concentration of
NaOH readily affects the reaction dramatically; the higher
concentration of base increases the amount of isopropoxide,
favoring the hydride formation then accelerating the
dehalogenation process.5b,9
Comparison of Various Metal Complexes in Catalytic
Activity. With a series of new heterobimetallic complexes in
hand, their catalytic activities on the tandem reactions
described in the previous section were also examined. For
comparison, a few monometallic complexes were also prepared
according to the reported methods (Figure 7).
As can be seen from Table 5, the best results regarding the
production of the desired coupling/transfer hydrogenation
compound II were obtained by using complexes Ru−Pd and
Ir−Pd (entries 1 and 3) as catalysts. Mononuclear palladium
complexes [Pd(ppy)Cl]2 and Pd(bpy)Cl2 show good activity
Figure 5. Reaction profile for debromination/transfer hydrogenation
of p-bromoacetophenone. Reaction conditions: p-bromoacetophe-
none (0.4 mmol), NaOH (0.8 mmol), and Ru−Pd (1.5 mol %) in i-
PrOH (1 mL) at 80 °C.
not requiring a significant induction period. The amount of
substrate decreased dramatically within the first hour to
generate intermediate A accompanied by trace amount of final
product C (less than 10%). Interestingly, the reaction is very
clean even without any formation of the other possible
intermediate, B (debromination product). The hydrogen
transfer reduction appears to be faster as seen by the formation
of p-(bromophenyl)ethan-1-ol (A) in a maximum yield of 80%
after 2.5 h of reaction. The production of 1-phenylethanol (C)
is gradual, starting from the beginning of the catalysis. To our
surprise, the major pathway of tandem reaction in this work is
different from the observations by other groups, in which the
E
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mixtures of related mononuclear complexes. A significant
decrease in catalytic activity of mixtures of two monometallic
complexes [Pd(ppy)Cl]2/1 (entry 4) and Pd(bpy)Cl2/[Ru-
(bpy)(p-cymene)Cl]Cl (entry 5) on the production of II as
compared to the activities of Ru−Pd and Ir−Pd. In this study,
we also learned that the ruthenium complex does assist the
hydrogen transfer reaction, but it is inactive to the coupling
reaction (entry 8). Unlike Ru−Pd and Ir−Pd, complex Rh−
Pd gave an unsatisfied outcome on the Suzuki−Miyaura
coupling reaction. For comparison, the use of a mixture of
[Pd(ppy)Cl]2 and [Rh(bpy)(Cp*)Cl]Cl as the catalyst
delivered 80% of coupling/transfer hydrogenation product II.
Apparently, the rhodium center in Rh−Pd seems to be the key
to the deactivation of the complex in the Suzuki−Miyaura
coupling reaction. All these observations distinctly indicates
the effect of the heterobimetallic complexes, which might be
due to the cooperative interaction of metal ions. However, at
this moment, we do not understand how the interaction
operates in the molecules, particularly the deactivation of
rhodium ion in Rh−Pd on the coupling reaction. By
comparison of 13C NMR shifts of Pd−C in these three
heterobimetallic complexes, there is no significant difference.
Even in the structural parameters in crystallography around the
palladium centers of these three complexes (Table 2), no
obvious difference is observed.
Figure 6. Reaction profile for a mixture of A and B catalyzed by Ru−
Pd at a concentration of NaOH 0.4 M (blue line) and 0.8 M (red
line). Reaction conditions: p-bromoacetophenone (0.4 mmol),
acetophenone (0.4 mmol), NaOH, and Ru−Pd (1.5 mol %) in i-
PrOH (1 mL) at 80 °C.
For the debromination/transfer hydrogenation, the catalytic
activities of three heterobimetallic complexes behave quite
similar to those for the tandem coupling/transfer hydro-
genation. Both Ru−Pd and Ir−Pd demonstrate excellent
activity on this tandem debromination/transfer hydrogenation
(Table 6, entries 1 and 3), but Rh−Pd shows poor activity
Figure 7. Structures of mononuclear complexes used as catalysts.
Table 6. Debromination/Transfer Hydrogenation Catalyzed
by Various Complexes
a
Table 5. Coupling/Transfer Hydrogenation Catalyzed by
a
b
Various Complexes
yields (%)
b
entry
metal complex (mol %)
Ru−Pd (1.5 mol %)
Rh−Pd (1.5 mol %)
Ir−Pd (1.5 mol %)
[Pd(ppy)Cl]2(0.75) + [Ru(bpy)(p-cymene)Cl]Cl 10
(1.5)
[Pd(ppy)Cl]2(0.75)
Pd(bpy)Cl2 (1.5)
A
B
C
yields (%)
1
2
3
4
100
29
78
entry
metal complex (mol %)
Ru−Pd (1)
Rh−Pd (1)
Ir−Pd (1)
[Pd(ppy)Cl]2(0.5) + 1 (1)
A
C
I
II
67
15
1
2
3
4
5
96
19
82
25
52
48
34
18
11
14
87
3
5
6
7
8
11
37
30
21
86
57
65
73
Pd(ppy)Cl]2(0.5) + [Ru(bpy)(p-cymene)
Cl]Cl (1)
24
[Pd(ppy)Cl]2(0.75) + Rh(bpy)(Cp*)Cl2 (1.5)
[Pd(ppy)Cl]2(0.75) + Ir(bpy)(Cp*)Cl2 (1.5)
6
7
8
9
[Pd(ppy)Cl]2(1)
Pd(bpy)Cl2 (1)
[Ru(bpy)(p-cymene)Cl]Cl (1)
[Pd(ppy)Cl]2(0.5) + [Rh(bpy)(Cp*)Cl]
Cl (1)
[Pd(ppy)Cl]2(0.5) + [Ir(bpy)(Cp*)Cl]
Cl (1)
10
12
62
38
18
59
10
7
a
Reaction conditions: a mixture of 4-bromoacetophenone (0.4
mmol), NaOH (0.8 mmol) and complex in i-PrOH (1 mL) at 80
°C for 20 h. NMR yields.
5
5
80
72
b
10
12
11
12
[Pd(ppy)Cl]2(0.5) + 2 (1)
[Pd(ppy)Cl]2(0.5) + 3 (1)
37
22
33
30
23
46
(Table 6, entry 2). The results obtained from the related
mononuclear complexes and mixtures of two monometallic
complexes (Table 6, entries 4−10) highlight the effect of
heterobimetallic effect on the catalysis.
a
Reaction conditions: a mixture of 4-bromoacetophenone (0.4
mmol), phenylboronic acid (0.6 mmol), NaOH (1.6 mmol) and
complex in i-PrOH (1 mL) for 20 h. NMR yields based on the
b
Finally, we screened various arylbromides and arylboronic
acid under Ru−Pd catalyzed reactions in isopropanol (Scheme
5). General speaking, various arylboronic acid underwent the
coupling with p-bromoacetophenone smoothly even with the
electron-withdrawing substituents such as halogens. p-
Bromobenzophenone proceeded the coupling reaction with
PhB(OH)2 followed by transfer hydrogenation giving 92%
yield.
average of two runs.
on Suzuki−Miyaura coupling reactions (Table 5, entries 6 and
7), whereas [Ru(bpy)(p-cymene)Cl]Cl exhibits activity on
hydrogen-transfer reduction (Table 5, entry 8). For evaluation
the effect of heterobimetallic complexes, we also compared the
catalytic activities of the dinuclear complexes to the activity of
F
DOI: 10.1021/acs.organomet.9b00682
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
a
Scheme 5. Various Substrates in Tandem Reactions of Coupling/Transfer Hydrogenation
a
Reaction conditions: mixture of ArBr (0.4 mmol), ArB(OH)2 (0.6 mmol), NaOH (1.6 mmol), and Ru−Pd (1 mol %) in 2-propanol (1 mL);
isolated yield.
Scheme 6. Reactivity of 1-(p-Bromophenyl)acetone in the Tandem Reactions
One would expect that the reactivity of 1-(p-bromophenyl)-
acetone in the tandem Suzuki−Miyaura coupling/transfer
hydrogenation or debromination/transfer hydrogenation is
lower than that of p-bromoacetophenone (Scheme 6). Indeed,
under the optimized conditions in both tandem reactions
studied for p-BrC6H4COCH3, 1-(p-bromophenyl)acetone gave
various side products and lower yields of the desired products.
Again, the deactivated aryl bromide should lead to a decrease
in the oxidative addition and transmetalation, then favoring the
transfer hydrogenation.
EXPERIMENTAL SECTION
■
General Information. Chemicals and solvents were of analytical
grade and were used without further purification. Nuclear magnetic
resonance spectra were recorded on a 400 MHz spectrometer.
Chemical shifts are given in parts per million relative to Me4Si for 1H
and 13C NMR. Ligand L was prepared according to the reported
methods.6d
Complex 1. A mixture of L (79.6 mg, 0.19 mmol) and [Ru(η6-
cymene)Cl2]2 (53.3 mg, 0.087 mmol) in a 25 mL round-bottomed
flask was flashed with nitrogen. Anhydrous acetonitrile (5 mL) was
syringed into the mixture, and the resulting solution was heated under
refluxing conditions for 16 h. Upon cooling, ether was added to the
reaction mixture to yield orange precipitate, which was reprecipitated
from ether/dichloromethane three times to give the desired complex
as orange-red solid (110 mg, 81%). Recrystallization from ether/
methanol gave the complex as crystalline solid, which is suitable for X-
CONCLUSIONS
■
In summary, we reported the preparation and characterization
of a series of bimetallic complexes bearing Ru−Pd, Rh−Pd
and Ir−Pd, via sequential complexation of metal ions with an
anthyridine ligand L. Among these complexes, Ru−Pd was
found highly active in tandem Suzuki−Miyaura/transfer
hydrogenation reaction of bromoacetophenones and boronic
acids with a high selectivity. In addition, Ru−Pd also shows an
excellent activity in debromination/transfer hydrogenation of
bromoacetophenone. By comparison study, it was revealed that
the bimetallic catalyst is superior to the combination of the
related monometallic species, leading to a higher selectivity in
the tandem reaction. The nature of the base strongly affects the
selectivity and pathway of the tandem reaction. Further
investigations are currently underway to extend the use of
bimetallic catalysts in other tandem transformations.
1
ray crystallographic analysis. H NMR (400 MHz, DMSO-d6) δ =
9.86 (d, J = 5.3 Hz, 1H), 9.04 (d, J = 8.0 Hz, 1H), 8.99 (d, J = 7.9 Hz,
1H), 8.93−8.84 (m, 2H), 8.81 (d, J = 9.1 Hz, 1H), 8.51−8.38 (m,
3H), 8.24 (t, J = 7.6 Hz, 1H), 8.01 (t, J = 6.5 Hz, 1H), 7.85−7.75 (m,
3H), 7.74−7.64 (m, 3H), 6.78 (d, J = 5.9 Hz, 1H), 6.71 (d, J = 5.8
Hz, 1H), 6.51 (d, J = 5.9 Hz, 1H), 6.19 (d, J = 5.9 Hz, 1H), 2.75−
2.63 (m, 1H), 2.43 (s, 1H), 1.03 (d, J = 6.3 Hz, 3H), 0.91 (d, J = 6.3
Hz, 3H). 13C NMR (100 MHz) δ = 162.8, 160.9, 156.8, 155.4, 154.8,
153.9, 153.4, 153.0, 149.9, 140.1, 139.9, 138.0, 137.8, 132.7, 130.9,
130.5, 129.9, 129.11, 129.10, 129.0, 127.3, 126.3, 122.5, 121.5, 121.3,
120.7, 119.7, 89.3, 87.8, 85.6, 83.1, 30.5, 22.0, 21.7, 18.9. HRMS (ESI,
m/z) [M − Cl]+ calcd for C37H31ClN5Ru, 682.1313. Found,
682.1318. UV−vis (MeOH) λmax (ε) 204 (58.4 × 103), 251 (43.3
× 103), 293 (38.2 × 103), 386 (31.3 × 103), 406 (42.9 × 103), 470
(4.8 × 103) nm.
G
DOI: 10.1021/acs.organomet.9b00682
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Complex 2. A mixture of L (9.9 mg, 0.024 mmol) and
[RhCp*Cl2]2(7.4 mg, 0.012 mmol) in a 10 mL reaction tube was
flashed with nitrogen. Dichloromethane (1 mL) was syringed into the
mixture, and the resulting solution was stirred at ambient temperature
for 16 h. Upon addition of ether, the desired complex was precipitated
HRMS (ESI, m/z) [M − Cl]+ calcd for C37H31Cl2N5PdRh, 824.0041.
Found, 824.0028. [M − 2Cl + H2O]2+ calcd for C37H33ClN5OPdRh,
403.5226. Found, 403.5244. UV−vis (DMF) λmax (ε) 268 (ε = 30.8 ×
103), 292 (ε = 32.2 × 103), 347 (ε = 26.7 × 103), 445 (ε = 17.7 ×
103) nm.
1
as orange solid (15.8 mg, 91%). H NMR (400 MHz, CDCl3) δ =
Complex Ir−Pd. The procedure for preparation of this complex is
similar to that of Rh−Pd. Yield: 20.5 mg, 92%. 1H NMR (400 MHz,
DMSO-d6) δ = 9.26 (d, J = 5.2 Hz, 1H), 9.02 (d, J = 8.1 Hz, 1H),
8,75 (d, J = 9.2 Hz, 1H), 8.56 (brs, 1H), 8.52−8.32 (m, 5H), 8.04−
7.97 (m, 1H), 7.86−7.78 (m, 1H), 7.79−7.68 (m, 3H), 7.66−7.54
(m, 2H), 1.78 (s, 15H); 13C NMR (100 MHz, DMF-d7) δ = 172.8,
161.2, 160.4, 156.6, 155.0, 153.7, 151.4, 151.0, 150.8, 141.22, 141.16,
140.6, 140.1, 134.1, 131.2, 130.9, 130.2, 130.0, 129.7, 129.05, 129.0,
127.6, 127.2, 123.6, 122.9, 122.0, 119.5, 90.8. HRMS (ESI, m/z) [M
− Cl]+ calcd for C37H31Cl2IrN5Pd, 914.0615. Found, 914.0590. [M −
2Cl + H2O]2+ calcd for C37H33ClN5OPdIr, 448.5513. Found,
448.5534. UV−vis (DMF) λmax (ε) 290 (ε = 34.8 × 103), 346 (ε =
25.5 × 103), 406 (ε = 15.9 × 103), 449 (ε = 16.3 × 103) nm.
Catalysis: Suzuki−Miyaura Coupling/Transfer Hydrogena-
tion of p-Bromoacetophenone with Arylboronic Acid. A
mixture of 4-bromoacetophenone (0.4 mmol), phenylboronic acid
(0.6 mmol), base, and catalyst in isopropanol (1 mL) was placed in a
reaction tube. The reaction vessel was flushed with nitrogen gas and
then heated at 80 °C for a period of time. After the reaction, the
solvent was removed under reduced pressure. The residue was
analyzed by NMR spectroscopy. For the purification, chromatography
on silica gels provided the desired compound in the pure form. The
spectral data of the organic products are essentially identical to those
reported ones.
Catalysis: Debromination/Transfer Hydrogenation of p-
Bromoacetophenone. A mixture of 4-bromoacetophenone (0.4
mmol), base, and catalyst in isopropanol (1 mL) was placed in a
reaction tube. The reaction vessel was flushed with nitrogen gas and
then heated at 80 °C for a period of time. After the reaction, the
purification and identification is similar to that for coupling/transfer
hydrogenation described above.
General Kinetic Procedures. The procedure is similar to that
described for the catalysis. However, at appropriate time intervals, 0.1
mL aliquots were removed using a syringe and quickly passed through
Celite to remove the metal complexes with elution of ether. The
filtrate was then concentrated under reduced pressure and analyzed by
1H NMR spectroscopy.
9.67 (d, J = 8.0 Hz, 1H), 9.42 (d, J = 9.2 Hz, 1H), 8.98 (d, J = 5.1 Hz,
1H), 8.83 (m, 3H), 8.63 (d, J = 9.0 Hz, 1H), 8.43 (t, J = 7.4 Hz, 1H),
8.32 (d, J = 9.0 Hz, 1H), 7.98 (m,1H), 7.83 (t, J = 6.5 Hz, 1H), 7.71
(m, 3H), 7.55 (m, 1H), 7.50 (m, 1H), 7.36 (m, 1H), 1.90 (s, 15H).
13C NMR (100 MHz) δ = 163.4, 161.1, 156.2, 155.5, 154.8, 153.5,
152.5, 151.9, 149.6, 141.8, 141.2, 137.3, 137.2, 132.7, 130.5, 130.3,
130.1, 129.5, 129.3, 129.2, 129.0, 125.7, 122.9, 122.0, 121.9, 121.3,
98.1 (d, JRh−C = 32.8 Hz). HRMS (ESI, m/z) [M − Cl]+ calcd for
C37H32ClN5Rh, 684.1396. Found, 684.1374. [M − 2Cl]2+ calcd for
C37H32N5Rh, 324.5851. Found, 324.5865. UV−vis (DMF) λmax (ε)
267 (ε = 25.8 × 103), 297 (ε = 26.8 × 103), 406 (ε = 25.6 × 103) nm.
Complex 3. A mixture of L (9.7 mg, 0.024 mmol) and
[Cp*IrCl2]2(9.4 mg, 0.012 mmol) in a 10 mL reaction tube was
flashed with nitrogen. Dichloromethane (1 mL) was syringed into the
mixture and the resulting solution was stirred at ambient temperature
for 18 h. Upon addition of ether, the desired complex was precipitated
as orange-red powder (17.7 mg, 93%). 1H NMR (400 MHz, CDCl3)
δ = 9.90 (d, J = 7.6 Hz, 1H), 9.65 (d, J = 9.1 Hz, 1H), 8.94 (d, J = 5.3
Hz, 1H), 8.83 (m, 3H), 8.61 (d, J = 9.0 Hz, 1H), 8.42 (t, J = 7.5 Hz,
1H), 8.32 (d, J = 9.0 Hz, 1H), 7.99 (m,1H), 7.78 (m, 1H), 7.70 (m,
3H), 7.53 (m, 1H), 7.49 (m, 1H), 7.37 (m, 1H), 1.88 (s, 15H). 13C
NMR (100 MHz, DMSO-d6) δ = 162.8, 161.3, 155.5, 155.4, 154.0,
153.6, 153.4, 151.7, 149.9, 141.2, 140.5, 138.14, 138.10, 132.7, 130.9,
130.6, 130.4, 129.9, 129.6, 129.1, 127.4, 126.4, 122.4, 121.7, 121.3,
121.1, 120.0, 90.3. HRMS (ESI, m/z) [M − Cl]+ calcd for
C37H32ClN5Ir, 774.1970. Found, 774.1950. [M − 2Cl]2+ calcd for
C37H32N5Ir, 369.6138. Found, 369.6171. UV−vis (DMF) λmax (ε)
267 (ε = 31.1 × 103), 296 (ε = 35.8 × 103), 390 (ε = 25.6 × 103), 408
(ε = 34.2 × 103) nm.
Complex Ru−Pd. A mixture of 1 (59.9 mg, 0.083 mmol) and
Pd(MeCN)2Cl2 (21.7 mg, 0.083 mmol) in a flask was flashed with
nitrogen. Dimethylformamide (5 mL) was added, and the solution
was heated at 80 °C in an oil bath for 20 h. Upon cooling, ether was
added to give Ru−Pd as orange-red solid (55.8 mg, 92%).
Recrystallization from DMF/methanol gave the complex as crystalline
1
solid, which is suitable for X-ray crystallographic analysis. H NMR
(400 MHz, DMSO-d6) δ = 9.80 (d, J = 5.4 Hz, 1H), 8.94 (d, J = 8.1
Hz, 1H), 8.71−8.64 (m, 3H), 8.43−8.37 (m, 4H), 7.97 (t, J = 6.5 Hz,
1H), 7.87−7.80 (m, 1H), 7.78−7.70 (m, 3H), 7.64−7.57 (m, 2H),
6.75 (d, J = 6.0 Hz, 1H), 6.64 (d, J = 5.9 Hz, 1H), 6.48 (d, J = 5.9 Hz,
1H), 6.15 (d, J = 5.9 Hz, 1H), 6.19 (d, J = 5.9 Hz, 1H), 2.69−2.58
(m, 1H), 2.43 (s, 1H), 0.99 (d, J = 6.3 Hz, 3H), 0.88 (d, J = 6.48 Hz,
3H). 13C NMR (100 MHz, DMF-d7) δ = 173.0, 162.6, 161.3, 160.1,
157.3, 156.0, 155.0, 152.8, 151.53, 151.51, 150.4, 140.9, 140.5, 140.3,
140.0, 134.3, 131.4, 131.0, 129.7, 129.2, 129.1, 127.6, 127.3, 124.0,
122.9, 121.5, 119.4, 90.1, 88.1, 86.8, 83.4, 31.3, 22.2, 21.9, 19.0. UV−
vis (DMF) λmax (ε) 286 (31.4 × 103), 347 (24.4 × 103), 390 (12.6 ×
103), 411 (17.9 × 103), 457 (12.0 × 103) nm.
Complex Rh−Pd. A mixture of L (9.5 mg, 0.023 mmol) and
[RhCp*Cl2]2 (7.1 mg, 0.012 mmol) in a 10 mL reaction tube was
flashed with nitrogen. Dichloromethane (1 mL) was syringed into the
mixture, and the resulting solution was stirred at ambient temperature
for 18 h. Removal of solvent, the residue was mixed with
Pd(MeCN)2Cl2 (6.0 mg, 0.023 mmol), and then DMF (1 mL) was
added. The resulting mixture was heated at 80 °C for 18 h. After
cooling to room temperature, ether was added to yield orange-yellow
powder (17.5 mg, 88%) as the desired complex. 1H NMR (400 MHz,
DMSO-d6) δ = 9.25 (d, J = 5.2 Hz, 1H), 8.94 (d, J = 8.0 Hz, 1H),
8,69 (d, J = 9.1 Hz, 1H), 8.55 (brs, 1H), 8.49−8.39 (m, 5H), 8.05−
7.98 (m, 1H), 7.87−7.79 (m, 1H), 7.75−7.67 (m, 3H), 7.62−7.52
(m, 2H), 1.75 (s, 15H). 13C NMR (100 MHz, DMF-d7) δ = 172.9,
159.4, 155.0, 154.0, 151.7, 151.6, 151.0, 141.2, 141.1, 141.0, 140.11,
140.06, 134.3, 131.3, 131.0, 130.0, 129.7, 129.6, 129.1, 129.0, 127.5,
126.9, 124.1, 123.6, 122.9, 122.2, 119.6, 98.5 (d, JRh−C = 8.0 Hz).
Crystallography. Crystals suitable for X-ray determination were
obtained: 1·(CH3OH), [2·(CHCl3)(H2O)2], [Ru−Pd·(Et2O)·
(CH2Cl2)0.25], [Rh−Pd·(DMF)1.5·(PrOH)], and [Ir−Pd·(DMF)0.5·
(PrOH)0.5]. The structure was solved using the SHELXS-97pro-
gram10 and refined using the SHELXL-97program11 by full-matrix
least-squares on F2 values. All crystal data and other parameters are
collected in the Supporting Information. CCDC numbers for these
complexes are 1954850−1954854. All crystallographic data of
complexes are deposited in the Supporting Information.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.9b00682.
Crystal data of complexes 1, 2,Ru−Pd, Rh−Pd·, and Ir−
Pd; spectral data for catalysis products (PDF)
Accession Codes
CCDC 1954850−1954854 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.
H
DOI: 10.1021/acs.organomet.9b00682
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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AUTHOR INFORMATION
Corresponding Author
*E-mail: stliu@ntu.edu.tw.
ORCID
Shiuh-Tzung Liu: 0000-0002-0544-006X
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank the Ministry of Science and Technology, Taiwan, for
financial support (MOST107-2113-M002-005).
■
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DOI: 10.1021/acs.organomet.9b00682
Organometallics XXXX, XXX, XXX−XXX