Dicopper Complexes with Anthyridine-Based Ligands: Coordination and Catalytic Activity

Article abstract of DOI:10.1021/acs.organomet.5b00895

Two new anthyridine-based ligands, 5-phenyl-2,8-bis(2-pyridinyl)-1,9,10-anthyridine (L₃) and 5-phenyl-2,8-bis(6′-bipyridinyl)-1,9,10-anthyridine (L₄), were designed for accommodation of dimetallic systems with the metal ions separated by 5 ?. Complexation of Cu(ClO₄)₂ with L₃ and L₄ provided the corresponding dicopper complexes [{Cu₂(L₃)(H₂O)₄(CH₃CN)₂}(ClO₄)₄] (3) and [{Cu₂(L₄)(μ-ClO₄)₂}(PF₆)₂] (4), respectively. Both complexes were characterized by spectroscopic methods, and the detail structural features were further confirmed by X-ray crystallography. Structural analyses of 3 and 4 reveal the Cu···Cu distances in the complexes being 4.9612(7) and 5.013 (2) ?, respectively. Both complexes are active in the catalytic oxidation of benzyl alcohols into the corresponding aldehydes. Furthermore, complex 4 appears to be a good catalyst for the oxidative coupling of primary alcohols into the corresponding esters with the use of hydrogen peroxide as the oxidant in an aqueous medium. The possible cooperative interactions between the metal ions during the catalysis are discussed.

Full text of DOI:10.1021/acs.organomet.5b00895

Article
pubs.acs.org/Organometallics
Dicopper Complexes with Anthyridine-Based Ligands: Coordination
and Catalytic Activity
Da-wei Huang, Yi-Hung Liu, Shie-Ming Peng, and Shiuh-Tzung Liu*
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
S
* Supporting Information
ABSTRACT: Two new anthyridine-based ligands, 5-phenyl-2,8-bis(2-pyridinyl)-
1,9,10-anthyridine (L3) and 5-phenyl-2,8-bis(6′-bipyridinyl)-1,9,10-anthyridine (L4),
were designed for accommodation of dimetallic systems with the metal ions separated
by ∼5 Å. Complexation of Cu(ClO4)2 with L3 and L4 provided the corresponding
dicopper complexes [{Cu2(L3)(H2O)4(CH3CN)2}(ClO4)4] (3) and [{Cu2(L4)-
(μ-ClO4)2}(PF6)2] (4), respectively. Both complexes were characterized by
spectroscopic methods, and the detail structural features were further confirmed by
X-ray crystallography. Structural analyses of 3 and 4 reveal the Cu···Cu distances in the
complexes being 4.9612(7) and 5.013 (2) Å, respectively. Both complexes are active
in the catalytic oxidation of benzyl alcohols into the corresponding aldehydes.
Furthermore, complex 4 appears to be a good catalyst for the oxidative coupling of
primary alcohols into the corresponding esters with the use of hydrogen peroxide as the
oxidant in an aqueous medium. The possible cooperative interactions between the
metal ions during the catalysis are discussed.
Scheme 2. Anthyridine-Based Multiple Dentates
INTRODUCTION
■
Dimetallic complexes containing metal centers confined in
close proximity are a topic of active research.1−5 Such systems
are exploited to study the metal−metal interactions,1 synergistic
effect in catalysis,2 and mimicking enzymes.3 When studying
these topics, it is inevitable to mention the ligand design in
which the multidonors accommodate the metal ions within
a short distance. As shown in Scheme 1, 2,7-disubstituted
Scheme 1. Naphthyridine-Based Multiple Dentates
donors at both sides, is designed to allow two pincer cores
covalently linked via the anthyridine moiety for the enhance-
ment of the chelation effect. This presentation will highlight our
work on the preparation, characterization, and catalytic activity
of dicopper complexes with L3 and L4.
RESULTS AND DISCUSSION
Preparation and Characterization. The desired ligand,
5-phenyl-2,8-bis(2-pyridinyl)-1,9,10-anthyridine (L3), was pre-
■
naphthyridine-based ligands such as L1 and L2 are appropriate
tetradentate ligands for the coordination of two metal ions
with a short metal−metal distance (ca. 3 Å). Indeed, crystal
structures of a series of dimetallic complexes based on these
ligands reveal this anticipation.6,7a−d The catalytic activities of
dicopper complexes with ligand L1 and dinickel complexes with
ligand L2 do show synergistic effects in the oxidative coupling
reaction of 2,6-disubstitued phenols into diphenoquinones and
terminal alkynes into diynes, respectively.7a,b
As a continuation of our interest in the development of new
dimetallic complexes, herein we report the preparation of
anthyridine-based ligands (Scheme 2), which allows the
pyridinyl-nitrogen to be separated by ca. 9.2 Å, longer than
those in L1 and L2. In addition, L4, with two extra pyridinyl
pared by a double Friedlander reaction according to a modified
̈
procedure (Scheme 3).8 Reaction of 2,6-diamino-3,5-pyridine-
dicarboxaldehyde 1 with 2-acetylpyridine proceeded smoothly
to give L3 in 73% isolated yield. Similarly, 5-phenyl-2,8-bis-
(6′-bipyridinyl)-1,9,10-anthyridine (L4) was obtained via the
reaction of 1 with 6-acetyl-2,2′-bipyridine (2). These
compounds were characterized by NMR and mass spectros-
1
copies. The H NMR signal corresponding to protons of the
anthyridine ring appear as two sets of doublets at δ 8.68 and
Received: October 24, 2015
© XXXX American Chemical Society
A
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Scheme 3. Preparation of L3 and L4
8.23 ppm with JH−H = 8 Hz in L3 and δ 8.88 and 8.29 ppm with
JH−H = 8 Hz in L4, clearly indicating the symmetrical nature of
1
the compound. Table 1 summarizes the H shifts correspond-
ing to protons of pyridinyl rings in L3 and L4. Furthermore,
ESI-HRMS spectra of L3 and L4 showing peaks at m/z
412.1565 and 566.2096 [M + H] ions, respectively, confirm the
molecular formula of each compound.
Figure 1. ORTEP plot of the cationic part of 3.
The dicopper(II) complex of L3 was prepared by treatment
of the corresponding ligand with a two equimolar amount of
[Cu(ClO4)2·6H2O] in acetonitrile under refluxing conditions
(eq 1). Subsequent crystallization from CH3CN/hexane/Et2O
almost coplanar, and the ligand pocket accommodates two
copper(II) ions, which are separated by 4.9612(7) Å. The two
copper(II) centers are embedded in a distorted trigonal
bipyrimidal environment, where the coordination geometry of
each copper is completed by the “bipyridine” moiety, two water
molecules, and acetonitrile with the structural index parameter
(τ) 0.48 and 0.68 for Cu(1) and Cu(2), respectively.9b Two
water molecules and a pyridinyl-nitrogen of L3 compose the
basal plane, and the axial coordination sites are occupied by
the nitrogen atom of acetonitrile and the anthyridine-nitrogen
of L3. The average distance of Cu−N(ligand L3) is 1.996 Å, similar
to those in copper(II) bipyrdine complexes.10 Bond lengths of
Cu−O(water) lie in the range 1.948(3)−2.211(3) Å. The central
nitrogen donor N(3) of anthyridine remains uncoordinated.
The distances of N(3) to Owater lie in the range 2.938(5)−
2.944(5) Å, showing the hydrogen-bonding interaction of water
molecules with the N(3) donor. The bite angles N(1)−Cu(1)−
N(2) and N(4)−Cu(2)−N(5) are 81.6(1)° and 82.0(2)°,
respectively, which are typical in bipyridine complexes.
Reaction of L4 with [Cu(ClO4)2·6H2O] followed by the
treatment with KPF6 proceeded smoothly to give the desired
complex 4 as green crystalline solids (95%) (eq 2). The
replacement of the anion by hexafluorophosphate gives an easy
crystallization. Twelve resonances for the paramagnetic 1H
NMR spectrum of 4 were observed (see Experimental Section).
The visible spectrum of 4 showed an absorption maximum at
λ = 681 (log ε = 2.23) nm, which implies a square pyramidal
geometry around the metal. The molar conductivity of 4 in
acetonitrile (ΛM ≈ 232 cm2 Ω−1 mol−1) is consistent with the
formation of a 2:1 electrolyte. Furthermore, complex 4 exhibits
afforded complex 3 as green crystalline solids. The para-
1
magnetic H NMR spectrum of 3 showed nine broad signals
for aromatic regions at δ 60.55 (2H), 48.78 (2H), 38.43
(2H), 30.18 (2H), 29.35 (2H), 10.85 (2H), 10.33 (2H),
6.23 (2H), and 5.43 (1H). Conductivity measurement for 3
using acetonitrile as the solvent provided the data Λ = 486
(ohm−1 cm2 mol−1), suggesting that complex 3 is a 4:1
electrolyte. The UV−vis spectrum of 3 in CH3CN has absorp-
tion maxima at λ = 227 (log ε = 4.60), 291 (4.34), 403 (4.35),
and 802 (2.3) nm. The absorption at λ = 802 nm is characteristic
9a
2
2
2
(dxy, dx −y → dz ) for a trigonal bipyramidal Cu(II) species.
The detailed coordination configurations were confirmed by
X-ray diffraction analysis on its single crystal.
The coordination characteristic to complex 3 is depicted in
Figure 1, while selected bond lengths and angles are presented
in Table 2. The anthyridine and pyridine rings in ligand L3 are
1
Table 1. H NMR shifts of L3 and L4
ligand
H-3
H-4
H-3′
H-4′
H-5′
H-6′
H-3″
H-4″
H-5″
H-6″
L3
L4
8.23
8.29
8.68
8.88
8.75
8.71
7.92
7.84
7.40
8.56
9.07
8.60
8.08
7.33
9.10
B
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Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for 3
Cu(1)−N(1)
1.985(3)
Cu(2)−N(4)
1.989(4)
Cu(1)−N(2)
2.021(4)
Cu(2)−N(5)
1.990(4)
Cu(1)−N(6)
2.005(4)
Cu(2)−N(7)
1.963(4)
Cu(1)−O(1)
1.948(3)
Cu(2)−O(3)
2.211(3)
Cu(1)−O(2)
2.197(3)
Cu(2)−O(4)
1.988(3)
N(1)−Cu(1)−N(2)
N(1)−Cu(1)−N(6)
N(1)−Cu(1)−O(1)
N(1)−Cu(1)−O(2)
O(1)−Cu(1)−O(2)
N(2)−Cu(1)−O(1)
N(2)−Cu(1)−O(2)
N(2)−Cu(1)−N(6)
81.64(14)
93.45(15)
146.05(17)
109.98(14)
103.97(16)
98.79(15)
89.27(14)
175.05(14)
N(4)−Cu(2)−N(5)
N(4)−Cu(2)−N(7)
N(4)−Cu(2)−O(3)
N(4)−Cu(2)−O(4)
O(3)−Cu(2)−O(4)
N(4)−Cu(2)−O(3)
N(4)−Cu(2)−O(4)
N(4)−Cu(2)−N(7)
82.01(15)
175.41(15)
88.03(13)
95.92(14)
109.98(13)
88.03(13)
95.92(14)
175.41(15)
Table 3. Selected Bond Distances (Å) and Bond Angles
(deg) for 4
Cu(1)−N(1)
Cu(1)−N(2)
Cu(1)−N(3)
Cu(1)−O(1)
Cu(1)−O(7)
N(1)−Cu(1)−N(2) 79.5(3)
N(1)−Cu(1)−N(3) 159.2(3)
N(2)−Cu(1)−N(3) 79.9(3)
N(1)−Cu(1)−O(1) 90.3(3)
N(1)−Cu(1)−O(7) 99.0(3)
N(2)−Cu(1)−O(1) 152.6(3)
N(2)−Cu(1)−O(7) 114.1(3)
N(3)−Cu(1)−O(1) 106.9(3)
N(3)−Cu(1)−O(7) 91.9(3)
O(1)−Cu(1)−O(7) 92.5(3)
2.042(8)
1.941(6)
2.081(7)
1.950(6)
Cu(2)−N(5)
Cu(2)−N(6)
Cu(2)−N(7)
Cu(2)−O(4)(apical)
2.048(7)
1.938(7)
2.040(7)
2.066(6)
1.975(7)
79.4(3)
158.9(3)
79.5(3)
92.2(3)
93.0(3)
118.4(3)
141.4(3)
96.9(3)
104.2(3)
99.5(3)
2.166(6)(apical) Cu(2)−O(8)
N(7)−Cu(2)−N(6)
N(7)−Cu(2)−N(5)
N(6)−Cu(2)−N(5)
N(7)−Cu(2)−O(4)
N(7)−Cu(2)−O(8)
N(6)−Cu(2)−O(4)
N(6)−Cu(2)−O(8)
N(5)−Cu(2)−O(4)
N(5)−Cu(2)−O(8)
O(4)−Cu(2)−O(8)
infrared absorptions for the ClO4− at ν = 1088, 1042, 927, 621,
and 504 cm−1, suggesting a bidentate coordination mode of the
perchlorate.11 Although spectroscopic data provided the struc-
tural information on 4, the detailed structural features were
confirmed by X-ray crystallography.
The molecular structure of 4 consists of a discrete [Cu2(L4)-
(μ-ClO4)2]2+ cation (Figure 2) and two hexafluorophosphate
lengths of Cu−O in apical positions are slightly longer than
those in basal ones (2.116 vs 1.962 Å), as expected. The angles
of N(1)−Cu(1)−N(2) [79.5(3)°], N(2)−Cu(1)−N(3)
[79.9(3)°], N(6)−Cu(2)−N(5) [79.5(3)°], and N(7)−
Cu(2)−N(6) [79.4(3)°] in the chelating rings are much
deviated from 90° due to the ligand constraint.
Cyclic voltammetry measurements on 3 and 4 in CH3CN
are summarized in Table 4. For both complexes 3 and 4, two
a
Table 4. Cyclic Voltammetry Data for Complexes 3 and 4
b
E1/2 (V)
complex CuIICuII → CuIICuI CuIICuI → CuICuI CuIICuII → CuIICuIII
3
4
−0.77
−0.60
−1.02
−0.92
1.00
1.15
Figure 2. ORTEP plot of the cationic part of 4.
a
Conditions: in CH3CN containing 0.1 M tetrabutylammonium
hexafluorophosphate at rt with a scan rate of 0.1 V/s. Relative to
b
anions. The coordination geometry about the Cu(II) atoms is
best described as distorted square pyramidal. Three nitrogen
donors of a “terpyidine” moiety and one oxygen atom of
perchlorate form the base of a square pyramid with the oxygen
atom from the other bridging perchlorate occupying the apical
position. The structural index parameter (τ) for both copper
ions are 0.12 and 0.30, respectively, demonstrating the distorted
coordination environment. Two copper ions are separated by
5.013 (2) Å, similar to that in 3. The average Cu−N distances
around Cu(1) and Cu(2) are 2.021 and 2.009 Å (Table 3),
respectively, which compare well to those observed in the
related copper(II) terpyridine complexes.12 The average bond
Ag/AgNO3.
reversible one-electron reductions corresponding to reductions
of CuIICuII to CuIICuI and then CuIICuI to CuICuI were observed.
By comparison of the reduction potentials, the terpyridine
chelation appears to be more efficient in stabilization of the
metal ions in lower oxidation states than the bipyridine one.
Catalysis: Oxidation of Benzyl Alcohols Leading to
Benzaldehydes. It is well-documented that copper complexes
are frequently used as catalysts for dehydrogenative oxidation
of alcohols.13 Thus, the activity of these dicopper complexes
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a
Table 5. Oxidation of Benzyl Alcohols Catalyzed by Complexes 3 and 4
Y
H
p-MeO
p-OH
p-Me
p-Br
p-NO2
o-Me
o-OH
b
yield (3 as catalyst)
91%
88%
93%
93%
94%
92%
87%
84%
74%
81%
74%
99%
99%
b
yield (4 as catalyst)
a
b
Reaction conditions: benzyl alcohol (0.2 mmol), complex 3 (5 × 10−3 mmol), TEMPO (1 × 10−2 mmol) in toluene at 100 °C 12 h. Isolated
yields.
toward oxidation of benzyl alcohols was investigated. First, the
catalytic activity of the dicopper complex 3 was tested in the
TEMPO-cocatalyzed oxidation of benzyl alcohols with
dioxygen (Table 5). The oxidation proceeded smoothly to
provide the corresponding aldehyde in excellent yields except
the nitro-substituted one. Similarly, complex 4 is active in
the catalytic oxidation (Table 5). However, the yield from
o-methylbenzyl alcohol appears to be lower than that of 3.
Presumably, this is due to the steric congestion around the
metal center in 4 due to the terpyridinyl ligand.
Among the oxidations, we noticed that the oxidation of
o-hydroxybenzyl alcohol proceeded faster than other substrates.
A comparison of the reaction rates of various substrates
catalyzed by 3 was investigated, and the conversions of various
substrates versus reaction time are summarized in Figure 3.
Catalysis: Oxidation of Primary Alcohols Leading to
Esters. In the catalytic oxidation of benzyl alcohols, we learned
that the oxidation product was readily affected by the oxidant
used (eq 3). A summary of the oxidation of C6H5CH2OH
Table 6. Oxidation of Benzyl Alcohol Catalyzed by Complex
a
4 under Different Conditions
b
b
b
conv
(%)
yield
yield
yield
entry
oxidant
TBHP
solvent
of A
of B
of C
1
2
3
4
5
6
7
8
9
MeOH
MeOH
MeOH
MeOH
IPA
40
50
86
10
35
0
14%
trace
26%
trace
20%
trace
22%
46%
15%
trace
12%
H2O2
oxone
O2
41%
trace
H2O2
H2O2
H2O2
H2O2
CH3CN
DMF
74
80
50
53%
trace
trace
trace
15%
75%
46%
H2O
H2O2 (0.4 mmol) H2O
a
Reaction conditions: benzyl alcohol (0.2 mmol), complex 4 (5 ×
10−3 mmol), oxidant (1 mmol) in solvent (0.5 mL) at 70 °C under an
air atmosphere for 12 h. NMR yields.
b
catalyzed by 4 with various oxidants is listed in Table 6.
Oxidants such t-BuOOH and oxone used in this oxidation
provided a mixture of benzaldehyde, benzoic acid, and benzyl
benzoate, whereas hydrogen peroxide gave majorly the ester
product (Table 6, entry 2). It is noted that the solvents used
for the reaction affected the product formation dramatically
(Table 6, entries 5−9). Water appears to be the best choice for
this catalysis for the formation of an ester product, which meets
the environmental criterion.
Figure 3. Conversions of various substrates versus reaction time
catalyzed by 3.
We also set up a series of experiments to evaluate the ability
of various copper complexes in this oxidation coupling process
(Table 7). Quite obviously, complex 4 proved to have the best
activity in this oxidation. Furthermore, carrying out the reaction
in the presence of sodium acetate gave an even better result
(Table 7, entry 3). Other copper complexes were tested as catalyst
precursors and exhibited some activity (Table 7, entries 4−7), but
not as good as complex 4. The poor activities of the mononuclear
species are quite similar to those reported in the literature.14 To our
surprise, the activity and selectivity of 3 in the oxidative coupling of
benzyl alcohol leading to the corresponding ester are poor, clearly
indicating that the ligand plays important roles in the catalysis.
To validate the ligand effect on the catalysis, oxidations of
benzyl alcohol were monitored by NMR spectroscopy. The
Based on the pseudo-first-order approximation, the values of
kobs for Y = o-OH, o-Me, p-OH, p-OMe, and H were estimated
to be 2.8 × 10−4, 2.9 × 10−5, 5.5 × 10−5, 5.7 × 10−5, and
3.7 × 10−5 (s−1), respectively. The reaction rate for the
oxidation of o-hydroxybenzyl alcohol is about 5 times faster
than those of p-hydroxy and p-methoxybenzyl alcohols, whereas
other substrates show similar reaction rates. This observation
indicates that the o-hydroxyl group might play a role in this rate
enhancement. We speculated that the coordination of the
o-hydroxyl moiety to one of the copper ions in 3 may accelerate
the oxidation of benzylic alcohol on the other metal center.
This rate enhancement is also observed in the oxidation of
o-hydroxybenzyl alcohol catalyzed by complex 4 (kobs = 2.5 ×
10−4 s−1).
D
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Table 7. Oxidation of Benzyl Alcohol Catalyzed by Various
ab
,
Copper Complexes
b
b
b
yield
yield
yield
entry
catalyst
conv
of A
of B
of C
1
2
3
3
4
62%
77%
95%
37%
trace
trace
trace
18%
d
0
0
71%
88%
d
4 + CH3COONa
(2 × 10−2 mmol)
c
4
Cu(ClO4)2·6H2O
Cu(CH3COO)2
20%
15%
70%
58%
15%
12%
50%
41%
0
trace
trace
5%
c
5
0
c
6
[Cu(terpy)(H2O)(ClO4)2]
15%
12%
c
7
[Cu(bipy)
trace
(CH3CN)2(ClO4)2] (5)
8
Cu(ClO4)2/L1 (1:1)
Cu(ClO4)2/L1 (2:1)
Cu(ClO4)2/L4 (1:1)
no catalyst
53%
85%
56%
trace
32%
45%
43%
13%
25%
trace
trace
12%
8%
9
Figure 6. Time-course of transformation of PhCH2OH to
PhCO2CH2Ph catalyzed by 4.
10
11
a
Reaction conditions: benzyl alcohol (0.2 mmol), complex (5 ×
10−3 mmol), H2O2 (0.6 mL) in water (0.5 mL) at 70 °C under an
benzyl alcohol catalyzed by 3 proceeded smoothly, and the
yield of benzaldehyde gradually increased to 42% in about 10 h
accompanied by the formation benzyl benzoate (11%). For the
mononuclear copper complex 5 as the catalyst, benzaldehyde
and benzoic acid were the major products in a ratio of 3:1. In
this catalysis, only trace benzyl benzoate was formed. However,
the activity of complex 4 behaved quite differently from those
of 3 and 5 (Figure 6). Benzyl benzoate was the major product
in the reaction, whereas the benzaldehyde remained in low
concentration. In addition, the kinetic data for the rate of the
formation of ester catalyzed by 3, 4, and [Cu(terpy)(H2O)
(ClO4)2] were determined to be 6.69 × 10−5, 6.83 × 10−6, and
2.86 × 10−6 (s−1), respectively. The above observations indicate
that the presence of a two-in-one terpyridine ligand in 4 might
play an important role in a cooperative effect during the
catalysis.
b
c
air atmosphere for 12 h. NMR yields. Complex 1 × 10−2 mmol
(5 mol %). Isolated yields.
d
conversion of C6H5CH2OH and the yields of various oxidation
products catalyzed by 3, [Cu(bipy) (CH3CN)2(ClO4)2] (5),
and 4 were plotted in Figures 4−6, respectively. Oxidation of
A plausible mechanistic pathway for the oxidative coupling
of benzyl alcohol leading to benzoate ester catalyzed by
a mononuclear copper complex is shown in Scheme 4.14
Scheme 4. Mechanistic Pathway for the Oxidative Coupling
of Benzyl Alcohol
Figure 4. Time-course of transformation of PhCH2OH to
PhCO2CH2Ph catalyzed by 3.
Oxidation of the primary alcohol to give benzaldehyde is the
first step (Scheme 4, step i). The aldehyde molecule reacts with
another molecule of alcohol in the presence of copper ion to
yield the hemiacetal copper complex I (Scheme 4, step ii),
which then undergoes β-elimination to yield the corresponding
ester (Scheme 4, step iii). Complex I is the key intermediate in
the transformation leading to the ester. If the formation of
hemiacetal copper complex (step ii) is a slow reaction,
β-elimination may take place on the coordinating alcohol to
give the aldehdyde (Scheme 4, step i). Apparently, the forma-
tion of the hemiacetal in complex 4 is accelerated. Presumably,
the coordination environment around the two copper centers
in 4 readily sets up for an intramolecular attack to form the
Figure 5. Time-course of transformation of PhCH2OH to
PhCO2CH2Ph catalyzed by 5.
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Scheme 5. Possible Intermediates for the Oxidative
Coupling of Benzyl Alcohol
SUMMARY
■
In summary, novel anthyridine-based ligands with two pyridinyl
and terpyridinyl compartments covalently linked via the anthyridine
moiety are presented. With these ligands (L3 and L4), we have
successfully prepared and characterized the dicopper complexes
3 and 4, where the metal centers are separated by ca. 5 Å.
Further utilization of these complexes as the catalysts for the
oxidation of alcohols was examined. Complex 3 is a good
catalyst for the oxidation of benzyl alcohols, leading to the
corresponding aldehydes under mild conditions, whereas com-
plex 4 is active in the oxidative coupling of alcohols leading to
the corresponding esters. The activity and selectivity of the
catalysts have been rationalized on the basis the cooperative
interaction of dimetallic centers through the analyses of the
kinetic study and mass spectra of intermediates. More catalytic
activity studies involving 3 and 4 in other transformations are
currently in progress.
acetal complex II, as shown in Scheme 5. The mass spectrum of
the catalytic reaction mixture showed a peak corresponding
to the species [Cu2(L4)(μ-OAc)(PhCHO)(PhCH2O)]2+
[C53H39Cu2N7O4; m/z found 481.57, calcd 481.58), providing
evidence for the formation of a catalytic intermediate II during
the catalysis. On the other hand, the extra available coordina-
tion sites on the metal centers in complex 3 allow several
competing coordination pathways, which make the intra-
molecular attack less accessible (Scheme 5, III).
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. Compound 1,15a [Cu(bipy)(CH3CN)2(ClO4)2]
(5),15b and [Cu(terpy)(H2O)(ClO4)2]15c were prepared according to
the reported methods.
1-(2,2′-Bipyridin-6-yl)ethanone. A mixture of 1-(6-bromopyr-
idin-2-yl)ethanone (2.00 g, 10 mmol), 2-(tributylstannyl)pyridine
(7.36 g, 20 mmol), and PdCl2(PPh3)2 (0.35 g, 0.5 mmol, 5 mol %) in
toluene (20 mL) was heated under reflux for 24 h. Water (20 mL) was
added, and the mixture was extracted with CH2Cl2 (30 mL × 3). Aftre
removal of solvents, the residue was chromatographed on silica gel
with elution of hexane/ethyl acetate. The desired compound was
With the optimized conditions, the oxidative coupling of a
variety of substituted benzyl alcohols and alkyl alcohols
catalyzed by 4 was investigated (Table 8). All benzyl alcohols
a
Table 8. Oxidative Coupling of Alcohol Catalyzed by 4
b
entry
substrate
C6H5CH2OH
product
yield
1
2
C6H5COOCH2C6H5
88%
82%
p-MeOC6H4CH2OH
p-MeOC6H4COOCH2C6H4-
(p-OMe)
3
4
5
6
p-MeC6H4CH2OH
p-ClC6H4CH2OH
p-BrC6H4CH2OH
p-NO2C6H4CH2OH
p-MeC6H4COOCH2C6H4(p-Me) 91%
p-ClC6H4COOCH2C6H4(p-Cl)
p-BrC6H4COOCH2C6H4(p-Br)
74%
67%
60%
1
obtained as white solids (1.63 g, 83%). H NMR (400 MHz, CDCl3,
p-NO2C6H4COOCH2C6H4-
(p-NO2)
298 K): δ 8.68 (d, J = 4 Hz, 1H), 8.60 (d, J = 8.4 Hz, 1H), 8.51 (d, J =
8.2 Hz, 1H), 8.03 (d, J = 8.4 Hz, 1H), 7.94 (t, J = 8.2 Hz, 1H), 7.84
(t, J = 8 Hz, 1H), 7.33 (m, 1H). The spectral data are essentially
idential to the reported ones.16
Preparation of Ligand L3. A mixture of 1 and 2-acetylpyridine
(0.16 g, 1.3 mmol) in ethanol (4 mL) was heated at 60 °C for 2 h.
Then 10% KOH in EtOH (0.1 mL) was added to the above solution,
and the color of the solution turned deep brown. The resulting
mixture was heated to reflux for 12 h. The reaction mixture was
extracted with CH2Cl2 (30 mL × 3). The organic extracts were dried
and concentrated. The residue was reprecipitated from CH2Cl2/
hexane twice. After filtration and drying under vacuum, the desired
7
HO(CH2)3OH
HO(CH2)4OH
HO(CH2)5OH
HO(CH2)6OH
o-C6H4(CH2OH)2
8
γ-butyrolactone
δ-valerolactone
ε-caprolactone
phthalide
91%
76%
trace
78%
63%
9
10
11
12
HOCH2CH
2-buteno-4-lactone
CHCH2OH
13
14
15
CH3CH2CH2OH
CH3(CH2)3OH
CH3(CH2)4OH
CH3CH2COOCH2CH2CH3
CH3(CH2)2COO(CH2)3CH3
CH3(CH2)3COO(CH2)4CH3
35%
42%
15%
1
compound was obtained as yellow solids (0.13 g, 73%). H NMR
a
Reaction conditions: alcohol (0.2 mmol), complex (5 × 10−3 mmol),
(400 MHz, CDCl3): δ 9.07 (d, J = 8.6 Hz, 2H), 8.75 (d, J = 8.6 Hz,
2H), 8.68 (d, J = 8 Hz, 2H), 8.23 (d, J = 8 Hz, 2H), 7.92 (t, J = 8.4 Hz,
2H), 7.65 (m, 3H), 7.49 (m, 2H), 7.40 (m, 2H). 13C NMR
(100 MHz): δ 161.8, 156.0, 155.2, 151.0, 149.1, 137.1, 136.6, 134.1,
130.7, 129.2, 128.9, 125.1, 123.3, 120.6, 119.7. ESI-HRMS: m/z =
[M + H]+ calcd for C27H18N5, 412.1562; found 412.1568.
Preparation of Ligand L4. The procedure for the preparation of
L4 is similar to that for L3. Compound L4 was obtained as light yellow
solids (0.21 g, 87%). H NMR (400 MHz, CDCl3, 298 K): δ 9.10 (d,
J = 8 Hz, 2H), 8.88 (d, J = 9 Hz, 2H), 8.71 (d, J = 4.6 Hz, 4H), 8.60
(d, J = 8 Hz, 4H), 8.56 (d, J = 8 Hz, 2H), 8.29 (d, J = 9 Hz, 2H), 8.08
(t, J = 8 Hz, 2H), 7.84 (t, J = 8 Hz, 2H), 7.68 (m, 3H), 7.53 (m, 2H),
7.33 (dd, J = 7.2, 4.6 Hz, 2H). 13C NMR (200 MHz, CD2Cl2/CDCl3):
δ 162.5, 156.8, 156.5, 156.2, 155.2, 151.7, 149.9, 138.8, 137.5, 137.4,
134.9, 131.4, 129.9, 129.5, 124.6, 123.6, 123.0, 121.6, 121.4, 120.4. ESI-
HRMS: m/z = [M + H]+ calcd for C37H24N7, 566.2093; found
566.2096. Anal. Calcd for C37H23N7: C, 78.57; H, 4.10; N, 17.33.
Found: C, 77.91; H, 4.21; N, 17.05.
NaOAc (2 × 10−2 mmol), H2O2 (0.6 mL) in water (0.5 mL) at 70 °C
under an air atmosphere for 12 h. Isolated yield.
b
were found to undergo the oxidative coupling to afford the
corresponding ester in good to excellent yields (Table 8, entries
1−6). For the diols, the oxidation couplings of 1,4-butanediol
and 1,5-pentanediol proceeded smoothly to give γ-butyrolactone
and δ-valerolactone, respectively (Table 8, entries 8 and 9).
However, 1,3- and 1,6-alkanediols did not provide the correspond-
ing lactone presumably due to the ring strain. This system was
successfully applied to the oxidation of o-C6H4(CH2OH)2 to
provide phthalide (Table 8, entry 11). The unsaturated
lactone, 2-buteno-4-lactone, can be obtained from the oxida-
tion of cis-2-butene-1,4-diol in 63% yield (Table 8, entry 12).
Simple alkyl alcohols do undergo this kind oxidative coupling
to yield the corresponding ester, but in low yields (Table 8,
entries 13−15).
1
Preparation of Complex 3. A mixture of L3 (50 mg, 0.12 mmol)
and Cu(ClO4)2·H2O (96.3 mg, 0.26 mmol) in CH3CN (3 mL) was
F
DOI: 10.1021/acs.organomet.5b00895
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
heated to reflux under a nitrogen atmosphere for 24 h. The reaction
mixture was filtered through Celite (0.5 g). Ether (10 mL) was added
to the filtrate to give dark green precipitates, which were recrystallized
from CH3CN/ether to give green crystalline solids (120 mg, 92%).
1H NMR (400 MHz, methanol-d4, 298 K): δ 60.55 (br, 2H), 48.78 (br,
2H), 38.43 (br, 2H), 30.18 (br, 2H), 29.35 (br, 2H), 10.85 (br, 2H),
10.33 (br, 2H), 6.23 (br, 2H), 5.43 (br, 1H). IR (KBr) ν 1091 (ClO4),
626 cm−1 (ClO4). Anal. Calcd for C31H33Cl4Cu2N7O21 (3+H2O):
C, 33.59; H, 3.00; N, 8.84. Found: C, 33.43; H, 3.12; N, 8.57.
Crystals suitable for X-ray determination were obtained for 3 by
recrystallization from CH2Cl2/MeOH at room temperature. The
structure of 3 was further confirmed by crystallography.
Preparation of Complex 4. A mixture of L4 (50 mg, 0.09 mmol)
and Cu(ClO4)2·H2O (70 mg, 0.19 mmol) in CH3CN (3 mL) was
heated to reflux under a nitrogen atmosphere for 24 h. The reaction
mixture was filtered through Celite (0.5 g). KPF6 (35 mg, 0.19 mmol)
was added to the filtrate, and the mixture was stirred for 12 h. Ether
(20 mL) was added slowly to the reaction mixture. The desired
complex precipitated as green crystalline solids (101 mg, 95%).
1H NMR (400 MHz, methanol-d4, 298 K): δ 75.25 (br, 2H), 72.84
(br, 2H), 57.47 (br, 2H), 56.67 (br, 2H), 41.73 (br, 2H), 25.52 (br,
2H), 24.81 (br, 2H), 9.48 (br, 2H), 7.67 (br, 2H), 6.19 (br, 2H),
5.58 (br, 2H), 5.33 (br, 1H). IR (KBr) ν 1088 (ClO4), 1042 (ClO4),
927 (ClO4), 621 (ClO4), 504 cm−1 (ClO4). Anal. Calcd for
C37H23Cl2Cu2F12N7O8P2: C, 37.61; H, 1.96; N, 8.30. Found: C,
37.82; H, 2.25; N, 8.16.
elution of ether. The filtrate was then concentrated under reduced
pressure and analyzed by H NMR spectroscopy.
1
Catalysis: Oxidative Coupling of Primary Alcohols. A mixture
of alcohol (0.2 mmol), complex 4 (5 × 10−3 mmol), sodium acetate
(2 × 10−2 mmol), and 30% H2O2 (0.6 mL) in water (0.5 mL) was
loaded in a reaction vessel with a stirring bar. This reaction mixture
was heated at 70 °C under an air atmosphere for 12 h. The desired
ester product was extracted with ether (5 mL × 3). The combined
organic extracts were dried and concentrated. The residue was
analyzed by H1 NMR spectroscopy. For further purification, the
residue was chromatographed on silica gels with elution of hexane/
ethyl acetate to yield the pure ester. The spectral data of the organic
products are essentially identical to those reported.
1
Benzyl Benzoate. H NMR (400 MHz, CDCl3): δ 8.10 (d, J =
8 Hz, 2H), 7.56 (t, J = 8 Hz, 1H), 7.43 (m, 7H), 5.38 (s, 2H).
13C NMR (100 MHz): δ 166.7, 136.3, 133.3, 130.4, 130.0, 128.9,
128.7, 128.5, 128.4, 67.0.
4-Methoxybenzyl 4-Methoxybenzoate. 1H NMR (400 MHz,
CDCl3): δ (ppm) 8.01 (d, J = 8 Hz, 2H), 7.34 (d, J = 8 Hz, 2H),
6.85−6.91 (m, 4H), 5.26 (s, 2H), 3.83 (s, 3H), 3.78 (s, 3H). 13C NMR
(100 MHz): δ 166.1, 163.4, 159.6, 131.8, 129.9, 128.4, 123.0, 114.3,
113.4, 66.5, 55.7, 55.1.
4-Methylbenzyl 4-Methylbenzoate. 1H NMR (400 MHz, CDCl3):
δ 7.94 (d, J = 8.4 Hz, 2H), 7.32 (d, J = 8.4 Hz, 2H), 7.18 (m, 4H), 5.29
(s, 2H), 2.38 (s, 3H), 2.34 (s, 3H). 13C NMR (100 MHz): δ 166.5,
143.5, 137.9, 133.1, 129.6, 129.2, 129.0, 128.2, 127.4, 66.4, 21.6, 21.1.
1
Crystals suitable for X-ray determination were obtained for 4 by
recrystallization from CH3CN/hexane at room temperature. The
structure of 4 was further confirmed by crystallography.
4-Chlorobenzyl 4-Chlorobenzoate. H NMR (400 MHz, CDCl3):
δ 7.96 (d, J = 8 Hz, 2H), 7.35 (m, 6H), 5.28 (s, 2H). 13C NMR
(100 MHz): δ 165.3, 139.6, 134.2, 131.0, 130.2, 129.6, 128.8, 128.7,
128.3, 66.0.
Catalysis: Oxidation of Benzyl Alcohols. A mixture of benzyl
alcohol (0.2 mmol), complex 3 (5 × 10−3 mmol), and TEMPO (1 ×
10−2 mmol) in toluene (2 mL) was loaded in a reaction vessel with a
stirring bar. The mixture was heated at 100 °C for 12 h. After the
reaction, brine (2 mL) was added to the reaction mixture and then
extracted with ether (3 mL × 3). The combined organic extracts were
dried and concentrated. The residue was analyzed by NMR spec-
troscopy. For the purification, chromatography on silica gels provided
the desired compound in pure form. The spectral data of the organic
products are essentially identical to those reported.
1
4-Bromobenzyl 4-Bromobenzoate. H NMR (400 MHz, CDCl3):
δ 7.88 (d, J = 8 Hz, 2H), 7.54 (d, J = 8 Hz, 2H), 7.47 (d, J = 8 Hz,
2H), 7.27 (d, J = 8 Hz, 2H), 5.26 (s, 2H). 13C NMR (100 MHz): δ
165.4, 134.7, 131.7, 131.1, 129.8, 128.7, 128.3, 122.4, 66.1.
1
4-Nitrobenzyl 4-Nitrobenzoate. H NMR (400 MHz, CDCl3): δ
8.25 (m, 6H), 7.60 (d, J = 8 Hz, 2H), 5.47 (s, 2H). 13C NMR
(100 MHz): δ 164.2, 150.7, 147.3, 142.3, 134.7, 130.8, 128.7, 123.9,
123.6, 66.0.
γ-Butyrolactone. 1H NMR (400 MHz, CDCl3): δ 4.31 (t, J = 8 Hz,
2H), 2.45(t, J = 8 Hz, 2H), 2.20−2.26 (m, 2H). 13C NMR
(100 MHz): δ 177.6, 68.4, 27.7, 22.1.
Benzaldehyde. 1H NMR (400 MHz, CDCl3): δ 10.20 (s, 1H), 7.87
(m, 2H), 7.64 (m, 1H), 7.44−7.51 (m, 2H). 13C NMR (100 MHz): δ
192.1, 136.6, 134.1, 130.0, 128.9.
δ-Valerolactone. 1H NMR (400 MHz, CDCl3): δ 4.36 (t, J = 8 Hz,
2H), 2.51 (t, J = 8 Hz, 2H), 1.85 (m, 4H). 13C NMR (100 MHz): δ
171.6, 69.7, 29.4, 22.3, 19.1.
1
p-Methoxybenzaldehyde. H NMR (400 MHz, CDCl3): δ 9.87
(s, 1H), 7.87 (d, J = 8.4 Hz, 2H), 6.98 (d, J = 8.4 Hz, 2H), 3.84 (s,
3H). 13C NMR (100 MHz): δ 190.5, 164.4, 131.8, 130.1, 114.1, 55.7.
Phthalide. 1H NMR (400 MHz, CDCl3): δ 7.83 (d, J = 8 Hz, 2H),
7.64 (d, J = 8 Hz, 2H), 7.46 (m, 4H), 5.26 (s, 2H). 13C NMR
(100 MHz): δ 171.0, 146.5, 133.9, 128.9, 125.5, 122.1, 69.6.
2-Buteno-4-lactone. 1H NMR (400 MHz, CDCl3): δ 7.61 (dt,
J = 6, 2 Hz, 1H), 6.13 (dt, J = 6, 2 Hz, 1H), 4.88 (m, 2H). 13C NMR
(100 MHz): δ 173.6, 152.7, 121.4, 72.5.
1
p-Hydroxybenzaldehyde. H NMR (400 MHz, CDCl3): δ 9.88 (s,
1H), 7.85 (d, J = 8.4 Hz, 2H), 6.94 (d, J = 8.4 Hz, 2H). 13C NMR
(100 MHz): δ 191.7, 161.5, 132.8, 130.1, 115.9.
p-Methylbenzaldehyde. 1H NMR (400 MHz, CDCl3): δ 9.94
(s, 1H), 7.75 (d, J = 7.8 Hz, 2H), 7.30 (d, J = 7.8 Hz, 2H), 2.47 (s,
3H). 13C NMR (100 MHz): δ 191.9, 145.8, 134.0, 130.2, 129.7, 22.2.
p-Bromobenzaldehyde. 1H NMR (400 MHz, CDCl3): δ 9.95
(s, 1H), 7.77 (d, J = 8.0 Hz, 2H), 7.68 (d, J = 8.0 Hz, 2H). 13C NMR
(100 MHz): δ 191.4, 134.8, 132.3, 131.1, 129.5.
1
Propyl Propionate. H NMR (400 MHz, CDCl3): δ 4.01 (t, J = 8
Hz, 2H), 2.31 (q, J = 8 Hz, 2H), 1.63 (m, 2H), 1.12 (t, J = 8 Hz, 3H),
0.92 (t, J = 8 Hz, 3H). 13C NMR (100 MHz): δ 174.5, 65.8, 27.6, 22.0,
10.3, 9.1.
p-Nitrobenzaldehyde. 1H NMR (400 MHz, CDCl3): δ 10.21
(s, 1H), 8.38 (d, J = 8.2 Hz, 2H), 8.05 (d, J = 8.2 Hz, 2H). 13C NMR
(100 MHz): δ 191.0, 151.4, 139.8, 130.7, 124.6.
Butyl Butyrate. H NMR (400 MHz, CDCl3): δ 4.05 (t, J = 8 Hz,
1
2H), 2.26 (t, J = 8 Hz, 2H), 1.66−1.57 (m, 4H), 1.36 (m, 2H), 0.92
(m, 6H). 13C NMR (100 MHz): δ 173.7,64.0, 36.2, 30.6, 19.0, 18.4,
13.6, 13.5.
o-Methylbenzaldehyde. 1H NMR (400 MHz, CDCl3): δ 10.22
(s, 1H), 7.83 (d, J = 7.4 Hz, 1H), 7.41 (m, 1H), 7.32 (m, 1H), 7.22 (d,
J = 7.4 Hz, 1H), 2.62 (s, 3H). 13C NMR (100 MHz): δ 192.8, 140.5,
134.2, 133.4, 132.4, 131.7, 126.3, 19.5.
Crystallography. Crystals suitable for X-ray determination were
obtained for 3·(CH2Cl2)2 and 4·(CH3CN)1.5(C6H14)0.5 by recrystal-
lization from dichloromethane and CH3CN/hexane solutions,
respectively. Cell parameters were determined by a Siemens SMART
CCD diffractometer. The structure was solved using the SHELXS-97
program17 and refined using the SHELXL-97 program18 by full-matrix
least-squares on F2 values. Crystal data for 3 and 4 are listed in the
Supporting Information (Table S1). Other crystallographic data are
deposited as Supporting Information.
1
o-Hydroxybenzaldehyde. H NMR (400 MHz, CDCl3): δ 10.98
(s, 1H), 9.91 (s, 1H), 7.56−7.49 (m, 2H), 7.03−6.98 (m, 2H).
13C NMR (100 MHz): δ 196.8, 161.1, 137.2, 133.9, 120.7, 120.1, 117.3.
General Kinetic Procedures. A mixture of benzyl alcohol
(0.2 mmol), dicopper complex (5 × 10−3 mmol), and TEMPO
(1 × 10−2 mmol) in toluene (2 mL) was loaded in a reaction vessel
with a stirring bar. The mixture was heated at 100 °C. At appropriate
time intervals, 0.2 mL aliquots were removed using a syringe and
quickly passed through Celite to remove the metal complexes with
Electrochemistry. Cyclic voltammetry was carried out with a CH
Instrument 750 electrochemistry analyzer. The sample cell used was a
standard three-electrode system with a platinum wire auxiliary as the
G
DOI: 10.1021/acs.organomet.5b00895
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
counter electrode. The reference electrode was Ag/Ag+. CH3CN as
solvent was deoxygenated by nitrogen bubbling. The measurements
were performed at rt in CH3CN containing 0.1 M tetrabutylammo-
nium hexafluorophosphate.
Trans. 2012, 41, 5782−5784. (d) Lan, Y.-S.; Liao, B.-S.; Liu, Y.-H.;
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(8) Murray, T. J.; Zimmerman, S. C.; Kolotuchin, S. V. Tetrahedron
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.organomet.5b00895.
■
S
(9) (a) McLachlan, G. A.; Fallon, G. D.; Martin, R. L.; Spiccia, L.
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1984, 1349−1356.
1H NMR spectra of complexes 3 and 4; in situ mass
spectra for the catalysis catalyzed by 4; table of crystal
data for 3 and 4; tables providing bond distances and
angles for complexes 3 and 4 (PDF)
Cartesian coordinates of complexes 3 and 4 (XYZ)
X-ray crystallographic data for complexes 3 and 4 (CIF)
(10) Ghosh, M.; Biswas, P.; Floerke, U.; Nag, K. Inorg. Chem. 2008,
47, 281−296.
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(c) Cao, Q.; Dornan, L. M.; Rogan, L.; Hughes, N. L.; Muldoon, M. J.
Chem. Commun. 2014, 50, 4524−4543 and references therein.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: stliu@ntu.edu.tw.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the Ministry of Science and Technology, Taiwan, for
the financial support (MOST103-2113-M-002-MY3).
■
(16) Mutai, T.; Cheon, J.-D.; Arita, S.; Araki, K. J. Chem. Soc., Perkin
Trans. 2 2001, 1045−1050.
(17) Sheldrick, G. M. SHELXS-97. Acta Crystallogr., Sect. A: Found.
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DOI: 10.1021/acs.organomet.5b00895
Organometallics XXXX, XXX, XXX−XXX