Effects of appended hydroxyl groups and ligand chain length on copper coordination and oxidation activity

Article abstract of DOI:10.1039/c7nj03113k

Treatment of a series of (imino)pyridine ligands bearing appended hydroxyl groups 2-((pyridin-2-ylmethylene)amino)phenol (Hpyph), 2-((pyridin-2-ylmethylene)amino)ethanol (Hpyet), and 3-((pyridin-2-ylmethylene)amino)propanol (Hpypr) with one equiv. of CuCl₂·2H₂O afforded the corresponding Cu(ii) complexes in low to moderate yields. The crystal structure of (μ-Cl)₂[CuCl(κ²-N,N-Hpyet)]₂ reveals a symmetric dinuclear structure with the bidentate N,N-coordination mode of (imino)pyridine with no Cu-OH interaction. On the other hand, the dinuclear Cu(ii) complex of the related propyl ligand Hpypr possesses a significantly different crystal structure involving nucleophilic addition of the hydroxyl group to the aldehyde group of 2-pyridinecarboxaldehyde. The Cu complex/Cu⁰/TEMPO/Na₂CO₃ (TEMPO = 2,2,6,6-tetramethylpiperidinyl-1-oxyl) catalyst system generally exhibited good activity for aerobic oxidation of benzyl alcohol to benzaldehyde in H₂O at room temperature. The dinuclear Cu(ii) complex (μ-Cl)₂[CuCl(κ²-N,N-Hpyet)]₂ was demonstrated as an effective catalyst toward aerobic oxidation of various benzyl alcohol derivatives, cinnamyl alcohol, and 2-thiophenemethanol.

Full text of DOI:10.1039/c7nj03113k

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DOI: 10.1039/C7NJ03113K
Effects of Appended Hydroxyl Groups and Ligand Chain Length
on Copper Coordination and Oxidation Activity
,
,§
Attawit Jehdaramarn,† ‡ Soraya Pornsuwan, Phongnarin Chumsaeng,†
†
§
, ,
Khamphee Phomphrai, and Preeyanuch Sangtrirutnugul† ‡
*
*
Corresponding author. Email: preeyanuch.san@mahidol.ac.th
†
Center of Excellence for Innovation in Chemistry (PERCHꢀCIC), Department of
Chemistry, Faculty of Science, Mahidol University, 272 Rama VI Rd. Bangkok,
0400, Thailand.
1
‡
Center for Catalysis, Department of Chemistry, Faculty of Science, Mahidol
University, 272 Rama VI Rd. Bangkok, 10400, Thailand.
§
Department of Materials Science and Engineering, School of Molecular Science and
Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong, 21210,
Thailand
Abstract
Treatment of a series of (imino)pyridine ligands bearing appended hydroxyl groups 2ꢀ
((pyridinꢀ2ꢀylmethylene)amino)phenol (Hpyph), 2ꢀ((pyridinꢀ2ꢀylmethylene)amino)ꢀ
ethanol (Hpyet), and 3ꢀ((pyridinꢀ2ꢀylmethylene)amino)propanol (Hpypr) with one
equiv of CuCl
2
ꢀ2H
2
O afforded the corresponding Cu(II) complexes in low to
2
moderate yields. The crystal structure of (ꢀꢀCl)
2
[CuCl(κ ꢀN,NꢀHpyet)]
2
reveals a
symmetric dinuclear structure with the bidentate N,N coordination mode of
imino)pyridine with no CuꢀꢀOH interaction. On the other hand, the dinuclear Cu(II)
(
complex of the related propyl ligand Hpypr possesses a significantly different crystal
structure involving nucleophilic addition of the hydroxyl group to the aldehyde group
0
of 2ꢀpyridinecarboxaldehyde. The catalyst system Cu complex/Cu /TEMPO/Na
2
CO
3
(TEMPO = 2,2,6,6ꢀtetramethylpiperidinylꢀ1ꢀoxyl) generally exhibited good activities
for aerobic oxidation of benzyl alcohol to benzaldehyde in H O at room temperature.
2
2
The dinuclear Cu(II) complex (ꢀꢀCl)
2
[CuCl(κ ꢀN,NꢀHpyet)]
2
was demonstrated as an
effective catalyst toward aerobic oxidation of various benzyl alcohol derivatives,
cinnamyl alcohol, and 2ꢀthiophenemethanol.
1

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Keywords: Appended hydroxyl group, copper catalyst, aerobic alcohol oxidation,
water soluble catalyst
Introduction
Copperꢀcatalyzed oxidation of alcohols to the corresponding aldehydes using
oxygen or air as an oxidant is generally highly efficient and can be performed under
1
environmentally friendly conditions. Sammelhack and coꢀworkers first reported the
catalyst system CuCl/TEMPO/DMF (TEMPO = 2,2,6,6ꢀtetramethylpiperidinylꢀ1ꢀ
oxyl, DMF = N,Nꢀdimethylformamide), which were effective for oxidation of primary
2
3
4
2
alcohols to aldehydes under an atmospheric O . Later works by Sheldon, Koshinen,
5
and Stahl involved copper catalyst systems supported by 2,2'ꢀbipyridine ligand in the
t
presence of an additional base including KO Bu, 1,8ꢀdiazabicyclo[5.4.0]undecꢀ7ꢀene
(
DBU), and Nꢀmethylimidazole (NMI) with O or ambient air as an oxidant in
2
3
CH CN. Other related Nꢀbased ligands have also been investigated as catalyst
6
ꢀ9
supports by our group and others. For environmental reasons, there have recently
been a growing interest in copperꢀcatalyzed aerobic alcohol oxidation in water.
9
10
Zhang and Rheingold reported waterꢀsoluble copper catalysts supported by Nꢀ
based ligands, which were active toward aerobic oxidation of benzyl alcohols in
1
1
water. Lipshutz also demonstrated that the designer surfactant TPGSꢀ750ꢀM was
effective at facilitating Cuꢀcatalyzed aerobic alcohol oxidation in an aqueous solution
(Table 1).
Notably, active copper species for aerobic oxidation of alcohols is a key
1
2
component in the enzyme Galactose Oxidase (GOase). The distorted square
pyramidal copper(II) active site in GOase consists of two nitrogen donors from two
histidine imidazoles and, at the axial position, a phenol group of tyrosine, which
1
3, 14
facilitates oxidative transformation.
Further evidences for the role of hydroxyl
group in oxidation were reported by Puddephatt and coꢀworkers, in which appended
phenol and hydroxyethyl groups in the ligand framework were shown to promote
1
5, 16
reactions between a dimethylplatinum(II) complex and O
2
.
Despite these findings,
no related studies have been carried out on molecular copper catalyst systems. Based
on established oxidation activities of copper catalysts and potential involvement of
appended hydroxyl groups in metalꢀcatalyzed oxidation, we prepared multidentate
ligands featuring N,N,O donors from pyridine, Schiff base, and hydroxyl groups with
three different ligand backbones. The corresponding Cu(II) complexes were used as
2

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catalysts for aerobic alcohol oxidation in an alkaline aqueous solution and their
catalytic activities were compared. In particular, this work aims to evaluate the effects
of appended hydroxyl groups associated with ligand rigidity and chain length on the
coordination environments at the Cu(II) center and their subsequent catalytic
activities toward oxidation reactions.
Table 1. Known Cu catalyst systems for aerobic oxidation of benzyl alcohol in water
Temp.
Cu catalyst
Solvent
Ref.
o
(
C)
Cu(OAc)
ꢀꢀCl) [Cu(bzm)Cl]
CuBr/bpy/NMI
2
•H
2
O/pytlꢀβꢀCD/Na
2
CO
3
H
2
2
O
O
100
RT
RT
9
(
2
2
H
10
11
2
2 wt% TPGSꢀ750ꢀM in H O
Experimental Section
Materials. 2ꢀPyridinecarboxaldehyde, 2ꢀaminophenol, 2ꢀaminoethanol, and 3ꢀ
aminopropanol (Aldrich) were of reagent grade and used as received. The commercial
grade ethyl acetate, dichloromethane, and hexane were distilled prior to use. All other
3

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chemicals and solvents used in this work are of reagent or analytical grade and were
used as received.
Physical Measurements. Fourier transform nuclear magnetic resonance spectra were
recorded by a Bruker’s Ascend 400 highꢀresolution magnetic resonance spectrometer.
Chemical shifts were reported in δ units (parts per million) using residual solvents
peaks as references. Mass spectrometric analysis was carried out using a Bruker micro
TOF spectrometer in the ESI mode whereas elemental analyses were performed on a
Perkin Elmer 2400 CHN. Fourier transform infrared spectroscopy (FTIR) were
collected on Bruker Alpha instrument (Bruker Optics GmbH, Ettlingen, Germany)
ꢀ1
(
400–4000 cm ). UVꢀvisible spectra of the aqueous solutions of the Cu(II) complexes
1
–3 were obtained from JASCO model Vꢀ530 and recorded in the range 200–800 nm.
Powder Xꢀray diffraction (PXRD) was performed by Bruker AXS model D8 Discover
o
with a monochromatic Cu K
α
(40 kV, 40 mA) source with a step size of 0.020 and a
ꢀ1
step time of 0.2 sꢀstep . The electron spin resonance (ESR) spectra were obtained at
room temperature using a Bruker Elexys500 Xꢀband spectrometer. The solid powder
samples were packed in 1.5 mm OD capillary tubes. All measurements were
performed at a frequency of ~9.8 GHz and microwave power of 2 mW. The center
field was set at 3200 G with 1500 G sweep width and the modulation amplitude was
at 4 G. Single crystal Xꢀray analyses of the complexes 2 and 3 were carried out at
VISTEC. The Xꢀray crystallographic data were collected at low temperatures on a
Bruker D8 venture using Photon II detector and IꢀS 3.0 Microfocus source, Mo K
α
radiation (λ = 0.71073 Å). Data integration was performed using SAINT and intensity
data were corrected based on the intensities symmetryꢀrelated reflections measured at
different angular setting (SADABS). The space group were determined using XPREP
whereas the structure was solved by intrinsic phasing method (XT program) and
2
refined by fullꢀmatrix least squares against F using the XL program based on ShelXle
engine. Product yields and percent substrate conversions obtained from catalytic
experiments were analyzed by GLC on a 6890N Agilent Technologies gas
chromatograph equipped with a 5973N Agilent Technologies quadrupole mass
detector.
Synthesis of the Schiff base ligands. To a 40 mL EtOH solution of 2ꢀ
pyridinecarboxaldehyde (950 ꢀL, 10 mmol) was added aminoalcohol (10 mmol). The
4

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reaction mixture was refluxed for a certain amount of time, after which all volatiles
were removed under vacuum.
2-((Pyridin-2-ylmethylene)amino)phenol (Hpyph). 2ꢀAminophenol (1.09 g, 10
mmol) was used. The mixture was refluxed for 1 h. Crystallization from slow
evaporation of diethyl ether solution gave a yellow powder in 60% yield (1.190 g, 6.0
1
mmol). H NMR (400 MHz, DMSOꢀd ): δ 9.23 (s, 1 H, ArOH), 8.71 (s, 1 H, =NH),
6
8.69 (m, 1 H, ArH), 8.38 (d J = 8 Hz, 1 H, ArH), 7.94 (t J = 8 Hz, 1 H, ArH), 7.50 (m,
1
H, ArH), 7.29 (m, 1 H, ArH), 6.92 (d J = 8 Hz, 1 H, ArH), 6.85 (m, 1 H, ArH).
1
3
1
C{ H} NMR (100 MHz, DMSOꢀd
6
): δ 159.4, 154.5, 151.4, 149.5, 136.8, 128.3,
ꢀ
1
1
1
2
25.4, 121.5, 119.7, 119.6, 116.3, 114.4 (aromatic Cs). FTꢀIR (cm ): 3378 (br, O–H),
628 (C=N), 1151 (C–OH).
-((Pyridin-2-ylmethylene)amino)ethanol (Hpyet). 2ꢀAminoethanol (600 ꢀL, 10
mmol) was used. The mixture was refluxed for 24 h. Solvent evaporation under
reduced pressure gave yellow oil in 93% yield (1.40 g, 9.3 mmol). The product was
1
pure based on the H NMR spectrum and was used in the next step without further
1
purifications. H NMR (400 MHz, DMSOꢀd
6
): δ 8.63 (m, 1 H, pyH), 8.31 (s, 1 H,
=
NH), 7.96 (m, 1 H, pyH), 7.87 (m, 1 H, pyH), 7.46 (m, 1 H, pyH), 4.65 (br s, 1 H,
1
3
1
OH), 3.68 (br s, 4 H, CH
2
CH
2
). C{ H} NMR (100 MHz, DMSOꢀd
6
): δ 162.7, 154.2,
ꢀ1
149.3, 136.8, 125.0, 120.4 (aromatic Cs), 63.1 60.5 (CH
2
CH ). FTꢀIR (cm ): 3278
2
(br, O–H), 1649 (C=N), 1059 (C–OH).
3-((Pyridin-2-ylmethylene)amino)propanol [Hpypr (a)] and tetrahydro-2-(2-
pyridinyl)-2H-1,3-oxazine [Hpypr (b)]. 3ꢀAminopropanol (765 ꢀL, 10 mmol) was
used. The reaction mixture was refluxed for 24 h. Solvent evaporation under reduced
pressure gave yellow oil (1.51 g). A mixture of open chain form and cyclic form was
obtained in ca. 8:1 ratio.
1
Hpypr (a): H NMR (400 MHz, DMSOꢀd ): δ 8.63 (d J = 4 Hz, 1 H, ArH), 8.34 (s, 1
6
H, =NH), 7.94 (d J = 8 Hz, 1 H, ArH), 7.86 (dt J = 8, 2 Hz, 1 H, ArH), 7.45 (m, 1 H,
ArH), 4.50 (t J = 5 Hz, 1 H, OH), 3.68 (dt J = 8, 1 Hz, 2 H, =NCH ), 3.48 (m, 2 H,
2
1
3
1
CH
δ 161.8, 154.2, 149.3, 136.8, 125.0, 120.4 (aromatic Cs), 58.5, 57.2, 33.6
CH CH CH ).
Hpypr (b): H NMR (400 MHz, DMSOꢀd
2 2 6
OH), 1.78 (quin J = 6 Hz, 2 H, CH ). C{ H} NMR (100 MHz, DMSOꢀd ):
(
2
2
2
1
6
): δ 8.50 (d J = 4 Hz, 1 H, ArH), 8.34 (s, 1
H, =NH), 7.79 (dt J = 8, 2 Hz, 1 H, ArH), 7.32 (m, 1 H, ArH), 5.06 (s, 1 H, pyCH),
5

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1
3
1
1
.31–4.09 (m, 6 H, CH
2
). C{ H} NMR (100 MHz, DMSOꢀd
6
): δ 148.5, 136.7,
ꢀ
2 2 2
123.4, 121.2 (aromatic Cs), 89.6 (pyCH), 67.3, 43.6, 26.5 (CH CH CH ). FTꢀIR (cm
1
): 3285 (br, O–H), 1649 (str, C=N), 1058 (str, C–OH).
Synthesis of the Cu(II) complexes (1–3). A equimolar mixture of CuCl
2 2
ꢀ2H O and
ligands 1–3 was stirred in 40 mL of CH Cl at room temperature for 24 h. All
2
2
volatiles were then removed in vacuo and the remaining solids were crystallized from
suitable solvents.
[
2-((Pyridin-2-ylmethylene)amino)phenol]copper(II) Chloride (1). Hpyph (0.22 g,
.1 mmol) and CuCl ꢀ2H O (0.20 g, 1.1 mmol) were used. Crystallization from slow
evaporation of the CH CN solution afforded brown powder in 46% yield (0.17 g, 0.51
mmol). FTꢀIR (cm ): 3432 (br, O–H), 1648 (C=N).
2-((Pyridin-2-ylmethylene)amino)ethanol]copper(II) Chloride (2). Hpyet (0.20 g,
.3 mmol) and CuCl ꢀ2H O (0.22 g, 1.3 mmol) were used. A layer diffusion of Et
onto the DMF solution of 2 at room temperature afforded a green microcrystalline
solid in 35% yield (0.27 g, 0.46 mmol). Anal. Calcd. for C16 Cu Cl : C, 33.76;
H, 3.54; N, 9.84 Found: C, 33.56; H, 3.53; N, 9.95. FTꢀIR (cm ): 3430 (br, O–H),
648 (C=N), 1058 (C–OH).
3-((Pyridin-2-ylmethylene)amino)propanol]copper(II) Chloride (3). Hpypr (0.20
g, 1.2 mol) and CuCl ꢀ2H O (0.21 g, 1.2 mol) were used. A layer diffusion of EtOAc
onto the DMF solution of 3 at room temperature afforded a green microcrystalline
solid in 30% yield (0.18 g, 0.36 mmol). Anal. Calcd. for C15 Cu Cl : C, 35.76;
H, 3.20; N, 8.34 Found: C, 36.13; H, 3.04; N, 8.21. FTꢀIR (cm ): 1607 (C=N), 1042
C–O–C).
1
2
2
3
ꢀ1
[
1
2
2
2
O
H
20
N
4
O
2
2
4
ꢀ1
1
[
2
2
H
16
N
3
O
2
2
3
ꢀ1
(
General procedure for alcohol oxidation
0
To a 3 mL of an aqueous solution of the Cu(II) complex (0.050 mmol Cu) and Cu
2
sheets (a total surface area = 1.0 cm ) was added 1.0 mmol of alcohol. To ensure the
same concentration of the Cu active species, 0.050 mmol Cu was calculated based on
0.025 mmol of the dinuclear copper complexes 1–3. A 2 mL aqueous solution of
TEMPO (0.050 mmol) and Na CO (0.10 mmol) was added and the reaction mixture
2
3
was stirred in air at room temperature for a given time. Then, the aqueous solution
was extracted with 3x10 mL of CH Cl . The combined organic layers were dried
2
2
using anhydrous Na
2
SO
4
and filtered, after which the solvent was removed. The
6

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percent conversions were quantified using the GCꢀMS method, in which EtOAc was
used as a solvent in the presence of anisole (0.10 mmol) as the internal standard.
Results and Discussion
Synthesis of Ligands and the Cu(II) Complexes.
The (imino)pyridine ligands Hpyph, Hpyet, Hpypr were prepared according
1
7, 18
to previous literatures,
with slight modifications via condensation of 2ꢀ
pyridinecarboxaldehyde and their respective aminoalcohols in EtOH under refluxed
conditions (Scheme 1). The phenyleneꢀcontaining product Hpyph appeared as a
yellow solid which was purified and isolated in 60% yield by crystallization in diethyl
ether. Meanwhile, ligands with an alkyl framework Hpyet and Hpypr were obtained
1
as yellow oils. Based on their H NMR spectra in DMSOꢀd , the ethylene linker
6
Hpyet exists only in an open chain form whereas the propyl derivative Hpypr is in
1
equilibrium between the open chain (a) and the cyclic (b) forms. The H NMR
integration reveals that the crude product of Hpypr has the open chain form as a major
species (ca. 8:1 ratio of (a):(b) in DMSOꢀd
6
, Figure S5). On the contrary, Puddephatt
et. al., whose work was published during the course of our study, reported the cyclic
1
6
3
form of Hpypr (b) as a dominant product (ca. 80%) in CDCl solution.
Scheme 1. Ligand synthesis
Treatments of CuCl
2
2 2 2
ꢀ2H O with an equimolar of Hpyph in CH Cl at room
temperature for 24 h resulted in a brown solid, while, under the same conditions,
reactions with Hpyet or Hpypr gave green crystals (Scheme 2). The phenyleneꢀ
3
containing Cu(II) complex 1 was isolated via slow evaporation of the CH OH
solution at room temperature, which afforded a brown crystalline solid in 46% yield.
7

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Attempts to obtain a solid state structure of 1 have so far been unsuccessful. On the
other hand, layering the DMF solutions of Cu(II) complexes 2 and 3 with Et O (for 2)
2
or EtOAc (for 3) at room temperature gave green crystalline solids suitable for Xꢀray
analysis in 35% and 30% yields, respectively. The FTꢀIR spectra of 1 and 2 contain a
ꢀ1
broad absorption band at ca. 3430 cm whereas no absorption band corresponding to
O–H stretching was observed in that of 3 (Figure S8, S10, and S12). Furthermore, the
powder XRD analyses of 2 and 3, both from the crude samples and the isolated
crystalline solids, reveal the same XRD patterns confirming that the obtained crystals
represent the major products from Cu complexation (Figure S14 and S15). It should
be noted that Pal and coꢀworkers have previously reported the synthesis and crystal
structure of Cu(pyph)Cl, generated from a similar reaction of Hpyph with
1
7
CuCl ꢀ2H O and KOH in refluxing CH OH. For our experiment, even in the
2
2
3
absence of a base, it is possible that the acidic phenolic proton may be deprotonated.
To gain insights into the structure of 1, the powder XRD spectrum of the isolated
complex 1 and that simulated from single crystal data of Cu(pyph)Cl were compared,
showing different XRD patterns (Figure S13). On the basis of powder XRD results
and the presence of OH absorption band in the FTꢀIR spectrum, the dimeric structure
similar to that of 2 is proposed for the phenylene derivative 1 (Scheme 2 and Figure
1).
Scheme 2. Synthesis of Cu(II) complexes 1–3.
Structure description of complexes 2 and 3
Xꢀray data of 2 reveal a chlorideꢀbridged asymmetric dinuclear Cu(II)
complex with a square pyramidal geometry at the copper center (
τ
= 0.06, where
τ = 0
1
9
for a square pyramid and
τ = 1 for a trigonal bipyramid), as shown in Figure 1. The
Cu(1) and Cu(1A) ions are related by inversion and each copper center is chelated by
8

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pyridine and imine N donors, both of which are at basal sites. A terminal Cl and a
bridging Cl ion complete basal positions whereas another bridging Cl locates at the
o
axial position. The bite angle Npy(2)–Cu(1)–Nim(1) is small at 80.26(4) whereas the
bond distances of Cu(1)–Npy(2) (2.030(1) Å) and Cu(1)–Nim(1) (2.032(1) Å) are
closely similar. The terminal Cu(1)–Cl(2) and bridging Cu(1)–Cl(1) bond distances at
basal sites are 2.2833(6) and 2.2679(6) Å, respectively, which are considerably
shorter than that of the bridging, axial Cu(1)–Cl(1A) bond (2.791(1) Å). Furthermore,
through bridging Cl ligands, the intramolecular CuꢀꢀꢀCu separation is 3.647(1) Å,
2
0, 21
comparable to other related dinuclear complexes with the Cu(
ꢀ
ꢀCl) Cu core.
It is
2
worth mentioning that Puddephatt and coꢀworkers have recently reported results from
1
6
DFT calculations of the related complexes Pt(L)Me (L = Hpyet and Hpypr). The
2
calculations suggest that the stable conformations involve a hydroxyl substituent
directing toward the Pt(II) center and forming a weak hydrogen bond. On the
contrary, the related crystal structure of the Cu(II) complex 2 exhibits no apparent Cuꢀ
ꢀOH interactions.
O1
Cl2
N2A
Cl1A
N1
Cu1A
N1A
Cu1
N2
Cl1
Cl2A
O1A
Figure 1. The ORTEP representation of 2 with thermal ellipsoid at the 50%
probability. Selected bond parameters: Cu1–N1 = 2.032(1), Cu1–N2 = 2.030(1),
Cu1–Cl1 = 2.2833(6), Cu1–Cl2 = 2.2679(6), Cu1–Cl1A = 2.791(1) Å; N1–Cu1–N2 =
8
1
0.26(3), Cl1–Cu1–Cl2 = 92.06(1), N1–Cu1–Cl1 = 173.07(3), N2–Cu1–Cl2 =
o
69.55(3), Cl1–Cu1–Cl1A = 88.33(1) .
Unlike 2, the solid state structure of 3 reveals an unsymmetric dinuclear
Cu(II) complexes, featuring five coordinate Cu1 and four coordinate Cu2 (Figure 2).
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Both Cu1 and Cu2 ions are bridged by the oxygen atom O2 of the alkoxy group and a
o
chloride ion. The bridging Cu1–O2–Cu2 bond angle is 110.96(9) . The Cu1 ion has a
1
9
distorted square pyramidal geometry based on has the
τ value of 0.26 and is
supported by a tridentate N,N,O ligand, a terminal Cl, and a bridging Cl ion. On the
other hand, the Cu2 ion possesses a distorted square planar geometry and is
o
surrounded by the bidentate N,O ligand with the bite angle O2–Cu2–N3 of 81.73(9) ,
a terminal Cl, and a bridging Cl. The sum of six independent angles at Cu2 is ca.
o
o
7
03 , significantly deviated from the expected 720 for the square planar structure.
Interestingly, the CuꢀꢀꢀCu separation in 3 is 3.199(2) Å, which is significantly shorter
than that in 2. In addition, despite the presence of chiral centers such as Cu1 and C10
atoms in the molecule, only one diastereomer crystallized from a mixture of
DMF:EtOAc solvent.
Figure 2. The ORTEP representation of 3 with thermal ellipsoid at the 50%
probability. Selected bond parameters: Cu1–N1 = 2.007(3), Cu1–N2 = 2.004(3),
Cu1–O2 = 1.962(2), Cu2–O2 = 1.920(2), Cu2–N3 = 2.004(3), Cu1–Cl1 = 2.242(1),
Cu1–Cl2 = 2.591(2), Cu2–Cl2 = 2.275(1), Cu2–Cl3 = 2.209(1) Å; N1–Cu1–N2 =
80.8(1), Cl1–Cu1–O2 = 94.67(6), Cl2–Cu1–O2 = 77.08(6), O2–Cu1–N2 = 91.04(9),
Cl2–Cu2–Cl3 = 96.36(3), O2–Cu2–N3 = 81.73(9), Cl2–Cu2–O2 = 86.22(6), N3–
o
Cu2–Cl3 = 95.57(7), Cu1–O2–Cu2 = 110.96(9), Cu1–Cl2–Cu2 = 81.92(3) .
For the (imino)pyridine ligands, the coordination behaviors of the appended
hydroxyl group are rather unexpected. The related NNO and ONO ligand systems
featuring an appended alcohol functional group have been crystallographically
16, 18, 22, 23
characterized or proposed as tridentate ligands with MꢀꢀOH interactions.
In
contrast, our results have revealed no CuꢀꢀOH interaction in the solid state structure,
1
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for the ethyleneꢀbased ligand. Furthermore, for the slightly longer propylene linker,
the intermolecular nucleophilic addition of OH group was evident according to the
crystal structure. In particular, formation of the dinuclear Cu(II) complex 6 is likely a
result of Cuꢀcatalyzed imine hydrolysis to generate Cuꢀcoordinated 2ꢀ
pyridinecarboxaldehyde followed by nucleophilic addition of the appended propyl
alcohol at the aldehyde group. Several works have previously documented copperꢀ
promoted hydrolysis of the Schiff base ligands to generate the constituent aldehyde
24, 25
and amine precursors.
Hammershøi and Cuevas also independently demonstrated
26ꢀ28
that the coordinated imine ligands were susceptible to nucleophilic attacks.
However, to the best of our knowledge, Cuꢀcatalyzed imine hydrolysis followed by
nucleophilic addition at aldehyde to afford the 8ꢀmembered dinuclear metallacycle is
unprecedented.
The effect of linker chain length of pyridineꢀdiimineꢀbased ligands on
complex's stability has been previously investigated by Ghosh and coꢀworkers. It was
found that while the Cu(II) complex of the ethyleneꢀlinked pyridineꢀdiimine ligand
was stable, the related propyleneꢀlinked ligand underwent hydrolysis and
+
25
rearrangement in the presence of Cu(II) or H ion. Lower stability of the propyleneꢀ
based chelate ligand was attributed to more steric strain, as a result of two adjacent
sixꢀmembered metallacyclic rings. However, in this work, since the hydroxyl groups
of the (imino)pyridine ligands do not coordinate to the Cu(II) centers, we reason that
the drastic differences in the ligand activity stems from the alkyl chain length. In
particular, the longer alkyl chain of the propyl alcohol enables the hydroxyl group to
reach farther away from the Cu coordination sphere for subsequent nucleophilic
addition.
Spectroscopic and ESR study.
UVꢀvis spectra of the Cu(II) complexes in DMSO contain a weak absorption
ꢀ1
band at 447, 709, and 714 cm assignable to Cu(II) dꢀd transitions for 1, 2, and 3,
respectively (Figure S16–S18). Meanwhile, intense absorption bands between 200–
ꢀ
1
3
50 cm were attributed to
π
ꢀ
π
* transition (intraligand) and ligandꢀtoꢀmetal charge
The ESR powder spectra of 1–3 contain the two signals, ꢀꢁ and ꢀ∥, as
ꢀvalues of
ꢀꢁ > 2.0023 is
2
9, 30
transfer.
illustrated in Figure 3. Table 2 shows the magnetic parameters for the
each Cu complex. The elongated axial symmetry characterized by ꢀ∥
ꢀ
>
1
1

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consistent with distorted square pyramid and square planar geometry at Cu(II) ions, as
3
1
confirmed by the solid state structures of 2 and 3 (vide supra). The ꢀ ꢀvalue
parameters obtained from 1–3 are comparable to our previously reported ESR data for
the mononuclear distorted square planar complex of the type (NN')CuCl (ꢀ ~ 2.07–
ꢁ
2
7
2
.08 and ꢀ ~ 2.25–2.28), suggesting weak exchange interactions between copper
∥
ions in the dinuclear Cu complexes. Another interesting feature of these ESR data is a
significantly smaller separation between ꢀ and ꢀ for 3 (∆ꢀ = 0.125 compared to
ꢁ
∥
0.204 (for 1) and 0.188 (for 2); Table 2). The lowering in ∆ꢀ value is attributed to a
9
stronger interaction at the d Cu electron, as a result of a bridging paramagnetic O
donor compared to a bridging Cl atom (Figure 1 and 2). Furthermore, based on the
closely related ꢀꢀvalue parameters of 1 and 2 together with their FTꢀIR and powder
XRD data, we propose that the solid state structure of 1 possesses similar
coordination environment to that of 2 (vide supra).
Figure 3. ESR powder spectra of the Cu complexes 1–3 at room temperature. The gꢀ
values of each complexes were shown as ꢀ and ꢀ indicated in arrow signs.
ꢁ
∥
Table 2. Magnetic parameters obtained from the ESR spectra of 1–3 in solid powder.
Cu complex
ꢀꢁ
ꢀ∥
∆ꢀ*
1
2
3
2.091
2.068
2.091
2.295
2.256
2.216
0.204
0.188
0.125
1
2

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*
∆ꢀ is calculated by ꢀ − ꢀ .
∥ ꢁ
Aerobic alcohol oxidation in water
The Cu(II) complexes 1–3 were investigated as catalysts for aerobic alcohol
oxidation in an aqueous solution. First, the catalytic activities of 1–3 were compared
using 1.0 mmol of benzyl alcohol as the model substrate, catalyzed by 5 mol% Cu(II)
0
2
species, 5 mol% TEMPO, Cu sheets with a total area of 1 cm , and 10 mol% Na CO
2
3
2
under air at room temperature with the H O solvent (Table 3). In comparison, the
activities of 2 and 3 are comparable, giving benzaldehyde as a sole product in
excellent conversions after 16 h, and higher than that of 1 (entries 1–3). Interestingly,
different coordination environments at the Cu centers in the dinuclear Cu(II)
complexes 2 and 3 did not have a significant impact on oxidation activities. The
reduced catalytic activities of 1 was explained by a decrease in the electron density at
3
2
the copper center as a result of the more electronꢀwithdrawing phenylene groups.
Next, the effect of base was investigated using the catalyst 2. Under similar
conditions, 10 mol% of Na CO at room temperature furnished a near quantitative
conversion of benzyl alcohol to benzaldehyde whereas other bases including Cs CO
K PO , and Nꢀmethylimidazole (NMI) resulted in lower conversions (entries 4–6).
2
3
2
3
,
3
4
Decreased conversions were also obtained with reduced catalyst loadings (from 5
mol% to 1 mol% Cu), the use of CuCl with no ligand, and the in situ generated
2
Cu(II) complex (entries 7–9). Similarly, lower conversions were observed with the
0
catalyst systems in the absence of either Cu sheets (75%, entry 10) or Cu(II) catalyst
(18%, entry 11). These results suggest that the most effective catalyst system requires
0
33, 34
both Cu(II) species and Cu to undergo a comproportionation reaction
generate the Cu(I) species, which are more active toward catalytic oxidation.
and
3
5
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3

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a
Table 3. Catalyst comparison for aerobic alcohol oxidation
b
Entry
Cu catalyst
Base
Conversion (%)
c
1
1
2
3
2
2
2
2
Na
2
2
CO
3
3
83
93
94
84
58
87
86
37
62
75
18
2
3
4
5
Na
CO
Na CO3
2
2 3
Cs CO
K PO4
3
c
6
NMI
d
7
Na CO3
2
8
CuCl
2
Na
2
CO
3
9
CuCl /Hpyet
Na CO3
2
2
e
1
0
2
Na
2
CO
3
1
1
ꢀ
Na CO3
2
a
Reaction Conditions: Benzyl alcohol (1.0 mmol), Cu catalyst (5 mol% based on Cu),
0
2
TEMPO (0.050 mmol), base (0.10 mmol), four Cu sheets with a total area of 1 cm in H O (5
2
mL) under aerobic conditions with 0.010 mmol of anisole as an internal standard, room
b
c
d
temperature, 16 h. Determined by GCꢀMS. Reaction time = 24 h. The catalyst loading = 1
e
0
mol% of 2. Without Cu sheet.
To investigate the substrate scope, the catalyst 2 was selected for further
studies. Aerobic oxidation of benzylic alcohol derivatives were evaluated as shown in
Table 4. Electronꢀdonating OMe and OH groups generally gave good to excellent
conversions after 16 h at room temperature (entries 1–3). On the other hand, the
2
electronꢀwithdrawing NO group on benzyl alcohol afforded slower reactions as only
moderate conversions were observed after 48 h (entries 4 and 5). Halogenꢀsubstituted
benzyl alcohols gave mixed results: 4ꢀiodobenzyl alcohol was completely converted
to the oxidized product after 16 h whereas 4ꢀbromo and 4ꢀchlorobenzyl alcohols
needed 48 h to reach high conversions (87% and 93%, respectively; entries 6–8).
Cinnamyl alcohol was oxidized to the corresponding aldehyde in a quantitative
conversion after 8 h (entry 9). Sulfur atom in 2ꢀthiophenemethanol is well tolerated
1
4

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giving a complete conversion after 24 h (entry 10). Attempts to oxidize aliphatic
primary and secondary alcohols including cyclohexane, 1ꢀhexanol, 2ꢀmethylꢀ1ꢀ
o
pentanol under similar catalytic conditions at room temperature and at 100 C were
unsuccessful.
a
Table 4. Substrate scope study
b
Entry
aldehyde
Time (h) Conversion (%)
1
16
16
16
>99
69
2
3
4
>99
42
4
8
8
4
51
5
6
7
8
16
16
16
>99
c
c
59 (87)
58 (93)
9
8
>99
d
1
0
16
76 (>99)
a
Reaction Conditions: Alcohol substrate (1.0 mmol), Cu complex 2 (0.025 mmol), TEMPO
0.050 mmol), Na CO (0.10 mmol), four Cu sheets with a total area of 1 cm in H O (5 mL)
2 3 2
0
2
(
under aerobic conditions at room temperature with 0.010 mmol of anisole as an internal
b
standard. Based on GC analysis. Since the corresponding aldehyde is the only product
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5

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c
observed, % conversion = the ratio of product x 100/(product + starting material). Reaction
d
time = 48 h. Reaction time = 24 h.
Conclusion
The appended hydroxyl substituents of the (imino)pyridine ligands Hpyph,
2
Hpyet, and Hpypr behave differently in the reaction with CuCl under the same
conditions. In the case of the ethyleneꢀbased ligand Hpyet, the solid structure of 2
reveals a bidentate N,N binding with no CuꢀꢀOH interactions. Surprisingly, a slightly
longer propyl analogue yielded a considerably different crystal structure for the
dinuclear Cu(II) complex 3, as a result of a nucleophilic addition of the appended
propyl alcohol to the aldehyde group of 2ꢀpyridinecarboxaldehyde. Despite the lack
of Xꢀray data for the phenyl derivative 1, the symmetrical dimeric structure similar to
that of 2 was proposed based on FTꢀIR, PXRD, and ESR data. Activity comparisons
0
of the catalyst system Cu(II) complex/Cu /TEMPO/Na
2
CO
3
in water revealed that
different coordination environments at the dinuclear copper centers in 2 and 3 do not
have a significant impact on catalytic oxidation of benzyl alcohol. On the other hand,
lower activities of 1 was contributed to the presence of the more electronꢀwithdrawing
phenylene groups in the ligand framework. In general, this work provides insights
into different chemical behaviors of the appended hydroxyl groups based on slightly
different ligand chain lengths. Lessons learned from the corresponding Cu
coordination and subsequent catalytic alcohol oxidation activities in water of these
complexes should be applicable to other metal complexes of related hydroxylꢀ
containing ligand systems.
1
6

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Acknowledgements
This research is funded by the Thailand Research Fund (Grant No. RSA6080082) The
authors also gratefully acknowledge Center for Innovation in Chemistry (PERCHꢀ
CIC), Faculty of Science, Mahidol University and Mahidol University under the
National Research Universities Initiative. A.J. also appreciates financial assistance
from Science Achievement Scholarship of Thailand (SAST).
Supplementary Materials. Characterization data for all new compounds reported in
this work, crystallographic data contained in a CIF file of the Cu(II) complexes 2
(CCDC 1564878) and 3 (CCDC 1564879) are available free of charge via the Internet
at xxxxx.
References
1
.
O. Das and T. K. Paine, in Transition metal catalysis in aerobic alcohol
oxidation, eds. F. Cardona and C. Parmeggiani, The Royal Society of
Chemistry, 2015, DOI: 10.1039/9781782621652ꢀ00040, ch. 2, pp. 40ꢀ69.
M. F. Sammelhack, C. R. Schmid, D. A. Cortes and C. S. Chou, J. Am. Chem.
Soc., 1984, 106, 3374ꢀ3376.
2.
3.
4.
P. Gamez, I. W. C. E. Arends, J. Reedijk and R. A. Sheldon, Chem. Commun.,
2
003, 2414ꢀ2415.
E. T. T. Kumpulainen and A. M. P. Koskinen, Chem. Eur. J., 2009, 15, 10901ꢀ
0911.
1
5
6
.
.
J. M. Hoover, J. E. Steves and S. S. Stahl, Nat. Protoc., 2012, 7, 1161ꢀ1166.
P. Thongkam, S. Jindabot, S. Prabpai, P. Kongsaeree, T. Wititsuwannakul, P.
Surawatanawong and P. Sangtrirutnugul, RSC Adv., 2015, 5, 55847ꢀ55855.
M. Kongkaew, K. Sitthisuwannakul, V. Nakarajouyphon, S. Pornsuwan, P.
Kongsaeree and P. Sangtrirutnugul, Dalton Trans., 2016, 45, 16810ꢀ16819.
P. J. Figiel, A. Sibaouih, J. U. Ahmad, M. Nieger, M. T. Räisänen, M. Leskelä
and T. Repo, Adv. Synth. Catal., 2009, 351, 2625ꢀ2632.
G. Zhang, X. Han, Y. Luan, Y. Wang, X. Wen, L. Xu, C. Ding and J. Gao,
RSC Adv., 2013, 3, 19255ꢀ19258.
G. Zhang, Y. Z. Zhang, W.ꢀF. Lo, J. Jiang, J. A. Golen and A. L. Rheingold,
Polyhedron, 2016, 103, 227ꢀ234.
7
8
9
1
1
1
.
.
.
0.
1.
2.
B. H. Lipshutz, M. Hageman, J. C. Fennewald, R. Linstadt, E. Slack and K.
Voigtritter, Chem. Commun., 2014, 50, 11378ꢀ11381.
J. A. D. Cooper, W. Smith, M. Bacila and H. Medina, J. Biol. Chem., 1959,
234, 445ꢀ448.
13.
14.
15.
J. W. Whittaker, Chem. Rev., 2003, 103, 2347ꢀ2363.
P. Gamez, I. A. Koval and J. Reedijk, Dalton Trans., 2004, 4079ꢀ4088.
M. E. Moustafa, P. D. Boyle and R. J. Puddephatt, Chem. Commun., 2015, 51,
10334ꢀ10336.
1
7

New Journal of Chemistry
Page 18 of 19
View Article Online
DOI: 10.1039/C7NJ03113K
1
6.
7.
K. A. Thompson, C. Kadwell, P. D. Boyle and R. J. Puddephatt, J.
Organomet. Chem., 2017, 829, 22ꢀ30.
D. Sunirban, S. A. Maloor, S. Pal and S. Pal, Cryst. Growth Des., 2006, 6,
1
2013ꢀ2018.
1
1
8.
9.
S. Striegler and M. Dittel, Inorg. Chem., 2005, 44, 2728ꢀ2733.
A. W. Addison, T. N. Rao, J. Reedijk, J. v. Rijn and G. C. Verschoor, J.
Chem. Soc. Dalton Trans., 1984, 1349ꢀ1356.
20.
M. Rodríguez, A. Llobet, M. Corbella, A. E. Martell and J. Reibenspies,
Inorg. Chem., 1999, 38, 2328ꢀ2334.
2
2
1.
2.
C. P. Pradeep and S. K. Das, Polyhedron, 2009, 28, 630ꢀ636.
G. S. Nyamato, S. O. Ojwach and M. P. Akerman, Dalton Trans., 2016, 45,
3407ꢀ3416.
2
2
3.
4.
C. P. Pradeep and S. K. Das, Coord. Chem. Rev., 2013, 257, 1699.
X. R. Bu, C. R. Jackson, D. V. Derveer, X. Z. You, Q. J. Meng and R. X.
Wang, Polyhedron, 1997, 16, 2991ꢀ3001.
25.
26.
27.
28.
29.
30.
M. S. Ray, R. Bhattacharya, S. Chaudhuri, L. Righi, G. Bocelli, G.
Mukhopadhyay and A. Ghosh, Polyhedron, 2003, 22, 617ꢀ624.
A. M. Arnaiz, A. Carbayo, J. V. Cuevas, V. Diez, G. GarcíaꢀHerbosa, R.
González, A. Martínez and A. Muñoz, Eur. J. Inorg. Chem., 2007, 4637ꢀ4644.
L. Bendahl, A. Hammershøi, D. K. Jensen, S. Larsen, A. Riisager, A. M.
Sargeson and H. O. Sørensen, J. Chem. Soc. Dalton Trans., 2002, 3054ꢀ3064.
T. Birkle, A. Carbayo, J. V. Cuevas, G. GarcíaꢀHerbosa and A. Muñoz, Eur. J.
Inorg. Chem., 2012, 2259ꢀ2266.
N. A. Bailey, D. E. Fenton, R. Moody, C. O. Rodriguez de Barbarin and I. N.
Sciambarella, J. Chem. Soc. Dalton Trans., 1987, 2519ꢀ1529.
K. R. J. Thomas, P. Tharmaraj and V. Chandrasekhar, Polyhedron, 1995, 14,
977ꢀ982.
31.
32.
33.
E. Garribba and G. Micera, J. Chem. Ed., 2006, 83, 1229ꢀ1232.
J. E. Steves and S. S. Stahl, J. Am. Chem. Soc., 2013, 135, 15742ꢀ15745.
S. Harrisson, P. Couvreur and J. Nicolas, Macromolecules, 2012, 45, 7388ꢀ
7396.
3
3
4.
5.
L.ꢀM. Zhang and T. C. W. Mak, J. Am. Chem. Soc., 2016, 138, 2909ꢀ2912.
B. L. Ryland, S. D. McCann, T. C. Brunold and S. S. Stahl, J. Am. Chem.
Soc., 2014, 136, 12166ꢀ12173.
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A table of content:
Effects of appended hydroxyl group's chain length from a series of
imino)pyridine ligands on copper coordination and copper-catalyzed
aerobic oxidation of alcohols were investigated.
(