One-, two- and three-dimensional coordination polymers based on copper paddle-wheel SBUs as selective catalysts for benzyl alcohol oxidation

Article abstract of DOI:10.1016/j.jssc.2019.06.011

Copper-based coordination polymers, Cu₂(μ₂-MeCO₂)₄(DABCO) (1), [Cu(1,4-BDC-Br)(DABCO)_0.5]. xDMF.yH₂O (2) and Cu(1,4-BDC-OH)(DMF) (3) (1,4-BDC-Br = 2-bromoterephthalate, DABCO = triethylenediamine, 1,4-BDC-OH = 2-hydroxyterephthalate) were solvothermally synthesized. Two polymorphs of 1 (1a and 1b) were obtained which both contain 1-D chains with similar connectivity and different packing. Compound 2 is a 3-D metal-organic framework (MOF) and 3 exhibits a 2-D structure. Structural analyses of the coordination polymers showed that they all have similar paddle-wheel secondary building units (SBUs). The coordination polymers 1–3 were characterized by FT-IR, thermogravimetric analysis (TGA), powder X-ray diffraction (XRD) and atomic absorption spectroscopy. Compound 2 is further analyzed by nitrogen adsorption/desorption technique. The obtained coordination compounds were employed as heterogeneous catalysts for selective oxidation of benzyl alcohol to benzoic acid using CH₂Cl₂ as solvent and tert-butyl hydroperoxide (TBHP) as oxidant. The catalysts displayed good activity in the oxidation reaction and could be reused without noticeable loss of activity.

Full text of DOI:10.1016/j.jssc.2019.06.011

Accepted Manuscript
One-, two- and three-dimensional coordination polymers based on copper paddle-
wheel SBUs as selective catalysts for benzyl alcohol oxidation
Leila Asgharnejad, Alireza Abbasi, Mahnaz Najafi, Jan Janczak
PII:
S0022-4596(19)30298-1
https://doi.org/10.1016/j.jssc.2019.06.011
YJSSC 20806
DOI:
Reference:
To appear in: Journal of Solid State Chemistry
Received Date: 26 January 2019
Revised Date: 24 April 2019
Accepted Date: 5 June 2019
Please cite this article as: L. Asgharnejad, A. Abbasi, M. Najafi, J. Janczak, One-, two- and three-
dimensional coordination polymers based on copper paddle-wheel SBUs as selective catalysts
for benzyl alcohol oxidation, Journal of Solid State Chemistry (2019), doi: https://doi.org/10.1016/
j.jssc.2019.06.011.
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Graphical abstract:
Cu(II)-coordination polymers with binuclear paddle-wheel SBUs as heterogeneous catalysts
for benzyl alcohol oxidation

ACCEPTED MANUSCRIPT
One-, two- and three-dimensional coordination polymers based on copper paddle-wheel
SBUs as selective catalysts for benzyl alcohol oxidation
Leila Asgharnejada, Alireza Abbasib*, Mahnaz Najafib and Jan Janczakc
aSchool of Chemistry, Alborz Campus, University of Tehran, Tehran, Iran.
bSchool of Chemistry, College of Science, University of Tehran, P.O. Box 14155-6455, Tehran, Iran.
E-mail: aabbasi@khayam.ut.ac.ir.
cInstitute of Low Temperature and Structure Research, Polish Academy of Sciences, PO Box 1410,
50-950 Wrocław, Poland.
Abstract
Copper-based
coordination
polymers,
Cu2(µ2-MeCO2)4(DABCO)
(1),
[Cu(1,4-BDC-
Br)(DABCO)0.5].xDMF.yH2O (2) and Cu(1,4-BDC-OH)(DMF) (3) (1,4-BDC-Br = 2-bromoterephthalate,
DABCO = triethylenediamine, 1,4-BDC-OH = 2-hydroxyterephthalate) were solvothermally synthesized. Two
polymorphs of 1 (1a and 1b) were obtained which both contain 1-D chains with similar connectivity and
different packing. Compound 2 is a 3-D metal-organic framework (MOF) and 3 exhibits a 2-D structure.
Structural analyses of the coordination polymers showed that they all have similar paddle-wheel secondary
building units (SBUs). The coordination polymers 1-3 were characterized by FT-IR, thermogravimetric analysis
(TGA), powder X-ray diffraction (XRD) and atomic absorption spectroscopy. Compound 2 is further analyzed
by nitrogen adsorption/desorption techniques. The obtained coordination compounds were employed as
heterogeneous catalysts for selective oxidation of benzyl alcohol to benzoic acid using CH2Cl2 as solvent and
tert-butyl hydroperoxide (TBHP) as oxidant. The catalysts displayed good activity in the oxidation reaction and
could be reused without noticeable loss of activity.
Keywords: Cu-coordination polymers, crystal structure, selective catalysts, benzyl alcohol oxidation
1

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1. Introduction
Selective oxidation of alcohols to aldehyde, ketone or acid products is a pivotal transformation in fine chemicals
and pharmaceutical industries. Catalytic oxidation systems have been focused from industrial and academic
views. [1, 2] Traditionally, stoichiometric amounts of oxidizing agents (e.g. MnO2, CrO3, K2Cr2O7, SeO2, etc)
have been utilized under harsh reaction industrial conditions which caused environmental issues and increased
production costs. Therefore, development of active and selective catalytic systems is a continuing demand for
economic and sustainable chemical transformations. [1, 3]
There are some reports about development of catalysts based on supported noble metals such as Au,[4] Pd,[5]
Ru[6] or a combination of them.[7] However, these catalysts are expensive and suffer from sustainable points of
views.[8] Transition metal-based catalysts were also immobilized using supports including graphene oxide,[9]
polymers,[10] metal nanoparticles[11] and silica.[12] For instance, there have been many reports on the catalytic
oxidation of alcohols over supported Cu(II) complexes.[13-16]
Coordination polymers and metal-organic frameworks (MOFs), with adjustable molecular structures and
electronic properties, offer a plentiful supply of transition metals with catalytic characteristics. These materials
are inorganic-organic hybrids assembled from metal ions or clusters which are held by organic linkers. [17, 18]
Unique features of these crystalline compounds such as uniform arrangement of catalytic sites with high
concentration, stable architecture and reusability make them suitable candidates in the field of heterogeneous
catalysis. [19-21]
Copper (II)-coordination polymers and MOFs have been already proved to be promising catalysts for alcohol
oxidation.[22-26] In this regard, we previously reported copper molybdate [CuMoO4(1,10-Phen)].H2O (Phen =
1,10-phenanthroline) coordination polymer and Cu3(BTC)2 MOF (BTC = 1,3,5-tricarboxylate) as heterogeneous
catalysts for epoxidation of allylic alcohols. [27, 28] Recently, Rajeev Gupta’s group synthesized Cu(II)-
coordination polymers containing metalloligands and applied them as heterogeneous catalysts for the oxidation
of benzyl alcohol. [29] Catalytic oxidation of benzyl alcohol was also reported over other Cu(II)-based MOFs
and coordination polymers such as Cu(1,4-BDC)(DABCO)0.5 (1,4-BDC = terephthalic acid) [30] and Cu3(BTC)2
[23], {[Cu(fcz)2(H2O)].SO4.DMF.2CH3OH·2H2O}n and {[Cu(fcz)2Cl2].2CH3OH}n (fcz
difluorophenyl)-1,1-bis[(1H-1,2,4-triazol-l-yl)methyl]ethanol).[22]
=
1-(2,4
Herein, we report solvothermal self-assembly synthesis and structural features of new 1-D, 2-D and 3-D
coordination polymers based on Cu(II) paddle-wheel SBUs including Cu2(µ2-MeCO2)4(DABCO) (1), [Cu(1,4-
BDC-Br)(DABCO)0.5].xDMF.yH2O (2) and Cu(1,4-BDC-OH)(DMF) (3). The prepared coordination polymers
were applied as heterogeneous catalysts for oxidation of benzyl alcohol.
2. Experimental
2.1. Materials and characterization
All reagents were purchased from commercial sources and used without further purification.
FT-IR spectra were acquired by means of a Bruker Equinox 55 spectrometer equipped with a single reflection
diamond ATR system. Powder X-ray diffraction (PXRD) patterns were recorded on a PANalytical X'Pert PRO
instrument using Cu Kα radiation (λ = 1.5406 Å). Thermogravimetric analysis (TGA) was measured in N2
atmosphere using Dupont 951 Thermogravimetric Analyzer. The surface area and pore volume of MOF 2 were
2

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identified by nitrogen adsorption/desorption measurements at liquid nitrogen temperature utilizing BELSORP-
mini II instrument. The sample was degassed under vacuum at 130 °C for 4 h before the experiments. The
oxidation products were determined using a gas chromatograph (HP, Agilent 5890) equipped with a capillary
column (HP-1) and a flame ionization detector (FID). The copper content in the catalysts was measured by AA-
400 atomic absorption spectrometer.
2.2. Synthesis of 1a and 2
A mixture of Cu(NO3)2.3H2O (0.06 g, 0.25 mmol), DABCO (0.07 g, 0.62 mmol), 1,4-BDC-Br (0.044 g, 0.18
mmol) and CH3COOH (0.08 mL) was dissolved in DMF (5 mL) and then stirred for 10 min. The suspension
was transferred into a Teflon-lined autoclave and kept at 110 ºC for 24 h to afford a mixture of 1a and 2. Green
parallelepiped crystals of 1a were settled at the bottom of the vessel and the above solution containing small
crystals of 2 were isolated by decantation. Crystals of 1a and 2 were washed with fresh DMF and dried in air.
2.3. Synthesis of 1b
Coordination polymer 1b was synthesized by solvothermal reaction of Cu(NO3)2.3H2O (0.016 g, 0.07 mmol),
DABCO (0.056 g, 0.5 mmol) and CH3COOH (0.08 mL) in DMF (5 mL). After stirring of the reagents for 10
min, the mixture was placed into a Teflon-lined autoclave and kept at 120 ºC for 24 h. Blue parallelepiped
crystals of 1b were separated by filtration, washed with DMF and then dried in air.
2.4. Synthesis of 3
The 1,4-BDC-OH ligand was synthesized according to previous method reported in the literature.[31]
To synthesize coordination polymer 3, Cu(NO3)2.3H2O (0.08 g, 0.33 mmol) and 1,4-BDC-OH (0.046 g,
0.25 mmol) were added to a mixture of DMF (4 mL) and EtOH (1 mL) in a 25 mL Teflon container and
then stirred for 10 min at room conditions. The mixture was kept in a stainless-steel autoclave at 80 ºC
for 24 h. After slow cooling of the vessel to room temperature, green crystals of
3 were collected and
washed with DMF.
2.5. Catalytic procedure
The oxidation of benzyl alcohol was carried out as follows: A mixture containing tert-butyl hydroperoxide
(TBHP, 2.26 mmol), benzyl alcohol (1 mmol) and catalyst (0.063 mmol based on Cu) in solvent (1 mL) was
prepared. TBHP (70% in H2O) was dried prior to use based on the procedure reported in the literature using
CH2Cl2.[32] The mixture was stirred for proper time either at room temperature or at 40 ºC and then the
oxidation products were analyzed by GC.
2.6. X-ray single crystal data collection and refinement
Single-crystal structure analyses were carried out on a four-circle κ geometry KUMA KM-4 diffractometer
equipped with a two-dimensional area CCD detector using graphite-monochromated Mo Kα radiation. The
structures were solved by the direct method using SHELXS-97 and refined using SHELXL-2014/7
program.[33] All non-hydrogen atoms were refined anisotropically. The CCDC numbers are 1571179 (1a),
1571178 (1b) and 1860730 (3).
3. Result and Discussion
3.1 Crystal structures
3.1.1. Structure of Cu2(µ2-MeCO2)4(DABCO) (1a)
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Crystallographic data of the structures are given in Table 1. The ORTEP diagram of 1a is illustrated in Fig. 1a.
Two Cu2+ ions, one DABCO molecule and four µ2-MeCO2 ligands exist in the asymmetric unit of this structure.
There are two crystallographically distinct Cu2+ ions which are joined through linear DABCO ligand. Square
pyramidal configuration around each Cu2+ center contains one N atom from a DABCO molecule in the axial site
(Cu1-N1, 2.195(3); Cu2-N2, 2.196(3) Å) and four O atoms from four different µ2-MeCO2 ions. The Cu1-O and
Cu2-O bond lengths are within the range of 1.966(2)-1.984(2) and 1.972(2)-1.989(2) Å, respectively (Table S1).
The Cu2+ ions are further joined each other via bridging DABCO and µ2-MeCO2 ligands to form the 1-D chain
depicted in Fig 1b. The Cu1 …. Cu1 and Cu2 …. Cu2 distances within {Cu2(µ2-MeCO2)4} units are 2.6155(8)
and 2.6142(8) Å, respectively. Some hydrogen bonding interactions are observed between H atoms of DABCO
and O atoms of µ2-MeCO2 ligands in the chains (Table S2). In this structure, the chains grow along two different
directions which led to the packing exhibited in Fig 1c. The PXRD pattern of 1a, presented in Fig. 2a, is similar
to that of the simulated pattern, indicating the purity of the sample.
4

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Fig. 1 a) ORTEP, b) 1-D chain and c) packing of 1a
5

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Table 1 Crystal structure and data refinement for 1a, 1b and 3
Compound
1a
1b
3
Empirical formula
Molecular weight
Temperature (K)
Wavelength, Mo Kα (Å)
Space group
C14H24Cu2N2O8
475.43
C14H18Cu2N2O8
469.40
C11H11CuNO6
316.75
100(2)
100(2)
100(2)
0.71073
0.71073
0.71073
P21/
c
C2/m
I2/m
a
b
c
α
β
γ
(Å)
(Å)
(Å)
(°)
10.1816(8)
16.239(2)
11.6419(9)
90
8.0473(5)
15.3181(8)
7.6289(4)
90
7.6618(4)
15.3855(7)
11.0185(7)
90
(°)
97.660(10)
90
104.346(8)
90
107.978(7)
90
(°)
Cell volume (Å3)
1907.7(3)
4
911.08(9)
2
1235.45(13)
4
Z
ρ (g cm-3)
1.655
1.704
1.703
µ (mm-1)
2.272
2.377
1.790
Total reflections
Unique reflections
Observed reflections [
Rint
38375
5275
11410
4988
1202
1668
F
2> 2σ(
F
2)]
3003
1066
1315
0.0650
0.0364
0.0672
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
4988/0/239
1.005
1202/3/84
1.003
1668/0/110
1.002
R [
F
2> 2σ(
F
2)] (R1, wR2)
0.0425, 0.0683
0.0915, 0.0758
0.410, -0.413
0.0328, 0.0729
0.0400, 0.0755
0.612, -0.469
0.0502, 0.0936
0.0752, 0.1026
0.616, -0.699
R (all data) (R1, wR2)
∆ρmax, ∆ρmin (e Å–3)
Fig. 2. XRD patterns of 1-3
3.1.2. Structure of [Cu(1,4-BDC-Br)(DABCO)0.5].xDMF.yH2O (2)
Since our effort for obtaining crystals suitable for structure determination by single-crystal X-ray diffraction resulted
in failure, phase and purity of this compound were studied using PXRD. The pattern of is similar to that of [Zn(1,4-
2
BDC)(DABCO)0.5] MOF previously reported by Danil N. Dybtsev et al.,[34] confirming that these two compounds
6

are isostructural and the structure of the MOF is not affected by functionalization of 1,4-BDC (Fig. 2b). The
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molecular structure and 3D packing of [Zn(1,4-BDC)(DABCO)0.5] MOF are shown in Fig. 3. The MOF contains
binuclear-paddle wheel units bridged by 1,4-BDC and form 2D layers which are further connected by pillared
DABCO ligands.
Fig. 3. a) Paddle-wheel and b) 3-D structure of [Zn(1,4-BDC)(DABCO)0.5][34]
3.1.3. Structure of Cu2(µ2-MeCO2)4(DABCO) (1b)
This structure is a polymorph of 1a. The molecular structure of binuclear 1b is shown in Fig. 4a. The asymmetric unit of 1b is
composed of one Cu2+ ion, one µ2-MeCO2 ion and half of a DABCO molecule. There is one crystallographically distinct Cu2+
ion which is coordinated to N atom of a DABCO molecule in the axial site. The carbon atoms of DABCO are disordered. The
structure of 1b showed similar connectivity to that of 1a and Cu2+ centres are five-coordinated by N from DABCO ligand
(Cu1-N1, 2.205(2) Å) and four O atoms from µ2-MeCO2 molecules with the Cu1-O bond lengths of 1.9684(16) and 1.9732(17)
Å (Table S3).
The binuclear units are joined together through linear DABCO molecules and form 1-D chain seen in Fig. 4b. The Cu …. Cu
distance in the chain is 2.632(6) Å which is slightly longer compared to those in 1a. Unlike 1a, the chains in 1b are all aligned
in a similar direction to afford the packing demonstrated in Fig. 4c.
Similar paddle wheel units were reported in some MOFs synthesized based on DABCO and 1,4-BDC or functionalized 1,4-
BDC including [Co2(1,4-BDC)2(DABCO)].4DMF.H2O,[31, 35] [Ni2(1,4-BDC)2(DABCO)].4DMF.H2O,[36] [Cu2(1,4-BDC-
NH2)2(DABCO)].2DMF.2H2O
(1,4-BDC-NH2
=
2-aminoterephthalate)[37]
and
[Zn2(1,4-BDC-
OH)2(DABCO)0.5].1.5DMF.0.3H2O.[31] Acetate ions as monotopic carboxylate ligands in 1a and 1b afforded the formation of
1-D chains while the ditopic functionalized 1,4-BDC ligands in 2 as well as the previously reported structures mentioned above
led to the formation of 3-D MOFs. Although 1,4-BDC-Br, DABCO and acetic acid were utilized in the solvothermal synthesis
of 1a, the 1,4-BDC-Br does not appear in this structure and we obtained a mixture containing coordination polymer 1a and
MOF 2. To investigate how 1,4-BDC-Br affects the final products, the synthesis was carried out under similar conditions to
those mentioned above, except that 1,4-BDC-Br was omitted. Rod-shape crystals were formed which were not suitable for X-
ray single crystal analysis. However, the PXRD pattern of these crystals is not similar to that of 1a (Fig. S1). Thus, we made an
attempt to obtain better crystals for X-ray analysis by changing the reaction conditions such as molar ratio of the starting
materials and the reaction temperature and this resulted in compound 1b.
7

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Fig. 4. a) ORTEP, b) 1-D chain and c) packing of 1b
3.1.4. Structure of Cu(1,4-BDC-OH)(DMF) (3)
The ORTEP view of coordination polymer
3
is displayed in Fig. 5a. The asymmetric unit in this structure is
composed of one Cu2+ ion, half of a 1,4-BDC-OH and one DMF molecule. The OH groups of 1,4-BDC-OH ligand
occupies two positions (both with the occupancy of 0.5). The DMF molecules have also two orientations (occupancy
of 0.5 for each) with two common atoms (O4 and C6). Each Cu2+ ion exhibits a square pyramidal configuration and is
coordinated to four O atoms from four 1,4-BDC-OH ligands (Cu-O1, 1.948(2) Å; Cu-O2i, 1.971(2) Å; Cu-O2ii,
1.971(2) Å; Cu-O1iii, 1.948(2) Å (symmetry codes: (i) 2-x, y, 1-z; (ii) 2-x, 1-y, 1-z; (iii) x, 1-y, z) and one O atom of a
DMF molecule in the axial position (Cu-O4, 2.135(4) Å). The bond lengths and angles for
within the range of those reported for similar paddle-wheel structures.[38]
3, shown in Table S4, lie
8

The paddle-wheel units with Cu….Cu distances of 2.642 Å, are connected through the carboxylate groups of 1,4-
ACCEPTED MANUSCRIPT
BDC-OH ligand to form a 2D layer shown in Fig 5b.
Fig. 5. a) ORTEP, b) 2-D chain structure of 3
3.2. Spectroscopic analysis
The FT-IR spectra of 1-3 are presented in Fig. 6. In the FT-IR spectra of fresh 1a and 1b, the symmetric and asymmetric
vibrations of COO groups for µ2-MeCO2 ligand are observed at 1417 and 1612 cm-1, respectively.[39] The ν (C-N) of DABCO
ligand is seen at 1060 cm-1. In the FT-IR spectrum of MOF 2, νas (C-N) and νs (C-N) for DABCO appeared at 1089 and 1058
cm-1, respectively. Moreover, the bands located at 1623 and 1672 cm-1 are ascribed to the νas (COO) vibrations of 1,4-BDC-Br
9

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ligand. The peak at 1383 cm-1 can be assigned to νs (COO) of this ligand.[36, 40, 41] Similar to 2, the FT-IR spectra of 3
exhibited the bands for ν (COO) vibrations exist in the structure of 1,4-BDC-OH ligand.
Fig. 6. FT-IR spectra of 1-3
3.3. Thermal stability
Thermogravimetric analyses were conducted to evaluate thermal stability of 1-3 as shown in Fig. 7. In coordination polymer
1a, the weight loss of approximately 4 % up to 190 ºC is probably due to adsorbed solvent molecules (DMF and/or H2O). One
distinct weight loss step occurred upon heating to 300 ºC owing to the decomposition of organic ligands. The residue of about
34 % (calculated 33.46 %) is attributed to copper oxide. In the TGA curve of MOF 2, the weight loss observed up to 230 ºC is
related to the solvent molecules occupying the pores of the MOF. Then the guest-free framework of 2 decomposes up to 510
ºC due to the elimination of organic molecules. Coordination polymer 1b is stable up to 170 ºC, followed by a one-step weight
loss by increasing in the temperature to 270 ºC, which is attributed to the removal of organic molecules. TG curve of 3 exhibits
no weight loss till 210 ºC and then is decomposed in two steps. The first weight loss of 22.66 % (calculated 23 %) observed at
210-280 ºC can be due to one DMF molecule. The residue of 24.99 % (calculated 25.11 %) at 600 ºC is related to copper
oxide. Powder XRD analyses for 1-3 after annealing at 600 ºC confirm that the residue is CuO (Fig. S2-S5).
Fig. 7. TGA curves of 1-3
3.4. Textural analysis
10

The N adsorption/desorption isotherm and pore size distribution of MOF 2 are shown in Fig. 8. The BET and Langmuir
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2
surface areas are estimated to be 1106 and 1211 m2g-1, respectively. Functionalization of 1,4-BDC by Br groups in MOF 2
provides a lower surface area compared to similar structures containing non-functionalized 1,4-BDC.[42] Moreover, pore
diameter and pore volume for 2, determined by MP (Micropore analysis) method are found to be 0.6 nm and 0.454 cm3g-1,
respectively.
Fig. 8. Nitrogen sorption isotherm of MOF
2
3.5. Catalytic tests
The catalytic performance of the prepared Cu-based coordination polymers was studied in the oxidation of benzyl alcohol
using TBHP as oxidant. All four Cu-coordination polymers exhibited good catalytic activity and displayed selectivity towards
the formation of benzoic acid at 40 ºC using CH2Cl2 as solvent.
Different factors such as solvent, temperature, time, etc can influence catalytic oxidation of organic substrates.[43, 44]
Therefore, we have studies the effect of some parameters to obtain the optimal reaction conditions for benzyl alcohol oxidation
in the presence of 1-3. The results are summarized in Table 2 and 3.
At first, the oxidation was investigated in CH2Cl2 at room temperature. Catalysts 1a, 2 and 3 selectively oxidized benzyl
alcohol to benzaldehyde while a very small amount of benzoic acid was also formed when 1b used as catalyst (Table 2, Entry
1-4). Since conversion of these reactions was 17-23 %, the reaction time was extended to 8 h (Table 2, Entry 5-8) to improve
the conversion. The results show that the reaction conversion for all catalysts was not significantly improved upon time
extension (23-28 %) although they were all still selective towards the formation of benzaldehyde.
To obtain optimal reaction conditions with good conversion and selectivity, DMF is used instead of CH2Cl2. Considering
polarity of the solvents, DMF with dipole moment of 3.86 D can facilitate solubility of both TBHP and benzyl alcohol
compared to CH2Cl2, which has dipole moment of 1.6 D.[44] The oxidation reactions were performed in DMF for 4 h using
the catalysts. The results revealed that reaction conversions were improved to 30-54 % after 4 h for all catalysts and
benzaldehyde and benzyl benzoate were formed as the products (Table 2, Entry 9-12). By increasing the time of the reactions
to 12 h using DMF, conversions increased to 58-62 % while selectivity to benzaldehyde decreased (Table 2, Entry 13-16).
Since all four catalysts displayed selectivity to benzaldehyde in CH2Cl2, this solvent was selected for further study of benzyl
alcohol oxidation. The reactions were conducted at 40 ºC to obtain better results (Table 3). As seen, the conversion of the
11

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oxidation reaction is above 82 % for 1a, 1b and 2 after 8 h. In the presence of catalyst 3, the reaction led to 97 % conversion
after 14 h. All catalysts in this condition afforded benzoic acid as the main product.
Table 2a Catalytic activity of coordination polymers 1-3 in the oxidation of benzyl alcohol
Entry Catalyst Solvent Time (h)
Selectivity (%)
Benzaldehyde Benzoic
Conversion (%)b
Benzyl benzoate
acid
0
1
1a
1b
2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DMF
4
100
98.5
100
100
100
95
0
18
23
17
17
23
28
25
28
30
49
38
54
62
58
64
58
11.7
2
4
0.2
0
0
3
4
0
4
3
4
0
0
5
1a
1b
2
8
0
0
6
8
5
0
7
8
100
100
94
0
0
8
3
8
0
0
9
1a
1b
2
4
0
6
10
11
12
13
14
15
16
17
DMF
4
60
0
40
22
11
72
59
80
51
0
DMF
4
78
0
3
DMF
4
89
0
1a
1b
2
DMF
12
12
12
12
12
28
0
DMF
41
0
DMF
20
0
3
DMF
49
0
-
DMF
100
0
aReaction conditions: benzyl alcohol (1 mmol), TBHP (2.26 mmol), Solvent (1 ml), catalyst (0.063 mmol based on
Cu), room temperature.
bGC yield is based on starting substrate.
Table 3a Catalytic activity of coordination polymers 1-3 in the oxidation of benzyl alcohol
Entry
Catalyst
Time (h)
Conversion
Selectivity
(%)b
65
82
67
85
65
95
30
68
97
20
(%)c
86
100
90
100
86
99
2
1
1a
1a
1b
1b
2
4
2
8
3
4
4
8
5
4
6
2
8
7
3
4
8
3
8
73
97
0
9
3
14
14
10
-
aReaction conditions: benzyl alcohol (1 mmol), TBHP (2.26 mmol),
catalyst (0.063 mmol based on Cu), CH2Cl2 (1 ml), 40 ºC.
bGC yield is based on starting substrate.
cSelectivity was determined towards the formation of benzoic acid.
To assess the catalytic effect of 1-3 catalysts on the oxidation process, the reaction was conducted without the catalysts under
similar conditions both at room temperature and 40 ºC. The results show that benzyl alcohol undergoes insubstantial oxidation
to benzaldehyde (with the conversion of 11.7 %) at room temperature using DMF solvent (Table 2, Entry 17). Performing the
blank test at 40 ºC for 14 h in CH2Cl2 also led to low conversion of 20 % for oxidation of benzyl alcohol to benzaldehyde
(Table 3, Entry 10).
Further studies of the catalytic parameters such as oxidant and catalyst amounts as well as substrate nature were carried out and
the results are given in Table 4. By increasing the amount of oxidant from 2.26 mmol to 4.52 mmol, the reaction conversion in
12

the presence of 1-3 reached 63-91 % after 8 h (Table 4, Entries 1-4). Also, the reaction conversion of 76-88 % was obtained
ACCEPTED MANUSCRIPT
after 6 h upon increasing of the catalyst amounts from 0.063 mmol to 0.126 mmol (Table 4, Entries 5-8). Comparing these
results with the conversions reported in Table 3 shows that increasing the amounts of the catalysts led to the improvement of
the reaction conversions.
In order to investigate the electronic effects of substrates on the oxidation reaction, 4-methoxybenzyl alcohol and 4-
chlorobenzyl alcohol were tested as representative substrate models. After 4 h, the reaction conversion of 99 % was obtained
using 4-methoxybenzyl alcohol (Table 4, Entries 9-12). More reactivity of this substrate compared to benzyl alcohol is due to
the presence of methoxy group which supply more electrons. When 4-chlorobenzyl alcohol is used as the substrate, the
oxidation reaction required more time (8 h) and the selectivity towards formation of the corresponding acid decreased (Table 4,
Entries 13-16).
Table 4a Effects of oxidant and catalyst amounts as well as substrate nature on the oxidation reaction catalyzed by 1-3
Entry
Substrate
Catalyst
Amount of
catalyst (mmol)
0.063
0.063
0.063
Amount of
TBHP (mmol)
4.52
4.52
4.52
Time
(h)
8
8
8
Conversion
(%)b
88
91
72
Selectivity
(%)c
94
96
86
1
2
3
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
1a
1b
2
4
Benzyl alcohol
3
0.063
4.52
8
63
77
5
6
7
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
1a
1b
2
0.126
0.126
0.126
2.26
2.26
2.26
6
6
6
83
88
76
88
88
91
8
Benzyl alcohol
3
0.126
2.26
6
78
86
9
4-Methoxybenzyl alcohol
4-Methoxybenzyl alcohol
4-Methoxybenzyl alcohol
4-Methoxybenzyl alcohol
4-Chlorobenzyl alcohol
4-Chlorobenzyl alcohol
4-Chlorobenzyl alcohol
4-Chlorobenzyl alcohol
1a
1b
2
0.063
0.063
0.063
0.063
0.063
0.063
0.063
0.063
2.26
2.26
2.26
2.26
2.26
2.26
2.26
2.26
4
4
4
4
8
8
8
8
100
100
100
100
100
62
100
100
100
100
37
53
48
11
10
11
12
13
14
15
16
3
1a
1b
2
100
100
3
aReaction conditions: Substrate (1 mmol), CH2Cl2 (1 ml), 40 ºC
bGC yield is based on starting substrate.
cSelectivity was determined towards the formation of acid.
The reusability of the obtained catalysts was examined in the oxidation of benzyl alcohol. After completion of the reaction, the
catalysts were separated and washed with DMF. The reaction conditions were similar to those reported in Table 3 at 40 ºC
using CH2Cl2 as solvent. The oxidation reactions were conducted for 14 h in the presence of reused 3 and for 8 h using
recycled 1a, 2 and 1b. The results, given in Fig 9, displayed no appreciable loss in the activity and selectivity towards benzoic
acid. Copper content of the catalysts before and after the reaction was determined by atomic absorption spectroscopy (Table 5).
The results show negligible leaching of the catalysts. Moreover, the PXRD patterns of the fresh and recycled coordination
polymers are similar (Fig. 2). These observations clearly confirm that the crystal structure of 1-3 catalysts remained intact upon
the oxidation reactions, exhibiting the structural stability of the coordination polymers.
In the FT-IR spectra of 1-3 which are shown in Fig. 6, the characteristic peaks for the reused catalysts resemble those found in
the spectra of their corresponding pristine coordination polymers. This observation points out that the structure of the catalysts
has not been changed upon oxidation of benzyl alcohol.
13

ACCEPTED MANUSCRIPT
Fig. 9. Reusability of catalysts 1-3
Table 5 Copper content of the catalysts which is determined by atomic absorption spectroscopy
Catalysts
Cu content in the catalysts (%)
Fresh
23.27
18
Reused
1a
2
22.25
16.24
30.64
17.50
1b
3
33.63
21.48
Based on the reported literature [45], benzyl alcohol oxidation over Cu2+ catalytic sites are attributed to a free radical pathway. The Cu2+ sites
react with TBHP and form peroxo (t-BuOO.) and alkoxo (t-BuO.) radical species. The peroxo radicals react with benzyl alcohol and form the
corresponding alcohol radical intermediate upon abstraction of benzylic hydrogen from the substrate. The intermediate reacts with t-BuO.
radical to generate t-BuOH and the oxidation products.
Benzyl alcohol oxidation was reported over MOF catalysts. Ling Peng et al. investigated the catalytic oxidation of benzyl alcohol to
benzaldehyde over isostructural MOFs M(1,4-BDC)(DABCO)0.5 [M = Co, Zn, Ni, Cu][30] with paddle wheel units. By employing Co(1,4-
BDC)(DABCO)0.5 as catalyst, 81.8 % conversion was obtained. Whereas in the presence of Cu(1,4-BDC)(DABCO)0.5, a very low conversion
(8.8 %) was achieved after 8 h. However, in our work, the Cu-based compounds with similar dinuclear units exhibited high conversions at
lower temperature using less amounts of catalysts.
4. Conclusions
This study presents four new Cu(II)-based coordination polymers with similar SBUs. Compounds 1a and 1b are formed based on a mixture
of µ2-MeCO2 and linear DABCO ligands. Coordination polymers 1a and 1b are polymorphous and comprise 1-D chains with similar
connectivity and different packing structures. Compound 2 which is isostructural with the reported [Zn(1,4-BDC)(DABCO)0.5], contains
1,4-BDC-Br and DABCO ligands. Compound 3 is a 2-D coordination polymer formed using 1,4-BDC-OH ligand. The resulting compounds
were applied as heterogeneous catalysts for the oxidation of benzyl alcohol. They displayed selectivity to the formation of benzoic acid at 40
ºC in CH2Cl2 solvent and could be recycled and reused with maintenance of catalytic performance.
Acknowledgements
We gratefully acknowledge University of Tehran and Iran National Science Foundation: INSF [post-doctoral grant no.
95002447] for the financial support.
14

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Highlights:
•
New coordination polymers with paddle-wheel SBUs were solvothermally
synthesized.
•
•
They were applied as heterogeneous catalysts for the oxidation of benzyl alcohol.
The catalysts displayed good activity and stability in the oxidation reaction.