One-dimensional copper(II) coordination polymers built on 4′-substituted 4,2′:6′,4- and 3,2′:6′,3-terpyridines: Syntheses, structures and catalytic properties

Article abstract of DOI:10.1016/j.poly.2015.12.042

Divergent ligands containing either 4,2′:6′,4- or 3,2′:6′,3-terpyridine domains (1-4) with various 4′-substituents are synthesized. The new ligand 4 was fully characterized by standard spectroscopic and mass spectrometric technique. All ligands were then

Full text of DOI:10.1016/j.poly.2015.12.042

Accepted Manuscript
One-dimensional copper(II) coordination polymers built on 4’-substituted
4,2’:6’,4’ ’- and 3,2’:6’,3’ ’-terpyridines: Syntheses, structures and catalytic
properties
Li Li, Yuan Zhuo Zhang, Chengxiong Yang, E. Liu, James A. Golen, Guoqi
Zhang
PII:
S0277-5387(15)00792-5
DOI:
http://dx.doi.org/10.1016/j.poly.2015.12.042
POLY 11741
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
26 October 2015
18 December 2015
Please cite this article as: L. Li, Y.Z. Zhang, C. Yang, E. Liu, J.A. Golen, G. Zhang, One-dimensional copper(II)
coordination polymers built on 4’-substituted 4,2’:6’,4’ ’- and 3,2’:6’,3’ ’-terpyridines: Syntheses, structures and
catalytic properties, Polyhedron (2015), doi: http://dx.doi.org/10.1016/j.poly.2015.12.042
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One-dimensional copper(II) coordination polymers built on 4'-
substituted 4,2':6',4''- and 3,2':6',3''-terpyridines: Syntheses,
structures and catalytic properties
Li Li,a Yuan Zhuo Zhang,b Chengxiong Yang,a E Liu,a James A. Golenc and Guoqi Zhangb*
a College of Chemical Engineering and Pharmacy, Jingchu University of Technology, Jingmen 448000, China
b
Department of Sciences, John Jay College and The Graduate Center, The City University of New York,
New York, NY 10019, USA
c
Department of Chemistry, University of Massachusetts, Dartmouth, North Dartmouth, MA 02747, USA
* Corresponding author. Email address: guzhang@jjay.cuny.edu
Keywords: 3,2':6',3''-terpyridine; Copper(II); Coordination polymer; Crystal structure;
Catalysis
ABSTRACT: Divergent ligands containing either 4,2':6',4''- or 3,2':6',3''-terpyridine
domains (
fully characterized by standard spectroscopic and mass spectrometric technique. All
ligands were then used to prepare four new copper(II) coordination polymers (
and good quality single crystals were obtained by the layering method. Single crystal
X-ray structural analysis reveals that while the structures of compounds and built
1
-
4
) with various 4
'
-substituents are synthesized. The new ligand 4 was
5-8)
5
6
on 4,2':6',4''-terpyridine ligands feature similar zigzag chains composed of paddle-
wheel {Cu2(µ-OAc)4} units, the use of 3,2':6',3''-terpyridine ligands bearing various
4'-substituents lead to different coordination polymers 7 and 8 in which the ligands
adopt remarkably distinct binding modes. The copper(II) coordination polymers were
employed to the catalytic aerobic oxidations of alcohols mediated by TEMPO radical
(TEMPO = 2,2,6,6-tetramethylpiperidinyl-1-oxyl) and the results revealed that the
polymers display slightly different catalytic activity towards the oxidation of benzylic
alcohol under heterogeneous conditions, and compound
8 was observed to catalyze
the reaction in water with almost quantitative yield when a base additive is present.
1

1. Introduction
Oligopyridines [1] are one class of frequently utilized molecular building blocks in
metal-mediated self-assembly of functional coordination polymers (CPs) or
frameworks which have found significant applications in supramolecular chemistry,
materials and catalysis science [2-8]. 4,2':6',4''-terpyridine (4,2':6',4''-tpy) is a
divergent tris-pyridine ligand that differs from the conventional 2,2':6',2''-terpyridine
(2,2':6',2''-tpy) by the positions of N-donors in two side pyridyl rings [9]. Since its first
employment in the synthesis of a CP in 1998 [10], 4,2':6',4''-tpy and its derivatives
have shown increasing importance in metal-mediated self-assembly owing to their
versatile coordination chemistry and tunable materials properties in the recent five
years [11-16]. Thanks to the facile synthetic protocol of 4'-substituted 4,2':6',4''-tpy
ligands, i.e. the one-pot Kröhnke condensation of various aldehydes and 4-
acetylpyridine, a number of metal assemblies of these ligands including discrete
supramolecules and infinite CPs have been prepared and structurally characterized [11,
12]. Typically, in all known coordination compounds of 4,2':6',4''-tpy derivatives, the
parent 4,2':6',4''-tpy unit behaves as a V-shaped bidentate
an internal angle of 120°), remaining the central pyridine non-coordinated. Despite the
restricted directionality of the '-donor set of 4,2':6',4''-tpy backbone in coordination
N,N-donating ligand (with
N,N
chemistry, variations in metal salts (metals and counter ions), 4'-substitutents on ligand
and reaction conditions (solvent, temperature, reaction time and methods of
crystallization) were found to largely influence the structures of the resulting metal-
organic architectures [11-16]. In contrast, 3,2':6',3''-tpy as an isomer of 4,2':6',4''-tpy
was surprisingly only little explored and its coordination versatility is not well
established [12, 17-24]. Unlike 4,2':6',4''-tpy, 3,2':6',3''-tpy can adopt different ligand
conformations upon rotating about the inter-ring C-C bonds, which would lead to
various directionalities between metal-coordinating
N,N'-donors as shown in Scheme
1. Therefore, rich coordination chemistry would be anticipated on this type of ligand,
in particular if different 4'-substitutents are introduced.
In continuation with our recent work on zinc(II) or cobalt(II)-driven self-
assembly of diverse CPs based on 4,2':6',4''-tpy derivatives [13-16, 24, 25], in this
2

paper we focus on the copper coordination chemistry of four 4'-substituted 4,2':6',4''-
or 3,2':6',3''-tpys ( , Scheme 2), of which ligand was unexplored. Because of the
significant impact of 4'-substituents on the structures of the resulting coordination
assemblies [11-13], variously substituted 4,2':6',4''-tpys ( and ) and 3,2':6',3''-tpys (
and ) were chosen, of which ligands and contain the same 4'-substituent, allowing
for a direct comparison upon copper(II) complexation. We report the reactions of
Cu(OAc)2 with and describe the solid-state structures of four new one-dimensional
copper(II) CPs resulting from these reactions. Compared to other -block metals,
1
-
4
4
1
2
3
4
2
3
1-4
d
copper was scarcely investigated in the coordination chemistry of 4,2':6',4''-tpy and not
even known for 3,2':6',3''-tpy [26, 27]. Built on our previous report on the catalytic
applications of Zn-CPs with 4,2':6',4''-tpy derivatives in heterogeneous esterification
reactions, herein we also investigate the catalytic ability of new Cu-CPs for TEMPO-
mediated aerobic alcohol oxidation, in order to extend the applications of new hybrid
materials derived from tpy ligands.
Scheme 1. The schematic representation of three possible metal binding modes of 3,2':6',3''-tpy.
3

Scheme 2. The structures of 4,2':6',4''- and 3,2':6',3''-tpy ligands
1-4 and atomic labelling (4) for NMR
spectroscopic assignments.
2. Experimental Section
2.1. General
Solvents and reagents were purchased from Fisher Scientific or Sigma-Aldrich in the US. All
1
13
reactions were performed under ambient conditions. Solution H and C NMR spectra were
obtained at room temperature on a Bruker III 500 MHz spectrometer with TMS as an internal
standard. FT-IR spectra were measured on a Shimadzu 8400S instrument with solid samples
using a Golden Gate ATR accessory. Elemental analyses were performed by Midwest
Microlab LLC in Indianapolis, and GC-MS analysis was carried out on a Shimadzu GCMS-
QP2010S gas chromatography mass spectrometer. Ligands 1-3 were synthesized according
the literature procedure [13-16, 24].
2.2. Synthesis of 4'-(4-hexyloxyphenyl)-3,2':6',3''-terpyridine (4)
In a 250 cm3 round-bottom flask equipped with a magnetic stirrer, 3-acetylpyridine (2.42 g,
20.0 mmol) was added to a solution of 4-hexyloxybenzaldehyde (2.06 g, 10.0 mmol) in EtOH
(40 cm3). KOH pellets (0.56 g, 10 mmol) were then added, followed by aqueous NH3 (28%,
4

40 cm3). The resulting orange solution was stirred at room temperature overnight, during
which time a white suspension had formed. White solid was collected by filtration, washed
with H2O and EtOH, and dried in vacuo over P2O5. Yield: 0.95 g (23%). 1H NMR (500 MHz,
CDCl3) δ 9.39 (d, J = 2.0 Hz, 2 H, HA2), 8.72 (dd, J = 4.5, 1.5 Hz, 2 H, HA6), 8.54 (dt, J = 8.0,
2.0 Hz, 2 H, HA4), 7.95 (s, 2 H, HB3), 7.73 (dd, J = 7.0, 2.0 Hz, 2 H, HC2), 7.49 (dd, J = 13.0,
5.0 Hz, 2 H, HA5), 7.08 (d, J = 8.5 Hz, 2 H, HC3), 4.07 (t, J = 6.5 Hz, 2 H, HOCH2), 1.86 (m, 2
13
H, HCH2), 1.52 (m, 2 H, HCH2), 1.39 (m, 4 H, HCH2), 0.95 (t, J = 7.0 Hz, 3 H, HCH3) ppm. C
NMR (125 MHz, CDCl3) δ 160.48, 155.21, 150.45, 149.97, 148.25, 134.89, 134.73, 130.04,
128.33, 123.71, 117.23, 115.26, 68.26, 31.60, 29.19, 25.74, 22.64, 14.09 ppm. FT-IR (solid,
cm-1): 1602s, 1516s, 1476m, 1416m, 1397m, 1239s, 1186s, 1119m, 1023s, 832m, 809s, 702s,
657w, 601m. Anal. Calcd. for C27H27N3O: C 79.19, H 6.65, N 10.26%. Found C 78.85, H
6.56, N 9.87%.
2.3. Preparation of [Cu2(1)(OAc)4]n (5)
A solution of 1 (30.9 mg, 0.050 mmol) in MeOH/CH2Cl2 (10 cm3, 1: 4, v/v) was placed in a
test tube. A mixture of MeOH and CH2Cl2 (5 cm3, 1: 1, v/v) was layered on the top of this
solution, followed by the solution of Cu(OAc)2·H2O (19.9 mg, 0.100 mmol) in MeOH (10
cm3). The tube was sealed and allowed to stand at room temperature for two weeks, during
which time X-ray quality green crystals had formed on the wall of the tube. The crystals were
collected by decanting the solvent and were washed with MeOH and dried in air. Yield: 41.0
mg (61.1%). FT-IR (solid, cm-1) 1638s, 1617s, 1591s, 1426s, 1223m, 1067m, 1027m, 850s,
825s, 665s, 650m, 619s, 522s. Anal. Calcd. for C29H27Cu2N3O8: C 51.78, H 4.05, N 6.25%.
Found C 52.01, H 4.12, N 6.21%.
2.4. Preparation of [Cu2(2)(OAc)4]n (6)
The same procedure as for 5 was applied to the synthesis of CP 6, except that ligand 2 was
used (35.2 mg, 0.100 mmol). Green block-like crystals were isolated after two weeks. Yield:
49.0 mg (68.6%). FT-IR (solid, cm-1) 1622s, 1591s, 1530s, 1503w, 1428s, 1372m, 1207w,
1173w, 1066m, 1019m, 948w, 841m, 811s, 681s, 624m. Anal. Calcd. for C31H32Cu2N4O8: C
52.02, H 4.51, N 7.83%. Found C 52.11, H 4.56, N 7.85%.
5

2.5. Preparation of [Cu2(3)(OAc)4]n (7)
The same procedure as for 5 was applied to the synthesis of CP 7, except that ligand 3 was
used (35.2 mg, 0.100 mmol). Green block-like crystals were isolated after two weeks. Yield:
52.0 mg (72.7%). FT-IR (solid, cm-1) 1619s, 1595s, 1529m, 1484w, 1429s, 1366w, 1251w,
1198w, 1056w, 1029m, 946w, 829m, 809s, 704m, 681s, 660m, 628w. Anal. Calcd. for
C31H32Cu2N4O8: C 52.02, H 4.51, N 7.83%. Found C 51.76, H 4.40, N 7.60%.
2.6. Preparation of [Cu2(4)(OAc)4]n (8)
The same procedure as for 5 was applied to the synthesis of CP 8, except that ligand 4 was
used (40.9 mg, 0.100 mmol). Green block-like crystals were isolated after two weeks. Yield:
55.5 mg (71.8%). FT-IR (solid, cm-1) 2932w, 1621s, 1602s, 1546w, 1516m, 1427s, 1291w,
1239m, 1186m, 1122w, 1028m, 937w, 836m, 812s, 700s, 680s, 643w, 625w. Anal. Calcd. for
C35H39Cu2N3O9: C 54.40, H 5.09, N 5.44%. Found C 53.93, H 4.98, N 5.36%.
Table 1. Structural refinement data for compounds 5-8.
Compound
5
6
7
8
Formula
Formula Weight
C29H27Cu2N3O8
672.62
C31H32Cu2N4O8
715.71
C31H32Cu2N4O8
715.69
C35H39Cu2N3O9
772.77
Crystal system
Space group
Monoclinic
I2/a
Monoclinic
I2/a
Monoclinic
I2/a
Triclinic
P-1
a/Å
b/Å
c/Å
α/º
7.9006(18)
15.207(3)
24.943(6)
90
91.166(4)
90
2996.3(12)
1.491
4
8.035(2)
14.857(4)
25.782(8)
90
91.109(4)
90
3077.1(16)
1.545
4
8.3609(7)
14.2179(12)
26.739(2)
90
91.758(2)
90
3177.1(5)
1.496
4
8.3546(6)
14.5546(9)
16.3920(12)
72.836(3)
77.803(3)
74.607(3)
1816.9(2)
1.413
β/º
γ/º
U/Å 3
Dc/Mg m-3
Z
2
µ/mm-1
1.472
1.440
1.394
1.226
T/K
100(2)
90(2)
129(2)
120(2)
Reflections/unique
Parameters
42802/3804
194
0.0286, 0.0347
0.0820, 0.0860
13641/3533
209
30368/3041
209
0.0252, 0.0323
0.0639, 0.0675
1.021
6670/5445
461
0.0431, 0.0568
0.1265, 0.1332
1.023
a
b
R1 , wR2 [I > 2σ(I)]
0.0352, 0.0380
0.1062, 0.1094
1.102
a
R1 , wR2b (all data)
GOF
1.035
aR1 = (Fo
–
Fc)/Fo
.
bwR2 = [
w
(
Fo
–
Fc2)2/
w
(
Fo)2]1/2
2
6

Table 2. Selected bond lengths (Å) and angles (°) for compounds 5-8.
5
Cu1–N1
Cu1–O2a
Cu1–O4a
N1–Cu1–Cu1a
6
2.1535(15)
1.9691(13)
1.9820(14)
178.60(4)
Cu1–O1
Cu1–O3
Cu1–Cu1a
1.9724(14)
1.9757(15)
2.6344(6)
Cu1–N1
Cu1–O2b
Cu1–O4b
N1–Cu1–Cu1b
7
2.1433(18)
1.9767(18)
1.9769(18)
177.07(5)
Cu1–O1
Cu1–O3
Cu1–Cu1b
1.9772(14)
1.9771(18)
2.6285(7)
Cu1–N1
Cu1–O1a
Cu1–O3a
N1–Cu1–Cu1a
8
2.1591(15)
1.9744(15)
1.9627(14)
178.21(4)
Cu1–O2
Cu1–O4
Cu1–Cu1a
1.9688(14)
1.9588(14)
2.5979(4)
Cu1–N1
Cu1–O3
Cu1–O4c
Cu2–N3
Cu1–O8
Cu2–O9c
N1–Cu1–Cu1c
2.154(3)
1.971(3)
1.973(2)
2.157(3)
1.965(3)
1.965(3)
178.41(8)
Cu1–O1
1.975(3)
1.978(3)
2.6134(8)
1.971(3)
1.967(3)
2.6047(8)
175.84(8)
Cu1–O2c
Cu1–Cu1c
Cu2–O6
Cu2–O7c
Cu1–Cu1c
N3–Cu1–Cu2c
Symmetry transformations used to generate equivalent atoms: a –x+ 1/2, – y + 1/2, – z; b – x + 1/2, – y
+ 1/2, – z + ½; c –x, – y, – z.
2.7. General Procedure for Catalytic Aerobic Oxidation
Under typical conditions, the catalytic reactions were performed in a small vial
equiped with magnetic stirers and open to the air. 1.0 mmol of benzyl alcohol, 0.010
mmol (1 mol% based on ligand) of copper(II) CP, TEMPO (0.050 mmol, 5 mol%) and
DMAP (5 mol%) were placed in the vial, to which 5 cm3 of water was sequentially
7

added. The reaction was allowed to stir at room temperature for 10 hours, after which
time the products were extracted with ethyl acetate. The organic layer was diluted with
dichloromethane and analysed by GC-MS to give the indicated conversions.
2.8. Crystal Structure Determinations
Suitable crystals of 5-8 were mounted on Cryoloops with Paratone-N oil. Data were collected
with a Bruker APEX II CCD or a Bruker D8 Venture X-ray instrument using Mo-Kα
radiation. Empirical absorption corrections were applied using the SADABS program [28].
The structures were solved by direct methods and refined based on F2 by the full matrix least-
squares methods using SHELXTL [29]. All non-hydrogen atoms were refined anisotropically
by full-matrix least squares on F2 and all hydrogen atoms were placed in calculated positions
with appropriate riding parameters. PLATON program SQUEEZE was used to address
diffused solvent that are disorder in the structure of 8 [30]. In 8, positional disorder (with
63.4/36.6 ratio) associated with carbon atoms C24-C27 in the hexyloxy chain of ligand was
treated with C-C distance restraints for C23-C24, C24-C25, C25-C26 and C26-C27 set to
1.54 Å and with equal anisotropic displacement parameters for each pair of those carbons. In
addition, anisotropic Uij components of C25/C25A and C27/C27A were refined isotropically.
All molecular structures and packing figures were drawn with the program Mercury v. 2.4
[31]. The refinement details are summarized in Table 1 and selected bond parameters are
listed in Table 2.
3. Results and Discussion
3.1. Synthesis and characterization
Ligands 1-4 were prepared according to the Kröhnke condensation procedure reported in
the literature and the synthesis of new ligand 4 is described here [13-16, 24]. The room-
temperature reaction of 4-hexyloxybenzaldehyde with two equiv. of 3-acetylpyridine in the
presence of KOH followed by addition of excess aqueous ammonia afforded pure 4 as a
white solid after isolation by filtration. 4 is well soluble in halogenated solvents such as
CH2Cl2 and CHCl3, DMF and DMSO, yet slightly soluble in MeOH, EtOH and MeCN. The
elemental analysis data match with that calculated, in agreement with the expected structure.
8

1H and 13C NMR spectra of 4 were recorded in CDCl3 and are also consistent with the
molecular structure of the ligand as illustrated in Scheme 1. Good quality single crystals of 5-
8 were obtained in reasonable yields by using a layering technique that allows slow diffusion
between solutions of the ligands and Cu(OAc)2·H2O in the CH2Cl2-MeOH system, while the
solution reactions of the metal-ligand mixture in the same solvent system gave immediately
precipitates that prevent from further characterization. The bulk samples of 5-8 are insoluble
in water and common organic solvents and they are characterized by FT-IR spectroscopy,
elemental analysis and X-ray diffraction analysis.
(a)
(b)
9

Fig. 1. (a) The structure of the repeat unit in
showing the coordination environment around copper(II) centers and bond expansion. (b) The 3D
crystal packing model in viewing down the crystallographic axis.
5 with ellipsoids plotted at the 50% probability level,
5
c
3.2. Crystal Structure of [Cu2(1)(OAc)4]n (5) and [Cu2(2)(OAc)4]n (6)
Green blocks of 5 that are suitable for X-ray diffraction analysis were obtained by the
layering method described above. 5 crystallizes in the monoclinic space group I2/a as a one-
dimensional coordination polymer with a formula of [Cu2(OAc)4(1)]n (Table 1), which is in
agreement with the results of elemental analysis. An ORTEP representation of the repeat unit
is depicted in Fig. 1a. The asymmetric unit contains only half of the ligand and the second
1
half was generated by symmetry operation (symmetry code: – x + 1/2, – y + /2, – z + ½). In
the repeat structural unit, a paddle-wheel {Cu2(µ-OAc)4} motif is observed, which links the
ligand L1 with two of its terminal pyridine-N atoms through its apical positions to form an
infinite zigzag chain (Fig. 1b). Similar to the well-known {Zn2(µ-OAc)4} [11], the paddle-
wheel {Cu2(µ-OAc)4} unit is common and has been previously found in copper acetate
assemblies of 4,2':6',4''-tpys containing various 4'-substituents such as 4'-(biphenyl-4-yl)-4,
2':6',4''-tpy and 4'-(pentafluorobiphenyl-4-yl)-4,2':6',4''-tpy [26, 27]. The comparison of cell
parameters for crystals of 5 with the known copper CPs, [Cu2(µ-OAc)4(4′-(biphenyl-4-yl)-
4,2′:6′,4″-tpy)]n and [Cu2(µ-OAc)4(4′-(pentafluorobiphenyl-4-yl)-4,2′:6′,4″-tpy)]n confirms
that they are isomorphic. The relevant bond lengths and angles for 5 listed in Table 2 are
unexceptional and close to those reported [26, 27]. The Cu…Cu distance of the {Cu2(µ-
OAc)4} unit in 5 is 2.6344(6) Å, while that between two copper centers coordinating to the
outer-N atoms of one ligand is 12.975(4) Å. The rings of the 4,2':6',4''-tpy domain of ligand
are almost planar, and the torsion angle between the outer and central pyridines is only 2.79º,
which is in contrast with the large deviation of the 4'-phenyl ring from the plane (torsion
angle = 30.02º). The co-planarity of 4,2':6',4''-tpy domains is reflected by the packing mode
between the adjacent 1D zigzag chains in 5. Effective π-stacking interactions of 4,2':6',4''-tpy
domains were observed as usually seen in 1D zigzag CPs of Zn(II)- and Cu(II)-4,2':6',4''-tpys.
Specifically, both the central pyridine ring and one of the side pyridine rings stack with one
another from adjacent polymeric chains with a centroid…centroid distance of 3.655 Å, and
10

the slip angel between two rings for the π-stacking is found to be 64.43º. Unlike the
additional inter-chain π…π interactions due to biphenyl…biphenyl stacking found in [Cu2(µ-
OAc)4(4′-(biphenyl-4-yl)-4,2′:6′,4″-tpy)]n, the phenyl groups between chains do not
participate in significant π-stacking interactions [26, 27]. The 3-D molecular packing along
the crystallographic c axis clearly presents the arrangement of chains, showing separate zones
of inorganic {Cu2(µ-OAc)4} units and organic ligands (Fig. 1b).
We next replaced the 4'-phenyl group with 4'-(4-dimethylaminophenyl) group on the 4,
2':6',4''-tpy backbone and tested the reaction of 2 with Cu(OAc)2·H2O by the same layering
method as for 5. As a result, green blocks of [Cu2(2)(OAc)4]n (6) have been harvested and
structurally characterized. X-ray structural analysis reveals that 6 is isomorphic to 5 and a
similar metal-ligand coordination mode to those found in 5 and other related structures, as
illustrated in Fig. 2a. Again 1-D zigzag chains composed of both paddle-wheel {Cu2(µ-
OAc)4} units and C2-symmetry ligands were observed in the crystal of 6 (Fig. 2b). In 6,
Cu1…Cu1i separation of 2.6285(7) Å was observed in the {Cu2(µ-OAc)4} unit and the
distance between the adjacent Cu2 cores is 12.862(3) Å, both close to those found in 5.
Similar to 5, inter-chain π-stacking of the 4,2':6',4''-tpy domains (the centroid…centroid
distance is 3.731 Å, with the slip angle being 61.25º) governs the crystal packing in 6 in the
solid state (Fig. 2b).
(a)
11

(b)
Fig. 2. (a) The crystal structure of the repeat unit in
6 with ellipsoids plotted at the 50% probability
level, showing the coordination environment around copper(II) centers and bond expansion. Hydrogen
atoms are omitted for clarity. (b) The packing mode of two adjacent zigzag chains showing inter-chain
π-stacking interactions of the tpy domains that are displayed in red and blue, respectively.
(a)
12

(b)
Fig. 3. (a) The crystal structure of the repeat unit in
7 with ellipsoids plotted at the 50% probability
level, showing the coordination environment around copper(II) centers and bond expansion. Hydrogen
atoms are omitted for clarity. (b) The 3D crystal packing in along the crystallographic axis.
7
c
3.3. Crystal Structure of [Cu2(3)(OAc)4]n (7) and [Cu2(4)(OAc)4]n (8)
Since the structures of 5 and 6 are similar to the known copper CPs in the literature, we
moved to the related ligand isomers, 3,2':6',3''-tpys. Accordingly, the reaction of 3 with
Cu(OAc)2·H2O through the same layering method using MeOH/CH2Cl2 solvent system gave
good-quality crystals of 7. Elemental analysis data indicates a metal-ligand ratio of 2:1 in
bulk sample of 7. 7 also crystallizes in the monoclinic space group I2/a and the cell
parameters are similar to those of 5 and 6, revealing isomorphism in these structures,
although the ligands used are essentially different (4,2':6',4''-tpy vs. 3,2':6',3''-tpy). The
common paddle-wheel {Cu2(µ-OAc)4} structure is observed again in the repeat unit of the
cell and is utilized to bind the outer 3-pyridyl units to form an infinite zigzag chain, in which
3,2':6',3''-tpy moieties adopt a (trans, trans)-type I binding mode (Scheme 1). The structural
similarity observed in CPs 5 and 7 is probably due to the similar V-shaped bidentate binding
mode of 3,2':6',3''-tpy (type I) and 4,2':6',4''-tpy (both have internal angles of 120o). In 7, a
slightly shorter Cu1…Cu1i contact in the {Cu2(µ-OAc)4} core is observed (2.5979(4) Å vs.
2.6344(5) in 5), while the distance between two Cu centers bonded to the same ligand is a
little longer (13.281(2) vs. 12.862(3) Å in 5). However, what is most distinct between these
13

two structures is the inter-chain molecular packing. Relatively weaker inter-chain π-stacking
occurs in the regions of the 4'-(4-dimethylaminophenyl) group and central pyridine of the
ligands from adjacent polymeric chains. The centroid…centroid distance between the phenyl
rings involved in the π-stacking is 4.430 Å, while the shortest interatomic distance is 3.563 Å.
Interestingly, it was observed that in the ligand molecule of 7 the methyl groups are slightly
deviated from the of phenyl ring by approximately 4.73º and the torsion angle between
phenyl and central pyridyl rings is 6.21º, while the outer 3-pyridyl groups are severely
twisted from the central pyridine by a dihedral angle of 20.45º. Furthermore, 1D chains pack
in an alternate head-to-tail fashion to form a 3D framework as shown in Fig. 3b. Again,
separate organic and inorganic regions are formed upon viewing down the crystallographic c
axis.
To further examine the influence of 4'-substituents on the copper coordination chemistry
with 3,2':6',3''-tpys, we introduced a long alkoxy chain onto the 3,2':6',3''-tpy backbone and
prepared the new ligand 4 (Scheme 1). Previously, the 4,2':6',4''-tpy analogue of 4 has been
utilized for zinc(II) acetate-mediated self assembly and a discrete complex was formed [25],
rather than a coordination polymer. The effect of the chain length of 4'-alkoxylphenyl groups
in the 4,2':6',4''-tpy on controlling the resulting ZnII assemblies as coordination polymers or
discrete complexes was also systematically investigated [32]. The reaction of 4 with
Cu(OAc)2·H2O under the same conditions by the layering method afforded green crystals of
8 that are suitable for X-ray structural analysis. 8 crystallizes in the triclinic space group P-1
and the repeat structural unit contains one independant ligand molecule and two CuII centers
that constitute similar paddle-wheel {Cu2(µ-OAc)4} units as found in 5-7 (Fig. 4a). Note that
the presence of a long alkoxy tail in 4 breaks the molecular symmetry of 3,2':6',3''-tpy, and
more interestingly, the 3,2':6',3''-tpy motif in 8 coordinates to CuII centers by the (trans, cis)-
type II binding mode, in sharp contrast to the situation in 7. This indicates that
functionalization on the 4'-position of 3,2':6',3''-tpy did play a crucial role in altering the
ligand conformation and hence changing metal coordination modes. In 8, the Cu…Cu
contacts in two independent {Cu2(µ-OAc)4} units are 2.6047(8) and 2.6134(8) Å,
respectively, falling in the range of those observed in 5-7 (Table 2). However, the distance
between Cu1 and Cu2 centers binding to one ligand is significantly shorter (10.932(3) Å)
than those in other zigzag chains, due to the trans, cis-binding mode of 4. Bond expansion of
14

the repeat structure in Fig. 4 results in the formation of 1D polymeric chain that is clearly
different from the zigzag chains described previously. Again, π-stacking interactions between
the 1D chains also occur in the domains of the phenyl group and central pyridine of the
ligands in a head-to-tail fashion (the shortest interatomic distance for the π…π interaction is
3.574 Å), resulting in the formation of 2D sheets, although the planes of both aromatic rings
are twisted with respect to each other with an dihedral angle of 18.45º. Furthermore, π-
stacking of the central pyridine domains between 2D sheets leads to a 3D network that is
distinct with those of 5-7 described above (Fig. 4b).
(a)
(b)
15

Fig. 4. (a) The crystal structure of the repeat unit in
8 with ellipsoids plotted at the 50% probability
level, showing the coordination environment around two distinct copper(II) centers. Hydrogen atoms
are omitted for clarity. (b) The 3D packing model in crystals of
8 along the crystallographic c axis.
3.4. Catalytic Properties
The catalytic aerobic oxidation of alcohols to aldehyde or ketone products is a critical organic
transformation that has received considerable attention [33, 34]. Copper-catalyzed TEMPO-
mediated aerobic oxidation of alcohols have proven to be the most efficient methodology for
this transformation and great attention has been paid to the search of effective combination
of copper sources and organic ligands. It has been established that the homogeneous
CuI/bpy/TEMPO system is the best choice for the oxidation of a large variety of alcohols
[34]. Our group has been recently interested in exploring the influence of both different
nitrogen ligands and the structures of copper complexes on the catalytic reactivity [35-41]
and found that a CuII/DMAP/TEMPO catalytic system enables aerobic oxidation of a range
of alcohols in aqueous media [36, 39]. Despite the significant progress in this field,
heterogeneous catalysts involving active Cu-pyridine substructure are little investigated. Due
to the poor solubility of CPs 5-8 in common solvents and water, we explore the catalytic
ability of these organic-inorganic hybrid materials for aerobic alcohol oxidation under
heterogeneous conditions, and herein some preliminary results are presented in Table 3.
Initially, we tested the reaction of benzyl alcohol in the presence of copper CPs (5-8, 1 mol%,
calculated based on the ligand) and TEMPO (5 mol%) in the air at room temperature when
acetonitrile was used as solvent, only poor reactivity (<10% conversion) was observed after
10 h for all the catalysts (entries 1-4). Interestingly, switching the solvent to water improved
the performance of all catalysts and the CP 8 was found to be most effective, as 20%
conversion of the substrate to benzaldehyde was detected by the GC-MS analysis (entries 5-
8). Increasing the catalyst loading of 8 to 5 mol% furnished the reaction with 82% conversion
(entry 9). Furthermore, we investigated the effect of a base additive, which has been
generally utilized in some effective catalyst systems reported previously. It was found that the
addition of DMAP (4-dimethylaminopyridine, 5 mol%) resulted in almost quantitative
conversion of benzyl alcohol (entry 10), even though the loading of 8 was low (1 mol%). In
contrast, another base additive, triethylamine (TEA) showed a negative influence on the
16

catalytic efficacy (entry 11). It was noted that CP 7 also performed well when DMAP was
added under the same conditions. Control experiment using in situ combination of free ligand
4 and Cu(OAc)2 in water gave much inferior results (entry 13), indicating the crucial role of
metal-organic coordination materials in promoting reactivity. The reaction using ligand 4
without the addition of copper salt gave no reaction, although the combination of Cu(OAc)2
(1 mol%) with DMAP was found to be moderately active (entries 14 and 15) [36]. In
addition, TEMPO radical was also required to gain good catalytic activity (entry 16).
Substituted benzylic alcohols such as 4-chlorobenzyl alcohol and 4-methoxybenzyl alcohol
were also catalytically oxidized to the corresponding aldehydes by using the
8/DMAP/TEMPO system under heterogeneous conditions, while the secondary alcohol 1-
phenylethanol did not proceed well under the optimized conditions (entries 17-19).
Table 3. Copper CPs-catalyzed, TEMPO-mediated aerobic oxidation of benzyl alcohol.a
Entry Copper CP
Additive
Solvent
Conv.b (%)
(5 mol%)
1
-
CH3CN
CH3CN
CH3CN
CH3CN
H2O
-
0
5
6
7
8
5
6
7
8
8
8
8
7
2
-
3
-
9
4
-
8
5
-
15
8
6
-
H2O
7
-
H2O
17
20
82
98
7
8
-
H2O
9c
10
11
12c
13d
14
-
H2O
DMAP
TEA
DMAP
H2O
H2O
H2O
96
6
4/Cu(OAc)2 DMAP
DMAP
H2O
H2O
0
4
17

15
Cu(OAc)2
DMAP
DMAP
DMAP
DMAP
DMAP
H2O
H2O
H2O
H2O
H2O
55
2
16e
17f
18g
19h
8
8
8
8
96
>99
5
a Condition: 1.0 mmol of the substrate, 1 mol% of copper CP, 5 mol% of TEMPO in solvents (5 mL), 1 atm. air,
room temperature, 10 h. b Conversions based on GC-MS analysis, aldehyde selectivity > 99% in all cases. c 5 mol%
of catalyst
8 4 were added sequentially to the reaction system,
was used. d 2 mol% of copper acetate and 1 mol% of
and a green suspension was developed. e Reaction run in the absence of TEMPO. f 4-chlorobenzyl alcohol (1.0
mmol) was used. g 4-methoxybenzyl alcohol (1.0 mmol) was used. h 1-phenylethanol (1.0 mmol) was used.
4. Conclusions
In summary, we present here the syntheses and structural characterization of four copper(II)
CPs based on 4,2':6',4''- or 3,2':6',3''-tpy derivatives including the new ligand 4. In the crystal
structures of CPs, a paddle-wheel {Cu2(µ-OAc)4} motif was observed in every case, which
linked the bidentate tpy ligands to form 1D polymeric chains with its remaining coordination
sites. Although the copper(II) CPs of 4,2':6',4''-tpys feature similar zigzag structures as that
reported previously, those derived from 3,2':6',3''-tpys display two different 1D chains,
resulting from the differing ligand binding modes of 3,2':6',3''-tpy unit. The effect of 4'-
substituents in 3,2':6',3''-tpy is believed to be the determining factor for distinct coordination
modes in the present work, although a systematic investigation on the utilization of a broader
range of substituents is desired in the future. In addition, these CPs were observed to
effectively catalyze the TEMPO-mediated aerobic oxidation of benzylic alcohols under
heterogeneous conditions in water.
Acknowledgements
We acknowledge the City University of New York (CUNY) Collaborative Incentive
Research Grant (CIRG#80209-06) and the Program for Research Initiatives for Science
Majors (PRISM) at John Jay College for support. The Natural Science Foundation of Hubei
Province in China (2015CFA130 and 2013CFA015) is gratefully acknowledged.
18

Appendix A. Supplementary data
CCDC Nos. 1430536-1430539 contain the supplementary crystallographic data for this
paper.
These
data
can
be
obtained
free
of
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.poly.2015.xx.xxxx.
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Graphic Abstract - Pictogram
21

Abstract - Synopsis
Four new Cu(II) coordination polymers based on the divergent ligands containing either
4,2':6',4''- or 3,2':6',3''-terpyridine domains with various 4'-substituents are synthesized
and structurally characterized. 4'-Substituents in the 3,2':6',3''-terpyridines are found to
affect the ligand binding modes and distinct coordination assemblies are revealed. The
catalytic aerobic alcohol oxidation using the crystalline materials was explored under
heterogeneous conditions.
22