Facile synthesis of new divergent imidazole-containing ligands for a 1-D cobalt(II) coordination polymer

Article abstract of DOI:10.1016/j.poly.2017.02.028

A new divergent ligand (3a) containing a triaryl core and two imidazole side-arms has been designed and synthesized. The synthetic procedure utilizes a two-step strategy, i.e. the Kr?hnke condensation, followed by a Cu-catalyzed Ullman-type coupling. This synthetic strategy allows the facile introduction of various substituents on the 4-position of the central pyridine in the ligands, which was proved by the synthesis of a ligand analog (3b). The new ligands were well characterized by spectroscopic methods and X-ray structural analysis was carried out for 3a. Reaction of 3a with Co(NCS)₂ by a layering method afforded single crystals of a coordination polymer 5, which was structurally characterized by X-ray diffraction. 5 is composed of two ligands and one Co(NCS)₂ in its asymmetric unit and features a 1-D coordination polymer with nano-scale metallomacrocyles. The structure of 5 is found to be quite different from all known coordination networks composed of Co(NCS)₂ and closely related divergent ligands.

Full text of DOI:10.1016/j.poly.2017.02.028

Polyhedron 127 (2017) 355–360
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Facile synthesis of new divergent imidazole-containing ligands for a 1-D
cobalt(II) coordination polymer
a
a
a
a
b
b
c
E Liu , Hangxing Xiong , Li Li , Chengxiong Yang , Zhiwei Yin , Anthony Chang , David R. Manke ,
c
b,
⇑
James A. Golen , Guoqi Zhang
a
College of Chemical Engineering and Pharmacy, Jingchu University of Technology, Jingmen 448000, China
Department of Sciences, John Jay College and The Graduate Center, The City University of New York, New York, NY 10019, USA
Department of Chemistry and Biochemistry, University of Massachusetts Dartmouth, North Dartmouth, MA 02747, USA
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
A new divergent ligand (3a) containing a triaryl core and two imidazole side-arms has been designed and
synthesized. The synthetic procedure utilizes a two-step strategy, i.e. the Kröhnke condensation, followed
by a Cu-catalyzed Ullman-type coupling. This synthetic strategy allows the facile introduction of various
substituents on the 4-position of the central pyridine in the ligands, which was proved by the synthesis of
a ligand analog (3b). The new ligands were well characterized by spectroscopic methods and X-ray struc-
Received 21 December 2016
Accepted 17 February 2017
Available online 24 February 2017
Keywords:
Divergent ligand
Imidazole
Cobalt(II)
Crystal structure
Coordination polymer
tural analysis was carried out for 3a. Reaction of 3a with Co(NCS)
crystals of a coordination polymer 5, which was structurally characterized by X-ray diffraction. 5 is com-
posed of two ligands and one Co(NCS) in its asymmetric unit and features a 1-D coordination polymer
with nano-scale metallomacrocyles. The structure of 5 is found to be quite different from all known coor-
dination networks composed of Co(NCS) and closely related divergent ligands.
Ó 2017 Elsevier Ltd. All rights reserved.
2
by a layering method afforded single
2
2
0
0
00
1
. Introduction
In search of other divergent ligand analogs to 4,2 :6 ,4 -tpys, we
found that 2,6-bis(imidazole-1-yl)pyridine (2, Scheme 1) bearing
N-imidazolyl side-arms instead of 4-pyridyl group could exhibit
similarly interesting coordination chemistry, even though the V-
shaped N-donors in 2 adopt a smaller angle. In addition, rotations
about the C–N bonds between the pyridyl core and imidazolyl
arms would lead to different ligand conformations during the pro-
cess of metal–ligand self-assembly, whereas rotations about the
In recent years, the divergent analogues of the classical triden-
0
0
00
0
0
00
0
0
00
tate 2,2 :6 ,2 -terpyridine (2,2 :6 ,2 -tpy) derived ligands, 4,2 :6 ,4 -
tpy (1, Scheme 1) have become a popular choice for the construc-
tion of novel metal–organic coordination polymers or networks
that displayed not only interesting supramolecular structures but
0
also intriguing materials properties [1–6]. Generally, 4 -aryl-
0
0
00
0
0
00
4
,2 :6 ,4 -tpys could be readily synthesized and purified in reason-
inter-ring C–C bond in 4,2 :6 ,4 -tpy has no effect on the direction-
alities of the peripheral N-donors. However, up to date there were
only a few metal coordination networks based upon this ligand
that have been reported [11], although it was recently revealed
that this compound comprises of a perfect building block for the
construction of various molecular boxes and covalent macrocycles
[12]. Indeed, the difficulty in modifying the pyridine core with dif-
ferent substituents has largely limited its applications in coordina-
tion and supramolecular chemistry.
able yields through the conventional one-pot Kröhnke condensa-
tion that has been employed for the preparation of a majority of
0
0
0
II
00
4
-aryl-2,2 :6 ,2 -tpys [7]. A variety of transition metals (such as
II
II
II
II
II
Zn , Cd , Hg , Cu , Co and Mn etc.) have been explored by the
Constable, our and other groups through coordination-driven
self-assembly to construct diverse metal–organic network struc-
tures [8–10], where the 4 -position substituent effect has been
extensively studied, thanks to the easiness on ligand modification.
Apparently, the divergent N-donors in 4,2 :6 ,4 -tpys open new
opportunities for the formation of more complicated supramolecu-
lar structures upon coordinating to metal ions, in comparison with
0
0
0
00
In continuation with our recent work on transition metal chem-
0
0
0
00
istry based on the divergent ligands, 4 -aryl-4,2 :6 ,4 -tpys and
other N-rich analogs [9,13], we were interested in further modify-
ing these ligands by varying the angle between the V-shaped N
coordination sites through the introduction of imidazolyl side-
arms. We envisaged that one could construct the triaryl core sim-
0
0
00
3
the chelating N cavity of 2,2 :6 ,2 -tpys.
⇑
Corresponding author.
E-mail address: guzhang@jjay.cuny.edu (G. Zhang).
0
0
00
ilar to 4,2 :6 ,4 -tpy by taking advantage of the easiness of modifi-
http://dx.doi.org/10.1016/j.poly.2017.02.028
277-5387/Ó 2017 Elsevier Ltd. All rights reserved.
0

3
56
E Liu et al. / Polyhedron 127 (2017) 355–360
1
(
23.83, 117.36 ppm. HR-MS (ESI, positive): m/z 465.9621 [M+H]+
the only peak, calc. 465.9629).
2.3. Synthesis of 4b
4
b was synthesized according to the same procedure as for 4a,
except the replacement of benzaldehyde with pyridine-4-carbox-
aldehyde (1.06 g, 10.0 mmol). Off-white solid of 4b was collected
1
in 52% yield (2.42 g). H NMR (500 MHz, CDCl
8
3
) d 8.84 (s, 2 H),
13
.09 (d, J = 8.5 Hz, 4 H), 7.90 (s, 2 H), 7.69 (m, 6 H) ppm;
C
3
NMR (125 MHz, CDCl ) d 156.99, 150.38, 147.75, 146.48, 137.66,
1
32.04, 128.66, 124.12, 121.83, 116.74 ppm. Anal. Calcd. for C22-
2 2
H14Br N : C, 56.68; H, 3.03; N, 6.01. Found: C, 56.59; H, 3.01; N,
6
.18%.
Scheme 1. Structures and various metal coordination directionalities of divergent
ligands 1–3.
2.4. Synthesis of 3a
To a 100 mL Pyrex tube was added a mixture of 4a (1.00 g,
2
0
2
.16 mmol), imidazole (2.94 g, 43.2 mmol), Cu (0.030 g,
2
O
cation on the 4-position of central pyridine through one-pot
Kröhnke condensation, and then imidazolyl units can be readily
anchored on the para-position of the side phenyl rings, should
halo-substituents were present in the phenyl rings of the starting
materials. Therefore, we report herein the successful synthesis of
a novel divergent ligand bearing imidazolyl side-arms with a larger
distance being between the divergent N-donating sites (Scheme 1).
It is revealed that the R substituents in the 4-position of the central
pyridine core can be readily altered as we anticipated. The applica-
tion of this type of ligands in the synthesis of metal–organic coor-
dination polymers/networks has been demonstrated through the
synthesis and X-ray structural characterization of a one-dimen-
sional (1-D) cobalt(II) coordination polymer containing nano-scale
metallomacrocyclic motifs.
.2 mmol), CuSO (0.070 mg, 0.2 mmol) and K CO (3.00 g,
4
2 3
1.6 mmol) under Ar atmosphere, and then anhydrous DMF
(
10 mL) was added. The suspension was heated to reflux and stir-
red overnight. The reaction mixture was cooled to room tempera-
ture and filtered, the solid was washed with CH Cl
(10 mL ꢀ 2).
Water (100 mL) was added to the filtrate which was then extracted
with CH Cl
(50 mL ꢀ 2). The organic layer was collected and
washed with water for three times. The solution was dried over
anhydrous Na SO and filtered. The solvent was removed under
2
2
2
2
2
4
reduced pressure. The product was purified through a flash column
chromatography (eluent: ethyl acetate/methanol = 1:10, v/v) to
give a white solid. Colorless single crystals were obtained by slow
evaporation of a CH
2
Cl
.86 g (91%). FT-IR (solid, cm ): 1610m, 1548w, 1522s, 1484s,
396m, 1301s, 1247s, 1186w, 1110w, 1053s, 961w, 901w, 817s,
2
-MeOH of 3a at room temperature. Yield:
ꢁ1
0
1
7
1
3
73s, 727m, 700m, 654s. H NMR (400 MHz, CDCl ) d 8.34 (d,
2
. Experimental
J = 6.0 Hz, 4 H), 8.00 (s, 2 H), 7.94 (s, 2 H), 7.76 (d, J = 4.8 Hz, 2
H),7.58–7.52 (m, 7 H), 7.38 (s, 2 H), 7.27 (s, 2 H) ppm; 13C NMR
2.1. General
(
100 MHz, CDCl
3
)
d
156.22, 150.84, 138.60, 138.59, 137.90,
35.50, 130.46, 139.36, 129.36, 129.28, 128.66, 127.19, 121.52,
18.16, 117.40 ppm. HR-MS (ESI, positive): m/z 462.1692 [M
1
1
Solvents and reagents were purchased from Fisher Scientific or
Sigma–Aldrich in the US. All reactions were performed under
ambient conditions (no inert atmosphere). FT-IR spectra were
measured on a Shimadzu 8400S instrument with solid samples
using a Golden Gate ATR accessory. Thermogravimetric analysis
+
+
+Na] (calc. 462.1695), 440.1873 [M+H] (base peak, calc.
4
40.1875).
2.5. Synthesis of 3b
(
TGA) was carried out on a Shimadzu TG-50 analyzer under N
2
1
13
atmosphere with a heating rate of 20 °C/min. H and C NMR
3
b was prepared according to the same procedure as for 3a,
spectra were obtained at room temperature on a Bruker III
except the use of 4b (1.01 g, 2.16 mmol) as the precursor. 3b was
isolated as white solid in 76% yield (0.72 g) after column chro-
matography. FT-IR (solid, cm ): 1740m, 1592s, 1521s, 1484m,
5
00 MHz spectrometer with TMS as an internal standard. High res-
olution mass spectra were recorded on an Agilent 6550 iFunnel
ESI-QTOF-LC/MS instrument. Elemental Analyses were performed
by Midwest Microlab LLC in Indianapolis.
ꢁ
1
1
6
8
397m, 1302s, 1246m, 1112w, 1053s, 962m, 902w, 825s, 727m,
54m, 622m. H NMR (400 MHz, CDCl
.35 (d, J = 8.8 Hz, 4 H), 7.99 (s, 2 H), 7.95 (s, 2 H), 7.68 (d,
1
3
) d 8.83 (d, J = 6.0 Hz, 2 H),
2.2. Synthesis of 4a
J = 6.0 Hz, 2 H), 7.59 (d, J = 8.8 Hz, 4 H), 7.39 (s, 2 H), 7.27 (s, 2 H)
1
3
ppm;
3
C NMR (100 MHz, CDCl ) d 156.69, 150.81, 148.06,
In a 250 mL round-bottom flask equipped with a magnetic stir-
146.01, 138.19, 137.93, 135.51, 130.70, 128.68, 121.63, 121.51,
rer, 4-bromoacetophenone (3.94 g, 20.0 mmol) was added to a
solution of benzaldehyde (1.06 g, 10.0 mmol) in EtOH (80 mL).
KOH pellets (0.84 g, 15 mmol) were then added, followed by aque-
118.06, 116.94 ppm. HR-MS (ESI, positive): m/z 463.1664 [M
+Na] (calc. 463.1647), 441.1828 [M+H] (base peak, calc.
441.1828).
+
+
3
ous NH (28%, 50 mL). The resulting reaction mixture was heated
to 70 °C upon rigorous stirring for 24 h. The suspension was cooled
2.6. Synthesis of 5
to room temperature and filtered. Off-white solid was collected,
washed with EtOH and dried in a desiccator over P
2
O
5
. Yield:
) d 8.10 (d, J = 8.0 Hz, 4 H),
.90 (s, 2 H), 7.76 (d, J = 7.5 Hz, 2 H), 7.68 (d, J = 8.5 Hz, 4 H),
.59–7.53 (m, 3 H) ppm; 13C NMR (125 MHz, CDCl
) d 156.39,
50.88, 138.54, 137.91, 131.93, 129.35, 129.26, 128.77, 127.22,
A solution of 3a (43.9 mg, 0.100 mmol) dissolved in MeOH/CH
(10 mL, 1: 4, v/v) was placed in a long test tube. A mixture of
MeOH and CH Cl (6 mL, 1: 1, v/v) was then layered on the top
of this solution, followed by another solution of Co(NCS)
(17.5 mg, 0.100 mmol) in MeOH (10 mL). The tube was sealed
2
-
1
2
7
7
1
.60 g (56%). H NMR (500 MHz, CDCl
3
Cl
2
2
2
3
2

E Liu et al. / Polyhedron 127 (2017) 355–360
357
and allowed to stand at room temperature for about 15 days, dur-
ing which time X-ray quality pinkish crystals grew on the wall of
the tube. After X-ray diffraction measurement crystals that were
used for analysis were collected by decanting the solvent, washed
with MeOH and dried in the air. The crystals became much paler
after being moved out from the solution and exposure to air, due
to the loss of interstitial solvents. Yield: 41.0 mg (78% based on
ꢁ
1
3
1
6
6
a). FT-IR (solid, cm ): 2063s, 1597m, 1549m, 1522s, 1498m,
429w, 1302m, 1244m, 1122w, 1066s, 964m, 934m, 833s, 764s,
88w, 659m, 623w, 528m. Anal. Calcd. for C60H42CoN12S : C,
2
8.37; H, 4.02; N, 15.95. Found: C, 67.98; H, 3.95; N, 15.72%.
2.7. X-ray structural determinations
Suitable single crystals were mounted on Cryoloops with oil.
Data were collected for 3a and 5 with a Bruker APEX II CCD using
Mo K
a
radiation and a Bruker D8 Venture X-ray instrument using
radiation, respectively. Data were corrected for absorption
Scheme 2. The synthetic route to 3a and 3b.
Cu Ka
with SADABS and structures solved by direct methods. All non-
idyl derived analog 4b was also prepared in 52% yield by following
the same procedure, except the use of pyridine-4-carboxaldehyde
as a starting material. This indicates that the modification on the
hydrogen atoms were refined anisotropically by full-matrix least
2
squares on F . Hydrogen atoms for 3a were found from Fourier dif-
ference maps and refined isotropically. In the structure of 5, sulfur
atoms S1 and S2 were disordered over two positions with occupan-
cies 63.1/38.9 and 94.7/5.3%, respectively. All hydrogen atoms
were placed in calculated positions with appropriate riding param-
eters. Large solvent accessible voids were observed in the structure
4
-position of the central pyridine ring is feasible, opening opportu-
nities for the facile introduction of various substituents.
In the next step, we utilize the di-bromo substituted precursors
to prepare imidazole-containing ligands 3a through an Ullman-
type C–N bond coupling reaction. Although it was unsuccessful
2 2
and the diffused solvent was assumed to be CH Cl and was treated
when a catalytic method (conditions: CuI/L-proline, K
0 °C) used for the synthesis of 1 in the literature [12], we eventu-
ally observed that the use of Cu O/CuSO as a catalyst combined
with K CO in DMF at 150 °C was effective for the coupling
2 3
CO , DMSO,
using the PLATON program SQUEEZE [14]. Crystal structures and pack-
ing figures were drawn with the program MERCURY v. 2.4 [15]. The
crystallographic refinement data are listed below.
9
2
4
2
3
3
21 5
a: C29H N , M = 439.51, colorless block, monoclinic, space
between 4a and imidazole in high yields [16]. As a result, the
desired ligand 3a was obtained in 91% yield after purification by
column chromatography (Scheme 2). Likewise, the derivative 3b
containing an additional 4-pyridyl group was synthesized in 76%
yield by using the same procedure. Both 3a and 3b are well soluble
group P2/n, a = 19.4272(5), b = 10.8084(3), c = 20.7906(5) Å,
3
ꢁ3
b = 100.8370(10), V = 4287.69(19) Å , Z = 8, D
c
= 1.362 mg m
T = 120(2) K. Total 26982 reflections,
923 unique. Refinement of 7840 reflections (617 parameters)
(I) converged at final R = 0.0365 (R all data = 0.0418),
= 00908 (wR all data = 0.0944), GOF = 1.037.
CoN12 2, M = 1139.03, colorless plate, monoclinic,
, l
ꢁ1
(
Mo Ka) = 0.650 mm ,
6
with I > 2
r
1
1
in CH
2 2 3
Cl , CHCl , DMF and DMSO, yet poorly soluble in MeOH, ace-
wR
2
2
1
tone and acetonitrile. They were fully characterized by FT-IR,
H
5
: C66
H44Cl
2
S
1
3
and C NMR spectroscopies, and HR-MS. Mass spectrum of 3a dis-
plays peak envelops at m/z 462.1692 and 440.1873, corresponding
space group P2(1)/c, a = 18.7769(5), b = 22.0438(6), c = 14.0169
3
–3
(
l
4) Å, b = 96.7920(10), V = 5761.1(3) Å , Z = 4, D
c
= 1.313 mg m
,
+
+
ꢁ
1
to the species [M+Na] and [M+H] , respectively, which matched
with those calculated and whose isotope modes are consistent
with that simulated as well. Similar results were also observed in
the mass spectrum of 3b. The H and C NMR spectra of 3a and
b recorded in CDCl reveal signals in agreement with their molec-
(Mo Ka) = 4.264 mm , T = 120(2) K. Total 10504 reflections,
8
376 unique. Refinement of 10504 reflections (684 parameters)
with I > 2 (I) converged at final R = 0.0645 (R all data = 0.0803),
wR = 0.1636 (wR all data = 0.1742), GOF = 1.016.
r
1
1
1
13
2
2
3
3
ular structures as illustrated in Scheme 1. In addition, the crystal
structure of 3a was confirmed by X-ray diffraction analysis.
3
. Results and discussion
3.1. Ligand synthesis and characterization
3.2. Crystal structure of 3a
The synthesis of the divergent ligand (3a) was carried out in
two steps (Scheme 2). Initially, we attempted to synthesize the
dibromo-containing precursor 4a by employing the facile one-pot
Kröhnke condensation as reported for tpy ligands [7–9]. However,
the room-temperature reaction of benzaldehyde with two equiva-
lent of 4 -bromoacetophenone in the presence of KOH followed by
3
addition of excess aqueous NH did not provide the desired pro-
2
Colorless plates of 3a were grown by slow evaporation of a CH -
Cl -MeOH solution within three days. X-ray structural analysis
2
reveals that 3a crystallizes in the monoclinic space group P2/n.
An ORTEP representation of one molecule of 3a is shown in Fig. 1a.
There exists one independent molecule of 3a in the asymmetric
unit. Due to the flexibility of bonds between the phenyl and imida-
0
duct after 24 h, yet a chalcone compound as a major product was
isolated as confirmed by NMR, resulting from the 1: 1 condensa-
tion of 4 -bromoacetophenone and benzaldehyde. Elevating the
1
zolyl rings, molecule 3a in the solid state adopts a C -symmetry
and the two imidazole side-arms are twisted about the C–N bond
with different orientations, unlike the situations often observed
0
0
0
00
reaction temperature to 70 °C was found to be crucial for the for-
mation of product 4a in a yield (56%) close to those for relevant
in ligands and coordination complexes containing 2,2 :6 ,2 - or
0
0
00
2
4,2 :6 ,4 -tpy moiety, where the tpy domains are usually of C -
0
0
00
4
,2 :6 ,4 -tpys. Thus, the white solid of 4a was readily isolated from
symmetry. The bond lengths in structure 3a are unexceptional.
All peripheral rings are out of the plane of the central pyridine ring
because of bond twisting. The dihedral angles between the phenyl
rings (containing C7, C14 and C24) and pyridine ring are 23.71,
the one-pot reaction after 24 h at 70 °C by filtration (Scheme 2),
1
13
and its purity was established by H and C NMR spectroscopies,
and high resolution mass spectrometry (HR-MS). Similarly, 4-pyr-

3
58
E Liu et al. / Polyhedron 127 (2017) 355–360
porous framework [8c]. The bulk sample of 5 was characterized
by IR spectrum and elemental analysis, and structurally analysed
by X-ray crystallography. The IR spectrum reveals a characteristic
ꢁ
1
strong absorption at 2063 cm , indicating the presence of NCS
group. The elemental analysis data is in good agreement with the
formation of a 1: 2 Co-ligand complex. X-ray structural analysis
clearly confirmed the formation of
a coordination polymer,
although the existence of disordered solvent molecules in the
unit cell that have been treated using the PLATON program SQUEEZE
[
15]. 5 crystallizes in the monoclinic space group P2(1)/c and in
the asymmetric unit there contains two independent ligand mole-
cules and one cobalt(II) coordination sphere surrounded with two
NCS groups. The ORTEP structure of two repeating structural units
is presented in Fig. 2a. The cobalt center is hexa-coordinate with
six nitrogen atoms, of which four are from distinct 3a molecules
(
a)
(
b)
Fig. 1. (a) The ORTEP representation of molecule of 3a in the crystals with thermal
ellipsoids plotted at the 50% probability level. (b) The molecular packing along the
crystallographic a axis in the unit cell of 3a.
4
3.79 and 25.16°, respectively, while those between the imidazolyl
rings (N2 and N4) and adjacent phenyl ring are 30.37 and 40.23°,
respectively. Due to the severe twisting of the aromatic rings
through the correspondent C–C bonds out of the central plane in
the molecule of 3a, the intermolecular packing as revealed in
Fig. 1b shows only very weak
p p stacking interaction (the short-
ꢂ ꢂ ꢂ
est intermolecular Cꢂ ꢂ ꢂC distance is more than 3.8 Å), and the
molecules in the asymmetric unit reside in four different
orientations.
3.3. Synthesis and crystal structure of 5
To examine the ability of the new divergent ligand 3a in form-
ing coordination polymers or networks while reacting with metal
ions, we conducted the reaction by choosing Co(NCS) as a precur-
sor, as it has been often utilized for the construction of 2-D or 3-D
2
0
0
0
00
frameworks involving 4 -substituted 4,2 :6 ,4 -tpys [8d,8e,17]. A
layering method that was previously applied to grow crystals of
coordination polymers with related ligands has been used for the
reaction between 3a and Co(NCS)
solution of Co(NCS) onto the top of a solution of 3a in a CH
MeOH solution with a metal–ligand ratio of 1: 1 in a test tube,
separated with a blank CH Cl -MeOH middle layer, led to good
quality pinkish crystals of 5 after two weeks. However, when the
same method was employed for the reaction of 3b with Co
2
. Thus, carefully layering a
2
2
Cl -
2
Fig. 2. (a) ORTEP structure of the repeat unit in 5 with thermal ellipsoids plotted at
the 50% probability level. H atoms are omitted for clarity. (b) One of the 1-D
polymeric chains found in 5. (c) The packing mode between 1-D chains in 5
2
2
showing the inter-hain
p-stacking interactions between central pyridine rings as
(
2
NCS) , no suitable crystals were harvested, although the
space-filling balls in pink and blue, respectively. NCS groups and H atoms are
tridentate ligand 3b could be attractive in generating novel
omitted for clarity. (Color online.)

E Liu et al. / Polyhedron 127 (2017) 355–360
359
and two from NCS anions (each S atom is disordered over two posi-
tions, see experimental section), adopting an octahedral geometry.
As expected, 3a behaves as a bidentate ligand to bind to the metal
center with its two imidazolyl-N atoms, remaining the central pyr-
idine uncoordinated. This is reminiscent of the coordination mode
0
0
00
of 4,2 :6 ,4 -tpys in all coordination structures thus far reported
[
II
5,6,8–10]. All bond angles and lengths around the Co center are
unexceptional and close to those in known Co(NCS) networks of
,2 :6 ,4 -tpys [8d,8e,17]. The phenyl rings in the ligand are less
2
0
0
00
4
twisted than those found in the structure of free ligand. The
corresponding dihedral angles fall into the ranges of 3.19–20.97°
and 2.53–24.39° with respect to the central pyridine ring, for two
independent ligand molecules, respectively, whilst those between
the imidazolyl rings and adjacent phenyl ring are 30.83 and
4
3.42°, which are still close to that observed in the free ligand.
Interestingly, two centrosymmetrical ligands bridge two cobalt
II) centers (Co1 and Co1A) to form a [2 + 2] metallomacrocyclic
(
subunit. In the macrocycle, the distance between metal centers is
approximately 15.58 Å, while the distance between central
pyridyl-N atoms of two ligands is about 12.10 Å, indicating the
size of the macrocyle is in a nano-scale. Furthermore, the
macrocyclic motifs are extended through the metal nodes to
form a 1-D polymeric chain (Fig. 2b). Significant inter-chain
stacking interactions were observed and serve to expand the
chains to a 2-D sheet (Fig. 2c). Specifically, the -stacking occurs
p-
p
between the central pyridine rings of adjacent 1-D chains in a
head-to-tail manner, and the closest inter-atomic distance for the
p
ꢂ ꢂ ꢂ
p
packing is approximately 3.402 Å. The 2-D sheets are
-stacking
further packed layer-by-layer through additional
p
interactions of aromatic regions in the ligands to form a 3-D
framework.
Fig. 3. The 2-D cobalt(II) networks assembled from Co(NCS)
4 -phenyl-4,2 :6 ,4 -tpy (b) [CCDC No.856247, 8e]. The structures were regenerated
based upon symmetry operations.
2
with 2 (a) [11a] and
0
0
0
00
A comparison of the structure of 5 revealed here with previ-
ously reported Co(NCS)
ligands 1 and 2 is instructive. First, in 2003 Chung’s group reported
the reaction of Co(NCS) with 2 in a MeOH-H O system by the lay-
2
networks based on structurally related
2
2
ering method and the resultant product was characterized as a 2-D
coordination polymer (6) containing interesting cylindrical chan-
nels (Fig. 3a) [11a]. Our independent experiment using the same
2 2
reaction procedure as for 5 (in CH Cl -MeOH) also led to single
crystals with a structure identical to the reported one [11a].
II
Although the coordination environments around Co in 5 and 6
are similar, the linking modes of ligands are essentially different
and as a result, distinct coordination networks (1-D vs. 2-D) were
obtained. We propose that the strong
p-stacking interactions
between large aromatic regions are the driving forces for the
formation of simple 1-D looped chain in 5. Next, a coordination
0
0
0
00
polymer built from Co(NCS)
derivative of 1) has also been reported by Constable and
coworkers, where 2-D network containing [4 + 4]
metallomacrocycles was disclosed and significant -stacking
2
and 4 -phenyl-4,2 :6 ,4 -tpy (a
a
p
interactions of aromatic regions were observed between the 2-D
networks. In addition, different 2-D or 3-D network structures
0
were reported for cobalt(II) complexes with other 4 -aryl-
4
Fig. 4. The TG curve for 5.
0
0
00
0
0
0
00
,2 :6 ,4 -tpys [8e,17] and 4 -tert-butyl-4,2 :6 ,4 -tpy [8d], yet
none has revealed a structure found in 5.
The thermogravimetric analysis (TGA) of 5 under N
sphere (Fig. 4) reveals that 5 is stable up to 345 °C and then expe-
riences continuous decomposition of the framework until over
00 °C. In contrast, the 2-D network 6 was only stable up to
56 °C as reported [11a].
2
atmo-
two such derivatives in good yields through a facile two-step strat-
egy which allows for easy modifications in the ligand backbone. A
proof of principle for the utilization of the new ligands to prepare
novel metal–organic coordination polymers has been carried out
8
2
2
through the self-assembly of one of the ligands with Co(NCS) . X-
ray structural analysis of the resultant coordination polymer
reveals a 1-D chain composed of nano-scale [2 + 2] metallomacro-
cycles, which is, however, essentially different from the known
4
. Conclusion
2
coordination networks of Co(NCS) with structurally related
In summary, we have designed a new class of divergent nitro-
gen ligands containing two imidazolyl side-arms and synthesized
ligands. The extended aromatic domain in the new ligand should
be responsible for the structural difference observed here upon

3
60
E Liu et al. / Polyhedron 127 (2017) 355–360
coordinating to cobalt(II) ions. This work opens opportunities for
the synthesis of a variety of divergent bis-imidazole-based ligands
with different substituents, as well as the construction of novel
metal–organic coordination polymers with these ligands.
Zhang, CrystEngComm 14 (2012) 446;
e) E.C. Constable, C.E. Housecroft, M. Neuburger, S. Vujovic, J.A. Zampese, G.
Zhang, CrystEngComm 14 (2012) 3554;
f) Y.M. Klein, E.C. Constable, C.E. Housecroft, J.A. Zampese, Polyhedron 81
(2014) 98;
(
(
(
(
(
g) E.C. Constable, C.E. Housecroft, S. Vujovic, J.A. Zampese, CrystEngComm 16
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Acknowledgements
17 (2015) 6483.
[
9] (a) G. Zhang, Y. Jia, W. Chen, W.F. Lo, N. Brathwaite, J.A. Golen, A.L. Rheingold,
RSC Adv. 5 (2015) 15870;
We are grateful to a PSC-CUNY award (69069-0047), A ‘‘Seed
Grant” from the Office for Advanced Research at John Jay College
and the Program for Research Initiatives for Science Majors
(
b) Z. Yin, S. Zhang, S. Zheng, J.A. Golen, A.L. Rheingold, G. Zhang, Polyhedron
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c) G. Zhang, Y. Jia, T. Phoenix, D.R. Manke, J.A. Golen, A.L. Rheingold, RSC Adv.
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d) L. Li, Y.Z. Zhang, E. Liu, C. Yang, J.A. Golen, G. Zhang, Polyhedron 105 (2016)
115.
(
6
(
(
PRISM) funded by the Title V, HSI-STEM and MSEIP programs
within the U.S. Department of Education; the PAESMEM program
through the National Science Foundation; and New York State’s
Graduate Research and Teaching Initiative. The National Science
Foundation (CHE-1429086) is acknowledged for the X-ray diffrac-
tometer. The Natural Science Foundation of Hubei Province in
China (2015CFA130 and 2013CFA015) is also acknowledged for
partial support.
[
10] (a) F. Yuan, X. Wang, H.-M. Hu, S.-S. Shen, R. An, G.-L. Xue, Inorg. Chem.
Commun. 48 (2014) 26;
(
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