A tetragonal molecular cage and polymeric macrocyclic complex based on a clip-like bis-pyridyl-bis-amide ligand

Article abstract of DOI:10.1080/00958972.2012.703778

Reactions of a bis-pyridyl-bis-amide ligand, N,N′-bis(pyridin-3-yl)- 2,6-pyridinedicarboxamide (bppdca) with nickel(II) and cobalt(II) salts resulted in a tetragonal molecular cage, [Ni₂(bppdca)₄(SCN) ₄] (1), and a 1-D polymeric macrocyclic complex, [Co(bppdca)(SO ₄)(CH₃OH)(DMF)]_n ·nCH₃OH (2) (DMF=N,N′-dimethylformamide). Compound 1 features a tetragonal cage-like structure. In each cage, four bppdca ligands are twisted in the same direction, which endow the cage with chirality. However, because the twist or screw directions of adjacent cages are contrary, 1 is a mesomer and exhibits no chirality on the whole. Compound 2 possesses a 1-D, double-chain structure containing Co₄(SO₄)₄(bppdca)₂ macrocyclic subunits.

Full text of DOI:10.1080/00958972.2012.703778

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A tetragonal molecular cage and
polymeric macrocyclic complex based
on a clip-like bis-pyridyl-bis-amide
ligand
a
a
a
a
Ya-Meng Li , Ya-Hui Yu , Yu-Han Peng , Ben-Lai Wu & Hong-
a
Yun Zhang
a
Department of Chemistry, Zhengzhou University, Zhengzhou
450052, P. R. China
Accepted author version posted online: 18 Jun 2012.Version of
record first published: 10 Jul 2012.
To cite this article: Ya-Meng Li, Ya-Hui Yu, Yu-Han Peng, Ben-Lai Wu & Hong-Yun Zhang (2012): A
tetragonal molecular cage and polymeric macrocyclic complex based on a clip-like bis-pyridyl-bis-
amide ligand, Journal of Coordination Chemistry, 65:16, 2771-2779
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Journal of Coordination Chemistry
Vol. 65, No. 16, 20 August 2012, 2771–2779
A tetragonal molecular cage and polymeric macrocyclic complex
based on a clip-like bis-pyridyl-bis-amide ligand
YA-MENG LI, YA-HUI YU, YU-HAN PENG, BEN-LAI WU* and
HONG-YUN ZHANG
Department of Chemistry, Zhengzhou University, Zhengzhou 450052, P. R. China
(
Received 13 December 2011; in final form 9 April 2012)
0
Reactions of a bis-pyridyl-bis-amide ligand, N,N -bis(pyridin-3-yl)-2,6-pyridinedicarboxamide
(bppdca) with nickel(II) and cobalt(II) salts resulted in a tetragonal molecular cage,
[
(
Ni
2
(bppdca)
CH OH)(DMF)]
4
(SCN)
4
] (1), and a 1-D polymeric macrocyclic complex, [Co(bppdca)(SO
0
4
)
3
n
ꢀnCH
3
OH (2) (DMF ¼ N,N -dimethylformamide). Compound 1 features a
tetragonal cage-like structure. In each cage, four bppdca ligands are twisted in the same
direction, which endow the cage with chirality. However, because the twist or screw directions
of adjacent cages are contrary, 1 is a mesomer and exhibits no chirality on the whole.
4 4 4 2
Compound 2 possesses a 1-D, double-chain structure containing Co (SO ) (bppdca)
macrocyclic subunits.
Keywords: Cage; Helicate; Metalated macrocycle; Crystal structure; Coordination polymer
1. Introduction
Synthesis of metalated macrocyclic and cage-like molecules has received attention for its
structural diversity and rich physical and chemical properties [1]. To construct these
supramolecular complexes, selection of appropriate organic linkers with given shape
and function is a crucial step. Clip-like ligands, which are ditopic linkers possessing one
coordinating arm on each side of the organic space, are good candidates for self-
assembly of molecular macrocycles and cages together with transition metal ions in
different coordination configurations [2]. When assembled with linearly coordinating
metal ions, two clips can connect two metal nodes to form a rectangular macrocycle [3].
Further, if the metal ions are triangularly coordinated, three clips together with two
metal ions can form a trigonal prism [4]. Finally, four clips together with two
square-planar metal ions, such as Pd(II), can fabricate a tetragonal prism [5].
The ditopic bis-pyridyl-bis-amide ligand has flexible configurations owing to free
rotation of the Namide–Opyridyl bonds, creating structural diversity upon assembling with
transition metals. Additionally, accessible hydrogen-bonding sites at amides can
strengthen the stability of the obtained structures. Recently, we reported two 1-D
0
chain-like complexes based on N,N -bis(pyridin-4-yl)-2,6-pyridinedicarboxamide, which
*
Corresponding author. Email: wbl@zzu.edu.cn
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.703778

2
772
Y.-M. Li et al.
Scheme 1. Schematic representation of the synthesis of bppdca.
assemble into 3-D supermolecules through hydrogen bonding [6]. As further work,
we wish to report a discrete tetragonal molecular cage with interesting chiral void and a
polymeric macrocyclic complex containing Co (SO ) (bppdca) subunits based on a
4
4 4
2
0
conformationally flexible clip-like ligand N,N -bis(pyridin-3-yl)-2,6-pyridinedicarbox-
amide (bppdca).
2
2
. Experimental
.1. General information and materials
All reagents were of analytical grade and used as obtained from commercial sources.
The ligand was synthesized according to a reported method [6].
2
2
.2. Syntheses
.2.1. Synthesis of [Ni (bppdca) (SCN) ] (1). A solution of 63.8 mg (0.2 mmol) bppdca
2
4
4
in 5 mL DMF was added slowly to a solution of 53 mg (0.2 mmol) NiSO ꢀ 6H O in
4
2
1
0 mL methanol at room temperature. A 19.4 mg (0.2 mmol) sample of KSCN in 10 mL
methanol was then added to the clear pale green solution and stirred for 30 min. Finally,
the mixture was filtered and the filtrate was allowed to stand undisturbed at room
temperature. Blue crystals suitable for single-crystal X-ray diffraction were obtained
after 3 weeks. The crystals were filtered, washed with methanol thrice, and dried in air
ꢁ
1
(
1
yield 38% based on bppdca). Infrared (IR) (KBr, ꢀ cm ): 3425(m), 2084(vs), 1655(s),
591(s), 1526(s), 1448(s), 1309(s), 1241(s), 1128(s), 801(m), 702(m) (scheme 1).
2
.2.2. Synthesis of [Co(bppdca)(SO )(CH OH)(DMF)] nCH OH (2). A solution of
n 3
.
4
3
3
1.9 mg (0.1 mmol) bppdca in 5 mL DMF was added slowly to a solution of 28 mg
(
0.1 mmol) CoSO ꢀ 7H O in 5 mL methanol at room temperature. A 9.7 mg (0.1 mmol)
4
2
sample of KSCN in 5 mL methanol was then added to the clear purple solution and
stirred for 30 min. Finally, the mixture was filtered and the filtrate was allowed to stand
undisturbed at room temperature. Red crystals suitable for single-crystal X-ray
diffraction were obtained after 3 months. The crystals were filtered, washed with
ꢁ1
methanol thrice, and dried in air (yield 32% based on bppdca). IR (KBr, ꢀ cm ):
3
1
492(m), 3252(m), 1682(s), 1651(s), 1589(s), 1533(s), 1449(s), 1319(s), 1244(s), 1200(s),
095(s), 799(m), 694(m).

Tetragonal molecular cage
Table 1. Crystallographic data for 1 and 2.
2773
1
2
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C
72
H
52
N
24Ni
2
O
8
S
4
22 6 9
C H28CoN O S
1627.04
298
Tetragonal
611.49
298
Triclinic
I42d
P1ꢀ
˚
ꢂ
Unit cell dimensions (A, )
ꢂ
b
c
ꢂ
ꢃ
ꢄ
21.6215(8)
21.6215(8)
46.266(3)
90
90
90
21629(2), 8
0.999
1.021
0.0647
0.1692
8.9248(11)
11.5507(14)
13.9825(17)
105.855(2)
104.196(2)
96.307(2)
1319.8(3), 2
1.539
ꢁ3
˚
Volume (A ), Z
Calculated density (g cm
Goodness-of-fit on F
Final R indices [I 4 2ꢅ(I)]
Largest difference peak and hole (e A
ꢁ3
)
2
1.028
0.0422
0.0864
ꢁ3
˚
)
2.3. Single-crystal structure determination and magnetic susceptibility measurements
Data collection was performed at 293 K on a Mercury CCD diffractometer with
˚
graphite monochromated Mo-Ka radiation (ꢁ ¼ 0.71073 A). The structures were solved
by direct methods and all calculations were performed using the SHELXL package [7].
All non-hydrogen atoms were refined anisotropically. In 1, to remove the high residual
peaks, the SQUEEZE routine in PLATON was used and structures were then refined
again using the data generated. The carbon and sulfur of thiocyanate inside the cage are
both disordered over two sites and refined anisotropically. The crystal data for 1 and 2
are summarized in table 1 and selected bond lengths and angles of 1 and 2 are listed in
tables 2 and 3, respectively. For 2, the polycrystalline magnetic susceptibility data were
collected on a Quantum Design MPMS (SQUID)-XL magnetometer at temperatures
ranging from 2 to 300 K.
3
3
. Results and discussion
.1. Syntheses and IR spectra
The tetragonal molecular cage (1) and polymeric macrocyclic (2) were synthesized
ꢁ
2
ꢁ
under similar conditions. SO4 and SCN are present in the solution in the syntheses of
1
1
and 2, respectively. However, upon crystallization different anions are incorporated in
2
and 2. Under the condition, the anion effect for Ni and Co is identical. Therefore,
þ
2þ
the different structures in 1 and 2 originate from coordinating preferences of the metal
ꢁ
centers. In the absence of SCN , the reaction of bppdca with NiSO does not give a
4
ꢁ
tetragonal molecular cage-like complex (1). However, in the absence of SCN , the
reaction of bppdca with CoSO4 results in a polymeric macrocyclic complex (2).
ꢁ
Therefore, we observe the synthesis of 2 in the presence of SCN . We consider that the
coordinating preference of the metal center is dominant in the assembly of 1 and 2.

2
774
Y.-M. Li et al.
˚
Table 2. Selected bond lengths (A) and angles ( ) for 1.
ꢂ
i
Ni1–N12
Ni1–N1
Ni1–N6
2.044(4)
2.091(4)
2.106(4)
Ni1–N5
Ni1–N9
Ni1–N13
2.118(4)
2.126(4)
2.148(3)
N12–Ni1–N1
N12–Ni1–N6
N1–Ni1–N6 i
N12–Ni1–N5
N1–Ni1–N5i
N6–Ni1–N5
N12–Ni1–N9
N1–Ni1–N9
91.33(17)
90.95(16)
91.31(15)
89.87(16)
177.62(17)
90.71(16)
90.79(16)
88.50(16)
N6–Ni1–N9
N5 –Ni1–N9
178.25(14)
89.44(16)
179.50(18)
88.73(14)
89.54(13)
90.05(14)
88.72(14)
91.33(17)
i
N12–Ni1–N13
N1–Ni1–N13
N6–Ni1–N13
i
i
N5 –Ni1–N13
N9–Ni1–N13
N12–Ni1–N1
i
Symmetry code: x, 1/2 ꢁ y, 1/4 ꢁ z.
˚
Table 3. Selected bond lengths (A) and angles ( ) for 2.
ꢂ
i
Co1–O8
Co1–O5
Co1–O3
2.032(2)
2.0849(19)
2.118(2)
Co1–O4
Co1–N5
Co1–N1
2.142(2)
2.151(2)
2.164(2)
ii
i
i
ii
ii
ii
O8 –Co1–O5
O8 –Co1–O3
O5–Co1–O3
O8 –Co1–O4
O5–Co1–O4
O3–Co1–O4
O8 –Co1–N5
O8 –Co1–O5
O8 –Co1–O3
O3–Co1–N5
O4–Co1–N5
O3–Co1–N5
ii
O4—Co1–N5
i
i
i
i
O5–Co1–O3
O8 –Co1–O4
O8 –Co1–N1
O5–Co1–N1
O3–Co1–N1
O4–Co1–N1
O8 –Co1–N1
i
i
O5–Co1–N1
O3–Co1–N1
O4–Co1–N1
O5–Co1–O4
O3–Co1–O4
O8 –Co1–N5
i
ii
i
ii
ii
ii
N5 –Co1–N1
N5 –Co1–N1
ii
ii
O5–Co1–N5
O5–Co1–N5
i
ii
Symmetry codes: 1 ꢁ x, 1 ꢁ y, ꢁz; x, 1 þ y, z.
ꢁ
Indeed, for 1, SCN also have an influence on the formation of tetragonal molecular
ꢁ1
ꢁ1
cage. In the IR spectra, a strong band emerging around 2084 cm for 1 and 1095 cm
ꢁ
2ꢁ
for 2 indicate the existence of SCN and SO4 in 1 and 2, respectively. The results of the
IR spectra of 1 and 2 are consistent with the single-crystal X-ray diffraction analyses.
3
.2. Crystal structures of 1 and 2
Single-crystal X-ray diffraction analysis confirms that 1 crystallizes in a non-
centrosymmetrical space group I42d and features a tetragonal molecular cage.
As shown in figure 1, the cage consists of two Ni(II), four clip-like bppdca ligands,
and four thiocyanates. Each Ni(II) in a slightly distorted octahedral coordination
environment is completed by four nitrogen atoms from four bppdca to construct the
equatorial plane and two nitrogen atoms from two thiocyanates occupying the axial
˚
sites (figure 1). The Ni–N distances are 2.050(6)–2.165(4) A and the N–Ni–N angles are
ꢂ ꢂ
8
8.6(2)–91.5(2) and 177.6(2)–179.3(3) , which are all in agreement with the reported
values [8]. Of the four thiocyanates, two are outside the tetragonal cage and the other
two are inside the cage.
In comparison with the reported dimeric tetragonal prisms [5], the tetragonal prism in
1
displays lower symmetry. In the cage, only a two-fold axis passes through the vector

Tetragonal molecular cage
2775
Figure 1. View of the coordination environment of Ni(II) with hydrogen atoms as well as carbon and sulfur
of the thiocyanates omitted for clarity. Symmetry code: A, x, 1/2 ꢁ y, 1/4 ꢁ z.
Figure 2. The two types of chiral helicates in 1: (P, P)-twisted helicates (left) and (M, M)-twisted helicates
(right). The middle insert shows the definition of the azimuthal angle ꢆ.
of C26–N11–N8–C35. However, four bppdca ligands are arranged in such a way that
pseudo C4h symmetry could be imagined to impose on the tetragonal prisms.
The deviation from a highly-symmetric tetragonal prism is probably due to symmetry
lowering of bppdca. In 1, the bppdca ligands are severely twisted and display a non-
planar conformation due to free rotation of Namide–Opyridyl bonds. The dihedral angles
ꢂ
involving terminal pyridyl rings and the central pyridyl ring are 23.15–49.20 . In each
cage four twisted bppdca ligands are screwed in the same direction, which endow the
cage with chirality. As seen in figure 2, the azimuthal angles ꢆ relating the two {NiN }
4
ꢂ
units are 64.93, 65.68, and 63.87 , respectively. The pitch of the helicate can be

2
776
Y.-M. Li et al.
Figure 3. The coordination environments of the dinuclear Co(II) ions with hydrogen atoms omitted for
clarity. Symmetry codes: A, 1 ꢁ x, 1 ꢁ y, ꢁz; B, x, 1 þ y, z.
˚
estimated by the Ni ꢀ ꢀ ꢀ Ni separation of 10.386(8) A. However, since the screw
directions of adjacent cages are contrary, we obtain two types of chiral isomers,
P, P)-twisted helicates and (M, M)-twisted helicates (figure 2). Upon stacking, the
(
layers, which are constructed by the homo-chiral (P, P)-twisted helicates or (M, M)-
twisted helicates, pack themselves alternately and form an ABAB arrangement. On the
whole, 1 is a mesomer and shows no chirality.
Similar quadruple-stranded helicates remain rare. The first example is confirmed by
electrospray mass spectrometry as a double-charged anion, which was constructed by
two Eu(III) cations and four bis-diketone ligands [9a]. However, there was no structural
evidence. The first fully structurally characterized quadruple-stranded helicates were
reported by Raymond in 2006, which were Th(III)-bis-diketone complexes [9b].
Recently two examples of planar four-coordinate Pd(II) quadruple-stranded helicates
were also characterized [9c, 9d]. However, the quadruple-stranded helicates based on 3d
metal ions are not yet reported.
2
4
ꢁ
In 2, two bppdca ligands, four Co(II) ions, and four SO ions form a 36-membered
ring. Each Co(II) is six-coordinate N O octahedral (figures 3 and 4). Two oxygen
2
4
2ꢁ
atoms from two SO4 , one oxygen atom from DMF, and a nitrogen atom from a
pyridyl group of bppdca form the equatorial plane. One oxygen atom from one
methanol and one nitrogen atom of bppdca complete the coordination sphere of
˚
cobalt(II) at the axial sites. The Co–N bond lengths are 2.151(2) and 2.164(2) A, and
˚
Co–O bond lengths are 2.032(2)–2.142(2) A, which are comparable with the reported
values [10]. The two centrosymmetric cobalt(II) ions are bridged by two bidentate-
ꢁ
2
4
bridging SO , which can be abstracted to a 4-connected node. By the coordination of
ditopic bppdca to cobalt(II), a polymeric chain based on metalated macrocycles is
formed. Two free methanols are trapped inside the 36-member macrocycle through
2
4
ꢁ
˚
hydrogen bonding with an oxygen atom of SO , with an O ꢀ ꢀ ꢀ O distance of 2.795 A.

Tetragonal molecular cage
2777
Figure 4. View of the polymeric macrocycle chain linked by the 4-connected node Co
coordinating methanol and DMF omitted for clarity.
2
(SO
4
)
2
with a
Figure 5. Temperature dependence of the magnetic susceptibility curves for 2. The solid line shows the
Curie–Weiss fitting.
3.3. Magnetism of 2
The magnetic susceptibility was measured on a polycrystalline sample of 2 under an
applied field of 1000 Oe, as shown in figure 5. The ꢇ T value at 300 K is
M
3
ꢁ1
.36 cm mol K (5.18ꢈ ) per Co(II) ion, which is much higher than the expected
B
3
3
ꢁ1
value (1.88 cm mol K, 3.87ꢈ ) for a magnetically isolated spin-only Co(II) ion with
B
S ¼ 3/2 and g ¼ 2.00 but close to the value expected when the spin momentum and

2
778
Y.-M. Li et al.
1
/2
orbital momentum exist independently [5.20ꢈ ; ꢈLS ¼ [L(L þ 1) þ 4 S(S þ 1)] ; L ¼ 3,
B
S ¼ 3/2] [11]. This clearly indicates the presence of significant orbital angular
contributions, common for high-spin Co(II) in an octahedral coordination environ-
ment. Along with the temperature decreasing from 300 K, the magnetic moments
decrease gradually and reach a minimum of 2.48 cm mol K at 40 K, due to
depopulation of the Stark sublevels of octahedral Co(II), which display a strong
spin–orbit coupling [12]. The reciprocal molar magnetic susceptibility data obey the
Curie–Weiss law in the high temperature region of 50–300 K with a Curie constant of
3
ꢁ1
3
ꢁ1
C ¼ 3.65 cm mol K and a Weiss constant of ꢆ ¼ ꢁ22.67 K. The negative value of the
Weiss temperature indicates that the dominant interactions between Co(II) ions are
antiferromagnetic. As the temperature is lowered, ꢇ T increases abruptly to a
M
3
maximum of 6.41 m mol K at 24 K, and then drops quickly and goes down to a
ꢁ1
3
ꢁ1
minimum of 2.03 m mol K at 2 K. This deviation below 40 K is mainly the result of
the presence of intramolecular weak ferromagnetism caused by small uncompensated
AF spin-canting or some impurity [13].
4
. Conclusions
Reactions of a clip-like bis-pyridyl-bis-amide ligand with nickel(II) and cobalt(II) salts,
we synthesized a tetragonal molecular cage 1 and a polymeric metalated macrocycle 2.
A comparison between the structures of 1 and 2 suggests that the coordination
configurations of the metal ions play an important role in the process of self-assembly.
In 1, if we omit the two terminal thiocyanates sitting at the axial sites and the nickel(II)
ion is square-planar. Therefore, four bppdca ligands clamp two nickel(II) ions to form a
tetragonal prism. Notably, as four severely twisted bppdca ligands in each cage are
screwed in the same direction in 1, each cage is deemed as a rare homochiral quadruple-
stranded helicate. In 2, the 4-connected node Co (SO ) subunit has a severe deviation
2
4 2
ꢂ
from planar (the N–centroid(
two bppdca connect two Co (SO ) subunits into a macrocycle and further expand into
–N angles are 45.58 and 134.42 ). Accordingly,
Coꢀ ꢀ ꢀCo)
4 2
2
a polymeric chain. Perhaps, this work will enhance rational design and synthesis of
metalated macrocyclic and cage-like molecules. Further research is under way and
systematic results will be reported later.
Acknowledgments
We gratefully acknowledge the financial support from the National Natural Science
Foundation of China (20771094).
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