A flexible bipyridyl-type linking ligand and its coordination polymers of cobalt, zinc and cadmium: [Co(L)(NO3)2], [Zn(L)(NO3)2], [CdL2(NO3)2] and [CdL2(NO3)2](C6H6 )2 (L = (4-py)-CH2-NH-(CH3)C6H3 -C6H

Article abstract of DOI:10.1016/j.poly.2008.03.024

A long, flexible bipyridyltype linking ligand L [(4py)-CH₂-NH-(CH₃)C₆H₃ -C₆H₃(CH₃)-NH-CH₂-(4-py)], which possesses two terminal 4-pyridyl groups, was prepared by reducing its parent diimine ligand (4-py)-CH{double bond, long}N-(CH₃)C₆H₃-C₆ H₃(CH₃)-N{double bond, long}CH-(4-py). Four novel polymeric species based on L, [Co(L)(NO₃)₂] (1), [Zn(L)(NO₃)₂] (2), [CdL₂(NO₃)₂] (3) and [CdL₂(NO₃]₂](C₆H₆ )₂ (4), could be synthesized under solvothermal conditions. Whereas polymers 1 and 2 have a 1D zigzag-chain structure, polymers 3 and 4 have a 2D network structure. The N?N separation between the two pyridyl groups in L (the free ligand) is 19.871(3) ?, whereas this distance varies from 11.934(4) to 16.373(8) ? in its coordination polymers.

Full text of DOI:10.1016/j.poly.2008.03.024

Polyhedron 27 (2008) 2083–2090
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
A flexible bipyridyl-type linking ligand and its coordination polymers of cobalt,
zinc and cadmium: [Co(L)(NO3)2], [Zn(L)(NO3)2], [CdL2(NO3)2] and
[CdL2(NO3)2](C6H6)2 (L = (4-py)–CH2–NH–(CH3)C6H3–C6H3(CH3)–NH–CH2–(4-py))
*
Dongwon Min, Hyun Sue Huh, Soon W. Lee
Department of Chemistry (BK21), Sungkyunkwan University, Natural Science Campus, Suwon 440-746, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A long, flexible bipyridyltype linking ligand L [(4py)–CH2–NH–(CH3)C6H3–C6H3(CH3)–NH–CH2–(4-py)],
which possesses two terminal 4-pyridyl groups, was prepared by reducing its parent diimine ligand
(4-py)–CH@N–(CH3)C6H3–C6H3(CH3)–N@CH–(4-py). Four novel polymeric species based on L, [Co(L)-
(NO3)2] (1), [Zn(L)(NO3)2] (2), [CdL2(NO3)2] (3) and [CdL2(NO3]2](C6H6)2 (4), could be synthesized
under solvothermal conditions. Whereas polymers 1 and 2 have a 1D zigzag-chain structure, polymers
3 and 4 have a 2D network structure. The Nꢀ ꢀ ꢀN separation between the two pyridyl groups in L (the free
ligand) is 19.871(3) Å, whereas this distance varies from 11.934(4) to 16.373(8) Å in its coordination
polymers.
Received 28 February 2008
Accepted 14 March 2008
Available online 16 May 2008
Keywords:
Flexible bipyridyl-type linking ligand
Coordination polymer
Reduction
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
other hand, ligands L1–L4 are structurally rigid because of the pres-
ence of the diimine fragments (–CH@N–) in their backbones, that
Construction of coordination polymers, also called metal–or-
ganic frameworks, by combining appropriate linking ligands and
transition metals is currently under intensive study due to their
desirable properties applicable to magnetism, photoluminescence,
catalytic properties and gas storage [1–6]. Relatively short bipyri-
dyl-type ligands have been used extensively in preparing various
metal–organic networks [7–10] as well as pure organic networks
[11]. Very long linking ligands containing terminal pyridyls, how-
ever, were recently employed in preparing coordination polymers
with various topologies [12–19]. We also prepared several long
bipyridyl-type ligands L1–L6 (Chart 1) and their coordination poly-
mers [20–25].
In controlling the network topologies of coordination polymers,
several experimental parameters are known, including the size or
polarity of the counterion [26–28], the length of the spacer [29],
the pH value of the reaction solution [30], and the nature of the sol-
vents used [31]. Furthermore, it is also known that ligands contain-
ing a flexible backbone provide conformational freedom to create
greater structural diversity [31,32]. Among the ligands L1–L6, li-
gands L5 and L6 turned out to be flexible due to the ring flip of
the intervening cyclohexyl fragment in these ligands [24]. On the
is, a complete conjugated p system. Considering our previous
works related to ligands L1–L6, we decided to reduce this type of
ligands to enhance their structural flexibility that would lead to
the creation of novel coordination polymers, which cannot be ob-
tained from their parent diimine ligands. With this aim in mind,
we reduced the diimine ligand L0 [(4-py)–CH@N–(CH3)C6H3–(CH3)-
C6H3–N@CH–(4-py)] with sodium borohydride to produce the
corresponding flexible ligand
L [(4-py)–CH2–NH–(CH3)C6H3–
(CH3)C6H3–NH–CH2–(4-py)], in which free rotations about the
Ar–NH, NH–CH2 and CH2–(4-py) bonds may be possible. Herein
we report the preparation of this flexible ligand (L) and its coordi-
nation polymers, [Co(L)(NO3)2] (1), [Zn(L)(NO3)2] (2), [CdL2(NO3)2]
(3) and [CdL2(NO3]2](C6H6)2 (4).
2. Experimental
All the solid chemicals were purified by recrystallization, and
all solvents were distilled and stored over molecular sieves.
Infrared (IR) samples were prepared as KBr pellets, and their
spectra were obtained in the range 400–4000 cmꢁ1 on a Nicolet
320 FTIR spectrophotometer. Elemental analyses were performed
with EA1110 (CE instrument, Italy) at the Korea Basic Science
Institute. Thermogravimetric analysis (TGA) was conducted on a
TA4000/SDT 2960 instrument at the Cooperative Center for Re-
search Facilities (CCRF) in Sungkyunkwan University. 1H and
13C{1H} NMR data were collected on a 500 MHz Varian Inova
spectrometer.
* Corresponding author. Tel.: +82 31 290 7066; fax: +82 31 290 7075.
E-mail address: soonwlee@skku.edu (S.W. Lee).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.03.024

2084
D. Min et al. / Polyhedron 27 (2008) 2083–2090
N
N
N
N
N
N
N
L1
N
L2
N
N
N
N
N
N
N
N
N
N
N
N
N
L3
L4
N
N
N
L5
L6
N
N
N
N
HN
NH
N
N
L'
L
Chart 1.
2.1. Preparation of [(4-py)–CH@N–(CH3)C6H3–C6H3(CH3)–
N@CH–(4-py)] (L0)
1308 (s), 1254 (s), 1140 (w), 1065 (w), 992 (w), 804 (w). Anal.
Calc. for C26H26N4: C, 79.16; H, 6.64; N, 14.20. Found: C, 79.30;
H, 6.62; N, 14.70%.
To 4-pyridine carboxaldehyde (6.30 ml, 66.8 mmol) in metha-
nol (200 ml) was added o-tolidine dihydrochloride hydrate
{[–C6H3(CH3)-4-NH2]2 ꢀ 2HCl ꢀ H2O, 9.52 g, 33.4 mmol)} at room
temperature. After the addition of dichlomethane (400 ml) and
then several drops of formic acid, the mixture was stirred at room
temperature for 24 h. The solvent was removed under vacuum, and
the residue was extracted with dichloromethane (500 ml), and
washed with a saturated Na2CO3 solution (500 ml) and then with
water (500 ml ꢂ 2). The organic phase was dried over MgSO4 and
filtered. Upon removal of the solvent, an analytically pure yellow
crystalline solid was obtained in 86% yield. 1H NMR (500 MHz,
CDCl3) d: 8.77 (m, 4H), 8.46 (m, 2H), 7.80 (m, 2H), 7.54–7.32
(m, 4H), 7.07 (m, 2H), 2.49 (s, 6H). 13C{1H} NMR (125 MHz, CDCl3)
d: 157.0, 150.8, 149.2, 143.2, 139.3, 133.5, 129.3, 125.5, 122.5,
118.0, 18.7. IR (KBr, cmꢁ1): 1622 (s), 1595 (s), 1546 (w),
1484 (w), 1414 (w), 1322 (w), 1220 (w), 1124 (s), 1059 (s), 989
(s), 822 (s), 645 (s), 556 (s), 518 (s), 486 (s). Anal. Calc. for
2.3. Preparation of [Co(L)(NO3)2] (1)
A mixture of Co(NO3)2 ꢀ 6H2O (29.7 mg, 0.10 mmol), ligand L
(39.4 mg, 0.1 mmol), methanol (10 ml) and benzene (3 ml) was
heated in a vial at 70 °C for 3 days and then cooled to room tem-
perature by air-cooling. The crystalline product was collected by
filtration, washed with methanol, and air-dried to give violet
crystals of polymer 1 (25 mg, 0.043 mmol, 43%). IR (KBr, cmꢁ1):
3427 (w), 1618 (s), 1502 (s), 1429 (w), 1382 (w), 1292 (s),
1261 (s), 1145 (w), 1018 (w), 806 (w). Anal. Calc. for C26H26Co-
N6O6: C, 54.18; H, 4.54; N, 14.55. Found: C, 54.06; H, 4.58; N,
14.18%.
2.4. Preparation of [Zn(L)(NO3)2] (2)
Polymer 2 was prepared similarly to polymer 1. A mixture of
C
26H22N4: C, 78.97; H, 5.68; N, 14.35. Found: C, 78.94; H, 6.07; N,
Zn(NO3)2 ꢀ 6H2O (29.7 mg, 0.10 mmol), ligand
L
(39.4 mg,
14.82%.
0.10 mmol), methanol (10 ml) and benzene (5 ml) was heated at
70 °C for 10 days. Yield: 5.1%. IR (KBr, cmꢁ1): 3426 (w), 1622
(s), 1505 (s), 1494 (s), 1432 (s), 1383 (s), 1305 (s), 1263 (s),
1146 (w), 1063 (w), 1031 (w), 805 (w). Anal. Calc. for
2.2. Preparation of [(4-py)–CH2–NH–(CH3)C6H3–C6H3(CH3)–NH–
CH2–(4-py)] (L)
C
26H26ZnN6O6: C, 53.48; H, 4.49; N, 14.39. Found: C, 53.31; H,
To compound L0 (7.0 g, 17.9 mmol) in methanol (200 ml) and
dichlomethane (300 ml) was added sodium borohydride (6.78 g,
179 mmol) in small portions with stirring at room temperature.
The resulting mixture was stirred at room temperature for
12 h. The solvent was removed under vacuum, and the residue
was extracted with dichloromethane (300 ml), and washed with
water (300 ml ꢂ 3). The organic phase was dried over MgSO4 and
filtered, and upon removal of the solvent, an analytically pure
yellow crystalline solid was obtained (6.22 g, 15.8 mmol, 88%
yield). 1H NMR (500 MHz, CDCl3) d: 8.58 (m, 4H), 7.34–7.21
(m, 10H), 6.47 (dd, 2H), 4.47 (s, 4H), 4.07 (br s, 2H), 2.28 (s,
6H). 13C NMR (125 MHz, CDCl3)d: 150.2, 149.4, 144.2, 131.3,
128.8, 125.3, 122.6, 122.3, 110.7, 47.4, 18.0. IR (KBr, cmꢁ1):
3322 (w), 1603 (s), 1503 (s), 1470 (w), 1440 (w), 1414 (w),
4.70; N, 14.60%.
2.5. Preparation of [CdL2(NO3)2] (3)
A mixture of Cd(NO3)2 ꢀ 4H2O (30.0 mg, 0.10 mmol), ligand L
(39.0 mg, 0.10 mmol) and methanol (10 ml) was heated in a vial
at 70 °C for 3 days and then cooled to room temperature by air-
cooling. The crystalline product was collected by filtration, washed
with methanol, and air-dried to give yellow crystals of polymer 3
(25 mg, 0.0244 mmol, 49%). IR (KBr, cmꢁ1): 3434 (w), 3408 (w),
1611 (s), 1584 (w), 1506 (s), 1425 (s), 1383 (s), 1309 (s), 1268
(s), 1145 (w), 1102 (w), 1013 (w), 876 (w), 823 (w), 801 (w). Anal.
Calc. for C52H52CdN10O6: C, 60.91; H, 5.11; N, 13.66. Found: C,
60.12; H, 4.98; N, 14.01%.

D. Min et al. / Polyhedron 27 (2008) 2083–2090
2085
Table 1
X-ray data collection and structure refinement details
L
1
2
3
4
Empirical formula
Formula weight
C
26H26N4
C26H26CoN6O6
577.46
C26H26ZnN6O6
583.90
C52H52CdN10O6
1025.44
C64H64CdN10O6
1181.65
394.51
Temperature (K)
296(2)
293(2)
293(2)
296(2)
293(2)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a (°)
b (°)
monoclinic
P21/c
10.9086(2)
17.3633(4)
11.3236(3)
monoclinic
C2/c
27.619(7)
7.683(2)
12.507(4)
monoclinic
C2/c
27.745(6)
7.734(2)
12.468(4)
triclinic
P1
monoclinic
P21/n
12.241(3)
15.970(3)
15.141(3)
ꢀ
9.2849(10)
9.5473(9)
15.163(2)
95.80(1)
100.53(1)
116.78(1)
1153.8(2)
1
1.476
0.538
530
0.8909
0.9788
18365
5632
2352
313
99.822(2)
98.86(2)
99.42(2)
93.80(2)
c (°)
V (Å3)
Z
2113.36(8)
4
1.240
0.075
840
2622(1)
4
1.463
0.707
1196
0.3995
0.4408
2353
2301
1882
181
3.5–50
0.299
ꢁ0.283
1.038
0.0431
0.1037
2639(1)
4
1.469
0.983
1208
0.6064
0.6733
2300
2248
1293
181
3.5–50
0.867
ꢁ0.801
1.082
0.0782
0.1762
2953.4(9)
2
1.329
0.430
1228
0.6132
0.6309
5445
5188
3199
345
3.5–50
0.997
ꢁ0.859
1.039
0.0749
0.1875
Dcalc (g cmꢁ3
l (mm)
F(000)
Tmin
)
Tmax
Number of reflections measured
Number of reflections unique
Number of reflections with I > 2r(I)
Number of parameters refined
2h Range (°)
16944
5223
1592
375
3.5–50
0.134
ꢁ0.151
0.762
0.0482
0.0970
4.8–57.0
0.715
Maximum, in Dq (e Åꢁ3
)
)
Minimum, in Dq (e Åꢁ3
ꢁ0.481
Goodness-of-fit on F2
0.976
a
R1
0.0631
0.1429
b
wR2
P
P
a
R1
wR2
=
[|Fo| ꢁ |Fc|]/ |Fo|].
P
P
b
=
[w(F2o ꢁ F2c)2]/ [w(F2o)2]1/2
.
2.7. X-ray structure determination
Table 2
Selected bond lengths (Å) and angles (°)
All X-ray data were collected with a Siemans P4 diffractometer
(1, 2 and 4) or a Bruker Smart APEX2 diffractometer (L and 3)
equipped with a Mo X-ray tube. No absorption corrections were
made for the ligand L, but intensity data were empirically
corrected for absorption with w-scan data (1, 2, 4) or SADABS (3)
[33]. All calculations were carried out with SHELXTL programs [34].
All structures were solved by direct methods. All non-hydrogen
atoms were refined anisotropically. All hydrogen atoms were gen-
erated in idealized positions and refined using the riding model.
A colorless crystal of L of approximate dimensions 0.18 ꢂ
0.12 ꢂ 0.10 mm3, shaped as a block, was used for crystal and inten-
sity data collection. A violet crystal of 1 (a block, 0.36 ꢂ 0.28 ꢂ
0.10 mm3), a yellow crystal of 2 (a block, 0.26 ꢂ 0.24 ꢂ 0.06 mm3),
a yellow crystal of 3 (0.22 ꢂ 0.12 ꢂ 0.04 mm3) and a yellow crystal
of 4 (a block, 0.42 ꢂ 0.12 ꢂ 0.10 mm3) were used. Details of the
crystal data, intensity collection and refinement details are given
in Table 1. Selected bond lengths and angles are given in Table 2.
Ligand L
N2–C7
N3–C21
N2–C6–C3
1.395(2)
1.425(3)
110.9(2)
N2–C6
C7–N2–C6
N3–C21–C22
1.438(3)
120.2(2)
111.5(2)
N3–C16
C16–N3–C21
1.392(2)
121.3(2)
Polymer 1
Co1–O2
N2–C7
O2–Co1–O1
N2–C6–C3
2.070(2)
1.395(4)
84.8(1)
Co1–O1
N2–C6
N1–Co1–O1
2.286(2)
1.431(4)
160.6(1)
Co1–N1
O2–Co1–N1
C7–N2–C6
2.072(2)
113.1(1)
123.6(3)
117.5(2)
Polymer 2
Zn1–O1
N2–C7
O1–Zn1–O3
N2–C7–C8
2.039(7)
1.407(10)
54.5(3)
Zn1–N1
N2–C6
N1–Zn1–O3
2.052(6)
1.434(12)
88.7(3)
Zn1–O3
O1–Zn1–N1
C7–N2–C6
2.492(9)
117.1(3)
123.2(9)
118.7(7)
Polymer 3
Cd1–O1
N2–C7
N3–Cd1–O1
N2–C6–C3
2.354(5)
1.381(6)
83.5(2)
Cd1–N3
N2–C6
N1–Cd1–O1
2.338(6)
1.427(6)
88.1(2)
Cd1–N1
N3–Cd1–N1
C7–N2–C6
2.345(4)
92.1(2)
124.1(4)
116.2(4)
Polymer 4
Cd1–O1
N2–C7
O1–Cd1–N4
N2–C6–C3
2.333(9)
1.403(8)
84.2(4)
Cd1–N1
N2–C6
O1–Cd1–N1
2.380(5)
1.440(9)
84.8(3)
Cd1–N4
O1–Cd1–N4
C7–N2–C6
2.364(5)
95.8(4)
121.3(6)
3. Results and discussion
3.1. Synthesis
115.3(6)
A long, flexible bis(monodentate) linking ligand C26H26N4 (L),
which possesses two terminal pyridyl groups with their nitrogen
atoms in the 4,40-positions, was prepared in two steps on a rela-
tively large scale (Eq. (1)). In the first step, the diimine species
[(4-py)–CH@N–(CH3)C6H3–C6H3(CH3)–N@CH–(4-py)] (L0), a posi-
tional isomer of the ligand L2 (Chart 1), was prepared in 86% yield
by the condensation of a diamine (o-tolidine dihydrochloride
hydrate) and a pyridinealdehyde (4-pyridine carboxaldehyde) at
room temperature. In the second step, L0 was reduced with sodium
borohydride to give [(4-py)–CH2–NH–(CH3)C6H3–C6H3(CH3)–NH–
CH2–(4-py)] (L) in 88% yield.
2.6. Preparation of [CdL2(NO3)2](C6H6)2 (4)
Polymer 4 was prepared similarly to polymer 1. A mixture of
(39.0 mg,
Cd(NO3)2 ꢀ 4H2O (30.0 mg, 0.10 mmol), ligand
L
0.10 mmol), methanol (10 ml) and benzene (5 ml) was heated at
70 °C for 3 days. Yield: 10%. IR (KBr, cmꢁ1): 3408 (w), 1611 (s),
1510 (s), 1424 (s), 1383 (s), 1309 (s), 1264 (s), 1218 (w), 1142
(w), 1062 (w), 1039 (w), 1014 (w), 992 (w), 803 (w), 687 (w). Anal.
Calc. for C64H64CdN10O6: C, 65.05; H, 5.46; N, 11.85. Found: C,
64.80; H, 5.33; N, 12.01%.

2086
D. Min et al. / Polyhedron 27 (2008) 2083–2090
CHO
N
It would be meaningful to consider the preparation of polymers
1 and 2. Our initial goal was to prepare coordination polymers that
contained guest (benzene) molecules, which might be able to act as
a template to enhance the dimensionality of the target polymers.
Inconsistent with our expectation, reactions only in methanol did
not produce suitable crystals for X-ray diffraction. Only the reac-
tions in the presence of benzene led to the formation of single crys-
talline products, which do not contain the benzene molecules.
Although we do not know the exact function of benzene in our
reaction system, benzene appears to have promoted the formation
of good crystals. Similar observations were previously reported in
the preparation of [Cu3(C6H8O4)3(H2O)2(C6H11OH)] [36], [M3(2,6-
NDC)3(bipy)1.5] (M = Ni, Co; 2,6-NDC = 2,6-naphthalenedicarboxy-
late; bipy = 4,40-bipyridyl) [37] and [Cu(1,3-BDC)(H2O)] ꢀ 2H2O
(1,3-BDC = 1,3-benzenedicarboxylate) [38].
+ 2
H2N
NH2
N
MeOH
N
N
HCOOH/CH2Cl2
L'
N
N
H
N
NaBH4
N
CH2Cl2
L
H
N
ð1Þ
Because of the possible rotations about the Ar–NH, NH–CH2 and
CH2–(4-py) single bonds, the ligand L is expected to be consider-
ably more flexible than L1–L4 [20–23]. However, X-ray structural
analysis of ligand L and its coordination polymers 1–4 revealed
that this ligand exhibits only two out of several possible conforma-
tions, in which the two terminal 4-pyridyl groups (or two central
methyl groups) are located on either the opposite (Z-mode) or
the same (U-mode) side of the central biphenyl fragment [35].
The Z-mode conformer can be transformed to the U-mode one by
rotation about the central C–C bond, and vice versa.
3.2. Structure
The molecular structure of L, with the atom-numbering scheme,
is shown in Fig. 1, in which two terminal pyridyl groups are linked
by a diamide moiety (–CH2–NH–(CH3)C6H3–(CH3)C6H3–NH–CH2–)
and have their pyridyl nitrogen atoms at the para positions (4,40).
Ligand L has a Z-conformation, in which two methyl substituents
are directed to opposite directions (anti orientation). The Nꢀ ꢀ ꢀN
separation between the two terminal pyridyl rings is 19.871(3) Å,
and these rings are essentially parallel to each other with a dihe-
dral angle of 3.7(2)°. The two phenyl rings in the central biphenyl
fragment are significantly twisted from each other by 39.17(6)°.
The N2–C6 (1.438(3) Å) and N3–C21 (1.425(3) Å) bond lengths
indicate N–C single bonds.
N
HN
HN
NH
NH
A monomer unit of polymer 1 is shown in Fig. 2, which shows that
the Co(II) metal is 6-coordinate and its coordination sphere consists
of two pyridyl nitrogens and four g2-nitrato oxygen atoms. The Co
metal lies on the crystallographic C2-axis, which explains the Z value
of 4 rather than 8 (Table 1). The N–H hydrogen is involved in inter-
molecular N–Hꢀ ꢀ ꢀO hydrogen bonding: N2–H2 = 0.73 Å; N2ꢀ ꢀ ꢀO1
(ꢁx, y + 1, ꢁz + 1/2) = 2.66 Å; N2–H2ꢀ ꢀ ꢀO1 = 136°. The projection of
polymer 1 along the b-axis demonstrates its one-dimensional zigzag
structure (Fig. 3). This chain is formed by the bridging L ligands
linking Co metals in the [101] direction, with a Nꢀ ꢀ ꢀN separation
of 16.112(6) Å and a Coꢀ ꢀ ꢀCo separation of 19.275(4) Å. The L
ligands adopt the Z-conformation, which is quite similar to that
found in the molecular ladder prepared from Cd(NO3)2 and 1,4-
bis(4-pyridylmethyl)benzene in a 2:3 molar ratio [39].
N
N
N
Z
-mode
U-mode
Solvothermal treatment of cobalt(II) nitrate and zinc(II) nitrate
with the ligand L in methanol–benzene at 70 °C produced a 1D co-
balt [Co(L)(NO3)2] (1) and
a 1D zinc coordination polymer
[Zn(L)(NO3)2] (2), respectively (Eqs. 2 and 3). In addition, a 2D grid
cadmium polymer [CdL2(NO3)2] (3) was prepared from cad-
mium(II) nitrate and the ligand L in methanol at 70 °C (Eq. (4)).
Interestingly, when the last reaction was carried out in a mixed sol-
vent (methanol–benzene), a guest-containing cadmium polymer,
[CdL2(NO3]2](C6H6)2 (4), was prepared (Eq. (5)).
Polymer 2 is isostructural with polymer 1, and its monomer
unit and projection along the b-axis are presented in Figs. 4 and
5, respectively. The Zn metal lies on the crystallographic C2-axis,
and the nitrato ligand g2-coordinates in an asymmetric mode
(Zn1–O1 = 2.039(7) Å; Zn1–O3 = 2.492(9) Å). The Nꢀ ꢀ ꢀN separation
in the one-dimensional zigzag chain is 16.083(11) Å, and the
Znꢀ ꢀ ꢀZn separation is 19.302(4) Å.
6H6
CoðNO3Þ þ L MeOꢁH!=C ½CoðLÞðNO3Þ ꢃ ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
2
2
6H6
ZnðNO3Þ þ L MeOꢁH!=C ½ZnðLÞðNO3Þ ꢃ ð2Þ
2
2
CdðNO3Þ þ L Mꢁe!OH½CdL2ðNO3Þ ꢃ ð3Þ
2
2
6H6
CdðNO3Þ þ L MeOꢁH!=C ½CdL2ðNO3Þ ꢃðC6H6Þ ð4Þ
2
2
Fig. 1. ORTEP drawing of ligand L with 50% probability thermal ellipsoids.

D. Min et al. / Polyhedron 27 (2008) 2083–2090
2087
Fig. 2. Local coordination environment around the cobalt atom in polymer 1. Unlabelled atoms are related to labelled atoms by the inversion symmetry.
A monomer unit of polymer 3 is given in Fig. 6, which shows
that the Cd(II) metal is 6-coordinate and its coordination sphere
consists of four pyridyl nitrogens and two oxygens of two g1-
NO3 ligands. The Cd metal is located on the crystallographic inver-
sion center. The N2–C6 and N2–C7 bond lengths are 1.427(6) and
1.381(6) Å, respectively. Fig. 7 shows a projection along the a-axis.
Polymer 3 has a 2D structure that extends approximately in the
[011] direction, and consists of layers of grids. Each grid consists
of four Cd metals and four L ligands, forming a large 84-membered
(80 ligand atoms and 4 Co metals) ring with dimensions of ca.
20.2 ꢂ 20.3 Å2 (based on the Cdꢀ ꢀ ꢀCd separations). Despite the
presence of large repeating rings, polymer 3 does not exhibit the
interpenetration phenomenon and does not contain guest
molecules, either. In polymer 3, the ligands have practically the
same Nꢀ ꢀ ꢀN separation [16.363(8) and 16.373(10) Å]. In addition,
the metals also have practically the same Cdꢀ ꢀ ꢀCd separation
[20.274(3) and 20.201(3) Å].
A monomer unit of polymer 4 is shown in Fig. 8. The Cd(II) me-
tal is 6-coordinate with a [CdN4O2] core, and its coordination
sphere consists of four pyridyl nitrogens and two oxygens of two
g1-NO3 ligands. The Nꢀ ꢀ ꢀN separation between the terminal pyridyl
rings is 11.934(8) Å. The N3–H3 hydrogen is involved in intermo-
lecular N–Hꢀ ꢀ ꢀO hydrogen bonding: N3–H3 = 0.72 Å; N2ꢀ ꢀ ꢀO3
(ꢁx + 2, ꢁy, ꢁz + 2) = 2.46 Å; N3–H3ꢀ ꢀ ꢀO3 = 163°.
Fig. 9 shows a projection of polymer 4 approximately perpen-
dicular to the (011) plane, which shows a 2D compact network
containing benzene guest molecules. It is worth noting that all
the L ligands in polymer 4 adopt the U-conformation (syn-orienta-
tion of pyridyl groups). This type of coordination of bipyridyl-based
ligands is rather unusual and was previously reported for [Cd(3-
pmpmd)1.5(NO3)2] ꢀ EtOH and [Zn(3-pmpmd)1.5(NO3)2] ꢀ MeOH (3-
Fig. 3. Projection of polymer 1 along the b-axis, showing a linear zigzag chain
structure.
Fig. 4. ORTEP drawing of local environment of zinc in polymer 2.

2088
D. Min et al. / Polyhedron 27 (2008) 2083–2090
Fig. 7. Projection of polymer 3 along the a-axis, showing a 2D grid network.
pmpmd = N,N0-bis(3-pyridylmethyl)pyromellitic diimide) [35]. The
benzene guest molecules are located inside the cavities of the 2D
framework. Each cadmium metal is linked by four ligands that
approximately form a square plane with 90° to one another. If
the four ligands are taken as two independent sets of two ligands,
Fig. 5. Projection of polymer 2 along the b-axis, showing a linear zigzag chain
structure.
Fig. 6. Local coordination environment of cadmium in polymer 3.

D. Min et al. / Polyhedron 27 (2008) 2083–2090
2089
3.3. Thermogravimetric analyses
Thermogravimetric analyses (TGA) were performed on poly-
mers 1–4 by heating them up to 800 °C. The framework of polymer
1 is stable up to 202 °C, above which an abrupt weight loss occurs
between 202 and 275 °C, followed by another abrupt weight loss
between 373 and 416 °C, which corresponds to the breakdown of
its framework. Polymer 2 is stable up to 261 °C, and two subse-
quent weight losses occur at 261–264 and 373–449 °C. These data
indicate that the thermal stability of the framework of polymer 2 is
higher than that of polymer 1. The TGA of polymer 3 displays two
steps of weight loss: an abrupt weight loss at 256–523 °C and a
gradual weight loss above 582 °C. The guest-containing polymer
4 exhibits a gradual weight loss (about 16%) in the range 178–
215 °C, corresponding to the loss of the guest benzene molecules.
The framework of polymer 4 decomposes above 273 °C. When
heated at 220 °C for 2 h, polymer 4 gave its guest-free form, which
is amorphous. Therefore, we gave up desorption–resorption exper-
iments for this polymer.
Fig. 8. Local coordination environment around the cadmium atom in polymer 4.
Fig. 6 looks like a combination of two honeycombs (red and black),
each of which is generated by each ligand set. Nevertheless, in fact,
the actual unit in polymer 4 is a 84-membered ring, composed of
four Cd atoms and four ligands (M4L4). Although polymers 3 and
4 have the same framework formula {[CdL2(NO3]2]}, their frame-
work structures are essentially different, depending on the absence
(3) or presence (4) of guest molecules.
4. Conclusion
In summary, we prepared a long, flexible (4,40-bipyridyl)-type
linking ligand
L
[(4-py)–CH2–NH–(CH3)C6H3–C6H3(CH3)–NH–
CH2–(4-py)] by reducing its parent diimine species (4-py)–
CH@N–(CH3)C6H3–C6H3(CH3)–N@CH–(4-py) (L0). Novel zinc (1),
cobalt (2) and cadmium (3 and 4) coordination polymers were pre-
pared from the corresponding metal nitrates and the ligand L un-
der solvothermal conditions. The Nꢀ ꢀ ꢀN separation between the
two pyridyl groups in L (in the free state) is 19.871(3) Å, and this
distance varies from 11.934(4) to 16.373(8) Å in the polymers 1–
4. In addition, L coordinates to the metal via its terminal pyridyl
nitrogen atoms either in the Z-mode (in polymers 1–3) or in the
U-mode (in polymer 4). Whereas polymers 1 and 2 have a 1D zig-
zag-chain structure, polymers 3 and 4 have a 2D network struc-
ture. Several conclusions may be drawn from our work: (1)
Reduction of diimine with NaBH4 to give its reduced form proceeds
readily at room temperature in high yield. (2) Benzene promoted
the formation of high-quality crystals of polymers 1 and 2. (3)
The various Nꢀ ꢀ ꢀN separations and two conformation modes in
Cd
Fig. 9. Projection of polymer 4, showing the U-mode of ligand L.

2090
D. Min et al. / Polyhedron 27 (2008) 2083–2090
[14] M.B. Zaman, M.D. Smith, D.M. Curtin, H.-C. zur Loye, Inorg. Chem. 41 (2002)
4895.
[15] E. Bosch, C.L. Barnes, New J. Chem. 25 (2001) 1376.
[16] Y.-B. Dong, M.D. Smith, R.C. Layland, H.-C. zur Loye, Chem. Mater. 12 (2000)
1156.
the coordination polymers indicate the high flexibility of ligand L.
(4) The amine nitrogen atoms in L do not bind to the metal in all
the polymers.
[17] Y.-B. Dong, M.D. Smith, H.-C. zur Loye, Inorg. Chem. 39 (2000) 4927.
[18] Y.-B. Dong, M.D. Smith, H.-C. zur Loye, J. Solid State Chem. 155 (2000) 143.
[19] M.A. Withersby, A.L. Blake, N.R. Champness, P.A. Cooke, P. Hubberstey, W.-S. Li,
M. Schröder, Inorg. Chem. 38 (1999) 2259.
5. Supplementary data
CCDC 672275, 672276, 672277, 672278 and 672279 contain the
supplementary crystallographic data for L, 1, 2, 3 and 4. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/con-
ts/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.
[20] H.N. Kim, H.K. Lee, S.W. Lee, Bull. Korean Chem. Soc. 26 (2005) 6.
[21] D. Min, Y.-B. Cho, H.K. Lee, S.W. Lee, Inorg. Chim. Acta 359 (2006) 577.
[22] Y.-B. Cho, D. Min, H.K. Lee, S.W. Lee, Cryst. Growth Des. 6 (2006) 342.
[23] S.H. Kim, H.S. Huh, S.W. Lee, J. Mol. Struct. 841 (2007) 78.
[24] H.S. Huh, S.W. Lee, Inorg. Chem. Commun. 10 (2007) 1244.
[25] H.S. Huh, H.J. Yun, S.W. Lee, Inorg. Chim. Acta, doi:10.1016/j.ica.2007.10.024.
[26] M. Du, X.-H. Bu, Y.-M. Guo, H. Liu, Inorg. Chem. 41 (2002) 4904.
[27] S.M. Kil, P.S. Myunghyun, J. Am. Chem. Soc. 122 (2000) 6834.
[28] A.J. Blake, N.R. Champness, P.A. Cooke, J.E.B. Nicolson, C. Wilson, J. Chem. Soc.,
Dalton Trans. (2000) 3811.
References
[29] X.-H. Bu, W. Chen, S.-L. Lu, R.-H. Zhang, D.-Z. Liao, W.-M. Bu, M. Shionoya, F.
Brisse, J. Ribas, Angew. Chem., Int. Ed. 40 (2001) 3201.
[30] M. Du, X.H. Bu, Y.M. Guo, J. Ribas, C. Diaz, Chem. Commun. (2002) 2550.
[31] J. Yang, J.-F. Ma, Y.Y. Liu, S.-L. Li, G.-L. Zheng, Eur. J. Inorg. Chem. (2005)
2174.
[1] P.J. Hagrman, D. Hagrman, J. Zubieta, Angew. Chem., Int. Ed. 38 (1999) 2638.
[2] M.J. Zaworotko, B. Moulton, Chem. Rev. 101 (2001) 1629.
[3] M. Eddaoudi, D.B. Moler, H. Li, B. Chen, T.M. Reineke, M.O. Keeffe, O.M. Yaghi,
Acc. Chem. Res. 34 (2001) 319.
[4] K. Nomiya, H. Yokoyama, J. Chem. Soc., Dalton Trans. (2002) 2483.
[5] A.Y. Robin, K.M. Fromm, Coord. Chem. Rev. 250 (2006) 2127.
[6] J.J. Vittal, Coord. Chem. Rev. 251 (2007) 1781.
[7] O.M. Yaghi, H. Li, C. Davis, D. Richardson, T.L. Groy, Acc. Chem. Res. 31 (1998)
474.
[8] S.R. Batten, R. Robson, Angew. Chem., Int. Ed. 37 (1998) 1460.
[9] A.J. Blake, N.R. Champness, P. Hubberstey, W.-S. Li, M.A. Withersby, M.
Schröder, Coord. Chem. Rev. 183 (1999) 117.
[10] S.A. Barnett, N.R. Champness, Coord. Chem. Rev. 246 (2003) 145.
[11] M.B. Zaman, M. Tomura, Y. Yamashita, Org. Lett. 2 (2000) 273.
[12] H.-F. Zhu, L.-Y. Kong, T.-A. Okamura, J. Fan, W.-Y. Sun, N. Ueyama, Eur. J. Inorg.
Chem. (2004) 1465.
[32] R.-Q. Zou, C.-S. Liu, Z. Huang, T.-L. Hu, X.-H. Bu, Cryst. Growth Des. 6 (2006)
99.
[33] G.M. Sheldrick, SADABS
Göttingen, 1996.
, Program for Absorption Correction, University of
[34] Bruker, SHELXTL, Structure Determination Software Programs, Bruker Analytical
X-ray Instruments Inc., Madison, WI, USA, 1997.
[35] X.Q. Lu, J.J. Jiang, H.-C. zur Loye, B.S. Kang, C.Y. Su, Inorg. Chem. 44 (2005) 1810.
[36] P.M. Forster, P.M. Thomas, A.K. Cheetham, Chem. Mater. 14 (2002) 17.
[37] S.W. Lee, H.J. Kim, Y.K. Lee, K. Park, J.-H. Son, Y.-U. Kwon, Inorg. Chim. Acta 353
(2003) 151.
[38] H.K. Lee, D. Min, B.-Y. Cho, S.W. Lee, Bull. Korean Chem. Soc. 25 (2004)
1955.
[39] M. Fujita, Y.J. Kwon, O. Sasaki, K. Yamaguchi, K. Ogura, J. Am. Chem. Soc. 117
(1995) 7287.
[13] X.-M. Quyang, B.-L. Fei, T.-A. Okamura, H.-W. Bu, W.-Y. Sun, W.-X. Tang, N.
Ueyama, Eur. J. Inorg. Chem. (2003) 618.