1D helix, 2D brick-wall and herringbone, and 3D interpenetration d 10 metal-organic framework structures assembled from pyridine-2,6-dicarboxylic acid N-oxide

Article abstract of DOI:10.1021/ic0509985

Five novel interesting d¹⁰ metal coordination polymers, [Zn(PDCO)(H₂O)₂]_n (PDCO = pyridine-2,6- dicarboxylic acid N-oxide) (1), [Zn₂(PDCO)₂(4,4′- bpy)₂(H₂O)₂·3H₂O] _n (bpy = bipyridine) (2), [Zn(PDCO)(bix)]_n (bix = 1,4-bis(imidazol-1-ylmethyl)benzene) (3), [Zn(PDCO)(bbi)·0.5H ₂O]_n (bbi = 1,1′-(1,4-butanediyl)bis(imidazole)) (4), and [Cd(PDCO)-(bix)_1.5·1.5H₂O]_n (5), have been synthesized under hydrothermal conditions and structurally characterized. Polymer 1 possesses a one-dimensional (1D) helical chainlike structure with 4₁ helices running along the c-axis with a pitch of 10.090 A. Polymer 2 has an infinite chiral two-dimensional (2D) brick-wall-like layer structure in the ac plane built from achiral components, while both 3 and 4 exhibit an infinite 2D herringbone architecture, respectively extended in the ac and ab plane. Polymer 5 features a most remarkable and unique three-dimensional (3D) porous framework with 2-fold interpenetration related by symmetry, which contains channels in the b and c directions, both distributed in a rectangular grid fashion. Compounds 1-5, with systematic variation in dimensionality from 1D to 2D to 3D, are the first examples of d¹⁰ metal coordination polymers into which pyridinedicarboxylic acid N-oxide has been introduced. In addition, polymers 1, 4, and 5 display strong blue fluorescent emissions in the solid state. Polymer 3 exhibits a strong SHG response, estimated to be approximately 0.9 times that of urea.

Full text of DOI:10.1021/ic0509985

Inorg. Chem. 2005, 44, 7161−7170
1D Helix, 2D Brick-Wall and Herringbone, and 3D Interpenetration d10
Metal Organic Framework Structures Assembled from
Pyridine-2,6-dicarboxylic Acid N-Oxide
−
Li-Li Wen, Dong-Bin Dang, Chun-Ying Duan, Yi-Zhi Li, Zheng-Fang Tian, and Qing-Jin Meng*
Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry,
Nanjing UniVersity, Nanjing 210093, P.R. China
Received June 18, 2005
Five novel interesting d10 metal coordination polymers, [Zn(PDCO)(H2O)2]n (PDCO
)
pyridine-2,6-dicarboxylic acid
bipyridine) (2), [Zn(PDCO)(bix)]n (bix 1,4-bis(imidazol-
1,1 -(1,4-butanediyl)bis(imidazole)) (4), and [Cd(PDCO)-
N-oxide) (1), [Zn2(PDCO)2(4,4
1-ylmethyl)benzene) (3), [Zn(PDCO)(bbi)
(bix)1.5
′
-bpy)2(H2O)2
‚
3H2O]n (bpy
)
)
)
‚
0.5H2O]n (bbi
′
‚
1.5H2O]n (5), have been synthesized under hydrothermal conditions and structurally characterized. Polymer
1 possesses a one-dimensional (1D) helical chainlike structure with 41 helices running along the c-axis with a pitch
of 10.090 Å. Polymer 2 has an infinite chiral two-dimensional (2D) brick-wall-like layer structure in the ac plane
built from achiral components, while both 3 and 4 exhibit an infinite 2D herringbone architecture, respectively
extended in the ac and ab plane. Polymer 5 features a most remarkable and unique three-dimensional (3D) porous
framework with 2-fold interpenetration related by symmetry, which contains channels in the b and c directions, both
distributed in a rectangular grid fashion. Compounds 1−5, with systematic variation in dimensionality from 1D to 2D
to 3D, are the first examples of d10 metal coordination polymers into which pyridinedicarboxylic acid N-oxide has
been introduced. In addition, polymers 1, 4, and 5 display strong blue fluorescent emissions in the solid state.
Polymer 3 exhibits a strong SHG response, estimated to be approximately 0.9 times that of urea.
Introduction
ion exchange, catalysis, and the development of optical,
electronic, and magnetic devices.4,5
The rational design of novel metal-organic frameworks
(MOFs) has attracted great interest from chemists in recent
years,1,2 and considerable efforts have been focused on the
design synthesis and characterization of novel multidimen-
sional not only because of their intriguing variety of
architectures and topologies but also because of their
fascinating potential applications in functional solid materials,
The MOFs are essentially assembled by very strong and
highly directional coordinative interactions between metal
centers and multitopic organic ligands, thus combining
properties of both purely organic and inorganic compounds.6
The multidentate O-donor organic aromatic polycarboxylate
ligands, such as 1,4-benzenedicarboxylate, 1,3,5-benzene-
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* To whom correspondence should be addressed. E-mail: mengqj@
nju.edu.cn. Phone: +86 25 83597266. Fax: +86 25 83314502.
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10.1021/ic0509985 CCC: $30.25
Published on Web 08/26/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 20, 2005 7161

Wen et al.
Chart 1
tricarboxylate, and 1,2,4,5-benzenetetracarboxylate, have
been extensively employed in the construction of a rich
variety of high-dimensional structures.7,8 However, pyridine-
2,6-dicarboxylic acid N-oxide (PDCO) (Chart 1), which has
limited steric hindrance and weak stacking interactions and
can offer possibilities to form coordination polymers through
a carboxylate and N-oxide bridge, remains largely unex-
plored. To the best of our knowledge, there has been only
one reported polymer based on the PDCO ligand hitherto.9
The N-oxide group of PDCO is a far better electron donor
than the ring nitrogen atom of pyridine-2,6-dicarboxylic
acid.10 Therefore, with the aim of preparing novel materials
with beautiful architecture and excellent physical properties,
we start to elaborate new high-dimensional coordination
polymers constructed from pyridine-2,6-dicarboxylic acid
N-oxide, with the introduction of N-containing auxiliary
ligands, such as rigid pyridyl-based 4,4′-bpy11 and highly
flexible imidazole-based bix and bbi (Chart 1),12,13 into
{M-PDCO} (M ) Zn2+, Cd2+), which afford the new
structures and fine-tune the structural motif of these MOFs.
In this paper, we report five interesting coordination poly-
meric complexes [Zn(PDCO)(H2O)2]n (1), [Zn2(PDCO)2(4,4′-
bpy)2(H2O)2‚3H2O]n (2), [Zn(PDCO)(bix)]n (3), [Zn(PDCO)-
(bbi)‚0.5H2O]n (4), and [Cd(PDCO)(bix)1.5‚1.5H2O]n (5).
These MOFs have several unusual features: (i) They are
the first examples of d10 metal coordination polymers into
which pyridine dicarboxylic acid N-oxide has been intro-
duced. (ii) d10 center metals and the conjugated π systems
containing aromatic rings favor the development of fluores-
cent materials.14,15 (iii) Compounds 1-3 are absent of a
center of symmetry, which meets the essential requirement
of a second harmonic generation (SHG) response.16 3 exhibits
a strong SHG response relative to urea; thus, it can be used
to form novel hybrid inorganic-organic NLO materials
which are currently investigated widely due to their possible
applications in optical switching, optical data processing,
image processing, etc.17,18
Experimental Section
Materials and Measurements. The reagents and solvents
employed were commercially available and used as received without
further purification. PDCO, bix, and bbi were synthesized as
reported previously.19-21 The C, H, and N microanalyses were
carried out with a Perkin-Elmer 240 elemental analyzer. The IR
spectra were recorded on KBr disks on a Bruker Vector 22
spectrophotometer in the 4000-400 cm-1 region. Luminescence
spectra for the solid samples were recorded with a Hitachi 850
fluorescence spectrophotometer. Thermogravimetric analyses were
performed on a simultaneous SDT 2960 thermal analyzer under
flowing N2 with a heating rate of 10 °C/min between ambient
temperature and 800 °C. Powder X-ray diffraction patterns were
recorded on a RigakuD/max-RA rotating anode X-ray diffractometer
with graphite monochromatic Cu KR (λ ) 1.542 Å) radiation at
room temperature. A pulsed Q-switched Nd:YAG laser at a
wavelength of 1064 nm was used to generate an SHG signal from
samples. The backward scattered SHG light was collected using a
spherical concave mirror and passed through a filter which transmits
only 532 nm radiation.
Hydrothermal Syntheses. [Zn(PDCO)(H2O)2]n (1). A mixture
of Zn(NO3)2‚6H2O (0.6 mmol), PDCO(0.6 mmol), triethylamine
(1.2 mmol), and H2O (4 mL) was placed in a Parr Teflon-lined
stainless steel vessel (25 cm3), and then the vessel was sealed and
heated at 120 °C for 3 days. After the mixture was slowly cooled
to room temperature, colorless crystals of 1 were obtained (yield:
84% based on Zn). Anal. Calcd for C7H7NO7Zn: C, 29.76; H, 2.50;
N, 4.96. Found: C, 29.72; H, 2.55; N, 5.01. IR spectrum (cm-1):
3386 (s), 3181 (s), 1672 (s), 1625 (s), 1477 (m), 1408 (s), 1359
(s), 1247 (s), 1197 (s), 1168 (w), 1091 (w), 1008 (w), 913 (m),
857 (m), 776 (s), 735 (m), 720 (m), 630 (m), 598 (m), 532 (m),
485 (m), 429 (w).
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[Zn2(PDCO)2(4,4′-bpy)2(H2O)2‚3H2O]n (2). A mixture of Zn-
(NO3)2‚6H2O (0.6 mmol), PDCO (0.6 mmol), 4,4′-bpy (0.6 mmol),
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M. L.; Chen, X. M. Eur. J. Inorg. Chem. 2003, 138. Wang, X. L.;
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Grachvogel, D. A.; Williams, D. J. Polyhedron 1998, 17, 3613.
7162 Inorganic Chemistry, Vol. 44, No. 20, 2005

Metal-Organic Structures Assembled from PDCO
Table 1. Crystal Data and Structure Refinement Information for Compounds 1-5
param
1
2
3
4
5
empirical formula
fw
cryst syst
space group
a (Å)
C7H7NO7Zn
282.51
tetragonal
I41cd
16.675(3)
16.675(3)
13.222(3)
90.00
3676.6(11)
16
C34H32N6O15Zn2
895.40
monoclinic
P21
10.825(2)
16.860(3)
11.004(2)
99.03(3)
1983.4(7)
2
C21H17N5O5Zn
484.77
orthorhombic
Pca21
20.740(2)
8.441(1)
11.486(1)
90.00
2011.0(4)
4
C68H72N20O22Zn4
1782.94
orthorhombic
Pbca
12.807(3)
16.145(4)
17.415(4)
90.00
C112H108Cd4N28O26
2711.86
monoclinic
P2/c
16.457(4)
9.193(2)
21.244(4)
113.52(1)
2946.9(11)
1
b (Å)
c (Å)
â (deg)
V (Å3)
3600.7(15)
2
1.409
Z
abs coeff (mm-1
F(000)
)
2.695
2272
1.284
916
1.268
992
0.796
1376
1832
limiting indices
-20 e h e 20
-20 e k e 12
-15 e l e 16
1786
1644
1.043
-13 e h e 13
-20 e k e 20
-13 e l e 13
7765
6731
1.029
-26 e h e 23
-10 e k e 9
-14 e l e 14
4338
3731
1.054
-13 e h e 15
-12 e k e 19
-20 e l e 20
3169
2362
0.976
-20 e h e 20
-11 e k e 11
-26 e l e 20
5767
3777
0.986
reflcns collcd
indep reflcns
GOF on F2
R1 [I > 2σ (I)]
wR2
0.0589
0.1468
0.0426
0.0929
0.0500
0.1026
0.0467
0.0928
0.0484
0.0885
NaOH (1.2 mmol), and H2O (5 mL) was heated at 120 °C for 3
days in a procedure analogous to that for 1. After the reactant
mixture was slowly cooled to room temperature, colorless crystals
of 2 were obtained (yield: 65% based on Zn). Anal. Calcd for
C34H32N6O15Zn2: C, 45.61; H, 3.60; N, 9.39. Found: C, 45.70; H,
3.68; N, 9.43. IR spectrum (cm-1): 3425 (m), 1612 (s), 1535 (w),
1489 (w), 1415 (m), 1384 (s), 1216 (m), 1071 (m), 1046 (w), 1015
(w), 860 (s), 808 (s), 774 (m), 727 (w), 705 (w), 674 (w), 642 (m),
609 (w), 570 (w), 493 (w), 465 (w).
methods with SHELXTL version 5.1.23 Anisotropic thermal pa-
rameters were refined for the non-hydrogen atoms. Hydrogen atoms
were localized in their calculation positions and refined using a
riding model.
Intensities of the complex 2 were collected on a Siemens P4
four-circle diffractometer with graphite monochromatic Mo KR
radiation (λ ) 0.710 73 Å) using the ω-2θ scan mode. Data were
corrected for Lorenz-polarization effects during data reduction
using XSCANS,24 and a semiempirical absorption correction from
ψ-scans was applied.25
Cavity dimensions were calculated by overlapping rigid spheres
with van der Waals radii for each element: O, 1.52 Å; N, 1.55 Å;
C, 1.7 Å; Cd, 2.2 Å (hydrogen atoms were omitted in all cases for
simplicity). Crystallographic data and other pertinent information
for 1-5 are summarized in Table 1. Selected bond lengths and
bond angles are listed in Table 2. More details on the crystal-
lographic studies as well as atomic displacement parameters are
given in the Supporting Information as CIF files.
[Zn(PDCO)(bix)]n (3). The synthesis was similar to that
described for 2 except using bix (0.6 mmol) instead of 4,4′-bpy,
and colorless crystals of 3 were obtained (yield: 54% based on
Zn). Anal. Calcd for C21H17N5O5Zn: C, 52.03; H, 3.53; N, 14.45.
Found: C, 52.06; H, 3.62; N, 14.51. IR spectrum (cm-1): 3423
(m), 3111 (s), 1654 (m), 1640 (s), 1595 (m), 1531 (m), 1520 (m),
1444 (m), 1394 (s), 1368 (m), 1339 (s), 1253 (w), 1234 (m), 1190
(m), 1110 (s), 1092 (s), 957 (m), 898 (m), 847 (m), 811 (m), 795
(w), 766 (m), 757 (m), 732 (m), 707 (s), 659 (s), 624 (m), 595
(m), 535 (w), 516 (w), 483 (w), 452 (w).
Results and Discussion
[Zn(PDCO)(bbi)‚0.5H2O]n (4). The synthesis was similar to that
described for 2 except using bbi (0.6 mmol) instead of 4,4′-bpy,
and colorless crystals of 4 were obtained (yield: 62% based on
Zn). Anal. Calcd for C68H72N20O22Zn4: C, 45.81; H, 4.07; N, 15.71.
Found: C, 45.76; H, 4.14; N, 15.62. IR spectrum (cm-1): 3419
(m), 3129 (s), 1650 (s), 1592 (m), 1528 (m), 1466 (w), 1390 (s),
1372 (s), 1331 (s), 1276 (m), 1247 (m), 1210 (m), 1177 (w), 1101
(s), 949 (m), 903 (w), 851 (s), 785 (s), 764 (w), 735 (w), 709 (s),
661 (m), 622 (w), 595 (w), 527 (w), 453 (w).
Crystal Structures. [Zn(PDCO)(H2O)2]n (1). The struc-
ture of compound 1 crystallizes in a noncentrosymmetric
space group I41cd, and the coordination geometry around
zincII center is a slightly distorted octahedron, the equatorial
plane of which comprises four oxygen atoms from the
carboxylate groups and N-oxide moieties of two different
PDCO anions; two coordinated water molecules occupy the
remaining apical coordination sites, as shown in Figure 1a.
The Zn-O distances range from 2.036(7) to 2.121(5) Å, in
the order Zn-Omean(aqua) > Zn-Omean(N-oxide) > Zn-
[Cd(PDCO)(bix)1.5‚1.5H2O]n (5). The synthesis was similar to
that described for 3 except using CdCl2‚2H2O (0.6 mmol) instead
of Zn(NO3)2‚6H2O, and colorless crystals of 5 were obtained
(yield: 27% based on Cd). Anal. Calcd for C112H108Cd4N28O26:
C, 49.60; H, 4.01; N, 14.46. Found: C, 49.68; H, 4.05; N, 14.51.
IR spectrum (cm-1): 3447 (m), 3105 (s), 1637 (s), 1513 (s), 1446
(m), 1429 (w), 1396 (w), 1356 (m), 1279 (s), 1234 (s), 1109 (s),
1090 (s), 1023 (w), 935 (m), 854 (w), 835 (m), 764 (s), 718 (s),
652 (s), 625 (m), 587 (w), 509 (w), 474 (w).
Omean(carboxylate), with the O-Zn-O bond angles varying
from 83.0(3) to 177.3 (3)°. The Zn-O(aqua) distance is
slightly shorter than the reported value,26 while the
Zn-O(carboxylate) distance is within the normal range.27
(22) SMART and SAINT, Area Detector Control and Integration Software;
Siemens Analytical X-ray Systems, Inc.: Madison, WI, 1996.
(23) Sheldrick, G. M. SHELXTL V5.1, Software Reference Manual; Bruker
AXS, Inc.: Madison, WI, 1997.
(24) XSCANS, version 2.1; Siemens Analytical X-ray Instruments, Inc.:
Madison, WI, 1994.
X-ray Structural Studies. Intensities of complexes 1 and 3-5
were collected on a Siemens SMART-CCD diffractometer with
graphite monochromatic Mo KR radiation (λ ) 0.710 73 Å) using
the SMART and SAINT22 programs. The structures were solved
by direct methods and refined on F2 using full-matrix least-squares
(25) Blessing, R. H. Acta Crystallogr. 1995, A51, 33. Blessing, R. H. J.
Appl. Crystallogr. 1997, 30, 421.
Inorganic Chemistry, Vol. 44, No. 20, 2005 7163

Wen et al.
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for 1-5
Complex 1a
Zn(1)-O(1)
Zn(1)-O(2)
2.074(6)
2.121(5)
Zn(1)-O(3)
Zn(1)-O(4)
2.089(7)
2.036(7)
Zn(1)-O(1)i
Zn(1)-O(6)i
2.106(7)
2.047(7)
O(1)-Zn(1)-O(4)
O(1)-Zn(1)-O(1)i
O(1)-Zn(1)-O(6)i
86.7(2)
174.6(2)
91.9(2)
O(2)-Zn(1)-O(3)
O(1)i-Zn(1)-O(4)
175.2(3)
98.3(3)
O(4)-Zn(1)-O(6)i
O(1)i-Zn(1)-O(6)i
177.3(3)
83.0(3)
Complex 2b
Zn(1)-O(1)
2.078(3)
2.149(4)
2.151(3)
Zn(1)-N(1)
2.149(3)
2.156(4)
2.044(3)
Zn(2)-O(8)
2.098(3)
2.089(3)
2.194(3)
Zn(1)-O(2)
Zn(1)-N(5)i
Zn(2)-O(9)
Zn(1)-O(3)
Zn(1)-O(12)
Zn(2)-O(6)
Zn(2)-O(7)
Zn(2)-N(4)
2.064(3)
2.083(3)
Zn(2)-N(2)ii
2.183(3)
O(1)-Zn(1)-O(2)
O(1)-Zn(1)-O(3)
O(1)-Zn(1)-O(12)
O(2)-Zn(1)-O(3)
O(2)-Zn(1)-O(12)
167.07(15)
87.86(13)
97.37(13)
79.27(14)
95.56(15)
O(3)-Zn(1)-O(12)
N(1)-Zn(1)-N(5)i
O(6)-Zn(2)-O(7)
O(6)-Zn(2)-O(8)
O(6)-Zn(2)-O(9)
172.88(13)
178.47(16)
98.20(14)
87.54(13)
170.71(13)
O(7)-Zn(2)-O(8)
O(7)-Zn(2)-O(9)
O(8)-Zn(2)-O(9)
N(2)ii-Zn(2)-N(4)
174.12(13)
90.90(13)
83.41(13)
173.01(19)
Complex 3c
Zn(1)-O(1)
1.955(3)
2.360(2)
119.13(13)
112.82(14)
Zn(1)-N(1)
1.972(3)
1.988(3)
Zn(1)-O(4)i
2.053(3)
Zn(1)-O(3)
Zn(1)-N(3)
O(1)-Zn(1)-N(1)
O(1)-Zn(1)-N(3)
O(3)-Zn(1)-O(4)i
164.30(11)
N(1)-Zn(1)-N(3)
125.40(13)
Complex 4
Zn(1)-O(1)
2.011(2)
2.155(2)
136.64(11)
112.74(11)
Zn(1)-O(4)
2.049(2)
2.022(3)
168.59(9)
Zn(1)-N(3)
2.034(3)
Zn(1)-O(3)
Zn(1)-N(1)
O(1)-Zn(1)-N(1)
O(1)-Zn(1)-N(3)
O(3)-Zn(1)-O(4)
N(1)-Zn(1)-N(3)
108.53(11)
Complex 5d
Cd(1)-O(2)
2.466(3)
2.324(3)
72.94(10)
88.69(11)
161.69(10)
Cd(1)-N(1)
2.298(4)
2.292(4)
159.83(11)
90.06(10)
Cd(1)-O(4)i
2.225(3)
2.320(4)
106.93(12)
172.56(12)
Cd(1)-O(3)
Cd(1)-N(5)
Cd(1)-N(4)ii
O(2)-Cd(1)-O(3)
O(2)-Cd(1)-N(5)
O(2)-Cd(1)-O(4)i
O(3)-Cd(1)-N(5)
O(4)i-Cd(1)-N(5)
N(1)-Cd(1)-N(4)ii
O(3)-Cd(1)-O(4)i
a Symmetry codes: (i) 1/2 + y, 1 - x, -1/4 + z. b Symmetry codes: (i) 1 + x, y, 1 + z; (ii) x, y, 1 + z. c Symmetry codes: (i) 1 - x, 1 - y, -1/2 +
z. d Symmetry codes: (i) 2 - x, 1 - y, -z; (ii) x, 1 - y, -1/2 + z.
Each carboxylate group is coordinated to Zn atom in a
monodentate fashion. The two carboxylate groups are out
of the plane of correspondingly linking pyridine rings, with
the dihedral angles between them being ca. 38 and 134°,
respectively, which are very distinct from those in the free
PDCO, in which the carboxylate groups are found to be
essentially coplanar with the pyridine rings.9
atom exhibits a distorted octahedron coordination environ-
ment formed by two oxygen atoms from a PDCO ligand,
one oxygen atom from the bridging carboxylate of another
PDCO, one coordinated aqua molecule occupying the basal
plane, and two individual 4,4′-bpy nitrogen atoms occupying
the axial sites, as illustrated in Figure 2a. The average Zn-O
bond length for the N-oxide groups amounts to 2.123 Å,
which is close to those involving the coordinated carboxylate
oxygens(2.087 Å) and the water molecules (2.081 Å). The
O-Zn-O bond angles range from 79.27 to 172.88°. The
average Zn-N bond lengths are 2.171 Å, with the N-Zn-N
bond angles varying from 173.01 to 178.47°, which is very
close to the linearity. The torsion angles between the COO-
group and corresponding pyridine ring range from ca. 33 to
128°. Furthermore, two pairs of pyridyl rings in the rigid
4,4′-bpy spacers are out of coplanarity, twisted by 27 and
21°, respectively. The neighboring 4,4′-bpy ligands orient
almost perpendicularly to each other so that the side C-H
bonds of one 4,4′-bpy are directed to the pyridyl plane of
the other. A synergistic distortion effect of PDCO and 4,4′-
bpy ligands leads to the formation of a chiral framework for
2.
The salient structural feature of the compound 1 is it
possesses a 1D helical chainlike structure with 41 helices
along the c-axis with a pitch of 10.090 Å. The twists of
PDCO2- in 1 may be responsible for the formation of the
helical structure. Because left-handed and right-handed
helical chains coexist in the crystal structure, the whole
structure is mesomeric and does not exhibit chirality (Figure
1b). The nearest Zn‚‚‚Zn separation of the adjacent helical
chains is 7.635 Å. Interchain hydrogen-bonding interactions
further extend the 1D helical chain to generate a 3-D
framework (Table 3, Figure S1).
[Zn2(PDCO)2(4,4′-bpy)2(H2O)2‚3H2O]n (2). Compound
2 crystallizes in the chiral space group P21, and each ZnII
(26) Gable, R. W.; Hoskins, B. F.; Robson, R. Chem. Commun. 1990, 1677.
Carlucci, L.; Ciani, G.; Gudenberg, D. W.; Proserpio, D. M.; Sironi,
A. J. Chem. Soc., Dalton Trans. 1997, 1801.
(27) MacDonald, J. C.; Dorrestein, P. C.; Pilley, M. M.; Foote, M. M.;
Lundburg, J. L.; Henning, R. W.; Schultz, A. J.; Manson, J. L. J. Am.
Chem. Soc. 2000, 122, 11692. Zheng, S. L.; Yang, J. H.; Yu, X. L.;
Chen, X. M.; Wong, W. T. Inorg. Chem. 2004, 43, 830.
Compound 2 is an infinite chiral two-dimensional brick-
wall-like layer structure in the ac plane built from achiral
components, which is composed of {Zn2(PDCO)2} dimer
units and 4,4′-bpy linkages, with 21 helices running along
7164 Inorganic Chemistry, Vol. 44, No. 20, 2005

Metal-Organic Structures Assembled from PDCO
Table 3. Distances (Å) and Angles (deg) of Hydrogen Bonding for
Complexes 1 and 2
D-H‚‚‚A
d(D‚‚‚A)
Compound 1a
-D-H‚‚‚A
O(2)-H(2A)‚‚‚N(1)i
O(2)-H(2B)‚‚‚O(7)ii
O(3)-H(3A)‚‚‚O(6)iii
O(3)-H(3B)‚‚‚O(4)iv
C(2)-H(2)‚‚‚O(7)v
C(3)-H(3)‚‚‚O(5)vi
2.921(9)
2.862(9)
2.676(10)
2.778(10)
3.320(11)
3.369(12)
122.00
122.00
139.00
149.00
143.00
166.00
Compound 2b
O(1)-H(1B)‚‚‚O(9)i
O(7)-H(7A)‚‚‚O(3)ii
O(7)-H(7B)‚‚‚O(5)
2.693(5)
2.767(5)
2.784(5)
2.835(3)
2.905(4)
2.945(5)
2.792(4)
2.792(4)
3.007(6)
3.450(6)
3.158(5)
3.422(6)
3.248(6)
120.00
136.00
135.00
157.00
116.00
128.00
158.00
157.00
123.00
161.00
148.00
168.00
134.00
O(13)-H(13A)‚‚‚O(15)iii
O(13)-H(13B)‚‚‚O(5)
O(14)-H(14A)‚‚‚O(11)iv
O(15)-H(15A)‚‚‚O(1)ii
O(15)-H(15B)‚‚‚O(11)ii
C(5)-H(5)‚‚‚O(6)
C(5)-H(5)‚‚‚O(8)
C(10)-H(10)‚‚‚O(12)vi
C(19)-H(19)‚‚‚O(14)
C(31)-H(31)‚‚‚O(5)vii
a Symmetry codes: (i) 1/2 + y, 1 - x, -1/4 + z; (ii) 2 - x, y, -1/2 +
z; (iii) 3/2 - x, 1/2 - y, -1/2 + z; (iv) 1 - y, -1/2 + x, 1/4 + z; (v) 2 -
x, 1 - y, z; (vi) 1/2 + y, x, 1/4 + z. b Symmetry codes: (i) 1 + x, y, z; (ii)
-1 + x, y, z; (iii) 1 - x, -1/2 + y, 1 - z; (iv) 2 - x, -1/2 + y, 1 - z; (v)
1 - x, -1/2 + y, 1 - z; (vi) x, y, -1 + z; (vii) 2 - x, 1/2 + y, 2 - z.
are different and are more flexible than 4,4′-bpy, in addition,
bbi is more variable than bix; both structures have only one
repeating metallic structural motif, with each crystallographi-
cally unique ZnII metal center in a less-common distorted
trigonal bipyramidal coordination sphere, {ZnN2O3}, which
is defined by two oxygen donors respectively from a
carboxylate and a N-oxide group of two distinct PDCO’s in
the axial positions while the equatorial positions are furnished
by two different bix (or bbi) nitrogen donors and one
carboxylate oxygen donor, as displayed in Figure 3a and
Figure 4a.
In 3, the Zn-O bond lengths for the coordinated carboxy-
late groups range from 1.955(3) to 2.053(3) Å, which are
obviously shorter than the Zn-O(N-oxide) distance of
2.360(2) Å, and the Zn-N bond lengths are 1.972(3) and
1.988(3) Å, respectively. Two carboxylate groups form
dihedral angles of ca. 66 and 31° with the correspondingly
pyridine rings plane. The bix ligands adopt a cis conforma-
tion, with the dihedral angles between the imidazole rings
of ca. 46°, while the distortion angles between the imidazole
rings and the average plane of phenyl group are ca. 103 and
79°, respectively.
In 4, the Zn-O bond lengths range between 2.011(2) and
2.155(2) Å. The Zn-N bond lengths are 2.022(3) and
2.034(3) Å, which are slightly longer with respect to those
involved in 3. The distortion angles between COO- groups
and pyridine rings range from ca. 32 to 134°. The bbi ligand
has an extended geometry in which the N-(CH2)4-N chain
has an all anti geometry, and the planes of the imidazole
rings are inclined by ca. 62° to each other.
Figure 1. (a) ORTEP view of 1 with hydrogen atoms omitted for clarity.
Thermal ellipsoids are drawn at the 30% probability level. (b) Stereoview
of adjacent two helices for 1. Hydrogen atoms and coordinated water
molecules have been omitted for clarity.
the c-axis (Figure 2b,c). All the helices lie along the same
direction and exhibit the same chirality. Each rectangular
grid of the brick-wall-like layer is organized by four PDCO
ligands acting as the two shorter edges (5.551 Å), four 4,4′-
bpy groups acting as the two longer edges (22.951 Å), four
Zn atoms representing the four vertexes, and two more Zn
atoms serving as the midpoints of the longer edges. The free
water molecules are included between the layers, and the
layers are stacked in an -ABAB- sequence along the b
axis with strong interlayer hydrogen-bond interactions by the
coordinated and free water molecules linking the carboxylic
acid moieties, thus leading to the construction of a three-
dimensional host-guest network (Table 3, Figure S2).
[Zn(PDCO)(bix)]n (3) and [Zn(PDCO)(bbi)‚0.5H2O]n
(4). For compounds 3 and 4, though the bridging ligands
For both compounds, the fully deprotonated PDCO2-
anions act as effective tridentate bridging ligands linking the
Zn metal centers by monodentate carboxylate and N-oxide
groups, imposing separations of Zn‚‚‚Zni 6.008 Å and
Inorganic Chemistry, Vol. 44, No. 20, 2005 7165

Wen et al.
Figure 2. (a) ORTEP view of 2 with hydrogen atoms and free water molecules omitted for clarity (30% probability ellipsoids). (b) View toward the ac
plane of the 2D brick-wall-like framework. (c) Space-filling model for a helical stereoview of 2.
Zn‚‚‚Znii 6.499 Å for 3 and 4, respectively [symmetry code:
(i) 1 - x, 1 - y, -1/2 + z; (ii) -1/2 + x, y, 3/2 - z]. All
the bidentate bix or bbi molecules establish a physical bridge
between Zn atoms, imposing Zn‚‚‚Zniii separations of 10.475
Å for 3 and Zn‚‚‚Zniv separations of 13.446 Å for 4
[symmetry code: (iii) 1 - x, 1 - y, 1/2 + z; (iv) -x, 1/2 +
y, 3/2 - z].
It is noteworthy that the repetition of the {ZnN2O3}
metallic structural motif of 3 generates an infinite 2D
herringbone architecture extended in the ac plane of the unit
cell (Figure 3b). The resulting 2D structure is cross-linked
by weaker hydrogen-bond interactions between C-H groups
from a bix ligand and uncoordinated carboxylate or N-oxide
oxygen atoms, thus producing a 3D framework (Table 4,
Figure S3).
connecting nodes, also distributed in the herringbone manner
(Figure 4b). Lattice water molecules are also included
between the layers, and the relatively weaker Owater-H‚‚‚N
and C-H‚‚‚Ocarboxylate hydrogen bonds complete the final 3D
architecture (Table 4, Figure S4).
Complexes 3 and 4 are similar in their coordination mode
and network structure. Both compounds crystallize in the
orthorhombic crystal system; however, noncentrosymmetric
space group Pca21 for 3 and centrosymmetric space group
Pbca for 4 are observed. The difference may originate from
the different conformations of the bix and bbi ligands in the
structures.28
[Cd(PDCO)(bix)1.5‚1.5H2O]n (5). Though the ligands are
the same as in 3, the structure of 5 is significantly different
from 3 because of its different metal centers. It is a most
Interestingly, there is a 34-membered metallocycle in the
2D layer developed in the ab plane for 4, which is composed
of two PDCO, two bbi units, and four Zn atoms acting as
(28) Fan, J.; Sun, W. Y.; Okamura, T.-a.; Tang, W. X.; Ueyama, N. Inorg.
Chem. 2003, 42, 3168. Fan, J.; Sui, B.; Okamura, T.-a.; Sun, W. Y.;
Tang, W. X.; Ueyama, N. J. Chem. Soc., Dalton Trans. 2002, 3868.
7166 Inorganic Chemistry, Vol. 44, No. 20, 2005

Metal-Organic Structures Assembled from PDCO
Figure 3. (a) ORTEP view of 3 with hydrogen atoms omitted for clarity. (b) View toward the ac plane of the 2D herringbone architecture.
remarkable and unique three-dimensional coordination poly-
mer where each CdII atom exhibits a distorted hexahedral
configuration with a N3O3 donor set, as depicted in Figure
5. The Cd atom is coordinated by three individual bix
nitrogen atoms with Cd-N bond distances varying from
2.292(4) to 2.320(4) Å. Three additional positions are
occupied by oxygen atoms from two individual PDCO
groups with Cd-O bond distances varying from 2.225(3)
to 2.466(3) Å. These bond lengths are comparable to those
reported values.29,30 The COO- groups deviate from the plane
of correspondingly pyridine rings with the dihedral angles
between them being ca. 57 and 80°. For the bix ligand
containing N1, two terminal imidazole groups are in cis
conformation and the dihedral angles between the imidazole
ring planes and the least-squares plane of the phenyl group
are 79 and 110°, respectively, while the two imidazole groups
twist by 45°. However, in the case of the bix containing N5,
residing at crystallographic inversion center, two terminal
imidazole groups assume a trans conformation and have their
planes steeply inclined, by 74°, to the average plane of the
phenyl group.
c axis, which is constructed from cis-bix and PDCO ligands,
and the 1D structure is further stacked along the a and b
directions, with the bidentate trans-bix spacers occupying
the void spaces, thus leading to the formation of a three-
dimensional porous framework. The crystal structure of 5
contains channels in two directions, both distributed in a
rectangular grid fashion, which are filled by the 3 crystal-
lographically unique uncoordinated water molecules involved
in C-H‚‚‚O and O-H‚‚‚O hydrogen bonds (Table 4). The
most prominent cavties are ca. 10.4 × 9.0 Å in cross section
and run along the c axis; other rectangular-shaped, but
smaller, channels with cross section of ca. 7.3 × 9.0 Å are
parallel to the b direction, as indicated in Figure 6.
Interestingly, the presence of large free voids in the
[Cd(PDCO)(bix)1.5‚1.5H2O]n single framework result in the
formation of interpenetrations. The channels of [Cd(PDCO)-
(bix)1.5‚1.5H2O]n are filled by another identical framework
in a typical 2-fold interpenetration fashion, displaced by the
usual half of the length of the unit, as illustrated in Figure
7. As a result, 5 exhibits a 2-fold self-interpenetration 3D
framework related by symmetry. Usually, the coexistence
of both syn and anti conformations of bix leads to an unusual
interpenetrating network.31 Although many entangled struc-
It is worth noting that the structure is composed of a one-
dimensional wavelike architecture with a period equal to the
(29) Tao, J.; Tong, M. L.; Chen, X. M. J. Chem. Soc., Dalton Trans. 2000,
(31) Abrahams, B. F.; Hoskins, B. F.; Robson, R.; Slizys, D. A. Crys-
tEngComm 2002, 4, 478. Carlucci, L.; Ciani, G.; Proserpio, D. M.;
Spadacini, L. CrystEngComm 2004, 6, 96. Gao, E. Q.; Xu, Y. X.;
Yan, C. H. CrystEngComm 2004, 6, 298.
3669.
(30) Fujita, M.; Aoyagi, M.; Ogura, K. Bull. Chem. Soc. Jpn. 1998, 71,
1799.
Inorganic Chemistry, Vol. 44, No. 20, 2005 7167

Wen et al.
Apparently, the formation of MOFs depends on the
combination of several factors, such as the coordination
geometry of metal ions, the nature of ligands,33 the ratio
between metal salt and ligand,34 and reaction conditions such
as solvent system and templates. In this study, the hydro-
thermal reaction of PDCO and Zn(NO3)2 produces 1, which
generates a 1D helical chainlike structure. When the neutral
bridging ligands 4,4′-bpy, bix, and bbi are introduced into
the reaction system, compounds 2-4 give rise to a diversity
of frameworks: 2 is an infinite chiral two-dimensional brick-
wall-like layer structure; both 3 and 4 exhibit infinite 2D
herringbone architectures. Complexes 1-4 all form a 3D
array considering the hydrogen-bonding interactions. Most
interestingly, the hydrothermal reaction of PDCO and bix
in the presence of CdCl2‚2H2O affords 5, which is a 2-fold
interpenetration three-dimensional coordination polymer.
In these five compounds, all the carboxyl groups adopt a
monodentate coordination mode to connect metal atoms. This
type of coordination fashion is similar to that for {Ln2(pdc)3‚
3H2O}35 and {[Ln(pdc)3Mn1.5(H2O)3]‚nH2O} (pdc ) pyri-
dine-2,6-dicarboxylic acid).36 For 2, the 4,4′-bpy ligand is
rigid, which may facilitate the formation of a homohanded
helix, hence, giving rising to the chiral framework. The bix
ligand is highly variable, which makes it adaptable to
different polymeric coordination structures. It assumes a cis
conformation in 3 vs both cis and trans conformations in
the case of 5, according to the different geometric needs of
metal ions. Consequently, the versatile conformation of bix
ligand intricately influences the crystal packing. Although
bbi is more variable than bix, it only exhibits an anti
conformation in 4 and the other previously reported com-
pounds containing bbi.37 Indeed, due to a different degree
in rigidity/flexibility of the neutral bridging ligands and in
atom radius of metal centers, systematic variations in
dimensionality (1D f 2D f 3D) of the coordination polymer
frameworks can be obtained, indicating ligand-directed and
metal-directed synthesis.
Figure 4. (a) ORTEP view of 4 with hydrogen atoms and free water
molecules omitted for clarity. (b) View toward the ab plane of the 2D
herringbone architecture.
Table 4. Distances (Å) and Angles (deg) of Hydrogen Bonding for
Complexes 3-5
D-H‚‚‚A
d(D‚‚‚A)
Compound 3a
-D-H‚‚‚A
C(1)-H(1)‚‚‚O(4)
3.099(4)
3.362(6)
3.192(6)
3.196(6)
154.00
142.00
141.00
125.00
Thermal Analysis. For 1, the weight loss of 12.82%
below 207 °C (calcd 12.74%) corresponds to the loss of two
coordinated water molecules/formula. Then the compound
begins to decompose. For 2, the weight loss attributed to
the gradual release of free aqua ligands is observed below
80 °C (obsd 5.66%, calcd 6.03%). The second loss corre-
sponding to the release of almost all coordinated water
molecules is detected between 80 and 160 °C (obsd 4.29%,
calcd 4.02%). Two consecutive decompositions suggest the
C(4)-H(4B)‚‚‚O(3)i
C(12)-H(12A)‚‚‚O(2)ii
C(18)-H(18)‚‚‚O(5)iii
Compound 4b
O(6)-H(6F)‚‚‚N(5)i
C(1)-H(1)‚‚‚O(5)
C(8)-H(8)‚‚‚O(5)ii
C(14)-H(14)‚‚‚O(2)iii
3.338(14)
3.096(5)
3.105(6)
3.126(5)
161.00
127.00
146.00
133.00
Compound 5c
O(6)-H(6B)‚‚‚O(7)
O(7)-H(7A)‚‚‚O(8)
O(7)-H(7B)‚‚‚O(1)i
O(8)-H(8A)‚‚‚O(1)ii
C(1)-H(1)‚‚‚O(4)iii
C(11)-H(11A)‚‚‚O(3)iv
C(15)-H(15)‚‚‚O(7)ii
3.233(11)
3.135(10)
2.776(7)
2.967(9)
3.062(5)
3.324(6)
3.169(7)
138.00
150.00
113.00
116.00
118.00
159.00
150.00
(32) Paz, F. A. A.; Klinowski J. Inorg. Chem. 2004, 43, 3882. Paz, F. A.
A.; Klinowski, J. Inorg. Chem. 2004, 43, 3948.
(33) Hirsch, K. A.; Wilson, S R.; Moore, J. S. Inorg. Chem. 1997, 36,
2960. Bu, X. H.; Chen, W.; Lu, S. L.; Zhang, R. H.; Liao, D. Z.; Bu,
W. M.; Shionoya, M.; Brisse, F.; Ribas, J. Angew. Chem., Int. Ed.
2001, 40, 3201. Kitaura, R.; Fujimoto, K.; Noro, S.; Kondo, M.;
Kitagawa, S. Angew. Chem., Int. Ed. 2002, 41, 133. Kitaura, R.; Seki,
K.; Akiyama, G.; Kitagawa, S. Angew. Chem., Int. Ed. 2003, 42, 428.
(34) Carlucci, L.; Ciani, G.; Gudenberg, D. W.; Proserpio, D. M.; Sironi,
A. Chem. Commun. 1997, 631.
a Symmetry codes: (i) x, 1 + y, z; (ii) 1 - x, -y, 1/2 + z; (iii) 3/2 - x,
y, -1/2 + z. b Symmetry codes: (i) -1/2 + x, y, 3/2 - z; (ii) 3/2 - x, 1/2
+ y, z; (iii) -1/2 + x, 1/2 - y, 2 -z. c Symmetry codes: (i) x, y, 1 + z; (ii)
1 - x, y, 1/2 - z; (iii) 2 - x, 1 - y, -z; (iv) x, -y, 1/2 + z.
(35) Ghosh, S. K.; Bharadwaj, P. K. Inorg. Chem. 2004, 43, 2293.
(36) Zhao, B.; Cheng, P.; Dai, Y.; Cheng, C.; Liao, D. Z.; Yan, S. P.;
Jiang, Z. H.; Wang, G. L. Angew. Chem., Int. Ed. 2003, 42, 934.
(37) Ma, J. F.; Yang, J., Zheng, G. L.; Li, L.; Zhang, Y. M.; Li, F. F.; Liu,
J. F. Polyhedron 2004, 23, 553.
tures have been reported, those constructed by mixed ligands
are rather few.32
7168 Inorganic Chemistry, Vol. 44, No. 20, 2005

Metal-Organic Structures Assembled from PDCO
Figure 5. ORTEP view of 5 with hydrogen atoms and water molecules omitted for clarity.
Figure 7. (a) Packing of the adjacent [Cd(PDCO)(bix)1.5‚1.5H2O]n
framework (in green- and blue-filled bonds) along the c direction. Hydrogen
atoms and water molecules have been omitted for clarity. (b) Topological
view of the two interpenetrating nets in [Cd(PDCO)(bix)1.5‚1.5H2O]n. Cd2+
cations and the PDCO2- anionic ligands are taken as nodes, and bix ligands
are substituted by a connection between metals.
Figure 6. Projections along the (a) c direction and (b) b direction of the
3D [Cd(PDCO)(bix)1.5‚1.5H2O]n framework in 5 showing the cavities.
Hydrogen atoms and water molecules have been omitted for clarity, and
cross sections have been calculated on the basis of van der Waals radii.
(3.98%). The second weight loss occurs in the range 250-
494 °C, which is attributed to the elimination of the PDCO
ligand (obsd 26.74%, calcd 26.70%). The guest molecules
are difficult to remove from the host molecules due to the
existence of strong hydrogen bonds between the host and
guest molecules.
Guest water molecules are removed by heating 2 and 5 at
80 and 250 °C for 2 h under N2, respectively. Powder X-ray
diffraction study of 2 and 5 before and after water expulsion
shows only minor changes in the diffraction patterns, which
indicate the chiral MOFs of 2 and 3D 2-fold interpenetration
MOFs of 5 are intact after removal of the guest water
molecule.
total destruction of the framework by the oxidation of the
organic component, leading to the formation of the stoichio-
metric amount of zinc oxide as the residues (obsd 18.68%,
calcd 18.18%). Compounds 3 and 4 are stable up to 233
and 220 °C, respectively, where the framework structures
begin to collapse. A plateau region is observed for 4 from
520 to 800 °C and a brown residue of ZnO (obsd 18.67%,
calcd 18.26%) remained. For 5, the first weight loss of 4.07%
below 250 °C is assigned to the liberation of free water
molecules, which is in agreement with the calculated value
Inorganic Chemistry, Vol. 44, No. 20, 2005 7169

Wen et al.
Photoluminescence Properties. The emission spectra of
complexes 1-5 in the solid state at room temperature are
investigated. Excitation at 342 nm leads to stronger blue-
fluorescent emission bands at 417 nm for 1, 447 nm for 4,
and 418 nm for 5 while weaker blue-fluorescent emission
bands are observed at 420 nm for 2 and 458 nm for 3,
respectively, under the same conditions. These emissions are
neither metal-to-ligand charge transfer (MLCT) nor ligand-
to-metal transfer (LMCT) in nature since the Zn2+ or Cd2+
ions are difficult to oxidize or to reduce due to their d10
configuration which can probably be assigned to the intrali-
gand (π-π*) fluorescent emission because similar emissions
are observed for the free PDCO at 416 nm. It is clear that
bathochromic shift of emission occurs in 3 and 4, compared
with other compounds, which is probably due to the
differences of ligands and coordination environment around
central metal ions, because photoluminescence behavior is
closely associated with the local environments around metal
ions.38 Complexes 1, 4, and 5 may be suitable as excellent
candidates of blue-fluorescent materials, since they are highly
thermally stable and insoluble in common solvents.39
SHG Measurement. Given that 1-3 all crystallize in a
noncentrosymmetric space group, preliminary SHG responses
of these complexes are estimated by measuring a powder
sample (ca. 80-120 µm in diameter, Kurtz powder test),40
relative to urea. The SHG efficiency of 1-3 is estimated to
be approximately 0.3, 0.3, and 0.9 times of that of urea,
respectively, which confirm their noncentric frameworks.
These SHG properties reflect the molecular arrangements
in the crystals. Compound 3 displays higher SHG efficiency
compared with 1 and 2, and the reason may be attributed to
the herringbone arrangement in the lattice of 3, which induces
the large charge-transfer interaction to result in the higher
SHG efficiency.41 Similar phenomena have been observed
in the cocrystals of 2-amino-3-nitropyridine or 2-amino-5-
nitropyridine with achiral benzenesulfonic acids.42 Thus, 3
appears to be a good candidate of novel hybrid inorganic-
organic NLO materials. 1-3 display different NLO proper-
ties owing to them having different architectures. It is obvious
that the NLO properties of coordination polymers can be
altered through structural manipulation.
Conclusion
In summary, five novel polymers with different architec-
tures, [Zn(PDCO)(H2O)2]n (1), [Zn2(PDCO)2(4,4′-bpy)2-
(H2O)2‚3H2O]n (2), [Zn(PDCO)(bix)]n (3), [Zn(PDCO)(bbi)‚
0.5H2O]n (4), and [Cd(PDCO)(bix)1.5‚1.5H2O]n (5), were
constructed from the PDCO ligand, in the presence of
different N-heterocycles used as spacers under mild hydro-
thermal conditions. For the first time, pyridine dicarboxylic
acid N-oxide has been introduced into the d10 metal coor-
dination polymer system. This study not only demonstrates
that the nature of ligands and the geometric needs of metal
atoms play an important role in the crystal packing of MOFs
but also illustrates that the hydrogen bonds affect the
formation of the supramolecular architecture. In addition,
complexes 1, 4, and 5 may be excellent candidates for blue-
fluorescent materials and 3 appears to be a good candidate
of novel hybrid inorganic-organic NLO materials.
Acknowledgment. We thank the National Natural Sci-
ence Foundation of China (Grant No.20490218), Jiangsu
Science & Technology Department, and the Center of
Analysis and Determination of Nanjing University for the
financial support.
Supporting Information Available: X-ray crystallographic files
in CIF format, figures of crystal packing, TG curves, photo-
luminescent spectra, and an X-ray power diffraction diagram.
This material is available free of charge via the Internet at
http://pubs.acs.org.
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7170 Inorganic Chemistry, Vol. 44, No. 20, 2005