Controllable assembly of metal-directed coordination polymers under diverse conditions: A case study of the MII-H3tma/Bpt mixed-ligand system

Article abstract of DOI:10.1021/ic060001d

A series of new metal-organic polymeric complexes, {[Co(bpt)(Htma)(H ₂O)₃]·2.25H₂O}_n (1), [Co(bpt)(Htma)(H₂O)]_n (2), [Ni(bpt)(Htma)(H ₂O)]_n (3), [Zn(bpt)₂(H₂tma) ₂]·6H₂O (4), {[Cd(bpt)(Htma)(H₂O) ·(C₂H₅OH)(H₂O)_1.5} _n (5), and {-[Cd(bpt)(Htma)(H₂O)₂]·5. 5H₂O}_n (6), was prepared from solution reactions of 4-amino-3,5-bis(4-pyridyl)-1,2,4-triazole (bpt) and trimesic acid (H ₃tma) with different metal salts under diverse conditions. All these compounds were structurally determined by X-ray single-crystal diffraction, and the bulk new materials were further identified by X-ray powder diffraction. Complexes 1 and 6 show 1-D zigzag or linear Htma-bridged polymeric chains, with the terminal bpt ligands as pendants, which are extended to 2-D hydrogen-bonded arrays with 4.8² or (6,3) network topology. Coordination polymers 2 and 3, in which the 2-D corrugated metal-organic frameworks make the interdigitated 3-D packing, are isostructural. Complex 4 has a mononuclear structure, and its subunits are hydrogen-bonded to each other to give a 2-D gridlike net. For complex 5, the Cd^II centers are linked by bpt/Htma ligands to form a 2-D (4,4) coordination layer, and these layers are interdigitated in pairs. Notably, secondary noncovalent forces, such as hydrogen bonds, play an important role in extending and stabilizing these structural topologies. Interestingly, distinct products are obtained for Co^II (1 and 2) and Cd^II (5 and 6) under ambient or hydrothermal conditions; however, for Ni^II and Zn^II, single products, 3 and 4, are generated. The thermal stabilities of 1-6 were studied by thermogravimetric analysis of mass loss. The desorption/adsorption properties of the porous material 5 are also discussed. Solid-state luminescent spectra of the Zn ^II and Cd^II complexes, 4-6, indicate intense fluorescent emissions at ca. 380 nm.

Full text of DOI:10.1021/ic060001d

Inorg. Chem. 2006, 45, 3998−4006
Controllable Assembly of Metal-Directed Coordination Polymers under
Diverse Conditions: A Case Study of the MII
System
−H3tma/Bpt Mixed-Ligand
Miao Du,* Xiu-Juan Jiang, and Xiao-Jun Zhao
College of Chemistry and Life Science, Tianjin Normal UniVersity, Tianjin 300074, P. R. China
Received January 1, 2006
A series of new metal
(2), [Ni(bpt)(Htma)(H2O)]n (3), [Zn(bpt)2(H2tma)2]
[Cd(bpt)(Htma)(H2O)2] 5.5H2O n (6), was prepared from solution reactions of 4-amino-3,5-bis(4-pyridyl)-1,2,4-triazole
−organic polymeric complexes,
{
[Co(bpt)(Htma)(H2O)3]
‚2.25H2O} (1), [Co(bpt)(Htma)(H2O)]n
n
‚
6H2O (4),
{
[Cd(bpt)(Htma)(H2O)]
‚(C2H5OH)(H2O)1.5} (5), and {-
n
‚
}
(bpt) and trimesic acid (H3tma) with different metal salts under diverse conditions. All these compounds were
structurally determined by X-ray single-crystal diffraction, and the bulk new materials were further identified by
X-ray powder diffraction. Complexes 1 and 6 show 1-D zigzag or linear Htma-bridged polymeric chains, with the
terminal bpt ligands as pendants, which are extended to 2-D hydrogen-bonded arrays with 4.82 or (6,3) network
topology. Coordination polymers 2 and 3, in which the 2-D corrugated metal−organic frameworks make the
interdigitated 3-D packing, are isostructural. Complex 4 has a mononuclear structure, and its subunits are hydrogen-
bonded to each other to give a 2-D gridlike net. For complex 5, the CdII centers are linked by bpt/Htma ligands to
form a 2-D (4,4) coordination layer, and these layers are interdigitated in pairs. Notably, secondary noncovalent
forces, such as hydrogen bonds, play an important role in extending and stabilizing these structural topologies.
Interestingly, distinct products are obtained for CoII (1 and 2) and CdII (5 and 6) under ambient or hydrothermal
conditions; however, for NiII and ZnII, single products, 3 and 4, are generated. The thermal stabilities of 1
−6 were
studied by thermogravimetric analysis of mass loss. The desorption/adsorption properties of the porous material 5
are also discussed. Solid-state luminescent spectra of the ZnII and CdII complexes, 4
−6, indicate intense fluorescent
emissions at ca. 380 nm.
Introduction
in the framework structures of such materials greatly depends
on the selection of the metal centers and organic spacers, as
well as on the reaction pathways.1 Accordingly, ligands with
certain functional groups, such as carboxylate, pyridyl, and
phosphate, are especially crucial and have been widely
explored.4,5
The design and construction of polymeric metal-organic
hybrid complexes are of considerable interest in recent years;
the interest results from their appealing structural topologies
and potential application in catalysis, adsorption/separation,
host-guest chemistry, and the promising photo-, electro-,
and magnetofunctional materials.1-3 Generally, the diversity
Several rational synthetic strategies have been proposed
to achieve the metallosupramolecular arrays, and the most
* To whom correspondence should be addressed. Fax: 86-22-23540315.
Tel: 86-22-23538221. E-mail: dumiao@public.tpt.tj.cn.
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3998 Inorganic Chemistry, Vol. 45, No. 10, 2006
10.1021/ic060001d CCC: $33.50
© 2006 American Chemical Society
Published on Web 04/18/2006

Assembly of Metal-Directed Coordination Polymers
effective one is to employ appropriate bridging building
blocks capable of binding metal centers through direct dative
bonds. As already known, the aromatic multicarboxyl
ligands, such as phthalic, isophthalic, terephthalic, trimesic,
and pyromellitic acid, are versatile modular components for
realization of these extended systems.6 Trimesic acid (H3-
tma), which possesses three symmetric exo-carboxyl groups
around the benzene ring, has attracted considerable interest
as a host for inclusion compounds with a wide variety of
species7 and also as an excellent organic spacer, which can
exhibit diverse coordination fashions and bridge metal centers
to assemble multidimensional architectures.8 H3tma can also
behave as a trianion, dianion, or monoanion under different
conditions and its degree of deprotonation would have great
influence on the structures of the resulting coordination
complexes.8a
On the other hand, rodlike N,N′-donor building blocks,
such as the traditionally employed 4,4′-bipyridine, have been
extensively studied in coordination chemistry,9 and modifica-
tion by introducing spacers between the two 4-pyridyl groups
results in distinct spatial effects to produce unexpected
architectures upon metal complexation.10 We have focused
intensively on the crystal engineering of an angular dipyridyl
derivative, namely, 2,5-bis(4-pyridyl)-1,3,4-oxadiazole (bpo),
from organic hydrogen-bonded solids to coordination poly-
mers.11 Recently, a related triazole-containing compound
4-amino-3,5-bis(4-pyridyl)-1,2,4-triazole (bpt) has attracted
our attention as a continuation of this program.12 Compara-
tively, bpt has a more bent backbone than bpo: the angle
subtended at the center of the five-membered heterocyclic
spacer and two pyridyl N-donors is 152° for bpt and 137°
for bpo. Furthermore, the amino donor group can provide
potential coordination or hydrogen bonding sites that will
influence the final coordination architectures.
As far as the reaction pathways, a conventional synthetic
method of solvent assembly has been carried out to obtain
the target compounds. At one time, hydrothermal synthesis
at mild temperature (100-200 °C) under autogeous pressure
was proven to be a powerful approach in the preparation of
low-soluble organic-inorganic hybrid materials, although the
mechanism of the hydrothermal technique is still poorly
understood.13 As known, the resultant materials are also
significantly dependent on a variety of factors, such as
solvent medium, temperature, pH value, and molar ratio of
reactants, and in some cases, two or more diverse species
could be isolated under different synthetic conditions or
strategies.14 Thus, in this context, it is still a great challenge
to obtain the predicted products at this stage.
Currently, the rational construction of new structurally
defined metal-organic frameworks using the mixed-ligand
strategy seems to be a marvelous success.4c,12,13a,15 In our
recent research, we have initiated a synthetic approach
employing aromatic multicarboxylates and bpt upon reaction
with different metal ions to construct new functional
frameworks. A new type of 1-D + 2-D f 3-D polythreading
topology with finite components and a 2-fold interpenetrating
porous network assembled from CuII or CdII with mixed
ligands bpt/terephthalate have been reported in the latest
communication.12 In this contribution, we will describe a
series of MII-H3tma/bpt mixed-ligand metal-organic frame-
works, which are interestingly regulated by metal ions and
specific reactive conditions. The thermal stability, desorption/
adsorption, and fluorescent emission of these new materials
have also been discussed in detail.
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Experimental Section
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General Materials and Methods. With the exception of the
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Inorganic Chemistry, Vol. 45, No. 10, 2006 3999

Du et al.
dure,16 all reagents and solvents for the synthesis and analysis were
commercially available and used as received. Fourier transform (FT)
IR spectra (KBr pellets) were taken on an AVATAR-370 (Nicolet)
spectrometer. Carbon, hydrogen, and nitrogen analyses were
performed on a CE-440 (Leemanlabs) analyzer. Thermogravimetric
analysis (TGA) experiments were carried out on a Dupont thermal
analyzer from room temperature to 800 °C under N2 atmosphere
at a heating rate of 10 °C/min. X-ray powder diffraction (XRPD)
data were recorded on a Rigaku D/max-2500 diffractometer at 60
kV and 300 mA for Cu KR radiation (λ ) 1.5406 Å), with a scan
speed of 2 deg/min and a step size of 0.02° in 2θ. The calculated
XRPD patterns were produced using the software PowderCell and
single-crystal diffraction data. Fluorescence spectra of the poly-
crystalline powder samples were performed on a Cary Eclipse
spectrofluorimeter (Varian) equipped with a xenon lamp and quartz
carrier at room temperature.
Found: C, 48.02; H, 3.02; N, 16.14. IR (KBr, cm-1): 3338m,
3083bs, 1689s, 1611s, 1542s, 1443s, 1368vs, 1281s, 1218m, 843m,
751m, 727m, 702m, 670m, 506w. Anal. Calcd for C21H29CdN6O13.5
(6): C, 36.30; H, 4.21; N, 12.11. Found: C, 36.97; H, 4.69; N,
12.01. IR (KBr, cm-1): 3301s, 3173s, 1634w, 1602vs, 1554m,
1455s, 1403m, 1365m, 1332w, 1274w, 1222m, 1066w, 978m, 938s,
831vs, 733s, 701s, 607m, 511s.
Caution: Metal perchlorate complexes in the presence of organic
ligand are potentially explosive. Only a small amount of material
should be handled with care.
X-ray Crystallography. X-ray single-crystal diffraction data for
1-6 were collected on a Bruker Apex II CCD diffractometer at
ambient temperature with Mo KR radiation (λ ) 0.71073 Å). There
was no evidence of crystal decay during data collection. Semiem-
pirical absorption corrections were applied (SADABS), and the
program SAINT was used for integration of the diffraction
profiles.17a All structures were solved by direct methods using the
SHELXS program of the SHELXTL package and refined with
SHELXL.17b The final refinements were performed by full-matrix
least-squares methods with anisotropic thermal parameters for all
non-hydrogen atoms on F2. Generally, C-bound hydrogen atoms
were placed geometrically and refined as riding atoms. For all the
cases, the starting positions for hydroxyl hydrogen atoms were
located in difference Fourier syntheses, and refinement proceeded
using a rigid model, allowed to rotate but not tip. After they were
located on the difference Fourier map, all amino H atoms were
positioned geometrically and treated as riding atoms. With the
exception of the solvate H atoms in 5 and 6 and all water hydrogens
in 1, which were not located, all other H atoms of the lattice and
coordinated water in the structures of 2-6 were first located on
difference Fourier maps, and then fixed at calculated positions and
included in the final refinement. Isotropic displacement parameters
of all H atoms were derived from the parent atoms. Further
crystallographic details are summarized in Table 1.
Syntheses of the Complexes. {[Co(bpt)(Htma)(H2O)3]‚2.25-
H2O}n (1). An aqua solution (5 mL) of Co(OAc)2‚4H2O (25.0 mg,
0.10 mmol) was added to a hot EtOH (30 mL) solution of bpt (24.0
mg, 0.10 mmol) with stirring for 10 min. Then, a solution of H3-
tma (22.0 mg, 0.10 mmol) in EtOH (5 mL) was added to the above
mixture with continuous stirring for about 30 min. The resultant
solution was filtered and left to stand at room temperature. Pink
block crystals suitable for X-ray analysis were produced by slow
evaporation of the solvent for 5 days in a 15% yield (9.0 mg, based
on bpt). Anal. Calcd for C21H24.5CoN6O11.25: C, 42.36; H, 4.12; N,
14.01. Found: C, 42.44; H, 4.00; N, 14.21%. IR (KBr, cm-1):
3338m, 3100w, 1699vs, 1631vs, 1540w, 1458w, 1252vs, 1214m,
1175vs, 1100vs, 926m, 836s, 772m, 747s, 684s, 625m.
[Co(bpt)(Htma)(H2O)]n (2). A mixture of Co(OAc)2‚4H2O (30.0
mg, 0.12 mmol), bpt (24.0 mg, 0.10 mmol), H3tma (21.0 mg, 0.10
mmol), and water (10 mL) was sealed in a Teflon-lined stainless
steel vessel (20 mL), which was heated at 140 °C for 5 days and
cooled to room temperature at a rate of 5 °C/h. Purple block crystals
of 2 were collected in a 67% yield (35 mg, based on bpt). Anal.
Calcd for C21H16CoN6O7: C, 48.20; H, 3.08; N, 16.06. Found: C,
47.83; H, 3.02; N, 15.96. IR (KBr, cm-1): 3329m, 3083bs, 1690vs,
1611vs, 1548s, 1443s, 1365vs, 1280s, 1095w, 923w, 1720m,
1610vs, 1555s, 1362s, 1259s, 1097m, 1065w, 933m, 833s, 752s,
695s, 510s. 841m, 701m.
Results and Discussion
Preparation and General Characterization of Com-
plexes 1-6. Two different synthetic methods were applied
in this work: solvent evaporation of the MII-H3tma/bpt
mixture at room temperature and hydrothermal conditions.
For CoII and CdII, two different products (1 and 2 for CoII,
5 and 6 for CdII) were obtained via the two approaches, and
for NiII and ZnII, only species 3 and 4 were produced.
Interestingly, if the purple crystals of 2 were kept in the
mother liquid, after a period of ca. 1 week, a small amount
of pink crystals of 1 were observed (identified by single-
crystal X-ray diffraction). The compositions of all the new
materials were determined by microanalytical and IR tech-
niques. The phase purities of the bulk samples were identified
by X-ray powder diffraction (XRPD, see Supporting Infor-
mation, Figures S1-S6). Complexes 1-6 are air stable and
can retain their structural integrity at room temperature for
a considerable length of time. Notably, ZnII complex 4 could
also be isolated using the same preparation method; however,
Zn(OAc)2 was employed as the source of zinc (confirmed
by X-ray diffraction, IR spectra, and elemental analyses).
[Zn(bpt)2(H2tma)2]‚6H2O (4) and {[Cd(bpt)(Htma)(H2O)]‚
(C2H5OH)(H2O)1.5}n (5). The same synthetic procedure as that for
1 was used except that Co(OAc)2‚4H2O was replaced by Zn(ClO4)2‚
6H2O for 4 and Cd(NO3)2‚4H2O for 5, giving colorless block
crystals, in 47% and 30% yields, respectively. Anal. Calcd for
C42H42ZnN12O18 (4): C, 47.22; H, 3.96; N, 15.73. Found: C, 47.04;
H, 3.94; N, 15.68. IR (KBr, cm-1): 3637m, 3409s, 3336s, 3154m,
1700vs, 1626vs, 1585m, 1443m, 1386m, 1243s, 1173m, 1102m,
1063m, 1006m, 841m, 746s, 706m, 683m, 615m, 569m, 439m.
Anal. Calcd for C23H25CdN6O9.5 (5): C, 42.51; H, 3.88; N, 12.94.
Found: C, 42.10; H, 3.46; N, 12.88. IR (KBr, cm-1): 3637m,
3409s, 3336s, 3154m, 1700vs, 1626vs, 1585m, 1443m, 1386m,
1243s, 1173m, 1102w, 1063w, 1006s, 841s, 746s, 706s, 683m,
615m, 569m, 507m.
[Ni(bpt)(Htma)(H2O)]n (3) and {[Cd(bpt)(Htma)(H2O)2]‚
5.5H2O}n (6). The same synthetic method as that for 2 was used
except that Co(OAc)2‚4H2O was replaced by Ni(OAc)2‚4H2O for
3 and Cd(NO3)2‚4H2O for 6, yielding green block crystals for 3
and colorless needle crystals for 6. Yield: 57% for 3; 29% for 6.
Anal. Calcd for C21H16NiN6O7 (3): C, 48.22; H, 3.08; N, 16.07.
(17) SAINT; Bruker AXS: Madison, WI, 1998. (b) Sheldrick, G. M.
SHELXTL NT, version 5.1; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(16) Bentiss, F.; Lagrenee, M.; Traisnel, M.; Mernari, B.; Elattari, H. J.
Heterocycl. Chem. 1999, 36, 149.
4000 Inorganic Chemistry, Vol. 45, No. 10, 2006

Assembly of Metal-Directed Coordination Polymers
Table 1. Crystallography Data and Structure Refinement Summary for
Complexes 1-6
1
2
3
chemical formula
fw
cryst size (mm)
C
599.9
21H24.5CoN6O11.25
C
21H16CoN6O7
C
21H16N6NiO7
523.33
523.11
0.37 × 0.15 × 0.11 0.37 × 0.29 × 0.09 0.23 × 0.22 ×
0.09
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/n
10.5223(12)
19.109(2)
13.7250(15)
90
91.488(2)
90
2758.8(5)
4
monoclinic
P21/c
8.591(3)
14.942(4)
16.870(5)
90
94.883(4)
90
2157.9(11)
4
monoclinic
P21/c
8.5256(8)
15.0853(14)
16.7476(16)
90
94.5770(10)
90
2147.1(3)
4
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
F
calcd (g/cm3)
1.444
0.689
1238
1.611
0.854
1068
1.618
0.963
1072
µ (mm-1
F(000)
)
total/independent 14 711/4826
reflns
11 548/3802
11 524/3782
params
389
0.0362
0.0611, 0.1848
1.054
0.867, -0.328
317
0.0213
0.0289, 0.0882
1.090
0.325, -0.234
321
0.0249
0.0299, 0.0846
1.037
0.326, -0.287
Rint
Ra, Rw
b
GOFc
residuals (e Å-3
)
4
5
6
chemical formula
fw
cryst size (mm)
C
42H42ZnN12O18
C
23H25CdN6O9.5
C
21H29CdN6O13.5
1068.25
649.89
693.90
0.30 × 0.19 × 0.10 0.30 × 0.22 × 0.10 0.28 × 0.17 ×
0.06
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
C2/c
31.351(5)
8.4407(12)
20.373(3)
90
122.321(2)
90
4556.0(11)
4
triclinic
P1h
triclinic
P1h
10.2402(11)
11.0268(10)
13.0807(12)
75.9540(10)
70.6970(10)
72.9590(10)
1315.3(2)
2
10.1568(18)
12.287(2)
13.938(2)
114.155(4)
100.316(7)
102.903(8)
1473.8(4)
2
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
F
calcd (g/cm3)
1.557
0.631
2208
1.641
0.895
658
1.564
0.814
706
µ (cm-1
F(000)
)
total/independent 12 774/4004
reflns
7224/4584
7828/5094
params
332
0.0267
0.0314, 0.0902
1.066
0.289, -0.338
371
0.0126
0.0234, 0.0632
0.994
0.441, -0.339
388
0.0299
0.0543, 0.1497
1.025
0.846, -0.913
Rint
Ra, Rw
b
GOFc
residuals (e Å-3
)
2 2 1/2
a R ) ∑||Fo| - |Fc||/∑|Fo|. b Rw ) [∑[w(Fo2 - Fc2)2]/∑w(Fo ) ]
.
c GOF
2
2
) {∑[w(Fo - Fc )2]/(n - p)}1/2
.
Figure 1. (a) Portion view of 1 with atom labeling of the asymmetric
unit and metal coordination (lattice water molecules are omitted for clarity).
(b) 1-D Htma-bridged chain with bpt decorated around each side. (c) A
perspective view of the 2-D layered structure of 1 via interchain hydrogen
bonds.
This is also the case for 5 and 6, suggesting that the final
products are independent of the counteranions of the metal
salts.
Structural Analysis of Complexes 1-6. Selected bond
distances and angles for complexes 1-6 are listed in Table
S1, and possible hydrogen-bonded parameters for these
structures are presented in Table S2 (see Supporting Infor-
mation).
the crystallographic [010] direction. The crooked angle of
the two carboxylates within each Htma makes the chain a
wavelike arrangement, and the adjacent Co‚‚‚Co separation
is 9.723(1) Å. Notably, the terminal bpt ligands decorate the
chain alternately in an outward fashion along two sides.
These coordinated chains are extended into a 2-D sheet
through interchain O-H‚‚‚N hydrogen bonds between the
carboxyl and “free” pyridyl nitrogen of bpt (see Figure 1c).
Thus, if the strong hydrogen bonding is also considered, then
both the Htma ligands and metal centers act as three-
connecting nodes to produce an unusual 2-D 4.82 network.18
Furthermore, such sheets are stacked in a parallel fashion
{[Co(bpt)(Htma)(H2O)3]‚2.25H2O}n (1) and [Co(bpt)-
(Htma)(H2O)]n (2). In complex 1, each CoII atom is six-
coordinated and exhibits a distorted octahedral geometry.
Three oxygens and a nitrogen atom from two Htma, one
water donor, and a monodentate bpt ligand form the
equatorial plane, and the other two water ligands occupy axial
positions with an O7-Co1-O8 angle of 176.76(14)° (see
Figure 1a). As depicted in Figure 1b, the Htma dianions act
as bidentate bridging ligands, jointing the CoII centers
together to result in a 1-D polymeric [Co(Htma)]n chain along
(18) Wells, A. F. Three-Dimensional Nets and Polyhedra; Wiley: New
York, 1977.
Inorganic Chemistry, Vol. 45, No. 10, 2006 4001

Du et al.
with a little offset. As illustrated in Figure S7 (see Supporting
Information), the centroid-to-centroid and face-to-face dis-
tances between the neighboring bpt and Htma segments are
ca. 3.63 and 3.24/3.39 Å (with a dihedral angle of 5.8°),
indicating the presence of significant interlayer π-π stacking
interactions.
In the 2-D metal-organic network of 2, the coordination
sphere of each central CoII atom is provided by four oxygens
and two nitrogens from two Htma, one water molecule, and
two bpt ligands, displaying a distorted octahedral geometry
(Figure 2a). The deviation of CoII from the mean coordination
plane, O5B-O6B-O1-N6A, is 0.004 Å, and the axial O7-
Co1-N1 angle is 172.14(6)°. The CoII centers are bridged
by Htma (Co‚‚‚Co distance ) 8.756(2) Å), in which the
carboxylates take the chelating bidentate and monodentate
coordination modes, to produce wavelike 1-D polymeric
chains, which are further extended by exo-bidentate bpt
connectors (Co‚‚‚Co distance ) 14.226(10) Å) to produce a
greatly corrugated 2-D (4,4) net (Figure 2b). Interestingly,
investigation of the crystal packing shows that the Htma
moieties, located up and down each 2-D array, insert into
the lateral voids of the adjacent networks to produce an
unusual interdigitated tactic motif (Figure 2c). In addition,
the whole supramolecular lattice is further fixed via interlayer
(O3-H3A‚‚‚O4) and intralayer (O7-H7A‚‚‚O2, O7-H7B‚
‚‚N3, N5-H5A‚‚‚O5, and N5-H5B‚‚‚O2) hydrogen bonds
between the water, carboxylate, and amido groups (see Table
S2 for details).
[Ni(bpt)(Htma)(H2O)]n (3). Single-crystal X-ray diffrac-
tion analysis shows that complex 3 (Figure S8) is isomor-
phous to 2. Two related 3-D porous frameworks [Ni6(tma)4-
(bipy)6(CH3OH)3(H2O)9‚(guest)] and [Ni3(tma)2(bipy)3(C2-
H6O2)3(H2O)3‚(guest)] (bipy ) 4,4′-bipyridine) have been
prepared by a layer-separation diffusion method.19a In both
structures, the fully deprotonated tma trianions bridge the
NiII centers to form rippled [Ni3(tma)2]n (6,3) layers, which
are further extended by bipy pillars to generate 3-D
architectures with large channels of 74% extraframework
volume. This significant difference in structure of 3 may be
attributed to the deprotonated levels of H3tma. A more related
example, namely, [Ni2(bipy)3(Htma)2(H2O)](H2O), was syn-
thesized under conditions similar to those for 3, and it also
displays a 2-D coordinated net bridged by Htma and bipy;
however, it is not as corrugated as 3.19b This should result
from the bent backbone of bpt. Of further interest, when
naphthalene is introduced in the synthetic procedure, diver-
sification of the hydrothermal products from 1-D chains, 2-D
layers, and 3-D pillar-layered porous frameworks was
found,19b,19c in which naphthalene plays key and different
roles on the basis of the compositions of the starting reagents.
[Zn(bpt)2(H2tma)2]‚6H2O (4). Complex 4 has a windmill-
like momonuclear structure that is disposed about a 2-fold
axis. The central ZnII atom is tetrahedrally coordinated
(ZnO2N2) to two H2tma and two bpt units, adopting the
Figure 2. (a) View of 2 with atom labeling of the asymmetric unit and
metal coordination. (b) 2-D corrugated coordination network of 2. (c) A
packing diagram of 2 showing the interdigitation of the 2-D nets.
unidentate binding form and a trans arrangement (see Figure
4a). In each monodeprotonated H2tma anion, one carboxyl
group (O3-C17-O4) is hydrogen bonded (O3-H3‚‚‚N1)
to a “free” pyridyl nitrogen terminal. Thus, these monomeric
molecules are linked to result in a 2-D (4,4) layered net with
large rhombus grids (21.9 × 21.9 Å2), as illustrated in Figure
4b. Interlayer N5-H5A‚‚‚O5 hydrogen bonds are detected
between the carboxyl and amido groups. The lattice water
entities are located up and down the layers and form multiple
(19) Prior, T. J.; Bradshaw, D.; Teat, S. J.; Rosseinsky, M. J. Chem.
Commun. 2003, 500. (b) Choi, E. Y.; Kwon, Y. U. Inorg. Chem. 2005,
44, 538. (c) Choi, E. Y.; Kwon, Y. U. Inorg. Chem. Commun. 2004,
7, 942.
4002 Inorganic Chemistry, Vol. 45, No. 10, 2006

Assembly of Metal-Directed Coordination Polymers
Figure 3. (a) Molecular structure of 4 with atom labeling of the asymmetric
unit and metal coordination (lattice water moieties are omitted for clarity).
(b) A perspective view of the 2-D hydrogen-bonded network of complex
4.
water-water and water-layer hydrogen bonding interactions
(see Table S2 for details), which further stabilize this
structure.
{[Cd(bpt)(Htma)(H2O)]‚(C2H5OH)(H2O)1.5}n (5) and
[Cd(bpt)(Htma)(H2O)2]‚5.5H2O}n (6). Complex 5 features
a 3-D porous framework arising from 2-D coordination sheets
interlinked through interlayer hydrogen bonds. The central
CdII atom has a distorted octahedral coordination sphere
defined by three oxygens from two separated Htma ligands,
one aqua donor, and two nitrogens from a pair of bpt
molecules (see Figure 4a). The O1, O2, O4A, and O7 atoms
form the basal equatorial plane, and two nitrogen atoms take
up the apical positions with a N6-Cd1-N1B angle of
164.44(10)°. Such fundamental building units are linked to
furnish a 2-D layer with (4,4) topology via metal coordina-
tion, in which the Cd‚‚‚Cd lengths bridged by Htma and bpt
are 10.240(1) and 14.923(9) Å, respectively. In addition, a
pair of adjacent 2-D patterns are antiparallel and interdigi-
tated; the lateral Htma anions that orient on the same side
of each layer penetrate into the 2-D grids, forming a
supramolecular “double-layer” motif (see Figure 4b). Inter-
layer N5-H5B‚‚‚O4 hydrogen bonds are detected to sustain
Figure 4. (a) View of 5 with atom labeling of the asymmetric unit and
metal coordination (lattice solvates are omitted for clarity). (b) A view of
a pair of opposite 2-D (4,4) coordination nets with interdigitated alignment.
(c) Space filling model of the 3-D packing of 5 showing rectangular
channels. The adjacent double-layer arrays are represented in different colors.
this array (see Figure S9). Interestingly, the water ligands
of each double-layer motif are hydrogen bonded (O7-H7A‚
‚‚O6 and O7-H7B‚‚‚O3) to the carboxyl groups of an
adjacent such motif (Figure S9) to generate an extended 3-D
framework (see Figure 4c). When viewed along the [010]
direction, square channels are observed which are occupied
by the disordered solvates. Computation of the channel space
using PLATON20 suggests a value of 279 Å3, corresponding
to 21.2% of the unit-cell volume. When an allowance is made
(20) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.
Inorganic Chemistry, Vol. 45, No. 10, 2006 4003

Du et al.
form a 1-D linear array with bpt decorated on one side as
terminal ligands (Figure 5b). The adjacent intrachain Cd‚‚‚
Cd distance is 10.157(2) Å. Similar to 1, interchain O3-
H3A‚‚‚N1 hydrogen bonds between the carboxyl and pyridyl
groups also link these 1-D chains to generate a 2-D hydrogen-
bonded pattern, which, however, has a (6,3) network topol-
ogy this time because of the discrepancy of two 1-D arrays
in 1 and 6. Moreover, the adjacent 2-D nets are combined
into a double-layered architecture via interlayer O7-H7A‚
‚‚O3 interactions (see Figure 5c).
Obviously, the structural diversification of this series of
mixed-ligand polymeric complexes should be attributed to
the nature of the metal coordination as well as the synthetic
approaches. The distinct structures of the CoII and CdII
complexes obtained under ambient or hydrothermal processes
suggest that the synthetic condition can remarkably influence
the formation of the final crystalline products. As a whole,
from the viewpoint of coordination chemistry, Htma and bpt
can behave as both bridging and terminal ligands in these
complexes. For 2-D coordination polymers 2, 3, and 5, the
metal surroundings are taken up by two pairs of bpt/Htma
and a water ligand, and the expansion of the 2-D nets along
two directions is fulfilled via bpt/Htma connectors. For 1
and 6, two Htma, one bpt, and water molecules occupy the
metal coordination sites to reduce the valid linkage of the
metal nodes in network generation. As a result, 1-D
polymeric Htma-bridged chains are shaped with bpt only
acting as terminal pendants. Additionally, the monodentate
binding of H2tma and bpt with ZnII lead to the formation of
mononuclear complex 4. Investigation of these structures at
a supramolecular level shows that different packing modes
of 2 (or 3) and 5 produce 3-D interdigitated or porous
architectures. Interestingly, in the other three cases, hydrogen
bonding between the carboxyl of Htma/H2tma and the free
pyridyl nitrogen of bpt link the 1-D or discrete coordination
motifs to result in 2-D networks, however, with distinct 4.82,
(4,4) or (6,3) topology, respectively. The synergistic effect
of coordinative and second-sphere interactions will be
responsible for the achievement of these well-regulated
supramolecular structures.
Thermal Stability of the New Materials. Thermogravi-
metric experiments were conducted to study the thermal
stability of 1-6, which is an important parameter for metal-
organic framework materials. The TGA curve of 1 suggests
that the first weight loss of 7.02% in the region of 40-123
°C (peak at 114 °C) corresponds to the expulsion of the
lattice water molecules (calculated 6.75%). The residual
framework starts to decompose beyond 262 °C with a series
of complicated weight losses (peaks at 328, 381, 463 and
704 °C) and does not stop until heating ends at 800 °C. The
TGA curves of 2 and 3 are similar, probably because of their
isostructural nature. The first weight loss of 3.78% in the
152-220 °C range for 2 (3.63%, 122-227 °C for 3) is
attributed to the removal of the coordinated water molecules
(calculated 3.44%). The remaining substance is stable up to
310 °C for 2 (308 °C for 3) followed by three consecutive
steps of weight loss (peaks at 381, 449 and 708 °C for 2
and 408, 433 and 703 °C for 3) and does not stop until
Figure 5. (a) View of 6 with atom labeling of the asymmetric unit and
metal coordination (lattice water molecules are omitted for clarity). (b) 1-D
Htma-bridged coordination chain with bpt decorated around one side. (c)
A perspective view of the 2-D hydrogen-bonded double-layered network
(the two layers are represented in different colors).
for the volume of the included solvate molecules, there is
only 17 Å3 of void space remaining. Additionally, there also
are other O-H‚‚‚O and N-H‚‚‚O interactions in this
structure (see Table S2 for details).
Complex 6 displays a polymeric chain coordination motif.
As depicted in Figure 5a, the distorted pentagonal-dipyra-
midal coordination environment of the central CdII atom is
provided by a pair of chelated carboxylate groups, one
pyridyl nitrogen from bpt in the equatorial plane and two
aqua moieties from apical sites. The CdII centers are
propagated along the [100] direction via Htma bridges to
4004 Inorganic Chemistry, Vol. 45, No. 10, 2006

Assembly of Metal-Directed Coordination Polymers
heating ends at 800 °C. For the TGA curve of 4, the loss of
lattice water moieties is not observed before 282 °C, where
the decomposition starts, maybe because of the multiple
strong hydrogen bonding interactions in the crystalline lattice.
Further heating of the sample to 800 °C only reveals a
continuous and slow weight loss. For the 3-D porous network
5, a weight loss of 11.60% in the 46-165 °C (peak at 95
°C) corresponds to the release of the lattice water and ethanol
guests (calculated 11.23%). Then the decomposition of the
residual components starts beyond 195 °C with a series of
complicated weight losses (peaks at 336, 382, 411 and 507
°C) and does not end until heating stops at 800 °C. For 6,
the weight loss of the lattice water (13.81%) occurs in the
range of 45-260 °C (calculated 14.27%). The main frame-
work remains intact until it is heated to 308 °C and then
there are four steps of weight loss (peaks at 332, 386, 436
and 520 °C) that do not end until 800 °C.
Removal and Reintroduction of Guest Solvates. Metal-
organic coordination polymers with specific channels or
cavities have attracted considerable interest because of their
potential applications in catalysis, adsorption, and ion
exchange.21 Complex 5 represents a fascinating 3-D su-
pramolecular array exhibiting 1-D open cavities filled with
solvates, as described above. The TGA results suggest that
the ethanol and water guests could be excluded from the
host framework upon heating. To further investigate the
desorption/absorption properties of 5, a freshly ground
sample (97.6 mg) was placed inside a vacuum oven at 170
°C for 8 h. The sample experienced a weight loss of 11.2
mg, consistent with the removal of all included lattice
solvates (1.5H2O + C2H5OH) per formular unit (calculated,
11.0 mg). The XRPD pattern recorded at this point (see
Figure 6) shows significant shifts of the sharp diffraction
peaks compared with that of the original product. This
indicates that the structure of the desolvated solid of 5 is
changed; however, the high periodicity of this crystalline
material still remains. Interestingly, the desolvated sample
can regain the guest molecules and revert to the original
structure by soaking in ethanol/water for 24 h. This is
confirmed by XRPD (see Figure 6), the weight gain of the
material (10.8 mg), and elemental analyses. Therefore, the
framework integrity of 5 can be maintained after a desorp-
tion/adsorption cycle, and the desolvated solid of 5 may be
useful as a potential adsorbent material for small guest
molecules.
Figure 6. XPRD patterns for 5 recorded (green) at room temperature,
(red) after removal of the guest solvates, and (blue) after reintroduction of
the guest molecules.
nm, corresponding to the π* f n transitions. For complexes
4, 5, and 6, the maximum emission bands are at 380, 381
and 382 nm, respectively, which should come from intrali-
gand π f π* transitions. The blue-shift for such fluorescence
peaks, compared to that of free bpt (388 nm), is the result
of the incorporation of metal-ligand coordination interac-
tions. Furthermore, considerable enhancement of the intensity
for these peaks in the metal complexes may be attributed to
the increased rigidity of the ligand when it is bound to a
metal center, compared with that of the free one, which
effectively reduces the loss of energy.22 Additionally, the
spectra of 4-6 also display a similar shoulder peak at ca.
360 nm, which probably results from the interplay of the
intraligand transitions of two types of ligands. These results
Photoluminescence Properties. The emission spectra of
ZnII and CdII (d10 metal) complexes 4-6, together with those
of the organic ligands bpt and H3tma, in the solid state at
room temperature are shown in Figure 7. The photolumi-
nescence properties of bpt suggest a weak emission peak
and an intense emission peak at 362 and 388 nm, respec-
tively. The strongest emission peak for H3tma appears at 365
(21) Sun, J.; Weng, L.; Zhou, Y.; Chen, J.; Chen, Z.; Liu, Z.; Zhao, D.
Angew. Chem., Int. Ed. 2002, 41, 4471. (b) Pan, L.; Liu, H.-M.; Lei,
X.-G.; Huang, X.-Y.; Olson, D. H.; Turro, N. J.; Li, J. Angew. Chem.,
Int. Ed. 2003, 42, 542. (c) Cui, Y.; Lee, S. J.; Lin, W. J. Am. Chem.
Soc. 2003, 125, 6014. (d) Kitagawa, S.; Kitaura, R.; Noro, S. Angew.
Chem., Int. Ed. 2004, 43, 2334. (e) Du, M.; Zhao, X.-J.; Guo, J.-H.;
Batten, S. R. Chem. Commun. 2005, 4836.
(22) Yi, L.; Zhu, L.-N.; Ding, B.; Cheng, P.; Liao, D.-Z.; Yan, S.-P.; Jiang,
Z.-H. Inorg. Chem. Commun. 2003, 6, 1209. (b) Cai, L.-Z.; Chen, W.-
T.; Wang, M.-S.; Guo, G.-C.; Huang, J.-S. Inorg. Chem. Commun.
2004, 7, 611.
Inorganic Chemistry, Vol. 45, No. 10, 2006 4005

Du et al.
discrete or infinite coordination structures. On the other hand,
the “free” carboxyl/pyridyl groups can also take part in the
formation of specific hydrogen bonds to extend and sustain
the final architectures. At this juncture, coordinative forces,
as well as weak interactions, play a key role in structural
assembly. Without doubt, this investigation tells us that the
synthetic conditions can significantly influence the target
products. However, at this stage, how to manipulate them
in a certain direction is still a great challenge and more work
is still need to extend this knowledge. In summary, this work
is expected to provide new and important information for
understanding and developing crystal engineering of metal-
organic frameworks. From this perspective, further systematic
studies for the design and synthesis, such coordination
crystalline materials with bpt and other aromatic multicar-
boxylic acids, are under way in our lab.
Figure 7. Solid-state fluorescence spectra recorded at room temperature
for bpt, H3tma, and complexes 4-6.
Acknowledgment. This work was financially supported
by the National Natural Science Foundation of China
(20401012), the National Fundamental Research Project of
China (2005CCA01200), the Key Project of Tianjin Natural
Science Foundation (043804111), and the Key Project of
Chinese Ministry of Education (205008).
indicate that such metal complexes could be good candidates
for potential photoactive materials.
Conclusions and Perspectives
A new family of six metal-directed polymeric coordination
complexes with mixed-ligand H3tma and bpt has been
prepared using diverse synthetic approaches, which exhibit
interesting structural diversification. The mixed-ligand strat-
egy seems to be a successful way to assemble programmed
coordination frameworks. On one hand, H3tma can be
deprotonated at different levels, so it may behave as either
a terminal or bridging connector (as well as bpt) to construct
Supporting Information Available: X-ray crystallographic files
(CIF) for 1-6, calculated and experimental X-ray powder diffrac-
tion (XRPD) patterns for 1-6, three additional figures for
complexes 1, 3, and 5, and tables of bond parameters and hydrogen-
bonded geometries for complexes 1-6. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC060001D
4006 Inorganic Chemistry, Vol. 45, No. 10, 2006