Syntheses, structures and properties of three novel coordination polymers with a flexible asymmetrical bridging ligand

Article abstract of DOI:10.1016/j.ica.2005.12.077

Three novel coordination polymers with different structural motifs, [Ag(pmtmb)]_n (1), [Cd(pmtmb)₂(H₂O)]_n (2) and [Cu(pmtmb)₂]_n (3), have been synthesized with a flexible asymmetrical bridging ligand, 4-(2-pyrimidylthiomethyl)benzoic acid (Hpmtmb). X-ray diffraction analyses show that 1 is a 2D layer containing unusual zigzag Ag chains based on mixed ligand-supported and ligand-unsupported Ag-Ag interactions, 2 is a necklace structure that further linked through hydrogen bonding to form a 2D sheet, and 3 is a 3D compact framework full assembled from 1D centipede-like chains via π-π stacking interactions. Interestingly, the ligand pmtmb adopts different configurations and coordination modes in the solid structures of these complexes. Furthermore, three complexes exhibit remarkable thermal stability and the complexes 1 and 2 exhibit intense green and purple luminescence, respectively.

Full text of DOI:10.1016/j.ica.2005.12.077

Inorganica Chimica Acta 359 (2006) 2232–2240
www.elsevier.com/locate/ica
Syntheses, structures and properties of three novel
coordination polymers with a flexible asymmetrical bridging ligand
*
Lei Han, Daqiang Yuan, Benlai Wu, Caiping Liu, Maochun Hong
State Key Laboratory of Structural Chemistry, Fujian Institute of the Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, Fujian 350002, China
Received 14 November 2005; received in revised form 26 December 2005; accepted 26 December 2005
Available online 13 March 2006
Abstract
Three novel coordination polymers with different structural motifs, [Ag(pmtmb)]n (1), [Cd(pmtmb)2(H2O)]n (2) and [Cu(pmtmb)2]n
(3), have been synthesized with a flexible asymmetrical bridging ligand, 4-(2-pyrimidylthiomethyl)benzoic acid (Hpmtmb). X-ray diffrac-
tion analyses show that 1 is a 2D layer containing unusual zigzag Ag chains based on mixed ligand-supported and ligand-unsupported
Ag–Ag interactions, 2 is a necklace structure that further linked through hydrogen bonding to form a 2D sheet, and 3 is a 3D compact
framework full assembled from 1D centipede-like chains via p–p stacking interactions. Interestingly, the ligand pmtmb adopts different
configurations and coordination modes in the solid structures of these complexes. Furthermore, three complexes exhibit remarkable ther-
mal stability and the complexes 1 and 2 exhibit intense green and purple luminescence, respectively.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Coordination polymer; Crystal structures; Coordination modes; Luminescence
1. Introduction
of complex structures [6–9]. However, the crystal
engineering of coordination frameworks with desired
topologies and specific properties still remains a difficult
challenge since it depends on a variety of factors that
can influence the self-assembly process [10–14]. One of
the rare and interesting phenomena occurring during
self-assembly is a conformational change of a flexible
building block, which may result in the increase of
enthalpy of the building block [15–21]. For example, com-
pared to a rigid building block, a flexible building block
with conformational freedom often shows a supramolecu-
lar isomerism by altering the geometry of the building
block [22]. In this respect, we are currently using
asymmetrical bridging ligands as building blocks and
investigating their crystal engineering via metal-directed
self-assembly [23–27] because the asymmetrically ligands
can be assembled around metal centers in diverse arrange-
ments, consequently resulting in unprecedented topologi-
cal features with novel photophysical properties in
asymmetric heterogeneous catalysis and second-order
nonlinear optical materials [28–34].
Since the synthesis of the first metal–organic frame-
work prior to the mid 1990s was driven mostly by topo-
logical interest [1],
a large number of prominent
coordination polymers with cationic, anionic, and neutral
frameworks have been successfully designed and synthe-
sized through judicious combination of metal ‘node’ and
ligand ‘spacer’ owing to their aesthetic structural and
fascinating functional motifs involving magnetism, cataly-
sis, nonlinear optics, and molecular sensing [2–5]. The
selection or design of suitable ligands containing certain
features, such as flexibility, versatile binding modes, and
the ability to form hydrogen bonding, is crucial to the
construction of metal–organic frameworks, and that the
combination of both p–p interactions and hydrogen
bonding has proved to be particularly useful for assembly
*
Corresponding author. Tel.: +86 591 83792460; fax: +86 591
83714946.
E-mail address: hmc@ms.fjirsm.ac.cn (M. Hong).
0020-1693/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.12.077

L. Han et al. / Inorganica Chimica Acta 359 (2006) 2232–2240
2233
10 °C/min. Emission spectra were recorded on a Perkin–
Elmer LS55 Luminescence spectrometer.
2.2. Synthesis of the ligand Hpmtmb
2-Mercaptopyrimidine (1.122 g, 10 mmol) was added to
a stirred solution of KOH (0.56 g, 10 mmol) in H2O
(20 mL). The mixture was warmed to reflux, then a-
bromo-p-toluic acid (2.15 g, 10 mmol) was added and the
mixture was refluxed for about 10 h with vigorous stirring.
After being cooled, the mixture was left to stand overnight.
The precipitate was filtered off and washed with water, giv-
ing a fine white powder in 90% yield. Anal. Calc. for
C12H10N2O2S: C, 58.52; H, 4.09; N, 11.37. Found: C,
58.56; H, 4.11; N, 11.30%. IR (KBr) m: 3120 (w), 2914
(m), 2788 (m), 2626 (m), 2516 (m), 1710 (vs), 1610 (m),
1558 (vs), 1421 (m), 1378 (vs), 1280 (s), 1188 (s), 1115
1
(m), 812 (m), 767 (s), 733 (m), 642 (m) cmꢀ1. H NMR
(500 MHz, DMSO-d6): d 4.475 (s, 2H, CH2), 7.239–7.258
(t, J1 = 4.5 Hz, J2 = 5 Hz, 1H, pym), 7.546–7.563 (d,
J = 8.5 Hz, 2H, ph), 7.870-7.887 (d, J = 8.5 Hz, 2H, ph),
8.658–8.668 (d, J = 5 Hz, 2H, pym), 12.916 (s, 1H,
COOH).
Scheme 1. The ligand Hpmtmb and its coordination modes in the
reported structures.
2.3. Synthesis of the complex 1
A mixed CH3OH/CHCl3 (1:1) solution (10 ml) of
Hpmtmb (50 mg, 0.2 mmol) was transferred to a glass tube
in presence of NEt3. A CH3CN solution (10 ml) of AgNO3
(35 mg 0.2 mmol) was poured into the tube without mixing
the two solutions. After 5 days at room temperature, a
quantity of colorless single crystals appeared at the bound-
ary between the two solutions. The yield of 1 is ca. 70%
based on Hpmtmb. Anal. Calc. for [C12H9AgN2O2S]: C,
40.81; H, 2.57; N, 7.93. Found: C, 40.73; H, 2.61; N,
7.88%. IR (KBr) m: 2921 (m), 1586 (s), 1567 (vs), 1537
(s), 1380 (vs), 1206 (m), 1186 (s), 1020 (m), 794 (m), 769
In our continuous investigations on self-assembly with
asymmetrical bridging ligands, herein, we wish to report
on the exploitation of a new bridging ligand, 4-(2-pyr-
imidylthiomethyl)benzoic acid (Hpmtmb) (Scheme 1).
Thus, the assembly of Hpmtmb with transition metal ions
under appropriate conditions afforded three novel coordi-
nation polymers with different structural motifs: 2D layer
[Ag(pmtmb)]n (1), 1D necklace [Cd(pmtmb)2(H2O)]n (2)
and 3D compact framework [Cu(pmtmb)2]n (3), respec-
tively, in which a variety of coordination modes and differ-
ent conformations of pmtmb were observed. The
photoluminescent properties for 1 and 2, and thermal sta-
bility of these complexes have also been measured,
respectively.
(m), 719 (m), 632 (w) cmꢀ1
.
2.4. Synthesis of the complex 2
A mixture of Cd(NO3)2 Æ 4H2O (31 mg, 0.1 mmol) with
Hpmtmb (50 mg, 0.2 mmol) in 10 mL of H2O was sealed
in a stainless-steel reactor with a Teflon liner and heated
at 110 °C for 72 h. A quantity of colorless single crystals
was obtained after the solution was cooled to room temper-
ature at a rate of 10 °C/h. The yield of 2 is ca. 65% based
on Hpmtmb. Anal. Calc. for [C12H10Cd0.5N2O2.5S]: C,
46.42; H, 3.25; N, 9.02. Found: C, 46.50; H, 3.32; N,
9.06%. IR (KBr) m: 3062 (w), 2924 (w), 1611 (m), 1588
(s), 1568 (s), 1548 (vs), 1524 (vs), 1401 (s), 1379 (vs), 1255
2. Experimental
2.1. Materials and physical techniques
All the reagents for syntheses were commercially avail-
able and employed without further purification or with
purification by standard methods prior to use. Elemental
analyses (C, H, N) were performed on a Perkin–Elmer
model 240C automatic instrument. IR spectra were
recorded on a Magna 750 FT-IR spectrophotometer with
(w), 1179 (s), 863 (m), 768 (m), 638 (m) cmꢀ1
.
1
KBr pellet. H NMR was measured on a Bruker AM500
spectrometer with SiMe4 as the internal reference. Thermal
analyses were performed on a Thermal Analyzer 2100TA
Instrument and SDT2960 Simultaneous TGA–DTA
Instrument under flowing N2 with a heating rate of
2.5. Synthesis of the complex 3
A mixture of Cu(CH3COO)2 Æ H2O (20 mg, 0.1 mmol)
with Hpmtmb (50 mg, 0.2 mmol) in 10 mL of H2O was

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L. Han et al. / Inorganica Chimica Acta 359 (2006) 2232–2240
Table 2
sealed in a stainless-steel reactor with a Teflon liner and
heated at 110 °C for 72 h. A quantity of green single crys-
tals was obtained after the solution was cooled to room
temperature at a rate of 10 °C/h. The yield of 3 is ca.
55% based on Hpmtmb. Anal. Calc. for [C24H18Cu-
N4O4S2]: C, 52.02; H, 3.27; N, 10.11. Found: C, 52.10;
H, 3.34; N, 10.06%. IR (KBr) m: 3124 (w), 3066 (w), 3021
(w), 2916 (w), 1710 (w), 1621 (vs), 1566 (vs), 1552 (vs),
1408 (vs), 1382 (vs), 1180 (s), 1020 (w), 800 (m), 750 (m),
˚
Selected bond lengths (A) and angles (°) for complexes 1–3
Complex 1a
Ag(1)–O(1A)
Ag(1)–O(2A)
Ag(1)–N(1)
2.208(4) Ag(1)–Ag(1C)
2.248(4) Ag(1)–Ag(1D)
2.484(4)
2.9137(10)
3.3362(11)
O(1A)–Ag(1)–O(2B)
O(1A)–Ag(1)–N(1)
O(2B)–Ag(1)–N(1)
O(1A)–Ag(1)–Ag(1C)
O(2B)–Ag(1)–Ag(1C)
162.62(15) O(1A)–Ag(1)–Ag(1D)
109.02(15) O(2B)–Ag(1)–Ag(1D)
87.31(15) N(1)–Ag(1)–Ag(1D)
58.60(12)
128.69(11)
92.22(11)
82.68(11) Ag(1C)–Ag(1)–Ag(1D) 105.47(3)
80.07(10) N(1)–Ag(1)–Ag(1C)
638 (m) cmꢀ1
.
162.19(11)
Complex 2b
Cd(1)–O(3)
Cd(1)–O(2)
2.6. X-ray crystal structural determination
2.264(8) Cd(1)–O(1)
2.330(5) Cd(1)–N(1B)
2.460(5)
2.483(5)
Single crystals of these complexes with approximate
dimensions were mounted on a glass fiber and used for
data collection, respectively. All intensity data were col-
lected on a Siemens Smart CCD diffractometer by x scan
technique at room temperature using graphite-monochro-
O(3)–Cd(1)–O(2)
O(2A)–Cd(1)–O(2)
O(3)–Cd(1)–O(1)
O(2A)–Cd(1)–O(1)
O(2)–Cd(1)–O(1)
O(1)–Cd(1)–O(1A)
83.35(13) O(3)–Cd(1)–N(1B)
96.22(14)
92.13(18)
89.31(18)
92.27(19)
78.58(19)
167.6(3)
166.7(3)
O(2A)–Cd(1)–N(1B)
136.93(11) O(2)–Cd(1)–N(1B)
138.52(17) O(1)–Cd(1)–N(1B)
54.54(17) O(1A)–Cd(1)–N(1B)
˚
86.1(2)
N(1B)–Cd(1)–N(1C)
mated Mo Ka (k = 0.71073 A) radiation. All empirical
absorption corrections were applied by using the SADABS
program [35]. The structures were solved by direct methods
[36] and refined using the SHELXL-97 program package [37].
Metal atoms in each complex were located from E-maps,
and other non-hydrogen atoms were located in successive
difference Fourier syntheses. The final refinement was per-
formed by full-matrix least-squares methods with aniso-
tropic thermal parameters for non-hydrogen atoms on F2.
The positions of H atoms were generated geometrically,
assigned isotropic thermal parameters, and allowed to ride
on their parent carbon atoms before the final cycle of
refinement. Crystal data and structure determination sum-
mary for these complexes are listed in the Table 1. The
selected bond lengths and angles for these complexes are
listed in the Table 2.
Complex 3c
Cu(1)–O(2)
Cu(1)–O(4)
Cu(1)–O(1A)
1.948(5) Cu(1)–O(3A)
1.952(5) Cu(1)–N(3B)
1.958(5) Cu(1)–Cu(1A)
1.985(5)
2.331(6)
2.693(2)
O(2)–Cu(1)–O(4)
86.3(2)
166.9(2)
91.8(2)
90.9(2)
166.7(2)
88.0(2)
99.4(2)
106.8(2)
O(1A)–Cu(1)–N(3B)
93.5(2)
86.5(2)
O(2)–Cu(1)–O(1A)
O(4)–Cu(1)–O(1A)
O(2)–Cu(1)–O(3A)
O(4)–Cu(1)–O(3A)
O(1A)–Cu(1)–O(3A)
O(2)–Cu(1)–N(3B)
O(4)–Cu(1)–N(3B)
O(3A)–Cu(1)–N(3B)
O(2)–Cu(1)–Cu(1A)
O(4)–Cu(1)–Cu(1A)
O(1A)–Cu(1)–Cu(1A)
O(3A)–Cu(1)–Cu(1A)
N(3B)–Cu(1)–Cu(1A)
82.15(16)
85.37(15)
84.77(16)
81.34(15)
167.73(15)
Symmetry transformations used to generate equivalent atoms: aA ꢀx,
y + 1/2, ꢀz ꢀ 1/2; B x, ꢀy + 1/2, z + 1/2; C ꢀx, ꢀy + 1, ꢀz; D ꢀx, ꢀy,
ꢀz. bA ꢀx, y, ꢀz ꢀ 1/2; B x, ꢀy + 1, z ꢀ 1/2; C ꢀx, ꢀy + 1, ꢀz. cA
ꢀx + 1, ꢀy + 1, ꢀz + 1; B ꢀx + 1, ꢀy + 1, ꢀz + 2.
Table 1
Crystallographic data and structure refinement for 1–3
Complex
Empirical formula
Fw
1
2
3
C
12H9AgN2O2S
C12H10Cd0.5N2O2.5
310.48
S
C24H18CuN4O4S2
554.08
green
353.14
colorless
P21/c
12.5640(6)
4.9805(2)
21.0407(5)
90
115.301(2)
90
1190.32(8)
4
1.971
1.862
696
0.0301
1.171
Color
colorless
C2/c
16.6795(11)
6.1977(4)
23.5700(15)
90
93.853(2)
90
2431.0(3)
8
1.697
1.114
1248
0.0453
1.191
0.0523/0.1175
ꢀ
Space group
P1
˚
a (A)
10.050(2)
11.218(2)
12.479(3)
97.78(3)
104.57(3)
114.76(3)
1189.2(4)
2
1.547
1.134
566
0.0400
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
V (A )
Z
Dc (g cmꢀ3
)
l (mmꢀ1
F(000)
Rint
)
S (F2)
R1a/wR2
1.202
0.0742/0.1537
b
0.0447/0.1185
P
P
a
R1 = ||Fo| ꢀ |Fc||/ |Fo|.
P
P
2
2 1=2
b
wR2 ¼ ½ wðF 2o ꢀ F c2Þ = wðF 2oÞ ꢁ
.

L. Han et al. / Inorganica Chimica Acta 359 (2006) 2232–2240
2235
3. Results and discussion
respectively, which are similar to the documented Ag–N
and Ag–O bonds [27]. Though the Ag–N bond lengths
in 1 are longer than those found in silver complex with
triangle geometry, the Ag–N distances are within the
expected range, which indicate the weak coordination of
the pyrimidine N to silver(I) ion. Each ligand acts as a tri-
dentate ligand [Scheme 1 (I)], in which two oxygen atoms
and one nitrogen atom coordinate to three Ag atoms in
two different directions, respectively. Thus, the array of
ligands linked by Ag centers forms a two-dimensional lay-
ered structure (Fig. 2a). The interestingly structural char-
acteristic of the layer is a novel inorganic–organic
coordination polymer with unusual 1D zigzag Ag chains
formed by Ag–Ag interactions. As well known, M(I)–
M(I) (M = Cu, Ag, Au) interactions in reported com-
plexes are generally ligand-supported, the M(I)–M(I)
geometry is maintained by bridging or capping ligands.
Ligand-unsupported M(I)–M(I) (M = Cu, Ag, Au) inter-
actions are scarce and only a few examples of ligand-
unsupported Ag(I)–Ag(I) interactions have been reported
in the literature [38–46], when there are d10–d10 metal–
metal interactions. The structural feature of this Ag chain
is that both the ligand-supported and the ligand-unsup-
ported Ag–Ag interactions exist simultaneously. To our
best knowledge, this zigzag Ag chains based on mixed
ligand-supported and ligand-unsupported Ag–Ag interac-
tions is the first example, although a zigzag Ag chain with
only ligand-unsupported Ag–Ag interactions has been
reported recently [46]. The distance of ligand-supported
Ag–Ag with bridging -CO2 from pmtmb ion is
3.1. Syntheses of complexes
The ligand Hpmtmb was conveniently prepared from
the substitution reaction between 2-mercaptopyrimidine
and a-bromo-p-toluic acid under strong alkaline (KOH)
conditions with higher yield, which followed our previous
strategy utilized to synthesize the ligand 4-(4-pyridylthiom-
ethyl)benzoic acid (Hptmb) [26]. The resulting white pow-
1
der was characterized by elemental analysis, IR and H
NMR. The reaction of Hpmtmb and AgNO3 in the pres-
ence of triethylamine as the proton scavenger through dif-
fusion approach gave rise to the neutral complex 1. The
complexes 2 and 3 were obtained as neutral polymers by
self-assembly of Hpmtmb and Cu2+ or Cd2+ salts under
hydrothermal conditions (110 °C, 72 h), respectively. It is
interestingly noted that compound 2 and 3 could also be
isolated according to the same synthetic procedure, how-
ever, using M(NO3)2, M(Cl)2 or M(OAc)2 (M = Cu, Cd)
as the source of metals, which indicates that the final prod-
ucts are independent of the counter-anions of the metal
salts. Furthermore, complexes 1–3 are air stable and insol-
uble in water or common organic solvents. Complexes 1–3
were also characterized by elemental analysis, IR and X-
ray diffraction.
3.2. Description of crystal structures
3.2.1. Structure of [Ag(pmtmb)]n (1)
˚
The X-ray crystallographic analysis reveals that 1 crys-
tallizes in the monoclinic unit cell space group P21/c with
an asymmetric unit consisting of one formula unit and
therefore there is only one metal environment. As shown
in Fig. 1, the silver(I) ion is coordinated to one pyrimidine
N atom and two carboxylate O atoms from three different
ligands in a distorted triangle geometry. The distances of
2.9137(10) A, which much shorter than the sum of van
˚
der Waals radii (3.40 A) [47]. This indicates the presence
of strong Ag–Ag interactions [38–46]. However, the
ligand-unsupported Ag–Ag interaction is weaker with
˚
the distance being 3.3362(11) A. The angle of Ag–Ag–
Ag in the zigzag chain is 105.47(3)°. In fact, this 2D
polymer can also be considered a floor slab structure con-
taining regular channels, as shown in Fig. 2b.
˚
Ag–N and Ag–O are 2.484(4), 2.208(4) and 2.248(4) A,
3.2.2. Structure of [Cd(pmtmb)2(H2O)]n (2)
X-ray single-crystal determination indicates that com-
plex 2 crystallizes in the monoclinic unit cell space group
C2/c and is a neutral 1D coordination polymer. As shown
in Fig. 3a, each Cd center is in a distorted pentagonal bi-
pyramidal geometry with two asymmetrically chelating car-
boxylate groups from different pmtmb ligands and one
oxygen atom from coordinated water molecule occupying
the equatorial basal plane. The pentagonal bi-pyramidal
around the Cd center is axially elongated and involves
˚
two longer Cd–N bonds [2.483(5) A] with the angle of N–
Cd–N being 167.6(3)°. Two O atoms from the carboxylate
group chelate to the same Cd(II) with considerably differ-
˚
ent Cd–O bond lengths [Cd1–O2 2.330(5) A and Cd1–O1
˚
2.460(5) A] in good agreement with those of those of
˚
2.391(8)–2.462(10) A found in other Cd(II) complexes
Fig. 1. The asymmetric unit representation of 1 showing the local
geometry around the metal center and ligand. Thermal ellipsoids were
drawn at the 50% probability level.
˚
[48]. The Cd–Owater distance [2.264(8) A] is shorter than
the Cd–Ocarboxylate bonds. Different in 1, the carboxylate

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L. Han et al. / Inorganica Chimica Acta 359 (2006) 2232–2240
Fig. 2. (a) Top view of the two-dimensional inorganic–organic layer with infinite Ag chains. (b) Side view of the floor slab with channels.
shown in Fig. 3b. However, in this case the asymmetry
and flexibility of the pmtmb ligand result in alternating
rhombus-shaped units being orientated perpendicular to
each other in adjacent links of the necklace. The adjacent
˚
Cd–Cd separation in the necklace is 11.790 A. The rhom-
bus-shaped cavities are packed to form channels that run
parallel to the b-axis. Moreover, the adjacent necklaces
are linked through hydrogen bonding between the coordi-
nated waters and carboxylate groups in view of Oꢂ ꢂ ꢂO
˚
2.718 A, which results in a 2D sheet network.
3.2.3. Structure of [Cu(pmtmb)2]n (3)
Single-crystal X-ray diffraction analysis reveals that 3
ꢀ
crystallizes in triclinic space group P1. The crystal structure
consists of a Cu2+ cation and two crystallographically inde-
Fig. 3. (a) The asymmetric unit representation of 2 showing the local
geometry around the metal center and ligand. Thermal ellipsoids were
drawn at the 50% probability level. (b) The necklace structure of 2
showing alternating rhombus-shaped units perpendicular to each other.
pendent pmtmb ligands, which exhibit two kinds of confor-
mations. The basic unit of the structure can be best
described as a paddle-wheel SBUs based on M2(CO2)4
building block. As shown in Fig. 4a, the dinuclear Cu unit
˚
group of pmtmb ligand adopts a chelating bidentate mode
[Scheme 1 (II)]. Thus, the array of ligands linked by Cd
centers gives a one-dimensional necklace structure, as
with Cu–Cu distance being 2.693(2) A is supported by four
carboxylate groups from four pmtmb ligands. The Cu–O
˚
distances range from 1.948(5) to 1.985(5) A. Two apical

L. Han et al. / Inorganica Chimica Acta 359 (2006) 2232–2240
2237
Fig. 4. (a) The paddle-wheel dinuclear Cu unit showing the local geometry around the metal center. Thermal ellipsoids were drawn at the 50% probability
level. Hydrogen atoms were omitted for clarity. (b) The centipede-like structure of 3.
˚
sites of the paddle-wheel are occupied by two N atoms
from two identical conformation pmtmb ligands, respec-
tively. The angle of N–Cu–Cu is 167.73(15)°. These pad-
dle-wheel units bridged by pmtmb ligands to give a 1D
centipede-like structure (Fig. 4b), in which there are two
kinds of conformations and coordination modes. One dis-
plays meso-planar conformation and acts as a tridentate
ligand [Scheme 1 (I)] through NO2 coordinating three Cu
atoms, while the other displays folded conformation and
coordinates to Cu atoms only by two O atoms, the N
atoms of pyrimidine ring do not take part in coordination
[Scheme 1 (III)]. Consequently, the non-coordinated
pyrimidine rings can be considered the feet of the centi-
pede-like structure. Very fascinatingly, these feet appear
important recognition functions in the self-assembly via
p–p stacking interactions to construct self-complemental
high-dimensional motifs. As shown in Fig. 5, adjacent
chains are interlocking through significant p–p stacking
interactions between the central phenyl rings and the pyri-
dine groups with ring–ring distances at 3.52–3.80 A to form
two-dimensional layered network. Furthermore, adjacent
layers are also interlocking complementarity through p–p
stacking interactions to form a three-dimensional compact
framework.
3.3. Luminescent and thermal properties
As well known, metal–organic polymeric compounds
with d10 configuration have been found to exhibit photolu-
minescence properties and have potential applications in
optoelectronic devices and as fluorescent sensors and
probes [49–53]. Thus, the photoluminescence of the com-
pounds 1 and 2 in the solid state at room temperature were
examined and the interesting results were obtained. At
ambient temperature, the free ligand Hpmtmb shows no
detectable luminescence. However, excitation of solid sam-
ples of 1 at k = 370 nm produces an intense green emission
with peak maximum at 530 nm, as shown in Fig. 6. The

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L. Han et al. / Inorganica Chimica Acta 359 (2006) 2232–2240
Fig. 6. Luminescence spectra for complex 1 (ex = 370 nm) in solid state at
room temperature.
Fig. 7. Luminescence spectra for complex 2 (ex = 320 nm) in solid state at
room temperature.
Fig. 5. Space-filling diagrams showing the interlocking 2D sheet (top) and
3D dense stacking structure (down) of 3 formed by p–p interactions.
version of porous coordination frameworks from labora-
tory
curiosities
to
practical
materials.
Thus,
interesting emission of 1 is tentatively assigned to originat-
ing from the ligand-to-metal charge-transfer (LMCT) and/
or metal-to-ligand charge-transfer (MLCT) characters,
mixed with metal-centered (ds/dp) states modified by Ag–
Ag interactions. Similarly excitation of solid samples of 2
at k = 320 nm (Fig. 7), but produces an intense purple
emission with peak maximum at 366 nm. This emission is
different to the former, which maybe originates from a
ligand-centered n–p* or p–p* process including significant
charge-transfer character induced by the polar Cd centers,
although the free ligand shows no emission. The interesting
luminescent properties of 1 and 2 endow their potential
applications as optical devices. On the other hand, high
thermal stability is an important precondition in the con-
thermogravimetric analyses (TGA) were conducted to
determine the thermal stability of complexes 1–3. The
results show that the complexes 1, 2 and 3 exhibit remark-
able thermal stability and have an onset temperature for
decomposition at 250, 285 and 310 °C, respectively. These
results also indicate that the complex 3 is very stable
because of its three-dimensional compact framework full
supported by p–p stacking interactions.
4. Conclusion
In summary, three novel coordination polymers with
different structural motifs have been synthesized from a
flexible asymmetrical bridging ligand, 4-(2-pyrimidylthio-

L. Han et al. / Inorganica Chimica Acta 359 (2006) 2232–2240
2239
[12] M. Oh, G.B. Carpenter, D.A. Sweigart, Acc. Chem. Res. 37 (2004) 1.
methyl)benzoic acid, and structurally characterized by X-
ray diffraction analyses. 1 is a 2D layer containing unusual
zigzag Ag chains based on mixed ligand-supported and
ligand-unsupported Ag–Ag interactions, 2 is a necklace
structure, which further linked through hydrogen bonding
to form a 2D sheet, and 3 is a 3D compact framework full
assembled from 1D centipede-like chains via p–p stacking
interactions. Interestingly, the ligand pmtmb adopts differ-
ent configurations and coordination modes when coordi-
nating to metal centers in these complexes. This research
demonstrates that the organic building block presents con-
formational flexibility that can be utilized in metal-assisted
assembly to affect the coordination architectures. Further
exploitation of such asymmetrical bridging ligands in crys-
tal engineering and functional material is underway. We
anticipate that this type of organic ligands with conforma-
tional flexibility will result in a variety of coordination
polymers or supramolecules with unique topologies and
fascinating properties.
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Acknowledgments
We are thankful for the financial support of the Na-
tional Nature Science Foundation of China (No.
20231020) and the Nature Science Foundation of Fujian
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Crystallographic details and complete listings of the
complexes have been deposited in the Cambridge Crystal-
lographic Data Center with CCDC Nos. 256261–256263
for 1–3, respectively. Copies of the data can be obtained
free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: +44 1223 336033; e-mail:
deposit@ccdc.cam.ac.uk). Further details are presented in
the Supporting Information. Supplementary data associ-
ated with this article can be found, in the online version,
at doi:10.1016/j.ica.2005.12.077.
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