Controlled assembly of zero-, one- and two-dimensional metal-organic frameworks involving in situ ligand synthesis under different reaction pH

Article abstract of DOI:10.1016/j.poly.2008.04.035

Five cadmium(II) metal-organic frameworks, namely [Cd(BIPA)(daf)(H₂O)₃] · 2H₂O (1), Cd₂(BDC)₂(pdon)₂(H₂O)₂ (2), Cd(BIPA)(pdon) (3), Cd(BIPA)(daf) (4) and [Cd₂

Full text of DOI:10.1016/j.poly.2008.04.035

Polyhedron 27 (2008) 2327–2336
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Controlled assembly of zero-, one- and two-dimensional metal-organic
frameworks involving in situ ligand synthesis under different reaction pH
*
Guang-Xiang Liu , Rong-Yi Huang, Heng Xu, Xue-Jun Kong, Liang-Fang Huang, Kun Zhu, Xiao-Ming Ren
Anhui Key Laboratory of Functional Coordination Compounds, School of Chemistry and Chemical Engineering, Anqing Normal University, Anqing 246003, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Five cadmium(II) metal-organic frameworks, namely [Cd(BIPA)(daf)(H2O)3] ꢀ 2H2O (1), Cd2(BDC)2(pdon)2-
(H2O)2 (2), Cd(BIPA)(pdon) (3), Cd(BIPA)(daf) (4) and [Cd2(BIPA)2(pdon)2] ꢀ H2O (5) (H2BIPA = 5-bromoi-
sophthalic acid, H2BDC = terephthalic acid, pdon = 1,10-phenanthroline-5,6-dione, daf = 4,5-diaza-fluo-
ren-9-one), have been constructed from cadmium(II) salts with multi-carboxylate ligands and pdon
ligands under different reaction pH. The framework structures of these polymeric complexes have been
determined by the X-ray single crystal diffraction technique. The differences of the five metal-organic
frameworks demonstrate that the reaction pH has an important effect on the structure of these com-
plexes. The thermal analyses of these five complexes have been measured and discussed. Additionally,
four complexes show strong fluorescence in the solid state at room temperature.
Received 2 February 2008
Accepted 15 April 2008
Available online 12 June 2008
Keywords:
Coordination polymers
In situ synthesis
Carboxylate ligand
Cadmium(II)
Photoluminescence property
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
sents a potential new direction for novel inorganic–organic hybrid
network construction through crystal engineering [7,8]. Examples
The rational design and synthesis of metal-organic frameworks
have been of increasing interest recently in materials science and
chemical research since they may provide a new strategy for
achieving solid functional materials with potential applications in
the areas of catalysis, gas storage, magnetism and optics [1,2]. Con-
sequently, a series of metal-organic frameworks with various
structural motifs including honeycomb, brick wall, bilayer, ladder,
herringbone, diamondoid and rectangular grid, have been deliber-
ately designed and synthesized. Being an important subclass of
metal-organic polymers, d10 metal complexes have been found to
exhibit intriguing structures and photoluminescent properties
[3]. A series of d10 metal-organic frameworks with bridging carb-
oxyate ligands and phenanthroline-like terminal chelating ligands
have been described recently [4]. So far, many efforts in this field
have focused on the influence of ligand spacers of flexible bridging
ligands on framework formation of their coordination polymers
[5]. However, studies on the effects of the reaction pH on frame-
work formation of complexes have rarely been reported [6].
Additionally, recent studies have revealed that crystal growth
under hydrothermal conditions occasionally involves in situ ligand
synthesis, which not only provides a powerful synthesis method
for organic ligands that are difficult to synthesize but also repre-
of in situ ligand syntheses under hydrothermal conditions include
hydrolysis of –CN and –COOR groups [7], hydroxylation [8] and
carbon–carbon bond formation by reductive coupling or oxidative
coupling [9].
Taking into account the factors mentioned above, we have paid
considerable interest to the reactions of various metal salts with
multi-carboxylate ligands and the influence of the reaction pH on
the structure of the resultant complexes, and we have found that
the reaction pH is one of the key factors in the formation of the
coordination architectures [10]. As an extension of our previous
work, we are paying attention to 1,10-phenanthroline-5,6-dione
(pdon) because of its peculiar reactivity (Scheme 1) in organic
chemistry [11]. Two nucleophilic centers (nitrogen and oxygen
lone pairs) are contained in the quinone-like pdon molecule, with
all the non-hydrogen atoms being sp2-hybridized. The presence of
two electronegative heteroatoms not only creates basic properties
in the Lewis sense but also, because of conjugation, makes it pos-
sible to alter the electron density in different parts of the molecule.
As a result, a dramatic change in reactivity can occur, which is not
observed in the case of the corresponding aromatic homocyclic
analogue, phenanthrenequinone. Herein we report on five new
coordination polymers, namely [Cd(BIPA)(daf)(H2O)3] ꢀ 2H2O (1),
Cd2(BDC)2(pdon)2(H2O)2 (2), Cd(BIPA)(pdon) (3), Cd(BIPA)(daf)
(4) and [Cd2(BIPA)2(pdon)2] ꢀ H2O (5), obtained by the reaction of
cadmium(II) salts with multi-carboxylate ligands and pdon ligands
under different reaction pH, which involve in situ synthesis of 4,5-
diaza-fluoren-9-one (daf) from pdon.
* Corresponding author. Tel./fax: +86 556 5500090.
E-mail address: liugx@live.com (G.-X. Liu).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.04.035

2328
G.-X. Liu et al. / Polyhedron 27 (2008) 2327–2336
O
O
HO
O
OH
O
OH-
OH-
CH3OH
CH3OH
N
N
N
N
N
N
Scheme 1. Schematic representation of the possible transformation mechanism of pdon to daf.
less steel vessel under autogenous pressure. After the reaction mix-
ture was slowly cooled down to room temperature, yellow platelet
2. Experimental
crystals were produced in
a yield of 54%. Anal. Calc. for
2.1. Materials and methods
C
19H19N2BrO10Cd: C, 36.36; H, 3.05; N, 4.46. Found: C, 36.33; H,
3.04; N, 4.44%. IR (KBr pellet, cmꢁ1): 3397 (br), 3057 (w), 1607
(s), 1582 (s), 1434 (m), 1376 (s), 1307 (m), 1183 (m), 1034 (w),
939 (w), 834 (m), 772 (m), 688 (m), 565 (m), 437 (m).
All commercially available chemicals and solvents were of re-
agent grade and were used as received without further purifica-
tion. 1,10-Phenanthroline-5,6-dione (pdon) was synthesized
following the published procedure [12]. Elemental analyses for C,
H and N were performed on a Perkin–Elmer 240C Elemental Ana-
lyzer at the Analysis Center of Nanjing University. FT-IR spectra
were recorded in the range 400–4000 cmꢁ1 on a Bruker Vector22
FT-IR spectrophotometer as KBr pellets. The luminescent spectra
for the powdered solid samples were recorded at room tempera-
ture on an Aminco Bowman Series 2 spectrofluorometer with a Xe-
non arc lamp as the light source. In the measurements of emission
and excitation spectra the pass width was 5.0 nm. All the measure-
ments were carried out under the same experimental conditions.
TG analyses were performed on a Perkin–Elmer TGA7 instrument
2.2.2. Synthesis of Cd2(BDC)2(pdon)2(H2O)2 (2)
A mixture containing Cd(NO3)2 ꢀ 4H2O (30.8 mg, 0.1 mmol),
pdon (21.0 mg, 0.1 mmol), H2BDC (16.6 mg, 0.1 mmol), NaOH
(8.0 mg, 0.2 mmol) and 10 ml H2O (the pH value was 7) was heated
at 160 °C for 3 days in a 25 ml Teflon lined stainless steel vessel un-
der autogenous pressure. After the reaction mixture was slowly
cooled down to room temperature, yellow platelet crystals were
produced in a yield of 58%. Anal. Calc. for C40H24N4O14Cd2: C,
47.59; H, 2.40; N, 5.55. Found: C, 47.61; H, 2.43; N, 5.57%. IR
(KBr pellet, cmꢁ1): 3431 (br), 3053 (m), 2358 (w), 1663 (s), 1610
(s), 1576 (s), 1516 (m), 1496 (w), 1442 (m), 1404 (s), 1354 (s),
1311 (s), 1284 (s), 1226 (w), 1144 (w), 1125 (m), 1104 (w), 1047
(w), 1031 (w), 936 (w), 918 (w), 866 (w), 846 (s), 818 (w), 771
(s), 725 (s), 715 (m), 696 (m), 641 (w), 610 (w), 564 (w), 438 (w).
in flowing N2 with a heating rate of 20 °C minꢁ1
.
2.2. Synthesis of the complexes
2.2.1. Synthesis of [Cd(BIPA)(daf)(H2O)3] ꢀ 2H2O (1)
A mixture containing Cd(NO3)2 ꢀ 4H2O (30.8 mg, 0.1 mmol),
pdon (21.0 mg, 0.1 mmol), H2BIPA (16.6 mg, 0.1 mmol), NaOH
(16.0 mg, 0.4 mmol), 5 ml H2O and 5 ml CH3OH (the pH value
was 9) was heated at 160 °C for 3 days in a 25 ml Teflon lined stain-
2.2.3. Synthesis of Cd(BIPA)(pdon) (3)
An identical procedure to 2 was followed to prepare 3 except
that H2BDC was replaced by H2BIPA, and the pH value was ad-
justed to 7 with NaOH, yield 64%. Anal. Calc. for C20H9BrN2O6Cd:
Table 1
Crystallographic data for complexes 1–5
Compound
1
2
3
4
5
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
19H19N2BrO10Cd
C40H24N4O14Cd2
1009.43
monoclinic
P21/c
10.7507(8)
14.5947(11)
24.4109(18)
90
98.4480(10)
90
3788.6(5)
4
C20H9BrN2O6Cd
565.60
monoclinic
P21/c
13.9849(19)
7.8845(11)
16.731(2)
90
100.079(2)
90
1816.4(4)
4
C19H9BrN2O5Cd
537.59
monoclinic
P21/c
14.131(3)
8.0869(15)
16.515(3)
90
114.070(2)
90
1723.2(5)
4
C40H20Br2N4O13Cd2
1149.22
orthorhombic
Pbcn
14.0658(14)
9.5474(10)
28.168(3)
90
90
90
3782.7(7)
4
627.67
triclinic
P-1
6.9994(17)
10.781(3)
15.018(4)
93.050(3)
94.117(3)
101.272(3)
1105.9(5)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
T (K)
293(2)
1.885
2.853
293(2)
1.770
1.200
293(2)
2.068
3.446
293(2)
2.072
3.623
293(2)
2.018
3.313
Dcacl (g cmꢁ3
)
l
(mmꢁ1
)
F(000)
620
2000
1096
1040
2232
2hmax (°)
25.05
25.00
25.05
25.05
25.04
Data collected
Independent data
Rint
5238
3760
18651
6657
0.0828
6657/4/541
0.934
8630
3208
8164
3031
17591
3306
0.0356
3760/2/306
1.014
0.0585
0.1569
0.0197
3208/0/271
1.003
0.0238
0.0651
0.0255
3031/0/253
1.003
0.0245
0.0656
0.1156
3306/0/280
1.002
0.0337
0.0659
Data/restraints/parameters
Goodness-of-fit
R1 [I > 2
wR2 [I > 2
r
(I)]a
(I)]b
0.0541
0.1215
r
P
P
a
R1
=
||Fo| ꢁ |Fc||/ |Fo|.
P
P
b
wR2 = | w(|Fo|2 ꢁ |Fc|2)|/ |w(Fo)2|1/2, w = 1/[
r
2(F2o) + (aP)2 + bP]. P = (Fo2 + 2F2c)/3.

G.-X. Liu et al. / Polyhedron 27 (2008) 2327–2336
2329
C, 42.47; H, 1.60; N, 4.95. Found: C, 42.51; H, 1.63; N, 4.98%. IR (KBr
pellet, cmꢁ1): 3422 (br), 1609 (vs), 1566 (s), 1489 (w), 1478 (w),
1438 (m), 1397 (m), 1375 (s), 1282 (w), 1151 (w), 1125 (w), 932
(w), 771 (s), 738 (m), 696 (m), 524 (m), 435 (m).
slowly cooled down to room temperature, yellow pillar crystals
were produced in a yield of 71%. Anal. Calc. for C40H20Br2N4O13Cd2:
C, 41.80; H, 1.75; N, 4.86. Found: C, 41.82; H, 1.77; N, 4.83%. IR (KBr
pellet, cmꢁ1): 3455 (br), 3056 (m), 2357 (w), 1667 (s), 1608 (s),
1574 (s), 1513 (m), 1495 (w), 1439 (m), 1405 (s), 1356 (s), 1314
(s), 1286 (s), 1227 (w), 1147 (w), 1123 (m), 1107 (w), 1043 (w),
1032 (w), 935 (w), 919 (w), 864 (w), 843 (s), 821 (w), 768 (s),
723 (s), 719 (m), 694 (m), 637 (w), 607 (w), 562 (w), 441 (w).
2.2.4. Synthesis of Cd(BIPA)(daf) (4)
The preparation of 4 was similar to that described for 1 except
that the pH of the solution of H2BIPA was adjusted to 8 rather than
9, yield: 51%. Anal. Calc. for C19H9BrN2O5Cd: C, 42.45; H, 1.69; N,
5.21. Found: C, 42.48; H, 1.67; N, 5.19%. IR (KBr pellet, cmꢁ1):
3436 (br), 3051 (m), 2361 (w), 1657 (s), 1608 (s), 1576 (s), 1519
(m), 1485 (w), 1444 (m), 1403 (s), 1371 (s), 1321 (s), 1286 (s),
1233 (w), 1121 (w), 1109 (w), 1045 (w), 1033 (w), 935 (w), 921
(w), 867 (w), 839 (s), 821 (w), 773 (s), 722 (s), 691 (m), 646 (w),
608 (w), 562 (w).
2.3. X-ray crystallography
The crystallographic data collections for complexes1–5 were
carried out on a Bruker Smart Apex II CCD with graphite-mono-
chromated Mo K
a radiation (k = 0.71073 Å) at 293(2) K using the
x-scan technique. The data were integrated using the SAINT pro-
gram, which also corrected the intensities for Lorentz and polariza-
tion effects [13]. An empirical absorption correction was applied
using the SADABS program [14]. The structures were solved by direct
methods using the program SHELXS-97 and all non-hydrogen atoms
were refined anisotropically on F2 by the full-matrix least-squares
technique using the SHELXL-97 crystallographic software package
[15,16]. The hydrogen atoms of water molecules were located from
difference Fourier maps and the other hydrogen atoms were gener-
ated geometrically. All calculations were performed on a personal
computer with the SHELXL-97 crystallographic software package.
The details of the crystal parameters, data collection and refine-
ment for four complexes are summarized in Table 1. Selected bond
lengths and angles with their estimated standard deviations for 1,
2, 4 and 5 are shown in Tables 2–5, respectively.
2.2.5. Synthesis of [Cd2(BIPA)2(pdon)2] ꢀ H2O (5)
A mixture containing Cd(NO3)2 ꢀ 4H2O (30.8 mg, 0.1 mmol),
pdon (21.0 mg, 0.1 mmol), H2BIPA (24.5 mg, 0.2 mmol), NaOH
(6.0 mg, 0.15 mmol) and 10 ml H2O (the pH value was 6) was
heated at 180 °C for 3 days in a 25 ml Teflon lined stainless steel
vessel under autogenous pressure. After the reaction mixture was
Table 2
Selected bond lengths (Å) and angles (°) for complex 1
Cd(1)–O(2W)
Cd(1)–O(1W)
Cd(1)–O(2)
2.294(4)
2.345(4)
2.350(4)
2.361(4)
Cd(1)–O(1)
Cd(1)–N(1)
Cd(1)–N(2)
2.467(4)
2.467(4)
2.515(5)
Cd(1)–O(3W)
O(2W)–Cd(1)–O(1W)
O(2W)–Cd(1)–O(2)
O(1W)–Cd(1)–O(2)
O(2W)–Cd(1)–O(3W)
O(1W)–Cd(1)–O(3W)
O(2)–Cd(1)–O(3W)
O(2W)–Cd(1)–O(1)
O(1W)–Cd(1)–O(1)
O(2)–Cd(1)–O(1)
175(1)
93(1)
89(1)
98(1)
85(1)
81(1)
89(1)
90(1)
54(1)
135(1)
82(1)
O(1W)–Cd(1)–N(1)
O(2)–Cd(1)–N(1)
O(3W)–Cd(1)–N(1)
O(1)–Cd(1)–N(1)
O(2W)–Cd(1)–N(2)
O(1W)–Cd(1)–N(2)
O(2)–Cd(1)–N(2)
O(3W)–Cd(1)–N(2)
O(1)–Cd(1)–N(2)
N(1)–Cd(1)–N(2)
95(1)
155(1)
74(1)
149(1)
93(1)
82(1)
132(1)
142(1)
79(1)
72(1)
3. Results and discussion
3.1. Crystal structures of complexes 1–5
3.1.1. [Cd(BIPA)(daf)(H2O)3] ꢀ 2H2O (1)
X-ray single-crystal structural analysis revealed that complex 1
ꢀ
crystallizes in the triclinic space group P1 and that the pdon ligand
O(3W)–Cd(1)–O(1)
O(2W)–Cd(1)–N(1)
Table 4
Selected bond lengths (Å) and angles (°) for complex 4
Table 3
Cd(1)–O(1)
Cd(1)–O(3)#1
Cd(1)–O(2)#2
2.222(2)
2.228(2)
2.298(2)
Cd(1)–N(2)
Cd(1)–N(1)
2.378(3)
2.477(3)
Selected bond lengths (Å) and angles (°) for complex 2
Cd(1)–O(1)
Cd(1)–O(1W)
Cd(1)–N(1)
Cd(1)–O(6)
Cd(1)–N(2)
Cd(1)–O(5)
Cd(2)–O(4)
2.163(6)
2.221(6)
2.328(7)
2.327(6)
2.397(6)
2.398(5)
2.204(5)
Cd(2)–O(2W)
Cd(2)–O(8)#1
Cd(2)–N(3)
Cd(2)–N(4)
Cd(2)–O(7)#1
Cd(2)–C(40)#1
2.243(6)
2.337(6)
2.345(7)
2.355(8)
2.398(5)
2.734(8)
O(1)–Cd(1)–O(3)#1
O(1)–Cd(1)–O(2)#2
O(3)#1–Cd(1)–O(2)#2
O(1)–Cd(1)–N(2)
89.71(9)
101.12(8)
84.55(8)
151.48(9)
92.88(9)
O(2)#2–Cd(1)–N(2)
O(1)–Cd(1)–N(1)
O(3)#1–Cd(1)–N(1)
O(2)#2–Cd(1)–N(1)
N(2)–Cd(1)–N(1)
107.41(8)
108.39(8)
161.06(9)
86.46(8)
73.99(9)
O(3)#1–Cd(1)–N(2)
Symmetry transformations used to generate equivalent atoms: #1 ꢁx + 1, y ꢁ 1/2,
ꢁz + 3/2; #2 ꢁx + 1, ꢁy + 1, ꢁz + 1.
O(1)–Cd(1)–O(1W)
O(1)–Cd(1)–N(1)
O(1W)–Cd(1)–N(1)
O(1)–Cd(1)–O(6)
O(1W)–Cd(1)–O(6)
N(1)–Cd(1)–O(6)
O(1)–Cd(1)–N(2)
O(1W)–Cd(1)–N(2)
N(1)–Cd(1)–N(2)
O(6)–Cd(1)–N(2)
O(1)–Cd(1)–O(5)
O(1W)–Cd(1)–O(5)
N(1)–Cd(1)–O(5)
O(6)–Cd(1)–O(5)
N(2)–Cd(1)–O(5)
O(4)–Cd(2)–O(2W)
O(4)–Cd(2)–O(8)#1
O(2W)–Cd(2)–O(8)#1
91.2(2)
87.6(2)
114.8(2)
96.3(2)
149.9(2)
94.7(2)
158.1(2)
94.8(2)
70.7(2)
89.1(2)
110.1(2)
94.9(2)
145.4(2)
55(1)
O(4)–Cd(2)–N(3)
155.1(3)
89.9(2)
95.8(2)
90.8(2)
124.0(2)
90.1(2)
69.7(3)
115.9(2)
91.3(2)
54.5(2)
89.0(2)
137.2(2)
111.0(2)
118.8(2)
26.9(2)
91.4(2)
113.5(2)
O(2W)–Cd(2)–N(3)
O(8)#1–Cd(2)–N(3)
O(4)–Cd(2)–N(4)
O(2W)–Cd(2)–N(4)
O(8)#1–Cd(2)–N(4)
N(3)–Cd(2)–N(4)
O(4)–Cd(2)–O(7)#1
O(2W)–Cd(2)–O(7)#1
O(8)#1–Cd(2)–O(7)#1
N(3)–Cd(2)–O(7)#1
N(4)–Cd(2)–O(7)#1
O(4)–Cd(2)–C(40)#1
O(2W)–Cd(2)–C(40)#1
O(8)#1–Cd(2)–C(40)#1
N(3)–Cd(2)–C(40)#1
N(4)–Cd(2)–C(40)#1
Table 5
Selected bond lengths (Å) and angles (°) for complex 5
Cd(1)–O(4)#1
Cd(1)–O(3)#2
Cd(1)–O(2)
2.185(3)
2.254(2)
2.257(2)
Cd(1)–N(1)
Cd(1)–N(2)
Cd(1)–O(1)
2.322(3)
2.352(2)
2.561(2)
O(4)#1–Cd(1)–O(3)#2
O(4)#1–Cd(1)–O(2)
O(3)#2–Cd(1)–O(2)
O(4)#1–Cd(1)–N(1)
O(3)#2–Cd(1)–N(1)
O(2)–Cd(1)–N(1)
96.7(1)
O(2)–Cd(1)–N(2)
N(1)–Cd(1)–N(2)
O(4)#1–Cd(1)–O(1)
O(3)#2–Cd(1)–O(1)
O(2)–Cd(1)–O(1)
N(1)–Cd(1)–O(1)
N(2)–Cd(1)–O(1)
176.30(9)
70.64(9)
137.21(9)
88.90(9)
53.36(8)
82.16(8)
130.05(9)
83.94(9)
92.90(9)
114.3 (1)
142.81(9)
109.69(9)
92.6(1)
90.4(2)
88.4(2)
99.7(2)
145.0(2)
O(4)#1–Cd(1)–N(2)
O(3)#2–Cd(1)–N(2)
88.70(9)
Symmetry transformations used to generate equivalent atoms: #1 x ꢁ 1/2, y ꢁ 1/2,
ꢁz + 1/2; #2 ꢁx + 2, y, ꢁz + 1/2.
Symmetry transformations used to generate equivalent atoms: #1 x + 1, y ꢁ 1, z; #2
x ꢁ 1, y + 1, z.

2330
G.-X. Liu et al. / Polyhedron 27 (2008) 2327–2336
was transformed in situ into daf during the hydrothermal process.
The asymmetric unit in 1 consists of one Cd(II) atom, one BIPA li-
gand, three coordinated water molecules and two lattice water
molecules. Each Cd(II) atom is seven-coordinated with a distorted
pentagonal bipyramidal geometry, being coordinated by two nitro-
gen atoms of one daf in a chelate mode, two oxygen atoms of one
carboxylate group and three aqua ligands. Two nitrogen atoms (N1,
N2) and three oxygen atoms (O1, O2, O3W) lie in the equatorial
plane, while the other two other aqua ligands occupy the apical
positions, as shown in Fig. 1a. The coordination bond lengths and
angles around the Cd(II) centre are in the range 2.294(4)–
2.515(5) Å and 54.08(14)–175.65(13)°, respectively, as listed in Ta-
ble 2.
group remains depronated and does not take part in the coordina-
tion. As a result complex 1 is a zero-dimensional complex and is
further linked into a 1D chain structure through
(Fig. 2b).
pꢀ ꢀ ꢀp interactions
In the packing structure of 1, as shown in Fig. 1c, the 1D chains
are firstly linked into a 2D layer by O–Hꢀ ꢀ ꢀO hydrogen bonds be-
tween the oxygen atoms of the coordinated water molecules
(O2W and O3W) and the carboxylate group from adjacent chains,
with Oꢀ ꢀ ꢀO distances ranging from 2.744(6) to 2.874(6) Å (Table
S1). Then the 2D layers are further connected through O–Hꢀ ꢀ ꢀO
hydrogen bonds between the oxygen atoms of the coordinated
water molecules (O1W and O2W) and the oxygen atoms of the lat-
tice water molecules (O4W and O5W) with Oꢀ ꢀ ꢀO distances of
2.744(6) and 2.756(3) Å (Fig. 1c, Table S1).
The coordination modes of BIPA and BDC in this work are sum-
marized in Scheme 2. As can be seen in Scheme 2a, one carboxylate
group adopts a bidentate chelating mode and the other carboxylate
Although pdon was used as the original organic reagent in the
preparation of 1, the daf ligand was unexpectedly found in the final
Fig. 1. (a) Coordination environment of the Cd(II) atom in complex 1 with the ellipsoids drawn at the 30% probability level, hydrogen atoms and water molecules are omitted
for clarity. (b) Side view of 1 showing the 1D chain structure through interactions (the coordinated water molecules and lattice water molecules are omitted for clarity).
ꢀ ꢀ ꢀ
(c) Projection of the structure of 1 along a-axis (dotted lines represent hydrogen-bonding).
p
p

G.-X. Liu et al. / Polyhedron 27 (2008) 2327–2336
2331
products, which indicates an in situ transformation of pdon into
daf during the course of the hydrothermal treatment. As shown
in the literature [8] on ligand reactions under hydrothermal condi-
tions in the presence of metal ions, the ligation of the metal ion will
cause some decrease of electron density on the aromatic ring of the
organic ligand, and hence promote nucleophilic attack by water
molecules or hydroxy groups. In this case, it seems most probable
that the hydroxy groups act effectively as nucleophiles, attacking
the carbonyl carbon atoms of pdon. As a result, the hydration of
the carbonyl groups of pdon furnishes an intermediate diol or even
a double diol. When the sp2 carbonyl carbon atoms become sp3-
hybridized, the carbon–carbon bond is then cleaved, probably be-
cause of the crowding, resulting in the formation of daf. The forma-
tion mechanism of daf in 1 shows some similarity to that of
dimethyl 2,20-bipyridine-3,30-dicarboxylate in a previously re-
ported pure organic reaction [11b]. Although the exact mechanism
of ligand transformation has not yet been proved, slow hydrolysis
of the carbonyl groups ensures that an excess of NaOH remains in
solution, thereby promoting the formation of less soluble poly-
meric networks. Equally important, the in situ slow daf formation
also ensures the growth of large single crystals that are suitable for
X-ray structure determination.
bonding interactions between the coordinated water molecule
and the carboxylate group (Table S1). Adjacent 2D networks are
ultimately extended into a 3D supramolecular structure through
p–p stacking interactions with a centroid-to-centroid distance of
3.775(3) Å between pdon ligands (Fig. 2c).
3.1.3. Cd(BIPA)(pdon) (3) and Cd(BIPA)(daf) (4)
The results of X-ray crystallographic analysis revealed that
complexes 3 and 4 crystallize in the same monoclinic P21/c space
group with similar cell parameters (Table 1), which indicate that
they are isomorphous and isostructural. Thus, as a typical example,
only the structure of 4 is described here in detail. The asymmetric
unit of 4 consists of one Cd(II) atom, one BIPA ligand and one daf
ligand. The Cd(II) atom is in a square-pyramidal coordinated geom-
etry with three oxygen atoms and two nitrogen atoms. Two oxygen
atoms (Cd1–O1 = 2.222(2) Å, Cd1–O2 = 2.298(2) Å) from two car-
boxylate groups of the BIPA ligands and two nitrogen atoms
(Cd1–N1 = 2.477(3) Å, Cd1–N2 = 2.378(3) Å) from one def ligands
are ligated to the Cd(II) ion in the quasi-plane, with another oxygen
atom belonging to the third BIPA ligand (Cd1–O3 = 2.228(2) Å)
being located in the axial position (Table 4), as displayed in Fig. 3a.
Different from that of complex 1, each BIPA ligand acts as a tri-
connector linking three Cd(II) atoms in complex 4 (Scheme 2c), one
of carboxylate groups is connected with one Cd(II) atom through a
monodentate O atom, while the other bonds with two Cd(II) atoms,
consequently resulting in the formation of a 2D layer (Fig. 3b). The
2D layer is developed in an undulated bilayer pattern parallel to
the bc-plane. By treating the Cd(II) atom as a single node and con-
necting the nodes according to the connectivity defined by the geo-
metrical centers of the benzene rings of the BIPA ligands, a
distorted 2D (4.82) network (denoted the feb net) [18] sustained
by 3-connected nodes is then yielded, as illustrated in Fig. 3c.
The van de Waals interactions extend the 2D layer into a 3D supra-
molecular net.
3.1.2. Cd2(BDC)2(pdon)2(H2O)2 (2)
Single-crystal X-ray diffraction analysis revealed that complex 2
is a 1D zigzag coordination polymer. The asymmetric unit of 2 con-
tains two cadmium atoms, two BDC ligands, two pdon ligands and
two coordinated water molecules. As illustrated in Fig. 2a, the two
Cd atoms are both six-coordinated by three oxygen atoms from
two different BDC ligands, two nitrogen atoms from one pdon mol-
ecule and one oxygen atom from a coordination water molecule to
give a distorted octahedral geometry. The Cd–O bond lengths fall in
the range 2.163(6)–2.398(5) Å and the Cd–N bond lengths fall in
the range 2.328(7)–2.397(6) Å (Table 3), which are in good agree-
ment with those found in other Cd-containing coordination frame-
works [17]. The BDC2ꢁ ligands acts as a bidentate bridging ligand,
but the coordination modes of the carboxyl groups are different,
one adopt a bis(bidentate) chelating mode, while the other adopts
a bis(monodentate) mode (Scheme 2d and e). Each BDC ligand
bridges two cadmium atoms to form a 1D zigzag infinite chain
(Fig. 2b) with Cdꢀ ꢀ ꢀCd distances of 11.551 and 11.191 Å. The 1D
zigzag chains form a 2D undulating network due to hydrogen-
3.1.4. [Cd2(BIPA)2(pdon)2] ꢀ H2O (5)
In order to further investigate the pH influence on the formation
and structure of the complex, the pH value of the ligand solution
was carefully adjusted to 6.0, while the other experimental condi-
tions remained the same as those used for the preparation of 3. As
a result, complex 5 was obtained with an entirely different struc-
ture. The asymmetric unit of complex 5 contains one Cd ion, one
Br
Br
Br
O
O
O
O
O
O
Cd
Cd
Cd
Cd
O
O
O
O
O
O
Cd
Cd
Cd
(a)
(b)
(c)
O
O
O
O
O
O
O
Cd
Cd
Cd
Cd
O
(d)
(e)
Scheme 2. Coordination modes of the BIPA and BDC ligands in complexes 1–5.

2332
G.-X. Liu et al. / Polyhedron 27 (2008) 2327–2336
a
b
c
Fig. 2. (a) Coordination environment of the Cd(II) atom in complex 2 with the ellipsoids drawn at the 30% probability level, hydrogen atoms are omitted for clarity. (b) One-
dimensional zigzag chain structure of complex 2. (c) Projection of the structure of 2 along a-axis (dotted lines represent hydrogen-bonding).
pdon ligand, one BIPA ligand and half of the lattice water molecule.
As shown in Fig. 4a, the Cd atom shows a distorted-octahedral
geometry, being chelated by two N atoms from one pdon ligand.
The remaining four coordination sites are occupied by four oxygen

G.-X. Liu et al. / Polyhedron 27 (2008) 2327–2336
2333
Fig. 3. (a) Coordination environment of the Cd(II) atom in complex 4 with the ellipsoids drawn at the 30% probability level, hydrogen atoms are omitted for clarity. (b) 2D
layer structure of complex 4 along the a-axis. (c) Projection of the structure of 4 showing the (4.82) net based on Cd atoms and BIPA ligands.
atoms from three BIPA ligands. The Cd–O bond lengths are in the
range 2.185(3)–2.561(2) Å and the Cd–N bond lengths are
2.322(3) and 2.352(2) Å (Table 5). In complex 5, the BIPA ligands
adopt a coordination mode different from those in complexes 1,
3 and 4: it acts as a tri-connector to link three Cd atoms through
one bis-monodentate carboxylic O atom from one carboxylate
group and a bidentate carboxylic O atom from the other carboxyl-
ate group, as depicted in Scheme 2b.
and it is accompanied by an in situ reaction. While in 4, obtained at
a slightly lower pH than 1, a more complicated 2D network was
generated, also accompanied by an in situ reaction. However for
3, obtained at a pH close to 7, its framework is similar to that of
4, but the in situ reaction did not happen in the absence of alkali.
Therefore, a rough conclusion could be made that the pdon ligand
can be converted into the daf ligand in the presence of alkali, which
is in agreement with the previous reports [21].
As shown in Fig. 4b, the bidentate carboxylic group links two Cd
atoms in a syn-anti fashion to form a one-dimensional zigzag chain
structure with a Cdꢀ ꢀ ꢀCd separation of 5.135(3) Å, in which the dis-
tances are in the normal range compared with those reported
(4.972–5.256 Å) in the literature [19]. The 1D Cd(CO2) chains are
further bridged into a two-dimensional undulating bilayer struc-
ture through the other carboxylate group. Interestingly, when
viewed along the b-axis, the two-dimensional layer contains rect-
angular windows with dimensions of ca. 4.974 ꢂ 5.535 Å2 that
are occupied by pdon ligands and the lattice water molecules. By
treating the Cd(II) atom as a single node and connecting the nodes
according to the connectivity defined by the geometrical centers of
the benzene rings of the BIPA ligands, a distorted 2D (4.102) net-
work (denote the KIb net) [20] sustained by three-connected nodes
is then yielded, as illustrated in Fig. 4c. The van de Waals interac-
tions extend the 2D layer into a 3D supramolecular framework.
In addition, it might be ascribed to the fact that the carboxylate
ligands were deprotonated before the in situ reaction by a compar-
ison of 3 and 4. From the points discussed above, it is suggested
that the increase of reaction pH may result in a different connectiv-
ity of the carboxylate ligand, controlling the in situ reaction and
further fabricating a more complicated structure.
3.3. Properties
3.3.1. Luminescent properties
Inorganic–organic hybrid coordination complexes, especially
with d10 metal centers, have been investigated for fluorescence
properties owing to their potential applications as luminescent
materials, such as light-emitting diodes (LEDs) [22]. Therefore, in
the present study, the photoluminescence properties of 2–5 as well
as free H2BIPA and the H2BDC ligand were investigated in the solid
state at room temperature. As shown in Fig. 5, there are two in-
tense emission peaks at 413 and 435 nm (kex = 310 nm) for 2,
421 and 442 nm (kex = 330 nm) for 3, 422 and 450 nm
(kex = 334 nm) for 5. For excitation wavelengths between 280 and
480 nm, there is no obvious emission observed for free H2BIPA
and H2BDC under the same experimental conditions, while it is re-
ported that free pdon shows fluorescence emission in the solid
state. The emission bands for phen are at 371 and 395 nm
(kex = 290 nm) [23], and compared with the pdon ligand a clear
3.2. Effect of synthetic conditions on the structures of the complexes
It has been reported that synthetic conditions, such as pH value,
reaction temperature, solvent, etc., are the key factors in assem-
bling MOFs. As to the synthesis of 1, 3 and 4, all the reaction con-
ditions are the same except for the difference of the reaction pH. As
a result, complex 1, obtained at a relatively high pH value, is a 0D
structure with the Cd(II) center bonded by three water molecules,

2334
G.-X. Liu et al. / Polyhedron 27 (2008) 2327–2336
Fig. 4. (a) Coordination environment of the Cd(II) atom in complex 5 with the ellipsoids drawn at the 30% probability level, hydrogen atoms are omitted for clarity. (b) Bilayer
structure of complex 5 along the a-axis. (c) Projection of the structure of 5 showing the (4 ꢂ 102) net based on Cd atoms and BIPA ligands.
red shift occurs in 2, 3 and 5, which might also be attributed to a
ligand-to-metal charge transfer (LMCT) transition [24]. There is
only a intense emission peak at 406 nm for 4, which means a red
shift of ca. 40 nm relative to that of the free daf (kmax = 375 nm)
[25]. We tentatively assign it to a LMCT transition [26].
ble up to 96 °C. A weight loss of 5.6% occurred in the temperature
range 96–128 °C, presumably due to the removal of two lattice
water molecules (calcd. 5.7%). A total weight loss of 8.4% occurred
in the temperature range 136–200 °C, mainly corresponding to the
removal of the one coordinated water molecule per formula unit
(calcd. 8.6%). Another weight loss of 65.7% occurred in the temper-
ature range 300–465 °C, corresponding to the removal of one BIPA
ligand and one pdon ligand per formula unit (calcd. 66.2%), no
weight losses were observed up to 800 °C, indicating the complete
deposition of the complex with the formation of CdO as the final
product. The TGA curve of 2 shows that the first weight loss of
3.4% between 110 and 160 °C corresponds to the loss of the two
coordinated water molecule per formula unit (calcd. 3.56%), then
3.3.2. Thermal properties
Thermal gravimetric analyses (TGA) were carried out to exam-
ine the thermal stabilities of 1–5. The results show that complexes
1–5 are very stable in air at ambient temperature, which make
them potential candidates for practical applications. The samples
were heated up in air at 1 atm pressure with a heating rate of
20 °C minꢁ1, as shown in Fig. 6. The TGA curve shows that1 is sta-

G.-X. Liu et al. / Polyhedron 27 (2008) 2327–2336
2335
can be obtained free of charge via http://www.ccdc.cam.ac.uk/con-
ts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (No. 20731004) and the Natural Science Founda-
tion of Education Commission of Anhui Province, China (Nos.
KJ2007B092, KJ2008B004).
3
5
4
2
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