Divalent zinc and cadmium coordination polymers of a new flexible tetracarboxylate ligand: Syntheses, crystal structures and properties

Article abstract of DOI:10.1039/c0dt00374c

A series of seven metal-organic frameworks, namely, {[Zn ₄(bptc)₂(bpp)₂]·3.5H₂O} _n (1), {[Zn₂(bptc)(bix)]·H₂O} _n (2), {[Zn₂(bptc)(bpe)_1.5(H₂O) ₃]·H₂O}_n (3), [Zn₂(Hbptc) (μ₃-OH)(4,4′-bpy)]_n (4), [Cd₂(bptc) (bpa)₂(H₂O)]_n (5), [Cd₂(bptc)(bpp) ₂(H₂O)₂]_n (6), {[Cd ₄(bptc)₂(bix)₂(H₂O) ₂]·4.5H₂O}_n (7) (bptc = 2,2′,3,3′benzophenonetetracarboxylate), have been hydro(solvo)thermally synthesized through the reaction of 2,2′,3,3′- benzophenone tetracarboxylic dianhydride (2,2′,3,3′-bptda) with divalent zinc and cadmium salts in the presence of ancillary nitrogen ligands (bpa = 1,2-bis(4-pyridyl)ethane, bpe = 1,2-bis(4-pyridyl)ethene, bpp = 1,3-bi(4-pyridyl)propane, bix = 1,4-bis(imidazol-1-ylmethyl)benzene, and bpy = 4,4′bipyridine). Due to various coordination modes and conformations of the versatile 2,2′,3,3′-bptc ligand and co-ligands, these complexes exhibit structural and dimensional diversity. In compounds 1-5, M-bptc (M = Zn or Cd) ribbons are connected together through bpp, bix, bpe, bpy and bpa, respectively, to form metal-organic sheets, which are then united together to generate 3D supramolecular structures through interlayer hydrogen bonding, C-H...π, or π...π interactions. In the structure of 4, the ribbon is formed from interlaced coaxial meso-helical Zn-Hbptc chains, and the meso-helical chains are stabilized by the bpa co-ligand in 5. Compound 6 features a 3D metal-organic framework in which [Cd₂(bptc)] _nmeso-helices are bridged by bpp ligands to exhibit a (2,6,2)-connected self-penetrating network with the Schlaefli symbol (12)(4²12⁹16⁴)(4)₂. Compound 7 also exhibits a 3D metal-organic framework in which [Cd₂(bptc)] _n layers are connected by pillared bix ligands to yield a (4,4,8)-connected network with the Schlaefli symbol (4¹⁶6 ¹²)(4⁴6²)(4⁴6²) ₂. Compounds 1 and 7 display intense blue fluorescence emissions at 460 and 465 nm, respectively, and may be suitable as excellent candidates of blue fluorescent materials.

Full text of DOI:10.1039/c0dt00374c

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www.rsc.org/dalton | Dalton Transactions
Divalent zinc and cadmium coordination polymers of a new flexible
tetracarboxylate ligand: syntheses, crystal structures and properties†
Shuang-Quan Zang,*a Ran Liang,a Ya-Juan Fan,a Hong-Wei Houa and Thomas C. W. Maka,b
Received 27th April 2010, Accepted 9th June 2010
First published as an Advance Article on the web 26th July 2010
DOI: 10.1039/c0dt00374c
A series of seven metal–organic frameworks, namely, {[Zn4(bptc)2(bpp)2]·3.5H2O}n (1),
{[Zn2(bptc)(bix)]·H2O}n (2), {[Zn2(bptc)(bpe)1.5(H2O)3]·H2O}n (3), [Zn2(Hbptc)(m3-OH)(4,4¢-bpy)]n (4),
[Cd2(bptc)(bpa)2(H2O)]n (5), [Cd2(bptc)(bpp)2(H2O)2]n (6), {[Cd4(bptc)2(bix)2(H2O)2]·4.5H2O}n (7)
(bptc = 2,2¢,3,3¢benzophenonetetracarboxylate), have been hydro(solvo)thermally synthesized through
the reaction of 2,2¢,3,3¢-benzophenone tetracarboxylic dianhydride (2,2¢,3,3¢-bptda) with divalent zinc
and cadmium salts in the presence of ancillary nitrogen ligands (bpa = 1,2-bis(4-pyridyl)ethane, bpe =
1,2-bis(4-pyridyl)ethene, bpp = 1,3-bi(4-pyridyl)propane, bix = 1,4-bis(imidazol-1-ylmethyl)benzene,
and bpy = 4,4¢bipyridine). Due to various coordination modes and conformations of the versatile
2,2¢,3,3¢-bptc ligand and co-ligands, these complexes exhibit structural and dimensional diversity. In
compounds 1–5, M–bptc (M = Zn or Cd) ribbons are connected together through bpp, bix, bpe, bpy
and bpa, respectively, to form metal–organic sheets, which are then united together to generate 3D
supramolecular structures through interlayer hydrogen bonding, C–H ◊ ◊ ◊ p, or p ◊ ◊ ◊ p interactions. In
the structure of 4, the ribbon is formed from interlaced coaxial meso-helical Zn–Hbptc chains, and the
meso-helical chains are stabilized by the bpa co-ligand in 5. Compound 6 features a 3D metal–organic
framework in which [Cd2(bptc)]n meso-helices are bridged by bpp ligands to exhibit a (2,6,2)-connected
self-penetrating network with the Schla¨fli symbol (12)(42129164)(4)2. Compound 7 also exhibits a 3D
metal–organic framework in which [Cd2(bptc)]n layers are connected by pillared bix ligands to yield a
(4,4,8)-connected network with the Schla¨fli symbol (416612)(4462)(4462)2. Compounds 1 and 7 display
intense blue fluorescence emissions at 460 and 465 nm, respectively, and may be suitable as excellent
candidates of blue fluorescent materials.
2,2¢3,3¢-tetracarboxyl acid, 2,2¢,3,3¢-oxidiphthalic acid, isomeric
2,2¢,3,3¢- and 2,3,3¢,4¢-thiaphthalic dianhydride, as well as 4,4¢-
(hexafluoroisopropylidene) diphthalic acid, have been used to
construct MOFs, leading to interesting structures including helices
and interpenetrating networks that have potential applications in
the field of separation, absorption, catalysis, and sensing.7–9
In the present study, we constructed a series of coordination
polymers of the new ligand 2,2¢,3,3¢-benzophenone tetracarboxylic
acid (H4bptc), which was generated from 2,2¢,3,3¢-bptda (2,2¢,3,3¢-
benzophenone tetracarboxylic dianhydride)10 (Scheme 1) under
hydro(solvo)thermal conditions. Ligand H4bptc possesses the fol-
lowing advantageous characteristics. (a) Its four carboxyl groups
may be completely or partially deprotonated, allowing it to exhibit
a rich variety of coordination modes in generating interesting
structural types of higher dimensions. (b) Possessing possible
syn and anti conformations (Scheme 2) and multiple bridging
moieties, bptc may be an excellent candidate for the construction
of multi-dimensional coordination polymers, especially 2D and
Introduction
The construction of coordination polymers with helical topologies
has aroused a great deal of attention1,2 owing to the wide ap-
pearance of helical structures in, for example, proteins, collagens,
quartz, single-walled carbon nanotubes (SWNTs) and many more
natural or artificial fiber-type derivatives.3 Its correlative topology
is the meso-helix which can be described as resulting from helical
reversal every half-turn of the helix.4 The composite chain is
formed by the alternation of helical conformers of opposite
chirality instead of molecules of the same chirality in a typical
helix. Up to now, knowledge about the meso-helical system has
remained largely unexplored and only limited examples have been
reported.4,5
Carboxylate-based ligands have been successfully employed
in the generation of many interesting systems exhibiting multi-
dimensional networks and useful properties.6 In the last few years,
a new family of multidentate O-donor ligands with a semi-
rigid V-shaped molecular conformation, including 1,1¢-biphenyl-
aDepartment of Chemistry, Zhengzhou University, Zhengzhou, 450001,
P. R. China
bDepartment of Chemistry, The Chinese University of Hong Kong, Shatin,
New territories, Hong Kong SAR, P. R. China
† Electronic supplementary information (ESI) available: 3D supramolec-
ular structures, TG curves and IR spectra. CCDC reference numbers
774752–774758 for 1–7. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0dt00374c
Scheme 1 Structural formula of 2,2¢,3,3¢-bptda (2,2¢,3,3¢-benzophenone
tetracarboxylic dianhydride).
8022 | Dalton Trans., 2010, 39, 8022–8032
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C, 50.69; H, 3.48; N, 5.01%. IR/cm-1 (KBr): 3431(m), 3160(w),
1666(w), 1614(s), 1580(w), 1453(m), 1365(s), 1268(m), 831(s),
777(m).
[Zn2(Hbptc)(l3-OH)(4,4¢-bpy)]n (4). The procedure is similar
to the synthesis of 1 except that 4,4¢-bpy (0.0192 g, 0.1 mmol)
was used instead of bpp. Yield: 92% based on zinc. Anal. Calc.
for C27H16N2O10Zn2 (659.16): C, 49.19; H, 2.45; N, 4.25%. Found:
C, 49.10; H, 2.38; N, 4.19%. IR/cm-1 (KBr): 3563(s), 3448(w),
3082(w), 1664(m), 1603(s), 1566(s), 1398(m), 1384(s), 1263(m),
813(m), 639(m).
Scheme
2
Syn and anti conformations of 2,2¢,3,3¢-benzophenone
[Cd2(bptc)(bpa)2(H2O)]n (5). The procedure is similar to the
synthesis of 1 except that Cd(NO3)2·4H2O (0.0308 g, 0.1 mmol)
and bpa (0.0184 g, 0.1 mmol) was used instead of Zn(NO3)2·6H2O
and bpp. Yield: 75% based on cadmium. Anal. Calc. for
C41H34Cd2N4O11 (983.52): C, 50.07; H, 3.48; N, 5.70%. Found:
C, 50.13; H,3.39; N, 5.61%. IR/cm-1 (KBr): 3365(s), 1673(m),
1612(w), 1569(s), 1385(s), 1268(s), 1227(m), 1017(s), 767(m),
705(s).
tetracarboxylic acid (H4bptc).
3D metal–organic frameworks. (c) Steric hindrance of the 2-
and 2¢-positioned carboxylate groups and the skew coordination
orientation of the carboxylate groups tend to induce the formation
of helical segments. (d) Though a few coordination polymers
based on its isomer ligand 3,3¢,4,4¢-bptc (3,3¢,4,4¢-benzophenone
tetracarboxylic acid)4 have been reported, the metal–organic
frameworks constructed from 2,2¢,3,3¢-bptc have not been re-
ported to date. Herein we undertake synthetic and structural
studies on a series of seven divalent zinc and cadmium complexes
of 2,2¢,3,3¢-bptc with co-existing nitrogen heterocyclic ligands:
{[Zn4(bptc)2(bpp)2]·3.5H2O}n(1), {[Zn2(bptc)(bix)]·H2O}n (2),
{[Zn2(bptc)(bpe)1.5(H2O)3]·H2O}n (3), [Zn2(Hbptc)(m3-OH)(4,4¢-
bpy)]n (4), [Cd2(bptc)(bpa)2(H2O)]n (5), [Cd2(bptc)(bpp)2(H2O)2]n
(6), {[Cd4(bptc)2(bix)2(H2O)2]·4.5H2O}n (7).
[Cd2(bptc)(bpp)2(H2O)2]n (6). The procedure is similar to the
synthesis of 1 except that Cd(NO3)2·4H2O (0.0308 g, 0.1 mmol) was
used instead of Zn(NO3)2·6H2O. Yield: 78% based on cadmium.
Anal. Calc. for C43H38Cd2N4O11 (1011.57): C, 51.05; H, 3.79;
N, 5.54%. Found: C, 50.98; H, 3.83; N, 5.49%. IR/cm-1 (KBr):
3405(s), 1672(w), 1612(m), 1592(w), 1572(s), 1457(w), 1430(w),
1384(s), 1281(w), 1018(w), 760(w), 703(w).
{[Cd4(bptc)2(bix)2(H2O)2]·4.5H2O}n (7). The procedure is sim-
ilar to the synthesis of 1 except that Cd(NO3)2·4H2O (0.0308 g,
0.1 mmol) and bix (0.0238 g, 0.1 mmol) was used instead of
Zn(NO3)2·6H2O and bpp. Yield: 83% based on cadmium. Anal.
Calc. for C62H53Cd4N8O24.50(1751.72): C, 42.51; H, 3.05; N, 6.40%.
Found: C, 42.59; H, 2.99; N, 6.33%. IR/cm-1 (KBr): 3423(m),
3113(s), 2938(w), 1684(m), 1557(s), 1454(m), 1385(s), 1266(m),
1083(w), 1016(w), 774(w), 720(m).
Experimental
Materials and general procedures
Ligand 2,2¢,3,3¢-bptda was prepared according to the literature
procedure,10 and all other starting materials were of analytical
grade as obtained from commercial sources without further
purification.
Physical measurements. Elemental analysis for C, H and N
were performed on a Perkin-Elmer 240 elemental analyzer. The
FT-IR spectra were recorded from KBr pellets in the range from
4000 to 400 cm-1 on a Bruker VECTOR 22 spectrometer. Thermal
analysis was performed on a SDT 2960 thermal analy◦zer from
Syntheses of metal complexes
{[Zn4(bptc)2(bpp)2]·3.5H2O}n (1). A mixture of 2,2¢,3,3¢-bptda
(0.0170 g, 0.05 mmol), bpp (0.0198 g, 0.1 mmol), Zn(NO3)2·6H2O
(0.0297 g, 0.1 mmol), and NaOH (0.0080 g, 0.2 mmol) in distilled
water (7 mL) was placed in a Teflon-lined stainless steel vessel,
heated to 120 ◦C for 3 days, and then cooled to room temper-
ature. Colorless block-like crystals of 1 were obtained in 84%
yield based on zinc. Anal. Calc. for C60H47N4O21.5Zn4(1429.50):
C, 50.40; H, 3.31; N, 3.92%. Found: C, 50.35; H, 3.27; N,
3.95%. IR/cm-1 (KBr): 3447(s), 2937(w), 1622(s), 1554(s), 1392(s),
1268(w), 1018(w), 778(m), 706(m), 670(m).
◦
room temperature to 800 C with a heating rate of 20 C min-1
under nitrogen flow. Luminescence spectra for the solid samples
were recorded on a Hitachi 850 fluorescence spectrophotometer.
X-Ray crystallography. Single-crystal X-ray analyses were
conducted on a Bruker SMART APEX CCD diffractometer11
using graphite-monochromatized Mo-Ka radiation (l = 0.71073
˚
A) at room temperature using the w-scan technique. Lorentz po-
larization and absorption corrections were applied. The structures
were solved by direct methods with SHELXS-9712 and refined with
the full-matrix least-squares technique using the SHELXL-9713
program. All nonhydrogen atoms were refined anisotropically. The
hydrogen atoms of the coordination water molecules, hydroxide
groups and ligands were included in the structure factor calcu-
lation at idealized positions by using a riding model and refined
isotropically. The hydrogen atoms of the solvent water molecules
were located from the difference Fourier maps, then restrained at
fixed positions and refined isotropically. Analytical expressions of
neutral atom scattering factors were employed, and anomalous
{[Zn2(bptc)(bix)]·H2O}n (2). The procedure is similar to the
synthesis of 1 except that bix (0.0238 g, 0.1 mmol) was used instead
of bpp. Yield: 69% based on zinc. Anal. Calc. for C31H22N4O10Zn2
(741.27): C, 50.23; H, 2.99; N, 7.56%. Found: C, 50.17; H, 2.95;
N, 7.51%. IR/cm-1 (KBr): 3045(m), 3108(m), 1684(m), 1617(s),
1560(s), 1399(s), 1379(s), 1242(w), 1115(w), 764(w), 710(w).
[Zn2(bptc)(bpe)1.5(H2O)3·H2O]n (3). The procedure is similar
to the synthesis of 1 except that bpe (0.0182 g, 0.1 mmol) was
used instead of bpp. Yield: 82% based on zinc. Anal. Calc. for
C35H29N3O13Zn2 (830.35): C, 50.62; H, 3.52; N, 5.06%. Found:
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Table 1 Crystal data and structure refinement details for 1–7
1
2
3
4
5
6
7
Formula
Mr
C60H47N4O21.50Zn4 C31H22N4O10Zn2 C35H29N3O13Zn2 C27H16N2O10Zn2 C41H34Cd2N4O11 C43H38Cd2N4O11 C62H53Cd4N8O24.50
1429.50
Triclinic
P1
741.27
Triclinic
¯
P1
830.35
Triclinic
¯
P1
659.16
Monoclinic
P21/c
983.52
Monoclinic
C2/c
1011.57
Triclinic
P1
1751.72
Triclinic
P1
Crystal system
Space group
¯
¯
¯
˚
a/A
11.4663(14)
11.4985(14)
12.4691(15)
93.944(2)
108.388(2)
94.247(2)
1548.4(3)
1
10.9889(7)
11.4124(7)
12.3293(8)
80.8810(10)
84.8950(10)
68.2990(10)
1417.69(16)
2
10.966(2)
10.974(2)
14.562(3)
84.32(3)
87.14(3)
71.83(3)
1656.5(5)
2
11.0753(7)
8.7427(6)
24.9801(16)
90
97.0080(10)
90
2400.7(3)
4
1.824
21.3161(10)
12.4028(10)
18.6010(10)
90
111.7400(10)
90
4567.9(5)
4
1.430
11.6833(13)
12.4887(14)
16.2276(18)
71.931(2)
74.323(2)
69.001(2)
2067.5(4)
2
11.659(2)
11.789(2)
12.945(3)
70.39(3)
67.92(3)
85.77(3)
1550.5(5)
1
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
Dc/g cm-3
F(000)
1.533
1.736
1.665
1.625
1.876
727
7905
752
7247
848
16776
1328
11731
1968
11270
1016
10447
869
15743
Reflns collected
Independent reflns 5402
4929
0.0581
0.878
0.0431
0.0681
0.444/-0.549
5831
0.0266
1.043
0.0328
0.0806
0.962/-0.354
4225
0.0913
0.896
0.0457
0.0872
0.825/-0.674
4027
0.0710
1.071
0.0290
0.0738
0.659/-0.848
7160
0.0795
0.960
0.0462
0.1093
1.154/-0.860
5440
0.0342
1.167
0.0487
0.1400
0.902/-0.787
Rint
0.1042
1.058
0.0662
GOF on F2
a
R1 (I > 2s (I))
a
wR2 (I > 2s(I)) 0.1652
-3
˚
Drmax/min/e A
1.150/-0.730
ꢀ
ꢀ
ꢀ
ꢀ
a R1 = ꢀFo| - |Fcꢀ/ |Fo|; wR2= { [w(Fo - Fc2)2]/ (Fo2)2}
.
2
1/2
dispersion corrections were incorporated. The crystallographic
data and selected bond lengths for 1–7 are listed in Table 1 and 2.
Results and discussion
Syntheses
The reaction of 2,2¢,3,3¢-H4bptc with divalent zinc and cadmium
ions under hydro(solvo)thermal conditions usually yielded only
white precipitates. Although detailed studies are still required to
better understand this phenomenon, we propose that the rate
of polymerization of M(II) (M = Zn or Cd) ions with 2,2¢,3,3¢-
H4bptc is too fast to obtain crystalline products. Hydrolysis of
2,2¢,3,3¢-bptda not only reduces the rate of crystallization that
may promote crystal growth of the product formed, but may also
produce H3bptc-, H2bptc2-, Hbptc3- and bptc4- species, which
facilitate the construction of high-dimensional metal–organic
hybrid compounds. In the “mix-ligand” method for the design
of high-dimensional MOFs,14 judicial selection of a co-ligand is
important in the construction of the resulting architecture. In
our investigation of the coordination chemistry of 2,2¢,3,3¢-bptda,
we have selected a series of five nitrogen heterocyclic ligands as
ancillaries, namely bpy, bpe, bpa, bpp and bix.
Scheme 3 Coordination modes of the bptc4- anions in 1 and 2 (a), 4 (c),
5 and 6 (d) and 7 (e), and the Hbptc3- anion in 3 (b). Color code: C gray,
O red, H green, Zn turquoise, Cd celatom.
Crystal structures
rings is 82.2◦, and the four carboxylate groups (2-, 2¢-, 3- and
3¢-COO-) have dihedral angles of 89.2, 76.8, 4.2 and 22.8◦ with
the planes of corresponding linked benzene rings, respectively.
The Zn1 center takes a distorted-tetrahedral geometry involving
three carboxylate O atoms from two different bptc4- anions and
one N atom from a bpp ligand. In contrast, Zn2 adopts a
distorted square-pyramidal geometry with a Addison trigonality
factor t = 0.53,16 being surrounded by four carboxylate O atoms
from two different bptc4- anions and one N atom from a bpp
ligand.
{[Zn4(bptc)2(bpp)2]·3.5H2O}n (1). As shown in Fig. 1a, this
compound consists of two types of ZnII cations, one type of
bptc4- anion, one type of bpp ligand, and uncoordinated water
molecules. The coordination mode of m5-h :h :h :h for 2,2¢,3,3¢-
bptc4- is found in compound 1, in which the anion takes a
trans-conformation with four carboxylate groups coordinating to
five ZnII cations (Scheme 3a). The 2-, 2¢- and 3¢-COO- groups
of the bptc4- anion are exobidentate and the 3-COO- group is
monodentate. As for bptc4-, the dihedral angle between two phenyl
2
2
2
1
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˚
Table 2 Selected bond lengths (A) for 1–7
Compound 1a
Zn(1)–O(3)#1
Zn(1)–N(2)
1.933(5)
2.011(6)
2.023(7)
Zn(1)–O(2)
Zn(2)–O(7)#3
Zn(2)–O(1)#3
1.976(5)
2.000(5)
2.096(5)
Zn(1)–O(9)#2
Zn(2)–O(8)
Zn(2)–O(6)
1.978(5)
2.005(5)
2.150(5)
Zn(2)–N(1)#4
Compound 2b
Zn(1)–O(1)
Zn(1)–O(6)
Zn(2)–O(7)
2.006(3)
2.076(3)
1.959(3)
Zn(1)–O(3)#1
Zn(1)–O(2)#1
Zn(2)–O(4)#3
2.008(3)
2.087(3)
1.984(3)
Zn(1)–N(4)
Zn(2)–O(8)#2
Zn(2)–N(1)#4
2.008(4)
1.939(3)
2.006(3)
Compound 3c
Zn(1)–O(4)
Zn(1)–O(1 W)
Zn(2)–O(7)
Zn(2)–N(1)
2.061(2)
2.132(2)
1.9561(2)
2.040(2)
Zn(1)–O(2 W)
Zn(1)–O(6)
Zn(2)–O(8)#1
2.071(2)
2.139(2)
1.972 (2)
Zn(1)–O(3 W)
Zn(1)–N(3)
Zn(2)–O(2)#2
2.098(2)
2.147(2)
1.982(2)
Compound 4d
Zn(1)–O(7)#1
Zn(1)–N(1)
Zn(2)–O(10)
Zn(2)–N(2)#4
2.058(3)
2.128(4)
1.915(3)
2.067(4)
Zn(1)–O(6)#2
Zn(1)–O(10)
Zn(2)–O(2)
2.084(3)
2.149(3)
1.941(3)
Zn(1)–O(1)
Zn(1)–O(10)#3
Zn(2)–O(8)#2
2.127(3)
2.209(3)
1.967(4)
Compound 5e
Cd(1)–N(1)
Cd(1)–O(1)#1
Cd(1)–O(2)#1
2.308(2)
2.380(2)
2.558 (2)
Cd(1)–N(2)
Cd(1)–O(3)
2.320(2)
2.384 (2)
Cd(1)–O(1 W)
Cd(1)–O(4)
2.362 (2)
2.495(2)
Compound 6f
Cd(1)–N(1)
Cd(1)–O(7)#2
Cd(1)–O(8)
Cd(2)–O(2 W)
Cd(2)–O(1)
2.333(4)
2.388(4)
2.515(4)
2.377(4)
2.467(4)
Cd(1)–N(2)#1
Cd(1)–O(9)
Cd(2)–N(3)
Cd(2)–O(2)
2.339(4)
2.418(4)
2.304(4)
2.416(4)
2.520(4)
Cd(1)–O(1 W)
Cd(1)–O(6)#2
Cd(2)–N(4)#3
Cd(2)–O(4)#4
2.384(3)
2.513(4)
2.348(4)
2.432(4)
Cd(2)–O(3)#4
Compound 7g
Cd(1)–N(3)
Cd(1)–O(7)#2
Cd(2)–O(4)
2.202(5)
2.343(4)
2.252(4)
2.369(5)
Cd(1)–O(8)#1
Cd(1)–O(2)
Cd(2)–O(1 W)
Cd(2)–O(6)#4
2.246(4)
2.484(4)
2.253(4)
2.392(5)
Cd(1)–N(1)
Cd(1)–O(3)
Cd(2)–O(2)#3
Cd(2)–O(1)#3
2.246(5)
2.590(4)
2.283(4)
2.510(5)
Cd(2)–O(5)#4
Symmetry codes:a #1 -x, -y + 2, -z + 1; #2 x, y + 1, z; #3 -x, -y + 1, -z + 1; #4 x - 1, y - 1, z - 1. b #1 -x + 2, -y + 1, -z + 1; #2 -x + 2, -y, -z + 1; #3
x, y - 1, z; #4 x + 1, y, z. c #1 -x + 2, -y, -z; #2 x, y - 1, z. d #1 -x + 1, -y + 1, -z + 1; #2 x, y + 1, z; #3 -x + 1, -y + 2, -z + 1; #4 x + 1, y - 1, z. e #1 -x
+ 3/2, -y + 3/2, -z + 1. f #1 x - 1, y, z; #2 -x + 1, -y + 1, -z + 1; #3 x, y + 1, z; #4 -x + 2, -y + 2, -z. g #1 -x, -y + 1, -z; #2 x + 1, y, z; #3 -x + 1, -y,
-z; #4 -x, -y, -z.
In the crystal structure of complex 1, two Zn2 atoms are bridged
by symmetry-related bptc4- ligands to form a (Zn2-bptc)2 binuclear
cluster, and such units are further connected together by Zn1
atoms to form a ribbon (Fig. 1b). Adjacent ribbons are linked
by bpp ligands in syn-fashion to form a metal–organic layer with
independent ZnII centers, one bptc4- anion, one bix ligand and
one solvent water molecule. As to bptc4-, the coordination mode
of the anion is the same as that in 1, the dihedral angle between
two phenyl rings is 74.4◦, and the four carboxylate groups
(2-, 2¢-, 3- and 3¢-COO-) have dihedral angles of 71.8, 85.5, 3.96
and 20.9◦ with the planes of corresponding linked benzene rings,
respectively. Zn1 is five-coordinated by four O atoms from four
carboxylate groups of two different bptc4- ligands and one N
atom from a bix ligand to form a square-pyramidal geometry with
Addison trigonality factor t = 0.53,16 In contrast, the Zn2 center
adopts a distorted-tetrahedral geometry: three carboxylate O
atoms from two different bptc4- anions and one N atom from a bix
ligand.
This structure is similar to that of 1, Zn1 and Zn2 atoms are
first connected together through 2,2¢,3,3¢-bptc ligands to give a
1D Zn-bptc ribbon (Fig. 2b) and such ribbons are interconnected
together by anti-bix to generate a 2D sheet (Fig. 2c). Noteworthy of
mentioning here is the solvent water molecules play an important
role to bridge adjacent sheets to form a 3D supramolecular struc-
ture through the formation of O1W–H ◊ ◊ ◊ O9, O1W–H ◊ ◊ ◊ O1#1,
2
˚
a hole (ca. 9.45 ¥ 7.24 A ) (Fig. 1c) in which (H2O)8 clusters are
present. Two pairs of symmetry-related water molecules O3W and
O4W form a planar square, and each O3W is further connected to
O1W and O2W through hydrogen bonds to form an octameric
water cluster.15 Water O1W is further involved in hydrogen-
bonding interactions with O4 and O7 from two neighboring
carboxylate groups in the covalent framework to stabilize this
octameric water cluster. Adjacent layers are further associated
through face-to-face p ◊ ◊ ◊ p interactions (inter-centroid distance
˚
3.59 A) between neighboring aromatic rings from two different
bptc4- ligands in adjacent layers to yield a 3D supramolecular
structure (Fig. S1, ESI†).
{[Zn2(bptc)(bix)]·(H2O)}n (2). As illustrated in Fig. 2a, the
fundamental building unit consists of two crystallographically
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◦
˚
Table 3 Selected hydrogen-bonding geometries for 1–4 (in A and
)
D–H ◊ ◊ ◊ A
d(D ◊ ◊ ◊ H)
d(H ◊ ◊ ◊ A)
d(D ◊ ◊ ◊ A)
∠DHA
Compound 1a
O(1 W)–H(1WA) ◊ ◊ ◊ O(4)#5
O(1 W)–H(1WB) ◊ ◊ ◊ O(7)#6
O(2 W)–H(2WB) ◊ ◊ ◊ O(3 W)#7
O(3 W)–H(3WA) ◊ ◊ ◊ O(4 W)
O(3 W)–H(3WB) ◊ ◊ ◊ O(1 W)#6
O(4 W)–H(4WA) ◊ ◊ ◊ O(3 W)#8
0.85
0.85
0.85
0.85
0.85
0.85
2.15
2.36
1.99
2.20
2.29
1.48
2.97(1)
3.08(1)
2.71(3)
2.79(3)
2.74(2)
2.20(3)
162.8
142.9
142.2
126.2
112.9
141.4
Compound 2b
O(1 W)–H(1WA) ◊ ◊ ◊ O(9)
O(1 W)–H(1WB) ◊ ◊ ◊ O(1)#1
C(26)–H(26) ◊ ◊ ◊ O(1 W)#5
C(29)–H(29) ◊ ◊ ◊ O(1 W)#5
0.86
0.85
0.93
0.93
2.05
2.25
2.69
2.68
2.873(4)
3.101(4)
3.475(1)
3.542(1)
159.5
172.5
143.3
154.5
Compound 3c
O(4 W)–H(4WA) ◊ ◊ ◊ O(3)#4
O(3 W)–H(3WB) ◊ ◊ ◊ O(4 W)#5
O(3 W)–H(3WA) ◊ ◊ ◊ O(9)#1
O(2 W)–H(2WB) ◊ ◊ ◊ O(8)
O(2 W)–H(2WA) ◊ ◊ ◊ O(3)#6
O(1 W)–H(1WB) ◊ ◊ ◊ O(3)#6
O(1 W)–H(1WA) ◊ ◊ ◊ N(2)#3
C(31)–H(31) ◊ ◊ ◊ O(4 W)
0.85
0.85
0.85
0.85
0.85
0.85
0.85
0.93
2.08
1.85
1.98
1.88
1.80
2.34
1.98
2.61
2.705(3)
2.696(3)
2.798(3)
2.723(3)
2.646(3)
2.965(3)
2.816(3)
3.348(1)
129.9
175.1
161.3
173.1
170.6
130.5
167.9
137.3
Compound 4d
O(4)–H(4) ◊ ◊ ◊ O(9)#5
0.82
1.71
2.500(5)
160.4
Symmetry codes: a #5 x + 1, y, z; #6 -x + 1, -y + 1, -z + 1; #7 x, y, z - 1; #8 -x + 1, -y + 1, -z + 2. b #1 -x + 2, -y + 1, -z + 1. #5 x, y, z - 1. c #1 -x +
2, -y, -z; #3 - x + 1, -y + 1, -z + 1; #4 x-1, y, z; #5 -x + 1, -y + 1, -z; #6 -x + 2, -y + 1, -z. d #5 x - 1, y + 1, z.
C26#6–H ◊ ◊ ◊ O1W and C29#6–H ◊ ◊ ◊ O1W inter-layer hydrogen
˚
face-to-face p ◊ ◊ ◊ p interactions (inter-centroid distances: 3.92 A
bonds (Table 3),17 as shown in Fig. S2 (ESI†).
˚
for type A and 3.84 A for type B) (Fig. 3c). The solvent water
molecules are first fixed in one sheet through O4W–H4 ◊ ◊ ◊ O3#4
and O3W#5–H3 ◊ ◊ ◊ O4W hydrogen bonds, and further linked with
another sheet through weak C31–H31 ◊ ◊ ◊ O4W hydrogen bonds.
Through such hydrogen bonds adjacent sheets are united together
to give a 3D supramolecular structure (Fig. S3, ESI†).
{Zn2(bptc)(bpe)1.5(H2O)3·H2O}n (3). The asymmetric unit of 3
contains two crystallographically non-equivalent ZnII atoms, one
bptc4- anion, one and a half bpe ligands, three coordinated and one
solvent water molecules (Fig. 3a). The 2,2¢,3,3¢-bptc4- anion adopts
a trans-conformation with four carboxylate groups to coordinate
to four ZnII cations with the m4-h :h :h :h coordination mode as
shown in Scheme 3b. The dihedral angle between the two phenyl
rings is 73.8◦. The four (2-, 2¢-, 3- and 3¢-COO-) carboxylate
groups make dihedral angles of 63.7, 62.8, 32.6 and 59.3◦ with
the corresponding linked benzene rings, respectively. The whole
ligand exhibits a m4-bridging coordination mode which is shown
in Scheme 3b. Zn1 is six-coordinated by two carboxylate O atoms
from one bptc4- anion, three water molecules and one N atom
from a bpe ligand, and it displays distorted octahedral geometry.
Zn2 shows a distorted tetrahedral geometry, being surrounded by
three carboxylate O atoms from two bptc4- anions and one N atom
from one bpe ligand.
Compound 3 also exhibits a similar 3D supramolecular struc-
ture. The Zn1 and Zn2 atoms are first connected together through
2,2¢,3,3¢-bptc ligands to give a Zn-bptc ribbon which is further
stabilized through the formation of hydrogen bonds, as shown in
Fig. 3b. The bpe ligands adopt different coordination modes: one
bridges two Zn1 atoms from adjacent ribbons to form a sheet;
the other has one terminal N atom binding one Zn2 atom while
the other N end is fixed by hydrogen bonding with a coordinated
water molecule. It is interesting to notice that the bidentate bpe
ligand is sandwiched between two monodentate bpe ligands with
[Zn2(Hbptc)(l3-OH)(4,4¢-bpy)]n (4). The 2,2¢,3,3¢-Hbptc3-
species occurs in compound 4 with an intact 3-COOH group. The
whole ligand adopts a trans-conformation with three carboxylate
and one carboxyl group attached to five ZnII cations in the
2
1
1
1
2
2
1
0
m5-h :h :h :h coordination mode (Scheme 3c).As illustrated in
Fig. 4a, the Zn1 cation is coordinated by three oxygen atoms from
two Hbptc3- anions, two hydroxyl groups, and one nitrogen atom
from a bpy ligand in a distorted octahedral geometry. In contrast,
Zn2 exhibits slightly distorted tetrahedral coordination geometry,
being surrounded by two oxygen atoms from separate Hbptc3-
ligands, the hydroxyl group, and one nitrogen atom from a bpy
ligand. In 2,2¢,3,3¢-Hbptc3-, the dihedral angle between the two
aromatic rings is about 83.5◦. The carboxylic/carboxylate groups
at the 2-, 2¢-, 3- and 3¢-positions make dihedral angles of 45.4,
64.8, 64.7 and 39.9◦, respectively, with the corresponding linked
benzene rings.
Two Zn1 atoms and two Zn2 atoms are associated together by
two pairs of symmetry-related hydroxyl groups and carboxylic
groups (2¢-COO-) to form a tetranuclear unit, and such units
are linked by Hbptc3- ligands to form a chain structure. The
most fascinating feature is that such a chain is not the common
helix but exhibits a meso-helical4 structure (Fig. 4b). Actually,
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Fig. 2 (a) Metal coordination and atom labeling in complex 2 (ellip-
soids at 50% probability). All hydrogen atoms are omitted for clarity.
(b) Metal–organic ribbon structure of 2. (c) Sheet structure for 2.
(Connecting bix ligands are marked as purple).
Fig. 1 (a) Metal coordination and atom labeling in complex 1 (thermal
ellipsoids at 50% probability level). All hydrogen atoms are omitted for
clarity. (b) Metal–organic ribbon along the b-axis. (c) Sheet structure with
(H2O)8 cluster in each cavity.
[Cd2(bptc)(bpa)2(H2O)]n (5). In this compound, the bptc4- an-
ion lies on a crystallographic 2-axis, and there are two independent
bpa ligands each occuping an inversion center (Fig. 5a). The
dihe◦dral angle between the two bptc4- aromatic rings is about
89.2 , and the four COO- groups make dihedral angles of 61.1◦ (3-
and 3¢-COO-) and 46.7◦ (2- and 2¢ -COO-) with the corresponding
linked benzene rings, respectively. The 2,2¢,3,3¢-bptc4- ligand
the bptc ligand readily forms helical segments because of the
twist of the two phenyl rings and/or the skew coordination
orientation of the carboxylate groups. In the crystal structure
of 4, there are two types of ligands with opposite chirality
(M and P) due to the reverse twisting direction of the two
phenyl rings. More interestingly, such meso-helical chains are
interlaced together to form a co-axial double meso-helical chain
(Fig. 4c). To date, a few examples of meso-helical coordination
polymers have been reported.5 However, the interlacing of two
meso-helical chains presents an unprecedented example among
multicarbocyclic acid based coordination polymers. Such double
meso-helical are connected together by bpy ligands to give a layer
structure, which is further stabilized by inter-chain hydrogen bonds
(Fig. 4d).
1
1
1
1
acts as a m4-bridge (m4-h :h :h :h ) with all carboxylate groups
adopting the bidentate chelating mode (Scheme 3d). The Cd1
atom is seven-coordinated by four carboxylate O atoms from two
different bptc4- anions, two N atoms from two bpa ligands, and
the water molecule.
The phthalate groups of two bptc4- anions link two CdII cations
to form a binuclear cluster, and such clusters are linked together
by bptc4- to form an infinite Cd-bptc chain (Fig. 5b). This
chain is also not a classical enantiomeric helix, but a meso-helix
because of the alternation of two types of bptc4- ligands with
opposite chirality (P and M) around [Cd2(CO2)2] bimetallic units.
The bpa ligand with syn-conformation (type A) binds two Cd
atoms along the chain with its pyridine rings twisted by 75.5◦
(Fig. 5c) to form a ribbon together with the Cd-bptc meso-
helix. Such ribbons are linked together by the other type of bpa
(type B) with Z-shaped conformation to form a metal–organic
Interestingly, adjacent phenyl rings from different layers are
arranged in an almost mutually perpendicular manner, which
is ideally situated for the formation of C–H ◊ ◊ ◊ p interactions
18
˚
(H11–centroid, 2.85 A). Adjacent layers are further associated
together through such C–H ◊ ◊ ◊ p interactions to generate a 3D
supramolecular structure (Fig. S4, ESI†).
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Fig. 3 (a) Metal coordination and atom labeling in complex 3 (thermal
ellipsoids at 50% probability level). All hydrogen atoms are omitted
for clarity. (b) Metal–organic ribbon (dotted lines represent H-bonds.)
(c) Sheet structure showing p ◊ ◊ ◊ p stacking interactions and O–H ◊ ◊ ◊ O,
O–H ◊ ◊ ◊ N hydrogen bonds.
Fig. 4 (a) Metal coordination and atom labeling in complex 4 (thermal
ellipsoids at 50% probability level). All hydrogen atoms are omitted for
clarity. (b) meso-Helix along the b-axis: Zn atoms (turquoise), others
(purple). (c) Interlace of two meso-helical chains: Zn atoms (turquoise).
(d) Metal–organic sheet structure in the ab-plane; connecting 4,4¢-bpy
ligands are marked in blue, and dotted lines represent H-bonds.
layer (Fig. 5d). Adjacent layers are further associated together
˚
by face-to-face p ◊ ◊ ◊ p interactions (inter-centroid distance 3.82 A)
between neighboring bpa ligands (type A and B) to yield a 3D
44.0 and 40.9◦ with the corresponding linked benzene rings,
respectively.
supramolecular structure (Fig. S5, ESI†).
Adjacent Cd atoms are bridged by two different bptc4- ligands
to form a binuclear cluster, and such clusters are linked by adjacent
bptc4- ligands to form an infinite [Cd2(bptc)]n chain. As expected,
this chain also displays an unusual meso-helical character (Fig. 6b).
The adjacent chains are connected together by one type of bpp
ligands (N1–N2) coordinating to Cd1 cations along the a-axis
to give a sheet structure (Fig. 6c). Based on this arrangement,
the neighboring aromatic rings from two different bptc4- ligands
are oriented parallel to each other with a face-to-face distance of
[Cd2(bptc)(bpp)2(H2O)2]n (6). Compound 6 contains two inde-
pendent CdII atoms (Cd1 and Cd2) both exhibiting pentagonal-
bipyramidal coordination geometry (Fig. 6a). Each Cd atom is
coordinated by four oxygen atoms of two chelating bidentate
carboxylic groups from two different bptc4- anions, two nitrogen
atoms from different bpp ligands, and a water molecule with
˚
bond lengths ranging from 2.304 to 2.520 A. The coordination
mode of the anion is same with that in 5, the dihedral angle
between two phenyl rings is 82.7◦, and the four (2-, 2¢-, 3- and
3¢-COO-) carboxylate groups make dihedral angles of 60.4, 65.8,
˚
3.593 A, which is indicative of p ◊ ◊ ◊ p stacking interaction (Fig. 6c).
Such sheets are united together by another bpp ligand (N3–N4)
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Fig. 5 (a) Metal coordination and atom labeling in complex 5 (thermal
ellipsoids at 50% probability level). All hydrogen atoms are omitted for
clarity. (b) Cd-bptc meso-helical chain. (c) Ball-and-stick representation of
the interlacing of unequal coaxial meso-helical chains: purple represents
bpa. (d) Layer structure of 5: purple, pink and blue represent bpa, bptc
and bridging bpa ligands, respectively.
bridging two Cd2 ions along the b-axis to form a robust 3D metal–
organic framework (Fig. 6d). A better understanding of this com-
plicated structure can be achieved via topological considerations.19
If the bpp and the bptc4- are considered as individual connecting
rods, and the bimetallic [Cd2(CO2)2] unit is regarded as a six-
connected node, the 3D structure of 6 can be simplified as a simple
6-connected lattice. However, this lattice cannot exhibit the real
chemical system of the structure. Instead, the bpp and the bptc4-
are considered as double-connected nodes20 and the bimetallic unit
is regarded as a six-connected node (connecting to two bptc4-
and four bpp ligands), then the complex 6 can be classified as
Fig. 6 (a) Metal coordination and atom labeling in complex 6 (thermal
ellipsoids at 50% probability level). All hydrogen atoms are omitted for
clarity. (b) [Cd2(bptc)]n meso-helical chain. (c) Layer structure. (d) 3D
Structure view down the a-axis: purple coat and turquoise chains represent
layer and connecting bpp ligands, respectively. (e) Schematic presentations
of a 3D net: bright green, turquoise and pink balls represent binuclear
Cd-cluster, bpp and bdta4- ligands, respectively. The self-penetrated
networks are highlighted in red and blue.
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Fig. 7 (a) Metal coordination and atom labeling in complex 7 (thermal ellipsoids at 50% probability level). All hydrogen atoms are omitted for clarity; (b)
chain structure; (c) metal–organic layer; (d) 3D structure viewed down the a-axis; (e) (4,4,8)-connected network with Schla¨fli symbol of (416612)(4462)(4462)2
topology: bptc (red), Cd1 cluster (green), Cd2 cluster (turquoise).
a (2,6,2)-connected lattice with a (12)(42129164)(4)2 Schla¨fli symbol
(Fig. 6e). Moreover compound 6 exhibits another entry of an
unusual but growing set of self-penetrated networks: two four-
member rings formed by two six-connected nodes and two double-
connected nodes which is simplified by bpp perpendicularly
penetrating through each other, forming a self-penetrated lattice
(Fig. 6e).
chain (Fig. 7b). Adjacent chains are interconnected to each other
by bptc4- ligands to form a metal–organic sheet (Fig. 7c). Such
sheets are stabilized by p ◊ ◊ ◊ p interactions between phenyl rings
˚
of neighboring ligands (inter-centroid distance 3.65 A). The bix
ligand exhibits an S-shaped conformation and connects the 2D
sheets to generate a 3D structure (Fig. 7d). If the bptc4- ligand
is considered as a four-connected node (connecting to two Cd1
cations and two Cd2 cations), the bimetallic unit [Cd12(CO2)2] as
an eight-connected node (connecting four bptc4- and four bix), a
pair of Cd2 bridges as a four-connected node (connecting four
bptc4- anions), the structure of 7 can be described as a (4,4,8)-
connected network with a Schla¨fli symbol of (416612)(4462)(4462)2
topology (Fig. 7e).
[Cd4(bptc)2(bix)2(H2O)2·4.5H2O]n (7). Both independent CdII
centers adopt a distorted octahedral geometry: Cd1 is coordinated
by four O atoms from three bptc4- anions and two N atoms from
two different bix ligands; Cd2 is surrounded by two chelating
carboxylate groups from different bptc4- ligands, one carboxylate
O atom from the third bptc4- ligand, and awater molecule (Fig. 7a).
The 2,2¢,3,3¢-bptc4- anion adopts a syn-conformation with four
Formation of meso-helix.
3
2
2
1
carboxylate groups linked to six CdII cations in the m6-h :h :h :h
coordination mode, as shown in Scheme 3e. The dihedral angle
between the two bptc4- aromatic rings is about 67.8◦, and the four
carboxylate groups (2-, 2¢-, 3- and 3¢-COO-) make dihedral angles
of 72.2, 88.0, 39.4 and 10.9◦ with the corresponding linked benzene
rings, respectively.
Two Cd1 cations are bridged by two bidentate 3-COO- groups
from symmetry-related bptc4- ligands to form a centrosymmetric
[Cd12(CO2)2] unit, and adjacent units are linked by m2-O and
m2-COO- bridges to a pair of Cd2 centers to form an infinite
In compounds 4–6, the coordination mode of the bptc4- ligands
leads to the formation of helical segments with chirality because
of the twist of the two phenyl rings and the skew coordina-
tion orientation of the carboxylate groups, and two types of
ligands with opposite chirality (M and P) could be formed
due to the reverse twisting direction of the two phenyl rings.
To form centrosymmetric structures, meso-helices are formed
by the alternation of helical conformers with opposite chirality
instead of molecules of the same chirality in the typical helix.
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In summary, the bptc ligand shows different coordination modes
in these complexes, exerting an important influence on the final
architectures, which demonstrated that bptc could be a potential
candidate for constructing novel coordination polymers.
Thermogravimetric analysis.
Thermogravimetric analysis (TG) was performed on complexes
1–7 to study the thermal stability of these polymers. TG curves
for complexes 1–7 are shown in Fig. S6 (ESI†). For complex 1,
a gradual weight loss between 20 and 100 ◦C is attributed to
the release of uncoordinated water molecules (observed, 4.65%;
calculated, 4.41%). Decomposition of the anhydrous residue was
observed at 400 ◦C. For compound 2, weight loss corresponding
to the release of water molecules occurred from 110 to 155 ◦C
(observed, 2.51%; calculated, 2.43%), and then the host framework
started to decompose beyond 380 ◦C. For compound 3, a weight
loss of 8.44% indicated that both the solvated and coordinated
water molecules (calculated, 8.67%) were lost in the range of
Fig. 8 Fluorescent emission spectra of complexes 1–7 and free ligand.
candidates for the exploration of blue-fluorescent materials, since
they are highly thermally stable and insoluble in common solvents.
Conclusions
◦
◦
60–170 C. A gradual weight loss from 180 C corresponded to
removal of the organic components. In the case of compound
4, no obvious weight loss was observed until the temperature
reached 345 ◦C, indicating that this metal–organic framework
material has high thermal stability. As for compound 5, weight
loss of coordinated water molecules took place in the range of 50–
In this paper, we have presented the results of our investigation
on a series of seven MOFs exhibiting various types of architec-
tures constructed from deprotonated forms of the new aromatic
tetracarboxylic acid ligand H4bptc, which adopt syn and anti
conformations and a variety of modes in coordination to divalent
zinc and cadmium metal centers. The complexes were crystallized
with nitrogen heterocycles as co-ligands, and the bptc4- anion is
found in all except compound 3, which alone contains the Hbptc3-
anion. The most notable structural feature is that two types of
bptc4- ligands with opposite chirality (M and P) are arranged
in an alternating manner to form a meso-helix, as found in 4, 5
and 6, which deserve further investigation for the construction of
MOFs with meso-helix systems. Compounds 6 and 7 possess novel
3D metal–organic networks with different interesting topologies.
Subsequent work will focus on the construction of new coordi-
nation polymers by reacting the present ligand and its positional
isomer 2,3,3¢,4¢-bptc with a wide range of metal salts.
◦
150 C (observed, 1.90%; calculated, 1.83%), and the anhydrous
component began to decompose at 285 ◦C. The TG curve of 6
showed that this complex lost its coordination water molecules
between 148 and 180 ◦C (observed, 3.31%; calculated, 3.56%).
Rapid weight loss was observed from 286 ◦C, which indicated
collapse of the whole structure. For 7, the weight loss of 4.32% in
the range of 35–130 ◦C is attributable to the uncoordinated water
molecules (calculated, 4.62%), whereas the loss of coordinated
water occurred above 175 ◦C. The anhydrous component of
compound 7 began to decompose at 285 ◦C.
Photoluminescent properties
Acknowledgements
As complexes 1–7 are hybrid inorganic–organic coordination
polymers with d10 metal centers,21 their emission spectra in the solid
state were investigated at room temperature. As shown in Fig. 8,
excitation at 352 nm leads to intense blue fluorescent emission
bands at 460 nm for 1 and 465 nm for 7 while weaker fluorescence
emissions occur at 474 nm for 2, 397 nm for 3, 473 nm for 4 and
396 nm for 5, respectively. Compound 6 gives two peaks at 459 and
490 nm with a weak shoulder appearing at 432 nm when irradiated
at 352 nm. The emissions of 1–7 can probably be assigned to the
intra-ligand (p–p*) fluorescent emission because similar emissions
are observed at 396 nm for the free bptda ligand. Compared to
the weak emission peak for free bptda ligand, the emissions of 1
and 7 with the enhancement of luminescence may be attributed to
ligand chelation at the metal center, which effectively increases the
rigidity of the ligand and reduces the loss of energy by radiationless
decay.22 Bathochromic shifts of emission occurring in 1, 2, 4, 6
and 7 are possibly assigned to different coordination environments
around the metal ions, since photoluminescence behavior is closely
associated with the metal center and the ligands coordinated
around it.23 Compounds 1 and 7 may be suitable as excellent
We gratefully acknowledge financial support by the National
Natural Science Foundation of China (No. 20901070), the
China Postdoctoral Science Foundation (Grant 20090460859) and
Zhengzhou University (P. R. China).
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