Central-metal exchange, improved catalytic activity, photoluminescence properties of a new family of d10 coordination polymers based on the 5,5′-(1: H -2,3,5-triazole-1,4-diyl)diisophthalic acid ligand

Article abstract of DOI:10.1039/c6dt00726k

The rigid and planar tetracarboxylic acid 5,5′-(1H-2,3,5-triazole-1,4-diyl)diisophthalic acid (H₄L), incorporating a triazole group, has been used with no or different pyridine-based linkers to construct a family of d¹⁰ coordination

Full text of DOI:10.1039/c6dt00726k

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ARTICLE
Central-metal Exchange, Improved Catalytic Activity,
Photoluminescence Properties of A New Family of d10
Coordination Polymers Based on 5, 5’-(1H-2, 3, 5-triazole-1, 4-
diyl)diisophthalic Acid Ligand
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Huarui Wanga,b, Chao Huanga, Yanbing Hana, Zhichao Shaoa, Hongwei Houa,*, Yaoting Fana
www.rsc.org/
The rigid and planar tetracarboxylic acid 5,5'-(1H-2,3,5-triazole-1,4-diyl)diisophthalic acid (H4L), incorporating a triazole
group, has been used with no or different pyridine-based linkers to construct a family of d10 coordination polymers,
namely, {[H2N(CH3)2]3[Cd3(L)2(HCOO)]}n (1), {[Cd2(L)(py)6]·H2O}n (2), {[H2N(CH3)2] [Cd2(L)(HCOO)(H2O)4] }n (3), {[Zn(H2L)]·H2O}n
(4), {[Zn(H2L)(4,4'-bipy)0.5]·C2H5OH·H2O}n (5) (py = pyridine, 4,4'-bipy = 4,4'-bipyridine). 1 constructs a 3D porous network
containing two kinds of channels: one are filled with coordinated HCOO− anions, and the other with [H2N(CH3)2]+ cations.
The framework of
1 can be described as a rare (5,6,7)-connected net with the Schläfli symbol of
(412.5.62)(45.53.62)2(48.53.68.82)2. The Cd(II) ions in 2 are connected through the carboxylate ligands to form a 2D layer, with
aperture dimensions of ~15.1 Å × 16.2 Å. The network of 3 features a 3D (3,4)-connected (6.8.10)2(6.83.102) topology. 3D
network with (42.6.83) topology of 4 possesses open 1D channel with the free volume of 29.2 %. 5 is a 2D layer structure
with the (42.63.8)(42.6) topology. The fluorescence lifetime τ values of 1-5 are on the nanosecond timescale at room
temperature. In particular, central-metal exchange in 2 leads to a series of isostructural M(II)-Cd frameworks [M = Cu (2a),
Co (2b), Ni (2c)] showing improved catalytic activity for the synthesis of 1,4,5,6-tetrahedropyrimidine derivatives. Based on
that, a plausible mechanism for the catalytic reaction has been proposed and the reactivity-structure relationship has been
further
clarified.
corresponding functionality within CPs. As a way to produce
coordination polymer materials with more sophisticated properties,
postsynthetic modification (PSM) has the potential to enhance the
performance of CPs for catalysis by adding functionality that could
(1) dock substrates by molecular recognition,17 (2) act as acid/base
catalytic sites,18,19 or (3) bind transition metals for tandem catalysis
with the metal core.20 With respect to catalytic applications CPs
possessing highly accessible, redox active metal centers are
particularly interesting.21 Central-metal exchange offers an
attractive strategy for the incorporation of catalytically active metal
centers in CPs. For instance, after replacing the inactive CdII ions in
porph@MOM-10 with MnII or CuII, the product could catalyze the
epoxidation of trans-stilbene with 75% conversion (compared to
only 7% conversion provided by the original CdII containing
porph@MOM-10).22 Improving catalytic activity through central-
metal exchange is only just emerging. Because of excluding the
effect of ligands and geometries, the research of isostructural CPs
before and after central-metal exchange is beneficial to find out the
relationship between the central-metal ions and corresponding
catalytic property of CPs.
Introduction
Coordination polymers (CPs) have been suggested for potential
applications in different fields, especially catalysis,1−3 gas storage
and separation,4−6 metal sensing and sequestration.7,8 The effective
approach to the design of these CPs is usually the fabrication of
metal ion as nodes and multidentate ligands as bridges or terminal
groups. Normally, metal ions possess variational coordination
geometries in different coordination cases and organic ligands such
as carboxylic species possess a variety of coordination modes under
different reaction/coordination conditions.9 Furthermore, the
coordination or combination of given metal ions and organic ligands
can also be affected by some other factors, including reaction
temperature, reactant concentration, solvents used, template, and
so on.10−16 Explorations on the factors that contribute to a given
structure are important to prepare the desired CPs.
On the other hand, the functionalization of target architectures
with specific properties and applications has become one of the
hottest topics in CPs research. In principle, specific potential
applications are tightly linked to the ability to present
a
The emergence of central-metal exchange synthetic pathway in
CPs has been relatively recent, with around thirty reports of
successful cases. The complete exchange cases are relatively few.23
Most of exchange examples in CPs take place between the
transition metal ions, such as MnII, CoII, NiII, CuII, ZnII, and CdII ions.
Based on previous studies from our group, as well as work of others,
we believe that the central-metal exchange would be a favorable
a
College of Chemistry and Molecular Engineering, Zhengzhou University,
Zhengzhou, Henan, 450001, P. R. China.
b College of Chemistry and Chemical Engineering, Luoyang Normal University,
Luoyang, Henan, 471022, P. R. China.
Electronic Supplementary Information (ESI) available: Crystallographic parameters,
PXRD, TGA, 1H NMR data. CCDC 1402574, 1445518-1445521. For ESI and
crystallographic data in CIF See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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process in porous coordination polymers (PCPs), because exchange spectrometer.
process must occur in the CPs pores. The process is therefore likely
influenced by diffusion and pore size effects.23
Taking the aforementioned idea into consideration, we choose
rigid and planar tetracarboxylic acid 5,5'-(1H-2,3,5-triazole-1,4-
diyl)diisophthalic acid (H4L, Scheme 1) as the ligand. H4L is an
excellent candidate for the preparation of porous CPs, with three
Scheme 1. Structure of the H4L ligand used in this work.
obvious characteristics: first, H4L has four carboxylate groups at two
ends of the ligand and a triazole group in the middle, which is
difficult to bind with metal ions during the de novo synthesis of CPs.
This is conducive to construct PCPs; second, the C–C bonds between
the rigid benzene rings and the triazole ring can
rotate to some degree to accommodate various coordination
environments; and third, it can be partially or completely
deprotonated, allowing its different coordination models to the
metal ion connectors. Then, reactions of Zn(II) or Cd(II) salts and H4L
with no or different pyridine-based linkers were carried out under
hydrothermal condition. In this work, we present the syntheses,
Preparation of complexes
Synthesis of {[H2N(CH3)2]3[Cd3(L)2(HCO2)]}n (1). H4L (19.8 mg, 0.05
mmol) and Cd(NO3)2·4H2O (23.6 mg, 0.1 mmol) were dissolved in
DMF (5 mL) in a capped vial, then H2O (1 mL) and 0.1 mL HNO3 were
added into the mixture solution. The mixture was placed in a
capped 10 mL vial and heated at 100 °C for 3 d. Colorless block
crystals suitable for X-ray diffraction were obtained with a yield of
78% (based on H4L). Anal. calcd for C43H39N9O18Cd3 (%): C, 39.52; H,
3.01; N, 9.65. Found: C, 39.81; H, 3.03; N, 9.43. IR (KBr, cm–1):
crystal
structures
of
five
new
CPs,
namely,
{[H2N(CH3)2]3[Cd3(L)2(HCO2)]}n
{[H2N(CH3)2][Cd2(L)(HCO2)(H2O)4]}n
(1),
{[Cd2(L)(py)6]·H2O}n
(3), {[Zn(H2L)]·H2O}n
(2),
(4),
3156(w),
3076(w),
1614(m),
1539(s),
1405(m),
{[Zn(H2L)(4,4'-bipy)0.5]·C2H5OH·H2O}n (5). The thermal stability and
solid-state photoluminescence of the complexes 1−5 were also
reported. The fluorescence lifetime τ values of 1-5 are on the
nanosecond timescale at room temperature. Moreover, the metal-
1356(s),1107(w),1021(w),918(w), 728(s), 669(w).
Synthesis of {[Cd2(L)(py)6]·H2O}n (2). H4L (19.8 mg, 0.05 mmol) and
Cd(NO3)2·4H2O (23.6 mg, 0.1 mmol) were dissolved in DMF (5 mL) in
a capped vial, then pyridine (1 mL) and 0.05 mL HNO3 were added
into the mixture solution. The mixture was placed in a capped 10
mL vial and heated at 100 °C for 4 d. The crystal products of 2 were
obtained with a yield of 65% (based on H4L). Anal. calcd for
C48H38N9O9Cd2 (%): C, 51.95; H, 3.45; N, 11.36. Found: C, 51.81; H,
3.57; N, 11.21. IR (KBr, cm–1): 3423(w), 3072(w), 1602(s), 1448(m),
1363(s), 1038(m), 723(s), 696(s), 630(m).
ion exchange behavior for
suspension/M(NO3)2 (M
2
was studied. Heating up
a 2
=
CuII, CoII, NiII) solution in N,N-
dimethylformamide (DMF) leads to isostructural replacement of
cadmium by M(II) centres. The exchange products showed
improved catalytic activity for the synthesis of 1,4,5,6-
tetrahedropyrimidine derivatives. Furthermore, the reactivity-
structure relationship has also been illuminated.
Synthesis of {[Cd0.78Cu1.22(L)(py)6]·H2O}n (2a). Compound 2a was
prepared by immersing crystals of 2 in an DMF solution of Cu(NO3)2
(0.05 M) and heating to 50 °C for 48 h. The exchange products were
proved by ICP atomic emission spectrometric analysis, which
showed that 61% replacement of CdII ions by CuII ions. Anal. calcd
for C48H38N9O9Cd0.78Cu1.22 (%): C, 54.90; H, 3.65; N, 12.01. Found: C,
54.62; H, 3.85; N, 11.83. IR (KBr, cm–1): 3418(w), 2927(w), 1652(s),
1544(m), 1435(m), 1373(s), 1105(m), 783(m), 759(m), 735(m).
Experimental
Materials and physical measurements
All reagents and solvents were commercially available except for
H4L, which was prepared according to the literature procedure.24
FT-IR spectra were recorded on a Bruker-ALPHA spectrophotometer
with KBr pellets in 400−4000 cm-1 region. Elemental analyses (C, H,
and N) were carried out on a FLASH EA 1112 elemental analyzer.
Powder X-ray diffraction (PXRD) patterns were recorded using Cu
Kα1 radiation on a PANalytical X’Pert PRO diffractometer. Thermal
analyses were performed on a Netzsch STA 449C thermal analyzer
from room temperature at a heating rate of 10 ºC min-1 in air. The
luminescence spectra for the powdered solid samples were
measured at room temperature on a Hitachi 850 fluorescence
spectrophotometer. The excitation slit and emission slit were both
2.5 nm. Fluorescence lifetimes were obtained on Edinburgh FlS-980
fluorescence spectrometer. The amounts of metal ions exchanged
were determined by inductively coupled plasma spectroscopy (ICP).
Synthesis of {[Cd0.9Co1.1(L)(py)6]·H2O}n (2b). Compound 2b was
prepared by immersing crystals of 2 in an DMF solution of Co(NO3)2
(0.05M) and heating to 50 °C for 48 h. The exchange products were
proved by ICP atomic emission spectrometric analysis, which
showed that 55% replacement of CdII ions by CoII ions. Anal. calcd
for C48H38N9O9Cd0.9Co1.1 (%): C, 54.86; H, 3.64; N, 12.00. Found: C,
54.74; H, 3.81; N, 11.88. IR (KBr, cm–1): 3202(w), 2979(w), 1651(s),
1609(w), 1562(s), 1411(s), 1105(m), 787(m), 758(s), 735(s).
Synthesis of {[Cd1.84Ni0.16(L)(py)6]·H2O}n (2c). Compound 2c was
prepared by immersing crystals of 2 in an DMF solution of Ni(NO3)2
(0.05M) and heating to 50 °C for 48 h. The exchange products were
proved by ICP atomic emission spectrometric analysis, which
showed that 8% replacement of CdII ions by NiII ions. Anal. calcd for
C48H38N9O9Cd1.84Ni0.16 (%): C, 52.36; H, 3.48; N, 11.45. Found: C,
52.14; H, 3.61; N, 11.33. IR (KBr, cm–1): 3427(m), 3019(w), 1652(m),
1541(s), 1411(s), 1371(s), 1309(m), 1037(m), 765(m), 729(m).
ICP was performed on
a
Thermo ICAP 6500 DUO
2 | J. Name., 2012, 00, 1-3
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Synthesis of {[H2N(CH3)2][Cd2(L)(HCO2)(H2O)4]}n (3). H4L (19.8 mg, Typical procedure of the synthesis of 1,4,5,6-tetrahydropyrimidine
0.05 mmol), malonic acid (5.2 mg, 0.05 mmol) and Cd(NO3)2·4H2O derivatives (7a-7b) with catalysts 2 and 2a-2c
(23.6 mg, 0.1 mmol) were dissolved in DMF (5 mL) in a capped vial,
Aromatic nitrile (1.0 mmol, 1.0 equiv), 1,3-diaminopropane (0.5 mL)
and catalyst (0.1 mmol, 0.1 equiv based on M(II) ions, 2 and 2a-2c)
were mixed with toluene reflux. After 4 h, the bulk of the cool
solvent was filtrated and evaporated in vacuo and then the residue
was partitioned between chloroform (25 mL) and H2O (25 mL). The
organic phase was washed by saturated aqueous NaCl, dried by
Na2SO4, filtered and evaporated in vacuo. The residue was purified
by column chromatography on silica gel (EtOAc–MeOH, 3:1).
then H2O (1 mL) and 0.1 mL HNO3 were added into the mixture
solution. The mixture was placed in a capped 10 mL vial and heated
at 100 °C for 3 d. The crystal products of 3 were obtained with a
yield of 23% (based on H4L). Anal. calcd for C21H16N4O14Cd2 (%): C,
32.62; H, 2.09; N, 7.25. Found: C, 32.31; H, 2.23; N, 7.13. IR (KBr,
cm–1): 3346(w), 3176(w), 2810(w), 1616(m), 1540(s), 1409(m),
1362(s), 1280(m), 1020(w), 916(w), 766(w), 727(s).
Synthesis of {[Zn(H2L)]·H2O}n (4). H4L (19.8 mg, 0.05 mmol) and Column chromatography was performed with silica gel (200–300
Zn(NO3)2·4H2O (18.9 mg, 0.1 mmol) were dissolved in DMF (5 mL) in mesh). 1H NMR spectra were recorded on 300 MHz instrument.
a capped vial, then H2O (1 mL), CH3CH2OH (1 mL) and 0.1 mL HNO3
Single crystal X-ray crystallography
were added into the mixture solution. The mixture was placed in a
capped 10 mL vial and heated at 85 °C for 4 d. The crystal products
of 4 were obtained with a yield of 72% (based on H4L). Anal. calcd
for C18H11N3O9Zn (%): C, 45.17; H, 2.32; N, 8.78. Found: C,44.86; H,
2.41; N, 8.90. IR (KBr, cm–1): 3068(w), 2800(w), 1710(w), 1618(m),
1574(m), 1470(w), 1334(s), 1270(m), 1021(w), 783(w), 755(m),
725(s).
The data of the 1 and 3−5, were collected on a Rigaku Saturn 724
CCD diffractomer (Mo-Kα, λ = 0.71073 Å) at temperature of 20 ± 1
°C. The data of the 2 was collected on a SuperNova diffractometer
with graphite monochromated Cu Kα radiation (λ = 1.54178 Å) at
160(2) K. The empirical absorption corrections were applied by
using multi-scan program. The data were corrected for Lorentz and
polarization effects. The structures were solved by direct methods
and refined with a full-matrix least-squares technique based on F2
with the SHELXL-97 crystallographic software package.25 The
hydrogen atoms were placed at calculated positions and refined as
riding atoms with isotropic displacement parameters. There is a
large solvent accessible void volume in 4 which is occupied by highly
disordered free solvent molecules. No satisfactory disorder model
could be achieved, and therefore the SQUEEZE program
implemented in PLATON was used to remove its electron
densities.26 Crystallographic crystal data and structure processing
parameters for 1-5 are summarized in Table 1. Selected bond
lengths and bond angles for 1-5 are listed in Table S1 (Supporting
Information).
Synthesis of {[Zn(H2L)(4,4'-bipy)0.5]·C2H5OH·H2O}n (5). H4L (19.8 mg,
0.05 mmol), 4,4'-bipy (7.8 mg, 0.05 mmol) and Zn(NO3)2·4H2O (18.9
mg, 0.1 mmol) were dissolved in DMF (5 mL) in a capped vial, then
H2O (1 mL), CH3CH2OH (1 mL) and 0.1 mL HNO3 were added into the
mixture solution. The mixture was placed in a capped 10 mL vial
and heated at 100 °C for 3 d. The crystal products of 5 were
obtained with a yield of 86% (based on H4L). Anal. calcd for
C25H21N4O10Zn (%): C, 49.81; H, 3.51; N, 9.29. Found: C, 48.87; H,
3.63; N, 9.53. IR (KBr, cm–1): 3053(w), 2800(w), 1720(w), 1615(m),
1549(m), 1426(w), 1359(s), 1228(m), 1021(w), 780(w), 729(s),
682(m).
Table 1 Crystallographic data and structure refinement details for complexes 1-5.
2
3
4
C18H11N3O9Zn
478.67
Compound
Formula
Fw
1
5
C43H39N9O18Cd3
1307.03
C48H38O9N9Cd2
1109.67
C21H16N4O14Cd2
773.18
C25H21N4O10Zn
602.83
293(2)
0.71073
Monoclinic
C2/c
160(10)
293(2)
293(2)
293(2)
0.71073
Triclinic
Pī
Temp(K)
Wavelength(Å)
Crystal system
Space group
a (Å)
1.54178
0.71073
Monoclinic
P21/c
0.71073
Monoclinic
P21/c
Monoclinic
C2/c
18.944(4)
23.156(5)
14.411(3)
90
25.1439(5)
18.3001(3)
16.0070(3)
104.32(3)
126.410(2)
109.51(3)
5927.59(19)
4
7.7476(15)
18.393(4)
18.753(4)
90
8.0098(16)
13.813(3)
22.126(4)
90
8.2275(16)
11.413(2)
14.634(3)
72.56(3)
86.08(3)
74.18(3)
1261.1(4)
2
b (Å)
c (Å)
α
(deg)
(deg)
(deg)
126.63(3)
90
101.72(3)
90
98.25
β
90
γ
V (Å3)
5073.0(18)
4
2616.6(9)
4
2422.6(8)
4
Z
Dc (g⋅cm-3)
ꢀ (mm-1)
1.711
1.243
1.963
1.312
1.587
1.330
6.183
1.704
1.061
1.041
2592
2228
1512
968
618
F(000)
GOF on F2
1.168
1.043
1.159
1.078
1.134
Reflections collected /
17262 / 4682
0.0723,0.2001
0.0817,0.2151
11990 / 5087
0.0929, 0.2481
0.1016, 0.2631
20639 / 4882
0.0829, 0.2028
0.1167, 0.2353
4508 / 4508
0.0781, 0.1888
0.1069, 0.2050
12853 / 4670
0.0567, 0.1581
0.0613, 0.1655
R1, wR2[I>2σ(I)]a
R1, wR2(all data)b
aR1 = ∑׀׀Fo׀-׀Fc׀׀/∑׀Fo׀. bwR2 = [∑w(Fo2 -Fc2)2/∑w(Fo2)2]1/2
.
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[51.44(11)−180.00(13)
°] are in
the normal range.
Results and discussion
Synthesis. The addition of nitric acid is important for the synthesis
of complex 1-5 while its absence will lead to a large amount of
precipitate and are not preferred for X-ray crystallographic study,
although it did not appear in their final frameworks. The formate
anions and [H2N(CH3)2]+ in 1 are generated by the in situ hydrolysis
of DMF under acid conditions and incorporated into the
compound.27 In consideration of the bridging capability of formic
ligand in the structure of 1, attempts to replace the formic ligand by
other aliphatic groups, flexible carboxylic ligands such as acetic acid,
propionic acid, butyric acid, oxalic acid, malonic acid and succinic
acid were conducted by addition of the corresponding acid to the
reaction system under the same reaction conditions. The result
showed that the formic ligand could not be replaced and 1 always
generated except for the case of malonic acid. However, the
malonic acid ligand was not coordinated to metal ions but only
adjusted the pH value of the system, which resulted in structural
transformation from 1 to 3. It should be noted that there does not
exist [H2N(CH3)2]+ counter cations in the structure of 2 and 4-5. For
2, [H2N(CH3)2]+ cations can't generate with no water involed. For 4-5,
this phenomenon may be ascribed to the addition of ethanol, which
restrains the hydrolysis of DMF. The free [H2N(CH3)2]+ cations could
be assigned by crystallographic analysis in spite of their disorder,
which were corroborated by thermogravimetric analyses (TGA) and
elemental analysis (EA).
Scheme 2. Diverse Coordination Modes of H4L in Complexes 1−5.
In the structure of 1, the L4- ligands are completely deprotonated,
exhibiting one kind of coordination mode: coordination mode
(Mode I of Scheme 2). In this mode, four carboxylic groups act as μ1-
η1:η1, μ2-η1:η1, and μ2-η1:η2 modes, respectively, to bridge seven
Cd(II) ions together. On the basis of these connection modes, all the
Cd(II) ions are linked together to generate the 3D network of 1. The
solvent is one of the most important parameters in the synthesis of
CPs, where they act as counterions for charge balance, as space-
filling molecules, or as “true” templates, in the case of well-defined
host-guest interactions.28,29 The [H2N(CH3)2]+ cations and HCOO−
anions generated via decomposition of the DMF solvent, act not
only as charge balance ions, but also as templates to construct the
3D porous network. A view along the crystallographic [101] axis
(Fig. 1b) shows two kinds of channels: one are filled with
coordinated HCOO− anions, and the other with [H2N(CH3)2]+ cations.
The effective free volume of 1 was calculated by PLATON30 analysis
as 34.3 % of the crystal volume (1739.3 out of the 5073.0 Å3 unit
cell volume). To fully understand the structure of 1, the topological
approach is applied to simplify such a 3D coordination framework.
From the viewpoint of topology, Cd1, Cd2 and L4- ligand can be
considered as five-, six- and seven-connected nodes, respectively,
thus, the framework of 1 can be described as a rare (5,6,7)-
Crystal structures
Structural description of {[H2N(CH3)2]3[Cd3(L)2(HCO2)]}n (1). Single
crystal X-ray diffraction analysis reveals that complex 1 crystallizes
in the monoclinic space group P21/c and exhibits a 3D porous
network. There are two crystallographically independent Cd(II)
centers in 1, as shown in Fig. 1a, Cd1 ion is seven-coordinated by six
oxygen atoms from four L4- ligands, and one oxygen atom from one
HCOO− anion. Cd2 ion has
a slightly distorted octahedral
coordination geometry, formed by six carboxylate oxygen atoms
from six different L4- ligands. All the Cd−O [2.163(2)−2.592(4) Å]
bond distances as well as the bond angles around each Cd(II) atom
connected
net
with
the
Schläfli
symbol
of
(Fig.
(412.5.62)(45.53.62)2(48.53.68.82)2
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1c).
(a)
(a)
(b)
(b)
(c)
Fig. 2 (a) Coordination environments of the Cd(II) ions in 2. Symmetry
code: #1 = -x+1/2, y+1/2, -z+1/2. (b) Ball-and-stick view of the 2D network
of 2 along a direction. (c) Ball-and-stick view of the 3D superamolecular
network of 2 along [110] direction.
(c)
Fig. 1 (a) Coordination environments of the Cd(II) ions in 1. Symmetry
code: #1 = x+1, -y+2, z+1/2, #2 = x+1/2, y+1/2, z, #3 = x, -y+2, z-1/2, #4 -x,
y, -z+3/2, #5 = -x+1, -y+2, -z+2, #6 = -x+1, y, -z+5/2. (b) Ball-and-stick view
of the 3D network of 1 along a direction. (c) Schematic representation of
the 3D (5,6,7)-connected (412.5.62)(45.53.62)2(48.53.68.82)2 topology in
complex 1.
Structural description of {[H2N(CH3)] [Cd2(L)(HCO2)(H2O)4]}n (3).
The results of crystallographic analysis revealed that the
asymmetric unit of complex 3 consists of two Cd(II) atoms, one L4-
anion, one HCOO− anion, four coordinated water molecules as well
as [H2N(CH3)2]+ cations as charge balance ions. As shown in Fig. 3a,
Cd1 center adopts a distorted pentagonal bipyramidal coordination
environment and is coordinated with four carboxylic oxygen atoms
from two L4- ligands, one oxygen atom from one HCOO− anion and
two coordinated water molecules. Cd2 center is coordinated by
three carboxylic oxygen atoms from two L4- ligands, one oxygen
atom from one HCOO− anion and two coordinated water molecules,
showing a distorted octahedral geometry. All the Cd−O [2.230(7)−
2.502(7) Å] bond distances as well as the bond angles around each
Cd(II) atom [54.0(2)−163.6(3) °] are in the normal range.
Structural description of {[Cd2(L)(py)6]·H2O}n (2). When pyridine
was added, a structurally different 2 was isolated. Complex 2
crystallizes in the monoclinic space group C2/c. The coordination
geometry of Cd can be described as a distorted pentagonal
bipyramid containing (N3O4) from two chelating L4- and three
pyridine ligands. Two oxygen atoms from the chelating L4-
asymmetrically coordinate to the Cd atoms (Cd−O
2.281(6)−2.389(5) and 2.512(8)−2.529(6) Å), and this distortion of
chelation may be attributed to the seven-coordination in 2.31 Two
nitrogen atoms from two different pyridine ligands occupy axial
positions, while one nitrogen atom from the third pyridine ligand
occupies a horizontal position. The Cd−N bond distances range from
2.362(6) to 2.383(4) Å, which are similar to those in the related
complexes.32 Each L4- ligand is completely deprotonated and
coordinates to four Cd(II) ions through its terminal carboxylate
groups in chelating mode (Mode II of Scheme 2). These Cd(II) ions
are bridged by the long aromatic backbone of the carboxylate
ligands to generate 2D layer, with aperture dimensions of ~15.1 Å ×
16.2 Å. However, the pyridine ligands which reside in the horizontal
position diminish the void of the layer significantly (Fig. 2b). In
addition, the extended 3D supramolecular network is formed
through C–H···O hydrogen bonds interactions (C13···O5a: 3.452 Å,
C1–H13···O5a angle 163 °. Symmetry codes a = x, 1-y, -1/2+z)
between pyridine rings and carboxylic groups (Fig. 2c).
Four carboxylate groups of L4- are deprotonated and adopt μ1-η1:η1
and μ1-η1:η0 coordination modes, respectively (Mode III of Scheme
2). The Cd(II) atoms are bridged by L4- and HCOO− ligands to form a
complicated 3D framwork (Fig. 3b). The [H2N(CH3)2]+ cations act as
charger balance ions filling in the voids of these channels. From the
topological perspective, each L4- Ligand coordinates to four Cd(II)
ions and therefore the ligand can be considered as 4-connected
node. Both of Cd1 and Cd2 can be considered as 3-connected node.
The network of 3 features a (3,4)-connected (6.8.10)2(6.83.102)
topology (Fig. 3c).
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(a)
(b)
(a)
(b)
(c)
Fig. 4 (a) Coordination environments of the Zn(II) ions in 4. Symmetry
code: #1 = x+1, y+1, z, #2 = -x+2, y+1/2, -z+1/2, #3 = -x+2, -y+2, -z.
Hydrogen atoms are omitted for clarity. (b) Ball-and-stick view of the 3D
network of 4 along a direction. (c) Schematic representation of the 3D 4-
connected (42.6.83) topology in complex 4.
(c)
Fig. 3 (a) Coordination environments of the Cd(II) ions in 3. Symmetry
code: #1 = x, -y+1/2, z+1/2, #2 = -x, y-1/2, -z+1/2. (b) Ball-and-stick view of
the 3D network of 3 along a direction. (c) Schematic representation of
the 3D (3,4)-connected (6.8.10)2(6.83.102) topology in complex 3.
Structural description of {[Zn(H2L)(4,4'-bipy)0.5]·C2H5OH·H2O}n (5).
The results of crystallographic analysis revealed that complex 5
crystallizes in the triclinic space group Pī and the asymmetric unit of
which consists of one Zn(II) atom, one H2L2- anion, one of a half 4,4'-
bipy ligand, one guest water as well as one alcohol molecule. As
shown in Fig. 5a, each Zn(II) atom is coordinated by three
carboxylate oxygen atoms from three different H2L2- ligands (Zn–O =
1.940(3), 1.961(3), and 2.001(3) Å) and one nitrogen atom from one
4,4'-bipy ligand (Zn–N = 2.018(3) Å) to constitute a (ZnO3N)
tetrahedral geometry. In the structure of 5, each partly
deprotonated H2L2- ligand utilizes its three carboxylic groups, with
monodentate coordination mode, to bridge three Zn(II) atoms,
generating a 1D polymer chain and then a 2D layer extended by
4,4'-bipy ligands (Fig. 5b). Owing to the long H2L2- ligand, the 2D
layer of 5 shows big window with the Zn···Zn separation through
4,4'-bipy is 11.083 Å, which is shorter than the one through the
H2L2- ligand (17.272 Å). The 2D layer structure of 5 topologically
possesses a (3,4)-connected network with the Schläfli symbol of
(42.63.8)(42.6), as illustrated in Fig. 5c. Analysis of the crystal packing
of complex 5 reveals that the 2D layer are further linked together to
generate an overall 3D network (Fig. 5d) by the co-effects of the
inter-layer π···π stacking between completely parallel triazole and
phenyl rings (triazole ring: N1-C9-N2-N3-C10; phenyl ring: C2-C3-C4-
C5-C6-C7) of H2L2- ligands in an off-set fashion with the centroid-
centroid separations of 3.401–3.688 Å and O–H···O hydrogen-
bonding interactions (O1···O5a: 2.576 Å, O1–H1···O5a angle 154 °.
Symmetry codes a = 1-x, 3-y, 1-z) between carboxylic groups of two
H2L2- ligands.
Structural description of {[Zn(H2L)]·H2O}n (4). When Cd(II) salt was
replaced by Zn(II) salt, two new Zn complexes 4 and 5 were isolated.
The fundamental building unit of
4
consists of one
crystallographically unique Zn(II) ions, one H2L2- ligand, and one
guest water molecule. As illustrated in Fig. 4a, each Zn(II) ion is
four-coordinated with a distorted square pyramidal geometry by
four carboxylic oxygen atoms from two different H2L2- ligands. All
carboxylic groups of H2L2- ligand exhibits μ1-η1:η0 modes to bridge
four Zn(II) ions giving ride to a 3D network (Fig. 4b). View along a
direction, 3D network possesses open 1D channel with the free
volume of 29.2 % (707.6 out of the 2422.7 Å3 unit cell volume). To
better understand the nature of this intricate framework, topology
analysis is provided: both of the H2L2- anions and Zn(II) ions can be
regarded as 4-connected nodes. Thus, the whole 3D framework can
be represented as a 4-connected net with (42.6.83) topology (Fig.
4c).
6 | J. Name., 2012, 00, 1-3
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The solid-state photoluminescent spectra of Zn(II)/Cd(II) complexes
1−5, the free H4L ligand were investigated at ambient temperature,
as depicted in Fig. 6. The free ligand H4L displays a relatively weak
emission band at 425 nm (λex = 359 nm). Complexes 1−5 exhibit the
emission maxima at 439 nm (λex = 359 nm) for 1, 423 nm (λex = 359
nm) for 2, 441 nm (λex = 359 nm) for 3, 442 nm (λex = 359 nm) for 4,
448 nm (λex = 359 nm) for 5, respectively. Since the Zn(II)/Cd(II) ions
are difficult to oxidize or to reduce because of the d10 configuration,
the emission of these complexes are mainly based on the
luminescence of ligands.33 For 1 and 3-5, the emission bands are
red-shifted compared with that of the free ligand H4L, which may be
assigned to the intraligand emission of carboxylate ligand. It is
possible that the coordination of the carboxylate groups to the
Zn(II)/Cd(II) ion decreases the π*→n gap of the carboxylate ligand,
leading to the red-shift of the emission peak.34−36 The emission
spectra of 2 is parallel to that of the H4L ligand, which could be
(a)
(b)
(c)
(d)
mainly
H4L.
ascribed
to
the
intraligand
emission
of
Fig. 5 (a) Coordination environments of the Zn(II) ions in 5. Symmetry
code: #1 = x+1, y, z+1, #2 = -x+2, -y+2, -z+2. (b) Ball-and-stick view of the
2D network of 5. (c) Schematic illustrating the 2D (3,4)-connected
(42.63.8)(42.6) topology in complex 5. (d) View of 3D superamolecular
network of 5
.
Thermal properties and PXRD
Powder X-ray diffraction (PXRD) has been conducted to confirm the
phase purity of these compounds. For complexes 1–5, the
measured PXRD patterns were well comparable to the
corresponding simulated one based on the single-crystal X-ray data,
indicative of a pure phase of each product (Fig. S1).
To evaluate the stability of the coordination architectures with
entangled structures, TGA was carried out to examine the thermal
stability of compounds 1–5 in the temperature range of 30–800 °C
as shown in Fig. S2 (Supporting Information). For 1, the first weight
loss of 10.85% is close to the calculated value (10.33%)
corresponding to the loss of the dimethylamine in the range of 100
and 200 °C. It should be noted that [H2NMe2]+ cations is removed
but the proton is needed for charge balance. The second weight
loss of 3.51% between 280 °C and 310 °C is attributed to the loss of
formic ligands composite (calculated: 3.67%), there is a sudden
weight loss of L4- ligand above 350 °C, giving final metal oxides. 2 is
stable up to 120°C and then loses weight to 490 °C, corresponding
to the release of coordinated pyridine molecules and
decomposition of organic ligands. For 3, the first weight loss of
15.02% is close to the calculated value (15.14%) corresponding to
the loss of the dimethylamine and the coordinated H2O molecule in
the range of 100 and 280 °C, and the second weight loss of 54.19%
between 300 °C and 500 °C is attributed to the loss of ligand
composite (calculated: 53.49%). Compound 4 loses the weight of
3.65% ranging from 100 to 260 °C, which can be attributed to the
removal of lattice water molecules (calc. 3.75%). Then it has
undergone a sudden weight loss above 350 °C, giving final metal
oxides. For 5, the first weight loss of 10.27% ranging from 90 to 340
°C, which can be ascribed to the removal of lattice water and
ethanol molecules (calc. 10.61%). With temperature increasing,
there is a sudden weight loss of 4,4′-bipy and H2L2- ligands above
350 °C, giving final metal oxides.
Fig. 6 The emission spectra of H4L and 1-5 excited at 359 nm in the solid
state at room temperature.
The fluorescence lifetime τ values of complexes 1-5 and the free
ligand are on the nanosecond timescale at room temperature, as
shown in Table 2 and Fig. S3-S8 (Supporting Information). The τ
values of these complexes are fitted to two components by a
biexponential decay curve except that of 3, which is fitted to three
components triexponentially. The fluorescence lifetime of 1-5 is
similar. This reveals that different crystal structures and
coordination models in 1-5 have no significant effect on the
fluorescence lifetime. The nanosecond timescale of lifetime shows
that the emission is fluorescent in nature. 35,37−39
Photoluminescence Properties of 1-5
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play crucial roles in determining the properties of CPs. Excluding the
effects of ligands and geometries, research into isostructural CPs is
beneficial for determining the relationships between metal ions and
the corresponding properties of CPs.47−52 As to isostructural 2 and
2a-2c, they consist of same secondary building units
with different proportions of CdII and CuII, CoII, NiII ions,
respectively. To determine the relationships between metal ions
and the corresponding catalytic properties of isostructural 2 and 2a-
2c, the catalyzed reactions of aromatic nitriles with 1,3-
diaminopropane were performed.
Table 2 Summary of Solid-State Photoluminescent
Data for of the H4L Ligand and Complexes 1-5.
χ2
Ligand/complex λem (nm)
τ, ns; (weight)
426
τ1 = 0.9507 (0.52)
τ2 = 3.9482 (0.48)
H4L
0.926
1.086
1.180
0.962
439
τ1 = 1.8121 (0.40)
τ2 = 5.2321 (0.60)
1
423
τ1 = 1.7702 (0.43)
τ2 = 4.9348 (0.57)
2
441
τ1 = 0.9725 (0.20)
τ2 = 3.3635 (0.65)
τ3 = 8.3612 (0.15)
3
As shown in Table 3, 2a and 2b are effective catalysts for the
cascade cyclozation of 4-pyridyl nitriles with 1,3-diaminopropane to
generate 2-(pyridin-4-yl)-1,4,5,6-tetrahydropyrimidine. 2a was
higher active than 2b, giving a conversion of 57 % and 43 %,
respectively. By contrast, 2 exhibits poor yield, which may originate
from the inactive CdII central-metal ions. Meanwhile, 2c indicates
lower catalytic capacity than 2a-2b, which can be attributed to the
low degree of exchange extent. Similarly, 2a and 2b are relative
effective, while 2 and 2c give poor conversions, towards the
442
τ1 = 2.1979 (0.68)
τ2 = 4.7857 (0.32)
4
1.162
1.066
448
τ1 = 2.4945 (0.67)
τ2 = 5.6240 (0.33)
5
Measured at λex = 359 nm.
synthesis
of
2-phenyl-1,4,5,6-tetrahydropyrimidine.
Central-metal exchange of 2
After 1-5 were synthesized, a further investigation was performed
to determine which types of metal cations can generate CPs with
similar structures. Under the similar conditions, divalent Cu(II),
Co(II), and Ni(II) cations were selected to synthesize CPs, but all
attempts failed. In view of its ability to prepare CPs featuring metals
that are difficult to obtain for specific compounds through direct
syntheses,40−45 we explored the possibility of replacing the Zn(II),
Cd(II) cations in CPs 1-5 with Cu(II), Co(II), and Ni(II) cations by
central-metal exchange method. Fortunately, colorless crystals of 2
turned green in a few hours when they were immersed in a 0.05
mol·L-1 solution of Cu(NO3)2 in DMF and heated in a 10 mL capped
vial at 50 °C. Similar phenomenon were also observed when crystals
of 2 were immersed in a 0.05 mol·L-1 solution of Co(NO3)2 or
Ni(NO3)2 in DMF and heated at 50 °C. The exchange products were
proved by ICP atomic emission spectrometric analysis, which
showed that 61% replacement of CdII ions by CuII ions (the
incomplete exchange product labeled as 2a) after 72 h, 55% CdII
ions were replaced by CoII ions (the incomplete exchange product
labeled as 2b) and only 8% CdII ions were replaced in NiII case (the
incomplete exchange product labeled as 2c). Obviously, relative
stabilities of the 2 analogues were deduced to be in the order of CuII
> CoII >> NiII. CuII typically inserts to the greatest extent. The high
electronegativity of CuII would enable it to form bonds that are
more covalent and thermodynamically stable. PXRD study (Fig. S9 of
the Supporting Information) indicated that the incompletely
exchanged products 2a-2c had the same framework as 2.
Table 3 Synthesis of 1,4,5,6-tetrahydropyrimidine
derivatives with Different Catalysts.
Entry
Ar
Ph
Product
7a
Catalyst
Yield (%)
trace
36
1
2
3
4
5
6
7
8
2
2a
2b
2c
2
15
trace
trace
57
2a
2b
2c
4-pyridyl
7b
43
12
As observed in the structure of 2, all CdII ions are seven-coordinated
and are coordinated with not only carboxylic groups but also
pyridine molecules. Since pyridine molecules are good leaving
groups, they may departure during catalytic process, leaving
unsaturated metal sites to be exposed to aromatic nitriles. Central-
metal ions can be used as unsaturated metal centers providing
vacant coordination sites/Lewis acid sites. The catalytic capacity of
2 and 2a-2c increases in the order of 2 < 2c< 2b < 2a, consistent
with the expansion of their Lewis acid strength. This trend indicates
that the electron accepting ability of central-metal ions can be a
crucial factor in determining the reaction rate for this catalyzed
reaction. The catalytic activity of a metal ion is largely governed by
its Lewis acidity. The stronger the Lewis acidity, the larger the effect
The catalytic capacity of 2 and 2a-2c
Although large amounts of CPs have been reported, their huge
potential in heterogeneous catalysis is still not fully appreciated and
far less explored. In recent work of our group,46 from the study of
three heterogeneous catalysts synthesized by CoII and pdpa4-
(H4pdpa = 5,5'-(pentane-1,2-diyl)-bis(oxy)diisophthalic acid) for the
synthesis of 2-imidazoline derivatives, we have gained some
understanding of the relationship between their structural features
and catalytic capacities. For instance, accessible catalytic sites and
suitable channel size and shape have positive effect on the catalytic
ability for heterogeneous catalysts. As well as the use of versatile
connectors during construction, central-metal ions undoubtedly
8 | J. Name., 2012, 00, 1-3
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ion
on
substrate
activation.53
Acknowledgements
This work was financially supported by the National Natural Science
Foundation (No. 21371155) and Research Fund for the Doctoral
Program of Higher Education of China (20124101110002).
Scheme 3. Proposed mechanism for the synthesis of 1,4,5,6-
tetrahydropyrimidine derivatives catalyzed by MII-based CPs.
Notes and references
1
2
3
4
5
6
7
8
9
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1,4,5,6-tetrahedropyrimidines catalyzed by the MII-based CPs is
proposed (Scheme 3). The nitrile is first activated by the catalyst to
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reation closely matched that of the pristine solid of 2a, strongly
proving that 2a is stable under the catalytic condition (Fig. S10). In
addition, 2a recovered from the catalytic reaction via centrifugation
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metal ions and corresponding catalytic property of CPs. Thus,
the effect of active metal centers as Lewis acid can be clarified
in the catalytic reaction process. However, improving catalytic
activity through central-metal exchange is a very young field.
More practices should be done to find out how the new
central metal activates the substrate more effectively compare
with the parent central metal for a given catalytic reaction.
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Central-metal Exchange, Improved Catalytic Activity, Photoluminescence
Properties of A New Family of d10 Coordination Polymers Based on 5, 5’-(1H-
2, 3, 5-triazole-1, 4-diyl)diisophthalic Acid Ligand
Huarui Wanga,b, Chao Huanga, Yanbing Hana, Zhichao Shaoa, Hongwei Houa,*, Yaoting Fana
Five d10 coordination polymers (CPs) have been successfully isolated under hydrothermal conditions by the reaction of
polycarboxylate ligand H4L together with Zn(II)/Cd(II) salts in the presence of no or different pyridine-based linkers. Central-metal exchange
in CP 2 leads to a series of isostructural M(II)-Cd CPs (M = Cu, Co, Ni) showing improved catalytic activity for the synthesis of 1,4,5,6-
tetrahedropyrimidine derivatives.
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