In situ hydrothermal syntheses of five new cadmium(ii) coordination polymers based on 3-(1H-tetrazol-5-yl)benzoate ligand

Article abstract of DOI:10.1039/c2ce26720a

Five new cadmium(ii) tetrazole-based compounds, {[Cd₄(3-tzba) ₄(H₂O)₂]·2H₂O (1), [Cd(3-tzba)(2,2′-bipy)₂] (2), [Cd(3-tzba)(1,10-phen)] (3), [Cd₃(3-tzba)₂(NA)₂(H₂O)] (4) and [Cd(3-tzba)(BIO)] (5), where 3-tzba = 3-(1H-tetrazol-5-yl)benzoate, 2,2′-bipy = 2,2′-bipyridine, 1,10-phen = 1,10-phenanthroline, NA = nicotinic acid, BIO = benzo-biimidazolediones}, were successfully synthesized involving the in situ generation of ligand under hydrothermal conditions. The compounds were thoroughly characterized by elemental analyses, Fourier transform infrared spectroscopy, thermal studies, and single crystal and powder X-ray diffraction. Compound 1 represents a three-dimensional (3D) coordination framework built up by the assembly of two-dimensional (2D) Cd-3-tzba wheel layers and 3-tzba linkers, exhibiting a pcu (4¹²·6 ³) topological net. Compound 2 displays a one-dimensional (1D) zigzag chain formed by the linkage of Cd(ii) centers and 3-tzba ligands. Compound 3 shows a 1D pearl-necklace-like chain built by the interconnection of Cd(ii) centers and 3-tzba ligands. Compound 4 is a 3D coordination framework which is constructed by 1D [Cd₃] chains with 3-tzba and NA ligands linkers displaying a hxl (3⁶) type net. Compound 5 exhibits a 3D sql (4 ⁴) topological network, with 1D Cd chains linking the adjacent 3-tzba and BIO ligands. It is noted that the 3-tzba and BIO ligands are generated in situ from the precursor 3-cyanobenzoic acid and 1,2,4,5-benzenetetracarboxylic acid (H₄bta), respectively. In addition, the luminescent properties of these compounds were investigated.

Full text of DOI:10.1039/c2ce26720a

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In situ hydrothermal syntheses of five new cadmium(II)
coordination polymers based on 3-(1H-tetrazol-5-
yl)benzoate ligand3
Cite this: CrystEngComm, 2013, 15,
365
Li Liang, Chengfeng Yang, Yingzhao Ma and Hong Deng*
Five new cadmium(II) tetrazole-based compounds, {[Cd4(3-tzba)4(H2O)2]?2H2O (1), [Cd(3-tzba)(2,29-bipy)2]
(2), [Cd(3-tzba)(1,10-phen)] (3), [Cd3(3-tzba)2(NA)2(H2O)] (4) and [Cd(3-tzba)(BIO)] (5), where 3-tzba = 3-
(1H-tetrazol-5-yl)benzoate, 2,29-bipy = 2,29-bipyridine, 1,10-phen = 1,10-phenanthroline, NA = nicotinic
acid, BIO = benzo-biimidazolediones}, were successfully synthesized involving the in situ generation of
ligand under hydrothermal conditions. The compounds were thoroughly characterized by elemental
analyses, Fourier transform infrared spectroscopy, thermal studies, and single crystal and powder X-ray
diffraction. Compound 1 represents a three-dimensional (3D) coordination framework built up by the
assembly of two-dimensional (2D) Cd-3-tzba wheel layers and 3-tzba linkers, exhibiting a pcu (412?63)
topological net. Compound 2 displays a one-dimensional (1D) zigzag chain formed by the linkage of Cd(II)
centers and 3-tzba ligands. Compound 3 shows a 1D pearl-necklace-like chain built by the interconnection
of Cd(II) centers and 3-tzba ligands. Compound 4 is a 3D coordination framework which is constructed by
1D [Cd3] chains with 3-tzba and NA ligands linkers displaying a hxl (36) type net. Compound 5 exhibits a 3D
sql (44) topological network, with 1D Cd chains linking the adjacent 3-tzba and BIO ligands. It is noted that
the 3-tzba and BIO ligands are generated in situ from the precursor 3-cyanobenzoic acid and 1,2,4,5-
benzenetetracarboxylic acid (H4bta), respectively. In addition, the luminescent properties of these
compounds were investigated.
Received 11th August 2012,
Accepted 26th October 2012
DOI: 10.1039/c2ce26720a
www.rsc.org/crystengcomm
via hydrothermal synthesis.5 With the development of this
method, a variety of tetrazole-based coordination frameworks
Introduction
Over the past decades, there has been a dramatic increase in
the synthesis of tetrazole compounds, not only owing to their
novel structural architectures and topologies, but also to their
potential applications in speciality explosives, adsorption,
sensors, magnetic and optical materials.1 Since the first
tetrazole compound was found by Bladin,2 the synthesis of
5-substituted 1H-tetrazole derivatives has been an active field
of study in the development supramolecular chemistry and
crystal engineering.3 It is a landmark breakthrough that
Demko and Sharpless pioneered a safe, convenient and
environmentally friendly synthetic method for the synthesis
which present various novel coordination networks with
diverse topologies and interesting functional properties have
been reported via in situ solvo-hydrothermal synthesis.6 For
example, Long et al. systematically synthesized a series of
highly porous 3,8-connected frameworks by linking cubic
M4Cl(BTT)8(H2O)4 (H3BTT = 1,3,5-benzenetristetrazolate; M =
Mn2+, Cu2+, Fe2+) clusters with different triangular polytetra-
zolate ligands, while Zubieta et al. reported a microporous
material
[Co2(H0.67bdt)3]?20H2O
[H2bdt
=
5,59-(1,4-
phenylene)bis(1H-tetrazole)] exhibiting single chain magnet-
ism and reversible changes in magnetism upon desolvation
and resolvation.7,8 Bu et al. obtained a microporous com-
pound, [Zn(tdp)] (H2tdp = 2,3-di-1H-tetrazol-5-ylpyrazine), in
which each Zn(II) ion is chelated by three tdp22 ligands and
of 5-substituted 1H-tetrazoles derivatives via
a [2 + 3]
cycloaddition reaction of azide anions and nitriles in water
with the aid of Lewis acid as a catalyst.4 Xiong et al. improved
this method using Zn(II) as a catalyst for in situ cycloaddition
and the formation of zinc tetrazole coordination compounds
each ligand bridges three Zn(II) ions, giving
a chiral
3-connected etd network.9 We also constructed a luminescent
microporous cadmium–organic compound [Cd(m2-Cl)(m4-
5MT)]n (5MT = 5-methyl-1H-tetrazole) that exhibits high
sensitivity to nitrite in both DMF and water.10
School of Chemistry & Environment, South China Normal University, Key Laboratory
of Theoretical Chemistry of Environment, Ministry of Education, Guangzhou 510006,
P. R. China. E-mail: dh@scnu.edu.cn
Recently, bridging ligands, for example, multidentate
aromatic ligands containing multiple coordination sites such
as the N-heterocyclic group6 and carboxylate group11 have
Electronic supplementary information (ESI) available. CCDC 854939–854943.
3
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2ce26720a
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been extensively studied and proved to be a rational choice to
synthesize coordination compounds possessing unique struc-
tures and potential applications. The tetrazolate–benzoate ligand
containing tetrazolate and carboxyl groups with four nitrogen
and two oxygen potential coordination sites has excellent
coordination ability and displays diverse coordination
modes.12,13 However, the investigation of bifunctional tetrazo-
late–benzoate ligand as building blocks for constructing coordi-
nation frameworks has been less developed so far.14 On the other
hand, the introduction of auxiliary ligands has an important
influence on structural and functional diversity as well.
Generally, the chelating auxiliary ligands (such as 2,29-bipy,
1,10-phen) manipulate the structures in terminal mode forming
lower dimension frameworks,13e,14a,15 while some other bridging
ligands (such as 4,49-bipy, nicotinic acid, isonicotinic acid)
control the structures in bridging mode, forming higher
dimension frameworks.13b,16 Therefore, to a certain extent, the
crystal structures with different dimensions can be obtained by
the introduction of suitable ancillary ligands.
Based on the above considerations and in our continuing
research on the assembly of tetrazole-based coordination
compounds,17 we used 3-cyanobenzoic acid, sodium azide and
different auxiliary ligands (2,29-bipyridine, 1,10-phenanthroline,
nicotinic acid and 1,2,4,5-benzenetetracarboxylic acid) with
different Cd(II) salts to synthesize novel tetrazole-based metal–
organic frameworks (MOFs), since 3-cyanobenzoic acid can form
3-(1H-tetrazol-5-yl)benzoic acid (3-H2tzba) ligand through the [2 +
3] cycloaddition reaction of azide anions and nitriles in water
with the aid of Lewis acid Cd(II) salts as the catalysts. The
3-H2tzba ligand containing tetrazole and carboxylate functional
groups permits a range of versatile bridging modes. Meanwhile,
the addition of different auxiliary ligands can make the
structures of the targeted products more versatile, due to the
chelating ligands (2,29-bipyridine and 1,10-phenanthroline) and
the bridging ligands (nicotinic acid and 1,2,4,5-benzenetetracar-
boxylic acid) having different coordination modes. Furthermore,
the cadmium metal ions not only can adopt various coordination
numbers and geometries, but also exhibit luminescent proper-
ties when bound to functional ligands.18
In this paper, by the in situ ligand reaction and by the
presence/absence of secondary ligands, five cadmium(II)
tetrazole-based
coordination
compounds:
[Cd4(3-
tzba)4(H2O)2]?2H2O (1); [Cd(3-tzba)(2,29-bipy)2] (2); [Cd(3-
tzba)(1,10-phen)] (3); [Cd3(3-tzba)2(NA)2(H2O)] (4) and [Cd(3-
tzba)(BIO)] (5) were obtained through hydrothermal synthesis.
In these compounds, the tetrazole and carboxylate groups of
the tetrazolate–benzoate adopt different coordination modes
generating different coordination motifs, including 1D chains
(2, 3) and 3D frameworks (1, 4, 5), respectively. It is interesting
that 3-tzba and BIO ligands were generated in situ. In
particular, in the case of the BIO, an unexpected ligand was
generated via the Schmidt reaction of H4bta and NaN3 in the in
situ formation of compound 5. To the best our knowledge, the
BIO ligand was first prepared by hydrothermal technique and
used for constructing coordination compounds. The thermal
stabilities and the solid-state photoluminescence properties of
these cadmium(II) compounds were also investigated.
Experimental section
Materials and physical measurements
All the materials and reagents were obtained commercially
and used without further purification. Elemental (C, H, N)
Table 1 Crystallographic data and structure refinement summary for compounds 1–5
Compound
1
2
3
4
5
Formula
Mr
T/K
C32H24Cd4N16O12
1274.31
296
C28H20CdN8O2
612.93
296
C20H12CdN6O2
480.77
296
C28H18Cd3N10O9
975.75
296
C16H10CdN8O4
490.73
296
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Orthorhombic
P21/n
12.380(3)
16.488(3)
21.246(4)
90
Orthorhombic
Pbca
10.984(10)
17.667(16)
26.08(2)
90
90
90
5061(8)
8
1.609
Monoclinic
C2/c
25.919(4)
9.9766(16)
17.701(3)
90
128.489(2)
90
3582.7(10)
8
1.783
1.250
1904.0
8754
3216
Monoclinic
P21/c
13.6219(13)
16.5119(16)
13.4293(13)
90
93.153(1)
90
3016.0(5)
4
2.149
2.170
1888.0
15 162
5400
Orthorhombic
Pna21
18.220(3)
13.269(2)
6.8783(11)
90
90
90
1662.9(5)
4
1.960
a (u)
b (u)
90
90
c (u)
V (Å3)
Z
4336.8(15)
4
1.952
rcalcd (g cm23
)
M (mm21
F(000)
)
2.011
0.907
1.361
968.0
8231
2958
2464.0
21 808
7791
2464.0
24 237
4541
Reflns collected
Unique reflns
GOF
1.077
1.031
1.067
1.191
1.020
Rint
0.0596
0.0682
0.2049
0.0418
0.0307
0.0732
0.0236
0.0289
0.0685
0.0229
0.0273
0.0615
0.0392
0.0278
0.0603
R1 [I . 2s(I)]a
wR2 (all data)b
a
b
2
R1 = g||Fo| 2 |Fc||/g|Fo|. wR2 = {g[w(Fo 2 Fc2)2]/g(w(Fo2)2}1/2
.
366 | CrystEngComm, 2013, 15, 365–375
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analyses were performed on a Perkin-Elmer 2400 element
analyzer. Infrared (IR) samples were prepared as KBr pellets,
and spectra were obtained in the 4000–400 cm21 range using a
(0.033 g, 0.5 mmol) and H2O (10 mL) was stirred for 30 min at
room temperature and kept in a 23 mL Teflon-lined autoclave
at 150 uC for 3 days. The mixtures were cooled to room
temperature at 5 uC h21. Colorless block single crystals of 4
were obtained (yield: 42% based on Cd). Elemental Anal. Calcd
(%) for 4, C28H18Cd3N10O9: C, 34.47; H, 1.86; N, 14.36. Found:
C, 34.50; H, 1.92; N, 14.30. IR data (KBr, cm21): 3424 (br), 1614
Nicolet
Avatar
360
FT-IR
spectrophotometer.
Thermogravimetric analysis (TGA) experiments were carried
out on a Perkin-Elmer TGA 7 thermogravimetric analyzer with
the heating rate of 10 uC min21 from 35 to 900 uC under dry air
atmosphere. Powder XRD investigations were carried out on a
Philips PW-1830 X-ray diffractometer with Cu Ka radiation.
Fluorescence spectra were recorded with an Edinburgh FLS920
spectrophotometer analyzer.
Table 2 Selected bond distances (Å) for compounds 1–5
Compound 1a
Syntheses
Cd(1)–N(13)
Cd(1)–N(2)#1
Cd(1)–O(7)
2.282(8)
2.412(9)
2.444(8)
2.274(9)
2.304(9)
2.332(8)
2.287(8)
2.316(9)
2.377(9)
2.246(10)
2.342(8)
2.431(10)
2.627(8)
Cd(1)–O(5)
Cd(1)–O(8)
Cd(1)–O(6)
Cd(2)–O(3W)
Cd(2)–O(8)
Cd(2)–O(10)#2
Cd(3)–O(6)
Cd(3)–N(7)#4
Cd(3)–O(11)#1
Cd(4)–O(11)
Cd(4)–O(10)
Cd(4)–N(10)#6
2.316(7)
2.412(7)
2.447(8)
2.279(11)
2.330(7)
2.350(8)
2.297(7)
2.356(9)
2.447(8)
2.297(8)
2.394(8)
2.553(9)
CAUTION! Azido and tetrazolate compounds are potentially
explosive under the hydrothermal reaction conditions. Only a
small amount of compound should be prepared and they
should be handled with extreme care.
Cd(2)–N(9)
Cd(2)–N(3)#1
Cd(2)–N(14)
Cd(3)–N(11)#3
Cd(3)–N(1)#1
Cd(3)–O(12)#1
Cd(4)–N(6)#5
Cd(4)–O(9)
Synthesis of [Cd4(3-tzba)4(H2O)2]?2H2O (1)
A mixture of CdCl2 (0.092 g, 0.5 mmol), 3-cyanobenzoic acid
(0.074 g, 0.5 mmol), NaN3 (0.033 g, 0.5 mmol) and H2O (10 mL)
was stirred for 30 min at room temperature and kept in a 23
mL Teflon-lined autoclave at 150 uC for 3 days. The mixtures
were cooled to room temperature at 5 uC h21. Colorless block
single crystals of 1 were obtained (yield: 45% based on Cd).
Elemental Anal. Calcd (%) for 1, C32H24Cd4N16O12: C, 30.16; H,
1.90; N, 17.59. Found: C, 30.10; H, 1.98; N, 17.50. IR data (KBr,
cm21): 3412 (br), 1609 (s), 1585 (s), 1508 (s), 1475 (s), 1457(s),
1408 (s), 1168 (m), 1082 (m), 885 (w), 745 (w), 683 (w).
Cd(4)–O(4W)
Cd(4)–O(5)
Compound 2b
Cd(1)–N(6)#1
Cd(1)–N(2)
Cd(1)–N(4)
Cd(1)–O(2)
2.272(3)
2.385(3)
2.412(3)
2.574(3)
Cd(1)–O(1)
Cd(1)–N(1)
Cd(1)–N(3)
2.325(3)
2.407(3)
2.522(3)
Synthesis of [Cd(3-tzba)(2,29-bipy)2] (2)
Compound 3c
A mixture of CdCl2 (0.092 g, 0.5 mmol), 3-cyanobenzoic acid
(0.074 g, 0.5 mmol), 2,29-bipy (0.078 g, 0.5 mmol), NaN3 (0.033
g, 0.5 mmol) and H2O (10 mL) was stirred for 30 min at room
temperature and kept in a 23 mL Teflon-lined autoclave at 150
uC for 3 days. The mixtures were cooled to room temperature
at 5 uC h21. Colorless block single crystals of 2 were obtained
(yield: 35% based on Cd). Elemental Anal. Calcd (%) for 2,
C28H20CdN8O2: C, 54.87; H, 3.29; N, 18.28. Found: C, 54.35; H,
3.34; N, 18.25. IR data (KBr, cm21): 3099 (br), 1623 (s), 1591 (s),
1503 (s), 1467 (s), 1447 (s), 1383 (s), 1155 (m), 1011 (m), 877
(w), 748 (w), 667 (w).
Cd(1)–O(1)
Cd(1)–N(2)
Cd(1)–N(1)
2.242(3)
2.341(3)
2.371(3)
Cd(1)–N(4)#1
Cd(1)–N(5)#2
Cd(1)–O(2)
2.297(3)
2.366(3)
2.440(3)
Compound 4d
Cd(1)–O(8)#1
Cd(1)–N(4)#2
Cd(1)–O(2)#3
Cd(2)–N(5)
2.223(3)
2.332(3)
2.365(3)
2.293(3)
2.380(3)
2.558(3)
2.640(3)
2.240(3)
2.311(2)
2.348(3)
Cd(1)–N(10)
Cd(1)–N(6)
2.286(3)
2.356(3)
2.401(3)
2.273(3)
2.295(3)
2.439(3)
2.274(3)
2.331(4)
2.383(3)
Cd(1)–O(3)#3
Cd(2)–O(6)#4
Cd(2)–O(4)#3
Cd(2)–N(9)#5
Cd(3)–O(5)#3
Cd(3)–O(1)
Cd(2)–N(1)
Cd(2)–O(7)#4
Cd(2)–O(5)#3
Cd(3)–O(9)#1
Cd(3)–O(3)#3
Cd(3)–N(2)
Synthesis of [Cd(3-tzba)(1,10-phen)] (3)
Cd(3)–N(7)
A mixture of CdSO4 (0.128 g, 0.5 mmol), 3-cyanobenzoic acid
(0.074 g, 0.5 mmol), 1,10-phen (0.099 g, 0.5 mmol), NaN3
(0.033 g, 0.5 mmol) and H2O (10 mL) was stirred for 30 min at
room temperature and kept in a 23 mL Teflon-lined autoclave
at 150 uC for 3 days. The mixtures were cooled to room
temperature at 5 uC h21. Colorless block single crystals of 3
were obtained (yield: 32% based on Cd). Elemental Anal. Calcd
(%) for 3, C20H12CdN6O2: C, 49.97; H, 2.52; N, 17.48. Found: C,
49.60; H, 2.40; N, 17.50. IR data (KBr, cm21): 3068 (br), 1611
(s), 1594 (s), 1504 (s), 1473 (s), 1445 (s), 1396 (s), 1145 (m), 943
(m), 881 (w), 745 (w), 694 (w).
Compound 5e
Cd(1)–O(3)
Cd(1)–N(6)#2
Cd(1)–O(2)
Cd(1)–O(1)
2.287(3)
2.327(4)
2.358(2)
2.554(3)
Cd(1)–N(7)#1
Cd(1)–O(4)#3
Cd(1)–O(4)#4
2.297(4)
2.354(3)
2.538(3)
a
Symmetry codes: #1: 2x + 5/2, y + 1/2, 2z + 1/2; #2: 2x + 3/2, y +
1/2, 2z + 1/2; #3: x + 1, y, z; #4: x + 1/2, 2y + 3/2, z 2 1/2; #5: 2x +
b
2, 2y + 1, 2z + 1; #6: 2x + 3/2, y 2 1/2, 2z + 1/2. Symmetry codes:
#1: 2x + 1/2, y + 1/2, z. Symmetry codes: #1: x 2 1/2, 2y + 1/2, z 2
1/2; #2: 2x + 1/2, 2y + 1/2, 2z + 1. Symmetry codes: #1: x, 2y + 5/
c
d
2, z 2 1/2; #2: 2x + 2, 2y + 2, 2z + 1; #3: x, 2y + 3/2, z + 1/2; #4: x,
y, z 2 1; #5: 2x + 1, 2y + 2, 2z + 1. Symmetry codes: #1: x 2 1/2,
e
Synthesis of [Cd3(3-tzba)2(NA)2(H2O)] (4)
2y + 1/2, z; #2: 2x + 3/2, y + 1/2, z + 1/2; #3: x + 1/2, 2y + 1/2, z 2
1; #4: 2x + 1/2, y + 1/2, z 2 1/2.
A mixture of CdCl2 (0.092 g, 0.5 mmol), 3-cyanobenzoic acid
(0.074 g, 0.5 mmol), nicotinic acid (0.062 g, 0.5 mmol), NaN3
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(s), 1593 (s), 1500 (s), 1481 (s), 1448 (s), 1403 (s), 1165 (m), 1030
(m), 878 (w), 752 (s), 694 (w).
bifunctional bridging main ligand, the 3-H2tzba ligands
possessing the four nitrogen atoms of tetrazole and the two
oxygen atoms of carboxylate serve as either bridging building
blocks or as multidentate ligands, which can lead the
Synthesis of [Cd(3-tzba)(BIO)] (5)
A mixture of CdCl2 (0.092 g, 0.5 mmol), 3-cyanobenzoic acid
(0.074 g, 0.5 mmol), H4bta (1.27 g, 0.5 mmol), NaN3 (0.033 g,
0.5 mmol) and H2O (10 mL) was stirred for 30 min at room
temperature and kept in a 23 mL Teflon-lined autoclave at 150
uC for 3 days. The mixtures were cooled to room temperature
at 5 uC h21. Red prismatic single crystals of 5 were obtained
(yield: 45% based on Cd). Elemental Anal. Calcd (%) for 5,
C16H10CdN8O4: C, 39.16; H, 2.05; N, 22.83. Found: C, 39.10; H,
2.10; N, 22.90. IR data (KBr, cm21): 3082 (br), 1693 (s), 1639 (s),
1581 (s), 1510 (s), 1483 (s), 1451 (s), 1410 (s), 1167 (m), 926 (m),
895 (w), 743 (w), 687 (w).
formation of
a variety of coordination frameworks. In
compounds 1–5, the 3-tzba ligand presents eight kinds of
coordination modes (Scheme 2). In particular, it is the first
time that the coordination modes a–d and f–h are observed in
tetrazole-5-carboxylate ligands.
In the absence of a secondary ligand, a 3D molecular
configuration of compound
1 exhibiting a
pcu (412?63)
topological net was obtained. As we know, the addition of
small auxiliary ligands can lead to the formation of different
coordination frameworks, so the influence of secondary
ligands on the architecture of coordination compounds was
investigated. By the addition of bidentate chelating ligand,
2,29-bipyridine for 2 and 1,10-phenanthroline for 3, into the
reaction system, the 1D frameworks were obtained. As NA and
BIO secondary ligands as building units can build a bridge to
connect metal ions at both ends, thus compounds 4 and 5 all
display a 3D network. These results indicate that the choice of
different auxiliary ligands has significant effect on structural
diversity.
The solvo/hydrothermal method has been widely used in the
preparation of high quality stable single crystals.20 Many
coordination compounds, which are difficult to obtain by
routine synthetic methods, can be formed in situ via a facile
solvo/hydrothermal strategy. In particular, these reactions are
not always simple coordination between metal ion and
ligands, some unique reaction may occur that involves in situ
hydrolysis, oxidation, and ligand synthesis.20 For example, Xu
et al. reported a new route for preparing a 2D coordination
compounds [Co(m3-Hbcbh)(bpy)]n (H3bcbh = benzene-4-carbox-
ylate-1,2-bihydrazide). The H3bcbh is derived from the in situ
acylation reaction of H4bta with N2H4?H2O.21 In our experi-
ment, reaction of 3-cyanobenzoic acid, H4bta, NaN3 with
cadmium salt under hydrothermal conditions results in the
formation of compound 5 exhibiting a 3D network based on
3-tzba and BIO and Cd(II) center. It is interesting that both
3-tzba and BIO were generated via in situ ligand synthesis.
Undoubtedly, as discussed earlier, 3-tzba was formed via the [2
+ 3] cycloaddition reaction of an azide with nitriles in water
with the aid of a Lewis acid.4 But how was the BIO ligand
generated?
Crystal structure determination
Single crystal X-ray diffraction data collections of 1–5 were
performed on a Bruker Apex II CCD diffractometer operating
at 50 kV and 30 mA using Mo Ka radiation (l = 0.71073 Å).
Data collection and reduction were performed using the APEX
II software.19 Multiscan absorption corrections were applied
for all the data sets using the APEX II program.19 The
structures were solved by direct methods and refined by full-
matrix least-squares on F2 using the SHELXL program
package.19 All non-hydrogen atoms were refined with aniso-
tropic displacement parameters. Hydrogen atoms attached to
carbon were placed in geometrically idealized positions and
refined using a riding model. Hydrogen atoms on water
molecules were located from difference Fourier maps and were
also refined using a riding model. Crystallographic data for
compounds 1–5 are listed in Table 1. Selected bond lengths
and angles are given in Table 2 and Table S1 (ESI ) for
3
compounds 1–5.
Results and discussion
Synthesis
Compounds 1–5 were obtained through the hydrothermal
reaction of 3-cyanobenzoic acid, sodium azide and different
auxiliary ligands with the aid of cadmium salts. The synthetic
route to the five compounds is shown in Scheme 1. With the
aid of cadmium salts, the in situ [2 + 3] cycloaddition reactions
of 3-cyanobenzoic acid and sodium azide took place. As a
Scheme 1 In situ hydrothermal synthesis of compounds 1–5.
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acylium ion I obtained from protonation and loss of water.
Reaction with hydrazoic acid gets the protonated azido ketone
II, which goes through a rearrangement reaction with the alkyl
group R, migrating over the C–N bond with expulsion of
nitrogen. The protonated III is attacked by water forming
carbamate IV, which after deprotonation loses CO2 to obtain
the amine V (Scheme S1, ESI ). According to our reaction,
3
H4bta goes through a rearrangement reaction with expulsion
of nitrogen and CO2 obtaining (a) and loses the water forming
(b), which automatically change to (c) through tautomerism
(Scheme 3). Due to the complexities involved in the in situ
ligand syntheses and supramolecular assemblies, it is difficult
to explain the exact reaction mechanism in the one-pot, black-
box-like hydrothermal metal–ligand reactions.20
Description of crystal structures
[CD4(3-TZBA)4(H2O)2]?2H2O (1). Compound 1 represents a 3D
coordination framework constructed by the assembly of 2D
Cd-3-tzba wheel layers and 3-tzba linkers, exhibiting the
orthorhombic space group P21/n. The asymmetric unit
contains four crystallographically independent Cd(II) ions,
four types of 3-tzba ligands, two coordinated water molecules
and two free water molecules (Fig. 1a). As shown in Fig. S1,
ESI, among the four unique Cd(II) metal ions, the Cd1, Cd2
3
and Cd3 ions adopt slightly distorted octahedral coordination
environments: Cd1 ion is coordinated by four oxygen atoms
from two different 3-tzba ligands, and two nitrogen atoms
from two different 3-tzba ligands; Cd2 ion is surrounded by
three nitrogen donors of three different 3-tzba ligands, and
three oxygen donors of two different 3-tzba ligands and one
coordinated water; Cd3 ion is bound to three nitrogen atoms
of three different 3-tzba ligands, and three oxygen atoms of
two different 3-tzba ligands. Meanwhile the Cd4 center has a
slightly distorted monocapped trigonal prism coordination
geometry that is defined by five oxygen atoms of three
different 3-tzba ligands and one coordinated water, and two
nitrogen atoms of two different 3-tzba ligands. This kind
coordination mode of Cd(II) ion has been found in many
metal–organic frameworks,23 but it is the first time that the
Cd(II) ion has been observed in a tetrazolate compound so far.
The Cd–N/O bond lengths range from 2.274(9) to 2.627(8) Å
and the N–Cd–N/O bond angles fall between 53.4(3) and
179.2(3)u. The bridging 3-tzba ligands adopt four different
coordination
kN6:kN7:kO7:kO8;
modes:
m5-kN1:kN2:kN3:kO5:kO6;
m4-
m4-
m6-kN9:kN10:kN11:kO9:kO10;
kN13:kN14:kO11:kO12 (Scheme 2, a–d), all of which are
observed in tetrazole-5-carboxylate compounds at the first
time.
On the basis of the connectivity of the multifunctional
3-tzba ligands, a 10-membered ring is generated containing
three Cd1 ions, two Cd2 ions, two Cd3 ions and three Cd4
ions, where each Cd(II) ion is linked by 3-tzba ligands. Each 10-
membered ring is further connected to the adjacent rings by
sharing three Cd(II) ions, resulting in a highly ordered layered
wheel network along the c-axis (Fig. 1b). The adjacent wheel
layers are interconnected through 3-tzba ligands to form a 3D
coordination framework (Fig. 1c and d). To better understand
the nature of the intricate framework, a topological approach
to simplify the 3D frameworks can be accomplished. Thus, if
Scheme 2 Coordination modes of ligands observed in this paper.
According to the synthetic systems we adopted, the Schmidt
reaction mechanism for the formation of BIO ligand can be
proposed.22 The carboxylic acid Schmidt reaction starts with
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Scheme 3 The probable reaction mechanism of 1,2,4,5-benzenetetracarboxylic acid (H4bta) in compound 5.
the 10-membered rings are considered as the six-connected
Cd(II) ion, one 3-tzba ligand and one 1,10-phen ligand. Each
Cd(II) ion is coordinated by four nitrogen atoms from two
different 3-tzba ligands and one 1,10-phen ligand, and two
oxygen atoms from carboxyl group of the 3-tzba ligand in a
slightly distorted octahedral coordination environment. The
Cd–N and Cd–O bond lengths range from 2.242(3)–2.440(3) Å,
and the N–Cd–N/O bond angles fall between 53.87(9)–
142.06(9)u. The 3-tzba ligands acting in m3-kN4:kN5:kO1:kO2
coordination mode are connected to three Cd(II) ions
(Scheme 2f), and it is the first time observed in tetrazolate-5-
carboxylate compounds as well. Whereas the 1,10-phen ligand
is involved in chelating coordination mode with nitrogen
atoms of pyridine rings linking one Cd(II) ion.
nodes (Fig. S2, ESI ), the whole framework of 1 can be
3
described as a pcu net with Schlafli symbol (412?63) (Fig. 1e).
¨
[CD(3-TZBA)(2,29-BIPY)2] (2). Compound 2 represents a 1D zigzag
chain constructed by the linkage of the Cd(II) centers and
3-tzba ligands and crystallizes in the orthorhombic space
group Pbca. As shown in Fig. 2a, in the asymmetric unit of 2
there are one crystallographically independent Cd(II) ion, one
3-tzba ligand and two 2,29-bipy ligands. Each Cd(II) ion is
coordinated to five nitrogen atoms from two different 2,29-bipy
ligands and one 3-tzba ligand, and two oxygen atoms from the
carboxyl group of 3-tzba ligand exhibiting a slightly distorted
monocapped trigonal prism coordination geometries, which
has been found in a tetrazole compound for the first time so
far as well. The Cd–N bond lengths range from 2.272(3) to
2.522(3) Å and the N–Cd–N bond angles fall between 66.99(10)
and 165.52(9)u. The Cd–O bond lengths are 2.325(3) and
2.574(3) Å and the N–Cd–O bond angles range from 52.64(8) to
146.50(9)u. The Cd(II) centers are spanned by a 3-tzba ligand
with a distance of 11.83 Å between the metal ions. All 3-tzba
ligands coordinated to the two Cd(II) ions are involved in m2-
kN6:kO1:kO2 bridging (Scheme 2e), while the 2,29-bipy ligand
displays a chelating coordination mode, the nitrogen atoms of
pyridine rings connecting one Cd(II) ion.
It is noted that the tetrazole groups of 3-tzba ligands connect
two neighboring Cd(II) ions forming a dimeric unit [Cd2N4]
…
with the Cd Cd distance of 4.05 Å, far different to that in
compound 2. The dimeric unit [Cd2N4] is doubly bridged by
two 3-tzba ligands generated an 18-membered ring. Finally,
each 18-membered ring is further linked to the adjacent rings
by sharing two Cd(II) octahedrons, resulting in a 1D pearl-
necklace-like chain running in the ab-plane (Fig. 3b). These
…
chains are in turn interconnected by C–H N hydrogen bonds
…
[C6–H6 N3: 3.501(18) Å] and extend to form a 2D supramo-
lecular network (Table 3, Fig. 3c).
Compared with 2 and 3, two similar frameworks have been
recently reported:13e [Zn(3-tzba)(2,2-bipy)(H2O)]?3H2O (29) and
[Zn2(3-tzba)2(1,10-phen)2]?3H2O (39), in which the central
metal ions differ from those in 2 and 3. In compounds 2
and 3, the central metal ions are Cd(II) ions, while in 29 and 39
the central metal ions are Zn(II) ions. So the coordination
geometries are different: monocapped trigonal prism
[CdN5O2] in 2; octahedron [CdN4O2] in 3; octahedron
[ZnN3O3] in 29; square-pyramidal [ZnN3O2] in 39. The coordi-
nation modes of 3-tzba and 2,29-bipy ligands in 2 and 29 are
the same, while in 3 and 39, the 3-tzba ligand has different
modes: m3-kN4:kN5:kO1:kO2 in 3; m3-kN3:kO1:kO2 in 39.
It is very interesting that the 3-tzba ligands ligate Cd(II) ions,
forming an infinite zigzag cadmium chain running along the
c-axis (Fig. 2b). In addition, hydrogen bonding interactions
play an important role in forming this structure. The adjacent
chains are assembled by two kinds of hydrogen bonding
…
…
interactions [C(4)–H(4) O(2): 3.140(6) Å; C(13)–H(13) O(2):
3.401(6) Å] forming a 3D supramolecular framework along the
c direction (Table 3, Fig. 2c, Fig. S2, ESI ).
3
[CD(3-TZBA)(1,10-PHEN)] (3). Compared with the structure of 2,
compound 3 crystallizes in the monoclinic system space group
C2/c and displays a 1D pearl-necklace-like chain constructed
by the interconnection of the Cd(II) centers and 3-tzba ligands.
As depicted in Fig. 3a, the asymmetric unit consists of one
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Fig. 2 (a) Ball and stick plot showing the asymmetric unit of compound 2. All H
atoms are omitted for clarity. Symmetry code: a: 0.5 2 x, 0.5 + y, z. (b) View of
the 1D zigzag chain constructed by the linkage of the Cd(II) centers and 3-tzba
ligands along the c-axis. (c) View of the 3D supramolecular framework of
compound 2 formed by hydrogen bonding.
Fig. 1 (a) Ball and stick plot showing the asymmetric unit of compound 1. All H
atoms and free water molecules are omitted for clarity. (b) A polyhedral view of
the 2D Cd-3-tzba wheel layer of compound 1 in the c-axis direction. All 3-tzba
ligands are omitted for clarity. (c) A polyhedral view of 3D coordination
frameworks constructed by the assembly of 2D Cd-3-tzba wheel layers and
3-tzba linkers in the ac-plane and (d) in the ab-plane. (e) View of the pcu net
when each 10-membered ring is considered as a node.
The bridging 3-tzba ligands adopt two different coordination
modes: m5-kN6:kN7:kN9:kO6:kO7; m4-kN1:kN2:kN4:kO2:kO3
(Scheme 2, g and h), both of which are also found for the
first time in tetrazolate-5-carboxylate compounds. And NA
ligands are also involved in two different bridging coordina-
tion modes: m3-kN5:kO4:kO5; m3-kN10:kO8:kO9 (Scheme 2, i
and j).
It is noted that the structure of 4 contains a [Cd3] chain
which is built by the interconnection Cd(II) ions and 3-tzba
ligands (Fig. 4b). When viewed along the b/c axis, it is noted
that these parallel 1D chains are alternately connected through
m4/m5-3-tzba ligands and m3-NA ligands. Therefore, an inter-
[CD3(3-TZBA)2(NA)2(H2O)] (4). Single crystal X-ray diffraction
structure analysis reveals that compound 4 crystallizes in the
monoclinic space group P21/c and features a 3D coordination
framework constructed by 1D [Cd3] chains, 3-tzba and NA
linkers. As shown in Fig. 4a, the asymmetric unit of 4 contains
three crystallographically independent Cd(II) ions, two types of
3-tzba ligands, two NA ligands and one coordinated water. The
two unique Cd(II) ions, Cd1 and Cd3, both have a distorted
octahedral coordination geometry: Cd1 is coordinated by three
nitrogen atoms from two different 3-tzba ligands and one NA
ligand, and three oxygen atoms from one 3-tzba ligand and
one NA ligand; Cd3 is surrounded by two nitrogen donors of
two different 3-tzba ligands, and four oxygen atoms from two
different NA ligands, one 3-tzba ligand and one coordinated
water. Cd2 has a slightly distorted monocapped trigonal prism
coordination geometry which is bound to three nitrogen
donors of two different 3-tzba ligands and one NA ligand, and
four oxygen atoms from one 3-tzba ligand and one NA ligand.
The Cd–N/O bond lengths fall between 2.223(3)–2.640(3) Å and
the N–Cd–N/O bond angles range from 52.32(9) to 175.82(12)u.
Table 3 Distance (Å) and angles (u) of hydrogen bonds for compounds 2 and 3
…
…
…
D–H
A
Distance (D A)
Angle (D–H A)
Compound 2a
…
C(4)–H(4) O(2)#1
3.140(6)
3.401(6)
165
148
…
C(13)–H(13) O(2)#2
Compound 3b
…
C(6)–H(6) N(3)#1
3.501(8)
160
a
Symmetry codes: #1: 1/2 + x, y, 1/2 2 z; #2: 3/2 2 x, 1/2 + y, z.
Symmetry codes: #1: 21/2 + x, 3/2 2 y, 21/2 + z.
b
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Fig. 3 (a) Ball and stick plot showing the asymmetric unit of compound 3. All H
atoms are omitted for clarity. Symmetry code: a: 20.5 + x, 0.5 2 y, 20.5 + z; b:
0.5 + x, 0.5 2 y, 1 2 z. (b) View of the 1D pearl-necklace-like chain constructed
from the interconnection of the Cd(II) centers and 3-tzba ligands in the ab-
plane. (c) View of the 2D supramolecular framework of compound 3 formed by
hydrogen bonding.
Fig. 4 (a) Ball and stick plot showing the asymmetric unit of compound 4. All H
atoms are omitted for clarity. Symmetry code: a: x, 2.5 2 y, 20.5 + z; b: 2 2 x, 2
2 y, 1 2 z; c: x, 1.5 2 y, 0.5 + z; d: x, y, 21+ z; e: 1 2 x, 2 2 y, 1 2 z. (b) View of
the infinite [Cd3] chain for compound 4 constructed from each set of three Cd(II)
ions linked by the tetrazole group of 3-tzba ligands in ac-plane. (c) View of the
3D coordination framework constructed from 1D [Cd3] chains and 3-tzba and
NA linkers along the c-axis, b-axis (d) and a-axis (e). The ligands were simplified
as lines for clarity. (f) A hxl type net when the [Cd3] chains are regarded as finite
‘‘node’’.
locked 3D network is produced (Fig. 4c and d). It is difficult to
assign a topological network to this structure. Since a series of
metal carboxylate coordination frameworks in terms of the
…
packing and interconnection of 1D rods are proposed by Yaghi
atom of BIO ligands with Cd Cd separation of around 4.088 Å
6
and O’Keeffe,24 4 has a hxl type net with Schlafli symbol (3 )
forming a 1D Cd chain along the c-axis. The chains are
alternately connected by 3-tzba and BIO ligands generating a
3D framework (Fig. 5b). If the chains are considered as finite
¨
when the [Cd3] chains are regarded as finite ‘‘nodes’’ (Fig. 4e
and f).
four-connected nodes, the whole structure of 5 can also be
[CD(3-TZBA)(BIO)] (5). Compound 5 crystallizes in an orthorhom-
bic space group Pna21. X-ray crystallographic analysis shows
that it is a 3D coordination framework with 1D Cd chains
linking the adjacent 3-tzba and BIO ligands. As shown in
Fig. 5a, the asymmetric unit of 5 contains one crystal-
lographically independent Cd(II) ions, one 3-tzba ligand and
one BIO ligand. The seven-coordinated Cd(II) ion has a slightly
distorted monocapped trigonal prism coordination geometries
with two nitrogen atoms from two different 3-tzba ligands, and
five oxygen atoms from one 3-tzba ligand and three BIO
ligands. The bond lengths of Cd–O/N fall between 2.287(3)–
2.554(3) Å and the O–Cd–N/O bond angles range from
53.22(10) to 162.91(11)u. All bridging 3-tzba ligands involved
in m3-kN6:kN7:kO1:kO2 mode (Scheme 2f) and the BIO ligands
also show the bridging coordination mode in m3-kO3:kO4
(Scheme 2k).
4
¨
described as an sql net with Schlafli symbol (4 ) (Fig. 5c).
Thermal analyses and powder X-ray diffraction (PXRD)
measurements
To examine the thermal stabilities of compounds 1–5, the
thermogravimetric analyses of these compounds were carried
out from 35 to 900 uC at a heating rate of 10 uC min21 in dry air
atmosphere. As shown in Fig. S4, ESI, compounds 2–5 are
3
stable up to approximately 300 uC, while beyond that, their
frameworks start the decomposition of the host network. The
first weight loss for compound 1 occurs below 300 uC,
corresponding to the loss of free or coordinated water
molecules.
Simulated and experimental powder X-ray diffraction
(PXRD) patterns of 1–5 are shown in Fig. S5–S9, ESI. They
3
are in fairly good agreement with the experimental patterns,
It is noted that the adjacent Cd(II) ions are interconnected
through the tetrazole group of 3-tzba ligands and the oxygen
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compounds 1–5 have main peaks at 434, 439, 539, 436 and 451
nm, with the excitation at 372, 376, 382, 346 and 374 nm,
respectively (Fig. S10, ESI ).
3
There are typical two types of electronic excited state
transition: ligand-to-ligand charge transfer (LLCT) and
ligand-to-metal charge transfer (LMCT) in ZnII/CdII com-
plexes.26 In order to better understand the room-temperature
fluorescent mechanism, Zheng et al. demonstrated the origin
of the spectral emissions observed in the spectra of the
complexes can be attributed to ligand-centered charge transi-
tions based on the 3-tzba and mixed-ligand base on the
theoretical calculated results by evaluation of the density of
states (DOS).13e From the reported literature, the free 3-H2tzba
ligands present the photoluminescence emission centered at
407 nm with the excitation at 362 nm at room temperature.13e
In our case, it is noted that the emission spectra of compound
3 shows a main peak at 539 nm with a shoulder peak at 457
nm excited at 382 nm. The twin peaks appeared in the
emission spectra may originate from the mixed-ligand system
(3-tzba and 1,10-phen ligands) and supramolecular interac-
tions such as p–p stacking and hydrogen bonds interaction,
leading to multiple electronic transitions with close transition
energies. Compared with the free 3-H2tzba ligands emitted at
407 nm, the fluorescence emission spectra of compounds 1–5
show the fluorescence intensity increased and exhibits a
significant red-shift in the emission maximum, which may be
ascribed to the cooperative effects of intraligand emission.27
The variation of photoluminescence may be attributed to the
difference of the auxiliary ligands to cadmium metal ions and/
or their local coordination environment.
Fig. 5 (a) Ball and stick plot showing the asymmetric unit of compound 5. All H
atoms are omitted for clarity. Symmetry code: a: 0.5 2 x, 0.5 + y, 20.5 + z; b: 0.5
+ x, 0.5 2 y, 21 + z; c: 1.5 2 x, 0.5 + y, 0.5 + z; d: 20.5 + x, 0.5 2 y, z. (b) View of
the 3D coordination framework built from 1D Cd chains linked by the 3-tzba
ligands and BIO ligands. (c) An sql type net when 1D Cd chains are regarded as
finite ‘‘node’’.
Conclusion
In summary, we have synthesized five new cadmium(II)
tetrazole-based coordination polymers using one-pot hydro-
thermal methods involving the in situ generation of 3-tzba and
BIO ligands. The 3-tzba ligand was prepared via the [2 + 3]
cycloaddition reaction of 3-cyanobenzonic acid and azide with
the aid of Cd(II) salts, while the BIO ligand was obtained via
the Schmidt reaction of H4bta and azide. These experimental
results demonstrate in situ ligand synthesis under solvo/
hydrothermal conditions can generate new ligands and
unexpected structures, which cannot be obtained by the direct
reaction of metal salts and ligands under normal conditions.
Undoubtedly, in situ metal–ligand reactions open a new
avenue for the construction of new metal–organic compounds
under a solvo/hydrothermal condition. In addition, by the
absence of an auxiliary ligand, compound 1 produces a 3D
molecular configuration exhibiting a pcu (412?63) topology.
With the introduction of bidentate 2,29-bipyridine and 1,10-
phenanthroline, both compounds 2 and 3 exhibit 1D frame-
works, while by the addition of bridging NA and H4bta ligands
compounds 4 and 5 show 3D structures featuring the hxl (36)
and sql (44) topologies, respectively. These results indicated
that the introduction of appropriate secondary ligands cannot
only control synthesis of the different dimension crystal
structures but also generate versatile tetrazole-based MOFs.
which clearly confirms the phase purity of the as-prepared
products.
Infrared spectroscopy
The FT-IR spectra of compounds 1–5 are similar, the broad
absorption bands around 3400 cm21 in 1 and 4 are ascribed to
the stretching vibrations of O–H, suggesting the presence of
free and/or coordinated water molecules. The emergence of
peaks 1400–1500 cm21 clearly confirms the formation of
tetrazole groups.18 The strong peaks of carboxyl groups in 1–5
appear in the region of 1581–1639 cm21 (antisymmetric
stretching vibrations) and 1383–1457 cm21 (symmetric stretch-
ing vibrations). The strong band in
5
at 1693 cm21
corresponds to the CLO stretching vibration. No peaks around
1700 cm21 are found, illustrating complete deprotonation of
the carboxylic acid groups in all compounds.25
Photoluminescence properties
It is universally acknowledged that many Cd(II) compounds
show excellent luminescence properties.18 Herein, the photo-
luminescence measurement of all compounds were carried out
in the solid state at room temperature. The emission spectra of
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