Use of polyazaheterocycles in the assembly of new cadmium sulfate frameworks: Synthesis, structure, and properties

Article abstract of DOI:10.1021/cg100865v

The reaction of cadmium sulfate in the presence of polyazaheterocyclic organic molecules gave rise to a variety of new cadmium sulfate phases in water containing solvothermal reaction. The compounds have two- (I) and three-dimensionally (II-VI) extended s

Full text of DOI:10.1021/cg100865v

DOI: 10.1021/cg100865v
Use of Polyazaheterocycles in the Assembly of New Cadmium Sulfate
Frameworks: Synthesis, Structure, and Properties
2010, Vol. 10
4161–4175
Avijit Kumar Paul,† Udishnu Sanyal,‡ and Srinivasan Natarajan*,†
†Framework Solids Laboratory, Solid State and Structural Chemistry Unit, Indian Institute of Science,
Bangalore-560012, India, and ‡Department of Inorganic and Physical Chemistry, Indian Institute of
Science, Bangalore-560012, India
Received June 30, 2010
ABSTRACT: The reaction of cadmium sulfate in the presence of polyazaheterocyclic organic molecules gave rise to a variety of
new cadmium sulfate phases in water containing solvothermal reaction. The compounds have two- (I) and three-dimensionally
(II-VI) extended structures. All the compounds have structures built up by the connectivity involving the cadmium octahedra
and the sulfate tetrahedra in which the heterocyclic organic molecules act as the ligand. The linkages between the Cd2þ and
(SO4)2- ions form one- (II), two- (I, III, and IV), and three- (V and VI) dimensionally extended cadmium sulfate phases. The
connectivity between Cd2þ ion and the heterocyclic ligand also gives rise to one- and two-dimensional structures. The inter-
connectivity between the two units gives rise to the observed structures. The presence of Cd-O-Cd chains and Cd-O-Cd
layers in some of the structures is noteworthy. The adsorption/desorption studies suggest that the cadmium sulfate phases
adsorb/desorb anionic dyes selectively in the presence of water/ethanol, respectively. The photocatalytic degradation studies on
cationic dyes under UV-irradiation indicate modest activity. The cyanosilylation of imines using the present compounds as
heterogeneous catalyst indicate good catalytic behavior. The various properties exhibited by the cadmium sulfate phases suggest
that these compounds are versatile. All the compounds were characterized by powder X-ray diffraction, thermogravimetric
analysis, infrared (IR) and UV-visible studies.
Introduction
has been attempted with reasonable success.11 It appears that
a fine balance exists between the reactivity of the carboxylates
and the basicity of the heterocycles in the formation of such
structures. Itoccurred tous thatitmay beprofitable toinvesti-
gate the formation of new open-structured solids using the
polyazaheterocycles along with a tetrahedral anion, such as
the sulfates. It may be noted that reports of sulfate-based open-
framework compounds are not numerous.4a,12 The use of aza-
heterocycles in the formation of sulfate frameworks resulted
in only limited success.13
Our current research focuses on the synthesis, structure,
and possibleapplicationsof newframework materials. Recent
studies on framework cadmium thiosulfate compounds in-
dicated interesting structures and catalytic properties.8,14 The
wastewater treatment is important and photocatalytic decom-
position of the organic pollutants in water has attracted con-
siderable attention in recent years.15 We have been interested
in the study of new materials that could be used in wastewater
treatment photocatalytically. To this end, we have made at-
tempts to prepare a family of cadmium sulfate phases in the
presence of polyazaheterocyclic ligands. During the course
of our investigations, we have prepared a series of new frame-
work cadmium sulfate compounds with two- and three-
dimensional structures. Thus, the compound, [(C3H7N6)2]-
[Cd3(C3H6N6)2(H2O)4(SO4)4] I, has a two-dimensional struc-
ture, and the remaining compounds [Cd(C5H5N5)(SO4)] II,
[Cd3(H2O)2(C2H2N3)2(SO4)2] III, [Cd2(C2H5N5)(SO4)2] IV,
[Cd5(OH)2(CH2N5)4(SO4)2] V, and [Cd2(OH)(H2O)(C2H3N4)-
Compounds exhibiting open structures provide immense
opportunities for exploring many important properties in the
areas of catalysis, sorption, and separation processes.1 The
potential as well as actual applications of the open-framework
compounds provide significant opportunities for the synthetic
chemists. Persistent efforts over the years, by many research
groups, resulted in the incorporation of most of the elements
of the periodic table as part of this family of compounds.2-5
Though the silicate and the phosphate anions have been the
dominant building units in most of the synthesized open-
framework solids,3 recent research has demonstrated that
other anions such as sulfates/sulfites,4 selenates/selinites,5
etc. can also be incorporated as part of the open structures.
The open-framework compounds are, generally, synthe-
sized in the presence of nitrogen-containing organic amine
molecules, which act as the templating or structure-directing
agent. It has been shown, in some cases, that the amine mole-
cules also act as a ligand binding with the metals.6 In addition,
the nitrogen-containing heterocyclic compounds can also act
as ligands for the metal ions, and such ligands have been em-
ployed for enhancing the dimensionality of the open-framework
compounds.7 We have recently employed this strategy for the
preparation of a new family of framework compounds based on
thiosulfate anions.8
Nitrogen-containing heterocyclic organic molecules have
been employed for thepreparation of a number of compounds
in the presence of suitable metal ions.9 The resulting solids ex-
hibit considerable variety in their structures and properties.10
Use of polyazaheterocycles along with aromatic carboxylates
(SO4)] 2H2O VI have three-dimensionally extended struc-
3
tures. The compounds are examined for the adsorption of
the organicdyesfrom water. Inthis paper, wedescribethe syn-
thesis, structure, adsorption, and preliminary studies on the
possible applications of the synthesized cadmium sulfate
phases as heterogeneous catalysts.
*To whom correspondence should be addressed. E-mail: snatarajan@
sscu.iisc.ernet.in.
r
2010 American Chemical Society
Published on Web 08/04/2010
pubs.acs.org/crystal

4162 Crystal Growth & Design, Vol. 10, No. 9, 2010
Paul et al.
Table 1. Synthetic Composition and Conditions Employed for the Preparation of the Cadmium Sulfate Compounds, I-VIa
time
pH
S. no.
1
composition
temp (°C) (day) (initial, final)
product
yield (%)
92
0.33(3CdSO4 8H2O) þ 1(melamine) þ
125
125
125
125
125
125
6
6
6
6
6
6
6.6, 6.0
4.3, 5.4
3.5, 5.3
4.1, 7.2
2.9, 4.7
5.2, 6.0
[(C3H7N6)2][Cd3(C3H6N6)2(H2O)4(SO4)4], I
3
95CH3CN þ 51EtOH þ 388H2O
2
3
4
5
6
0.33(3CdSO4 8H2O) þ 1(adenine) þ
[Cd(C5H5N5)(SO4)], II
88
91
90
85
89
3
95CH3CN þ 51EtOH þ 388H2O
0.33(3CdSO4 8H2O) þ 1(1,2,4-triazole) þ
[Cd3(H2O)2(C2H2N3)2(SO4)2], III
[Cd2(C2H5N5)(SO4)2], IV
3
95CH3CN þ 51EtOH þ 388H2O
0.33(3CdSO4 8H2O) þ 1(guanazole) þ
3
95CH3CN þ 51EtOH þ 388H2O
0.33(3CdSO4 8H2O) þ 1(5-aminotetrazole) þ
[Cd5(OH)2(CH2N5)4(SO4)2], V
3
95CH3CN þ 51EtOH þ 388H2O
0.33(3CdSO4 8H2O) þ 0.5(Cd(NO3)2 4H2O) þ
[Cd2(OH)(H2O)(C2H3N4)(SO4)] 2H2O, VI
3
3
3
1(NaN3) þ 95CH3CN þ 51EtOH þ 388H2O
a Elemental analysis calcd (%) for I: C 11.07, H 2.63, N 25.91; Found: C 11.12, H 2.68, N 25.97; Calcd (%) for II: C 17.46, H 1.46, N 20.38; Found: C
17.39, H 1.52, N 20.31; Calcd (%) for III: C 6.84, H 1.14, N 11.97; Found 6.78, H 1.19, N 11.91; Calcd (%) for IV: C 4.65, H 0.97, N 13.56; Found: C 4.71,
H 0.91, N 13.61; Calcd (%) for V: C 4.27, H 0.89, N 24.91; Found: C 4.35, H 0.92, N 24.99; Calcd (%) for VI: C 5.05, H 2.12, N 11.79; Found: C 5.15, H
2.19, N 11.71.
Synthesis and Initial Characterization. All the compounds
were prepared by using solvothermal conditions in the pre-
sence of water at 125 °C in a Teflon-lined autoclave. The
reagents, 3CdSO4 8H2O (Merck, 98%), Cd(NO3)2 4H2O
(Merck, 99%), NaN3 (SD FINE, 99%) melamine (CDH, 99%),
adenine (SD FINE, 99%), 1,2,4-triazole (CDH, 99%), 3,5-
diamino 1,2,4-triazole or guanazole (CDH, 99%), and 5-ami-
notetrazole (CDH, 99%), were used as received and without
any further purifications.
KBr pellets (Perkin-Elmer, SPECTRUM 1000). IR spectra
of all the compounds were comparable, and six distinct regions
can be identified (see Supporting Information, Figure S2). (i)
The compounds have bands at the 3450-3350 cm-1 region
indicative of the presence of water molecules (-OH unit
also), and the bands in the region 3360-3260 cm-1 can be
assigned to the υ(N-H) modes of the NH2 group; (ii) a band
at 3100 cm-1 can be assigned to aromatic υ(C-H) modes of
the heterocyclic ring; (iii) the bands in the region 1687-1550
cm-1 can be assigned to the C-H bending vibrations and
due to the asymmetric and symmetric modes of the hetero-
cyclic ring; (iv) the bands are in the 1280-1200 cm-1 region
for υs (C-C) or υs (C-N) of the heterocyclic rings; (v) 1120-
1050 cm-1 region, with a sharp band at 1080 cm-1 is due to υs
(S-O); and (vi) the sharp bands in the region of 620 cm-1 are
due to the bending mode of mono- or tridentate oxygen of
SO4 units.
UV-vis Spectroscopy. The solid-state UV-visible spectra
of all the samples were recorded in the diffuse reflectance
mode atroom temperature (Perkin-Elmer model Lambda 35).
The powdered sample was placed inside a cuvette (75 mm ꢀ
80 mm ꢀ 20 mm), and the spectrum was recorded using an
integrating sphere of radius 50 mm. The reflectance spectra
were converted into an absorption like spectra using the
Kubelka-Munk equation (see Supporting Information,
Figure S3). The diffuse reflectance spectra [F(R) vs
wavelength] of the compounds exhibited a sharp peak cen-
tered at the 250-300 nm region, which may be due to the
intraligand charge-transfer. Compound IV exhibits an addi-
tional peak centered around 385 nm, which could be due to
the charge transfer from the guanazole unit to the metal.
From the UV-vis spectra, estimates for the band gaps were
obtained. The band gaps for the compounds were found to
be in the range of 4.13-4.96 eV. For compound IV, the
band gap for the charge-transfer band was calculated to be
3.26 eV.
3
3
In a typical synthesis, for [(C3H7N6)2][Cd3(C3H6N6)2-
(H2O)4(SO4)4] I, 3CdSO4 8H2O (0.257 g, 0.33 mM) and
3
melamine (0.126 g, 1 mM) were dissolved in 7 mL of distilled
water. To this, 5 mL of CH3CN and 3 mL of EtOH were
added under continuous stirring. The mixture was homo-
genized for 30 min at room temperature. The final mixture
with the composition, 3CdSO4 8H2O/3C3H6N6/285CH3CN/
3
153EtOH/1164H2O was sealed in a 23 mL PTFE-lined
autoclave and heated at 125 °C for 6 days under autogenous
pressure. The initial pH of the reaction mixture was 6.6 and
the final pH was observed to be 6.0. The final product, con-
taining large quantities of colorless rectangular crystals, was
filtered, washed with deionized water under a vacuum, and
dried at ambient conditions (yield ∼ 90% based on Cd). A
similar synthesis procedure was followed for the preparation
of all the other compounds (Table 1). For the preparation of
[Cd2(OH)(H2O)(C2H3N4)(SO4)] 2H2O, VI, we also added
3
0.154 g of Cd(NO3)2 4H2O (0.5 mM) and 0.065 g NaN3
3
(1 mM). In all the cases, we obtained good quality single cry-
stals with good yields. Thus, blocklike crystals of III, IV and
rodlike crystals of II, V, and VI were collected by filtration,
washed with deionized water, and dried at ambient condi-
tions. CHN analysis was performed for all the compounds
using a Thermo Finnigan FLASH EA 1112 CHN analyzer
(Table 1).
The compounds were characterized by powder X-ray
diffraction (XRD), IR, UV-vis, and TGA studies. The
powder X-ray diffraction (PXRD) patterns (Philips, X’pert-
Pro) were recorded in the 2θ range of 5-50° by using
Thermal Studies. TGA studies (Mettler-Toledo TG850)
of all the compounds were carried out in an atmosphere of
flowing oxygen (flow rate = 50 mL min-1) in the tempera-
ture range 30-850 °C (heating rate = 5 °C min-1) (see
Supporting Information, Figure S4). The studies indicate
that the compounds II, IV, and V appear to be stable up to
360 °C. The compounds I, III, and VI exhibit gradual
weight loss below the 300 °C, which may be due to the loss
of water molecules from the structures. The TGA studies
˚
Cu-KR radiation (λ = 1.5418 A). The XRD patterns
indicated that the products were new and the observed
XRD patterns were entirely consistent with the simulated
XRD pattern generated based on the structures determined
using the single crystal XRD studies (see Supporting Infor-
mation, Figure S1). Infrared (IR) spectroscopic studies have
been carried out in the mid-IR region (4000 to 400 cm-1) on

Article
Crystal Growth & Design, Vol. 10, No. 9, 2010 4163
Table 2. Crystal Data and Structure Refinement Parameters for I-VIa
structural parameter
I
II
III
IV
V
VI
empirical
formula
[(C3H7N6)2][Cd3-
(C3H6N6)2(H2O)4-
(SO4)4]
[Cd(C5H5N5)-
(SO4)]
[Cd3(H2O)2-
(C2H2N3)2
(SO4)2]
[Cd2(C2H5N5)-
(SO4)2]
[Cd5(OH)2
(CH2N5)4(SO4)2]
[Cd2(OH)(H2O)
(C2H3N4)-
(SO4)] 2H2O
3
formula
weight
1300.14
343.62
701.53
516.07
1124.49
474.95
crystal system
space group
triclinic
monoclinic
P21/c (No. 14)
6.7010(13)
11.860(2)
11.190(2)
90.000
103.73(3)
90.000
863.9(3)
4
2.642
monoclinic
P21/c (No. 14)
9.8131(4)
6.4065(3)
12.9635(6)
90.000
110.819(2)
90.000
761.77(6)
2
3.058
triclinic
monoclinic
P21/c (No. 14)
8.8562(6)
9.5734(7)
13.3383(9)
90.000
102.9140(10)
90.000
1102.27(13)
2
3.382
orthorhombic
Pna21 (No. 33)
9.6227(3)
16.7625(5)
7.1301(2)
90.000
90.000
90.000
1150.09(6)
4
2.708
P1 (No. 2)
6.8147(10)
10.3938(15)
14.487(2)
106.352(2)
97.049(2)
107.237(2)
916.3(2)
1
P1 (No. 2)
6.65880(10)
9.2325(2)
9.7163(2)
65.8430(10)
73.3020(10)
71.0830(10)
507.242(17)
2
˚
a (A)
˚
b (A)
˚
c (A)
R (°)
β (°)
γ (°)
3
˚
V (A )
Z
D (calc/
2.356
3.379
g cm-3
)
μ (mm-1
)
2.068
2.778
4.489
4.656
5.032
3.915
˚
λ (Mo KR/A)
θ range (°)
total data
collected
0.71073
1.50-26.00
9477
0.71073
2.54-26.00
6510
0.71073
2.22-26.00
5771
0.71073
2.34-26.00
7640
0.71073
2.36-26.00
8246
0.71073
2.43-25.99
6544
unique data
Rint
3571
0.078
1696
0.0255
1478
0.0346
1994
0.0242
2156
0.0220
2227
0.0315
R indexes
[I > 2σ(I)]
R indexes
(all data)
R1 = 0.0368;
wR2 = 0.0721
R1 = 0.0516;
wR2 = 0.0763
R1 = 0.0244;
wR2 = 0.0557
R1 = 0.0297;
wR2 = 0.0575
R1 = 0.0189;
wR2 = 0.0457
R1 = 0.0205;
wR2 = 0.0465
R1 = 0.0171;
wR2 = 0.0423
R1 = 0.0184;
wR2 = 0.0429
R1 = 0.0268;
wR2 = 0.0661
R1 = 0.0297;
wR2 = 0.0671
R1 = 0.0280;
wR2 = 0.0597
R1 = 0.0395;
wR2 = 0.0622
a R1 = Σ F0| - |Fc /Σ|F0|; wR2 = {Σ[w(F02 - Fc2)2]/Σ[w(F02)2]}1/2. w = 1/[σ2(F0)2 þ (aP)2 þ bP], P = [max(F02,0) þ 2(Fc)2]/3, where a = 0.0271
and b = 0.9445 for I, a = 0.0289 and b = 0.6661 for II, a = 0.0194 and b = 0.5264 for III, a = 0.0178 and b = 0.7575 for IV, a = 0.0273 and b = 7.3658
for V, a = 0.0175 and b = 5.5577 for VI.
indicate an initial weight loss of ∼5% in the range 200-250
°C for I, which may be due to the loss of the coordinated
water molecules (calc 5%). The second continuous weight
loss was observed to be 57% in the temperature range of
300-620 °C. For compound II, only one weight loss of
57% was observed in the range 420-630 °C. For III, we
observed an initial weight loss of 3% in the range 100-300
°C, which may be due to the loss of the water molecules
(calc 5%). The second weight loss of 33% was observed in
the range 320-650 °C. For compound IV, we observed a
single step weight loss of 39% in the range 420-580 °C. For
compound V, we observed the total weight loss of 30% in
the range 350-600 °C. For VI, we observed an initial
weight loss of 10% in the range 100-320 °C, which may
be due to the loss of the water molecules (calc 11%). The
second weight loss of 28% was observed in the range
320-650 °C. In all the cases, the calcined products were
found to be poorly crystalline by PXRD, and the majority
of the lines appear to correspond to a mixture of CdO
(ICDD No. 01-1049) and Cd3O2SO4 phases (ICDD No. 32-
0140).
solved and refined using SHELXL9718 present in the WinGx
suit of programs (Version 1.63.04a).19 The hydrogen posi-
tion of the μ3-OH group in the compound V and the water
molecules in the compound VI could not be located from
the difference Fourier maps. The possible hydrogen posi-
tion was arrived based on the bond valence sum (BVS)
calculations20 for the μ3-OH group. All the other hydrogen
positions of the heterocyclic ring for the compounds, the
hydrogen position of the lattice water molecules (I and III)
were located from the difference Fourier maps and for the
final refinement the hydrogen positions were placed in
geometrically ideal positions and refined in the riding
mode. The final refinement included atomic positions for
all the atoms, anisotropic thermal parameters for all the
non-hydrogen atoms, and isotropic thermal parameters for
all the hydrogen atoms. Full-matrix least-squares refine-
ment against |F2| was carried out using the WinGx package
of programs. Details of the structure solution and final
refinements for all the structures are given in Table 2. The
selected bond distances are given in Table 3. The other
geometrical parameters for all the compounds are given as
Supporting Information (Table S1-S6). The crystallo-
graphic data for the compounds, I-VI, were deposited
with the Cambridge Crystallographic Data Center
(CCDC) and can be downloaded free of charge by quoting
the numbers (CCDC: 777059-777064) via www.ccdc.cam.
ac.uk/data_request/cif.
Single-Crystal Structure Determination. A suitable single
crystal of each compound was carefully selected under a
polarizing microscope and glued to a thin glass fiber with a
cyanoacrylate (superglue) adhesive. The single crystal data
were collected on a Bruker AXS smart Apex CCD diffracto-
meter at 293(2) K. The X-ray generator was operated at 50 kV
˚
and 35 mA using Mo KR(λ= 0.71073 A) radiation. Data were
Adsorption and Photocatalytic Experiments. The organic
dyes for the adsorption studies, Congo red (CR), Coomassie
Brilliant Blue R-250 (CBB), and Alizarin red S (ARS) (S. D.
Fine-Chem. Ltd., India) were used as received. The water
used was double distilled, filtered through a Millipore mem-
brane. All the dye molecules, investigated in the present
study, were stirred with the prepared compounds in the dark
collected with ω scan width of 0.3°. A total of 606 frames were
collected at three different settings of j (0, 90, 180°) keeping
the sample-to-detector distance fixed at 6.03 cm and the
detector position (2θ) fixed at -25°. The data were reduced
using SAINTPLUS,16 and an empirical absorption correction
was applied using the SADABS program.17 The structure was

4164 Crystal Growth & Design, Vol. 10, No. 9, 2010
Paul et al.
Table 3. Selected Bond Distances Observed in
[(C3H7N6)2][Cd3(C3H6N6)2(H2O)4(SO4)4], I; [Cd(C5H5N5)(SO4)], II;
[Cd3(H2O)2(C2H2N3)2(SO4)2], III; [Cd2(C2H5N5)(SO4)2], IV;
[Cd5(OH)2(CH2N5)4(SO4)2], V; and
to explore surface adsorption, if any. In a typical experiment,
200 mg of the cadmium sulfate compound was added to 100 mL
of the dye solutions (conc. 100 ppm) and stirred for 90 min. The
supernatant liquid samples were collected at different time
intervals (0, 15, 30, 45, 60, 90 min, respectively), filtered thr-
ough Millipore membrane filters, and centrifuged to remove
the catalyst particles prior to analysis. The concentration of
the dye solution was determined using UV-vis spectroscopic
investigations.
The details of the experimental set up for the photocata-
lytic experiments have been described earlier.14 The photo-
catalytic degradation studies were carried out at 25 °C. The
organic dyes were dissolved in double distilled Millipore
filtered water. The dye solutions were collected at regular
time intervals and analyzed, after filtration and centrifuging
the solutions, using a UV-vis spectroscopic method.
Heterogeneous Catalytic Studies. The reagents for the
heterogeneous cyanosilylation experiments, N-benzilidine
aniline and trimethylsilyl cyanide (Aldrich), were used as
received. The reagents were taken in a 100 mL round-bottom
flask with the DCM solvent. After the addition of the solid
catalyst cadmium sulfate, the mixture was stirred for 2 h at
0 °C in N2 atmosphere. The product was filtered through
Millipore membrane filters to remove the catalyst particles.
The supernatant liquid was dried under a vacuum to remove
the solvent. The product was analyzed and evaluated for the
conversion of the reactants.
[Cd2(OH)(H2O)(C2H3N4)(SO4)] 2H2O, VIa
3
bond
˚
distance (A)
˚
distance (A)
bond
I
Cd(1)-O(1)
Cd(1)-O(2)
Cd(1)-O(3)
Cd(1)-O(1)#1
Cd(1)-O(2)#1
Cd(1)-O(3)#1
Cd(2)-O(4)#2
Cd(2)-O(5)
Cd(2)-O(6)
Cd(2)-O(7)
2.284(3)
2.281(4)
2.300(3)
2.284(3)
2.281(4)
2.300(3)
2.262(3)
2.310(4)
2.323(3)
2.332(3)
Cd(2)-O(8)
Cd(2)-N(1)
S(1)-O(1)
S(1)-O(4)
S(1)-O(7)
S(1)-O(8)#3
S(2)-O(3)#4
S(2)-O(6)
S(2)-O(9)
S(2)-O(10)
2.362(3)
2.326(4)
1.471(3)
1.466(3)
1.464(3)
1.485(3)
1.490(3)
1.480(4)
1.441(4)
1.448(4)
II
Cd(1)-O(1)
Cd(1)-O(2)
Cd(1)-O(3)#2
Cd(1)-O(4)#3
Cd(1)-N(1)
2.539(3)
2.320(2)
2.433(2)
2.433(2)
2.204(3)
Cd(1)-N(2)#1
S(1)-O(1)
S(1)-O(2)
S(1)-O(3)
S(1)-O(4)
2.239(3)
1.460(3)
1.485(2)
1.469(2)
1.477(2)
III
Cd(1)-O(1)
Cd(1)-O(1)#1
Cd(1)-O(2)#2
Cd(1)-O(4)
Cd(1)-N(1)
Cd(1)-N(3)
Cd(2)-O(4)
Cd(2)-O(4)#3
2.439(2)
2.221(2)
2.375(2)
2.670(2)
2.193(3)
2.192(3)
2.294(2)
2.294(2)
Cd(2)-O(5)
Cd(2)-O(5)#3
Cd(2)-N(2)
Cd(2)-N(2)#3
S(1)-O(1)
S(1)-O(2)
S(1)-O(3)
2.342(3)
2.342(3)
2.246(2)
2.246(2)
1.500(2)
1.468(2)
1.436(2)
1.479(2)
S(1)-O(4)
IV
Results
Cd(1)-O(1)
Cd(1)-O(2)
Cd(1)-O(3)
Cd(1)-O(4)
Cd(1)-O(5)
Cd(1)-N(1)
Cd(2)-O(4)
Cd(2)-O(5)
Cd(2)-O(6)
Cd(2)-O(8)
2.222(2)
2.306(2)
2.346(2)
2.362(2)
2.414(2)
2.265(2)
2.349(2)
2.365(1)
2.281(2)
2.296(1)
Cd(2)-O(8)#2
Cd(2)-N(2)#1
S(1)-O(1)
S(1)-O(2)#3
S(1)-O(5)#5
S(1)-O(6)#4
S(2)-O(3)
S(2)-O(7)
S(2)-O(8)
S(2)-O(4)#6
2.332(1)
2.199(2)
1.471(2)
1.458(2)
1.490(2)
1.476(2)
1.472(2)
1.446(2)
1.488(1)
1.492(2)
Structure of [(C3H7N6)2][Cd3(C3H6N6)2(H2O)4(SO4)4], I.
The asymmetric unit of I contains 32 non-hydrogen atoms.
There are two crystallographically independent Cd2þ ions
and sulfate units present in the asymmetric unit. The Cd(1)
is coordinated by four oxygen atom of the sulfate units and
two terminal water molecules forming a distorted octahe-
dral coordination (CdO4(H2O)2, CN = 6). The Cd(2) is
coordinated by four oxygen atom of the sulfate units, one
terminal water molecule, and one nitrogen atom of the
melamine unit forming a distorted octahedral coordination
(CdO4N(H2O), CN = 6). The average Cd-O distance is
V
Cd(1)-O(1)
Cd(1)-O(1)#2
Cd(1)-O(5)
Cd(1)-N(1)
Cd(1)-N(7)
Cd(1)-N(9)#1
Cd(2)-O(2)
Cd(2)-O(2)#4
Cd(2)-O(5)#3
Cd(2)-O(5)#2
Cd(2)-N(2)
2.406(4)
2.471(4)
2.293(4)
2.270(5)
2.279(4)
2.359(4)
2.283(4)
2.283(4)
2.266(4)
2.266(4)
2.312(5)
Cd(2)-N(2)#4
Cd(3)-O(3)#5
Cd(3)-O(4)
Cd(3)-O(5)#2
Cd(3)-N(3)
Cd(3)-N(6)
Cd(3)-N(8)
S(1)-O(1)
2.312(5)
2.254(4)
2.284(4)
2.291(4)
2.377(5)
2.350(4)
2.558(4)
1.480(4)
1.463(4)
1.478(4)
1.466(4)
˚
2.301 A. The cadmium centers are connected with the
sulfate units through a Cd-O-S bond with an average
angle of 136.2°. The sulfate units have an average S-O
bond distance of 1.468 A and an average O-S-O angle of
109.4°. The various geometric parameters observed in I are
comparable to other similar cadmium sulfate structures
reported in the literature.12,13 Selected bond distances are
listed in Table 3.
The octahedral Cd and the tetrahedral sulfate units are
connected together to form a two-dimensional cadmium-
sulfate layer structure (Figure 1a). Each cadmium ion is bon-
ded with four sulfate units. Of the two sulfate units, S(1)O4
unit binds with four cadmium ions through four oxygen
atoms in a η4, μ4-coordination mode, whereas the S(2)O4 unit
binds with only two cadmium ions through the two oxygen
atoms in a η2, μ2-coordination mode (see Supporting Infor-
mation, Figure S5). This connectivity between the Cd2þ ions
and the sulfate anions gives rise to an anionic layer structure
with four- and eight-membered rings. The layers are arran-
ged in a ABAB... fashion. The axial positions of the Cd(1)
octahedra have the water molecules, while a melamine and a
water molecule occupy a similar position for Cd(2). The
melamine unit is terminal and hangs in the interlayer spaces.
˚
S(1)-O(2)
S(1)-O(3)
S(1)-O(4)
VI
Cd(1)-O(1)
Cd(1)-O(2)
Cd(1)-O(5)
Cd(1)-O(6)
Cd(1)-N(1)
Cd(1)-N(4)#1
Cd(2)-O(3)
Cd(2)-O(5)
2.268(4)
2.294(4)
2.269(4)
2.251(5)
2.423(15)
2.494(15)
2.283(11)
2.318(11)
Cd(2)-O(4)#4
Cd(2)-O(5)#2
Cd(2)-N(2)
Cd(2)-N(3)#3
S(1)-O(1)
S(1)-O(2)#5
S(1)-O(3)#6
S(1)-O(4)
2.347(10)
2.205(11)
2.320(13)
2.315(13)
1.469(4)
1.471(5)
1.498(10)
1.440(11)
a Symmetry transformations used to generate equivalent atoms. I: #1
-x, -y þ 1, -z þ 1; #2 -x þ 1, -y þ 2, -z þ 1; #3 x - 1, y, z; #4 -x þ 1,
-y þ 1, -z þ 1. II: #1 x, -y - 1/2, z - 1/2; #2 -x þ 1, -y, -z þ 1; #3 -x
þ 2, -y, -z þ 1. III: #1 -x þ 1, y - 1/2, -z þ 1/2; #2 x, y - 1, z; #3 -x,
-y þ 1, -z. IV: #1 -x þ 2, -y þ 1, -z þ 2; #2 -x þ 3, -y þ 1, -z þ 1; #3
-x þ 1, -y þ 2, -z þ 1; #4 x - 1, y þ 1, z; #5 -x þ 2, -y þ 2, -z þ 1; #6
-x þ 2, -y þ 1, -z þ 1. V: #1 x, -y þ 3/2, z þ 1/2; #2 -x þ 1, -y þ 2, -z
þ 1; #3 x þ 1, y, z; #4 -x þ 2, -y þ 2, -z þ 1; #5 x, -y þ 3/2, z - 1/2. VI:
#1 x, y, z þ 1; #2 -x þ 1, -y þ 1, z - 1/2; #3 -x þ 1, -y þ 1, z þ 1/2; #4 x
- 1/2, -y þ 1/2, z; #5 x þ 1/2, -y þ 1/2, z; #6 -x þ 3/2, y - 1/2, z þ 1/2.

Article
Crystal Growth & Design, Vol. 10, No. 9, 2010 4165
Figure 1. (a) The two-dimensional cadmium sulfate layer observed
in the structure [(C3H7N6)2][Cd3(C3H6N6)2(H2O)4(SO4)4], I. Note
the difference in the connectivity of the sulfate units. The hydro-
gen atoms of the water molecules are not shown. (b) View of the
arrangement of the cadmium-sulfate layers in I. Note that the free
and the bound melamine molecules are located in the interlamellar
spaces.
The interlayer spaces also have free protonated melamine
units, which act as the charge-balancing cations. Thus, the
melamine units in I perform a dual role of a ligand as well as
the charge-balancing cations (Figure 1b). A similar role for
aliphatic amines has been observed earlier.21 The close
proximity between the cations and anions gives rise to a
Figure 2. (a) View of the one-dimensional cadmium-sulfate ladder
in [Cd(C5H5N5)(SO4)], II. (b) View of the two-dimensional layer
formed by the connectivity between the cadmium-sulfate ladder and
the adenine unit. (c) View of the three-dimensional structure in the
“bc” plane.
number of N-H N and N-H O type hydrogen bonds
3 3 3
3 3 3
(see Supporting Information, Table S7).
Structure of [Cd(C5H5N5)(SO4)], II. The asymmetric unit
of II contains 16 non-hydrogen atoms, of which one Cd atom
and a sulfate unit is crystallographically independent. The
Cd2þ ion is octahedrally coordinated by four oxygen atoms of
the sulfate units and two nitrogen atoms of the adenine mole-
cules to form a distorted octahedral environment (CdO4N2,
the three Cd2þ ions. The connectivity of the sulfate units can
be considered to be a η4, μ3-coordination mode (see Support-
ing Information, Figure S6a). The CdO4N2 and SO4 units are
connected to form one-dimensional ladder-like chains
(Figure 2a). The adenine unit connects the one-dimensional
ladders through the nitrogen atoms forming the two-dimen-
sional structure (Figure 2b). The layers are further connected
through another nitrogen atom of the adenine ligand to form
the three-dimensional structure (Figure 2c). The connectivity
between the adenine ligand and the cadmium centers forms a
˚
CN = 6). The average Cd-O bond distance is 2.431 A. The
Cd centers are connected through Cd-O-S bonds (ave:
107.1°) with the sulfate units and through Cd-N-C bonds
(ave: 127.8°) with the adenine molecules. The average S-O
˚
bond distance in the sulfate unit is 1.472 A (Table 3).
In II, the octahedral Cd2þ ions are bonded with the three
different sulfate units, and each sulfate unit is connected with

4166 Crystal Growth & Design, Vol. 10, No. 9, 2010
Paul et al.
Figure 3. (a) View of the two-dimensional Cd-O-Cd layer observed in [Cd3(H2O)2(C2H2N3)2(SO4)2], III. (b) The two-dimensional cadmium-
sulfate layer observed in III. The 21 axis is highlighted (cyan colored bonds). (c) The cadmium triazolate corrugated layer observed in III.
(d) View of the connectivity between the cadmium sulfate layers and the cadmium triazolate layers.
one-dimensional wirelike structure (see Supporting Informa-
tion, Figure S6b). Thus, the three-dimensional structure of II
is formed by the cross-coupling between the cadmium-
sulfate ladders and the cadmium-adenine wires. The free
125.4°) and with the triazole units through Cd-N-C/N
linkages (ave: 126.6°). The sulfate unit has an average S-O
bond distance of 1.470 A (Table 3).
˚
The three-dimensional structure of III can be considered
to be a cross-linking between two independent layered
structures, viz., the cadmium sulfate and the cadmium
triazolate layers. The sulfate unit binds with four cadmium
ions through a η3, μ4-coordination mode (see Supporting
Information, Figure S7). The cadmium centers are con-
nected through the μ3-oxygen, [O(1)], of the sulfate unit,
giving rise to infinite Cd-O-Cd one-dimensional (1D)
chains. The Cd-O-Cd chains are arranged along the 21
screw axis and are also connected with the Cd(2) centers
through another μ3-oxygen, [O(4)], forming a two-dimen-
sional Cd-O-Cd layer with apertures bound by 8-T (T =
Cd) atoms (Figure 3a). The sulfate units are bonded with the
Cd-O-Cd layers in such a way that it forms a cadmium
sulfate layers with three- and six-membered apertures
(Figure 3b). The formation of Cd-O-Cd layers in this
structure is unique and noteworthy. The triazolate units
are monoanionic and are connected to three cadmium centers
forming a corrugated cadmium-triazolate layer (Figure 3c).
The cadmium sulfate layers and the cadmium triazolate layers
amino group of the adenine unit participates in N-H
hydrogen bonding interactions (see Supporting Informa-
tion, Table S7).
O
3 3 3
Structure of [Cd2(H2O)(C2H2N3)(SO4)], III. The asym-
metric unit of III contains 13 non-hydrogen atoms. There are
two crystallographically independent Cd2þ ions and one
sulfate unit present in the asymmetric unit. Of the two
cadmium atoms, Cd(2) occupies a special position (2c) with
a site multiplicity of 0.5. Both the Cd atoms have octahedral
coordination. While the Cd(1) is coordinated by the four
oxygen atoms from three sulfate units and two nitrogen
atoms from two different triazolate units, the Cd(2) is
coordinated by two oxygen atoms from sulfate unit, two
nitrogen from two different triazolate units, and two term-
inal water molecules. The average Cd-O and Cd-N bond
˚
distances are 2.334 and 2.210 A, respectively. The oxygen
atoms, O(1) and O(4) were found to have μ3-connectivity,
binding with two Cd and a sulfur. The Cd centers are bonded
with the sulfate units through Cd-O-S linkages (ave:

Article
Crystal Growth & Design, Vol. 10, No. 9, 2010 4167
are interconnected forming the three-dimensional structure
(Figure 3d). O-H O type hydrogen bond interactions
3 3 3
have been observed due to the interaction between the co-
ordinated water molecules and the oxygen atom of the
sulfate units (see Supporting Information, Table S7).
Structure of [Cd2(C2H5N5)(SO4)2], IV. The asymmetric
unit of IV contains 19 non-hydrogen atoms, of which two
cadmium and sulfate units are crystallographically indepen-
dent. Both the cadmium ions are octahedrally coordinated
by five oxygen atoms from the sulfate units and one nitrogen
atom of the guanazole unit. The average Cd-O and Cd-N
˚
bond distances are 2.327 and 2.232 A, respectively. The Cd
centers are connected with the sulfate units through Cd-
O-S linkages (ave: 128.4°) and with the guanazole unit thro-
ugh Cd-N-C/N linkages (ave: 127.6°). The sulfate units
˚
have average S-O bond distances of 1.474 A (Table 3). The
oxygen atoms, O(4), O(5), and O(8), exhibit μ3-coordination
connectivity connecting Cd(1) and Cd(2) centers. This con-
nectivity gives rise to a tetrameric unit (see Supporting
Information, Figure S8c).
The rather complex three-dimensional structure of IV can
be understood by considering simpler building units invol-
ving the connectivity between the cadmium and the sulfate
as well as the cadmium and the guanazole units. In IV, each
cadmium center is connected to five sulfate units and vice
versa. The sulfate units exhibit differences in their bonding.
Thus, S(1)O4 unit binds with five cadmium ions in a η4, μ5-
coordination mode, and S(2)O4 unit binds with a η3, μ5-
coordination mode (see Supporting Information, Figure S8).
The connectivity between Cd2þ ions and (SO4)2- units forms
the secondary building unit, SBU-4 (Figure 4a).22 The SBU-
4 units are separated and connected by the four-membered
cadmium sulfate rings (Figure 4b). To the best of our knowl-
edge, this structural feature has been observed for the first
time in a cadmium sulfate structure. This arrangement gives
rise to a one-dimensional columnar unit, which are connec-
ted together through the terminal oxygens forming a two-
dimensional cadmium sulfate layers (Figure 4c). Each cad-
mium center of the cadmium sulfate layer has one bonded
nitrogen atom of the guanazole unit. Thus, the guanazole
units link the two layers to form the three-dimensional struc-
ture, which can be considered to be a pillared layer arrange-
ment (Figure 4d). The hydrogen atoms of the terminal amino
groups from the guanazole unit participate in the N-H
type hydrogen bond interactions (see Supporting Informa-
O
3 3 3
tion, Table S7).
Structure of [Cd5(OH)2(CH2N5)4(SO4)2], V. The asym-
metric unit of V contains 21 non-hydrogen atoms, of which
three Cd atoms and one sulfate unit are crystallographically
independent. One of the cadmium atoms, Cd(2), occupies a
special position (2c) with a site multiplicity of 0.5. All the
three cadmium centers have octahedral coordination. Cd(1)
and Cd(3) have a chemically similar environment formed by
one μ3-OH unit, two oxygen atoms of the sulfate units and
three nitrogen atoms of the 5-aminotetrazolate units. Cd(2)
is coordinated with the two μ3-OH units, two oxygen atoms
from the sulfate units, and two nitrogen atoms of the
5-aminotatrazolate units. The average Cd-O and Cd-N
Figure 4. (a) The SBU-4 unit observed in the structure
[Cd2(C2H5N5)(SO4)2], IV. (b) Figure shows the connectivity be-
tween the SBU-4 units through the four-membered rings. (c) The
two-dimensional cadmium sulfate layer observed in the structure
IV. (d) Figure shows the connectivity between the cadmium sulfate
layers and the guanazole units forming the three-dimensional
structure in IV.
˚
bond distances are 2.309 and 2.352 A, respectively. The oxy-
gen atoms, O(1) and O(5), have μ3-connectivity. While O(1)
binds to two Cd atoms [two Cd(1), symmetry generated] and
one SO4 unit, O(5) binds three Cd centers [Cd(1), Cd(2), and
Cd(3)]. In addition, BVS calculations20 (see Supporting Infor-
mation, Table S8) indicate that O(5) oxygen is, in fact, a -OH
group. The presence of the three-coordinated oxygen atoms
in V gives rise to one-dimensional Cd-O-Cd chains. The Cd
atoms are connected to the sulfate units via Cd-O-S bonds

4168 Crystal Growth & Design, Vol. 10, No. 9, 2010
Paul et al.
(ave: 125.2°) and to the 5-aminotetrazole units through
Cd-N-C/N bonds (ave: 125.1°). The sulfate units have an
˚
average S-O bond distance of 1.471 A (Table 3).
The structure of V is different from the other cadmium
sulfate phases described here as it has a three-dimensionally
extended cadmium sulfate structure in which cadmium tet-
razolate layers are also present. The cadmium sulfate frame-
work can be understood by considering simple building
units. Thus, the Cd-O-Cd chains, formed by the μ3-OH
groups, are connected by the sulfate units to form a layer (see
Supporting Information, Figure S9). The layers are cross-
linked through the sulfate oxygens giving rise to the three-
dimensional cadmium sulfate structure (Figure 5a). Consid-
ering the connectivity of the sulfate units, we observed that
each sulfate unit binds five cadmium ions through a η4, μ5-
coordination mode (see Supporting Information, Figure S9a).
The structure of V contains two different monoanionic
tetrazolate units, of which one is coordinated to three Cd
centers while the other binds with four Cd centers. The
connectivity between the cadmium octahedra and the tetra-
zolate units gives rise to a two-dimensional cadmium-tetra-
zolate layer (Figure 5b). The cadmium-tetrazolate layer and
the three-dimensional cadmium sulfate units interpenetrate
to form the overall, rather complex, three-dimensional cad-
mium sulfate structure (Figure 5c). We have observed
N-H O and N-H N type hydrogen bond interactions
3 3 3
3 3 3
in V (see Supporting Information, Table S7).
Structure of [Cd2(OH)(H2O)(C2H3N4)(SO4)] 2H2O, VI.
3
The asymmetric unit of VI contains 17 non-hydrogen atoms,
of which two Cd2þ ions and one sulfate unit are crystal-
lographically independent. Both the cadmium centers have
octahedral coordination. Cd(1) is coordinated to one μ3-OH
unit, one terminal water molecule, two oxygen atoms of the
sulfate units, and two nitrogen atoms of the 5-methyltetra-
zolate units. Cd(2) is coordinated to the two μ3-OH units,
two oxygen atoms from the sulfate units, and two nitrogen
atoms of the 5-methyltatrazolate units. One of the oxygen
atoms, O(5), connects to three different cadmium centers and
also is found to be a μ3-OH group. The presence of the μ3-OH
units gives rise to one-dimensional Cd-O-Cd chains. The
average Cd-O and Cd-N bond distances are 2.279 and
˚
2.388 A, respectively. The presence of a μ3-OH group in VI
was also confirmed by the bond valence sum calculations (see
Supporting Information, Table S8).20 The Cd atoms are
connected to the sulfate units via Cd-O-S bonds (ave:
128.2°) and to the 5-methyltetrazole units through Cd-
N-C/N bonds (ave: 126.3°). The sulfate unit has an average
˚
S-O bond distance of 1.469 A (Table 3).
Similar to V, the structure of VI also has a three-dimen-
sional cadmium sulfate structure and the cadmium tetrazo-
lates have formed a simple one-dimensional ladder structure
instead of the cadmium tetrazolate layers observed in V. All
the sulfate units have a similar connectivity of binding with
four cadmium centers through a η4, μ4-coordination mode
(see Supporting Information, Figure S10a). The three-
dimensional structure of VI can be understood by considering
simpler building units. Thus, the connectivity involving the
μ3-OH unit forms a one-dimensional Cd-O-Cd chain (see
Supporting Information, Figure S10b). The sulfate tetrahe-
dra bonds with the Cd-O-Cd chains to form a cadmium
sulfate layer, which are connected through the oxygen atoms
to form the three-dimensional cadmium sulfate structure
(Figure 6a). A closer examination of the cadmium sulfate
structure reveals the presence of a well-established building
Figure 5. (a) View of the three-dimensional cadmium-sulfate struc-
ture observed in [Cd5(OH)2(CH2N5)4(SO4)2], V in the “ac” plane.
(b) Figure shows the two-dimensional layer formed by the con-
nectivity between the 5-aminotetrazolate units and the Cd2þ ions.
Trz3 and Trz4 imply the 3-connected and 4-connected aminotetra-
zolate units (see text). (c) The three-dimensional structure of V in the
“bc” plane.
unit, Spiro-5, known in traditional tetrahedral frameworks
such as the alumino-silicate zeolites.23 The Spiro-5 unit is
formed by the connectivity involving Cd(2)O4N2, Cd(1)O4N2

Article
Crystal Growth & Design, Vol. 10, No. 9, 2010 4169
dimensional structure (Figure 6b). The one-dimensional
cadmium tetrazolate units viewed individually exhibit a her-
ringbone arrangement, which are linked by the sulfate tetra-
hedra (see Supporting Information, Figure S10f). The cross
coupling between the cadmium tetrazolate one-dimensional
units and the cadmium sulfate structures gives rise to the
framework of VI (Figure 6c).
Discussion
Six different cadmium sulfate phases with two- (I) and three-
dimensional (II-VI) structures have been prepared under
mild solvothermal conditions in the presence of water. All
the compounds were prepared using cadmium sulfate as the
starting material (see Supporting Information, Figure S11),
which acts as the source for both the cadmium as well as the
sulfate species. It is also possible to prepare the present com-
pounds using other salts of cadmium along with a source for
sulfate - such as sulphuric acid. We, however, observed that
good quality single crystals were formed only when cadmium
sulfate was used as the combined source for cadmium and sul-
fate. In all the cases, the heterocyclic amine molecules were
found to behave as a ligand, but the connectivity exhibits dif-
ferences among the structures. During the preparations, we
consistently observed an increase in the pH value. This
suggests that the amine molecule was not fully utilized in the
formation of the final solid product, whereas more cad-
mium sulfate would have been consumed. There appears to
be no particular correlation between the starting composi-
tion and the final product, which is typical of solvothermal
preparations.24
Structurally, all the compounds are unique, being formed
by the connectivity between the octahedral Cd centers, tetra-
hedral sulfate, and the heterocylic ligands. A detailed analysis
of the structures indicates that in most of the compounds the
fundamental building unit, four-membered ring, is present. In
addition, the observation of two-dimensional Cd-O-Cd
layers in III and Cd-O-Cd chains in V and VI is noteworthy.
Formation of a metal-oxide layer is not common and only a
few reports are known.25 In VI, the 5-methyltetrazole ligand
was formed in situ, generated from the starting materials,
azide anions and acetonitrile. The possible mechanism of the
formation of 5-methyltetrazole is given in Scheme 1.26 As can
be noted, the reaction proceeds via a [2 þ 3] dipolar cycload-
dition reaction in the presence of the Lewis acid, (cadmium
center). Similar in situ formation of 5-methyltetrazole has
been observed during the preparation of other cadmium
sulfate phases.13c The observation of SBU-4 units (IV) and
spiro-5 units (VI) indicates a rich diversity in the present
cadmium sulfate structures. From the various structures
observed in the present study, one can visualize and simplify
the formation of the different frameworks schematically
(Scheme 2).
The present compounds also exhibit another feature where-
in the cadmium sulfate itself forms one- (II), two- (I, III, and
IV), and three-dimensionally (V and VI) extended structures.
The heterocyclic ligand molecules bind with the cadmium and
give rise to the observed higher dimensional structures except
in I, where the amine molecules hang in the interlayer spaces
(Table 4). Increasing the dimensionality of a particular phase
using heterocyclic as well as aliphatic amines has been shown
elegantly earlier.6,7,27 The situation in the present compounds,
however, is different as the metal-amine connectivity also
gives rise to a one- (II, IV, VI) and two- (III, V) dimensional
Figure 6. (a) The three-dimensional cadmium-sulfate structure ob-
served in [Cd2(OH)(H2O)(C2H3N4)(SO4)] 2H2O, VI. Note the for-
mation of the Spiro-5 units (see text). (b) Figure shows the cadmium-
tetrazolate structure observed in VI. Note that the μ3-OH connects
the Cd centers forming Cd-O-Cd chains. (c) The three-dimen-
sional structure of VI in the “bc” plane.
3
and S(1)O4 tetrahedral units (see Supporting Information,
Figure S10e). The spiro-5 units are connected together
through the sulfate units forming the cadmium sulfate
structure. This is indeed a unique structure as such building
units are not common in compounds having both the
octahedral as well as the tetrahedral units. To the best of
our knowledge, this is the first observation of spiro-5 units in
a cadmium sulfate phase. The 5-methyltetrazole units con-
nect with four different cadmium centers giving rise to a one-

4170 Crystal Growth & Design, Vol. 10, No. 9, 2010
Paul et al.
arrangement. This type of interpretation of two different
sublattices has been observed for the first time in sulfate-based
open structures.
A careful study of the present structures reveals that the
structural features observed in the present compounds may be
compared with other related reported structures. The inor-
ganic cadmium sulfate ladder, observed in II, has a close
resemblance to many similar structures reported in the
literature.3a,b In the iron-sulfate structure, [C2N2H10][FeF3-
(SO4)],28 the FeF3O3 octahedra share a vertex with triply
bridging SO4 tetrahedra to form a one-dimensional ladder
structure (Figure 7a). In the present compound II, we obser-
ved a similar ladder, but the connectivityof the SO4 tetrahedra
is different (Figure 7b). In compound III, we have the un-
usual formation of Cd-O-Cd layers. If we consider the
sulfate connectivity also, one can simplify the structure into a
Scheme 1. Schematic of the Possible Pathway for the Formation
of 5-Methyltetrazole Ligand in [Cd2(OH)(H2O)(C2H3N4)-
(SO4)] 2H2O, VI. (i) a Two-Step Mechanism (ii) a
Concerted Mechanism
3
Table 4. The Observed Coordination Behavior of the Heterocyclic Rings
in the Cadmium Sulfate Compounds, I-VI
no. of ring
nitrogen Cd2þ/Na
compound
amine
[(C3H7N6)2][Cd3(C3H6N6)2- melamine
(H2O)4(SO4)4], I
3
1/3 þ 0
[Cd(C5H5N5)(SO4)], II
[Cd3(H2O)2(C2H2N3)2-
(SO4)2], III
adenine
1,2,4-triazole
4
3
2/4
3/3
[Cd2(C2H5N5)(SO4)2], IV
[Cd5(OH)2(CH2N5)4-
(SO4)2], V
guanazole
5-aminotetrazole
3
4
2/3
3/4 þ 4/4
[Cd2(OH)(H2O)(C2H3N4)- 5-methyltetrazole
(SO4)] 2H2O, VI
4
4/4
3
a The ratio is calculated by the number of ring nitrogen atoms
attached to the metal centers; for example, in structure I, one melamine
unit (3 ring nitrogens) is coordinated to the one metal center (i.e., 1/3)
and one unit is free from coordination (i.e., 0).
Scheme 2. Schematic of the Structures I-VI Depicting the Connectivity of the Organic Ligand Part with the Inorganic Cadmium
Sulfate Parta
a Two different colors used for the cadmium sulfate framework and the organic ligand part.

Article
Crystal Growth & Design, Vol. 10, No. 9, 2010 4171
Figure 7. (a) View of the one-dimensional ladder observed in the
structure of [C2N2H10][FeF3(SO4)]. (b) View of the one-dimensional
cadmium sulfate ladder observed in the structure of [Cd(C5H5N5)-
(SO4)], II.
Figure 9. (a) The T-atom connectivity observed in K4[Cd3(HPO4)4-
(H2PO4)2]. Note that the layers are formed by the Spiro-5 units (see
text). (b) The Spiro-5 units observed in the three-dimensional
cadmium sulfate structure [Cd2(OH)(H2O)(C2H3N4)(SO4)] 2H2O,
VI. Note the difference between the Spiro-5 units.
3
one-dimensional ladder connected by Cd centers. The nodal
representation between the Cd centers and the sulfate units
gives rise to a layer with four- and eight-membered apertures
(Figure 8a). This arrangement has close resemblance to the
cadmium sulfate structure, [Cd(Htrz)(SO4)(H2O)].29 The
four-membered rings in the reported structure, [Cd(Htrz)-
(SO4)(H2O)],29 share the corners forming different chain
units, which are connected by the Cd centers forming a similar
arrangement (Figure 8b). Though the layers have a close simi-
larity between the two structures, the connectivity involving
the triazole ligand between the structures is different. In [Cd-
(Htrz)(SO4)(H2O)],29 the triazole ligand is neutral, whereas in
III the triazole units are anionic. The Spiro-5 unit observed in
VI can be compared to the Spiro-5 unit present in the two-
dimensional cadmium phosphate structure, K4[Cd3(HPO4)4-
(H2PO4)2].30 Both the compounds have similar Spiro-5 build-
ing units connected through the terminal MO4 (M = S, P)
units (Figure 9). In VI, the connectivity between the Spiro-5
units forms a three-dimensional structure, whereas it has only
a two-dimensional structure in K4[Cd3(HPO4)4(H2PO4)2].30
Adsorption-Desorption Studies. Preliminary studies on
the adsorption/desorption behavior using organic dyes were
carried out on a few cadmium sulfate compounds prepared
in the present study. For this, we took both the anionic (e.g.,
Figure 8. (a) Figure shows the connectivity between the sulfate and
cadmium nodes in the cadmium sulfate layer of [Cd3(H2O)2-
(C2H2N3)2(SO4)2], III. Note the formation of edge-shared four-
membered rings. (b) Figure shows the cadmium sulfate layer
observed in [Cd(Htrz)(SO4)(H2O)]. Note the close resemblance
between the structures. Also note the formation of corner-shared
four-membered rings.

4172 Crystal Growth & Design, Vol. 10, No. 9, 2010
Paul et al.
Figure 10. (a) Adsorption profile of Congo red (CR) by compounds I-VI (2 g/L in each case). (b) Adsorption profile of Coomassie Brilliant
Blue R-250 (CBB) by compounds III, IV, and VI (2 g/L in each case). (c) Adsorption profile of Alizarin red S (ARS) by compounds III, IV, and
VI (2 g/L in each case).
Congo Red, Coomassie Brilliant Blue R-250 and Alizarin
Red S) as well as the cationic (e.g., Methylene Blue and
Methyl Violet) dyes. The dyes were taken in aqueous solu-
tion and the studies were performed in the dark in the
presence of compounds III, IV, and VI. In all the cases, we
observed that the adsorption of the cationic dyes were
negligible compared to the anionic dyes. The initial calibra-
tion for the anionic dyes, Congo Red (CR), Coomassie
Brilliant Blue R-250 (CBB), and Alizarin Red S (ARS),
was based on the Beer-Lambert law at its maximum absorp-
tion wavelength, λmax of 493, 555, and 428 nm, respectively.
The UV spectrum of the dye solution exhibited a continuous
decrease in the peak intensity, which corresponded to the
decrease in the concentration of the dye in the solution. We
have not observed any additional peaks in the UV-vis
spectra, which suggests that there are no detectable inter-
mediates formed during the adsorption of the organic dyes.
The adsorption studies indicated that the Congo red (CR)
adsorbs more efficiently compared to the other two dyes. We
have observed a similar behavior in cadmium thiosulfate
compounds recently.14 The studies also indicated that the
compounds III and VI exhibit reasonable adsorption of the
CR dye (Figure 10a). The adsorption of the CR dye resulted
in a reduction of the concentration from 100 to 5 ppm in 1 h
by the compounds III and VI, while it decreased from 100 to
15 ppm in the case of IV. Since compounds III and VI
exhibited reasonable adsorption behavior, we investigated
the adsorption studies with other related anionic dyes, CBB

Article
Crystal Growth & Design, Vol. 10, No. 9, 2010 4173
and ARS. The studies reveal that the rate of adsorption of
CBB is slower than that for CR and ARS. The concentration
of CBB is reduced from 100 ppm to 30 ppm after 1 h, while
the concentration of ARS is reduced from 100 to 12 ppm
after 1 h (Figure 10). The present studies show that the rate of
adsorption follows the order CR > ARS > CBB. The dyes
are physically adsorbed on the compounds, and such physi-
sorption of the anionic dyes on the cadmium centers could
depend on many factors. For example, the physisorption
depends on the contact time between the dye and the adsor-
bent and the availability of a free-site on the surface of the
solid adsorbent. It is known that the cadmium can possess
coordination numbers that could be more than six.31 Thus,
the approaching anionic dye could, then, loosely bind with
the cadmium centers on the surface of the compounds. The
compounds III and VI, which exhibit reasonable physisorp-
tion, have water molecules coordinated with the cadmium
centers. The coordinated water molecules would make
spaces for the adsorption of the anionic dye molecule at
the surface. This mechanistic view could be plausible as phy-
sisorption of dye molecules is a surface phenomenon and
depends on the type and nature of the medium employed for
the adsorption studies. If this assumption is true, then it may
be possible to desorp the adsorbed dye molecules by modify-
ing the experimental conditions. Thus, desorption studies
were carried out using an alcoholic medium. Studies of this
nature are required to understand the adsorption/desorption
behavior as well as to examine the reusability of the same
solid compound repeatedly.
Figure 11. Desorption profile of (i) CR, (ii) ARS, and (iii) CBB by
compound III in ethanol (2 g/L in each case).
The desorption of the dyes was studied by treating the dye-
adsorbed compounds with ethanol. For the desorption
experiments, 100 ppm of the dyes (initial concentration) were
taken with compound III and stirred for 2 h. The contact
time between the dye and the solid phase was enhanced to
2 h, as the adsorption of the dye molecules reaches a constant
value at ∼90 min of exposure (Figure 10a). The increase in
time of exposure is to ascertain that the dye molecules have
adsorbed and equilibrated fully on the cadmium sulfate
compounds. After the adsorption was completed, the solu-
tions were filtered and the dye adsorbed solids were recov-
ered and dried at ambient conditions. Then, the compound
was taken in ethanol for the desorption experiments. When
the dye-adsorbed compounds were stirred in ethanol for 2 h
at room temperature, the desorption of the dye molecules
were observed (Figure 11). The desorption behavior was also
investigated similar to the adsorption studies using the
UV-vis spectroscopy. Similar desorption studies were also
carried out for compounds IV and VI. The desorption
studies suggested that the CR desorbs only 63% (total adsor-
bed = 95%), while ARS desorbs 80% (total adsorbed =
88%) and CBB desorbs 81% (total adsorbed = 84%) after
2 h of stirring in ethanol (Figure 11). Compound III, after the
adsorption/desorption, were also examined using PXRD.
No change in the structure was observed indicating that the
prepared compounds are stable under experimental condi-
tions (see Supporting Information, Figure S12). The changes
in the color of the solutions during the adsorption and the
desorption can also be observed visually (see Supporting
Information, Figure S13).
For this, we employed compound IV as it exhibited an
additional band in the UV spectra (see Supporting Informa-
tion, Figure S3), which corresponds to 3.26 eV. The 385 nm
(3.26 eV) band could be due to the charge transfer from
guanazole ligand to the cadmium center (LMCT) in the solid
phase. Since this band is near the UV region, we explored the
photodecomposition of the organic dyes. Two cationic dyes,
viz., methylene blue (MB) and methyl violet (MV), were in-
vestigated in the presence of UV light. The photocatalytic
studies indicated that the concentration of MB was reduced
from 100 to 42 ppm, while the concentration of MV was re-
duced from 100 to 50 ppm after 90 min (Figure 12). The
control experiment on the pure dye solutions (without the
catalyst) indicates that the degradation of the dyes under UV
radiation is negligible. The cadmium sulfate compound was
examined after catalytic studies using PXRD, the unchanged
PXRD pattern suggests that the compound is stable and can
be used repetitively (see Supporting Information, Figure S14).
The degradation studies were also compared with Degussa
P-25 (TiO2) catalyst, which exhibits a complete degradation
of the dyes (100 to 5 ppm) within 1 h. The present study
indicates that the cadmium sulfate (IV) compounds ex-
hibit marginal photocatalytic activity when exposed to the
UV-light.
The photocatalytic activity of the compound IV can be
explained based on our earlier observation of cadmium
thiosulfates.14 It is likely that the additional band at 385
nm (3.26 eV) observed in our UV spectra could be respon-
sible for the observed photocatalytic behavior. The other
cadmium sulfate phases in the present study did not exhibit
any appreciable photocatalytic activity as there are no such
bands in the UV-region, which can be used for UV-assisted
photocatalytic purposes. From the molecular orbital point
of view, the HOMO has filled d-orbital (d10 in Cd2þ) and the
Photocatalytic Studies. The adsorption studies indicated
that the sulfonated dyes (anionic) exhibited a reasonable
physisorption behavior. It occurred to us that the non-
sulfonated dyes (cationic) could undergo photocatalytic
decomposition under the exposure of the UV radiation.

4174 Crystal Growth & Design, Vol. 10, No. 9, 2010
Paul et al.
in CH2Cl2 solution (10 mL) of imine (A, 0.09 g, 0.5 mmol).
Trimethylsilyl cyanide (0.075 g, 0.75 mmol) was added at
0 °C, and the reaction mixture was stirred for 2 h. Similar
molar concentrations have been employed for other cad-
mium sulfate phases. The product, aminonitrile (B), was iso-
lated and analyzed by 1H NMR spectra. From the catalytic
studies, we observed that compounds III, V, and VI exhibi-
ted a near quantitative conversion of the imine to the amin-
onitrile (∼98%). We observed poorer conversion (∼50%)
for the other compounds (see Supporting Information,
Table S9). The control experiments in the absence of cad-
mium sulfate compounds have also been carried out to
ensure that the reaction proceeds catalytically. We have also
carried out the reaction in the presence of Cd(NO3)2 4H2O,
3
which gave a conversion of ∼20%. The present study
suggests that the sulfate compounds are active for the
heterogeneous catalytic cyanosilylation of the imines. It is
likely that the presence of the Lewis acidity may be respon-
sible for the observed catalytic behavior. Similar catalytic
conversion has also been made earlier, though the reaction
times were shorter.33
Figure 12. Degradation profile of methyl violet (MV) and methy-
lene blue (MB) under UV illumination. (a) and (b) without the
catalyst, (c) and (d) with the catalyst (2 g/L of IV in each case).
LUMO (free s-orbital) is vacant in IV. In the presence of UV
light there would be an electron transfer from the HOMO to
the LUMO. The transferred electron of the LUMO can be
easily lost from the excited state, whereas the HOMO would
require the electron to return to its original state. Thus, the
excited Cd2þ center would decay to the ground state quickly.
During the decay process, if any organic molecule is present
within a reasonable energy range and appropriate orienta-
tion, then it can form an activated complex, which could be
transient. From the structure of the organic dye, one can
state that the electron-withdrawing group attached to the
carbon center can receive the electron. During this electron
transfer process, the hydrogen atom of the methyl group in
MB can sacrifice its electron and leave as Hþ species. The
electron could be abstracted by the excited Cd2þ, which
results in the cleavage of the C-N bond in a stepwise manner
to the total degradation of MB. The other dye, MV, has
similar groups which offer the cleavage as C-O, N-N, and
C-N bonds in the structure. Similar photoassisted decom-
position mechanisms have been proposed earlier.32
Heterogeneous Catalytic Studies. Heterogeneous catalysis
using open-framework compounds are being investigated
continuously over the years.1a Catalytic studies on com-
pounds with metal ion having filled d10 shell have not been
examined, though the open metal sites could provide the
Lewis acidic sites. Recent studies on cadmium thiosulfate
framework compounds showed that the Lewis acidity arising
from coordinatively unsaturated cadmium centers can be
exploited fruitfully in the cyanosilylation of imines.8b It
occurred to us that it may be feasible to carry out a similar
study on these cadmium sulfate phases. The heterogeneous
catalytic studies were performed on all the compounds
prepared in the present study. In a typical experiment,
powdered catalyst (0.07 g of III, 0.1 mmol) was suspended
The cadmium sulfate compounds were examined after the
catalytic studies using PXRD, which did not exhibit obser-
vable differences. This indicates that the cadmium sulfate
phases are stable under the experimental conditions. We
have also repeated the cyanosilylation studies on the used
catalyst, which also indicated that the heterogeneous cata-
lytic conversion of imines were comparable to the studies
performed using the fresh catalyst. This study suggests that
the cadmium sulfate phases are stable and can be used
recyclably.
Conclusions
The synthesis, structure, and characterization of a family of
sulfate phases have been accomplished. The polyazahetero-
cyclic organic molecules act as the ligand linking one- (II),
two- (I, III, and IV), and three- (V and VI) dimensional cad-
mium sulfate units within the structures. The observation of
Cd-O-Cdtwo-dimensionallayersincompoundIII isunique
and noteworthy. Some of the structures possess well-estab-
lished secondary building units such as SBU-4 and Spiro-5.
The adsorption studies indicate that the cadmium sulfates
selectively adsorb/desorb anionic dyes, the behavior depend-
ing on the reaction medium. This suggests that the cadmium
sulfates can be employed for the selective scavenging of the
anionic dyes in a reaction mixture. The photocatalytic studies
indicate a modest activity in the photodegradation of the
cationic dyes under UV irradiation. The cyanosilylation
heterogeneous catalytic studies suggest that the cadmium
sulfate phases can be employed as an effective heterogeneous
catalyst. The diversity of the structures along with the variety
in the properties exhibited by the cadmium sulfate phases
suggests that this area has considerable potential for further
investigations.

Article
Acknowledgment. The authors thank Prof. B. R. Jagirdar,
Crystal Growth & Design, Vol. 10, No. 9, 2010 4175
B.; Knobler, C.; Furukawa, H.; O'Keeffe, M.; Yaghi, O. M. Science
2008, 319, 939. (c) Xu, G.-C.; Hua, Q.; Okamura, T.-a.; Bai, Z.-S.;
Ding, Y.-J.; Huang, Y.-Q.; Liu, G.-X.; Sun, W.-Y.; Ueyama, N.
CrystEngComm 2009, 11, 261.
Department of Inorganic and Physical Chemistry, Indian
Institute of Science, for help with the catalytic studies. The
authors thank the Department of Science and Technology
(DST), Government of India, for the award of a research
grant. The Council of Scientific and Industrial Research
(CSIR), Government of India, is thanked for the award of a
research grant (S.N.) and a fellowship (A.K.P.). S.N. also
thanks DST for the award of a RAMANNA Fellowship.
(10) (a) Zhu, A.-X.; Lin, J.-B.; Zhang, J.-P.; Chen, X.-M. Inorg. Chem.
2009, 48, 3882. (b) Tang, S.; Liu, M. R.; Lu, P.; Xia, H.; Li, M.; Xie,
Z. Q.; Shen, F. Z.; Gu, C.; Wang, H. P.; Yang, B. Adv. Funct. Mater.
2007, 17, 2869. (c) Chen, H. Y.; Chi, Y.; Liu, C. S.; Yu, J. K.; Cheng,
Y. M.; Chen, K. S.; Chou, P. T.; Peng, S. M.; Lee, G. H.; Carty, A. J. Adv.
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9, 3683. (b) Vaidhyanathan, R.; Iremonger, S. S.; Dawson, K. W.;
Shimizu, G. K. H. Chem. Commun. 2009, 5230.
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Supporting Information Available: Selected bond angles in I-VI;
hydrogen bond interactions in I-VI; selected bond distances and
bond valance sums in I-VI; product obtained from the cyanosilyla-
tion reaction catalyzed by compounds I-VI; powder XRD (CuKR)
patterns of compounds I-VI; IR spectra of I-VI; diffuse reflec-
tance spectra of compounds I-VI; TGA studies of I-VI; various
compound structures; powder XRD patterns of III and IV; dye
solution images. This material is available free of charge via the
Internet at http://pubs.acs.org.
(14) Paul, A. K.; Madras, G.; Natarajan, S. Phys. Chem. Chem. Phys.
2009, 11, 11285.
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