Synthesis, structure, transformation studies and catalytic properties of open-framework cadmium thiosulfate compounds

Article abstract of DOI:10.1039/b908995k

Five new thiosulfate based inorganic-organic hybrid open-framework compounds have been synthesized employing mild reaction conditions. Of the five compounds, [Na₂(H₂O)₈][Cd(C₁₀H ₈N₂)(S₂O₃)₂] ·2H₂O, I and [Cd₂(C₁₀H₈N ₂)₂(HS₂O₃)₂(S ₂O₃)₂][(C₁₀H₉N ₂)₂(C₁₀H₈N₂) ₂]·8H₂O, II have one-dimensional (1D) structures and [Cd(C₁₀H₈N₂)(H₂O) ₂(S₂O₃)]·2H₂O, III, [Cd ₂(C₁₀H₈N₂)₃(S ₂O₃)₂], IV and [Cd₂(C ₁₀H₈N₂)_2.5(S₂O ₃)₂], V have three-dimensional (3D) structures. The 1D structures are somewhat related, formed by the bonding between tetrahedral Cd centers (CdN₂S₂) and 4,4′-bipyridine (bpy) units. The inter-chain spaces are occupied by the hanging thiosulfate units in both the cases along with Na(H₂O)₆ chains in I and free bpy units in II. The three 3D structures have one-dimensional cadmium thiosulfate chains linked by bpy units. Interpenetration has been observed in all the 3D structures. The 3D structures appear to be related and can be derived from fgs net. Transformation studies on the 1D compound, [Na₂(H ₂O)₈][Cd(C₁₀H₈N₂)(S ₂O₃)₂]·2H₂O, I, indicated a facile formation of [Cd(C₁₀H₈N₂)(H ₂O)₂(S₂O₃)]·2H₂O, III. Prolonged heating of I gave rise to a 3D cadmium sulfate phase, [Cd ₂(C₁₀H₈N₂)₂(H ₂O)₃(SO₄)₂]·2H₂O, VI. Compound VI has one-dimensional cadmium sulfate chains formed by six-membered rings connected by bpy units to form a 3D structure, which appears to resemble the topological arrangement of III. Transformation studies of III indicates the formation of IV and V, and at a higher temperature a new 3D cadmium sulfate, [Cd(C₁₀H₈N₂)(SO₄)], VII. Compound VII has a 4 × 4 grid cadmium sulfate layers pillared by bpy units. All the compounds were characterized by PXRD, TGA, IR and UV-visible studies. Preliminary studies on the possible use of the 3D compounds (III-VII) in heterogeneous cyanosilylation of imines appear to be promising. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b908995k

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis, structure, transformation studies and catalytic properties of
open-framework cadmium thiosulfate compounds†
Avijit Kumar Paul,a Giridhar Madras*b and Srinivasan Natarajan*a
Received 7th May 2009, Accepted 2nd November 2009
First published as an Advance Article on the web 12th January 2010
DOI: 10.1039/b908995k
Five new thiosulfate based inorganic–organic hybrid open-framework compounds have been
synthesized employing mild reaction conditions. Of the five compounds, [Na2(H2O)8]-
[Cd(C10H8N2)(S2O3)2]·2H2O, I and [Cd2(C10H8N2)2(HS2O3)2(S2O3)2][(C10H9N2)2(C10H8N2)2]·8H2O, II
have one-dimensional (1D) structures and [Cd(C10H8N2)(H2O)2(S2O3)]·2H2O, III,
[Cd2(C10H8N2)3(S2O3)2], IV and [Cd2(C10H8N2)2.5(S2O3)2], V have three-dimensional (3D) structures. The
1D structures are somewhat related, formed by the bonding between tetrahedral Cd centers (CdN2S2)
and 4,4¢-bipyridine (bpy) units. The inter-chain spaces are occupied by the hanging thiosulfate units in
both the cases along with Na(H2O)6 chains in I and free bpy units in II. The three 3D structures have
one-dimensional cadmium thiosulfate chains linked by bpy units. Interpenetration has been observed in
all the 3D structures. The 3D structures appear to be related and can be derived from fgs net.
Transformation studies on the 1D compound, [Na2(H2O)8][Cd(C10H8N2)(S2O3)2]·2H2O, I, indicated a
facile formation of [Cd(C10H8N2)(H2O)2(S2O3)]·2H2O, III. Prolonged heating of I gave rise to a 3D
cadmium sulfate phase, [Cd2(C10H8N2)2(H2O)3(SO4)2]·2H2O, VI. Compound VI has one-dimensional
cadmium sulfate chains formed by six-membered rings connected by bpy units to form a 3D structure,
which appears to resemble the topological arrangement of III. Transformation studies of III indicates
the formation of IV and V, and at a higher temperature a new 3D cadmium sulfate, [Cd(C10H8N2)(SO4)],
VII. Compound VII has a 4 ¥ 4 grid cadmium sulfate layers pillared by bpy units. All the compounds
were characterized by PXRD, TGA, IR and UV-visible studies. Preliminary studies on the possible use
of the 3D compounds (III–VII) in heterogeneous cyanosilylation of imines appear to be promising.
the last few years has yielded several organic amine templated
Introduction
open-framework metal sulfates.3
Inorganic open-framework compounds are an important class of
Generally the open-framework structures are formed by bond-
solids, which have attracted much attention due to their impor-
ing between the metal oxygen polyhedra and the anion tetrahedra,
tant potential applications in the areas of sorption, separation
which essentially have M–O–X type bonding only. It seemed
and catalysis.1 Most of the open-framework solids investigated
rational to examine whether such open-framework compounds
and reported in the literature pertains to the silicates and the
can be prepared by employing a tetrahedral anion that contain
phosphates.2 Recently, it has been shown that the oxy-anions of
sulfur and selenium can also form open networks.3 One of the
important issues with regards to the sulfate and selenate anions
compared to the phosphate and silicate ones, is the difference
in the formal oxidation state of the central atom, which would
lead to an increased charge on the X–O–M bonds (X = Si, P, S,
Se and M = metal atom) within the framework. In addition, it
is well known that the sulfates and the selenates do not readily
form polymerisable ring structures, unlike the phosphates and the
silicates. It has been suggested that the sulfates can form stable
structures with low valent metal ions. Persistent research during
more than one ligand, viz., thiosulfate (S2O3)2-.
One of the problems with the thiosulfate is its lack of stability
under acidic conditions. Many of the open-framework compounds
are prepared under acidic conditions employing hydrothermal
methods.2 The thiosulfate anion (S2O3)2- has sulfur both in the
+6 as well as in the -2 oxidation states, which renders the (S2O3)2-
ion unstable. So, under the reaction conditions, the thiosulfate
decomposes to form elemental sulfur (generally as cyclic S8
polymorph) or reacts further giving rise to metal sulfides. The
other possibility would be that the thiosulfate decomposes forming
either the sulfite or the sulfate anions, which eventually react with
the metal ions to form the corresponding phases. Thus, the low
stability of the thiosulfate anion creates a slight impediment for
the synthetic chemist and could be the reason for the lack of open-
framework thiosulfate phases in the literature.
aFramework Solids Laboratory, Solid State and Structural Chemistry Unit,
Indian Institute of Science, Bangalore-560012, India. E-mail: snatarajan@
sscu.iisc.ernet.in
bDepartment of Chemical Engineering, Indian Institute of Science,
Bangalore-560012, India. E-mail: giridhar@chemeng.iisc.ernet.in
The low stability coupled with the lack of reactivity of the
thiosulfate anion necessitates newer approaches in the synthesis.
Since the sulfates predominantly form low-dimensional structures,
it is expected that the thiosulfate would also form similar ones.
It has also been shown that the use of thiosulfate under mild
† Electronic supplementary information (ESI) available: Experimental
data. CCDC reference numbers 697711–697713 and 727266–727269. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/b908995k
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conditions gives rise to one-dimensional structures.4 Increasing
the dimensionality of a solid has been elegantly established
by Lii and coworkers by the use of 4,4¢-bipyridine as a rigid
linker in many open-framework structures.5 We explored the
preparation of the thiosulfate phases by employing reaction
conditions that facilitates self assembly as well as solvothermal
methods in combination with the use of 4,4¢-bipyridine in the
reaction mixture. We have chosen cadmium as the element
of choice as it is likely to bind with sulfur more easily due
to its larger size. We have employed self assembly approaches
(room temperature or lower) to study the possible evolution of
structures as a function of temperature. Such studies in MOF
compounds exhibit profound effects in both the dimensionality
as well as structures.6 This combined strategy yielded new
inorganic–organic hybrid phases with thiosulfate as part of the
network structure.7 In the present study, we have prepared five
thiosulfate based hybrid compounds of varying dimensionalities.
The compounds, [Na2(H2O)8][Cd(C10H8N2)(S2O3)2]·2H2O, I and
[Cd2 (C10H8N2 )2 (HS2O3 )2 (S2O3 )2 ][(C10H9N2 )2 (C10H8N2 )2 ]·8H2O,
II have one-dimensional structures, and [Cd(C10H8N2)-
(H2O)2(S2O3)]·2H2O,
III,
[Cd2(C10H8N2)3(S2O3)2],
IV
and [Cd2(C10H8N2)2.5(S2O3)2],
V
have three-dimensional
structures. Two three-dimensional sulfate based compounds
[Cd2(C10H8N2)2(H2O)3(SO4)2]·2H2O, VI, and [Cd(C10H8N2)(SO4)],
VII, have also been isolated during the study of the interconversion
of the thiosulfate compounds. The structure of the cadmium
sulfate compound [Cd2(C10H8N2)2(H2O)3(SO4)2]·2H2O, VI, was
already known, though with different water content.8 In this
paper, we describe the synthesis, structure, transformation
studies and heterogeneous catalytic properties of the thiosulfate
phases.
Results and discussion
Structure of [Na2(H2O)8][Cd(C10H8N2)(S2O3)2]·2H2O, I
The asymmetric unit contains 35 non-hydrogen atoms. The
crystallographically independent Cd centers are tetrahedrally
coordinated by two sulfur atoms of the thiosulfate unit and two
nitrogen atoms of the 4,4¢-bipyridine (CdN2S2, CN = 4). The
asymmetric unit contains two thiosulfate units, both of which are
disordered over two positions with a site occupancy of 70 : 30 (see
ESI†). The average S–S and S–O bond distances are 1.991 and
Fig. 1 (a) One-dimensional structure of [Na2(H2O)8][Cd(C10H8N2)-
(S2O3)2]·2H2O, I. Note that the thiosulfate unit hangs from the Cd–center.
Only one sulfur atom and the oxygen atoms (S1A, S2A, O2B, O3B, O4B
and O5B; 70% occupancy) of each thiosulfate unit have been shown for
clarity. (b) One-dimensional sodium–water chain observed in I. (c) The
structure of I showing the arrangement of the anionic and the cationic
chains. Note that the cationic chains propagate orthogonal to the Cd-bpy
chains (see text). The hydrogen atoms of the bpy units and the water
molecules are omitted in the figures for clarity.
˚
1.462 A, respectively. The thiosulfate unit possesses an average
angle of 106.1◦, which is indicative of a distorted tetrahedral
arrangement. The N–Cd–N bond angle is 84.9(2)◦, and the S(1A)–
Cd–S(2A) bond angle is 103.8(1)◦. The structure of I consists
of one-dimensional chains formed by the linking of cadmium
centers by bipyridine units (Fig. 1(a)). The 4,4¢-bpy units are
twisted around the central C–C bond with an average torsion
angle of 60.0◦. The two thiosulfate units are bonded with the Cd
through Cd–S linkages and hang as a pendent from the chain
in the inter-chain region. The anionic chains are separated by
the presence of one-dimensional cationic chains formed by the
sodium atoms, which are octahedrally coordinated and bonded
together by the water molecules (Fig. 1(b)). The interesting point
about this structure is, while the anionic cadmium thiosulfate
chains propagate along the “b” axis, the sodium–water chain
propagates along the “a” axis (Fig. 1(c)). Thus, the two chains
are orthogonal to each other and the spaces between the chains
are occupied by the water molecules. The presence of [Na(H2O)4]n
=
chains, water molecules and terminal S O units in close proximity
in I indicates the possibility of strong hydrogen bond interactions.
Since, we could not locate the hydrogen positions of the water
molecules from the single-crystal study, it was not possible to
evaluate the nature and strength of the hydrogen bond interactions.
The O–O distances, however, can provide some clues to the
possible nature of such interactions. Thus, we have average O–O
˚
distances of 2.78 A between the [Na(H2O)4]n chains and the
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˚
Structure of [Cd(C10H8N2)(H2O)2(S2O3)]·2H2O, III
=
lattice water molecules, 2.77 A between the S O and the water
˚
=
molecules, and 2.65 A between the S O and [Na(H2O)4]n chains.
These are typical distances that indicates strong hydrogen bond
interactions.9
The asymmetric unit of III contains 22 non-hydrogen atoms.
There is only one crystallographically independent Cd2+ ion,
which is coordinated by one sulfur and one oxygen atom of
the thiosulfate unit, two nitrogen atoms of the bipyridine and
two terminal water molecules forming a distorted octahedral
environment, (Cd(H2O)2N2SO, CN = 6). The average Cd–N and
Structure of [Cd2(C10H8N2)2(HS2O3)2(S2O3)2][(C10H9N2)2-
(C10H8N2)2]·8H2O, II
˚
Cd–O bond distances are 2.324 and 2.350 A, respectively. The Cd–
˚
S bond distance is 2.644 (1) A. The thiosulfate has average S–O
The asymmetric unit contains 52 non-hydrogen atoms. There
are two crystallographically distinct cadmium ions and both
occupy special positions (2f and 2e) with a site multiplicity of
0.5. Both the cadmium ions are tetrahedrally coordinated with
two sulfur from the thiosulfate units and two nitrogen atoms
from the 4,4¢-bipyridine units (CdN2S2, CN = 4). The Cd–N
and Cd–S bond distances have an average value of 2.346 and
˚
bond distances of 1.445 and the S–S bond distance is 2.016 A.
The thiosulfate unit has an average angle of 109.5◦, which is
typical for the tetrahedral arrangement. The O/S–Cd–S/O bond
angles are in the range 95.66(8)–173.75(9)◦ and the N–Cd–N
bond angle is 176.61(11)◦. The Cd center is connected with one
sulfur and one oxygen from two different thiosulfate units and
forms a one-dimensional chain, which is arranged as a right
handed helix (Fig. 3(a)). The 4,4¢-bipyridine units link the chain
by bonding with the Cd-centers completing the three-dimensional
connectivity. The 4,4¢-bipyridine connectivity is such that only
the alternate Cd centers are linked to form the two-dimensional
layer like arrangement (Fig. 3(b)). The two-dimensional layers
interpenetrate each other forming the three-dimensional structure
(Fig. 3(c)). The most important aspect of the structure is that
all the bpy units are planner and the central C–C bonds are not
twisted. There are two extra-framework water molecules in the
asymmetric unit and two other water molecules are generated by
the symmetry, which occupy the middle of the one-dimensional
channels. The water molecules form a cyclic tetrameric cluster
˚
2.486 A, respectively. The two thiosulfate units have average
S–S and S–O bond distances of 2.029 and 1.464 A, respectively.
˚
The thiosulfate unit has an average angle of 109.4◦, which is
indicative of a regular tetrahedral arrangement. The N–Cd–N
bond angles are in the range 94.0(2)–94.5(2)◦ and the S–Cd–S
bond angles are in the range 142.5(1)–144.4(1)◦. Compound
II also has a one-dimensional structure formed by the linking
of the cadmium centers by the bipyridine units, similar to I
(Fig. 2(a)). The C–C bond of the 4,4¢-bipyridine, in II, is not
twisted and the torsion angle is only 1.4◦ between the two
benzene rings. The thiosulfate units in II have both protonated and
deprotonated species and are bonded with cadmium through the
monodentate sulfur atom. Out of two distinct cadmium centers,
one is coordinated exclusively with two (HS2O3)- units while the
other is coordinated with two (S2O3)2-. This gives rise to two
distinct chains, which are stacked alternatively in the “ab” plane
(Fig. 2(a)). The (HS2O3)- and (S2O3)2- units hang as a pendent in
the inter-chain region from the cadmium center. The structure of
II also possesses two free 4,4¢-bipyridine molecules, one of which
is protonated, and occupies the inter-chain spaces along with
four lattice water molecules. The bonding of two (S2O3)2- units
with the cadmium center renders one of the one-dimensional
chains anionic, which is neutralized by the protonated bpy unit.
It may be noted that the position of the bpy molecules in II is
similar to the arrangement of the one-dimensional sodium–water
chain in I (Fig. 2(b)). The free 4,4¢-bipyridine units are twisted
around the central C–C bond with an average torsion angle of
27.4◦. The twisting of the free bpy appears to maximize the
interactions with the lattice water molecules. Here, we have both
N–H ◊ ◊ ◊ N and C–H ◊ ◊ ◊ O interactions. The possible O–H ◊ ◊ ◊ O and
O–H ◊ ◊ ◊ N interactions in this structure could not be evaluated as
we have not been successful in finding the actual positions of
the hydrogen atoms bound with the lattice water molecules. The
N ◊ ◊ ◊ N (donor–acceptor) distance between the protonated end
˚
arrangement with an average distance of 2.78 A (Fig. 3(d)) between
the water molecules, which is often observed in many hybrid
compounds including MOFs.10
Structure of [Cd2(C10H8N2)3(S2O3)2], IV
The asymmetric unit of IV contains 25 non-hydrogen atoms.
There are two crystallography independent Cd2+ ions, Cd(1)
and Cd(2), both occupying special positions (4e) with a site
multiplicity of 0.5. The Cd(1) is coordinated by two oxygen
atoms from two thiosulfate units and four nitrogen atoms from
the 4,4¢-bipyridine units forming a distorted octahedral environ-
ment (Cd(1)N4O2, CN = 6) and Cd(2) is coordinated by two
sulfur atoms from the thiosulfate units and two nitrogen atoms
from the 4,4¢-bipyridine units forming a tetrahedral environment
(Cd(2)N2S2, CN = 4). The average Cd–N, Cd–O and Cd–S
˚
bond distances are 2.352, 2.307 and 2.524 A, respectively. The
thiosulfate unit has an average S–O bond distance of 1.454
˚
and S–S bond distance of 2.044 A. The thiosulfate unit has an
average angle of 109.4◦, which is indicative of the tetrahedral
arrangement. The O–Cd–O bond angle is 177.3(1)◦, the S–Cd–S
bond angle is 103.9(1)◦ and the N–Cd–N bond angles are in
the range 93.02(9)–167.50(8)◦. The structure of IV consists of
the connectivity involving the tetrahedral and the octahedral Cd
centers, the thiosulfate unit and the 4,4¢-bipyridine unit. Thus,
the tetrahedral and the octahedral Cd centers are connected
through the thiosulfate units to form a one-dimensional chain
structure (Fig. 4(a)). The cadmium thiosulfate chains are bonded
with the 4,4¢-bipyridine units giving rise to the three-dimensional
structure. The octahedral and the tetrahedral Cd centers from the
˚
and the free end of the bpy is 2.74(1) A and the N–H ◊ ◊ ◊ N bond
angle is 175.0◦, suggesting favorable hydrogen bond interactions,
paving the formation of a dimer. Similarly, the C ◊ ◊ ◊ O distances
˚
are in the range 3.24(1)–3.45(1) A while the C–H ◊ ◊ ◊ O bond
angles are in the range 155.0–173.0◦. The observed O ◊ ◊ ◊ N and
˚
O ◊ ◊ ◊ O distances are in the range 2.60(1)–2.78(1) A. The O ◊ ◊ ◊ O
and O ◊ ◊ ◊ N interactions appear to be strong in this structure
also and the C–H ◊ ◊ ◊ O interactions can provide active secondary
supporting role.9b
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Fig. 2 (a) One-dimensional structure of [Cd2(C10H8N2)2(HS2O3)2(S2O3)2][(C10H9N2)2(C10H8N2)2]·8H2O, II. Note that two (S2O3)2- units hang from the
Cd-center of chain A and two (HS2O3)- units hang from the Cd-center of chain B. The hydrogen atoms are omitted for clarity. (b) The structure of II
showing the arrangement of the Cd-bpy chains along with the free bpy molecules. Dotted lines represent possible hydrogen bond interactions and the
free bpy units have been shown in a different color.
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Fig. 3 (a) The one-dimensional cadmium thiosulfate helical chain in [Cd(C10H8N2)(H2O)2(S2O3)]·2H2O, III. (b) Figure shows the connectivity between
the adjacent chains through the bpy units in the bc plane forming the two-dimensional layer arrangement. (c) The three-dimensional structure of III. The
different colors of the bpy units indicate connectivity between two different layers. The hydrogen atoms of bpy and water are omitted for clarity. (d) The
cyclic water tetramer found in the channels in III.
adjacent chains are connected through the bpy units forming a
two-dimensional layer (Fig. 4(b)) where the bpy units are twisted
around the central C–C bond with an average torsion angle of
11.3◦. The other nitrogen coordination of the octahedral Cd
centers are utilized to form the three-dimensional structure via
bonding with 4,4¢-bipyridine units (Fig. 4(c)) which are not twisted
around the central C–C bond. In this structure interpenetration is
also observed.
to form a distorted trigonal bipyramidal (TBP) environment
(Cd(2)N3O2, CN = 5). The average Cd–N, Cd–O and Cd–S bond
˚
distances are 2.321, 2.326 and 2.491 A, respectively. The average S–
˚
O and S–S bond distances in the thiosulfate are 1.443 and 2.060 A,
respectively. The thiosulfate unit has an average angle of 109.4◦.
The O–Cd–O bond angle is 75.2(1)◦, the S–Cd–S bond angle is
119.2(1)◦ and the N–Cd–N bond angles are in the range of 88.01(9)
to 173.59 (10)◦. The strictly alternating tetrahedral and TBP Cd
centers are connected through the thiosulfate unit forming one-
dimensional cadmium thiosulfate chains (Fig. 5(a)). The cadmium
thiosulfate one-dimensional chains are connected by the 4,4¢-
bipyridine units forming the three-dimensional structure. The
tetrahedral and TBP Cd centers from the adjacent chains are
connected through the bpy units to form the two-dimensional layer
(Fig. 5(b)). Here, the layer is formed by two different bpy units with
an average torsion angle of 24.7◦ and 3.1◦ around the central C–C
Structure of [Cd2(C10H8N2)2.5(S2O3)2], V
The asymmetric unit of V contains 42 non-hydrogen atoms. There
are two crystallography independent Cd2+ ions and both occupy
normal positions. The Cd(1), is tetrahedrally coordinated by two
sulfur and two nitrogen atoms (Cd(1)N2S2, CN = 4), and Cd(2)
is coordinated by two oxygen atoms and three nitrogen atoms
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Fig. 4 (a) The one-dimensional cadmium thiosulfate chain in [Cd2(C10H8N2)3(S2O3)2], IV. (b) Figure shows the connectivity between the chains by the
bpy units in the ab plane. (c) The three-dimensional structure of IV. The hydrogen atoms of the bpy units are omitted for clarity.
bond. The one extra nitrogen coordination of TBP Cd centers is
utilized to form the three-dimensional structure (Fig. 5(c)) where
the bpy units are not twisted around the central C–C bond. In
this structure we also observed interpenetration similar to that
observed in IV.
part of the one-dimensional structural arrangement, but hang
in the inter-chain spaces and participate in hydrogen bond
interactions, which could lend some stability to these low-
dimensional structures. The one-dimensional structures may also
be compared with the earlier reported zinc thiosulfate structure,
{[Zn(C10H8N2)(H2O)4][Zn2(S2O3)3(C10H8N2)2]·2H2O}n.4a The zinc
thiosulfate compound is formed by the linking of zinc centers
by bipyridine as well as the thiosulfate units (Fig. 6(a)). This
gives rise to a double chain structure instead of a simple one-
dimensional structure observed for I and II. The zinc thiosulfate
compound also possesses one terminal thiosulfate unit, where
both I and II have two terminal thiosulfate units. Similar to
the zinc thiosulfate compound, I and II are also anionic. The
charge compensation is achieved by the presence of [Na(H2O)4]n
and protonated bipyridine units in I and II, respectively, whereas
the [Zn(bpy)(H2O)4]n chain acts as the compensating unit in
Structural comparison
Among all the cadmium thiosulfate phases prepared in the
present study, I and II have one-dimensional structures, whereas
III, IV and V have three-dimensional structures. All the struc-
tures have connectivity that involves cadmium ions, thiosul-
fate and bipyridine units. The Cd centers have different co-
ordination environments. The one-dimensional structures of I
and II are formed by the connectivity between the tetrahe-
dral cadmium and bpy units. The thiosulfate units are not
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Fig. 5 (a) The one-dimensional cadmium thiosulfate chain in [Cd2(C10H8N2)2.5(S2O3)2], V. (b) Figure shows the connectivity between the chains by the
bpy units in the ab plane. (c) The three-dimensional structure of V. The hydrogen atoms of the bpy units are omitted for clarity.
the reported structure.4a A closer examination also reveals
that the positions of the [Na(H2O)4]n and bpy units in I
and II are comparable to the [Zn(bpy)(H2O)4]n chains in the
{[Zn(C10H8N2)(H2O)4][Zn2(S2O3)3(C10H8N2)2]·2H2O}n compound
(Fig. 6(b)). In the zinc thiosulfate structure, the [Zn(bpy)(H2O)4]n
chains are mutually perpendicular (two chains), which is observed
in II but not in I (Fig. 6(c) and 6(d)). The one-dimensional
structures have close structural features to that reported earlier.4a
The cadmium thiosulfate chains observed in the three-
dimensional structures, III, IV and V, have also been ob-
served in a manganese thiosulfate compound, [Mn(S2O3)2-
(C12H8N2)(H2O)2].4c In the manganese thiosulfate compound, the
use of 1,10-phenanthroline resulted in a simple chain structure
supported by p ◊ ◊ ◊ p interactions (see ESI,† Fig. S12). In
structures, III, IV and V, the use of 4,4¢-bipyridine resulted in
three-dimensional structures. The three-dimensional structures,
III, IV and V, also have comparable structural features. The
structures of the compounds have been reported recently by us.7
The three structures, however, appear to be related even though
the coordination around the Cd atoms differ between them. The
three 3D structures, III, IV and V, have interpenetrations arising
primarily due to the connectivity involving the bpy units. We
have performed a detailed structural analysis using the TOPOS40
program,11 to understand the structural interpenetration in III, IV
and V. The analysis indicated that compound III has cadmium
thiosulfate chains cross-linking through the bpy units (Fig. 7(a)).
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Fig. 6 (a) The zinc thiosulfate one-dimensional double-chain structure of {[Zn(C10H8N2)(H2O)4][Zn2(S2O3)3(C10H8N2)2]·2H2O}n. Note that the
thiosulfate units hang from the Zn-center. (b) The packing diagram of {[Zn(C10H8N2)(H2O)4][Zn2(S2O3)3(C10H8N2)2]·2H2O}n. Note the presence of
the zinc–water–bpy chains along the two mutually perpendicular directions. (c) View of the structure of I in the “bc” plane. (d) The packing diagram of
compound II. Note that the free bipyridine units along the two mutually perpendicular directions. Note also the close structural similarity between the
structures. Hydrogen atoms have been omitted for clarity.
A similar analysis on IV revealed the presence of two completely
interpenetrated three-dimensional structures (Fig. 7(b), 7(c)).
Each net that sustains the structure have similar building units
(viewed along the bc plane) and the inter-penetration occurs only
due to the bpy linkages between the two nets. The analysis of V also
indicates the presence of two interpenetrated three-dimensional
structures, but is different to IV. Here, the structure interpenetrates
perpendicularly between the two three-dimensional structures
(Fig. 7(d) and 7(e)), again mediated by the bpy units.
In addition to the interesting interpenetrations observed in the
structures of III, IV and V, there are other structural relationships
that exist between the three structures. All the three structures
have different Cd : bpy ratios (1 : 1 in III, 1 : 1.5 in IV and 1 : 1.25
in V) but the Cd : S2O3 ratios are similar. This suggests close
structural relationships between the three structures. The presence
of one-dimensional cadmium thiosulfate chains in III–V also
suggests the possibility for structural relationships. The main
difference between the structures arises due to the differences in
the coordination geometry of the Cd centers. A careful study of
the three structures suggested that they can be derived from the
primitive cubic (pcu) structural network. The pcu net is generally
based on an octahedrally connected structure (see ESI,† Fig.
S13(a)). Of the three structures, only compound III contains
octahedral Cd, but of the six connections, two are terminal water
molecules, rendering the Cd centers as essentially four connected.
Compound IV, on the other hand, has both octahedral as well as
four-connected cadmium centers and can be used as the starting
point to visualize the present structures as derived from the pcu
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Fig. 7 (a) Figure shows the connectivity between the nodes (Cd centers) through the bpy and the S2O3 units in III. Note that the bpy units link the layers.
(b) Figure shows the connectivity between the nodes (Cd centers) through the bpy and the S2O3 units in IV. Note that the connectivity forms a square-box
like unit. (c) Figure shows the interpenetration observed in IV. Note that the two square grids interpenetrate perpendicularly (see text). (d) Figure shows
the connectivity between the nodes (Cd centers) through the bpy and the S2O3 units in V. Note that the connectivity gives rise to a rectangular-box
like unit. (e) Figure shows the interpenetration observed in V. Note that the rectangular-box units interpenetrate perpendicularly (see text).
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net. It has been suggested that the combination of octahedral and
four-coordinated metal centers can give rise to a pcu derived net
known as the fgs net (see ESI,† Fig. S13(b)).12 This fgs net has been
observed earlier in Mn2(dca)3(NO3)(Mepyz)2.13 Our structure IV
has similar connections to that of Mn2(dca)3(NO3)(Mepyz)2 and
can be related to the fgs net. In V, we have four and five connected
Cd centers, which could distort the fgs net further resulting in
another derived net (see ESI, Fig. S13(c)). Compound III, which
essentially has only four-connections would further distort the fgs
net (see ESI,† Fig. S13(d)). Thus, the three-dimensional thiosulfate
structures III, IV and V are related and can be generated from the
pcu net topology.
transformation reactions do proceed rapidly in a water–ethanol
solvent mixture. The reactions carried out at 60 ◦C showed a
completely different behavior and we observed newer peaks in the
PXRD pattern (Fig. 8). As can be noted, we do not observe the
formation of III at 60 ◦C, but I transforms directly to V after 6 h of
heating at 60 ◦C. After 36 h, we observed new peaks, which were
later identified as corresponding to the cadmium sulfate phase
(compound VI). This suggests that the compound undergoes
partial decomposition at elevated temperature in a water–ethanol
mixture, with the (S2O3)2- unit losing some sulfur to form the
(SO4)2- units. The transformation reactions in a water–ethanol
mixture also yielded good quality single-crystals of the new phase,
which were employed for the structural studies. The single-crystal
structural study showed that this phase is a cadmium sulfate
[Cd2(C10H8N2)2(H2O)3(SO4)2]·2H2O. The cadmium sulfate phase
was prepared, subsequently, starting from individual components
and is discussed later.
Transformation reactions
It has been shown earlier that the low-dimensional structures could
be a precursor for structures of higher dimensionality.14 Since, in
present compounds I and II, the low-dimensional structures do
not have the continuous connectivity between the cadmium centers
and the thiosulfate units, we believe that it would be interesting to
examine the reactive nature of the 1D cadmium thiosulfate phases.
Of the two compounds, II could not be obtained in pure form in
large enough quantity for investigating the reactivity and hence
only compound I was studied. For this, we have taken compound
I and reacted at varying times and temperatures. The various
experimental conditions employed for the transformation studies
have been summarized in Table S9 (see ESI†).
For the transformation study, 0.18 g of I was heated in 2.4 ml
of water at 30, 45 and 60 ◦C simultaneously in many closed
containers. The reaction vessels were removed periodically at
different time intervals, the products recovered by filtration and
analyzed using powder X-ray diffraction (see ESI,† Fig. S14). As
can be noted, compound III starts to form after about 12 h at
30 ◦C. The presence of compound I persists up to 48 h, after
which we observe only compound III. On heating I at 45 ◦C,
we observe the formation of compound III at about 6 h and
compound I completely vanishes after 12 h. This suggests that
I transforms to III at a much faster rate at elevated temperatures
(45 ◦C compared to 30 ◦C). At 60 ◦C, though we observed a similar
behavior of the formation of III after 6 h of reaction, at longer
duration we observed the formation of V. Compound III persists
up to 36 h and after that only the pure phase of V was observed.
This suggests the possibility of III being an intermediate phase in
the formation of V. In this context, it is worthwhile to note that
we have prepared the pure phase of III at 30 ◦C in a water–ethanol
mixture compared to 75 ◦C required to form V in pure water
medium. Since compound I has been synthesized in a mixture of
solvents (water–ethanol), we wanted to study the transformation
of I in a water–ethanol mixture by employing similar conditions
as before. For these set of reactions, we have taken 0.18 g of I in a
solvent mixture containing 1.2 ml of water and 1.2 ml of ethanol
(see ESI,† Fig. S15). The studies at 30 ◦C show the formation
of III from I, but the trend is considerably different compared to
that observed using only water as the solvent. Here, compound
III starts to form even at 6 h and after 12 h there is no trace
of I suggesting that the transformation of I to III is much more
facile in a water–ethanol solvent mixture. At 45 ◦C, we do observe
a similar trend, but in addition we also observe the formation
of V during longer heating (after 36 h). This suggests that the
Fig. 8 Powder XRD pattern of the transformation reactions of I in a
water–ethanol mixture at 60 ◦C. Note that the different phases start to
form at different time intervals. (* = I, ꢀ = V and + = VI).
The transformation reactions confirmed the reactive nature of I.
It is also possible to visualize the formation of structures III and V
from I by simple manipulations (see ESI,† Scheme 1). The removal
of the (S2O3)2- unit from I can form the simple two-dimensional
layer observed in III and the further addition of bpy units can give
rise to V. This indicates that the reaction pathway is not a simple
solid state type reaction, but suggestive of the dissolution and
recrystallization type reaction. Similar mechanistic pathways have
been observed and postulated for the transformation reactions of
one-dimensional zinc-phosphate phases.15
The transformation study of I suggested the possibility of
III being an important intermediate in the formation of other
cadmium thiosulfate phases. In addition, our structural evaluation
also suggested that compound III has the most distorted structure
when one derives from the pcu net. So, it occurred to us that
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we could investigate the possible reactivity of III. The various
experimental conditions employed for the transformation studies
have been summarized in Table S10 (ESI).†
For the transformation studies, we have taken 0.15 gm of III and
heated in water at 30, 45 and 60 ◦C (see ESI,† Fig. S16). At 30 ◦C,
we did not observe any new products even after 60 h. The reactions
at 45 ◦C, however, indicated the formation of compound V after
48 h but the transformation was not complete as compound III
persisted up to 60 h. At 60 ◦C, the formation of V was seen after
24 h and compound III disappeared completely after 36 h. We did
not observe any other phase other than V up to 60 h. These studies
showed that III, indeed, transforms to another related phase
(V) as a function of time and temperature. Since, the reactivity
of I was observed to be much more facile in a water–ethanol
mixture, we investigated the transformation of III in a water–
ethanol mixture at 30, 45 and 60 ◦C following a similar procedure
as described earlier (see ESI,† Fig. S17). The studies indicated
that compound III did not undergo any transformations at 30 ◦C
irrespective of the duration of the reaction. At 45 ◦C, compound
III starts to transform to V after 12 h and the transformation was
completed after 24 h. We did not observe the formation of any
other phase up to 60 h. At 60 ◦C, the formation of compound
V was observed within 6 h. After 24 h, we observed additional
new phases in the PXRD, which corresponded to the cadmium
sulfate phase observed during the transformation of I in a water–
ethanol mixture at 60 ◦C. The transformation studies of III appear
to confirm our view that III could be an intermediate and also
support the results of the transformation reactions of I.
Since, the bpy : Cd ratio among all the three 3D compounds
are different, we wanted to examine the possible conversion of III
to IV and/or V, as the latter two have higher bpy/Cd ratios (1.5
and 1.25, respectively, compared to 1 : 1 for III). For this, III was
reacted with stoichiometric quantities of bpy (0.25 and 0.5 mol)
for varying times and temperatures. As before, the products were
isolated and examined by PXRD studies (see ESI,† Fig.◦S18, S19).
The results indicated that there was no reaction at 30 C, but at
45 ◦C in the presence 0.5 mol bpy the formation of compound IV
was seen after 8◦h. Pure phase of IV was observed only after 20 h.
Similarly, at 45 C in the presence of 0.25 mol bpy, we observed
the formation of both IV and V after 8 h. We did not observe the
formation of any one of the pure phases (IV or V) even after 20 h
when the reactions were carried out using 0.25 mol bpy. At 60 ◦C,
both IV and V form after 4 h in the presence of 0.25 and 0.5 mol
of bpy. The pure phase of V was obtained after 20 h when the
reactions were carried out using 0.25 mol bpy, but only a mixture
of IV and V, while using 0.5 mol bpy.
Fig. 9 Powder XRD pattern of the transformation reaction of III in
ethanol medium at 110 ◦C (᭺ = III, ꢀ = VII).
that was isolated during the transformation reactions. From the
transformation reactions, we believe the reaction proceeds via
a dissolution and re-crystallization pathway. The formation of
sulfate from thiosulfate is not surprising as the S–S bond is quite
weak and tends to form the sulfate species under the reaction
conditions.
(S2O3)2- → (SO4)2- + S
The pathway can be substantiated as we observed the formation
of sulfur (as S8 species) and CdS phase. The results of the various
transformation studies can be summarized schematically as shown
in Scheme 1. The single-crystal study gave the stoichiometric
compositions of the cadmium sulfate phases and we sought to
prepare them starting from the individual components (Table 1).
Structure of [Cd2(C10H8N2)2(H2O)3(SO4)2]·2H2O, VI
The structure of compound VI is comparable to the cadmium sul-
fate [Cd2(C10H8N2)2(H2O)3(SO4)2]·3H2O, reported previously.8 In
the present structure, we observed less framework water molecules
than the reported structure. Three CdO4N2 and SO4 units are
connected together forming a six-membered ring, which are
further linked to form a one dimensional ladder-like chain
along the “b” axis (Fig. 10(a)). The one-dimensional ladder-
like units are connected by 4,4¢-bipyridine ligands forming the
three-dimensional structure (Fig. 10(b)). The structure of VI has
features that are comparable to the structure of III, especially
with regard to the bpy connectivity. The bpy connectivity in this
structure has close resemblance to the interpenetration observed in
III. The topological analysis, however, exhibits differences between
the sulfate and thiosulfate chain in the structure VI and III,
respectively (Fig. 10(c)).
Since, we have derived the structures of III, IV and V from the
fgs net, it can be very easily shown schematically how the addition
of bpy to the most distorted structure III can be effected (see ESI,†
Scheme 2). This must involve changes in the bonding as well as
the coordination environment around Cd.
We have also carried out one set of experiments at elevated
temperature (110 ◦C) to investigate the transformation of III.
After 12 h, the sample becomes grossly amorphous and the XRD
pattern (Fig. 9) matched with that of CdS with a particle size
of ~100 nm. On heating further, we observed an entirely new
phase. Few single crystals were found in the product after 48 h
and the structural determination indicated that it is a cadmium
sulfate phase, VII. This is the second cadmium sulfate phase
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Table 1 Synthetic compositions and conditions employed for compounds I–VIIa
pH
S. No
Composition
Temp/◦C
Time/h
72
(range)
6.0–8.0
6.0
Product
Yield (%)
1
2
3
4
5
6
7
1Cd(NO3)2·4H2O + 2Na2S2O3·5H2O +
1bpy + 51EtOH + 166H2O
5
[Na2(H2O)8][Cd(C10H8N2)(S2O3)2]·2H2O, I
[Cd(C10H8N2)(HS2O3)2][(C10H8N2)2]·4H2O, II
[Cd(C10H8N2)(H2O)2(S2O3)]·2H2O, III
[Cd2(C10H8N2)3(S2O3)2], IV
90
70
90
90
85
90
85
1Cd(NO3)2·4H2O + 2Na2S2O3·5H2O +
75
25
75
75
60
2bpy + 166H2O
.
1Cd(NO3)2.4H2O + 2Na2S2O3 5H2O +
60
6.0–8.0
6.0–8.0
8.0
1bpy + 51EtOH + 166H2O
1Cd(NO3)2·4H2O + 2Na2S2O3·5H2O +
2bpy + 166H2O
12–24
60
1Cd(NO3)2·4H2O + 2Na2S2O·5H2O +
[Cd2(C10H8N2)2.5(S2O3)2], V
2bpy + 166H2O
1Cd(NO3)2·4H2O + 1Na2SO4 + 1bpy + 75
166H2O
60
5.0
[Cd2(C10H8N2)2(H2O)3(SO4)2]·2H2O, VI
[Cd(C10H8N2)(SO4)], VII
1Cd(NO3)2·4H2O + 1Na2SO4 + 1bpy + 110
60
5.0
26EtOH + 83H2O
a pH was adjusted by the addition of NH4OH. Elemental analysis calcd (%) for I: C 16.69, H 3.89, N 2.78; found 16.48, H 3.96, N 2.88; Elemental analysis
calcd (%) for II: C 40.94, H 3.87, N 9.55; found: C 40.81, H 3.69, N 9.48; Elemental analysis calcd (%) for III: C 26.50, H 3.53, N 6.18; found: C 26.54, H
3.54, N 6.15; Elemental analysis calcd (%) for IV: C 39.23, H 2.61, N 9.15; found: C 39.30, H 2.64, N 9.11; Elemental analysis calcd (%) for V: C 35.73,
H 2.38, N 8.34; found: C 35.81, H 2.35, N 8.38; Elemental analysis calcd (%) for VI: C 29.29, H 3.17, N 6.83; found: C 29.44, H 3.11, N 6.88; Elemental
analysis calcd (%) for VII: C 32.90, H 2.19, N 7.68; found: C 32.81, H 2.25, N 7.59.
Scheme 1 Scheme summarizing the transformation reactions. Only the Cd coordination sphere is shown for simplicity. Note that red atoms are the
oxygen atoms of the thiosulfate and sulfate units. For compounds IV, V and VI, one Cd center has been shown for clarity.
Three-dimensional cadmium sulfate, VII, [Cd(C10H8N2)(SO4)]
˚
average Cd–O and Cd–N bond distances of 2.289 and 2.374 A,
respectively. The S atom is tetrahedrally coordinated by four
˚
The asymmetric unit contains 11 non-hydrogen atoms, of which
one Cd and S are crystallographically independent. Both Cd and
S occupy special positions (2c and 2b, respectively), with a site
multiplicity of 0.25. The Cd atom is octahedrally coordinated
with four oxygen atoms and two nitrogen atoms (CdN2O4), with
oxygen atoms with an average S–O bond distance of 1.440 A. The
sulfate possesses an average O–S–O bond angle of 110.6◦, which
is indicative of the tetrahedral arrangement. The O–Cd–O bond
angles are in the range 83.5(6)–160.8(1)◦ and the N–Cd–N bond
angle is 180.0(1)◦. The connectivity between CdN2O4 octahedra
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and SO4 tetrahedra give rise to a two-dimensional cadmium sulfate
layers (Fig. 11(a)), which are pillared by the 4,4¢-bipyridine units
giving rise to the three-dimensional structure (Fig. 11(b)). The
cadmium sulfate layers resemble a typical 4 ¥ 4 square grid. From
the topological point of view, the cadmium sulfate layers form the
4-4 nets (Fig. 11(c)) which are connected by bpy units.
Both the cadmium sulfate structures, VI and VII, have three-
dimensional connectivity involving the sulfate and the bipyri-
dine unit. The bpy units connect the one-dimensional cadmium
sulfate ladder-like units forming the 3D structure in VI, whereas
it links a square grid cadmium sulfate layers in VII. The bpy
connectivity of structure VI appears to have some resemblance to
that found in III. The inorganic cadmium sulfate layer, observed in
VII, has close resemblance to many similar structures reported in
the literature. Thus, the cadmium sulfate layers are comparable
to the two-dimensional layers observed in the zinc-arsenate,
{[C4N2H12][Zn(AsO4)]2},16 and also with the cadmium sulfate
17
layers in {[H3N(CH2)3NH3][Cd2(H2O)2(SO4)3]} (Fig. 12). The
later structure has layers formed by the connectivity between
CdO6 octahedral and SO4 tetrahedral units, but the presence of
two terminal water molecules and a terminal sulfate unit reduces
the Cd as a 4-connected center giving rise to the 4 ¥ 4 grid (see
ESI,† Fig. S20). Similarly, the gallium arsenate and phosphate
structures {Ga2(4,4¢-bpy)(MO4)2, (M= As, P)},18 have GaO4N
trigonal bipyramid units bound to MO4 tetrahedra forming the
4 ¥ 4 grid (see ESI,† Fig. S21). Since, the Ga-centers are only
5-coordinated the bipyridine units link only alternate Ga centers
giving rise to the 3D structure. Thus, the layer in VII is comparable
to many structures in the literature, but the connectivity between
the layers gives rise to the observed differences.
Catalytic studies
The formation of new types of structures in the family of
inorganic–organic hybrids with thiosulfate as a component
prompted us to investigate the efficacy of these solids for
possible catalytic purposes. Recently, the cyanosilylation of
imines has been carried out in the presence of {[Cd(4,4¢-
bpy)2(H2O)2](NO3)2·4H2O}n, heterogeneously.19 Since, the present
compounds also involves bonding between the cadmium centers
and the bipyridine units, we sought to perform a preliminary study
of the catalytic reactivity of the 3D thiosulfate and sulfate com-
pounds. For this, we employed similar experimental conditions as
used in the earlier study.19 Typically a powdered catalyst (0.1 mmol)
was suspended in CH2Cl2 solution (1.5 ml) of imine (A, 0.5 mmol).
Trimethylsilyl cyanide (0.75 mmol) was added at 0 ◦C and the
reaction mixture was stirred for 1 h. The product, aminonitrile (B),
was isolated and analyzed. Of the three 3D thiosulfate compounds,
we found that compound III shows a quantitative conversion of
the imine to the aminonitrile (~98%). Compounds IV and V show
only about 40% conversion. The cadmium sulfate phases (VI and
VII), on the other hand, exhibited a conversion of 60–63% (see
ESI,† Table S12). In order to check the efficacy of the thiosulfate
as the catalyst, we have also carried out control experiments in
the absence of thiosulfate in the suspension. We did not observe
any reaction between the reactants in the absence of catalysts.
We have also carried out a similar experiment in the presence of
Cd(NO3)2·4H2O, which gave a conversion of ~20%. This suggests
that the thiosulfates are active as the heterogeneous catalyst for
Fig. 10 (a) The one-dimensional cadmium sulfate chain observed in
[Cd2(C10H8N2)2(H2O)3(SO4)2]·2H2O, VI. Note that the chains are formed
by the six-membered units. (b) The three-dimensional structure of VI
showing the connectivity between the cadmium sulfate chains and the
bpy units. The different colors of the bpy units indicate connectivity
between two different layers. The hydrogen atoms of the bpy units are
omitted for clarity. (c) Figure shows the connectivity between the chains
and the bpy units. Note the similarity in the interpenetration with that of
III (Fig. 6(a)).
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Fig. 12 (a) The two-dimensional 4 ¥ 4 layer observed in the zinc-arsenate,
{[C4N2H12][Zn(AsO4)]2}. (b) The two-dimensional 4 ¥ 4 layer observed
in the cadmium-sulfate, {[H3N(CH2)3NH3][Cd2(H2O)2(SO4)3]} (c) The
two-dimensional 4 ¥ 4 layer observed in VII.
Fig. 11 (a) Figure shows the connectivity between CdO4N2 octahedra and
SO4 tetrahedra in [Cd(C10H8N2)(SO4)], VII. Note the formation of a 4 ¥
4 two-dimensional square grid. (b) Figure shows the connectivity between
the cadmium sulfate layers and the bpy units in VII. The hydrogen atoms
of the bpy units are omitted for clarity. (c) Figure shows the connectivity
between the nodes (Cd-center) through the SO4 and the bpy units.
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the cyanosilylation of imines. We believe that the presence of Lewis
acidity associated with the Cd center may be responsible for the
observed catalytic behavior. Similar observations have also been
made earlier.19
and kept at 5 ◦C inside the refrigerator. Plate like crystals of I were
obtained after ~72 h (yield = 90%).
Synthesis of [Cd(C10H8N2)(HS2O3)2][(C10H8N2)2]·4H2O, II.
Cd(NO3)2·4H2O (0.309 g, 1 mM) and Na2S2O3·5H2O (0.496 g,
2 mM) were dissolved in 3 ml of distilled water. Then, 4,4¢-
bipyridine (0.312 g, 2 mM) was added and the mixture was stirred
continuously for 30 min at room temperature. This mixture was
transferred into a 7 ml Teflon-lined stainless steel autoclave and
◦
heated at 75 C for 60 h to result in block like crystals (yield =
70%).
Synthesis of [Cd(C10H8N2)(H2O)2(S2O3)]·2H2O, III. A mix-
ture of Cd(NO3)2·4H2O (0.309 g, 1 mM) and Na2S2O3·5H2O
(0.496 g, 2 mM) was dissolved in 3 ml of distilled water and
4,4¢-bipyridine (0.156 g, 1 mM) was taken in 3 ml of EtOH.
The alcoholic solution was layered carefully on top of the salt
solution, similar to the synthesis of I and the mixture was allowed
The compounds, after the reactions, were also examined using
powder XRD for phase stability. The PXRD patterns did not
show any appreciable change, which implies that the compounds
are stable (see ESI,† Fig. S22, for compound III). This study
suggests that cadmium thiosulfate hybrid compounds are not
only stable, but also recyclable for catalytic purpose. The bound
water molecules and the unsaturated Cd-coordination in III could
have played a subtle role in the conversion of the imine, but this
hypothesis needs to be confirmed by carrying out other related
and detailed catalytic experiments.
◦
to evaporate slowly at room temperature (25 C) to result in rod
like crystals of III after ~60 h (yield = 90%).
Synthesis of [Cd2(C10H8N2)3(S2O3)2], IV. Cd(NO3)2·4H2O
(0.309 g, 1 mM) and Na2S2O3·5H2O (0.496 g, 2 mM) were dissolved
in 3 ml of distilled water. Then, 4,4¢-bipyridine (0.312 g, 2 mM)
was added and the mixture was stirred continuously for 30 min
at room temperature. This mixture was transferred into a 7 ml
Teflon-lined autoclave and heated at 75 ◦C for 12 h to result in
plate like crystals (yield = 90%).
Synthesis of [Cd2(C10H8N2)2.5(S2O3)2], V. Cd(NO3)2·4H2O
(0.309 g, 1 mM), Na2S2O3·5H2O (0.496 g, 2 mM) and 4,4¢-
bipyridine (0.312 g, 2 mM) were dissolved in 3 ml water. NH4OH
was slowly added to the mixture to obtain a pH of 8.0 and stirred
for 30 min at room temperature. This mixture was transferred into
a 7 ml Teflon-lined autoclave and heated at 75 ◦C for 60 h to result
in rectangular block like crystals (yield = 85%).
Conclusions
The synthesis and structure of a new family of thiosulfate
phases has been accomplished. The formation of one- and
three-dimensional thiosulfate phases suggest that the thiosulfate
compounds would also lead to a rich variety and diversity in
the structures. The transformation reactions clearly reveal that
the lower-dimensional structures are indeed reactive and can act
as a precursor for phases of higher dimensionality. The close
structural relationships between the three-dimensional structures
and the transformation of one into another also suggest that the
energy of formation of such structures could be comparable. The
formation of cadmium sulfate phases during the transformation
studies was unexpected, though with hindsight one can rationalize
the formation of such phases. The preliminary catalytic conversion
of the imines suggests that we are investigating a series of
compounds with interesting possibilities. It would be desirable
to prepare compounds that have three-dimensional connectivity
arising entirely through the thiosulfate units as such structures
could be distinctly different from the conventional ones. Efforts
are under way towards this goal.
Synthesis of [Cd2(C10H8N2)2(H2O)3(SO4)2]·2H2O, VI. A mix-
ture of Cd(NO3)2·4H2O (0.309 g, 1 mM), Na2SO4 (0.142 g, 1 mM)
and 4,4¢-bipyridine (0.156 g, 1 mM) was dissolved in 3 ml of
distilled water and heated at 75 ◦C for 60 h to result in block like
crystals.
Synthesis of [Cd(C10H8N2)(SO4)], VII.
A
mixture of
Cd(NO3)2·4H2O (0.309 g, 1 mM) and Na2SO4 (0.142 g, 1 mM) was
dissolved in 1.5 ml of distilled water and 4,4¢-bipyridine (0.156 g,
1 mM) was taken in 1.5 ml of EtOH. The alcoholic solution was
mixed with the water mixture and heated in a 7 ml Teflon-lined
autoclave at 110 ◦C for 60 h resulting in plate like crystals.
The synthetic compositions and conditions employed for the
preparation of the phases are given in Table 1. Elemental analyses
were carried out using a Thermo Finnigan FLASH EA 1112 CHN
analyzer (Table 1).
All the compounds were characterized by powder X-ray diffrac-
tion (XRD), IR, UV-visible and TGA studies. The powder XRD
patterns were recorded in the 2q range 5–50◦ by using Cu Ka
radiation (Philips, X’pert-Pro). The XRD patterns indicated that
the products were new and the patterns were entirely consistent
with the simulated XRD pattern generated based on the structures
determined using the single-crystal XRD (see ESI,† Fig. S1–S7).
Infrared (IR) spectroscopic studies have been carried out in the
mid-IR region (4000 to 400 cm-1) on KBr pellets (Perkin Elmer,
Experimental
Synthesis and initial characterization.
All the compounds were prepared by using self assembly methods
(I, III) at room temperature or even lower and solvothermal
conditions for the other compounds in a Teflon-lined autoclave.
The reagents, Cd(NO3)2·4H2O (Aldrich 98%), Na2S2O3·5H2O
(Fluka 99%) and 4,4¢-bipyridine (Aldrich 98%) were used as
received without any further purification.
Synthesis of [Na2(H2O)8][Cd(C10H8N2)(S2O3)2]·2H2O, I.
A
mixture of Cd(NO3)2·4H2O (0.309 g, 1 mM) and Na2S2O3·5H2O
(0.496 g, 2 mM) was dissolved in 3 ml of distilled water and
4,4¢-bipyridine (0.156 g, 1 mM) was taken in 3 ml of EtOH. The
alcoholic solution was layered carefully on top of the salt solution
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SPECTRUM 1000). IR spectra of all the compounds were more
or less similar and can be described in terms of five distinct regions
(see ESI,† Fig. S8). (i) Compounds I, II, III and VI have bands at
3600–3300 cm-1, dominated by a broad band centered at 3435 cm-1
indicative of the presence of water molecules. A band at 3375 cm-1
can be assigned to the protonated OH group of the thiosulfate unit
in II; (ii) a band at 3045 cm-1 can be assigned to aromatic u(CH)
modes of the bipyridine ring; (iii) a band in the region 1650–
1200 cm-1 can be assigned to the C–H bending vibrations due to
the asymmetric and symmetric modes of the pyridine skeletal; (iv)
1150–1000 cm-1 region, with sharp band at 1130 cm-1 is due to us
(S–O) and (v) the bands in the region of 805–535 cm-1 are due to
the bending mode of S–O and us (S–S) of the thiosulfate unit.
The solid state UV-visible absorbance spectroscopic studies
were carried out for all compounds I–VII at room temperature
using a Perkin-Elmer model Lambda 35 UV-visible spectrometer
(see ESI,† Fig. S9). The surface area of the three thiosulfate
compounds (III, IV and V) was determined using porosimeter
(Belsorp, Japan) in the presence of N2. The adsorption–desorption
isotherm has shown that the compounds are very less porous.
Compound III has the highest surface area (3 m2 g-1) than
compounds IV and V (1.4 m2 g-1 and 1.3 m2 g-1, respectively,
see ESI,† Fig. S10).
with a cyanoacrylate (superglue) adhesive. Single-crystal data
were collected on a Bruker AXS smart Apex CCD diffractometer
at 293(2) K. The X-ray generator was operated at 50 kV and
˚
35 mA using Mo Ka (l = 0.71073 A) radiation. Data were
collected with w 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 (2q) fixed at -25◦. The data were reduced
using SAINTPLUS,20 and an empirical absorption correction was
applied using the SADABS program.21 The structure was solved
and refined using SHELXL9722 present in the WinGx suit of
programs (Version 1.63.04a).23 The hydrogen positions of the two
lattice water molecules and eight coordinated water molecules of
the sodium, present in compound I, could not be located. All the
hydrogen positions of the 4,4¢-bipyridine for the compounds and
the hydrogen position of the hydroxyl group in II could be located
but the hydrogen positions of the four lattice water molecules in
II, the hydrogen positions of one lattice water molecule in III, two
lattice water molecules and three coordinated water molecules
in VI could not be located. Where the hydrogen positions were
located, restraints for the bond distances were used during the
refinement to keep the hydrogen atoms bonded with the three
water molecules in III. The hanging thiosulfate groups in I appear
to be disordered with a mixed occupancy of 70 : 30 (see ESI†). The
corresponding positions were refined isotropically. Full-matrix
least-squares refinement 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.
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
Table 2 Crystal data and structure refinement parameters for I–VII
Structural parameter
Empirical formula
I
II
III
IV
V
VI
VII
[Na2(H2O)8]-
[Cd(C10H8N2)- (HS2O3)2(S2O3)2]-
(S2O3)2]·2H2O [(C10H9N2)2(C10H8- (S2O3)]·2H2O
[Cd2(C10H8N2)2-
[Cd(C10H8N2)- [Cd(C10H8N2)1.5
(H2O)2-
-
[Cd2(C10H8-
N2)2.5·(S2O3)2] (H2O)3(SO4)2]·
[Cd2(C10H8N2)2- [Cd(C10H8N2)-
(S2O3)]
(SO4)]
2H2O
N2)2]·8H2O
Formula weight
Crystal system
Space group
718.85
Monoclinic
P21 (No. 4)
1758.42
452.75
Monoclinic
P21/n (No.14) C2/c (No. 15)
458.80
Monoclinic
839.50
Monoclinic
P21/c (No. 14) C2 (No. 5)
819.28
Monoclinic
364.64
Monoclinic
P2/n (No. 13)
Tetragonal
¯
P421m (No.
113)
˚
a/A
6.9673(2)
14.1333(4)
14.3657(4)
90.000
104.0350(10)
90.000
1372.37(7)
2
1.691
13.420(3)
17.361(4)
15.840(3)
90.000
105.228(3)
90.000
3560.8(12)
2
1.625
7.1760(13)
10.5145(18)
20.899(4)
90.000
99.300(3)
90.000
1556.1(5)
4
1.924
12.628(2)
15.081(2)
17.362(3)
90.000
107.076(3)
90.000
3160.8(9)
8
1.928
11.0343(17)
15.894(2)
15.823(2)
90.000
91.969(3)
90.000
2773.3(7)
4
2.011
24.5534(6)
9.7877(2)
13.5716(3)
90.000
120.579(2)
90.000
2807.96(11)
4
1.914
6.8212(7)
6.8212(7)
11.838(2)
90.000
90.000
90.000
550.81(14)
2
2.199
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
D
calc/g cm-3
m/mm-1
1.198
0.911
1.705
1.665
1.887
1.733
2.179
˚
l(Mo Ka)/A
0.71073
2.05–30.67
14 333
7626
0.0487
0.71073
1.96–28.07
37 720
8352
0.0357
0.71073
2.17–26.37
12 147
3160
0.0249
0.71073
2.16–28.08
13 654
3729
0.0305
0.71073
1.82–28.08
23 968
6541
0.0310
0.71073
1.74–30.57
30 229
8579
0.0417
-0.03(4)
R1 = 0.0485
0.71073
1.72–26.30
4320
627
0.1463
q range/◦
Total data collected
Unique data
Rint
Flack parameter
R indexes [I > 2s (I)] R1 = 0.0630
0.00(1)
0.0(3)
R1 = 0.0677
R1 = 0.0323
wR2 = 0.0792
R1 = 0.0374
wR2 = 0.0823
R1 = 0.0332
wR2 = 0.0700
R1 = 0.0433
wR2 = 0.0742
R1 = 0.0372
R1 = 0.0451
wR2 = 0.0792
R1 = 0.0503
wR2 = 0.0812
wR2 = 0.1632 wR2 = 0.1585
wR2 = 0.0646 wR2 = 0.1311
R indexes (all data)
R1 = 0.0689
R1 = 0.1060
R1 = 0.0558
R1 = 0.0687
wR2 = 0.1687 wR2 = 0.1814
wR2 = 0.0705 wR2 = 0.1435
ꢀ
ꢀ
ꢀ
ꢀ
R1 = ꢀFo| - |Fcꢀ/ |Fo|; wR2 = { [w(Fo - Fc2)2]/ [w(Fo2)2]}1/2. w = 1/[s (Fo)2 + (aP)2 + bP], P = [max.(Fo2,0) + 2(Fc)2]/3, where a = 0.0501
and b = 6.0399 for I, a = 0.0661 and b = 15.5599 for II, a = 0.0379 and b = 2.3594 for III, a = 0.0277 and b = 3.3819 for IV, a = 0.0213 and b = 2.1270
for V, a = 0.0762 and b = 7.0806 for VI, and a = 0.0275 and b = 0.4698 for VII.
2
2
2278 | Dalton Trans., 2010, 39, 2263–2279
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©

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The selected bond distances and angles are given in Tables S1–
S7 (see ESI†) for all the structures. The observed hydrogen bond
interactions are listed in Table S8 for compounds I, II and III. The
crystallographic data for compounds can be found in the ESI.†
R. Baggio and A. Mombru, Acta Crystallogr., Sect. C: Cryst. Struct.
Commun., 2001, 57, 14.
5 (a) W.-J. Chang, Y.-C. Jiang, S.-L. Wang and K.-H. Lii, Inorg. Chem.,
2006, 45, 6586; (b) L.-H. Huang, H.-M. Kao and K.-H. Lii, Inorg.
Chem., 2002, 41, 2936; (c) L.-I. Hung, S.-L. Wang, H.-M Kao and K.-
H. Lii, Inorg. Chem., 2002, 41, 3929; (d) W.-K. Chang, R.-K. Chiang,
Y.-C. Jiang, S.-L. Wang, S.-F. Lee and K.-H. Lii, Inorg. Chem., 2004,
43, 2564.
Thermal studies
6 (a) P. Mahata, M. Prabu and S. Natarajan, Inorg. Chem., 2008, 47, 8451;
(b) P. Mahata, A. Sundaresan and S. Natarajan, Chem. Commun., 2007,
4471; (c) A. J. Bailey, C. Lee, R. K. Feller, J. B. Orton, C. M. Draznieks,
B. Slater, W. T. A. Harrison, P. Simoncic, A. Navrotsky, M. C. Grossel
and A. K. Cheetham, Angew. Chem., Int. Ed., 2008, 47, 8634; (d) A. K.
Cheetham, C. N. R. Rao and R. K. Feller, Chem. Commun., 2006, 4780.
7 A. K. Paul, G. Madras and S. Natarajan, CrystEngComm, 2009, 11,
55.
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 temperature range 30–800 ◦C (heating rate =
◦
2 C min-1). The results of the TGA studies (see ESI,† Fig. S11)
are summarized in Table S11 (see ESI†).
8 S. D. Huang, R.-G. Xiong, J. Han and B. R. Weiner, Inorg. Chim. Acta,
1999, 294, 95.
Acknowledgements
9 (a) G. R. Desiraju, Angew. Chem., Int. Ed. Engl., 1995, 34, 2311; (b) Per-
spectives in Supramolecular Chemistry: The Crystal as a Supramolecular
Entity, ed. G. R. Desiraju, Wiley, Chichester, 1996, vol. 2.
10 O. Fabelo, J. Pasa´n, L. Canadillas-Delgado, F. S. Delgado, A. Labrador,
F. Lloret, M. Julve and C. Ruiz-Pe´rez, CrystEngComm, 2008, 10, 1743.
11 (a) V. A. Blatov, 2006, http://www.topos.ssu.samara.ru/; (b) V. A.
Blatov, A. P. Shevchenko and V. N. Serezhkin, J. Appl. Crystallogr.,
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12 L. Ohrstrom and K. Larsson, Molecule-Based Materials-The Structural
Network Approach, Elsevier, 2005.
13 A. M. Kutasi, A. R. Harris, S. R. Batten, B. Moubaraki and K. S.
Murray, Cryst. Growth Des., 2004, 4, 605.
The authors thank Mr Udishnu Sanyal and Prof. B. R. Jagirdar
of the 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 (SN) and a
fellowship (AKP). SN thanks DST for the award of a RAMANNA
fellowship.
14 A. A. Ayi, A. Choudhury, S. Natarajan, S. Neeraj and C. N. R. Rao,
J. Mater. Chem., 2001, 11, 1181.
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©