Cu6S6 Clusters as a Building Block for the Stabilization of Coordination Polymers with NiAs, NaCl, and Related Structures: Synthesis, Structure, and Catalytic Studies

Article abstract of DOI:10.1002/ejic.201701262

Four new three-dimensional heterometallic coordination polymers (CPs), [{Zn₃(Hen)(OH)}{Cu₆(6-mna)₆}·(H₂O)₆] (I), [{Zn₂(OH)(en)}{Cu₆(6-mna)₆}_0.5·(H₂

Full text of DOI:10.1002/ejic.201701262

A Journal of
Accepted Article
Title: Cu6S6 clusters as a building block for the stabilization of
coordination polymers with NiAs, NaCl and related structures:
synthesis, structure and catalytic studies
Authors: Srinivasan Natarajan and Ajay Kumar Jana
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201701262
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10.1002/ejic.201701262
European Journal of Inorganic Chemistry
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Cu6S6 clusters as a building block for the stabilization of
coordination polymers with NiAs, NaCl and related structures:
synthesis, structure and catalytic studies
Ajay Kumar Jana[a] and Srinivasan Natarajan*[a]
Abstract: New three-dimensional heterometallic coordination
polymers (CPs), [{Zn3(Hen)(OH)}{Cu6(6-mna)6}.(H2O)6], (I),
cluster, Cu6S6, as a building unit in preparing new compounds. It
may be noted that the Cu6S6 cluster has been prepared earlier
and employed as the building node for inorganic-organic
coordination polymer compounds.[26,27] The Cu6S6 cluster has
been prepared by reacting the soft acid Cu1+ ions with soft bases
such as –SH and –N present in the mercaptonicotinic acid.[26] It
is known that the oxygens of the carboxylic acid is harder
compare to the –SH and –N units. We wanted to explore the
step-wise reaction in which the soft centers react first followed
by reaction with hard centers. This approach was beneficial
during our earlier studies with the [Cu6S6]6- cluster.[26] The
previous work employed 2-mercaptonicotinic acid and in the
present study the same cluster was prepared employing 6-
mercaptonicotinic acid. This Cu6S6 cluster on reaction with Zn2+
ions gave rise to four new compounds: [{Zn3(Hen)(OH)}{Cu6(6-
mna)6}.(H2O)6], (I), [{Zn2(OH)(en)}{Cu6(6-mna)6}0.5.(H2O)2.EtOH],
(II), [{Zn3(dap)3(H2O)}{Cu6(6-mna)6}.(H2O)4], (III) and [{Zn2(4,4’-
[{Zn2(OH)(en)}{Cu6(6-mna)6}0.5.(H2O)2.EtOH], (II), [{Zn3(dap)3(H2O)}
{Cu6(6-mna)6}.(H2O)4], (III) and [{Zn2(4,4’-bpy)0.5(OH)(H2O)}{Cu6(6-
mna)6}0.5.(H2O)], (IV), (en
= ethylenediamine, 6-H2mna = 6-
mercaptonicotinic acid, dap = 1,2-diamino propane, 4,4’-bpy = 4,4’-
bipyridine), were synthesized and their structures determined by
single crystal X-ray crystallography. The structures of the
compounds have Cu6S6 octahedral clusters linked through the
carboxylates with different zinc-oxo clusters. The compounds have
CuI based clusters and ZnII based clusters, which are not observed
commonly in coordination polymer structures. Compound I stabilizes
in NiAs-related structure, compound II and IV in a NaCl related
structure and compound III forms a new type of network structure.
All the compounds were active for the heterogeneous nitroaldol
reaction.
bpy)0.5(OH)(H2O)}{Cu6(6-mna)6}0.5.(H2O)],
(IV).
The
Introduction
heterometallic (Cu1+ and Zn2+ ions) coordination polymers have
three-dimensional structures. In this manuscript, we present the
synthesis, structure and the use of these compounds in
heterogeneous catalytic nitroaldol reaction.
Inorganic-organic coordination polymer compounds, formed by
the connectivity between the metal centers and bridging organic
ligands, have been a topic of immense research during the last
two decades.[1-5] The prospect of employing metal-oxo clusters
instead of simple metal ions as part of this family of compounds
have also been explored.[6-10] A prodigality of papers now exist
where the metal-oxo clusters have been linked with organic
ligands forming interesting structures.[11,12] During the last
decade or so, the HSAB theory of Pearson[13] was utilized
towards the design and synthesis of new coordination
polymers.[14] A number of examples now exist where selective
binding of metal ions were established based on some of the
these ideas for successful design and synthesis of coordination
polymers.
Over the years, many inorganic-organic coordination
polymers have been prepared employing different metal clusters
as the nodes.[15-19] For example, silver clusters such as Ag6S6,
Ag9S9 etc. have been incorporated as part of the framework
during the preparation of coordination polymer compounds.[20-25]
We have been intrigued by the possibility of utilizing the ideas of
HSAB theory towards the design of building units, which may
lead to newer inorganic-organic coordination polymer
compounds. To this end, we have employed the octahedral
Results and Discussion
The Cu6S6 cluster was prepared employing a reported
procedure (see experimental section).[26] The heterometallic
compounds, reported herein, were prepared by employing a
three-layer approach. The first layer containing Cu6S6 was
prepared by solubilizing the compound by employing bases such
as ethylenediamine (en), 1,2-diaminopropane (dap), 4,4’-
bipyridine and ammonium hydroxide. This solution was basic
with a pH of 8.5. The secondary metal employed in the present
study is zinc, which appear to form interesting open-framework
structures under basic conditions exemplified by the preparation
of a number of open-framework zinc-phosphates.[28-30] The basic
pH may also prove beneficial in deprotonating the carboxylic
acid and facilitating the binding of Zn2+ ions with the carboxylate
anions. In addition, we wanted to explore the possible
importance of temperature in the synthesis of these compounds
employing the layered approach. The usefulness of temperature
in the bi-layer approach has been attempted before.[31] During
the present study the layered reaction vessel was kept at 60°C
and at 100°C (Table 4), which resulted in the compounds
reported in this study. We also made attempts to prepare the
present compounds without the layering approach, which
resulted in mostly non-crystalline product. It is likely that the slow
diffusion of the different ions helps in forming the products.
[a]
A. K. Jana, Prof. Dr. S. Natarajan
Framework Solids laboratory
Solid State and Structural Chemistry Unit
Indian Institute of Science, Bangalore–560012 (India)
Fax: (+91) 80-2361-1310
E-mail: snatarajan@.iisc.ac.in
Structure of [{Zn3(Hen)(OH)}{Cu6(6-mna)6}.(H2O)6], (I) The
Supporting information for this article is given via a link at the end of
the document.
asymmetric unit of
I
consists of six Cu(I), six 6-
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10.1002/ejic.201701262
European Journal of Inorganic Chemistry
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mercaptonicotinate ions, three Zn2+ ions, one ethylene diamine,
one hydroxyl group and six water molecules (Figure S1 in the
Supporting Information). The copper atoms are coordinated by
two sulfur atoms and one nitrogen atom forming a triangular
coordination from three different 6-mercaptonicotinate ligands.
The average Cu−N and Cu−S bond lengths are 2.017 Å and
2.252 Å, respectively (Table S1 in the Supporting Information).
The distance between the two Cu(I) species within the Cu6
cluster is in the range of 2.721(2) Å – 3.219(6) Å (Figure S2 in
the Supporting Information). The [Cu6(6-mna)6]6− cluster has six
carboxylic acid groups; of which three carboxylate groups are
connected with Zn2+ ions via both the oxygen atoms and the
other three carboxylates connect through one oxygen atom. This
connectivity gives rise to a μ9-η2:η2:η2:η1:η1:η1 binding mode
(Figure 1). Among the three Zn2+ ions, Zn(1) and Zn(3) are
tetrahedrally corordinated by three carboxylate oxygens and one
hydroxyl group, whereas Zn(2) is octahedrally corordinated with
three carboxylate oxygens, one hydroxyl group and two nitrogen
atoms [N(7), N(8)] of the ethylene diamine molecule ( Figure S3
in the Supporting Information). The various bond distances and
angles observed in I are within the expected ranges (Table S1 in
the Supporting Information). It may be noted that the hydroxyl
group (-OH) connects all the three zinc centers forming a Zn3
trimer (Figure S4 in the Supporting Information). The various
interactions with the carboxylate oxygen atoms (Table S3 and
Figure S6 in the supporting information).
(a)
units would result in
a
formula, [{Zn3(en)(OH)}{Cu6(6-
mna)6}.(H2O)6]−, which is negatively charged. The negative
charge is balanced by protonation of one of the nitrogen of the
en molecule. We have employed
a bond-valence sum
calculation to ascertain the protonated nitrogen of the en
molecule. Thus, from the bond valence sum calculation, atoms
{N(7), N(8)}, it is likely that the N(8) would be protonated (Table
S2 in the Supporting Information). The protonation of the
nitrogen is also confirmed from the IR spectra, where a band at
~ 1451 cm-1 is obtained, which corresponds to the asymmetric
bending mode of the ammonium group.[32]
The structure of I can be considered to be built up from the
connectivity between Zn3 trimer units and Cu6 octahedral units.
Thus, the Zn3 trimer units are connected with six different [Cu6(6-
mna)6]6− cluster units (Figure S5 in the Supporting Information).
The [Cu6(6-mna)6]6− cluster unit is also connected with six Zn3
trimer units (Figure 1). Among the six carboxylate groups of the
[Cu6(6-mna)6]6− cluster, four caroboxylate groups connect with
four Zn3 trimer units in a plane forming a two dimensional sheet.
The remaining carboxylate groups are connected with two Zn3
trimer units from the adjacent layers above and below giving rise
to the three dimensional structure (Figure 2).
To understand the structure better, we carried out a
topological analysis using TOPOS.[33-35] Thus the Zn3 trimer and
Cu6(6-mna)6]6− cluster were considered as two distinct nodes.
This resulted in a 2-nodal net closely related to the NiAs
structure (nia topology). The Schlafli symbol for this net is
(412.63)(49.66). Another way to interpret this net is to consider a
hexagonal close packing (hcp) of Zn3 units in which the
octahedral voids are filled by the Cu6 units. The structure
(b)
Figure 1. (a) The [Cu6(6-mna)6]6− cluster and its connectivity with the Zn3
cluster in I. (b) The [Cu6(6-mna)6]6− cluster (pink) octahedral cluster and the
Zn3 trimer cluster (brown) and their connectivity. Note that the Cu6S6 cluster
connect with four Zn3 trimers in-plane and two Zn3 trimers out-of-plane (see
text).
Structure of [{Zn2(OH)(en)}{Cu6(6-mna)6}0.5.(H2O)2.EtOH], (II)
The asymmetric unit of II consists of three Cu(I), three 6-
mercaptonicotinate ions, two Zn2+ ions, one ethylene diamine,
one ethanol, one hydroxyl ion and two water molecules (Figure
S7 in the Supporting Information). Similar to the structure of I,
the octahedral [Cu6(6-mna)6]6− cluster was formed by the
connectivity between sulfur and nitrogen atoms from the
contains
a number of non-bonded extra-framework water
mercaptonicotinate anions. The Cu(I) has
coordination formed by two sulfur atoms and one nitrogen atom
a
triangular
molecules, which actively participate in hydrogen-bond
from three 6-mercaptonicotinate units, with average Cu−N and
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10.1002/ejic.201701262
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Cu−S bond lengths of 2.019 Å and 2.244 Å, respectively (Table
S1 in the Supporting Information). The Cu(I) atoms, within the
cluster, are separated from each other with distances in the
range of 2.674(3) Å - 3.313(1) Å (Figure S8 in the Supporting
Information). Of the six carboxylate units of the [Cu6(6-mna)6]6−
cluster, four are bound with eight Zn2+ ions through both the
oxygen atoms, while the other two connect with two Zn2+ ions
through one oxygen atom. This connectivity gives rise to a μ10-
η2:η2:η2:η2:η1:η1 binding mode (Figure 3). Of the two Zn2+ ions,
Zn(1) is octahedrally corordinated with two carboxylate oxygen
[O(4), O(6)], two hydroxyl oxygens [O(7), O(7a)] and two
nitrogen [N(4), N(5)] from the ethylenediamine molecule; and
Zn(2) is tetrahedrally corordinated with three carboxylate oxygen
[O(2), O(3), O(5)] and one hydroxyl oxygen [O(7)] ( Figure S9 in
the Supporting Information). The Zn(1) and Zn(2) atoms are
connected through the hydroxyl oxygens [O(7)] forming a Zn4
tetramer unit (Figure S10 in the Supporting Information).
(a)
(a)
(b)
Figure 2. (a) Figure shows the three-dimensional connectivity in I. The Cu6S6
cluster (pink) and the Zn3 trimer (brown). (b) The NiAs structure. Note the
close similarity between the two structures (Ni ≡ Cu6S6, As ≡ Zn3).
Similar to the structure of I, we can understand this
structure also by considering the connectivity between the Zn4
tetramer units and the Cu6 octahedral units. Thus, the Zn4
tetramer units are connected with six different Cu6(6-mna)6]6−
clusters (Figure 4). Similarly each [Cu6(6-mna)6]6− cluster unit is
also connected with six Zn4 tetramer units (Figure 3). It may be
noted that four carboxylate units bind with four Zn4 tetramer units
within a plane forming a two dimensional layer like arrangement.
The other two carboxylate groups connect with Zn4 tetramer
units of the adjacent layer on both the sides. This connectivity
results in a 6:6 binding between the Zn4 tetramer and the Cu6S6
octahedral cluster. A close examination of this bonding reveals a
resemblance to the three dimensional NaCl - like structure
(Figure 5). To understand the structure further, we have carried
out the TOPOS analysis.[33-35] For the topological analysis, the
Zn4 tetramer and [Cu6(6-mna)6]6− cluster units were considered
as two distinct nodes. This resulted in an uninodal net; with a α-
Po topology. The Schlafli symbol for this net is (412.63). The
guest molecules, water and ethanol participates in hydrogen-
bond interactions (Table S3 and Figure S11 in the supporting
information).
(b)
Figure 3. (a) The [Cu6(6-mna)6]6− cluster and its connectivity with Zn4
tetramers in II. (b) The octahedral Cu6S6 cluster (pink) and the Zn4 tetramer
(brown) connectivity in II. Note that the octahedral Cu6S6 cluster connects with
six Zn4 tetramers (see text).
Structure of [{Zn3(dap)3(H2O)}{Cu6(6-mna)6}.(H2O)4], (III) The
asymmetric unit consists of six Cu(I), six 6-mercaptonicotinate
ions, three Zn2+ ions, three 1,2-diaminopropane and five water
molecules (Figure S12 in the Supporting Information). The
structure contains two independent [Cu6(6-mna)6]6− cluster units:
2 x Cu(1), Cu(2), Cu(3) (cluster-1) and 2 x Cu(4), Cu(5), Cu(6)
(cluster-2). Similar to the previous structures, the copper atoms
have the trigonal coordination formed by two sulfur atoms and
one nitrogen atom from three 6-mercaptonicotinate ligands
(Table S1 in the Supporting Information). The Cu-atoms within
the cluster are separated from each other with distances in the
range 2.729(4)Å – 3.076(7) Å (Figure S13 in the Supporting
Information).
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10.1002/ejic.201701262
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dimensional chain like arrangement. Similarly, cluster-2
connects with four Zn(3) ions forming
a one-dimensional
arrangement (Figure 7). The two chain structures are cross-
linked by Zn(1) ion forming the extended three-dimensional
structure (Figure 8). Though the connectivity appears simple, the
relative disposition of the independent copper cluster-Zn chain
units make the understanding of the structure difficult. To have a
better understanding of the structure, TOPOS analysis with the
three Zn centers (Zn1, Zn2 and Zn3) as well as cluster-1 and
cluster-2 as independent nodes were considered. The TOPOS
analysis gave a new topology with Schlafli symbol of (12)
(42.129.164) (4)2. The presence of a number of extra-framework
water molecules gives rise to hydrogen-bond interactions (Table
S3 and Figure S17 in the supporting information).
Figure 4. The Zn4 tetramer (brown) connectivity with Cu6S6 cluster (pink). Note
that the Zn4 tetramer connects with six Cu6S6 cluster (see text).
(a)
(b)
Figure 5. (a) Three dimensional connectivity between Cu6S6 cluster (pink) and
Zn4 tetramers (brown). (b) The NaCl structure. Note the close similarly
between the structures (see text).
Among the three Zn2+ ions, Zn(1) and Zn(3) are tetrahedrally
corordinated with two carboxylate oxygens and two nitrogen
from the 1,2-diaminopropane molecule (Figure S14-S15 in the
Supporting Information) and Zn(2) has a distorted trigonal
bipyramidal geometry from two carboxylate oxygens [O(3), O(5)],
one water molecule [O(13)] and two nitrogen atoms from the
1,2-diaminopropane molecule (Figure S16 in the Supporting
Information). The geometry of the Zn(2) ions was determined by
employing the method described by earlier.36 The parameter, τ,
suggests the possible geometry for the central metal ion. Thus, τ
= 1 for a perfect trigonal bipyramidal geometry and τ = 0 for a
perfect square pyramidal geometry. The value of τ parameter
was found 0.52 for Zn(2), which indicates that geometry around
Zn(2) ion is a highly distorted. The connectivity between the
three Zn2+ ions and the octahedral [Cu6(6-mna)6]6− clusters are
similar. Thus, cluster-1 binds with four Zn(2) ions and two Zn(1)
ions while cluster-2 binds with four Zn(3) ions and two Zn(1) ions
through the mono-dentate coordination of the carboxylate unit
(Figure 7). This binding gives rise to a μ6-η1:η1:η1:η1:η1:η1 binding
mode (Figure 6). The structure of III can be understood better by
considering individual connectivities of each Cu6 cluster. Thus
Figure 6. The two [Cu6(6-mna)6]6− clusters and their connectivity with the Zn in
III.
cluster-1 connects with four Zn(2) ions to form
a one-
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10.1002/ejic.201701262
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connectivity gives rise to μ10-η2:η2:η2:η2:η1:η1 binding mode
(Figure 9). Of the two Zn2+ ions, Zn(1) has a tetrahedral
coordination and Zn(2) is octahedrally coordinated. The zinc
ions are connected through the hydroxyl oxygen [O(7)] forming a
Zn4 tetramer (Figure S20 in the Supporting Information). This
Zn4 tetramer closely resembles the Zn4 tetramer units observed
in II (Figure S21 in the Supporting Information).
(a)
(b)
Figure 7. (a) Figure shows the connectivity between [Cu6S6] cluster and Zn
atoms forming a one-dimensional chain. (b) Similar connectivity involving
cluster-2.
(a)
(b)
Figure 9. (a) The connectivity between [Cu6(6-mna)6]6− cluster and Zn4
tetramer units in IV. (b) The octahedral Cu6S6 cluster (pink) and the Zn4
tetramer (brown, in-plane; cyan, out-of-plane) connectivity in IV. Note that the
octahedral Cu6S6 cluster connects with six Zn4 tetramers.
Figure 8. The connectivity between the two chain structures through the Zn(1)
(green). Note that the chains are not parallel to each other (see text).
Structure
of
[{Zn2(4,4’-bpy)0.5(OH)(H2O)}{Cu6(6-
The Cu6 octahedral clusters and the Zn4 tetramer units are
connected together forming the structure of IV. The Zn4 tetramer
units are connected with six carboxylate units from six different
[Cu6(6-mna)6]6− clusters. The 4,4’-bipyridine ligand links the Zn4
mna)6}0.5.(H2O)], (IV) The asymmetric unit consists of three
Cu(I), three 6-mercaptonicotinate ions, two Zn2+ ions, one lattice
water, one coordinated water, one hydroxyl ion and half 4,4’-
bipyridine ligand (Figure S18 in the Supporting Information).
The Cu(I) centers have a trigonal coordination (Table S1 in the
Supporting Information) and the Cu-Cu distances within the Cu6
octahedral cluster are in the range of 2.793(5) Å - 3.501(11) Å
(Figure S19 in the Supporting Information). Of the six
carboxylate units attached with the [Cu6(6-mna)6]6− cluster, four
bonds with eight Zn2+ ions via both the oxygen atoms and two
carboxylates connect to two Zn2+ ions via one oxygen atom. This
tetramers along the [020] direction (Figure 10). The [Cu6(6-mna)-
6−
]
cluster units are also connected with six Zn4 tetramer units
6
(Figure S22 in the Supporting Information). The connectivity
between Cu6 cluster units and Zn4 tetramer units are similar to
that observed in the structure of II. Thus, four Zn4 tetramer units
bind with four carboxylates of Cu6 clusters within the plane and
two carboxylate units connect with Zn4 tetramer units present in
adjacent layers giving rise to the three-dimensional structure
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(Figure S23 in the Supporting Information). Thus the structure
of IV also has a 6:6 connectivity between the Cu6 and Zn4 units.
The presence of 4,4’-bipyridine and its connection with Zn4
tetramer, however adds an additional twist to the structure. A
TOPOS[33-35] analysis with the Zn4 tetramer and Cu6(6-mna)6]6−
Structural comparison: From the synthesis conditions, the
longer duration of the reaction time forms a structure that is
more stable. The synthesis conditions for I and II are similar but
during the formation of II, the reaction mixture was allowed to
react for 96h at 100°C. As described above, compound I
cluster as two different nodes results in a uninodal α-Po topology. stabilized in NiAs-related structure whereas compound II
If the 4,4’-bipyridine link between the Zn-center is also
considered, a new topology would result. The Schlafli symbol for
this new net would be (32.410.52.6)(34.412.58.64). The extra-
framework water molecule, participates in hydrogen-bond
interactions (Table S3 and Figure S24 in the supporting
information).
stabilizes in NaCl-related structure. From the solid state
chemistry point of view, the NaCl structure is more stable
compared to NiAs structure.[37] Similarly, it is likely that the longer
duration of the reaction gave rise to a thermodynamically more
stable phase in II (NaCl-related) compared to I (NiAs-related).
Similarly, in the formation of IV also one can make such
observation. The presence of 4,4’-bipyridine actually links two
Zn-centers creating a disruption in the overall structure, though
the overall topology is NaCl-related. The new network observed
for III could be due to the shorter duration as well as lower
temperature. It has been established both in framework
compounds such as zeolites, aluminophosphates etc[38] and
inorganic-organic hybrids[39] that higher temperature and / or
longer duration during the synthesis results in dense framework
that are thermodynamically stable.
As mentioned earlier, the Cu6S6 octahedral units have
been stabilized before by the use of other ligands.[26,27] As part of
the present study, we have employed the [Cu6(mna)6]6- ions as a
ligand to react with a secondary metal ion (Zn2+ ion) to convert
the molecular structure of [Cu6(mna)6] clusters into three-
dimensional ones. Thus, four new Cu1+-Zn2+ heterometallic
compounds with three – dimensional structures have been
isolated. In an earlier study, we employed Mn2+ ions as the
secondary metal ions to stabilize the [Cu6(mna)6]6- clusters into
extended structures.[26] It is interesting to note that the presence
of M (M = Mn, Zn) tetramer clusters appear to form a 6:6
connectivity with the [Cu6(mna)6]6- clusters giving rise to a α-Po
(NaCl-related) structure. A similar observation was also made
(a)
with
[Ag6(mna)6]6-
cluster
with
Na+
ions
in
4Na+.2[C4O3H12N]+.[Ag6(mna)6]6-.10H2O.[40] The connectivity
between [Ag6(mna)6]6- clusters with alkaline earth metal ions (Ca,
Sr, Ba), on the other hand, forms many different structures.[25] Of
these, the connectivity involving Sr2 dimers and Sr4 tetramers
with [Ag6(mna)6]6- gives rise to the α-Po structure (Figure 11). It
appears that the octahedral molecular clusters [Cu6(mna)6]6- and
[Ag6(mna)6]6- are versatile binding units for the preparation of
new inorganic coordination polymers.
The thermogravimetric analysis (TGA):
The thermogravimetric analysis (TGA) studies were carried out
in the temperature range of 30 - 900°C (heating rate = 10°C min-
1) under a flowing N2 atmosphere (40 mL min-1) (Figure 12). For
compound I, a weight loss of ~10.6% was observed in the
temperature range of 30-250°C. This would correspond to the
loss of seven water molecules and one ethylenediamine
molecule (cal. 11.1%). The broad weight loss in the temperature
(b)
Figure 10. (a) Figure shows the Zn4 tetramer connectivity with Cu6S6 cluster.
Note that four Cu6S6 clusters connect in-plane (pink) and two Cu6S6 clusters
out-of-plane (cyan). Note also that the 4,4’-bipyridine unit connects two Zn4
tetramers. (b) The structure of IV showing the connectivity between octahedral
Cu6S6 (pink) and Zn4 tetramer (brown). The blue line represents connectivity
involving the 4,4’-bipyridine moiety. Note the close similarity to structure of II
and NaCl structure (Figure 5b).
range of 250-730°C was found to be
~ 55.8%, which
corresponds to the loss of six mercaptonicotinate moieties (cal.
55%). For compound II, the observed weight loss in the
temperature range of 30-340°C was 21.7%, which corresponds
to the loss of two water molecules, one hydroxyl ion, one ethanol
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Figure 11. (a) Comparison of structures involving octahedral Cu6 or Ag6 clusters and different metal-oxo clusters. (a) Zn4 tetramer in II, (c) Zn4 tetramer in IV, (e)
Mn4 tetramer in Ref. 26, (g) Na4 tetramer in Ref. 40, (i) Sr2 dimer and Sr4 tetramer in Ref. 25; The node representation of the observed structures (b) in II, (d) in IV,
(f) in Ref. 26, (h) in Ref. 40, (j) in Ref. 25 respectively.
molecule and one ethylenediamine molecule (cal. 16.9%). The
observed weight loss in the temperature range 340-715°C was
46.2%, which corresponds to the loss of three
mercaptonicotinate moieties (cal. 48.9%). For compound III, in
the temperature range of 30-260°C, the observed weight loss
was 5.9%, which corresponds to the loss of one coordinated
water and four lattice water molecules (cal. 4.98%). The second
weight loss in the temperature range of 260-720°C was found to
be ~58.1%, which corresponds to the loss of three 1,2-
diaminopropane molecules and six mercaptonicotinate moieties
(cal. 63.1%). For compound IV, in the temperature range 30-
350°C, the observed weight loss 9.9%, which corresponds to the
loss of one lattice water molecule, one coordinated water
molecule and the 4,4’-bipyridine moiety (cal. 12.5%). The weight
loss in the temperature range 350-780°C was ~ 53.1%, which
corresponds to the loss of three mercaptonicotinate moities (cal.
50.3%). The TGA results are represented in Table 1. The
calcined compound, after TGA studies in all the case was a
mixture of CuO and ZnO (JCPDS No: 89-5895 and 89-1397)
(Figure S25 in the Supporting Information).
Nitroaldol reaction:
The Nitroaldol or Henry reaction is an important C-C bond
coupling reactions employing nitroalkane and aldehyde.[41-54] We
desired to explore this reaction employing the present
compounds as heterogeneous catalysts. Previous studies
suggested that the tetrahedral Zn2+ ions may act as a Lewis acid
center catalyzing this reaction.[46-54] In the present study, all the
four compounds have tetrahedral Zn(II) ions as part of the
structure and it occurred to us that the present compounds could
also be active for the nitroaldol reaction. Thus, the catalytic
activities of all the compounds (I–IV) were examined using
nitromethane along with different aldehydes. The products of the
reaction were characterized by 1H-NMR spectroscopy.[52,53] In all
the cases, the reaction proceeded as expected and the yield of
the reaction varied from 41-94%. The yield appears to depend
on the aldehyde employed in the reaction (Table 2). The yield
was higher when electronegative groups are present as part of
the aromatic ring of the aldehyde. It is likely that the
electronegative groups stabilizes the transition state of the
reaction (Scheme 1).[47]
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10.1002/ejic.201701262
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100
90
80
70
60
50
40
30
100
90
80
70
60
50
40
30
II
I
100 200 300 400 500 600 700 800 900
100 200 300 400 500 600 700 800 900
Temperature (oC)
o
Temperature ( C)
100
90
80
70
60
50
40
30
100
90
80
70
60
50
40
30
IV
III
100 200 300 400 500 600 700 800 900
100 200 300 400 500 600 700 800 900
o
Temperature ( C)
Temperature (oC)
Figure 12. TGA of compounds I-IV.
Table 1. The results for the TGA studies for compounds I-IV.
Compound
Temperature
range (°C)
Weight loss (%)
Possible loss
Observed
Calculated
11.1
[{Zn3(Hen)(OH)}{Cu6(6-
mna)6}.(H2O)6], (I)
30-250
10.6
55.8
21.7
46.2
5.9
6H2O and one ethylenediamine (en)
six mercaptonicotinate moieties
2H2O, OH-, EtOH, en
250-730
30-340
55
[{Zn2(OH)(en)}{Cu6(6-
mna)6}0.5.(H2O)2.EtOH], (II)
16.9
340-715
30-260
48.9
three mercaptonicotinate moieties
5H2O
[{Zn3(dap)3(H2O)}{Cu6(6-
4.98
mna)6}.(H2O)4], (III)
260-720
58.1
63.1
three 1,2-diaminopropane molecules and six
mercaptonicotinate moieties
2H2O and the 4,4’-bipyridine moiety
three mercaptonicotinate moities
[{Zn2(4,4’-bpy)0.5(OH) (H2O)}{Cu6(6-
mna)6}0.5. (H2O)], (IV)
30-350
9.9
12.5
50.3
350-780
53.1
The tetrahedrally coordinated Zn(II) ions indeed act as a Lewis
acid center by accepting the lone pair of electrons from the
oxygen atom of the aldehyde as well as the electron density
from the nitro group of nitromethane (Scheme 1B). The catalytic
activities and yield of the products observed employing the
present compounds are comparable with those reported in the
literature (Table 3).[49-54] In order to investigate the possible
reuse of the catalyst, we isolated the catalyst after the reaction
and reemployed for the same reaction for up to 3 cycles. The re-
used catalyst exhibits comparable activity with reasonable yields
of 71% (Figure 13).
We have also carried out the reaction between
benzaldehyde and nitromethane in the absence of the present
compounds as well as in the presence of ZnCl2, Zn(NO3)2 and
free metalloligand [Cu6(Hmna)6]. In all the cases, we did not
observe the formation of the desired nitroaldol product. This
suggests that the present compounds act heterogeneous
catalyst for this reaction. We examined the structural integrity of
the catalyst during the repeated use. The PXRD pattern
indicated that the structural integrity of the compound remains
the same after 3-cycles (Figure S26-S27 in the Supporting
Information).
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Table 2. Nitroaldol reaction between aromatic aldehyde and nitromethane employing I-IV as a catalyst.
OH
Catalyst
CH3OH
Ar-CHO
NO2
CH NO
+
3
2
Ar
65°C
Product
Entry
Substrate
Reaction
time (h)
Yield %
III
I
II
IV
HO
HO
CHO
NO2
1
48
24
72
79
63
78
42
CHO
NO2
NO2
NO2
2
3
4
68
70
81
76
51
Br
Br
HO
HO
CHO
24
81
75
44
Cl
Cl
CHO
24
92
94
56
NO2
NO2
CHO
HO
HO
NO2
NO2
5
6
24
24
84
78
76
62
82
68
54
41
NO2
CHO
NO2
CN
CN
Reaction conditions: Aromatic aldehyde (0.5 mmol), nitromethane (1.5 mmol), MeOH (2.5 mL), and catalyst (10 mol %). Yield based on column chromatography.
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HO
Ar
NO2
(5)
H
O
Ar
NO2
O
L4Zn
H3C
H
H
(A)
H
H
(1)
O
Ar
N
O
L4Zn
L4Zn
O2N
HO
H
O
H
H
H
(E)
(B)
Ar
Ar
(2)
H+ shift
O
(4)
Ar
H
O
H
H
N
O
L4Zn
L4Zn
O2N
HO
H
H
H
(3)
O
H
Ar
C-C bond formation
/ H+ shift
Figure 13. The yield for recyclability test up to 3rd cycles using compound-I as
Ar
a catalyst.
(D)
(C)
Scheme 1. The proposed mechanism of nitroaldol reaction.
Table 3. Nitroaldol reaction between aryl aldehyde and nitromethane catalyzed by Zn(II) based catalysts reported in the literature.
Aryl aldehyde
Catalyst
Reaction conditions
% Yield
55
Ref.
49
Benzaldehyde
Zn(L)Cl2
MeOH/25°C/7d/10 mol%
CH3NO2/60°C/120h/1.6 mol%
EtOH/25°C/5h/10 mol%
EtOH/25°C/5h/10 mol%
THF/-50°C/24h/54 mol%
THF/-50°C/24h/54 mol%
THF/-50°C/24h/54 mol%
THF/-35°C/24h/10 mol%
THF/-35°C/24h/5 mol%
THF/-35°C/24h/5 mol%
MeOH/65°C/48h/10 mol%
4-Nitrobenzaldehyde
4-Chlorobenzaldehyde
4-Nitrobenzaldehyde
Benzaldehyde
Zn-MOF 1
80
50
ZnLs
85
51
ZnLs
96
51
Zinc-Fam Catalyst
Zinc-Fam Catalyst
Zinc-Fam Catalyst
Et2Zn/1f
83
52
4-Nitrobenzaldehyde
4-Bromobenzaldehyde
Benzaldehyde
84
52
73
52
52
53
Benzaldehyde
Zn Catalyst (2)
Zn Catalyst (2)
Compound-I
75
54
1-Naphthaldehyde
Benzaldehyde
71
54
72
This
study
This
study
4-Nitrobenzaldehyde
Compound-III
MeOH/65°C/24h/10 mol%
94
Conclusions
on Cu6S6 cluster in {[Mn4(OH)2(H2O)10][(Cu6(2-mna)6]·8H2O}[26]
and {[Sr3(H2O)10][Ag6(2-mna)6]·12H2O}.[25] The Lewis acid
nitroaldol reactions were shown to be promoted by these
compounds with very good catalytic activity. From the present
study, it is clear that the [Cu6(mna)6] is a good building block to
investigate with many other elements as the softer –SH and –N
were employed first to stabilize the Cu6S6 cluster. The number of
hard acids such as Fe2+/Al3+ etc can also be attempted to form
new type of extended structures. Studies towards this goal are
being perused and the results would be reported in future.
The synthesis, structure and heterogeneous catalytic
activity of four three-dimensional heterometallic coordination
polymer compounds have been accomplished. All the
compounds have Cu6S6 octahedral clusters connected with
different Zn-oxo clusters forming the extended 3D structures.
The Cu6S6 octahedral cluster appears to be a versatile building
block in forming newer assemblies. In the present study, we
stabilized the Cu6S6 cluster in NiAs-related (I) and NaCl-related
structures (II and IV) along with a new type of network structure
(III). We have earlier reported a NaCl network structure based
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10.1002/ejic.201701262
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was removed from the reaction mixture by centrifugation and the yield of
the reaction was continuously monitored. The yield was found to be 44 %
and after the removal of the catalyst the reaction was continued for 24h.
At the end of the reaction, we did not observe any significant increase in
the yield of the product (Figure S30 in the supporting information).
Experimental Section
Materials: 6-mercaptonicotinic acid was acquired from Sigma-Aldrich.
Cul, 4,4’-bipyridine, N,N’-dimethylformamide, Zn(NO3)2.6H2O, Zn(OAc)2.
2H2O, EtOH, ethylenediamine, 1,2-diaminopropane nitromethane,
methanol, benzaldehyde, 4-bromobenzaldehyde, 4-chlorobenzaldehyde,
4-nitrobenzaldehyde, 2-nitrobenzaldehyde, 3-cyanobenzaldehyde were
obtained from SDfine (India). All the chemicals are used without further
purification.
Characterization techniques:
Initial characterizations were carried out by powder X-ray
diffraction (PXRD), infrared spectroscopy (IR), UVVis spectroscopy,
photoluminescence studies (PL). FTIR spectra were recorded as KBr
pellets of the samples on Perkin Elmer L125000P machine using the
Spectrum 10TM software. For the compounds (I-IV) the broad band at ~
3200-3480 cm-1 represent the presence of water molecules in the
structure, the bands ~ 1600 cm-1, ~1580 cm-1 represents the asymmetric
stretching of coordinated −COO− and the C-C stretching of the aromatic
ring respectively.[32] Other typical bands have also been observed (Figure
S31 in the Supporting Information). Important IR bands have been
tabulated (Table S4 in the Supporting Information). The solid state
UV−vis absorption spectra of compounds were recorded at room
temperature using Perkin Elmer Lambda 35 spectrophotometer. For the
compounds (I-IV) the band at ~ 406-413 nm observed, which are due to
the π−π* transition of the ligand (Figure S32, Table S5 in the Supporting
Information). The solid state emission study was carried out in
PerkinElmer LS 55 luminescence spectrophotometer. The emission
spectra were recorded employing the excitation wavelength of 400 nm
(Figure S33, Table S5 in the Supporting Information). The emission
bands at ~ 443-563 nm are may be due to the π*−π transition of the
ligand. The bands ~590-649 nm are due to the [Cu6(6-mna)6]6-cluster
centered transitions mixed with ligand-to-metal charge transfer (LMCT)
transitions.[26,55,56]. The PXRD data were recorded in the 2θ range 550°
using Cu Kα radiation (Philips X’pert). The observed PXRD patterns for
all the compounds were found to be consistent with the PXRD patterns
simulated from the single crystal structure study (Figure S34 in the
Supporting Information). The nitroaldol compounds were characterized
by 1H-NMR spectroscopy employing a Bruker 400 MHz spectrometer.
Chemical shifts in ppm are reported using tetramethylsilane (TMS) as the
internal standard.
Synthesis:
Preparation of [Cu6(6-Hmna)6] metalloligand: We have earlier
prepared the metalloligand, [Cu6(2-Hmna)6].[26] We have adopted the
same procedure for the synthesis of [Cu6(6-Hmna)6] metalloligand. In a
typical procedure,
a
mixture of CuI (0.019 g, 0.1 mmol),
6mercaptonicotinic acid (0.031 g, 0.1 mmol) in 1 mL of water were
sonicated at room temperature for 15 min. After the sonication bright
orange colored precipitate was observed, which was dissolved by adding
appropriate bases (for
I and II: ethylenediamine, for III: 1,2-
diaminopropane, for IV: 4,4’-bipyridine and aqueous NH3). The resulting
orange colored solution was employed for the preparation of the
compounds reported in this paper.
Synthesis of [{Zn3(Hen)(OH)}{Cu6(6-mna)6}.(H2O)6], (I): In a 6 mL test
tube 1mL of the metalloligand solution (ethylene diamine as the base)
was taken and 1:1 water/ethanol mixture (1mL) was carefully layered on
top. A solution containing Zn(NO3)2.6H2O (0.03 g, 0.1 mmol) in 1 mL
ethanol was layered on top of the water/ethanol mixture. The test tube
was carefully sealed by a screw cap and the reaction vessel was kept at
100°C. After two days, we observed the formation of brown colored rod-
shaped crystals (Figure S28 in the Supporting Information), which were
carefully harvested and washed with a mixture of H2O/EtOH and stored
at room temperature for further characterizations. (Yield = 54 % based on
Cu). A schematic of the reaction set up is shown in ESI (Figure S29 in
the Supporting Information). A similar synthetic approach was employed
for the preparation of all the compounds and the synthetic conditions and
composition are presented in Table 4.
Single crystal structure determination: The single crystal data were
collected at 120 K on an Oxford Xcalibur (Mova) diffractometer equipped
with an EOS CCD detector. The Xray generator was operated at 50 kV
and 0.8 mA using Mo Kα (λ= 0.71073 Å) radiation. The cell refinement
and data reduction were accomplished using CrysAlis RED.[57] The
structures were solved by direct methods and refined using SHELX97
present in the WinGX suit of programs (version 1.63.04a).[58] The
hydrogen positions were initially located in the difference Fourier map
and for the final refinement, the hydrogen positions were fixed in a
geometrically ideal position and refined employing riding model. The
solvent molecules were found to be disorder in compound I and III were
accounted by using the SQUEEZE option within the WinGX-platon
program.[59] The potential solvent accessible voids (for I 10.67% and for
III 9.4% of the total volume of the unit cell) were determined from the
SQUEEZE results. The final refinements included atomic positions for all
the atoms, anisotropic thermal parameters for all the nonhydrogen
atoms except C6 (for IV) and isotropic thermal parameters for all the
hydrogen atoms. The CCDC numbers for the compounds 1555266 (I),
1555267 (II), 1555268 (III) and 1555269 (IV). The details of the structure
solution and final refinement parameters are listed in Table 5.
Catalysis: In a 5 mL flask, aromatic aldehyde (0.5 mmol) and MeOH (2.5
mL) were taken and stirred for 10 minutes. To this mixture, 1.5 mmol of
nitromethane (80 μL) and 10 mol% of the catalyst (10 mol %) were added.
The entire reaction mixture was kept at 65°C under atmospheric
conditions. The progress of the reaction was monitored by employing thin
layer chromatography. After the completion of the reaction, the catalyst
was separated by filtration and washed with methanol. The filtrate was
concentrated employing a rotary evaporator and the crude product was
purified by column chromatography (in column chromatography silica gel
was taken in the solid phase and hexane / ethyl acetate mixture was
employed as the eluent).
Hot Filtration Studies: To examine the possible leaching of the catalyst
into the reaction mixture, we have carried out the hot filtration test. In the
hot filration test, 4-nitrobenzaldehyde (0.5 mmol, 0.076 g), 1.5 mmol of
nitromethane (80 μL), catalyst (III, 10 mol %) were taken in 2.5 mL MeOH
and stirred at 65°C under atmospheric conditions. After 6h the catalyst
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Table 4. The synthetic conditions of compounds I - IV.
Compound
[{Zn3 (Hen)(OH)}{Cu6(6-
mna)6}.(H2O)6], (I)
[{Zn2(OH)(en)}{Cu6(6-
mna)6}0.5.(H2O)2.EtOH],
(II)
[{ Zn3(dap)3(H2 O)} {Cu6(6-
mna)6}. (H2O)4], (III)
[{Zn2 (4,4’-bpy)0.5(OH)(H2O)} {Cu6(6-
mna)6 }0.5. (H2O)], (IV)
Compositions
BL: Metalloligand (1 mmol) +
H2O (1mL) + en (15 μL)
ML: [H2O + EtOH] (1:1) 1mL
TL: Zn(NO3)2. 6H2O (0.1
mmol) + EtOH (1mL)
BL: Metalloligand (1
mmol) + H2O (1mL) +
en (15 μL)
ML: [H2O + EtOH] (1:1)
1mL
TL: Zn(OAc)2. 2H2O
(0.1 mmol) + EtOH
(1mL) + H2O (0.25mL)
100
BL: Metalloligand (1 mmol)
+ H2O (1mL) + dap (20 μL)
ML: [H2O + EtOH] (1:1) 1mL
TL: Zn(NO3)2. 6H2O (0.1
mmol) + EtOH (1mL)
BL: Metalloligand (1 mmol) + H2O
(1mL) + NH4OH (20 μL)
ML: [H2O + EtOH] (1:1) 1mL
TL: Zn(NO3)2. 6H2O (0.1 mmol) +
EtOH (1mL) +4,4’-bpy (1mmol) +
DMF (0.5mL)
Temp. (°C)
100
60
100
Time (h)
Yield (%)
48
54
96
42
48
26
96
76
BL = bottom layer, ML = middle layer, TL= top layer, en = ethylenediamine, dap = 1,2-diaminopropane, 4,4ʹ-bpy = 4,4ʹ-bipyridine, DMF = N,N’-dimethylformamide.
Elemental analysis: Anal. Calculated (%) for I: C 27.35, H 1.57, N 6.72, S 11.53; found: C 25.54, H 1.98, N 6.12, S 11.03; for II: C 28.23, H 2.58, N 7.48, S 10.28;
found: C 27.22, H 2.95, N 6.87, S 11.14; for III: C 30.05, H 2.69, N 9.34, S 10.70; found: C 29.76, H 2.98, N 9.74, S 10.18; for IV: C 30.36, H 1.77, N 6.16, S
10.57; found: C 31.14, H 2.18, N 6.77, S 10.49
Table 5. Crystal data and structure refinement parameters for I - IV.
Structural
I
II
III
IV
parameter
Empirical formula
[{Zn3(Hen)(OH)}{Cu6
(6-mna)6}.(H2O)6]
[{Zn2(OH)(en)}{Cu6(6-
mna)6}0.5.(H2O)2.EtOH]
[{Zn3(dap)3(H2O)}{Cu6(6-
mna)6}.(H2O)4]
[{Zn2(4,4’-
bpy)0.5(OH)(H2O)}{Cu6(6-
mna)6}0.5.(H2O)]
913.01
Formula weight
Crystal system
Space group
a (Å)
1682.60
Monoclinic
P21/c
940.10
Monoclinic
C2/c
1808.85
Triclinic
P-1
Triclinic
P-1
12.8700(5)
20.7985(10)
24.0234(12)
90
13.9429(3)
19.5452(4)
26.0040(5)
90
11.1148(5)
16.4516(8)
20.8107(10)
77.806(4)
86.676(4)
77.019(4)
3624.2(3)
2
10.4475(5)
11.9018(7)
13.7950(7)
81.153(5)
86.546(4)
64.284(5)
1527.01(14)
2
b (Å)
c (Å)
α (°)
β (°)
93.165(4)
90
98.909(2)
90
γ (°)
V (Å3)
6420.7(5)
4
7001.0(2)
8
Z
T/K
120
120
120
120
ρ(calc/gcm-3)
μ (mm-1)
(Mo Kα/Å)
θ range ( °)
R indexes [ I > 2
(I)]
1.773
1.776
1.648
1.979
3.307
3.376
2.935
3.864
0.71073
0.71073
0.71073
0.71073
2.52 - 25.00
R1 = 0.0676
wR2 = 0.1505
2.55 - 25.00
R1 = 0.0340
wR2 = 0.0811
2.51 - 25.00
R1 = 0.0644
wR2 = 0.1598
2.50 - 25.00
R1 = 0.0631
wR2 = 0.1415
R
indexes (all
R1 = 0.1058
R1 = 0.0379
R1 = 0.0908
R1 = 0.0904
data)
wR2 = 0.1727
wR2 = 0.0832
wR2 = 0.1764
wR2 = 0.1572
,
aR1 = Σ||Fo| − |Fc||/Σ|Fo|; wR2 = {Σ|w(Fo2 − Fc2)]/Σ [w(Fo2)2]}1/2. w = 1/[ρ2(Fo)2 + (aP)2 + bP]. P = [max (Fo O) + 2(Fc)2]/3, where a = 0.0679 and b = 0.0000 for I, a =
0.0289 and b = 37.0595 for II, a = 0.0789 and b = 3.2775 for III and a = 0.0605 and b = 1.5764 for IV.
Research (CSIR), Government of India, is thanked for the award
of a research grant (S.N.) and a research fellowship (A.K.J.).
The authors thank the Science and Engineering Research Board
Acknowledgements
(SERB), Government of India, for the award of a J. C. Bose
National Fellowship. The Council of Scientific and Industrial
Keywords: Cu6S6 cluster • Zn-oxo cluster • NiAs-related
structure • NaCl-related structure • Nitroaldol reaction
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10.1002/ejic.201701262
European Journal of Inorganic Chemistry
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10.1002/ejic.201701262
European Journal of Inorganic Chemistry
FULL PAPER
FULL PAPER
The molecular [Cu6(6-mna)6]6- clusters
have been assembled through
different Zn-O clusters forming new
three-dimensional structures
resembling NiAs and NaCl-related
structures.
Ajay Kumar Jana[a] and Srinivasan
Natarajan*[a]
Page No. – Page No.
Cu6S6 clusters as a building block for
the stabilization of coordination
polymers with NiAs, NaCl and related
structures: synthesis, structure and
catalytic studies
This article is protected by copyright. All rights reserved.