Design and synthesis of mixed valent coordination networks containing pyridine appended terpyridyl, halide, and dicarboxylates

Article abstract of DOI:10.1021/cg300770b

The ability of 4′-(4-pyridyl)-2,2′:6′,2′- terpyridine (L), which contains a chelating center as well as an exodentate center for coordination, to form mixed-valent coordination polymers together with bromide and organic dicarboxylates as bridges has been explored with Cu(I), Cu(II), Co(II), and Ni(II) metal salts using hydrothermal reactions. The mixed-valent complexes containing Cu(I) and Cu(II) with L were formed only when the organic anions are glutarate (GA) or 1,3-dicarboxylate (BDC). The glutarate-containing complex exhibits a double helix in which the opposite handed helices interact with each other via π-π interactions between the two L-units. In the case of the BDC-containing complex, the Br^- joins the two Cu(I) and Cu(II) centers to form a one-dimensional (1D) network containing Cu₂L₂Br₂ boxes. The HBDC units, which are connected to the above-mentioned 1D network, extend the dimensionality of the network to a three-dimensional (3D) network through catemeric -COOH interactions. The presence of succinate (SA) or biphenyl 2,6,2′,6′- tetracraboxylate (TCA) produced the complexes containing only Cu(II) but not Cu(I). The SA acts as a bridge between two Cu(II) centers to form one-dimensional chains containing Cu₂L₂SA₂ boxes. On the other hand, the presence of TCA resulted in the formation of a 1D chain of CuL without TCA inclusion in the crystal lattice. The reactions of L in the presence of BDC with Ni(II) and Co(II) resulted in the formation of similar 1D isomeric chains containing ML units. All the six complexes were characterized by single crystal X-ray diffraction, IR, and diffuse reflectance spectroscopy studies. The mixed valence complexes were characterized by cyclic voltammetry.

Full text of DOI:10.1021/cg300770b

Article
pubs.acs.org/crystal
Design and Synthesis of Mixed Valent Coordination Networks
Containing Pyridine Appended Terpyridyl, Halide, and
Dicarboxylates
Published as part of the Crystal Growth & Design virtual special issue In Honor of Prof. G. R. Desiraju
Kaustuv Banerjee and Kumar Biradha*
Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India
S
* Supporting Information
ABSTRACT: The ability of 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine (L),
which contains a chelating center as well as an exodentate center for
coordination, to form mixed-valent coordination polymers together with
bromide and organic dicarboxylates as bridges has been explored with
Cu(I), Cu(II), Co(II), and Ni(II) metal salts using hydrothermal reactions.
The mixed-valent complexes containing Cu(I) and Cu(II) with L were
formed only when the organic anions are glutarate (GA) or 1,3-
dicarboxylate (BDC). The glutarate-containing complex exhibits a double
helix in which the opposite handed helices interact with each other via π−π
interactions between the two L-units. In the case of the BDC-containing
complex, the Br− joins the two Cu(I) and Cu(II) centers to form a one-
dimensional (1D) network containing Cu2L2Br2 boxes. The HBDC units,
which are connected to the above-mentioned 1D network, extend the
dimensionality of the network to a three-dimensional (3D) network through catemeric -COOH interactions. The presence of
succinate (SA) or biphenyl 2,6,2′,6′-tetracraboxylate (TCA) produced the complexes containing only Cu(II) but not Cu(I). The
SA acts as a bridge between two Cu(II) centers to form one-dimensional chains containing Cu2L2SA2 boxes. On the other hand,
the presence of TCA resulted in the formation of a 1D chain of CuL without TCA inclusion in the crystal lattice. The reactions
of L in the presence of BDC with Ni(II) and Co(II) resulted in the formation of similar 1D isomeric chains containing ML units.
All the six complexes were characterized by single crystal X-ray diffraction, IR, and diffuse reflectance spectroscopy studies. The
mixed valence complexes were characterized by cyclic voltammetry.
(Scheme 1). For example, it was shown that L forms mixed
valence rectangular grid complexes with Cu(I) and Cu(II)
metal centers via halogen or SCN bridges.8 Further, it was also
shown that the reaction of L and organic anion such as
terephthalate with Cu(II) salt resulted in the complex
containing only Cu(II) but not Cu(I). The terephthalate
units bridge two CuL units to form a discrete species and the 4-
pyridyl group of L remained uncoordinated. In both cases, the
metal salts used for the complexation reactions are Cu(II), but
in the case of inorganic anions some of the Cu(II) was reduced
to Cu(I), which leads to the formation a mixed valence
complex. Given these results, we aim to synthesize the mixed
valence networks of L that contain halides or dicarboxylate
bridges between the Cu(I) and Cu(II) centers. Three types of
bridges (L, inorganic ion, and organic ion) in the same network
are expected to result in increased dimensionality of the
network, the versatile topologies, cavities, and properties. In
anticipation of such versatile functional materials, hydrothermal
INTRODUCTION
■
Exodentate ligands containing pyridine or −COOH groups are
in regular use for the generation of coordination networks or
metal−organic frameworks (MOFs).1 All these networks have
gained particular interest due to their fascinating structures,
porous nature,2 guest inclusion phenomenon,3 selective gas
adsorption and desorption,4 and magnetic properties.5 On the
other hand, the reactions of multidentate chelating ligands with
transition metals are traditionally known to form complexes
containing discrete assemblies. In several instances, the
combination of chelating ligands and exodentate ancillary
ligands was shown to produce coordination networks of varied
topologies. Despite a plethora of literature, the construction of
coordination networks using the ligands containing both
exodentate and chelating features are very rare. In the present
contribution, we wish to present our studies on one such
ligand, namely, 4′-(4-pyridyl) 2,2′:6′,2″-terpyridine, L.
The interesting feature of L is that it contains a chelating
center such as terpyridine6 and a unidentate coordination
center such as pendant pyridine. This type of ligand is expected
to coordinate to the metal centers with the varied oxidation
states7 due to the presence of two types of binding centers
Received: June 5, 2012
Revised: July 3, 2012
Published: July 6, 2012
© 2012 American Chemical Society
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Scheme 1. Various Possible Networks by the Ligand L with Transition Metals with (b, d−f, and h) or without (a, c, and g)
Linking of Anions
Table 1. Crystallographic Parameters for the Crystal Structures of 1−6
compound
formula
1
2
3
4
5
6
Cu3Br2C48H50N8O15
1329.40
293(2)
Cu3Br3C45H38N8 O6
1217.18
293(2)
Cu3Br2C56H42N8O10
1337.42
293(2)
CuBr2C20H18N4O2
569.74
NiC28H20N4O5
551.19
Co2C56H48N8O14
1174.88
mw
T (K)
system
space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
293(2)
293(2)
293(2)
monoclinic
P21/n
monoclinic
C2/c
monoclinic
P2/c
monoclinic
P21/c
monoclinic
P21/n
triclinic
P1
̅
16.528(3)
8.653(14)
18.604(3)
90.00
20.984(3)
21.858(3)
20.929(3)
90.00
12.353(3)
10.952(2)
19.413(4)
90.00
8.667(8)
10.805(11)
21.815(2)
90.00
8.796(17)
13.421(3)
20.615(4)
90.00
8.841(4)
10.104(5)
14.995(7)
99.32(2)
90.67(2)
108.53(1)
1250.4(1)
1
104.030(5)
90.00
111.872(5)
90.00
102.469(6)
90.00
99.205(3)
90.00
92.277(6)
90.00
γ (°)
vol (Å3)
Z
2581.3(8)
2
8909(2)
8
2564.4(9)
2
2016.7(3)
4
2431.9(9)
4
D
calc (mg/m3)
1.692
1.838
1.682
1.547
1.499
1.863
R1 (I > 2σ(I))
wR2 (on F2, all data)
0.0529
0.0621
0.0657
0.0406
0.0411
0.0906
0.1351
0.1519
0.1008
0.1144
0.1141
0.1885
reactions of L and organic dicarboxylates with CuBr and CuBr2
have been performed. The crystal structures of these materials
have been characterized and analyzed in terms of various
interactions, coordination geometries of the metal, and network
geometries. Further these materials have also been charac-
terized by elemental analysis, IR, diffuse reflectance spectros-
copy (DRS), and cyclic voltammetry (CV).
were considered as colinkers of metal centers with L. The slow
diffusion by layering at room temperature yielded precipitates
in all cases. Single crystals suitable for X-ray diffraction were
obtained for complexes 1, 2, and 3 in the case of three
dicarboxylates, namely, succinate (SA), glutarate (GA), and iso-
phthalate (BDC), respectively, by using hydrothermal method-
ology. The use of the nonplanar tetracarboxylate such as
biphenyl-2,6,2′,6′-tetracarboxylic acid resulted in single crystals
of complex 4 without inclusion of TCA in the crystal lattice.
The reaction of L with Ni(II) and Co(II) salts were tried in the
presence of Cu(I) halide and disodium salt of isophthalate to
produce mixed metallic complexes. These reactions resulted in
RESULTS AND DISCUSSION
The ligand L was prepared by the modified Krohnke reaction of
4-pyridine carboxaldehyde with 2-acetyl pyridine in the
■
̈
presence of base.9 Several aromatic and aliphatic dicarboxylates
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Figure 1. Illustrations for the crystal structure of 1: (a) 1D chain containing the boxes of Cu2(L)2(SA)2, (b) assembling of 1D chains via O−H···Br
and weak aromatic C−H···Br interactions.
the single crystals of complexes 5 and 6 with Ni(II) and Co(II)
respectively. Pertinent crystallographic information for all these
complexes was given in Table 1. The crystal structures of 1−6
were determined and it found that the complexes 2 and 3 have
both Cu(I) and Cu(II), while all the other complexes contain
bivalent metal centers (1, 4−6).
positions. On the other hand, the Cu2 center sits on the
inversion symmetry and exhibits elongated octahedral geometry
with two each pendant pyridine N-atoms and O-atoms of SA in
the equatorial plane. The axial positions of Cu2 are very weakly
coordinated to water with a Cu−O distance of 2.292(4) Å. In
the crystal lattice, these 1D chains are assembled via a plethora
of O−H···O (2.745(6) and 2.766(5) Å), O−H···Br (3.240(4)
Å) and weak aromatic C−H···Br (2.88, 3.03 Å) interactions
(Figure 1b). It is interesting to note here that carboxylates of
succinate exhibit a diversified nature in the sense that they
coordinate as monodentate with Cu2, while as bidentate with
Cu1, and further SA exhibits a gauche conformation with a C−
C−C−C torsion of 72.74°.
{[Cu3L2(SA)2(Br)2(H2O)4]·3H2O} , 1
n
{[Cu3L2(GA)Br ]·2H2O}n, 2
3
{[Cu3L2(HBDC)2Br2]·2H2O}n, 3
{[CuLBr2]·2H2O}n, 4
The crystal structure analysis of 2 reveals that it contains
Cu(I) as well as Cu(II) centers. The asymmetric unit of 2 has
two Cu(II) centers, and one Cu(I) center, two units of L, three
Br− anions, one unit of GA, and two uncoordinated water
molecules. The Cu(II) centers exhibit distorted square pyramid
geometry, while the Cu(I) exhibits triangular coordination
geometry. The square pyramid geometry was formed by the
coordination of three terpyridyl N atoms and carboxylate O
atom in equatorial positions and the Br− ion in the axial
position. The triangular coordination around Cu(I) is formed
by two pendant pyridyl units of L and one Br− anion. It is
interesting to note here that, unlike SA in 1, the dicarboxylate
links the two Cu(II) centers that are involved in chelation with
L to form a 1D network. The GA exhibits gauche−gauche
conformation with C−C−C−C torsions of 72.06° and 68.13°.
As a result the 1D network exhibits a helicity (Figure 2).10
Further, two of such opposite handed helices form a double
helical chain through aromatic π···π interactions between the
units of L.
[NiL(BDC)(H2O)]n , 5
{[CoL(BDC)]·3H2O}n, 6
Aliphatic Dicarboxylates as Linkers (1 and 2). Although
several aliphatic dicarboxylates have been employed for this
purpose, only succinate (SA) and glutarate (GA) have shown
such linking ability to give complexes 1 and 2 respectively. The
complex 1 crystallizes in the P21/n space group and the
asymmetric unit contains two symmetry independent Cu(II)
centers (one with full occupancy, Cu1, and the other with half
occupancy, Cu2), one unit each of L, SA, and Br−, two units of
coordinated water molecules, and 1.5 units of uncoordinated
water. Interestingly, the Br− ion does not coordinate to metal
atoms, but it hydrogen bonds with coordinated water
molecules. The metal centers are bridged by succinate such
that it forms a linear one-dimensional (1D) chain containing
the boxes of Cu2(L)2(SA)2 (Figure 1a). Within the 1D chain,
water molecules are hydrogen bonded to axially coordinated O-
atom of carboxylate. In this chain, the Cu1 exhibits a distorted
octahedral geometry with three N-atoms of terpyridine and O-
atom of carboxylate in the equatorial plane, while the other O
atom of carboxylate and coordinated water occupies the axial
1,3-Benzene Dicarboxylate as a Linker (3). Although
several aromatic polycarboxylates such as terphthalate,
trimesate, and pyromelitate have been tried for generating the
complexes with L and copper metals, only BDC has shown the
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Figure 2. Illustrations for the crystal structure of 2: (a) One-dimensional helical chain; double helix through aromatic π···π interactions between the
units of L: (b) cylinder mode and (c) space-filling mode.
ability to give mixed metallic complex 3 and also to bridge 1D
clusters of metal, L, and Br− into a three-dimensional (3D)
network via catemeric O−H···O hydrogen bonds. The crystal
structure of 3 reveals that it contains both Cu(I) and Cu(II),
and the Br− also acts as a bridge between them. The
asymmetric unit of 3 contains Cu(I) with full and Cu(II)
with half occupancies, one L, one each of HBDC, Br− anion,
and one water molecule. It is interesting to note here that
although disodium salt was used in the reaction one of the
carboxylate was protonated during the course of the reaction.
The bridging of Cu(I) and Cu(II) centers by Br− leads to the
formation of a zigzag chain that contains Cu2L2Br2 boxes
(Figure 3). The Cu(I) center exhibits a pseudotetrahedral
geometry with the coordination of two pendant pyridyl groups
of L and two bridging bromide ions. On the other hand, Cu(II)
exhibits distorted square pyramidal geometry with the
coordination of three terpyridyl N atoms, one bridging Br−
ion, and one carboxylate O atom of HBDC. The axial position
of Cu(II) is weakly coordinated to the Br− anion with the Cu−
Br distance of 2.658(4) Å. In Cu2L2Br2 boxes, the Cu(I) and
Cu(II) centers are connected through the Br− bridges such that
the two L ligands are in a head-to-tail arrangement leading to
the formation of a mixed-valence tetrameric pseudo rectangular
grid of size 11.056 × 4.362 Å2 with the Cu(II)−Cu(I)−Cu(II)
angle of 87.29°. The ligand L is somewhat twisted with a
torsion angle of 19.92°. Interligand face-to-face π−π stacking
(centroid−centroid) interactions are observed with a distance
of 4.078 Å. The pendant pyridine N atoms from adjacent grids
coordinated to the Cu(I) center to extend the rectangular boxes
into a 1D zigzag chain. In the crystal lattice two adjacent
HBDC units are also assembled via a plethora of O−H···O
interactions with a neighboring water molecule (O···O
2.902(5) and 2.787(4) Å). In the crystal lattice, the 1D chains
are assembled into an extended hydrogen bonded 3D network
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Figure 3. Illustrations for the crystal structure of 3: (a) coordination around Cu(I) and Cu(II) centers, (b) 1D zigzag chain that contains Cu2(L)2Br2
boxes, (c) side view of 1D zigzag chains.
via catemeric O−H···O interactions (O···O: 2.506(4) Å)
having huge and continuous channels which are filled by the
4-fold self-interpenetration (Figure 4). Earlier, we have shown
in a number of examples that hydrogen bonding can be used to
assemble the coordination networks/complexes into higher
dimensional aggregates.11
Linking of Cu(II) Centers by L Leading to 1D-Chain (4).
The asymmetric unit of complex 4 contains one each of L and
Cu(II), two each of Br− ions and water molecules. The Cu(II)
center exhibits square planar geometry with the coordination of
four nitrogen atoms, three from terpyridine and one from
pendant pyridine (Cu−Nter 2.049(3), 1.933(3), 2.047(3) Å and
Cu−N(pen) 1.980(3) Å) (Figure 5). The edges of the square
N4 differ largely because three of the nitrogen atoms come from
the chelation of terpyridine units. The two N−N edges formed
by chelated terpyridyl are shorter (2.561 and 2.553 Å) than the
two N−N edges which involve the pendant N-atom (3.075 Å
and 3.079 Å). The square contains N−Cu−N bond angles that
range from 79.70° and 99.63°. Overall, the Cu(II) exhibits
distorted and elongated octahedral geometry as the epical
positions occupied by Br− with a Cu(II)−Br distance of
2.726(7) Å. This type of coordination around Cu(II) generates
1D chains that are joined in three dimensions with hydrogen
bonds such as C−H···Br (2.99 Å) O−H···O (2.933(11) Å) and
O−H···Br (3.340(4) Å) and π···π interactions (3.723 Å).
Linking of Ni(II)L/Co(II)L Units by BDC (5 and 6). The
complex 5 crystallizes in the P2(1)/n space group and
asymmetric unit of 5 is constituted by one each of L and
Ni(II), BDC ions, and one water molecule. The Ni(II) exhibits
perfect octahedral geometry, the three nitrogen atoms from
terpyridine (Ni−Nter 2.110(3), 1.991(2), 2.108(3) Å) and
water molecule occupies an equatorial position and the axial
position is occupied by the monocoordination of carboxylates
of BDC (Ni−OBDC 2.134(19) and 2.040(19) Å and Ni−Owater
2.027(2) Å) (Figure 6). The resultant network can be described
as a 1D chain in which BDC links Ni(L)H2O units using the
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Figure 4. Illustrations for the crystal structure of 3: (a) three-dimensional network formed by the linking the 1D chains via a catemer type of
−COOH hydrogen bonding; 4-fold interpenetration of the 3D networks: (b) view without hydrogen atoms; (c) line drawing by representing metal
atoms and hydrogen bonding synthon as nodes and L and Br atoms as connectors.
inversion symmetry. The 4-pyridyl of L remains uncoordinated
and hangs on both sides of the chain and connects the chains
into a two-dimensional (2D) layer via O−H···N (O···N
2.742(3) Å).
are interlinked by water molecules via O−H···N, O−H···O
(O···N, 2.993(10) Å and O···O, 2.998(8), 2.955(10) Å) and
C−H···O interaction (C···O 3.335(12) Å).
Characterization of Cu(I)−Cu(II) Complexes by Cyclic
Voltammetric Studies. The CV of the complexes 1−4 were
recorded in order to understand the differences in oxidation−
reduction behaviors of the complexes containing only Cu(II)
(1 and 4) and Cu(II)−Cu(I) (2 and 3) in CH3CN. The peaks
observed in the cyclic voltammograms for the complexes 1−4
are given in Table S2.
The complex 6 crystallizes in P1 space group and its
̅
asymmetric unit contains one each of L and Co(II), BDC ions
and three uncoordinated water molecules. The Co(II) center
exhibits highly distorted square pyramidal geometry with the
coordination of three nitrogen atoms from terpyridine (Co−
Nter: 2.137(6) Å, 2.149(6) Å, 2.040(6) Å) and two carboxylates
in mondentate fashion (Co−OBDC 2.061(6) and 2.058(5) Å)
(Figure 7). This type of coordination results in the formation of
a 1D network that is formed by the linking of Co(L) units by
the BDC units. Here it is interesting to note that, unlike in 5,
the 1D chain is propagated using the translational symmetry
such that the uncoordinated pyridines of L hang on only one
side of the chain. This chain can be regarded as a
supramolecular isomer to the one observed in 5. These chains
The cyclic voltammograms of complexes 1 and 4 (Figure S1
and S4) exhibit irreversibility and typical behavior of Cu(II)
complexes in the forward scans for both 1 and 4. On the other
hand, CVs for the mixed valence (Cu(I) and Cu(II))
complexes 2 and 3 exhibit reversibility (Figure S2 and S3).
In the forward scans of 2, the close proximity between the two
peaks may be attributed to the fact that the SET process12 from
Cu(II) to Cu(I) may be sluggish13 in nature due to its
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Figure 5. Illustrations for the crystal structure of 4: (a) coordination around Cu(II), assembling 1D chains: (b) via O−H···Br and O−H···O and (c)
C−H···Br and π···π interactions.
resistance by the steric effects of the ligand for required
structural changes.14 The CV of 3 contains a single broadened
peak both in forward and reverse scans (Figure S3). The
broadening of peaks for both 2 and 3 may be attributed to the
increase in chain length and the aromatic nature of the
dicarboxylate linker which decreases the difference in potential
between two adjacent redox states that in turn results in the
merging of peaks into a single broad peak.15 Thus a clear
indication from the CV plots that the mixed valence complexes
tend to attain reversibility and peaks tend to merge with each
other to broaden the voltammogram.
DRS Studies for the Complexes. The DRS studies of the
complexes were carried out using a Cary 5000 UV−vis-NIR
analyzer. All the complexes were ground into a fine powder,
and DRS studies were performed with these powdered samples
taking the corresponding metal salts as the reference. The DRS
spectra of the complexes showed good agreement with those of
the constituent metal salts in the visible region. The peaks
observed in the DRS spectra for the complexes are given in the
Table S3.
Further in DRS, all the complexes exhibited charge transfer
bands in the visible region (600−700 nm) possibly due to
LMCT transition from filled orbital of the dicarboxylate to the
low lined vacant d-orbital of the transition metals. Among the
complexes 1−4, only the complex 2 exhibited distinct LMCT
transition in the visible region, which may be attributed to the
two distinct coordination environments of copper in 2.
Notably, the DRS spectra for the two isomeric complexes (5
and 6) containing Ni(II) or Co(II) did not show distinct
LMCT bands in the visible region.
CONCLUSIONS
■
Coordination networks of L with transition metal salts using
organic dicarboxylate and Br− ions as linkers have been
synthesized via hydrothermal methodology, and their crystal
structures have been analyzed in terms of network geometry
and bridging modes of the linkers. The mixed valent complexes
were obtained only in the case of 2 and 3 when the organic
linkers are glutarate and isophthalate, both the complexes
contain Br− in addition to the organic carboxylate anion. In
complex 2, the ligand L bridges Cu(I) and Cu(II) centers,
while the organic anion bridges two Cu(II) centers to form a
helical structure that further assembles into a double helix via
The DRS and absorption spectra in the solid state (null
transmittance) of all the copper complexes (1−4) are found to
be identical signifying the presence of similar chromophores.16
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Figure 6. Illustrations for the crystal structure of 5: (a) coordination around Ni(II), (b) linking the Ni(II) centers by the BDC dianion to form a 1D
chain; (c) assembling 1D chains into a 2D layer via O−H···N hydrogen bonds.
BRUKER-AC 200 MHz spectrometer. Elemental analyses were carried
aromatic π−π interactions. In the case of 3, the inorganic anion,
Br−, bridges Cu(I) and Cu(II) centers, while HBDC links the
out with a Perkin-Elmer Series II 2400, and melting points were taken
using a Fisher Scientific melting point apparatus cat. no. 12-144-1.
1D chains into a 3d network via catemeric −COOH hydrogen
Crystal Structure Determination. The single crystal X-ray
bonds. Complexes 5 and 6 exemplifies the phenomenon of
diffraction data for all structures were collected on Bruker-APEX-2
supramolecular isomerism due to the isomeric bridging of
X-ray diffractometer that uses graphite monochromated Mo Kα
radiation (λ = 0.71073 Å) using the hemisphere method. The
dicraboxylate linker in a sense that they have similar network
components but not geometries.17
structures were solved by direct methods and refined by least-squares
methods on F2 using SHELX-97.18 Non-hydrogen atoms were refined
EXPERIMENTAL SECTION
anisotropically and hydrogen atoms attached to C-atom and N-atom
were fixed at calculated positions and refined using a riding model.
The pertinent crystallographic details for the complexes are given in
Table 1.
The cyclic voltammograms were recorded in an Electrochemical
analyzer- CH Instrument 10688. All the solutions were deaerated at
■
All the chemicals were used as received without further purification. 4-
Pyridine carboxaldehyde was purchased from Aldrich Chemicals.
Other chemicals were bought from a local chemical company. FTIR
spectra were recorded with a Perkin-Elmer Instrument Spectrum Rx
1
serial no. 73713. H NMR (200 MHz) spectra was recorded on a
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Figure 7. Illustrations for the crystal structure of 6: (a) coordination around central Co(II), (b) linking of CoL units by BDC; compare with Figure
6b, (c) linking of the chains by water molecules via O−H···N and O−H···O hydrogen bonds.
least for 30 min with pure argon. All the potentials were coated against
a Ag/AgCl electrode, the counter electrode was selected to be a Pt rod,
and all experiments were performed at room temperature. The
samples were prepared in CH3CN, and NaClO4 was used as
nonaqueous electrolyte. The peaks observed in the voltammograms
for the mixed valence complexes are given in Table S2. The cyclic
voltammograms for the complexes 1−4 are given in Figures S1−S4.
The thermogravimetric analyses for the complexes were performed
using LUXX STA 409PC analyzer in the temperature range of 0−500
°C at a scan rate of 5 °C/min. The TGA curves for the complexes are
given in Figures S5−S10. The experimental and calculated X-ray
powder patterns of the complexes are given in Figures S11−S14.
The DRS studies of the complexes were performed using a Cary
5000 UV−vis-NIR analyzer. All the complexes were ground into fine
powder and recorded by taking the corresponding metal salts as the
reference. The DRS spectra and the peaks observed in the DRS spectra
for all the complexes are given in Figures S15−S20 and Table S3
respectively. The corresponding absorption spectra are given in
Figures S21−S26.
Synthesis of L. The solution of NaOH (0.075 mM, 3.0 g in 3.0 mL
of water) was added to 40 mL of EtOH in a two neck round-bottom
flask. To this solution, 5.4 mL of 2-acetyl-pyridine and 2.26 mL of 4-
pyridine carboxaldehyde were added and stirred for 10 min. A
yellowish color developed immediately after which 15 mL of aqueous
NH3 (28%) was added to the yellow colored solution and stirred for
48 h. The resulting precipitate was filtered and recrystallized from
(1:1) v/v hot (EtOH + MeOH) mixture. Yield: 31.2% based on 4-
pyridine carboxaldehyde, mp 228.2−228.6 °C. 1H NMR (CDCl3) δH:
3
3
4
7.37 (2H, d of d, JHH = 4.75 Hz, JHH = 6 Hz, JHH = 1 Hz, arom);
7.79 (2H, d, 3JHH = 4.5 Hz, arom); 8.09 (2H, d of t, 3JHH = 6 Hz, 3JHH
= 8 Hz, 4JHH = 1.75 Hz, arom); 8.70 (2H, d, 3JHH = 8 Hz, arom); 8.78
(2H, d, 3JHH = 4.75 Hz, arom); 8.82 (2H, s, arom); 8.91 (2H, d, 3JHH
=
4.5 Hz, arom).
Synthesis of Metal Complexes. Preparation of
{[Cu3L2(SA)2(Br)2(H2O)4]·3H2O}n, 1. Ethanol solution of L (5 mL,
0.016 g., 0.05 mmol) and aqueous solution of Na-succinate (2 mL,
0.0081 g, 0.05 mmol) were mixed with 6 mL of CH3CN solution of
CuBr (0.007 g, 0.05 mmol) and CuBr2 (0.011 g, 0.05 mmol) in warm
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conditions and to this mixture 0.9 mL of DMA (dimethylacetamide)
was added. The resultant solution was sealed in a 15 mL Teflon
reactor and was heated up to 100 °C for 3 days and then gradually
cooled to room temperature. The dark green crystals were obtained
and dried in the air to give 0.0061 g. Yield: 38.13% based on L.
Elemental analysis: (%) Calc. for C48H50Br2Cu3N8O15: C, 59.99; H,
5.49; N, 12.72. Found C, 58.89; H, 5.45; N, 12.22. IR (cm−1):
3431.48(m), 3032.46(s), 2365.62(s), 1602.98(s), 1406.96(s),
1247.98(s), 1018.65(s), 837.11(s), 801.39(s), 667.92(m).
Preparation of {[Cu3L2(GA)Br3]·2H2O}n, 2. Complex 2 was prepared
by using a similar procedure as for 1, except that Na glutarate (0.0088
g, 0.05 mmol) was taken in place of Na-succinate. The dark green
crystals were obtained and dried in the air to give 0.0066 g. Yield:
41.25% based on L. Elemental analysis: (%) Calc. for
C45H38Br3Cu3N8O6: C, 68.77; H, 5.40; N, 13.62. Found C, 68.57;
H, 5.45; N, 13.66. IR (cm−1): 3431.84(m), 3031.59(m), 2362.92(s),
2344.64(s), 1609.76(s), 1408.15(s), 1248.54(s), 1018.37(s),
834.27(s), 801.79(s), 655.76(m).
Preparation of {[Cu3L2(HBDC)2Br2]·2H2O}n, 3. Complex 3 was
prepared by using a similar procedure as for 1, except that Na-BDC
(0.011 g, 0.05 mmol) was used in place of Na-succinate. The dark
green crystals were obtained and dried in the air to give 0.0075 g.
Yield: 46.88% based on L. Elemental analysis: (%) Calc.for
C56H42Br2Cu3N8O10: C, 68.29; H, 4.03; N, 11.78. Found C, 68.30;
H, 4.10; N, 11.50. IR (cm−1): 3422.43(m), 3032.09(s), 2365.15(s),
1609.71(s), 1413.57(s), 1248.20(s), 1018.83(s), 838.01(s), 802.01(s),
667.64(m).
Preparation {[CuLBr2]·2H2O}n, 4. Complex 4 was prepared by
following a similar procedure as for 1 except bipyridine 2,6,2′,6′-
tetracarboxylic acid (0.017 g, 0.05 mmol), and a calculated amount of
Et3N was taken in place of Na-succinate. The dark green crystals were
obtained and dried in the air to give 0.0048 g. Yield 30% based on L.
Elemental analysis: (%) Calc. for C20H18Br2 CuN4O2: C, 64.29; H,
3.60; N, 8.33. Found C, 69.35; H, 5.24; N, 16.17. IR (cm−1):
3422.19(m), 3032.06(s), 2365.40(s), 1560.28(m), 1405.99(s),
1248.30(s), 970.27(s), 841.10(s), 802.01(s), 667.80(s).
Preparation of [NiL(BDC)(H2O)]n, 5. Complex 5 was prepared by
following a similar procedure as for 1 except that Na-BDC (0.011 g,
0.05 mmol) and NiCl2 (0.024 g, 0.1 mmol) were used in place of Na-
succinate and CuBr2. Light green crystals were obtained and dried in
the air to give 0.0087 g. Yield: 54.38% based on L. Elemental analysis:
(%) Calc. for C28H20N4NiO5: C, 68.29; H, 4.09; N, 11.38. Found C,
68.22; H, 4.10; N, 10.68. IR (cm−1): 3055.88(m), 1611.55(s),
1545.25(m), 1361.68(s), 1013.90(s), 794.02(s), 748.72(s), 708.02(m).
Preparation of {[CoL(BDC)]·3H2O}n, 6. Complex 6 was prepared
following a similar procedure as for 1 except for Na-BDC (0.011 g,
0.05 mmol) and CoCl2 (0.028 g, 0.1 mmol) was used in place of Na-
succinate and CuBr2. Dark red crystals were obtained and dried in the
air to give 0.0143 g. Yield: 89.375% based on L. Elemental analysis:
(%) Calc. for C56H48Co2N8O14: C, 63.63; H, 4.58; N, 10.60. Found C,
63.65; H, 4.45; N, 10.50. IR (cm−1): 3055.12(s), 1610.07(s),
1542.81(m), 1361.90(s), 794.19(s), 744.16(m), 709.38(s).
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from DST, and
DST-FIST for the single-crystal X-ray facility. K.B. thanks CSIR
for a junior research fellowship.
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ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR of L, XRPD-patterns, IR, TGA, CV, DRS spectra, and
crystallographic information for the complexes 1−6 (CIF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kbiradha@chem.iitkgp.ernet.in. Fax: +91-3222-
282252. Tel: +91-3222-283346.
Notes
The authors declare no competing financial interest.
̀
32, 5818. (e) Fang, Y. Q.; Taylor, N. J.; Laverdiere, F.; Hanan, G. S.;
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