Synthesis, characterization and electrochemical investigations of mixed-ligand copper(II)-organic supramolecular frameworks

Article abstract of DOI:10.1016/j.molstruc.2017.06.128

Two mixed-ligand copper(II)-organic coordination compounds with 5,5′-dimethyl-2,2′-bipyridine (5,5′-Me₂bpy) as a primary ligand while aliphatic malonate (Hmal) and aromatic 2-hydroxynicotinate (2-OHNA) as secondary ligands, were synthesized. These complexes are formulated as: [Cu(Hmal)(5,5′-Me₂bpy)(H₂O)](ClO₄) 1 and [Cu₂(2-OHNA)₂(5,5′-Me₂bpy)₂(NO₃)](NO₃) 2. These two complexes were structurally characterized by single crystal X-ray diffraction analysis. Characterization was further supported by powder X-ray diffraction analysis, elemental analyses, FT-IR, FAB-MASS and TGA, DSC studies. Cyclic voltammetric and UV-visible spectral studies of these two complexes have also been done. The electrochemical studies of complex 1 in DMSO and DMF have shown that this complex undergoes quasi-reversible diffusion-controlled one-electron transfer reaction without any chemical complication while complex 2 in DMSO undergoes quasi-reversible diffusion-controlled one electron transfer reaction, following EC mechanism. The electrochemical behaviour of complex 2 in DMF is complicated probably due to presence of more than one species in solution phase.

Full text of DOI:10.1016/j.molstruc.2017.06.128

Accepted Manuscript
Synthesis, characterization and electrochemical investigations of mixed-ligand
copper(II)-organic supramolecular frameworks
Sandeep K. Singh, Ashish Kumar Srivastava, Krishna Srivastava, Rahul Banerjee,
Jagdish Prasad
PII:
S0022-2860(17)30909-2
10.1016/j.molstruc.2017.06.128
MOLSTR 24014
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 25 April 2017
Revised Date: 17 June 2017
Accepted Date: 28 June 2017
Please cite this article as: S.K. Singh, A.K. Srivastava, K. Srivastava, R. Banerjee, J. Prasad, Synthesis,
characterization and electrochemical investigations of mixed-ligand copper(II)-organic supramolecular
frameworks, Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.06.128.
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ACCEPTED MANUSCRIPT
Graphical abstract

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Synthesis, characterization and electrochemical investigations of
mixed-ligand copper(II)-organic supramolecular frameworks
Sandeep K. Singh*a,b, Ashish Kumar Srivastavab, Krishna Srivastavab, Rahul Banerjeec
and Jagdish Prasadb
*Author to whom all correspondence should be made.
*aRegional Water Analysis & Research Laboratory, U. P. Jal Nigam, Lanka, Varanasi-221005 (U.P.),
India
Tel: +91-9450750601; E-mail: skssrinet@gmail.com
bNanoscience and Electrochemical Research Laboratory, Department of Chemistry, University of
Allahabad, Allahabad-211002 (U.P.), India
Fax: +91-532-2462241; Tel: +91-9415247536; E-mail: dr_jagdish_p@yahoo.co.in
cPhysical Chemistry Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411008
(Maharashtra), India
Fax: +91-20-25902636; Tel: +91-020-2590-2535; E-mail: r.banerjee@ncl.res.in
Abstract: Two mixed-ligand copper(II)-organic coordination compounds with 5,5'-
dimethyl-2,2'-bipyridine (5,5'-Me2bpy) as a primary ligand while aliphatic malonate (Hmal)
and aromatic 2-hydroxynicotinate (2-OHNA) as secondary ligands, were synthesized. These
complexes are formulated as: [Cu(Hmal)(5,5'-Me2bpy)(H2O)](ClO4) 1 and [Cu2(2-
OHNA)2(5,5'-Me2bpy)2(NO3)](NO3) 2. These two complexes were structurally characterized
by single crystal X-ray diffraction analysis. Characterization was further supported by powder
X-ray diffraction analysis, elemental analyses, FT-IR, FAB-MASS and TGA, DSC studies.
Cyclic voltammetric and UV-visible spectral studies of these two complexes have also been
done. The electrochemical studies of complex 1 in DMSO and DMF have shown that this
complex undergoes quasi-reversible diffusion-controlled one-electron transfer reaction
without any chemical complication while complex 2 in DMSO undergoes quasi-reversible
diffusion-controlled one electron transfer reaction, following EC mechanism. The
electrochemical behaviour of complex 2 in DMF is complicated probably due to presence of
more than one species in solution phase.
Keywords: Mixed-ligand complex; Supramolecular framework; X-ray diffraction; Cyclic
voltammetry; 5,5'-dimethyl-2,2'-bipyridine; π-π stacking interaction.
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1.Introduction
Recent years have witnessed rapid progress in the construction of
metallosupramolecular frameworks due to their variety of architectures, enchanting topology
[1-7], and the potential applications as functional materials [8-14]. Early reports have shown
that carboxylate compounds and nitrogen heterocyclic ligands have been successfully
employed in the generation of many novel structures [15]. Cui and his co-workers have
extensively studied the application of nitrogen-heterocyclic ligand based metal-organic
coordination compounds in the field of photo-catalysis, electrochemistry, luminescence,
fluorescence and dye adsorption [16-21]. Further, it has also been recognized for many years
that mixed-ligand copper(II) complexes containing an aromatic amine or its derivatives and
an anionic auxiliary ligand are much more stable than would be expected on statistical
grounds [22]. Earlier studies have also demonstrated that the aromatic ligand receives π-
electron density from the metal ions and that this process is augmented by the binding of
anionic auxiliary ligands to the metal ions [23]. These electronic effects have been suggested
to lead to the enhanced stability of the complex species. In contrast to this, the stability of
mixed-ligand copper(II) complexes, containing neutral donor ligands as a moiety, are of the
order expected on statistical ground [24].
Moreover, copper(II)-organic frameworks possess enormous structural diversity due
to its flexible coordination sphere around the copper(II) ion, in combination with steric and
packing forces [25]. Copper(II) is one of the most important transition metal ions, not only
because of its relevance as the active-site structure of metalloproteins [26], but also due to its
magnetochemistry [27].
In this respect polycarboxylates along with copper(II) ions are widely used in the
construction of supramolecular architectures organised through coordination bonds, π-π
stacking interaction and/or hydrogen bondings. For this purpose dicarboxylate ligands are
frequently used as bridging and chelating linkers. Among these malonate is a dicarboxylate
with singular behaviour different from the other dicarboxylates. It shows a number of
coordination modes with transition metal ions [28]. It also acts as a blocking agent through
bidentate chelation and produces an isolated structure which is further extended by
supramolecular interactions [29]. Mono-deprotonated malonate frequently enhances the
structures with low dimensionality to supramolecular architectures via the assembly of
hydrogen bonding [30]. Further, functionalized carboxylates can easily be applied to construct
functionalized metal-organic coordination compounds with supramolecular architectures.
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Functional groups like –OH, -NH2 are frequently used for this purpose [31]. 2-
hydroxynicotinic acid (2-OHNAH) is one of the most successful functionalized carboxylates
used for the purpose due to its versatile modes of coordination [32,33].
A fascinating supramolecular architecture can easily be achieved via the combination
of carboxylate ligands with other ligands having N-donor chelating bipyridine moiety. 2,2'-
bipyridine and its derivatives are frequently used for the synthesis of low dimensional
molecular architectures [34-38], which can be further extended to high dimensional
supramolecular architectures via supramolecular interactions.
Scheme 1. Ligands used for the synthesis of mixed-ligand complexes.
Scheme 2. General synthetic routes for the complexes [Cu(Hmal)(5,5'-Me2bpy)(H2O)](ClO4)
1 and [Cu2(2-OHNA)2(5,5'-Me2bpy)2(NO3)](NO3) 2.
Thus keeping in mind the above mentioned importance and properties a successful
attempt has been made for synthesizing two mixed-ligand copper(II)-organic coordination
compounds, taking 5,5'-dimethyl-2,2'-bipyridine (5,5'-Me2bpy) as a primary ligand while
aliphatic malonate (Hmal) and aromatic 2-hydroxynicotinate (2-OHNA) as secondary ligands
(Scheme 1). These complexes are formulated as: [Cu(Hmal)(5,5'-Me2bpy)(H2O)](ClO4) 1 and
[Cu2(2-OHNA)2(5,5'-Me2bpy)2(NO3)](NO3) 2. These two complexes were procured by
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traditional room temperature synthesis (Scheme 2) and were structurally characterized by
single crystal X-ray diffraction analysis. Characterization was further supported by powder X-
ray diffraction, elemental analyses, FT-IR, FAB-MASS, UV-Visible spectral analyses, TGA,
DSC and cyclic voltammetric (CV) studies.
2.Experimental
2.1. Materials and physical measurements
All the chemicals used in the synthesis were purchased from vendors and were used
as such. Metal salts, i.e., Cu(NO3)2.3H2O, Cu(ClO4)2.6H2O and organic ligands 5,5'-dimethyl-
2,2'-bipyridine (5,5'-Me2bpy), malonic acid (H2mal) and 2-chloronicotinic acid (2-ClNAH)
were purchased from Sigma Aldrich Chemicals Pvt. Ltd., New Delhi. Ethanol was purchased
from Bengal Chemicals, Kolkata, India and methanol, dimethylsulfoxide (DMSO) and
dimethylformamide (DMF) were procured from Merck Specialities Pvt. Ltd. All experimental
operations were performed in air. FT-IR spectra were taken on a Bruker Optics ALPHA-E
spectrometer with a Universal Zn-Se ATR accessory in the 600–4000 cm-1 region. Powder X-
ray diffraction (PXRD) patterns were recorded on a Phillips PANalytical diffractometer for
Cu-Kα radiation (λ = 1.5406 Å), with a scan speed of 2o min-1 and a step size of 0.02o in 2θ.
Elemental (C, H, N) analyses were done on Elemental Analyzers Elementar Vario EL III
Carlo Erba 1108. FAB-MASS spectra were recorded on a Jeol SX 102/Da-600 mass
spectrometer/Data System using Argon/Xenon (6kv, 10mA) as the FAB gas. The accelerating
voltage was 10 kv and spectra were recorded at room temperature. UV-Visible spectra were
recorded at Perkin-Elmer UV-Visible spectrophotometer model Lambda-35. Cyclic
voltammetric studies were carried out at BAS electrochemical system model EPSILON at
room temperature. Thermo-gravimetric analyses (TGA) and DSC experiments were carried
out in the temperature range of 50-400oC on a SDT Q600 TG-DTA analyzer under N2
atmosphere at a heating rate of 10 oC min-1.
2.2. Synthesis of Cu(II)-mixed-ligand complexes
2.2.1. Synthesis of [Cu(Hmal)(5,5'-Me2bpy)(H2O)](ClO4), 1
40 mL aqueous solution of malonic acid (2 mM, 0.208 g) was added to another 20
mL aqueous solution of Cu(ClO4)2.6H2O (2 mM, 0.741 g), followed by 30 mL ethanolic
solution of 5,5'-dimethyl-2,2'-bipyridine (2 mM, 0.368 g) with constant stirring. Initially deep
blue coloured solution was obtained. After continuous stirring (30 min.) green coloured
precipitate with blue coloured supernatant solution was obtained. Precipitate turned blue after
3h continuous stirring. Filtered the content and kept the turquoise coloured solution for
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evaporation at room temperature. Blue coloured shining single crystals of [Cu(Hmal)(5,5'-
Me2bpy)(H2O)](ClO4), suitable for X-ray diffraction were obtained after two days. Resulting
crystals were washed with water followed by ether and dried at room temperature (yield: 289
mg, 39% based on Cu). FTIR (KBr 4000–600 cm-1): 3482m(b), 3221w, 3051w, 1627s, 1479s,
1391m, 1095s, 846m, 722m, 625m. Anal. Calcd for C15H17CuN2O9Cl (%): C, 38.44; H, 3.63;
N, 5.98. Found: C, 38.32; H, 3.49; N, 6.07.
2.2.2.Synthesis of [Cu2(2-OHNA)2(5,5'-Me2bpy)2(NO3)](NO3), 2
50 mL methanolic solution of 2-chloronicotinic acid (6.67 mM, 1.039 g) was slowly
added to 20 mL aqueous solution of Cu(NO3)2.3H2O (3.33 mM, 0.805 g) with continuous
stirring. After 20 min. of continuous stirring another 20 mL aqueous-methanolic (1:1) solution
of 5,5'-dimethyl-2,2'-bipyridine (3.33 mM, 0.615 g) was added to the above mixed solution,
resulting in a deep blue coloured solution. Whole content was stirred for another 1h with
constant heating at water bath. Now cooled the solution and raised the pH from 1.85 to 4.5
and heated over water bath for 1h. Filtered the content and kept the blue coloured solution for
evaporation. Blue coloured shining crystals of [Cu2(2-OHNA)2(5,5'-Me2bpy)2(NO3)](NO3),
were obtained after three days. Resulting crystals were washed with 1:1 aqueous-methanol
solution followed by ether and dried at room temperature (yield: 459 mg, 57% based on Cu).
FTIR (KBr 4000–600 cm-1): 3454m(sh), 3064w, 2922w, 2758w, 2474m(br), 1875m(sh),
1717m(sh), 901m, 774s, 667s. Anal. Calcd for C36H32Cu2N8O12 (%): C, 48.23; H, 3.57; N,
12.50. Found: C, 48.16; H, 3.52; N, 12.52.
2.3. Instrumentation
2.3.1. Single crystal X-ray diffraction data collection of 1 and 2
All single crystal data were collected on a Bruker SMART APEX three circle
diffractometer equipped with a CCD area detector (Bruker Systems Inc., 1999a) [39] and
operated at 1500W power (50kV, 30 mA) to generate Mo-Kα radiation (λ = 0.71073Å).
Crystals of the coordination compounds reported herein were mounted on nylon CryoLoops
(Hampton Research) with Paraton-N (Hampton Research). Data were integrated using Bruker
SAINT Software [40]. Data were subsequently corrected for absorption by the program
SADABS [41]. The space group determinations and tests for merohedral twinning were
carried out using XPREP [42]. All structures were solved by direct methods and refined using
the SHELXTL 97 software suite [42]. Atoms were located from iterative examination of
difference F-maps following least squares refinements of the earlier models. Hydrogen atoms
were placed in calculated positions and included as riding atoms with isotropic displacement
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parameters 1.2–1.5 times Ueq of the attached C atoms. Data were collected at 150(2) K for
[Cu(Hmal)(5,5’-Me2bpy)(H2O)](ClO4) 1, and [Cu2(2-OHNA)2(5,5’-Me2bpy)2(NO3)](NO3) 2.
All structures were examined using the ADDSYM subroutine of PLATON [43] to ensure that
no additional symmetry could be applied to the models. Crystallographic data of all these
complexes are summarized in Table 1. CCDC 756584 and 756654 contains the supplementary
crystallographic data (excluding structure factors) for complexes 1 and 2, respectively. Copies
of the data can be obtained, free of charge, on application to the CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: + 44 (1223) 336 033; e-mail: deposit@ccdc.cam.ac.uk).
2.3.2.Electrochemical measurements
Cyclic voltammetric experiments were carried out with a Bio-Analytical Systems
(BAS) model EPSILON (Bioanalytical Systems, Inc, USA). Ag/AgCl in saturated KCl (E0 =
+0.199V vs NHE) was used as reference electrode. The working electrode was a platinum
microelectrode (MF-2013), platinum wire was used as an auxiliary electrode. All the cyclic
voltammetric experiments were done in an inert atmosphere achieved by purging the cell
solution with nitrogen (99.999% pure) for about 20 minutes at 20±0.5oC. An inert atmosphere
of nitrogen was also maintained over the cell solution during the recording of the
voltammograms. Great care was taken in the electrode pre-treatment. Mechanical polishing of
the working electrode (GCE) was done over a velvet micro-cloth with an alumina suspension.
3. Results and discussion
As mentioned earlier, these two complexes were synthesized by using traditional or
solvent evaporation method. Blue coloured shining crystals of complex 1 were obtained by
mixing Cu(ClO4)2.6H2O:H2mal:5,5'-Me2bpy in 1:1:1 molar ratio into 2:1 water-ethanol
mixture. Interestingly, in this complex malonic acid is monodeprotonated being only one
carboxylic group dissociated. Crystals for complex 2 were obtained by mixing
Cu(NO3)2.3H2O:2-ClNAH:5,5'-Me2bpy in 1:2:1 molar ratio into 1:2 water-methanol mixture
under basic condition. In this complex the original 2-ClNAH ligand underwent to a base
hydrolysis reaction followed by keto-enol tautomerization (Scheme 3).
Scheme 3. Hydrolysis of 2-chloronicotinic acid in presence of base and successive keto-enol
tautomerization.
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3.1. Crystal structure of 1
Mixed-ligand complex 1 crystallizes in monoclinic crystal system with space group
P21/n. The structure of this complex consists of [Cu(Hmal)(5,5'-Me2bpy)(H2O)]+, monocation
unit and a non-coordinating ClO4¯ anion. The metal ion in this complex is penta-coordinated
with distorted square-pyramidal geometry. The base of square-pyramid is built by the N-
atoms of the chelating 5,5'-Me2bpy ligand and two O-atoms of the chelating mono-
deprotonated H2mal ligand, while the apical position is occupied by the O-atom of a
coordinated water molecule. The ORTEP diagram with 50% probability and atom numbering
scheme of the crystallographic independent unit is depicted in fig. 1. The two Cu-N bond
distances present same value (1.980(3) Ǻ) with a chelating angle of 81.60(11)o. The two
oxygen atoms from Hmal ligand are at 1.937(2) and 1.926(2) Ǻ, with a O-Cu-O bond angle of
91.11(10)o, while the Cu-O5 distance for the axially coordinated water is significantly longer,
being 2.239(3) Ǻ.
Table 1. Crystallographic data and structure refinement for complexes 1 and 2
Compounds
CCDC No.
Complex 1
756584
Complex 2
756654
Formula
Mr
C15H17CuN2O5ClO4
468.30
C36H32Cu2N8O12
895.80
Crystal system
Space group
a (Å)
Monoclinic
P 21/n
8.1563(10)
17.7192(17)
12.892(2)
90.00
105.584(15)
90.00
1794.7(4)
4
Triclinic
P-1
10.590(2)
12.0717(17)
15.056(3)
70.614(16)
80.063(18)
76.954(16)
1758.8(6)
2
b (Å)
c (Å)
α (o)
β (o)
γ (o)
V (Å3)
Z
Crystal size
F(000)
0.30x0.20x0.10 mm
956
0.23x0.18x0.15 mm
916
Dc (g cm-3)
ꢀ (mm-1)
Reflections collected
Unique reflections
Rint
GOF on F2
RI [I>2σ(I)]
wR2 (all data)
1.733
1.420
12026
5899
0.0901
1.080
0.0601
0.1612
1.691
1.290
22472
12742
0.0724
0.766
0.0503
0.1381
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All these bond lengths fall in the common range observed for penta-coordinated
copper(II) ion [44]. Selected bond lengths and bond angles are given in table 2.
The lattice structure is further stabilized and extends to a 3D supramolecular
framework by weak π-π stacking interactions and hydrogen bonding. The π-π stacking
interactions occur between symmetry related pyridine rings of the 5,5'-Me2bpy ligand, with
centroid distance in the range 3.94 - 4.35 Ǻ. The intermolecular hydrogen bonds occur
between the coordinated water with the oxygen atoms of symmetry related perchlorate anions
(O···O distances of 2.829(4) and 2.953(5) Ǻ). Further a strong interaction is observed
between the carboxylic O(2)-H with carboxylate oxygen O(4) (at ½+x, ½-y, ½+z) of another
complex. The O···O donor-acceptor distances is 2.475(3) Ǻ. Selected hydrogen bond
geometry is listed in table 3. The hydrogen bondings lead the structure to a one dimensional
linear polymeric chain. This supramolecular 1D chain is expanded to 2D and further
expanded to 3D supramolecular network by the combination of π-π stacking interactions. 1D,
2D and 3D supramolecular network structures of complex 1 are given in figure 2.
Fig. 1. ORTEP diagram (50% probability) of [Cu(Hmal)(5,5'- Me2bpy)(H2O)](ClO4) 1, with
atom numbering.
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(a)
(b)
(c)
Fig. 2. Supramolecular network structure of complex 1, (a) 1D chain along b-axis; (b) 2D
sheet viewed down a-axis; and (c) 3D network structure showing π-π stacking interactions
and H-bondings.
3.2. Crystal structure of 2
Mixed-ligand complex 2 crystallizes in triclinic crystal system with space group P-1.
The crystal structure of this complex consists of [Cu2(2-OHNA)2(5,5'-Me2bpy)2(NO3)]+ and a
non-coordinated NO3¯ anion. It should be noted that in complex 2 the asymmetric unit
comprises two copper complexes. Out of these the first complex is neutral where the
copper(II) ion is penta-coordinated with distorted square-pyramidal geometry. The
coordination set of the copper(II) ion is satisfied by the two N-atoms from bidentate chelating
5,5'-Me2bpy ligand, two O-atoms from 2-OHNA ligand, these four coordinate bonds are lying
in one plane and constitute the base of square-pyramid, and one O-atom from axially
coordinated nitrate anion (Fig. 3).
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Table 2. Selected bond lengths and bond angles in complexes 1 and 2
Complex 1
Cu1-O3
Cu1-O1
Cu1-N2
Cu1-N1
Cu1-O5
O3-Cu1-O1
O3-Cu1-N2
O1-Cu1-N2
1.926(2)
1.937(2)
1.980(3)
1.980(3)
2.239(3)
91.11(10)
166.88(10)
93.77(10)
O3-Cu1-N1
O1-Cu1-N1
N2-Cu1-N1
O3-Cu1-O5
O1-Cu1-O5
N2-Cu1-O5
N1-Cu1-O5
90.85(10)
166.30(10)
81.60(11)
98.19(10)
92.31(10)
93.77(10)
100.83(11)
Complex 2
Cu1-O1A
Cu1-O2A
Cu1-N1A
Cu1-N2A
Cu1-O13
Cu2-O1B
Cu2-O2B
Cu2-N1B
Cu2-N2B
1.910(2)
1.912(2)
1.982(2)
1.985(3)
2.487(3)
1.889(2)
1.901(2)
1.969(2)
1.982(2)
O2A Cu1 N2A
N1A Cu1 N2A
O1A Cu1 O13
O2A Cu1 O13
N1A Cu1 O13
N2A Cu1 O13
O1B Cu2 O2B
O1B Cu2 N1B
O2B Cu2 N1B
166.66(11)
81.83(10)
93.23(10)
107.35(11)
86.83(10)
84.83(11)
93.39(9)
171.62(9)
94.14(10)
O1A-Cu1-
O2A
93.13(9)
O1B Cu2 N2B
90.96(10)
O1A Cu1 N1A
O2A Cu1 N1A
O1A Cu1 N2A
173.35(10)
93.20(10)
91.55(9)
O2B Cu2 N2B
N1B Cu2 N2B
175.65(9)
81.53(10)
The second complex is mono-cationic, in which the copper(II) ion is in square
planar geometry. Four coordination around the central metal ion are satisfied by the two
N-atoms from chelating 5,5'-Me2bpy and two O-atoms from 2-OHNA ligands. These four
coordinate bonds lie in the same plane and constitute the square-plane. The extra cationic
charge of this unit is balanced by the counter nitrate anion. ORTEP diagram with 50%
probability and atom numbering scheme of crystallographic independent unit of complex
2, is depicted in figure 3. In this complex, ligand 2-OHNA exists in keto form and
bidentately coordinates to the metal centre in O,O’-salicylate manner. In this way the
ligand 2-OHNA constituting a six membered rings with the central metal ion. Cu-N bond
distances in this complex lie in the range 1.969 to 1.985 Ǻ, while Cu-O bond distances are
in the range 1.889 to 2.487 Ǻ. All these bond lengths fall in the common range observed
for the square-pyramidal and square planar copper(II) complexes [44]. In addition to
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these, weak bonding interactions are also observed between the Cu1-O2b (3.082(3) Ǻ)
and Cu2-O2a (2.856(3) Ǻ). Selected bond lengths and bond angles are listed in table 2.
Further, the weak supramolecular interactions such as hydrogen bonding, π-π
stacking interaction etc. stabilize the lattice structure. The π-π stacking interaction is
observed between the pyridine rings of the 5,5’-Me2bpy and 2-OHNA ligands. The
shortest π-π stacking interacting distances between centroids are between pyridine rings
N(2a) and N(3) (at 1-x, 1-y, 1-z) of 3.706 Ǻ and between pyridine rings N(2b) and N(4) at
(1-x, 1-y, -z) of 3.859 Ǻ. Hydrogen bondings are observed between the oxygens of nitrate
ions and the pyridine N-H of the 2-OHNA ligand. First type of intermolecular hydrogen
bonding is observed between O(12) of the coordinated nitrate ion and pyridyl N(3)-H of
2-OHNA ligand associated with square-pyramidal complex. N···O donor-acceptor
distances is 2.940(4) Ǻ (at 1-x, 1-y, 1-z). Second type of hydrogen bonding is observed
between O(23) of free nitrate ion and pyridyl N(4)-H of 2-OHNA ligand associated with
square-planar complex. In this case N···O donor-acceptor distances is 2.838(4) Ǻ (at x, y,
z). Selected hydrogen bond geometry is shown in table 3. These weak interactions
produce 3D supramolecular architectures as shown in figure 4.
Fig. 3. ORTEP diagram (50% probability) of [Cu2(2-OHNA)2(5,5'-Me2bpy)2(NO3)](NO3)
2, with atom numbering (Hs are omitted due to clarity).
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Table 3. Selected hydrogen bonds geometry (Å, °) in complexes 1 and 2
D-H····A
Complex 1
D····H
H····A
∟DHA
D····A
O2-H2····O4
O2-H2····O3
O5-H101····O9
0.84
0.84
1.64
2.53
170.24
119.34
2.475(3)
3.035(3)
2.829(4)
2.953(5)
0.82(3)
0.80(3)
2.02(3)
2.19(3)
170.0(3)
160.0(4)
O5-H102····O6
Complex 2
N3-H3····O12
N3-H3····O13
N4-H4····O21
N4-H4····O23
0.88
0.88
0.88
0.88
2.17
2.42
2.44
1.99
145.3
158.6
140.0
162.9
2.940(4)
3.251(4)
3.164(4)
2.838(4)
(a)
(b)
Fig. 4. Supramolecular interactions in complex 2, (a) π-π stacking interactions (centroid
distances shown in bold yellow dotted line); (b) 3D network structure showing π-π
stacking interaction and H-bonding.
3.3. Vibrational studies
Vibrational study of complex 1 reveals a broad band at 3482 cm-1, which simply
indicates the presence of hydrogen bonded water molecule. Presence of water in coordination
is further supported by the bands at 846 and 772 cm-1 [45]. Further, weak bands at 3221 and
3051 cm-1, correspond to the hydrogen bonded carboxylic O-H stretching and aromatic C-H
stretching, respectively. The presence of a sharp band at 1479 cm-1 may be designated for the
C=N stretching of the pyridine ring. Finally, the presence of bands at 1627 and 1391 cm-1 are
12

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assigned to asymmetric and symmetric C=O stretching of the carboxylate group. Their
difference is above 200 cm-1, i.e., 236 cm-1 confirms the monodentate coordination of the
carboxylate group in this complex [46].
In the FT-IR spectrum of complex 2 a sharp medium band at 3454 cm-1 is assigned to
the aromatic N-H stretching frequency. Two successive bands at 3064 and 2922 cm-1 are
assigned to the aromatic (pyridine ring) and aliphatic C-H (-CH3) stretching, respectively.
Further, the presence of pyridine rings are also confirmed by the weak bands at 1477 and
1450 cm-1, which are assigned to C=N and C=C stretching, respectively. Two sharp bands at
1651 and 1378 cm-1, respectively, assigned for the asymmetric and symmetric stretching of
the carboxylate group. Their difference is above 200 cm-1, i.e., 273 cm-1 confirms the
monodentate coordination of the carboxylate group [46]. Presence of another strong band at
1579 cm-1 indicates the availability of C=O group directly associated with the aromatic ring.
Moreover, the strong and sharp bands at 1259 and 1153 cm-1 are assigned for asymmetric and
symmetric stretching frequency of coordinated nitrate ion. Difference of asymmetric to
symmetric stretching frequency comes out to be around 106 cm-1 confirms the monodentate
coordination of nitrate ion [45]. Again a sharp band at 1062 cm-1 is assigned for the
coordinated nitrate ion [47].
Finally, the presence of un-coordinated nitrate ion represents the bands at 1875, 1717
and 838 cm-1 in this complex [45]. FT-IR spectra of the complexes 1 and 2 are given in
supplementary information (SI) as figures S1 and S2.
3.4. FAB-MASS studies
FAB-MASS spectra of the complexes 1 and 2 are given in SI as figures S3 and S4,
respectively. MASS spectral study of complex 1, shows a peak at m/z 467 assigned for
molecular ion peak (Calc. 468). Another peak is observed after the expulsion of perchlorate
radical ion at m/z 369 (Calc. 368.5). Further successive fragmentation produces a number of
peaks at m/z 355 (Calc. 350.5), 307 (Calc. 306.5), 247 (Calc. 247.5), 221 (Calc. 217.5) and
136 (Calc. 140) assigned for radical ions [C15H15CuN2O4], [C14H15CuN2O2], [C12H12CuN2],
[C10H6CuN2] and [C5H3CuN] obtained after the expulsion of a water, CO2, CH2COOH, two
CH3 and a pyridine radical ions, respectively. The peak at m/z 136 is assigned as base peak.
Moreover, a peak at 154 (Calc. 154), shows the presence of bipyridine radical ion which is
obtained by the combination of two pyridine radical ions.
In the MASS spectrum of complex 2, the molecular ion peak appears at m/z 893
(Calc. 895.8). Two other successive peaks appeared at m/z 845 (Calc. 833.8) and 764 (Calc.
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771.8) obtained after the two successive expulsion of two nitrate ion. Further, successive
fragmentation of the radical ion produces a number of peaks at m/z 744 (Calc. 741.8), 700
(Calc. 697.8), 663 (Calc. 667.8), 577 (Calc. 573.8), 523 (Calc. 529.8), 431 (Calc. 435.8), 358
(Calc. 369.8), 307 (Calc. 308.8), 289 (Calc. 280.8), 247 (Calc. 247.8) and 137 (Calc. 140.8)
assigned for [C34H26Cu2N6O6], [C33H26Cu2N6O4], [C31H20Cu2N6O4], [C26H16Cu2N5O3],
[C25H16Cu2N5O], [C20H12Cu2N4], [C15H9Cu2N3], [C20H12N4], [C10H6Cu2N2], [C12H12CuN4]
and [C5H3CuN] radical ions, respectively. Moreover, a peak with 100% abundance is
observed at m/z 154 assigned for bipyridine radical ions obtained after the combination of two
pyridine radical ions.
3.5. Electrochemical studies
The electrochemical behaviours of mixed-ligand copper(II) complexes
[Cu(Hmal)(5,5'-Me2bpy)(H2O)](ClO4) 1, and [Cu2(2-OHNA)2(5,5'-Me2bpy)2(NO3)] (NO3) 2,
have been studied in dimethylsulfoxide (DMSO) and dimethylformamide (DMF) containing
0.2M NaClO4 as the supporting electrolyte at a glassy carbon working electrode (GCE) using
cyclic voltammetry. CV data for complexes 1 and 2 in DMSO and DMF are given in SI as
Table S1 and S2, and the representative cyclic voltammograms are displayed in figures S5
and S6.
Cyclic voltammogram of complex 1 in DMSO showed a single well defined quasi-
reversible one electron reduction couple (c/a) associated with Cu2+/+ change, with Epc = 26
mV, Epa = 108 mV, E0’ = 67 mV vs Ag/AgCl and ∆Ep = 82 mV at 100 mVs-1. The peak
current ratio Ipa/Ipc is equal to unity, clearly indicating that the reduction process is
reversible. Further, the plot of Ipc or Ipa vs square root of scan rate (υ1/2) gives a straight line
passing through the origin, indicating that the redox process is diffusion controlled (Fig. 5(a)).
This complex shows almost similar behaviour in DMF also, with Epc = 79 mV, Epa
= 163 mV, E0’ = 120 vs Ag/AgCl and ∆Ep = 84 mV at 100 mVs-1. Here again, the peak
current ratio (Ipa/Ipc) is equal to unity, and the plot of Ipc or Ipa vs square root of scan rate
(υ1/2) gives a straight line without any intercept, showing that the redox process is reversible
and diffusion controlled (Fig. 5(b)).
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The cyclic voltammogram of complex 2, in DMSO showed a well-defined quasi-
reversible one electron redox couple associated with Cu2+/+ change, with Epc1 = 41 mV, Epa1
0’
= 157 mV, E1 = 99 mV vs Ag/AgCl and ∆Ep1 = 116 mV. The plot of Ipc1 vs square root of
scan rate (υ1/2) gives a straight line passing through origin, indicating that the redox process is
diffusion controlled (Fig. 5(c)). Further, the peak current ratio (Ipa1/Ipc1) is below unity
(Ipa1/Ipc1<1), suggesting that the redox process follows EC mechanism [48]. The
electrochemical behaviour of complex 2 appears to be different in DMF as evident from the
cyclic voltammogram (Figure S6(b) in SI). The cyclic voltammogram shows two cathodic
waves (c1 and c2) in the forward negative scan while the reverse anodic scan exhibited two ill-
defined anodic peaks (a2 and a1). It must be noted that a1 is not the corresponding peak of c1.
This may be due to the presence of two complex species in equilibrium in DMF solution.
Fig. 5. Plot of Ipc (ꢀA) vs square root of scan rate (υ1/2) (a) complex 1 in DMSO, (b) complex
1 in DMF and (c) complex 2 in DMSO.
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3.6. UV-Visible spectral studies
UV-Visible spectra of complexes 1 and 2 have also been recorded in DMSO and
DMF. For complex 1, a weak band is observed at 663 and 649 nm in DMSO and DMF,
respectively. Complex 2 also exhibited a weak band at 698 and 676 nm in DMSO and DMF,
respectively (Table S3, Figures S7 and S8 in SI). All these bands are due to d-d transitions.
This type of spectra is typical for tetragonally distorted octahedral copper(II) complexes [49-
51]. This indicates the coordination of the solvent molecule to the metal centre.
3.7. Thermal studies
o
Thermal study of complex 1 shows two steps degradations in the range 30-400 C,
one related to the water and other related to the rest of the compound. The first one between
80-120 oC related to the expulsion of coordinated water molecule and other between 190-350
oC. The overall loss of weight is around 75 percent at the temperature of 400 oC. There is still
o
a residue equivalent to 25 percent which has not degraded even upto 400 C. Likewise
complex 1, complex 2 also shows two steps degradation. The first one is very sharp between
o
o
185-190 C. There is very strong indication of phase change at around 188 C and loss of
o
matter is around 18 percent. The second degradation starts at 190 oC and ends around 390 C
but there could be one smaller step in this complex as we can see a clear phase change at 240
oC. The weight loss between 190-245 oC is being around 33 percent. The total weight loss at
240 oC is 50 percent (half of the sample). If we reject the in between phase then the complex
undergoes a total weight loss equal to around 75 percent. This is akin to the complex 1, which
shows similar loss at 400oC.
Another interesting observation about complexes 1 and 2 is that the phase change is
observed in both these complexes. Phase change is endothermic for complex 1 and
exothermic for complex 2, and is accompanied by a heat flow of 15 and 1 watt per gram,
respectively. TGA profiles of these two complexes are given in SI as figures S9 and S10.
3.8. Powder X-ray diffraction patterns
A comparison of the experimental X-ray powder pattern of both complexes with
respective simulated X-ray powder pattern shows the similarity in the major peaks. It also
confirms the phase purity and crystallinity of the complexes in bulk. Powder X-ray patterns of
these complexes are given as supplementary material (Figs. S11 and S12).
4. Conclusion
In summary, synthesized mixed-ligand complexes viz.; [Cu(Hmal)(5,5'-
Me2bpy)(H2O)] (ClO4) 1 and [Cu2(2-OHNA)2(5,5'-Me2bpy)2(NO3)](NO3) 2 were structurally
16

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characterized by single crystal X-ray diffraction analysis and different spectral techniques
such as FT-IR, UV-Visible and FAB-MASS analyses. The electrochemical studies of
complex 1 in DMSO and DMF containing 0.2M NaClO4 have shown that this complex
undergoes quasi-reversible diffusion-controlled one-electron transfer reaction (Cu2+/+) without
any chemical complication while complex 2 in DMSO undergoes quasi-reversible diffusion-
controlled one electron transfer reaction, following EC mechanism. The electrochemical
behaviour of complex 2 in DMF is complicated probably due to presence of more than one
species in solution.
Appendix A. Supplementary data
CCDC Nos. 756584 and 756654 contain the supplementary crystallographic data
(excluding structure factors) for complexes 1 and 2, respectively. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-
033; or e-mail:deposite@ccdc.cam.ac.uk.
Acknowledgements
The author (SKS) is thankful to University Grant Commission, New Delhi for providing
financial support as SRF. We are also thankful to Director, NCL, Pune & Head, Department of
Chemistry, Panjab University, Chandigarh, Panjab for their outstanding support in providing
facilities for characterization of complexes.
†Supporting Information
Supplementary information contains figures, including IR spectra, FAB-MASS, Cyclic
Voltammetry, UV-Visible, TGA profiles and PXRD patterns, tables of Cyclic Voltammetric data
and UV-Visible data.
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Highlights:
ꢀ Two new mixed-ligand complexes were synthesized.
ꢀ Electrochemical studies of these complexes have been done.
ꢀ These complexes involve supramolecular interactions.
ꢀ FAB-MASS studies have also been carried out.