Synthesis and characterization of mononuclear copper(II) complexes of pyridine 2-carboxamide: Their application as catalyst in peroxidative oxidation and antimicrobial agents

Article abstract of DOI:10.1007/s12039-015-0909-8

Four water soluble copper(II) complexes, [Cu(HL)₂(H₂O)₂]Cl₂ (1), [Cu(HL)₂(ClO₄)₂] (2), [Cu(HL)₂(SCN)₂] (3) and [CuL₂]8H₂O (4), where H

Full text of DOI:10.1007/s12039-015-0909-8

c
J. Chem. Sci. Vol. 127, No. 8, August 2015, pp. 1451–1463. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0909-8
Synthesis and characterization of mononuclear copper(II) complexes
of pyridine 2-carboxamide: Their application as catalyst in peroxidative
oxidation and antimicrobial agents
SUVENDU SAMANTAa,∗ , SHOUNAK RAYa, SUTAPA JOARDARb,∗ and SUPRIYA DUTTAc
aDepartment of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur,
Howrah 711 103, India
bDepartment of Biotechnology, Neotia Institute of Technology, Management and Science, Jhinga,
Diamond Harbour Road, Amira, South 24 Parganas 743 368, India
cDepartment of Chemistry, Nistarini College, Deshbandhu Road, Purulia, West Bengal 723 101, India
e-mail: samanta.suvendu88@gmail.com; sutapajor@yahoo.co.in
MS received 20 March 2015; revised 25 May 2015; accepted 30 May 2015
Abstract. Four water soluble copper(II) complexes, [Cu(HL)2(H2O)2]Cl2 (1), [Cu(HL)2(ClO4)2] (2),
[Cu(HL)2(SCN)2] (3) and [CuL2]·8H2O (4), where HL is pyridine 2–carboxamide, have been synthesized and
characterized by various spectroscopic techniques. Structures have been determined by single crystal X-ray
crystallography. The pH induced inter-conversion of Cu(HL)2(H2O)2]Cl2 (1) and [CuL2]·8H2O (4) through
co-ordinationmode switching was investigated thoroughly with the help of absorption spectroscopy. Complexes
1–3 were found to be active catalysts for the oxidation of toluene, ethyl benzene and cyclohexane in the pres-
ence of hydrogen peroxide as the oxidant under mild conditions. Toluene was oxidized to benzyl alcohol and
benzaldehyde, ethyl benzene was oxidized to 1-phenylethanol and acetophenone and cyclohexane was oxidized
to yield cyclohexanol and cyclohexanone Antimicrobial activities have been investigated with these copper(II)
complexes against gram +ve bacteria, gram −ve bacterial and fungal species.
Keywords. Homogeneous catalysis; Copper; Amide ligand; Oxidation; Antimicrobial activity.
1. Introduction
reactions include the poorly characterized particulate
methane monooxygenase (pMMO) which is composed
of tri- or multinuclear clusters of copper that catalyze
conversion of alkanes and alkenes.1f−h Several cop-
per(II) complexes have been exploited as catalyst in
various liquid-phase oxidation reactions,5 e.g., catechol
oxidation,6 alkane oxidation (especially cycloalkane),7
oxidation of aromatic hydrocarbons,7i,8 sulfoxidation,9
epoxidation,10 etc.
The antibacterial properties of copper(II) have also
been known for thousands of years.11 Copper(II) com-
plexes with diverse drugs have been the subject of a
large number of research studies,12 apparently due to the
biological role of copper(II) and its synergetic activity
with the drug.13 The antifungal and antibacterial proper-
ties of a range of copper(II) complexes have been evalu-
ated against several pathogenic fungi and bacteria.14 In
the literature, it has also been reported that organic lig-
ands exhibit enhanced antibacterial activity once coor-
dinated to copper.14b
Oxidation reactions play a significant role in organic syn-
thesis and presently there is a demand for more selective
and efficient oxidation methods.1 In recent times, much
emphasis has been given to the need for sustainable and
environmentally friendly processes. So, the use of oxi-
dants such as molecular oxygen and hydrogen peroxide
which are environmentally friendly and produce no toxic
waste is highly desirable. The use of molecular oxygen
and hydrogen peroxide as the primary oxidant has sev-
eral benefits such as low cost, improved safety, “green”
and water as the sole byproduct.2
The oxidation of organic compounds catalyzed by
copper(II) complexes under mild condition has also
drawn significant attention over the last few decades.3
Relatively high abundance of copper in the Earth’s crust
and its redox properties make it ideally suited for cat-
alytic oxidation processes. In natural enzymes, several
copper containing enzymes are known with mono-, di-,
tri- or polynuclear Cu centers that catalyze mild and
highly selective oxidative transformations.4 These
The carboxamide group, [–C(O)NH–], ubiquitous
throughout nature in the primary structure of proteins,
is versatile and important ligand construction unit for
synthetic coordination chemists.15 It is noteworthy to
∗For correspondence
1451

1452
Suvendu Samanta et al.
mention that neutral amides predominately coordinate 1126(m), 1033(m), 783(m), 761(m), 657(m), 503(m).
to metal ion via the lone pair on the carbonyl oxygen UV-Vis (in MeOH) [λmax, nm (ε/M−1 cm−1)]: 756 (86).
atoms whereas deprotonated amides preferably coor-
dinate to metal ions via their amido nitrogen atom.16
2.2b [Cu(HL)2(ClO4)2](2): Ligand HL (2 mmol, 0.23 g)
The presence of deprotonated carboxamido-N coordi-
was dissolved in 25 mL of methanol and a 10 mL solu-
nation in some metalloenzymes has also motivated the
tion of Cu(ClO4)2·6H2O (1 mmol, 0.37 g) in methanol
synthetic chemists to design ligand systems with the
was added to it. Solution was stirred for 1 h during
amide functional group.17 A wide variety of pyridine 2-
which time a deep blue compound appeared. This was
carboxamide ligands have been synthesized for inves-
filtered and recrystallized from acetonitrile. Yield 0.35 g
tigating their coordination properties with metal.18–23
(80%). Found. C, 28.56; H, 2.51; N, 11.23. Calcd. For
Structural investigation of some copper(II) complexes
C12H12Cl2CuN4O10: C, 28.45; H, 2.39; N, 11.06. FT-
of the simplest pyridine 2–carboxamide ligand (HL)
IR (KBr, ν/cm−1) 3215(w, br), 3026(s, br), 1666(s),
have been reported previously.24 But systematic studies
1560(s), 1438(m), 1037(w), 1145(s), 1110(s), 1087(s),
of their spectroscopic and electrochemical behaviours
786(m), 676(m), 661(m), 630(m). UV-Vis (in MeOH)
are lacking. In the present work, we report X-ray struc-
tural characterization, pH induced co-ordination mode
[λmax, nm (ε/M−1 cm−1)]: 748 (115).
switching phenomenon, peroxidative oxidation catalysis,
antimicrobial activity of water soluble discrete mono-
nuclear complexes of copper(II) using the simplest pyri-
dine 2–carboxamide ligand (HL) and its deprotonated
form.
2.2c [Cu(HL)2(SCN)2] (3): 5 mL methanolic solution
ofCuCl2·6H2O (0.24 g,1 mmol) was added to a solution
ofligandHL(2mmol,0.23g) in 20 mL methanol. 10 mL
aqueous solution of NH4SCN (2.1 mmol, 0.16 g) was then
added to the resulting faint blue mixture and solution
turned to deep green. Solution was stirred for another
20 min at room temperature. The deep green product
was filtered, dissolved in methanol and allowed to stand
at room temperature for slow evaporation. Green crys-
tals were obtained after three days. Yield: 0.32 g, (74%).
Found. C, 39.52; H, 2.81; N, 19.90. Calcd. For C14H12
CuN6O2S2: C, 39.66; H, 2.85; N, 19.82. FT-IR (KBr,
ν/cm−1) 3269(w, br), 3056(m, br), 2048(s), 1670(s),
1568(m),1494(w),1436(s), 1305(m), 1276(w), 1137(m),
1107(w) 1031(m), 952(w), 817(w), 792(w), 754(m),
732(m), 678(m), 663(m). UV-Vis (in MeOH) [λmax, nm
(ε/M−1 cm−1)]: 672 (60).
NH2
N
O
HL
2. Experimental
2.1 Materials
All reagents and solvents were purchased from commer-
cialsourcesandwereusedasreceived.Thesolventswere
purified and dried according to standard methods.25
Ligand pyridine 2–carboxamide (HL) was synthesized
according to the method reported earlier.26
2.2d [CuL2]·8H2O (4): 1 mL of aqueous solution of
sodium hydroxide (2 mmol, 0.08 g) was added slowly
to aqueous solution of complex 1 (1 mmol, 0.40 g) and
stirred for about 5 min. Violet colored crystalline com-
pound that appeared was filtered off and washed with
cold ethanol and diethyl ether. Yield 0.35g (85%).
Anal. Calcd. for C12H26CuN4O10: C, 32.12; H, 5.80;
N, 12.55. Found: C, 32.04; H, 5.83; N, 12.45. FT–IR
(KBr, ν/cm−1) 3431(s, br), 3307 (w), 3240(w), 1631(s),
1597(s),1566(m),1440(s), 1271(m), 1047(w), 1022(w),
786(m), 649(w), 607(w), 567(w). UV-Vis (in MeOH)
[λmax, nm (ε/M−1 cm−1)]: 570 (80).
2.2 Synthesis of the metal complexes
2.2a [Cu(HL)2(H2O)2]Cl2 (1): 10 mL solution of CuCl2·
6H2O (1 mmol, 0.24 g) in methanol was added dropwise
to a methanolic solution (25 mL) of pyridine 2–carbox-
amide(HL,2 mmol, 0.23 g) with stirring at room temper-
ature. The stirring was continued for 1 h, during which
time a light blue compound was separated out. The pro-
duct was filtered off and recrystallized from methanol.
Yield 0.35 g (85%). Anal. Calcd. for C12H16CuCl2N4O4:
C, 34.92; H, 3.99; N, 13.36. Found: C, 34.75; H, 3.89;
2.3 Catalytic activity studies
N, 13.51. FT–IR (KBr, ν/cm−1) 3448(s), 3269(s, br), 2–12 mmol of hydrogen peroxide (30% in H2O) was
3078(s,br),1666(s),1568(s),1438(s),1309(w),1278(w), added to the catalyst (0.02 mmol) in 5 mL of acetonitrile

Catalytic activity studies
1453
in a two-neck round bottom flask fitted with a con- array spectrophotometer. Gas chromatographic analyses
denser. To this, HNO3 (0.2 mmol) was added followed were conducted using an Agilent Technologies 6890
by the addition of 1.0 mmol of substrate (toluene, ethyl N network GC system equipped with a fused silica
benzene or cyclohexane). The reaction mixture was capillary HP–5 column (30 m × 0.32 mm) and a FID
stirred for 6–14 h at 60◦C under atmospheric pressure. detector. X-band EPR measurements were carried out
After the reaction was over 90 μL of chlorobenzene using a JEOL JES-FA 200 instrument. 1H NMR spectra
was added as an internal standard and the substrate and were obtained using a Bruker Avance DPX 300 MHz
products from the reaction mixture were extracted with spectrometer. Electrochemical measurements were car-
10 mL diethyl ether and then triphenylphosphine (PPh3) ried out in N, N-dimethylformamide (DMF) at 25◦C
(1.0 g) was added to reduce the organo-hydroperoxides. under nitrogen atmosphere using a Bioanalytical Systems
Theresultantmixturewas stirred for 15min and then the BAS 100B electrochemical analyzer. The concentration
sample taken from the organic phase was analyzed by of the supporting electrolyte, tetramethylammonium
gas chromatography. The identification was done by the perchlorate (TEAP), was 0.1 M, while that of the com-
comparison with known standards. Blank experiments plex was 1 mM. Cyclic voltammetric (CV) and square
for the oxidation of substrates were carried out with- wave voltammetric (SWV) measurements were carried
out any catalyst keeping other experimental conditions outusingathree-electrodeassemblycomprisingaglassy
unaltered.
carbon or platinum working electrode, a platinum aux-
iliary electrode, and an aqueous Ag/AgCl reference
electrode. Under the given experimental conditions, the
potential of the external standard ferrocene/ferrocenium
(Fc/Fc+)couplewasmeasuredat+0.390 Vvs. Ag/AgCl.
2.4 Antimicrobial activity
The antimicrobial activities of all the synthesized cop-
per(II) complexes were investigated against bacterial
strains Gram-positive Bacillus subtilis (MTCC 441),
Staphylococcus aureus (MTCC 96), Gram-negative
Escherichia coli (MTCC 2939), Pseudomonas aerug-
inosa (MTCC 2453), Klebsiella pneumonia (MTCC
618) and yeast Saccharomyces cerevisiae (MTCC 170),
Candida albicans (MTCC 227) following reported
method.27 The stock solutions (1 mg mL−1) of the com-
plexes were prepared by dissolving 10 mg of the test
compound in 10 mL of water. The stock solution was
suitably diluted with sterilized distilled water to get
dilution in between 400–3 μg mL−1.
The bacteria were sub-cultured in Müller-Hinton
agar. The Petri dishes were incubated for 24 h at 37◦C.
The fungi were sub-cultured in potato dextrose agar
medium. The Petri dishes were incubated for 48 h at
37◦C. Activity was determined by measuring the diam-
eter of the zone (mm) showing complete inhibition of
microbial growth that a clear zone surrounding the test
sample (in sterile disc) where bacterial growth does
not occur (or is inhibited). The growth of the bacteria
and fungi were measured by observing the minimum
inhibitory concentration.
2.6 X-ray crystallography
Crystals suitable for structure determinations of 1, 2, 3
and 4 were obtained by slow evaporation of their water-
methanol solutions. The crystals were mounted on glass
fibers using perfluoropolyether oil. Intensity data were
collected on a Bruker–AXS SMART APEX diffrac-
tometer at 123(2) K using graphite-monochromated
Mo-Kα radiation (λ = 0.71073 Å). The data were pro-
cessed with SAINT28 and absorption corrections were
made with SADABS software.28 The structures were
solved by direct and Fourier methods and refined by
full-matrix least-squares methods based on F2 using
SHELX-97.29 For the structure solutions and refine-
ments the SHELX-TL software package30 was used.
The non-hydrogen atoms were refined anisotropically,
while the hydrogen atoms were placed at geometrically
calculated positions with fixed thermal parameters.
Crystal data and details of structure determination for
complex 4 are summarized in table 1.
3. Results and Discussion
2.5 Physical measurements
3.1 Synthesis and characterization
Elemental (C, H and N) analyses were performed on The synthesis of ligand pyridine 2-carboximidine was
a Perkin-Elmer 2400 II elemental analyzer. IR spectra carried out following the method reported earlier.26 The
were recorded using KBr disks on a Shimadzu FTIR mononuclear copper(II)complexes, [Cu(HL)2(H2O)2]Cl2
8400Sspectrometer. Theelectronic spectra were record- (1)and[Cu(HL)2(ClO4)2](2)areobtainedbythereacting
ed at room temperature using an Agilent 8453 diode methanolic solution of ligand with CuCl2·6H2O or

1454
Suvendu Samanta et al.
Table 1. Crystallographic data for [Cu(L)2]·8H2O (4)a.
partial single bond character, C=O vibration appears at
much lower frequency (1631 cm−1) and a strong peak
due to C=N is also observed at 1597 cm−1.
4
The compound 2 shows four characteristic ClO4−
vibrations for coordinated perchlorate at 1145, 1115,
1088 and 627 cm−1. On the other hand, compound 3
exhibits a strong band at 2048 cm−1 with an ill-defined
shoulder at lower energy side due to the presence of
thiocyanate group.
Empirical formula
M
C12H18N4O6Cu
377.84
T , K
Crystal system
Space group
298(2)
Triclinic
P-1
a/Å
b/Å
10.6100(8)
11.1360(9)
14.7224(11)
68.237(2)
73.419(2)
83.203(2)
1548.2(2)
4
c/Å
α/◦◦
γ/
3.2 Description of crystal structure
β/◦
The X-ray crystal structures of complexes 1–4 have
been determined. It should be mentioned that after
determination of the structures of the compounds 1, 2
and 3, we found that the X-ray structure of these three
compounds have been reported earlier.24 The struc-
ture determination of 2 by us has been made at 120 K
whereas the reported one was made at 293 K. Accord-
ingly, the unit cell parameters found in the present
case are relatively shorter as compared to the earlier
reported values.24 The influence of temperature can be
appreciated by comparing the unit cell volume, which
is 442.3(2) Å3 at 120 K as against 457.26(15) Å3 at
293 K. In terms of Cu–N and Cu–O(amide) distances,
the difference observed in the two sets of studies are
insignificant, albeit the Cu–O(ClO4) distance reported
here [2.612 (2) Å] is somewhat shorter compared to the
reported value [2.649(3) Å]. The thermal ellipsoid plot
of the compounds 1–3 are shown in figures S1–S3 and
relevant bond distances and bond angles are given in
tables S1–S3. Crystal data and details of structure deter-
minations for complex 1–3 are summarized in table S4.
Figure S4–S6 show intermolecular hydrogen bonding
network in compounds 1–3.
U/Å3
Z
D/g cm−3
1.621
1.448
μ/mm−1
F(000)
780.0
Crystal size/mm
No. of measured reflections
No. of observed reflections
Parmeter refined
No. of reflections [I > 2σ (I)]
Goodness of fit, S[a]
0.38×0.22×0.18
21791
6941
486
4963
1.005
Final R1[b], wR2[c] [I > 2σ (I)]
R1[b], wR2[c] (all data)
ꢀ
0.0373 , 0.0917
0.0607, 0.1027
aS = [ w(Fo2–Fc2)/(N − P)]1/2 where N is the number
of data and P the total number of parameters refined. [b]
2
R1(F) = ꢁFo| – |Fcꢁ |Fo|. [c] wR2(F2) = [ w(Fo
–
2 2
2 2 1/2
Fc ) / w(Fo ) ]
.
Cu(ClO4)3·6H2O, respectively, while the complex
[Cu(HL)2(SCN)2] (3) is prepared by the direct reaction
between the ligand HL, CuCl2·6H2O and NaSCN in
a 1:2:1 ratio in methanol medium. Both compounds 1
and 2 upon treatment with 2 equiv of aqueous alkali,
undergo deprotonation to produce the compound
[CuL2]·8H2O (4).
3.2a [CuL2]·8H2O(4): In compound 4,the asymmetric
The IR spectra of the complexes exhibit several unit contains three independent [CuL2] molecules and
diagnostic features. A weak band observed between eight water molecules of crystallization. A thermal ellip
3080 and 3025 cm−1 in 1–3 is due to the hydrogen soid plot of the structural arrangement of the asymmet-
bonded N-H stretching vibration of the amide –NH2 ric unit is shown in figure 1 and the relevant metrical
group. The free ligand has characteristic IR band at parameters involving the metal centers are given in
1680 cm−1 due to amide I [(C=O)] vibration. The table 2. In all the three units, the coordination environ-
metal-coordinated C=O vibration in the compounds ment around the four-coordinated metal centre [CuN4]
1–3 are observed in between 1670–1665 cm−1. On the may be considered perfect square planar. The Cu-N
other hand, compound 4 exhibits two strong bands at (pyridine) distances are almost equal lying in all the
1631 and 1597 cm−1, due to C=O and C=N vibrations, units between 2.0179 (19) to 2.027 (2) Å but somewhat
respectively. In a deprotonated N-coordinated amide longer relative to the Cu-N (amide) distances which
group it is expected that the negative charge would be are also nearly identical, lying in all the units between
delocalized along the amide C–N and C–O bonds, lead- 1.924 (2) to 1.932 (2) Å. The cisoid angles vary from
ing to two resonance forms which have different C–N 81.26(8)◦ to 98.89(8)◦, while the transoid angles are
and C–O bond lengths. These bonds can also be inter- 175.24(7)◦ and 180.00(2)◦. Thus, copper centres have a
mediate between double and single bonds. Due to this near perfect square planar geometry.

Catalytic activity studies
1455
connecting dangling water molecules. Both values are
comparable to the average Owater · · · ·Owater contact of
ca. 2.85 Å found in liquid water and in other H2O
clusters hosted by metal-organic frameworks.31 The
O· · · ·O· · · ·O angles range from 84.24◦ to 131.02◦
(table S5), considerably deviating from the preferred
ideal tetrahedral geometry of water. The tetradecameric
water clusters are further linked by hydrogen bonds
to form infinite parallel two dimensional sheet inter-
calated in voids of the host metal-organic matrix of 4
(figure S7). The four dangling H2O molecules of every
water cluster also have a structure-stabilizing effect,
each of them being hydrogen bonded to oxygen atoms
of amide group of a pyridine 2-carboxamide ligand.
Thus, (H2O)14 cluster forms two-dimensional sheets,
and copper complexes act as connectors to those sheets
to form three-dimensional packing arrays (figure S7).
The packing diagram of compound 4 also reveals
the presence of two intermolecular π–π interaction
between the pyridine ring C1, C2, C3, C4, C5, and N1
with the pyridine ring C13, C14, C15, C16, C17, and
N5 [the distance between the two centroids is 3.701 Å]
and second one is between the pyridine ring C13, C14,
C15, C16, C17, and N5 with the pyridine ring C7, C8,
C9, C10, C11, and N3 [the distance between the two
centroids is 3.693 Å].
The N, N-coordination mode of pyridine-2-carbox-
amide is quite rare.24d,e The structure of [Cu(pia)2]·
4H2O24d and [Cu(pia)2]·2H2O24e have been reported
previously. The main difference between the previous
two complexes with our complex, apart from the num-
ber of co-crystallized water molecules, is the presence
of three independent [CuL2] molecules in an asymmet-
ric unit.
3.3 Electronic spectra
Figure 1. An ORTEP representation of the molecule
[Cu(L)2]·8H2O (4) showing 50% probability displacement
ellipsoids. Hydrogen atoms and waters of crystallization are
removed for clarity.
The absorption spectroscopic behaviour of compounds
1–4 have been studied in methanol. The Vis–NIR spec-
tral data for these compounds in methanol are given
in Experimental Section.The six-coordinated octahedral
Interestingly, this framework hosts chains of hydrogen- copper(II) complexes, 1–3 exhibit a broad band in the
bonded clusters of molecules of crystallization water visible and Near IR range due to d–d transition (675–
(figure S7). Each tetradecameric, (H2O)14, cluster is form- 760 nm) though three transition bands are expected for
ed by a cyclic water decamer and four pendent water tetragonally elongated octahedral geometry. The broad-
molecules (figure 2a).Among the eight water molecules, ness of these bands with a low energy tail are indicative
five water molecules are generated by an inversion cen- of the presence of more than one transitions at lower
tre to form a cyclic water decamer. The cyclic decamer energies, as expected for copper(II) in a tetragonally elon-
assumes a boat-chair-boat conformation (figure 2b). In gated octahedral environment. In general, the electronic
the (H2O)14 cluster, the average Owater · · · ·Owater sepa- spectra of octahedrally coordinated copper(II) complexes
ration of ca. 2.789 Å (table S5) within the decameric are dominated by Jahn-Teller-induced tetragonal distor-
core is almost same as that of ca. 2.829 Å [O(7)– tions, which give rise to a characteristic broad band
H(7B)· · · O(5) and O(12)–H(12A)· · · O(11), table S5] formed due to overlap of the three bands. Indeed,

1456
Suvendu Samanta et al.
Table 2. Selected bond lengths [Å] and angles [◦] for [Cu(L)2]·8H2O (4)a.
4
Cu1–N1
Cu1–N2
Cu1–N3
Cu1–N4
2.0179(19)
1.928(2)
2.0233(19)
1.924(2)
Cu2–N5
Cu2–N6
Cu2–N5A
Cu2–N6A
2.027(2)
1.929(2)
2.027(2)
1.930(2)
Cu3–N7
Cu3–N8
Cu3–N7B
Cu3–N8B
2.013(2)
1.931(2)
2.013(2)
1.932(2)
N1–Cu1–N2
N1–Cu1–N3
N1–Cu1–N4
N2–Cu1–N3
N2–Cu1–N4
N3–Cu1–N4
81.39(8)
175.24(7)
98.89(8)
98.34(8)
178.45(9)
81.26(8)
N5–Cu2–N6
N5–Cu2–N6A
N6–Cu2–N6A
N5A–Cu2–N6A
N5–Cu2–N5A
N5A–Cu2–N6
81.49(8)
98.51(8)
180.00(12)
81.49(8)
180.00(9)
98.51(8)
N7–Cu3–N8
N7–Cu3–N8B
N7 –Cu3–N7B
N7B–Cu3–N8
N8–Cu3–N8B
N7B–Cu3–N8B
81.99(9)
98.01(9)
180.00(19)
98.01(9)
180.00(2)
81.99(9)
[a] ‘A’ indicates atoms at (-x, -y, 1-z) and ‘B’ indicates atoms at (2-x, -y, -z).
Figure 2. (a) Perspective representations of the tetradecameric, (H2O)14,
cluster formed by a cyclic water decamer and four pendent water molecules.
(b) The boat-chair-boat conformation of cyclic water decamer.
Figure 3. Visible and near infrared absorption spectra of the compounds (a)
[Cu(HL)2(H2O)2]Cl2 (1) and (b) [Cu(L)2]·8H2O (4) in methanol (1×10−2 M).
The three bands below the spectrum are obtained by Gaussian line-shape
analysis and added together give the experimental line.
deconvolution of the absorption spectra of 1–3 in metha- 3.4 EPR spectra
nol by Gaussian line-shape analysis gives rise to three
X-band EPR spectrum of the complexes 1–4, have been
carried out in 4:1 ethanol-methanol solution at 77 K
(figure 4). EPR spectra of complexes 1–3, show axially
symmetric copper(II) centres32 with g⊥ = 2.073, gꢁ =
2.31; g⊥ = 2.045, gꢁ = 2.19; g⊥ = 2.046, gꢁ = 2.34 for
complexes 1, 2 and 3, respectively. These values also
indicate that the ground state of Cu(II) is predominantly
peaks. The spectral data and the deconvoluted peak
positions of 1–4 in methanol are listed in table S6.
Square planar copper(II) complex, 4, also shows one
broad band at relatively higher energy (570 nm) due to
d–d transition.Deconvolution of the absorption spectrum
of 4 in methanol by Gaussian line-shape analysis gives
rise to three peaks as expected for square planar
Cu(II) complexes. Figure 3 shows deconvoluted peaks
in compounds 1 and 4.
2
dx−y2. Complex 4 exhibits a slight rhombic signal with
gꢁ (2.24) >g⊥(2.063)andgav = 2.063. This type of signal

Catalytic activity studies
1457
due to d–d transition. Upon addition of sodium hydrox-
ide solution to an aqueous solution of compound 1, the
light blue solution (λmax = 685 nm) turned gradually
violet (λmax = 575 nm) in accordance with the forma-
tion of the [CuL2]·8H2O (4) complex (figure 5). No fur-
ther change in spectrum was observed after addition of
two equivalent of alkali. Conversely, upon the addition
of hydrochloric acid to a violet solution of [CuL2]·8H2O
(4), the initial absorption at λmax = 575 nm was replaced
by a band at 685 nm characteristic of the compound 1
in aqueous solution.
We have also carried out potentiometric titration to
obtain the pKa values for the amide hydrogens. The
potentiometric titration was carried out over a pH range
of 3.5–10.5. The titration of [Cu(HL)2(H2O)2]Cl2 (1)
(10 mM) in water at 25◦C and I = 0.1 M (sodium per-
chlorate) gave one well-defined inflection. The pKa
value was determined from the titration curve to be 7.15
and the deprotonating number was determined to be two
at the inflection point as shown in figure S9.
Figure 4. X-band EPR spectra of the complexes 1–4 under
nitrogen in a frozen EtOH–MeOH (4:1) solvent mixture at
77 K.
3.7 Oxidation of Toluene, Ethyl Benzene and
Cyclohexane
is common for square planar copper(II) complexes with
2
(dx−y2) magnetic orbital.32
We investigated the catalytic potential of compounds 1,
2 and 3 for the oxidation of various organic substrates
such as toluene, ethyl benzene and cyclohexane by
aqueous hydrogen peroxide under mild conditions. The
reaction occurs in acetonitrile solution, and nitric acid
in low concentration is a necessary component of the
reaction mixture. It has been reported earlier33 that the
nitric acid increases the unsaturation at the metal cen-
ter by protonation of the ligand of the catalyst and
hence increases oxidative properties of the catalyst. In
3.5 Electrochemistry
The electrochemical characteristics of complexes 1–4
in dimethyl formamide have been investigated by cyclic
and square wave voltammetric methods. The E1/2 values
obtained by the two methods agree within 5 mV. The
conventional accuracy of the E1/2 values by these tech-
niques, of course, is taken as 10 mV. In case of com-
plexes 1–3, the metal-centered reductions take place
quasi-reversibly. The relevant electrochemical data for
the complexes 1–4 are given in table S7. In dimethyl
formamide, the E1/2 value of [CuL2(H2O)2]Cl2 (1) is
435 mV with ꢀEp = 130 mV (figure S8a), that of
[Cu(HL)2(ClO4)2] (2) is 60 mV with ꢀEp = 115 mV
(figure S8b) and finally for the [Cu(HL)2(SCN)2] (3)
the E1/2 and ꢀEp values are 390 mV and 135 mV
(figure S8c). Square planar complex [CuL2]·8H2O (4)
exhibits one irreversible reduction peak (figure S8d) at
about 570 mV (Ep,c).
3.6 pH induced co-ordination mode switching
The possible inter-conversion between [Cu(HL)2(H2
O)2]Cl2 (1) and [CuL2]·8H2O (4) through co-ordination
mode switching was then investigated. In aqueous
medium, [Cu(HL)2(H2O)2]Cl2 (1) and [CuL2]·8H2O (4)
show one broad band at 685 and 575 nm, respectively,
Figure 5. Change in absorption spectra of 1 in aqueous
solution upon the addition of OH− ion.

1458
Suvendu Samanta et al.
Table 3. Oxidation[a] of toluene by complexes 1, 2 and 3.
Yield (%)[b]
n(catalyst) Time/h Benzyl Alcohol Benzaldehyde Total Selectivity (%) TON[c]
n (H2O2)/
Benzyl Alcohol
Entry
Catalysts
1
1
2
3
4
5
6
7
100
200
400
500
600
600
600
10
10
10
10
10
8
46.4
51.2
49.1
49.9
51.0
51.4
51.1
24.1
30.8
31.9
36.4
41.4
40.6
40.9
70.5
78.4
81.0
86.3
92.4
92.0
92.0
65.8
65.3
60.6
57.8
55.2
55.9
55.6
35.25
39.20
40.50
43.15
46.20
46.00
46.00
6
8
9
10
11
12
13
14
2
3
100
200
400
500
600
600
600
12
12
12
12
12
10
8
48.1
49.5
49.8
50.3
53.1
54.5
52.9
19.9
23.0
26.1
29.8
36.6
36.4
37.3
68.0
72.5
75.9
80.1
89.7
90.9
90.2
70.7
68.3
65.6
62.8
59.2
59.9
58.6
34.00
36.25
37.95
40.05
44.85
45.45
45.10
15
16
17
18
19
22
21
100
200
400
500
600
600
600
14
14
14
14
14
12
11
28.2
35.4
36.8
37.7
40.6
40.0
39.9
20.0
25.9
29.9
33.3
36.2
35.2
33.8
48.2
61.3
66.7
71.0
76.8
75.2
73.7
58.4
57.7
55.2
53.1
52.9
53.2
54.1
24.10
30.65
33.35
35.50
38.40
37.60
36.85
22
23
24
Cu(ClO4)2·6H2O
CuCl2·6H2O
None
600
600
600
12
12
12
5.9
6.1
2.7
3.1
3.3
2.1
9.0
9.4
4.8
–
–
–
4.50
4.70
2.40
[a] solvent = CH3CN, 333 K, oxidant = hydrogen peroxide. [b] Calculated after treatment with PPh3. [c] TON: turn over
number = moles of product/mole of catalyst.
presence of nitric acid, decomposition of peroxide, substrate is carried out without nitric acid, it results in
which is present in the reaction medium, is slowed much lower yields for all complexes. The amount of
down and thus, the stability of peroxo intermediate is oxidized products increased drastically with addition
enhanced. The yield has been optimized by varying the of acid up to 10 equiv, beyond which the yield drops.
relative proportions of nitric acid and hydrogen perox- The relatively low amount of acid required to reach a
ide with respect to the catalysts, and also by varying the maximum of activity of 1–3 is in agreement35 with the
reaction time. The reaction gives alkyl or aryl hydroper- low-coordination n−umber of copper and the presence
oxides which are gradually transformed into the corre- of labile H2O, ClO4 or SCN− ligands. On the basis of
sponding ketones (aldehydes) and alcohols. The final these observations, further catalytic activity tests were
concentrations of the ketones (aldehydes) and alcohols run at the n(HNO3)/n(catalyst) molar ratio of 10. Sim-
were measured after the addition of PPh3 in accord with ple copper salts, like Cu(NO3)2·6H2O or CuCl2·6H2O,
34
´
the method developed earlier by Shulpin. As cata- under the same reaction conditions exhibit a much lower
lytic reaction was done in presence of acidic medium, activity towards oxidation of all substrates (entries 22,
compound 4 has not been considered as catalyst.
23 and 24 of table 6), under the same experimental con-
The influence of various parameters such as the rel- ditions. So it is evident that the presence of N and O
ative amounts of nitric acid, hydrogen peroxide, and donor ligands is quite relevant. The oxidation reactions
catalyst on the catalytic activity has been investigated were also carried out at 333 K. On increasing tempera-
aiming at the optimization of the oxidation process. No ture from room temperature to 333 K, yield increases
oxidation products (or only traces) were obtained in significantly. When the temperature of the oxidation reac-
the absence of catalyst or hydrogen peroxide. We have tions was increased further, no significant improvement
also verified that the presence of nitric acid is important in yield was observed. So we have performed all the
in such oxidation reactions. When the oxidation of the catalytic reactions at 333 K.

Catalytic activity studies
1459
The results of the oxidation of toluene, ethyl benzene The results of cyclohexane oxidation are shown in
and cyclohexane are shown in tables 3, 4 and 5, respec- table 5. It is clearly seen that the conversion of cyclohex
tively. It can be clearly seen from table 3 that among all ane is influenced by the relative amounts of hydrogen
the catalysts, complex 1 is the most efficient catalyst for peroxide and the reaction period. The corresponding
the oxidation of toluene with 92.0% (entry 7, table 3) products are cyclohexanol and cyclohexanone in the
conversion of toluene when the n(H2O2)/n(catalyst) case of cyclohexane oxidation. Among three catalysts,
ratio is 600, with a reaction time of 6 h. The maximum again complexes 1 and 2 are almost equally efficient
conversions of toluene achieved are 90.2% (entry 14, catalyst for the oxidation of cyclohexane with 862%
table 3) and 73.7% (entry 21, table 3) with complexes 2 (entry 6, table 5) and 841% (entry 12, table 5) conver-
and 3, respectively, in the presence of different amounts sion of cyclohexane when the n (H2O2)/n(catalyst) ratio
of hydrogen peroxide and with somewhat longer reac- is 500, with a reaction time of 6 h. The maximum con-
tion time. It has been observed for all the catalytic con- version of cyclohexane achieved is 73.0% (entry 18,
versions that the yield increases with time. Catalytic table 5) with complex 3, in the presence of different
conversions are also dependent on the amount of oxi- amounts of hydrogen peroxide.
dant used. The results of the oxidation of toluene show
good conversion rate as well as TON and TOF (table 3, is schematically given in scheme S1. Metal-assisted
entries 7, 14 and 21). decomposition of H2O2 could lead to the formation
The proposed mechanism of the catalytic conversion
Among three catalysts, complexes 1 and 2 are almost of hydroxyl radical (HO·) which, upon H-abstraction
equally efficient catalyst for the oxidation of ethyl ben- from RH, would form the alkyl radical (R·). The for-
zene with 74.0% (entry 7, table 4) and 72.2% (entry 14, mation of the HO· radical involves proton-transfer steps
table 4) conversion of ethyl benzene when the n (H2O2)/ among H2O2, hydroperoxo and peroxo metal-species,
n(catalyst) ratio is 600, with a reaction time of 8 h. as suggested earlier,35b,f which can be promoted by the
The maximum conversion of ethyl benzene achieved N,O-donor pyridine 2–carboxamide ligand. The alkyl
is 57.6% (entry 21, table 4) with complex 3, in the radical, on reaction with a metal-peroxo intermediate
presence of different amounts of hydrogen peroxide.
species LCu–OOH, could form ROOH which, upon
Table 4. Oxidation[a] of ethyl benzene by complexes 1, 2 and 3.
Yield (%)[b]
n (H2O2)/
1-phenylethanol
Entry
Catalysts
1
n(catalyst)
Time/h
1-phenylethanol
Acetophenone
Total
Selectivity (%)
TON[c]
1
2
3
4
5
6
7
100
200
400
500
600
600
600
10
10
10
10
10
9
36.7
39.1
40.1
41.4
41.9
41.2
41.2
17.4
19.9
24.0
29.0
34.0
33.8
32.8
54.1
59.0
64.1
70.4
75.9
75.0
74.0
67.8
66.3
62.6
58.8
55.2
54.9
55.7
27.05
29.50
32.05
35.20
37.95
37.50
37.00
8
8
9
10
11
12
13
14
2
3
100
200
400
500
600
600
600
10
10
10
10
10
9
33.7
36.2
39.9
44.6
47.2
48.0
47.9
12.7
15.4
17.5
21.2
27.5
25.7
24.3
46.4
51.6
57.4
65.8
74.7
73.7
72.2
72.7
70.1
69.5
67.8
63.2
65.1
66.3
23.20
25.80
28.70
32.90
37.35
36.85
36.10
8
15
16
17
18
19
20
21
100
200
400
500
600
600
600
12
12
12
12
12
11
10
23.2
26.6
27.9
31.4
32.7
31.5
32.1
11.0
14.9
17.7
22.9
26.6
26.7
25.5
34.2
41.5
45.6
54.3
59.3
58.2
57.6
67.7
64.2
61.1
57.9
55.2
54.1
54.1
17.10
20.75
22.80
27.15
29.65
29.10
28.80
[a] solvent = CH3CN, 333 K, oxidant = hydrogen peroxide. [b] Calculated after treatment with PPh3. [c] TON: turn over
number = moles of product/mole of catalyst.

1460
Suvendu Samanta et al.
Table 5. Oxidation[a] of cyclohexane by complexes 1, 2 and 3.
Yield (%)[b]
n(H2O2)/
n(catalyst)
Cyclohexanol
Selectivity (%)
Entry
Catalysts
1
Time/h
Cyclohexanol
Cyclohexanone
Total
TON[c]
1
2
3
4
5
6
100
200
400
500
500
500
10
10
10
10
9
39.5
42.8
43.8
45.8
46.8
48.0
26.6
29.7
33.1
35.7
40.1
38.2
66.1
72.5
76.9
81.5
86.9
86.2
59.7
59.1
56.9
56.2
53.8
55.7
33.05
36.25
38.45
40.75
43.45
43.10
8
7
8
9
10
11
12
2
3
100
200
400
500
500
500
10
10
10
10
9
43.5
45.7
45.7
51.3
58.1
58.1
16.0
17.9
20.8
24.3
26.7
26.0
59.5
63.6
66.5
75.6
84.8
84.1
73.1
71.9
68.7
67.9
68.5
69.1
29.75
31.80
33.25
37.80
42.40
42.05
8
13
14
15
16
17
18
100
200
400
500
500
500
11
11
11
11
11
10
23.5
31.0
37.1
38.4
44.2
41.8
13.0
17.4
22.4
25.2
29.7
31.2
36.5
48.4
59.5
63.6
73.9
73.0
64.3
64.1
62.3
60.3
59.8
57.2
18.25
24.20
29.75
31.80
36.95
36.50
[a] solvent = CH3CN, 333 K, oxidant = hydrogen peroxide. [b] Calculated after treatment with PPh3. [c] TON: turn over
number = moles of product/mole of catalyst.
Table 6. Minimum inhibitory concentrations (MIC) for the complexes 1–4 (μg ml−1).
Antibacterial activity
Antifungal activity
C. albicans S. cerevisiae
Compound
S. aureus
B. subtilis
E. coli
P. aeruginosa
K. pneumonia
1
2
3
4
>50
25
>50
>50
>50
12.5
6.25
>50
25
50
>50
>50
50
>50
50
>50
>50
>50
>50
50
>50
>50
6.25
12.5
6.25
12.5
>50
25
metal-promoted homolytic decomposition, would lead 3.8 Antimicrobial activity
to O-centred organoradicals. These are the oxyl (RO·)
and the peroxyl (ROO·) radicals formed upon O-O
and O-H bond cleavage, respectively, from which the
final oxidation products could be obtained. Alcohols
(ROH) could then be formed by H-abstraction from
RH by RO· or upon decomposition of ROO· to both
alcohol and aldehyde/ketone.34a,36b,d,37 The presence of
ROOH at the end of the reaction is shown by the
increase of the amount of alcohol with a correspond-
ing decrease in that of the aldehyde/ketone, upon treat-
ment of the final reaction solution with an excess of
PPh3 prior to the GC analysis, according to the method
The amide ligand and its copper(II) complexes (1–
4) were evaluated for in vitro antibacterial activity
against Gram-positive Bacillus subtilis, Staphylococcus
aureus, Gram-negative Escherichia coli, Pseudomonas
aeruginosa, Klebsiella pneumonia and in vitro antifun-
gal activity against Saccharomyces cerevisiae, Candida
albicans. Muller Hinton, Potato dextrose broth and agar
were employed for bacterial and fungal growth, respec-
tively. Minimum Inhibitory Concentrations (MIC) were
determined by disc diffusion method27 and are pre-
sented in table 6. Neither the free ligand nor the cop-
per(II) salts inhibited growth of the tested organisms at
concentrations below 500 μg mL−1.
´
reported by Shulpin. Reduction of ROOH by PPh3
gives alcohol, thus eliminating the decomposition of
ROOH to both alcohol and aldehyde/ketone in the gas
chromatograph.
Table 6 indicates that metal complex 2 and 3 showed
good antibacterial activity against Bacillus subtilis and

Catalytic activity studies
1461
acknowledge DST-India for the Fast Track Project
(No.SR/FT/CS-022/2009). S.S. is indebted to CSIR,
India for his JRF [08/003(0083)/2011-EMR-1]. Thanks
are due to the Department of Science and Technology,
Government of India for establishing the National X-ray
diffractometer facility at the Department of Inorganic
Chemistry, Indian Association for the Cultivation of
Science.
Pseudomonas aeruginosa. Complex 2 showed no activ-
ity against fungi whereas complex 4 exhibited no
antibacterial activity. It can be noted that complex 3
inhibited growth of both bacterial and fungal strains.
On the other hand, complex 1 behaves neither as good
antimicrobial agent nor as antifungal agent. Complexes
3 and 4 showed good antifungal activity against Sac-
charomyces cerevisiae. All the metal complexes exhib-
ited better activity than ligand and copper(II) salts such
as Cu(ClO4)2·6H2O and CuCl2·6H2O.
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Supplementary Information
ORTEP representations and H-bonding (figures S1–
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tables S1–S7, scheme S1, synthesis and characteriza-
tion of the ligand HL, and crystal structure descriptions
for complexes 1 and 3 are available at www.ias.ac.in/
chemsci. CCDC-1011671(1), 1011672(2), 1011673(3)
and 1011674(4) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
We gratefully acknowledge Dr. Papu Biswas for his
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