Synthesis, spectral investigation of Ni(II) schiff base complexes: Antimicrobial activities and catalytic oxidation of alcohols

Article abstract of DOI:10.14233/ajchem.2016.19784

Air stable Ni(II) Schiff base complexes viz. [Ni(L¹)(PPh₃)] and [Ni(L²)(PPh₃)] [where L¹ and L² are dianions of Schiff base ligands, respectively] have been synthesized and characterized by analytical and spectral (electronic, FT-IR, ¹H, ¹³C and ³¹P NMR) methods. The assignment of all the aromatic carbon-hydrogen resonances is made on the basis of ¹H-¹³C HSQC spectrum of the complexes. The Schiff base ligands behave as a bibasic tridentate ligands and bonded through ONO and ONS mode. A square planar structure has been proposed on the basis of spectral data. Novel Ni(II) Schiff base complexes exhibited good antimicrobial activity towards the strains Staphylococcus epidermidis and Escherichia coli. Thermal and air stability of the complexes offer the advantage of oxidation of alcohols.

Full text of DOI:10.14233/ajchem.2016.19784

Asian Journal of Chemistry; Vol. 28, No. 8 (2016), 1682-1686
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2016.19784
Synthesis, Spectral Investigation of Ni(II) Schiff Base Complexes:
Antimicrobial Activities and Catalytic Oxidation of Alcohols
1,2
3
2,3
4
4
2,3,*
R. MADASELVI , N. PADMA PRIYA , M. JEYARAJ , C. ARUN PAUL , K. KALAIVANI , H. SHAHUL MEERAN2,5 and S. ARUNACHALAM
1Department of Chemistry, Arulmigu Kalasalingam College of Education, Virudhunagar-626 126, India
2Research and Development Centre, Bharathiar University, Coimbatore-641046, India
3Department of Chemistry, Sri Krishna College of Engineering and Technology, Coimbatore-641 008, India
4Department of Physics, Sri Krishna College of Engineering and Technology, Coimbatore-641 008, India
5Department of Chemistry, United Institute of Technology, Coimbatore-641 032, India
*Corresponding author: E-mail: drarunachalam.s@gmail.com
Received: 18 December 2015;
Accepted: 19 February 2016;
Published online: 30 April 2016;
AJC-17873
Air stable Ni(II) Schiff base complexes viz. [Ni(L1)(PPh3)] and [Ni(L2)(PPh3)] [where L1 and L2 are dianions of Schiff base ligands,
respectively] have been synthesized and characterized by analytical and spectral (electronic, FT-IR, 1H, 13C and 31P NMR) methods. The
assignment of all the aromatic carbon-hydrogen resonances is made on the basis of 1H-13C HSQC spectrum of the complexes. The Schiff
base ligands behave as a bibasic tridentate ligands and bonded through ONO and ONS mode. A square planar structure has been proposed
on the basis of spectral data. Novel Ni(II) Schiff base complexes exhibited good antimicrobial activity towards the strains Staphylococcus
epidermidis and Escherichia coli. Thermal and air stability of the complexes offer the advantage of oxidation of alcohols.
Keywords: Nickel(II), Schiff base complexes, Square planar nickel, Tridentate ligands, Oxidation of alcohols.
India. Infrared spectra were recorded as KBr pellets in the
4000-400 cm-1 region using a Perkin Elmer FT-IR 8000
spectrophotometer. Electronic spectra of all the complexes
were taken in dichloromethane solution in quartz cells. The
concentration of the complexes ranges around 0.02-0.3 N. The
spectra were recorded on a Systronics double beam UV-visible
spectrophotometer 2202 in the range 200-800 nm at room
INTRODUCTION
The chemistry of transition metal complexes with coordi-
nated phosphorous ligands has been known for a long time.
During the previous years we have been working on the coor-
dination chemistry and biological properties of different metal
complexes of isatin and other Schiff base ligands in order to
establish a possible relationship between chemical structure
and biological activity. Schiff bases have been reported to show
a variety of biological actions by virtue of the azomethine
linkage, which is responsible for various antibacterial, anti-
fungal, herbicidal, clinical and analytical activities [1-7].
Following this research line, here we report the synthesis and
characterization of Schiff bases of isatin with o-aminophenol
and o-aminothiophenol (Scheme-I) as well as some of their
nickel(II) complexes.
1
temperature. H and 13C NMR spectra for the ligand were
recorded using Bruker 500 MHz instrument in CDCl3 at room
temperature in Indian Institute of Science, Bangalore. Minimum
quantities of ligands were dissolved in deuterated CDCl3. 1H
NMR chemical shifts were referenced to tetramethylsilane
(TMS) as an internal solvent standard resonance and 13C NMR
chemical shifts were referenced to the internal solvent resonance.
Signals are quoted in parts per million as δ downfield from
internal reference.
Preparation of new tridentate Schiff base ligands:To an
ethanolic solution of isatin (0.147 g: 0.1 mmol) with o-amino-
phenol (0.109 g: 0.1 mmol)/o-aminothiophenol (0.125 g: 0.1
mmol) was added in 1:1 ratio with stirring in the magnetic
stirrer for about 0.5 h and then refluxed in a round bottomed
flask fitted with double surface condenser on the water bath
for about 6 h (Scheme-I). The resultant product was washed
with ethanol and the purity of the ligands were checked by TLC
EXPERIMENTAL
All the reagents used were ofAnalaR grade and purchased
from Sigma Aldrich chemicals and was used without further
purification. Melting points were recorded on a Veego VMP-
DS melting point apparatus and are uncorrected. The analysis
of carbon, hydrogen, nitrogen and sulphur were performed in
Vario EL III CHNS analyzer at Cochin University, Kerala,

Vol. 28, No. 8 (2016)
Synthesis of Ni(II) Schiff Base Complexes 1683
temperature (Scheme-III).A typical reaction using the comp-
lexes [Ni(L)(PPh3)] as a catalyst and alcohols as substrates at
a 1:100 molar ratio is described as follows.A solution of Ni(II)
Schiff base complexes (0.01 mmol) in 20 cm3 CH2Cl2 was
added to the solution of alcohol (1 mmol) under 1 atm oxygen
atmosphere at ambient temperature. The solution mixture was
stirred at room temperature for 6 h and the solvent was then
evaporated from the mother liquor under reduced pressure.
The residue was then extracted with petroleum ether (60-80 °C)
(20 cm3) and the carbonyl compounds were treated with 2,4-
dinitrophenylhydrazine, methanol and few drops of sulphuric
acid. The yellow colour product obtained is quantified as 2,4-
dinitrophenylhydrazone derivatives [5-7].
NH2
O
Reflux
6 h/EtOH
XH
XH
XH
+
N
N
O
keto-enol
N
H
O
N
OH
N
H
X = O / S
Scheme-I: Synthesis and keto-enol form of Schiff base ligands
and was column chromatographed on silica gel (petroleum
ether:ethyl acetate) (Yield ≈ 79 %). The C, H, N, S analysis
were recorded.
Preparation of new Ni(II) Schiff base complexes: To a
solution of NiCl2·6H2O (0.1 mmol) in ethanol (20 cm3) the
appropriate Schiff base (0.1 mmol) was added with PPh3 (0.1
mmol) in 1:1 molar ratio and heated under reflux in a round
bottomed flask fitted with a double surface condenser for 6 h
under anhydrous condition (Scheme-II). The solution was
then concentrated on the water bath to 3 cm3 and cooled. The
complex was precipitated by the addition of small quantity of
petroleum ether (60-80 °C) and recrystallized from CH2Cl2/
petroleum ether and dried in vaccuo (Yield about 76 %).
O
OH
NiII complex/6 h stirring
CH2Cl2/O2/RT
NiII complex/6 h stirring
OH
O
CH2Cl2/O2/RT
NiII complex/6 h stirring
CH2Cl2/O2/RT
OH
O
Scheme-III: Catalytic oxidation of alcohols
XH
XH
N
O
X
N
O
N
O
Ni
Reflux
RESULTS AND DISCUSSION
keto-enol
N
P
6 h/EtOH
N
H
N
H
+
The Ni(II) Schiff base complexes of the type [Ni(PPh3)(L)]
were synthesized by reacting equimolar solution of NiCl2·6H2O,
Schiff base ligands (H2L1 and H2L2) and triphenyl phosphine
in ethanolic medium, refluxed for 6 h. The complexes are non-
hygroscopic and are soluble in common organic solvents. In all
the reactions, it was found that Schiff bases behave as binega-
tive tridentate ligands. The analytical data obtained for the new
Ni(II) Schiff base complexes are agreed well with proposed
molecular formulae (Table-1).
Infrared spectra: The IR spectra of the ligands were
compared with those of the nickel complexes in order to
confirm the binding mode of the Schiff base ligands to the
nickel(II) atom in the complexes (Table-2). The free Schiff
base ligands showed a strong band in the region 1617-1615
cm-1, which is characteristic of the azomethine ν(C=N) group.
Coordination of the Schiff bases to the metal through the
nitrogen atom is expected to reduce the electron density in the
azomethine link and lower the ν(C=N) absorption frequency.
The band due to ν(C=N) is shifted to lower frequencies and
appears around 1592-1580 cm-1, indicating coordination of the
azomethine nitrogen to the nickel metal [1-6]. A strong band
observed at 1343 cm-1 in the free Schiff base H2L1 has been
assigned to phenolic C-O stretching. On complexation, this
band is shifted to a higher frequency at 1438 cm-1, indicating
.
NiCl2 6H2O + PPh3
X= O / S
Scheme-II: Synthesis of new Ni(II) Schiff base complexes
Antimicrobial activities: The in vitro cytotoxicity of the
complexes were screened in order to evaluate activity against
Staphylococcus epidermidis and Escherichia coli at 0.25 %,
0.50 % and 1 % concentration by disc diffusion method. Strep-
tomycin was used as a standard. The bacteria (Staphylococcus
epidermidis, Escherichia coli) were grown in nutrient broth
and incubated at 37 °C for 48 h followed by frequent subculture
to fresh medium and were used as test bacteria. Then the
petriplates were inoculated with a loop full of bacterial culture
and spread throughout the petriplates uniformly with a sterile
glass spreader. To each disc, the test samples and reference anti-
biotic (streptomycin) were added with a sterile micropipette.
The plates were then incubated at 35 2 °C for 24 h for
bacteria. Plates with disc containing respective solvents served
as control. Inhibition was recorded by measuring the diameter
of the inhibitory zone after the period of incubation [6].
Catalytic oxidation reactions by Ni(II) Schiff base
complexes: Catalytic oxidation of alcohols to corresponding
carbonyl compounds by Ni(II) Schiff base complexes was
carried out in the presence of oxygen atmosphere at ambient
TABLE-1
ANALYTICAL DATA OF Ni(II) SCHIFF BASE COMPLEXES
Elemental analysis (%): Calcd. (Found)
Ligands and
Complexes
Colour
m.p. (°C)
m.f.
m.w.
C
H
N
S
–
H2L1
Brown
Orange
Green
128
135
230
283
C14H10N2O2
C14H10N2OS
C32H23N2O2PNi
C32H23N2OPSNi
238.24
254.31
557.20
573.27
70.57 (70.21)
66.11 (66.01)
68.98 (68.86)
7.04 (66.97)
4.22 (4.15)
3.95 (3.76)
4.16 (4.09)
4.04 (3.91)
11.75 (11.56)
11.01 (10.93)
5.03 (4.91)
4.89 (4.79)
H2L2
12.6 (12.34)
–
5.59 (5.49)
[Ni(L1)(PPh3)]
[Ni(L2)(PPh3)]
Brown

1684 Madaselvi et al.
Asian J. Chem.
coordination through the phenolic oxygen. This has been
further supported by the disappearance of the broad band
ν(OH)around 3000 cm-1 in the complex [Ni(PPh3)(L1)], indi-
cating deprotonation of the phenolic proton prior to coordi-
nation [4-7]. In the IR spectra of the Schiff base H2L2, a very
weak absorption band appeared at 2834 cm-1 corresponding
to ν(S-H) disappeared in the spectra of the complexes due to
the fact that coordination has taken place through the sulphur
atom after deprotonation. Moreover, the absorption due to
ν(C-S) of the ligand at 1224 cm-1 is shifted to a higher frequency
1256 cm-1 in the complexes [Ni(PPh3)(L2)], indicating that the
other coordination is through thiophenolic sulphur atom. The
characteristic bands due to triphenylphosphine were observed
in the expected region. The characteristic band for ν(C=O)
and ν(NH) disappears on complexation. This may be due to
the enolization and subsequent coordination through the
deprotonated oxygen atom [8].
The 13C NMR data for the complexes have been recorded
and summarized in Table-3. The assignment of all the aromatic
1
carbon resonances is made on the basis of H-13C HSQC
spectrum of the complex [Ni(PPh3)(L1)] and [Ni(PPh3)(L2)] are
given in Figs. 1 and 2, respectively. The chemical shifts for
the aromatic carbon atoms of triphenylphosphine and phenyl
groups in the complexes appear at 109-137 ppm. In all the
complexes, Ph-C-O and Ph-C-S appears at 137 and 134 ppm,
respectively and also for Ph-C=N-Ph and Ph-N=C-O in the
complexes appears in the range 136-153 ppm and 145-159
ppm, respectively.
0
Ultraviolet-visible spectra: The electronic spectra of both
ligands and their Ni2+ complexes in dichloromethane showed
four to seven bands in the 230-494 nm regions. The electronic
spectra of Ni(II) Schiff base complexes and the positions are
similar to the one that have been observed for other nickel(II)
square planar complexes showed one to four bands at the region
20
40
60
80
2
2
230-440 nm due to B1g→ A1g transition in a square planar
geometry. The results are in accordance with the electronic
spectra of other similar square planar Ni(II) complexes [9,10].
The UV-visible spectral data are given in Table-2.
100
120
140
1H, 13C and 31P NMR spectra of the Ni(II) complexes:
1
The H NMR spectra of all the complexes were recorded to
confirm the binding of Schiff bases to the nickel ion (Table-3).
Multiplets are observed around 5.4-7.6 ppm in all the complexes
have been assigned to aromatic protons of triphenylphosphine
and Schiff base ligands. The disappearance of signals due to
phenolic, enolic and thiolato hydrogen atoms in the 1H NMR
spectra of all the Ni(II) complexes indicates the deprotonation
of these groups and the Schiff bases are coordinated to metal
ions as dianionic ligands, which indicates the coordination of
nickel(II) through the Ph-O, Ph-S and enolic-O atoms.
11
10
9
8
7
6
5
4
3
2
ppm
Fig. 1. 1H-13C HSQC Spectrum of the complex [Ni(L1)(PPh3)]
31P NMR spectra were recorded for two complexes in order
to confirm the presence of triphenylphosphine group of the
complexes. The complexes [Ni(PPh3)(L1)] and [Ni(PPh3)(L2)]
exhibits only one signal at 23.91 and 26.00 ppm confirming
the presence of triphenylphosphine, respectively (Fig. 3).
TABLE-2
FT-IR SPECTRAL AND UV-VISIBLE DATA OF NEW Ni(II) SCHIFF BASE COMPLEXES
IR spectra (cm-1)
Ligands & Complexes
UV-visible λmax (nm)
255, 296, 369, 401
256, 297, 369, 404, 435, 472, 494
290, 330, 322, 412
ν(C=N)
1615
1617
1592
1580
ν(C-O)
1343
–
1438
–
ν(C-S)
–
1224
–
H2L1
H2L2
[Ni(L1)(PPh3)]
[Ni(L2)(PPh3)]
1260
266, 322, 428
TABLE-3
1H, 13C AND 31P NMR SPECTRAL DATA OF Ni(II) SCHIFF BASE COMPLEXES
Ligands and
complexes
1H NMR (ppm)
13C NMR (ppm)
6.2-7.6 (m, aromatic), 10.8 (s, Ph-OH),
12.8 (s, enolic-OH)
110, 114, 116, 122, 136, 143, 153 (aromatic C), 155 (Ph-C-OH),
158 (Ph-C=N-Ph), 163 (Ph-N=C-OH)
H2L1
6.4-7.8 (m, aromatic), 3.4 (s, Ph-SH),
10.4 (s, enolic-OH)
108, 116, 121, 125, 130, 135, 141 (aromatic C), 154 (Ph-C-SH),
158 (Ph-C=N-Ph), 176 (Ph-N=C-OH)
H2L2
111, 115, 118, 123, 128, 129, 137 (aromatic C), 137 (Ph-C-O),
153 (Ph-C=N-Ph), 159 (Ph-N=C-O)
[Ni(L1)(PPh3)]
[Ni(L2)(PPh3)]
6.3-7.6 (m, aromatic)
5.4-7.5 (m, aromatic)
109, 120, 124, 126, 129, 130, 131, 132 (aromatic C), 134
(Ph-C-SH), 136 (Ph-C=N-Ph), 145 (Ph-N=C-O)

Vol. 28, No. 8 (2016)
Synthesis of Ni(II) Schiff Base Complexes 1685
cell membrane and can be explained by Tweedy’s chelation
theory [11]. Chelation considerably reduces the polarity of
the metal ion because of partial sharing of its positive charge
with donor groups and possible π-electron delocalization over
the whole chelate ring. Such a chelation could enhance the
lipophilic character of the central metal atom, which subse-
quently favours its permeation through the lipid layer of the cell
membrane. This increased lipophilicity enhances the pene-
tration of the complexes into lipid membrane and blocking
the metal binding sites on enzymes of microorganism. These
complexes also disturb the respiration process of the cell and
thus block the synthesis of proteins, which restrict the further
growth of the organism [11-13]. The variation in the effec-
tiveness of different compounds against different organisms
depends either on the impermeability of the cells of the
microbes or on differences in the ribosome of microbial cells.
A number of new nickel(II) Schiff base complexes exhibits a
greater activity when compared with the Ru(III) Schiff base
complexes[6], which contains the same ligands.
0
20
40
60
80
100
120
140
11 10
9
8
7
6
5
4
3
2
1
ppm
Fig. 2. 1H-13C HSQC Spectrum of the complex [Ni(L2)(PPh3)]
Oxidation of alcohols: Catalytic oxidation of cyclohexanol,
benzyl alcohol and cinnamyl alcohol by the synthesized
nickel(II) Schiff base complexes were carried out in CH2Cl2
under an oxygen atmosphere at ambient temperature which is
a green technique and the results are summarized in Table-4.
Activation of molecular oxygen by transition metals for the
catalytic oxidation of organic substrates has been of continued
interest in organic synthesis [14]. Cyclohexanone, benzaldehyde
and cinnamaldehyde were formed from cyclohexanol, benzyl
alcohol and cinnamyl aclchol, respectively, after stirring for
about 6 h and the carbonyl compounds were quantified as 2,4-
dinitrophenylhydrazone derivatives. Only a very little amount
of carbonyl compound is formed when the reaction is carried
out without the catalyst in presence of oxygen atmosphere at
ambient temperature. This is an insignificant amount compared
with the yields of carbonyl compounds that have been obtained
from the reaction catalyzed by nickel complexes. The relatively
higher product yield obtained for oxidation of cinnamyl alcohol
and benzyl alcohol compared with cyclohexanol is due to
the fact that α-CH unit of benzyl alcohol is more acidic than
cyclohexanol [14].
[Ni(L1)(PPh3)]
100
50
0
-50
-100
-150
-200
ppm
[Ni(L2)(PPh3)]
100
50
0
-50
ppm
-100
-150
-200
Fig. 3. 31P NMR Spectra of the complexes
Microbial studies: The in vitro cytotoxicity of ligands
and the complexes were screened in order to evaluate activity
against Staphylococcus epidermidis and Escherichia coli at
0.25 %, 0.50 % and 1 % concentration and the results are
shown in Table-4. From the results, it is inferred that the Ni(II)
Schiff base complexes show higher efficiency when compared
with the standard (co-trimoxazole), parent ligands, nickel(II)
precursors and standard reference against same microbes under
identical experimental conditions. This would suggest that the
chelation could facilitate the ability of a complex to cross a
Conclusion
Nickel(II) complexes with dibasic tridentate Schiff bases
have been prepared and characterized by (FT-IR, UV-visible
and NMR). The Ni(II) ion is assigned a square planar geometry
and coordination of the ligands through ONO/ONS atoms
(Scheme-IV). The [Ni(L1)(PPh3)] complex can serve as good
catalyst for the oxidation of alcohols to corresponding carbonyl
compounds reactions. All complexes were screened for their
TABLE-4
OXIDATION OF ALCOHOLS AND BIOCIDAL ACTIVITY OF NEW Ni(II) SCHIFF BASE COMPLEXES
Inhibition zone (mm)
Benzyl alcohol
Cyclohexanol
Cinnamyl alcohol
Staphylococcus epidermidis
Escherichia coli
Complexes
Yield
(%)
Turnover
number
Yield
(%)
Turnover
number
Yield
(%)
Turnover
number
0.25 %
0.5 %
1 %
0.25 %
0.5 %
1 %
[Ni(L1)(PPh3)]
[Ni(L2)(PPh3)]
Standard
71
69
73
71
59
59
61
61
47
43
49
45
24
26
24
27
26
27
27
29
27
29
29
30
Co-trimoxazole (21)
Co-trimoxazole (22)

1686 Madaselvi et al.
Asian J. Chem.
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ACKNOWLEDGEMENTS
One of the authors (SA) is thankful to Mrs. S. Malarvizhi,
Chair Person and Managing Trustee, Sri Krishna Group of
Institutions, Coimbatore, India and Dr. P.N. Magudeswaran,
Head, Department of Chemistry for irrecoverable support and
providing the facilities.
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