New acyclic schiff-base nickel(II) complexes and their electrochemical, kinetic, and antimicrobial studies

Article abstract of DOI:10.1080/15533174.2011.591328

Five new nickel(II) complexes were synthesized. All the complexes were characterized by elemental and spectral analysis. Electronic spectra of the complexes show d-d transition in the range of 500-800 nm. Electrochemical studies of the complexes show an i

Full text of DOI:10.1080/15533174.2011.591328

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 41:963–972, 2011
Copyright ꢀ Taylor & Francis Group, LLC
C
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2011.591328
New Acyclic Schiff-Base Nickel(II) Complexes and their
Electrochemical, Kinetic, and Antimicrobial Studies
A. Vijayaraj,1 R. Prabu,1 R. Suresh,1 G. Jayanthi,2
J. Muthumary,2 and V. Narayanan1
1Department of Inorganic Chemistry, School of Chemical Sciences, University of Madras, Guindy
Maraimalai Campus, Chennai, India
2Centre for Advanced Studies in Botany, University of Madras, Guindy Maraimalai Campus, Chennai,
India
mononuclear coordination compounds of Schiff bases are taken
into account. Thereupon, a systematization of various types of
Schiff-base complexes is offered.
Five new nickel(II) complexes were synthesized. All the com-
plexes were characterized by elemental and spectral analysis. Elec-
tronic spectra of the complexes show d–d transition in the range of
500–800nm. Electrochemicalstudies ofthecomplexes showan irre-
versible one-electron reduction process around –0.65 to –0.91 V and
an irreversible one-electron oxidation process around 0.87 to 1.00
V. The reduction and oxidation potential of nickel(II) complexes
shifts toward cathodic and anodic directions, respectively, upon
increasing the chain length. The catalytic activity of the nickel(II)
complexes on the hydrolysis of 4-nitrophenylphosphate was deter-
mined. All the nickel(II) complexes were screened for antibacterial
activity.
The field of acyclic chemistry of metals is developing very
rapidly because of its application[25,26] and importance in the
area of coordination chemistry.[27,28] Studies on complexes of
acyclic Schiff-base ligands with different size, and number and
donor atoms for coordination with a variety of metal centers
have been published.[29–31] Many transition metal ions in living
systems work as enzymes or carriers in a macrocyclic ligand en-
vironment. Meaningful research in this direction might generate
simple models for biologically occurring metalloenzymes,[32–34]
and thus help in further understanding biological systems. The
development of the field of bioinorganic chemistry has been
another important factor in spurring the growth of interest in
macrocyclic compounds.[35] The stability of acyclic metal com-
plexes depends upon a number of factors, including the number
and types of donor atoms present in the ligand and their relative
positions within the macrocyclic skeleton, as well as the number
and size of the chelate rings formed on complexation.
Nickel is recognized as an essential trace element for bacteria,
plants andanimals.[36–39] Theactivesitesof enzymes suchasure-
ase, carbon monoxide dehydrogenase, [NiFe]-hydrogenase, and
methyl-S-coenzyme-M methylreductase are known to contain
nickel centers, which are intimately involved in the catalytic cy-
cles.[40] Until the recent reports of the structures of urease[41] and
[NiFe]-hydrogenase,[42] x-ray structural data on nickel enzymes
was lacking. The quest for information about the structures and
modes of action of nickel active sites continues to generate inter-
est in the development of synthetic models for the various nickel
biosites and in the coordination chemistry of nickel in gen-
eral.[40,43–48] Apart from their potential to form the basis of mod-
els for the polynuclear active sites of Ni containing enzymes,
complexes of macrocyclic ligands and their derivatives are ideal
for studying magnetic exchange interactions[44,45] and the redox
properties of nickel(II) centers in close proximity.[46] Parashar
et al.49 reported copper(II), cobalt(II), and nickel(II) complexes
of the Schiff-base ligands obtained from 2-substituted anilines
Keywords antimicrobial activity, cyclic voltammetry, nickel(II)
complexes, Schiff-base ligand, urease activity
INTRODUCTION
Schiff bases and their structural analogues, as ligating com-
pounds containing acyclic and cyclic imine (C = N) bonds,
are of great importance in modern coordination chemistry.[1–6]
Interest in metal complexes of these ligands[7–10] is related
to wide variability of their fine structures (rational design[5])
and obtaining of polyfunctional materials. Among them, we
note luminescent complexes,[11,12] magnetoactive[13–15] and liq-
uid crystal[16,17] structures, chemosensors[[18–20] and other useful
metal-containing azomethine compounds.[21] Bioinorganic and
biomimetics are widely represented.[22–24] Practically all coor-
dination compounds of Schiff bases were synthesized using
chemical and electrochemical techniques.[2] All factors forming
Received 4 February 2011; accepted 3 March 2011.
Financial support from CSIR, New Delhi, is gratefully acknowl-
edged.
Address correspondence to V. Narayanan, Assistant Professor, De-
partment of Inorganic Chemistry, School of Chemical Sciences, Uni-
versity of Madras, Guindy Maraimalai Campus, Chennai–25, Tamil
Nadu, India. E-mail: vnnara@yahoo.co.in
963

964
A. VIJAYARAJ ET AL.
and salicylaldehyde, which exhibit good antifungal and an- of time. A plot of log[Aα/(Aα – At)] versus time was made for
tibacterial activity. Tagenine et al.[50] reported antibacterial ac- each complex and the rate constants for the catalytic oxidations
tivity of Zn(II), Cu(II), and Ni(II) Schiff-base complexes de- and the hydrolysis of 4-nitrophenylphsophate were calculated.
rived from 2,3-diamino pyridine and salicylaldehyde. Julieta
Gradinaru et al.[51] reported Ni(II), Cu(II), Zn(II), and VO(II)
Schiff-base complexes from 1-phenylbutane-1,3-dione mono-S-
Synthesis of Ligands
Synthesis of L1 [55]
methylisothiosemicarbazone with o-hydroxybenzaldehyde that
An absolute methanol solution (10 mL) containing diethylen-
exihibit nonlinear optical (NLO) properties. Kianfar et al.[52] re-
etriamine (0.1030 g, 1 mmol) was added dropwise to a stirred
ported the electrochemical activity of VO(IV) Schiff-base com-
solution of 5-methylsalicylaldehyde (0.2720 g, 2 mmol). The
plexes, salen type of aldehydes with diamine. Rahimi et al.[53]
solution was refluxed for 8 h (Scheme 1). The resulting yel-
reported Cu(II) Schiff-base complexes derived from ethylene di-
low solution was cooled in ice. The yellow precipitate formed
amine, 2,3-diaminobenzene with 5-methylsalicylaldehyde that
was filtered and washed with hexane and dried under vacuum.
exhibit good electrochemical activity. We reported recently
The crude product on recrystalization from THF gave yellow
Cu(II) Schiff base complexes in 2011.[55] The present work deals
crystals.
with the influence of ligand modification on spectral, electro-
Yield (54.8%). m.p.: 163◦C. Anal.: Calcd. for C20H25N3O2
chemical, and kinetic studies, reporting the synthesis and char-
(%): C, 70.77; H, 7.42; N, 12.38. Found: C, 70.72; H, 7.31; N,
acterization of acyclic mononuclear nickel(II) complexes.
12.30. ESI MS: (m/z) 339.19 [MH+]. Calcd. av. m/z 338.10. IR
data (KBr) (ν, cm−1): 3405 ν [b,(OH)], 1637 [s, ν(C = N)]. 1H-
NMR (ppm in CDCl3, 400 MHz): 2.94 (2CH3, s, 6H), 3.11(N-H,
d, 1H, J = 8 Hz), 3.24 (N-CH2, t, 8H, J = 12 Hz), 6.84 (Ar-H,
t, 6H, J = 8Hz), 8.29 (-CH = N, d, 2H, J = 12 Hz), 9.78 (2Ar-
OH, s, 2H). 13C-NMR (CDCl3, 400 MHz): 20.15 (2CH3), 55.98
(2CH2), 57.98 (2CH2), Ar(CH) 116.42, 118.40, 127.28, 131.59,
132.76, 158.59, 166.28.
EXPERIMENTAL
Chemicals and Reagents
5-Methylsalicylaldehyde[54] was prepared following the lit-
erature method. Analytical-grade methanol, acetonitrile, and
dimethyl formamide were purchased from Qualigens. TBAP
(tetra(n-butyl)ammonium perchlorate) used as supporting elec-
trolyte in electrochemical measurements was purchased from
Fluka and recrystallized from hot methanol. N,N-Bis-(3-
aminopropyl) piperazine, N,N-bis-(3-aminopropyl) ethylene di-
amine, and tris-(2-aminoethyl)amine were purchased from
Aldrich. Triethylenetetramine and diethylenetriamine were pur-
chased from Qualigens.
Ligands L2, L3, L4, and L5 were synthesized by following
the already-described procedure using tris-(2-aminoethyl)amine
(0.1490 g, 1 mmol), triethylenetetramine (0.1420 g, 1 mmol),
N,N-bis-(3-aminopropyl)ethylenediamine (0.1740 g, 1 mmol),
and N,N-bis-(3-aminopropyl)piperazine (0.2000 g, 1 mmol), re-
spectively, instead of diethylenetriamine.
Synthesis of L2
Physical Measurements
Yield (35.2%). m.p.: 138◦C. Anal.: Calcd. for C22H30N4O2
(%): C, 69.08; H, 7.91; N, 14.65. Found: C, 69.01; H, 7.34; N,
14.61. ESI MS: (m/z) 382.24. Calcd. av. m/z 382.10. Selected
IR data (KBr) (ν, cm−1): 3485 [b, ν(OH)], 1635 [s, ν(C = N)].
1H-NMR (ppm in CDCl3 400 MHz): 2.35 (2CH3, s, 6H), 3.49
(N-CH2, t, 10H, J = 8 Hz), 5.82 (NH2, s, 2H), 6.94 (Ar-H, t,
6H, J = = 8 Hz), 7.93 (2CH2, s, 4H), 8.24 (-CH = N, d, 2H, J
= 12 Hz), 9.89 (2Ar-OH, s, 2H). 13C-NMR (CDCl3, 400 MHz):
21.43 (2CH3), 35.28 (CH2-NH2) 54.59 (2CH2), 57.29 (CH2-N),
59.10 (2CH2), Ar(CH) 115.39, 119.49, 128.24, 134.59, 135.10,
159.43, 164.59.
Elemental analysis of the complexes was obtained using
a Haereus CHN rapid analyzer. IR spectra were recorded on
a PerkinElmer FT-IR 8300 series spectrophotometer on KBr
disks from 4000 to 400 cm−1. 1H-Nuclear magnetic resonance
(NMR) spectra were recorded using a JEOL GSX 400-MHz
NMR spectrometer. Electrospray ionization (ESI) mass spectra
were obtained on a JEOL DX-303 mass spectrometer. Elec-
tronic spectral studies were carried out on a PerkinElmer 320
spectrophotometer. Cyclic voltammograms were obtained on
a CHI-600A electrochemical analyzer under oxygen-free con-
ditions using a three-electrode cell in which a glassy carbon
electrode was the working electrode, a saturated Ag/AgCl elec-
trode was the reference electrode, and platinum wire was the
auxiliary electrode. A ferrocene/ferrocenium couple was used
as an internal standard and E1/2 of the ferrocene/ferrocenium
(Fc/Fc+) couple under the experimental condition was 470 mV.
Synthesis of L3
Yield (41.5%). m.p.: 125◦C. Anal.: Calcd. for C22H30N4O2
(%): C, 69.08; H, 7.81; N, 15.65. Found (%): C, 69.01; H, 7.64;
N, 15.61. ESI MS: (m/z) 383.24 [MH+]. Calcd. av. m/z 382.10.
Selected IR data (KBr) (ν, cm−1): 3445 ν(OH), 1630 [s, ν(C =
N)]. 1H-NMR (ppm in CDCl3, 400 MHz): 2.32 (2CH3, s, 6H),
2.97 (2N-H, d, 2H, J = 8 Hz), 3.77 (2N-(CH2)2, d, 12H, J = 12
Hz), 6.94 (Ar-H, t, 6H, J = 8 Hz), 8.32 (-CH = N, d, 2H, J = 8
Hz), 9.87 (2Ar-OH, s, 2H). 13C-NMR (CDCl3, 400 MHz): 20.88
Tetra(n-butyl)ammonium perchlorate (TBAP) at 1 × 10−1
M
was used as the supporting electrolyte. Catalytic hydrolysis of
4-nitrophenylphosphate by the nickel complexes was monitored
by following the ultraviolet–visible (UV-vis) absorbance change
at 420 nm (assigned to the 4-nitrophenolate anion) as a function

NEW ACYCLIC SCHIFF-BASE NICKEL(II) COMPLEXES
965
TABLE 1
Physical properties and analytical data of [Ni(II)L1] to [Ni(II)L5]
Elemental analysis (%)
Mol. wt.
found
(calcd.)
C found
(calcd.)
H found N found Ni found
Complexes
[Ni(II)L1]
Molecular formula Yield (%) M.P. (◦C)
(calcd.)
(calcd.)
(calcd.)
C20H23NiN3O2
C22H28NiN4O2
C22H28NiN4O2
C24H31NiN4O2
C26H34NiN4O2
59
71
69
74
61
312
321
325
329
341
60.12
(60.65)
60.02
(60.17)
60.30
(59.51)
61.11
(61.77)
63.12
5.72
(5.88)
6.25
(6.42)
6.20
(6.36)
6.56
(6.72)
6.80
10.22
(10.61)
12.49
(12.72)
12.80
(12.62)
12.11
(12.02)
11.17
14.61
(14.85) (393.95)
13.26
(13.31) (438.10)
13.39
(14.31) (438.69)
12.13
(12.45) (466.28)
394.12[MH+]
[Ni(II)L2]
[Ni(II)L3]
[Ni(II)L4]
[Ni(II)L5]
437.15[MH−]
436.20[MH−]
464.20[MH−]
11.69
490.27[MH−]
(63.31)
(6.97)
(11.31)
(11.94) (492.25)
(2CH3), 49.82 (2CH2), 51.39 (2CH2), 56.91 (2CH2), Ar(CH) crude product was recrystallized from methanol and acetonitrile
115.88, 118.39, 129.39, 132.81, 134.73,158.90, 163.89.
(1:3, v/v). [Ni(II)L2], [Ni(II)L3], [Ni(II)L4], and [Ni(II)L5] were
synthesized by following the already-described procedure using
L2 (0.038 g, 0.1 mmol), L3 (0.0382 g, 0.1 mmol), L4 (0.0426 g,
Synthesis of L4
Yield (29.5%). m.p.: 141◦C. Anal.: Calcd. for C24H34N4O2 0.1 mmol), and L5 (0.0468 g, 0.1 mmol). Physical and analytical
(%): C, 70.21; H, 8.34; N, 13.69. Found (%): C, 70.13; H, 8.29; data of the complexes are given in Table 1. All the nickel(II)
N, 13.53. ESI MS: (m/z) 410.30. Calcd. av. m/z 410.10. Selected complexes are neutral.
IR data (KBr) (ν, cm−1): 3475 ν(OH), 1637 [s, ν(C = N)]. 1H-
Synthesis of [Ni(II)L1]
NMR (ppm in CDCl3, 400 MHz): 2.60(2CH3, s, 6H), 2.88 (2N-
H, d,2H, J = 8 Hz), 3.71 (N-(CH2)2, d, 4H, J = 8 Hz), 7.09
(Ar-H, s, 6H), 7.81 (CH2, s,12H), 8.16 (-CH = N, s, 2H), 9.81
(2Ar-OH, s, 2H). 13C-NMR (CDCl3, 400 MHz): 20.98 (2CH3),
32.49 (2CH2), 46.89 (2CH2), 49.48 (2CH2), Ar(CH) 116.81,
125.39, 131.49, 133.53, 135.33,158.89, 163.84.
1H-NMR (ppm in CDCl3,, 400 MHz): 2.94 (2CH3, s, 6H),
3.53 (N-CH2, t, 8H, J = 12 Hz), 6.94 (Ar-H, t, 6H, J = 8 Hz),
8.12 (-CH = N, d, 2H, J = 12 Hz).
Synthesis of [Ni(II)L2]
1H-NMR (ppm in CDCl3, 400 MHz): 2.30 (2CH3, s, 6H),
2.95 (N-CH2, t, 12H, J = 8 Hz), 2.1 (NH,s,IH), 7.25 (Ar-H, t,
6H, J = 8 Hz), 8.20 (-CH = N, d, 2H, J = 12 Hz).
Synthesis of L5
Yield (29.5%). m.p.: 171◦C. Anal.: Calcd. for C26H36N4O2
(%): C, 71.53; H, 8.32; N, 12.80. Found (%): C, 71.41; H, 8.24;
N, 12.77. ESI MS: (m/z) 436.35 [MH+]. Calcd. av. m/z 435.20.
Selected IR data (KBr) (ν, cm−1): 3435 ν(OH), 1631 [s, ν(C =
N)]. 1H-NMR (ppm in CDCl3, 400 MHz): 2.28 (2CH3, s, 6H),
3.54 (2N-(CH2)2, d, 8H, J = 12 Hz), 5.71 (CH2, s, 12H), 7.24
(Ar-H, s, 6H), 8.25 (-CH = N, s, 2H), 9.79 (2Ar-OH, s, 2H). 13C-
NMR (CDCl3, 400 MHz): 22.49 (2CH3), 31.10 (2CH2), 50.39
(2CH2),53.39(4CH2), Ar(CH) 118.39, 124.39, 132.41, 134.49,
137.10, 157.79, 160.83.
Synthesis of [Ni(II)L3]
1H-NMR (ppm in CDCl3, 400 MHz): 2.35 (2CH3, s, 6H),
2.79 (2N-(CH2)2, d, 12H, J = 12 Hz), 6.94 (Ar-H, t, 6H, J = 8
Hz), 8.12 (-CH = N, d, 2H, J = 8 Hz).
Synthesis of [Ni(II)L4]
1H-NMR (ppm in CDCl3, 400 MHz): 2.55 (2CH3, s, 6H),
2.81 (N-(CH2)2, d, 4H, J = 8 Hz), 3.05 (6CH2, s, 12H), 7.20
(Ar-H, s, 6H), 8.16 (-CH = N, s, 2H).
Synthesis of Metal Complexes
An absolute methanol solution containing Ni(ClO4)2·6H2O
(0.036 g, 0.1mmol) was added dropwise to a stirring solution of
L1 (0.0339 g, 0.1 mmol) in 20 mL of absolute methanol. The so-
lution was refluxed for 8 h. On cooling the solution, green color
microcrystals were formed, which were filtered and washed with
methanol followed by diethyl ether and dried in vacuum. The
Synthesis of [Ni(II)L5]
1H-NMR (ppm in CDCl3, 400 MHz): 2.22 (2CH3, s, 6H),
2.53 (2N-(CH2)2, d, 8H, J = 12 Hz), 2.82 (6CH2, s, 12H), 7.33
(Ar-H, s, 6H), 8.29 (-CH = N, s, 2H).

966
A. VIJAYARAJ ET AL.
CH3
by the diffusion of diethyl ether vapor into DMF solutions or
recrystallization of the complexes from acetonitrile) have failed
and only green powder or micro crystals were obtained. Spec-
tral, electrochemical, catalytic, and antimicrobial studies of the
complexes were carried out.
CH3
NH2
NH2
OH
OH
CH3OH
N
N
Spectroscopic Studies
2
1 R
+
R
The Fourier-transform infrared (FT-IR) spectra of the ligands
show a broad band around 3400 cm−1 due to the presence of
the phenolic OH group. 5-Methy1salicylaldehyde shows a sharp
band peak at 1655 cm−1 corresponding to ν(-CHO). Ligands and
complexes show a sharp band at 1620–1640 cm −1 due to the C =
N group. The presence of this new peak and the absence of ν(C
= O) in the complexes indicate Schiff-base condensation.[56–61]
The absence of a peak around 3400 cm−1 in all the complexes in-
dicates the absence of ν(-OH) due to deprotonation followed by
complexation. For the complexes, bands at 420–460 cm−1 could
be assigned to the -(M–O) bond. Other weak bands at lower fre-
quency could be assigned to the -(M–N) bond.[62] The spectra of
all the complexes are dominated by bands at 3150–3070 cm−1
due to the aromatic C–H stretching vibration. A strong band at
1260 cm−1 in the free Schiff bases has been assigned to phenolic
C–O stretching. Upon complexation, this band shifts to a higher
frequency (1300 cm−1), showing coordination through pheno-
lic oxygen[63]; the IR data are given in Table 2. In the proton
NMR spectra of all the nickel(II) complexes the peak due to
phenolic OH group is absent. This indicates that the phenolic
protons are deprotonated during complexation. The nickel(II)
complexes are all neutral.
Reflux 8 hours
OH
O
CH3
N
N
NH
NH
NH
NH
NH
L1
N
=
R
NH2
L5
L3
L4
L2
SCH. 1. Schematic diagram for the synthesis of ligands.
RESULTS AND DISCUSSION
A series of acyclic mononuclear nickel(II) complexes was
synthesized by the Schiff-base condensation of the precursor
compounds with diamines in presence of metal ion. The syn-
thetic pathway of mononuclear complexes is shown in Scheme
2. All attempts to grow single crystals of the complexes (e.g.,
CH3
CH3
ESI Mass Spectral Analysis
The ESI mass spectra of the mononuclear complexes
[Ni(II)L3], [Ni(II)L4], and [Ni(II)L5] show the molecular ion
peak (M+) at m/z = 436.20, 464.20, and 490.27 respectively
The spectra show some prominent peaks corresponding to the
various fragments of the complexes. The appearance of many
peaks is due to the presence of isotopic atoms. The ESI mass
spectra of the mononuclear complexes [Ni(II)L3], [Ni(II)L4],
and [Ni(II)L5] are shown in Figure 1.
.
Ni(ClO4)2 6H2O
O
O
N
N
OH
OH
N
N
R
R
Ni
CH3OH
Reflux 8 hours
Electronic Spectra
Electronic spectra for all the complexes were obtained in
DMF medium. The electronic spectra of all the complexes show
a single weak d–d band in the region 410–750 nm due to 1A1g
→ A2g associated with square-planar geometry.[64] A red-shift
1
in the λmax value of the d–d band[65] with increase in the chain
length between imine nitrogen has been observed. This red-
shift may be due to the distortion from planar geometry as the
chain length increases. The moderately intense band observed
in the region of 260–330 nm is associated with ligand-to-metal
charge transfer transition. An intense band observed in the re-
gion 210–240 nm is associated with intraligands transition. The
absorption spectral data are given in Table 2.
CH3
CH3
N
NH
NH
NH
N
R =
NH
N
NH
NH2
L1
L2
L5
L3
L4
SCH. 2. Schematic diagram for the synthesis of complexes.

NEW ACYCLIC SCHIFF-BASE NICKEL(II) COMPLEXES
967
FIG. 1. (a) ESI mass spectrum of [Ni(II)L3] complex m/z 436.20 [MH−]; (b) ESI mass spectrum of [Ni(II)L4] complex m/z 464.20 [MH−]; (c) ESI mass spectrum
of [Ni(II)L5] complex m/z 490.27 [MH−] (continued on next page).

968
A. VIJAYARAJ ET AL.
FIG. 1. (Continued.)
duction wave at a negative potential in the range –0.400 V
Electrochemistry of the Complexes
to –1.100 V. The controlled potential electrolysis carried out at
100 mV more negative than the reduction wave conveys the con-
sumption of one electron per molecule. It is very interesting to
compare the electrochemical behavior of [Ni(II)L] complexes.
The reduction potential shifts toward the anodic direction for the
complexes [Ni(II)L1] to [Ni(II)L5] from –0.910 V to –0.650 V,
as the number of methylene groups increases.[66,67] This shows
that as the number of methylene groups between the imine ni-
trogen (chain length) increases, the entire acyclic ring becomes
more flexible, which causes a distortion of the geometry of the
Reduction Process at Negative Potential
The electrochemical properties of the mononuclear com-
plexes were studied by cyclic voltammetry in DMF solution
containing TBAP as supporting electrolyte in the potential
range to 0 to −1.200 V (Figure 2). The electrochemical data
of nickel(II) complexes are given in Table 3. Generally the elec-
trochemical properties of the complexes depend on a number of
factors, such as chelate ring/size, axial ligation, degree and dis-
tribution of unsaturation, and substitution pattern in the chelate
ring. Each voltammogram shows a one-electron irreversible re-
TABLE 2
Characteristic IR bands (cm−1) of metal complexes and UV-Vis data of the prepared complexes
Complexes
ν(C = N) (cm−1)
ν(N-H) (cm−1)
ν(M-N) (cm−1)
ν(M-O) (cm−1)
λmax (nm) (ε (M−1cm−1) in DMF
[Ni(II)L1]
[Ni(II)L2]
[Ni(II)L3]
[Ni(II)L4]
[Ni(II)L5]
1624 s
1635 s
1629 s
1632 s
1621 s
3375 s
—
3359 s
3370 m
—
616 s
623 s
609 s
611 s
605 s
436 s
421 s
431 s
459 s
440 s
541 (251), 320 (17,843)
573 (191), 339 (17,230)
631 (178), 351 (17,020)
650 (148), 390 (16,710)
725 (129), 410 (16,430)
S = strong, w = weak, m = medium.

NEW ACYCLIC SCHIFF-BASE NICKEL(II) COMPLEXES
969
TABLE 3
Electrochemical data for the complexes’ reduction (at cathodic
potential), oxidation (at anodic potential), and hydrolysis of
4-nitrophenylphosphate data for the complexes
Reduction
(at cathodic)
(V)
Oxidation
(at anodic)
(V)
Rate constant (k)
× 10− 3 min−1
NPP
Complexes
[Ni(II)L1]
[Ni(II)L2]
[Ni(II)L3]
[Ni(II)L4]
[Ni(II)L5]
–0.910
–0.810
–0.750
–0.700
–0.650
0.870
0.920
0.950
0.970
1.000
2.688
4.516
5.822
6.888
8.533
Measured by CV at 50 mV s−1 scan rate. E vs. Ag/AgCl conditions:
GC working electrode and Ag/AgCl reference electrodes; supporting
electrolyte TBAP; concentration of complex 1 × 10−3 M, concentra-
tion of TBAP 1 × 10−1 M. Measured spectrophotometrically in DMF.
Concentration of the complexes: 1 × 10−3 M. Concentration of 4-
nitrophenylphosphate: 1 × 10−1 M.
FIG. 3. Voltammogram of mononuclear nickel(II) complexes: (a)
[Ni(II)L1],(b) [Ni(II)L2], (c) [Ni(II)L3], (d) [Ni(II)L4], and (e) [Ni(II)L5] (oxi-
dation process).
oxidation process occurs with difficulty. The electrochemical
data are given in Table 3.
nickel(II) complexes and makes the system more flexible, which
stabilizes the low-valent Ni(I).
Kinetic Studies of Hydrolysis of 4-Nitrophenylphosphate
The catalytic activity of the nickel(II) complexes on the hy-
drolysis of 4-nitrophenylphosphate was determined spectropho-
tometrically by monitoring the increase in the characteristic ab-
sorbance of the 4-nitrophenolate anion at 420 nm over time in
dimethylformamide at 25◦C. For this purpose, 10−3 mol dm−3
solutions of complexes in dimethylformamide were treated with
100 equivalents of 4-nitrophenyl phosphate in the presence of
Oxidation Process at Positive Potential
The oxidation potential shifts toward the cathodic direction
for the complexes [Ni(II)L1] to [Ni(II)L5] from 0.870 V to 1.000
V as the number of methylene groups is increased (Figure 3).
This is because as the ring size increases the flexibility the
planarity of the complexes decreases, and the electrochemical
FIG. 4. Catalysis of 4-nitrophenylphosphate by the nickel(II) complexes: (a)
[Ni(II)L1], (b) [Ni(II)L2], (c) [Ni(II)L3], (d) [Ni(II)L4], and (e) [Ni(II)L5]. The
inset is the time-dependent growth of p-nitrophenolate chromophore in the
presence of [Ni(II)L5].
FIG. 2. Voltammogram of mononuclear nickel(II) complexes: (a) [Ni(II)L1],
(b) Ni(II)L2], (c) [Ni(II)L3], (d) [Ni(II)L4], and (e) [Ni(II)L5] (reduction pro-
cess).

970
A. VIJAYARAJ ET AL.
TABLE 4
Antibacterial activity data of nickel(II) complexes
Zone of inhibition (mm)
Bacillus subtilis
Staphylococcus aureus
Compound
10 mg
20 mg
30 mg
10 mg
20 mg
30 mg
[Ni(II)L1]
[Ni(II)L2]
[Ni(II)L3]
[Ni(II)L4]
[Ni(II)L5]
Std-1
24.0 0.80
29.0 0.69
20.0 1.16
21.0 1.20
—
29.0 1.0
32.0 1.0
25.0 1.0
22.0 1.0
—
30.0 1.0
33.0 1.0
26.0 1.0
25.0 1.0
13.0 1.0
14.0 0.80
12.0 0.69
24.0 1.38
22.0 1.26
14.0 1.80
14.0 0.80
—
33.0 1.0
31.0 1.0
17.0 1.0
15.0 1.0
—
—
—
—
43.0
38.0
Std-1 = Azithromycin (15 mg/mL).
air. The course of the reaction was followed at 420 nm for nearly The pH range was maintained between 7.0 and 7.5. Incubation
45 min at regular time intervals. The slope was determined by the was maintained at 37◦C for 24 h.
method of initial rates by monitoring the growth of the 420 nm
Nutrient agar (NA) plates were seeded separately with active
band of the product 4-nitrophenolate anion. A linear relationship growth of 105 test organisms such as B. subtilis, S. aureus,
for all the complexes shows a first-order dependence on the com- K. pneumonia, P. aeruginosa, or C. albicans, distributed by a
plex concentration for the systems. Plots of log[Aα/(Aα – At)] sterile cotton swab on the surface of the MHA medium and left
versus time for hydrolysis of 4-nitrophenylphosphate activity of for 5 min in a laminar airflow cabinet to dry. Yeast was grown on
the complexes are obtained and shown in Figure 4. The inset in SDA medium for antimicrobial activity. After drying, a well was
Figure 4 shows the time dependent growth of p-nitrophenolate created by a cork borer (9 mm) and 10, 20, and 30 mg/well was
chromophore in the presence of [Ni(II)L5]. The observed initial separately transferred to the well and left for 30 min at 5◦C until
rate constant values for all the nickel(II) complexes are given in the diffusion of compounds was completed. A well received a
Table 3. The catalytic activities of the nickel(II) complexes are similar volume of 0.4% DMSO, which served as control.[71] The
founded to increase as the chain length increases due to the flex- plates were incubated for 24 h at 37◦C. The development of an
ibility resulting from the distraction of the coordination sphere; inhibition zone around the well was measured and recorded.
i.e., increasing in the chain length enhances the rate constant
of hydrolysis fairly well by producing distortion in the geome-
try around the metal ion that enhances the accessibility of the
Results
The antibacterial activity of the complexes [Ni(II)L1] to
[Ni(II)L5] against human bacterial and yeast pathogens was
determined by agar diffusion method with azithromycin as ref-
erence control and was represented in the Table 4. For the
antibacterial activity against S. aureus the zone of inhibition
observed for the compound [Ni(II)L1] was found to be maxi-
mum, followed by [Ni(II)L2], [Ni(II)L3], and [Ni(II)L4]. Zone
of inhibition was absent in [Ni(II)L5]. For the antibacterial ac-
tivity against B. subtilis the zone of inhibition observed for
the compound [Ni(II)L2] is found to be maximum, followed
by [Ni(II)L1], [Ni(II)L4], [Ni(II)L3], and [Ni(II)L5]. [Ni(II)L2]
and [Ni(II)L1] showed similar antibacterial activity with a zone
of inhibition 33 1.0 mm at 30 mg concentration in both B.
subtilis and S. aureus. [Ni(II)L5] showed less antibacterial ac-
tivity against B. subtilis and had no activity against S. aureus.
The nickel complexes showed no antibacterial activity against
metal ion for the bonding of phosphate and OH group.[68–70]
The rate constant value for the mononuclear nickel(II) complex
[Ni(II)L5] is higher (8.533 × 10−3 min−1) than that of the com-
plex [Ni(II)L4] (6.888 × 10−3 min−1), which in turn is higher
than the nickel(II) complex [Ni(II)L1] (2.688 × 10−3 min−1).
Antimicrobial Activity
Test Organisms
The axenic cultures Bacillus subtilis (MTCC 441), Staphylo-
coccus aureus (MTCC 96), Pseudomonas aeruginosa (MTCC
424), K. pnemonia (MTCC), and Candida albicans (MTCC
227) were obtained from Microbial Type Culture Collection
(MTCC), Chandigarh, India.
Muller Hinton agar (MHA)
Commercially available Muller Hinton agar (MHA) and P. aeruginosa, K. pnemonia, and Candida albicans even at 30
sabouraud dextrose agar (SDA) purchased from Himedia Pvt. mg/mL concentration. The results show that all derivative com-
Ltd, Mumbai, India were used in this study. Agar was suspended pounds were able to inhibit gram-positive bacterial organisms
in 1600 mL of distilled water in a 2-L flask, stirred, boiled to but there was no zone of inhibition in gram-negative bacterial
dissolve, and then autoclaved at 15 lb and at 121◦C for 15 min. and yeast organisms. Therefore, the inhibition mode of these

NEW ACYCLIC SCHIFF-BASE NICKEL(II) COMPLEXES
971
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of [Ni(II)L] complexes indicates square planar geometry, and
there is a red-shift due to the increase in the chain length. Cyclic
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