Electrochemical, catalytic and antimicrobial activities of N-functionalized cyclam based unsymmetrical dicompartmental binuclear nickel(II) complexes

Article abstract of DOI:10.1016/j.saa.2009.08.008

Five binuclear nickel(II) complexes have been prepared by simple Schiff base condensation of the compound 1,8-[bis(3-formyl-2-hydroxy-5-bromo)benzyl]-l,4,8,11-tetraazacyclotetradecane (L) with appropriate aliphatic or aromatic diamine, nickel(II) perchlor

Full text of DOI:10.1016/j.saa.2009.08.008

Spectrochimica Acta Part A 74 (2009) 849–854
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Electrochemical, catalytic and antimicrobial activities of N-functionalized cyclam
based unsymmetrical dicompartmental binuclear nickel(II) complexes
S. Sreedarana, K. Shanmuga Bharathia, A. Kalilur Rahimana, R. Suresha,
R. Jegadeeshb, N. Raamanb, V. Narayanana,∗
a Department of Inorganic Chemistry, School of Chemical Sciences, University of Madras, Guindy Campus, Chennai 600 025, Tamil Nadu, India
b Centre for Advanced Studies in Botany, University of Madras, Guindy Campus, Chennai 600 025, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Five binuclear nickel(II) complexes have been prepared by simple Schiff base condensation of
the compound 1,8-[bis(3-formyl-2-hydroxy-5-bromo)benzyl]-l,4,8,11-tetraazacyclotetradecane (L) with
appropriate aliphatic or aromatic diamine, nickel(II) perchlorate and triethylamine. All the complexes
were characterized by elemental and spectral analysis. Positive ion FAB mass spectra show the pres-
ence of dinickel core in the complexes. The electronic spectra of the complexes show red shift in the
d–d transition. Electrochemical studies of the complexes show two irreversible one electron reduction
processes in the range of 0 to −1.4 V. The reduction potential of the complexes shifts towards anodically
upon increasing chain length of the macrocyclic ring. All the nickel(II) complexes show two irreversible
one electron oxidation waves in the range 0.4–1.6 V. The observed rate constant values for catalysis of
the hydrolysis of 4-nitrophenyl phosphate are in the range of 1.36 × 10−2–9.14 × 10−2 min−1. The rate
constant values for the complexes containing aliphatic diimines are found to be higher than the com-
plexes containing aromatic diimines. Spectral, electrochemical and catalytic studies of the complexes
were compared on the basis of increasing chain length of the imine compartment. All the complexes
show higher antimicrobial activity than the ligand and metal salt.
Received 28 December 2008
Received in revised form 3 July 2009
Accepted 5 August 2009
Keywords:
Dicompartmental ligand
Cyclam
Nickel(II) complexes
Hydrolysis of nitrophenylphosphate
Cyclic voltammetry
Antifungal activity
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
mimic the physical and chemical properties of the active sites
present in metalloenzymes is very essential and the studies on such
N-functionalized cyclam ligands exhibit very rich coordination
chemistry with a variety of transition metal ions [1,2]. The prop-
erties of these ligands can, in principle, be modified by varying
the donor groups, size of the macrocycle, nature of the metal ions,
etc. By this way the ligand assemblies can be tuned to give control
over various aspects of metal–metal interactions, redox properties
and exogenous ligand binding. There is currently intense interest
in dinickel phenolate-bridged complexes as models for the active
sites in enzymes [3,4]. The design of dinucleating compartmental
ligands, whose two metal-binding sites are unsymmetrical with
respect to the cavity size, coordination number, geometric require-
ment, or the nature of the donor atom, can serve as a muse for
the bioinorganic chemist to inspire the generation of a range of
bio-resemblant coordination chemistry [5]. These unsymmetrical
compartmental ligands provide adjacent, dissimilar binding sites
which can each accommodate a metal and so produce binuclear
complexes with coordination environments resembling the active
site in urease [6–8]. Hence, synthesis of model compounds that
compounds is becoming increasingly important in understanding
biological functions of the bimetallic cores [9].
Several N-functionalized macrocyclic systems bearing tetra and
hexa coordination sites have been discussed (Chart I) in the litera-
ture [10]. Our research focuses on the synthesis of unsymmetrical
dicompartmental ligands (Chart I) bearing N-functionalized cyclam
groups (Scheme 1). This ligand contains hexa (N4O2, amine com-
partment) and tetra (N2O2, imine compartment) coordination sites.
In these systems, one metal is in 6-coordinate site and the other
−
is in 4-coordinate site, if a ligand like ClO4 is coordinated, this
site can be made 5-coordinate. The advantage of this ligand is
that the macrocyclic ring size can be controlled in order to study
the spectral, electrochemical, catalytic properties. Additionally the
presence of high valued cyclam group allows us to use these ligands
and their complexes as potential antimicrobial agents.
2. Experimental
2.1. Analytical and physical measurements
∗
Elemental analysis of the complexes was obtained using
Haereus CHN rapid analyzer. 1H NMR spectra were recorded using
Corresponding author. Tel.: +91 44 2230 0488; fax: +91 44 2230 0488.
E-mail address: vnnara@yahoo.co.in (V. Narayanan).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.08.008

850
S. Sreedaran et al. / Spectrochimica Acta Part A 74 (2009) 849–854
Chart I.
JEOL GSX 400 MHz NMR spectrometer. Electronic spectral stud-
ies were carried out on a Hitachi 320 spectrophotometer in the
range 200–1100 nm. IR spectra were recorded on a Shimadzu
FTIR 8300 series spectrophotometer on KBr disks in the range
4000–400 cm−1. Molar conductivity was measured by using an
Elico digital conductivity bridge model CM-88 using freshly pre-
pared solution of the complex in dimethylformamide. The atomic
absorption spectral data were recorded using Varian spectra AA-
200 model atomic absorption spectrophotometer. Mass spectra
were obtained on a JEOL SX-102 (FAB) mass spectrometer. Cyclic
voltammograms were obtained on CHI-600A electrochemical ana-
lyzer. The measurements were carried out under oxygen free
condition using a three-electrode cell in which a glassy carbon elec-
trode was the working electrode, a saturated Ag/AgCl electrode was
the reference electrode and platinum wire was used as the auxil-
iary electrode. A ferrocene/ferrocenium (1+) couple was used as
an internal standard and E1/2 of the ferrocene/ferrocenium (Fc/Fc+)
couple under the experimental condition is 470 mV. Tetra(n-
butyl)ammonium perchlorate (TBAP) was used as the supporting
electrolyte. Room temperature magnetic moment was measured
on a PAR vibrating sample magnetometer Model-155. The hydrol-
ysis of 4-nitrophenylphosphate by the nickel(II) complexes were
Scheme 1.

S. Sreedaran et al. / Spectrochimica Acta Part A 74 (2009) 849–854
851
studied in a 10−3 M dimethylformamide by spectrophotometri-
2.3.3. [Ni2L1b(ClO4)](ClO4)
cally.
Dark green compound. Yield: 0.97 g (62%). Analytical data for
29H38N6O10Cl2Br2Ni2: Calculated (%): C, 35.59; H, 3.91; N, 8.57;
C
Ni, 11.99; Found (%): C, 35.57; H, 3.89; N, 8.60; Ni, 11.96; con-
2.1.1. Chemicals and reagents
ductance (ꢁm, S cm2 mol−1) in DMF: 78; selected IR data (KBr)
3-Chloromethyl-5-bromolsalicylaldehyde [11], 1,4,8,11-tetra-
azatricyclo[9.3.1.14,8]hexadecane [12] and 1,8-[bis(3-formyl-2-
hydroxy-5-brmol)benzyl]-4,11-diazaniatricyclo[9.3.1.14,8]hexad-
ecane dichloride [13] were prepared by following the literature
method using 5-bromo salicylaldehyde instead of 5-methyl sali-
cylaldehyde. 5-Bromo salicylaldehyde, analytical grade methanol,
acetonitrile and dimethylformamide were purchased from
Qualigens and used as such. TBAP used as supporting elec-
trolyte in electrochemical measurement was purchased from
Fluka and recrystallized from hot methanol. (Caution! TBAP is
potentially explosive; hence care should be taken in handling
the compound). All other chemicals and solvents were of ana-
lytical grade and were used as received without any further
purification.
(␯/cm−1): 3349 ␯(NH), 1634 [s, ␯(C N), 1092, 1089 (w) [␯(ClO4
coordinated], 1105 (w) [␯(ClO4−) uncoordinated], 629 (s).
)
−
2.3.4. [Ni2L1c(ClO4)](ClO4)
Dark green compound. Yield: 0.98 g (62%). Analytical data for
30H40N6O10Cl2Br2Ni2: Calculated (%): C, 36.30; H, 4.06; N, 8.47;
C
Ni, 11.82; Found (%): C, 36.28; H, 4.09; N, 8.44; Ni, 11.80; FAB mass
(m/z) (%): [Ni2L1c–2ClO4]+ 794; Conductance (ꢁm, S cm2 mol−1
)
in DMF: 81; selected IR data (KBr) (ꢂ/cm−1): 3357 ␯(NH), 1637
[s, ␯(C N)], 1098, 1091 (w) [␯(ClO4−) coordinated], 1101 (w)
[␯(ClO4−) uncoordinated], 624 (s).
2.3.5. [Ni2L1d(ClO4)](ClO4)
Dark green compound. Yield: 1.03 g (63%). Analytical data for
C
32H36N6O10Cl2Br2Ni2: Calculated (%): C, 37.93; H, 3.58; N, 8.30;
2.2. Synthesis of ligand (L)
Ni, 11.59; Found (%): C, 37.90; H, 3.56; N, 8.33; Ni, 11.57; FAB mass
(m/z) (%):[Ni2L1dClO4–ClO4]+ 913;Conductance (ꢁm, S cm2 mol−1
)
2.2.1. Synthesis of 1,8-[bis(3-formyl-2-hydroxy-5-
in DMF: 95; Selected IR data (KBr) (ꢂ/cm−1): 3335 ␯(NH), 1643
[s, ␯(C N)], 1098, 1087 (w) [␯(ClO4−) coordinated], 1110 (w)
[␯(ClO4−) uncoordinated], 626 (s).
bromo)benzyl]-1,4,8,11-tetraazacyclotetradecane (L)
The compound 1,8-[bis(3-formyl-2-hydroxy-5-bromo)benzyl]-
4,11-diazaniatricyclo[9.3.1.14,8]hexadecane
dichloride
(1 g,
0.0014 mol) was dissolved in 200 ml of an aqueous NaOH
solution (0.3 M) with stirring. After stirring for 4 h, the solu-
tion was extracted with CHCl3 (5 × 30 mL). The combined CHCl3
extracts were dried with anhydrous MgSO4, and concentrated
under vacuum to give the expected compound 1,8-[bis(3-formyl-
2-hydroxy-5-bromo)benzyl]-1,4,8,11-tetraazacyclotetradecane.
Yield: 75%; m.p.: 300 ◦C (dec). Analytical data for C26H34N4O4Br2:
Calculated: C, 49.86; H, 5.47; N, 8.95; Found: C, 49.78; H, 5.42;
N, 8.87. Selected IR (KBr): 3407 cm−1 ␯(OH), 3289 cm−1 ␯(NH),
1680 cm−1 ␯(C O); 1H NMR ı (ppm in CDCl3): 1.49 (q, 4H, ␤-CH2),
2.56 (br, s, 2H, NH), 2.38 (m, 8H, ␣-CH2), 3.39 (s, 8H, H2C–N–CH2)
4.1 (s, 4H, N–CH2–Ar), 7.76 (d, 4H, Ar–H), 10.20 (s, 2H, Ar–CHO),
12.64 (br, s, Ar–OH). 13C NMR ı (ppm in DMSO-D6): 29.8, 49.7,
53.7, 55.0, 55.6, 61.6 122.6, 124.8, 126.0, 131.0, 136.1, 159.0, 196.
ꢀmax, nm (ε, M−1 cm−1) in DMF: 289 (22,760).
2.3.6. [Ni2L1e(ClO4)](ClO4)
Dark green compound. Yield: 0.93 g (55%). Analytical data for
C36H38N6O10Cl2Br2Ni2: Calculated (%): C, 40.68; H, 3.60; N, 7.91;
Ni, 11.04; Found (%): C, 40.66; H, 3.57; N, 7.94; Ni, 11.00; Con-
ductance (ꢁm, S cm2 mol−1) in DMF: 95; Selected IR data (KBr)
(ꢂ/cm−1): 3300 ␯(NH), 1650 [s, ␯(C N), 1091, 1087 (w) [␯(ClO4
coordinated], 1103 (w) [␯(ClO4−) uncoordinated], 626 (s).
)
−
3. Results and discussion
Five macrocyclic binuclear nickel(II) complexes were synthe-
sized by Schiff’s base condensation of the ligand with diamines in
presence of metal ion (Scheme 1). In all the prepared nickel(II) com-
plexes, one of the nickel(II) ion in the amine compartment is six
coordinated, and the other in the imine compartment is five coordi-
nated. Conductivity measurements of (ꢁm, 75–95 S cm2 mol−1) all
the complexes show that they are 1:1 conductors in DMF solution
[14]. The effective magnetic moment values for the nickel(II) com-
plexes at room temperature are in the range of 2.58–2.74ꢃB, which
is normally observed for octahedral coordinated nickel(II) ions due
to the presence of strong nickel–nickel interaction. All attempts
to grow single crystals of the complexes (e.g. by the diffusion of
2.3. Synthesis of the macrobicyclic binuclear nickel(II) complexes
2.3.1. [Ni2L1a(ClO4)](ClO4)
A methanolic solution of nickel(II) perchlorate hexahydrate
(0.74 g, 0.002 mol) was added to a hot solution of ligand compo-
nent (L) (1.00 g, 0.002 mol) in methanol, followed by the addition
of 1,2-diaminoethane (0.12 g, 0.002 mol) and triethylamine (0.41 g,
0.004 mol) in methanol. After an hour, another 1 equiv. of nickel(II)
perchlorate (0.74 g, 0.002 mol) was added and the solution was
refluxed in a water bath for 24 h. The resulting solution was fil-
tered whilst hot and allowed to stand at room temperature. After
slow evaporation of the solvent at 25 ◦C, dark green compound was
collected by filtration, which was recrystallised in acetonitrile, and
dried in vacuum.
2.3.2. [Ni2L1a(ClO4)](ClO4)
Dark green compound. Yield: 1.00 g (65%). Analytical data
for C28H36N6O10Cl2Br2Ni2: Calculated (%): C, 34.86; H, 3.76; N,
8.71; Ni, 12.17; Found (%): C, 34.83; H, 3.74; N, 8.69; Ni, 12.15;
FAB mass (m/z) (%): [Ni2L1a(ClO4)-ClO4]+ 865; conductance (ꢁm
,
S cm2 mol−1) in DMF: 75. Selected IR data (KBr) (ꢂ/cm−1): 3355
␯(NH), 1630 [s, ␯(C N)], 1107, 1105 (w) [␯(ClO4−) coordinated],
1100 (w) [␯(ClO4−) uncoordinated], 630 (s).
Fig. 1. Cyclic voltammogram of the complexes: (a) [Ni2L1a(ClO4)](ClO4), (b)
[Ni2L1b(ClO4)](ClO4) and (c) [Ni2L1c(ClO4)](ClO4) (reduction process).

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S. Sreedaran et al. / Spectrochimica Acta Part A 74 (2009) 849–854
Table 1
Electronic spectral data of binuclear nickel(II) complexes.
No.
Complexes
ꢀmax, nm (ε, M−1 cm−1
)
d–d
Charge transfer
1
2
3
4
5
[Ni2L1a(ClO4)](ClO4)
[Ni2L1b(ClO4)](ClO4)
[Ni2L1c(ClO4)](ClO4)
[Ni2L1d(ClO4)](ClO4)
[Ni2L1eClO4)](ClO4)
985 (42), 726 (101), 612 (278)
1022 (50), 708 (87), 658 (164)
1048 (34), 850 (75), 704 (205)
956 (63), 730 (68), 600 (193)
995 (55), 756 (85), 690 (178)
382 (10 800), 300(17 600)
396 (10 700), 315 (18 400)
421 (11 900), 342 (15 200)
371 (14 300), 304 (18 200)
373 (13 600), 305 (21 00)
Table 2
Electrochemical* and hydrolysis of 4-nitrophenylphosphate** data for the complexes.
No.
Complexes
Reduction (at cathodic)
Oxidation (at anodic)
Rate constant (k)
E1 (V)
E2 (V)
E1 (V)
E2 (V)
(×10−2 min−1
)
pc
pc
pc
pc
1
2
3
4
5
[Ni2L1a(ClO4)](ClO4)
[Ni2L1b(ClO4)](ClO4)
[Ni2L1c(ClO4)](ClO4)
[Ni2L1d(ClO4)](ClO4)
[Ni2L1eClO4)](ClO4)
−0.79
−0.64
−0.60
−0.77
−0.60
−1.23
−1.17
−1.10
−1.24
−1.18
0.86
0.95
0.98
0.77
0.84
1.22
1.26
1.36
1.23
1.32
5.85
7.48
9.14
1.36
3.09
*Measured by CV at 50 mV/s E vs Ag/AgCl conditions: GC working and Ag/AgCl reference electrodes; supporting electrolyte TBAP; concentration of complex 1 × 10−3 M,
concentration 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. The rate constant values are the average of three experiments.
diethyl ether vapor into DMF solutions or recrystallization of the
complexes from acetonitrile) have failed and only green powder or
micro-crystals were obtained. Spectral, electrochemical, catalytic
and antimicrobial studies of the complexes were carried out.
pyramidal or trigonal bipyramidal coordination environment of at
least one of the nickel(II) ions [21]. The electronic spectral studies
inferred that an increase in ꢀmax (red shift) of the d–d transition
of nickel(II) ion in the ligand L1a to L1c and L1d to L1e. This is due
to the flexibility of the macrocyclic ring that is imparted by the
distortion of the geometry of the complexes due to the increase in
macrocyclic ring size, which causes more distortion of the geometry
[22,23].
3.1. Spectral studies
The FT IR spectra of the binuclear nickel(II) complexes show
bands in the region 3300–3357 cm−1, indicating the presence of
NH groups in the complexes. The IR spectra of ligand L shows a
band at 1680 cm−1 due to the presence of C O (–CHO) group. All
the complexes show a sharp band in the region of 1630–1650 cm−1
due to the presence of C N in the complexes. The complete disap-
3.2. Electrochemical properties of the complexes
3.2.1. Reduction process at negative potential
The electrochemical data of the complexes are summarized
in Table 2. The electrochemical behavior was studied by cyclic
voltammetry in DMF containing 10−1 M tetra(n-butyl)ammonium
perchlorate over the range of 0 to −1.30 V. Cyclic voltammograms
for the complexes are shown in Fig. 1. The macrocyclic doubly
phenoxo-bridged dinickel complexes typically undergo two well-
separated one electron reductions [24,25]. It is observed that all
the binuclear complexes show two irreversible reduction waves in
the cathodic potential region. The first reduction potential ranges
from −0.60 to −0.79 V and the second reduction potential lies in
the range of −1.10 to −1.24 V. Controlled potential electrolysis
pearance of C O peak (–CHO) and the appearance of a new C
N
peak shows the effective Schiff base condensation between the
aldehyde and the amines [15–17]. All the binuclear nickel(II) com-
plexes showed two sharp peaks at near 1100 cm−1 and in the region
of 624–630 cm−1, which are assigned to perchlorate ions [18]. The
peak around 1100 cm−1 is split, which clearly explains the presence
of a coordinated perchlorate ion [19], while the other peak which
does not show any splitting, indicate the presence of an uncoordi-
nated perchlorate ion. The IR spectral data of the complexes show
the presence of two different types of perchlorate ions. Further, the
appearance of new bands in the 1537–1555 cm−1 region in all the
complexes suggests phenoxide bridging with the metal ions [20].
Electronic spectra of five dinickel complexes were obtained in
DMF medium (Table 1). The electronic spectra of all the complexes
exhibit three main features. (i) One or two peaks in the range of
300–342 nm is assigned to the intra-ligand charge transfer tran-
sition (␲–␲*). (ii) An intense peak in the range of 371–421 nm is
due to ligand-to-metal charge transfer transition, and (iii) the d–d
transition for the nickel(II) complexes show three main bands in
the range of 612–1048 nm, which is characteristic of Ni2+ in the
5/6 coordination environment [21]. Although the appearance of
the spectra bear similarities with those for octahedrally coordi-
nated Ni(II) ions, the assignment of the three most intense bands
3
3
3
3
3
3A2g → T2g(P), A2g → T1g(F), A2g → T1g(P) to the spin allowed
transitions of an octahedrally coordinated d8 [21] ion would be
a misinterpretation. Further, the higher molar extinction coeffi-
cients of the absorption band maxima in fact indicate a square
Fig. 2. Cyclic voltammogram of the complexes: (a) [Ni2L1a(ClO4)](ClO4), (b)
[Ni2L1b(ClO4)](ClO4) and (c) [Ni2L1c(ClO4)](ClO4) (oxidation process).

S. Sreedaran et al. / Spectrochimica Acta Part A 74 (2009) 849–854
853
Fig. 4. Hydrolysis of 4-nitrophenylphosphate by binuclear nickel(II) complexes:
(a) [Ni2L1a(ClO4)](ClO4), (b) [Ni2L1b(ClO4)](ClO4), (c) [Ni2L1c(ClO4)](ClO4), (d)
[Ni2L1d(ClO4)](ClO4) and (e) [Ni2L1e(ClO4)](ClO4). The inset is the time dependent
growth of p-nitrophenolate chromophore in the presence of [Ni2L1a(ClO4)](ClO4).
Fig. 3. Cyclic voltammogram of the complexes: (a) [Ni2L1dClO4)](ClO4) and (b)
[Ni2L1e(ClO4)](ClO4) (oxidation process).
was also carried out and the experiment reports that each couple
corresponds to one electron transfer process. The two reduction
processes are assigned as follows:
nitude of the bimetallic reduction process is subject to vary due to
changes caused by the nature of the ligand, geometry and by the
presence of the other metal [34].
NiIINiII → NiIINiI → NiINiI
3.2.2. Oxidation process at anodic potential
The first reduction potential of the Ni2+/Ni+ couple for the
complexes of L1a–c in the range of −0.60 to −0.79 (Table 2) are
attributed to the reduction of nickel(II) in the imine compartment
(N2O2). The second reduction wave for the nickel(II) complexes
of L1a–c in the range −1.10 to −1.23 V, is attributed reduction of
nickel(II) ion in the amine compartment (N4O2) [25]. From the
result it is observed that, both first and second reduction potentials
shift towards anodically, from −0.79 to −0.60 V and from −1.23
to −1.10 V, respectively, as the number of methylene groups is
increased [26–28]. For example, the reduction potential for the
nickel(II) complex of L1a is (E1 −0.79 V and E2 −1.23 V), which
All the nickel complexes show two oxidation processes in
the range 0.77–1.36 V. The cyclic voltammogram of the binuclear
nickel(II) complexesare shown in Figs. 2and3 andthe dataaresum-
marized in Table 2. The oxidation process is irreversible in nature.
Controlled potential electrolysis experiment indicates that the two
oxidation peaks are associated with stepwise oxidation process at
nickel(II) center
NiIINiII → NiIINiIII → NiIIINiIII
The first and second oxidation potential of the complexes of
pc
pc
L1a−c shifts towards more positive value [28] (from Epc 0.86 to
1
are more negative in comparison to those of the complex of L1b
2
(E1 −0.64 V and E2 −1.17 V), which, in turn, are more negative
0.98 V and Epc 1.22 to 1.36 V) as the chain length increases in the
pc
pc
in comparison to those of the complex of L1c (E1pc −0.60 V and E2
imine compartment. This is because as the ring size increases due to
flexibility, the planarity of the complex decreases and the electro-
chemical oxidation process occurs with difficult. For example the
pc
−1.10 V). This shows that as the chain length increases, the entire
macrocyclic ring becomes more flexible, which causes easy reduc-
tion. Thus, the large size of the cavity easily holds the reduced cation
and stabilizes the formation of Ni(I) in both compartments. The
first and second oxidation potentials of the complex of L1c is (Epc
1
2
0.98 and Epc 1.36 V) which are more positive when compared to
the complex of L1b (Epc1 0.95 V and Epc2 1.26 V) which is more pos-
same trend is also observed for the complexes of the ligands L1d–e
.
itive than complexes of L1a (Epc 0.86 V and Epc 1.22 V). Similar
trend has been observed for complexes of L1d–e. This is because for
complexes with aromatic diimines, an increase in unsaturation will
decrease the electron on the metal through delocalization, on to the
ligand and this increases the difficulty to oxidize the metal ion.
1
2
The reduction of complexes of L1d and L1e is rather difficult when
compared to the reduction of the complexes of the ligands L1a–c
due to the planarity induced by the aromatic ring, which makes the
system more rigid.
Further, the presence of electron-withdrawing substituent (Br)
at the para position to the phenoxide oxygen in the phenyl ring
plays an important role in the reduction process [29,30–33]. The
nickel(II) complexes of the ligands L1a−e reduced at lower nega-
tive potential than the complexes which contains electron donating
(−CH3) para substituent. This may be due to the poor electron
donating nature of the bromide ion, which reduces the electron
density on the metal ion [33b]. These results show that the mag-
3.3. Kinetic studies of hydrolysis of 4-nitrophenylphosphate
The catalytic activity of the nickel(II) complexes on the
hydrolysis of 4-nitrophenylphosphate was determined spec-
trophotometrically by following our previous literature reports
[29]. Plots of log (A␣/A␣ − At) vs time for hydrolysis of 4-
Table 3
Antibacterial and antifungal screening data of complexes.
No
Complexes
Representation zone of inhibition (100 ␮g/ml)
Antibacterial
Antifungal
S.a.
B.c.
K.p.
P.a.
E.c.
C.a.
1
2
3
4
5
[Ni2L1a(ClO4)](ClO4)
[Ni2L1b(ClO4)](ClO4)
[Ni2L1c(ClO4)](ClO4)
[Ni2L1d(ClO4)](ClO4)
[Ni2L1eClO4)](ClO4)
11
13.5
11
12.5
14
10
11
14.5
13
14.5
14
10
14
12
15
10
14
13.5
19
17
–
–
–
–
–
10
11
13
17
18.5
S.a.: Staphylococcus aureus; B.a.: Bacillus subtilis; K.p.: Klebsiella pneumonia; P.a.: Pseudomonas aeruginosa; E.c.: Escherichia coli; C.a.: Candida albicans.

854
S. Sreedaran et al. / Spectrochimica Acta Part A 74 (2009) 849–854
nitrophenylphosphate activity of the complexes are obtained and
shown in Fig. 4. The observed initial rate constant values for all the
nickel(II) complexes are given in Table 2. The observed rate constant
values for catalysis of the hydrolysis of 4-nitrophenyl phosphate are
Acknowledgements
Financial support from University Grants Commission (RGN-
SRF), New Delhi, is gratefully acknowledged. We are thankful to
CDRI-Lucknow for providing FAB Mass spectral analysis.
in the range of 1.36 × 10−2–9.14 × 10−2 min−1
.
The catalytic activities of the binuclear complexes are found to
increase as the macrocyclic ring size increases due to the intrinsic
flexibility, i.e. increase in the chelate ring size enhances the rate
constant of hydrolysis fairly well by producing distortion in the
geometry around the metal ion that enhances the accessibility of
the metal ion for the bonding of phosphate and OH groups. The cat-
alytic activity of the complexes containing aromatic diimines (L1d
and L1e) is found to be less than that of the complexes contain-
ing aliphatic diimines. This may be due to the planarity, which is
associated with aromatic ring, imparts less catalytic efficiency due
to the rigidity of the systems as observed in the case of previous
literature reports [35,36,10b]. It is seen that if the reduction poten-
tial is too negative, the complex has a decreased catalytic activity
due to a more difficult reduction to metal(I), and a less negative
reduction potential of the complex gives a higher catalytic activity
since the donor atoms stabilize metal(I) at the expense of metal(II)
[37].
The presence of electron-withdrawing substituent (Br) at the
para position to the phenoxide oxygen in the phenyl ring is the rea-
son for the observed higher hydrolysis activity than the complexes
containing electron donating –CH3 groups at the para position
[30–33]. The literature reports [33b] also show that the complexes
containing electron-withdrawing groups shows higher catalytic
activity than complexes, which contain electron donating sub-
stituent.
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