Spectroscopic studies, cyclic voltammetry and synthesis of nickel(II) complexes with N4, N2O2 and N4S 2 donor macrocyclic ligands

Article abstract of DOI:10.1016/j.saa.2005.02.006

Nickel(II) complexes of the general composition Ni(L)X₂ (where X = SCN, NO₃ and 1/2SO₄ and ligands = L¹ L ² and L³) have been synthesized and characterized by elemental analyses, magnetic m

Full text of DOI:10.1016/j.saa.2005.02.006

Spectrochimica Acta Part A 62 (2005) 518–525
Spectroscopic studies, cyclic voltammetry and synthesis of
nickel(II) complexes with N4, N2O2 and N4S2 donor
macrocyclic ligands
Sulekh Chandra∗, Rajiv Kumar
Department of Chemistry, Zakir Husain College, University of Delhi, Jawahar Lal Nehru Marg, New Delhi 110002, India
Received 31 January 2005; received in revised form 31 January 2005; accepted 7 February 2005
Abstract
Nickel(II) complexes of the general composition Ni(L)X2 (where X = SCN, NO3 and 1/2SO4 and ligands = L1 L2 and L3) have
been synthesized and characterized by elemental analyses, magnetic moments, IR, 1H NMR, 13C NMR and electronic spectral stud-
ies. Nickel(II) ions, such as nitrates, thiocyantes and sulphates were found to act as templates for the cyclic condensations [1 + 1] and
[2 + 2] of NH2 C6H4 O CH2 CH2 O C6H4 NH2, NH2 (CH2)2 NH2 and NH2 CH(CH3) CH2 NH2 with C6H5 CO CO C6H5,
C6H5 CO CH2 CO C6H5 and (COOH CH2 CH2)2S.
All the complexes show magnetic moments corresponding to two unpaired electrons except [Ni(L1)](NO3)2 and [Ni(L2)](NO3)2 complexes
which are diamagnetic. Electronic spectroscopy was used to analyse the differences between the paramagnetic and diamagnetic forms.
Electrochemical properties have been studied extensively for Ni(III/II) and Ni(II/I) couples. The equilibrium between the paramagnetic and
diamagnetic forms and the nickel(III/II) couple are strongly dependent on the electrolyte. It has been observed that the sulphate group
coordinated selectively on the apical position of the nickel(II) centers of the compounds. The structural and electrochemical studies suggest
that cooperative effects, involving coordination of sulphate to one nickel center, is responsible for the recognition of this anion. Various ligand
field parameters have been calculated and discussed.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Macrocyclic ligands; Nickel(II); Mass spectroscopy; 1H NMR; 13C NMR; Electronic; IR; Cyclic voltammetry
1. Introduction
coordination sites lead to the formation of homo and het-
eroploynuclear complexes [7,8]. This opens opportunity to
design the macrocyclic ligands containing additional donor
atoms such as N2O2, N4, or S2N4 in the chain. A variety
of possible applications, including crystal engineering, the
preparation and stabilization of anionic metal complexes, as
well as cooperatively and selectivity of anion binding have
been described with macrocyclic ligands complexes. In a
series of recent papers, the synthetic and characterization
aspects of a number of transition metal complexes of dif-
ferent donor atoms have been reported [9,10]. The electro-
chemical properties have also been studied extensively for
Ni(III)/Ni(II) and Ni(II)/Ni(I) couples which were affected
by coordination sites and donor environments. This paper
gives an insight about the preparation, spectral, characteri-
zation and cyclic voltammetry studies of some N4, N2O2,
Molecules possessing O, N and S donor sites are impor-
tant in the development of coordination chemistry as well
in the bio-mimetic chemistry of a number of metals, par-
ticularly in transition metals [1–3]. It is known from many
reports that nickel(II) complexes of macrocyclic ligands can
act as versatile supramolecular receptors for globular guests
like carborane, C60, C70 and S8 [4–6]. A rapidly emerging
area of chemical interest in resent year is in the synthesis
of nickel(II) complexes derived from such macrocyclic lig-
ands which contain polydentate coordination modes. These
∗
Corresponding author. Tel.: +91 9719102121; fax: +91 1122911267.
E-mail addresses: schandra 00@yahoo.com (S. Chandra),
chemistry rajiv@hotmail.com (R. Kumar).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.02.006

S. Chandra, R. Kumar / Spectrochimica Acta Part A 62 (2005) 518–525
519
or N4S2 donor macrocyclic ligands and their nickel(II) com-
plexes.
2. Experimental
All chemicals used were of AR grade and were purchased
from Sigma Chemical Co., U.S.A., E. Merck, Germany or
Sarabhai Merck Company, India. Ethanol used was of ana-
lytical grade procured from S.D. Fine Chemicals Pvt. Ltd. It
was dried by storing over clean, dried sodium wire, refluxed
for 30 min and distilled using a double-walled condenser at
78 ◦C.
Fig. 1. IR spectrum of macrocyclic ligand (L1).
due to the C N group is observed [11,12], which indicates
that the ligands are macrocyclic in nature.
1H NMR: (CDCl3) δ: 7.32 (10H, m), 6.81 (2H, d,
J = 7.2), 7.1 (2H, m), 6.6 (2H, m), 7.0 (2H, d, J = 7.4), 3.20
(4H, m, O CH2−). 13C NMR: 121.90, 122.15, 122.25,
123.07, (C6H4); 128.20, 128.30, 128.4, 130.07–136.09
(C6H5); 147.20, 147.30 (o-C6H4); 157.30, 155.50 (C N);
60.46–64.50 (CH2). Mass spectrum of ligand shows a peak
at 417 corresponding to molecular ion (M+ + 1). Mass spec-
trum, EIMS m/z (%) 417 (M+, 100) for ligand (L1) (Fig. 2).
1H NMR: (CDCl3) δ: 7.2–7.3 (20H, m), 7.1 (8H, m),
7.0 (4H, m); 13C NMR: 121.94, 122.10, 122.30, 123.10,
(C6H4); 128.22, 128.35, 128.96, 130.07, 136.90 (C6H5);
147.15, 147.32 (o-C6H4); 155.60, 156.48, 156.60, 156.90
(C N); 63.60, 64.60–66.65 (CH2) (Fig. 3). Mass spectrum
of ligand shows a peak at 523 corresponding molecular ion
(M+ + 1). Mass spectrum, EIMS m/z (%) 523 (M+, 100) for
ligand (L2).
3. Synthesis of the ligands
3.1. Synthesis of diamines
1,2-Di(o-aminophenoxy)ethane diamine was prepared by
the method reported [1].
3.2. Preparation of macrocyclic ligands
(L1)
(2,3-diphenyl-1,4-diaza-7,10-dioxa-5,6:11,12-
dibenzocyclododoca-1,3-diene[N2O2]) was prepared by
method earlier reported [1] and ligand L2 (2,4,9,11-
tetraphenyl-1,5,8,12-tetraazacyclotetradeca-1,4,8,11-
tertaene [N4]) is prepared by the method as given below.
An ethanolic solution (25 ml) of 1,2-diaminoethane
(0.001 mol, 0.06 g) was added to an ethanolic solution (25 ml)
of dibenzoylmethane (0.001 mol, 0.224 g) in the presence of
1 ml concentrated HCl. The resulting solution was boiled un-
der reflux for 3–6 h. On cooling, light yellowish crystals sepa-
rated out, which were filtered, washed with ethanol and dried
under vacuum over P4O10.
The bands corresponding to free COOH and NH2
groups are not observed in the IR spectrum of L3. It con-
firms complete condensation [7] between thiodiglycolic acid
and 1,2-diaminopropane. Some new bands appear at 1680,
1620, 1520 and 1325 cm−1 corresponding new groups. A sin-
gle, sharp band also observed in the region 3340–3380 cm−1
which may be assigned to ν(N H). It indicates secondary
moderate-intensity absorptions in amino group [8].
Ligand L3 (8,17-dimethyl-1,7,10,16-tetraaza-4,13-ditha-
cyclooctadeca-2,6,11,15-teraone[N4S2]).
1H NMR: (CDCl3) δ: 14.31 (4H, s), 4.54 (2H, m), 2.8
(8H, s), 1.5 (4H, m), 0.51 (6H, d, J = 4.7 Hz); 13C NMR:
13.50, 13.62 (CH3) 60.45, 61.40, 62.40, 63.90, 66.60, 67.65
To an ethanolic solution (25 ml) of thiodiglycolic acid
(0.002 mol, 0.30 g), an ethanolic solution (25 ml) of 1,2-
diaminopropane (0.002 mol, 0.15 g) in absolute ethanol
(25 ml) was added drop-wise. The resulting solution was re-
fluxed on water bath with constant stirring at a temperature of
75 ◦C. The solution was then concentrated to half of its vol-
ume under reduced pressure and kept for two days at room
temperature. Light yellowish crystals separated out. The re-
sulting solution was filtered, washed with ethanol and dried
under vacuum over P4O10.
3.3. Characterization of macrocyclic ligands
The bands corresponding to NH2 and >CO groups are
not observed in the IR spectra of L1 and L2. It indicates
that the condensation takes place between the 1,2-di(o-
aminophenoxy)ethane or 1,2-diaminoethane and benzyl or
dibenzoylmethane. A sharp band at 1599–1595 cm−1 (Fig. 1)
Fig. 2. 1H NMR spectrum of macrocyclic ligand (L2).

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S. Chandra, R. Kumar / Spectrochimica Acta Part A 62 (2005) 518–525
Fig. 3. 13C NMR spectrum of macrocyclic ligand (L2).
( CH2 ) 67.85, 67.90 ( CH ); 168.50, 169.90, 170.55,
170.60 (C O). Mass spectrum of ligand shows a peak at
377 corresponding molecular ion (M+ + 1). Mass spectrum,
EIMS m/z (%) 375 (M+, 70) for ligand (L3) (Fig. 4). Sug-
gested structures of macrocyclic ligands are given in Fig. 5.
3.4. Preparation of the complexes
A general method is used for the preparation of the
complexes. To a hot (90–95 ◦C) aqueous ethanolic solution
(25 ml) of the NiX2·xH2O (where X = NCS, NO3 and SO4)
(0.001 mol) and a hot ethanolic solution (25 ml) of the respec-
tive ligand (0.001 mol) was added. The mixture was refluxed
for about 6–5 h at 75–80 ◦C. On cooling to 6 ◦C, the com-
plexes were separated out. They were filtered, washed with
98% ethanol and dried over P4O10.
Fig. 5. Suggested structures of macrocyclic ligands.
tra of the complexes were recorded on a Perkin-Elmer FTIR
1710 automatic recording spectrophotometer in KBr. Elec-
tronic spectra of the complexes were recorded on a DMR-
21 automatic recording spectrophotometer in DMF solution.
Conductance measurements in nitromethane were carried out
on a Leeds Northup Model 4995 conductivity bridge. Anal-
4. Physical measurements
The magnetic susceptibilities were measured on a Gouy
balance using Hg[Co(NCS)4] as a calibrant. Infrared spec-
Fig. 4. Mass spectrum of macrocyclic ligand (L3).

S. Chandra, R. Kumar / Spectrochimica Acta Part A 62 (2005) 518–525
521
Table 1
Analytical data of nickel(II) complexes
Complexes
Yield
(%)
mp
Molar conductance
Colour
White
Light Blue 9.89 (9.80)
Light Blue 9.77 (9.72)
Light Blue 10.24 (10.10) 58.67 (58.50) 3.87 (3.60)
White
Sky Blue
Sky Blue
Green
Elemental analysis calculated (found) %
(◦C)
(ꢀ−1 cm2 mol−1
)
Ni
–
C
H
N
L1 C28H22N2O2
[Ni(L1)(NCS)2], NiC30H22N4O2S2 45
40
190
232
219
222
186
240
265
242
189
210
215
217
–
18.0
165
06
80.0 (79.5)
60.73 (60.7)
55.9 (55.80)
5.3 (5.0)
3.74 (3.70)
3.69 (3.60)
6.6 (6.1)
9.44 (9.40)
9.32 (9.30)
4.89 (4.75)
[Ni(L1)](NO3)2, NiC28H22N4O8
[Ni(L1)SO4], NiC28 22N2O6S
L2 C36 36N4
[Ni(L2)(NCS)2], NiC38H22N6S2
[Ni(L2)](NO3)2, NiC36 22N6O6
50
65
55
58
61
63
52
H
H
–
–
82.4 (82.0)
6.92 (6.50) 10.68 (10.16)
12.0
195
12.0
8.56 (8.50)
8.47 (8.41)
8.82 (8.75)
–
66.59 (66.30) 3.24 (3.16) 12.26 (12.10)
62.37 (62.23) 3.20 (3.00) 12.12 (12.0)
64.99 (64.50) 3.33 (3.20)
44.66 (44.0) 6.43 (5.90) 14.88 (14.20)
34.85 (34.65) 4.39 (4.26) 15.24 (15.0)
10.50 (1025) 30.07 (29.80) 4.33 (4.23) 15.03 (14.90)
H
[Ni(L2)SO4], NiC36H22N4O4S
L3 C14H24N4O4S2
[Ni(L3)(NCS)2], NiC16H24N6O4S4 57
[Ni(L3)](NO3)2, NiC14H22N6O10S2 68
8.42 (8.32)
White
180
195
80
Light Blue 10.65 (9.99)
Sky Blue
Light Blue 11.05 (10.90) 31.65 (31.61) 4.55 (3.44) 10.55 (1024)
[Ni(L3)SO4], NiC14H22N4O8S3
61
The ν(C N) bands in the infrared spectra of [Ni(L1)(NCS)2]
and [Ni(L2)(NCS)2] appeared at 1597 and 1601 cm−1, re-
spectively [10]. They also show a ν(CN) band at 2087 and
2089 cm−1, respectively (Figs. 6 and 7) corresponding to co-
ordinated NCS group.
yses of carbon and hydrogen were performed at the Mi-
croanalytical Laboratory of the Central Drug Research In-
stitute, Lucknow, India. The nitrogen contents of the com-
1
plexes were determined using Kjeldahl’s method. The H
NMR and 13C NMR spectra of the macrocyclic ligands
were recorded on Bruker Avance 300 spectrophotometer at
100 kHz modulation and higher frequency also at room tem-
perature. The voltammograms and the simultaneous current
intensity-time plots for electrolysis were registered in a x-
y Houston-Ommigraphic 2000 recorder. The values of EF
for reversible or quasi-reversible redox transformations were
calculated as the midpoints between the anodic and cathodic
peaks. The distances between peaks (ꢁE) were used as the
parameters for the characterization of the reversibility of the
electrochemical transformation.
Electronic spectra of [Ni(L1)(NCS)2] and[Ni(L2)(NCS)2]
display [11] bands at 14,285–14,290, 16,000–16,994 and
30,258–32,250 cm−1. These bands may be assigned to the
3
3
3
3
following transitions A2g → T2g(F) (ν1), A2g → T1g(F)
(ν2) and 3A2g → T1g(P) (ν3), in order of increasing energy,
3
5. Result and discussion
5.1. Nickel(II) complexes with the ligand L1
(2,3-diphenyl-1,4-diaza-7,10-dioxa-5,6:11,12-
dibenzocyclododoca-1,3-diene[N2O2]) and
L2(2,4,9,11-tetraiphenyl-1,5,8,12-
tetraazacyclotetradeca-1,4,8,11-tertaene[N4])
Fig. 6. IR spectrum of [Ni(L1)(NCS)2].
All the nickel(II) complexes with these ligands have
the composition [Ni(L)X2] (X = SCN and 1/2SO4) and
[Ni(L)](NO3)2 (where L = ligands L1 and L2). The magnetic
moments of the complexes at room temperature (300 K) lie in
the range 2.90–2.98 B.M. [13] corresponding to two unpaired
electrons, except for [Ni(L1)](NO3)2 and [Ni(L2)](NO3)2
which are diamagnetic in nature. The molar conductance
measurements lie in the range of 6–18 ꢀ−1 cm2 mol−1
for all the complexes expect for [Ni(L1)](NO3)2 and
[Ni(L2)](NO3)2 which show molar conductance in the range
of 165–195 ꢀ−1 cm2 mol−1 (Table 1) corresponding to a 1:2
electrolyte [14].
5.1.1. Thiocyanato complexes
Both the nickel(II) thiocyanato complexes with ligands L1
and L2 have the composition Ni(L1or L2)(NCS)2 (Table 1).
Fig. 7. IR spectrum of [Ni(L2)(NCS)2].

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S. Chandra, R. Kumar / Spectrochimica Acta Part A 62 (2005) 518–525
Fig. 10. IR spectrum of [Ni(L1)](NO3)2.
Fig. 8. UV spectrum of A and B are [Ni(L1and L2)(NCS)2] and C is
[Ni(L1)](NO3)2.
trons)inthesolidaswellasinbenzenesolution. Thissuggests
a square-planar geometry around nickel(II) [17].
The electronic spectra of these complexes display
three well-defined bands in the range of 18,115–19,200,
22,540–23,540 and 26,240–27,450 cm−1 these may be
respectively, characteristic of an octahedral geometry [15]
(Fig. 8).
On the basis of above studies, a six-coordinated octahe-
dral geometry may be suggested for these thiocyanato com-
plexes. Suggested structures of the complexes are given be-
low (Fig. 9).
1
1
1
1
assigned, to A1g → A2g (ν1), A1g → B1g (ν2) and
1A1g → Eg (ν3) transitions, respectively characteristic to
1
square-planar geometry. The first two bands are purely d–d
transitions, whereas ν3 obviously is enveloped by strong
charge transfer [15]. On the basis of above studies, four-
coordinate square-planar geometry may be suggested for
[Ni(L1)](NO3)2 and [Ni(L2)](NO3)2. Suggested structures
of the complexes are given below (Fig. 11).
5.1.2. Nitrate complexes
Both the nickel(II) nitrate complexes with ligands L1 and
L2 have the composition Ni(L1or L2) (NO3)2 (Table 1).The
ν(C N) bands in the infrared spectra of [Ni(L1)](NO3)2 and
[Ni(L2)](NO3)2 appear at 1590 and 1594 cm−1, respectively
[14] and also show bands at 1384 and 1385 cm−1 (Fig. 10)
corresponding to an un-coordinated nitrate group. Further,
the complexes have diamagnetic character (no unpaired elec-
5.1.3. Sulphate complexes
Both the nickel(II) sulphate complexes with ligands L1
and L2 have the composition [Ni(L1)SO4] and [Ni(L2)SO4]
Fig. 9. Suggested structure of [Ni(L1or L2)(NCS)2].
Fig. 11. Suggested structures of [Ni(L1or L2)] (NO3)2.

S. Chandra, R. Kumar / Spectrochimica Acta Part A 62 (2005) 518–525
523
Fig. 14. IR spectrum of [Ni(L3)](SO4).
Fig. 12. Suggested structure of [Ni(L1)](SO4).
(Table 1). The ν(C N) bands in the infrared spectra
of [Ni(L1)SO4] and [Ni(L2)SO4] appeared at 1590 and
1594 cm−1, respectively [8]. These complexes also show
bands at 940 (ν1), 1050 (ν2) and 1150 (ν3) cm−1 and 942 (ν1),
1055 (ν2) and 1160 (ν3) cm−1 corresponds to the unidentate
sulphate group [14].
may be assigned to ν(N H) which indicates secondary
moderate-intensity absorptions in amino group shifted to
lower side as comparative to the macrocyclic ligand [8,16].
5.2.1. Thiocyanate complex
On the basis elemental analysis this complex has the com-
position [NiL3](NCS)2 (Table 1). IR spectrum shows ν(CN)
absorptions at 2020 cm−1 indicating un-coordinated NCS
group [14]. The electronic spectrum of this complex exhibits
three strong bands at 11,111 (ν1), 15,384 (ν2) and 27,777
The electronic spectra of the complexes show two ab-
sorption bands at 10,425 and 12,475 cm−1 these may be as-
3
3
3
3
signed to E → A2 (F) (ν1) and E → B2 (F) (ν2) transi-
tion, respectively corresponding to a five-coordinate square-
pyramidal geometry [17]. Suggested structures of the com-
plexes are given below (Figs. 12 and 13).
3
(ν3) cm−1, [17]. These bands may be assigned to 3B1g → Eg
3
3
(ν1) and 1B1g → B2g (ν2) and 3B1g → A2g (ν3) transition,
respectively (Fig. 15) on the basis of symmetry arguments
[18–20].
5.2. Nickel(II) complexes with the ligand L3
(8,17-dimethyl-1,7,10,16-tetraaza-4,13-
dithiacyclooctadeca-2,6,11,15-teraone[N4S2])
5.2.2. Nitrate complex
On the basis elemental analysis this nitrate complex has
the composition [Ni(L3)](NO3)2. IR spectrum shows ν(CN)
absorptions at 1385 cm−1 corresponding to un-coordinate ni-
trate group [14]. The electronic spectrum of this complex
exhibits three strong bands at 11,000 (ν1), 15,258 (ν2) and
26,766 (ν3) cm−1, [17]. These bands may be assigned to
All the complexes have the composition Ni(L3)X2
(X = NO3 or NCS) and Ni(L3)SO4 (Table 1). The magnetic
moments of the complexes lie in the range of 2.90–3.01 B.M.
corresponding to two unpaired electrons.
The molar conductance of Ni[(L3)]X2 (X = NO3
or NCS) in nitrobenzene was found in the range
180–195 ꢀ−1 cm−1 mol−1 which indicates that the
complexes are 1:2 electrolyte and may be formulated
as [Ni(L3)]X2 (X = NO3 and SCN). But the molar conduc-
tivity measurement of Ni[(L3)]SO4 in nitrobenzene was
found 80 ꢀ−1 cm−1 mol−1 which indicates that the complex
is 1:1electrolyte and may be formulated as [Ni(L3)]SO4.
A single, sharp bands in the IR region 3375–3340 cm−1
3
3
3
3B1g → Eg (ν1)and1B1g → B2g (ν2)and3B1g → A2g (ν3)
transition, respectively (Fig. 15) on the basis of symmetry ar-
guments [18–20].
5.2.3. Sulphate complex
On the basis elemental analysis this sulphate complex
has the composition [Ni(L3)]SO4. IR spectrum shows ab-
sorption at 1133 (ν2) and 1005 (ν1) cm−1 (Fig. 14) indicat-
ing un-coordinated nature of sulphate group [14]. The elec-
tronic spectrum of this complex exhibits three strong bands
at 10,526 (ν1), 14,285 (ν2) and 25,000 (ν3) cm−1, [17]. These
bands may be assigned to 3B1g → Eg (ν1) and 1B1g → B2g
3
3
(ν2) and 3B1g → A2g (ν3) transition, respectively (Fig. 15)
3
on the basis of symmetry arguments. Suggested structures of
the complexes are given below (Fig. 16) [18–20].
6. Ligands field parameter
Lever [17] has devised normalized spherical harmonic
Hamiltonian parameters applicable to molecules of D4h sym-
metry. The values of ligand field parameters Dt (540–300),
Fig. 13. Suggested structure of [Ni(L2)](SO4).

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S. Chandra, R. Kumar / Spectrochimica Acta Part A 62 (2005) 518–525
Fig. 17. Cyclic voltammogram [Ni(L1)(NCS)2] in the anodic region.
Fig. 15. UV spectrum of [Ni(L3)]X2 X = CI, NCS and SO4.
Dqz (645–610), Dqxy (1245–1232), and McClure’s [21] pa-
rameters (δt2g (1011–310) and δeg) are calculated. ␴-bonding
along the xy-plane is strong. It is related to the negative sign
of the δσ (723–127) values. But the δπ (575–190) values are
positive in our result. It indicates that the ␲-bonding effect
is relatively more important along the axial direction than
␴-bonding.
7. Cyclic voltammetry
Perhaps as a consequence of the complicated nature of
these reductions, we observed minimal catalytic activity for
the one-electron reduction of the nickel(II) complex. The
electrochemical behaviour of the nickel(II) complexes was
studied in acetonitrile solution. The reduction potential ob-
served for nickel(II) complexes have a slight difference com-
paratively non-complex nickel(II) ion.
Fig. 18. Cyclic voltammogram [Ni(L2)(NCS)2] in the anodic region.
Cyclic voltammetry spectra show that the oxidation and
reduction potentials of the nickel(II) complexes of all lig-
ands lie in the range = +0.040 to +0.045 (V) and −1.60 to
−1.70 (V) quasi-reversible redox process, assigned to the
Ni(III)/Ni(II) couple was observed (Figs. 17–20). EF [16]
(binding constant) = 1.085 V (ꢁE = 86 mV). An irreversible
process where Ecat = −1.48 V is attributed to the Ni(II)/Ni(I)
couple [22] which indicates two electron electrochemical re-
dox behaviour.
Fig. 19. Cyclic voltammogram [Ni(L1)(NCS)2] in the cathodic region.
Fig. 16. Suggested structure of [Ni(L3)X]X, where X = SCN, NO3 and
1/2SO4.
Fig. 20. Cyclic voltammogram [Ni(L2)(NCS)2] in the cathodic region.

S. Chandra, R. Kumar / Spectrochimica Acta Part A 62 (2005) 518–525
525
According to Nernst equation, the greater the difference
between the values of the reduction potential of the metal
complexes comparatively to the metal ions, the higher is the
complexation constant. It can be determine that the more neg-
ative the potential is, the greater the constant. It may be as-
sumed that the ligand forms stable complexes with nickel(II)
comparatively other metal salts. During cyclic voltammetry
studies, it is find out that the oxidation and the reduction po-
tential of the nickel(II) complex is corresponding more neg-
ative that for square planar complexes of nickel(II). This fact
can be attributed to the coordination of the axial groups of the
macrocyclic nickel(II) are coordinated to the metal ion. The
EF values for the single quasi-reversible redox transformation
of the nickel(II) complexes are strongly anion dependent and
decrease in the order SCN > NO3 > SO4. This sequence is
typical for Ni(III)/Ni(II) couples of azamacrocyclic ligands
and reflects the increasing stabilization of nickel(III) by elec-
trostatic effects [23,24].
field parameters are also calculated and their values in our
result help to explain the stereochemistry of the complexes.
Acknowledgements
Oneoftheauthors(RajivKumar)gratefullyacknowledges
his younger brother Bitto for motivation. Thanks are also
due to the Principal of Zakir Husain College, New Delhi,
for providing laboratory facilities, Nitin Kampani, Central
Science Librarian, Delhi University for providing guidance
in computer software programming, to the University Grants
Commission, New Delhi for financial assistance and the Uni-
versity Science Instrumentation Center, Delhi University, for
recording IR spectra. Thanks are also due to the Solid State
Physics Laboratory India for recording magnetic moments.
References
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8. Conclusion
The size of cavity and different donor atoms in the macro-
cyclic ligands affected the stability of complexes, which can
be easily explained by comparing between the values of crys-
tal field splitting energy. The redox properties and stability
of the complexes towards oxidation wave explored by cyclic
voltammetry are related to the electron withdrawing or releas-
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Proposed structures of the ligands and their nickel(II) com-
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the complexes is depended on the number of coordination
sites of the ligands and different anion. All the complexes
are showing diamagnetic character comparatively other an-
ions in L1 ad L2. Magnetic moment data are supported by the
infrared information about the diamagnetic nickel(II) com-
plexes which show uncoordinated behaviour of nitrate group.
The detailed information about the geometrical arrange-
ments of atoms in a cyclic environment has been discussed on
the basis of 1H NMR and 13C NMR data. The cyclic voltam-
metry of the complexes showed that the nickel(II) complexes
undergo one-electron reduction and oxidation to from the cor-
responding nickel(I) and nickel(III). These kinds of different
oxidizing compounds may be found being used as one elec-
tron redox reagent since the former is a strong reducing agent
and the latter is strong oxidizing agent. All the complexes
in present study show kinetic property due to the chemical
reaction coupled between two charges transfer processes in
which irreversible first order chemical reaction is interposed
between two successive one-electron transfers. The EF val-
ues of the complexes are strongly anion dependent. Ligands
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