Synthesis, electrochemistry and spectral studies on cobalt(II) and manganese (II) complexes with 12-,14-,15-, and 18-membered N4, N 2O2, N2O2S, N6 donor macrocyclic ligands

Article abstract of DOI:10.1081/SIM-200035709

13-, 14-, 15-, and 18-membered N₂O₂, N₄, N₂O₂S, and N₆ donor macrocyclic ligands and their complexes with cobalt(II) and manganese(II) of the general composition ML,(NCS)₂ (where M = cobalt(II) and manganese(II) and ligands L = L¹, L², L³, and L⁴ have been synthesized. All of the macrocyclic ligands and their complexes have been characterized by elemental analyses, IR, mass, ¹H NMR, magnetic moments, electronic and EPR spectral studies. The structures of the complexes and the various coordinating groups such as >C=N, >CONH, Ph-O-CH ₂- and >NH are discussed. The redox properties of the complexes were examined by cyclic voltammetery. They exhibit a high potential Co(III)/Co(II) couple along with ligand reduction. Copyright

Full text of DOI:10.1081/SIM-200035709

Synthesis and Reactivity in Inorganic, Metal-Organic and Nano-Metal Chemistry, 35:161–170, 2005
Copyright # 2005 Taylor & Francis, Inc.
ISSN: 0094-5714 print/1532-2440 online
DOI: 10.1081/SIM-200035709
Synthesis, Electrochemistry and Spectral Studies on
Cobalt(II) and Manganese (II) Complexes with 12-,14-,15-,
and 18-membered N4, N2O2, N2O2S, N6 Donor
Macrocyclic Ligands
Sulekh Chandra and Rajiv Kumar
Department of Chemistry, Zakir Husain College, University of Delhi, New Delhi, India
1993) and anticancer drugs (Mishra et al., 2003). Interest in such
13-, 14-, 15-, and 18-membered N2O2, N4, N2O2S, and N6 donor macrocycles is growing on account of their utility in enzyme
macrocyclic ligands and their complexes with cobalt(II) and
manganese(II) of the general composition ML,(NCS)2 (where
mimicking studies and catalysis, and their rapidly growing
applications as radiopharmaceuticals, magnetic resonance
M 5 cobalt(II) and manganese(II) and ligands L 5 L1, L2, L3,
imaging reagents, and fluorescent probes (Herrera et al., 2003).
The activities of such groups depends on the nature of the
and L4 have been synthesized. All of the macrocyclic ligands
and their complexes have been characterized by elemental
1
analyses, IR, mass, H NMR, magnetic moments, electronic and substituents, the number of heteroatoms and ring size of the
EPR spectral studies. The structures of the complexes and
the various coordinating groups such as >C55N, >CONH,
Ph22O22CH222 and >NH are discussed. The redox properties of
the complexes were examined by cyclic voltammetery. They
macrocycle (Chandra and Kumar 2004; Salavati-Niasari and
Najafian 2003). The most significant change has been observed
on changing the ring size of the ligand (Autzen et al., 2003;
Cruz et al., 2003). Macrocycles with a large cavity, which
can accommodate two metal ions, can be used to bind the
metal center at fixed distances. In these systems, there is
often an additional internal or external bridging group, which
completes the structure of the binuclear species which has
the advantage of being relatively rigid and thus give structu-
rally well-defined moieties (Lodeiro et al., 2003). Due to our
continuous research in this field we expanded the synthesis
of new series of macrocyclic ligands and their metal complexes
which may be widely used in chemistry, molecular electronics,
and biology due to their appearance in biosystems (Gao and
Martell 2003; Sengupta et al., 2003). The macrocycles as
well as their manganese(II) and cobalt(II) complexes play
important roles as catalysts in the activation of small mol-
ecules. In view of the above applications, in this article we
report the synthesis of cobalt(II) and manganese(II) complexes
with the macrocyclic ligands L1, L2, L3, and L4 (Figure 1).
exhibit a high potential Co(III)/Co(II) couple along with ligand
reduction.
Keywords Magnetic moments, IR, 1H NMR, Cobalt(II), Manga-
nese(II), Macrocyclic ligands, Cyclic voltammetry
INTRODUCTION
Interest in the transition metal coordination chemistry of
.C55N, 22CONH, and .NH groups is rapidly expanding in
several areas ranging from organometallic chemistry, metal-
assisted organic transformation, radical chemistry, stabilization of
low-valent metal oxidation state to DNA labeling (Fang et al.,
Accepted 8 July 2004.
EXPERIMENTAL
One of the authors (Rajiv Kumar) is greatly, indebted to his
younger brother Bitto for motivation. Thanks are also due to the
Principal of Zakir Husain College, New Delhi for providing laboratory
All the chemicals used in this investigation were of AR
grade, and were purchased from Sigma Chemical Co.,
facilities, the University Grants Commission, Delhi for financial U.S.A., E. Merck, Germany, or Sarabhai Merck Company,
assistance and the University Science Instrumentation Centre, Delhi
India. Ethanol used was of analytical grade procured from
University for recording IR spectra. Thanks are also due to the Solid
S. D. Fine Chemicals Pvt. Ltd. All the solvents were dried
before use by passing over clean, dried sodium wire, refluxed
State Physics Laboratory, Delhi for recording magnetic moments.
Address correspondence to Sulekh Chandra, Department of Chem-
istry, Zakir Husain College, University of Delhi, Jawaher Lal Nehru
Marg, New Delhi 110002, India. E-mail: schandra_00@yahoo.com
for 30 minutes and distilled at 78 8C using a double-walled
condenser.
161

162
S. CHANDRA AND R. KUMAR
overnight at ꢀ5 8C. The white crystals formed were filtered,
washed with EtOH, and dried under vacuum over P4O10.
Characterization of Macrocylic Ligands
Ligand L1
The infrared spectrum of this ligand (Figure 2) shows bands at
1610 and 1510cm21, which may be assigned to C55N and
Ph22O22CH222 groups, respectively. The mass spectrum of the
ligand shows a peak at m/z 431 corresponding to the molecular
ion (Mþ þ 1). EIMS m/z: 30 (Mþ, 60%), 239 (40%), 149 (65%)
and 105 (45%).
Ligand L2
The infrared spectrum of the ligand shows a band at
1595 cm21, which may be assigned to the C55N group
(Figure 3). The mass spectrum of the ligand shows a peak at
m/z 495 corresponding to the molecular ion (Mþ þ 1). EIMS
m/z: 495 (Mþ, 75%), 419 (60%), 342 (65%), 266 (70%),
and 182 (45%).
FIG. 1. Structures of macrocyclic ligands.
Ligand L3
The infrared spectrum of the ligand shows various bands at
1637–1645, 1551–1575, 1371–1382, and 690–771 cm21
Preparation of Macrocyclic Ligands
.
Ligand L1 (2,4-Diphenyl-1,5-diaza-8,11-dioxo-6,7:12,13-
dibenzocyclotrideca-1,4-diene[N2O2]ane) and Ligand L2
(2,4,9,11-Tetraphenyl-1,5,8,12-traazacyclotertr-adeca-
1,4,8,11-tetraene[N4]ane
These bands may be assigned to amide I [n(C55O)], amide II
[n(C22N)], amide III [d(N22H) and amide IV [d(C55O)],
respectively. The mass spectrum of the ligand shows a peak
at m/z 357 corresponding to molecular ion (Mþ þ 1). EIMS
m/z: 365 (Mþ, 70%), 259 (45%), 183 (70%), and 139 (65%).
To an EtOH solution of (25 mL) dibenzoylmethane
(0.05 mole, 11.2g), an EtOH solution (25 mL) of the diamine,
i.e., 1,2-di(o-aminophenoxy)ethane (0.05 mole, 12.2g) or diami-
noethane (0.05 mole, 3.0 g) was added in the presence of a few
drops (ꢀ1 mL) of conc. HCI. The solution was refluxed on a
water bath at 75–80 8C for 2–3 h. The solution was then concen-
trated to half its volume under reduced pressure and kept over
night at ꢀ5 8C. The off-white crystals formed were filtered,
washed with EtOH, and dried under vacuum over P4O10.
Ligand L4
The infrared spectrum of the ligand shows bands at 3330
and 3310cm21, which may be assigned to amide II (NH). The
mass spectrum of ligand shows a peak at m/z 257 correspond-
ing to molecular ion (Mþ þ 1). EIMS m/z: 257(Mþ, 60%). 195
(40%), 165 (20%), 109 (80%), and 70 (50%). All the ligands
Ligand L3 (1,7-Diaza-4-monothia-10,13-dioxo-8,9:14,15-
cyclopetdeca-2,6-di-one [N2O2S]ane
To an EtOH solution (25 mL) of thiodiglycolic acid
(0.05 mole, 7.49 g), an EtOH solution (25 mL) of 1,2-di(o-amino-
phenoxy)ethane (0.05 mole, 12.21 g) was added. The resulting
solution was refluxed on a water bath at 75 8C for 6–7 h. The
solution was then concentrated to half of its volume under
reduced pressure and kept overnight at ꢀ5 8C. The off-white
crystals formed were filtered, washed with EtOH, and dried
under vacuum over P4O10.
Ligand L4 (1,4,7,10,13,16-Hexaazacyclooctadecane[N6]ane
To an EtOH solution (20 mL) of Bis(2-chloroethyle)amine
(0.05 mole, 7.12g), an EtOH solution (20mL) of diaminoethane
(0.05 mole, 3.0 g) was added. The resulting solution was refluxed
on a water bath at 808C for 4–5 h. The solution was then concen-
trated to half of its volume under reduced pressure and kept
FIG. 2. IR spectrum of the macrocyclic ligand L21
.

STUDIES ON COBALT(II) AND MANGANESE(II) COMPLEXES
163
RESULTS AND DISCUSSION
Cobalt(II) Complexes with Ligands L1 and L2
The formation of the complexes may be represented by the
following equation:
Co(NCS)2 ꢀ nH2O þ L ꢁ! CoL(NCS)2 þ nH2O
On the basis of the elemental analyses, the cobalt(II) com-
plexes have the general composition CoL(NCS)2 (where
L ¼ ligand L1 or L2). All of the complexes show molar conduc-
tances in the range 5–16 V21cm2mol21 corresponding to non-
electrolytes (Table 2). On complexation the positions of the
important bands in the infrared spectrum due to the groups
Ph22O22CH222 and C55N were shifted to lower frequency
compared to the macrocyclic ligands (Mandal and Nag,
1983; Ray et al., 2003). The infrared spectra of the
Co(L1)(NCS)2 and Co(L2)(NCS)2 complexes show sharp
peaks at 2080 and 2085 cm21 respectively, indicating that the
thiocyanate groups coordinate through the nitrogen of the
NCS groups (Bentini and Sabatini, 1966). Thus, these com-
plexes may be formulated as [Co(L1)(NCS)2] and
[Co(L2)(NCS)2], respectively.
FIG. 3. IR spectrum of the macrocyclic ligand L22
.
were also characterized by 1H NMR and 1H NMR spectral data
are given in Table 1.
Preparation of the Complexes
To an EtOH solution of (20 mL) containing the macrocyclic
ligand (0.05 mole), was added an EtOH solution 20 mL of the
metal salt (0.05 mole), and the resulting solution was refluxed
on a water bath at 75–85 8C for 3–5 h. The solution was
then concentrated to half of its volume under reduced
pressure and kept overnight at ꢀ5 8C. The colored crystals
formed were filtered, washed with EtOH and dried under
vacuum over P4O10
The magnetic moments of the complexes at room tempera-
ture (300 K) are 4 85–5.0 B.M. (Cotton and Willcinson, 1988;
Ray et al., 2003) corresponding to three unpaired electrons.
The electronic spectra of the cobalt(II) complexes display
three well-defined bands at 8,870–8,890 (n1,), 16,345–
16,395 (n2), and 20,110–20,225 (n3) cm21, which may be
4
assigned to the transitions, 4T1g(F) ! T2g(F) (n1),
4
4
4
4T1g(F) ! A2g(F) (n2), and T1g(F) ! T1g(P) (n3), respect-
ively, corresponding to a six-coordinate (Figure 4) octahedral
geometry (Lever, 1984; Malik and Phillips, 1975).
Physical Measurements
The magnetic susceptibilities were measured on a Gouy
balance using Hg[Co(NCS)4] as a calibrating agent. KBr
pellets of the complexes were prepared and their IR spectra
of the complexes were recorded on a Perkin-Elmer FTIR
1710 automatic-recording spectrophotometer. Electronic
Spectra (200–1100 nm) of the complexes were recorded on
a Shimadzu DMR-21 automatic-recording spectrophotometer
in Nujol mulls. Mass spectra were carried out on a Jeol,
JMX, DX-303 mass spectrophotometer. 1H NMR spectra
Cobalt(II) Complexes with Ligand L3.
The formation of the complex may be represented following
equation:
Co(NCS)2 ꢀ nH2O þ L3 ꢁ! CoL(NCS)2 þ nH2O
On the basis of elemental analyses, the cobalt(II) complex of
were recorded on a Bruker Advance 300 spectrometer at the ligand L3 has the general composition CoL3(NCS)2. This
100 kHz modulation at room temperature. The EPR spectra complex shows a molar conductance at 150 V21cm2mol21 cor-
of the complexes were recorded as polycrystalline samples at responding to a 1 : 2 electrolytes (Chandra, 1982) (Table 2).
room temperature (25 8C) on a Varian E-4 EPR spectrometer This complex thus may be formulated as [Co(L3)](NCS)2.
operating at 9.4 GHz and 100 kHz field modulation and phase On complexation, the position of the important bands in the
sensitive detection. Conductance measurements in nitro- infrared spectrum due to the following groups: amide II
methane were carried out on a Leeds Northrup model 4995 [n(C22N)], amide III [d N22H)], Ph22O22CH2 and
conductivity bridge. Analyses of carbon and hydrogen were CH222S22CH222 groups, are shifted to lower frequency
performed by the Microanalytical Laboratory of the Central compared to the free macrocyclic ligand (Abu Ismaiel et al.,
Drug Research Institute, Lucknow. The nitrogen content of 2001) (Table 3). The infrared spectrum of [Co(L3)](NCS)2
the complexes was determined using Kjeldahl’s method. The displays a sharp band at 2050 cm21 corresponding to uncoordi-
cobalt content in the complexes was determined volumetri- nated thiocyanate groups (Abu Ismaiel et al., 2001; Nakamoto,
cally, and the manganese content gravimetrically as 1978). The magnetic moment of this complex is 4.85 B.M.
Mn2P2O7. The voltammograms were recorded on an x-y indicating a high-spin configuration (Ray et al., 2003). The elec-
Houston-Ommigraphic 2000 recorder.
tronic spectrum of this complex displays well-defined bands at

TABLE 1
1H NMR spectral data of the macrocyclic ligands L1, L2, L3, and L4
22CH222
(C55N)
S. no.
Compound
Phenyl
7.2–7.3
(10H, m)
Benzene (subst.)
22NH
Ph22CH222Ph Ph22O22CH2
22S22CH2 22NH22CH2
(1)
L1
C29H24N2O2
7.1, 7.0, 6.9, 6.6
(2H, m) (2H, d,
J ¼ 7.3) (2H, d,
J ¼ 7.1) (2H, m)
—
—
4.1
(2H, s)
4.0
(4H, m)
2.8
(8H, m)
—
—
(2)
(3)
L2
C34H32N4
6.8, 6.4, 6.1
(4H, m) (8H, m)
(8H, d, J ¼ 6.5)
—
—
4.4
(4H, m)
—
—
—
—
—
L3
C18H18N2O4S
7.2, 6.8, 6.5 (4H, 5.1
—
3.0
(4H, m)
—
2.8
(4H, s)
m) (2H, d,
J ¼ 7.1) (2H, d,
J ¼ 6.8)
(2H, s)
(4)
L4
C12H30N6
—
—
9.1
(6H, s)
—
—
—
—
3.2
(24H, m)

STUDIES ON COBALT(II) AND MANGANESE(II) COMPLEXES
165
4,765, 5,560, 12,940 and 16,590 cm21, which are assigned to to uncoordinated thiocyanate groups (Table 3). This complex
4
4
4
4
4
4
0
0
A2(F) ! E0, A2 (F) ! E0(P), and A2 (F) ! A20 (P). A shows a magnetic moment of 4.53 B.M. (Ray et al., 2003).
similar spectrum has been reported for other high-spin five- The electronic spectrum of the [CoL4](NCS)2 complex
coordinated (Figure 5) cobalt(II) complexes (Sacconi and displays three well-defined bands at 8,870, 15,995 and
Speroni, 1976).
20,190 cm21, which may be assigned to the following tran-
4
4
4
4
sitions, T1g(F) ! T2g(F) (n1), T1g(F) ! T2g(F) (n2) and
4T1g(F) ! T2g(P) (n3), corresponding to six-coordinated
4
Cobalt(II) Complexes with Ligand L4
The formation of the complex may be represented by the
following equation:
(Figure 6) octahedral geometry (Chandra and Singh, 1985;
Lever, 1984.; Malik and Phillips, 1975).
Co(NCS)2 ꢀ nH2O þ L4 ꢁ! CoL4ðNCSÞ2 þ nH2O
EPR Spectra
On the basis of elemental analyses, the cobalt(II) complex of
the ligand L4 has the general composition [CoL4](NCS)2.
The EPR spectra of the complexes under study were
Molar conductance of this complex is 146 V 21 cm2 mol21 recorded at liquid nitrogen temperature because the rapid
(Table 2) corresponding to a 1:2 electrolyte (Chandra, 1982). spin lattice relaxation of cobalt(II) broadens the lines at
On complexation, the position of the important bands in the higher temperature. The g-values are given in Table 4. The
infrared spectrum due to .NH groups are shifted to lower fre- higher deviation of the g values from the free electron value
quency compared to the macrocyclic ligands (Chandra and (g ¼ 2.0023) is due to orbital contribution (Bencini and
Kumar, 2004; Raman et al., 2003). The infrared spectrum of Gatteschi, 1990; Brown and West, 2003; Chandra et al., 1991)
this complex displays a sharp peak at 2055 cm21 corresponding and g and g values were found in the range of 3.45–4.50.
k
?
TABLE 2
Elemental analyses and magnetic moments of cobalt(II) and manganese(II) complexes
Elemental analyses
(%) found/(calcd.)
Decomp.
Temp.
( 8C)
Formula
weight
Yield
(%)
meff
(B.M.)
S. no.
Compound
Color
White
M
C
N
H
(1)
L1
435.51
607.56
603.57
496.66
671.71
667.72
358.06
533.11
529.12
258.42
433.47
429.48
180
210
215
190
225
215
211
226
231
201
206
217
40
50
55
65
60
55
60
55
50
40
45
35
80.00
6.41
5.65
(5.65)
3.90
C29H24N2O2
(80.05) (6.50)
61.01 9.11
(2)
(3)
[Co(L1)(SCN)2]
C31H24CON4O2S2
[Mn(L1)(SCN)2]
C31H24MnN4O2S2
L2
Pink
9.60
4.85
5.90
(9.70) (61.28) (9.22)
9.06 61.30 9.22
(9.10) (61.68) (9.30)
81.15 11.11
(4.00)
3.83
Light
yellow
White
(4.43)
6.40
(4)
C34H32N4
(82.26) (11.26) (6.55)
64.15 12.42 4.65
(8.79) (64.38) (12.50) (4.48)
8.12 64.30 12.14 4.65
(8.23) (64.77) (12.58) (4.84)
(5)
[Co(L2)(SCN)2]
C36H32CoN6S2
[Mn(L2)(SCN)2]
C36H32MnN6S2
L3
Pink
White
White
8.72
5.01
5.92
(6)
(7)
60.20
(60.35) (7.80)
45.00 10.4
(11.07) (45.06) (10.5)
7.50
6.10
(5.07)
3.30
C18H18N2O4S
[Co(L3)](SNC)2
C20H18CoN4O4S3
[Mn(L3)(SCN)](SNC)
C20H18MnN4O4S3
L4
(8)
Light
pink
Yellow
11.00
5.00
5.95
(3.4)
3.40
(9)
10.30
45.3
10.5
(10.6)
32.10
(10.40) (45.4)
55.00
(3.44)
11.70
(10)
(11)
(12)
White
Pink
C12H30N6
(55.77) (32.55) (11.73)
[Co(L4)](SCN)2
C14H30CoN8S2
[Mn(L4)](SCN)2
C14H30MnN8S2
13.50
(13.59) (38.8)
12.60 38.99
(12.79) (39.2)
38.5
25.5
(25.8)
26.0
6.75
(7.0)
6.7
4.90
5.92
Yellow
(26.1)
(7.0)

166
S. CHANDRA AND R. KUMAR
L3 and L4). In the complex of L1, coordination occurs through
the oxygen of Ph22O22CH222 and the nitrogen of .C55N,
for L2, the nitrogen of .C55N and for L3 through the
nitrogen of 22CONH and for L4 through the nitrogen of .NH
(Brown and West, 2003; Ray et al., 2003). The molar conduc-
tance data (Table 2) are 06, 10, 107, and 150 V21 cm22mol21
for the complexes of the ligands L1, L2, L3, and L4, respectively.
The infrared spectra (Figure 7) of the thiocyanato complexes
of the ligands L1 and L2 show sharp peaks at 2085 and
2087cm21 due to the N-bonded thiocyanato group (Bentini
and Sabatini, 1966). But the thiocyanato complex of L3 shows
two sharp peaks at 2080 and 2050cm21, one corresponding to
coordinated N-bonded thiocyanate and the other corresponding
to an uncoordinated thiocyanato group (Ferguson et al., 1963).
The thiocyanoto complex of L4 shows a peak at 2045cm21
suggesting the uncoordinated nature of the thiocyanato groups.
On the basis of the above studies these complexes may be for-
mulated as [Mn(L1 or L2)(NCS)2], [Mn(L3)(NCS)](NCS), and
[Mn(L4)](NCS)2, respectively.
The present complexes show magnetic moments in the range
5.86–6.00B.M. The electronic spectra of the manganese(II)
complexes show bands at 17,565–17,615 (n1), 24,550–24,610
(n2), 28,640–28,715 (n3), and 31,250–31,315 cm21 (n4), which
6
4
6
4
may be assigned as A1g ! T1g(4G) (n1), A1g ! Eg(4G)
6
4
6
4
(n2), A1g ! Eg(4D) (n3), and A1g ! T1g(4P) (n4) tran-
sitions, respectively. The ligand field parameter values Dq,
B, C, b, F4, and F2 were calculated and are given in Table 5.
The electron-electron repulsion in the complexes is less
than that in the free ion, resulting in an increased distance
between electrons and thus an effective increase in the size of
the orbital. On increasing delocalization the value of b decreases
and is less than one in the complexes. The numerical value
786 cm21 for B of the free manganese(II) ion has been
used to calculate the value of b. The calculated values of b
FIG. 4. Suggested structure of [Co(L1)(NCS)2] and [Co(L2)(NCS)2].
Manganese(II) Complexes with Ligands L1, L2, L3, and L4
The formation of the complexes may be represented by the
following equation:
Mn(NCS)2 ꢀ nH2O þ L ꢁ! Mn(L)(NCS)2 þ nH2O
Elemental analyses suggest that all the manganese(II) com-
plexes have the general composition MnL(NCS)2 (L ¼ L1, L2,
TABLE 3
IR spectral data of the macrocylic ligands L1, L2, L3, and L4 and their manganese(II) and cobalt(II) complexes (cm21
)
Amide(I) Amide(II)
n(C55O)
S. no.
Compound
n(NH)
n(C22N)
n(C55N) Ph22CO22CH222
n(SCN)
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
L1
—
—
—
—
—
—
1,610 m
1,608 m
1,607 m
1,595 s
1,590 s
1,593 s
—
1,510 s
1,507 s
1,508 s
—
—
—
—
[Co(L1)(SCN)2]
[Mn(L1)(SCN)2]
L2
2,080 s
2,085 s
—
—
—
—
—
—
[Co(L2)(SCN)2]
[Mn(L2)(SCN)2]
L3
[Co(L3)](SNC)2
[Mn(L4)(SCN)](SNC)
L4
[Co(L4)](SCN)2
[Mn(L4)](SCN)2
—
—
—
—
—
—
—
—
2,085 s
2,087 s
—
—
1,645 m
1,640 m
1,637 m
—
1,575 m
1,570 m
1,500 m
—
1,515 m
1,520 m
1,512 m
—
—
—
2,050 m
2,080 s, 2,050 m
—
(9)
—
(10)
(11)
(12)
3,330 s, 3,310 m
3,326 s, 3,300 m
3,329 s, 3,309 m
—
—
—
—
—
—
—
—
2,055 s
2,045 s
—

STUDIES ON COBALT(II) AND MANGANESE(II) COMPLEXES
167
TABLE 4
Ligand field parameters and EPR data of cobalt(II) complexes
with the ligands L1, L2, L3, and L4
B
S. no
Complex
Dq (cm21
)
b
C.F.S.E
g
g
k
?
(2) [Co(L1)(SCN)2] 1120 641 0.57 108 2.61 4.50
(5) [Co(L2)(SCN)2] 1060 685 0.61 102 2.00 4.23
(8) [Co(L3)](SCN)2 1105 648 0.57 106 2.71 4.40
(11) [Co(L4)](SCN)2 1090 656 0.50 105 2.53 3.45
FIG. 5. Suggested structure of [Co(L3)](NCS)2.
Cyclic Voltammetry
(0.71–0.80) indicate that the complexes under study have
appreciable covalent character.
The complexes of cobalt(II) exhibit both metal and ligand-
centered electrochemistry in the potential range +1.7 V versus
the Ag/AgCl, CI2 electrode. All the complexes exhibit one
quasi-reverisible oxidation process as evident by the peak-
to-peak separation, DEp ꢂ 100 mV, and show redox potentials
in the range of 1.0–1.25 V. In all cobalt(II) complexes, the
cathodic and anodic peak heights (Ipc and Ipa) are the same.
The one-electron nature of the couple is confirmed when
compared with the current height of the Fe(CN)362/Fe(CN)642
system. The electrochemical behavior (Figures 9 and 10) of
EPR Spectra
The EPR spectra of the complexes have been recorded as
polycrystalline samples at room temperature and g values are
given in Table 5. Tile polycrystalline samples give one broad
isotropic signal centered at ca. 3200. The free electron g-value
is g0 ¼ 2.0023. In DMF, the EPR spectra of the complexes
clearly show that they exist as monomeric units (Ferguson
et al., 1963). The nuclear magnetic quantum number, MI, corre-
sponding to these lines are 25/2, 23/2, 21/2, þ1/2, þ3/2,
þ5/2 from low to high field (Hankare and Chavan, 2003).
Thus, on the basis of above studies the structures in Figure 8
may be suggested for the manganese(II) complexes.
FIG. 7. IR spectrum of [Mn(L2)(NCS)2].
FIG. 6. Suggested structure of [Co(L)](NCS)2.
TABLE 5
Ligand field parameters and EPR data of manganese(II) complexes with the ligands L1, L2, L3, and L4
S. No
Complex
Dq
B (cm21
)
C (cm21
)
b
F4
F2
hx
gav
(3)
(6)
(9)
(12)
[Mn(L1) (SCN)2]
[Mn(L2) (SCN)2]
[Mn(L3) (SCN)] (SNC)
[Mn(L4)] (SCN)2
1760
1756
1761
1759
584
585
585
586
3742
2674
2678
2678
0.74
0.74
0.74
0.74
107
76
1119
965
965
966
3.7
3.7
3.7
3.7
1.92
1.99
1.95
1.93
76
76

168
S. CHANDRA AND R. KUMAR
FIG. 8. Continued.
transfer centers (Basosi et al., 1975; Mayer et al., 2003). We
observed two one-electron quasi-reversible reductions
(DEp ꢂ 100 mV) in the potential range 0.2 to 20.3 and 20.4
to 20.6 V along with a two-electron reduction at 21.0 to 21.3 V.
The redox property of the manganese(II) complexes was
studied in the potential range of þ1.0 to 21.6 V. Cyclic vol-
tammograms of Mn(II) complexes are shown in Figure 11
for the cathodic region and Figure 12 for the anodic region.
All the complexes show two reduction waves in the cathodic
region at different potentials. During the reverse sweep,
the complexes exhibit two oxidation waves at different poten-
tials (Donzello et al., 2003). It indicates that each reduction is
associated with a single-electron transfer process (Rossister
and Hamilton, 1985). DEp (DEp ¼ Epc 2 Epa) values for the
FIG. 8. Suggested structures of [Mn(L1)(NCS)2], [Mn(L2)(NCS)2],
[Mn(L3)(NCS)]NCS, and [Mn(L4)] (NCS)2.
the cobalt(II) complexes is due to the Co(III)/Co(II) couple.
The more positive the potential, the stronger is the binding to
the lower oxidation state. The redox responses, which are
negative relative to the reference electrode, are due to ligand
reductions. The .C55N, 22CONH, Ph22CO22CH222 and
.NH groups in the ligands are known as potential electron
FIG. 10. Cyclic voltammogram of [Co(L1)(NCS)2] in the cathodic region.
FIG. 9. Cyclic voltammogram for [Co(L1)(NCS)2] in the anodic region. Scan
Scan rate 50 m V s21
.
rate 50 m Vs21
.

STUDIES ON COBALT(II) AND MANGANESE(II) COMPLEXES
169
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