Spectroscopic techniques and cyclic voltammetry with synthesis: Manganese(II) coordination stability and its ligand field parameters effect on macrocyclic ligands

Article abstract of DOI:10.1016/j.saa.2006.07.001

Manganese(II) macrocyclic complexes are prepared with different macrocyclic ligands, containing cyclic skeleton bearing organic components which have different chromospheres like N, O and S donor atoms and stereochemistry. Thus, six macrocyclic ligands, w

Full text of DOI:10.1016/j.saa.2006.07.001

Spectrochimica Acta Part A 67 (2007) 188–195
Spectroscopic techniques and cyclic voltammetry with synthesis:
Manganese(II) coordination stability and its ligand field
parameters effect on macrocyclic ligands
Rajiv Kumara,b,∗, Sulekh Chandrab
a Department of Chemistry, University of Delhi, New Delhi 110007, India
b Department of Chemistry, Zakir Husain College, University of Delhi, Jawahar Lal Nehru Marg, New Delhi 110002, India
Received 11 January 2006; accepted 3 July 2006
Abstract
Manganese(II) macrocyclic complexes are prepared with different macrocyclic ligands, containing cyclic skeleton bearing organic components
which have different chromospheres like N, O and S donor atoms and stereochemistry. Thus, six macrocyclic ligands, were prepared and their
capacity to retain the manganese(II) ion in solid as well as in aqueous solution was determined and characterized by elemental analyses, molar
conductance measurements, magnetic susceptibility measurements, mass, 1H NMR, IR, electronic spectral and cyclic voltammetric studies.
The electronic spectrum of this system showed a dependence that may be consistent with the formation of stable complexes and coordination
behaviour of the ions. ESR spectra of all the complexes are recorded in solid as well as solution, which show the oxidation state of the manganese(II).
Spin Hamiltonian manganese(II), which can be defined as the magnetic field vector (Ᏼ):
ꢀ
ꢁ
35
12
1
6
Ᏼ = gβeHS + D Sz2 −
+ E[Sz2 − Sy2] + ASI +
a
ꢀ
ꢁ
ꢀ
ꢁ
35Sz2 − 475
707
16
1
Sx4 + Sy4 + Sz4 −
+
F
180
2Sz2 + 3255/10
Significant distortion of the manganese(II) ion in observed geometry is evident from the angle subtended by the different membered chelate rings
and the angles spanned by trans donor atoms octahedral geometry. Cyclic voltammetric studies indicate that complexes with all ligands undergoes
one electron oxidation from manganese(II) to manganese(III) followed by a further oxidation to manganese(IV) at a significantly more positive
potential.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Macrocyclic ligands; EPR; IR; Cyclic voltammetry; Mn(II)complexes
1. Introduction
able to develop such different chromospheres containing organic
cyclic ligands as reported earlier [2–5].
Manganese and its compounds find very historical impor-
tance in medicine and also have some biological importance.
Manganese plays a significant role in enzyme activation. Cit-
rate cyclase catalyzes the cleavage of citrate to oxaloacetate and
acetate in the presence of manganese(II) or magnesium(II) [1].
In order to obtain new model of such complexes, here we are
It is well known that manganese plays an important role in
many biological redox processes including disproportionation
of hydrogen peroxide (cat−alase activity) in micro-organisms
[6], decomposition of O2 radicals catalyzed by superox-
ide dismutases (SODs) [7] and water oxidation by photo-
synthetic enzymes (photosystem II) [8]. Superoxide dismu-
tase catalyses the dismutation of superoxide radical anions to
the non-radical products oxygen and hydrogen peroxide and
protects the living cells against the toxicity of hydroxia [9]
O2− + H2O + H+ → O2 + H2O2.
∗
Corresponding author. Tel.: +91 1234276530; fax: +91 1234276530.
E-mail address: chemistry rajiv@hotmail.com (R. Kumar).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.07.001

R. Kumar, S. Chandra / Spectrochimica Acta Part A 67 (2007) 188–195
189
Also manganese(II) can replace magnesium(II) in a num-
2.2. Isolation and preparation of macrocyclic ligands
ber of biological system [10–13]. A number of reviews are
available on the physiology and biochemistry of manganese
in mammals [14]. DNA and RNA polymerases [15] catalyze
the replication and transcription of DNA, have a specific
requirement for manganese(II). The binding of manganese(II)
in these systems have been characterized by the using EPR.
There are some examples of enzymes, which bind manganese
tightly such as pseudocatalase [16], ribonucleotide reducatse
[17,18] and oxygen evolving centre in photosystem-II [19].
On the basis of above importance which manganese has
in biological systems? The synthesis and characterization
studies on macrocyclic complexes of manganese(II) are
highly desirable with these six macrocylic ligands: 2,4,10,12-
tetraphenyl-1,5,7,9,13,15-hexaazatricyclo[15,3,1]-octadeca-1,
4,7,9,12,14-hexaene[N6]ane(L1); 2,4,10,12-tetraphenyl-1,5,7,
13,-tetraazacyclohexadeca-1,4,9,12-teraene [N4]ane (L2);2,4,
9,11-tetraphenyl-1,5,8,12-tetraazacycloteradeca-1,4,8,11-tetra-
ene[N4]ane (L3); 1,4,7,10,13,16-hexaaza-cyclooctadecane[N6]
ane (L4); 1,8-diaza-4,5,11-trithia-2,3:6,7-dibenzo[b,h]-cyclo-
pentadeca-9,13-dione[S3N2]ane (L5); 9,18-dimethyl-1,7,10,16-
tetraza-4,13-dithia-cyclooctadecane-2,6,11,16-teraone (L6).
Ligand L2, L4, L5 and L6 are prepared and characterized
as reported earlier [2–5]. Ligand L1 and L3 were synthe-
sized by refluxing an ethanolic solution of dibenzoylmethane
(0.01 mol) with an ethanolic solution of 2,6-diaminopyridine or
diaminoethane (0.01 mol) for 5 h in the presence of few drops
of conc. HCl. On cooling the contents overnight a dull white
crystalline compound separated out. This was filtered, washed
with ethanol and dried over P4O10. Molecular formula (L1):
C40H30N6 (Found: C% = 80.20, H% = 4.92, N% = 30.91, Calcd.
C% = 80.81, H% = 5. 05, N% = 14.13). Molecular formula (L3):
C36H36N4 (Found: C% = 81.80, H% = 6.13, N% = 10.91, Calcd.
C% = 82.25, H% = 6. 45, N% = 11.28).
2.3. Characterization of macrocyclic ligands
2.3.1. 1H NMR spectra of macrocyclic ligands
These ligands are characterized on the basis of elemental
analysis, mass spectra and infrared spectra. The electron impact
mass spectra of the ligands confirm the proposed formula by
showing different peaks corresponding to the macrocyclic moi-
ety corresponding to various fragments. Their intensity revealed
the stability of these fragments. The 1H NMR spectra of the lig-
and(L1 andL3)donotshowsignalcorrespondstoprimaryamine
and ethyl protons. Other signals appear in the regions at 1.53 and
7.22 ppm. These signals correspond to methyne (–CH–CH2–)
and benzenoid protons, respectively.
2. Experimental
All the chemicals used were of AnalaR grade and procured
from Fluka. Metal salts were purchased from E. Merck and were
used as received. All solvents used were purified before use
according to standard procedures.
2.3.2. IR spectra of macrocyclic ligands
IR spectral studies of isolated ligands show different char-
acteristic bands according their functional group present. The
main characteristic, strong band C N lies in the range of
1599–1628 cm−1. This band confirms the reaction between pri-
mary amine group –NH2 of diamine and carbonyl group of
diketone or any other carbonyl compounds. So the bands due to
–NH2 group of diamine in 3200–3350 cm−1 region and C O
group of carbonyl compounds above 1700 cm−1 are absent.
These confirm the elimination of water molecule and also con-
firm complete condensation. The absorptions in the IR spectrum
ofL3 intherangeof720–780and1430–1630 cm−1 areduetothe
presence of a phenyl group. IR spectrum shows a new medium
to strong band in region 1531–1465 cm−1 as expected for the
highest energy pyridine ring vibrations. IR spectra of all ligands
show a strong band in region 2950–3010 cm−1, which confirms
the C–H stretching due to the –CH2– group. The ligand may be
racemic or meso; however, this will not affect the geometry of
the complexes. Thin-layer chromatography has been performed
which indicates that the ligand is not a mixture (Fig. 1).
2.1. Physical measurements
C, H and N were analysed on a Carlo-Erba 1106 ele-
mental analyzer. Molar conductances were measured on an
Elico (CM82T) conductivity bridge. Magnetic susceptibilities
were measured at room temperature on a Gouy balance using
CuSO4·5H2O as a calibrant. Electron impact mass spectra were
recorded on a Jeol, JMS, DX-303 mass spectrometer. 1H NMR
spectra were recorded on a model R-600 Hitachi FT-NMR,
model R-600 spectrometer using deuterated DMF as solvent,
chemical shifts are given in ppm relative to tetramethylsilane.
IR spectra (KBr) were recorded on a FTIR Spectrum BX-II spec-
trophotometer. The electronic spectra were recorded in DMSO
on a Shimadzu UV mini-1240 spectrophotometer. EPR spectra
of the complexes were recorded as polycrystalline samples and
in solution in DMSO at room temperature for maganease(II)
complexes on an E4 EPR spectrometer using DPPH as the g-
marker. Cyclic voltammetry of the complexes was recorded in
DMF at a scan rate of 100 mV s−1. The redox potential was
recorded with Ag/AgCl as reference electrode and platinum as
working electrode, tetrabutylammonium phosphate was the sup-
porting electrolyte.
2.4. Preparation of manganese(II) complexes (1 + 1)
condensation
A hot ethanolic solution (20.0 ml) of MnCl2 or Mn(SCN)2
is added to the ethanolic solution (20.0 ml) of ligand. The mix-
ture was refluxed on water bath at 65 ◦C for 4–5 h. The color of
the solution changed in to pale yellow, off white and brown
The E0.5 values were also equal to the E0.5 values measured
under steady-state conditions at a microelectrode as the potential
at half the limiting current value (IL/2).

190
R. Kumar, S. Chandra / Spectrochimica Acta Part A 67 (2007) 188–195
according macrocyclic ligands. On cooling, precipitate, so
obtained, filtered, washed with ethanol and dried over P4O10
under vacuum.
3. Result discussion
On the basis of elemental analysis, complexes under study
were found to have general composition MLX2 (L = ligand and
X = Cl−, SCN− and 1/2SO42−) and related data are presented
in Table 1.
Chloro complexes of L2, and L3 are non-electrolytes but the
chloro complexes of ligands L1, L4, L6 and L5 are 1:2 and
1:1 electrolytes, respectively. Thiocyanato complexes of ligands
L2 and L3 are non-electrolytes but complexes of ligands L1,
L4, L6 and L5 are 1:2 and 1:1 electrolytes, respectively. Thus,
the complexes may be formulated as [MnLX2] [MnLX]X, and
[MnL]X2.
3.1. IR spectra
IR spectra of chloro complexes of all ligands show that the
shifting in C N band to lower side in most complexes. It con-
firms, the coordination takes place between the metal ion and
donating atoms.
3.1.1. Confirmation and coordination ability of sulphato
anion by IR spectra
Spectral studies of ligands L1 and L6 sulphato complexes
show the sharp band in the range 1101–1115 cm−1, which shows
ionic behaviour or uncoordinate sulphate of sulphato group.
According to Nakamoto [20] ionic sulphato group belongs to
high symmetry point groups Td and shows two bands at 1104
and 613 cm−1 denoted as ν3 and ν4 do not split and ν2 do not
appear, although if observed, it is very weak. He suggests ν3
appeared at approximately 1104 cm−1 and weak band 613 cm−1
for ionic sulphate.
Sulphato complex of L5 shows that ν3 band at
1030–1140 cm−1(s) and other in the range 604–645 cm−1(ν4).
This indicates unidentate behaviour of sulphate group [21].
According to Nakamoto [20] monodentate sulphate group
belongs to C3v symmetry and shows bands with medium
intensity, moreover, ␯ and ν4 each splits in to two bands. This
3
result can be explained by assuming a lowering of symmetry
from Td to C3v (unidentate co-ordination).
3.1.2. Confirmation and coordination ability of
pseudohalide anion by IR spectra
Coordination ability of thiocyanate ion, which is also, called
pseudohalide ion. This ion may coordinate to a metal through
either one of the end atoms. It may be bonded to metal ion
through sulphur atom or nitrogen atom. As a result two linkage
isomers are possible, one is M–SCN, which is brown as thio-
cyanate complexes and second is M–NCS, which is known as
isothiocyanato complexes.
Fig. 1. Structure of the macrocyclic ligands.
In general, first transition form M–N bonded whereas sec-
ond and third transition series form M–S-bonded [22]. The
CN stretching frequencies are observed in L5 complex near

R. Kumar, S. Chandra / Spectrochimica Acta Part A 67 (2007) 188–195
191
Table 1
Molar conductance and elemental analyses data of the manganese(II) complexes
Complexes/empirical formula
Yield
(%)
Molar conductance
Color
Elemental analysis calculated (found) (%)
(ꢀ−1 cm2 mol−1
)
Mn
7.64 (7.14)
7.35 (6.85)
7.18 (6.75)
8.45 (8.15)
C
H
N
[Mn(L1)]Cl2/MnC40H30N6Cl2
[Mn(L1)]SO4/MnC40H30N6SO4
[Mn(L1)](SCN)2/MnC42H30N8S2
[Mn(L2)Cl2]/MnC36H36N4Cl2
[Mn(L3)Cl2]/MnC34H32N4Cl2
[Mn(L4)]Cl2/MnC12H30N6Cl2
[Mn(L5)Cl]Cl/MnC18H18N2S3O2Cl2
70
40
47
65
62
66
61
52
198.0
95.0
190.0
14.0
Brown
White
White
Brown
66.70 (66.12)
64.45 (64.00)
65.89 (64.19)
66.49 (61.12)
65.60 (65.42)
37.53 (37.12)
4.17 (3.84) 11.67 (11.17)
4.04 (3.74) 11.27 (10.83)
3.92 (3.34) 25.61 (24.11)
5.54 (5.16)
5.18 (5.15)
8.62 (8.12)
9.00 (8.55)
17.0
Reddish brown 8.83 (8.49)
205.0
105.0
15.0
Off brown
Brown
Black brown
White
White
Brown
14.30 (13.8)
10.64 (10.51) 41.88 (41.62.) 3.48 (3.18)
10.15 (9.91)
9.79 (9.65)
10.94 (10.75) 33.49 (33.28)
10.42 (10.26) 31.90 (31.71)
7.82 (7.26) 21.88 (21.34)
5.42 (5.12)
5.44 (5.24)
9.97 (9.88)
[Mn(L5)SO4]/MnC18
H18N2S4O6
39.94 (39.84)
42.79 (42.61)
3.33 (3.21)
3.21 (2.99)
4.78 (4.61) 11.15 (10.82)
4.55 (4.25) 10.62 (10.51)
4.39 (4.21) 15.35 (15.15)
[Mn(L5)(SCN)](SCN)/MnC22H22N4S5O3 47
92.0
[Mn(L6)]Cl2/MnC14H24N4S2O4Cl2
[Mn(L6)]SO4/MnC14H24N4S2O8
[Mn(L6)](SCN)2/MnC16H24N6S4O4
35
71
60
201.0
106.0
195.0
White
10.04 (9.91)
35.11 (34.98)
2070 cm−1, which is corresponds to N-bonded behaviour of this
group but the thiocyanate complexes of ligands L1 and L6 under
study show ionic behaviour of thiocyanato group or uncoordi-
nate nature of this group [23].
17280–18,950; 23,923–28,380; 28,450–28,980; 31,055–
31,600 cm−1, which may be assigned to the transitions: 6A1g
→
4T1g(4G), 6A1g → Eg, 4A1g(4G) (10B + 5C), 6A1g → Eg(4D)
4
4
(17B + 5C) and 6A1g → T1g(4P) (7B + 7C), respectively[2,26].
4
3.2. Magnetic moment
3.3.1. Ligands field parameters of manganese(II)
complexes
The ground state term of manganese(II) is 6A1g. There is no
temperature-independent paramagnetic effect and no reduction
of the moment below the spin only value by spin–orbit cou-
pling with higher ligand filed terms. Magnetic movement of the
complexes recorded at room temperature (298 K). The present
complexes show magnetic moment in the range 5.85–6.02 BM,
which are corresponding spin-only values, suggesting an octa-
hedral geometry around manganese ion. Magnetic moment cal-
culated from the susceptibility measurements after correcting
diamagnetic contribution [24,25]. Magnetic moment values of
the complexes under study are given in Table 2.
The parameters B and C were calculated and reported which
have a good agreement with data reported earlier [24]. The cal-
culated values of the ligand field parameters are given in Table 3.
These values are calculated from the second and third transitions
because these transitions are free from the crystal field splitting
and depend on B and C parameters. Orgen [27] calculate the
values of Dq with the help of curve, transition energies ver-
sus Dq, as given by the using the energy due to the transition
6A1g → T1g (4G). Parameters B and C are linear combinations
4
of certain coulombs and exchange integral, which are generally,
treated empirical parameters obtained from the spectra of the
free ions. Slater Condon-shortly repulsion parameters F2 and
F4 are related to Racah parameters B and C as: B = F2 − 5F4
and C = 35F4, The electron–electron repulsion in the complexes
is more than in the free ion, resulting in an increased distance
between electrons, and thus, affects the size of the orbital. On
3.3. Electronic spectra of manganese(II) complexes
The electronic spectra of the manganese(II) complexes
exhibit four weak-intensity absorption bands in the ranges
Table 2
Magnetic moment and electronic spectra of the manganese(II) complexes with L1 and L2
Complex
␮ efficiency (BM)
υ1 (cm−1
)
υ2 (cm−1
)
υ3 (cm−1
)
υ4 (cm−1
)
[Mn(L1)]Cl2
5.89
5.98
5.93
5.89
5.89
6.02
6.01
6.00
5.99
5.95
5.8
17,750
17,300
17,950
18,950
17,950
18,867
17,600
17,900
17,660
17,280
18,540
17,600
24,200
24,650
24,700
24,300
23,923
24,449
24,850
24,810
24,800
24,100
28,380
24,950
28,600
28,950
28,980
28,740
27,840
28,901
28,900
28,200
28,980
28,650
28,750
28,450
–
[Mn(L1)]SO4
[Mn(L1)](SCN)2
[Mn(L2)Cl2]
31,600
–
–
–
31,055
32,000
31,800
–
31,400
–
[Mn(L3)Cl2]
[Mn(L4)]Cl2
[Mn(L5)Cl]Cl
[Mn(L5)SO4]
[Mn(L5)(SCN)](SCN)
[Mn(L6)]Cl2
[Mn(L6)]SO4
[Mn(L6)](SCN)2
5.8
–

192
R. Kumar, S. Chandra / Spectrochimica Acta Part A 67 (2007) 188–195
Table 3
Ligand field parameters of the manganese(II) complex
Complex
Dq (cm−1
)
B (cm−1
)
C (cm−1
)
β
F4
F2
hx
[Mn(L1)]Cl2
1775
1780
1730
1795
1795
1886
1830
1760
1790
1766
1828
1854
628
614
611
634
560
636
617
484
597
650
624
500
3584
3702
3694
3592
3664
3617
3682
3994
3766
3520
3628
3990
0.79
0.78
0.77
0.80
0.69
0.80
0.78
0.61
0.75
0.82
0.79
0.63
102
105
105
102
104
103
105
114
107
100
103
114
1138
1139
1136
1144
1080
1151
1142
1054
1132
1150
1139
1074
3.00
4.00
3.28
3.86
4.40
2.80
3.14
5.57
3.57
2.57
3.00
5.29
[Mn(L1)]SO4
[Mn(L1)](SCN)2
[Mn(L2)Cl2]
[Mn(L3)Cl2]
[Mn(L4)]Cl2
[Mn(L5)Cl]Cl
[Mn(L5)SO4]
[Mn(L5)(SCN)](SCN)
[Mn(L6)]Cl2
[Mn(L6)]SO4
[Mn(L6)](SCN)2
ꢂ
ꢃ
ꢀ
ꢁ
35Sz2 − 475
increasing delocalization, the value of β decreases up to less
than one in the complexes.
707
16
1
Sx4 + Sy4 + Sz4 −
+
F
180
2Sz2 + 3255/10
The value of β can be calculated from the Nephelauxetic
parameter for the ligand (hx) and the Nephelauxetic parameter
for the metal ion (km) as (1 − β) = hx × km. The value of the
parameter hx for manganese(II) complexes have been calculated
by using the co-valency contribution of manganese(II), while
for the calculation of β, we used the numerical value of B for
manganese(II) free ion which is 786 cm−1. The observed values
for parameter β and hx suggest that the complexes, reported
here, have appreciable ionic character [28].
where Ᏼ is the magnetic field vector, g the spectroscopic split-
ting factor, β the Bohr magnetron, A the manganese hyperfine
splitting constant, S the electron spin vector, I the nuclear spin
vector, S = 5/2 and Sz is the diagonal spin operator.
Further, a resonance is readily detected even for large zero-
field splitting, because d5 is an odd-electron system whose
ground state is a Kramer’s doublet and whose degeneracy is
only completely removed by a magnetic field. In polycrys-
talline samples, manganese(II) complexes usually gives broad
3.4. EPR spectra of manganese(II) complexes
signals attributed to forbidden transitions where ꢁm =
M
(M = electron spin quantum number) and ꢁm = 0 (m = nuclear
EPR spectra were recorded at room temperature for a poly-
crystalline sample as well as DMSO and calculated data are
reported in Table 4. Ground term of manganese(II) high spin is
6S5/2. However, the combined action of the electric field gradient
and the spin–spin interaction produces splitting of the energy
levels due to second order spin–orbital coupling between the
spin quantum number).
The broadening of the spectrum in powder sample is analo-
gous to that observed in the case of immobilized free-radicals,
e.g. manganese(II) complexes of Concanavalin broadening due
to immobilization of M = M ion in the ligand results because
the rotational motion of manganese(II) is highly restricted.
In DMSO solution, the manganese(II) complexes give EPR
spectra containing the six lines arising due to the hyperfine inter-
action between the unpaired electron with the 55Mn nucleus
(1 = 5/2). The nuclear magnetic quantum number M1, corre-
sponding to these lines, are −5/2, −3/2, −1/2, +1/2, +3/2 and
+5/2 from low to high field [2,30] (Figs. 2–4).
4
6A1g ground state and the lowest level of the A2g state [29].
For a weak crystal field (t2g Eg2, S = 5/2) the EPR results on d5
ions have fitted to the spin Hamiltonian manganese(II) which
can be defined as
ꢀ
ꢁ
35
12
1
6
Ᏼ = gβeHS + D Sz2 −
+ E [Sz2 − Sy2] + ASI +
a
Table 4
EPR parameters of the manganese(II) complex
Complex
g
g
⊥
A0
giso
ꢀ
[Mn(L1)]Cl2
3.95
3.64
3.94
3.77
3.99
3.95
3.65
3.94
3.99
3.93
3.91
3.98
1.89
1.87
1.99
1.91
1.93
1.89
1.87
1.99
1.99
1.95
1.96
1.98
111
110
105
103
108
109
110
121
118
117
113
114
2.58
2.46
2.64
2.69
2.66
2.58
2.46
2.64
2.66
2.65
2.63
2.64
[Mn(L1)]SO4
[Mn(L1)](SCN)2
[Mn(L2)Cl2]
[Mn(L3)Cl2]
[Mn(L4)]Cl2
[Mn(L5)Cl]Cl
[Mn(L5)SO4]
[Mn(L5)(SCN)](SCN)
[Mn(L6)]Cl2
[Mn(L6)]SO4
[Mn(L6)](SCN)2
Fig. 2. EPR spectrum of Mn(L1)Cl2.

R. Kumar, S. Chandra / Spectrochimica Acta Part A 67 (2007) 188–195
193
closely spaced one electron processes, but with a separation of
25 10 mV.
The average E0.5 values for the first unresolved two electrons
oxidation responses of L1, L4, L5 and specially L6 decrease with
increasing E1/2 = +0.55 to + 0.48 V according to chain length as
do those for the second oxidation +1.33 to +1.23 V. The inves-
tigated data explain the probability that, as the alkyl chain has a
slight effect on cyclic potentials of metal centre [4]. These com-
plexes show (E0.5 values of +0.41 and +1.03 V), which reflects
the absence of electrostatic effects that would be caused by a
second metal center in large ring containing macrocyclic lig-
ands systems. The aromatic part of such ligands systems may
be reducing the binding ability of the bridgehead nitrogen and
hence the overall field strength of the ligand, and this could be
creating problems in oxidation process in the complexes of L1,
L2, L3, L5, L6 comparatively L4.
Fig. 3. EPR spectrum of Mn(L4)Cl2.
Acknowledgements
One of the authors (Rajiv Kumar) gratefully acknowledges
his younger brother Bitto for motivation. Specially, thanks to
Mohit Krishan, Computer Programmer, CSL Delhi University
Delhi for providing computer facilities. Thanks to the University
Grants Commission, New Delhi for financial assistance. Thanks
Fig. 4. EPR spectrum of Mn(L5)Cl2.
4. FAB mass spectrum
The FAB mass spectrum gives additional structural informa-
tion about the stereochemistry of the studies complexes. The
FAB mass spectrum of MnC34H32N4Cl2 shows a molecular ion
(M+) peak at m/z = 384.25 a.m.u., which suggests the monomeric
nature of the complex and confirm the proposed formula. The
spectrum of complex also shows a series of peaks corresponding
to various fragments. Their intensity gives an idea of the stability
of the fragments and also about the geometrical configuration.
5. Redox behaviour by cyclic voltammetry
measurements
Fig. 5. Structure of [MnL1]Cl2 complex where (X = Cl).
The redox behaviour of all the complexes displays a
chemically reversible wave at a scan rate of 100 mV s−1
.
This E0.5 value is assumed to be the reversible poten-
tial because it is in agreement with the Ep. In the
cyclic voltammetry measurements, the oxidation of man-
ganese(II)/manganese(III) state in two unresolved one electron
processes [Mn(II) → Mn(II)Mn(III) → Mn(III)] and then to the
manganese(IV) state [Mn(III) → Mn(III)Mn(IV) → Mn(IV)].
CV signals are relatively sharp compared to those for these
complexes, as evidenced by the smaller ꢁEp values of 72
and 78 mV, respectively. However, these ꢁEp values are still
larger than expected for a simultaneous two electron pro-
cess (i.e., ꢁEp = 57 mV/n), indicating that oxidation of the
two manganese(II) centers to manganese(III) occurs via two
Fig. 6. Structure of [MnL1](NCS)2 complex.

194
R. Kumar, S. Chandra / Spectrochimica Acta Part A 67 (2007) 188–195
Fig. 10. Structure of [MnL4Cl2] complex.
Fig. 7. Structure of [MnL1]SO4 complex.
to IIT Bombay for recording EPR spectra. Thanks are also due to
the Solid State Physics Laboratory, India for recording magnetic
moments.
Thus, on the basis of above discussion which is based upon
elemental analysis, molar conductance, magnetic susceptibility,
IR, electronic, EPR spectral studies, the following structures are
investigated for the complexes (Figs. 5–15).
Fig. 11. Structure of [MnL4(NCS)2] complex.
Fig. 12. Structure of [MnL5Cl]Cl complex with L5.
Fig. 8. Structure of [MnL2Cl2] complex.
Fig. 13. Structure of [MnL5(NCS)]NCS complex.
Fig. 9. Structure of [MnL3Cl2] complex.

R. Kumar, S. Chandra / Spectrochimica Acta Part A 67 (2007) 188–195
195
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Fig. 14. Structure of [MnL5SO4] complex.
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Fig. 15. Structure of [MnL6]X2 complex, where X = NCS, CI and NO3.
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