Synthesis, physicochemical and electrochemical studies on Mn(II), Co(II), Ni(II), and Cu(II) complexes with an N-donor tetradentate (N4) macrocycle ligand derived from ethyl cinnamate

Article abstract of DOI:10.1081/SIM-200026596

Manganese(II), cobalt(II), nickel(II), and copper(II) complexes with a new tetradentate ligand, 1,3,7,9-tetraaza-4,10-diketo-6,12-diphenyl-2,8- dithiocyclododecane (L), were synthesized and characterized by elemental analyses, molar conductance measurements, magnetic susceptibility measurements, mass, ¹H NMR, IR, electronic, EPR spectral and cyclic voltammetric studies. On the basis of the IR, electronic, and EPR spectral studies, an octahedral geometry has been assigned for the Mn(II) and Co(II) complexes, a square-planar one for the Ni(II) and a tetragonal one for the Cu(II) complexes.

Full text of DOI:10.1081/SIM-200026596

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Synthesis, Physicochemical
and Electrochemical Studies
on Mn(II), Co(II), Ni(II), and
Cu(II) Complexes with an
N‐Donor Tetradentate (N4)
Macrocycle Ligand Derived
from Ethyl Cinnamate
Sulekh Chandra a , Lokesh Kumar Gupta a &
Sangeetika a
a Department of Chemistry, Zakir Husain
College , University of Delhi , JLN‐Marg, New
Delhi, 110 002, India
Published online: 16 Nov 2010.
To cite this article: Sulekh Chandra , Lokesh Kumar Gupta & Sangeetika (2004)
Synthesis, Physicochemical and Electrochemical Studies on Mn(II), Co(II), Ni(II),
and Cu(II) Complexes with an N‐Donor Tetradentate (N4) Macrocycle Ligand
Derived from Ethyl Cinnamate, Synthesis and Reactivity in Inorganic and Metal-
Organic Chemistry, 34:9, 1591-1604, DOI: 10.1081/SIM-200026596
To link to this article: http://dx.doi.org/10.1081/SIM-200026596
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SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 34, No. 9, pp. 1591–1604, 2004
Synthesis, Physicochemical and
Electrochemical Studies on Mn(II), Co(II),
Ni(II), and Cu(II) Complexes with an
N-Donor Tetradentate (N4) Macrocycle
Ligand Derived from Ethyl Cinnamate
*
Sulekh Chandra, Lokesh Kumar Gupta,
and Sangeetika
Department of Chemistry, Zakir Husain College, University of Delhi,
New Delhi, India
ABSTRACT
Manganese(II), cobalt(II), nickel(II), and copper(II) complexes with a
new tetradentate ligand, 1,3,7,9-tetraaza-4,10-diketo-6,12-diphenyl-2,8-
dithiocyclododecane (L), were synthesized and characterized by elemen-
tal analyses, molar conductance measurements, magnetic susceptibility
1
measurements, mass, H NMR, IR, electronic, EPR spectral and cyclic
voltammetric studies. On the basis of the IR, electronic, and EPR spectral
studies, an octahedral geometry has been assigned for the Mn(II) and
*
Correspondence: Sulekh Chandra, Department of Chemistry, Zakir Husain College,
University of Delhi, JLN-Marg, New Delhi 110 002, India; Fax: 91-11-23215-906;
E-mail: schandra_00@yahoo.com.
1591
DOI: 10.1081/SIM-200026596
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

1592
Chandra, Gupta, and Sangeetika
Co(II) complexes, a square-planar one for the Ni(II) and a tetragonal one
for the Cu(II) complexes.
Key Words: Tetradentate ligand; Complexes; Ligand field parameters;
Electrochemical behavior.
INTRODUCTION
Coordination compounds containing macrocyclic ligands have been
studied during the past decades owing to their wide applications in biological
and sensor fields.[1,2] Different neutral donors have been incorporated into the
ligand backbone.[3–6] Nature prefers macrocyclic derivatives for many funda-
mental biological functional systems such as photosynthesis and transpor-
tation of oxygen in mammalian and other respiratory systems.[7,8] In the
present paper, we report the synthesis and characterization of Mn(II),
Co(II), Ni(II), and Cu(II) complexes with a 12-membered tetradentate (N4)
macrocyclic ligand, 1,3,7,9-tetraaza-4, 10-diketo-6,12-diphenyl-2,8-dithiocyclo-
dodecane (L), shown in Fig. 1.
EXPERIMENTAL
Ethyl cinnamate and thiourea used were of AnalaR grade and procured
from Fluka. Metal salts were purchased from E. Merck and were used as
Figure 1. Synthesis and structure of the ligand.

Mn(II), Co(II), Ni(II), and Cu(II) Complexes with Tetradentate Ligand
1593
received. All solvents used were purified before use according to standard
procedures.[9]
Synthesis of Ligand
A hot ethanolic solution (20 mL) of ethyl cinnamate (3.52 g, 0.02 mol)
and a hot ethanolic solution (20 mL) of thiourea (1.52 g, 0.02 mol) were
mixed slowly with constant stirring. This mixture was refluxed at 82 8C for
12 hr in the presence of a few drops of concentrated hydrochloric acid.
Cooling at 5 8C gave a white precipitate, which was filtered, washed with
cold EtOH, and dried under vacuum over P4O10.
Synthesis of Complexes
A hot ethanolic solution (20 mL) of the ligand (0.82 g, 0.002 mol) and a
hot ethanolic solution (20 mL) of the metal salt (0.002 mol) were mixed
together with constant stirring. The mixture was refluxed for 8–16 hr
at 65–90 8C. On cooling to 5 8C, the colored complexes precipitated out.
They were filtered, washed with cold EtOH, and dried under vacuum
over P4O10.
Physical Measurements
C, H, and N were analyzed on a Carlo-Erba 1106 elemental 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 spectrometers. 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 spectrophotometer. 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 liquid nitrogen temperature for the Co(II) and at room
temperature for the Mn(II) and Cu(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 sec21. The redox potential was

1594
Chandra, Gupta, and Sangeetika
recorded with Ag/AgCl as reference electrode and platinum as working
electrode, tetrabutylammonium phosphate was the supporting electrolyte
and the concentration of the complex was 1 ꢀ 1023 M.
RESULTS AND DISCUSSION
The formation of the ligand and the complexes may be represented by the
following reactions:
Reflux 12hr
2PhCH55CH–COOEtþ 2SCðNH2Þ2 ꢁꢁꢁꢁ! C20H20N4O2S2 þ2C2H5OH
Conc: HCl
Ligand (L)
Reflux
LþMX2 ꢁꢁꢁ!MLX2
Ligand
The electron impact mass spectrum of the ligand (L) confirmed the pro-
posed formula by showing a peak at 412 amu corresponding to the macro-
cyclic moiety (C20H20N4O2S2). It also showed a series of peaks at 74, 78,
132, 162, 206, 212, 250, 280, and 363 amu corresponding to various
fragments. Their intensity revealed the stability of these fragments (Fig. 2).
The 1H NMR spectrum of the ligand (L) does not give any signal correspond-
ing to primary amine and ethyl protons. It gives a doublet at 8.57 ppm, which
is due to the thio-amide group (–NH–CS–, 2H) protons.[10] Other signals
appear at 1.53, 2.39, and 7.22 ppm. These signals correspond to methyne
(–CH–CH2–), methlene (–CH2–CO–), and benzenoid protons, respectively.
The presence of a broad signal at d 5.18 ppm is due to the protons of the –NH
groups.[11] The IR spectrum of the ligand does not exhibit any band cor-
responding to the free primary amino and ketonic groups. The absorptions
in the IR spectrum of L in the range of 720–780 and 1430–1630 cm21
are due to the presence of a phenyl group. The bands in the region ꢀ809–
1478 cm21 are due to the thioamide-I and thioamide-II bands. Four new
bands in the spectrum of the free ligand at 1620, 1590, 1248, and 780 cm21
are assignable to amide-I n(C55O), amide-II [n(C–N) þ d(N–H)], amide-III
[d(N–H)], and amide-IV [d(C55O)] bands, respectively.[8,12] A sharp
band observed at 3278 cm21 may be assigned to n(N–H) of the secondary
amido group.[10] 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.

Mn(II), Co(II), Ni(II), and Cu(II) Complexes with Tetradentate Ligand
1595
Figure 2. Electron impact mass spectrum of the ligand (L).
Complexes
On the basis of the elemental analyses, the complexes were assigned the
compositions shown in Table 1. The molar conductance measurements of the
complexes in DMSO correspond to the non-electrolytic nature for the Mn(II),
Co(II), and Cu(II) complexes and a 1 : 2 electrolyte for the Ni(II) complex.
Thus, these complexes may be formulated as [M(L)X2] and [Ni(L)]X2
[where M ¼ Mn(II), Co(II), and Cu(II) and X ¼ Cl2 and NO32]. On com-
plexation, the IR bands corresponding to n(NH), amide-I, and amide-II are
shifted to lower frequency when compared with the macrocyclic ligand.
This indicates that the coordination of the ligand has taken place through
the nitrogen of –NH group [N4].
IR Bands Due to Anions
The IR spectra (Fig. 3) of nitrato complexes display three (N–O) stretch-
ing bands [Fig. 3(a), (b), and (d)] at ꢀ1418–1427 cm21 (n5), 1303–
1311 cm21 (n1), and 1003–1012 cm21 (n2). The position of bands in the IR
spectra of the complexes indicates that both nitrate groups are coordinated
to the central metal ion in an unidentate manner.[13]

1596
Chandra, Gupta, and Sangeetika

Mn(II), Co(II), Ni(II), and Cu(II) Complexes with Tetradentate Ligand
1597
Figure 3. IR Spectral bands of the anions.
However, the IR spectrum of the Ni(II) nitrate complex displays a sharp
and strong band [Fig. 3(c)] at 1388 cm21, which suggests that the nitrate
groups are uncoordinated.[13]
Manganese(II) Complexes
The manganese(II) complexes show a magnetic moment at room
temperature in the range of 5.83–5.97 BM corresponding to five unpaired
electrons. The electronic spectra of the Mn(II) complexes exhibit
four weak-intensity absorption bands in the ranges 18,519–18,911
(1 ¼ 41–55 L mol21 cm21), 22,553–23,105 (1 ¼ 41–43 L mol21 cm21),
29,349–29,733 (1 ¼ 69–74 L mol12 cm21), and 37,583–38,129 cm21

1598
Chandra, Gupta, and Sangeetika
(1 ¼ 127–131 L mol21 cm21), Table 2. These bands may be assigned to the
6
4
6
4
4
6
transitions: A1g ! T1g(4G), A1g ! Eg, A1g(4G) (10B þ 5C), A1g
!
4Eg(4D) (17B þ 5C), and A1g ! T1g(4P) (7B þ 7C), respectively.[14,15]
6
4
The values of the Racah parameters B and C were calculated from the
second and third transitions because these transitions are free of crystal-field
splitting and depend only on the B and C parameters.[8] The calculated
values of the ligand field parameters are given in Table 5.
EPR spectra were recorded at room temperature for a polycrystalline
sample and in a solution of DMSO. The polycrystalline spectra were isotropic
and exhibit the ‘g’ value in the range 2.0054–2.0099 (Table 3). In DMSO sol-
ution, the Mn(II) complexes give EPR spectra (Table 4) containing the six
lines arising due to the hyperfine interaction between the unpaired electron
with the 55Mn nucleus (1 ¼ 5/2). The nuclear magnetic quantum number
M1, corresponding to these lines, are 25/2, 23/2, 21/2, þ1/2, þ3/2,
and þ5/2 from low to high field.
Cobalt(II) Complexes
At room temperature, the magnetic moment measurements of cobalt(II)
complexes lie in the range 4.89–4.97 BM[16] corresponding to three unpaired
electrons, Table 2.
The electronic spectra of all the cobalt(II) complexes exhibit absorptions
in the regions 10,173–11,198 (1 ¼ 72–79 L mol21 cm21), 14,620–14,793
(1 ¼ 81–84 L mol21 cm21), 18,657–18,692 (1 ¼ 90–92 L mol21 cm21),
Table 2. Magnetic moment and electronic spectral data of the complexes.
Complex
meff (BM)
5.83
lmax (cm21) 1 (L mol21 cm21 a
)
[Mn(L)Cl2]
18,519 (49), 22,553 (55), 29,349 (74),
37,593 (131)
[Mn(L)(NO3)2]
[Co(L)Cl2]
5.97
4.89
4.97
18,911 (47), 23,105 (51), 29,733 (69),
38,129 (127)
10,173 (79), 14,620 (81), 18,692 (90),
26,596 (147)
[Co(L)(NO3)2]
11,198 (72), 14,793 (84), 18,657 (92),
32,680 (139)
[Ni(L)]Cl2
Diamagnetic
Diamagnetic
1.93
16,178 (61), 22,090 (93), 29,543 (137)
16,548 (57), 21,307 (95), 29,825 (132)
10,905 (61), 11,249 (63), 16,447 (159)
10,183 (57), 11,223 (61), 16,260 (153)
[Ni(L)](NO3)2
[Cu(L)Cl2]
[Cu(L)(NO3)2]
2.04
a1 Values in parentheses.

Mn(II), Co(II), Ni(II), and Cu(II) Complexes with Tetradentate Ligand
1599
Table 3. EPR spectral data of the complexes (as polycrystalline sample).
Complexes
Temperature
g
k
g
?
giso
G
[Mn(L)Cl2]
[Mn(L)(NO3)2]
[Co(L)Cl2]
RT
RT
—
—
—
—
2.0054
2.0099
2.1155
2.1245
2.0682
2.0840
—
—
LNT
LNT
RT
2.3268
2.3387
2.0853
2.1182
2.0098
2.0174
2.0597
2.0669
—
—
[Co(L)(NO3)2]
[Cu(L)Cl2]
[Cu(L)(NO3)2]
1.4288
1.7668
RT
Note: RT, room temperature and LNT, liquid nitrogen temperature.
and 26,596–32,680cm21(1 ¼ 139–147 L mol21 cm21). These bands may be
4
4
4
4
assigned to the transitions: T1g(F) ! T2g(F) (n1), T1g ! A2g (n2), and
4
4T1g(F) ! T1g(P) (n3), respectively. The fourth band may be due to charge
transfer. The position of these bands indicates that these complexes have a dis-
torted octahedral geometry,[17,18] Fig. 4, and might possess D4h symmetry. The
4
d7 system has the ground state term F in an octahedral crystal-field and is
split into three states. Since these states are connected by spin–orbit coupling,
the spin–lattice relaxation times are short, making EPR measurements pos-
sible only at very low temperature. That is why EPR spectra of the cobalt(II)
complexes were recorded as polycrystalline samples and in DMSO solutions
at liquid nitrogen temperature. The ‘g’ values were found to be almost the
same in the polycrystalline form as well as in solution (Table 3).
Nickel(II) Complexes
At room temperature, these complexes show diamagnetic behavior,
indicating a square-planar environment around the Ni(II) ion, Fig. 4.
Table 4. EPR spectral data of the complexes (in DMSO solution).
Complexes
Temperature
g
k
g
?
giso
G
[Mn(L)Cl2]
[Mn(L)(NO3)2]
[Co(L)Cl2]
RT
RT
—
—
—
—
2.0017
2.0020
2.1205
2.1135
2.0910
2.0737
—
—
LNT
LNT
RT
2.3256
2.3298
2.1329
2.0831
2.0179
2.0053
2.0701
2.0690
—
—
[Co(L)(NO3)2]
[Cu(L)Cl2]
[Cu(L)(NO3)2]
1.8959
1.2043
LNT
Note: RT, room temperature and LNT, liquid nitrogen temperature.

1600
Chandra, Gupta, and Sangeetika
Figure 4. Suggested structures of the complexes.
The electronic spectra of the Ni(II) complexes exhibit three absorption
bands in the ranges 16,178–16,548 (1 ¼ 57–61 L mol21 cm21), 21,307–
22,090 (1 ¼ 93–95 L mol21 cm21), and 29,543–29,825 cm21 (1 ¼ 132–
137 L mol21 cm21). An examination of these bands indicates that the com-
plexes have square-planar geometry. For diamagnetic square-planar Ni(II)
complexes; the five degenerate d-orbitals are split as follows- dx22y2(b1g),
dxy(b2g), dz2(A1g), dxz, dyz (eg). The electronic ground state in a spin triplet
3
1
state is A2g and in an excited state A1g. The relative stabilities of the
3
1A1g and A2g states are determined by the energy separation D1, of the dxy
and dx22y2 orbitals. Thus, these bands may be assigned to the three spin-
1
1
1
1
allowed transitions: A1g(D) ! A2g(G) (n1), A1g(D) ! B2g(G) (n2), and
1A1g(D) ! Eg(G) (n3), respectively.
1
Copper(II) Complexes
The magnetic moment measurements of the Cu(II) complexes at room
temperature lie in the range 1.97–2.09 BM corresponding to one unpaired
electron.[19,20]
Electronic spectra of copper(II) complexes display bands in the
ranges 10,183–10,905 (1 ¼ 57–61 L mol21 cm21) (n1), 11,223–11,249
(1 ¼ 61–63 L mol21 cm21
)
(n2) and 16,260–16,447 cm21 (1 ¼ 153–
159 L mol21 cm21) (n3), corresponding to the following transitions[18]
:
2
2
2
2B1g ! A1g (dx22y2 ! dz2
)
n1, B1g ! B2g (dx22y2 ! dzy) n2, and
2
2B1g ! Eg (dx22y2 ! dzy, dyz) n3, respectively.

Mn(II), Co(II), Ni(II), and Cu(II) Complexes with Tetradentate Ligand
1601
Room temperature EPR spectra of Cu(II) complexes were recorded as a
polycrystalline sample and in DMSO solution, on X-band at a frequency of
9.5 GHz and magnetic field strength of 3400 + 2000 G. Polycrystalline
spectra exhibit a single broad signal resulting from the interaction of the
unpaired electron. No band corresponding to DMs ¼ +2 transitions was
observed in the spectrum to suggest Cu–Cu interaction.[21,22] The absence
of this transition can be explained by proposing that interaction between
two paramagnetic centers is negligible. The analysis of the spectra gives
g ¼ 2.0831–2.1329 and g ¼ 2.0597–2.0701. The observed g values for
k
?
k
the complexes are less than 2.3 in agreement with the covalent character of
the metal–ligand bond. The trend g . g . 2.0023 observed for the com-
k
?
plexes indicates that the unpaired electron is localized in a dx22y2 orbital of
the Cu(II) ion and the spectra are characteristic of axial symmetry. Tetra-
gonally elongated structures are thus confirmed for the Cu(II) complexes.
Electrochemical Behavior
The cyclic voltammogram of the copper(II) complexes in DMSO were
recorded at room temperature using a glassy carbon as working electrode
and TBAP as supporting electrolyte. Both complexes of Cu(II) show a
quasi-reversible peak.
The chloro complex of copper gives a peak at 0.72 V, which corresponds
to copper(II) ! copper(III), and a cathodic peak at Epc ¼ 0.39 V, which
corresponds to Cu(III) ! Cu(II). While the nitrato complex gives peaks at
Epc ¼ 0.69 and 0.42 V.
The complexes exhibit two irreversible peaks characteristic for
copper(II) ! copper(I), [CuLCl2] (Epc ¼ 20.69 V) and [CuL(NO3)2]
(Epc ¼ 20.56 V) and copper(I) ! copper(0) for [CuLCl2] (Epc ¼ 20.95 V)
and for [CuL(NO3)2] (Epc ¼ 20.91 V). These peaks suggest single-electron
reduction processes. On the anodic side, two peaks are also observed, which
correspond to oxidation of Cu(0) ! Cu(II). This quasi-reversible behavior
of the Cu(II)/Cu(III) couple was observed by varying the scan rates with
peak potentials.
The E1/2 values are independent of the scan rate. It has been observed that
DEp increases with the increasing scan rate. Ipc/Ipa being equal to unity indi-
cates the chemical reversibility of the redox process. The reversibility of the
reduction behavior on the DEp value may arise due to the stereochemical
changes in the complexes. The Cu(II) to Cu(I) redox processes are influenced
by the coordination number, stereochemistry, and the hard/soft character of
the ligand donor atoms.

1602
Chandra, Gupta, and Sangeetika
Table 5. Ligand field parameters of the complexes.
Dq
(cm21
B
C
(cm21
LFSE
(kJ mol21
)
Complex
)
(cm21
)
)
b
[Mn(L)Cl2]
[Mn(L)(NO3)2]
[Co(L)Cl2]
1851.90
1891.10
1271.62
1399.75
1617.80
1654.80
613.71
518.29
644.55
574.06
735.36
752.18
3060.44
3581.01
—
0.78
0.66
0.58
0.52
0.71
0.72
00
00
121.54
133.79
231.94
237.25
[Co(L)(NO3)2]
[Ni(L)]Cl2
[Ni(L)](NO3)2
—
—
—
On comparing the cyclic voltammograms, we observe that the variation in
the oxidation and reduction potentials may be due to distortion in the geometry
that arises due to different coordinated anions.
Ligand Field Parameters
Various ligand field parameters were calculated for the complexes and are
listed in Table 5. The values of Dq for the Co(II) complexes were calculated
from the transition energy ratio diagram using the n3/n2 ratios.[18] Our results
of ligand field parameters are in agreement with the results reported
earlier.[14,15,23] The Nephelauxetic parameter b was readily obtained by
using the relation: b ¼ B (Complex)/B (Free ion) where B (Free ion) for
Mn(II) is 786 cm21, for Ni(II) is 1041 cm21, and for Co(II) is 1120 cm21
.
The values of b lie in the range of 0.52–0.78. These values indicate appreci-
able covalent character of the metal–ligand ‘s’ bonds.
ACKNOWLEDGMENT
We are thankful to the Principal, Zakir Hussain College, for providing
research facilities, the Regional sophisticated Instrumentation Center, Indian
Institute of Technology, Mumbai, for recording EPR spectra and the Univer-
sity Grant Commission, New Delhi, for financial assistance.
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Received November 14, 2003
Accepted May 6, 2004