Spectral studies, cyclic voltammetry and synthesis of cobalt(II) and ruthenium(III) complexes with symmetric and asymmetric ring containing membered N2S2, N4, and N5 donor macrocyclic ligands

Article abstract of DOI:10.1016/j.saa.2005.04.002

Reaction of divalent cobalt(II) and trivalent ruthenium(III) salts (NO ₃, SCN and SO₄) with macrocyclic ligands L¹, L² and L³ having N₂S₂, N₄ and N₅ core, have been designed and carry out. All these three macrocyclic ligands and their complexes were obtained in pure form. Their structures were investigated by using microanalytical analyses, IR, mass, magnetic moments, electronic and EPR spectral studies. The redox properties of the complexes were also examined by cyclic voltammetry. An interesting feature of complexes is that the relatively large rings of macrocyclic ligands prevent the macrocyclic rings from approaching the metal center as closely as they would, if they were not constrained. So the Ru-N distances are longer than expected due to ring size. Electrochemical studies show that the macrocyclic ligand L¹ is more effective electron donors to ruthenium than of L² and L³. Electronic spectral properties also show that the sulphur donor atom of L¹ weakens the ligand field with respect to ligand-to-metal charge-transfer band. However it is expected that second-row transition metal-ligand bonds tend to be weaker than third-row transition metal-ligand bonds. There are well-established examples of reactions in which decreased of reactivity down a triad of transition metals is not observed. These novelties are usually attributed to π-bonding effects for ligands such as carbon monoxide, solvent effects, or a change in mechanism.

Full text of DOI:10.1016/j.saa.2005.04.002

Spectrochimica Acta Part A 62 (2005) 1050–1057
Spectral studies, cyclic voltammetry and synthesis of cobalt(II) and
ruthenium(III) complexes with symmetric and asymmetric ring
containing membered N2S2, N4, and N5 donor macrocyclic ligands
Sulekh Chandra∗, Rajiv Kumar
Department of Chemistry, Zakir Husain College (Delhi University), J.L. Nehru Marg, New Delhi 110002, India
Received 9 December 2004; received in revised form 21 March 2005; accepted 14 April 2005
Abstract
Reaction of divalent cobalt(II) and trivalent ruthenium(III) salts (NO3, SCN and SO4) with macrocyclic ligands L1, L2 and L3 having N2S2,
N4 and N5 core, have been designed and carry out. All these three macrocyclic ligands and their complexes were obtained in pure form. Their
structures were investigated by using microanalytical analyses, IR, mass, magnetic moments, electronic and EPR spectral studies. The redox
properties of the complexes were also examined by cyclic voltammetry. An interesting feature of complexes is that the relatively large rings of
macrocyclic ligands prevent the macrocyclic rings from approaching the metal center as closely as they would, if they were not constrained.
So the Ru–N distances are longer than expected due to ring size. Electrochemical studies show that the macrocyclic ligand L1 is more effective
electron donors to ruthenium than of L2 and L3.
Electronic spectral properties also show that the sulphur donor atom of L1 weakens the ligand field with respect to ligand-to-metal charge-
transfer band. However it is expected that second-row transition metal–ligand bonds tend to be weaker than third-row transition metal–ligand
bonds. There are well-established examples of reactions in which decreased of reactivity down a triad of transition metals is not observed.
These novelties are usually attributed to ␲-bonding effects for ligands such as carbon monoxide, solvent effects, or a change in mechanism.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Macrocyclic ligands; Mass spectroscopy; Cobalt(II); Ruthenium(III); IR; UV; Cyclic voltammetry
1. Introduction
metal binding sites are extremely effective in terms of cav-
ity size, geometrical requirements, coordination number, and
nature of the donor atoms to selectively form complexes. In
fact, by changing the nature of macrocyclic frameworks sym-
metrically or asymmetrically introduced with in the macro-
cyclic structure, selectively could be reach in the binding
properties of the two different donating atoms. An important
role is of course played by the flexibility of the large lig-
and ring. These aspects were addressed in a symmetric man-
ner by developing three types of ligands, in which these lig-
ands are tetradentate, and pentadentate, having either N2S2,
N4, and N5 core. In the present process, the reaction prod-
uct in each case was isolated, purified and characterized. In
this paper, we report the preparation and characterization
of cobalt(II) and ruthenium(III) complexes with multiden-
tate ligands having N2S2, N4, and N5 core. The geometry of
the present complexes is highly affected from a number of
The rational design and synthesis of macrocyclic ligands
systems, capable of accommodating metal centers in close
proximity and their spatial arrangement is an effective area
of research [1]. Indeed, macrocyclic ligands and their com-
plexes in which donor atoms can be tuned via variation func-
tionality in the ligand structure, also find application as mod-
els for important metallobiosities [2], as catalysts [3] and in
the investigation of the mutual influence of proximal metal
centers on the electronic, magnetic and redox properties of
such systems [4]. This may represent a very promising new
synthetic approach to design the macrocyclic ligands whose
∗
Corresponding author. Tel.: +91 1 234276530; fax:+91 1 234276530.
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.2005.04.002

S. Chandra, R. Kumar / Spectrochimica Acta Part A 62 (2005) 1050–1057
1051
Table 1
Analytical data of cobalt(II) complexes
Complexes
Empirical formula Yield
(%)
m.p.
(◦C)
Molar conductance
Color
Elemental analysis calculated (found) (%)
(ꢀ−1 cm2 mol−1
)
Co
C
H
N
CO(L1)(NCS)2 COC31H24N4S4
36
49
57
211
205
221
240
231
250
247
237
231
11.0
17.0
95.0
195
190
185
12.0
19.0
85.0
Brown
9.21 (9.14)
8.42 (8.40)
58.20 (58.10) 3.78 (3.60) 8.76 (8.74)
65.22 (65.02) 5.19 (5.10) 12.01 (11.85)
CO(L2)(NCS)2 COC38
CO(L3)(NCS)2 COC20
H
H
36N6S2
25N7S2
Reddish
Light red
Light red
Reddish brown 8.33 (8.16)
Red
Blackish red
Light red
Pink
12.11 (12.0) 49.37 (49.30) 5.18 (5.09) 20.15 (20.0)
9.10 (8.90)
CO(L1)(NO3)2 COC29H24N4S2O6 51
53.79 (53.42) 3.74 (3.65) 8.65 (8.32)
61.10 (61.0) 5.13 (5.02) 11.88 (11.23)
CO(L2)(NO3)2 COC36
H
36N6O6
55
CO(L3)(NO3)2 COC18H25N7S2O6 44
10.55 (10.10) 38.71 (38.50) 4.51 (4.25) 17.56 (17.26)
9.51 (9.20)
8.67 (8.12)
CO(L1)SO4
CO(L2)SO4
CO(L3)SO4
COC29H24N2S3O4 39
COC36 36N4SO4 58
COC18H25N5SO4 50
56.21 (56.02) 3.90 (3.75) 4.52 (4.25)
63.61 (63.20) 5.34 (5.12) 8.24 (8.00)
H
12.64 (12.24) 46.35 (46.22) 5.40 (5.36) 15.02 (14.85)
Table 2
Analytical data of ruthenium(III) complexes
Complexes
Empirical formula Yield
(%)
m.p.
Molar conductance
Color
Elemental analysis calculated (found) (%)
(◦C)
(ꢀ−1 cm2 mol−1
)
Ru
13.66 (13.52) 51.94 (51.63) 3.27 (3.10) 9.46 (9.12)
Brownish yellow 12.63 (12.10) 58.55 (58.10) 4.54 (4.12) 12.26 (12.05)
C
H
N
Ru(L1)(NCS)2 RuC32H24N5S5
Ru(L2)(NCS)2 RuC39H36N7S3
Ru(L3)(NCS)2 RuC21H25N8S3
42
41
52
190
198
185
210
205
200
84.0
96.0
190
90.0
105
Light yellow
Light yellow
Yellow
Light yellow
Light yellow
17.23 (17.10) 42.99 (42.63) 4.29 (4.00) 19.10 (18.9)
13.44 (13.13) 46.33 (46.21) 3.22 (3.10) 9.32 (9.00)
12.45 (12.32) 53.21 (5310) 4.42 (4.32) 12.08 (11.76)
16.89 (16.23) 36.12 (33.96) 4.21 (4.06) 18.72 (18.63)
Ru(L1)(NO3)2 RuC29H24N5S2O9 57
Ru(L2)(NO3)2 RuC36H36N7O9
Ru(L3)(NO3)2 RuC18H25N8O9
16
39
110
atoms and the anions of coordinated metal. Molar conduc-
tance measurements provide important information regard-
ing the position of anion in the coordination sphere (Tables 1
and 2).
To an EtOH solution (25 ml) of C6H5COCH2COC6H5
(0.005 mol), an EtOH solution (25 ml) of NH2C6H4SCH2
CH2SC6H4NH2 (0.005 mol) or NH2CH2CH(CH3)NH2
(0.005 mol) was added. The resulting solution was refluxed
on water bath at 70–80 ◦C for 6–7 h. The solution was then
concentrated to half of its volume under reduced pressure
and kept overnight at ∼5 ◦C. The white crystals formed were
filtered, washed with EtOH, and dried under vacuum over
P4O10.
2. Experimental
All the chemicals used in this investigation were of AR
grade, and were purchased from Sigma Aldrich Chemical
Co., USA. Ethanol used was of analytical grade procured
from S.D. Fine Chemicals Pvt. Ltd. All the solvents were
dried before use by passing over clean, dried sodium wire,
refluxed for 30 min and distilled using a double-walled con-
denser at 78 ◦C.
Ligand L3: 1,4,7,10,13-pentaazacyclopentadecane [N5].
To an EtOH solution (25 ml) of NH2C6H4NHCH2CH2
NHC6H4NH2 (0.002 mol), an EtOH solution (25 ml) of
ClCH2CH2NHCH2CH2Cl (0.002 mol) was added. The re-
sulting solution was refluxed on water bath at 75–80 ◦C for
4–5 h. The solution was then concentrated to half of its vol-
ume under reduced pressure and kept overnight at ∼5 ◦C. The
white-off crystals formed were filtered, washed with EtOH,
and dried under vacuum over P4O10.
3. Synthesis of macrocyclic ligands
3.1. Preparation of diamines
3.3. Characterization of macrocyclic ligands
Ligand L1 found C, 79.0, H, 5.0, N, 6.0% cald. for
C29H24N2S2C, 74.9, H, 5.2, N, 6.0% and for ligand L2 found
C, 82.0, H, 6.5, N, 22.0% cald. for C36H36N4C, 82.41, H,
6.92, N, 10.64%. The bands corresponding to NH2 and CO
are not observed in the IR spectrum of these ligands, but
shows sharp and new bands at 1596 and 1597 cm−1 respec-
tively which may be assigned to the C N group. It indicates
that complete condensation takes place. The mass spectrum
The diamines used are prepared as reported earlier [4].
3.2. Preparation of macrocyclic ligands
Ligand L1—1,4-diphenyl-1,5-diaza-8,11-dithiacyclotri-
deca-1,4-diene [N2S2] and ligand L2—2,4,9,11-tetraphelyl-
6,13-dimethyl-1,5,8,12-tetraazacyclotetradeca-1,4,8,11-tet-
etraene [N4] are prepared by the method given below.

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S. Chandra, R. Kumar / Spectrochimica Acta Part A 62 (2005) 1050–1057
sitive detection. Conductance measurements in nitromethane
were carried out on a Leeds Northrup model 4995 conductiv-
ity bridge. Analyses of carbon and hydrogen were performed
by the Microanalytical Laboratory of the Central Drug Re-
search Institute, Lucknow. The nitrogen content of the com-
plexes was determined using Kjeldahl’s method. The cobalt
content in the complexes was determined volumetrically. The
voltammogram were recorded under nitrogen using freshly
distilled, degassed DMF. A glass carbon electrode was used
as the counter electrode. A non-aqueous reference electrode
was employed. The electrode solution for non-aqueous ref-
erence electrode was prepared by dissolving 0.01 m AgNO3
in 0.1 M tetramethylammonium perchlorate (TEAP) in DMF
solution resulting an Ag/Ag+ electrode. TEAP was applied
as electrode on an x–y Houston-Ommigraphic 2000 recorder.
Fig. 1. Suggested structures of macrocyclic ligands.
5. Result and discussion
of ligand L1 shows a peak at 463 corresponding to the molec-
ular ion (M+ + 1). EIMS m/z (%) 463 (M+, 75%) and the mass
spectrum of the L2 shows a peak at m/z 523 corresponding to
the molecular ion (M+ + 1). EIMS m/z (%) 523 (M+, 70%).
Ligand L3 found C, 69.5, H, 7.5, N, 22.0% cald. for
C18H25N5 C, 69.42, H, 8.09 N, 22.40%. The infrared spec-
trum of this ligand shows a band at 3250 cm−1 which may be
assigned to the secondary amino group. The mass spectrum
of the ligand shows a peak at 309 corresponding to the molec-
ular ion (M+ + 1). EIMS m/z (%) 310 (M+, 65%) (Fig. 1).
5.1. Cobalt(II) complexes with ligands L1 and L2
On the basis of the elemental analyses, the cobalt(II) com-
plexes have the general composition Co(L1 or L2)X2 (where
X = NCS, NO3, and 1/2SO4). All of the complexes show
molar conductance 190, 95, and 10 ꢀ−1 cm2 mol−1 corre-
sponding to 1:2, 1:1 and non-electrolytes (Table 1). On com-
plexation the important ν(C N) band in the infrared spectra
are shifted to lower frequency as compared to the macrocyclic
ligands [4].
3.4. Preparation of the complexes
This indicates that these ligands coordinated to the
cobalt(II) through nitrogen of C N. These complexes show
magnetic moments corresponding to three unpaired electrons
in the range 4.75–4.82 B.M. at room temperature (300 K)
[5,6,13].
EtOH solution of (25 ml) of the corresponding macro-
cyclic ligand (0.002 mol), was added to an EtOH solution
of the (25 ml) hydrated cobalt(II) or ruthenium(III) salts
(0.002 mol). The resulting solution was refluxed on a wa-
ter bath at 70–80 ◦C for 7–8 h. The solution was then con-
centrated to half of its volume under reduced pressure and
kept overnight at ∼5 ◦C. The colored crystals formed were
filtered, washed with EtOH and dried under vacuum over
P4O10.
The infrared spectra of the Co(L1)(NCS)2 and
Co(L2)(NCS)2 show sharp peaks at 2082 and 2081 cm−1
,
indicating that the thiocyanate groups are coordinated
through the nitrogen of the NCS group [7]. The electronic
spectra of these complexes display three well-defined bands
at 9510–9523 (ν1), 16,395–16,445 (ν2), and 23,225–23,809
(ν3) cm−1 which may be assigned to the transitions,
4T1g(F) → T2g(F) (ν1), T1g(F) → A2g(F) (ν2), and
4
4
4
4T1g(F) → T1g(P) (ν3), respectively (Fig. 2) corresponding
4
4. Physical measurements
to a six-coordinated geometry [6] (Fig. 3).
The magnetic susceptibilities were measured on a Gouy
balance using Hg[Co(NCS)4] as calibrating agent. These in-
frared spectra of the complexes were recorded on a Perkin-
Elmer FTIR 1710 automatic-recording spectrophotometer in
KBr pellets. Electronic Spectra (200–1100 nm) of the com-
plexes were recorded on a Shimadzu DMR-21 automatic-
recording spectrophotometer in DMSO. Mass spectra were
carried out on a Jeol, JMX, DX-303 mass spectrophotometer.
The EPR spectra of the complexes were recorded as poly-
crystalline samples at room temperature (25 ◦C) as well as in
nitrogen atmosphere on a Varian E-4 EPR spectrometer oper-
ating at 9.4 GHz and 100 kHz field modulation and phase sen-
In the infrared spectra of Co(L1 or L2)(NO3)2 complexes
show sharp bands at 1383 and 1382 cm−1 corresponding to
an uncoordinated nitrate group [6,7] respectively. These com-
plexesshowmagneticmomentcorrespondingtooneunpaired
electron in the range of 1.99–2.00 B.M. Electronic spectrum
of these complexes display two bands at 10,250–10,300 and
20,730–20,840 cm−1 corresponding to a four coordinated ge-
ometry (Fig. 4).
Co(L1)SO4 and Co(L2)SO4 show magnetic moment in
the range 4.85–4.90 B.M. indicating a high-spin config-
uration [9]. IR spectra Co(L1 or L2)SO4 show bands at
945-41 (ν1), 1055-51 (ν2) and 1160-58 (ν3) cm−1 and

S. Chandra, R. Kumar / Spectrochimica Acta Part A 62 (2005) 1050–1057
1053
Fig. 5. Suggested structures of [Co(L1 or L2)SO4].
Fig. 2. Electronic spectrum of A and B are for [Co(L1)(NCS)2] and
[Co(L2)(NCS)2].
show molar conductance 195–180 ꢀ−1 cm2 mol−1 corre-
sponding to 1:2 electrolytes but sulphate complex is non-
electrolyte [12]. So that these complexes may be formulated
as [Co(L3)](SCN)2, [Co(L3)](NO3)2 and [Co(L3)(SO4)]. On
complexation, the position of the important infrared spectral
bands (amide II [(N H)], and CH2 NH CH2 ) are shifted
to lower frequency as comparative the free macrocyclic lig-
and [7].
The infrared spectra of [Co(L3)](NO3)2 and
[Co(L2)](NCS)2 complexes show bands at 1383 and
2050 cm−1 corresponding to an uncoordinated nitrate
and thiocyanate groups [2,6]. The electronic spectra of
these complexes display well-defined bands at 4700–4866,
9990–9741, 12,345–12,500 and 16,650–16,949 cm−1
Fig. 3. Suggested structures of [Co(L1 or L2)(NCS)2].
4
4
4
4
942-40 (ν1), 1055-51 (ν2) and 1165-62 (ν3) cm−1 cor-
responding to unidentate nature of sulphato group [10].
The electronic spectra of these complexes display well-
defined bands at 4765–4850, 5560–5665, 12,940–13,010 and
these may assigned to A2(F) → E, A2(F) → E(P) and
4A2(F) → A2(P) (Fig. 6) transition respectively. Thus,
4
a five-coordinate geometry may be suggested for these
complexes (Fig. 7).
In the infrared spectrum of [Co(L3)]SO4 shows absorp-
tion at 1103 and 930 cm−1 indicating coordinated nature of
sulphate group. The electronic spectrum of the [Co(L3)]SO4
display three well-defined bands at 9017 (ν1), 16,640 (ν2),
and 21,210 (ν3) cm−1 which may be assigned to the transi-
4
4
19,352–20,980 cm−1 these may assigned to A2(F) → E,
4
4
4A2(F) → E(P) and 4A2(F) → A2(P) transition respec-
tively according to five coordinated geometry. On the basis of
above studies, a five-coordinate square pyramidal geometry
(Fig. 5) may be suggested for these complexes.
tions, 4T1g(F) → T2g(F) (ν1), 4T1g(F) → A2g(F) (ν2), and
4
4
4T1g(F) → T1g(P) (ν3), respectively, corresponding to a six-
4
5.2. Cobalt(II) complexes with the ligand L3
coordinate geometry [5,6] (Figs. 8 and 9).
On the basis of elemental analyses, these complexes have
the general composition CoL3(X)2 (where L = NCS, and
NO3). These complexes show magnetic moment in the range
4.75–4.82 B.M. at room temperature (300 K) [10–13] cor-
responding to three unpaired electrons. These complexes
Fig. 6. Electronic spectra of A and B of [Co(L3)](X)2 where (X = SCN and
NO3).
Fig. 4. Suggested structures of [Co(L1 or L2)](NO3)2.

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S. Chandra, R. Kumar / Spectrochimica Acta Part A 62 (2005) 1050–1057
7. Cyclic voltammetry
Cyclic voltammetry was used to confirm the oxidation lev-
els of the metal centers in the cobalt(II) complexes. The elec-
trochemical properties of metal complexes, particularly with
sulphur donor atoms have been studied in order to consider
spectral and structural changes accompanying with electron
transfer. The redox behaviour of the cobalt(II) complexes has
been examined in DMSO at a glassy carbon as working elec-
trode using electrochemical analyzer. All these complexes
are electroactive only with respect to metal center. The ob-
servation of a single, strong peak which is not exhibited by
the corresponding ligand indicates oxidation of cobalt(II) to
cobalt(III) on the cyclic voltammetric time scale and suggests
that in one complex, the metal center are symmetrically lo-
cated. Comparison of the oxidation potential of the nitrogen
or sulphur donor complexes reveals that both have different
oxidation potentials. It is due to the greater difference in the
electron withdrawing ability of donor atoms. The complexes
of cobalt(II) exhibit both metal and ligand-centered electro-
chemistry in the potential range 1.7 V versus the Ag/AgCl,
CI− electrode. All the complexes exhibit one quasi-reversible
oxidation process as evident by the peak to peak separation,
ꢁEp > 100 mV, and show redox potentials in the range of
1.0–1.25 V. In all cobalt(II) complexes the cathodic and an-
odic peak heights (Ipc and Ipa) are the same. The one-electron
nature of the couple is confirmed on comparing with the cur-
Fig. 7. Suggested structure of [Co(L3)](X)2 where (X = SCN and NO3).
Fig. 8. Electronic spectrum of [Co(L3)SO4].
4−
rent height of the Fe(CN)63−/Fe(CN)6 system. The elec-
trochemical behaviour of the cobalt(II) complexes is due to
theCo(III)/Co(II)couple. Thepositivepotentialindicatesthat
metal in lower oxidation state is strongly bound to the ligand.
7.1. Reduction process
Cobalt(II) complexes show a quasi-reversible two step sin-
gle electron transfer process. E1/2 values are independent of
scan rate. The ꢁEp increases with increasing scan rate and is
always greater than 60 mV.
Voltammetric parameters are studied in the scan range of
60–800 mV s−1. The ratio between the cathodic peak current
and square root of the scan rate (Ipc/v1/2) is approximately
constant [10,14]. The peak potential shows a small depen-
dence with the scan rate. The ratio (Ipa/Ipc) is close to unity.
Form these data, it can be deduced that this redox couple is re-
lated to a reversible one-electron transfer process controlled
by diffusion (Figs. 10 and 11).
Fig. 9. Suggested structure of [Co(L3)(SO4)].
6. EPR spectra
7.2. Oxidation process
The EPR spectra of the complexes under study were
recorded at liquid nitrogen temperature because the rapid spin
lattice relaxation of cobalt(II) broadens the lines at higher
temperature. The higher deviation of the g values from the
free electron value (g = 2.0023) is due to orbital contributions
[8,9]. g and g values were calculated and found in the range
The cyclic voltammogram of cobalt(II) complexes shows
two peaks corresponding to two single electron transfer pro-
cess.
The difference between the potential of the anodic peak
and cathodic peak remains constant. Also, the ratio between
the cathodic peak current and square root of the scan rate
is practically constant in the range studied. All the data are
||
⊥
of 3.40–4.55.

S. Chandra, R. Kumar / Spectrochimica Acta Part A 62 (2005) 1050–1057
1055
Fig. 10. Cyclic voltammogram of A and B are for [Co(L1 or L2)(NCS)2] in
anodic region.
Fig. 13. Cyclic voltammogram of A and B are for [Co(L1 or L2)](NO3)2 in
cathodic region.
8. Ruthenium(III) complexes with ligands L1, L2 and
L3
On the basis of elemental analyses, all the complexes were
found to have general composition RuL(X)3. The molar con-
ductance measurement indicates that the complexes are 1:1
and 1:2 electrolyte in nature. Thus, these complexes may
be formulated as [RuL1 or L2(X)2](X) and [RuL3(X)](X)2
(where X = NO3 and SCN) respectively (Table 2). The Ru(III)
complexes show magnetic moments at room temperature in
the range of 1.76–1.80 B.M., which is lower than the pre-
dicted value of 2.10–2.15 B.M. [13,15] The lowering in µeff
values may be due to either lower symmetry ligand fields,
metal–metal interaction or extensive electron delocalization
[13].
The infrared spectra of the thiocyanate and nitrate com-
plexes of ruthenium show bands corresponding to coordi-
nated as well as uncoordinated nature of both groups and also
indicates that the thiocyanate groups is coordinated through
the nitrogen of the NCS group [7,16].
Fig. 11. Cyclic voltammogram of A and B are for [Co(L1 or L2)(NO3)2] in
anodic region.
diagnostic for a simple quasi-reversible one-electron charge
transfer controlled by diffusion method.
The redox responses are negative which are relative to ref-
erence electrode due to ligand reductions. The C N, NH ,
and Ph S CH2 groups in the ligands are known as potential
electron transfer centers [14]. We observed two one-electron
quasi-reversible reductions (ꢁEp > 100 mV) in the potential
range 0.2 to −0.3 and −0.4 to −0.6 V along with a two-
electron reduction at −1.0 to −1.3 V (Figs. 12 and 13).
The electronic spectra of ruthenium(III) complexes dis-
play three bands at 15,625–15,873 (ν1), 17,241–16,522 (ν2)
and 25,000–26,315 cm−1 (ν3). These bands may be assigned
4
4
4
to 2T2g → T1g, 2T2g → T2g and 2T2g → A1g transitions in
order of increasing energy [13] (Figs. 14 and 15).
The position of bands is in tune with the prediction of
octahedral arrangement around of the metal ion. The ligand
field parameters ∆, B and β have been calculated by using the
relation ν1 = ∆ − 4B + 86(B)2/∆ and ν2 = ∆ + 12B +2(B)2/∆
[5,6]. The value of B in free ion is ruthenium(III)
630 cm−1
.
The value of β indicates that there is low covalency in the
metal ligand ␴-band (Figs. 16 and 17). Suggested structures
of these complexes are given in figures.
9. Electrochemical studies
Fig. 12. Cyclic voltammogram of A and B are for [Co(L1 or L2)(NCS)2] in
cathodic region.
The electrochemical properties of complexes show a
variety of chemically reversible processes i.e. cathodic and

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S. Chandra, R. Kumar / Spectrochimica Acta Part A 62 (2005) 1050–1057
Fig. 17. Suggested structure of [Ru(L3)(X)](X)2.
Fig. 14. Electronic spectra of A and B are [Ru(L1or L2)(NCS)2](NCS) and
[Ru(L3)(NCS)2](NCS).
of L1 as compared to the +2 charge L2. It means that removal
of an electron will be electro statically much easier in the
former case, irrespective of the nature of the donor atoms of
the ligands.
The second contribution arises from the inherently differ-
ent electron donor/acceptor properties of a ligand as com-
pared to an L2 and L3 ligands. Although it is not possible
to separate the two effects, but it has been known that the
electrostatic effect is the more significant, with changes in
the nature of the ligands. It suggests that the electrostatic
contribution to stabilization of the ruthenium(III) state will
be similar in both cases. The additional +0.1 V stabilization
of the ruthenium(III) state in L1 as compared to L2, may be
due to the different electronic properties of the sulphur lig-
ands, and apparently suggests that the electron donor ability
is dependent upon the nature of donor atoms. This is inconsis-
tent with the known sulphur donor abilities of these ligands
that indicates the stronger electron donor property of ligand.
However, such a simplistic electrostatic picture ignores the
effects of covalency, in particular the greater polarisability
of the lone-pair electrons on sulphur compared to those on
nitrogen, which is clearly a dominant effect here.
Fig. 15. Electronic spectra of A, B and C [Ru(L1or L2)(NO3)2](NO3) and
[Ru(L3)(NO3)2](NO3).
anodic peak currents whereas equal intensity over a wide
range of scan rates.
9.1. Metal-based couples
For complex L1 (N2S2 donor set) there are, surprisingly,
two one-electron redox processes at moderate potentials
which shows two waves corresponding to Ru(II)/Ru(III) and
Ru(III)/Ru(IV) couples. The Ru(II)/Ru(III) couple of L2 may
be ascribed to the process at 21.43 V. The difference be-
tween the electronic effects of sulphur and nitrogen donors
is less obvious here, with the two complexes having their
Ru(II)/Ru(III)couplesat verysimilarpotentials. The negative
shift of 2.32 V with respect to parent complex corresponds
to 0.76 V, which is somewhat smaller than the shift of 0.96 V
observed for L2.
The metal-based couples are most significant for assess-
ing the electronic properties of the ligands and metal com-
plexes. For the N4 coordinated, ruthenium(III) couple occurs
at 20.07 V versus the ferrocene–ferrocenium couple, a sub-
stantial shift from the value of +0.89 V for (L2). The first
change is in the overall charge on the complex; the +1 charge
When significant covalency is involved we would not nec-
essarily expect the effects to be exactly additive. For example,
in complex L1, where there are two sulphur donors, it is quite
reasonable that each one should donate rather less electron
density to the metal than does the single as in donor of L2
(the electro neutrality principle), and this is reflected in the
electrochemical results.
Fig. 16. Suggested structures of [Ru(L1 or L2)(X)2](X).

S. Chandra, R. Kumar / Spectrochimica Acta Part A 62 (2005) 1050–1057
1057
The electrochemical properties of ruthenium(III) com-
plexes in which two sulphur atoms present in ligand were
attached to a ruthenium(L3) fragment, and these provide a
convenient basis for comparison with L2 and L1. The cou-
ple at 20.21 V versus ferrocene/ferrocenium for complex L1
we assign to the expected Ru(III)/Ru(IV) couple, by com-
parison with ruthenium(L2) for which this couple occurs at
+0.14 V. The ruthenium(IV) state is therefore stabilized in L1
by 0.35 V more than it is in L2. This difference in potential
between the Ru(III)/Ru(IV) couples is much larger than the
difference between the Ru(II)/Ru(III) couples for the same
pair of complexes. The electron-donating properties to ruthe-
nium become more pronounced as the oxidation state of the
metal center increases, which is consistent with the fact that
the electrons on sulphur are more polarisable than those on
other atoms: the ligands can adjust to the higher oxidation
state by transferring more electron density to the metal, which
the more electronegative and less polarisable sulphur atoms
are unable to do so well.
The coordination behaviour of the nitrato and thiocyanato
groups in L3 confirms the stability of metal ion in the ring.
The stability of the complexes is also depending on the num-
ber of donating atoms in the macrocyclic ring and upon the
nature transition metal. In the cobalt(II) and ruthenium(III)
complexes of L3, which is a pentadentate ligand, the anions
are coordinated as well as uncoordinated. Cyclic voltamme-
try explain the relation between the stability of complexes
and the donor atoms which is highly dependable on electro
negativity of donor atoms and the nature of metal ion.
Acknowledgements
One of the authors (Rajiv Kumar) is greatly indebted to his
younger brother Bitto for motivation. The University Grants
Commission, Delhi for financial assistance and the Univer-
sity Science Instrumentation Centre, New Delhi University,
for recording IR spectra. Thanks to Nitin Kampani, Cen-
tral Science Library, Delhi University for providing guid-
ance in computer software programming and the Solid State
Physics Laboratory, Delhi for recording magnetic moments
and thanks to I.I.T. Bombay for recording EPR spectra.
9.2. Ligand-based couples
The third redox process of complex L1, at +0.56 V, was
entirely unexpected as it has no counterpart as in RuL2. It
is highly unlikely to be a Ru(IV)/Ru(V) couple as ruthe-
nium(V) complexes are very rare and are unknown with this
type of ligands. If two atomic orbitals, each containing a lone
pair, overlap slightly then bonding and antibonding sum-and-
difference combinations will form. Normally both of these
contain two electrons so there is no net bonding interaction,
but one-electron oxidation of the pair will give a this inter-
action which will be particularly strong for a pair of sulphur
atoms.
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In this paper we reported the synthesis and characteri-
zation of 13-, 14- and 15-membered macrocyclic ligands.
The size is effected the complexation behaviour of these lig-
ands. Recently we published the complexation behaviour of
12-membered ligands with first row transition metals. But
in this paper, the nature of the ligands and its effect on the
stability of the complexes are observed constantly by using
12–15-membered macrocyclic ligands.
The second important behaviour of macrocyclic ligands
with different oxidation state metals i.e. first row transi-
tion metals cobalt(II) and second-row transition metal ruthe-
nium(III) are observed. Both metals oxidation states are also
examined by cyclic voltammetry. The oxidation and reduc-
tion behaviour of our complexes explains the stability of
metal ions in complexes in present study.