Pseudohalide-induced structural variations in hydrazone-based metal complexes: Syntheses, electrochemical studies and structural aspects

Article abstract of DOI:10.1016/j.ica.2008.01.019

The effects of pseudohalides on the modes of binding of a hydrazone ligand are investigated and we report here four new hydrazone complexes [Co(L)₂] · ClO₄ (1), [Co(L)₂]₂[Co(SCN)₄] (2), [Mn(L)₂] (3) and [Mn(N₃)₂ (LH)₂] · H₂O (4) [where LH = condensation product of pyridine-2-carboxaldehyde and benzhydrazide]. The hydrazone ligand is quite diverse in its chelating ability and can act as a neutral or mononegative ligand and as a bidentate or tridentate unit. In this paper, we report structures showing different denticities of the ligand having different charges. The complexes are characterized by IR, UV-Vis spectroscopy and cyclic voltammetric studies and the structures are conclusively established by single crystal X-ray diffraction study. Of the complexes, 1 and 3 are purely metal hydrazone complexes whereas 2 and 4 incorporate coordinated pseudohalides in their crystal structures, and 4 shows different coordination modes of the hydrazone ligand.

Full text of DOI:10.1016/j.ica.2008.01.019

Available online at www.sciencedirect.com
Inorganica Chimica Acta 361 (2008) 2692–2700
www.elsevier.com/locate/ica
Pseudohalide-induced structural variations in
hydrazone-based metal complexes: Syntheses, electrochemical
studies and structural aspects
a
a
a
a,
b
*
Sambuddha Banerjee , Aurkie Ray , Soma Sen , Samiran Mitra , David L. Hughes ,
Ray J. Butcher c, Stuart R. Batten d, David R. Turner d
a Department of Chemistry, Jadavpur University, Raja S.C. Mallick Road, Kolkata, West Bengal 700 032, India
b School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich NR4 7TJ, UK
c Department of Chemistry, Howard University, 2400 Sixth Street, NW, Washington, DC 200 59, USA
d School of Chemistry, Monash University, 3800, Australia
Received 30 August 2007; received in revised form 19 January 2008; accepted 23 January 2008
Available online 1 February 2008
Abstract
The effects of pseudohalides on the modes of binding of a hydrazone ligand are investigated and we report here four new hydrazone
complexes [Co(L)2] ꢀ ClO4 (1), [Co(L)2]2[Co(SCN)4] (2), [Mn(L)2] (3) and [Mn(N3)2 (LH)2] ꢀ H2O (4) [where LH = condensation product
of pyridine-2-carboxaldehyde and benzhydrazide]. The hydrazone ligand is quite diverse in its chelating ability and can act as a neutral or
mononegative ligand and as a bidentate or tridentate unit. In this paper, we report structures showing different denticities of the ligand
having different charges. The complexes are characterized by IR, UV–Vis spectroscopy and cyclic voltammetric studies and the structures
are conclusively established by single crystal X-ray diffraction study. Of the complexes, 1 and 3 are purely metal hydrazone complexes
whereas 2 and 4 incorporate coordinated pseudohalides in their crystal structures, and 4 shows different coordination modes of the
hydrazone ligand.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Hydrazone complexes; Synthesis; X-ray crystal structures; Electrochemical studies; Spectral studies; H-bonded polynuclear structure
1. Introduction
[2,3]. Studies have also shown that the azomethine N,
which has a lone pair of electrons in an sp2 hybridized orbi-
tal, has considerable biological importance [4]. Aroylhyd-
razone complexes of transition metal ions are known to
provide useful models for elucidation of the mechanism
of enzyme inhibition by hydrazine derivatives [5] and for
their possible pharmacological applications [6]. Addition-
ally, hydrazone complexes have been the subject of studies
over many years for their anti-microbial and anti-tumor
activities [7–13]. Benzoylhydrazone complexes of copper
[14], vanadium [15,16], ruthenium [17] and manganese
[18] can be found in the literature.
Hydrazone ligands have similarities in their donor prop-
erties with unsymmetrical salen (condensation product of
salicylaldehyde and 1,2-diaminoethane) type ligands.
Recently it was reported that such ligands, like non-sym-
metrical salens, can act as effective catalysts towards alkene
epoxidation [1]. Due to the short N–N bond length, the
benzoyl hydrazone ligands act mostly as tridentate moieties
though they have the potential to act as bridging tetraden-
tate ligands. In analytical chemistry also, hydrazone
ligands find wide applications as transition metal binders
The various modes of coordination of the hydrazone
ligand, LH, the condensation product of pyridine-2-car-
boxaldehyde and benzhydrazide, are shown in Scheme 1.
*
Corresponding author. Tel.: +91 33 2668 2017; fax: +91 33 2414 6414.
E-mail address: smitra_2002@yahoo.com (S. Mitra).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.01.019

S. Banerjee et al. / Inorganica Chimica Acta 361 (2008) 2692–2700
2693
Scheme 1. Different modes of coordination of the ligand LH.
In this work, our aim was to exploit the versatility of coor-
dination behaviour of LH by incorporating pseudohalides
(azide and thiocyanate) into manganese and cobalt precur-
sor complexes. Herein we report four new hydrazone com-
plexes [Co(L)2] ꢀ ClO4 (1), [Co(L)2]2[Co(SCN)4] (2),
[Mn(L)2] (3) and [Mn(N3)2(LH)2] ꢀ H2O (4), where the
same hydrazone ligand binds, depending upon the metals
and pseudohalides used, in tridentate mononegative
NNO donor mode for 1, 2 and 3, and bidentate neutral
NO and tridentate neutral NNO donor modes in 4. In this
paper, we describe the IR, UV–Vis spectra, electrochemical
studies and X-ray crystal structures of these four
complexes.
were recorded on a Perkin–Elmer RX 1 FT-IR spectropho-
tometer with a KBr disc. The electronic spectra were
recorded, in acetonitrile, on a Perkin–Elmer Lambda 40
(UV–Vis) spectrophotometer. Electrochemical studies of
complexes 1–3 were performed using acetonitrile solution
on a Versastat-Potentiostat II electrochemical analyser
against a standard calomel electrode as reference and plat-
inum as the working electrode.
Caution! Perchlorate salts are potentially explosive and
should be prepared in small quantities and handled with
much care.
2.3. Synthesis
2. Experimental
2.3.1. Synthesis of the hydrazone,
[C6H5C(O)NHN@C(H)C5H4N] (LH)
The pro-ligand was synthesized by condensing benzhy-
2.1. Materials
drazide with pyridine-2-carboxaldehyde following
a
Benzhydrazide, pyridine-2-carboxaldehyde, manganous
chloride hexahydrate, cobalt(II) perchlorate hexahydrate,
sodium azide, and sodium thiocyanate (Fluka) were used
as received without further purification. All the solvents
were of reagent grade.
method reported elsewhere [19]. The resulting light yellow
liquid was used without purification. Different modes of
coordination of the ligand LH is presented in Scheme 1.
2.3.2. Synthesis of [Co(L)2] ꢀ ClO4 (1)
To a methanolic solution (30 ml) of 1 mmol of Co(II)
perchlorate hexahydrate (0.365 g), a solution of the hydra-
zone (2 mmol) was added and the solution was stirred at
room temperature for 45 min. The solution yielded crystals
suitable for X-ray diffraction after 3 days. Yield: 78%.
2.2. Physical measurements
Elemental analyses were carried out using a Perkin–
Elmer 2400 II elemental analyzer. The infrared spectra

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Anal. Calc. for C26H20CoN6O6Cl: C, 51.45; H, 3.06; N,
2.3.4. Synthesis of [Mn(L)2] (3)
17.15. Found: C, 51.41; H, 3.01; N, 17.12%.
To a methanolic solution (30 ml) of 1 mmol of manga-
nous chloride (0.197 g), the liquid hydrazone LH (2 mmol)
was added with constant stirring. The solution was stirred
at room temperature for 30 min and then kept in a refrig-
erator. Crystals suitable for X-ray diffraction appeared
after 7 days. Yield: 80%.
2.3.3. Synthesis of [Co(L)2]2[Co(SCN)4] (2)
A methanolic solution (30 ml) of 1 (1 mmol, 0.606 g)
was made slightly alkaline and refluxed on a water bath
for half an hour. The precipitate thus formed was dis-
carded. To the filtrate, 2 mmol (0.160 g) of sodium thiocy-
anate, dissolved in minimum amount of water, was added
with constant stirring. Single crystals suitable for X-ray dif-
fraction were obtained after 3 weeks on keeping the mother
liquor in a refrigerator. Yield: 82%.
Anal. Calc. for C26H20MnN6O2: C, 61.97; H, 3.97; N,
16.68. Found: C, 61.94; H, 3.95; N, 16.66%.
2.3.5. Synthesis of [Mn(N3)2(LH)2] ꢀ H2O (4)
To a methanolic solution (30 ml) of 3 (1 mmol, 0.503 g),
sodium azide (2 mmol, 0.130 g), in minimum amount of
water, was added. The solution was refluxed for 30 min
Anal. Calc. for C56H40Co3N16O4S4: C, 51.45; H, 3.06; N,
17.15. Found: C, 51.42; H, 3.03; N, 17.14%.
Table 1
Crystallographic and refinement data of complexes 1, 2, 3 and 4
Compound
1
2
3
4
Elemental formula
C
ClO4
26H20CoN6O2,
2(C26H20CoN6O2),
C4CoN4S4
C26H20MnN6O2
C26H22MnN12O2, H2O
Formula weight
Crystal system
Space group
606.86
1306.07
monoclinic
C2/c (No. 15)
503.42
monoclinic
P21/n (equiv. to No. 14)
607.5
monoclinic
P21/c (No. 14)
monoclinic
P21/n (equiv. to No.
14)
10.5363(2)
17.6654(4)
14.0139(4)
90
101.923(3)
90
2552.11(10)
4
˚
a (A)
32.1665(14)
22.3903(13)
16.2540(8)
90
92.926(2)
90
11691.2(10)
8
1.484
9.8375(13)
23.9336(16)
10.0407(3)
90
103.736(1)
90
2296.4(3)
4
1.456
9.3296(6)
33.887(2)
9.7560(3)
90
117.711(4)
90
2729.8(3)
4
1.478
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
V (A )
Z
D (Mg mꢁ3
)
1.579
0.833
1240
l (mmꢁ1
F(000)
)
1.045
5320
0.612
1036
0.537
1252
Diffractometer used
Bruker SMARTCCD Bruker X8 Apex II CCD
Oxford Diffraction
Xcalibur-3 CCD
140(1)
Oxford Diffraction
Xcalibur-3 CCD
140(1)
T (K)
173(2)
123(1)
Crystal size (mm)
h Range for data collection (°) 4.6–30.7
0.65 ꢂ 0.35 ꢂ 0.15
0.12 ꢂ 0.10 ꢂ 0.03
0.68 ꢂ 0.32 ꢂ 0.12
3.7–30.0
35020
0.48 ꢂ 0.38 ꢂ 0.12
3.5–27.5
42166
2.1–0.0
No. of reflections collected
No. of independent reflections 7124 (0.036)
(Rint
21680
15423
5432 (0.061)
6662 (0.049)
6245 (0.061)
)
Goodness-of-fit
0.965
1.068
1.040
1.181
R indices (all data)
R1 = 0.070,
wR2 = 0.099
R1 = 0.114,
wR2 = 0.221
R1 = 0.061,
wR2 = 0.075
R1 = 0.082,
wR2 = 0.100
Table 2
˚
Selected bond lengths (A) about the metals in complexes 1, 2, 3 and 4
Complex 1
Complex 2
Complex 3
Complex 4
Molecule A
Molecule B
Co–O(1)
Co–N(2)
Co–N(22)
Co–O(3)
Co–N(4)
Co–N(42)
1.9052(12)
1.8421(16)
1.9271(15)
1.8981(13)
1.8480(16)
1.9192(15)
Co(1)–O(1)
Co(1)–N(2)
Co(1)–N(22)
Co(1)–O(3)
Co(1)–N(4)
Co(1)–N(42)
Co(3)–N(91)
Co(3)–N(92)
1.908(7)
1.864(9)
1.939(9)
1.912(6)
1.840(8)
1.929(8)
2.105(18)
1.951(16)
Co(2)–O(5)
Co(2)–N(6)
Co(2)–N(62)
Co(2)–O(7)
Co(2)–N(8)
Co(2)–N(82)
Co(4)–N(93)
Co(4)–N(94)
1.906(7)
Mn–O(1)
Mn–N(2)
Mn–N(22)
Mn–O(3)
Mn–N(4)
Mn–N(42)
2.1610(10)
Mn–O(1)
Mn–N(2)
Mn–N(22)
Mn–O(3)
Mn–N(4)
Mn–N(51)
Mn –N(61)
2.3193(17)
1.846(10)
1.939(10)
1.899(7)
1.871(9)
1.924(10)
1.946(13)
1.88(2)
2.1874(11)
2.3577(11)
2.1382(10)
2.1950(11)
2.3191(12)
2.303(2)
2.382(2)
2.2706(17)
2.376(2)
2.204(2)
2.210(2)

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2695
and then kept in a refrigerator. Crystals suitable for X-ray
diffraction appeared after seven days. Yield: 65%. Anal.
Calc. for C26H24MnN12O3: C, 51.36; H, 3.62; N, 27.65.
Found: C, 51.32; H, 3.59; N, 27.61%.
2.4. X-ray crystallography
The crystal structure analyses of the complexes 1–4 were
performed by several of the authors, in different laborato-
ries; the equipment used for each sample is listed with the
crystallographic data and the refinement results in Table
1. The structures of all the complexes were determined by
direct method procedures in SHELXS-97, and refined in
SHELXL-97 [20]. The hydrogen atoms bonded to water oxy-
gens in compound 4 were freely refined; all other hydrogen
atoms were included in idealized positions and set to ride
on the parent carbon and nitrogen atoms.
3. Results and discussion
3.1. Description of the crystal structures
3.1.1. Description of the structure of [Co(L)2] ꢀ ClO4 (1)
Relevant bond length and bond angle data for complex
1 are listed, together with corresponding dimensions in the
other complexes, in Tables 2 and 3 respectively.
Complex 1, shown in Fig. 1, consists of a Co(III) center
surrounded by two tridentate hydrazone ligands, with the
metal in a distorted octahedral environment [21]. The
dimensions in the two ligands are very similar and indicate
that the ligands adopt the enolate form (IV in Scheme 1).
Fig. 1. View of a cation and anion of compound 1, indicating the atom
numbering scheme. Thermal ellipsoids are drawn at the 50% probability
level.

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The two central coordinating bonds, Co–N2 and Co–N4,
are shortest, providing a z-in type of distortion in the octa-
hedral arrangement. The N–Co–N trans angle is close to
the ideal value of 180°, but the other trans angles are con-
strained within the meridional ligands, and are ca 166°. The
intraligand bite angles lie in the 82.36(6)–83.55(6)° range.
The angle between the normals to the mean-plane of the
O–C–N–N–C–C–N chains in the hydrazone ligands, the
‘interligand angle’, is 86.1°. Each hydrazone ligand acts
as a tridentate mononegative unit, and there is one uncoor-
dinated perchlorate anion present to neutralize the 1+
charge on the Co(III) complex cation.
The phenyl ring of C(11–16) is overlapped by an almost
parallel pyridyl ring C(21–26) of a neighbouring molecule
(related by n-glide-plane symmetry), and chains through
such overlaps are thus formed through the crystal. Pairs
of the other ligands are formed about centres of symmetry,
but the overlap here, of the N(3)ꢀ ꢀ ꢀC(41) section of the cen-
tral chain, is less pronounced.
of neighbouring phenyl groups, e.g. the ring of C(11–16)
and C(710–760) are close to parallel and overlapping, as
are a pair of C(51–56) rings about a centre of symmetry;
in these molecules, the pyridinyl rings are not involved in
this type of interaction, and there is no extended linking
of molecules.
The two [Co(SCN)4]2ꢁ anions lie on symmetry elements,
so that only half of each anion is unique. These anions have
Co(II) centres with tetrahedral arrangements of NCSꢁ
ligands. The NCSꢁ groups are almost linear but there is
slight bending at the N-atom in each. There is disorder in
a pair of the NCSꢁ ligands on Co(4), with alternative sites
for the C and S atoms.
3.1.3. Description of the structure of [Mn(L)2] (3)
An ORTEP diagram of 3 is presented in Fig. 3 with its
atom numbering scheme and bond dimensions are listed
in Tables 2 and 3. The manganese(II) ion is in an N4O2
coordination sphere [1,18]. Each of the two meridionally
spanning ligands coordinates the metal ion as in complexes
1 and 2, with an ‘interligand angle’ of 85.65(3)°. However,
the distortion of the MnN4O2 octahedron is rather larger
than in 1 and 2, arising from the longer Mn–N and Mn–
O distances; also the trans angles are much further from
180° and the intraligand bite angles are correspondingly
more acute, lying in the range 70.89(4)–71.64(4)°. The bond
lengths and angles in the hydrazone chain conform, as in 1
and 2, to the enolate form of the ligand.
3.1.2. Description of the structure of
[Co(L)2]2[Co(SCN)4] (2)
An ORTEP diagram of 2 is presented in Fig. 2 and bond
dimensions are listed in Tables 2 and 3. A complex with
similar structure with the same metal but with a completely
different ligand system has been reported by our group ear-
lier [22]. In 2, there are four different Co atoms, two inde-
pendent [Co(III)(L)2]2+ cation complexes, and two separate
[Co(II)(SCN)4]2ꢁ anions. In the cations, both the Co(III)
centers are six-coordinate, having distorted octahedral
geometry with two hydrazone ligands disposed in mer-
arrangement, similar to compound 1; the ‘interligand
angles’, here, are 89.4° and 89.0°. As in 1, there is overlap
The hydrazone ligands of N(1) are linked in chains, as in
compound 1, by the overlap of nearly parallel, neighbour-
ing C(11–16) and C(21–26) rings. The pyridinyl ring and
three atoms of the bridging chain of the second hydrazone
ligand, viz. C(41–46), C(4), N(4) and N(3), overlap the
Fig. 2. View of the two independent cations and one of the anions of compound 2, indicating the atom numbering scheme. Hydrogen atoms have been
omitted for clarity. Thermal ellipsoids are drawn at the 30% probability level.

S. Banerjee et al. / Inorganica Chimica Acta 361 (2008) 2692–2700
2697
Fig. 3. View of a molecule of compound 3, indicating the atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level.
group inverted about a centre of symmetry, and thus link
molecules in pairs.
(I in Scheme 1) and O3 and N4 of the second (II in Scheme
1). The two axial positions are occupied by N51 and N61 of
the two azido groups. The C1–O1 and C3–O3 bond lengths
˚
3.1.4. Description of the structure of
[both 1.230(3) A] in the hydrazone ligands indicate that they
[Mn(N3)2(LH)2] ꢀ H2O (4)
coordinate the metal center in the keto form; the locating
and refinement of the hydrogen atoms on both N1 and
N3 confirm this and show these ligands to be neutral
donors.
Both the N–H groups are involved in hydrogen bonds,
that of N1–H1 linking to a water molecule, whilst N3–
H3 bonds intramolecularly to the uncoordinated pyridyl
This structural study reveals that 4 consists of a mononu-
clear unit having a seven-coordinate Mn(II) ion in a pentag-
onal bipyramidal pattern, Fig. 4 and bond dimensions are
listed in Tables 2 and 3. There are two hydrazone ligands
in the molecule showing different denticities. The pentago-
nal plane is occupied by O1, N2, N22 of the first LH ligand
Fig. 4. View of a molecule of compound 4 with the hydrogen-bonded water molecule, indicating the atom numbering scheme. Thermal ellipsoids are
drawn at the 50% probability level.

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S. Banerjee et al. / Inorganica Chimica Acta 361 (2008) 2692–2700
Fig. 5. The hydrogen-bonded polymeric ladder-like structure in compound 4. Thermal ellipsoids are drawn at the 30% probability level.
nitrogen atom, giving the E isomer (with respect to the
imine bond) of the hydrazone ligand extra stability. The
water molecule is involved in further intermolecular hydro-
gen-bonding to N53 and N61 of symmetry-related azido
ligands, forming a ladder-like structure (Fig. 5 and Table
4).
the hydrazone ligand has undergone deprotonation on
complexation. For complex 4, the N–H absorption band
appears at 3010 cmꢁ1 whereas the C–O band appears at
1663 cmꢁ1; both these bands indicate that in this case the
ligand has not undergone the keto–enol tautomerisation
and has coordinated the metal in its original keto form.
These data provide evidence for the coordination of the
ligand to the respective metal ions [Mn(II) and Co(III)]
via two nitrogen atoms and one oxygen atom for complexes
1, 2 and 3. The infrared spectra of complexes 1–4, respec-
tively, display IR absorption bands at 1603, 1604, 1593
and 1637 cmꢁ1 which can be assigned to the C@N stretch-
ing frequencies of the coordinated ligand [23], whereas for
3.2. IR, UV–Vis and Electrochemical studies
3.2.1. Infrared spectra
The IR spectra of all the complexes were analyzed in
comparison with that of the free molecule, LH, in the
region 4000–250 cmꢁ1. The IR spectrum of the hydrazone
molecule contains a strong C–O absorption band at 1651–
1659 cmꢁ1 and an N–H absorption band at 3187–
3250 cmꢁ1. For complexes 1, 2 and 3, both these bands
disappear on complexation and a new C–O absorption
band appears at 1040–1089 cmꢁ1, which indicates that
the free molecule, the same band is observed at 1663 cmꢁ1
.
The shift of this band on complexation towards lower wave
number indicates coordination of the azomethine nitrogen
to the metal center [24]. The ligand coordination is substan-
tiated by two bands appearing at 460, 369 cmꢁ1 for 1, 480,
372 cmꢁ1 for 2, 444, 379 cmꢁ1 for 3, and 492, 375 cmꢁ1 for
4; these are mainly attributed to mM–N and mM–O, respec-
tively. For 2 there is one strong band at 2065 cmꢁ1, which
indicates the presence of coordinated SCNꢁ in monoden-
tate fashion. For 4 there is one strong band at 2057 cmꢁ1
indicating the presence of coordinated azide ligand.
Table 4
H-bond dimensions for 4
D–Hꢀ ꢀ ꢀA
D–H
Aꢀ ꢀ ꢀH
Dꢀ ꢀ ꢀA
\(D–Hꢀ ꢀ ꢀA)
N(1)–H(1)ꢀ ꢀ ꢀO(7)
N(3)–H(3)ꢀ ꢀ ꢀN(42)
(7)–H(71)ꢀ ꢀ ꢀN(610)
O(7)–H(72)ꢀ ꢀ ꢀN(5300)
0.86
0.86
0.82(4)
0.80(3)
1.92
1.96
2.04(4)
2.05(4)
2.742(3)
2.633(4)
2.859(3)
2.843(3)
160
134
176(4)
173(4)
The low energy pyridine ring in-plane and out-of-plane
vibrations are observed in the spectrum of the LH molecule
at 625 and 406 cmꢁ1, respectively, whereas the correspond-
ing bands for the complexes are shifted to higher
Symmetry operations: : 1 ꢁ x, 1 ꢁ y, 1 ꢁ z; 00: ꢁx, 1 ꢁ y, ꢁz.
0

S. Banerjee et al. / Inorganica Chimica Acta 361 (2008) 2692–2700
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frequencies at 638–640 and 419–422 cmꢁ1, respectively,
which are good indications of the coordination of the het-
erocyclic nitrogen atom [25].
sponding to the oxidation of Co(II) to Co(III), is observed
at ꢁ0.085 V. The separation between the two redox peaks
is greater than 50 mV (DEp = 78 mV) and hence the system
corresponds to an irreversible oxidation reduction couple.
Complex 3 shows (Fig. 6) an oxidative response at 0.51 V
versus SCE, which is assigned to the Mn(II) to Mn(III) oxi-
dation, and a reductive response at 0.44 V versus SCE,
assigned to Mn(III) to Mn(II) reduction. This oxidation
is also reversible, characterized by a peak-to-peak separa-
tion (DEp) of 70 mV, which remains unchanged upon vary-
ing the scan rate. The anodic peak current (ipa) is again
almost equal to the cathodic peak current (ipc), as expected
for a reversible electron transfer process.
3.2.2. UV–Vis spectra
UV–Vis spectra of 1, 2 and 3 were recorded at 300 K in
HPLC grade acetonitrile solution. For 1, containing dis-
torted octahedral Co(III) ion, the band at 586 nm appeared
1
due to A1g ? 1T1g transition. The higher energy band
1A1g ? 1T2g was obscured by ligand-to-metal charge trans-
fer bands. For 2, two very broad d–d transition bands are
observed at 625 and 574 nm, and several CT bands can be
observed in the region 442–382 nm.
For 3, with distorted octahedral Mn(II) coordination,
we obtained two bands in the UV region. The band at
374 nm is considerably stronger than the band at 247 nm
and these bands may be assigned to n ? p* (forbidden)
and p ? p* transitions in the ligand. Complex 3 does not
show any d–d transition [26]. Complex 4 was insoluble in
all available common solvents and the UV–Vis spectrum
of this compound in Nujol showed no d–d band, but a ser-
ies of charge transfer bands, as for 3, corresponding to
p ? p* transitions of the ligand, was observed.
4. Conclusion
In this paper, we have reported four new complexes of a
hydrazone ligand with manganese and cobalt metal cen-
ters. In complexes 1, 2 and 3, the ligand acts a mononega-
tive, tridentate NNO donor, whereas in 4 there is one
neutral tridentate NNO and one neutral bidentate NO
donor ligand.
Acknowledgements
3.2.3. Cyclic voltammetry study
Electrochemical properties of 1, 2 and 3 were studied in
HPLC grade acetonitrile medium with tetrabutylammo-
nium perchlorate as supporting electrolyte at a scan rate
50 mV sꢁ1. The voltammogram of 1 shows a peak at
ꢁ0.15 V (versus SCE) which can be attributed to the reduc-
tion of Co(III) to Co(II). Another peak, at ꢁ0.12 V (versus
SCE), was assigned to the oxidation of Co(II) to Co(III).
This process is thus reversible, characterized by a peak-
to-peak separation (DEp) of 30 mV, which remains
unchanged upon varying the scan rate. The anodic peak
current (ipa) is almost equal to the cathodic peak current
(ipc), as expected for a reversible electron transfer process.
For 2, the first reductive response, corresponding to the
reduction of Co(III) to Co(II), appears at ꢁ0.163 V (in
the same region as for 1), whereas the second peak, corre-
We are grateful to DRDO New Delhi and AICTE New
Delhi, India, for funding this research.
Appendix A. Supplementary data
CCDC 648267, CCDC 642868, CCDC 657006 and
CCDC 651331 contain the crystallographic data for 1, 2,
3, 4. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version,
at doi:10.1016/j.ica.2008.01.019.
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