Versatility in the coordination behavior of a hexatopic compartmental Schiff-base ligand in the architecture of binuclear transition metal(II) complexes

Article abstract of DOI:10.1080/00958971003802075

Condensation of ¹H-pyrazole-3,5-dicarboxylic hydrazide with 1H-indole-2,3-dione (isatin) yield the compartmental ligand, which is capable of encapsulating two transition metal ions namely Co^II, Ni ^II, Cu^II, and Zn^II. The ligand is a binuclear hexadentate chelate with N₄O₂ donating sites. The pyrazole core provides the diazine fragment, which serves as an endogenous bridge between the two metal centers. In Co^II and Ni^II complexes, the ligand is in the imidol form and the subsequent coordination through the imidol oxygen. In other complexes, the lactonic oxygen takes part in ligation. All the complexes are non-electrolytes and soluble in DMSO, DMF, and acetonitrile. Spectral and magnetic studies along with analytical data suggest octahedral geometry for the Co^II and Ni^II complexes, whereas the Cu^II and Zn^II complexes are assigned square pyramidal geometry. The Cu^II and Ni^II complexes show one electron redox behavior and the rest are electrochemically inactive.

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Versatility in the coordination behavior
of a hexatopic compartmental Schiff-
base ligand in the architecture
of binuclear transition metal(II)
complexes
Srinivasa Budagumpi a , Gurunath S. Kurdekar a , Ganesh S. Hegde
a , Nagaraj H. Bevinahalli a & Vidyanand K. Revankar a
a Department of Chemistry , Karnatak University , Pavate Nagar,
Dharwad 580 003, Karnataka, India
Published online: 28 Apr 2010.
To cite this article: Srinivasa Budagumpi , Gurunath S. Kurdekar , Ganesh S. Hegde , Nagaraj H.
Bevinahalli & Vidyanand K. Revankar (2010) Versatility in the coordination behavior of a hexatopic
compartmental Schiff-base ligand in the architecture of binuclear transition metal(II) complexes,
Journal of Coordination Chemistry, 63:8, 1430-1439, DOI: 10.1080/00958971003802075
To link to this article: http://dx.doi.org/10.1080/00958971003802075
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Journal of Coordination Chemistry
Vol. 63, No. 8, 20 April 2010, 1430–1439
Versatility in the coordination behavior of a hexatopic
compartmental Schiff-base ligand in the architecture
of binuclear transition metal(II) complexes
SRINIVASA BUDAGUMPI, GURUNATH S. KURDEKAR,
GANESH S. HEGDE, NAGARAJ H. BEVINAHALLI
and VIDYANAND K. REVANKAR*
Department of Chemistry, Karnatak University, Pavate Nagar,
Dharwad 580 003, Karnataka, India
(Received 22 September 2009; in final form 11 January 2010)
Condensation of 1H-pyrazole-3,5-dicarboxylic hydrazide with 1H-indole-2,3-dione (isatin)
yield the compartmental ligand, which is capable of encapsulating two transition metal ions
namely CoII, NiII, CuII, and ZnII. The ligand is a binuclear hexadentate chelate with N4O2
donating sites. The pyrazole core provides the diazine fragment, which serves as an endogenous
bridge between the two metal centers. In CoII and NiII complexes, the ligand is in the imidol
form and the subsequent coordination through the imidol oxygen. In other complexes, the
lactonic oxygen takes part in ligation. All the complexes are non-electrolytes and soluble in
DMSO, DMF, and acetonitrile. Spectral and magnetic studies along with analytical data
suggest octahedral geometry for the CoII and NiII complexes, whereas the CuII and ZnII
complexes are assigned square pyramidal geometry. The CuII and NiII complexes show one
electron redox behavior and the rest are electrochemically inactive.
Keywords: Pyrazole; Indole; Endogenous; Diazine; Octahedral
1. Introduction
Pyrazole with 1,2-diazine unit provides endogenous bridging between two similar/
dissimilar transition/inner transition metal ions [1–6]. With the aid of alkyl or aryl
pendent arms at the 3- and 5- positions of the pyrazole core, it can form stable
transition metal complexes through chelation. Mixed-valence diazine-bridged coordi-
nation compounds involving smaller chloride, bromide, etc., for exogenous bridging
are of escalating interest in biology due to the total delocalization of the metal valencies
over the cluster, which makes them proficient in electron transfer processes [7–9].
Diazine-bridged compounds have interesting physical properties, such as magnetic
behavior, electrical conductivity, stability towards air and moisture, capability of
electron transfer, etc., which makes these compounds of preference for applications
*Corresponding author. Email: vkrevankar@rediffmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958971003802075

Hexatopic compartmental Schiff base
1431
in magnetic materials, metalloenzymes, molecular electronics, and also in the multi-
electron transfer processes in biology [10].
1H-indole-2,3-dione (isatin) has two dissimilar carbonyl groups, a lactam-carbonyl
(ꢀ), and a keto-carbonyl (ꢁ). The difference in functionality of two carbonyl groups
provides many applications in synthetic chemistry [11, 12]. Isatin exhibits versatile
biological activities [13] and also is an important biochemical reagent used in the
determination of amino acids [14, 15]. Isatin forms Schiff-base ligands by involving
ꢁ carbonyl with amine functionalities in a selective manner. These ligands, upon
interaction with transition metal ions, yield numerous complexes with a variety of
physico-chemical properties [16]. Metal–isatin binary complexes were advantageous
over simple isatin in chemotherapy and found to act as anticancer agents, especially
Schiff-base transition metal complexes derived from isatin [16]. In continuation of our
research on the coordination chemistry of pyrazole and isatin, we have synthesized a
series of coordination compounds of later first row transition metals.
2. Experimental
1H-Pyrazole-3,5-dicarboxylic acid, isatin, hydrazine hydrate, and thionyl chloride were
purchased from S.D. Fine Chem. Ltd. and used without purification. Analytical grade
absolute ethanol was purchased from the same agency, further dried and distilled
before use according to standard procedure. 1H-Pyrazole-3,5-dicarboxylic hydrazide
was prepared with slight modifications according to the reported method [17]. The
preparation of ligand is outlined in figure 1. The estimation of metal(II) ions was
carried out by standard methods [18] after decomposing the complexes in nitric acid.
The molar conductivity measurements were made on ELICO-CM-82 Conductivity
Bridge. Magnetic susceptibility measurements were made on a Faraday balance at
1
room temperature using Hg[Co(SCN)4] as calibrant. H NMR spectra were recorded
in DMSO-d6 on a Bruker-300 MHz spectrometer at room temperature using TMS as
internal reference. IR spectra were recorded in a KBr matrix using an Impact-410
Nicolet (USA) FT-IR spectrometer from 4000 to 400 cmꢀ1. The electronic spectra
of the complexes were recorded on a Hitachi 150-20 from 1000 to 200 nm. The ESR
spectrum of the copper complex was carried out on a Varian E-4X-band EPR
spectrometer using TCNE as g-marker. The TG and DTA measurements of the
complexes were recorded under nitrogen on a Universal V2.4F TA Instrument keeping
final temperature at 800ꢁC and heating rate was 10ꢁC minꢀ1. Cyclic voltammetric
studies were performed at room temperature in DMF under O2 free conditions using
CH instruments Electrochemical analyzer, CHI-1110A (USA), using tetramethylam-
monium chloride as supporting electrolyte. FAB mass spectra were obtained from
a JEOL SX 102/DA-6000 mass spectrometer using Argon/Xenon (6 kV, 10 mA) as the
FAB gas.
2.1. Preparation of 1H-pyrazole-3,5-dicarbohydrazide
Pyrazole-3,5-dicarboxylic acid (0.312 g, 0.02 mol) was refluxed with 10 mL of thionyl
chloride for about 4 h at 110–115ꢁC under anhydrous conditions. Excess thionyl

1432
S. Budagumpi et al.
O
O
HO
OH
N
N
H
Pyrazole-3,5-dicarboxylic acid
SOCl2 Reflux
O
O
Cl
Cl
N
N
H
3,5-Dichloroformyl pyrazole
.
.
NH2 NH2 H2O
Reflux in
dry ethanol
O
O
HN
NH2
Pyrazole-3,5-dicarboxylic hydrazide
NH
N
N
H2N
H
O
+ 2
O
N
H
Isatin
Reflux in
absolute Ethanol
O
O
HN
NH
N
N
N
N
H
N
N
O
O
LH
H
H
Figure 1. Preparation of LH.
chloride was removed under reduced pressure. The resultant white pasty solid was
cooled in an ice bath for 15 min and the hydrazine monohydrate (2.24 mL, 0.04 mol)
was added and further refluxed for about 4 h producing a clear colorless solution.
The resultant solution was then cooled at 0ꢁC to get colorless solid. Yield: 77%,
m.p.: 212–214ꢁC.

Hexatopic compartmental Schiff base
2.2. Preparation of the Schiff-base ligand (LH)
1433
A mixture of 1H-pyrazole-3,5-dicarboxylic hydrazide (1.84 g, 0.01 mol) and isatin
(2.94 g, 0.02 mol) in absolute ethanol (50 cm3) was stirred at room temperature for
about 15 min, then refluxed for 3–4 h on a water bath. LH was obtained as a pale yellow
precipitate, separated by filtration under suction, washed with ethanol and then air
dried. Yield: 84%; m.p. 286–288ꢁC.
2.3. Preparation of the complexes
Metal(II) chloride {CoCl2 ꢂ 6H2O (0.475 g, 0.002 mol), NiCl2 ꢂ 6H2O (0.474 g, 0.002 mol).
CuCl2 ꢂ 2H2O (0.341 g, 0.002 mol) and ZnCl2 (0.271 g, 0.002 mol)} in EtOH (25 cm3) was
added with stirring to an ethanolic solution (25 cm3) of LH (0.433 g, 0.001 mol) and
refluxed at water bath temperature for 3–4 h. The obtained solid was separated by
filtration under suction, washed with hot ethanol and dried in vacuo.
3. Results and discussion
Analytical, magnetic, and conductivity data for the complexes are given in table 1.
All the complexes are insoluble in water, sparingly soluble in common organic solvents
and completely soluble in DMF, DMSO, and acetonitrile.
3.1. FTIR spectral studies
Infrared frequencies and their assignments for the different functional groups are given
in table 2. The characteristic lactam stretching frequency of the isatin ring system is
observed at 1708 and 1679 cmꢀ1 as strong bands due to the molecular dissymmetry.
It shifts to the lower frequency in CuII and ZnII complexes and is absent in CoII and NiII
complexes. This shift with reduction in intensity is attributed to the participation
of lactam oxygen in coordination; the absence of lactam stretching frequency suggests
the imidol form of the ligand with subsequent oxygen coordination. A band at
1665 cmꢀ1 in the free ligand assigned to the ꢂ(C¼O) of amide remains unchanged
in spectra of the complexes. Azomethine and pyrazole ring (C¼N) vibrations were
observed at 1611 and 1557 cmꢀ1, respectively, in the ligand. Upon complexation,
azomethine shifts about 7–14 cmꢀ1 to higher frequency, indicating its involvement in
coordination [19]. The increase in frequency of ꢂ(N–N) also supports coordination
through azomethine nitrogen in all complexes [20]. Pyrazole ring vibrations shift
both directions signifying diazine bridging to the metals. The magnitude and direction
of shift may be attributed to the extent of overlapping of ꢃ-1,2-diazine lone pair with
metal orbitals [21]. The presence of coordinated/lattice waters is confirmed by a band
at 3350–3400 cmꢀ1. ꢂ(–NH) appear at 3150 cmꢀ1, unaltered in the complex spectra
[22, 23]. The ꢂ(M–N) are observed at 456–466 cmꢀ1 [24]. IR spectra of ligand and CuII
complex are provided in Supplementary Material.

1434
S. Budagumpi et al.

Hexatopic compartmental Schiff base
Table 2. Infrared spectral data of ligand and complexes in cmꢀ1
1435
.
ꢂ(N–H) ꢂ(N–H) ꢂ(4C¼O) ꢂ(4C¼O) ꢂ(4C¼N) ꢂ(4C¼N)
ꢂ(O–H) Isatin Amide Lactum Amide Azomethine Pyrazole ꢂ(N–N) ꢂ(M–N)
Compounds
LH
[Co2L(m-Cl)(H2O)4]
[Ni2L(m-Cl)(H2O)4]
–
3168
–
–
3162
3118 1709, 1679
1655
b, 1660
1657
s, 1652
1658
1611
1625#
1621#
1618
1619
1557
1505
1502
1546
1567
1017
1063
1052
1063
1054
–
b, 3430
3424
b, 3100
b, 3110
3115
–
–
1700
1702
461
466
458
456
[Cu2L(m-Cl)Cl2] ꢂ H2O 3318
[Zn2L(m-Cl)Cl2] ꢂ H2O b, 3340 b, 3170 b, 3100
b: broad band; s: shoulder.
#Around 1600 cmꢀ1 a shoulder band is observed due to the formation of new azomethine fragment in the isatin ring system.
3.2. 1H NMR studies
1
The H NMR spectrum of LH exhibits sharp singlets at 14.7, 14.1, and 10.8 ppm,
assigned to pyrazole ring proton (ꢀNH) [21, 25], isatin ring protons (ꢀNH) [16], and
amide protons, respectively. Isatin ring protons at such a downfield region confirm its
existence as the lactam tautomer. Multiplets at 6.9–7.8 ppm are from aromatic protons.
The absence of pyrazole ring proton (ꢀNH) in the ZnII complex spectrum indicates
deprotonation, confirming the endogenous bridging of pyrazole diazine. The other
protons experience a small change in the chemical shift values, attributable to the
variation in electron density and steric constraints brought about by chelation.
3.3. Magnetic susceptibility and absorption spectral studies
Magnetic susceptibility values summarized in table 1 show little or no antiferromagnetic
exchange interaction between metal centers. Endogenous diazine bridging can bring the
metal ions closer together, but reduction in electron density on the metal centers by
electron-withdrawing of bridging exogenous chloride is less favorable for effective spin
exchange interactions, hence the exchange interaction decreases [26, 27].
The electronic absorption spectra of the compounds were recorded in DMF at 200–
1000 nm. LH shows bands at 313 and 342 nm which correspond to intraligand, ꢄ ! ꢄ*
and n ! ꢄ* transitions. These transitions have been red shifted by 17–25 and 30–35 nm,
respectively, upon complexation indicating coordination of azomethine group. The
electronic spectrum of CoII complex shows an intense band at 560 nm, which is a
characteristic d–d transition band for CoII complexes with octahedral geometry [28].
Three distinguishable d–d bands at 495, 550, and 665 nm are observed for the NiII
complex suggesting octahedral geometry [29, 30]. The CuII complex exhibits three main
bands at around 615, 350, and 320 nm. The lowest energy asymmetric band is assigned
to d–d transition. Broad asymmetric bands at around 600 nm, with a low-energy
shoulder band, have been observed for cyclic and acyclic Schiff-base binuclear CuII
complexes with square-pyramidal geometry [31, 32]. ZnII complex shows absorptions
around 350–365 nm, which correspond to intraligand transitions. The electronic spectra
of ligand and complexes are provided in ‘‘Supplementary material’’.
3.4. Molar conductivity measurements
The molar conductance values of complexes were obtained at room temperature
in DMSO solution with 10ꢀ3 mol dmꢀ3 concentration. The molar conductivity values

1436
S. Budagumpi et al.
of all the complexes (table 1) fall in the range 2.8 to 6.1 Ohm cm2 molꢀ1, indicating the
non-electrolytic nature of the complexes [33].
3.5. ESR spectral studies
X-band ESR spectra of polycrystalline CuII complex recorded at room temperature
shows an isotropic intense broad signal at giso ¼ 2.07 with no response at hyperfine
splitting. No half-field absorption rules out the existence of spin–spin interaction, which
is further supported by the normal magnetic moment value. The ESR spectrum of the
binuclear CuII complex of this type is characteristic of a square pyramid [27].
3.6. FAB-mass spectral studies
The elemental and analytical data suggest [M2L] ꢂ nH2O for all the complexes; this is
supported by FAB mass spectra. The peaks at highest m/z value cannot always be
assigned with certainty, but the isotopic pattern is consistent with the binuclear
complexes. A peak at m/z 707 was observed in the CuII complex, which is consistent
with the mass of the entire binuclear, monomeric complex including coordinated
chloride ions and lattice-celled water molecules. Apart from this, spectrum shows some
other peaks which are due to the molecular cations of various fragments of complex.
By comparing the analytical and spectral data of cobalt, nickel, and zinc complexes,
it is evident that these are monomeric and binuclear complexes.
3.7. Thermal studies
Thermogram of [CoL(ꢃ-Cl)(H2O)4] complex in the range 30 to 800ꢁC under nitrogen
shows decomposition in two steps. In the first step, loss of 9.46% was observed at 60–
210ꢁC from loss of four coordinated waters and a chloride, confirmed by the
appearance of endothermic and exothermic DTA signals at 100 and 210ꢁC, respectively.
In the next step, the total weight loss is due to the elimination of ligand and formation
of stable metal oxide. Thermograms of the remaining complexes are in agreement with
the proposed structures of the complexes.
3.8. Electrochemistry
The electrochemical behavior of metal complexes containing two or more chemically
equivalent electroactive sites have been the subject of numerous studies. It is reported
[34] that for reversible sequential transfer of two electrons (each in oxidation and
reduction process) the DEp in the cyclic voltammogram would be 42 mV. In the present
investigation the compounds were scanned for the cyclic voltammetric studies. Ligand
and its CoII and ZnII complexes are found to be electrochemically innocent in the
working potential range (þ1 to ꢀ1 V); in remaining cases the redox processes are
unambiguously metal based.
The typical cyclic voltammogram of CuII complex is shown in figure 2(a). The present
CuII complex shows one electron transfer redox behavior with oxidation potential at
0.15, 0.16, and 0.17 V and there is no reduction peak observed with varying scan rates of

Hexatopic compartmental Schiff base
1437
Figure 2. Cyclic voltammogram of CuII (a) and NiII (b) complexes at the scan rate of 0.1 V sꢀ1
.
0.05, 0.1, and 0.15 V sꢀ1, respectively. Analysis of the cyclic voltammogram for
CuII–CuIII oxidation at varying scan rates reveals that it is irreversible one electron
transfer. A quasireversible cyclic voltammogram (figure 2b) was obtained for NiII
complex. The oxidation potential is observed at 0.40, 0.41, and 0.42 V and is assigned to
NiII–NiIII. Subsequently in the opposite scan, a peak is observed at 0.28, 0.29, and
0.31 V indicating reduction of NiIII–NiII. By analyzing the variation of peak position as
a function of scan rates, it is possible to predict the nature of the electron transfer
behavior. The peak difference in the potential of oxidation and reduction process
on average is around 120 mV, suggesting quasireversible redox process. The oxidation
step and the corresponding reduction obey the diagnostic criteria that is, Ipc/Ipa almost
constant but not unity which further supports the quasireversible electron transfer
process [19, 21].

1438
S. Budagumpi et al.
O
O
HN
NH
N
N
.
N
N
H2O
M
Cl
M
Cl
N
H
N
O
O
Cl
H
M: Cu and Zn
O
O
HN
NH
N
N
OH2
OH2
N
N
M
M
N
N
O
O
Cl
OH2
OH2
M: Co and Ni
Figure 3. Proposed structures of the complexes.
4. Conclusion
Ligand formed by the condensation of 1H-pyrazole-3,5-dicarboxylic hydrazide with
two equivalent molecules of isatin is a hexadentate chelate towards later first row
transition metal ions. All synthesized complexes have [M2L] composition. The diazine
group acts as endogenous bridge between metal centers and successive coordination
occurred through the azomethine nitrogen and lactam/imidol oxygen of isatin. Copper
and zinc complexes have square pyramidal geometries, while nickel and cobalt
complexes are octahedral (figure 3). All complexes are monomers and non-electrolytes.
A weak/non-antiferromagnetic interaction is present between the metal ions. An
irreversible and quasireversible cyclic voltammograms are obtained for the copper
and nickel complexes, respectively.
Acknowledgments
The authors thank Department of Chemistry and USIC, Karnatak University,
Dharwad for the spectral facility. Recording of ESR spectrum (IIT Bombay) and
the FAB-mass spectra (CDRI) Lucknow are gratefully acknowledged. One of the
authors Srinivasa Budagumpi thanks UGC for awarding Research Fellowship in
Science for Meritorious Students.

Hexatopic compartmental Schiff base
1439
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