Ligational behavior of S, N, and O donor quinoxaline derivatives toward the later first-row transition metal ions

Article abstract of DOI:10.1080/00958972.2010.487938

Ligands derived from quinoxaline-2,3-(1,4H)-dithione are treated with Co(II), Ni(II), Cu(II), and Zn(II) chlorides to yield stable complexes. The prepared compounds are characterized by spectro-analytical techniques and magnetic susceptibility measurements, and the coordination behavior of ligands is discussed. All the complexes are octahedral and mononuclear with general formula [MLCl₂(H2O)]. The electrochemical behavior of the synthesized compounds was investigated by cyclic voltammetry studies and the redox activity is explained.

Full text of DOI:10.1080/00958972.2010.487938

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Ligational behavior of S, N, and O
donor quinoxaline derivatives toward
the later first-row transition metal ions
Naveen V. Kulkarni a , Nagaraj H. Bevinahalli a & Vidyanand K.
Revankar a
a Department of Studies in Chemistry , Karnatak University ,
Pavatenagar, Dharwad 580003, Karnataka, India
Published online: 11 Jun 2010.
To cite this article: Naveen V. Kulkarni , Nagaraj H. Bevinahalli & Vidyanand K. Revankar
(2010) Ligational behavior of S, N, and O donor quinoxaline derivatives toward the later
first-row transition metal ions, Journal of Coordination Chemistry, 63:10, 1785-1794, DOI:
10.1080/00958972.2010.487938
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Journal of Coordination Chemistry
Vol. 63, No. 10, 20 May 2010, 1785–1794
Ligational behavior of S, N, and O donor quinoxaline
derivatives toward the later first-row transition metal ions
NAVEEN V. KULKARNI, NAGARAJ H. BEVINAHALLI
and VIDYANAND K. REVANKAR*
Department of Studies in Chemistry, Karnatak University, Pavatenagar,
Dharwad 580003, Karnataka, India
(Received 30 November 2009; in final form 8 February 2010)
Ligands derived from quinoxaline-2,3-(1,4H)-dithione are treated with Co(II), Ni(II), Cu(II),
and Zn(II) chlorides to yield stable complexes. The prepared compounds are characterized by
spectro-analytical techniques and magnetic susceptibility measurements, and the coordination
behavior of ligands is discussed. All the complexes are octahedral and mononuclear with
general formula [MLCl2(H2O)]. The electrochemical behavior of the synthesized compounds
was investigated by cyclic voltammetry studies and the redox activity is explained.
Keywords: Coordination compound; Quinoxaline-2,3-(1,4H)-dithione; Quinoxaline-2,3-
(1,4H)-dione; Isatin
1. Introduction
In coordination chemistry, ligational behavior of multidentate ligands, especially with
different donors is of interest. A coordination cavity created by oxygen, nitrogen, and
sulfur donors can provide a better fit in terms of size and flexibility for transition
metal(II) ions. Structural features (stereochemical arrangement) of a heterocyclic or
acyclic core molecule which contain these donors play an important role in the
coordination behavior [1]. Metal complexes derived from the S, N, and O donor ligands
are of interest because of their versatile structural and functional properties, and their
applications in biochemistry, catalysis, medicine, and material research [2–4]. The
task of constructing bicompartmental SNO donor ligands and their complexes is of
immense interest.
Quinoxaline derivatives are a very important class of nitrogen-containing compounds
and have been widely used in dyes [5], pharmaceuticals [6, 7], and electrical/
photochemical materials [8–10]. In particular, thione-substituted quinoxaline
derivatives show applications in medicinal chemistry.
In this work, we select quinoxaline-2,3-(1,4H)-dithione for construction of ligand
systems. One of the two thione functionalities was replaced by the hydrazine sidearm
*Corresponding author. Email: vkrevankar@rediffmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.487938

1786
N.V. Kulkarni et al.
and is used for condensation with the ketone-functionality of isatin and quinoxaline-
dione. One of the two ketonic groups of isatin and quinoxaline-dione interacts with the
hydrazine sidearm and constructs L1 and L2, respectively (figure 1). The free thione
group of quinoxaline and the free ketonic oxygen of isatin, and another quinoxaline
ring along with the nitrogen constitute the SNO cavity. Hence, formed ligands interact
with later first-row transition metal chlorides to yield complexes. The compounds are
characterized by the spectro-analytical techniques and the electrochemical behavior was
investigated.
2. Experimental
2.1. Reagents and apparatus
The chemicals used were of reagent grade. Purified solvents were used for the synthesis
of ligands and complexes. Syntheses of quinoxaline-2,3-(1,4H)-dione [11] and
quinoxaline-2,3-(1,4H)-dithione [12] were done according to the literature with slight
modification. Isatin was purchased from Sigma Aldrich and the metal chlorides
{CoCl2 ꢀ 6H2O, NiCl2 ꢀ 6H2O, CuCl2 ꢀ 2H2O, and ZnCl2} were used for the complex
formation. C, H, N, and S analyses were carried out on a ThermoQuest elemental
analyzer. Metal and chloride estimations were done by standard procedures after
decomposition in nitric acid [13]. Molar conductivity measurements in dimethylforma-
mide (DMF) were made on an ELICO-CM-82 conductivity bridge with a conductivity
H
N
N
NH
2
N
H
S
O
H
N
3-Hydrazinylquinoxaline-2(1H)-thione
Reflux in dry methanol
O
O
O
N
H
N
H
Quinoxaline-2,3(1,4H)-dione
Isatin
N
H
N
NH
H
N
N
N
NH
N
N
H
O
O
N
H
S
N
H
S
1
2
L
L
Figure 1. Schematic representation of preparation of ligands.

S, N, and O donor quinoxalines
1787
cell having cell constant 0.51 cmꢁ1. Magnetic susceptibility measurements were
made using a Faraday balance at room temperature with Hg[Co(SCN)4] as calibrant.
1H-NMR spectra were recorded in DMSO-d6 on a Bruker-300 MHz spectrometer at
room temperature using tetramethylsilane (TMS) as internal reference. Infrared (IR)
spectra were recorded in KBr using an Impact-410 Nicolet (USA) FT-IR spectrometer
from 4000 to 400 cmꢁ1. Electronic spectra of the complexes were recorded on a Hitachi
150–20 from 1000–200 nm. Cyclic voltammetric studies were performed at room
temperature in DMF under pure nitrogen with a CHI1110A electrochemical analyzer
(USA) comprising a three electrode assembly of glassy carbon working electrode,
platinum auxiliary electrode, and non-aqueous Agþ/AgCl reference electrode.
Tetramethylammonium chloride (0.01 mol Lꢁ1) was used as supporting electrolyte
and the instrument was standardized by ferrocene/ferrocenium redox couple. Electron
spin resonance (ESR) of the copper complexes was carried out on a Varian E-4X-band
electron paramagnetic resonance (EPR) spectrometer using tetracyanoethylene (TCNE)
as the g-marker. Fast atom bombardment (FAB) mass spectra were recorded from a
JEOL SX 102/DA-6000 mass spectrometer using Argon/Xenon (6 kV, 10 mA) as the
FAB gas and m-nitrobenzyl alcohol as matrix. Thermogravimetric (TG) and differential
thermal analysis (DTA) measurements of theꢁco1 mplexes were recorded in nitrogen on a
Universal V2 4F TA instrument at 10ꢂC min and scan range of 25–800ꢂC.
2.2. Synthesis
2.2.1. Synthesis of 3-hydrazinoquinoxaline-2-thione. Hydrazine hydrate in methanol
(0.6 mL, 0.1 mol Lꢁ1) is added dropwise to a methanolic solution of quinoxaline-2,3-
(1,4H)-dithione (1.94 g, 0.1 mol Lꢁ1) with constant stirring. The mixture is then refluxed
for 2 h over a steam bath. The product formed as a shining dark yellow solid was
filtered, thoroughly washed with methanol, and then air dried (Yield: 83%, m.p.: 182–
184ꢂC) (figure 2).
2.2.2. Synthesis of L1 and L2. To a hot dry methanolic solution of 3-hydrazinoqui-
noxaline-2-thione (1.92 g, 0.1 mol Lꢁ1), a hot methanolic solution of isatin (1.51 g,
0.1 mol Lꢁ1) or quinoxaline-2,3(1,4H)-dione (1.62 g, 0.1 mol Lꢁ1) (for L1 and L2,
respectively) was added dropwise with constant stirring at room temperature.
The mixture was refluxed for 1 h and the solid was filtered and washed with dry
methanol and then dried (figure 1). The elemental analysis data and melting points are
given in table 1.
H
N
H
N
S
S
N
Dry methanol
Reflux
NH2
.
+
NH2NH2 H2O
N
H
N
H
S
Quinoxaline-2,3(1,4H)-dithione
Hydrazine hydrate
3-Hydrazinylquinoxaline-2(1H)-thione
Figure 2. Schematic representation of preparation of 3-hydrazinylquinoxaline-2-thione.

1788
N.V. Kulkarni et al.

S, N, and O donor quinoxalines
1789
2.2.3. Synthesis of complexes. Metal(II) chloride {CoCl2 ꢀ 6H2O (0.237 g, 0.01 mol Lꢁ1),
NiCl2 ꢀ 6H2O (0.237 g, 0.01 mol Lꢁ1), CuCl2 ꢀ 2H2O (0.171 g, 0.01 mol Lꢁ1), and ZnCl2
(0.136 g, 0.01 mol Lꢁ1)} in EtOH (100 cm3) was added with stirring to an ethanolic
solution of ligand {L1(0.314 g, 0.01 mol Lꢁ1), L2 (0.329 g, 0.01 mol Lꢁ1)} and refluxed
at water bath temperature for 3–4 h. The solid complex obtained by this process
was separated by filtration under suction, washed with hot ethanol, and then dried
in vacuo.
3. Results and discussion
The transition metal complexes are non-hygroscopic (stable at room temperature)
amorphous solids, soluble in dimethyl sulfoxide (DMSO) and DMF, sparingly soluble
in ethanol and methanol, and insoluble in chlorinated hydrocarbons. The complexes
melt with decomposition above 280ꢂC. The elemental analysis data of the ligands and
their complexes along with molar conductivity values are compiled in table 1.
3.1. Molar conductivity measurements
The molar conductance values of the complexes measured at room temperature in
DMSO solution at 10ꢁ3 mol dmꢁ3 fall in the range 14.3–18.7 ꢀꢁ1 cm2 molꢁ1 (table 1)
indicating nonelectrolytic nature of the complexes [14]. Comparatively high values are
due to the solvation of DMSO, which replaces anions from the coordination sphere.
3.2. IR spectral studies
In both ligands, the amine ꢀ(NH) absorptions are broad bands around 3180 cmꢁ1 with
a little shift and line broadening upon complexation. For L1, an intense band at
1704 cmꢁ1 is ascribed to the lactonic carbonyl group of isatin. Due to coordination of
oxygen in complexes, a reduction of intensity and a shift in ꢀ(C¼O) is observed. For L2,
ꢀ(C¼O) is observed at 1681 cmꢁ1 and experiences a negative shift upon complexation.
The ꢀ(C¼N) observed at 1675 cmꢁ1 in L1 shifts to a lower frequency by ꢃ10–20 cmꢁ1
upon complexation due to coordination of azomethine nitrogen. In both ligands ring
(C¼N) stretching vibrations are observed around 1600 cmꢁ1 and remain almost
unaltered upon complexation. Thioamide bands having a major contribution from
ꢀ(C¼S) are at 1293 and 767 cmꢁ1 as sharp bands, which undergo considerable reduction
in intensity and shift to lower frequency upon complexation, suggesting coordination
of thione sulfur [15]. The low frequency non-ligand bands at 470 and 420 cmꢁ1
are assigned to ꢀ(M–N) and ꢀ(M–S), respectively. The IR spectral data are provided
in table 2.
3.3. 1H-NMR studies
1H-NMR spectra of ligands and zinc complexes were analyzed. L1 displays two singlets
(D2O exchangeable) at 11.91 and 11.02 ppm assigned to isatin and quinoxaline

1790
N.V. Kulkarni et al.
Table 2. IR spectral data of ligands and complexes in cmꢁ1
.
Thioamide bands
ꢀ(OH)
water ꢀ(NH) ꢀ(C¼O) ꢀ(C¼N) ꢀ(C¼N) ring
Compound
ꢀ(C¼S)
ꢀ(M–N) ꢀ(M–S)
L1
–
3180b 1704s
3182b
1715s
1675s
1662s
1613m
1617s
1293s
767s
–
–
[CoL1Cl2(H2O)]
3417b
1284m
733m
472s
420m
3250b
[NiL1Cl2(H2O)]
[CuL1Cl2(H2O)]
[ZnL1 Cl2(H2O)]
L2H2
3407b 3176b 1713s 1665m
3439b 3200b 1686m 1651m
3400b 3209b 1684m 1650m
1614m
1614s
1612s
1593s
1550s
1283m
1277m
1263m
1299s
743m
750m
753m
815s
465s
471s
460s
–
418m
416s
417s
–
–
3210b 1681s
–
–
[Co2L32 (H2O)4] ꢀ 2H2O 3416b 3200b 1650s
1251m
755m
493s
416s
[Ni2L22 (H2O)4] ꢀ 2H2O 3417b 3216b 1646s
–
–
–
1548m
1598s
1251m
1250m
1252m
756m
756m
754m
486s
471s
470s
425s
416m
421s
[Cu2L22 (H2O)4]
3422b 3200b 1652s
[Zn2L22 (H2O)4] ꢀ H2O 3442b 3220b 1669s
1556m
s, sharp; m, medium; b, broad.
ring NH, respectively. They shift (12.13 and 10.99 ppm) upon complexation, suggesting
coordination of oxygen and sulfur through ketone and thione. Hydrazine NH is
observed at 8.53 ppm and shifts downfield in the respective zinc complex (9.01 ppm).
The aromatic protons resonate in the range 6.61–7.58 ppm and undergo a little
alteration in chemical shift value (6.80–8.04 ppm). The coordinated water resonates at
3.50 ppm and broadens the DMSO peak. In L2, two singlets at 13.52 and 11.88 ppm
were ascribed to ring NH of quinoxaline 2,3-(1,4H)-dione and 3-hydrazinyl quinoxaline
2-thione, respectively. In the zinc complex, they shift downfield (13.82 and 12.18 ppm)
due to the variation in electron density and steric constraints brought about by
chelation [16]. The hydrazine protons resonate at 9.50 ppm as doublet, suggesting their
chemical equivalence. In the complex, since one NH participates in coordination,
chemical equivalence is not maintained hence the peaks get separated. The coordinated
NH shifts upfield at 10.00 ppm as doublet, whereas the non-coordinating NH is at
8.99 ppm. The aromatic protons (7.08–8.13 ppm) undergo a little change in chemical
shift upon complexation (6.91–8.05 ppm) and coordinated water resonates at 3.50 ppm.
3.4. Electronic spectral studies
Electronic spectra of ligands and their complexes were recorded in DMF from
200–1000 nm. Both ligands exhibit similar electronic transitions with strong bands at
230–260 nm (" ꢃ 15,000 L cmꢁ1 molꢁ1), assigned to intraligand ꢁ ! ꢁ* transitions,
which remain almost unchanged in the complexes. The transitions at 275–300 and
350 nm (" ꢃ 10,000 L cmꢁ1 molꢁ1) are assigned to n ! ꢁ* transitions of carbonyl and
imine, respectively [17]; these peaks redshift upon complexation.
In all the complexes, an intense peak is observed at 400–420 nm with
" ꢃ 30,000 L cmꢁ1 molꢁ1, assigned to ligand-to-metal CT transition (S ! M) [17]. The
cobalt(II) complexes show absorptions at 450 nm (" ꢃ 250 L cmꢁ1molꢁ1), 570 nm
(" ꢃ 200 L cmꢁ1 molꢁ1), and 830 nm (" ꢃ 150 L cmꢁ1molꢁ1), assignable to d–d transi-
4
4
4
4
4
4
tions, T1g(F) ! T1g(P), T1g ! A2g, and T1g(F) ! T2g, respectively, evidencing
octahedral complexes. In Ni(II) complexes, the lowest energy band at 860 nm

S, N, and O donor quinoxalines
1791
(" ꢃ 150 L cmꢁ1 molꢁ1) was due to A2g ! T2g (ꢀ1) and bands near 600 nm
3
3
(" ꢃ 200 L cmꢁ1 molꢁ1), and 520 nm (" ꢃ 250 L cmꢁ1 molꢁ1
)
were assigned to
3A2g ! T1g (ꢀ2) and A2g ! T1g(P) (ꢀ3), respectively. Appearance of these spin-
allowed d–d bands indicates octahedral geometry around Ni [18]. Both Cu(II)
complexes display closely spaced absorptions around 500 and 550 nm with low "
values (" ꢃ 100–120 L cmꢁ1 molꢁ1), which are attributed to d–d transitions of d9 system.
Zn(II) complexes show only the absorptions around 420 nm (" ꢃ 15,000 Lcmꢁ1 molꢁ1),
corresponding to the intraligand transitions. The electronic spectral data of the ligands
and their complexes along with the magnetic moment values are tabulated in
‘‘Supplementary material’’ (table S1).
3
3
3
3.5. Magnetic properties
The room temperature magnetic moment values of nickel and cobalt complexes were in
the range 2.82–2.87 and 4.32–4.38 BM, respectively, suggesting the six-coordinate
distorted octahedral geometry [19, 20]; copper complexes exhibit magnetic moment
values (ꢂeff) of 1.60–1.62 BM, which is subnormal even though there is no possibility of
spin–spin interaction between the metal centers (since complexes are mononuclear). It is
presumed that the higher covalency of Cu–S bond and low spin–orbit coupling of sulfur
is responsible for the decrease in the effective magnetic moment values [21, 22].
3.6. EPR spectral analysis
Solid state X-band EPR spectra of the copper complexes exhibit isotropic intense broad
signals with giso of 2.09 and 2.05. No hyperfine splitting in g or g region and half-field
k
?
absorption are observed. This type of spectrum was reported earlier for complexes
bearing large organic ligand substituents having covalent metal–ligand bonds [22]. In
this case, the higher covalency of sulfur–metal bond and the lower spin–orbit coupling
of sulfur ligand may be the main factors affecting the EPR spectrum. The observations
made in this study suggest the dependence of EPR parameters on coordinating atoms.
3.7. Mass spectral analysis
The spectral and analytical data suggest the empirical formula [MLCl2(H2O)] for all the
complexes (proposed structures are given in figure 3). The FAB mass spectral results are
in agreement with the elemental analyses. The molecular formula [MLCl2(H2O)] is
hence assigned to all the complexes. In the mass spectra, the peaks at highest m/z value
can be assigned for molecular mass with certainty, with the aid of consistent isotopic
pattern. Hence, in this case, intense peaks at m/z 474 and 489 for Cu(II) complexes of L1
and L2 are consistent with the total molecular mass of the coordination complexes.
Along with the molecular ion peak, spectra show some other peaks which are attributed
to molecular cations of various fragments of complexes.

1792
N.V. Kulkarni et al.
N
H
N
H
N
NH
N
NH
N
NH
N
O
O
M
N
H
S
M
N
H
S
Cl
Cl
Cl
OH
2
Cl
OH
2
1
2
[ML Cl (H O)]
[ML Cl (H O)]
2 2
2
2
M =Co, Ni, Cu and Zn
Figure 3. Tentative structures for complexes.
3.8. Thermal studies
The thermal stability and decomposition pattern of the complexes was analyzed by TG
and DTA studies in nitrogen with a heating rate of 10ꢂC minꢁ1. Decomposition of
[CuL1Cl2(H2O)] takes place in three stages. The first corresponding to a mass loss of
ꢃ3.80% at 100–140ꢂC is attributed to loss of one coordinated water molecule. The
corresponding differential peak (DTA) at ꢃ120ꢂC signifies an endothermic process.
The second stage of decomposition is accompanied with liberation of two chlorides
from 200 to 250ꢂC with DTA curve at ꢃ220ꢂC representing an exothermic process.
Further reduction of mass at higher temperature is ascribed to ligand decomposition.
The final product was metal oxide. The copper complex [CuL2Cl2(H2O)] also exhibits a
similar pattern of thermal decomposition with 3.68% weight loss at 150ꢂC and 14.6%
weight loss at 250ꢂC corresponding to loss of one water molecule and two chlorides.
3.9. Cyclic voltammetric studies
The ligand and complexes (0.001 mol Lꢁ1 in DMSO) were scanned in the potential
range ꢁ1.0 to 1.0 V in deareated conditions with different scan rates (0.05, 0.1, and
0.15 V sꢁ1). The copper and nickel complexes show redox reactions in the applied
potential range, whereas the ligand and other complexes do not show electrochemical
response, revealing that the redox property exhibited by copper and nickel complexes is
purely metal based.
Both the copper complexes exhibit similar redox behavior. During the positive
potential scan, the voltammograms display an anodic peak in the range Epa ¼ 0.43 to
0.55 V, which is assigned to oxidation of Cu(II) ! Cu(III). The corresponding reverse
scan shows one cathodic peak in the potential range Epc ¼ 0.33 to 0.41 V, attributable to
subsequent reduction (Cu(III) ! Cu(II)). The separation between cathodic and anodic
peak potentials (DEp ¼ Epa ꢁ Epc) for the redox couple Cu(II)/Cu(III) is greater than
60 mV, which indicates quasi-reversible redox process. Along with this, the dependence
of peak potentials on scan rate and values of peak current ratio (Ipc/Ipa) (almost

S, N, and O donor quinoxalines
1793
constant but not unity) show quasi-reversible nature of the redox processes [23]. The
nickel complexes of both ligands show almost the same electrochemical behavior,
undergoing oxidation, Ni(II) ! Ni(III), during the anodic scan (0.50–0.60 V), followed
by the two cathodic peak potentials in the range 0.24–0.30 V and ꢁ0.45 to ꢁ0.55 V,
attributed to two step reduction of Ni(III) species, Ni(III) ! Ni(II), and Ni(II) ! Ni(I).
The redox couples Ni(II)/Ni(III) of both the nickel complexes exhibit the characteristic
features of quasi-reversiblity similar to that of the corresponding copper complexes [23].
For reduction of Cu(II) (d9) to Cu(I) (d10), increase in the radius and change in
configuration from square planar to tetrahedral is needed. On the other hand, Cu(III)
(d8) assumes square-planar geometry with a low-spin ground state and oxidation of
Cu(II) to Cu(III) involves reduction in ionic radius [24]. The constrained cavity fit
Cu(III) better and thus stabilizes the complex. The extra stability achieved by the
complex on oxidation influence redox and slows down the rate of reduction. Detailed
studies are required to understand the nature of chemical reactions following electron
transfers which are helpful in enzyme catalysis. The representative voltammograms are
given in ‘‘Supplementary material’’ (figure S4) and the numerical data in table S2.
4. Conclusion
Spectro-analytical investigation of the complexes reveals tridentate nature of the ligands
with S, N, and O donors. In complexes of L1, coordination occurs through quinoxaline
thione group, azomethine nitrogen, and ketonic oxygen from isatin ring. L2 coordinates
through quinoxaline thione, hydrazine nitrogen, and ketonic oxygen of a second
quinoxaline ring. In both cases, though different heterocyclic motifs are used to
construct the ligands, the ligational behavior is almost the same and the SNO cavity fits
transition metal(II) ions. The structural difference of ligands has no significant
influence on the coordination behavior. The electrochemical behavior exhibited by
copper and nickel complexes is assigned to redox of metal ion. These compounds can be
used as structural and functional models for various metallobiosites. Detailed studies
are required to analyze the utility of these compounds as biomimetics.
Acknowledgments
The authors thank the Department of Chemistry and USIC, Karnatak University,
Dharwad for providing spectral and analytical facility. Recording of FAB mass spectra
(CDRI, Lucknow) and ESR spectra (IIT, Mumbai) are gratefully acknowledged.
The author, Naveen Kulkarni, thanks the Karnatak University, Dharwad for providing
the Nilekani Research Scholarship.
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