Construction of mononuclear transition metal(II) complexes with bi- and tridentate, neutral hydrazone ligands with a quinoxaline hub

Article abstract of DOI:10.1080/00958972.2010.497553

A series of Co(II), Ni(II), Cu(II), and Zn(II) complexes of bi- and tridentate hydrazones were prepared. Ligands L¹ and L² were synthesized by the condensation of 2-mercapto-3-hydrazinoquinoxaline with 2-hydroxy-3-formylquinoline and 3-acetylcoumarin, respectively. The compounds were characterized by various spectro-analytical techniques and magnetic moment studies. The complexes are found to be monomeric and non-electrolytes. In these complexes, [CuL¹Cl²] has square pyramidal geometry and others have octahedral. The copper complexes are electrochemically active in the applied potential range.

Full text of DOI:10.1080/00958972.2010.497553

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Construction of mononuclear transition
metal(II) complexes with bi- and
tridentate, neutral hydrazone ligands
with a quinoxaline hub
Gurunath S. Kurdekar a , Naveen V. Kulkarni a , Srinivasa
Budagumpi a , M.P. Sathisha a , Nagaraj H. Bevinhalli a &
Vidyanand K. Revankar a
a Department of Chemistry , Karnatak University , Pavate Nagar,
Dharwad–580 003, Karnataka, India
Published online: 06 Jul 2010.
To cite this article: Gurunath S. Kurdekar , Naveen V. Kulkarni , Srinivasa Budagumpi , M.P.
Sathisha , Nagaraj H. Bevinhalli & Vidyanand K. Revankar (2010) Construction of mononuclear
transition metal(II) complexes with bi- and tridentate, neutral hydrazone ligands with a quinoxaline
hub, Journal of Coordination Chemistry, 63:12, 2172-2180, DOI: 10.1080/00958972.2010.497553
To link to this article: http://dx.doi.org/10.1080/00958972.2010.497553
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Journal of Coordination Chemistry
Vol. 63, No. 12, 20 June 2010, 2172–2180
Construction of mononuclear transition metal(II) complexes
with bi- and tridentate, neutral hydrazone ligands with a
quinoxaline hub
GURUNATH S. KURDEKAR, NAVEEN V. KULKARNI,
SRINIVASA BUDAGUMPI, M.P. SATHISHA,
NAGARAJ H. BEVINHALLI and VIDYANAND K. REVANKAR*
Department of Chemistry, Karnatak University, Pavate Nagar,
Dharwad – 580 003, Karnataka, India
(Received 5 October 2009; in final form 30 March 2010)
A series of Co(II), Ni(II), Cu(II), and Zn(II) complexes of bi- and tridentate hydrazones were
prepared. Ligands L1 and L2 were synthesized by the condensation of 2-mercapto-3-
hydrazinoquinoxaline with 2-hydroxy-3-formylquinoline and 3-acetylcoumarin, respectively.
The compounds were characterized by various spectro-analytical techniques and magnetic
moment studies. The complexes are found to be monomeric and non-electrolytes. In these
complexes, [CuL1Cl2] has square pyramidal geometry and others have octahedral. The copper
complexes are electrochemically active in the applied potential range.
Keywords: Quinoxaline; Quinoline; Coumarin; SNO donor system
1. Introduction
Hydrazones have versatility in coordinating metals and exhibit pharmacological and
biological activities [1, 2]. The biological activities range from antibacterial to
chemotherapy [3, 4]. The coordination chemistry of heterocyclic hydrazones constitutes
one of the most important classes of sulfur, oxygen, and nitrogen donor ligands [5]. The
formation of many metal complexes from these ligands shows their importance in
coordination and bioinorganic chemistry. These hydrazones have also outstanding
potential for inhibiting the growth of various pathogenic microorganisms, providing
insight into new drug systems. Ligational behavior of multidentate ligands, especially
chelates with different donors, is very interesting in coordination chemistry. The most
popular donor set involves ONS or NNS displaying versatile coordination with
monomers having one or two ligands attached to the metal, or dimers, in which the
metal centers can be bridged through either the sulfur or oxygen of ligand skeleton or
co-ligand (i.e., Cl, N3, NCS, and CH3COO) [6]. Structural features (stereochemical
arrangement) of heterocyclic or acyclic cores which contain these donors play an
*Corresponding author. Email: vkrevankar@rediffmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.497553

Construction of mononuclear transition metal(II) complexes
2173
important role in the coordination behavior [7]. Numerous examples of metal
complexes derived from S, N, and O donors are of immense interest because of their
versatile structural and functional properties and their applications in biochemistry,
catalysis, medicine, and material research [8–10]. With all these findings, we prefer
quinoxaline derivatives as the core moiety for the synthesis of metal complexes.
Quinoxaline derivatives are a very important class of nitrogen-heterocycles, widely
used as pharmaceutical [11, 12] and electrical/photochemical materials [13–15]. Also,
they can serve as a backbone for the construction of multidentate ligands [16, 17].
Quinoline, being the backbone for most of the natural products, is used for designing
many compounds that have diverse pharmacological applications [18, 19]. Coumarin
derivatives are well-known precursors for the design of ligands and complexes with
multiple applications [20]. We have selected 3-hydrazinoquinoxaline-2-thione as a core
for the synthesis of SNO donors. 3-Formylquinolinone and 3-acetylcoumarin scaffolds
are condensed with the hydrazinoquinoxaline via Schiff base condensation and the
resulting ligands are treated with transition metal(II) ions to form complexes. The
ligational behavior of the ligands, the structural features and electrochemical behavior
of the compounds are investigated.
2. Experimental
All chemicals used were of reagent grade and the solvents were distilled prior to use.
The metal estimation was done by standard methods [21]. Carbon, hydrogen, nitrogen,
and sulfur analyses were carried out on a Thermo quest elemental analyzer. Molar
conductance measurements were made on an ELICO-CM-82 conductivity bridge with
a cell having cell constant 0.51 cmꢀ1. Magnetic measurements were made on a Faraday
balance at room temperature by using Hg[Co(SCN)4] as calibrant. Electronic spectra of
the compounds in dimethyl sulfoxide (DMSO) were recorded using a VARIAN CARY
50 Bio UV-Vis spectrophotometer. Infrared (IR) spectra of ligands and their complexes
were recorded as KBr pellets from 4000 to 400 cmꢀ1 on a Nicolet 170 SX FT-IR
1
spectrometer. H-NMR spectra of ligands and zinc(II) complexes were recorded in
DMSO-d6 on a Bruker 300 MHz spectrometer using tetramethylsilane (TMS) as an
internal standard. Electron paramagnetic resonance (EPR) spectra of copper(II)
complexes were recorded at room temperature on a Varian E-4 X-band spectrometer
using tetracyanoethylene (TCNE) as g-marker. Electrochemical measurements were
carried out on a CHI 1110A electrochemical analyzer (USA) at room temperature.
Glassy carbon, Pt wire, and non-aqueous Agþ/AgCl were used in the analysis as
working electrode, counter electrode, and reference electrode, respectively, and
tetramethylammonium chloride was used as the supporting electrolyte. Precursors,
2,3-dimercaptoquinoxaline [22], 2-hydroxy-3-formylquinoline [23], 3-acetylcoumarin
[24], were prepared by the method reported earlier.
2.1. Preparation of 2-mercapto-3-hydrazinoquinoxaline
Hydrazine hydrate (0.025 mol, 50 mL) was added dropwise to hot ethanolic solution of
2,3-dimercaptoquinoxaline (0.005 mol, 50 mL) and then the reaction mixture was

2174
G.S. Kurdekar et al.
H
N
N
N
(III)
N
H
S
O
N
H
H
H
N
S
N
N
L1
NH
2NH2C
·H O
2
NH2
Ethanol
Ethanol
N
H
S
CH3
O
N
H
S
H
N
N
(II)
(I)
(IV)
N
(I) 2, 3-Dimercaptoquinoxaline
N
H
S
O
(II ) 3-Hydrazinoquinoxaline-2-thione
(III) 2-Hydroxy-3-formylquinoline
L2
(IV) 3-Acetylcoumarin
Scheme 1. Schematic representation for the preparation of ligands.
refluxed for 3–4 h in a water bath. The resulting light orange solid was filtered hot,
washed with hot alcohol, and dried.
2.2. Preparation of the ligand
An ethanolic solution of 2-hydroxy-3-formylquinoline (0.005 mol, 0.86 g) was added
to a hot ethanolic solution of 3-hydrazinoquinoxaline-2-thione (0.005 mol, 0.96 g)
and refluxed for 3–4 h in a water bath; the resulting orange solid product was filtered
hot, then washed with hot alcohol and dried (scheme 1). The same procedure was
followed for the preparation of L2 by using 3-acetylcoumarin (0.005 mol, 0.94 g;
scheme 1).
2.3. Synthesis of the complexes
Appropriate amount of L1 (0.002 mol, 0.69 g) was taken in 30–40 mL of hot ethanol.
To this mixture, hot ethanolic solution of metal(II) chloride {CoCl2 ꢁ 6H2O (0.002 mol,
0.475 g), NiCl2 ꢁ 6H2O (0.002 mol, 0.475 g), CuCl2 ꢁ 2H2O (0.002 mol, 0.341 g), and
ZnCl2 (0.002 mol, 0.272 g)} was added dropwise with stirring at 60–65ꢂC. After
completely adding the metal salt solution, the reaction mixture was stirred for another
30–40 min at the same temperature and refluxed for 3–4 h in a water bath. The isolated
complexes were filtered while hot, washed with hot aqueous ethanol and dried. The
same procedure was followed for the preparation of complexes of L2 (0.002 mol,
0.756 g). Data for C, H, and N analysis are given in table 1.
3. Results and discussion
3.1. IR spectral studies
IR absorptions of both L1 and L2 and their metal complexes along with assignments
are summarized in table 2. L1 and L2 show sharp bands at 1608 and 1596 cmꢀ1
,

Construction of mononuclear transition metal(II) complexes
2175

2176
G.S. Kurdekar et al.
Table 2. IR spectral data of the ligands and their complexes.
ꢁ(C¼N)
(cmꢀ1
ꢁ(C¼O)
(cmꢀ1
ꢁ(C¼O)
(cmꢀ1
(Azomethine)
)
(Quinoline)
)
ꢁ(C¼S)
(Coumarin)
)
ꢁ(H2O)
Compound
(cmꢀ1
)
(cmꢀ1
)
L1
1
2
3
1608
1556
1597
1545
1598
1660
1636
1638
1639
1638
1255
1228
1222
1220
1223
–
–
–
–
–
3431
3406
3385
3419
3401
4
L2
5
6
7
1596
1609
1609
1609
1608
–
–
–
–
–
1246
1236
1218
1230
1211
1725
1728
1729
1724
1728
–
3432
3434
3430
3402
8
respectively, assigned to ꢁ(C¼N). This band shifts to low frequency for 1–4 and to high
frequency for 5–8 due to the coordination of azomethine nitrogen to metal [25]. Bands
at 1660 and 1725 cmꢀ1 are attributed to ꢁ(C¼O) of quinoline [26] and coumarin [20],
respectively. The absence of a band at 2500 cmꢀ1 attributable to ꢁ(S–H) in both ligands
rules out thione–thiol tautomerism and suggests the stable thione form of the ligand.
The coordination of the quinoline carbonyl is evidenced by negative shift for ꢁ(C¼O) in
complexes. The ꢁ(C¼O) of coumarin remains unaltered upon complexation suggesting
non-involvement in coordination. The ꢁ(C¼S) in 1–8 shifts lower and decreases in
intensity due to the thione sulfur coordination [27], further supported by the presence of
1
ꢁ(N–H) in spectra of the complexes and H-NMR. Bands at 3300–3400 cmꢀ1 indicate
the presence of coordinated/crystal water.
3.2. 1H-NMR study
1H-NMR spectra of the ligands and Zn(II) complexes were recorded in DMSO-d6 from
0 to 16 ppm. The peak observed at 8.62 ppm in L1 is assigned to azomethine proton,
which shifts downfield upon complexation, confirming coordination of the azomethine
nitrogen [28]. Quinoxaline ring –NH resonates at 11.62 and 10.64 ppm, respectively, for
L1 and L2, and quinoline ring –NH for L1 resonates at 12.02 ppm. A multiplet at 6.93–
7.91 ppm is assigned to aromatic protons of the ligands. Relative to the free ligands,
zinc complexes show small shifts in proton resonance frequencies, attributable to
variation in electron density and steric constraints brought about in the compounds
upon complexation [29]. 1H-NMR data of ligands and complexes are given in
‘‘Supplementary material’’ (table 3).
3.3. Molar conductivity
Molar conductance values of the complexes measured at room temperature in DMSO
at 10ꢀ3 mol dmꢀ3 concentration fall in the range of 6.9–15.4 ꢀꢀ1 cm2 molꢀ1 (table 1),
which is in agreement with non-electrolytes [30].

Construction of mononuclear transition metal(II) complexes
3.4. Electronic spectra
2177
Electronic spectra of the ligands and their complexes were measured in dimethylfor-
mamide (DMF). The free ligands exhibit strong absorptions at 275–292 nm
(" ꢃ 15,000 L cmꢀ1 molꢀ1) due to intraligand ꢂ–ꢂ* transitions; these bands are
almost unchanged in spectra of complexes. Absorptions at 350–370 nm
(" ꢃ 12,000 L cmꢀ1 molꢀ1) are attributed to the n–ꢂ* transition associated with
azomethine, and a red shift of these absorptions upon complexation indicates the
coordination of azomethine nitrogen [31, 32]. In the electronic spectra of cobalt (1 and
5), bands at 465–434 nm (" ꢃ 20,000 L cmꢀ1 molꢀ1) are assigned for ligand-to-metal
charge transfer (LMCT) transitions. A broad distinct peak observed at 550 nm
4
4
(" ꢃ 200 L cmꢀ1 molꢀ1) assignable to T1g ! A2g suggests octahedral structure for the
complexes [33]. Both the nickel complexes (2 and 6) have LMCT transitions at 423 nm
)
(" ꢃ 20,000 L cmꢀ1 molꢀ1) and d–d transitions around 600 nm (" ꢃ 150 L cmꢀ1 molꢀ1
3
3
[34] assigned to A2g ! T1g indicating octahedral geometry. Electronic spectra of
copper complexes exhibit a broad intense peak at 445 nm (" ꢃ 25,000 L cmꢀ1 molꢀ1
)
assignable to S ! Cu(II) LMCT and
3 exhibits a broad peak at 565 nm
(" ꢃ 100 L cmꢀ1 molꢀ1) with low energy shoulder, assigned for square pyramidal
geometry [35]. In 7, a band at 565 nm is attributed to d–d transition. Cu(II) can adopt
square-planar, square-pyramidal, trigonal-bipyramidal, octahedral, and tetrahedral
geometries, which are generally distorted from idealized structures. The d–d spectra
shown by these coordination geometries are distinctive only in the case of the
tetrahedral environment where the absorptions occur at much lower energies and
generally show well-separated absorptions. For all other distorted geometries, spectra
show closely spaced absorption manifolds. Hence it is difficult to predict the accurate
structure for the complexes only on the basis of electronic spectra. Zinc(II) complexes
(4 and 8) show strong absorptions at 370–390 nm for charge transfer.
3.5. Magnetic studies
The magnetic moment values of the synthesized metal complexes are given in table 1.
The magnetic moments of 1 and 5 are 5.22 and 5.18 B.M. at room temperature,
indicating high-spin octahedral cobalt(II) complexes [36, 37]. For 2 and 6, magnetic
moment values are 2.95 and 3.2 B.M., respectively, indicating octahedral configuration
[38], with two unpaired electrons. The effective magnetic moment values of copper 3
and 7 are 1.73 and 1.98 B.M. The higher magnetic moment values of these complexes
can be rationalized in terms of the orbital contribution of donor atoms toward the
spin-only value [36, 37].
3.6. EPR spectral studies
The solid state X-band EPR spectra of copper complexes 3 and 7 exhibit isotropic
intense broad signals with giso values 2.06 and 2.08, respectively, without hyperfine
splitting. No half field absorption was observed indicating the absence of metal–metal
interaction. Similar type of spectra were reported for complexes bearing large organic
ligand substituents having considerable covalent character for metal–ligand bonds [39].

2178
G.S. Kurdekar et al.
H
H
N
N
N
N
N
N
Cl
O
Cl
O
Cu
M
S
S
Cl
N
H
N
N
H
N
OH2
H
Cl
H
Where M = Co, Ni and Zn
O
O
H
N
N
CH
. H2O
3 H
N
Cl
N
S
M
N
S
N
Cl
H
H3C
N
H
N
O
O
Where M = Co, Ni, Cu and Zn
Figure 1. Proposed structures of complexes.
3.7. Fast atom bombardment mass study
Fast atom bombardment (FAB) mass spectral studies provide supporting evidence for
the proposed complexes (figure 1). The two copper complexes 3 and 7 show molecular
ion peaks at m/z 486 and 882 that correspond to the formula weight [CuL1Cl2] and
[Cu(L2)2Cl2] ꢁ H2O, respectively. Besides, the spectra show some prominent peaks
corresponding to various molecular ion fragments. It is difficult to assign the
fragmentation pattern and isotopic ratio of metal and chlorine due to the complex
nature of the spectra. FAB mass spectra of both complexes are given in ‘‘Supplementary
material’’ (figures 2 and 3).
3.8. Cyclic voltammetry study
Cyclic voltammetry is the most versatile electro-analytical technique used for the study
of electrochemical properties of metallo-organic species. Important parameters of cyclic
voltammograms are the magnitudes of the peak current (Ipa, Ipc) and peak potential
(Epa, Epc). Electrochemical properties of L1 and L2 and 1–8 were studied in the potential
range 1.2 to ꢀ1.6 V. Except for 3 and 7, all the compounds are electrochemically inert
in the potential range. Cyclic voltammograms of 3 and 7 show a well-defined redox
process corresponding to the Cu(II)/Cu(I) couple. Complex 3 exhibits quasi-reversible
process with cathodic peak at Epc ¼ 0.51 V corresponding to Cu(II) ! Cu(I) and the

Construction of mononuclear transition metal(II) complexes
2179
respective anodic peak for Cu(I) ! Cu(II) at Epa ¼ 0.55 V. In 7, Epa and Epc are 0.47
and 0.38 V, respectively. In both 3 and 7, the value of DEp, i.e., the separation between
the anodic and cathodic peak potentials, is greater than 59 mV indicating quasi-
reversible redox [40]. The dependency of peak potentials on scan rates and value (ꢃ1)
for Ia/Ic (ratio of oxidative to reductive peak currents) indicates simple one-electron
transfer [41]. Representative cyclic voltammogram of 3 is given in ‘‘Supplementary
material’’ (figure 4).
4. Conclusion
In this study, the synthesis and structural investigations of sulfur-containing hydrazone
ligands and their transition metal(II) complexes are presented. Structures of the ligands
and complexes were confirmed by spectral and analytical techniques. The ligand-to-
metal (L : M) stoichiometry is 1 : 1 and is 2 : 1 in case of L1 and L2, respectively. The
ligands coordinate to metal in thione rather than thiol form and give evidence for non-
involvement of lactonic oxygen (5–8). All the complexes have octahedral geometry
except [CuL1Cl2], which is square pyramidal. Only the copper complexes are
electrochemically active in the applied potential range. Further studies are needed to
understand the mechanism of electron transfer and to check their applicability as
pharmaceuticals. These compounds can be used as structural and functional models for
various metallobiosites. However, 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 research and spectral facilities. Recording of FAB mass spectra
(CDRI Lucknow) and EPR spectra (IIT Bombay) are gratefully acknowledged.
Further, the authors thank UGC and Karnatak University for providing research
fellowships.
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