Binuclear transition metal complexes of bicompartmental SNO donor ligands: Synthesis, characterization, and electrochemistry

Article abstract of DOI:10.1080/00958971003770405

A series of new binucleating Co^II, Ni^II, Cu ^II, and Zn^II complexes of bicompartmental ligands with SNO donors was prepared. The Schiff bases were obtained by the condensation of 4,6-diacetylresorcinol and mercapto-substituted 1,2,4-triazoles. The ligands and their complexes were characterized by elemental analysis, infrared, ¹H-NMR, UV-Vis, FAB-mass, and ESR spectral studies, magnetic susceptibility, and conductivity measurements. All the complexes were monomeric and binuclear. Ni^II and Co^II complexes were octahedral, whereas Cu^II and Zn^II complexes were square planar and tetrahedral, respectively. The compounds are investigated for electrochemical activity.

Full text of DOI:10.1080/00958971003770405

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Binuclear transition metal complexes
of bicompartmental SNO donor ligands:
synthesis, characterization, and
electrochemistry
Naveen V. Kulkarni a , M.P. Sathisha a , Srinivasa Budagumpi a ,
Gurunath S. Kurdekar a & Vidyanand K. Revankar a
a Department of Studies in Chemistry , Karnatak University ,
Pavate Nagar, Dharwad 580 003, Karnataka, India
Published online: 09 Apr 2010.
To cite this article: Naveen V. Kulkarni , M.P. Sathisha , Srinivasa Budagumpi , Gurunath S. Kurdekar
& Vidyanand K. Revankar (2010) Binuclear transition metal complexes of bicompartmental SNO
donor ligands: synthesis, characterization, and electrochemistry, Journal of Coordination Chemistry,
63:8, 1451-1461, DOI: 10.1080/00958971003770405
To link to this article: http://dx.doi.org/10.1080/00958971003770405
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Journal of Coordination Chemistry
Vol. 63, No. 8, 20 April 2010, 1451–1461
Binuclear transition metal complexes of bicompartmental
SNO donor ligands: synthesis, characterization,
and electrochemistry
NAVEEN V. KULKARNI, M.P. SATHISHA, SRINIVASA BUDAGUMPI,
GURUNATH S. KURDEKAR and VIDYANAND K. REVANKAR*
Department of Studies in Chemistry, Karnatak University, Pavate Nagar,
Dharwad 580 003, Karnataka, India
(Received 4 October 2009; in final form 23 December 2009)
A series of new binucleating CoII, NiII, CuII, and ZnII complexes of bicompartmental ligands
with SNO donors was prepared. The Schiff bases were obtained by the condensation of
4,6-diacetylresorcinol and mercapto-substituted 1,2,4-triazoles. The ligands and their com-
plexes were characterized by elemental analysis, infrared, 1H-NMR, UV-Vis, FAB-mass, and
ESR spectral studies, magnetic susceptibility, and conductivity measurements. All the
complexes were monomeric and binuclear. NiII and CoII complexes were octahedral, whereas
CuII and ZnII complexes were square planar and tetrahedral, respectively. The compounds are
investigated for electrochemical activity.
Keywords: 4,6-Diacetylresorcinol; Triazole; Schiff base; Binuclear complex
1. Introduction
Synthesis of binuclear complexes in which a ligand structure maintains two metal
centers in close proximity but in different compartments separated by an intervening
group represents an important current objective in transition-metal systems. These
complexes serve as simple models for multi-metal-centered catalysts and
multielectron-transfer reagents [1–4]. The orientation of the metal centers, and hence
the nature of the metal–metal interactions, are controlled via appropriate bridging
ligands [5, 6]. The bifunctional carbonyl compound, 4,6-diacetylresorcinol, serves as
precursor for the formation of different polydentate ligands [7–11] and as primary
ligand in the various mixed-ligand complexes [12, 13]. 4,6-Diacetylresorcinol is also
employed in the construction of SNO chelating ligands by the condensation of
thiosemicarbazides [14] and thiocarbohydrazide [15]. These ligands are employed to
synthesize mono-, bi-, and poly-nuclear complexes with different binding modes and the
structural and functional features were explored.
*Corresponding author. Email: vkrevankar@rediffmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958971003770405

1452
N.V. Kulkarni et al.
In this study, 4,6-diacetylresorcinol is selected as precursor for the construction of
bicompartmental ligands. The two carbonyl groups at the 4- and 6-positions are
expected to form Schiff bases with amines and the adjacent hydroxy groups may
construct two coordination compartments with the help of azomethine nitrogen and
other functionalities present in amines. The aromatic ring acts as a bridge as well as a
rigid separator between the two compartments.
Amine- and mercapto-substituted triazoles have versatile coordination behavior
toward transition metal ions by providing several monodentate binding modes and SN
bidentate coordination through S¼C–NH–NH2, which results in an enormous number
of complexes [5, 16]. Incorporation of an additional coordinating functionality to the
amino-mercapto triazoles via Schiff base formation generates a group of multidentate
organic hosts with considerable coordination potential. These undergo chelation with
transition metal ions and exhibit versatile stereo- and electro-chemical properties
[17, 18]. To utilize these features, amino-mercapto-substituted triazoles are employed
for the construction of bicompartmental SNO donors.
Complexes of SNO donors have versatile structural and functional properties and
applications in the field of biochemistry and catalysis [19–21]. Construction of
bicompartmental SNO donor ligands and their complexes is, hence, of interest. The
rigid aromatic spacer separates the two cavities and avoids the possibility of direct spin–
spin interaction between metal centers. Therefore, the direct influence of one metal site
on the steric and electronic properties of another is avoided, making the structural and
functional properties interesting.
2. Experimental
2.1. Materials and methods
The chemicals used were of reagent grade and the solvents were dried and distilled
before use according to standard procedures; 3-methyl-5-mercapto-4-amino-1,2,4-
triazole (MMAT), 3-methylsulfhydryl-4-amino-5-mercapto-1,2,4-triazole (MAMT)
[22], and 4,6-diacetylresorcinol [23] were synthesized according to the methods in
literature. The metal chlorides used were in the hydrated form. Elemental analysis was
carried out on a Thermo quest elemental analyzer; metal and chloride analyses were
done by following standard procedures. The molar conductivity measurements in
dimethylformamide (DMF) were made on an ELICO-CM-82 conductivity bridge with
conductivity cell having cell constant 0.51 cmꢀ1. The numerical details of elemental
analysis and conductivity measurements are shown in table 1. Magnetic susceptibility
measurements were made using a Faraday balance at room temperature using
Hg[Co(SCN)4] as calibrant. 1H-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 KBr using an Impact-410 Nicolet (USA) FT-IR
spectrometer from 4000 to 400 cmꢀ1. Electronic spectra of the complexes in DMF
were recorded on a Hitachi 150-20 spectrophotometer in the range 1000–200 nm. Cyclic
voltammetric studies were performed at room temperature in DMF under oxygen-free
conditions created by purging pure nitrogen gas with a CHI1110A electrochemical
analyzer (USA) comprising a three electrode assembly of glassy carbon working

Bicompartmental SNO donors
1453

1454
N.V. Kulkarni et al.
electrode, platinum auxiliary electrode, and Agþ/AgCl reference electrode.
Tetramethylammoniumchloride (0.1 mol dmꢀ3) was used as supporting electrolyte
and the instrument was standardized by ferrocene/ferrocenium redox couple. ESR
study of the copper complexes was carried out on a Varian E-4X-band EPR
spectrometer with field intensity at ꢃ3000 G, using TCNE as the g-marker. FAB
mass spectra were recorded on a JEOL SX 102/DA-6000 mass spectrometer using
Argon/Xenon (6 kV, 10 mA) as the FAB gas and 3-nitrobenzylalcohol as matrix. TG
analysis of tꢀh1e complexes was done in nitrogen on a Universal V2 4F TA instrument
at 10 ꢁC min and scan range of 25–800ꢁC.
2.2. General procedure for the preparation of ligands
Hot solution of triazole (0.02 mol, 2.68 g (MMAT) or 3.32 g (MAMT) in 100 mL
of methanol was treated with methanolic solution of 4,6-diacetylresorcinol
(0.01 mol, 1.94 g). The mixture was stirred and refluxed for 4–5 h on a steam bath.
The solid product was filtered, washed with methanol, and dried over anhydrous CaCl2.
The reaction pathway is represented in figure 1.
2.3. General procedure for the preparation of 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 methanol was
added with stirring to 50 mL of methanolic solution (0.001 mol) of L1H4 (0.294 g) or
L2H4 (0.326 g, 0.001 mol). The mixture was then refluxed for 2 h on a water bath; the
obtained complexes were filtered off and dried over anhydrous CaCl2.
3. Results and discussion
3.1. Molar conductivity measurements
The molar conductance values of the complexes measured at room temperature in
DMSO solution at 10ꢀ3 mol dmꢀ3 were in the range 5.3–10.1 ohmꢀ1 cm2 molꢀ1
indicating non-electrolytic nature for the complexes.
,
CH3
CH3
R
R
NH2
N
N
N
S
CH3
CH3
N
N
O
R
S
H
O
H
N
N
EtOH
+
N
N
Reflux
N
N
HO
OH
S
H
HO
OH
4,6-Diacetylresorcinol
Where R = CH3 for L1H4 and CH2SH for L2H4
Amino-mercaptotriazole
Figure 1. Schematic representation of the preparation of ligands.

Bicompartmental SNO donors
1455
3.2. Infrared spectral studies
IR spectra of the free ligands show a band of medium intensity at 3140–3290 cmꢀ1
assigned to ꢀ(NH). The absence of a band at 2400–2600 cmꢀ1 due to ꢀ(SH) in L1H4 and
L2H4 confirms that the ligands exist in the thioketo form. The sharp band around
1630 cmꢀ1 assigned to ꢀ(C¼N) in the free ligands shifts to lower frequency in spectra of
the complexes, suggesting the coordination of both azomethine nitrogens to the metal.
The broad ꢀ(O–H) at ꢃ3400 cmꢀ1 in the free ligands is absent in spectra of all the
complexes except [Zn2L1H2Cl2] ꢂ H2O, suggesting deprotonation of the phenolic oxygen
on coordination. The vibrational couplings among thioamide group are around 1550,
1450, 1320, and 975 cmꢀ1. In [Co2L2H2Cl2(H2O)4] ꢂ 2H2O, [Ni2L2H2Cl2(H2O)4] ꢂ 2H2O,
and [Zn2L2H2Cl2] ꢂ 2H2O the signal due to ꢀ(NH) is retained and thioamide bands
which have major contribution of ꢀ(C¼S) are reduced in intensity, which supports the
thioketo mode [18] of coordination of sulfur. In all other complexes no thioamide bands
indicates thioenolization and subsequent coordination to metal, which is supported by
the appearance of a weak band at 670 cmꢀ1 due to ꢀ(C–S) and disappearance of
ꢀ(N–H). The sulfhydryl ꢀ(SH) in L2H4 at 2600 cmꢀ1 is further confirmed by a signal at
1
5.4 ppm in H-NMR spectrum. Generally –CH2SH is a good coordinating site, but in
the present case the non-participation of –CH2SH in coordination may be due to the
presence of stronger coordinating mercapto- and thioketo-group, suppressing –CH2SH
coordination [18]. Further, the possibility of –CH2SH acting as an additional
binding site to the metal is ruled out due to steric constraints which may affect
complex formation. The low-frequency bands at 500–465 and 440–410 cmꢀ1 are
assigned to ꢀ(M–N) and ꢀ(M–S), respectively. All the complexes exhibit a broad peak at
3400 cmꢀ1 suggesting the presence of coordinated water. The IR spectral data are
given in table 2.
3.3. 1H-NMR studies
Proton-NMR data for the ligands and binuclear ZnII complexes were recorded.
In spectra of ligands, the signals around 12.80 and 14.00 ppm are assigned to –OH
and –NH of diacetophenone and triazole. For [Zn2L1H2Cl2] ꢂ H2O, phenolic –OH is
retained, but the –NH signal disappears on complexation, owing to the thioenolization
followed by deprotonation. Resonance due to the methyl attached to the triazole ring at
2.4 ppm in L1H4 remains unaltered in the complex. In [Zn2L2H2Cl2] ꢂ 2H2O, disap-
pearance of signal at 12.80 ppm indicates chelation of the ligand accompanied with the
deprotonation of phenolic –OH [24]. The retention of signal corresponding to –NH
(D2O exchangeable) in the spectrum suggest that the complexation is via thioketo
sulfur. Resonance due to sulfhydryl –SH in L2H4, which appears at 5.48 ppm (D2O
exchangeable) and the methylene group attached to –SH group identified at 3.90 ppm
are also retained in the complex. Further, the methyl of acetyl and the diacetophenone
ring protons are at 2.60 and 6.00–8.00 ppm, respectively, in both ligands and their
corresponding complexes. A considerable degree of symmetry is present in these
compounds so that the protons in the two halves of the molecules are magnetically
equivalent. Relative to the free ligands, the dizinc complexes show small shift in proton
resonance frequencies, attributable to variation in electron density and steric constraints
brought about in the compounds upon complexation [24].

1456
N.V. Kulkarni et al.

Bicompartmental SNO donors
1457
3.4. Magnetic properties, electronic, and EPR spectral studies
Electronic spectra of the ligands in DMF show bands in the range 275–345 nm. The
higher energy bands in the region 275–290 nm are assigned to ꢁ–ꢁ* transitions of the
aromatic ring. The medium energy bands in the region 290–330 nm are assigned to n–ꢁ*
transitions of the azomethine nitrogen and thione sulfur. Lower energy bands with
molar extinction coefficient " ꢃ 25,000 L cmꢀ1 molꢀ1 at 330–345 nm are attributed to
intraligand charge transfer (CT) transitions [25]. The UV-Vis spectra of CuII complexes
in DMF have bands around 400 nm with " ꢃ 25,000 L cmꢀ1 molꢀ1 and 600 nm with
" ꢃ 300 L cmꢀ1 molꢀ1 attributable to CT transition of S ! CuII [18] and the d–d
2
2
transition, B1g ! E2g, dx2ꢀy2 ! dz2 corresponding to square-planar structure [26]. The
magnetic moments of 1.84 and 1.86 BM, respectively, above the spin-only value for CuII
suggest square-planar structure for the complex [27]. The broad isotropic peak observed
with giso 2.01 and 2.03 in the X-band EPR spectra suggest the absence of spin–spin
interaction in the complexes. In the electronic spectra of nickel complexes three d–d
bands were observed at 840, 520, and 450 nm with " ꢃ 300 L cmꢀ1 molꢀ1 attributable to
3
3
3
3
3
3
spin-allowed transitions A2g ! T2g, A2g ! T1g, and A2g ! T1g(P), respectively,
representing octahedral complex [25]; the magnetic moments (3.18 and 3.24 BM)
obtained for both complexes suggest the same [28]. The octahedral CoII complexes
usually show three bands [26], but in the present case only one peak corresponding to
4
4T1g(F) ! T1g(P) was observed at 500 nm (" ꢃ 300 L cmꢀ1 molꢀ1) in both cobalt
4
4
complexes due to the fact that the band due to T1g(F) ! T2g occurs in near infrared
4
4
region and the band due to T1g ! A2g involves a two-electron transition. Octahedral
geometry was assigned to the complexes by considering the magnetic moment values
4.20 and 4.26 BM, respectively [28, 29]. The diamagnetic zinc complexes show
absorptions only in the higher frequency region with high extension coefficient values
attributed to the ligand electron transitions.
3.5. FAB mass spectral studies
FAB mass spectral studies provide supporting evidence for the proposed constitutions
of the complexes (represented in figure 2). The two copper complexes selected as
representative show the molecular ion peaks at m/z 599 and m/z 643 that correspond to
the formula weight [Cu2L1(H2O)2] ꢂ H2O and [Cu2L2(H2O)2], respectively, for binuclear
monomeric complexes.
3.6. Thermogravimetric analysis
The thermal stability and decomposition pattern of the complexes were analyzed ꢀb1y
thermogravimetric studies under nitrogen with a heating rate of 10ꢁC min
.
Decomposition of [Cu2L1(H2O)2] ꢂ H2O takes place in three stages. The first stage
corresponds to a mass loss of ꢃ2.8% around 91ꢁC, attributed to the elimination of
solvated water. The second stage of a mass loss of 6.62% taking place at 185ꢁC
corresponds to the loss of two coordinated water molecules. Further reduction of mass
in a higher temperature range (around 250ꢁC) is ascribed to ligand decomposition. The
final product was metal-oxide. The copper complex [Cu2L2(H2O)2] also exhibits
three-step decomposition pattern with 6.67% weight loss at 150ꢁC corresponding to the

1458
N.V. Kulkarni et al.
Complexes of L1H4
CH3
CH3
CH3
CH3
N
S
N
S
N
S
N
S
CH3
CH3
CH3
CH3
N
N
N
N
N
N
N
N
N
N
N
N
. H2O
. H2O
Zn
Cu
Zn
Cu
O
H
O
H
O
O
Cl
H2O
Cl
OH2
1
.
1
.
[Cu2L (H2O)2] H2O
[Zn2L H2Cl2] H2O
CH3
CH3
N
S
N
N
CH3
CH3
N
N
H2O
N
OH2
N
N
S
. H2O
M
M
O
O
H2O
OH2
OH2
H2O
1
.
[M2L (H2O)6] H2O
Where M = Ni, Co
Complexes of L2H4
SH
HS
N
N
N
S
CH3
CH3
N
N
H
H
N
OH2
N
N
H2O
. 2H2O
S
M
M
O
O
H2O
Cl
OH2
Cl
2
.
[M2L H2Cl2(H2O)4] H2O
Where M = Ni, Co
SH
HS
SH
HS
N
N
N
S
N
S
N
S
CH3
CH3
CH3
CH3
N
N
H
N
H
N
N
N
N
N
N
N
N
. 2H2O
Zn
Zn
S
Cu
Cu
OH2
O
O
O
O
Cl
Cl
OH2
[Cu2L2(H2O)2]
.
2
[Zn2L H2Cl2] 2H2O
Figure 2. Proposed structures of complexes [L1H4] and [L2H4].
elimination of two water molecules. The weight loss of 7.02% and 25.57% observed
around 200ꢁC and 350ꢁC is attributed to the decomposition of ligand.
3.7. Electrochemical measurements
The cyclic voltammetric study of ligands and complexes in DMSO (0.001 mol) was
carried out in the potential range of ꢀ0.1 to 1.0 V in O2 free condition with different

Bicompartmental SNO donors
1459
Figure 3. Cyclic voltammograms of copper complexes at scan rate of 0.1 V sꢀ1
.
scan rates (0.15, 0.1, and 0.05 V sꢀ1). Only the copper complexes exhibited redox
properties. This suggests that the electrochemical activity of the copper complexes is
purely based on metal.
The voltammograms of the two copper complexes are shown in figure 3 and
numerical results are given in table 3. In the electrochemical investigation of
[Cu2L1(H2O)2] ꢂ H2O anodic peak (Epa) observed in the voltammogram in the potential
range 0.44–0.46 V represents oxidation (CuII ! CuIII). The corresponding cathodic

1460
N.V. Kulkarni et al.
Table 3. Cyclic voltammetry results.
Complex
Scan rate (V sꢀ1
)
Epa (V)
Epc (V)
DEp (V)
E1/2 (V)
Ipc/Ipa
[Cu2L1(H2O)2] ꢂ H2O
0.15
0.1
0.05
0.15
0.1
0.46
0.45
0.44
0.47
0.46
0.43
0.34
0.35
0.36
0.32
0.33
0.34
0.12
0.10
0.08
0.15
0.13
0.09
0.40
0.40
0.40
0.39
0.39
0.38
0.72
0.74
0.77
0.73
0.75
0.78
[Cu2L2(H2O)2]
0.05
DEp ¼ Epa ꢀ Epc and E1/2 ¼ [Epc þ Epa]/2.
potential scan gives a peak (Epc) with potential in the range 0.34–0.36 V representing
reduction (CuIII ! CuII). [Cu2L2(H2O)2] shows similar behavior, with corresponding
anodic peak, Epa, in the range 0.43–0.47 V and cathodic peak, Epc, in the range 0.32–
0.34. The high value of DEp, separation between the cathodic and anodic peak
potentials (Epa ꢀ Epc), in both cases greater than 60 mV (which varies with scan rate),
indicates quasi-reversible nature of the redox process [30, 31].
4. Conclusion
L1H4 is hexadentate tetrabasic for CoII, NiII, and CuII complexes and hexadentate
dibasic chelate for ZnII. On the other hand, L2H4 is hexadentate tetrabasic for CuII and
hexadentate dibasic in other complexes. Both ligands provide SNO donating sites to
each metal ion in the binuclear complexes. CuII and ZnII complexes of both ligands
have square-planar and tetrahedral geometry, respectively, whereas CoII and NiII
complexes are octahedral. The CuII complexes exhibit quasi-reversible redox activity in
the applied potential range. Further studies are necessary to understand catalytic
behavior and multielectron-transfer mechanism of compounds.
Acknowledgements
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, Bombay) are gratefully acknowledged.
N.V. Kulkarni and M.P. Satisha thank Karnatak University, Dharwad, for providing
the Nilekani and University Research Scholarship, and S. Budagumpi thanks the
University Grant Commission for providing RFSMS.
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