Synthesis and characterization of some mixed-ligand picrate complexes of nickel(II) involving heterocyclic nitrogen donors

Article abstract of DOI:10.1081/SIM-120020332

The present communication describes the synthesis of some mixed-ligand hexacoordinated picrato complexes of nickel(II) of the composition [Ni(PA)₂(L-L)₂], where PAH = picric acid and L-L = 2-aminoben-zothiazole (ABZT), 2-(2′-aminophenyl)benzothiazole (2′-AMPHBZT), 2-(3′-aminophenyl)benzothiazole (3′-AMPHBZT), bipyridine (bpy), o-phenanthroline (phen) or 2,6-bis(benzimidazyl)pyridine (BBZLPY). These complexes were obtained by the interaction of nickel(II) picrate with the corresponding ligands L-L in methanol. The resulting compounds have been characterized by elemental analyses, magnetic, molar conductance and decomposition temperature measurements, electron spin resonance, infrared and electronic spectral studies. All the complexes are thermally stable and behave as non-electrolytes. An octahedral structure has been tentatively proposed for these complexes.

Full text of DOI:10.1081/SIM-120020332

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Synthesis and Characterization of Some Mixed‐Ligand
Picrate Complexes of Nickel(II) Involving Heterocyclic
Nitrogen Donors
R. C. Maurya a , P. Sharma a & S. Roy a
a Coordination Chemistry Laboratory , Department of P.G. Studies and Research in
Chemistry , R.D. University , Jabalpur, 482‐001, India
Published online: 15 Feb 2007.
To cite this article: R. C. Maurya , P. Sharma & S. Roy (2003) Synthesis and Characterization of Some Mixed‐Ligand Picrate
Complexes of Nickel(II) Involving Heterocyclic Nitrogen Donors, Synthesis and Reactivity in Inorganic and Metal-Organic
Chemistry, 33:4, 683-698, DOI: 10.1081/SIM-120020332
To link to this article: http://dx.doi.org/10.1081/SIM-120020332
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SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 33, No. 4, pp. 683–698, 2003
Synthesis and Characterization of Some
Mixed-Ligand Picrate Complexes of Nickel(II)
Involving Heterocyclic Nitrogen Donors
*
R. C. Maurya, P. Sharma, and S. Roy
Coordination Chemistry Laboratory, Department of P. G. Studies and
Research in Chemistry, R. D. University, Jabalpur, India
ABSTRACT
The present communication describes the synthesis of some mixed-
ligand hexacoordinated picrato complexes of nickel(II) of the composi-
tion [Ni(PA)2(L-L)2], where PAH = picric acid and L–L = 2-aminoben-
zothiazole (ABZT), 2-(2’-aminophenyl)benzothiazole (2’-AMPHBZT),
2-(3’-aminophenyl)benzothiazole (3’-AMPHBZT), bipyridine (bpy),
*
Correspondence: R. C. Maurya, Coordination Chemistry Laboratory, Department of
P. G. Studies and Research in Chemistry, R. D. University, Jabalpur 482-001, India;
E-mail: rcmaurya@sancharnet.in.
683
DOI: 10.1081/SIM-120020332
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright D 2003 by Marcel Dekker, Inc.

684
Maurya, Sharma, and Roy
o-phenanthroline (phen) or 2,6-bis(benzimidazyl)pyridine (BBZLPY).
These complexes were obtained by the interaction of nickel(II) picrate
with the corresponding ligands L-L in methanol. The resulting com-
pounds have been characterized by elemental analyses, magnetic, molar
conductance and decomposition temperature measurements, electron
spin resonance, infrared and electronic spectral studies. All the
complexes are thermally stable and behave as non-electrolytes. An
octahedral structure has been tentatively proposed for these complexes.
Key Words: Mixed-ligand; Ni(II) picrate complexes; Spin free
octahedral; Medicine in homeopathy.
INTRODUCTION
Picric acid was found to stain proteins yellow colour and was
introduced as dye for silk in 1849. It has bactericidal activity and was
formerly used in the treatment of burns. It is employed in the laboratory for
the characterization of organic bases and polynuclear hydrocarbons.[1] It is
also a good antiseptic and astringent agent.[2] The metal derivatives of
picric acid are also well known as medicines in homeopathy.[3] Silver
picrate is a good antimicrobial agent. Zinc picrate is used in Bright’s
disease, headache, exhaustion, facial paralysis, nymphomania, paralysis
astringents, priapism, satyriasis, seminal emissions, spiral weakness, loss of
memory and energy. Potassium picrate in 3ꢀ potency or above is good for
jaundice and violent eructations. Ferric picrate is used in clinical
albuminuria, corns, deafness, ear diseases and fatigue. Ammonium picrate
is a remedy for malarial fever and neuralgia, whooping cough, pain in the
occiput and mastoid regions.[3,4] Benzothiazole derivatives have been
reported to be of medicinal importance as local anaesthestics.[5] 2-(2’-
Pyridyl)benzothiazole and other benzothiazoles have been investigated as
fungicidal agents.[6] The ligational behaviour of benzothiazole and its
derivatives have already been reported from our Coordination Chemistry
Laboratory[5,7,8] by Maurya et al. and others also.[9,10]
Benzimidazole and its derivatives have been shown to possess anti-
bacterial and antimalarial activities.[11–14] A large number of benzimidazole
derivatives[15,16] has been synthesized and patented as fungicides.
Aware with biomedicinal importance of picric acid, metal picrates
and benzimidazole/benzothiazole derivatives, it was thought of interest to
synthesize and characterize some hitherto unknown mixed-ligand com-
plexes of Ni(II) picrate with benzothiazole, benzimidazole derivatives or
some heterocyclic nitrogen donors involving the pyridine nucleus. The

Mixed-Ligand Picrate Complexes of Nickel(II)
685
ligands used in the present investigation are 2-aminobenzothiazole (ABZT),
2-(2’-aminophenyl)benzothiazole (2’-AMPHBZT), 2-(3’-aminophenyl)ben-
zothiazole (3’-AMPHBZT), bipyridine (bpy), o-phenanthroline (phen) and
2,6-bis(benzimidazole-2’-yl)pyridine (BBZLPY). As the biological activity
is often augmented when a ligand forms complexes, the resulting mixed-
ligand complexes may be of potential biological importance. The ligands
along with picric acid are shown in Figure 1.
Figure 1. Structures of ligands.

686
Maurya, Sharma, and Roy
EXPERIMENTAL
Materials
NiCl2ꢁ6H2O, picric acid, AgNO3 and polyphosphoric acid (B.D.H.
Chemicals, Bombay, India), 2-aminobenzothiazole (Aldrich Chemical
Company, U.S.A.), o-toluic acid, m-toluic acid and pyridine-2,6-dicar-
boxylic acid (Sisco Chem. Industries, Bombay) and o-phenylenediamine (E.
Merck, India), A.R. grade, were used as supplied. All other chemicals used
were of A.R. Grade.
Synthesis of Silver Picrate
A quantity of 6 g (150 mmol) of NaOH dissolved in 125 mL of water
was added to a solution of 25 g ( ꢂ 150 mmol) of AgNO3 in 125 mL of
water.[17] The precipitated AgOH was collected by suction filtration in a
sintered glass crucible and washed several times with distilled water. The
AgOH so obtained was transferred to a 500 mL round bottom flask and
refluxed with picric acid (32.5 g, 142 mmol) previously dried over H2SO4
in 1:1 methanol–water (125 mL) on a water bath for 2 hours. The hot
solution of silver picrate was filtered for removing any undissolved matters.
On cooling orange yellow crystals of silver picrate were obtained. They
were filtered by suction, washed several times with petroleum ether and
recrystallized from 95% ethanol and completely dried in vacuo over conc.
H2SO4.Yield: 35.85 g (75%).
Synthesis of Nickel Picrate
When an aqueous solution of NiCl2ꢁ6H2O (2.37 g, 10 mmol) was
treated with an aqueous solution (20 mL) of silvers picrate (6.72 g, 20
mmol)), AgCl was precipitated, leaving nickel picrate in solution.[17] The
precipitated AgCl was removed by suction filtration and the filtrate on
concentration to half of its volume and cooling gave nickel picrate crystals.
The product was washed with benzene and chloroform and recystallized
from 1:1 methanol–water and dried in vacuo over conc. H2SO4. Yield: 3.08
g (60%) (bright green).
Preparation of 2’-AMPHBZY and 3’-AMPHBZT
These benzothiazole derivatives were prepared by the method reported
elsewhere.[18]

Mixed-Ligand Picrate Complexes of Nickel(II)
Synthesis of BBZLPY
687
It was prepared by the usual method[19] of condensation of o-phenyl-
enediamine with pyridine-2,6-dicarboxylic acid in the presence of polypho-
sphoric acid.
Synthesis of Mixed-Ligand Complexes
To a methanolic solution (25 mL) of nickel picrate (5.14 g, 10 mmol),
a methanolic solution (25 mL) of the corresponding ligand (20 mmol) was
added with shaking. The resulting solution was refluxed for 1 to 2 hours
over a hot plate at 80 °C, when a coloured solid precipitated. The pre-
cipitate was filtered by suction, washed 4 to 5 times with methanol and
dried in vacuo over anhydrous CaCl2 at room temperature.
Analyses
For the determination of nickel, the mixed-ligand complexes were
decomposed by digestion with perchloric acid and nitric acid (1:3), until the
resulting residue gave a transparent solution in dilute HCl. Nickel was
determined as nickel dimethylglyoximate by precipitation with a 2% etha-
nolic solution of dimethylglyoxime in the presence of ammonia.[20] C, H
and N were determined microanalytically.
Physical Methods
Magnetic susceptibilities were measured at room temperature on a
Gouy balance using mercury(II) tetrathiocyanatocobaltate(II) as the ca-
librant. Molar conductances were measured in dimethylformamide on a
Toshniwal Conductivity bridge. Infrared spectra (4000–450 cm–1) of the
ligands and complexes were recorded as Nujol mulls on a Perkin Elmer
model 1620 FT-IR spectrophotometer. Electronic spectra of the complexes
were recorded in 10–3 M solutions in dimethylformamide on a Shimadzu
model UV-160 spectrophotometer. ESR spectra were recorded on an EPR
E-112 spectrometer using powdered samples at the Regional Sophisticated
Instrumentation Centre, Indian Institute of Technology, Madras. Decom-
position temperatures of the complexes were determined using an
electrothermal apparatus having the capacity to record temperatures up to
360 °C.

688
Maurya, Sharma, and Roy
RESULTS AND DISCUSSION
The nickel(II) picrate mixed-ligand complexes were synthesized ac-
cording to the following equation:
Methanol
NiðPAÞ2 þ 2ðL À LÞ ÀÀÀ! ½NiðPAÞ2ðL À LÞ2Š
ð1Þ
Reflux
PAH =picric acid, L–L=ABZT (1), 2’-AMPHBZT (2), 3’-AMPHBZT
(3), bpy (4), phen (5) or BBZLPY (6).
The resulting products are non-hygroscopic, air-stable, coloured solids.
The analytical data, colours, decomposition temperatures and % yields of
these complexes are given in Table 1. The compounds were characterized
on the basis of the following studies.
Infrared Spectral Studies
The Important IR spectral bands and their tentative assignments are
presented in Table 2. The characteristic absorption bands at 3385 and 1565
cm–1 in picric acid are assigned to n(OH) (hydrogen bonded) and
nas(NO2), respectively. The disappearance of n(OH) at 3385 cm–1 and
the appearance of a medium band at 1550–1566 cm–1 in all the complexes
suggest the presence of picrate ion in a monodentate fashion in these com-
plexes. The appearance of a new medium band at 1255–1273 cm–1 in the
complexes is attributed to the n(C–O) mode due to coordination of the
phenolic oxygen[21] after deprotonation.
It is observed that the CSC bands of ABZT, 2’-AMPHBZT and 3’-
AMPHBZT occurring at approximately 700 cm–1 are not affected
appreciably in the respective complexes, indicating that the benzothia-
zole-S is not involve in bonding. The coordination of the amino nitrogen of
these three ligands is inferred by a shift to lower wave numbers of n(N–H),
which appears at 3290–3310 and 3180–3215 cm–1 in the complexes (1),
(2) and (3). These three ligands displayed a strong band at approximately
1615 cm–1 attributable to the n(C=N) vibration. Shifting of this band to
lower frequency (1600–1610 cm–1) in the complexes indicates the in-
volvement of the benzothiazole nitrogen in bonding.[22]
The appearance of two characteristic bands at 1606–1610 cm–1 and
1023–1040 cm–1 assignable to ring n(C=N) and ring breathing, res-
pectively, in the compounds (4) and (5) suggest the bidentate (N,N) coor-
dination of bpy and phen in compounds (4) and (5), respectively.
The NH stretching and bending vibrations of the ligand BBZLPY,
which occur at 3100 and 1455 cm–1, respectively, remain almost

Mixed-Ligand Picrate Complexes of Nickel(II)
689

690
Maurya, Sharma, and Roy

Mixed-Ligand Picrate Complexes of Nickel(II)
691
Figure 2. EPR Spectrum of [Ni(PA)2(phen)2] (5).
unchanged, suggesting that the NH nitrogen of the imidazole nucleus is not
involved in coordination. The n(C=N) band which occurs at 1635 cm–1 in
the ligand undergoes a low-energy shift and then appears at 1611 cm–1
after complexation. This suggests the coordination of at least one of the
tertiary nitrogens of the benzimidazole moiety. An additional band observed
at 1635 cm–1 for the free n(C=N) mode (imidazole tertiary nitrogen) in-
dicates that one of the imidazole tertiary nitrogens is not involved in bond-
ing in the compound (6). These results suggest the bidentate coordination
of BBZLPY.
Electron Paramagnetic Resonance Spectra
The room temperature X-band EPR spectrum of one representative
compound, [Ni(PA)2(phen)2], was recorded for a powdered sample at
the microwave frequency of 9.42 GHz. (Figure 2) with the field set at
2500 G and scan range at 4.0 KG. The ‘g’ value was calculated using the
following relationship:
ꢀ
ꢁ
DH
Hsample
g ¼ 2:0036 ꢀ 1 À
ð2Þ
ð3Þ
where
DH ¼ Hsample À HDPPH

692
Maurya, Sharma, and Roy
or
HDPPH
Hsample
g ¼ 2:0036 ꢀ
ð4Þ
where HDPPH =field in Gauss for the standard, 1,1-biphenyl-2-picrylhy-
drazyl free radical (DPPH), and Hsample = field in Gauss for the sample.
The ‘g’ value [calculation by Eq. 5] is 2.552.
3210
g ¼ 2:0036 ꢀ
¼ 2:552
ð5Þ
2520
The shifting of the observed g value to 2.55 from the free electron
value 2.0023 suggests the covalent[23] nature of metal-ligand bonds in
the complex.
Magnetic Measurements
The observed magnetic moments of these mixed-ligand picrate
complexes are given in Table 2. The effective magnetic moments,
Figure 3. Electronic spectrum of [Ni(PA)2(bpy)2] (4).

Mixed-Ligand Picrate Complexes of Nickel(II)
693
3.06–3.20 B.M., suggest the presence of two unpaired electrons in these
complexes. These results indicate the spin-free octahedral geometry for
Ni(II) complexes.[24] The EPR results support this. The possibility of
spin-paired octahedral complexes is ruled out as the d[8] arrangement in a
very strong octahedral field may result in tetragonal[25] distortion, making
the complexes diamagnetic.
Electronic Spectra
The electronic spectra of two representative compounds, [Ni(PA)2-
(bpy)2] and [Ni(PA)2(BBZLPY)2], were recorded in DMF in the wave
length range 320–800 nm. These compounds exhibit three absorption bands
at 360–364, 430–468 and 556–592 nm. The first intense UV band may be
due to a charge transfer transition. The remaining two bands at 430–468
and 556–592 nm are most probably due to d–d transitions assignable to
3
3
3
3A2g(F)! T1g(P) and A2g(F)! T1g(F), respectively. The positions and
assignments of these bands suggest an octahedral environment around the
Figure 4. Electronic spectrum of [Ni(PA)2(BBZLPY)2] (6).

694
Maurya, Sharma, and Roy

Mixed-Ligand Picrate Complexes of Nickel(II)
695
Figure 5. Suggested cis-structure of the complexes.
Figure 6. Suggested trans-structure of the complexes.

696
Maurya, Sharma, and Roy
nickel(II) ion[26] in the adducts. The electronic spectra of these two
compounds are given in Figures 3 and 4.
CONCLUSION
The satisfactory analytical data and the results discussed above suggest
that the complexes are of the general composition [Ni(PA)2(L-L)2], where
PAH = picric acid and L–L= ABZT, 2’-AMPHBZT, 3’-AMPHBZT, bpy,
phen or BBZLPY (Table 3). The overall IR data suggest that 1) PAH acts
as a monobasic mono-dentate ligand coordinating through the phenolic
oxygen after deprotonation, 2) the ligands ABZT, 2’-AMPHBZT, 3’-
AMPHBZT coordinate to the central metal ion through amino and
benzothiazole nitrogens, 3) bpy and phen behave as bidentate (N,N) donors
and 4) BBZLPY exhibits bidentate coordination in its complex, one donor
site being the pyridine ring nitrogen, and the other the imidazole tertiary
nitrogens. The composition of these complexes suggests the possibility of
cis and trans isomers for these complexes as shown in Figures 5 and 6. In
view of the steric demand of the three nitro groups of the picric acid in the
cis-structure (Figure 5), the trans-structure seems to be more favourable
(Figure 6). X-ray crystallographic studies, which might confirm the trans-
structure, could not be carried out as we failed to grow crystals of any of
these complexes.
ACKNOWLEDGMENTS
The authors are thankful to Professor S.D. Tripathi, Vice-chancellor of
this University, for encouragement, and Professor K.K. Mishra, Head,
Department of Chemistry, for laboratory facilities and helpful discussion.
Analytical facilities provided by the Central Drug Research Institute,
Lucknow, India, are gratefully acknowledged. Thanks are also due to
Referee II for constructive comments.
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Received: August 17, 1998
Accepted: January 17, 2003
Referee I: K. Moedritzer
Referee II: G. R. Van Hecke