Metal thiocyanato complexes: Synthesis, magnetic, and spectral studies of some mixed-ligand thiocyanato complexes of nickel(II) involving benzothiazole and benzimidazole derivatives

Article abstract of DOI:10.1081/SIM-120019992

A new series of mixed-ligand thiocyanato complexes of nickel(II) of the compositions [Ni(NCS)₂(L)₄], where L = 2-mercaptobenzothiazole (2-MBZT), 2-(2′-hydroxyphenyl)-benzothiazole (2′-HPBZT), 2-(2′-tolyl)-benzothiazole (2′-TOBZT), 2-(3′-tolyl)benzothiazole (3′-TOBZT), [Ni(NCS)₂(2-AMBZT)₂] and [Ni(NCS)₂(BBZLY)(H₂O)₂], where 2-AMBZT = 2-aminobenzothiazole and BBZLPY = 2,6-bis(benzimidazole-2′-yl)pyridine, has been synthesized by the interaction of nickel(II) thiocyanate with the respective ligands. The compounds so obtained were characterized by analytical data, molar conductances, magnetic measurements, infrared and electronic spectral studies. Suitable octahedral structures have been suggested for these complexes.

Full text of DOI:10.1081/SIM-120019992

SYNTHESIS AND REACTIVITY IN INORGANIC AND METAL-ORGANIC CHEMISTRY
Vol. 33, No. 3, pp. 387–401, 2003
Metal Thiocyanato Complexes: Synthesis,
Magnetic, and Spectral Studies of Some
Mixed-Ligand Thiocyanato Complexes of
Nickel(II) Involving Benzothiazole and
Benzimidazole Derivatives
*
R. C. Maurya, P. Sharma, and D. Sutradhar
Coordination Chemistry Laboratory, Department of P. G. Studies and
Research in Chemistry, R. D. University, Jabalpur, India
ABSTRACT
A new series of mixed-ligand thiocyanato complexes of nickel(II) of the
compositions [Ni(NCS)2(L)4], where L = 2-mercaptobenzothiazole (2-
MBZT), 2-(2’-hydroxyphenyl)-benzothiazole (2’-HPBZT), 2-(2’-tolyl)-
benzothiazole (2’-TOBZT), 2-(3’-tolyl)benzothiazole (3’-TOBZT),
[Ni(NCS)2(2-AMBZT)2] and [Ni(NCS)2(BBZLY)(H2O)2], where 2-
AMBZT = 2-aminobenzothiazole and BBZLPY = 2,6-bis(benzimida-
zole-2’-yl)pyridine, has been synthesized by the interaction of nickel(II)
thiocyanate with the respective ligands. The compounds so obtained
were characterized by analytical data, molar conductances, magnetic
*
Correspondence: R. C. Maurya, Coordination Chemistry Laboratory, Department of
P. G. Studies and Research in Chemistry, R. D. University, Jabalpur 482001, India;
E-mail: rcmaurya@sancharnet.in.
387
DOI: 10.1081/SIM-120019992
0094-5714 (Print); 1532-2440 (Online)
www.dekker.com
Copyright D 2003 by Marcel Dekker, Inc.

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Maurya, Sharma, and Sutradhar
measurements, infrared and electronic spectral studies. Suitable octa-
hedral structures have been suggested for these complexes.
INTRODUCTION
In continuation of our interest to synthesize and characterize mixed-
ligand thiocyanato[1–3] complexes of bipositive metal ions involving hete-
rocyclic ligands, our studies have been extended so as to observe the
ligational behaviour at some benzothiazole and benzimidazole derivatives
towards nickel(II) thiocyanate.
Benzothiazole derivatives have been reported to be of medicinal
importance as local anaesthestics.[4] 2-(2’-Pyridyl)benzothiazole and other
benzothiazoles have been investigated as fungicidal agents.[5] The ligational
behaviour of benzothiazole and its derivatives have been reported earlier
from our coordination chemistry laboratory[4,6–8] by Maurya et al. and also
by others.[9,10] Benzimidazole and its derivatives have been shown to
possess antibacterial and antimalarial activities.[11–14] The benzimidazole
moiety is reported to be toxic to fungi. A large number of benzimidazole
derivatives[15] has been synthesized and patented as fungicides.
Methyl-1-butyl carbamoyl-2-benzimidazole carbonate is a novel
fungicide which has been used successfully to control Losse smut (a type
of plant disease caused by fungi, and characterized by the production of
masses of usually black spores within host) on wheat and barley.[16]
The thiocyanate group may coordinate to the metal through either the
nitrogen or the sulfur atoms or both. IR spectra have been used frequently
to indicate the bonding of the thiocyanate group to the metal ions.[17,18]
The thiocyanate ion should have three fundamental IR absorption
frequencies: 1) a strong band due to the C–N stretch, 2) a weak C–S
stretch, and 3) a weak NCS bending. In the recent past, there has been
continuing interest in the synthesis and spectral studies of some mixed-
ligand thiocyanato complexes.[19–27] Maurya et al.[1] recently have reported
the synthesis and characterization of some mixed-ligand thiocyanato
complexes of cadmium(II), mercury(II) and zinc(II) with 2- or 3-pyrazo-
line-5-one derivatives, viz., 3-methyl-1-phenyl-2-pyrazoline-5-one, 2,3-
dimethyl-4-methylaminomethanesulfonate sodium monohydrate-1-phenyl-
3-pyrazoline-5-one and 4-amino-2,3-dimethyl-1-phenyl-3-pyrazoline-5-one.
However, there is no report on the synthesis and characterization of mixed-
ligand derivatives of nickel(II) thiocyanate with benzothiazole and ben-
zimidazole derivatives, viz., 2-aminobenzothiazole (2-AMBZT), 2-mercap-
tobenzothiazole (2-MBZT), 2-(2’-hydroxyphenyl)-benzothiazole (2’HPBZT),
2-(2’tolyl)benzothiazole (2’-TOBZT), 2-(3’-tolyl)benzothiazole (3’-TOBZT)

Metal Thiocyanato Complexes
389
and 2,6-bis(benzimidazole-2’-yl)pyridine (BBZLPY). As the biological
activity is often augmented when the ligand forms complexes, the resulting
mixed-ligand complexes may be of potential biological importance. The
structures of these ligands are shown in Figure 1.
EXPERIMENTAL
Materials
Nickel(II) chloride hexahydrate (B. D. H. Chemicals, Bombay), po-
tassium thiocyanate (E. Merck, Germany), 2-aminobenzothiazole (Sisco-
Chem. Industries, Bombay), 2-mercaptobenzothiazole (Aldrich Chemical
Company, U. S. A.), salicylamide (E. Merck, India), o-toluic acid and m-
toluic acid (Sisco-Chem Industries, Bombay) were used as supplied.
Nickel(II) thiocyanate was prepared by the method of Maurya et al.[1] as
reported elsewhere. All other chemicals used were of A.R. grade.
Synthesis of HPBZT, 2’-TOBZT and 2’-TOBZT
The benzothiazole derivatives were prepared by the reported method of
condensation of o-aminothiophenol with o-toluic acid for 2’-TOBZT or with
m-toluic acid for 3’-TOBZT in the presence of a sufficient quantity of
polyphosphoric acid to give a suitable paste.[28] The mixture was heated
slowly to 250 °C on an oil bath and the resulting solution was stirred at
250 5 °C for 4–5 h, permitted to cool to about 100 °C, and then poured
in a thin stream into a large volume of rapidly stirred water. The resulting
solid was collected by filtration, washed with a small amount at water and
reslurried in an excess at 10% sodium carbonate solution. The alkaline
slurry was filtered and the product washed thoroughly with water and dried
at 60 °C. The crude product was recrystallized from ethyl alcohol or an
aqueous-alcohol mixture (1:1), subsequent to treatment with a small amount
of activated charcoal. The purity of these compounds was checked by their
literature melting points.[28] Synthesis of the benzothiazole derivatives
occured as shown in the reaction scheme 1a in Figure 2.
Synthesis of 2,6-Bis(benzimidazole-2’ -yl)pyridine
It was prepared by the interaction of pyridine-2,6-dicarboxylic acid and
o-phenylenediamine as reported elsewhere.[29] The synthesis is shown in the
reaction scheme 1b Figure 2.

390
Maurya, Sharma, and Sutradhar
Figure 1. Ligand structures.
Synthesis of the Mixed-Ligand Complexes
A stoichiometric quantity of nickel(II) thiocyanate (0.01 M, 1.75 g) dis-
solved in ꢀ 30 mL of ethanol was added to the solution of the appropriate
ligand (0.01 M for BBZLPY, 0.02 M for 2-AMBZT and 0.04 M for 2-MBZT,
2’-HPBZT, 2’-TOBZT or 3’-TOBZT) dissolved in ethanol (ꢀ 20 mL). The
resulting mixture was refluxed for 4 to 6 h at 80 °C over a hotplate. During this

Metal Thiocyanato Complexes
391
Figure 2. Reaction scheme for the preparation of the ligands.
time coloured solids separated out. They were filtered by suction, washed
several times with ethanol and dried in vacuo. The analytical data are given in
Table 1.
Analysis
For the determination of nickel, a weighed amount (ꢀ 150 mg) of
the mixed-ligand complex was decomposed by digestion with perchloric
acid and nitric acid, until the resulting residue gave a transparent solution
in dilute HCl. Nickel was determined as nickel dimethylglyoximate by

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Maurya, Sharma, and Sutradhar

Metal Thiocyanato Complexes
393
precipitation with a 2% ethanolic solution of dimethylglyoxime in the
presence of ammonia.[30] C, H and N were determined microanalytically.
Physical Methods
The solid state infrared spectra was recorded in Nujol on a Perkin
Elmer model 1620 FT-IR spectrophotometer at the National Chemical
Laboratory, Pune. ESR spectra were recorded on an EPR E-112 spec-
trometer using powdered samples at the Regional Sophisticated Instrumen-
tation Centre, Indian Institute of Technology, Madras. Electronic spectra of
the complexes were recorded on a Shimadzu model UV-160 spectropho-
tometer. Magnetic measurements were performed by Gouy’s method using
mercury(II) tetrathiocyanatocobaltate(II) as the calibrant at the Central Salt
and Marine Chemicals Research Institute, Bhavnagar. Conductance mea-
surements were performed at room temperature in dimethylformamide
using a Toshniwal conductivity bridge and dip-type cell with a smooth
platinum electrode of cell constant 1.3. Decomposition temperatures of the
complexes were determined using an electrothermal apparatus having the
capacity to record temperatures up to 360 °C.
RESULTS AND DISCUSSION
The mixed-ligand nickel(II) thiocyanato complexes under investigation
were prepared according to the following equations:
Ethanol ð90%Þ
Reflux
NiðNCSÞ2 þ 2ð2-AMBZTÞ
½NiðNCSÞ2ð2-AMBZTÞ2ꢁ ð1Þ
Ethanol ð90%Þ
NiðNCSÞ2 þ 4L
Reflux
½NiðNCSÞ2ðLÞ4ꢁ
Where L = 2-MBZT (2), 2’-HPBZT (3), 2’-TOBZT (4) or 3’-TOBZT
(5) and
Ethanol ð90%Þ
Reflux
NiðNCSÞ2 þ BBZLPY
½NiðNCSÞ2ðBBZLPYÞðH2OÞ2ꢁ ð6Þ
The resulting products are non-hygroscopic, air-stable, coloured solids.
The colours, decomposition temperatures and % yields of these complexes
are given in Table 1. The compounds were characterized on the basis of the
following studies.

394
Maurya, Sharma, and Sutradhar
Infrared Spectra
The important IR spectral bands and tentative assignments are
presented in Table 2. The thiocyanate group is known to be coordinated
to the metal ion through either the sulfur atom or the nitrogen atom or both.
Nickel(II), being in the 3d-transition series, has been associated with ‘‘Class
A’’ metals, where the coordination of the thiocyanate group to nickel(II) is
of the Ni-NCS type,[18] that is, coordination through the nitrogen atom. The
appearance of n(C–N) and n(C–S) modes at 2080 to 2115 cm–1 and 780 to
812 cm–1 indicates that the bonding of the thiocyanate group towards
nickel(II) ion is of the Ni–NCS type.[19]
The ligands 2-MBZT, 2’-TOBZT and 3’-TOBZT used in the present study
possess two potential donor sites: 1) the benzothiazole nitrogen and 2) the
benzothiazole sulfur, while the ligands 2-AMBZT and 2’-HPBZT are
potentially tridentate: 1) the benzothiazole nitrogen, 2) the benzothiazole
sulphur, and 3) the amino nitrogen in the case of AMBZT or the phenolic
oxygen in the case of 2’-HPBZT. The benzimidazole derivative BBZLPY
contains five potential donor sites: 1) and 2) the imidazole tertiary nitrogens,
3) and 4) the imidazole NH-nitrogens, and 5) the pyridine ring nitrogen.
The n(OH) band at 2540 cm–1 in the free 2’-HPBZT is unaffected
after complexation suggesting that the phenolic OH group is not involved in
the bonding. The possibility of amino nitrogen (–NH2) coordination in the
ligand 2-AMBZT towards nickel was also examined. The appearance of
nas(N–H) and ns(N–H) modes of the amino group at 3376 and 3283 cm–1,
respectively, in the complex (1), suggests coordination of this amino
nitrogen towards nickel. It is observed that the CSC band of the ligands 2-
AMBZT, 2-MBZT, 2’-HPBZT, 2’TOBZT and 2’-TOBZT occurring at 700–
720 cmꢂ1 is not affected appreciably in the respective complexes. This
indicates that the benzothiazole sulfur is not taking part in the coor-
dination. The appearance of a band at 1620–1640 cm–1 assignable to
n(C=N) + n(C=C) modes indicates the bonding of the benzothiazole
tertiary nitrogen[2,8] to nickel in the complexes (1)–(5).
The NH stretching and bending vibrations of the ligand BBZLPY which
occur at 3100 and 1455 cm–1, respectively, remain almost unchanged,
suggesting that the NH nitrogen of the imidazole nucleus[30] is not involved in
coordination. The n(C=N) band which occurs at 1635 cm–1 in the ligand
undergoes a shift to lower energy and then appears at 1608 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 uncomplexed n(C=N) mode (imidazole tertiary nitrogen)
indicates that one of the imidazole tertiary nitrogens is reluctant to coordinate
in the compound (6). The assignment of bands due to n(C=N) (pyridine

Metal Thiocyanato Complexes
395

396
Maurya, Sharma, and Sutradhar
Table 3. Solubility of the synthesized complexes in different solvents.
Sr.
no.
Ace- Eth- Meth- Ethyl
tone CCl4 CHCl3 DMF anol anol acetate
Compound
(1) [Ni(NCS)2(2-AMBZT)2]
(2) [Ni(NCS)2(2-MBZT)4]
(3) [Ni(NCS)2(2’-HPBZT)4]
(4) [Ni(NCS)2(2’-TOBZT)4]
(5) [Ni(NCS)2(3’-TOBZT)4]
PS
PS
S
IS
IS
IS
IS
IS
IS
IS
IS
IS
IS
IS
IS
S
IS
IS
PS
PS
PS
S
IS
IS
IS
IS
IS
PS
S
S
PS
PS
PS
PS
PS
S
PS
S
S
(6) [Ni(NCS)2(BBZLPY)H2] PS
S
S
Abbreviation: S = soluble (80–100%); PS = partially soluble (ꢀ 50%); IS =
Insoluble (5–10%). Solubility was tested by taking 100 mg of each compound in
5 mL of the solvent.
moiety) of the ligand and complex is somewhat difficult due to the appearance
of this band along with the bands of the tertiary nitrogen of the benzimidazole
moiety. However, the appearance of a band at 1044 cm–1 due to the ring
breathing mode of the pyridine moiety indicates the coordination of the
pyridinic nitrogen.[31,32] These observations suggest the bidentate coordina-
tion of BBZLPY. This is due to the rigidity and non-planarity of the two
benzimidazole moieties. The appearance of a band at 3392 cm–1 due to the
n(OH) mode suggests the presence of coordinated water in this complex.
Figure 3. Electronic spectrum of [Ni(NCS)2(BBZLPY)(H2O)2].

Metal Thiocyanato Complexes
397
Figure 4. Proposed structures of the complexes.

398
Maurya, Sharma, and Sutradhar
Conductance Measurements
The molar conductances of all the mixed-ligand complexes in 10–3
M
DMF are given in Table 2. These values are indicative of the non-
electrolytic[33] nature of these complexes (Table 3).
Magnetic Measurements
The observed magnetic moments, 3.02–3.27 B.M. (Table 2), of the
complexes under investigation, correspond to two unpaired electron and
give no specific information about their stereochemistry.
Electronic Spectra
The electronic spectrum of one representative compound, (6), was
recorded in 10–3 M DMF solution (Figure 3). This compound shows
spectral peaks at 368, 418, 525 and 750 nm. The first intense UV band is
attributed to ligand ! metal charge transfer transition. We are unable to
give any explanation for the band shapes of the two high energy signals in
the electronic spectrum. The remaining three peaks at 418, 525 and 750 nm
may be assigned to the spin-allowed transitions from the ground state level,
3
3
3
3
3
3A2g ! T1g (P), A2g ! T1g (F) and A2g ! T2g, respectively. This is in
conformity with the expected d–d transitions for octahedrally coordinated
Ni(II) complexes.[34]
CONCLUSIONS
The analytical data and the above physico-chemical studies suggest
that the mixed-ligand complexes may be formulated as [Ni(NCS)2(L)4],
where L = 2-MBZT, 2’-HPBZT, 2’-TOBZT or 3’-TOBZT, [Ni(NCS)2(2-
AMBZT)2] and [Ni(NCS)2(BBZLPY)(H2O)2]. Suitable octahedral structures
are proposed for these complexes as shown in Figure 4.
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,

Metal Thiocyanato Complexes
399
Lucknow, India, and the Regional Sophisticated Instrumentation Centre,
Indian Institute of Technology, Chennai, India, are gratefully acknowledged.
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Received: August 17, 1998
Accepted: October 28, 2002
Referee I: C. M. Hockensmith
Referee II: J. J Alexander