Synthesis, coordination behavior, and investigations of pharmacological effects of some transition metal complexes with isoniazid Schiff bases

Article abstract of DOI:10.1080/00958972.2010.504986

Two isoniazid Schiff bases, N-isonicotinamido-2-furanketimine (INH-F ¹) and N-isonicotinamido-5-methyl-2-furanketimine (INH-F ²), possessing potential N and O coordination sites have been prepared by the reaction of isoniazid with 2-acetylfuran and 2-acetyl-5-methylfuran, respectively. Complexes of Pd(II) and Pt(II) have been prepared and characterized by elemental analyses, melting point determinations and electronic, infrared, ¹H NMR, ¹³C NMR spectral studies, and X-ray powder diffraction studies. In all the complexes, the monobasic bidentate nature of the ligand is evident. Antibacterial and antifungal studies of these compounds against various pathogenic bacterial and fungal strains have been carried out. Both the ligands and their metal chelates were active against all the microbial strains investigated. However, the chelates were found to be more active than the ligands. The antimycobacterial activity of the ligands and their metal complexes has been evaluated against Mycobacterium smegmatis, which showed clear enhancement in this activity upon metal complexation with Schiff bases.

Full text of DOI:10.1080/00958972.2010.504986

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Synthesis, coordination behavior,
and investigations of pharmacological
effects of some transition metal
complexes with isoniazid Schiff bases
Krishna K. Sharma a , Ritu Singh a , Nighat Fahmi a & R.V. Singh a
a Department of Chemistry, University of Rajasthan,
Jaipur–302055, India
Version of record first published: 28 Jul 2010.
To cite this article: Krishna K. Sharma , Ritu Singh , Nighat Fahmi & R.V. Singh (2010): Synthesis,
coordination behavior, and investigations of pharmacological effects of some transition metal
complexes with isoniazid Schiff bases, Journal of Coordination Chemistry, 63:17, 3071-3082
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Journal of Coordination Chemistry
Vol. 63, No. 17, 10 September 2010, 3071–3082
Synthesis, coordination behavior, and investigations of
pharmacological effects of some transition metal complexes
with isoniazid Schiff bases
KRISHNA K. SHARMA, RITU SINGH,
NIGHAT FAHMI and R.V. SINGH*
Department of Chemistry, University of Rajasthan, Jaipur – 302055, India
(Received 21 May 2009; in final form 5 May 2010)
Two isoniazid Schiff bases, N-isonicotinamido-2-furanketimine (INH-F1) and N-isonicotina-
mido-5-methyl-2-furanketimine (INH-F2), possessing potential N and O coordination sites
have been prepared by the reaction of isoniazid with 2-acetylfuran and 2-acetyl-5-methylfuran,
respectively. Complexes of Pd(II) and Pt(II) have been prepared and characterized by elemental
analyses, melting point determinations and electronic, infrared, 1H NMR, 13C NMR spectral
studies, and X-ray powder diffraction studies. In all the complexes, the monobasic bidentate
nature of the ligand is evident. Antibacterial and antifungal studies of these compounds against
various pathogenic bacterial and fungal strains have been carried out. Both the ligands and
their metal chelates were active against all the microbial strains investigated. However, the
chelates were found to be more active than the ligands. The antimycobacterial activity of the
ligands and their metal complexes has been evaluated against Mycobacterium smegmatis, which
showed clear enhancement in this activity upon metal complexation with Schiff bases.
Keywords: Isoniazid Schiff bases; Pd(II) and Pt(II) complexes; Antifungal; Antibacterial and
antimycobacterial activity
1. Introduction
Heterocyclic compounds are widely distributed in nature and are essential for many
biochemical processes. The past few decades have seen the introduction of a number of
pharmaceutical compounds which contain five, six, and seven-membered rings such as
piperazines, piperidines, imidazoles, benzodiazepines, and other heterocycles containing
nitrogen, sulfur, and oxygen [1]. Systems of this kind play a significant role in many
biological processes due to their metal-coordinating ability [2]. Bioinorganic com-
pounds and biomimetics, containing a C¼N bond, are widely represented [3]. Schiff
bases have been intensively investigated for their strong coordination capability and
diverse biological activities [4–7]. The tuberculostatic activity of isonicotinic acid
hydrazide and its aroylhydrazones containing azomethine nitrogen is attributed to their
ability to form stable complexes with d- and f-block metal ions [8–10]. Interest in the
study of hydrazones has been growing because of their antimicrobial, antituberculosis,
*Corresponding author. Email: rvsjpr@hotmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.504986

3072
K.K. Sharma et al.
and antitumour activity [11–14]. Isoniazid Schiff bases have been found to show better
anticancer and antitubercular activity than isoniazid [15, 16].
The remarkable biological activity of acid hydrazides RꢀCOꢀNHꢀNH2 as well as
their corresponding aroylhydrazones RꢀCOꢀNHꢀN¼CHꢀR, and the dependence of
their mode of chelation with transition metal ions present in the living system have been
of great importance [17–20]. Enzymatic acetylation of the antitubercular isoniazid by
N-acetyltransferase represents a major metabolic pathway for isoniazid in humans.
Acetylation greatly reduces the therapeutic activity of the drug, resulting in
underdosing, decreased bioavailability, and acquired isoniazid resistance. Chemical
modification of the hydrazine unit of isoniazid with a functional group that blocks
acetylation, while maintaining strong antimycobacterial action, has the potential to
improve clinical outcomes and to reduce the emergence in patients of acquired isoniazid
resistance. Isoniazid Schiff bases and their metal complexes exhibit better antimicrobial
activity than isonicotinic acid hydrazide [21, 22].
Considering the constant emergence of antibiotic resistance to clinically used
compounds, it is critical to develop new antibiotic classes, which eventually would
target the lipoid layer of the organisms and other aspects of the pathogen life cycle.
Interest in platinum and palladium complexes is still rapidly growing because of the
attempts to find complexes with greater potency and less toxicity than the existing
clinical drugs [23, 24]. A careful literature survey indicated that very little work is
known about Pd(II) and Pt(II) complexes of hydrazones having an isoniazid moiety. In
the quest for biologically more potent antibacterial and antitubercular compounds, we
envisioned two isoniazid Schiff bases, N-isonicotinamido-2-furanketimine (INH-F1)
and N-isonicotinamido-5-methyl-2-furanketimine (INH-F2) and their Pd(II) and Pt(II)
complexes. The results of preparation, spectroscopic investigations, antibacterial,
antifungal, and anti-mycobacterial activities of the synthesized ligands and their metal
chelates are discussed in this article.
2. Experimental
2.1. Chemicals
Palladium and platinum salts, PdCl2 and PtCl2 as well as 2-acetyl-5-methylfuran were
purchased from Alfa Aesar and isoniazid was purchased from SD Fine Chemicals Ltd.
(Mumbai, India). Solvents of AR grade were distilled from appropriate drying agents
prior to use.
2.2. Physical measurements and analyses
Molecular weights of the synthesized ligands and their metal complexes were
determined by the Rast Camphor method. Chlorine was estimated by Volhard’s
method and nitrogen was estimated by the Kjeldahl’s method. Carbon and hydrogen
analyses were performed at the CDRI, Lucknow. Pd(II) and Pt(II) were estimated
gravimetrically. Electronic spectra were recorded on a Varian–Cary/5E spectro-
photometer at SAIF, IIT, Madras, Chennai. IR spectra of the ligands and their
complexes were recorded with the help of
a
Nicolet Magna FTIR-550

Isoniazid Schiff bases
3073
spectrophotometer from 4000–200 cmꢀ1 in KBr pellets. 1H and 13C NMR spectra were
recorded on a JEOL-AL-300 FT NMR spectrometer in DMSO-d6 using TMS as the
internal standard. X-ray powder diffractograms of the compounds were obtained on a
Philip Model PW 1840 automatic diffractogram using Fe (Ka) target with Mg filter.
˚
The wavelength used was 1.93604 A.
2.3. Synthesis of aroylhydrazones
Both of the ligands, N-isonicotinamido-2-furanketimine (INH-F1) and N-isonicotina-
mido-5-methyl-2-furanketimine (INH-F2), were prepared as reported earlier [25] and
their physical properties and analytical data are given in table 1. The synthetic route of
the ligands is shown in figure 1.
Table 1. Analytical data and physical properties of the ligands and their complexes.
Found (Calcd) (%)
Color and Yield
Molecular weight
Found (Calcd)
Compound
m.p. (ꢁC)
(%)
C
H
N
Cl
M
(1) INH-F1
Brown
232–235
Yellow
239–242
Brown
165–167
Yellow
172–174
Yellow
244–246
Green
183–185
Brown
198–200(d)
Coffee
205–208(d)
Brown
70
76
72
67
68
72
64
66
74
70
57.02
3.99
19.75
–
–
205.34
(229.23)
298.35
(243.26)
599.01
(635.77)
530.08
(562.85)
624.38
(663.83)
549.65
(590.91)
685.29
(724.45)
612.12
(651.53)
703.98
(57.67) (4.83) (18.33)
58.99 5.03 16.99
(59.25) (5.38) (17.29)
41.04 2.98 12.97
(2) INH-F2
–
–
(3) [Pd(INH-F1)2]Cl2
(4) [Pd(IN-F1)2]
[Pd(INH-F2)2]Cl2
[Pd(IN-F2)2]
11.02
16.24
(41.56) (3.48) (13.21) (11.15) (16.73)
46.12
(46.94) (3.58) (14.93)
43.03 3.32 11.99
3.01
14.68
–
18.36
(18.90)
26.98
9.99
(43.42) (3.94) (12.65) (10.68) (26.70)
48.09
(48.78) (4.09) (14.22)
36.01 2.95 10.89
(36.47) (3.06) (11.60)
40.00 2.99 11.78
(40.55) (3.09) (12.89)
37.98 3.02 10.96
(38.30) (3.48) (11.16)
42.02 3.10 11.98
(42.41) (3.55) (12.36)
3.97
13.99
–
29.95
(30.00)
26.89
[Pt(INH-F1)2]Cl2
[Pt(IN-F1)2]
9.10
(9.78) (26.92)
–
29.84
(29.94)
25.20
(5) [Pt(INH-F2)2]Cl2
(6) [Pt(IN-F2)2]
8.98
(9.42) (25.92)
–
202–205(d)
Brown
164–166(d)
(752.52)
635.24
(679.60)
28.10
(28.70)
d ¼ Decomposition point. Compounds 1–6 were tested for their antimicobacterial activity.
NH
O
NH2
C
O
O
N
C
N
C
Ethanol
C
NH
R
R
O
–H O
2
O
H3C
CH3
N
Isoniazid
Substituted furan
Isoniazid Schiff base
Figure 1. Synthetic route of the ligands, where R ¼ H(INH-F1) and CH3(INH-F2).

3074
K.K. Sharma et al.
2.4. Synthesis of the complexes
For the preparation of [Pd(IN-F)2] complexes, methanol solution of PdCl2 (0.42 g,
2.37 mmol) was mixed with methanol solution of INH-F1 (1.08 g, 4.74 mmol) and
INH-F2 (1.15 g, 4.74 mmol) in 1 : 2 molar ratios, respectively. Aqueous ammonia was
added drop wise to the reaction mixture until it was weakly alkaline (pH ca 8.0) and this
reaction mixture was then heated under reflux for 1 h. On cooling, the complexes
separated, were filtered, washed with methanol, and dried in vacuum. To obtain
[Pd(INH-F)2]Cl2, methanol solution of PdCl2 and the ligand in 1 : 2 molar ratio was
stirred on a magnetic stirrer for 2–3 h in presence of a few drops of concentrated HCl.
For the preparation of platinum complexes, 1 : 1 water–ethanol solution of PtCl2
(0.43 g, 1.61 mmol) was mixed with an ethanol solution of INH-F (0.74 g, 3.23 mmol)
and INH-F2 (0.78 g, 3.23 mmol) in 1 : 2 molar ratio, respectively. To obtain the [Pt(IN-
F)2] aqueous ammonia was added dropwise to the reaction mixture until it was weakly
alkaline (pH ca 8.0) and the reaction mixture was then heated under reflux for 1 h.
[Pt(INH-F)2]Cl2 complexes have been synthesized by stirring the above reaction
mixture on a magnetic stirrer for 2–3 h in the presence of a few drops of concentrated
HCl in place of aqueous ammonia. The resulting products were washed with ethanol
and dried in vacuum.
2.5. Microbiology
2.5.1. Antifungal activity. The ligands as well as their complexes were screened for
their antifungal activity against Aspergillus niger and Fusarium oxysporum using potato
dextrose agar (potato 250 g þ dextrose 20 g þ agar 20 g) medium [26]. Solutions at 0.5, 1,
and 1.5 mg mLꢀ1 of each compound in DMSO were prepared for testing against spore
germination. A drop of the solution of each concentration was kept separately on glass
slides. Conidia, the fungal reproducing spores (approximately 200), were lifted with the
help of an inoculating needle and were mixed in every drop of each compound
separately. Each treatment was replicated thrice and a parallel DMSO solvent control
set was run concurrently on separate glass slides. All the slides were incubated in humid
chambers at 25 ꢂ 2ꢁC for 24 h. Each slide was observed under the microscope for spore
germination, and the percentage of germination was finally calculated. The results were
also compared with a standard antifungal drug Flucanazole at the same concentrations.
2.5.2. Antibacterial activity. In vitro antibacterial screening is generally performed by
disc diffusion methods for selection of compounds as therapeutic agents [26]. In this
method, the activity of the test compounds is expressed by measuring the diameter of
zone of inhibition. Generally, the more susceptible the organisms, the bigger the zone of
inhibition. Each compound was dissolved in DMSO and solutions of concentration 2
and 1 mg mLꢀ1 were prepared separately. Paper discs of Whatman filter paper (No. 42)
of uniform diameter (2 cm) were cut and sterilized in an autoclave. The paper discs
soaked in the desired concentration of the complex solutions were placed aseptically in
Petri dishes containing nutrient agar media (agar 20 g þ beef extract 3 g þ peptone 5 g)
seeded with Escherichia coli (ꢀ) and Staphylococcus aureus (þ) bacteria separately. The
Petri dishes were incubated at 37ꢁC and the inhibition zones were recorded after 24 h of
incubation. The antibacterial activity of a common standard antibiotic Ampicillin was

Isoniazid Schiff bases
3075
also recorded using the same procedure as above at the same concentrations and using
same the solvent. The % Activity Index for the complex was calculated by the formula:
Zone of inhibilition by test compound ðdiamterÞ
% Activity Index ¼
ꢃ 100
Zone of inhibilition by standard ðdiamterÞ
2.5.3. Minimum inhibitory concentration (MIC) determination. The MIC of the
compound is defined as the lowest concentration of that compound in a medium
without visible growth of the test organisms. The MIC was determined using the disc
diffusion technique by preparing discs containing 0.1–1.0 mg mLꢀ1 of each compound
against both bacteria and applying the protocol. All the compounds were more effective
at 1.0 and 2.0 mg mLꢀ1 concentrations. Consequently, all the compounds were screened
at these concentrations against both bacteria.
2.5.4. In vitro antimycobacterial activity. YT agar medium was prepared using 1%
yeast extract, 2% trypton, 1.5% agar, 1% NaCl in 250 mL distilled water by
maintaining the pH of the medium at 7 using 10% NaOH solution. This medium was
then sterilized by autoclaving at 120ꢁC for 15 min. After cooling to 50ꢁC the medium
was poured into 85 mm diameter Petri dishes (approx. 25 mL each) and setting aside at
37ꢁC. After a few hours, Petri dishes were stored in the cold room at 4ꢁC. Freshly
prepared 100 mL of inoculum of Micobacterium smegmatis was spread in each dish and
20 mL (100 mg) solution of the test compound was poured in each well. Twenty
milliliter DMSO was used as negative control. The plates were kept at 37ꢁC for 24 h
after which the diameter of the inhibition zones was measured [27]. Ciprofloxacin was
used as a standard reference drug for comparison.
3. Results and discussion
It appears from the analytical data that the ligands react with metal salts in 1 : 2 (M : L)
molar ratio to form complexes of general compositions [M(INH-F)2]Cl2 and [M(IN-
F)2]. The metal chloride interacts with the ligands in presence of a few drops of
concentrated HCl to form [M(INH-F)2]Cl2. However, complexes of the type [M(IN-
F)2] were obtained when reactions were carried out in the presence of aqueous NH4OH.
The reactions may be written as:
Â
Ã
MCl2 þ 2INH-F ꢀꢀꢀꢀꢀꢀꢀ! MðINH-FÞ2 Cl2
Â
Ã
MCl2 þ 2INH-F þ 2NH4OH ꢀꢀꢀꢀꢀꢀꢀ! MðIN-FÞ2 þ 2NH4Cl þ 2H2O
where M ¼ Pd(II) and Pt(II) and INH-F is the isoniazid Schiff base. The reactions
proceed easily and all the complexes are soluble in DMSO, DMF, and CHCl3. The molar
conductance values of 10ꢀ3 mol Lꢀ1 solutions of [M(IN-F)2] complexes lie in the range
12–15 Ohmꢀ1 cm2 molꢀ1 in dry DMF indicating nonelectrolytes. However, [M(INH-
F)2]Cl2 complexes are 1 : 2 electrolytes, with conductance values of 205–220 Ohmꢀ1
cm2 molꢀ1. The metal complexes are diamagnetic, as expected for square planar d8
complexes. Their magnetic susceptibilities lie in the range 0.3–0.8 ꢃ 10ꢀ6 c.g.s. units.

3076
K.K. Sharma et al.
Table 2. Electronic spectral data (cmꢀ1) of the palladium(II) and platinum(II) complexes.
Complex
Spectral bands (cmꢀ1
)
Transitions
D1 (cmꢀ1
)
D2 (cmꢀ1
)
D3 (cmꢀ1
)
ꢀ2/ꢀ1
1
[Pd(INH-F1)2]Cl2
22,222
24,390
26,666
21,598
23,923
26,881
19,230
23,809
28,985
19,120
23,529
28,735
1A1g ! A2g (ꢀ1)
24,322
3368
1976
1.09
1
1A1g ! B1g (ꢀ2)
1
1A1g ! E1g (ꢀ3)
1
[Pd(IN-F1)2]
1A1g ! A2g (ꢀ1)
23,698
21,330
21,220
3530
5779
5609
2653
4876
4906
1.10
1.23
1.23
1
1A1g ! B1g (ꢀ2)
1
1A1g ! E1g (ꢀ3)
1
[Pt(INH-F2)2]Cl2
[Pt(IN-F2)2]
1A1g ! A2g (ꢀ1)
1
1A1g ! B1g (ꢀ2)
1
1A1g ! E1g (ꢀ3)
1
1A1g ! A2g (ꢀ1)
1
1A1g ! B1g (ꢀ2)
1
1A1g ! E1g (ꢀ3)
The complexes are monomers as revealed by their molecular weight determinations. The
data from analytical and physicochemical studies were correlated to explain the
properties and natures of the complexes (table 1).
3.1. Spectroscopic characterization
3.1.1. Electronic spectra. The electronic spectra of the metal complexes were recorded
in distilled DMSO so as to confirm their square-planar geometry. The spectra of the
complexes show three bands due to three d–d spin-allowed transitions from the three
lower lying ‘‘d’’ orbitals to the empty dx2–y2 orbital. The ground state is 1A1g and excited
1
1
1
states corresponding to the above transitions are A2g, B1g, and E1g in order of
increasing energy. By assuming a value of F2 ¼ 10F4 ¼ 600 cmꢀ1 for Slater Condon
interelectronic repulsion parameters (F2 and F4), and subsequently the equations
suggested by Gray and Ballhausen [28], it is possible to calculate the single electron
parameters: The ꢀ2/ꢀ1 values (table 2) lie in the 1.09–1.23 range which are in close
agreement with those reported earlier for square-planar complexes [28, 29].
3.1.2. IR spectra. A comparison of the IR spectra of the complexes and the ligands
INH-F1 and INH-F2 shows that v(C¼O) of the ligands at 1665–1675 cmꢀ1 and v(NH)
at 3240–3246 cmꢀ1 are absent in the spectra of the respective complexes (table 3). This is
presumably due to amide–imidol tautomerism (figure 2) and their subsequent
coordination through the imidol oxygen [30]. This is supported by appearance of a
new band in spectra of the complexes at 1520–1550 cmꢀ1 attributed to the azine group,
4C¼N–N¼C5, which is absent in spectra of the hydrazones. Bands at 1600–1612 cmꢀ1
due to v(C¼N) of the hydrazones shifts to lower wavenumbers indicating coordination
of the azomethine nitrogen. Nonligand bands at 415–428 and 352–456 cmꢀ1 have been
assigned to ꢀ(M–O) and ꢀ(M–N), respectively. In both hydrazones bands at 1480–
1500 cmꢀ1 due to the pyridine ring nitrogen remain unchanged on complexation,
indicating noninvolvement of the ring nitrogen in complex formation [31]. The overall
IR spectral evidence suggests that both ligands are bidentate, coordinating through
amide-oxygen and azomethine-nitrogen forming a five-membered chelate ring.

Isoniazid Schiff bases
3077
Table 3. IR (cmꢀ1) spectral data of the ligands and their metal complexes.
IR spectral data (cmꢀ1
)
Compound
ꢀ(NH)
ꢀ(C¼O)
ꢀ(C¼N)
4C¼N–N¼C5
ꢀ(M ! O)
INH-F1
3240
3246
1665
1675
1290
1293
1320
1318
1298
1300
1309
1312
1600
1612
1590
1585
1595
1588
1592
1589
1597
1594
–
–
–
–
INH-F2
[Pd(INH-F1)2]Cl2
[Pd(IN-F1)2]
[Pd(INH-F2)2]Cl2
[Pd(IN-F2)2]
[Pt(INH-F1)2]Cl2
[Pt(IN-F1)2]
–
–
–
–
–
–
–
–
1520
1526
1537
1540
1542
1546
1548
1550
415
418
422
425
423
424
423
428
[Pt(INH-F2)2]Cl2
[Pt(IN-F2)2]
OH
O
C
N
N
N
C
N
C
C
NH
N
R
R
O
O
CH3
H3C
Amide form
Imidol form
Figure 2. Tautomeric form of the ligands.
3.1.3. 1H NMR spectra. The 1H NMR spectral data of the ligands and their
corresponding metal complexes were recorded in DMSO-d6 with TMS as an internal
standard. A comparison of 1H NMR spectra of the metal complexes and their
respective ligands is shown in Supplementary material. Disappearance of the –NH
proton signal at ꢁ 10.06–11.02 ppm in the complexes is considered as an additional
evidence of enolization of the ligand during complexation. A singlet at 8.90–8.92 ppm
may be assigned to ꢂ-protons of the pyridine ring. The multiplet at 6.97–8.27 ppm may
be assigned to ꢃ-protons of the pyridine ring and protons of the aromatic ring. In the
spectrum of the metal complexes, the signals due to ꢂ- and ꢃ-protons of the pyridine
ring and protons of the aromatic ring were downfield. A sharp singlet at ꢁ 2.34–
2.51 ppm due to the methyl protons attached to azomethine of the ligands undergoes a
downfield shift due to the coordination of the azomethine nitrogen.
3.1.4. 13C NMR spectra. 13C NMR data recorded for both the ligands and their
complexes support the proposed structures (figure S1 in Supplementary material and
figure 3). Signals due to carbons attached to the azomethine nitrogen and enolic oxygen
appear at ꢁ 157.60–162.82 and ꢁ 166.62–175.78 ppm, respectively. However, in the
spectra of corresponding metal complexes, a considerable downfield shift is observed in
these signals. The shifts in the positions of carbons adjacent to the coordinating atoms
clearly indicate bonding of the azomethine nitrogen and enolic oxygen to the metal.
1
Thus, the H and 13C NMR spectra confirm the monobasic bidentate nature of the
ligands, already suggested by the IR spectral studies.

3078
K.K. Sharma et al.
H C
3
R
C
N
O
OH
N
C
N
N
C
Cl
2
M
N
C
HO
O
N
R
CH
3
Addition complex
Figure 3. Proposed structures for addition complexes, M ¼ Pd(II) and Pt(II); R ¼ H(INH-F1) and
CH3(INH-F2).
3.1.5. Mass spectra. Mass spectroscopy, which is mainly applied in the analysis of
biomolecules, has been increasingly used as a powerful structural characterization
technique in coordination chemistry. The FAB mass spectrum of the [Pd(INH-F1)2]Cl2
complex 3 was studied as a representative case. Peaks of appreciable intensity were
observed at m/z values 635.9, 564.5, and 229.3 (Supplementary material). The molecular
ion peak for the complex [Pd(INH-F1)2]Cl2 observed at m/z 635.9 is in good agreement
with its molecular weight, which suggests the monomeric nature of the complex and
confirms the proposed formula. The peak at m/z 564.5 indicates that the molecular ion
gave a fragment ion [Pd(INH-F1)2]þ by losing two chlorines. This fragment ion
undergoes demetallation to form the species [INH-F1]þ that gave fragment ion peak at
m/z 229.3 as follows:
Â
Therefore, these results complemented previous characterizations of the complexes as
Ã
Â
Ãþ
ꢀ2Cl
dematallation
PdðINH-F1Þ2 Cl2 ꢀꢀꢀꢀ! Pd ðINH-F1Þ2 ꢀꢀꢀꢀꢀꢀꢀꢀꢀ! ½INH-F1ꢄþ
M=Z¼229:3
M=Z¼635:9
M=Z¼564:5
being square planar with bidentate Schiff-base ligands.
The structures in figures 3 and 4 are proposed for the complexes.
3.2. X-ray diffraction studies
The possible geometry of the finely powdered product, [Pd(INH-F1)2]Cl2, has been
deduced on the basis of X-ray powder diffraction pattern (Supplementary material).
˚
The observed interplanar spacing values (‘‘d’’ in A) have been measured from the
diffractogram of the compound and the Miller indices h, k, and l have been assigned to
each d value and 2ꢄ angles are reported in Supplementary material. The results show
that the compound belongs to ‘‘orthorhombic’’ crystal system [32] having unit cell
parameters as a ¼ 25.420, b ¼ 17.320, c ¼ 10.220, maximum deviation of 2ꢄ ¼ 0.045ꢁ and
ꢂ ¼ 90ꢁ, ꢃ ¼ 90ꢁ, ꢅ ¼ 90ꢁ.

Isoniazid Schiff bases
3079
H C
3
R
C
N
O
N
O
C
N
N
C
M
N
C
O
O
N
R
CH
3
Figure 4. Proposed structures for substitution complexes, M ¼ Pd(II) and Pt(II); R ¼ H(INH-F1) and
CH3(INH-F2).
Table 4. Antifungal screening data for the ligands and their metal complexes.
(%) Inhibition after 96 h (Conc. in mg mLꢀ1
)
Aspergillus niger
Fusarium oxysporum
Compound
0.5
1.0
1.5
0.5
1.0
1.5
INH-F1
39 ꢂ 0.4
42 ꢂ 0.3
52 ꢂ 0.1
50 ꢂ 0.6
54 ꢂ 0.3
52 ꢂ 0.3
54 ꢂ 0.4
53 ꢂ 0.5
58 ꢂ 0.7
56 ꢂ 1.1
82 ꢂ 0.8
–
54 ꢂ 0.5
57 ꢂ 0.3
62 ꢂ 0.5
60 ꢂ 0.2
65 ꢂ 0.4
63 ꢂ 0.6
65 ꢂ 0.4
62 ꢂ 0.5
69 ꢂ 0.4
67 ꢂ 0.4
99 ꢂ 0.4
–
65 ꢂ 0.5
68 ꢂ 0.6
69 ꢂ 0.4
66 ꢂ 0.5
73 ꢂ 0.5
71 ꢂ 0.3
74 ꢂ 0.5
72 ꢂ 0.5
79 ꢂ 0.3
76 ꢂ 0.7
100 ꢂ 1.2
–
40 ꢂ 1.2
42 ꢂ 0.4
50 ꢂ 0.8
48 ꢂ 0.4
52 ꢂ 0.5
50 ꢂ 1.1
55 ꢂ 0.5
53 ꢂ 0.3
59 ꢂ 0.4
56 ꢂ 0.4
86 ꢂ 0.5
–
55 ꢂ 0.6
57 ꢂ 0.3
64 ꢂ 0.5
62 ꢂ 0.8
65 ꢂ 0.6
61 ꢂ 0.5
66 ꢂ 0.2
64 ꢂ 0.5
70 ꢂ 0.3
68 ꢂ 0.4
98 ꢂ 0.6
–
67 ꢂ 0.6
69 ꢂ 0.5
70 ꢂ 0.4
69 ꢂ 0.5
74 ꢂ 0.3
72 ꢂ 0.6
78 ꢂ 0.4
76 ꢂ 0.5
80 ꢂ 0.6
77 ꢂ 0.5
100 ꢂ 0.6
–
INH-F2
[Pd(INH-F1)2]Cl2
[Pd(IN-F1)2]
[Pd(INH-F2)2]Cl2
[Pd(IN-F2)2]
[Pt(INH-F1)2]Cl2
[Pt(IN-F1)2]
[Pt(INH-F2)2]Cl2
[Pt(IN-F2)2]
Flucanazone
Control
3.3. Bioassay
The antimicrobial studies showed that the ligands and their metal complexes exhibit
considerable activity against all the pathogenic bacterial and fungal strains (tables 4
and 5). The results show that the complexes are more active than their parent ligands
against the same microorganisms and as the concentration increases the activity also
increases. MIC values (table 5) also show that metal complexes are more potent than
both ligands. Chelation theory [33] accounts for the increased activity of the metal
complexes. The enhanced activity could also be due to inherent properties of the metal
ion in precipitating or denaturing proteins. As enzymes are proteins, it would be
expected that the heavy metal would inactivate these catalysts [34].
The mode of action of antimicrobials may involve various targets in microorganisms,
e.g. interference with cell wall synthesis, damage to the cytoplasmic membrane, causing

3080
K.K. Sharma et al.

Isoniazid Schiff bases
3081
Figure 5. Antimycobacterial assay of the ligands and their metal complexes against M. smegmatis.
an alternation of the cell permeability, or a disorganization of the lipoproteins leading
to cell death. Antimicrobials can bind to ribosome and may interfere with peptide chain
formation in microorganisms or with the transcription mechanisms. At lower
concentration, inhibition is less severe because the activities of the organisms will be
slowed down, while at higher concentration, more enzymes will become inhibited
leading to a quicker death of the organisms [35].
From bactericidal activity, it is apparent that the complexes were more active towards
Gram (þ) than Gram (ꢀ) strain. The reason is the difference in the structures of the cell
walls. The walls of Gram (ꢀ) cells are more complex than those of Gram (þ) cells.
Lipopolysaccharides form an outer lipid membrane and contribute to the complex
antigenic specificity of Gram (ꢀ) cells [36].
The antimycobacterial assay for the ligands and their metal complexes against
M. smegmatis are shown in figure 5, which indicates that metal conjugation enhances
the antitubercular activity of the parent ligand by three- or four-fold, making it
comparable with that of Ciprofloxacin. Such an enhancement may be due to greater
lipophilicity of the metal conjugates, promoting facile intracellular metal transport.
4. Conclusion
Based on various physicochemical and structural investigations, the ligands are
bidentate forming square-planar complexes with Pd(II) and Pt(II). All the compounds
showed appreciable antifungal and antibacterial activity. The present study shows that
metal conjugation may be advantageous in designing highly effective drugs in
antitubercular therapy.
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