Synthesis, structural characterization and biological studies of neodymium(III) and samarium(III) complexes with mercaptotriazole Schiff bases

Article abstract of DOI:10.1002/aoc.3405

A series of neodymium(III) and samarium(III) complexes of type [Ln(L)Cl(H₂O)₃] have been synthesized with Schiff bases (LH₂) derived from 3-(phenyl/substituted phenyl)-4-amino-5-mercapto-1,2,4-triazoles and isatin. The str

Full text of DOI:10.1002/aoc.3405

Full paper
Received: 29 July 2015
Revised: 16 September 2015
Accepted: 18 October 2015
Published online in Wiley Online Library: 20 November 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3405
Synthesis, structural characterization and
biological studies of neodymium(III) and
samarium(III) complexes with
mercaptotriazole Schiff bases
Qurratul Ain, Sarvesh Kumar Pandey, Om Prakash Pandey
and Soumitra Kumar Sengupta*
A series of neodymium(III) and samarium(III) complexes of type [Ln(L)Cl(H2O)3] have been synthesized with Schiff bases (LH2) de-
rived from 3-(phenyl/substituted phenyl)-4-amino-5-mercapto-1,2,4-triazoles and isatin. The structures of the complexes were
established using elemental analysis, molar conductivities, magnetic moments, infrared, NMR (1H, 13C) and UV–visible spectra,
X-ray diffraction and mass spectrometry. The thermal behaviour of these compounds under non-isothermal conditions was inves-
tigated using thermogravimetry and differential thermogravimetry. The intermediates obtained at the end of various thermal
decomposition steps were identified from elemental analysis and infrared spectral studies. All the ligands and their complexes
were also screened for their antibacterial activity against Staphylococcus aureus and Bacillus subtilis and antifungal activity against
Aspergillus niger, Aspergillus flavus and Colletotrichum capsici. The screening results were correlated with the structural features of
the compounds. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: 1,2,4-triazoles; Schiff bases; neodymium(III); samarium(III); antimicrobial
spectroscopic properties, thermal behaviour and antimicrobial (anti-
bacterial and antifungal) activities of neodymium(III) and samarium
Introduction
Schiff bases are very accomplished ligands in transition metal as well
as inner-transition metal complexes with suitable properties for theo-
retical and practical applications.[1] Heterocyclic Schiff bases derived
from 1,2,4-triazoles have received much attention, particularly in the
study of complex formation because they exhibit interesting biologi-
cal activities. 1,2,4-Triazole moieties have been incorporated into a
wide variety of therapeutically interesting drug candidates including
anti-inflammatories, central nervous system stimulants, sedatives,
anti-anxiety agents and antimicrobial agents.[2,3] There are various
known drugs containing 1,2,4-triazole moieties, such as fluconazole,
itraconazole, voriconazole,[4,5] triazolam,[6] alprazolam,[7] etizolam,[8]
ribavirin,[9] mycobutanil,[10] rizatriptan[11] and fluotrimazole.[12] 1,2,4-
Triazole ligands are considered to be good coordinating ligands be-
cause they involve both hard nitrogen and soft sulfur atoms in a
thioamide group. These ligands have donor groups that coordinate
with a wide range of metal ions. The diverse applications are at least
in part a consequence of the numerous approaches that are available
for the insertion of specific functionalities into the 1,2,4-triazole
nucleus.[13–15] However, lanthanide(III) complexes formed with hetero-
cyclic Schiff bases, particularly those containing a 1,2,4-triazole ring
system, have received comparatively less attention. Apart from struc-
tural diversities and bonding interactions, the multitude of applica-
tions of lanthanide complexes makes them an exciting area of
coordination chemistry.
(III) complexes with Schiff bases derived from 3-(phenyl/substituted
phenyl)-4-amino-5-mercapto-1,2,4-triazole and isatin.
Experimental
Materials and spectral measurements
All reagents were of analytical grade and were used without further
purification. Neodymium(III) chloride and samarium(III) chloride
were purchased from Merck.
Nitrogen was estimated using the Kjeldahl method and neo-
dymium and samarium were determined gravimetrically as neo-
dymium oxide and samarium oxide. Elemental analysis (C, H and
N) was conducted using a Coleman CHN analyser. Melting points
were determined with a Buchi 530 apparatus in open capillary
tubes. Molar conductance of 10ꢀ3 M solutions of the complexes
in dimethylsulfoxide (DMSO) was recorded with a Hanna EC 215
conductivity meter using 0.01 M KCl water solution as calibrant.
*
Correspondence to: Soumitra Kumar Sengupta, Chemistry Department, DDU
Gorakhpur University, Gorakhpur – 273009, India.
E-mail: sengupta@hotmail.co.uk
Keeping in view the above facts and in continuation of our research
on biologically potent molecules, we herein report the synthesis,
Chemistry Department, DDU Gorakhpur University, Gorakhpur - 273009, India
Appl. Organometal. Chem. 2016, 30, 102–108
Copyright © 2015 John Wiley & Sons, Ltd.

Neodymium(III) and samarium(III) complexes as antimicrobial agents
Magnetic susceptibility measurements of the complexes in pow-
der form were carried out at room temperature using a Gouy
balance. The effective magnetic moments (μeff) were calculated
using the formula
6.0 using sodium hydroxide solution. The reaction mixture was
refluxed for 12–20 h. The product formed was filtered, washed with
water followed by ethanol and dried in vacuo.
pffiffiffiffiffiffiffiffiffiffiffiffi
μeff ¼ 2:238 χmꢁT
(1)
Table 1. Antibacterial and antifungal activities of Schiff bases, Nd(III)
and Sm(III) complexes and standards
where T is the absolute temperature and χm the molar susceptibility
obtained after employing diamagnetic corrections. The magnetic
moment is usually expressed in units called Bohr magnetons (BM).
Infrared (IR) spectra were recorded with a Matson model 1000
FTIR spectrophotometer using KBr discs. 1H NMR and 13C NMR
spectra were recorded with a Jeol AL300 spectrometer using
DMSO-d6 as solvent. Chemical shifts (δ) were measured in parts
per million (ppm) relative to an internal standard of Me4Si. A Hitachi
(Japan) model U-2000 spectrophotometer was used to record the
electronic spectra. The particle size of the complexes was calculated
by analysis of X-ray diffraction patterns, obtained using a powder X-
ray diffractometer (Rigaku Geigerflex) with a Cu Kα (λ = 1.54060 Å)
source. Thermogravimetric analyses were conducted from 50 to
830°C at a heating rate of 10°C minꢀ1. The data were obtained
using a PerkinElmer Diamond instrument. High-resolution mass
spectra were recorded with a Waters Micromass Q-Tof Micro.
Compound
Conc.
Antibacterial
Antifungal activity:
zone of inhibition (%)
(μg mlꢀ1) activity: zone of
inhibition (%)
S. aureus B. subtilis A. niger A. flavus C. capsici
L1H2
100
50
40.4
30.5
18.0
68.5
57.2
46.3
60.0
50.3
38.5
49.4
38.3
27.8
30.4
21.3
10.4
58.3
36.4
25.6
75.7
66.2
55.4
68.5
55.7
45.3
57.0
45.7
35.2
38.5
28.2
17.5
60.5
35.8
26.2
75.0
65.7
56.2
68.8
56.5
45.0
57.1
46.0
35.9
39.6
28.5
17.3
49.3
39.5
27.7
75.0
61.0
50.4
70.2
60.1
46.5
60.6
49.7
35.5
40.2
29.0
19.5
60.8
47.5
35.6
86.5
71.5
59.8
79.5
67.3
53.6
70.1
57.5
45.1
48.5
38.5
27.0
60.5
47.7
34.8
86.2
70.9
58.9
78.5
66.7
53.5
70.4
58.8
45.4
49.7
38.1
27.5
52.5
41.6
30.5
80.5
69.7
58.2
72.7
61.5
50.0
61.3
50.7
40.4
45.8
33.1
25.7
61.8
50.1
39.4
88.3
77.4
65.7
80.2
70.5
59.5
70.2
58.0
47.0
52.1
41.3
30.5
60.5
51.0
38.8
87.2
75.8
64.5
78.8
69.4
59.0
70.6
58.8
47.3
52.3
41.0
30.2
38.5
29.3
18.1
64.5
53.8
42.0
56.7
45.3
36.7
47.6
38.2
28.7
28.5
18.0
10.4
46.3
35.0
26.5
72.1
63.3
52.6
64.2
52.0
45.1
55.3
45.4
34.4
37.0
26.0
17.0
46.7
35.4
25.5
72.3
62.7
51.9
64.0
52.7
40.7
55.1
45.0
34.7
37.4
26.2
17.6
—
—
25
—
L2H2
100
50
52.3
42.8
30.6
43.0
30.3
22.4
33.2
22.8
15.9
—
25
L3H2
100
50
25
L4H2
100
50
General method for synthesis of 3-(phenyl/substituted phe-
nyl)-4-amino-5-mercapto-1,2,4-triazoles
25
L5H2
100
50
—
The method used was that of Tomayo.[16] Carbon disulfide
(38.4mmol) was slowly added in a methanolic solution of the ap-
propriate hydrazide (36.5mmol) and KOH (36.5mmol) with con-
stant stirring. This solution was stirred further for 2–3 h and left
overnight. The mixture was then treated with hydrazine hydrate
(36.5mmol) and refluxed for 4 h. The resulting mixture was cooled
and filtered. The filtrate was acidified with dilute HCl to afford the
triazoles, which were recrystallized from aqueous ethanol.
25
—
[Nd(L1)Cl(H2O)
100
50
—
]
3
—
25
—
[Nd(L2)Cl(H2O)
100
50
60.1
50.5
38.3
51.0
40.2
28.6
40.0
29.0
20.0
—
]
3
25
[Nd(L3)Cl(H2O)
100
50
]
3
Synthesis of Schiff bases
25
[Nd(L4)Cl(H2O)
100
50
The Schiff bases (LH2) were prepared by the condensation of
3-(phenyl/substituted phenyl)-4-amino-5-mercapto-1,2,4-triazoles
with isatin as reported in the literature.[17–21] The general procedure
for synthesis of the Schiff bases is shown in Scheme 1.
]
3
25
[Nd(L5)Cl(H2O)
100
50
]
3
—
25
—
Synthesis of Nd(III) and Sm(III) complexes (1–10)
[Sm(L1)Cl
(H2O)3]
100
50
—
To a solution of appropriate ligand (1mmol) in ethanol (20 cm3), the
appropriate LnCl3ꢂ6H2O (Ln = Nd, Sm; 1 mmol) in ethanol (10 cm3)
was added and the pH of the reaction mixture was adjusted to ca
—
25
—
[Sm(L2)Cl
(H2O)3]
100
50
59.7
51.2
38.3
50.6
41.1
28.5
40.5
29.7
20.3
—
25
[Sm(L3)Cl
(H2O)3]
100
50
25
[Sm(L4)Cl
(H2O)3]
100
50
25
[Sm(L5)Cl
(H2O)3]
100
50
—
25
—
Standard
100
50
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
25
Scheme 1. Synthesis of Schiff bases.
Appl. Organometal. Chem. 2016, 30, 102–108
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

Q. Ain et al.
In vitro antibacterial and antifungal activity
Results and discussion
The synthesized Schiff bases and complexes were screened in vitro
for their antimicrobial activity against two bacteria, namely Staphy-
lococcus aureus and Bacillus subtilis, and three fungi, namely
Aspergillus niger, Aspergillus flavus and Colletotricum capsici, using
reported methods.[22] A stock solution (1 mgmlꢀ1) of the test
chemical was prepared by dissolving 10mg of the test compound
in 10 ml of N,N-dimethylformamide (DMF) solvent. The stock solu-
tion was suitably diluted with sterilized distilled water to get dilu-
tions of 100, 50 and 25 μg mlꢀ1. Control for each dilution was
prepared by diluting 10 ml of solvent instead of stock solution with
sterilized distilled water.
The bacteria were subcultured in agar medium. Petri dishes were
incubated for 24 h at 37°C. Standard antibacterial drug (gentamicin)
was also screened under similar conditions for comparison. The
fungi were subcultured in potato dextrose agar medium. Standard
antifungal drug (fluconazole) was used for comparison. Petri dishes
were incubated for 48 h at 37°C. Wells were dug in the agar media
using a sterile metallic borer. Activity was determined by measuring
the diameter (mm) of the zone showing complete inhibition
(Table 1). Growth inhibition was compared with that of the stan-
dard drugs. The percentage inhibition of the growth of the test or-
ganism was calculated using the following formula:
The Schiff bases are soluble in DMF and DMSO. All the neodymium
(III) and samarium(III) complexes are stable at room temperature
and are non-hygroscopic. The results of the elemental analyses
are summarized in Table 2. The molar conductance values in DMF
are in the range 9–12 Ωꢀ1 cm2 molꢀ1 (Table 2), indicating the
non-electrolytic behaviour of the complexes in solution.
IR spectra
The IR spectral data for all complexes are summarized in Table 3. Or-
ganic compounds having a thioamide group (H–N–C¼S) give rise
to four thioamide bands (ca 1520, ca 1300–1400, 1000 and 700–
850 cmꢀ1) in IR spectra. These bands have contributions from
δ(C–H)+ δ(N–H), ν(C¼S)+ ν(C¼N)+ δ(C–H) and ν(C–N). For the li-
gands under investigation here, these bands appear at 1510–
1530, 1280–1320, 1040–1060 and 780–820 cmꢀ1. The bands are ex-
pected to be affected differently by different modes of coordina-
tion. It has been suggested that the coordination of amido
nitrogen is accompanied by a small increase in the thioamide I
band. Coordination through the thioketosulfur causes a decrease
in the frequency of the thioamide IV band. However, in the com-
plexes under investigation, all thioamide bands disappear indicat-
ing that mixing of ν(C–N), δ(N–H) and ν(C¼S) vibrations may be
absent. In addition, a weak band at 2560–2585 cmꢀ1 in the spectra
of the ligands (in solution) due to the –SH group vibration further
suggests the thione–thiol tautomerism. However, in the spectra of
Cd ꢀ Td
Inhibition ð%Þ ¼ 100ꢁ
(2)
Cd
where Cd is the colony diameter of the control and Td the colony di-
ameter of the treated set. Each set was kept in triplicate.
Table 2. Elemental analyses of neodymium(III) and samarium(III) complexes
Compound Empirical formula Yield (%) M.p. (°C)
C (%)
H (%)
N (%)
Ln (%)
μeff (BM) Molar conductance
(Ωꢀ1 cm2 molꢀ1
)
Found (calcd) Found (calcd) Found (calcd)
Found (calcd)
1
2
C16H15N5SO4ClNd
68
58
70
75
65
65
60
74
72
69
160
175
230
190
195
170
182
220
185
210
34.50 (34.74)
32.46 (32.69)
32.47 (32.69)
32.05 (32.12)
35.00 (35.01)
34.16 (34.36)
32.19 (32.36)
32.20 (32.36)
31.57 (31.79)
34.45 (34.64)
2.56 (2.71)
2.13 (2.38)
2.11 (2.38)
2.25 (2.34)
2.75 (2.92)
2.45 (2.68)
2.22 (2.36)
2.23 (2.36)
2.18 (2.32)
2.68 (2.89)
12.49 (12.66)
11.75 (11.92)
11.78 (11.92)
14.00 (14.05)
11.97 (12.01)
12.41 (12.53)
11.55 (11.79)
11.58 (11.79)
13.80 (13.91)
11.70 (11.89)
26.05 (26.09)
24.32 (24.56)
24.34 (24.56)
24.06 (24.13)
24.53 (24.75)
26.78 (26.90)
25.28 (25.34)
25.21 (25.34)
24.70 (24.89)
25.32 (25.53)
3.55
3.58
3.40
3.50
3.45
1.45
1.40
1.45
1.50
1.42
12.0
10.6
10.4
9.8
C16H14N5SO4Cl2Nd
3
C16H14N5SO4Cl2Nd
4
C
16H14N6SO6ClNd
17H17N5SO5ClNd
C16H15N5SO4ClSm
5
C
12.0
10.5
11.7
11.1
9.0
6
7
C
16H14N5SO4Cl2Sm
16H14N5SO4Cl2Sm
8
C
9
C16H14N6SO6ClSm
10
C17H17N5SO5ClSm
10.5
Table 3. Important IR frequencies (cmꢀ1) of neodymium(III) and samarium(III) complexes
Compound
Formula
ν(OH)
ν(C–O)
ν(C¼N)
ν(Ln–N)
ν(Ln–S)
ν(Ln–O)
1
2
C16H15N5SO4ClNd
C16H14N5SO4Cl2Nd
C16H14N5SO4Cl2Nd
C16H14N6SO6ClNd
C17H17N5SO5ClNd
C16H15N5SO4ClSm
C16H14N5SO4Cl2Sm
C16H14N5SO4Cl2Sm
C16H14N6SO6ClSm
C17H17N5SO5ClSm
3424
3460
3495
3440
3450
3420
3475
3485
3448
3445
1240
1245
1245
1235
1238
1232
1245
1240
1235
1230
1605
1610
1600
1605
1610
1605
1610
1600
1610
1615
415
425
420
435
440
430
445
415
428
420
390
380
388
378
390
375
385
390
385
385
510
515
520
510
510
500
500
500
515
500
3
4
5
6
7
8
9
10
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Appl. Organometal. Chem. 2016, 30, 102–108

Neodymium(III) and samarium(III) complexes as antimicrobial agents
the complexes these bands disappear, indicating the coordination
through sulfur after deprotonation. The IR spectra of the complexes
show a band at 620 cmꢀ1 which is due to conversion of C¼S to C–
NMR spectra (in DMSO-d6) of Schiff bases exhibit a singlet at
8.5 ppm due to thiol proton (s, 1H, –SH) and 10.5 ppm assigned to
enolic proton (s, 1H, –OH).[25,26] These signals disappear in the spec-
tra of the neodymium(III) complexes indicating deprotonation of
thiol and enolic groups. The chemical shift covering a range of
7.2–8.3ppm in the spectra of Schiff bases and their neodymium
(III) complexes is due to benzene ring (attached to triazole nucleus
and isatin) protons. Neodymium(III) complexes show a new signal
at 5.4ppm due to H2O (s, 6H, –OH).
S–. The ν(Ln–S) vibration appears at 375–390 cmꢀ1
.
The bands observed around 1620–1640, 1570–1590 and 1500–
1550 cmꢀ1 in the spectra of the ligands may be assigned to
ν(C¼N), ν(C¼N) (triazole ring) and ν(C¼C) (phenyl) vibrations, re-
spectively. Strong vibrations in the region 1550–1600 cmꢀ1 are usu-
ally associated with phenyl and C¼N groups and it is normally not
possible to identify such bands with particular C¼C or C¼N linkage.
However, a stringent assignment of C¼N group can be made by
comparing the spectrum of the parent mercaptotriazole containing
N–NH2 group with that of the ligands in which N–NH2 group is re-
placed by N–N¼CH group. Following this method, the strong band
observed at 1620–1640 cmꢀ1 is assigned to the ν(C¼N) vibration.
The spectra of the complexes show the shifting of this band to
lower frequency (ca 20cmꢀ1) indicating the coordination of the
azomethine nitrogen to the metal atom. The bands observed at
415–445 cmꢀ1 may be assigned to ν(Ln–N).
13C NMR spectra
The 13C NMR spectra of neodymium(III) complexes were recorded
in DMSO-d6 (Table 5). In the spectra of the Schiff bases, the signals
are observed in the region 161.8–163.0 ppm are due to azomethine
carbon (C¼N, 1C). This signal is shifted downfield in the spectra of
the complexes due to the coordination through azomethine nitro-
gen to the central metal ion. In the spectra of all Schiff bases and
their corresponding neodymium(III) complexes, the peaks observed
at 158.5 and 152.0 ppm are assignable to triazole ring carbons (2C).
For the benzene ring, a number of signals appear at 123.4–149.5 ppm
(12C). In the spectra of Schiff bases, the signal at 159.2 ppm may be
assigned to C–O (1C) of isatin moiety which is shifted downfield in
the spectra of the complexes. This shifting is due to the decrease of
the electron density in the enolate anions on coordination to the lan-
thanide atom moiety during complexation.
The IR spectra of the ligands LH2 show a broad band at about
3320–3335 cmꢀ1 due to ν(–OH) (in solution), which disappears in
the spectra of the corresponding complexes indicating the depro-
tonation of O–H group.[23,24] The ν(Ln–O) vibration band appears
at 500–520 cmꢀ1. The presence of coordinated water molecules in
the complexes is indicated by a broad band in the region 3400–
3500 cmꢀ1
.
1H NMR spectra
Magnetic moments and electronic spectra
The proton magnetic resonance spectra of neodymium(III) com-
plexes were recorded in DMSO-d6. Assignments of chemical shifts
for protons in various environments are given in Table 4. The H
The magnetic moment values for the neodymium(III) and samarium
(III) complexes lie in the range 3.40–3.58 and 1.40–1.50 BM,
1
Table 4. 1H NMR data for neodymium(III) complexes
Compound
Formula
Aromatic protons (δ, ppm)
H2O protons –CH3 protons
(δ, ppm)
(δ, ppm)
1
2
3
4
5
C16H15N5SO4ClNd
7.31–7.50 (4H, m, Ar–H); 7.62–8.30 (5H, m, Ar–H)
5.40 (6H, s)
5.42 (6H, s)
5.45 (6H, s)
5.48 (6H, s)
5.40 (6H, s)
—
C16H14N5SO4Cl2Nd
C16H14N5SO4Cl2Nd
C16H14N6SO6ClNd
C17H17N5SO5ClNd
7.25–7.58 (4H, m, Ar–H); 7.74–7.85 (4H, m, Ar–H)
—
—
7.35–7.61 (4H, m, Ar–H); 7.66 (d, 2H, J = 7.8 Hz, Ar–H); 8.20 (d, 2H, J = 7.8 Hz, Ar–H)
7.35–7.50 (4H, m, Ar–H); 8.26 (d,-2H, J = 7.8 Hz, Ar–H); 8.30 (d, 2H, J = 7.8 Hz, Ar–H)
7.29–7.58 (4H, m, Ar–H); 7.60–7.89 (4H, m, Ar–H)
—
2.37 (3H, s)
Table 5. 13C NMR data for neodymium(III) complexes
Compound
Formula
Phenyl carbons (δ, ppm)a
Triazole carbons
–C¼N
–CH3
(δ, ppm)
C–O
(δ, ppm)
(δ, ppm)
(δ, ppm)
1
2
C
16H15N5SO4ClNd 123.5, 126.8, 127.0, 127.5, (2C, Ar), 129.6 (2C, Ar),
130.2 (Ar), 130.9 (Ar), 131.8, 132.2, 148.0 (–O–C¼N–)
16H14N5SO4Cl2Nd 123.4, 126.5, 127.3, 127.9 (Ar), 128.9 (Ar), 129.2
(Ar), 129.8 (Ar), 130.9 (Ar), 131.5, 132.0, 132.9 (Ar),
148.5 (–O–C¼N–)
152.0, 158.5 (2C)
152.5, 158.6 (2C)
167.7 (1C)
—
—
159.2 (1C)
159.6 (1C)
C
167.8 (1C)
3
4
C
16H14N5SO4Cl2Nd 124.2, 126.9, 127.8, 128.6, (2C, Ar), 130.6 (Ar),131.0
(Ar), 131.7, 132.6, 133.5 (2C, Ar), 148.7 (–O–C¼N–)
16H14N6SO6ClNd 123.9, 126.6, 126.9, 127.0 (2C, Ar), 128.7
(2C, Ar), 130.2 (Ar), 131.8, 132.2, 145.3 (Ar), 148.7
(–O–C¼N–)
152.8, 158.5 (2C)
152.5, 158.5 (2C)
167.8 (1C)
167.3 (1C)
—
—
159.5 (1C)
159.0 (1C)
C
5
C
17H17N5SO5ClNd 124.5, 126.8, 127.0, 127.7 (Ar), 127.9 (Ar), 129.6 (Ar),
130.5 (Ar), 131.2 (Ar), 132.7, 133.6, 135.2 (Ar),
149.5 (–O–C¼N–)
152.2, 158.7 (2C)
167.5 (1C)
19.5 (1C) 159.1 (1C)
aAr = phenyl ring attached to triazole ring.
Appl. Organometal. Chem. 2016, 30, 102–108
Copyright © 2015 John Wiley & Sons, Ltd.
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Q. Ain et al.
respectively (Table 2). These values show little deviation from van
Vleck values and those of hydrated sulfates.
parameter Ek) in the complex and in the free ion:
À
Á
Ek
Ek
ðFkÞcomplex
ðFkÞfree-ion
complex
Tripositive neodymium and samarium ions have the outer con-
figuration 4f35s25p6 and 4f55s25p6, respectively, and follow the
Russell–Saunders L, S, J coupling scheme but with a certain amount
of configuration interaction. The 5 s and 5p subshells have a radial
dispersion which is greater than that of the 4f electrons, and hence
shield the latter from the effect of coordinated ligands to a very
large extent, but not completely.[27] Thus, the electronic spectra of
neodymium(III) and samarium(III) can be considered as derived
from the spectra of gaseous ions by a fairly small perturbation. This
perturbation may be of two general types. The first is a general
nephelauxetic effect owing to a drift of ligand or 5 s, 5p electron
density into the metal ion, slightly expanding the 4f subshell and re-
ducing the 4f–4f interactions. The second is a crystal field splitting
effect, which removes the degeneracy of the free ion levels, and
which can be represented with fair success by a point negative
charge model having the appropriate molecular symmetry.[28–30]
The f3 configuration of a free neodymium(III) ion involves 41 en-
ergy levels. The absorption spectra of neodymium(III) complexes
(Fig. 1) in crystal form and solution contain many bands due to tran-
sitions from the ground level 4I9/2 to excited levels of the f3 config-
uration. The absorption spectra of neodymium(III) complexes
derived from isatin show bands around 12 450, 12 800–12 900,
13 000–13 600, 13 800, 17 510, 19 500, 20 500–21 500 and 23 000–
23 250 cmꢀ1 corresponding to the transitions from the ground level
4I9/2 to excited levels 4F5/2, 2H9/2, 4F7/2, 2S3/2, 2G7/2, 4G9/2, 2G9/2 and
2P1/2, respectively. The f–f transitions are weakly allowed due to
some mixing of the excited state of the opposite parity into the
ground state and a marked enhancement in the intensity of bands
upon complexation is observed. This is due to an increase in the
configuration interaction upon complexation.
À
Á
β ¼
or β ¼
(3)
free-ion
As the naphelauxetic effect value (1 ꢀ β) is small in lanthanide
complexes, it can, to a fairly good approximation, be defined from
the ratio of the wavenumbers of f–f transitions in the spectra of
the complex and the free ion:
Vcomplex
β≈
(4)
Vfree-ion
Since, in general, the energy levels in the lanthanide free ions are
unknown, the relative nephelauxetic effect values β are usually de-
termined from experimental data using the spectra of the lantha-
nide aqua ions as standards:
n
∑
1
Vcomplex
β ¼
(5)
n n¼1 Vaqua
From the meanβ values, Sinha’s covalency parameter (δ) and bond-
ing parameter (b1/2) are calculated using the following formulas:
ꢀ
ꢁ
1=2
1 ꢀ β
1 ꢀ β
δ ¼
ꢁ100 and b1=2
¼
(6)
2
β
These parameters measure the amount of 4f–ligand mixing, i.e.
covalency.
The values of the parameters β, δ and b1/2 for the complexes are
given in Table 6. The values of β being less than unity and the pos-
itive values of δ and b1/2 support the partial covalent nature of
bonding between metal and ligand.
The f5 configuration of samarium(III) ion contains 198 levels with
various J. The ground state level is 6H5/2. The absorption spectra of
samarium(III) complexes (Fig. 2) show bands due to transitions from
Thermal studies
the ground level 6H5/2 to excited energy levels 4G5/2, 4F3/2, 4G7/2, 4I9/2
,
The thermal stabilities of the neodymium and samarium com-
plexes were examined using the thermogravimetry (TG) and differ-
ential thermogravimetry (DTG) techniques in the temperature
range 50–830°C at a heating rate of 10°C minꢀ1. The complexes de-
compose via intermediates to give the respective metal
oxide.[32,33] For all complexes, two steps are apparent in the TG–
DTG curves (Fig. 3). The first step corresponds to the loss of all wa-
ter molecules in the temperature range 165–195°C, accompanied
by an endothermic effect. Higher temperature of this stage
6
4
4I13/2, P5/2 and F9/2. All the transitions corresponds to bands at
17 200–17 350, 18 510, 20 000, 20 500–20 600, 21 510, 23 600–
23 900, 25 000–25 300 cmꢀ1, respectively.
A general feature in the spectra of lanthanides is a shift of absorp-
tion bands towards lower energy on complex formation, i.e. the
nephelauxetic effect.[31] This effect is quantitatively described by a
naphelauxetic parameter (β) equal to the ratio of the interelectronic
repulsion parameter (either Slater’s integrals Fk or Racah’s
Figure 1. Electronic spectrum of [Nd(L1)Cl(H2O)3].
Figure 2. Electronic spectrum of [Sm(L1)Cl(H2O)3].
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Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2016, 30, 102–108

Neodymium(III) and samarium(III) complexes as antimicrobial agents
Table 6. Electronic spectral parameters of neodymium(III) and samarium
(III) complexes
Compound
Formula
β
δ
b1/2
1
2
3
4
5
6
7
8
9
10
C16H15N5SO4ClNd
0.9868
0.9870
0.9860
0.9858
0.9874
0.9792
0.9793
0.9799
0.9795
0.9788
1.3377
1.3171
1.4199
1.4404
1.2761
2.1242
2.1138
2.0512
2.0929
2.0616
0.0812
0.0806
0.0837
0.0843
0.0794
0.1019
0.1017
0.1002
0.1012
0.1029
C16H14N5SO4Cl2Nd
C16H14N5SO4Cl2Nd
C16H14N6SO6ClNd
C17H17N5SO5ClNd
C16H15N5SO4ClSm
C16H14N5SO4Cl2Sm
C16H14N5SO4Cl2Sm
C16H14N6SO6ClSm
C17H17N5SO5ClSm
Figure 4. Powder X-ray diffraction pattern of [Sm(L1)Cl(H2O)3].
peak broadening and θ is the Bragg angle. The size of the particles is
found to be in the range 16–28 nm which falls in the nanoscale range.
Pharmacology results
All the synthesized Schiff bases and their corresponding neody-
mium(III) and samarium(III) complexes were screened in vitro for
their antimicrobial activity against two bacteria and three fungi.
The antimicrobial results are summarized in Table 1. All the com-
plexes possess high antibacterial activity against Gram-positive (B.
subtilis) bacteria and antifungal activity against A. flavus. Complexes
[Nd(L2)Cl(H2O)3], [Nd(L3)Cl(H2O)3] and [Nd(L4)Cl(H2O)3] show zones
of inhibition of 86.5, 79.5 and 70.0% at 100μg mlꢀ1 against B.
subtilis which are quite good. The same complexes show good
zones of inhibition against A. flavus: 86.2, 78.5 and 70.4%, respec-
tively. The antimicrobial studies show that the activity of the ligands
increases on coordination with metal ion, i.e. the metal complexes
are more active than the free organic ligands. This pronounced in-
crease in activity may be due to the formation of a chelate, a less
polar form of the metalloelement, which thereby increases the lipo-
philic character. The increased lipophilic character of the chelate fa-
vours the interaction of these complexes with cell constituents,
resulting in interference with normal cell processes. According to
Overton’s concept of cell permeability, the lipid membrane that sur-
rounds a cell favours the passage of only lipid-soluble materials so
that liposolubility is an important factor that controls antimicrobial
activity.[36] The widespread interaction of metal ions with cellular
components is owing to the fact that lipids and polysaccharides
are some important constituents of cell walls and membranes,
which are preferred for metal ion interaction. In addition to this, cell
walls also contain several aminophosphates and carbonyl and
cysteinyl ligands, which maintain the integrity of the membrane
by acting as a diffusion barrier and also provide suitable sites for
binding. If the geometry and charge distribution around a molecule
are incompatible with the geometry and charge distribution around
the pores of a cell wall, penetration through the wall by the toxic
agent cannot take place and this will prevent a toxic reaction within
the pores.[37] Chelation is not the only criterion for antimicrobial ac-
tivity. The nature of the metal ion and ligand, the coordinating sites,
the geometry of the complex, the concentration, the hydrophilicity,
the lipophilicity and the presence of co-ligands also influence the ac-
tivity. In addition to this, steric and pharmacokinetic factors also play
decisive roles in deciding the potency of an antimicrobial agent.
Figure 3. TG–DTG curves of [Nd(L1)Cl(H2O)3].
suggests a stronger association of complex molecule through in-
termolecular hydrogen bonding between coordinated water and
bonded ligands. The second stage corresponds to the decomposi-
tion of the anhydrous complexes to the oxides and is accompanied
by a strong exothermic effect with two peaks in the temperature
range 345–670°C. The final products of the decomposition of the
lanthanide complexes are oxides: ½Nd2O3 and ½Sm2O3.
X-ray diffraction
Our efforts to prepare single crystals of these complexes were un-
successful due to the high molecular mass of the metal ions in-
volved. Therefore, powder X-ray diffraction patterns were
recorded. Powder X-ray diffraction is a rapid analytical technique.
This technique is based on constructive interference of monochro-
matic X-rays and a crystalline sample.[34,35] These X-rays are gener-
ated by a cathode ray tube, filtered to produce monochromatic
radiation to concentrate and directed towards the sample.
The powder X-ray diffraction pattern of one representative com-
plex, i.e. [Sm(L1)Cl(H2O)3], is shown in Fig. 4. The peak broadening at
lower angle is significant for the calculation of particle size. There-
fore the size of the particles was calculated using the Debye–
Scherrer formula:
D ¼ 0:94λ=β cos θ
(7)
where D is the size of the particle, λ is the wavelength of X-rays, β (in
rad) is the full width at half maximum after correcting the instrument
Appl. Organometal. Chem. 2016, 30, 102–108
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

Q. Ain et al.
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Figure 5. Tentative structure of the complexes.
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High-resolution mass spectra were recorded, and, within acceptable
limits, the benefits of mass spectrometry in the study of organometal-
lic chemistry are that investigation of molecules and complexes in the
gas phase allows determination of the intrinsic properties without in-
terference from solvent or counter ion. TOF-MS spectra of representa-
tive complexes show peaks at (m/z) 610.9641 and 600.9453,
assignable to [M + NH3]+ (Figs S5 and S6 of the supporting informa-
tion), and various other peaks corresponding to the fragmentation
of degraded species. The patterns of peaks give a clear impression
of the successive degradation of the target compound with the series
of peaks corresponding to the various fragments with different inten-
sities. The intensity detected for a particular peak is directly related to
the stability of degraded fragments. These peaks corresponding to
the fragments are observed to confirm the molecular formula of the
selected complexes. HRMS (m/z): [M + NH3]+ calcd for C16H14Cl2-
N5O4SNd, 600.9486; found, 600.9453; [M + NH3]+ calcd for C16H14Cl2-
N5O4SSm, 610.9606; found, 610.9641.
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The synthesized 3-(phenyl/substituted phenyl)-4-amino-5-mercap-
to-1,2,4-triazoles act as tridentate Schiff bases. The metals are coor-
dinated to thiol sulfur, azomethine nitrogen and phenolic oxygen
atoms. The analytical, IR, NMR and electronic spectral, magnetic,
thermal and X-ray studies confirm the bonding of Schiff bases to
metal ions. All Schiff bases and complexes are found to potentially
active towards all microbial strains investigated.
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All these observation put together lead us to propose a structure
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Acknowledgements
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The authors are grateful to SAIF, Chandigarh, STIC Cochin for pro-
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financial support. The authors are also grateful to Head, Chemistry
Department, DDU Gorakhpur University for providing necessary
facilities.
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