Synthesis, characterization and antimicrobial activity of nickel(II) complexes of tridentate N3 ligands

Article abstract of DOI:10.1016/j.ica.2021.120515

The desire to develop new antimicrobial agents against the rising threat of extensive antibiotic resistance led to the exploration of many natural and synthetic compounds. Herein, we report the isolation and structural characterization of nickel(II) complexes of the type [Ni(L)₂]²⁺ (1 and 2) of tridentate N3 ligands such as bis(pyridin-2-ylmethyl)amine (L1) and 1-(1-methyl-1H-imidazol-2-yl)-N-(pyridin-2-ylmethyl)methanamine (L2). Both the complex cations of [Ni(L1)₂]²⁺ (1) and [Ni(L2)₂]²⁺ (2) possess a distorted octahedral coordination geometry in which Ni(II) is chelated to two equivalents of the tridentate L1 and L2 ligand in facial coordination, respectively. Complexes 1 and 2 exhibit the bands in the visible region in UV–Vis spectra and peaks in the downfield region in paramagnetic NMR spectra suggesting the octahedral coordination geometry of the complexes in solution as well. Further, complexes 1 and 2 were evaluated for their antimicrobial activity against various clinical pathogens. With in vitro studies, complex 1 displayed activity against Vibrio parahaemolyticus and V. alginolyticus, Staphylococcus aureus, Candida albicans, and Salmonella typhi, and complex 2 displayed activity against S. aureus, S. typhi, and C. albicans. In addition, with in vivo studies, complex 1 was effective in inhibiting Vibrio infection in brine shrimp larvae while having no toxicity on the larvae. We believe these results will encourage medicinal chemists to further the study on metal complexes against multi-resistant pathogens.

Full text of DOI:10.1016/j.ica.2021.120515

Inorganica Chimica Acta 526 (2021) 120515
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, characterization and antimicrobial activity of nickel(II)
complexes of tridentate N3 ligands
Athulya Dasa, Anjana Rajeeva, Sarmistha Bhuniab, Manivel Arunkumarc, Nithya Charic,
,
*
Muniyandi Sankaralingama
a Bioinspired & Biomimetic Inorganic Chemistry Lab, Department of Chemistry, National Institute of Technology Calicut, Kozhikode 673601, Kerala, India
b School of Chemical Sciences, Indian Association for the Cultivation of Science, Kolkata 700032, India
c Department of Microbiology, School of Life Sciences, Bharathidasan University, Tiruchirappalli 620024, Tamil Nadu, India
A R T I C L E I N F O
A B S T R A C T
The desire to develop new antimicrobial agents against the rising threat of extensive antibiotic resistance led to
the exploration of many natural and synthetic compounds. Herein, we report the isolation and structural
characterization of nickel(II) complexes of the type [Ni(L)2]2+ (1 and 2) of tridentate N3 ligands such as bis
(pyridin-2-ylmethyl)amine (L1) and 1-(1-methyl-1H-imidazol-2-yl)-N-(pyridin-2-ylmethyl)methanamine (L2).
Both the complex cations of [Ni(L1)2]2+ (1) and [Ni(L2)2]2+ (2) possess a distorted octahedral coordination
geometry in which Ni(II) is chelated to two equivalents of the tridentate L1 and L2 ligand in facial coordination,
respectively. Complexes 1 and 2 exhibit the bands in the visible region in UV–Vis spectra and peaks in the
downfield region in paramagnetic NMR spectra suggesting the octahedral coordination geometry of the com-
plexes in solution as well. Further, complexes 1 and 2 were evaluated for their antimicrobial activity against
various clinical pathogens. With in vitro studies, complex 1 displayed activity against Vibrio parahaemolyticus and
V. alginolyticus, Staphylococcus aureus, Candida albicans, and Salmonella typhi, and complex 2 displayed activity
against S. aureus, S. typhi, and C. albicans. In addition, with in vivo studies, complex 1 was effective in inhibiting
Vibrio infection in brine shrimp larvae while having no toxicity on the larvae. We believe these results will
encourage medicinal chemists to further the study on metal complexes against multi-resistant pathogens.
Dedicated to Professor Dr. Mallayan Pala-
niandavar on the occasion of his 70th birthday.
Keywords:
Medicinal inorganic chemistry
Nickel(II) complexes
Heterocyclic compounds
Spectroscopy
Antimicrobial agents
1. Introduction
metal complexes play a major role in biological functions (enzymes) and
their potential against various diseases have inspired bioinorganic
High mortality rate by pathogenic diseases due to antimicrobial
resistance (AMR) has become a grave threat for human health and
economic development, and AMR has been included in the top 10 global
public health threats to humanity by World Health Organization (WHO)
[1,2]. Microbes develop AMR through random mutation of the genome
and accepting antimicrobial-resistant genes from other microbes. These
strains develop ability to reduce or eliminate the effect of antimicrobial
agents [3]. To spread the seriousness of AMR, WHO has updated the
slogan for a global public awareness campaign, World Antimicrobial
Awareness Week (WAAW), as “Antimicrobials: Handle with Care” in
2020 [2]. “Superbugs” formed as a result of AMR, will rule the world
within no time if more promising antimicrobial agents are not discov-
ered [4]. Medicinal chemists are poised to play a central role in
addressing the key scientific challenges toward the development of
metal-based antimicrobial agents. Especially, earth-abundant transition
chemists to explore transition metal complexes for improved pharma-
cological activities [5–9]. A huge amount of antimicrobial data reveals
that the activity of the ligand is enhanced when it is chelated with metal
ion [10–12]. Further, a possible mode of increased activity of metal
chelates can be explained on the basis of Overtone’s concept and
Tweedy’s chelation theory [13]. In addition, chelation could enhance
the lipophilic character of the central metal to favour its permeation
through lipid bilayers of the cell membrane and that results in an
increased uptake of the metal chelates [14].
Among the transition metal complexes, the significance of nickel in
bioinorganic chemistry has rapidly increased after the discovery of the
nickel enzyme, urease [15,16]. Following to this, a number of nickel
complexes have been reported with a broad-spectrum of activity against
pathogens, which is expected due to the ability of such complexes can
penetrate into the microbial cells and affect the enzyme activity
* Corresponding author.
E-mail address: msankaralingam@nitc.ac.in (M. Sankaralingam).
https://doi.org/10.1016/j.ica.2021.120515
Received 20 February 2021; Received in revised form 6 June 2021; Accepted 11 June 2021
Available online 17 July 2021
0020-1693/© 2021 Elsevier B.V. All rights reserved.

A. Das et al.
Inorganica Chimica Acta 526 (2021) 120515
Scheme 1. Reported nickel complexes and ligand (E) having antimicrobial activity [17–23].
Gram-positive and Gram-negative bacteria except for MRSA with MIC
ranging between 0.025 - 1000 µg/mL [23]. Thus, there is still much to be
investigated in this area to understand the mechanism of action of these
nickel complexes by tuning the ligand systems.
More importantly, the ligand system should be tuned with hetero-
cyclic compounds, because more than 90% of the drugs contain het-
erocycles in which a wide range of them contain five or six-membered
nitrogen-containing imidazole and pyridine framework [24–37]. Hence,
the present work has been designed to explore pyridyl and imidazolyl
nitrogen-containing nickel(II) complexes (Scheme 2) as potential anti-
microbial agents. These complexes have been characterized by various
spectroscopic techniques and tested for their antimicrobial activity
against human and aquaculture pathogens. Therefore, this work is
intended to highlight the potential of nickel complexes as antimicrobial
agents.
Scheme 2. Nickel(II) complexes used in this study.
[17–23]. For example, in 2001, Nomiya et al reported a series of nickel
complexes with thiosemicarbazone and semicarbazone ligands for
antimicrobial studies. Among them complex A in Scheme 1 showed se-
lective and effective activity against Bacillus subtilis and S. aureus (Gram-
positive) with a minimum inhibitory concentration (MIC) of 31.3 µg/mL
[17,18]. Later in 2003, Kessissoglou et al studied the relationship be-
tween nuclearity and antimicrobial activity using the mixed ligand
nickel complexes with salicylhydroxamic acid and di-2-pyridyl-
ketonoxime, and among their complexes, B in Scheme 1 showed activ-
ity against B. subtilis with a MIC of 12 µg/mL [19]. Kurtaran et al in 2005
reported a heteronuclear nickel complex of N,N’-bis(salicylidene)-1,3-
diaminopropane, (C in Scheme 1), which showed activity against
Enterobacter aerogenes with a MIC of 31.25 µg/mL [20]. Psomas et al
reported a series of mixed ligand nickel complexes with diimines and 3rd
generation quinolone antibacterial agent sparfloxacin, (D in Scheme 1)
against E. coli, P aeruginosa, and S. aureus, for which MIC range between
16 - 128 µg/mL [21]. Recently, Singh et al reported nickel complexes
based on imine donors and observed reasonable MIC between 15 - 31
µg/mL towards S. aureus, which is comparable to the standard drug
ciprofloxacin (E in Scheme 1) [22]. Very recently, Omondi et al reported
several nickel(II) complexes of formamidine-based dithiocarbamate li-
gands (F in Scheme 1), in which the complexes containing electron-
withdrawing substituents showed antimicrobial activity against both
2. Experimental
2.1. Materials
All chemicals were purchased from commercial sources as reagent
grade and used directly without further purification unless otherwise
specified.
Nickel(II)
perchlorate
hexahydrate,
pyridine-2-
carboxaldehyde and sodium sulfate were purchased from Avra. So-
dium borohydride and 2-picolylamine was purchased from Alfa Aesar
and 1-methylimidazole-2-carboxaldehyde was purchased from TCI, and
were used as such. Methanol was purchased from Qualigens and
acetonitrile from Fischer and both were distilled over calcium hydride
before use.
2.2. Physical measurements
The electronic spectra of the complexes were recorded using Agilent
Cary 8454 UV–Visible diode array spectrophotometer, using distilled
acetonitrile as the solvent at 298 K. The ATR-FTIR spectra of the ligands
and complexes were recorded using JASCO 4700, in 4000 to 400 cmꢀ 1
range. The 1H NMR spectra of the complexes were recorded using Bruker
2

A. Das et al.
Inorganica Chimica Acta 526 (2021) 120515
400 MHz, using deuterated acetonitrile as the solvent and tetrame-
thylsilane as the internal reference. The mass data of the complexes were
recorded using Waters QTOF Micro YA263. The crystal structure of
complex 2 was determined by carrying out the diffraction experiments
using the Bruker SMART APEX diffractometer. The TGA and DSC anal-
ysis were carried out on a TGA Q50 thermal analyser.
Table 1
Selected bond lengths [Å] and bond angles [◦] for complex
2.
2
Bond lengths/Å
Ni(1)-N(1)
Ni(1)-N(2)
Ni(1)-N(3)
Ni(1)-N(5)
Ni(1)-N(6)
Ni(1)-N(7)
2.080(5)
2.166(5)
2.072(5)
2.093(5)
2.159(5)
2.070(5)
Bond angles [o]
175.8(2)
101.3(2)
81.36(18)
96.68(18)
174.24(19)
97.0(2)
2.3. Synthesis of ligands
The present ligands L1 and L2 have been synthesized according to
the literature methods [38–42].
N(2)-Ni(1)-N(7)
N(3)-Ni(1)-N(7)
N(6)-Ni(1)-N(7)
N(2)-Ni(1)-N(6)
N(3)-Ni(1)-N(6)
N(1)-Ni(1)-N(7)
N(1)-Ni(1)-N(6)
N(1)-Ni(1)-N(2)
N(1)-Ni(1)-N(5)
N(1)-Ni(1)-N(3)
N(5)-Ni(1)-N(7)
N(5)-Ni(1)-N(6)
N(5)-Ni(1)-N(2)
N(5)-Ni(1)-N(3)
N(2)-Ni(1)-N(3)
2.3.1. Bis(pyridin-2-ylmethyl)amine (L1)
Yield 0.993 g (99.6%). 1H NMR (400 MHz, CD3CN) (ppm) (Fig. S1A):
5.45 (s, 1H), 7.2–8.6 (m, 4H), 3.95 (s, 4H). IR data (ATR, cmꢀ 1
)
(Fig. S2A): 3336 ν(NH), 745 ν(CN).
92.75(18)
79.3(2)
2.3.2. 1-(1-methyl-1H-imidazol-2-yl)-N-(pyridin-2-ylmethyl)methanamine
(L2)
167.48(19)
91.99(19)
91.50(18)
79.47(17)
91.76(19)
95.29(19)
81.0(2)
Yield 0.583 g (63.6 %). 1H NMR (400 MHz, CD3CN) (ppm): (Fig. S1B)
5.45 (s, 1H), 7.2–8.6 (m, 4H), 3.63 (s, 3H), 3.85 (s, 2H), 3.81 (s, 2H). IR
data (ATR, cmꢀ 1) (Fig. 2): 3347
ν(NH), 752 ν(CN).
2.4. Isolation of the nickel(II) complexes
SMART APEX diffractometer equipped with a CCD area detector at 273
K. The data integration and reduction was processed with SAINT soft-
ware [43]. An empirical absorption correction was applied to the
collected reflections with SADABS [44]. The structure was solved by
direct methods using SHELXTL [45] and refined on F2 by the full-matrix
least-squares technique using the SHELXL-97 package [46]. Other non-
hydrogen atoms were located in successive difference Fourier syntheses.
The final refinement was performed by full-matrix least-squares anal-
ysis. Hydrogen atoms attached to the ligand moiety were located from
the difference Fourier map. Crystal data and additional details of the
data collection and refinement of the structure are presented in Table S1.
The selected bond lengths and bond angles are listed in Table 1.
The present complexes 1 and 2 have been isolated according to the
literature methods [39].
Ni(ClO4)2⋅6H2O (91 mg, 0.25 mmol) was stirred and dissolved in 1
mL of methanol and to this, a methanolic solution (1 mL) of ligand (0.5
mmol) was added dropwise with constant stirring. After 1 h, the pink
precipitate formed was filtered through a G4 crucible, washed with cold
methanol and diethylether. The pink precipitate was dried under a
vacuum.
2.4.1. [Ni(L1)2](ClO4)2 (1). Complex 1 was prepared using the above
general procedure using L1
Yield 90%. TOF MS m/z (Fig. S3): 228.09 [Ni(L1)2]2+, 256.05 [Ni
(L1)]+, 356.01 {[Ni(L1)](ClO4)}+ 555.14 {[Ni(L1)2](ClO4)}+, 200.13
(L1H)+. IR data (ATR, cmꢀ 1) (Fig.,S2B): 3282
ν(NH), 753 ν(CN), 1052,
2.6. In vitro antimicrobial activity
911
ν
(ClO-4). UV–Vis. λmax (nm) (
ε
, in Mꢀ 1 cmꢀ 1) in CH3CN (Fig. 1): 801
Antimicrobial activity was assayed in vitro by the disc diffusion
susceptibility test according to the recommendations of the National
Committee for Clinical Laboratory Standards [47]. Screening for anti-
bacterial activity was performed against common pathogenic bacteria
and fungi, including Pseudomonas aeruginosa PAO1, Pseudomonas aeru-
ginosa PA14, and Staphylococcus aureus (ATCC11632), Salmonella typhi
(MTCC733), Candida albicans as well as the marine pathogens, Vibrio
parahaemolyticus (TFM1) and Vibrio alginolyticus (TFM17). Overnight
cultures (100 mL) of each pathogen were added to 3 mL of soft Mueller-
Hinton agar (MHA; 0.7% w/v agar) and poured onto the surface of MHA
plates (15 mL). The complexes were dispensed in sterile distilled water
in the concentration of 50, 100, 150, 200 µg/mL and 1 mL were added to
5 mm diameter wells punched with a glass pipette, and plates were
incubated at 37 ◦C up to 72 h. Zones of clear inhibition were measured
from the edge of the well [48].
(10), 511 (7), Elemental analysis calcd (%) for; C24H26N6Cl2O8Ni: C
43.94, H 3.99, N 12.81; found: C 43.84, H 3.89, N 12.84.
2.4.2. [Ni(L2)2](ClO4)2 (2): Complex 2 was prepared using the above
general procedure using L2
Yield 35.8%. TOF MS m/z (Fig. S4): 231.11 [Ni(L2)2]2+, 259.07 [Ni
(L2)]+, 561.19 {[Ni(L2)2](ClO4)}+, 203.15 (L2H)+. IR data (ATR, cmꢀ 1
)
(Fig. 2): 3304
ν(NH), 760
ν(CN), 1062, 901
ν
(ClO-4). UV–Vis. λmax (nm)
(
ε
, in Mꢀ 1 cmꢀ 1) in CH3CN (Fig. 1): 832 (7), 700 (2), 536 (11), Elemental
analysis calcd (%) for; C22H28N8Cl2O8Ni: C 39.91, H 4.26, N 16.92;
found: C 39.95, H 4.29, N 16.96. Single crystals suitable for X-ray
crystallographic analysis were obtained by slow evaporation of a solu-
tion of the complex in acetonitrile.
Caution! The perchlorate salts of the compounds are potentially explo-
sive! Only small quantities of these compounds should be prepared and
necessary precautions should be taken when they are handled.
2.7. In vivo antimicrobial acitivity
2.5. Crystal data collection and structure refinement
Antimicrobial activity of complex 1 was determined in vivo using
brine shrimp (Artemia franciscana) against V. parahaemolyticus (TFM1)
and V. alginolyticus (TFM17). In this experiment, the bacterial density
104 CFU mLꢀ 1 was used in all challenge tests with Artemia culture.
Artemia cultures were tested in four groups. Group 1: Artemia were
maintained with complex 1 and exposed to Vibrio spp. Group 2: Artemia
were maintained with complex 1 only. Group 3: Artemia were main-
tained with Vibrio spp. in the absence of complex 1 as positive control.
Single crystals of complex 2, suitable for X-ray crystallographic
analysis were obtained by slow evaporation of a complex solution in
acetonitrile. The diffraction experiments were carried out on a Bruker
SMART APEX diffractometer equipped with a CCD area detector. High-
quality crystals, suitable for X-ray diffraction were chosen after careful
examination under an optical microscope. Intensity data for the crystal
was collected using Mo-Kα (k = 0.71073 Å) radiation on a Bruker
3

A. Das et al.
Inorganica Chimica Acta 526 (2021) 120515
Table 2
UV–Vis spectral data (λmax in nm;
ε
in Mꢀ 1 cmꢀ 1 in parenthesis) of Ni(II) com-
plexes 1 and 2 in acetonitrile at 298 K.
Complex
3A2g
→
3T1g(P)
3A2g
→
3A2g
→
B′
bβ
(%)
(
ν3) (nm)
3T1g(F) (ν2
)
3T2g(F) (ν1
)
(cm-
1
(nm)
(nm)
)
Found
Calcda
1
2
–
–
320
344
511 (7)
801 (10)
794 (5)
888
665
0.82
0.62
534 (10)
a,bCalculated by solving the quadratic equation and using B as 1080 cmꢀ 1
.
Fig. 2. ATR IR spectra of the ligand L2 (top) and [Ni(L2)2](ClO4)2 2 (bottom).
3.2. Electronic absorption spectral properties
Fig. 1. UV–Visible spectra of the complexes (1, and 2; 1.0 × 10-2 M) were
Although complex 1 has been reported previously, we also attempted
to characterize it with a variety of spectroscopic techniques to have a
better comparison with 2. In the following section, we have discussed
the results.
recorded in acetonitrile, at 298 K.
Group 4: Artemia were maintained alone for control. The mortality rate
was monitored every 1 h upto 72 h. Each group has maintained as 20
individuals in 20 mL of sterile seawater and this experiment was done in
triplicate. The sterility of this experiment was checked at the end of the
challenge; 1 mL of Artemia culture was mixed with 9 mL of marine broth
in a test tube and incubated for 48 h at 30 ◦C. The percentage of mor-
tality was calculated by using the formula [(number of mortality/Initial
number of survival) × 100)] [49].
The electronic absorption spectra of complexes 1 and 2 were recor-
ded in acetonitrile, at 298 K and the data are summarized in Table 2. The
typical spectra are presented in Fig. 1. Both the complexes exhibit two
broad absorption bands in the range of λ(nm), 511–534 and λ(nm),
801–794. The peaks are assigned to various transitions that are usually
observed in typical Ni(II) complexes, with the help of the Tanabe-
Sugano diagram. UV–Vis spectra of complex 1 perfectly matches with
the reported wavelengths and it reveals that we also isolated the same
octahedral coordinated complex 1 [39]. The lowest energy transition is
the 3A2g(F)→3T2g(F) (ν1), which gives rise to the broad absorption bands
in the range of 801–794 nm, while the 3A2g(F)→3T1g(F) (ν2) transition
gives rise to the bands in the range of 511–534 nm. Using ν1 and ν2
3. Results and discussion
3.1. Synthesis and characterization of the N3 ligands and nickel(II)
complexes
3
transitions, we have calculated the A2g(F)→3T1g(P) (ν3) transition for
The tridentate N3 ligands L1 and L2 were synthesized according to
the known procedures [38–42], which involves Schiff base condensation
of the corresponding aldehydes with amines followed by reduction with
sodium borohydride. The nickel(II) complexes 1 and 2 were prepared by
adding two equivalents of the ligand to an aqueous methanolic solution
of Ni(ClO4)2⋅6H2O. Both the complexes are formulated as [Ni(L)2]
(ClO4)2 based on UV–Vis, IR, paramagnetic NMR spectroscopic tech-
niques, ESI-MS and single-crystal X-ray structure analysis. In fact,
complex 1, has been reported previously in the literature [39]. Single-
crystal x-ray structure determination reveals that complex 2 also pos-
sesses distorted octahedral coordination geometry with two facial tri-
dentate ligand coordination as similar to complex 1 [39]. The thermal
behaviour of complexes 1 and 2 was examined using TGA and DSC and
they were found to be thermally stable up to 201˚C and 217˚C, respec-
tively. Both the complexes are expected to show promising antimicro-
bial activity against different pathogens (Gram-negative and Gram-
positive) due to the coordination of metal ion to heterocyclic ring
nitrogen.
both the complexes (1, 320: 2, 344 nm), however, it was not observed
experimentally and it might be overlapped with the ligand-based high
energy transitions. This data is in good accordance with that of the
previously reported octahedral Ni(II) complexes [39,50–55]. The ν1
band position depends on the donor atom type; thus, when the pyr-
idylmethyl arm in complex 1 is replaced an imidazolylmethyl arm to
obtain complex 2, the ν1 band is shifted slightly to higher energy because
of the strongly σ-bonding imidazole donor atom as expected based on
the literature [50–55]. The metal–ligand covalency parameter β was
calculated for both complexes (1, 0.82; 2, 0.62) [54]. The higher β value
for complex 1 can be attributed to the strong π-back donating ability of
the pyridine N atoms, as has been observed previously in other reported
complexes [39,50–55]. These observations confirm that the Ni(II)
complexes 1 and 2 have octahedral geometry in solution. In addition, we
1
have recorded the paramagnetic H NMR for both complexes and the
peaks were observed in the more downfield region, which again con-
firms the octahedral coordination geometry in solution (Figs. S5, and S6)
[39].
4

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Inorganica Chimica Acta 526 (2021) 120515
Fig. 3. (A) ORTEP diagram of [Ni(L2)2]2+ 2 showings 50% probability thermal ellipsoids and the labelling scheme for selected atoms. All the hydrogen atoms are
omitted for clarity. (B) A crystal packing diagram of 2, viewed along the c axis.
3.3. Infrared Spectroscopy
The infrared (IR) spectra of the ligands and complexes were recorded
by employing the Attenuated Total Reflection (ATR) technique. Both the
ligands exhibit the principal peaks in the range of 3333 – 3342 cmꢀ 1 and
748 – 750 cmꢀ 1, that have been assigned to the N H and C
–
–
N
stretching frequencies, respectively (Figs. 2 and S2). The peak at 3333
cmꢀ 1 of Ligand L1 and 3342 cmꢀ 1 of Ligand L2 is very broad, owing to
–
hydrogen bonding between the N H and H2O. These peaks were
observed to be slightly shifted in the corresponding complexes 1 and 2.
The decrease in N H stretching frequency (1, 3282 from 3333 cmꢀ 1; 2,
–
3304 from 3342 cmꢀ 1) and its sharp nature indicates complex formation
–
without hydrogen bonding and the increase in C N stretching fre-
quency to (1, 753 from 748 cmꢀ 1; 2, 758 from 750 cmꢀ 1) is explained by
the attraction of the aromatic ring electrons toward nitrogen, due to a
deficiency in its electron density upon coordination to the nickel(II)
center. Also, the broad intense peak around for 1, 1052 cmꢀ 1; 2, 1060
cmꢀ 1, sharp intense peak at 618 nm and the weaker absorption peak
around the range (1, 910 cmꢀ 1; 2, 901 cmꢀ 1) indicates the presence of
the uncoordinated ClO-4 ions [56].
3.4. Description of the structure of [Ni(L2)2](ClO4)2 2 and comparison
with [Ni(L1)2](ClO4)2
1
The molecular structure of complex cation of [Ni(L2)2](ClO4)2 2 is
shown in Fig. 3A, together with the atom numbering scheme and the
principal bond length and bond angles are collected in Table 1. The
complex cation of 2 possesses a NiN6 coordination sphere with distorted
octahedral coordination geometry where two pyridine of the 1 is
replaced by two imidazole groups. Both the ligands are facially coordi-
nated with the two coordinating secondary amino- and two imidazolyl
groups being cis to each other. The Ni–Npy (2.080, 2.093 Å), Ni–Namine
(2.159, 2.166 Å) and Ni–Nim (2.070, 2.072 Å) bond distances fall in the
ranges observed for the previously reported nickel(II) complexes (Ni–
Fig. 4. (A) Thermogravimetric curve for 2, Derivative (weight%) curve for 2,
under inert N2 atmosphere up to 600˚C (Inset) (B) DSC curve for 2 under inert N2
atmosphere up to 350˚C.
N
im, 1.999–2.134 Å) [50,54,55,57,58]. The Ni–N3im bond length (2.072
Å) is shorter than the Ni–Npy bond lengths (2.080 Å) as the coordination
of imidazole is stronger than that of pyridine donor [pKa (BH+): pyri-
dine, 5.2; imidazole, 7.01]. The Ni–Npy (2.080, 2.093 Å) and Ni-
(2)◦) bond angles deviate from the ideal octahedral angles of 90◦ and
180◦, respectively, revealing that there is significant distortion in octa-
hedral coordination geometry around Ni(II). A crystalline arrangement
of complex 2 viewed along the c axis is illustrated in Fig. 3B. It is
noteworthy that the oxygen atoms in one of the ClO-4 anions were found
N
im(2.070, 2.072 Å) bond lengths are shorter than the Ni–Namine bond
lengths (2.159, 2.116 Å) due to sp2 and sp3 hybridization of pyridyl,
imidazolyl and amine nitrogen donors, respectively.
The N–Ni–N (79.3(2)◦–101.3(2)◦) and N–Ni–N (167.48(19)◦– 175.8
5

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Inorganica Chimica Acta 526 (2021) 120515
Fig. 5. In vitro antimicrobial studies of complex 1 with (A) Vibrio parahaemolyticus, (B) Vibrio alginolyticus, (C) Pseudomonas aeruginosa PA14, (D) Pseudo-
monas aeruginosa PAO1, (E) Staphylococcus aureus (MRSA), (F) Candida albicans, (G) Salmonella typhi.
to exhibit multiple equilibria.
Table 3
The complex cation of 1 possesses a NiN6 coordination sphere with a
The minimal inhibitory concentration of complexes 1 and 2.
distorted octahedral coordination geometry constituted by four pyridyl
S.
Organisms
MIC of 1 (µg/
MIC of 2 (µg/
and two secondary amine nitrogen atoms from two tridentate ligands
[39]. The ligand L1 is facially coordinated, conferring a cis coordination
geometry on Ni(II). The Ni–Npy bond length (2.102(5) Å) is shorter than
the Ni–Namine bond lengths (2.106(4), 2.113(5) Å) due to sp2 and sp3
hybridization of pyridyl and amine nitrogen donors, respectively [39].
The N–Ni–N (80.77(17)◦–104.21(16)◦) and N–Ni–N (167.56(14)◦–
173.93(19)◦) bond angles deviate from the ideal octahedral angles of
90◦ and 180◦, respectively, revealing that the octahedral coordination
geometry around Ni(II) is significantly distorted [39].
No.
mL)
mL)
1
2
3
V. parahaemolyticus (TFM 1)
V. Alginolyticus (TFM 17)
Staphylococcus aureus
(ATCC11632)
125 ± 13
90 ± 7
–
–
140 ± 12
85 ± 9
4
5
Salmonella typhi (MTCC733)
Candida albicans
80 ± 9
20 ± 5
130 ± 10
110 ± 7
aeruginosa (PAO1), Pseudomonas aeruginosa (PA14), and Salmonella typhi
(MTCC733), Gram-negative marine pathogens: Vibrio parahaemolyticus
(TFM1) and Vibrio alginolyticus (TFM17), Gram-positive bacterium
Staphylococcus aureus (ATCC11632), as well as fungi, Candida albicans.
In the present study, both the complexes exhibited antibacterial activity
against both Gram-positive and Gram-negative pathogens (Fig. 5 and
Fig. S8). The molecules that show wide activity against bacteria and
fungi will have a broader application. Complex 1 displayed activity
against all the pathogens tested except for P. aeruginosa. Whereas,
complex 2 displayed activity against S. typhi, S. aureus and C. albicans.
Both the complexes inhibited the growth of C. albicans while none of the
complexes inhibited the growth of P. aeruginosa. As the concentration
increases, there was a significant increase in antimicrobial activity for
both the complexes. Table 3 shows the MIC of both the complexes
against different pathogens tested. The antifungal activity of complex 1
and 2 is more pronounced than their antibacterial activity. Complex 1
inhibited the growth of C. albicans and has a better MIC when compared
to the reported nickel complexes of C and E (Scheme 1). However,
complex 2 has a reasonable MIC against C. albicans when compared with
these complexes [20,22].
3.5. Thermal studies (TGA/DSC)
The thermal analysis of complexes [Ni(L1)2](ClO4)2 1 and [Ni(L2)2]
(ClO4)2 2 was carried out under an inert atmosphere by heating at a rate
◦
of 5 C. According to the TG curve, both the nickel(II) complexes are
◦
stable upto 200 C. Complex 1 loses weight in two steps as shown in
◦
◦
Fig. S7A. The first step spans between 201 C and 278 C, which cor-
responds to 1.4%. The second thermolytic step starts at 206 ◦C and ends
◦
at 414 C with 48% weight loss and it can be due to the loss of four
pyridine molecules (Fig. S7A). The sharp exothermic peak at 272 ◦C in
the DSC curve of 1 gives the total heat effect of the process (Fig. S7B).
For complex 2, the first step spans between 217 ◦C and 328 ◦C, corre-
sponds to a weightloss of 31.8% and can be due to the loss of two
perchlorate molecules (Fig. 4A). The second thermolytic step starts at
328 ◦C and ends at 427 ◦C with 13.6% weight loss. The total heat effect
of the above process is observed as a sharp exothermic peak at 272 ◦C in
the DSC curve of 1 and 302 ◦C in the DSC curve of 2 shown in Fig. 4B.
The derivative (W%) curve in Fig. S7A (Inset) and Fig. 4A (Inset) gives
the exact temperature value in the form of the endothermic peak for
complex 1 and 2 respectively.
One of the most common modes of action of antimicrobials is cell
membrane disruption and here the increased lipophilicity of the central
metal atom by chelation would facilitate the penetration through the
lipid bilayer of cell membrane [20,22]. The relation between the ligand
system and antimicrobial activity is evident from the MIC value
(Table 3), which increased in the case of S.typhi and C.albicans and
decreased for S. aureus when one of the pyridyl donor in 1 was replaced
3.6. Antimicrobial activity
Complexes 1 and 2 were tested for the in vitro antimicrobial activity
by well diffusion method against Gram-negative bacteria: Pseudomonas
6

A. Das et al.
Inorganica Chimica Acta 526 (2021) 120515
Fig. 6. Survival graphs of TFM1 and TFM17 showing the rescued survival of brine shrimp larvae supplemented with 150
μ
g mLꢀ 1of 1.
by imidazolyl donor to obtain 2. Further, imidazole interacts with re-
ceptors and enzymes of microbes through different interactions such as
inhibiting NO dioxygenase (NOD) function by coordinating to fla-
vohemoglobin (flavoHb) which induce antimicrobial activity and
particularly the fungicidal effect is correlated with the inhibition of the
dimorphic transition, ergosterol biosynthesis and mitochondrial activity
[59–61].
reported Ni(II) complexes along with the paramagnetic 1H NMR spectra
helps in confirming the octahedral geometry of complexes 1 and 2 in
solution. The higher β value for complex 1 is attributed to the strong
π
-back donating ability than complex 2 and, it is in accordance with the
literature [39]. Further, degradation of the complexes occurs in
consecutive reactions, which is determined by thermal analysis. Overall,
the present study concluded that both complexes 1 and 2 have antibiotic
activity against the tested pathogens. The antifungal activity of complex
1 is more pronounced. In vivo studies showed that complex 1 is effective
in preventing Vibrio infection in brine shrimp. This could ultimately
improve the aquaculture productivity by preventing Vibrio infections in
Artemia.
3.7. Toxicity and therapeutic potential of complex 1 to brine shrimp
larvae against Vibrio spp.
Here, the ability of complex 1 to protect the model animal gnoto-
biotic Artemia franciscana from pathogenic Vibrio isolates TFM1 and
TFM17 was investigated. Besides, toxicity of complex 1 on Artemia also
CRediT authorship contribution statement
examined. Complex 1 showed no toxicity on Artemia at 20
μ
g mLꢀ 1 and
Athulya Das: Data curation, Formal analysis, Investigation, Meth-
odology, Project administration, Resources, Software, Writing – original
draft, Writing – review & editing. Anjana Rajeev: Data curation,
Investigation, Methodology, Project administration, Resources, Soft-
ware. Sarmistha Bhunia: Data curation, Investigation, Methodology,
Project administration, Resources, Software. Manivel Arunkumar:
Data curation, Investigation, Methodology, Project administration, Re-
sources, Software. Nithya Chari: Formal analysis, Investigation, Meth-
odology, Project administration, Resources, Software, Supervision,
Validation, Visualization, Writing – original draft, Writing – review &
editing. Muniyandi Sankaralingam: Conceptualization, Formal anal-
ysis, Funding acquisition, Investigation, Methodology, Project admin-
istration, Resources, Software, Supervision, Validation, Visualization,
Writing – original draft, Writing – review & editing.
150
μ
g mLꢀ 1. Vibrio spp. TFM1 and TFM17 caused 80–100% and
90–100% mortality respectively in Artemia in the absence of complex 1.
Complex 1 radically improved the survival of Artemia up to 75%
(against TFM1) and 85% (against TFM17) (Fig. 6). For in vivo challenges,
the brine shrimp larvae were used as a bio-monitor animal model system
to detect the impact of complex 1 against Vibrio infection. Due to the
antimicrobial activity of complex 1 against Vibrio spp, the survival rate
of brine shrimp larvae was not affected in the presence of complex 1.
Moreover, complex 1 also protected the larvae from the colonization of
Vibrio pathogens in their gut. These initial findings would be helpful to
guide the future research in finding the most optimal application in the
field of aquaculture to combat against multi-drug resistant pathogens.
4. Conclusion
Declaration of Competing Interest
The tridentate (N3) ligands bis(pyridin-2-ylmethyl)amine (L1) and
1-(1-methyl-1H-imidazol-2-yl)-N-(pyridin-2-ylmethyl)methanamine
(L2) and their corresponding Ni(II) complexes have been synthesized
and characterized by UV–Visible, IR, paramagnetic 1H NMR spectro-
scopic techniques, mass spectrometry and single-crystal X-ray and
thermal analysis. The crystal structure of complex 2 has been found to
possess a distorted octahedral geometry by the facial coordination of
two tridentate N3 ligands to nickel(II) center as similar to the complex 1
[39]. UV–Visible spectral pattern which is similar to that of previously
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
AD and AR acknowledge the DST-Inspire (IF-170136), and UGC-JRF
7

A. Das et al.
Inorganica Chimica Acta 526 (2021) 120515
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8