Half-sandwich platinum group metal complexes containing coumarin-N-acylhydrazone hybrid ligands: Synthesis and biological evaluation studies

Article abstract of DOI:10.1016/j.ica.2021.120459

A series of half-sandwich platinum group metal complexes containing coumarin-N-acylhydrazone ligands have been prepared. The metal precursors of the type [(p-cymene)RuCl₂]₂ and [Cp*MCl₂]₂ (M = Rh/Ir) and coumarin-N-acylhydrazone ligands (L1, L2 and L3) were reacted in the ratio of 1:2 (M:L), forming neutral bidentate (N ∩ O) complexes (1–9). The complexes are of the general formula [(arene)M{κ²_(N∩O)L}Cl]. All these complexes have been characterized by analytical, spectroscopic and single-crystal X-ray diffraction studies. The complexes and ligands were then carried out for antibacterial, antioxidant and DNA binding studies. The results show that both ligands and complexes possess potent antibacterial and antioxidant properties. ___________________________________________________________________________

Full text of DOI:10.1016/j.ica.2021.120459

Inorganica Chimica Acta 525 (2021) 120459
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Inorganica Chimica Acta
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Research paper
Half-sandwich platinum group metal complexes containing
coumarin-N-acylhydrazone hybrid ligands: Synthesis and biological
evaluation studies☆
Carley Giffert L. Nongpiura, Lincoln Dkhara, Deepak Kumar Tripathib, Krishna Mohan Polurib,
,
*
Werner Kaminskyc, Mohan Rao Kolliparaa
a Center for Advanced Studies in Chemistry, North-Eastern Hill University, Shillong 793 022, India
b Department of Biotechnology and Centre for Nanotechnology, Indian Institute of Technology Roorkee, Roorkee 247 667, India
c Department of Chemistry, University of Washington, Seattle, WA 98195, USA
A R T I C L E I N F O
A B S T R A C T
Keywords:
A series of half-sandwich platinum group metal complexes containing coumarin-N-acylhydrazone ligands have
been prepared. The metal precursors of the type [(p-cymene)RuCl2]2 and [Cp*MCl2]2 (M = Rh/Ir) and coumarin-
N-acylhydrazone ligands (L1, L2 and L3) were reacted in the ratio of 1:2 (M:L), forming neutral bidentate (N ∩
O) complexes (1–9). The complexes are of the general formula [(arene)M{κ2(N∩O)L}Cl]. All these complexes have
been characterized by analytical, spectroscopic and single-crystal X-ray diffraction studies. The complexes and
ligands were then carried out for antibacterial, antioxidant and DNA binding studies. The results show that both
ligands and complexes possess potent antibacterial and antioxidant properties.
Rhodium
Iridium
coumarin-N-acylhydrazone
Antibacterial
Antioxidant studies
___________________________________________________________________________
1. Introduction
to their relative chemical inertness [13].
The use of heterocyclic compounds as chelating ligands provides a
Over the years, there has been an enormous expansion in the field of
organometallic chemistry due to the diverse roles that metals play in
biological systems [1–4]. Several contributing factors such as high sta-
bility, high yields, less toxicity, robustness, versatility to exhibit various
coordination modes and mild reaction conditions have led to an increase
in the study of metals like ruthenium, rhodium and iridium [5]. In
particular, arene ruthenium complexes, owing to their promising anti-
cancer activity both in vitro and in vivo, have attracted great interest
[6–9]. They have also found applications in catalysis, supramolecular
assemblies and molecular devices and have shown antiviral, antibiotic
and antibacterial activities [10–12]. Transition metal drug complexes
are important in medical research due to their interaction with DNA.
These metal complexes can target selectively particular DNA sites,
which form the basis of many anticancer and antiviral drugs. Of late,
several pentamethylcyclopentadienyl rhodium and iridium complexes
have also been examined as prospective anticancer drugs on account of
their exciting properties. Besides, they offer a high level of stability due
new strategy in the design of new metallodrugs. Coumarins are an
important class of naturally occurring heterocyclic compounds also
known as chromen-2-one or benzopyrone. They can also be considered
as lactones or cyclic esters. More than 1000 different types of coumarins
have been isolated from natural sources [14]. The biological interest in
coumarins derives from their versatile applications such as anticonvul-
sant, antidepressant, analgesic, anti-inflammatory, antimicrobial, anti-
malarial, antitumoral, antileukemic, antiviral, antitubercular as well as
antioxidant activity [15–16]. They are also known to exhibit excellent
fluorescence and DNA binding properties. Moreover, the derivatives of
coumarin are considered essential for presenting anticancer properties.
More specifically, 3-acetylcoumarin [17] shows a variety of biological
properties and attracted intense interest as the acetyl group can be
involved in Schiff base condensation with amines to give interesting
biologically active compounds.
Considering the potential significance of coumarins and aromatic
acid hydrazides, we are encouraged to synthesize coumarin-N-
☆
Dedicated to Prof. M. Palaniandavar to recognize the contributions on the occasion of his 70th birthday.
* Corresponding author.
E-mail address: mohanrao59@gmail.com (M.R. Kollipara).
https://doi.org/10.1016/j.ica.2021.120459
Received 17 February 2021; Received in revised form 21 April 2021; Accepted 14 May 2021
Available online 16 May 2021
0020-1693/© 2021 Elsevier B.V. All rights reserved.

C.G.L. Nongpiur et al.
Inorganica Chimica Acta 525 (2021) 120459
acylhydrazone ligands, which can act as bidentate NO or tridentate ONO
donor ligands upon complexation with metals. Herein, we report the
synthesis of three coumarin-N-acylhydrazone ligands and their corre-
sponding ruthenium, rhodium and iridium complexes. Since the ligands
and complexes exhibit efficient fluorescent property, their binding
property towards DNA has been studied which can prove beneficial in
medicinal fields. In this present work, we have also investigated the
antimicrobial and antioxidant studies for all the synthesized complexes.
Ligands used in this study are presented in Chart 1.
Pseudomonas aeruginosa) bacterial strains. The agar nutrient broth was
prepared and sterilized at 121 ◦C for 15 min. The chosen bacterial strains
were inoculated in nutrient broth and incubated overnight. Petri plates
containing 30 ml of fresh Muller Hinton (M.H.) agar medium was seeded
with a 24 h grown culture of bacterial strains. Wells of 5 mm diameter
were cut and 100 ml of each test compound was added. Following an
incubation period of the plates at 37 ◦C for 24 h, the antibacterial ac-
tivities were evaluated by measuring the diameter of the inhibition zone
formed around the well. Each experiment was performed in triplicate.
For well diffusion assay 5 mg mLꢀ 1 of test compounds were used. DMSO
was used as a negative control and the antibiotic kanamycin was applied
as a positive control drug.
2. Experimental section
2.1. Physical methods and materials
2.3. DPPH free radical scavenging assay
All the reagents required were obtained from commercial sources
and used as received. Salicylaldehyde, 4-(Diethylamino) salicylalde-
hyde, 2-hydroxy-1-naphthaldehyde, ethyl acetoacetate, piperidine, 4-
hydroxybenzhydrazide and 3-methoxybenzhydrazide were acquired
from Alfa Aesar. The solvents were purified and dried before use ac-
cording to the standard procedures. Ligands were prepared according to
a known method (Scheme 1). Starting metal precursor complexes
[Cp*MCl2]2 (M = Rh/Ir) were prepared according to the new procedure
[18] by Anton Paar Monowave 50. 1H NMR spectra were recorded on a
Bruker Avance II 400 MHz spectrometer using CDCl3 and DMSO‑d6 as
solvents; chemical shifts were referenced to TMS. Infrared spectra (KBr
pellets; 400–4000 cmꢀ 1) were recorded on a Perkin-Elmer 983 spec-
trophotometer. Mass spectra were recorded with a Waters UPLC-TQD
Mass spectrometer using acetonitrile as solvent. Absorption spectra
were recorded on a Perkin – Elmer Lambda 25 U.V./Visible spectro-
photometer in the range of 200–800 nm at room temperature in
acetonitrile.
DPPH (2,2-diphenyl-1-picryl-hydrazyl) free radical scavenging assay
was performed as described by Blois [19]. This method is based on the
measurement of the scavenging capacity of antioxidants towards DPPH.
Methanol solution of DPPH (1 ml of 0.004%) was added to 250 ml of the
tested compounds dissolved in DMSO at a concentration of 1 mg/ml.
The mixtures were vortexed thoroughly and kept in the dark for 30 min
at room temperature. Then absorbance was measured at 517 nm using a
UV–VIS spectrophotometer. For control, DMSO was added to DPPH. An
ascorbic acid solution at the same concentration (1 mg/mL) was used as
a standard. All the tests were performed in triplicates. The potency of
scavenging the DPPH radical was calculated by measuring the per-
centage inhibition, i.e., % DRSA = {(A0 -A1)/A0) × 100} where A0 is the
absorbance of the control reaction, and A1 is the absorbance of the
sample considered.
2.4. Fluorescence studies on DNA interaction
2.2. In vitro antimicrobial assay
DNA binding experiments of the ligand (L2) and complexes (4, 5 and
6) were carried out using a Fluorometric assay. All experiments were
done with a fixed concentration (10 µM) of the compounds under study
while gradually increasing the Salmon milt (S.M.) DNA concentration
(5–100 µM). For the acquisition of fluorescence emission spectrum at
each concentration of DNA, the parameters used were, λex = 429 nm;
The antimicrobial activity of the ligands L1-L3 and the newly syn-
thesized complexes was evaluated by the agar well diffusion method.
The test organisms included two Gram-positive (Staphylococcus aureus
and Bacillus thuringiensis) and two Gram-negative (Escherichia coli and
Chart 1. Ligands used in this study.
2

C.G.L. Nongpiur et al.
Inorganica Chimica Acta 525 (2021) 120459
Scheme 1. Preparation of Ligands.
λem = 473 nm for ligand L2, and λex = 430 nm; λem = 473 nm for
complexes 4, 5 and 6. The dissociation constant was quantitatively
analyzed using a ligand-binding model as described elsewhere [20].
temperature in dry methanol (10 ml) for 4 h (Scheme 2). The product
precipitated out from the reaction mixture after stirring for 1 h and
stirring was continued for another 3 h to complete the reaction. The
precipitate was centrifuged, washed with cold methanol (2–5 ml) and
diethyl ether (2–10 ml) and air-dried.
2.5. General procedure for the synthesis of neutral metal complexes (1–9)
2.5.1. [(p-cymene)Ru(κ2(N∩O)L1)Cl] (1)
To
a solution of metal precursor [(p-cymene)RuCl2]2 and
Yield : 85%; Color: Orange; IR (KBr, cmꢀ 1): 3449 ν(OH), 1712 ν(C
,
O)
–
–
[Cp*MCl2]2 (M = Rh/Ir) complexes (0.1 mmol), coumarin hydrazone
ligand (L1, L2 and L3) (0.2 mmol) were added and stirred at room
1
–
–
1608 ν(C O), 1567 ν(C N); H NMR (400 MHz, DMSO‑d6, ppm) δ 10.58
–
–
Scheme 2. Schematic representation for the syntheses of complexes.
3

C.G.L. Nongpiur et al.
Inorganica Chimica Acta 525 (2021) 120459
(s, 1H), 10.02 (s, 1H), 9.86 (d, J = 8 Hz, 1H), 7.77 (d, J = 8 Hz, 1H), 7.69
(d, J = 8 Hz, 1H), 7.62 (d, J = 8 Hz, 1H), 6.87 (d, J = 8 Hz, 1H), 6.78 (d,
J = 12 Hz, 1H), 6.00 (d, J = 4 Hz, 1H), 5.88 (d, J = 8 Hz, 1H), 5.81 (d, J
= 4 Hz, 1H), 5.74 (t, J = 8 Hz, 2H), 5.64 (d, J = 4 Hz, 1H), 2.74 (sept, J =
8 Hz, 1H), 2.15 (s, 1H), 2.09 (s, 3H), 1.25 (d, J = 8 Hz, 2H), 1.21 (d, J =
4 Hz, 2H), 1.18 (d, J = 8 Hz, 2H); 13C NMR (100 MHz, CDCl3): δ 159.50,
155.56, 154.09, 147.74, 134.65, 131.06, 130.47, 129.71, 125.22,
118.49, 116.93, 115.01, 30.78, 22.26, 21.98, 19.17; UV–Vis {Acetoni-
Hz, 1H), 7.38 (s, 1H), 7.25 (t, J = 8 Hz, 1H), 6.95 (d, J = 8 Hz, 1H), 5.56
(d, J = 4 Hz, 1H), 5.32 (d, J = 4 Hz, 1H), 5.20 (d, J = 4 Hz, 1H), 4.09 (d,
J = 4 Hz, 1H), 3.86 (s, 3H), 2.85 (s, 3H), 2.53 (sept, J = 8 Hz 1H), 1.92 (s,
3H), 1.05 (d, J = 8 Hz, 2H), 1.11 (d, J = 8 Hz, 2H); UV–Vis {Acetonitrile,
λmax nm (
ε
, 104 Mꢀ 1 cmꢀ 1)}: 253 (3.755), 321 (2.670), 365 (2.619));
ESI-MS (m/z): 657.3 [M + 1]+, 621.3 [Mꢀ Cl]+.
2.5.8. [Cp*Rh(κ2(N∩O)L3)Cl] (8)
–
trile, λmax nm (
ε
,104 Mꢀ 1 cmꢀ 1)}: 266 (2.495), 361 (1.352), 441 (1.235).
Yield : 85%; Color: Orange; IR (KBr, cmꢀ 1): 1721 ν(C O), 1630
–
1
–
–
–
–
ν(C O), 1590 ν(C N), 1489–1454 ν(C C), 1138 ν(C O); H NMR (400 MHz,
–
–
–
2.5.2. [Cp*Rh(κ2(N∩O)L1)Cl] (2)
CDCl3 + DMSO‑d6, ppm) δ 10.05 (s, 1H), 8.89 (d, J = 8 Hz, 1H), 8.21 (d,
J = 4 Hz, 1H), 8.02 (d, J = 12 Hz, 2H), 7.81 (t, J = 8 Hz, 1H), 7.74 (d, J =
4 Hz, 1H), 7.68 (s, 1H), 7.64 (d, J = 4 Hz, 1H), 7.59 (d, J = 8 Hz, 1H),
7.29 (t, J = 8 Hz, 1H), 6.99 (d, J = 8 Hz, 1H), 3.85 (s, 3H), 2.75 (s, 3H),
Yield : 90%; Color: Orange; IR (KBr, cmꢀ 1): 3445 ν(OH), 1723 ν(C
,
–
O)
–
1
–
–
1608 ν(C O), 1569 ν(C N); H NMR (400 MHz, CDCl3 + DMSO‑d6, ppm) δ
–
–
8.51 (s, 2H), 7.64 (m, (d, J = 4 Hz, 5H), 7.35 (d, J = 12 Hz, 2H), 7.32 (d,
J = 12 Hz, 1H), 2.73 (s, 3H), 1.58 (s, 15H); 13C NMR (100 MHz, CDCl3): δ
147.73, 144.24, 134.65, 131.04, 130.66, 130.46, 128.94, 125.22,
118.48, 116.93, 116.83, 115.12, 30.78, 27.33, 9.59, 9.24, 9.09; UV–Vis
1.37 (s, 15H); UV–Vis {Acetonitrile, λmax nm (
ε
,104 Mꢀ 1 cmꢀ 1)}: 252
(4.317), 375 (3.023); ESI-MS (m/z): 659.3 [M + 1]+, 623.3 [Mꢀ Cl]+.
2.5.9. [Cp*Ir(κ2(N∩O)L3)Cl] (9)
{Acetonitrile, λmax nm (
ε
,104 Mꢀ 1 cmꢀ 1)}: 260 (3.646), 330 (2.848), 432
Yield : 75%; Color: Yellow; IR (KBr, cmꢀ 1): 1720 ν(C O), 1629
–
(1.434).
–
1H NMR (400
–
–
–
–
;
O)
ν(C O), 1590 ν(C N), 1489–1454 ν(C
C),
–
1138 ν(C
–
–
2.5.3. [Cp*Ir(κ2(N∩O)L1)Cl] (3)
MHz, CDCl3 + DMSO‑d6, ppm) δ 9.91 (s, 1H), 8.69 (d, J = 8 Hz, 1H),
8.29 (d, J = 12 Hz, 1H), 8.25 (d, J = 4 Hz, 1H), 8.09 (d, J = 8 Hz, 1H),
7.82 (t, J = 8 Hz, 1H), 7.72 (d, J = 8 Hz, 1H), 7.66 (t, J = 8 Hz, 2H), 7.34
(t, J = 8 Hz, 1H), 7.05 (d, J = 8 Hz, 1H), 3.84 (s, 3H), 2.86 (s, 3H), 1.31
Yield : 87%; Color: Yellow; IR (KBr, cmꢀ 1): 3440 ν(OH), 1718 ν(C
,
–
O)
–
1
–
–
1608 ν(C O), 1564 ν(C N); H NMR (400 MHz, CDCl3 + DMSO‑d6, ppm) δ
–
–
10.12 (s, 1H), 9.56 (d, J = 16 Hz, 1H), 9.27 (d, J = 8 Hz, 1H), 7.83 (d, J
= 8 Hz, 2H), 7.71 (d, J = 8 Hz, 1H), 7.56 (d, J = 8 Hz, 1H), 6.83 (m, J =
8 Hz, 3H), 2.59 (s, 3H), 1.74 (s, 15H); 13C NMR (100 MHz, CDCl3): δ
147.74, 134.65, 131.30, 130.66, 130.47, 125.23, 124.76,118.49,
116.93, 115.12, 100.21, 86.06, 85.81, 30.78, 9.30, 9.11, 8.36; UV–Vis
(s, 15H); UV–Vis {Acetonitrile, λmax nm (
ε
, 104 Mꢀ 1 cmꢀ 1)}: 214
(9.864), 304 (3.197), 366 (3.161); ESI-MS (m/z): 749.4 [M + 1]+, 713.4
[Mꢀ Cl]+.
{Acetonitrile, λmax nm (
ε
, 104 Mꢀ 1 cmꢀ 1)}: 275 (3.454), 312 (2.917),
2.6. Structure determination by single-crystal X-ray analyses
445 (1.304).
The solvent diffusion method was used for growing single crystals.
Suitable single crystals for X-ray analysis have been obtained for 1, 7, 8
and 9 in a dichloromethane-hexane mixture. Single crystal data for the
complexes were collected with an Oxford Diffraction Xcalibur Eos
2.5.4. [(p-cymene)Ru(κ2(N∩O)L2)Cl] (4)
Yield : 85%; Color: Orange; IR (KBr, cmꢀ 1): 1723 ν(C O), 1663 ν(C
,
O)
–
–
–
–
1
–
1570–1617 ν(C N); H NMR (400 MHz, CDCl3, ppm) δ 8.52 (s, 1H), 7.48
–
(d, J = 8 Hz, 2H), 7.20 (s, 1H), 6.70 (d, J = 8 Hz, 2H), 6.55 (s, 2H), 5.72
(d, J = 4 Hz, 2H), 5.64 (d, J = 4 Hz, 2H), 3.97 (s, 3H), 3.54 (q, J = 8 Hz,
4H), 3.12 (sept, J = 8 Hz, 1H), 2.76 (s, 6H), 2.32 (t, J = 12 Hz, 6H), 1.38
(d, J = 4 Hz, 6H); 13C NMR (100 MHz, CDCl3): δ 173.11, 161.03, 159.76,
158.91, 153.19, 148.03, 132.07, 129.98, 127.54, 121.10, 120.71,
116.27, 112.99, 110.03, 108.32, 102.01, 96.74, 84.30, 81.83, 79.93,
55.90, 45.30, 31.17, 30.72, 23.05, 22.35, 18.56, 12.60; UV–Vis
Gemini diffractometer using graphite monochromated Mo-Kα radiation
(λ = 0.71073 Å). The strategy for data collection was evaluated using the
CrysAlisPro CCD software [21]. Crystal data were collected by standard
“phi–omega scan” techniques and were scaled and reduced using Cry-
sAlisPro RED software. The structures were solved by direct methods
using SHELXS-97 and refined by full-matrix least-squares with SHELXL-
97, refining on F2 [22,23]. The positions of all atoms were obtained by
direct methods. Metal atoms in the complex were located from the E-
maps and all non-hydrogen atoms were refined anisotropically by full-
matrix least squares. Hydrogens were placed in geometrically ideal-
–
{Acetonitrile, λmax nm (
ε
,104 Mꢀ 1 cmꢀ 1)}: 256 (2.652), 428 (5.726).
2.5.5. [Cp*Rh(κ2(N∩O)L2)Cl] (5)
Yield : 90%; Color: Orange; IR (KBr, cmꢀ 1): 1718 ν(C O), 1660 ν(C
–
–
,
O)
–
ized positions and constrained to ride on their parent atoms with C
H
–
1
–
1570 ν(C N); H NMR (400 MHz, CDCl3, ppm) δ 8.43 (s, 1H), 7.48 (s,
distances of 0.95–1.00 Angstrom. Isotropic thermal parameters Ueq were
fixed such that they were 1.2Ueq of their parent atom; Ueq for C.H.’s are
1.5Ueq of their parent atom Ueq in case of methyl groups. Crystallo-
graphic and structure refinement parameters for the complexes are
summarized in Table 1 and selected bond lengths and angles are pre-
sented in Table 2. Fig. 1 was drawn with the ORTEP3 program [24].
–
1H), 7.44 (d, J = 4 Hz, 1H), 7.42 (s, 1H), 7.35 (t, J = 8 Hz, 1H), 7.12 (d, J
= 8 Hz, 1H), 6.64 (d, J = 12 Hz, 1H), 6.47 (s, 1H), 3.85 (s, 3H), 3.45 (q, J
= 8 Hz, 4H), 2.66 (s, 3H), 1.76 (s, 15H), 1.23 (t, J = 8 Hz, 6H); UV–Vis
{Acetonitrile, λmax nm (
ε
, 104 Mꢀ 1 cmꢀ 1)}: 248 (3.292), 421 (4.546);
ESI-MS (m/z): 644.59 [Mꢀ Cl]+, 541.1 [Mꢀ Rhꢀ HCl]+.
2.5.6. [Cp*Ir(κ2(N∩O)L2)Cl] (6)
3. Results and discussions
Yield : 82%; Color: Yellow; IR (KBr, cmꢀ 1): 1723 ν(C O), 1663 ν(C
–
–
,
O)
–
–
1
–
1565 ν(C N); H NMR (400 MHz, CDCl3, ppm) δ 8.44 (s, 1H), 7.41 (s,
3.1. Synthesis of metal complexes
–
1H), 7.39 (s, 1H), 7.05 (d, J = 8 Hz, 1H), 6.63 (d, J = 4 Hz, 1H), 6.60 (d,
J = 4 Hz, 1H), 6.47 (d, J = 4 Hz, 2H), 3.87 (s, 3H), 3.43 (q, J = 12 Hz,
4H), 2.68 (s, 3H), 1.63 (s, 15H), 1.22 (t, J = 12 Hz, 6H); UV–Vis
Treatment of d6 halide-bridged metal dimers [(p-cymene)RuCl2]2
and [Cp*MCl2]2 (M = Rh and Ir) with the coumarin hydrazone ligands in
1:2 ratio resulted in the formation of neutral mononuclear bidentate
chelated complexes 1–9. Earlier reports [25] have shown that ruthe-
nium complexes of hydrazone derivatives exhibited N∩O coordination
mode as neutral complexes by deprotonation of NH proton using a base
such as Et3N. But in our case, ruthenium complexes exhibited N∩O
bonding mode even without the usage of a base, as is seen in other
complexes of hydrazone derivatives [26]. The same is observed for
rhodium and iridium complexes. All these complexes were obtained in
{Acetonitrile, λmax nm (
ε
,104 Mꢀ 1 cmꢀ 1)}:247 (2.641), 420 (4.476).
2.5.7. [(p-cymene)Ru(κ2(N∩O)L3)Cl] (7)
Yield : 80%; Color: Yellow; IR (KBr, cmꢀ 1): 1711 ν(C O), 1634
–
–
ν(C O), 1615–1545 ν(C N), 1489–1454 ν(C
C),
–
1138 ν(C
1H NMR
–
;
O)
–
–
–
–
–
(400 MHz, CDCl3 + DMSO‑d6, ppm) δ 9.82 (s, 1H), 8.71 (d, J = 8 Hz,
1H), 8.14 (d, J = 8 Hz, 1H), 7.98 (d, J = 8 Hz, 1H), 7.83 (t, J = 8 Hz, 1H),
7.76 (d, J = 8 Hz, 1H), 7.71 (s, 1H), 7.64 (t, J = 8 Hz, 1H), 7.58 (d, J = 8
4

C.G.L. Nongpiur et al.
Inorganica Chimica Acta 525 (2021) 120459
Table 1
Crystal structure data and refinement parameters of complexes 1, 7, 8 and 9.
Complexes
[1]
[7]
[8]
[9]
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C
28H27ClN2O4Ru
C33 H31 Cl N2 O4 Ru
656.12
C33 H32 Cl N2 O4 Rh
658.96
C33 H32 Cl Ir N2 O4
748.25
592.03
293(2)
100(2)
100(2)
90(2)
0.71073
0.71073
0.71073
0.71073
triclinic
monoclinic
Orthorhombic
P b c a
Orthorhombic
P b c a
P
C 2/c
a (Å)/
α
(◦)
10.762(2)/ 66.10(3)
23.0837(4)/ 90
12.6818(3)/ 109.782(2)
20.5436(4)/ 90
5659.1(2)
12.7090(3)/ 90
21.1480(5)/ 90
21.6980(6)/ 90
5831.8(3)
12.7895(4)/ 90
21.1851(7)/ 90
21.6163(8)/ 90
5856.9(3)
b (Å)/β (◦)
11.002(2)/ 80.20(3)
c (Å)/γ (◦)
12.049(2)/ 71.13(3)
Volume (Å3)
Z
1233.0(6)
2
8
8
8
Density (calc) (Mg/mꢀ 3
Absorption coefficient
F(000)
)
1.595
1.540
1.501
1.697
0.783
0.691
0.718
4.691
604.0
2688
2704
2960
Crystal size (mm3)
0.35 × 0.19 × 0.05
3.702 to 53.768◦
ꢀ 13 ≤ h ≤ 13, ꢀ 13 ≤ k ≤ 13,ꢀ 15
≤ l ≤ 15
0.250 × 0.200 × 0.150
2.562 to 28.273◦.
ꢀ 30≤h≤30,
ꢀ 16≤k≤15,ꢀ 27≤l≤27
12,512
0.100 × 0.070 × 0.050
2.092 to 28.286◦
ꢀ 16≤h≤16,
ꢀ 28≤k≤28,ꢀ 28≤l≤28
13,539
0.250 × 0.200 × 0.100
1.884 to 28.294◦
ꢀ 17≤h≤17,
ꢀ 28≤k≤28,ꢀ 28≤l≤28
13,805
Theta range for data collection
Index ranges
Reflections collected
10,201
Independent reflections
5208 [Rint = 0.0598, Rsigma
=
6977 [R(int) = 0.0340]
7190 [R(int) = 0.0936]
7242 [R(int) = 0.0180]
0.0990]
97.9%
Completeness to theta = 25.00◦
99.7%
99.8%
99.7%
Absorption correction
Semi-empirical from equivalents
Semi-empirical from
equivalents
Semi-empirical from
equivalents
Semi-empirical from
equivalents
Refinement method
Full-matrix least-squares on F2
5208 / 0 /333
Full-matrix least-squares on F2
6977 / 0 / 375
Full-matrix least-squares on F2
7190 / 0 / 377
Full-matrix least-squares on F2
7242 / 0 / 377
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I greater than
2sigma(I)]
0.833
0.961
0.926
1.104
R1 = 0.0393, wR2 = 0.0741
R1 = 0.0341, wR2 = 0.0867
R1 = 0.0467, wR2 = 0.0746
R1 = 0.0240, wR2 = 0.0555
R indices (all data)Absolute
structure parameter
R1 = 0.0789, wR2 = 0.0837n/a
R1 = 0.0494, wR2 = 0.0950n/a
R1 = 0.1163, wR2 = 0.0884n/a
R1 = 0.0392, wR2 = 0.0651n/a
Largest diff. peak and hole (e.Åꢀ 3
CCDC No.
)
0.53 and ꢀ 1.17
0.911 and ꢀ 0.793
0.784 and ꢀ 0.959
2.370 and ꢀ 0.823
2,059,269
2,059,270
2,059,271
2,059,272
Structures were refined on F20: wR2 = [Σ[w(F20 - F2c)2] / Σw(F02)2]1/2, where w-1 = [Σ(F20)+(aP)2 + bP] and P = [max(F02, 0) + 2F2c]/3
and complexes, suggests that it is not involved in coordination, also
Table 2
confirmed from the crystal structures. In the case of free ligands,
Selected bond lengths (Å) and bond angles (◦) of complexes.
–
stretching frequencies corresponding to ν(C
of the amide group are
O)
–
found in 1663–1698 cmꢀ 1, whereas stretching frequencies due to ν(C
Complexes
1
7
8
9
–
N)
–
M(1)-CNT
1.770
1.677
1.770
1.773
occur in the range of 1554–1635 cmꢀ 1. These frequencies decrease upon
complexation, which suggests coordination takes place through the
azomethine nitrogen and carbonyl oxygen. The broad peak around
3428–3449 cmꢀ 1 can be attributed to the presence of O.H. groups in
ligand L1 and complexes 1–3. In all the free ligands, stretching fre-
quencies corresponding to the NH proton occur in the range of
3336–3463 cmꢀ 1 confirming that the ligands are present in the keto
form. It is noteworthy that in all the synthesized complexes, a band in
the range of 1608–1635 cmꢀ 1 characteristic of the azomethine group
M(1)-Cl(1)
2.4217(17)
2.098(3)
2.4231(6)
2.4240(8)
2.4227(7)
2.112(3)
M(1)-N(1)
M(1)-N(2)
2.1045(18)
2.0641(15)
2.133(3)
2.056(2)
M(1)-O(1)
2.054(3)
M(1)-O(3)
2.063(2)
N(1)- M(1)-O(1)
N(1)- M(1)-O(3)
N(2)- M(1)-O(1)
Cl(1)- M(1)-N(1)
Cl(1)- M(1)-N(2)
Cl(1)- M(1)-O(1)
Cl(1)- M(1)-O(3)
76.38(11)
75.62(9)
86.33(7)
75.68(6)
76.51(9)
86.33(10)
84.59(9)
88.10(5)
85.32(5)
88.62(7)
88.65(6)
– –
– –N-acylhydrazone
(-C N-N C-) is observed due to deprotonation and enolization of the
86.53(6)
coumarin
ligands and subsequent coordination
–
–
through the R = N–N C–O enolate form [27].
–
CNT represents the centroid of the p-cymene/Cp* ring and (M = Ru, Rh and Ir)
3.2.2. 1H NMR studies of complexes
good yields and are yellow or orange. They are non-hygroscopic and are
stable in the air as well as in a solution. These complexes show good
solubility in polar organic solvents like DCM, chloroform and acetoni-
trile, whereas they are insoluble in non-polar solvents like hexane,
diethyl ether and petroleum ether. The analytical data of these com-
pounds are consistent with the formulations. All the coumarin hydra-
zone complexes were characterized spectroscopically and the molecular
structures of the representative complexes were confirmed by single-
crystal X-ray analysis.
The 1H NMR of the ligands L1-L3 shows a singlet in the range of
9.99–10.87 ppm, which is assigned to the N.H. proton. For L1, the
singlet corresponding to the O.H. group is observed at 10.45 ppm. The
methoxy group OMe in L2 and L3 are observed as singlets in 3.83–3.84
ppm. The methyl protons are observed as a singlet in the range of
2.22–2.40 ppm. The signals for the diethylamino group are observed as a
triplet and a quartet at 1.13 ppm and 3.44 ppm, respectively. The signals
in the range of 6.57–9.30 ppm are assigned to the protons of the aro-
matic rings. The 1H NMR spectra of all the complexes 1–9 are in good
agreement with the proposed structures. The spectra of the metal com-
plexes exhibit that the ligand resonance signals are shifted either
downfield or upfield than that of the free ligands. This shift of proton
signals is because of the ligand coordination to the metal atom. The
absence of the N.H. peaks indicates the deprotonation of the ligands and
3.2. Spectral studies of the complexes
3.2.1. FT-IR studies
A sharp intensity band in 1711–1726 cmꢀ 1 is characteristic of ν(C
–
O)
–
stretching of the lactone ring. It is present in the spectra of all ligands
5

C.G.L. Nongpiur et al.
Inorganica Chimica Acta 525 (2021) 120459
Fig. 1. ORTEP generated molecular structures of complexes 1, 7, 8 and 9 with 50% thermal ellipsoid probability. Hydrogen atoms (except on O2) are omitted for
clarity purposes.
the existence of enolate form upon complexation. This confirmed the
coordination of the ligands to the metals through the enolate-Oꢀ , as is
evident from the crystal structures. For complexes 1 and 3, the signal
assignable to the O.H. group is observed as singlets in the range of
10.12–10.58 ppm. For complex 2, this signal is observed at 8.51 ppm,
overlapping with the C.H. signal of coumarin moiety. The appearance of
the OH signal indicates that the hydroxyl group is not involved in
bonding to the metal atom. The signals for the diethylamino group are
observed as triplets and quartets in the range of 1.22–2.32 ppm and
3.43–3.54 ppm, respectively. For methoxy group OMe, a sharp singlet is
observed in the range of 3.84–3.97 ppm. The methyl group protons of
the ligand part appeared as a singlet around 2.15–2.97 ppm. For com-
plex 4, this signal overlap with the methyl group protons of a p-cymene
moiety at 2.76 ppm. Resonances due to all the aromatic protons of the
ligands are observed around 5.74–10.02 ppm. In p-cymene complexes 1,
4 and 7, a sharp singlet due to the methyl group of the p-cymene moiety
is observed in the range of 1.92–2.76 ppm. The methine proton
exhibited a septet around 2.52–3.12 ppm. Usually, the two methyl
groups of isopropyl substituents are observed as one doublet and the
aromatic protons of the p-cymene ring are observed as two doublets, as
seen in the spectrum of complex 4. But in complex 1, the methyl protons
of the isopropyl group appear as three doublets around 1.18 ppm, while
for complex 7, the methyl protons of the isopropyl group appear as two
doublets around 1.06 ppm. Also, the aromatic protons of the p-cymene
ring appeared as four doublets in the range 4.10–6.01 ppm. This unusual
splitting pattern is ascribed to the diastereotopic and chiral nature of the
metal center or loss of symmetry of the p-cymene group upon coordi-
nation with the ligands [28,29]. In addition to these proton signals, a
sharp singlet is observed around 1.31–1.76 ppm for the methyl protons
of the Cp* ligand for the rhodium and iridium complexes. The 1H NMR
spectra of all the ligands and complexes are given in supplementary data
(Figure S1-S12).
the supplementary data (Figures S13-16). In the 13C NMR spectra of the
complexes, the signals in the range of 155.56–173.11 ppm are related to
–
the ester C O group while the signals in the range of 147.74–158.91
–
–
–
ppm are related to the imine C N group. For complex 4, the signal due
to methoxy carbon is observed at 55.90 ppm. The signals due to meth-
ylene carbons and methyl carbons of the diethylamino group are
observed at 45.30 and 12.60 ppm, respectively. The aromatic carbon
resonances are observed in the range of 112.99–134.65 ppm. The ring
carbon resonances of the p-cymene ligand are observed around
96.74–118.49 ppm. The methyl, methine and isopropyl carbons of p-
cymene ligand exhibit signals in the range between 9.11 and 31.17 ppm.
For complexes 2 and 3, the signals associated with the ring carbons of
the Cp* ligand are observed in the range of 85.81–94.40 ppm.
3.2.4. Mass studies of the complexes
The formation of the synthesized complexes is further supported by
studying the mass spectra of some of the complexes. This study helps in
establishing the structure by giving the exact molecular mass or by
revealing the presence of certain structural units in a molecule. The mass
spectra obtained agree well with the theoretical masses of the com-
plexes. The mass spectra of complexes 5, 7–9 are given in supplementary
data (Figures S17-20). Complex 5 shows a molecular ion peak at m/z
value 644.59, which corresponds to [Mꢀ Cl]+ ion peak. Complexes 7–9
shows prominent molecular ion peaks at m/z values 657.3 and 621.3(7),
659.3 and 623.3(8) and 749.4 and 713.4 (9) identifiable to [M + 1]+
and [Mꢀ Cl]+ ion respectively. Also, in the case of complexes 8 and 9,
[Mꢀ Cl]+ is most abundant, i.e., the base peak showing maximum in-
tensity in the spectra.
3.2.5. UV–Visible studies of the complexes
The electronic spectra of the ligands L1-L3 and complexes 1–9 were
recorded in acetonitrile at 10-4M concentration at room temperature and
scanned in the region 200–550 nm and the plot is shown in supple-
mentary data (Figures S21-22). The electronic spectra of the ligands L1-
L3 display two absorption bands at 230–265 nm and 328–426 nm,
3.2.3. 13C NMR spectra of complexes
The 13C NMR spectra of the representative complexes are provided in
6

C.G.L. Nongpiur et al.
Inorganica Chimica Acta 525 (2021) 120459
which, based on extinction coefficients, are assigned to intra-ligand
π
-
π
*
complexes are in the range of 84.59◦-88.65◦ whereas the Cl(1)-M(1)-N
(1) and Cl(1)-M(1)-N(2) bond angles are in the range of 86.33◦-88.62◦
which are consistent with the piano stool arrangement of various groups
around the metal center are comparable to previously reported values
[33].
and n-π* transitions respectively. The complexes displayed absorption
bands at λ = 230–315 nm and 360–445 nm. Based on extinction coef-
ficient, the higher energy absorbance band at 230–315 nm is ascribed to
the intra-ligand or ligand centered transition, i.e., π-π* and n-π* transi-
tions, whereas the lower energy absorbance band at 360–445 nm is
attributed to the metal to ligand charge transfer (MLCT) transition.
These metal centers, low spin Ru(II), Rh(III), and Ir(III), have filled
3.4. In vitro antimicrobial activity
dπ (t2g) orbitals of proper geometry which can interact or donate elec-
This study is focused on exploring the antimicrobial properties of the
ligands as well as the complexes. Bacterial species like Staphylococcus
aureus, Escherichia coli and Pseudomonas aeruginosa can cause many
foods borne diseases like diarrhea, vomiting, etc., and also healthcare-
associated infections [34]. Therefore, it becomes essential to develop
new metal-based antibacterial drugs that can inhibit the growth of
bacteria. The antimicrobial activity of the synthesized ligands L1-L3,
metal precursors and metal complexes along with the standard drug
kanamycin was determined against two Gram-positive (Staphylococcus
aureus and Bacillus thuringiensis) and two Gram-negative (Escherichia coli
and Pseudomonas aeruginosa) bacterial strains. The results are expressed
as the zone of inhibition at concentration 5 mg/ml and are given in
Table 3. From the agar well diffusion study, the results unveiled that
none of the compounds showed any inhibitory activity towards the
Gram-negative bacterial strains (E. coli and P. aeruginosa) whilst some of
the compounds showed positive results towards the Gram-positive
bacterial strains (S. aureus and B. thuringiensis) (Figures S25-S26). Out
of all the compounds analyzed, only complexes 3, 4, 5, 6 and ligand L3
are observed to be showing antimicrobial activity towards the Gram-
positive bacterial strains, indicating selective antibacterial agents.
Ligand L3 showed activity only against B. thuringiensis with an inhibition
value of 16 ± 1 mm. Complex 3 of L1 showed significant activity against
both S. aureus and B. thuringiensis with an inhibition value of 18 ± 1 mm,
respectively. Both complex 4 and complex 6 with the same inhibition
value of 19 ± 1 mm showed higher inhibitory activity towards S. aureus
than B. thuringiensis. The results revealed that the coumarin hydrazone
ligands showed little or no activity against all the tested organisms while
some of the complexes were found to be toxic against the tested bacteria.
The presence of the electron-donating groups like diethylamino and
methoxy in complexes 4, 5 and 6 might have contributed to increasing
the antibacterial activity in these complexes [35]. It may be indicated
that the toxicity of the metal complexes is increased compared to that of
the parent ligands upon chelation. However, the activity of the com-
plexes did not supersede the activity of the standard kanamycin. This
study shows that complex 6 has the potential to be used as an antibac-
terial agent against Gram-positive bacteria.
trons to the empty low lying π* orbitals of the ligands [30]. For com-
plexes 1–3, the MLCT bands appear very weak due to the low
concentration of the solution and are obscured by the high intense
ligand bands [31]. It may also be noted that in complexes 4–6, the
charge transfer (C.T.) bands are shifted to longer wavelength (Red shift)
and are more intense (Hyperchromic shift) compared to the other
complexes with the introduction of the electron-donating auxochrome,
diethylamino group as a substituent on ligand L2. The auxochrome
provides an additional opportunity for charge delocalization and also
stabilization of π* orbitals thus permitting lower energy (longer wave-
length) for a transition. The stability of the complexes was examined
using UV–Visible analysis by monitoring the solutions at different time
intervals up to 48 h. No changes were noticed in the UV–Visible spectra
signifying that all the complexes are stable at room temperature
(Figures S23-24).
3.3. Description of the molecular structures of complexes
The molecular structures of the representative complexes were
established by single-crystal X-ray crystallography. Atom numbering of
the complexes was done by using the ORTEP program, as shown in
Fig. 1. The summary of the crystal data, data collection and structure
refinement parameters are summarized in Table 1. Selected bond
lengths, bond angles and metal atom involving ring centroid values are
listed in Table 2.
The geometry of the complexes obtained from single-crystal analyses
revealed the coordination behavior of the ligands towards the metal.
Single crystal analyses of the complexes also confirmed the formation of
the desired product as well as the bonding modes associated with the
ligand. Single crystals suitable for X-ray diffraction analysis were ob-
tained for complexes 1, 7, 8 and 9. These crystals are yellow, orange and
red and were obtained by the solvent diffusion method for all the
complexes. In all the complexes with both ligands, the preferable mode
of coordination of the metal is through the imine nitrogen and the ox-
ygen atom with the deprotonation of the NH proton forming neutral
bidentate mononuclear complexes (1–9). All these half-sandwich com-
plexes adopt a typical three-legged “piano-stool” geometry through N ∩
O and one terminal chloride representing the three legs of a piano while
the arene ring (arene = p-cymene, Cp*) represents the seat of a piano.
Complex 1 crystallizes in the triclinic system with P space group, com-
plex 7 crystallizes in monoclinic system with C2/c space group and
complexes 8 and 9 crystallizes in an orthorhombic system with Pbca
space group. The distance between the metal to the centroid of the p-
cymene/Cp* ring of complexes 1, 7, 8 and 9 are 1. 770 Å, 1.677 Å,
1.770 Å and 1.773 Å, respectively. The M(1)-N(1) bond distances in
complexes 1 and 9 are 2.098(3) Å and 2.112(3), respectively, whereas
the M(1)-N(2) bond distances in complexes 7 and 8 are 2.1047(18) Å
and 2.133(3) Å respectively. The M(1)-O(1) bond distances for com-
plexes 1, 7 and 8 are 2.054(3) Å, 2.0641(15) Å and 2.056(2) Å respec-
tively, while the M(1)-O(3) bond distance for complex 9 is 2.063(2) Å.
The M(1)-Cl(1) bond distances for these complexes are 2.4217(17) Å,
2.4231(6) Å, 2.4240(8) Å and 2.4227(7) Å, respectively. These data are
consistent with the previously reported arene metal complexes [32]. The
N(1)-M(1)-O(1) bond angle value of complex 1 is 76.38(11)◦ and the N
(1)-M(1)-O(3) bond angle value of complex 9 is 75.62(9)◦. Similarly, the
N(2)-M(1)-O(1) bond angles of complexes 7 and 8 are 75.68(6)◦ and
76.51(9)◦ respectively. The Cl(1)-M(1)-O(1) bond angles of these
The MIC value of the tested compounds ranges from 1.25 to 2.5 mg/
ml against the Gram-positive bacterial strains S. aureus and B.
Table 3
Antibacterial activity (Agar well) of tested compounds at concentration 5 mg/
mL against different bacterial strains.
Sl.
Compounds
Bacterial Strains
No.
Gram –ve
Gram + ve
E. coli
P. aeruginosa
S. aureus
B. thuringiensis
Kanamycin (+ve
control)
20 ±
22 ± 1
21 ± 1
20 ± 1
1
–
–
–
–
–
–
–
–
–
–
–
–
1
Ligand 1
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
2
Complex 1
Complex 2
Complex 3
Ligand 2
–
3
–
4
18 ± 1
18
1
5
–
–
6
Complex 4
Complex 5
Complex 6
Ligand 3
19
1
16 ± 1
14 ± 1
17 ± 1
16 ± 1
–
–
–
7
15 ± 1
8
19
–
1
9
10
11
12
Complex 7
Complex 8
Complex 9
–
–
–
7

C.G.L. Nongpiur et al.
Inorganica Chimica Acta 525 (2021) 120459
thuringiensis, whereas there is no inhibition observed against the Gram-
Table 4
DPPH radical scavenging activity of tested compounds.
negative bacterial strains by the ligands or complexes.
Sl. No.
Compound
% DRSA
Std. error
3.5. Antioxidative activity
1
AA
99.9
39.7
51.4
29.9
85.0
18.8
50.6
21.7
37.6
12.5
10.6
± 0.2
± 3.9
± 1.6
± 2.3
± 0.4
± 3.2
± 1.7
± 0.5
±1.6
2
Ligand 1
3
Complex 1
Complex 2
Complex 3
Complex 5
Complex 6
Ligand 3
Free radicals are highly reactive species that play an important role
in biological processes such as metabolic pathways, cell signaling, im-
mune response and various kinds of pathophysiological conditions. They
can be both beneficial and deleterious. Aging and degenerative diseases
such as heart disease, cancer, cataracts, brain dysfunction and arthritis
can occur when there is oxidative damage of biomolecules such as DNA
induced by excess free radicals [36]. The most effective way to eliminate
free radicals that cause oxidative stress is with the help of antioxidants.
Antioxidants or inhibitors of oxidation are compounds that inhibit or
prevent the process of oxidation that can produce free radicals. Hence,
antioxidants can act as free radical scavengers in the defense system.
One of the most widely used methods to determine antioxidant activity
is DPPH (2, 2-diphenyl-1-picryl-hydrazyl) free radical scavenging
method. The DPPH radical scavenging assay is a decolorization assay
that depends on the capacity of antioxidants to scavenge DPPH radicals.
DPPH is a stable organic nitrogen-centered free radical, which is a deep
purple that will decolorize when reduced into non-radical form by an-
tioxidants. More the antioxidant activity more will be the number of
DPPH radicals that will be scavenged by the antioxidants. This will make
the compound an excellent antioxidant or scavenger in preventing cell
damage. The percentage radical scavenging ability of the compounds
was tested based on the radical scavenging effect on the DPPH free
radical. Amongst the tested compounds, at a concentration of 1 mg/mL,
the DPPH radical scavenging activity (DRSA%) of complexes 1, 3,
and 6 reached more prominent than or equivalent to 50% as shown in
Fig. 2 and Table 4. The iridium metal complex of L1, i.e., complex 3
showed the highest scavenging activity at 85% while complex 1 and
complex 6 showed moderate scavenging activity at 51.4% and 50.6%
respectively. Other compounds also showed appreciable radical scav-
enging activities. The high radical scavenging activity in complex 3 may
be due to the phenolic hydroxyl group, which is present as a substituent
in ligand L1, which can provide the necessary component as a radical
scavenger and hence influence the antioxidant activity. Phenolic com-
pounds, on account of their redox properties, act as reducing agents,
hydrogen donors and metal chelators and hence are considered anti-
oxidants as they can neutralize the free radicals and inhibit the propa-
gation of the chain reactions [37]. It may also be suggested that the
chelation of the ligand with metal ions plays an important role in
increasing the antioxidative properties, as is evident from the higher
4
5
6
7
8
9
Complex 7
Complex 8
Complex 9
10
11
±1.6
± 1.7
activities portrayed by the complexes in comparison to the parent
ligands.
3.6. DNA binding properties
A large number of compounds can bind with the double-stranded
DNA molecules through covalent and non-covalent interactions. The
negatively charged phosphate backbone, the hydrogen accepting-
donating sites and aromatic hydrophobic components are parts of a
DNA molecule that can interact with the compounds [38]. The UV–Vis
absorption spectroscopy is one of the most universally employed
methods to study the binding modes and binding extent of compounds to
DNA. A fixed concentration of the ligand L2, complexes 4, 5 and 6 (10
μ
M) was titrated against increasing concentrations of Salmon milt (S.M.)
DNA. To remove the absorbance of DNA itself, an equal amount of DNA
was added to both the compound solution and reference solution while
taking the absorption spectra. Upon addition of SM DNA to ligand L2
and complexes 4 and 5, no change in their fluorescence spectrum was
observed, indicating that they do not bind to SM DNA (Fig. 3). On the
other hand, the MLCT band of complex 6 exhibited a substantial increase
in the absorbance (‘hyperchromic effect’) upon the addition of DNA,
which reflects the significant binding tendency of the complex for DNA
(Fig. 4). However, no change was observed in the position of the ab-
sorption band of the complex in the presence of DNA, suggesting the
possibility of electrostatic interactions and groove (surface) binding of
the metal complex. The hyperchromic effect arises mainly due to the
electrostatic attraction between the charged cations which bind to DNA
and the phosphate group of the DNA backbone, thereby causing a
contraction and overall damage to the secondary structure of DNA [39].
To further assess the affinity of complex 6 towards double-stranded
SM DNA, the intrinsic DNA dissociation constant (Kd) was determined
by monitoring changes in absorbance in the MLCT band with increasing
concentrations of SM DNA. The obtained dissociation constant is 37 ±
03 µM which suggests that the complex binds to DNA with moderate
affinity [40]. However, the exact mode of binding to DNA cannot be
concluded only by this method and more experiments are required to
further clarify the binding mode.
4. Conclusion
The present study depicts the synthesis, characterization and bio-
logical evaluation of the half-sandwich complexes of coumarin-N-acyl-
hydrazone ligands (L1 ¡ L3). The ligands were found to be excellent
chelators for the ruthenium, rhodium and iridium metals, coordinating
ꢀ
–
preferably in the enolate form (R = N ꢀ N C ꢀ O ) through the azo-
–
methine nitrogen and the enolate-Oꢀ , forming a 5-membered ring
around the metal by deprotonation of NH proton. All the complexes
were isolated as neutral complexes. The exact structures were deter-
mined by X-ray crystallography. The results obtained from the antimi-
crobial activities showed that the ligands and complexes exhibit
interesting selectivity in antibacterial studies, selectively inhibiting only
Fig. 2. Histogram of the DPPH radical scavenging activity of ligands and
complexes in comparison with Ascorbic acid.
8

C.G.L. Nongpiur et al.
Inorganica Chimica Acta 525 (2021) 120459
Fig. 3. The emission profile of ligand (L2), complex 4 and complex 5 at 10 µM in the presence of increasing concentrations of SM-DNA.
Fig. 4. The emission profile of complex 6 at 10 µM in the presence of increasing concentrations of SM-DNA. The arrow represents the changes in emission intensity
upon the addition of SM-DNA.
Gram-positive bacterial strains (S. aureus and B. thuringiensis) thereby
acting as selective antibacterial agents. The DPPH radical scavenging
activity of the compounds revealed that complex 3 showed the highest
antioxidant activity owing to the presence of the phenolic hydroxyl
group attached to the ligand moiety. DNA binding studies of the fluo-
rescent compounds with SM DNA suggest that complex 6 binds to DNA
with moderate affinity via electrostatic interactions and groove (surface)
binding. In particular, it is worth mentioning that complex 6 is observed
to be a potent antimicrobial, antioxidant and DNA binding compound
that can be further assessed for future use in pharmaceutical fields.
CRediT authorship contribution statement
Carley Giffert L. Nongpiur: Conceptualization, Methodology,
Writing - original draft. Lincoln Dkhar: Formal analysis, Validation.
Deepak Kumar Tripathi: Supervision, Visualization, Investigation.
Krishna Mohan Poluri: . Werner Kaminsky: Crystal data collection,
Structure solving. Mohan Rao Kollipara: Supervision, Visualization.
Declaration of Competing Interest
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.
9

C.G.L. Nongpiur et al.
Inorganica Chimica Acta 525 (2021) 120459
[19] M.S. Blois, Nature 181 (1958) 1199.
Acknowledgment
[20] K. Gulati, K. Gangele, D. Kumar, K.M. Poluri, Arch. Biochem. Biophys. 662 (2019)
121.
Carley Giffert L Nongpiur thanks Professor A. K. Singh, Department
of Biochemistry, NEHU, SAIF-CDRI, Lucknow and SAIF-NEHU, Shillong
for spectral analyses. Carley also thank Sergei Novikov, Department of
Chemistry and Chemical Biology, McMaster University, Ontario, Canada
for his help with one of the crystal analyses.
[21] Crysalis P R O, release (2012) Version 1.171.36.20. Agilent Technologies, Yarnton.
[22] G.M. Sheldrick, Acta. Cryst. A71 (2015) 3.
[23] G.M. Sheldrick, Acta. Cryst. C71 (2015) 3.
[24] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565.
[25] Y. Li, Z. Yang, M. Zhou, Y. Li, J. He, X. Wang, Z. Lin, RSC Adv. 7 (2017) 41527.
[26] (a) S. Aslkhademi, N. Noshiranzadeh, M.S. Sadjadi, K. Mehrani, N. Farhadyar,
Polyhedron, 160 (2019) 115. (b) K. Gudasi, R. Vadavi, R. Shenoy, S. Patil, M.
Nethaji, J. Coord. Chem., 60, 14 (2007) 1547. (c) R. Bikas, V. Lippolis, N.
Noshiranzadeh, H. Farzaneh-Bonab, A.J. Blake, M. Siczek, H. Hosseini-Monfared,
T. Lis, Eur. J. Inorg. Chem., 6 (2017) 999.
Appendix A. Supplementary data
´
[27] I.P. Ferreira, E.D.L. Pilo, A.A. Recio-Despaigne, J.G. Da Silva, J.P. Ramos, L.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.ica.2021.120459.
B. Marques, P.H.D.M. Prazeres, J.A. Takahashi, E.M. Souza-Fagundes, W. Rocha,
H. Beraldo, Bioorg. Med. Chem. 24 (2016) 2988.
ˇ
[28] P. Govindaswamy, B. Therrien, G. Süss-Fink, P. Stepnicka, J. Ludvík, J. Organomet.
Chem. 692 (2007) 1661.
References:
[29] R. Lalrempuia, M.R. Kollipara, P.J. Carroll, Polyhedron 22 (2003) 605.
[30] (a) T. Tsolis, M.J. Manos, S. Karkabounas, I. Zelovitis, J. Organomet. Chem., 768
(2014) 1. (b) P. Govindaswamy, J. Canivet, B. Therrien, G. Suss-Fink, P. Stepnicka,
J. Ludvik, J. Organomet. Chem., 692 (2007) 3664.
[1] G. Gasser, I. Ott, N. Metzler-Nolte, J. Med. Chem. 54 (2011) 3.
[2] L. Ronconi, P.J. Sadler, Coord. Chem. Rev. 251 (2007) 1633.
[3] Y.K. Yan, M. Melchart, A. Habtemariam, P.J. Sadler, Chem. Commun. (2005) 4764.
[4] B. Therrien, Coord. Chem. Rev. 253 (2009) 493.
[31] (a) J. Pitchaimani, M.R.C. Raja, S. Sujatha, S.K. Mahapatra, D. Moon, S.P. Anthony,
V. Madhu, RSC Adv., 6 (2016) 90982. (b) M.K.M. Subarkhan, R. Ramesh, Y. Liu,
New J. Chem., 40 (2016) 9813.
[5] A. Mandal, S. Dasgupta, S. Ganguly, A. Bauz, A. Frontera, D. Das, Dalton Trans. 46
(2017) 15257.
[32] (a) R. Payne, P. Govender, B. Therrien, C.M. Clavel, P.J. Dyson, G.S. Smith, J.
Organomet. Chem., 729 (2013) 20. (b) H.S. Calik, E. Ispir, S. Karabuga, M.
Aslantas, J. Organomet. Chem., 801 (2016) 122.
[6] H. Chen, J.A. Parkinson, S. Parsons, R.A. Coxall, R.O. Gould, P.J. Sadler, J. Am.
Chem. Soc. 124 (2002) 3064.
[7] R.E. Morris, R.E. Aird, P.S. Murdoch, H. Chen, J. Cummings, N.D. Hughes,
S. Parsons, A. Parkin, G. Boyd, D.I. Jodrell, P.J. Sadler, J. Med. Chem. 44 (2001)
3616.
[33] (a) G. Gupta, S. Park, S.S. Lee, J. Kim, Z. Anorg. Allg. Chem., 637 (2011) 1516. (b)
T. Tsolis, M.J. Manos, S. Karkabounas, I. Zelovitis, A. Garoufis, J. Organomet.
Chem., 768 (2014) 1. (c) L. Dkhar, V. Banothu, W. Kaminsky, M.R. Kollipara, J.
Organomet. Chem., 914 (2020) 121225.
[8] C.S. Allardyce, P.J. Dyson, D.J. Ellis, S.L. Heath, Chem. Commun. 1396 (2001).
[9] C. Scolaro, A. Bergamo, L. Brescacin, R. Delfino, M. Cocchietto, G. Laurenczy, T.
J. Geldbach, G. Sava, P.J. Dyson, J. Med. Chem. 48 (2005) 4161.
[10] R. Lalrempuia, M.R. Kollipara, Polyhedron 23 (2003) 3155.
[11] K.S. Singh, Y.A. Mozharivskyj, M.R. Kollipara, Z. Anorg, Allg. Chem. 632 (2006)
172.
[34] (a) M.B. Ferrari, S. Capacchi, G. Pelosi, G. Reffo, P. Tarasconi, R. Al-bertini, S.
Pinelli, P.L. Helicin, Inorg. Chim. Acta, 286 (1999) 134. (b) L.R. Dinelli, A.A.
Batista, K. Wohnrath, M.P. de Araujo, S.L. Queiroz, M.R. Bonfadini, G. Oliva, O.R.
Nascimento, P.W. Cyr, K.S. MacFarlane, B.R. James, Inorg. Chem., 38 (1999) 5341.
[35] (a) A.R. Kaneria, R.R. Giri, V.G. Bhila, H.J. Prajapati, D.I. Brahmbhatt, Arab. J.
Chem., 10 (2017) 1100. (b) N.C. Desai, H.M. Satodiya, K.M. Rajpara, V.V. Joshi, H.
V. Vaghani, J. Saudi Chem. Soc., 21 (2017) 153.
[12] K.T. Prasad, G. Gupta, A.K. Chandra, M.P. Pavan, M.R. Kollipara, J. Organomet.
Chem. 695 (2010) 707.
[13] Y. Geldmacher, M. Oleszak, W.S. Sheldrick, Inorg. Chim. Acta 393 (2012) 84.
[14] A. Kotali, D.A. Nasiopoulou, C.A. Tsoleridis, P.A. Harris, C.A. Kontogiorgis, D.
J. Hadjipavloulitina, Molecules 21 (2016) 138.
[36] K. Tsai, T.G. Hsu, K.M. Hsu, H. Cheng, T.Y. Liu, C.F. Hsu, C.W. Kong, Free Radical
Biol. Med. 31 (2001) 1465.
[37] N. Ozsoy, E. Candoken, N. Akev, Oxid. Med. Cell. Longev. 2 (2009) 99.
[38] M.M. Aleksic, V. Kapetanovic, Acta Chim. Slov. 61 (2014) 555.
[39] F. Arjmand, A. Jamsheera, Spectrochim. Acta A 78 (2011) 45.
[40] M.R. Kollipara, L. Shadap, V. Banothu, N. Agarwal, K.M. Poluri, W. Kaminsky,
J. Organomet. Chem. 915 (2020), 121246.
[15] S. Emami, S. Dadashpour, Eur. J. Med. Chem. 102 (2015) 611.
[16] H.A. Elshemy, M.A. Zaki, Bioorg. Med. Chem. 25 (2017) 1066.
[17] G. Feurer, Prog. Med. Chem. 85 (1974) 10.
[18] L. Dkhar, W. Kaminsky, K.M. Poluri, M.R. Kollipara, J. Organomet. Chem. 891
(2019) 54.
10