Synthesis, structure, spectral, redox properties and anti-cancer activity of Ruthenium(II) Arene complexes with substituted Triazole Ligands

Article abstract of DOI:10.1016/j.jorganchem.2021.122074

Three versatile half-sandwich ruthenium(II) p-cymene complexes bearing substituted triazole ligands exhibit promising cancer cell growth inhibition activity towards A549 lung adenocarcinoma and MDA-MB-231 breast adenocarcinoma cells. In this context, the triazole based phthalimide protected new ligand (2-(3, 5-di(pyridin-2-yl)-4H-1,2,4-triazol-4-yl) isoindoline-1,3-dione) (L¹) was prepared. Three ruthenium(II) p-cymene complexes [Ru(η⁶-p-cymene)(L¹)Cl]Cl: [1]Cl, L¹: (2-(3,5-di(pyridin-2-yl)-4H-1,2,4-triazol-4-yl)isoindoline-1,3-dione), [Ru(η⁶-p-cymene)(L²)Cl]Cl: [2]Cl and [Ru(η⁶-p-cymene)(L²)Cl](PF₆): [2](PF₆), L² (2,2′-(4-(1H-pyrrol-1-yl)-4H-1,2,4-triazole-3,5-diyl) dipyridine) have been successfully synthesized and characterized by different spectral and analytical tools. Pyrrole protected substituted ruthenium complexes [2]Cl and [2](PF₆) have been successfully identified structurally by single-crystal X-ray diffraction studies and confirmed the successful anion exchange. The redox properties of the ligands and the targeted metal complexes have been carefully examined. Cellular staining, live-cell imaging and MTT assay have been performed for all the complexes. We have demonstrated that our synthesized ruthenium(II) p-cymene complexes are capable of inducing significant cytotoxicity in A549 lung cancer cell lines, with an IC₅₀ values of 6.56 ± 0.31 μM, 4.74 ± 0.2 μM and 13.67 ± 0.64 μM and in MDA-MB-231 breast cancer cell lines with an IC₅₀ values of 1.13 ± 0.046 μM, 0.36 ± 0.016 μM and 11.32 ± 0.49 μM for [1]Cl, [2]Cl and [2](PF₆) respectively.

Full text of DOI:10.1016/j.jorganchem.2021.122074

Journal of Organometallic Chemistry 954–955 (2021) 122074
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, structure, spectral, redox properties and anti-cancer activity
of Ruthenium(II) Arene complexes with substituted Triazole Ligands
Arabinda Muleya, Kalai Selvan Karumbana, Parth Guptab, Sadananda Kumbhakara,
Bishnubasu Giria, Rajnikant Rautb, Ashish Misrab,∗, Somnath Majia,∗
a Department of Chemistry, Indian Institute of Technology, Hyderabad, Kandi, Sangareddy 502284, Telangana, India
b Department of Biotechnology, Indian Institute of Technology, Hyderabad, Kandi, Sangareddy 502284, Telangana, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Three versatile half-sandwich ruthenium(II) p-cymene complexes bearing substituted triazole ligands ex-
hibit promising cancer cell growth inhibition activity towards A549 lung adenocarcinoma and MDA-
MB-231 breast adenocarcinoma cells. In this context, the triazole based phthalimide protected new
ligand (2-(3, 5-di(pyridin-2-yl)-4H-1,2,4-triazol-4-yl) isoindoline-1,3-dione) (L1) was prepared. Three
ruthenium(II) p-cymene complexes [Ru(η6-p-cymene)(L1)Cl]Cl: [1]Cl, L1: (2-(3,5-di(pyridin-2-yl)-4H-1,2,4-
triazol-4-yl)isoindoline-1,3-dione), [Ru(η6-p-cymene)(L2)Cl]Cl: [2]Cl and [Ru(η6-p-cymene)(L2)Cl](PF6):
Received 15 July 2021
Revised 26 August 2021
Accepted 2 September 2021
Available online 8 September 2021
Keywords:
ꢀ
Redox-active ligand
Ruthenium-arene complexes
Electrochemistry
[2](PF6), L2 (2,2 -(4-(1H-pyrrol-1-yl)-4H-1,2,4-triazole-3,5-diyl) dipyridine) have been successfully synthe-
sized and characterized by different spectral and analytical tools. Pyrrole protected substituted ruthenium
complexes [2]Cl and [2](PF6) have been successfully identified structurally by single-crystal X-ray diffrac-
tion studies and confirmed the successful anion exchange. The redox properties of the ligands and the
targeted metal complexes have been carefully examined. Cellular staining, live-cell imaging and MTT as-
say have been performed for all the complexes. We have demonstrated that our synthesized ruthenium(II)
p-cymene complexes are capable of inducing significant cytotoxicity in A549 lung cancer cell lines, with
Crystal structures
Anticancer activity
In vitro cytotoxicity
an IC50 values of 6.56
0.31 μM, 4.74
0.2 μM and 13.67
0.64 μM and in MDA-MB-231 breast cancer
0.016 μM and 11.32 0.49 μM for [1]Cl, [2]Cl
cell lines with an IC50 values of 1.13
0.046 μM, 0.36
and [2](PF6) respectively.
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
their severe side effects like nerve damage, nephrotoxicity and
myelotoxicity have made them unsuitable for broader application
[6]. Also, the acquired drug resistance of cisplatin and analogues
remains a barrier towards its further improvement and clinical us-
age [7]. Optimization of these factors requires specific attention to-
wards designing other metal-based cost-effective drugs for the de-
velopment of anticancer agents [8].
Cancer persists to be a major concern for humankind for long
times and the pursuit of anticancer agents has an evolving and
ever-growing history over the last few decades [1]. The increase of
population density, industrialization and other environmental fac-
tors have directly contributed to the spike of cancer cases world-
wide [2] demanding the scientific community to design and de-
velop potent anticancer drugs with low toxicity and high selec-
tivity and efficacy [3]. The major issues with the traditional can-
cer treatment approach, such as radiation therapy, chemother-
apy, surgery, etc., lies in molecular oxygen dependency, biocom-
patibility, selectivity, expense, insufficient optimization protocols,
etc., together with controlled clinical trials [4,5]. In this regard,
chemotherapeutic agents based on platinum (cisplatin, carboplatin,
oxaliplatin, etc.) have gained special attention over the years, but
However, ruthenium complexes could be promising alternatives
due to their lower toxicity towards healthy cells in the biological
environment [9–11]. Two major classes of ruthenium-based com-
plexes have gained enormous attention in the pharmaceutical field.
Primarily robust oxidation catalysts, ruthenium polypyridyls are
also employed as anticancer drugs [12,13] and ruthenium arenes,
with their half sandwich structures are highly suitable for in vitro
studies [14–17]. Due to their structural identity, the distinct class
of arene ruthenium(II) complexes are commonly known as ’pi-
ano stool’ complexes. They have a general formula with a core of
[(η6−arene)Ru] with either a bidentate ligand system accompanied
by mono dentate ligands, solvents, or a tridentate ligand to satisfy
∗
Corresponding authors.
E-mail address: smaji@chy.iith.ac.in (S. Maji).
https://doi.org/10.1016/j.jorganchem.2021.122074
0022-328X/© 2021 Elsevier B.V. All rights reserved.

A. Muley, K.S. Karumban, P. Gupta et al.
Journal of Organometallic Chemistry 954–955 (2021) 122074
Fig. 1. Selected Arene compounds with N,N−chelating ligands reported in the literature.
[Ru(η6-p-cymene)(L2)Cl](PF6): [2](PF6) towards potent application
of anticancer drug (Fig. 2). Extended conjugation and protected
ring have been introduced to the structural unit to increase hy-
drophobicity and improve transport to the cells. Thorough charac-
terization of the new ligand L1, complex [1]Cl and [2]Cl have been
achieved via 1H NMR, 13C NMR, two-dimensional NMR spectral
studies (HSQC, COSY and HMBC). Furthermore, we have exchanged
the counterion for varying the solubilities in the aqueous medium.
Detailed spectroscopic and redox properties have been investigated
for all the prepared metal complexes along with the ligands. The
single-crystal X-ray structures have elucidated the molecular struc-
tures of [2]Cl and [2](PF6). Finally, we performed detailed analysis
of the potency of these ruthenium complexes in A549 lung ade-
nocarcinoma and MDA-MB-231 breast adenocarcinoma cells. Our
results revealed that these complexes exhibit substantial cytotoxi-
city in cancer cells and could potentially be used for development
of drugs against cancer.
the coordination environment of central ruthenium [18]. The bal-
ance between hydrophobic (arene part) and hydrophilic (anionic
part) nature finely tunes their activity for in vitro studies. [19].
Common bidentate ligands used in the systems are N,N−chelating
(aliphatic and aromatic diamines, pyridine derivatives) [16,20–22].
However, there are multiple examples of N,O− (amino acid lig-
ands, amine alcohols, pyridine derivatives of aldehydes, acids, etc.),
[15] O,O− (mainly β-diketones), [19] and C, N− (benzimidazole
based) chelating ligands, [23,24] and they also showed promising
antitumor and DNA cleavage activity. The attachment of the 1,3,5-
triaza-7-phosphaadamantane (PTA) ligand to central ruthenium as
a mono dentate ligand, well-known as RAPTA (ruthenium arene
PTA) complexes, [11] has opened a new spectrum for cancer treat-
ment. The hydrophilicity of these classes of compounds and proto-
nation in lower pH allows them in vitro hydrolysis and efficient
generation of the active intermediates [25,26]. Several modes of
action for cell death are found in the derivatives of RAPTA com-
plexes [27–29]. Multinuclear arene complexes were also investi-
gated, [30–32] and their unique self-assembly properties gener-
ated two- and three-dimensional structures with good solubility
and the large cavity helps to encapsulate small molecules [33,34].
Recently, the ruthenium arene complexes have been exploited as
powerful catalysts for water oxidation, proton reduction and car-
bon dioxide reduction, which is frequently proposed as a promis-
ing option for harvesting solar energy aiming to mimic the critical
aspects of the natural processes. Some selected mononuclear arene
complexes with N,N− chelating ligands employed earlier for anti-
cancer drug has been summarized in Fig 1.
2. Experimental
2.1. Materials
RuCl3.3H2O, 2-cyanopyridine, hydrazine hydrate and α-
terpinene were purchased from Sigma-Aldrich (Merck). Phthalic
anhydride, 2, 5-dimethoxytetrahydrofuran and sodium hydride
were purchased from SRL chemicals. Dry acetonitrile was obtained
by distillation over CaH2, whereas methanol and ethanol were
dried by using sodium following standard laboratory protocol. All
the chemicals and reagents were used as received without further
purification unless otherwise mentioned. The precursor dichloro(p-
cymene)ruthenium(II) dimer [35] and L2 [36] were synthesized
following the previously reported literature procedure with slight
modifications. Human lung adenocarcinoma cells; A549 and breast
adenocarcinoma cells; MDA-MB-231 were procured from Amer-
Herein we report the synthesis and characterization of two ap-
propriately designed bis bidentate ligands L1 (2-(3,5-di(pyridin-2-
ꢀ
yl)-4H-1,2,4-triazol-4-yl)isoindoline-1,3-dione) and L2 (2,2 -(4-(1H-
pyrrol-1-yl)-4H-1,2,4-triazole-3,5-diyl)dipyridine) and their corre-
sponding mononuclear ruthenium chloro arene complexes [Ru(η6-
p-cymene)(L1)Cl]Cl: [1]Cl, [Ru(η6-p-cymene)(L2)Cl]Cl: [2]Cl and
2

A. Muley, K.S. Karumban, P. Gupta et al.
Journal of Organometallic Chemistry 954–955 (2021) 122074
Fig. 2. Complexes used in this study
yield and used for the next step. Both the ligands L1 (2-(3,5-
di(pyridin-2-yl)-4H-1,2,4-triazol-4-yl)isoindoline-1,3-dione) and L2
ican Type Culture Collection (ATCC®). DMEM, RPMI-1640 and
Penicillin-Streptomycin solution were obtained from Lonza. Fetal
Bovine Serum was purchased from Himedia. Tissue culture plates
and reagents including MTT reagent, hydrochloric acid, isopropanol
and methanol were procured from Thermo Fisher Scientific. DMSO,
crystal violet stain and NP-40 were obtained from Sigma-Aldrich
(Merck).
ꢀ
(2,2 -(4-(1H-pyrrol-1-yl)-4H-1,2,4-triazole-3,5-diyl)dipyridine) have
been prepared from this common synthon.
2.3.1. Preparation of L1
3,5-Di(2-pyridyl)-4-amino-1,2,4-triazole (500 mg, 2.1 mmol)
was taken in a round-bottom flask and dissolved in 3 mL acetic
acid. Then phthalic anhydride (311 mg, 2.1 mmol) was added, and
the solution was refluxed overnight to obtain white precipitate
L1 which was further crystallized from dichloromethane. Yield:
590 mg (76%). 1H NMR (400 MHz, CDCl3): δ (ppm) = 8.39 (2H,
2.2. Physical measurements
BRUKER AVANCE III-400 MHz spectrometer was used to record
1H, 13C NMR, two-dimensional NMR spectra at 295 K in CDCl3;
chemical shift values (δ, ppm) and coupling constants (Hz) are
reported in the standard fashion with reference to either tetram-
ethylsilane (TMS) (δ(H) 0.00 ppm) or CHCl3 (δ(H) 7.26 ppm). In 1H
NMR, the following abbreviations are used throughout: s = singlet,
d = doublet, t = triplet, m = multiplet and dd = doublet of dou-
blet, td = triplet of doublet. High-resolution mass spectra (HR-MS)
were recorded in Q-TOF electron spray ionization (ESI) and atmo-
spheric pressure chemical ionization (APCI) modes (Model: HRMS
Q-TOF 6538). FT-IR measurement (KBr) was carried out on a Jasco
spectrometer (Model: FTIR 4600 ST LE). The UV-vis spectra were
collected using a Jasco V-730 UV-vis spectrophotometer. All cyclic
voltammetry (CV) experiments were performed on CHI-660 elec-
trochemical analyzer under nitrogen atmosphere at scan rate of
100 mV s−1 at room temperature in acetonitrile using tetra butyl
ammonium perchlorate (TBAP) as supporting electrolyte in three-
electrode one-component system. A glassy carbon electrode (3 mm
diameter) was used as working electrode, platinum wire as auxil-
iary electrode and saturated calomel electrode (SCE) as a reference
electrode. The elemental analysis was performed on a BRUKER
EURO EA machine. Thermo Fisher Scientific Steri Cycle CO2 incu-
bator was used to culture the cells. Live-cell imaging was carried
out using Nikon ECLIPSE TS2R-FL Inverted Research Microscope.
ꢀ
ꢀ
d, J = 7.9 Hz, H1 and H1 ), 8.25 (2H, d, J = 4.8 Hz, H4 and H4 ),
ꢀ
8.00 (2H, dd, J = 5.5, 3.0 Hz, H3 and H3 ), 7.85−7.92 (2H, m, H6
ꢀ
ꢀ
and H6 ), 7.80 (2H, td, J = 7.8, 1.8 Hz, H5 and H5 ), 7.24 (2H,
ddd, J = 7.6, 4.9, 1.2 Hz, H2 and H2 ). 13C NMR of L1, (101 MHz,
ꢀ
CDCl3): δ (ppm) = 163.55, 151.59, 149.37, 145.81, 136.94, 134.78,
130.49, 124.79, 124.38, 123.54. Mp: 239−242 °C. FT-IR (KBr, cm−1):
1750 (s) ν(C=O), 1698 (s) ν(C=C), 1470 (m) ν(C=N) (m, medium; s,
strong). UV-vis (MeOH) λ (nm) [ε (M−1 cm−1)]: 240 [27496], 290
[35850].
2.3.2. Preparation of L2
3, 5-di (pyridin-2-yl)-4H-1, 2, 4-triazol-4-amine (500 mg,
2.1 mmol) was taken in
a
round-bottom flask and dis-
solved in 1, 4-dioxane-acetic acid (1:1) mixture. Next, 2, 5-
dimethoxytetrahydrofuran (330 mg, 2.5 mmol) was added and the
solution was refluxed for 24 h. After work up the final product
L2 was purified by column chromatography over neutral alumina
by 1% ethanol-chloroform eluent. Yield: 484 mg (84%). 1H NMR
ꢀ
(400 MHz, CDCl3): δ (ppm) = 8.42 (2H, d, J = 2.4 Hz, H1 and H1 ),
ꢀ
7.73 (2H, d, J = 3.9 Hz, H4 and H4 ), 7.65 (2H, dt, J = 7.8, 1.0 Hz,
ꢀ
ꢀ
H3 and H3 ), 7.20 (2H, dt, J = 4.1, 2.5 Hz, H2 and H2 ), 6.78 (2H,
ꢀ
ꢀ
t, J = 1.1 Hz, H5 and H5 ), 6.13 (2H, t, J = 1.1 Hz, H6 and H6 ). FT-
IR (KBr, cm−1): 1587 (s) ν(C=C), 1466 (m) ν(C=N) (m, medium; s,
strong). UV-vis (MeOH) λ (nm) [ε (M−1 cm−1)]: 247 [39882], 279
[49890].
2.3. Preparation of the ligands
2-Cyanopyridine (5 mL, 52 mmol) and hydrazine hydrate (8 mL
of 50−60 %) were taken in a round-bottomed flask in 10 mL of
absolute ethanol. Sulfur powder (200 mg) was added to the so-
lution and kept under reflux for 3 h. Initially, the solution turned
brown and orange crystals start precipitating after 2 h. The crys-
talline product was filtered, which was air-dried and used with-
out further purification. After that, the orange product was boiled
with 2 M HCl solution for 1 h and the initially formed brown so-
lution became colorless. The solution was then cooled and basi-
fied with ammonium hydroxide solution and a white curd-like pre-
cipitate of 3,5-Di(2-pyridyl)-4-amino-1,2,4-triazole (L) was formed.
The precipitate was recrystallized from dichloromethane in good
2.4. Synthesis of complexes
2.4.1. Preparation of Ruthenium(II) complex [1]Cl
In argon atmosphere, L1 (37 mg, 0.1 mmol) was dissolved in
15 mL dry methanol in a 50 mL round-bottom flask. Dichloro(p-
cymene)ruthenium(II) dimer (70 mg, 0.11 mmol) was added and
stirred in room temperature for 8 h. The brown solution obtained
were dried in vacuo and purified by column chromatography over
neutral alumina column using 3% ethanol/dichloromethane as elu-
ent. Yield: 58 mg (67%). Anal. Calc. (%) for C30H26Cl2N6O2Ru C,
53.42; H, 3.89; N, 12.46; Found (%): C, 52.99; H, 4.08; N, 12.24;
1H NMR (400 MHz, CDCl3): δ (ppm) = 9.14 (1H, d, J = 6.3 Hz, H8),
3

A. Muley, K.S. Karumban, P. Gupta et al.
Journal of Organometallic Chemistry 954–955 (2021) 122074
Scheme 1. Synthesis of L, L1 and L2.
H, 4.41; N, 14.14; Found (%): C, 52.48; H, 4.79; N, 13.94; 1H NMR
of [2]Cl, (400 MHz, CDCl3): δ (ppm) = 9.71 (1H, d, J = 2.9 Hz,
H8), 8.52 (1H, d, J = 6.2 Hz, H19), 8.16 (1H, d, J = 5.2 Hz, H11),
7.85 (2H, dt, J = 9.8 Hz, H16), 7.70 (1H, t, J = 2.9 Hz, H10), 7.64
(1H, d, J = 5.1 Hz, H9 and H17), 7.40 (1H, m, H18), 6.89 (1H, t,
J = 6.4 Hz, H12), 6.40 (2H, m, H13 and H15), 6.30 (3H, m, H4, H7
and H14), 6.12 (1H, d, J = 8.1 Hz, H5), 5.94 (1H, d, J = 5.6 Hz, H6),
3.09 (1H, m, p-cym H2), 2.33 (3H, s, p-cym H3), 1.73 (6H, s, p-cym
H1). 13C NMR of [2]Cl, (101 MHz, CDCl3): δ (ppm) = 157.98, 153.61,
152.86, 150.01, 143.18, 142.21, 139.67, 137.24, 128.79, 126.14, 124.70,
122.53, 122.18, 120.64, 110.55, 110.40, 104.71, 102.01, 87.90, 85.72,
83.90, 83.83, 31.06, 22.94, 21.66, 18.74. ESI−MS(+) mass spectrum
in MeOH: [M − Cl]+, m/z: 559.10 (calc. 559.10). FT-IR (KBr, cm−1):
1613 (m) ν(C=C), 1450 (m) ν(C=N) (m, medium; s, strong). UV-vis
(MeOH) λ (nm) [ε (M−1 cm−1)]: 294 [26657], 348 [6722].
8.8 (1H, d, J = 5.3 Hz, H19), 8.63 (1H, d, J = 9.6 Hz, H11), 8.45
(1H, d, J = 9.5 Hz, H16), 8.02 (1H, d, J = 8.7 Hz, H13), 7.95 (1H, t,
J = 7.6 Hz, H14), 7.79 (1H, t, J = 8.3 Hz, H10), 7.34−7.55 (5H, m, H9,
H12, H15, H17 and H18), 5.91 (1H, d, J = 4.7 Hz, H4), 5.76 (1H, d,
J = 7.7 Hz, H7), 5.69 (1H, d, J = 2.9 Hz, H6), 5.59 (1H, d, J = 6.5 Hz,
H5), 2.87 (1H, dd, J = 3.4 Hz, p-cym H2), 2.27 (3H, s, p-cym H3),
1.17−1.22 (6H, m, p-cym H1). 13C NMR of [1]Cl, (101 MHz, CDCl3):
δ (ppm) = 172.16, 170.15, 154.80, 154.54, 150.63, 149.94, 145.68,
145.25, 139.58, 137.55, 136.80, 132.76, 130.40, 129.11, 128.08,
126.46, 124.83, 124.35, 123.99, 115.99, 104.74, 101.35, 86.01, 84.61,
84.08, 82.76, 30.95, 22.33, 22.01, 18.61. ESI−MS(+) mass spectrum
in MeOH: ([M + MeOH]+), m/z: 671.11 (calc. 671.11). FT-IR (KBr,
cm−1): 1721 (s) ν(C=O), 1614 (m) ν(C=C), 1450 (m) ν(C=N) (m,
medium; s, strong). UV-vis (MeOH) λ (nm) [ε (M−1 cm−1)]: 292
[36064], 343 [10257].
[2](PF6): The precursor [2]Cl was dissolved in methanol and
saturated methanolic KPF6 was added and stirred for 2 h. The so-
lution was filtered and kept for crystallization to obtain [2](PF6).
Yield: 54%. Anal. Calc. (%) for C26H26ClF6N6PRu C, 44.36; H, 3.72;
N, 11.94; Found (%): C, 44.14; H, 3.95; N, 11.42; ESI−MS(+) mass
spectrum in MeOH: [M − PF6]+, m/z: 559.10 (calc. 559.10). FT-IR
(KBr, cm−1): 1616 (m) ν(C=C), 1450 (m) ν(C=N), 836 (s) ν(PF6)
(m, medium; s, strong). UV-vis (MeOH) λ (nm) [ε (M−1 cm−1)]:
294 [25861], 348 [5961], 557 [420].
2.4.2. Preparation of Ru(II) complexes [2]Cl and [2](PF6)
[2]Cl: In argon atmosphere, L2 (33 mg, 0.1 mmol) was dissolved
in 15 mL dry methanol in a 50 mL round-bottom flask. Dichloro(p-
cymene)ruthenium(II) dimer (70 mg, 0.11 mmol) was added and
stirred in room temperature for 8 h. The brown solution obtained
were dried in vacuo and purified by column chromatography over
neutral alumina column using 5% ethanol/dichloromethane eluent.
Yield: 60 mg (71%) [2]Cl. Anal. Calc. (%) for C26H26Cl2N6Ru C, 52.53;
4

A. Muley, K.S. Karumban, P. Gupta et al.
Journal of Organometallic Chemistry 954–955 (2021) 122074
Scheme 2. Synthesis of the [1]Cl, [2]Cl and [2](PF6).
2.5. Crystallography
2.6. Cell culture
Single crystals of both the complexes [2]Cl and [2](PF6)
were crystallized by the slow evaporation of a 1:2 methanol-
dichloromethane solvent mixture. A crystal of suitable dimensions
was mounted on a CryoLoop (Hampton Research Corp.) with a
layer of light mineral oil. The crystallographic data were mea-
sured with an Agilent Supernova dual-source diffractometer. Sin-
gle crystal X-ray diffraction study was carried out on a Rigaku Su-
pernova Xcalibur Eos CCD detector with graphite-monochromatic
Human lung adenocarcinoma cell line; A549 was cultured in
Dulbecco’s Modified Eagle Medium (Lonza) and breast adenocarci-
noma cell line; MDA-MB-231 was cultured in RPMI-1640 medium
(Lonza), both containing 10% Fetal Bovine Serum (Himedia) and 1%
antibiotic mixture of Penicillin-Streptomycin (Lonza). These cells
were seeded in 12-well and/or 96-well plates (Thermo Fisher Sci-
entific) depending on the experiment to be done and were cul-
tured in humid conditions with constant 5% CO2 at 37 °C.
˚
˚
CuKα (1.54184 A) radiation for [2]Cl and MoKα (λ = 0.71073 A) ra-
diation for [2](PF6). The structures were solved by direct methods
SHELXT [37] and refined on F2 using the full-matrix least-squares
methods with SHELXL [38]. Absorption corrections were performed
based on multi-scans. All hydrogen atoms were placed at idealized
positions, while all other atoms were refined anisotropically. The
MERCURY program was used to draw the molecules.
2.7. Live-cell imaging
To study the effects of ruthenium(II) arene complexes, can-
cer cells were seeded in a 12-well plate at a cell density of 104
cells/well, in DMEM/RPMI-1640 + 10% FBS + 1% Pen/Strep media
in a 5% CO2 incubator at 37 °C. The complexes were dissolved in
dimethyl sulfoxide (DMSO) to carry out the experiments. The cells
were treated with [1]Cl, [2]Cl and [2](PF6) at indicated concentra-
5

A. Muley, K.S. Karumban, P. Gupta et al.
Journal of Organometallic Chemistry 954–955 (2021) 122074
1
Fig. 3. H NMR of ligand L1 in CDCl3.
Fig. 4. 1H NMR of [1]Cl in CDCl3.
6

A. Muley, K.S. Karumban, P. Gupta et al.
Journal of Organometallic Chemistry 954–955 (2021) 122074
Fig. 5. 1H NMR of [2]Cl in CDCl3.
Table 1
Selected crystallographic data for [2]Cl and [2](PF6)
Complex
[2]Cl
[2](PF6)
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
C
26H26Cl2N6Ru
C26H26ClF6N6PRu
594.5
273
704.02
273
triclinic
P-1
monoclinic
P21/n
˚
a/A
8.6716(3)
16.4053(4)
˚
b/A
12.4944(6)
10.5310(3)
˚
c/A
13.9241(7)
16.9552(4)
α/°
β/°
γ /°
113.755(5)
90
91.254(3)
100.972(2)
104.937(3)
90
3
˚
Volume/A
1320.60(11)
2875.70(13)
Z
2
4
ρ
calcg/cm3
1.495
1.626
μ/mm-1
6.871
0.761
F(000)
604.0
1416.0
Crystal size/mm3
0.06 × 0.05 × 0.02
CuKα (λ = 1.54184)
7.008 to 139.986
0.04 × 0.03 × 0.02
MoKα (λ = 0.71073)
4.576 to 58.426
Radiation
2θ range for data collection/°
Index ranges
-8 ≤ h ≤ 10, -15 ≤ k ≤ 15, -16 ≤ l ≤ 16
12158
-22 ≤ h ≤ 22, -12 ≤ k ≤ 14, -21 ≤ l ≤ 19
12492
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I>=2σ (I)]
Final R indexes [all data]
4982 [Rint = 0.0423, Rsigma = 0.0392]
4982/0/319
6650 [Rint = 0.0257, Rsigma = 0.0420]
6650/0/373
1.028
1.043
R1 = 0.0320, wR2 = 0.0840
R1 = 0.0345, wR2 = 0.0863
0.70/-0.76
R1 = 0.0389, wR2 = 0.0908
R1 = 0.0541, wR2 = .1021
0.67/-0.44
−3
˚
Largest diff. peak/hole / e A
tions for 72 h once 70−75% cell density was achieved. Afterwards,
the growth medium was removed and cells were washed multiple
times with 1X Phosphate Buffered Saline (PBS) to remove resid-
ual ruthenium complexes and bright field images were captured
using Nikon Inverted Research Microscope at 20 ms exposure. Fur-
thermore, to observe relative cell death, the cells were stained us-
ing 0.1% Crystal Violet stain (prepared in 20% Methanol v/v) and
the image was captured using Bio-RAD Chemidoc XRS+ imaging
system after multiple washes with 1X PBS to remove the residual
stain.
7

A. Muley, K.S. Karumban, P. Gupta et al.
Journal of Organometallic Chemistry 954–955 (2021) 122074
Fig. 6. UV-vis spectra of L1, L2, [1]Cl, [2]Cl and [2](PF6) in methanol solvent at room temperatur-e.
Fig. 7. Single crystal X-ray crystal structures of [2]Cl. The thermal ellipsoids are drawn with 40% probability. Hydrogen atoms are omitted for clarity.
8

A. Muley, K.S. Karumban, P. Gupta et al.
Journal of Organometallic Chemistry 954–955 (2021) 122074
Fig. 8. Single crystal X-ray crystal structures of [2](PF6). The thermal ellipsoids are drawn with 40% probability. Hydrogen atoms are omitted for clarity.
2.8. Cell cytotoxicity experiment
sponding ligands with the standard starting material dichloro(p-
cymene)ruthenium(II) dimer in methanol at room temperature
with moderately high yield. For complex [2](PF6), anion exchange
was performed with methanolic KPF6 solution. The formation
of all these complexes has been authenticated by their satisfac-
tory 1H and 13C NMR spectra, mass spectra, FT-IR spectra and
UV-vis spectral studies. Moreover, for [2]Cl and [2](PF6), suitable
single-crystal structures have been grown by the slow evapora-
tion from methanol-dichloromethane mixture and have been struc-
turally elucidated by single-crystal X-ray diffraction studies.
103 cancer cells were seeded in each well of a 96-well plate
and cultured for 16 h in media containing DMEM/RPMI-1640 + 10%
FBS + 1% Pen/Strep at 37 °C in a 5% CO2 incubator. To examine
the cytotoxic effects, the cells were treated with [1]Cl, [2]Cl and
[2](PF6), for 72 h. Thereafter, the media containing the compounds
was removed and the cells were washed using 1X PBS. Subse-
quently, 100 μL of 0.5 mg/mL MTT reagent (3-(4,5-dimethylthiazol-
2-yl)-2,5-diphenyltetrazolium bromide) was added onto the cells
and incubated for 4 h. Afterwards, MTT reagent was aspirated and
100 μL MTT solvent (4 mM HCl, 0.1% Nonidet P-40 (NP-40) pre-
pared in isopropanol) was added to each well of the 96-well plate
and rotated gently for 20 min on an orbital shaker to dissolve the
formed formazan crystals. Absorbance was recorded at 595 nm us-
ing iMARK micro plate reader (Bio-Rad). The cellular cytotoxicities
of [1]Cl, [2]Cl and [2](PF6) were calculated as a ratio of recorded
absorbance at 595 nm of treated cells to the DMSO control and
IC50 values and standard deviation were determined using Graph-
Pad Prism.
3.2. Spectral Aspects
NMR Spectroscopy: 1H NMR spectra in CDCl3 exhibit total twelve
aromatic protons with six distinguishable resonances for both the
ligands L1 and L2 in the range of 6.12 to 8.43 ppm. For L1, the
characteristic doublets of pyridyl −CH groups appear at 8.25 and
8.39 ppm and the multi-plates at 7.25 and 8.01 ppm corresponds
to the other two pyridyl −CH from the pyridine moiety. Benzene
protons of the extended phthalic moiety match well with doublets
of triplets at 7.81 and 7.88 ppm (Fig. 3). In the 13C NMR spectrum
in CDCl3, phthalimide −C=O resonance appears at 163 ppm (Fig.
S1). All the protons have been successfully identified with the help
of two-dimensional NMR spectral studies (HSQC, COSY and HMBC)
(Fig S2, S3 and S4). For L2, the peaks are deshielded compared to
L1 due to the presence of pyrrole moiety. Pyridyl −CH protons are
in the range of 7.20 to 8.45 ppm, whereas the two protons of the
extended pyrrole moiety appear as triplets at 6.13 and 6.78 ppm
(Fig. S5). The unsymmetrical nature of [1]Cl and [2]Cl gives rise
to total twenty-six protons; out of which sixteen aromatic protons
appear in the range of 5.58 to 9.72 ppm and the rest ten aliphatic
protons appear in the range of 1.17 to 3.12 ppm, revealing the dia-
magnetic nature of Ru(II) with low-spin d6 configuration for both
the metal complexes. Ten aliphatic protons of p-cymene groups are
in the range of 1.20 to 3.09 ppm. The typical four aromatic pro-
3. Results and discussion
3.1. Synthesis and characterization
The triazole-based phthalimide protected new ligand L1 was
synthesized by reaction of phthalic anhydride and 3,5-Di(2-
pyridyl)-4-amino-1,2,4-triazole, which is a rearrangement prod-
uct of 3,6-Di(2-pyridyl)-1,2-dihydro-1,2,4,5-tetrazine. Similarly, an-
other triazole-based pyrrole protected ligand L2 was prepared
by the reaction of 3,5-Di(2-pyridyl)-4-amino-1,2,4-triazole with
2, 5-dimethoxytetrahydrofuran following previously reported lit-
erature procedure with slight modification (Scheme 1 and 2).
All the ruthenium complexes [1]Cl, [2]Cl and [2](PF6) were syn-
thesized following conventional method, by reacting the corre-
9

A. Muley, K.S. Karumban, P. Gupta et al.
Journal of Organometallic Chemistry 954–955 (2021) 122074
Fig. 9. Bright-field images of A549 adenocarcinoma cells treated with various ruthenium complexes for 72 h. (A) DMSO-treated A549 cells. (B) 5 μM [1]Cl treated A549
cells. (C) 10 μM [1]Cl treated A549 cells. (D) 20 μM [1]Cl treated A549 cells. (E) 5 μM [2]Cl treated A549 cells. (F) 10 μM [2]Cl treated A549 cells. (G) 20 μM [2]Cl treated
A549 cells. (H) 5 μM [2](PF6) treated A549 cells. (I) 10 μM [2](PF6) treated A549 cells. (J) 20 μM [2](PF6) treated A549 cells.
tons of arene moiety are within the range of 5.58 to 6.41 ppm
for both the complexes, which are in good agreement with the
previously reported analogous ruthenium(II) p-cymene complexes
[39,40]. The distinct isopropyl −CH proton for [1]Cl and [2]Cl ap-
pears at 2.86 and 3.09 ppm, respectively (Figs. 4 and 5). In the 13C
NMR spectrum of both [1]Cl and [2]Cl, the complexes showed 30
and 26 peaks respectively due to the unsymmetrical nature (Figs.
S6 and S8). The two distinct phthalimide −C=O resonance of [1]Cl
appears at 172 ppm and 170 ppm. Additionally, both the complexes
are identified by two-dimensional NMR spectral studies (Figs. S7
and S9).
Vibrational spectroscopy: In the solid-state FT-IR spectra of L1, an
intense sharp band was observed at 1750 cm−1 that corresponds
to the C=O stretching vibration of the phthalic moiety attached to
the triazole ring. The C=O band shifted to high energy 1721 cm−1
in [1]Cl, suggesting the successful incorporation of Ru(II). The char-
tronic transition near 350 nm, which could be attributed to Ru(II)
based metal to ligand charge transfer (MLCT) transition to the π∗
orbital of the respective ligand and intense ligand-based multiple
transitions in the high energy UV region [13,42,43]. The band en-
ergy (MLCT) decreasing in the order [2]Cl ≈ [2](PF6) < [1]Cl due
to the relative stabilization of dπ {Ru(II)} energy level (Fig. 6).
Mass spectroscopy: The electrospray mass spectra of complexes
[1]Cl, [2]Cl and [2](PF6) were recorded in methanol solvent and
[1]Cl showed molecular ion peak associated with methanol solvent
centered at m/z 671.11, calculated: 671.11. Both the complexes [2]Cl
and [2](PF6) showed their molecular ion peaks at 559.10; calcu-
lated: 559.10. Thus, satisfying their molecular integrity. No addi-
tional significant peaks are observed beyond the molecular peaks
which confirm the formation of monomer complexes. Moreover,
the relative isotopic abundances match well with the simulated
mass spectra (Fig. S11).
−
acteristic sharp peak at 836 cm−1 indicates the presence of PF6 in
3.3. Redox properties
[2](PF6) via the counterion exchange from [2]Cl (Fig. S10).
UV-vis spectroscopy: UV-vis spectroscopy of all the compounds
were recorded in methanol at room temperature. For L1 and L2,
the π to π∗ transitions were observed at 292 and 280 nm, re-
spectively [41]. The complexes [1]Cl, [2]Cl and [2](PF6) exhibit elec-
The redox properties of all the complexes and ligands have
been investigated in degassed acetonitrile solution at room tem-
perature. Both the ligands are redox-active and the irreversible
oxidations are observed at 1.20 V and 1.25 V for L1 and L2, re-
10

A. Muley, K.S. Karumban, P. Gupta et al.
Journal of Organometallic Chemistry 954–955 (2021) 122074
Fig. 10. Bright-field images of MDA-MB-231 adenocarcinoma cells treated with various ruthenium complexes for 72 h. (A) DMSO-treated MDA-MB-231 cells. (B) 2.5 μM
[1]Cl treated MDA-MB-231 cells. (C) 5 μM [1]Cl treated MDA-MB-231 cells. (D) 10 μM [1]Cl treated MDA-MB-231 cells. (E) 2.5 μM [2]Cl treated MDA-MB-231 cells. (F)
5 μM [2]Cl treated MDA-MB-231 cells. (G) 10 μM [2]Cl treated MDA-MB-231 cells. (H) 2.5 μM [2](PF6) treated MDA-MB-231 cells. (I) 5 μM [2](PF6) treated MDA-MB-231
cells. (J) 10 μM [2](PF6) treated MDA-MB-231 cells.
spectively [44,45]. Due to the different electronic nature of the
mononuclear Ru(II) complexes, the Ru(II)/(III) couples in acetoni-
trile Eo298K/V are observed at 1.58 V, 1.65 V and 1.63 V respectively
for [1]Cl, [2]Cl and [2](PF6) while the ligand-based oxidations are
observed at 1.14 V, 1.17 V and 1.32 V (Fig. S12) [13,42,43]. The more
positive potential value in [2]Cl/[2](PF6) compared to [1]Cl implies
the additional stability of the Ru(II) state because of the electronic
nature of the pyrrole-based ligand L2.
the triazole Ru−N2 bond, as expected from the structural overview.
This is due to an unoccupied pyridine ring on the other side of the
triazole bridge. Steric factors working on the p-cymene moiety re-
sulted in longer Ru−C distances to the three bonds closer to the
isopropyl group (Ru−C4, Ru−C5, Ru−C6) than the carbon centers
in proximity to the methyl group. (Ru−C2, Ru−C3, Ru−C7). The
crystal structures show a planar rotation of the triazole ligand sys-
tem to the rest of the molecule in the case of [2]Cl to [2](PF6). All
the bond angles and bond distances are in very good agreement
with the previously reported related ruthenium(II) p-cymene com-
plexes [46,47].
3.4. Crystal structures
The molecular structures of [2]Cl and [2](PF6) were elucidated
The complex [2]Cl shows intra-molecular hydrogen bonding
by single-crystal X-ray diffraction and the selected crystallographic
˚
•
• •
N3) (distance 2.654 A). These
in C−H hydrogen (C1−H1C
˚
data and important bond distances (A) and bond angles (°) are
•
• •
molecules are connected via intermolecular weak C13−H13
given in Tables 1 and 2, respectively. Both the complexes display
the usual piano stool geometry coordinated via arene ring car-
bons in η6− fashion, two nitrogen atoms of the chelating ligand
and the other position is occupied by chlorine. And the charge
has been balanced by either Cl− or PF6−ions. The complex [2]Cl
crystalized in triclinic system with P-1 space group and the com-
plex [2](PF6) crystallized in monoclinic system with P21/n space
group. ORTEP representations have been shown in Figs. 7 and 8.
˚
Cl1 hydrogen bonds (distance 2.672 A, angle 154.44°). The dis-
placed stacking π−π interactions are also found within the crystal
symmetry between the coordinated pyridine groups of neighbor-
˚
ing−metal centers (distance 3.830 A). In the complex [2](PF6), the
•
PF6 involves hydrogen bonding with C−H hydrogen (C18−H18
˚
•
•
F6) (distance 2.391 A). Intermolecular weak hydrogen bonds
˚
•
• •
Cl1 (distance 2.692 A, angle 135.48°) is also
present in the structure like its precursor with increased distance
in C13−H13
˚
The Ru−Cl bond in the case of [2]Cl (2.4011 A) is slightly longer
and closer angle. In this case, the displaced stacking π−π interac-
˚
than [2](PF6) (2.3968 A). The pyridyl Ru−N1 bond is longer than
11

A. Muley, K.S. Karumban, P. Gupta et al.
Journal of Organometallic Chemistry 954–955 (2021) 122074
Fig. 11. IC50 values showing effect of treatment of [1]Cl, [2]Cl and [2](PF6) on A549 lung adenocarcinoma and MDA-MB-231 breast adenocarcinoma cells.
˚
centrations of all of them for 72 h. Treatment of A549 and MDA-
MB-231 cells with [1]Cl, [2]Cl and [2](PF6) result in significant cy-
totoxicity (Figs. 9 and 10). Significant cell death was not observed
when the cells were treated with ruthenium(II) complexes for 24 h
while moderate death was observed upon treatment for 48 h (Figs.
S16−S19). Additionally, consistent cell death was observed upon
crystal violet staining of the A549 cells treated with [1]Cl, [2]Cl
and [2](PF6) (Figs. S20 and S21). These observations suggest that
the ruthenium complexes are highly efficient in killing cancer cells.
tions are longer (distance 3.920 A) (Figs. S13 and S14). The crystal
packing along with hydrogen bonding in the crystal structures of
[2]Cl and [2](PF6) are given in Supporting Information (Fig. S15).
3.5. Live-cell imaging
Cell cytotoxicity of ruthenium(II) arene complexes were deter-
mined in A549; lung adenocarcinoma and MDA-MB-231; breast
adenocarcinoma cells. The cells were treated with multiple con-
12

A. Muley, K.S. Karumban, P. Gupta et al.
Journal of Organometallic Chemistry 954–955 (2021) 122074
Table 2
ized by different spectral and analytical tools. Both the pyrrole sub-
stituted ruthenium complexes [2]Cl and [2](PF6) have been struc-
turally elucidated by single-crystal X-ray diffraction studies and
authenticated the successful anion exchange. The electrochemical
study exhibits the non-innocent nature of the targeted metal com-
plexes and ligands. Cellular staining, live-cell imaging and MTT as-
say have shown remarkable cell cytotoxic effects of [1]Cl, [2]Cl
and [2](PF6) in A549 and MDA-MB-231 cells, ascertaining the
anti-oncogenic properties of ruthenium(II) p-cymene complexes.
Our designed complexes show remarkably better efficacy towards
killing cancer cells compared to earlier reported analogous ruthe-
nium(II) p-cymene derivatives. Overall, our study provides insights
into the role of ligand design of ruthenium(II) p-cymene complexes
for the development of potential chemotherapeutic agents for the
treatment of cancer. This will significantly advance the understand-
ing about the structure-activity relationship to help in the future
efficient design of anticancer agents.
˚
Selected bond length (A) and bond angles (°) for [2]Cl and
[2](PF6)
Bond length / Bond angle
[2]Cl
[2](PF6)
Ru1−Cl1
2.4011(7)
2.135(2)
2.3968(8)
2.119(2)
Ru1−N1
Ru1−N2
2.062(2)
2.057(2)
Ru1−C2
2.189(3)
2.193(3)
Ru1−C3
2.189(3)
2.161(3)
Ru1−C4
2.196(3)
2.169(3)
Ru1−C5
2.209(3)
2.218(3)
Ru1−C6
2.201(3)
2.200(3)
Ru1−C7
2.170(3)
2.168(3)
N1−Ru1−N2
N1−Ru1−Cl1
N1−Ru1−C2
N1−Ru1−C3
N1−Ru1−C4
N1−Ru1−C5
N1−Ru1−C6
N1−Ru1−C7
N2−Ru1−Cl1
N2−Ru1−C2
N2−Ru1−C3
N2−Ru1−C4
N2−Ru1−C5
N2−Ru1−C6
N2−Ru1−C7
75.84(8)
76.03(9)
86.24(6)
84.75(7)
119.37(10)
156.55(11)
159.50(10)
121.84(9)
96.86(9)
115.01(11)
150.93(12)
166.39(11)
128.38(12)
101.44(11)
95.29(11)
82.64(7)
95.32(10)
85.22(7)
Declaration of Competing Interest
92.53(10)
96.80(10)
124.21(10)
162.02(10)
153.30(10)
116.21(10)
95.65(11)
93.79(11)
117.23(11)
154.17(12)
161.61(12)
123.89(12)
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
Acknowledgment
3.6. Cell cytotoxicity in A549 and MDA-MB-231 cells
Financial support received from Science and Engineering Re-
search Board (SERB), Department of Science and Technology, India:
project no. ECR/2016/000382 (fellowship to S.K.); Council of Sci-
entific and Industrial Research (CSIR) (fellowship to A.M., K.S., B.G.
and R.R.) and Indian Institute of Technology Hyderabad are grate-
fully acknowledged. P.G. acknowledges fellowship support from
Ministry of Education. A.M. acknowledges funding support from
DST SERB (ECR/2017/002544) and IIT Hyderabad.
A colorimetric MTT assay was performed to examine the cy-
totoxicity induced by ruthenium complexes in A549 lung adeno-
carcinoma and MDA-MB-231 breast adenocarcinoma cells. The as-
say was performed using a yellow-colored MTT reagent which
gets converted to formazan crystals in live-cells. These crystals
were readily dissolved in an acidified isopropanol solvent to give
a violet-colored complex which was measured at 595 nm. This
conversion takes place in mitochondria and hence the assay de-
termines the viability of cells. We have examined the viability
of cancer cells upon treatment of all the complexes after 72 h.
The A549 cells treated with [1]Cl, [2]Cl and [2](PF6) exhibited
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.2021.
122074.
substantial cell cytotoxicity with IC50 values of 6.56
0.31 μM,
4.74 0.2 μM and 13.67 0.64, respectively. Moreover, these com-
plexes showed remarkable cell growth inhibition of MDA-MB-231
cells with an IC50 values of 1.13
0.046 μM, 0.36
0.016 μM
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