Aarene platinum group metal complexes containing imino-quinolyl ligands: synthesis and antibacterial studies

Article abstract of DOI:10.1080/00958972.2020.1753037

Imino-quinolyl Schiff-base ligands have been prepared by the condensation reaction of substituted 2-aminopyridine and quinoline-2-carbaldehyde. The reaction of [(arene)MCl₂]₂ with imino-quinolyl Schiff-base ligands leads to the forma

Full text of DOI:10.1080/00958972.2020.1753037

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Aarene platinum group metal complexes
containing imino-quinolyl ligands: synthesis and
antibacterial studies
Agreeda Lapasam, Sanjay Adhikari, Venkanna Banothu, Uma Addepally &
Mohan Rao Kollipara
To cite this article: Agreeda Lapasam, Sanjay Adhikari, Venkanna Banothu, Uma Addepally
& Mohan Rao Kollipara (2020): Aarene platinum group metal complexes containing imino-
quinolyl ligands: synthesis and antibacterial studies, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2020.1753037
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2020.1753037
Aarene platinum group metal complexes containing
imino-quinolyl ligands: synthesis and antibacterial studies
Agreeda Lapasama, Sanjay Adhikaria, Venkanna Banothub, Uma Addepallyb and
Mohan Rao Kolliparaa
b
aCentre for Advanced Studies in Chemistry, North Eastern Hill University, Shillong, India; Centre for
Biotechnology (CBT), Institute of Science & Technology (IST), Jawaharlal Nehru Technological
University Hyderabad (JNTUH), Hyderabad, India
ABSTRACT
ARTICLE HISTORY
Received 12 November 2019
Accepted 27 March 2020
Imino-quinolyl Schiff-base ligands have been prepared by the con-
densation reaction of substituted 2-aminopyridine and quinoline-2-
carbaldehyde. The reaction of [(arene)MCl2]2 with imino-quinolyl
Schiff-base ligands leads to the formation of cationic complexes
[(arene)M(L)Cl]þ (1–12). Single crystal X-ray diffraction studies were
used to confirm the coordination mode and structures of these com-
plexes. The molecular structures of these complexes revealed that
they adopt characteristic three-legged piano stool geometry with
the metal coordinating through a terminal chloride and imino-qui-
nolyl ligands in a bidentate chelating NN0 fashion. The ligand coordi-
nates to the metal center through the nitrogen of the quinoline and
the imine nitrogen forming a five-membered metallacycle. These
compounds were evaluated for their in vitro antibacterial activity by
the agar well diffusion method against Staphylococcus aureus,
Escherichia coli and Klebsiella pneumoniae strains. Results show that
all the ligands and complexes inhibited the growth of bacteria.
KEYWORDS
Ruthenium; rhodium;
iridium; imino-quinolyl;
antibacterial activity
CONTACT Mohan Rao Kollipara
mohanrao59@gmail.com
Centre for Advanced Studies in Chemistry, North
Eastern Hill University, Shillong, 793022, India.
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2020.1753037.
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
A. LAPASAM ET AL.
1. Introduction
One of the major advances in medical science over the last century has been the
development of antimicrobials. Due to challenges that current drugs face today, the
development of significant antibacterial compounds remains one of the attractive
areas. Drug-resistant pathogens have become an important cause of morbidity and
mortality worldwide; methicillin-resistant Staphylococcus aureus, vancomycin-resistant
Enterococcus, and fluoroquinolone resistant Pseudomonas aeruginosa show rapidly
increasing incidence of infection with treatment failure leading to high mortality rates
[1]. Extensive attention has been paid to the chemistry of half-sandwich organometal-
lic complexes because of their broad spectrum in biological and medicinal fields [2–5].
Ruthenium complexes of organic drugs can overcome resistance developed by the
microbe to the organic compound alone as ruthenium offers numerous advantages
including low micromolar potency, scope of controlling the shape and the chemical
and pharmacological properties of the complex by the abundant selection of the
arene and the ligands at the “legs” of the “piano-stool” structure [6]. Ruthenium com-
plexes show some potential against drug-resistant bacterial strains, including examples
of Ru ꢀ polypyridyl complexes and RAPTA-type complexes showing promising antibac-
terial activity [7]. Ruthenium mononuclear complexes have shown good antibacterial
activity, particularly against Gram-positive bacteria. Dinuclear ruthenium complexes
appear to show greater potential as their activity is generally maintained against anti-
biotic-resistant bacterial strains [8]. Binuclear rhodium(II) complexes [Rh2(OOCR)2(N-
N)2(H2O)2]2þ (N-N ¼ bi-py, phen.) are effective antibacterial and antitumor agents
[9, 10]. Syntheses of organoiridium(III) complexes have given rise to a broad range of
biological activities such as antiproliferative, antibacterial activity and the delocaliza-
tion of the p-electrons over the chelate ring helps to increase the lipophilicity of the
complexes and enables their penetration into bacterial cell membranes, hence hinder-
ing the growth of bacteria [11].
Quinoline (nitrogen-containing heterocycle) and its derivatives represent a class of
organic compounds having wide optical and bioactive properties. Large varieties of
quinoline core moieties can be repeatedly found in the structure of several naturally
occurring alkaloids. They have been correlated with a wide range of biological activ-
ities including malaria, bacterial infection and tuberculosis [12]. A quinoline based lig-
and has been employed as a fluorescent probe of Agþ ion in aqueous media [13].
Transition metal complexes of quinoline have also displayed their remarkable activity
as catalysts towards polymerization reactions [14]. Organoruthenium complexes of 8-
hydroxy quinoline have been reported which possess excellent anticancer properties,
thus making these complexes potential candidates as anticancer drugs [15].
Our group has reported many half-sandwich metal complexes bearing Schiff-base
ligands with interesting coordination modes of the ligands [16, 17]. Stimulated by the
success of both Schiff base and ruthenium complexes in biological activity, we have
combined both types to prepare arene metal complexes bearing an imino-quinolyl
Schiff base ligand. Keeping our interest with Schiff base ligands, herein we report the
synthesis, structural and biological aspects of 12 new conformationally rigid organo-
metallic half-sandwich complexes synthesized by the reaction of precursor complexes

JOURNAL OF COORDINATION CHEMISTRY
3
Chart 1. Ligands used in the present study.
with ligands in 1:2 ratios in the presence of ammonium hexafluorophosphate. Chart 1
represents the ligands used in the present study.
2. Experimental
2.1. Physical methods and materials
All reagents were purchased from commercial sources and used as received without
purification. 2-Quinolinecarboxaldehyde, 4-nitroaniline, and 2-aminopyridine were
obtained from Sigma-Aldrich. The solvents were purified and dried according to stand-
ard procedures [18]. All reactions were carried out under normal conditions. All gram-
positive and gram-negative bacterial strains used were obtained from the Department
of Microbiology, Osmania General Hospital, Hyderabad. The starting precursor metal
ꢁ
complexes [(p-cymene)RuCl2]2, [(benzene)RuCl2]2, and [Cp MCl2]2 (M ¼ Rh/Ir) were pre-
pared according to the literature methods [19, 20]. Infrared spectra were recorded on
1
a Perkin-Elmer 983 spectrophotometer by using KBr pellets from 400 to 4000 cmꢀ1. H
NMR spectra were recorded on a Bruker Avance II 400 MHz spectrometer using DMSO-
d6 and CDCl3 as solvents. Absorption spectra were recorded on a Perkin-Elmer
Lambda 25 UV/Visible spectrophotometer from 200 to 600 nm at room temperature in
acetonitrile. Mass spectra of 2–4 were recorded using a Q-Tof APCI-MS instrument
(model HAB 273). Mass spectra of 7, 8, 9, 11 and 12 were recorded using an Agilent
6540 UHD Accurate-Mass Q-TOF LC/MS instrument. Elemental analyses of the c were
performed on a Perkin-Elmer 2400 CHN analyzer.
2.2. Single-crystal X-ray structure analyses
Suitable single crystals of 1, 2 and 4 were obtained by slow diffusion of hexane into acet-
one or dichloromethane solution. Single crystal X-ray diffraction data for the complexes
were collected on an Oxford Diffraction Xcalibur Eos Gemini diffractometer at 293 K using
graphite monochromated Mo-Ka radiation (k ¼ 0.71073 Å). The strategy for the data col-
lection was evaluated using the CrysAlisPro CCD software. Crystal data were collected by
standard “phi–omega scan” techniques and were scaled and reduced using CrysAlisPro
RED software. The structures were solved with SHELXT-2016 [21] solution program using
the direct method and refined full-matrix least-squares with SHELXL-2016/SHELXL-2018/
3 refining on F2 [22]. Software packages used: CrysAlisPRO for data collection, for cell
refinement, data reduction and absorption correction [23]. The positions of all atoms
were obtained by direct methods. Metal ions in the complex were located from the
e-maps and non-hydrogen atoms were refined anisotropically. Crystallographic and

4
A. LAPASAM ET AL.
Table 1. Crystal data and structure refinement details of 1 and 2.
1
2
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)/a (ꢂ)
C50H50Cl2F12N6P2Ru2
1297.94
C21H17ClF6RuN3P
592.87
294(2)
0.17073
Monoclinic
275(2)
0.71073
Triclinic
P
1
P21/c
7.1052(3)/90
22.6904(11)/98.371(4)
13.7378(7)/90
2191.21(18)
8.9599(4)/77.655(4)
13.7443(5)/86.070(4)
21.5051(11)/85.436(3)
2575.3(2)
b (Å)/b (ꢂ)
c (Å)/c (ꢂ)
Volume (Å3)
Z
2
4
Density (calc) (Mg/mꢀ3
)
1.674
0.839
1304
1.797
0.976
1176
Absorption coeff. (m) (mmꢀ1
F(000)
)
Crystal size (mm3)
0.25 ꢃ 0.11 ꢃ 0.09
3.041 to 29.040
ꢀ11< ¼h< ¼10,
ꢀ18< ¼k< ¼16, ꢀ29< ¼l < 21
13601
0.26 ꢃ 0.23 ꢃ 0.15
3.08 to 29.03
ꢀ9< ¼h< ¼6,
ꢀ28< ¼k< ¼30, ꢀ18< ¼l < 14
8434
Theta range for data collection (ꢂ)
Index ranges
Reflections collected
Independent reflections
Completeness to theta ¼ 25.00ꢂ
Absorption correction
Refinement method
10645 [R(int) ¼ 0.0233]
4991 [R(int) ¼ 0.0343]
93.5%
99.6%
Semi-empirical from equivalents
Full-matrix least-squares on F2
10645/0/667
Semi-empirical from equivalents
Full-matrix least-squares on F2
4991/0/298
Data/restraints/parameters
Goodness-of-fit on F2
1.022
1.059
Final R indices [I > 2sigma(I)]
R indices (all data)
R1 ¼ 0.0483, wR2 ¼ 0.1102
R1 ¼ 0.0715, wR2 ¼ 0.1242
0.872 and ꢀ0.616
1953782
R1 ¼ 0.0453, wR2 ¼ 0.0917
R1 ¼ 0.0667, wR2 ¼ 0.1032
0.527 and ꢀ0.600
1953783
Largest diff. peak and hole (e.Åꢀ3
CCDC No.
)
Structures were refined on F02: wR2 ¼ [R[w(F0 ꢀ Fc ) ]/Rw(F02)2]1/2, where wꢀ1 ¼ [R(F02)þ(aP)2þbP] and P ¼
2
2 2
2
[max(F02, 0)þ2Fc ]/3.
structure refinement parameters for the complexes are summarized in Table 1, and
selected bond lengths and angles are presented in Table 2. The molecular structures of
the complexes are presented as thermal ellipsoid plots in Figures 1 and 2 [24].
2.3. Antibacterial activity
An agar-well diffusion method was employed for the evaluation of antibacterial activ-
ities of test compounds [25, 26]. All gram-positive and gram-negative bacterial strains
used in the present study were obtained from the Department of Microbiology,
Osmania General Hospital, Hyderabad. All strains were tested for purity by standard
microbiological methods. The bacterial stock cultures were maintained on Mueller-
Hinton agar slants and stored at 4 ꢂC. An agar-well diffusion method was employed
for the evaluation of antibacterial activities of test compounds. DMSO was used as a
negative control. The bacterial strains were reactivated from stock cultures by transfer-
ring into Mueller-Hinton broth and incubating at 37 ꢂC for 18 h. A final inoculum con-
taining 106 colonies forming units (1 ꢃ 106 CFU/ml) was added aseptically to MHA
medium and poured into sterile petri dishes. Different test compounds at a concentra-
tion of 200 mg per well were added to the wells (8 mm in diameter). Plates were incu-
bated overnight at 37 ꢂC and the zone of inhibition was measured by considering the
diameter around each well (mm). Experiments were performed in triplicate.

JOURNAL OF COORDINATION CHEMISTRY
5
Table 2. Selected bond lengths (Å) and angles (ꢂ) of 1 and 2.
1
2
M1-CNT
M1-N1
M1-N2
1.695
1.688
2.118(4)
2.085(4)
2.392(1)
76.6(1)
85.7(1)
87.3(1)
2.127(3)
2.075(3)
2.378(9)
76.6(1)
84.24(8)
86.99(8)
M1-Cl1
N1-M1-N2
N1-M1-Cl1
N2-M1-Cl1
CNT represents the centroid of the arene ring (arene ¼ p-cymene, ben-
zene) and M ¼ Ru.
Figure 1. (a) Thermal ellipsoid plot of 1. (b) Thermal ellipsoid plot of 2 with 50% probability.
Hydrogens are omitted for clarity.
2.4. MIC and MBC
The minimum inhibitory concentration (MIC) and minimum bactericidal concentration
(MBC) were determined according to a standard protocol [27]. The minimum inhibitory
concentration (MIC) and minimum bactericidal concentration (MBC) was determined
by the micro-broth dilution method done in 96 well plates according to standard
protocol. A 2-fold serial dilution of the compounds, with the appropriate antibiotic,
was prepared. Initially, 100 ml of MH broth was added to each well plate. Then 100 ml
of compound or antibiotic was taken from the stock solution and dissolved in the first
well plate. Serial dilution was done to obtain different concentrations. The stock con-
centrations of 2.0 mg/ml 24-hour culture turbidity were adjusted to match 0.5
McFarland standards which correspond to 1 ꢃ 108 CFU/ml. The standardized suspen-
sion (100 ml) of bacteria was added to all the wells except the antibiotic control well
and the 96 well plates were incubated at 37 ꢂC for 24 h. After 24 h of incubation, 40 ml
of MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) reagent (0.1 mg/
ml in 1x PBS) was added to all the wells. MIC was taken as the lowest concentration,
which did not show any growth, which was visually noted from the blue color, devel-
oped by MTT. Subcultures were made from clear wells and the lowest concentration
that yielded no growth after subculturing was taken as the MBC.

6
A. LAPASAM ET AL.
Figure 2. Ball and stick plot of 4. Hydrogens are omitted for clarity. This complex crystallized in
monoclinic system with the following unit cell a ¼ c ¼ 90ꢂ, b ¼ 108.467(8), a ¼ 12.0587(11),
b ¼ 14.9542(10) and c ¼ 15.2979(11). Because of the disorder in the crystal the structure of this
complex presented here to only confirm the composition of molecule.
2.5. Synthesis of imino-quinolyl ligands
The imino-quinolyl Schiff base ligands L1 and L2 were prepared by the reaction of quin-
oline-2-carbaldehyde (471.5 mg, 3.000 mmol) and the desired substituted 2-aminopyri-
dine (282.4 mg, 3.000 mmol (L1), 324.5 mg, 3.000 mmol (L2)). Ligand L3 was prepared by
condensation of quinoline-2-carbaldehyde (471.5 mg, 3.0 mmol) and 4-nitroaniline
(414.5 mg, 3.001 mmol). Quinoline-2-carbaldehyde was dissolved in 10 ml of ethanol to
this solution, added substituted 2-aminopyridine or 4-nitroaniline and this reaction
mixture was heated under reflux for 24 h. The solvent was evaporated under reduced
pressure; the product was obtained and used without further purification (Scheme 1).
2.6. General procedure for preparation of imino-quinolyl metal complexes 1–12
A mixture of metal precursor [(arene)RuCl2]2 {arene ¼ p-cymene (61.2 mg, 0.100 mmol),
ꢁ
benzene (50.0 mg, 0.099 mmol)} or [Cp MCl2]2 {M ¼ Rh (61.8 mg, 0.099 mmol)/Ir
(79.7 mg, 0.100 mmol) and imino-quinolyl ligand {L1 (46.6 mg) L2 (49.4 mg) L3
(55.4 mg)} (0.200 mmol) was dissolved in dry methanol (5 ml) and stirred at room tem-
perature for 1 h. Then 5 equivalents of NH4PF6 dissolved in dry methanol (2 ml) was
added dropwise to the reaction mixture and stirring continued for a further 5–6 h
whereupon a yellow, red or brown solid precipitated out from the reaction mixture
(Scheme 2). The precipitate was collected by filtration, washed with cold methanol
and diethyl ether and air dried.
2.6.1. [(P-cymene)Ru(L1)Cl]PF6 (1)
Red solid, Yield: 44.2 mg (68%); IR (KBr, cmꢀ1): 3447(m), 1590(m), 1568(m), 1462(m),
1437(m), 845(s); 1H NMR (400 MHz, CDCl3, ppm): 8.98 (s, 1H, CH(imine)), 8.60 (d, 1H,

JOURNAL OF COORDINATION CHEMISTRY
7
Scheme 1. Synthesis of L1-L3.
J ¼ 12 Hz), 8.52 (d, 1H, J ¼ 4 Hz), 8.46 (d, 1H, J ¼ 8 Hz), 8.10 (d, 1H, J ¼ 8 Hz), 7.91–8.00
(m, 3H), 7.86 (t, 1H, J ¼ 8 Hz), 7.77 (t, 1H, J ¼ 8 Hz), 7.40 (t, 1H, J ¼ 4 Hz), 5.69 (d, 1H,
J ¼ 4 Hz, CH(p-cym)), 5.59 (dd, 2H, J ¼ 4 and 4 Hz, CH(p-cym)), 5.26 (d, 1H, J ¼ 8 Hz, CH(p-
cym)), 2.27 (sept, 1H, CH(p-cym)), 2.11 (s, 3H, CH(p-cym)), 0.86 (d, 3H, J ¼ 8 Hz, CH(p-cym)),
0.73 (d, 3H, J ¼ 8 Hz, CH(p-cym)); 13C NMR (100 MHz, CDCl3, ppm): 167.04, 159.87, 154.34,
148.96, 148.74, 140.54, 139.14, 133.44, 130.62, 129.30, 128.94, 128.22, 125.56, 124.38,
118.76, 106.13, 105.37, 86.01, 85.85, 84.63, 84.56, 30.60, 21.67, 21.06, 18.29; UV-Vis
{Acetonitrile, kmax nm (e/10ꢀ4
M
ꢀ1 cmꢀ1)}: 263 (1.80), 302 (1.19), 346 (1.18), 462 (0.36);
Anal. Calc for C25H25ClN3F6PRu (648.97): C, 46.27; H, 3.88; N, 6.47. Found: C, 46.39; H,
3.95; N, 6.63%.
2.6.2. [(Benzene)Ru(L1)Cl]PF6 (2)
Red solid, Yield: 43.8 mg (74%); IR (KBr, cmꢀ1): 3102(m), 3033(m), 1592(m), 1568(w),
1
1461(m), 1438(m), 838(s); H NMR (400 MHz, DMSO-d6, ppm): 9.57 (s, 1H) 8.94 (d, 1H,
J ¼ 8 Hz), 8.86 (d, 1H, J ¼ 8 Hz), 8.75 (d, 1H, J ¼ 4 Hz), 8.51 (d, 1H, J ¼ 8 Hz), 8.32 (d, 1H,
J ¼ 12 Hz), 8.15–8.21 (m, 3H), 8.02 (t, 1H, J ¼ 8 Hz), 7.71 (t, 1H, J ¼ 8 Hz), 6.10 (s, 6H,
CH(benzene)); 13C NMR (100 MHz, DMSO-d6, ppm): 168.74, 159.68, 155.41, 148.76, 148.63,
140.74, 139.38, 133.20, 130.22, 129.50, 129.07, 128.96, 125.58, 124.99, 119.76, 87.50;
UV-Vis {Acetonitrile, kmax nm (e/10ꢀ4
M
ꢀ1 cmꢀ1)}: 263 (2.54), 302 (1.97), 346 (1.98), 450
(0.57); HRMS-APCI (m/z): 448.0139 [M-PF6]þ; Anal. Calc. for C21H17ClN3F6PRu (592.86): C,
42.54; H, 2.89; N, 7.09. Found: C, 42.68; H, 2.81; N, 7.18%.
ꢁ
2.6.3. [Cp Rh(L1)Cl]PF6 (3)
Brown powder, Yield 46.9 mg (72%); IR (KBr, cmꢀ1): 3368(m), 2924(m), 1590(m),
1567(m), 1460(m), 1436(w), 845(s); 1H NMR (400 MHz, CDCl3, ppm): 9.27 (s, 1H,
CH(imine)), 8.69 (t, 2H, J ¼ 8 Hz), 8.61 (d, 2H, J ¼ 8 Hz), 8.28 (d, 1H, J ¼ 4 Hz), 8.14 (d, 1H,
J ¼ 12 Hz), 7.97–8.07 (m, 2H), 7.91 (t, 1H, J ¼ 8 Hz), 7.53–7.56 (m, 1H), 1.43 (s, 15H,
CH(Cp )); 13C NMR (100 MHz, CDCl3, ppm): 168.23, 158.51, 156.41, 148.28, 147.63,
ꢁ
140.63, 138.36, 134.20, 131.42, 129.71, 129.18, 128.35, 125.17, 122.28, 119.57, 87.38,
8.23; UV-Vis {Acetonitrile, kmax nm (e/10ꢀ4
M
ꢀ1 cmꢀ1)}: 262 (1.54), 292 (0.95), 347

8
A. LAPASAM ET AL.
Scheme 2. Synthesis of metal complexes 1–12.
(0.85), 409 (0.38); HRMS-APCI (m/z): 506.0846 [M-PF6]þ; Anal. Calc for C25H26ClN3F6PRh
(651.81): C, 46.07; H, 4.02; N, 6.45. Found: C, 46.14; H, 4.11; N, 6.58%.
ꢁ
2.6.4. [Cp Ir(L1)Cl]PF6 (4)
Brown powder, Yield 51.1 mg (69%); IR (KBr, cmꢀ1): 3135(m), 1590(m), 1563(m),
1
1468(w), 1436(m), 844(s); H NMR (400 MHz, CDCl3, ppm): 9.61 (s, 1H, CH(imine)), 8.70 (t,
1H, J ¼ 4 Hz), 8.66 (d, 1H, J ¼ 8 Hz), 8.57 (d, 1H, J ¼ 8 Hz), 8.42 (d, 1H, J ¼ 8 Hz), 8.23 (d,
1H, J ¼ 8 Hz), 8.13 (d, 1H, J ¼ 8 Hz), 7.99 (t, 2H, J ¼ 8 Hz), 7.91 (t, 1H, J ¼ 8 Hz), 7.53–7.56
(m, 1H), 1.45 (s, 15H, CHCp ); 13C NMR (100 MHz, CDCl3, ppm): 167.15, 159.54, 157.33,
ꢁ
149.28, 146.13, 139.25, 138.36, 135.75, 130.42, 129.88, 129.08, 128.23, 126.48, 123.65,
119.13, 86.16, 8.07; UV-Vis {Acetonitrile, kmax nm (e/10ꢀ4
M
ꢀ1 cmꢀ1)}: 264 (1.37), 312
(0.83), 348 (0.88), 428 (0.28); HRMS-APCI (m/z): 596.1563 [M-PF6]þ; Anal. Calc for
C25H26ClN3F6PIr (741.12): C, 40.51; H, 3.54; N, 5.67. Found: C, 40.72; H, 3.62; N, 5.76%.
2.6.5. [(P-cymene)Ru(L2)Cl]PF6 (5)
Brown solid, Yield 48.3 mg (73%); IR (KBr, cmꢀ1): 2967(w), 1606(m), 1513(m), 1469(w),
1
1407(m), 843(s); H NMR (400 MHz, CDCl3, ppm): 9.41 (s, 1H, CH(imine)), 7.99 (s, 1H), 7.85
(d, 3H, J ¼ 8 Hz), 7.35 (d, 2H, J ¼ 8 Hz), 6.89 (s, 2H), 6.67 (s, 1H), 5.49 (d, 2H, J ¼ 4 Hz,
CH(p-cym)), 5.36 (d, 2H, J ¼ 4 Hz, CH(p-cym)), 2.87–2.98 (sept, 1H, CH(p-cym)), 2.43 (s, 3H),
2.16 (s, 3H, CH(p-cym)), 1.28 (d, 6H, J ¼ 8 Hz, CH(p-cym)); 13C NMR (100 MHz, CDCl3, ppm):

JOURNAL OF COORDINATION CHEMISTRY
9
167.17, 160.70, 157.43, 154.86, 151.68, 148.83, 141.02, 134.01, 131.10, 129.80, 129.42,
128.77, 126.98, 124.81, 119.49, 86.32, 86.09, 86.03, 85.45, 31.20, 22.29, 21.39, 18.69,
14.00; UV-Vis {Acetonitrile, kmax nm (e/10ꢀ4
M
ꢀ1 cmꢀ1)}: 262 (0.451), 396 (0.128); Anal.
Calc for C26H27ClN3RuF6P (663.00): C, 47.10; H, 4.10; N, 6.34. Found: C, 47.18; H, 4.06;
N, 6.41%.
2.6.6. [(Benzene)Ru(L2)Cl]PF6 (6)
Yellow solid, Yield 42.4 mg (70%); IR (KBr, cmꢀ1): 2966(m), 1605(m), 1536(m), 1469(m),
1
1439(m), 842(s); H NMR (400 MHz, CDCl3, ppm): 9.54 (s, 1H), 8.38 (s, 2H), 7.88 (d, 1H,
J ¼ 8 Hz), 7.67 (d, 2H, J ¼ 12 Hz), 7.57 (d, 1H, J ¼ 4 Hz), 7.00 (d, 2H, J ¼ 8 Hz), 6.48 (s, 1H),
6.09 (s, 6H, CH(benzene)), 2.44 (s, 6H); UV-Vis {Acetonitrile, kmax nm (e/10ꢀ4
M
ꢀ1 cmꢀ1)}:
236 (0.452), 320 (0.402), 389 (0.256); Anal. Calc for C22H19ClN3F6PRu (606.89): C, 43.54;
H, 3.16; N, 6.92. Found: C, 43.61; H, 3.23; N, 6.97%.
ꢁ
2.6.7. [Cp Rh(L2)Cl]PF6 (7)
Yellow solid, Yield 45.2 mg (68%); IR (KBr, cmꢀ1): 3154(m), 1603(s), 1511(m), 1402(s),
1
842(s); H NMR (400 MHz, CDCl3, ppm): 9.78 (s, 1H, CH(imine)), 8.21 (s, 1H), 7.98 (s, 1H),
7.84 (d, 2H, J ¼ 8 Hz), 7.34 (d, 2H, J ¼ 8 Hz), 6.88 (d, 2H, J ¼ 4 Hz), 6.66 (s, 1H), 2.42 (s,
3H), 1.60 (s, 15H, CH(Cp )); 13C NMR (100 MHz, CDCl3, ppm): 168.85, 161.08, 157.95,
ꢁ
154.14, 153.79, 152.63, 138.69, 138.35, 133.83, 130.14, 127.44, 127.18, 126.58, 125.99,
119.06, 95.97, 19.99, 7.34; UV-Vis {Acetonitrile, kmax nm (e/10ꢀ4
M
ꢀ1 cmꢀ1)}: 223
(0.762), 497 (0.125); ESI-MS (m/z): 520.119 [M-PF6]þ. Anal. Calc for C26H28ClN3F6PRh
(665.84): C, 46.90; H, 4.24; N, 6.31. Found: C, 46.89; H, 4.29; N, 6.27%.
ꢁ
2.6.8. [Cp Ir(L2)Cl]PF6 (8)
Red solid, Yield 48.3 mg (64%); IR (KBr, cmꢀ1): 2965(m), 1604(m), 1513(m), 1456(w),
1408(m), 844(s); 1H NMR (400 MHz, CDCl3, ppm): 8.99 (s, 1H, CH(imine)), 8.38 (d, 1H,
J ¼ 8 Hz), 8.09 (d, 1H, J ¼ 8 Hz), 8.02 (d, 1H, J ¼ 8 Hz), 7.80 (d, 1H, J ¼ 8 Hz), 7.62 (d, 2H,
J ¼ 8 Hz), 7.35 (t, 1H, J ¼ 8 Hz), 7.29 (d, 1H, J ¼ 8 Hz), 6.51 (s, 1H), 2.39 (s, 3H), 1.53 (s,
15H, CH(Cp )); 13C NMR (100 MHz, DMSO-d6, ppm): 168.88, 156.74, 153.96, 151.33,
ꢁ
148.90, 146.09, 142.07, 134.22, 133.59, 131.21, 129.70, 129.01, 127.00, 124.80, 120.13,
90.87, 21.95, 8.75; UV-Vis {Acetonitrile, kmax nm (e/10ꢀ4
M
ꢀ1 cmꢀ1)}: 242 (0.577), 288
(0.456), 502 (0.113); ESI-MS (m/z): 610.160 [M-PF6]þ. Anal. Calc for C26H28ClN3F6PIr
(755.15): C, 41.35; H, 3.74; N, 5.56. Found: C, 41.72; H, 3.71; N, 5.69%.
2.6.9. [(P-cymene)Ru(L3)Cl]PF6 (9)
Yellow solid, Yield 56.8 mg (82%); IR (KBr, cmꢀ1): 2922(m), 1642(m), 1524(s), 1347(m),
1
844(s); H NMR (400 MHz, CDCl3 þ DMSO-d6, ppm): 8.78 (s, 1H, CH(imine)), 8.66 (d, 1H,
J ¼ 8 Hz), 8.56 (d, 1H, J ¼ 8 Hz), 8.36 (d, 2H, J ¼ 8 Hz), 8.28 (d, 1H, J ¼ 8 Hz), 8.19 (d, 2H,
J ¼ 12 Hz), 8.04 (t, 1H, J ¼ 8 Hz), 7.97 (d, 1H, J ¼ 8 Hz), 7.86 (t, 1H, J ¼ 8 Hz), 5.69 (d, 1H,
J ¼ 4 Hz, CH(p-cym)), 5.64 (d, 1H, J ¼ 4 Hz, CH(p-cym)), 5.39 (d, 1H, J ¼ 4 Hz, CH(p-cym)), 5.20
(d, 1H, J ¼ 4 Hz, CH(p-cym)), 2.51 (sept, 1H, CH(p-cym)), 2.03 (s, 3H), 0.78 (d, 6H); 13C NMR
(100 MHz, CDCl3 þ DMSO-d6, ppm): 175.15, 161.32, 160.02, 152.94, 145.84, 138.58,
135.79, 134.34, 133.91, 130.14, 128.61, 117.53, 91.28, 90.75, 90.66, 89.92, 35.61, 27.15,
26.26, 23.45; UV-Vis {Acetonitrile, kmax nm (e/10ꢀ4
M
ꢀ1 cmꢀ1)}: 232 (0.217), 312 (0.196),

10
A. LAPASAM ET AL.
496 (0.110); ESI-MS (m/z): 548.105 [M-PF6]þ. Anal. Calc for C26H25ClN3O2RuPF6 (692.98):
C, 45.06; H, 3.64; N, 6.06. Found: C, 45.02; H, 3.67; N, 5.99%.
2.6.10. [(Benzene)Ru(L3)Cl]PF6 (10)
Yellow solid, Yield 41.3 mg (65%); IR (KBr, cmꢀ1): 2926(m), 1637(w), 1525(s), 1461(w),
1
1384(s), 851(s); H NMR (400 MHz, CDCl3, ppm): 9.25 (s, 1H), 8.85 (t, 2H, J ¼ 8 Hz), 8.49
(d, 2H, J ¼ 4 Hz), 8.27 (d, 3H, J ¼ 4 Hz), 8.19 (s, 1H) 8.00 (d, 1H, J ¼ 4 Hz), 7.93 (d, 1H,
J ¼ 4 Hz), 5.99 (s, 6H, CH(benzene)); 13C NMR (100 MHz, CDCl3 þ DMSO-d6, ppm): 171.18,
156.66, 155.88, 149.14, 148.11, 141.24, 133.77, 130.77, 129.60, 129.47, 125.33, 124.37,
88.14; UV-Vis {Acetonitrile, kmax nm (e/10ꢀ4
M
ꢀ1 cmꢀ1)}: 223 (0.382), 376 (0.482), 503
(0.132); Anal. Calc for C22H17ClN3O2RuPF6 (636.87): C, 41.49; H, 2.69; N, 6.60. Found: C,
41.52; H, 2.73; N, 6.68%.
ꢁ
2.6.11. [Cp Rh(L3)Cl]PF6 (11)
Orange solid, Yield 55.0 mg (79%); IR (KBr, cmꢀ1): 2924(m), 1635(m), 1521(s), 1382(w),
1347(s), 841(s); 1H NMR (400 MHz, CDCl3, ppm): 9.03 (s, 1H, CH(imine)), 8.73 (d, 1H,
J ¼ 8 Hz), 8.53 (d, 1H, J ¼ 8 Hz), 8.46 (d, 2H, J ¼ 8 Hz), 8.41 (d, 1H, J ¼ 8 Hz), 8.30 (d, 2H,
J ¼ 8 Hz), 8.16 (d, 1H, J ¼ 8 Hz), 8.07 (t, 1H, J ¼ 8 Hz), 7.93 (t, 1H, J ¼ 8 Hz), 1.43 (s, 15H,
CH(Cp )); 13C NMR (100 MHz, CDCl3 þ DMSO-d6, ppm): 169.36, 153.17, 151.99, 147.11,
ꢁ
145.72, 140.52, 132.16, 130.09, 128.59, 128.10, 124.38, 123.44, 97.25, 8.16; UV-Vis
{Acetonitrile, kmax nm (e/10ꢀ4
M
ꢀ1 cmꢀ1)}: 318(0.789), 512 (0.214); ESI-MS (m/z):
550.078 [M-PF6]þ. Anal. Calc for C26H26ClN3O2RhPF6 (695.82): C, 44.88; H, 3.77; N, 6.04.
Found: C, 44.90; H, 3.81; N, 6.03%.
ꢁ
2.6.12. [Cp Ir(L3)Cl]PF6 (12)
Orange solid, Yield 49.2 mg (65%); IR (KBr, cmꢀ1): 2938(m), 1634(m), 1522(s), 1406(w),
1352(s), 844(s); 1H NMR (400 MHz, CDCl3, ppm): 9.75 (s, 1H, CH(imine)), 8.82 (d, 2H,
J ¼ 8 Hz), 8.49 (d, 3H, J ¼ 8 Hz), 8.24 (d, 3H, J ¼ 8 Hz), 8.11 (t, 1H, J ¼ 4 Hz), 7.99 (t, 1H,
J ¼ 8 Hz), 1.43 (s, 15H, CH(Cp )); 13C NMR (100 MHz, CDCl3 þ DMSO-d6, ppm): 170.72,
ꢁ
153.58, 145.69, 140.83, 133.46, 132.30, 130.24, 129.16, 128.68, 127.08, 124.92, 120.56,
97.73, 8.57; UV-Vis {Acetonitrile, kmax nm (e/10ꢀ4
M
ꢀ1 cmꢀ1)}: 220 (0.421), 352 (0.423),
521 (0.231); ESI-MS (m/z): 640.134 [M-PF6]þ. Anal. Calc for C26H26ClN3O2IrPF6 (785.13):
C, 39.77; H, 3.34; N, 5.35. Found: C, 39.83; H, 3.41; N, 5.39%.
3. Results and discussion
3.1. Synthesis of complexes
Metal complexes 1–12 were synthesized by the reaction of metal precursors with an
imino-quinolyl ligand in methanol. The complexes were isolated as cationic salts with
ꢀ
PF6 counter ions. All these metal complexes were obtained in good yields and are
yellow, red or brown in color. They are stable in air as well as in the solid state, and
are non-hygroscopic. These complexes are soluble in common organic solvents such
as dichloromethane, acetonitrile, acetone and DMSO, but insoluble in diethyl ether
and hexane. The metal complexes were fully characterized by spectroscopic

JOURNAL OF COORDINATION CHEMISTRY
11
techniques. The molecular structures of some of the complexes were established by
single crystal X-ray analysis.
3.2. Spectroscopic characterization of imino-quinolyl metal complexes
3.2.1. Ir studies of metal complexes
In the IR spectra of the metal complexes the C ¼ N stretching frequencies were
detected in lower frequency region around 1563–1592 cmꢀ1 which suggest coordin-
ation of the imino-quinolyl ligand occurs through the imine and quinoline nitrogen
atoms. A strong band is also observed in the spectra of 9–12 in the region
1521–1525 cmꢀ1, corresponding to the asymmetric stretching vibration of N-O. All the
metal complexes displayed a sharp band around 838–851 cmꢀ1 corresponding to the
P-F stretching frequency of the counter ion [28].
1
3.2.2. H NMR studies of metal complexes
The proton NMR spectra of the metal complexes further confirm formation of the
complexes. In the metal complexes, the signals associated with the ligand protons
were observed in the downfield region as compared to the free ligand which suggests
the coordination of the imino-quinolyl ligand to the metal ion. The aromatic proton
signals were observed in the range 6.51–8.94 ppm. The imine proton signal was
observed in the downfield region 8.98–9.78 ppm as compared to the free ligand at
8.06 ppm, which indicates the coordination of the imine bond to the metal center. The
aromatic protons of the p-cymene ligand of 1, 5 and 9 are doublets in the range
5.20–5.69 ppm. The isopropyl group of the p-cymene ligand was observed as doublets
around 1.28–0.73 ppm, the methyl protons of the p-cymene ligand were observed as a
singlet and methine protons of the p-cymene ligand were observed as a heptet. This
splitting pattern of p-cymene ligand protons is due to the desymmetrization of the p-
cymene moiety upon coordination of the imino-quinolyl ligand, which correlates well
with our previous reported complexes [29]. The aromatic protons of the benzene
ruthenium complexes 2, 6 and 10 displayed singlets at 6.10, 6.09 and 5.99 ppm,
ꢁ
respectively, corresponding to the six protons of the benzene ring. The Cp Rh and
ꢁ
Cp Ir complexes 3, 4, 7, 8, 11 and 12 exhibited singlets at 1.43, 1.45, 1.60, 1.53, 1.43
ꢁ
and 1.43 ppm, respectively, corresponding to the methyl protons of the Cp group.
Thus, the proton NMR spectra of the complexes strongly support the formation of the
metal complexes.
3.2.3. 13C NMR studies of the metal complexes
Further confirmation of the formation of complexes was established by measuring the
13C NMR spectra of the complexes. The carbon resonances associated with the ligand
were observed in the downfield region 118 to 175 ppm. In 1, 5 and 9 the aromatic
carbon signals for the p-cymene ligand were observed in the region 84–109 ppm while
the methine, methyl and isopropyl carbon resonances were observed in the region
18–31 ppm. Complexes 2 and 10 displayed sharp signals at 87.50 and 88.14 ppm for
ꢁ
the carbon of benzene ring. The ring carbons of the Cp ligand for 3, 4, 7, 8, 11 and

12
A. LAPASAM ET AL.
ꢁ
12 were observed around 97.73–86.16 ppm and the signal for methyl carbons of Cp
was observed in the range 8.75–7.34 ppm.
3.2.4. Electronic spectra of metal complexes
The electronic spectra of the imino-quinolyl metal complexes were recorded in aceto-
nitrile solution at room temperature and the respective plots are shown in supporting
information Figures S20 and S21. These complexes displayed three absorption bands
in the region around 225–500 nm. The band in the higher energy region around
ꢁ
ꢁ
225–320 nm and 300–350 nm can be assigned as ligand centered p-p and n-p transi-
tion. The low oxidation state d6 metal complexes provide filled dp (t2g) orbitals, which
ꢁ
can interact with low-lying p orbitals of the imino-quinolyl ligand and therefore one
can expect a metal to ligand (MLCT) charge transfer band. The broad absorption band
in the visible region around 405–500 nm has been attributed as metal to ligand charge
ꢁ
transfer (MLCT) band arising from M (dp) to p (ligand) transition.
The m/z values of the representative complexes are listed in the Experimental sec-
tion and are presented in the Supporting Information, supporting information Figures
S22–S29. Complexes 2, 3, 4, 7, 8, 9, 11 and 12 display predominant peaks at m/z
448.0139, 506.0846, 596.1563, 520.119, 610.160, 548.105, 550.078 and 640.134, respect-
ively, corresponding to [M-PF6]þ ion peak.
3.3. Description of the crystal structures of complexes
In addition to the spectroscopic analysis, we were also able to establish the molecular
ꢁ
structures of some of 1, 2 and 4. The methyl groups of Cp in 4 are disordered result-
ing in the methyl groups having large thermal ellipsoids. Complex 4’s crystal structure
is presented to only confirm the composition of molecule. Single crystal X-ray diffrac-
tion analysis was carried out to confirm the coordination of the imino-quinolyl ligand
to the metal ion and to understand the geometry of the complexes. Thermal ellipsoid
plots of the complexes along with the atom numbering scheme are shown in Figures
1 and 2, respectively. The detailed summary of the crystal data including data collec-
tion and structure refinement parameters are summarized in Table 1 and selected
bond lengths and angles, including those involving the metal ion and ring centroids,
are listed in Table 2. Complex 1 crystallized in the triclinic system with space group
P
1 and 2 crystallized in the monoclinic crystal system with space group P21/c.
All these complexes have a cationic species with the general formula [(arene)M(L)Cl]
with counter-ion PF6ꢀ. These complexes adopt a typical three-legged piano-stool geom-
etry (a description commonly employed for half-sandwich complexes) around the metal
center with coordination sites occupied by an arene ring (arene ¼ p-cymene and ben-
zene) in a g6 manner, two nitrogen donor atoms from an imino-quinolyl ligand in a che-
lating bidentate j2 NNʹ fashion and a terminal chloride ion. The coordination around the
metal center is pseudo-octahedral wherein the arene ligand forms the seat and imino-
quinolyl ligand and terminal chloride forms the legs, thus satisfying a pseudo-octahedral
arrangement. The imino-quinolyl ligand coordinates to the metal center through quinolyl
nitrogen N(1) and imine nitrogen (N2) in a bidentate chelating manner, thus forming a
five-membered chelate ring (Figures 1 and 2). The pyridine nitrogen N(3) is not involved

JOURNAL OF COORDINATION CHEMISTRY
13
in coordination. The distances between the metal and centroid of the arene ring (arene ¼
p-cymene and benzene) are 1.695 (1) and 1.688 (2) Å. The Ru-N(1) bond lengths from the
quinoline {2.188(4) (1) and 2.127(3) (2) Å} are comparatively longer than the imine nitro-
gen-metal N(2)-Ru distances {2.085(4) (1) and 2.075(3) (2) Å}. The observed Ru-N and Ru-
Cl bond lengths in these imino-quinolyl complexes are in close agreement with similarly
reported complexes with Schiff-base ligands [30]. The bond angle values N-Ru-Cl are
close to 90ꢂ and are consistent with the piano-stool arrangement of the various group
about the metal center (Table 2).
3.4. Antibacterial activity
The antibacterial potential of ligands and 1–12 were evaluated in vitro against three
tested organisms: Gram-positive S. aureus, Gram-negative Escherichia coli and Klebsiella
pneumoniae strains. The results in terms of zone of inhibition (mm) are compared with
the activity of the standards ciprofloxacin (positive control). Inhibition results are meas-
ured to the nearest millimeter, which is tabulated in supporting information Table S1.
The histogram of the zone of inhibition is presented in Figure 3.
The antibacterial activity of the starting metal precursors of [(p-cymene)RuCl2]2,
ꢁ
ꢁ
[(benzene)RuCl2]2, [Cp RhCl2]2 and [Cp IrCl2]2 were found to be inactive as previously
reported [31]. All the complexes displayed effective antibacterial activity against all the
studied bacterial strains, but p-cymene ruthenium chelates have high antibacterial
activity. The slightly better activity for some metal chelates compared to the free
ligands can be elucidated on the basis of chelation theory [32]. In vitro assay results
revealed that ligands and complexes were more active against the Gram-negative bac-
teria than against the Gram-positive bacterium. The variance in the effectiveness of
the complexes against the tested organisms is based on the difference in the cell wall
structure of the Gram-negative and Gram-positive bacteria, ribosome of the microbial
cells or impermeability of the cells of microbes [33]. A Gram-negative bacterium has a
thin peptidoglycan layer and an outer membrane that contains proteins, lipopolysac-
charide, and phospholipids, while a Gram-positive bacterium has a thick peptidoglycan
layer that contains lipoteichoic acid. Hence, the cell wall of a Gram-negative bacterium
is more polar, and the permeation of complexes into the microorganism is enabled by
this polarity. Therefore, the effectiveness of the investigated compounds against K.
pneumoniae is greater than against S. aureus. Furthermore, the results show that p-
ꢁ
cymene ruthenium complex 1, complex 5, complex 9 and Cp rhodium complex 7
showed good activity whereas benzene ruthenium complex 2 and 6 showed the least
activity compared to other complexes. The comparison of metal ion with free ligands
has been made and the results are presented in Figure 4.
It is also worth comparing these results with the antibacterial activity that was deter-
mined for some of the previously prepared ruthenium complexes. In our previous study
antibacterial activities of ruthenium, rhodium and iridium complex containing azine ligands
were determined and showed lower activities to those obtained in our present study [34].
Comparing these results to other reported ruthenium(II) complexes the inhibition proper-
ties of these complex are better [35]. It is clear that many factors govern the antibacterial
properties of ruthenium complexes (e.g. oxidation state of metal; type of ligand, etc.) and

14
A. LAPASAM ET AL.
Figure 3. Histogram of the zone of inhibition (mm) of the ligands and complexes 1–12 in com-
parison with ciprofloxacin. All the complexes data are means (n ¼ 3)
Standard deviation of
three replicates.
more research is needed to get a stronger depiction. However, the activities of all the
tested complexes are less effective compared to the positive control ciprofloxacin.
3.5. MIC and MBC
The minimum inhibitory concentration (MIC) and minimum bactericidal concentration
(MBC) results are listed in supporting information Table S2 and Figure 5. The MIC and
MBC values of ligands and complexes 1–12 ranged from 0.015 to 0.5 mg/ml against all
three organisms. The MIC and MBC values of 1 ranged from 0.031 to 0.062 mg/ml for
S. aureus and K. pneumoniae and 0.015 to 0.031 mg/ml for E. coli and the values of 7
ranged from 0.062 to 0.125 mg/ml for S. aureus and 0.031 to 0.062 mg/ml for E. coli
and K. pneumoniae. The MIC and MBC values of standard ciprofloxacin, which range
from 0.031 to 0.062 mg/ml and 0.062 to 0.0125 mg/ml against the tested organisms,
were taken as standards. It was found that the MBC values attained for the ligand and
complexes are twice that of the corresponding MIC values. As the MBC values were
twice the MIC values it can be concluded that the ligand and complexes are bacterio-
static rather than bactericidal.
4. Conclusion
We have synthesized new imino-quinolyl ligands and their corresponding ruthenium,
rhodium and iridium imino-quinolyl complexes. All these complexes and ligands were
fully characterized by spectroscopic analysis. These complexes were isolated as cat-
ꢀ
ionic salts with PF6 counter ion. The molecular structures of some of the complexes
were established by single-crystal X-ray diffraction studies. The imino-quinolyl ligand
preferably binds to the metal in a bidentate NNꢀ fashion using quinoline and imine

JOURNAL OF COORDINATION CHEMISTRY
15
Figure 4. Histogram of the zone of inhibition (mm) of the metal ions in comparison with free
ꢁ
ligands (where “ ” indicated no zone of inhibition).
Figure 5. MIC and MBC of the ligands and complexes 1–12.
nitrogen atoms forming a five-membered ring. The pyridine nitrogen N(3) is not
involved in coordination to the metal center. All the compounds investigated in the
present study were tested for their antibacterial properties against three strains of
bacterial microorganisms, S. aureus (þve), K. pneumoniae (-ve) and E. coli (-ve). The
compounds showed activity against both Gram-negative bacteria and Gram-positive
bacteria.

16
A. LAPASAM ET AL.
Acknowledgements
Agreeda Lapasam acknowledges the financial support of CSIR-HRDG, New Delhi in the form of
SRF-NET fellowship award no. 09/347(0223)/2017-EMR-I. We thank DST-PURSE SCXRD, NEHU-
SAIF, Shillong, India for providing single-crystal X-ray analysis and other spectral studies.
Disclosure statement
No potential conflict of interest was reported by the authors.
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