Synthesis and structural characterization of arene d6 metal complexes of sulfonohydrazone and triazolo ligands: High potency of triazolo derivatives towards DNA binding

Article abstract of DOI:10.1016/j.poly.2018.08.051

Complexation of [(p-cymene)RuCl₂]₂ and [Cp?MCl₂]₂ (M = Rh/Ir) with chelating ligand 4-methylbenzenesulfonohydrazone ligands (L1 and L3) resulted in the formation of mononuclear cationic complexes having PF₆

Full text of DOI:10.1016/j.poly.2018.08.051

Accepted Manuscript
Synthesis and structural characterization of arene d6 metal complexes of sulfo-
nohydrazone and triazolo ligands: high potency of triazolo derivatives towards
DNA binding
Lathewdeipor Shadap, Nidhi Joshi, Krishna Mohan Poluri, Mohan Rao
Kollipara, Werner Kaminsky
PII:
S0277-5387(18)30519-9
https://doi.org/10.1016/j.poly.2018.08.051
POLY 13374
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
5 July 2018
17 August 2018
20 August 2018
Please cite this article as: L. Shadap, N. Joshi, K.M. Poluri, M.R. Kollipara, W. Kaminsky, Synthesis and structural
characterization of arene d6 metal complexes of sulfonohydrazone and triazolo ligands: high potency of triazolo
derivatives towards DNA binding, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.08.051
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Synthesis and structural characterization of arene d6 metal complexes of sulfonohydrazone and
triazolo ligands: high potency of triazolo derivatives towards DNA binding
Lathewdeipor Shadapa, Nidhi Joshib, Krishna Mohan Polurib, Mohan Rao Kolliparaa*,
Werner Kaminskyc
aCentre for Advanced Studies in Chemistry, North-Eastern Hill University, Shillong 793 022,
India. E-mail: mohanrao59@gmail.com
bDepartment of Biotechnology and Centre for Nanotechnology, Indian Institute of Technology
Roorkee, Roorkee- 247 667, India.
cDepartment of Chemistry, University of Washington, Seattle, WA 98195, USA
1

Graphical Abstract
Reaction of 4-methyl benzenesulfonohydrazone ligands (L1 and L3) with metal precursors
resulted in the formation of cationic complexes with PF6 as the counter ion. The triazolo ligand
(L2) with metal precursor resulted in the formation of neutral complexes.
ORTEP view of Complex 2
2

Abstract
Complexation of [(p-cymene)RuCl2]2 and [Cp*MCl2]2 (M = Rh/Ir) with chelating ligand 4-
methylbenzenesulfonohydrazone ligands (L1 and L3) resulted in the formation of mononuclear
cationic complexes having PF6 as the counter ion whereas, complexation with the triazolo
pyridine based ligand (L2) resulted in neutral complexes with mono-dentate binding fashion to
the metal center. All these complexes were fully characterized by analytical, spectroscopic and
X-ray diffraction studies. The complexes showed typical piano stool geometry around the metal
center with the ligands acting as NN’ donor chelating ligand (for L1 and L3) and mono-dentate
ligand (for L2). Biological studies such as antibacterial and DNA binding studies were screened
for the ligands as well as for the complexes. The complexes as well as ligands have not indicated
any antibacterial activity but the triazolo ligand (L2) as well as complex 4 and complex 6
exhibited DNA binding activity.
Keywords: 4-methylbenzenesulfonohydrazone, triazolo, DNA binding studies, antibacterial
study
3

1.
Introduction
Half sandwich complexes of some platinum group metals (ruthenium, rhodium, iridium)
containing heterocyclic nitrogen based ligands have emerged to be popular and well- studied in
the field of organometallics due to their readiness to form stable complexes [1, 2], unique
properties; mild reaction conditions, photochemical properties, catalytic activities,
electrochemical behavior as well as biological activities such as cytotoxic, antibacterial,
antimalarial activities, etc., [3-10].
The heterocyclic nitrogen based ligands such as pyridine-based hydrazones and triazolo
derivatives have been found to be the fundamental building blocks for numerous pharmaceutical
and functional materials. In fact, the hydrazone derivatives can behave as bidentate / tridentate
chelating ligands due to the presence of lone pair of electrons on the nitrogen atom which can
bind to the metal center and have found their applications in coordination chemistry, florescence
sensors and chemosensors chemistry, showing catalytic properties and behaving as
pharmaceutical active agents [11-14]. Biologically, hydrazones show a wide range of biological
activities such as cytotoxicity, antitumor, antimicrobial, and anti-inflammatory [15-19].
The properties of platinum group metals such as ruthenium, rhodium and iridium as well as
hydrazones and environmental remediation curved our minds to synthesize and study the
following reported benzenesulfonohydrazone ligand complexes with p-cymene ruthenium, Cp*
rhodium and Cp* iridium to further explore the coordination of the hydrazone derivative ligands
to the metal as well as to investigate the biological activity of the complexes.
In our path to synthesize the reported hydrazone ligands according to the reported procedure [20,
21], we were not only able to synthesize our desired ligands L1 and L3 (Scheme 1 and 3) but
unexpectedly, ligand L2 (which is a triazolo derivative ligand) was also obtained which had
resulted from the side product of ligand L1 (Scheme 1).
4

Triazolopyridines derivatives are an important class of compounds and have been widely
explored by Jones et al. [22] and Abarca et al. [23], since the early 1980s. Traditionally, triazolo
pyridine based derivatives have been synthesized by oxidative cyclization of 2- pyridyl ketone
hydrazones by using at least a stoichiometric of oxidants such as copper salts which have shown
to be more effective in providing appropriate yield. Other oxidants may include Ag2O, MnO2,
hypervalent iodine and Ni peroxide [24-28].
The formation of ligand L2 provides an alternative route to the synthesis of triazolo pyridine
based ligands by decomposition of the hydrazones. This has also occurred in the case of
dipyridyl ketone, which in this case resulted in the formation a bidentate chelating triazolo ligand
[29]. Compounds containing triazolo moiety have been found to be highly interesting due to
readiness to complex with heavy metals, cationic, neutral or anionic species of biomedical or
environmental relevance. They can also show interesting magnetic properties as well as
fluorescent properties [30].
With such properties possessed by the hydrazone as well as the triazolo ligands, we are
determined to explore the coordination of the complexes of these ligands with the platinum
group metal precursors of ruthenium, rhodium and iridium as well as to investigate their
biological activities.
As far as our knowledge is concern, no other work as such has been reported to carry out the
synthesis and studies of metal complexes 1-9 comprising of the ligands L1, L2 and L3 for which
the ligands have been prepared according to the reported procedure [20, 21].
2. Experimental
2.1 Materials and methods
2-Acetylpyridine and 2-benzoylpyridine were purchased from Aldrich and used as such without
further purification; the solvents were purified and dried according to standard procedures [31].
5

The starting precursor metal complexes [(p-cymene)RuCl2]2 and [Cp*MCl2]2 (M = Rh/Ir) were
prepared according to the reported literature [32, 33]. Ligands (L1 and L3) were prepared
according to the reported procedures [21] whereas the formation of triazolo ligand L2 was
unexpected. During the course of preparation of ligand L1 following the reported procedure, two
distinct spots and one spot (unreacted reactant) were observed at the bottom by TLC. The upper
spot corresponds to ligand L1 which precipitates out as orange crystalline solids out of the
reaction mixture on cooling, while the lower spot corresponds to ligand L2 which was obtained
after passing through column as creamy white solids. The ligands (L1 and L2) were washed with
diethyl ether and air-dried.
2.2
Single crystal X-ray diffraction
Crystal’s suitable for X-ray analyses for complexes 2, 3, 4, 6, 7, 8, and 9 were obtained
by slow diffusion of hexane/diethyl ether into DCM/CHCl3 solution and their molecular
structures have been given in Figure 1-3 while for complex 9, the ORTEP view has been given
just to show the mode of binding and the composition of the metal complex due to low theta
value. The single crystals of complexes were attached to a glass fiber and placed into the Oxford
Diffraction Xcalibur Eos Gemini diffractometer. Single crystal X-ray diffraction data for the
complexes were collected on an Oxford Diffraction Xcalibur Eos Gemini diffractometer using
graphite monochromatic Mo-Kα radiation (λ = 0.71073 Å). The strategy for the data collection
was evaluated using the CrysAlisPro CCD software [34]. Crystal data were collected by standard
‘‘phi–omega scan’’ techniques and were scaled and reduced using CrysAlisPro RED software.
The structures were solved using SHELX and refined by full-matrix least squares with SHELX
refining on F2 [35, 36]. The metal atoms in the complex were located from the E-maps and non-
hydrogen atoms were refined anisotropically. The hydrogen atoms bound to the carbon were
placed in geometrically constrained positions and refined with isotropic temperature factors,
generally 1.2 or 1.5 Ueq of their parent atoms. The crystallographic and structure refinement
parameters for the complexes are listed in Table 1 and 2, and selected bond lengths and bond
6

angles are presented in Table 3 and 4. The molecular structures were drawn with ORTEP3
program [37] whereas packing pattern and interactions like H- bonding, π---π, etc, were drawn
using MERCURY [38].
2.3. In-vitro antibacterial studies: The bactericidal efficacy of the synthesized compounds was
evaluated by agar well diffusion method as described elsewhere. Each well diffusion experiment
was performed in triplicate with 1 mg mL-1 concentration of complexes. Dimethylsulphoxide
(DMSO) was used as a solvent and as a negative control, whereas gentamycin antibiotic was
used as positive control.
2.4. Fluorescence studies on DNA interaction: Experiments involving the interaction of
complexes with SM-DNA (SM - Salmon milt) were carried-out in 5 mM Tris–HCl and 50 mM
NaCl buffer of pH 7.4. The concentration of SM-DNA per nucleotide was determined by
absorption spectroscopy at 260 nm. All florescence titration experiments were carried out with
fixed concentration (20 µM) of ligand (L2), complex 4 and complex 6 while gradually increasing
the DNA concentration (10 µM - 200 µM). For all complexes, emission spectra were recorded at
300-650 nm by exciting them at 275 nm. Complexes were incubated for 15 min at room
temperature to achieve thermodynamic equilibrium before spectral recording. Experiments were
repeated twice for reproducibility. To elucidate this quenching phenomenon, the quenching data
were analyzed with Stern–Volmer equation.
F0/F= 1+K τ [Q] =1 + KSV [Q]…………………(1)
q
0
Where F0 and F represent the fluorescence intensities in the absence and in the presence of
quencher, K is the quenching rate constant of the bimolecular, K is the dynamic quenching
q
SV
constant, τ0 is the average lifetime of the molecule without quencher and [Q] is the concentration
of the quencher.
Furthermore, the relationship between fluorescence quenching intensity and the
concentration of quenchers can be described by the binding constant equation (2).
7

log [F0-F/F] = log Ka +n log [Q]……………. (2)
From this equation, a double logarithm plot can be obtained which gives us the necessary
binding / dissociation constants.
2.5. General procedure for synthesis of mononuclear ruthenium complexes of L1, L2 and L3
Metal precursor [(p-cymene)RuCl2]2 (0.1 mmol), ligands (L1, L2 and L3) (0.2 mmol)
and 3mmol of NH4PF6 were dissolved in dry methanol (10 mL) and stirred for about 4 hours at
room temperature. After 1 hour, a yellowish-orange precipitate appeared in the reaction mixture.
The precipitate was filtered, washed with cold methanol (2×5 mL) and then with diethyl ether
(3×5 mL) and was air- dried.
2.5.1. [(p-cymene)Ru(L1)Cl]PF6 (1)
Yield (76%); Orange; IR (KBr, cm-1): 3447 ν(N-H), 1597 ν(C=N), 1469 ν(N-H)bending, 1350 ν(S=O)asym
1
and 1307 ν(S=O)asym, 1232 ν(C-N), 1150 ν(S=O)sym, 871 ν(P-F), 768 ν(S-N), 674 ν(N-H)wagging; H NMR
(400 MHz, CDCl3): δ = 9.28 (d, 1H, J = 8Hz ), 7.85 (d, 2H, J = 12Hz), 7.58(t, 1H, J = 8Hz), 7.30
(t, 3H, J = 4Hz), 7.2-7.13 (m, 7H), 5.85 (d, 1H, J = 4Hz, CH(p-cym)), 5.69 (d, 2H, J = 4Hz, CH(p-
cym)), 5.67 (d, 1H, J = 8Hz, CH(p-cym)), 5.55 (d, 1H, J = 8Hz), 2.84 (sept, 1H, CH(p-cym)), 2.35(s,
3H), 2.31 (s, 3H, CH(p-cym)), 1.25(d, 3H, J = 8Hz, CH(p-cym) ), 1.15 (d, 3H, J = 4Hz, CH(p-cym));
Mass- ESI (m/z): 586.14 [M-PF6-Cl]2+; UV–Vis {Acetonitrile, λmax, nm (ε/10-4 M-1 cm-1)}: 235
(3.413), 264 (2.133), 309 (1.277), 408 (0.327).
2.5.2. [(p-cymene)Ru(L2)Cl2 (4)
Yield (72%); Reddish orange; IR (KBr, cm-1): 1632 ν(C=C), 1606 ν(C=C), 1494 ν(C=N), 1471 ν(C=C)
,
1
1454 ν(C=C), 1432 ν(N=N), 1224 ν(C-N); H NMR (400 MHz, CDCl3): δ = 8.79 (d, 1H, J = 8Hz),
8.04 (d, 1H, J = 8Hz), 7.97 (d, 2H, J = 4Hz), 7.53 (t, 2H, J = 8Hz), 7.42- 7.40 (m, 1H), 7.35(d,
1H, J = 8Hz), 7.06 (t, 1H, J = 8Hz), 5.67 (d, 2H, J = 8Hz,), 5.60 (d, 2H, J = 8Hz), 3.04 (sept, 1H,
CH(p-cym)), 2.28 (s, 3H, CH(p-cym)), 1.31 (d, 6H, J = 8Hz, CH(p-cym)); Mass- ESI (m/z): 270.91 [M-
8

triazole-Cl]2+; UV–Vis {Acetonitrile, λmax, nm (ε/10-4 M-1 cm-1)}: 218 (4.399), 272 (4.201), 330
(1.126), 405 (0.114).
2.5.3. [(p-cymene)Ru(L3)Cl]PF6 (7)
Yield (80%); Red; IR (KBr, cm-1): 3437 ν(N-H), 1573 ν(C=N), 1452 ν(N-H)bending, 1357 ν(S=O)asym and
1
1302 ν(S=O)sym, 1239 ν(C-N), 1159 ν(S=O)sym, 842 ν(P-F), 778ν(S-N), 684 ν(N-H)wagging; H NMR (400
MHz, CDCl3): δ = 7.50 (d, 1H, J = 4Hz), 8.23 (d, 2H, J = 4Hz), 7.94 (d, 3H J = 8Hz ), 7.81- 7.77
(m, 1H,), 7.42 (d, 2H, J = 8Hz), 6.14 (d, 2H, J = 4Hz), 5.73 (d, 1H, J = 4Hz, CH(p-cym)), 5.64 (d,
1H, J = 4 Hz, CH(p-cym)), 2.65 (sept, 1H, CH(p-cym)), 2.47 (s,3H), 2.36 (s,3H, CH(p-cym)), 2.16 (s,
3H,) 1.00 (d, 3H, J = 8Hz, CH(p-cym)), 0.94 (d, 3H, J = 8 Hz, CH(p-cym)); Mass- ESI (m/z): 562.04
[M-PF6+2H]+; UV–Vis {Acetonitrile, λmax, nm (ε/10-4 M-1 cm-1)}: 227 (5.203), 278 (2.075), 337
(0.617), 408 (0.544).
2.6. General procedure for preparation of mononuclear Cp*Rh and Cp*Ir complexes of L1, L2
and L3
Metal precursor [Cp*MCl2]2 (M = Rh/Ir) (0.1 mmol), ligands (L1, L2, L3) (0.2 mmol) and 3
equivalents of NH4PF6 were dissolved in dry methanol (10 mL) and stirred at room temperature
for 4 hours. After 1-2 hours a yellow/orange compound precipitated out from the reaction
mixture. The precipitate was filtered, washed with cold methanol (2 x 5 mL) and diethyl ether (3
x 10 mL) and air-dried.
2.6.1. [Cp*Rh(L1)Cl]PF6 (2)
Yield (82%); Orange; IR (KBr, cm-1): 3441 ν(N-H), 1583 ν(C=N), 1461ν(N-H)bending, 1360 ν(S=O)asym
1
and 1305 ν(S=O)asym, 1231 ν(C-N), 1169 ν(S=O)sym, 871 ν(P-F), 764 ν(S-N), 671 ν(N-H)wagging; H NMR
(400 MHz, CDCl3): δ = 8.87 (d, 1H, J = 4Hz), 8.01 (t, 1H, J = 4Hz), 7.88- 7.82 (m, 3H), 7.72 (t,
1H, J = 4Hz), 7.57 (t, 3H, J = 8Hz), 7.43 (s, broad, 1H), 7.33 (d, 2H, J = 4Hz), 7.07 (t, 2H, J =
4Hz), 2.74 (s, 3H,), 1.75 (s, 15H, CH(Cp*)); Mass- ESI (m/z): 623.99 [M-PF6-2H] and 588.07 [M-
PF6-Cl-2H]2+; UV–Vis {Acetonitrile, λmax, nm (ε/10-4 M-1 cm-1)}: 229 (6.695), 278 (2.429), 389
(0.754).
9

2.6.2. [Cp*Rh(L2)Cl2] (5)
Yield (77%); Orange; IR (KBr, cm-1): 1633 ν(C=C), 1610 ν(C=C), 1541 ν(C=N), 1458 ν(C=C), 1414
νC=C), 1327 ν(N=N), 1230 ν(C-N); 1H NMR (400 MHz, CDCl3 + DMSO): δ = 8.86 (d, 1H, J = 8Hz),
8.09 (d, 1H, J = 8Hz), 7.96 (d, 1H, J = 8Hz), 7.72 (s,1H), 7.53 (t, 2H, J = 8Hz), 7.44- 7.38 (m,
2H), 7.13 (t, 1H, J = 4Hz), 1.75 (s, 15H, CH(Cp*)); Mass- ESI (m/z): 274.89 [M-triazole-Cl]+;
UV–Vis {Acetonitrile, λmax, nm (ε/10-4 M-1 cm-1)}: 231(3.808), 266 (1.547), 406 (0.613).
2.6.3. [Cp*Rh(L3)Cl]PF6 (8)
Yield (77%); Orange; IR (KBr, cm-1): 3412 ν(N-H), 1599 ν(C=N), 1459 ν(N-H)bending, 1350 ν(S=O)asym
,
1
1218 ν(C-N), 1170 ν(S=O)sym, 849 ν(P-F), 777 ν(S-N), 679 ν(N-H)wagging; H NMR (400 MHz, CDCl3): δ
= 8.96 (d, 1H, J = 4Hz), 8.16 (t, 1H, J = 8Hz), 8.05 (d, 1H, J = 8Hz), 7.92 (d, 2H, J = 8Hz),
7.85 (t, 1H, J = 8Hz), 7.23 (d, 3H, J = 8Hz), 2.27 (s, 3H), 2.06(s, 3H) 1.67 (s, 15H, CH(Cp*));
Mass- ESI (m/z): 561.99 [M-PF6-2H]+ and 526.06 [M-PF6-Cl-2H]2+; UV–Vis {Acetonitrile, λmax
nm (ε/10-4 M-1 cm-1)}:227 (5.793), 278 (3.163), 376 (0.487), 386 (0.462).
2.6.4. [Cp*Ir(L1)Cl]PF6 (3)
,
Yield (70%); Yellowish orange; IR (KBr, cm-1): 3446 ν(N-H), 1582 ν(C=N), 1450 ν(N-H)bending, 1359
1
ν(S=O)asym, 1309 ν(S=O)sym, 1159 ν(S=O)sym, 868 ν(P-F), 761 ν(S-N), 672 ν(N-H)wagging; H NMR (400
MHz, CDCl3): δ = 8.90 (d, 1H, J = 8Hz), 7.87 (d, 2H, J = 8Hz), 7.67- 7.60 (m, 1H), 7.36- 7.31
(m, 3H), 7.28 (m, 3H), 7.21 (d, 2H, J = 8Hz), 7.16 (d, 2H, J = 8 Hz), 2.33 (s, 3H), 1.66 (s, 15H,
CH(Cp*)); Mass- ESI (m/z): 714.07 [M-PF6-H]+and 678.15 [M-PF6-Cl+H]2+; UV–Vis
{Acetonitrile, λmax, nm (ε/10-4 M-1 cm-1)}: 223 (5.420), 265 (2.682), 429 (0.503).
2.6.5. [Cp*Ir(L2)Cl2 (6)
Yield (78%); yellow; IR (KBr, cm-1): 1621 ν(C=C), 1609 ν(C=C), 1542 ν(C=N), 1479 ν(C=C), 1434
ν(C=C), 1422 ν(N=N), 1217 ν(C-N); 1H NMR (400 MHz, CDCl3): δ = 8.00 (d, 1H, J = 8Hz), 7.91 (d,
2H, J = 8Hz), 7.55 (t, 3H, J = 8Hz), 7.44 (t, 1H, J = 8Hz), 7.37 (t, 1H, J = 4Hz), 7.04 (t, 1H, J =
8Hz), 1.63 (s, 15H, CH(Cp*)); Mass- ESI (m/z): 522.13 [M-2HCl]2+ and 362.98 [M-triazole+2H-
10

Cl]+; UV–Vis {Acetonitrile, ν λmax, nm (ε/10-4 M-1 cm-1)}: 218 (3.821), 261 (1.698), 331 (1.219),
409 (0.166).
2.6.6. [Cp*Ir(L3)Cl]PF6 (9)
Yield (74%); Yellow; IR (KBr, cm-1): 3422 ν(N-H), 1598 ν(C=N), 1459 ν(N-H)bending, 1351 ν(S=O)asym
,
1
1171 ν(S=O)sym, 851ν(P-F), 778 ν(S-N), 667 ν(N-H)wagging; H NMR (400 MHz, CDCl3): δ = 8.63 (d,
1H, J = 4Hz), 8.04 (t, 1H, J = 8Hz), 7.94 (d, 2H, J = 8Hz), 7.90 (d, 1H, J = 8Hz ), 7.73 (t, 1H, J
= 8Hz), 7.29- 7.26 (m, 3H,), 2.41 (s, 3H), 2.03(s, 3H) 1.74 (s, 15H, CH(Cp*)); Mass- ESI
(m/z):652.14 [M-PF6-H]+; UV–Vis {Acetonitrile, λmax, nm (ε/10-4 M-1 cm-1)}: 228 (6.632), 277
(3.812), 383 (0.768).
3. Results and discussion
3.1. Synthesis of the complexes
The reaction of metal precursors with benzenesulfonohydrazone ligands (L1 and L3) with
appropriate ratio in dry methanol resulted in the formation of mononuclear complexes. In the
formed complexes with ligand L1 and L3, the ligands act as bidentate NN’ chelating ligands
whereas the reaction of metal precursors with the unexpected formed triazolo ligand (L2)
resulted in the formation of neutral complexes with the ligand acting as a mono-dentate ligand
Scheme 3. The molecular structures of the complexes (2, 3, 4, 6, 7, 8, 9) were determined by
single crystal X-ray analysis. Complexes (2, 3, 7, 8, 9) were isolated as cationic complexes with
PF6 as the counter ion whereas complexes (4 and 6) were isolated as neutral complexes. The
ORTEP view of complex 9 has been given just to show the mode of binding and the composition
of the metal complex due to low theta value. All the complexes were isolated as yellow to orange
solids and were found to be stable in air and non- hygroscopic. These complexes are soluble in
acetonitrile, dichloromethane, DMSO, chloroform but insoluble in hexane, diethyl ether and pet
ether.
3.2.
Spectral studies of the complexes
11

Information about the ligands and the complexes has been studied by IR spectra, which revealed
significant information. Stretching frequencies of NH is observed around 3412 - 3447 cm-1
indicating that there is no bonding to the metal atom. The stretching frequency of S=O in free
ligands is around in the range 1225 - 980 cm-1 whereas in the complexes, we observed symmetric
as well as asymmetric stretching; the symmetric stretching frequency ʋ(S=O)sym is observed in the
range 1360 - 1350 cm-1 and the asymmetric stretching frequency ʋ(S=O) asym is observed in the
range 1171-1150 cm-1 indicating that there is no bonding through oxygen atom to the metal
center. In case of ligand (L2), the stretching frequency of S=O is absent which indicates the
dissociation of the sulfonyl group resulting in the formation of the triazolo ligand. Similarly, in
complexes 4, 5 and 6 the stretching frequency of S=O is absent. The C=N stretching frequency
for the ligands lies in the range of 1600-1650 cm-1 while for the complexes, the C=N stretching
frequency is observed in the range 1599-1573 cm-1. This decrease in the stretching frequency of
C=N indicates the binding of nitrogen to the metal atom. In all the complexes except for
complexes 4, 5 and 6, the presence of the counter ion PF6 has been observed in the range 871 -
842 cm-1 due to P-F stretching frequency [40]. The stretching frequency of aromatic C=C has
been observed in the range 1640- 1600 cm-1 and 1494- 1414 cm-1 while that of N=N has been
observed in the range 1432-1327 cm-1.
1H NMR studies of the complexes
1
The H NMR spectra of the complexes have been provided in the supplementary information
(Figure S1-S10). To confirm the formation of the complexes have been confirmed through NMR
studies which showed the appearance of the ligand proton signals as well as p-cymene and Cp*
ring protons. The aromatic protons of the ligands can be observed in the downfield region
between 7.06-9.28 ppm. The N-H proton signal for all the complexes except 4, 5 and 6 has been
observed to couple with the other aromatic ligand protons. For complexes 4, 5 and 6 N-H proton
is absent. It has also been observed that there is an unusual splitting pattern of signal for p-
12

cymene moiety in all the three complexes (1, 4, 7). In complex 1, we observe the aromatic proton
signals of p-cymene ring splits into four doublets in the range 5.55- 5.85, while in complex 4, we
observed two doublets around 5.67 ppm and 5.60 ppm and in complex 7, we observed three
doublets in the range 5.64-6.14 ppm. In complex 4, we observed a doublet at 1.31 ppm while in
complexes 1 and 7, we observed two doublets for the six methyl protons of p-cymene instead of
one doublet in the range 0.95-1.25 ppm. This unusual pattern can be attributed to the
diastereotopic effect of the methyl protons of the isopropyl group and the methine protons of the
p-cymene ring as well as the chirality of the metal upon coordination with hydrazone ligands
[41]. Septet signal for complexes 1, 4, 7 have been observed at 2.84 ppm, 3.04 ppm and 2.65
ppm respectively and singlet signal for methine protons of p-cymene have been observed at 2.31
ppm, 2.28 ppm and 2.16 ppm respectively. In rhodium and iridium complexes, in addition to the
protons of the ligands, we have also observed a sharp singlet around 1.63-1.75 ppm for
complexes 2, 3, 5, 6, 8, 9 corresponding to Cp* protons. By and large, the NMR data of all the
complexes are with expected resonance and integration with the formulated complexes.
Mass spectral studies of the complexes
Mass spectral data of the complexes have been provided in the supplementary data (Figure S11-
S19) and their values are given in the experimental section. The analysis of these complexes
were done in acetonitrile. The analyzed masses of the complexes are found to be consistent and
tallied with the calculated masses. The molecular ion peaks of all the complexes are found to be
in well agreement with the calculated masses. The found molecular ion peaks (for each complex)
are displayed at m/z: 586.14(1), m/z: 588.07(2), m/z: 679.87 (3), m/z: 270.91 (4), m/z:
274.89(5), m/z: 522.13 (6), m/z: 560.09 (7), m/z: 526.06 (8), m/z: 652.14 (9). In all the cationic
complexes, fragmentation of the counter ion as well as the chloride was observed while in the
neutral complexes of triazole, we observed fragmentations of triazole ligand from the metal
13

center. The consistent molecular ion peaks of the all complexes with the calculated mass, shows
that there is a strong bonding of the arene ring (arene= p-cymene, Cp*) to the metal atom.
UV- Visible description of metal complexes
The electronic spectra of the metal complexes were recorded in acetonitrile with 10 μM
concentrations at room temperature and these spectra have been provided in the supplementary
data (Figure S20). These complexes are d6 low spin metal complexes containing filled orbitals of
proper geometry at the metal centers which can interact with the low lying π * orbitals of the
ligands which may result in metal- to- ligand charge transfer (MLCT) transitions [42, 43]. The
low energy absorption band observed in the range 385- 408 nm is assigned to metal- to- ligand
charge transition (MLCT) dπ(M) to π*(L) transitions while the high energy absorption band
observed in the range 218- 235 nm and 265-331 nm may be attributed to ligand- center π- π* / n-
π* transitions [44, 45].
Description of molecular structures of metal complexes
To get a deeper and broader understanding about the spatial arrangement and interactions among
the molecules in a unit cell as well as the information about the coordination in metal complexes,
metal complexes are crystallized; which in our case the metal complexes have been crystallized
by using slow diffusion method of hexane/ diethyl ether into a saturated solution of metal
complexes in DCM / CHCl3. Through crystallography studies, a variety of binding modes and
coordination in the metal complexes can be studied and known which other spectroscopic studies
are unable to do so. In this structural analysis, we have been able to establish the crystal
structures of some of the metal complexes. The ORTEP view of the isolated crystal structures (2,
3, 4, 6, 7, 8, 9) with atom numbering are presented in Figures 1-3 and the relevant
crystallographic parameters along with the details of bond lengths; bond angles are listed in
Table 1, 2, 3 and 4. Complex 6 crystallized along with metal precursor, which may be due to
presence of excess metal precursor, which was able to stay even after washing with cold
14

methanol. For complex 9, its ORTEP view has been given just to present the structure,
composition and confirmation of the metal complex as the crystal has a low theta value.
Single crystals were attached to a glass fiber and transferred to the Oxford Diffraction
Xcalibur Eos Gemini diffractometer. For complexes 2, 3, 7, 8 and 9, X- ray studies showed that
these complexes are cationic complexes bearing the general formula [(arene)M(L)Cl]PF6. The
metal complexes featured a regular three-legged “ piano stool” geometry in which the arene ring
(arene= p-cymene, Cp*) occupied the coordination sites around the metal in 휂5/휂6 manner,
terminal chloride and chelating N, N donor ligand. The metal atom shows pseudo octahedral
geometry with the arene ring occupying the three facial geometry acting as the seat of a piano
and the nitrogen donor atoms from hydrazone ligands and chloride atom acting as the three legs
of a piano. The molecular structures of these complexes (2, 3, 7, 8 and 9) revealed the hydrazone
derivative ligands bind to the metal in a bidentate manner through imine nitrogen and nitrogen of
the pyridine ring leading to the formation of a five membered chelated ring with PF6 as the
counter ion.
In the case of complexes 4 and 6 we observed the binding of the ligand (L2) to metal atom takes
place only through the nitrogen atom of the triazolo ligand, which resulted in the formation of
mono-dentate neutral complexes. The crystallized systems of the complexes 2, 3, 4, 6, 7 and 8
are given in Table 1 and 2. The distances between the metal (M) center to the centroid of the p-
cymene/Cp* ring of the complexes has been provided in Table 3 and 4. For complexes 2, 3, 7,
and 8, the metal to pyridyl nitrogen [M(1)-N(1)], the metal to imine nitrogen [M(1)-N(2)] and
the metal to chloride [M(1)-Cl(1)] bond lengths as well as their respective bond angle values
[N(1)-M(1)-N(2)], [N(1)-M(1)-Cl(1)] and [N(2)-M(1)-Cl(1)] of these complexes are given in
Table 3 and are found to be comparable to the previous reported values [46, 47] . The S=O bond
distances for complexes 2, 3, 7 and 8 are found to be in the range 1.415-1.435 Å. Similarly, in
case of complexes 4 and 6, the metal to triazolo nitrogen [M(1)-N(2)], the metal to chloride
15

[M(1)-Cl(1)] and [M(1)-Cl(2)] bond lengths as well as their respective bond angle values are
given in Table 4. The bond angle values of these complexes 2, 3, 4, 6, 7 and 8 are comparable to
the piano stool arrangement at the metal atom and are comparable to reported values for closely
related systems [48-50].
The crystal packing of these complexes shows some inter molecular hydrogen bonding
such as C-H---Cl, O---H and π---π that contributes towards the stability of the complexes.
Complex 2 and 3 show two types of inter hydrogen bonding i.e for complex 2, O(2)---(3)H and
H(29)---Cl(1) distanced at 2.325 Å and 2.905 Å respectively and for complex 3 O(2)---(3)H and
H(17)---Cl(1) distanced at 2.330 Å and 2.843 Å respectively (Figure 4). In complex 4, we
observed one inter hydrogen bonding only i.e H(18)---Cl(2) distanced at 2.759 Å (Figure 5)
which is due to the interaction of chlorine and C-H of the phenyl ring while in complex 6, we
observed two types of interactions i.e, inter hydrogen bonding H(14)---Cl(1) and H(22)---Cl(1)
distanced at 2.863 Å and 2.868 Å respectively and C to π interaction which values at 3.589 Å
(Figure 5). The H---Cl interactions in complex 6 is due to the interaction of the chlorine atom
(Cl1) and hydrogen atom H(22) of the phenyl ring (2.868 Å) and chlorine atom (Cl1) with the
hydrogen atom H (14) of the phenyl fused triazole ring (2.863 Å). Complex 7 shows similar inter
hydrogen bonding H(17C)---O(2) distanced at 2.678 Å (Figure 6) as that of complexes 2 and 3
while in complex 8, in addition to inter hydrogen bonding O(2)---H(3N) distanced at 2.199 Å,
we also observed O(2)---N(3) interactions distanced at 2.933 Å as shown in figure 6. These types
of interactions pave way for supramolecular motifs. Interestingly, in complex 9, we observed π-
π interaction at 3.988 Å (Figure 6) between the pyridyl rings leading to the formation of dimeric
unit.
3.3. Biological studies on metal-ligand complexes
(i) In- vitro antibacterial assay: Due to the massive resistance of bacteria to various drugs has
raised a considerable interest in developing of new classes of antimicrobial agents. Here in
16

present study the synthesized complexes were investigated using zone inhibition assay to exert
their activity against two Gram positive bacteria S. aureus and B. thuringiensis and two Gram
negative bacteria E. coli and P. aeruginosa. None of the tested complexes show any significant
bactericidal activity against the tested Gram positive and Gram negative bacteria with reference
to gentamycin which is used as a positive control (Figure S21).
(ii) DNA binding studies: The search for alternative drugs to the well-known anti-cancer agent
Cisplatin is in progress considering the potency of the platinum group metallodrugs (51, 52). The
anti-cancer action of these metallocomplexes is based on the direct binding and DNA damage
(53). Hence, the binding studies between a novel transition metal-ligand complexes with DNA
may pave the way for exploration and characterization that further can translate into a therapeutic
lead (54, 55). In the current study the binding propensity between synthesized complexes with
SM-DNA were explored by florescence spectroscopy using quenching experiments. The
fluorescence spectra of synthesized complexes (4 and 6) and ligand (L2) with increasing
concentration of SM-DNA were quenched remarkably and significant decrease in the emission
profile intensities were observed upon adding SM-DNA indicating for a specific interaction
between SM-DNA and the ligand (L2) and complexes (4 and 6) (Figure 7 A-C). Using a double
logarithmic plot (Figure 7 D-F), the binding constants were calculated as discussed in the
materials and methods (Table 5). The dissociation constant of ligand (L1) and complex 6
suggested weak binding to DNA. Amongst all, complex 4 binds strongly to DNA with a sub-
micro molar affinity, thus indicating its potency to be further evaluated for in-vitro and in-vivo
studies as an anti-cancer agent.
Conclusion
In summary, we have successfully synthesized the complexes of ruthenium, rhodium and iridium
with hydrazones (L1 and L3) and triazolo (L2) as ligands. All these complexes have been
characterized by various spectroscopic techniques and molecular structures have been
17

determined by X-ray diffraction study. These complexes as well as the respective ligands, though
they did not display any antibacterial activity, yet, ligand L2, complex 4 and complex 6 have
revealed the binding to DNA for which their florescence spectra with increase in concentration
of SM-DNA were quenched remarkably and the emission profile intensities decrease on adding
SM-DNA indicating interaction with DNA. Among ligand L2, complex 4 and complex 6,
complex 4 binds strongly to DNA with a sub-micro molar affinity, which shows its potential as
anti-cancer agent for in-vitro and in-vivo studies.
Acknowledgement
Lathewdeipor Shadap thanks Dr. Narasinga Palepu Rao for his help, DST-PURSE SCXRD,
NEHU-SAIF, Shillong, India for providing Single Crystal X-ray analysis and other spectral
studies as well as UGC, New Delhi, India for providing financial assistance in the form of
university fellowship (non-Net).
Supplementary material
CCDC 1853646 (2), 1853647 (3), 1853645 (4), 1853642 (6), 1853643 (7) and 1853644
(8) contains the supplementary crystallographic data for this paper. These data can be obtained
free
of
charge
via
www.ccdc.cam.ac.uk/data_request/cif,
by
e-mailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre,
12, Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033.
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23

Chart 1: Ligands used in study
Scheme 1: Preparation of ligands (L1 and L2)
Scheme 2: Preparation of ligand (L3)
Scheme 3: Preparation of complexes.
24

Figure 1: (a) ORTEP diagram of complex (2) and (b) ORTEP diagram of complex (3)
with 50% probability thermal ellipsoids. Hydrogen atoms and counter ions have been
omitted for clarity.
Figure 2: (a) ORTEP diagram of complex (4) and (b) ORTEP diagram of complex (6)
with 50% probability thermal ellipsoids. Hydrogen atoms, counter ions and residual
metal precursor (in case of complex 6) have been omitted for clarity.
25

Figure 3: (a) ORTEP diagram of complex (7) and (b) ORTEP diagram of complex (8)
(c) ORTEP diagram of complex (9) with 50% probability thermal ellipsoids. Solvent
molecules, counter ions and hydrogen atoms are omitted for clarity. ORTEP diagram of
Complex 9 has just been given to shown the mode of binding and composition of the
complex.
Figure 4: (a) Complex 2 showing inter hydrogen bonding O(2)---(3)H and H(29)---Cl(1)
(b) complex 3 showing inter hydrogen bonding O(2)---(3)H and H(17)---Cl(1).
26

Figure 5: (a) Complex 4 showing inter hydrogen bonding H(18)---Cl(2) and (b) Complex 6
showing inter hydrogen bonding H(14)---Cl(1) and H(22)---Cl(1) and C---π interactions.
Figure 6: (a) Complex 7 showing inter hydrogen bonding H(17C)---O(2), (b) Complex 8
showing inter hydrogen bonding O(2)---H(3N) and covalent interaction O(2)---N(3) (c) Complex
9 showing π---π interaction.
27

Figure 7: (A-C): The emission profile (λex= 275 nm; λem= 300-650 nm) of ligand (L2) (A),
complex 4 (B), and complex 6 (C) at 20 µM in the presence of increasing concentrations of SM-
DNA (10 µM-200 µM). The arrows represent the changes in emission intensity upon addition of
SM DNA. D-F represents F0/F vs [Q] plots for complexes ligand (L2), complex 4 and complex
6 respectively.
28

Table 1: Crystal structure data and refinement of complexes 2, 3 and 4.
[2]PF6
[3]PF6
[4]
Complexes
Empirical formula
C29H32ClF6N3O2PRhS
C29H32ClF6IrN3O2PS
C22H23Cl2N3Ru
Formula weight
769.96
859.25
501.40
Temperature (K)
Wavelength (Å)
293(2) K
0.71073
292.2(7)
0.71073
292(2)
0.71073
Crystal system
Space group
a (Å)/α (°)
monoclinic
P 21/c
monoclinic
P 21/c
triclinic
P -1
14.3284(9)/90
15.8911(13)/115.152(7)
15.6936(9)/90
3234.5(4)
4
14.4592(6)/90
15.5533(5)/115.559(5)
15.7417(6)/90
3193.7(2)
4
7.6746(6)/93.200(6)
10.1164(8)/90.474(7)
13.8190(11)/105.681(7)
1031.05(15)
2
b (Å)/β (°)
c (Å)/γ (°)
Volume (Å3)
Z
Density (calc) (Mg/m-3)
Absorption coefficient
1.581
1.787
1.615
0.791
4.447
1.032
(µ) (mm-1)
F(000)
1560
1688
508
Crystal size (mm3)
0.29 x 0.25 x 0.12
3.393 to 28.844
0.21 x 0.15 x 0.11
3.123 to 25.349
0.25 x 0.13 x 0.12
3.160 to 29.120
Theta range for data
collection
Index ranges
-17<=h<=18, -
20<=k<=20, -20<=l<=20
13016
-17<=h<=16, -
13<=k<=18, -18<=l<=14
11735
-10<=h<=7, -12<=k<=13, -
18<=l<=18
7016
Reflections collected
Independent reflections
7315 [R(int) = 0.0355]
5811 [R(int) = 0.0312]
4652 [R(int) = 0.0363]
Completeness to theta =
25.00°
99.5
99.4
99.1
Absorption correction
Semi-empirical from
equivalents
Semi-empirical from
equivalents
Semi-empirical from
equivalents
Refinement method
Full-matrix least-squares
Full-matrix least-squares
Full-matrix least-squares on
2
2
2
on F
on F
F
Data/restraints/parameter 7315/93/461
s
5811/93/442
4652/ 0/256
Goodness-of-fit on F2
1.063
1.052
1.008
Final R indices
[I>2sigma(I)]
R1 = 0.0551, wR2 =
0.1053
R1 = 0.0339, wR2 =
0.0769
R1 = 0.0319, wR2 = 0.0688
R indices (all data)
R1 = 0.0844, wR2
=0.1196
1.046 and -0.5538
R1 = 0.0496, wR2 =
0.0861
1.094 and -1.097
R1 = 0.0377, wR2 = 0.0710
0.528 and -0.793
1853645
Largest diff. peak and
hole( e.Å-3)
CCDC No.
1853646
1853647
29