Synthesis, crystal structures and antioxidant studies of Pd(II) and Ru(II) complexes of 2-(4-methoxyphenyltelluro) ethanol

Article abstract of DOI:10.1016/j.jorganchem.2019.120967

Pd(II) (1) and Ru(II) (2) complexes of 2-(4-methoxyphenyltelluro)ethanol (L), were synthesized using Na₂ [PdCl₄] and [Ru (η⁶-p-cymene)Cl₂]₂ as metal ion sources. The new complexes (1 and 2) were chara

Full text of DOI:10.1016/j.jorganchem.2019.120967

Journal of Organometallic Chemistry 902 (2019) 120967
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, crystal structures and antioxidant studies of Pd(II) and
Ru(II) complexes of 2-(4-methoxyphenyltelluro) ethanol
,
K.M. PrabhuKumar a, H.R. Rajegowda a, P. Raghavendra Kumar a *, R.J. Butcher b
a Department of Studies and Research in Chemistry, Tumkur University, Tumkur, 572 103, Karnataka, India
b Department of Chemistry, Howard University, Washington, D.C, 20059, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Pd(II) (1) and Ru(II) (2) complexes of 2-(4-methoxyphenyltelluro)ethanol (L), were synthesized using Na2
[PdCl4] and [Ru (h
6-p-cymene)Cl2]2 as metal ion sources. The new complexes (1 and 2) were charac-
Received 14 June 2019
Received in revised form
27 September 2019
Accepted 30 September 2019
Available online 3 October 2019
terized by 1H, 13C{1H} NMR, FT-IR, UVeVisible spectroscopy and elemental analyses. Their structures
were also confirmed by single crystal X-Ray diffraction. The characterizationdata revealed that the ligand
L coordinated to M(II) ion as a monodentate telluroether ligand in both 1 and 2. The structure of cis-
[PdCl2(L)2] (1) was square planar about Pd in which two molecules of L coordinated to palladium (II) in
cis-fashion. Interestingly, two molecules of 1 were held by strong Te/Cl and aromatic p/
p
secondary
interactions and oriented in staggered conformation with short Pd-Pd [3.1712 (5) Å] distance forming a
dinuclear palladium cluster (cis-[PdCl2(L)2])2 (1)2. The complex [Ru (
6-p-cymene)Cl2(L)] (2) has half-
Keywords:
Tellurium
Palladium
Ruthenium
Pd-Pd bond
Antioxidant
h
sandwich, pueudo-octahedral structure. There exists several intermolecular hydrogen bonds of type:
OH/Cl and OH/O in 1 and CH/Cl, CH/O and OH/Cl in 2. These interactions results in supramolecular
assemblies in the new complexes 1 and 2. In-vitro antioxidant activity of L, 1 and 2 was investigated using
DPPH assay. Vitamin-C was used as a standard antioxidant. All three compounds showed to exhibit
antioxidant activity. The free radical scavenging activity was quantitatively expressed in terms of their
IC50 values which found in the order: Vitamin-C ~2 > L > 1.
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
ruthenium (Ru) complexes of ligands containing different donor
atoms has been increased because of their potential biological [8]
Synthesisedtelluroether and hybrid organotellurium ligands
and their coordination chemistry have been extensively studied
with transition metals [1]. Though tellurium (Te) was long been
considered a non-essential trace element but the biological role of
organotellurium compounds has still been not completely estab-
lished [2] in comparison to those of its congeners S and Se [3] and it
can't be left unconcern. Organotellurium compounds are promising
because some of the current studies have demonstrated that they
are potential enzyme mimetics [4], antioxidants [5] and have
shown to overshine as free radical scavengers [6] under biologically
relevant conditions when compared to those of the corresponding
S and Se compounds and presence of other functional groups also
expected to be more pronounced on their activity [7].
and efficient catalytic activities [9], mimic platinum (Pt) and iron
(Fe) respectively under similar physiological conditions [10], Pd is
cost effective and Ru exhibit easily accessible oxidation states (II/III)
[11], improved solubility, high reactivity, less toxicity to normal
tissues and high activity of their complexes [10e12]. Recently, the
quasi-octahedral half-sandwich complexes containing [(h
6-arene)
Ru(II)] species have been demonstrated to exhibit excellent bio-
logical activity [8c,12,13,14]. The Ru and Pd complexes have been
investigated in the treatment of cancer [8a,c,d] but are yet to be
explored in chemotherapy to a large extent [15].
[1] Organotellurium ligands have shown potential biological
activity [5] and have been proved to be excellent antioxidants [16].
They behaves as hemilabile ligands with soft metals especially Ru
and Pd, their complexes containing metal-tellurium (M ꢀ Te) bond
have been explored as efficient catalysts for various organic
transformations [1,9]. They show high trans influence and stabilise
the unstable organotellurium compounds even in aqueous solution
[1g,17]. However, the metal complexes containing M ꢀ Te bond
Since the discovery of cis-platin, an anticancer drug, the interest
in the synthesis and characterization of palladium (Pd) and
* Corresponding author.
E-mail address: raghukp1@gmail.com (P. Raghavendra Kumar).
https://doi.org/10.1016/j.jorganchem.2019.120967
0022-328X/© 2019 Elsevier B.V. All rights reserved.

2
K.M. PrabhuKumar et al. / Journal of Organometallic Chemistry 902 (2019) 120967
have never been explored in biological activities [8]. Therefore, the
designing and synthesis of potential Ru and Pd complexes of
organotellurium ligands as antioxidants would be very much
interesting.
methylenedichloride (2 ꢁ 25 mL). The organic layer was dried un-
der anhydrous sodium sulfate. When the solvent was distilled off
under reduced pressure using rotary evaporator resulted an
orange-red solid. Orange-red single crystals of complex 1 were
grown after 3e4 days when the solution of above solid in a 1:1
mixture of chloroform and n-hexane was stored at 4e5 ꢂC in
refrigerator. The crystal were filtered and air dried.
The synthesis and structural characterization of metal com-
plexes of hybrid organotellurium ligands of (Te, O) type containing
ethereal oxygen (R-O-R) have been well studied [18,1c] in which
they behaved as monodentate telluroethers, but those containing
alcoholic (ReOH) group are not fully explored [19]. Hence, herein
we have explored the coordination chemistry of 2-(4-
methoxyphenyltelluro)ethanol (L) with Pd(II) and Ru(II) species
Yield: 75%; M.P: 122e125 ꢂC; Elemental analysis: Found
(Calculated) for C18H24Cl2O4PdTe2: C, 29.34 (29.31); H, 3.28 (3.25);
FT-IR (KBr,
y
cmꢀ1): 3446 (OH), 1586 (C]C), 590 (C-Te, aryl), 507 (C-
Te, alkyl); 1H NMR (DMSO‑d6,
d
ppm): 3.273 (bs, 2H, TeCH2), 3.643
using Na2PdCl4 and [Ru (
h
6-p-cymene)Cl2]2 respectively as metal
(bs 2H, OCH2), 3.785 (s, 3H, OCH3), 5.21 (s, 1H, OH), 6.921e6.941 (d,
ion sourses. The resulted complexes, cis-[Pd(L)2Cl2] (1) and [Ru (h6
-
J ¼ 8.0 Hz, 2H, ArH ortho to Te), 7.60 (bs, ArH meta to Te); 13C{1H}
p-cymene)Cl2(L)] (2) were structurally characterized by 1H, 13C{1H}
NMR, FT-IR, UV Visible spectroscopy, elemental analysis and single
crystal X-ray diffraction techniques. These compounds (L, 1 and 2)
were also explored in-vitro free radical scavenging activity studies
using DPPH radicals. The results of these investigations are pre-
sented in this article.
NMR (DMSO‑d6, d ppm): 25.1 (Te-CH2), 55.33 (OCH3), 58.25 (O-
CH2), 106.13 (ArC-Te), 115.64 (ArC meta to Te), 138.04 (ArC ortho to
Te), 160.72 (ArC-OCH3).
Synthesis of complex [Ru(p-cymene)Cl2(L)]
(2)
A solution of [RuCl2(p-cymene)]2 (0.612 g, 1.0 mmol) in 10 mL of
dry MDC was stirred for 2e3 min and then was added a solution of
L (0.560 g, 2.0 mmol) in 20 mL of dry MDC at room temperature.
The resulting orange solution was stirred further for 1 h. The con-
sumption of the ligand during this time was steadily monitored by
TLC. Then, the solution was concentrated to 2e3 mL on rotary
evaporator. An orange-red precipitate of 2 was obtained by slow
addition of n-hexane (3 mL) with vigorous magnetic stirring. The
precipitate was filtered and then recrystallized from a 1:1 mixture
of dry MDC and n-hexane at 4e5 ꢂC in a refrigerator. The orange-
red crystals of 2 obtained after 3 days were filtered and air dried.
Yield: 65%; M.P: 117e120 ꢂC; Elemental analyses: Found
(Calculated) for C19H26Cl2O2RuTe: C, 38.90 (38.94); H, 4.47 (4.47);
2. Experimental section
2.1. Reagents and analytical methods
Tellurium (Te), sodium tetrachloropalladate (Na2PdCl4), dichloro
(p-cymene)ruthenium (II) dimer ([RuCl2(p-cymene)]2), 1,1-
diphenyl-2-picrylhydrazyl (DPPH), 2-chloroethanol and anisole
were purchased from Sigma Aldrich Ind. Ltd. All other chemicals
and reagents used were of analytical reagent (AR) grade and the
common reagents and solvents were purchased from Spectrochem
Ind. Pvt. Ltd. and used without further purification. All the chemical
reactions were carried out under oxygen and moisture free nitro-
gen (N2) atmosphere. Analytical thin layer chromatography (TLC)
was conducted on Merck-60 F254 silica gel pre-coated on
aluminum sheets. TLC plates were viewed under UV-light, using
potassium permanganate solution, ninhydrin stain and/or iodine
spray. Flash column chromatography was performed using silica gel
230e400 mesh obtained from Merck Ind. Pvt. Ltd. The tellurium
precursor compounds, bis-(4-methoxyphenyl)ditelluride (Ar2Te2)
was prepared as per the reported procedure [20]. Melting points
were determined in open capillary tubes closed at one-end and
were reported uncorrected. Infrared spectra were recorded on a
Jasco FT-IR-4100 instrument in the wavenumber range of 4000-
400 cmꢀ1. 1H and 13C{1H} NMR spectra were recorded at 400 and
100 MHz respectively with TMS as an internal standard on
AVANCE-II Bruker 400 MHz spectrometer and the chemical shifts
FT-IR (KBr,
y
cmꢀ1): 3429 (OH), 1581 (C]C), 590 (C-Te aryl), 521 (C-
ppm), 1.225e1.250 (d, J ¼ 10.0 Hz, 3H,
Te alkyl); 1H NMR (CDCl3,
d
CH3 of i-Pr), 1.266e1.292 (d, J ¼ 10.4 Hz, 3H, CH3 of i-Pr), 2.104 (s,
3H, CH3 para to i-Pr),2.715 (bs, 1H, TeCH2), 2.802e2.856 (m, 1H, CH
of i-Pr), 3.41 (bs, 1H, TeCH2), 3.481 (bs, 1H, OCH2), 3.726 (bs, 1H,
OCH2), 3.848 (s, 3H, OCH3), 4.90 (bs, 1H, ArH of p-cymene), 5.252,
5.257, and 5.422 (3bs, 3H, ArH of p-cymene), 6.901e6.919 (d,
J ¼ 8.0 Hz, 2H, ArH ortho to Te), 7.882e7.901 (d, J ¼ 8.0 Hz, 2H, ArH
meta to Te); 13C{1H} NMR: (CDCl3,
d ppm), 18.45 (Te-CH2), 18.48 (p-
cymene CH3), 22.41 and 23.64 (CH3 of i-Pr of p-cymene), 30.86, (CH
of i-Pr of p-cymene), 55.34 (OCH3), 59.68 (O-CH2), 81.59 (ArC of p-
cymene meta to i-Pr), 84.95 (ArC of p-cymene ortho to i-Pr), 98.20
(ArC-CH3 of p-cymene), 104.52 (ArC-Te), 106.38 (ArC-i-Pr of p-
cymene), 115.45 (ArC meta to Te), 137.48 (ArC ortho to Te), 161.36
(ArC-OCH3).
are given in
d ppm. UVeVisible absorption spectra were recorded
using Thermo scientific Evolution-220 spectrophotometer in the
spectral window of 200e800 nm. Bruker APEX-II CCD has been
used in the single crystal X-ray data collection of complex 1, and
Rigaku CrysAlisPro for that of complex 2. The structures were
solved using Olex2 [21] and SHELXTL [22] structure solution pro-
grams by direct methods. SADABS has been used in the absorption
correction of complex 1 and the software incorporated in Crysa-
lisPro for that of complex 2.
2.2. Free radical scavenging activity (DPPH assay)
A stock solution of DPPH (0.1 mM) was made in 50% methanol.
The solutions of different concentrations (10, 20, 30, 40, 50 and
60
mM) of compounds (L, 1 and 2) were prepared in 50% methanol.
Then, to each of these solutions 140
mL of above DPPH solution was
added and then incubated at 37 ꢂC for 30 min in dark. The absor-
bance was measured at lmax, 517 nm against 50% methanol as
blank. The actual absorbance was recorded as the difference in the
absorbance of the control (absorbance without test sample) and the
test sample.
Synthesis of complex cis-[PdCl2(L)2]
(1)
Na2PdCl4 (0.294 g, 1.0 mmol) was dissolved in 5 mL of distilled
water and stirred magnetically for 2e3 min. A solution of ligand L
(0.560 g, 2.0 mmol) made in 10 mL of acetone was added to the
above solution under vigorous stirring. After which the resulting
solution was stirred further 2 h for completion of complexation
reaction. The orange-red solution so obtained was poured into
50 mL of distilled water from this, complex 1 was extracted into
3. Results and discussion
3.1. Synthesis of ligand (L) and complexes (1-2)
The ligand, 4-MeOC6H4TeCH2CH2OH (L) was synthesized

K.M. PrabhuKumar et al. / Journal of Organometallic Chemistry 902 (2019) 120967
3
according to the procedure reported in the literature by Singhet al.
[19] who explored the coordination chemistry of ligand (L) with
Pd(II), Hg(II) and Pt (II) species wherein these complexes were
synthesized by reaction of ligand in the form of sodium alkoxide (4-
MeOC6H4TeCH2CH2ONa) with the corresponding metal chlorides
(PdCl2 and HgCl2) and K2PtCl4by reacting them (L:M) in 1:1 ratio.
They have reported that the L behaved as monobasic bidentate
(Te,Oꢀ) ligand with M-Cl-M bridging in complexes. In this paper,
we have synthesized the Pd(II) (1) and Ru(II) (2) complexes by
521 cmꢀ1 respectively in 1 and 2 as expected for a coordinated
organotelluroether ligand [23,1h]. The other IR bands were
appeared at characteristic wavenumbers. The representative FT-IR
spectra of 1 and 2 are given in Fig. S1 and Fig. S2 respectively as
an supplementary information (SI).
3.4. 1H and 13C{1H} NMR spectra
direct reaction of L with Na2PdCl4 and [Ru (h
6-p-cymene)Cl2]2
The 1H and 13C{1H} NMR spectra of 1 and 2 are represented in
Figs. S3, S4, S5.1andS6 respectively asSI. In the 1H NMR spectrum of
1, the peaks appeared broad. The CH2Te and aromatic (ortho and
respectively in 2:1 (L:M) ratio at room temperature (27e28 ꢂC). The
chemical equations for the reactions involved in the synthesis of
ligand Land its new complexes 1 and 2 are shown in Scheme 1.
meta to Te) protons showed downfield shift of about
d,
0.21e0.26 ppm respectively in comparison with those signals in the
1H NMR spectrum of ligand [19]. In the 1H NMR spectrum of
complex 2, the CH2Te protons and aromatic (ortho and meta to Te)
3.2. Characterization of complexes (1-2)
The composition of complexes 1 and 2 determined by elemental
analysis was in agreement with that calculated according to their
empirical formulae. The complexes were found soluble in chloro-
form, dichloromethane (MDC), ethanol, methanol, tetrahydrofuran
(THF), acetonitrile, dimethylformamide (DMF) and dimethyl sulf-
oxide (DMSO) but were found insoluble in water, diethyl ether, n-
hexane, n-heptane and toluene. The structural characterization of
complexes 1 and 2 was carried out by FT-IR, 1H, 13C{1H} NMR,
UVeVisible spectroscopy and single crystal X-ray diffraction.
protons showed downfield shift of d, 0.15e0.20 ppm [19]. The sig-
nals for O-H proton was observed at 5.21 in 1 and was significantly
deshielded due to its involvement in strong secondary interactions
and no signal was observed for O-H proton in 2. The signals for
other protons in 1 and 2 appeared at characteristic chemical shifts
even though the peaks were broad.
In the 13C{1H} NMR spectrum of 2, the peaks for C-1 and C-3
carbons appeared at 22.41 and 104.52 ppm whereas in 1, they
appeared at 25.1 and 106.13 ppm respectively. These signals
showed downfield shift of about
d, 5 and 8 ppm in 1 and 2
3.3. FT-IR spectra
respectively when compared to the corresponding signals in free
ligand [19]. But these signals in complex 1 were found in very low
intensity as indicated in Fig. S5.2 which may be due to the
involvement of Te in Te/Cl secondary interactions. These shifts
were in agreement with the Pd(II) and Ru(II) complexes of similar
ligand, 2-(4-methoxyphenyltelluro)ethyl amine [1h, 23].
The results observed from the in the FT-IR, 1H and 13C{1H} NMR
spectra revealed that the ligand (L) coordinated to the palladium
(II) and ruthenium (II) centers through Te hence, acting as a mon-
odentate telluroether in both 1 and 2 and the OH group remained
uncoordinated.
In the IR spectra of complexes 1 and 2, a strong absorption band
was observed at n
, 3446 and 3429 cmꢀ1 respectively which was
attributed to the O-H stretching and this band showed blue shift of
above 200 cmꢀ1 when compared to the IR spectrum of free ligand
(n
, 3200 cmꢀ1). This observation indicated that the presence of free
OH group and not invoved in the coordination with metal unlike
reported before [19] but the shift was due to the strong secondary
interactions. The ArC-Te stretching vibrational band was appeared
at n n, 507 and
, ~590 cmꢀ1 while the RC-Te band was appeared at
Scheme 1. Synthesis of ligand, L and its complexes, cis-[PdCl2(L)2] (1), and [Ru (ƞ6-p-cymene)Cl2(L)] (2).

4
K.M. PrabhuKumar et al. / Journal of Organometallic Chemistry 902 (2019) 120967
3.5. Electronic spectra
The UVeVisible spectra of the 2-(4-methoxyphenyltelluro)
ethanol (L) and its palladium (II) (1) and ruthenium (II) (2) com-
plexes were recorded in DMSO and are shown in SI (Fig. S7). In the
electronic absorption spectra of L, 1 and 2, there exists an intense
absorption band at lmax, ꢃ253 nm. This prominent band arises from
the ligand centered (LC)
p - p* intra-ligand charge transfer (ILCT)
transition. A weak absorption band observed at lmax, ꢃ320 nm in
the ligand L and in its complexes 1 and 2 was also attributed to the
n - p* ILCT transition.
3.6. Single crystal X-ray diffraction
Single crystals of palladium (1) and ruthenium (2) complexes of
4-MeOC6H4TeCH2CH2OH (L), were obtained from a solution of
chloroform:n-hexane (1:1) for complex 1 and from MDC:diethyl
ether (1:1) for complex 2. The orange-red crystals were formed
when these solutions stored at 4e5 ꢂC for 3e4 days in the refrig-
erator. The crystal data and structure refinement parameters
collected for the complexes 1 and 2 are given in Table 1. The crystal
data revealed that both the complexes crystallized in monoclinic
crystal system. The molecular structures of 1 and 2 are shown in
Fig. 1 and Fig. 2 respectively. The selected bond lengths and bond
angles of 1 and 2 are given in Table 2.
Fig. 1. (a) Molecular structure of complexcis-[PdCl2(L)2] (1).
bond lengths are in agreement with those reported for cis-Pd(II)
complexes of monodentate telluroether ligands, (C4H3E)TeMe
(where E ¼ O and S) [24]. However, the Pd-Te [2.5817 (3) to 2.6052
(2) Å] bonds shorter and Pd-Cl [2.290 (2) to 2.329 (2) Å] are longer
than those observed in some of the trans-Pd(II) complexes of
monodentate telluroether ligands [25]. The short Pd-Te and long
Pd-Cl bonds observed in complex 1 were due to the strong trans
influence of Te of L [1g] which is higher than that reported for cis-
complexes of other telluroether ligands [24].
3.7. Crystal structure of 1
The molecular structure of 1 demonstrated that the two mole-
cules of ligand L coordinated to palladium (II) center through the Te
only in cis-fashion resulting the complex as cis-[PdCl2(L)2] (1) and
acquired square planar geometry at Pd(II) ion. The Pd (1)eTe (1), Pd
(1)eTe (2), Pd (1)eCl (1) and Pd (1)eCl (2) bond lengths found were
2.5373(7), 2.5405(6), 2.3823(12) and 2.3615 (13) Å respectively. The
Pd-Te [2.530 (1) to 2.546 (1) Å] and [Pd-Cl 2.351 (1) to 2.359 (1) Å]
In crystalline solid of complex 1, there were three significant
weak Te/Cl [3.332and 3.452 Å] and two aromatic p/
p [3.744(3)-
3.745(3) Å] secondary interactions as per the classification by
Table 1
Crystal data and structure refinement parameters for 1 and 2.
Compound
1
2
Empirical formula
Formula weight
Temperature, K
Wavelength, Å
Crystal system
Space group
a, Å
C18H24Cl2O4Te2Pd
735.86
293 (2)
1.54178
Monoclinic
P21/n
10.384 (2)
8. 6405 (17)
24.827 (5)
90
94.18 (3)
C19 H26 Cl2 O2RuTe
585.97
295 (2)
0.71073
Monoclinic
P21/c
9.9617 (9)
8.3670 (7)
25.303 (2)
90
90.736 (8)
2108.8 (3)
4
b, Å
c, Å
a
bꢂ
,
gꢂ
Volume, Å3
2221.6 (8)
4
2.203
29.444
Z
Density (cal.), Mg/m3
1.846
2.363
m
/mmꢀ1
F (000)
1392
1144
Size, mm3
0.22 ꢁ 0.25 ꢁ 0.27
6.672 to 64.57
ꢀ11 ˂ ¼ h ˂ ¼ 12
ꢀ10˂ ¼ k ˂ ¼ 9
ꢀ28˂ ¼ l ˂ ¼ 15
13072
0.29 ꢁ 0.22 ꢁ 0.17
2.045 to 33.362
ꢀ14 ꢄ h<¼14,
0 ꢄ k<¼12,
ꢀ16 ꢄ l<¼38
7470
Theta (q
) range (ꢂ)
Index ranges
Reflections
Independent reflections
Data/Restraints/Parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
3639 [Rint ¼ 0.0374]
3639/3/253
1.054
R1 ¼ 0.0317, wR2 ¼ 0.0811
R1 ¼ 0.0332, wR2 ¼ 0.0823
1.212 and ꢀ1.015
1040585
7439 [Rint ¼ 0.09494]
7439/21/243
1.299
R1 ¼ 0.0987, wR2 ¼ 0.1509
R1 ¼ 0.1107, wR2 ¼ 0.1551
1.088 and ꢀ1.806
1852878
Largest diff. peak and hole, eAꢀ3
CCDC No.

K.M. PrabhuKumar et al. / Journal of Organometallic Chemistry 902 (2019) 120967
5
Further, complex 1 contain moderate to strong intermolecular
secondary interactions of OH/O [2.35 Å], CH/O [2.64 Å] and CH/Cl
[2.51 to 2.95 Å] type as shown in Fig. S9 and its packing diagram is
shown in Fig. S10 (SI). These bonding parameters present in com-
plex 1 are given in Table 3.
3.8. Crystal structure of 2
The crystal structure of complex 2 also revealed that the ligand L
found to act as monodentate telluroether resulting [Ru (h
6-p-
cymene)Cl2(L)] (2). In 2, the Ru-Te (1), Ru-Cl (1) and Ru-Cl (2) bond
lengths found were 2.644(7), 2.421(2) and 2.424 (2) Å respectively.
These bond lengths are in agreement with the reported values of
Ru(II) complexes of other monodentate organotellurium ligands
[18f,32,25d].The complex 2 has pseudo-octahedral geometry at
Ru(II) ion and acquired half-sandwich also known as piano-stool
structure with
h
6-p-cymene, two chloro (Clꢀ) and L. In 2, there
exists significant intermolecular secondary interactions as given in
Table 3 such as OH/Cl [2.36 to 2.88 Å], CH/Cl [2.74e2.93 Å] and C-
H/O [2.27 Å] which results in supramolecular assembly. These in-
teractions are shorter than sum of their corresponding van der
Waals radii 2.95 Å (H/Cl) and 2.72 Å (H/O) respectively. Inter-
molecularsecondary interactions and crystal packing present in [Ru
Fig. 2. Molecular structure of complex [RuCl2(p-cymene) (L)] (2).
(h
6-p-cymene)Cl2(L)] (2)are shown in Figs. S11 and S12 respectively
(SI).
Secondary interactions between chalcogens [E/E] or chalcogen
Table 2
Selected bond lengths (Å) and bond angles (ꢂ) of 1 and 2.
1
and heteroatoms [E/X] (where, X ¼ N, O, S, F, Cl, Br and I) in
organochalcogen compounds or their complexes results in supra-
molecular structures [33]. These non-bonded interactions reported
to play important roles in biological applications particularly in the
DNA binding [34], catalytic antioxidant activity [5] and/or other-
wise enhance the stability.
Pd-Cl (1)
Pd-Cl (2)
Pd-Te (2)
Pd-Te (1)
2.3816 (12)
2.3613 (13)
2.5405 (6)
2.5374 (7)
3.1712 (10)
92.30 (4)
Cl (2)-Pd-Te (2)
Cl (1)-Pd-Te (2)
Te (1)-Pd-Te (2)
Cl (2)-Pd-Pd#1
Cl (1)-Pd-Pd#1
Te (1)-Pd-Pd#1
C (1A)-Te (1)-Pd
C (8A)-Te (1)-Pd
94.29 (4)
172.15 (3)
91.358 (12)
99.29 (5)
88.31 (3)
87.37 (3)
Pd-Pd#1
Cl (2)-Pd-Cl (1)
Cl (2)-Pd-Te (1)
Cl (1)-Pd-Te (1)
170.86 (4)
81.58 (3)
95.30 (12)
111.86 (12)
3.9. DPPH free radical scavenging activity
Symmetry transformations used to generate equivalent atoms: #1 -xþ1,-yþ1,-
zþ1
2, 2-diphenyl-1-picrylhydrazyl (DPPH) [35] is a stable free
radical that has long been used widely as an antioxidant assay for
the evaluation of in-vitro free radical scavenging capacity of newly
synthesized compounds. In the presence of an antioxidant, the
DPPH radical quenched to reduced form with the loss of its violet
color [36] and then the change in absorbance at 517 nm was
2
Ru-Te
2.6438 (7)
2.421 (2)
2.424 (2)
Cl (2)-Ru-Te
C (18)-Te-Ru
C (18A)-Te-Ru
Cl (1)-Ru-Te
84.97 (6)
108.4 (8)
99.1 (7)
Ru-Cl (1)
Ru-Cl (2)
C (11)-Te-Ru
104.64 (18)
82.27 (5)
Jeffrey [26] forming a dinuclear palladium cluster (cis-[PdCl2(L)2])2
(1)2 and the cis-[PdCl2(L)2] molecules were oriented in staggered
conformation with short Pd-Pd [3.1712 (5) Å] bonds as shown in
Fig. S8.
Table 3
Hydrogen-bonds (Å, ꢂ) in 1 and 2.
DdH$$$A
DdH H$$$A D$$$A
DdH$$$A
1
One of the eCH2-CH2-OH group was placed over two positions
due to dynamic disorderness rising from thermal vibrations. The
two positions of the group were treated by using PART command to
obtain site occupancy ratio 0.60(3):0.40(3). During final stages of
the refinement all the disordered atoms C8BA, C8BB, C9BA, C9BB,
O2BA and O2BB were refined anisotropically and their anisotropic
displacement factors were treated as equivalent to the corre-
sponding carbon/oxygen atoms in the other eCH2-CH2-OH group.
The cis-complexes of palladium (II) with monodentate tellur-
oether ligands are rare [24]. Few cis-complexes of Pd(II) were re-
ported for ditelluroether [27,25b] and hybrid telluroether [28]
ligands. The Pd-Pd distance [3.171 (7) Å] in 1 was comparatively
shorter and stronger than that of trans-complexes of palladium (II)
with monodentate telluroether [3.214 (1) Å] [25d], and hybrid tri-
dentate organotellurium [3.203 (1) Å] [29] ligands as well as that of
cis-complexes of palladium (II) with bidentate ditelluroether
[3.1993 (3) Å] [30] and other hybrid organotellurim [3.4156 (6) Å]
[31] ligands.
O (2A)-H (2A) … O (1B)#3
C (8A)-H (8AA) … O (2BB^b)#2 0.97
C (7B)-H (7BA) … Cl (2)#4
C (8BA^a)-H (8BD^a) … Cl (2)
C (9BA^a)-H (9BB^a) … Cl (1)#5 0.97
C (8BB^b)-H (8BC^b) … Cl (2) 0.97
0.82
2.35
2.64
2.94
2.95
2.95
2.51
2.951 (6)
3.440 (13)
3.795 (6)
3.569 (14)
3.654 (13)
3.263 (15)
130.3
140.4
148.7
123.1
129.9
134.7
0.96
0.97
Symmetry codes: #1 -xþ1,-yþ1,-zþ1; #2 -xþ1,-yþ2,-zþ1; #3 -xþ1/2,y-1/2,-
zþ1/2; #4 -xþ3/2,yþ1/2,-zþ1/2; #5 x,yþ1,z
2
O (2)eH (2) … Cl (2)#1
O (2A)-H (2A1) … Cl (1)#1
O (2A)-H (2A1) … Cl (2)#1
C (19)-H (19A) … O (1)#2
C (18)-H (18A) … Cl (2)
C (18A)-H (18C)/Cl (2)
C (19A)-H (19C)/Cl (2)#1
0.82
0.82
0.82
0.97
0.97
0.97
0.97
2.55
2.88
2.36
2.27
2.93
2.74
2.79
3.30 (2)
3.455 (18) 128.9
152.4
3.11 (2)
3.11 (3)
3.59 (4)
3.43 (3)
3.40 (2)
153.0
145.3
126.4
128.0
121.8
Symmetry transformations used to generate equivalent atoms: #1 -xþ1,y-1/2,-
zþ3/2 #2 -x,yþ1/2,-zþ3/2

6
K.M. PrabhuKumar et al. / Journal of Organometallic Chemistry 902 (2019) 120967
Table 4
p/
p secondary interactions between benzene ring of ligand in 1.
Cg(I) Res(I) Cg(J)
Cg/Cg
Slippage
Cg1 [1] - > Cg2
Cg2 [1] - > Cg1
3.745 (3)
3.744 (3)
0.564
0.602
measured by UVeVisible spectrophotometer. In-vitro antioxidant
activity of 2-(4-methoxyphenylrelluro)ethanol (L), an organo-
tellurium ligand and its newly synthesized Pd(II) and Ru(II) com-
plexes (1 and 2) was assessed in accordance with the method of
Choi et al. [36] using DPPH assay. The percentage inhibition of free
radicals generated from DPPH by varying concentrations
(10e60 mM) with an increment of 10 mM of compounds (L, 1 and 2)
was measured. The measurements were made similarly for a
standard compound Vitamin-C (Vit-C). A graph was plotted for %
inhibition against concentration of each compound and Vit-C. The
graph obtained is shown in Fig. 3 from which the concentration of a
compound required to inhibit 50% of the radicals (IC50) was
determined. The IC50 values of L, 1 and 2 were found in the range of
Fig. 4. IC50 values of L, 1, 2 and Vit-C.
secondary interactions. The two molecules of 1 configured in
staggered conformation held through a short Pd-Pd bond resulted
in di-nuclear palladium clusters, (cis-[PdCl2(L)2])2 (1)2. The com-
plex 2 exhibit pseudo-octahedral geometry and half-sandwich
structure about the central Ru(II) ion. In the crystals of 1 and 2
the complex molecules were held by strong intermolecular sec-
ondary interactions of type OH/Cl, CH/Cl and C ꢀ H/O resulted in
supramolecular structures. The ligand and complexes were
explored for the antioxidant activities for first time. All the com-
pounds (L, 1 and 2) showed excellent free radical scavenging ac-
9e33 mM also represented by bar diagram in Fig. 4. These results
suggested that the above organotellurium ligand and its palladium
and ruthenium complexes containing M ꢀ Te bond(s) showed
antioxidant activity. Among the compounds (L, 1 and 2) tested, the
ruthenium complex, 2 (IC50, 9.8) found an effective antioxidant in
compared with standard, Vit-C (IC50, 11.9).
4. Conclusions
tivity and the IC50 values (9e33 mM) demonstrated that these
The Pd(II) (1) and Ru(II) (2) complexes of 2-(4-
methoxyphenyltelluro)ethanol, a telluroether ligand (L) contain-
ing OH functionality were synthesized and structurally character-
ized by 1H, 13C{1H} NMR, UVeVisible, FT-IR spectroscopy, elemental
compounds can act as moderate to strong antioxidants among
which the ruthenium complex (2) could be an effective antioxidant
and is highly in agreement with standard antioxidant Vit-C.
analysis. The molecular structures of cis-[PdCl2(L)2] (1), [Ru (h
6-p-
Acknowledgments
cymene)Cl2(L)] (2) were further confirmed by single crystal X-ray
diffraction. The data revealed that in both 1 and 2, the L coordinated
through Te only and acting as a monodentate telluroether ligand. In
crystalline solid of 1, the two monodentate telluroether (L) were
found coordinated to Pd(II) ion in cis-fashion and between which
P. Raghavendra Kumar thanks DST-SERB, New Delhi, India, for
funding the major research project No. SR/S1/IC-76/2010(G).
Appendix A. Supplementary data
there were strong intermolecular Te/Cl and aromatic
p
/p
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jorganchem.2019.120967.
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