Synthesis, spectroscopic characterization, catalytic and antibacterial studies of ruthenium(III) Schiff base complexes

Article abstract of DOI:10.1016/j.molliq.2015.07.001

The bidentate Schiff base ligands (HL_n) have been synthesized by condensation of 2-hydroxy-l-naphthaldehyde with aniline and its p-substituted derivatives in ethanol. Ruthenium(III) complexes of the type [Ru(L_n)₂(H₂O)₂]Cl have been synthesized by the reaction of RuCl₃·nH₂O with the Schiff base ligands (in a molar ratio 1:2) in ethanol. The ligands and their Ru(III) complexes have been characterized by elemental analysis, magnetic susceptibility, spectroscopic (FTIR, UV-vis, ¹H NMR and X-ray diffraction) and thermal analysis techniques. All the ruthenium(III) complexes are found to be stable, paramagnetic, low spin and octahedrally coordinated by the ligands through the nitrogen atom of the azomethine (-CN-) group and the oxygen atom of the deprotonated phenolic group. The molecular and electronic structures of the investigated ligands (HL_n) were also studied using quantum chemical calculations. The complexes (1, 3 and 5) exhibited a catalytic activity for the oxidation of benzoin to benzil with moderate to high yield in the presence of sodium periodate as co-oxidant. The antibacterial activities of the ligands (HL_n) and their Ru(III) complexes towards Gram positive and Gram negative bacteria have been investigated.

Full text of DOI:10.1016/j.molliq.2015.07.001

Journal of Molecular Liquids 211 (2015) 217–227
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis, spectroscopic characterization, catalytic and antibacterial
studies of ruthenium(III) Schiff base complexes
1
⁎
A.F. Shoair , A.R. El-Shobaky, H.R. Abo-Yassin
Department of Chemistry, Faculty of Science, Damietta University, Damietta 34517, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
The bidentate Schiff base ligands (HLn) have been synthesized by condensation of 2-hydroxy-L-naphthaldehyde
with aniline and its p-substituted derivatives in ethanol. Ruthenium(III) complexes of the type [Ru(Ln)2(H2O)2]Cl
have been synthesized by the reaction of RuCl3·nH2O with the Schiff base ligands (in a molar ratio 1:2) in ethanol.
The ligands and their Ru(III) complexes have been characterized by elemental analysis, magnetic susceptibility,
spectroscopic (FTIR, UV–vis, 1H NMR and X-ray diffraction) and thermal analysis techniques. All the
ruthenium(III) complexes are found to be stable, paramagnetic, low spin and octahedrally coordinated by the li-
gands through the nitrogen atom of the azomethine (–C_N–) group and the oxygen atom of the deprotonated
phenolic group. The molecular and electronic structures of the investigated ligands (HLn) were also studied using
quantum chemical calculations. The complexes (1, 3 and 5) exhibited a catalytic activity for the oxidation of ben-
zoin to benzil with moderate to high yield in the presence of sodium periodate as co-oxidant. The antibacterial
activities of the ligands (HLn) and their Ru(III) complexes towards Gram positive and Gram negative bacteria
have been investigated.
Received 30 January 2015
Received in revised form 30 May 2015
Accepted 1 July 2015
Available online xxxx
Keywords:
Schiff base
Ruthenium(III)
Catalytic oxidation
Antibacterial
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
In continuation of our earlier work [13,14], we report the synthesis
and characterization of the Schiff base ligands (HLn) derived from
Schiff bases form an interesting class of chelating ligands that has
enjoyed popular use in the coordination chemistry of transition
elements [1,2]. A large number of Schiff bases and their metal com-
plexes have been studied because of their interesting and important
properties such as their ability to reversibly bind oxygen and their
use in catalyses [3,4] and biological systems [5,6]. Aromatic Schiff
bases with an o-hydroxy substituent possess a very interesting
characteristic-reversible color changes induced by irradiation (photo-
chromism) or by a change in temperature (thermochromism). The
change of color of thermochromic compounds is ascribed to the tautom-
erism between the OH and NH groups resulting from intramolecular hy-
drogen transfer between an enolimine and ketoamine tautomer, which
can be cis or trans relative to the C_N bond [7]. We have reported a
number of ruthenium complexes containing N,O-donor ligands and in-
vestigated their catalytic activity for alcohol oxidation in combination
with various oxidants such as K2S2O8, IO(OH)5, H2O2 and NMO [8–11].
Recently, we have synthesized a new class of ruthenium(III) complexes,
[RuIIICl(L)2(H2O)] and [RuIIICl2(L)2]Cl (L = 2-aminophenol, 8-
hydroxyquinoline and 4-aminoantipyrine). Elemental analysis and
spectroscopic characterization showed an octahedral geometry
around ruthenium(III) ion [12].
the condensation of 2-hydroxy-1-naphthaldehyde with aniline and its
p-substituted derivatives. Their ruthenium(III) complexes were synthe-
sized and characterized by different spectroscopic techniques. The opti-
mized bond lengths, bond angles and the calculated quantum chemical
parameters for the ligands (HLn) were investigated. Also, we investigat-
ed the catalytic activity of these complexes towards the oxidation of
benzoin to benzil at different temperatures. The antibacterial activities
of the ligands and their Ru(III) complexes towards Gram positive and
Gram negative bacteria have been investigated.
2. Experimental
2.1. Chemicals and physical measurements
All the chemicals and solvents were purchased from Sigma-Aldrich
Chemicals Company (USA) and used without further purification. Mi-
croanalytical data (C, H and N) were collected on Automatic Analyzer
CHNS Vario ELIII, Germany. Spectroscopic data were obtained using
the following instruments: FTIR spectra (KBr disks, 4000–400 cm−1
)
by Jasco FTIR-4100 spectrophotometer; the 1H NMR spectra by Bruker
WP 300 MHz using DMSO-d6 as a solvent containing TMS as the internal
standard. UV–visible spectra by Perkin-Elmer AA800 spectrophotome-
ter Model AAS. Thermal analysis of the Schiff base ligands and their
Ru(III) complexes were carried out using a Shimadzu thermogravimet-
ric analyzer under a nitrogen atmosphere with heating rate of 10 °C/min
⁎
Corresponding author.
E-mail address: abdel_shoair@yahoo.com (A.F. Shoair).
Abstracted from her M.Sc. Thesis.
1
http://dx.doi.org/10.1016/j.molliq.2015.07.001
0167-7322/© 2015 Elsevier B.V. All rights reserved.

218
A.F. Shoair et al. / Journal of Molecular Liquids 211 (2015) 217–227
over a temperature range from room temperature up to 800 °C.
Magnetic susceptibility measurements were determined at room tem-
perature on a Johnson Matthey magnetic susceptibility balance using
Hg[Co(SCN)4] as calibrant. Conductivity measurements of the com-
plexes at 25 1 °C were determined in DMF (10−3 M) using conductiv-
ity/TDS meter model Lutron YK-22CT. X-ray diffraction analysis of
compounds were recorded on X-ray diffractometer in the range of
diffraction angle 2θ = 8–40°. This analysis was carried out using CuKα
radiation (λ = 1.54098 Å). The applied voltage and the tube current
are 40 kV and 30 mA, respectively. The molecular structures of the in-
vestigated compounds were optimized by HF method with 3-21G
basis set. The molecules were built with the Perkin Elmer ChemBio
Draw and optimized using Perkin Elmer ChemBio3D software [15].
Fig. 2. The proposed structure of the complexes (1–5) [Ru(Ln)2(H2O)2]Cl.
2.2. Preparation of Schiff base ligands (HLn)
concentrated to half of its original volume by evaporation and allowed
to cool at room temperature. During this, a microcrystalline solid was
separated, which was isolated by filtration, washed with hot ethanol,
ether and dried in a vacuum desiccator over anhydrous Na2SO4.
The Schiff base ligands (HLn) were prepared (Fig. 1) by ethanolic so-
lution of 2-hydroxy-1-naphthaldehyde (20 mmol in 15 cm3 of ethanol
to ethanolic solution of aniline and its p-derivatives (20 mmol,
15 cm3) of ethanol. The mixture was refluxed for 4 h with stirring, dur-
ing this time yellow to orange precipitates are formed. The precipitates
were collected by filtration, washed twice with hot ethanol and dried in
a vacuum desiccator over anhydrous CaCl2.
RuCl3·nH2O + 2HLn → [Ru(Ln)2(H2O)2]Cl + 2HCl
The resulting formed ligands are:
2.4. Catalytic oxidation of benzoin by ruthenium(III) complexes
HL1
HL2
HL3
HL4
HL5
2-hydroxy-1-naphthylidene-4-methoxyaniline
2-hydroxy-1-naphthylidene-4-methylaniline
2-hydroxy-1-naphthylideneaniline
2-hydroxy-1-naphthylidene-4-chloroaniline
2-hydroxy-1-naphthylidene-4-nitroaniline.
The complex [Ru(Ln)2(H2O)2]Cl (n = 1, 3 or 5) was dissolved in a
mixture of 5 cm3 of DMF and 5 cm3 CH3CN (0.01 mmol). To which ben-
zoin (2 mmol) was added with stirring, then an aqueous solution of
NaIO4 (5 mmol, 5 ml water) was added dropwisely within half an
hour and the reaction mixture was further stirred for 4 h at 70 °C. The
reaction mixture was extracted with diethyl ether to collect benzil and
dried over anhydrous Na2SO4 and weighed.
2.3. Preparation of ruthenium(III) Schiff base complexes (1–5)
All Ru(III) complexes (1–5) (Fig. 2) were synthesized according to
the general procedure [13]: a stoichiometric amount of the desired
ligand (2 mmol, 20 cm3) in ethanol was added dropwisely to a hot
ethanolic solution of RuCl3·nH2O (1 mmol, 20 cm3) with stirring
and the reaction mixture was refluxed for 3 h. The solution was
2.5. Antibacterial investigation
In vitro antibacterial activity studies were carried out using the stan-
dardized disk–agar diffusion method [16] to investigate the inhibitory
effect of the synthesized Schiff base ligand and their Ru(III) complexes
(A)
(B)
Fig. 1. The structure of the Schiff base ligands (HLn).

A.F. Shoair et al. / Journal of Molecular Liquids 211 (2015) 217–227
219
against Gram-positive bacteria (e.g., Bacillus cereus and Staphylococcus
aureus) Gram-negative bacteria (e.g., Escherichia coli and Klebsiella
pneumoniae) as a kind of fungi. An inhibition zone diameter indicates
that the tested compounds are active against the used kinds of the bac-
teria and fungus. The tested compounds were dissolved in DMF (which
have no inhibition activity). Also, the antifungal activities were tested
against four local fungal species (Aspergillus niger, Alternaria alternata,
Penicillium italicum and Fusarium oxysporium) on DOX agar medium.
The concentrations of each solution were 50, 100 and 150 μg/ml. By
using a sterile cork borer (10 mm diameter), wells were made in agar
medium plates previously seeded with the test microorganism. 200 μl
of each compound was applied in each well. The agar plates were kept
at 4 °C for at least 30 min to allow the diffusion of the compound to
agar medium. The plates were then incubated at 37 °C or 30 °C for
bacteria and fungi, respectively. The diameters of inhibition zone were
determined after 24 h and 7 days for bacteria and fungi, respectively,
taking the consideration of the control values (DMF). Penicillin and mi-
conazole were used as reference substances against bacteria and fungi,
respectively.
unoccupied molecular orbital (LUMO) are the main orbitals take part in
chemical stability. The HOMO represents the ability to donate an elec-
tron, LUMO as an electron acceptor represents the ability to obtain an
electron. The HOMO and LUMO for ligands tautomers (A) are shown
in Fig. S1 in the supplementary. The calculated quantum chemical pa-
rameters are given in Table 2. Additional parameters such as ΔE, abso-
lute electronegativities, χ, chemical potentials, Pi, absolute hardness, η,
absolute softness, σ, global electrophilicity, ω, global softness, S, and ad-
ditional electronic charge, ΔNmax, have been calculated [15] according
to the following Eqs. (1)–(8):
ΔE ¼ ELUMO−EHOMO
ð1Þ
ð2Þ
−ðEHOMO þ ELUMO
Þ
χ ¼
2
ELUMO−EHOMO
η ¼
ð3Þ
2
1
σ ¼
η
ð4Þ
ð5Þ
ð6Þ
3. Results and discussion
Pi ¼ −χ
The results of physical properties of the prepared ligands (HLn) and
their Ru(III) complexes (1–5) along with their elemental analysis are
collected in Table 1. The analytical data of Ru(III) complexes indicated
that the complexes have 1:2 (metal:ligand) stoichiometry. The com-
plexes of the type [Ru(Ln)2(H2O)2]Cl (where Ln = mono anion of the
bidentate Schiff base ligands) are stable in air and soluble in most com-
mon organic solvents. All the complexes are soluble in highly coordinat-
ing solvents like DMSO and DMF. The ligands are asymmetrical
bidentate and coordinate through the nitrogen atom of the azomethine
(–C_N–) group and the oxygen atom of the deprotonated phenolic
group. Hence, the complexes [Ru(Ln)2(H2O)2]Cl have a D2h-symmetry
(Fig. 2). The composition of these complexes has been confirmed by el-
emental analysis and spectroscopic and thermal techniques. The molar
conductance values of the Ru(III) complexes (10−3 M) are measured
in DMF and these values are (51–59 Ω−1 cm2 mol−1) range indicating
the electrolytic nature of the complexes (presence of Cl ion) [1]. The
magnetic susceptibility measurements show that the complexes
[Ru(Ln)2(H2O)2]Cl are paramagnetic (μeff = 1.8–2.1 BM, low spin d5,
S = ½), as is normal for ruthenium(III) complexes in an octahedral
environment [13].
1
S ¼
2η
Pi2
ω ¼
ð7Þ
ð8Þ
2η
Pi
η
ΔNmax ¼ −
:
The HOMO–LUMO energy gap, ΔE, which is an important stability
index, is applied to develop theoretical models for explaining the struc-
ture and conformation barriers in many molecular systems [15]. The
calculations indicated that the form (A) is more stable form and highly
reactive than form (B).
3.2. Infrared spectra
The FTIR spectral data of the Schiff base ligands (HLn) and their
Ru(III) complexes (1–5) are listed in Table S1 in the supplementary.
On the basis of the similarity of the spectra of the complexes (1–5), it
may be assumed that they have the similar coordination structures.
Comparison of the IR spectra of Ru(III) complexes with that of the free
Schiff base ligands revealed that:
3.1. Molecular structure of the ligands
The selected geometrical structures of the investigated ligands are
calculated by optimizing their bond lengths and bond angles. The calcu-
lated molecular structures for ligands tautomers (A & B) are shown in
Fig. 3. Both the highest occupied molecular orbital (HOMO) and lowest
(1) The IR spectra of Schiff base ligands exhibited a broad band of
medium intensity at the regions 3424–3445 cm−1, strong band
at 1607–1619 cm−1, and a medium band at 1230–1255 cm−1
,
which were assigned to H-bonded OH stretching ν(OH),
azomethine ν(C_N) group and phenolic oxygen ν(C–O) group
vibrations, respectively.
Table 1
Physical properties and elemental analysis data of Schiff base ligands (HLn) and their
Ru(III) complexes (1–5).
(2) The broad band due to υ(OH) group which appear in the spectra
of the free ligands at 3424–3445 cm−1 region, which associated
with the complexes are confirmed by elemental and thermal
analyses for the coordinated or uncoordinated water molecules.
(3) The strong band at 1607–1619 cm−1 region assigned to υ(C_N)
in the free ligands were shifted to the lower frequencies by
20–25 cm−1 indicating the participation of the azomethine
group in chelation [17,18].
(4) Furthermore, on complexation, the medium band corresponding
to phenolic oxygen ν(C–O) in the free ligands is shifted to higher
wavenumber in the range 1245–1265 cm−1 for all the com-
plexes indicating that, the ligands coordinate through their
deprotonated form and formation of metal–oxygen bonds.
Compound
M.p. Yield% μeff
.
% found (calc.)
(°C)
(BM)
C
H
N
HL1
110 54.17
–
77.78(77.98) 5.28(5.42) 4.77(5.05)
(1) [Ru(L1)2(H2O)2]Cl 126 45.06 1.95 59.48(59.62) 3.76(3.86) 3.57(3.86)
HL2 122 78.69 82.66(82.76) 5.65(5.75) 5.07(5.36)
(2) [Ru(L2)2(H2O)2]Cl 175 40.12 2.01 62.19(62.38) 3.89(4.04) 3.77(4.04)
HL3 240 17.62 82.45(82.59) 5.15(5.26) 5.44(5.67)
(3) [Ru(L3)2(H2O)2]Cl 130 29.33 2.09 61.25(61.39) 3.54(3.61) 3.88(4.21)
HL4 140 65.91 72.34(72.47) 4.14(4.26) 4.76(4.97)
(4) [Ru(L4)2(H2O)2]Cl 120 51.64 1.99 55.54(55.62) 2,77(3.00) 3.64(3.82)
–
–
–
HL5
220 73.55
–
69.76(69.86) 3.89(4.11) 9.34(9.59)
(5) [Ru(L5)2(H2O)2]Cl
96 62.96 2.10 53.87(54.07) 2.86(2.92) 7.32(7.42)

220
A.F. Shoair et al. / Journal of Molecular Liquids 211 (2015) 217–227
HL1
HL2
HL3
HL4
HL5
Fig. 3. The calculated molecular structures of the investigated ligands (HLn).
(5) In addition, new bands were observed in the region 526–545
δ (ppm) 3.73 (s, 3H, OCH3) and 2.35 (s, 3H, CH3), respectively. The
signal at the region 8.30–8.49 ppm is assigned to the azomethine
group (–HC_N–). Furthermore, the ligands (HLn) showed a broad
peak in the range 9.63–9.68 ppm for OH proton of 2-hydroxy-1-
naphthldehyde ring.
and 474–482 cm−1, which were assigned to the formation of
υ(Ru–O) and υ(Ru–N), respectively [19] which further sup-
ports the coordination of the azomethine nitrogen and the
phenolic oxygen.
(6) Finally, the presence of coordinated water was suggested by the
very broad absorption band in the region 3405–3440 cm−1 in
the IR spectra of complexes [20].
3.4. X-ray diffraction
The X-ray diffraction (XRD) pattern powder forms of HL2 and
[Ru(L2)2(H2O)2]Cl are presented in Fig. 4. The XRD of HL2 shows many
diffraction peaks which indicate the polycrystalline phase. The average
crystallite size (ξ) can be calculated from the XRD pattern according to
Debye–Scherrer Eq. (9) [21,22]:
3.3. 1H NMR spectra
The 1H NMR spectra of Schiff base ligands (HLn) were recorded in
d6-DMSO at room temperature. The 1H NMR spectral data showed a
multiplet signal of the aromatic protons (m, Ar–H) at 6.90–7.57 and
7.27–7.94 ppm regions of naphthalene and benzene rings, respectively.
In the meantime, the 1H NMR of the HL1 and HL2 exhibits signals at
Kλ
ξ ¼
:
ð9Þ
β1=2 cos θ

A.F. Shoair et al. / Journal of Molecular Liquids 211 (2015) 217–227
221
Table 2
The calculated quantum chemical parameters of the investigated Schiff base ligands (HLn).
Comp.
E
HOMO (a.u)
ELUMO (a.u)
ΔE (a.u)
χ (a.u)
η (a.u)
σ (a.u)−1
Pi (a.u)
S (a.u)−1
ω (a.u)
ΔNmax
HL1
(A)
(B)
HL2
(A)
(B)
HL3
(A)
(B)
HL4
(A)
(B)
HL5
(A)
(B)
−0.2689
−0.3812
−0.1663
−0.1539
0.1026
0.2273
0.2176
0.2675
0.0513
0.1136
19.4856
8.7981
−0.2176
−0.2675
9.7428
4.3991
0.4614
0.3149
4.2403
2.3538
−0.2694
−0.3818
−0.1721
−0.1592
0.0973
0.2226
0.2207
0.2705
0.0487
0.1113
20.5528
8.9834
−0.2207
−0.2705
10.2764
4.4918
0.5006
0.3286
4.5363
2.4301
−0.2694
−0.3818
−0.1757
−0.1626
0.0937
0.2192
0.2226
0.2722
0.0468
0.1096
21.3515
9.1237
−0.2225
−0.2722
10.6758
4.5618
0.5288
0.3380
4.7521
2.4836
−0.2691
−0.3728
−0.1641
−0.1519
0.1050
0.2209
0.2166
0.2624
0.0525
0.1104
19.0422
9.0559
−0.2165
−0.2623
9.5211
4.5279
0.4467
0.3117
4.1244
2.3761
−0.2707
−0.3822
−0.2263
−0.2170
0.0444
0.1652
0.2485
0.2996
0.0222
0.0826
45.0146
12.1080
−0.2484
−0.2996
22.5073
6.0540
1.3896
0.5435
11.185
3.6279
The equation uses the reference peak width at angle (θ), where λ is
wavelength of X-ray radiation (1.541874 Å), K is constant taken as 0.95
for organic compounds [22] and β1/2 is the width at half maximum of
the reference diffraction peak measured in radians. The dislocation den-
sity, δ, is the number of dislocation lines per unit area of the crystal. The
value of δ is related to the average particle diameter (ξ) by relation (10)
[21,22]:
3.5. Electronic spectra
The electronic spectra of the free ligands and the complexes were
carried out in DMF solution. The free ligands showed two types of tran-
sitions, at the range 218–234 nm (π–π*) transitions for the benzene ring
and azomethine group and the other bands in the 285–310 nm region
(n–π*) transition of non-bonding electrons present on the nitrogen of
the azomethine group. These peaks exhibited bathochromic shift upon
complex formation, which supported the coordination of the ligands
to Ru(III) ion. The ground state of Ru(III) in an octahedral environment
is 2T2g and the first excited doublet levels in the order of increasing en-
1
δ ¼
:
ð10Þ
ξ2
2
2
ergy are A2g and T1g, which arise from t4 e1 configuration [25].
2g
g
Hence, two bands corresponding to 2T2g → 2A2g and 2T2g → 2T1g are pos-
sible. UV–vis spectra of the reported Ru(III) complexes show a third in-
tense absorption band in the region 314–316 nm, which can be assigned
to charge transfer (CT) transitions of the type Lπ → 2T2g [26–28]. Similar
observations have been made for other ruthenium(III) octahedral
complexes [29,30] and in most ruthenium(III) Schiff base chelates [31].
The value of ξ is calculated and found to be 32.1 nm and the value of
δ is 9.70 × 10−4 nm−2 for ligand (HL2).
The diffraction peaks in powder spectra are indexed and the lattice
parameters are determined with the aid of CRYSFIRE computer program
[23,24]. The values of interplanar spacing (d) and Miller indices (hkl) for
each diffraction peak before and after refinement are determined by
using CHEKCELL program [23,24]. The values of d and corresponding
hkl for HL2 are listed in Table 3. The results show that start 2 has
orthorhombic crystal structure with space group PCA21. The lattice
parameters are estimated as:
3.6. Thermal analyses
The TGA analysis was carried out to know the actual loss of organic
moiety present in the ligands HL1, HL3 and HL5 and their Ru(III) com-
plexes (1, 3 and 5). It is clear that the change of substituent affects the
thermal properties of the ligands and complexes. The temperature
intervals and the percentage of loss of masses are listed in Table S2 in
the supplementary.
The TGA curve of ligand (HL1)shows two decomposition steps, the
first stage occur in the temperature range 180–308 °C is attributed to
loss of C17H12NO (found 88.43% and calc. 88.80%). The second stage
occurs in the temperature range 308–732 °C corresponding to loss of
CH3O (found 11.57%; calc. 11.19%).
a ¼ 20:1660 Å; b ¼ 6:8910 Å; c ¼ 4:5890 Å; α ¼ γ ¼ β ¼ 90ꢀ:
(3)
(2)
Table 3
X-ray data of ligand (HL2).
(8)
(4)
(1)
(5)
(6)
(10)
(7)
20
(a)
(9)
Peak no.
2θ (obs.)
2θ (calc.)
d
hkl (obs.)
dhkl (calc.)
h k l
1
2
3
4
5
6
7
8
9
10
8.763
12.828
13.559
15.544
17.571
18.417
21.243
29.093
32.788
35.156
8.765
12.839
13.572
15.567
17.582
18.428
21.261
29.107
32.787
35.182
10.24
6.89
6.52
5.69
5.04
4.81
4.18
3.067
2.72
2.55
8.63
6.89
6.52
6.68
5.04
4.81
4.17
3.06
2.72
2.54
2 0 0
0 1 0
1 1 0
2 1 0
4 0 0
3 1 0
2 0 1
3 2 0
1 2 1
3 2 1
(b)
10
30
40
2
θo
Fig. 4. X-ray diffraction (XRD) patterns powder forms of: a) ligand (HL2) and b) complex
(2).

222
A.F. Shoair et al. / Journal of Molecular Liquids 211 (2015) 217–227
The ligand (HL3) shows loss in the temperature range 190–290 °C
θ was drawn and found to be linear from the slope of which Ea was cal-
corresponding to loss of C10H7O (found 57.01%, calc. 57.8%). While the
weight loss in the temperature range 290–752 °C (found 42.75%, calc.
42.11%), which is attributed to loss of a part of the ligand (C7H6N).
TG curve of the (HL5) ligand shows three steps of decomposition.
The first stage of decomposition occur in the temperature range of
120–331 °C and are associated with the loss of a part of the ligand
(C7H6) with an estimated weight loss of 30.95% (calcd. 30.82%). The
second stage of decomposition occur in the temperature range of
331–760 °C and are associated with the loss of C9H6N2O3 atoms and
with an estimated weight loss of 64.88% (calcd. 65.07%). The finally
stage of decomposition associated with the loss of 4C atoms with an
estimated weight loss of 4.20% (calcd. 4.11%).
All Ru(III) complexes showed TG curves in the temperature range
~100–209 °C loss of coordinated water molecules. The second and
third stages are related to loss of a part of the ligand. The final weight
losses are due to the decomposition of the rest of the ligand leaving
metal oxides residue (Table S2 in the supplementary).
culated. The pre-exponential factor, A, calculated from Eq. (16):
Ea
A
ꢂ
ꢀ
ꢁꢃ
¼
:
ð16Þ
2
Ea
RTs
RTs
φ exp
−
⁎
The entropy of activation, ΔS , is calculated from Eq. (13). The
⁎
⁎
enthalpy activation, ΔH , and Gibbs free energy, ΔG , calculated from:
ΔHꢁ ¼ Ea−RT
ð17Þ
ð18Þ
ΔGꢁ ¼ ΔHꢁ−TΔSꢁ:
⁎
⁎
⁎
The calculated values of Ea, A, ΔS , ΔH and ΔG for the decomposi-
tion steps for ligands (HL1, HL3 and HL5) and their Ru(III) complexes
(1, 3 and 5) are summarized in Table 4. The kinetic data obtained
from the two methods are comparable and can be considered in good
agreement with each other. The entropy of activation energies reflects
the thermal stability of the compounds. The entropy of activation is
found to be of negative values in the ligands (HLn) and their Ru(III)
complexes (1, 3 and 5) which indicate that the decomposition reactions
3.6.1. Calculation of activation thermodynamic parameters
The thermodynamic activation parameters of decomposition pro-
cesses of the ligands (HL1, HL3 and HL5) and their Ru(III) complexes
⁎
(1, 3 and 5) namely activation energy (Ea), enthalpy (ΔH ), entropy
⁎
⁎
⁎
(ΔS ), and Gibbs free energy change of the decomposition (ΔG ) are
evaluated graphically by employing the Coast–Redfern [32] and
Horowitz–Metzger [33] methods.
proceed spontaneously [34,35]. The values of ΔG are positive consid-
⁎
ered as favorable or spontaneous reaction. The values of ΔH are
positive indicate that the reaction is endothermic.
From the values of the energy of activation (Ea) of the ligands (HLn)
and their Ru(III) complexes (1, 3 and 5), it is observed that all the Ru(III)
complexes are less stable than the ligands (HLn). As shown in Fig. S2 in
the supplementary, the Ea values for the ligand (HL5) and its Ru(III)
complex (5) is higher values compared to the other compounds. This
can be attributed to the fact that the effective charge increased due to
the electron withdrawing p-substituent (HL5) while it decreased by
the electrons donating character of (HL1). This is in accordance with
that expected from Hammett's constant (σR) (Fig. S2).
3.6.1.1. Coast–Redfern equation. The Coast–Redfern Eq. (11), which is a
typical integral method, can represent as:
ꢀ
ꢁ
Z
Z
a
T2
T1
dx
A
φ
Ea
RT
¼
exp
−
dt:
ð11Þ
n
0 ð1−αÞ
For convenience of integration, the lower limit T1 usually taken as
zero. This equation on integration gives:
ꢂ
ꢃ
ꢂ
ꢃ
lnð1−αÞ
Ea
RT
AR
φEa
3.7. Catalytic oxidation of benzoin
ln −
¼ −
þ ln
:
ð12Þ
T2
The catalytic activity of ruthenium complexes for the oxidation of
benzoin to benzil has been studied and the effect of time, solvent and
temperature were optimized to produce maximum yield. The electronic
spectrum of benzil is characterized by absorption at 305 nm, which is
readily differentiated from the absorption bands of benzoin at 247 nm
[36]. Accordingly, the band at 305 nm was used to determine the
concentration of the produced benzil. Three ruthenium(III) complexes
(1, 3 and 5) were tested as catalyst for oxidation of benzoin using
NaIO4 as an oxidant under different reaction conditions viz., tempera-
ture, reaction time and type of solvent in order to optimize the condi-
tions to find out the best catalyst.
A plot of left-hand side (LHS) against 1/T was drawn (Fig. 5). Ea is the
energy of activation in J mol−1 and calculated from the slope and A in
(s−1) from the intercept value. The entropy of activation (ΔS ) in
⁎
(J mol−1 K−1) calculated by using Eq. (13):
ꢂ
ꢀ
ꢁꢃ
Ah
kBTs
ΔSꢁ ¼ 2:303 log
R
ð13Þ
where kB is the Boltzmann constant, h is the Plank's constant and Ts is
the TG peak temperature.
The absorbance at 305 nm increases with reaction time as an
indication of the increasing concentration of benzil by using one of the
catalysts, complex (5). The maximum benzil yield was found to depend
on the type of catalyst used. It is clear from Fig. 7 that complex (5) and
complex (3) give higher benzil yields than complex (1) with the same
reaction time. Complex (3) gave 50% yield after 2 h, while complex
(5) gave 68% after a longer reaction time (4 h). The order of catalytic ac-
tivity was found to be as follows: complex (5) N complex (3) N complex
(1). It was found that the values of yield (%) and TOF values are related
to the nature of the p-substituent as they increase according to the
following order p-NO2 N H N OCH3. This can be attributed to the
fact that the effective charge increased due to the electron withdrawing
p-substituent (HL5) while it decreased by the electrons donating char-
acter of (HL1). This is in accordance with that expected from Hammett's
constant (σR) as shown in Fig. S3 in the supplementary. Correlation of
the yield (%) and TOF values Hammett's constant (σR) showed the
incensement of these values with increasing σR.
3.6.1.2. Horowitz–Metzger equation. The Horowitz–Metzger equation is
an illustrative of the approximation methods. These authors derived
the relation:
"
#
1−n
1−ð1−αÞ
1−n
Eaθ
log
¼
for n≠1
ð14Þ
2
2:303RTs
when n = 1, the LHS of Eq. (14) would be log[−log(1 − α)] (Fig. 6). For
a first order kinetic process, the Horowitz–Metzger equation may write
in the form:
ꢂ
ꢀ
ꢁꢃ
Wα
Wγ
Eaθ
log log
¼
2 − log2:303
ð15Þ
2:303RTs
where θ = T − Ts, wγ = wα − w, wα = mass loss at the completion
reaction; w = mass loss up to time t. The plot of log [log (wα / wγ)] vs.

A.F. Shoair et al. / Journal of Molecular Liquids 211 (2015) 217–227
223
-11.0
-12.0
-12.5
-13.0
-13.5
-14.0
-14.5
-15.0
-15.5
-16.0
-16.5
(1)
-11.5
-12.0
-12.5
-13.0
-13.5
-14.0
-14.5
HL
1
o
o
105-260 C
190-230 C
0.00175
0.00182
0.00189
0.00196
0.00203
0.00210
0.0018
0.0020
0.0022
1/T(K-1
0.0024
0.0026
)
(1/T(K))
-11.0
-11.5
-12.0
-12.5
-13.0
-13.5
-14.0
-14.5
-15.0
-15.5
-16.0
-11.5
-12.0
-12.5
-13.0
-13.5
-14.0
-14.5
(3)
HL
3
o
o
110-315 C
170-200 C
0.0018
0.0020
0.0022
0.0024
0.00182
0.00189
0.00196
(1/T(K))
0.00203
0.00210
1/T (K -1
)
-11.4
-12.0
-12.6
-13.2
-13.8
-14.4
-15.0
(5)
HL
-12.0
5
-12.6
-13.2
-13.8
-14.4
-15.0
-15.6
-16.2
o
o
170-320 C
240-280 C
0.0017
0.0018
0.0019
1/T(K -1
0.0020
0.0021
0.0022
0.00161
0.00168
0.00175
(1/T(K))
0.00182
0.00189
)
Fig. 5. Coats–Redfern (CR) of the ligands (HL1, HL3 and HL5) and their Ru(III) complexes (1, 3 and 5).
3.7.1. Effect of solvent
(1), (3) and (5) when the reaction temperature was increased from
30 to 70 °C. At 30 °C, moderate yields of benzil were obtained. However,
increasing the temperature to 70 °C led to increasing the yield to 70% of
benzil. This indicates that at 70 °C the best yield was obtained under
these reaction conditions.
The influence of three different solvents viz. benzene, toluene and
acetonitrile on the yield of benzoin oxidation was studied using the
complexes 1, 3 and 5.
3.7.2. Effect of temperature
3.7.3. Effect of time
The performance of the catalysts was investigated at three different
temperatures viz. 30, 50 and 70 °C in acetonitrile after 3 h reaction time
(Table S3 in the supplementary). In general, the catalytic activity of the
three catalysts increased with increasing temperature. Fig. 7 showed
that there was a significant increase in benzil yield using complexes
The catalytic oxidation of benzoin using NaIO4 as oxidant in the pres-
ence of the three complexes was followed as a function of time in aceto-
nitrile at 70 °C. The reaction profiles showed that the yield of benzil
increased with increasing reaction time until a steady state was reached
after 4 h (Fig. 8).

224
A.F. Shoair et al. / Journal of Molecular Liquids 211 (2015) 217–227
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
-2.0
0.4
(1)
HL
1
0.0
-0.4
-0.8
-1.2
o
o
190-230 C
105-260 C
-1.6
-50 -40 -30 -20 -10
0
10
20
30
40
-60
-40
-20
0
20
40
60
80
θ
(K)
θ
(K)
0.4
0.2
0.0
(3)
0.0
-0.4
-0.8
-1.2
HL
3
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
-1.8
o
110-315 C
o
170-200 C
-80 -60 -40 -20
0
20 40 60 80 100
-30
-20
-10
0
θ
10
20 30
θ
(K)
(K)
0.4
0.0
(5)
0.0
-0.3
-0.6
-0.9
-1.2
-1.5
-1.8
HL
5
-0.4
-0.8
-1.2
-1.6
o
o
170-320 C
349-459 C
-40 -30 -20 -10
10
20
30
40
θ0(K)
-60
-40
-20
0
θ
20
40
60
(K)
Fig. 6. Horowitz–Metzger (HM) of the ligands (HL1, HL3 and HL5) and their Ru(III) complexes (1, 3 and 5).
3.7.4. The mechanism for the catalytic oxidation of benzoin to benzil by
[Ru(Ln)2(H2O)2]Cl/NaIO4
followed by hydride abstraction to form the unstable intermediate,
[RuOV(L1)2HOCHPhCOPh(H2O)]+(Eq. (20)). This intermediate loses
water molecule which in turn associates with ruthenium ion to yield
benzil, and the starting complex to complete the catalytic cycle
(Eqs. (21) and (22)). This catalytic cycle continues until all the benzoin
completely consumed as shown in Fig. 9.
The oxidation of alcohols to the corresponding carbonyl compound is
well-known to take place by high-valent metal oxo complexes. Therefore,
The catalytic oxidation of benzoin to benzil by [Ru(Ln)2(H2O)2]Cl/NaIO4
could be achieved via high valent ruthenium oxo species [37]. The hydride
abstraction mechanism is suggested here for the process of catalytic oxi-
dation of benzoin to benzil by the new Schiff base ruthenium(III) com-
plexes. An accurate mechanistic study for reactions containing low-
valent ruthenium complexes as catalysts suggests the formation of
metal-oxo intermediates [38]. The complex [Ru(Ln)2(H2O)2]Cl reacts
with NaIO4 to produce [RuOV(L1)2(H2O)]2+, NaIO3 and water (Eq. (19)).
Association of benzoin to the resulting ruthenium oxo complex occurs,
½RuðL1Þ ðH2OÞ ꢂþ þ NaIO4→½RuOðL1Þ ðH2OÞꢂþ þ H2O þ NaIO3
ð19Þ
2
2
2
½RuOðL1Þ ðH2OÞꢂþ þ Ph–CO–CHðOHÞPh→½RuOVÞL1Þ HOCHPhCOPhðH2OÞꢂþ
2
2
ð20Þ

A.F. Shoair et al. / Journal of Molecular Liquids 211 (2015) 217–227
225
Table 4
The kinetic parameters of the Schiff base ligands (HL1, HL3 and HL5) and their Ru(III) complexes (1, 3 and 5).
Compounda
Decomp. temp. (°C)
Method
Parameter
a (kJ mol−1
Correlation coefficient (r)
E
)
A (s−1
)
ΔS* (J mol−1 K−1
)
ΔH* (kJ mol−1
)
ΔG* (kJ mol−1
)
HL1
(1)
HL3
(3)
HL5
(5)
190–230
105–260
170–200
110–315
240–280
170–320
CR
HM
CR
HM
CR
HM
CR
HM
CR
HM
CR
121
132
1.94E+09
7.36E+10
1.04E+02
1.25E+03
3.56E+13
6.11E+14
1.12E+00
9.97E+00
7.02E+11
1.56E+13
9.68E+03
1.23E+05
−7.20E+01
−4.17E+0
−2.10E+02
−1.89E+02
1.01E+01
116
127
36.1
43.6
151
158
22.8
29.0
135
163
60.4
68.6
155
149
132
130
146
141
143
141
166
162
150
147
0.98146
0.98894
0.99772
0.98986
0.99581
0.9952
0.99396
0.99478
0.99392
0.99104
0.97074
0.98316
39.8
47.4
155
162
26.9
33.0
157
168
64.7
72.9
3.37E+01
−2.48E+02
−2.30E+02
−2.36E+01
2.17E+00
−1.73E+02
−1.52E+02
HM
a
Numbers as given in Table 1.
½RuOvðL1Þ2HOPhCOPhðH2OÞꢂ→½RuðL1Þ2OCPhCOPhðH2OÞꢂþ þ H2O
ð21Þ
3.8. Antibacterial studies
The antimicrobial activity of ligands (HLn) and their Ru(III)
complexes was tested against bacteria and fungi; we used more than
one test organism to increase the chance of detecting their antimicrobial
activities. The organisms used in the present investigations included
two Gram positive bacteria (B. cereus and S. aureus) and two Gram neg-
ative bacteria (E. coli and K. pneumoniae) in addition to different kinds of
fungi (A. niger, F. oxysporium, P. italicum and A. alternata).
½RuðL1Þ OCPhCOPhðH2OÞꢂþ þ H2O→½RuðL1Þ ðH2OÞ ꢂþ þ PhCO–COPh ð22Þ
2
2
2
(1)
(3)
(5)
The results of the antibacterial activity of ligands (HLn) and their
ruthenium(III) complexes (1–5) are tested against bacterial species as
shown in Table 5. The ligand HL3 has antibacterial activity against
B. cereus (inhibition zone of HL3 = 8 and 7 mm at concentrations =
100 and 150 μg/ml, respectively) and S. aureus (inhibition zone of
HL3 = 4 and 5 mm at concentrations = 100 and 150 μg/ml, respective-
ly), but low or no effects were recorded against Gram negative bacteria.
For the complexes, all of them were found to have high antibacterial
activity against Gram positive bacteria namely; B. cereus (inhibition
zone of complex (1) = 8 and 10 mm at concentrations = 50
and 100 μg/ml, respectively; inhibition zone of complex (3) = 7
and 10 mm at concentrations = 50 and 100 μg/ml, respectively)
and S. aureus (inhibition zone of complex (1) = 4 and 6 mm at
concentrations = 100 and 150 μg/ml, respectively) when comparing
with penicillin except complex (5).
80
60
40
20
20
40
60
80
Temp
Ligands HL1, HL2, HL3, HL4 and HL5 and complexes (1–5) have no
effect against B. cereus, S. aureus, E. coli and K. pneumoniae when
comparing with penicillin.
Fig. 7. Effect of temperature on benzil yield using complexes (1, 3 and 5) as a catalyst.
Comparative analysis for antibacterial study of ligands (HLn) and
their ruthenium(III) complexes (1–5) is shown in Fig. S4 in the supple-
mentary. It is observed that the ligand (HL3) and all complex are more
potent antibacterial than other ligands against B. cereus and S. aureus.
This may support the argument that some type of biomolecular binding
70
(1)
(3)
(5)
60
50
40
30
20
2
3
4
Time
Fig. 8. Effect of time on benzil yield using complexes (1, 3 and 5) as a catalyst.
Fig. 9. Mechanism for oxidation of benzoin to benzil by [Ru(Ln)2(H2O)2]Cl/NaIO4.

226
A.F. Shoair et al. / Journal of Molecular Liquids 211 (2015) 217–227
Table 5
Table 6
Antibacterial activity data of Schiff base ligands (HLn) and their Ru(III) (1–5). The results
Antifungal activity data of Schiff base ligands (HLn) and their Ru(III) complexes (1–5). The
were recorded as the diameter of inhibition zone (mm).
results were recorded as the diameter of inhibition zone (mm).
Compounda
Conc.
(μg/ml)
Gram positive bacteria
Gram negative bacteria
Compounda
Conc.
(μg/ml)
Aspergillus
niger
Fusarium
oxysporum
Alternaria
alternata
Penicillium
italicum
Bacillus
cereus
Staphylococcus
aureus
Escherichia
coli
Klebsiella
pneumoniae
HL1
50
100
150
50
100
150
50
100
150
50
100
150
50
100
150
50
100
150
50
100
150
50
100
150
50
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
1
−ve
2
2
2
2
2
1
4
4
1
1
1
1
1
1
1
2
2
2
2
5
1
2
3
3
3
3
3
3
3
2
3
3
−ve
−ve
−ve
−ve
−ve
2
−ve
3
−ve
3
−ve
−ve
3
3
1
1
3
3
3
−ve
−ve
−ve
5
5
3
−ve
−ve
−ve
−ve
−ve
−ve
2
HL1
HL2
HL3
HL4
HL5
(1)
50
100
150
50
100
150
50
100
150
50
100
150
50
100
150
50
100
150
50
100
150
50
100
150
50
−ve
−ve
−ve
−ve
−ve
−ve
8
−ve
−ve
−ve
−ve
−ve
−ve
3
−ve
−ve
−ve
−ve
−ve
−ve
1
−ve
−ve
1
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
1
HL2
HL3
2
2
8
4
HL4
−ve
−ve
−ve
−ve
−ve
−ve
4
7
5
−ve
−ve
3
−ve
−ve
−ve
1
1
HL5
−ve
−ve
−ve
8
−ve
−ve
−ve
−ve
−ve
−ve
1
−ve
−ve
−ve
1
1
1
−ve
−ve
4
(1)
4
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
−ve
1
10
11
6
4
6
4
(2)
(2)
−ve
2
−ve
1
9
6
(3)
9
7
2
1
(3)
7
4
−ve
−ve
−ve
−ve
1
1
1
1
−ve
2
10
10
8
4
6
4
(4)
100
150
50
100
150
50
4
4
−ve
−ve
−ve
5
(4)
100
150
50
100
150
50
7
6
(5)
11
2
6
2
2
2
(5)
−ve
−ve
−ve
1
3
3
−ve
−ve
−ve
−ve
−ve
−ve
3
2
Miconazole
4
2
100
150
3
4
6
6
1
2
Penicillin
1
2
100
150
3
2
a
3
2
Numbers as given in Table 1.
a
Numbers as given in Table 1.
64.7 kJ/mol for the complexes 1, 3 and 5, respectively. These complexes
were found to be effective catalyst for the oxidation of benzoin to benzil
in the presence of NaIO4 as co-oxidant. The antifungal activity of all the
ligands (HLn) and their Ru(III) complexes (1–5) were investigated. It
was found that the ligand (HL3) has the best antibacterial activity
(S. aureus and B. cereus) and all Ru(III) complexes have higher antifun-
gal activity than the ligands against F. oxysporium.
to the metal ions or interchelation or electrostatic interactions causes
the inhibition of biological synthesis [39].
The results of the antifungal activity of ligands (HLn) and their
ruthenium(III) complexes are listed in Table 6. The results of the exam-
ination of antifungal activity revealed that A. niger is resistant to all li-
gands and complexes when compared with Miconazole. HLn ligands
and complexes are moderately effective against F. oxysporium (inhibi-
tion zone of HL3 (4 and 4 mm) at concentrations (100 and 150 μg/ml),
respectively) (Table 6). Complex (1) showed good effect against
P. italicum (inhibition zone of (1)) (4 and 4 mm) at concentrations
(50 and 100 μg/ml respectively) when compared with Miconazole.
Complexes (2–5) have no effect against P. italicum. The enhancement
in antifungal activity is rationalized on the basis of the partial sharing
of the positive charge of metal ions with donor groups [40]. So it can
be concluded that some complexes exhibits higher antimicrobial
activity than the free ligand [41].
Acknowledgements
The authors would like to thank Prof. M.I. Abou-Dobara and Miss.
N.F. Omar (Botany Department, Faculty of Science, Damietta University,
Egypt) for their help during testing antibacterial activity.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.molliq.2015.07.001.
4. Conclusion
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