Ruthenium?p-cymene complexes with acylthiourea, and its heterogenized form on graphene oxide act as catalysts for the synthesis of quinoxaline derivatives

Article abstract of DOI:10.1016/j.jorganchem.2021.121933

Synthesis of a series of half-sandwich Ru(II) complexes (1-5) containing acylthiourea ligand is reported herein. All the Ru(II) complexes were well characterized by analytical and spectroscopic (UV-Vis, FT-IR, NMR and mass spectrometry) methods. Molecular structures of two (2 and 3) of the complexes were confirmed by single crystal X-ray diffraction, and the complexes adopted pseudo-octahedral geometry around Ru. Catalytic ability of the Ru complexes was evaluated in the synthesis of quinoxaline compounds from various 2-nitroaniline and hydroxy ketone derivatives via transfer hydrogenation approach. Active homogeneous catalyst was heterogenized by supporting it on graphene oxide, and the heterogeneous equivalent was characterized by Raman, XPS, TEM, SEM and ICP-OES techniques. Activity of the heterogeneous catalyst was tested, and it can be reused up to five cycles without any loss in activity.

Full text of DOI:10.1016/j.jorganchem.2021.121933

Journal of Organometallic Chemistry 949 (2021) 121933
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Ruthenium−p-cymene complexes with acylthiourea, and its
heterogenized form on graphene oxide act as catalysts for the
synthesis of quinoxaline derivatives
Dharmalingam Sindhujaa, Mayakrishnan Gopiramanb, Punitharaj Vasanthakumara,
Nattamai Bhuvaneshc, Ramasamy Karvembua,∗
a Department of Chemistry, National Institute of Technology, Tiruchirappalli, Tamil Nadu 620015, India
b Department of Applied Bioscience, College of Life and Environmental Science, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 05029, South
Korea
c Department of Chemistry, Texas A&M University, College Station, TX 77842, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis of a series of half-sandwich Ru(II) complexes (1-5) containing acylthiourea ligand is reported
herein. All the Ru(II) complexes were well characterized by analytical and spectroscopic (UV-Vis, FT-IR,
NMR and mass spectrometry) methods. Molecular structures of two (2 and 3) of the complexes were
confirmed by single crystal X-ray diffraction, and the complexes adopted pseudo-octahedral geometry
around Ru. Catalytic ability of the Ru complexes was evaluated in the synthesis of quinoxaline com-
pounds from various 2-nitroaniline and hydroxy ketone derivatives via transfer hydrogenation approach.
Active homogeneous catalyst was heterogenized by supporting it on graphene oxide, and the heteroge-
neous equivalent was characterized by Raman, XPS, TEM, SEM and ICP-OES techniques. Activity of the
heterogeneous catalyst was tested, and it can be reused up to five cycles without any loss in activity.
Received 1 March 2021
Revised 14 May 2021
Accepted 7 June 2021
Available online 11 June 2021
Keywords:
Ruthenium complexes
Acylthiourea ligand
Graphene oxide
Heterogeneous catalysis
Quinoxalines
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
heterocycles, etc [12–16]. In the recent past, various half-sandwich
Ru-arene catalysts have been used in many useful organic transfor-
Quinoxalines or benzopyrazines are significant core units, act-
ing as effective pharmacophores, imparting noticeable biological
activities such as antibacterial, antiviral, anti-inflammatory, an-
ticancer etc., which make them to be a part of many widely
used therapeutic agents [1,2]. Molecules bearing quinoxaline mo-
tifs, such as varenicline, brimonidine, quinacillin, chloroquinoxa-
line sulphonamide, botryllazine B, riboflavin etc., are therapeuti-
cally significant (Fig. 1). Besides biological applications, quinoxa-
lines have been used as a basic building block in preparing various
dyes, semiconductors, luminescent materials, cavitands, chemically
controllable switches, etc [3–8]. Synthetic methodologies widely
used so far for preparing quinoxaline derivatives are reported by
Xie et al [9]. However, simple and environmentally friendly proce-
dures have received great attention for the past few years [10]. For
this purpose, transfer hydrogenation strategy is very useful [11].
Ru compounds are established as active catalysts for transfer hy-
drogenation and acceptor-less dehydrogenation for the synthesis of
mations like intermolecular Markovnikov selective hydroacylation
of olefins, biomass conversions, N-alkylation of amines, etc [17–
19]. The arene ligand in Ru complexes is essential for restricting
further oxidation of metal in octahedral complexes. Furthermore,
preparation of half-sandwich Ru complexes is feasible under mild
conditions in high yields with good purity, and they are soluble in
many organic solvents [20]. Even though the homogeneous cata-
lysts are highly active and selective, catalyst contamination is of-
ten seen in the final product due to the complications involved in
the separation processes. On the other hand, heterogeneous cat-
alysts overcome the problem of separation. To maintain the ad-
vantages of both homogeneous and heterogeneous systems, active
complexes are anchored on a solid support such as silica, alumina,
carbon materials etc [21–23]. Carbon materials are found to have
good tensile strength, large surface area, promising thermal stabil-
ity and an ability to be functionalized. Many noble metal catalysts
were immobilized on carbon materials and used in various organic
transformations and electrocatalysis. Graphene is the basic building
block for carbon materials of different dimensionalities: fullerenes
(0D), nanotubes (1D) or graphite (3D). Synthetic approach widely
adopted for graphene is the chemical oxidation and exfoliation
∗
Corresponding author.
E-mail address: kar@nitt.edu (R. Karvembu).
https://doi.org/10.1016/j.jorganchem.2021.121933
0022-328X/© 2021 Elsevier B.V. All rights reserved.

D. Sindhuja, M. Gopiraman, P. Vasanthakumar et al.
Journal of Organometallic Chemistry 949 (2021) 121933
Fig. 1. Some bioactive molecules composing quinoxaline scaffold.
of graphite to graphene oxide (GO) followed by reduction. Impor-
tantly, graphene can be produced in a relatively larger scale. More-
over, its surface properties can be easily modified by introduc-
ing a variety of defects and oxygen-containing groups. Although
the residual defects and oxygen-containing groups are detrimen-
tal to the electronic properties of graphene, they are important for
other applications since the defects can play a role as anchoring
sites and enhance the interaction with a second component [24–
29]. Hence, graphene surface can be decorated with metal com-
plexes or metal nanoparticles, and employed as catalysts for vari-
ous reactions [30–34]. Recently, Peris and his co-workers reported
the immobilization of Pd and Ru complexes on graphene surface
through non-covalent interactions, and their catalytic activity was
demonstrated in olefin reduction and dehydrogenation of alcohol
[35]. The stable Ru-arene complexes containing graphene oxide
supported chiral acylthiourea ligand were reported as catalysts for
transfer hydrogenation of carbonyl compounds [36]. The stability
and activity of these graphene-based Ru-arene complexes moti-
vated us to develop analogues homogeneous and heterogeneous
catalysts for one-pot quinoxaline synthesis from 2-nitroaniline
and hydroxy ketone by implementing transfer hydrogenation
approach.
scopic (XPS) analyses were done on a Kratos Axis-Ultra DLD in-
strument. During the analyses, samples were irradiated with Mg
Kα X-ray source. The scanning electron microscopic (SEM) im-
ages were obtained by using a Bruker microscope. The transmis-
sion electron microscopic (TEM) images were seen by using a JEOL
JEM 2100 microscope. Raman spectra were recorded by using a
Bruker RFS 27 spectrometer. The quinoxaline derivatives were an-
alyzed using Shimadzu GC 2010 and Shimadzu GCMS-QP2010 Ul-
tra instruments equipped with a 60 m × 0.32 mm Restek Rtx-5
column.
2.1. Synthesis of the ligands
Ligands were synthesized according to the reported procedure.
A solution of 2,4-dichlorobenzoyl chloride or cyclohexane carbonyl
chloride in acetone (15 mL) was added dropwise to the suspen-
sion of potassium thiocyanate in acetone (15 mL) under stirring,
and the solution was refluxed for an hour. The mixture was cooled
to room temperature, and liquid NH3 was added slowly. The reac-
tion mixture was poured into 300 mL of deionised water, and the
formed precipitate was collected by filtration. The ligands (L2 and
L5) were washed with water and dried. Ligands L1, L3 and L4 were
reported by us earlier [37,38].
2. Experimental section
N-carbamothioyl-2,4-dichlorobenzamide (L2): Yield: 78%. White
solid. Mp: 169°C. UV-vis (CH3CN): λmax (nm) 203, 282. FT-IR (KBr,
cm−1): 3345 and 3151 (s; ν(NH2)), 3254 (s; ν(N−H)), 1688 (s;
ν(C=O)), 1122 (s; ν(C=S)). 1H NMR (500 MHz, DMSO-d6): δ, ppm
11.64 (s, 1H), 9.60 (s, 1H), 7.91 (s, 1H), 7.73-7.51 (m, 3H). 13C NMR
(125 MHz, DMSO-d6): δ, ppm 181.9, 167.7, 136.4, 134.7, 131, 130.5,
129.5, 127.7.
Reagent grade chemicals were used without any further purifi-
cation. The solvents were purified by standard procedures. Melt-
ing points were measured in open capillary tubes in a Sigma
melting point apparatus. UV-visible spectra were recorded using a
Shimadzu UV-2600 instrument. Fourier transform infrared (FT-IR)
spectra were recorded as KBr pellets on Thermo Scientific Nico-
N-carbamothioylcyclohexanecarboxamide (L5): Yield: 82%. White
solid. Mp: 164°C. UV-vis (CH3CN): λmax (nm) 208, 274. FT-IR (KBr,
cm−1): 3344 and 3194 (s; ν(NH2)), 3243 (s; ν(N−H)), 1693 (s;
ν(C=O)), 1185 (s; ν(C=S)). 1H NMR (500 MHz, DMSO-d6): δ, ppm
10.91 (s, 1H), 9.59 (s, 1H), 9.25 (s, 1H), 2.48-2.34 (m, 1H), 1.66 (t,
J = 14.6 Hz, 4H), 1.27-1.01 (m, 6H). 13C NMR (125 MHz, DMSO-d6):
δ, ppm 182.4, 178.0, 44.2, 29, 25.6, 25.3.
let iS5 FT-IR spectrometer in the range of 550-4000 cm−1 1H
.
and 13C nuclear magnetic resonance (NMR) spectra were recorded
on a Bruker 500 MHz spectrometer in DMSO-d6 or CDCl3 with
tetramethyl silane as an internal reference. Ruthenium was quan-
tified by using PerkinElmer Optima 5300 DV inductively cou-
pled optical emission spectrometer. X-ray photoelectron spectro-
2

D. Sindhuja, M. Gopiraman, P. Vasanthakumar et al.
Journal of Organometallic Chemistry 949 (2021) 121933
2.2. Synthesis of the Ru complexes
A mixture of [RuCl2(η6-p-cymene)]2 (0.2 mmol, 122.4 mg) and
ligand (L1-L5) (68-99 mg, 0.4 mmol) in toluene (30 mL) was stirred
for 6-9 h at 27 °C. The reaction mixture was concentrated, and
hexane was added to get orange coloured precipitate. The obtained
complex was filtered, washed with hexane and diethyl ether, and
dried in vacuo.
[RuCl2(η6-p-cymene)L1] (1): Yield: 92%. Orange solid. Mp: 238°C.
UV-vis (CHCl3): λmax (nm) 279, 335, 434. FT-IR (KBr, cm−1): 3353
and 3167 (s; ν(NH2)), 3237 (s; ν(N−H)), 1683 (s; ν(C=O)), 1112 (s;
ν(C=S)). 1H NMR (500 MHz, CDCl3): δ, ppm 10.40 (s, 1H), 10.06
(s, 1H), 7.94 (s, 1H), 7.78 (d, J = 3.6 Hz, 1H), 7.63 (d, J = 1.0 Hz,
1H), 6.55 (dd, J = 3.6, 1.6 Hz, 1H), 5.45 (d, J = 5.9 Hz, 2H), 5.27 (d,
J = 5.9 Hz, 2H), 3.14-2.94 (m, 1H), 2.28 (s, 3H), 1.35 (d, J = 6.9 Hz,
5H). 13C NMR (126 MHz, CDCl3): δ, ppm 180.8, 157.3, 147.6, 144.5,
121.3, 113.0, 103.6, 99.7, 84.1, 82.5, 30.5, 22.2, 18.3. ESI-MS (m/z):
found 405.0174 [M-2HCl+H]+ (calcd. 405.0210).
˚
Fig. 2. Molecular structure of L2. Selected bond lengths (A) and angles (°):
C(7)−O(1) 1.219(3), C(8)−S(1) 1.689(2), N(1)−H(1) 0.88, N(2)−H(2A) 0.88,
N(2)−H(2B) 0.88, S(1)−C(8)−N(1) 117.94(16), S(1)−C(8)−N(2) 123.37(17),
N(2)−C(8)−N(1) 118.7(2), O(1) −C(7)−N(1) 123.9(2).
[RuCl2(η6-p-cymene)L2] (2): Yield: 70%. Orange solid. Mp: 164°C.
UV-vis (CHCl3): λmax (nm) 254, 335, 432. FT-IR (KBr, cm−1): 3342
and 3168 (s; ν(NH2)), 3238 (s; ν(N−H)), 1694 (s; ν(C=O)), 1107
(s; ν(C=S)). 1H NMR (500 MHz, CDCl3): δ, ppm 11.10 (s, 1H), 10.11
(s, 1H), 7.77 (s, 1H), 8.04 (d, J = 8.4 Hz, 1H), 7.43 (d, J = 2.0 Hz,
1H), 7.36 (dd, J = 8.4, 2.0 Hz, 1H), 5.42 (d, J = 5.9 Hz, 2H), 5.25
(d, J = 5.9 Hz, 2H), 3.03-2.95 (m, 1H), 2.26 (s, 3H), 1.33 (d, J = 6.9
Hz, 6H). 13C NMR (126 MHz, CDCl3): δ, ppm 181.8, 166, 139, 137.8,
133.5, 132.5, 130.7, 127.5, 83.9, 82.5, 30.5, 22.2, 18.3. ESI-MS (m/z):
found 482.9532 [M-2HCl+H]+ (calcd. 485.9636).
yield graphene oxide appended with acid chloride (GO-Cl). GO-Cl
precipitate was washed thoroughly with diethyl ether and acetone
to remove unreacted thionyl chloride, and dried under vacuum for
6 h. GO-Cl was modified by treating it (200 mg) with ligand L4 (2
mmol) in N,N-dimethylformamide (25 mL) at 100 °C for 24 h to get
GO-L4 which was filtered, washed with ethanol and diethyl ether,
and dried under vacuum. Ru−p-cymene dimer (1 mmol) dissolved
in toluene (15 mL) was added to the dispersion of GO-L4 (200 mg)
in toluene (15 mL), and the resulting mixture was stirred at room
temperature for 10 h. The formed heterogeneous complex (6) was
removed by means of filtration, and washed with toluene (3 × 10
mL) and hexane (3 × 10 mL) to remove the excess Ru−p-cymene
dimer.
[RuCl2(η6-p-cymene)L3] (3): Yield: 85%. Orange solid. Mp.:
217°C. UV-vis (CHCl3): λmax (nm) 251, 336, 433. FT-IR (KBr, cm−1):
3327 and 3197 (s; ν(NH2)), 3229 (s; ν(N−H)), 1682 (s; ν(C=O)),
1248 (s; ν(C=S)). 1H NMR (500 MHz, DMSO-d6): δ, ppm 11.18 (s,
1H), 9.79 (s, 1H), 9.50 (s, 1H), 7.86 (d, J = 7.4 Hz, 2H), 7.57 (t,
J = 7.4 Hz, 1H), 7.44 (t, J = 7.8 Hz, 2H), 5.75 (d, J = 6.3 Hz, 2H), 5.71
(d, J = 6.2 Hz, 2H), 2.81-2.71 (m, 1H), 2.02 (s, 2H), 1.13 (d, J = 6.9
Hz, 6H). 13C NMR (126 MHz, DMSO-d6): δ, ppm 182.5, 168.2, 133.4,
129, 128.8, 106.8, 86.8, 85.9, 30.4, 21.9, 18.3. ESI-MS (m/z): found
415.1483 [M-2HCl+H]+ (calcd. 415.0418).
2.4. General procedure for the synthesis of quinoxaline derivatives
Initially, catalyst 4 (0.5 mol%) or 6 (0.5 mol%) and hydroxy ke-
tone (benzoin / furoin / acetoin) were taken in formic acid / tri-
ethyl amine mixture (0.75 molar ratio), and stirred at 70 °C. 2-
nitroaniline (or its derivative) was added to the above mixture,
and the reaction was continued until nitroaniline was fully con-
sumed (monitored by TLC). Then, the reaction mixture was diluted
with water, and extracted with ethyl acetate. The combined organic
layer was dried over anhydrous Na2SO4, filtered, and concentrated
under vacuum. The crude was purified by column chromatography
using hexane-ethyl acetate mixture (9:1) as eluent to afford pure
quinoxaline derivative.
[RuCl2(η6-p-cymene)L4] (4): Yield: 82%. Orange solid. Mp.:
252°C. UV-vis (CHCl3): λmax (nm) 253, 294, 342, 434. FT-IR (KBr,
cm−1): 3342 and 3164 (s; ν(NH2)), 3223 (s; ν(N−H)), 1665 (s;
ν(C=O)), 1257 (s; ν(C=S)). 1H NMR (500 MHz, CDCl3): δ, ppm
11.02 (s, 1H), 10.21 (s, 1H), 8.43 (dd, J = 3.9, 1.0 Hz, 1H), 7.63 (dd,
J = 5.0, 1.0 Hz, 1H), 7.16 (s, 1H), 7.13 (dd, J = 4.9, 4.0 Hz, 1H), 5.45
(d, J = 6.0 Hz, 2H), 5.28 (d, J = 6.0 Hz, 2H), 3.06-2.97 (m, 1H), 2.29
(s, 3H), 1.35 (d, J = 6.9 Hz, 6H). 13C NMR (126 MHz, CDCl3): δ, ppm
181.4 (C=S), 162.4 (C=O), 135.8, 135.4, 134.8, 129, 103.4, 99.8, 84.1,
82.5, 30.5, 22.2, 18.3. ESI-MS (m/z): found 421.9982 [M-2HCl+H]+
(calcd. 421.0126).
3. Results and discussion
[RuCl2(η6-p-cymene)L5] (5): Yield: 71%. Orange solid. Mp.:
172°C. UV-vis (CHCl3): λmax (nm) 279, 336, 439. FT-IR (KBr, cm−1):
3341 and 3169 (s; ν(NH2)), 3243 (s; ν(N−H)), 1635 (s; ν(C=O)),
1161 (s; ν(C=S). 1H NMR (500 MHz, CDCl3): δ, ppm 10.68 (s, 1H),
9.99 (s, 1H), 7.67 (s, 1H), 5.43 (d, J = 5.6 Hz, 2H), 5.26 (d, J = 5.7
Hz, 2H), 3.02-2.97 (m, 1H), 2.26 (s, 3H), 1.91 (dd, J = 17.8, 6.5
Hz, 4H), 1.82-1.73 (m, 4H), 1.47-1.40 (m, 2H), 1.33 (d, J = 6.9 Hz,
6H). 13C NMR (126 MHz, CDCl3): δ, ppm 181.2, 178.4, 101.2, 96.7,
83.8, 82.4, 44.7, 30.5, 29.6, 25.4, 25, 22.2, 18.4. ESI-MS (m/z): found
420.0887 [M-2HCl+H]+ (calcd. 421.0863).
3.1. Synthesis of the ligands
The ligands (L1-L5) were prepared by reacting corresponding
acyl chlorides with potassium thiocyanate (1:1 stoichiometry) and
liquid ammonia as per the reported procedure [37] and character-
ized by spectroscopic and analytical techniques. Ligands L2 and L5
were not yet reported, and hence they were crystallized to get sin-
gle crystals.
3.2. Solid state structures of the ligands
2.3. Immobilization of Ru complex 4 on graphene oxide to get 6
Molecular structures of the ligands (L2 and L5) are shown in
Figs. 2 and 3. Good quality crystals were grown from acetonitrile
solutions of the ligands. The crystallographic and refinement pa-
rameters are summarized in Table S1. Ligands L2 and L5 crystal-
lized in monoclinic (C12/c1) and triclinic (P-1) systems, respec-
tively. C(7)−O(1) and C(8)−S(1) bond lengths were in the ranges of
Steps involved in the immobilization are shown in Scheme 2.
Graphene oxide was synthesized as per the modified Hummers
method [39]. For further functionalization, graphene oxide (250
mg) was treated with excess thionyl chloride at 80 °C for 24 h to
3

D. Sindhuja, M. Gopiraman, P. Vasanthakumar et al.
Journal of Organometallic Chemistry 949 (2021) 121933
˚
Fig. 3. Molecular structure of L5. Selected bond lengths (A) and angles
(°): C(7)−O(1) 1.2228(19), C(8)−S(1) 1.6843(16), N(1)−H(1) 0.88, N(2)−H(2A)
0.88, N(2)−H(2B) 0.88, S(1)−C(8)−N(1) 119.05(11), S(1)−C(8)−N(2) 123.24(12),
N(2)−C(8)−N(1) 117.69(13), O(1)−C(7)−N(1) 122.57(14).
˚
Fig. 5. Molecular structure of 3. Selected bond lengths (A) and angles (°):
Ru(1)−Cl(1) 2.4193(4), Ru(1)−Cl(2) 2.4268(12), Ru(1)−S(1) 2.4136(12), S(1)−C(1)
1.711(5), O(1)−C(2) 1.226(6), N(1)−H(1A) 0.8800, N(2)−H(1A) 0.8800, N(2)−H(2)
0.8800, Cl(1)−Ru(1)−Cl(2) 85.845(5), Cl(1)−Cu(1)−S(1) 92.19(4), S(1)−Ru(1)−Cl(2)
91.65(4), C(1)−S(1)−Ru(1) 118.88(17), H(1A)−N(1)−H(1B) 120, C(1)−N(1)−H(1A)
120, C(1)−N(1)−H(1B) 120, N(1)−C(1)−S(1) 119.4(4), N(1)−C(1)−N(2) 119.5(4),
N(2)−C(1)−S(1) 121.1(3), O(1)−C(2)−N(2) 122.0(4).
ligands to Ru. 1H NMR spectra of the complexes displayed signals
corresponding to N−H and NH2 protons as singlets at 10.48-11.18
and 7.16-10.21 ppm, respectively. In addition to the signals due to
acylthiourea protons, new signals observed in the regions 5.25-
5.75 (two doublets), 2.02-2.29 (singlet), 2.76-3.02 (multiplet) and
1.13-1.35 (doublet) ppm were assigned to p-cymene ring, methyl,
isopropyl methine and isopropyl methyl protons, respectively. 13C
NMR spectra of the complexes revealed the presence of thiocar-
bonyl (180.8-182.5 ppm) and carbonyl carbons (157.3-178.4 ppm),
along with aromatic carbons of the ligands. The p-cymene ring car-
bons resonated in the region 82.4-106.8 ppm, and the aliphatic car-
bons gave signals in the region 18.3-30.5 ppm.
˚
Fig. 4. Molecular structure of 2. Selected bond lengths (A) and angles (°):
Ru(1)−Cl(1) 2.44465(9), Ru(1)−Cl(2) 2.4269(9), Ru(1)−S(1) 2.4079(10), S(1)−C(1)
1.691(4), O(1)−C(2) 1.218(5), N(1)−H(1A) 0.8800, N(2)−H(1A) 0.8800, N(2)−H(2)
0.8800, Cl(1)−Ru(1)−Cl(2) 87.49(3), Cl(1)−Ru(1)−S(1) 94.31(3), S(1)−Ru(1)−Cl(2)
89.39(3), C(1)−S(1)−Ru(1) 116.94(14), H(1A)−N(1)−H(1B) 120, C(1)−N(1)−H(1A)
120, C(1)−N(1)−H(1B) 120, C(1)−N(1)−H(2) 116.6, N(1)−C(1)−S(1) 120.7(3),
N(1)−C(1)−N(2) 118.9(3), N(2)−C(1)−S(1) 120.1(3), O(1)−C(2)−N(2) 123.5(4).
3.4. Solid state structures of the complexes
Single crystals of complexes 2 and 3 were obtained by slow
evaporation of chloroform/acetonitrile solutions of the complexes.
Molecular structures of the complexes are displayed in Figs. 4 and
5. A summary of crystallographic data, selected bond lengths and
hydrogen bonding interactions are given in Table S2. Crystal sys-
tem of complexes 2 and 3 was found to be monoclinic and or-
thorhombic, with P121/c1 and Pna21 space groups, respectively.
Ru occupied the centre of pseudo-octahedron, with two chloride,
one acylthiourea and one p-cymene ligands. p-Cymene occupied
three coordination sites of Ru, leading to half-sandwich piano-
stool structure. The acylthiourea ligands coordinated to Ru through
S atom, and Ru−S bond length was measured as 2.4079(10) and
˚
1.219(3)-1.2228(19) and 1.689(2)-1.6843(16) A, respectively [37,38].
N(1)−H(1), N(1)−H(2A) and N(1)−H(2B) bond lengths in L2 and L5
˚
were fixed to be 0.88, 0.88 and 0.88 A, respectively [37,40]. Two
intramolecular hydrogen bonding interactions [N(2)−H(2A)…O(1),
˚
˚
1.98-2.04 A and N(2)−H(2B)…S(1), 2.62-2.66 A] were noted in both
the ligands.
3.3. Synthesis and characterization of the Ru(II) complexes
Five Ru−p-cymene complexes (1-5) were synthesized by the
treatment of one equivalent of [RuCl2(η6-p-cymene)]2 with two
equivalents of corresponding acylthiourea ligands (L1-L5) in
toluene at room temperature for 6-9 h (Scheme 1). The com-
plexes were orange coloured, soluble in chloroform, acetoni-
trile, dichloromethane, dimethyl sulfoxide, dimethylacetamide and
dimethylformamide, and less soluble in diethyl ether, hexane and
water.
˚
2.4136(12) A in 2 and 3, respectively, which is in accordance with
the earlier reports [40–42]. Moreover, C=S bond length in the com-
plexes was greater than that in the ligands. Also, intramolecular
hydrogen bonding interactions of the type N(2)−H(2)…Cl(1) (2.32-
˚
˚
2.499 A) and N(1)−H(1A)…O(1) (1.946-2.01 A) were seen in both
the complexes. Ru−p-cymene centroid distance in 2 and 3 was
˚
FT-IR spectra of the complexes showed stretching frequencies of
NH2 and N−H moieties in the ranges of 3164-3353 and 3223-3238
cm−1, respectively. Similarly, carbonyl and thiocarbonyl stretching
frequencies were observed at 1107-1257 and 1635-1694 cm−1, re-
spectively. Stretching frequencies of NH2, N−H and C=O were al-
most unaltered when compared to those of the free ligands. But
there was a decrease in C=S stretching frequency on complexa-
tion, which indicated the coordination of neutral S atom of the
found to be 1.661 and 1.662 A, respectively.
3.5. Synthesis of quinoxaline derivatives from 2-nitroaniline and
hydroxy ketone
3.5.1. Optimization of the reaction conditions
The reaction conditions were optimized by taking 2-nitroaniline
and benzoin as substrates. Initially, the molar ratio of 2-
4

D. Sindhuja, M. Gopiraman, P. Vasanthakumar et al.
Journal of Organometallic Chemistry 949 (2021) 121933
Scheme 1. Synthesis of the Ru complexes.
Scheme 2. Synthesis of the Ru complex supported on graphene oxide.
nitroaniline and benzoin was varied from 0.2 to 0.5, and com-
plete conversion of 2-nitroaniline was obtained in 0.3 molar ra-
tio. Influence of temperature on the conversion was investigated,
and complete conversion was attained at 70 °C. Among the five
complexes, complex 4 showed better activity with an isolated
yield of 83% of 2,3-diphenylquinoxaline. Hence, complex 4 was
used further for extending scope of the substrates. Since the use
of 2-propanol as hydrogen donor/solvent did not lead to product
formation, formic acid/triethyl amine system was used. The mo-
lar ratio of formic acid/triethyl amine was varied from 0.25 to
1.5; quantitative conversion (>99%) was achieved while using 0.75
molar ratio. Fixing the molar ratio of formic acid/triethyl amine
as 0.75, catalyst amount and time were varied. Good conversion
of 2-nitroaniline was observed with 0.5 mol% of the catalyst in
8 h. Control experiments were performed to establish the cata-
lyst role; no conversion was observed in the absence of the cat-
alyst or hydrogen donor. Also, the reaction did not proceed in
the presence of Ru−p-cymene dimer. These results elucidated the
role of Ru−p-cymene complex of acylthiourea in catalytic transfer
hydrogenation.
3.5.2. Catalytic performance by complex 4
Table summarizes the results of reactions between vari-
1
ous 2-nitroaniline compounds and benzoin. Complex 4 catalyzed
the conversion of 2-nitroaniline and benzoin into 2,3-diphenyl
quinoxaline (9a) in 8 h with the yield of 83%. Catalytic efficiency
was tested with electron releasing or electron withdrawing sub-
stituent in the substrates. 6-methoxy-2,3-diphenylquinoxaline (9b)
and 6,7-dimethyl-2,3-diphenylquinoxaline (9c) were obtained in
66 and 68% yields, respectively. When nitroaniline with methyl
group was employed, the reaction did not complete even after
24 h, and the maximum conversion was 54% with 100% selectiv-
ity (9d). Similarly, nitroaniline derivatives with halide (Br and Cl)
were studied. 4-bromonitroaniline was coupled with benzoin to
give 6-bromo-2,3-diphenylquinoxaline (9e) selectively (100%) with
87% conversion. The conversion (99%) was excellent in the case
of chloro substituted nitroaniline, and the formation of 5-chloro-
2,3-diphenylquinoxaline (9f) was observed with 100% selectiv-
ity. Also, reaction between nitroaniline (with trifluoromethyl sub-
stituent) and benzoin completed in 3 h with the selective forma-
tion of 2,3-diphenyl-6-(trifluoromethyl)quinoxaline (9g). Nitroani-
5

D. Sindhuja, M. Gopiraman, P. Vasanthakumar et al.
Journal of Organometallic Chemistry 949 (2021) 121933
Table 1
Synthesis of quinoxaline derivatives from benzoin using catalyst 4a,b,c
.
a
b
c
Reaction conditions: 7 (0.5 mmol), 8 (1.4 mmol), 4 (0.5 mol%), HCOOH/NEt3 (0.75 molar ratio), 70 °C.
Isolated yield.
Conversion/selectivity determined by GC-MS.
line with both trifluoromethyl and chloro substituents was used to
produce 5-chloro-2,3-diphenyl-7-(trifluoromethyl)quinoxaline (9h)
quantitatively in 2 h. 4,5-dichloro-2-nitroaniline was reacted with
benzoin to give 6,7-dichloro-2,3-diphenylquinoxaline (9i). 2,3-
diphenylpyrido[2,3-b]pyrazine (9j) and 2,3-diphenylquinoxaline-6-
ol (9k) were formed with 100% selectivity.
with 2-nitroaniline or 4-methyl-2-nitroaniline gave 2,3-di(furan-
2-yl)quinoxaline (11a) or 2,3-di(furan-2-yl)-6-methylquinoxaline
(11b), respectively with excellent conversion and selectivity.
Coupling of trifluoromethyl substituted nitroaniline with 2,2’-
furoin yielded 2,3-di(furan-2-yl)-6-trifluoromethyl-quinoxaline
(81%) (11c). Likewise, the yields of 5-chloro-2,3-di(furan-2-yl)-
Scope of the catalytic system was extended to the coupling
of nitroaniline derivatives with 2,2’-furoin, and the details are
provided in Table 2. All the nitroaniline substrates were smoothly
combined with 2,2’-furoin in presence of catalyst 4, and the
products were obtained in 65-81% yields. Reaction of 2,2’-furoin
7-trifluoromethyl-quinoxaline
(11d),
6-chloro-2,3-di(furan-2-
yl)quinoxaline (11e) and 6-bromo-2,3-di(furan-2-yl)quinoxaline
(11f) were 79, 65 and 72%, respectively.
Less reactive aliphatic hydroxy ketone (acetoin) was employed
in the same reaction, and the details of conversions and selectivi-
6

D. Sindhuja, M. Gopiraman, P. Vasanthakumar et al.
Journal of Organometallic Chemistry 949 (2021) 121933
Table 2
Synthesis of quinoxaline derivatives from furoin using catalyst 4a,b,c
.
a
b
c
Reaction conditions: 7 (0.5 mmol), 10 (1.4 mmol), 4 (0.5 mol%), HCOOH/NEt3 (0.75 molar ratio), 70 °C.
Conversion/selectivity determined by GC-MS.
Isolated yield.
Table 3
Synthesis of quinoxaline derivatives from acetoin using catalyst 4a,b
.
a
Reaction conditions: 7 (0.5 mmol), 12 (1.4 mmol), 4 (0.5 mol%), HCOOH/NEt3 (0.75 molar ratio), 70 °C.
b
Conversion/selectivity determined by GC-MS.
ties are listed in Table 3. The reaction was slower compared to that
with the other hydroxy ketones. Selectivity of quinoxaline deriva-
tives was maintained in all the reactions. 2,3-dimethylquinoxaline
(13a) and 5-chloro-2,3-dimethyl-7-(trifluoromethyl) quinoxaline
(13b) were obtained with 99% conversion. The conversion of 2-
nitro-5-(trifluoromethyl)aniline was only 69% with the selective
formation of 2,3-dimethyl-6-(trifluoromethyl)quinoxaline (13c).
was observed by TEM and SEM as presented in Fig. 6 and S17 re-
spectively, which showed the presence of graphene layers. SEM-
EDAX spectrum (Fig. S17) confirmed the presence of various ele-
ments in the catalyst. TEM images of the catalyst revealed the lay-
ered structure of graphene, which also established the fact that the
morphology of graphene oxide was unaltered even after complex-
ation. In addition, Raman analysis provided the structural charac-
teristics and properties of the graphene oxide support. The G line
is usually assigned to the first order scattering of the E2g phonons
of sp2 C atoms. The D line is the breathing mode of the j-point
phonons of A1g symmetry [43–45]. Raman spectrum (Fig. S16) of 6
3.6. Synthesis and characterization of the graphene supported Ru
catalyst (6)
showed two bands i.e., D band at 1375 and G band at 1595 cm−1
.
Active Ru−p-cymene complex was supported on graphene ox-
ide (6) as shown in Scheme 2. The morphology of graphene in 6
The relative intensity ratio is ~1.3 which was due to the increase
7

D. Sindhuja, M. Gopiraman, P. Vasanthakumar et al.
Journal of Organometallic Chemistry 949 (2021) 121933
Table 4
Synthesis of quinoxaline derivatives from benzoin/furoin using heterogenized catalyst 6a,b
.
a
Reaction conditions: 7 (0.5 mmol), 8 or 10 (1.4 mmol), 6 (0.15 mol%), HCOOH/NEt3 (0.75 molar ratio), 70
°C.
b
Isolated yield.
Fig. 6. TEM images of 6.
in the degree of disorder. The disorder nature was due to the in-
teraction between Ru complex and graphene oxide, which in turn
confirmed the anchoring of Ru complex on graphene. XPS analyses
were done to elucidate the chemical nature of the catalyst and the
spectrum is shown in Fig. 7. The spectrum showed the presence
of elements like Ru, O, S, N and Cl in 6. The signals at 464.1 and
486.1 eV can be attributed to Ru 3p3/2 and Ru 3p1/2 core levels,
respectively. The signal around 280 eV appeared as partially over-
lapped (C 1s and Ru 3d). Hence, Ru 3p levels were focussed pri-
marily to confirm the presence of Ru. Furthermore, the Ru 3p core
level peaks were best fitted to elucidate the +2 oxidation state of
Ru in 6 [20,44]. The binding energies corresponding to Ru(0) and
RuO2 were not observed in Fig. 7, which confirmed that Ru loaded
was only in the form of complex. XPS spectrum of 6 also possessed
peaks due to N 1s, O 1s, S 2p and Cl 2p. ICP-MS study established
3.1 wt% loading of Ru over graphene oxide.
8

D. Sindhuja, M. Gopiraman, P. Vasanthakumar et al.
Journal of Organometallic Chemistry 949 (2021) 121933
Fig. 7. XPS spectra (a) wide spectrum of 6 (b) Cl2p region (c) N1s region (d) O1s region (e) Ru3p region and (f) S2p region.
3.7. Performance by catalyst 6
give the corresponding products in moderate to good yields.
The present catalytic system yielded 2,3-diphenylquinoxaline (14a)
from 2-nitroaniline and benzoin in 80% yield. Similarly, yields
of 6,7-dimethyl-2,3-diphenylquinoxaline (14b) and 6-methoxy-2,3-
diphenylquinoxaline (14c) were found to be 73 and 59%, respec-
tively. Further, 78% of 5-chloro-2,3-diphenylquinoxaline (14d) was
obtained in 12 h, and the yield was comparable with that ob-
tained from carbon nanotube-Au nanohybrid catalytic system [46].
Also, yield of 5-chloro-2,3-diphenyl-7-(trifluoromethyl)quinoxaline
(14e) was 79%. Like benzoin, 2,2’-furoin was also coupled with
Optimum catalyst loading was found by varying the amount
of the catalyst in the reaction between 2-nitroaniline and ben-
zoin, and good conversion of 2-nitroaniline was observed when
it was 0.15 mol%. With the same optimum conditions, scope
was extended. Catalyst
6
was used for the coupling of var-
furoin, and the
ious 2-nitroaniline derivatives with benzoin
/
yields of the isolated products are summarized in Table 4. All
the substrates were coupled smoothly in the presence of 6 to
9

D. Sindhuja, M. Gopiraman, P. Vasanthakumar et al.
Journal of Organometallic Chemistry 949 (2021) 121933
Fig. 8. Possible pathway for the synthesis of quinoxaline derivatives.
Fig. 9. Reusability of 6.
various 2-nitroaniline derivatives to produce quinoxaline deriva-
tives. The yields of 2,3-di(furan-yl)quinoxaline (14f), 2,3-di(furan-
2-yl)-6-trifluoromethyl-quinoxaline (14g), 5-chloro-2,3-di(furan-2-
yl)-7-trifluoromethyl-quinoxaline (14h) and 6-chloro-2,3-di(furan-
2-yl)quinoxaline (14i) were 82, 85, 87 and 68%, respectively. In-
terestingly, this catalytic system is compatible for the substrates
having electron withdrawing or electron donating substituent(s).
From GC and GC-MS analyses, it was found that there was no
formation of diamine from 2-nitroaniline, which confirmed that
the first step was imine formation, followed by transfer hydrogena-
tion and cyclization. The possible pathway for quinoxaline forma-
tion is shown in Fig. 8.
4. Conclusions
Synthesis and characterization of air-stable half-sandwich Ru(II)
complexes (1-5) are reported. Crystal structures of complexes 2
and 3 are described. Catalytic efficacy of 4 was better towards the
synthesis of quinoxaline derivatives. Hence, it was made hetero-
geneous (6) by immobilizing it on graphene oxide and character-
ized. Scope of both homogeneous (4) and heterogeneous (6) cata-
lysts has been extended to various 2-nitroaniline and hydroxy ke-
tone derivatives. The present system has been proven to be versa-
tile and efficient for the synthesis of quinoxaline derivatives. Also,
heterogenous catalyst 6 is recyclable and it can be reused up to
five cycles with good activity.
3.7.1. Reusability of the heterogenized catalyst (6)
Declaration of Competing Interest
The authors declare no conflict of interest
Acknowledgement
Catalyst 6 was used for the next cycle after washing it with
water, ethanol and diethyl ether. Benzoin and 2-nitroaniline were
used as substrates for the reusability tests. The reactions were car-
ried out under the same optimized conditions. The yield of 2,3-
diphenylquinoxaline was noted as 80% in the first run, and there
was a little reduction in the yields after the third cycle. Catalyst
was reused up to five cycles (Fig. 9).
We thank DAE-BRNS (34/14/52/2014-BRNS/2069) for financial
support.
10

D. Sindhuja, M. Gopiraman, P. Vasanthakumar et al.
Journal of Organometallic Chemistry 949 (2021) 121933
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11