Half-sandwich ruthenium(II) complexes containing 4-substituted aniline derivatives: structural characterizations and catalytic properties in transfer hydrogenation of ketones

Article abstract of DOI:10.1007/s11243-021-00461-9

Four half-sandwich Ru(II) complexes (1)–(4) with the general formulae [Ru(η⁶-p-cymene)(L)Cl₂] were synthesized by the reaction of one equivalent of the Ru(II) p-cymene dimer with two equivalents of a p-substituted aniline derivative L (where L is p-methyl, p-isopropyl, p-methoxy, or p-hydroxy aniline). The structures of complexes (2)–(4) were determined by single-crystal X-ray diffraction studies. The structural analysis revealed piano-stool geometry at the Ru(II) ions which are coordinated to the η⁶-p-cymene, two chloride anions and the amine group of the aniline ligand. In the structure of (2)–(4), the coordinated chloride ions make intermolecular hydrogen bonding with the –NH₂ group of an adjacent molecules (NH–Cl) resulting in hydrogen bond networks. The catalytic activities of the complexes in transfer hydrogenation of acetophenone were studied. Complex [Ru(η⁶-p-cymene)(p-methylaniline)Cl₂] (1) showed the best catalytic performance in the transfer hydrogenation of acetophenone. The presence and positions of methyl and bromide groups on the acetophenone have an impact on the catalytic activity in transfer hydrogenation properties of the complex (1). Moreover, catalytic activity of the complex (1) is significantly higher in the transfers hydrogenation of cyclohexanone than 2-hexanone.

Full text of DOI:10.1007/s11243-021-00461-9

Transition Metal Chemistry
https://doi.org/10.1007/s11243-021-00461-9
Half‑sandwich ruthenium(II) complexes containing 4‑substituted
aniline derivatives: structural characterizations and catalytic
properties in transfer hydrogenation of ketones
Mean Sadık1 · Muharrem Karabork1 · Irfan Sahin1 · Muhammet Kose1
Received: 14 March 2021 / Accepted: 30 April 2021
© The Author(s), under exclusive licence to Springer Nature Switzerland AG 2021
Abstract
Four half-sandwich Ru(II) complexes (1)–(4) with the general formulae [Ru(η6-p-cymene)(L)Cl2] were synthesized by the
reaction of one equivalent of the Ru(II) p-cymene dimer with two equivalents of a p-substituted aniline derivative L (where
L is p-methyl, p-isopropyl, p-methoxy, or p-hydroxy aniline). The structures of complexes (2)–(4) were determined by
single-crystal X-ray difraction studies. The structural analysis revealed piano-stool geometry at the Ru(II) ions which are
coordinated to the η6-p-cymene, two chloride anions and the amine group of the aniline ligand. In the structure of (2)–(4), the
coordinated chloride ions make intermolecular hydrogen bonding with the –NH2 group of an adjacent molecules (NH–Cl)
resulting in hydrogen bond networks. The catalytic activities of the complexes in transfer hydrogenation of acetophenone
were studied. Complex [Ru(η6-p-cymene)(p-methylaniline)Cl2] (1) showed the best catalytic performance in the transfer
hydrogenation of acetophenone. The presence and positions of methyl and bromide groups on the acetophenone have an
impact on the catalytic activity in transfer hydrogenation properties of the complex (1). Moreover, catalytic activity of the
complex (1) is signifcantly higher in the transfers hydrogenation of cyclohexanone than 2-hexanone.
Introduction
those disadvantages [10]; the hydrogen atoms transferred in
the reaction are generally provided from another molecule
(hydrogen donor), which also acts as a solvent in the pres-
ence of a catalyst [11]. Suitable hydrogen donors include
propan-2-ol, glycerol and formic acid/triethylamine mixtures
[12–14]; propan-2-ol is usually preferred due to its lower
environmental impact and because many catalysts can sur-
vive even in refuxing 2-propanol [15]. While aluminium
alkoxide was previously used to catalyze the reduction of
ketones to the corresponding alcohols, more recent powerful
catalysts for transfer hydrogenation of ketones are based on
transition metals such as rhodium, iridium, and ruthenium
[16–18]. The development of novel catalysts for catalytic
transfer hydrogenation of ketones is now focussed on the
use of ruthenium-based complexes as ruthenium is rela-
tively cheaper than rhodium and iridium. However, in many
cases the Ru-catalysts required are complicated and require
multi-step ligand design [19–26]. Noyori and co-workers
reported extremely efcient ruthenium bifunctional catalyst
[RuCl2(diphosphane)(diamine)] for the enantioselective
reduction of ketones in the presence of a base [27]. This
pioneering catalytic system has allowed the preparation of
alcohols with high optical purity [28]. In this catalytic sys-
tem, the proper matching of a chiral ruthenium diphosphine
Reduction of aldehydes and ketones to alcohols is an impor-
tant transformation for the production of many industrial
chemicals such as esters, acetals, and acids, which are com-
monly used in the chemical industry and in pharmaceuti-
cal applications [1–4]. Metal hydride reduction (often with
NaBH4 and LiAlH4), direct hydrogenation, and catalytic
transfer hydrogenation are the three general methods used
to achieve this transformation. Hydride reagents are very
efective; however, the metal hydride reduction produces sto-
ichiometric amounts of waste requiring appropriate disposal,
which is neither environmentally nor economically sustain-
able [5–8]. Direct hydrogenation is also a popular method
that provides clean reaction and proceeds in high yield, yet
it has some drawbacks such as the use of hydrogen gas and
requirement for high-pressure reactors [8, 9]. Catalytic trans-
fer hydrogenation has been developed in order to overcome
* Muhammet Kose
muhammetkose@ksu.edu.tr
1
Chemistry Department, Kahramanmaras Sutcu Imam
University, 46100 Kahramanmaras, Turkey
Vol.:(0123456789)
1 3

Transition Metal Chemistry
with the correct enantiomer of diamine is the key point for
enantioselective catalysts [29]. It was discussed that the
presence of primary amino donors is important for the cata-
lytic cycle where the hydride on ruthenium and proton on
nitrogen attack the polar bond [30].
analysis (Calculated for C19H27NCl2Ru): C: 51.70, H: 6.17;
N: 3.17; Found: C: 51.22, H: 6.02, N: 3.02, 1H-NMR: 1.13
(6H, d, C(CH3)2 aniline), 1.20 (6H, d, C(CH3)2 p-cymene),
2.09 (3H, s, Ph–CH3 p-cymene), 2.71 (1H, septet, CH(CH3)2
p-cymene), 2.84 (1H, septet, CH(CH3)2 p-cymene), 4.88
(2H, s, NH2 aniline) 5,79 (4H, d, CH p-cymene), 6.50 (2H,
d, CH aniline), 6.87 (2H, d, CH aniline). FT-IR (cm−1):
3203, 3104, 2953, 2865, 1608, 1600, 1505, 1465, 1378,
1230, 1119, 1053,1005, 882, 839, 568.
Due to the importance of the primary amine group in
the catalytic transfer hydrogenation of ketones, we prepared
four half-sandwich Ru(II) complexes (1)-(4) derived from
[Ru(η6-p-cymene)Cl2]2 and p-substituted aniline derivatives
with the general formulae [Ru(η6-p-cymene)(L)Cl2] (where
L is p-methyl, p-isopropyl, p-methoxy, or p-hydroxy ani-
line) and used as catalysts for the transfer hydrogenation
of ketones. The complexes were characterized by FT-IR,
UV–Vis. and 1H-NMR spectroscopies, and the structures of
(2)–(4) were determined by X-ray crystallography.
[Ru(η6-p-cymene)(p-methoxyaniline)Cl2] (3): Yield:
87%, Colour: Red, M.p. 199 °C (decomposed), Elemental
analysis (Calculated for C17H23NOCl2Ru): C: 47.56, H: 5.40;
N: 3.26; Found: C: 47.31, H: 5.12, N: 3.01, 1H-NMR: 1.19
(6H, d, C(CH3)2 p-cymene), 2.09 (3H, s, Ph–CH3 p-cymene),
2.84 (1H, septet, CH(CH3)2 p-cymene), 3.62 (3H, s, OCH3),
4.75 (2H, s, NH2), 5.81 (4H, d, CH p-cymene), 6.64 (2H, d,
CH aniline), 6.66 (2H, d, CH aniline). FT-IR (cm−1): 3283,
3810, 3100, 2947, 1596, 1579, 1510, 1453, 1305, 1247,
1167, 1096, 1024, 876, 827, 736, 544.
Experimental
Starting materials [Ru(η6-p-cymene)Cl2]2, p-methylaniline,
p-isopropyl aniline, p-methoxy aniline, and p-hydroxyl ani-
line were purchased from commercial sources and used as
received. FT-IR spectra of the complexes recorded taken on
a PerkinElmer Spectrum 100 FT-IR, and FT-IR spectra are
given in the supplementary documents (Figs. S1–S4). The
UV–Vis. absorption spectra were recorded on a PerkinElmer
Lambda 45 spectrophotometer. 1H-NMR spectra were meas-
ured on Bruker Avance III 400 MHz spectrometer using
d6-DMSO as solvent. Catalytic conversion of ketones to
alcohols was monitored by GC using a YL6500 Instrument.
[Ru(η6-p-cymene)(p-hydroxyaniline)Cl2] (4): Yield: 85%,
Colour: Red, M.p.: 195 °C (decomposed), Elemental analy-
sis (Calculated for C16H21NOCl2Ru): C: 46.27, H: 5.10; N:
3.37; Found: C: 46.05, H: 4.95, N: 3.12, 1H-NMR: 1.20 (6H,
d, C(CH3)2 p-cymene), 2.09 (3H, s, Ph–CH3 p-cymene), 2.82
(1H, septet, CH(CH3)2 p-cymene),4.43(2H, s, NH2 aniline),
5.81 (4H, d, CH p-cymene), 6.41–6.43(4H, d, CH aniline),
8.34 (1H, b, OH aniline). FT-IR (cm−1): 3341, 3255, 3187,
3101, 2960, 1612, 1600, 1569, 1501, 1511, 1458, 1375,
1357, 1255, 1214, 1119, 834, 803, 759, 663,573, 542.
X‐ray crystallography
Synthesis of the complexes
X‐ray crystallographic data for complexes (2)-(4) were col-
lected at 293 (2) K on a Bruker D8 QUEST difractometer
using Mo‐Kα radiation (λ = 0.71073 Å). Data reduction
was performed using Bruker SAINT [31, 32]. SHELXT
2018/2 was used to solve and SHELXL-2018/3 to refne
the structures [33, 34]. The structures were solved by direct
methods and refned on F2 using all the refections. All the
non‐hydrogen atoms were refned using anisotropic atomic
displacement parameters, and hydrogen atoms bonded to
carbon, nitrogen, and oxygen atoms were inserted at calcu-
lated positions using a riding model and refned with tem-
perature factors. The crystal data and refnement details are
given in Table 1. Hydrogen bond parameters are given in the
supplementary fle (Tables S1–S3).
The para-substituted aniline derivative (0.326 mmol) in etha-
nol (5 mL) was added dropwise to a refuxing solution of
[Ru(η6-p-cymene)Cl2]2 (100 mg, 0.16 mmol) in ethanol (15
mL). The reaction solutions were refuxed for 24 h. Upon
cooling to 298 K, the solvent volume was reduced to 5 mL.
Red crystals formed in three days and were fltered and dried
in air.
[Ru(η6-p-cymene)(p-methylaniline)Cl2] (1): Yield:
84%, Colour: Red, M.p.: 229 °C (decomposed), Elemen-
tal analysis (Calculated for C17H23NCl2Ru): C: 49.40, H:
5.61; N: 3.39; Found: C: 49.18, H: 5.42, N: 3.11, 1H-NMR:
1.21 (6H, d, C(CH3)2 p-cymene), 2.10 (3H, s, Ph–CH3
p-cymene), 2.12 (3H, s, Ph–CH3 aniline), 2.83 (1H, septet,
CH(CH3)2 p-cymene), 4.84 (2H, s, NH2 aniline), 5.82 (4H,
d, CH p-cymene), 6.48 (2H, d, (CH aniline), 6.83 (2H, d,
(CH aniline), FT-IR (cm−1): 3323, 3200, 3120, 3040, 2956,
2861 1610, 1578, 1515, 1467, 1375, 1225, 1135, 1105,1086,
1049, 876, 819, 7450, 668, 546, 450.
Transfer hydrogenation of ketones
The acetophenone derivative (1 mmol) was dissolved in in
2-propanol (9.4 mL) followed by addition of base and cata-
lyst (5 mg). The reaction mixture was stirred at 82 °C. Sam-
ples from the reaction solution were collected periodically,
[Ru(η6-p-cymene)(p-isopropylaniline)Cl2] (2): Yield
82%, Colour: Red, M.p. 199 °C (decomposed), Elemental
1 3

Transition Metal Chemistry
Table 1 X-ray crystallographic data for the compounds
Complex
(2)
(3)
(4)
Formulae
Molecular weight
Temperature
C19H27Cl2NRu
441.38
293(2) K
C17H23Cl2Nru
429.33
293(2) K
C16H21Cl2NORu
415.31
293(2) K
Wavelength
Crystal system
Space group
0.71073 Å
Triclinic
P-1
0.71073 Å
Monoclinic
C2/c
0.71073 Å
Monoclinic
P21/n
Unit cell parameters
a=7.81510(10) Å α=96.0000(10)°
b=9.04110(10) Å β=98.9440(10)°
c=15.4559(2) Å γ=108.2440(10)°
1010.81(2)
a=15.8030(5) Å α=90°
b=27.4023(6) Å β=97.455(3)°
c=17.5361(5) Å γ=90°
7529.6(4)
a=9.9076(4) Å α=90°
b=28.1896(11) Å β=105.837(5)°
c=12.5573(6) Å γ=90°
3374.0(3)
Volume (Å3)
Z
2
16
1.515
1.117
0.15×0.11×0.09
51,662
8
Density(calc.) (Mg/m3)
Absorption coefcient (mm1)
Crystal size (mm3)
Ref. collected
1.450
1.039
0.22×0.15×0.11
14,026
1.635
1.244
0.16×0.14×0.13
13,183
Independent ref
4751 [R(int)=0.0282]
R1=0.0251, wR2=0.0624
R1=0.0276, wR2=0.0643
99.8%
9208 [R(int)=0.0467]
R1=0.0417, wR2=0.0815
R1=0.0597, wR2=0.0880
99.8%
7627 [R(int)=0.0602]
R1=0.0697, wR2=0.1700
R1=0.1083, wR2=0.2023
99.1%
Final R values [I>2sigma(I)]
R indices (all data)
Completeness to θ=25.242°
CCDC Number
1,956,820
1,956,821
1,956,822
Single crystals of the synthesized complexes were obtained,
and their structures were illuminated by single-crystal X-ray
difraction studies.
1
In order to characterize the new complexes, H-NMR
spectra were recorded in d6-DMSO solvent (Figs. S5–S8).
Hydrogen signals due to the p-cymene ligands were observed
in similar regions in each complex. The hydrogen signals
due to the isopropyl (–CH(CH3)2) group in the p-cymene
ligand were observed as multiplets (CH septet and –CH3
doublet) in the ranges 1.20–1.21 and 2.82–2.84 ppm, respec-
tively. The methyl group hydrogens (–CH3) on p-cymene
ligand were observed as singlets at around 2.09–2.12 ppm.
Multiplets at 5.79–5.82 ppm are due to the aromatic hydro-
gens of p-cymene. The coordinated NH2 groups in the ani-
line ligands resulted in broad signals in the range 4.43–4.88
ppm. In the spectrum of [Ru(η6-p-cymene)( p-methylaniline)
Cl2] (1), the singlet signal at 2.12 ppm is due to the methyl
group hydrogens on the aniline ring. A doublet at 1.13 and
a septet at 2.71 ppm are seen in the spectrum of [Ru(η6-
p-cymene)(p-isopropylaniline)Cl2] (2) due to the isopropyl
hydrogens on the aniline ring (CH and CH3), respectively.
The methoxy group hydrogens on the aniline ring in the
spectrum of [Ru(η6-p-cymene)(p-methoxyaniline)Cl2] (3)
are seen as a singlet at 3.62 ppm. The phenolic OH hydrogen
in the spectrum of [Ru(η6-p-cymene)(p-hydroxyaniline)Cl2]
(4) appears as a singlet at 8.34 ppm. The 1H NMR spectra
and integration suggest that there were no signifcant organic
impurities in the samples.
Fig. 1 The structures of half-sandwich Ru(II) (1)–(4) complexes
and conversion rates were determined on GC using a HP-5
column.
Results and discussion
In this work, four mononuclear Ru(II) complexes were
obtained from the reaction of one equivalent of [Ru(η6-p-
cymene)Cl2]2 dimer with two equivalents of p-substituted
aniline derivatives [p-methyl, p-isopropyl, p-methoxy, or
p-hydroxyl aniline] (Fig. 1) in high yield and purity. The
structures of the synthesized complexes were determined
by 1H-NMR, FT-IR (Figs. S1–S4) and elemental analysis.
1 3

Transition Metal Chemistry
UV–Vis absorption spectra of the complexes were studied
in MeOH (10–4 mol L−1), and the spectra of the complexes
are shown in Fig. S9. The complexes exhibit two broad
absorption bands: the frst band was observed in the 370–550
nm range and can be assigned to π-π* electronic transitions;
weaker band observed at 550–700 nm is assigned to the
ligand–metal charge transfer transition.
(4), the phenol units of the two independent complex mol-
ecules make diferent hydrogen bonding interactions. While
the phenol unit (O1H) of complex containing Ru1 involves
only one hydrogen bonding interaction (as donor) with the
coordinated chloride (Cl4) anion of an adjacent molecule,
the phenol unit (O2H) of the other molecule containing Ru2
makes two hydrogen bonds (as donor with the coordinated
chloride (Cl3) anion of an adjacent molecule and as accep-
tor with the amine NH2 group of a neighbouring molecule
forming a 2D hydrogen bond network (Fig. S12).
Molecular structures of complexes (2)–(4)
The structure of complex (1) was previously reported [36].
Here, we report the structures of complexes (2)–(4) for the
frst time. Quality crystals of complexes (2)–(4) suitable for
single-crystal X-ray difraction study were obtained from
slow evaporation of solvent from the reaction solution over
a few days. The X-ray crystallographic data for complexes
(2)-(4) are summarized in Table 1. The structure of (2) was
solved in triclinic unit cell, while the structures of complexes
(3) and (4) were solved in monoclinic unit cell. The asym-
metric unit of (2) contains a single molecule; however, there
are two independent molecules in the asymmetric units of
complexes (3) and (4). The structures of the complexes are
shown in Fig. 2. The bond lengths and angles in the inde-
pendent molecules are almost identical with diferent orien-
tation of methoxy group in (3) and opposite alignment of the
phenol unit in (4). In the structures of all three complexes,
each Ru(II) coordinates to the η6-p-cymene, two chloride
anions and the amine group of p-substituted aniline ligands
with piano-stool geometry. The ruthenium Ru(II) ion is
π-bonded to the p-cymene ligand. The Ru–C distances in
the complexes are in the range of 2.139(7) and 2.2284(19)
Å (Table 2) and the distance between the centroid of phenyl
ring (C1/C6) and Ru(II) is about 1.66 Å which is similar
to those reported Ru(II) η6-arene complexes [35, 36]. The
Cl–Ru–Cl and Cl–Ru–N bond angles are comparable with
the literature values [35, 36].
Catalytic studies
The catalytic activities of the half-sandwich Ru(II) com-
plexes were investigated in transfer hydrogenation reac-
tions. In order to determine the best catalysts among the
complexes, the acetophenone 1-phenylethanol reduction was
chosen as a model reaction. It was performed with 100:1
substrate/complex ratios in the presence of a base (0.1 mol
L−1 NaOH) with isopropanol as a solvent and hydrogen
source (Fig. 3). The reactions were stirred at 82 °C for 90
min. and samples from the reaction solutions were subjected
to GC analysis to determine percentage (%) conversion. The
catalytic performances of the complexes for acetophenone
1-phenylethanol transformation are given in Table S4. The
data showed that the substituent groups in the para position
of the aniline ligands have an impact in the catalytic transfer
hydrogenation reaction. Complex (1) exhibited the highest
catalytic activity for acetophenone 1-phenylethanol with
56% conversion at the end of 90 min. Therefore, complex (1)
was chosen to optimize the catalytic reaction conditions. The
efects of base type, amounts, and substituent groups on the
ketone compounds were studied in order to establish the best
catalytic conditions. The lowest activity (17%) of compound
(4) may be assigned to the phenolic –OH group in the para
position of aniline ligand. The phenolic group can involve in
hydrogen bonding interaction with acetophenone, which can
hinder the interaction of the metal centre with the substrate.
In the catalytic transfer hydrogenation reactions, a base
additive is used in order to stimulate metal hydride forma-
tion to initiate transfer hydrogenation reactions [37]. The
Ru(II)-based catalytic transfer hydrogenation mechanism
was well established in the literature [38]. The N–H func-
tionality is important as they activate the C=O double bond
towards H− acceptance through N–H–O hydrogen bonding
interactions [39]. In the absence of a base, the same reac-
tion occurs with only very small conversion rates. Bases
Although the coordination cores of the complexes are
similar, the molecular packing shows distinct features due
to diferent intermolecular interactions. In the structure of
complex (2), the coordinated chloride ions are involved in
intermolecular hydrogen bonding with the NH2 group of an
adjacent molecule (NH–Cl) and these hydrogen bond con-
tacts make 1D hydrogen bond chains along the ac plane
(Fig. S10). The hydrogen bonded chains are further linked
by CH–Cl interactions. In the structure of complex (3),
while one of the coordinated chloride ions (Cl1) makes an
intermolecular hydrogen bond with the –NH2 group of an
adjacent molecule (Cl–HN), the other chloride ion (Cl2)
makes two hydrogen bonds with the –NH2 groups of two
adjacent molecules. These hydrogen bond contacts connect
four complex molecules into hydrogen bond cluster (Fig.
S11a). The hydrogen bonded clusters are further linked by
Cl–HC and O–HC (Fig. S11b). In the structure of complex
NaOH, KOH, NaOiPr, or KOtBu (0.4 mL, 0.1 mol L−1
)
were used for acetophenone reduction to 1-phenylethanol
under the reaction conditions described above (Table S5).
It was seen that the introduction of a base to the reaction
vessel considerably increases the conversion rate. The
conversion rates were found to vary in the following order
1 3

Transition Metal Chemistry
Fig. 2 Molecular structures
of the complexes (2)–(4) with
atom numbering. Hydrogen
atoms are omitted for clarity
KOtBu>NaOH>KOH>NaOiPr. The amount of base also
infuences the catalytic activity of the complex. Increasing
the amount of base KOtBu (0.1molL−1) from 0.2 to 0.6 mL
improved catalytic activity observed in the presence of 0.6
mL KOtBu (Table S6), but further increase in the amount of
base did not afect the catalytic activity.
Methyl groups (–CH3) at the ortho, meta, and para positions
of the acetophenone phenyl ring were tested, and % conver-
sions (from ketone to corresponding alcohol) were measured
at the end of four hours. The electron-donating methyl group
at the ortho, meta, and para positions slightly decreased the
catalytic activity when compared to the non-substituted ace-
tophenone. The presence and position of a bromide group on
the acetophenone also had an impact on the catalytic activ-
ity. Although the bromide group has electron withdrawing
The catalytic activity of the complex (1) was investi-
gated for transfer hydrogenation reactions of acetophenone
derivatives containing diferent substituent groups (Table 3).
1 3

Transition Metal Chemistry
Fig. 3 Catalytic transfer hydrogenation reaction of acetophenone derivatives
Table 2 Selected bond lengths (Å) and angles (°) for complexes
for 2-hexanone to product 2-hexanol and cyclohexanone to
product cyclohexanol for complex (1) are given in Table 3.
For transfer hydrogenation of 2-hexanone, %conversion was
67% at the end of four hours. Under the same reaction condi-
tions, the conversion of cyclohexanone to cyclohexanol was
almost completed (98%).
(2)
(3)
(4)
Ru(1)-C(1)
Ru(1)-C(2)
Ru(1)-C(3)
Ru(1)-C(4)
Ru(1)-C(5)
Ru(1)-C(6)
Ru(1)-N(1)
Ru(1)-Cl(1)
Ru(1)-Cl(2)
Cl(1)-Ru(1)-Cl(2)
N(1)-Ru(1)-Cl(1)
N(1)-Ru(1)-Cl(2)
2.2284(19)
2.1924(19)
2.1514(19)
2.1864(19)
2.1420(19)
2.1885(19)
2.1793(15)
2.4024(5)
2.4189(5)
87.078(18)
83.65(4)
2.192(4)
2.175(4)
2.171(4)
2.202(4)
2.160(4)
2.148(4)
2.179(3)
2.4181(9)
2.4104(10)
88.05(4)
84.93(8)
80.64(8)
2.221(7)
2.171(7)
2.139(7)
2.221(7)
2.165(7)
2.198(8)
2.164(6)
2.4167(18)
2.4208(19)
86.78(7)
79.39(16)
84.05(17)
Conclusions
In summary, in the course of this work, we prepared four
half-sandwich Ru(II) complexes using simple p-substituted
aniline derivatives and characterized them by spectroscopic
and analytical methods. The solid-state structures of com-
plexes (2)–(4) were investigated by single-crystal X-ray
difraction studies. In each case, the Ru(II) centre shows a
piano-stool geometry and coordination sphere consists of
a η6-p-cymene, two chloride anions and the amine group
of p-substituted aniline ligands. Though the coordination
geometry and bond distances around the Ru(II) centre are
similar, the complex molecules showed distinct intermo-
lecular interactions. The catalytic properties of the synthe-
sized half-sandwich complexes were studied for transfer
hydrogenation reactions of ketones. The complexes were
found to catalyze the transfer hydrogenation of acetophe-
none derivatives, cyclic and acyclic ketones in the presence
of isopropanol (as hydrogen source) and a base. Complex
80.28(4)
properties, the bromide group at the para position of aceto-
phenone did not signifcantly change the %conversion when
compared to the para methyl-substituted acetophenone.
However, when the bromide group is located at the meta
position, the catalytic activity dropped dramatically and 53%
conversion was obtained at the end of four hours.
Complex [Ru(η6-p-cymene)(p-methylaniline)Cl2] (1) was
further investigated for its catalytic activity in transfer hydro-
genation of aliphatic (2-hexanone) and its cyclic analogue
(cyclohexanone) (Fig. 3). The catalytic conversion values
1 3

Transition Metal Chemistry
Table 3 Transfer hydrogenation of ketones catalyzed by complex (1)*
Ketone
Product
Time (hour)
4h
% Conversion**
96
4h
90
4h
4h
90
85
4h
53
4h
4 h
4 h
87
67
98
*: Reaction conditions: ketone, (1.0 mmol), i−PrOH (9.4 mL), KOtBu (0.6 mL, 0.1 M), 0.5 mg complex (1) and 82 °C. **: GC yield
[Ru(η6-p-cymene)(p-methylaniline)Cl2] (1) showed the
highest catalytic activity for transfer hydrogenation of ace-
tophenone. The substituent groups and their positions on the
acetophenone are important factors for the catalytic activity.
Complex (1) was found to catalyze the transfer hydrogena-
tion of cyclic (cyclohexanone) and acyclic (2-hexanone)
ketones. The activity of the complex (1) is signifcantly
higher for transfer hydrogenation of cyclohexanone than
acyclic analogue 2-hexanone.
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