Synthesis and characterization of new ruthenium(III) complexes derived from fluoreneamine-based Schiff base ligands and their catalytic activity in transfer hydrogenation of ketones

Article abstract of DOI:10.1080/00958972.2020.1718665

An easy and convenient synthesis of a new series of octahedral ruthenium(III) complexes bearing Schiff base ligands of general formula [RuCl₂(EPh₃)₂(L)] (where E = P, As and L = O,N-donor Schiff bases) has been reported. The composition of all complexes has been unequivocally characterized by spectral (IR, UV-vis, EPR) and ESI-MS techniques. The substituted Schiff base ligands behave as bidentate O,N-donors and coordinate to ruthenium via the phenolic oxygen, the azomethine nitrogen. Complexes 1–6 have been proven to catalyze the transfer hydrogenation of linear, cyclic and aromatic ketones to their corresponding secondary alcohols in the presence of i-PrOH/KOH at 80 °C with conversion up to 99%. The effect of other variables on the transfer hydrogenation reaction such as solvent, base, and catalyst loading is also reported.

Full text of DOI:10.1080/00958972.2020.1718665

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Synthesis and characterization of new
ruthenium(III) complexes derived from
fluoreneamine-based Schiff base ligands and their
catalytic activity in transfer hydrogenation of
ketones
Veerasamy Nagalakshmi, Raja Nandhini, Galmari Venkatachalam & Kasturi
Balasubramani
To cite this article: Veerasamy Nagalakshmi, Raja Nandhini, Galmari Venkatachalam & Kasturi
Balasubramani (2020): Synthesis and characterization of new ruthenium(III) complexes derived
from fluoreneamine-based Schiff base ligands and their catalytic activity in transfer hydrogenation
of ketones, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2020.1718665
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2020.1718665
Synthesis and characterization of new ruthenium(III)
complexes derived from fluoreneamine-based Schiff base
ligands and their catalytic activity in transfer
hydrogenation of ketones
Veerasamy Nagalakshmia,b, Raja Nandhinia,b, Galmari Venkatachalama,b and
Kasturi Balasubramania,b
aPG & Research Department of Chemistry, Government Arts College, Dharmapuri, Tamilnadu, India;
bDepartment of Chemistry, Govt Arts College (Autonomous), Karur, Tamilnadu, India
ABSTRACT
ARTICLE HISTORY
Received 23 October 2019
Accepted 11 January 2020
An easy and convenient synthesis of a new series of octahedral
ruthenium(III) complexes bearing Schiff base ligands of general
formula [RuCl2(EPh3)2(L)] (where E ¼ P, As and L ¼ O,N-donor Schiff
bases) has been reported. The composition of all complexes has
been unequivocally characterized by spectral (IR, UV-vis, EPR) and
ESI-MS techniques. The substituted Schiff base ligands behave as
bidentate O,N-donors and coordinate to ruthenium via the phen-
olic oxygen, the azomethine nitrogen. Complexes 1–6 have been
proven to catalyze the transfer hydrogenation of linear, cyclic and
aromatic ketones to their corresponding secondary alcohols in the
presence of i-PrOH/KOH at 80ꢀC with conversion up to 99%. The
effect of other variables on the transfer hydrogenation reaction
such as solvent, base, and catalyst loading is also reported.
KEYWORDS
Ruthenium(III); Schiff base
ligands; Synthesis;
Characterization; Transfer
hydrogenation
CONTACT Galmari Venkatachalam
gvchem@gmail.com, manavaibala@gmail.com
PG & Research Department
of Chemistry, Government Arts College, Dharmapuri 636 705, Tamilnadu, India
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2020.1718665.
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
R. NANDHINI AND G. VENKATACHALAM
1. Introduction
Schiff base ligands, considered as privileged ligands and attractive due to their stabil-
ity and the ease by which modified variations can be obtained, are once again topical
in connection with a diverse range of applications such as organic synthesis [1, 2]. For
several reasons, Schiff bases have been found to be among the most convenient and
attractive ligands for ruthenium complexes [3–7]. Schiff base ligands have been used
extensively in coordination chemistry to build complexes with transition and main
group metals. The proper choice of ligands is key in manipulating the activity and
selectivity of catalysts. Steric and electronic effects around the metal center can be
finely adjusted through an appropriate selection of the Schiff base ligands.
Experimental data suggests that the steric bulk of the Schiff base has a greater impact
on catalytic performance compared to the electronic influence of Schiff base ligands
[8]. The two donor atoms O,N in the ligated Schiff base exert opposite electronic
effects, thus a flexible interplay between these two binding sites can be achieved and
it triggers catalytic activity. Schiff base complexes of transition metals [9] having O,N-
donor atoms have shown an exponential increase as inorganic catalysts for various
organic transformations.
Ruthenium complexes have been recently gaining importance in a broad range of
applications, and consequently there is a continuous endeavor to synthesize new com-
plexes of ruthenium with different types of ligands, such as Schiff base [10], azo [11],
pincer [12], tripodal [13] and carbenes [14, 15]. In particular, emphasis has been placed
on ligands containing a phenolate oxygen, which is a recognized hard donor, and
hence coordination by a phenolate oxygen is of importance with regard to stabiliza-
tion of the higher oxidation states of ruthenium [16–19].
Transition metal triphenylphosphine/triphenylarsine complexes, especially ruthe-
nium complexes, find application in classical catalytic process such as hydrogenation,
isomerization, decarbonylation, reductive elimination, oxidative addition and in making
C-C bonds [20]. Ruthenium complexes are currently being investigated because of
their rich structural, electrochemical, catalytic and biological properties [21–29].
Furthermore, transfer hydrogenation is one of the most important methods for the
reduction of ketones to alcohols, due to its valuable and atom-efficient reaction and
simpler experimental procedure [30]. Ruthenium-based catalytic systems are found to
be effective in the transfer hydrogenation reactions [31].
Herein we report on the reactions of the fluoreneamine Schiff base ligands with
ruthenium precursor complexes. The newly synthesized complexes were fully charac-
terized by spectroscopic and mass techniques and their ability to serve on precursors
in the catalytic transfer hydrogenation of a wide range of substrates is discussed.
2. Experimental
2.1. Materials and physical measurements
All chemicals and solvents were obtained from commercial sources and used
without further purification. RuCl3·3H2O and different aromatic aldehydes for catalytic
studies were purchased from Sigma-Aldrich. Schiff base ligands were prepared by

JOURNAL OF COORDINATION CHEMISTRY
3
2-aminofluorene with corresponding aldehydes according to reported procedures [32].
The microanalysis of carbon, hydrogen and nitrogen was carried out using a Vario EL
III Elemental analyzer at SAIF, Cochin, India. The FT-IR spectra of the compounds were
recorded on an Agilent Resolution pro system spectrometer from 4000 to 400 cm–1.
Electronic spectra of the complexes were recorded in CHCl3 solution on a Cary 300 Bio
UV-vis Varian spectrophotometer from 800 to 200 nm. ESR spectra were recorded in
the X-band frequency on an ESR-JEOL/82 ESR spectrometer with a microwave power
of 1 mW and a modulation amplitude of 160.00. The starting ruthenium(III) precursors,
[RuCl3(PPh3)3] [33] and [RuCl3(AsPh3)3] [34], were prepared according to the litera-
ture reports.
2.2. Synthesis of ruthenium(III) Schiff base complexes (1-6)
Complexes 1–6 were prepared by the following general procedure. To a benzene
(20 mL) solution of [RuX3(EPh3)3] (0.0993 g, 0.1 mmol) (X ¼ Cl, E ¼ P or As) was added
the Schiff base ligands (0.0253-0.0335 g, 0.1 mmol) (HL1-HL3) in the presence of trie-
thylamine. The solution was allowed to heat under reflux for 5 h. The solution mixture
was concentrated to 3 mL under reduced pressure and cooled. The complex was sepa-
rated out upon the addition of 10 mL petroleum ether (60-80 ꢀC) and reprecipitated
from CH2Cl2/petroleum ether and dried under vacuum.
2.2.1. [RuCl2(PPh3)2L1] (1)
Green solid, M.p. 230 ꢀC. Anal calcd. for C56H44ONCl2P2Ru: C: 68.78; H: 4.50; N: 1.43.
Found: C: 68.79; H: 4.50; N: 1.44. IR (cmꢁ1): 1596 m(–C¼N), 1300 m(C–O). UV–vis, kmax
(nm): 430, 320, 260.
2.2.2. [RuCl2(AsPh3)2L1] (2)
Pale-green solid, M.p. 220 ꢀC. Anal calcd. for C56H44ONCl2As2Ru: C: 62.98; H: 4.12; N:
1.31. Found: C: 62.97; H: 4.11; N: 1.30. IR (cmꢁ1): 1597 m(–C¼N), 1305 m(C–O). UV-vis,
kmax (nm): 430, 330, 250.
2.2.3. [RuCl2(PPh3)2L2] (3)
Green solid, M.p. 250 ꢀC. Anal calcd. for C57H46O2NCl2P2Ru: C: 67.92; H: 4.56; N: 1.39.
Found: C: 67.93; H: 4.56; N: 1.40. IR (cmꢁ1): 1586 m(–C¼N), 1305 m(C–O). UV-vis, kmax
(nm): 410, 310, 270.
2.2.4. [RuCl2(AsPh3)2L2] (4)
Green solid, M.p. 230 ꢀC. Anal calcd. for C57H46O2NCl2As2Ru: C: 63.27; H: 4.25; N: 1.29.
Found: C: 63.28; H: 4.24; N: 1.28. IR (cmꢁ1): 1578 m(–C¼N), 1306 m(C–O). UV-vis, kmax
(nm): 440, 320, 250.
2.2.5. [RuCl2(PPh3)2L3] (5)
Green solid, M.p. 210 ꢀC. Anal calcd. for C60H47ONCl2P2Ru: C: 70.03; H: 4.57; N: 1.36.
Found: C: 70.02; H: 4.56; N: 1.37. IR (cmꢁ1): 1597 m(–C¼N), 1336 m(C–O). UV-vis, kmax
(nm): 440, 310, 265.

4
R. NANDHINI AND G. VENKATACHALAM
EPh3
Ru
Cl
Cl
RuCl3(EPh3)3
N
E = P or As
N
H
H
Benzene/ Et3N/ 5h
OH
R
O
R
EPh3
R = H (L1);
P (1) or As (2)
R = H (HL1)
R = OCH3 (HL2)
R = OCH3 (L2); P (3) or As (4)
EPh3
Cl
RuCl3(EPh3)3
E = P or As
Benzene/ Et3N/ 5h
H
N
H
N
Ru
O
OH
Cl
EPh3
L3; P (5) or As (6)
HL3
Scheme 1. Synthesis of ruthenium(III) Schiff base complexes.
2.2.6. [RuCl2(AsPh3)2L3] (6)
Green solid, M.p. 200 ꢀC. Anal calcd. for C60H47ONCl2As2Ru: C: 64.40; H: 4.20; N: 1.25.
Found: C: 64.41; H: 4.20; N: 1.25. IR (cmꢁ1): 1597 m(–C¼N), 1336 m(C–O). UV-vis, kmax
(nm): 420, 310, 272.
2.3. Typical procedure for transfer hydrogenation of ketones
The mixture of a ketone (5.0 mmol) and base (0.03 mmol) containing the catalyst
(0.2 mol %), i-PrOH (6 mL) was refluxed for 5 h. After completion of the reaction, the
solvent was removed under reduced pressure. The catalyst was removed by the add-
ition of 15 mL of petroleum ether followed by filtration and subsequent neutralization
with dilute HCl. The combined organic fractions were dried over anhydrous Na2SO4.
Percentage of conversion was determined by GC analysis of the crude mixture and
compared with the authentic samples.
3. Results and discussion
The reactions of 1:1 molar ratio of [RuX3(EPh3)3] (X ¼ Cl, E ¼ P or As) and Schiff base
ligands (HL1-HL3) in dry benzene afforded new six-coordinate low spin ruthenium(III)
Schiff base complexes. The reaction proceeds as shown in Scheme 1. All the

JOURNAL OF COORDINATION CHEMISTRY
5
1
2
3
4
5
6
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
Wavelength (nm)
Figure 1. UV-vis spectra of 1–6 in CHCl3 (10ꢁ5 M).
complexes are air-stable, non-hygroscopic in nature, insoluble in water and highly sol-
uble in common solvents such as dichloromethane, acetonitrile and chloroform. The
data obtained from elemental analysis are in good agreement with the compositions
proposed for the molecular structure of the ruthenium complexes.
3.1. Characterization of the complexes
The IR spectra of the free Schiff base ligands display a strong band at 1610-1615 cmꢁ1
that is attributed to the m(C¼N) stretching. Coordination of the Schiff bases to the
ruthenium ion through the azomethine nitrogen atom is expected to reduce the elec-
tron density in the azomethine link. Hence, this band undergoes a shift to lower fre-
quency to 1578–1597 cmꢁ1 after complexation, indicating coordination of the
azomethine nitrogen to ruthenium atom [35, 36]. This fact is further supported by
increase in the absorption frequency of the phenolic m(C-O) band from
1276–1296 cmꢁ1 in the free ligands to 1300–1336 cmꢁ1 in the ruthenium complexes,
confirming that the other coordination site of Schiff base is phenolic oxygen in all the
complexes. FT-IR of 1–6 are given in Figures S1–S6.
Electronic spectra of 1-6 have been recorded in chloroform and the representative
spectra are depicted in Figure 1. Complexes showed a moderate intense band in the
visible region. The absorption bands around 420–440 nm are due to metal-to-ligand
charge-transfer (MLCT) transition [37, 38]. The high intensity bands around
ꢂ
ꢂ
250–270 nm and 310–330 nm were assigned to p–p and n–p transitions, respectively,
due to non-bonding electron present on the nitrogen of the azomethine group of
the ligands.
The electron paramagnetic resonance spectral studies are performed both in pure
powder samples and in solution at room temperature and liquid nitrogen

6
R. NANDHINI AND G. VENKATACHALAM
Table 1. Transfer hydrogenation of acetophenone to 1-phenylethanol catalyzed by com-
plexes (1–6).a
OH
O
complexes (1-6)
0.2 mol%
i-PrOH/ KOH/ 5h/ 80oC
Entry
Complex
Conversionb (%)
1
2
3
4
5
6
7
1
2
3
4
5
6
–
99
85
80
75
72
70
No desired product
aReaction conditions: reactions were carried out using acetophenone (5 mmol), catalyst (0.2 mol%), base (0.03 mmol).
bConversion was monitored by GC analysis and are average of two runs.
temperature, respectively, at X-band frequency. Complexes 1-3 exhibit well-defined
single isotropic feature near g ¼ 2.10 to 2.17 at room temperature. However, the EPR
spectral profiles of the complexes in dichloromethane solution at 77 K show rhombic
spectra with three different ‘g’ values (gx¼ gy¼ gz) (Figures S7–S9). Overall the pos-
ition of lines and nature of the EPR spectra of the complexes are characteristic of low
spin ruthenium(III) octahedral complexes [39–42]. The electrospray ionization (ESI)
mass spectra of 1 and 6, obtained in positive mode, give peaks at m/z ¼ 977 and
1118, respectively (Figures S10 and S11). The loss of one chloride atom for ruthenium
complexes is quite common under ESI-MS conditions [43–45].
3.2. Catalytic transfer hydrogenation of ketones
Catalytic transfer hydrogenation is being increasingly used in industry because of its
selectivity, efficiency, scope, simplicity, economic viability and also the growing aware-
ness of the need for green chemistry. In general, recent aims have been to improve
catalyst turnover and selectivity, either through more active catalysts or ones that can
be recycled, to enable lower costs and higher purity products, as well as more pro-
ductive, less wasteful processes. Ruthenium complexes catalyzed transfer hydrogen-
ation reactions require the Ru-H an intermediate species and hence complexes with
Ru-H bond or with potential for the in-situ formation of such a bond are suitable can-
didates for these catalytic reactions. Given the propensity toward in situ formation of
ruthenium-hydrido species in the present family of complexes, we wish to explore
their catalytic activity in transfer hydrogenation of ketones. In order to optimize the
reaction conditions, the effect of solvents, bases, time, temperature and catalyst:sub-
strate ratios were studied.
We started our study by examining the transfer hydrogenation of acetophenone to
1-phenylethanol using synthesized ruthenium Schiff base complexes. All complexes
efficiently catalyze the transfer hydrogenation of acetophenone with maximum con-
version within 5 h. Among the tested complexes, 1 is highly efficient in the transfer
hydrogenation of ketones to alcohol with high conversion of 99%. The result of

JOURNAL OF COORDINATION CHEMISTRY
7
Table 2. Effect of the catalyst, solvent and base.a
O
OH
Complex 1
solvent/ base/ 5h/ 80oC
Entry
Catalyst (mol%)
Solvent
Base
Conversionb(%)
1
2
3
4
5
6
7
8
0.2
0.1
0.05
0.02
0.2
–
0.2
0.2
0.2
0.2
0.2
0.2
0.2
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
Methanol
Ethanol
KOH
KOH
KOH
KOH
–
99
81
70
45
0
KOH
0c
NaOH
Na2CO3
K2CO3
Et3N
Pyridine
KOH
96
65
53
Trace
Trace
45
9
10
11
12
13
KOH
30
aReaction conditions: reactions were carried out using acetophenone (5 mmol), catalyst, base (0.03 mmol), solv-
ent (6 mL).
bConversion determined by GC analysis.
cReaction carried out in the absence of catalyst.
transformations is given in Table 1. The lowest conversion observed is 72% for 6. The
catalytic process is more efficient with 1 followed by 2, 3, 4, 5 and 6 showing the
least activity among all six complexes. It was observed that the efficiency of catalysts
was simply dependent on the structure of Schiff base ligands. The electron donating
substituent (-OCH3) group present in the Schiff base ligand in 2 and bulky naphthyl
group in 3 shows less activity than the catalyst 1. Further, it is observed that the com-
plex catalyst containing PPh3 group shows higher activity than the complexes possess-
ing AsPh3.
The effect of methanol, ethanol and isopropanol on the hydrogenation of aceto-
phenone was investigated. When no catalyst was added, the blank reaction with solv-
ent isopropanol exhibited no conversion in 5 h (entry 7, Table 1).
The base also plays an essential role, and in the absence of a base, no product was
formed (entry 5, Table 2). KOH was found to be the effective base and its substitution
by NaOH, Na2CO3 and K2CO3 led to a significant drop in yield. The results propose
that KOH/isopropanol was preferred base/solvent system for transfer hydrogenation of
ketones under our experimental conditions.
The reaction proceeds with high conversion 99% when the catalyst loading was
0.2 mol %. When decreasing the catalyst loading to 0.1, 0.05 mol %, the reaction pro-
ceeds with good conversions 81% and 70%, respectively. Further, the catalysis works
well with low loading of catalyst 0.02 mol % and shows conversion of 45%. Thus, it
was concluded that the catalyst loading of 0.2 mol % of 1 is the best compromise
between optimum reaction rates. The results are collected in Table 2 and the conver-
sions reported are averages of two runs i7n the case of all catalytic reaction.
To examine the generality of 1 as a catalyst for transfer hydrogenation, the scope
of ketones with different electronic effect was investigated under similar conditions;
the results are summarized in Table 3. Acetophenone with substituents of varying
electronic properties were efficiently reduced to the corresponding secondary alcohols

8
R. NANDHINI AND G. VENKATACHALAM
Table 3. Transfer hydrogenation of various ketones with complex (1).a
OH
O
Complex catalyst 1
(0.2mol%)
i-PrOH/ KOH/ 5h/ 80oC
R1
R1
R2
R2
Entry
1
Substrate
Product
Conversionb (%)
TONc
495
O
OH
99
92
80
O
OH
2
3
460
400
O
OH
H3C
H3C
O
OH
4
5
6
78
92
95
390
460
475
H3CO
H3CO
O
OH
Cl
Br
Cl
O
OH
Br
O
OH
7
8
9
98
89
93
490
445
465
O2N
O2N
O
O
OH
OH
O
OH
OH
OH
OH
10
11
12
13
91
96
85
82
455
480
425
410
O
O
O
aReaction conditions: ketone (5 mmol), catalyst (0.2 mol%, base (0.03 mmol), i-PrOH (6 mL).
bConversion was monitored by GC analysis and are average of two runs.
cTON ¼ Turnover number ¼ ratio of moles of product formed to moles of catalyst used.

JOURNAL OF COORDINATION CHEMISTRY
9
in good conversions. The conversion of acetophenone to 1-phenyl ethanol is obtained
in 99% conversion (entry 1). The presence of electron-withdrawing or electron-donat-
ing substituents on the aromatic ring has significant effect in the conversion. Aromatic
halo-substituents have an enhancing effect on the catalytic activity, showing quantita-
tive results on the formation of the corresponding alcohols (entries 5, 6 and 7) and
catalyzed with good conversions of 92%, 95% and 98%, respectively. Whereas, elec-
tron-donating substituents (entries 3 and 4) on the ring gave lower conversions to the
corresponding alcohols are 80% and 78%, respectively, than that of acetophenone.
Cyclic ketones containing five- and six-membered rings were reduced effectively to
give the corresponding alcohols with conversions of up to 89% and 93%. The complex
catalyst also efficiently catalyzed the reduction of aliphatic ketones such as 2–buta-
none, 2–pentanone, diethyl ketone and 4–methylpentan–2–one to their corresponding
secondary alcohols in 91%, 96%, 85% and 8 2% yield, respectively.
It is well known from the literature that the mechanism of transfer hydrogenation
of ketones by ruthenium complexes has been proposed to involve the formation of
ruthenium(II)-hydride as catalytic intermediate [46] and the mechanism involving
Ru(III) complexes is not known. However, the present Ru(III) Schiff base complexes
show an excellent catalytic performance with high conversions and the catalytic activ-
ity is comparable to the reported ruthenium complexes in terms of conversion, reac-
tion time and catalyst loading [47].
4. Conclusion
The new family of ruthenium(III) fluoreneamine based Schiff base complexes of the
type [RuCl2(EPh3)2L] (E ¼ P, As and L ¼ O,N-donor) were synthesized. The characteriza-
tion of the complexes was accomplished by analytical, spectral (FT-IR, UV-vis, EPR) and
ESI-MS analyses. The bulky bidentate fluoreneamine Schiff base ligand, twoPPh3/AsPh3
and two chlorine atoms are coordinated to ruthenium metal which forms octahedral
geometry in these complexes. The catalytic studies showed that the synthesized ruthe-
nium complexes are effective for the transfer hydrogenation of a range of ketones.
Further, the synthesis of ruthenium complexes containing different types of fluorene-
amine-based bulky Schiff base ligands are under process.
Disclosure statement
No potential conflict of interest was reported by the author(s).
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