Ru(II) mediated C[sbnd]H activation of 1-(biphenylazo)naphtholSynthesis and catalytic evaluation for transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.inoche.2017.03.029

New cyclometalated ruthenium(II) complexes of the type [Ru(L)(CO)(EPh₃)₂] (L?=?di-anionic CNO- donor of 1-(biphenylazo)naphthol; E?=?P, As) have been synthesized by the reaction using [RuHCl(CO)(EPh₃)₃] (E?=?P, As) with 1-(biphenylazo)naphthol ligand (H₂L). The 1-(biphenylazo)naphthol ligand and ruthenium complexes are characterized by analytical, spectral (FT–IR, UV–Vis, ¹H NMR and ³¹P NMR) methods. The molecular structure of ruthenium complex 1 was further confirmed by single crystal X-ray diffraction method. The catalytic efficiency of ruthenium complex 1 was evaluated for the transfer hydrogenation of various ketones to alcohols with excellent conversion up to 99% in the presence of i-PrOH/KOH at 82?°C.

Full text of DOI:10.1016/j.inoche.2017.03.029

Accepted Manuscript
Ru(II) mediated C-H activation of 1-(biphenylazo)naphthol:
Synthesis and catalytic evaluation for transfer hydrogenation of
ketones
Madhan Ramesh, Ganesan Prabusankar, Galmari Venkatachalam
PII:
DOI:
Reference:
S1387-7003(17)30163-6
doi: 10.1016/j.inoche.2017.03.029
INOCHE 6596
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
24 February 2017
15 March 2017
25 March 2017
Please cite this article as: Madhan Ramesh, Ganesan Prabusankar, Galmari Venkatachalam
, Ru(II) mediated C-H activation of 1-(biphenylazo)naphthol: Synthesis and catalytic
evaluation for transfer hydrogenation of ketones. The address for the corresponding author
was captured as affiliation for all authors. Please check if appropriate. Inoche(2016), doi:
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ACCEPTED MANUSCRIPT
Ru(II) mediated C‒H activation of 1-(biphenylazo)naphthol: Synthesis and
catalytic evaluation for transfer hydrogenation of ketones
Madhan Ramesha, Ganesan Prabusankarb, Galmari Venkatachalam a, *
aPG & Research Department of Chemistry, Government Arts College, Dharmapuri – 636 705,
Tamil Nadu, India.
bDepartment of Chemistry, Indian Institute of Technology Hyderabad, ODF Estate,
Yeddumailaram, TS 502 205, India.
To whom correspondence should be addressed, e-mail: gvchem@gmail.com [G. Venkatachalam]
Abstract
New cyclometalated ruthenium(II) complexes of the type [Ru(L)(CO)(EPh3)2] (L =
di-anionic CNO- donor of 1-(biphenylazo)naphthol; E = P, As) have been synthesized by the
reaction using [RuHCl(CO)(EPh3)3] (E = P, As) with 1-(biphenylazo)naphthol ligand (H2L).
The 1-(biphenylazo)naphthol ligand and ruthenium complexes are characterized by analytical,
1
spectral (FT‒IR, UV‒Vis, H NMR and 31P NMR) methods. The molecular structure of
ruthenium complex 1 was further confirmed by single crystal X–ray diffraction method. The
catalytic efficiency of ruthenium complex 1 was evaluated for the transfer hydrogenation of
various ketones to alcohols with excellent conversion up to 99% in the presence of i-PrOH/KOH
at 82 oC.
Keywords: C−H activation; Ruthenium(II) complexes; Crystal structure; Transfer hydrogenation

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Cyclometalation has become an important part of organometallic chemistry and the
application of organometallic compounds to organic transformation has considerable interest
[1‒4]. Over the past few years, cyclometalated ruthenium complexes have opened up enormous
possibilities for variety of organic processes [5] due to great advances in the design and synthesis
of new and efficient catalysts. The responsible for these progresses are mainly by introduced
ligands, particularly with O- and N- donors. Some ligands induce unprecedented catalyst
tolerance towards organic functional groups, air and moisture, thus expanding the scope of
utilization of corresponding metal complexes. It is reported that the O- and N- donors of the azo
ligands play an important role in the catalytic reactions particularly with transfer hydrogenation
reactions [6]. Further, substituents on the nitrogen atom of the azo moiety also play an important
role in tuning steric and electronic properties of the molecule [7].
The transfer hydrogenation of carbonyl compounds using 2-propanol as a hydrogen
source to alcohols is one of the most applied processes in organic chemistry. Ruthenium-based
catalytic systems are found to be effective in the transfer hydrogenation of ketones [8] and
imines [9,10]. Verpoort and co-workers reported that ruthenium azonaphthol complexes were
used as efficient catalysts for olefin isomerization reactions [11]. Furthermore, ruthenium
complexes containing CNN- donor ligands (A & B) have been used as catalysts for the transfer
hydrogenation of ketones [12] (Fig. 1). When compared to catalyst A, catalyst B shows poor
catalytic activity and it is due to less labile nature of ligand in the catalyst. The interest behind
the choice of present 1-(biphenylazo)naphthol ligand in the cyclometalated ruthenium(II)
complexes is that it can also act as di-anionic tridentate CNO- donor which generates a five-
membered metallacycles. The bulky biphenyl and naphthol moiety provides greater steric
crowding compared to the phenyl ring in these complexes.

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O
Ph3P
Ru
N
N
N
N
N
I
N
PPh3
N
H2N
CO
Ru
Ru
N
Ph3P
I
PPh3
Cl
CO
nC7H15
A
B
C (this work)
Fig. 1. Models of Ru-complexes.
Herein, we describe the synthesis of new air stable ruthenium(II)
1-(biphenylazo)naphtholate complexes incorporated with carbonyl and
triphenylphosphine/triphenylarsine as ancillary ligands. The characterization of the
1
complexes was investigated by spectral (FT-IR, UV–Vis, H NMR and 31P NMR) and single
crystal X–ray diffraction method. Further, the catalytic efficiency of complex 1 was
examined for the transfer hydrogenation of various ketones. To the best of our knowledge,
there are no reports available for catalytic transfer hydrogenation of ketones by this type
ruthenium(II) 1-(biphenylazo)naphthol complexes so far.
EPh3
EPh3
OH
O
O
RuHCl(CO)(EPh3)3
E = P, As
N
N
CHCl3/ light
N
N
Ru
N
Ru
CO
Cl
N
Toluene/ 6 h
Et3N
EPh3
E = P (1), As (2)
EPh3
E = P (3), As (4)
H2L
Scheme 1. Synthesis of cyclometalated ruthenium(II) 1-(biphenylazo)naphtholate complexes.
New 1-(biphenylazo)naphthol ligand (H2L) was prepared by coupling diazotized
2-aminobiphenyl with β-naphthol (see Supplementary material). The reaction of

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[RuHCl(CO)(EPh3)3] (E = P, As) [13,14] with 1-(biphenylazo)naphthol ligand (H2L) in 1:1
molar ratio refluxing in toluene for 6 h in the presence of Et3N afforded a green colored
cycometalated complexes of the composition [Ru(L)(CO)(EPh3)2] (E = P, As) (L = di-anionic
tridentate CNO- ligand (Scheme 1). The oxidation state of ruthenium remains unchanged during
the formation of cyclometalated species. The exclusive formation cyclometalated complexes are
due to steric hindrance of bulkier ligand which did not allow another (H2L) within the
coordination sphere. The successful ruthenium(II) mediated C‒H activation of the biphenyl
moiety in the 1-(biphenylazo)phenol ligand (H2L) and trapping of the resulting CNO- ligated
ruthenium(L) fragment by triphenylphosphines/triphenylarsine prompted us to check the
feasibility of donor atoms. These complexes are readily soluble in common organic solvents like
CHCl3, CH2Cl2, CH3CN and C6H6 etc., Preliminary characterization by microanalysis of the
isolated complexes supports the expected composition.
The IR spectra of the complexes display a band of azo group ν(N=N) around 1399 cm‒1
which is slightly lower frequency than that of free 1-(biphenylazo)naphthol ligand (H2L)
1447 cm‒1 indicating coordination of azo nitrogen to ruthenium(II) metal ion. The band
corresponding to naphtholic ν(C‒O) stretching frequency 1275 cm‒1 in (H2L) is shifted to higher
frequency in the region 1298 cm‒1 in these complexes is further conforming the coordination of
oxygen atom to ruthenium(II) ion. In addition, a strong sharp signal observed at 1903 cm‒1 has
been assigned to carbonyl group ν(C≡O) in these complexes [15]. The IR spectra of the ligand
(H2L) and complexes are given in Figs. S1–S3 (see supplementary material). The UV–Vis
spectra of the complexes 1 and 2 in CHCl3 solution exhibited well characteristic absorptions near
450 nm and 700 nm regions. The absorption appears at 450 nm is probably due to MLCT
transitions and the intense absorptions in the visible region of 700 nm are attributed to d–d

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transitions. Representative UV–Vis spectrum of the complexes 1 and 2 are given in Figs. S4‒S5
(see supplementary material).
The 1H NMR spectra of the complexes were recorded in CDCl3. The aromatic protons of
the complexes appeared in the range δ = 7.4–6.9 ppm. A sharp singlet appeared for OH proton of
1-(biphenylazo)naphthol ligand in the region δ = 11.9 ppm is absent in these complexes
indicating the coordination of oxygen atom. Further, 31P NMR spectrum of complex 1 was
recorded to confirm the presence of PPh3 groups which was observed as a sharp singlet at
1
29.13 ppm. Therefore, the H NMR spectral data of the complexes confirms the coordination of
tridentate CNO- donor mode of the 1-(biphenylazo)naphthol ligand to ruthenium(II) ion.
Relevant 1H NMR & 31P NMR spectra are given in Figs. S6–S9 (see supplementary material).
Attempts made to obtain a single crystals of complex 1 suitable for X‒ray structure
determination by recrystallization using chloroform as solvent over a long period of time and in
the presence of light resulted in oxidation Ru(II) to Ru(III) (3) and the replacement of CO group
by Cl ligand (Scheme 1). The molecular structure of 3 is depicted (Fig. 2). Crystal data and
structure refinement for molecule 3 is shown (Table S10 see supplementary material). Molecule
3 is a monomer. The ruthenium center in 3 is in distorted octahedral geometry. The coordination
environment of ruthenium is satisfied by two PPh3, one Cl-, and L2- ligands. The two PPh3
groups are in trans positions. As shown (Fig. 2) the L2- ligand is coordinated to the ruthenium
metal via O, N and C- donors as tridentate fashion. The bond angles for ruthenium metal center
at two five membered chelate rings with bite angles of 77.8(4)o for N(1)–Ru(1)–O(1) and
88.15(19)o for Cl(1)–Ru(1)–P(2). The Ru–C(11) bond length is 2.112(13) Å. The Ru–O(1) bond
distance is 2.140(8) Å. The Ru–N(1) bond distance is 2.049(9) Å. The Ru–P bond distances are
nearly comparable (Ru(1)–P(1), 2.370(3) Å and Ru(1)–P(2), 2.376(3) Å.

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Fig. 2. (I) The molecular structure of 3. The hydrogen atoms have been omitted for clarity. (II)
The core structure of 3. The non-coordinating atoms were omitted for clarity.
The catalytic transfer hydrogenation of ketones to alcohols using complex 1 was
performed in the presence of i-PrOH/KOH. In order to prove the optimized reaction conditions,
different catalyst: substrate ratios were tested and the results summarized in Table 1.
Benzophenone was chosen as a test-substrate to explore the catalytic activity of complex 1. It
was observed that C/S ratio of 1:500 is best compromise for good conversion (99%) under mild
optimal reaction conditions. A blank experiment was carried out either in the absence of catalyst

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or base gave hydrogenation of benzophenone. According to optimization results, the catalytic
transfer hydrogenation involving a variety of aliphatic and aromatic ketones was carried out
under inert atmosphere by a mixture containing the ketone (1 mmol), complex catalyst (0.2
o
mol%) and base (0.005 mmol), heated to reflux for 2 h in 5 mL of i-PrOH at 82 C. The
catalyst was removed from the reaction mixture by the addition of light petroleum followed by
filtration and subsequent neutralization with 1M HCl. The ether layer was filtered through a
short path of silica gel and condensed under reduced pressure in vacuum to afford corresponding
alcohols, conversion of the product was determined by comparison with GC analysis of authentic
samples. The conversion of benzophenone to benzhydrol catalyzed by complex 1 versus reaction
time was plotted in Fig. 3. The yield of benzhydrol gradually increased to 42% in 0.5 h and
reached 99% conversion within 2 h.
Table 1. Effect of catalyst/substrate/base ratioa
O
OH
Complex catalyst (1)
(0.2 mol%)
i-PrOH/KOH/2h
Entry Substrate/ catalyst/ base ratios Time (h) Conv. (%) TONb
1
2
3
4
5
1500
1000
500
1
1
1
‒
1
2.5
2.5
2.5
2.5
‒
12
12
2
32
67
99
‒
160
335
495
‒
500
2
500
2
‒
‒

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a Conditions: reactions were carried out at 82 oC using different catalyst/substrate ratio in 5 mL
of isopropanol and KOH.
b GC analysis with authentic samples.
c TON = ratio of moles of product formed to moles of catalyst used.
100
90
80
70
60
50
40
0.5
1.0
1.5
2.0
2.5
3.0
Reaction time (h)
Figure 3. Influence of reaction time on the formation of benzhydrol.
The choice of solvent influences was chosen for the optimization reaction condition. The
rates of reaction were strongly dependent on the solvent as it is employed as hydrogen donor.
The influence of solvents was investigated by use of methanol, ethanol and isopropanol. As seen
(Table 2) the use of isopropanol is found to be efficient one to obtain the maximum conversion
of benzhydrol from benzophenone.

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Table 2. Effect of solventsa
O
OH
Complex catalyst (1)
(0.2 mol%)
i-PrOH/KOH/2h
___________________________________________
Entry Solvents
Conv. (%) b TONc
___________________________________________
1
2
3
Methanol
Ethanol
18
24
99
90
120
495
Isopropanol
__________________________________________
a Conditions: reactions were carried out at 82 oC using substrate (1mmol), catalyst (0.2 mol%) in
5 mL of solvents and base (0.005 mmol).
b GC analysis with authentic samples.
c TON = ratio of moles of product formed to moles of catalyst used.
A plausible mechanism is proposed (Scheme 2) to demonstrate Ru‒hydride is the
catalytic active species. To support the mechanistic proposal, the complex 1 was refluxed for 2 h
1
with KOH in i-PrOH to yield the complex 5. The H NMR spectrum of this complex shows a
triplet at δ = -8.5 ppm, indicating the presence of Ru‒H bond in the complex 5 (Fig. S11 see
supplementary material). The results of the catalytic activity are shown (Table 3). The complex 1
catalyze the ketones to corresponding alcohols with good to excellent conversion (>90%) in most
cases within 2 h with TON/TOF value up to 495/247 h‒1 (entry 2). Electron withdrawing group
present in the aryl ring of acetophenone increases the rate of the reaction (entry 5‒7) whereas
donating group (entry 3 and 4) decrease it reasonably. Further, benzophenone is reduced into
corresponding alcohol up to 99% conversion. In addition, the complex efficiently catalyzes the

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reduction of five, six-membered cyclic and aliphatic ketones into the corresponding alcohols up
to 98% conversion.
The work up process is very simple for this catalytic system as the catalyst is stable in all
organic solvents and it can be recovered. L. T. Ghoochany et al.[16] reported that ruthenium
complex containing symmetrical NNN‒ donors and CO ligand shows no catalytic activity for
transfer hydrogenation reaction and it is due to unavailability of labile PPh3 ligands. Further, the
present catalytic system exhibited remarkably high catalytic activity when compared to the
catalytic activity of the catalyst B (Fig. 1) containing CNN‒ donor ligand and other ruthenium
complexes [17] under similar conditions. The enhancement of the catalytic activity of the present
complex 1 is may be due to the unsymmetrical environment around the metal center by the
tridentate CNO- donor ligand in addition to labile PPh3 ligands.
HO
H
O
HO
Ph3P
Ru
H
R'
N
R
N
CH3
CO
H3C
PPh3
1
H
CH3
CH3
H
O
O
Ph3P
Ru
Ph3P
Ru
N
N
O
O
CO
R'
N
N
H
H
R
CO
PPh3
PPh3
O
Ph3P
N
H
O
R
N
R'
Ru
H
CO
O
CH3
PPh3
H3C
5
Scheme 2. Probable mechanism for the observed transfer hydrogenation by the complex
[Ru(L)(CO)(PPh3)2] (1).

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Table 3. Transfer hydrogenation of ketones using [Ru(L)(CO)(PPh3)2] (1)a.
Complex catalyst (1)
OH
0.2 mol%
O
i-PrOH/ KOH/2h
R1
R1
R2
R2
Entry
Substrates
O
Products
Conv. (%)
96
TONb
TOFc
OH
240
1
2
480
OH
O
247
195
495
390
99
78
OH
O
3
H3C
H3C
OH
OH
O
O
380
485
190
242
76
91
4
5
H3CO
Cl
H3CO
Cl
OH
OH
O
O
222
445
89
6
7
Br
Br
225
230
235
450
460
90
92
O2N
O2N
O
O
OH
OH
8
9
470
94
O
O
OH
OH
10
11
92
98
460
490
230
245

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a Conditions: reactions were carried out at 82 oC using 1 mmol of ketone, 0.2 mol% catalyst in
5 mL i-PrOH.
Catalyst/substrate/KOH ratio is 1:500:2.5.
Conversion was determined by GC analysis with an authentic samples.
b TON = ratio of moles of product formed to moles of catalyst used.
c TOF = TON/h.
In conclusion, the ruthenium precursor [RuHCl(CO)(PPh3)3] and [RuHCl(CO)(AsPh3)3]
are effectively mediates C‒H activation of 1-(biphenylazo)naphthol and two new cyclometalated
ruthenium(II) complexes have been obtained. The characterization of the complexes was
accomplished by analytical, spectral (FT–IR, UV–Vis, 1H NMR and 31P NMR) and single crystal
XRD analysis. Ruthenium complex 1 was investigated as catalyst and turned out to be efficient
catalyst for the transfer hydrogenation of various ketones.
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Supplementary data
The experimental section, single crystal X-ray study, crystal data and structure
refinement parameters for the molecule 3, FT-IR, UV-Vis, 1H NMR and 31P NMR spectra of the
ligand and complexes are associated with this article. CCDC 1530992 contains the
supplementary crystallographic data for this paper. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif or
from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.

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Graphical Abstract
______________________________________________________________________________
Ru(II) mediated C‒H activation of 1-(biphenylazo)naphthol: Synthesis and
catalytic evaluation for transfer hydrogenation of ketones
Madhan Ramesha, Ganesan Prabusankarb, Galmari Venkatachalam a, *

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Highlights
 New air stable of Ru(II) 1-(biphenylazo)naphtholate complexes has been reported.
 Carbonyl group replaced by using CHCl3/light, recrystallization method.
 Structure of the complex was confirmed by single crystal XRD.
 Synthesized complex as efficient catalyst (0.2 mol%) for transfer hydrogenation.