Arene-ruthenium complexes with 2-(arylazo)phenol as ancillary ligand: Synthesis, characterization, and utilization in catalytic transfer-hydrogenation

Article abstract of DOI:10.1016/j.poly.2019.02.046

Reaction of 2-(arylazo)phenols (HL-R, where H represents the phenolic proton and, R = CH₃, H and Cl) with [{Ru(p-cymene)Cl₂}₂] in the presence of triethylamine affords a group of three reddish-brown complexes of type [Ru(p-cymene)(L-R)Cl] in good yields. Structure of [Ru(p-cymene)(L-CH₃)Cl] has been determined by X-ray crystallography. The 2-(arylazo)phenolate ligand (L-R) is coordinated to the metal center as a mono-anionic bidentate N,O-donor forming five-membered chelate ring. All the complexes are diamagnetic, and show characteristic ¹H NMR signals. They also show intense absorptions in the visible and ultraviolet regions, which have been analyzed by TDDFT calculations. Cyclic voltammetry on the complexes shows two successive irreversible oxidations within 1.10–1.40 V versus SCE. The [Ru(p-cymene)(L-R)Cl] complexes are found to serve as efficient catalyst-precursor for the transfer-hydrogenation of aldehydes.

Full text of DOI:10.1016/j.poly.2019.02.046

Accepted Manuscript
Arene-ruthenium complexes with 2-(arylazo)phenol as ancillary ligand: Syn-
thesis, characterization, and utilization in catalytic transfer-hydrogenation
Jit Karmakar, Samaresh Bhattacharya
PII:
S0277-5387(19)30147-0
https://doi.org/10.1016/j.poly.2019.02.046
POLY 13790
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
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23 February 2019
Please cite this article as: J. Karmakar, S. Bhattacharya, Arene-ruthenium complexes with 2-(arylazo)phenol as
ancillary ligand: Synthesis, characterization, and utilization in catalytic transfer-hydrogenation, Polyhedron (2019),
doi: https://doi.org/10.1016/j.poly.2019.02.046
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Manuscript No.:
POLY-D-19-00042
Revised)
(
Arene-ruthenium complexes with 2-(arylazo)phenol as
ancillary ligand: Synthesis, characterization, and utilization
in catalytic transfer-hydrogenation
Jit Karmakar, Samaresh Bhattacharya*
Department of Chemistry, Inorganic Chemistry Section,
Jadavpur University, Kolkata 700 032, India
ABSTRACT
Reaction of 2-(arylazo)phenols (HL-R, where H represents the phenolic proton
and, R = CH , H and Cl) with [{Ru(p-cymene)Cl } ] in the presence of triethylamine
3
2 2
affords a group of three reddish-brown complexes of type [Ru(p-cymene)(L-R)Cl] in
good yields. Structure of [Ru(p-cymene)(L-CH )Cl] has been determined by X-ray
3
crystallography. The 2-(arylazo)phenolate ligand (L-R) is coordinated to the metal center
as a mono-anionic bidentate N,O-donor forming five-membered chelate ring. All the
1
complexes are diamagnetic, and show characteristic H NMR signals. They also show
intense absorptions in the visible and ultraviolet regions, which have been analyzed by
TDDFT calculations. Cyclic voltammetry on the complexes shows two successive
irreversible oxidations within 1.10 - 1.40 V vs. SCE. The [Ru(p-cymene)(L-R)Cl]
complexes are found to serve as efficient catalyst-precursor for the transfer-
hydrogenation of aldehydes.
Keywords:
2
-(arylazo)phenols (HL-R)
[Ru(p-cymene)(L-R)Cl] complexes
Crystal structure
Electronic spectra
Catalytic transfer-hydrogenation
*
Corresponding author.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).

2
1
. Introduction
The current interest in the coordination complexes of ruthenium is largely due to
their catalytic and biological applications [1-6]. Such properties are dependent primarily
on the coordination environment around the metal center, and hence binding of
ruthenium by ligands of selected types is of significant importance. Herein we have
chosen a group of three 2-(arylazo)phenols as the principal ligand, which are abbreviated
in general as HL-R, where H depicts the phenolic proton and R the para-substituent in
the pendent phenyl ring. The three different substituents (R = CH , H and Cl), with
3
different electron-withdrawing properties, have been chosen to study their influence, if
any, on the redox properties of the resulting complexes. The 2-(arylazo)phenols usually
bind to a metal center, via dissociation of the phenolic proton, as bidentate N,O-donors
forming either five-membered (I) or six-membered (II) chelate rings [7-10]. In the
H C
3
O
N
OH
O
M
M
N
H C
3
N
N
H C
3
N
N
R = CH , H, Cl
3
R
R
R
HL-R
I
II
present work, which has originated from our continued interest in ruthenium complexes
having targeted coordination environments, with particular reference to their possible
utilization in catalysis [11-16], the primary objective has been to synthesize a family of
mixed-ligand arene-ruthenium complexes incorporating the chosen 2-(arylazo)phenols,
and explore their catalytic potential. In order to prepare the targeted Ru-arene-2-
(
arylazo)phenol complexes, [{Ru(p-cymene)Cl } ] has been selected as the source of
2 2
ruthenium-arene moiety because of its demonstrated ability to serve as an efficient
starting material for the synthesis of mixed-ligand complexes of type [Ru(p-cymene)(L-
Lʹ)Cl] (L-Lʹ depicts a bidentate chelating ligand) [17-20]. Reaction of the selected 2-
(
arylazo)phenols with [{Ru(p-cymene)Cl } ] has indeed afforded a group of complexes
2 2
of the expected type. The chemistry of all these complexes is reported herein, with
special reference to their formation, characterization, and catalytic application.

3
2
. Experimental
2
.1. Materials
Ruthenium trichloride was purchased from Arora Matthey, Kolkata, India, and α-
phellandrene was procured from Sigma-Aldrich, USA. [{Ru(p-cymene)Cl } ] was
2
2
synthesized by following a reported procedure [21]. Aniline, p-toluidine, p-chloroaniline
and p-cresol were purchased from S.D. Fine-Chem Limited, Mumbai, India. The 2-
(arylazo)phenols were prepared by coupling respective diazotized aniline with p-cresol.
Tetrabutylammonium hexaflurophosphate (TBHP), obtained from Aldrich, and AR grade
acetonitrile, procured from Merck, India, were used for electrochemical work. All other
chemicals and solvents were reagent grade commercial materials and were used as
received.
2
.2. Syntheses of complexes
The [Ru(p-cymene)(L-R)Cl] complexes were prepared by following a general
procedure. Specific details are given below for a particular complex.
[
Ru(p-cymene)(L-CH )Cl]: The HL-CH ligand (38 mg, 0.17 mmol) was
3 3
dissolved in 30 mL of methanol, and to it was added a solution of [{Ru(p-cymene)Cl } ]
2
2
1
(
50 mg, 0.08 mmol) in chloroform (30 mL), followed by triethylamine (17 mg, 0.17
mmol). The reaction mixture was refluxed for 4 h. The solvents were removed under
reduced pressure to yield a dark solid, which was subjected to purification by thin layer
chromatography on a silica plate. With 1:5 acetonitrile-benzene as the eluant, a reddish-
brown band separated, which was extracted with acetonitrile. Evaporation of the
acetonitrile extract gave [Ru(p-cymene)(L-CH )Cl] as a dark crystalline solid. Yield:
3
7
5
1
8%. Anal. Calcd for C H N O Cl Ru: C, 58.12; H, 5.44; N, 5.65. Found: C, 58.06; H,
24 27 2 1 1
+
.46; N, 5.58 %. MS (ESI), positive mode: [M - Cl] , 460. IR: 2958, 2920, 1614, 1491,
-1
1
2
384, 1310, 1242, 1136 and 1111 cm . H NMR: 7.78 (d, 1H, J = 8.5 Hz), 7.60 (d, 1H, J
8.0 Hz), 7.48 (s, 1H), 7.34 (d, 1H, J = 9.0 Hz), 7.17 (d, 1H, J = 7.0 Hz), 6.96 (d, 1H, J =
.5 Hz), 6.89 (d, 1H, J = 8.5 Hz), 5.47 (d, 1H, J = 6.0 Hz), 5.12 (d, 1H, J = 6 Hz), 4.70
=
8
(
d, 1H, J = 5.5 Hz), 4.07 (d, 1H, J = 5.5 Hz), 3.47 (q, 1H, J = 7.0 Hz), 2.48 (s, CH ), 2.17
3
(
s, CH ), 2.00 (s, CH ), 1.22 (m, 2CH ).
3
3
3

4
[
Ru(p-cymene)(L-H)Cl]: Yield: 72%. Anal. Calcd for C H N O Cl Ru: C,
2
3
25
2
1
1
5
7.32; H, 5.19; N, 5.81. Found: C, 57.05; H, 5.32; N, 5.80 %. MS (ESI), positive mode:
+
-1 1
[
M - Cl] , 446. IR: 2960, 2920, 1613, 1490, 1384, 1311, 1241, 1135 and 1109 cm . H
NMR: 7.91 (d, 1H, J = 8.0 Hz), 7.71 (d, 1H, J = 7.0 Hz), 7.41 (s, 1H), 7.54-7.44 (3H)*,
.99 (d, 1H, J = 7.6 Hz), 6.89 (d, 1H, J = 8.8 Hz), 5.52 (d, 1H, J = 5.5 Hz), 5.17 (d, 1H, J
5.7 Hz), 4.72 (d, 1H, J = 5.5 Hz), 4.16 (d, 1H, J = 5.7 Hz), 3.50 (q, 1H), 2.18 (s, CH3),
.03 (s, CH ), 1.15 (m, 2CH ).
6
=
2
3
3
[
Ru(p-cymene)(L-Cl)Cl]: Yield: 74%. Anal. Calcd for C H N O Cl Ru: C,
23 24 2 1 2
5
3.48; H, 4.65; N, 5.42. Found: C, 53.40; H, 4.58; N, 5.39 %. MS (ESI), positive mode:
+
+
[
M - Cl] , 480. [M − Cl] . IR: 2924, 2854, 1575, 1496, 1429, 1273, 1141, 1083 and 516
-1 1
cm . H NMR: 7.90 (d, 1H, J = 8.6 Hz), 7.70 (d, 1H, J = 8.1 Hz), 7.52 (d, 1H, J = 8.5
Hz), 7.45 (d, 1H, J = 7.4 Hz), 7.40 (s, 1H), 6.95 (d, 1H, J = 9.4 Hz), 6.88 (d, 1H, J = 8.8
Hz), 5.51 (d, 1H, J = 6.0 Hz), 5.16 (d, 1H, J = 5.6 Hz), 4.71 (d, 1H, J = 6.0 Hz), 4.15 (d,
1
H, J = 6.0 Hz), 3.49 (m, 1H), 2.22 (s, CH ), 2.18 (s, CH ), 1.12 (m, 2CH ).
3 3 3
2
.3. Physical measurements
Microanalyses (C, H, N) were performed on a Heraeus Carlo Erba 1108 elemental
1
analyzer. Magnetic susceptibilities were measured using a Sherwood MK-1 balance. H
NMR spectra recorded in CDCl solution on a Bruker Avance DPX 300 NMR
3
spectrometer using TMS as the internal standard. IR spectra were obtained on a Perkin
Elmer Spectrum Two spectrometer with samples prepared as KBr pellets. Mass spectra
were recorded with a Micromass LCT electrospray (Qtof Micro YA263) mass
spectrometer. Electronic spectra were recorded on a PerkinElmer LAMBDA 25
spectrophotometer. Electrochemical measurements were made using a CH Instruments
model 600A electrochemical analyzer. A platinum disc working electrode, a platinum
wire auxiliary electrode and an aqueous saturated calomel reference electrode (SCE)
were used in the cyclic voltammetry experiments. All electrochemical experiments were
performed under a dinitrogen atmosphere, and the electrochemical data were collected at
2
98 K. Geometry optimization by density functional theory (DFT) method and electronic
spectral analysis by TDDFT calculation were performed using the Gaussian 09

5
(B3LYP/SDD-6-31G) package [22]. GC-MS analyses were performed using a Perkin
Elmer CLARUS 680 instrument.
2
.4. X-ray crystallography
Single crystals of [Ru(p-cymene)(L-CH )Cl] were obtained by slow diffusion of
3
toluene into a chloroform solution of the complex. Selected crystal data and data
collection parameters are given in Table 1. Data were collected on a Bruker SMART
CCD diffractometer using graphite monochromated MoK radiation ( = 0.71073 Å). X-
ray data reduction and, structure solution and refinement were done using SHELXS-97
and SHELXL-97 programs [23,24]. The structures were solved by the direct methods.
The structure refined fairly well, however, the azo (-N=N-) fragment was found to suffer
from disorder problem.
2
.5. General procedure for the catalytic transfer-hydrogenation reactions
In a typical run, an oven-dried 10 ml round-bottomed flask was charged with the
aldehyde (1 mmol), a known mol percent of the catalyst, and KOH (0.5 mmol) dissolved
in 2-propanol (5 ml). The flask was placed in a preheated oil bath at the required
temperature. After the specified time, the flask was removed from the oil bath and water
(20 ml) was added, and extracted with diethyl ether (4-10 ml). The combined organic
layers were washed with water (3-10 ml), dried with anhydrous Na SO , and filtered.
2
4
Diethyl ether was removed under vacuum and the residue obtained dissolved in hexane
and analyzed by GC-MS.

6
3
. Results and discussion
3
.1. Synthesis and characterization
As outlined in the introduction, the initial goal of the present work was to
synthesize a group of mixed-ligand arene-ruthenium complexes containing the N,O-
donor 2-(arylazo)phenolate ligand. Accordingly, reactions of the chosen 2-
(
arylazo)phenols (HL-R) were carried out with [{Ru(p-cymene)Cl } ] in the presence of
2 2
triethylamine, which afforded the targeted complexes of type [Ru(p-cymene)(L-R)Cl] in
good yields. Preliminary characterizations on the complexes were found to be consistent
with their compositions. However, for an unambiguous identification of these complexes,
with particular regard to coordination mode of the 2-(arylazo)phenolate ligand, structure
of a selected complex, viz. [Ru(p-cymene)(L-CH )Cl], was determined by X-ray
3
crystallography. The structure is shown in Fig. 1 and some relevant bond parameters are
Fig. 1
given in Table 2. The structure shows that the 2-(arylazo)phenolate ligand is coordinated
to ruthenium as a mono-anionic N,O-donor forming a five-membered chelate ring (I, M =
Ru). The p-cymene moiety is linked to ruthenium in the usual -fashion, and a chloride is
3
also bound to the metal center. All the bond distances are observed to be quite usual. As
all three [Ru(p-cymene)(L-R)Cl] complexes were synthesized similarly, and they show
similar properties (vide infra), the remaining two complexes, viz. [Ru(p-cymene)(L-H)Cl]
and [Ru(p-cymene)(L-Cl)Cl], are assumed to have similar structure as [Ru(p-cymene)(L-
CH )Cl].
3
3
.2. Spectral studies
Magnetic susceptibility measurements show that the [Ru(p-cymene)(L-R)Cl]
complexes are diamagnetic, which is consistent with the +2 oxidation state of ruthenium
6
1
(
low-spin d , S = 0) in them. H NMR spectra of the [Ru(p-cymene)(L-R)Cl] complexes
show many signals arising from the coordinated p-cymene and 2-(arylazo)phenolate
ligands, most of which could be easily identified, while few could not be clearly detected
due to overlap problem. For example, all the signals for the p-cymene ligand could be
distinctly observed in all three complexes. The methyl group in the phenolic fragment of
the 2-(arylazo)phenolate ligands is observed near 2.18 ppm in all the complexes. In the

7
[
Ru(p-cymene)(L-CH )Cl] complex, an additional methyl signal is observed at 2.48 ppm,
3
which is due to the methyl group in the arylazo fragment.
Infrared spectra of the [Ru(p-cymene)(L-R)Cl] complexes show many bands of
-1
different intensities in the 450-4000 cm region, and assignment of the bands to specific
vibrations has not been attempted. However, comparison with the spectrum of the starting
[
{Ru(p-cymene)Cl } ] complex shows the presence of several additional bands (near
2 2
-1
1
491, 1314, 1244, 1135 and 1110 cm ), which are attributable to the coordinated 2-
-1
(
arylazo)phenolate ligand. Among these bands, the one near 1491 cm and the other one
-1
near 1110 cm are assignable respectively to the azo (N=N) and phenolic C-O stretches.
1
The H NMR and infrared spectral data are therefore in good agreement with the
composition of the complexes.
The [Ru(p-cymene)(L-R)Cl] complexes are found to be readily soluble in polar
organic solvents, such as: methanol, ethanol, dichloromethane, chloroform and
acetonitrile, producing intense pinkish-brown solutions. Electronic spectra of the
complexes were recorded in methanol solutions. Spectral data are presented in Table 3.
Each complex shows three absorptions spanning the visible and ultraviolet regions. The
two absorptions in the visible region near 550 nm and 388 nm are believed to be due to a
metal-to-ligand charge-transfer transition. The intense absorption in the ultraviolet region
around 320 nm is attributable to transitions within the ligand orbitals. To have an insight
into the nature of these absorptions, TDDFT calculations have been performed on all
three [Ru(p-cymene)(L-R)Cl] complexes, using the Gaussian 09 package [21], and the
results are found to be similar for all the complexes. The DFT-optimized structures of the
complexes are shown in Fig. S1 and some computed bond parameters are listed in Table
S1 (Supplementary material). The computed bond parameters for [Ru(p-cymene)(L-
CH )Cl] compare well with those found in its crystal structure. The main calculated
3
transitions for [Ru(p-cymene)(L-CH )Cl] and compositions of the molecular orbitals
3
associated with these transitions are presented respectively in Table 4 and Table 5, and
contour plots of the same molecular orbitals are shown in Fig. 2. Similar data and
Fig. 2

8
diagrams for the other two [Ru(p-cymene)(L-R)Cl] (R = H and Cl) complexes are
deposited in Table S2-S5 and Fig. S2-S3 (Supplementary material). Since the computed
orbital and optical transition data are qualitatively similar for this group of complexes,
only the results for [Ru(p-cymene)(L-CH )Cl] are discussed here as a representative case.
3
Plots of experimental and theoretical spectra for this complex are shown in Fig. S4
(Supplementary material). The lowest energy absorption at 547 nm is attributable
primarily to a HOMO → LUMO transition, with much less HOMO-1 → LUMO,
HOMO-3 → LUMO and HOMO → LUMO+2 character, and based on the nature of the
participating orbitals the electronic excitation is assignable to a combination of MLCT,
LMCT, LLCT and ILCT transitions. The next absorption at 388 nm has dominant
HOMO-1 → LUMO character, while the third absorption at 323 nm is largely due to
HOMO-3 → LUMO and HOMO-1 → LUMO+1 transitions, and both of these
absorptions are again assignable to a combination of MLCT, LMCT, LLCT and ILCT
transitions.
3
.3. Electrochemical properties
Electrochemical properties of the [Ru(p-cymene)(L-R)Cl] complexes have been
studied by cyclic voltammetry in acetonitrile solution (0.1 M TBHP). Each complex
shows two oxidative responses on the positive side of SCE, which are found to be
irreversible in nature. Cyclic voltammetric data are given in Table 3. The first oxidative
response near 1.10 V vs SCE is assigned to Ru(II)-Ru(III) oxidation. The second
oxidation, that takes place near 1.40 V vs SCE, is too close to the first Ru(II)-Ru(III)
oxidation, and hence it is unlikely to be Ru(III)-Ru(IV) oxidation. Therefore this second
oxidation is tentatively assigned to oxidation of the coordinated 2-(arylazo)phenolate
ligand. Potential of the oxidative responses in these [Ru(p-cymene)(L-R)Cl] complexes
does not show any significant variation with the nature of the substituent R in the 2-
(arylazo)phenolate ligands.
3
.4. Catalytic transfer-hydrogenation
Catalytic transfer-hydrogenation of organic substrates has been receiving
considerable attention [25-28], primarily because of the green nature of the methodology.
Such reactions are known to be efficiently catalyzed by a variety of ruthenium(II)

9
complexes [25,27,28], where the reactions usually proceed through the intermediacy of a
ruthenium-hydrido species. Hence, complexes having a pre-existing Ru-H bond, or with
the potential of formation of a Ru-H bond in situ, are suitable for trial as transfer-
hydrogenation catalyst. As complexes with a Ru-X (X = halide, pseudohalide, or other
anionic ligand) bond are known to provide a Ru-H fragment upon reaction with primary
or secondary alcohols [12,13,15,29,30], and complexes of type [Ru(p-cymene)(L-Lʹ)Cl]
(L-Lʹ depicting a bidentate chelating ligand) are being routinely utilized for such
reactions [31-42], we wanted to explore such possibility in the present group of [Ru(p-
cymene)(L-R)Cl] complexes.
We began our study by examining the transfer hydrogenation of 4-
chlorobenzaldehyde to 4-chlorobenzyl alcohol using [Ru(p-cymene)(L-CH )Cl] as the
3
catalyst. After extensive optimization (Table S6; Supplementary material) it was found
that 0.00001 mol% catalyst, 0.5 mol% KOH, 2-propanol as solvent, a reaction
temperature of 100 °C, and a reaction time of 30 min furnished an excellent yield (97%)
of the expected product (Table 6, entry 1). As all the three [Ru(p-cymene)(L-R)Cl]
complexes were found to show comparable catalytic efficiency (Table S6;
Supplementary material), only the results obtained with [Ru(p-cymene)(L-CH )Cl] as the
3
catalyst are presented here. Using the optimized reaction conditions, transfer-
hydrogenation of twelve different aldehydes have been performed (Table 6, entries 1-
1
2), and the corresponding product alcohols were obtained in good to excellent yields for
all the substrates (entries 1-9) except three, which have a donor site next to the -CHO
group, where the product alcohols were obtained in negligible yield (entries 10-12). It is
interesting to mention here that when ketones were taken as substrates, their
hydrogenation to the corresponding secondary alcohol was not observed at all, which
indicates that the present group of [Ru(p-cymene)(L-R)Cl] complexes can selectively,
and efficiently, catalyze hydrogenation of alhedydes to the corresponding primary
alcohols.
The observed Ru-catalyzed transfer-hydrogenation of aldehydes is believed to
follow the sequences illustrated in Scheme 1, which are essentially drawn from those
proposed earlier [31-42]. The [Ru(p-cymene)(L-R)Cl] catalyst-precursor, which is

1
0
depicted as (i) with only the Ru-Cl fragment shown, undergoes reaction with 2-propanol
in the initial step to generate the intermediate (ii), in which the isopropoxide ion is
Scheme 1
coordinated to ruthenium through oxygen, and thus entry into the catalytic cycle takes
place. This species (ii) then undergoes a -hydride elimination from the coordinated
isopropoxide ligand, which is quite usual [29,43-47], to afford the hydrido species (iii).
Attachment of the aldehyde to the metal center is believed to take place next to generate
species (iv), which is followed by insertion of the aldehyde substrate into the Ru–H bond
to generate the corresponding aryloxo species (v). In the final step, elimination of the
product alcohol takes place with simultaneous regeneration of the active catalyst (ii). In
the light of the proposed mechanism, the negligible yield observed for the hydrogenation
of pyrrole-2-aldehyde, pyridine-2-aldehyde and 2-hydroxynaphthaldehyde (entries 9-12)
is probably due to catalyst deactivation through the coordination of these substrates, their
potential to form chelates being well known. The absence of any catalysis for
hydrogenation of ketones is attributable to the steric inhibition involved in the formation
of species of type (iv). Though catalytic transfer-hydrogenation with [Ru(p-cymene)(L-
Lʹ)Cl] type complexes as pre-catalyst is precedent in the literature [31-42], the observed
catalytic efficiency of the present group of [Ru(p-cymene)(L-R)Cl] complexes is
remarkable, with particular reference to very low catalyst loading, high yield, very large
7
turn-over number (~ 10 ), and low reaction time.
4
. Conclusion
The present study shows that the 2-(arylazo)phenols (HL-R) undergo facile
reaction with [{Ru(p-cymene)Cl } ] in the presence of triethylamine to afford mixed-
2
2
ligand complexes of type [Ru(p-cymene)(L-R)Cl]. This study also demonstrates that
these [Ru(p-cymene)(L-R)Cl] complexes serve as excellent precursor for catalytic
transfer-hydrogenation of aldehydes.

1
1
Acknowledgments
The authors thank the reviewers for their constructive comments, which have
been very helpful in preparing the revised manuscript. Financial assistance received from
the UGC-CAS program and the DST-PURSE program of the Department of Chemistry,
Jadavpur University, Kolkata 700032, is gratefully acknowledged. J. K. thanks the
Council of Scientific and Industrial Research (CSIR), New Delhi, for a fellowship [grant
number 09/096(0842)/2015-EMR-I]. The authors thank Prof. Saurabh Das (Department
of Chemistry, Jadavpur University) for his help in recording the electronic spectra.
Appendix A. Supplementary data
CCDC 1889794 contains the supplementary crystallographic data for [Ru(p-
cymene)(L-R)Cl]. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html,
or
from
the
Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)
1
223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in the online
version, at doi:--------------------------.

1
2
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Footnotes
1
The mmol calculation was been done taking [{Ru(p-cymene)Cl } ] as the
2 2
molecular formula.
2
Chemical shifts for all NMR data are given in ppm and the multiplicity of the
signals, along with the associated coupling constant, is given in parentheses.
Overlapping signals are marked with an asterisk (*).
3
Bond parameters around the azo (-N=N-) fragment, which had disorder problem,
are found to be bit different than usual values.
*
*********

1
5
Table 1
Crystal data and structure refinement parameters for
[
Ru(p-cymene)(L-CH )Cl].
3
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C H N O Cl Ru
24 27 2 1 1
496.00
Monoclinic
P2 /n
1
12.7479 (8)
13.6907 (9)
13.0168 (9)
90.00
b (Å)
c (Å)

(º)
β (º)
(º)
100.075 (4)
90.00

V (Å3)
Z
2236.8 (3)
4
Dcalcd (mg/m3)
F(000)
1.473
1016
Crystal size (mm)
T (K)
0.12  0.16  0.24
296
-
1
µ (mm )
0.837
Collected reflections
31557
Rint
0.074
Independent reflections
5122
R1a
0.0481
0.2059
0.75
wR2b
GOFc
a
b
c
R =  F -  F / F 
1
o
c
o
2
2 2
2 2 1/2
wR = [ [ w( F - F ) ] /  [w( F ) ] ]
2
o
2
c
o
2 2
1/2
GOF = [[w(F -F ) ]/(M-N)] , where M is the number of reflections
o
c
and N is the number of parameters refined.

1
6
Table 2
Selected bond distances and bond angels for [Ru(p-cymene)(L-CH )Cl].
3
Bond distances (Ǻ)
Ru1-Cl1
Ru1-O1
Ru1-N1
2.4101(14)
2.039(4)
2.130(6)
C1-O1
C2-N1
N1-N2
N2-C8
1.310(7)
1.389(8)
1.306(11)
1.427(12)
Bond angles (°)
84.57(12) O1-Ru1-N1
84.55(16)
Cl1-Ru1-O1
Cl1-Ru1-N1
74.0(2)

1
7
Table 3
Electronic spectral and cyclic voltammetric data.
a
Cyclic voltammetric datab
E, V vs. SCE
Complex
Electronic spectral data
max, nm (, M cm )
-1
-1
c
c
c
c
[
[
[
Ru(p-cymene)(L-CH )Cl] 547 (2700), 388 (5000), 323 1.11 , 1.36
3
(
8300)
546 (2700), 388 (4900), 316 1.12 , 1.38
10000)
557 (2200), 384 (3900), 321 1.14 , 1.40
7400)
c
Ru(p-cymene)(L-H)Cl]
Ru(p-cymene)(L-Cl)Cl]
(
c
(
a
In methanol.
b
c
Solvent, acetonitrile; supporting electrolyte, TBHP; scan rate, 50 mVs-1.
Epa (anodic peak potential) value.

1
8
Table 4
Main calculated transitions for [Ru(p-cymene)(L-CH )Cl] with composition in terms of
3
molecular orbital contribution of the transition, excitation energies, and oscillator strength
in methanol.
Transi- Nature of
CI
E (eV)
2.5706
Oscillator λtheo (nm) Assignments
tion
No.
transition
strength
(λexp(nm))
(f)
1
H-3 → L
H-1 → L
H → L
-0.14270
-0.16198
0.62242
-0.15597
-0.27939
0.0038
518.32
547)
MLCT/LLCT/ILCT
MLCT/LLCT/ILCT
MLCT/ILCT
(
H → L+2
LMCT/LLCT/MLCT
MLCT/LLCT/ILCT
LMCT/MLCT/LLCT
LMCT/MLCT/LLCT
MLCT/ILCT
MLCT/LMCT/LLCT
LLCT/ILCT
MLCT/LLCT/ILCT
MLCT/LLCT/ILCT
MLCT/ILCT
2
3
H-3 → L
H-3 → L+1 -0.18172
H-2 → L+1 -0.16122
H-1 → L
H-1 → L+1 0.29659
H-5 → L
H-3 → L
H-2 → L
H-1 → L
3.1674
3.7349
0.0048
0.0256
391.44
388)
(
0.44296
0.10854
0.34398
-0.28828
-0.17738
331.96
323)
(
H-1 → L+1 0.43232
H → L+2 -0.13053
MLCT/LMCT/LLCT
LMCT/LLCT/MLCT

1
9
Table 5
Compositions of selected molecular orbitals of [Ru(p-cymene)(L-CH )Cl].
3
Fragments
Contribution (%) of fragments to
H-3 H-2 H-1 HOMO LUMO L+1
L+2
53
17
29
0
(H)
(L)
Ru
25
54
5
30
50
8
44
24
16
16
13
81
4
5
52
12
26
10
L-CH3
p-cymene
Cl
89
4
16
12
2
2

2
0
Table 6
Catalytic transfer-hydrogenation of aldehydes.a
O
H
OH
H
Catalyst
R
H
2-propanol
KOH
R
b
Entry
1
Substrate
Yield (%)
97
CHO
Cl
CHO
2
3
96
98
O2N
F
CHO
CHO
4
5
93
94
CHO
H3C
CHO
6
7
95
94
H3CO
CHO
CHO
8
95
9
CHO
91
0
CHO
1
0
N
H
CHO
1
1
1
2
2
2
N
CHO
OH
a
b
Reaction conditions: aldehyde (1.0 mmol), KOH (0.5 mmol), 2-propanol (5 ml),
catalyst: [Ru(p-cymene)(L-CH )Cl] (0.00001 mol%), bath temperature (100º C),
3
time (30 min).
Determined by GC-MS.

2
1
Fig. 1. Crystal structure of [Ru(p-cymene)(L-CH )Cl] (hydrogen atoms are omitted for
3
clarity).

2
2
H (HOMO)
L (LUMO)
H-1
L+1
H-2
L+2
H-3
Fig. 2. Contour plots of molecular orbitals of [Ru(p-cymene)(L-CH )Cl], which
3
are associated with the observed electronic spectral transitions.

2
3
Ru Cl
i)
(
H
HO
CH3
CH3
HCl
H
R
H
HO
CH3
CH3
H
O
Ru
O
CH3
CH3
(ii)
H
HO
CH3
CH3
H
R
H
Ru
v)
O
Ru H
(iii)
(
H
R
O
H
H
R
Ru
O
(iv)
Scheme 1.
Probable mechanism for the observed transfer hydrogenation reaction. In
the pre-catalyst (i), besides the Ru-Cl fragment no other coordinated
ligand is shown.

2
4
Manuscript title: Arene-ruthenium complexes with 2-(arylazo)phenol as
ancillary ligand: Synthesis, characterization, and
utilization in catalytic transfer-hydrogenation
Authors:
Jit Karmakar, Samaresh Bhattacharya*
Graphical Abstract
Synopsis
2 2
Reaction of 2-(arylazo)phenols (HL-R) with [{Ru(p-cymene)Cl } ] in the presence of triethylamine
afforded a group of complexes of type [Ru(p-cymene)(L-R)Cl], which are found to efficiently catalyze
transfer-hydrogenation of aldehydes.
Manuscript title: Arene-ruthenium complexes with 2-(arylazo)phenol as
ancillary ligand: Synthesis, characterization, and
utilization in catalytic transfer-hydrogenation
Authors:
Jit Karmakar, Samaresh Bhattacharya*
Graphical Abstract
Picture
H
R
OH
HO
O
H
2 2
[{Ru(p-cymene)Cl } ]
H C
3
N
N
H
R = CH , H, Cl
3
R
R

2
5