Synthesis of a rhodium(III) triphenylphosphine complex via C–S bond cleavage of an azo-thioether ligand: X-ray structure, electrochemistry and catalysis towards transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.poly.2018.10.061

A new rhodium(III) triphenylphosphine complex having the general formula [Rh(PPh₃)₂(L)Cl] (1) was synthesized by C–S bond cleavage of an ONS donor azo-thioether ligand (L-CH₂Ph). The complex was thoroughly characterized by various spectroscopic techniques. Its single crystal X-ray structure exhibits an octahedral geometry around the rhodium(III) center. A cyclic voltammogram of the complex exhibits ligand based quasi-irreversible oxidative and reductive responses. The electronic structure, redox properties and electronic excitations in the complex were interpreted by DFT and TDDFT calculations. The complex effectively catalyzed the transfer hydrogenation reaction of ketones with high yields in i-PrOH in the presence of a base.

Full text of DOI:10.1016/j.poly.2018.10.061

Accepted Manuscript
Synthesis of a rhodium(III) triphenylphosphine complex via C-S bond cleavage
of an azo-thiether ligand: X-ray structure, electrochemistry and catalysis to-
wards transfer hydrogenation of ketones
Puspendu Roy, Chandan Kumar Manna, Rahul Naskar, Tapan Kumar Mondal
PII:
S0277-5387(18)30697-1
https://doi.org/10.1016/j.poly.2018.10.061
POLY 13535
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
12 July 2018
11 October 2018
13 October 2018
Please cite this article as: P. Roy, C.K. Manna, R. Naskar, T.K. Mondal, Synthesis of a rhodium(III)
triphenylphosphine complex via C-S bond cleavage of an azo-thiether ligand: X-ray structure, electrochemistry and
catalysis towards transfer hydrogenation of ketones, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.
2
018.10.061
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1
Synthesis of a rhodium(III) triphenylphosphine complex via C-S bond
cleavage of an azo-thiether ligand: X-ray structure, electrochemistry and
catalysis towards transfer hydrogenation of ketones
a,b
a
a
a*
Puspendu Roy , Chandan Kumar Manna , Rahul Naskar and Tapan Kumar Mondal
aDepartment of Chemistry, Jadavpur University, Kolkata-700032, India.
bDepartment of Chemistry, Netaji Nagar Day College, University of Calcutta, Kolkata 700092,
India
Abstract
A new rhodium(III) triphenylphosphine complex having the general formula [Rh(PPh
3
)
2
(L)Cl]
Ph).
(
1) was synthesized by C-S bond cleavage of an ONS donor azo-thioether ligand (L-CH
2
The complex was thoroughly characterized by various spectroscopic techniques. Its single crystal
X-ray structure exhibits an octahedral geometry around the rhodium(III) center. A cyclic
voltammogram of the complex exhibits ligand based quasi-irreversible oxidative and reductive
responses. The electronic structure, redox properties and electronic excitations in the complex
were interpreted by DFT and TDDFT calculations. The complex effectively catalyzed the
transfer hydrogenation reaction of ketones with high yields in i-PrOH in the presence of a base.
Keywords:
Rhodium(III) complex; C-S bond cleavage; Electrochemistry; Transfer
hydrogenation; DFT calculation.
Corresponding author: Email: tkmondal@chemistry.jdvu.ac.in

2
1
. Introduction
There has been growing interest in transition metal mediated activation of carbon-sulfur bonds
due to its importance in the fuel and coal industry for decades [1-4]. Transition metal mediated
C-S bond activation and cleavage also play a vital role in the field of bioorganic and synthetic
chemistry [5,6]. In order to improve the desired reactivities of metal catalyzed C-S bond
activation, the design of ligand systems are important. By the adjustment of the binding affinity
and the size of the ligand, both the electron density and steric factors at the metal center can be
tuned in the complexes. The phosphine group PR
important ligands used in organometallic chemistry for C-H and C-S activations. The effects on
M-L bonds with the variation of the electronic and steric natures of the R group of PR ligands
3
(R= alkyl, aryl, alkoxy etc.) is one of the
3
are well quantified by Tolman [7,8]. Moreover, transition metal mediated C-S bond activation
along with the transformation processes are of stimulating interest from the perspective of
synthetic, mechanistic as well as catalytic aspects [9-12].
In recent years, catalytic transfer hydrogenation has received considerable attention
because of its capacity to hydrogenate substrates under mild conditions and it avoids the use of
high pressure vessels and compressed H or moisture-sensitive hydride reagents [13,14].
2
Continuous research is going on to develop new technologies for future industrial applications in
terms of selectivity, efficiency, scope, simplicity and economic viability [15,16]. So, the
development of new catalysts for the transfer hydrogenation of ketones is still of considerable
interest. Currently, we are working on the design and development of ruthenium, rhodium and
osmium metal complexes as catalysts in the transfer hydrogenation of ketones [17-19]. In the
present contribution, we have synthesized a new rhodium(III) triphenylphospine complex,
[
Rh(PPh
3
2
) (L)Cl] (1), via C-S bond cleavage of a thioether containing ligand, 3-(2-(2-((pyridin-

3
2
-ylmethyl)thio)phenyl)hydrazono)pentane-2,4-dione (L-CH
3
). The complex effectively
catalyzes the transfer hydrogenation reaction of ketones in i-PrOH. The structure of the complex
was confirmed by the single crystal X-ray diffraction method. The electronic structure and redox
properties of the complex are interpreted by DFT calculations.
2. Experimental
2
.1. Material and methods
All the reagents and solvents were purchased from commercial sources and used as received.
Acetylacetone and the inorganic metal salts were obtained from E. Merck, India. 2-
Aminothiophenol, 2-(chloromethyl)pyridine hydrochloride and [Rh(PPh
from Sigma Aldrich. The ligand L-CH Ph was prepared by following the published procedure
20].
3
)
3
Cl] were purchased
2
[
Microanalyses (C, H, N) data were obtained using a PerkinElmer Series-II CHN-2400
CHNS/O elemental analyzer. Electronic spectra were measured on a Lambda 750 PerkinElmer
spectrophotometer in acetonitrile. IR spectra were recorded on a RX-1 PerkinElmer spectrometer
-
1
in the range 4000-400 cm with the samples in the form of KBr pellets. The NMR spectrum was
recorded in CDCl on a Bruker (AC) 300 MHz FT-NMR spectrometer in the presence of TMS as
3
an internal standard. HRMS mass spectra were obtained on a Waters (Xevo G2 Q-TOF) mass
spectrometer. Cyclic voltammetric measurements were carried out using a CHI electrochemical
workstation. A platinum wire working electrode, a platinum wire auxiliary electrode and
Ag/AgCl reference electrode filled with saturated KCl solution were used in a standard three-
electrode configuration. The supporting electrolyte was 0.1 M tetrabutylammonium
, and the solute concentration was ca 10 3 M. Before

hexafluorophosphate, [Bu
4
N]PF
6

4
measurement, the solutions were degassed by bubbling N
2
through them for 5 minutes. The half-
wave potential E1/2 was set equal to 1/2(Epa + Epc), where Epa and Epc are anodic and cathodic
cyclic voltammetric peak potentials, respectively.
2
.2. Synthesis of [Rh(PPh
.190 g (0.2 mmol) Rh(PPh
Ph in 10 mL of acetonitrile was added. The reaction mixture was refluxed for
atmosphere. The solvent was then removed under reduced pressure in a rotary
3
)
2
(L)Cl] (1)
0
3 3
)
Cl was dissolved in 20 mL of acetonitrile. To this solution 0.065 g
(
0.2 mmol) L-CH
2
1
0 h under a N
2
evaporator. The crude product was dissolved in dichloromethane and purified by column
chromatography using silica gel (mesh 60–120). A deep red coloured complex (1) was eluted by
a 50% (v/v) ethyl acetate-petroleum ether mixture. On removal of the solvent under reduced
pressure, the pure complex 1 was obtained as a red solid which was further dried under vacuum.
Yield: 0.114 g, 64%.
Anal. Calc. for C47
H40ClN
2
O
2
P
2
RhS (1): C, 62.91; H, 4.51; N, 3.11; Found: C, 62.92; H, 4.49; N,
-1
1
3
.12%; IR data (KBr, cm ): 1654 (C=O), 1355 (N=N). H NMR data (CDCl
3
, δ, ppm): 2.53
-3
-1
-
(
3H, s), 2.67 (3H, s), 7.22-7.93 (34H, m). UV-Vis (in acetonitrile), max (nm) ( × 10 , mol cm
1)
: 530(6.13), 425(sh.), 344(11.44), 281(25.38). HRMS m/z, 920.3256 [M] . Electrochemistry
+

1
(
in acetonitrile) (scan rate: 100 mV s ): E1/2 = 1.06 V (E = 116 mV) and Epc = -1.12 V (E,
1
40 mV).
2
.3. Crystallography
Single crystals of 1 were obtained by slow diffusion of n-hexane into a dichloromethane solution
of the complex. Single crystal data collection was performed with an automated Bruker SMART
APEX CCD diffractometer using graphite monochromatized Mo K radiation ( = 0.71073 Å).
Reflection data were recorded using the  scan technique. Unit cell parameters were determined

5
from least-squares refinement of the setting angles with  in the range 1.71    27.54. Out of
6
6540 collected data, 9423 with I  2 (I) within the -30  h  31, -12  k  12, -46  l  46
were used for the structure solution. The structure was solved and refined by full-matrix least-
2
squares techniques on F using SHELXL-2016/6 [46,47]. The absorption corrections were done
by multi-scan (SHELXTL program package) and all the data were corrected for the Lorentz,
polarization effect. Hydrogen atoms were included in the refinement process as per the riding
model. Details of the crystal analysis, data collection and structure refinement data for 1 are
given in Table 1.
2
.4. Computational method
+
Geometry optimizations of [Rh(PPh
3
) (L)Cl] (1) and the oxidized species 1 were carried out by
2
the DFT/B3LYP and DFT/UB3LYP methods, respectively [22, 23]. All elements, except
rhodium, were assigned the 6-31G(d) basis set. The LanL2DZ basis set with an effective core
potential was employed for the rhodium atom [24-26]. A vibrational frequency calculation was
performed to ensure that the optimized geometry represents the local minima and there are only
positive eigenvalues. Vertical electronic excitations were computed using the time-dependent
density functional theory (TDDFT) formalism [27-29] for 1 in acetonitrile using a conductor-like
polarizable continuum model (CPCM) [30-32]. All computations were carried out using the
Gaussian09 (G09) program [33]. GaussSum [34] software was used to calculate the fractional
contributions of various groups to each molecular orbital.
2
.5. General procedure for the transfer hydrogenation of ketones
Typical procedure for the catalytic transfer hydrogenation reaction: A solution of the
rhodium(III) complex (1) (0.0075 mmol), KOH (0.03 mmol) and the corresponding ketone (3
mmol) in 10 mL degassed i-PrOH was refluxed at 80 °C under stirring conditions. The reaction

6
was monitored at various time intervals by GC. After the reaction was complete, 2-propanol was
removed under reduced pressure on a rotary evaporator and the resulting semi-solid was
extracted with diethyl ether (5 × 10 mL). To separate the catalyst, the ether extract was then
passed through a short silica gel column and condensed under reduced pressure; conversion was
determined by GC equipped with a flame ionization detector (FID) and an HP-5 column of 30 m
length, 0.53 mm diameter and 5.00 m film thickness. The column, injector and detector
temperatures were 200, 250 and 250 ºC, respectively. The carrier gas was N (UHP grade) at a
2
flow rate of 30 mL/min. The injection volume of the sample was 2 L. The alcohols were
identified by GC using undecane as an internal standard and each of the catalytic runs was
performed three times.
3. Results and discussion
3
.1. Synthesis and spectral characterization
The octahedral rhodium(III) triphenylphosphine complex, [Rh(PPh
by the reaction of Rh(PPh Cl and the azo-thioether ligand L-CH Ph under refluxing conditions
in acetonitrile (Scheme 1). The complex was thoroughly characterized by several spectroscopic
3
2
) (L)Cl] (1) was synthesized
3
)
3
2
-1
techniques. The IR spectrum of the complex exhibits a sharp signal at 1654 cm , which
-1
corresponds to (C=O), along with a weak peak at 1355 cm , corresponding to (N=N). The

(N=N) band is significantly shifted to a lower frequency region compare to the free ligand
1
value, suggesting the coordination of azo-N atom to the rhodium ion [20]. The H NMR
spectrum of the complex was taken in CDCl
aromatic protons, were resolved, except the S-CH
complex. A series of overlapping multiplet signals appeared in the region δ 7.2 to 7.9 ppm,
3
. All the expected signals of the ligands, including
2
Ph peaks, suggesting C-S bond cleavage in the

7
which correspond to the protons of the coordinated triphenylphosphine ligand in the complex.
The UV-vis spectrum of the complex in acetonitrile exhibits a low energy broad peak at 530 nm
-3
-1
-1
(
 × 10 , 6.13 M cm ) along with a shoulder at 425 nm. In addition, two ligand centered peaks
-3
-1
-1
-3
-1
-1
appeared at 344 nm ( × 10 , 11.44 M cm ) and 281 nm ( × 10 , 25.38 M cm ) in
acetonitrile (Fig. 1).
Scheme 1. Synthesis of the rhodium(III) complex, [Rh(PPh
3
2
) (L)Cl] (1)
3
.2. Molecular structure
The molecular structure of the complex [Rh(PPh
3
2
) (L)Cl] (1) was confirmed by the single crystal
X-ray diffraction method. Single crystals suitable for structure determination were grown by
slow diffusion of n-hexane into a dichloromethane solution of the complex. The molecule
crystallized in the monoclinic crystal system with C2/c symmetry. Selected bond lengths and
bond angles are given in Table 2. An ORTEP plot with the atomic numbering scheme is shown
in Fig. 2. The molecule consists of a central rhodium atom surrounded by six donor centers and
it adopts a distorted octahedral geometry. The atomic arrangement involves two trans-PPh
3
ligands, chelating O,N,S donors and a Cl atom trans to the azo-N atom within the RhP ONSCl
2

8
coordination sphere. The O,N,S donor thioether ligand binds through the O, azo-N and
thiophenolato-S (formed by C-S bond cleavage) atoms to the rhodium centre in the complex. The
rhodium coordination sphere deviates from an ideal octahedron because of the deviation of the
chelate bite angle of the five membered ring (Rh1-N1-C6-C1-S1) [86.50(6)°] from 90°. The Rh-
P (Rh1-P1, 2.4109(7) and Rh1-P2, 2.3978(7) Å), Rh-Cl (Rh1-Cl1, 2.3676(7) Å) and Rh-S (Rh1-
S1, 2.3021(7) Å) bond distances are found to be as expected by comparison to similar rhodium
triphenylphosphine complexes [18,35,36].
3
.3. DFT and TDDFT calculations
To interpret the electronic structure of the complex, the geometry of 1 was optimized by the
DFT/B3LYP method. DFT optimized bond distances and angles are well correlated with the X-
ray crystal structure data (Table 2). The energy and compositions of selected molecular orbitals
of 1 are summarized in Table 3. The HOMO of the complex has 88% ligand character and is
mostly concentrated on the S atom. HOMO-1 has 79% π(L) character along with a reduced
contribution of dπ(Rh) (15%). HOMO-2 has mixed contributions of π(L), dπ(Rh) and pπ(Cl)
*
orbitals. The LUMO of the complex has 80% π (L) character, while LUMO+1 and LUMO+2
*
*
have mixed contributions of dπ(Rh) (30-50%), π (L) (24-37%) and π (PPh ) orbitals (Fig. 3).
3
The UV-vis spectrum of the complex was interpreted using calculated vertical electronic
transitions by the TDDFT method using the CPCM model in acetonitrile. The calculated vertical
electronic transitions are summarized in Table 4. The experimental broad band at 530 nm
corresponds to the admixture of HOMO  LUMO and HOMO  LUMO+1 transitions, having
mixed ILCT (intra-ligand charge transfer transition) and LMCT (ligand to metal charge transfer
*
transition) character. The low energy bands at 344 and 281 nm correspond to π(L)  π (L)
transitions, having ILCT character.

9
3
.4. Electrochemistry
The electrochemical behavior of complex 1 was investigated by a cyclic voltammetric study in
acetonitrile using tetrabutylammonium hexafluorophosphate [Bu N]PF (0.1 M) as the
4
6
supporting electrolyte and a Ag/AgCl reference electrode. The complex exhibits one quasi-
reversible reduction couple with E1/2 = -1.12 V (E = 140 mV) along with a quasi-reversible
oxidation couple with E1/2 = 1.06 V (E = 116 mV) (Fig. 4) within the experimental potential
range of ± 2.0 V versus the Ag/AgCl reference electrode in CH CN. The peak at -1.12 V
3
corresponds to the reduction of the azo(N=N) bond of the ligand as the LUMO of the complex
*
has 80%  (L) character with a major contribution of the azo(N=N) function. The quasi-
reversible oxidation of 1 is likely to be associated with the Rh(III) to Rh(IV) oxidation [36] or
oxidation of the thiophenolato-S to a thiyl radical [37-39]. The HOMO of complex 1 has 88%
ligand character with a major contribution of the pπ(S) orbital (51%). So, the oxidation process is

better assigned as the formation of the thiyl radical (S ) in comparison to the alternate possibility
of Rh(III) to Rh(IV) oxidation. This was also further evidenced from DFT calculations of the
+
+
oxidized species (1 ) by the UB3LYP method. The -spin LUMO of 1 has 91% ligand character
+
with a major contribution from the pπ(S) orbital (53%) (Fig. 5). A spin density plot of 1 is
majorly concentrated on the S atom and has only 1% contribution from the Rh atom, suggesting
the oxidation takes place on the thiophenolato-S atom to form the thiyl radical (S).
3
.5. Catalytic transfer hydrogenation reactions
The transfer hydrogenation reaction is a significant and useful process in organic synthesis. The
platinum group metal complexes have been used extensively as effective catalysts for transfer
hydrogenation reactions [19,40-48]. In the present work, the catalytic properties of the

10
synthesized rhodium(III) complex 1 towards transfer hydrogenation reactions of ketones were
tested in i-PrOH in the presence of a base.
Complex 1 catalyzed the transfer hydrogenation of ketones at 80 ºC with i-PrOH as the
hydrogen source in the presence of a base. To evaluate the catalytic efficiency of the complex,
the reaction conditions were optimized with variation of the base, reaction time and
catalyst:substrate (C:S) ratios. The transfer hydrogenation of acetophenone to 1-phenylethanol
was chosen as a standard reaction in i-PrOH and the results are summarized in Table 5. With a
catalyst:substrate (C:S) ratio of 1:300, the conversion rates were significantly varied (39-98%)
with the base employed. In the case of weak bases, such as Na
2
CO
3
and CH COONa, the
3
conversions were significantly low, whereas a moderate conversion was observed in the case of
t
KO Bu. Significant catalytic conversions were achieved for strong bases such as KOH or NaOH.
The progress of the reaction was also monitored over different time intervals and it was found
that the formation of 1-phenylethanol initially increased and after 5 h there was no appreciable
change in the conversion rate. Again, to understand the catalytic efficiency of the complex 1,
different catalyst:substrate (C:S) ratios were tested. It was found that the conversions were
excellent with C:S ratios of 1:300 to 1:400. With a decrease of the catalyst concentration, with
C:S ratios of 1:500, 1:600, 1:800, 1:1000 or 1:1500, the transfer hydrogenation reactions proceed
but a sharp decrease in the rates of conversion was observed for C:S ratios of 1:600 to 1:1500.
The excellent conversion (96%) with a moderate turnover number (TON) was observed with the
C:S ratio of 1:400. A series of ketones were screened for transfer hydrogenation reactions using a
C:S ratio of 1:400 and KOH as the base in i-PrOH. The results are summarized in Table 6. The
maximum conversion was observed for p-methoxycetophenone (98%) to the corresponding
alcohol. The conversions of other acetophenones were found to be in the range 89-97%.

11
Excellent catalytic conversions were also acheived for cyclic ketones, such as cyclopentanone
95%), cyclohexanone (92%) and cycloheptanone (89%). Moreover, the catalytic efficiency of
(
the present rhodium(III) complex towards transfer hydrogenation of ketones are comparable to
reported rhodium catalysts [49-51].
4. Conclusion
Herein, a new rhodium(III) triphenylphosphine complex, [Rh(PPh
3
)
2
(L)Cl] (1) was synthesized
by C-S cleavage of L-CH Ph. X-ray structure analysis confirms the distorted octahedral
2
geometry around the rhodium ion. In a cyclic voltammetric study, the complex exhibits a

thiophenolato-S to thiyl (S ) oxidation process and azo(N=N) bond reduction. The redox
properties are well interpreted by DFT studies. Vertical electronic transitions calculated by the
TDDFT/CPCM method were used to interpret the UV-Vis spectrum of the complex and a good
correlation between the experimental bands and the calculated transitions were observed.
Moreover, the complex effectively catalyzed the transfer hydrogenation reaction of ketones with
high yields in i-PrOH in the presence of a base.
Acknowledgment
Financial supports received from the Science and Engineering Research Board (SERB), New
Delhi, India (YSS/2015/001533) is gratefully acknowledged. P. Roy and R. Naskar are thankful
to UGC, New Delhi, India for fellowships.
Supplementary materials

12
Crystallographic data for the structure of 1 has been deposited with the Cambridge
Crystallographic Data center, CCDC No. 1495887. Copies of this information may be obtained
free of charge from the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (e-mail:
deposit@ccdc.cam.ac.uk or www:htpp://www.ccdc.cam.ac.uk).
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16
Table 1. Crystallographic data for [Rh(PPh
3
2
) (L)Cl] (1)
Empirical formula
Formula weight
Crystal system
Space group
a / Å
C
47
H
40ClN
2
O
2
2
P RhS
897.17
Monoclinic
C2/c
24.2017(6)
9.5376(3)
b / Å
c / Å
35.9841(10)
100.072(2)
o

( )
V / Å3
8178.1(4)
8
Z
3

calcd / g cm
1.457
0.654
293(2)


/ mm 1
T / K
hkl range
F(000)
-30 to 31, -12 to 12, -46 to 46
3680

range (o)
1.709 to 27.539
66260
Reflns collected
Unique reflns (Rint
)
9391
Observed data (I2(I))
7797
Data/restraints/parameters
9391 / 0 / 505
0.0389, 0.0832
0.0498, 0.0891
0.977
a
b
R1 , wR2 (I2(I))
R1, wR2 (all data)
GOFc
Largest diff. peak/hole / e Å
3
0.433/ -594
a
R
1
= ∑ |(|F
o
| – |F
c
|)| / ∑ |F
o
|
b
2–F 2)2] / ∑ [w (F
2 2
1/2
2
2
2
wR
2
= { ∑ [w (F
o
c
o
) ] } , w = 1/[σ (F ) + (0.0312P) + 25.0278P], where P
o
2
+ 2F
2)/3
=
(F
o
c
c
2–F 2)2] / (n–p)}1/2 , where n = number of measured data and p = number of
GOF = {∑ [w(F
o
c
parameters.

17
Table 2. X-ray and DFT calculated selected bond distances (Å) and angles (°)
Bonds (Å)
X-ray
Calc.
1
1+
Rh1- N1
Rh1- O1
Rh1- S1
Rh1- Cl1
Rh1- P1
Rh1- P2
N1- N2
O1- C10
S1-C1
1.996(2)
2.0659(18)
2.3021(7)
2.3676(7)
2.4109(7)
2.3978(7)
1.266(3)
1.257(3)
1.746(3)
2.035
2.095
2.347
2.437
2.481
2.442
1.272
1.269
1.756
2.038
2.103
2.308
2.422
2.504
2.522
1.292
1.256
1.725
Bond angles (°)
N1- Rh1- O1
N1- Rh1- S1
N1- Rh1- P1
N1- Rh1- P2
N1- Rh1- Cl1
O1- Rh1- S1
O1- Rh1- P1
O1- Rh1- P2
O1- Rh1- Cl1
S1- Rh1- P1
S1- Rh1- P2
S1- Rh1- Cl1
P1-Rh1-P2
90.23(8)
86.50(6)
89.78(6)
91.21(6)
179.78(8)
176.72(5)
89.72(5)
89.11(5)
89.96(5)
90.44(3)
90.79(3)
93.31(3)
178.47(2)
90.12(2)
88.89(2)
97.19(9)
89.022
85.912
92.459
92.251
178.910
174.855
90.277
89.039
90.204
89.043
90.054
94.873
175.227
88.302
86.977
96.906
88.476
85.408
93.962
93.248
179.156
173.883
89.066
87.344
92.301
91.143
93.203
93.812
171.855
86.440
86.401
98.357
P1-Rh1-Cl1
P2-Rh1-Cl1
C1-S1-Rh1

18
Table 3. Energy and composition of selected molecular orbitals of [Rh(PPh
3
2
) (L)Cl] (1)
MO
Energy (eV)
% of composition
Rh
0
L
PPh
3
Cl
0
LUMO+5
LUMO+4
LUMO+3
LUMO+2
LUMO+1
LUMO
-0.65
-0.65
-0.71
-1.19
-1.76
-1.88
-4.84
-5.64
-5.97
-6.06
-6.22
-6.45
-6.64
-6.7
01
09
0
98
88
99
04
44
11
08
03
05
40
04
66
95
90
47
88
82
02
01
50
30
09
04
15
25
09
08
10
0
0
0
37
24
80
88
79
39
08
70
14
04
07
34
11
13
9
02
0
HOMO
0
HOMO-1
HOMO-2
HOMO-3
HOMO-4
HOMO-5
HOMO-6
HOMO-7
HOMO-8
HOMO-9
HOMO-10
03
30
43
18
10
0
02
06
01
02
01
14
0
-6.75
-6.78
-6.82
04

19
Table 4. Vertical electronic excitations calculated by the TDDFT/CPCM method for 1
E
excitation
λ
excitation
Osc.
expt. (nm)
a
-3
-1
-1
(
eV)
(nm)
Strength (f)
Key transitions
Character
(
×10 , M cm )
2
.2471
551.8
0.0242
(67%) HOMOLUMO+1
π(L)→π*(L)/dπ(Rh) 530 (6.13)
ILCT/LMCT)
(
(
28%) HOMOLUMO
(72%) HOMOLUMO
26%) HOMOLUMO+1
2.4725
501.4
0.0888
π(L)→π*(L)/dπ(Rh)
ILCT/LMCT)
(
(
2
3
3
.8431
.1573
.4871
436.1
392.7
355.6
0.0145
0.0545
0.1307
(69%) HOMO-1LUMO+1 π(L)→π*(L)/dπ(Rh) 425 (sh.)
ILCT/LMCT)
(
(62%) HOMO-1LUMO
π(L)→π*(L)
ILCT)
π(L)→π*(L)
ILCT)
(
(63%)HOMO-4LUMO
344 (11.44)
(
aLMCT: Ligand to metal charge transfer transition and ILCT: Intra ligand charge transfer
transition.

20
Table 5. Effect of the C:S ratio, base and reaction time on the transfer hydrogenation of
acetophenonea
b
TONc
213
246
294
294
294
285
147
117
201
384
450
510
528
430
315
Entry
C:S ratio
1:300
1:300
1:300
1:300
1:300
1:300
1:300
1:300
1:300
1:400
1:500
1:600
1:800
1:1000
1:1500
Base
Time (h)
Conversion (%)
1
2
3
4
5
6
7
8
9
KOH
KOH
KOH
KOH
KOH
NaOH
3
4
5
7
10
5
5
5
5
5
5
5
5
5
5
71
82
98
98
98
95
49
39
67
96
90
85
66
43
21
Na
2
CO
3
CH
3
t
COONa
KO Bu
KOH
KOH
KOH
KOH
KOH
KOH
1
1
1
1
1
1
0
1
2
3
4
5
a
Reaction conditions: acetophenone (3 mmol), complex 1 (10-2 mol), catalyst:KOH 1:4 in i-
b
PrOH (10 mL) at 80 ºC; Conversion was determined by GC with undecane as an internal standard;
cTurnover number (TON) = mole of product/mol of catalyst.

21
Table 6. Transfer hydrogenation of ketones using complex 1a
Entry Ketones
1
Alcohols
Conversion
TONc
Entry
7
Ketones
Alcohols
Conversion
(%)b
TONc
b
(
%)
96.13  0.51
385  2
89.34  0.25
357  1
2
3
92.25  0.50
96.31  0.24
369  2
385  1
8
9
91.42  0.25
95.12  0.27
366  1
380  1
4
5
94.33  0.36
96.89  0.50
97.76  0.40
377  1
388  2
391  2
10
11
12
92.13  0.22
89.34  0.51
85.32  0.50
369  1
357  2
341  2
6
a
Experimental conditions: reactions were carried out at 80 °C, ketone (3 mmol), Rh(III) complex (0.25 mol%), KOH (0.1 mmol), i-PrOH (10
b
mL); Conversions were determined by GC with undecane as an internal standard and were reported mean values with standard error of three
c
runs; Turnover number (TON) = mole of product/mol of catalyst.

22
Figure captions:
Fig. 1. UV-Vis spectrum of complex [Rh(PPh
3
2
) (L)Cl] (1) in acetonitrile.
Fig. 2. ORTEP plot of 1 with 35% ellipsoidal probability.
Fig. 3. Contour plots of selected molecular orbitals of 1. Isodensity value 0.04 e Bohr .
Fig. 4. Cyclic voltammogram of 1 in acetonitrile (Supporting electrolyte: 0.1 M
-3

3
tetrabutylammonium hexafluorophosphate, [Bu
4
N]PF ; solute concentration: ~10 M; scan rate:
6

1
1
00 mV s )
+
-3
Fig. 5. Contour plots of selected molecular orbitals of 1 . Isodensity value 0.04 e Bohr .
+
-3
Fig. 6. Spin density plot of 1 (Rh, 2%; S, 57%). Isodensity value 0.004 e Bohr .

23
Fig. 1

24
Fig. 2

25
HOMO
HOMO-1
HOMO-2
LUMO
LUMO+1
LUMO+2
Fig. 3

26
Fig. 4

27
HOMO(-spin)
LUMO(-spin)
HOMO(-spin)
LUMO(-spin)
Fig. 5

28
Fig. 6

29
GRAPHICAL ABSTRACT
A new rhodium(III) triphenylphosphine complex having the general formula [Rh(PPh
3
)
2
(L)Cl]
Ph).
(
1) was synthesized by C-S bond cleavage of an ONS donor azo-thioether ligand (L-CH
2
The complex was thoroughly characterized by various spectroscopic techniques. Its single crystal
X-ray structure exhibits an octahedral geometry around the rhodium(III) center. A cyclic
voltammogram of the complex exhibits ligand based quasi-irreversible oxidative and reductive
responses. The electronic structure, redox properties and electronic excitations in the complex
were interpreted by DFT and TDDFT calculations. The complex effectively catalyzed the
transfer hydrogenation reaction of ketones with high yields.