Mixed-ligand 1, 3-diaryltriazenide complexes of ruthenium: Synthesis, structure and catalytic properties

Article abstract of DOI:10.1016/j.ica.2013.06.043

Reaction of 1, 3-diaryltriazenes (abbreviated in general as HL-R, where H represents the dissociable N-H proton and R is the para substituent (R = OCH₃, CH₃, H, Cl, NO₂) on the aryl fragment) with [Ru(PPh₃)₂ (CO)₂Cl₂] in 2-methoxyethanol in the presence of a base (NEt₃) affords a family of yellow complexes of the type [Ru(PPh₃)₂(L-R)(CO)(H)], where the 1, 3-diaryltriazenes are coordinated as monoanionic biden-tate NN-donors. Two triphenylphosphines, a hydride and a carbonyl are also coordinated to ruthenium. The triphenylphosphines are mutually trans, and the hydride and carbonyl are mutually cis. Structure of the [Ru(PPh ₃)₂(L-H)(CO)(H)] complex has been determined by X-ray crystallography. All the complexes show intense absorptions in the visible region, which are assigned, based on DFT calculations, to transitions within orbitals of the triazenide ligand. The complexes exhibit an irreversible oxidation on the positive side of SCE and an irreversible reduction on the negative side. All the complexes are found to efficiently catalyze transfer hydrogenation reactions.

Full text of DOI:10.1016/j.ica.2013.06.043

Inorganica Chimica Acta 406 (2013) 20–26
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Mixed-ligand 1,3-diaryltriazenide complexes of ruthenium: Synthesis,
structure and catalytic properties
Nabanita Saha Chowdhury a, Chhandasi GuhaRoy a, Ray J. Butcher b, Samaresh Bhattacharya a,
⇑
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India
b Department of Chemistry, Howard University, Washington, DC 20059, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 May 2013
Received in revised form 24 June 2013
Accepted 25 June 2013
Available online 4 July 2013
Reaction of 1,3-diaryltriazenes (abbreviated in general as HL-R, where H represents the dissociable N–H
proton and R is the para substituent (R = OCH3, CH3, H, Cl, NO2) on the aryl fragment) with [Ru(PPh3)2
(CO)2Cl2] in 2-methoxyethanol in the presence of a base (NEt3) affords a family of yellow complexes of
the type [Ru(PPh3)2(L-R)(CO)(H)], where the 1,3-diaryltriazenes are coordinated as monoanionic biden-
tate NN-donors. Two triphenylphosphines, a hydride and a carbonyl are also coordinated to ruthenium.
The triphenylphosphines are mutually trans, and the hydride and carbonyl are mutually cis. Structure of
the [Ru(PPh3)2(L-H)(CO)(H)] complex has been determined by X-ray crystallography. All the complexes
show intense absorptions in the visible region, which are assigned, based on DFT calculations, to transi-
tions within orbitals of the triazenide ligand. The complexes exhibit an irreversible oxidation on the posi-
tive side of SCE and an irreversible reduction on the negative side. All the complexes are found to
efficiently catalyze transfer hydrogenation reactions.
Keywords:
1,3-Diaryltriazenes
Ruthenium complexes
Crystal structure
Catalytic properties
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
undertaken study has been to synthesize a series of mixed-ligand
ruthenium complexes of 1,3-diaryltriazenes, and find out the coor-
There has been considerable current attention on the coordina-
tion chemistry of the 1,3-diaryltriazenes primarily because of their
varied modes of binding [1–3]. The 1,3-diaryltriazenide anion,
formed in situ via loss of the acidic N–H proton, is a short-bite li-
gand, which is known to serve as monodentate ligand [1], chelating
bidentate ligand [2], and also as bridging ligand [3]. However, in
most of the cases it usually coordinates to a metal center as biden-
tate NN-donor forming a four-membered chelate ring (1) [2]. It is
also worth mentioning that the 1,3-diaryltriazenes find important
applications in biological field [4]. We have recently explored the
chemistry of two groups of 1,3-diaryltriazenide complexes of rho-
dium [5], and the present work has originated from our continued
interest in the triazenide complexes of other platinum group of
metals. For the present study we have chosen a family of five
1,3-diaryltriazenes (HL-R) differing in the para-substituent R on
the aryl fragment, and selected ruthenium as the metal center.
The chosen ligands are abbreviated in general as HL-R, where H
stands for the dissociable N–H hydrogen and R for the para-substi-
tuent. As the ruthenium starting material [Ru(PPh3)2(CO)2Cl2] has
been selected, because of its demonstrated ability to readily
accommodate ligands of different types via displacement of some
of the pre-coordinated ligands [6]. The primary objective of the
dination mode of the triazenes in the complexes. Reaction of the
selected 1,3-diaryltriazenes (HL-R) with [Ru(PPh3)2(CO)2Cl2] has
indeed afforded a group of interesting mixed-ligand complexes,
and herein we describe the chemistry of these complexes, with
special reference to their formation, structure, spectral and electro-
chemical properties, and catalytic activity.
R = OCH3, CH3,
R
R
H, Cl, NO2
N
N
N
.
.
H
.
N
N
M
.
.
N
R
R
1
HL-R
⇑
Corresponding author. Tel./fax: +91 33 24146223.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.06.043

N.S. Chowdhury et al. / Inorganica Chimica Acta 406 (2013) 20–26
21
2.3. Physical measurements
2. Experimental
Microanalyses (C, H and N) were performed using a Heraeus
Carlo Erba 1108 elemental analyzer. Mass spectra were recorded
with a Micromass LCT electrospray (Qtof Micro YA263) mass spec-
trometer by electrospray ionization method. Magnetic susceptibil-
ities were measured using a Sherwood MK-1 balance. 1H NMR
spectra were recorded in CDCl3 solution on a Bruker Avance DPX
300 NMR spectrometer using TMS as the internal standard. IR spec-
tra were obtained on a Shimadzu FTIR-8300 spectrometer with
samples prepared as KBr pellets. Electronic spectra were recorded
on a JASCO V-570 spectrophotometer. Optimization of ground-
state structures and energy calculations for all the complexes were
carried out by density functional theory (DFT) method using the
GAUSSIAN 03 package [10], where B3LYP was chosen as the basis
function and 631g(d,p) basis set was taken for H, C and N, and
SDD basis set for Ru and P. Electrochemical measurements were
made using a CH Instruments model 600A electrochemical ana-
lyzer. 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 elec-
trochemical experiments were performed under a dinitrogen
atmosphere. All electrochemical data were collected at 298 K and
are uncorrected for junction potentials. GC–MS analyses were per-
formed using a Perkin Elmer CLARUS 680 instrument.
2.1. Materials
Commercial ruthenium trichloride, purchased from Arora Mat-
they, Kolkata, India, was converted to RuCl3Á3H2O by repeated evap-
oration with concentrated hydrochloric acid. Triphenylphosphine
was purchased from Loba Chemie, Mumbai, India. The para-substi-
tuted anilines were obtained from S.D. Fine-Chem, Mumbai, India.
[Ru(PPh3)2(CO)2Cl2] and the 1,3-diaryltriazenes were prepared by
following reported procedures [7,8]. Tetrabutylammonium hexafl-
urophosphate (TBHP), obtained from Sigma–Aldrich, was used for
electrochemical work. Purification of dichloromethane and acetoni-
trile for electrochemical work was performed as reported in the lit-
erature [9]. All other chemicals and solvents were reagent grade
commercial materials and were used as received.
2.2. Synthesis of the complexes
All the [Ru(PPh3)2(L-R)(CO)(H)] complexes were synthesized by
following a general procedure. Specific details are given below for a
particular complex.
[Ru(PPh3)2(L-OCH3)(CO)(H)]. To a solution of HL-OCH3 (33 mg,
0.13 mmol) in 2-methoxyethanol (40 mL) was added triethylamine
(13 mg, 0.13 mmol), followed by [Ru(PPh3)2(CO)2Cl2] (100 mg,
0.13 mmol). The mixture was refluxed for 24 h, which yielded a yel-
low solution. The solution was evaporated in air to afford a yellow
solid, which was purified by thin layer chromatography on a silica
plate. With 1:1 hexane–benzene as eluant a yellow band separated,
which was extracted by acetonitrile. Evaporation of this acetonitrile
extract yielded [Ru(PPh3)2(L-OCH3)(CO)(H)] as a yellow crystalline
solid. Yield: 67%. Anal. Calc. for C51H45N3 O3P2Ru: C, 67.25; H, 4.95;
N, 4.62. Found: C, 67.47; H, 4.91; N, 4.67%. Mass: 934, [M+Na]+. 1H
NMR in CDCl3, d ppm1: À12.90 (t, hydride, J = 20.6); 3.46 (s, OCH3);
2.4. Crystallography
Single crystals of [Ru(PPh3)2(L-H)(CO)(H)] were obtained by
slow diffusion of hexane into a solution of the complex in dichlo-
romethane. Selected crystal data and data collection parameters
are given in Table 1. Data were collected on a Bruker SMART Apex
CCD area detector using graphite monochromated Mo Ka radiation
(k = 0.71073 Å). X-ray data reduction, structure solution and
refinement were done using SHELXS-97 and SHELXL-97 programs
[11]. The structure was solved by the direct methods.
3.51 (s, OCH3); 6.64 (d, 2H, J = 8.9); 6.76 (d, 2H, J = 9.0); 6.90 (d, 2H,
J = 8.9); 7.01 (d, 2H, J = 9.3); 7.14–7.70 (30H ). IR, m cmÀ1: 1926,
⁄
ꢀ
1634, 1501, 1438, 1280, 1098, 818, 745, 694 and 518.
[Ru(PPh3)2(L-CH3)(CO)(H)]. Yield: 72%. Anal. Calc. for C51H45N3-
O1P2Ru: C, 69.70; H, 5.13; N, 4.78. Found: C, 69.67; H, 5.02; N,
4.80%. Mass: 902, [M+Na]+. 1H NMR in CDCl3, d ppm: À12.87 (t, hy-
dride, J = 20.6); 2.17 (s, CH3); 2.23 (s, CH3); 6.33 (d, 2H, J = 7.2); 6.46
(d, 2H, J = 7.4); 6.79 (d, 2H, J = 7.3); 6.87 (d, 2H, J = 7.6); 6.95–7.70
2.5. Catalysis: general procedure for the transfer hydrogenation
reactions
A mixture of ketone (1 mmol), a known mole percent of the cat-
alyst and KOH (0.06 mmol) was dissolved in 2-propanol (5 mL),
⁄
(30H ). IR,
m
cmÀ1: 1928, 1636, 1502, 1436, 1279, 1094, 819, 746,
ꢀ
694 and 516.
Table 1
Crystallographic data for [Ru(PPh3)2(L-H)(CO)(H)].
[Ru(PPh3)2(L-H)(CO)(H)]. Yield: 70%. Anal. Calc. for C49H41N3O1-
P2Ru: C, 69.18; H, 4.82; N, 4.94. Found: C, 69.46; H, 4.74; N, 4.98%.
Mass: 874, [M + Na]+. 1H NMR in CDCl3, d ppm: À12.85 (t, hydride,
J = 20.6); 6.94 (t, 1H, J = 6.8); 7.04 (t, 1H, J = 7.3); 7.36 (d, 2H,
ꢀ
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C49H41N3O1P2Ru1
850.86
triclinic
J = 7.5); 7.26 (d, 2H, J = 7.2); 7.20–7.70 (34H⁄). IR,
m
cmÀ1: 1928,
ꢀ
P1
11.8767(17)
14.0894(15)
14.9356(15)
66.122(10)
69.210(11)
65.226(12)
2022.7(5)
2
1636, 1500,1435, 1280, 1097, 817, 745, 695 and 517.
b (Å)
c (Å)
[Ru(PPh3)2(L-Cl)(CO)(H)]. Yield: 72%. Anal. Calc. for C49H39N3O1-
P2Cl2Ru: C, 63.98; H, 4.24; N, 4.57. Found: C, 64.10; H, 4.16; N,
4.63%. Mass: 943, [M+Na]+. 1H NMR in CDCl3, d ppm: À12.93 (t, hy-
dride, J = 20.6); 6.75 (d, 2H, J = 7.4); 7.00 (d, 2H, J = 8.6); 7.22–7.70
a
(°)
b (°)
c
(°)
V (Å3)
⁄
cmÀ1: 1929, 1634, 1502, 1436, 1279, 1096, 816, 746,
ꢀ
m
(34H ). IR,
Z
695 and 516.
k (Å)
0.71073
[Ru(PPh3)2(L-NO2)(CO)(H)]. Yield: 69%. Anal. Calc. for C49H39N5-
O5P2Ru: C, 62.55; H, 4.15; N, 7.45. Found: C, 62.61; H, 4.09; N,
7.49%. Mass: 964, [M+Na]+. 1H NMR in CDCl3, d ppm: À12.95 (t, hy-
dride, J = 20.6); 7.03 (d, 2H, J = 8.2); 7.54 (d, 2H, J = 8.3); 7.20–7.89
Crystal size (mm)
0.23 Â 0.36 Â 0.39
T (K)
203
l
R1
wR2
(mmÀ1
)
0.508
0.0369
0.1074
1.04
a
b
⁄
(34H ). IR,
m
cmÀ1: 1930, 1635, 1503, 1438, 1278, 1094, 820, 744,
c
ꢀ
Goodness-of-fit
694 and 515.
a
b
c
R1
wR2 = [
Goodness-of-fit = [
=
R
||Fo| À |Fc||/
R|Fo|.
R
{w(Fo À Fc2)2}/
R .
{w(Fo2)}]1/2
2
Chemical shifts are given in ppm and multiplicity of the signals along with the
associated coupling constants (J in Hz) are given in parentheses. Overlapping signals
are marked with an asterisk.
R
(w(Fo À Fc2)2)/(M À N)]1/2
,
1
2
where M is the number of reflections and N is the number
of parameters refined.

22
N.S. Chowdhury et al. / Inorganica Chimica Acta 406 (2013) 20–26
of type [Ru(PPh3)2(L-R)(CO)(H)] in good yields. Elemental (C, H,
N) analytical data of the complexes are found to be in good agree-
ment with their compositions. The molecular formulae of the com-
plexes are further authenticated by their mass spectral data. In
view of the probable relative dispositions of ligands present in
the [Ru(PPh3)2(L-R)(CO)(H)] complexes, four geometrical isomers
(I–IV) are, in principle, possible for them. To find out the
stereochemistry of these complexes, as well as to ascertain the
coordination mode of the 1,3-diaryltriazenes in them, structure
of
a representative member of this family, viz. [Ru(PPh3)2
(L-H)(CO)(H)], has been determined by X-ray crystallography.
The structure is shown in Fig. 1, and some relevant bond parame-
ters are listed in Table 2. The structure shows that 1,3-diaryltria-
zene is coordinated to ruthenium, via loss of the N-H proton, in
the monoanionic bidentate NN-fashion forming a four-membered
chelate ring (I) with a bite angle of 57.74(6)°. Two triphenylphos-
phines, a hydride and a carbonyl are also coordinated to the metal
center. The hydride and the carbonyl are sharing the same equato-
rial plane with the NN-coordinated ligand, and the two triphenyl-
phosphines have taken up the remaining two axial positions, and
hence they are mutually trans. Therefore, out of the four possible
geometrical isomers (I–IV), the [Ru(PPh3)2(L-H)(CO)(H)] complex
has the stereochemistry I. Ruthenium is thus nested in a HCN2P2
coordination sphere in this complex, which is distorted signifi-
cantly from ideal octahedral geometry, as reflected in the bond
parameters around the metal center (Table 2). The Ru–H, Ru–C,
Ru–N and Ru–P distances are all quite normal [12], and so are
the bond distances in the NN-coordinated ligand [5]. As all the
[Ru(PPh3)2(L-R)(CO)(H)] complexes have been synthesized simi-
larly, and they show similar properties (vide infra), the other four
[Ru(PPh3)2(L-R)(CO)(H)] (R – H) complexes are assumed to have
similar structures as the [Ru(PPh3)2(L-H)(CO)(H)] complex. The
Ru–H bond in these [Ru(PPh3)2(L-R)(CO)(H)] complexes is believed
to result from the interaction of a ruthenium-bound chloride, in
any of the possible intermediates, with 2-methoxyethanol. Such
conversion of a Ru–Cl bond into a Ru–H bond in alcoholic solvent
in the presence of base is well documented in the literature [13].
Fig. 1. View of the crystal structure of [Ru(PPh3)2(L–H)(CO)(H)].
and the mixture was heated under reflux for the desired period of
time. The catalyst was removed as precipitate from the reaction
mixture by the addition of diethyl ether followed by filtration
and subsequent neutralization with 5 mL of 1 M HCl. Then the
ether layer was passed through a short path of silica gel and the fil-
trate was subjected to GC analysis for identification and
quantification.
PPh
CO
3
3. Results and discussion
N
CO
N
PPh
3
3.1. Synthesis and structure
Ru
Ru
As outlined in the introduction, the initial aim of the present
study has been to synthesize a group of mixed-ligand ruthenium
complexes via interaction of [Ru(PPh3)2(CO)2Cl2] with a group of
selected 1,3-diaryltriazenes (HL-R). Accordingly, reactions of the
chosen 1,3-diaryltriazenes have been carried out with [Ru(PPh3)2
(CO)2Cl2] in refluxing 2-methoxyethanol in the presence of trieth-
ylamine, which proceed smoothly to afford a family of complexes
N
H
N
PPh
3
PPh
H
3
I
II
PPh
PPh
3
3
Table 2
Selected bond distances and bond angles for [Ru(PPh3)2(L-H)(CO)(H)].
N
CO
N
H
Bond lengths (Å)
Ru–P1
Ru–P2
Ru–N1
Ru–N3
Ru–C1S
Ru–H
2.3468(7)
2.3612(7)
2.1907(18)
2.1467(18)
1.840(2)
C1–N1
N1–N2
N2–N3
N3–C7
C1S–O1S
1.408(2)
1.310(2)
1.324(2)
1.401(3)
1.156(3)
Ru
Ru
N
PPh
N
PPh
3
3
1.61(2)
Bond angles (°)
P1–Ru–P2
C1S–Ru–N3
N1–Ru–H
H
CO
169.42(3)
167.34(9)
155.2(9)
N1–Ru–N3
Ru–N1–N2
N1–N2–N3
Ru–N3–N2
Ru–C1S–O1S
57.74(6)
97.59(12)
105.36(17)
99.24(12)
177.7(2)
III
IV

N.S. Chowdhury et al. / Inorganica Chimica Acta 406 (2013) 20–26
23
Table 3
Electronic spectral and cyclic voltammetric data.
a
Compound
Electronic spectral data kmax, nm (
e
, MÀ1 cmÀ1
)
Cyclic voltammetric datab E, V vs SCE
[Ru(PPh3)2(L-OCH3)(CO)(H)]
[Ru(PPh3)2(L-CH3)(CO)(H)]
[Ru(PPh3)2(L-H)(CO)(H)]
[Ru(PPh3)2(L-Cl)(CO)(H)]
[Ru(PPh3)2(L-NO2)(CO)(H)]
431(4500), 330(5200)c, 259(12700)c
426(7700), 314(4600)c, 265(12000)c
419(6500), 308(5400)c, 259(11,600)c
422(5300), 309(2600)c, 258(8900)c
354(3900), 241(3200)c
0.53d, À1.05e
0.60d, À1.03e
0.75d, À1.01e
1.11d, À1.08e
1.40d, À0.99e
a
In dichloromethane.
b
Solvent, 1:9 dichloromethane–acetonitrile; supporting electrolyte, TBHP; scan rate 50 m VsÀ1
.
c
d
e
Shoulder.
Anodic peak potential (Epa) value.
Cathodic peak potential (Epc) value.
LUMO
Table 4
Composition of selected molecular orbital of the complexes.
Complex
Contributing fragments % Contribution of
fragments to
HOMO
LUMO
[Ru(PPh3)2(L-OCH3)(CO)(H)] Ru
CO
11.23
4.16
19.42
4.22
L-OCH3
84.61
76.36
HOMO
[Ru(PPh3)2(L-CH3)(CO)(H)]
[Ru(PPh3)2(L-H)(CO)(H)]
[Ru(PPh3)2(L-Cl)(CO)(H)]
[Ru(PPh3)2(L-NO2)(CO)(H)]
Ru
CO
10.74
4.25
17.94
4.01
L-CH3
85.01
78.05
Ru
CO
L-H
9.86
4.01
86.13
16.19
4.38
79.43
Ru
CO
L-Cl
13.62
4.39
81.99
19.95
3.97
76.08
Fig. 2. Frontier orbital diagram of [Ru(PH3)2(L–H)(CO)(H)].
Ru
CO
16.02
4.68
22.56
4.77
L-NO2
79.30
72.67
observed at around 516, 694 and 746 cmÀ1, in the spectra of all
the complexes are attributable to the coordinated PPh3 ligands. A
distinct band observed near 1926 cmÀ1 in all the complexes is
due to the coordinated carbonyl ligand. The N–H stretch, observed
near 3200 cmÀ1 in the uncoordinated 1,3-diaryltriazenes, is found
to be absent in the spectra of all the complexes, as expected. Com-
parison of the spectrum of each [Ru(PPh3)2(L-R)(CO)(H)] complex
with that of [Ru(PPh3)2(CO)2Cl2] shows the presence of some
new bands near 1636, 1502, 1436, 1279, 1094 and 819 cmÀ1 in
the former, which are attributable to the coordinated 1,3-diary-
ltriazenide ligand.
3.2. Spectral properties
Magnetic susceptibility measurements show that the
[Ru(PPh3)2(L-R)(CO)(H)] complexes are diamagnetic, which corre-
sponds to the +2 oxidation state of ruthenium (low-spin d6, S = 0)
in them. 1H NMR spectra of the [Ru(PPh3)2(L-R)(CO)(H)] complexes
have been recorded in the CDCl3 solution. In all the [Ru(PPh3)2(L-
R)(CO)(H)] complexes
a distinct triplet is observed around
À12.9 ppm indicating the presence of a coordinated hydride. The
triplet nature of this signal, with a coupling constant of about
21.0 Hz, is attributable to coupling with the two magnetically
equivalent phosphorus nuclei. The signals for the coordinated tri-
phenylphosphines are broad in nature and appear within 7.0–
7.6 ppm. In the [Ru(PPh3)2(L-CH3)(CO)(H)] complex, signals for
the two methyl substituents in the aryl fragments of the coordi-
nated 1,3-diaryltriazenide ligand are observed at 2.10 and
2.28 ppm, while the methoxy-methyl signals in the [Ru(PPh3)2(L-
OCH3)(CO)(H)] complex are observed at 3.46 and 3.51 ppm. Out
of the expected signals for the aromatic protons in the coordinated
1,3-diaryltriazenide ligand, most could be clearly identified in all
the complexes within 6.3–7.5 ppm, while few could not be traced
in some of the complexes due to their overlap with the broad sig-
nals from the triphenylphosphines. Thus the observed NMR spec-
tral data of the [Ru(PPh3)2(L-R)(CO)(H)] complexes are found to
be in well agreement with their composition and stereochemistry.
Infrared spectra of the [Ru(PPh3)2(L-R)(CO)(H)] complexes show
many vibrations of different intensities in the 4000–400 cmÀ1 re-
gion. Assignment of each individual band to a specific vibration
has not been attempted. However, three prominent bands,
The [Ru(PPh3)2(L-R)(CO)(H)] complexes are readily soluble in
dichloromethane, chloroform, acetone, acetonitrile, ethanol, meth-
anol, etc., producing intense yellow solutions. Electronic spectra of
the complexes have been recorded in dichloromethane solution.
All the complexes show intense absorptions in the visible and
ultraviolet regions (Table 3). The absorptions in the ultraviolet re-
gion are believed to be due to transitions within the ligand orbitals.
To have an insight into the nature of absorptions in the visible re-
gion, DFT calculations have been performed on all the complexes,
where the phenyl rings of the triphenylphosphines have been re-
placed by hydrogens. Structures of all the [Ru(PH3)2(L-R)(CO)(H)]
complexes have been optimized, and the optimized structures
are found to be very similar to the crystal structure of [Ru(PPh3)2
(L-H)(CO)(H)]. For comparison, the DFT-optimized structure of
[Ru(PH3)2(L-H)(CO)(H)] is shown in Fig. S1 (Supporting informa-
tion) and the computed bond parameters are presented in
Table S1 (Supporting information). The results are found to be
qualitatively similar for all the complexes. The highest occupied
molecular orbital (HOMO) is found to be localized primarily
(>79%) on the 1,3-diaryltriazenide ligand, with much less contribu-
tion (<17%) coming from the metal center. The lowest unoccupied

24
N.S. Chowdhury et al. / Inorganica Chimica Acta 406 (2013) 20–26
Table 5
Table 6
Screening of experimental conditions.a
Catalytic transfer hydrogenation of ketones and aldehydes.a
catalystb
O
OH
catalystb
solvent, base
O
OH
R'
2-propanol, KOH
R
R'
R
Entry
1
Substrate
Yieldc (%)
99
O
Entry
Mol% of catalyst
Solvent
Base
Time (h)
Yieldc (%)
1
2
3
4
5
6
7
8
9
0.001
0.001
0.001
0.001
0.001
–
0.001
0.001
0.0001
2-propanol
2-propanol
2-propanol
Ethanol
KOH
K3PO4
–
KOH
KOH
KOH
NaOH
KOH
KOH
2
2
8
8
8
8
2
1
99
30
N.O.d
45
2
3
62
92
O
PEG
58
N.O.d
74
CH3
O
2-propanol
2-propanol
2-propanol
2-propanol
62
44
10
a
Reaction conditions: benzophenone (1.0 mmol), base (0.06 mmol), solvent
CH3
(5 mL), reaction temperature (80 °C).
b
Catalyst: [Ru(PPh3)2(L-CH3)(CO)(H)].
Determined by GC–MS.
Not observed.
Br
c
4
5
96
58
O
d
H
molecular orbital (LUMO) is also observed to have less (<23%)
ruthenium character and dominant (>72%) 1,3-diaryltriazenide li-
gand character. Compositions of the HOMO and LUMO for all the
complexes are given in Table 4, and contour plots of these orbitals
O
H
H3CO
for
a representative complex, viz. [Ru(PPh3)2(L-H)(CO)(H)], is
6
7
99
90
O
shown in Fig. 2. The lowest-energy absorption in the visible region
in all these complexes is therefore assignable to a transition from a
filled orbital of mixed (ruthenium and 1,3-diaryltriazenide ligand)
nature to a vacant orbital of similar nature.
H
Br
O
3.3. Electrochemical properties
H
Redox properties of the [Ru(PPh3)2(L-R)(CO)(H)] complexes
have been studied by cyclic voltammetry in 1:9 dichlorometh-
ane-acetonitrile solution (0.1 M TBAP)2. The cyclic voltammetric
data are presented in Table 3, and a representative cyclic voltammo-
gram is shown in Fig. S2 (Supporting information). All the complexes
exhibit an irreversible oxidation on the positive side of SCE and one
irreversible reduction on the negative side. In view of composition of
the HOMO, the oxidation, observed within 0.50–1.40 V, is assignable
to an oxidation of the coordinated triazenide ligand. Similarly, based
on composition of the LUMO, the reductive response, observed
around À1.0 V, may be assigned to a reduction of the triazene frag-
ment in the NN-coordinated ligand. Potential of the oxidation is
found to be sensitive to the nature of the substituent R in the
coordinated triazenide ligand, and it increases with increasing elec-
tron-withdrawing character of the substituent R. However, such a
systematic variation of potential has not been observed for the
reductive response.
Cl
8
51
O
H
N
N
9
37
64
O
CH3
10
O
a
Reaction conditions: aldehyde or ketone (1.0 mmol), KOH (0.06 mmol), 2-pro-
panol (5 mL), reaction temperature (80 °C), reaction time (2 h).
b
Catalyst: [Ru(PPh3)2(L-CH3)(CO)(H)] (0.001 mol%).
c
Determined by GC–MS.
3.4. Catalysis
Ru–H bond in the [Ru(PPh3)2(L-R)(CO)(H)] complexes has encour-
aged us to explore their catalytic potential towards transfer hydro-
genation reaction. Accordingly, hydrogenation of various types of
aldehydes and ketones was carried out in 2-propanol using
0.001 mol% of the complexes, in the presence of KOH. The products
were isolated in moderate to high yield and characterized by GC
method. We began our study by examining transfer hydrogenation
of benzophenone using the [Ru(PPh3)2(L-CH3)(CO)(H)] complex as
catalyst to furnish benzhydrol. Table 5 provides information on the
impact of various reaction parameters on the efficiency of this pro-
cess. After extensive optimization, we found that 0.001 mol%
[Ru(PPh3)2(L-CH3)(CO)(H)] catalyst, 0.06 mmol KOH, 2-propanol
Transfer hydrogenation of carbonyl compounds using 2-propa-
nol as reducing agent, as well as solvent, has attracted a lot of
attention over last few years due to the comparatively benign nat-
ure of the reagents [14]. While a number of transition metal cata-
lysts have been used for this transformation, the main focus has
always been on ruthenium(II) complexes [15]. All these transfer
hydrogenation reactions are known to proceed through the inter-
mediacy of a ruthenium–hydride species, and availability of a
2
A
little dichloromethane was necessary to take the complex into solution.
Addition of large excess of acetonitrile was necessary to record the redox responses in
proper shape.

N.S. Chowdhury et al. / Inorganica Chimica Acta 406 (2013) 20–26
25
as solvent, reaction temperature 80 °C, reaction time 2 h, furnish
an excellent yield (99%) of the targeted product (entry 1). As ex-
pected, in the absence of ruthenium catalyst the desired reaction
does not proceed (entry 6). It has been observed that the presence
of base plays an essential role in the reaction, and KOH is proved to
be most effective base here. Replacement of KOH by K3PO4 or
NaOH leads to a drop in yield (entries 2 and 7). Moreover, choice
of appropriate solvent plays a crucial role to get the desired prod-
uct in satisfactory yield. Among the different solvents examined,
2-propanol was the most suitable for the catalytic reaction. We
also tested the reaction at lower catalyst loading and found the
reaction to be incomplete even after 10 h (entry 9).
The scope of the reaction is shown in Table 6. All the
[Ru(PPh3)2(L-R)(CO)(H)] complexes have shown comparable cata-
lytic efficiency, and only the results obtained with [Ru(PPh3)2
(L-CH3)(CO)(H)] as the catalyst are highlighted here. The same
transfer hydrogenation reaction, when carried out using acetophe-
none instead of benzophenone, showed much lower yield (entry
2). When p-bromoacetophenone was used instead of acetophenone
the same reaction was achieved in higher yield (entry 3). Use of
benzaldehyde as substrate, increases the yield to a great extent
(entry 4). The yield varies significantly with electronic effect of
the substituents at the para position (entries 5 and 6). When pyri-
dyl group was used instead of phenyl group in the aldehyde part,
the yield was lower (entry 8). Again, substitution of the aldehy-
dic-hydrogen by a methyl group causes sharp decrease in the yield
(entry 9). Use of cyclic aliphatic ketone decreases the yield of prod-
uct alcohol to some extent (entry 10). The observed catalytic activ-
ity of the complexes is found to be comparable to that of some
recently reported species [16]. The present group of mixed-ligand
ruthenium 1,3-diaryltriazenide complexes, viz. [Ru(PPh3)2(L-
R)(CO)(H)], are thus found to be efficient catalysts for transfer-
hydrogenation of aldehydes and ketones, using 2-propanol as the
hydrogen-provider. The noticeable aspects of the observed cataly-
sis are (i) good yields of the products, (ii) low catalyst loading, (iii)
relatively mild reaction conditions, and (iv) short reaction time.
rate of 50 m VsÀ1 (Fig. S2) have been deposited. Supplementary
data associated with this article can be found, in the online version,
at http://dx.doi.org/10.1016/j.ica.2013.06.043.
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Acknowledgments
We thank the reviewers for their constructive comments, which
have been helpful in preparing the revised manuscript. We thank
the Department of Science and Technology, New Delhi, India
(Grant No. SR/S1/IC-29/2009) for financial assistance. NSC thanks
the University Grants Commission, New Delhi, for her fellowship.
Appendix A. Supplementary material
CCDC 894827 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/
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H)(CO)(H)] (Fig. S1) and some computed bond parameters
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