Ruthenium(III)/phosphine/pyridine complexes applied in the hydrogenation reactions of polar and apolar double bonds

Article abstract of DOI:10.1016/j.molstruc.2016.01.080

In this work, five ruthenium(III) complexes containing phosphine and pyridine based ligands with general formula mer-[RuCl₃(dppb)(N)] [where dppb = 1,4-bis(diphenylphosphino)butane and N = pyridine (py), 4-methylpyridine (4-Mepy), 4-vinylpyridine (4-Vpy), 4-tert-butylpyridine (4-tBupy) and 4-phenylpyridine (4-Phpy)] were synthesized and characterized using spectroscopic and electrochemical techniques, as well as magnetic susceptibility to check the paramagnetism of these compounds. These complexes were tested as catalytic precursors in hydrogenation reactions with cyclohexene, undecanal and cyclohexanecarboxaldehyde, as compounds bearing C=C and C=O groups. Broad screening was carried out in order to find the optimal reaction conditions with the highest conversion. It was found that by using a ratio of Ru-catalyst/substrate = 1:530 at 80°C and 15 bar of H₂ for 24 h, cyclohexene can be reduced. Hydrogenation of undecanal was possible using a Ru-catalyst/substrate ratio of 1:100 at 160°C and 100 bar for 24 h, and for the reduction of cyclohexanecarboxaldehyde the reaction conditions were Ru-catalyst/substrate ratio of 1:100 at 160°C and 50 bar for 24 h.

Full text of DOI:10.1016/j.molstruc.2016.01.080

Journal of Molecular Structure 1111 (2016) 84e89
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Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Ruthenium(III)/phosphine/pyridine complexes applied in the
hydrogenation reactions of polar and apolar double bonds
,
1
a
a
b
Claudia Rodrigues a , Fabio G. Delolo , Lucas M. Ferreira , Pedro I. da S. Maia ,
ꢀ
b
c
c
c
c
€
Victor M. Deflon , Jabor Rabeah , Angelika Brückner , Jakob Norinder , Armin Borner ,
Andre L. Bogado **, Alzir A. Batista a
d,
, *
ꢀ
a
~
~
Departamento de Química, Universidade Federal de Sao Carlos, Rod. Washington Luís, km 235 e SP-310, CEP 13565-905 Sao Carlos, SP, Brazil
Instituto de Química de Sao Carlos, Universidade de Sao Paulo, Av. Trabalhador Sao-carlense, 400, CEP 13566-590 Sao Carlos, SP, Brazil
Leibniz-Institut für Katalyse an der Universitat Rostock e.V., A.-Einstein-Str. 29a, 18059 Rostock, Germany
Faculdade de Ciencias Integradas do Pontal, Universidade Federal de Uberlandia, Rua vinte, 1600, CEP 38304-402 Ituiutaba, MG, Brazil
b
c
~
~
~
~
€
d
^
^
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work, five ruthenium(III) complexes containing phosphine and pyridine based ligands with
Received 3 November 2015
Received in revised form
27 January 2016
Accepted 28 January 2016
Available online 29 January 2016
general formula mer-[RuCl3(dppb)(N)] [where dppb
¼
1,4-bis(diphenylphosphino)butane and
N ¼ pyridine (py), 4-methylpyridine (4-Mepy), 4-vinylpyridine (4-Vpy), 4-tert-butylpyridine (4-tBupy)
and 4-phenylpyridine (4-Phpy)] were synthesized and characterized using spectroscopic and electro-
chemical techniques, as well as magnetic susceptibility to check the paramagnetism of these compounds.
These complexes were tested as catalytic precursors in hydrogenation reactions with cyclohexene,
undecanal and cyclohexanecarboxaldehyde, as compounds bearing C]C and C]O groups. Broad
screening was carried out in order to find the optimal reaction conditions with the highest conversion. It
was found that by using a ratio of Ru-catalyst/substrate ¼ 1:530 at 80 ꢀC and 15 bar of H2 for 24 h,
cyclohexene can be reduced. Hydrogenation of undecanal was possible using a Ru-catalyst/substrate
ratio of 1:100 at 160 ꢀC and 100 bar for 24 h, and for the reduction of cyclohexanecarboxaldehyde the
reaction conditions were Ru-catalyst/substrate ratio of 1:100 at 160 ꢀC and 50 bar for 24 h.
© 2016 Elsevier B.V. All rights reserved.
Keywords:
Ruthenium(III) complexes
Phosphine-pyridine ligands
Homogeneous catalysis
Hydrogenation reactions
1. Introduction
electrochemical properties of this aqua-complex make it a unique
compound to be used in different applications, such as a potential
The chemistry of ruthenium complexes is very attractive due to
the wide range of applications of these compounds in electron
transfer reactions [1], as homogeneous catalysts in numerous types
of organic reactions [2e8], as thin films [9e12], as compounds with
potential anti-Trypanosomal activity [13e17], antitumor [18e22],
antiparasitic [14] and anti-tuberculosis [23,24].
The aqua-complex mer-[RuCl3(dppb)(H2O)] [25] is frequently
used in our group due to its versatility as a precursor for the syn-
thesis of various new [Run-Cln(dppb)] containing compounds with
different dimensionality, such as mononuclear, binuclear and su-
anticancer drug [29], electrochemical devices [30,31], and as cata-
lysts in the hydrogenation of ketones [32].
Thus, having in mind that ruthenium complexes have been used
as homogeneous catalysts for a series of alkene reactions, in this
study we decided to synthesize and characterize some 6-
coordinate, octahedral, mer-[RuCl3(dppb)(N)] (N ¼ pyridine and
derivatives) complexes and to study their catalytic properties in
hydrogenation reactions of cyclohexene, undecanal and cyclo-
hexanecarboxaldehyde. Therefore, five Ru complexes with phos-
phine and pyridine based ligands, namely mer-[RuCl3(dppb)(N)]
[dppb ¼ 1,4-bis(diphenylphosphino)butane and N ¼ pyridine (py)
1, 4-methylpyridine (4-Mepy) 2, 4-vinylpyridine (4-Vpy) 3, 4-tert-
butylpyridine (4-tBupy) 4 and 4-phenylpyridine (4-Phpy) 5], were
synthesized. The complexes with pyridine, 4-methylpyridine and
4-vinylpyridine were previously reported [26], but nothing is
known about their catalytic properties. Pivotal catalytic parameters
such as reaction time, catalyst concentration, gas pressure and
pramolecular
complexes
[26e28].
The
structural
and
* Corresponding author.
** Corresponding author.
E-mail addresses: albogado@pontal.ufu.br (A.L. Bogado), daab@ufscar.br
(A.A. Batista).
1
In memoriam of Claudia Rodrigues.
http://dx.doi.org/10.1016/j.molstruc.2016.01.080
0022-2860/© 2016 Elsevier B.V. All rights reserved.

C. Rodrigues et al. / Journal of Molecular Structure 1111 (2016) 84e89
85
temperature were studied in order to identify the optimal condi-
tions for activity of the catalysts.
N, 1.94%. IR (KBr): 695 and 498 cmꢁ1
,
n
(C¼Npy), 512 cmꢁ1
,
n
(RueP),
413 cmꢁ1 (RueN), 335, 313 and 286 cmꢁ1
n , n(RueCl). UVeVis
(CH2Cl2): 528 (1248), 450 (800) and 349 (1125) Mꢁ1 cmꢁ1
.
2. Experimental section
mer-[RuCl3(dppb)(4-tBupy)] 4 Yield: 87 mg (83%). Calc. for
C
37H41NP2Cl3Ru: C, 57.78; H, 5.37; N, 1.82%. Found: C, 57.51; H, 5.43;
2.1. Materials and methods
N, 1.77%. IR (KBr): 694 and 500 cmꢁ1
,
n
(C¼Npy), 514 cmꢁ1
, n(RueP),
416 cmꢁ1 (RueN), 336, 316 and 287 cmꢁ1
, n , n(RueCl). UVeVis
2.1.1. Chemistry
(CH2Cl2): 530 (1860), 441 (1200) and 350 (1750) Mꢁ1 cmꢁ1
.
Solvents were purified by standard methods. All chemicals used
were of reagent grade or comparable purity. RuCl3$3H2O, triphe-
nylphosphine (PPh3), 1,4-bis(diphenylphosphino)butane (dppb),
pyridine (py), 4-methylpyridine (4-Mepy), 4-vinylpyridine (4-Vpy),
4-tert-butylpyridine (4-tBupy) and 4-phenylpyridine (4-Phpy)
were used as received from Aldrich. The precursors [RuCl2(PPh3)3],
[RuCl2(dppb)(PPh3)] and mer-[RuCl3(dppb)(H2O)] were prepared
according to the literature [25,33,34]. The mer-[RuCl3(dppb)(py)] 1,
mer-[RuCl3(dppb)(4-Mepy)] 2, mer-[RuCl3(dppb)(4-Vpy)] 3, mer-
[RuCl3(dppb)(4-tBupy)] 4 and mer-[RuCl3(dppb)(4-Phpy)] 5 com-
plexes were prepared according to the modified procedures [26].
mer-[RuCl3(dppb)(4-Phpy)] 5 Yield: 89 mg (84%). Calc. for
C
39H37NP2Cl3Ru: C, 59.36; H, 4.73; N, 1.78%. Found: C, 59.66; H,
4.63; N, 1.77%. IR (KBr): 695 and 495 cmꢁ1
,
n
(C¼Npy), 513 cmꢁ1
,
n
(RueP), 413 cmꢁ1 (RueN), 336, 313 and 290 cmꢁ1
, n , n(RueCl).
UVeVis (CH2Cl2): 536 (1250), 449 (850) and 353 (1450) Mꢁ1 cmꢁ1
.
2.1.5. Instrumentation
Elemental analyses were performed in a Fison EA 1108 model.
The FTIR spectra of the powder complexes were recorded from
KBr pellets in the 4000e200 cmꢁ1 range, in a BomeneMichelson FT
MB-102 instrument.
The UVeVis spectra of the complexes, in dichloromethane so-
lution, concentration of 1 ꢂ 10ꢁ3 mol Lꢁ1, were recorded with a
Hewlett Packard diode array e 8452 A.
2.1.2. X-ray crystallography
Red crystals of mer-[RuCl3(dppb)(4-Phpy)] were grown by slow
evaporation of a dichloromethane/n-hexane/diethyl ether solution
(2:1:1) at room temperature. The data collection was performed
The Electron Paramagnetic Resonance (EPR) spectra in the solid
state was measured at 77 K using a Varian E-109 instrument
operating at the X band frequency, within a rectangular cavity (E-
248) fitted with a temperature controller. X-band (9.4 GHz) EPR
spectra were recorded at 100 K on an EMX cw-spectrometer
(Bruker) equipped with a liquid N2 cryostat and a temperature
controller with a modulation frequency of 100 kHz, a microwave
power 6.3 mW and modulation amplitude of 5 G.
using Mo-K
a
radiation (
l
¼ 71.073 pm) on a BRUKER APEX II Duo
diffractometer. Data reduction and absorption correction were
carried out with the Bruker SAINT package. The structure was
solved with SHELXS97 using direct methods [35] and all non-
hydrogen atoms were refined with anisotropic displacement pa-
rameters with SHELXL97 [36]. The hydrogen atoms were calculated
at idealized positions using the riding model option of SHELXL97
[36]. See in supporting information Table 1S, the detailed infor-
mation about the structure determination.
Cyclic voltammetry (CV) experiments of the complexes, in so-
lution, were conducted in an electrochemical analyzer BAS model
100B Instrument. These experiments were carried out at room
temperature in CH2Cl2 containing 0.10 mol Lꢁ1 Bu4NClO4 (TBAP)
(Fluka Purum) as a support electrolyte using a one-compartment
cell, where the working and auxiliary electrodes were stationary
Pt foils, and the reference electrode was Ag/AgCl, 0.10 mol Lꢁ1 TBAP
in CH2Cl2. Under these conditions ferrocene was oxidized at 0.43 V
(Fcþ/Fc).
2.1.3. Catalytic studies
Hydrogenation reactions were performed in 25 and 75 mL
stainless steel autoclaves equipped with an overhead magnetic
stirrer, a pressure indicator and a thermocouple for temperature
registration. The autoclaves were equipped with an electrical
heating/cooling system to control the temperature inside the
vessel. The hydrogenation active catalyst substrate/Ru-complex
was prepared in situ, once the Ru-complexes used were pre-
catalysts. The autoclave was charged with Ru-complex (0.013,
0.015 or 0.026 mmol), substrate (1.5 or 6.9 mmol), MeOH (20 or
6 mL). The system was flushed three times with H2. Then, the
autoclave was pressurized with H2 (15, 50 or 100 bar) and heated to
a temperature of 80 or 160 ꢀC, for 15 or 24 h. After the reaction, the
homogeneous reaction mixture was cooled down in an ice bath to
room temperature, and the upper organic layer was analyzed by
GC-FID and GC-MS.
All the NMR experiments were recorded on BRUKER
DRX400 MHz equipment; in a BBO 5 mm probe at 298 K, using
CDCl3 (1H) and CH2Cl2
(
31P{1H}) as solvents, TMS for internal
reference for 1H.
GC analysis was run on a Shimadzu CLASS-VPTM instrument
(50 m capillary column, carrier gas: 3 atm N2 and FID detector).
3. Results and discussion
3.1. Characterization of complexes containing Ru(III)
IR spectra for all complexes show the typical band of coordi-
2.1.4. Preparation of ruthenium (III) complexes
nated phosphine 512e514 cmꢁ1
413e421 cmꢁ1
n
(RueP) and pyridine ligands
mer-[RuCl3(dppb)(N)], N ¼ 4-vinylpyridine (4-Vpy), 4-tert-butyl-
pyridine (4-tBupy) and 4-phenylpyridine (4-Phpy): Ruthenium (III)
complexes 3, 4 and 5 were synthesized similar to described in the
literature for the synthesis of the 1 and 2 [26], reacting an equi-
molar amount of N-heterocyclic ligands (0.153 mmol) with the mer-
[RuCl3(dppb)(H2O)] precursor (0.100 g, 0.153 mmol). The reaction
mixture was refluxed and stirred for 6 h, under Ar atmosphere, in a
Schlenk flask. The final pink solutions were concentrated to ca.
2 mL and 10 mL of n-hexane, previously degassed, were added in
order to obtain pink precipitates. The solids were filtered off, well
rinsed with n-hexane and diethyl ether and dried in vacuum.
n(RueN). Three bands were observed, from 336 to
286 cmꢁ1 n(RueCl), suggesting a meridional (mer) arrangement of
the chlorine ligands [37]. The electronic spectra of the complexes in
CH2Cl2 solutions in the UVeVis region, showed three characteristic
bands. The bands near 530, 450, and 350 nm can be tentatively
attributed to Cl/Ru, N/Ru and P/Ru, ligand-metal charge
transfer (LMCT), respectively [25,26].
The presence of ruthenium(III) species was confirmed by mag-
netic susceptibility experiments and EPR measurements (see Fig.1).
The EPR spectra of 1e3 complexes measured in frozen dichloro-
methane solution are shown in Fig. 1. All these complexes show
rhombic signals, typical for isolated Ru(III) ions in a rhombic
environment [38,39]. The positions of the signals were reproduced
mer-[RuCl3(dppb)(4-Vpy)]
3 Yield: 90 mg (85%). Calc. for
C
35H35NP2Cl3Ru: C, 56.88; H, 4.77; N,1.90%. Found: C, 57.10; H, 5.03;

86
C. Rodrigues et al. / Journal of Molecular Structure 1111 (2016) 84e89
Scheme 1. Redox mechanism of mer-[RuCl3(dppb)(N)] complexes, where N ¼ pyridine
and derivatives.
Table 1
Redox potentials of cyclic voltammograms of mer-[RuCl3(dppb) (N)] complexes (Ag/
AgCl, 1.0 ꢂ 10ꢁ3 mol Lꢁ1, 0.1 mol Lꢁ1 TBAP, CH2Cl2; sweep rate 100 mV sꢁ1).
Complexes
Epa (V)
Epc(V)
(1)
(3)
(2)
(10)
(30)
(20)
Fig. 1. EPR spectra of mer-[RuCl3(dppb)(py)] 1 (AQPy), mer-[RuCl3(dppb)(4-Mepy)] 2
(AQPic) and mer-[RuCl3(dppb)(4-tBupy)] 3 (AQ4tBu) in frozen dichloromethane solu-
tion recorded at 100 K.
1
2
3
4
5
0.15
0.12
0.12
0.18
0.19
0.50
0.48
0.42
0.50
0.52
0.60
0.65
0.54
0.65
0.65
ꢁ0.01
ꢁ0.07
ꢁ0.07
0.03
0.41
0.42
0.38
0.45
0.45
0.57
0.58
0.49
0.61
0.59
by simulating the corresponding experimental spectra [40]
[AQPy ¼ 1 (g1 ¼ 2.774, g2 ¼ 2.053 g3 ¼ 1.635), AQ4Pic ¼ 2
(g1 ¼ 2.786, g2 ¼ 2.066 g3 ¼ 1.646), and AQ4tBu ¼ 4 (g1 ¼ 2.71,
g2 ¼ 2.093 g3 ¼ 1.716)] (see Fig.1). These spectra are similar to those
of mer-[RuCl3(dppb)(py)] (g1 ¼ 2.928, g2 ¼ 2.037, g3 ¼ 1.607) and
mer-[RuCl3(dppb)(4-Mepy)] (g1 ¼ 2.487, g2 ¼ 2.101, g3 ¼ 1.866),
previously reported in the literature [26]. The g-anisotropy of the
Ru complexes increases in the order of AQPy < AQ4Pic < AQ4tBu,
more probably due to the shape anisotropy imposed by the pres-
ence of different organic substituents in the pyridine ligand.
Cyclic voltammograms of the complexes, in CH2Cl2 solutions, at
room temperature, reveal an irreversible Ru(III) / Ru(II) process
(only one electron involved). In the first cycle, starting at 0.50 V, a
reduction process was observed at ꢁ0.09 V (peak 10) (See Fig. 2). In
the second cycle, after inversion of the scan rate (at ꢁ0.40 V), two
poorly defined oxidation processes were derived characterized by
anodic peak potential (Epa) at 0.52 V and 0.65 V, peaks 3 and 2,
respectively. This electrochemical behavior can be explained
considering that with the reduction of Ru III/II, at ꢁ0.07 to 0.03 V (in
the first cycle), the species [Ru2Cl5(dppb)2] (2), [Ru2Cl4(dppb)2(L)]
(3) and [RuCl2(dppb)(L)2] (4) should be generated on the electrode
surface. In this case [Ru2Cl5(dppb)2] (eq. 2), [Ru2Cl4(dppb)2(N)] (eq.
ꢁ0.09
Epa ¼ anodic peak potential, Epc ¼ cathodic peak potential. Labels of the studied
complexes: mer-[RuCl3(dppb)(py)] 1, mer-[RuCl3(dppb)(4-Mepy)] 2, mer-
[RuCl3(dppb)(4-Vpy)] 3, mer-[RuCl3(dppb)(4-tBupy)]
Phpy)] 5.
4 and mer-[RuCl3(dppb)(4-
3) and [RuCl2(dppb)(N)2] (eq. 4) complexes were generated, ac-
cording to a suggested mechanism (see Scheme 1) (Table 1)[26].
An ORTEP representation of the molecular structure of mer-
[RuCl3(dppb)(4-Phpy)], as well as the numbering scheme are pre-
sented in Fig. 3. Selected bond lengths and angles for this com-
pound are shown in the caption of Fig. 3. The crystal structure
presents a 6-coordinated complex where the metal center is sur-
rounded by two phosphorous atoms, P(1) and P(2) of the bidentate
dppb ligand, the pyridine nitrogen atom N(1) of the 4-Phpy ligand
and three monoanionic chloride ligands in mer configuration,
similar to those found in mer-[RuCl3(dppb)(4-Mepy)] [26]. After
coordination, the N-heterocyclic ligand occupies the position that
was previously occupied by H2O in the precursor, trans positioned
Fig. 3. Ellipsoid representation of the molecular structure of mer-[RuCl3(dppb)(4-
Phpy)], 50% of probability ellipsoids. Hydrogen atoms have been omitted for clarity.
Selected bond lengths [Å]: Ru(1)-N(1) 2.1920(19), Ru(1)-Cl(1) 2.3304(6), Ru(1)-Cl(2)
2.3373(6), Ru(1)-Cl(3) 2.4056(6), Ru(1)-P(1) 2.4166(6), Ru(1)-P(2) 2.3438(6). Selected
Bond Angles [ꢀ]: N(1)-Ru(1)-Cl(1) 88.26(5), N(1)-Ru(1)-Cl(2) 89.45(5), Cl(1)-Ru(1)-
Cl(2) 175.50(2), N(1)-Ru(1)-P(2) 174.67(5), Cl(1)-Ru(1)-P(2) 91.73(2), Cl(2)-Ru(1)-P(2)
90.91(2), N(1)-Ru(1)-Cl(3) 82.54(5), Cl(3)-Ru(1)-P(1) 173.27(2), Cl(1)-Ru(1)-Cl(3)
90.86(2), Cl(2)-Ru(1)-Cl(3) 92.68(2), P(2)-Ru(1)-Cl(3) 92.13(2), N(1)-Ru(1)-P(1)
90.82(5), Cl(1)-Ru(1)-P(1) 87.84(2), Cl(2)-Ru(1)-P(1) 88.33(2), P(2)-Ru(1)-P(1) 94.50(2).
Fig. 2. Cyclic voltammograms of mer-[RuCl3(dppb)(4-Phpy)] 5, 1.0 ꢂ 10ꢁ3 mol Lꢁ1, for
potentials between ꢁ400 and 1000 mV, vs. Ag/AgCl, recorded in CH2Cl2, in 0.1 mol Lꢁ1
TBAP; sweep rate 100 mV sꢁ1
.

C. Rodrigues et al. / Journal of Molecular Structure 1111 (2016) 84e89
87
Table 2
to the P(2) of the dppb ligand, while one chloride, Cl(3), is in trans
position to the P(1) atom. The slightly longer RueP(1) distance
(2.4166 Å) compared to the RueP(2) bond length (2.3438 Å) can be
Catalytic activity of the complexes containing Ru (III) in the hydrogenation of
cyclohexene (1a), undecanal (1b) and cyclohexanecarboxaldehyde (1c).
Entry
Catalytic precursor
Substrate
Yields (%)
TOF (hꢁ1
)
attributed to the better s-donor properties of the chloride ligand,
1
2
3
4
5
6
7
8
mer-[RuCl3(dppb)(py)]A
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
84
73
78
83
76
87
2
86
96
87
93
73
30
26
28
29
27
31
0.71
15
34
31
4
when compared with the pyridine ring from 4-phenylpyridine,
trans to the P(2) atom. The RueCl(3) bond distance (2.4046 Å) is
longer than that of RueCl(1) and RueCl(2) (2.3304 and 2.3373 Å,
respectively) due to the trans effect of the phosphorus atom, P(2).
The angles Cl(1)-Ru(1)-Cl(2), N(1)-Ru(1)-P(2) and Cl(3)-Ru(1)-P(1),
all around 174ꢀ, show a low distortion for the octahedral environ-
ment in the region of the ruthenium(III) metal center.
mer-[RuCl3(dppb)(4-Mepy)]A
mer-[RuCl3(dppb)(4-Vpy)]A
mer-[RuCl3(dppb)(4-tBupy)]A
mer-[RuCl3(dppb)(4-Phpy)]A
mer-[RuCl3(dppb)(4-Vpy)]B
mer-[RuCl3(dppb)(4-Vpy)]C
mer-[RuCl3(dppb)(4-Vpy)]D
mer-[RuCl3(dppb)(4-Vpy)]E
mer-[RuCl3(dppb)(4-Vpy)]F
mer-[RuCl3(dppb)(4-Vpy)]G
mer-[RuCl3(dppb)(4-Vpy)]H
9
10
11
12
3.2. Catalytic activity in hydrogenation reactions
3
A
reaction conditions: Ru-cat 0.013 mmol, H2 pressure 7.5 bar, 20 mL of MeOH,
The catalytically active ruthenium species for hydrogenation
reactions were in situ prepared by the reaction of the substrate with
the Ru(III) precursor in the presence of molecular hydrogen.
Scheme 2 describes the general conditions to obtain the hydroge-
nated products from cyclohexene (1c), undecanal (1b) and cyclo-
hexanecarboxaldehyde(1c). In order to determine the best reaction
conditions, a series of experiments was performed varying pres-
sure, temperature and catalyst concentration. The results for these
reactions are present in Table 2.
B
mechanical stirring 150 rpm, 15 h, temperature 80 ꢀC; addition of 15
mL of trie-
thylamine (NEt3); C addition of an excess (2:1) of ligand 4-Vpy; D using 0.026 mmol
E
F
G
of catalyst; using 15 bar H2 pressure; RT; Ru-cat 0.015 mmol, H2 pressure
100 bar, 6 mL of MeOH as solvent, mechanical stirring 150 rpm, 24 h reaction time,
H
temperature 160 ꢀC; Ru-cat 0.015 mmol, H2 pressure 50 bar, 6 mL of MeOH as
solvent, mechanical stirring 150 rpm, 24 h reaction time, temperature 160 ꢀC. TOF
(hꢁ1) turnover frequency ¼ npro/(ncat ꢂ t); where npro ¼ mols number of product,
ncat ¼ mols number of catalyst and t ¼ reaction time (h).
It is possible to observe in the Table 2 for the hydrogenation of
cyclohexene (1a), that there is no substantial difference among the
catalytic activity of the ruthenium complexes 1e5. All complexes
were active in the reaction conditions of 7.5 bar H2, 80 ꢀC, 150 rpm,
1:530 Ru-complex/(1a) and they showed a good performance with
an average of 78.8% of the hydrogenated product. The reduction of
the C]C in the cyclohexene was obtained with rates between 26
and 30 hꢁ1 (turnover frequency - TOF) (see Table 2, entries from 1 to
5).
To better understand the influence of precursors on the catalytic
activity, the solvent from the reaction with mer-[RuCl3(dppb)(4-
Vpy)] 3 was removed under vaccum and the isolated yellow pow-
der was characterized by FTIR, conductivity and NMR spectroscopy.
The FTIR (KBr, solid-state) showed a characteristic band due the
n
(RueH) stretching at 1945 cmꢁ1, and the H1 NMR (CDCl3 solution)
spectrum showed a broad sign at
molecule. When the complex contains a terminal hydride ligand in
addition to a coordinated
2-H2 molecule, the terminal hydride is
d
ꢁ2.42 ppm, attributed to
h
2-H2
h
usually found at lower frequency, where the chemical shifts are
usually in the range
coordinated
d
ꢁ5 to ꢁ45 ppm. The 1H chemical shifts for the
h
2-H2 molecule are often found at low frequency with
respect to TMS and can be relatively broad due to inter and intra-
molecular exchange dynamics [41]. Additionally it is also observed
in the 1H NMR data a chemical shift related to NeH bond at
d
11.48 ppm (supporting information, Fig. 1S). Therefore, it suggests
the presence of the protonated 4-VpyHþ, and a cationic metal
complex with formula [RuCl(
2-H2)(dppb)(cxe)2]þ, obtained by
h
chloride displacement {where cxe ¼ cyclohexene}.
In towards of this view, the conductivity value of the
1.0 ꢂ 10ꢁ3 mol Lꢁ1 solution of mer-[RuCl3(dppb)(4-Vpy)] is neutral,
1.60 m m
S cmꢁ1, in CH2Cl2 (1:1 electrolyte range 12e77 S cmꢁ1), but
the conductivity value in the same concentration of the residual
solution after catalysis presented 1:1 conductivity value,
12.78 m
S cmꢁ1, which is in agreement with the presence of the salts
described above.
The 31P {1H} NMR (CDCl3) of the residual complex showed a
singlet signal at
and it corresponded with the formation of a kind of Ru(II), attrib-
uted to [RuCl(
2-H2)(dppb)(cxe)2]þ (supporting information,
d 34 ppm, due the magnetic equivalence of the P,
h
Fig. 2S). The dichloride complex trans-[RuCl2(dppb)(cxe)2] was
synthesized from mer-[RuCl3(dppb)(H2O)] under H2 atmosphere
and an excess (3:1) of cyclohexene within 7 h. The yellow solution
was reduced to ca. 2 mL, and dimethyl ether was added to pre-
cipitate the complex. A pale yellow solid was obtained quantita-
tively and it was filtered off by cannula filtration. The 31P {1H} NMR
data showed a singlet at
which is close to the chemical shift of the residual complex after the
catalytic reaction, labeled as [RuCl(
2-H2)(dppb)(cxe)2]þ.
d 37 ppm (supporting information, Fig. 3S),
h
This effort gives support to the similar catalytic behavior
observed for the complex 1e5 in the hydrogenation of cyclohexene
(entries from 1 to 5 in the Table 2), suggesting no dependence
between N-groups coordinated to the metal center and the cata-
lytic results. This attempt contributed to understand the drastic
decrease in the catalytic performance when mer-[RuCl3(dppb)(4-
Scheme 2. Hydrogenation of cyclohexene (1a), undecanal (1b) and cyclo-
hexanecarboxaldehyde (1c) with mer-[RuCl3(dppb)(N)] complexes.

88
C. Rodrigues et al. / Journal of Molecular Structure 1111 (2016) 84e89
Vpy)] was applied in the presence of an excess of free 4-Vpy (2:1)
(entry 7, Table 2). The TOF was reduced to 0.71 hꢁ1, demonstrating
the competition of the olefinic substrate with N-groups to the same
site of coordination.
from CNPq, CAPES, FAPESP and FAPEMIG. They would also like to
€
acknowledge the Leibniz-Institut für Katalyse an der Universitat
Rostock e.V., Germany, for the support of EPR analyses and alde-
hydes hydrogenation.
It is well know in the catalytic hydrogenation systems that the
presence of triethylamine (NEt3) can promote the release of halo-
genated ligands, such as chloride ions. However, the addition of
Appendix A. Supplementary data
15 mL of NEt3 (entry 6, Table 2), does not improve the activity of
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.molstruc.2016.01.080.
precursor as expected, only 87% of the hydrogenated product was
obtained. This result is similar in absence of NEt3 (entry 3, Table 2),
where 78% of hydrogenated product was obtained. It is interesting
to observe, that complex with general formula mer-
[RuCl3(dppb)(L)] {L ¼ H2O [25] or N-donor groups} has the RueCl
bond length trans to P atom longer than the others (see the caption
of Fig. 3), which suggested a weaker bond with the metal center. In
the presence of a chloride scavenger, such as NEt3, the chloride ion
could be released easier, to promote the NEt3HþClꢁ salt. However, it
does not improve the performance of the precursors 1e5 applied in
the catalytic system. The release of chloride is a kinetic pathway,
and it should improve the initial rates of the reaction, but after 15 h
of reaction, it showed no improvement in the yields of the product.
Comparing the reactions containing twofold catalyst concentration,
0.026 mmol (entry 8, Table 2) no advantage was observed in the
yield. However, with the reaction using a twice hydrogen pressure
higher, 15 bar (entry 9, Table 2), the quantitative amount of the
product was obtained.
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