A.O. Ogweno et al. / Applied Catalysis A: General 486 (2014) 250–258
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cationic 6-(2-phenoxyethanol) ruthenium complexes containing
2-(2-pyridyl)benzoazole ligands and their successful application in
catalytic hydrogenation of styrene, 1-hexene, 1-octene, 1-decene,
1-hexyne and 1-octyne in methanol. Biphasic catalytic experiments
were conducted using water and toluene medium to demon-
strate the recyclability of these catalysts. The influence of reactions
parameters such as time, substrate/catalyst ratio, temperature,
aqueous/organic phase ratio and pressure have been investigated
and are herein discussed.
Their positions were calculated using a standard riding model with
C–Haromatic distances of 0.95 A and Uiso = 1.2 Ueq and C–Hmethylene
˚
˚
distances of 0.99 A and Uiso = 1.2 Ueq. The positions of the hydroxyl
˚
O–H of 1 and 3 were constrained using O–H distances of 0.84 A and
Uiso = 1.5 Ueq. The hydroxyl O–H of 2, all hydrogen atoms of the
water solvate molecules and the imidazole N-H of 1 were located
in the difference density map, and refined isotropically. Disordered
solvent molecules were removed from the lattices of 1 and 3, using
Platon SQUEEZE [27]. This leaves solvent accessible voids of 46 and
3
˚
65 A in the lattices of 1 and 3, respectively.
2. Experimental
2.4. Hydrogenation reactions
2.1. Materials and instrumentation
2.4.1. Homogeneous experiments
niques unless stated otherwise. Dichloromethane was distilled
from P2O5 prior to use. Ru(II) starting material, [6-(2-
phenoxyethanol)RuCl2]2 was prepared according to literature
procedure [21,22]. The ligands, 2-(2-pyridyl)benzimidazole (L1),
2-(2-pyridyl)bezothiazole (L2), 2-(2-pyridyl)bezoxazole (L3) were
also prepared according to previous methods [23]. NMR spectra
were recorded on a Bruker 400 Ultrashield instrument at room tem-
perature in DMSO-d6 solvent. The 1H (400 MHz) and 13C (100 MHz)
chemical shifts are reported in ı (ppm) and referenced to the resid-
ual proton in the solvents for 1H and to solvent signals for 13C.
Coupling constants (J) are measured in Hertz (Hz). Elemental anal-
yses were performed on Thermal Scientific Flash 2000 and mass
spectra were recorded on LC Premier micro-mass Spectrometer in
the micro-analysis Laboratory at the University of KwaZulu-Natal,
South Africa. GC analyses were carried out on Varian CP-3800 GC
(ZB-5HT column 30 m × 0.25 mm × 0.10 m) instrument.
A typical procedure for the catalytic hydrogenation of alkenes
was as follows. A Parr High pressure reactor with an in built cooling,
heating and stirring systems was charged with styrene (0.56 mL,
5.00 mmol), catalyst (5 mg, 0.01 mmol) and methanol (50 mL) and
sealed. It was then evacuated, flushed with H2 three times and the
pressure adjusted to 10 bar. The mixture was stirred at 500 rpm
under constant hydrogen pressure for the duration of the reac-
tion period. After the reaction time, the autoclave was vented and
samples drawn for GC analyses. The samples were filtered using
0.45 m micro filters and the solutions analyzed by Varian CP-
3800 GC (ZB-5HT column 30 m × 0.25 mm × 0.10 m). Commercial
ethylbenzene was used as an authentic standard to determine
the percentage conversion of styrene to ethylbenzene. Percent-
age conversions were determined by comparing the peak areas of
ethylbenzene (product) and styrene substrate.
2.4.2. Biphasic hydrogenation experiments
These reactions were carried in
a mixture of water and
toluene biphasic system. In a typical experiment, complex 1 (5 mg,
0.01 mmol, equivalent to a substrate/catalyst ratio of 500 was
weighed and dissolved in water (25 mL) in a two neck round bot-
tom flask. A solution of styrene 0.56 mL (5.00 mmol) in toluene
(25 mL) was added and the mixture transferred to the reactor via
canula and sealed. The autoclave was evacuated and flushed with
H2 three times, filled with H2 and the pressure adjusted to 10 bar
and the stirring speed set at 500 rpm. After the reaction period, the
reactor was depressurized and the mixture allowed to settle for
approximately 5 min. The aqueous layer was then separated from
the organic layer using separating funnel. The organic layer was
filtered and analyzed by GC to determine the percentage conver-
sion of the substrate to the products. In the recycling experiments,
a fresh solution of styrene (0.56 mL, 5.00 mmol) in toluene (25 mL)
was added without addition of the catalyst in the aqueous phase.
This experiment was repeated for six consecutive cycles.
2.2. Syntheses of cationic Ru(II) complexes 1–3
In
a typical synthetic approach, a
suspension of [6-(2-
phenoxyethanol)RuCl2]2 (0.10 g, 0.16 mmol) in CH2Cl2 (10 mL) was
added two molar equivalents of solutions of L1 (0.06 g, 0.31 mmol)
in CH2Cl2 (5 mL) and mixture stirred for 24 h at room tempera-
ture. After the reaction period, the yellow precipitate formed was
filtered, washed with CH2Cl2 (15 mL) to obtain compound 1 as a
yellow solid. Recrystallization by slow evaporation of the methanol
solution of 1 at room temperature afforded single crystals suitable
for X-ray analyses. Complexes 2 and 3 were prepared following the
same procedure adopted for 1. All the compounds were character-
ized by 1H, 13C, mass spectroscopy and microanalyses. The spectral
and analytical data obtained are given as supplementary material.
2.3. X-ray crystallography analyses of complexes of 1–3
X-ray data were recorded on a Bruker Apex Duo diffractometer
equipped with an Oxford Instruments Cryojet operating at 100(2)
3. Results and discussion
3.1. Synthesis of cationic ruthenium(II) complexes
˚
detector distance of 50 mm. The data collections were performed
using omega and phi scans with exposures taken at 30 W X-ray
power and 0.50◦ frame widths using APEX2 [24]. The data were
correction [24] was applied to the data. Direct methods, SHELXS-97
[25] and WinGX [26] were used to solve both structures. All non-
hydrogen atoms were located in the difference density map and
refined anisotropically with SHELXL-97 [23]. All hydrogen atoms
were included as idealized contributors in the least squares process.
Compounds
2-(2-pyridyl)benzoimidazole
(L1),
2-(2-
pyridyl)benzothiazole (L2) and 2-(2-pyridyl)benzoxazole (L3)
were synthesized according to literature procedures [23]. Treat-
ment of [6-(2-phenoxyethanol)RuCl2]2 dimer with two molar
equivalents of L1-L3 in CH2Cl2 gave the corresponding monometal-
lic complexes ruthenium(II) [6-(2-phenoxyethanol)RuCl(L1)]Cl
(1),
[6-(2-phenoxyethanol)RuCl(L2)]Cl
(2)
and
[6-(2-
phenoxyethanol)RuCl(L3)]Cl (3) as yellow solids in high yields of
78–88% (Scheme 1). All the complexes were insoluble in chlori-
nated solvents but were highly soluble in methanol, ethanol and
water.