2
B. Sun et al. / Journal of Organometallic Chemistry xxx (2016) 1e9
intercalation of cationic benzene ruthenium complexes into hec-
torite, followed by reduction with molecular hydrogen to give a
black solid containing metallic ruthenium nanoparticles interca-
lated in hectorite (nanoRu@hectorite) [27e29]. This material was
the CuK
scattering planes.
a
,
l
¼ 1.5418 Å).
q is the angle between incident beam and
Transmission electron microscopy (TEM) was conducted in
CSEM on a Philips CM 200 Transmission Electron Microscope
(operating at 200 kV) coupled with Energy Dispersive X-ray spec-
trometry (EDS) for chemical analysis. The solid catalyst samples are
thoroughly dispersed in ethanol and deposited on carbon film
coated square mesh copper grids. The calculation of the nano-
particle size was obtained from TEM images with a number of over
100 nanoparticles by using the software ImageJ [35].
Inductively coupled plasma optical emission spectrometry (ICP-
OES, Perkin-Elmer Optima 3300 DV) was employed to analyze the
amount of in situ formed ruthenium in hectorite and ruthenium
leaching in the centrifuged supernatant after the catalytic run.
found to catalyze the hydrogenation of quinoline with switchable
ꢀ
selectivity, the reaction in water at 60 C and 30 bar H
2
giving THQ,
conversion and selectivity being at 99% [30]. However, high-
pressure equipment is required for this reaction.
Inspired by a recent paper by M. M. Dell’Anna et al. on the hy-
drogenation of quinoline by sodium borohydride in water catalyzed
by polymer-supported palladium nanoparticles [31], we modified
our nanoRu@hectorite catalyst system, so that it also works with
NaBH
quinoline. The simple intercalation of RuCl
gives a black precatalyst, which is stable in air and which catalyzes
the selective hydrogenation of quinoline to THQ with NaBH in
4
and H
2
O as the hydrogen source for the hydrogenation of
3
, n H O in hectorite
2
4
2.4. Catalytic reactions
water under mild conditions in an open reaction vessel; no pres-
sure equipment is required. The actual catalyst, metallic ruthenium
nanoparticles intercalated in hectorite (nanoRu @hectorite), can be
recovered and reused. Herein, we report the preparation of the
precatalyst, the characterization of the catalyst and its performance
for N-cycle hydrogenation of quinoline and its derivatives including
isoquinoline and quinoxalines.
Precatalyst (50 mg), NaBH
the N-cyclic substrate and 5 ml deionized and degassed water (or
2
D O) were placed in a 25 ml three-necked flask equipped with a
4 4
(or NaBD ) (3e12 mmol), 1 mmol of
0
reflux condenser and a pressure release valve to discharge the
hydrogen gas self-generated during the reactions. The operation
was carried out under inert atmosphere. The reaction mixture was
ꢀ
vigorously stirred at different temperatures (25e60 C) for the time
2
. Experimental
selected. The complete conversion of substrate was determined by
submitting small samples to spot thin layer chromatography (TLC).
After completion, the slurry was centrifuged to separate the cata-
2.1. General
2
lyst. The solid phase obtained was washed with deionized H O and
Deionized water was made from tap water by ionic exchange
then several times with ethyl acetate to remove all organic residue.
resins and degassed before use. All the N-cyclic chemicals were
The filtrate was collected, extracted with ethyl acetate and the
purchased from commercially available sources and used as
4
extract dried over anhydrous MgSO . After removal of the solvent in
received. RuCl
Research Center. NaBH
from Aldrich, and
Laboratories.
3
, n H
2
O was loaned by the Johnson Matthey
and NaBD (D, 98 atom %) were purchased
2
D O (D, 99.9%) from Cambridge Isotope
vacuo, the corresponding product was obtained. In some cases, a
silica-gel column chromatography was used to purify the product
(isolated yield). The product analysis and identification was con-
ducted by comparing the NMR spectral data with those of the
published pure substances (all analyzed by H NMR and C NMR
on Bruker Avance II 400 MHz spectrometer). The reaction selec-
tivities were obtained from the NMR spectra by integration of
characteristic peaks for the product and reactant.
4
4
1
13
2.2. Preparation of the precatalyst
White sodium hectorite powder was synthesized according to
the method of Bergk and Woldt [32]. The sodium cation exchange
capacity, determined under the method of Lagaly and Tributh [33],
was found to be 1.04 mEq/g. White hectorite powder (1 g) was
Alternatively, the reaction was performed in a magnetically
stirred stainless-steel autoclave (100 ml) in cases where the reac-
tion failed to be complete under atmospheric pressure. The loading
procedure was the same as that in a flask. After purging three times
with nitrogen, the autoclave was quickly fixed in the preheated oil
bath. Once the reaction was complete, the autoclave was quenched
in cold water, the pressure was released, and the product was
isolated as described above.
degassed in vacuo for 1 h and followed by the N
the calculated amount of RuCl $ n H O (0.40 mmol) dissolved in
5 ml H O (black solution) was transferred dropwise to the hec-
2
saturation, then
3
2
8
2
torite powder. The suspension was stirred for 3 h at room tem-
perature, then treated by filtration and washing with deionized
H
2
O (black solid) until no chloride ion was detected. The obtained
Isotope labeling experiments were done for the hydrogenation
ꢀ
solid product was dried in vacuo at 50 C for 12 h and then ground
of quinoline with NaBH
4
/D
2
O and with NaBD /H
4
2
O (and for com-
to give a fine powder, containing 0.39
mmol/mg Ru (2 mol%) based
parison with NaBH /H O and with NaBD
4
2
4
2
/D O), under the same
on ICP-OES analysis.
conditions as those of entry 7 in Table 1 (see below). All NMR
spectra were recorded with a narrow-bore Bruker 400 spectrom-
0
1
2
.3. Characterization of the catalyst nanoRu @hectorite formed in
eter (9.4 T) operating at
u
0
/2
p
¼ 400.0 and 100.6 MHz for H and
13
situ during the catalytic hydrogenation reactions
C, respectively, and equipped with an AVANCE-II console and a
mm double-resonance probe. The rf-field strengths of all hard /2
5
p
1
13
The powder X-ray diffraction (PXRD) patterns of the catalysts
were collected by XRD Application LAB in CSEM (Switzerland). The
samples were measured in air at 20 C on a STOE STADIP high-
and
channels, respectively. Quantitative proton spectra were acquired
with a recycling delay d , with longitudinal relaxation time
¼ 5 ꢁ T
constraints T of the as measured inversion-recovery experiments.
p
pulses were
u
1
/2
p
¼ 26 kHz and 27 kHz, for H and
C
ꢀ
1
1
resolution X-ray diffractometer using CuK
d) determination of the interlamellar spacing in hectorite, based
on hectorite (001) reflection, was calculated from Bragg's law [34]:
a
radiation. D-spacing
1
(
Semi-empirical calculations were performed with the PM7 method
as implemented in MOPAC2012 [36]. The optimized geometries
were used for calculations of the electrostatic potentials by means
of the keyword ESP.
n
l
¼ 2 d sin
q
0
The recyclability of the nanoRu @hectorite catalyst was exam-
where n is an integer (herein n ¼ 1),
l
is the X-ray wavelength (for
ined for the hydrogenation of quinoline under atmospheric
Please cite this article in press as: B. Sun, et al., Journal of Organometallic Chemistry (2016), http://dx.doi.org/10.1016/j.jorganchem.2016.07.010