2
B. Sun, G. Süss-Fink / Journal of Organometallic Chemistry xxx (2015) 1e6
role in the hydrogenation of aromatic amino acids. For example, for
the hydrogenation of (R)-phenylglycine, palladium on charcoal
under basic conditions gave only phenylacetic acid [33], however,
L ¼ Kl=ðb cos qÞ
where is the X-ray wavelength (
related to crystallite shape, here taken as 0.94.
half maximum (FWHM) of the peak profile, and
l
l
¼ 1.5418 Å), K is a constant
2
the expected (R)-cyclohexylglycine can be formed over Pd(OH) on
b
is the full width at
is the Bragg angle.
charcoal in the pH range of 4.5e8.0 with 66% conversion and 84%
e.e [22]. To the best of our knowledge, a very limited number of
supported catalysts have been reported to selectively hydrogenate
phenyl-substituted amino acids into cyclohexyl-substituted amino
acids with high e.e., such as Ru on carbon [24,26], Rh on carbon [25]
q
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 total number of
2 3
and Rh on Al O [27]. Among these catalytic systems, only Rh on
carbon seems to be recyclable.
Herein, we report metallic ruthenium nanoparticles intercalated
in hectorite (nanoRu@hectorite) as a highly active and selective
catalyst for the hydrogenation of aromatic amino acids, a reaction
which works in aqueous solution. The effect of the pH on the hy-
drogenation processes is thoroughly studied. The nano-
Ru@hectorite catalyst can be recovered and reused for further runs.
100 nanoparticles by using the software ImageJ [39].
2.3. Catalysis
The selective hydrogenation of the optically active phenyl amino
acid was carried out in a magnetically stirred stainless-steel auto-
clave (100 ml). Prior to the loading of the catalyst, the autoclave was
purged three times with hydrogen to expel the air. Typically, a
freshly prepared suspension of nanoRu@hectorite (0.01592 mmol
2
. Experimental
2.1. Syntheses
Ru, 10 ml H
carefully transferred into the autoclave under inert atmosphere,
and then the autoclave was charged with H to the desired pres-
2
O) and the appropriate amount of the substrate were
White sodium hectorite powder was synthesized according to
the method of Bergk and Woldt [34]. The sodium cation exchange
capacity, determined under the method of Lagaly and Tributh [35],
2
sure. The autoclave was placed into the pre-heated heating mantle
and the magnetic stirring was started for the indicated reaction
time. After the reaction, the autoclave was cooled down and the
pressure was released. The reactor was thoroughly rinsed with 2 N
NaOH solution to wash out the entire product (in the case of acidic
system, 2 N HCl was used). All the collected solutions were filtered
was found to be 104 mEq per 100 g. The dimeric complex [(C
RuCl was synthesized following the procedure reported by
Arthur and Stephenson [36].
6 6
H )
2 2
]
2
.1.1. Preparation of the ruthenium(II)-containing catalyst
precursor
The neutral complex [(C
dissolved in distilled and N
(0.22 mm, PTFE) to remove the catalyst and then treated with
6
6 2 2
H )RuCl ] (83.8 mg, 0.17 mmol) was
diluted HCl (or NaOH) solution to adjust the pH to 5.5, which
caused the partial precipitation of the product. The suspension was
then reduced in vacuo to 10 ml in order to complete the precipi-
tation. The precipitate was filtered off, washed with distilled water
and dried in vacuo for 24 h.
2
-saturated water (50 ml), giving a clear
yellow solution after vigorous stirring for 1 h. The pH of this solu-
tion was adjusted to 8 (using a glass electrode) by adding the
appropriate amount of 0.1 M NaOH. After filtration this solution was
added to 1 g of finely powdered and degassed (1 h under high
The white product was analyzed through 1H and 13C NMR in
vacuum, then N
2
-saturated) sodium hectorite. The resulting sus-
ꢀ
4 2
methanol-d or D O using a Bruker Avance II 400 MHz spectrom-
pension was stirred for 4 h at 20 C. Then the yellow ruthenium(II)-
eter using tetramethylsilane (TMS) as internal standard. IR spectra
were recorded with a PerkinElmer FT-IR 1720 X spectrometer.
Optical rotation was measured by a SCHMIDT HAENSCH Polartronic
H532 polarimeter. The optical purity of the product was further
examined by HPLC-UV technique (Ultimate 3000RS Dionex system
with Acquity UPLC® BEH HILIC column). Electrospray ionization
mass spectra (ESI-MS) were obtained in negative ion mode on a
Bruker FTMS 4.7T BioAPEX II mass spectrometer. Inductively
coupled plasma optical emission spectrometry (ICP-OES, Perkin-
Elmer Optima 3300 DV) was used to analysis the ruthenium
leaching after the catalytic run.
containing hectorite was filtered off and dried in vacuo for 12 h.
2.1.2. Preparation of the nanoRu@hectorite catalyst
The ruthenium(0)-containing hectorite was obtained by react-
ing a suspension of the yellow ruthenium(II)-containing hectorite
50 mg, 0.01592 mmol Ru) in water (10 ml) in a magnetically stirred
(
stainless-steel autoclave (volume 100 ml) under a pressure of H
2
ꢀ
(
50 bar) at 100 C for 14 h. After pressure release and cooling, the
nanoRu@hectorite catalyst was isolated as a black material.
.2. Methods
The powder X-ray diffraction (XRD) patterns of the catalysts
2
2.4. Catalyst recycling and recovery
were collected by XRD Application LAB in CSEM (Switzerland). The
samples were measured in air at 20 C on a STOE STADIP high-
ꢀ
After a catalytic run, the nanoRu@hectorite catalyst was sepa-
rated by decantation from the centrifuged reaction mixture. The
supernatant was analyzed by ICP-OES to detect the Ru leaching. The
catalyst was washed with 2 N NaOH (in the case of acidic system,
resolution X-ray diffractometer using CuKa radiation. D-spacing
(d) determination of the interlamellar spacing in hectorite, based
on hectorite (001) reflection, was calculated from Bragg's law [37]:
2
N HCl was used) solution and then with degassed water to extract
nl ¼ 2d sin q
traces of the catalytic product. After drying in vacuo for 12 h, the
recycled catalyst was dispersed in the reaction medium under ul-
where n is an integer (herein n ¼ 1),
l
is the X-ray wavelength (for
trasonic conditions and reactivated in the autoclave under a H
2
ꢀ
the CuK is the angle between incident beam and
a
,
l
¼ 1.5418 Å).
q
pressure of 50 bar at 100 C for 14 h. After pressure release and
cooling, the amino acid substrate, the amount of which was
calculated from the weight of the corresponding recycled catalyst,
was added for the next catalytic run.
scattering planes. Based on the Ru(011) reflection with the Si
standard as a reference for the instrument peak broadening, the
crystallite size L was calculated using the Scherrer equation [38]:
Please cite this article in press as: B. Sun, G. Süss-Fink, Journal of Organometallic Chemistry (2015), http://dx.doi.org/10.1016/
j.jorganchem.2015.09.011