A. Tavares et al. / Journal of Catalysis 254 (2008) 374–382
375
cationic rhodium complex, [(η5-Cp∗)2Rh2(μ2-Cl)3]PF6, where
Cp∗ = pentamethylcyclopentadienyl, as a precatalyst to help
overcome the phosphine leaching process. This complex con-
tains a Cp∗ ligand that could be an excellent substitute for the
phosphine ligands, helping to stabilize the catalyst and allow-
ing easy product separation and catalyst phase recycling. In
this paper, we report our findings on the hydrogenation of 1,7-
octadiene, 1,9-decadiene, and 1,5-cyclooctadiene catalyzed by
[(η5-Cp∗)2Rh2(μ2-Cl)3]PF6 in PEO 3350/MeOH polar phase.
atom coupled to six fluorine nuclei, confirming the presence
of PF−6 anion in the complex. The presence of chlorine was
detected, with formation of a white precipitate of AgCl ob-
served on the addition of an excess of aqueous AgNO3 to a
solution of the complex in MeCN. The weight of the AgCl
obtained corresponded to 14.00% chlorine in the complex,
very close to the expected value of 14.62%. The results of its
elemental analysis (Perkin–Elmer 2400 CHN Elemental Ana-
lyzer) were C = 30.09%, H = 3.86%, N = 6.53% (Anal. calcd:
C = 29.51%, H = 3.71%, N = 6.45%). These results lead us to
2. Experimental
conclude that the replacement of the cation NH+4 by K+ on the
hexafluorophosphate salt led to the same rhodium complex, as
could be predicted.
2.1. Materials
2.4. Characterization of the complex
Dienes (Aldrich, 98%) were passed through a column of
activated neutral alumina to eliminate peroxides and stocked
over 3A activated molecular sieves under argon atmosphere.
Heptane was distilled over Na/benzophenone, and MeOH was
distilled over CaH2. PEO (Alfa) 3350 was evacuated for 12 h
under dynamic vacuum at room temperature and stocked un-
der argon. Hydrogen (White Martins, 99.999%) was used as
received. Argon (White Martins, 99.998%) was passed through
two traps filled with 3A activated molecular sieves and one trap
containing BASF catalyst R3-11 to remove traces of water and
oxygen.
[(η5-Cp∗)Rh(MeOH)3](PF6)2
IV (DRIFTS): νO–H = 3427 cm−1, νC–H
= 2960 cm−1
aliphatic
and 2929 cm−1, νC=C = 1469 cm−1, νC–C = 1020 cm−1
,
νP–F = 873 cm−1, 862 cm−1 and 559 cm−1
;
1H NMR (CD2Cl2, 200 MHz, δ in ppm referred by TMS):
δ 3.49 (CH3–CH3OH), δ 1.68 (CH3–Cp∗);
C, H, N: observed: C = 25.24%, H = 4.20%. Anal. calcd:
C = 25.01%, H = 4.36%; UV/vis (CH3OH): 351 nm (sh),
394 nm and 505 nm.
2.2. Characterizations
2.5. Catalytic experiments
All NMR spectra were obtained using a Varian VXR
Multinuclear spectrometer. H NMR spectra were obtained at
All catalytic systems were prepared under argon atmosphere.
Reactions were performed under 40 bar hydrogen pressure
in the MeOH/PEO polar phase, and the products were ex-
tracted at the end of the reaction using n-heptane. Samples for
GC or GC/MS were withdrawn directly from the n-heptane
phase. In a typical experiment, 1.4 × 10−2 mmol (10.2 mg)
of [(η5-Cp∗)2Rh2(μ2-Cl)3]PF6 was added to an argon-purged
Schlenk tube, followed by 1.8 g of PEO 3350 (Sigma). These
solids were dissolved by adding 7 ml of MeOH and 3.1 mmol
(∼0.4 ml) of diene under magnetic stirring. This solution was
then transferred by cannula to a stainless steel reactor that had
been purged by vacuum/argon cycles. The reactor was closed,
purged 3 times with H2, and then pressurized to 40 bar. The
products were analyzed by GC and GC/MS.
1
200 MHz using C3D6O (d6-acetone) or CDCl3 as solvents and
calibrated by the TMS signal. 31P{1H} NMR analysis were car-
ried out at 121 MHz using a capillary tube filled with H3PO4 as
reference and C3D6O as solvent. Elemental analyses were run
in a Perkin–Elmer 2400 CHN Elemental Analyzer. UV spec-
tra were performed between 300 and 800 nm in a Shimadzu
(UV–1601 PC) spectrometer, using a quartz cell with an optical
path length of 1 cm. IR spectra were collected in solid phase
by diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS), 32 scans, in a Bomen MB102 spectrometer. GC/MS
analyses were done using a Shimadzu 17A gas chromatograph
equipped with a capillary column DB-5 (30.0 m × 0.25 mm),
using He as carrier gas, coupled to a Shimadzu GCMS-QP5050
mass spectrometer operating at 70 eV (EI).
2.6. Filtration test for an active heterogeneous component in
the catalysis
2.3. Catalyst precursor preparation
Maitlis and coworkers have developed a simple and very
convenient test to verify whether small metal aggregates are
involved in the catalytic activity of a homogeneously cat-
alyzed reaction conducted under reducing conditions [24]. This
method involves adding an inert filter aid, such as cellulose, to
the reaction solution to adsorb small metal particles that could
be very active catalysts in many processes. Then the mixture is
filtered, and the catalytic activity of the filter aid and the filtered
solution are evaluated. The catalytic experiments were done
as described above using 1,7-octadiene (0.4 ml, 3 mmol) as a
substrate and [(η5-Cp∗)2Rh2(μ2-Cl)3]PF6 (1.4 × 10−2 mmol,
[(η5-Cp∗)Rh(μ2-Cl)Cl]2 was prepared from RhCl3·3H2O
and pentamethyl–cyclopentadiene (Aldrich, 99%) as stated in
the literature, in almost quantitative yield [22]. The mono-
cationic complex [(η5-Cp∗)2Rh2(μ2-Cl)3]PF6 was prepared
from [(η5-Cp∗)Rh(μ2-Cl)Cl]2 as described previously, replac-
ing NH4PF6 with KPF6 [23]. The yield of the recrystallized
product (orange-red crystals) was 60%. Its 1H NMR spectrum
demonstrating one singlet at 1.73 ppm related to the methyl
groups of Cp∗. The spectrum of phosphorus resonance pre-
sented a heptuplet at −144 ppm related to the phosphorus