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which enabled the observation of the diphenylphosphine vi-
brational modes in Figure 3d. The RR spectrum agrees well
with the SERS spectrum of the PPh3–Ag NP system previously
reported in the literature.[22]
Experimental Section
Preparation of rhodium NPs
Rhodium NPs were synthesized by using the modified method de-
scribed by Brust et al.[13] Rhodium(III) chloride hydrate (30 mg,
0.11 mmol of Rh) and TOAB (130 mg, 0.23 mmol) were dissolved in
deionized water (30 mL) and toluene (30 mL), respectively, at RT.
The aqueous phase was adjusted to pH 6 by the addition of aque-
ous NaOH. The phase transfer reagent solution was added drop-
wise to the Rh3+ aqueous solution, and the mixture was stirred for
30 min. An aqueous sodium borohydride solution (5 mL,
1.36 mmol) was freshly prepared and added dropwise to the mix-
ture. The organic layer turned black, and the system was further
stirred for 3 h. Then, the organic phase containing rhodium NPs
was separated and washed twice with water. The NPs synthesized
using this procedure were labelled as Rh-TOAB NPs.
Raman scattering was also used to investigate the spent cat-
alyst recovered after the hydroformylation reaction. The
Raman spectrum shown in Figure 3e exhibits many changes
compared with that of the as-prepared catalyst shown in Fig-
ure 3d. The band assigned to the P=O stretching mode at
1212 cmÀ1 is absent, which is an evidence that diphenylphos-
phine oxide was reduced under reaction conditions; however,
the stability of the phosphorus ligand requires more detailed
studies. Although the main bands attributed to the phosphine
ligand are still observed (1001, 1028, and 1592 cmÀ1), the
bands previously assigned to an enhancement by the interac-
tion with the metal NP surface (521 and 787 cmÀ1) are absent.
This observation provides additional evidence for a corrosive
chemisorption of rhodium NPs under hydroformylation condi-
tions, which would lead to the formation of a molecular rhodi-
um complex still attached to the support owing to the interac-
tion with the phosphine ligand grafted on the silica matrix. For
comparison, a rhodium complex was prepared by reacting the
Fe3O4@SiO2-N(CH2PPh2)2 support with Rh3+ ions. The Raman
spectrum of the supported rhodium complex (Figure 3 f) re-
vealed similarities with the spectrum of the spent catalyst (Fig-
ure 3e), which corroborates the role of Rh-TOAB NPs as precur-
sors for the formation of a rhodium–phosphine complex on
the support surface under catalytic reaction conditions.
Preparation of the catalyst support
The catalyst support consists of silica-coated magnetite NPs
(Fe3O4@SiO2) prepared following the procedure described else-
where.[14] The solid was calcined for 2 h at 5408C, and then the
silica surface was modified with terminal amino groups using (3-
aminopropyl)triethoxysilane. The solid (200 mg) was added to the
(3-aminopropyl)triethoxysilane solution (1% v/v) in dry toluene
(200 mL) under N2 atmosphere. The mixture was stirred at RT for
2 h, and then the solid was separated magnetically. The material,
labelled as Fe3O4@SiO2-NH2, was washed twice with toluene and
acetone and dried at 1008C for 20 h.
In the next step, terminal amino groups in Fe3O4@SiO2-NH2 were
subjected to phosphinomethylation.[15] Under an inert atmosphere,
the mixture of paraformaldehyde (1.82 mmol) and diphenylphos-
phine (2.00 mmol) in methanol (5 mL) was heated at 608C for 1 h.
Then, the suspension of Fe3O4@SiO2-NH2 (1.00 g, 0.5 mmol of NH2)
in a toluene (20 mL)–methanol (10 mL) solution was added to the
reaction mixture, which was stirred overnight at RT (258C). Then,
the solid, labelled as Fe3O4@SiO2-N(CH2PPh2)2, was washed five
times with toluene and dried under vacuum. Both Fe3O4@SiO2-NH2
and Fe3O4@SiO2-N(CH2PPh2)2 materials were further used as sup-
ports for the immobilization of rhodium NPs.
Conclusions
The immobilization of rhodium nanoparticles (NPs) stabilized
by tetraoctylammonium bromide (Rh-TOAB NPs) on a magnetic
support with diphenylphosphine ligands grafted on its surface
enabled the preparation of an air-stable, easily recoverable,
and reusable catalyst for the hydroformylation of olefins. We
observed a strong interaction between the rhodium NPs and
the phosphine ligands grafted on the support surface, which
caused an enhancement in the Raman spectrum of the ligand.
Analysis of the recovered catalyst after the hydroformylation
reaction reveals that such interaction is no longer present,
which indicates the dissolution of rhodium NPs and formation
of molecular rhodium species. These active species are formed
in situ and maintained on the magnetic support surface after
magnetic separation. The results of Raman spectroscopy also
indicate the oxidation of the diphenylphosphine ligand grafted
on the silica matrix after exposure to air and its possible reduc-
tion under reaction conditions. Recycling studies show negligi-
ble metal leaching and high activity, which are maintained in
successive reactions. It is important to mention that both the
Rh-TOAB NPs and the supported Fe3O4@SiO2-N(CH2PPh2)2Rh
catalyst are stable in air and can be exposed to air during the
preparation steps and work-up procedures in recycling studies.
This is a special feature of our catalyst and a distinct behavior
as compared with many air-sensitive homogeneous and
heterogeneous catalyst counterparts.
Preparation of supported rhodium NP catalysts
A toluene solution containing Rh-TOAB NPs (ꢀ30 mL, 0.11 mmol
of Rh) was added to the Fe3O4@SiO2-NH2 or Fe3O4@SiO2-
N(CH2PPh2)2 support (500 mg). The mixtures were stirred overnight,
and then the solids were separated magnetically. After the separa-
tion, the catalysts were washed several times with toluene and
dried under vacuum. The materials obtained are labelled as
Fe3O4@SiO2-NH2Rh and Fe3O4@SiO2-N(CH2PPh2)2Rh, respectively.
The ICP–OES analysis showed that the rhodium content was
0.7 wt% in Fe3O4@SiO2-NH2Rh and 0.2 wt% in Fe3O4@SiO2-
N(CH2PPh2)2Rh.
Catalytic reactions
The hydroformylation reactions were performed in a home-made
100 mL stainless steel reactor. In a typical run, the mixture of tolu-
ene (10 mL), the rhodium catalyst (50 mg, 1.0–3.4 mmol of Rh), 1a
or 2a (2.0 mmol), and dodecane (internal standard, 1 mmol) was
transferred to the reactor, which was pressurized to 60 atm
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