L. Huang et al.
Bull. Chem. Soc. Jpn., 77, No. 2 (2004)
301
In the first hour, the homogeneous [{RhCl(CO)2}2] system was
the most active and all the supported catalyst systems were little
active. Then the activity of the former decreased but its turn-
over still increased with reaction time till 9 h of total time,
whereas the activities of the supported catalyst systems increas-
ed gradually. After 9 h, the homogeneous [{RhCl(CO)2}2] sys-
tem was almost not active. At the same time, the reaction solu-
tion changed from yellow to colorless in color, with the con-
comitant formation of black precipitate, which was indicative
of the metallic aggregation from [{RhCl(CO)2}2] under pres-
surized (CO + H2). This phenomenon did not occur with the
supported catalyst systems. The turnover of aldehyde formed
on [{RhCl(CO)2}2]/MCM-41(NH2) increased linearly with re-
action time from the second reaction cycle. This indicates that
the MCM-41(NH2)-tethered catalyst can maintain its activity
unchanged in a prolonged reaction.
charge from the rhodium centre to the sulphur, which impedes
the formation of an active hydridic complex by hydrogenolysis
of the Rh–Cl bond under hydroformylation conditions. This
renders the [{RhCl(CO)2}2]/MCM-41(SH) catalyst system in-
active. In contrast, (Os)3Si{(CH2)3NH2} has no dꢃ orbitals.
However, a strong N–Rh bond may be formed and the transfer
of the negative charge from the rhodium centre to other ligands
can be controlled by the strong electronegativity of the nitro-
gen. It may be assumed that the presence of an amine ligand
in the tethered complex helps the transformation of the Rh–
Cl bond into the Rh–H bond proceed to a satisfactory extent.
The complexation of a supported amine with [{RhCl(CO)2}2]
not only produces a tight N–Rh bond to immobilize the com-
plex, but ensures stable and high catalytic activity for hydrofor-
mylation.
The electronic effects of supported donor ligands on the cat-
alytic activity and stability of a metallic carbonyl complex can
also cause spectral variation in the vibration of carbonyl groups
in the complex. An IR carbonyl spectral change does occur af-
ter [{RhCl(CO)2}2] has been tethered to MCM-41 via phos-
phine, amine, and thiol ligands in the forms of MCM-
41{PPh2cis-RhCl(CO)2}, MCM-41{NH2cis-RhCl(CO)2} and
[MCM-41{SRh(CO)2}]2 plus MCM-41{SRh2Cl(CO)4}. How-
ever, the spectral comparison is significant only for MCM-
41{PPh2RhCl(CO)2} and MCM-41{NH2RhCl(CO)2} with re-
spect to [{RhCl(CO)2}2]/MCM-41. The coordination of sup-
ported phosphine or amine results in the transfer of the increas-
ed negative charge to the carbonyls via the rhodium by ꢃ-back
donation and thus the weakening of the C=O bond vibration. In
The catalytic results demonstrate that the [{RhCl(CO)2}2]/
MCM-41(NH2)-derived catalyst is not only highly active but
quite stable for recycling. These results also demonstrate that
the [{RhCl(CO)2}2]/MCM-41(SH)-derived catalyst is inactive
though very stable, and that the [{RhCl(CO)2}2]/MCM-
4
1(PPh2)-derived catalyst suffers from heavy rhodium leach-
ing. The [{RhCl(CO)2}2]/MCM-41(NH2)-derived catalyst
which has good performances, exhibits obvious advantages
over the [{RhCl(CO)2}2]-derived homogeneous catalyst in
both activity and stability. Among the three tethered catalysts
via different donor ligands, the [{RhCl(CO)2}2]/MCM-
4
1(SH)-derived catalyst is the most resistant to rhodium leach-
ing, and the [{RhCl(CO)2}2]/MCM-41(PPh2)-derived catalyst
is the less resistant to rhodium leaching. The [{RhCl(CO)2}2]/
MCM-41(NH2)-derived catalyst also turns out to have much
better resistance to rhodium leaching than the [{RhCl-
ꢂ1
fact, the gem-dicarbonyl bands at 2096 and 2017 cm for
MCM-41{NH2RhCl(CO)2} have a significant downward shift
ꢂ1
compared to those at 2113sh, 2098, and 2042 cm
for
(
CO)2}2]/MCM-41(PPh2)-derived catalyst. This had not been
[{RhCl(CO)2}2]/MCM-41, as shown in Fig. 1. The gem-dicar-
bonyl bands at 2081 and 2003 cm for MCM-41{PPh2cis-
ꢂ1
clarified in the previous studies concerning 1-hexene hydrofor-
mylation with [{RhCl(CO)2}2]-derived SiO2-tethered catalysts
via donor ligands, as SiO2{PPh2RhCl(CO)2} has never been
compared with others.5 It follows that the effect of donor li-
gands on the immobility of the rhodium complex has, from
strongest to weakest –SH > –NH2 > –PPh2.
The strength of complexation of the supported donor ligands
with the rhodium centre determines the catalytic properties of
the tethered catalysts, along with their resistance to rhodium
leaching. It is known that phosphine, amine, and thiol ligands
are all strong ꢂ-electron donors. In the meantime, (Os)3Si-
RhCl(CO)2} observed in our previous work show a stronger
downward shift than those for MCM-41{NH2RhCl(CO)2}.25
It is evident that both supported phosphine and supported amine
donate their negative charge to the rhodium complex, but more
negative charge is transferred to the carbonyls from supported
phosphine than from supported amine. The IR observations are
in accord with the above explanation regarding the influences
of supported phosphine and amine ligands on the catalytic ac-
tivity and stability of tethered rhodium complexes. The un-
availability of MCM-41{SHRhCl(CO)2} does not permit us
to examine the shift of gem-dicarbonyl bands with supported
thiols coordinated under identical conditions. Therefore, in or-
der to demonstrate the electronic contributions of these ligands
to a complex with regards to their promoting actions on catalyt-
ic activity and stability, we intend to study the trend in the car-
bonyl vibration in MCM-41-tethered rhodium monocarbonyl
complexes via these ligands in complexes such as MCM-
41{PPh2RhH(CO)(PPh3)2}, MCM-41{NH2RhH(CO)(PPh3)2}
and MCM-41{SHRhH(CO)(PPh3)2}.
,6
{
(CH2)3PPh2} and (Os)3Si{(CH2)3SH} (Os: surface oxygen)
are poor and strong ꢃ-electron acceptors, respectively. As a re-
sult, the weak dꢃ–pꢃ bonding between the phosphorus and the
rhodium centre results in the transfer of the negative charge
from the phosphorus to the rhodium complex in the case of
MCM-41{PPh2cis-RhCl(CO)2}. This favors the formation of
a hydridic complex necessary for hydroformylation by hydro-
genolysis of the Rh–Cl bond under hydroformylation condi-
tions, and thus renders the MCM-41{PPh2cis-RhCl(CO)2}-de-
rived catalyst active. Meanwhile, the weak dꢃ–pꢃ bonding
forms a weak P–Rh bond, so that a supported phosphine coor-
dinated to the rhodium complex is readily replaced by CO with
the occurrence of rhodium leaching under hydroformylation
conditions. In the case of [{RhCl(CO)2}2]/MCM-41(SH), the
strong dꢃ–pꢃ bonding leads to the transfer of the negative
Finally, it is important to mention that the XRD spectrum of
the [{RhCl(CO)2}2]/MCM-41(NH2)-derived catalyst main-
tained the initial peak intensities for mesoporous MCM-41 after
the fourth reaction cycle, as shown in Fig. 10. This demon-
strates that the mesoporous structure of the MCM-41-based cat-
alysts is not affected by operating catalytic conditions.