Verankerung von Rhodium-Hydrierkatalysatoren an Alkylthiolat-beschichtete Goldkolloide

In this project, a new approach to heterogeneous catalysts based on selfassembled
monolayers has been investigated. Catalytically active metal complexes,
containing a linker with a thiol group, were attached to the surface of gold particles or
colloids and were embedded in an environment of spacers.
Gold as a solid support
Gold surfaces have the advantage that they are inert and do not interact with the
active metal center. Gold particles or colloids can be prepared with a variable diameter
and surface area and catalyst immobilization takes place under mild conditions.
Gold particles
We have developed a general synthesis of a range of thiols, symmetrical and unsymmetrical
disulfides (chapters 2.2 and 2.3).
To determine the amount of metal complex attached to the gold surface, we
attached p-nitrophenylester groups to the linkers, which are easily cleavable under
certain conditions. With this system we were able to study the immobilization and
loading under different conditions, e.g. solvent, time, concentration and gold core size
(chapters 4.3, 4.4, 4.5 and 5.4).
We have also immobilized a metal complex on the gold surface (without spacers)
and used it in catalysis. The system was active, but the gold particles were too large
(d = 1000 nm) and 5.40 g of gold had to be used for a reaction with 0.5 mmol
substrate (chapter 4.8).
Therefore, we decided to investigate the immobilization on gold colloids, which
are approximately 1000 times smaller and therefore have a much larger surface area
per gram of gold.
Gold colloids
A series of n-alkanethiolate-protected gold colloids with diameters between 2.3 -
3.5 ± 0.6 nm were prepared by the sodium borohydride reduction of AuCl4
- in the
presence of an n-alkylthiol. Thermogravimetric analysis and XPS-measurements
proved that the S-Au bond is stable up to 150 °C (chapters 5.5.5 and 5.5.7).
Gold colloids were modified with a monolayer of apolar (methyl and adamantyl),
polar (OH and NH2) or chiral (norephedrine derived) thiolates. The apolar and chiral
colloids were purified by precipitation from toluene with ethanol. Rhodium(I)-
complexes 49 and 74 were introduced according to the "place exchange" methodology described by Murray and co-workers. The resulting functionalized colloids were
purified by precipitation from ethanol with hexane.
Asymmetric Hydrogenation
The catalytic activity and selectivity of the colloids 98 - 100 were similar to the
corresponding homogeneous catalyst 49. At the end of the reaction, the modified
colloids could be recovered by filtration after the addition of 2 eq. of cyclooctadiene.
The activity and selectivity of the colloids 101 and 102 were lower than those of the
corresponding homogeneous catalyst 49.
STM-Investigations
The exchange reaction was also carried out on (111) oriented Au surfaces by
immersing a gold film in n-octanethiol solutions. The obtained films were investigated
with STM under ambient conditions. The best conditions, giving a highly ordered
upright standing arrangement of thiolates, were found in ethanol with a concentration
of 10-1 M, a immobilization temperature of 55 °C and an incubation time of 8 h
(chapter 6.9). STM studies showed that n-octanethiolates were exchanged by functionalized
thiols, which contain a longer chain length than C8, and that the catalytic sites
are distributed statistically (chapter 6.12). The exchange reaction with the rhodiumpyrphos-
complex 49 showed much perturbation on the (111) gold surface during the
measurements, probably due to the flexible head-group of the immobilized catalyst
(chapter 6.13).