Immobilisierung von chiralen Hydrierkatalysatoren auf Goldkolloiden

The aim of the presented thesis was the immobilisation of chiral metal complexes onto gold colloids. By anchoring known, homogeneous catalysts onto a solid support, the advantages of homogeneous and heterogeneous catalysis should be combined.
On the one hand, the immobilized catalysts can be used as “classic” immobilized catalysts (Figure 1, left), on the other hand, a direct interference with the environment in terms of substrate scope (Figure 1, center) or selectivity (Figure 1, right), is possible.
Based on the obtained results by Belser in the immobilisation of rhodium complexes49 the aim of this work was to extend the concept to iridium complexes.
Initially, chiral iridium complexes were immobilised onto gold colloids. For that, the ThreoPHOX and PHOX ligands were modified with a thiol-containing linker, with which the iridium complexes were attached to the gold colloids.
It was found, that the Iridium-ThreoPHOX complexes were too sensitive towards thiol impurities to be successfully used as heterogenized catalyst. When the immobilized complexes were used in catalysis, no results comparable to the homogeneous control experiment could be obtained. Only marginal activity could be observed in the iridium-catalysed hydrogenation of imines. The products were isolated in low yields as racemic mixtures which indicated decomposition of the complexes during the reaction.
After immobilisation of the more robust Iridium-PHOX complexes, activity in the hydrogenation of alkenes and imines could be observed. However only if an amout of colloids
equalling 23 mol percent of catalyst was used, comparable results to the homogeneous hydrogenation were obtained (Scheme 1).
The results show that in principle the immobilisation of iridium-PHOX complexes is possible; the sensitivity of the catalysts towards impurities however, remain an unsolved problem. Considering the results obtained in the immobilisation of iridium catalysts onto silica gel,100 which gave similar results as the gold colloids, the iridium complexes studied in this thesis seem unsuitable for immobilisation on solid supports.
For investigating the influence of the environment on the metal complexes pictured in Figure 1, the focus of research was changed to the already successfully immobilized rhodium complexes. These complexes were anchored within the monolayer and furthermore the polarity of the surrounding monolayer was altered (Scheme 2).
The polarity of the monolayer had a profound impact on catalysis. Especially in polar-protic solvents, the functionalized head groups prevented catalysis completely. When the catalysts were placed closer to the surface of the monolayer, the activity of the complexes was regained (Scheme 2).
The substrate size dependence was also investigated. With complexes embedded deeply in an alkanethiol monolayer, it was observed that short chain alkyl esters of acetamidocinnamic acid were hydrogenated faster then long chain alkyl esters. Especially the elongation from methyl to ethyl ester had a significant impact, further elongation led to smaller effects (Scheme 3).
The introduction of polar chiral headgroups in the monolayer showed an inhibiting effect on catalysis, which can presumably be attributed to coordination of the polar groups to the catalyst.
When applying chiral apolar spacers, an impact on the selectivity of the catalytic reaction could be observed. The use of BINOL-derived spacers led to a decrease in enantioselectivity by ten percent without decrease in activity. The same effect could be observed for both BINOL enantiomers, regardless of their configuration. The effect can therefore not be attributed to a match/mismatch interaction between the chiral ligand and the chiral spacer (Scheme 4).
In addition to the primary task of applying goldc olloids as supports in asymmetric catalysis, the use of gold colloids as potential circuit paths in molecular electronics was investigated in collaboration with the group of Schönenberger in the Department of Physics of the University of Basel. Colloids arrays printed on silicon wafers could be obtained and after immobilisation of conjugated aryl thiols, the circuits could be addressed and a current flow could be measured when applying a bias voltage to the circuits (Figure 2).
For these experiments conjugated molecules (OPE) were synthesized and immobilized in the intercolloidal spaces. The colloid arrays exhibited after treatment with OPE a diminished resistance by three orders of magnitude (Scheme 5).
This exchange was reversible and it could be distinguished between octanethiol stabilized arrays and the functionalized arrays by the difference in conductance.
The successful construction of these systems is a further step in the fundamental research of molecular electronics.