Syntheses and catalytic applications of immobilized ruthenium complexes

A ruthenium meso-tetra[4-(vinylbenzoxy)benzyl]porphyrin catalyst with four meso-polymerizable side chains was synthesized in three steps from commercially available starting materials. The polymerizable side chains allowed the incorporation of this complex in various types of highly cross-linked mesoporous co-polymers. Core-shell microspheres were prepared in a two-step precipitation polymerization procedure. The porous shell contains the Ru-catalyst and divinylbenzene while the dense core is constituted only of divinylbenzene. A block polymer and molecularly imprinted polymers containing the Ru-catalyst were prepared by co-polymerization with ethyleneglycol dimethacrylate. The imprinted polymers were prepared in the presence of primary amines as templates, which create a free substrate pocket next to the catalytically active center upon removal of the template. All polymers displayed high catalytic activity for the oxidation of alcohols and alkanes to the corresponding ketones with 2,6-dichloropyridine-N-oxide as the oxidant. A comparison of the catalytic activity of the Ru-catalyst, the block polymer and the imprinted polymer was made. While the homogeneous Ru-catalyst showed no activity, the block polymer was moderately active and the imprinted polymer displayed the highest activity. The increase of activity of the block polymer compared to the homogeneous Ru-catalyst can be explained by the site-isolation effect which prevents the irreversible dimerisation of the ruthenium porphyrin catalysts upon oxidation. The further increase of activity displayed by the imprinted polymer can be attributed to the free substrate pocket next to the active site which allows a better access of the substrates to the ruthenium center. With the polymer imprinted with 1-adamantylamine, a substrate selectivity of 1.4 in favor of 2-adamantanol was observed for the oxidation of 2-adamantanol in competition with 2-octanol. However, with the other polymer imprinted with aminodiphenylmethane no such selectivity was recorded. It was possible to modulate both the activity and the selectivity of the block polymer using a fluorous solvent, perfluoromethylcyclohexane (PFMC). A large increase of catalytic activity was measured upon use of PFMC as a reaction co-solvent. Some substrate selectivity was also observed for substrates with different polarities. Both phenomena can be attributed to the increase of the local concentration of substrate in the polymer pores, which occurs to avoid contact with the very apolar PFMC solvent.