N-containing covalent organic frameworks as supports for rhodium as transition-metal catalysts in hydroformylation reactions

Abstract

Porous cross-linked polymers containing nitrogen donor functionalities are a promising class of materials for the immobilization of transition metal catalysts. These highly porous materials are built from ligand-containing building blocks using a bottom-up approach. Utilizing dinitrile containing monomers, highly porous covalent triazine frameworks (CTFs) are formed at elevated temperatures in molten zinc chloride. Containing a large number of accessible nitrogen coordination sites, metal species can be stabilized within these materials. The materials' porosity can be tuned, depending on the monomeric building block used and synthesis conditions. By immobilizing rhodium species within the framework followed by pre-activation, Rh@CTF materials are suitable as catalysts in the solvent-free hydroformylation of 1-octene. These Rh@CTF catalysts are not as active as conventional homogeneous benchmark catalysts; however, their activity and selectivity clearly outperform comparable carbon-based catalysts. A correlation between porosity and polarity of the support material and the observed catalytic activity could be established. Furthermore, CTF-based catalysts could be recycled easily with no further treatment for up to five runs with just a gradual deactivation. The leaching of active species has been studied intensively to elucidate the strength of the metal-support-interactions in this rather challenging reaction for an immobilized catalyst.