Encapsulation of Transition Metal Catalysts by Ligand-Template Directed Assembly

Abstract

Encapsulated transition metal catalysts are presented that are formed by templated self-assembly processes of simple building blocks such as porphyrins and pyridylphosphine and phosphite ligands, using selective metal-ligand interactions. These ligand assemblies coordinate to transition metals, leading to a new class of transition metal catalysts. The assembled catalyst systems were characterized using NMR and UV-vis spectroscopy and were identified under catalytic conditions using high-pressure infrared spectroscopy. Tris-3-pyridylphosphine binds three mesophenyl zinc(II) porphyrin units and consequently forms an assembly with the phosphorus donor atom completely encapsulated. The encapsulated phosphines lead exclusively to monoligated transition metal complexes, and in the rhodium-catalyzed hydroformylation of 1-octene the encapsulation of the catalysts resulted in a 10-fold increase in activity. In addition, the branched aldehyde was formed preferentially (l/b = 0.6), a selectivity that is highly unusual for this substrate, which is attributed to the encapsulation of the transition metal catalysts. An encapsulated rhodium catalyst based on ruthenium(II) porphyrins and tris-meta-pyridyl phosphine resulted in an even larger selectivity for the branched product (l/b = 0.4). These encapsulated catalysts can be prepared easily, and various template ligands and porphyrins, such as tris-3-pyridyl phosphite and ruthenium(II) porphyrins, have been explored, leading to catalysts with different properties.

Self-assembled biomimetic [2Fe2S]-hydrogenase-based photocatalyst for molecular hydrogen evolution

“…The addition of Zn(II)TPP (5a) to complex 1 in CDCl 3 caused a typical coordinationinduced shift in the 1 H NMR (⌬␦ ϭ 6.0 ppm) for the orthoprotons of the pyridine moiety, indicative of selective binding of the chromophore to the nitrogen-donor atom of complex 1 (27 for the assembly 1⅐5a in toluene, which is typical for this type of dynamic system. The binding of 5a to 1 is only slightly weaker than determined for free PPh 2 (4-py) (K ass ϭ 6.3⅐10 3 M Ϫ1 ) (24), and there is no significant difference between association to 5a or 5b (K ass 1⅐5b ϭ 1.0⅐10 3 M Ϫ1 ).…”

ZnII–Salphen Complexes as Versatile Building Blocks for the Construction of Supramolecular Box Assemblies

“…[13] Interestingly, an association constant of 1.2 10 6 m À1 was determined for 6·pyridine; this illustrates that the stability of the complex can be fine-tuned by installing electron-withdrawing groups on the salphen structure.…”

Selective Molecular Recognition, C−H Bond Activation, and Catalysis in Nanoscale Reaction Vessels

“…[22][23][24][25][26][27][28][29] Since this Account is not intended to be a comprehensive review, but rather an illustration of a rapidly developing topic, only a few examples of such studies are discussed below.…”

“…In particular, in rhodium-catalyzed hydroformylation reactions carried out at 25°C, the porphyrin encapsulated complex showed up to 10-fold higher activity and a 0.6:1 selectivity for the linear over the branched product. 22,24 Hupp and co-workers have also utilized porphyrins as molecular panels around a reactive metal center. 25,26 When coordinative directed assembly is used with octahedral rhenium metal centers at the corners, in combination with zinc porphyrin walls, a supramolecular square with a large internal cavity is formed, where a catalytically active manganese porphyrin moiety binds to the zinc walls.…”