Preparation, characterization, and performance of tripodal polyphosphine rhodium catalysts immobilized on silica via hydrogen bonding

The heterogenization of the zwitterionic Rh(I) catalysts (sulfos)Rh(cod) (1) and (sulfos)Rh(CO)(2) (2) [sulfos = -O3S (C6H4)CH2C(CH2PPh2)(3); cod = cycloocta-1,5-diene] is performed by controlled adsorption on partially dehydroxylated high surface area silica. The immobilization procedure is based uniquely on the capability of the sulfonate tail of sulfos to link the silanol groups of the support via hydrogen bonding. Experimental evidence of the -SO3... HOSi- interaction between 1 or 2 and silica has been obtained from IR, Rh K-edge EXAFS, and CP MAS P-31 NMR studies. The grafted catalyst (sulfos)Rh(cod)/SiO2 (1/SiO2) is active for the hydrogenation of alkenes in either flow reactors (ethene, propene) or batch reactors (styrene) in hydrocarbon solvents. The hydroformylation of alkenes, here exemplified by I-hexene, is catalyzed exclusively in solid-liquid conditions. No Rh leaching is observed in either case. In solid-gas conditions, the catalyst 1/SiO2 is converted by syngas to the catalytically inactive, dicarbonyl derivative (sulfos)Rh(CO)(2)/SiO2 (2/SiO2). The termination metal products of the solid-gas reactions have been studied by EXAFS, while those of the batch reactions have been authenticated by NMR spectroscopy after extraction with methanol. In all of the cases investigated there was no evidence of the formation of contiguous Rh-Rh sites, indicating that the catalytic active sites are isolated Rh atoms, as in homogeneous phase. A comparison with analogous hydrogenation and hydroformylation reactions catalyzed by the soluble complex 1 in liquid-biphase conditions shows that the immobilized catalyst is more chemoselective and more easily recyclable than the unsupported analogue.