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 = cy cloocta-1,5-diene] is performed by controlled adsorption on partially dehyd roxylated high surface area silica. The immobilization procedure is based u niquely on the capability of the sulfonate tail of sulfos to link the silan ol 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 (su lfos)Rh(cod)/SiO2 (1/SiO2) is active for the hydrogenation of alkenes in ei ther flow reactors (ethene, propene) or batch reactors (styrene) in hydroca rbon solvents. The hydroformylation of alkenes, here exemplified by I-hexen e, is catalyzed exclusively in solid-liquid conditions. No Rh leaching is o bserved in either case. In solid-gas conditions, the catalyst 1/SiO2 is con verted by syngas to the catalytically inactive, dicarbonyl derivative (sulf os)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 h ave 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 isol ated Rh atoms, as in homogeneous phase. A comparison with analogous hydroge nation and hydroformylation reactions catalyzed by the soluble complex 1 in liquid-biphase conditions shows that the immobilized catalyst is more chem oselective and more easily recyclable than the unsupported analogue.