METAL-ASSISTED HYDROFORMYLATION ON A SIO2-ATTACHED RH DIMER - INSITU EXAFS AND FT-IR OBSERVATIONS OF THE DYNAMIC BEHAVIORS OF THE DIMER SITE

Trans-[Rh(C5Me5)(CH3)]2(μ-CH2)2 was supported on the SiO2 surface by the reaction with OH groups of SiO2 at 313 K, followed by evacuation at 373 K. In this attaching reaction, the Rh dimer complexes were bound to the SiO2 surface through Rh-O(surface) bonds, losing one CH3 ligand and one C5Me5 ligand per Rh dimer. The attached Rh dimers were found to be more active and selective than a conventionally prepared Rh/SiO2 catalyst. The structure change of the Rh dimer sites in each reaction step for catalytic hydroformylation was followed by means of in situ FT-IR and in situ EXAFS techniques. The Rh-Rh bond in the attached Rh dimers (Rh-Rh = 0.262 nm) was cleaved by CO adsorption to form monomer pairs [Rh(C2H5)(CO)2(O-Si) + Rh(C5Me5)(O-Si)]. Heating the monomer pairs to 423 K under vacuum resulted in CO insertion, with new peaks exhibited at 1710 and 1394 cm-1 due to the acyl ligand. The insertion was promoted by rebonding of the two adjacent Rh atoms observed at 0.270 nm. The Rh-acyl dimers were reversibly converted to the previous monomer pairs without Rh-Rh bonding by CO admission. These behaviors of Rh dimers on SiO2 are entirely different from usual Rh monomer chemistry. A new mctal-assistcd reaction mechanism is described. © 1990, American Chemical Society. All rights reserved.