Rh2Cl2(CO)4adsorbed and tethered on gold powder: IR spectroscopic characterization and olefin hydrogenation activity

Abstract

Catalysts were prepared by adsorbing Rh2Cl2(CO)4directly on gold powder or on gold that contained the tethered ligands 2-(diphenylphosphino)ethane-1-thiol (DPET) or methyl 2-mercaptonicotinate (MMNT). Infrared (IR) studies (diffuse reflectance infrared Fourier transform (DRIFT)) of the catalyst Rh–Au prepared by adsorbing Rh2Cl2(CO)4directly on Au indicate that a RhI(CO)2species is present. IR studies of Rh–DPET-Au suggest that tethered cis-Rh(DPET)(CO)2Cl is the major species at relatively high Rh2Cl2(CO)4loadings, but trans-Rh(DPET)2(CO)Cl is observable at low Rh2Cl2(CO)4loadings. Spectral investigations of the catalyst Rh–MMNT-Au prepared by adsorbing Rh2Cl2(CO)4on MMNT-Au suggest that tethered [cis-Rh(MMNT)2(CO)2]+Cl–and (or) Rh(MMNT)(CO)2Cl are the major species at low Rh2Cl2(CO)4loadings, while a new unidentified species predominates at high Rh2Cl2(CO)4loadings. All three catalysts are active 1-hexene hydrogenation catalysts under the mild conditions of 40°C and 1 atm of H2; they are much more active than Au powder or Rh2Cl2(CO)4in solution. Of the three catalysts, Rh–Au is the most active with a maximum turnover frequency (TOF) of 800 mol H2per mol Rh per min while its turnover (TO) is 29 600 mol H2per mol Rh during a 2-hour run. Under the conditions of 1-hexene hydrogenation, the catalysts lose their CO ligands. Thus, it appears that a form of Rh metal on Au is the catalytically active species.Key words: catalysis, olefin hydrogenation, gold powder, tethered rhodium complexes, infrared studies, adsorption, rhodium complexes.