Pentamethylcyclopentadienyl-rhodium and -iridium complexes. Part 16. Homogeneous hydrogenation catalysts

Abstract

A number of pentamethylcyclopentadienyl-rhodium and -iridium complexes catalyse the hydrogenation of olefins under ambient conditions. The complexes [{M(C₅Me₅)X₂}₂](X = Cl, Br, or I, and M = Rh or Ir; M = Rh and X = NO₃), [{M(C₅Me₅)}₂HX₃](M = Rh and X = Cl or Br; M = Ir and X = Cl, Br, or 1) and [{Ir(C₅Me₅)HX}₂](X = Cl or Br) are, in the presence of triethylamine, particularly efficient catalysts in propan-2-ol and, to a lesser extent, in dichloromethane. In the presence of base the µ-chloro-complexes [{M(C₅Me₅)Cl₂}₂](M = Rh or Ir) are more active than the µ–hydrido-complexes [{M(C₅Me₅)}₂H_xC1_4–x](M = Rh, x= 1; M = Ir, x= 1 or 2) and the tri-µ- hydrido-complex [{ Ir(C₅Me₅)}₂H₃]+ Cl^– is inactive. Dienes, acetylenes, and olefins bearing functional groups are readily reduced by [{M(C₅Me₅)}₂HCl₃](M = Rh or Ir) at 100 atm and 20 °C; the functional groups were not reduced. The results of preliminary kinetic studies on these systems suggest that alkene hydrogenations in the presence of [{M(C₅Me₅)Cl₂}₂](M = Rh or Ir) and [{Ir(C₅Me₅)HCl}₂] involve monomeric active species whereas those carried out using [{M(C₅Me₅)}₂HC1₃](M = Rh or Ir) proceed via dimeric intermediates.