Homogeneous hydrogenation reaction with two tridentate rhodium(I) chelate complexes of various ring size

Abstract

Each kinetically examined catalyst of the type LRhCl (L = bis(3-(diphenylphosphino)propyl)amine, PPA, and bis(4-(diphenylphosphino-butyl)benzyl)amine, PBA) was prepared through a reaction between the tridentate ligands and [Rh(COD)Cl] 2 in a polar solvent. The products obtained were both identified by means of elemental analysis, mass spectroscopic, 1H, 13C and 31P NMR and IR measurements. The geometry of the ligand donor atoms (2 × P, N and Cl) is square planar in these complexes, as is normally observed in Rh(I) coordination compounds. Via kinetic measurements of hydrogenation reactions of cyclohexene as substrate in the range of 20–50 °C and 800–1200 hPa H 2 pressure, it was found that the catalyst (PPA) RhCl has an activation energy E A of 52.3 kJ mol −1 and that of (PBA)RhCl is 45.3 kJ mol −1. The observed difference of 7.0 kJ mol −1 relates to a chelate ring size dependency, decreasing on going from a six- to a seven-membered chelate ring. The rate-determining step of the overall hydrogenation process with both catalysts probably is dependent on a chloride dissociation step. Consequently, on going from nonpolar to polar solvents for the examined hydrogenation processes under comparable reaction conditions, the observed hydrogenation rate (TON) of both catalysts increases. The substrate dependency of the two catalysts determined in a hydrogenation reaction is increased as follows: terminal olefins > cyclic olefins ≈ chelating olefins > cis -olefins > trans -olefins > multiply-substituted olefins; alkynes as substrates also achieved good initial hydrogenation rates.