Thermodynamics of homogeneous hydrogenation: Part III. Oxidative addition of H₂ to Rh(I) complexes: correlation of dihydrides structure to the thermodynamics of the catalysed homogeneous hydrogenation of cyclohexane

Abstract

The oxidative addition of H₂ to the Rh(I) chelates RhCl(PPh₃)₃1, [RhCl(diphos)]₂2 and RhCl(NP₂) (NP₂=H-N(CH₂CH₂PPh₂)₂ proceeds at 30°C and 1 atm of molecular hydrogen in ethanol-benzene with the formation of exclusively cis-dihydride products elucidated by their proton and ³¹P {¹H} NMR spectra in situ. Complex 1 affords the cis-dihydrides RhCl-(H)₂ (PPh₃)₂(S) 1a and RhCl(H)₂(PPh₃)₃1b with the preponderance of 1a over 1b. Complex 2 dissociates in solution to form an active RhCl(diphos)(S) species which oxidatively adds H₂ to form the isomeric dihydrido complexes RhCl(diphos)(H)₂(S) 2a-2c. Complex 2a is the kinetically controlled dihydride isomer with both H⁻ trans to P, whereas 2b and 2c are more stable isomers with the hydrides trans to P and (S) in 2b and P and (Cl) in 2c, respectively. Complex 2a is completely converted to 2b+2c in about 24 h. Complex 3 afford the most stable mer-Rh(NP₂)Cl(H)₂3a with H⁻ trans to N and (Cl). The less stable fac-Rh(NP₂)Cl(H)₂3c species in which one H⁻ is trans to P and the other to N and the least stable 3b where both H⁻ are trans to P. The homogeneous hydrogenation of cyclohexene catalyzed by complexes 1-3 was investigated in the temperature range 10-40°C at 0.6-1 atm of hydrogen partial pressure. Thermodynamic parameters for the formation of the dihydrido complexes of 1, 2 and 3 and the monoolefin complexes of Rh(I) were computed. The activation parameters corresponding to the rate constant k for the homogeneous hydrogenation of cyclohexene were also calculated. The enthalpy of formation of the dihydrido complexes ΔH⁰ is more favourable (exothermic) for those hydrido species where there are minimal changes in the configuration of the dihydrido complex in oxidative addition reaction. The enthalpies ΔH⁰ increase in the order 3 < 2 < 1, in accord with the configurational changes observed in NMR. Complex 1 has an unfavourably large positive ΔH^‡ of reaction. The highest catalytic activity of 1 is thus entirely due to entropy effect where the ΔS^‡ is about 50 e.u. more positive than complexes 2 and 3.