Thermodynamics of homogeneous hydrogenation: Part IV. Synthesis, characterization and hydride formation of the pd(ii) complexes chloro-bis(diphenyl- phosphinoethyl)aminepalladium(ii) chloride and chloro-tris(diphenylphosphinoethyl)aminepalladium(II) chloride and their catalysis in the homogeneous hydrogenation of cyclohexene

Abstract

The synthesis and characterization of [Pd(DPEA)Cl]Cl 1 and [Pd-(TPEA)C1]Cl 2, where DPEA and TPEA are bis(2-diphenylphosphino-ethyl) amine NH(CH₂CH₂PPh₂)₂ and tris(2-diphenylphosphinoethyl)amine N(CH₂CH₂PPh₂)₃ respectively, is reported. The catalytic activity of 1 and 2 in the homogeneous hydrogenation of cyclohexene over the temperature range 10-40°C and 0.4 to 1 atm of hydrogen partial pressure has been investigated. The rate of hydrogenation of cyclohexene catalyzed by 1 and 2 is found to be first order with respect to the catalyst, fractional order with respect to the substrate concentration and independent of H₂ partial pressure. A suitable mechanistic pathway has been proposed in which the catalysts 1 and 2 activate molecular hydrogen by forming hydrido species [Pd(DPEA)(H)]⁺ 1a and [Pd(TPEA)(H)]⁺ 2a, respectively. The formation of an unusual Pd(IV) trihydrido intermediate, [Pd(DPEA)(H)₃⁺ 1b and the dihydride [Pd(TPEA)(H)₂]²⁺ 2b by an oxidative addition of H₂ of [Pd(DPEA)(H)⁺ 1a and 2, respectively, is also reported. The hydrido species 1a and 1b, 2a and 2b were characterized by hydrido proton and ³¹P{¹H} NMR spectra. The activation parameters ΔS‡ and ΔH‡ for the hydrogenation of cyclohexene catalyzed by 1 and 2, were evaluated.