Diphosphine and Dithiolate Rhodium Complexes:  Characterization of the Species under Hydroformylation Conditions

The solution structures of the dominant species present during the styrene hydroformylation using rhodium precursors [Rh(μ-OMe)(cod)]2 (1) and the dithiolate-bridged complex [Rh2(μ-BCOS)(cod)2]2 (9) in the presence of BDPP [(2S,4S)-bis(diphenylphosphine)pentane] have been determined. The high-pressure spectroscopic studies have revealed that the mononuclear complex [HRh(BDPP)(CO)2] (7) is the predominant species during the hydroformylation process, but the dimeric species [Rh(BDPP)(CO)2]2 (8) is also present and in equilibrium with 7. Equilibrium constants were measured by 31P NMR spectroscopy (at [Rh] = 0.0138−0.0157 grat·L-1, 5−50 bar of H2, 5 bar of CO, 298−343 K, the molar ratio of 7:8 ranges from 1 to 20). Kinetic measurements have shown that the half-life of the species at ambient temperature is on the order of magnitude of 1 min. Variable-temperature 31P and 1H NMR spectra (293−193 K) revealed fluxional behavior in complex 7 where the bidentate diphosphine BDPP adopts equatorial−axial coordination to rhodium in the trigonal-bipyramidal complex [HRh(BDPP)(CO)2] (bite angle ≈90°). A low free energy of activation (ΔG ⧧ 193K = 44 kJ·mol-1 and ΔG ⧧ 293K = 44 kJ·mol-1) for phosphorus exchange has been measured for the fluxional process observed in complex 7.