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(mu-OMe)(cod)](2) (1) and the dithiolate-bridged complex [Rh-2(mu-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 P-31 NMR spectroscopy (at [Rh] = 0.0138-0.0157 grat.L-1, 5-50 bar of H-2, 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 P-31 and H-1 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 approximate to 90 degrees). A low free energy of activation (Delta G double dagger(193K) = 44 kJ.mol(-1) and Delta G double dagger(293K) = 44 kJ.mol(-1)) for phosphorus exchange has been measured for the fluxional process observed in complex 7.