DIPHOSPHINE AND DITHIOLATE RHODIUM COMPLEXES - CHARACTERIZATION OF THE SPECIES UNDER HYDROFORMYLATION CONDITIONS

The solution structures of the dominant species present during the sty rene 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 bee n determined. The high-pressure spectroscopic studies have revealed th at the mononuclear complex [HRh(BDPP)(CO)(2)] (7) is the predominant s pecies during the hydroformylation process, but the dimeric species [R h(BDPP)(CO)(2)](2) (8) is also present and in equilibrium with 7. Equi librium 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 mol ar 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 ord er of magnitude of 1. min. Variable-temperature P-31 and H-1 NMR spect ra (293-193 K) revealed fluxional behavior in complex 7 where the bide ntate diphosphine BDPP adopts equatorial-axial coordination to rhodium in the trigonal-bipyramidal complex [HRh(BDPP)(CO)(2)] (bite angle ap proximate to 90 degrees). A low free energy of activation (Delta G dou ble dagger(193K) = 44 kJ.mol(-1) and Delta G double dagger(293K) = 44 kJ.mol(-1)) for phosphorus exchange has been measured for the fluxiona l process observed in complex 7.