Online Monitoring of Hydroformylation Intermediates by ESI-MS

Self-assembling ligands bearing permanently charged moieties have been synthesized and investigated in the Rh-catalyzed hydroformylation of terminal alkenes. By coupling a high-pressure autoclave directly to an ESI mass spectrometer hydroformylation reactions applying self-assembling 6-DPPon ligands could be studied in an online fashion. The live-streaming of the reaction mixture to the spectrometer revealed a series of different complexes not observed by other methods before, the structures of which were corroborated by CID experiments. Under CO/H-2 atmosphere, new complexes that are predicted by the Wilkinson catalytic cycle could be identified and studied by CID experiments, too. Especially the ion at m/z 848, a square-planar hydrido-carbonyl complex that is normally not detectable by other methods, was investigated in detail. Collision experiments of this complex resulted in the loss of CO and H-2, the latter being quite unusual, and points to the involvement of the hydrogen bond framework. These findings were further supported by deuteration experiments that revealed a clear incorporation of deuterium into the ligands. From these findings a new hydrogen-activation mechanism was proposed. Furthermore, substrate-containing complexes could be generated too, though a huge excess of substrate was necessary. CID experiments either with D-2 or Ar yielded nearly identical spectra, hinting at a complex that might result either from a beta-hydride elimination or from intramolecular oxidative addition of one of the ligands.