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/H2 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 H2, 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 D2 or Ar yielded nearly identical spectra, hinting at a complex that might result either from a β-hydride elimination or from intramolecular oxidative addition of one of the ligands.