Mono-alkylated bisphosphines as dopants for ESI-MS analysis of catalytic reactions

Bisphosphines Ph2P(CH2)(n)PPh2 (n = 1, 2, 4, 6) may be easily monobenzylated to generate cationic phosphine/phosphonium ligands [Ph2P(CH2)(n)PPh2CH2Ph](+). These ligands may be doped into a catalytic reaction involving neutral complexes with labile phosphine ligands, and replacement of a neutral phosphine with a charged analogue renders the resulting complex amenable to electrospray ionisation mass spectrometry (ESI-MS). Examination of olefin hydrogenation with Wilkinson's catalyst, RhCl(PPh3)(3), revealed that this approach yielded rapid identification of all off-cycle solution species as well as catalyst poisons. Reactive intermediates could be generated using collision-induced dissociation (CID) of a triphenylphosphine ligand to make three-coordinate RhClP2 species, and these react with alkenes in the gas phase to form RhClP2(alkene). The solution speciation and gas phase behaviour revealed by ESI-MS match closely to what is already known about the system from kinetic and NMR studies.