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

Bisphosphines Ph(2)P(CH(2))(n)PPh(2) (n = 1, 2, 4, 6) may be easily monobenzylated to generate cationic phosphine/phosphonium ligands [Ph(2)P(CH(2))(n)PPh(2)CH(2)Ph](+). 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(PPh(3))(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 RhClP(2) species, and these react with alkenes in the gas phase to form RhClP(2)(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.