Disulfonated xantphos for mass spectrometric mechanistic analysis

Xantphos is a wide bite angle bisphosphine ligand that finds wide application in catalysis. Tracking its behavior during reactions under realistic reaction conditions can be difficult at low concentrations, and although electrospray ionization mass spectrometry (ESI-MS) is effective at real-time monitoring of catalytic reactions, it can only observe ions. Accordingly, we experimented with the dianionic disulfonated version of xantphos as a charged tag for mechanistic analysis. It proved to behave exactly as hoped, providing good intensity and enabled the direct study of both an initial binding event (to copper, very fast) and a subsequent transfer to another metal (palladium). Its dianionic nature makes it especially promising for the study of reactions in which metals change charge state, because a cationic metal complex with an anionic ligand is an invisible zwitterion, whereas a dianionic ligand would instead make the same cationic complex appear due to the overall charge of −1. As such, disulfonated xantphos holds genuine promise as a mechanistic probe in real time analysis using mass spectrometry.