Disulfonated xantphos for mass spectrometric mechanistic analysis

Abstract

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 while 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.

Comparative Assessment of ESI‐MS Softness for Inorganic Complexes: How Soft Is Your ESI‐MS?

“…Given that Pd(PPh 3 ) 4 primarily exists in solution as the trisligated Pd(PPh 3 ) 3 species formed following ligand dissociation, [39] the introduction of a charge‐tagged analogue of PPh 3 facilitates the generation of anionic species through ligand exchange, making the complex detectable in negative mode ESI‐MS. Previous work has demonstrated the utility of charge‐tagged analogues in illuminating reaction intermediates in ESI‐MS studies, with their reactivity closely mirroring that of their neutral analogues [40–42] . In this context, detection of the n =2 trisligated ion implies preservation of weaker metal‐ligand interactions in a “softer” source.…”

“…Previous work has demonstrated the utility of chargetagged analogues in illuminating reaction intermediates in ESI-MS studies, with their reactivity closely mirroring that of their neutral analogues. [40][41][42] In this context, detection of the n = 2 trisligated ion implies preservation of weaker metal-ligand interactions in a "softer" source.…”

How soft is your ESI-MS anyway?

“…Previous work has demonstrated the utility of charge-tagged analogues in illuminating reaction intermediates in ESI-MS studies, with their reactivity closely mirroring that of their neutral analogues. 29,39,40 In this context, detection of the n = 2 (3-coordinate) ion implies preservation of weaker metal-ligand interactions in a "softer" ESI-MS source.…”