The use of mass spectrometry in mechanistic studies allows for detection, characterisation and investigation of short-lived intermediates. However, the gap between solution-phase environment and gas-phase observations inherent to mass spectrometry adds a layer of complexity to the interpretation of the results.

Our work[1] presents a multifaceted investigation, exploring a ligand-based divergent selectivity in the reaction of [(NHC)Au-aryl] complexes with aryldiazenyl radicals to form either biaryl- or azoarene products. While electrospray ionization mass spectrometry (ESI-MS) serves as a powerful tool for detecting the putative gold-diazenyl intermediates, we reveal a critical challenge: the propensity for these species to undergo significant structural rearrangements during their transfer into the gas phase. Through the synergistic application of ion mobility spectrometry (IMS) and infrared photodissociation (IRPD) spectroscopy, we demonstrate that the detected ions correspond to isobaric gold(I)-azoarene product complexes, formed in situ during the ESI-MS process, rather than the true solution-phase intermediates.

Despite these gas-phase transformations, kinetic studies utilizing isotopic labeling provide compelling evidence for the transient nature of genuine solution-phase intermediates, indicating sub-second lifetimes. Furthermore, our findings highlight the role of gold-chloride complexes in facilitating the oxidative addition of aryldiazonium ions, leading to distinct gold(III) intermediates detectable in the gas phase.

Our efforts to bridge the gap “from solution to spectrometer” reveal lessons about the necessity of rigorous analytical validation in mechanistic studies and offer new avenues for rational ligand design to control selectivity in gold catalysis.

[1] F. Ruepp, J. Mehara, P. White, A. Y. Pereverzev, J. Roithová, Helvetica Chimica Acta, 2025, e202500057