Contrasting Solution and Gas-Phase: Understanding Structure and Reactivity of Gold-Diazenyl Intermediates

Abstract

Gold(I)-catalyzed cross-coupling reactions with aryldiazonium salts are instrumental in forming C─C and C─N bonds. However, mechanistic details remain unclear, particularly the role of diazenyl intermediates. We employ stoichiometric reactions, NMR spectroscopy, electrospray ionization mass spectrometry, and infrared photodissociation spectroscopy to investigate the reactivity of gold-aryl complexes with aryldiazenyl radicals and aryldiazonium ions. Our results reveal that [(NHC)Au(Ar)] complexes form short-lived electron donor-acceptor (EDA) complexes [(NHC)Au(Ar)(Ar'N₂)]⁺ in solution, which undergo a C─N coupling reaction during the transfer to the gas phase to form gold(I) azoarene complexes. The rearrangements of the gold complexes during the transfer from the solution to the gas phase were studied by ion-mobility separation, IRPD spectra, and delayed reactant labeling experiments. We also show that gold-chloride complexes facilitate the oxidative addition of aryldiazonium ions to form [(NHC)Au^III(Cl)(Ar)(N₂)]⁺, suggesting an alternative non-radical mechanism initiating the C─C bond formation reactions. In solution-phase reactivity, ligand steric effects are crucial in steering the reaction selectivity: IMes ligands favor N₂ expulsion followed by the C─C coupling, while bulkier IPr and IPent ligands promote the C─N coupling. These findings refine our understanding of gold(I)-mediated cross-coupling reactions, and the observed solution-gas phase discrepancies highlight the need for careful interpretation of ESI-MS data in mechanistic studies.