Gas-Phase Synthesis and Soft-Landing Deposition of Silver-Coumarin Complexes [Ag(C₉H₆O₂)₂]⁺ within Weakly Bonded Interactions

Coumarin and its derivatives are common in biomolecules and function as pharmacophores in diverse medications. Here we studied the gas-phase reactions of coumarin with silver (Ag) and copper (Cu) using a compact tube reactor coupled to a custom reflection time-of-flight mass spectrometer (Re-TOFMS) featuring dual ionization sources of magnetron sputtering (MagS) and thermal evaporation (TVa). Interestingly, both Ag⁺ and Cu⁺ facilitated hydrogenated coumarin formation, yielding mono- and bis-coumarin complexes in varying ratios at different TVa temperatures. DFT calculations and topology analysis showed that [Cu(C₉H₆O₂)_1,2]⁺ primarily involves conventional Cu-O coordination bonds, but [Ag(C₉H₆O₂)_1,2]⁺ exhibits weaker metal-ligand interactions. In [Ag(C₉H₆O₂)₂]⁺, Ag coordinates with coumarin's oxygen via weakly bonded interactions, allowing a low-lying isomer to form a noncovalent Ag-C═C interaction and intermolecular CH···O hydrogen bonds that bridge the two coumarin molecules. Using soft-landing deposition (SLD), we isolated [Ag(C₉H₆O₂)₂]⁺ and characterized it by XPS and Raman spectroscopies. XPS analysis revealed an Ag(0) state of the deposited [Ag(C₉H₆O₂)₂]⁺ clusters, illustrating significant electron density redistribution and validating the ligand protection to prevent oxidation of Ag(0) during vacuum sampling. Raman spectra aligned with DFT prediction, confirming structural integrity postdeposition. These results advance our understanding of metal-ligand coordination and noncovalent stabilization mechanisms.