Effect of the Charge State (z =-1, 0,+1) on the Nuclear Magnetic Resonance of Monodisperse Au(25)[S(CH(2))(2)Ph](18)(z) Clusters

Monodisperse Au25L180 (L = S(CH2)2Ph) and [n-Oct4N+][Au25L18] clusters were synthesized in tetrahydrofuran. Monodispersity was evaluated by matrix-assisted laser desorption ionization mass spectroscopy, high-angle annular dark-field scanning transmission electron microscopy, UV-vis absorption spectroscopy, and electrochemical methods, the latter also used to assess the cluster’s charge state. An original strategy was devised to oxidize Au25L180: in the presence of bis(pentafluorobenzoyl) peroxide, the neutral cluster acts as an electron donor in a fast dissociative electron transfer (ET) process yielding [Au25L18+][C6F5CO2]. As opposed to other reported redox methods, this dissociative ET reaction is irreversible, easily controllable, and clean, particularly for NMR purposes as no hydrogen atoms are introduced. It was also used to charge [n-Oct4N+][Au25L18] and the progressive transition Au25L18  Au25L180  Au25L18+ was monitored by UV-Vis spectroscopy. Further information of the dissociative ET process was obtained by electrochemistry and dissociative ET theory. Pure samples featuring the three charge states of Au25L18 were fully characterized by 1H and 13C NMR spectrometry, using one- and two-dimensional techniques, various solvents, and as a function of temperature. The results led us to obtain new insights into the NMR behavior of Au25L18 and to unravel a fascinating picture for Au25L180 and Au25L18+. For all charge states, the NMR results and analysis nicely match recent finding about the presence of two different (inner and outer) ligand populations in the capping monolayer, each resonance of the two ligand families displaying distinct NMR patterns. The radical nature of Au25L180 is particularly evident in the 1H and 13C NMR patterns of the inner ligands. The NMR behavior of radical Au25L180 was also simulated by DFT calculations. The interplay between theory and experiments reveals a fundamental paramagnetic contribution coming from Fermi contact shifts. Interestingly, the NMR patterns of Au25L18 and Au25L18+ were found to be quite similar, pointing to the latter cluster form as a diamagnetic species.