Effect of the Charge State (z =-1, 0,+1) on the Nuclear Magnetic Resonance of Monodisperse Au25[S(CH2)2Ph]18z Clusters

Monodisperse Au25L180 (L = S(CH2)(2)Ph) and [n-Oct(4)N(+)][Au25L18,(-)]clusters were synthesized in tetrahydrofuran. An original strategy was then devised to oxidize them: in the presence of bis(pentafluorobenzoyl) peroxide, the neutral or the negatively charged dusters react as efficient electron donors in a dissociative electron-transfer (ET) process, in the former case yielding [Au25L18+][C6F5CO2-]. As opposed to, other reported redox methods, this dissociative ET approach is irreversible, easily controllable, and clean, particularly for NMR purposes, as no hydrogen atoms are introduced. By using this approach, the -1, 0, and +1 charge states of Au25L18 could be fully characterized by H-1 and C-13 NMR spectroscopy, using one- and two-dimensional techniques, in various solvents, and as a function of temperature. For all charge states, the NMR results and analysis nicely match recent structural findings about the presence of two different 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 H-1 and C-13 NMR patterns of the inner ligands. The NMR behavior of radical (AuL180)-L-25 was also simulated by DFT calculations, and the interplay between theory and experiments revealed 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 duster form as a diamagnetic species.