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

Abstract

Monodisperse Au(25)L(18)(0) (L = S(CH(2))(2)Ph) and [n-Oct(4)N(+)][Au(25)L(18)(-)] 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 clusters react as efficient electron donors in a dissociative electron-transfer (ET) process, in the former case yielding [Au(25)L(18)(+)][C(6)F(5)CO(2)(-)]. 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 Au(25)L(18) could be fully characterized by (1)H and (13)C 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 Au(25)L(18)(0) is particularly evident in the (1)H and (13)C NMR patterns of the inner ligands. The NMR behavior of radical Au(25)L(18)(0) 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 Au(25)L(18)(-) and Au(25)L(18)(+) were found to be quite similar, pointing to the latter cluster form as a diamagnetic species.

Enhancing stability through ligand-shell engineering: A case study with Au25(SR)18 nanoclusters

“…1(a), the absorption spectra of the four samples display step-like features, suggesting the formation of molecular-like NC species. In particular, a main peak at ~670 nm along with three shoulder peaks at ~440, ~540, and ~780 nm are present in each UV-vis spectrum, which is the characteristic absorption of thiolated Au 25 NCs [36][37][38][39][40][41][42][43][44][45][46][47][48]. These data suggest the formation of atomically precise Au 25 NCs.…”

“…For proof-of-concept, the well-studied Au 25 NC was selected as the NC model [36][37][38][39][40][41][42][43][44][45][46][47][48]. In particular, we synthesized four types of thiolated Au 25 NCs: (1) Au 25 NC protected by a short-chain mono-thiolateligand containing carboxyl groups (i.e., C 3 -chain 3-mercaptopropionic acid (MPA, see insets of Fig.…”

Electron Transfer between [Au25(SC2H4Ph)18]−TOA+ and Oxoammonium Cations

“…Of note, it came to our attention that very recently Maran and co-workers have reported a different approach to the preparation of Au 25 + clusters using a special peroxide oxidant. 63 In summary, this work demonstrates that Au 25 (SC 2 H 4 -Ph) 18 À TOA + nanoclusters can work as electron donors in the electron transfer reaction, as revealed by UVÀvis, 1 H NMR, and ESR analyses. The intermolecular electron transfer between Au 25 À and TEMPO + BF 4 À occurs in a quantitative manner: the cationic Au 25 cluster can be conveniently prepared in its pure form via the two single-electron oxidation steps due to the presence of a kinetic plateau (or "buffer zone").…”

A QM/MM approach for the study of monolayer-protected gold clusters

“…Guided by the benchmark studies of Cheeseman et al [32], our calculations of NMR chemical shifts are performed with the B3LYP hybrid functional, using the LANL2DZ basis set for Au, and the 6-31G(d,p) for S, C and H. We see that the NMR isotropic shieldings obtained using the QM/MM model compare very well to that found using full DFT on the same [Au 25 (SCH 3 ) 12 (SCH 2 CH 2 Ph) 6 ] − structure. We also calculate the mean unsigned error (MUE) to compare theory with NMR experiments reported recently [10]. The mean unsigned error is defined as…”

“…Such structures are characterized by surface gold atoms in the so called "staple" (-S-Au-S-) or "V-shape" motifs (-S-Au-S-Au-S-), as opposed to commonly assumed structures in which thiolates only passivate a high symmetry gold cluster. Along these experimental studies, Density Functional Theory has been crucial to interpret structural data under the light of NMR spectroscopy [9,10], optical absorption [11], and electrochemical experiments [12]. Due to the size of these clusters, augmented by thiolated ligands, theoretical characterization (e.g calculation of spectra) requires an enormous amount of computational resources (both in time and memory).…”