Interplay of Charge State, Lability, and Magnetism in the Molecule-like Au₂₅(SR)₁₈ Cluster

Au₂₅(SR)₁₈ (R = −CH₂–CH₂–Ph) is a molecule-like nanocluster displaying distinct electrochemical and optical features. Although it is often taken as an example of a particularly well-understood cluster, very recent literature has provided a quite unclear or even a controversial description of its properties. We prepared monodisperse Au₂₅(SR)₁₈⁰ and studied by cyclic voltammetry, under particularly controlled conditions, the kinetics of its reduction or oxidation to a series of charge states, −2, −1, +1, +2, and +3. For each electrode process, we determined the standard heterogeneous electron-transfer (ET) rate constants and the reorganization energies. The latter points to a relatively large inner reorganization. Reduction to form Au₂₅(SR)₁₈^2– and oxidation to form Au₂₅(SR)₁₈²⁺ and Au₂₅(SR)₁₈³⁺ are chemically irreversible. The corresponding decay rate constants and lifetimes are incompatible with interpretations of very recent literature reports. The problem of how ET affects the Au₂₅ magnetism was addressed by comparing the continuous-wave electron paramagnetic resonance (cw-EPR) behaviors of radical Au₂₅(SR)₁₈⁰ and its oxidation product, Au₂₅(SR)₁₈⁺. As opposed to recent experimental and computational results, our study provides compelling evidence that the latter is a diamagnetic species. The DFT-computed optical absorption spectra and density of states of the −1, 0, and +1 charge states nicely reproduced the experimentally estimated dependence of the HOMO–LUMO energy gap on the actual charge carried by the cluster. The conclusions about the magnetism of the 0 and +1 charge states were also reproduced, stressing that the three HOMOs are not virtually degenerate as routinely assumed: In particular, the splitting of the HOMO manifold in the cation species is severe, suggesting that the usefulness of the superatom interpretation is limited. The electrochemical, EPR, and computational results thus provide a self-consistent picture of the properties of Au₂₅(SR)₁₈ as a function of its charge state and may furnish a methodology blueprint for understanding the redox and magnetic behaviors of similar molecule-like gold nanoclusters

Gold Nanowired: A Linear (Au₂₅)_<i>n</i> Polymer from Au₂₅ Molecular Clusters

Au₂₅(SR)₁₈ has provided fundamental insights into the properties of clusters protected by monolayers of thiolated ligands (SR). Because of its ultrasmall core, 1 nm, Au₂₅(SR)₁₈ displays molecular behavior. We prepared a Au₂₅ cluster capped by <i>n</i>-butanethiolates (SBu), obtained its structure by single-crystal X-ray crystallography, and studied its properties both experimentally and theoretically. Whereas in solution Au₂₅(SBu)₁₈⁰ is a paramagnetic molecule, in the crystal it becomes a linear polymer of Au₂₅ clusters connected <i>via</i> single Au–Au bonds and stabilized by proper orientation of clusters and interdigitation of ligands. At low temperature, [Au₂₅(SBu)₁₈⁰]_<i>n</i> has a nonmagnetic ground state and can be described as a one-dimensional antiferromagnetic system. These findings provide a breakthrough into the properties and possible solid-state applications of molecular gold nanowires

ENDOR Evidence of Electron–H₂ Interaction in a Fulleride Embedding H₂

An endofulleropyrrolidine, with H₂ as a guest, has been reduced to a paramagnetic endofulleride radical anion. The magnetic interaction between the electron delocalized on the fullerene cage and the guest H₂ has been probed by pulsed ENDOR. The experimental hyperfine couplings between the electron and the H₂ guest were measured, and their values agree very well with DFT calculations. This agreement provides clear evidence of magnetic communication between the electron density of the fullerene host cage and H₂ guest. The <i>ortho-H</i>_<i>2</i>/<i>para-H</i>_<i>2</i> interconversion is revealed by temperature-dependent ENDOR measurements at low temperature. The conversion of the paramagnetic <i>ortho-H</i>_<i>2</i> to the diamagnetic <i>para-H</i>_<i>2</i> causes the ENDOR signal to decrease as the temperature is lowered due to the spin catalysis by the paramagnetic fullerene cage of the radical anion fulleride

Au₂₅(SEt)₁₈, a Nearly Naked Thiolate-Protected Au₂₅ Cluster: Structural Analysis by Single Crystal X‑ray Crystallography and Electron Nuclear Double Resonance

X-ray crystallography has been fundamental in discovering fine structural features of ultrasmall gold clusters capped by thiolated ligands. For still unknown structures, however, new tools capable of providing relevant structural information are sought. We prepared a 25-gold atom nanocluster protected by the smallest ligand ever used, ethanethiol. This cluster displays the electrochemistry, mass spectrometry, and UV–vis absorption spectroscopy features of similar Au₂₅ clusters protected by 18 thiolated ligands. The anionic and the neutral form of Au₂₅(SEt)₁₈ were fully characterized by ¹H and ¹³C NMR spectroscopy, which confirmed the monolayer’s properties and the paramagnetism of neutral Au₂₅(SEt)₁₈⁰. X-ray crystallography analysis of the latter provided the first known structure of a gold cluster protected by a simple, linear alkanethiolate. Here, we also report the direct observation by electron nuclear double resonance (ENDOR) of hyperfine interactions between a surface-delocalized unpaired electron and the gold atoms of a nanocluster. The advantages of knowing the exact molecular structure and having used such a small ligand allowed us to compare the experimental values of hyperfine couplings with DFT calculations unaffected by structure’s approximations or omissions

Au₂₅(SEt)₁₈, a Nearly Naked Thiolate-Protected Au₂₅ Cluster: Structural Analysis by Single Crystal X‑ray Crystallography and Electron Nuclear Double Resonance

X-ray crystallography has been fundamental in discovering fine structural features of ultrasmall gold clusters capped by thiolated ligands. For still unknown structures, however, new tools capable of providing relevant structural information are sought. We prepared a 25-gold atom nanocluster protected by the smallest ligand ever used, ethanethiol. This cluster displays the electrochemistry, mass spectrometry, and UV–vis absorption spectroscopy features of similar Au₂₅ clusters protected by 18 thiolated ligands. The anionic and the neutral form of Au₂₅(SEt)₁₈ were fully characterized by ¹H and ¹³C NMR spectroscopy, which confirmed the monolayer’s properties and the paramagnetism of neutral Au₂₅(SEt)₁₈⁰. X-ray crystallography analysis of the latter provided the first known structure of a gold cluster protected by a simple, linear alkanethiolate. Here, we also report the direct observation by electron nuclear double resonance (ENDOR) of hyperfine interactions between a surface-delocalized unpaired electron and the gold atoms of a nanocluster. The advantages of knowing the exact molecular structure and having used such a small ligand allowed us to compare the experimental values of hyperfine couplings with DFT calculations unaffected by structure’s approximations or omissions