Interplay
of Charge State, Lability, and Magnetism
in the Molecule-like Au<sub>25</sub>(SR)<sub>18</sub> Cluster
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Abstract
Au<sub>25</sub>(SR)<sub>18</sub> (R = −CH<sub>2</sub>–CH<sub>2</sub>–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 Au<sub>25</sub>(SR)<sub>18</sub><sup>0</sup> 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 Au<sub>25</sub>(SR)<sub>18</sub><sup>2–</sup> and oxidation
to form Au<sub>25</sub>(SR)<sub>18</sub><sup>2+</sup> and Au<sub>25</sub>(SR)<sub>18</sub><sup>3+</sup> 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
Au<sub>25</sub> magnetism was addressed by comparing the continuous-wave
electron paramagnetic resonance (cw-EPR) behaviors of radical Au<sub>25</sub>(SR)<sub>18</sub><sup>0</sup> and its oxidation product,
Au<sub>25</sub>(SR)<sub>18</sub><sup>+</sup>. 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 Au<sub>25</sub>(SR)<sub>18</sub> 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