The study of the structures and properties of atomically precise gold nanoclusters is the object of active research worldwide. Recently, research has been also focusing on the doping of metal ...nanoclusters through introduction of noble metals, such as platinum, and less noble metals, such as cadmium and mercury. Previous studies, which relied extensively on the use of mass spectrometry and single-crystal X-ray crystallography, led to the assignment of the location of each of these foreign-metal atoms. Our study provides new insights into this topic and, particularly, compelling evidence about the actual position of the selected metal atoms M = Pt, Pd, Hg, and Cd in the structure of Au24M(SR)18 0. To make sure that the results were not dependent on the thiolate, for SR we used both butanethiolate and phenylethanethiolate. The clusters were prepared according to different literature procedures that were supposed to lead to different doping positions. Use of NMR spectroscopy and isotope effects, with the support of mass spectrometry, electrochemistry, and single-crystal X-ray crystallography, led us to confirm that noble metals indeed dope the cluster at its central position, whereas no matter how the doping reaction is conducted and the nature of the ligand, the position of both Cd and Hg is always on the icosahedron shell, rather than at the central or staple position, as often reported. Our results not only provide a reassessment of previous conclusions, but also highlight the importance of NMR spectroscopy studies and cast doubts on drawing conclusions mostly based on single-crystal X-ray crystallography.
In applications of monolayer-protected gold clusters, assessing the dynamic behavior of the capping monolayer is crucial, as this determines the cluster’s effective size and electron-transfer (ET) ...properties. Here, we describe a systematic study on the effect of the monolayer thickness on the ET between molecular Au25(SR)18 0 nanoclusters in films. The length of the ligands protecting the Au core was varied by using a series of linear-chain thiolate (SC n H2n+1) protected Au25 clusters, where n = 3, 4, 5, 6, 7, 8, and 10. The effect of branching was assessed by using the 2-methyl-1-butanethiolate-protected Au25 cluster. Conductivity measurements were carried out on dry films obtained by drop-casting Au25(SR)18 0 solutions onto interdigitated gold electrodes. Changing the alkyl chain from C3 to C10 induces a smooth decrease in the film conductivity by 3.5 orders of magnitude. By using electrochemically determined Stokes radii, the conductivity results were transformed into the corresponding ET rate constants (k ETs) between neighboring clusters. k ET exponentially depends on the distance, and the data show that the average Au center-to-center distance in the film is not larger than the sum of two Stokes radii. This indicates that linear alkyl chains hold a detectable degree of fluidity in films, although not as much as in solution, as shown by the trend of the corresponding heterogeneous standard rate-constant values. For both physical states, these conclusions are precisely confirmed by the much slower ET rate determined for the Au25 cluster protected by the hindered thiolate. ET in films was also studied as a function of temperature, and the results were analyzed in the framework of the ET theory. We found that the outer and inner reorganization energies increase with n. For these ETs, the electronic coupling resonance between the reactant and product states, H RP, ranges from 12 (n = 3) to 0.7 cm–1 (n = 10) and therefore ETs are nonadiabatic. The data also show that 3/4 of the effect of distance on k ET is related to H RP, while the rest is associated with the intrinsic barrier and thus the distance dependence of the outer and inner reorganization energies.
Thiolate-protected metal clusters are materials of ever-growing importance in fundamental and applied research. Knowledge of their single-crystal X-ray structures has been instrumental to enable ...advanced molecular understanding of their intriguing properties. So far, however, a general, reliable, chemically clean approach to prepare single crystals suitable for accurate crystallographic analysis was missing. Here we show that single crystals of thiolate-protected clusters can be grown in large quantity and very high quality by electrocrystallization. This method relies on the fact that charged clusters display a higher solubility in polar solvents than their neutral counterparts. Nucleation of the electrogenerated insoluble clusters directly onto the electrode surface eventually leads to the formation of a dense forest of millimeter-long single crystals. Electrocrystallization of three known Au25(SR)18 0 clusters is described. A new cluster, Au25(S-nC5H11)18, was also prepared and found to crystallize by forming bundles of millimeter-long Au25 polymers.
The study of the molecular cluster Au25(SR)18 has provided a wealth of fundamental insights into the properties of clusters protected by thiolated ligands (SR). This is also because this cluster has ...been particularly stable under a number of experimental conditions. Very unexpectedly, we found that paramagnetic Au25(SR)18 0 undergoes a spontaneous bimolecular fusion to form another benchmark gold nanocluster, Au38(SR)24. We tested this reaction with a series of Au25 clusters. The fusion was confirmed and characterized by UV–vis absorption spectroscopy, ESI mass spectrometry, 1H and 13C NMR spectroscopy, and electrochemistry. NMR evidences the presence of four types of ligand and, for the same proton type, double signals caused by the diastereotopicity arising from the chirality of the capping shell. This effect propagates up to the third carbon atom along the ligand chain. Electrochemistry provides a particularly convenient way to study the evolution process and determine the fusion rate constant, which decreases as the ligand length increases. No reaction is observed for the anionic clusters, whereas the radical nature of Au25(SR)18 0 appears to play an important role. This transformation of a stable cluster into a larger stable cluster without addition of any co-reagent also features the bottom-up assembly of the Au13 building block in solution. This very unexpected result could modify our view of the relative stability of molecular gold nanoclusters.
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)18 0 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)18 2+ and Au25(SR)18 3+ 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)18 0 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.
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)18 0. 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.
Several research groups have observed magnetism in monolayer-protected gold cluster samples, but the results were often contradictory, and thus, a clear understanding of this phenomenon is still ...missing. We used Au25(SCH2CH2Ph)18 0, which is a paramagnetic cluster that can be prepared with atomic precision and whose structure is known precisely. Previous magnetometry studies only detected paramagnetism. We used samples representing a range of crystallographic orders and studied their magnetic behaviors using electron paramagnetic resonance (EPR). As a film, Au25(SCH2CH2Ph)18 0 exhibits a paramagnetic behavior, but at low temperature, ferromagnetic interactions are detectable. One or few single crystals undergo physical reorientation with the applied field and exhibit ferromagnetism, as detected through hysteresis experiments. A large collection of microcrystals is magnetic even at room temperature and shows distinct paramagnetic, superparamagnetic, and ferromagnetic behaviors. Simulation of the EPR spectra shows that both spin−orbit (SO) coupling and crystal distortion are important to determine the observed magnetic behaviors. Density functional theory calculations carried out on single cluster and periodic models predict the values of SO coupling and crystal-splitting effects in agreement with the EPR-derived quantities. Magnetism in gold nanoclusters is thus demonstrated to be the outcome of a very delicate balance of factors. To obtain reproducible results, the samples must be (i) controlled for composition and thus be monodisperse with atomic precision, (ii) of known charge state, and (iii) well-defined in terms of crystallinity and experimental conditions.
The preparation and properties of a series of gold nanoclusters protected by thiolated peptides based on the α-aminoisobutyric acid (Aib) unit are described. The peptides were devised to form 0−3 ...CO···H−N intramolecular hydrogen bonds, as required by their 310-helical structure. The monolayer-protected clusters (MPCs) were prepared, using a modified version of the two-phase Brust−Schiffrin preparation, and fully characterized with 1H NMR spectrometry, IR and UV−vis absorption spectroscopies, transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). The MPCs were obtained with core diameters in the range of 1.1−2.3 nm, depending on the reaction conditions. Structured peptides formed smaller clusters. The smallest MPC obtained is in agreement with the average formula Au38Pep18. The results showed that the chemical integrity of the peptide is maintained upon monolayer formation and that the average number of peptide ligands per gold cluster is typically 75−85% the value calculated for alkanethiolate MPCs of similar sizes. The IR and NMR spectra indicated that in the monolayer the peptides are involved in both intra- and interligand CO···H−N hydrogen bonds.
The rate constant of intramolecular electron transfer through oligopeptides based on the α-aminoisobutyric acid residue was determined as a function of the peptide length and found to depend weakly ...on the donor−acceptor separation. By measuring the electron-transfer activation energy and estimating the energy gap between donor and bridge, we were able to discard the electron hopping mechanism.
We investigated the reducing properties of a series of 1,2-diaryl-1,2-disodiumethanes by means of equilibration reactions. The electron-donor power of these vic-diorganometals is strongly affected by ...the nature of substituents present either on the aromatic ring(s) or on the carbanionic centers, and it can be correlated with their ability to delocalize the arylmethyl carbanions. These findings are supported by electrochemical analysis of the reduction behavior of the parent 1,2-diarylalkene. Applications of these results to the reduction of selected substrates are described.
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