Intermediate Intercluster Bond Orders. Electronic Communication in Au38(SR)24 Superatomic Molecules

Ligand-protected gold clusters remain potential building blocks for envisaged molecular materials. The archetypal Au38(SR)24 cluster can be viewed as a robust template for the fusion of two Au25(SR)18 - cluster units, retaining a bi-icosahedral Au23 core. Via electrochemical properties, the overall charge state can be selectively tuned, enabling the access of 14 valence electron (ve) species featuring a single intercluster bond and nearby charge from -1 to +3, achieving related species bearing 15- to 11-ve with variable intercluster bond orders. Here, we explore the characteristics of intermediate intercluster bond orders in order to provide insights into the plausible electron communication between the constituent building blocks, with Au38(SR)24, as a representative template. Our results denote a small structural variation along -1 to +3 charge states, provided by the core-protecting ligand interaction, which is enhanced towards more oxidized species. The remaining unpaired electron from intermediate intercluster bond orders of 1.5 for Au38(SR)24 1-, 1.5 for Au38(SR)24 1+, and 2.5 for Au38(SR)24 3+, holds delocalized characteristics between the building block units, favoring electron communication for conductive and cooperative cluster aggregates. Such features are relevant for the formation of molecular electronic device applications, favoring the rationalization prior to engaging in explorative synthesis of larger ligand-protected cluster aggregates.

Ligand-free supermolecules: [Pd2@Ge18]4- and [Pd2@Sn18]4- as multiple-bonded Zintl-ion clusters based on Pd@Ge9 and Pd@Sn9 assembled units

Understanding intercluster bonding interactions is important in the rational synthesis of building blocks for molecular materials. Such characteristics have been developed for coinage metal clusters resembling single-, double-, and triple-bonded species, coined as supermolecules. Herein, we extend such an approach for understanding main-group clusters, thus evaluating [Pd2@E18]4- clusters (E = Ge, Sn) involving the fusion of parent spherical aromatic [Pd@E12]2- building units. Our results indicate intercluster bonding provided by contribution from 2P and 1G shells centered at each building motif, leading to an overall bond order of 2.70 and 2.31 for [Pd2@Ge18]4- and [Pd2@Sn18]4-, respectively. In addition, 119Sn-NMR patterns were evaluated to complement the experimental characterization of a single peak owing to the insolution fluxional behavior of [Pd2@Sn18]4- as three peaks owing to the three sets of unique Sn atoms within the structure. Magnetic response properties revealed that spherical aromatic characteristics from parent [Pd@E12]2- building units are retained in the overall [Pd2@E18]4- oblate cluster as two spherical aromatic units. Hence, the notion of superatomic molecules is extended to Zintl-ion clusters, favoring further rationalization for the fabrication of cluster-assembled solids.

Atom-Precise Ligated Copper and Copper-Rich Nanoclusters with Mixed-Valent Cu(I)/Cu(0) Character: Structure-Electron Count Relationships

Copper homometallic and copper-rich heterometallic nanoclusters with some Cu(0) character are reviewed. Their structure and stability are discussed in terms of their number of "free" electrons. In many aspects, this structural chemistry differs from that of their silver or copper homologs. Whereas the two-electron species are by far the most numerous, only one eight-electron species is known, but more electron-rich nanoclusters have also been reported. Owing to the relatively recent development of this chemistry, it is likely that more electron-rich species will be reported in the future.

Second-order superatoms: Au52-PAP featuring a three-dimensional cluster-of-clusters core

The recent characterization of Au52-PAP cluster can be viewed as a three-dimensional arrangement featuring four Au13 motifs. As a result, a new set of superatomic orbitals are built up from the superatomic shell of each constituent unit, denoted by 1S'21P'62S'21D'102P'61F'6 and, thus, referred to as a second-order superatomic shell structure. This favors the rationalization of larger species toward the formation of cluster-assembled materials of different sizes.

Supervalence Bonding in Bi-icosahedral Cores of [M1Au37(SC2H4Ph)24]- (M = Pd and Pt): Fusion-Mediated Synthesis and Anion Photoelectron Spectroscopy

Au₃₈(PET)₂₄ (PET = SC₂H₄Ph) is known to have a bi-icosahedral Au₂₃ core consisting of two Au₁₃ icosahedrons by sharing three Au atoms. Previous theoretical studies based on a supervalence bond (SVB) model have demonstrated that the bonding scheme in the Au₂₃ core is similar to that in the F₂ molecule. The SVB model predicted that the electron configuration of the Au₂₃ core with 14 valence electrons is expressed as (1Σ)²(1Σ*)²(1Π)⁴(2Σ)²(1Π*)⁴ where each orbital is created by the bonding and antibonding interactions between the 1S and 1P superatomic orbitals of the icosahedral Au₁₃ units. Therefore, the bi-icosahedral Au₂₃ can be viewed as a di-superatomic molecule. To validate the SVB model, we herein conducted anion photoelectron spectroscopy (PES) on [M₁Au₃₇(PET)₂₄]^- (M = Pd and Pt), which are isoelectronic and isostructural with Au₃₈(PET)₂₄. To this end, the neutral precursors [M₁Au₃₇(PET)₂₄]⁰ were first synthesized by fusion reactions between hydride-doped clusters [HAu₉(PPh₃)₈]²⁺ and [M₁Au₂₄(PET)₁₈]^-. The formation of bi-icosahedral M₁Au₂₂ cores with open electronic structure in [M₁Au₃₇(PET)₂₄]⁰ was confirmed by single-crystal X-ray diffraction analysis and electron paramagnetic resonance measurement. Then, the target anions [M₁Au₃₇(PET)₂₄]^- were obtained by reducing [M₁Au₃₇(PET)₂₄]⁰ with NaBH₄, and isoelectronicity with [Au₃₈(PET)₂₄]⁰ was confirmed by optical spectroscopy and density functional theory calculations. Finally, anion PES on [M₁Au₃₇(PET)₂₄]^- observed two distinctive peaks as predicted by the SVB model: one from the nearly degenerate 1Π* orbitals and the other from the nearly degenarate 1Π and 2Σ orbitals.

Superatomic Three-Center Bond in a Tri-Icosahedral Au36Ag2(SR)18 Cluster: Analogue of 3c-2e Bond in Molecules

Probing the nature of electronic stability for ligand-protected gold clusters is important in gold chemistry. A thermally stable Au₃₆Ag₂(SR)₁₈ nanocluster was synthesized recently. It has a D_3h tri-icosahedral [Au₃₀Ag₂]¹²⁺ core with 20 valence electrons, which does not follow the magic number of gold superatoms. Herein, we propose a superatomic three-center bond to unveil its electronic stability. The [Au₃₀Ag₂]¹²⁺ core is viewed as a union of three face-fused superatoms, and chemical bonding analysis suggests a three-superatom-center two-electron (3sc-2e) bond for the octet rule of each superatom, which mimics the bonding framework of the D_3h O₃^2- molecule. Moreover, a liganded tri-icosahedral [Au₂₇Pt₃Ag₂]¹¹⁺ core with 18 valence electrons is predicted, and three 2sc-2e bonds are formed between each of two superatoms to satisfy the octet rule (analogue of D_3h O₃), indicating the flexibility of superatomic bonding. Such a superatomic three-center bond extends the community of superatomic bonding and gives a new perspective for superatom assembling.

Asymmetrically Doping a Platinum Atom into a Au38 Nanocluster for Changing the Electron Configuration and Reactivity in Electrocatalysis

It is an obstacle to precisely manipulate a doped heteroatom into a desired position in a metal nanocluster. Herein, we overcome this difficulty to obtain Pt₁ Au₃₇ (SCH₂ Ph^t Bu)₂₄ and Pt₂ Au₃₆ (SCH₂ Ph^t Bu)₂₄ nanoclusters via controllably doping Pt atoms into the kernels of Au₃₈ (SCH₂ Ph^t Bu)₂₄ . We reveal that asymmetrical doping of one Pt atom into either of the cores of Au₃₈ (SCH₂ Ph^t Bu)₂₄ elevates the relative energy of the HOMO (highest occupied molecular orbital) accompanied by one valence electron loss of Pt₁ Au₃₇ (SCH₂ Ph^t Bu)₂₄ , compared to Au₃₈ (SCH₂ Ph^t Bu)₂₄ with 14 electrons, while symmetrical doping of two Pt atoms into the cores of Au₃₈ (SCH₂ Ph^t Bu)₂₄ narrows the HOMO-LUMO gap (LUMO: lowest unoccupied molecular orbital) of Pt₂ Au₃₆ (SCH₂ Ph^t Bu)₂₄ with two valence electrons less. Consequently, Pt₁ Au₃₇ (SCH₂ Ph^t Bu)₂₄ shows an electron-spin-induced high activity for CO₂ electroreduction, whereas Pt₂ Au₃₆ (SCH₂ Ph^t Bu)₂₄ is least efficient and Au₃₈ (SCH₂ Ph^t Bu)₂₄ has a decent performance.

The Factors Dictating Properties of Atomically Precise Metal Nanocluster Electrocatalysts

Metal nanoparticles occupy an important position in electrocatalysis. Unfortunately, by using conventional synthetic methodology, it is a great challenge to realize the monodisperse composition/structure of metal nanoparticles at the atomic level, and to establish correlations between the catalytic properties and the structure of individual catalyst particles. For the study of well-defined nanocatalysts, great advances have been made for the successful synthesis of nanoparticles with atomic precision, notably ligand-passivated metal nanoclusters. Such well-defined metal nanoclusters have become a type of model catalyst and have shown great potential in catalysis research. In this review, the authors summarize the advances in the utilization of atomically precise metal nanoclusters for electrocatalysis. In particular, the factors (e.g., size, metal doping/alloying, ligand engineering, support materials as well as charge state of clusters) affecting selectivity and activity of catalysts are highlighted. The authors aim to provide insightful guidelines for the rational design of electrocatalysts with high performance and perspectives on potential challenges and opportunities in this emerging field.

Tri- and Tetra-superatomic Molecules in Ligand-Protected Face-Fused Icosahedral (M@Au12)n (M = Au, Pt, Ir, and Os, and n = 3 and 4) Clusters

Cluster assembling has been one of the hottest topics in nanochemistry. In certain ligand-protected gold clusters, bi-icosahedral cores assembled from Au₁₃ superatoms were found to be analogues of diatomic molecules F₂, N₂, and singlet O₂, respectively, in electronic shells, depending upon the super valence bond (SVB) model. However, challenges still remain for extending the scale in cluster assembling via the SVB model. In this work, ligand-protected tri- and tetra-superatomic clusters composed of icosahedral M@Au₁₂ (M = Au, Pt, Ir, and Os) units are theoretically predicted. These clusters are stable with reasonable highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps and proven to be analogues of simple triatomic (Cl₃^-, OCl₂, O₃, and CO₂) and tetra-atomic (N≡C-C≡N, and Cl-C≡C-Cl) molecules in both geometric and electronic structures. Moreover, a stable cluster-assembling gold nanowire is predicted following the same rules. This work provides effective electronic rules for cluster assembling on a larger scale and gives references for their experimental synthesis.

Superatomic Orbital Splitting in Coinage Metal Nanoclusters

The superatomic orbital splitting (SOS) method is developed to understand the electronic structures of coinage metal nanoclusters, in which delocalized electron counts are not magic numbers. Because the symmetry of a metal core can significantly affect the electronic structure of a nanocluster, this method takes the shape of the core into account in determining the order of group orbital levels. By taking nanoclusters as superatoms, a highly positively charged core is established by removing the ligands and staples. The superatomic orbitals split into group orbitals at different energy levels because of the nonspherical shape of the cluster core. Therefore, the electron configuration of the nonmagic-number nanocluster can be qualitatively analyzed without quantum chemical calculations, which is very important for understanding the stability of the cluster.

Interaction potential energy surface between superatoms

In this work, we report the potential energy surface between superatoms (SPES) based on first-principles theory. The calculations show that superatoms have SPES crossing behavior between different electronic states similar to an atomic potential energy surface (PES). However, unlike atoms, the relative rotation between superatoms results in new dimensions for the SPES. The rotation is divided into two types, around the direction of the line between two superatoms and perpendicular to it. At the equilibrium distance, the former rotation results in maximum energy and charge changes of 0.03 eV and 10^-2 e respectively. However, the latter rotation yields changes that are 17 times and 7 times those of the former. These findings help promote the understanding of intermolecular interactions, and will contribute to the bottom-up superatomic-based assembly of novel materials.

Controlled Dimerization and Bonding Scheme of Icosahedral M@Au12 (M=Pd, Pt) Superatoms

Targeted syntheses of MM'Au₃₆ (PET)₂₄ (M, M'=Pd, Pt; PET=SC₂ H₄ Ph) were achieved by hydride-mediated fusion reactions between [MAu₈ (PPh₃ )₈ ]²⁺ and [M'Au₂₄ (PET)₁₈ ]^- . Single-crystal X-ray diffraction analysis indicated that the products have bi-icosahedral MM'Au₂₁ cores composed of M@Au₁₂ and M'@Au₁₂ superatoms. Although the MM'Au₂₁ superatomic molecules correspond to O₂ in terms of the number of valence electrons (12 e), the distances between the icosahedrons were larger than that in the bi-icosahedral Au₂₃ core of Au₃₈ (PET)₂₄ corresponding to F₂ and the spin state was singlet. These counterintuitive results were explained by a "bent bonding model" based on tilted (non-orthogonal) bonding interaction between the 1P superatomic orbitals of M@Au₁₂ and M'@Au₁₂ superatoms.

Electron counting and bonding patterns in assemblies of three and more silver-rich superatoms

DFT calculations were carried out on a series of cluster cores, the framework of which was made of the condensation of several Pt@Ag12-centered icosahedra. Icosahedral condensations through vertex-sharing, face-sharing, and interpenetration were considered and their favored electron counts were determined from their stable closed-shell configurations. A large number of the computed assemblies of n icosahedral superatomic units can be considered as isolobal analogs of stable, closed-shell n-atom molecules, most of them obeying the octet rule. The larger the degree of fusion between icosahedra, the stronger the interaction between them. For example, it was possible to design 3-icosahedral supermolecular cores analogous to CO2, SF2, or [I3]-, but also to the not-yet-isolated cyclic O3. Supermolecules equivalent to non-stable molecules can also be designed. Indeed, differences exist between atoms and superatoms, and original icosahedra assemblies with no "molecular" analogs are also likely to exist, especially with compact structures and/or systems made of a large number of fused superatoms.

Multiple d-d bonds between early transition metals in TM2Lin (TM = Sc, Ti) superatomic molecule clusters

The synthesis and application of compounds with Cr-Cr and V-V d-d quintuple bonds (σ, 2π, 2δ) have led to new thinking about whether d-d multiple bonds also exist between early transition metals such as Sc-Sc and Ti-Ti. In this study, by extensive unbiased global search at the density functional theory level, the low-energy structures of 26e and 30e TM2Lin clusters were obtained. Based on the super valence bond (SVB) theory, the prolate double-core structure of TM2Lin clusters was regarded as a superatomic molecule, of which each half was regarded as an open-shell superatom, and the electronic shell-closure was realized by forming multiple bonds between superatoms. Then, the quintuple super bonds (2δ, 2π, σ) of the Li18Ti2, Li20Sc2, [Li17V2]+, [Li17Ti2]- clusters and the triple super bonds (2π, σ) of the Li24Sc2 and Li24Y2 clusters were confirmed via chemical-bonding analysis. This way of forming multiple bonds between early transition metals through superatomic bonding has promoted the experimental synthesis and application of early transition metal multiple bond compounds.

Atomically precise alloy nanoclusters: syntheses, structures, and properties

Metal nanoclusters fill the gap between discrete atoms and plasmonic nanoparticles, providing unique opportunities for investigating the quantum effects and precise structure-property correlations at the atomic level. As a versatile strategy, alloying can largely improve the physicochemical performances compared to the corresponding homo-metal nanoclusters, and thus benefit the applications of such nanomaterials. In this review, we highlight the achievements of atomically precise alloy nanoclusters, and summarize the alloying principles and fundamentals, including the synthetic methods, site-preferences for different heteroatoms in the templates, and alloying-induced structure and property changes. First, based on various Au or Ag nanocluster templates, heteroatom doping modes are presented. The templates with electronic shell-closing configurations tend to maintain their structures during doping, while the others may undergo transformation and give rise to alloy nanoclusters with new structures. Second, alloy nanoclusters of specific magic sizes are reviewed. The arrangement of different atoms is related to the symmetry of the structures; that is, different atoms are symmetrically located in the nanoclusters of smaller sizes, and evolve into shell-by-shell structures at larger sizes. Then, we elaborate on the alloying effects in terms of optical, electrochemical, electroluminescent, magnetic and chiral properties, as well as the stability and reactivity via comparisons between the doped nanoclusters and their homo-metal counterparts. For example, central heteroatom-induced photoluminescence enhancement is emphasized. The applications of alloy nanoclusters in catalysis, chemical sensing, bio-labeling, and other fields are further discussed. Finally, we provide perspectives on existing issues and future efforts. Overall, this review provides a comprehensive synthetic toolbox and controllable doping modes so as to achieve more alloy nanoclusters with customized compositions, structures, and properties for applications. This review is based on publications available up to February 2020.

Size Evolution Dynamics of Gold Nanoclusters at an Atom-Precision Level: Ligand Exchange, Growth Mechanism, Electrochemical, and Photophysical Properties

Interpretation of size evolution is an essential part of nanocluster transformation processes for unraveling the mechanism at an atom-precision level. Here we report the transformation of a non-superatomic Au₂₃ to a superatomic Au₃₆ nanocluster via Au₂₈ cluster formation, activated by the bulky 4-tert-butylbenzenethiol ligand. Time-dependent matrix-assisted laser desorption ionization mass spectrometry data revealed that the conversion proceeds through ligand exchange followed by the size focusing method, ultimately leading to size growth. We also validated this transformation through time-dependent ultraviolet-visible data. Density functional theory calculations predicted that the kernel of the Au₂₈ cluster evolved through a linear combination of molecular orbitals of the fragment of 2e^- units (Au₄²⁺ and Au₃⁺) from the kernel of the Au₂₃ cluster. Periodic growth of gold cores through continuous growth of Au₄ tetrahedral unit leads to the formation of the Au₃₆ cluster from the Au₂₈ cluster. These results reinforce the plausibility of size evolution through the growth mechanism during the transformation process. Differential pulse voltammetry studies showed that the highest occupied molecular orbital-lowest unoccupied molecular orbital gap inversely varies with the kernel size of these clusters. Photophysical experiments support the molecular-like intersystem crossing rather than core-shell relaxation to these clusters. The trends of photoluminescence lifetime were found to be the reverse of those of the energy gap law. The increment of lifetimes for the larger cluster can be mainly due to the contribution of both hot carriers and band-edge carriers.

Triple 1D[triple bond, length as m-dash]1D superatomic bonding. Au22(dppo)6 as a Π4- and Δ2-triply bonded cluster based on Au11 assembled units

δ-Bonds have been intimately related to metal-metal bonds of d-elements since the archetypal d⁴-d⁴ [Re₂Cl₈]^2- ion with a σ²π⁴δ² bond. Currently, the notion of multiple superatom arrangements as initial steps toward molecular materials within the building-up approach is dominated by P-shell characteristics, as given in the well-described Au₂₅(SR)₁₈ ligand protected cluster. In this work we rationalize the Au₂₂(dppo)₆ cluster as a triple-bonded 22-valence electron (ve) supermolecule, featuring a bonding scheme based on 1D + 1D shell combinations, which largely contrasts with the 14-ve Au₃₈(SR)₂₄ with mainly 1P + 1P patterns mimicking a F₂ molecule. The resulting Π⁴Δ²-bonding pattern shows an unprecedented superatomic counterpart of a d-shell based bond inherently related to transition-metal dimers, adding useful key aspects to the understanding of species based on cluster-assembly.