Superatomic Aromaticity in Cyclic Superatomic Molecules: Ligand-Protected Penta-Icosahedral [M@Au11]5 (M = Au, Pt) Clusters

Pure or doped gold icosahedra, which can be generally viewed as superatoms, are promising candidates for cluster-assembled structures. As the first large-scale ring-like gold cluster, the report of [Au60Se2(Ph3P)10(SeR)15]+ has arisen much interest, where its Au60 core is composed of five vertex-sharing gold icosahedra in a cyclic way. From electronic characters, this Au60 core is a 40e cyclic penta-superatom network formed by five 8e closed-shell superatoms (S2P6). When more valence electrons are introduced into the penta-superatom network by atomic doping, global delocalized bonds are induced in its bonding framework. In the 42e Au60 core of the [Au60Se2Cl15]- cluster, two extra electrons occupy one delocalized π-bonding orbital formed by super D orbitals of five superatoms, resulting in superatomic π aromaticity. In the 46e [Pt@Au11]5 core of [(Pt@Au11)5Ga2Cl15] cluster, three delocalized super-π bonds are formed, which are organized in the similar way as the aromatic C5H5- molecule. The unveiling of superatomic aromaticity promotes our understanding of the stability of cyclic superatom assemblies and extends the family of superatomic bonding patterns.

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.

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.

The Covalent AuI-AuI Bond in (AuF)n (n = 2∼4): A Perspective to Understand the Closed-Shell AuI···AuI Interaction

The nature of closed-shell Au^I···Au^I attraction is still a conundrum in theoretical chemistry. However, for Au₂F₂ with a zigzag conformation, the d¹⁰-d¹⁰ closed-shell interaction between the AuF monomers is demonstrated as a coordinate covalent bond. Chemical bonding analysis reveals that the strong Au^I···Au^I attraction is caused by the participation of the extraordinary active 5d orbital of Au. Based on our study, one of the 5d orbitals of the Au atom is activated to hybridize with its 6s and 6p orbitals to form hybridized dsp² orbitals, where each Au atom is both an electron donor (Lewis base) and acceptor (Lewis Acid) in dimerization. Actually, the closed-shell Au^I···Au^I interaction in the zigzag conformation of Au₂X₂ (X = F, Cl, Br, I, or NH₂) is covalent. Our results provide a rather simple but clear-cut example, where mysterious Au^I···Au^I attractions can be possibly explained by the covalent bond theory.

Tetrahedral Pt10- Cluster with Unique Beta Aromaticity and Superatomic Feature in Mimicking Methane

Utilizing a customized metal cluster source in tandem with a flow tube reactor and a reflectron time-of-flight mass spectrometer, we have obtained well-resolved pure metal clusters Pt_n^- and observed their gas-phase reactions with a few small gas molecules. Interestingly, the remarkable inertness of Pt₁₀^- was repeatedly observed in different reactions. Meanwhile, we have determined the structure of Pt₁₀^- within a regular tetrahedron. Considering that Pt possesses 5d⁹6s¹ electron configuration, the tetrahedral Pt₁₀^- exhibits unexpected stability at neither a magic number of valence electrons nor a shell closure of geometric structure. Comprehensive theoretical calculations unveil the stability of Pt₁₀^- is significantly associated with the all-metal aromaticity. In addition to the classical total aromaticity, which is mainly due to 6s electrons, there is unique beta-aromaticity ascribed to spin-polarized beta 5d electrons pertaining to singly occupied multicenter bonds. Further, we demonstrate the superatomic feature of such a transition metal cluster Pt₁₀^-, as Pt₆@Pt₄^-, in mimicking methane.

Low-lying states of MX2 (M = Ag, Au; X = Cl, Br and I) with coupled-cluster approaches: effect of the basis set, high level correlation and spin–orbit coupling

Spin–orbit coupling, electron correlation level and basis set are important in describing Renner–Teller and pseudo-Jahn–Teller effects and properties of MX₂.

Prediction of the Au4S crystal via a superatom network model: from clusters to solids

Prediction of the Au₄S crystal on the basis of the structural character of the Au₂₂(μ₄-S)(SH)₁₂ cluster.

Electronic shells of a tubular Au26 cluster: a cage-cage superatomic molecule based on spherical aromaticity

Gold clusters, which display a variety of unusual geometric structures due to their strong relativistic effects, have attracted much attention. Among them, Au₂₆ has a high-symmetry tubular structure (D_6d) with a large HOMO-LUMO energy gap, but its electronic stability still remains unclear. In this paper, the electronic nature of the Au₂₆ cluster is investigated using the density functional theory method. Depending on the super valence bond model, the tubular Au₂₆ cluster with 26 valence electrons could be viewed as a superatomic molecule composed of two open cages based on spherical aromaticity, and its molecule-like electronic shell closure is achieved via a super triple bond (σ, 2π) between the two cages. Based on this new cage-cage superatomic structural model, a series of similar tubular clusters are predicted from the Au₂₆ skeleton. The two capped Au atoms are replaced by Cu, Ag and In atoms, respectively, to form tubular D_6d Au₂₄Cu₂ and Au₂₄Ag₂ (26e) and Au₂₄In₂ (30e) clusters, where the super triple bonds also exist. Moreover, tubular D_5d Au₂₀In₂ (26e) is obtained by replacing hexatomic Au₆ rings in the bulk of Au₂₄In₂ with pentagonal Au₅ rings. Chemical bonding analysis reveals that there is a super quintuple bond (σ, 2π, 2δ) between two open (Au₁₀In) cages, in accordance with the 26e Li₂₀Mg₃ superatomic molecule composed of two icosahedral superatoms. Our study proposes the new cage-cage structural model of superatomic molecules based on spherical aromaticity, which extends the range of the super valence bonding pattern and gives inferences for further study of superatomic clusters.

Evidence for the Superatom-Superatom Bonding from Bond Energies

Metal clusters with specific number of valence electrons are described as superatoms. Super valence bond (SVB) model points out that superatoms could form the superatomic molecules through SVBs by sharing nucleus and electrons. The existence of superatom-superatom bonding was verified by the shape of their orbitals in former studies. In this paper, another important evidence-bond energy is studied as the criterion for the SVBs using the density functional theory method. In order to get the reliable values of bond energies, the series of Zn-Cu and Mg-Li superatomic molecules composed of two tetrahedral superatoms which do not share their nucleus are designed. Considering the number of the valence electrons in one tetrahedral superatomic unit, (Zn₄)₂/(Mg₄)₂, (Zn₃Cu)₂/(Mg₃Li)₂, (Zn₂Cu₂)₂/(Mg₂Li₂)₂, and (ZnCu₃)₂/(MgLi₃)₂ clusters are 8e-8e, 7e-7e, 6e-6e, and 5e-5e binary superatomic molecules with super nonbond, single bond, double bond, and triple bond, respectively, which are verified by chemical bonding analysis depending on the SVB model. Further calculations reveal that the bond energies increase and the bond lengths decrease along with the bond orders in Zn-Cu and Mg-Li systems which is in accordance with the classical nonbond, single bond, double bond, and triple bond in C-H systems. Thus, these values of bond energies confirm the existence of the SVBs. Moreover, electron localization function analysis is also carried on to describe the similarity between the superatomic bonds and atomic bonds in simple molecules directly. This study reveals the new evidence for the existence of the superatom-superatom bonding depending on the bond energies, which gives the new insight for the further investigation of the superatomic clusters.

Theoretical investigation on the covalence in AgRnX and XAgRn (X = F - I)

CCSD(T) calculations were performed to investigate the stabilities and interaction mechanisms of the AgRnX and XAgRn (X = F - I) series. Dissociation energies and frontier orbital properties demonstrate an increased trend of stabilities. Ag spd hybrids and Rn/X sp hybrids come into the σ_Ag-Rn and σ_Ag-X bonding orbital. The nature of Ag-Rn, Ag-X and Rn-X interactions were investigated by atoms in molecules (AIM) theory. The negative energy density and positive Laplacian values, as well as small electron densities at bond critical points (BCPs), characterize the moderate strength with partial covalence of interactions. BCP properties (-G/V and G/ρ), electron density deformations and natural resonance theory (NRT) results display increased covalence down the periodic table.

Size evolution and ligand effects on the structures and stability of (AuL)n (L = Cl, SH, SCH3, PH2, P(CH3)2, n = 1–13) clusters

Size evolution on the global minimum structures of (AuCl)_n clusters at n = 1–13.

First principles study on the structural evolution and properties of (MCl)n (n = 1–12, M = Cu, Ag) clusters

Energetic gaps (E − E_fit) and second differences of binding energies (Δ₂E) for (CuCl)_n and (AgCl)_n clusters as a function of cluster size, n.