Vertex‐Shared Linear Superatomic Molecules: Stepping Stones to Novel Materials Composed of Noble Metal Clusters

Extremely small metal clusters composed of noble metal atoms (M) have orbitals similar to those of atoms and therefore can be thought of as artificial atoms or superatoms. If these superatoms can be assembled into molecular analogs, it might be possible to create materials with new characteristics and properties that are different from those of existing substances. Therefore, the concept of superatomic molecules has attracted significant attention. The present review focuses on vertex‐shared linear M12n+1 superatomic molecules formed via the sharing of a single metal atom between M13 superatoms having icosahedral cores and summarizes the knowledge obtained to date in this regard. This summary discusses the most suitable ligand combinations for the synthesis of M12n+1 superatomic molecules along with the valence electron numbers, stability, optical absorption characteristics, and luminescence properties of the M12n+1 superatomic molecules fabricated to date. This information is expected to assist in the production of many M12n+1 superatomic molecules with novel structures and physicochemical properties in the future.


Introduction
Metal clusters, which can be represented as M n where M is gold (Au), silver (Ag), copper (Cu), or other metals, and n is the number of metal atoms in the cluster, have unique electron orbitals. In these M n clusters, the valence electrons can be regarded as being confined in spherically symmetrical regions within a positive charge potential field (representing the so-called jellium model [1] ). The orbitals (S, P, D, F, etc.) in this model are analogs to those of the corresponding atomic orbitals (s, p, d, f, etc.). [2] For these reasons, M n clusters are sometimes referred to as superatoms ( Figure  1). [3] Examples include [Au 13 (PMe 2 Ph)] 10 Cl 2 ] 3þ (Pme 2 Ph = dimethylphenylphosphine, Cl = chloride), [4] [Au 13 (dppe) 5 Cl 2 ] 3þ (dppe = bis(diphenylphosphino)ethane), [5] [Au 9 M 4 (PmePh 2 ) 8 Cl 4 ] þ (M = Au, Ag, Cu; PmePh 2 = methyldiphenylphosphine), [6] [Au 12 Pd(dppe)(PPh 3 ) 6 Cl 4 ] 0 (Pd = palladium, PPh 3 = triphenylphosphine), [7] and [Ag 12 Pt(dppm) 5 (DMBT) 2 ] 2þ (Pt = platinum, dppm = bis(diphenylphosphino)methane; DMBT = 2,4-dimethylbenzenethiolate), [8] in which an icosahedral M 13 core is surrounded by ligands. The formation of molecular analogs from such superatoms could potentially create materials with new properties and functions different from those of existing substances ( Figure 1). [3a] Several methods describing the connection of superatoms have been reported. Among these techniques, the connection of superatoms via shared metal atoms is of interest in that this process promotes the mixing of orbitals between superatoms. As such, electronic structures that are different from those of each individual superatom can be obtained. [3b,9] In the case of M 13 clusters, the superatoms can be connected by sharing one (via a vertex; Figure 2a), [10] two (via an edge; Figure 2b), or three (via a face; Figure 2c) [11] atoms. These processes create superatomic molecules consisting of 2 Â 13 À 1 = 25 atoms, 2 Â 13 À 2 = 24 atoms, or 2 Â 13 À 3 = 23 atoms, respectively ( Figure 2). This review summarizes the state-of-the-art concerning research into vertex-sharing linear M 12nþ1 superatomic molecules (n ≥ 2) formed by the sharing of a single metal atom, which has been the most studied of these superatomic molecules. This article presents the latest knowledge regarding such molecules and describes the novel structures and physicochemical properties that can be obtained.

M 12nþ1 Superatomic Molecules (n ≥ 2)
The basic geometry of an M 25 superatomic molecule (n = 2) is shown in Figure 3. The M 25 core is formed by two icosahedral M 13 superatomic molecules sharing one metal atom and has a fivefold axis of symmetry in the long axis direction of the molecule. In this structure, more than five L 1 ligands ( Figure 3a) are DOI: 10.1002/smsc.202300024 Extremely small metal clusters composed of noble metal atoms (M) have orbitals similar to those of atoms and therefore can be thought of as artificial atoms or superatoms. If these superatoms can be assembled into molecular analogs, it might be possible to create materials with new characteristics and properties that are different from those of existing substances. Therefore, the concept of superatomic molecules has attracted significant attention. The present review focuses on vertex-shared linear M 12nþ1 superatomic molecules formed via the sharing of a single metal atom between M 13 superatoms having icosahedral cores and summarizes the knowledge obtained to date in this regard. This summary discusses the most suitable ligand combinations for the synthesis of M 12nþ1 superatomic molecules along with the valence electron numbers, stability, optical absorption characteristics, and luminescence properties of the M 12nþ1 superatomic molecules fabricated to date. This information is expected to assist in the production of many M 12nþ1 superatomic molecules with novel structures and physicochemical properties in the future.
coordinated to the metal atom at the M L1 site, two L 2 ligands ( Figure 3a) are coordinated to the metal atom at the M L3 site, and approximately 10 L 3 ligands ( Figure 3a) are coordinated to the metal atom at the M L3 site (Figure 3b). The ligands of superatomic molecules having M 12nþ1 cores (n ≥ 2) for which geometric structures have been determined are categorized in the Venn diagram, [12] as presented in Figure 4. In many cases, anionic ligands such as halogens (X) or chalcogenides (ER) are used as the L 1 and L 2 ligands (areas A, D, G, and F in Figure 4). The L 3 ligands are typically neutral with an unshared electron pair, such as a phosphine (PR 3 ) or N-heterocyclic carbene (NHC) (area C in Figure 4). The coordination styles of the ligands are basically the same in linear M 37 (n = 3) and M 49 (n = 4) superatomic molecules as M 25 superatomic molecules. The Venn diagram presented here demonstrates that there are currently no ligands that coordinate only to the L 2 site (area B in Figure 4) or to both L 2 and L 3 sites (area E in Figure 4). Because there is a wide range of other ligands that could potentially be used to produce ligand-protected metal clusters, [13] including acetylide (C ≡ CR), [14] arsine (AsR 3 ), [15] and tellurolate (TeR) moieties, [16] it is expected that ligands satisfying these conditions will be discovered in the future.
The number of valence electrons (n*) in these superatomic molecules can be estimated using a method based on the jellium model proposed by Mingos et al. [17] or the superatom model more recently developed by Häkkinen et al. [18] and Cheng and Yang et al. [19] Table 1 summarizes the chemical compositions, geometric structure parameters, and numbers of valence electrons for representative superatomic molecules. [10] Here, the torsion angle (φ, the average of five dihedral angles) and the bond angle (θ) between the two M 13 cores (see Figure 3c) are also provided in addition to the symmetry of the structure and the ligands. The following text discusses the knowledge obtained to date while classifying superatomic molecules according to the type of ligand incorporated.

M 25 Protected by PR 3 and X Ligands
Since the 1980s, Teo et al. have successfully synthesized a number of novel M 25 superatomic molecules and determined the corresponding geometric structures using single-crystal X-ray diffraction (SC-XRD). In many cases, these structures incorporated PR 3 and X ligands, and the superatomic molecules were obtained by reducing metal salts (that is, complexes consisting of metal ions, PR 3 and/or X) or smaller metal clusters. Figure 5a-e,g,i provides the geometric structures of representative M 25 superatomic molecules fabricated in this prior work (entries 1-5, 7, and 9). Interestingly, the number of L 1 ligands and the bridging structures in these molecules were found to  vary with changes in the type of ligand and the counter ion. These phenomena were attributed to the steric effects of the various ligands as well as to electronic factors. [9] The M 25 core was also found to undergo torsion depending on the type and number of L 1 and L 2 ligands, producing various symmetries in the core, such as D 5h or D 5d symmetries or their subgroups (Table 1). Additionally, the charge state of the metal cluster changed from þ2 to 0 as the number of L 1 ligands was increased. In the case of an M 25 superatomic molecule, a closed-shell electronic structure (2 Â (1S 2 1P 6 )) was obtained when the total number of valence electrons was 16. [17] Therefore, varying the quantity of L 1 ligands (anionic ligands) produced different charge states in the metal clusters. For entry 6 ( Figure 5f ), a selective synthesis method was established in a later study by Jin et al. [10f] Note that this structure is isomeric with entry 5. Jin demonstrated a reversible structural transition between these isomers based on temperature, suggesting that M 25 superatomic molecules could have applications as molecular motors. [ 10 Cl 10 ] 0 (entry 10) (Figure 5j), which had a different geometric structure compared with those in entries 1-9. [10j] In this structure, eight Ag atoms in the Ag 25 cluster were replaced with Au atoms such that the M 25 core did not have fivefold symmetry. In addition, two Cl atoms were terminally coordinated to the metal atom at the M L1 site without bridging, in contrast to the metal clusters in entries 1-9. The bond angle (θ) between the M 13 units in this structure was 173°, meaning that the long axis exhibits a greater degree of bending than those in the other superatomic molecules. [10j] Bakr et al. produced the Ag-based superatomic molecule [Ag 23 Pt 2 (PPh 3 ) 10 Cl 7 ] 0 (entry 14) [10m] in which the two central Ag atoms of each Ag 13 cluster were replaced with Pt atoms. The author's group also recently synthesized three novel Ag-based superatomic molecules having the formula [Ag 23 M 2 (PPh 3 ) 10 X 7 ] 0 (M/X = Pd/Br (entry 11); Pd/Cl (entry 12); Pt/Br (entry 13)) ( Figure 6). [10k,l] In the case of entry 13 ([Ag 23 Pt 2 (PPh 3 ) 10 Br 7 ] 0 ), as in entry 14 ([Ag 23 Pt 2 (PPh 3 ) 10 Cl 2 ] 0 ), the two central Ag atoms of the Ag 13 cluster were replaced with Pt atoms although the L 1 and L 2 ligands were Br rather than Cl (Figure 6c, d). Entry 14 had an achiral geometric structure because its M 25 core exhibited a mirror plane. In contrast, in entry 13 the dihedral angle φ between the two Ag 12 Pt cores was twisted by approximately 10°such that there was no mirror plane in the M 25 core or in the entire molecule, leading to chirality. Although the AgÀBr bond is longer than the AgÀCl bond, the distance between the two Ag 12 Pt units in this structure was kept by this torsion. Similar torsion was also observed in entry 11 ([Ag 23 Pd 2 (PPh 3 ) 10 Br 7 ] 0 ). Studies have shown that the stability of these Ag 23 M 2 superatomic molecules having Ag as the base element together with PR 3 and X as ligands (entries 11-14) can be greatly improved by using Cl atoms (which have a high affinity for Ag) as the L 1 ligands and by Figure 4. Classification of ligands used for the formation of superatomic molecules based on a Venn diagram. L 1 , L 2 , and L 3 represent characteristic ligand sites (see also Figure 3). a) Ethanethiolate, b) 2-phenylethanethiolate, c) benzenethiolate, d) benzeneselenolate, e) triphenylphosphine, f ) tri(p-tolyl)phosphine, g) methyldiphenylphosphine, h) bis(diphenylphosphino)methane, i) 1,1 0 -bis(diphenylphosphino)ferrocene, j) 1,3-di-i-propylbenzimidazol-2-ylidene, k) 1,3-di (2,4,6-trimethylbenzyl)benzimidazol-2-ylidene, l) bromide, m) chloride, n) 1-adamantanethiolate, o) O,O-dipropyldithiophosphate, p) 4-tert-buthylbenzenethiolate, and q) dipyridylaminide ligands. Table 1. Chemical compositions, core metals, molecular symmetries, core symmetries, torsion, and bond angles between the superatoms, L 1 , L 2 , and L 3 ligands, charge states, and numbers of valence electrons of representative vertex-shared linear superatomic molecules.
a) SbF 6 = hexafluoroantimonate ion, PF 6 = hexafluorophosphate ion, Br = bromide ion, NO 3 = nitrate ion, B(C 6 F 5 ) 4 = tetrakis(pentafluorophenyl)borate ion, X = halogen, Ni = nickel, Cd = cadmium. b) The molecular symmetry of the framework structure. c) Because the dihedral angle φ is close to 0°or 36°(=72°/2), some of these molecules exhibit imperfect symmetry: h, d, and v indicate a horizontal, dihedral, or vertical mirror plane, respectively. The symmetry was not determined in the case that the synthesis gave a mixture of products. d) The torsion angle (the average dihedral angle) between the two M 13 groups (see Figure 3). e) The bond angle between the two M 13 groups (see Figure 3). f ) The charge. g) The number of valence electrons. h) A summed point group indicates that the product was a mixture of molecules for which the symmetries were C 5v and C s .
replacing Ag atoms with other metals to strengthen the metallic bonds and thereby form a more rigid metallic core. [10k,l] Zhu et al. reported the synthesis of [Au 23 Pd 2 (PPh 3 ) 10 Br 7 ] 0 (entry 15) with Au as the base element instead of Ag ( Figure 7a). Both Cl-substituted ([Au 23 Pd 2 (PPh 3 ) 10 Cl 7 ] 0 ) and Pt-substituted ([Au 23 Pt 2 (PPh 3 ) 10 Br 7 ] 0 ) analogs were also obtained ( Figure 7b), although the geometric structures of these compounds were not been determined by SC-XRD. [10n] To the best of our knowledge, there have been no reports of [Au 25 (PR 3 ) 10 X 7 ] þ molecules with cores composed solely of Au. It is thought that an X-bridged Au 25 superatomic molecule would not be sufficiently stable to be isolated unless the metal core was strengthened by doping with different metals.
In addition to the M 25 superatomic molecules containing two metal elements described above, Teo et al. and Steggerda et al. have also reported the synthesis of several structures incorporating three metals (entries 16-19; Figure 8). [10o-r] These studies established the existence of preferential sites for each metal element. [9] 2.2. M 25 Protected by PR 3 , ER, and X Ligands Au and Ag can form strong bonds with thiolate (SR) ligands. [20] Therefore, in the case that such ligands are included in superatomic molecules, the structure tends to be relatively stable. Tsukuda et al. produced [Au 25 (PPh 3 ) 10 (SR) 5 Cl 2 ] 2þ (R = C n H 2nþ1 ; n = 2 for entry 20) containing only Au in the metal core using SR ligands in addition to PR 3 and X (Figure 9a). [10s] In this  These [Au 25 (PPh 3 ) 10 (SR) 5 Cl 2 ] 2þ molecules were also studied extensively with respect to their optical properties and electronic structures. [22] As a result, [Au 25 (PPh 3 ) 10 (SR) 5 Cl 2 ] 2þ was found to absorb light over a wide range from the near-infrared region (approximately 900 nm) to the ultraviolet (UV) region [10s] and also to exhibit photoluminescence (PL) at approximately 990 nm [23] (Figure 9b). Analyses of the PL characteristics of [Au 25 (PPh 3 ) 10 (PET) 5 Cl 2 ] 2þ (Figure 9c) found a quantum yield (Φ PL ) and lifetime (τ PL ) of approximately 8% and 3.2 μs, respectively, indicating that this molecule could be better suited to certain applications compared with other secondary near-infrared chromophores. [23] The spin multiplicity of the excited state of [Au 25 (PPh 3 ) 10 (PET) 5 Cl 2 ] 2þ was recently reported to be 3 by Mitsui et al. [24] The same group also showed that the intersystem crossing yield (Φ ISC ) in [Au 25 (PPh 3 ) 10 (PET) 5 Cl 2 ] 2þ is approximately 1 using the triplet-triplet annihilation-based photon upconversion phenomenon in conjunction with fluorescent dyes. These results suggest that [Au 25 (PPh 3 ) 10 (SR) 5 Cl 2 ] 2þ could be utilized as a phosphorescent material with a dark-excited singlet state and a bright-excited triplet state at room temperature. [24] The [Au 25 (PPh 3 ) 10 (SR) 5 Cl 2 ] 2þ species have also been reported to exhibit various catalytic activities, including electrochemical CO 2 reduction catalysis, selective aerobic oxidation photocatalysis of amines to imines, and aerobic oxidation of glucose to gluconic acid. [10u,25] The syntheses of [Au 24 (PPh 3 ) 10 (SR) 5 X 2 ] þ (X = Br or Cl) molecules lacking an Au atom at the M C1 site were reported by Jin et al. (Figure 10a). [26] Both [Au 25 (PPh 3 ) 10 (PET) 5 Cl 2 ] 2þ and [Au 24 (PPh 3 ) 10 (PET) 5 X 2 ] þ had similar geometric structures (Figure 9a and 10a) together with very similar light absorption and PL characteristics (Figure 9b and 10b). [23,27] However, the Φ PL of [Au 24 (PPh 3 ) 10 (PET) 5 X 2 ] þ was only on the order   [23] Copyright 2021, Wiley-VCH. c) Excited state deactivation pathway of [Au 25 (PPh 3 ) 10 (SR) 5 Cl 2 ] 2þ . Reproduced with permission. [24] Copyright 2022, Royal Society of Chemistry.  (Figure 10c). [23] The alloying strategy has also been implemented for [Au 25 (PPh 3 ) 10 (SR) 5 Cl 2 ] 2þ . Jin et al. and Zhu et al. synthesized [Ag x Au 25Àx (PPh 3 ) 10 (PET) 5 Cl 2 ] 2þ (x ≤ 13, entry 23) and [Au 25Àx Cu x (PPh 3 ) 10 (PET) 5 Cl 2 ] 2þ (entry 24), in which multiple Au atoms were replaced with Ag or Cu atoms, and investigated the Ag and Cu substitution sites ( Figure 11A). [10v,w] The results showed that in the case of [Ag x Au 25Àx (PPh 3 ) 10 (PET) 5 Cl 2 ] 2þ , Ag atoms were substituted at the M L1 or M L2 sites up to the 12 th Ag, while the 13 th Ag was substituted at an M C1 site. [10v] In [Au 25Àx Cu x (PPh 3 ) 10 (PET) 5 Cl 2 ] 2þ , Cu was substituted at the M L1 and M L2 sites. [10w] The absorption spectra of [Ag x Au 25Àx (PPh 3 ) 10 (PET) 5 Cl 2 ] 2þ gradually changed when the number of Ag atoms increased ( Figure 11B). In the same work, they found that the substitution of up to 12 Ag atoms did not significantly change the PL properties of [Ag x Au 25Àx (PPh 3 ) 10 (PET) 5 Cl 2 ] 2þ , while the Φ PL of the molecule was increased to approximately 40% after the 13th Ag was substituted at the M C1 site ( Figure 11C). Using density functional theory (DFT) calculations, Muniz-Miranda et al. reported that the oscillator strength of the S 0 ! S 1 transition was enhanced when the 12th or 13th Au atom in [Au 25 (PPh 3 ) 10 (PET) 5 Cl 2 ] 2þ was replaced. [28] Mitsui et al. reported that these emission characteristics could be attributed to phosphorescence and that the 13th Ag substitution shifted the excited triplet state to a higher energy, thereby suppressing the T 1 ! S 0 intersystem crossing, such that entry 23 exhibited a higher Φ PL value than [Ag x Au 25Àx (PPh 3 ) 10 (PET) 5 Cl 2 ] 2þ (x ≤ 12). [29] During the synthesis of the [Au 25Àx M x (PPh 3 ) 10 (PET) 5 Cl 2 ] 2þ (M = Ag, Cu) molecules noted above, the number of Ag or Cu atoms that were inserted could not be controlled with atomic precision. However, in later work, Jin et al. synthesized Consequently, the Cu-substituted form was obtained as a mixture of two isomers (Figure 12b). The same group also demonstrated the precise synthesis of [Au 23 Ag 2 (PPh 3 ) 10 (PET) 5 Cl 2 ] 2þ , whose Ag atoms are doped into the M L2 sites by controlling the reaction mechanism. In the case of the former, unlike the latter, the absorption peak at longer wavelengths was clearly split into two peaks (Figure 12d). This splitting was shown by DFT calculations performed by Jiang et al. to be attributed to the splitting of the orbital near the highest occupied molecular orbital (HOMO) due to the monoatomic Pd Figure 10. a) Framework structure of [Au 24 (PPh 3 ) 10 (PET) 5 X 2 ] þ (X = Br or Cl) [26] and b) UV-vis absorption/PL spectra of [Au 24 (PPh 3 ) 10 (PET) 5 Cl 2 ] þ . b) Reproduced with permission. [23] Copyright 2021, Wiley-VCH. c) The effect of a central Au atom on the excitation relaxation rate. [23] Figure 4d); entries 28 and 29) molecules, each of which had a selenolate (SeR) ligand at the L 1 site (Figure 13a). [10z] Both these materials were synthesized using PPh 3 -protected Au clusters (Au n (PPh 3 ) m ) as a precursor while controlling the charge state of the product by optimizing the reaction temperature, the amount of reducing agent, the solvent, and the amount of benzeneselenol (PhSeH). The geometrical structures of these molecules were similar to one another, although the arrangements of the Ph groups attached to the Se atoms differed ( Figure 13a). In addition, the structure in entry 28 had a closed-shell electronic structure while that in entry 29 contained one extra electron. Therefore, the electron paramagnetic resonance spectrum of the latter had peaks related to S = 1/2 at g values of 2.40, 2.26, and 1.78 (Figure 13b 5 Cl 2 ] 2þ/þ molecules. The results demonstrated that the relaxation lifetimes of less than 1 ps and %1 μs were associated with the internal conversion from higher to lower excited state and lower excited state to ground state, respectively. However, an intermediate time constant of approximately %100 ps was observed only in the case of the open-shell system and originated from the presence of singly occupied molecular orbital (SMO) (Figure 13d). [30] DFT calculations have also suggested that ligand exchange reactions may occur between these M 25 superatomic molecules in solution. [31] 2.3. M 25 Protected by PR 3 , C ≡ CR, and X Ligands The synthesis of M 25 superatomic molecules containing acetylide ligands, C ≡ CR, has thus far been limited to [Au 25 (PPh 3 ) 10 (PA) 5 X 2 ] 2þ (PA = phenylacetylide), which was   reported by Jin et al. in 2014. [14r] The formation of this molecule was confirmed by electrospray ionization-mass spectrometry and optical absorption spectroscopy (Figure 14). A geometrical structure in which the SR ligands at the L 1 sites in [Au 25 (PPh 3 ) 10 (SR) 5 X 2 ] 2þ were replaced by PA ligands was predicted. [14r] Thus, the C ≡ CR ligand evidently appears in area A in the ligand classification system, as shown in Figure 4. The same group also found that [Au 25 (PPh 3 ) 10 (PA) 5  . Because the AuÀC bond is stronger than the AuÀP bond, [32] those molecules containing the former bonds could be more robust. [10aa] The [Au 25 ( i Pr 2bimy) 10 Br 7 ] 2þ molecule has also been shown to function as a highly active catalyst for the cyclic isomerization of alkynyl amines to indoles. [10aa] The [Au 25 ( MesCH2 bimy) 10 Br 7 ] 2þ molecule (entry 31), which contains a bulky 1,3-di (2,4,6-trimethylbenzyl)benzimidazol-2ylidene ( MesCH2 bimy) group (Figure 4k) as the NHC ligand, was also synthesized by Crudden et al. (Figure 15b,c). [10ab] In the case of entry 31, the ticosahedral Au 13 are twisted by 10°to give D 5 symmetry. The same group synthesized [Au 25 ( MesCH2 bimy) 10 Br 8 ] þ (entry 32), in which not five but six Br are coordinated to the metal at the M L1 site. [10ab] In this case, the L 1 ligands include four μ-Br and two terminal Br and this is the second-ever example of a superatomic molecule having a terminal halide coordinated to the M L1 site, following entry 10 ([Ag 17 Au 8 (PPh 3 ) 10 Cl 10 ] 0 ). The number of valence electrons in entries 31 and 32 was estimated to be 16, indicating that a closed shell structure was formed in conjunction with the NHC ligands. [10ab] The entry 31 exhibited PL with a higher Φ PL value of 15% compared with other Au-based superatomic molecules. In this molecule, the rigidity of the ligand layer was increased as a result of CH-π or π-π interactions between the surface ligands, resulting in a higher Φ PL . [10ab]
[10ac] These two metal clusters had a more complex geometry than those described in Section 2.1À2.4 although the number of valence electrons in these structures was estimated to be 16, similar to other Au 25 superatomic molecules. Liu et al. also reported the synthesis of a metal cluster with the formula [Ag 33 Pt 2 (dpt) 17 ] 0 (dpt = dipropyl dithiophosphate (Figure 4o; entry 35)) ( Figure 16c). A SC-XRD analysis established that an AgÀdpt network surrounded the Ag 23 Pt 2 core in this structure. [10ad] These M 25 superatomic molecules incorporating metal complexes may exhibit physicochemical properties different from  www.advancedsciencenews.com www.small-science-journal.com those of M 25 superatomic molecules protected only by ligands, because the ligand layers in the former contain metal atoms. Therefore, it is expected that future studies of these superatomic molecules will focus on not only the stability but also catalytic activity of these materials.

Longer Linear M 12nþ1 Superatomic Molecules (n ≥ 3)
Organic dyes such as cyanines and acenes comprise 1D conjugated systems and the absorption of these materials at longer wavelengths tends to be red-shifted as the conjugation is extended. [33] Similarly, in the case of vertex-sharing 1D M 12nþ1 superatomic molecules, a shift to longer wavelengths occurs in conjunction with increments in the number of the connection. Jin et al. synthesized [Au 37 (PPh 3 ) 10 (PET) 10 X 2 ] þ (X = Br or Cl; entry 37) ( Figure 17A), in which three Au 13 units were linearly connected through the sharing of a vertex Au atom.
[10ae] These molecules were found to have geometries similar to that of [Au 37 (PH 3 ) 10 (SCH 3 ) 10 Cl 2 ] þ (PH 3 = phosphine, SCH 3 = methanethiolate) [34] which had been theoretically predicted by Nobusada et al. 7 years prior. The [Au 37 (PR 3 ) 10 (SR) 10 X 2 ] þ structure had 24 valence electrons and the [Au 37 ] 13þ core was isolobal to Ne 3 . [35] As shown in Figure 17B, the peaks at longer wavelengths of [Au 13 (dppe) 5 Cl 2 ] 3þ , [Au 25 (PPh 3 ) 10 (PET) 5 Cl 2 ] 2þ , and [Au 37 (PPh 3 ) 10 (PET) 10 X 2 ] þ were red-shifted as the number of Au 13 icosahedra were increased. Consequently, absorption was obtained up to approximately 1,200 nm in the spectrum acquired from the [Au 37 (PPh 3 ) 10 (PET) 10 X 2 ] þ . [10ae] These linearly connected superatoms have also shown red-shifted PL emission wavelengths ( Figure 17C). [23,27] In the case of [Au 37 (PPh 3 ) 10 (PET) 10 X 2 ] þ , the linear M 37 core is covered only by simple ligands. However, there have recently been several reports of superatomic molecules incorporating metal complexes that surround a 1D M 12nþ1 (n ≥ 3) core. As an example, Liu et al. produced [Ag 44 Pt 3 (dpt) 22 ] 0 (entry 36), in which an Ag 10 (dpt) 22 shell surrounds a linear Ag 34 Pt 3 core formed by the vertex sharing of three Ag 12 Pt groups ( Figure 18A). [10ad] This metal cluster is considered part of a series of superatomic families comprising [Ag 20 Pt(dpt) 12 ] 0 and [Ag 33 Pt 2 (dpt) 17 ] 0 (entry 35) ( Figure 18A). The number of valence [10t] B) UV-vis-NIR absorption and C) PL spectra of these compounds. Note that each PL spectra was recorded on a different spectrometer and so these spectra cannot be directly compared. Data in (B) and (C) are replotted from refs. [10ae] (red lines), [23] (green lines), and [27] (blue lines).  22 ] 0 indicates that absorption at long wavelengths was redshifted with increasing the number of the connection ( Figure 18B), as shown in Figure 18A. [10ad] Wang et al. synthesized [Ag 61 (dpa) 27 ] 4þ (dpa = dipyridylaminide (Figure 4q; entry 38), which had a linear Ag 49 core formed by four Ag 13 ( Figure 19A). [10af] This superatomic molecule can be considered as a connected structure made from the [Ag 21 (dpa) 12 ] þ superatoms previously reported by the same group. [36] The dpa ligand has three N atoms and so these ligands had various coordination forms when incorporated into the [Ag 21 (dpa) 12 ] þ and [Ag 61 (dpa) 27 ] 4þ molecules ( Figure 19B). The number of valence electrons in [Ag 61 (dpa) 27 ] 4þ was estimated to be 30 and the electronic structure of the [Ag 49 ] 19þ core of this structure was equivalent to that of [I 4 ] 2À . [10af] The Ag 49 core of [Ag 61 (dpa) 27 ] 4þ was determined to have a length of 2.11 nm along its long axis, equivalent to the distance of approximately 2 nm over which localized surface plasmon resonance occurs. [37] However, [Ag 61 (dpa) 27 ] 4þ has a molecular-like electronic structure and the absorption peak at 1,170 nm in the optical absorption spectrum of this material ( Figure 19C) is not attributed to plasmons but rather to an electronic transition from the HOMO to LUMO, whose electron distributions are localized in the Ag 49 core. [10af] The peak at 819 nm (with a molar absorption coefficient of ε = 6.2 Â 10 4 M À1 cm À1 ) was attributed to the ligand to metal charge transfer (LMCT) from a motif to the core, similar to the peak at 512 nm (ε = 2.0 Â 10 4 M À1 cm À1 ) in the spectrum of [Ag 21 (dpa) 12 ] þ . [10af] Figure 19C shows that the oscillator strength of these LMCT peaks was enhanced by a factor of approximately 3 in the case of multiple connections. [10af] As noted, the optical absorbance at longer wavelengths exhibited by linear superatomic molecules is primarily the result of transitions from the orbitals close to the HOMO to the orbitals close to LUMO, which are symmetrical to the fivefold symmetric long axis of the molecule. As an example, [Au 25 (PPh 3 ) 10 (SR) 5 Cl 2 ] 2þ (entries [20][21][22] and [Au 37 (PPh 3 ) 10 (PET) 5 X 2 ] þ (entry 37), both of which have D 5h symmetry, absorb at longer wavelengths because of an orbital transition from the occupied a 2 "(Σ) to the virtual a 1´( Σ*). As the number of connections increases, the energy gap between these orbitals decreases, resulting in redshifts of the absorption peaks of these species. [10aa,24,34] In the case of [Ag 61 (dpa) 27 ] 4þ (entry 38), the absorption peak at 1,200 nm originated from the transition between orbitals having nodes vertical to the long axis (six nodes ! seven nodes), corresponding to an   12 ] þ superatom [36]  Even organic dyes with linearly connected conjugated systems show narrowing of the energy gaps between orbitals as the number of connections increases. However, it is difficult to obtain infrared fluorescence from these compounds with high quantum yields because of the energy gap law [23,38] and the photobleaching effect associated with the instability of the photoexcited states. [39] In contrast, superatoms and superatomic molecules composed of metal atoms are highly stable in response to light exposure and so no significant destabilization of the excited state occurs even as the number of connections is increased. In the future, it is expected that a deeper understanding of the excited states of linear superatomic molecules will be obtained, [13bp,14n,40] and thereby the research on their application as infrared fluorescent materials will become more active than at present.

Conclusion and Perspectives
This review summarized the geometries and optical properties of linear M 12nþ1 superatomic molecules (n ≥ 2) formed by the vertex sharing of M 13 superatoms. The following key points are emphasized:

Length of the M 12nþ1 Core
Linear M 12nþ1 superatomic molecules with n = 2-4 have been reported to date.

Appropriate Ligand Combinations
Specific ligand combinations have been found to produce linear M 12nþ1 superatomic molecules. The ligands employed thus far comprise PR 3 and X, PR 3 , ER and X, PR 3 , C ≡ CR and X, NHC and X, SR, metal complexes and X, SR and metal complexes, and metal complexes alone.

Stability
The stability of superatomic molecules can be enhanced by selecting the second to sixth ligand combinations noted in point (b) above, rather than the first combination. Stability can also be improved by strengthening the M 12nþ1 core through heteroatom substitution.

Optical Absorption
Light absorption at long wavelengths by these materials can be attributed primarily to transitions from orbitals close to the HOMO to orbitals close to the LUMO, both of which are symmetric to the long axis of the molecule, which has fivefold symmetry. As the number of connections increases, the energy gap between these orbitals decreases, resulting in a red shift in optical absorption. In addition, increasing the number of connections enhances the oscillator strength associated with the LMCT.

PL Properties
M 12nþ1 superatomic molecules (n ≥ 2) exhibit PL and the PL emission wavelengths of these molecules have been shown to undergo a redshift with increases in the number of the connection. In the case of [Au 25 (PPh 3 ) 10 (SR) 5 Cl 2 ] 2þ , higher Φ PL and longer τ PL values have been observed compared with those for other secondary near-infrared chromophores, suggesting the potential for practical applications. Increasing the rigidity of the ligand layer via interactions between surface ligands can further enhance Φ PL .

Catalytic Activities
The catalytic activity of M 12nþ1 superatomic molecules (n ≥ 2) depends on the protective ligand. The reported catalytic activities include oxidation of organic molecules, catalysis of terminal alkynes to alkenes, the cyclic isomerization of alkynyl amines to indoles, and the electrochemical reduction of inorganic compound.
As described in the Introduction, it is expected that the assembly of superatomic molecules from superatoms will lead to the creation of new materials with unique physical properties and functions that are different from those of existing substances. As an example, the chirality resulting from the structures of certain superatomic molecules cannot be obtained in molecules having isoelectronic structures. The creation of stable and highly efficient near-infrared luminescent materials by increasing the number of the connection of superatomic molecules is another unique phenomenon. It is expected that many M 12nþ1 superatomic molecules having novel structures and physicochemical properties will be created in the future by making good use of the above information. www.advancedsciencenews.com www.small-science-journal.com Although this review focused solely on linear connections of superatomic molecules, ring formations are also possible. [10w,41] In superatomic molecules, as is the case for more typical molecules, both the electronic structure and physicochemical properties are changed as a consequence of ring formation. [41c] Although there have been only a few reports to date concerning cyclic superatomic molecules, it is expected that many studies will be conducted in the future.
Yoshiki Niihori is a junior associate professor in Prof. Negishi group at Tokyo University of Science (TUS). He received a Ph.D. degree in chemistry (2014) from TUS under the supervision of Prof. Yuichi Negishi. Before his current position, he was employed as an assistant professor in Prof. Mitsui's group at Rikkyo University. His research interests include the development of precise synthesis methods and characterization of photoexcited state for noble metal nanoclusters.
Sayuri Miyajima is a master's course student in the Negishi group at TUS. She received her B.Sc. (2021) in chemistry from TUS. Her research interests include the creation of novel superatomic molecules.
Ayaka Ikeda is a master's course student in the Negishi group at TUS. She received her B.Sc. (2022) in chemistry from TUS. Her research interests include the creation of novel superatomic molecules.
Taiga Kosaka is an undergraduate's course student in the Negishi group at TUS. His research interests include the establishment of the methods for connecting metal clusters.
Yuichi Negishi is a professor at the Department of Applied Chemistry at TUS. He received his Ph.D. degree in chemistry (2001) from Keio University under the supervision of Prof. Atsushi Nakajima. Before joining TUS in 2008, he was employed as an assistant professor at Keio University and the Institute for Molecular Science. His current research interests include the precise synthesis of stable and functionalized metal nanoclusters, metal nanocluster-connected structures, and covalent organic frameworks.