Recent advances in atomic cluster synthesis: a perspective from chemical elements

Takamasa Tsukamoto abc
aInstitute of Industrial Science, The University of Tokyo, 4-6-1 Komaba Meguro-Ku, Tokyo 153-8505, Japan. E-mail: ttsukamo@iis.u-tokyo.ac.jp
bDepartment of Applied Chemistry, School of Engineering, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo, 153-8505, Japan
cJST PRESTO, Honcho, Kawaguchi, Saitama 332-0012, Japan

Received 21st December 2023 , Accepted 5th April 2024

First published on 11th April 2024


Abstract

Despite its potential significance, “cluster chemistry” remains a somewhat marginalized topic within the chemistry field. However, atomic clusters with their unusual and unique structures and properties represent a novel material group situated between molecules and nanoparticles or solid matter, judging from both scientific standpoints and historical backgrounds. Surveying an entire material group, including all substances that can be regarded as a cluster, is essential for establishing cluster chemistry as a more prominent chemistry field. This review aims to provide a comprehensive understanding by categorizing, summarizing, and reviewing clusters, focusing on their constituent elements in the periodic table. However, because numerous disparate synthetic processes have been individually developed to date, their straightforward and uniform classification is a challenging task. As such, comprehensively reviewing this field from a chemical composition viewpoint presents significant obstacles. It should be therefore noted that despite adopting a synthetic method-based classification in this review, the discussions presented herein could entail inaccuracies. Nevertheless, this unorthodox viewpoint unfolds a new scientific perspective which accentuates the common ground between different development processes by emphasizing the lack of a definitive border between their synthetic methods and material groups, thus opening new avenues for cementing cluster chemistry as an attractive chemistry field.


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Takamasa Tsukamoto

Takamasa Tsukamoto received his PhD degree in Eng. from Tokyo Metropolitan University in 2015. After being a research fellow of the Japan Society for the Promotion of Science (2014–2016), a visiting researcher at the University of Miami (2014), a doctoral research fellow at the University of Tokyo (2015–2016), and a postdoctoral researcher and an assistant professor at the Tokyo Institute of Technology (2016–2023), he joined the Institute of Industrial Science at the University of Tokyo as a lecturer (2023) and the Japan Science and Technology Agency as a PRESTO researcher (2020–2023). His current research interests include developing organic–inorganic hybrid compounds for cluster synthesis, theoretical studies on clusters involving superatoms, the interpretation of the super-degeneracy phenomenon, and proposing a higher-order periodic table of superatoms.


1. Introduction

Atomic clusters comprising a few, a few dozen, or hundred atoms, with particle diameters of typically ∼1 nm and reaching up to ∼3 nm fall under an ultrasmall particulate material group (Fig. 1).
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Fig. 1 Classification of materials based on the size scale.

Compared to conventional nanoparticles (5–500 nm), their atypical smallness exhibits certain eccentric properties not found in nanoparticles, originating from the remarkable quantum size effect. Regarding general nanoparticles, physical properties and chemical reactivities are determined by their surface structures and geometrical shapes.1–3 On the other hand, their electronic states of clusters resemble more closely those of molecules than bulk materials (including nanoparticles), leading to their unique characteristics.4–8 Such clusters contribute to the manifestation of highly efficient and sophisticated biological functions in nature, such as water oxidation in photosynthesis, electron transfer, nitrogen fixation, and breathing.9–13 Moreover, clusters possess various structural characteristics that are not observed in other chemicals, including the number of constituent atoms, types of constituent elements, elemental ratios, atomic arrangements, and geometrical symmetry. Such a feature affords a great degree of freedom in terms of material design. Furthermore, their steric and electronic structures on a molecular scale offer significant potential for achieving novel properties and functions. Therefore, this sub-nanometer material group has attracted worldwide attention as a candidate for next-generation post-nanotechnology materials.

However, the desired progress in the cluster chemistry field has been hindered by the inherent challenges entailed in synthesizing clusters, as compared to nanoparticles. This issue originates from the critical factors differentiating the properties of nanoparticles and clusters. Particularly, in contrast to nanoparticles, the structures, properties, and stabilities of clusters depend significantly on the number of constituent atoms. Since precise atom manipulation techniques (within 1 nm of the space in cluster particles) are necessary for the synthesis of clusters, applying conventional synthetic methods lacking atomic-level precision is unsuitable for forming such sub-nanosized materials. Therefore, developing new clustering techniques that are independent of these methods is essential. Atomic-level synthesis is significantly influenced by the nature of the constituent elements; however, in many cases, the types of elements that can be treated using a single method tend to be limited. By considering this aspect, synthetic methods dedicated to certain elements or elemental groups have been established, with the regions of the corresponding elements distributed on the periodic table like islands (Fig. 2). Notable developments in cluster chemistry have been achieved using these methods as starting points.


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Fig. 2 A rough sketch of a map for the classification of suitable methods for atomic cluster synthesis depending on the chemical element composition.

As noted above, synthetic methods for clusters, unlike those for nanoparticles, tend to be inflexible in terms of element selectivity and applicability, despite their high efficacy for a certain region in the periodic table. This limitation has hindered cross-cutting discussions focusing on constituent elements in the topic of cluster synthesis. In this review, with the aim of providing a comprehensive discussion and understanding of the entire field of cluster synthesis, we categorize, summarize, and review these methods by focusing on the constituent elements of clusters in the periodic table.

2. Methods for cluster synthesis

Metal carbonyl clusters composed of single or multiple transition metal elements and carbonyl ligands represent one of the most well-established material groups among chemical clusters. They possess the most fundamental structures in coordination chemistry and serve as precursors in various chemistry fields, typified by organometallic chemistry.14 These clusters are synthesized by common chemical reactions and their reaction formulas are relatively simple. Moreover, borane clusters are also well synthesized mainly by the pyrolysis of diborane, and their atomicity and shape are classified as obeying the Wade–Mingos rule15,16 or the Jemmis mno rule.17

However, in such cases, the degrees of freedom of design and synthesis are reduced and limited from the perspective of expanding the applicable elements. For example, in thermodynamically stable clusters obtained as metal carbonyls, the number of metal atoms, carbonyl ligands, and structures (hapticity) are uniquely determined by the electronic state of the transition metals. Therefore, the introduction of new factors that can be actively added or modified into the synthesis process of clusters is key for obtaining clusters composed of the desired number of atoms and element types. Based on this concept, various synthetic approaches using gas-, liquid-, and solid-phase reactions have been investigated.

3. Gas-phase synthesis

Fullerenes stand as the most well-known clusters synthesized in the gas-phase.18–20 In an arc reactor or through laser vaporization in vacuum, fullerenes, typified by commercially available C60, C70, and C84 are obtained from carbon sources as a mixture and subsequently separated and purified by chromatography.

Endohedral fullerenes in which other elements are constrained in their cages, such as [Li@C60]+, can also be prepared using carbon mixed with metal sources.21,22

Although gas-phase synthesis is also effective in the case of heavier metal elements,23 metal clusters tend to easily undergo aggregation and particle enlargement after production, in contrast to lighter elements. Therefore, techniques for separating clusters from mixtures directly after synthesis are crucial. In particular, gas-phase reaction systems combined with mass-separation units are often employed, leading to numerous reports on metal clusters typified by Nax.24 This method not only effectively yields single elemental clusters such as superatomic [Al13],25 stannaspherene [Sn12]2−,26 borospherene [B40],27 and tetrahedral Au20,28 but also heteroelemental clusters such as M@Sn1226 and M@Si16.29 On the other hand, stable extraction of clusters produced from the gas phase has recently been investigated, including the soft-landing method, in which clusters are deposited intact on a self-assembled monolayer (SAM) of alkyl groups formed on a substrate.30

As mentioned above, gas-phase preparation techniques offer the merit of involving the use of many elements, including both metal and nonmetal elements, but are fundamentally not conducive to mass production. Therefore, although these methods are significantly effective for analyzing physical properties, including electronic states, they are not suitable for investigations requiring large amounts of clusters, such as chemical reactivity. Additionally, because it is necessary to provide a positive or negative charge to clusters for mass separation, the properties of charged clusters are preferentially observed. The latter can be addressed by establishing a neutralization method for obtaining neutral clusters after mass separation.31

4. Liquid-phase synthesis

Various liquid-phase synthesis methods have been explored due to their practicality. The principal approaches for stabilizing clusters involve methods adopting ligand protection, employing polyoxometalate anions, and applying the Zintl phase. Each method targets specific regions of elements in the periodic table, leading to differences in the properties of the obtained clusters. These disparities often arise from variations in the chemical states of the clusters’ surface or interior. Therefore, given the present circumstances, a suitable synthetic method should be carefully selected that not only considers the employed elements, but that can also induce the desired properties.

4.1. Clusters stabilized by ligand protection

In this method, the clusters are chemically stabilized by the steric and electronic protection effect of the organic ligands, which cover their surface atoms with functional groups. Applicable metal elements vary depending on the functional groups of the ligands. Negatively charged alkylthiolate-type (SR) ligands typically used for group 11 element clusters (Au, Ag, Cu) are among the most well-investigated representatives of this method.32–36 Alkyl selenide-type analog structures (SeR) have also been reported.37 The steric structure of ligands modifies the number of constituent atoms and their geometric arrangements, which are often based on highly symmetric core structures.38 In this regard, Au102(SR)44,39 Au144(SR)60,40,41 [Ag180(SR)90(CH3SO3)44]46+,42 and Ag374(SR)113Br2Cl243 stand as the largest well-known clusters with numerous constituent atoms (Fig. 3A). Recent reports have revealed that this method is also applicable to the synthesis of alloy clusters composed of group 11 elements, such as [Au12Ag32(SR)30]4− (ref. 44) (Fig. 3B), despite the composition and arrangement of multiple elements in these clusters tending to be uncontrollable because of the preferential generation of energetically stable clusters. Additionally, an alloying method for doping with other metal elements has been developed by utilizing the stability of ligand-protected Au or Ag clusters.45 This approach enables the use of metals that do not achieve ligand stabilization, such as [M@Au25(SR)18] (M = Pd, Pt, Cd, Hg)46 and Pd@Ag20(S2PR2)12[thin space (1/6-em)]47 (S2P(OR)2 = dialkoxyldithiophosphinate). A recently reported anion-templated synthesis method of such clusters (especially for Ag clusters) also provides various geometric structures containing non-metallic elements or anionic species (atomic anion: H, D, F, Cl, Br, I, S2−, Se2−, and Te2−; oxoanion: CO32−, C2O42−, C4O42−, C5O52−, NO3, AsO43−, SO32−, SO42−, SeO32−, SeO42−, TeO32−, TeO66−, ClO4, VO43−, CrO42−, MoO42−, and WO42−; polyoxometalate anion: [V10O28]6−, [Mo6O22]8−, [EuW10O36]9−, etc.) in the center of the structure,48 such as [X@Ag8(S2P(OR)2)6]+/0 (X = F, Cl, Br, S).49,50 Other elements in group 11 have also been investigated, such as [Cl@Cu14(S(CH3)2(CH2NH2))12]7+.51 Thiolate-type ligands have also been used to stabilize metal sulfide clusters. For example, group 10 elements (Ni, Pd, Pt) form tiara-shaped cluster complexes, such as [M(SR)2]n (M = Ni, Pd, Pt)52–54 (Fig. 3C), that often encapsulate guest metal ions or molecules in their ring centers (I2, Ag+).55,56 In the case of other group elements typified by Mn, Co, Zn, and Cd, a combination of monoanionic SR and dianionic S2− provides sulfide clusters including tetrahedral [Mn8Se(SePh)16]2−,57 octahedral [Co8S6(SPh)8]4−,58 tetrahedral [Zn10S4(SPh)16]4−,59 and [Cd54S32(SPh)48(DMF)4]4− (ref. 60) (Fig. 3D).
image file: d3nr06522g-f3.tif
Fig. 3 Crystal structures of: (A) Ag374(SR)113Br2Cl2 (R = 4-tert-butylphenyl),43 Ag: gray, S: yellow, Cl: green, Br: brown, and C: grayish bone; (B) [Au12Ag32(SR)30]4− (R = 4-fluorophenyl),44 Au: orange-yellow, Ag: gray, S: yellow, C: gray bone, and F: pale blue bone; (C) Ni10(SR)20 (R = 2-pyridylethyl),52 Ni: gray, S: yellow, C: grayish bone, and N: pale blue bone; (D) [Mn8Se(SePh)16]2−,56 Mn: violet, Se: yellow-green, C: gray bone; (E) [Au13(Ph2PCH2PPh2)6]5+,66 Au: orange-yellow, P: grayish pink, and C: grayish bone; (F) [Pd8(Ph2P(CH2PPh)2CH2PPh2)4]4+,70 Pt: gray, P: grayish pink, C: grayish bone; and (G) [Be4(NH2)6(NH3)4]2+,75 Be: green, N: pale blue, and H: white. Figures are reproduced from (A) CCDC 1496141, (B) CCDC 953881, (C) CCDC 743607, (D) CCDC 612696, (E) CCDC 1577669, (F) CCDC 1023572, and (G) CCDC 1982295.

Among other functional groups used for ligand protection, various neutral, cationic, and anionic ligands have been investigated. Neutral ligands typically include monodentate or multidentate organophosphines (PR3), amines (NR3), and imines (NR([double bond, length as m-dash]R)). Analogs with heavier atoms, such as organostibines (SbR3), have recently been reported.61 In the case of phosphines, clusters with group 11 elements, especially gold62–65 (sometimes group 10 elements, such as Ni3(PPh)(PPh2)2(PPh3)366), typified by commercially available [Au3O(PPh3)3]+ and icosahedral [Au13(Ph2PCH2PPh2)6]5+ (ref. 67) are principally explored in the same manner as alkylthiolate (Fig. 3E). In particular, precisely designed multidentate phosphine ligands provide and control unique arrangements of metal atoms in a cluster, such as icosahedron-based [Au20(P(CH2CH2PPh2)3)]4+,68 linear [Au4(Ph2P(CH2PPh)2CH2PPh2)2]4+ (ref. 69) and linear [Pd8(Ph2P(CH2PPh)2CH2PPh2)4]4+ with tetraphosphines70 (Fig. 3F). Anion-templated synthesis is also used for clusters with phosphine ligands such as Cl@Ag12@Ag48(Ph2PCH2PPh2)12.71 Another example is transition-metal chalcogenide clusters with protective ligands, such as Co6Te8(PR3)6.72 The clusters were stabilized by amine or imine ligands: cubic Cu4I4(C5H5N)4 with pyridine ligands,73 rhombic [In4(C10H8N2)6]4+ with bipyridyl ligands,74 and [Be4(NH2)6(NH3)4]2+ with ammonia ligands75 (Fig. 3G).

Anionic ligands, including carboxylate (OCOR), represent a well-known example, especially for clusters with low atomicity. Examples include dinuclear clusters such as M2(OCOR)4(L)2 (where M = Cr, Mo, Rh, Cu, Bi)76 and trinuclear clusters such as [M3O(OCOR)6(L)3]+ (where M = V, Cr, Fe, Ru, Co, Rh, Ir).77,78 Notably, such clusters are often commercially available. For the formation of these complex clusters, the oxidation number of the transition metal ions is an important factor; for example, dinuclear and trinuclear carboxylate clusters require metal ions with stable valences of +II and +III, respectively. In this regard, square-planar V4(OCOCH3)4(OH)4(H2O)8,77 tetrahedral Zn4O(OCOCH3)6, Zn10O4(OCOCH3)1279 and In37P20(OCOCH2Ph)51[thin space (1/6-em)]80 have been successfully reported. Alkoxides (OR), amides (NR2), and methides (CR3) have also been used as simple protecting ligands. In particular, research has primarily focused on group 13 and 14 elements (Al, Ga, In, and Sn).81 Examples include [Al77(N(SiMe3)2)20]2− (ref. 82) and Sn15(NR2)6,83 while transition metal elements with structures resembling oxides are often crucial for alkoxides, as seen in compounds like W4(OR)16.84

Organometallic approaches are also effective in stabilizing clusters, with the formation of tetrahedral Li4R4 or octahedral Li6R6 clusters standing as a typical example.85 In particular, cyclopentadienyl-type or pentamethylcyclopentadienyl-type ligands (C5H5 or C5Me5) have been extensively investigated for the stabilization of clusters composed of various elements. Typically, clusters of group 11, 12, and 13 elements (Cu, Zn, Al, Ga, and In) tend to have structures with large atomicity compared to transition metal elements (Co, Rh, Ir, etc.),86 such as trigonal-bipyramidal Zn9(C5Me5)6[thin space (1/6-em)]87 and tetrahedral Al4(C5Me5)4[thin space (1/6-em)]88 (Fig. 4A). Alloy clusters have also been obtained using this method, such as trigonal-bipyramidal Cu3Zn4(C5Me5)5,89 with icosahedral Cu43Al12(C5Me5)12 being one of the largest clusters reported to date90 (Fig. 4B and C). Moreover, some transition metal halides, chalcogenides, and pnictogenides are also stabilized by cyclopentadienyl ligands like pseudo-tridecahedral La9I18(C5Me5)9,91 trigonal-bipyramidal Rh3Se2(C5EtMe4)9,92 and tetrahedral Cr4P4(C5Me5)4[thin space (1/6-em)]93 or Mn4P4(C5H5)4[thin space (1/6-em)]94 (Fig. 4D and E). Using this synthetic approach, model clusters mimicking the biochemical functions associated with nitrogen fixation were recently created (e.g., FeMo3S4(C5H4SiR3)3).95 Additionally, the aromatic rings with large π-conjugated systems also serve as a planar protection ligand and often provide sophisticated highly-symmetric clusters such as cuboctahedral [Pd13(C7H7)6]2+ stabilized by cycloheptatrienylium (C7H7+)96 and square-planar [Pd4(C8H8)(C9H9)]+ stabilized by cyclooctatetraene (C8H8) and cyclononatetraenyl (C9H9)97 (Fig. 5A). Conversely, bridged aromatic ligands can effectively stabilize group 11 elements (Au, Ag, and Cu), yielding star-shaped clusters typified by pentagonal planar Au5(C6H3Me3)5, Cu5(C6H3Me3)5, and square-planar Ag4(C6H3Me3)4 with mesityl anions (C6H2Me3)98,99 (Fig. 5B). Additionally, tetramesityl diiron Fe2(C6H3Me3)4 has also been reported, albeit not among group 11 elements.100 Recently, by utilizing such bridged aromatic ligands, other atomic arrangements including alloys are obtained, like octahedral [Au4Ag2(C6H4PR2)4]2+.101 In contrast, aromatic ligands with bulky substituents also stabilize unique cluster structures by the steric protection effect, such as adamantane-shaped Al4(PH)6(2,6-C6H4(C6Me3H2)2)4.102


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Fig. 4 Crystal structures of (A) Zn9(C5Me5)6.87 (B) Cu3Zn4(C5Me5)5,89 (C) Cu43Al12(C5Me5)12,90 (D) La9I18(C5Me5)9,91 and (E) Cr4P4(C5Me5)4.93 Zn: gray, Cu: blue, Al: light blue, La: green, I: violet, Cr: dark blue, P: grayish pink, and C: grayish bone. Figures are reproduced from (A) CCDC 1434844, (B) CCDC 1854852, (C) CCDC 1845365, (D) CCDC 1992039, and (E) CCDC 984791.

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Fig. 5 Crystal structures of (A) [Pd13(C7H7)6]2+,96 (B) Au5(C6H3Me3)5,98 (C) Pd4(SiPh2)3((CH2PMe2)2)3,103 (D) Ge5Ph10,109 and (E) Ir4Au2(CO)11(PPh3)2.118 Pd: gray, Au: orange-yellow, Si: blue-violet, P: grayish pink, Ge: grayish blue-violet, Ir: light green-yellow, C: gray bone, and O: red bone. Figures are reproduced from (A) CCDC 1813458, (B) CCDC 1119882, (C) CCDC 702935, (D) CCDC 1137518, and (E) CCDC 924993.

Other organometallic approaches for controlling the number and arrangement of metal atoms in clusters have also been developed. For example, organosilicon or organogermanium ligands containing Si and Ge as metalloid elements yield group 10 metal clusters with unusual geometric structures, such as hexagonal planar Pd4(SiPh2)3((CH2PH2)2)3 and hexagonal-bipyramidal Pd6(GePh2)2(CNC6H3Me2)10[thin space (1/6-em)]103–106 (Fig. 5C). Otherwise, the organometallic ligands form clusters, such as cyclic (GeMes2)3, (GePh2)4, and (GePh2)5 with Ge–Ge bonds107–109 (Fig. 5D). Organic or organosilicon ligands have also been employed to synthesize clusters comprising nonmetallic C and semimetallic Si. For example, employing a low-temperature photochemical process or a pure organometallic process in a solvent can yield tetrahedral structures such as C4(CR3)4[thin space (1/6-em)]110 and Si4(SiR3)4.111

In other cases, methods for achieving both the stabilization and alloying of metal clusters using transition metal carbonyl complexes as ligands have been reported, such as octahedral [Ag13Fe8(CO)32]4− (ref. 112) and three pentagonal-antiprismatic [Sb3Rh20(CO)36]3− (ref. 113) complexes. It is worth mentioning that as a technique for clustering certain metals or mixing different metal elements in a cluster, the method using multiple ligands with different functional groups has been investigated, such as Fe8(N[o-H2NC6H4NH(CH2)2]3)2(PMe2Ph)2 with tetraamine and phosphine ligands,114 Ga6(C6Me3H2)4(C3N2Me2Pr2)2 with mesityl and tetraalkylimidazol-2-ylidene ligands,115 [Mg16(C5Me5)8(NEt3)2Br4]2− with cyclopentadienyl and amine ligands,116 Cu10Zn2(C6Me3H2)6(C5Me5)2 with mesityl and cyclopentadienyl ligands,117 Ir4Au2(CO)11(PPh3)2,118 Pt13Au4(CO)10(PPh3)8,119 and Pd145(CO)x(PEt3)30 (x ≈ 60)120 with carbonyl and phosphine ligands (Fig. 5E), and co-crystalized [(AuAg)267(SPhMe2)80]·[(AuAg)45(SPhMe2)27(PPh3)6] with thiolate and phosphine ligands.121 This approach has recently led to the development of model clusters for photosynthetic systems (Mn3Ca clusters with carboxylate and pyridine ligands).122

By surveying ligand protection methods, we can broadly summarize the tendencies of the applicable elements for each method. The most extensively studied elements in ligand protection methods are noble metals (Pd, Pt, Au, and Ag). Following closely are other late transition metal elements (Fe, Co, Rh, Ni, Cu) and post-transition elements (Zn, Cd, Al, Ga, and In). Remarkably, the bond type between the metal atoms directly influences the diversity of the cluster structures. The fact that the most varied cluster structures were found for the metal elements in group 11 could be rationalized by their valence s-electrons forming relatively free bonds and resistance to oxidation. In contrast, other elements tend to be in charged states with the ligands. Although there are relatively few reports on minor elements, a few atoms are often implemented in a cluster by doping the surrounding environment where the stabilization effect of cluster structures composed of major elements is dominant. The types of elements stabilized as a cluster significantly differ by and depend on the functional groups of the organic ligands, which are frequently observed in the case of clusters with single-bond-mediated ligand protection. However, only cyclopentadienyl ligands differ from these ligands and are utilized for applying a broad range of elements by stabilizing clusters with both electrostatic and steric effects. Therefore, such an approach, without obvious single-bond-mediated ligand protection, is expected to be effective for developing versatile synthetic methods from the viewpoint of applicable elements.

Although simple functional groups such as thiolates, carboxylates, and phosphines are used in conventional methods, suitable ligands for this approach are being explored and developed. In particular, new ligands with more complicated structures and compositions containing multidentate ligands afford unique stability to cluster structures.123 Such a method to stabilize a certain geometric structure using custom-designed multidentate ligands is similar to the approach for controlling crystal polymorphs of nanoparticles, in which modification of the surface of a nanoparticle by multidentate ligands induces unusual crystal structures in the whole nanoparticle.124 These reports also indicated that an approach that induces multipoint interactions between a cluster and its ligands is effective for developing versatile synthetic methods.

4.2. Clusters stabilized as ionic compounds

Clusters solely stabilized without protecting ligands have also been reported. They are generally obtained as ionic compounds that undergo electronic stabilization by bonding with pnictogens, chalcogens, halides, and metals with lower electronegativity. Polyoxometalate (POM) clusters are well-known materials. These clusters are almost negatively charged metal oxides and tend to be composed of metal atoms with high oxidation numbers.125–127 In many cases, group 5 and 6 elements (V, Nb, Ta, Mo, and W) in the +V or +VI state prefer a six-coordination state. Some species such as paramolybdate [Mo7O24]6−, decatungstate [W10O32]4−, metatungstate [W12O40]8−, paratungstate [W12O42]12−, molybdenum blue reagents [Mo154O462H14(H2O)70]14− and [Mo152O457H14(H2O)68]16− are commercially available.128 [HxMo368O1032(H2O)240(SO4)48]48− was one of the largest POM clusters129 (Fig. 6A). Similarly, group 3 superheavy elements (U, Np, Pu, and partially Am) in the +V or +VI state also form POM structures, such as [U60O240(OH)60]60− with fullerene topologies130–132 (Fig. 6B). The atomicity and arrangement of POM clusters can often be tuned by introducing other elements as the central core. For example, 4-coordinating species mainly in group 13, 14, and 15 elements (B, Al, Ga, C, Si, Ge, P, and As), such as PO4, SiO42−, and AlO43− species, form unique structures typified by tungstophosphate [PMo12O40]3− called Keggin-type structures.125,126 Fe, Co, Cu, and Zn in the +II or +III state, and S and Se in the +VI state have also been reported as tetrahedral centers. In the case of 6-coordinating species of elements (Cr, Mn, Fe, Co, Rh, Ir, Ni, Pd, Pt, Cu, Zn, Al, Ga, Sb, Te, I), more planar Anderson–Evans-type POM clusters are constructed by a central EO6 unit that is often in highly oxidized states, such as [CoMo6O18(OH)6]6− with Co(+III), [PtW6O24]8− with Pt(+IV), and [IMo6O24]8− with I(+VII).124,133 Moreover, icosahedrally 12-coordinating EO12 central units containing group 3 and 4 heavy elements (Ce, Th, U, Np, and Zr in the +IV or +V state) provide unique Dexter–Silverton-type structures such as pyritohedral [CeMo12O42]8−.134 By introducing two or more core units, POMs lead to the construction of more complicated structures called Wells–Dawson-type or Preyssler–Pope–Jeannin-type structures.125 Additionally, multiple POMs form sandwich-type or capsule-type structures intercalating group 1, 2, and 3 elements and low valent transition metals as a cation,125,135 and they are also found in natural minerals, typified by the [Zn2Mn2(H2O)2(FeW9O34)2]12− cluster in ophirite.136 By applying this unique property that partially incorporates other elements in or between POMs, it is possible to synthesize rarer, larger and more complicated structures, such as tetrapod-shaped [Ti4Cl(OH)12(P2W15Ti3O62)4]45− where Ti atoms are sandwiched by four POM cluster units,137 wheel-shaped [Cu20Cl(OH)24(H2O)12(P8W48O184)]25− where Cu atoms are encapsulated in a circular POM cluster host,138 and a tetrahedral layered superstructure of [La10Ni48W140Sb16P12O568(OH)24(H2O)20]86−.139 Recently, it was reported that large capsule-like POM clusters can also incorporate other small metal clusters into their centers, such as [Ag27@(Si2W18O66)3]31−.140
image file: d3nr06522g-f6.tif
Fig. 6 Crystal structures of (A) a unit in [HxMo368O1032(H2O)240(SO4)48]48− with a structure showing molybdenum oxide polyhedra,129 (B) a unit in [U60O240(OH)60]60− with a structure showing uranium oxide polyhedra,130 and (C) a unit in [Fe13O16(OH)12(OCOR)12]17− with a structure showing iron oxide polyhedra (below).142 Mo: reddish gray polyhedron, O: red, U: bluish gray polyhedron, Fe: brown, and O: red. Figures are reproduced from (A) CCDC 1727624, (B) CCDC 1732644, and (C) CCDC 1525080.

With the exception of V, Mo, and W, advanced synthesis techniques have been developed. In particular, the polycationic Keggin-type structures of aluminum [Al30O8(OH)56(H2O)24]18+ containing 6-coordinating AlO6 sites and central 4-coordinating AlO4 sites are well known.141 However, in such cases, co-adoption of the concept of ligand protection in these clusters is also effective in synthesizing POMs composed of minor elements, such as Keggin-type [Fe13O16(OH)12(OCOR)12]17− containing an Fe(+II) center and Fe(+III) shells with carboxylate ligands142 (Fig. 6C), Keggin-type [Mn13O6(OH)2(OMe)4L6]4+ containing a Mn(+III) center and Mn(+IV) shells with alkoxylate and imine ligands (L = 2,6-bis[N-(2-hydroxyethyl)iminomethyl]-4-methylphenol),143 Ti17O24(OiPr)20 with alkoxylate ligands,144 and octahedral [(BuSn)12O14(OH)6]2+ with alkyl ligands.145 In contrast, a few POMs composed of only four-coordinating units are reported including [Mn39O55]26− (ref. 146) (Fig. 7A). Interestingly, the anion-templated synthesis described in the section on ligand-protected noble metal clusters is also effective for the synthesis of POM clusters. The geometries and charges of the template anion provide clusters with various sophisticated core–shell structures, such as [(SCN)@HV22O54]6−, [(CH3COO)@H2V22O54]7−,147 [Cl@Eu15(OH)20]24+,148 and [(SO4)@As4Mo6V7O39]4−.149 Even electronically neutral chemical species, such as solvent molecules, often serve as templates. For example, water and acetonitrile molecules compose [(H2O)@V18O42]12− (ref. 150) and [(CH3CN)@V12O32]4−,151 respectively.


image file: d3nr06522g-f7.tif
Fig. 7 Crystal structures of (A) a unit in [Mn39O55]26− with a structure showing manganese oxide polyhedra (right),146 (B) [Nb6Cl18]2−,152 (C) a unit in [In35S48O8(Im)2]9−,153 and (D) [AsNi12As20]3−.155 Mn: reddish violet, O: red, Nb: bluish green, Cl: light green, In: pale magenta, S: yellow, Ni: gray, As: green, C: grayish bone, and N: light blue bone. Figures are reproduced from (A) CCDC 1728352, (B) CCDC 654117, (C) CCDC 1862551, and (D) CCDC 206301.

Except for the stabilization of clusters, such as the oxides mentioned above, other pnictogenides, chalcogenides, and halides can induce such stabilization. Polyhalometalate clusters, especially of early transition metals (Zr, Hf, Nb, Ta, Mo, W, Re), are well-known, such as [Nb6Cl18]2− (ref. 152) (Fig. 7B). The compound clusters also often have larger layered structures, such as tetrahedral [In35S48O8(Im)2]9−,153 tetrahedral [Sn10O4S20]8−,154 icosahedral [AsNi12As20]3−,155 and tetrahedral Sc4C10Sc20I30[thin space (1/6-em)]156 (Fig. 7C and D). Additionally, it has been reported that the cation-templated synthesis of some chalcogenide clusters provides rare geometric structures, such as [(NH4)@Pd2S28]4− (ref. 157) and [Na2@Fe18S30]8−.158

In summary, in the case of clusters obtained as ionic compounds, the oxide clusters of the early transition metals (V, Mo, and W) are the most investigated, and the runner-up is another base metal element with a relatively high valence (Al, Ti, Mn, Fe, and U). Halide clusters of early transition metals (Zr, Nb, Mo, Hf, Ta, W, and Re) are the main compounds used in this field. Similar to ligand-protected clusters, the introduction of minor elements is achieved by doping a few atoms into the cluster structure composed of major elements. Particularly for POM clusters, it appears that metal elements in remarkably high oxidation states play an important role in diversifying their steric structures. One factor is that metals with high oxidation states maintain the charge balance of the entire cluster structure by reducing the negative charge on their own metalate units. If the metalate anion is represented as {MnO6}(12−n)−, its total negative charge is smaller when oxidation state n is higher. On the other hand, because the stability of these clusters is significantly associated with the affinity of the metal elements for pnictogens, chalcogens, and halogens, clusters composed of noble metals have rarely been reported. As represented by POMs, these clusters often have unique symmetric structures, whereas in the case of different metal elements, these metal oxide units tend to undergo atomic-level phase separation in a cluster and do not form true complex oxides with a uniform atomic co-arrangement. This suggests that the variety of ionic compound clusters can be expanded by improving the synthetic schemes.

4.3. Naked elementary clusters

In contrast, naked clusters composed only of icosagens, tetrels, pnictogens, chalcogens, and halogens (groups 13, 14, 15, 16, and 17) are also well known. Although white phosphorus (tetrahedral P4) is the most famous example, it is generally an ionic species called Zintl clusters or Zintl ions that is stabilized in crystals.159 In particular, there are many reports of Zintl clusters of group 13, 14, 15, and 16 metal elements with various polyhedral structures, such as icosahedral [Tl13]10−,160 trigonal-bypyramidal [Sn5]2−,161 bicapped-square-antiprismatic [Pb10]2−,162 square antiprismatic [Bi8]2+,163 square-planar [Te4]2+, and barrel-shaped [Te8]2+ (ref. 164) (Fig. 8A–C). In other cases, examples of clusters made of group 11, 12, and 17 elements are slightly known like icosahedral [Ag13]4+ (ref. 165) or polyhalogen cations and anions typified by rectangular [Cl4]+ (ref. 166) and trigonal-pyramidal [I7] (ref. 167) (Fig. 8D and E). Similar to other methods, Zintl clusters also form molecular alloys (intermetalloids) of such elements including [BiIn8Bi12]3−/5− (ref. 168) and [K2Zn20Bi16]6−,169 or clusters incorporating a few other transition metal elements170 including [Sn9Pt2(PPh3)]2−, [Sn9Ni2(CO)]3− (ref. 171) and [Th@Bi12]4− (ref. 172) with and without protecting ligands, respectively (Fig. 8F and G).
image file: d3nr06522g-f8.tif
Fig. 8 Crystal structures of (A) [Pb10]2−,162 (B) [Bi8]2+,163 (C) [Te8]2+,164 (D) [Ag13]4+,165 (E) [Cl4]+,166 (F) [K2Zn20Bi16]6−,169 and (G) [Th@Bi12]4−.172 Pb: black, Bi: purple, Te: grayish yellow, Ag: pale gray, Cl: light green, K: violet, Zn: gray, and Th: bluish green. Figures are reproduced from (A) CCDC 288640, (B) CCDC 1728864, (C) CCDC 1471867, (D) CCDC 1728248, (E) CCDC 1726993, (F) CCDC 1969162, and (G) CCDC 1983072.

As mentioned above, these clusters are mainly composed of post-transition elements with many valence electrons, and in many cases, they are positively or negatively charged structures. This tendency is rationalized by the thermodynamic stability based on the electronic configurations unique to clusters typified by superatoms.6–8,25,32,36,47,90 These clusters generally have relatively rigid structures with large bond angles of up to 90° originating from p-electrons and the ability to contain a few atoms of other group elements in electronically stabilized skeletons. In particular, the endohedral structures typified by stannaspherenes mentioned in the gas-phase synthesis section enable the incorporation of other groups of elements without ligands. It should be noted that the accurate choice of counter ions is an important factor in the stable design of these clusters because most clusters are obtained only in the crystal form.

4.4. Clusters obtained by nanospace-assisted template synthesis

The third method utilizes nanospace as reaction fields for cluster synthesis. Because the aforementioned synthetic methods depend on the thermodynamic stability of the cluster structure, only certain clusters with energetically suitable atomicity, atomic arrangement, and composition ratios were obtained. However, in this method, the reaction field is confined within a nanosized space using various approaches, and unique clusters in metastable states that are not found using other methods are forcibly obtained. The pores of zeolites serve as typical capsules for cluster synthesis, such as tetrahedral [Ag4(H2O)4]2+ in their cages.173–177 Metal–organic frameworks (MOFs) are also used for nanosized cages, and templated synthesis of various clusters, including Pt quasi-linear [Pd4]2+,178 tetrahedral Ir4,179 Pt12±x,180 and triple-decker trigonal [Au3L3-Ag-Au3L3-Ag-Au3L3]2+,181 has been reported. Recently, MOFs have been used to cluster carbon as a nonmetal element through selective template synthesis of polyacenes using reactants.182 Flexible supramolecular capsules realize the synthesis of such metastable clusters, including some allotropes of nonmetal elements, such as phosphorus P4[thin space (1/6-em)]183 and sulfur S6 or S12,184 where the catenation behavior is controlled. In other cases, cages of crystalline proteins and nanospaces of macromolecules have been used as templates and protectors for clusters.185,186 In such cases, because the precision of cluster synthesis depends on the operation of the atomic assembly, it is expected that only one method will allow us to use a broad range of elements by adopting the same chemical principle for the assembly. Indeed, the atom hybridization method utilizing multimetallic multinuclear complexes of dendrimers as macromolecular templates for cluster synthesis affords the highest degrees of freedom in the design of multielemental clusters.187–191 Because it adopts the coordination behavior of guests for metal assembly based on the simple principle of acid–base chemistry, all elements can be treated under the same conditions during cluster synthesis. This approach led to the first synthesis of multimetallic clusters containing five or six metal elements like GaInAu3Bi2Sn6.187

In contrast to other liquid-phase synthesis methods, template synthesis provides clusters with relatively metastable structures, which are also regarded as intermediates in the process producing more stable clusters. Capsules need to be individually designed for each chemical element, while this approach, which reduces the direct influence originating from the properties of the elements, enables the use of a broad range of elements. Alternatively, in this method, the interaction between the capsules and precursor compounds, such as electrostatic interactions, acid–base reactions (including coordination), and hydrophobic interactions, is a key factor for accumulating atoms. However, it is important to stabilize such clusters in order to hold them in capsules or fix them on/in support materials. Moreover, in many cases, it is difficult to conduct the usual identification analyses, such as single-crystal X-ray diffraction and direct observation by STEM, SPM, and MS, which are often effective.

5. Solid-phase synthesis

Several methods have been reported for the synthesis of clusters in the solid phase. The mechanochemical synthesis of general nanoparticles has already been established, but that of clusters remains limited.192 For instance, Keggin-type POM of aluminum [Al13O4(OH)24(H2O)12]7+ can be obtained by a mechanochemical reaction between the [Al(H2O)6]Cl3 and (NH4)2CO3 reactants.193 Dimensional reduction of solid-state precursors is effective for the precise synthesis of clusters. Some metal chalcogenide or halide clusters typified by [Re6S6Cl8]2− are stoichiometrically synthesized by dimensional reduction of an extended solid-state structure.194,195 As an ultimate physical method, a technique for the mechanical synthesis of clusters by directly manipulating atoms using scanning probe microscopy was recently developed. Indeed, metal clusters such as Au12, Ag12, and Au5Pb have been synthesized directly on Si surfaces.196

As mentioned previously, the advantages of synthetic methods involving solid-phase processes are extremely limited in terms of the selectability of elements. However, bottom–up synthesis involving mechanochemical reactions and top–down synthesis involving dimensional reduction are significantly more effective for mass production than liquid-phase synthesis. This suggests that expanding the applicable elements in solid-phase synthesis will contribute to promoting clusters as more general and valuable materials in the future.

6. Conclusions and perspectives

As discussed above, the optimal method for synthesizing clusters varies significantly depending on the chemical element involved. Consequently, research on cluster synthesis has been pursued individually, with each method serving as a starting point. Notably, in liquid-phase processes, doping other group elements using the stable structures and atomic-level space obtained by each method has proven highly effective. Recently, these methods have been increasingly interconnected based on this approach.

However, according to the classification of clusters based on chemical elements, it is evident that there is no accurate border between cluster structures, even though each adopts a different synthetic method. For example, certain representatives among ligand-protected clusters and ionic compound clusters, namely [Be4(NH2)6(NH3)4]2+,75 [Zn10S4(SR)16]4−,59 Zn10O4(OCOR)12,79 In37P20(OCOCH2Ph)51,80 [Mn13O6(OH)2(OMe)4L6]4+,143 and [In35S48O8(Im)2]9−,153 partially contain both their two structural factors for cluster stabilization. [Sn9Pt2(PPh3)]2− (ref. 171) can be regarded as a cluster undergoing stabilizing effects by both ligand-protection and Zintl anions. Interestingly, the clusters developed using different methods often exhibit similar structures with reversed atomic arrangements and elemental ratios. Some of the simplest examples are [Co8S6(SPh)8]4− (ref. 58) and Co6Te8(PR3)6,72 where the transition metal and chalcogen atoms occupy reversed positions in the same symmetric structure. Moreover, [Sb3Rh20(CO)36]3− reported as a ligand-protected cluster with a few free post-transition metal atoms113 and [Sn9Ni2(CO)]3− reported as a Zintl cluster with a few ligand-protected transition metal atoms171 have a complementary relationship (Fig. 9A). Additionally, the [Mo6O22@Ag58S2(SPhtBu)36(OCOCF3)10] cluster obtained by the anion-template synthesis of ligand-protected clusters48,197 and the [Ag27@(Si2W18O66)3]31− clusters obtained by the expansion of the synthesis method of POMs140 also have complementary structures. In their skeletons, where the [Mo6O22]8− POM unit was encapsulated in a ligand-protected Ag58 cluster and the [Ag27]17+ unit was encapsulated in three open-Dawson-type POMs [Si2W18O66]16−, the core and shell structures were completely inverted (Fig. 9B). Ag6@(MoO4)7@Ag56(MoO4)2(SiPr)28(p-SO3PhMe)14 with a nested structure was recently synthesized.198 This similarity and reversibility of the cluster structures indicate that the area of elements covered by each synthetic method has been extended widely enough to be close to each other in recent investigations. This suggests that a more flexible cluster design can be realized in the future by improving synthetic techniques.


image file: d3nr06522g-f9.tif
Fig. 9 (A) Crystal structures of [Sb3Rh20(CO)36]3− (ref. 113) and [Sn9Ni2(CO)]3−,171 C: gray, O: red, Ni: light gray, Rh: grayish green, Sn: pale purple, and Sb: brown. (B) Crystal structures of Mo6O22@Ag58S2(SPhtBu)36(OCOCF3)10 showing molybdenum oxide polyhedra197 and [Ag27@(Si2W18O66)3]31− showing silicon oxide and tungsten oxide polyhedra.140 C: grayish bone, O: red, Si: pale blue polyhedron, S: yellow, Mo: reddish gray polyhedron, Ag: pale gray, and W: gray polyhedron. Part A reproduced from CCDC 1960490 and 185824. Part B reproduced from CCDC 1050793 and 1949947.

However, there are uninvestigated clusters for which precise synthetic methods that allow mass production, such as multimetallic clusters, have not yet been established. In light of the tendencies mentioned above, blending these individually developed synthetic methods is expected to make a breakthrough in revealing as-yet-unknown clusters. Although such an attempt has already been made, for example, the combination of ligand protection and nanospace-assisted synthesis methods,199 reports are still rare.

The electronic analogy between the clusters obtained using different methods suggests the possibility of new cluster structures by isomorphic substitution. For example, the ligand-protected [M(SR)2]n (M(II) = Ni, Pd, Pt) in section 4.1 might provide as-yet-unknown tiara-type bimetallic clusters, such as [M(SR)2]n[M'S2]x (M(III) = Mn, Fe, In), by doping based on the concept of chalcogenide clusters. Borane and Zintl clusters often have very similar geometric structures according to the Wade–Mingos rule,15,16 such as square pyramidal B5H9[thin space (1/6-em)]200 and [In5]9− (ref. 201) or bicapped-square-antiprismatic [B10H10]2− (ref. 202) and [Pb10]2−.162 As some transition-metal elements doped with ligands have been reported,171,203 mutual feedback from each chemistry associated with the synthetic methods will become an effective approach for designing new cluster structures. Cation- or anion-templated methods will also contribute to the creation of such hybridized clusters because they are reported in both ligand-protected clusters and ionic compound clusters. Although conventional clusters obtained by ion-templated synthesis are composed of relatively simple anions as core structures, undeveloped ionic species of metals or metal oxides allow expandability in their structures. For example, by tracing these methods, the templated synthesis of clusters adopting uninvestigated ions, such as Ni2+, Zn2+, VO2+, RuO4, and [Pt(OH)6]2−, may contribute to creating clusters with new combinations of elements. Additionally, the secondary chemical treatment of these clusters, inspired by nanospace-assisted synthesis, is expected to add new lineups, even in Zintl clusters. Unrevealed pure metal clusters typified by such Zintl clusters will be obtained by the reduction of ionic compound clusters with diverse structures typified by POMs as precursors.

In contrast to clusters, synthesis methods for nanoparticles cover a broad range of chemical elements and chemical fields.1 Therefore, hybridizing conventional synthesis methods for nanoparticles and clusters is expected to contribute significantly and provide new guidelines for cluster synthesis. As described in the sections on ligand-protected clusters, modifying the cluster structures with multidentate ligands is a strong candidate.123,124 The effectiveness of such an approach following the methods for nanoparticles might ultimately be extended to bulk materials containing clusters as partial structures, such as clathrate compounds, gas hydrates, or minerals204–208 and cluster units in metallic glasses or quasi-crystals.209–213

In conclusion, a comprehensive categorization of clusters based on their constituent chemical elements allowed us to understand which method covered which elements and revealed the common points of the structures synthesized by different methods. It is suggested that blending chemical concepts for stabilizing clusters by combining these methods or introducing them into nanoparticles will enable the synthesis of as-yet-unknown clusters not obtained by conventional techniques, affording us many bridgeheads for the next stage of developing cluster fields. Finally, the one-size-fits-all synthetic method constructed using such an approach is a new strategy to promote this cluster chemistry to a more central and larger chemistry field in the future, leading to the creation of next-generation functional materials, such as quantum-sized catalysts.

Author contributions

T. T. composed the entire manuscript.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The author acknowledges grants from JST PRESTO Grant Number JPMJPR20AA, JSPS KAKENHI Grant Number JP22H01904, the Advanced Technology Institute, the Yoshinori Ohsumi Fund for Fundamental Research, the JGC-Saneyoshi Scholarship Foundation, the Toshiaki Ogasawara Memorial Foundation, the Iwatani Naoji Foundation, the Inamori Foundation, Mizuho Foundation for the Promotion of Sciences, Iketani Science and Technology Foundation, and the Special Fund of Institute of Industrial Science, University of Tokyo. Drawings of the crystal structures of the clusters were reproduced using VESTA214 according to the Cambridge Crystallographic Data Centre (CCDC) database.

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