Large Metal Chalcogenide Clusters and Their Ordered Superstructures via Solvothermal and Ionothermal Syntheses

Levchenko, Tetyana I.; Huang, Yining; Corrigan, John F.

doi:10.1007/430_2016_5

Large Metal Chalcogenide Clusters and Their Ordered Superstructures via Solvothermal and Ionothermal Syntheses

Tetyana I. Levchenko¹⁰,
Yining Huang¹⁰ &
John F. Corrigan¹⁰

Chapter
First Online: 21 July 2016

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7 Citations

Part of the book series: Structure and Bonding ((STRUCTURE,volume 174))

Abstract

Nanometre-scale metal chalcogenide clusters and materials derived from their regular spatial organization via covalent or other bonding interactions represent an important area of research, encompassing intricate structures and unique size-related electronic and physical properties. This chapter will summarize the structure and bonding principles in these systems, focusing on high nuclearity and discrete metal chalcogenide clusters, and will review the recent progress in their preparation using solvothermal and ionothermal approaches.

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1 Introduction

Polynuclear metal chalcogenides ME (where M = metal, E = group 16 element heavier than oxygen) often possess unique structures due to the bonding modes inherent to the chalcogen sites [1–7]. Metal chalcogenide clusters with well-defined sizes and chemical composition can contain tens to hundreds of metal core atoms, organized with a high level of symmetry, reaching several nanometres in size [6, 8–10]. Many of these nanoclusters can be prepared and isolated as single crystals, making it possible to obtain complete structural information through single-crystal X-ray diffraction [11]. Other powerful tools, such as electron tomography, can help significantly in the analysis of cluster (super)structures [12–15]. Knowledge of the exact structure of such clusters provides valuable insight into structure–property relationship in nanodimensional systems without obscuring effects of size polydispersity and structure ambiguity, often inherent to colloidal nanoparticles. Generally, clusters of semiconductor metal chalcogenides have size-related electronic and photophysical properties due to quantum confinement effects [16–18]. Thus, a systematic blue shift of the optical absorption band is observed with decreasing CdSe cluster size [19, 20]. The properties of the clusters can also be tuned by the substitution of M and E, by combining several different metals or chalcogens, with site-selective distribution of the components in a cluster core, and/or fitting organic ligands on a cluster surface. Long-range order is present with certain secondary structures (1D, 2D and 3D arrangements), maintained by electrostatic (Coulomb) interactions and/or relatively weak (e.g. van der Waals) forces or, alternatively, by covalent bonding (with or without auxiliary organic linkers) between metal chalcogenide clusters [21]. Such multilevel, hierarchical structures of metal chalcogenide clusters have multiple attractive features: materials containing metal chalcogenide clusters can be engineered at several different length scales, from atomic level (size and composition of cluster core) to the superstructure level (intercluster bonding type and connectivity patterns), which provides an additional opportunity to control their properties [22]. Moreover, hybrid materials can be created based on molecular-level integration of anionic metal chalcogenide clusters with cations having special functions [23, 24], or even by the combination (crystallization) of cluster superstructures with other advanced materials. One of the recent examples of the latter is the cluster-based material (C₅H₁₂N)₁₂[Zn₄Ga₁₄Sn₂Se₃₅]@reduced graphene oxide, where C₅H₁₂N is piperidinium cation [25]. The properties of the materials containing metal chalcogenide clusters encompass such research areas as photophysics, photoelectrochemistry, photocatalysis, etc., as they are promising candidates for application as advanced energy conversion materials and bio-labels [26–30].

Historically, a coordination chemistry approach has been used for the preparation and crystallization of metal chalcogenide clusters from solutions [1, 6, 9]. This powerful approach utilizes different sources of chalcogenides and surface chalcogenolates (among them silylated reagents E(SiMe₃)₂ and RESiMe₃) [11] and has been proven to give access to, for instance, unprecedented large sizes (e.g. [Ag₄₉₀S₁₈₈(S^tC₅H₁₁)₁₁₄]) [31] as well as opportunities for unique surface functionalization (e.g. ferrocene (fc) decorated [Ag₇₄S₁₉(dppp)₆(fc(C{O}OCH₂CH₂S)₂)₁₈], where dppp=1,3-bis(diphenylphosphino)propane) [32] to group 11 metal chalcogenide clusters. Materials containing metal chalcogenide clusters have also been prepared by solid-state chemistry approaches from high-temperature reactions in polychalcogenide flux (e.g. a series of discrete clusters [M₄Sn₄S₁₇]¹⁰⁻ with M=Mn, Fe, Co, Zn in a K₂S_x flux) [33, 34]. More recently, solvothermal approaches have been exploited [35, 36], conducting synthesis using relatively simple reagents (e.g. elemental forms and inorganic salts) in an appropriate organic solvent in a sealed vessel at moderately high temperature and autogenous pressure. A related approach, utilizing ionic liquids as reaction media is also a focus of research efforts for the preparation of metal chalcogenide clusters [37]. During the last decade solvothermal and ionothermal approaches have yielded, for example, new metal chalcogenide clusters with unprecedented structures [38–40], the ability for precise and uniform one-atom doping of clusters with vacant sites [41, 42] and the preparation of large, discrete clusters, previously accessible only in covalently bonded 2D and 3D superlattices [43, 44].

Several research groups have been developing solvothermal and ionothermal approaches towards the preparation of large metal chalcogenide clusters and materials derived from their regular spatial organization. The general synthetic routes and the structures and properties of these materials will be summarized in this review. Note that the main focus is on discrete clusters (mainly tetrahedral) and their non-covalent 3D superstructures, while extended framework superstructures (both zeolite and metal-organic framework analogues with inorganic linkers and organic ligand connection between clusters, respectively) were previously covered in several reviews [45–48]. Relatively large metal chalcogenide clusters, mainly containing ≥ 8 metal sites in the core, are the focus herein.

2 Bonding in Metal Chalcogenide Clusters

2.1 Metal–Chalcogen Bonding

Chalcogenides E²⁻ form stable bonds with many metals, adopting several different bridging coordination modes, with μ₃ and μ₄ being the most common [2, 4, 6]. Thus, the coordination number of sulfur reaches 4 even with a relatively large metal cation such as Cd²⁺. The ability to bridge metals with high coordination numbers is attributed to the large ionic radii, high polarizability, more delocalized electron orbitals and the anionic nature of chalcogenide ligands [6]. The bridging ability increases on going down group 16 from sulfur to selenium to tellurium [3, 5, 7]. Metal cations in cluster chalcogenides can be in one particular oxidation state (M²⁺, M³⁺ or M⁴⁺) or different combinations of two cations (e.g. M³⁺/M⁺ or M⁴⁺/M²⁺) or exhibit even more complex composition. A recent example of such multinary compounds is a family of discrete M₂₀E₃₅ clusters, combining, for instance, five metals with different oxidation states (i.e. Cu, Zn, Mn, Ga and Sn) in one cluster, as confirmed by EDX analysis; for clusters with quaternary composition (e.g. [Cu₂Ga₁₆Sn₂Se₃₅]¹²⁻ or [Zn₄Ga₁₄Sn₂Se₃₅]¹²⁻), single-crystal structure refinement results are in good agreement with atomic absorption spectroscopy analysis [49]. As a consequence of the high bridging ability of E²⁻ with high coordination numbers for M^x+, in many metal chalcogenide frameworks, cations and anions both adopt tetrahedral coordination, which makes tetrahedral unit {ME₄} the most basic building block in these materials. A distinct structural feature is the overall tetrahedral shape of many such clusters. The covalent character of bonding in the tetrahedral units {ME₄} reflects the relative position of the composing metals in the periodic table. Most often, metals in these tetrahedral clusters belong to groups 12, 13 and 14 (e.g. Zn, Cd, Hg; Ga, In; Ge, and Sn) and late first-row transition metals (e.g. Mn, Fe, Co and Cu); however, this does not exclude the possibility of doping by other metals (e.g. Li). Many of the tetrahedral metal chalcogenide clusters, originally prepared by other approaches, have been reproduced solvothermally. Even more clusters have proven accessible by solvothermal and ionothermal approaches, including those with completely new structure types. The group 11 metal chalcogenide clusters, prepared by coordination chemistry approaches, are numerous and structurally diverse [50–52], but such discrete clusters are typically not accessible via solvothermal or ionothermal approaches. Although large cluster cores composed of tetrahedral units {ME₄} and an overall tetrahedral shape are characteristic to metal chalcogenides, some examples are also known for oxides (e.g. tetrahedral clusters [Mn^II ₂₉Mn^III ₆O₅₆]³⁶⁻ or [Ln₂₀O₁₁]³⁸⁺, where Ln=lanthanoid metal) [53, 54]. With adamantoid (cubic) (Fig. 1, left) and barrelanoid (hexagonal) (Fig. 1, right) crystalline cages both being possible with the tetrahedral coordination of atoms (corresponding to zinc blende and wurtzite crystal structures, respectively, well known for bulk crystalline metal chalcogenides), the recognized structural variations of tetrahedral metal chalcogenide cluster arise from different combinations of cubic and hexagonal cages in the ME frameworks. Thus, by the nature of intra-cluster connectivity, tetrahedral metal chalcogenide clusters can be classified as belonging to three particular structural series, (basic) supertetrahedral, penta supertetrahedral and capped supertetrahedral (Fig. 2) [47, 57]; these are considered in detail below in Sects. 2.3, 5.2 and 5.3. Although large, non-tetrahedral clusters [6, 9] are less often prepared via solvothermal and ionothermal syntheses, some fascinating examples of discrete ring- and cagelike frameworks formed by vertex and edge sharing of basic tetrahedra {ME₄} have been reported recently. These clusters are discussed below in Sects. 2.5, 5.5 and 5.6.

2.2 Local Electroneutrality in the Cluster Core

One of the most important factors affecting the size and connectivity of metal chalcogenide clusters is the charge on the constituent metal cations. As the tetrahedral clusters display a clear structural relationship with the corresponding crystalline solids, they are found to obey the same rules surrounding their bonding. Generally, the charge of metal cations appearing in particular sites of tetrahedral metal chalcogenide clusters is found to follow Pauling’s electrostatic valence rule. According to this rule, in order to keep local electroneutrality (local charge balance), the sum of the strengths of the electrostatic bonds to E²⁻ anion should be equal to the charge on the anion, i.e. 2. The electrostatic bond strength can be calculated as the ratio of the charge on adjacent metal cations to its coordination number. From this it follows, for example, that each tetrahedral E²⁻ site could be either surrounded by four tetrahedral M²⁺ or two tetrahedral M³⁺ plus two tetrahedral M⁺. More specific cases are addressed below when considering the tetrahedral cluster series. Pauling’s electrostatic valence rule works most obviously for the inner sites in the cluster, although it is not always applicable to surface sites (at vertexes, edges and faces of tetrahedral clusters). This is because E²⁻ sites on the surface may receive additional bond valence from cationic species that are not part of the cluster. A few exceptions to Pauling’s electrostatic valence rule (e.g. a tetrahedral cluster with a core E²⁻ site bonded to four M³⁺) [58] can be rationalized considering cluster stabilization from additional lattice species.

2.3 Series of Tetrahedral Clusters

In a basic supertetrahedral series, each molecular cluster consists of a regular tetrahedral-shaped fragment of the zinc blende-type lattice (cubic, adamantoid cages) (Fig. 3). Larger clusters in this series are formed by fusion of adamantoid cages only. This is the most fundamental type of connectivity; other series of clusters can be geometrically derived from the basic supertetrahedral building units. The difference between clusters within the series lies in the size of the framework. This is reflected in conventional notation for the clusters in the supertetrahedral series, T_n, where the integer n indicates the number of individual {ME₄} tetrahedra along each edge (Fig. 3). The integer n is also equivalent to the number of metal layers within a particular cluster. Thus, a T₃ cluster with a M₁₀E₂₀ core contains four fused adamantoid cages and has three {ME₄} tetrahedra along each edge (or three metal layers) (Fig. 3, top right). The composition of an idealized core M_xE_y of any T_n cluster is strictly defined (see formulae in Table 1). It can be seen that the number of E sites in a T_n cluster is equal to the number of M sites in the next larger T_(n+1) cluster. The peculiarity of large T_n clusters is the presence of tetrahedrally coordinated (inner) anions, while smaller clusters (T₁, T₂ and T₃) consist of μ- and μ₃-anions only. To maintain the local electroneutrality, in large metal chalcogenide clusters containing two or more types of metal cations, site-selective distribution of metals will be one that better balances the tetrahedrally coordinated anion sites E²⁻, which occur inside clusters ≥T₄. In multinary clusters with more than one type of chalcogenide (e.g. both Se and S), the appearance of E, E′, M and M′ at inner or surface sites may be governed by multiple factors [59]. The largest reported discrete supertetrahedral clusters are T₅; for instance, [Cd₁₃In₂₂S₅₂L₄]¹²⁻ cluster, where L is neutral organic ligand 1-methylimidazole, mim, capping four cluster vertexes through In−N coordination bonds (Fig. 2, left); this was prepared using solvothermal methods [43].

Table 1 Series of tetrahedral metal chalcogenide clusters^a

Full size table

Clusters with a void or cavity in the core, i.e. hierarchical and coreless clusters, can be considered as structure variations of a supertetrahedral series rather than a separate connectivity type. Hierarchical supertetrahedral clusters (denoted T_p,q) consist of four supertetrahedral T_p units assembled (through vertex sharing by bridging E²⁻ or ER⁻ sites) into a self-closed T_q cluster with a central void of size T_p. Hierarchical T_p,q clusters can also be viewed as T_n-like clusters of a larger size (n = p*q) with a well-defined tetrahedral void in a core, created by the systematic absence of M and E atoms. In hierarchical clusters, the presence of an inner tetrahedral void ensures a decrease of the coordination number of some of the internal anions; the structure is favourable under conditions of an appropriate combination of constituent elements and a structure-directing agent that optimizes both local and total charge balances. Hierarchical clusters with large T_p units (and, consequently, large voids) are rare, as T_n clusters preferentially self-assemble into extended lattices (an extraordinary example is dual hierarchical covalently bonded 3D superstructure T_5,∞ [60]) instead of forming discrete self-closed T_p,q clusters. An example of large hierarchical cluster is the solvothermally prepared discrete T_4,2 [Cd₁₆In₆₄S₁₃₄]⁴⁴⁻ (Fig. 4) [61]. More recently, solvothermal synthesis also resulted in the preparation of the anionic T_2,2 cluster [M₁₆Se₃₄]^x− (M=Ge/In mixed sites) covalently linked with T₃ clusters in a 3D framework [62]. Hierarchical supertetrahedral clusters can be prepared while systematically hosting a particular chemical species (e.g. alkali metal cations) [63].

The other set of clusters with a central void are coreless clusters, having in their otherwise regular T_n lattice a single metal tetrahedral site vacant, surrounded by four core E²⁻ ions. Examples are the solvothermally prepared coreless T₅ [Cd₆In₂₈S₅₆]¹²⁻ which are arranged in a covalently bonded 2D superstructure [64] and coreless T₅ [In₃₄S₅₆]⁶⁻ which form a covalently bonded 3D co-assembly with regular T₃ [In₁₀S₁₈]⁶⁻ units [65]. These large clusters with one metal cation missing appear since such a structure allows for a reduction in the coordination number of four inner chalcogenide anions from four to three, helping to maintain local electroneutrality. The void in as-prepared coreless clusters is occupied by various (highly disordered) guest species [64]. At the same time, a coreless structure provides a unique possibility for precise doping with carefully chosen metal cations (e.g. by Cu⁺ or Mn²⁺), which was shown to change dramatically the photophysical properties versus the pristine metal chalcogenide frameworks [41, 42].

Similar to the main structural feature in coreless clusters that results from metal atom elimination, uncommon stuffed clusters can be viewed as a product of the addition of extra atoms to regular T_n frameworks. Recent examples of solvothermally prepared stuffed clusters include [Sn₁₀S₂₀O₄]⁸⁻ and [Sn₁₀Se₂₀O₄]⁸⁻ with extra oxygen atoms in each cubic cage of the T₃ units (Fig. 5); both S- and Se-containing analogues are covalently linked in co-assemblies of clusters of different sizes [66, 67]. The formation of such oxychalcogenide units allows for the stabilization of a Sn⁴⁺-containing T₃ framework, which is otherwise unlikely to form: according to Pauling’s electrostatic valence rule, μ₃-E²⁻ sites do not match with tetrahedral Sn⁴⁺ sites and the largest possible supertetrahedral cluster in the pure system M_x ⁴⁺E_y is T₂.

Penta supertetrahedral cluster series (denoted P_n) are formed by coupling four T_n supertetrahedral units onto the faces of an anti-supertetrahedral unit of the same order. The central anti-supertetrahedral unit has the M and E positions exchanged in comparison to a regular one; e.g. anti-T₂ unit has composition {E₄M₁₀} (Fig. 6, top left). In this way, P_n clusters contain both cubic and hexagonal cages, and the latter appear on fused faces (Fig. 6, top centre). Thus, in a P₁ cluster four hexagonal cages are sharing a single tetrahedral E site, also each containing three M sites of the same {EM₄} unit. In a P₂ cluster there are three hexagonal cages on each of four faces of anti-T₂ unit, twelve in total (Fig. 6, top right). The structural relation between P_n and T_n with the same n is reflected in the composition of an idealized P_n core M_xE_y (see formulae in Table 1), as formulae can be derived using the known composition law for T_n. The largest solvothermally prepared penta supertetrahedral cluster P₂ with composition [Li₄In₂₂S₄₄]¹⁸⁻ exhibits corner sharing in a covalently bonded 3D structure [68]. This large cluster contains four tetrahedrally coordinated S²⁻ sites, located in the central anti-T₂ unit. To satisfy Pauling’s electrostatic valence rule, each such S²⁻ site should be surrounded by two Li⁺ and two In³⁺, giving together a bond valence sum of +2. These two metals are statistically distributed over six symmetry equivalent inner metal sites (located in central anti-T₂ unit) with 2/3 occupancy by Li⁺ and 1/3 occupancy by In³⁺ (Fig. 6, bottom) [68]. In the discrete cluster of the same size P₂, prepared using a coordination chemistry approach [55], a statistical distribution of Cu⁺ and In³⁺ cations over six symmetry equivalent inner metal sites was also found (Fig. 2, centre). With four vertex metal positions in the central anti-T₂ unit, as well as four metal positions at P₂ cluster vertexes solely occupied by Cu⁺, results of elemental analysis are in a good agreement with the disordered model and a neutral formula [Cu₁₁In₁₅Se₁₆(SePh)₂₄(PPh₃)₄], featuring PhSe⁻ ligands on edges and PPh₃ ligands at cluster vertexes [55].

Capped supertetrahedral cluster series (denoted C_n) consist of a core, which is a regular fragment of the cubic lattice, and four hexagonal (barrelanoid) cages capping the vertexes. Another way to view clusters of the C_n series, better showing their relation with T_n series, is as follows: a regular supertetrahedral unit T_n at the core is covered on each face with a single “layer” of vertex-sharing basic {ME₄} units (see Fig. 7, top centre) and each vertex is completed by a {M₄E₄} group to form a hexagonal cage. In this way, the composition of an idealized C_n core can be derived using formulae for T_n with the same n (Table 1). The structural feature of C_n clusters is the open cleft that runs along each of the tetrahedral edges (see Fig. 7, bottom). Like in T_n clusters, the number of E sites in a C_n cluster is equal to the number of M sites in the next larger C_(n+1) in the series. In C_n clusters, each hexagonal cage (more precisely, a M₄E₅ unit) at one of four vertexes can also be independently rotated (around the threefold axis of the tetrahedron) by 60°. This results in additional variation (isomerism) in the capped supertetrahedral series, denoted as C_n,m where m refers to the number of corners that have been rotated from their original position in the parent C_n. This variant does not usually change either cluster or superstructure properties significantly, so vertex rotation will not be mentioned below while referring the cluster type and size. Discrete capped supertetrahedral clusters with sizes up to C₃ were synthesized solvothermally; some examples are [Cd₅₄S₃₂(SPh)₄₈(H₂O)₄]⁴⁻ (Fig. 2, right) and [Cd₅₄Se₃₂(SPh)₄₈(H₂O)₄]⁴⁻ (Fig. 7, bottom) [56]. The core of these C₃ clusters is formed by ten tetra-coordinated cadmium and twenty tetra-coordinated chalcogenide sites in a cubic arrangement (forming a regular T₃ unit) (Fig. 7, top left). The inner tetrahedron is covered on each face with seven {CdE₄} units fused through vertexes by rows 2-3-2 to form a single cubic sheet (Fig. 7, top centre), resulting in four times three μ₃-E²⁻ sites (twelve in total). Capping each vertex with a hexagonal cage (Fig. 7, top right) increases the number of edge μ-PhS⁻ sites to eight per each of the six edges (48 in total).

From the description above, it can be seen that in the vast majority of these tetrahedral clusters, the number of E sites exceeds the number of M sites; this follows from having the tetrahedral {ME₄} unit as a building block. The presence of an inner anti-T_n unit (derived from a {EM₄} unit) in the structure of P_n cluster series is an exception. Interestingly, the preparation and structural characterization of several large tetrahedral “quantum dots” with crystalline CdSe cores corresponding entirely to anti-T_n clusters was recently reported [69]. The metal chalcogenide core structure with unusual metal-terminated {111} facets was derived using a combination of single and powder X-ray diffraction data and atomic pair distribution function analysis. These quantum dots have approximate formulae Cd₃₅Se₂₀X₃₀L₃₀, Cd₅₆Se₃₅X₄₂L₄₂ and Cd₈₄Se₅₆X₅₆L₅₆, with benzoate and n-butylamine ligands (X = O₂CPh, L = H₂N-Bu), and can be viewed as anti-T₄, anti-T₅ and anti-T₆, respectively.

2.4 Ligands on Tetrahedral Clusters

Some metal chalcogenide clusters, such as those with group 13 and 14 metals, may be prepared as purely inorganic (anionic) frameworks. This is in accordance with Pauling’s electrostatic valence rule, as tetrahedrally coordinated M³⁺ or M⁴⁺ cations can balance edge or corner E²⁻ anions with low coordination numbers. For metal chalcogenide clusters with surface M²⁺ sites, the sum of the strengths of the electrostatic bonds to edge or vertex E²⁻ sites is too low to reach local electroneutrality. To overcome this, the coordination numbers of such E²⁻ sites are found to increase. In other words, clusters require the incorporation of an encapsulating and stabilizing shell of organic ligands. The ligands on a metal chalcogenide core also kinetically protect the cluster and prevent further condensation to the thermodynamically favoured infinite crystalline lattice of the related solid. Organic ligands serving in this capacity include various phosphines PR₃, amines (especially, N-containing aromatic heterocycles), halides (Hal) and organo-chalcogenolate anions RE⁻ [6, 9]. While the majority of these ligands replace surface E²⁻ sites, creating M−P, M−N and M−Hal coordination, chalcogenolates at edges and vertexes do not alter the M_xE_y stoichiometry of the idealized cluster core. For chalcogenolate ligands the most common bonding mode is the doubly bridging μ; triply and higher bridging coordination modes are more often observed for selenolate and tellurolate ligands than for thiolates, reflecting their larger size. In discrete metal chalcogenide systems with mixed ligands, bridging chalcogenolate ligands preferentially occupy edge positions, while other ligands are bonded to metals at vertex positions.

In coordination chemistry approaches for cluster formation, the use of coordinating and chelating solvents to increase the solubility of reactants and/or products simultaneously can lead to the preparation of metal chalcogenide clusters containing solvent molecules as ligands (e.g. pyridine, dmf) [70, 71]. Higher reactivity under solvothermal or ionothermal conditions may also cause some side reactions to occur. Consequently, products of the decomposition/conversion of solvent (or additive) may serve as ligands. Examples include the coordination of dimethylamine from DMF, piperidine from dipiperidinomethane and 1-butyl-2-methyl-imidazole, Bim, from 1-butyl-2,3-dimethylimidazolium chloride, [Bmmim]Cl [44, 49, 72, 73]. An interesting case of ligand conversion during hydrothermal synthesis is the hydrolysis of the cyano group of 3-pyridinecarbonitrile, which resulted in the preparation of 1D covalently bonded clusters [Zn₈S(SPh)₁₃L(H₂O)], with bidentate L = 3-carboxypyridyl bridging two adjacent clusters via M−N and M−O coordination (Fig. 8) [74]. Another possibility for “by-product” ligands to appear in the coordination sphere of metals is from the reaction of solvent with some precursor (e.g. MeOCS₂ ⁻ ligand formed from reaction of MeOH and CS₂, used as sulfur source) [75]. The concept of intentional ligand modification during the assembly of metal chalcogenide clusters via solvothermal and ionothermal approaches has been developed recently, aiming at broadening the range of possible ligands and gaining access to new moieties that are unreachable under milder synthetic conditions. In this vein, a C−S cross-coupling reaction under hydrothermal conditions was systematically studied for in situ ligand reactions between mono-halide-substituted pyridines (L=Hal-C₅H₄N) and thiophenol during the preparation of [Zn₈S(SPh)₁₄L₂] [76]. Varying the nature and position of the halide substituent allowed to observe that ligands containing iodine as a substituent were, unexpectedly, unreactive under the conditions explored, despite the fact that iodide is the best leaving group in comparison to F⁻, Cl⁻ or Br⁻. The lack of reactivity of iodide-substituted pyridines was attributed to the higher energy barrier for iodide elimination during the hydrothermal process in comparison to the other halide-substituted pyridines. It was also found that with a ligand containing the substituent in the ortho-position, no crystalline product was obtained, whereas the use of ligands with substituents in meta- and para-positions (e.g. 3-chloropyridine and 4-chloropyridine) led to the crystallization of clusters with in situ prepared ligands at the vertexes (L=m-C₆H₅SC₅H₄N and p-C₆H₅SC₅H₄N, respectively). Such selectivity was attributed to the spatial hindrance induced by the cluster [Zn₈S(SPh)₁₄L₂]. Overall, the successful one-pot-synthesis of clusters with tailored ligands demonstrates the potential of in situ ligand-generating reactions under solvothermal and ionothermal conditions in constructing functional metal chalcogenide clusters, simultaneously building a new bridge between coordination chemistry and synthetic organic chemistry.

The selection and in-situ design of ligands provide potential to modify metal chalcogenide clusters on several levels, tailoring cluster size and composition by adjusting the coordinating ability of the ligands and regulating superstructure topology by changing cluster–cluster interactions. The latter can be illustrated on the example of the neutral C₂ clusters [Cd₃₂S₁₄(SR)₃₆L₄], where R is either the phenyl [70] or the 2-hydroxypropyl [77] group and L is dmf or water respectively. The strong influence of ligands on the superstructure packing is such that the thiophenolate-stabilized Cd32 clusters crystallize into cubic superstructure (space group P32) sustained by van der Waals ligand–ligand intercluster interactions, whereas the thiopropanol-stabilized Cd32 clusters crystallize into a double layer superstructure (space group \( R\overline{3} \)) with a continuous network of hydrogen bonding. As another important function, an increased solubility of clusters due to the presence of organic surface ligands (especially those with modified properties, such as fluorinated ligands [78–80]) can also enhance the crystallization of clusters into superlattices [81]. The recent preparation of various mononuclear metal complexes with the perfluorinated chalcogenolate ligands [82, 83] that potentially can be used as precursors for the large cluster synthesis lays the foundation for future progress in this field.

Ligands are also known to influence the photophysical properties of metal chalcogenide clusters. For instance, phenylchalcogenolate ligands were reported to quench CdE clusters emission at room temperature, which was attributed to the existence of non-radiative relaxation mechanism that involves vibrating modes of the bridging μ-PhE⁻ ligands [20]. In contrast to this, the replacement of PhE⁻ by Hal⁻ ligands results in red shifts and significant enhancements of the emission [84] and absorption [85] peaks. Moreover, optical properties of clusters can be affected by trapping of organic species in ligand shell via cation–π interactions [86], which may potentially be used in various sensing systems. Generally, the electronic and photophysical properties of smaller clusters were found to be more sensitive to changes in the ligand shell. The influence of ligands becomes less pronounced with increasing cluster size; this was observed experimentally and confirmed by theoretical calculations at DFT and TDDFT levels for tetrahedral clusters belonging to different series (e.g. see [87]). The incorporation of ligands with special functionality (such as those containing ferrocene derivatives) can also introduce electrochemical functionality onto the clusters [88–90]. Ligand exchange reactions provide even more opportunities for tailoring metal chalcogenide clusters; the approach was proven to be efficient for the preparation of neutral Cd₁₀E_x clusters with dendritic thiolate ligands [91] or with poly(ethylene glycol) units directly attached to the core [92], featuring high solubility in organic solvents and water, respectively, as well as modified photophysical properties.

2.5 Non-tetrahedral Clusters

Non-tetrahedral clusters possess diverse frameworks and have no obvious structural similarity with the corresponding bulk crystalline metal chalcogenides [6, 9]. In this review (see Sects. 5.5 and 5.6), the focus will be on the discrete assemblies where basic tetrahedral {ME₄} units are joined together into polymeric fragments through sharing of vertexes and/or edges so as to form one or several rings. For instance, large, “double-decker” rings and complex cages have been prepared recently using solvothermal and ionothermal approaches. Metal cations here belong to groups 13 (In³⁺) and 14 (Ge⁴⁺, Sn⁴⁺), or transition metals (Mn²⁺), and E is a heavier (Se, Te) chalcogen. Such clusters can be viewed as molecular analogues of polymeric 1D chains [93, 94], typical for compounds of group 13 and 14 elements, and more unusual 1D ribbons [95], also prepared under solvothermal and ionothermal conditions. Complex vertex-linkage or the coexistence of vertex- and edge-linked basic tetrahedral {ME₄} units was previously also found in some 3D metal chalcogenides [96, 97]. The tendency of the repeating fragments, composed of linked {MSe₄} or {MTe₄} units, to cyclize can be attributed to the larger atomic size and, as a consequence, the higher structural flexibility of Se²⁻ and Te²⁻ in comparison with S²⁻. These clusters are typically charge-balanced, templated and stabilized by bulky imidazolium-based cations or other organic amines (see Sect. 5.6).

3 Bonding in Materials Containing Metal Chalcogenide Clusters

3.1 Bonding in Cluster Superstructures

Crystalline solids containing spatially organized metal chalcogenide clusters can be categorized into several classes depending on the nature of the bonding in the superstructure. Clusters may form covalently linked “continuous” frameworks of various types (i.e. 3D networks, 2D layers or 1D chains), or alternatively, with an absence of such interconnected species, metal chalcogenide clusters are “isolated” or “discrete” (0D) in their crystalline superstructures.

The covalent linkage of clusters can be realized through inorganic bridges (most often, corner-sharing clusters connected at vertexes with a single E²⁻ or RE⁻ bridge) [98, 99] or through the use of organic multidentate ligands (e.g. bi- or even tetradentate tetrahedral linkers) [100–102]. In some superstructures, both inorganic and organic connectivities can coexist [103], and such covalent linkages can also be realized via more unusual species, e.g. metal complexes [104, 105]. Superstructures with covalent linkages between tetrahedral clusters have been extensively studied and several reviews were published [47, 48]. They are not the main subject of this review and only selected cases (featuring exceptional clusters, prepared under solvothermal or ionothermal conditions) are discussed in the following sections. Large tetrahedral metal chalcogenide clusters (e.g. T₄ and T₅), covalently linked into superstructures, are well established, while the preparation of the corresponding discrete analogues remained a formidable challenge until recently.

An interesting type of bonding in such superstructures is realized when metal chalcogenide clusters form dimers, i.e. two clusters are linked via covalent bonds, and then such dimers are self-assembled into a non-covalent superstructure. This type of bonding of two clusters was achieved, for example, under solvothermal conditions using 1,2-di(4-pyridyl)ethylene (dpe) ligands as the organic linker, covalently bonding two vertices of two T₃ clusters (Fig. 9) [106]. In each “half” of such T₃-T₃ two-cluster anion [Ga₁₀S₁₇HL₂-dpe-Ga₁₀S₁₇HL₂]⁶⁻, the remaining two vertexes are terminated by L=3,5-dimethylpyridine, while the fourth vertex contains a SH⁻ anion. Total electroneutrality is achieved via 3,5-dimethylpyridinium cations. The self-assembly into a non-covalent superstructure (space group \( P\overline{1} \)) is realized though π–π interactions between aromatic rings and N−H∙∙∙S hydrogen bonding between protonated organic cations and surface S atoms in clusters. Even more sophisticated coupling is realized in solvothermally prepared crystalline solids containing a C₁–C₁ two-cluster neutral component, double bridged by the more flexible bifunctional organic ligand 1,3-di(4-pyridyl)propane (dpp) [Cd₁₇Se₄(SPh)₂₆-(dpp)₂-Cd₁₇Se₄(SPh)₂₆] (Fig. 10) [107]. Such dimers are subsequently assembled into a non-covalent superstructure (space group P2 ₁/c). Non-covalent superstructures, containing cluster dimers, allow the intercluster connectivity with organic linkers to adjust system performance (through the combination of the size-related properties of nanodimensional clusters with functionality of bifunctional ligands), at the same time preserving the solubility of individual components.

As opposed to covalent intercluster bonding, metal chalcogenide clusters can be considered as being discrete molecular entities when the superstructure is formed only via electrostatic bonding and/or other cluster–cluster interactions, e.g. hydrogen bonding and dispersion (van der Waals) forces. In such cases, the superstructures of metal chalcogenide clusters can be referred to as molecular crystals [21]. Such superstructures of smaller clusters prepared by a coordination chemistry approach are especially well documented [70, 77, 108–117]. In contrast, the preparation of progressively larger, discrete metal chalcogenide clusters (with several composition restrictions related with maintaining both local and total electroneutrality, in addition to low solubility of formed clusters) requires special conditions for superlattice formation. Recent successes (e.g. a superlattice of discrete “full-core” T₅ clusters) [43, 44] are closely connected with developing solvothermal and ionothermal approaches together with a better understanding of the role of various factors associated with these synthetic routes.

Since anionic clusters dominate this area, discrete ionic superstructures are most likely to form. Less common, neutral metal chalcogenide clusters, typically with phenylchalcogenolate ligands and/or aromatic ring-containing structure-directing and stabilizing species, can form discrete superstructures through relatively weak ligand–ligand and ligand–template–ligand interactions. The intercluster bonding (for instance, hydrogen or π–π interactions) is such that connection between the building blocks into a superstructure is reversible [118–121]. The key factor is whether superstructure disassembly (e.g. via dissolving in a suitable solvent) would be possible in such a way that the core and ligand shell of individual clusters does not change. Several cases of complete recrystallization of superstructures consisting of large discrete clusters were reported under relatively mild solvothermal conditions. Disassembly of the crystalline solid, while clusters went into solution at elevated temperature and pressure, was followed by recurring superstructure formation upon cooling [41, 122].

The solubility of the discrete large tetrahedral clusters broadens their potential for application, making possible, for instance, solution processing to achieve new advanced materials. Thus, mesostructured materials and even porous gels and aerogels were prepared using small metal chalcogenide clusters (e.g. [Ge₄S₁₀]^4–) as building blocks; such materials may be useful in photocatalysis or in the removal of heavy metals from water [123]. The production of semiconductor-doped thin-film materials for optics and electronics has also been proposed [70]. Thus, polyvinylcarbazole films, functionalized by [Cd₃₂S₁₄(SPh)₃₆(dmf)₄], can be spin-coated from a pyridine solution.

The nature of the bonding in superstructures is known to influence the physical properties of cluster assembly. In some cases, the effect of connectivity of the clusters is less pronounced in comparison with the effect of cluster size and composition, as it can be followed, for example, for the optical properties of the systems [41, 49]. In other cases, these (inter)cluster features are found to be of comparable importance: it was shown that the photocurrent response of solvothermally prepared material containing a 3D covalent framework of [Cd₃₂S₁₄(SPh)₄₀]⁴⁻ clusters (corner sharing through PhS⁻ ligands) synthetically integrated with a metal-complex dye is seven times larger than that of the material where the identical clusters are discrete. This was attributed to the facilitated transfer of photo-induced electrons in the 3D framework [75].

3.2 Topology of Superstructures

For topological consideration on the level of superstructure, it is convenient to view each tetrahedral cluster as a tetrahedral pseudo-atom (T) or, alternatively, to consider only the positions of the barycentres of the clusters. The covalent linkage of four-vertex-connected tetrahedral clusters (often realized by a single E²⁻ bridge) is known to give a limited number of topologies for 3D superstructures [47], which is related to the limited flexibility of the T−E−T angles [46, 124]. The common topological types for covalently linked large tetrahedral clusters are (cubic) single and double diamond, as well as cubic carbon nitride. In the latter, four connected clusters are combined with tri-connected S²⁻ sites that bridge the corners of three adjacent clusters [125]. The covalent linkage of tetrahedral clusters with auxiliary organic ligands L, most often pyridyl-based ones, helps to increase the flexibility of the T−L−T connection, which potentially broadens the range of the possible topologies. The cluster connectivity in such cases rarely reaches four and the coordination polymers are most often prepared as 1D and 2D superstructures. An exception is a series of 3D four connected covalent superlattices where T₃ or T₄ units are linked by imidazolate ligands [126].

The wide variety of the nature and relative weakness of interactions leading to the formation of superstructures from discrete clusters leads to the remarkable diversity in connectivities and makes it more difficult to generalize corresponding topological types. Various distortions also complicate this assignment. For instance, considering the barycentre positions, superlattices with distorted cubic diamond and hexagonal diamond topologies have been often reported for large anionic tetrahedral clusters (Fig. 11) (e.g. see [44]). This means that intra- and inter-cluster connectivities are the same, and the clusters behave like artificial atoms in zinc blende- and wurtzite-like crystal structures.

Unlike 2D and 3D covalent superstructures formed via corner sharing through inorganic linker, where topologies combining two tetrahedral clusters of different size, structure or composition are not that rare (e.g. P₁-T₂, T₂-T₅, or T_2,2-T₃ hybrid covalent superstructures) [58, 62, 65, 127–129], there are a limited number of examples of superstructures combining two different discrete clusters. Thus, ionic superstructure with a cubic [Cd₈L₁₂(NO₃)(dmf)₈]³⁺ cluster as a cation and a dumbbell-shaped [Cd₆L₁₄]^2– cluster as an anion (L = 2,5-dimethylphenylthiolate) was prepared under ambient conditions [130]. Even more unusual cases of two-cluster-anion superstructures via solvothermal preparation (e.g. co-crystallization of tetrahedral T₄ [Cu₄In₁₆S₃₅H₄]¹⁴⁻ and cubic [Cu₁₂S₈]⁴⁻ discrete anionic clusters) [122] are considered below.

In the superstructures of neutral discrete clusters, multilevel organization often takes place with the participation of several different interactions. Thus neighbouring clusters may be arranged into layer-like formation via intercluster N−H∙∙∙E or C−H∙∙∙E hydrogen bonding, with such layers further combined into superstructure through van der Waals forces [131].

3.3 Total Electroneutrality in Superstructures

As opposed to the local electroneutrality, total electroneutrality (global charge balance) refers to the overall charge density match between clusters and charge-balancing species. As was discussed above, local electroneutrality generally follows Pauling’s electrostatic valence rule, making relatively straightforward calculations possible (e.g. using Brown’s bond valence model) [132, 133] to explain/predict the arrangement of metal cations of different valence in particular cluster. In contrast, with total electroneutrality there are many different factors (among them, partial atomic charges on cluster core atoms and protonation ability of charge-balancing species) to be taken into account simultaneously, making any attempt of its quantitative representation more difficult. Thus, an additional stabilization of superstructures assembled via electrostatic (Coulomb) forces can be achieved while charge-balancing species are also capable of other interactions with clusters, e.g. N−H···E and C−H···E hydrogen bonding, π–π, anion-π and hydrophobic interactions. Aromatic quaternary ammonium cations and protonated organic amines are most important in this capacity. Some effects related with maintaining total electroneutrality are discussed below.

Even in solvothermally prepared covalently bonded 3D and 2D superstructures of clusters, where charge-balancing species are most often highly disordered, alternating the charge-balancing cations was reported to cause changes in cluster arrangement, varying from different unit cell parameters to the different packing of clusters in a superstructure. For instance, the use of the larger Et₄N⁺ cation instead of Me₄N⁺ results in a change of stacking pattern for the 2D covalently bonded superstructure of T₅ clusters [Cu₅In₃₀S₅₄]¹³⁻ (space groups Pm and C2/c, respectively) [134]. It was proposed that even small quaternary alkyl ammonium cations may show structure-directing effect in addition to charge compensation. Different protonated organic amines with well-known structure-directing ability may display even more remarkable effects: thus, under the same synthetic conditions, the addition of dipiperidinomethane instead of 1,4-bis(3-aminopropyl)piperazine leads to the solvothermal preparation of a 3D covalent superstructure of two clusters, T₃ and coreless T₅ as [In₁₀S₂₀]¹⁰⁻ and [In₃₄S₅₆]¹⁰⁻, respectively, versus that of the single T₄ cluster as [Zn₄In₁₆S₃₅]¹⁴⁻ (space groups I4 ₁/a and I4 ₁/acd) [65, 135]. It is interesting that a source of a M²⁺ d-block metal is present in the reaction mixtures probed with all amines, but M²⁺ only becomes incorporated into T₄ clusters. The formation of superstructures with substantially different charge densities (the overall framework negative charge per metal site is −0.273 vs. −0.5 for T₃–coreless T₅ and T₄, respectively) was discussed in terms of the charge densities of the incorporated protonated amine molecules, approximated by their C/N ratio (5.5 vs. 2.5 for dipiperidinomethane and 1,4-bis(3-aminopropyl)piperazine, respectively). Such an approximation is rough and cannot be generalized; for instance, the same 3D covalent superstructure of T₄ clusters [Zn₄In₁₆S₃₅]¹⁴⁻ (space group \( I\ \overline{42} d \)) was also reported with other protonated amine species, including 4,4′-trimethylenedipiperidine which has a C/N ratio of 6.5 [135].

In superstructures containing discrete clusters, additional interactions helping in stabilizing negative charges are of even greater significance. Their assembly may depend to a large extent not only on the electrostatic interactions but on hydrogen bonding as well. Though N−H···S or N−H···Se hydrogen bonding is weaker in comparison with N−H···O that is known to direct the assembly of oxide frameworks (e.g. zeolites), charge-balancing protonated organic amines in 0D superstructures of metal chalcogenide clusters are often found to be ordered and shown to play an important role in cluster formation and crystallization. A close match of charge density, geometry and additional interactions should exist between anionic clusters and cationic species in superstructures to make the formation of particular discrete clusters more favourable. The preparation of covalently bonded 3D frameworks is typically more tolerant of small variations in the size and shape of amines. For example, varying the protonation ability or steric hindrance by using similar amines (piperidine derivatives and related compounds) under the same solvothermal conditions was shown to result in the formation of different superstructures [49]. Thus, comparing o-, m- and p-methyl piperidines with the unsubstituted one indicates that the substituent in the p-position gives a superstructure of discrete T₄ clusters [Zn₄Ga₁₄Sn₂Se₃₅]¹²⁻ with a significantly larger unit cell parameter (19.2020(3) Å vs. 18.8951(1) Å for substituted and unsubstituted piperidine, respectively, space group \( I\ \overline{43} m \)). The weaker bonding in the superstructure containing protonated p-methyl piperidine is reflected, for instance, in the faster dissolution rate and increased solubility of the product, as well as in its band gap change. Both o- and m-methyl piperidines lead to the formation of related 3D covalently bonded T₄ clusters (space group I4 ₁/acd) as minor and exclusive products, respectively. It was concluded that m-position substitution creates the highest steric hindrance in comparison with o- or p-positions, not allowing such an arrangement of protonated amines around the discrete cluster, while hydrogen bonding allows for additional stabilization [49].

4 Synthetic Approaches: Solvothermal and Ionothermal Routes

Generally, a solvothermal approach refers to conducting reactions in an appropriate solvent with the aid of suitable additives in a sealed vessel at elevated temperature and autogenous pressure. If the process is done in water, the process is differentiated as hydrothermal, and in the case of other (organic) solvents, it is referred to as solvothermal. Some organic solvents widely used for the preparation of metal chalcogenide clusters are methanol, acetonitrile, DMF and organic amines. The importance of the latter (e.g. N-containing aromatic heterocycles) as solvents and additives is related to the fact that organic amines can act as ligands, stabilizers and (in a protonated form) charge-balancing species for large anionic metal chalcogenide clusters. The most recently explored variation, ionothermal process, utilizes more thermally and chemically stable ionic liquids as a reaction medium. Reaction vessels may vary from sealed thick-walled glass tubes to stainless steal autoclaves with an inert lining or inner container; a combination of the sealed in glass tube with an autoclave with some liquid for counter pressure is also possible. Under solvothermal conditions a supercritical state can be achieved, when the liquid–vapour boundary disappears and the fluid achieves properties of both the liquid and the gas though for many reactions it is not necessary and rarely applied.

In a typical solvothermal or ionothermal process, the reagents are mixed with suitable additives in a chosen reaction medium and heated to moderately high temperature for a period of time from several hours to several days, cooled to room temperature with a desired rate, and products are isolated. Syntheses of metal chalcogenide clusters by these approaches are typically performed with small-scale reactions (product weight from tens to hundreds of mg). Reported yields (% based on a metal source used) vary, although they are generally higher for smaller clusters (e.g. ~65% for P₁ [74, 136] or even ~90% for T₃ [106]) but decreasing for larger systems. Optimization of reaction conditions (such as alternating metal or chalcogen source, addition of auxiliary solvents, changing reaction time or temperature [44, 59]) can help to enhance product purity and yield.

Although smaller metal chalcogenide clusters may be used as precursors for solvothermal or ionothermal conversion into larger ones [137, 138], the synthesis often starts with simple elementary forms and inorganic salts and involves redox chemistry for cluster formation. Various clusters with different sizes and compositions can be present in solution simultaneously, while upon cooling and crystallization, equilibrium shifts in favour of one (or more) product(s). In comparison with a solid state chemistry approach, where performing the reactions in molten media (e.g. polychalcogenide flux) requires high temperatures (>300°C, often 500–650°C), the solvothermal approach offers a significant reduction in the reaction temperatures (typically ≤200°C). The flexibility of the solvothermal approach also allows an adaption to large-scale synthesis or a combination with other techniques, e.g. microwave-assisted synthesis. The combination of elevated temperature and pressure during solvothermal synthesis often allows increased solubility of precursors, promoting diffusion in reaction mixtures, improving selectivity of conversion, speeding up reactions and facilitating crystallization of the product. Performing such synthesis in ionic liquids shares some advantages with those done in traditional organic solvents (solvothermal approach), where reaction media may simultaneously act as a structure-directing agent and as a template. In this vein, ionic liquids with voluminous quaternary ammonium and imidazolium-based cations are of particular interest [37, 139]. At the same time, the negligible vapour pressure of ionic liquids makes the use of autoclaves (and associated equipment cost and safety measures) unnecessary. Generally, reaction pathway and outcome may be quite different under solvothermal and ionothermal conditions, and selection of the particular synthetic approach for each system depends on multiple factors.

5 Structures of Materials Containing Metal Chalcogenide Clusters

5.1 General Comments

As was described above, there are certain limitations and conditions for metal chalcogenide cluster formation related to maintaining local and total electroneutrality. Since the preparation of discrete tetrahedral metal chalcogenide clusters meets particular (different) restrictions depending on cluster composition, i.e. the type of metal cations present, it is reasonable to consider solvothermal and ionothermal routes to (1) clusters with M²⁺ cations exclusively and (2) clusters with M³⁺ cations, both exclusively or doped with M⁴⁺, M²⁺ or M⁺ cations, separately. Reactions where tetrahedral clusters are taken as starting reagents resulting in the preparation of new clusters are also discussed separately. As the distinct group, discrete non-tetrahedral metal chalcogenide clusters with M²⁺, M³⁺ and M⁴⁺ cations (and mixes) are described as well.

5.2 Tetrahedral Clusters with M²⁺ Cations

M²⁺ cations of later d-block metals have been widely used for preparing metal chalcogenide clusters. Large, discrete tetrahedral clusters made of entirely group 12 metals are known for all tetrahedral cluster series. Moreover, only M²⁺ cations have been reported to yield any C_n clusters, and the largest known solvothermally prepared cluster is C₃, containing 54 metal sites.

The adjacent tetrahedral M²⁺ sites are ideal to charge-balance the inner (tetrahedral) E²⁻, which is essential for the formation of the core of large clusters. At the same time M²⁺ cations are not adequate for low-coordinated edge and vertex E²⁻ sites. Such sites tend to be occupied by chalcogenolate RE⁻ groups (most often, PhE⁻). Even with this substitution, maintaining the total electroneutrality of the clusters becomes problematic when cluster size gets larger, as the negative charge of the clusters increases considerably. This can be illustrated on C_n cluster series with M²⁺ cations [107], from C₂ to (theoretical) C₅ showing the negative charge increase of idealized clusters from 4 to 22:

C₂ [M₃₂E₁₄(EPh)₄₀]⁴⁻
C₃ [M₅₄E₃₂(EPh)₅₂]⁸⁻
C₄ [M₈₄E₅₉(EPh)₆₄]¹⁴⁻
C₅ [M₁₂₃E₉₆(EPh)₇₆]²²⁻

Note that M²⁺ cations in combination with a specific cluster geometry in the C_n series (i.e. higher ratio between low-coordinated edge and vertex and high-coordinated inner E sites) are much more favourable for preparing large tetrahedral clusters in comparison with other cluster series, where the negative charge would increase even more dramatically. This can be seen by comparing clusters with approximately the same number of metal and chalcogenide sites in the different series, e.g. C₂ [M₃₂E₅₄] and T₅ [M₃₅E₅₆]. With M²⁺ cations and all edge and vertex chalcogenide sites occupied by PhE⁻, the stoichiometry of these clusters is [M₃₂E₁₄(EPh)₄₀]⁴⁻ and [M₃₅E₂₈(EPh)₂₈]¹⁴⁻, respectively. The difference in negative charge (4 vs. 14) explains why there are multiple examples of C₂ clusters with exclusively M²⁺ cations, while the corresponding T₅ clusters are not yet known.

Thus, key synthetic strategies for large clusters with M²⁺ cations are (1) decreasing and/or (2) stabilizing the large negative charge. The first strategy can be realized by replacing four vertex negatively charged RE⁻ ligands with neutral ones (e.g. P-, N- or O-containing). The second requires using adequate charge-balancing species with charge density and geometry match, as well as complementary interactions (e.g. hydrogen, π–π and anion-π bonding) allowing them to perform roles of structure-directing and template agents for superlattice crystallization.

A number of M²⁺-containing tetrahedral clusters have been originally prepared by coordination chemistry approach and then were reproduced under solvothermal conditions. An example is the discrete neutral P₁ cluster [Cd₈Se(SePh)₁₂Cl₂L₂], where two vertexes are occupied with neutral ligands L=PCy₃, tricyclohexylphosphine, and the other two with Cl⁻ [140]. In this way, such a P₁ cluster consists of a tetrahedral anti-T₁ {SeCd₄} central unit capped by two tetrahedral {CdSe₃L} and two tetrahedral {CdSe₃Cl} units, with alkylphosphine or halogenide ligands replacing Se in regular T₁ {CdSe₄} unit. Using [Cd₄(SePh)₈]_∞ and CdCl₂ precursors with methanol as a solvent allowed rather unusual short reaction times and low temperatures (1 h at 130°C, respectively) in this case; very slow cooling to room temperature (0.3°C/min) helped product crystallization. A similar approach, based on “corner capping” with neutral ligands, was reported for the preparation of neutral discrete P₁ clusters [Zn₈S(SPh)₁₄L₂] using a series of substituted pyridine ligands, e.g. L=3-aminopyridine [136], or fused-ring heterocyclic N-containing aromatic ligands, e.g. L=4,7-phenanthroline, 5-aminoquinoline or 3-(2-thienyl)-pyridine (Fig. 12) [74]. Varying the capping ligands was shown to influence cluster–cluster interactions (leading to crystallization in different space groups belonging to triclinic or monoclinic crystal systems) and modification of the optical properties of the clusters. For instance, in room temperature PL spectra obtained in DMSO solutions, an emission band for [Zn₈S(SPh)₁₄L₂] with L=3-(2-thienyl)-pyridine is substantially narrower and blue shifted in comparison with the corresponding band for the clusters with L=5-aminoquinoline (~350 and 476 nm, respectively). In contrast, no emission was observed at room temperature for the clusters with L=4,7-phenanthroline [74], which demonstrates that photophysical properties of such clusters can be strongly influenced by ligands.

The “corner capping” with neutral ligands, occurring through the formation of M−O bonds at all four vertexes of a tetrahedral cluster, was also used to decrease the charge of even larger frameworks, resulting in the crystallization of discrete tetra-anionic C₃ clusters [Cd₅₄S₃₂(SPh)₄₈(H₂O)₄]⁴⁻ and [Cd₅₄Se₃₂(SPh)₄₈(H₂O)₄]⁴⁻ [56]. Water ligands (replacing PhS⁻ sites at each vertex) arise from the use of the mixed solvent system (acetonitrile–water) for solvothermal synthesis with [Cd₄(SePh)₈]_∞ and thiourea/selenourea precursors. These large tetrahedral clusters (edge length 1.97 nm as measured between vertex metal sites) crystallize into noncentrosymmetric superlattices, either primitive or face-centred (space groups P23 or \( F\ \overline{43} c \), respectively) (Fig. 13). [Cd₅₄Se₃₂(SPh)₄₈(H₂O)₄]⁴⁻ has μ₃- and μ₄-Se²⁻ sites that were formed by replacing thiourea with selenourea, while all edge ligands are μ-PhS⁻. Anionic clusters were prepared with a variety of charge-balancing alkylammonium cations, i.e. tetramethylammonium, Me₄N⁺; tetraphenylphosphonium, Ph₄P⁺; and n-octyltrimethylammonium, C₁₁H₂₆N⁺. These disordered species, along with disordered solvent molecules, occupy the large voids between Cd54 units.

The solvothermal preparation of various clusters belonging to the C_n series made it convenient to follow the influence of size and composition of clusters on their optical properties. Thus, a systematic blue shift of the low-energy absorption peak (from 353 through 327 to 291 nm) was observed with cluster size decrease from [Cd₅₄S₃₂(SPh)₄₈(H₂O)₄]⁴⁻ through [Cd₃₂S₁₄(SPh)₄₀]⁴⁻ to [Cd₁₇S₄(SPh)₂₆(H₂NCSNH₂)₂]. The effect of cluster composition (for a given [Cd₅₄E₃₂(SPh)₄₈(H₂O)₄]⁴⁻ cluster size) was demonstrated by a red shift (from 353 to 393 nm) upon changing from sulfur to the heavier selenium in the cluster core [56].

The use of (Me₄N)₂[Cd(EPh)₄] as a single source precursor in DMF solvent allowed for the solvothermal preparation of the all-selenium analogue [Cd₅₄Se₃₂(SePh)₄₈(dmf)₄]⁴⁻ (Fig. 14) (Levchenko TI, Huang Y, Corrigan JF, unpublished results) and even larger CdS clusters (with the size as large as C₄ and C₅ mentioned above) [141], although orientation flexibility of the latter within the superlattice hampers single-crystal characterization. Based on series of analyses, including TEM and electron tomography, these clusters break the trend in the capped tetrahedral series and have a truncated tetrahedral shape [141, 142].

The co-crystallization of anionic metal chalcogenide clusters with counterions having special functions (e.g. organic chromophores) enables uniform molecular-level integration of inorganic and organic components to obtain new functional materials with synergistic properties. For example, the solvothermally prepared combination of the discrete P₁ anionic cluster [Zn₈S(SPh)₁₅H₂O]⁻ with the fluorescent dye acridine yellow G through the formation of ion-pair charge transfer salt [C₁₅H₁₆N₃][Zn₈S(SPh)₁₅H₂O] gives rise to the new crystalline material (space group C2/c) (Fig. 15), in which the metal chalcogenide framework serves as the electron donor and augments the colour of the fluorescent dye [23]. Experiments on labelling bacteria (e.g. E. coli) using a suspension of this material show that a combination of fluorescent dye and metal chalcogenide cluster was efficient for staining under confocal microscopy conditions with minimal photobleaching over time, while fluorescent imaging of bacteria with acridine yellow G on its own was much less stable.

Although the co-crystallization of metal chalcogenide clusters and optically active species can also be achieved using conventional synthesis [143–145], such integration was shown to be enhanced even under mild-temperature solvothermal conditions. Moreover, an additional feature in the latter case is the possibility to realize a “one-pot synthesis”, when the assembly of large anionic clusters is combined with their co-crystallization with functional cations. When such cations represent fused-ring aromatic compounds, they can play an even more complex role, combining additional functionality, charge balancing and superlattice stabilization (e.g. through π–π interactions with PhE⁻ ligands of clusters). This was realized, for instance, with the solvothermal preparation of the discrete T₃ cluster [Zn₁₀S₄(SPh)₁₅Cl]⁴⁻, co-crystallized with methylviologen cation dye ([C₁₂H₁₄N₂]²⁺ or MV²⁺) to give ion-pair charge transfer salt (MV)₂[Zn₁₀S₄(SPh)₁₅Cl] [24]. The resulting crystalline material shows a remarkable red shift (>200 nm) of a broad absorption band in solid-state spectra in comparison with that of the individual components; such a shift was assigned to a charge transfer from the electron-rich metal chalcogenide cluster anions to MV²⁺ cations. Similar integration with the MV²⁺ cation was achieved for discrete C₁ clusters [Cd₁₇Se₄(SPh)₂₄Br₄]²⁻ [146]; cyclic voltammetry showed a low-potential shift of the MV²⁺ cations in this ion-pair charge transfer salt in comparison with MVBr₂, which indicates that strong cation–anion interaction was preserved even upon dissolving in DMF. Examination of photocurrent responses of (MV)[Cd₁₇S₄(SPh)₂₄Br₄] and (MV)[Cd₁₇Se₄(SPh)₂₄Br₄] showed that the current intensities of the ion-pair charge transfer salts are significantly larger than those of the similar clusters [Cd₁₇E₄(SPh)₂₈]²⁻ with (Me₄N)⁺ cations; the MV²⁺ cation was found to play different roles in electron transfer under visible light or UV irradiation [146].

Optically active metal-chelate dyes (e.g. complexes of M²⁺ with 1,10-phenanthroline, phen, or 2,2′-bipyridine, bpy, ligands) further extend the approach for the assembly of integrated materials through cation–anion interactions involving tetrahedral metal chalcogenide clusters. Bulky cations [M(phen)₃]²⁺ and [M(bpy)₃]²⁺, formed in situ during the solvothermal process, are comparable in size with large tetrahedral clusters and can additionally play the role of space-filling (template) species. Geometry match in this case is accompanied by charge density match: compared to widely used quaternary ammonium cations and protonated organic amines, the metal-chelate dyes possess both a large size and relatively low charge density, which fits the low charge density of large anionic tetrahedral clusters belonging to the C_n series. Hydrophobic and π–π interactions between fused-ring N-containing aromatic ligands of such cationic species and surface PhE⁻ ligands of anionic clusters also contribute to superlattice stabilization. Thus, the discrete C₂ anionic clusters [Cd₃₂S₁₄(SPh)₄₀]⁴⁻ were solvothermally prepared and integrated with the metal-chelate dye cations [Fe(phen)₃]²⁺ (Fig. 16) [147]. The use of a bulkier ligand (namely, 3,4,7,8-tetramethyl-1,10-phenanthroline, tmphen) instead of phen as in [Fe(phen)₃]₂[Cd₃₂S₁₄(SPh)₄₀] leads to crystallization of [Fe(tmphen)₃]₂[Cd₃₂S₁₄(SPh)₄₀], having different packing of the same tetrahedral clusters (space groups P2 ₁/c and \( P\overline{1} \), respectively). The optical properties and photoelectrochemical performance of the composite material can be tuned by varying the cluster size, changing the type of metal centres or organic chelating ligands; for instance, the advantage of Ru²⁺ over Fe²⁺ in metal-complex dyes was demonstrated [75].

5.3 Tetrahedral Clusters with M³⁺ and Mixed Cations

In contrast to M²⁺ cations, the formation of discrete tetrahedral clusters composed entirely of trivalent metal ions is limited to relatively small species. The observation that clusters having interstitial chalcogenide atoms (e.g. larger than T₃) are unlikely to form is in accordance with Pauling’s electrostatic valence rule, as the adjacent tetrahedrally coordinated M³⁺ sites would overburden the total bond valence of tetrahedrally coordinated E²⁻ sites. Therefore, access to large tetrahedral clusters with M³⁺ cations requires the presence of lower valence metals (M²⁺ or M⁺) in the inner sites to maintain the local electroneutrality. A classical example is the T₄ cluster [Cd₄In₁₆S₃₅]¹⁴⁻ present in 3D covalent superstructures [148]. At the same time, M³⁺ cations usually provide enough bond valence to balance low-coordinated surface E²⁻ sites, which eliminates (or decreases) the need for surface ligands. That is why tetrahedral clusters with M³⁺ surface sites can exist as “naked” species, although ligands at vertexes are still useful to prevent covalent linkage into 3D and 2D condensed frameworks. The common challenge for the preparation of large tetrahedral clusters, already addressed while discussing systems with M²⁺ cations, is related with maintaining the total electroneutrality, as the negative charge of the clusters increases with their size increase. The incorporation of lower valence metals into a M³⁺ system, unavoidable to keep the local electroneutrality in large tetrahedral clusters, simultaneously complicates maintaining the total electroneutrality by contributing to an increase in negative charge. This can be illustrated by comparison of the (hypothetical) binary and (isolated) ternary cluster compositions, e.g. T₄ [In₂₀E₃₅]¹⁰⁻ vs. T₄ [Cd₄In₁₆E₃₅]¹⁴⁻ and T₅ [In₃₅E₅₆]⁷⁻ vs. T₅ [Cd₁₃In₂₂E₅₆]²⁰⁻.

The synthetic strategies used with mixed-metal systems based on M³⁺ cations are also related to (1) decreasing and/or (2) stabilizing the large negative charge, as was discussed above for M²⁺ systems, while the arsenal of solutions is more diverse and includes both similar routes (as “corner capping” the cluster with neutral ligands) and those specific to mixed systems. Thus, introducing M⁴⁺ cations onto surface (most often, vertex) sites helps in reducing the overall cluster negative charge, also providing more flexibility to adjust charge density of the system. A general way towards large tetrahedral clusters here assumes varying the ratio between multiple metal ions in different oxidation states (e.g. M⁴⁺/M³⁺/M²⁺, M⁴⁺/M³⁺/M⁺ or even M⁴⁺/M³⁺/M²⁺/M⁺) and meticulous selection of charge-balancing species with geometrical, charge density and mutual interaction match. The preparation of tertiary (and more complex) metal chalcogenides can often be complicated by phase separation, with M⁴⁺, M³⁺ or M²⁺ cations forming stable chalcogenides on their own. Solvothermal and ionothermal synthesis with suitable additives (charge-balancing, structure-directing and space-filling species, with possibility to blend all those functions in just one compound) provide favourable conditions to facilitate integration of different metal cations into the same cluster. Some particular cases illustrating the mentioned synthetic strategies and approaches, starting from those common between M²⁺ and M³⁺ tetrahedral cluster systems, are described below.

The “corner capping” with neutral N-containing aromatic ligands in a purely M³⁺ system was achieved, for example, in the preparation of the discrete anionic T₃ cluster [Ga₁₀S₁₆L₄]²⁻, where all four vertexes are occupied by L=3,5-dimethylpyridine, covalently attached via the formation of Ga−N bonds [149]. Each anionic cluster is charge-balanced and additionally stabilized with two monoprotonated 3,5-dimethylpyridine cations; despite the disorder of the cationic species, the orientation of the heterocyclic aromatic ring parallel to cluster faces can be distinguished (Fig. 17).

The idea of using fused-ring heterocyclic N-containing additives to corner-cap, charge-balance and stabilize large tetrahedral clusters also resulted in the solvothermal preparation of several discrete clusters with size from T₃ to T₅ and edge lengths reaching 1.55 nm (as measured between vertex metal sites) [43]. Prior to this work, T₅ clusters were known only in 3D and 2D covalently linked superstructures. In the discrete anionic T₅ cluster [Cd₁₃In₂₂S₅₂L₄]¹²⁻, four vertexes are capped by L=1-methylimidazole (mim), ligands, and negative charge of the cluster is balanced by protonated forms of organic superbase 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU) and Li⁺ cations. The Cd²⁺ sites in the inner fragment {Cd₁₃S₄}, containing four tetrahedrally coordinated S²⁻ sites, are mandated by local electroneutrality requirement, while edge and corner In³⁺ sites alleviate the otherwise low-coordinated surface S²⁻ sites. The orientational disorder of mim ligands and charge-balancing H⁺-DBU species did not allow their precise location to be determined in the superstructure of the T₅ cluster (space group I4 ₁/amd), although their presence was confirmed with a series of analyses. Single-crystal X-ray diffraction analysis of the smaller T₄ anionic cluster [Cd₄In₁₆S₃₁L₄]⁶⁻, prepared by the same “superbase route”, allowed location of the capping ligands L=1,5-diazabicyclo[4.3.0]non-5-ene (DBN) at vertexes and charge-balancing H⁺-DBN species, which create a stabilizing “cocoon” around the cluster (space group I4 ₁/a, see Fig. 18).

The T₅ cluster [Cd₁₃In₂₂S₅₂(mim)₄]¹²⁻ exhibits distinct, broad emission in the solid state at room temperature with the maximum observed at 512 nm (fwhm ~70 nm); a band gap of 2.87 eV was calculated from the diffuse reflectance UV−vis data. Both absorption and emission bands were found to be red shifted in comparison with those of smaller clusters (e.g. T₄ [Cd₄In₁₆S₃₁(DBN)₄]⁶⁻ with a band gap 3.27 eV) as result of both size increase and composition change [43].

Other derivatives of imidazolium salts were also useful to provide access to extra-large supertetrahedral metal chalcogenide clusters in a “corner capping” approach. Performing syntheses in the ionic liquid [Bmmim]Cl (where Bmmim=1-butyl-2,3-dimethylimidazolium) allowed the combination of charge-decreasing (partially), charge-balancing and charge-stabilizing functions in one compound, which also served as the reaction medium. This resulted in the preparation of several discrete anionic T₅ clusters, including (Bmmim)₁₂(NH₄)[Cu₅In₃₀S₅₂(SH)₂Cl₂] and the first Ga-based T₅ cluster (Bmmim)₈(NH₄)₃[Cu₅Ga₃₀S₅₂(SH)₂(Bim)₂] [44]. In the latter, the corner-capping ligand Bim (1-butyl-2-methyl-imidazole) is generated by in situ decomposition of the IL. The relatively unusual precursor, [H⁺-en]₂[Ga₄S₇(en)₂], was separately prepared by solvothermal synthesis in ethylenediamine (en) and used as the Ga source, with In₂S₃ as the In source. In T₅ clusters with mixed M⁺ and M³⁺ cations, the central metal site, surrounded by four tetrahedrally coordinated S²⁻, should be a Cu⁺ cation, and each inner tetrahedrally coordinated S²⁻ anion should be bonded with two Cu⁺ and two M³⁺ cations in order to maintain local electroneutrality. According to this, in each cluster one Cu⁺ cation occupies solely the central metal site, while four Cu⁺ cations are statistically distributed along with M³⁺ cations in the other 12 metal sites of the inner {M₁₃S₄} fragment (Fig. 19). Most of the [Bmmim]⁺ cations are located between the tetrahedral faces of two T₅ clusters, and the imidazolium rings of [Bmmim]⁺ cations are oriented such to be parallel to the nearby cluster face (Fig. 19). The closest distances between S²⁻ on the face of the cluster and the centre of imidazolium rings are such that the presence of anion–π interaction was assumed. C−H···S hydrogen bonding and anion–π interactions also help to stabilize the large anionic clusters.

Both In- and Ga-based T₅ clusters show emission in solid state at room temperature but the obtained spectra are remarkably different. Thus, [Cu₅In₃₀S₅₂(SH)₂Cl₂]¹³⁻ shows a distinct asymmetric emission band at 540 nm (fwhm ~50 nm), while [Cu₅Ga₃₀S₅₂(SH)₂(Bim)₂]¹¹⁻ shows an unusual broad emission band at 630 nm with fwhm of ~180 nm. Calculated from the diffuse reflectance UV−vis data, band gaps are 2.28 and 3.68 eV for [Cu₅In₃₀S₅₂(SH)₂Cl₂]¹³⁻ and [Cu₅Ga₃₀S₅₂(SH)₂(Bim)₂]¹¹⁻, respectively, exhibiting a blue shift compared to the bulk CuInS₂ (1.53 eV) and CuGaS₂ (2.40 eV) [44].

An approach to decrease the charge of anionic clusters, complementary to the use of the “corner capping” neutral organic ligands, was realized via covalent termination of the cluster vertexes with complex metal cations. In this case, instead of replacing the vertex E²⁻ sites in tetrahedral clusters, longer E-ML_n units are formed with participation of four vertex E atoms, where M is a transition metal and L organic ligand. Thus in the discrete T₃ cluster [Zn₂Ga₄Sn₄Se₂₀]⁸⁻, introducing Sn⁴⁺ cations onto four vertex sites contributed to a decrease in the negative charge, while the attachment of four metal complexes [Mn(L)]²⁺ with the polydentate organic ligand L = C₈H₂₃N₅, tetraethylenepentamine (tepa), covalently terminates all cluster vertexes and charge-balances the framework [131]. In the in situ formed metal complex [Mn(tepa)]²⁺, the Mn atom is coordinated with five N sites from the organic ligand and one vertex Se site of the tetrahedral cluster, thus having a distorted octahedral environment. Hence, the distribution of Mn²⁺ and Zn²⁺ cations in the clusters (octahedral and tetrahedral coordination, respectively) results from the different coordination abilities of these metals. The ligand tepa also serves as the reaction medium in the solvothermal synthesis. The resulting neutral clusters with pendent metals, [Mn(tepa)]₄[Zn₂Ga₄Sn₄Se₂₀] (Fig. 20), assemble into a superlattice (space group \( P\ \overline{4} b 2 \)) with different levels of ordering provided by different intercluster forces: hydrogen bonding N−H···Se between tepa ligands on one cluster and Se sites on the face of the adjacent cluster give a layered arrangement parallel to the (001) plane, while the layers are further packed into 3D superlattice through van der Waals interactions. Hence, the metal complexes [Mn(tepa)]²⁺ at the four cluster vertexes not only allow charge balance but also act as structure-directing agents for superstructure assembly.

The isostructural [Mn(teta)]₄[Mn₂Ga₄Sn₄S₂₀], also covalently terminated with metal-complex cations ML_n, was solvothermally prepared using the shorter C₆H₁₈N₄, triethylenetetramine (teta), as both solvent and polydentate ligand [150]. Further shortening the length of the organic ligand in the metal-complex cation (L = C₄H₁₃N₃, diethylenetriamine (dien)) changes not only the hydrogen bonding-governed assembly of clusters into a superstructure (space group C2/c) but the cluster composition itself, leading to formation of discrete anionic T₃ clusters [Mn₂Ga₄Sn₄S₂₀]⁸⁻ charge-balanced and stabilized by [Mn(dien)₂]²⁺ cations (Fig. 21) with additional hydrogen N−H···S bonding (in the absence of covalent bonding) between negatively charged cluster and positively charged metal-complex. However, the use of a bidentate ligand as an extreme case of shortening (L = C₂H₈N₂, ethylenediamine (en)) under similar reaction conditions results in the formation of a covalently bonded 1D superstructure, where anionic clusters [Mn₂Ga₄Sn₄S₂₀]⁸⁻ are interlinked by two pairs of unsaturated metal-complex cations [Mn₂(en)₅]⁴⁺ via Sn–S–Mn covalent bonds.

While metal-complex cations such as [M(phen)₃]²⁺ and [M(bpy)₃]²⁺ are used to template, charge-balance and stabilize the formation of anionic metal chalcogenide clusters, enhanced optical properties (due to cation–anion charge transfer) are also incorporated. Such integrated materials are formed to a great extent in a similar manner as was discussed above for pure M²⁺ systems (with surface PhE⁻ ligands), except here there are no additional π–π and hydrophobic surface interactions in the case of naked T_n clusters. Some discrete anionic clusters prepared under solvothermal conditions using this approach are the In³⁺-containing T₃ clusters [Ni(phen)₃]₃[In₁₀S₂₀H₄] (Fig. 22, left) [73] and [Ni(bpy)₃]₃[In₁₀S₂₀H₄] [144], where phen and bpy ligands on three metal complex cations provide steric hindrance and an aromatic environment to template and stabilize the metal chalcogenide frameworks (Fig. 22, right). Similarly, the iron-doped T₄ cluster [Fe(bpy)₃]₃[Fe₄In₁₆S₃₅H₂]∙4H⁺-tea∙2H⁺-bpy can be prepared, with additional charge balance with protonated triethylamine (tea) and protonated bipyridine [151].

In the examples addressed above, decreasing and balancing the charge and stabilization of large anionic clusters was achieved by (1) covalent capping/terminating of cluster vertexes by neutral and cationic groups and/or by (2) non-covalent (e.g. ionic and hydrogen bonding or van der Waals forces) interactions with different species, i.e. P- or N-containing organic ligands and transition metal complexes with N-containing aliphatic or aromatic chelating ligands; often several routes are realized simultaneously. A particular case where stabilization of clusters is achieved via non-covalent interactions with only protonated forms of organic amines can also take place. Many protonated amines provide more flexibility in templating and charge-balancing of anionic metal chalcogenide clusters, in comparison, for example, with rigid metal-complex cations with phen and bpy ligands. Thus, the series of discrete anionic T₄ clusters [M_xGa_18-xSn₂E₃₅]¹²⁻, where x = 2 or 4; M = Mn, Cu and Zn; E = S and Se, was solvothermally prepared using piperidine (pr, C₅H₁₁N) as the reaction solvent [49]. Stabilization of the clusters is achieved, on the one hand, by varying the ratio between precursors (complex composition including M⁺, M²⁺, M³⁺ and M⁴⁺ metal sources) allowing charge tuning of the cluster and, on the other hand, by a perfect match of charge density, geometry and mutual interactions (electrostatic and hydrogen bonding) between the highly ordered protonated piperidine cations and the anionic clusters in the superstructure. Theoretical calculations at the DFT level show that the [Cu₂Ga₁₆Sn₂Se₃₅]¹²⁻ cluster has more negative charge centres at the Se²⁻ vertexes of the tetrahedron and at the central Se²⁻ site of each edge. In the superstructure of such clusters (space group \( I\ \overline{43} m \); body-centred cubic packing in unit cell), two piperidinium cations interact with Se²⁻ at each edge centre and three piperidinium cations – with each vertex Se²⁻ with the formation of strong electrostatic interactions and additional N−H···Se hydrogen bonds, so each discrete T₄ cluster is surrounded by and bonded with 24 piperidinium cations (Fig. 23). Since each piperidinium cation interacts with two adjacent metal chalcogenide clusters, it provides a total charge balance (H⁺-pr)₁₂[Cu₂Ga₁₆Sn₂Se₃₅]¹²⁻ for each cluster. The remarkable stability of such protonated amine-cluster “ion pair” was confirmed by the miniscule change of electrical conductivity upon dissolving the crystalline product in piperidine. While solvothermal synthesis was performed under similar reaction conditions but using other amines (piperidine derivatives and related compounds) possessing stronger protonation ability and/or higher steric hindrance, only the formation of 3D covalent superstructures took place, which proves the importance of a multilateral match between the protonated amine and cluster.

Solvothermal reactions in a mixed solvent system containing water and the organic “superbase” amine DBU allowed the preparation of very unusual large In³⁺-containing cluster [In₃₈S₆₅(H₂O)₆]¹⁶⁻ stabilized by H⁺-DBU [38]. This cluster is covalently bonded via dimeric [In₂S(H₂O)₂]⁴⁺ units into a 2D framework (space group Pnma). The structure of the cluster [In₃₈S₆₅(H₂O)₆]¹⁶⁻ with an overall tetrahedral shape (Fig. 24, left) is different from well-known T_n, P_n or C_n structures and can be described as a combination of an octahedral core unit {In₁₀S₁₃} (Fig. 24, right) with four tetrahedral T₂ corners {In₄S₁₀} and four hexagonal rings {In₃S₃} as faces. There are very few examples known for clusters containing both octahedral and tetrahedral coordination for metal sites; one example is the smaller anionic cluster [Mn₆Ge₄Se₁₇(H₂O)₆]⁶⁻ [152]. In the [In₃₈S₆₅(H₂O)₆]¹⁶⁻, the core unit of the cluster {In₁₀S₁₃} possesses an octahedral crystalline lattice of NaCl type and features a central μ₆-S²⁻ site. Six H₂O molecules complete the six In³⁺ sites at the face centre of the octahedral core unit. Four corner {In₄S₁₀} T₂ units are attached to the core unit {In₁₀S₁₃} via bonding between three S²⁻ sites on one face of the T₂ unit and the corner In³⁺ site of the central moiety, which enables all ten In³⁺ sites within the core to have an octahedral coordination. Therefore, both In³⁺ and S²⁻ sites in this framework have local coordination geometries that are unusual for tetrahedral metal chalcogenide clusters. A calculation of bond valence sums gives 2.078 for the central μ₆-S²⁻ site; such a value was previously considered unlikely to be found in stable systems as local electroneutrality is not maintained. Another rare exception to Pauling’s electrostatic valence rule in tetrahedral metal chalcogenide clusters is observed in the smaller covalently bonded P₁ cluster [In₈S₁₇H]⁹⁻ with a μ₄-S²⁻ site (calculated bond valence sum 2.28 instead of required 3) in the central anti-T₁ unit {SIn₄} [58]. From the number of both metal and chalcogen sites, the 2D covalently bonded cluster [In₃₈S₆₅(H₂O)₆]¹⁶⁻ (proposed notation TO₂ meant to stress the mixed tetrahedral (T)/octahedral (O) configuration of the core) exceeds the size of the discrete supertetrahedral T₅ clusters (e.g. [Cu₅In₃₀S₅₂(SH)₄]¹³⁻) [41]. Both of these tetrahedral metal chalcogenide clusters were formed due to a stabilizing “cocoon” of protonated organic “superbases”, H⁺-DBU and H⁺-DBN/H⁺-PR, respectively.

To conclude the overview of M³⁺-based tetrahedral metal chalcogenide clusters, it is worth mentioning the very unusual system where stabilization of a superstructure consisting of two different discrete anionic clusters is achieved with participation of protonated amines. Here, solvothermal synthesis in ethylenediamine results in the preparation of a binary superstructure, combining the tetrahedral T₄ [Cu₄In₁₆S₃₅H₄]¹⁴⁻ and cubic [Cu₁₂S₈]⁴⁻ discrete clusters (Fig. 25, left), with only protonated ethylenediamine species compensating the (high) charge of both anions [122]. It was proposed that [Cu₁₂S₈]⁴⁻ clusters may act as template during the formation and crystallization of [Cu₄In₁₆S₃₅H₄]¹⁴⁻. The overall ratio between these two anionic clusters in superlattice is 1:2 and each [Cu₁₂S₈]⁴⁻ is located in a cavity formed by six adjacent [Cu₄In₁₆S₃₅H₄]¹⁴⁻ (Fig. 25, right).

The red crystals of [Cu₄In₁₆S₃₅H₄]₂[Cu₁₂S₈]·32H⁺-en (space group \( R\overline{3} \)) are stable in their mother liquor in the sealed container, while they quickly degenerate to black product upon isolation from the solution [122]. The blackened crystals absorb intensely in the near-IR diapason; the absorption properties were found to be even better for the annealed product. Such remarkable near-infrared absorption properties along with photocurrent response may allow future application as a near-infrared protective material.

5.4 Reactions of Large Tetrahedral Clusters

Recently, several cases of “solvothermal insertion” have been described, where discrete tetrahedral clusters with available cavities envelop a size-fitting metal cation, leading to the formation of a new product. Precise doping is possible due to the two-step strategy, assuming (1) solvothermal preparation and isolation of host cluster crystals, followed with (2) metal insertion into the core and crystallization of a new host–guest cluster, again enhanced under solvothermal conditions. Using soluble clusters as a host is essential, as attempts of metal cation diffusion into coreless clusters covalently bonded into rigid 3D or 2D superstructures were reported to be incomplete and inhomogeneous. Doping with a single metal ion (realizing highly ordered distribution of multiple metal components in a tetrahedral cluster) is very unlikely to be achieved in a one-step preparation as multinary cluster systems often show statistical distribution of several metals over multiple possible sites to satisfy the local electroneutrality requirement. For instance, discrete T₅ clusters [Cu₅In₃₀S₅₂(SH)₄]¹³⁻ have only one central Cu site and yet 12 inner sites partially occupied by Cu⁺ (1/3 probability) and In³⁺ (2/3 probability). In contrast to this, monocopper doping into an In³⁺-based T₅ cluster was achieved in the two-step strategy, with metal solvothermal insertion into discrete coreless T₅ cluster [Cd₆In₂₈S₅₂(SH)₄]¹²⁻ (space group I4 ₁/amd) (Fig. 26, left) realized at relatively mild temperature (150 °C) in mixed solvent (DBN, PR and H₂O), leading to the preparation of the discrete T₅ cluster [CuCd₆In₂₈S₅₂(H₂O)₄]⁷⁻ (crystallized in the same space group I4 ₁/amd) (Fig. 26, right) [41]. The yield for the Cu⁺ insertion is ~70% based on the host cluster; the driving force for the reaction is proposed to be the reduction of the charge of anionic host. Also interesting is that metal insertion is accompanied by four-vertex HS⁻ sites being replaced with neutral water ligands, further decreasing the overall cluster charge. In a similar way, a single Mn²⁺ was inserted into the open T₅ [Cd₆In₂₈S₅₂(SH)₄]¹²⁻ or [Zn₆In₂₈S₅₂(SH)₄]¹²⁻ clusters, resulting in host–guest T₅ cluster with drastically changed optical properties [42]. Thus, the Mn²⁺-doped material shows a prominent red emission at room temperature with maximum at 630 nm, which is significantly red shifted in comparison with both host clusters with weak green emission (~490 nm), and traditional Mn²⁺-doped chalcogenides of group 12 metals with orange emission (~585 nm). An alkali metal cation (Cs⁺ or Rb⁺) was also ionothermally inserted into the central cavity of the hierarchical T_2,2 cluster [In₈Sn₈Se₃₄]¹²⁻ with polyselenium Se₄ chains interconnecting the clusters into a covalent 2D superstructure in a one-step process [63]. The larger size of the negatively charged cavity in the host cluster (with a “missing” {EM₄} unit in the centre in comparison with just a single M site in coreless T₅ examples above) fits alkali metal cations but not alkaline earth (Ca²⁺, Sr²⁺) or transition (Mn²⁺, Cu²⁺) metals.

5.5 Non-tetrahedral Clusters with M²⁺ Cations

Metal chalcogenide clusters with overall tetrahedral shape are the most common for large M²⁺ systems, especially those prepared by solvothermal approach, with only a few examples of other arrangements. One group of non-tetrahedral clusters includes relatively small, cagelike assemblies formed by group 12 metals where basic tetrahedra {ME₄} are linked by vertex sharing. For instance, the discrete cubic cluster [Cd₈L₁₄(dmf)₆(NO₃)]⁺ was prepared by a coordination chemistry approach using the fluorine-substituted ligand L = 3-fluorophenylthiolate [81]. In this “double four-ring” cationic cluster, eight Cd²⁺ are arranged at eight corners of a cube and bridged by twelve 3-fluorophenylthiolate ligands with S atoms being slightly out from the centre of each cubic edge. Corner Cd²⁺ sites within the cube are bonded to 3-fluorophenylthiolate, dmf and NO₃ ⁻ ligands. The related cubic [Cd₈(SPh)₁₂]⁴⁺ cluster (Fig. 27) was previously prepared solvothermally as a 3D covalently bonded MOF, linked by in situ generated tetradentate 1,2,4,5-tetra(4-pyridyl)benzene ligands, coordinated to cube vertexes via the formation of Cd−N bonds [153]. Both cagelike cationic clusters are found to contain trapped anions (NO₃ ⁻ or SO₄ ²⁻), which come from starting reagents and may additionally play the role of template and structure-directing species. The structurally related [Hg₈(μ₈-S)(SCH₃)₁₂]²⁺ cluster has an enclosed μ₈-S inside its cage [154]. It should be mentioned that such positively charged molecular clusters (as well as 3D covalently bonded frameworks of such clusters) are usually not accessible via solvothermal or ionothermal approaches. The likely reason is the difficulties with charge balancing and stabilization of the clusters and their superstructure in this case.

5.6 Ring- or Cagelike Clusters with M³⁺, M⁴⁺ and Mixed Cations

Discrete ring-shaped clusters, as well as cagelike assemblies in which metal cations are bridged by group 16 elements (oxygen or chalcogen), are relatively widespread for transition metals (e.g. some transition metal sulphide rings, giant oxomolybdate, oxothiomolybdate and polyoxometalate wheels or cages) [155–158]. In contrast, such large clusters are rather unusual for group 13 and 14 metals.

Unlike the large tetrahedral metal chalcogenide clusters which represent regular fragments of related solid-state ME, ring- and cagelike clusters possess laced structures: basic tetrahedral {ME₄} units are combined into polymeric formations (linear and branched, respectively) via vertex and/or edge sharing. The higher structural flexibility of the heavier chalcogenides allows geometrical adjustment in forming arching fragments. Chalcogenide sites are generally low coordinate (mostly μ-, seldom μ₃-E²⁻); local charge balance is maintained with high-valence metal ions. While the M:E ratio in these ring- and cagelike clusters is higher in comparison with large tetrahedral clusters (~1:2.0 vs. ~ 1:1.7, respectively), the presence of M³⁺ and M⁴⁺ cations contributes to a decrease of the negative charge. Tracery-like frameworks allow for an arrangement of a large number of charge-balancing species around the anionic cluster without steric hindrance. The effect of structure-directing and templating agents on the assembly of these structures is suggested to be of a great importance. A few known examples of their solvothermal and ionothermal preparation are described below.

The previously unknown group 15 metal ring-shaped anionic cluster [Sb₆S₁₂]⁶⁻ (formed by six corner-sharing SbS₃ pyramids) was solvothermally prepared using the multidentate amine N-(aminoethyl)-1,3-propanediamine (aepa) as a reaction solvent [159]. In situ formed [Ni(aepa)₂]²⁺ complexes serve to charge-balance, template and stabilize the ring-shaped clusters into a superstructure formed through hydrogen bonding and van der Waals interactions (space group \( R\overline{3} \), featuring two crystallographically independent ring-shaped anions with slightly different geometric parameters; see Fig. 28).

The much larger and structurally sophisticated cluster [In₁₈Te₃₀(dach)₆]⁶⁻ was solvothermally prepared in a mixed solvent of 1,2-diaminocyclohexane (dach) and water [39]. As opposed to single-chain rings like in the [Sb₆S₁₂]⁶⁻ anion, this cluster has a double-decker ring or wheel topology (Fig. 29). The structure of the highly symmetrical In₁₈Te₃₀ wheel (point group pseudo-D_3d when ignoring the dach ligands) can be viewed as a combination of six {In₂Te₆} (representing two edge-sharing basic tetrahedra ME₄) with six {InTe₃N₂} units. The latter unit is formed from the basic ME₄ tetrahedron, while one E site is replaced by two N from the chelating amine dach; it contains an unusual five-coordinated In³⁺ cation that possesses trigonal bipyramidal geometry. The organic ligand dach can be considered as “decorating”, in contrast with bridging ligands (e.g. μ-chalcogenolates) in some well-known [160, 161] or recently reported [162, 163] metal chalcogenide rings. The 2H⁺-dach∙H₂O unit, assembled by hydrogen bonding, was found positioned as an axle with H₂O molecule located exactly at the centre of the In₁₈Te₃₀ wheel. This unit is proposed to act as a template in the formation of the anionic cluster, while metal-complex cations [Mn(dach)₃]²⁺ provide additional charge-balancing, templating and stabilization of the superstructure with overall composition [Mn(dach)₃]₂[In₁₈Te₃₀(dach)₆]∙2H⁺-dach∙H₂O (space group Pnnm).

The analogous wheel-shaped cluster [In₁₈Te₃₀(dapn)₆]⁶⁻, where dapn = 1,3-diaminopropane, was prepared with such metal-complex cations as [Fe(phen)₃]²⁺ or [Ni(phen)₃]²⁺ and isolated as air-stable crystals [164]. Unlike the [In₁₈Te₃₀(dach)₆]⁶⁻ anion, where dach is chelated to the In³⁺ giving {InTe₃N₂} units, dapn was found to react as a monodentate ligand giving {InTe₃N} units with tetrahedral geometry in [In₁₈Te₃₀(dapn)₆]⁶⁻ cluster. In the superstructures with composition [M(phen)₃]₂[In₁₈Te₃₀(dapn)₆]∙2H⁺-dapn∙dapn with M = Fe or Ni (space group \( P\overline{1} \)), clockwise (Δ) [M(phen)₃]²⁺ cation couples with anticlockwise (Λ) [M(phen)₃]²⁺ through π–π interactions forming dimeric species. Such positively charged dimers are about the same size as the wheel-shaped anionic cluster [In₁₈Te₃₀(dapn)₆]⁶⁻ and bonded with the latter through electrostatic and additional anion–π interactions (Fig. 30). The solvothermal synthesis of [In₁₈Te₃₀(dapn)₆]⁶⁻ required substantially higher temperature and much longer reaction time in comparison with that of [In₁₈Te₃₀(dach)₆]⁶⁻: the optimized reaction conditions are 180°C/28–30 days and 140°C/4 days, respectively. This can be related to the use of elemental indium instead of InCl₃ and/or different properties of dapn as solvent (e.g. bp 140°C) in comparison with mixed system dach:H₂O = 7:3 (with bp of dach ~80°C). Dapn ligands are significantly disordered, while chelating and relatively more rigid dach molecules were located and refined using single-crystal X-ray analysis.

The combination of a mixed solvent of dach and H₂O with [Bmim]Br (Bmim=1-butyl-3-methyl-imidazolium) allowed for the solvothermal preparation of the binary superstructure, combining wheel-shaped [In₁₈Se₃₀(dach)₆]⁶⁻ with the triangular double-decker ring [Mn₉In₃₃Se₆₀(dach)₂₄]³⁻ clusters in a 1:2 ratio (Fig. 31, right) [165]. While the first cluster is the Se-containing analogue of [In₁₈Te₃₀(dach)₆]⁶⁻, the second is a novel discrete ring structure possessing a different topology and containing both M³⁺ group 13 and M²⁺ transition metal cations. The tangled structure of this triangular ring can be viewed as a complex combination of 27 basic {InSe₄} tetrahedral and 6 {InSe₃N₂} trigonal bipyramidal units through either vertex- or edge-sharing (Fig. 31, left). The outer diameter of resulting In₃₃Se₆₀ ring was calculated as ~2.5 nm (while measuring between two opposite Se²⁻ sites). The In₃₃Se₆₀ ring is further decorated by 9 {Mn(dach)₂} bridging units (distorted octahedral geometry for Mn), with three units on the inside, three on the outside and the other three on a same face as the ring. The discrete clusters of [Mn₉In₃₃Se₆₀(dach)₂₄]³⁻ are discernible on TEM images. The charge balance in the two-anion superstructure is achieved with combination of [Mn(dach)₃]²⁺, Mn²⁺, H⁺-dach and Cl⁻. The overall composition (deduced from both single-crystal X-ray diffraction data and a set of auxiliary analyses) is Mn₂[Mn(dach)₃]₃[Mn₉In₃₃Se₆₀(dach)₂₄]₂[In₁₈Se₃₀(dach)₆]∙(H⁺-dach)₁₁Cl₉ċ7H₂O, space group \( R\ \overline{3} c \). The assembly of the triangular double-decker ring [Mn₉In₃₃Se₆₀(dach)₂₄]³⁻ is proposed to be structure-directing and templated by a Mn²⁺ cation in the centre of the ring through Mn∙∙∙N inverse second-sphere coordination. The ionic solvent [Bmim]Br takes part in the formation of large ring-shaped anions by increasing the solubility of the products, but is not present in the final compound.

Probing the optical properties of the material containing [Mn₉In₃₃Se₆₀(dach)₂₄]³⁻ and [In₁₈Se₃₀(dach)₆]⁶⁻ clusters via UV–Vis diffuse reflectance spectroscopy showed that band gap (1.9 eV) is narrower than was expected for the nanodimensional In₂Se₃. This was attributed to a resonance effect due to a ringlike structure.

Discrete, cagelike anionic clusters of group 13 and 14 metal chalogenides are rare, especially those prepared under solvothermal or ionothermal conditions. For instance, the reaction of [K₄(H₂O)₃][Ge₄Se₁₀] and SnCl₄ċ5H₂O in [Bmmim][BF₄] with 2,6-dimethylmorpholine as an additive under ionothermal conditions yielded the discrete cagelike cluster [Sn₃₆Ge₂₄Se₁₃₂]^24– forming ordered superstructure (space group P2 ₁/c) [40]. This cluster anion is comprised of two different types of building blocks: {Ge₃Se₉}, which represents a trimer of corner-sharing basic tetrahedra GeSe₄ (Fig. 32, top left), and {Sn₆Se₁₈}, which contains a dimer of Sn₃Se₄ semicubes doubly bridged by two Se (Fig. 32, top right). A similar structural motif (i.e. {M₃Se₉} unit; see Fig. 32, top left) is also found in a smaller 72-atom supercubooctahedron cluster [Ga₁₅Ge₉Se₄₈]¹⁵⁻ _, prepared by the solid state reaction in a CsCl flux [166]. In 192-atom cluster [Sn₃₆Ge₂₄Se₁₃₂]^24–, eight {Ge₃Se₉} are located at the vertexes of a cube, while six {Sn₆Se₁₈} occupy the vertexes of an octahedron inscribed inside of this cube; the two types of units are linked via the sharing of common Se sites. The resulting cluster is nearly perfectly spherical in shape, with an outer diameter of 2.83 nm (including van der Waals radii of the surface atoms), a cavity with a diameter of 1.16 nm and 12 windows with cross sections of 0.56–0.88 nm (Fig. 32, bottom left). Similar, discrete cagelike clusters with partial metal site disorder [Bmim]₂₄[Sn_32.5Ge_27.5Se₁₃₂] was prepared in [Bmim][BF₄] and crystallized in the space group \( P\overline{1} \) (Fig. 32, bottom right). In this superstructure half of the 24 charge-balancing [Bmim]⁺ cations is arranged at the windows, while the other half is outside of the highly charged cagelike anion. The amine additive is proposed to participate in the formation of Sn-containing units, although the mechanism is not determined yet.

Potentially, such cagelike metal chalcogenide clusters with a large confined space can be used as “molecular flasks” to host species and perform reactions, as the windows of the cluster are not blocked by covalently bonded ligands. Indeed, preliminary results show that [Sn₃₆Ge₂₄Se₁₃₂]^24–can trap I₂ molecules and induce heterolytic I−I bond cleavage.

6 Summary and Concluding Remarks

In the present review, some light was shed on the preparation of large, metal chalcogenide clusters and their crystalline superstructures obtained via synthetic routes utilizing reactions in solution under increased temperature and pressure, i.e. solvothermal and ionothermal syntheses. Performing reactions in such conditions shows great potential for both tuning size and composition of a cluster core (e.g. by increased solubility and additional stabilization gained from mutual interactions with carefully chosen stabilizers and/or counterions) and a ligand shell (e.g. by ligand reactions enabled to occur in situ), which assures that materials containing metal chalcogenide clusters can be engineered at several levels.

Both solvothermal and ionothermal routes were shown to be very effective for the synthesis of new clusters with unique structural features and physical properties that are inaccessible using other techniques. An example is the highly ordered distribution of multiple metal components in a cluster, realized as doping a tetrahedral framework with a single metal ion in exact position (such as solvothermal insertion of a single Cu⁺ or Mn²⁺ into the host cluster [Cd₆In₂₈S₅₂(SH)₄]¹²⁻ [41, 42]). The Mn²⁺-doped cluster shows a prominent red emission at room temperature, which is significantly red shifted in comparison with orange emission observed in traditional Mn²⁺-doped group 12–16 semiconductors, related by size and composition but without such precisely defined order. An unusual binary superstructure, combining the tetrahedral [Cu₄In₁₆S₃₅H₄]¹⁴⁻ and cubic [Cu₁₂S₈]⁴⁻ clusters with protonated ethylenediamine species, was found to absorb intensely in the near-IR part of the electromagnetic spectrum [122].

Remarkable progress has been achieved with the preparation of progressively larger discrete clusters, with sizes that reach to several nanometers (e.g. tetrahedral clusters [Cd₁₃In₂₂S₅₂(mim)₄]¹²⁻ and [Cd₅₄S₃₂(SPh)₄₈(H₂O)₄]⁴⁻ with edge lengths measured between vertex metal sites 1.55 and 1.97 nm, respectively [43, 56]). The dimensions of such cluster cores, having structural similarity with the corresponding bulk crystalline metal chalcogenides, already overlap with those for some colloidal systems. Recently prepared ring- and cagelike clusters (e.g. [In₁₈Te₃₀(dach)₆]⁶⁻, [Mn₉In₃₃Se₆₀(dach)₂₄]³⁻ or [Sn₃₆Ge₂₄Se₁₃₂]^24– [39, 40, 165]) represent the largest non-tetrahedral frameworks. The presence of ringlike structure is associated with a resonance effect, contributing to the optical properties of such clusters.

The development of new reactants and synthetic procedures is closely connected with the availability of more advanced characterization techniques (such as sophisticated X-ray diffraction instrumentation and processing software, as well as auxiliary analyses), allowing detailed characterization of unusual structural types, e.g. confirming the nature and oxidation state of metals in multinary clusters [49].

Systematic investigation of bonding and structural principles, especially in new structure types, such as tetrahedral/octahedral cluster [In₃₈S₆₅(H₂O)₆]¹⁶⁻ [38], will provide useful guidance for the future discovery and development of new cluster-based materials for applications in various fields.

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Acknowledgements

The authors YH and JFC thank the Natural Sciences and Engineering Research Council (NSERC) of Canada for its continued support of their research programmes. TIL is most grateful to NSERC for a Canada Graduate Scholarship.

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Department of Chemistry and Centre for Advanced Materials and Biomaterials Research (CAMBR), The University of Western Ontario, London, ON, Canada, N6A 5B7
Tetyana I. Levchenko, Yining Huang & John F. Corrigan

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Tetyana I. Levchenko
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Yining Huang
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John F. Corrigan
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Correspondence to Yining Huang or John F. Corrigan .

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Fachbereich Chemie, Philipps-Universität Marburg and Wissenschaftliches Zentrum für Materialwissenschaften, Marburg, Germany
Stefanie Dehnen

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Dedicated to Prof. Dr. Hansgeorg Schnöckel on the occasion of his 75th birthday.

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Levchenko, T.I., Huang, Y., Corrigan, J.F. (2016). Large Metal Chalcogenide Clusters and Their Ordered Superstructures via Solvothermal and Ionothermal Syntheses. In: Dehnen, S. (eds) Clusters – Contemporary Insight in Structure and Bonding. Structure and Bonding, vol 174. Springer, Cham. https://doi.org/10.1007/430_2016_5

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DOI: https://doi.org/10.1007/430_2016_5
Published: 21 July 2016
Publisher Name: Springer, Cham
Print ISBN: 978-3-319-52294-4
Online ISBN: 978-3-319-52296-8
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