Intermetalloid and Heterometallic Clusters Combining p-Block (Semi

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Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

Intermetalloid and Heterometallic Clusters Combining p‑Block (Semi)Metals with d- or f-Block Metals Robert J. Wilson, Niels Lichtenberger, Bastian Weinert, and Stefanie Dehnen*

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Fachbereich Chemie und Wissenschaftliches Zentrum für Materialwissenschaften, Philipps-Universität Marburg, Hans-Meerwein-Straße 4, 35043 Marburg, Germany ABSTRACT: Clusters have been the subject of intense investigations since their famous definition launched by Cotton in 1963, and the area has expanded ever since. One obvious development addresses the widening of the definition of what to call a cluster: from purely (transition) metal−metal linked assemblies, as per Cotton’s early denomination, to nonmetal/metal clusters or purely nonmetal cages, like fullerenes, and even noncovalent aggregates such as water clusters. The other extension concerns the broadened spectrum of compositions within the aforementioned cluster types and their corresponding structures that range from trinuclear motifs to clusters with sizes in the range of the hemoglobin unit. This review article reports on one cluster family that has its origins in traditional Zintl anion chemistry but has undergone rapid development in recent years, namely, ligand-free clusters that combine main group and transition metal atoms. Depending on the position of the transition metal atom(s), one refers to such clusters as intermetalloid (endohedral) clusters or as a special type of heterometallic clusters. The predominant synthetic access makes use of soluble Zintl anions. Other pathways for their preparation include traditional solid state reactions of according element combinations or bottom-up syntheses employing low valent organo-main group element sources. This survey will shed light on all of these approaches, with an emphasis on the syntheses that employ soluble Zintl anion compounds. The article will give a comprehensive overview of the currently known compounds, their different synthesis protocols, and analytic techniques for determination of their compositions, structures, and further properties. Additionally, this survey will report peculiarities of bonding situations found within some of the cluster molecules, which were studied by means of sophisticated quantum chemical investigations.

CONTENTS 1. Introduction 1.1. Intermetalloid and Heterometallic Clusters 1.2. Synthetic Approaches 1.2.1. Preparation of Zintl Phase-Type Solids 1.2.2. Preparation of Salts of Zintl Anions 1.2.3. Preparation of (Element)Organic Derivatives of Zintl Anions 1.2.4. Preparation of Intermetalloid and Heterometallic Clusters 1.2.5. Preparation of Cationic Intermetalloid and Heterometallic Clusters in Solid State Reactions or Ionic Liquids 1.3. Experimental Characterization Methods 1.3.1. X-ray Diffraction Methods 1.3.2. X-ray Spectroscopy and Mass Spectrometry 1.3.3. Nuclear Magnetic and Electron Paramagnetic Resonance Spectroscopy 1.3.4. Quantum Chemical Studies 1.4. Concepts for Bonding in Intermetalloid and Heterometallic Clusters 1.4.1. Definition of Terms 1.4.2. Bonding Concepts 2. Binary Clusters © XXXX American Chemical Society

2.1. General Remarks 2.2. Binary Intermetalloid and Heterometallic Clusters of Group 13 Elements 2.3. Binary Intermetalloid Clusters of Group 14 Elements 2.3.1. Deltahedral Clusters with 9, 10, and 12 Vertices 2.3.2. Non-Deltahedral Clusters with 10 and 12 Vertices 2.3.3. Larger Intermetalloid Clusters of Group 14 Elements 2.4. Coordination Compounds and Heterometallic Clusters of Group 14 Elements 2.4.1. Coordination Compounds of Tt 4 4− Anions 2.4.2. Coordination Compounds of Tt94− and Tt52− Anions 2.4.3. More Complex Coordination Compounds and Heterometallic Clusters of Group 14 Elements

B B C C C C C

D D D D D E E E F F

F F H H J J L N N

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Special Issue: Frontiers in Main Group Chemistry Received: November 1, 2018

A

DOI: 10.1021/acs.chemrev.8b00658 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 2.5. Coordination Compounds and Heterometallic Clusters of Substituted Zintl Anions 2.6. Binary Clusters of Group 15 Elements 2.6.1. Binary Intermetalloid Clusters of Group 15 Elements 2.6.2. Coordination Compounds and Heterometallic Clusters of Group 15 Elements 2.6.3. Cationic Deltahedral Intermetalloid and Heterometallic Clusters 3. Ternary Clusters 3.1. Syntheses 3.2. Occupational Disorder 3.3. Ternary Heterometallic and Intermetalloid Clusters Containing Transition Metal Atoms from Groups 12 to 8 3.4. Ternary Intermetalloid Clusters Containing Lanthanide and Actinide Metal Atoms 3.5. Ternary Intermetalloid Clusters Containing Group 5 Metal Atoms 4. Reactivity and Formation Pathways of Intermetalloid Clusters 4.1. Basicity of Intermetalloid Clusters 4.2. Insertion of M−L Fragments into Intermetalloid Clusters and Ligand Exchange Reactions 4.3. Fusion of Intermetalloid Clusters 4.4. Formation and Growth of Intermetalloid Clusters 4.5. Decomposition of Intermetalloid and Heterometallic Clusters to Intermetallic Materials 5. Correlation of Electronic and Geometric Structures in Intermetalloid Clusters 5.1. Studies on Clusters with 8, 12, or 20 Vertices 5.2. Electronic Effects in Intermetalloid Cluster Families: Quantum Chemical Predictions 5.3. Electronic Effects in Intermetalloid Cluster Families with 10, 12, or 14 Vertices: Experimental Examples 6. Conclusion Author Information Corresponding Author ORCID Author Contributions Notes Biographies Acknowledgments Abbreviations References

Review

significantly larger number of (mainly main group) metal or semimetal atoms.4 They are thus reminiscent of the structural situation in intermetallic compounds, such as the Laves phases with their typically high coordination numbers. Another definition of “intermetalloid clusters” was introduced by Kempe in 2010. In this case, they were defined as polyatomic, metal−metal bonded assemblies combining dblock and f-block metal atoms, regardless of structural features or coordination numbers.5 In this article, the term will be used according to Fässler’s definition, hence classifying them as endohedral clusters only. Such intermetalloid clusters can be represented by the general formulas [Mx@Ey]q− or [Mx@ EyMz]q−, with M being a transition metal, lanthanide, or actinide atom and E representing one or more types of p-block (semi)metal atoms. The charge depends on many factors, including the cluster size, the identity of E, and the charge of the interstitial atom(s). In such [Mx@EyMz]q− clusters, x is typically much smaller than y (+z). All other combinations of main group and transition metal atoms [MxEy]q−, which lack an inner atom or an inner group of atoms, will be referred to in this article as “heterometallic clusters”. Since the seminal characterization of [Pt@Pb12]2− in 2004,6 dozens of such transition-metal-centered p-block element clusters have been reported. These have been astonishingly varied in terms of both their physical and electronic structures. Additionally, investigations into intermetalloid cluster syntheses have afforded many related heterometallic clusters. The vast majority of intermetalloid and p-block-atom-rich heterometallic clusters are synthesized using Zintl anion precursors. The field of Zintl anion chemistry has matured beyond exploratory synthesis and could be said to be in a “preapplication” phase, with their modification7,8 and controlled decomposition9−13 becoming routine. In contrast, the field of intermetalloid cluster research is still primarily one of exploratory synthesis and computational investigation. However, an impressive foundation of knowledge has been assembled, upon which future innovative research can build. One of the goals of this review is to stimulate such research by assembling all of the evidence available regarding the reactivity of these clusters, as well as the relationship between their electronic and physical structures. Many intermetalloid clusters are ligand free and can, due to their mixed-metallic nature, be viewed as molecular models of doped metals, intermetallic compounds, or alloys in general.4,14−21 Furthermore, as soluble, monodisperse, and redox active metal clusters, they are potentially interesting candidates for a variety of applications. One near-term possibility that is already being probed13 is the use of intermetalloid cluster compounds as precursors to new intermetallic phases. Such an application would be analogous to the oxidative formation of the clathrate-like germanium modification Ge(cF136) from the ubiquitous homoatomic Zintl anion Ge94−.9,22 Other potential applications could be facilitated by the postsynthetic modification of intermetalloid clusters. While Zintl anions have been extensively modified via the attachment of organic, organometallic, and element-organic ligands, success in this area is conspicuously absent for intermetalloid clusters. However, such an advancement would allow for the control of cluster solubility, the addition of functional moieties, or even the linking of clusters into a hybrid network. Progress on these and other fronts is currently hindered by experimental difficulties, such as low yields and the generation of unwanted byproducts. Therefore, fundamental research into the scalable

Q T V X Z AA AB AB

AB AD AE AF AF

AG AH AI

AJ AK AK AK

AL AN AN AN AN AN AN AN AO AO AO

1. INTRODUCTION 1.1. Intermetalloid and Heterometallic Clusters

In 1999, Schnöckel introduced the term “metalloid cluster” to describe subvalent, main group metal, molecular clusters. These feature central metal atoms that exclusively bond to other metal atoms and a total number of metal−metal contacts exceeding that of the metal−ligand contacts.1−3 Similar to this classification, yet concentrating on structural features, Fässler derived the term “intermetalloid cluster” in 2004. It denominates heteroatomic endohedral clusters with inner (usually transition) metal atoms, which are surrounded by a B


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the investigation of cluster formation reactions. In order to obtain soluble and well-defined precursors, it may therefore be desirable to prepare salts of molecular p-block polyatomic anions. To form such salts, Zintl phases are ground into a fine powder and extracted in polar amine solvents like liquid ammonia, en (1,2-diaminoethane, ethylenediamine), DMF (N,N-dimethylformamide), or py (pyridine), with a cation sequestering agent like 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) or crypt-222 (4,7,13,16,21,24-hexaoxa-1,10diazabicyclo[8.8.8]hexacosane). Examples are shown in eqs 6 and 7.64,65

synthesis of pure intermetalloid cluster compounds is still needed for the evolution of this field. As a note to the reader, this review will cover clusters in which d- and f-block metals are combined with metals and semimetals of groups 13−15 (Al−Tl, Si−Pb, As−Bi). In particular, we will focus on clusters that are majority p-block elements and are most often derived from Zintl anions. Thus, the class of transition metal carbonyl clusters comprising endohedral main group atoms falls outside the scope of this review.23−29 Furthermore, only selected examples of pnictogen−transition metal compounds will be covered, as much of this extensive field is outside the scope of this review and has been reviewed elsewhere. One recent review covers the chemistry of Pn73− (Pn = P, As, Sb),17 while another gives a survey of transition metal complexes of the pnictides.30 Furthermore, reactions of P4 and As4 with d-block metal compounds have also been reviewed.31−33 As a final note, another way to form and study intermetalloid clusters is through gas phase reactions in molecular beams. Such results are discussed elsewhere in the literature.34−56 Our focus here is on compounds that were isolated and characterized in the solid state and/or in solution.

crypt ‐ 222

“NaSb3” ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [Na(crypt‐222)]3 Sb7 en

crypt ‐ 222

“KSnBi” ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [K(crypt‐222)]2 (Sn2Bi 2) en

The access to intermetalloid and heterometallic clusters is as variable as are their compositions and structures. In the following, we will give an overview of typical preparation strategies, including the preparation of key precursors. 1.2.1. Preparation of Zintl Phase-Type Solids. Typically, well (and sometimes not so well) defined Zintl phases are the sources of choice for the p-block element atoms within intermetalloid and heterometallic clusters. These phases are composed of one or more alkali metalsmainly Na, K, or Rband one or more p-block (semi)metals. The favored synthetic procedure involves heating mixtures of the pure elements in niobium or tantalum ampules with a precise temperature program. Occasionally, direct heating of the elements in a silica ampule is desirable, for example, when the reaction mixtures contain arsenic, which is known to react with group 5 metals at high temperature.57−60 Example syntheses are shown in eqs 1 and 2.61,62 To prepare intermetalloid clusters from these solids, they are subject to solution reactions with d- or f-block metal compounds (section 1.2.4). (1)

2K + TI + 3Bi ⎯→ ⎯ KTIBi + KBi 2

(2)

ΔΔ

ΔΔ

crypt ‐ 222

K4Ge9 + HypCl ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ (Cat)(Ge9Hyp3) MeCN

(3)

8K + Nb + Pb + 5As ⎯→ ⎯ K 8NbPbAs5

(4)

ΔΔ

ΔΔ

crypt ‐ 222

⎯⎯⎯⎯⎯⎯⎯⎯⎯→ (Cat)3 (Sn 9CHCH 2) en,py

ΔΔ

(9)

Hyp = Si(SiMe3)3 , Cat = [K(crypt‐222)]+

Modified Zintl anions can also be prepared from metastable subhalides of p-block metals or semimetals. Solutions of the subhalides are treated with alkali metal salts of the desired substituents in tetrahydrofuran (THF) or toluene (tol) at low temperatures (down to −80 °C). The mixtures are then slowly allowed to warm to room temperature and the products purified by recrystallization (see eq 10 for an example).68 GeBr + LiHyp ⎯⎯⎯⎯⎯⎯⎯⎯ → [Li(thf)4 ](Ge9Hyp3) n tol,N Pr3 THF

(10)

Hyp = Si(SiMe3)3

1.2.4. Preparation of Intermetalloid and Heterometallic Clusters. The precursor compounds described above can be subsequently used in reactions with transition metal, lanthanide, or actinide compounds to afford intermetalloid or heterometallic clusters. Representative reactions are shown in eqs 11−13.6,69,70

Ta (ampule)

K + Ge + As ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ “KTa 0.1GeAs”

(8)

K4Sn 9 + Me3SiCCSiMe3

Solid mixtures that comprise a transition metal can also be prepared (see eqs 3−5 for examples).59,60,63 Extractions of these materials can sometimes directly afford intermetalloid clusters without addition of a reactive transition metal reagent (section 1.2.4). 4K + 3Ni + 9Sn ⎯→ ⎯ “K4Ni3Sn 9”

(7)

1.2.3. Preparation of (Element)Organic Derivatives of Zintl Anions. Organic or element-organic moieties can be attached to Zintl anions to alter their charges, solubilities, and reactivities. These modified clusters have been reacted with transition metal complexes to afford coordination compounds and heterometallic clusters (section 2.5). In these cases, the most commonly used types of substituents are silyl groups. Substituted clusters are synthesized either by metathesis reactions of Zintl phases with halide derivatives of the desired moieties or by addition of unsaturated organic molecules (see eqs 8 and 9 for examples).66,67 Note that functionalized Zintl anions prepared with unsaturated organic molecules have yet to be further reacted with transition metal complexes.

1.2. Synthetic Approaches

4K + 9Sn ⎯→ ⎯ K4Sn 9

(6)

[Pt(PPh3)4 ], crypt ‐ 222

(5)

K4Pb9 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ (Cat)2 [Pt@Pb12 ]

1.2.2. Preparation of Salts of Zintl Anions. Zintl phases are often based on extended anionic structures that undergo essentially unknown decompositions and rearrangements in solution. This may hinder the calculation of reaction yields or

(11)

en

[Pd(dppe)2 ]

(Cat)2 (Sn2Bi 2) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ (Cat)4 [Pd3@Sn8Bi6] en

C

(12)


Chemical Reviews

Review [Pd(PPh3)4 ], crypt ‐ 222

i

“K[Ge9(Sn Pr3)3 ]” ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ → (Cat)2 [Ge18Pd3(Sni Pr3)6 ] i MeCN, Pr2O

(13) +

Cat = [K(crypt‐222)]

As mentioned above, in a minority of cases, it is possible to prepare intermetalloid or heterometallic clusters directly by dissolution of solids comprising d-block metals (eqs 3−5) in the presence of cation sequestering agents. Examples of such reactions are shown in eqs 14 and 15.63,71 crypt ‐ 222

“K4Ni3Sn 9” ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [K(crypt‐222)]4 [Ni@Sn 9] en

(14)

crypt ‐ 222

“K 6ZnSb5” ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [K(crypt‐222)]4 [Zn2Sb12 ] en,tol

(15)

1.2.5. Preparation of Cationic Intermetalloid and Heterometallic Clusters in Solid State Reactions or Ionic Liquids. Heterometallic and intermetalloid clusters do not necessarily need to carry a negative charge. A limited number of polycationic bismuth clusters have been reported. The first of these clusters, Bi95+, was found in the subchloride Bi12Cl14 in 1963.72 Since then, this chemistry was further developed and more complex clusters containing transition metal atoms were found in mixed element subhalides prepared via classical solid state syntheses (see eq 16 for an example).73 More recently, ionic liquids (ILs) were utilized as reaction media to carry out these syntheses at lower temperatures (see eq 17 for an example).74 ILs have also been used to synthesize salts of cationic clusters from ternary subhalides (see eq 18 for an example).75 Bi + Bi 2Pd + Br ⎯→ ⎯ Bi14PdBr16 ΔΔ

Bi + Pt + BiBr3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [Pt@Bi10][AlBr4]4 [BMIm]Br·4AlBr3, Δ

Bi12 − aRhCl13 − a ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [Rh@Bi 9][AlCl4]4 [BMIm]Cl·4AlCl3, Δ

Figure 1. (a) The crystallographic modeling of the α (D2h, gold, minor) and β (C2h, green, major) isomers of [Co2@Ge16]4−. (b, c) The separated components of the disordered anion. Adapted with permission from ref 77. Copyright 2018 Wiley-VCH.

and 10a,b).77 Finally, different clusters with equivalent charges and similar sizes have been known to cocrystallize on the same site in the unit cell, for example, [La@Sn7Bi7]4− and [La@ Sn 4 Bi 9 ] 4 − in the compound [K(crypt-222)] 4 [La@ Sn7Bi7]0.7[La@Sn4Bi9]0.3·0.7en.78 Because of these difficulties, further analytical and quantum chemical methods are crucial to providing a complete picture of the clusters. The most powerful tools for elucidation of the composition of multinary clusters after SCXRD are elemental analysis and mass spectrometry. 1.3.2. X-ray Spectroscopy and Mass Spectrometry. Experimental realities, such as low yields, impurities, and highly air and moisture sensitive products make traditional elemental analysis challenging. Therefore, X-ray spectroscopic techniques, such as EDXS (energy-dispersive X-ray spectroscopy) and μ-XFS (micro-X-ray fluorescence spectroscopy), are employed to confirm the presence and determine the ratio of heavy elements. Because X-ray spectroscopic techniques are only semiquantitative and can be adversely affected by the presence of impurities or cocrystallized products, high-resolution mass spectrometry (MS) often provides experimental confirmation of exact cluster composition(s). Furthermore, mass spectrometry can differentiate between different clusters that may be cocrystallized in the same compound. It can also provide information about cluster fragmentation. The most commonly used ionization method is electrospray ionization (ESI). It is very common for intermetalloid cluster anions, aided by their multicenter bonding, to simply oxidize to the monoanion, making the detection of the intact cluster feasible. This is particularly true when the cluster comprises an endohedral atom, which usually endows it with greater gas phase stability. This behavior allows for the unambiguous identification of even exceptionally complicated cases, such as a mixture of protonated clusters (Figure 2). 1.3.3. Nuclear Magnetic and Electron Paramagnetic Resonance Spectroscopy. If suitable elements are present in the cluster shell (e.g., 29Si, 31P, 119Sn, 207Pb) or in an endohedral or coordinating position (e.g., 63Cu, 113Cd, 139La, 195 Pt), nuclear magnetic resonance (NMR) spectroscopy can

(16) (17) (18)

BMIm = 1‐butyl‐3‐methylimidazolium 1.3. Experimental Characterization Methods

1.3.1. X-ray Diffraction Methods. Most of the known compounds discussed herein have been characterized by means of single crystal X-ray diffraction (SCXRD). Crystallographic disorder can arise for a number of reasons. First, many clusters possess a (near) spherical shape; therefore, positional disorder via rotation of the cluster is common. Second, severe occupational disorder can arise in clusters with multielement shells, not only due to rotation of the clusters but also due to cocrystallization of energetically similar isomers with different distributions of the elements over the atomic positions. This hinders the elucidation of both the absolute composition of the cluster and the identification of the global minimum isomer (this point is discussed further in section 3.2). Neighboring elements in the periodic table are especially problematic, as they cannot be intrinsically distinguished by means of common X-ray diffraction techniques; application of anomalous X-ray scattering techniques has helped here in some cases.76 Third, two or more conformational isomers of the same cluster representing local minima that are close in energycan cocrystallize on the same site in the unit cell, as observed for the α (D2h) and β (C2h) isomers of [Co2@Ge16]4− (Figures 1 D


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Figure 4. Experimental (a, black) and simulated (b, gray) EPR spectra for [Ru@Ge12]3−. Adapted from ref 84. Copyright 2014 American Chemical Society. Figure 2. Segment of the high-resolution ESI mass spectrum of single crystals of [K(crypt-222)]3[Sm@Ga2HBi11]0.9[Sm@Ga3H3Bi10]0.1·en· tol in DMF, showing distinct mass envelopes for both protonated clusters. Reproduced with permission from ref 79. Copyright 2014 Wiley-VCH.

organometallic complexes. For this reason, quantum chemical calculations are critical to provide a clear picture from the assembled evidence. Computed structures can help to support a crystal structure that is riddled with disorder or provide a structure when none is obtainable. Calculated molecular orbitals (MOs) and localized molecular orbitals (LMOs) can provide an idea about the degree to which electrons participate in multicenter bonding. Natural population analysis (NPA),85 population analysis based on occupation numbers (PABOON),86 and Mulliken population analysis87 are used to gain information about charge distribution, the degree of actual bonding between atoms, and the atoms’ contributions to the bonds. In addition, chemical shifts and spin densities can be calculated to corroborate NMR, EPR, and SQUID data. Finally, in multi-isomer ternary systems, calculations must be employed to find the global minimum structure with respect to the architectures and to the distribution of the different atom types over the atomic sites. The latter applies especially to clusters in which the two main group elements involved are neighbors in the periodic table, and hence indistinguishable by (common) X-ray techniques (see section 3.2). Most investigations are undertaken with density functional theory (DFT) methods.88−90

provide (limited) information about the structural and electronic environment of the NMR-active nuclei. NMR can be used to discern low-symmetry and high-symmetry environments,80 detect or confirm the insertion of transition metal atoms,81 gain insight into dynamic processes,82 or detect multiple species in solution (Figure 3).83 NMR is probably

Figure 3. 139La NMR spectrum of single crystals of [K(crypt222)]3[La@Pb6Bi8]0.04[La@Pb3Bi10]0.96 dissolved in en. Reproduced with permission from ref 83. Copyright 2015 Wiley-VCH.

1.4. Concepts for Bonding in Intermetalloid and Heterometallic Clusters

most critical in the identification of protonated clusters, as protons are generally not discernible via SCXRD, and in certain cases, it is difficult to know if species detected by ESIMS were protonated before or after ionization. This topic is discussed in greater detail in section 4. Although intermetalloid clusters are often redox active, they are usually closed-shell compounds with the interstitial metal atoms in a d0 or d10 electronic configuration. However, in rare cases, the inner atoms exhibit open shell dn or fn configurations, and some clusters have been reported in which an unpaired electron was confirmed to reside on the main group element shell. Of course, experimental confirmation of paramagnetism is critical when a cluster appears to have an odd number of electrons, specifically because protonated clusters and deprotonated cocrystallized amine solvent molecules are difficult to detect. In these cases, electron paramagnetic resonance (EPR) spectroscopy (Figure 4) or magnetic measurements using a superconducting quantum interference device (SQUID) are employed to confirm the presence of unpaired electrons and to give clues as to the location of the spin density. 1.3.4. Quantum Chemical Studies. It is easy to see that all of the above-mentioned techniques involve a greater level of uncertainty when applied to the analysis of intermetalloid clusters as opposed to more conventional compounds, such as

1.4.1. Definition of Terms. Zintl and intermetalloid clusters can be classified in terms of the number of skeletal electrons, the total number of valence electrons, and the number of vertices in their p-block-element frameworks. The term skeletal electrons (typically abbreviated SE) refers to the number of valence electrons that are available for cluster bonding. For ligand-less clusters, total valence electrons (typically abbreviated VE) then refers to the number of skeletal electrons plus two electrons per p-block atom for either lone pairs or bonding with ligands. The number of cluster vertices is simply equal to the number of p-blockelement atoms in the cluster shell and is denoted by the letter “n”. For example, Pb102− (n = 10) has 42 valence electrons (4n + 2), 20 of which are lone pair electrons (2n) and 22 of which are skeletal electrons (2n + 2). For simplicity, in this review, we will generally describe clusters in terms of their total valence electron counts (VE). These terms are complicated in intermetalloid and heterometallic clusters by the presence of transition metals, particularly when they strongly interact with the cluster shell. Most transition metals in these clusters will formally have either filled or empty d-shells (i.e., d10 or d0). For clarity, we E


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Recently, attempts have been made to evaluate the bonding in intermetalloid clusters by applying the concepts of (anti)aromaticity in cases that show multicenter bonding.101,102 However, well-established quantum chemical concepts, including the superatom concept103,104 or the related jellium model,97,105 which rely on cluster orbitals, can be used more generally and more quantitatively for all kinds of multicenter bonding in polyatomic molecules.

will indicate this state using formal charges when applicable. In this case, we will refer to the number of valence electrons in the cluster shell after formal charges have been assigned, and not including d-shell electrons. For example, [Cu@Sn9]3− may be written as [Cu+@Sn94−]3−. It would then be said to have 40 valence electrons (4n + 4, n = 9), including four electrons from each tin atom and four electrons from the formal charge of the Sn9 moiety. 1.4.2. Bonding Concepts. The structure and bonding situation of many intermetalloid and heterometallic clusters can be understood in terms of either the Zintl−Klemm− Busmann pseudo-element concept or application of the Wade− Mingos rules. The Zintl−Klemm−Busmann concept addresses electron-precise structures comprising formal two-center−twoelectron (2c−2e) bonds, in which every atom achieves a full electron octet.91,92 In the field of intermetalloid cluster chemistry, “electron-precise” is in most cases used in conjunction with clusters comprising (pseudo)pnictogen atoms where VE = 5n. For example, the anion [Ta@ Ge8As4]3− can be better understood when written with formal charges, which highlights that in [Ta5+@(Ge−)8(As0)4]3− (VE = 60) all formal Ge− ions behave as “pseudo-arsenic” atoms that form three bonds (Figure 5a).58

2. BINARY CLUSTERS 2.1. General Remarks

Binary clusters are generally obtained from reactions of salts of homoatomic Zintl anions or binary Zintl phases with transition metal complexes (see section 1.2). The p-block element used often determines the general cluster type obtained. Tetrel elements (Tt = Si, Ge, Sn, Pb) tend to form deltahedral (or deltahedron-derived) clusters, based on three-atom faces, that conform to Wade−Mingos electron counting rules. Highly reduced clusters comprising triel elements (Tr = Al, Ga, In, Tl) often adopt related structures in intermetallic phases. In contrast, pnictogens (Pn = P, As, Sb, Bi) tend to form clusters with electron-precise bonds. In the following subsections, clusters are grouped according to main group elements. Endohedral clusters are presented first, followed by coordination-type compounds and heterometallic clusters. Tables 1−4, which are given in the corresponding subsections, summarize the different binary species along with the reactants used for their syntheses. 2.2. Binary Intermetalloid and Heterometallic Clusters of Group 13 Elements

Even though this review mostly focuses on intermetalloid and heterometallic clusters that are accessible through solution chemistry, no discussion of this family of compounds is complete without at least a brief mentioning of the group 13 elements and their clusters in the solid state. We therefore provide a short chapter on this chemistry and refer the reader to an extensive survey of this class of compounds by Corbett for further information.106 Group 13 elements can be reduced with alkali metals in the solid state to form deltahedral clusters that are reminiscent of the according group 14 clusters. Due to their high total charges, these clusters will always be a solid state phenomenon, as stabilization can only be achieved in effectively chargecompensating environments. However, despite a large number of additional electrons with regard to the neutral atoms, the species often fail to reach the valence electron numbers required for a closo-cluster. Therefore, their structures typically deviate from the analogous group 14 species, both because they may be more electron deficient than their group 14 counterparts and due to their high charges and closely packed cations. Besides a great variety of simple homoatomic Zintl clusters of Ga−Tl being found in solid-state compounds,106 several examples of intermetalloid and heterometallic clusters have been characterized as well. There are a few examples of heterometallic clusters with mixed shells that are highly electron deficient, such as 4n − 6 valence electrons in [Au2Tl9]9− and 4n − 4 valence electrons in [Cd3Tl8]10− and [HgIn10]8−.107−109 All three clusters adopt D3h-symmetric structures that can be derived from the same parent cluster type, Tr117− (Tr = Ga,110 In,111−113 Tl108,113,114), which has an all-face-capped trigonal prismatic structure and 4n − 4 valence electrons. The substitution of transition metal atoms for

Figure 5. Applicability of concepts for the correlation of electronic structures with geometric structures of intermetalloid clusters: (a) electron-precise pseudo-element case [Ta@Ge8As4]3−;58 (b) Wade− Mingos-type cluster [Ir@Sn12]3−;99 (c) [Fe@Ge10]3−,100 not conforming to either of the two concepts.

In intermetalloid cluster chemistry, the Wade−Mingos rules93−98 are applied to polyhedral clusters that are dominated by multicenter bonding. As a short reminder, for a given cluster, these rules connect the number of vertices (n) to the number of cluster valence electrons and classify them according to the number of vertices that are “missing” relative to a complete deltahedral structure. A complete cluster is classified as closo and has 4n + 2 valence electrons. Cluster fragments that lack one (nido), two (arachno), or three (hypho) vertices with respect to the parent deltahedron have 4n + 4, 4n + 6, or 4n + 8 valence electrons, respectively (and so on). For example, [Ir−@Sn122−]3− has 4n + 2 (n = 12) valence electrons in its cluster shell and is thus classified as a closo-cluster (Figure 5b).99 There are also many clusters for which neither of these concepts apply properly, as observed in [Fe@Ge10]3− (Figure 5c).100 The reason for this deviation may be a total electron count that is in between the two scenarios or a specific electronic structure (i.e., electron count plus energetics) that leads to distortions. In such complicated cases, the employment of quantum chemistry is mandatory to precisely understand the electronic situation that underlies structure and bonding. For a more detailed discussion of the factors influencing the geometric and electronic structures of intermetalloid clusters, see section 5 of this article. F


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Table 1. Binary Intermetalloid Clusters of the Group 14 Elements cluster [Ni@Ge9]3− [Co@Sn9]4− [Co@Sn9]5− a [Ni@Sn9]4− [Ni@Sn9]3− [Cu@Tt9]3− Tt = Sn, Pb [Ni@Pb10]2− [Co@Ge12]3− [Rh@Sn12]3− [Ir@Sn12]3− [Mn@Pb12]3− [Ni@Pb12]2− [Pd@Pb12]2− [Pt@Pb12]2− [Au@Pb12]3− [(Ni@Ge9)Ni(en)]3− [(Ni@Ge9)Ni(PPh3)]2− [(Ni@Ge9)Ni(CO)]2− [(Ni@Ge9)Ni(CCPh)]3− [(Ni@Sn9)Ni(CO)]3− [(Pt@Sn9)Pt(PPh3)]2− [(Ni@Ge9)Pd(PPh3)]2− [(Co@Sn9)Ni(CO)]3− [(Co@Sn9)Ni(C2H4)]3− [(Co@Sn9)Pt(PPh3)]3− [(Co@Sn9)AuPh]3− [Fe@Ge10]3− [Co@Ge10]3− [Fe@Sn10]3− [Rh@Sn10]3− [Ru@Ge12]3− [Co2@Ge16]4− [(Co@Sn8)Sn(Co@Sn8)]5− [(Ni@Sn8)Sn(Ni@Sn8)]4− [Rh2@Sn17]6− [Pt2@Sn17]4− [Pd2@Ge18]4− [Pd2@Sn18]4− [Rh3@Sn24]5− [(Ni@Ge9)Ni(Ni@Ge9)]4− [Ni2@Ge13Ni4(CO)5]4− [Ti@Sn15Ti3Cp5]4/5− [Sn@Cu12@Sn20]12− a

precursors K4Ge9/crypt-222 K4.79Co0.79Sn9/crypt-222 K, Sn K4Ni3Sn9/18-crown-6/crypt-222 in solution and K, Ni, Sn in solid state K4Sn9/crypt-222 K4Tt9/crypt-222

ref [Ni(cod)2]

8b 8a 8a 8a

[Ni(cod)2] [CuMes]5

126 80

8b 8b

[Ni(cod)2] [CoMe(PMe3)4] [Rh(coe)2(μ-Cl)]2 dppe [MnMes2]3 [Ni(cod)2] [Pd(PPh3)4] [Pt(PPh3)4] [Au(PPh3)Ph] [Ni(cod)2] [Ni(CO)2(PPh3)2] [Ni(cod)2]/[Ni(CO)2(PPh3)2] KCCPh/crypt-222 [Ni(CO)2(PPh3)2] [Pt(PPh3)4] [Ni(PPh3)4]/[Pd(PPh3)4] [Ni(PPh3)2(CO)2] [Ni(cod)2] [Pt(PPh3)4] [Au(PPh3)Ph] [Fe(2,6-Mes2C6H3)2] [Co(C8H12)(C8H13)] [FeMes2]2 [Rh(coe)2(μ-Cl)]2 [Ru(cod){η3-CH3C(CH2)2}2] [{(ArN)2CtBu}Co(η6-tol)]/ [Co(PPhEt2)2(Mes)2]

131, 132 133 134 99 135 132 132 6, 132 136 125 137 125 125 138 138 139 140 140 140 140 100 141 142 134 84 77, 143

8f 8i 8i 8i 8j 8i 8i 8i 8i 8e 8e 8e 8e 8d 8e 8e 8d 8d 8e 8e 8h 8h 8h 8g 8k 10a,b

127, 129 144 134 145 146 147 134 148 137 149 150

10d,e 10d 10e 10f 10h 10h 10c 10g 10j 10i 17f

Co

K4Pb9/crypt-222 K4Ge9/crypt-222 K4Sn9/crypt-222 [K(crypt-222)]3[Sn9Ir(cod)] K4Pb9/crypt-222 K4Pb9/crypt-222 K4Pb9/crypt-222 K4Pb9/crypt-222 K4Pb9/crypt-222 K4Ge9/crypt-222 K4Ge9/crypt-222 K4Ge9/crypt-222 [K(crypt-222)]2[Ni@Ge9Ni(CO)] K4Sn9/crypt-222 K4Sn9/crypt-222 K4Ge9/crypt-222 K5Co3Sn9/crypt-222 K5Co3Sn9/crypt-222 K5Co3Sn9/crypt-222 K5Co3Sn9/crypt-222 K4Ge9/crypt-222 K4Ge9/crypt-222 K4Sn9/crypt-222 K4Sn9/crypt-222 K4Ge9/crypt-222 K4Ge9/crypt-222 K4.79Co0.79Sn9/crypt-222 K4Sn9/crypt-222 K4Sn9/crypt-222 K4Sn9/crypt-222 K4Ge9/crypt-222 K4Sn9/crypt-222 K4Sn9/crypt-222 K4Ge9/crypt-222 K4Ge9/crypt-222 K12Sn17/18-crown-6 Na, Sn

Figure

125 127 81, 129, 130 63

[Ni(cod)2] [Rh(coe)2(μ-Cl)]2 [Pd(PPh3)4]/[Pt(norbornene)3] [Pd(PPh3)4] [Pd(PPh3)4] [Rh(coe)2(μ-Cl)]2 [Ni(cod)2] [Ni(CO)2(PPh3)2] [TiCl2Cp2] Cu

a

Found only in solid solution.

thallium atoms in the parent Tr117− cluster shell causes distortions, most noticeably in the [Au2Tl9]9− anion. Herein, the cluster is strongly compressed along the 3-fold rotation axis due to a Au···Au interaction. Besides these heterometallic clusters, several examples of intermetalloid clusters with 10 or 12 vertices that are centered by a d10 transition metal atom have been reported. The 10vertex clusters [M@Tr10]q− (M/Tt/q = Zn/In/8,115,116 Ni/ Ga/10,117 Ni−Pt/In,Tl/10;118 4n valence electrons) are hypoelectronic, whereas the 12-vertex cages [M@Tl12]12− (M = Mg, Zn, Cd, Hg; 2n + 2 skeletal electrons)119,120 are classic icosahedral closo-clusters. In the 10-vertex cages, a peculiarity can be observed. The clusters [Zn@Tr10]8− (Tr = In, Tl) both

adopt a bicapped square antiprismatic structure, while [Ni@ Ga10]10− forms a distorted, tetracapped trigonal prism. The same type of cluster framework is present in the “centaur polyhedron” [Ge10{Fe(CO)4}8]6−.121 Some structures of transition-metal-centered clusters of group 13 elements are shown in Figure 6. Solution syntheses of group 13 heterometallic or intermetalloid clusters have been only rarely reported. A molecular excerpt from a classical intermetallic phase can be found in the cluster [(Cp*AlCu)6H4], the first example of a Hume− Rothery-type heterometallic cluster (Figure 7).122 The compound is prepared from [(Ph3P)CuH]6 and (AlCp*)4 and is composed of a Cu6 fragment with a bicapped tetrahedral G


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shape that interpenetrates an aluminum octahedron. A Cp* ligand was retained on each of the Al atoms (Cp* = pentamethylcyclopentadienide). Reactive hydride ligands on the cluster surface readily interact with small unsaturated organic molecules. This was shown in a reaction of the cluster with benzonitrile, which includes a hydride migration onto the nitrile carbon atom to afford the final product [(Cp*AlCu)6H3(NCHPh)] (Figure 7). This is a rare case in which a well-defined molecular species serves as a model compound to study the surface reactivity of otherwise typically poorly defined catalysts.

Figure 6. Examples of transition-metal-centered clusters of group 13 elements: (a) [Cd3Tl8]10−,109 (b) [Ni@Ga10]10−,117 and (c) [M@ Tl12]12− (M = Zn, Cd, Hg).119

2.3. Binary Intermetalloid Clusters of Group 14 Elements

Binary clusters that consist of tetrel element and d-block atoms adopt deltahedral shapes in most cases, but non-deltahedral species are also known. All of these are discussed in the following subsections and summarized in Table 1. 2.3.1. Deltahedral Clusters with 9, 10, and 12 Vertices. The most prominent type of Zintl anions of group 14 elements are deltahedral clusters that can be understood by Wade−Mingos rules (e.g., closo-E52−, nido-E94−, or closo-E102−). This follows from a group 14 atom with a lone pair being isoelectronic to a B−H fragment, the building unit of the archetypical borane or borate cages. These simple Zintl clusters often react with transition metal complexes to afford

Figure 7. Molecular structure of [(Cp*AlCu)6H3(NCHPh)].122

Figure 8. Molecular structures of intermetalloid clusters with 9, 10, or 12 vertices, comprising group 14 atoms and endohedral transition metal atoms. Compounds with similar structures are grouped, and a representative cluster is shown. (a) [M@Sn9]4− (M = Co,127 Ni63), (b) [M@Tt9]3− (M/Tt = Ni/Ge,125 Ni/Sn,126 Cu/Sn,80 Cu/Pb80), (c) [(Pd@Sn9)SnCy3]3−,82 (d) [{η4-(M@Sn9)}Ni(L)]3− (M/L = Co/CO,140 Co/C2H4,140 Ni/CO138), (e) [{η3-(M@Tt9)}M′(L)]q− (M/Tt/M′ = Ni/Ge/Ni, L/q = PPh3/2,137 en/3,125 CO/2,125 CCPh/3;125 Tt = Sn, M(=M′)/L/q = Pt/ PPh3/2,138 Pd/SnCy3/3;82 M/Tt/M′/L/q = Ni/Ge/Pd/PPh3/2;139 M = Co, Tt = Sn, q = 3, M′/L = Pt/PPh3,140 Au/Ph),140 (f) [Ni@ Pb10]2−,131,132 (g) [Rh@Sn10]3−,134 (h) [M@Ge10]3− (M = Fe,100 Co141), (i) [M@Tt12]q− (M/Tt/q = Ni,Pd,Pt/Pb/2,6,132 Rh*,Ir/Sn/3,99,134 Co/ Ge/3*,133 Au/Pb/3*136), (j) [Mn@Pb12]3−,135 (k)[Ru@Ge12]3−.84 Compounds denoted with * can deviate from the ideal Ih symmetry for icosahedral Wade−Mingos clusters. H


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K[K(crypt-222)]3[Co0.68@Sn9], which contains a mixture of the empty Sn94− and [Co@Sn9]4−. The latter is oxidized by one electron with respect to the [Co@Sn9]5− anions present in the parent solid.127 The compound [K(crypt-222)]5[(Co@ Sn8)Sn(Co@Sn8)] was also isolated from K4.79Co0.79Sn9 (see section 2.3.3). In addition to crystallographic characterization, these clusters have been studied in the solid state by magic angle spinning (MAS) 119Sn NMR. In particular, spectra of two related solid compounds K13Co1Sn17 and K4.79Co0.79Sn9 were collected and compared with the parent compounds K12Sn17 and K4Sn9.81 Distinct signals were observed and assigned to filled [Co@Sn9]q−, as well as unfilled Sn9q− and Sn44−. The encapsulation of a d10 metal is a strong driving force for cluster formation. Presumably, the metal atoms act as a template for the cluster shell, particularly in regard to the formation of larger clusters with very flexible shells (number of vertices ≥10), which usually are not isolable without the endohedral transition metal. For example, there are no empty homoatomic 12-atom cages that have been crystallized from solution, while several 12-vertex icosahedral intermetalloid clusters have been fully characterized.6,99,132,136 The mechanisms by which the metal atoms are inserted into the cluster framework are not yet known.19 While a simple process can easily be imagined in the case of the 9-atom cages that involves a “breathing mechanism” of the cluster framework, the formation of the larger clusters from the 9-atom Zintl anion starting materials must involve a series of bond-breaking and bond-formation steps. It is difficult to probe the dynamic processes involved in this chemistry, especially if the elements that form the cluster frameworks have no NMR active nuclei or if their relaxation times do not allow for such studies. The formation of intermetalloid clusters may proceed within minutes, even though their crystallization may take between hours and weeks. In several cases, the addition of an M′−L fragment to [M@ Tt9]q− to afford [(M@Tt9)M′(L)]q− has been observed (Figure 8d,e). The reaction is analogous to the addition of an M(L) fragment to Tt9q− (section 2.4.2). A series of related anions, [(Ni@Ge9)Ni(L)]q− (L/q = PPh3/2,137 en/3,125 CO/ 2125) and [(M@Sn9)M(L)]q− (M/L/q = Ni/CO/3, Pt/PPh3/ 2),138 have been synthesized via reactions of K4Tt9 (Tt = Ge, Sn) and either [Ni(CO)2(PPh3)2], [Ni(cod)2] (cod = 1,5cyclooctadiene), or [Pt(PPh3)4] in en in the presence of crypt222. Unlike empty Tt9q−, which normally coordinates a transition metal in an η4 or η5 fashion (see section 2.4.2), [M@Tt9]q− primarily forms η3-type complexes (with only a couple of exceptions). The stepwise nature of these reactions has been demonstrated by the synthesis of the mixed metal clusters, [(Ni@Ge9)Pd(PPh3)]2− and [(Co@Sn9)M(L)]3− (M(L) = Ni(CO), Ni(C2H4), Pt(PPh3), Au(Ph)).139,140 These compounds are discussed further in section 4. The larger 10- and 12-vertex cages [M@Pb10]2− (M = Ni) and [M@Pb12]2− (M = Ni, Pd, Pt) are readily accessible through reactions of K4Pb9 with [Ni(cod)2] or [M(PPh3)4] (M = Pd, Pt) in en (Figure 8f,i).131,132 These anions are closoclusters that adopt either bicapped square antiprismatic or Ih topologies, respectively. Crystalline [K(crypt-222)]2[Pt@Pb12] has been exploited as a source for [Pt@Pb10]− and [Pt@Pb12]− ions in a gas phase photoelectron spectroscopy study.151 In addition, the vibrational and optical properties of [K(crypt222)]+ salts of [Ni@Pb10]2−, [Ni@Pb12]2−, and [Pt@Pb12]2−

deltahedral clusters with an interstitial transition metal atom, often with a concomitant rearrangement of their original cage structures. The anion [Pt@Pb12]2− was the first such cluster that was isolated, and the term intermetalloid cluster was suggested to describe this quickly growing class of compounds (see section 1.1).4,6 Deltahedral intermetalloid clusters that conform to the Wade−Mingos rules are typified by transition-metal-centered 9-vertex nido-cages or 10- and 12-vertex closo-clusters, respectively. The empty 9-vertex cages usually adopt either a capped square antiprismatic (C4v) or a tricapped trigonal prismatic (D3h) structure, and they are the smallest known polyhedra to encapsulate a transition metal atom. The activation barrier for the interconversion between both cluster shapes is very low.123 Furthermore, the 9-vertex cages can exist in multiple charge states, from −2 to −4 in solution.16,124 The 10- and 12-atom cages adopt bicapped square antiprismatic (D4d) and icosahedral (Ih) structures, respectively. Intermetalloid versions of all three types have been prepared with interstitial d10 transition metal atoms, M = Ni0−Pt0, Co−, Rh−, Ir−, Cu+, Au−. In one special case, M possesses formally a d8 configuration (Mn−); however, the cluster structure is drastically distorted from the idealized (Ih) structure by the donation of electron density from the Mn atom to the cluster shell, which results in an actual electron configuration that is between d5 (Mn2+) and d8 (Mn−) (see below). Filled 9-vertex cages can be obtained with first row transition metals in the endohedral position, as displayed by the syntheses of [M@Tt9]q− (M/Tt/q = Ni/Ge/3,125 Ni/Sn/ 3,126 Ni/Sn/4,63 Co/Sn/4,127 Cu/Sn/3,80 Cu/Pb/380). There are variations in the topologies of the clusters in this series. The only clusters in this series that adopt a capped square antiprismatic shape (Figure 8a) are [Ni@Sn9]4− and [Co@ Sn9]4−,63,127 while all others are much closer to a tricapped trigonal prism (Figure 8b). As stated previously, the energy barrier between these two conformations is low, and it can be the case that a one-electron oxidation leads to the distortion from capped square antiprismatic to tricapped trigonal prismatic,125 similar to what has been observed for the paramagnetic Sn93−.128 Still more unexpected deviations can occur. For instance, the anions [Cu@Sn9]3− and [Cu@Pb9]3− are nido-clusters according to their electron count, yet adopt a tricapped trigonal prismatic shape typical of closo-clusters. This results in an almost spherical coordination environment around the Cu+ ion.80 Solution 63Cu- and 119Sn-NMR studies on [Cu@Sn9]3− showed that the high symmetry of the 9-atom cage is retained in solution and that it is highly fluxional. Magnetic measurements indicated that the compound is diamagnetic, thereby confirming the presence of a d10 Cu+ ion inside the cluster. In contrast, the anion [Co@Sn9]4− adopts a capped square antiprismatic structure, despite being isovalent with Sn93− (the cobalt atom is formally Co−); however, this may be due to packing effects, since it is disordered in the solid state with Sn94−.127 The ubiquity of these clusters is further highlighted by the fact that they exist in intermetallic solids as well as in solution. Ternary solids have been reported that contain intermetalloid [M@Sn9]q− (M/q = Co/5,81,129,130 Ni/463,81) clusters. From these solids, the filled 9-vertex clusters can be extracted by dissolution of the transition-metal-containing compound in en in the presence of crypt-222. In the case of cobalt, the solid phase K4.79Co0.79Sn9 contains a mixture of filled and unfilled 9vertex cages. Extraction of this phase afforded the compound I


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2.3.2. Non-Deltahedral Clusters with 10 and 12 Vertices. Even though deltahedral structures dominate the intermetalloid group 14 clusters, some deviations from these structural motifs have been reported. Reactions of Tt94− anions with organometallic Fe and Co precursors afforded threeconnected pentagonal prismatic [M@Ge10]3− clusters (M = Fe,100 Co141) with pseudo D5h symmetry (Figure 8h). In both clusters, the Tt−Tt bonds between the two five-membered rings are only slightly elongated with respect to the ones within the rings. As neither Wade−Mingos rules nor an electronprecise bonding model are able to explain the electronic structure of these clusters, they were investigated by quantum chemical methods. Strong interactions between the central transition metal atom and the cluster shell were found to be present. A more in-depth analysis of 10-vertex intermetalloid clusters was prompted by the synthesis of [K(crypt222)]3[Fe@Sn10], in which the anion displayed heavy disorder in the solid state.142 The potential energy hypersurface for the [Fe@Sn10]3− anion was found to be flat, with little difference in energy between the three investigated symmetries (D4d, C3v, and C2v; see section 5 for more details). Another cluster with purely three-bonded Tt atoms was obtained upon reaction of K4Ge9 with [Ru(cod)(Me-Allyl)2], leading to the formation of the paramagnetic [Ru@Ge12]3− cluster anion.84 Its structure is near-D2d symmetric and can be viewed as two Dewar-benzene-like {Tt6} fragments that are rotated against each other by 90° (Figure 8k). The cluster anion has an odd number of electrons and was shown to be paramagnetic by EPR spectroscopy (Figure 4). A three-bonded structure, typical of electron-precise compounds, can be rationalized by imagining a complete transfer of eight valence electrons from the ruthenium atom to the Ge12 cage. This would result in a formal valence electron count of 59, one short of the 5n = 60 expected for an electron-precise cluster with n = 12 vertices. This is an oversimplified picture; however, DFT calculations indeed pointed to strong bonding interactions between the Ru 4d orbitals and orbitals of the Ge 12 polyhedron. The unpaired electron was found to be entirely delocalized over all 12 germanium atoms in the cluster shell without any contribution of the endohedral ruthenium atom. A thorough computational study of 12-vertex intermetalloid clusters of silicon and germanium showed that the formation of non-deltahedral cluster architectures is closely related to the amount of electron donation from the transition metal atom to the cluster shell (see section 5).156 The structural dependency of cluster cages on the identity of the endohedral metal atom has become increasingly evident, as the number of successfully incorporated transition metal atoms has increased. These findings sparked several quantum chemical studies that focused on this very question for several cage sizes of homoatomic group 14 clusters: [M@E10]q, [M@ E12]q, and [M@E14]q.142,156,157 In these studies, dependencies on total valence electron counts as well as elements in the cluster shell are found and discussed (see section 5). 2.3.3. Larger Intermetalloid Clusters of Group 14 Elements. In some cases, reactions of Tt94− with transition metal complexes afford larger deltahedral clusters (>12 vertices) that incorporate two endohedral transition metals and often adopt oblong shapes. The valence electron counts in these clusters can no longer be understood via the Wade− Mingos rules; however, most examples are deltahedral and their structural similarities to their smaller cousins can be striking. The clusters [Pd2@Tt18]4− (Tt = Ge,146,147 Sn147),

were thoroughly studied by means of UV−vis and FTIR spectroscopy.152 The first known deltahedral 12-vertex intermetalloid cluster of tin was prepared in a two-step reaction, in which [Sn9Ir(cod)]3− was first synthesized and isolated as its [K(crypt-222)]+ salt. Subsequent oxidation with PPh3 or dppe (1,2-bis(diphenylphosphino)ethane) at elevated temperatures then afforded [Ir@Sn12]3−.99 This cluster is isoelectronic and isostructural with [M@Pb12]2−,6,132 with the additional negative charge resulting from the formal Ir−. The consecutive characterization of these species provided a rare glimpse into the stepwise formation of ligand-less endohedral intermetalloid clusters (see also section 4). The cluster [Au@Pb12]3− was synthesized from the reaction of K4Pb9 with [Au(PPh3)Ph] in pyridine.136 This cluster is unusual for two reasons. First, an endohedral gold atom was previously unknown in anionic intermetalloid clusters (with the only other example being the cationic intermetalloid cluster [Au@Bi10]5+, see section 2.6.3).153 Second, the gold atom in this cluster is formally Au−, meaning that it has a filled 6s orbital (6s 2 5d 10 ), such as in aurides like CsAu or (NMe4)Au.154,155 This lone pair is not completely inactive, and the cluster is distorted from icosahedral symmetry due to second-order Jahn−Teller effects. This contrasts with the situation in [Pt@Pb12]2−, in which the endohedral Pt0 has an unfilled 6s orbital (6s05d10) and the Pb12 cage is not significantly distorted from the ideal icosahedral geometry. Recently, the related rhodium clusters [Rh@Sn10]3− and [Rh@Sn12]3− were isolated from reactions of K4Sn9 and [Rh(coe)2(μ-Cl)]2 (coe = cyclooctene).134 Crystallographic disorder prevented accurate structural characterization of [Rh@Sn10]3−; however, a distortion of the structure away from the ideal D4d toward a C2v symmetry is evident. Quantum chemical calculations indicated a small energy difference between both geometries, a very flat energy hypersurface and indeed a preference of the C2v symmetric cluster. The larger [Rh@Sn12]3− cluster crystallizes as a mixture of two isomers, one with almost ideal Ih symmetry and one with D3d symmetry. In all cases, the distortions can be attributed to electron donation from the endohedral, formally d10 Rh− atom into cluster orbitals, based on the DFT results. The lighter 12-vertex cluster [Co@Ge12]3− also distorts significantly from ideal icosahedral symmetry.133 This anion crystallizes in the salt [K(crypt-222)]3[Co@Ge12]0.76[Co@ Ge10]0.24·en upon reaction of [Co(PMe3)4Me] with K4Ge9 and crypt-222 in en. The cluster structure is elongated along one C5 axis, such that two 6-atom “caps” are separated by long contacts, resulting in a symmetry reduction (from Ih) to pseudo-D5d. Quantum chemical calculations indicated that multicenter bonding was present between the two “caps”. The synthesis of [Mn@Pb12]3− provided the first example of a transition metal with an open d-shell (dx, 0 < x < 10) being incorporated into a ligand-free intermetalloid cluster.135 DFT studies suggest that the formally Mn− ion is involved in substantial electron donation to empty Pb122− orbitals. As a consequence, the cluster distorts from an ideal Ih symmetry to D2h, as these orbitals have antibonding character with respect to some of the Pb−Pb bonds (Figure 8j). The calculated spin densities on the manganese atom and the Pb12 cage indicate that the actual distribution of charge lies between the limits of [(Mn−)@Pb122−]3− and [(Mn2+)@Pb125−]3−, with the latter representing a stable half-filled d-shell. J


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part in exchange until the surprisingly high temperature of 60 °C. A cluster with the same structure was obtained through two different extractions from the ternary solid K4.79Co0.79Sn9.127,129 The product [K(crypt-222)]5[(Co@ Sn8)Sn(Co@Sn8)] in both cases contains the same anion, although in slightly different conformations (Figure 10d,e). [(Co@Sn8)Sn(Co@Sn8)]5− has an odd number of electrons, and the paramagnetism of the cluster was confirmed by EPR spectroscopy. For the heavier congener of the interstitial atom, rhodium, the same main group atom shell was observed in [(Rh@ Sn8)Sn(Rh@Sn8)]6−.134 This cluster forms as a minor product from the reaction of K4Sn9 and [Rh(coe)2(μ-Cl)]2 and adopts the less symmetric conformation (Figure 10e). The cluster possesses a high charge of 6−, which is countered by close contacts to three K+ ions. In a DFT study, these contacts were found to cause the cluster distortion away from the ideal D2d symmetry, and a relationship between all known [M2@Sn17]q− cages was established, based on the M−Tt−M angle. Another example of both the flexibility of the simple 9-atom clusters and the likelihood of the formation of the aforementioned 17- and 18-atom cages via the fusion of two 9-atom cages is the anion [(Ni@Ge9)Ni(Ni@Ge9)]4−.148 In it, a linear array of three Ni atoms with an interatomic distance of 2.395 Å is inserted into two Ge9 subunits. These subunits adopt tricapped triangular prismatic structures, in which one of the two former base faces has been opened up due to the insertion of the Ni atoms (Figure 10g). It is worth noting that a potential intermediate on the path to [(Ni@Ge9)Ni(Ni@ Ge9)]4− was observed in the form of [(Ni@Ge9)Ni(PPh3)]2−, in which a Ni(PPh3) unit is coordinated by a Ni-centered Ge9 cluster in an η3 fashion (Figure 8e).137 Exchange of the PPh3 ligand for an additional [Ni@Ge9]x− would afford the [(Ni@ Ge9)Ni(Ni@Ge9)]4− cluster anion. A very interesting non-deltahedral cluster species is [Co2@ Ge16]4−, which forms as a mixture of two different isomers, α (D2h) and β (C2h) (parts a and b of Figure 10, respectively). It was first synthesized from K4Ge9 and [Co(PPhEt2)2(Mes)2] in 2015, but the experimental data did not allow for the unambiguous confirmation of both isomers, although they were predicted through extensive DFT studies.143 Three years later, a second synthetic route to these clusters with [{(DippN)2CtBu}Co(η6-tol)] as the Co source allowed for the structural characterization and unambiguous identification of both isomers with the energetically more favored α isomer being present as the major component.77 The α isomer is three-bonded and is formally the product of the fusion of two pentagonal prismatic {Co@Ge10} clusters at a 4-atom face. In contrast to this, the β isomer shows features of deltahedral cluster bonding. DFT studies underscore that the bonding in the α isomer is electron-precise, while the β isomer is characterized by multicenter bonding. Another, closely related cluster is [Rh3@Sn24]5− (Figure 10c). Its synthesis and key structural features are discussed in more detail in the context of cluster fusion (see section 4.3). It can be viewed as a trimer of the very same {M@Tt10} cages that are present in β-[Co2@Ge16]4−. A more complex fusion of three {Rh@Sn10} cages via edges of the square faces affords [Rh3@Sn24]5−. Larger intermetalloid clusters have also been characterized which have M−L fragments distributed throughout the cluster shell. The reaction of K4Ge9 with [Ni(CO)2(PPh3)2] in en at

prepared from K4Tt9 and [Pd(PPh3)4], adopt an ellipsoid shape with a (nonbonded) dumbbell of Pd atoms occupying the cage (Figure 10h). Even though the formation pathway is not clear, a simple fusion of two [M@Tt9]4− cluster halves can be assumed. The 17-vertex cluster [Pt2@Sn17]4− (Figure 10e), which is structurally related to [Pd2@Tt18]4−, is obtained upon reaction of K4Sn9 with [Pt(PPh3)4] or [Pt(norbornene)3].145 The solution behaviors of [Pd2@Sn18]4− and [Pt2@Sn17]4− were studied via 119Sn and 195Pt NMR spectroscopy.144,158 For both clusters, only one 119Sn and one 195Pt NMR signal were observed. This was true for [Pt2@Sn17]4− even at low temperatures (−50 to −60 °C, see Figure 9a). Thus, it was

Figure 9. 119Sn NMR studies of binary intermetalloid clusters. (a) Temperature-dependent 119Sn NMR spectra for [Pd2@Sn18]4−, recorded in the temperature range from −50 to −10 °C. Adapted from ref 158. Copyright 2008 American Chemical Society. (b) 119Sn NMR spectrum of [(Ni@Sn8)Sn(Ni@Sn8)]4−, recorded at −64 °C. Peaks are plotted on the same intensity scale. Adapted from ref 144. Copyright 2006 American Chemical Society.

concluded that the cluster shells are highly fluxional in solution. For all clusters, no evidence for a Pd−Pd or Pt−Pt interaction is found, hinting at a merely templating role of the transition metal atoms. In contrast, [(Ni@Sn8)Sn(Ni@Sn8)]4−, prepared from K4Sn9 and [Ni(cod)2], consists of two vertex-sharing {Ni@ Sn9} subunits (Figure 10d).144 This cluster shows no fluxionality on the NMR time scale at −64 °C, and resonances for all four different Sn environments can be observed in the expected ratio (Figure 9b). An increase in the temperature is accompanied by an intramolecular exchange process that involves all but the central Sn atoms. This atom does not take K


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Figure 10. Molecular structures of intermetalloid clusters with more than 12 vertices, comprising group 14 atoms and endohedral transition metal atoms. Compounds with similar structures are grouped, and a representative cluster is shown. (a) [Co2@Ge16]4− (α isomer),77 (b) [Co2@Ge16]4− (β isomer),77 (c) [Rh3@Sn24]5−,134 (d) [(M@Sn8)Sn(M@Sn8)]q− (M/q = Ni/4,144 Co/5129), (e) [(M@Sn8)Sn(M@Sn8)]q− (M/q = Co/5,127 Rh/6134), (f) [Pt2@Sn17]4−,145 (g) [(Ni@Ge9)Ni(Ni@Ge9)]4−,148 (h) [Pd2@Tt18]4− (Tt = Ge,146 Sn147), (i) [Ti@Sn15Ti3Cp5]4/5−,149 (j) [Ni2@ Ge13Ni4(CO)5]4−.137

room temperature afforded the 17-vertex deltahedral cluster, [Ni2@Ge13Ni4(CO)5]4−.137 This pill-shaped anion comprises two interstitial nickel atoms and an irregular shell of 13 germanium atoms, two {Ni(CO)} moieties, and one {Ni2(CO)3} moiety (Figure 10j). Viewed another way, the cluster comprises two interpenetrating icosahedra, {Ni@ Ge9Ni3(CO)2} and {Ni@Ge7Ni5(CO)5}, each with one of the two interstitial nickel atoms at one of its vertices. The anion [Ti@Sn15Ti3Cp5]4/5− was obtained from a liquid ammonia solution of K12Sn17, 18-crown-6, and [TiCl2Cp2].149 The charge situation is unclear, as there are nine cations and two clusters in the unit cell. Therefore, the two clusters have either two different charges, or there is an unidentified proton on the cluster, or a cocrystallized amide. The cluster is unique and appears only “half-formed”. Half of the cluster anion contains an endohedral titanium atom at the center of a partially distorted icosahedron that is missing two vertices. The

other half is a pentagonal bipyramid with one axial vertex replaced by {TiCp}, which can be seen as the base of an icosahedron with the endohedral metal atom in place (Figure 10i). These partially formed cluster fragments provide rare insight into potential intermediates on the path to intermetalloid clusters. See section 4 for further discussion of this topic. 2.4. Coordination Compounds and Heterometallic Clusters of Group 14 Elements

A key step in intermetalloid cluster formation is the coordination of a transition metal atom by one or more Zintl ions. Many such coordination compounds of both Tt44− and Tt94− have been structurally characterized. Zintl anions that are not directly isolable can also be trapped as part of such coordination compounds, providing direct evidence of otherwise difficult-to-characterize solution species. For group L


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Table 2. Coordination Compounds and Heterometallic Clusters of the Group 14 Elements cluster

precursors 4−

[(MesCu)Si4(CuMes)] [(MesCu)(Si3.3Ge0.7)(CuMes)]4− [(MesCu)Ge4(CuMes)]4− [{η2-(HGe4)}ZnPh2]3− [(η2-Ge4)Zn(η3-Ge4)]6− [(η3-Ge4)Zn(η3-Ge4)]6− [(η2-Sn4)Au(η2-Sn4)]7− 2 2 3− 1 ∞[Au(η :η -Sn4)] 2 2 3− 1 [Au(η :η -Pb )] ∞ 4 2 2 4− 1 [Au{η :η -(TlSn ∞ 3)}] [(Sn2Sb2)Au(Sn2Sb2)]3− [Cd3(Ge3P)3]3− [(η4-Tt9)Cr(CO)3]4− Tt = Sn, Pb [(η4-Tt9)M(CO)3]4− Tt = Sn, Pb; M = Mo, W [(η5-Sn9)W(CO)3]4− [(η5-Pb9)Mo(CO)3]4− [Tt9ZnPh]3− Tt = Si, Ge, Sn, Pb [Tt9ZnR]3− Tt = Ge, Sn, Pb; R = Mes, iPr [Tt9CdPh]3− Tt = Sn, Pb [Sn9Cd(SnnBu3)]3− [Ge9Cu(PR3)]3− R = Cy, iPr [Si9Cu(NHCDipp)]3− [Sn9M(NHCDipp)]3− M = Cu, Ag, Au [Ge9Ni(CO)]2− [Ge9Pd(PPh3)]3− [Tt9Ir(cod)]3− Tt = Sn, Pb [(Ge9)Zn(Ge9)]6− [(Tt9)M(Tt9)]7− M = Cu, Ag [(Sn9)Hg(Sn9)]6− [(Tt9)MM(Tt9)]6− Tt/M = Ge/Zn, Pb/Cd [(Ge9)Zn(Ge9)Zn(Ge9)]8− [(Ge9)Hg(Ge9)Hg(Ge9)Hg(Ge9)]10− 2− 1 ∞[Zn(Ge9)] 2− 1 ∞[Hg(Ge9)]

[Ge5Ni2(CO)3]2− [(CO)3Mo(η5:η5-Pb5)Mo(CO)3]4− [(tol)Nb(η3:η3-Sn6)Nb(tol)]2− [Ge8Fe(CO)3]3− [Ge8Mo2(CO)6]4− [Sn8TiCp]3− [Ag(Sn9Sn9)]5− [Sn14Ni(CO)]4− [(Ge9)Au3(Ge9)]5− [Au3Ge45]9− [Ge12{Fe(CO)3}8I4] [(SnI)6{Fe(CO)3}4I4]2−

K6Rb6Si17/18-crown-6 Rb12Si12Ge5/18-crown-6 K14ZnGe16/18-crown-6 K6Rb6Ge17 K/Rb, Ge Cs, Ge K12Sn17/18-crown-6 AN3 (A = K, Rb, Cs), Sn AN3 (A = Rb, Cs), Pb K, Tl, Sn [K(crypt-222)]2(Sn2Sb2)·en [K(crypt-222)]2(Ge2P2)·en K4Tt9/crypt-222 Tt = Sn, Pb KSn2.05/crypt-222 K4Sn9/crypt-222 K4Pb9/crypt-222 K4Sn9/crypt-222 K4Pb9/crypt-222 K12Si17/crypt-222 K4Tt9/crypt-222 Tt = Ge, Sn, Pb K4Tt9/crypt-222 Tt = Ge, Sn, Pb Tt4E9/crypt-222 Tt = Sn, Pb [Sn9CdPh]3− K4Ge9/crypt-22 K4Ge9/crypt-222 A12Si17/crypt A = K, K/Rb, Rb K4Sn9/crypt-222 K4Ge9/crypt-222 K4Ge9/crypt-222 K4Tt9/crypt-222 Tt = Sn, Pb K4Ge9/crypt-222 K4Tt9/crypt-222 K4Sn9/crypt-222 K4Ge9/crypt-222 K4Pb9/crypt-222 K4Ge9/crypt-222 K4Ge9/crypt-222 K4Ge9/18-crown-6 Rb4Ge9/crypt-222 K4Ge9/crypt-222 KGe1.67/crypt-222 K4Pb9/crypt-22 K4Sn9/crypt-222 K4Ge9/crypt-222 K4Ge9/18-crown-6 K4Sn9/18-crown-6 K4Sn9/crypt-222 K4Sn9/crypt-222 K4Ge9/crypt-222 K4Ge9/crypt-222 GeI4 SnI4

ref

Figure

[CuMes]5 [CuMes]5 [CuMes]5 ZnPh2 Zn Zn [AuCl(PtBu3)] Au Au Au [AuBr(PPh3)] CdPh2 [Cr(CO)3Mes]

160 161 162 163 162 164 165 166 166 167 168 169 159, 170

11f 11f 11f 11g 11b 11c 11a 11d 11d 11e 23b 23a 12b

[M(CO)3(R)] (M = Mo, R = Mes/C7H8; M = W, R = Mes)

171, 172

12b

[W(CO)3Mes] [Mo(CO)3(MeCN)3] ZnPh2

171 173 174

12c 12c 12e

ZnMes2, ZniPr2

175

12d

CdPh2

176

12d

HSnnBu3 [CuCl(PR3)] R = Cy, iPr [CuCl(NHCDipp)]

176 177

12d 12d

178

12d

179

12d

125 139 99, 180

12d 12d 12d

181 179

12i,j 12e,f

182 183, 184

12g 12k

183 185 183

12m 12n 12o

186 187 188 189 190 191 149 192 193 194 195 196 197

12p 12a 14a 14b 14c 14e 14d 12l 14f 12h 14g 14h

[MCl(NHCDipp)] M = Cu, Ag, Au [Ni(CO)2(PPh3)2] [Pd(PPh3)4] [Ir(cod)Cl]2 [Zn2{HC(Ph2PNPh)2}2], ZnCp*2 [CuCl(PR3)], R = Cy, iPr [AgCl(NHCDipp)] Hg [Zn2{HC(Ph2PNPh)2}2] CdPh2 [Zn2{HC(Ph2PNPh)2}2] HgPh2 ZnCp*2 Hg [Ni(CO)2(PPh3)2] [Mo(CO)3Mes] [Nb(tol)2] [Fe(CO)3(cot)] [Mo(CO)6], [Mo(CO)3(C7H8)] [TiCl2Cp2] [Ag4Mes4] [Ni(CO)2(PPh3)2] [AuCl(PPh3)] [AuCl(PPh3)] [Fe2(CO)9] [Fe(CO)5] M


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Figure 11. Molecular structures of coordination compounds formed by tetrahedra of group 14 atoms with transition metal atoms. Compounds with similar structures are grouped, and a representative structure is shown. (a) [Au(η2-Sn4)2]7−,165 (b) [(η2-Tt4)Zn(η3-Tt4)]6− (Tt = Ge,162,164 Sn198), (c) [(η3-Ge4)Zn(η3-Ge4)]6−,164 (d) 1∞[Au(η2:η2-Tt4)]6− (Tt = Sn,166 Pb166), (e) 1∞[Au(η2:η2-TlSn3)]4−,167 (f) [(MesCu)Tt4(CuMes)]4− (Tt4 = Si4,160 Ge4,162 Si3.3Ge0.7161), (g) [{η2-(HGe4)}ZnPh2]3−.163 Hydrogen atoms are omitted for clarity.

anions. Homoatomic tetrahedral Tt44− anions have too great a charge density for their salts to be soluble in or isolated from en or DMF; however, complexes of Tt44− have been crystallized from liquid ammonia solutions. Single Tt44− clusters can form a bridge between two Cu complex fragments in an η3:η3 fashion, as seen in the anions [MesCu(Si4−xGex)CuMes]4− (x = 0,160 0.7,161 4;162 Figure 11f). In [{η2-(HGe4)}ZnPh2]3−, a protonated tetrahedron, HGe43−, coordinates a ZnPh2 moiety in an η2 fashion.163 Quantum chemical calculations in conjunction with X-ray structural data suggested that the hydrogen atom bridges two germanium atoms opposite the ZnPh2 group (Figure 11g). The inverse situation, in which two Tt44− anions coordinate to a transition metal, has been observed for the anions [(η3Ge 4 )Zn(η 3 -Ge 4 )] 6− and [(η 2 -Tt 4 )Zn(η 3 -Tt 4 )] 6− (Tt = Ge,162,164 Sn198). These are present in the compounds A14ZnGe16 (A = K, Rb) and Cs6ZnGe8 or were prepared in liquid ammonia from Rb4Sn4 and ZnPh2. The compound [(η2Sn4)Au(η2-Sn4)]7− was isolated from a liquid ammonia solution of K12Sn17 and [AuCl(PtBu3)] (Figure 11a−c).165 Two coordination compounds of the related group 14/15 anions (Sn2Sb2)2− and (Ge3P)3− have been reported as [(η2Sn 2 Sb 2 )Au(η 2 -Sn 2 Sb 2 )] 3− 168 and the unique trimer [Cd3(Ge3P)3]3−,169 both of which will be discussed in more detail below (see section 3.3 and Figure 23a and b). The lower charges of these binary tetrahedral anions enabled their syntheses in en, rather than necessitating the use of ammonia. Related compounds containing extended one-dimensional anions have also been reported. The solids A3[AuSn4] (A = K−Cs), A3[AuPb4] (A = Rb, Cs), and K4[AuTlSn3] all contain linear chains of tetrahedral anions that are bridged through Au+ ions (Figure 11d and e).166,167 In all cases, the bridging mode is exclusively η2:η2 via the edges of the tetrahedra. 2.4.2. Coordination Compounds of Tt94− and Tt52− Anions. Reports of coordination compounds of Tt94− cages are far more common than those of Tt44−, which can be

14 cluster anions in particular, the line between coordination and heterometallic cluster formation is blurry; in one sense, the Zintl anion is acting as a ligand to a transition metal, while, in another sense, the transition metal is an additional vertex in a new cluster. For example, the reaction of Sn94− with [Cr(CO)3(Mes)] affords [(η4-Sn9)Cr(CO)3]4−.159 The anion Sn94− displaces the mesityl ligand and acts as a 6-electron donor to the chromium atom, while the Cr(CO)3 moietya zero-electron-donor according to the isolobal analogyadds a vertex to the nido-Sn 9 4− to afford a new 10-vertex heterometallic closo-cluster. This line is further blurred by the addition of more transition metals, leading to novel structure types. A brief overview of these compounds is given here. We note that a more detailed discussion of Zintl ions as mere ligands in coordination compounds is to be found elsewhere.15,16,124 Table 2 contains a list of these compounds and the reagents used in their preparation. 2.4.1. Coordination Compounds of Tt44− Anions. The simplest types of heterometallic clusters, in one sense, are those in which the intact anions merely act as polydentate ligands. Coordination compounds of the known group 14 Zintl anions Tt44−, Tt52−, and Tt9q− have been structurally characterized. In all of these compounds, the anion has several possible coordination modes: the tetrahedral Tt44− anions coordinate via either an edge in an η2 fashion or a face in an η3 fashion. The 9-atom Tt94− cages can coordinate as η1 ligands via a single vertex, in an η3 mode via a deltahedral face, or in an η4 manner via the open square face of a capped square antiprismatic cage. Homoatomic Tt94− anions are typically accessible from solutions of the compounds A4Tt9 (A = alkali metal). While respective A4Tt4 compounds exist as well, they have very low solubilities in en and DMF and can only be used as sources of Tt 4 4− in liquid ammonia. The more soluble A 12 Tt 17 compounds (which contain both Tt44− and Tt94− in a 2:1 ratio) have proven to be more practical sources for Tt44− N


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Figure 12. Molecular structures of coordination compounds formed by 5-vertex and 9-vertex cages of group 14 atoms with transition metal atoms. Compounds with similar structures are grouped, and a representative anion is shown. (a) [Ge5Ni2(CO)3]2−,187 (b) [(η4-Tt9)M(CO)3]4− (Tt = Sn, Pb; M = Cr, Mo, W),159,170,172,173,199 (c) [(η5-Tt9)M(CO)3]4− (Tt/M = Sn/W,171 Pb/Mo173), (d) [(η4-Tt9)M(L)]q− (Tt/M/L/q = Si/Cu/ NHCDipp/3,178 Si,Ge,Sn,Pb/Zn/Ph/3,174 Ge,Sn,Pb/Zn/Mes,iPr/3,175 Sn,Pb/Cd/Ph/3,176 Ge/Cu/PiPr3,PCy3/3,177 Sn/Ag/NHCDipp/3,179 Ge/ Ni/CO/3,125 Ge/Pd/PPh3/3,139 Sn/Ir/cod/3,99,180 Pb/Ir/cod/3,180 (e) [(Ge9)Cu(Ge9)]7−,177 (f) [(Sn9)Ag(Sn9)]7−,179 (g) [(Sn9)Hg(Sn9)]6−,182 (h) [(Ge9)Au3(Ge9)]5−,194 (i) [(η3-Ge9)Zn(η3-Ge9)]4−,181 (j) [(η3-Ge9)Zn(η4-Ge9)]4−,181 (k) [(Tt9)M−M(Tt9)]6− (M/Tt = Zn/Ge,183 Pb/ Cd184), (l) [Ag(Sn9−Sn9)]7−,192 (m) [(Ge9)Zn(Ge9)Zn(Ge9)]8−,183 (n) [(Ge9)Hg(Ge9)Hg(Ge9)Hg(Ge9)]10−,185 (o) 1∞[Zn(Ge9)]2−,183 (p) 1 2− 185,186,200 ∞[Hg(Ge9)] .

syntheses of [(Ge9)Zn(Ge9)]6−,181 [(Sn9)Hg(Sn9)]6−,182 and [(Ge9)M(Ge9)]7− (M = Cu,177 Ag;179 Figures 12e−g,i,j). Remarkably, this was also demonstrated for Zn(I) and Cd(I) dumbbells with the syntheses of [(Tt9)M−M(Tt9)]6− (Tt/M = Ge/Zn,183 Pb/Cd;184 Figure 12k). Metal-cluster coordination can continue with the formation of oligomers, such as [(Ge9)Zn(Ge9)Zn(Ge9)]8− and [(Ge9)Hg(Ge9)Hg(Ge9)Hg(Ge9)]10− (Figures 12m,n) and even polymeric chains like 2− 1 and 1∞[HgGe9]2− (Figure 12o,p).183,185,186 ∞[ZnGe9] The variation of binding modes when two 9-atom cages coordinate one M atom was investigated for [(η4-Ge9)Sn(η3Ge 9)] 4− , [(η 4-Ge9 )Zn(η 3-Ge9 )] 6− , and [(η 3 -Ge9 )Zn(η 3 Ge9)]6− (Figure 12i,j).181 Their electronic structures were studied by means of quantum chemical methods, which

explained by the good solubility of the A4Tt9 precursor salts at room temperature in solvents like en and DMF. Several metal carbonyl complexes of the form [(η4/5-Tt9)M(CO)3]4− (M = Cr, Mo, W; Tt = Sn, Pb) have been synthesized by reacting K4Tt9 with [M(CO)3(L)] (Figure 12b,c).159,170−173,199 K4Tt9 (Tt = Ge, Sn, Pb) and K12Si17 also react with a multitude of transition metal complexes to afford the related clusters [(η4Tt9)M(L)]q− (M(L) = Zn(Ph),174 Zn(Mes),175 Zn(iPr),175 Cd(Ph),176 Cu(PR3),177 Cu(NHCDipp),178 Ag(NHCDipp),179 Ni(CO),125 Pd(PPh3),139 Ir(cod);99,180 Figure 12d). Complexation can sometimes stabilize otherwise difficult to isolate clusters, such as Si94− in [(η4-Si9)Cu(NHCDipp)]3−.178 Further coordination of the transition metal atom by a second anion can occur, as has been demonstrated by the O


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simple {Ge9} units and one {Ge18} fragment (see Figure 14g).195 The {Ge18} moiety contains yet another {Ge9} unit that is bonded to every other atom of a chairlike Ge6 ring through a triangular face. This {Ge6} ring is in turn bonded to a {Ge3Au3} ring via the Ge atoms. The coordination environment of the Au atoms is planar, which would suggest their oxidation state to be +III, but computational results supported an assignment of +I. Long interatomic Au−Au distances of around 3.7 Å hint at a lack of aurophilic interactions. The formation of this very complex main group substructure once more highlights the extensive abilities of Zintl anions to rearrange in solution to form novel clusters. As shown for the [Au3Ge45]9− anion, Zintl ions that have not been isolated as bare clusters can sometimes be realized as part of a coordination compound. In another example, a pentagonal planar Pb54− unit is found in the inverse sandwich complex [(CO)3Mo(η5:η5-Pb5)Mo(CO)3]4− (Figure 14a), which was obtained upon reacting K4Pb9, crypt-22, and [Mo(CO)3Mes]. As with [Ge5Ni2(CO)3]2−, the cluster can be described as a pentagonal bipyramid.188 Similarly, a (formally) highly charged six-membered ring Sn612− is trapped in a chair conformation in the complex [(tol)Nb(η3:η3-Sn6)Nb(tol)]2− (Figure 14b). This is possible because the electron density can effectively be shifted to the highly electrophilic group 5 metal atoms.189 Analogues of 9- and 10-vertex Zintl ions have been isolated as [Ge8Fe(CO)3]3− and [Ge8Mo2(CO)6]4−. In [Ge8Fe(CO)3]3−, the iron atom occupies the vertex of the 9-atom cluster, trapping a distorted square antiprismatic Ge8 moiety (Figure 14c),190 similar to Sn86−, found in A4Li2Sn8 (A = K, Rb).207 The {Fe(CO)3} fragment acts as a 2-electron donor, making it an isolobal replacement for a Ge vertex. The cluster thus has 21 skeletal electrons and would be expected to be paramagnetic. This was confirmed by EPR spectroscopy, with calculations indicating that the SOMO is localized on the Ge8 framework (Figure 13).

showed that, depending on the cluster face that coordinates the metal atom M (Sn, Zn), the type of interaction changes. Interactions between the open square face of a {Ge9} capped square antiprismatic cluster and the coordinated metal M are strongly covalent and characterized by multicenter bonding, so that M becomes part of the cluster. In contrast, coordination through a triangular face can be viewed as a Lewis acid−base interaction between an orbital located at M radially pointing outward and a second orbital located on a triangular face. Again, the type of M dictates the nature of this interaction. In the case of [(η4-Ge9)Sn(η3-Ge9)]4−, the {(η4-Ge9)Sn} moiety acts as the Lewis base, with a lone pair on the tin atom interacting with the unoccupied molecular orbital at a triangular face of the other {Ge9} unit. For [(η4-Ge9)Zn(η3Ge9)]6−, the reverse is true. The {(η4-Ge9)Zn} moiety acts as a Lewis acid and accepts electron density via the zinc atom. In contrast to the plethora of compounds containing or derived from Tt44− and Tt9q−, the well-known Tt52− (Tt = Si− Pb)201−203 anions had long been neglected. However, Ge52− was just recently employed as a precursor in a reaction with [Ni(CO)2(PPh3) 2], affording [Ge5Ni2 (CO) 3]2− (Figure 12a).187 In this cluster, the Ge52− cage retains its trigonal bipyramidal shape. Two Ni(CO) fragments cap neighboring trigonal faces, and a third CO ligand bridges a Ni−Ni contact, creating a pseudopentagonal bipyramid. Thus, the cluster may be viewed as a heterometallic structural analogue of the Zintl ions Tl77− and (Tl4Bi3)3−.62,204 According to computational analyses, the cluster is best described as the coordination of {(μ-CO)(NiCO)2}0 by Ge52−, with 4c−2e bonds formed between the Ge3 faces and the Ni atoms. 2.4.3. More Complex Coordination Compounds and Heterometallic Clusters of Group 14 Elements. All of the anions presented in the preceding sections 2.4.1 and 2.4.2 can easily be interpreted as simple coordination compounds in which the Zintl anions have replaced one or more ligands at the transition metal atom but have not undergone a rearrangement process themselves. However, these Zintl anion ligands can also react themselves, as clearly evident from their potential to fully rearrange to facilitate the formation of (larger) intermetalloid clusters (see section 2.3). The simplest transformation possible is the oxidative formation of a Tt−Tt bond, as observed in the reaction of K4Sn9 with [AgMes]4. This reaction affords the anionic cluster [Ag(Sn9−Sn9)]5−, wherein the two former Sn94− fragments are still intact but share a common Sn−Sn bond.192 The newly formed (Sn9−Sn9)6− unit in the [Ag(Sn9−Sn9)]5− anion then acts like a pincer-type ligand, with Ag+ occupying a pocket between the two cages (see Figure 12l). This kind of bond formation has also been observed before for Ge94− clusters without the clusters coordinating a transition metal.205,206 In a similar manner, the Ni atom in [Sn14Ni(CO)]4−synthesized by heating a DMF solution of [(Ni@Sn9)Ni(CO)]3−can be viewed as being bound inside a pocket formed by two fused, edge-sharing, Sn8 cages (Figure 14f). Reactions of K4Ge9 with [AuCl(PPh3)] yielded two products, in which Ge clusters coordinate Au+ ions. The anion [(Ge9)Au3(Ge9)]5− comprises one capped square antiprismatic and one tricapped trigonal prismatic Ge9 cluster unit, which coordinate a Au33+ triangle in an η3:η3 fashion (see Figure 12h).194 The Au atoms within the triangle are 2.9−3.0 Å apart, but quantum chemical studies showed that there is only a weak Au−Au interaction. The second product is the much more complex anionic cluster [Au3Ge45]9−, which can be viewed as being composed of three

Figure 13. X-band CW EPR spectrum of the [K(crypt-222)]+ salt of [Ge8Fe(CO)3]3− at 20 K (black) and the simulated spectrum of the anion (blue). The simulated spectrum has been calculated for the three most abundant cluster anions (73Ge, I = 9/2): [nGe8Fe(CO)3]3− (52%), [73GenGe7Fe(CO)3]3− (35%), [73Ge2nGe6Fe(CO)3]3− (10%), where n represents all of the remaining naturally occurring germanium isotopes (70Ge, 72Ge, 74Ge, 76Ge: all with I = 0). Reproduced with permission from ref 190. Copyright 2010 Wiley-VCH. P


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Figure 14. Molecular structures of heterometallic clusters formed by group 14 polyatomic units with transition metal atoms. (a) [(CO)3Mo(η5:η5Pb5)Mo(CO)3]4−,188 (b) [(tol)Nb(η3:η3-Sn6)Nb(tol)]2−,189 (c) [Ge8Fe(CO)3]3−,190 (d) [Sn8TiCp]3−,149 (e) [Ge8Mo2(CO)6]4−,191 (f) [Sn14Ni(CO)]4−,193 (g) K[Au3Ge45]8−,195 (h) [Ge12{Fe(CO)3}8(μ-I)4].196 Hydrogen atoms are omitted for clarity.

I)4].196 This species comprises a Ge4 ring, two Ge2 dumbbells, and four Ge atoms that bridge the four {Fe(CO)3}2 units (Figure 14h). Quantum chemical calculations indicated that the bonds are primarily localized 2c−2e interactions; however, there is some delocalization between the Ge4 ring and the bridging Ge atoms. A similar reaction between SnI4 and [Fe(CO)5] afforded the adamantane-like cluster, [{Fe(CO)3}4{SnI}6I4]2−.197 Here again, the bonding was shown to be mostly localized, although results of the localized molecular orbital analysis differed depending on the method. This localization, as well as the positive oxidation states of the group 14 elements, sets these clusters apart from those derived from Zintl ions.

The anion [Ge8Mo2(CO)6]4− forms a distorted bicapped square antiprism, in which {Mo(CO)3} moieties are inserted into two adjacent vertices with a long Mo−Mo contact (Figure 14e).191 Formally, the cluster has only 20 skeletal electrons2 short of the 22 required for a closo-cluster. This electron deficiency explains strong distortions of the Ge framework and the resulting variation in the Ge−Ge bond lengths. The reaction of [TiCl2Cp2] and Sn94− in liquid ammonia afforded a coordination complex of another 8-atom cluster, [Sn8TiCp]3− (Figure 14d).149 In this case, however, the anion does not represent an analogue of a known Zintl ion but is rather reminiscent of a fragment of a larger intermetalloid cluster, as discussed in section 4. Heterometallic clusters of group 14 (semi)metal and transition metal atoms have also been synthesized without Zintl ion precursors. For example, GeI4 and [Fe2(CO)9] were reacted in the ionic liquid system, [BMIm]Cl/AlCl3, resulting in the formation of the neutral cluster [Ge12{Fe(CO)3}8(μ-

2.5. Coordination Compounds and Heterometallic Clusters of Substituted Zintl Anions

Simple Zintl anions, such as Ge94− and Sn94−, can be modified via the attachment of organic or organoelement ligands in Q


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Figure 15. Molecular structures of coordination compounds or heterometallic clusters formed from silyl-, stannyl-, or organyl-substituted 9-vertex cages of group 14 atoms with transition metal atoms. Compounds with similar structures are grouped, and a representative molecule is shown. (a) (Ge9Hyp3)−,66,68,215,232 (b) [{Ge9R2R′}M(L)] (R = R′ = Hyp/M(L) = Ni(dppe),215 Cu(PiPr3),216 ZnCp*,216 (Cu,Ag,Au)NHCDipp,217 Cu(MIC/ CAAC);218 R = R′ = Si(iPr)3, Si(iBu)3, P(NiPr2)2, P(NiPr2)tBu/M(L) = CuNHCDipp),220−222 R = Hyp, R′ = P(Mes)2/q = 0/M(L) = Cu(NHCDipp),219 (c) [(η5-Ge9Hyp3)M(CO)3]− (M = Cr, Mo, W),214,233 (d) [(η5-Ge9Hyp3Et)M(PPh3)] (M = Ni, Pd, Pt),223,224 (e) [(L)M(Ge9R2R′)M(L)] (R = R′ = Hyp, M = Ni, L = dppe;215 R = Hyp, R′ = -, L = NHCDipp, M = Cu, Ag, Au;220 L = CAAC, M = Cu,218 R = P(NiPr2)2, R′ = Cr(CO)5, L = NHCDipp, M = Cu),222 (f) [{Ge9(SiR3)3}M{Ge9(SiR3)3}]q (M/R/q = Mn/TMS/0,215 Zn,Cd,Hg/TMS/0,216,225 Cu,Ag,Au/TMS/1−,226 Au/iBu/1−,221 Pd/TMS/2−),228 (g) [{(Ge9Hyp2)Cu(PtBu2)}2],219 (h) [(Ge9Hyp3)Cu(Ge9Hyp3)Cu(PPh3)],228 (i) [{Ge9(SiPh3)2}{Cu(PiPr3)}2]2,216 (j) [HypM′(Tt9Hyp3)M(Tt9Hyp3)M′Hyp}]q (Tt/M/M′/q = Ge/Pt/Zn/0,229 Sn/Au/Au/−1230), (k) [{Ge9(SniPr3)3}Pd3{Ge9(SniPr3)3}],70 (l) [{Ge9(SiiPr3)3}Pd3{Ge9(SiiPr3)3}].231 All hydrogen atoms and selected carbon atoms are omitted for clarity.

Ge94− leads to a clear energetic preference of the C3v conformation of the 9-vertex cage. Hence, only 3-atom faces remain accessible, leading with few exceptions to η3-like coordination of transition metals, instead of the η4 or η5 modes of coordination described above for some of the “naked” 9vertex cages with C4v symmetry. The reactivities of these compounds with complexes of low oxidation state (0, +I, and +II) transition metals have been extensively explored. Those that have resulted in coordination compounds or hetero-

order to alter the charge, solubility, and stability of the cluster67,208−211 or to introduce functionality.212,213 In particular, these modified Zintl anions tend to be soluble in typical organic solvents, such as THF or acetonitrile (MeCN). Alternatively, ligand decorated Zintl clusters can be synthesized by salt metathesis of subvalent element halides (e.g., GeBr) and an alkali metal silyl compound (e.g., LiHyp; Hyp = Si(SiMe3)3).68 Both methods allow for the preparation of salts of the trisilylated monoanion [Ge9(SiR3)3]− (R = TMS, Et, iPr, i Bu; Figure 15a). The attachment of three substituents to R


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Table 3. Coordination Compounds of Substituted Zintl Anions of the Group 14 Elements cluster

precursors

[(η5-Ge9Hyp3)M(CO)3]− M = Cr, Mo, W

(Ge9Hyp3)−

[(Ge9Hyp3)Ni(dppe)] [(Ge9Hyp3)ZnCp*] [(Ge9Hyp3)Cu(PiPr3)] [(Ge9Hyp3)M(NHCDipp)] M = Cu, Ag, Au [(Ge9Hyp3)M(LC)] LC = MIC, CAAC [{Ge9Hyp2PMes2}Cu(NHCDipp)] [(Ge9{SiiPr3}3)Cu(NHCDipp)] [(Ge9{SiiBu3}3)Cu(NHCDipp)] [{Ge9(P{NiPr2}3)3}Cu(NHCDipp)] [(dppe)Ni(Ge9Hyp3)Ni(dppe)]+ [(LC)Cu(Ge9Hyp2)Cu(LC)] LC = NHCDipp, CAAC C [(L )Cu{Ge9RP2Cr(CO)5}Cu(LC)] LC = NHCDipp, RP = P(NiPr2)2 [{(Ge9Hyp2)Cu(PtBu2)}2] [{Ge9(SiPh3)2}{Cu(PiPr3)}2]2 [(η5-Ge9Hyp3Et)M(PPh3)] M = Ni, Pd, Pt [(Ge9Hyp3)M(Ge9Hyp3)] M = Mn, Zn, Cd, Hg [(Ge9Hyp3)M(Ge9Hyp3)]− M = Cu, Ag [(Ge9Hyp3)Ag(Ge9Hyp3)]− [(Ge9Hyp3)Au(Ge9Hyp3)]− [(Ge9{Si(iBu)3}3)Au(Ge9{Si(iBu)3}3)]− [(Ge9Hyp3)Pd(Ge9Hyp3)]2− [(Ge9Hyp3)Cu(Ge9Hyp3)2Cu(PPh3)] [HypZn(Ge9Hyp3)Pt(Ge9Hyp3)ZnHyp] [HypAu(Sn9Hyp3)Au(Sn9Hyp3)AuHyp]− [{Ge9(SnR3)3}Pd3{Ge9(SnR3)3}]2− R = iPr, Cy [(Ge9{Si(iPr)3}3)Pd3(Ge9{Si(iPr)3}3)]2− [(Pd@Sn9)SnCy3]3− [(Pd@Sn9)Pd(SnCy3)]3−

(Ge9Hyp3)− (Ge9Hyp3)− (Ge9Hyp3)− (Ge9Hyp3)−

ref

Figure

[{Ge9RP3}Cu(LC)]

[M(CO)3(RCN)] M = Cr, W; R = Me M = Mo; R = Et [NiCl2(dppe)] ZnCp*2 [CuCl(PiPr3)] [MCl(NHCDipp)] M = Cu, Ag, Au [CuCl(LC)] LC = MIC, CAAC [CuCl(NHCDipp)] [CuCl(NHCDipp)] [CuCl(NHCDipp)] [CuCl(NHCDipp)] [NiCl2(dppe)] [CuCl(LC)] LC = NHCDipp, CAAC [Cr(CO)5(thf)]

(Ge9Hyp2)2−, tBu2PCl [Ge9(SiPh3)3]− [Ge9Hyp3(Et)]

[CuCl(PCy3)] [CuCl(PiPr3)] [M(PPh3)2(C2H4)] (M = Ni, Pt)/[Pd(PPh3)4]

219 216 223, 224

15g 15i 15d

(Ge9Hyp3)−

MCl2 (M = Mn, Zn, Cd, Hg)

215, 225

15f

(Ge9Hyp3)−

226

15f

(Ge9Hyp3)− (Ge9Hyp3)− K4Ge9/SiiBu3Cl K4Ge9/HypCl K4Ge9/HypCl [Zn(Ge9Hyp3)2], LiHyp [Sn10Hyp4]2− K4Ge9/SnR3Cl/crypt-222

[Cu[Al(OCH(CF3)2)4]2 [Ag(Al(OC4F9)4] [AgCl(NHCDipp)] [AuCl(PPh3)] [AuCl(PPh3)] [Pd(PPh3)4] [Cu(PPh3)3Br] [Pt(PPh4)4], ZnCl2 [Au(SHyp)(PPh3)] [Pd(PPh3)4]

217 227 221 228 228 229 230 70, 231

15f 15f 15f 15f 15h 15j 15j 15k

K4Ge9/Si(iPr)3Cl/crypt-222 [Sn9(SnCy3)]3− [Sn9(SnCy3)]3−

[Pd(PPh3)4] [Pd(PPh3)4] [Pd(PPh3)4]

231 82 82

15l 8c 8e

(Ge9Hyp3)− (Ge9Hyp2PMes2)− [Ge9{SiiPr3}3]− [Ge9{SiiBu3}3]− [Ge9{P(NiPr2)3}3]− (Ge9Hyp3)− (Ge9Hyp2)2−

214

15c

215 216 216 217

15b 15b 15b 15b

218

15b

219 220 221 222 215 218, 220

15b 15b 15b 15b 15e 15e

222

15e

Ge9RSi3)M(CO)3]−, in which the transition metal atom occupies a five-bonded vertex in a bicapped square antiprismatic architecture (Figure 15c).214 The authors presented a possible reaction sequence, which includes the stepwise loss of individual acetonitrile ligands and the extension of hapticity from a single germanium atom ligand to a triangle of germanium atoms coordinated to the transition metal. In the last step, the {M(CO)3} fragment is integrated into the cluster shell. Gas phase collision induced dissociation of [(Ge9RSi3)Cr(CO)3]− afforded the carbonyl-free cluster, [(Ge9RSi3)Cr]−, prior to the elimination of other (mainly silyl) substituents.46 Coordination of one or two transition metal atoms by one (Ge9RSi,P3)− cluster has been achieved via metathesis reactions of K(Ge9RSi,P3) and a series of transition metal complexes of nickel, zinc, and the coinage metals. These reactions afford the smallest possible coordination compounds [(Ge9RSi,P3)M(L)] (M = Ni,215 Zn,216 Cu,216,218−222 Ag,217 Au217; Figure 15b). From similar reactions, disubstituted clusters, [(L)M(Ge9RSi2R′)M(L)]q (M = Ni,215 Cu,218,220 Ag,220 Au220), can

metallic cluster complexes are presented here and summarized in Table 3. In the following section, we will discuss several series of compounds that share common structural motifs. The main building units are (Ge9R2)2− and (Ge9R3)− cages and their respective coordination compounds. In these series, the compounds mainly differ in the substituents (R) on the cage or the nature of the transition metal atoms (M) and ligands (L). For the sake of readability, we introduce abbreviations to group compounds wherever reasonable. The structures of these compounds are shown in Figure 15. A full list of all compounds discussed in the following, along with the individual references for each compound, is provided in its caption. The following abbreviations will be used: RSi (silyl groups, SiR3), LC (carbene ligands: NHCDipp, MIC, CAAC), RP (phosphine groups, PR2). In THF, [M(CO)3(NCMe)3] (M = Cr, Mo, W) reacts with (Ge9RSi3)− via displacement of the labile NCMe ligands. Insertion of the [M(CO)3] fragment into the cluster at a Ge3 face results in rearrangement to a 10-atom cluster, [{(η5S


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(Ge9Hyp3)Cu(PPh3)] (Figure 15h), was synthesized by reacting K(Ge9Hyp3) with [CuBr(PPh3)3] in acetonitrile.228 The longest “chains” to date, though, are [(L)M′(Tt9RSi3)M(Tt9RSi3)M′(L)}]q (Tt/M/M′/q = Ge/Pt/Zn/0,229 Sn/Au/ Au/−1;230 Figure 15j). [HypAu(Sn9Hyp3)Au(Sn9Hyp3)AuHyp]− is the only such compound containing (Sn9RSi3)− subunits.230 The precursors in this case were the metalloid cluster (Sn10Hyp4)2− and [Au(PPh3)Hyp], which were reacted in THF. Unlike the analogous compounds containing (Ge9RSi3)− moieties, significant rearrangement of both precursors had to occur prior to product formation. [HypZn(Ge9Hyp3)Pt(Ge9Hyp3)ZnHyp] forms in a complex rearrangement reaction of [(Ge9Hyp3)Zn(Ge9Hyp3)] and [Pt(PPh3)4]. This is the first example of a compound comprising two different types of transition metal atoms in this chemistry.229 (Ge9R3)− can also trap small transition metal clusters. In a series of reports, it was demonstrated that K[Ge9(EiPr3)3] (E = Si, Sn) reacts with [Pd(PPh3)4] to afford [{Ge9(EiPr3)3}Pd3{Ge9(EiPr3)3}]2−.70,231 These clusters comprise two icosahedral halves, which share a Pd3 face or, alternatively, in which two 9-atom fragments of an icosahedron coordinate a Pd3 cluster. Notably, the silyl and stannyl derivatives were characterized as positional isomers, with the SiiPr3 and SniPr3 ligands leading to eclipsed and staggered conformations, respectively (Figure 15k,l). Quantum chemical calculations and NMR spectroscopy showed that the different isomers were not simply a result of crystal packing but were rather the most stable conformations instead. Compared to Ge94−, it is more difficult to add substituents to Sn94−.67 Consequently, the chemistry of the hypothetical “(Sn9R3)−” has yet to be explored. However, the reaction behavior of the singly substituted version of the heavier 9-atom cage, (Sn9SnCy3)3− (Cy = cyclohexyl), has been investigated. The anion was reacted with a stoichiometric amount of [Pd(PPh3)4] in a mixed en/py solution. This did not directly result in the insertion of an ML fragment but instead in the uptake of a naked Pd atom to form [(Pd@Sn9)SnCy3]3− (Figure 8c).82 When the reaction is carried out with 2 equiv of [Pd(PPh3)4], a second Pd atom is inserted into the Sn9 cage to afford the functionalized 10-vertex cluster [(Pd@Sn9)Pd(SnCy3)]3− (Figure 8d). The formation of the cluster anion can be viewed as an oxidative insertion of a Pd atom, with two electrons being removed from cluster bonding to form the exo Pd−SnCy3 bond. NMR spectroscopy revealed that, while the SnCy3 moiety scrambles around the cage in [(Pd@Sn9)SnCy3]3−, it remains bound to the second Pd atom in [(Pd@ Sn9)Pd(SnCy3)]3−.

result as well. The most efficient way to prepare such compounds is through the use of the bis-hypersilyl compound K2(Ge9Hyp2) with respective carbene complexes [Cu(LC)X] in acetonitrile (Figure 15e).220 Analogous Ag and Au compounds were not structurally characterized but were detected by NMR spectroscopy. A phosphine-substituted cluster [(Ge9RP3)Cu(LC)] was shown to react with [Cr(CO)5(thf)] to afford [(LC)Cu{Ge9RP2Cr(CO)5}Cu(LC)].222 In this compound, the {Ge9} cluster coordinates two {Cu(LC)}+ fragments via two opposing 3-atom faces and one RP unit is replaced by a neutral {Cr(CO)5} fragment (Figure 15e). The treatment of K2(Ge9Hyp2) with tBu2PCl in MeCN, followed by addition of [Cu(PCy3)Cl], led to the formation of a dimer, [{(Ge9Hyp2)Cu(PtBu2)}2]. In the first step, a phosphine-functionalized cluster ligand, {Ge9Hyp2(PtBu2)}, is formed, which then further reacts with the copper complex [Cu(PCy3)Cl]. This complex undergoes a complete ligand exchange: the chlorine ligand is displaced by a {Ge9} unit that coordinates through a 3-atom face, and the PCy3 ligand on the Cu atoms undergoes an exchange reaction with the PtBu2 phosphine bound to a second {Ge9} cluster. This results in the formation of the final dicopper complex (Figure 15g).219 A related compound was synthesized using the less bulky anion [Ge9(SiPh3)3]−. Its reaction with [Cu(PiPr3)Cl] in MeCN resulted in the loss of one SiPh3 substituent and the formation of the dimer [{Ge9(SiPh3)2}{Cu(PiPr3)}2]2. The 4-atom faces of the square antiprismatic {Ge9} cages are oriented outward, and each one coordinates one {Cu(PiPr3)} moiety. The loss of one SiPh3 substituent per {Ge9} cluster allows for the coordination of a second {Cu(PiPr3)} unit, this time via one 3-atom face adjacent to the square face. In addition, each of these Cu atoms takes a bridging role and is coordinated by the capping Ge atom of the other {Ge9} cluster, thereby leading to the formation of the dimer (Figure 15i).216 The synthesis of neutral derivatives, (Ge9Hyp3R) (R = SnPh3, SnnBu3, Et), of (Ge9Hyp3)− has been achieved through addition of a fourth substituent via salt metathesis reactions.7,211 The neutral cluster (Ge9Hyp3Et) was shown to react with [Pd(PPh3)4] and [M(PPh3)2(C2H4)] (M = Ni, Pt) in benzene or toluene to afford the pentafunctionalized 10vertex clusters, [(η5 -Ge9 Hyp3Et)M(PPh 3)] (M = Ni− Pt).223,224 These clusters and the (Ge9Hyp3Et) precursor were analyzed by variable temperature 1H NMR spectroscopy. The data showed that the insertion of {M(PPh3)} (M = Pd, Pt) units into the cluster shell hinders the dynamic surface processes observable in (Ge9Hyp3)−, while the Ni analogue showed some evidence of dynamic behavior. Reactions of (Ge9RSi3)− with various transition metal salts and complexes often lead to the displacement of all ligands to afford cluster “sandwich” complexes of the form [{Ge9RSi3}M{Ge9RSi3}]q (M = Mn,215 Zn,216,225 Cd,216,225 Hg,216,225 Cu,226 Ag,226 Au,221,226 Pd228). In all of these dimers, the transition metal is 6-coordinate and sandwiched between two Ge3 faces, and the RSi groups on opposite clusters are staggered to varying degrees (Figure 15f). The above cluster sandwich complexes still have accessible Ge3 faces, which has led to the suggestion that supramolecular chains may be synthesized with alternating {Tt9R3} units and transition metal atoms or complex fragments.227 This has yet to be realized; however, additional coordination of metal− ligand fragments to cluster−transition metal complexes has been observed. The neutral complex, [(Ge 9 Hyp 3)Cu-

2.6. Binary Clusters of Group 15 Elements

Mixed clusters of group 15 elements and transition metals can also be synthesized using homoatomic Zintl anions as precursors. The most prominent Zintl anions of group 15 elements used in synthesis are the 7-atom nortricyclane-like cages Pn73− (Pn = P−Sb).17 All of these are readily available from binary solids A3Pn7 (A = alkali metal). The bismuthanalogue, Bi73−, was not synthesized until recently,234 and instead, K5Bi4 has been regularly used in cluster synthesis. In addition to Zintl ions, P4 and As4 have been widely used as precursors. While bonding in clusters composed of group 14 elements is marked by electron delocalization, the situation is drastically different for clusters of group 15 elements. Bonds are primarily electron-precise (2c−2e), and the number of valence electrons equals 5n for cages with n vertices, at least for T


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Table 4. Binary Intermetalloid and Selected Heterometallic Clusters of the Group 15 Elements cluster

precursors

2− 1 ∞[Rb(Nb@As8)] 3− [Nb@As8] 2− 1 ∞[K(Cr@As8)] [Cr@As8]3− [Mo@As8]2−

Rb, As K3As7/crypt-222 K3As7/crypt-222 Na3As7/crypt-222 K3As7/crypt-222

[Mo@Sb8]3− [Nb@Sb8]3− [Ln@Sb12]3− Ln = Y, La, Ho, Er, Lu [U@Bi12]3− [As@Ni12@As20]3− “[Sb@Ni12@Sb20]q−”a [Sb@Pd12@Sb20]3−

K3Sb7/crypt-222 K3Sb7/crypt-222 K5Sb4/crypt-222

[Sb@Pd12@Sb20]4− [Zn@Zn8Bi11]5− [Ni@Bi6Ni6(CO)8]4− [Ni@Bi6Ni6(Bi3)2(CO)4]4− [(As7)Zn(As7)]4− [(η4-Pn7)M(CO)3]3− Pn = P, As, Sb M = Cr, Mo, W [(As7)Pd2(As7)]4− [(As7)Cu2(As7)]4− [(As7)Au2(As7)]4− [(As7)Hg2(As7)]4− [ZnAs15]3− [HgAs15]3− [Au2Sb16]4− [Sb3Au3Sb3]3− [Pd7As16]4− [Ni5Sb17]4− [Sb6{RuCp*}2]2− [Bi3M2(CO)6]3− M = Cr, Mo [Pn3Ni4(CO)6]3− Pn = Sb, Bi [{Ni2(CO)3}Bi4{Ni2(CO)3}]2− [Bi6Ni6(CO)8]4− [Bi3Ni6(CO)9]3− [Zn2Sb12]4− [Bi9{Ru(cod)}2]3− [CuBi8]3+ [Au2Bi10]6+ [Rh@Bi9]4+ [Pd@Bi10]4+

[Pt@Bi10]4+ [Au@Bi10]5+ [(Bi8)Ru(Bi8)]6+ [(Bi8)Au(Bi8)]5+ [Ni2@Bi12]4+ [Rh2@Bi12]4+

Nb2O5 [Nb(tol)2] [Cr(naphthalene)2] [Cr(naphthalene)2] [Mo(Me-naphthalene)2] [Mo(naphthalene)2] [Mo(Me-naphthalene)2] [Nb(tol)2] [Ln(benzyl)3(thf)3] Ln = Y, La, Ho, Er, Lu [U(C5Me4H)3]/[UCl(C5Me4H)3] [Ni(cod)2] [Ni(cod)2] [Pd(PPh3)4]

[K(crypt-222)]2(GaBi3)·en K3As7/[PnBu4]Br K3Sb7/[PnBu4]Br K3Sb7/[PnBu4]Br KSb/crypt-222 K5Sb4/[PnBu4]Br KSb/18-crown-6 K5Bi4/crypt-222 K5Bi4/18-crown-6 K5Bi4/crypt-222 K3As7/crypt-222 Rb3As7/crypt-222 Na3Sb7/crypt-222 K3Pn7/crypt-222 K3As7/crypt-222 K3As7/crypt-222 A3As7/crypt-222 (A = K, Rb, Cs) K3As7/crypt-222 Rb3As7/crypt-222/18-crown-6 K3As7/crypt-222 K3Sb7/crypt-222 K5Sb4/crypt-222 K3As7/crypt-222 K3Sb7/crypt-222 K3Sb/crypt-222 K5Bi4/crypt-222

[Pd(PPh3)4]

ref

Figure

57 235 236 236 237, 236

17a 17a 17a 17a 17a

236 235 238

17a 17a 17b

239 240 241 241 242 241 242 243 244 245 246 247−249

17c 17d 17d 17d 17d

ZnPh2 [Ni(CO)2(PPh3)2] [Ni(CO)2(PPh3)2] ZnPh2 [Cr(CO)3Mes] [Mo(bipy)(CO)4] [M(CO)3L] (L = Mes (M = Cr, W); C7H8 (M = Mo)) [Pd(PCy3)2] [CuMes]5 [AuCl(PPh3)] ZnPh2 ZnPh2 HgPh2 [AuPh(PPh3)] [AuPh(PPh3)] [Pd(PCy3)2] [Ni(cod)2] [Ru(cod)Cp*Cl] [M(CO)3L] (L = Mes (M = Cr); C7H8 (M = Mo))

250 246 251 252 253 254 255 256 250 257 258 259

18a 18a 18b 18d 18e 18f 18k 18g

K5Pn4/crypt-222

[Ni(CO)2(PPh3)2]

244

18h

[K(crypt-222)]3[Bi3Ni4(CO)6]·en·tol K5Bi4/18-crown-6 K5Bi4/18-crown-6 K6ZnSb5/crypt-222 [K(crypt-222)]2(TlBi3)·0.5en Bi, BiCl3, [BMIm]Cl·4 AlCl3 Sb, Bi, BiBr3 As/Bi, Bi, BiX3 (X = Cl, Br) Bi12−xRhCl13−x or Bi12−xRhBr13−x in [BMIm]Cl Bi2Pd, Bi, Br2 Bi, Cl2 Sn, Bi, BiCl3 Bi16PdCl22, AlBr3, [BMIm]Br Bi, BiBr3, AlBr3, [BMIm]Br “AuBi8SnX9” Bi, BiBr3 Bi, BiCl3, [BMIm]Cl, AlCl3 Bi, [BMIm]Cl, AlCl3 Bi12−xRhCl13−x

pyridine [Ni(CO)2(PPh3)2] [Ni(CO)2(PPh3)2]

244 244 244 71 76 260 261 262 75

18j 17g 18i 18c 18l 19a 19b

73 263 153 74 74 153 264 265 266 266

19d

[Ru(cod)(H2CC(Me)CH2)2] CuCl Au Au

Pd Pd Pt Ru AuCl NiCl2

17f 17g 17h

19c

19d 19d 19g 19h 19f 19e

a

Only identified by mass spectrometry. U


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Pn = P, As, and Sb. Consequently, clusters containing group 15 element atoms (except Bi, see below) tend not to incorporate endohedral metal atoms. Instead, most clusters are either heterometallic or simple coordination compounds. Multitudes of compounds have been synthesized that occupy the gray area between cluster and coordination compound. As the focus of this review is clusters, those derived from Zintl ions in particular, not all compounds containing pnictogen−transition metal bonds will be covered. All known binary intermetalloid clusters comprising group 15 element atoms and selected heterometallic clusters are summarized in Table 4. For further examples, the reader is referred to two recent reviews: one on the chemistry of Pn73− (Pn = P−Sb)17 and the other on transition metal complexes of the pnictides.30 In addition, the reactivity of P4 and As4 toward transition metal compounds has also been reviewed.31−33 2.6.1. Binary Intermetalloid Clusters of Group 15 Elements. As stated above, clusters containing group 15 elements are far less likely than their group 14 cousins to comprise endohedral transition metals. That is not to say, however, that there are no instances of interstitial transition metals. The first such example arose from a high temperature (925 K) reaction of rubidium and arsenic in a niobium ampule. Subsequent extraction of “RbAs” with en and crypt-222 afforded crystals of [Rb(crypt-222)]2[Rb{Nb@As8}].57 The anion is composed of a puckered ring of eight arsenic atoms coordinating a central niobium atom (Figure 17a). These rings combine with the free Rb+ to form a one-dimensional coordination polymer. Formally, each arsenic atom has a negative charge, making the As88− ring isoelectronic with Se8. The niobium atom is formally in the +V oxidation state, and its inclusion is the result of the oxidative attack of the ampule material by arsenic at high temperatures.57−59 In later studies, the isoelectronic anions [Nb@Sb8]3− and [Mo@As8]2− were characterized in salts isolated from reactions of K3Pn7 (Pn = As, Sb) and aromatic sandwich complexes of transition metal atoms.235−237 Additionally, the reduced radicals [Cr@As8]3− and [Mo@Sb8]3− have also been characterized. This reduction does not, as might be expected, take place on the main group ring. EPR spectroscopy found the radicals to be mainly transition metal based with couplings to the main group atoms (Figure 16).236 A series of real and hypothetical clusters [MV@As8]3−, [MVI@As8]2−, and [MVII@As8]− were analyzed by DFT methods. Bonding between the transition metal atoms and the As8 rings is discussed with the use of detailed MO diagrams, population analyses, atomization energies, and optimized structural information.267 The stabilization of the rings discussed above with high negative charges was enabled by the high oxidation state of the central metal atom. Ring-type cluster shells have also been shown to encapsulate f-block atoms with +III and +IV oxidation states. A series of novel Ln-containing clusters [Ln@Sb12]3− (Ln = La, Y, Ho, Er, Lu), as their [K(crypt-222)]+ salts, were prepared via the reaction of [Ln(benzyl)3(thf)3] with K5Sb4 in pyridine in the presence of crypt-222.238 The clusters are isostructural and are comprised of a formal Ln3+ ion coordinated by three nearly planar Sb4 rings, which are in turn weakly bound to each other to form a Sb126− doughnut-shaped polyantimonide ligand (Figure 17b). The unique electronic properties of this new cluster type become evident when the central metal atom is exchanged for one that is more redox active. This was achieved by reacting

Figure 16. EPR spectrum for K[Cr@As8]2− ion recorded from DMF solutions at 25 °C. The simulated spectrum is given in the inset. Reproduced from ref 236. Copyright 2003 American Chemical Society.

[U(C 5 Me 4 H) 3 Cl] or [U(C 5 Me 4 H) 3 ] with [K(crypt222)]2(GaBi3)·en in en, which led to the formation of the intermetalloid cluster anion [U@Bi12]3− (Figure 17c).239 At first glance, the obvious assignment of oxidation states would be [U3+@Bi126−]3−, but further reactions of the uranium complexes with other binary Zintl anions, magnetic measurements, and a detailed quantum chemical analysis all suggested that the correct assignment was rather [U4+@Bi127−]3−. The calculations indicated that the open-shell Bi127−• ligand interacts with the 5f orbitals of U4+ in a donor−acceptortype manner, leading to covalent bonding. This is in contrast to the essentially ionic interaction found in the [Ln@Sb12]3− species. A more detailed discussion of this cluster anion is provided in sections 3.4 and 5.3 below. A dramatic rearrangement of Pn73− (Pn = As, Sb) precursors as a result of reactions with complexes of nickel and palladium, as well as cation exchange, led to the synthesis of so-called “onion-type” or “matryoshka-type” clusters. The first such example came from the reaction of K3As7 with [Ni(cod)2] in the presence of [nBu4P]Br, which afforded the compound [nBu4P]3[As@Ni12@As20]·1.5en.240 The cluster anion can be described as comprising an As20 dodecahedron encapsulating an As-centered Ni12 icosahedron of the formula [As@Ni12]3− (Figure 17d). A similar result was achieved by reacting K3Sb7, [Pd(PPh3)4], and [nBu4P]Br, which yielded [nBu4P]3[Sb@ Pd12@Sb20]·3.5en.241 When the reaction was instead carried out using the more reduced precursor K5Sb4, a salt of the reduced radical [Sb@Pd12@Sb20]4− was isolated. LDI-TOF mass spectrometry additionally shows signals for both [Pd12Sb21]+ and [Pd12Sb21]−. The four observed charge states of [Sb@Pd12@Sb20]q (q = +1, −1, −3, −4) demonstrate the exceptional redox activity of this large cluster. Attempts to synthesize a Ni analogue failed to produce a crystalline product; however, [Sb@Ni12@Sb20]± were identified by mass spectrometry. A separate study noted that the same Pd/Sb cluster could be synthesized using conventional cation sequestering agents, the identity of which also influenced the charge. The use of the smaller crown ether ligand 18-crown-6 stabilized [Sb@Pd12@Sb20]4−, while addition of the larger sequestration agent crypt-222 favored [Sb@Pd12@Sb20]3−.242 It is worth noting that this cluster type is not limited to group V


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Figure 17. Molecular structures of intermetalloid clusters comprising group 15 atoms and endohedral transition metal atoms. Compounds with similar structures are grouped, and a representative cluster is shown. (a) [M@Pn8]q− (M/Pn/q = Mo/As/2,237 Mo/Sb/3,236 Nb/As/3,235 Nb/Sb/ 3,235 Cr/As/3),236 (b) [Ln@Sb12]3− (Ln = Y, La, Ho, Er, Lu),238 (c) [U@Bi12]3−,239 (d) [Pn@M12@Pn20]q− (M/Pn/q = Ni/As/3,240 Pd/Sb/ 3,4),241,242 (e) topological representation of [Sn@Cu12@Sn20]12−,150 (f) [Zn@Zn8Bi11]5−,243 (g) [Nix@Bi6Ni6(CO)8]4− (x = 0, 1),244 (h) [Ni@ Bi6Ni6(Bi3)2(CO)4]4−.245

15 elements but was also observed in the isoelectronic group 14 cluster [Sn@Cu12@Sn20]12−.150 The high charge that results from this elemental substitution, however, means that such a cluster can only occur in Zintl phases. Additionally, the internal p-block-metal-centered icosahedra are similar to a series of main-group-atom-centered metal−carbonyl clusters, [E@ M12(CO)n]q (E = Ga, Ge, Sn, Sb, Bi; M = Ni, Rh; n = 22− 27), and the binary main group metal cluster [Tl@ Tl4Pb8]4−.23−29,62 Given the electron-precise bonding in the large “onion-type” clusters, one might suspect strong interactions between the pnictogen atoms. However, the Pn−Pn bonds within the outer shell appear to be weak, whereas the M−Pn interactions are much more pronounced. This can be seen in the interatomic distances, where the Pn− Pn distances are much longer than what is expected for 2c−2e bonds, while the Pn−M distances are within the range that is observed for according clusters and intermetallic compounds. The bonding within these clusters was analyzed in terms of the superatom concept in two quantum chemical studies. Both found that the shells of these clusters can effectively be

interpreted by this model and that they indeed show superatomic behavior.268,269 The few examples of group 15 binary clusters in which an endohedral transition metal atom is encapsulated by a single main group atom shell all contain bismuth. Bismuth is unique among pnictogens, in that it is far more likely to partake in multicenter bondingin agreement with its more metallic character. The first such cluster, [Zn@Zn8Bi11]5−, was formed when an en solution of K5Bi4 and crypt-222 was reacted with ZnPh2.243 The resulting cluster anion is remarkably unsymmetrical. It comprises a distorted icosahedral {Zn@Zn8Bi4} core unit, with 7 Bi atoms capping irregularly 7 of the 20, 3atom faces (Figure 17f). The central {Zn@Zn8Bi4} unit comprises 43 valence electrons, including 5 electrons from the cluster charge and 2 from the endohedral zinc atom. This is 7 electrons short of the 50 valence electrons that would be expected for a 12-vertex closo Wade−Mingos cluster (4n + 2). These 7 electrons are provided by the 7 exo-Bi atoms, each of which apparently acts as a one-electron-donor ligand. This cluster remains one of the few binary molecular compounds of Zn and Bi that could give insight into the bonding between W


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Figure 18. Molecular structures of selected coordination compounds and heterometallic clusters comprising group 15 elements and transition metal atoms. Compounds with similar structures are grouped, and a representative cluster is shown. (a) [MAs15]3− (M = Zn,253 Hg254), (b) [Au2Sb16]4−,255 (c) [Zn2Sb12]4−,71 (d) [Sb3Au3Sb3]3−,256 (e) [Pd7As16]4−,250 (f) [Ni5Sb17]4−,257 (g) [Bi3M2(CO)6)]3− (M = Cr, Mo),259 (h) [Pn3Ni4(CO)6]3− (Pn = Sb, Bi),244 (i) [Bi3Ni6(CO)9]3−,244 (j) [Bi4{Ni2(CO)3}2]2−,244 (k) [Sb6(RuCp*)2]2−,258 (l) [Bi9{Ru(cod)}2]3−.76 Hydrogen atoms are omitted for clarity.

complex is the displacement of one or more labile ligands from the latter to give [Pn7M(L)x]q− or [(Pn7)M(Pn7)]q− (Pn = P, As, Sb, Bi).17,270 In such complexes, the Pn73− anions can act as 4- or 6-e− donors, coordinating metal atoms in either an η2 or η4 fashion, respectively. In the latter, a Pn−Pn bond is broken, leading to a conversion of the nortricyclane-type structure to a norbornadiene-type structure. This is observed in the formation of [(η4-As7)Cr(CO)3]3− from the reaction of Rb3As7 and [Cr(Mes)(CO)3].247 These Zintl anions may also coordinate M2 dumbbells, in a host of different topologies, to give [(Pn7)M2(Pn7)]q− (M = Pd,250 Cu,246 Au,251 Hg252). All of these straightforward coordination compounds were recently summarized and discussed in a comprehensive review by Turbervill and Goicoechea.17 Significant rearrangements of the Pn73− precursor upon reaction or the insertion of additional Pn atoms again begin to blur the line between Zintl ion coordination and cluster formation. For example, the reactions of A3As7 and MPh2 (A/ M = Rb/Zn,253 K/Hg254) afforded salts of [MAs15]3− (Figure

these two elements, particularly as there is no known binary alloy. Carrying out a similar reaction between K5Bi4 and [Ni(CO)2(PPh3)2] in the presence of 18-crown-6 yielded [Ni@Bi6Ni6(CO)8]4−, which also has a pseudo-icosahedral structure, yet with much less distortion (Figure 17g).244 The crystal structure showed statistical disorder of the anion with an identical “empty” cluster, lacking the interstitial Ni atom. Heating a solution of K5Bi4 and [Ni(CO)2(PPh3)2] in en, this time with crypt-222, instead afforded [Ni@ Bi6Ni6(Bi3)2(CO)4]4−.245 This cluster again has a pseudoicosahedral cage but with two Ni atoms capped by rare η3-Bi3 ligands instead of CO (Figure 17h). Its structure and electron count were rationalized by viewing the cluster as being composed of eight fused tetrahedra, which share vertices, edges, and faces. 2.6.2. Coordination Compounds and Heterometallic Clusters of Group 15 Elements. As with the Tt94− anions, the simplest reaction of a Pn73− cage with a transition metal X


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18a). These clusters are reminiscent of the [(As7)Au2(As7)]4− anion quoted above,251 in which a Au22+ dumbbell is trapped side-on between two As73− clusters; however, the Au22+ dumbbell is replaced by a ZnAs fragment. Another recent example of moderate Pn73− rearrangement is [Au2Sb16]4−, which was synthesized by the reaction of an en solution of K3Sb7 and crypt-222 with [Au(PPh3)Ph].255 In this cluster, one Au2 and one Sb2 dumbbell have been inserted between two Sb7 cages (Figure 18b). Each gold atom is at the center of a notably distorted square plane of Sb atoms. Quantum chemical calculations showed a degree of multicenter bonding within the {AuSb4} subunits, involving a total of 6 electrons for σ-type bonding. In contrast, the same reaction carried out with K5Sb4 afforded the heterometallic cluster anion [Sb3Au3Sb3]3−, in which a formal Au33+ triangle is coordinated by Sb33− subunits, forming a near perfect trigonal prism (Figure 18d).256 In this all-metal sandwich complex, the Sb33− rings are Hückel-aromatic, and a detailed quantum chemical analysis suggested strong electron donation from the ligands to the Au3 ring as well as back-donations that greatly stabilize the cluster. When reacted with complexes of group 10 metals, Pn73− (Pn = As, Sb) can radically rearrange to form more complex heterometallic clusters. The reaction of K3As7 with [Pd(PCy3)2] in en afforded [(As7)Pd2(As7)]4−, as well as the 23atom heterometallic cluster [Pd7As16]4− in low yields (Figure 18e).250 In the latter, several different types of (poly)arsenide anions (2 × planar As5−, 2 × As22−, and 2 × As3−) are present that link one Pd2+ and six Pd+ cations together into a complex arrangement. The structure can be interpreted with a very localized electronic picture in which several Pd coordination complexes form one large polynuclear complex. A similar phenomenon was observed in the reaction of K3Sb7 and [Ni(cod)2], which afforded the complex cluster anion [Ni5Sb17]4− (Figure 18f).257 The structure can be described as a Ni-centered Ni4Sb4 ring that sits atop an Sb13 bowl. The cluster is similar to that of [Pd7As16]4−, which has a {Pd@ As4Pd4} subunit. The [Ni5Sb17]4− anion has 139 valence electrons and calculations predicted an S = 3/2 spin ground state. EPR measurements, however, failed to confirm paramagnetism. Until recently, Bi73− was unknown and thus inaccessible as a potential precursor to intermetalloid and heterometallic clusters. Instead, the phase K5Bi4 has been employed in reactions with transition metal complexes to great effect. K5Bi4 comprises Bi44− zigzag chains and an extra delocalized electron in the solid state.271 It has been shown to form Bi42− rings upon dissolution in en.272 Using the K5Bi4 precursor, a series of metal carbonyl clusters of bismuth were prepared that are all isoelectronic and isostructural with closo-Ttn2− (n = 5, 7, 8, 12). The reaction of an en solution of K5Bi4 and crypt-222 with [M(CO)3(L)] (M/L = Cr/Mes, Mo/cycloheptatriene) afforded the trigonal bipyramidal [Bi3M2(CO)6]3− (Figure 18g).259 In this cluster, each bismuth atom contributes 3 skeletal electrons, the M(CO)3 moieties none, and 3 electrons are added for the charge. Thus, the anion has 12 electrons available for cluster bonding, and is therefore isoelectronic with closo-Pb52−. Interestingly, the bent Bi33− subunit is a heavy analogue of ozone. The same reaction with [Ni(CO)2(PPh3)2] afforded pentagonal bipyramidal [Bi3Ni4(CO)6]3− (Figure 18h).244 The electron counting is the same here but with the two bridging carbonyl ligands contributing two electrons apiece: 3 × 3 (Bi) + 2 × 2 (CO) + 3 (charge) = 16 electrons, or 2n + 2 (n = 7), available to occupy bonding MOs. Thus, this

cluster is again a closo-type and is isoelectronic with the recently characterized binary Zintl anion (Tl4Bi3)3−.62 The isostructural Sb cluster is accessible upon use of K5Sb4 as a starting material.244 Dissolution of [Bi3Ni4(CO)6]3− in pyridine led to the oxidative addition of another bismuth atom to give [{(CO)3Ni2}Bi4{Ni2(CO)3}]2− (Figure 18j). This 8-vertex cluster adopts a bisdisphenoidal structure, hence another closo-topology with 18 skeleton electrons (2n + 2, n = 8) in bonding MOs. We note here that, in contrast to [Ge5Ni2(CO)3]2− (see section 2.4.2 and Figure 12a), the {Ni2(CO)3} units do not bind externally to a closed polyhedron but are clearly part of it themselves. Employment of 18-crown-6 as the cation sequestering agent afforded the closo-icosahedron [Bi6Ni6(CO)9]4−, which in the solid state is statistically disordered on the same position as a Ni-centered analogue (see section 2.6.1 and Figure 17g). The same reaction, however, also produced [Bi3Ni6(CO)9]4− (Figure 18i), which can be viewed as derived from the filled icosahedral cluster [Ni@Bi6Ni6(CO)8]4− (see Figure 17g), upon removal of {Bi3[Ni(CO)]} and the addition of two additional bridging carbonyl ligands. This cluster is both isoelectronic and isostructural to Tl 9 9 − found in the Zintl phase Na12K38Au2Tl48.204 Recently, the 14-atom cluster [Zn2Sb12]4− was synthesized by direct extraction of the ternary phase “K6ZnSb5” in en in the presence of crypt-222.71 The structure of [Zn2Sb12]4− was previously unknown and can be described as resulting from the fusion of two “(ZnSb6)2−” norbornadiene-type clusters (Figure 18c). This overall structure is distorted by the near planarity of the {ZnSb3} subunits. A novel bismuth-containing cluster [Bi9{Ru(cod)}2]3− crystallized as the [K(crypt-222)]+ salt from a reaction of [K(crypt-222)]2(TlBi3)·0.5en with [Ru(cod)(Me-allyl)2] in en, after layering with THF.76 The {Ru2Bi9} core of [Bi9{Ru(cod)}2]3− is quite complex but can in one sense be viewed as a Bi-capped {Ru2Bi4} hexagon that sits atop a {Bi4} zigzag chain (Figure 18l). Quantum chemical calculations revealed a mixed picture of the bonding, with the Bi−Bi bonds in the capping {Ru2Bi4} hexagon being multicentered (3c−2e), while all other Bi−Bi contacts are 2c−2e bonds. An alternative view concentrates on the Ru atoms, which are regarded as Ru2+ ions (d6 configuration). Beside the 4 electrons donated from the cod ligand, each of the Ru atoms requires another 8 electrons for their desired 18-electron situation. These are provided by the four coordinating Bi atoms per Ru atom from the Bi97− unit, in which all Bi atoms except the apical one are regarded as (two-bonded) Bi− and the apical one as (mostly isolated) Bi+. A second product was also identified from this reaction. Concentration of the mother liquor followed by layering with toluene afforded the ternary anion [Tl2Bi6{Ru(cod)}]2− as the [K(crypt-222)]+ salt. A two-electron donation from the ruthenium atom makes this 9-vertex cluster isoelectronic with nido-Pb94− and has one more valence electron than the related species [Ge8Fe(CO)3]3−. Indeed, [Tl2Bi6{Ru(cod)}]2− adopts the expected capped square antiprismatic structure, with some distortion (Figure 23d). Recently, another heterometallic ruthenium−pnictide cluster, [Sb6(RuCp*)2]2−, was isolated from the reaction of an en solution of “K3Sb” and crypt-222 with a toluene solution of [Ru(cod)Cp*Cl].258 This cluster consists of a Sb64− “ring” (in the boat conformation) that coordinates two {Cp*Ru}+ fragments (Figure 18k). The coordination modes of the two ruthenium atoms are different; however, 1H and 13C NMR Y


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Figure 19. Molecular structures of polycationic intermetalloid clusters, heterometallic clusters, and coordination compounds comprising group 15 atoms and transition metal atoms. Compounds with similar structures are grouped, and a representative cation is shown. (a) [CuBi8]3+,260 (b) [Au2Bi10]6+,261,262 (c) [Rh@Bi9]4+,75 (d) [M@Bi10]q+ (M/q = Pd,Pt/4;73,74,153,263 Au/5153), (e) [Rh2@Bi12]4+,266 (f) [Ni2@Bi12]4+,266 (g) [(Bi8)Ru(Bi8)]6+,264 (h) [(Bi8)Au(Bi8)]5+.265

ionic liquids. It was postulated that this was due to the formation of larger halido-aluminates [AlnX3n+1]− (n > 1).74 Using these methodologies, several polycationic heterometallic and intermetalloid clusters have been prepared and characterized to date. The two cations [CuBi8]3+ and [Au2Bi10]6+ are binary Wade−Mingos clusters with capped square antiprismatic and compressed icosahedral structures, respectively (Figure 19a and b).260−262 The Cu and Au atoms in these clusters form close contacts to the counteranions. In contrast to this, the cluster cations [Rh@Bi9]4+,75 [M@Bi10]4+ (M = Pd, Pt),73,74,153,263 and [Au@Bi10]5+ all encapsulate the transition metal atoms within a polycationic Bix deltahedron.153 In [Rh@Bi9] 4+ , a formal Rh− ion with a d 10 configuration is inserted into the well-known Bi95+ cluster structure (Figure 19c), analogous to the intermetalloid cluster chemistry of anionic 9-atom cages of group 14 atoms discussed above (see section 2.3.1 and Figure 8a,b). The cations [M@ Bi10]4+ (M = Pd, Pt) and [Au@Bi10]5+ adopt structures that have not been observed in the chemistry of the analogous clusters of the group 14 elements (Figure 19d). As discussed in section 2.3.2, two polyanions of similar compositions, namely, [M@Tt10]3− (M/Tt = Fe/Ge,100 Co/Ge,141), have pentagonal prismatic Tt10 shells (see section 2.3.2 and Figure 8h), with nonclassical electron numbers and bonding. In contrast, the 10 Bi atoms in [M0@Bi104+]4+ (M = Pd, Pt) and [Au+@Bi104+]5+ form pentagonal antiprisms, much like the Bi10 fragment in the [(Au+)2Bi104+]6+ cation.261,262 The Bi104+ cages in these species are arachno-clusters (VE = 4n + 6, n = 10), with two missing vertices relative to an icosahedron. Further uncapped antiprisms that lack interstitial atoms are present in the polycations [(Bi8)Ru(Bi8)]6+ and [(Bi8)Au(Bi8)]5+ (Figure 19g,h).264,265 Again, these are in accordance with an arachnolike electron count. In the [(Bi8)Au(Bi8)]5+ cation, the two

showed that these two positions exchange on the NMR time scale in solution. 2.6.3. Cationic Deltahedral Intermetalloid and Heterometallic Clusters. The group 15 elements are in a very special position in the periodic table with respect to their potential to form cluster structures. As discussed above, they can easily be reduced to form polypnictides, Pnxq−. Additionally, they can be substituted into Ttxq− polyanions to give binary clusters of reduced charge, (Ttx−yPny)(q−y)−. Under certain conditions, group 15 elements can also be oxidized to afford cationic clusters, Pnxq+, that are reminiscent of deltahedral anions of the group 14 elements (i.e., Tt94−). Such a chemical oxidation of the group 15 element must be possible at reasonable potentials. It is for this reason that all reports of cationic intermetalloid and heterometallic clusters have been limited to the element bismuth. The potential for the formation of such polycationic clusters has long been an experimental reality. In 1963, Bi95+ was identified in the bismuth subchloride Bi12Cl14, which was synthesized by reacting bismuth metal with BiCl3 at high temperatures.72 Subhalides that comprise intermetalloid cluster cations can be similarly made. The reaction of bismuth metal with Bi2Pd and Br2 at 1000 °C affords Bi14PdBr16, which contains [Pd@Bi10]4+ cations.73 It has recently been reported that similar compounds can be accessed at much lower temperatures using ionic liquids. For example, [Pt@Bi10][AlBr4]4 is formed from the reaction of platinum metal with BiBr3 in [BMIm]Br·4AlBr3 (BMIm = 1-butyl-3-methylimidazolium) at 140 °C.74 A third route combines these two approaches; a ternary subhalide may be dissolved in an IL to effect either an exchange of anions or a rearrangement of the cationic components. As a note, an excess of AlX3 (X = halogen) was found to be necessary in reactions employing Z


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Table 5. Ternary Intermetalloid and Heterometallic Clusters cluster [PhZn(Sn2Sb5)ZnPh]3− [(CuSn5Sb3)2]4− [Co@Sn6Sb6]3− [Co2@Sn5Sb7]3− [Ni2@Sn7Sb5]3− [Zn@Zn5Sn3Bi8]4− [Ni2@Sn7Bi5]3− [Zn@Zn5Pb3Bi8]4− [Ni2@Pb7Bi5]3− [Pd3@Sn8Bi6]4− [Pd@Pd2Pb10(Bi3)2]4− [Ni@TlSn9]3− [(Ni@Sn8)Ge(Ni@Sn8)]4− [Eu@Sn6Bi8]4− [Sm@Ga2HBi10]3− [Sm@Ga3H3Bi10]3− [(La@In2Bi11)(μ-Bi)2(La@In2Bi11)]6− [Ln@Sn4Bi9]4− (Ln = La, Ce) [Ln@Sn7Bi7]4− (Ln = La, Ce) [Ln@Pb3Bi10]3− (Ln = La, Ce, Nd) [Ln@Pb6Bi8]3− (Ln = La, Nd) [Ln@Pb4Bi9]4− (Ln = La, Nd, Gd, Sm, Tb) [La@Pb7Bi7]4− [U@Tl2Bi11]3− [U@Pb4Bi9]3− [U@Pb7Bi7]3− [Tl2Bi6{Ru(cod)}]2− [Tl(TaAs4)Tl]5− [(NbAs4)Tl]6− [(NbAs4)PbAs]8− [NbGaAs6]10− [NbInAs6]10− [(In3NbAs4)As]7− [{(In3NbAs4)As}2]13− [As(In3NbAs4)(In3As4)2(In3NbAs4)As]24− [V@Ge8As4]3− [Nb@Ge8As6]3− [Ta@Ge6As4]3− [Ta@Ge8As4]3− [Ta@Ge8As6]3− [(As3)Nb(As3Sn3)]3−

K8SnSb4/crypt-222 [K(crypt-222)]2(Sn2Sb2)·en [K(crypt-222)]2(Sn2Sb2)·en [K(crypt-222)]2(Sn2Sb2)·en [K(crypt-222)]2(Sn2Sb2)·en [K(crypt-222)]2(Sn2Bi2)·en [K(crypt-222)]2(Sn2Bi2)·en [K(crypt-222)]2(Pb2Bi2)·en [K(crypt-222)]2(Pb2Bi2)·en [K(crypt-222)]2(Sn2Bi2)·en [K(crypt-222)]2(Pb2Bi2)·en K4Sn9/crypt-222, TlCp “K4Ge4.5Sn4.5”/crypt-222 [K(crypt-222)]2(Sn2Bi2)·en [K(crypt-222)]2(GaBi3)·en [K(crypt-222)]2(GaBi3)·en [K(crypt-222)]2(InBi3)·en [K(crypt-222)]2(Sn2Bi2)·en [K(crypt-222)]2(Sn2Bi2)·en KPbBi/crypt-222 KPbBi/crypt-222 KPbBi/crypt-222 KPbBi/crypt-222 [K(crypt-222)]2(TlBi3)·0.5en KPbBi/crypt-222 KPbBi/crypt-222 [K(crypt-222)]2(TlBi3)·0.5en K or Rb, As, Tl K, As, Tl K, As, Pb Na, As, Ga K, As, In Cs, As, In Cs, As, In Cs, As, In K, Ge, As, crypt-222 K, Ge, As, crypt-222 K, Ge, As, crypt-222 K, Ge, As, crypt-222 K, Ge, As, crypt-222 K8NbSnAs5/crypt-222

Bi82+ cages coordinate the d10 Au+ cation in two different ways: one in an η2 manner and one as an η4 ligand. In contrast, the [(Bi8)Ru(Bi8)]6+ cation is more symmetric, with both units bonding in an η4 fashion. It does not contain a d 10 metal atom, though. Its electronic structure was investigated through quantum chemical studies. According to these, the Bi82+ units act as six-electron-donors, so that a filled 18 electron shell for the Ru atom is achievedsimilar to the situation in polyanionic [Bi9{Ru(cod)}2]3− (see section 2.6.2 and Figure 18l). The cluster is thereby related to ruthenocene, and also to the metal-bridged dimeric group 14 polyanions, such as [(Ge9)Zn(Ge9)]4−,181 as well as some related structures with main group metals only, [(Ge9)In(Ge9)]5− or [(As7)Sn(As7)]4−.273,274 In contrast to the species discussed above, the clusters [Rh2@Bi12]4+ and [Ni2@Bi12]4+ both comprise two endohedral metal atoms.266 The cation [Ni2@Bi12]4+ has a fused square antiprism structure with D4h symmetry, in which each nickel

precursors

ref

Figure

ZnPh2 [CuL(NCMe)] (L = nacnac) [K(thf)x][Co(cod)2] [K(thf)x][Co(cod)2] [Ni(cod)2] ZnPh2 [Ni(cod)2] ZnPh2 [Ni(cod)2] [Pd(dppe)2] [Pd(PPh3)4] [Ni(cod)2] [Ni(cod)2] [Eu(C5Me4H)3] [Sm(C5Me4H)3] [Sm(C5Me4H)3] [La(C5Me4H)3] [Ln(C5Me4H)3] (Ln = La, Ce) [Ln(C5Me4H)3] (Ln = La, Ce) [Ln(C5Me4H)3] (Ln = La, Ce, Nd) [Ln(C5Me4H)3] (Ln = La, Ce, Nd) [Ln(C5Me4H)3] (Ln = La, Nd, Gd, Sm, Tb) [La(C5Me4H)3] [U(C5Me4H)3]/[U(C5Me4H)3Cl] [U(C5Me4H)3]/[U(C5Me4H)3Cl] [U(C5Me4H)3]/[U(C5Me4H)3Cl] [Ru(cod)(H2CC(Me)CH2)2] Ta Nb Nb Nb Nb Nb Nb Nb V Nb Ta Ta Ta

278 279 277 277 277 280 275 276 276 69 281 125 63 282 79 79 283 78 78 83 83 83 83 239 239 239 76 284 60 60 285 286 287 288 288 59 59 58 58 58 289

23c 23e 23h 23i 23i 23f 23i 23f 23i 23j 23k 23g 10d 26f 26h 26i 26j 26e 26g 26e 26g 26e 26g 26k 26l 26m 23d

26c 26d 26b 26c 26e 26a

atom is 8-coordinate (Figure 19f). It is iso(valence)electronic and isostructural with the family of clusters, [Ni2@Tt7Pn5]3− (Tt/Pn = Sn/Sb, Sn/Bi, Pb/Bi) and [Co2@Sn5Sb7]3−.275−277 In comparison to [Ni2@Bi12]4+, the cluster [Rh2@Bi12]4+ lacks two valence electrons, which causes a significant structural distortion from D4h to D2h (Figure 19e). See section 5.3 for a further discussion of the influence of electron count on the structures of 12-vertex clusters.

3. TERNARY CLUSTERS In this section, we focus on examples of ternary intermetalloid clusters that combine one type of a transition metal (including lanthanides and actinides) with a pair of elements of groups 13/14, 13/15, 14/14, or 14/15. Primarily, the compounds have been derived by solution means; however, some discrete ternary cluster anions present in quaternary Zintl phases are also included. Table 5 summarizes the species discussed herein. The subsequent sections will inform about specific synthetic AA


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multimetallic clusters, even without previous knowledge of the most stable cluster topology (Figure 20).296

approaches to these multinary clusters and about methods of how to cope with the frequently observed atomic disorder in the crystal structures. 3.1. Syntheses

The synthesis of ternary clusters from solution is primarily achieved by reacting a salt of a binary Zintl ion, [K(crypt222)]2(TrBi3)·en (Tr = Ga,290 In,290 Tl239) or [K(crypt222)]2(Tt2Pn2)·en (Tt/Pn = Ge/P,169 Sn/Sb,278,279 Sn/Bi,65 Pb/Bi276), with a transition metal, lanthanide, or actinide organometallic complex. The solvent of choice is usually ethylenediamine. Zintl phases, such as K8SnSb4 or KPbBi, have been dissolved in en along with crypt-222 to produce binary Zintl anion precursors in situ. The formation of larger clusters from 4-atom Zintl anions is generally oxidative, with either the metals, solvent, or organic ligands being reduced. Common byproducts of this oxidation are binary 9-atom Zintl ions, (Tr4Bi5)2− (Tr = In,290 Tl76) and (Tt7Pn2)2− (Tt/Pn = Ge/ P,169 Ge/As,58 Sn/Bi,280 Pb/Bi276), which are isoelectronic to Tt94− (Tt = Si−Pb). None of these anions have been applied as precursors to ternary clusters, though they presumably play a role in their formation.58 Further binary main group (semi) metal anions are also known, like (Sn3Bi3)5−,291 (Sn3Bi5)3−,291 (Sn4Bi4)4−,292 or (Ge4Bi14)4−.293 Their reactivities have not thus far been explored, in part due to their limited solubilities and low synthetic yields. There is one instance of a ternary Zintl phase with a mixture of group 14 elements, K4Ge9−xSnx, being used as a precursor to a salt of the ternary anion, [(Ni@ Sn8)Ge(Ni@Sn8)]4−.63 Ternary clusters have also been extracted from quaternary Zintl phases by dissolution in en in the presence of cation sequestering agents, such as crypt-222. One element that is often present in these systems is arsenic, which reacts with group 5 metals at temperatures above 600 °C, leading to their incorporation into the solid.57

Figure 20. Graphical results of a genetic algorithm (GA) study of the cluster [La@Pb7Bi7]4−, combined with reassignment of atomic positions by means of first-order perturbation theory in the nuclear charge (RP).83 The graphic shows the average values over five different runs for the mean (upper part) and best (lower part) energies (relative to the global minimum), Emean and Ebest, for each generation for each variant of the GA. For the global minimum, Bi occupies positions 1−7 in the displayed structure. Reproduced with permission from ref 296. Copyright 2014 American Institute of Physics.

3.2. Occupational Disorder

The presence of two p-block elements in ternary intermetalloid clusters often results in several energetically similar isostructural isomers. This can manifest as severe occupational disorder in the crystal structure. In cases where the elements are from the same period of the periodic table, and thus differ by only 1−2 valence electrons, the relative atomic occupations of these elements cannot be refined using standard crystallographic methods. Even in cases where there is a significant difference in the atomic number of the two p-block elements, the simultaneous refinement of their relative occupations over numerous atomic sites produces dubious results. A combination of auxiliary experimental data and quantum chemical calculations must therefore be employed to identify both the correct elemental composition and the lowest energy isomers that contribute to the averaged structure. Electrospray ionization mass spectrometry (ESI-MS) and energy dispersive X-ray (EDX) or micro X-ray fluorescence (μ-XRF) analysis are standard means of confirming the cluster composition. Once this is established, computational methods are necessary to properly assign atomic positions in the cluster. This can be tedious for systems with often >100 potential isomers. This problem has been addressed by a reassignment of atomic positions (RP) with DFT-based first-order perturbation theory in the nuclear charge.294,295 In combination with a DFT-based genetic algorithm (GA-RP), this procedure can be applied to the automatic determination of the global minima in

3.3. Ternary Heterometallic and Intermetalloid Clusters Containing Transition Metal Atoms from Groups 12 to 8

Initial investigations into ternary intermetalloid cluster syntheses focused on the element combinations of Sn/Sb and Sn/Bi. It was found that layering an en solution of K8SnSb4 and crypt-222 with toluene afforded crystals of [K(crypt-222)]2(Sn2Sb2)·en. Addition of ZnPh2 to this solution gave the ternary anion, [Sn2Sb5(ZnPh)2]3−, as the [K(crypt-222)]+ salt (Figure 23c).278 Despite (Sn2Sb2)2− being observed, the main group metal core of [Sn2Sb5(ZnPh2)2]3− is clearly derived from (Sn2Sb5)5− (isoelectronic with Sb73−), which has been coordinated by two {PhZn}+ fragments. This is likely due to the excess Sb in the K8SnSb4 precursor. The Zintl phase K2SnSb was later found to be a more atom-efficient precursor to [K(crypt-222)]2(Sn2Sb2)·en.279 The latter has been reacted directly with [CuL(NCMe)] (L = nacnac = [(N(C6H3iPr2-2,6)C(Me))CH]−) in both en and DMF to afford the ternary dimer [{CuSn5Sb3}2]4− as the [K(crypt222)]+ salt.279 The two “{CuSn5Sb3}2−” monomers are bridged via two 3c−2e bonds, each involving a Cu atom on one side and two Sn atoms on the other (Figure 23e). Calculations indicated almost no Cu−Cu interaction. When reacted with [Au(PPh3)Ph], (Sn2Sb2)2− was found to coordinate Au+ without rearrangement to yield the complex [Au(η2-Sn2Sb2)2]3− (Figure 23b).168 The (Sn2Sb2)2− units AB


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coordinate the gold atom in a planar fashion, with DFT calculations indicating that this occurs via the Sn atoms. The compound K4AuTlSn3 was reported by Corbett and coworkers, in which (η2:η2-TlSn3)5− subunits act as bridges between Au+ ions in the infinite chain, 1∞[Au(TlSn3)]4−. Here, however, the (TlSn3)5− tetrahedra are rotated 90° relative to each other. Coordination of intact binary tetrahedral Zintl ions to a transition metal was also observed in the trimeric cluster [Cd3(η2:η3-Ge3P)3]3−. In this case, however, the (Ge2P2)2− precursor anions rearrange in the presence of CdPh2 in en to form (Ge3P)3− anions, which coordinate a Cd36+ triangle in the product cluster (Figure 23a).169 Each Cd atom is coordinated to two (Ge3P)3− anions via the Ge3 face on one cluster and the Ge2 edge on the other in a pseudolinear fashion. ZnPh 2 and [Ni(cod) 2 ] both react with [K(crypt222)]2(Sn2Bi2)·en in en to afford the ternary intermetalloid clusters [Zn@Zn5Sn3Bi8]4− and [Ni2@Sn7Bi5]3−, as their [K(crypt-222)]+ salts, respectively. The structure of [Zn@ Zn5Sn3Bi8]4− comprises an 11-vertex {Zn@Zn5Sn3Bi3} core that is capped by five Bi atoms (Figure 23f).280 This cluster is closely related to the binary anion [Zn@Zn8Bi11]5− presented above (see section 2.6.1 and Figure 17f) in three regards.243 First, it is another rare example of a heterometallic or intermetalloid cluster comprising Zn and Bi. Second, the {Zn@Zn5Sn3Bi3} core is in accord with Wade−Mingos rules for an 11-atom nido-cluster, similar to the icosahedral closocage core of [Zn@Zn8Bi11]5−. Third, the 48 valence electrons expected for a nido-cluster (i.e., 4n + 4, n = 11) are also realized by a 1-electron donation from each external Bi atom (5 here, 7 in the case of [Zn@Zn8Bi11]5−). [Ni2@Sn7Bi5]3− is a 12-vertex cluster constructed from two Ni-centered square antiprisms that share a 4-atom face (Figure 23j).275 The cluster shell has a total of 56 valence electrons and, standing alone, is neither electron-precise nor can it be described by Wade−Mingos rules.275 Rather, the structure is a blend of (weak) multicenter (3c−2e and 4c−2e) and localized (2c−2e) bonding. Viewed another way, the encapsulated transition metal atoms have a full 18-electron shell: 10 e− from Ni0 and 2 e− each from each neighboring main group atom in the square antiprismatic pocket. The structure of [Ni2@ Sn7Bi5]3− is related to a family of carbide-centered transition metal−carbonyl clusters (e.g., [C2@Rh12(CO)24]2−)297−300 as well as structural motifs found in the solid state, such as in Ir3E7 (E = Ge, Sn) and FeBi2.301−303 The antimony-containing [Ni2@Sn7Sb5]3− was recently isolated in a similar manner to the bismuth analogue by using the precursor [K(crypt-222)]2(Sn2Sb2)·en.277 Reaction of this same precursor with [K(thf)x][Co(cod)2] in DMF at 5 °C led to the isolation of the mixed-cluster compound [K(crypt-222)]3[Co@Sn6Sb6]0.83[Co2@Sn5Sb7]0.17·2dmf·2tol. At higher reaction temperatures, only [K(crypt-222)]3[Co2@ Sn5Sb7] was formed (Figure 21). The [Co2@Sn5Sb7]3− is isostructural to [Ni2@Sn7Sb5]3− (Figure 23j); however, [Co@ Sn6Sb6]3− maintains the same fused-antiprism structure but has one unoccupied 8-atom “pocket” (Figure 23i). Both species were characterized by NMR and showed surprising stability in DMF. The three different ratios of tin to antimony in the aforementioned clusters are due to the incorporation of two Ni 0 , one Co − , or two Co − . The resulting series, [(Ni 0 ) 2 @(Sn 7 Sb 5 ) 3 − ] 3 − , [Co 1 − @(Sn 6 Sb 6 ) 2 − ] 3 − , and [(Co1−)2@(Sn7Sb5)1−]3−, can be contrasted with that of [Cu 1+ @Sn 9 4− ] 3− , [Ni 0 @Sn 9 3−/4− ] 3−/4− , and [Co 1− @

Figure 21. Segments of high-resolution ESI mass spectra recorded on DMF solutions of (a) single crystals of [K(crypt-222)]3[Co@ Sn6Sb6]0.83[Co2@Sn5Sb7]0.17·2dmf·2tol and (b) a powder containing the salt of the [Co2@Sn5Sb7]3− anion. Red lines are simulated mass envelopes for [CoSn6Sb6]− (left) and [Co2Sn5Sb7]− (right). The conditions for crystallization (top) or precipitation (bottom) are indicated in the legend. Reproduced with permission from ref 277. Copyright 2018 Wiley-VCH.

Sn93−/4−]4−/5−.63,80,81,126,127,129 For the binary 9-vertex clusters, an increase in the negative charge of the endohedral atom leads to an increase in the overall charge (density) of the anion, which can only be relieved by oxidation126,127 or protonation.304 In the case of the ternary 12-vertex clusters, charge is simply shifted between the endohedral metal atom(s) and the binary Sn/Sb shell by changes in stoichiometry, without any overall oxidation occurring. Applying (Pb2Bi2)2− as a precursor, rather than (Sn2Bi2)2−, affords isostructural clusters in higher yields. In reactions with ZnPh2 and [Ni(cod)2], [K(crypt-222)]4[Zn@Zn5Pb3Bi8] and [K(crypt-222)]3[Ni2@Pb7Bi5]·2en·2tol were obtained in yields of 55% and 64%, respectively, compared to ∼15% for both of the Sn analogues (Figure 23f and j).276 However, the same Sn-to-Pb substitution leads to very different outcomes when the transition metal is Pd. [K(crypt222)]2(Sn2Bi2)·en reacts with [Pd(dppe)2] in en to afford a salt of the 14-vertex [Pd3@Sn8Bi6]4− cluster, which comprises a Pd3 triangle surrounded by an oblate {Sn8Bi6} cage (Figure 23k).69 Quantum chemical calculations including difference electron density analyses (see Figure 22) point to a [(Pd3)0@(Sn8Bi6)4−]4− charge distribution and a weak interaction of the Pd30 triangle with the cage. The axially compressed structure of [Pd3@Sn8Bi6]4− is related to a deficiency of valence electrons when compared to that expected for an electron-precise cluster (see section 5.3). In contrast, the reaction of [K(crypt-222)]2(Pb2Bi2)·en with [Pd(PPh 3) 4 ] in en leads to the formation of [Pd@ Pd2Pb10(Bi3)2]4−.281 Although this anion also comprises 3 Pd atoms, it adopts a completely different form. It is a Pdcentered, trimetallic icosahedron, in which 2 vertices are Pd atoms, which are in turn capped with Bi3 triangles (Figure 23l). A similar structural feature was later observed in the binary cluster [Ni@Ni6Bi6(Bi3)2(CO)4]4−, in which the Bi3 moieties cap Ni atoms (see section 2.6.1 and Figure 17h).245 Quantum chemical calculations using perturbation theory and 207Pb NMR spectroscopy point to a mixture of two compositions with several possible isomers, [Pd0@(Pd2Pb10)6−(Bi3+)2]4− and [Pd0@(Pd2Pb8Bi2)4−(PbBi2)02]4−. In both compositions, the cluster structures can be understood in terms of Wade− AC


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[Ln3+@(Tt−)7(Bi0)7]4−. The second structure type is a 13vertex cluster with the general formulas [Ln3+@Tt3Bi10]3−, [Ln3+@Tt4Bi9]4−, or [Ln3+@Tr2Bi11]4− (Figure 26e). The assignment of formal charges in this structure is more complicated but has been done with the aid of DFT calculations.78,83 The results can be interpreted to give a simplified picture of the distribution of formal charges and to rationalize the overall charge. Starting from the bottom of the cluster, as shown in Figure 26e, the basal 4-atom ring and the next 4 atoms attached to this ring act as (pseudo)pnictogens (i.e., Bi0, Tt−, Tr2−). The next four atoms (attached to the apical atom) behave as pseudochalcogens, with these sites being occupied exclusively by four “Bi−” atoms. Lastly, the apical atomic site is occupied by a formal “Bi+” atom, which has a (second) lone pair oriented toward the interstitial lanthanide. Thus, a detailed formula of [Ln@Tt4Bi9]4− could be written as [Ln3+@(Tt−)4(Bi0)4(Bi−)4Bi+]4−. Understanding the formal charges of these ternary clusters allows educated predictions to be made regarding unknown element compositions or charges. For example, rationalization of the total charge of the anion [Eu@Sn6Bi8]4− requires that europium have a +II oxidation state, as in [Eu2+@(Sn−)6(Bi0)8]4−. Magnetic measurements, which indicated a 7/2 ground state, supported this assumption.282 Lanthanide-centered clusters which contain triel (Tr = Ga, In) atoms display greater basicity at these atomic sites, due to the charge accumulation at the formal Tr2− pseudo-group 15 atoms. The cluster “[Sm@GaxBi13−x]3−” was shown by mass spectrometry and NMR to actually be a mixture of [Sm@ Ga2HBi11]3− and [Sm@Ga3H3Bi10]3− in a 9:1 ratio, with protonation occurring on the Ga atoms (Figure 26h,i).79 Similarly, the In atoms in “{La@In2Bi11}4−” act as Lewis donors toward bridging “Bi+” ions, leading to the formation of the μ-Bi-bridged dimer [(La@In2Bi11)(μ-Bi)2(La@In2Bi11)]6− (Figure 26j).283 The 13- and 14-vertex Ln/Tt/Bi clusters often crystallize as statistically disordered pairs.78,83,239 This behavior was investigated in a series of [Ln@PbxBiy]q− clusters where Ln = La, Ce, Nd, Sm, Gd, and Tb.83 X-ray crystallography, mass spectrometry, and 139La NMR spectroscopy were employed to study the coexistence of the two cluster types (Figure 24, see section 1.3.3, and Figure 3). It is posited that the ionic radius of Ln3+ plays a role in the relative ratios of the 13- and 14vertex clusters, in that an atom with a larger radius will lead to a greater prevalence of the larger 14-atom cage and vice versa.83 This chemistry has also recently been extended to actinides. [K(crypt-222)]2(Pb2Bi2) (formed in situ from a mixture of KPbBi and crypt-222) and [K(crypt-222)]2(TrBi3)·en (Tr = Ga, Tl) were reacted in en with [U(C5Me4H)3].239 Salts of the ternary clusters [U@Pb4Bi9]3−, [U@Pb7Bi7]3−, and [U@ Tl2Bi11]3− were isolated from these reactions (Figure 26k− m). The elemental ratios (determined from mass spectrometry and μ-XFS) and charges of the anions pointed to a +IV oxidation state for the uranium atoms. No evidence was found for the protonation of the Tl atoms. This unusual oxidation of the guest metal under what are generally reductive conditions was further supported by magnetic measurements (see Figure 25) and quantum chemical calculations on the binary cluster [U@Bi12]3−, the product of the reaction with [K(crypt222)]2(GaBi3)·en. The evidence pointed to U4+ and thus a radical Bi127− shell (as discussed in section 2.6.1.). Lastly, it was

Figure 22. Difference electron densities Δρ(“0 + 4”) = ρ{[Pd3Sn8Bi6]4−} − {ρ[Pd3] + ρ([Sn8Bi6]4−)} (left) or Δρ(“2 + 2”) = ρ{[Pd3Sn8Bi6]4−} − {ρ([Pd3]2−) + ρ([Sn8Bi6]2−)} (right), based on DFT calculations of the ternary cluster anion and the according fragments on the cluster atomic positions. Electron densities are drawn from 2 × 10−3 au (red) to −2 × 10−3 au (blue). Reproduced from ref 69. Copyright 2011 American Chemical Society.

Mingos rules by counting {Bi3}+ and {PbBi2}0 as 2-electron donors. Both cluster cores in this case have 50 valence electrons (4n + 2, n = 12), as expected for icosahedral closoclusters. The only example of a ternary heterometallic cluster that comprises a group 8 metal is [Tl2Bi6{Ru(cod)}]2−, which was isolated as the [K(crypt-222)]+ salt from the reaction of [K(crypt-222)]2(TlBi3)·0.5en with [Ru(cod)(Me-Allyl)2] in en (along with [Bi9{Ru(cod)}2]3−, see section 2.6.2).76 The [Tl2Bi6{Ru(cod)}]2− anion adopts a distorted capped square antiprismatic structure (Figure 23d), which is typical of nidoTt94− (Tt = Si−Pb). The reaction of [K(crypt-222)]2(TlBi3)·0.5en with ZnPh2 in en afforded the ternary heterometallic cluster [(Bi 6)Zn3(TlBi5)]4− (Figure 23h).270 This anion features a mixture of 2c−2e and multicenter bonding, comparable to the situation in [Bi9{Ru(cod)}2]3− (see section 2.6.2 and Figure 18l).76 Two Ni-centered ternary clusters have been synthesized by routes that do not make use of 4-atom binary precursors. The anion [Ni@TlSn9]3− was made by first preparing [Ni@Sn9]4− and then reacting it in situ with TlCp (Figure 23g).126 The ternary Zintl phase K 4 Ge 9−x Sn x , from which binary [Ge9−xSnx]q− clusters can be extracted,305 was reacted with [Ni(cod)2] to afford [(Ni@Sn8)Ge(Ni@Sn8)]4−.63 This anion consists of two 9-atom endohedral clusters that share a germanium atom vertex; it is isostructural with the binary cluster [(Ni@Sn8)Sn(Ni@Sn8)]4− (Figure 10d).144 3.4. Ternary Intermetalloid Clusters Containing Lanthanide and Actinide Metal Atoms

A large series of clusters with similar structures have been isolated by reacting either [K(crypt-222)]2(Tt2Bi2)·en (Tt = Sn, Pb) or [K(crypt-222)]2(TrBi3)·en (Tr = Ga, In) with [Ln(C5Me4H)3] (Ln = lanthanide). Two types of structures are observed in the products of these reactions. The first is a 14-vertex cluster with the general formulas [Ln3+@Tt6Bi8]3− or [Ln3+@Tt7Bi7]4− (Figure 26g). These feature electron-precise 3-connected main group metal shells; that is, they have 2 e− per bond and 5n = 70 (n = 14) valence electrons in their cluster shells. Their charges and structure can also be explained via the pseudoelement concept, wherein each group 14 atom has a formal −1 charge and behaves as a pseudopnictogen atom. For example, [Ln@Tt7Bi7]4− may be understood as AD


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Figure 23. Molecular structures of ternary heterometallic and intermetalloid clusters with electron-rich transition metal atoms from groups 12 to 8. Compounds with similar structures are grouped, and a representative anion is shown. (a) [Cd3(Ge3P)3]3−,169 (b) [Au(Sn2Sb2)]3−,168 (c) [Sn2Sb5(ZnPh)2]3−,278 (d) [Tl2Bi6{Ru(cod)}]2−,76 (e) [(CuSn5Sb3)2]4−,279 (f) [Zn@Zn5Tt3Bi8]4− (Tt = Sn,280 Pb276), (g) [Ni@TlSn9]3−,126 (h) [(Bi6)Zn3(TlBi5)]4−,270 (i) [Co@Sn6Sb6]3−,277 (j) [M2@Tt7−2xPn5+2x]3− (M/Tt/Pn/x = Ni/Sn/Bi/0,275 Ni/Pb/Bi/0,276 Co/Sn/Sb/1277), (k) [Pd3@Sn8Bi6]4−,69 (l) [Pd@Pd2Pb10(Bi3)2]4−.281 Positions that are affected by disorder and/or indistinguishability of neighboring atoms are drawn in two-colored mode, with the more probable atom type (according to quantum chemical analyses) shown as the dominant color.

Using this method, several quaternary phases have been synthesized that contain discrete ternary anions. None of these anions, however, have the transition metal in an endohedral position. The quaternary phases A5Tl2TaAs4 (A = Rb, K),284 K 6 TlNbAs 4 , 6 0 K 8 PbNbAs 5 , 6 0 K 1 0 InNbAs 6 , 2 8 6 and Na10GaNbAs6285 all feature tetrahedral [MAs4]8− (M = Ta, Nb) subunits, which are attached to p-block metals via bridging arsenic atoms. The compound Cs7In3NbAs5 contains a cubane-like anion [(In3NbAs4)As]7− with a terminal arsenic “handle”. 287 A dimer of this cluster is present in Cs13In6Nb2As10, and two of these clusters are bridged by defect cubanes in Cs24In12Nb2As18.288 Salts of ternary intermetalloid clusters have been prepared by extraction of quaternary phases that contain V, Nb, and Ta

demonstrated that all reactions were reproducible using the U4+ precursor, [U(C5Me4H)3Cl]. 3.5. Ternary Intermetalloid Clusters Containing Group 5 Metal Atoms

Zintl phases are often synthesized by high-temperature fusion of the component elements in sealed niobium or tantalum ampules. It has been observed that arsenic reacts with these metals at temperatures greater than 600 °C, leading to the unintended incorporation of niobium or tantalum in the product.57−60 When desired, this reactivity can be exploited by direct addition of powders of the group 5 metals to reactions that include arsenic. AE


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germanium atoms act as pseudo-pnictogens with formal charges of −1. The 12-vertex clusters [M@Ge8As4]3− (M = V, Nb) can be similarly described (Figure 26c). They are isostructural with the D2d symmetric [Ru@Ge12]3− mentioned above (see section 2.3.2 and Figure 8k).84 Most notably, the 10-vertex cluster [Ta@Ge6As4]3− displayed an unusual structure and was identified as a possible intermediate in the formation of [Ta@Ge8As4]3− and [Ta@Ge8As6]3− (Figure 26b). The implications of this finding are discussed in section 4.4. The novel intermetalloid cluster [(As3)Nb(As3Sn3)]3− was isolated as the [K(crypt-222)]+ salt by extraction of a phase with the nominal composition “K8NbSnAs5”.289 The cluster consists of a formal Nb5+, which is coordinated by As33− and (Sn3As3)5− fragments (Figure 26a). The latter is related to the binary Zintl ion (Sn3Bi3)5−, which was reported in the double salt Rb6(Sn3Bi3)(Sn4)0.25·6.75NH3.291

4. REACTIVITY AND FORMATION PATHWAYS OF INTERMETALLOID CLUSTERS Reactions of Zintl ions, such as Tt9q− and Pn73−, have been thoroughly studied and reviewed.16,17,20,124 Reactions of the corresponding intermetalloid clusters, however, have only been intermittently explored. This is in part due to the nascence of the field and in part due to experimental difficulties inherent to intermetalloid clusters; their syntheses are often plagued with low yields and impurities. That being said, there is some concrete evidence of their chemical behavior. In this section, reactions that occurred with preformed intermetalloid clusters are presented. This includes Lewis and Brønsted basic behavior, the insertion of metal complex fragments, and the fusion and decomposition of intermetalloid clusters. Additionally, evidence regarding the formation of intermetalloid clusters will be discussed. As NMR is an important tool to identify modifications and monitor cluster dynamics in solution, we provide a table of heteronuclear NMR shifts (Table 6).

Figure 24. ESI mass spectra of fresh DMF solutions of single crystals containing salts of [Ln@PbxBiy]q− clusters where Ln = La, Ce, Nd, Sm, Gd, and Tb, indicating the coexistence of 13-vertex and 14-vertex endohedral clusters, yet with different relative amounts depending on the ionic radius of the interstitial Ln3+ ion. Reproduced with permission from ref 83. Copyright 2015 Wiley-VCH.

4.1. Basicity of Intermetalloid Clusters

Many intermetalloid clusters comprise a shell of p-block elements which carry (partial) negative charges. Unsurprisingly, this can lead to Lewis and Brønsted basic behavior, which is more pronounced going from the heavier to lighter elements. Protonated clusters have been identified by combining spectroscopic, spectrometric, and crystallographic evidence with quantum chemical calculations. For example, the proton on the cluster [Ni@Sn9H]3− is not discernible in the crystal structure due to its proximity to heavy elements and positional disorder. It was hence discussed whether or not the observed species was the unprotonated paramagnetic “[Ni@ Sn9]3−”, which would be chemically reasonable, as the paramagnetic Zintl anion analogue Sn93− is known to exist.128 However, 1H−119Sn coupling was clearly observed in both the 1H and 119Sn NMR spectra. Comparisons were also made between the molecular structure obtained from X-ray crystallography and the optimized DFT structure of [Ni@ Sn9H]3−(Figure 27). A similar observation arose during the synthesis of [Sm@Ga3−xH3−2xBi10+x]3− (x = 0, 1).79 The 3− charge determined crystallographically did not match with that expected for the formulas [Sm@Ga2Bi11]4− or [Sm@Ga3Bi10]4− that were supported by energy-dispersive X-ray spectroscopy. A combination of mass spectrometry (see Figure 2), 1H NMR spectroscopy, and quantum chemical calculations determined that two clusters, [Sm@Ga2HBi11]3− and [Sm@Ga 3H 3Bi 10 ]3−, were present in the sample.

Figure 25. Temperature dependence of χT (magnetic susceptibility χ = M/H per mole of cluster at 0.1 and 1 T below and above 100 K, respectively) for [K(crypt-222)]3[U@Bi12]·tol·1.5en (1), [K(crypt222)]2 [K(crypt-222)(en)][U@Tl2 Bi 11]·tol (2), and [K(crypt222)]3[U@Pb7Bi7]0.66[U@Pb4Bi9]0.34·2tol (3). The measurements support a +IV oxidation state of the interstitial U atom and antiferromagetic coupling of a U4+ cation with an open-shell Bi127− ligand in the case of the [U@Bi12]3− anion. Reproduced from ref 239. Copyright 2016 American Chemical Society.

besides K, Ge, and As. Solid solutions with the nominal formula “KMxGeAs” (M = V, Nb, Ta; x = 0.09−0.1) were synthesized either by the high-temperature reaction of elemental K, Ge, and As in niobium or tantalum ampules or by mixing these elements with metal powder and heating them in a silica ampule with an oxygen torch. Extractions of these phases with the cation sequestration agent crypt-222 in en afforded the endohedral clusters [V@Ge8As4]3−, [Nb@ Ge8As6]3−, [Ta@Ge6As4]3−, [Ta@Ge8As4]3−, and [Ta@ Ge8As6]3− as well as the binary Zintl ions (Ge2As2)2− and (Ge7As2)2−.58,59 The 14-vertex clusters, [M@Ge8As6]3− (M = Nb, Ta; Figure 26d), are isostructural and isoelectronic with the aforementioned 14-vertex lanthanide and uranium clusters, and thus have 5n valence electrons in the cluster shell. The endohedral M atom has a formal +V oxidation state, and the AF


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Figure 26. Molecular structures of ternary intermetalloid clusters with electron-poor transition metal atoms or f-block atoms. Compounds with similar structures are grouped, and a representative structure is shown. (a) [(As3)Nb(As3Sn3)]3−,289 (b) [Ta@Ge6As4]3−,58 (c) [M@Ge8As4]3− (M = V,59 Ta58), (d) [M@Ge8As6]3− (M = Nb,59 Ta58), (e) [Ln@Tt4Bi9]4− (Tt = Sn, Ln = La, Ce;78 Tt = Pb, Ln = La, Ce, Nd, Sm, Gd, Tb),83 (f) [Eu@Sn6Bi8]4−,282 (g) [Ln@Tt7Bi7]4− (Tt = Sn, Ln = La, Ce;78 Tt = Pb, Ln = La, Nd, Sm, Gd, Tb),83 (h) [Sm@Ga2HBi11]3−,79 (i) [Sm@ Ga3H3Bi10]3−,79 (j) [(La@In2Bi11)2(μ-Bi)2]6−,283 (k) [U@Tl2Bi11]3−,239 (l) [U@Pb4Bi9]3−,239 (m) [U@Pb7Bi7]3−.239

new species with an additional vertex. This has been demonstrated indirectly by the stepwise addition of [Ni(cod)2] and [Ni(CO)2(PPh3)2] to an en solution of K4Ge9, which afforded [{η3-(Ni@Ge9)}Ni(CO)]2−.125 The reaction of K4Ge9 with [Ni(CO)2(PPh3)2] produced only [(η4-Ge9)Ni(CO)]3−, while [Ni(cod)2] afforded a mixture of [{η3-(Ni@ Ge9)}Ni(en)]3− and [Ni@Ge9]3−. Thus, it is plausible that [Ni@Ge9]3− is formed in situ upon addition of [Ni(cod)2] and subsequently undergoes oxidative insertion of a {Ni(CO)} fragment after the addition of [Ni(CO)2(PPh3)2]. It should be noted that the addition of a p-block metal atom to [Ni@Sn9]4− was also accomplished by the subsequent addition of [Ni(cod)2] and TlCp to an en solution of K4Sn9 and crypt222, yielding [Ni@TlSn9]3− (see section 3.3 and Figure 23g), though the addition of Tl+ was not accompanied by an oxidation of the cluster.126 The reverse behavior was also

According to quantum chemical studies, protonation occurs exclusively at the gallium atoms, as they have formal 2− charges and thus possess the highest basicity. In both of these cases, the likely source of the protons is the en solvent. Similar Lewis basic behavior was exhibited by “[La@In2Bi11]4−”, which crystallized as part of the dimer [(La@In2Bi11)2(μ-Bi)2]6− (Figure 26j).283 In this dimer, the formal In2− atoms donated electron density to bridging Lewis acidic Bi+ atoms, leading to essentially homopolar bonds in the resulting “In−−Bi−−In−” bridges. 4.2. Insertion of M−L Fragments into Intermetalloid Clusters and Ligand Exchange Reactions

Intermetalloid clusters can displace ligands on transition metal complexes, in a manner similar to Zintl ions (see section 2.4). Alternatively, this can be seen as the addition of a transition metal complex fragment to an intermetalloid cluster to form a AG


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Table 6. Reported Heteronuclear NMR Shifts of Intermetalloid and Heterometallic Clusters compound

solvent

[(Sn9)Cr(CO)3]4− [(Sn9)Mo(CO)3]4− [(Sn9)W(CO)3]4− [Ni@Pb10]2− [(Pta@Sn9)Ptb(PPh3)]2− [Pt@Pb12]2− [Pt2@Sn17]4− [Pt@Sn9H]3− [Cu@Sn9]3− [Cd3(Ge3P)3]3− [Au@Pb12]3− [Rh@Sn10]3− [Rh@Sn12]3− [Co2@Sn5Sb7]3− [La@Pb6Bi8]3− [La@Pb3Bi10]3− [Pb@Pd2Pb10(Bi3)2]4−

DMF DMF DMF DMF DMF DMF DMF en/tol MeCN DMF DMF en en DMF DMF

119

DMF

207

δ (ppm)

nucleus Sn 119 Sn 119 Sn 207 Pb 119 Sn 207 Pb 119 Sn 119 Sn 119 Sn 31 P 207 Pb 119 Sn 119 Sn 119 Sn

2327, −180, −447 1988, −606, −361 2279, −662, −443 −996 −862 1780 −742 −368 −1440 −129 −785 −1313 −460 −635, −857, −879

Pt Pt 195 Pt 195 Pt 63 Cu 113 Cd

a, −5270; b, −6010 −4527 −5713 −5303 −330 636

139

−330 −380

195 195

La

Pb

δ (ppm)

nucleus

2400, 530, −1420

ref 171 171 171 131 138 6 145 145 80 169 136 134 134 277 83 281

(PPh3)Ph] to afford [{η4-(Co@Sn9)}Ni(CO)]3−, [{η4-(Co@ Sn9)}Ni(C2H4)]3−, [{η3-(Co@Sn9)}Pt(PPh3)]3−, and [{η3(Co@Sn9)}AuPh]3−, respectively (Figure 8d,e).140 The insertions of all of these neutral fragments were accompanied by one-electron oxidations of the clusters. The structures of the Sn9 cages were all different and represented a quasi-continuum from C4v (η4) to C3v (η3). With the aid of quantum chemical calculations, it was suggested that this was the result of a shift between two limiting resonance forms, in which electron density is more localized either on the Co−M bond (C3v, [Co(d10)@(Sn92−)M′(d10)−L]3−) or on the Sn9 cage (C4v, [Co(d9)@(Sn94−)M′(d9)−L]3−). The key frontier orbitals of these two resonance forms for [(Co@Sn9)Ni(CO)]3− are shown in Figure 28. Cluster complexes [Tt9M(L)]q− and [(M@Tt9)M(L)]q− can also undergo exchange of the ligand, L. The “capped” cluster [Sn9CdPh]3− reacts with HSntBu3 in pyridine to afford [Sn9Cd(SntBu3)]3− and benzene.176 Similarly, a crystalline sample of [K(crypt-222)]2[(Ni@Ge9)Ni(CO)] was reacted with KCCPh in en, yielding the substituted cluster complex [(Ni@Ge9)Ni(CCPh)]3− (Figure 29).125 Similar behavior has been indirectly observed in the gas phase. Under ESI-MS conditions, the clusters [Bi9{Ru(cod)}2]3− and [Tl2Bi6{Ru(cod)}]2− form dioxygen adducts upon loss of one or more cod ligands (Figure 30).76 The O2 molecules presumably coordinate to the ruthenium atoms.

Figure 27. Molecular structure of [Ni@Sn9H]3− obtained by (a) Xray crystallography and (b) geometry optimization using DFT methods. (c) 119Sn NMR, (d) {1H}119Sn NMR, and (e) 1H NMR spectra of [Ni@Sn9H]3−. Adapted from ref 304. Copyright 2012 American Chemical Society.

indirectly observed. Addition of [Pd(PPh3)4] to an en solution of K4Ge9 and crypt-222 afforded the coordination compound [(η4-Ge9)Pd(PPh3)]3−, while the subsequent addition of [Ni(PPh3)4] in a parallel reaction resulted in the isolation of [{η3-(Ni@Ge9)}Pd(PPh3)]2−.139 Thus, it was inferred that [(η4-Ge9)Pd(PPh3)]3− undergoes oxidative insertion of a Ni atom, with a concomitant topology change of Ge9 from a monocapped square antiprism (C4v) to a distorted tricapped trigonal prism (C3v). A recent study provided more definitive evidence that a preformed intermetalloid cluster can undergo oxidative insertion of M−L fragments. Previously, it was reported that a salt of the intermetalloid cluster [Co@Sn9]4− can be prepared through extraction of the ternary phase K4.79Co0.79Sn9.127 It has now also been demonstrated that [Co@Sn9]4− is produced when a phase of the nominal composition “K5Co3Sn9” and crypt-222 are dissolved in en, both by the isolation of K[K(crypt-222)]3[Co0.87@Sn9] and by the observation of [Co@Sn9]q− as the dominant species in the mass spectrum. Solutions of “K5Co3Sn9” were then reacted with [Ni(CO)2(PPh3)2], [Ni(cod)2], [Pt(PPh3)4], and [Au-

4.3. Fusion of Intermetalloid Clusters

Intermetalloid clusters have also been shown to fuse at elevated temperatures. Reacting an en solution of K5Bi4 and crypt-222 with [Ni(CO)2(PPh3)2] at room temperature yields the heterometallic cluster [Bi3Ni4(CO)6]3− (see Figure 18h).244 It was later demonstrated that, when heated to 60 °C for 6 h, a salt of the aggregated cluster [Ni@Bi6Ni6(Bi3)2(CO)4]4− could be isolated from the same solution (discussed in section 2.6.1, see Figure 17h).245 A similar reaction was carried out using [K(crypt-222)]3[Ni@Sn9Ni(CO)] (Figure 8d),138 which was prepared as a powder and then redissolved in DMF and heated to 60 °C for 30 min. A salt containing the anion [Sn14Ni(CO)]4− was crystallized from this solution (see Figure 14f). The cluster consists of two {Sn8Ni} halves that share a common NiSn2 face but contain no interstitial Ni atoms.193 AH


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Figure 30. High-resolution ESI mass spectrum of [RuTl2Bi6]− (left) and [RuTl2Bi6O2]− (right), both obtained from a solution of single crystals of [K(crypt-222)]2[Tl2Bi6{Ru(cod)}] in DMF. Reproduced with permission from ref 76. Copyright 2017 Wiley-VCH.

Figure 28. Selected frontier Kohn−Sham orbitals of the two isomers of [(Co@Sn9)Ni(CO)]3−. Top: In the C4v structure, electron density from the Co−Ni bond is distributed onto the Sn9 cage, representing the [M(d9)@(Sn94−)M′(d9)−L]3− resonance form. Bottom: In the C3v structure, the electron density of the Co−Ni bond is localized between the two metals, representing the [M(d10)@(Sn92−)M′(d10)− L]3− resonance form. Reproduced from ref 140. Copyright 2018 American Chemical Society.

solution of the salt in DMF, again followed by heating of the solution, affords the final product in much higher yields. Even though this is not direct proof of a formation mechanism, this result suggests an involvement of [Rh@Sn10]3− clusters, or their fragmentation products, in the formation of [Rh3@ Sn24]5−. 4.4. Formation and Growth of Intermetalloid Clusters

It is clear from the sometimes dramatic differences in the structures and electron counts of Zintl ion reactants and their intermetalloid cluster products that complex processes are occurring in solution. Often, two or more clustersmany of which are never isolated or identifiedmust come together to form larger aggregates. In addition, cluster formation is often oxidative, with the solvents being the most probable oxidant(s).79,138 These issues are compounded by a lack of good spectroscopic handles as well as the dynamic behavior of many clusters in solution. Because of this, definitive evidence regarding mechanisms of intermetalloid cluster formation is rare. It is therefore noteworthy when species are structurally characterized that may be intermediates or odd “fragments” that are structurally related to a known cluster or cluster type. The direct insights provided by the isolation of such species have also been augmented by in-depth quantum chemical studies. For a more thorough treatment of this subject, see a recent review article on multimetallic cluster growth.19 An early example of the identification of an intermetalloid cluster intermediate came from the reaction of [IrCl(cod)] with Sn94−. The Zintl ion displaces a Cl− ligand to give [Sn9Ir(cod)]3−.99,180 Crystalline [K(crypt-222)]3[Sn9Ir(cod)] was subsequently dissolved in en and heated in the presence of an oxidant (dppe) to afford [K(crypt-222)]3[Ir@Sn12],99 a structural and electronic analogue of several known endohedral clusters.6,132,136 Additionally, 31P NMR spectroscopy was used to confirm the reduction of dppe. Thus, a plausible

Figure 29. Reaction cascade from Ge93− through [Ni@Ge9]3− to [(Ni@Ge9)Ni(CO)]2−, which then undergoes a ligand exchange reaction with KCCPh in en to yield [(Ni@Ge9)Ni(CCPh)]3−. Adapted from ref 125. Copyright 2006 American Chemical Society.

Another interesting study was carried out using K4Sn9 and [Rh(coe)2(μ-Cl)]2 as starting materials.134 Simply mixing the two reactants in en affords salts of the intermetalloid clusters [Rh@Sn10]3− and [Rh@Sn12]3− as major products and [Rh2@ Sn17]6− as a side product after prolonged crystallization times (see sections 2.3.1 and 2.3.3 and Figures 8g,i and 10e). Dissolving the reactants in en, removing the solvent, and then redissolving and heating the resulting residue in DMF at 50 °C for 5 h affords [Rh3@Sn24]5− in low yields. This cluster can be viewed as a combination of three {Rh@Sn10} cages, in which three square faces of these clusters are fused together to form a central trigonal prism (see section 2.3.3 and Figure 10c). An alternative synthesis was described, in which the cluster is prepared directly from [K(crypt-222)]3[Rh@Sn10]·2en. DisAI


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intermediate was identified that highlights the stepwise nature of cluster formation (Figure 31).

Figure 31. Synthesis of [Sn9Ir(cod)]3− and [Ir@Sn12]3− from Sn94−. Adapted with permission from ref 99. Copyright 2010 Wiley-VCH. Figure 33. Outline of the stepwise formation of a non-deltahedral intermetalloid Ta/Ge/As cluster.58,306 The pathway shown starts out from (Ge2As2)2− and (Ge10)2− anions under consideration of all isolable species: (Ge7As2)2−, [Ta@Ge6As4]3−, and the final cluster [Ta@Ge8As6]3−. The cascade additionally considers the calculated anions [Ta@Ge4As2]− and (TaGe3)−. Adapted with permission from ref 306. Copyright 2018 Royal Society of Chemistry.

In two other cases, the isolation of unusual cluster fragments provided hints as to potential intermediates. The anion [Sn8TiCp]3− was characterized during investigations of the reaction of [TiCl2Cp2] and Sn94− in liquid ammonia.149 Like [Sn9Ir(cod)]3−, the {Sn8Ti} moiety in [Sn8TiCp]3− may represent a step in the formation of endohedral Sn clusters, such as [Ir@Sn12]3− and [(M@Sn8)Sn(M@Sn8)]q− (M/q = Ni/4,144 Co/5,127,129 Rh/6;134 Figure 32). A larger cluster, [Ti@Sn15Ti3Cp5]4/5−, was also structurally characterized (Figure 10i), and structural relationships between this and [Sn8TiCp]3− were evaluated.

In order to better understand how the structurally characterized [Ta@Ge6As4]3− species may relate to a reactive intermediate to larger intermetalloid clusters in solution, it was necessary to study its isomerization. This was done by means of a pathfinder tool.307 As an example, the pathway leading from the global minimum structure of [Ta@Ge6As4]3− to a reactive intermediate to [Ta@Ge8As4]3− is shown in Figure 34.

Figure 34. Isomerization pathway from the [Ta@Ge 6 As 4 ] 3− minimum structure toward a higher-energy isomer that represents an exact fragment of [Ta@Ge8As4]3−, and thus may act as a reactive, direct precursor to this cluster upon reaction with (Ge2As2)2−. The pathway was studied by means of DFT calculations. Reproduced with permission from ref 58. Copyright 2016 Springer-Nature.

Figure 32. Comparison of MSn8 fragments of the structures of (a) [Ir@Sn12]3−,99 (b) [(M@Sn8)Sn(M@Sn8)]q− (M/q = Ni/4,144 Co/ 5,127,129 Rh/6134), and (c) [Sn8TiCp]3−.149 Adapted with permission from ref 149. Copyright 2015 Wiley-VCH.

A similar situation occurred with the characterization of the 10-vertex cluster fragment [Ta@Ge6As4]3−, which crystallized as the [K(crypt-222)]+ salt from a solution of “KTa0.1GeAs” and crypt-222 in en (Figure 26b).58 In addition, salts of [Ta@ Ge8As6]3−, [Ta@Ge8As4]3−, (Ge7As2)2−, and (Ge2As2)2− were characterized crystallographically, and the presence of Ge102− ions in solution was confirmed by mass spectrometry. Using DFT methods, plausible reaction pathways were calculated from the binary Zintl anion (Ge2As2)2− to the 12- and 14vertex intermetalloid clusters, [Ta@Ge8As4]3− and [Ta@ Ge8As6]3−, via the isolated 10-vertex cluster fragment, [Ta@ Ge6As4]3−. It begins with the reaction of (Ge2As2)2− with Ge102− to afford 2 equiv of (Ge7As2)2−. This is then combined with Ta and additional (Ge2As2)2− to produce the structurally characterized [Ta@Ge6As4]3− species. In the final steps, [Ta@Ge6As4]3− combines with (Ge2As2)2−, leading to either [Ta@Ge8As6]3− upon the loss of two electrons (Figure 33) or [Ta@Ge8As4]3− upon release of As22−.

The cluster anion [Co2@Ge16]4− was predicted to have two isomers which are close in energy (see Figure 10a,b). This was manifested in the observation of severe disorder of these two in the solid state (see Figure 1).77,143 A reasonable isomerization pathway was investigated by means of quantum chemistry (Figure 35). 4.5. Decomposition of Intermetalloid and Heterometallic Clusters to Intermetallic Materials

As mentioned in the Introduction, a realistic near-term area of investigation regarding intermetalloid and heterometallic clusters is their controlled oxidation to intermetallic materials. Of particular interest is the relationship between a precursor cluster’s structure and that of the intermetallic product. Such studies have been carried out with homoatomic Zintl anions as precursors to porous materials9,10,12,308,309 and nanoparticles.11 Only one related study on intermetalloid and heterometallic clusters has been published to date. The authors reported that AJ


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structures were those in which the transition metal atom sits inside a crown-like As88− ring, as observed experimentally. The study concludes that covalent M−As interactions increase going from group 5 to group 7 within a period, while the overall stability of the cluster increases going from 3d, to 4d, to 5d transition metals within a group. The onion-type clusters [E@M12@E20]q− (E/M/q = Sn/ Cu/12,150 As/Ni/3,240 Sb/Pd/3,241,242 Sb/Pd/4;241,242 see Figure 17d,e) are intriguing targets for quantum chemical studies, both because of their unique multilayer structures and because of their redox activity. As shown experimentally (see section 2.6.1), they can exist in multiple oxidation states.241 The electronic structure in the cluster anion [Sn@Cu12@ Sn20]12− was investigated within the context of the superatom concept. 269 The authors found that, for a [{Sn@ Cu12}4−@(Sn20)8−]12− charge distribution, the inner centered icosahedron has a filled electronic shell that engages in radial bonding to the outer Sn20 shell. A series of clusters [M@M12@ Bi20]− were investigated for M = 3d and 4d metals.268 A detailed look at the electronic structures of these clusters was presented, also with respect to the superatom concept. Due to the unpaired d-electrons that are present within these molecules, many of the clusters were proposed to possess magnetic moments, with a value as high as 36 μB for [Mn@ Mn12@Bi20]−. While many transition metal and lanthanide atoms have successfully been included in homo- or heteroatomic main group element cages, examples are rare for the inclusion of actinide atoms. Before the first synthesis of actinide-centered clusters was published,239 the question of whether such compounds would be stable was addressed by means of quantum chemical studies. Herein, the icosahedral Zintl anions Sn122− and Pb122− with endohedral actinide atoms have been investigated.310,311 Their icosahedral shape and the resulting cluster orbitals are a perfect match for the symmetry of forbitals. As a consequence, the well-known octet rule and the 18-electron rule for the stability of closed shell systems could be extended toward a 32-electron principle upon occupation of the f-shell with an additional 14 electrons. The model compound [Pu@Pb12] would fulfill this principle but has not yet been synthesized. A series of hypothetical clusters with different actinide elements, [An@Pb12]q (An = Th, U, Np, Pu, Am, Cm; q = 4− to 2+), was further studied with respect to their stability. None of these clusters have yet been synthesized; however, the successful preparation of the first Pu(II) compound [K(crypt-222)][Pu(Cp″)3] (Cp″ = 1,3C 5 H 3 (SiMe 3 ) 2 ) as well as the first actinide-centered intermetalloid clusters open the door for further studies in this field.239,312 In addition to the 12-vertex icosahedra, further intermetalloid clusters with Ih symmetry were studied in [An@ Si20]q− (An = U−Cm, q = 6−2), providing more examples of clusters with 32-electron shells that are predicted to be stable.313

Figure 35. Isomerization pathway from the C2h-symmetric β isomer of [Co2@Ge16]4− to the D2h-symmetric α isomer, studied by DFT calculations. Reproduced with permission from ref 77. Copyright 2018 Wiley-VCH.

[(Pt@Sn9)Pt(PPh3)]2− and [Sn9Ir(cod)]3− could be oxidized by I2 to afford PtSn4 and Ir3Sn7 nanoparticles.13 These reactions were carried out in ethylenediamine/toluene solutions using the [K(crypt-222)]+ salts of the anions (eqs 19 and 20). I2

[(Pt@Sn 9)Pt(PPh3)]2 − → 2PtSn4 + Sn + 2I− + PPh3 en

(19) 3 − 9/2I 2

3[Sn 9Ir(cod)]

⎯⎯⎯⎯⎯→ Ir3Sn 7 + 20Sn + 9I− + 3cod en/tol

(20)

It was noted that there are structural similarities between the cluster precursors and the intermetallic products. Additionally, [Sn9Ir(cod)]3− decomposed to the more structurally related Ir3Sn7, rather than the more stoichiometrically favorable IrSn4. Thus, it was posited that it may be possible to influence the structure of nanomaterials through the use of single-source cluster precursors. However, more research in this area is needed for a more thorough understanding of such effects.

5. CORRELATION OF ELECTRONIC AND GEOMETRIC STRUCTURES IN INTERMETALLOID CLUSTERS The following section will provide some more insight into intermetalloid cluster bonding, which will be illustrated by means of some selected examples. While the first characterized clusters mostly followed either the Wade−Mingos rules or the pseudo-element concept, ever more examples that deviate from these rules are being discovered. These deviations from predicted shapes can be minor, like the small distortion from ideal Ih symmetry in [Pd@Pb12]2− (see Figure 8j), or quite major, like the adoption of an unexpected cluster shape in [M@Ge10]3− (M = Fe, Co; see Figure 8h).100,132,141 In such cases, quantum chemical studies are of tremendous help to understand the electronic situations and facilitate a broader knowledge of cluster bonding. Most reports of new clusters include a quantum chemical study, either to demonstrate agreement with experimental data or to explain a finding for which insufficient experimental proof exists. In some cases, a distinct type of cluster was chosen as a subject of study and a more in-depth investigation into stability trends was provided.

5.2. Electronic Effects in Intermetalloid Cluster Families: Quantum Chemical Predictions

As evident from the studies referred to in the previous section, most of the quantum chemical investigations focus on one cluster type, which is analyzed thoroughly to gain deep insights into the prevailing electronic situations. This approach is useful to understand a particular compound, especially in cases where the bonding cannot be interpreted by simple concepts. However, this offers only limited information on general

5.1. Studies on Clusters with 8, 12, or 20 Vertices

Some of the simplest systems to be studied were the clusters [M@As8]q− (see Figure 17a), which were investigated for group 5−7 metals (M/q = V,Nb,Ta/3; Cr,Mo,W/2; Mn,Tc,Re/1).267 For the whole series, the most stable AK


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trends or the subtle electronic factors that govern the preference of one cluster type over another. The intermetalloid clusters comprising group 14 elements cover a large family of compounds with varying shapes that are ideal for a more systematic study. They were hence chosen for corresponding studies of intermetalloid 10-, 12-, and 14-vertex clusters comprising Si, Ge, or Sn atoms in the cluster shells.142,156,157 Herein, the cluster shape and bonding patterns were studied as a function of the total electron count of the cage. This number is closely related to the degree of orbital interaction between the endohedral transition metal and the surrounding cage, which has drastic effects on the observed cluster shape. For example, in the 12-vertex cluster family, the availability of noninnocent d-electrons leads to a stabilization of threeconnected clusters, like [Ru@Ge12]3− (see Figure 8k), while inert d-electrons facilitate (or do not hinder) the formation of deltahedral structures like [Ni@Pb12]2− (see Figure 8i). This can be attributed to the amount of electron delocalization from the transition metal atom into antibonding orbitals (with respect to the deltahedral structure) of the cluster shell, with the tendency of both the endohedral metal atom and the outer cage to achieve closed shells.156 Evidently, there are several factors that all influence the final cluster shape, some of which have opposing effects. This delicate interplay has led to inconsistencies in the literature, in which different global minima were found just by altering the respective exchangecorrelation functional.157 This highlights the necessity for broader studies with meticulous analyses of electronic structures. The question of how a particular cage type forms and why one structure is more stable than another is difficult to answer and can usually only be addressed in retrospect. An interesting insight into this question was recently provided by a computational study of [M@Si14]q− cages.157 In the case of these clusters, the potential energy surface is quite flat, with only small energy differences between deltahedral and nondeltahedral isomers. Starting from a fully electron-precise cage with 70 electronsas observed in the ternary intermetalloid clusters [M@Tt14−xBix]q− (M = Ln, U; Tt = Sn, Pb; see section 3.4 and below)there are two different paths of (formal) electron withdrawal (Figure 36). In the 70-electron cluster, each atom has one lone pair (28 electrons) and all other electrons (42 electrons) are used for 2c−2e bonds. Electrons can either be removed from the lone pairs, affording multiple bonds in the shell (to form fullerene-related structures), or from the 2c−2e bonds, which then leads to the formation of different types of deltahedral structures (Wade−Mingos clusters) corresponding to the degree of electron deficiency. This later path is clearly favored for clusters comprising germanium, tin, or lead as a result of the inert pair effect.

Figure 36. Linkage between fullerene-related clusters and Wade− Mingos-type cages with 14 vertices. In the “fullerene route”, electron deficiency is countered via the formation of multiple bonds. In the “Wade’s rule route”, electron deficiency is countered through the formation of multicenter bonds, leading to deltahedral structures. Note that a real unsaturated situation like in fullerenes would be reached at 56 electrons. Reproduced with permission from ref 157. Copyright 2017 Royal Society of Chemistry.

distributions are merely formalisms that represent an extreme yet not necessarily realistic case; they are nonetheless often instructive. A relationship between cluster geometry and cluster shell valence electron count can be observed nicely within the series of five 12-vertex clusters shown in Figure 37: [Ir−@Sn122−]3− (VE = 50),99 [Mn2+@Pb125−]3− (VE = 53),135 [(Rh−)2@ Bi126+]4+ (VE = 54),266 [(Co−)2@(Sn5Sb7)−]3− (VE = 56),277 and [Ta5+@(Ge8As4)8−]3− (VE = 60).58 The [Ir−@Sn122−]3− anion is a classic closo-cluster with 50 valence electrons in its cluster shell (4n + 2, n = 12) and virtually undistorted icosahedral symmetry. The anion is characterized by multicenter bonding, while localized 2c−2e bonds are not found. In the extreme case of a [Mn2+@ Pb125−]3− anion, significant electron density is transferred from the manganese atom to the antibonding orbitals of the cluster shell, leading to a (hypothetical) distortion from Ih to D2h symmetry. The anion [(Co−)2@(Sn5Sb7)−]3− formally has 56 valence electrons in its cluster shell, which is essentially the midpoint between a closo (VE = 50) and an electron-precise (VE = 60) configuration. Consequently, the structure of the cluster displays features of both extremes. It adopts a pseudo-D4hsymmetric shape of two face-sharing square antiprisms. The two outer four-atom rings of the Sn/Sb shell exhibit primarily 2c−2e bonding, while the interactions between the atoms within the cluster’s “waist” and the outer ring atoms are mostly 3c−2e or 4c−2e type bonds. In addition, there are strong interactions between the atoms of the cluster shell and the endohedral cobalt atoms. Because of this, the multicenter bonds are weak and the distances between the 4-atom rings are long. A deviation from the structure type can be observed in

5.3. Electronic Effects in Intermetalloid Cluster Families with 10, 12, or 14 Vertices: Experimental Examples

The preceding sections 5.1 and 5.2 highlight conclusions drawn from computational studies. However, the diverse array of clusters that have now been characterized provides tangible examples of the effects of electron count and individual element properties on cluster structure. The following section highlights these effects in four series of structures. For emphasis, some formulas will be written with formal charges. The valence electron (VE) count will refer to that of the cluster shell including its formal charge. Note that these charge AL


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Figure 37. Molecular structures of (a) [Ir−@Sn122−]3− (50 VE),99 (b) [Mn2+@Pb125−]3− (53 VE),135 (c) [(Rh−)2@Bi126+]4+ (54 VE),266 (d) [(Co−)2@(Sn5Sb7)−]3− (56 VE),277 and (e) [Ta5+@(Ge8As4)8−]3− (60 VE).58 VE indicates the number of valence electrons in the p-block element shell including its indicated formal charge.

the cation [(Rh−)2@Bi126+]4+, which formally has 54 valence electrons in its (formal) {Bi12}6+ shell. The two-electron deficiency manifests in the distortion of the formerly planar outer “rings”. Finally, further increase of the valence electron count leads to formally electron-precise cluster cages. In the [Ta5+@(Ge8As4)8−]3− anion, each germanium atom can be described as a pseudoarsenic atom (i.e., Ge−). As a result, the 60 valence electron (5n, n = 12) cluster adopts a structure with only three-connected atoms, fully eliminating deltahedral faces. A structural distortion due to a change in valence electron count is also visible in the 14-vertex clusters, [Eu@Sn8Bi6]4− and [Pd3@Sn8Bi6]4−. The anion [Eu2+@(Sn6Bi8)6−]4− has 70 valence electrons in its cluster shell (5n, n = 14).282 Similar to [Ta@Ge8As4]3−, all tin atoms in [Eu@Sn8Bi6]4− behave as pseudopnictogens (i.e., Sn−), all atoms are three-connected, and bonding is dominated by 2c−2e interactions. The anion [(Pd3)0@(Sn8Bi6)4−]4− has 66 valence electrons in its cluster shell.69 This four-electron deficit is visible in an overall compression of the cage and the formation of (weak) bonding contacts between the three pairs of Sn atoms, indicative of multicenter bonding. The metalloid cluster [Ge 14[Ge(SiMe3)3]5]3− has a highly related oblate structure.314 This can also be seen as a deviation from the 70 electron ideal. Treating the five germyl ligands as one-electron donors yields a cluster shell valence electron count of 64. This example additionally serves to demonstrate that endohedral metal atoms are not a necessary requirement for the formation of larger cluster shells. Figure 38 compares the 14-vertex architectures of [Ge14[Ge(SiMe3)3]5]3− (64 VE), [Pd3@ Sn8Bi6]4− (66 VE), and [Eu@Sn6Bi8]4− (70 VE). In the family of 10-vertex cages, [Ni@Pb10]2−, [Fe@Sn10]3−, and [Fe@Ge10]3− (Figure 39), the influence of the central atom as well as the element that forms the outer cage can be analyzed. The cluster [Ni@Pb10]2− adopts a perfect bicapped square antiprismatic closo-type structure.132 However, the cluster [Fe@Ge10]3− adopts a drastically different D5hsymmetric, pentagonal prismatic shape.100 An intermediate between these two structure types was found in the [Fe@ Sn10]3− anion, which is iso(valence)electronic with its lighter Ge homologue.142 This transition in cluster shape from a bicapped square antiprism to a pentagonal prism can be attributed to back-bonding from the central metal atoms into orbitals of the surrounding main group atom cage.156 There is greater overlap between cluster shell orbitals and transition metal d orbitals for lighter p-block elements. Additionally, back-bonding is more efficient in the pentagonal prismatic cage

Figure 38. Side views (top) and top views (bottom) of 14-vertex cluster structures (a) [Ge14[Ge(SiMe3)3]5]3− (64 VE, drawn without SiMe3 groups),314 (b) [Pd3@Sn8Bi6]4− (66 VE),69 and (c) [Eu@ Sn6Bi8]4− (70 VE).282 Adapted from ref 69. Copyright 2011 American Chemical Society.

Figure 39. Structural comparison between different structures of [M@Tt10]q− clusters: (a) [Ni@Pb10]2− (bicapped square antiprism),132 (b) [Fe@Sn10]3− (C2 symmetry, global minimum from DFT calculations; gray, dashed lines indicate broken bonds with Sn− Sn distances >3.4 Å),142 (c) [Fe@Ge10]3− (pentagonal prism).100

than in the purely deltahedral alternative. The combination of these two effects leads to the D5h extreme being reached for the germanium cage, while only a small distortion is observed for that of tin. Additionally, this effect is also stronger for electronpoor transition metals.156 Thus, the endohedral nickel atom (formally Ni0) in [Ni@Pb10]2− has almost no influence on the electronic structure of the surrounding Pb102− cage.315 A similar, yet less pronounced, effect can be observed in the icosahedral cluster series [Ir@Sn12]3−,99 [Pd@Pb12]2−,132 and [Co@Ge12]3−.133 All of these clusters are iso(valence)electronic with 50 valence electrons (4n + 2, n = 12) in their cluster shells. Hence, they should adopt icosahedral structures according to Wade−Mingos rules. However, this is AM


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strictly true only for the [Ir@Sn12]3− cluster, while [Pd@ Pb12]2− is slightly distorted. Most notably, [Co@Ge12]3− deviates quite drastically from the icosahedral structure through an elongation of the equatorial Ge−Ge bonds along one C5 axis, resulting in a pseudo-D5d structure. This is likely the result of back-donation of d-electrons from Co− to the {Ge12}2− cage, analogous to the situation described above for [Fe@Ge10]3−. As demonstrated above, the degree of covalent bonding interaction between the central atom and the surrounding cage strongly influences intermetalloid cluster structure. Another notable example is found in the cluster family [M@Pn12]3− (Pn = Sb, M = Y, La, Ho, Er, Lu;238 Pn = Bi, M = U;239 Figure 40). In the [M@Sb12]3− anions, all central atoms clearly

reproduce the experimental structural features. The combined evidence clearly indicated that the f-electrons of uranium contribute to the bonding between the central metal atom and the p-block element ring. This is in contrast to all intermetalloid clusters with endohedral lanthanide atoms, in which the corresponding interactions are largely electrostatic.

6. CONCLUSION The rapid development of intermetalloid and heterometallic cluster chemistry over the past two decades has provided a large and varied library of these fascinating species. Currently, the field is entering a new phase, in which researchers are beginning to probe cluster reactivities and investigate their formation. Such studies are hindered by the synthetic challenges, lack of spectroscopic handles, and dynamic behavior that is inherent to metal cluster chemistry. These difficulties have in part been addressed by using quantum chemical studies to fill in the gaps; however, more synthetic breakthroughs are needed. It is our hope that the research assembled here serves as an inspiration for more targeted synthetic and computational investigations. Furthermore, the large array of accessible clusters that we have outlined in this article can and should be considered as subjects for secondary studies of cluster properties; this includes their still widely unexplored use as precursors to new materials. To encourage these developments, we have assembled the current state of knowledge regarding cluster reactivity and have highlighted correlations between their electronic and physical structures. The goals of moving beyond “black box” cluster chemistry, enabling more rational synthetic designs, and revealing potential applications are ambitious. However, considering the incredible progress to date, these objectives are less farfetched than they may seem. AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Figure 40. Comparison between the structures of (a) [M@Sb12]3− and (b) [U@Bi12]3−.238,239 (c) Comparison of orbital schemes for a hypothetical, empty Bi126− cage and the [U@Bi12]3− cluster. Reproduced from ref 239. Copyright 2016 American Chemical Society.

Stefanie Dehnen: 0000-0002-1325-9228 Author Contributions

The manuscript was written through contributions of all authors. R.J.W. and N.L. contributed equally. Notes

possess a +III oxidation state, and the outer shell is understood as {Sb12}6− with almost planar Sb4 rings that are linked together by Sb−Sb bonds. The Sb−Sb distances between the Sb4 rings are larger than those within them. At first glance, the same could be assumed for [U@Bi12]3−, yet noticeable differences between the structures hint toward significant covalent U−Bi interactions. Quantum chemical studies showed that a HOMO(Bi12) → dz2(U) donation of the Bi12 ligand and a fz3(U) → LUMO(Bi12) back-donation from the U atom leads to an elongation of the Bi−Bi distances within the three Bi4 rings, while the distances between them get shorterin agreement with the experimental findings. Magnetic measurements and quantum chemical calculations further support a formulation of [U4+@Bi127−]3− with significant interactions between the endohedral U4+ and the radical Bi127−• ring. Furthermore, optimization of the cluster structure with a basis set that includes the f-electrons in an effective core potential (restricting uranium to a +III oxidation state) does not

The authors declare no competing financial interest. Biographies Dr. Robert J. Wilson obtained his bachelor’s degree in Chemistry from the California Polytechnic State University (Cal Poly), San Luis Obispo. He then went on to receive his doctoral degree in Chemistry in 2013 under the supervision of Miriam Bennett at San Diego State University (SDSU) in a joint program with the University of California, San Diego (UCSD). There his research focused on the synthesis of anionic gallium− and indium−nitrogen clusters and was in part supported by the ARCS foundation. He joined the group of Stefanie Dehnen in 2014 where he investigates the synthesis of binary Zintl anions and ternary intermetalloid clusters of the heavy group 14/15 elements. Dr. Niels Lichtenberger obtained his master’s degree in Chemistry from the Philipps-Universität Marburg. He finished his doctoral studies in 2018 under the supervision of Stefanie Dehnen and AN


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Cp* = pentamethylcyclopentadienyl cod = 1,5-cyclooctadiene coe = cyclooctene cot = 1,3,5,7-cyclooctatetraene crypt-222 = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo [8.8.8]hexacosane Cy = cyclohexyl DFT = density functional theory methods Dipp = 2,6-diisopropylphenyl DMF = N,N-dimethylformamide dppe = 1,2-bis(diphenylphosphino)ethane EDXS = energy-dispersive X-ray spectroscopy en = ethylenediamine, 1,2-diaminoethane EPR = electron paramagnetic resonance ESI = electrospray ionization Et = ethyl i Bu = iso-butyl i Pr = iso-propyl Hyp = hypersilyl, Si(SiMe3)3 L = any ligand Ln = lanthanide M = transition metal MeCN or NCMe = acetonitrile Mes = mesityl, 1,3,5-trimethylbenzyl μ-XFS = micro-X-ray fluorescence spectroscopy MIC = mesoionic carbene MO = molecular orbital MS = mass spectrometry n Bu = n-butyl nacnac = [(N(C6H3iPr2-2,6)C(Me))2CH]−) NHC = N-heterocyclic carbene NMR = nuclear magnetic resonance Pn = pnictogen element py = pyridine R = (metal)organic group SCXRD = single crystal X-ray diffraction SE = skeletal electron number seq = cation sequestering agent solv = solvent(s) SQUID = superconduction quantum interference device t Bu = tert-butyl THF = tetrahydrofuran TMS = trimethylsilyl, SiMe3 tol = toluene Tr = triel element Tt = tetrel element VE = valence electron number

currently works as a postdoctoral researcher in her group. His research focuses on binary and ternary intermetalloid and heterometallic clusters of heavy group 13 elements. During his studies, he was supported by a Ph.D. scholarship from the MArburg University Research Academy (MARA) and received a scholarship from the Deutscher Akademischer Austauschdienst to join the group of Stosh Kozimor at the Los Alamos National Laboratory (NM, USA) to further develop the field of intermetalloid clusters with actinide ions. Dr. Bastian Weinert obtained his diploma in 2011 and his doctoral degree in 2014 from the Philipps-Universität Marburg under the supervision of Stefanie Dehnen. He was supported by a ChemiefondStipendium from Verband der Chemischen Industrie (VCI). Since 2015, he has held a position as Akademischer Rat in the Dehnen group. His research contributes to the development of multimetallic and intermetalloid Zintl clusters of transition metals and group 13/15 elements. Prof. Dr. Stefanie Dehnen obtained her diploma in 1993 and her doctoral degree in 1996 from the University of Karlsruhe (now KIT) under the supervision of Dieter Fenske on experimental and theoretical investigations of copper sulfide and selenide clusters. After a postdoctoral stay with Reinhart Ahlrichs (1997), she completed her Habilitation in Inorganic Chemistry in 2004. In the same year, she was awarded the Wöhler Young Scientists Award from the German Chemical Society (Gesellschaft Deutscher Chemiker, GDCh). In 2005, she received a Heisenberg Grant from German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) and the State-of-Baden-Württemberg Teaching Award. Since 2006, she has been Full Professor of Inorganic Chemistry at Philipps University of Marburg. In 2006, she also became Director, and from 2012 to 2014, she was Executive Director of the Scientific Center of Materials Science at Philipps-Universität Marburg. She is currently an elected member of the Board of the Division for Inorganic Chemistry (Wöhler-Vereinigung für Anorganische Chemie) at GDCh, elected member and spokesperson of the Review Board (Fachkollegium) for Molecular Chemistry at DFG, and Editorial Board or Editorial Advisory Board Member of several scientific journals. Since 2016, she has been a full member of Göttingen Academy of Sciences and Humanity (Akademie der Wissenschaften zu Göttingen) and a full member of Academy of Sciences and Literature, Mainz (Akademie der Wissenschaften and der Literatur Mainz). Her current research interests comprise synthesis, formation mechanisms, stability, reactivity, and physical properties of compounds with binary and ternary chalcogenidometalate anions, organotetrel chalcogenide compounds, binary Zintl anions, and ternary intermetalloid clusters.

ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (DFG), the Alexander von Humboldt Foundation, the Friedrich Ebert Stiftung, the MArburg University Research Academy (MARA), the Deutscher Akademischer Austauschdienst (DAAD), and the Fonds der Chemischen Industrie (FCI).

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ABBREVIATIONS 18-crown-6 = 1,4,7,10,13,16-hexaoxacyclooctadecane An = actinide Ar = aryl bipy = 2,2′-bipyridine BMIm = 1-butyl-3-methylimidazolium CAAC = cyclic alkyl amino carbine Cp = cyclopentadienyl AO


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