(Ge4 Bi14 )4− : A Case of “Element Segregation” on the Molecular Level

[K2(Bi@Pd12@Bi20)]4-: An Endohedral Inorganic Fullerene with Spherical Aromaticity

Inorganic fullerene clusters have attracted widespread attention due to their highly symmetrical geometric structures and intrinsic electronic properties. However, cage-like clusters composed of heavy metal elements with high symmetry are rarely reported, and their synthesis is also highly challenging. In this study, we present the synthesis of a [K2(Bi@Pd12@Bi20)]4- cluster that incorporates a {Bi20} cage with pseudo-Ih symmetry, making it the largest main group metal cluster compound composed of the bismuth element to date. Magnetic characterization and theoretical calculations suggest that the spin state of the overall cluster is a quartet. Quantum chemical calculations reveal that the [Bi20]3- cluster has a similar electronic configuration to C606- and the [Bi@Pd12@Bi20]6- cluster exhibits a unique open-shell aromatic character.

Exploration of the potential energy surface in mixed Zintl clusters applying an automatic Johnson polyhedra generator: the case of arachno E6M24- (E = Si, Ge, Sn; M = Sb, Bi)

A new algorithm called Automatic Johnson Cluster Generator (AJCG) is presented, which, as its name indicates, allows the definition of the desired Johnson polyhedron to subsequently carry out all the possible permutations between the atoms that form this polyhedron. This new algorithm allows the exhaustive study of the structures' potential energy surface (PES). In addition, the AJCG algorithm is helpful for the study of three-dimensional compounds such as boranes or Zintl clusters and their structural derivatives with two or more different atoms. The automatic filling of vertices is particularly useful in mixed compounds because of the possibility of taking into account all possible configurations in the structure. As a test system, we investigated the arachno-type E6M24- (E = Si, Ge, Sn; M = Sb, Bi) structure which has eight vertices and complies with Wade-Mingos rules. Initially, we defined a bipyramidal structure (10 vertices), and filled the vertices with the atoms in all possible configurations. Since the selected system has eight atoms, the two remaining vertices were filled with pseudo atoms to complete the structure. After re-optimizing the initial population generated with AJCG, a large number of isomers with energy below 10 kcal mol-1 are identified. These results show that the most stable isomers possess homonuclear M-M bonds, except Sn6Bi24-. Although the overall putative minima differ at the PBE0-D3 and DLPNO-CCSD(T) levels, they are always competitive minima. In addition to using high-precision methodologies to correctly study relative energies, applying solvent effects in highly charged systems becomes mandatory. The aromatic character of these studied systems was demonstrated qualitatively with two- and three-dimensional mapping and quantitatively by calculating the value of the z-component of the induced magnetic field at the cage center, including scalar and spin-orbit correction for relativistic effects. The compounds studied have a high degree of aromaticity, which allows us to establish that despite structural modifications (i.e., from closo to arachno), the aromaticity is preserved.

Between Elemental Match and Mismatch: From K12 Ge3.5 Sb6 to Salts of (Ge2 Sb2 )2- , (Ge4 Sb12 )4- , and (Ge4 Sb14 )4

The solid mixture "K₂ GeSb" was shown to comprise single-crystalline K₁₂ Ge_3.5 Sb₆ (1), a double salt of K₅ [GeSb₃ ] with carbonate-like [GeSb₃ ]^5- anions, and the metallic Zintl phase K₂ Ge_1.5 . Extraction of 1 with ethane-1,2-diamine in the presence of crypt-222 afforded [K(crypt-222)]⁺ salts of several novel binary Zintl anions: (Ge₂ Sb₂ )^2- (in 2), (Ge₄ Sb₁₂ )^4- (in 3), and in the presence of [AuMePPh₃ ] also (Ge₄ Sb₁₄ )^4- (in 4). The anion in 2 represents a predicted, yet heretofore missing pseudo-tetrahedral anion. 4 comprises a cluster analogous to (Ge₄ Bi₁₄ )^4- and (Ga₂ Bi₁₆ )^4- , and thus one of the most Sb-rich binary p-block anions. The unprecedented cluster topology in 3 can be viewed as a defect-version of the one in 4 upon following a "dead end" of cluster growth. The findings indicate that Ge and Sb atoms are at the border of a well-matching and a mismatch elemental combination. We discuss the syntheses, the geometric structures, and the electronic structures of the new compounds.

Single-Electron Oxidation of Carbene-Coordinated Pnictinidenes-Entry into Heteroleptic Radical Cations and Metalloid Clusters

Stable heavy main group element radicals are challenging synthetic targets. Although several strategies have been developed to stabilize such odd-electron species, the number of heavier pnictogen-centered radicals is limited. We report on a series of two-coordinated pnictogen-centered radical cations [(^MecAAC)EGa(Cl)L][B(C₆F₅)₄] (^MecAAC = [H₂C(CMe₂)₂NDipp]C; Dipp = 2,6-i-Pr₂C₆H₃; E = As 1, Sb 2, Bi 3; L = HC[C(Me)NDipp]₂) synthesized by one-electron oxidation of L(Cl)Ga-substituted pnictinidenes (^MecAAC)EGa(Cl)L (E = As I, Sb II, Bi III). 1-3 were characterized by electron paramagnetic resonance (EPR) spectroscopy and single crystal X-ray diffraction (sc-XRD) (1, 2), while quantum chemical calculations support their description as carbene-coordinated pnictogen-centered radical cations. The low thermal stability of 3 enables access to metalloid bismuth clusters as shown by formation of [{LGa(Cl)}₃Bi₆][B(C₆F₅)₄] (4).

Electronic structure and bonding in endohedral Zintl clusters

Endohedral Zintl clusters-multi-metallic anionic molecules in which a d-block or f-block metal atom is enclosed by p-block (semi)metal atoms-are very topical in contemporary inorganic chemistry. Not only do they provide insight into the embryonic states of intermetallic compounds and show promise in catalytic applications, they also shed light on the nature of chemical bonding between metal atoms. Over the past two decades, a plethora of endohedral Zintl clusters have been synthesized, revealing a fascinating diversity of molecular architectures. Many different perspectives on the bonding in them have emerged in the literature, sometimes complementary and sometimes conflicting, and there has been no concerted effort to classify the entire family based on a small number of unifying principles. A closer look, however, reveals distinct patterns in structure and bonding that reflect the extent to which valence electrons are shared between the endohedral atom and the cluster shell. We show that there is a much more uniform relationship between the total valence electron count and the structure and bonding patterns of these clusters than previously anticipated. All of the p-block (semi)metal shells can be placed on a ladder of total valence electron count that ranges between 4n+2 (closo deltahedra), 5n (closed, three-bonded polyhedra) and 6n (crown-like structures). Although some structural isomerism can occur for a given electron count, the presence of a central metal cation imposes a preference for rather regular and approximately spherical structures which maximise electrostatic interactions between the metal and the shell. In cases where the endohedral metal has relatively accessible valence electrons (from the d or f shells), it can also contribute its valence electrons to the total electron count of the cluster shell, raising the effective electron count and often altering the structural preferences. The electronic situation in any given cluster is considered from different perspectives, some more physical and some more chemical, in a way that highlights the important point that, in the end, they explain the same situation. This article provides a unifying perspective of bonding that captures the structural diversity across this diverse family of multimetallic clusters.

From organotin hydrides to heteronuclear main group metal compounds: isolation of the first neutral bismuth/tin clusters

From conversions of Ar₂SnH₂ (Ar = Tripp, Dipp; Tripp = 2,4,6-Triisopropylphenyl, Dipp = 2,6-Diisopropylphenyl), and a bismuth(III) amide, Bi[N(SiMe₃)₂]₃, we isolated the first representatives of mixed, uncharged Bi/Sn clusters, Bi₈Sn₃Ar₆. Along with unprecedented bicyclo[2.2.0]hexanes, (HAr₂Sn)₂Sn₂Bi₄, these have been characterized by single crystal X-Ray diffraction, heteronuclear NMR, vibrational and UV-Vis spectroscopy. Quantum-chemical calculations were carried out in order to understand bonding within the isolated polyhedral compounds.

Insights into Formation and Relationship of Multimetallic Clusters: On the Way toward Bi-Rich Nanostructures

Bismuth-rich polyanions show a unique potential in constructing nanostructured bismuth-based materials, but they are still poorly investigated. We use a ternary precursor of the nominal composition "K₅Ga₂Bi₄" for the formation of [K(crypt-222)]⁺ salts of novel Bi-rich polyanions [Bi@Ga₈(Bi₂)₆]^q- (q = 3, 5; in 1), (Ga₂Bi₁₆)^4- (in 2), and [{Ru(cod)}₄Bi₁₈]^4- (in 3). Their bismuth contents exceed that of the largest homoatomic polyanion, Bi₁₁^3-. The numbers of bismuth atoms in the anions in 2 and 3 furthermore surmount that of the Bi-richest binary main-group anion, (Ge₄Bi₁₄)^4-, and they equal (2) or surmount (3) that reported for the anion and the cations with the largest number of Bi atoms so far, [K₂Zn₂₀Bi₁₆]^6-, [(Bi₈)Ru(Bi₈)]⁶⁺, and [(Bi₈)Au(Bi₈)]⁵⁺. Compounds 1 and 2 were obtained from reaction mixtures that contain [La(C₅Me₄H)₃], apparently assisting in the network formation without being included in the products. In the presence of [Ru(cod)(H₂CC(Me)CH₂)₂], yet another reaction pathway leads to the formation of the anions in 3 (conformers 3a and 3b), which are Bi-Bi linked dimers of two "[{Ru(cod)}₂Bi₉]^2-" subunits. They comprise the largest connected assemblies of Bi atoms within one molecule and may be viewed as snapshots on the way toward even larger polybismuthide units and, ultimately, new bismuth modifications. Mass spectrometry allowed insight into the processes in solution that precede the cluster formation. In-depth quantum chemical studies were applied to explain structural peculiarities, stabilities of the observed isomers, and bonding characteristics of these bismuth-rich nanoarchitectures. The work demonstrates the high potential of the method for the access of new Bi-based materials.

[Bi6Mo3(CO)9]4-: a multiple local σ-aromatic cluster containing a distorted Bi6 triangular prism

The first Zintl cluster containing a distorted Bi₆ triangular prism, [Bi₆Mo₃(CO)₉]^4-, has been synthesized and structurally characterized. Quantum chemical calculations indicated that the distorted cage cluster features multiple local σ-aromaticity.

Stabilizing a metalloid {Zn12} unit within a polymetallide environment in [K2Zn20Bi16]6

The access to molecules comprising direct Zn–Zn bonds has become very topical in recent years for various reasons. Low-valent organozinc compounds show remarkable reactivities, and larger Zn–Zn-bonded gas-phase species exhibit a very unusual coexistence of insulating and metallic properties. However, as Zn atoms do not show a high tendency to form clusters in condensed phases, synthetic approaches for generating purely inorganic metalloid Zn_x units under ambient conditions have been lacking so far. Here we show that the reaction of a highly reductive solid with the nominal composition K₅Ga₂Bi₄ with ZnPh₂ at room temperature yields the heterometallic cluster anion [K₂Zn₂₀Bi₁₆]^6–. A 24-atom polymetallide ring embeds a metalloid {Zn₁₂} unit. Density functional theory calculations reveal multicenter bonding, an essentially zero-valent situation in the cluster center, and weak aromaticity. The heterometallic character, the notable electron-delocalization, and the uncommon nano-architecture points at a high potential for nano-heterocatalysis.

Low-valent zinc clusters, though exceedingly rare, are appealing synthetic targets because there is evidence that they may show unconventional chemical and physical behavior. Here, the authors obtain a large heterometallic zinc-bismuth cluster anion and discover that it bears a metalloid {Zn₁₂} core with four-center bonding and essentially zero-valent character.

Systematic DFT Studies on Binary Pseudo-tetrahedral Zintl Anions: Relative Stabilities and Reactivities towards Protons, Trimethylsilyl Groups, and Iron Complex Fragments

Binary pseudo-tetrahedral Zintl anions composed of (semi)metal atoms of the p-block elements have proven to be excellent starting materials for the synthesis of a variety of heterometallic and intermetalloid transition metal-main group metal cluster anions. However, only ten of the theoretically possible 48 anions have been experimentally accessed to date as isolable salts. This brings up the question whether the other species are generally not achievable, or whether synthetic chemists just have not succeeded in their preparation so far. To contribute to a possible answer to this question, global minimum structures were calculated for all anions of the type (TrTt3 )5- , (TrPn3 )2- , and (Tt2 Pn2 )2- , comprising elements of periods 3 to 6 (Tr: triel, Al⋅⋅⋅Tl; Tt: tetrel, Si⋅⋅⋅Pb; Pn: pnictogen, P⋅⋅⋅Bi). By analyzing the computational results, a concept was developed to predict which of the yet missing anions should be synthesizable and why. Additionally, the results of an electrophilic attack by protons or trimethylsilyl groups or a nucleophilic attack by transition metal complex fragments are described. The latter yields butterfly-like structures that can be viewed as a new form of adaptable tridentate chelating ligands.

Recent developments in Zintl cluster chemistry

Zintl anions have been known for more than a century and were studied systematically by Eduard Zintl in the 1930s. Since then, they have been investigated for their interesting structures, bonding, and physical properties - in solid Zintl phases, in solvate salts, and in solution. While their popularity remained limited for several decades, Zintl ion chemistry has recently experienced a renaissance as a result of breakthroughs regarding their modifications into multinary anions that include transition metal atoms, their organic derivatization, and their oxidative linkage. A plethora of reports from the past two decades - demonstrating the ever growing variety of Zintl ion chemistry - have been since summarized in several review articles. Herein, we intend to present the most recent developments, which also shed light on Zintl anions and clusters as useful precursors for materials development, as illustrated by one recent example.

On the Nature of Bonding in Synthetic Charged Molecular Alloy [P7ZnP7]4- Cluster and Its Relevant [P7]3- Zintl Ion

Charged molecular alloys and Zintl ions are of interest in synthetic chemistry. However, their chemical bonding has seldom been elucidated using modern quantum chemistry tools. Herein, we report on in-depth chemical bonding analyses for a charged molecular alloy C ₂ [P₇ZnP₇]^4- cluster and its relevant Zintl ion C _3v [P₇]^3- ligand, making use of electronic structure calculations at PBE0/def2-TZVP level, natural bond orbital and orbital composition analyses, canonical molecular orbitals, and adaptive natural density partitioning (AdNDP). The computational data show that C _3v [P₇]^3- Zintl ion has three isolated, negatively charged, bridging P sites. Such charges are largely P 3p lone-pairs in nature, but they also participate in secondary P-P bonding along the bridging sites. C ₂ [P₇ZnP₇]^4- cluster is formulated as [P₇]^2-[Zn]⁰[P₇]^2-, in which [P₇]^2- ligands maintain the structural and bonding integrity of [P₇]^3- Zintl ion despite their difference in charge state. Two [P₇]^2- ligands collectively bind with Zn center via four bridging P sites, resulting in a quasi-tetrahedral ZnP₄ core with the eight-electron counting. This bonding picture can alternatively be rationalized using the superatom concept. The Zn-P bonds are weak with a bond order of around 0.5, because the P centers have partial nonbonding 3p character, akin to 3p² lone-pairs albeit with a lower occupation number.

Ge14 Br8 (PEt3 )4 : A Subhalide Cluster of Germanium

Heating a metastable solution of Ge^I Br to room temperature led to the first structurally characterized metalloid subhalide cluster Ge₁₄ Br₈ (PEt₃ )₄ (1). Furthermore 1 can be seen as the first isolated binary halide cluster on the way from Ge^I Br to elemental germanium, giving insight into the complex reaction mechanism of its disproportionation reaction. Quantum chemical calculations further indicate that a classical bonding situation is realized within 1 and that the last step of the formation of 1 might include the trapping of GeBr₂ units.

(Ge2P2)2−: a binary analogue of P4 as a precursor to the ternary cluster anion [Cd3(Ge3P)3]3−

The novel binary P₄ analogue (Ge₂P₂)²⁻ proved to be a suitable precursor for heteroatomic cluster synthesis. Upon reaction with CdPh₂, an unprecedented cluster anion is formed, [Cd₃(Ge₃P)₃]⁻.

[SnBi3 ]5- -A Carbonate Analogue Comprising Exclusively Metal Atoms

The new [SnBi₃ ]^5- polyanion is obtained by the reaction of K₃ Bi₂ with K₄ Sn₉ or K₁₂ Sn₁₇ in liquid ammonia. The anion is iso(valence)electronic with and structurally analogous to the carbonate ion. Despite the high negative charge of the anion, the Sn-Bi bond lengths range between single and double bonds. Quantum-chemical calculations at a DFT-PBE0/def2-TZVPP/COSMO level of theory reveal that the partial double bond character between the heavy main-group atoms Bi and Sn originates from a delocalized π-electronic system. The structure of the anion is determined by single-crystal X-ray diffraction analyses of the compounds K₅ [SnBi₃ ] 9 NH₃ (1) and K₉ [K(18-crown-6)][SnBi₃ ]₂ ⋅15 NH₃ (2). The [SnBi₃ ]^5- unit is the first example of a carbonate-like anion obtained from solution, and it consists exclusively of metal atoms and completes the series of metal analogues of CO and CO₂ .