Monocyclic π-aromatic compounds are ubiquitous throughout almost all fields of natural sciences—as synthons in industrial processes, as ligands of metal complexes for catalysis or sensing and as bioactive molecules. Planar organocycles stand out through their specific way of overcoming electron deficiency by a non-localizable set of (4n + 2)π electrons. By contrast, all-metal aromatic monocycles are still rare, as metal atoms prefer to form clusters with multiply bonded atoms instead. This limits the knowledge and potential of corresponding compounds in chemical syntheses or for innovative materials. Here we report the successful generation of Bi5−, the heaviest analogue of (C5H5)−. Its use as a ligand in [IMesCo2(µ,η5:η5-Bi5)] (1) was realized by reacting (TlBi3)2− with [(IMes)2CoCl] (where IMes is bis(1,3-(2,4,6-trimethylphenyl))imidazol-2-ylidene) in ortho-difluorobenzene. Compound 1 is mixed-valence Co0/CoI as verified by µ-SQUID measurements and density functional theory, and embeds the planar Bi5− cycle in an inverse-sandwich-type manner. Capturing Bi5− represents a landmark in the chemistry of all-metal aromatic molecules and defines a new era for aromatic compounds. All-metal aromatic monocycles are still rare, in contrast to their ubiquitous organic counterparts, because metal atoms tend to form clusters with multiply bonded atoms instead. Now a planar aromatic Bi5− ring has been synthesized as part of a mixed-valence Co0/CoI inverse-sandwich-type complex.

Treatment of the binary Zintl anion (Sn2Bi2)2− with phosphines as mild oxidation agents in ortho-difluorobenzene causes expansion to the new species (Sn4Bi4)2−, which crystallizes along with [K(crypt-222)]+ cations (crypt-222=4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane). The electron-count and architecture of the anion are consistent with a Wade-Mingos arachno-type cluster, obtained in its minimum-energy isomer according to DFT calculations (DOI: 10.1002/zaac.202300229).

The research into adamantane-type compounds has gained momentum in recent years, yielding remarkable new applications for this class of materials. In particular, organic adamantane derivatives (AdR4) or inorganic adamantane-type compounds of the general formula [(RT)4E6] (R: organic substituent; T: group 14 atom C, Si, Ge, Sn; E: chalcogenide atom S, Se, Te, or CH2) were shown to exhibit strong nonlinear optical (NLO) properties, either second-harmonic generation (SHG) or an unprecedented type of highly-directed white-light generation (WLG) - depending on their respective crystalline or amorphous nature. The (missing) crystallinity, as well as the maximum wavelengths of the optical transitions, are controlled by the clusters' elemental composition and by the nature of the organic groups R. Very recently, it has been additionally shown that cluster cores with increased inhomogeneity, like the one in compounds [RSi{CH2Sn(E)R '}3], not only affect the chemical properties, such as increased robustness and reversible melting behaviour, but that such 'cluster glasses' form a conceptually new basis for their use in light conversion devices. These findings are likely only the tip of the iceberg, as beside elemental combinations including group 14 and group 16 elements, many more adamantane-type clusters (on the one hand) and related architectures representing extensions of adamantane-type clusters (on the other hand) are known, but have not yet been addressed in terms of their opto-electronic properties. In this review, we therefore present a survey of all known classes of adanmantane-type compounds and their respective synthetic access as well as their optical properties, if reported. Adamantane-type clusters exhibit a huge diversity of chemical compositions, structural details, and resulting properties.

Reactions involving Zintl ions are typically carried out in N-based solvents such as liquid ammonia (NH3(l)), ethane-1,2-diamine (en), pyridine (py), or acetonitrile (CH3CN). We are currently investigating the application of other solvents as alternatives. Herein, we present the synthesis of the (Sn4Bi4)(2-) anion, obtained as its [K(crypt-222)](+) salt (crypt-222=4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane), from reactions of the binary, pseudo-tetrahedral Zintl anion (Sn2Bi2)(2-) in the presence of phosphines as oxidation agents in ortho-difluorobenzene (o-DFB). The cluster comprises 38 valence electrons, hence, represents an arachno-type architecture according to Wade-Mingos rules. Micro-X-ray fluorescence (mu-XFS) spectroscopy rationalized the elemental composition of the anion, and quantum chemical studies served to compare the stability of possible structural isomers of the global minimum structure.

We read the manuscript by Dariusz W. Szczepanik and Miquel Solà with interest, and recognized several misinterpretations (based on oversimplifications) of our work and also errors that results from inappropriate/insufficient methods applied in their follow-up studies. This led to erroneous statements, which the authors additionally mixed with statements on aromaticity, which does not fully comply with definitions that have been well-established, e.g., for benzene. In this comment, we outline the misinterpretations, errors, and questionable statements, thereby referring to our work and further literature to underline the facts.

Monocyclic π-aromatic compounds are ubiquitous throughout almost all fields of natural sciences-as synthons in industrial processes, as ligands of metal complexes for catalysis or sensing, and as bioactive molecules. They stand out through their specific way of overcoming electron deficiency as planar organocycles comprising a non-localizable set of (4n+2)π electrons.1 By contrast, all-metal aromatic monocycles with all C atoms replaced by metal atoms are still rare, as metal atoms prefer to form clusters instead of cycles with multiply-bonded atoms, making them a huge synthetic challenge.2–4 This limits the knowledge and potential of corresponding compounds in chemical syntheses or for innovative materials. Here, we show the first successful generation of Bi5–, the heaviest analogue of the aromatic ligand (C5H5)–. Its use as ligand in [{IMesCo}2(µ,η5:η5-Bi5)] (1) was realized by reacting (TlBi3)2– with [(IMes)2CoCl] (IMes: bis(1,3-(2,4,6-trimethylphenyl))imidazol-2-ylidene) in ortho-difluorobenzene. Compound 1 is mixed-valent (Co0/CoI) as verified by µ-SQUID measurements and DFT, and perfectly embeds the planar Bi5– cycle in an inverse-sandwich-type manner. Capturing Bi5– represents a landmark in the chemistry of all-metal aromatic molecules and defines a new era for aromatic compounds. Immediate implications include the systematic development of new complexes featuring Bi5– and its derivatives as innovative bismuth-based materials.

As a means of tuning the electronic properties of tin-chalcogenide-based compounds, we present a strategy for the compositional and structural expansion of selenido stannate frameworks under mild conditions by introducing Cu+ ions into binary anionic Sn/Se aggregates in ionothermal reactions. The variable coordination modes of Cu+-contrasting with tetrahedral {SnSe4} or trigonal bipyramidal {SnSe5} units-and corresponding expansion toward ternary Cu/Sn/Se substructures helped to add another degree of freedom to the nanoarchitectures. As desired, the variation of the structural features was accompanied by concomitant changes of the physical properties. Upon treatment of alkali metal salts of the [SnSe4]4- anion at slightly elevated temperatures (120 or 150 °C) in ionic liquids, we isolated a series of compounds comprising ternary or quaternary cluster molecules or networks of cluster units, (C2C2Im)9Li[Cu10Sn6Se22] (1), (C2C2Im)4[Cu8Sn6Se18] (2), (C2C1Im)3[Cu5Sn3Se10] (3), and (C2C2Im)5[Cu8Sn6Se18F]·(C2C2Im)[BF4] (4; C2C2Im = 1,3-diethyl-imidazolium, C2C1Im = 1-ethyl-3-methyl-imidazolium), which were investigated in terms of their optical gaps and photocurrent conversion properties. As illustrated by the synthesis and characterization of an additional salt that does not include Cu+, {(C2C2Im)2[Sn3Se7]}4·{(C2C2Im)[BF4]}2 (5), the significant role of Cu+ in this system was shown to be 3-fold: (a) structural expansion, (b) narrowing of the optical gap, and (c) photocurrent enhancement. By this three-in-one effect, the work offers an in-depth understanding of chalcogenido metalate chemistry with atomic precision.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Pseudo-tetrahedral units of p-block atoms proved to be excellent building blocks for novel molecular architectures and for introducing new elemental combinations which are not otherwise accessible. In this work, we present a series of clusters obtained by reactions of binary Ge/As anions with [MPh2] (M=Zn, Cd, Hg; Ph=phenyl). The study is grounded on the fact that the binary reactant gained by extracting the solid 'K2GeAs' with ethane-1,2-diamine (en) co-exists as (Ge2As2)(2-) and (Ge3As)(3-) in solution. This allows for a larger variety of products by 'selecting' the most suitable species for the final ternary complex to crystallize. The reactions afforded the unprecedented first step of the corresponding interaction, thus attachment of (MPh)(+) to a pseudo-tetrahedral unit in [PhZn(Ge3As)](2-) (1) and [PhHg(Ge3As)](2-) (2), and complex anions with two, three, or four units, [(Ge3As)Zn(Ge2As2)](3-) (3), [Cd-3(Ge3As)(3)](3-) (4), and [Zn-3(Ge3As)(4)](6-) (5). Quantum chemistry confirmed the compositions and the positions of the Ge or As atoms, beside explaining structural peculiarities. The subtle impact of different [MR2] reactants was additionally studied by corresponding reactions using [ZnMes(2)] (Mes=mesityl), which showed success in selectively crystallizing [MesZn(Ge3As)](2-) (6). Based on our findings, we derive a suggestion of the underlying reaction cascade.

So far, homoatomic polybismuthide Zintl anions, which can be used as starting materials for larger binary or ternary, bismuth-based clusters, have always been isolated from potassium-based Zintl phases using 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (crypt-222) to sequester the K+ counterions. The formation of larger clusters from such anions is still poorly understood, and a change of the sequestering agent might lead to the crystallization of missing links in the cluster formation pathways, which is why - in addition to cost considerations - such variants are being investigated. Herein, we present the synthesis and characterization of the first homoatomic polybismuthide salt with a 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) complex as counterion, [K(18crown-6)](2)Bi-2 (1). By means of mass spectrometry, we show that compound 1 behaves, generally, alike [K(crypt-222)](2)Bi-2 in reactions with [CpRu(NCMe)(3)](+), forming the previously reported cluster anion [{CpRu}(3)Bi-6]. However, crystallization of this cluster can only be achieved in the presence of [PF6] anions, yielding [{K(18-crown-6)}(2){PF6}][{CpRu}(3)Bi-6]center dot en (2). This shows that [K(18-crown-6)](+), if paired up with an additive like [PF6](-), indeed is a (cheaper) alternative for the crystallization of anionic bismuth-based clusters.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

ConspectusBismuth-based research has become a highly topical field in recent years, yielding remarkable prospects for new fundamental insights and new materials applications, ranging from innovative catalysts to novel pharmaceuticals, due to this heavy metal's virtually nonradioactive and nontoxic properties. Given that the 6s2 electron pair can be stereochemically active under certain circumstances, bismuth atoms adopt a variety of coordination modes and bonding environments with oxidation states ranging from (formally) +V to -III. As a consequence, bismuth-based compounds cover the entire spectrum from simple coordination compounds to much more unusual cluster cations and cluster anions exhibiting metal-metal bonding in a homoatomic manner, or in concert with other s-, d-, p-, or f-block metal atoms. Such bismuth clusters show high potential for the development of new bismuth-based materials, but they are also interesting objects by themselves. Given the relatively recent development of bismuth-rich cluster molecules, a deep understanding of their properties─including unprecedented structural features, complex electronic structures, substantial heavy metal aromaticity, as well as their formation pathways─is still in its infancy. The topic thus spans a broad range from highly sophisticated synthetic chemistry through interdisciplinary experimental and theoretical analyses to materials science.Based on our recent work and several notable reports from other groups, this article will highlight the successful access to a number of novel bismuth-rich cluster ions emerging from both solution-based approaches and solid-state chemistry. It will shed light on the unique structural and electronic properties that cause chemical and physical peculiarities of such compounds. Selected examples include, but are not limited to, (1) the first encapsulation of actinide ions in intermetalloid clusters which additionally served to manifest substantial all-metal π-aromaticity with a (calculated) record ring current per electron; (2) a large metalloid {Zn12} unit stabilized in a porphine-related {Zn8Bi16} moiety in [K2Zn20Bi16]6-; (3) the largest assembly of bismuth atoms within one molecule, observed in [{Ru(cod)}4Bi18]4- that consists of two Bi-Bi-linked "[{Ru(cod)}2Bi9]2-" subunits.Notably, cluster growth has remained largely a black box, which is starting to be revealed, however. We discuss possible formation pathways of such (multi)metallic nanoarchitectures on the basis of smaller subunits that were detected by mass spectrometric analyses and could also be captured upon reaction with organometallic complexes. In addition to the intrinsic structural and electronic properties of the cluster anions and cluster cations reviewed herein, we will briefly introduce the emerging usage of bismuth-based compounds in material science and give an outlook to future developments.

Key challenges in modern synthetic chemistry include the design of reliable, selective, and more sustainable synthetic methods, as well as the development of promising candidates for new materials. Molecular bismuth compounds offer valuable opportunities as they show an intriguing spectrum of properties that is yet to be fully exploited: a soft character, a rich coordination chemistry, the availability of a broad variety of oxidation states (at least +V to -I) and formal charges (at least +3 to -3) at the Bi atoms, and reversible switching between multiple oxidation states. All this is paired with the status of a non-precious (semi-)metal of good availability and a tendency towards low toxicity. Recent findings show that some of these properties only come into reach, or can be substantially optimized, when charged compounds are specifically addressed. In this review, essential contributions to the synthesis, analyses, and utilization of ionic bismuth compounds are highlighted.

Graphical Abstract Starting from the seminal potentiometric titration experiment by Zintl that led to the detection of the first three polybismuthide anions, the image gives an overview of all homoatomic bismuth polyanions that have been isolated thus far in isolated salts (DOI: 10.1002/zaac.202300121).

We read the Matters Arising article by Dariusz W. Szczepanik and Miquel Solà with interest, and recognized several misinterpretations (based on oversimplifications) of our work and also errors that results from inappropriate/insufficient methods applied in their follow-up studies. This led to erroneous statements, which the authors additionally mixed with statements on aromaticity, which does not fully comply with definitions that have been well-established, e.g., for benzene. In this comment, we outline the misinterpretations, errors, and questionable statements, thereby referring to our work and further literature to underline the facts.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Multimetallic clusters play a key role as models to doped metals, as candidates to new types of superatomic catalysts and as precursors to new multimetallic solids. Understanding formation pathways is an essential and necessary step forward in the development of cluster synthesis and research, yet remains considerably lacking owing to difficulty in identification of intermediates and the ill-defined nature of common starting materials. Here we show progress in this regard by investigating the reactivity of an intermetallic solid of nominal composition 'K5Ga2Bi4' with [W(cod)(CO)4] upon extraction with ethane-1,2-diamine (en) and 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (crypt-222). Several polybismuthide intermediates and by-products were identified along the reaction pathway, ultimately forming the new polybismuthide salt [K(crypt-222)]3[µ:η3-Bi3{W(CO)3}2]∙en∙tol. DFT calculations revealed plausible reaction schemes for the transformations taking place in the reaction mixture providing insight into the complex reactivity of 'K5Ga2Bi4' on the basis of in situ generation of Bi22-.

The occurrence of aromaticity in organic molecules is widely accepted, but its occurrence in purely metallic systems is less widespread. Molecules comprising only metal atoms (M) are known to be able to exhibit aromatic behaviour, sustaining ring currents inside an external magnetic field along M-M connection axes (σ-aromaticity) or above and below the plane (π-aromaticity) for cyclic or cage-type compounds. However, all-metal compounds provide an extension of the electrons' mobility also in other directions. Here, we show that regular {Bi6} prisms exhibit a non-localizable molecular orbital of f-type symmetry and generate a strong ring current that leads to a behaviour referred to as φ-aromaticity. The experimentally observed heterometallic cluster [{CpRu}3Bi6]-, based on a regular prismatic {Bi6} unit, displays aromatic behaviour; according to quantum chemical calculations, the corresponding hypothetical Bi62- prism shows a similar behaviour. By contrast, [{(cod)Ir}3Bi6] features a distorted Bi6 moiety that inhibits φ-aromaticity.

Experimental and theoretical studies are reported of the first two-coordinated Si0-isocyanide compound (SIDipp)Si═C═N-ArMes (1: SIDipp (NHC) = C[N(Dipp)CH2]2, ArMes = 2,6-dimesitylphenyl), supported by an N-heterocyclic carbene (NHC). A Si atom economic two-step synthesis of 1 involves a 2e reduction of the isocyanide-stabilized silyliumylidene salt [SiBr(CNArMes)(SIDipp)][B(ArF)4] (2[B(ArF)4], ArF = B(C6H3-3,5-(CF3)2)4) with KC8. 2[B(ArF)4] was obtained from SiBr2(SIDipp) after bromide abstraction with an equimolar mixture of Na[B(ArF)4] and ArMesNC. Exact adherence to the stoichiometry is crucial in the latter reaction, since 2[B(ArF)4] reacts with SiBr2(SIDipp) via isocyanide exchange to afford the disilicon(II) salt [Si2Br3(SIDipp)2)][B(ArF)4] (3[B(ArF)4]), the reaction leading to an equilibrium that favors 3[B(ArF)4] (Keq(298 K) = 10.6, ΔH° = -10.6 kJ mol-1; ΔS° = -16.0 J mol-1 K-1). 3[B(ArF)4] was obtained selectively from the 2:1 reaction of SiBr2(SIDipp) with Na[B(ArF)4] and fully characterized. Detailed studies of 1 reveal an intriguing structure featuring a planar CNHC-Si-C-N skeleton with a V-shaped geometry at the dicoordinated Si0 center, a slightly bent Si═C═N core, a CNHC-Si-CCNR 3c-2e out of plane π-bond (HOMO), and an anticlinal conformation of the SIDipp and ArMes substituents leading to axial chirality and the presence of two enantiomers, (Ra)-1 and (Sa)-1. Compound 1 displays structural dynamics in solution, rapidly interconverting the enantiomers. The silacumulene 1 is a potent Si(SIDipp) transfer agent as demonstrated by the synthesis and full characterization of the NHC-supported germasilyne (Z)-(SIDipp)(Cl)Si═GeArMes (4) from 1 and Ge(ArMes)Cl.