Three-dimensional Aromaticity Research Articles

Xanthene-Separated 24 π-Electron Antiaromatic Rosarin Dimer.

Antiaromatic molecules have recently received attention because of their intrinsic properties, such as high reactivity and their narrow HOMO-LUMO gaps. Stacking of antiaromatic molecules has been predicted to induce three-dimensional aromaticity via frontier orbital interactions. Here, we report a covalently linked π-π stacked rosarin dimer that has been examined experimentally by steady-state absorption and transient absorption measurements and theoretically by quantum chemical calculations, including time-dependent density functional theory, anisotropy of induced current density, and nucleus-independent chemical shift calculations. Relative to the corresponding monomer, the dimer exhibits diminished antiaromaticity upon lowering the temperature to 77 K, a finding ascribed to intramolecular interactions between the macrocyclic rosarin subunits.

Chemical Bonding and Dynamic Structural Fluxionality of a Boron-Based Na5B7 Sandwich Cluster.

Doping alkali metals into boron clusters can effectively compensate for the intrinsic electron deficiency of boron and lead to interesting boron-based binary clusters, owing to the small electronegativity of the former elements. We report on the computational design of a three-layered sandwich cluster, Na5B7, on the basis of global-minimum (GM) searches and electronic structure calculations. It is shown that the Na5B7 cluster can be described as a charge-transfer complex: [Na4]2+[B7]3-[Na]+. In this sandwich cluster, the [B7]3- core assumes a molecular wheel in shape and features in-plane hexagonal coordination. The magic 6π/6σ double aromaticity underlies the stability of the [B7]3- molecular wheel, following the (4n + 2) Hückel rule. The tetrahedral Na4 ligand in the sandwich has a [Na4]2+ charge-state, which is the simplest example of three-dimensional aromaticity, spherical aromaticity, or superatom. Its 2σ electron counting renders σ aromaticity for the ligand. Overall, the sandwich cluster has three-fold 6π/6σ/2σ aromaticity. Molecular dynamics simulation shows that the sandwich cluster is dynamically fluxional even at room temperature, with a negligible energy barrier for intramolecular twisting between the B7 wheel and the Na4 ligand. The Na5B7 cluster offers a new example for dynamic structural fluxionality in molecular systems.

Overcoming C60-induced interfacial recombination in inverted perovskite solar cells by electron-transporting carborane

Inverted perovskite solar cells still suffer from significant non-radiative recombination losses at the perovskite surface and across the perovskite/C60 interface, limiting the future development of perovskite-based single- and multi-junction photovoltaics. Therefore, more effective inter- or transport layers are urgently required. To tackle these recombination losses, we introduce ortho-carborane as an interlayer material that has a spherical molecular structure and a three-dimensional aromaticity. Based on a variety of experimental techniques, we show that ortho-carborane decorated with phenylamino groups effectively passivates the perovskite surface and essentially eliminates the non-radiative recombination loss across the perovskite/C60 interface with high thermal stability. We further demonstrate the potential of carborane as an electron transport material, facilitating electron extraction while blocking holes from the interface. The resulting inverted perovskite solar cells deliver a power conversion efficiency of over 23% with a low non-radiative voltage loss of 110 mV, and retain >97% of the initial efficiency after 400 h of maximum power point tracking. Overall, the designed carborane based interlayer simultaneously enables passivation, electron-transport and hole-blocking and paves the way toward more efficient and stable perovskite solar cells.

Electrochemical Cage Activation of Carboranes.

Carboranes are boron–carbon molecular clusters that possess unique properties, such as their icosahedron geometry, high boron content, and delocalized three‐dimensional aromaticity. These features render carboranes valuable building blocks for applications in supramolecular design, nanomaterials, optoelectronics, organometallic coordination chemistry, and as boron neutron capture therapy (BNCT) agents. Despite tremendous progress in this field, stoichiometric chemical redox reagents are largely required for the oxidative activation of carborane cages. In this context, electrosyntheses represent an alternative strategy for more sustainable molecular syntheses. It is only in recent few years that considerable progress has been made in electrochemical cage functionalization of carboranes, which are summarized in this Minireview. We anticipate that electrocatalysis will serve as an increasingly powerful stimulus within the current renaissance of carborane electrochemistry.

All are aromatic: A 3D globally aromatic cage containing five types of 2D aromatic macrocycles

All are aromatic: A 3D globally aromatic cage containing five types of 2D aromatic macrocycles

A Strategy for Selective Catalytic B-H Functionalization of o-Carboranes.

Carboranes are a class of polyhedral carbon-boron molecular clusters featuring three-dimensional aromaticity, which are often considered as 3D analogues of benzene. Their unique structural and electronic properties make them invaluable building blocks for applications ranging from functional materials to versatile ligands to pharmaceuticals. Thus, selective functionalization of carboranes has received tremendous research interest. In earlier days, the vast majority of the works in this area were focused on cage carbon functionalization via facile deprotonation of cage CH, followed by reaction with electrophiles. On the contrary, cage B-H activation is very challenging since the 10 B-H bonds on o-carborane are very similar, and how to achieve the desired transformation at specific boron vertex is a long-standing issue.As carbon is considered more electronegative than boron, this property results in different vertex charges on the o-carborane cage, which follow the order B(3,6)-H ≪ B(4,5,7,11)-H < B(8,10)-H < B(9,12)-H. We thought that this difference may trigger the favorite interaction of a proper transition metal complex with a specific B-H bond of carborane, which could be utilized to solve the selectivity issue. Accordingly, our strategy is described as follows: (1) electron-rich transition metal catalysts are good for the activation of the most electron-deficient B(3,6)-H bonds (connected to both cage C-H vertices); (2) electron-deficient transition metal catalysts are good for the activation of the relatively electron-rich B(8,9,10,12)-H bonds (with no bonding to either cage C-H vertices); and (3) directing-group-assisted transition metal catalysis is appropriate for the activation of the B(4,5,7,11)-H bonds (connected to only one cage C-H vertex), whose vertex charges lie in the middle of the range for the 10 B-H bonds. This strategy has been successfully applied by our laboratory and other groups in the development of a series of synthetic routes for catalytic selective activation of B-H bonds of the carborane cage, resulting in the synthesis of a large number of cage-boron-functionalized carborane derivatives in a regioselective and catalytic fashion. Subsequently, significant progress in this emerging area has been made.In 2013 we reported the selective tetrafluorination of o-carboranes at the B(8,9,10,12)-H bonds using an electron-deficient Pd(II) salt, [Pd(MeCN)4][BF4], as the catalyst. In 2014 we disclosed the first example of carboxy-directed alkenylation of o-carboranes at the B(4) vertex promoted by an Ir(III) catalyst. Subsequently, in 2017 we presented an electron-rich Ir(I)-catalyzed diborylation of o-carboranes at the B(3,6)-H bonds. We also uncovered the first example of Pd-catalyzed asymmetric synthesis of chiral-at-cage o-carboranes in 2018. These proof-of-principle studies have greatly stimulated research activities in selective B-H activation of carboranes and boron clusters enabled by transition metal catalysts. We have so far developed a toolbox of synthetic methods for selective catalytic cage B-olefination, -arylation, -alkenylation, -alkynylation, -oxygenation, -sulfenylation, -borylation, -halogenation, and -amination. We have recently expanded our research to base metal catalysis. As the field progresses, we expect that other methods for regioselective cage B-H activation will be invented, and the results detailed in this Account will promote these efforts.

Determinant Factors of Three-Dimensional Aromaticity in Antiaromatic Cyclophanes.

Three-dimensional aromaticity arising from the close stacking of two antiaromatic π-conjugated macrocycles has recently received considerable attention. Here, a cyclophane consisting of two antiaromatic Ni(II) norcorrole units tethered with two flexible alkyl chains was synthesized. The norcorrole cyclophane showed crystal polymorphism providing three different solid-state structures. Surprisingly, one of them adopted an aligned face-to-face stacking arrangement with negligible displacement along the slipping axis. Although the exchange repulsion between two π-clouds should be maximized in this orientation, the π-π distance is remarkably close (3.258 Å). Three-dimensional aromaticity in this conformation has been supported experimentally and theoretically as evidenced by small bond length alternations as well as the presence of a diatropic ring current. An analogous cyclophane with two aromatic Ni(II) porphyrin units was prepared for comparison. The porphyrin cyclophane exhibited a slipped-stacking conformation with a larger displacement (2.9 Å) and a larger interplanar distance (3.402 Å) without noticeable change of the aromaticity of each porphyrin unit. In solution, the norcorrole cyclophane forms a twist stacking arrangement with effective interplanar orbital overlap and exists in an equilibrium between stacked and nonstacked structures. Thermodynamic parameters of the stacking process were estimated, revealing an inherently large attractive interaction operating between two norcorrole units, which has been further supported by energy decomposition analysis.

Recent Advances in Transition Metal-Catalyzed Selective B-H Functionalization of o-Carboranes

Abstract Carboranes are a class of carbon-boron molecular clusters, possessing extraordinary characteristics including three-dimensional aromaticity conjugated by σ-bonds, icosahedral geometry and inherent robustness. They are finding growing applications as valuable building blocks in boron neutron capture therapy agents, pharmacophores, nanomaterials, optoelectronic, organometallic/coordination chemistry and more. Therefore, the effective and controlled functionalization of carboranes has attracted enormous research interests, particularly in regio- and enantio-selective cage BH derivatization among ten chemically similar BH vertices of o-carboranes. Only in the recent few years, significant progress has been made in transition metal catalyzed vertex-specific BH functionalization. This review summarizes recent advances in this research realm.

Synthesis and X-ray characterization of 15- and 16-vertex closo-carboranes

Carboranes are a class of carbon-boron molecular clusters with three-dimensional aromaticity, and inherent robustness. These endowments enable carboranes as valuable building blocks for applications ranging from functional materials to pharmaceuticals. Thus, the chemistry of carboranes has received tremendous research interest, and significant progress has been made in the past decades. However, many attempts to the synthesis of carboranes with more than 14 vertices had been unsuccessful since the report of a 14-vertex carborane in 2005. The question arises as to whether these long sought-after molecules exist. We describe in this article the synthesis and structural characterization of 15- and 16-vertex closo-carboranes as well as 16-vertex ruthenacarborane. Such a success relies on the introduction of silyl groups to both cage carbons, stabilizing the corresponding nido-carborane dianions and promoting the capitation reaction with HBBr2·SMe2. This work would shed some light on the preparation of carboranes with 17 vertices or more, and open the door for studying supercarborane chemistry.

De Novo Carborane-Containing Macrocyclic Peptides Targeting Human Epidermal Growth Factor Receptor.

l-Carboranylalanine (LCba) is a unique artificial amino acid containing a cluster of 10 boron atoms. Since the three-dimensional aromaticity and charge distributions of the carborane side chain are quite different from any side chains of proteinogenic amino acids, there is no report whether LCba can be a substrate for the translation machinery. Here, we report studies on the ribosomal incorporation of LCba into peptide via initiation and elongation using the flexizyme-assisted translation system. Our results indicate that only the initiation step could tolerate LCba incorporation, but the elongation steps could not, very likely due to its steric bulkiness of the side chain. Based on this knowledge, we have designed a library of macrocyclic peptides initiated by α-N-(2-choloroacetyl)-l-carboranylalanine (ClAc-LCba) and selected molecules capable of binding to human epidermal growth factor receptor (hEGFR). Two peptides that were forwarded to deeper studies exhibited affinities with KD values at 16 and 20 nM against hEGFR. Computational modeling of one of the peptides suggested that the carborane side chain might be directly involved in the interaction with the hydrophobic β-sheet core in the EGF binding site of hEGFR, which is consistent with the mutational data where replacing LCba residue with LPhe completely eliminated the binding activity. Cell lines that stably express hEGFR could be stained by incubation with the C-terminal fluorescein-labeled peptides, whereas hEGFR-negative cells could not be stained. This study provides a general strategy for the de novo discovery of carborane-containing macrocyclic peptides targeting various tumor biomarker proteins, potentially applicable to boron neutron capture therapy.

Three-dimensional aromaticity in an antiaromatic cyclophane

Understanding of interactions among molecules is essential to elucidate the binding of pharmaceuticals on receptors, the mechanism of protein folding and self-assembling of organic molecules. While interactions between two aromatic molecules have been examined extensively, little is known about the interactions between two antiaromatic molecules. Theoretical investigations have predicted that antiaromatic molecules should be stabilized when they stack with each other by attractive intermolecular interactions. Here, we report the synthesis of a cyclophane, in which two antiaromatic porphyrin moieties adopt a stacked face-to-face geometry with a distance shorter than the sum of the van der Waals radii of the atoms involved. The aromaticity in this cyclophane has been examined experimentally and theoretically. This cyclophane exhibits three-dimensional spatial current channels between the two subunits, which corroborates the existence of attractive interactions between two antiaromatic π-systems.

Global Aromaticity in Macrocyclic Polyradicaloids: Hückel's Rule or Baird's Rule?

Aromaticity is one of the most important concepts in organic chemistry to understand the electronic properties of cyclic π-conjugated molecules. Over a century, different aromaticity rules have been developed and validated. For planar monocyclic conjugated polyenes (also known as [n]annulenes), they will be aromatic if they contain [4N + 2] π electrons according to Hückel's rule, or antiaromatic if they have [4N] π electrons. Topological change from a planar to a half-twisted Möbius strip will lead to [4N] ([4N + 2]) aromaticity (antiaromaticity), which is just inverse to Hückel's rule. When the molecules are excited into the first triplet excited state, the Hückel (anti)aromaticity observed in the ground state will become reversed according to Baird's rule. Strictly speaking, these basic rules are only applicable for monocyclic conjugated systems, but some polycyclic systems such as porphyrinoids may also follow these rules if there is a dominant [n]annulene-like conjugation pathway. On the other hand, all-benzenoid polycyclic aromatic hydrocarbons usually display local aromaticity with π electrons predominantly localized at certain benzene rings according to Clar's aromatic sextet rule. In recent years, some proaromatic and antiaromatic molecules with even number of paired electrons have been found to exhibit open-shell diradical character and unique optical, electronic, and magnetic activities. One of the major driving forces is their intrinsic tendency to become aromatic in the open-shell diradical/polyradical forms. A number of stable diradicaloids and linear polyradicaloids have been successfully synthesized by using thermodynamic and kinetic stabilizing strategies. Herein, our particular interest is a type of macrocyclic polyradicaloid in which multiple frontier π-electrons are antiferromagnetically coupled with each other in a cyclic mode. Formally, these free electrons may behave like normal π-electrons in the [n]annulenes, and thus, it raises questions about their possible global aromaticity and which rule they will follow. In the past 5 years, our group has synthesized a series of macrocyclic polyradicaloids and systematically investigated their global aromaticity and electronic properties. Some important findings include: (1) global (anti)aromaticity is generally observed, but there is a balance between local aromaticity and global aromaticity; (2) most of these molecules follow Hückel's rule in the singlet state and display respective (anti)aromatic characteristics; (3) in some special cases, both Hückel's rule and Baird's rule can be applicable, and a unique annulene-within-an-annulene super-ring structure was demonstrated for the first time; (4) global antiaromaticity in the transition state is also important and a slow valence tautomerization process was observed in a supercyclobutadiene tetraradicaloid. These studies demonstrate how these open-shell macrocyclic polyradicaloids adapt their geometry and spin state to reach the lowest-energy state (aromatic). In this Account, we will mainly discuss their synthesis, global aromaticity, and the fundamental structure-radical character-aromaticity-properties relationships. Various experimental methods (e.g., NMR, X-ray crystallographic analysis, and electronic absorption spectroscopy) and theoretical calculations (e.g., anisotropy of the induced current density, nucleus independent chemical shift, and isochemical shielding surface) have been used to elaborate their (anti)aromatic character. At the end, a perspective on the possible three-dimensional global aromaticity in fully conjugated cagelike diradicaloids or polyradicaloids will be also discussed.

Structure, aromaticity and reactivity of corannulene and its analogues: a conceptual density functional theory and density functional reactivity theory study

ABSTRACTCorannulene is an interesting yet special molecule, which has witnessed widespread applications. It is aromatic, but not planar and the total number of 20 π electrons is in conflict with Hückel's 4n + 2 rule. In this work, we design a series of analogous model systems based on this molecule with the central ring size extended from five members to three to eight members. A number of theoretical and analytical tools available in the literature are employed to systematically examine their structure, aromaticity and reactivity properties. We found that structurally speaking, they change from bowl-like to planar and then to saddle shapes as the central ring size increases from three to eight. From the reactivity perspective, species with five and six-membered-rings in the centre are chemically more stable and less reactive, which are confirmed by the numerical results from aromaticity indexes and quantities from the information-theoretic approach. Overall, our results show that only corannulene and its six-membered-ring, coronene, analogue are aromatic. Even though these two systems are aromatic in nature, they are markedly different in a number of ways in structure, reactivity and other properties. These results should provide with us insights and understanding about the phenomenon of three-dimensional and non-planarity aromaticity.

Boron Chemistry – A Rapidly Expanding Research Field

Guest Editors Vladimir Bregadze and Zuowei Xie give a short introduction to the field and history of boron chemistry. This thematic issue is devoted to boron chemistry. This field of chemistry quickly started to develop when scientists tried to investigate boron hydrides in the middle of the 20th century after Alfred Stock's discovery. At that time, the leading world countries started to invest a lot of money in research into this field, in view of the good prospects for the application of boron hydrides as rocket fuel. However the toxicity and flammability of these compounds prevented the investigations in this field. So, attempts were directed to synthesize less explosive and less toxic boron hydride derivatives. It was found that polyhedral boron hydrides are less toxic and not flammable. The first polyhedral boron hydrides were described in 1959 and 1960 by Hawthorne and Pitochelli1 with the examples of decahydro-closo-decaborate [B10H10]2– and dodecahydro-closo-dodecaborate [B12H12]2–. In these clusters, the whole polyhedron consist of boron atoms. At the end of 1963 the first communications describing the synthesis of icosahedral carboranes, a new unusual class of polyhedral compounds whose cage consisted of ten boron atoms and two carbon atoms,2, 3 were simultaneously published in USSR and USA. Although papers on derivatives known as lower and medium carboranes had already been published,4 the publications on higher icosahedral carboranes made a great impression on the scientific community of that time. It became clear that a new, important page in the development of chemistry was thus opened. The subsequent vigorous development of this field demonstrated that the discovery of carboranes is as significant as the discovery of metallocenes and fullerenes for the development of modern chemistry or even science as a whole. The boron hydrides and polyhedral boron compounds proved to be interesting from both practical and theoretical points of view. The studies of their structure and reactivity formed the basis for important chemical concepts such as three-center two-electron bonds, multicenter bonds, and three-dimensional aromaticity. The unusual structures and properties of polyhedral borates and carboranes, and metallaboranes and metallacarboranes derived from them, caught the attention of scientists to this field and led to the preparation of a great number of new compounds. A comprehensive monograph on carboranes was recently presented by Grimes.5 In addition to boranes and carboranes, other types of boron compounds also received an enormous amount of attention. As shown in this thematic issue, a total of 41 articles highlight the different aspects of current boron chemistry and encompass a broad spectrum of compounds bearing one to ten boron atoms. This field continues to remain important and popular as can be seen by the contributions from all over the world. We hope that readers will find this timely cluster issue interesting and stimulating. We wish to thank all contributors, reviewers, and the EurJIC editorial team for their high-quality and professional work. Vladimir Bregadze graduated from the Chemistry Department of M.V. Lomonosov Moscow State University in 1960. He received his Ph.D. and D.Sc. degree, Professor of Chemistry in USSR Academy of Sciences. He is now Head of Laboratory of Organoaluminum and Boron Compounds of the A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences. In 1976 V.I.Bregadze received the USSR State Prize for the Development and Application of New Materials in Industry, and in 1996 he received the Russian Federation State Prize in Science and Technology for the “Chemistry of Carboranes and Polyhedral Boranes”. His fields of interest are organic and inorganic derivatives of boron and main group metals, the chemistry of polyhedral boranes and carboranes, the study of their reactivity and application in medicine (antitumour activity, boron neutron capture therapy) and for the design of materials. He is Adjunct Professor of the Beijing University of Chemical Technology. He has published over 400 research papers, reviews and chapters in books. Zuowei Xie obtained a B.Sc. from Hangzhou University in 1983, a M.Sc. from Shanghai Institute of Organic Chemistry (SIOC), the Chinese Academy of Sciences in 1986, and his Ph.D. in 1990, working in a joint program between SIOC and Technische Universität Berlin. After serving as a Research Associate in SIOC and a Postdoctoral Fellow at University of Southern California, he joined the chemistry faculty of The Chinese University of Hong Kong in 1995, where he is now a Choh-Ming Li Professor of Chemistry. He has co-authored over 270 papers and received several prestigious awards including the State Natural Science Prize in 2008 and the Chinese Chemical Society Yao-Zeng Huang Award in Organometallic Chemistry in 2010.

(Invited) Synthesis, Structures, and Properties of Stacked Antiaromatic Porphyrins

The stacking of antiaromatic compounds has not been well investigated. Theoretical calculations have proposed that staking of two antiaromatic systems lead to three-dimensional aromaticity, which results from mutual interactions between frontier orbitals of each π-system. However, there has been no experimental support for this interesting prediction. Norcorrole is a ring-contracted porphyrin, which shows distinct antiaromatic character. We have discovered that diphenylnorcorrole Ni(II) adopts a triple-decker π–π stacked structure in the solid state. A significant reduction of antiaromaticity of the norcorrole skeleton was observed. Figure 1

Stacked antiaromatic porphyrins

Aromaticity is a key concept in organic chemistry. Even though this concept has already been theoretically extrapolated to three dimensions, it usually still remains restricted to planar molecules in organic chemistry textbooks. Stacking of antiaromatic π-systems has been proposed to induce three-dimensional aromaticity as a result of strong frontier orbital interactions. However, experimental evidence to support this prediction still remains elusive so far. Here we report that close stacking of antiaromatic porphyrins diminishes their inherent antiaromaticity in the solid state as well as in solution. The antiaromatic stacking furthermore allows a delocalization of the π-electrons, which enhances the two-photon absorption cross-section values of the antiaromatic porphyrins. This feature enables the dynamic switching of the non-linear optical properties by controlling the arrangement of antiaromatic π-systems on the basis of intermolecular orbital interactions.

The study of bonding in pyramidanes [(Me3Si)4C4]E (E = Ge, Sn, Pb) by optical (Raman, UV–vis) spectroscopy and quantum-chemical methods

The nature of the apex-base bonds in organometallics of a novel class – pyramidanes [(Me3Si)4C4]E (E = Ge, Sn, Pb) was shown to be covalent but with a high degree of polarity on the basis of the Raman data and the results of QTAIM analysis. NICS data suggest three-dimensional aromaticity of the C4E pyramid.

Reaction Development on π- and σ-Conjugated Bonds and Creation of Innovative Functions.

Monocarba-closo-dodecaborate (1; [closo-CB11H12](-), or C1-carborane anion) is a symmetrical, stable anionic cluster, which possesses low nucleophilicity/basicity and exhibits three-dimensional aromaticity. In contrast to the rich applications of C2-carborane molecules (C2B10H12), the chemistry of the C1-carborane anion as a platform for functional molecules has not been thoroughly studied thus far due to the lack of its efficient functionalization. In particular, no efficient general methods are available for the introduction of aryl and sp(2)/sp-carbon groups at the carbon vertex of the C1-carborane anion. The unique electronic structure and potential applications of the C1-carborane anion prompted us to investigate methods to functionalize it. We developed a general, efficient C-C cross-coupling reaction of 1 under palladium catalysis which yields a variety of 1-C-functionalized C1-carborane derivatives. The use of copper(I) or lithium species as a transmetalating partner facilitated the cross-coupling process of the sterically hindered C1-carborane anion. The potential application of 1-C-arylated C1-carborane anion derivatives thus obtained were explored, some of which showed potential as pharmacophores and ionic liquid crystal behavior. Furthermore, conjugation between σ- and π-aromatic moieties in 1-C-arylated monocarba-closo-dodecaborate anion derivatives was identified by means of kinetic experimental studies combined with theoretical calculations.

Hückel's Rule of Aromaticity Categorizes Aromatic closo Boron Hydride Clusters.

A direct connection is established between three-dimensional aromatic closo boron hydride clusters and planar aromatic [n]annulenes for medium and large boron clusters. In particular, the results prove the existence of a link between the two-dimensional Hückel rule, as followed by aromatic [n]annulenes, and Wade-Mingos' rule of three-dimensional aromaticity, as applied to the aromatic [Bn Hn ](2-) closo boron hydride clusters. The closo boron hydride clusters can be categorized into different series, according to the n value of the Hückel (4 n+2) π rule. The distinct categories studied in this work correspond to n=1, 2, and 3. Each category increases in geometrical difficulty but, more importantly, it is possible to associate each category with the number of pentagonal layers in the structure perpendicular to the main axis. Category 1 has one pentagonal layer, category 2 has two, and category 3 has three.

Bismuth Undecahydro-closo-dodecaborane: A Retainable Intermediate of B-H Bond Activation by Bismuth(III) Cations.

The [B12 H12](2-) anion shows an extensive substitutional chemistry based on its three-dimensional aromaticity. The replacement of functional groups can be attained by electrophilically induced substitution caused by Brønsted or Lewis acidic electrophiles (e.g. Pt(2+)). Until now, it was impossible to structurally characterize a metal-substituted [B12 H12](2-) cage. When an aqueous solution containing both Bi(3+) cations and [B12 H12](2-) anions was heated, the charge-neutral bismuth undecahydro-closo-dodecaborane BiB12 H11 was obtained, representing a new class of metalated [B12 H12](2-) clusters. The title compound was characterized by single-crystal X-ray diffraction and NMR spectroscopic methods. Compared to the typical B-H bond, the short B-Bi single bond (230 pm) exhibits inverted polarity.