Boron is an interesting element with unusual polymorphism. While three-dimensional (3D) structural motifs are prevalent in bulk boron, atomic boron clusters are found to have planar or quasi-planar ...structures, stabilized by localized two-center–two-electron (2c–2e) σ bonds on the periphery and delocalized multicenter–two-electron (nc–2e) bonds in both σ and π frameworks. Electron delocalization is a result of boron’s electron deficiency and leads to fluxional behavior, which has been observed in B13 + and B19 –. A unique capability of the in-plane rotation of the inner atoms against the periphery of the cluster in a chosen direction by employing circularly polarized infrared radiation has been suggested. Such fluxional behaviors in boron clusters are interesting and have been proposed as molecular Wankel motors. The concepts of aromaticity and antiaromaticity have been extended beyond organic chemistry to planar boron clusters. The validity of these concepts in understanding the electronic structures of boron clusters is evident in the striking similarities of the π-systems of planar boron clusters to those of polycyclic aromatic hydrocarbons, such as benzene, naphthalene, coronene, anthracene, or phenanthrene. Chemical bonding models developed for boron clusters not only allowed the rationalization of the stability of boron clusters but also lead to the design of novel metal-centered boron wheels with a record-setting planar coordination number of 10. The unprecedented highly coordinated borometallic molecular wheels provide insights into the interactions between transition metals and boron and expand the frontier of boron chemistry. Another interesting feature discovered through cluster studies is boron transmutation. Even though it is well-known that B–, formed by adding one electron to boron, is isoelectronic to carbon, cluster studies have considerably expanded the possibilities of new structures and new materials using the B–/C analogy. It is believed that the electronic transmutation concept will be effective and valuable in aiding the design of new boride materials with predictable properties. The study of boron clusters with intermediate properties between those of individual atoms and bulk solids has given rise to a unique opportunity to broaden the frontier of boron chemistry. Understanding boron clusters has spurred experimentalists and theoreticians to find new boron-based nanomaterials, such as boron fullerenes, nanotubes, two-dimensional boron, and new compounds containing boron clusters as building blocks. Here, a brief and timely overview is presented addressing the recent progress made on boron clusters and the approaches used in the authors’ laboratories to determine the structure, stability, and chemical bonding of size-selected boron clusters by joint photoelectron spectroscopy and theoretical studies. Specifically, key findings on all-boron hydrocarbon analogues, metal-centered boron wheels, and electronic transmutation in boron clusters are summarized.
Synthetic strategies to yield molecular complexes of high-valent lanthanides, other than the ubiquitous Ce4+ ion, are exceptionally rare, and thorough, detailed characterization in these systems is ...limited by complex lifetime and reaction and isolation conditions. The synthesis of high-symmetry complexes in high purity with significant lifetimes in solution and the solid state is essential for determining the role of ligand-field splitting, multiconfigurational behavior, and covalency in governing the reactivity and physical properties of these potentially technologically transformative tetravalent ions. We report the synthesis and physical characterization of an S 4 symmetric, four-coordinate tetravalent terbium complex, Tb(NP(1,2-bis- t Bu-diamidoethane)(NEt2))4 (where Et is ethyl and t Bu is tert-butyl). The ligand field in this complex is weak and the metal–ligand bonds sufficiently covalent so that the tetravalent terbium ion is stable and accessible via a mild oxidant from the anionic, trivalent, terbium precursor, (Et2O)KTb(NP(1,2-bis- t Bu-diamidoethane)(NEt2))4. The significant stability of the tetravalent complex enables its thorough characterization. The stepwise development of the supporting ligand points to key ligand control elements for further extending the known tetravalent lanthanide ions in molecular complexes. Magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, X-ray absorption near-edge spectroscopy (XANES), and density functional theory studies indicate a 4f7 ground state for Tb(NP(1,2-bis- t Bu-diamidoethane)(NEt2))4 with considerable zero-field splitting, demonstrating that magnetic, tetravalent lanthanide ions engage in covalent metal–ligand bonds. This result has significant implications for the use of tetravalent lanthanide ions in magnetic applications since the observed zero-field splitting is intermediate between that observed for the trivalent lanthanides and for the transition metals. The similarity of the multiconfigurational behavior in the ground state of Tb(NP(1,2-bis- t Bu-diamidoethane)(NEt2))4 (measured by Tb L3-edge XAS) to that observed in TbO2 implicates ligand control of multiconfigurational behavior as a key component of the stability of the complex.
Abstract
In all known examples of metal–ligand (M–L)
δ
and
φ
bonds, the metal orbitals are aligned to the ligand orbitals in a “head-to-head” or “side-to-head” fashion. Here, we report two ...fundamentally new types of M–L
δ
and
φ
interactions; “head-to-side”
δ
and “side-to-side”
φ
back-bonding, found in complexes of metallacyclopropenes and metallacyclocumulenes of actinides (Pa–Pu) that makes them distinct from their corresponding Group 4 analogues. In addition to the known Th and U complexes, our calculations include complexes of Pa, Np, and Pu. In contrast with conventional An–C bond decreasing, due to the actinide contraction, the An–C distance increases from Pa to Pu. We demonstrate that the direct L–An
σ
and
π
donations combined with the An–L
δ
or
φ
back-donations are crucial in explaining this non-classical trend of the An–L bond lengths in both series, underscoring the significance of these
δ
/
φ
back-donation interactions, and their importance for complexes of Pa and U in particular.
The electron deficiency and strong bonding capacity of boron have led to a vast variety of molecular structures in chemistry and materials science. Here we report the observation of highly symmetric ...cobalt-centered boron drum-like structures of CoB16(-), characterized by photoelectron spectroscopy and ab initio calculations. The photoelectron spectra display a relatively simple spectral pattern, suggesting a high symmetry structure. Two nearly degenerate isomers with D8d (I) and C4v (II) symmetries are found computationally to compete for the global minimum. These drum-like structures consist of two B8 rings sandwiching a cobalt atom, which has the highest coordination number known heretofore in chemistry. We show that doping of boron clusters with a transition metal atom induces an earlier two-dimensional to three-dimensional structural transition. The CoB16(-) cluster is tested as a building block in a triple-decker sandwich, suggesting a promising route for its realization in the solid state.
Two-dimensional (2D) materials with planar hypercoordinate motifs are extremely rare due to the difficulty in stabilizing the planar hypercoordinate configurations in extended systems. Furthermore, ...such exotic motifs are often unstable. We predict a novel Cu2Si 2D monolayer featuring planar hexacoordinate copper and planar hexacoordinate silicon. This is a global minimum in 2D space which displays reduced dimensionality and rule-breaking chemical bonding. This system has been studied with density functional theory, including molecular dynamics simulations and electronic structure calculations. Bond order analysis and partitioning reveals 4c–2e σ bonds that stabilize the two-dimensional structure. We find that the system is quite stable during short annealing simulations up to 900 K, and predict that it is a nonmagnetic metal. This work opens up a new branch of hypercoordinate two-dimensional materials for study.
Originally, the concepts of aromaticity and antiaromaticity were introduced to explain the stability and reactivity of unsaturated organic compounds. Since then, they have been extended to other ...species with delocalized electrons including various saturated systems, organometallic compounds, and even inorganic clusters and molecules. In this Minireview, we focus on the most recent progress of using these concepts to guide experimental synthesis and rationalize geometrical and electronic structures of a particular family of polyanions composed of Group 14 and 15 elements, namely Zintl clusters.
Bonding in a cluster: The application of concepts of aromaticity and antiaromaticity towards Zintl clusters is discussed in this minireview.
Helium is generally understood to be chemically inert and this is due to its extremely stable closed-shell electronic configuration, zero electron affinity and an unsurpassed ionization potential. It ...is not known to form thermodynamically stable compounds, except a few inclusion compounds. Here, using the ab initio evolutionary algorithm USPEX and subsequent high-pressure synthesis in a diamond anvil cell, we report the discovery of a thermodynamically stable compound of helium and sodium, Na
He, which has a fluorite-type structure and is stable at pressures >113 GPa. We show that the presence of He atoms causes strong electron localization and makes this material insulating. This phase is an electride, with electron pairs localized in interstices, forming eight-centre two-electron bonds within empty Na
cubes. We also predict the existence of Na
HeO with a similar structure at pressures above 15 GPa.
Coronaviruses (CoVs) pose a huge threat to public health as emerging viruses. Bat-borne CoVs are especially unpredictable in their evolution due to some unique features of bat physiology boosting the ...rate of mutations in CoVs, which is already high by itself compared to other viruses. Among bats, a meta-analysis of overall CoVs epizootiology identified a nucleic acid observed prevalence of 9.8% (95% CI 8.7-10.9%). The main objectives of our study were to conduct a qPCR screening of CoVs' prevalence in the insectivorous bat population of Fore-Caucasus and perform their characterization based on the metagenomic NGS of samples with detected CoV RNA. According to the qPCR screening, CoV RNA was detected in 5 samples, resulting in a 3.33% (95% CI 1.1-7.6%) prevalence of CoVs in bats from these studied locations. BetaCoVs reads were identified in raw metagenomic NGS data, however, detailed characterization was not possible due to relatively low RNA concentration in samples. Our results correspond to other studies, although a lower prevalence in qPCR studies was observed compared to other regions and countries. Further studies should require deeper metagenomic NGS investigation, as a supplementary method, which will allow detailed CoV characterization.
Small boron clusters are known to be planar, and may be used as ligands to form novel coordination complexes with transition metals. Here we report a combined photoelectron spectroscopy and ab initio ...study of CoB12 – and RhB12 –. Photoelectron spectra of the two doped-B12 clusters show similar spectral patterns, suggesting they have similar structures. Global minimum searches reveal that both CoB12 – and RhB12 – possess half-sandwich-type structures with the quasi-planar B12 moiety coordinating to the metal atom. The B12 ligand is found to have similar structure as the bare B12 cluster with C 3v symmetry. Structures with Co or Rh inserted into the quasi-planar boron framework are found to be much higher in energy. Chemical bonding analyses of the two B12 half sandwiches reveal two sets of σ bonds on the boron unit: nine classical two-center–two-electron (2c–2e) σ bonds on the periphery of the B12 unit and four 3c–2e σ bonds within the boron unit. Both σ and π bonds are found between the metal and the B12 ligand: three M–B single σ bonds and one delocalized 4c–2e π bond. The exposed metal sites in these complexes can be further coordinated by other ligands or become reaction centers as model catalysts.
In this Review we present examples of clusters, molecules, and solid‐state compounds, for which the use of σ‐aromaticity and σ‐antiaromaticity concepts is essential for understanding of chemical ...bonding. We show that the bonding patterns in these σ‐aromatic and σ‐antiaromatic compounds are similar to those of the corresponding π‐aromatic and π‐antiaromatic chemical systems, respectively. Undoubtedly, σ‐aromaticity helps us understand why the high symmetry isomers are the most stable among myriads of other potential structures. We also show that besides systems exhibiting either σ‐ or π‐aromatic features, there are species, which can possess multiple aromaticity/antiaromaticity, or conflicting aromaticity patterns. We believe that the σ‐aromaticity and σ‐antiaromaticity concepts will be helpful in rationalizing chemical bonding, structure, stability, and molecular properties of chemical species in both organic and inorganic chemistry. We hope that they will also be useful for other areas of science such as material science, catalysis, nanotechnology, and biochemistry.
Understanding chemical bonding: This Review focuses on the usefulness of the concepts of σ‐aromaticity and σ‐antiaromaticity in chemistry: from small clusters to bulky solid‐state compounds. As one of the numerous examples, the counter‐intuitive geometry of the global minimum structure of the Li2Mg2 cluster is explained using the σ‐aromaticity based on the particular delocalized bonding pattern and energy considerations (see picture).
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