Probing the Electronic Structure of the CoB16- Drum Complex: Unusual Oxidation State of Co-1

Since the discovery of the first drum-like CoB₁₆^- complex, metal-doped drum-like boron nanotubular structures have been investigated with various metal dopants and different tubular size, forming a new class of novel nanostructures. The CoB₁₆^- cluster was found to be composed of a central Co atom coordinated by two fused B₈ rings in a tubular structure, representing the potential embryo of metal-filled boron nanotubes and providing opportunities to design one-dimensional metal-boron nanostructures. Here we report improved photoelectron spectroscopy and a more in-depth electronic structure analysis of CoB₁₆^-, providing further insight into the chemical bonding and stability of the drum-like doped boron tubular structures. Most interestingly, we find that the central Co atom has an unusually low oxidation state of ?1 and neutral CoB₁₆ can be viewed as a charge transfer complex (Co^-@BB₁₆⁺), suggesting both covalent and electrostatic interactions between the dopant and the boron drum.     

LiBC — ein vollständig interkalierter Heterographit

LiBC — A Completely Intercalated Heterographite LiBC is a new compound composed only from light main group elements. LiBC is synthesized from the elements in sealed niobium ampoules at 770 K, and short annealing at 1 770 K, forming hexagonal platelets with golden lustre. According to Li⁺(BC)^?, boron and carbon form planar hetero graphite layers of the isoelectronic hexagonal boron nitride type. The inter-layer regions are completely filled by lithium (P6₃/mmc; a = 275.2 pm; c = 705.8 pm; hP6; ZrBeSi type). The deformation density of the valence electrons prove the π character of the B? C bonds, as well as a polarization according to (BC^?). Chemical and physical properties indicate a certain range of homogeneity x(Li) ≤ 1. The thermal decomposition and chemical reactions lead to BC products not yet characterized. The oxidation of LiBC obviously runs by a mechanism similar to that of graphite.     

Structures,magnetic properties,and electronic counting rule of metals‐encapsulated cage‐like M2 Si18 (M = Ti‐Zn) clusters

The geometries, magnetic properties and stabilities of the transition metal (TM) atoms encapsulated M₂Si₁₈ (M = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) clusters have been systematically calculated by using the density function theory with generalized gradient approximation. Only when the doping metal atom has no more than half‐full d electronic shell, a double hexagonal prism cage‐like M₂Si₁₈ structure could form. The total moments of M₂Si₁₈ are either 0 or 2μ_B. Co₂Si₁₈ is the most stable cluster among all 3d doped M₂Si₁₈ clusters. The model of shell closure at the TM atom may be helpful to understand the stability of M₂Si₁₈ clusters. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Boron rings containing planar octacoordinate iron and cobalt

The boron rings containing planar octacoordinate transition metals, D _8h FeB₈ ²⁻, CoB₈ ⁻ and CoB₈ ³⁺, C _2v FeB₈, D _2h CoB₈ ⁺ and CoB₈, are optimized with all real vibrational frequencies at the B3LYP/6–311+G* level of the theory. The D _8h FeB₈ ²⁻ and CoB₈ ⁻ isomers are global minima, while D _8h CoB₈ ³⁺ is only local minimum. The electronic structure character of these systems is revealed by natural bond orbital (NBO) analysis, showing that the boron rings containing planar octacoordinate transition metals have stability and aromaticity with six π electrons. The aromaticity is confirmed by nucleus independent chemical shifts (NICS) calculations. Supported by the specialized research fund for the doctoral program of higher education (20060007030)     

Cross‐Linked Polyphosphazene Hollow Nanosphere‐Derived N/P‐Doped Porous Carbon with Single Nonprecious Metal Atoms for the Oxygen Reduction Reaction

Heteroatom‐doped polymers or carbon nanospheres have attracted broad research interest. However, rational synthesis of these nanospheres with controllable properties is still a great challenge. Herein, we develop a template‐free approach to construct cross‐linked polyphosphazene nanospheres with tunable hollow structures. As comonomers, hexachlorocyclotriphosphazene provides N and P atoms, tannic acid can coordinate with metal ions, and the replaceable third comonomer can endow the materials with various properties. After carbonization, N/P‐doped mesoporous carbon nanospheres were obtained with small particle size (≈50 nm) and high surface area (411.60 m² g^?1). Structural characterization confirmed uniform dispersion of the single atom transition metal sites (i.e., Co‐N₂P₂) with N and P dual coordination. Electrochemical measurements and theoretical simulations revealed the oxygen reduction reaction performance. This work provides a solution for fabricating diverse heteroatom‐containing polymer nanospheres and their derived single metal atom doped carbon catalysts.     

Directly Diphenylborane‐Fused Porphyrins

Mono‐ and bis(diphenylborane)‐fused porphyrins were synthesized from the corresponding β‐(2‐trimethylsilylphenyl)‐substituted porphyrins through the sequence of Si–B exchange reaction, intramolecular bora‐Friedel–Crafts reaction, and ring‐closing Si–B exchange reaction. Effective electronic interactions of the empty p‐orbital of the boron atom with the porphyrin π‐circuit lead to red‐shifted absorption spectra and substantially decreased LUMO energy levels. Pyridine adds at the boron center to cause disruption of the electronic interaction of the boron atom with large association constants (1.9–17×10⁴ m ^?1) depending on the central metal at the porphyrin. The Zn^II complex behaved as a hetero‐dinuclear Lewis acid, exhibiting regioselective binding of pyridines at the boron or the zinc center.     

Trimeric cluster of lithium amidoborane—the smallest unit for the modeling of hydrogen release mechanism

A detailed first‐principle DFT M06/6‐311++G(d.p) study of dehydrogenation mechanism of trimeric cluster of lithium amidoborane is presented. The first step of the reaction is association of two LiNH₂BH₃ molecules in the cluster. The dominant feature of the subsequent reaction pathway is activation of H atom of BH₃ group by three Li atoms with formation of unique Li₃H moiety. This Li₃H moiety is destroyed prior to dehydrogenation in favor of formation of a triangular Li₂H moiety, which interacts with protic H atom of NH₂ group. As a result of this interaction, Li₂H₂ moiety is produced. It features N^?? H⁺? H^? group suited near the middle plane between two Li⁺ in the transition state that leads to H₂ release. The transition states of association and hydrogen release steps are similar in energy. It is concluded that the trimer, (LiNH₂BH₃)₃, is the smallest cluster that captures the essence of the hydrogen release reaction. © 2016 Wiley Periodicals, Inc.     

Sodium‐Doped Mesoporous Ni2P2O7 Hexagonal Tablets for High‐Performance Flexible All‐Solid‐State Hybrid Supercapacitors

A simple hydrothermal method has been developed to prepare hexagonal tablet precursors, which are then transformed into porous sodium‐doped Ni₂P₂O₇ hexagonal tablets by a simple calcination method. The obtained samples were evaluated as electrode materials for supercapacitors. Electrochemical measurements show that the electrode based on the porous sodium‐doped Ni₂P₂O₇ hexagonal tablets exhibits a specific capacitance of 557.7 F g^?1 at a current density of 1.2 A g^?1. Furthermore, the porous sodium‐doped Ni₂P₂O₇ hexagonal tablets were successfully used to construct flexible solid‐state hybrid supercapacitors. The device is highly flexible and achieves a maximum energy density of 23.4 Wh kg^?1 and a good cycling stability after 5000 cycles, which confirms that the porous sodium‐doped Ni₂P₂O₇ hexagonal tablets are promising active materials for flexible supercapacitors.     

Alkali Metal Covalent Bonding in Nickel Carbonyl Complexes ENi(CO)3−

The alkali metal‐nickel carbonyl anions ENi(CO)₃^? with E=Li, Na, K, Rb, Cs have been produced and characterized by mass‐selected infrared photodissociation spectroscopy in the gas phase. The molecules are the first examples of 18‐electron transition metal complexes with alkali atoms as covalently bonded ligands. The calculated equilibrium structures possess C_3v geometry, where the alkali atom is located above a nearly planar Ni(CO)₃^? fragment. The analysis of the electronic structure reveals a peculiar bonding situation where the alkali atom is covalently bonded not only to Ni but also to the carbon atoms.     

Reactivity of [Ge9{Si(SiMe3)3}3]− Towards Transition‐Metal M2+ Cations: Coordination and Redox Chemistry

Recently the metalloid cluster compound [Ge₉Hyp₃]^? ( 1 ; Hyp=Si(SiMe₃)₃) was oxidatively coupled by an iron(II) salt to give the largest metalloid Group 14 cluster [Ge₁₈Hyp₆]. Such redox chemistry is also possible with different transition metal (TM) salts TM²⁺ (TM=Fe, Co, Ni) to give the TM⁺ complexes [Fe(dppe)₂][Ge₉Hyp₃] ( 3 ; dppe=1,2‐bis(diphenylphosphino)ethane), [Co(dppe)₂][Ge₉Hyp₃] ( 4 ), [Ni(dppe)(Ge₉Hyp₃)] ( 5 ) and [Ni(dppe)₂(Ge₉Hyp₃)]⁺ ( 6 ). Such a redox reaction does not proceed for Mn, for which a salt metathesis gives the first open shell [Hyp₃Ge₉‐M‐Ge₉Hyp₃] cluster ( 2 ; M=Mn). The bonding of the transition metal atom to 1 is also possible for Ni (e.g., compound 6 ), in which one or even two nickel atoms can bind to 1 . In contrast to this in case of the Fe and Co compounds 3 and 4 , respectively, the transition‐metal atom is not bound to the Ge₉ core of 1 . The synthesis and the experimentally determined structures of 2 – 6 are presented. Additionally the bonding within 2 – 6 is analyzed and discussed with the aid of EPR measurements and quantum chemical calculations.     

Pushing the Lewis Acidity Boundaries of Boron Compounds With Non‐Planar Triarylboranes Derived from Triptycenes

Bending the planar trigonal boron center of triphenylborane by connecting its aryl rings with carbon or phosphorus linkers gave access to a series of 9‐boratriptycene derivatives with unprecedented structures and reactivities. NMR spectroscopy and X‐ray diffraction of the Lewis adducts of these non‐planar boron Lewis acids with weak Lewis base revealed particularly strong covalent bond formation. The first Lewis adduct of a trivalent boron compounds with the Tf₂N^? anion illustrates the unrivaled Lewis acidity of these species. Increasing the pyramidalization of the boron center and using a cationic phosphonium linker resulted in an exceptional enhancement of Lewis acidity. Introduction of a phosphorus and a boron atom at each edge of a triptycene framework, allowed access to new bifunctional Lewis acid‐base 9‐phospha‐10‐boratriptycenes featuring promising reactivity for the activation of carbon‐halogen bonds.     

Extensive Structural Rearrangements upon Reduction of 9H‐9‐Borafluorene

Common wisdom has it that organoboranes are readily oxidized. Described herein is that also their reduction can result in remarkable chemistry. Treatment of dimeric 9H‐9‐borafluorene with Li metal in toluene yields two strikingly different classes of compounds. One part of the sample reacts in a way similar to B₂H₆, thus affording an aryl(hydro)borane cluster reminiscent of the [B₃H₈]^? anion. The other part furnishes a dianionic boron‐doped graphene flake devoid of hydrogen substituents at the boron centers and featuring a central B?B bond. A change in the solvent to THF allows an isolation of this dibenzo[g,p]chrysene analogue in good yields.     

Neue molekulare Indium‐Zinn‐Sauerstoff‐ und Indium‐Zinn‐Natrium‐Sauerstoff‐Chlor‐Cluster

Abstract. The five‐membered heteroelement cluster THF · Cl₂In(O^tBu)₃Sn reacts with the sodium stannate [Na(O^tBu)₃Sn]₂ to produce either the new oxo‐centered alkoxo cluster ClInO[Sn(O^tBu)₂]₃ ( 1 ) (in low yield) or the heteroleptic alkoxo cluster Sn(O^tBu)₃InCl₃Na[Sn(O^tBu)₂]₂ ( 2 ). X‐ray diffraction analyses reveal that in compound 1 the polycyclic entity is made of three tin atoms which together with a central oxygen atom form a trigonal, almost planar triangle, perpendicular to which a further indium atom is connected through the oxygen atom. The metal atoms thus are arranged in a Sn₃In pyramid, the edges of which are all saturated by bridging tert‐butoxy groups. The indium atom has a further chloride ligand. Compound 2 has two trigonal bipyramids as building blocks which are fused together at a six coordinate indium atom. One of the bipyramids is of the type SnO₃In with tert‐butyl groups on the oxygen atoms, while the other has the composition InCl₃Na with chlorine atoms connecting the two metals. The sodium atom in 2 has further contacts to two plus one alkoxide groups which are part of a[Sn(O^tBu)₂]₂ dimer disposing of a Sn₂O₂ central cycle. The hetero element cluster in 2 thus combines three closed entities and its skeleton SnO₃InCl₃NaO₂Sn₂O₂ consists of three different metallic and two different non‐metallic elements.     

Electrochemical Conversion of CO2 to Syngas with Controllable CO/H2 Ratios over Co and Ni Single‐Atom Catalysts

The electrochemical CO₂ reduction reaction (CO₂RR) to yield synthesis gas (syngas, CO and H₂) has been considered as a promising method to realize the net reduction in CO₂ emission. However, it is challenging to balance the CO₂RR activity and the CO/H₂ ratio. To address this issue, nitrogen‐doped carbon supported single‐atom catalysts are designed as electrocatalysts to produce syngas from CO₂RR. While Co and Ni single‐atom catalysts are selective in producing H₂ and CO, respectively, electrocatalysts containing both Co and Ni show a high syngas evolution (total current >74 mA cm^?2) with CO/H₂ ratios (0.23–2.26) that are suitable for typical downstream thermochemical reactions. Density functional theory calculations provide insights into the key intermediates on Co and Ni single‐atom configurations for the H₂ and CO evolution. The results present a useful case on how non‐precious transition metal species can maintain high CO₂RR activity with tunable CO/H₂ ratios.     

A spin‐frustrated cobalt(II) carbonate pyrochlore network

The crystal structure of the cobalt(II) carbonate‐based compound cobalt(II) dicarbonate trisodium chloride, Co(CO₃)₂Na₃Cl, grown from a water–ethanol mixture, exhibits a three‐dimensional network of corner‐sharing {Co₄(μ₃‐CO₃)₄} tetrahedral building blocks, in which the Co^II centres define a pyrochlore lattice and reside in a slightly distorted octahedral Co(O–CO₂)₆ environment. The space outside the hexagonal framework defined by these interlinked groups is occupied by Na⁺ and Cl⁻ ions. Antiferromagnetic coupling between adjacent Co^II centres, mediated by carbonate bridges, results in geometric spin frustration which is typical for pyrochlore networks. The Co and Cl atoms reside on the special position , one Na atom on position 2 and a carbonate C atom on position 3.     

Observation of Transition‐Metal–Boron Triple Bonds in IrB2O− and ReB2O−

Multiple bonds between boron and transition metals are known in many borylene (:BR) complexes via metal d_π→BR back‐donation, despite the electron deficiency of boron. An electron‐precise metal–boron triple bond was first observed in BiB₂O^? [Bi≡B?B≡O]^? in which both boron atoms can be viewed as sp‐hybridized and the [B?BO]^? fragment is isoelectronic to a carbyne (CR). To search for the first electron‐precise transition‐metal‐boron triple‐bond species, we have produced IrB₂O^? and ReB₂O^? and investigated them by photoelectron spectroscopy and quantum‐chemical calculations. The results allow to elucidate the structures and bonding in the two clusters. We find IrB₂O^? has a closed‐shell bent structure (C_s, ¹A′) with BO^? coordinated to an Ir≡B unit, (^?OB)Ir≡B, whereas ReB₂O^? is linear (C_∞v, ³Σ^?) with an electron‐precise Re≡B triple bond, [Re≡B?B≡O]^?. The results suggest the intriguing possibility of synthesizing compounds with electron‐precise M≡B triple bonds analogous to classical carbyne systems.     

Strong Exchange Coupling in a Trimetallic Radical‐Bridged Cobalt(II)‐Hexaazatrinaphthylene Complex

Reducing hexaazatrinaphthylene (HAN) with potassium in the presence of 18‐c‐6 produces [{K(18‐c‐6)}HAN], which contains the S=1/2 radical [HAN]^.?. The [HAN]^.? radical can be transferred to the cobalt(II) amide [Co{N(SiMe₃)₂}₂], forming [K(18‐c‐6)][(HAN){Co(N′′)₂}₃]; magnetic measurements on this compound reveal an S=4 spin system with strong cobalt–ligand antiferromagnetic exchange and J≈?290 cm^?1 (?2 J formalism). In contrast, the Co^II centres in the unreduced analogue [(HAN){Co(N′′)₂}₃] are weakly coupled (J≈?4.4 cm^?1). The finding that [HAN]^.? can be synthesized as a stable salt and transferred to cobalt introduces potential new routes to magnetic materials based on strongly coupled, triangular HAN building blocks.     

The Role of Holes in Borophenes: An Ab Initio Study of Their Structure and Stability with and without Metal Templates

An electron‐counting strategy starting from magnesium boride was used to show the inevitability of hexagonal holes in 2D borophene. The number (hole density, HD) and distribution of the hexagonal holes determine the binding energy per boron atom in monolayer borophenes. The relationship between binding energy and HD changes dramatically when the borophene is placed on a Ag(111) surface. The distribution of holes in borophenes on Ag(111) surfaces depends on the temperature. DFT calculations show that aside from the previously reported S1 and S2 borophene phases, other polymorphs may also be competitive. Plots of the electron density distribution of the boron sheets suggest that the observed STM image of an S2 phase corresponds to a sheet with a HD of 2/15 instead of a sheet with a HD of 1/5. The hole density and the hole distribution echo the distribution of vacancies and extra occupancies in complex β‐rhombohedral boron.     

A Boron Doped Diamond Electrode Modified with Nano‐carbon Black for the Sensitive Electrochemical Determination of Chlorogenic Acid

An electrochemical method was developed for the sensitive determination of chlorogenic acid using a boron doped diamond electrode (BDDE) modified with nano‐carbon black (nano‐CB). The active surface areas were found to be 0.059 and 0.146 cm² for the unmodified BDDE, and nano‐CB/BDDE, respectively. Compared with a BDDE, the nano‐CB/BDDE exhibited a well‐defined redox couple for chlorogenic acid. In addition, the plot of the peak current response changing from a square root to a linear dependence on scan rate is attributed to the transition from planar diffusion to surface behaviour. The anodic and cathodic peak separations (ΔE_p) were 97 mV and 14 mV at BDDE and nano‐CB/BDDE, respectively. The decrease in ΔE_p at the proposed electrode indicated that the process of chlorogenic acid was greatly accelerated. Square wave voltammetry (SWV) exhibited a dynamic range in which the current versus the concentration of chlorogenic acid were linear from 2.0×10^?8 to 2.0×10^?6 M with a LOD of 4.1×10^?9 M (based on 3S_b/m). The nano‐CB modified BDDE provided improved electrochemical behavior, high electrocatalytic activity, high sensitivity and good reproducibility.     

Templated Atom‐Precise Galvanic Synthesis and Structure Elucidation of a [Ag24Au(SR)18]− Nanocluster

Synthesis of atom‐precise alloy nanoclusters with uniform composition is challenging when the alloying atoms are similar in size (for example, Ag and Au). A galvanic exchange strategy has been devised to produce a compositionally uniform [Ag₂₄Au(SR)₁₈]^? cluster (SR: thiolate) using a pure [Ag₂₅(SR)₁₈]^? cluster as a template. Conversely, the direct synthesis of Ag₂₄Au cluster leads to a mixture of [Ag_25?xAu_x(SR)₁₈]^?, x=1–8. Mass spectrometry and crystallography of [Ag₂₄Au(SR)₁₈]^? reveal the presence of the Au heteroatom at the Ag₂₅ center, forming Ag₂₄Au. The successful exchange of the central Ag of Ag₂₅ with Au causes perturbations in the Ag₂₅ crystal structure, which are reflected in the absorption, luminescence, and ambient stability of the particle. These properties are compared with those of Ag₂₅ and Ag₂₄Pd clusters with same ligand and structural framework, providing new insights into the modulation of cluster properties with dopants at the single‐atom level.