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.     

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.     

Boron-Made N2: Realization of a B≡B Triple Bond in the B2Al3− Cluster

Until now, all B≡B triple bonds have been achieved by adopting two ligands in the L→B≡B←L manner. Herein, we report an alternative route of designing the B≡B bonds based on the assumption that by acquiring two extra electrons, an element with the atomic number Z can have properties similar to those of the element with the atomic number Z+2. Specifically, we show that due to the electron donation from Al to B, the negatively charged B≡B kernel in the B₂Al₃⁻ cluster mimics a triple N≡N bond. Comprehensive computational searches reveal that the global minimum structure of B₂Al₃⁻ exhibits a direct B–B distance of 1.553 Å, and its calculated electron vertical detachment energies are in excellent agreement with the corresponding values of the experimental photoelectron spectrum. Chemical bonding analysis revealed one σ and two π bonds between the two B atoms, thus confirming a classical textbook B≡B triple bond, similar to that of N₂.     

PrB7−: A Praseodymium‐Doped Boron Cluster with a PrII Center Coordinated by a Doubly Aromatic Planar η7‐B73− Ligand

The structure and bonding of a Pr‐doped boron cluster (PrB₇⁻) are investigated using photoelectron spectroscopy and quantum chemistry. The adiabatic electron detachment energy of PrB₇⁻ is found to be low [1.47(8) eV]. A large energy gap is observed between the first and second detachment features, indicating a highly stable neutral PrB₇. Global minimum searches and comparison between experiment and theory show that PrB₇⁻ has a half‐sandwich structure with C_6v symmetry. Chemical bonding analyses show that PrB₇⁻ can be viewed as a Pr^II[η⁷‐B₇³⁻] complex with three unpaired electrons, corresponding to a Pr (4f²6s¹) open‐shell configuration. Upon detachment of the 6s electron, the neutral PrB₇ cluster is a highly stable Pr^III[η⁷‐B₇³⁻] complex with Pr in its favorite +3 oxidation state. The B₇³⁻ ligand is found to be highly stable and doubly aromatic with six delocalized π and six delocalized σ electrons and should exist for a series of lanthanide M^III[η⁷‐B₇³⁻] complexes.     

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

The [B₁₂H₁₂]^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. Pt²⁺). Until now, it was impossible to structurally characterize a metal‐substituted [B₁₂H₁₂]^2? cage. When an aqueous solution containing both Bi³⁺ cations and [B₁₂H₁₂]^2? anions was heated, the charge‐neutral bismuth undecahydro‐closo‐dodecaborane BiB₁₂H₁₁ was obtained, representing a new class of metalated [B₁₂H₁₂]^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.     

Cyanoisocyanoacetylene,N≡C−C≡C−N≡C

Vacuum pyrolysis of the precursor complex [(CO)₅Cr(CN−CCl=CF−CN)] resulted in the isolation and structure elucidation by molecular spectroscopy of isomer 1 . According to ab initio calculations 1 is 109 kJ mol⁻¹ less stable than NC−C≡C−CN, which has been known for some time. An accurate equilibrium structure for 1 has been determined with mixed experimental and theoretical methods.     

The Hexacyanodiborane(6) Dianion [B2(CN)6]2−

Diborane(6) dianions with substituents that are bonded to boron via carbon are very reactive and therefore only a few examples are known. Diborane(6) derivatives are the simplest catenated boron compounds with an electron‐precise B–B σ‐bond that are of fundamental interest and of relevance for material applications. The homoleptic hexacyanodiborane(6) dianion [B₂(CN)₆]²⁻ that is chemically very robust is reported. The dianion is air‐stable and resistant against boiling water and anhydrous hydrogen fluoride. Its salts are thermally highly stable, for example, decomposition of (H₃O)₂[B₂(CN)₆] starts at 200 °C. The [B₂(CN)₆]²⁻ dianion is readily accessible starting from 1) B(CN)₃²⁻ and an oxidant, 2) [BF(CN)₃]⁻ and a reductant, or 3) by the reaction of B(CN)₃²⁻ with [BHal(CN)₃]⁻ (Hal=F, Br). The latter reaction was found to proceed via a triply negatively charged transition state according to an S_N2 mechanism.     

Triple Bonds Between Iron and Heavier Group 15 Elements in AFe(CO)3− (A=As,Sb, Bi) Complexes

Heteronuclear transition‐metal–main‐group‐element carbonyl complexes of AsFe(CO)₃⁻, SbFe(CO)₃⁻, and BiFe(CO)₃⁻ were produced by a laser vaporization supersonic ion source in the gas phase, and were studied by mass‐selected IR photodissociation spectroscopy and advanced quantum chemistry methods. These complexes have C_3v structures with all of the carbonyl ligands bonded on the iron center, and feature covalent triple bonds between bare Group 15 elements and Fe(CO)₃⁻. Chemical bonding analyses on the whole series of AFe(CO)₃⁻ (A=N, P, As, Sb, Bi, Mc) complexes indicate that the valence orbitals involved in the triple bonds are hybridized 3d and 4p atomic orbitals of iron, leading to an unusual (dp–p) type of transition‐metal–main‐group‐element multiple bonding. The σ‐type three‐orbital interaction between Fe 3d/4p and Group 15 np valence orbitals plays an important role in the bonding and stability of the heavier AFe(CO)₃⁻ (A=As, Sb, Bi) complexes.     

An Isolable Potassium Salt of a Borasilene–Chloride Adduct

Among the variety of isolable compounds with multiple bonds involving silicon, examples of compounds that contain silicon–boron double bonds (borasilenes) still remain relatively rare. Herein, we report the synthesis of the potassium salt of a chloride adduct of borasilene 1 ([ 2 ]⁻), which was obtained as an orange crystalline solid. Single‐crystal X‐ray diffraction analysis and reactivity studies on [ 2 ]⁻ confirmed the double‐bond character of the Si=B bond as well as the reduced Lewis acidity, which is due to the coordination of Cl⁻ to the boron center. A thermal reaction of [ 2 ]⁻ afforded a bicyclic product by formal intramolecular C−H insertion across the Si=B bond of 1 , which was corroborated by a theoretical study.     

[SnBi3]5− —A Carbonate Analogue Comprising Exclusively Metal Atoms

The new [SnBi₃]⁵⁻ polyanion is obtained by the reaction of K₃Bi₂ with K₄Sn₉ or K₁₂Sn₁₇ 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.     

Approach to thermal properties and electronic polarizability from average single bond strength in ZnOBi2O3B2O3 glasses

The glass transition temperature (T_g), density, refractive index, Raman scattering spectra, and X-ray photoelectron spectra (XPS) for xZnO-yBi₂O₃-zB₂O₃ glasses (x=10-65, y=10-50, z=25-60 mol%) are measured to clarify the bonding and structure features of the glasses with large amounts of ZnO. The average electronic polarizability of oxide ions (α_O2−) and optical basicity (Λ) of the glasses estimated using Lorentz-Lorenz equation increase with increasing ZnO or Bi₂O₃ content, giving the values of α_O2−=1.963 Å³ and Λ=0.819 for 60ZnO-10Bi₂O₃-30B₂O₃ glass. The formation of BOBi and BOZn bridging bonds in the glass structure is suggested from Raman and XPS spectra. The average single bond strength (B_MO) proposed by Dimitrov and Komatsu is applied to the glasses and is calculated using single bond strengths of 150.6 kJ/mol for ZnO bonds in ZnO₄ groups, 102.5 kJ/mol for BiO bonds in BiO₆ groups, 498 kJ/mol for BO bonds in BO₃ groups, and 373 kJ/mol for BO bonds in BO₄ groups. Good correlations are observed between T_g and B_MO, Λ and B_MO, and T_g and Λ, proposing that the average single bond strength is a good parameter for understanding thermal and optical properties of ZnOBi₂O₃B₂O₃ glasses.     

Tetrabutylammonium Salts of Aluminum(III) and Gallium(III) Phthalocyanine Radical Anions Bonded with Fluoren‐9‐olato− Anions and Indium(III) Phthalocyanine Bromide Radical Anions

Reduction of aluminum(III), gallium(III), and indium(III) phthalocyanine chlorides by sodium fluorenone ketyl in the presence of tetrabutylammonium cations yielded crystalline salts of the type (Bu₄N⁺)₂[M^III(HFl−O⁻)(Pc^.3−)]^.−(Br⁻) ⋅ 1.5 C₆H₄Cl₂ [M=Al ( 1 ), Ga ( 2 ); HFl−O⁻=fluoren‐9‐olato⁻ anion; Pc=phthalocyanine] and (Bu₄N⁺) [In^IIIBr(Pc^.3−)]^.− ⋅ 0.875 C₆H₄Cl₂ ⋅ 0.125 C₆H₁₄ ( 3 ). The salts were found to contain Pc^.3− radical anions with negatively charged phthalocyanine macrocycles, as evidenced by the presence of intense bands of Pc^.3− in the near‐IR region and a noticeable blueshift in both the Q and Soret bands of phthalocyanine. The metal(III) atoms coordinate HFl−O⁻ anions in 1 and 2 with short Al−O and Ga−O bond lengths of 1.749(2) and 1.836(6) Å, respectively. The C−O bonds [1.402(3) and 1.391(11) Å in 1 and 2 , respectively] in the HFl−O⁻ anions are longer than the same bond in the fluorenone ketyl (1.27–1.31 Å). Salts 1 – 3 show effective magnetic moments of 1.72, 1.66, and 1.79 μ_B at 300 K, respectively, owing to the presence of unpaired S= 1/2 spins on Pc^.3−. These spins are coupled antiferromagnetically with Weiss temperatures of −22, −14, and −30 K for 1 – 3 , respectively. Coupling can occur in the corrugated two‐dimensional phthalocyanine layers of 1 and 2 with an exchange interaction of J /k _B=−0.9 and −1.1 K, respectively, and in the π‐stacking {[In^IIIBr(Pc^.3−)]^.−}₂ dimers of 3 with an exchange interaction of J /k _B=−10.8 K. The salts show intense electron paramagnetic resonance (EPR) signals attributed to Pc^.3−. It was found that increasing the size of the central metal atom strongly broadened these EPR signals.     

The Na⋅⋅⋅B Bond in NaBH3−: A Different Type of Bond

A newly introduced Na−B bond in NaBH₃⁻ has been a challenge for the chemical bonding community. Here, a series of MBH₃⁻ (M=Li, Na, K) species and NaB(CN)₃⁻ are studied within the context of quantum chemical topology approaches. The analyses suggest that M–B interaction cannot be classified as an ordinary covalent, dative, or even simple ionic interaction. The interactions are controlled by coulombic forces between the metals and the substituents on boron, for example, H or CN, more than the direct M–B interaction. On the other hand, while the characteristics of the (3, −1) critical points of the bonds are comparable to weak hydrogen bonds, not covalent bonds, the metal and boron share a substantial sum of electrons. To the best of the author's knowledge, the characteristics of these bonds are unprecedented among known molecules. Considering all paradoxical properties of these bonds, they are herein described as ionic-enforced covalent bonds.     

Synthesis of a Doubly Boron‐Doped Perylene through NHC‐Borenium Hydroboration/C−H Borylation/Dehydrogenation

Reaction of an N‐heterocyclic carbene (NHC)–borenium ion with 9,10‐distyrylanthracene forms four B−C bonds through two selective, tandem hydroboration–electrophilic C−H borylations to yield an isolable, crystallographically characterizable polycyclic diborenium ion as its [NTf₂]⁻ salt ( 1 ). Dehydrogenation of 1 with TEMPO radical followed by acidic workup yields a 3,9‐diboraperylene as its corresponding borinic acid ( 2 ). This sequence can be performed in one pot to allow the facile, metal‐free conversion of an alkene into a small molecule containing a boron‐doped graphene substructure. Doubly boron‐doped perylene 2 exhibits visible range absorbance and fluorescence in chloroform solution (Φ =0.63) and undergoes two reversible one‐electron reductions at moderate potentials of −1.30 and −1.64 eV vs. ferrocenium/ferrocene in DMSO. Despite sterically accessible boron centers and facile electrochemical reductions, compound 2 is air‐, moisture‐, and silica gel‐stable.     

Synergy of Bi2O3 and RuO2 Nanocatalysts for Low-Overpotential and Wide pH-Window Electrochemical Ammonia Synthesis

Electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions is still seriously impeded by the inferior NH₃ yield and low Faradaic efficiency, especially at low overpotentials. Herein, we report the synthesis of nano-sized RuO₂ and Bi₂O₃ particles grown on functionalized exfoliated graphene (FEG) through in situ electrodeposition, denoted as RuO₂−Bi₂O₃/FEG. The prepared self-supporting RuO₂−Bi₂O₃/FEG hybrid with a Bi mass loading of 0.70 wt% and Ru mass loading of 0.04 wt% shows excellent NRR performance at low overpotentials in acidic, neutral and alkaline electrolytes. It achieves a large NH₃ yield of 4.58±0.16 μg_NH3 h⁻¹ cm⁻² with a high Faradaic efficiency of 14.6 % at −0.2 V versus reversible hydrogen electrode in 0.1 M Na₂SO₄ electrolyte. This performance benefits from the synergistic effect between Bi₂O₃ and RuO₂ which respectively have a fairly strong interaction of Bi 6p orbitals with the N 2p band and abundant supply of *H, as well as the binder-free characteristic and the convenient electron transfer via graphene nanosheets. This work highlights a new electrocatalyst design strategy that combines transition and main-group metal elements, which may provide some inspirations for designing low-cost and high-performance NRR electrocatalysts in the future.     

Coaxial Triple‐Layered versus Helical Be6B11− Clusters: Dual Structural Fluxionality and Multifold Aromaticity

Two low‐lying structures are unveiled for the Be₆B₁₁⁻ nanocluster system that are virtually isoenergetic. The first, triple‐layered cluster has a peripheral B₁₁ ring as central layer, being sandwiched by two Be₃ rings in a coaxial fashion, albeit with no discernible interlayer Be−Be bonding. The B₁₁ ring revolves like a flexible chain even at room temperature, gliding freely around the Be₆ prism. At elevated temperatures (1000 K), the Be₆ core itself also rotates; that is, two Be₃ rings undergo relative rotation or twisting with respect to each other. Bonding analyses suggest four‐fold (π and σ) aromaticity, offering a dilute and fluxional electron cloud that lubricates the dynamics. The second, helix‐type cluster contains a B₁₁ helical skeleton encompassing a distorted Be₆ prism. It is chiral and is the first nanosystem with a boron helix. Molecular dynamics also shows that at high temperature the helix cluster readily converts into the triple‐layered one.     

Hexaaquacobalt(II) and hexaaquanickel(II) bis(μ‐pyridine‐2,6‐dicarboxylato)bis[(pyridine‐2,6‐dicarboxylato)bismuthate(III)] dihydrate

The title complexes, hexaaquacobalt(II) bis(μ‐pyridine‐2,6‐dicarboxylato)bis[(pyridine‐2,6‐dicarboxylato)bismuthate(III)] dihydrate, [Co(H₂O)₆][Bi₂(C₇H₄NO₄)₄]·2H₂O, (I), and hexaaquanickel(II) bis(μ‐pyridine‐2,6‐dicarboxylato)bis[(pyridine‐2,6‐dicarboxylato)bismuthate(III)] dihydrate, [Ni(H₂O)₆][Bi₂(C₇H₄NO₄)₄]·2H₂O, (II), are isomorphous and crystallize in the triclinic space group P. The transition metal ions are located on the inversion centre and adopt slightly distorted MO₆ (M = Co or Ni) octahedral geometries. Two [Bi(pydc)₂]⁻ units (pydc is pyridine‐2,6‐dicarboxylate) are linked via bridging carboxylate groups into centrosymmetric [Bi₂(pydc)₄]²⁻ dianions. The crystal packing reveals that the [M(H₂O)₆]²⁺ cations, [Bi₂(pydc)₄]²⁻ anions and solvent water molecules form multiple hydrogen bonds to generate a supramolecular three‐dimensional network. The formation of secondary Bi...O bonds between adjacent [Bi₂(pydc)₄]²⁻ dimers provides an additional supramolecular synthon that directs and facilitates the crystal packing of both (I) and (II).     

A S2O32−‐Enhanced Fluorogenic Chemosensor for Fe3+ in Aqueous Media Based on a Boron–Fluorine Derivative

A fluorescent “turn‐on” probe for Fe³⁺ was investigated in an aqueous system based on a boron 2‐(2′‐pyridyl) imidazole complex (BOPIM‐dma). BOPIM‐dma shows weak or no fluorescence in polar solvents due to twisted intramolecular charge transfer, but the addition of Fe³⁺ to BOPIM‐dma leads to fluorescence switch‐on responses. The binding is highly selective to Fe³⁺ over other metal ions, indicating that BOPIM‐dma is a chemodosimeter for Fe³⁺. Furthermore, the existence of S₂O₃²⁻ could much enhance and stabilize the emission significantly, indicating that the BOPIM‐dma/Fe³⁺/S₂O₃²⁻ complexes are a strong fluorescence system, and can be used as a sensitive detector for Fe³⁺, with the limit of detection of 6.0 × 10⁻⁷ mol L⁻¹.     

High‐Resolution Photoelectron Imaging of IrB3−: Observation of a π‐Aromatic B3+ Ring Coordinated to a Transition Metal

In a high‐resolution photoelectron imaging and theoretical study of the IrB₃^? cluster, two isomers were observed experimentally with electron affinities (EAs) of 1.3147(8) and 1.937(4) eV. Quantum calculations revealed two nearly degenerate isomers competing for the global minimum, both with a B₃ ring coordinated with the Ir atom. The isomer with the higher EA consists of a B₃ ring with a bridge‐bonded Ir atom (C_s , ²A′), and the second isomer features a tetrahedral structure (C_3v , ²A₁). The neutral tetrahedral structure was predicted to be considerably more stable than all other isomers. Chemical bonding analysis showed that the neutral C_3v isomer involves significant covalent Ir?B bonding and weak ionic bonding with charge transfer from B₃ to Ir, and can be viewed as an Ir–(η³‐B₃⁺) complex. This study provides the first example of a boron‐to‐metal charge‐transfer complex and evidence of a π‐aromatic B₃⁺ ring coordinated to a transition metal.     

The Existence of a Designer Al=Al Double Bond in the LiAl2H4− Cluster Formed by Electronic Transmutation

The Al=Al double bond is elusive in chemistry. Herein we report the results obtained via combined photoelectron spectroscopy and ab initio studies of the LiAl₂H₄⁻ cluster that confirm the formation of a conventional Al=Al double bond. Comprehensive searches for the most stable structures of the LiAl₂H₄⁻ cluster have shown that the global minimum isomer I possesses a geometric structure which resembles that of Si₂H₄, demonstrating a successful example of the transmutation of Al atoms into Si atoms by electron donation. Theoretical simulations of the photoelectron spectrum discovered the coexistence of two isomers in the ion beam, including the one with the Al=Al double bond.